**Catalytic Conversions of Biomass-Derived Furaldehydes Toward Biofuels**

Shun Nishimura and Kohki Ebitani

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67805

#### **Abstract**

Upgrading of biomass resources toward high-energy compounds (biofuel) is a crucial technology for sustainable development because utilizations of biomass resources can contribute to the low CO2 emission on the basis of carbon neutral concept. In this chapter, recent advances on catalytic hydrogenation and hydrogenolysis of biomass-derived furaldehydes, dehydration products of saccharides, for example, called as hydroxymethylfuran (HMF) and furfural, toward biofuels over heterogeneous catalytic system are introduced. Some approaches on mechanistic study and reactor design are also mentioned in this chapter.

**Keywords:** biomass, furaldehydes, biofuel, hydrogenation, hydrogenolysis

#### **1. Introduction**

In the past decades, owing to the rise of a living standard through social infrastructure development in the world, the energy demand growth has been stronger with increase of global gross domestic product (GDP), and it would rise further 30% till 2040 [1, 2]. While concerns on global warming derived from CO2 emission have been debated in Intergovernmental Panel on Climate Change (IPCC) and Conference of the Parties (COP), they established global guidelines to participate in a sustainable development toward "zero emission." New energy sources and technologies have been current; however, it is still a great challenge to fabricate a low carbon society where high quality of life is constructed with low CO2 emission. The rate of renewable energy including solar, geothermal, waste, wind, biofuels, wood, and hydroelectric power was only below 10% in the total primary energy consumption diagram still now [1].

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A facile transformation of biomass-based resources is one of the crucial technologies to reduce the emission of CO2 according to the carbon neutral concept; that is, the amount of CO2 emission owing to biomass consumption (burning) is equal to the amount of CO2 storage during its growth (photosynthesis). Thus, the balance of CO2 concentration in the atmosphere on the basis of biomass utilization process is scarcely influenced. A lot of efforts have put into the investigations of new biomass transformation processes and derived technologies (e.g., see [3–8]). In this chapter, recent advanced catalytic transformations of biomass-based furaldehydes (furfurals) toward biofuels *via* hydrogenation/hydrogenolysis are introduced.

#### **2. Transfer hydrogenation of furaldehydes to biofuels**

A schematic reaction pathway for transfer hydrogenation/hydrogenolysis of biomass-based furaldehydes toward biofuels and chemicals is summarized in **Scheme 1**. Investigations on the synthesis of tetrahydrofuran (THF), tetrahydrofurfuryl alcohol (THFA) [9, 10], and 2-methyltetrahydrofuran (2-MTHF) [11] are attractive to serve biomass-based green solvent and precursor for aliphatic alcohols such as 1-butanol, 1,5-poentandiol, and 2-pentanol, respectively [12]. Moreover, furaldehyde-based diols of 2,5-bis(hydroxymethyl)furan (BHMF) [13–15] and 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF) [16–18] are the candidate for biomassderived monomer source. Note, BHMF is also called as 2,5-dihydroxymethylfuran (DHMF) as a popular style. Accordingly, the strategy to rule such competitive reactions among reduction,

**Scheme 1.** Schematic reaction pathway for transfer hydrogenation/hydrogenolysis of biomass-based furaldehydes toward biofuels and chemicals.

ring-hydrogenation, hydrogenolysis, and decarbonylation is the key factor in the biorefinery on the transfer hydrogenation of furaldehydes.

The products of 2-methylfuran (2-MF), 2,5-dimethylfuran (DMF), and 2,5-dimethyltetrahydrofuran (DMTHF) have been counted on as biomass-based fuels (biofuels) as well as conventional biomass-derived transportation fuels such as bioethanol and biodiesel. These furan-based biofuels possess high-energy density [lower heating value (LHV): 28–32 MJ L−1], low volatility [higher bp. (336–367 K) compared to ethanol], and good combustibility [research octane number (RON): 82–131]. Additionally, these are immiscible with water and stable compound in stage and thus would be an easier blender in gasoline than bioethanol. The DMF applications as automotive fuel have been challenged on a single-cylinder or multicylinder gasoline engines [19–21].

#### **2.1. Synthesis of DMF by HMF hydrogenolysis**

Synthesis of DMF from HMF is one of major researches for biofuel productions. First of all, studies on the synthesis of DMF from HMF hydrogenolysis (**Scheme 2**) have been introduced in the following five contents.

#### *2.1.1. At pressurized hydrogen condition*

A facile transformation of biomass-based resources is one of the crucial technologies to reduce

basis of biomass utilization process is scarcely influenced. A lot of efforts have put into the investigations of new biomass transformation processes and derived technologies (e.g., see [3–8]). In this chapter, recent advanced catalytic transformations of biomass-based furalde-

A schematic reaction pathway for transfer hydrogenation/hydrogenolysis of biomass-based furaldehydes toward biofuels and chemicals is summarized in **Scheme 1**. Investigations on the synthesis of tetrahydrofuran (THF), tetrahydrofurfuryl alcohol (THFA) [9, 10], and 2-methyltetrahydrofuran (2-MTHF) [11] are attractive to serve biomass-based green solvent and precursor for aliphatic alcohols such as 1-butanol, 1,5-poentandiol, and 2-pentanol, respectively [12]. Moreover, furaldehyde-based diols of 2,5-bis(hydroxymethyl)furan (BHMF) [13–15] and 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF) [16–18] are the candidate for biomassderived monomer source. Note, BHMF is also called as 2,5-dihydroxymethylfuran (DHMF) as a popular style. Accordingly, the strategy to rule such competitive reactions among reduction,

**Scheme 1.** Schematic reaction pathway for transfer hydrogenation/hydrogenolysis of biomass-based furaldehydes

sion owing to biomass consumption (burning) is equal to the amount of CO2

**2. Transfer hydrogenation of furaldehydes to biofuels**

hydes (furfurals) toward biofuels *via* hydrogenation/hydrogenolysis are introduced.

its growth (photosynthesis). Thus, the balance of CO2

according to the carbon neutral concept; that is, the amount of CO2

emis-

storage during

concentration in the atmosphere on the

the emission of CO2

52 Green Chemical Processing and Synthesis

toward biofuels and chemicals.

Chidambaran and Bell provided 15% yield of DMF from hydroxymethylfuran (HMF) over Pd/C catalyst in ionic liquid/acetonitrile mixed solution under pressured H2 (6.2 MPa) [22]. Recently, Saha and co-workers achieved 85% yield for DMF from HMF *via* BHMF intermediate with combined use of Pd/C and Lewis acid ZnCl2 at lower pressurized H2 (0.8 MPa) [23]. The improvement induced by ZnCl2 agent was due to the facilitation on the rate-determinating step of BHMF to DMF *via* cleavage of C─O bonds. The synergism with Zn2+ was varied by the metal center: Pd/C/ZnCl2 was faster than Ru/C/ZnCl2 , and Ni/C shows poor synergism. Interestingly, BHMF was identified as an intermediate product during the reaction, and it was 52% yield at maximum with the Ru/C/ZnCl2 system.

Ru-supported catalyst is examined by several groups in HMF hydrogenation at pressurized H2 in recent days. Zu *et al.* investigated Ru/Co3 O4 catalyst and conducted 93% yield for DMF at 403 K and 0.7 MPa H2 [24]. In this system, Ru is responsible for hydrogenation, while CoO*<sup>x</sup>* works for the adsorption of hydrogenation product and then breaks the C─O bond. Nagpure *et al.* suggested Ru-NaY zeolite composed with Ru particles (Av. 2.8 nm) on the external surface served 78% yield of DMF under H2 (1.5 MPa) at 493 K in a short duration of the reaction (1 h) [25]. According to the TOF value in DMF production at the

**Scheme 2.** Synthesis of DMF from HMF hydrogenolysis.

same condition, the reactivity of metal was expected as following order Ni (5.2 h−1) < Cu (6.4 h−1) < Au (16.4 h−1) < Pt (60.8 h−1) < Rh (80.2 h−1) < Pd (98.6 h−1) < Ru (156.0 h−1), though the detailed information of size or surface area of other metals was not shown. The Lewis acidity of NaY zeolite was expected to increase deoxygenation ability and helped to improve the DMF yield in the reaction. On the other hand, Hu *et al.* gave 95% for DMF production over Ru/C catalyst at 473 K and 2 MPa H2 [26]. The acid sites on carbon support supposedly promoted the hydrogenation of HMF to DMF. In addition, the authors also examined a combined use of ionic liquid (1-butyl-3-methylimidazolium chloride) and cellulose-derived sulfonated carbonaceous catalyst for HMF production from biomass-derived carbohydrates and successive hydrogenation of the extracted HMF to DMF over Ru/C catalyst, and then, more than 82% DMF yield was successfully obtained. It was generally known in the case of Ru catalyst that the reactivation by heating under H2 flow is required to remove the depositions of high-molecular-weight by-products blocking the active Ru sites [26, 27].

In the case of Ni catalyst, the acidity of support is also expected to be the key function for deoxygenation step. Huang *et al.* explained the Ni particles mainly played a role in the hydrogenation step but had limited deoxygenation, whereas W2 C particles mainly promoted the deoxygenation step of hydroxymethyl group but had limited hydrogenation ability. They have served 96% yield of DMF by using the synergy between Ni and W2 C particles at 4 MPa H2 [28]. It should be noted that the observed major intermediate was not BHMF but 5-MF in this study.

#### *2.1.2. With hydrogen donor agent*

Because the concentration of other undesirable byproducts *via* ring-open and ring hydrogenation such as BHMTHF, 5-methyltetrahydrofuran (5-MTHFA), DMTHF, and hexanediol increased at higher H2 pressure [24, 29, 30], several approaches instead of the utilization of pressurized H2 agent had been examined to build up further possible way in the catalytic system. Jae *et al.* applied 2-propanol as a hydride donor for Ru/C-catalyzed HMF hydrogenation and gave 81% yield for DMF at 463 K under pressurized N2 (2.04 MPa) [27]. Two ethers *via* etherification of 2-propanol and BHMF or 5-MFA (denoted as ethers 2 and 3 in **Scheme 3**) were detected in the reaction, and the expected reaction network in the presence of 2-propanol

**Scheme 3.** Reaction network of the hydrogenation of HMF into DMF in the presence of 2-propanol.

is described in **Scheme 3**. Nagpure *et al.* demonstrated Ru-Mg(Al)O catalyst with 58% yield of DMF at 493 K for 4 h at 1 MPa H2 assisted by hydrogen transfer from 2-propanol solvent [30]. Continuous-flow transfer hydrogenation/hydrogenolysis of HMF in 2-propanol yielded 72% DMF with Pd/Fe2 O3 catalyst at 2.5 MPa pressure and 453 K through BHMF production at an initial stage (70% sel. at 50% conv.) [31]. Utilization of hydrogen donor agent would be a versatile strategy for the reaction.

One of the dramatic effects was found when the formic acid (HCOOH) agent was applied for the reaction. Rauchfuss *et al.* had surprisingly announced that decarbonylation of HMF to DMF (>95% yield) *via* BHMF formation route was proceeded by Pd/C catalyst and formic acid (10 equiv.) in refluxing THF/H<sup>2</sup> SO4 (0.13 equiv.) or dioxane (at 393 K) through diformate ester form (**Scheme 4**). The formic acid agent plays the multiroles, inhibits the decarbonylation and ring-hydrogenation, and serves mild hydrogen source and a precursor to formate ester [32, 33]. De *et al.* described Ru/C-catalyzed DMF formation from fructose with similar catalytic system, and 30% yield was obtained [34]. They also announced the positive effect of irradiation treatment (at 300 W) during reaction to significantly decrease the time for reaction.

#### *2.1.3. Under supercritical conditions*

same condition, the reactivity of metal was expected as following order Ni (5.2 h−1) < Cu (6.4 h−1) < Au (16.4 h−1) < Pt (60.8 h−1) < Rh (80.2 h−1) < Pd (98.6 h−1) < Ru (156.0 h−1), though the detailed information of size or surface area of other metals was not shown. The Lewis acidity of NaY zeolite was expected to increase deoxygenation ability and helped to improve the DMF yield in the reaction. On the other hand, Hu *et al.* gave 95% for DMF production

promoted the hydrogenation of HMF to DMF. In addition, the authors also examined a combined use of ionic liquid (1-butyl-3-methylimidazolium chloride) and cellulose-derived sulfonated carbonaceous catalyst for HMF production from biomass-derived carbohydrates and successive hydrogenation of the extracted HMF to DMF over Ru/C catalyst, and then, more than 82% DMF yield was successfully obtained. It was generally known in the case of

In the case of Ni catalyst, the acidity of support is also expected to be the key function for deoxygenation step. Huang *et al.* explained the Ni particles mainly played a role in the hydro-

deoxygenation step of hydroxymethyl group but had limited hydrogenation ability. They have

It should be noted that the observed major intermediate was not BHMF but 5-MF in this study.

Because the concentration of other undesirable byproducts *via* ring-open and ring hydrogenation such as BHMTHF, 5-methyltetrahydrofuran (5-MTHFA), DMTHF, and hexanediol

system. Jae *et al.* applied 2-propanol as a hydride donor for Ru/C-catalyzed HMF hydrogena-

*via* etherification of 2-propanol and BHMF or 5-MFA (denoted as ethers 2 and 3 in **Scheme 3**) were detected in the reaction, and the expected reaction network in the presence of 2-propanol

pressure [24, 29, 30], several approaches instead of the utilization of

agent had been examined to build up further possible way in the catalytic

tions of high-molecular-weight by-products blocking the active Ru sites [26, 27].

[26]. The acid sites on carbon support supposedly

flow is required to remove the deposi-

C particles mainly promoted the

C particles at 4 MPa H2

(2.04 MPa) [27]. Two ethers

[28].

over Ru/C catalyst at 473 K and 2 MPa H2

54 Green Chemical Processing and Synthesis

*2.1.2. With hydrogen donor agent*

increased at higher H2

pressurized H2

Ru catalyst that the reactivation by heating under H2

genation step but had limited deoxygenation, whereas W2

served 96% yield of DMF by using the synergy between Ni and W2

tion and gave 81% yield for DMF at 463 K under pressurized N2

**Scheme 3.** Reaction network of the hydrogenation of HMF into DMF in the presence of 2-propanol.

As a different approach, Chatterjee *et al.* investigated the Pd/C catalyst under supercritical carbon dioxide (scCO2 ) condition with various operation conditions [29]. Interestingly, they found that the scCO2 pressure strongly contributed the selectivity for the HMF hydrogenation at 353 K and 1 MPa H2 for 2 h (**Figure 1**). The selectivity for DMF was increased from 42 to 100% along with scCO2 pressure increased from 4 to 10 MPa. 5-MTHFA was formed with a comparatively higher selectivity of 58% at 4 MPa and then decreased with pressure. At higher pressure (>14 MPa), DMTHF became the dominant product with about 70% selectivity. Controlled experiments with different CO<sup>2</sup> /H2 O molar ratio indicated that (*i*) in the absence of H2 O or CO2 , the reaction proceeds through the path of HMF → 5-MFA → 5-MTHFA, and (*ii*) combined effect of CO<sup>2</sup> and H2 O forced the reaction to move in the direct of HMF → DMF. Weak acidic condition derived from CO2 dissolved in water would be one of the crucial factors

**Scheme 4.** Reaction network of the hydrogenation of HMF into DMF in the presence of formic acid.

**Figure 1.** Effect of CO<sup>2</sup> pressure on the conversion and product profile. Reaction conditions: catalyst: substrate = 1:5, 353 K, 2 h, 1 MPa (H2 ), water (1 mL). Reproduced with permission from The Royal Society of Chemical (RSC) of Ref. [29].

behind the hydrogenation of HMF to DMF. This catalytic system could be applied for furfural hydrogenation and 100% yield of 5-MF or 2-MTHF was achieved under optimized condition.

Hansen *et al.* demonstrated Cu-doped porous Mg-Al-O*<sup>x</sup>* (Cu-PMO) catalyst with supercritical methanol in a stainless steel bomb reactor [35]. According to their concept, the rapid deoxygenation of HMF in the presence of Cu-PMO drastically diminished undesired side reactions such as polymerizations and condensations. Total yield of three main products of DMF, DMTHF, and 2-hexanol was reached to be 61%. The maximum yield for DMF was 48% at 533 K for 3 h reaction.

#### *2.1.4. Under ambient operation conditions*

In order to decrease the operation risks owing to the utilization of pressurized and/or hightemperature conditions, application of ambient condition for the target reaction is an ideal system. Moreover, as an additional issue, the undesired formation of insoluble humin is often observed and decreased the yield in furaldehyde utilizations at elevated reaction temperature [36]. One of impressive approaches under ambient operation conditions was reported by Bekkum *et al.* They have studied HMF hydrogenolysis under atmospheric H2 at 333 K with Pd/C catalyst and gave 35.7% yield of DMF in 1-propanol solvent through propyl ether intermediates formation; the 2-methyl-5-(propoxymethyl)furan was detected in the initial stage with high yield (>80%) [37], while in 1,4-dioxane, mainly BHMF is formed with 80% yield at the same reaction condition.

Bimetallic catalysts for upgrading of biomass resources into high-value fuels and chemicals were the one of research interests in biomass conversion in the last decade [38–40]. Such movements motivated researchers to investigate bimetallic catalytic system for HMF hydrogenation with an atmospheric pressurized H2 .

Ebitani *et al.* prepared the PdAu bimetallic nanoparticles supported carbon (Pd*<sup>x</sup>* Au*<sup>y</sup>* /C) catalysts with different Pd/Au molar ratio (*x*/*y*) and applied to the HMF hydrogenation in an atmospheric H2 in the presence of HCl agent [41]. In the monometallic catalysts, Pd100/C and Au100/C gave 60 and 0% yield for DMF yield, respectively. While, in the bimetallic PdAu series, increasing the Au contents in Pd*<sup>x</sup>* Au*<sup>y</sup>* leads to increase the DMF yield till Pd/Au = 50/50 (presumption) and then slightly decrease; that is, positive effect on the coexistence of Au in Pd active center in the reaction was observed. With consideration of XRD experiments, they proposed that the highest yield for DMF, 96% at ambient H2 condition with HCl agent at 333 K, would be served over the Pd73Au27 sites formed on as-prepared Pd50Au50/C catalyst. It is expected that electronic and/or geometric changes in Pd, that is, the internal electronic transfer from Pd to another atom in agreement with the Pauling electronegativity scale and/ or the modification of Pd-Pd atomic distance due to the insertion of another atom would play a crucial role for Pd-alloy mediated reactions [42, 43]; however, the detailed mechanism is a subject for a further study in HMF hydrogenation over PdAu/C catalyst.

#### *2.1.5. Over bimetallic active sites*

behind the hydrogenation of HMF to DMF. This catalytic system could be applied for furfural hydrogenation and 100% yield of 5-MF or 2-MTHF was achieved under optimized condition.

pressure on the conversion and product profile. Reaction conditions: catalyst: substrate = 1:5,

), water (1 mL). Reproduced with permission from The Royal Society of Chemical (RSC) of Ref. [29].

methanol in a stainless steel bomb reactor [35]. According to their concept, the rapid deoxygenation of HMF in the presence of Cu-PMO drastically diminished undesired side reactions such as polymerizations and condensations. Total yield of three main products of DMF, DMTHF, and 2-hexanol was reached to be 61%. The maximum yield for DMF was 48% at

In order to decrease the operation risks owing to the utilization of pressurized and/or hightemperature conditions, application of ambient condition for the target reaction is an ideal system. Moreover, as an additional issue, the undesired formation of insoluble humin is often observed and decreased the yield in furaldehyde utilizations at elevated reaction temperature [36]. One of impressive approaches under ambient operation conditions was reported by

Pd/C catalyst and gave 35.7% yield of DMF in 1-propanol solvent through propyl ether intermediates formation; the 2-methyl-5-(propoxymethyl)furan was detected in the initial stage with high yield (>80%) [37], while in 1,4-dioxane, mainly BHMF is formed with 80% yield at

Bekkum *et al.* They have studied HMF hydrogenolysis under atmospheric H2

(Cu-PMO) catalyst with supercritical

at 333 K with

Hansen *et al.* demonstrated Cu-doped porous Mg-Al-O*<sup>x</sup>*

533 K for 3 h reaction.

**Figure 1.** Effect of CO<sup>2</sup>

56 Green Chemical Processing and Synthesis

353 K, 2 h, 1 MPa (H2

the same reaction condition.

*2.1.4. Under ambient operation conditions*

Approaches with bimetallic catalyst have been also investigated by other researches. Dumesic and coworkers studied the bimetallic Ru catalytic system for the reaction. The 46% yield of DMF from HMF was produced over RuSn/C catalyst at 473 K in the presence of lactone using a Parr reactor [44]. They also reported a systematic production of DMF using bimetallic RuCu/C catalyst in 1-butanol phase to afford 71% yield for 10 h at 393 K and 0.68 MPa H<sup>2</sup> . This vapor-phase hydrogenolysis is capable for a two-step reaction from fructose connected with biphasic reactor for *in-situ* formed HMF feeder [45].

Interestingly, Yu *et al.* found the combination of low-selective Ni species in high conversion and inactive Fe species for HMF hydrogenation by alloying contributed to the significant catalytic performance at 3.0 MPa H2 in *n*-butanol [46]. The selectivity to DMF showed a volcano-type behavior with increasing Fe loading, and the Ni2 Fe1 /CNT catalyst gave the highest 91% yield for DMF at 473 K for 3 h. The author also reported 96% yield toward BHMF production at lower temperature (393 K); that is, the reaction temperature markedly affected the product distribution. It was expected that the longer C─O bond on Ni-Fe than that on Ni might facilitate the conversion of HMF to DMF as the main route. Wang *et al.* also induced the favorable adsorption of the C═O or C─O bond over that of the C═C by applying PtCo bimetallic sites. They achieved 98% yield of DMF formation from HMF at 453 K and 1 MPa H2 over PtCo encapsulated in hollow carbon sphere (PtCo@HCS) catalyst in 1-butanol [47]. The author also indicated that the lower reaction temperature at 393 K served 70% yield of BHMF with PtCo@HCS catalyst.

A robust and highly active CuZn nanopowder (60 mesh) catalyst for the conversion of HMF to DMF was examined by Barta and coworkers [48]. They achieved 90% yield for DMF in concentrated HMF (10 wt%) of cyclopentyl methyl ether (CPME) solution at 493 K under pressurized H2 (20 bar). Catalytic conversion of HMF toward BHMF was also tested with same system, and >95% yield and selectivity were obtained with the CuZn nanopowder catalyst at 393 K and H2 (70 bar) in ethanol solvent.

Recently, Yang and coworkers provided 90% yield of DMF from HMF at 483 K with formic acid in a stainless steel autoclave under self-generated pressure [49]. In this report, a reaction route involving 5-MF instead of BHMF as an intermediate has been ascertained over NiCo/C catalyst. The reaction system proceeded over alloys with non-noble metals would be a futural target for next generation.

#### **2.2. Synthesis of DMTHF from HMF hydrogenation**

Owing to the ring-hydrogenation step, direct synthesis of DMTHF from HMF (**Scheme 5**) would require different strategies. Yang and Sen applied the water-soluble RhCl3 species with HI acid agent in the transformation of fructose to DMTHF in chlorobenzene/water biphasic reaction system under H2 (2.07 MPa) [50]. This powerful catalytic system served the end product of DMTHF or 2-MTHF from various saccharides such as fructose, glucose, sucrose, inulin, cellulose, and corn stover with significant activity (*ca.* 50–80% yield) at 433 K. Further mechanistic study on HMF conversion [51] revealed that the HI agent behaved as both acid catalyst for the initial dehydration of saccharides to the corresponding furans and as reducing agent in hydrogenolysis step of carbinol group in HMF *via* the formation of 5-iodomethylfurfural intermediate (**Scheme 6**). While the Rh species catalyze not only the C═O hydrogenolysis and C═C hydrogenation but also re-hydrogenation of produced iodine to HI with hydrogen. Overall, such efficient catalytic system, that is, the dehydration/reduction ability of HI combined with the hydrogenation/hydrogenolysis ability of the Rh catalyst, effectively facilitated the conversion of fructose to DMTHF.

Mitra *et al.* carefully surveyed the effects of HMF concentration, Pd loading and kinds of acidic additive in Pd/C-catalyzed hydrogenation/ hydrogenolysis of HMF at 0.21 MPa H2 in

**Scheme 5.** Synthesis of DMTHF from HMF hydrogenation/hydrogenolysis.

**Scheme 6.** HI-assisted hydrogenolysis of carbinol group *via* formation of iodo intermediate.

dioxane. Under the optimized condition, DMTHF (>95% yield) and DMF (85% yield) was produced in the presence of acetic acid and dimethyldicarbonate, respectively. It is likely that the tendency for ring-hydrogenation against hydrogenation/hydrogenolysis was decreased in higher HMF concentration and amount of catalyst loading [32].

It would be noted that though at higher pressurized H2 condition (70 bar), Pd/Al2 O3 (>99% yield), Pd/C (89% yield), and Ru/C (88% yield) are also found to be a good catalyst for DMTHF production from HMF in ethanol solvent at 393 K, as reported by Bottari *et al.* [48].

#### **2.3. Synthesis of DMF from 5-MF or 5-MFA hydrogenation**

concentrated HMF (10 wt%) of cyclopentyl methyl ether (CPME) solution at 493 K under

same system, and >95% yield and selectivity were obtained with the CuZn nanopowder cata-

Recently, Yang and coworkers provided 90% yield of DMF from HMF at 483 K with formic acid in a stainless steel autoclave under self-generated pressure [49]. In this report, a reaction route involving 5-MF instead of BHMF as an intermediate has been ascertained over NiCo/C catalyst. The reaction system proceeded over alloys with non-noble metals would be a futural

Owing to the ring-hydrogenation step, direct synthesis of DMTHF from HMF (**Scheme 5**) would

agent in the transformation of fructose to DMTHF in chlorobenzene/water biphasic reaction sys-

or 2-MTHF from various saccharides such as fructose, glucose, sucrose, inulin, cellulose, and corn stover with significant activity (*ca.* 50–80% yield) at 433 K. Further mechanistic study on HMF conversion [51] revealed that the HI agent behaved as both acid catalyst for the initial dehydration of saccharides to the corresponding furans and as reducing agent in hydrogenolysis step of carbinol group in HMF *via* the formation of 5-iodomethylfurfural intermediate (**Scheme 6**). While the Rh species catalyze not only the C═O hydrogenolysis and C═C hydrogenation but also re-hydrogenation of produced iodine to HI with hydrogen. Overall, such efficient catalytic system, that is, the dehydration/reduction ability of HI combined with the hydrogenation/hydrogenolysis ability of the Rh catalyst, effectively facilitated the conversion of fructose to DMTHF. Mitra *et al.* carefully surveyed the effects of HMF concentration, Pd loading and kinds of acidic additive in Pd/C-catalyzed hydrogenation/ hydrogenolysis of HMF at 0.21 MPa H2

(2.07 MPa) [50]. This powerful catalytic system served the end product of DMTHF

(70 bar) in ethanol solvent.

require different strategies. Yang and Sen applied the water-soluble RhCl3

**2.2. Synthesis of DMTHF from HMF hydrogenation**

**Scheme 5.** Synthesis of DMTHF from HMF hydrogenation/hydrogenolysis.

**Scheme 6.** HI-assisted hydrogenolysis of carbinol group *via* formation of iodo intermediate.

(20 bar). Catalytic conversion of HMF toward BHMF was also tested with

species with HI acid

in

pressurized H2

tem under H2

lyst at 393 K and H2

58 Green Chemical Processing and Synthesis

target for next generation.

5-MF based transformations to DMF (**Scheme 7**) has received little attention. The condensedphase C─O hydrogenolysis of 5-MFA to DMF at 493 K was carried out by the copper chromite (CuCr2 O4 ·CuO) catalyst pre-reduced with H2 at various temperature (513–633 K) [52]. The authors selected the simplified system in order to examine the relationship of the Cu oxidation state and C─O hydrogenolysis activity which was the one of intermediate steps during hydrogenation/hydrogenolysis of biomass-derived HMF or 5-MF toward DMF. The surface concentration of Cu0 and Cu+ was varied by a reduction temperature, and reduction at 573 K caused the highest activity, an initial rate for the production of DMF and DMTHF divided by BET surface area of catalyst was 108 mmol m−2 min−1 and the highest concentration of Cu0 and Cu+ sites. It concluded that the Cu0 was primarily responsible for the activity; however, unfortunately, the detailed role of Cu+ could not be ruled out at that time.

#### **2.4. Synthesis of 2-MF and 2-MTHF from furfural or FFA hydrogenation**

Furfural derived reaction path for 2-MF and 2-MTHF production (**Scheme 8**) has attracted many scientists. The gas-phase selective cleaving the C═O bond outside the furan ring of furfural was performed on Mo2 C [53]. It was considered that the strong interaction between

**Scheme 8.** Synthesis of 2-MF and 2-MTHF from furfural or FFA hydrogenation.

C─O bond of furfural in the aldehyde group and Mo2 C surface-assisted selective deoxygenation affording to 60% selectivity for 2-MF production, although the conversion of furfural seems to be low. Zhu *et al.* investigated a fixed-bed reactor test of furfural hydrogenation over a commercial catalyst (Cu/Zn/Al/Ca/Na = 59/33/61/1/1, atomic ratio) [54]. The 87% yield from furfural and 93% yield from FFA were obtained toward 2-MF formation at 523 K. In the same group, recently, further advanced examination on a continuous two-step fixed-bed reactor system over catalysts utilizing a combination of acidic Hβ-zeolite for the former dehydrogenation step of xylose to furfural and Cu/ZnO/Al2 O3 for the latter hydrogenation steps of produced furfural in a mixture of γ-butyrolactone/water as solvent was attempted to the direct conversion of xylose to 2-MF *via* FFA formation [55]. It was observed that the main product can be simply tuned by the hydrogenation temperature, and the 87% yields of FFA and 2-MF was formed at 423 and 463 K under 0.1 MPa H2 , respectively. The 2-MF was frequently detected at higher reaction temperature in the FFA synthesis from hydrogenation of furfural with moderate yield (35%) [56].

In the FFA hydrogenation reaction, Pd/Fe2 O3 catalyst yielded 31% for 2-MF under batch conditions at a transfer hydrogenation of FFA in 2-propanol, though the conversion of furfural to 2-MF was scarcely occurred (13% yield) at 453 K [31]. The former value reached to be 76% in a continuous-flow reactor, while the Pd/TiO<sup>2</sup> catalyst showed 71 and 88% yield for 2-MF for constant and continuously controlled pressured H2 (0.3 MPa) condition in an autoclave reactor at room temperature (298 K) [57].

In order to avoid the use of high H2 pressure, Zhu *et al.* designed a two-stage packing fixedbed reactor system with Cu2 Si2 O5 (OH)2 (copper phyllosilicate) in the upper reactor conjugated with Pd/SiO2 catalyst in the bottom reactor [11]. The Cu2 Si2 O5 (OH)2 itself possessed an outstanding hydrogenation ability and gave 84.6% yield for hydrogenation-deoxygenation of furfural toward 2 MF, while Pd/SiO2 catalyst gave a significant hydrogenation performance for the C═C bonds in furan ring, serving 87.7% yield of ring-hydrogenation of 2 MF toward 2 MTHF at 443 K. Accordingly, such a dual solid catalyst system achieved up to 97.1% yield on the direct production for 2-MTHF with 100% conversion of furfural under atmospheric pressure (**Figure 2**). Thereafter, bimetallic CuPd-supported ZrO2 catalyzed hydrogenation by using a batch reactor in the presence of 2-propanol as hydrogen donor was highlighted by other research group [58]. The molecular ratio of Cu/Pd gave significant influences on the yields for 2-MF and 2-MTHF; for example, the highest yield for 2-MF (63.6%) and 2-MTHF (78.8%) was obtained over Cu10Pd1 /ZrO2 and Cu10Pd5 /ZrO2 catalyst, respectively, with 2-propanol (14 mL) at 493 K in 1-mmol scale operation. It was denoted that the reaction mainly proceed through furfural → FFA → 2-MF → 2-MTHF sequence.

To clarify the crucial factor to prevent C─C bonds breaking while effectively breaking the C─O bonds in formyl group in furfural to form 2-MF, suppressing the furan formation, Resasco *et al.* studied the addition of 5-wt% Fe into 2-wt% Ni/SiO2 catalyst with coimpregnation method formed Ni-Fe bimetallic alloys and yielded 39% of MF from furfural and *ca.* 78% from FFA on NiFe/SiO2 catalyst at 523 K in vapor-phase conversion of furfural [59]. Controlled

C─O bond of furfural in the aldehyde group and Mo2

60 Green Chemical Processing and Synthesis

drogenation step of xylose to furfural and Cu/ZnO/Al2

and 2-MF was formed at 423 and 463 K under 0.1 MPa H2

furfural with moderate yield (35%) [56].

In the FFA hydrogenation reaction, Pd/Fe2

a continuous-flow reactor, while the Pd/TiO<sup>2</sup>

tor at room temperature (298 K) [57].

In order to avoid the use of high H2

bed reactor system with Cu2

gated with Pd/SiO2

sequence.

from FFA on NiFe/SiO2

constant and continuously controlled pressured H2

ation of furfural toward 2 MF, while Pd/SiO2

Si2 O5 (OH)2

for 2-MF (63.6%) and 2-MTHF (78.8%) was obtained over Cu10Pd1

Resasco *et al.* studied the addition of 5-wt% Fe into 2-wt% Ni/SiO2

ation affording to 60% selectivity for 2-MF production, although the conversion of furfural seems to be low. Zhu *et al.* investigated a fixed-bed reactor test of furfural hydrogenation over a commercial catalyst (Cu/Zn/Al/Ca/Na = 59/33/61/1/1, atomic ratio) [54]. The 87% yield from furfural and 93% yield from FFA were obtained toward 2-MF formation at 523 K. In the same group, recently, further advanced examination on a continuous two-step fixed-bed reactor system over catalysts utilizing a combination of acidic Hβ-zeolite for the former dehy-

of produced furfural in a mixture of γ-butyrolactone/water as solvent was attempted to the direct conversion of xylose to 2-MF *via* FFA formation [55]. It was observed that the main product can be simply tuned by the hydrogenation temperature, and the 87% yields of FFA

quently detected at higher reaction temperature in the FFA synthesis from hydrogenation of

ditions at a transfer hydrogenation of FFA in 2-propanol, though the conversion of furfural to 2-MF was scarcely occurred (13% yield) at 453 K [31]. The former value reached to be 76% in

O3

catalyst in the bottom reactor [11]. The Cu2

an outstanding hydrogenation ability and gave 84.6% yield for hydrogenation-deoxygen-

performance for the C═C bonds in furan ring, serving 87.7% yield of ring-hydrogenation of 2 MF toward 2 MTHF at 443 K. Accordingly, such a dual solid catalyst system achieved up to 97.1% yield on the direct production for 2-MTHF with 100% conversion of furfural under atmospheric pressure (**Figure 2**). Thereafter, bimetallic CuPd-supported ZrO2

lyzed hydrogenation by using a batch reactor in the presence of 2-propanol as hydrogen donor was highlighted by other research group [58]. The molecular ratio of Cu/Pd gave significant influences on the yields for 2-MF and 2-MTHF; for example, the highest yield

catalyst, respectively, with 2-propanol (14 mL) at 493 K in 1-mmol scale operation. It was denoted that the reaction mainly proceed through furfural → FFA → 2-MF → 2-MTHF

To clarify the crucial factor to prevent C─C bonds breaking while effectively breaking the C─O bonds in formyl group in furfural to form 2-MF, suppressing the furan formation,

tion method formed Ni-Fe bimetallic alloys and yielded 39% of MF from furfural and *ca.* 78%

catalyst at 523 K in vapor-phase conversion of furfural [59]. Controlled

O3

C surface-assisted selective deoxygen-

for the latter hydrogenation steps

, respectively. The 2-MF was fre-

catalyst yielded 31% for 2-MF under batch con-

catalyst showed 71 and 88% yield for 2-MF for

pressure, Zhu *et al.* designed a two-stage packing fixed-

(copper phyllosilicate) in the upper reactor conju-

Si2 O5 (OH)2

(0.3 MPa) condition in an autoclave reac-

catalyst gave a significant hydrogenation

/ZrO2

and Cu10Pd5

catalyst with coimpregna-

itself possessed

cata-

/ZrO2

**Figure 2.** Systematic design for one-step conversion of furfural toward 2-MTHF with a dual solid catalyst in a fixed-bed reactor.

experiments with monitoring the yields of products in the conversion of furfural, FFA, and THFA suggested that enhancement of both hydrogenolysis and reduction in decarbonylation activity increased following to increase the Fe content in bimetallic catalyst. Notably, the authors further investigated the reaction with benzyl alcohol, which strongly supported that coexistence of Fe in Ni-Fe/SiO2 catalyst significantly promoted the hydrogenolysis path to toluene rather than the decarbonylation path to benzene (**Figure 3**). The DFT calculation approaches determined that configuration of the adsorbed furfural on NiFe was much differed from Ni, and the significant lengthening of the C─O bond in formyl group caused on bimetallic NiFe surface would be a crucial factor to facilitate the reaction for C─O hydrogenolysis rate observed experimentally.

The same group also has claimed the importance of the differences in the molecular-surface interactions between the aldehydes and active surfaces affording to furan or FFA during furfural hydrogenation over PdCu/SiO2 catalyst in a flow reactor [60]. In the case of PdCu/SiO2 , the formation of FFA *via* reduction gradually became dominant in the hydrogenation reaction of furfural in comparison with the formation of furan *via* decarbonylation as function of Cu loadings onto Pd/SiO2 prepared with co-impregnation method; that is, the decarbonylation rate is greatly reduced on PdCu catalyst, but the hydrogenation (reduction) rate is increased. The changes in electron structure of PdCu alloy different from that of pure Pd resulted in a lower extent of electron back-donation to the *π*\* system of the aldehydes and a less stability toward the η<sup>1</sup> (C)-acyl intermediate formation affording to furan, and these increased

**Figure 3.** Yield of products from the reaction of benzyl alcohol over Ni-Fe bimetallic catalysts as a function of Fe loading. Reaction conditions: W/F = 0.1 h, H<sup>2</sup> /Feed ratio = 25, 523 K, 1 atm. Reproduced with permission from Elsevier of Ref. [59].

**Figure 4.** An expected mechanism for furfural conversion over Pd catalyst. Reproduced with permission from Elsevier of Ref. [60].

the frequency for FFA production drastically through the hydroxyalkyl intermediate path (**Figure 4**).

Mechanistic studies for understanding and controlling the reactivity for hydrogenation reactions on metal catalysts have been an attractive subject. Advanced review articles for this area would be helpful for further study (e.g., see [61, 62]).

#### **3. Conclusions**

Due to the high oxygen content in biomass resources, investigations on deoxygenation *via* hydrogenation/hydrogenolysis reaction open up a lot of insights for catalytic and systematic design on biomass upgrading toward biofuel and blender. As introduced in this chapter, bimetallic catalyst and/or utilization of transfer hydrogen donor have been a growing interest instead of conventional system using pressurized H2 and monometalic catalyst. Mechanistic studies with systematic experiments have been a powerful tool to reveal the reaction path and optimize the reaction conditions to provide target compound selectively. As the next research generation, alloying of terminal precious sites with addition of transition metals would have been getting more attentions in hydrogenation/hydrogenolysis of furaldehydes. It needs to be underlined that pyrolysis of biomass resources toward gas, char, and/or biooil is an energetic movement [63–65]. These transformation technologies of biomass would be a key component to fabricate sustainable social design with low carbon emission.

## **Author details**

**Figure 3.** Yield of products from the reaction of benzyl alcohol over Ni-Fe bimetallic catalysts as a function of Fe loading.

**Figure 4.** An expected mechanism for furfural conversion over Pd catalyst. Reproduced with permission from Elsevier

/Feed ratio = 25, 523 K, 1 atm. Reproduced with permission from Elsevier of Ref. [59].

Reaction conditions: W/F = 0.1 h, H<sup>2</sup>

62 Green Chemical Processing and Synthesis

of Ref. [60].

Shun Nishimura\* and Kohki Ebitani

\*Address all correspondence to: s\_nishim@jaist.ac.jp

Graduate School of Advanced Science and Technology, Japan Advanced Institute of Science and Technology (JAIST), Nomi, Ishikawa, Japan

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## **Green Synthesis of Oligomer Calixarenes**

Ratnaningsih Eko Sardjono and Rahmi Rachmawati

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67804

#### **Abstract**

The synthesis of calixarenes can be conventionally done by heating at high temperature for a few hours and using various solvents in large quantities. The greener synthesis can be done with microwave-assisted synthesis and the solvent-free method, where both of these methods can reduce reaction time, energy use, solvent, and waste, with a higher percentage yield than that from the conventional synthesis method, making the synthesis of cyclic oligomer calixarenes and their derivatives more environmentally friendly.

**Keywords:** calixarenes, cyclic oligomer, green synthesis, microwave, solvent-free

#### **1. Introduction**

A calixarene is a cup-shaped supramolecule. The name is derived from the word "calix," which means a cup. Synthesis of calixarenes becomes very important because these molecules are widely used in various fields, especially in its use in a guest-host system molecule. The molecule was first reported in 1872 by Baeyer as a reaction product of aldehyde-phenol condensation. The same product can also be formed from benzaldehyde-pyrogallol (benzene-1,2,3-triol) condensation reaction. The structure of phenols used in this reaction has reactivity at ortho and para positions so that it can form a polymer cross-linked at three positions (two orthos and one para). In 1942, Zinke and Ziegler used para-substituted phenols so that the phenols could only react at the two ortho positions, thereby reducing the possibility of cross-link formation. By reacting *p*-*tert*-butylphenol with formaldehyde under a basic condition at 100–220°C, a waxy brown paste was produced, which was then recrystallized and formed a crystal decomposed at above 300°C. The results of the analysis showed that the product had a molecular weight that was in agreement with a cyclic tetrameric structure [1]. Similarly, Cornforth reacted *p-tert*-butylphenol

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

with formaldehyde. However, the reaction resulted in two crystal products that were rather soluble in water with high but nonidentical melting points. Likewise, when *p*-*tert*-butylphenol was replaced with *p*-1,1,3,3-tetramethylbutylphenol, two products with different melting points were obtained. The results of X-ray showed that the four condensation products had a cyclic tetrameric structure. The different melting points, according to Cornforth, indicated that the two compounds are diastereoisomers [1, 2].

Calixarene synthesis can be done through a noncomplicated process. The compound group is commonly synthesized from 4-alkylphenol, especially 4-*t*-buthylphenol, through a one-stage reaction with formaldehyde and NaOH or KOH as the base. The making of calixarenes, known as a base-induced process, results in calixarenes that may vary in their aromatic components. The reaction product between phenols or phenol derivatives and aldehydes is also known as calixarene, while the reaction product between resorcinol or resorcinol derivatives and aldehydes is known as calixresorcinarene. In addition, there are several products that use aromatic compounds in the forms of para-alkoxy-substituted benzyl alcohol derivatives, such as *p*-alkoxybenzyl alcohol, that when reacted with the aid of AlCl3 as a catalyst will result in non-hydroxylated calixarenes [3]. In general, these reactions are illustrated in **Figure 1**.

The basic structure of calixarenes consists of phenolic units or phenolic derivatives which are repeated and interconnected with Ar-C-Ar bridge to form a hollow cycle. The part with a broader hollow side is called the upper rim, and the narrow hollow part is called the "lower rim" as shown in **Figure 2** [4, 5].

The various conformations of calixarenes are caused by the free rotation of sigma bonds at methylene, Ar-C-Ar, bridges [6]. For example, calix[4]resorcinarenes have four prochiral centers on carbon atoms at the methylene bridges, causing the compound to form four different diastereoisomers. The relative configurations of the four R substituents at the methylene bridges can form a cis for all R substituents (rccc) or a cis-cis-trans to one R (rcct), cis-transtrans (rctt), and trans-cis-trans (rtct). The configurations are determined based on their configurations to one of the R substituents on the prochiral center in a clockwise direction [4, 7, 8]. **Figure 3** illustrates the relative configurations of the four R substituents.

In reality, the cyclic tetrameric ring of a calixarene is not a planar molecule and can adopt five conformations, namely, "crown" or C4v, "boat" or C2v, "chair" or C2h, "diamond" or C<sup>s</sup> , and "saddle" or S<sup>4</sup> [2, 8]. The five conformations of calixarene is shown in **Figure 4**. In fact, the rccc isomer where all R groups in the same position, so far only found with the macrocyclic ring adopted the "crown" or C4v and "boat" or C2v conformations. This is caused by the single bond with a tetrahedral shape of R groups so that it can rotate freely. The rccc isomers can form conformation "boat" or C2v with the fourth R possesses an all axial and all-cis configuration, while the macrocyclic ring in a boat conformation. In this case, the fourth R leads into the annulus of the calixarene (endo). It was also reinforced by the result of a condensation reaction between resorcinol and 4-bromobenzaldehyde with an acid catalyst, where produced the rccc isomer of calix[4]resorcinarene in "boat" or C2v conformation [7]. The rctt isomer is only found in "chair" or C2h conformation. In the calculation of energy, the rccc isomer in the crown conformation (C4v) is favored over the rctt isomer in the chair conformation (C2h).

**Figure 2.** The "upper rim" and "lower rim" of calixarenes.

with formaldehyde. However, the reaction resulted in two crystal products that were rather soluble in water with high but nonidentical melting points. Likewise, when *p*-*tert*-butylphenol was replaced with *p*-1,1,3,3-tetramethylbutylphenol, two products with different melting points were obtained. The results of X-ray showed that the four condensation products had a cyclic tetrameric structure. The different melting points, according to Cornforth, indicated that the two

Calixarene synthesis can be done through a noncomplicated process. The compound group is commonly synthesized from 4-alkylphenol, especially 4-*t*-buthylphenol, through a one-stage reaction with formaldehyde and NaOH or KOH as the base. The making of calixarenes, known as a base-induced process, results in calixarenes that may vary in their aromatic components. The reaction product between phenols or phenol derivatives and aldehydes is also known as calixarene, while the reaction product between resorcinol or resorcinol derivatives and aldehydes is known as calixresorcinarene. In addition, there are several products that use aromatic compounds in the forms of para-alkoxy-substituted benzyl alcohol derivatives, such as *p*-alkoxybenzyl alcohol, that when reacted with the aid of

as a catalyst will result in non-hydroxylated calixarenes [3]. In general, these reac-

The basic structure of calixarenes consists of phenolic units or phenolic derivatives which are repeated and interconnected with Ar-C-Ar bridge to form a hollow cycle. The part with a broader hollow side is called the upper rim, and the narrow hollow part is called the "lower

The various conformations of calixarenes are caused by the free rotation of sigma bonds at methylene, Ar-C-Ar, bridges [6]. For example, calix[4]resorcinarenes have four prochiral centers on carbon atoms at the methylene bridges, causing the compound to form four different diastereoisomers. The relative configurations of the four R substituents at the methylene bridges can form a cis for all R substituents (rccc) or a cis-cis-trans to one R (rcct), cis-transtrans (rctt), and trans-cis-trans (rtct). The configurations are determined based on their configurations to one of the R substituents on the prochiral center in a clockwise direction [4, 7,

In reality, the cyclic tetrameric ring of a calixarene is not a planar molecule and can adopt five conformations, namely, "crown" or C4v, "boat" or C2v, "chair" or C2h, "diamond" or C<sup>s</sup>

rccc isomer where all R groups in the same position, so far only found with the macrocyclic ring adopted the "crown" or C4v and "boat" or C2v conformations. This is caused by the single bond with a tetrahedral shape of R groups so that it can rotate freely. The rccc isomers can form conformation "boat" or C2v with the fourth R possesses an all axial and all-cis configuration, while the macrocyclic ring in a boat conformation. In this case, the fourth R leads into the annulus of the calixarene (endo). It was also reinforced by the result of a condensation reaction between resorcinol and 4-bromobenzaldehyde with an acid catalyst, where produced the rccc isomer of calix[4]resorcinarene in "boat" or C2v conformation [7]. The rctt isomer is only found in "chair" or C2h conformation. In the calculation of energy, the rccc isomer in the crown conformation (C4v) is favored over the rctt isomer in the chair conformation (C2h).

[2, 8]. The five conformations of calixarene is shown in **Figure 4**. In fact, the

, and

8]. **Figure 3** illustrates the relative configurations of the four R substituents.

compounds are diastereoisomers [1, 2].

72 Green Chemical Processing and Synthesis

tions are illustrated in **Figure 1**.

rim" as shown in **Figure 2** [4, 5].

AlCl3

"saddle" or S<sup>4</sup>

**Figure 3.** R relative configurations at the methylene bridge.

**Figure 4.** Calix[4]arene conformations.

Nevertheless, the rccc in boat conformation (C2v) has similar energy to the rctt in chair conformation [51]. The rcct isomer adopts the " diamond" or C<sup>s</sup> conformation [4].

Based on the projection of aryl group on the cyclic tetramer structure of calixarenes, four different conformations can be formed depending on the forms of aryl cluster projection, which can be upward (u) or downward (d), relative to the plane. The four conformations are a cone, partial cone, 1,2-alternate, and 1,3-alternate conformations (**Figure 5**), each with different thermodynamic stabilities [1, 5].

In their development, calixarenes have been successfully synthesized not only in the form of the tetramer but also hexamer and octamer. Therefore, the naming of calixarenes is developed by inserting a number inside brackets, showing the number of monomers they possess. For instance, a calixarene containing eight monomers will be written as calix[8]arene. Furthermore, to show the types of phenol used in the synthesis, para substituents are also included in the name. The cyclic hexamer of *p*-*tert*-butylphenol, for example, is named *p*-*tert*-butylcalix[6] arene. For resorcinol-derived calixarenes, the name is calix[n]resorcinarene, while the substituents of the methylene carbon are shown by the prefix of "C-substituent." For example, the reaction yield of resorcinol with acetaldehyde is named C-methylcalix[4]resorcinarene, indicating the presence of methyl substituents in the methylene carbon. For a more systematic

**Figure 5.** Conformation based on the projection of aryl group in tetramer structure of calixarenes.

naming, especially for scientific publication, the basic name of calix[n]arene is maintained, whereas the substituent identity is shown based on its position through the number, such as shown in **Figure 6** [9].

Calixarenes have been broadly used, among others, as liquid crystal materials [10–12], sensors [13, 14], catalysts [15, 16], the stationary phase of chromatography [17], host molecules [18–25], or even semiconductors [26]. As a role in the guest-host system molecule, calix[4] arene and 4-*tert*-butylcalix[6]arene can be the host molecules for compounds such as trifluoromethyl-benzene contained in pesticides [24]. In addition, some calixarene derivatives can also be the host for the dye molecules such as orange I (OI), methylene blue (MB), neutral red (NR), and brilliant green (BG) [18]. This finding makes calixarenes as a promising candidate as a host molecule to be used in chemical sensors to measure neutral organic molecules. In addition to being the host molecules for neutral compounds, calixarene and its derivatives were also proven to be a host for the molecular ion compounds such as heavy metal ions [19, 20]. It is no wonder that calix[n]arenes have been greatly synthesized and yielded various oligomers with n = 4–20 according to their purposes. Various modifications of functional clusters have also been developed in order to yield the desired molecular structures, for example, the modification of the aryl group [27] or the aldehyde. The modification can be in the form of the addition of a functional cluster such as alkyl cluster and hydroxyl cluster or the elimination of a functional cluster in the starting materials used or further modification of the calixarene molecules formed [19]. Some of the modifications make the calixarene structure more rigid,

**Figure 6.** Calixarene numbering system.

**Figure 4.** Calix[4]arene conformations.

ent thermodynamic stabilities [1, 5].

mation [51]. The rcct isomer adopts the " diamond" or C<sup>s</sup>

**Figure 3.** R relative configurations at the methylene bridge.

74 Green Chemical Processing and Synthesis

Nevertheless, the rccc in boat conformation (C2v) has similar energy to the rctt in chair confor-

Based on the projection of aryl group on the cyclic tetramer structure of calixarenes, four different conformations can be formed depending on the forms of aryl cluster projection, which can be upward (u) or downward (d), relative to the plane. The four conformations are a cone, partial cone, 1,2-alternate, and 1,3-alternate conformations (**Figure 5**), each with differ-

In their development, calixarenes have been successfully synthesized not only in the form of the tetramer but also hexamer and octamer. Therefore, the naming of calixarenes is developed by inserting a number inside brackets, showing the number of monomers they possess. For instance, a calixarene containing eight monomers will be written as calix[8]arene. Furthermore, to show the types of phenol used in the synthesis, para substituents are also included in the name. The cyclic hexamer of *p*-*tert*-butylphenol, for example, is named *p*-*tert*-butylcalix[6] arene. For resorcinol-derived calixarenes, the name is calix[n]resorcinarene, while the substituents of the methylene carbon are shown by the prefix of "C-substituent." For example, the reaction yield of resorcinol with acetaldehyde is named C-methylcalix[4]resorcinarene, indicating the presence of methyl substituents in the methylene carbon. For a more systematic

conformation [4].

thereby forming the expected conformations [28]. To cite an example, the addition of ketone group into aryl group was done to increase the affinity of calixarene compound to the complex formation of alkali cation [29]. The modification can also be made by adjusting the hollow size formed in calix[n]arenes to match the size of a certain molecule both by modifying the functional cluster and adding other components [2, 30].

### **2. Synthesis of oligomer calixarenes**

#### **2.1. Conventional synthesis procedures**

Various calixarene synthesis procedures and their derivations have been largely developed, for example, the procedure developed by Zinke-Ziegler, where the synthesis of calixarene started by mixing *p*-*tert*-butylphenol, 37% formaldehyde, and NaOH heated at 50–55°C for 45 h. To remove water content, the reaction compound was reheated at 110–120°C for 2 h. In the purification process, the yellow solid yielded was subsequently heated in linseed oil up to 200°C for several hours, and the process yielded tetramers. The modification to this procedure was made by Cornforth by directly heating the reaction compound (*p*-*tert*-butylphenol, formaldehyde, and NaOH) at 110–120°C for 2 h, thereby yielding a very thick product which was then refluxed with diphenyl ether for 2 h. Once the compound was cold, the product was separated with infiltration and recrystallized to produce white crystals with a melting point of 342–344°C for 50%. The procedure was later on known as the Zinke-Cornforth procedure [1, 2, 31]. The condensation of *p*-*tert*-butylphenol with formaldehyde under basic conditions (NaOH, KOH) can yield cyclic tetramers, hexamers, and even octamers with a percentage of 85–63%. **Figure 7** illustrates the reaction mechanisms of calix[n]arene formation under basic conditions.

The right mechanism to transform linear oligomer into a cyclic oligomer, according to Gutsche [1], can be predicted from the formation of hemicalixarene combined with a pair of linear oligomers. Under appropriate conditions, two linear methyl-hydroxylated tetramers combine to form hemicalix[8]arene, followed by the release of water to form calix[8]arene [1]. This process can also be seen in the synthesis of *p-*chlorocalix[4]arene, which is a "2 + 2" condensation product of *p*,*p*'-dichlorodiphenylmethane, such as illustrated in **Figure 8**.

Similarly, calix[6]arene formation is postulated to be the result of a combination of two linear trimers. Based on this postulation, the form of dianion hemicalix[6]arene can be the cause of the large amounts of bases required for hemicalix[6]arene synthesis. The following reaction in **Figure 9**, for example, adopts a "3 + 3" condensation that involves two trimers to obtain 9% calix[6]arene [32].

Gutsche synthesized *p-tert*-butylcalix[4]arene with the Zinke-Cornforth procedure to see the effect of basic conditions on the cyclic oligomer produced. The yellow thick paste ("precursor") formed after heating the compound for 1.5–2 h at 110–120°C was diluted with chloroform and washed with aqueous HCl and water several times to neutralize the base. The results of Gutsche's research showed that when the precursor containing a base of 0.03–0.04

**Figure 7.** Reaction mechanisms of calix[n]arene formation under basic conditions.

thereby forming the expected conformations [28]. To cite an example, the addition of ketone group into aryl group was done to increase the affinity of calixarene compound to the complex formation of alkali cation [29]. The modification can also be made by adjusting the hollow size formed in calix[n]arenes to match the size of a certain molecule both by modifying

Various calixarene synthesis procedures and their derivations have been largely developed, for example, the procedure developed by Zinke-Ziegler, where the synthesis of calixarene started by mixing *p*-*tert*-butylphenol, 37% formaldehyde, and NaOH heated at 50–55°C for 45 h. To remove water content, the reaction compound was reheated at 110–120°C for 2 h. In the purification process, the yellow solid yielded was subsequently heated in linseed oil up to 200°C for several hours, and the process yielded tetramers. The modification to this procedure was made by Cornforth by directly heating the reaction compound (*p*-*tert*-butylphenol, formaldehyde, and NaOH) at 110–120°C for 2 h, thereby yielding a very thick product which was then refluxed with diphenyl ether for 2 h. Once the compound was cold, the product was separated with infiltration and recrystallized to produce white crystals with a melting point of 342–344°C for 50%. The procedure was later on known as the Zinke-Cornforth procedure [1, 2, 31]. The condensation of *p*-*tert*-butylphenol with formaldehyde under basic conditions (NaOH, KOH) can yield cyclic tetramers, hexamers, and even octamers with a percentage of 85–63%. **Figure 7** illustrates the reaction mechanisms of calix[n]arene formation under basic

The right mechanism to transform linear oligomer into a cyclic oligomer, according to Gutsche [1], can be predicted from the formation of hemicalixarene combined with a pair of linear oligomers. Under appropriate conditions, two linear methyl-hydroxylated tetramers combine to form hemicalix[8]arene, followed by the release of water to form calix[8]arene [1]. This process can also be seen in the synthesis of *p-*chlorocalix[4]arene, which is a "2 + 2" condensation

Similarly, calix[6]arene formation is postulated to be the result of a combination of two linear trimers. Based on this postulation, the form of dianion hemicalix[6]arene can be the cause of the large amounts of bases required for hemicalix[6]arene synthesis. The following reaction in **Figure 9**, for example, adopts a "3 + 3" condensation that involves two trimers to obtain 9%

Gutsche synthesized *p-tert*-butylcalix[4]arene with the Zinke-Cornforth procedure to see the effect of basic conditions on the cyclic oligomer produced. The yellow thick paste ("precursor") formed after heating the compound for 1.5–2 h at 110–120°C was diluted with chloroform and washed with aqueous HCl and water several times to neutralize the base. The results of Gutsche's research showed that when the precursor containing a base of 0.03–0.04

product of *p*,*p*'-dichlorodiphenylmethane, such as illustrated in **Figure 8**.

the functional cluster and adding other components [2, 30].

**2. Synthesis of oligomer calixarenes**

**2.1. Conventional synthesis procedures**

76 Green Chemical Processing and Synthesis

conditions.

calix[6]arene [32].

**Figure 8.** Cyclic tetramer formation through "2 + 2" condensation.

or equiv. (relative to the number of phenols used) was refluxed for 2 h, it could yield cyclic tetramers. However, the lower the concentration of the base used, the lower the cyclic tetramer formations; on the other hand, a higher concentration of the base will increase the formation of cyclic hexamers. The findings of this research showed that bases play a role in inducing cyclization and are vital in determining the cyclic oligomer formed [33, 34]. The cations used as the base catalyst can play the role of a "template" which determines the size or number of monomers forming oligomers.

A large cyclic oligomer enables the formation of a larger hollow, and this can be used to absorb heavy metals, as can be found in the synthesis of calix[6]arene involving *p*-*tert*-butylphenol, paraformaldehyde, and bases (KOH) with a proportion of 0.45 mole relative to the *p*-*tert*butylphenol used. The reaction happened for 4 h by refluxing it in *p*-xylene. The compound

**Figure 9.** Cyclic hexamer formation through "3 + 3" condensation.

was separated with liquid-liquid extraction using chloroform. The organic phase obtained was then evaporated and recrystallized with methanol to yield 65.47% white solids with a melting point of 370–372°C. The analysis of the product showed that it is a cyclic hexamer of calix[6]arene [35].

Condensation of resorcinol with formaldehyde under acidic conditions formed a linier or cross-linked polymer between formaldehyde with resorcinol. This reaction also did not produce calixresorcinarene under alkaline conditions. It is not surprising since resorcinol and formaldehyde are highly reactive so that the reaction becomes uncontrollable and does not produce a cyclic oligomer. Base conditions can be applied to the synthesis of calixresorcinarene from 2-substituted resorcinols and formaldehyde. Nevertheless, resorcinol used should be substituted electron-withdrawing group at position 2 as shown in **Figure 10**. Electron-withdrawing group at position 2 will reduce the reactivity of resorcinol

**Figure 10.** Synthesis of resorcinarenes from 2-substituted resorcinols and formaldehyde in basic conditions.

so that the reaction will be controlled and it will be suitably carried out under alkaline conditions. The presence of electron-withdrawing substituents such as nitro group (−NO2 ) facilitates condensation reactions that occur by directing the reaction site at the meta position of the nitro group. The reaction rate decreased from electron-withdrawing substituent nitro > acetyl > carboxylate > H due to the ability of the groups to deactivate the resorcinol. In contrast, the reaction of resorcinol substituted electron-donating group such as 2-methylresorcinol or phloroglucinol (=benzene-1,3,5-triol) with formaldehyde under base conditions did not produce a derivative of calixresorcinarene. Therefore, the acidic conditions are more suitable when used resorcinol substituted electron-donating group in position 2. The condensation reaction between formaldehyde and 2-nitroresorcinol and its analogs seems to take place in the irreversible reaction, so it is unknown whether to form a tetramer, pentamer, or hexamer [36].

Besides adding bases, the synthesis of calix[n]arenes can also be done under acidic conditions. One such example is found in the procedure of calix[n]resorcinarene synthesis developed by Niederl-Hogberg, in which resorcinol was used as an aromatic unit in the reaction. Resorcinol is more reactive compared to phenol. Resorcinol was reacted with acetaldehyde by adding hydrochloric acid. The reaction took place at 80°C for 16 h and yielded C-methylcalix[4]resorcinarenes for 75%. The reaction follows the following mechanism which is illustrated in **Figure 11**.

was separated with liquid-liquid extraction using chloroform. The organic phase obtained was then evaporated and recrystallized with methanol to yield 65.47% white solids with a melting point of 370–372°C. The analysis of the product showed that it is a cyclic hexamer of

Condensation of resorcinol with formaldehyde under acidic conditions formed a linier or cross-linked polymer between formaldehyde with resorcinol. This reaction also did not produce calixresorcinarene under alkaline conditions. It is not surprising since resorcinol and formaldehyde are highly reactive so that the reaction becomes uncontrollable and does not produce a cyclic oligomer. Base conditions can be applied to the synthesis of calixresorcinarene from 2-substituted resorcinols and formaldehyde. Nevertheless, resorcinol used should be substituted electron-withdrawing group at position 2 as shown in **Figure 10**. Electron-withdrawing group at position 2 will reduce the reactivity of resorcinol

**Figure 10.** Synthesis of resorcinarenes from 2-substituted resorcinols and formaldehyde in basic conditions.

calix[6]arene [35].

78 Green Chemical Processing and Synthesis

**Figure 9.** Cyclic hexamer formation through "3 + 3" condensation.

**Figure 11.** Reaction mechanism of calix[n]resorcinarene formation under acidic conditions.

In the condensation reaction catalyzed by acids, kinetic and thermodynamic factors compete with each other. The kinetic factor controls the oligomer rate produced, while the thermodynamic factor controls the cyclic structure formation. In this reaction, controlling the thermodynamic factor will be very determining for the formation of a small cyclic oligomer, such as the formation of calix[4]resorcinarene. To control the thermodynamic factor, less reactive aldehydes can be used to slow down the reaction rate. If the aldehyde used is very reactive (such as formaldehyde), the reaction will happen very quickly so that the kinetic factor will dominate and the majority of the products formed will be polymers with linear chains. A modified aldehyde was used in the synthesis reaction of methoxyphenylcalix[4]resorcinarene from resorcinol with *p-*anisaldehyde with the addition of HCl. The reaction happened employing reflux for 30 h, yielding 4-methoxyphenylcalix[4]resorcinarene as much as 91.54% [37].

The synthesis of calix[4]resorcinarene from resorcinol with acetaldehyde was done by Petrova et al. in 2012. The reaction was made by varying the amounts of hydrochloric acid used. The reaction occurred at 75°C for 5–24 h. The reaction product was dependent upon the amounts of hydrochloric acid used. Increasing the amount of acid can cause the formation of linear oligomers. The amount of acid catalyst effective in this reaction was 4% of the resorcinol. In addition, the polarity of the solvents used affected the composition of the yielded product. An increase in the polarity of the solvent caused an increase in the oligo cyclization. Calix[4]resorcinarene with rccc conformation was quantitatively formed very well (76.89%) when 30% water-ethanol solvent was used [38].

Calix[4]resorcinarenes synthesized through a condensation reaction between resorcinol and an aliphatic or aromatic aldehyde under acidic conditions yield products in the form of cyclic tetramer calixresorcinarenes that theoretically can contain four diastereoisomer products. However, in reality, only three have been identified to be formed by the reaction with an aliphatic aldehyde and two from the reaction of an aromatic aldehyde with resorcinol. The products with crown conformation with all side chains in the axial position (rccc) are more desirable and stable thermodynamically. Hogberg reacted resorcinol with acetaldehyde by adding hydrochloric acid. The reaction occurred at 75°C for 1 h and yielded sediment that was subsequently acetylated. The acetylated product was in the form of two isomeric cyclic tetramers, one of which formed cis (rccc) conformation of up to 47%, while the other product formed rctt conformation with a percentage yield of 13%. This indicated that products with all side chains in the axial position (rccc) are more desirable and stable thermodynamically [39]. The same result can be obtained by selectively increasing the alcohol proportion in the solvent mixtures used and increasing the reaction temperatures and time [39, 40].

Several synthesis methods of calix[n]arenes and their derivatives have been largely developed to produce high yields. However, the methods require heating at high temperatures and a long reaction time. This requirement is disadvantageous, ultimately when the synthesis is done on a large scale. The required solvents will be in a very large amount, and the reaction time needed will be much longer. This makes the synthesis of calix[n]arenes and their derivatives not environmentally friendly.

#### **2.2. Green synthesis procedures**

In the condensation reaction catalyzed by acids, kinetic and thermodynamic factors compete with each other. The kinetic factor controls the oligomer rate produced, while the thermodynamic factor controls the cyclic structure formation. In this reaction, controlling the thermodynamic factor will be very determining for the formation of a small cyclic oligomer, such as the formation of calix[4]resorcinarene. To control the thermodynamic factor, less reactive aldehydes can be used to slow down the reaction rate. If the aldehyde used is very reactive (such as formaldehyde), the reaction will happen very quickly so that the kinetic factor will dominate and the majority of the products formed will be polymers with linear chains. A modified aldehyde was used in the synthesis reaction of methoxyphenylcalix[4]resorcinarene from resorcinol with *p-*anisaldehyde with the addition of HCl. The reaction happened employing reflux for 30 h, yielding 4-methoxyphenylcalix[4]resor-

The synthesis of calix[4]resorcinarene from resorcinol with acetaldehyde was done by Petrova et al. in 2012. The reaction was made by varying the amounts of hydrochloric acid used. The reaction occurred at 75°C for 5–24 h. The reaction product was dependent upon the amounts of hydrochloric acid used. Increasing the amount of acid can cause the formation of linear oligomers. The amount of acid catalyst effective in this reaction was 4% of the resorcinol. In addition, the polarity of the solvents used affected the composition of the yielded product. An increase in the polarity of the solvent caused an increase in the oligo cyclization. Calix[4]resorcinarene with rccc conformation was quantitatively formed very well (76.89%) when 30%

Calix[4]resorcinarenes synthesized through a condensation reaction between resorcinol and an aliphatic or aromatic aldehyde under acidic conditions yield products in the form of cyclic tetramer calixresorcinarenes that theoretically can contain four diastereoisomer products. However, in reality, only three have been identified to be formed by the reaction with an aliphatic aldehyde and two from the reaction of an aromatic aldehyde with resorcinol. The products with crown conformation with all side chains in the axial position (rccc) are more desirable and stable thermodynamically. Hogberg reacted resorcinol with acetaldehyde by adding hydrochloric acid. The reaction occurred at 75°C for 1 h and yielded sediment that was subsequently acetylated. The acetylated product was in the form of two isomeric cyclic tetramers, one of which formed cis (rccc) conformation of up to 47%, while the other product formed rctt conformation with a percentage yield of 13%. This indicated that products with all side chains in the axial position (rccc) are more desirable and stable thermodynamically [39]. The same result can be obtained by selectively increasing the alcohol proportion in the solvent mixtures used and increasing the reaction

Several synthesis methods of calix[n]arenes and their derivatives have been largely developed to produce high yields. However, the methods require heating at high temperatures and a long reaction time. This requirement is disadvantageous, ultimately when the synthesis is done on a large scale. The required solvents will be in a very large amount, and the reaction time needed will be much longer. This makes the synthesis of calix[n]arenes and their deriva-

cinarene as much as 91.54% [37].

80 Green Chemical Processing and Synthesis

water-ethanol solvent was used [38].

temperatures and time [39, 40].

tives not environmentally friendly.

Various approaches to calixarenes and their derivatives have been attempted, one of which is by implementing the concept of "green chemistry." The synthesis of calixarenes and their derivatives requires heating at high temperatures for quite a long time. The heating is usually done using conventional heating, where heat transfer occurs conductively. The method is relatively slow and inefficient to transfer energy into the reaction system, as it depends on the thermal conductivity of the materials used; consequently, the temperature of the reaction vessel will be greater than that in the reaction compound. Irradiation with a microwave can produce more efficient internal heating because microwave energy penetrates into the containers so that it can directly hit the reaction compound, and energy transfer becomes more efficient. The phenomenon will depend on the ability of the molecules (solvents or reagents) to absorb microwave energy and change it into heat. The thermal energy is obtained when the polar molecules try to align themselves with an electric field resulted from microwave irradiation. The amount of heat produced depends on the dielectric properties of the molecules. Dielectric constants indicate the ability of the molecules to get polarized by the electric field. The electric field received by the molecules causes reorientation of the dipolar molecules which in turn increases polarity and reactivity by reducing the activation energy. Consequently, the reaction can happen more quickly [41–43].

Organic synthesis using microwave technology has been largely employed, as it can accelerate the reaction time as a consequence of an increase in heating that cannot be achieved with conventional heating [44]. This is what happened in Baozhi's [45] synthesis of *p*- methylcalix[6] arene. A reaction compound consisting of formaldehyde, *p*-methylphenol, and KOH was heated with microwave for 2–8 min (output power at 100%). The required reaction time at this stage was faster than that when using conventional heating, which could take up to 2 h. After solids were formed, reflux was done for 40 min using dimethylbenzene. When cooled down, white crystals with a melting point of 361–362°C were yielded for up to 81.7%. The synthesis of *p*-*tert*-butylcalix[8]arene was done with the same procedure using the microwave as well, in which the compound was radiated for 4–8 min. To yield a larger cyclic reaction product, the amounts of bases used were increased with a longer reflux time. As previously discussed, bases and reflux time play a role in cyclic formation [45].

The same phenomenon was observed when calixarene synthesis was done using resorcinol, which is known to be more reactive than phenol. The reaction product in the form of cyclic tetramer calix[n]resorcinarene was obtained after the compound was heated with conventional heating for 5–10 h, with a reaction yield of around 60–90%. However, the reaction could be faster when the heating method was changed with microwave. One reaction only required 3–5 min with a greater reaction percentage yield, namely, around 80–90% [46]. This finding was put to use in the synthesis of thiacalix[n]arene with n = 4–8 [47].

Aldehyde modification can also be done by using the abundant natural products such as vanillin that can be derived from vanillin plant (*Vanilla planifolia*), cinnamaldehyde from cinnamon oil, and anisaldehyde from Indonesian fennel oil. The synthesis procedures using these materials have been developed for the synthesis of calix[n]arenes and their derivatives with "green chemistry" approach. The three compounds were reacted with resorcinol to yield cyclic tetramers, calix[4]resorcinarenes. As previously discussed, the synthesis reaction of calix[4]resorcinarenes can occur for 15–30 h using conventional heating. Sardjono et al. [48] developed a microwave-assisted synthesis procedure for calix[4]resorcinarenes, where resorcinol was reacted to vanillin or cinnamaldehyde. The calix[4]resorcinarenes derivatives yielded could be up to 97.9% in 8 min with a ratio of resorcinol to vanillin or cinnamaldehyde was 1:1, and the microwave power used 332 W. Another calix[4]resorcinarenes derivatives, namely C-anisalcalix[4]resorcinarene could also be yielded for up to 99.5% in 5 min from the reaction of resorcinol with anisaldehyde at the ratio of 1:1.2 and 264 W microwave power [48]. The three reactions of resorcinol with various aldehydes to form calix[4]resorcinarenes is illustrated in **Figure 12**.

Vanillin in the synthesis of calix[4]resorcinarene derivatives was also employed by Nakajima and Kobayashi [20], although they still used the conventional heating method (without a microwave). The reaction compound comprised resorcinol and aldehyde (vanillin, syringaldehyde, or *p-*hydroxybenzaldehyde) with a mole ratio of 1:1 and hydrochloric acid heated at 70°C and stirred for 12 h. The reaction yield was then washed with hot water and purified using methanol and ethanol. The third reaction yield formed cyclic tetramers, namely, vanillin calix[4]arene for 32%, syringaldehyde calix[4]arene for 28%, and *p-*hydroxybenzaldehyde calix[4]arene for 37% [20].

Calixarene synthesis commonly needs a lot of solvents which results in a large amount of waste. Therefore, a synthesis approach by minimizing the use of solvents has been attempted and proved to be more effective. The solvent-free reactions that have been tried, in addition

**Figure 12.** Synthesis reaction of calix[4]resorcinarene from resorcinol with various aldehydes.

to requiring a shorter amount of time, used less energy and could produce better and more effective and selective reactions than the conventional method [49]. Solvent-free reactions have also been used for calixresorcinarene synthesis, which is a condensation reaction between aldehyde and resorcinol. The method has also been widely used in several organic reactions and sometimes gives better results than the reaction method in solvents [50].

with "green chemistry" approach. The three compounds were reacted with resorcinol to yield cyclic tetramers, calix[4]resorcinarenes. As previously discussed, the synthesis reaction of calix[4]resorcinarenes can occur for 15–30 h using conventional heating. Sardjono et al. [48] developed a microwave-assisted synthesis procedure for calix[4]resorcinarenes, where resorcinol was reacted to vanillin or cinnamaldehyde. The calix[4]resorcinarenes derivatives yielded could be up to 97.9% in 8 min with a ratio of resorcinol to vanillin or cinnamaldehyde was 1:1, and the microwave power used 332 W. Another calix[4]resorcinarenes derivatives, namely C-anisalcalix[4]resorcinarene could also be yielded for up to 99.5% in 5 min from the reaction of resorcinol with anisaldehyde at the ratio of 1:1.2 and 264 W microwave power [48]. The three reactions of resorcinol with various aldehydes to form calix[4]resorcinarenes

Vanillin in the synthesis of calix[4]resorcinarene derivatives was also employed by Nakajima and Kobayashi [20], although they still used the conventional heating method (without a microwave). The reaction compound comprised resorcinol and aldehyde (vanillin, syringaldehyde, or *p-*hydroxybenzaldehyde) with a mole ratio of 1:1 and hydrochloric acid heated at 70°C and stirred for 12 h. The reaction yield was then washed with hot water and purified using methanol and ethanol. The third reaction yield formed cyclic tetramers, namely, vanillin calix[4]arene for 32%, syringaldehyde calix[4]arene for 28%, and *p-*hydroxybenzaldehyde

Calixarene synthesis commonly needs a lot of solvents which results in a large amount of waste. Therefore, a synthesis approach by minimizing the use of solvents has been attempted and proved to be more effective. The solvent-free reactions that have been tried, in addition

**Figure 12.** Synthesis reaction of calix[4]resorcinarene from resorcinol with various aldehydes.

is illustrated in **Figure 12**.

82 Green Chemical Processing and Synthesis

calix[4]arene for 37% [20].

In calixresorcinarene synthesis, the use of solvent-free method was carried out by mixing aldehyde and resorcinol (1:1) with a certain amount of acid catalysts and then ground simultaneously. Although all reagents were in solid forms, when mixed they could form a thick liquid or paste reaction compound. Isolation of the reaction product can be carried out by washing with water to remove the acid catalyst and recrystallize with the right solvent. The method was proven to effectively yield calixresorcinarene with various aldehydes. In this method, no external heating is needed; only consistent scouring is needed. The scouring can be done in order to reduce particle sizes so that the compounding will be efficient. Because the reaction forms a thick liquid, not powder compound, then scouring at high intensity is not necessary [51].

Using the solvent-free method, the reaction occurs in mere minutes, much faster than the reaction time using the conventional method, which requires hours or even days. The percentage yield of the solvent-free method is often greater than that of the conventional synthesis method. The synthesis of calix[4]resorcinarenes commonly yields isomeric products, with two forms of isomer, namely, cis-cis-cis (rccc) and cis-trans-trans (rctt) with "crown" (C*4v*), "chair" (C*2h*), or "boat" (C*2v*) conformation. Theoretically, without the effect of solvents, rccc isomers with "crown" (C*4v*) conformation are more desirable than the rctt isomers with "chair" (C*2h*) conformation [51]. Besides reduced reaction time, synthesis with this method uses a small amount of catalyst and does not produce solvent waste, hence meeting the principles of "green synthesis" [51, 52].

The condensation reaction between aldehyde and resorcinol (1:1) with *p*-toluenesulfonate acidic catalyst (5%) occurs for several seconds up to several minutes and yields derivatives of calix[4]resorcinarenes with rccc and rctt isomers. Four out of the six reactions were reported by Roberts et al. [51] to yield more rctt isomers with "chair" (C*2h*) conformation than rccc isomers with "crown" (C*4v*) conformation, with a ratio of 2:1. However, in one of the reactions, more rccc "crown" (C*4v*) isomers were yielded for up to 95%. The distribution of isomer products is very possibly related to solubility that also plays a role in the reaction with the solvent-free method. In this reaction, increased reaction or heating time of the initial materials up to 85°C for 5 h did not cause an increase in the amount of rccc isomers yielded with "crown" (C*4v*) conformation [51].

Employing the same method, Firdaus et al. [53] reacted vanillin with resorcinol (1:1) and yielded C-4-hydroxy-3-methoxyphenylcalix[4]resorcinarene for 52%, with a ratio of rccc isomers to rctt isomers of 1:5. However, when the aldehyde used was replaced with *p*-anisaldehyde, 63% of C-4-methoxyphenylcalix[4]resorcinarene was yielded with rccc isomers two times greater in number than rctt isomers. The distribution of the isomer products showed that each aldehyde has such a different characteristic that it can yield a different product composition. This difference may also be related to the solubility of the aldehyde used in the reaction compound. In addition, the reaction product demonstrated that rccc isomers with "crown" (C*4v*) conformation are more desirable [53].

The solvent-free method is significantly faster than the conventional synthesis method. This can also be seen in the synthesis of calix[4]pyrogallolarene that is commonly done with condensation reaction of pyrogallol and aldehyde using alcohol solvent and acid catalyst and yields in rccc "cone" isomers. In the synthesis of calix[4]pyrogallolarene using the solvent-free method, an amount of aldehyde was added drop by drop to pyrogallol (1:1) with *p*-toluenesulfonate (3.7%) catalyst and scoured constantly. The reaction yielded vulnerable white solids in 2 min. The solids were easily scoured into soft yellow powder in 5 min. As was the case with the synthesis reaction of calix[4]resorcinarene derivatives, no heating was used in this synthesis [52].

The synthesis of C-(*p*-substituted phenyl) calix[4]resorcinarenes was carried out to see the interaction between host and guest using DMSO by mixing resorcinol with halogen- substituted benzaldehyde in para positions (1:1) under acidic conditions with an addition of 5% solid *p*-toluenesulfonate. The synthesis yielded a product at a percentage of 90–95% by scouring without using any solvents [54].

In the synthesis of calixresorcinarenes from resorcinol and aromatic aldehyde, Lewis acid is frequently used, such as BF3 -OEt<sup>2</sup> , AlCl3 , and SnCl4 . For example, in the synthesis of resorcinarene *O*-acetates from 1,3-di(alkoxycarbonylmethoxy)benzenes with aromatic or aliphatic aldehydes (1:1) can be efficiently done at room temperature with the addition of BF3 -OEt<sup>2</sup> (boron trifluoride etherate) as a catalyst in order to produce high yields [55]. However, the amount of acid needed in this reaction is quite large (50–200% mole). Certainly, this is not good for the environment and safety, considering the sensitivity of Lewis acid to water that can yield side products in the forms of acidic oxides and metal oxides. Furthermore, the process is not efficient in terms of atom economy. The reaction also can yield two diastereoisomers. Lanthanide (III) triflates are selective acid catalysts that have been widely used for the formation of carbon-carbon and carbon-heteroatom bonds. Different from the common Lewis acid that is frequently used in stoichiometric numbers, lanthanide (III) triflates can be used in various reactions in catalytic numbers in the presence of solvents such as THF, DMSO, DMF, MeCN, and water. In addition, the stability and solubility of this catalyst in water make the compound easily separated and reused. One of the examples of lanthanide (III) triflate compounds that can be used as a catalyst is Ytterbium (III) triflate nonahydrate {[Yb(H2 O)9 ] (OTF)3 }. In the condensation reaction of resorcinol with various aldehydes (1:1), to produce derivatives of calix[n]resorcinarenes only needs 8% mole of Ytterbium (III) triflate nonahydrate as a catalyst. Although the amount of the catalyst is small, the reaction product yielded can reach up to 71–94% and this reaction is stereoselective. The reaction is done by refluxing for 48 h with ethanol as solvent [56].

Tungstate sulfuric acid (TSA) can also be used in the synthesis of calix[4]resorcinarene derivatives. In the reaction of an aldehyde with resorcinol at 120°C, the addition of TSA could produce around 78–95% in 25–80 min. The use of TSA as an acid catalyst is also more environmentally friendly because this acid can be recycled and reused for up to four reaction cycles with a decrease in reaction product yield around 3–11% [57].

### **3. Conclusion**

reaction compound. In addition, the reaction product demonstrated that rccc isomers with

The solvent-free method is significantly faster than the conventional synthesis method. This can also be seen in the synthesis of calix[4]pyrogallolarene that is commonly done with condensation reaction of pyrogallol and aldehyde using alcohol solvent and acid catalyst and yields in rccc "cone" isomers. In the synthesis of calix[4]pyrogallolarene using the solvent-free method, an amount of aldehyde was added drop by drop to pyrogallol (1:1) with *p*-toluenesulfonate (3.7%) catalyst and scoured constantly. The reaction yielded vulnerable white solids in 2 min. The solids were easily scoured into soft yellow powder in 5 min. As was the case with the synthesis reaction of calix[4]resorcinarene derivatives, no heating was used in this synthesis [52]. The synthesis of C-(*p*-substituted phenyl) calix[4]resorcinarenes was carried out to see the interaction between host and guest using DMSO by mixing resorcinol with halogen- substituted benzaldehyde in para positions (1:1) under acidic conditions with an addition of 5% solid *p*-toluenesulfonate. The synthesis yielded a product at a percentage of 90–95% by scouring

In the synthesis of calixresorcinarenes from resorcinol and aromatic aldehyde, Lewis acid is

narene *O*-acetates from 1,3-di(alkoxycarbonylmethoxy)benzenes with aromatic or aliphatic aldehydes (1:1) can be efficiently done at room temperature with the addition of BF3

(boron trifluoride etherate) as a catalyst in order to produce high yields [55]. However, the amount of acid needed in this reaction is quite large (50–200% mole). Certainly, this is not good for the environment and safety, considering the sensitivity of Lewis acid to water that can yield side products in the forms of acidic oxides and metal oxides. Furthermore, the process is not efficient in terms of atom economy. The reaction also can yield two diastereoisomers. Lanthanide (III) triflates are selective acid catalysts that have been widely used for the formation of carbon-carbon and carbon-heteroatom bonds. Different from the common Lewis acid that is frequently used in stoichiometric numbers, lanthanide (III) triflates can be used in various reactions in catalytic numbers in the presence of solvents such as THF, DMSO, DMF, MeCN, and water. In addition, the stability and solubility of this catalyst in water make the compound easily separated and reused. One of the examples of lanthanide (III) triflate compounds that can be used as a catalyst is Ytterbium (III) triflate nonahydrate {[Yb(H2

}. In the condensation reaction of resorcinol with various aldehydes (1:1), to produce

derivatives of calix[n]resorcinarenes only needs 8% mole of Ytterbium (III) triflate nonahydrate as a catalyst. Although the amount of the catalyst is small, the reaction product yielded can reach up to 71–94% and this reaction is stereoselective. The reaction is done by

Tungstate sulfuric acid (TSA) can also be used in the synthesis of calix[4]resorcinarene derivatives. In the reaction of an aldehyde with resorcinol at 120°C, the addition of TSA could produce around 78–95% in 25–80 min. The use of TSA as an acid catalyst is also more environmentally friendly because this acid can be recycled and reused for up to four reaction cycles

. For example, in the synthesis of resorci-


O)9 ]

, and SnCl4

"crown" (C*4v*) conformation are more desirable [53].


refluxing for 48 h with ethanol as solvent [56].

with a decrease in reaction product yield around 3–11% [57].

, AlCl3

without using any solvents [54].

frequently used, such as BF3

84 Green Chemical Processing and Synthesis

(OTF)3

Calixarenes and their derivatives can form a cyclic oligomer with a number of monomer units ranging from 4 to 20 and a basic structure consisting of phenolic units or phenol derivatives which are repetitive and interconnected by the Ar-C-Ar bridge to form a hollow cycle. Cyclic oligomer calixarene and its derivatives have been widely used in various fields, especially as host molecules in a guest-host system. Therefore, it takes a lot of effort to synthesize calixarenes and their derivatives. Various modifications of functional groups and synthesis methods were made in order to produce the expected configuration and conformation. These compounds have largely been synthesized by conventional methods by heating at 50–120°C for 1.5–45 h, either under acidic or basic conditions. The conventional method requires the use of various solvents in large amounts. The percentage yield of the reaction employing this method can range from 50 to 91%. The conventional synthesis method that requires heating at high temperatures, a fairly long reaction time, and large amounts of solvents has encouraged many researchers to develop more environmentally friendly synthesis methods, such as microwave-assisted synthesis and solvent-free methods.

Microwave-assisted synthesis of calixarenes and their derivatives requires 2–8 min with a percentage of reaction products of up to 99%. This method has been proven to accelerate reaction time without reducing the percentage of reaction products significantly. The synthesis of calixarenes and their derivatives can also be done with the solvent-free method. The method only relies on scouring process without heating. Nevertheless, the method requires a shorter reaction time with a higher percentage yield than that from the conventional synthesis method. Both microwave-assisted synthesis method and solvent-free method can reduce reaction time, energy use, solvent, and waste, making the synthesis of oligomer calixarenes and their derivatives more environmentally friendly.

#### **Author details**

Ratnaningsih Eko Sardjono\* and Rahmi Rachmawati

\*Address all correspondence to: ratnaeko@upi.edu

Indonesia University of Education, Bandung, Indonesia

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**Environment-Friendly Approach in the Synthesis of Metal/Polymeric Nanocomposite Particles and Their Catalytic Activities on the Reduction of** *p-***Nitrophenol to** *p-***Aminophenol**

Noel Peter Bengzon Tan and Cheng Hao Lee

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.68388

#### **Abstract**

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bkcs.2012.33.1.123

90 Green Chemical Processing and Synthesis

In this chapter, an environment‐friendly approach in synthesizing Au and Au@Ag metal nanoparticles using a microgel is demonstrated. Poly(*N*‐isopropyl acrylamide)/poly‐ ethyleneimine microgel was used as a multifunctional template to reduce metal ions to metal nanoparticles, stabilize and immobilize these metal nanoparticles, and regulate their accessibility within the template. Such multifunctional roles of microgel template were possible due to their unique properties (i.e., amino groups reducing capabil‐ ity, electrostatic and steric stabilizing properties, and swelling/deswelling properties). Characterizations of these metal/polymeric composite particles were also performed, such as scanning electron microscope (SEM), transmission electron microscope (TEM), Zeta‐potential, UV‐vis spectroscopy, X-ray Diffraction (XRD), and X‐ray photoelectron spectroscopy (XPS). To test the catalytic activities of both gold and gold@silver nanopar‐ ticles in the microgel template, a model reaction *(*i.e., reduction of *p*‐nitrophenol to *p*‐ami‐ nophenol) was performed. Results showed that bimetallic gold@silver gave 10 times higher catalytic activity compared to monometallic gold nanoparticles. With the simple one‐step synthesis, a highly scalable process is possible.

**Keywords:** green synthesis, gold nanoparticles, Au@Ag bimetallic nanoparticles, core‐ shell particles, smart microgel particles, smart materials

## **1. Introduction to metal/polymeric nanocomposite particles**

Metal/polymeric nanocomposite particles are a combination of both metal particles and polymers in nanoscale. They come in different terms and play of words. But simply they are

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

colloidal polymers with metal nanoparticles. Metal nanoparticles that can be incorporated into different colloidal polymeric systems are magnetic, semiconductor, and noble metals. On the other hand, colloidal polymers act as carriers of these metal nanoparticles. They are mostly referred to as polymer templates. These templates can either be soluble (i.e., colloi‐ dally soluble) or insoluble (i.e., solid or heterogeneous) polymers. **Figure 1** displays the dif‐ ferent conformations of metal nanoparticles with polymeric templates. For example, metal nanoparticles are seen as core (**Figure 1a** and **b** [1–2]) or part of the polymeric template shell (**Figure 1c** [3]) or attached to both the core and shell of the composite (**Figure 1d** [4]).

Applications of metal/polymeric nanocomposites vary from the fields of chemistry (e.g., catal‐ ysis, sensors, and polymers), physics (e.g., optics and electronics), biology (e.g., detection and control of microorganism), and nanomedicine (e.g., drug development and immunoassay).

There are two general approaches in synthesizing metal nanoparticles: top‐down and bottom‐up. Top‐down methods comprise physical methods such as lithography and etching of bulk metals to nanoscopic scale. Bottom-up approaches are more common these days than the top‐down. The bottom‐up approach also has an advantage of gener‐ ating uniform nanoparticles with controlled size and shape.

Bottom-up approaches or commonly referred to as wet chemical methods were pioneered for more than a century ago [5]. In particular, Michael Faraday's method used metal‐salt solution mixed with reducing agents (e.g., hydrogen, alcohol, hydrazine, or borohydride)

**Figure 1.** TEM images of metal nanoparticles (dark dots) encapsulated within its polymeric templates: (a) PNIPAm‐*b‐* PMOEGMA/Au [Taken from Ref. [1]], (b) PS*‐co*‐PGA/Au [Taken from Ref. [2]], (c) PS/poly (aminoethylmethacrylate HCl)/ gold particles PNIPAm‐*co‐*GMA/Au [Taken from Ref. [3]], and (d) PNIPAm*‐b‐*PMOEGMA/ Au with Ag [Taken from Ref. [4]].

colloidal polymers with metal nanoparticles. Metal nanoparticles that can be incorporated into different colloidal polymeric systems are magnetic, semiconductor, and noble metals. On the other hand, colloidal polymers act as carriers of these metal nanoparticles. They are mostly referred to as polymer templates. These templates can either be soluble (i.e., colloi‐ dally soluble) or insoluble (i.e., solid or heterogeneous) polymers. **Figure 1** displays the dif‐ ferent conformations of metal nanoparticles with polymeric templates. For example, metal nanoparticles are seen as core (**Figure 1a** and **b** [1–2]) or part of the polymeric template shell

(**Figure 1c** [3]) or attached to both the core and shell of the composite (**Figure 1d** [4]).

ating uniform nanoparticles with controlled size and shape.

92 Green Chemical Processing and Synthesis

Applications of metal/polymeric nanocomposites vary from the fields of chemistry (e.g., catal‐ ysis, sensors, and polymers), physics (e.g., optics and electronics), biology (e.g., detection and control of microorganism), and nanomedicine (e.g., drug development and immunoassay).

There are two general approaches in synthesizing metal nanoparticles: top‐down and bottom‐up. Top‐down methods comprise physical methods such as lithography and etching of bulk metals to nanoscopic scale. Bottom-up approaches are more common these days than the top‐down. The bottom‐up approach also has an advantage of gener‐

Bottom-up approaches or commonly referred to as wet chemical methods were pioneered for more than a century ago [5]. In particular, Michael Faraday's method used metal‐salt solution mixed with reducing agents (e.g., hydrogen, alcohol, hydrazine, or borohydride)

**Figure 1.** TEM images of metal nanoparticles (dark dots) encapsulated within its polymeric templates: (a) PNIPAm‐*b‐* PMOEGMA/Au [Taken from Ref. [1]], (b) PS*‐co*‐PGA/Au [Taken from Ref. [2]], (c) PS/poly (aminoethylmethacrylate HCl)/ gold particles PNIPAm‐*co‐*GMA/Au [Taken from Ref. [3]], and (d) PNIPAm*‐b‐*PMOEGMA/ Au with Ag [Taken from Ref. [4]]. and later, stabilizing agents (e.g., ligands, polymers, or surfactants). Turkevich et al. [6] and Brust-Schiffrin et al. [7] were able to use this similar approach by synthesizing gold nanopar‐ ticles. Their synthetic route involved the reaction of a chloroauric acid with sodium citrate solution at boiling temperature (HAuCl4 + Na3 C6 H5 O7 = Au<sup>0</sup> ). Later on, Frens [8] was able to control the size formation of gold nanoparticles by varying the reducing agent to gold‐salt ratio during the reduction process. Furthermore, Yonezawa and Kunitake [9] used sodium 3-mercaptopropionate to the prestabilized citrate gold nanoparticles. The Brust-Schiffrin's method involves the reduction of gold‐salt solution using a thiol‐based organic solvent in a two‐phase system. The organic layer is separated, evaporated, and mixed with ethanol to get rid of excess thiol. The crude product is further dissolved in toluene and precipitated in ethanol. A modified Brust-Schiffrin process was carried out by Murray et al. [10] or commonly called as "place exchange" process. This process used various functionalities, such as bromine, cyanide, ferrocenyl, alcohol, formaldehyde, and anthraquinone, in replace‐ ment of a simple alkane group. Sulfur ligands such as xanthates [11], disulfides [12], di and trithiols [13], and resorcinarene tetrathiols [14] have also been utilized for gold nanoparticles (AuNPs) syntheses. Biphasic methods of AuNP synthesis can also use similar ligands such as phosphine [15], amine [16], carboxylate [17], isocyanides [18], citrate with acetone [19], and iodine [20]. Structures of these ligands are shown in **Figure 2**.

In general, there are three classifications of approaches to prepare AuNPs: (1) nonchemical methods such as electrochemical [21] and thermal decomposition of a metal‐salt solution [22], photochemical [23], sonochemical [24], laser ablation synthesis [25], and microwave‐assisted technique [26]; (2) biological sources such as the use of plant extracts and microorganism‐ assisted formation of metal nanoparticles. Bio-reduction of metal ions involves both intracellu‐ lar and extracellular precipitations of metal nanoparticles within the microorganism (**Figure 3**) [27]. Biomolecules such as proteins are mainly responsible for the synthesis of gold nanopar‐ ticles while enzymes produced in the outer layer membrane of the microorganism are respon‐ sible for the reduction of gold ions. The biological pathways for metal nanoparticles synthesis

**Figure 2.** Different ligand molecular structures that can be used for gold nanoparticle synthesis.

**Figure 3.** Biosynthetic mechanism of metal nanoparticles using microorganism [Taken from Ref. [27]].

can be carried out by a microorganism (e.g., bacteria [28], yeast (*P. jadini*), and fungal (*V. luteo‐ album*) cultures [29]), plants [30–32] and plant extracts [33]. In recent years, development of plant extract‐based synthesis of metal nanoparticles has been investigated. Using plant‐based synthesis results into more stable and faster rate of synthesis compared in the case of micro‐ organism [34]. (3) Use of a polymer as a template for metal nanoparticles (MNPs) generation is commonly called polymer‐mediated synthesis. This emerging type of approach was con‐ ceived to solve the issue on MNP aggregation.

#### **1.1. Polymer‐mediated synthesis of metal nanoparticles**

Polymers that have both reducing and stabilizing properties have been developed to syn‐ thesize metal nanoparticles. Such dual properties give pure and homogenous products. The main feature of this approach lies on its low cost, high efficiency, and environmentally benign nature. Several existing polymers, which display these dual properties (e.g., reducing and stabilizing metal nanoparticles), have already been used in the synthesis of MNPs such as poly(N‐vinyl‐2‐pyrrolidone) (PVP) [35], poly(allylamine) (PAAm) [36], poly(o‐phenylenedi‐ amine) (PoPD) [37], polyethyleneimine (PEI) [38], and poly(4‐styrenesulfonic acid‐co‐maleic acid) (PSSMA) [39]. Mechanisms have been studied in the reducing capacity of the PVP. These include a free radical mechanism, oxidation of the hydroxyl end groups [40] and the C=O double bond [41]. Other factors include an abundance of amino groups in the PAAm and PEI molecules that drive the reduction of gold ions into metal nanoparticles and strong bonding between the electrons donor, π orbitals donor, and the lone pair orbitals of amine groups of PoPD with the electron-deficient orbitals of gold nanoclusters providing efficient stabilizing effect. Due to the high impact polymer-assisted approach on the synthesis of metal nanopar‐ ticles, several studies came up with some concluded advantages.

(1) Only a small concentration of polymer is used. (2) The functional groups in the polymer can serve for dual properties. (3) Polymer template itself can control the size and morphology of MNPs and its resultant composite.

#### **1.2. Core‐shell particles (CSP)**

can be carried out by a microorganism (e.g., bacteria [28], yeast (*P. jadini*), and fungal (*V. luteo‐ album*) cultures [29]), plants [30–32] and plant extracts [33]. In recent years, development of plant extract‐based synthesis of metal nanoparticles has been investigated. Using plant‐based synthesis results into more stable and faster rate of synthesis compared in the case of micro‐ organism [34]. (3) Use of a polymer as a template for metal nanoparticles (MNPs) generation is commonly called polymer‐mediated synthesis. This emerging type of approach was con‐

**Figure 3.** Biosynthetic mechanism of metal nanoparticles using microorganism [Taken from Ref. [27]].

Polymers that have both reducing and stabilizing properties have been developed to syn‐ thesize metal nanoparticles. Such dual properties give pure and homogenous products. The main feature of this approach lies on its low cost, high efficiency, and environmentally benign nature. Several existing polymers, which display these dual properties (e.g., reducing and stabilizing metal nanoparticles), have already been used in the synthesis of MNPs such as poly(N‐vinyl‐2‐pyrrolidone) (PVP) [35], poly(allylamine) (PAAm) [36], poly(o‐phenylenedi‐ amine) (PoPD) [37], polyethyleneimine (PEI) [38], and poly(4‐styrenesulfonic acid‐co‐maleic acid) (PSSMA) [39]. Mechanisms have been studied in the reducing capacity of the PVP. These include a free radical mechanism, oxidation of the hydroxyl end groups [40] and the C=O double bond [41]. Other factors include an abundance of amino groups in the PAAm and PEI molecules that drive the reduction of gold ions into metal nanoparticles and strong bonding between the electrons donor, π orbitals donor, and the lone pair orbitals of amine groups of PoPD with the electron-deficient orbitals of gold nanoclusters providing efficient stabilizing effect. Due to the high impact polymer-assisted approach on the synthesis of metal nanopar‐

ceived to solve the issue on MNP aggregation.

94 Green Chemical Processing and Synthesis

**1.1. Polymer‐mediated synthesis of metal nanoparticles**

ticles, several studies came up with some concluded advantages.

With the promising potential of the polymer‐assisted approach on the synthesis of metal nanoparticles, the authors make use of polymeric particles. Here, it is referred as core‐shell particles (CSP). Some of the commonly used polymers as templates and nanoreactors for metal nanoparticle formation are poly(glycidyl methacrylate‐co‐N‐isopro‐ pylacrylamide) [(poly(GMA‐co‐NIPAM))] [42], poly(N‐isopropylacrylamide)‐co‐poly(acrylic acid) (PNIPAM‐co‐PAA) [43], glycidyl methacrylate (GMA) and *N*‐ isopropyl acrylamide (NIPAM) [44], polystyrene (PS) core and a polyaniline (PANI) [45], (poly(N‐isopro‐ pylacrylamide‐acrylic acid) P(NIPAM‐AA) [46], long cationic polyelectrolyte chains of poly(2‐aminoethyl methacrylate hydrochloride) (PAEMH)) [47], and poly(ionic liquid) (PIL) [48].

Over the past decade, a metal‐salt reduction process is the most common method for generating metal nanoparticles. This type of reaction has shown reliability and uniformity of metal nanoparticles produced. However, environmental concerns are not well addressed or worst not met. For example, the use of different forms of energy (e.g., photoirradiation, ultrasound irradiation, and high temperature boiling process) in both electrochemical and thermal decomposition methods is far way exploited [49] in addition to long and tedious synthetic procedures. And worst, giving low yields [50] with a high polydispersity of metal nanoparticles. Such high polydispersity is mostly observed in a reverse microemulsion of metal nanocomposite [51]. For metal nanoparticles bound ligands, the consequence of the difficulty in dispersing in water hinders the surface modification and functionalization for further applications [7]. As a result of this water incompatibility, metal nanoparticle proper‐ ties are altered [52]. Also, some reducing agents such as sodium borohydride and hydrazine are considered toxic chemicals and not tolerable for future commercial scale‐up [53]. Else, defective products or impurities may arise from excess reducing agents [54]. As a result, impurities left behind may eventually affect the composite material's functionality and its potential applications.

With the existing and emerging technologies in the synthesis of metal/polymeric nanocom‐ posites, there is still a great challenge to the concept of Green Chemistry. This concept aims at the development of methods for the synthesis of metal/polymeric in this case, with the least impact on humans and environment as a whole. The challenges in creating novel metal/poly‐ mer nanocomposites are: (1) to create a unique template that is an all-in-one platform that can reduce metal ions to nanoparticles, immobilize the resultant nanoparticles, and stabilize the composite particle; (2) to regulate the accessibility of the metal nanoparticles through control‐ ling external stimuli such as pH, temperature, and electrolyte; (3) to immobilize other organic and biological molecules for protection and deliveries; (4) to easily be purified and recovered; (5) to efficiently scale up process for commercialization.

## **2. Core‐shell microgel template and metal/polymeric nanocomposite synthesis**

A novel approach was developed with a simple yet versatile synthesis of a variety of amphi‐ philic core‐shell particles [55]. This approach enables to synthesize a broad range of core‐ shell particles with different chemical structure, composition, size, and functionality. The process uses aqueous‐based Chemistry, which is environmentally benign, and the particles are easy to synthesize in high solids content (up to 30%) in the absence of surfactant. A novel feature of this synthetic approach is that it combines graft copolymerization, *in situ* self‐assembly of the resulting amphiphilic graft copolymers and emulsion polymerization in a one‐step synthesis. In this chapter, core‐shell microgels were used in the synthesis of mono (Au) and bimetallic (Au@Ag) nanoparticles. Briefly, the mechanism involved in the core-shell microgel synthesis combines graft copolymerization of vinyl monomer from a water‐soluble polymer containing an amino group and self‐assembly of the resulting particle. Graft polym‐ erization of vinyl monomer in water happens when amino radicals are formed. The electron transfer and loss of proton form amino radicals during the interaction of alkyl hydroper‐ oxide (ROOH) with the amino group of the polymer backbone (i.e., PEI is mostly used). Alkoxyl radicals (RO) are inevitably produced during this interaction. The resulting amphi‐ philic macroradicals undergo self‐assembly forming micelle‐like microdomains, where they become loci for the further polymerization of the monomers. The generated RO·radical on the other hand initiates homopolymerization of the vinyl monomer or creates radicals for further graft polymerization. This process results in well-defined core-shell particle structure with a hydrophilic shell and a hydrophobic core.

#### **2.1. Synthesis of AuNPs in PNIPAm/PEI microgel template**

The preparation of the Au nanocomposite (Au/(PNIPAm/PEI)) particles was carried out based on a previous method [56] developed by Tan et al., performed via the addition of hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 ·3H<sup>2</sup> O) solution into the as‐prepared PNIPAm/PEI. The mixture was continuously stirred and carried out at different temperatures and pHs for 2 hours and heated at 60°C for about an hour. The resulting gold loaded microgels were then purified by centrifugation.

Gold nanoparticle formation in a microgel template is shown in a schematic diagram (**Figure 4**). Such formation from its ionic form is considered to be thermodynamically stable, which does not need any activation energy to form gold nanoparticles even at room temperature. Two successive reactions occur to complete the gold generation. First is the interaction between the negatively charged gold chloride ions (AuCl4 − ) and the cationic microgels. Once the gold ions are attracted into the microgel, a subsequent redox reaction occurs between the gold ions and available amine groups in the microgel template. As a result, gold ions are reduced while the amine groups are oxidized. Amine oxidation allows transfer of electrons from the amine to the gold ions, thus, generating zero‐state AuNPs. Such reaction was reported by Lala et al. [57], wherein they have proposed that the AuCl4 − ions are electrostatically bound to Environment-Friendly Approach in the Synthesis of Metal/Polymeric Nanocomposite Particles and Their Catalytic... http://dx.doi.org/10.5772/intechopen.68388 97

**2. Core‐shell microgel template and metal/polymeric nanocomposite** 

A novel approach was developed with a simple yet versatile synthesis of a variety of amphi‐ philic core‐shell particles [55]. This approach enables to synthesize a broad range of core‐ shell particles with different chemical structure, composition, size, and functionality. The process uses aqueous‐based Chemistry, which is environmentally benign, and the particles are easy to synthesize in high solids content (up to 30%) in the absence of surfactant. A novel feature of this synthetic approach is that it combines graft copolymerization, *in situ* self‐assembly of the resulting amphiphilic graft copolymers and emulsion polymerization in a one‐step synthesis. In this chapter, core‐shell microgels were used in the synthesis of mono (Au) and bimetallic (Au@Ag) nanoparticles. Briefly, the mechanism involved in the core-shell microgel synthesis combines graft copolymerization of vinyl monomer from a water‐soluble polymer containing an amino group and self‐assembly of the resulting particle. Graft polym‐ erization of vinyl monomer in water happens when amino radicals are formed. The electron transfer and loss of proton form amino radicals during the interaction of alkyl hydroper‐ oxide (ROOH) with the amino group of the polymer backbone (i.e., PEI is mostly used). Alkoxyl radicals (RO) are inevitably produced during this interaction. The resulting amphi‐ philic macroradicals undergo self‐assembly forming micelle‐like microdomains, where they become loci for the further polymerization of the monomers. The generated RO·radical on the other hand initiates homopolymerization of the vinyl monomer or creates radicals for further graft polymerization. This process results in well-defined core-shell particle structure

The preparation of the Au nanocomposite (Au/(PNIPAm/PEI)) particles was carried out based on a previous method [56] developed by Tan et al., performed via the addition of hydrogen

The mixture was continuously stirred and carried out at different temperatures and pHs for 2 hours and heated at 60°C for about an hour. The resulting gold loaded microgels were then

Gold nanoparticle formation in a microgel template is shown in a schematic diagram (**Figure 4**). Such formation from its ionic form is considered to be thermodynamically stable, which does not need any activation energy to form gold nanoparticles even at room temperature. Two successive reactions occur to complete the gold generation. First is the interaction between

ions are attracted into the microgel, a subsequent redox reaction occurs between the gold ions and available amine groups in the microgel template. As a result, gold ions are reduced while the amine groups are oxidized. Amine oxidation allows transfer of electrons from the amine to the gold ions, thus, generating zero‐state AuNPs. Such reaction was reported by

−

−

O) solution into the as‐prepared PNIPAm/PEI.

) and the cationic microgels. Once the gold

ions are electrostatically bound to

·3H<sup>2</sup>

**synthesis**

96 Green Chemical Processing and Synthesis

with a hydrophilic shell and a hydrophobic core.

tetrachloroaurate(III) trihydrate (HAuCl4

the negatively charged gold chloride ions (AuCl4

Lala et al. [57], wherein they have proposed that the AuCl4

purified by centrifugation.

**2.1. Synthesis of AuNPs in PNIPAm/PEI microgel template**

**Figure 4.** Schematic diagram on the synthesis of gold nanoparticles (AuNPs) using PNIPAm/PEI microgel as a template.

the protonated amine group and simultaneously reduced by the unprotonated amine group. AuNPs clusters were allowed to grow by further heating or manipulating their temperature or pH conditions.

#### **2.2. Synthesis of Au@Ag/core‐shell PNIPAm/PEI microgel composite particles**

Au@Ag bimetallic nanoparticle synthesis was carried out through a progressive reduction of Au and Ag metal ions as performed previously by Tan et al. [58]. Gold metal ions were first reduced to the shell component of the microgel. These gold metal nanoparticles were then used as a seed for the successive reduction of the silver ions to silver nanoparticles. Appropriate molar ratios of Au3+ and Ag1+ ions were used and mixed for 30 minutes to reduce the silver ions to metal nanoparticles further, followed by heating at 60° C for 30 minutes.

Generating bimetallic nanoparticles in a microgel template is shown in a schematic diagram (**Figure 5a**). After the synthesis of microgel template through graft copolymerization, gold clusters were first generated on the shell layers of the templates. Such generation is possible due to hyperbranched PEI in the shell region, which contains amine groups that are known to have reducing ability to generate metal nanoparticles [59]. And through the chelating proper‐ ties of the same amino groups, PEI can also complex with metal ions and metal nanoparticles [60]. The preformed gold nanoparticles acted as seeds or nucleation sites for further bimetallic nanocrystals formation. Such formation of Au seeds occurred after 30–40 minutes of reaction at room temperature which was evident by the change of the solution color from turbid white to light pink. The transition of the solution color also signifies the change in ionization potential and electron affinity values of Au atoms. Au atoms' ionization potential becomes higher than those of Ag atoms. Such shift results to a larger electronegativity value for Au, wherein signifi‐ cant charge transfer may occur from silver to gold atoms [61]. Simultaneously, silver metal ions (Ag+) were reduced to silver nanoparticles through under‐potential deposition mechanism [62], or noble metal induced reduction (NMIR) method [63]. Further, illustration of this mecha‐ nism is displayed in **Figure 5b**. It is shown that gold nanoparticles used as a seed for further reduction of silver ions to silver nanoparticles. The AuNP with a bigger size attracts the silver ions resulting to a bimetallic alloy nanoparticle. Further heating was necessary to improve the crystallinity of the bimetallic nanoparticles. Consequently, heating of these composite particles removes partially the template resulting in the naked exposure bimetallic nanoparticles.

**Figure 5.** (a) Schematic diagram on the synthesis of bimetallic nanoparticles from Au/PNIPAm/PEI composite particles, (b) mechanism on the formation of Au@Ag nanoparticles from Au/PNIPAm/PEI nanocomposites.

#### **3. Multifunctional roles of PNIPAm/PEI microgel**

#### **3.1. Microgel as a nanoreactor**

Transmission electron microscope (TEM) images of both the empty PNIPAm/PEI microgel template and the gold nanoparticle-filled composite particles are shown in **Figure 4**. Herein, the empty PNIPAm/PEI microgel particles show a core‐shell structure (**Figure 6a**), while AuNP-filled microgel template (**Figure 6b**) shows darks spots around its perimeter. The gold nanoparticles within the microgel template look like clusters of small gold nanoparticles. When heated to 60°C for an hour, the gold nanoparticles further crystallized and became clearer. The size of the gold nanoparticles was roughly estimated at an average of 17.60 ± 2.34 nm with a narrow size distribution.

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**Figure 6.** TEM images of the (a) PNIPAm/PEI microgel template, (b) Au/(PNIPAm/PEI) composite particles synthesized at 25° C and pH 7.30, and (c) heated Au/PNIPAm/PEI composite particles.

The kinetics of the formation of gold nanoparticles was monitored through the UV‐vis absor‐ bance at 525 nm wavelength with time. Such adsorption at 525 nm wavelength is one charac‐ teristic of AuNPs. In **Figure 7**, the increase of the absorbance was fast in the first 30 minutes of reaction and became slower after that, until the third hour of reaction. The reaction started to cease after 3 hours, and no significant change of absorbance was further observed. This data concludes that both electrostatic interaction and reduction of gold ions to nanoparticles simultaneously occurred at a fast rate. This graph further proves that there is a rapid nucle‐ ation during the initial stage of gold‐ion reduction, resulting in numerous Au clusters. It was further concluded that the reduction of gold ions to AuNPs using microgel was 625 times faster than the naked hyperbranched PEI (linear curve with hollow points).

#### **3.2. Microgel as a stabilizer of AuNPs in composite particles**

**3. Multifunctional roles of PNIPAm/PEI microgel**

(b) mechanism on the formation of Au@Ag nanoparticles from Au/PNIPAm/PEI nanocomposites.

Transmission electron microscope (TEM) images of both the empty PNIPAm/PEI microgel template and the gold nanoparticle-filled composite particles are shown in **Figure 4**. Herein, the empty PNIPAm/PEI microgel particles show a core‐shell structure (**Figure 6a**), while AuNP-filled microgel template (**Figure 6b**) shows darks spots around its perimeter. The gold nanoparticles within the microgel template look like clusters of small gold nanoparticles. When heated to 60°C for an hour, the gold nanoparticles further crystallized and became clearer. The size of the gold nanoparticles was roughly estimated at an average of 17.60 ± 2.34

**Figure 5.** (a) Schematic diagram on the synthesis of bimetallic nanoparticles from Au/PNIPAm/PEI composite particles,

**3.1. Microgel as a nanoreactor**

98 Green Chemical Processing and Synthesis

nm with a narrow size distribution.

There are two kinds of stabilization that holds both the AuNPs and the composite material in suspension. Such stabilization is due to the microgel template's property to provide electro‐ static interaction between composite particles and steric effect of the PEI shell.

Electrostatic interaction between the composite particles and the gold nanoparticles within the template is the primary contributor to its stabilization. When Au nanoparticles are formed and immobilized in the particle template, the overall size of the composite particle becomes smaller than the pure template itself. This shrinkage is due to the formation of gold/amine complexes resulting in the contraction of the PEI shell. Such contraction of PEI shell reduces metal nanoparticles leaking from its template or its individual network‐cage‐like structure. Consequently, continuous leaking of naked AuNPs will form aggregates within the template. On the other hand, the same repulsion force acts between composite particles. Such force prevents them from getting attracted to each other preventing them from forming precipitates eventually.

Steric contribution to the stabilization of the AuNPs comes from the hyperbranched structure of the PEI‐shell component in the microgel template. This type of stabilization is known in a lot of amphiphilic graft copolymers [64]. Such property of amphiphilic copolymers is due to the hydrophobic‐hydrophilic interaction of the copolymers involved. This interaction is very

**Figure 7.** Time courses of the absorbance monitored at 525 nm during the formation of gold nanoparticles in the presence of PNIPAm/PEI microgel particles (25°C, at pH 5.6).

significant on the stability of the microgel template itself and in the formation of Au/microgel composite particles. When this interaction happens in the microgel template, the PEI shell anchors in the gold nanoparticles, while the PNIPAm core is kept together away from the shell. With such action, both the shape and stability of the composite particles are achieved.

#### **3.3. Microgel as AuNP immobilizer**

The generated gold nanoparticles using microgel template were immobilized through the template PEI shell's properties. Primarily, the weak bonding between the amino group and the gold nanoparticles is the primary source of immobilization [65, 66]. Such immobilization strongly supported by the hyperbranched nature of the PEI [67], which helps to shield AuNPs into a network‐cage like structure. Such construction provides bulkiness and prevents the AuNPs from aggregating with neighboring AuNPs or composite particles. Furthermore, PEI‐ shell structure can also link the gold nanoparticles intact [68] within its boundary template.

There are five pieces of evidence to demonstrate the microgel acting as an immobilizer of gold nanoparticles: (1) In **Figure 6b**, AuNPs are seen as fuzzy gray dots embedded within the circumference of the microgel, attached in the shell region. (2) There was a decrease in the size of the pure microgel template when loaded with AuNPs. The decrease in size was due to the encapsulation of the gold metal ions attracted to the template. Absorption of the gold metal ions leads to the shrinking of the composite material. (3) There was a decrease in the zeta-potential from 30 to 15 mV from a pure microgel to Au-loaded template, respectively. Such decrease of the zeta-potential is attributed to the partial consumption of the cationic ammonium ions during the gold‐ion adsorption stage. (4) X‐ray photoelectron spectroscopy (XPS) (**Figure 14**) result further shows proof of the immobilization of AuNP in microgel tem‐ plate. This result verifies the location of AuNPs which are found within 2—10 nm deep from the surface of the microgel template. (5) The ligand role of the PEI shell (i.e., complexation of the water-soluble PEI with metal ions) plays a significant part of the immobilization of AuNPs. This ligand role property results in some advantages of the composite material such as water solubility, high capacity for metal uptake, easy separation of polymer complexes, high flex‐ ibility of the molecular conformation, and good chemical and physical stability [69–71].

#### **3.4. Microgel as a smart controller of AuNP accessibility**

significant on the stability of the microgel template itself and in the formation of Au/microgel composite particles. When this interaction happens in the microgel template, the PEI shell anchors in the gold nanoparticles, while the PNIPAm core is kept together away from the shell. With such action, both the shape and stability of the composite particles are achieved.

**Figure 7.** Time courses of the absorbance monitored at 525 nm during the formation of gold nanoparticles in the presence

The generated gold nanoparticles using microgel template were immobilized through the template PEI shell's properties. Primarily, the weak bonding between the amino group and the gold nanoparticles is the primary source of immobilization [65, 66]. Such immobilization strongly supported by the hyperbranched nature of the PEI [67], which helps to shield AuNPs into a network‐cage like structure. Such construction provides bulkiness and prevents the AuNPs from aggregating with neighboring AuNPs or composite particles. Furthermore, PEI‐ shell structure can also link the gold nanoparticles intact [68] within its boundary template.

There are five pieces of evidence to demonstrate the microgel acting as an immobilizer of gold nanoparticles: (1) In **Figure 6b**, AuNPs are seen as fuzzy gray dots embedded within the circumference of the microgel, attached in the shell region. (2) There was a decrease in the size of the pure microgel template when loaded with AuNPs. The decrease in size was due to the encapsulation of the gold metal ions attracted to the template. Absorption of the gold metal ions leads to the shrinking of the composite material. (3) There was a decrease in the zeta-potential from 30 to 15 mV from a pure microgel to Au-loaded template, respectively. Such decrease of the zeta-potential is attributed to the partial consumption of the cationic

**3.3. Microgel as AuNP immobilizer**

of PNIPAm/PEI microgel particles (25°C, at pH 5.6).

100 Green Chemical Processing and Synthesis

One of the best features of PNIPAm/PEI microgel template is its ability to regulate its size. Such ability is useful in the accessibility of the gold nanoparticles generated within the microgel template. This ability of the microgel comes from the stimuli‐responsive nature of the PNIPAm or some refer them to smart materials. In the case of PNIPAm/PEI microgel, such sensitivity is based on both sensitive pH and temperature. The core part of the microgel, PNIPAm is temperature sensitive, while the PEI shell is pH sensitive. The response of this soft template to temperature or pH affects its conformational structure. The changes in the structure of the tem‐ plate result in the controlled accessibility of AuNPs as demonstrated in **Figure 8**. Herein, the microgel template loaded with AuNPs is in different sizes under different pH or temperatures. At low pH, the template gets protonated and swells. Such swelling exposes the encapsulated AuNPs. However, when pH increases, the microgel becomes deprotonated and deswelling of the template occurs. By this action of the microgel template, AuNPs embedded within are trapped. The same action also controls that degree of plasmon coupling of AuNPs. Such cou‐ pling property originates from the dipole interaction among gold nanoparticles, which allows the control of the interparticle distance between gold nanoparticles [72].

On the other hand, when the temperature of the microgel system reaches beyond the lower critical solution temperature (LCST) point of the core part, PNIPAm (i.e., 32°C), the entire template shrinks. Such shrinking leads to the trapping of AuNPs within the template. But when the temperature goes below the LCST of PNIPAm, the template is more open and loose than the original condition. This looseness results in easy accessibility of the AuNPs within the template.

Microgel particles were subjected to different temperature conditions at 29°C, 45°C, and back to 29°C in aqueous solution to demonstrate the smart properties of the template. Their cor‐ responding structural changes of the microgel particles under different temperatures were captured with AFM analysis. Original microgel template at 29°C in a fluid mode is shown in **Figure 8a** with sizes ranging from 100 to 150 nm with quasi-spherical morphologies. When the temperature was raised to 45o C (**Figure 8b**), the templates decreased in size showing porous surfaces. Such phenomenon is attributed to the shrinking of the templates as it goes beyond its volume phase transition temperature (VPTT). However, when restored to 29°C (**Figure 8c**), the smooth morphology and size of the templates were restored. Such restoration demonstrates that the conformational changes of the template triggered by the response to temperature are reversible.

**Figure 8.** Left side: conformational changes of microgel template from stimuli response to pH solution and temperature. Right side: AFM micrographs of PNIPAM/PEI microgel particles measured in a fluid mode at different temperatures: (a) 29°C; (b) 45°C; and (c) Cooled from 45 to 29°C. Scale bar: 200 nm.

#### **4. Measurements and characterization**

#### **4.1. Particle size and surface charge**

Dynamic Light spectrophotometer measured the sizes of both pure and gold‐loaded micro‐ gels. Synthesized PNIPAm/PEI microgels have an average hydrodynamic diameter of 402 nm while the gold‐microgel composite particles were measured at 298 nm as shown in **Figure 9a**. The polydispersity indices on both unloaded and gold-loaded particles were 0.050 and 0.055, respectively. As anticipated, the particle size of the gold‐loaded microgel is smaller than the pure microgel. This decline in size is due to the incorporation of the counterions into the microgel template during the metal ion absorption and reduction stages. Furthermore, when AuNPs are formed, the microgel network immobilizes *in‐situ* generated AuNPs by capturing them on its network-like structure, providing a steric effect on the metal nanoparticles.

The gold‐loaded microgel particles were further characterized based on its surface charge expressed in zeta‐potential. Gold‐loaded composite particles have an average zeta‐potential of 15 mV at pH 7.00 in an aqueous medium. At this state, composite particles were stable with no aggregation or precipitation occurred. However, zeta-potential can be affected by the pH solution in a colloidal system. To demonstrate this effect, **Figure 9b** demonstrates the change of the surface charge as a function of pH. In the same figure, gold-loaded microgels can be grouped into a three-phase behavior regarding zeta-potential versus pH solution. The first phase shows a constant zeta‐potential behavior at a pH range of 2–6.5. The second phase is between pH 6.5 and 9.0, which shows a noticeable decrease of zeta-potential. The third

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**Figure 9.** (a) Particle size and size distribution of pure microgel template and Au/(PNIPAm/PEI) composite particles synthesized at optimum conditions of 25°C and pH 7.30. (b) Zeta-potential profile of gold-loaded microgel (solid points) and pure microgel template (hollow points) in different pH solution.

phase between pH 9.0 and 11.5 gives a slight change of zeta-potential values. The constant zeta-potential in the first phase is attributed to the saturation of microgel template with AuNP at this certain range of pH. However, increasing the pH affects the composite material and decreases its surface charge surpassing the isoelectric point (i.e., pH 9.2). Further increase of pH at this stage may supersaturate the microgel template and then again give a very minimal or no effect on its zeta-potential.

**4. Measurements and characterization**

29°C; (b) 45°C; and (c) Cooled from 45 to 29°C. Scale bar: 200 nm.

Dynamic Light spectrophotometer measured the sizes of both pure and gold‐loaded micro‐ gels. Synthesized PNIPAm/PEI microgels have an average hydrodynamic diameter of 402 nm while the gold‐microgel composite particles were measured at 298 nm as shown in **Figure 9a**. The polydispersity indices on both unloaded and gold-loaded particles were 0.050 and 0.055, respectively. As anticipated, the particle size of the gold‐loaded microgel is smaller than the pure microgel. This decline in size is due to the incorporation of the counterions into the microgel template during the metal ion absorption and reduction stages. Furthermore, when AuNPs are formed, the microgel network immobilizes *in‐situ* generated AuNPs by capturing them on its network-like structure, providing a steric effect on the metal nanoparticles.

**Figure 8.** Left side: conformational changes of microgel template from stimuli response to pH solution and temperature. Right side: AFM micrographs of PNIPAM/PEI microgel particles measured in a fluid mode at different temperatures: (a)

The gold‐loaded microgel particles were further characterized based on its surface charge expressed in zeta‐potential. Gold‐loaded composite particles have an average zeta‐potential of 15 mV at pH 7.00 in an aqueous medium. At this state, composite particles were stable with no aggregation or precipitation occurred. However, zeta-potential can be affected by the pH solution in a colloidal system. To demonstrate this effect, **Figure 9b** demonstrates the change of the surface charge as a function of pH. In the same figure, gold-loaded microgels can be grouped into a three-phase behavior regarding zeta-potential versus pH solution. The first phase shows a constant zeta‐potential behavior at a pH range of 2–6.5. The second phase is between pH 6.5 and 9.0, which shows a noticeable decrease of zeta-potential. The third

**4.1. Particle size and surface charge**

102 Green Chemical Processing and Synthesis

To demonstrate the effect of temperature on its surface charge, **Figure 10** shows that vary‐ ing solution temperatures from 25 to 40°C strongly affect the zeta-potential of both the pure and gold‐loaded microgel particles. An abrupt change of surface charge in the temperature range between 29 and 34°C is obvious. This region crosses the VPTT region of the microgels. However, prior and after this temperature range, the zeta‐potential was more or less constant. Such behavior is attributed to the increase in the surface charge density of the composite par‐ ticles with the decrease in size. Smaller particles result in higher surface charge density, result‐ ing in the shrinking of the composite particles, as also observed in the work of Ou et al. [73].

#### **4.2. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images**

SEM and TEM images of the AuNP/(PNIPAm/PEI) composite particles are both shown in **Figure 11**. **Figure 11a** shows uniform spherical morphologies of the composite particles. Such morphologies are identical to that of the original microgel template (**Figure 11a** inset). However, partial agglomeration of the particles is also observed which may have occurred during the drying of the SEM sample treatment. **Figure 11b** shows the TEM image of the gold‐loaded microgel which shows clearly the location of the AuNPs within the microgel template. Specifically, AuNPs reside around the circumference of the microgel attached in the shell region. Apparently, these images also show the effectiveness of the immobilization of the gold nanoparticles within the microgel network.

**Figure 10.** Zeta-potential profile of gold-loaded microgel composite particles (solid points) and pure microgel template (hollow points) in different temperature conditions.

**Figure 11.** (a) SEM image of Au/ (PNIPAm/PEI) composite particles, inset is the original microgel template, (b) TEM image of Au/ (PNIPAm/PEI) composite particles. The particles were synthesized at 25° C and pH 7.30.

#### **4.3. UV‐vis spectroscopy**

The formation of gold nanoparticles in the presence of microgel template was monitored by a UV‐vis spectroscopy as a function of time. In this case, gold nanoparticle formation was evident at the absorbance wavelength of 525 nm as shown in **Figure 12**. Gold formation starts after 20 minutes of reaction together with the change in color from turbid white to light pink. An increase of absorbance happens as further reaction occurs for gold nanoparticle formation. This also gives rise on the concentration of gold nanoparticle at higher absorbance. After 4 hours of reaction, it was observed that there were no more significant changes in the absor‐ bance intensity, which signify that gold nanoparticles have ceased to grow or gold ions have ultimately been reduced to nanoparticles. Wavelength absorbance of gold nanoparticles rang‐ ing from 520 to 525 nm is a characteristic of Au nanoparticles with spherical shape with sizes ranging from 15 to 30 nm [74].

#### **4.4. High‐resolution TEM (HRTEM) and X‐ray diffraction**

To get a closer look at the image of AuNPs immobilized within the PNIPAm/PEI microgels template, an HRTEM analysis was performed as shown in **Figure 13a**. This image reveals a five-fold twinned Au nanocrystal with a diameter of 22.5 nm. Top inset of **Figure 13a** displays the selected-area of electron diffraction (SAED) pattern of AuNPs examined, which reveals ring patterns indexed as (111), (200), and (222) of a face-centered cubic (FCC) gold lattice. Furthermore, this five-fold twinned boundary at the center of an Au nanocrystal can suggest formation of a multiply twinned particle (MTP) close to an icosahedral gold nanostructure. The fuzzy portion observed on TEM image is attributed to the composite particle's sensitiv‐ ity to misorientation and distortion of the ideal icosahedrons. Aside from the twin boundary observed, the Au nanostructure is mainly composed of (111) planes with a d-spacing of 0.236 nm as shown in **Figure 13b**. The lattice plane is separated by a twin boundary indicated as a white line on the image. The crystallinity of AuNPs embedded in the microgel template is

**Figure 12.** UV-vis spectra profile for reduction of gold ions to nanoparticles using a PNIPAm/PEI microgels versus time (minute). Experiment was performed at 25°C, pH 5.6, 200 rpm, with N/Au molar ratio of 28.5.

**4.3. UV‐vis spectroscopy**

(hollow points) in different temperature conditions.

104 Green Chemical Processing and Synthesis

The formation of gold nanoparticles in the presence of microgel template was monitored by a UV‐vis spectroscopy as a function of time. In this case, gold nanoparticle formation was evident at the absorbance wavelength of 525 nm as shown in **Figure 12**. Gold formation starts

**Figure 11.** (a) SEM image of Au/ (PNIPAm/PEI) composite particles, inset is the original microgel template, (b) TEM

C and pH 7.30.

image of Au/ (PNIPAm/PEI) composite particles. The particles were synthesized at 25°

**Figure 10.** Zeta-potential profile of gold-loaded microgel composite particles (solid points) and pure microgel template

**Figure 13.** (a) HRTEM image of Au nanoparticle embedded within a microgel template, inset is the selected-area electron diffraction (SAED) pattern, (b) (111) planes of Au nanocrystal with a *d*-spacing of 0.236 nm, and (c) XRD spectra of Au/ PNIPAm/PEI composite particles.

analyzed through an X-ray diffractometer in **Figure 13c**. Au nanocrystals formed have lattice arrangements of (111), (200), and (220) at corresponding angles of 38, 44, and 65o. This result is consistent with the previous SAED analysis except for the (220) lattice with a dominant (111) arrangement.

#### **4.5. Surface composition using X‐ray photoelectron spectroscopy**

To further investigate the formation of AuNPs using a microgel template, X‐ray photoelectron spectroscopy (XPS) analysis at a depth of 10 nm was used. Results shown in **Figure 14** reveal an XPS spectra with binding energies of different elements present in the composite particles. The binding energies correspond to elements of C, O, N, and Au. Convoluted C1s spectra were fitted with peaks at 285.0 and 287.9 eV, assigned to C-C/C-H and C-O bonds, respectively. N1s peak fitted at 399.3 assigned to amines coordinated with AuNPs. O1s at 531.2 eV corresponds to the carbonyl functional group of the microgel. Zero‐valent AuNPs are observed from its two peak characteristics at 84.3 and 88 eV, consistent with literature [75]. Other characteristic of AuNP is its XPS spectra peak‐to‐peak distance of 3.7 eV on the Au 4f doublet which further gives a standard measure of the Au<sup>0</sup> oxidation state [76]. As the AuNPs attached to the amino groups, detection of N1s in the XPS analysis weakens due to the overlapping of AuNPs on the amine group [77], eventually strengthening the Au signal. This amine‐gold interaction was also observed in the work of Kumar et al. [78] and similar to Manna et al. [79]. Nitrogen peak was also curve fitted into two components at 399.3 and 401.2 eV. The first one corresponds to the amine (free and coordinated to gold) and the other corresponds to the protonated amine or ammonium. With the presence of these two peaks, AuNP binding to the amine group is more on metal‐ligand coordination (metallic gold atom and amine) than electrostatic interac‐ tion (between ammonium and the negative charges on the surface of the particles).

Further study on the same XPS spectra indicates that there was no significant change of O1s to C1s ratio before and after gold loading. Such insignificant change further proves the presence of the carbonyl functional group, PNIPAm, in the microgel. The XPS spectra also mean that at a depth of at most 10 nm, PNIPAm is present within the shell region partially overlapping the

Environment-Friendly Approach in the Synthesis of Metal/Polymeric Nanocomposite Particles and Their Catalytic... http://dx.doi.org/10.5772/intechopen.68388 107

**Figure 14.** XPS spectra of AuNP embedded in PNIPAm/PEI microgel template. Inset is the Au 4f core‐level spectra.

analyzed through an X-ray diffractometer in **Figure 13c**. Au nanocrystals formed have lattice arrangements of (111), (200), and (220) at corresponding angles of 38, 44, and 65o. This result is consistent with the previous SAED analysis except for the (220) lattice with a dominant

**Figure 13.** (a) HRTEM image of Au nanoparticle embedded within a microgel template, inset is the selected-area electron diffraction (SAED) pattern, (b) (111) planes of Au nanocrystal with a *d*-spacing of 0.236 nm, and (c) XRD spectra of Au/

To further investigate the formation of AuNPs using a microgel template, X‐ray photoelectron spectroscopy (XPS) analysis at a depth of 10 nm was used. Results shown in **Figure 14** reveal an XPS spectra with binding energies of different elements present in the composite particles. The binding energies correspond to elements of C, O, N, and Au. Convoluted C1s spectra were fitted with peaks at 285.0 and 287.9 eV, assigned to C-C/C-H and C-O bonds, respectively. N1s peak fitted at 399.3 assigned to amines coordinated with AuNPs. O1s at 531.2 eV corresponds to the carbonyl functional group of the microgel. Zero‐valent AuNPs are observed from its two peak characteristics at 84.3 and 88 eV, consistent with literature [75]. Other characteristic of AuNP is its XPS spectra peak‐to‐peak distance of 3.7 eV on the Au 4f doublet which further

groups, detection of N1s in the XPS analysis weakens due to the overlapping of AuNPs on the amine group [77], eventually strengthening the Au signal. This amine‐gold interaction was also observed in the work of Kumar et al. [78] and similar to Manna et al. [79]. Nitrogen peak was also curve fitted into two components at 399.3 and 401.2 eV. The first one corresponds to the amine (free and coordinated to gold) and the other corresponds to the protonated amine or ammonium. With the presence of these two peaks, AuNP binding to the amine group is more on metal‐ligand coordination (metallic gold atom and amine) than electrostatic interac‐

Further study on the same XPS spectra indicates that there was no significant change of O1s to C1s ratio before and after gold loading. Such insignificant change further proves the presence of the carbonyl functional group, PNIPAm, in the microgel. The XPS spectra also mean that at a depth of at most 10 nm, PNIPAm is present within the shell region partially overlapping the

tion (between ammonium and the negative charges on the surface of the particles).

oxidation state [76]. As the AuNPs attached to the amino

**4.5. Surface composition using X‐ray photoelectron spectroscopy**

(111) arrangement.

PNIPAm/PEI composite particles.

106 Green Chemical Processing and Synthesis

gives a standard measure of the Au<sup>0</sup>

core. With this overlapping of shell component to the core makes the whole microgel system shrink whenever PNIPAm shell becomes sensitive to temperature. Such phenomenon further verifies our claim that the amine group residing in the PEI shell l is mainly responsible for the formation and binding of the AuNPs.

#### **5. Catalytic activities of gold/microgel and gold@silver/microgel nanocomposite particles**

To demonstrate the catalytic activity of gold and gold@silver nanoparticles in a microgel template, a catalytic reduction first order kinetic model (i.e.*, p*‐nitrophenol reduction by sodium borohydride) was chosen. Silver and gold metal nanoparticles have a wide absorp‐ tion band in the visible region of the electromagnetic spectrum. Thus, they are easy to characterize and with a wide availability of related literature. These metal nanoparticles have also been involved in many catalytic organic reactions and synthesis in both pure and alloyed form. Previous study suggests that silver preserves the overall spherical morphology of the resultant bimetallic eventually prevents the tendency to phase segregate [85]. These are the primary reasons why these two metals have been chosen to demonstrate the metal nanoparticle forming capabilities of the microgel (i.e*.,* PNIPAm/PEI) template. The *p*‐nitro‐ phenol solution exhibits a typical absorption peak at around 320 nm under neutral or acidic condition. When sufficient amount of NaBH<sup>4</sup> is added, the nitrophenolate ions become the dominant species and reduce to aminophenol. Such conversion causes the absorption peak to shift to 400 nm. In the absence of any catalyst, the reduction of *p*-nitrophenol by NaBH<sup>4</sup>

cannot proceed based on a control experiment. And theoretically, this is because the *E*<sup>o</sup> value for the reduction of *p‐*nitrophenol to *p*-aminophenol was −0.76 V and that of H<sup>3</sup> BO<sup>3</sup> /BH<sup>4</sup> − was −1.33 V versus the standard hydrogen electrode (NHE). However, when a reduction of *p*‐nitrophenol starts, a new peak appears at about 310 nm, which corresponded to the typi‐ cal absorption peak of *p*‐aminophenol Physical change on the solution color is also obvious during the reaction [80]. For the case of gold@silver metal nanoparticle as catalyst, it only took 2 minutes to complete the catalytic reaction (**Figure 15a**). When using monometallic Au catalyst, catalytic reactions were completed in 15 minutes and 3.5 minutes using dif‐ ferent amine to gold ratios (i.e., 28.2 and 14.09, respectively). The catalytic activity of the bimetallic catalyst is obviously higher than that of the monometallic catalyst using the same template.

The kinetic rate constant, which is proportional to its overall kinetic rate in a first order reac‐ tion, was estimated from its slope. The control sample has a rate constant of 5.4 × 10−3 s−1. However, when monometallic gold nanoparticles (N/Au = 28.20 mole ratio) was used as a catalyst, the reaction proceeded approximately 10 times faster (i.e., with a rate constant of 2.44 × 10−2 s−1) than without catalysts. Moreover, when bimetallic gold@silver nanoparticles were used as a catalyst, the reaction rate was significantly enhanced. The enhancement in catalytic activity is attributed to the synergistic effects and the flexible design between the two metal nanoparticles [81], in this case gold and silver nanoparticles. The electronic and geometrical properties of the synthesized bimetallic nanoparticles can also affect the catalytic activity. Similar studies suggest that the increase in the number of low coordination number, edge and corner sites can also enhance catalytic activity [82]. Surface science studies conclude that the surface electronic structure can be modified by the interactions between the two kinds of atoms in the bimetallic alloy owing to ligand [83] and strain effects [84]. **Figure 15b** shows comparison of the different catalytic reaction rates constants by plotting ln (*C*<sup>t</sup> /*C*<sup>0</sup> ) versus

**Figure 15.** (a) UV-vis spectroscopy profile for the reduction of *p‐*nitrophenol to *p*‐aminophenol using Au@Ag/(PNIPAm/ PEI) composite particle as a catalyst. The different colored-curves refer to the different 30 second time intervals, (b) plot of ln (*C*<sup>t</sup> /*C*<sup>0</sup> ) as a function of time for the reaction catalyzed by Au/PNIPAm/PEI in different N/Au mole ratios and Au@Ag bimetallic nanoparticles in PNIPAm/PEI template. Inset is the reaction scheme of the catalytic reaction model used (i.e., reduction of *p*‐nitrophenol to *p*‐aminophenol).

reaction time for the reduction of *p*‐nitrophenol. The results demonstrate that the increase or incorporation of different metal nanoparticles can significantly increase the reduction rate.

#### **5.1. Modification of AuNP to Au@Ag bimetallic NP and its effect on catalysis**

cannot proceed based on a control experiment. And theoretically, this is because the *E*<sup>o</sup>

was −1.33 V versus the standard hydrogen electrode (NHE). However, when a reduction of *p*‐nitrophenol starts, a new peak appears at about 310 nm, which corresponded to the typi‐ cal absorption peak of *p*‐aminophenol Physical change on the solution color is also obvious during the reaction [80]. For the case of gold@silver metal nanoparticle as catalyst, it only took 2 minutes to complete the catalytic reaction (**Figure 15a**). When using monometallic Au catalyst, catalytic reactions were completed in 15 minutes and 3.5 minutes using dif‐ ferent amine to gold ratios (i.e., 28.2 and 14.09, respectively). The catalytic activity of the bimetallic catalyst is obviously higher than that of the monometallic catalyst using the

The kinetic rate constant, which is proportional to its overall kinetic rate in a first order reac‐ tion, was estimated from its slope. The control sample has a rate constant of 5.4 × 10−3 s−1. However, when monometallic gold nanoparticles (N/Au = 28.20 mole ratio) was used as a catalyst, the reaction proceeded approximately 10 times faster (i.e., with a rate constant of 2.44 × 10−2 s−1) than without catalysts. Moreover, when bimetallic gold@silver nanoparticles were used as a catalyst, the reaction rate was significantly enhanced. The enhancement in catalytic activity is attributed to the synergistic effects and the flexible design between the two metal nanoparticles [81], in this case gold and silver nanoparticles. The electronic and geometrical properties of the synthesized bimetallic nanoparticles can also affect the catalytic activity. Similar studies suggest that the increase in the number of low coordination number, edge and corner sites can also enhance catalytic activity [82]. Surface science studies conclude that the surface electronic structure can be modified by the interactions between the two kinds of atoms in the bimetallic alloy owing to ligand [83] and strain effects [84]. **Figure 15b** shows

comparison of the different catalytic reaction rates constants by plotting ln (*C*<sup>t</sup>

**Figure 15.** (a) UV-vis spectroscopy profile for the reduction of *p‐*nitrophenol to *p*‐aminophenol using Au@Ag/(PNIPAm/ PEI) composite particle as a catalyst. The different colored-curves refer to the different 30 second time intervals, (b) plot

) as a function of time for the reaction catalyzed by Au/PNIPAm/PEI in different N/Au mole ratios and Au@Ag bimetallic nanoparticles in PNIPAm/PEI template. Inset is the reaction scheme of the catalytic reaction model used (i.e.,

for the reduction of *p‐*nitrophenol to *p*-aminophenol was −0.76 V and that of H<sup>3</sup>

same template.

108 Green Chemical Processing and Synthesis

of ln (*C*<sup>t</sup> /*C*<sup>0</sup>

reduction of *p*‐nitrophenol to *p*‐aminophenol).

value

BO<sup>3</sup> /BH<sup>4</sup> −

/*C*<sup>0</sup>

) versus

The main goal in making bimetallic nanoparticles is to enhance the catalytic activity in the reduction of *p*‐nitrophenol to *p*‐aminophenol. Through the introduction of silver ions into the as-prepared seed gold nanoparticles, surface modification was achieved in the resultant bime‐ tallic Au@Ag nanoparticles. This modification affects the electronic properties of the resultant bimetallic nanoparticles affecting the catalytic activities. Thus, the role of the Ag in the bime‐ tallic structure is a co‐catalyst able to promote the ligand effect [85].

Ligand effect suggests that with the presence of a co-catalyst, Ag is important for the redox reaction (i.e., reduction of *p‐*nitrophenol) occurring on Ag@Au interfaces [86]. These Ag@Au inter‐ faces are the main actors in improving the catalytic activities. **Figure 16** demonstrates an Ag@Au interface with different work functions (i.e., Au (~5.3 eV) and Ag (~4.7 eV)). Since Ag has a lower work function compared to Au, electrons leave from the Ag atom side of the interface toward the Au side through a depleted region (Region D). As a result, the Au becomes an electron-rich region (Region E). The abundance of electrons on the Au side initiates the uptake of electron from the reactants (i.e., *p*‐nitrophenol) on top of the usual uptake from the depleted region. Thus, the more interfaces there are the more depletion and surplus of electron exist, resulting to increase the adsorption of reactants to be reduced on top of the interfacial regions. Such mechanism is consistent with the study of Zhang et al. [87] wherein the increasing electronegativity of Au with

**Figure 16.** Diagram on the transport of electron from an Ag‐Au interface bimetallic NP [Adopted from Ref. [87]].

respect to Ag facilitates adsorbate binding, increasing the electron transfer to *p*‐nitrophenol. As a result, this reduces the activation energy barrier, thus increasing the catalytic activity.

## **6. Conclusion**

Environment‐friendly approach on the synthesis of metal/polymeric nanocomposite particles was demonstrated in this chapter through the fabrication of Au and Au@Ag nanoparticles using a microgel template (i.e., PNIPAm/PEI). PNIPAm/PEI microgel template plays a crucial role in the reduction of metal salts, stabilization, and immobilization of the resulting metal/ polymer nanocomposites. Furthermore, it can also act as a regulator of metal nanoparticles. Catalytic activities of the Au and Au@Ag metal nanoparticles in microgel template were also demonstrated in the reduction of *p*‐nitrophenol to *p‐*aminophenol.

#### **Acknowledgements**

We highly acknowledge the Hong Kong Polytechnic University that has given us the chance to explore our interests in Applied Chemistry. And we are grateful to everyone who inspired us to write this chapter.

## **Author details**

Noel Peter Bengzon Tan1 \* and Cheng Hao Lee2

\*Address all correspondence to: bengzontan@nami.org.hk

1 Environmental Technologies Section, Nano and Advanced Materials Institute, Ltd., Hong Kong

2 Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hong Kong

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respect to Ag facilitates adsorbate binding, increasing the electron transfer to *p*‐nitrophenol. As a

Environment‐friendly approach on the synthesis of metal/polymeric nanocomposite particles was demonstrated in this chapter through the fabrication of Au and Au@Ag nanoparticles using a microgel template (i.e., PNIPAm/PEI). PNIPAm/PEI microgel template plays a crucial role in the reduction of metal salts, stabilization, and immobilization of the resulting metal/ polymer nanocomposites. Furthermore, it can also act as a regulator of metal nanoparticles. Catalytic activities of the Au and Au@Ag metal nanoparticles in microgel template were also

We highly acknowledge the Hong Kong Polytechnic University that has given us the chance to explore our interests in Applied Chemistry. And we are grateful to everyone who inspired

1 Environmental Technologies Section, Nano and Advanced Materials Institute, Ltd., Hong

2 Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic

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result, this reduces the activation energy barrier, thus increasing the catalytic activity.

demonstrated in the reduction of *p*‐nitrophenol to *p‐*aminophenol.

\* and Cheng Hao Lee2

\*Address all correspondence to: bengzontan@nami.org.hk

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**6. Conclusion**

110 Green Chemical Processing and Synthesis

**Acknowledgements**

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Noel Peter Bengzon Tan1

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