**4. Composites**

120 The Development and Application of Microwave Heating

power of 1100 W, according to the reaction (6):

2008) from KF and MCl2 (M = Zn, Mn, Co, Fe) in a Parr hydrothermal vessel, subjected to MW-heating in a domestic microwave operating for 4 min at 2.45 GHz with a maximum

2 3 3KF MCl KMF 2KCl (6)

**Figure 9.** The SEM images (a and b) for **T**-InP (obtained by a **traditional** solvothermal procedure) samples and (c and d) for **M**-InP (obtained by a **microwave** solvothermal procedure) samples after HCl

treatment. Inserted in (c) is the close-up of hollow nature for **M**-InP samples. With permission.

**Figure 10.** The proposed formation mechanism for InP hollow spheres. With permission.

The composites, reported as fabricated by H-MW technique, are mainly based on the simple, double oxides or salts described above. Thus, transition metal oxide/graphene composites are known (Kim et al., 2011). For single-phase unitary/binary oxides-graphene composites, a two-step strategy to prepare them is as follows: precipitation of hydroxides followed by H(S)-MW annealing (Chang et al., 2010). This method was applied to the preparation of Mn3O4-graphene and NiCo2O4-graphene composites. Metal oxide / CNT nano-hybrid materials (LiMn2O4/CNT, LiCoO2/CNT and Li4Ti5O12/CNT nanocomposites) were synthesized through selective heterogeneous nucleation and growth of the oxides on CNT surface using H-MW process (Ma et al., 2010). In the composites, CNTs acted as a substrate to deposit nano-sized metal oxide and to connect the nanoparticles along 1D conduction path. The nano-hybrid materials showed excellent high rate capability and good structural reversibility for energy storage applications. H-MW synthesis method was also applied (Guo et al., 2009) for fabrication of tin dioxide nanoparticles-multiwalled carbon nanotubes (MWCNTs) composite powder for preparation of gas sensor. The mixed solution of SnCl4. 5H2O and NaOH at mole ratio of 1:(8-12), NaCl and multiwalled carbon nanotubes (MWCNTs) were used as reactants, subjecting to ultrasonic dispersion, and reacting at 110- 180oC for 30-300 min to synthesize SnO2 nanoparticles-MWCNTs composite powder by microwave-assisted hydrothermal reaction. Se/C nanocomposite with core-shell structures was prepared through a facile one-pot H-MW process (Yu, J.C. et al., 2005). This material, consisting of a trigonal-Se (t-Se) core and an amorphous-C (a-C) shell, can be converted to hollow carbon capsules by thermal treatment. GaP nanocrystals/morin composite fluorescent materials were prepared by microwave hydrothermal synthesis method with Na3P, GaCl3 and morin as raw materials (Cui et al., 2001). It was shown that GaP nanocrystals underwent no structure transformation under the microwave hydrothermal condition, but they grew larger after the composite reaction with morin. The wavelength of

the composite materials blue-shifted and their luminescent efficiency increased when the particle size of GaP nanocrystals decreased.

Mesoporous composites of metal organic frameworks (MOFs; Cu-based) with boehmite and silica were prepared by one-pot H-MW synthesis in the presence of Pluronic-type triblock-copolymer (Gorka et al., 2010). Mesoporosity in these composites can be tailored by varying the MOF/oxide phase ratio. Fe-JLU-15 materials with different Si/Fe ratios (Si/Fe = 90, 50, 10) were synthesized by H-MW process (Bachari et al., 2009). These species correspond to hematite particles, very small "isolated" or oligomeric Fe(III) species possibly incorporated in the mesoporous silica wall, and Fe(III) oxide clusters either isolated or agglomerated, forming "rafts" at the surface of the silica and exhibiting ferromagnetic ordering. Applying similar Fe-FSM-16 materials, the liquid-phase benzylation of aromatic compounds with benzyl chloride (BC) was investigated (Bachari et al., 2010). Catalytic data in the benzylation of aromatic compounds such as benzene and toluene with BC show that Fe-FSM-16 samples synthesized by the M-H process are very active and recyclable catalysts. Nano-TiO2 and super fine Al2O3 composite powder was developed by H-MW method, and TiO2-Al2O3 semiconductor-dielectric composite ceramics with uniform fine grain structure were prepared by fast speed sintering in H2 atmosphere (Lu & Zhang, 2003). Titania-hydroxyapatite (TiO2-HAp) nanocomposite was produced by H-MW technique (Pushpakanth et al., 2008); but, in case of related Ca, Sr and Ca0.5Sr0.5 hydroxyapatites (Komarneni, Noh et al., 2010), microwave-assisted reactions did not lead to accelerated syntheses of hydroxyapatites in comparison with conventional-hydrothermal method, because the crystallization of these materials occurred at very low temperature.

In addition, the nano-sized BaTiO3 and NiCuZn ferrite powders (40-60 nm) were synthesized at 160oC for 45 min (Sadhana, Praveena et al., 2009). These *x*BaTiO3 + (1 *x*)NiCuZnFe2O4 nano-composites were prepared at different weight percentages. It was observed that these composites were useful for the fabrication of Multilayer Chip Inductors (MLCI). The related nanocomposites of NiCuZnFe2O4-SiO2 (particle size of 20 nm) were prepared using H-MW method in the same conditions (Praveena et al., 2010). Among a little of organic matter-containing composites, we note the synthesis of poly (3,4 ethylenedioxythiophene)/V2O5 (PEDOT/V2O5) by *in-situ* oxidation of monomer (3,4 ethylenedioxythiophene) into crystalline nanostrip V2O5 using H-MW technique (Ragupathy et al., 2011). It was observed that the interlayer spacing of V2O5 upon intercalation of the polymer expands from 4.3 to 14o.

#### **5. Ceramics**

The hydrothermal synthesis of oxidic ceramic powders was reviewed (Somiya et al., 2005) for the decomposition of complex oxides like ilmenite, the hydrothermal oxidation of metals, hydrothermal precipitation, combinations of electrochemical, mechanical, microwave and sonochemical methods with hydrothermal methods. Niobates are widely described as ceramic basis. Thus, using KNN ("kalium" (potassium) nitrate niobate) powder prepared by the H-MW method as raw material, traditional ceramic method was employed to fabricate the KNN based lead-free piezoelectric ceramic with 1 mol.% ZnO or 1 mol.% CuO sintering additives (Li, Y. et al., 2011). The preparation method of lithium-doped potassium sodium niobate-based lead-free piezoelectric ceramic powder was based on use of MOH (M=Li, Na, K) and Nb2O5 as raw materials in a H-MW reactor (Tan et al., 2011). The advantages of the offered technique were low reaction temperature, short reaction period, high reaction activity of obtained powder, low energy consumption, and environmental friendliness. In addition, K0.5Na0.5NbO3 lead-free piezoelectric ceramics were prepared from their powders which were synthesized by H-MW method (Zhou et al., 2010). The results indicated that phase of KNN ceramics were pure orthorhombic symmetry. When the powders synthesized at 160oC for 7 h, the final ceramic grains possess considerably better distribution and more homogonous in size.

Among other ceramics, indium vanadates for severe applications as photocatalysts, anodes for Li rechargeable batteries or electrochromic devices were prepared *via* H-MW synthesis performed at 220oC for different reaction times (Bartonickova et al., 2010). The H-MW method was applied to the preparation of strontium-doped lanthanum manganites with different stoichiometric ratio of the three oxides, La1-*x*Sr*x*MnO3 (*x* = 0.3, 0.5, 0.6) (Rizzuti & Leonelli, 2009). The complete chemistry, mineralogical and microstructural characterization of the powders revealed the same structural properties of the perovskite powders previously synthesized by ceramic and conventional hydrothermal routes. In addition, a series of Ni0.5-*x*Cu*x*Zn0.5Fe2O4 (*x* = 0.05, 0.10, 0.15, and 0.20) ferrites nanopowders were synthesized by H-MW method, and sintered into dense-ceramics under the conditions of 900oC/4 h (Zheng, Ya-lin et al., 2007). The performed studies showed within a limited Cu content range of *x* = 0.05-0.20, copper ions were present in different ionic states in the A- and B-sites which could influence the size of lattice parameter. It was also found that crystallite size, initial permeability, resistivity and quality factor were the highest when the Cu content was 0.20.

#### **6. Metal complexes**

122 The Development and Application of Microwave Heating

particle size of GaP nanocrystals decreased.

occurred at very low temperature.

polymer expands from 4.3 to 14o.

**5. Ceramics** 

the composite materials blue-shifted and their luminescent efficiency increased when the

Mesoporous composites of metal organic frameworks (MOFs; Cu-based) with boehmite and silica were prepared by one-pot H-MW synthesis in the presence of Pluronic-type triblock-copolymer (Gorka et al., 2010). Mesoporosity in these composites can be tailored by varying the MOF/oxide phase ratio. Fe-JLU-15 materials with different Si/Fe ratios (Si/Fe = 90, 50, 10) were synthesized by H-MW process (Bachari et al., 2009). These species correspond to hematite particles, very small "isolated" or oligomeric Fe(III) species possibly incorporated in the mesoporous silica wall, and Fe(III) oxide clusters either isolated or agglomerated, forming "rafts" at the surface of the silica and exhibiting ferromagnetic ordering. Applying similar Fe-FSM-16 materials, the liquid-phase benzylation of aromatic compounds with benzyl chloride (BC) was investigated (Bachari et al., 2010). Catalytic data in the benzylation of aromatic compounds such as benzene and toluene with BC show that Fe-FSM-16 samples synthesized by the M-H process are very active and recyclable catalysts. Nano-TiO2 and super fine Al2O3 composite powder was developed by H-MW method, and TiO2-Al2O3 semiconductor-dielectric composite ceramics with uniform fine grain structure were prepared by fast speed sintering in H2 atmosphere (Lu & Zhang, 2003). Titania-hydroxyapatite (TiO2-HAp) nanocomposite was produced by H-MW technique (Pushpakanth et al., 2008); but, in case of related Ca, Sr and Ca0.5Sr0.5 hydroxyapatites (Komarneni, Noh et al., 2010), microwave-assisted reactions did not lead to accelerated syntheses of hydroxyapatites in comparison with conventional-hydrothermal method, because the crystallization of these materials

In addition, the nano-sized BaTiO3 and NiCuZn ferrite powders (40-60 nm) were synthesized at 160oC for 45 min (Sadhana, Praveena et al., 2009). These *x*BaTiO3 + (1 *x*)NiCuZnFe2O4 nano-composites were prepared at different weight percentages. It was observed that these composites were useful for the fabrication of Multilayer Chip Inductors (MLCI). The related nanocomposites of NiCuZnFe2O4-SiO2 (particle size of 20 nm) were prepared using H-MW method in the same conditions (Praveena et al., 2010). Among a little of organic matter-containing composites, we note the synthesis of poly (3,4 ethylenedioxythiophene)/V2O5 (PEDOT/V2O5) by *in-situ* oxidation of monomer (3,4 ethylenedioxythiophene) into crystalline nanostrip V2O5 using H-MW technique (Ragupathy et al., 2011). It was observed that the interlayer spacing of V2O5 upon intercalation of the

The hydrothermal synthesis of oxidic ceramic powders was reviewed (Somiya et al., 2005) for the decomposition of complex oxides like ilmenite, the hydrothermal oxidation of metals, hydrothermal precipitation, combinations of electrochemical, mechanical, microwave and sonochemical methods with hydrothermal methods. Niobates are widely described as ceramic basis. Thus, using KNN ("kalium" (potassium) nitrate niobate) powder There are obviously no many examples of coordination compounds, obtained by H(S)-MW technique due to low stability of organic matter in hydrothermal conditions. Thus, an organic-inorganic UV absorber [Hgua]2. (Ti5O5F12) was obtained (Lhoste et al., 2011), whose 3D network is built up from infinite inorganic layers (Ti5O5F12) separated by guanidinium cations. Under UV irradiation at 254 nm for 40 h, the white microcrystalline powder turned to light purple-gray due to reduction of Ti(IV) to Ti(III), confirmed by magnetic measurements. Two extended solids displaying both 1D coordination polymer [Co(H2O)4(4,4'-bipy)](4,4'-bipyH2)2(SO4). 2H2O (bipy = 2,2'-bipyridine) and 2D H-bonded structural features [Co2(4,4'-bipy)2(SO4)2(H2O)6]. 4(H2O) were prepared (Prior et al., 2011) under H-MW conditions. Within the first framework is located a twice protonated 4,4' bipyridine molecule (C10N2H102+) which forms 2 short N-H…….O H-bonds and 8 further nonclassical C-H……O interactions. The second compound displays 1D chains of Co-bipyridine which are sinusoidal in nature. Three mixed-ligand Co(II) complexes (Shi et al., 2009)

[Na2Co(4-btec)(H2O)8]*n*, [Co2(2-btec)(bipy)2(H2O)6]. 2H2O, and [Co2(2 btec)(phen)2(H2O)6]. 2H2O (H4btec = 1,2,4,5-benzenetetracarboxylic acid, phen = 1,10 phenanthroline) are also known. A vanadium 2,6-naphthalenedicarboxylate, VIII(OH)(O2C-C10H6-CO2). H2O was synthesized under S-MW procedure (Liu, Y.-Ya et al., 2012). After calcination at 250oC in air, the VIII center was oxidized to VIV with the structure of VIVO(O2C-C10H6-CO2). The last compound, in the liquid-phase oxidation of cyclohexene, exhibited catalytic performance similar to [VO(O2C-C6H4-CO2)]. The compound is reusable and maintains its catalytic activity through several runs. Purinium, adeninium, and guaninium fluoroaluminates, [Hpur]2. (AlF5), [Hade]3. (AlF6). 6.5H2O and [Hguan]3. (Al3F12), were synthesized by H-MW synthesis at 120oC or 190oC (Cadiau et al., 2011). Authors commented that the crystallization was difficult; all crystals of the first two complexes were very small while only a microcrystalline powder of the third compound was obtained. The purine, adenine, and guanine amines were found to be monoprotonated and lie between the preceding chains or layers.

By treating Cu(NO3)2· 3H2O with a V-shaped ligand 4,4'-oxydibenzoic acid (H2oba), a dynamic metal-carboxylate framework [Cu2(oba)2(DMF)2]. 5.25DMF (MCF-23, Fig. 12) was synthesized, which features a wavelike layer with rhombic grids based on the paddle-wheel secondary building units (Xiao-Feng Wang et al., 2008). MCF-23 synthesized by conventional solvothermal methods always contained considerable and intractable impurities. In contrast, a S-MW method was proven to be a faster and greener approach to synthesize phase-pure MCF-23 in high yield. Larger crystals suitable for single-crystal diffraction could be obtained by the multistep microwave heating mode. Also, a 3D coordination copper polymer, [Cu2(pyz)2(SO4)(H2O)2]*n* (pyz = pyrazine), was synthesized under H-MW conditions (Amo-Ochoa et al., 2007). The authors especially note that microwave assisted synthesis produces monocrystal suitable for X-ray diffraction studies, reducing reaction time and with higher yield than the classical hydrothermal procedures.

**Figure 12.** SEM images of MCF-23 synthesized *via* single-step microwave-assisted solvothermal synthesis (MASS): (a) 1 min, (b) 5 min, (c) 10 min, (d) 30 min, (e) 150 min at 160 °C, and (f) photo of single crystals synthesized *via* multistep MASS. With permission.

Single crystals of [H3dien]. (FeF6). H2O and [H3dien]. (CrF6). H2O were obtained by S-MW (Ben Ali et al., 2007). These structures are built up from isolated FeF6 or CrF6 octahedra, water molecules and triprotonated amines; each octahedron is connected by hydrogen bonds to six organic cations and two water molecules. 57Fe Mossbauer spectrometry proved the hyperfine structure confirming the presence of Fe3+ in octahedral coordination and reveals the existence of paramagnetic spin fluctuations. Under H-MW conditions, two compounds Zn(pinH)(H2O) and Cd(pinH) were obtained through the reactions of Zn (or Cd) sulfate and 2-phosphonic-isonicotinic acid (pinH3) (Yang Yi-F. et al., 2007). The authors established that the first complex has a layer structure in which double chains composed of corner-sharing {ZnO5N} octahedra and {CPO3} tetrahedra are cross-linked through the carboxylate groups. The latter has a framework structure where the inorganic layers composed of edge-sharing {CdO5N} octahedra and {CPO3} tetrahedra are pillared by the pyridyl carboxylate groups. A 3D open-framework and a chain-structured zinc terephthalate were obtained by hydrothermal crystallization under microwave heating at 180oC (Rajic et al., 2006). These structures have rather similar principal building units, which facilitates the structure transformation from the low-dimensional to the open-framework one during crystallization. For the optimization of the H-MW synthesis of [Zn(BDC)(H2O)2]*n*, where H2BDC = 1,4 benzenedicarboxylic acid, the reactions were carried out at the fixed temperature of 120oC for 10, 20, 30 and 40 min (Wanderley et al., 2011). Pure crystalline [Zn(BDC)(H2O)2]*n* was obtained in high yield (90%) with a reaction time of 10 min.

#### **7. Other materials and processes**

124 The Development and Application of Microwave Heating

btec)(phen)2(H2O)6].

fluoroaluminates, [Hpur]2.

preceding chains or layers.

By treating Cu(NO3)2·

C10H6-CO2).

[Na2Co(4-btec)(H2O)8]*n*, [Co2(2-btec)(bipy)2(H2O)6].

dynamic metal-carboxylate framework [Cu2(oba)2(DMF)2].

2H2O, and [Co2(2-

5.25DMF (MCF-23, Fig. 12) was

(Al3F12), were

2H2O (H4btec = 1,2,4,5-benzenetetracarboxylic acid, phen = 1,10-

3H2O with a V-shaped ligand 4,4'-oxydibenzoic acid (H2oba), a

6.5H2O and [Hguan]3.

H2O was synthesized under S-MW procedure (Liu, Y.-Ya et al., 2012). After

phenanthroline) are also known. A vanadium 2,6-naphthalenedicarboxylate, VIII(OH)(O2C-

calcination at 250oC in air, the VIII center was oxidized to VIV with the structure of VIVO(O2C-C10H6-CO2). The last compound, in the liquid-phase oxidation of cyclohexene, exhibited catalytic performance similar to [VO(O2C-C6H4-CO2)]. The compound is reusable and maintains its catalytic activity through several runs. Purinium, adeninium, and guaninium

(AlF6).

synthesized by H-MW synthesis at 120oC or 190oC (Cadiau et al., 2011). Authors commented that the crystallization was difficult; all crystals of the first two complexes were very small while only a microcrystalline powder of the third compound was obtained. The purine, adenine, and guanine amines were found to be monoprotonated and lie between the

synthesized, which features a wavelike layer with rhombic grids based on the paddle-wheel secondary building units (Xiao-Feng Wang et al., 2008). MCF-23 synthesized by conventional solvothermal methods always contained considerable and intractable impurities. In contrast, a S-MW method was proven to be a faster and greener approach to synthesize phase-pure MCF-23 in high yield. Larger crystals suitable for single-crystal diffraction could be obtained by the multistep microwave heating mode. Also, a 3D coordination copper polymer, [Cu2(pyz)2(SO4)(H2O)2]*n* (pyz = pyrazine), was synthesized under H-MW conditions (Amo-Ochoa et al., 2007). The authors especially note that microwave assisted synthesis produces monocrystal suitable for X-ray diffraction studies, reducing reaction time and with higher yield than the classical hydrothermal procedures.

**Figure 12.** SEM images of MCF-23 synthesized *via* single-step microwave-assisted solvothermal synthesis (MASS): (a) 1 min, (b) 5 min, (c) 10 min, (d) 30 min, (e) 150 min at 160 °C, and (f) photo of

single crystals synthesized *via* multistep MASS. With permission.

(AlF5), [Hade]3.

Series of reports are devoted to H-MW fabrication of a variety of *molecular sieves* and other *adsorbents*. Thus, a range of nanosize alkaline-free gallosilicate mesoporous molecular sieves (GaMMS) were synthesized using H-MW method (Cheng et al., 2011). These nanosize GaMMSs exhibited high surface area (240-720 m2/g), pore volume (1.06-1.49 m3/g), narrow pore size distribution and nano-particle size between 20 and 100 nm and four-coordinated gallium site mainly. It was revealed that the nanosize GaMMS shows a much higher activity than that of conventional GaMCM-41, probably due to higher concentrations of H-form sites, external surface and fast diffusion. Mesoporous molecular sieves MCM-41 modified by single (Ti) and bimetal (Ti-V) ions with highly ordered hexagonal arrangement of their cylindrical channels were prepared by direct synthesis under H-MW conditions at 403 K (Guo, Y. et al., 2010). It was shown that Ti and V ions were introduced into MCM-41 under M-H conditions and Ti/V-Si bond was formed. The modified materials were high active and selective in the epoxidation of styrene at 343 K in comparison with single functional MCM-41. The applied method greatly improved the selectivity to styrene oxide. In addition, a rapid process to prepare cryptomelane-type octahedral molecular sieve (OMS-2) nanomaterials using a H-MW technique (MW-HT) for 10 s (in comparison with up to 4 days in a conventional hydrothermal reaction) was presented (Huang, H. et al., 2010). It was shown that the OMS-2 nanowires were produced from thin nano-flakes with increasing reaction temperatures. Carbon/silica adsorbents (carbosils) were prepared by pyrolisis of CH2Cl2 at 823 K and the reaction time from 0.5 to 6 h on the mesoporous silica gel surface

and then hydrothermally treated at 473 K with steam or liquid water by using the classical autoclave with traditional heating way or in the microwave reactor (Skubiszewska-Zieba, 2008). It was stated that hydrothermal treatment in the microwave reactor, contrary to that in the classical autoclave, allowed obtaining adsorbents with noticeably higher values of total pore volume in relation to the initial adsorbents and in majority with a higher specific surface area. Application of microwave energy allowed obtaining adsorbents with lower values of surface free energy in relation to the initial adsorbents and those modified in the autoclave.

*Carbon-based materials* are also produced by H-MW method. Thus, rice chaff can be converted into activated carbon by alkaline treatment at high temperature, in particular by H-MW treatment with NaOH solution (Inada et al., 2011). Rice chaff was heated at 700oC for 1 h in N2 after HCl treatment to remove impurity, such as alkali metals, resulting in the formation of charcoal. After further necessary steps, it was revealed that the specific surface area of product was 1060 m2/g under microwave irradiation and 690 m2/g by conventional heating with autoclave, which indicated that H-MW treatment is effective for activation of charcoal. In addition, acidic suspensions of multi-walled carbon nanotubes (MWCNTs) in 5 M HNO3 solutions were readily obtained by microwave-assisted hydrothermal digestion and their rapid surface functionalization with polyaniline could be achieved growing composite coatings *in situ* onto graphite electrodes in an acidified suspension of MWCNTs and aniline (Wu et al., 2007). The advantage of the integrity of the MWCNTs after microwave exposure was used to improve the mechanical strength of the composite coatings, especially when an utmost thick coating was attempted for a high capacitance.

*Materials and processes using iron-containing compounds.* Iron-FSM [folded-sheet mesoporous material]-16 materials with different Si/Fe ratio (Si/Fe = 90, 60, and 10) were synthesized by H-MW process (Bachari et al., 2011). These species correspond to hematite particles, small isolated Fe(III) species possibly incorporated in the mesoporous silica wall, and iron oxide clusters either isolated or agglomerated, forming rafts at the surface of the silica and exhibiting ferromagnetic ordering. It was shown that Fe-FSM-16 synthesized this way are active and recycle catalysts. The high yield and rapid precipitation of Fe(III) arsenate(V) dihydrate (identical to the mineral scorodite) from aqueous Fe(III)-As(V) solutions at pH 0.6-1.1 was achieved long ago using microwave dielectric heating (Baghurst et all., 1995). The process could be useful for As removal from industrial wastes. Metallic iron (- Fe)/manganese-zinc ferrite (Fe3-*x-y*Mn*x*Zn*y*O4) nanocomposites were synthesized for 15 s using H(S)-MW treatment of alcohol solutions of chloride precursors and sodium ethoxide (Caillot et al., 2011). For all samples, 20% of metallic iron was routinely obtained using the microwave flash synthesis. Consequently, the microwave heating appears to provide an efficient source of energy in producing metallic iron nanoparticles protected against oxidation by an oxide matrix. Additionally, as an improvement of the conventional hydrothermal reaction with iron powder, NaOH and H2O as reactants at 423 K, leading to iron oxide Fe3O4, NaFeO2 and hydrogen, MW-heating was adopted to induce the hydrothermal reaction (Liu, X. et al., 2011). Under MWs, NaOH and H2O absorbed microwave energy by space charge polarization and dipolar polarization and instantly converted it into thermal energy, which initiated the hydrothermal reaction that involved with zero-valent iron. The developed microwave-hydrothermal reaction was employed for the dechlorination of PCBs (polychlorinated biphenyls). For PCBs in 10 mL simulative transformer oil, almost complete dechlorination was achieved by 750 W MW irradiation for 10 min, with 0.3 g iron powder, 0.3 g NaOH and 0.6 mL H2O added. The authors stated that MW irradiation combined with the common and cheap materials, iron powder, NaOH and H2O, might provide a fast and cost-effective method for the treatment of PCBs-containing wastes.

126 The Development and Application of Microwave Heating

autoclave.

and then hydrothermally treated at 473 K with steam or liquid water by using the classical autoclave with traditional heating way or in the microwave reactor (Skubiszewska-Zieba, 2008). It was stated that hydrothermal treatment in the microwave reactor, contrary to that in the classical autoclave, allowed obtaining adsorbents with noticeably higher values of total pore volume in relation to the initial adsorbents and in majority with a higher specific surface area. Application of microwave energy allowed obtaining adsorbents with lower values of surface free energy in relation to the initial adsorbents and those modified in the

*Carbon-based materials* are also produced by H-MW method. Thus, rice chaff can be converted into activated carbon by alkaline treatment at high temperature, in particular by H-MW treatment with NaOH solution (Inada et al., 2011). Rice chaff was heated at 700oC for 1 h in N2 after HCl treatment to remove impurity, such as alkali metals, resulting in the formation of charcoal. After further necessary steps, it was revealed that the specific surface area of product was 1060 m2/g under microwave irradiation and 690 m2/g by conventional heating with autoclave, which indicated that H-MW treatment is effective for activation of charcoal. In addition, acidic suspensions of multi-walled carbon nanotubes (MWCNTs) in 5 M HNO3 solutions were readily obtained by microwave-assisted hydrothermal digestion and their rapid surface functionalization with polyaniline could be achieved growing composite coatings *in situ* onto graphite electrodes in an acidified suspension of MWCNTs and aniline (Wu et al., 2007). The advantage of the integrity of the MWCNTs after microwave exposure was used to improve the mechanical strength of the composite coatings, especially when an utmost thick coating was attempted for a high capacitance.

*Materials and processes using iron-containing compounds.* Iron-FSM [folded-sheet mesoporous material]-16 materials with different Si/Fe ratio (Si/Fe = 90, 60, and 10) were synthesized by H-MW process (Bachari et al., 2011). These species correspond to hematite particles, small isolated Fe(III) species possibly incorporated in the mesoporous silica wall, and iron oxide clusters either isolated or agglomerated, forming rafts at the surface of the silica and exhibiting ferromagnetic ordering. It was shown that Fe-FSM-16 synthesized this way are active and recycle catalysts. The high yield and rapid precipitation of Fe(III) arsenate(V) dihydrate (identical to the mineral scorodite) from aqueous Fe(III)-As(V) solutions at pH 0.6-1.1 was achieved long ago using microwave dielectric heating (Baghurst et all., 1995). The process could be useful for As removal from industrial wastes. Metallic iron (- Fe)/manganese-zinc ferrite (Fe3-*x-y*Mn*x*Zn*y*O4) nanocomposites were synthesized for 15 s using H(S)-MW treatment of alcohol solutions of chloride precursors and sodium ethoxide (Caillot et al., 2011). For all samples, 20% of metallic iron was routinely obtained using the microwave flash synthesis. Consequently, the microwave heating appears to provide an efficient source of energy in producing metallic iron nanoparticles protected against oxidation by an oxide matrix. Additionally, as an improvement of the conventional hydrothermal reaction with iron powder, NaOH and H2O as reactants at 423 K, leading to iron oxide Fe3O4, NaFeO2 and hydrogen, MW-heating was adopted to induce the hydrothermal reaction (Liu, X. et al., 2011). Under MWs, NaOH and H2O absorbed microwave energy by space charge polarization and dipolar polarization and instantly *Zeolites/alumosilicates*, *composites* and *membranes* on their basis are frequently (especially ZSM-5) obtained by the discussed method. Thus, evaluation of hydrothermal synthesis of the zeolite BZSM-5 was performed by treating the synthesis mixture by different aging processes, namely, ultrasonic, static, stirring, and microwave-assisted aging prior to the conventional hydrothermal treatment (Abrishamkar et al., 2010). That the ultrasonic and microwave assisted aging shortened the crystallization time and altered the crystal size and the morphology of the obtained products. Mullite (Al6Si2O13) powders were prepared by solgel combined with H-MW process using tetra-Et orthosilicate and Al(NO3)3. 9H2O as raw materials (Yang, Q. et al., 2010). It was shown that the increasing of microwave hydrothermal temperature can decrease the synthetic temperature of mullite. The initial temperature of the mullitization of gels was about 1046oC, but the temperature of preparation of pure mullite should be at 1300oC, which can be decreased to 1200oC through H-MW process. Using silatrane and alumatrane templates, remarkably uniform zeolites (NaA, EDI-type, ABW-type, FAU-type, K, and ZSM-5 zeolites) were obtained via the sol-gel process and H-MW treatment to obtain small and uniform zeolite crystals to prepare zeolite membranes (Wongkasemjit, 2009). The organic ligands of both precursors not only make the molecules stable to moisture, but also, after hydrolysis, provide trialkanolamine molecules which could function as another template for the reaction. The formation of ZSM-5 zeolite/porous carbon composite from carbonized rice husk was investigated using microwave- and conventional-hydrothermal reaction at 140-160oC (Katsuki et al., 2005). It was established that, compared to the conventional-hydrothermal (C-H) formation of ZSM-5 zeolite, the H-MW reaction led to increased rate of formation by 3-4 times at 150oC. The surface area of ZSM-5 zeolite (without template)/porous carbon composite was shown to be 485.4 m2/g and this composite had both micropores and mesopores. Zeolite nanocrystals (e.g., silicalite-1, ZSM-5, LTL, BEA and LTA) with controllable size, morphology and SiO2/Al2O3 ratio were prepared (Yuanyuan Hu et al., 2009). It is found that high synthesis temperature and long reaction time benefit the growth of all the referred zeolite nanocrystals. The authors established that for the nanozeolites crystallized in low alkalinity systems (e.g., ZSM-5, BEA and LTA), both increasing the alkalinity and decreasing the water content accelerate their nucleation process and thereby result in the decrease of their crystal size. On the contrary, for those prepared in high alkalinity systems (e.g., LTL and silicalite-1), an inversed trend could be observed. In addition, stable zeolite beta coatings with a thickness of 1-2 m were synthesized on a borosilicate glass substrate by H-MW synthesis (Muraza et al., 2008). In addition, a regular nanocrystalline supramolecular Mg-Al hydrotalcite was prepared *via* glycol-frequency H-MW reaction using MgCl2. 6H2O,

AlCl3. 6H2O as raw material and Na2CO3 as precipitator (Wu, J. et al., 2010). It was shown that the hydrotalcite exhibited as a homogeneous and hexagonal sheet.

A high quality pure hydroxy-sodalite Na8[AlSiO4]6(OH)2 zeolite membrane was H-MWsynthesized on an a-Al2O3 support (Xiaochun Xu et al., 2004). This process only needed 45 min and synthesis was more than 8 times faster than by the conventional hydrothermal synthesis method. The pure hydroxy-sodalite zeolite membrane method was found to be well inter-grown and the thickness of the membrane was 6–7 m. Gas permeation results showed that the hydrogen/n-butane permselectivity of the hydroxy-sodalite zeolite membrane was larger than 1000, being a promising candidate for the separation of hydrogen from gas mixtures and important for the emerging hydrogen energy fuel system. The potential of microwave heating for the rapid synthesis of thin silicalite-1 membranes by secondary growth from microwave-derived silicalite-1 seeds was evaluated (Motuzas et al. 2006). The morphology, thickness, homogeneity, crystal preferential orientation and single gas permeation properties of the silicalite-1 membranes were studied in relation to the synthesis parameters. Other related zeolite-ceramic membranes (titanosilicalite TS-1 (Sebastian, V.; Motuzas, J. et al., 2010) and MFI-type (Sebastian, V.; Mallada, R. et al., 2010)) are also known.

Other H-MW-synthesized materials and compounds are rare; some of them also found useful practical applications, as, for example, titanium-based nanometer pigments (Paskocimas et al., 2009). CdS/Titanate nanotubes (TNTs) were also prepared (Chen, Y.-C. et al., 2011). It was established that the CdS nanoparticles synthesized using a 140-W microwave irradiation power at 423 K photodegraded 26% ammonia in water, while the photocatalytic efficiency increased to 52.3% using the synthesized CdS/TNTs composites. So, it can be stated that the CdS/TNTs photocatalysts possess improved photocatalytic activity than that of CdS or TNTs materials alone. Mesoporous composites of metal organic frameworks (MOFs; Cu-based) with boehmite and silica were prepared by one-pot H-MW synthesis in the presence of Pluronic-type triblock-copolymer (Gorka, J.; Fulvio, P.F. et al., 2010). A variety of visible-light-driven silver vanadates, including -AgVO3, -AgVO3, and -Ag3VO4, were synthesized using a H-MW synthesis method (Pan et al., 2011). The - Ag3VO4 crystalline sample with rich hydroxyl functional groups on the surface exhibited the highest degree of photocatalytic activity. Thus, the reaction rates of the photodegradation of isopropanol (IPA) and benzene vapors were approximately 8 times higher than those of P25 under visible-light irradiation. In addition, the photocatalytic activities of H-MW samples were higher than those of samples produced by conventional hydrothermal techniques. The authors explained this due to an increase in the specific surface area and additional hydroxyl functional groups on the surface. Another application of the H-MW method for destruction of organic pollutants was reported in (Liu, X. et al., 2011). The reaction of reduced iron powder and NaOH or KOH led to obtaining iron oxides and/or ferrites, releasing hydrogen for quick dechlorination of persistent organic pollutants. The method was shown to have the advantages of short treatment time and high dechlorination efficiency and can be used for treating waste organochlorine pesticides, polychlorobiphenyl oil in transformer, soil and sediment heavily polluted by organochlorine pesticides and polychlorobiphenyl oil, and garbage burning fly ash and chlor-alkali brine sludge containing high-concentration dioxins. Ca(OH)2 alone and Ca(OH)2 with H3PO4 addition (Paddition) were effectively used to remove and recover boron from wastewater using hydrothermal methods (Tsai et al., 2011). A microwave hydrothermal method was also used and compared with the conventional heating method in batch experiments. For the case of Ca(OH)2 alone and the MW method, expterimental results showed that boron recovery efficiency reached 90% within 10 min, and crystals of Ca2B2O5. H2O were observed. For the case of P-addition and the MW method, boron recovery efficiency reached 99% within 10 min, and Ca phosphate species (CaHPO4. H2O, CaHPO4 and Ca10(PO4)6(OH)2) were formed. Hydrophobic organoclays (hybrids derived from an ion exchange of hydrophilic clays with quaternary ammonium salts and used as rheological additives in paints, inks, cosmetics, nanocomposites, and as pollutant absorbing agents in soil remediation programs) were studied using a natural (Na-montmorillonite) and several synthetic clays (Nafluorophlogopites) as precursors (Baldassari). These organoclays synthesized using both conventional hydrothermal and H-MW processes.

#### **8. Conclusions**

128 The Development and Application of Microwave Heating

6H2O as raw material and Na2CO3 as precipitator (Wu, J. et al., 2010). It was shown

A high quality pure hydroxy-sodalite Na8[AlSiO4]6(OH)2 zeolite membrane was H-MWsynthesized on an a-Al2O3 support (Xiaochun Xu et al., 2004). This process only needed 45 min and synthesis was more than 8 times faster than by the conventional hydrothermal synthesis method. The pure hydroxy-sodalite zeolite membrane method was found to be well inter-grown and the thickness of the membrane was 6–7 m. Gas permeation results showed that the hydrogen/n-butane permselectivity of the hydroxy-sodalite zeolite membrane was larger than 1000, being a promising candidate for the separation of hydrogen from gas mixtures and important for the emerging hydrogen energy fuel system. The potential of microwave heating for the rapid synthesis of thin silicalite-1 membranes by secondary growth from microwave-derived silicalite-1 seeds was evaluated (Motuzas et al. 2006). The morphology, thickness, homogeneity, crystal preferential orientation and single gas permeation properties of the silicalite-1 membranes were studied in relation to the synthesis parameters. Other related zeolite-ceramic membranes (titanosilicalite TS-1 (Sebastian, V.; Motuzas, J. et al., 2010) and MFI-type (Sebastian, V.; Mallada, R. et al., 2010))

Other H-MW-synthesized materials and compounds are rare; some of them also found useful practical applications, as, for example, titanium-based nanometer pigments (Paskocimas et al., 2009). CdS/Titanate nanotubes (TNTs) were also prepared (Chen, Y.-C. et al., 2011). It was established that the CdS nanoparticles synthesized using a 140-W microwave irradiation power at 423 K photodegraded 26% ammonia in water, while the photocatalytic efficiency increased to 52.3% using the synthesized CdS/TNTs composites. So, it can be stated that the CdS/TNTs photocatalysts possess improved photocatalytic activity than that of CdS or TNTs materials alone. Mesoporous composites of metal organic frameworks (MOFs; Cu-based) with boehmite and silica were prepared by one-pot H-MW synthesis in the presence of Pluronic-type triblock-copolymer (Gorka, J.; Fulvio, P.F. et al., 2010). A variety of visible-light-driven silver vanadates, including -AgVO3, -AgVO3, and -Ag3VO4, were synthesized using a H-MW synthesis method (Pan et al., 2011). The - Ag3VO4 crystalline sample with rich hydroxyl functional groups on the surface exhibited the highest degree of photocatalytic activity. Thus, the reaction rates of the photodegradation of isopropanol (IPA) and benzene vapors were approximately 8 times higher than those of P25 under visible-light irradiation. In addition, the photocatalytic activities of H-MW samples were higher than those of samples produced by conventional hydrothermal techniques. The authors explained this due to an increase in the specific surface area and additional hydroxyl functional groups on the surface. Another application of the H-MW method for destruction of organic pollutants was reported in (Liu, X. et al., 2011). The reaction of reduced iron powder and NaOH or KOH led to obtaining iron oxides and/or ferrites, releasing hydrogen for quick dechlorination of persistent organic pollutants. The method was shown to have the advantages of short treatment time and high dechlorination efficiency and can be used for treating waste organochlorine pesticides, polychlorobiphenyl oil in transformer, soil and sediment heavily polluted by organochlorine pesticides and

that the hydrotalcite exhibited as a homogeneous and hexagonal sheet.

AlCl3.

are also known.

Nowadays, the H(S)-MW techniques are already classic method for fabrication of distinct chemical compounds and materials, including nanomaterials. Inorganic compounds and materials are generally obtained by this route, although a certain number of organiccontaining compounds are also reported. Use of organic matter in this method can be considered as a careful pioneer experimentation in order to check its suitability for MWhydro(solvo)thermal reactions. Compounds, which are stable in hydrothermal conditions, such as metal oxides, oxygen-containing and other metal salts, a variety of zeolites, carbonbased materials, as well as composites on their basis, are classic synthesis objects by this route. It is expected that more organic/organometallic products could be tried to be prepared in H(S)-MW conditions, for example thermally stable (up to 500-600oC, that is rare for organic matter) aromatic macrocycles of phthalocyanine (Edrissi et al., 2007) type and related compounds.

#### **Author details**

Boris I. Kharisov, Oxana V. Kharissova and Ubaldo Ortiz Méndez *Universidad Autónoma de Nuevo León, Monterrey, México* 

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