2.2.3 Supported metal complexes: bridging homogeneous catalysis with heterogeneous catalysis

To increase the site efficiency of heterogeneous catalysis, metal complexes with 100% site efficiency are deposited on to the support surface. For example, Zweni et al. integrated palladium metal ion complex on silica gel supported dendron ligands with different generations (Figure 7) [22]. The catalytic reactions of 1,3 cyclohexadiene in various solvent systems are investigated to see the influence of solvent characteristics on the reactivity and selectivity of the catalyst. Methanol is found to be the optimized solvent system for this catalyst based on the selectivity to cyclohexene by mono-hydrogenation.

This silica-supported PAMAM-palladium complex would exhibit different catalytic reactivities and selectivities for hydrogenation of dienes depending on the dendrimer sizes and linker chain lengths (Table 4). Overall, the similar selectivity is observed for Entries 1 (shorter reaction time) and 2 (longer reaction time). Generation 0/complex 1 and generation 2/complex 6 exhibit higher reactivity than other generation/linker combinations. Entries 2, 4, and 6 show the results of the catalytic reactions by G1 catalysts at the first 30 min, which suggest the high initial selectivity of these catalysts toward cyclohexene. With the increased reaction time,

Figure 7. Silica-supported PAMAM-palladium complex catalyst. Reproduced from [22] with permission from WILEY.

to the confinement effect within the micropores of the alumina overcoat and the stronger adsorption of 1,3-butadiene than alkenes on Pd nanoparticle catalysts. Mesoporous carbon films as support to control the activity of Pd catalyst also reported for the selective hydrogenation of 1,3-butadiene [20]. The material synthesis involves the co-deposition of small polymeric carbon clusters, structure filling agents, and Pd ions on a substrate (Scheme 1). Thermal treatments converted these hybrids into graphitized microporous carbon with active Pd catalysts. These catalysts were highly active in the gas-phase mono-hydrogenation of 1,3-butadiene to butenes. The major product for these catalytic systems is 1-butene at 50% selectivity among butene isomers, which is very similar to the catalytic selectivity of

Catalytic property of microporous alumina-coated Pd/Al2O3 using atomic layer deposition (ALD) for 1,3 butadiene hydrogenation in the presence of an excess propene. Reproduced from [19] with permission from the

Product distribution

 1 1 3680 100 16.5 22 58 4 3 1 3680 100 17.5 23.5 53 5 1 0.25 3680 100 20 26 51 4.5 1 0.25 7360 91.5 9 64.5 25.5 1

The catalytic reactions of 2,5-dimethyl-2,4-hexadiene with Pd nanoparticle composite in 2 mL toluene at 70°C

Bimetallic Au-Pd alloy catalysts with low amount of Pd were prepared by either co-deposition–precipitation or co-impregnation procedure [21]. This approach is especially beneficial considering the low usage of somewhat toxic Pd metals. These bimetallic catalysts could selectively hydrogenate 1,3-butadiene in the presence of propene. By changing the Au/Pd ratio, the catalytic activity of bimetallic catalysts could be further controlled. The overall selectivity among butene isomers also depended on the reaction temperatures. At the lower temperature, 1-butene was

porous alumina-coated Pd catalysts described above.

Entry

Table 3.

Figure 6.

108

American Chemical Society.

Data reproduced from [18].

and under 3 MPa H2.

P, MPa t, h Sub/Pd Conv.,%

Gold Nanoparticles - Reaching New Heights


#### Table 4.

Hydrogenation of 1,3-cyclohexadiene with silica supported PAMAM-palladium complex catalyst (5.25mmol 1,3-hexadiene, 10 μmol Pd catalyst complex, 5 mL methanol, pressurized glass autoclave to 14 psi H2, 25° C).

the catalysis results shown in Entries 3, 5, and 7 indicate the increased conversion of reactants with the formation of some minor full hydrogenation products. The results suggest that the primary product, cyclohexene, would compete with the diene reactant for the catalytic active sites. When the concentration of 1,3 cyclohexadiene is high at the beginning of the reaction, diene would dominate the adsorption on the active sites and the reaction would maintain a good selectivity to cyclohexene. With the increased concentration of cyclohexene at the later stage of the reaction, the adsorption on active catalytic sites starts to take place. The reactions are generally slower for G1 catalysts compared to G0 catalyst especially with longer linkers. The G2-C12 catalyst, however, exhibits good activity and selectivity toward cyclohexene indicating the importance of right combination between dendrimer generations and linker lengths. A trace amount of benzene was also observed for some of the catalysts. The catalytic reactions of other acyclic dienes also are investigated to understand the effects of substrate structure. In general, the 1,2-addition hydrogenation of less hindered C=C is the most favorable compared to that of more hindered C=C, 1,4-addition hydrogenation, and full hydrogenation.

conjugated C=C bonds can be synthesized by the pyrolysis of β-pinene in nature. Despite the availability of myrcene, only little success in selective hydrogenation of the substrate to diene have been made. Gusevskaya et al. reported the hydrogenation of myrcene either by using metal complex ion or heterogeneous sol-gel catalysts [24, 25]. For 10% palladium on carbon (Pd/C) under 20 atm of H2 and cyclohexane solvent system at 80°C, full hydrogenation reaction would take place within 30 min, and the catalytic system does not have any selectivity toward diene

Reactant (1) and potential products (2–7) for myrcene hydrogenation. Products 4–7 are dienes. Reproduced

Selectivity plots for MgO-supported rhodium dimers in the absence (A) and in the presence (B) of CO ligands in the hydrogenation of 1,3-butadiene (filled circle: 1-butene, open diamond: trans-2-butene, blue square: cis-2 butene, red triangle: butane). (Reactions condition: 2 vol % 1,3-butadiene, balanced with H2, total pressure = 1 bar, room temp) Reproduced from [23] with permission from the American Chemical Society.

Selective Mono-Hydrogenation of Polyunsaturated Hydrocarbons: Traditional and Nanoscale…

[RuCl2(CO)2(PPh3)2], [RhH(CO)(PPh3)3], [IrCl(CO)(PPh3)2], and [Cr(CO)6] show the capability to form diene products in relatively good yields (Figure 9 and

From Entries 1–4 in Table 5, the reactivity of the metal complexes under the same condition turns out to be Ru < Cr < Ir < Rh. Ru and Rh complexes show slightly higher selectivity toward monohydrogenated products, dienes, than chromium and iridium complexes. Rh complex is further tested by adding extra PPh3 ligand in the reaction (Entry 5). The presence of extra PPh3 slows the reaction down requiring higher reaction temperature, but increases selectivity toward dienes. The similar selectivity of myrcene hydrogenation is achieved by simply changing solvent from cyclohexane to benzene even at the lower reaction temperature of 80°

The reaction only produces a trace amount of double or full hydrogenation products. However, the selectivity among different dienes (Entries 4–7) is still poor. The first hydrogenation of myrcene, a triene, takes place at the conjugated diene group instead of the isolated and hindered alkene group. The 1,2-addition of conjugated diene produces either compound 4 or compound 5. The 1,4-addition of conjugated diene involving Pd-allyl intermediates produce compounds 6 and 7, the

C.

or monoene. On the other hand, the transition metal complexes of

Table 5) [24].

111

Figure 8.

Figure 9.

from [24] with permission from the Sciencedirect.

DOI: http://dx.doi.org/10.5772/intechopen.81637

Zeolite- and magnesium oxide-supported molecular rhodium complexes are also tested for the hydrogenation of 1,3-butadiene [23]. The selectivity for monohydrogenation increases when the Rh species nucleation decreased from several atoms to dimeric clusters. The poisoning with CO ligands further increases the mono-hydrogenation selectivity, especially when electron donating MgO is used as a support (>99% selectivity for mono-hydrogenation as shown in Figure 8). This is attributed to limiting the activity for H2 dissociation and preventing the additional hydrogenation to butane.

#### 2.3 Metal complexes and supported materials for the hydrogenation of trienes

Selective hydrogenation of triene is also an important topic for fine chemicals and pharmaceutical industries [3]. Myrcene with one isolated C=C bond and two Selective Mono-Hydrogenation of Polyunsaturated Hydrocarbons: Traditional and Nanoscale… DOI: http://dx.doi.org/10.5772/intechopen.81637

Figure 8.

the catalysis results shown in Entries 3, 5, and 7 indicate the increased conversion of reactants with the formation of some minor full hydrogenation products. The results suggest that the primary product, cyclohexene, would compete with the diene reactant for the catalytic active sites. When the concentration of 1,3 cyclohexadiene is high at the beginning of the reaction, diene would dominate the adsorption on the active sites and the reaction would maintain a good selectivity to cyclohexene. With the increased concentration of cyclohexene at the later stage of the reaction, the adsorption on active catalytic sites starts to take place. The reactions are generally slower for G1 catalysts compared to G0 catalyst especially with longer linkers. The G2-C12 catalyst, however, exhibits good activity and selectivity toward cyclohexene indicating the importance of right combination between dendrimer generations and linker lengths. A trace amount of benzene was also observed for some of the catalysts. The catalytic reactions of other acyclic dienes also are investigated to understand the effects of substrate structure. In general, the 1,2-addition hydrogenation of less hindered C=C is the most favorable compared to that of more hindered C=C, 1,4-addition hydrogenation, and full hydrogenation. Zeolite- and magnesium oxide-supported molecular rhodium complexes are also

Hydrogenation of 1,3-cyclohexadiene with silica supported PAMAM-palladium complex catalyst (5.25mmol 1,3-hexadiene, 10 μmol Pd catalyst complex, 5 mL methanol, pressurized glass autoclave to 14 psi H2, 25°

1 G0, C1 1.75 >99 76 24 — 2 G1-C2, C2 0.5 21 >99 — — 3 20 >99 88 10 2 4 G1-C6, C3 0.5 11 >99 — 0 5 20 72 97 1.5 1.5 6 G1-C12, C4 0.5 15 >99 — — 7 20 20 >99 — — 8 G2-C6, C5 0.5 N.R ——— 9 20 >99 73 11 6 10 G2-C12, C6 0.5 24 >99 — — 11 5 >99 80 14 6

(%) (%) (%)

C).

Entry Catalyst Time (h) Conv. (%)

Gold Nanoparticles - Reaching New Heights

GC and NMR are used to monitor the reaction.

Data reproduced from [22].

Table 4.

tested for the hydrogenation of 1,3-butadiene [23]. The selectivity for monohydrogenation increases when the Rh species nucleation decreased from several atoms to dimeric clusters. The poisoning with CO ligands further increases the mono-hydrogenation selectivity, especially when electron donating MgO is used as a support (>99% selectivity for mono-hydrogenation as shown in Figure 8). This is attributed to limiting the activity for H2 dissociation and preventing the additional

2.3 Metal complexes and supported materials for the hydrogenation of trienes

Selective hydrogenation of triene is also an important topic for fine chemicals and pharmaceutical industries [3]. Myrcene with one isolated C=C bond and two

hydrogenation to butane.

110

Selectivity plots for MgO-supported rhodium dimers in the absence (A) and in the presence (B) of CO ligands in the hydrogenation of 1,3-butadiene (filled circle: 1-butene, open diamond: trans-2-butene, blue square: cis-2 butene, red triangle: butane). (Reactions condition: 2 vol % 1,3-butadiene, balanced with H2, total pressure = 1 bar, room temp) Reproduced from [23] with permission from the American Chemical Society.

Figure 9. Reactant (1) and potential products (2–7) for myrcene hydrogenation. Products 4–7 are dienes. Reproduced from [24] with permission from the Sciencedirect.

conjugated C=C bonds can be synthesized by the pyrolysis of β-pinene in nature. Despite the availability of myrcene, only little success in selective hydrogenation of the substrate to diene have been made. Gusevskaya et al. reported the hydrogenation of myrcene either by using metal complex ion or heterogeneous sol-gel catalysts [24, 25]. For 10% palladium on carbon (Pd/C) under 20 atm of H2 and cyclohexane solvent system at 80°C, full hydrogenation reaction would take place within 30 min, and the catalytic system does not have any selectivity toward diene or monoene. On the other hand, the transition metal complexes of [RuCl2(CO)2(PPh3)2], [RhH(CO)(PPh3)3], [IrCl(CO)(PPh3)2], and [Cr(CO)6] show the capability to form diene products in relatively good yields (Figure 9 and Table 5) [24].

From Entries 1–4 in Table 5, the reactivity of the metal complexes under the same condition turns out to be Ru < Cr < Ir < Rh. Ru and Rh complexes show slightly higher selectivity toward monohydrogenated products, dienes, than chromium and iridium complexes. Rh complex is further tested by adding extra PPh3 ligand in the reaction (Entry 5). The presence of extra PPh3 slows the reaction down requiring higher reaction temperature, but increases selectivity toward dienes. The similar selectivity of myrcene hydrogenation is achieved by simply changing solvent from cyclohexane to benzene even at the lower reaction temperature of 80° C. The reaction only produces a trace amount of double or full hydrogenation products. However, the selectivity among different dienes (Entries 4–7) is still poor.

The first hydrogenation of myrcene, a triene, takes place at the conjugated diene group instead of the isolated and hindered alkene group. The 1,2-addition of conjugated diene produces either compound 4 or compound 5. The 1,4-addition of conjugated diene involving Pd-allyl intermediates produce compounds 6 and 7, the


that the reaction temperature would greatly affect the reactivity of the Pd/SiO2 catalysts. BET surface area analysis shows the change in synthesis temperature that would cause some variations in the pore size of the catalyst. Since the pore is created by the presence of organic solvent during the synthesis process, the high tempera-

Selective Mono-Hydrogenation of Polyunsaturated Hydrocarbons: Traditional and Nanoscale…

pressure. However, unstable gaseous environment at higher temperature of

tivity toward dienes by mono-hydrogenation is higher than 90%. The Pd/SiO2

surface and undergo full hydrogenation. For the Pd/SiO2 heated at 1100°

form dienes from myrcene is observed for this Pd/SiO2 heated at 1100°

3.1 Semi-heterogeneous colloidal nanoparticles in ionic liquids

C causes the collapse in its pore resulting in the decreased surface area and

The catalysis results for myrcene hydrogenation in Table 6 show that the selec-

organic compound without forming any alkene. Due to the higher surface energy of this catalyst with a large pore, the substrates are readily adsorbed on the catalyst

lower surface energy of the catalyst makes the activity to be decreased. The hydrogenation of isolated alkene would become unfavorable and the high selectivity to

Colloidal nanoparticle catalysts are considered semi-heterogeneous due to their homogeneous characteristics (kinetic efficiency) accompanied by their heterogeneous surface property. Semi-heterogeneous catalyst can be benefited from the advantages that both homogenous and heterogeneous catalysts have. Nanoparticle catalysts can have a higher catalytic activity than their bulk and heterogeneous counterparts, especially with nano-effects of high surface area to volume ratio. They can also be easily separated from the products and recycled similar to other

Semi-heterogeneous nanoparticle catalysts used for the hydrogenation of dienes show the relatively good selectivity and reactivity even compared to the traditional homogeneous catalysts. Ionic liquid-stabilized Pd nanoparticle catalyst shown in Figure 10 is one of those examples used for diene hydrogenation [26]. The thin film

(Pd/sgPF6) enhances the solubility in methylene chloride and the selectivity of the catalyst. The ionic liquid on the surface acts like a cage to control which substrate would pass through the liquid ion film and reach the particle surface for catalytic reaction. The liquid ion film on catalyst creates the non-equilibrium environment,

Several conjugated and isolated diene compounds are tested for the catalytic hydrogenation using Pd/sgPF6, the hydrophobic ionic liquid nanoparticle catalyst (Table 7). The results show that the conjugated cyclodienes (Entries 1 and 3) are much more reactive than the isolated cyclodienes (Entries 2 and 4). The catalytic reactions of cyclohexa-1,3-diene 8 and cycloocta-1,3-diene 13 produce monohydrogenation products almost quantitatively. Once the monoene is formed, the further hydrogenation does not take place due to the weaker adsorption of

monoenes on Pd nanoparticle surface. The adsorption of dienes on Pd nanoparticle surface by two π bonds coordination is much more preferred because Pd atom is electron deficient. In addition, the mono-hydrogenation products have lower

C would generate the largest pore size due to increased gas

C converts the reactant to saturated

on the Pd nanoparticle surface

C, the

C. The ratio

ture at or above 300°

catalyst with a larger pore created at 300°

DOI: http://dx.doi.org/10.5772/intechopen.81637

of the E/Z isomer (compound 6 and 7) is 0.176.

3. Semi-heterogeneous nanoscale catalysts

heterogeneous catalytic systems.

113

of ionic liquid with the hydrophobic anion PF6

so that the rate determining step could easily be observed.

1100°

pore size.

a Reaction time necessary for ca. 80% conversion.

b Selectivity for monohydrogenated products 4–7 at ca. 80% conversion. <sup>c</sup>

PPh3 was added (0.17 mmol). <sup>d</sup>

Benzene was used as a solvent.

Data reproduced from Ref [24].

#### Table 5.

The catalytic reaction of metal ion catalyst with myrcene, compound 1, under 20 atm H2 and cyclohexane solvent.

E-Z isomers. Based on the results in Table 5, the major diene products are compound 5 and 6. This is due to the higher reactivity of terminal alkene in myrcene, which undergoes the coordination of the primary alkene group followed by the hydrogen addition. These isolated diene products 4–7 are further hydrogenated to monoene 3 with the addition of hydrogen to less sterically hindered alkene. When the reaction is continued, the yield for full hydrogenation product 2 constantly increases. Since there are several pathways for the hydrogenation of myrcene and the reactions generate many different mono- and di-hydrogenation products, this reaction is extremely difficult to control and hard to predict with regarding the overall selectivity. However, there are some correlations between the yields of products and the kinetics/thermodynamics of intermediates and products. For example, the kinetic reactivity among diene products should follow the ensuing order: 4 > 5 > 6 > 7.

For the hydrogenation of myrcene by sol–gel Pd/SiO2 catalyst, the selectivity for dienes is higher than that of the metal complex catalyst [25]. Robles-Dutenhefner et al. used three different temperatures for the reactions, the catalysis results show


Data reproduced from Ref [25].

#### Table 6.

Hydrogenation of myrcene catalyzed by Pd/SiO2 in cyclohexane under 20 atm H2.

Selective Mono-Hydrogenation of Polyunsaturated Hydrocarbons: Traditional and Nanoscale… DOI: http://dx.doi.org/10.5772/intechopen.81637

that the reaction temperature would greatly affect the reactivity of the Pd/SiO2 catalysts. BET surface area analysis shows the change in synthesis temperature that would cause some variations in the pore size of the catalyst. Since the pore is created by the presence of organic solvent during the synthesis process, the high temperature at or above 300° C would generate the largest pore size due to increased gas pressure. However, unstable gaseous environment at higher temperature of 1100° C causes the collapse in its pore resulting in the decreased surface area and pore size.

The catalysis results for myrcene hydrogenation in Table 6 show that the selectivity toward dienes by mono-hydrogenation is higher than 90%. The Pd/SiO2 catalyst with a larger pore created at 300° C converts the reactant to saturated organic compound without forming any alkene. Due to the higher surface energy of this catalyst with a large pore, the substrates are readily adsorbed on the catalyst surface and undergo full hydrogenation. For the Pd/SiO2 heated at 1100° C, the lower surface energy of the catalyst makes the activity to be decreased. The hydrogenation of isolated alkene would become unfavorable and the high selectivity to form dienes from myrcene is observed for this Pd/SiO2 heated at 1100° C. The ratio of the E/Z isomer (compound 6 and 7) is 0.176.

## 3. Semi-heterogeneous nanoscale catalysts

## 3.1 Semi-heterogeneous colloidal nanoparticles in ionic liquids

Colloidal nanoparticle catalysts are considered semi-heterogeneous due to their homogeneous characteristics (kinetic efficiency) accompanied by their heterogeneous surface property. Semi-heterogeneous catalyst can be benefited from the advantages that both homogenous and heterogeneous catalysts have. Nanoparticle catalysts can have a higher catalytic activity than their bulk and heterogeneous counterparts, especially with nano-effects of high surface area to volume ratio. They can also be easily separated from the products and recycled similar to other heterogeneous catalytic systems.

Semi-heterogeneous nanoparticle catalysts used for the hydrogenation of dienes show the relatively good selectivity and reactivity even compared to the traditional homogeneous catalysts. Ionic liquid-stabilized Pd nanoparticle catalyst shown in Figure 10 is one of those examples used for diene hydrogenation [26]. The thin film of ionic liquid with the hydrophobic anion PF6 on the Pd nanoparticle surface (Pd/sgPF6) enhances the solubility in methylene chloride and the selectivity of the catalyst. The ionic liquid on the surface acts like a cage to control which substrate would pass through the liquid ion film and reach the particle surface for catalytic reaction. The liquid ion film on catalyst creates the non-equilibrium environment, so that the rate determining step could easily be observed.

Several conjugated and isolated diene compounds are tested for the catalytic hydrogenation using Pd/sgPF6, the hydrophobic ionic liquid nanoparticle catalyst (Table 7). The results show that the conjugated cyclodienes (Entries 1 and 3) are much more reactive than the isolated cyclodienes (Entries 2 and 4). The catalytic reactions of cyclohexa-1,3-diene 8 and cycloocta-1,3-diene 13 produce monohydrogenation products almost quantitatively. Once the monoene is formed, the further hydrogenation does not take place due to the weaker adsorption of monoenes on Pd nanoparticle surface. The adsorption of dienes on Pd nanoparticle surface by two π bonds coordination is much more preferred because Pd atom is electron deficient. In addition, the mono-hydrogenation products have lower

E-Z isomers. Based on the results in Table 5, the major diene products are compound 5 and 6. This is due to the higher reactivity of terminal alkene in myrcene, which undergoes the coordination of the primary alkene group followed by the hydrogen addition. These isolated diene products 4–7 are further hydrogenated to monoene 3 with the addition of hydrogen to less sterically hindered alkene. When the reaction is continued, the yield for full hydrogenation product 2 constantly increases. Since there are several pathways for the hydrogenation of myrcene and the reactions generate many different mono- and di-hydrogenation products, this reaction is extremely difficult to control and hard to predict with regarding the overall selectivity. However, there are some correlations between the yields of products and the kinetics/thermodynamics of intermediates and products. For example, the kinetic reactivity among diene products should follow the ensuing

The catalytic reaction of metal ion catalyst with myrcene, compound 1, under 20 atm H2 and cyclohexane

1 [RuCl2(CO)2(PPh3)2] 110 100 83 1 16 7 32 9 35 2 [Cr(CO)6] 45 100 74 4 22 8 26 29 11 3 [IrCl(CO)(PPh3)2] 15 100 76 4 20 8 22 33 13 4 [RhH(CO)(PPh3)3] 5 100 87 4 9 13 24 34 16

C) S (%)<sup>b</sup> Product distribution (%)

<sup>c</sup> 24 140 96 tr. 4 14 31 26 25

<sup>d</sup> 60 80 98 tr. 2 15 27 34 22

234567

Run Catalyst Time (min)a T (°

Gold Nanoparticles - Reaching New Heights

5 [RhH(CO)(PPh3)3]

6 [RhH(CO)(PPh3)3]

PPh3 was added (0.17 mmol). <sup>d</sup> Benzene was used as a solvent. Data reproduced from Ref [24].

Reaction time necessary for ca. 80% conversion.

Selectivity for monohydrogenated products 4–7 at ca. 80% conversion. <sup>c</sup>

a

b

Table 5.

solvent.

For the hydrogenation of myrcene by sol–gel Pd/SiO2 catalyst, the selectivity for dienes is higher than that of the metal complex catalyst [25]. Robles-Dutenhefner et al. used three different temperatures for the reactions, the catalysis results show

> Conv. (%)

C (0.5) 80 15 75 100 20 18 62

C (0.5) 100 15 80 99 1 18 15 66

C (2.5) 80 15 75 98 1 1 18 15 65

C (0.5) 80 60 96 97 1 2 20 15 62

S (%)a

60 96 94 1 5 16 16 62

Product distribution (%)

2 3 4 5 6+7

Time (min)

C (0.5) 80 15 100 0 100

order: 4 > 5 > 6 > 7.

Run Catalyst (wt. %) T

1 1% Pd/SiO2/300°

4 1% Pd/SiO2/1100°

5 1% Pd/SiO2/1100°

6 1% Pd/SiO2/1100°

8 3% Pd/SiO2/1100°

Data reproduced from Ref [25].

Selectivity for monohydrogenated products 4–7.

a

112

Table 6.

(° C)

Hydrogenation of myrcene catalyzed by Pd/SiO2 in cyclohexane under 20 atm H2.

#### Figure 10.

Synthesis of ionic liquid hybride palladium nanoparticle by sputtering-deposition. Reproduced from [26] with permission from the American Chemical Society.

solubility in ionic liquid than diene reactants, which readily transfer through the liquid ion film. This makes monoene to be evicted out from the ionic liquid inhibiting the second hydrogenation. For isolated cyclohexa-1,4-diene 12 and cycloocta-1,5-diene 16, the reactivity and selectivity to generate monoene are similar, but the overall conversion yields are extremely low. The low reactivity of monoene compounds is confirmed from the catalytic reactions of cyclohexene (Entry 5) and cyclooctene (Entry 6). Both substrates are unreactive for hydrogenation condition. The reaction of unsymmetrical cyclohexa-1,3-diene 17 (Entry 7) results in high conversion yields for mono-hydrogenation products with a preference for the hydrogenation of less hindered alkene group. Due to the steric hindrance of methyl and isopropyl groups, the 1,4-hydrogenation product is not formed. For the lager-conjugated dienes, the catalytic reactivity decreases with increased steric hindrance of alkyl substituent groups (Entries 9–11). However, the opposite trend for selectivity toward internal alkene is observed. As the number of alkyl substituents around C=C bonds increases, the yield for thermodynamic 1,4 hydrogenation product also increases. Moreover, the deuterium gas studies also show the actual mechanism for the cyclohexa-1,3-diene hydrogenation by ionic liquid catalyst. The conversion from cyclohexa-1,3-diene to cyclohexene is mainly through the 1,2-hydrogenation reaction with 92% selectivity and the ratio of 1,2 addition/1,4-addition around 1.7.

product on the Pd surface and minimize the further hydrogenation to

sgPF6 catalyst (0.1 μmol Pd), substrate/Pd = 5000, 10 mL of CH2Cl2, 4 bar H2, 40 °

Entry Diene TOF (Conv.)<sup>a</sup> Products (Selectivity/%)

Selective Mono-Hydrogenation of Polyunsaturated Hydrocarbons: Traditional and Nanoscale…

9 (97) 10 (2) 11 (1)

9 (>99) 11 (<1)

14 (>99) 15 (<1)

14 (>99) 15 (<1)

18 (67) 19 (33)

29 (19) 30 (81)

25(18) 26 (7) 27 (75)

C and 250 rpm).

3.0 (>99)

0.1 (<2)

13.0 (>99)

0.3 (<4)

6.2 (>99)

3.7 (>99)

0.7 (>99)

(<1) —

(<1) —

8

DOI: http://dx.doi.org/10.5772/intechopen.81637

12

13

16

9

14

17

24

28

TOF = mol substrate converted/(mol Pd surface ∙ s).

Other possible reasons for the high selectivity toward 1,4-addition product are also proposed. First, the initially produced 1,2-addition products could be isomerized to 1,4-addition product due to the higher stability of 2-methyl-2-butene. Second, the methyl group on isoprene would affect the stability of π-allyl intermediate (Figure 11) [6]. The intermediate A would generate two different products, 3-methyl-1-butene or 2-methyl-2-butene. In regards to the steric effect of intermediate A at C2 and C4, C4 has less substituted groups around C=C compared to C2. Therefore, it is easier for the second hydrogen to add on C4, and increase the yield of 2-methyl-2-butene. For the intermediate B, it is also easier for hydrogen atom to transfer to C1 since C3 is relatively more hindered than C1, which results mostly in 2-methyl-2-butene. The steric hindrance also directly influences the relative yield of 3-methyl-1-butene (higher) compared to that of 2-

Selective hydrogenation of dienes by Pd/sgPF6 catalyst under optimized conditions (Reaction condition: Pd/

<sup>9</sup> <sup>20</sup> 11.6 (>99) <sup>21</sup> (36) <sup>22</sup> (54) <sup>23</sup> (10)

2-methylbutane.

Conversion determined by GC.

Data reproduced from [26].

1

2

3

4

5

6

7

10

11

Table 7.

a

methyl-1-butene (lower).

115

#### 3.2 Semi-heterogeneous dendrimer-encapsulated metal nanoparticles

The catalytic property of polypropylenimine (PPI)-Pd nanoparticle hybrids is examined by the hydrogenation of isoprene substrate (Table 8) [27]. The reaction generates 2-methyl-2-butene (1,4-addition product) as the major product and 3 methyl-1-butene and 2-methyl-1-butene (1,2-addition products) as the minor products. The selectivity of the catalytic reaction is dependent upon the pressure of applied hydrogen gas and the ratio of substrate and catalyst. Higher hydrogen pressure and low substrate/catalyst ratio would result in decreased selectivity for monohydrogenation product. PPI dendrimer would enhance the catalytic selectivity to form monoene because it would limit the adsorption of the primary monoene

Selective Mono-Hydrogenation of Polyunsaturated Hydrocarbons: Traditional and Nanoscale… DOI: http://dx.doi.org/10.5772/intechopen.81637


Conversion determined by GC. a TOF = mol substrate converted/(mol Pd surface ∙ s). Data reproduced from [26].

#### Table 7.

solubility in ionic liquid than diene reactants, which readily transfer through the liquid ion film. This makes monoene to be evicted out from the ionic liquid inhibiting the second hydrogenation. For isolated cyclohexa-1,4-diene 12 and cycloocta-1,5-diene 16, the reactivity and selectivity to generate monoene are similar, but the overall conversion yields are extremely low. The low reactivity of monoene compounds is confirmed from the catalytic reactions of cyclohexene (Entry 5) and cyclooctene (Entry 6). Both substrates are unreactive for hydrogenation condition. The reaction of unsymmetrical cyclohexa-1,3-diene 17 (Entry 7) results in high conversion yields for mono-hydrogenation products with a preference for the hydrogenation of less hindered alkene group. Due to the steric hindrance of methyl and isopropyl groups, the 1,4-hydrogenation product is not formed. For the lager-conjugated dienes, the catalytic reactivity decreases with increased steric hindrance of alkyl substituent groups (Entries 9–11). However, the opposite trend for selectivity toward internal alkene is observed. As the number of alkyl substituents around C=C bonds increases, the yield for thermodynamic 1,4 hydrogenation product also increases. Moreover, the deuterium gas studies also show the actual mechanism for the cyclohexa-1,3-diene hydrogenation by ionic liquid catalyst. The conversion from cyclohexa-1,3-diene to cyclohexene is mainly through the 1,2-hydrogenation reaction with 92% selectivity and the ratio of 1,2-

Synthesis of ionic liquid hybride palladium nanoparticle by sputtering-deposition. Reproduced from [26] with

3.2 Semi-heterogeneous dendrimer-encapsulated metal nanoparticles

The catalytic property of polypropylenimine (PPI)-Pd nanoparticle hybrids is examined by the hydrogenation of isoprene substrate (Table 8) [27]. The reaction generates 2-methyl-2-butene (1,4-addition product) as the major product and 3 methyl-1-butene and 2-methyl-1-butene (1,2-addition products) as the minor products. The selectivity of the catalytic reaction is dependent upon the pressure of applied hydrogen gas and the ratio of substrate and catalyst. Higher hydrogen pressure and low substrate/catalyst ratio would result in decreased selectivity for monohydrogenation product. PPI dendrimer would enhance the catalytic selectivity to form monoene because it would limit the adsorption of the primary monoene

addition/1,4-addition around 1.7.

114

permission from the American Chemical Society.

Gold Nanoparticles - Reaching New Heights

Figure 10.

Selective hydrogenation of dienes by Pd/sgPF6 catalyst under optimized conditions (Reaction condition: Pd/ sgPF6 catalyst (0.1 μmol Pd), substrate/Pd = 5000, 10 mL of CH2Cl2, 4 bar H2, 40 ° C and 250 rpm).

product on the Pd surface and minimize the further hydrogenation to 2-methylbutane.

Other possible reasons for the high selectivity toward 1,4-addition product are also proposed. First, the initially produced 1,2-addition products could be isomerized to 1,4-addition product due to the higher stability of 2-methyl-2-butene. Second, the methyl group on isoprene would affect the stability of π-allyl intermediate (Figure 11) [6]. The intermediate A would generate two different products, 3-methyl-1-butene or 2-methyl-2-butene. In regards to the steric effect of intermediate A at C2 and C4, C4 has less substituted groups around C=C compared to C2. Therefore, it is easier for the second hydrogen to add on C4, and increase the yield of 2-methyl-2-butene. For the intermediate B, it is also easier for hydrogen atom to transfer to C1 since C3 is relatively more hindered than C1, which results mostly in 2-methyl-2-butene. The steric hindrance also directly influences the relative yield of 3-methyl-1-butene (higher) compared to that of 2 methyl-1-butene (lower).


Data reproduced from [27].

#### Table 8.

The catalytic reactions of isoprene with Pd nanoparticle composite in 2 mL toluene at 70°C and under 3 MPa H2.

PAMAM dendrimer-encapsulated Pd nanoparticle catalyst. Depending on the generation of the triazole dendrimer, the catalyst could develop into different morphologies that control the activity and selectivity. When the G0 dendrimers are used, the interdendrimer-stabilized palladium nanoparticle (DSN) is formed

(a) Generation 1, dsn, and (b) generation 1, den, Pd nanoparticle encapsulated by PAMAM. Reproduced

Selective Mono-Hydrogenation of Polyunsaturated Hydrocarbons: Traditional and Nanoscale…

encapsulated palladium nanoparticle (DEN) is produced. Due to the smaller size of the G0 dendrimer, the Pd nanoparticle cannot be encapsulated by dendrimer and needs to be stabilized by several G0 dendrimer. This makes the overall size of DSN relatively larger than the other higher generation dendrimer-capped catalysts. When the higher generation dendrimer is used, the large size of dendrimer allows enough Pd ions to be encapsulated in the interior of dendrimer and the following reduction generates dendrimer-encapsulated Pd nanoparticles. Unlike PAMAM-stabilized metal nanoparticle, 1,2,3-triazoleferrocenyl dendrimerstabilized Pd nanoparticle would have higher stability during the catalyst reaction [28]. The nature of the reducing agent and the generation of dendrimer are found to have noticeable influence on the reactivity and stability of each catalyst. DEN-G1 reduced by methanol has the best reactivity for the mono-hydrogenation of diene, which indicates that the smaller size of the Pd nanoparticle increases the reactivity. Moreover, the unique structure of 1,2,3-triazoleferrocenyl dendrimer is also found to be the another reason for catalyst to have higher reactivity. Table 9 shows DEN-G1 has higher reactivity to small dienes for mono-hydrogenation. However, the catalytic reactions of large dienes with steric bulkiness are slightly slower. Hydrogenation of trienes mostly results in the formation of monoene compounds indicating the high activity of diene intermediate after mono-hydrogenation. More substituted dienes tend to have a lower catalytic reactivity. Moreover, the isomerization of terminal monoenes and the trace amount of the 1,4-hydrogenation prod-

(Figure 12). For the higher generation dendrimers, the intradendrimer-

from [28] with permission from the Royal Society of Chemistry.

DOI: http://dx.doi.org/10.5772/intechopen.81637

uct from highly substituted dienes are also observed in the reaction.

as semi-heterogeneous catalysts

117

Figure 12.

3.3 Well-defined small organic ligand-capped palladium nanoparticles

Many ligand-passivated nanoparticles have been used as semi-heterogeneous catalysts. Since the surface ligands that stabilize nanoparticles from aggregation can have either hydrophobic or hydrophilic property, they can be soluble in various solvents including organic and aqueous solutions. Shon group has developed the thiosulfate protocol using alkanethiosulfate as a ligand precursor to passivate and

#### Figure 11.

Reduction pathway for isoprene to form monoene product. Reproduced from [6] with permission from the American Chemical Society.

Ornelas et al. also studied the semi-heterogeneous catalysis by using dendrimerpassivated palladium nanoparticle as a catalyst for the hydrogenation of dienes [28]. They synthesized the 1,2,3-triazole heterocycles-capped palladium nanoparticle catalyst that exhibits the higher reactivity to diene hydrogenation compared to the

Selective Mono-Hydrogenation of Polyunsaturated Hydrocarbons: Traditional and Nanoscale… DOI: http://dx.doi.org/10.5772/intechopen.81637

Figure 12.

(a) Generation 1, dsn, and (b) generation 1, den, Pd nanoparticle encapsulated by PAMAM. Reproduced from [28] with permission from the Royal Society of Chemistry.

PAMAM dendrimer-encapsulated Pd nanoparticle catalyst. Depending on the generation of the triazole dendrimer, the catalyst could develop into different morphologies that control the activity and selectivity. When the G0 dendrimers are used, the interdendrimer-stabilized palladium nanoparticle (DSN) is formed (Figure 12). For the higher generation dendrimers, the intradendrimerencapsulated palladium nanoparticle (DEN) is produced. Due to the smaller size of the G0 dendrimer, the Pd nanoparticle cannot be encapsulated by dendrimer and needs to be stabilized by several G0 dendrimer. This makes the overall size of DSN relatively larger than the other higher generation dendrimer-capped catalysts.

When the higher generation dendrimer is used, the large size of dendrimer allows enough Pd ions to be encapsulated in the interior of dendrimer and the following reduction generates dendrimer-encapsulated Pd nanoparticles. Unlike PAMAM-stabilized metal nanoparticle, 1,2,3-triazoleferrocenyl dendrimerstabilized Pd nanoparticle would have higher stability during the catalyst reaction [28]. The nature of the reducing agent and the generation of dendrimer are found to have noticeable influence on the reactivity and stability of each catalyst. DEN-G1 reduced by methanol has the best reactivity for the mono-hydrogenation of diene, which indicates that the smaller size of the Pd nanoparticle increases the reactivity. Moreover, the unique structure of 1,2,3-triazoleferrocenyl dendrimer is also found to be the another reason for catalyst to have higher reactivity. Table 9 shows DEN-G1 has higher reactivity to small dienes for mono-hydrogenation. However, the catalytic reactions of large dienes with steric bulkiness are slightly slower. Hydrogenation of trienes mostly results in the formation of monoene compounds indicating the high activity of diene intermediate after mono-hydrogenation. More substituted dienes tend to have a lower catalytic reactivity. Moreover, the isomerization of terminal monoenes and the trace amount of the 1,4-hydrogenation product from highly substituted dienes are also observed in the reaction.
