3.3 Well-defined small organic ligand-capped palladium nanoparticles as semi-heterogeneous catalysts

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

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

Product distribution

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

Figure 11.

116

Entry

a

Table 8.

3 MPa H2.

Mol/mol ratio. Data reproduced from [27].

P, MPa t, h Sub/ Pd<sup>a</sup>

Gold Nanoparticles - Reaching New Heights

Conv., %

Selec. on alkene, % 1 1 3680 100 96 16.5 22 58 4 3 1 3680 100 95 17.5 23.5 53 5 1 0.25 3680 100 95.5 20 26 51 4.5 1 0.25 7360 91.5 99 9 64.5 25.5 1 3 0.25 7360 100 91 16.5 24 50.5 9 3 0.25 14,720 100 93.5 20.5 25 48 6.5 3 0.25 22,080 100 94 22 24.5 47.5 6 3 0.25 66,240 83 98.5 19 30 49 2

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

American Chemical Society.

The C8 PdNP is used as the catalyst for the hydrogenation of various conjugated diene and triene substrates as shown in Table 10 [33]. The catalytic reactions of conjugated dienes with different substitution patterns around C=C bonds produce almost exclusively the mono-hydrogenation products (Table 10, Entries 1–5). In addition, the hydrogenation of trienes (Entries 6 and 7) also results in the high yields of isolated dienes, the mono-hydrogenation products. This reactivity confirms the important role of alkanethiolate ligands on controlling the activity of Pd catalyst surface. With its surface passivated by organic ligands, nanoparticle could maintain a good stability, so that it could avoid aggregation and keep the large surface area intact. The surface ligand could block the more active site (terrace surface) for hydrogenation and promotes selective hydrogenation of dienes. The analysis of final monoene compositions showed that the major product is the 1,4 addition product and the minor product is the 1,2-addition product. The kinetic study of diene to monoene proved that the high selectivity for the 1,4-addition product is the result of both initial 1,4-addition reaction and the subsequent isom-

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

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

erization of terminal alkene, the 1,2-addition product, into internal alkene.

Entry Substrate Reaction condition Major product (%)

24 h

Catalysis results of various dienes and trienes with 5 mol% of octanethiolate-capped Pd nanoparticle in CDCl3

24 h (91% 1,4-) + (9% 1,2-)

24 h (93% 1,4-) + (7% 1,2-)

24 h (90% 1,4-) + (7% 1,2-)

24 h (90% 1,4-) + (5% 1,2-)

24 h (59% 1,4-) + (41% 1,2-)

24 h (69% 1,4-) + (23% 1,2-)

(92% 1,4-) + (8% 1,2-)

1 C8 PdNP

2 C8 PdNP

3 C8 PdNP

4 C8 PdNP

5 C8 PdNP

6 C8 PdNP

7 C8 PdNP

Data reproduced from [29].

at 1 atm H2 after 24 h.

Table 10.

119

After the first hour of reaction, the conversion of diene in entry 1 reaches over 50% with the ratio of 1,4-/1,2-addition products at 3.43. The consumption of reactant is almost complete after 5 h reaction with the ratio of 1,4-/1,2-addition products at 4.90. The yield of the 1,4-addition product is continuously increasing after 5 h reaction with the ratio of 1,4-/1,2-addition products increasing to 10.30 at the 24 h reaction. This clearly indicated that the isomerization of the terminal alkene to the internal alkene is the reason for the high selectivity of 1,4-addition product. The selectivity between the 1,4- and 1,2-addition products are also summarized for other diene and triene substrates as shown in Table 10. C8 PdNP clearly showed high selectivity for the 1,4-addition product, the thermodynamically more stable product. C8 PdNP clearly exhibits excellent selectivity to form internal alkene, the mono-hydrogenation and 1,4-addition product, compared to other reported catalytic systems. Not only the conversion yields and selectivity are superior but also the reaction condition (room temperature and atmospheric pressure) is much

#### Table 9.

Hydrogenation of olefins catalyzed by methanol reduced DEN-G1 (Reaction condition: 25° C, 1 atm H2, and substrate/Pd ratio is 1000 in mixed solvent CHCl3/MeOH = 2).

stabilize the catalytically active palladium nanoparticle surface [29–33]. The hydrophobic alkanethiolate ligand gives the nanoparticle high solubility in nonpolar organic solvent. Since alkanethiosulfate offers slower passivation activity, the surface ligand density of alkanethiolate on Pd nanoparticles can be controlled. Therefore, the alkanethiolate-capped Pd nanoparticles generated from alkylthiosulfate exhibit good catalytic activity and selectivity toward various organic reactions including isomerization and hydrogenation. Hexanethiolate- and dodecanethiolatecapped Pd nanoparticles show unique catalytic properties for the reaction of allyl alcohol under the atmospheric pressure of hydrogen gas at room temperature [30, 31]. Allyl alcohol can undergo either hydrogenation to 1-propanol or isomerization to propanal.

Octanethiolate-capped Pd nanoparticle (C8 PdNP) with average core size of 2.3 nm is synthesized and its composition and structure are confirmed by various instruments. Alkanethiolate-capped Pd nanoparticles are investigated for the catalytic reaction of 1,4-pentadiene [32]. The mechanistic studies show that monohydrogenation of isolated dienes would take place on one of the terminal C=C bond (Figure 13). The di-σ-bonded Pd intermediate A would form after the di-π-bond adsorption on Pd, which leads to the mono-hydrogenation and the formation of 1-pentene. The further isomerization via mono-σ-bonded Pd intermediate converts 1-pentene to the isomerized product, 2-pentene.

#### Figure 13.

The proposed mechanism (from A to E) for 1,4-pentadiene catalytic reaction under H2 environment. Reproduced from [32] with permission from the Royal Society of Chemistry.

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

The C8 PdNP is used as the catalyst for the hydrogenation of various conjugated diene and triene substrates as shown in Table 10 [33]. The catalytic reactions of conjugated dienes with different substitution patterns around C=C bonds produce almost exclusively the mono-hydrogenation products (Table 10, Entries 1–5). In addition, the hydrogenation of trienes (Entries 6 and 7) also results in the high yields of isolated dienes, the mono-hydrogenation products. This reactivity confirms the important role of alkanethiolate ligands on controlling the activity of Pd catalyst surface. With its surface passivated by organic ligands, nanoparticle could maintain a good stability, so that it could avoid aggregation and keep the large surface area intact. The surface ligand could block the more active site (terrace surface) for hydrogenation and promotes selective hydrogenation of dienes. The analysis of final monoene compositions showed that the major product is the 1,4 addition product and the minor product is the 1,2-addition product. The kinetic study of diene to monoene proved that the high selectivity for the 1,4-addition product is the result of both initial 1,4-addition reaction and the subsequent isomerization of terminal alkene, the 1,2-addition product, into internal alkene.

After the first hour of reaction, the conversion of diene in entry 1 reaches over 50% with the ratio of 1,4-/1,2-addition products at 3.43. The consumption of reactant is almost complete after 5 h reaction with the ratio of 1,4-/1,2-addition products at 4.90. The yield of the 1,4-addition product is continuously increasing after 5 h reaction with the ratio of 1,4-/1,2-addition products increasing to 10.30 at the 24 h reaction. This clearly indicated that the isomerization of the terminal alkene to the internal alkene is the reason for the high selectivity of 1,4-addition product. The selectivity between the 1,4- and 1,2-addition products are also summarized for other diene and triene substrates as shown in Table 10. C8 PdNP clearly showed high selectivity for the 1,4-addition product, the thermodynamically more stable product. C8 PdNP clearly exhibits excellent selectivity to form internal alkene, the mono-hydrogenation and 1,4-addition product, compared to other reported catalytic systems. Not only the conversion yields and selectivity are superior but also the reaction condition (room temperature and atmospheric pressure) is much


#### Table 10.

Catalysis results of various dienes and trienes with 5 mol% of octanethiolate-capped Pd nanoparticle in CDCl3 at 1 atm H2 after 24 h.

stabilize the catalytically active palladium nanoparticle surface [29–33]. The hydrophobic alkanethiolate ligand gives the nanoparticle high solubility in nonpolar organic solvent. Since alkanethiosulfate offers slower passivation activity, the surface ligand density of alkanethiolate on Pd nanoparticles can be controlled. Therefore, the alkanethiolate-capped Pd nanoparticles generated from alkylthiosulfate exhibit good catalytic activity and selectivity toward various organic reactions including isomerization and hydrogenation. Hexanethiolate- and dodecanethiolatecapped Pd nanoparticles show unique catalytic properties for the reaction of allyl alcohol under the atmospheric pressure of hydrogen gas at room temperature [30, 31]. Allyl alcohol can undergo either hydrogenation to 1-propanol or isomeri-

Hydrogenation of olefins catalyzed by methanol reduced DEN-G1 (Reaction condition: 25°

Substrate Product (yield) TOFa

(78%)

(22%)

(100%)

(100%)

810

230

1150

530

C, 1 atm H2, and

Octanethiolate-capped Pd nanoparticle (C8 PdNP) with average core size of 2.3 nm is synthesized and its composition and structure are confirmed by various instruments. Alkanethiolate-capped Pd nanoparticles are investigated for the catalytic reaction of 1,4-pentadiene [32]. The mechanistic studies show that monohydrogenation of isolated dienes would take place on one of the terminal C=C bond (Figure 13). The di-σ-bonded Pd intermediate A would form after the di-π-bond adsorption on Pd, which leads to the mono-hydrogenation and the formation of 1-pentene. The further isomerization via mono-σ-bonded Pd intermediate

The proposed mechanism (from A to E) for 1,4-pentadiene catalytic reaction under H2 environment.

converts 1-pentene to the isomerized product, 2-pentene.

TOF were determined by the yield of formation and final product.

Gold Nanoparticles - Reaching New Heights

substrate/Pd ratio is 1000 in mixed solvent CHCl3/MeOH = 2).

Reproduced from [32] with permission from the Royal Society of Chemistry.

zation to propanal.

a

Table 9.

Data reproduced from [28].

Figure 13.

118

friendlier than other homogenous and heterogeneous catalysts tested for diene hydrogenation.

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

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