2.2.2 Supported metal nanoparticle catalysts: the nano effects

More and more researchers consider nanoparticles as a better option for catalytic reactions due to their high surface area per volume characteristics. In the area of heterogeneous catalysis, the complete analyses of catalyst sizes, compositions, and distributions are now required and many well-known active solid-state catalysts including carbon-supported Pd are found to be actually in nanoscale. With the advancement of nanomaterials synthesis and characterization, the nanoscale catalysts are now designed and prepared to tune their activities for desired applications including diene hydrogenation. For example, Pd nanoparticles stabilized with dendrimers (polypropylenimine, PPI) deposited on a silica surface are used for catalysis application (Figure 5) [18]. The internal amine functional groups on PPI dendrimers are used as a ligand to encapsulate Pd nanoparticles and the external amine groups help grafting the dendrimers on a polyamine-modified silica surface to form the immobilized dendrimer catalyst composite. The dendrimers around the under-deposited nanoparticle increase the selectivity of the Pd nanoparticles and decrease the Pd metal leaching. The immobilized dendrimer catalyst reveals higher reactivity for the selective hydrogenation of dienes than the traditional heterogeneous catalysts. In this study, Karakhanov et al. further discussed the effects of size and substituent pattern of the substrate, 2,5-dimethyl-2,4-hexadiene, during the catalytic hydrogenation (Table 3). Since both C=C double bonds in 2,5-dimethyl-2, 4-hexadiene are internal and highly substituted at C2 and C5 positions, the rate of the reaction is relatively slow but the overall reactions result in the high yield of thermodynamic 1,4-addition product.

Instead of using modifier or poisoning agents to change the catalytic activity of heterogeneous metal substrates, the modification of support materials to induce the steric-related selectivity has been successfully attempted [19]. By overcoating Pd nanoparticle catalyst with porous alumina using atomic layer deposition, Yi et al. could produce a highly stable (against deactivation) and selectivity for monohydrogenation of 1,3-butadiene to butenes (Figure 6). The selective hydrogenation worked well even in the presence of excess propene. The alumina overcoat clearly suppressed the conversion of prepene to propane very efficiently while maintaining 100% butenes selectivity with 100% 1,3-butadiene conversion. This is attributed

#### Figure 5.

Polypropylenimine (PPI)-modified palladium nanoparticle catalyst composite. Reproduced from [18] with permission from the American Chemical Society.

fatty acid reactants based on the kinetic studies and ligand chain length studies reported in this work. The influence of ligand chemical functionality was also investigated in this study. The results showed that unlike hydrophobic

Kinetic data for linoleic acid hydrogenation over Pd/Al2O3 at 30°C and 6 bar H2. (a) Uncoated Pd/Al2O3 and (b) dodecanethiol-coated Pd/Al2O3. Reproduced from [17] with permission from the American Chemical

The working hypothesis of physical mixtures for 1,3-butadiene hydroisomerization. (a) Pd containing mixtures and (b) Pt containing mixtures. Reproduced from [13] with permission from the American Chemical Society.

Figure 4.

Figure 3.

Gold Nanoparticles - Reaching New Heights

Society.

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#### Table 3.

The catalytic reactions of 2,5-dimethyl-2,4-hexadiene with Pd nanoparticle composite in 2 mL toluene at 70°C and under 3 MPa H2.

> obtained as major product, reaching up to 76% selectivity. At the higher temperature, the selectivity for thermodynamically stable trans-2-butene increased with

Illustration of the synthetic approach to mesoporous Pd/OMC catalyst films. Reproduced from [20] with

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

2.2.3 Supported metal complexes: bridging homogeneous catalysis with heterogeneous

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

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,

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

the best selectivity at 60%.

permission from the American Chemical Society.

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

cyclohexene by mono-hydrogenation.

catalysis

Figure 7.

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Scheme 1.

#### Figure 6.

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 American Chemical Society.

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 porous alumina-coated Pd catalysts described above.

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

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

#### Scheme 1.

Illustration of the synthetic approach to mesoporous Pd/OMC catalyst films. Reproduced from [20] with permission from the American Chemical Society.

obtained as major product, reaching up to 76% selectivity. At the higher temperature, the selectivity for thermodynamically stable trans-2-butene increased with the best selectivity at 60%.
