2.2.1 Supported metal catalysts: different strategies to modify activity

Selective hydrogenation of 1,3-butadiene was studied using graphite-supported palladium and platinum and the influence of FeCe alloying to these heterogeneous catalysts was investigated [13]. The results showed that the mono-hydrogenation and subsequent isomerization to 2-butene takes better place when the alloying was limited to less than 1/20 (Figure 3). The monohydrogenation selectivity was ascribed to the depletion of hydrogen atoms away from palladium surfaces by spill over to alloyed metal surfaces. The overall catalytic activity has also been increased by alloying of FeCe to Pd or Pt catalysts, indicating the activation of FeCe by spill over hydrogen.

Similarly, alumina-supported palladium catalysts doped with either tin or silver were tested for the selective hydrogenation of 1,5-hexadiene and 1,3-hexadiene [14]. Palladium on alumina itself produced mono-hydrogenation products from both 1,5-hexadiene and 1,3-hexadiene with a high selectivity even at full conversions. However, the selectivity for 1-hexene (or 3-hexene from 1,3-hexadiene) over the isomerized 2-hexene (trans > cis) from 1,5-hexadiene started to decrease at conversions higher than 80%. The addition of tin or silver tends to significantly increase the selectivity for 1-hexene, but with the loss of overall activity for monohydrogenation. This indicated that the addition of doping metal causes a geometric dilution of active Pd adsorption sites for both double-bond isomerization and hydrogenation.

Sulfidation of supported Pd catalysts has also been identified as an efficient way to increase the selectivity for mono-hydrogenation of dienes [15]. Supported palladium sulfide catalysts could be prepared by the addition of H2S or Na2S or the treatment with fuming sulfuric acid [16]. The produced palladium sulfide (Pd4S) catalysts deposited on carbon nanofibers exhibited the mono-hydrogenation activity in the gas-phase butadiene reduction producing butenes of various forms in good yields (99% of butenes at 100% conversion: the selectivities among various butenes are not reported). In contrast to Pd metal-based catalysts, this Pd4S catalyst presented high stability under reaction conditions while having significant activity and appropriate selectivity for partial hydrogenation of dienes.

Thiolate self-assembled monolayers deposited on Pd/Al2O3 catalysts could also direct the catalytic activity of heterogeneous systems for fatty acid diene hydrogenation as shown in Figure 4 [17]. In comparison, the uncoated Pd/Al2O3 catalyst produced the fully hydrogenated fatty acids under the same hydrogenation condition. This selectivity is attributed to steric effects between thiolate monolayers and

of substrate at the lower temperature resulting in low yield for the hydrogenation

as shown in Figure 2 below [12]. The catalytic reactions produced mono-

corresponding chiral α,β-unsaturated ketones.

Gold Nanoparticles - Reaching New Heights

0.69

0.64

1.1

0.007

0.1<sup>b</sup>

Regioselective asymmetric monohydrogenation of 1,4-dienes has been studied using various organometallic catalysts including ruthenium, rhodium, and iridium complexes with N or P binding chiral ligands [9–12]. The iridium catalysts exhibited excellent enantioselectivity for the hydrogenation of disubstituted cyclohexadienes

hydrogenation products shown below as major products in the yields ranging from 45 to 99% depending on the structure of O-R group. Tetrahydropyranyl acetal (THP) and triisopropyl silyl ether (TIP) resulted in 99% monohydrogenation selectivity. The enantioselectivity of these two groups were 83 and 97%ee, respectively, indicating highly efficient regio- and enantioselectivity of Ir catalyst for the synthesis of silyl protected enol ethers. Oxidation of these chiral enol ethers led to the

k, h<sup>1</sup> Products (% at 6 h)

(5)

(45)

C; initial H2 pressure: 30 atm.).

Reproduced from [12] with permission from the American Chemical Society.

Catalytic hydrogenation of methyl-substituted dienes with 0.5 mmol chromium complex (solvent: n-pentane,

Asymmetric hydrogenation using iridium metal complex (0.5 mol% Ir catalyst, PhCF3, K3PO4, H2, rt., 12 h).

(82) (18)

(75) (17) (8)

(66) (14) (14) (6)

product.

Dienes, 9.5 mmol

<sup>b</sup> At 175° C. Data reproduced from [8].

Table 2.

Figure 2.

104

50 ml; temperature: 160 °

alkanethiolate ligand modifiers, the hydrophilic thioglycerol modifiers strongly

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

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

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

Polypropylenimine (PPI)-modified palladium nanoparticle catalyst composite. Reproduced from [18] with

inhibited the catalytic activity of Pd/Al2O3 surfaces.

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

thermodynamic 1,4-addition product.

permission from the American Chemical Society.

Figure 5.

107

2.2.2 Supported metal nanoparticle catalysts: the nano effects

Figure 3.

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.

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 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

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

alkanethiolate ligand modifiers, the hydrophilic thioglycerol modifiers strongly inhibited the catalytic activity of Pd/Al2O3 surfaces.
