2. Traditional catalysts

### 2.1 Metal complexes for hydrogenation of dienes in homogeneous system

Homogeneous system using metal complex usually exhibits high reactivity for the catalytic hydrogenation of dienes [7, 8]. Schrock et al. proposed the reaction mechanism for the catalysis of homogeneous rhodium metal complex ([RhLn] + ) that is efficient for the selective hydrogenation of norbornadiene (NDB). Based on the results of 1a–1d in Table 1, the diene reaction rate was not significantly affected by the addition of other reagents. Table 1 also presents the effect of the size of the ligand on metal complex (Rene values of 1a, 2, and 3 in Table 1). These results indicated that the reactivity decreases as the size of the ligand increases. This phenomenon is similar to other homogenous reactions of metal complex. Dienes could effectively chelate to the metal complex and form strong bonds with metal atoms, even under the presence of excess hydrogen gas. The strong bond formation between metal ion and diene could be visualized by the rapid color change of the complex after diene was added to the solution. The overall results indicated that the major path involves the coordination of olefins and the adsorption of hydrogen ([RhH2(NBD)Ln] + ), which are followed by the addition of hydride to form Rh-alkyl intermediate ([RhH(NBD-H)Ln] + ). After the reductive elimination of alkyl and hydride, rhodium metal complex ([RhLn] + ) is regenerated and is ready for recycle. The deuterium gas addition study for [Rh(NBD)Ln] <sup>+</sup> revealed that the substrate is chelated on metal ion by the two π bonds. Therefore, the primary hydrogenation reaction would have two deuterium on the endo side of the norbornene.


a A markedly nonlinear rate was observed. The behavior was more nearly first order in olefin (k <sup>¼</sup> <sup>4</sup>:<sup>4</sup> � <sup>10</sup>�<sup>4</sup><sup>s</sup> �<sup>1</sup>). b Catalyst precursor = 0.053 mmol.

Data reproduced from [7].

#### Table 1.

The hydrogenation of norbornadiene in acetone with Rh complex (in 10.0 ml of acetone, 1.0 ml of NBD, 30.0 � 0.5° C, 1 atm total pressure of H2, 0.026 mmol of catalyst precursor, R = rate in mmol/min).

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

The metal complex catalyst could also be deactivated once two bonds on dienes are chelated to one metal ion. Because the metal ion with a chelated diene compound is too stable to react, the bidentate ligand as shown in Figure 1 was essential in avoiding this deactivation by diene coordination. The catalysis results of substituted dienes by the rhodium complex with different ligands showed the influence of ligands on the catalytic selectivity for 1,2- and 1,4-addition products. The comparisons of results indicated that diphos and arphos favor the 1,4-addition product (80–90%), while dpea favors the 1,2-addition product (80%). Between diphos and arphos, arphos exhibits slightly higher selectivity toward the 1,4-addition product. The catalytic reaction of 1,4-cyclohexadiene begins with the isomerization to 1,3-cyclohexadiene. The produced 1,3-cyclohexadiene, however, would not dissociate from the metal ion, forming [Rh(diene)Ln] + , due to the strong bond formation between metal ion and diene compound. The monoene compound would dissociate from metal ion after its formation, because it forms a weakened bond with metal ion. The high conversion yields (>98%) of these catalytic reactions indicated the high reactivity of the [Rh(diene)Ln] <sup>+</sup> catalyst in general.

Frankel et al. showed that other metal complex catalysts such as chromium complex, methyl benzoate-Cr(CO)3, also favors the 1,4-addition reduction of dienes for the hydrogenation of 1,3- and 2,4-hexadiene (70–90%) [8]. In contrast, the formation of 1,4-addition hexene product was accompanied with the major formation of conjugated 1,3- and 2,4-diene products for the catalytic hydrogenation of 1,4- and 1,5-hexadiene indicating that the reaction would most likely involves the first isomerization of isolated dienes to conjugated intermediates. The catalytic reactions of conjugated dienes with methyl substituted group(s) also mostly produced the 1,4-addition hydrogenation products as shown in Table 2. The position of substituted methyl group would not have a major effect on the catalytic activity except the case for 2,5-dimethyl-2,4-hexadiene, which exhibits low reactivity due to the large steric interference of four methyl groups. The difficulty in generating the product with cis-conformation, which chromium complex catalyst favors, is considered to be the main reason. The 1,2-addition hydrogenation product generated from the catalytic reaction of 4-methyl-1,3-pentadiene is also turned out to be the 1,4-addition product involving H shift. The isomerization of 1,4-cyclohexadiene was also more favorable than the 1,4-addition hydrogenation, forming 1,3 cyclohexadiene as the major product. This result also proves that the isomerization would take place prior to the hydrogenation. When the reaction temperature is increased to 170° C, the hydrogenation product becomes the major (thermodynamic) product. The high stability of 1,3-cyclooctadiene also reduces the reactivity

Figure 1. Different bidentate ligands for diene catalysis reaction by rhodium complex [7].

perfume industry and more attentions are currently placed on the single hydrogenation products of the natural triene substrates as both perfume ingredients and pharmaceutical precursors [3, 4]. Many efforts have been carried out in the past for selective hydrogenation using either homogenous molecular catalysis or heterogeneous solid state reactions [5, 6]. With both the pros and cons of each approach, the semi-heterogeneous characteristics of soluble colloidal metal nanoparticles in addition to their large surface area to volume ratio have increased research interests on nanoparticle catalysts for selective organic reactions. This chapter reviews the upto-date progress on the selective hydrogenation of polyunsaturated olefins using

2.1 Metal complexes for hydrogenation of dienes in homogeneous system

Homogeneous system using metal complex usually exhibits high reactivity for the catalytic hydrogenation of dienes [7, 8]. Schrock et al. proposed the reaction mechanism for the catalysis of homogeneous rhodium metal complex ([RhLn]

that is efficient for the selective hydrogenation of norbornadiene (NDB). Based on the results of 1a–1d in Table 1, the diene reaction rate was not significantly affected by the addition of other reagents. Table 1 also presents the effect of the size of the ligand on metal complex (Rene values of 1a, 2, and 3 in Table 1). These results indicated that the reactivity decreases as the size of the ligand increases. This phenomenon is similar to other homogenous reactions of metal complex. Dienes could effectively chelate to the metal complex and form strong bonds with metal atoms, even under the presence of excess hydrogen gas. The strong bond formation between metal ion and diene could be visualized by the rapid color change of the complex after diene was added to the solution. The overall results indicated that the major path involves the coordination of olefins and the adsorption of hydrogen

+

chelated on metal ion by the two π bonds. Therefore, the primary hydrogenation

Run Catalyst Rdiene Rene Max % ene

1b 1a with 3.0 mol of HClO4 0.21 0.05 92 1c 1a with D2 0.22 (b) (b)

A markedly nonlinear rate was observed. The behavior was more nearly first order in olefin (k <sup>¼</sup> <sup>4</sup>:<sup>4</sup> � <sup>10</sup>�<sup>4</sup><sup>s</sup>

The hydrogenation of norbornadiene in acetone with Rh complex (in 10.0 ml of acetone, 1.0 ml of NBD,

C, 1 atm total pressure of H2, 0.026 mmol of catalyst precursor, R = rate in mmol/min).

reaction would have two deuterium on the endo side of the norbornene.

+

+ PF6

> + PF6

> + PF6

), which are followed by the addition of hydride to form Rh-alkyl

). After the reductive elimination of alkyl and

� 0.22 0.03 97

� 0.16 0.12 97

� 0.14 0.19 90

(initial)<sup>a</sup>

) is regenerated and is ready for recycle.

<sup>+</sup> revealed that the substrate is

(b) 80

�<sup>1</sup>).

+ )

both traditional and nanoscale catalysts.

Gold Nanoparticles - Reaching New Heights

2. Traditional catalysts

([RhH2(NBD)Ln]

a

b

Table 1.

102

30.0 � 0.5°

+

1a [Rh(NBD)(PPh3)2]

2 [Rh(NBD)(PPh2Me)2]

3<sup>b</sup> [Rh(NBD)(PPhMe2)2]

Catalyst precursor = 0.053 mmol. Data reproduced from [7].

hydride, rhodium metal complex ([RhLn]

The deuterium gas addition study for [Rh(NBD)Ln]

1d 1a with 2.0 mol of Et3N and D2 �0.21

intermediate ([RhH(NBD-H)Ln]

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

2.2 Supported materials for the hydrogenation of dienes in heterogeneous

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

materials are discussed here as traditional catalysts.

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

2.2.1 Supported metal catalysts: different strategies to modify activity

In terms of reactivity, heterogeneous catalysts are usually less reactive than homogeneous systems. However, heterogeneous catalysts only require simple separation processes for purification and can be more easily recycled compared to homogeneous catalysts. Therefore, many research groups have been working on advancing fundamental understanding on the structure/property relationship and technological applications of heterogeneous catalysts in the past decade. Many of these supported metal catalysts are in fact in nanoscale dimensions, but many earlier catalysis studies did not attempt detailed characterizations on material sizes and their distributions on the supports. Both materials with and without welldefined sizes and structures including the catalysts reported as nanoparticulate

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

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

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

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

and appropriate selectivity for partial hydrogenation of dienes.

system

over hydrogen.

hydrogenation.

105

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 as shown in Figure 2 below [12]. The catalytic reactions produced monohydrogenation 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 corresponding chiral α,β-unsaturated ketones.


#### Table 2.

Catalytic hydrogenation of methyl-substituted dienes with 0.5 mmol chromium complex (solvent: n-pentane, 50 ml; temperature: 160 ° C; initial H2 pressure: 30 atm.).

#### Figure 2.

Asymmetric hydrogenation using iridium metal complex (0.5 mol% Ir catalyst, PhCF3, K3PO4, H2, rt., 12 h). Reproduced from [12] with permission from the American Chemical Society.
