1. Introduction

Selective hydrogenation of polyunsaturated hydrocarbons including polyenes and alkynes are vitally important processes in fine chemical industries [1]. For instance, polymerization and hydroformylation reactions require a high purity of monomeric alkenes. However, light alkenes produced by catalytic cracking of petroleum often contain a high level of dienes or alkynes, making the selective hydrogenation of these compounds to monoenes in the presence of alkenes that are highly essential [2]. The removal of polyunsaturated hydrocarbons is also often important for catalysis applications, since their strong adsorption to metal surfaces would deactivate the catalyst during the reaction. In addition, the unique scent of natural polyunsaturated compounds makes them an important ingredient in

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 both traditional and nanoscale catalysts.

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

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

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

would not dissociate from the metal ion, forming [Rh(diene)Ln]

indicated the high reactivity of the [Rh(diene)Ln]

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

increased to 170°

Figure 1.

103

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,

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

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

namic) product. The high stability of 1,3-cyclooctadiene also reduces the reactivity

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

C, the hydrogenation product becomes the major (thermody-

+

<sup>+</sup> catalyst in general.

, due to the strong
