Supported Heterogeneous Catalyst Systems

Silica-supported heterogenous Cr systems, such as the Phillips247,248 and Union Carbide catalysts,249,250 are used in the commercial production of polyethylene. The active sites are widely agreed to contain low-valent Cr centers. The relatively ill-defined nature of these catalysts has led to considerable efforts to synthesize well-defined homogeneous Cr-based catalysts. 

Supported Pt [61] and Ti [62] catalysts have been used in heterogeneous catalyst systems with H202 as an oxidant. However, these catalysts suffered from low TON values and low selectivity. A Sn-substituted beta zeolite was recently applied as a... 

The active components of many commercial supported heterogeneous catalysts are oxides or salts. Even for many metal catalysts, the precursors of metallic particles are also oxides or salts in some dispersed form. Hence the preparation of heterogeneous catalysts is deeply concerned in one way or another about the dispersion of oxides or salts on support surfaces. Furthermore, promoters or additives added to heterogeneous catalyst systems are also oxides or salts. Therefore, the spontaneous monolayer dispersion of oxides or salts on supports with highly specific surfaces as a widespread phenomenon will find extensive application in heterogeneous catalysis. Examples illustrative of this viewpoint are cited in the following sections. 

Bochmann [184] developed a heterogeneous catalyst system of silica-supported neodymium amides (Scheme 28) that in conjunction of TIBA displayed activity for the copolymerization of ethylene and butadiene to give copolymer with relatively broad molecular weight distribution and fra i -l,4-regulatedbutadiene sequences. 

Epoxidation processes currently account for around 30 % of H2O2 consumption in the chemical industry. Present processes are mainly peracid based and have limitations in scope and safety, as well as incurring high capital investment. Consequently, we have attempted to develop heterogeneous catalyst systems for such transformations. A system which has been extensively studied has been the epoxidation of cyclic aikenes with H2O2 in the presence of supported phosphotungstic acid. Table 3 illustrates our results. 

F Zhao, M Shirai, M Aral. Heterogeneous catalyst system for Heck reaction using supported ethylene glycol phase Pd/TPPTS catalyst with inorganic base. J Mol Catal A Chem 154 39-44, 2000. 

The supported nanoparticle catalyst system was used for solvent-free hydrogenation reactions of cydohexene, 1-hexene, and 1,3-dicydohexadiene, respectively, and compared with a similar biphasic ionic liquid system and with a heterogeneous supported nanocatalyst (Table 5.6-6). 

A similar mode of operation, i.e. delivery of highly active palladium and efficient prevention of Pd agglomeration can be assumed for other heterogeneous catalyst systems that effectively activate aryl chlorides, like layered double hydroxides (LDH) or chosen polymers. Recent reports on interesting catalytic results using polymers as support unfortunately do not give clear and sufficient experimental results on mechanistic aspects or on metal leaching [163, 171). 

Table 3 Comonomer effect in ethene/a-olefin copolymerization using heterogeneous and supported ZN catalyst systems...

Diels-Alder cycloaddition is an organic chemical reaction between a conjugated diene and a substituted alkene to form a substituted cyclohexene system [62]. Polymer-supported chloroaluminates, poly(l-butyl-4-vinylpyridinium) chloroaluminate, poly(l-butyl-3-vinylimidazolium) chloroaluminate, and poly(l-ethyl-3-vinylimidazolium) chloroaluminate were synthesized, and their activities were tested for the Diels-Alder reaction of cydopentadiene with methyl methacrylate [63]. These polymer-supported heterogeneous catalysts exhibited activities comparable to their corresponding monomeric homogeneous analogs and could be reused with a moderate loss of initial activity up to five runs. 

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