Heterogeneous supported metal catalyst

Five- and six-membered oxygen-containing heterocycles include furan, benzofuran, pyran, flavanone, flavonol, and cyclic carbonates, etc. The synthesis of these heterocycles by heterogeneous supported metal catalysts will be discussed here. 

Heterogeneous vapor-phase fluorination of a chlorocarbon or chlorohydrocarbon with HP over a supported metal catalyst is an alternative to the hquid phase process. Salts of chromium, nickel, cobalt or iron on an A1P. support are considered viable catalysts in pellet or fluidized powder form. This process can be used to manufacture CPC-11 and CPC-12, but is hampered by the formation of over-fluorinated by-products with Httle to no commercial value. The most effective appHcation for vapor-phase fluorination is where all the halogens are to be replaced by fluorine, as in manufacture of 3,3,3-trifluoropropene [677-21 ] (14) for use in polyfluorosiHcones. 

The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. Most commercial titanium-based PE catalysts are supported on MgCl2. 

These were prepared by tethering Rh and Pt complexes to silica-supported metal catalysts (metal = Pd, Ni, Ru, Au). The catalysts are very active in the hydrogenation of benzene derivatives to the corresponding substituted cyclohexanes under mild conditions. The activities are higher than those of the separate homogeneous complexes, complexes just tethered to silica, or the silica-supported heterogeneous catalysts. When the sol-gel-entrapped [Rh2Co2(CO)12] complex was heat-treated at 100°C, immobilized metallic nanoparticles were formed.425 The catalyst thus prepared efficiently catalyzed substituted benzene derivatives. 

As for hydrogenation, heterogeneous catalytic oxidation of carbohydrates was essentially performed in the presence of carbon-supported metal catalysts, namely Pt, Pd or Bi-doped Pd.[57] Oxidation of glucose into gluconic acid, the worldwide production of which is around 60000 tons year 1,[52] is used in the food and pharmaceutical industry, and is produced today by enzymatic oxidation of D-glucose with a selectivity in gluconic acid close to 100%. 

Since the late 1960s there has been some interest in the concept of a structure-sensitive reaction in heterogeneous catalysis (177, 178). In the case of supported metal catalysts, structure sensitivity is visualized as a dependence of metal particle size and catalytic behavior in a given reaction (activity and selectivity). Almost all of the possible kinds of relationships were reported in the past. Recently, Che and Bennett reviewed this problem (161). Our intention here is not to repeat most of their analysis, rather we shall try to present our view on the general characteristics of palladium versus other platinum metals. 

The complexity of many heterogeneous systems used in multi-phase reactions, the use of a solid support, the difficulty in analyzing highly dispersed active sites and the bifunctional nature of many solid supported metal catalysts, make a detailed and complete study challenging. The simpler homogeneous systems teach many of the principles of catalysis active sites, reaction mechanisms, reaction kinetics and catalytic cycles, which can often be applied elsewhere. 

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