Heterogeneous catalysts advantages

The use of heterogeneous catalysts in the liquid phase offers several advantages compared with homogeneous counterparts, in that it facilitates ease of recovery and recyclidg. A chro-miiun-containingmediiun-pore molecular sieve fSi Cr> 140 1, CrS-2, efficiently catalyzes the direct oxidadon of various primary amines to the corresponclmg nitro compounds using 70% r-butylhy operoxide (TBHP. ... 

In comparison to heterogeneous catalyzed reactions, homogeneous catalysis offers several important advantages. The catalyst complex is usually well defined and can be rationally optimized by ligand modification. Every metal center can be active in the reaction. The reaction conditions are usually much milder (T usually < 200 °C), and selectivities are often much higher than with heterogeneous catalysts. 

Microporous catalysts are heterogeneous catalysts used in catalytic converters and for many other specialized applications, because of their very large surface areas and reaction specificity. Zeolites, for example, are microporous aluminosilicates (see Section 14.19) with three-dimensional structures riddled with hexagonal channels connected by tunnels (Fig. 13.38). The enclosed nature of the active sites in zeolites gives them a special advantage over other heterogeneous catalysts, because an intermediate can be held in place inside the channels until the products form. Moreover, the channels allow products to grow only to a particular size. 

Heterogeneous catalysts, in the general sense of catalysts placed in a phase different from that of the reagents and products, present clear advantages from a practical point of view, including ease of recovery and potential recycling and reuse. The latter point is especially important when the catalyst cost is high, as is the case for chiral catalysts [1]. 

Experiments have been carried out to compare nanoscale catalysts composed of Fe-, Ni-, and Co-complexes of several porphyrins or cyanocobalamin (Dror et al. 2005). A cobalt-porphyrin complex and cyanocobalamin in the presence of Ti(III)citrate reduced the initial concentrations of tetrachloromethane and tetrachloroethene by —99.5%, and the porphyrin was equally effective with trichloroethene. The advantage of using heterogeneous catalysts was shown by experiments in repetitive cycling of tetrachloromethane. Zero-valent metals degrade vicinal dichlorides such as tetrachloroethene by a-elimination to produce dichloroacetylene and hnally acetylene (Roberts et al. 1996). 

Finally, a second area of research for nanoparticles is their immobihza-tion on various supports. The deposition of well-defined nanoparticles on a support by different methods should advantageously replace traditional heterogeneous catalysts in terms of activity and selectivity. 

It is well known that microwave drying of many solid materials is a very efficient and widely used process even on an industrial scale [3] it is also an attractive means of drying of heterogeneous catalysts. Microwave drying of catalysts and supported sorbents has several advantages ... 

Despite this selectivity advantage of homogeneous catalysts, almost all of the industrial catalytic processes use heterogeneous catalysts, because of their one major advantage, their ease of separation form the reaction product. Being insoluble in the reaction... 

The obvious technological advantage of a heterogeneous catalyst is that it can be easily separated from reactants and products. However, the serious physical problem is diffusion of reactants to active centers on the surface of the catalyst and back diffusion of the formed intermediate and final products from the surface into the solution. This duffusion occurs much more slowly in the liquid phase compared to the gas phase. The problem of effectiveness of the heterogeneous catalyst in comparison with the homogeneous catalyst is closely connected with the problem of diffusion and sorption on the surface in the liquid phase. 

Corma and Renz6 developed an effective heterogeneous catalyst system. Incorporation of tin into a beta zeolite network (Sn-Beta) gave a catalyst that was used to convert citronellal 5 to racemic isopulegol (6, Equation (3)) with 85% diastereoselectivity. It was calculated that each metal site performed 11,500 reaction cycles. No leaching of the tin was detected. This catalyst system is advantageous over normal Lewis acids, since precautions against humidity are not required, and it is suitable for use in a fixed bed continuous reactor. 

The advantages that heterogeneous catalysts have is that they are easily separable from the product, and can be recycled. A number of studies have been conducted in which ligands have been attached or bound to polymeric material to provide an immobilized ligand, and these include polyacrylate and silica [27], polyurea [28], polythiourea [29], polyether [30, 31] and dendritic [32] systems. Upon metal coordination, the immobilized catalysts have retained most of the activity and selectivity, but they now provide the advantage of simple separation and recycling. For exam-... 

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