Homogeneous catalysts heterogeneous catalyst comparisons

A valuable indirect method of probing the Horiuti-Polanyi mechanism is the study and comparison of competitive rates of hydrogenation of olefins using both homogeneous and heterogeneous catalysts. Comparisons of individual rates... 

Pioneering works in this area started in the 1960s by polymer scientists like D. Ballard at ICI and Y. Yermakov at the Novosibirsk Institute of Catalysis [54,55]. The need for polymers with better properties and for better technologies (gas phase processes) has led to the development of various strategies to obtain supported catalytic systems [56-58]. Here, we will concentrate on reactions leading to basic chemicals, and a comparison between homogeneous and heterogeneous catalysts will be performed when possible. 

The most striking comparison between the homogeneous and heterogeneous catalysts was that four times more palladium was required in the homogeneously catalyzed reaction to give about the same rate as that of the Pd/Al203 promoted reaction. A tertiary amine is present in the reaction mixture to remove the HC1 from the catalyst and regenerate the catalytically active... 

Comparison oj Activation Energies for Methanol Carbonylation with Various Homogeneous and Heterogeneous Catalysts... 

Advances made in the area of mechanism have been presented so as to give a perspective on similarities and differences between different catalysts. Both classical and novel systems have been covered with considerable attention placed on comparisons. Less detail is given for certain homogeneous and heterogeneous catalyst systems reported elsewhere in this book, such as Au/titanosilicates. 

A comparison between homogeneous and heterogeneous catalysts from the viewpoint of a homogeneous catalysis expert is presented below... 

Blaser, H.U., Jalett, H.P., Spindler, F. (1996) Enantioselective hydrogenation of alpha-ketoesters comparison of homogeneous and heterogeneous catalysts, J. Mol. Catal. A. Chem. 107, 85-94. 

Make a table comparing advantages and disadvantages of enzymes with conventional homogeneous and heterogeneous catalysts. Include comparisons of catalyst activity, selectivity, stability, sensitivity to reaction environment and cost. 

Table 6-1. Comparison of homogeneous and heterogenized catalysts in industrial reactions...

Ab initio quantum-chemical methods can be used to calculate a range of relevant properties for homogeneous and heterogeneous catalysts. The size of the system that can be examined, however, is still quite small in comparison with the features that make up the actual system. 

In liquid-phase oxidations, POMs can be used as efficient homogeneous and heterogeneous catalysts in combination with the environment-friendly oxidants such as H2O2 and O2. Especially, the POM catalysts with multimetallic active sites show unique oxidation reactions in comparison with the conventional monometallic complexes. The activities and selectivities for the selective oxidation by multimetallic active sites of POMs can largely be enhanced by the cooperative activation of oxidants, simultaneous activation of oxidants and substrates, ... 

These factors—activity, selectivity, and catalyst recovery—are the ones on which comparisons between homogeneous and heterogeneous catalysts are usually made. Other important issues are catalyst life, susceptibility toward poisoning, diffusion, and, last but probably most important, control of performance through mechanistic understanding. 

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. 

Fig. 31. Selectivity comparison for the enantioselective addition of Et2Zn to benzaldehyde using different dendritic and non-dendritic homogeneous and heterogeneous Ti-TADDOLates as chiral catalysts [107,110], (S)-.(R) ratios refer to the 1-phenyl-propanol formed...

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. 

Catalytic homogeneous hydrogenation of cyclohexene has been claimed for simple systems such as nickel(II) acetylacetonate [39] or a nickel-chloride complex with two monodentate amines [40]. The latter complex was used as comparison for a heterogeneous catalyst obtained by impregnation of the complex on y-alu-mina [40]. SCRs of 100 were used at 30 atm. H2 and temperatures up to 100°C, resulting in conversions of only 20-35% after 1 h. 

The water-gas shift rates are obviously much lower when heterogenized in comparison with the Rh complexes in homogeneous solutions of the amines (also see Tables 30-33). Kinetics for nitrobenzene reduction were performed for the cis-[Rh(CO)2(2-picoline)2]PF6 catalyst, and reported in 2000. Kinetics displayed a first order dependence on Pco over the range 0-1.9 atm in the temperature range 80-120 °C. As with the kinetics previously reported by Lima Neto and coworkers,121 it was suggested that the CO addition preceded the rate limiting step. A non-linear dependence on the rate versus Rh concentration, as with the previous study, suggested participation by both mononuclear and polynuclear species. 

Table 3.6 Comparison of homogeneous and heterogeneous Re-based catalysts in methyl oleate metathesis.

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