Tailoring Heterogeneous Catalysts

The first high-activity catalyst for ethylene polymerization which avoided the necessity of washing was derived from titanium(III)alkoxy chloride and triisohexylaluminum (92) [for the patent literature, see (25)1. Another route starts with titanium trichloride. Thus, excellent results as regards activity are obtained with the so-called Stauffer TiCls which contains 30 mol% of aluminum, obtained according to the following reaction. 

A supported catalyst for ethylene polymerization which requires no alkyl aluminum for activation was first claimed by the Phillips Petroleum Company (32). It consists of chromium oxide on silica, reduced with hydrogen. Krauss and Stach (93) showed that the active sites are Cr(II) centers. The presence of solvent, or even aluminum alkyls, diminishes 

Important Combinations for the Design of High-Activity, High-Stereoselectivity Ziegler Catalyst Systems for Propylene Polymerization 

Reactants for TiCl4 reduction TiCl3 treatment Possible co-components during TiCl3 treatment 

Aluminum powder Aluminum alkyls Grinding Heating Carboxylic acid chlorides Ethers Tertiary amines Tertiary phosphines Metal chlorides 

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Titanium-silsesquioxanes tailored at modeling and understanding Ti-based heterogeneous catalysts form the largest family of M-POSS models. 

Largely out of the need to understand the mechanistic details of Ziegler-Natta polymerization in order to take rational steps to improve the performance of Ti-based heterogeneous catalysts, attention has turned to the properties of Group 4 (Ti, Zr, Hf) metallocenes as homogeneous polymerization catalysts.15-17 As noted above, homogeneous catalysts offer the chemist precise knowledge of the nature of the catalytic site, and they also allow the properties and performance of the catalyst to be tailored to meet requirements. 

Mother nature has resolved the various limitations involved in multi-electron processes. Unique assemblies composed of cofactors and enzymes provide the microscopic catalytic environments capable of activating the substrates, acting as multi-electron relay systems and inducing selectivity and specificity. Artificially tailored heterogeneous and homogeneous catalysts as well as biocatalysts (enzymes and cofactors) are, thus, essential ingredients of artificial photosynthetic devices. 

The combination of surface-associated reactants with surface-bound H-atoms, occasionally leads to poor photoinduced hydrogenation of the reactant and parallelly to inhibition of H2-evolution. For such systems, tailored bifunctional heterogeneous catalysts have been developed [141], where cooperative catalytic effects are observed in the photohydrogenation reactions. Substitution of ethylene by acetylene, C2H2, in the photosystem composed of Ru(bpy) +/MV2+/Na2EDTA and the Pt colloid results in inefficient hydrogenation of acetylene to ethylene,

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Ugo R, Dossi C, Psaro R (1996) Molecular metal carbonyl clusters and volatile organometallic compounds for tailored mono and bimetallic heterogeneous catalysts. 1 Mol Catal A 107 13... 

This concept allows the important advantages of homogeneous over heterogeneous catalysts to be utilized for the first time. Particular mention should be made here of the significantly greater variability of homogeneous catalysts which enables them to be tailored to the requirements of the desired reaction by means of steric and/or electronic modification (cf. Table 1). 

In conclusion, this report introduces a truly heterogeneous catalyst based on the in situ generation of active peroxotungstate complexes. By taking profit of the variability of the anion population, a new class of tailor-made catalysts is available. The hydrophobicity of the surface can be tuned, and heteronuclear anion oligomers can be synthesized at the sohd-liquid interphase. 

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