Transition metal catalysts 1770 INDEX

The molecular weight of polyethylene made with some late transition metal catalysts based on iron can be very high (> 1000000 g mol ) [30]. At low temperatures polymerization of a-oleftns with certain diimine-NiCl2 catalysts behave as a living polymerization that is, the number-average molecular weight increases linearly with polymerization time and the polydispersity index (PDI) is approximately 1. 

These oxidations suffer from the fact that the high selectivities are only observed at low conversions (<7%). At higher conversions, the carboxylic acid products leach the transition metals out of the zeolite framework into solution where the selectivity index is much lower [63]. As these reactions proceed, the 3 -I- oxidation states of the metal ions return to their 2 -I- states, accompanied by their characteristic color change. In the case of MnAlPO-18, the spent catalyst (Mn ) was washed with methanol and reactivated in dry air at 550°C and successfully recycled (Mn Mn ) twice without appreciable loss of activity [64]. 

In this paper, we propose to present a theoretical study of the catalytic hydrogenation of ethylene in order to compare the catalytic power of various transition metals. As a basic hypothesis, we have taken the fact that the ethylene molecule attaches itself to the metallic surface and this increases its reactivity. This is the same hypothesis as that of Coulson and Longuet-Higgins82 and Daudel and Sandorfy.38 These workers calculated the variation of the index of free valence. We have considered it more interesting to endeavour to calculate the electronic potential barrier. We have used the system of reaction levels as shown in Fig. 1, where V is the potential barrier in the absence of catalyst, Q the heat of chemisorption, and U the potential barrier in passing... 

Since the number of atoms on the surface of a bulk metal or metal oxide is extremely small compared to the number of atoms in the interior, bulk materials are often too costly to use in a catalytic process. One way to increase the effective surface area of a valuable catalytic material like a transition metal is to disperse it on a support. Figure 5.1.5 illustrates how Rh metal appears when it is supported as nanometer size crystallites on a silica carrier. High-resolution transmission electron microscopy reveals that metal crystallites, even as small as 10 nm, often expose the common low-index faces commonly associated with single crystals. However, the surface to volume ratio of the supported particles is many orders of magnitude higher than an equivalent amount of bulk metal. In fact, it is not uncommon to use catalysts with 1 nm sized metal particles where nearly every atom can be exposed to the reaction environment. 

Metallocene catalysis is an alternative to the traditional Ziegler-Natta vanadium-based catalysis for commercial polyolefin production, e.g. the use of metallocene-catalyzed ethylene alpha-olefin copolymers as viscosity index modifiers for lubricating oil compositions [23]. The catalyst is an activated metallocene transition metal, usually Ti, Zr or Hf, attached to one or two cyclopentadienyl rings and typically activated by methylaluminoxane. Metallocene catalysis achieves more stereo-regularity and also enables incorporation of higher alpha-olefins and/or other monomers into the polymer backbone. In addition, the low catalyst concentration does not require a cleanup step to remove ash. 

A heterogeneously catalyzed reaction takes place at the surface of a catalyst. Catalysts, their properties, and the nature of catalytic surfaces are discussed in Chapter 7. For this discussion, we approximate the surface as a single crystal with a known surface order. The density of atoms at the low-index planes of transition metals is on the order of. 10 cm 2. Figure 5.16 presents the atomic arrangement of low-index surfaces for various metals. This figure illustrates the packing arrange-... 

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