Heterogeneous model catalysts

Keywords Ethylene polymerization mechanisms Heterogeneous model catalysts Homogeneous model catalysts Molecular modeling Phillips Cr/silica catalyst Polyethylene Polymerization kinetics... 

During the last decade, increasing research efforts have been performed on Phillips catalysts through various approaches including spectroscopic methods, polymerizatiOTi kinetics, heterogeneous model catalysts, homogeneous model... 

In the previous section, investigations on heterogeneous model catalysts with more uniform and well-defined structure of surface chromium species supported on silica gel had been demonstrated as a powerful strategy for the basic study of Phillips catalysts. However, the complexity is still derived from the heterogeneity of the... 

Kunz, S., Haiti, K., Nesselberger, M., Schweinberger, F. R, Kwon, G., Hanzhk, M., et al. (2010). Size-selected clusters as heterogeneous model catalysts under applied reaction conditions. Physical Chemistry Chemical Physics, 12, 10288-10291. 

Model catalysts. Model catalysts usually feature with well-defined structures and could provide much clearer information of the active site to understand the PhiUips catalyst. Scheme 3.2 depicts some typical silica-supported heterogeneous model catalysts for the PhiUips chromium catalyst. S-2 catalyst prepared by wet impregnation of bis(triphenylsUyl) chromate onto thermaUy pretreated silica gel could be considered as a... 

Scheme 3.2 Heterogeneous model catalysts for Phillips chromium catalyst.

Schildenberger M, Bonetti YC, Gobrecht J, Prins R Nano-pits supports for heterogeneous model catalysts prepared by interference Hthograpy, Top Catal 13 109—120, 2000. 

As an example of the use of MIXCO.TRIAD, an analysis of comonomer triad distribution of several ethylene-propylene copolymer samples will be delineated. The theoretical triad Intensities corresponding to the 2-state B/B and 3-state B/B/B mixture models are given In Table VI. Abls, et al (19) had earlier published the HMR triad data on ethylene-propylene samples made through continuous polymerization with heterogeneous titanium catalysts. The data can be readily fitted to the two-state B/B model. The results for samples 2 and 5 are shown In Table VII. The mean deviation (R) between the observed and the calculated Intensities Is less than 1% absolute, and certainly less than the experimental error In the HMR Intensity determination. 

Heterogeneously catalyzed hydrogenation of alkenes is generally considered to be a structure-insensitive reaction, as was deduced from numerous studies on more or less complex model catalyst systems [40-54]. However, the following sections will give examples of the opposite case. 

A closer similarity exists between the C2-symmetric octahedral isospecific model sites, which have been proposed for the heterogeneous polymerization catalysts,13 15 and some slightly distorted octahedral metal complexes, including bidentate or tetradentate ligands, which have recently been described as active in isospecific olefin polymerization in the presence of MAO.128-130 In fact, all these catalytic systems can be described in terms of racemic mixtures of active species with A or A chiralities. 

The catalytic site is chiral and analogous to that proposed for the isospedfic heterogeneous catalysts (Section 3.2), but, unlike the heterogeneous model site, interconversion between enantiomeric complexes is assumed to be possible, after each insertion step, when the metal atom is pentacoordinated.147,149 The analysis of the nonbonded interactions at the catalytic site showed that a si chain favors the formation of the A complex, which in turn favors the re coordination and insertion of the successive monomer unit, thus assuring the syndiospedfic propagation of the chain. 

Among the large bulk of work regarding titanium(IV) silsesquioxanes, specific studies have been focused on synthesizing close molecular models of the active sites of titanium heterogeneous epoxidation catalysts in siUcaUte and in mesostruc-tured MCM-41 silica (Figure 14.3 gives some selected examples) [54, 61-66]. 

Industrial heterogeneous catalysts and laboratory-scale model catalysts are commonly prepared by first impregnating a support with simple transition metal complexes. Catalytically active metal nanoparticles (NPs) are subsequently prepared through a series of high temperature calcination and / or reduction steps. These methods are relatively inexpensive and can be readily applied to numerous metals and supports however, the NPs are prepared in-situ on the support via processes that are not necessarily well understood. These inherent problems with standard catalyst preparation techniques are considerable drawbacks to studying and understanding complex organic reaction mechanisms over supported catalysts. (4)... 

Mathematical models of packed bed reactors can be classified into two broad categories (1) one-phase, or pseudohomogeneous, models in which the reactor bed is approximated as a quasi-homogeneous medium and (2) two-phase, or heterogeneous, models in which the catalyst and fluid phases and the heat and mass transfer between phases are treated explicitly. Although the... 

Note that the results of our simulation via the pseudohomogeneous model tracks the actual plant very closely. However, since the effectiveness factors r]i were included in a lumped empirical fashion in the kinetic parameters, this model is not suitable for other reactors. A heterogeneous model, using intrinsic kinetics and a rigorous description of the diffusion and conduction, as well as the reactions in the catalyst pellet will be more reliable in general and can be used to extract intrinsic kinetic parameters from the industrial data. 

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