Two models are used to predict the expected molecular weight distribution for polyethylene and polypropylene formed through Ziegler-Natta catalysis. Realistic values of kinetic and physical parameters, taken from experimental studies, are used to provide quantitative results. The simple core model, which neglects catalyst particle breakup, does not allow significant MWD broadening due to diffusion effects. On the other hand, a more realistic multigrain model shows that significant MWD broadening could arise due to intraparticle monomer diffusion limitations.

This communication reports the first application of NMR imaging to study the progress of a multiphase heterogeneous catalytic reaction in situ. Various stationary regimes of alpha-methylstyrene (AMS) hydrogenation on a single Pt/gamma-Al2O3 catalyst pellet have been investigated. The two-dimensional maps of the liquid-phase distribution within the pellet have been obtained in the course of the catalytic reaction, with the pellet temperature rising up to 185 degrees C. The large liquid-phase concentration gradients have been shown to exist under conditions of liquid AMS evaporation accompanied by its vapor-phase hydrogenation. It has been demonstrated that despite a substantial broadening of the NMR lines of liquids permeating porous solids the quantification of the relative amounts of AMS and the major reaction product cumene with spatial resolution across the pellet is feasible.

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