THE EFFECT OF CRYSTALLITE SIZE ON KINETICS OF EXCHANGE OF NEOPENTANE AND BENZENE WITH DEUTERIUM OVER SUPPORTED PLATINUM CATALYSTS

The primary objective of this work was to determine the suitability of several reaction systems as probe reactions for studying both chemical and physical phenomena associated with heterogeneous catalysis. Two systems investigated include benzene-deuterium exchange as a new system and neopentane-deuterium exchange which has been used previously for such studies.;In the absence of intraparticle and interparticle mass transfer effects, the kinetics of the two reactions was described by a simple model based on a single-step surface exchange mechanism. Deviations of kinetic data for neopentane exchange from single-step behavior were successfully explained by Dwyer's model of single-step exchange influenced by desorption effects. Analysis of neopentane exchange activity as a function of platinum dispersion showed no specific trend while the selectivity defined in terms of (beta) exhibited a weak maximum at 40-50 Angstroms. Hence, the suitability of neopentane-deuterium exchange as a probe reaction for studying physical phenomena in commercial catalysts and catalytic reactors was established.;The deviations of the kinetic data from single-step exchange behavior for benzene-deuterium exchange reaction could not be adequately explained by Dwyer's model. The agreement between the experimental results and those predicted by Dwyer's model for benzene exchange was not as satisfactory as that obtained for neopentane exchange. In any case, analysis of activity and selectivity showed a "demanding" behavior with a maximum at 15-25 Angstroms. Thus, benzene-deuterium exchange proved to be less than ideal as a probe reaction.;It was suspected that the presence of kinetic isotope effects in benzene-deuterium exchange reaction may be responsible for the relatively poor fit obtained with Dwyer' model. A model which takes into account the change in the specific rate constants with the deuterium content of the benzene molecule was developed.;A series of catalysts with uniform distribution of platinum and ranging in metal content from 0.28 to 2.68% (wt) were prepared by impregnation of (gamma)-alumina support with chloroplatinic acid. The characterization of the catalysts was carried out using three techniques: hydrogen chemisorption, x-ray line broadening, and electron microscopy. The average crystallite size ranged from 8 to 176 Angstroms while the metal dispersion varied from 1.06 and 0.05.