An attempt to predict the optimum zeolite-based catalyst for selective cracking of naphtha-range hydrocarbons to light olefins

Theoretical calculations were performed in order to determine, a priori, which in a range of small-to-medium pore zeolites would be the most effective at suppressing hydride transfer reactions by estimating the magnitude of steric hindrance. Steric hindrance was estimated as the difference between the enthalpies of adsorption of the transition-state complex and those of the two reacting hydrocarbons, calculated using the configurational-bias Monte Carlo (CBMC) method. As the concentration of adsorbed heptane also affects reaction rates adsorption equilibria were also calculated using this same theory. Heptane cracking experiments were then performed using acidic catalysts of this same series of zeolites to test the validity of these predictions. Reaction products were sampled during the first seconds on stream and extrapolation was made to zero time on stream in order to determine the initial catalytic activities and product selectivities in the absence of deactivation. Turnover frequency (TOF) decreased with decreasing pore diameter in agreement with predicted heptane adsorption equilibria. Hydride transfer was predicted to be most restricted in FER and TON, however, lower heptane adsorption capacities suggested that MFI would be the best compromise in terms of both high activity and selectivity. Although differences in paraffin product differences were observed that were consistent with an increasing degree of protolytic cracking with decreasing pore size, no discernible pattern was observed in olefin product distributions (ethylene-butenes). (C) 2002 Elsevier Science B.V. All rights reserved.