KINETICS OF CRACKING OF NORMAL-DECANE AND NORMAL-HEXANE ON ZEOLITES H-ZSM-5 AND HY IN THE TEMPERATURE-RANGE 500-K TO 780-K

The kinetics of cracking of n-decane and n-hexane were investigated by observing the composition of the gas phase in contact with zeolite catalysts H-ZSM-5 and HY in a well-mixed closed system (gradientless batch-reactor). Catalyst temperature was varied from 200°C (473 K) to 500°C (773 K); partial pressure of reactant paraffin between 0.5 and 20 mbar. For ZSM-5 the influence of crystal size and of Si/Al-ratio on the kinetics was investigated. At temperatures above 400°C (673 K), the rates of cracking on H-ZSM-5 are first order in the partial pressures of paraffin; an estimate of diffusivity under reaction conditions is obtained from the influence of crystal size on observed rates at T > 700 K. Between 200°C (473 K) and 400°C (673 K), the rate of reaction of pure n-decane on H-ZSM-5 exhibits a maximum near 300°C and decreases with temperature to a minimum near 360°C. An induction period followed by autocatalysis was observed with n-decane as reactant in this temperature-range, depending on Si/Al ratio and degree of deactivation of the zeolite. It can be suppressed by addition of olefin to reactant paraffin. These phenomena are also observed in a less pronounced way with dealuminated HY, which deactivates more rapidly. A mathematical model based on a dual pathway of carbenium ion and protolytic cracking as postulated by Haag and Dessau (in "Proceedings 8th International Congress on Catalysis, Berlin 1984,"Vol. 11, p. 305, Verlag Chemie, Weinheim 1984) represents the observations qualitatively and in an approximate way also quantitatively. © 1992.