Consequences of Medium-Pore Zeolite Constraints for Alkene Cracking-The Case of n-Pentene.

The catalytic cracking of alkenes in zeolites is of fundamental and industrial significance, yet the elementary steps of the mechanism are surprisingly less well established than those of alkane cracking. Here, pentenes were employed as model alkenes to investigate cracking kinetics and pathways on H-ZSM-5 (MFI framework) at 703-843 K. Cracking is initiated from a hydrogen-bonded alkene, with specific carbenium ions acting as transition states or short-lived intermediates. Monomolecular cracking, quantified via ethene formation, has an intrinsic activation enthalpy (ΔHǂ°int.) of 167 kJ·mol-1, which is 26 kJ·mol-1 lower than for n-pentane, while maintaining comparable activation entropies (-3 vs. 3 J·mol-1·K-1), resulting in a 28-fold higher activity at 773 K. Butene formation follows two temperature-dependent pathways: dimerization cracking via tertiary-to-secondary carbenium ions at 703-733 K (ΔHǂ°int. = 64 kJ·mol-1) and monomolecular cracking involving CH3 + formation at 813-843 K (ΔHǂ°int. = 184 kJ·mol-1). Extending the analysis to other medium-pore zeolite frameworks such as TON and FER demonstrates that narrower pore systems suppress activity by increasing ΔHǂ°int., whereas extra-framework aluminum oxide promotes reactivity by entropically shifting the transition state to a later stage. Together, these results establish alkene cracking in zeolites as an enthalpy-entropy-controlled process dictated by topology and local chemical environment.