How Chain Length and Branching Influence the Alkene Cracking Reactivity on H-ZSM-5

Catalytic alkene cracking on H-ZSM-5 involves a complex reaction network with many possible reaction routes and often elusive intermediates. Herein, advanced molecular dynamics simulations at 773 K, a typical cracking temperature, are performed to clarify the nature of the intermediates and to elucidate dominant cracking pathways at operating conditions. A series of C-4-C-8 alkene intermediates are investigated to evaluate the influence of chain length and degree of branching on their stability. Our simulations reveal that linear, secondary carbenium ions are relatively unstable, although their lifetime increases with carbon number. Tertiary carbenium ions, on the other hand, are shown to be very stable, irrespective of the chain length. Highly branched carbenium ions, though, tend to rapidly rearrange into more stable cationic species, either via cracking or isomerization reactions. Dominant cracking pathways were determined by combining these insights on carbenium ion stability with intrinsic free energy barriers for various octene beta-scission reactions, determined via umbrella sampling simulations at operating temperature (773 K). Cracking modes A (3 degrees -> 3 degrees) and B-2 (3 degrees -> 2 degrees) are expected to be dominant at operating conditions, whereas modes B-1 (2 degrees -> 3 degrees), C (2 degrees -> 2 degrees), D-2 (2 degrees -> 1 degrees), and E-2 (3 degrees -> 1 degrees) are expected to be less important. All beta-scission modes in which a transition state with primary carbocation character is involved have high intrinsic free energy barriers. Reactions starting from secondary carbenium ions will contribute less as these intermediates are short living at the high cracking temperature. Our results show the importance of simulations at operating conditions to properly evaluate the carbenium ion stability for beta-scission reactions and to assess the mobility of all species in the pores of the zeolite.