Kinetics and thermochemistry of C₄-C₆ olefin cracking on H-ZSM-5

C₄-C₆ olefin β-scission rate constants were inferred from experimental studies at 773-813 K and <15% conversion by considering every C₄-C₆ olefin isomer and all available β-scission modes: 2° to 2° (C), 1° to 3° and 3° to 1° (E), 1° to 2° and 2° to 1° (D), and 1° to 1° (F). Group contribution methods were implemented to assess adsorption enthalpies and entropies of C₄-C₆ olefin isomers on H-ZSM-5 via the development of group correction terms for surface alkoxides; a linear dependence of enthalpy (or entropy) of formation difference between a surface alkoxide and a gas-phase alkane on carbon number was considered. Tertiary alkoxides have the smallest adsorption constants among surface adsorbates, and the resulting low coverage of highly substituted alkoxides restricts their contribution to alkene cracking pathways. Intrinsic β-scission rate constants (k _E:k_C:k_D:k_F ratio of 1094:21:8:1 at 783 K) and activation energies (E_inE < E_inC < E _inD < E_inF) from experimentally observed effluent compositions of C₄-C₆ olefin cracking consistent with computational studies were derived after rigorously accounting for adsorption constants and surface coverages of each C₄-C₆ olefin isomer. These results demonstrate that shape selectivity constraints prevent equilibration of surface alkoxides on surfaces under reaction conditions relevant for alkene cracking and present a quantitative description of C-C bond cracking reactions of olefins catalyzed by solid acids.