(513e) Chain-Length and Framework Effects in Alkane Cracking on Solid Acid Catalysts
Catalytic cracking is a cornerstone process for current petroleum refineries. Its various forms, such as fluid catalytic cracking and hydrocracking, are the major routes through which transportation fuels and lubricants are made. More recently, the endothermic nature of cracking is also being explored as an effective route for heat removal and temperature control in supersonic aircraft engines. In all these applications, nanoporous solid acids such as zeolites are often used as the catalysts, whose unique pore / channel systems afford the potential for a precise control of reactivity and selectivity through different degrees of “solvation” of the reactants and products, as well as transition states. Here we present a computational study using dispersion-corrected, periodic density-functional theory. We investigate the cracking of short alkanes (normal alkanes from C2—C4, isobutane and adamantane) in medium and large pore zeolites (framework types MFI, BEA, MOR, and FAU) and on a model Al/SiO2 surface. We discuss both the mono-molecular and bi-molecular cracking mechanisms and show how dispersion is critical in understanding the chain-length dependence and framework effects of the activation energies. In particular, the apparent activation energies decrease with carbon number and framework confinement, while the intrinsic activation energies exhibit a much weaker dependence on either factor. Electronic effects (e.g., from branching) affect the barriers in accordance with the rules of carbenium ion chemistry. These results provide a foundation for the use of more cost effective methods to screen different zeolite pore topologies and to investigate how one can control the product selectivity by varying the temperature and reactant distributions, ultimately a step towards the rational catalyst design and optimization.