First-Principles-Based Multiscale Modelling of Nonoxidative Butane Dehydrogenation on Cr₂O₃(0001)

Propane (C₃H₈) and butane (C₄H₁₀) are short straight-chain alkane molecules that are difficult to convert catalytically. Analogous to propane, butane can be dehydrogenated to butenes (also known as butylenes) or butadiene, which are used industrially as raw materials when synthesizing various chemicals (plastics, rubbers, etc.). In this study, we present results of detailed first-principles-based multiscale modelling of butane dehydrogenation, consisting of three size- and time-scales. The reaction is modelled over Cr₂O₃(0001) chromium oxide, which is commonly used in the industrial setting. A complete 108-step reaction pathway of butane (C₄H₁₀) dehydrogenation was studied, yielding 1-butene (CH₂CHCH₂CH₃) and 2-butene (CH₃CHCHCH₃), 1-butyne (CHCCH₂CH₃) and 2-butyne (CH₃CCCH₃), butadiene (CH₂CHCHCH₂), butenyne (CH₂CHCCH), and ultimately butadiyne (CHCCCH). We include cracking and coking reactions (yielding C₁, C₂, and C₃ hydrocarbons) in the model to provide a thorough description of catalyst deactivation as a function of the temperature and time. Density functional theory calculations with the Hubbard U model were used to study the reaction on the atomistic scale, resulting in the complete energetics and first-principles kinetic parameters for the dehydrogenation reaction. They were cast in a kinetic model using mean-field microkinetics and kinetic Monte Carlo simulations. The former was used to obtain gas equilibrium conditions in the steady-state regime, which were fed in the latter to provide accurate surface kinetics. A full reactor simulation was used to account for the macroscopic properties of the catalytic particles: their loading, specific surface area, and density and reactor parameters: size, design, and feed gas flow. With this approach, we obtained first-principles estimates of the catalytic conversion, selectivity to products, and time dependence of the catalyst activity, which can be paralleled to experimental data. We show that 2-butene is the most abundant product of dehydrogenation, with selectivity above 90% and turn-over frequency above 10^-3 s^-1 at T = 900 K. Butane conversion is below 5% at such low temperature, but rises above 40% at T > 1100 K. Activity starts to drop after ∼6 h because of surface poisoning with carbon. We conclude that the dehydrogenation of butane is a viable alternative to conventional olefin production processes.