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

Abstract

Propane (C 3 H 8 ) and butane (C 4 H 10 ) 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 Cr 2 O 3 (0001) chromium oxide, which is commonly used in the industrial setting. A complete 108-step reaction pathway of butane (C 4 H 10 ) dehydrogenation was studied, yielding 1-butene (CH 2 CHCH 2 CH 3 ) and 2-butene (CH 3 CHCHCH 3 ), 1-butyne (CHCCH 2 CH 3 ) and 2-butyne (CH 3 CCCH 3 ), butadiene (CH 2 CHCHCH 2 ), butenyne (CH 2 CHCCH), and ultimately butadiyne (CHCCCH). We include cracking and coking reactions (yielding C 1 , C 2 , and C 3 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 conventiona...

Effect of Surface Oxidation on Oxidative Propane Dehydrogenation over Chromia: An Ab Initio Multiscale Kinetic Study

“…On the reduced surface, hydrogen atoms bound to oxygen atoms can only recombine into H 2 and desorb. 28 , 29 However, the oxidized surface can lose its surface oxygen as it gets reduced. Recombination of hydrogen atoms on two adjacent oxygen surface atoms yields chemisorbed water, which can desorb.…”

“…Theoretical calculations of the electronic structure were performed with VASP 30 − 33 in the plane-wave approach. The simulation parameters chosen were consistent with our previous work 28 , 29 for comparability. A GGA functional by Perdew and Wang was used (PW91) 34 with the projector-augmented wave method.…”

“…Theoretical calculations of the electronic structure were performed with VASP − in the plane-wave approach. The simulation parameters chosen were consistent with our previous work , for comparability. A GGA functional by Perdew and Wang was used (PW91) with the projector-augmented wave method. , To mitigate the self-interaction error on Cr when using GGA functionals, the DFT + U approach was used with a Hubbard factor of D – J = 4 eV, , which had been proved to describe Cr 2 O 3 satisfactorily. , As Cr 2 O 3 is magnetic, spin-polarized calculations were performed with initial magnetic moments of 3.0 on chromium.…”

“…Noting that the B, C, and D surfaces are essentially the E surface of different surface oxygen (in the form of chromyl) coverages, we construct a simplified model as an alternative. In our model, we use the A surface, which was already used in our previous studies, 28 , 29 and the E surface in varying fractions. The model has three types of active sites: chromium atoms on A (Cr red ), oxygen atoms on A (O red ), and oxygen atoms on E (O ox ).…”

“…In our previous studies, we have investigated the non-oxidative dehydrogenation of propane on the reduced α-Cr 2 O 3 (0001) surface using kinetic Monte Carlo (KMC) on DFT-obtained data 28 and butane dehydrogenation in an idealized plug flow reactor using DFT-fed microkinetic modeling. 29 In this work, we used DFT calculations to construct a thorough reaction pathway for propane dehydrogenation when oxidants are co-fed on the reduced and oxidized surfaces of α-Cr 2 O 3 (0001). These are used as the two extrema of the realistic catalyst surface.…”