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

Abstract

An increasingly utilized way for the production of propene is propane dehydrogenation. The reaction presents an alternative to conventional processes based on petroleum resources. In this work, we investigate theoretically how Cr 2 O 3 catalyzes this reaction in oxidative and reducing environments. Although previous studies showed that the reduced catalyst is selective for the non-oxidative dehydrogenation of propane, real operating conditions are oxidative. Herein, we use multiscale modeling to investigate the difference between the oxidized and reduced catalyst and their performance. The complete reaction pathway for propane dehydrogenation, including C–C cracking, formation of side products (propyne, ethane, ethylene, acetylene, and methane), and catalyst coking on oxidized and reduced surfaces of α-Cr 2 O 3 (0001), is calculated using density functional theory with the Hubbard correction. Parameters describing adsorption, desorption, and surface reactions are used in a kinetic Monte Carlo simulation, which employed industrially relevant conditions (700–900 K, pressures up to 2 bar, and varying oxidants: N 2 O, O 2 , and none). We observe that over the reduced surface, propene and hydrogen form with high selectivity. When oxidants are used, the surface is oxidized, which changes the reaction mechanism and kinetics. During a much faster reaction, H 2 O forms as a coproduct in a Mars–van Krevelen cycle. Additionally, CO 2 is also formed, which represents waste and adversely affects the selectivity. It is shown that the oxidized surface is much more active but prone to the formation of CO 2 , while the reduced surface is less active but highly selective toward propene. Moreover, the effect of the oxidant used is investigated, showing that N 2 O is preferred to O 2 due to higher selectivity and less catalyst coking. We show that there exists an optimum degree of surface oxidation, where the yield of propene is maximized. The coke, which forms during the reaction, can be burnt away as CO 2 with oxygen.

Light olefin synthesis from a diversity of renewable and fossil feedstocks: state-of the-art and outlook

“…A less intensive oxygen treatment of the catalyst surface, achieved both by catalyst design and by reducing the oxygen concentration, promotes the dominance of nucleophilic sites. 133–136 This phenomenon is independent of the catalyst structure and works for different systems.…”

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“…In a recent work, Huš et al 301 investigated how Cr 2 O 3 catalyzes the propane dehydrogenation reaction in oxidative and reducing environments. An interesting contribution of this work is that the catalytic system was studied on a mixed surface, consisting of equal parts of oxidized and reduced areas.…”

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