Kinetics of non oxidative propane dehydrogenation on Cr2O3 and the nature of catalyst deactivation from first principles simulations
Abstract As the global demand for propene (propylene) is increasing, classic commercial production processes are becoming unable to keep up. Non oxidative dehydrogenation, although hitherto underutilised industrially, has been put forward as a viable and green alternative, which is already used in a few commercial processes. In this work, we present detailed first principles calculations of this reaction over a chromium oxide catalyst, which is the cornerstone of the Catofin (r) process. A complete reaction pathway for the dehydrogenation of propane to propene and ultimately to propyne (methylacetylene) was considered. Cracking, which can yield C1 and C2 hydrocarbons, and the deactivation of the catalyst because of coking were also included and modelled. We used density functional theory calculations with the Hubbard model to study the structure of the involved intermediates, their adsorption and their interconversion to explain how chromium oxide catalysts facilitate this reaction and which processes cause their deactivation. We showed that the interaction of the hydrocarbons and molecular hydrogen with the catalytic surface is rather weak, resulting in low surface coverages, but increasing with multiple bonds present in hydrocarbons. Having constructed the potential energy surface with all the intermediates and the transition states linking them, we proposed a kinetic model for the reaction. Kinetic Monte Carlo simulations were performed at experimentally relevant temperatures (700 1000 degC) pressures (up to 10 bar) and inlet mixture compositions to study the kinetics of the reaction and discover the rate determining steps. As the reaction is highly endothermic, considerable conversions only occur at high temperatures. The accumulation of propene and propyne in the reaction mixture adversely affects the reaction rate and selectivity. Higher pressures increase the reaction rate but also increase the rate of coke formation, which poisons the catalyst. Deactivation of the catalyst has a strong temperature dependence and is caused by the accumulation of C* and CH3CC* on the surface, which are hard to remove even with hydrogen.