Mechanistic insight into propane dehydrogenation into propylene over chromium (III) oxide by cluster approach and Density Functional Theory calculations

Abstract

A preliminary study to provides insight into the kinetic and thermodynamic assessment of the reaction mechanism involved in the non-oxidative dehydrogenation (NOD) of propane to propylene over Cr2O3, using a density functional theory (DFT) approach, has been undertaken. The result obtained from the study presents the number of steps involved in the reaction and their thermodynamic conditions across different routes. The rate-determining step (RDS) and a feasible reaction pathway to promote propylene production were also identified. The results obtained from the study of the 6-steps reaction mechanism for dehydrogenation of propane into propylene identified the first hydrogen abstraction and hydrogen desorption to be endothermic. In contrast, other steps that include propane’s adsorption, hydrogen diffusion, and the second stage of hydrogen abstraction were identified as exothermic. The study of different reaction routes presented in the energy profiles confirms the Cr-O (S1, that is, the reaction pathway that activates the propane across the Cr-O site at the alpha or the terminal carbon of the propane) pathway to be the thermodynamically feasible pathway for the production of propylene. The first hydrogen abstraction step was identified as the potential rate-determining step for defining the rate of the propane dehydrogenation process. This study also unveils that the significant participation of Cr sites in the propane dehydrogenation process and how the Cr high surface concentration would hinder the desorption of propylene and thereby promote the production of undesired products due to the stronger affinity that exists between the propylene and Cr-Cr site, which makes it more stable on the surface. These findings thereby result in Cr-site substitution suggestion to prevent deep dehydrogenation in propane conversion to propylene. This insight would aid in improving the catalyst performance.

Impact of Mo and W on CrXO3 (X = Cr, Mo, W) Catalytic Performance in a Propane Non-oxidative Dehydrogenation Process

“…The structures were built and minimised using the molecular mechanics (MMFF) method to remove strain energy, 38,89 after which the resulting structure was used as initial geometry during the use of DFT where B3LYP calculation method with 6-31G* and LANL2DZ basis sets were adopted, in line with the existing reports. 29,31 In using semi-empirical theory, the PM3 method was also used to optimise another set of the built structures where respective minimised structures were used as the structure for initiating the PM3 semi-empirical calculation. Existing literature 40,41 has justified using the PM3 and B3LYP method for the chromium and transition metals-based systems.…”

“…It was further reported that the reaction paths with moderate chromium site participation favoured better selectivity for propylene production. 31 The report of Hus et al 37 further indicated that high temperature, high pressure, and prolonged propylene accumulation on the catalyst promotes its deactivation. These reports suggest the need to investigate the impact of substituting the chromium site with other metals.…”

“…However, the literature shows that studies were primarily concentrating their investigations on using platinum-based catalysts. [8][9][10][11][12][13][14][15][16][17][18] Only a few studies gave preferential attention to chromium-based catalysts, [19][20][21][22][23][24][25][26][27][28][29][30][31][32] while some have investigated other catalyst forms like zeolite, nickel, gallium oxide, and many others. [33][34][35][36] Existing reports show that some works deployed an experimental approach to study the catalysts.…”