A lost piece of the puzzle of the alkane cracking mechanism: a carbanion pathway on a solid base catalyst

Abstract

The alkane cracking mechanism has been a subject of intense scrutiny, with carbonium and free radical mechanisms being two well-established pathways which correlate to solid acid catalysis and thermal cracking, respectively. However, despite an understanding of these two mechanisms, certain intricacies remain unexplored, especially when it comes to alternative reaction routes over solid base materials. This gap in the knowledge hinders optimization of the desired product selectivity of alkane cracking processes. In this work, solid superbases were first prepared by impregnation of NaNO₃ on MgO. The Na/MgO catalysts were characterized by XRD, BET, XPS and CO₂-TPD techniques. To investigate the role of solid base materials, propane cracking was conducted over MgO and Na/MgO. SiO₂ was chosen as a representative of thermal cracking. Na/MgO showed better selectivity for light olefins than MgO or SiO₂. Ethylene and light olefin selectivity could reach about 65.8% and 91.7%, respectively. Meanwhile, in terms of Na/MgO, the ratio of ethylene selectivity and propylene selectivity is greater than 2, exhibiting the advantage of selectivity for ethylene, which is obviously different from MgO and SiO₂. Propane cracking over Na/MgO with different loading amounts of NaNO₃ was investigated further. The conversion rates of the samples presented a “volcano curve” with increasing Na content. Furthermore, DFT calculation showed that the base-catalyzed process of the propane cracking reaction follows a carbanion mechanism. The better product distribution and stronger surface base sites can be ascribed to charge transfer arising from the loading of NaNO₃.