Selective Hydrodemethylation of Methylalkylbenzenes on Potassium Hydride

Abstract

Dealkylation and hydrodealkylation of alkylaromatics are key processes in the petrochemical industry. The typical susceptibility of alkyl groups to removal via both dealkylation and hydrodealkylation reactions in aromatic compounds follows the order CH3 < 1° < 2° < 3°, with methyl and tertiary alkyl groups having the slowest and fastest reaction rates, respectively. Here, we report the selective catalytic hydrodealkylation of methylalkylbenzenes using carbon-supported potassium hydride (KH/C) under a H2 atmosphere, which exhibits an opposite reactivity pattern compared with that described above. This trend is observed across various methylalkylbenzenes and also aligns with the higher reactivity of toluene compared to other monoalkylbenzenes. Density functional theory calculations on ortho-ethyltoluene as a model substrate rationalize the experimental results by indicating a lower rate-limiting energy barrier for the direct removal of the methyl group as methane relative to the removal of the ethyl group as ethane on the (001) facet of KH. Our study reveals that the potassium hydride surface exhibits a distinct hydrodealkylation mechanism, characterized by selectivity for hydrodemethylation, which contrasts with that of zeolites, Mo- or Pt-based hydrodealkylation catalysts, and homogeneous free-radical pathways.