The roles of gallium hydride and Brønsted acidity in light alkane dehydrogenation mechanisms using Ga-exchanged HZSM-5 catalysts: A DFT pathway analysis

Abstract

We have used density functional theory (DFT) to study light alkane dehydrogenation by Ga-exchanged HZSM-5 by considering two types of catalytic sites: a mono-Al site of the form Z−[HGaX]+ (X=H, CH3, OH, Cl) and a di-Al site of the form Z2−[GaH]2+. For the mono-Al site, we report a new, direct one-step dehydrogenation mechanism; however, we conclude in general that mono-Al sites in ZSM-5 are not likely responsible for alkane dehydrogentation, as calculated activation energies are too high compared to experimental values (∼60kcal/mol versus 39kcal/mol). Instead, we propose [GaH]2+ residing near di-Al sites (Z2−[GaH]2+) are more active sites for dehydrogenation. We report a three-step mechanism for di-Al sites consisting of (1) C–H activation, followed by (2) alkene desorption and (3) H2 removal. We find that as Al–Al distance increases, the activation barrier for C–H activation decreases (ranging from 85.72 to 38.38 to 19.69kcal/mol), while the barrier for H2 removal increases (ranging from 15.49 to 36.71 to 47.38kcal/mol)—resulting in an optimal Al–Al separation distance of 4.53Å arising from these competing trends. As a result, we propose a simple ‘structure-to-activity’ correlation based on the Sabatier principle, which could be used to model and design the catalyst with required dehydrogenation activity.