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

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 Z(2-) [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 (similar to 60 kcal/mol versus 39 kcal/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) H-2 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 19.69 kcal/mol), while the barrier for H2 removal increases (ranging from 15.49 to 36.71 to 47.38 kcal/mol)-resulting in an optimal Al-Al separation distance of 4.53 angstrom 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. (c) 2005 Published by Elsevier B.V.