Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
10243806 | Catalysis Today | 2005 | 16 Pages |
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 Zâ[HGaX]+ (XÂ =Â H, CH3, OH, Cl) and a di-Al site of the form Z2â[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 (â¼60Â kcal/mol versus 39Â kcal/mol). Instead, we propose [GaH]2+ residing near di-Al sites (Z2â[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 19.69Â kcal/mol), while the barrier for H2 removal increases (ranging from 15.49 to 36.71 to 47.38Â kcal/mol)-resulting in an optimal Al-Al separation distance of 4.53Â Ã
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.
Related Topics
Physical Sciences and Engineering
Chemical Engineering
Catalysis
Authors
Yogesh V. Joshi, Kendall T. Thomson,