(356f) Light Alkane Dehydrogenation Activity for [Gah]2+ Sites Involving Two Framework Aluminum in Ga-Hzsm-5 Catalysts: a Dft Pathway Analysis

Light alkane
aromatization using shape selective MFI based catalysts is an important
chemical process. Unfortunately, substantial cracking activity of unmodified
HZSM-5 catalyst leads to large amounts of methane and ethane as undesired byproducts
reducing the aromatization selectivity. This problem is overcome by incorporating
additional dehydrogenation function in the form of extra-framework species like
Ga, Zn and Pt.   However, there are varying opinions regarding the role of GaH_X
species in the alkane dehydrogenation.  Using electronic DFT methods, we have
studied the catalytic activity of extra-framework [GaH]²⁺ species in
the proximity of the two framework Al, and we propose this as a likely model
for the most active Ga-related sites for alkane dehydrogenation.  We find that
the dehydrogenation activation barriers correlate strongly with reducibility of
the [GaH]²⁺ site (and hence the Al-Al distance in the dual site
model)?consistent with Brønsted-Evans-Polanyi relationships.  The optimal Al-Al
separation is governed by the interplay between two compensating reaction steps
(C-H activation and H-H formation), exemplifying the applicability of the
Sabatier principle for a distribution of the Al-pair sites in the zeolite.