Through a full thermochemical analysis at reaction conditions, we studied ethane dehydrogenation activity of extra-framework [GaH](2+) in Ga/H-[Al]ZSM-5 at the 6-31g(d,p)/B3LYP level. We provide a theoretical evidence to Support the proposition of [GaH](+) near-pair framework-Al sites as being the active catalytic sites for ethane dehydrogenation. Dehydrogenation activity is governed by the reducibility of [GaH](2+) Z(d)(2-) site in the presence of H-2, which is largely dependent on a zeolite ring Structure consisting of two framework Al atoms. The Optimum sites are defined by the intersection of linear Bronsted-Evans-Polanyi relations between the activation energy of the structure sensitive steps and enthalpy of reduction for the catalytic site. A comparison of the carbenium and alkyl activation mechanism indicates that the activity of the alkyl activation mechanism will be limited by the removal of highly activated (Delta E-act similar to 55-60 kcal/mol) ethene from [HGaC2H5](+) species. Our thermochemical analysis also indicates that at higher temperatures, the pair-Al sites with larger Al-A] distances become more prevalent, increasing the number of optimum catalytic sites. This work suggests a potential optimal Si/Al ratio for a given Ga loading (c) 2006 Published by Elsevier Inc.