(461f) Combined Group Contribution and Structure-Activity Relations for Ga/H-[Al]Zsm-5 Catalyzed Dehydrogenation of Alkanes Using Density Functional Theory

For light alkane aromatization using acidic zeolites, the initial activation of alkane is the rate limiting step¹. To enhance the aromatization selectivity of the HZSM-5, additional dehydrogenation function is provided by the extraframework Ga. Until recently, a very little was known about the exact nature of the active site. Our recent publication² has shown that, only pair framework Al site with [GaH]²⁺ as the charge compensating species can explain the experimentally observed lower activation barrier for ethane dehydrogenation. In our previous investigation², we found the characteristic structure-activity relation for the [GaH]²⁺Z^2- sites with varying distance between the framework Al pair. We find that the activity of the catalytic site can be uniquely defined in terms of the reduction energy (ΔH_red) of the active site. Here we will compare the carbenium and alkyl activation mechanism for C-H activation. Although alkyl activation has lower activation barrier, the structure insensitive alkene desorption makes the apparent activation barrier larger than that for carbenium activation.

We have investigated the simultaneous proton and hydride abstraction mechanism³ for various alkanes such as ethane, propane, n-butane, isobutane, isopentane and 2,3-diemthylbutane (hexane) for different pair framework-Al sites. We find linear Brønsted-Evans-Polanyi relations between the reduction energy of the monohydride site ([GaH]²⁺Z^2-) and the activation barrier for C-H activation. We find systematic changes in the activation barrier for the simultaneous proton and hydride abstraction mechanism by changing the nature of the hydrogen donating carbon from primary to secondary to tertiary. These changes are quantified in terms of the group contribution relations to predict the activation barrier for the dehydrogenation reaction step. These combined group contribution and structure-activity relations enable use to predict the dehydrogenation activity corresponding to any combination of the active site structure and any given alkane.

References:

(1) Doolan, P. C.; Pujado, P. R. Hydrocarbon Processing 1989, 68, 72.

(2) Joshi, Y. V.; Thomson, K. T. Catalysis Today 2005, 105, 106.

(3) Mota, C. J. A.; Bhering, D. L.; Ramirez-Solis, A. International Journal of Quantum Chemistry 2005, 105, 174.