Influence of acid strength on the reactivity of alkane activation on solid acid catalysts: A theoretical calculation study

The influence of Bronsted acid strength on the reactivities of alkane activations has been systematically studied by density functional theory (OFT) calculations. Some typical reactions, such as methane hydrogen exchange (C-H bond activation), propane dehydrogenation (C-H bond dissociation and H-H bond formation), and propane cracking (C-C cleavage), were investigated on 8T solid acid models with varying acid strengths from weak-, strong-, to super-acid. According to our calculational results, it was revealed that the activation barriers (E-act) of all the reactions decrease linearly, while the rate coefficients (log k) increase linearly with increasing the acid strength, being indicative of the enhancement of reactivities. However, the reactivity of different reactions exhibits different sensitivity to acid strength: the propane cracking is most sensitive to acid strength, while the methane hydrogen exchange reaction is least sensitive to acid strength. On the basis of the natural charge on the organic fragment in the transition state, it was revealed that the sensitivity of the reactivity to acid strength could be related to the ionic character of the transition state. The propane cracking reaction possesses a more ionic transition state (0.913-0.964 vertical bar e vertical bar), while the methane hydrogen exchange has a less ionic transition state (0.646-0.773 vertical bar e vertical bar). (C) 2011 Elsevier Inc. All rights reserved.