Brønsted Acid Catalyzed Cyclization of C7 and C8 Dienes in HZSM-5: A Hybrid QM/MM Study and Comparison with C6 Diene Cyclization

We extend our previous work (Joshi, Y. V.; Thomson, K. T. J. Catal. 2005, 230, 440) on H-ZSM-5 catalyzed cyclization of C6 diene using hybrid QM/MM method by studying cyclization of C7 and C8 dienes by 1,6 ring closing mechanism at the B3LYP/6-31g(d,p):UFF hybrid ONIOM method. We find the activation barrier for C7 or C8 diene cyclization starting from secondary chemisorbed alkoxide is lower compared to that for C6 diene. This is expected according to carbenium chemistry rules, which marks secondary carbenium transition state of C7 or C8 diene more stable compared to the primary carbocationic transition state for C6 diene. The energy difference between the bare cluster (BC) and embedded cluster (EC) calculations clearly point out the shape selective exclusion of the C8+ chemisorbed alkoxides. Thus, calculated energetics evidence the C7 cyclic product as the primary product of cyclization for light alkane aromatization reaction and explains the dominance of toluene in the aromatics distribution.