To investigate the hydrocracking mechanisms of polycyclic aromatic hydrocarbons, we used HZSM-5 with one Al substitution (T1, T3, T5, T7, T11, and T12) as a solid acid catalyst and 9,10-dihydroanthracene as a model substrate on the basis of periodic density functional theory computations. It is found that the intersection of the straight and sinusoidal channels is energetically more favored than the individual sinusoidal and straight channels due to their different local environments. On the basis of the computed Gibbs free energy at 500 °C, the rate-determining step for the formation of 1-benzyl-2-methylbenzene as the first intermediate is the heterolytic activation of H 2 , which is encapsulated in a conjugated frustrated Lewis pair (FLP), generated from the acidic O−H proton transfer to the hydrocarbon substrate. For the subsequent formation of toluene or benzene and o-xylene from 1-benzyl-2-methylbenzene hydrocracking, the rate-determining step, which is the heterolytic activation of H 2 encapsulated in a conjugated FLP, determines the selectivity. On the basis of the apparent Gibbs free energy barriers at 500 °C, the activity has the decreasing order of T5 > T11 > T1 ≈ T12 ≈ T3 ≈ T7. It is noted that there is no correlation between the Gibbs free energy barriers and the acidic strength based on O−H deprotonation Gibbs free energy and the adsorption enthalpy of pyridine as a probe molecule.

Possible Al sites supported by ammonia and pyridine adsorption enthalpies from DFT calculations and experiment.

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