Hydrocracking of Fused Aromatic Hydrocarbons Catalyzed by Al-Substituted HZSM-5—A Case Study of 9,10-Dihydroanthracene

Abstract

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.

Frustrated Lewis Pair in Zeolite Cages for Alkane Activations

“…Moreover, the high activity of these FLPs on H 2 dissociation may be the other explanation for the effective effect of co‐feeding H 2 to prolong the lifetime of the catalysts in the MTO reaction by transforming active PMB + to relatively stable cyclic dienes [13] . The analogous reaction mechanism has been applied to the hydrocracking of 9,10‐dihydroanthracene in H‐ZSM‐5 [14] . Therefore, these supramolecular systems consisting of oxygen‐centric LB (Si−O−Al − ) and carbon‐centric LA (PMB + ) are undisputed FLPs with high activities due to their separated nature of negative/positive charged center by steric hindrance.…”

“…[13] The analogous reaction mechanism has been applied to the hydrocracking of 9,10dihydroanthracene in H-ZSM-5. [14] Therefore, these supramolecular systems consisting of oxygen-centric LB (SiÀ OÀ Al À ) and carbon-centric LA (PMB + ) are undisputed FLPs with high activities due to their separated nature of negative/positive charged center by steric hindrance.…”

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

“…Besides the FLP in the MTO reaction, Jiao and coworkers 124 treated the carbocation intermediates of 9,10-dihydroanthracene (DHA) hydrocracking encapsulated in ZSM-5 as a conjugated FLP as well (Fig. 12a).…”

High-efficiency hydrocracking of phenanthrene into BTX aromatics over a Ni-modified lamellar-crystal HY zeolite

“…10,11 It not only can make up for the shortage of petroleum products to a certain extent, [12][13][14] but also can allow utilization of these coalchemical wastes. 15,16 Currently, there are several conversion methods of polycyclic aromatic hydrocarbons, such as biodegradation, 17 adsorption, 18 photolysis, 19,20 catalytic oxidation, 21 hydrocracking, 22,23 etc. Compared with other methods, hydrocracking is superior because of its economy, high efficiency and simple operation.…”