Light
cycle oil (LCO) can no longer be used as fuel additives for
diesel upgrading, and hence, the catalytic cracking of LCO into light
aromatics has become an urgent job for its proper utilization. In
this work, toward the rational design of catalysts for LCO cracking,
various ZSM-5 zeolites were prepared and utilized in tetralin (as
a presentative compound of LCO) cracking to determine the essential
locations of active sites. Results show that tetralin could hardly
penetrate into the micropores constituted by the 10-member-ring channels
of ZSM-5 owing to the large molecular size. Instead, this reaction
majorly proceeds via the acid sites residing on the external surfaces
rather than those in the intracrystal mesopores. Meanwhile, the external
acid sites also possess the advantage of rapid diffusion, which significantly
decreases the formation of coke and promotes the catalytic lifetime.
Consequently, the morphologies of ZSM-5 play a pivotal role in catalytic
performances, among which the nanosized ZSM-5 catalyst was the most
outstanding candidate, exhibiting a tetralin conversion of 87.0%,
benzene/toluene/xylene total yields of 51.3%, and a lifetime of 22
h as a result of the most abundant external acid sites. These findings
disclosed the locations of real active sites in ZSM-5 zeolites in
the cracking of tetralin and developed methods to enhance the catalytic
performances, which are helpful for the rational design of catalysts
for tetralin cracking.
In alkylation of benzene with syngas, the inevitable converting of syngas into methane, as a side reaction, severely hinders the effective utilization of carbon monoxide (CO), which is a major challenge to circumvent. In this work, a series of bifunctional catalysts composed of various zinc/zirconium/cerium metal oxides and HZSM-5 zeolite were prepared in order to explore the key factor of methane formation and achieve high CO efficiency. The results show that methane selectivity mostly depends on the hydrogenation capability, which can be regulated via altering compositions of the metal oxides. Combined with density functional theory calculation and characterizations, it is found that the hydrogenation capability of metal oxide is related to the energy barrier of hydrogen (H 2 ) heterolysis. Compared to CeO 2 , the addition of zinc favors a lower energy barrier and a stronger ability toward H 2 heterolysis, while the addition of zirconium makes this ability weaker. Too strong heterolysis capability consequently leads to excessive hydrogenation, which further causes high methane selectivity; however, too weak heterolysis capability will lead to low catalytic activity. Hence, proper hydrogenation capability is an important element for low methane selectivity and high catalytic activity. Our findings reveal the formation mechanism of methane and provide a new strategy to reduce methane content.
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