Hierarchically micro-/mesoporous Pt/KL for alkane aromatization: Synergistic combination of high catalytic activity and suppressed hydrogenolysis

Abstract

Pt/KL is a highly active and selective monofunctional catalyst for aromatization of n-alkanes due to its 1-dimensionally connected cage-like micropores. The unique micropore structure of the KL zeolite, however, can also inhibit the diffusion of bulky aromatic products out of the catalyst, which can cause unwanted side reactions that can decrease the aromatic yields. In this work, we investigated the effect of secondary mesoporosity, which can substantially facilitate molecular diffusion in Pt/KL during C6–C8 alkane aromatizations. The results showed that the hierarchically micro-/mesoporous Pt/KL synthesized with a subsequent dealumination/desilication method exhibited enhanced aromatic yields, compared with a conventional Pt/KL. In particular, in C7 and C8 aromatizations, the hierarchical Pt/KL showed substantially less formation of dealkylated aromatic products than the Pt/KL due to suppressed secondary hydrogenolysis. Pt supported on a solely mesoporous γ-Al2O3 also showed significantly suppressed secondary hydrogenolysis (dealkylation), which indicates that fast diffusion of alkylated aromatics out of the catalyst structure is important for suppressing the secondary hydrogenolysis. However, the solely mesoporous Pt/γ-Al2O3 showed markedly lower aromatization activities than the KL-supported catalysts, which indicates that Pt located inside the zeolite micropores is crucial for obtaining high catalytic activity due to the preorganization of n-alkanes in the micropores. The present results showed that the hierarchical Pt/KL provides the synergistically combined benefits of the Pt/KL and the mesoporous Pt catalyst, namely high aromatization activity and suppressed secondary hydrogenolysis, respectively.