Dual-Acid-Site Engineering in Ti-β Zeolites for Enhanced Benzene Alkylation with Cyclohexene

초록

The alkylation of benzene with cyclohexene provides a direct and atom-efficient route to cyclohexylbenzene (CHB), a key intermediate for advanced chemical manufacturing. The role of Lewis acid sites (LAS) in homogeneous alkylation is well established; however, their function and synergy with Br & oslash;nsted acid sites (BAS) in solid acid catalysts remain elusive. Herein, we proposed a dual-acid-site engineering strategy to construct titanium-incorporated beta zeolites (H beta-at-x) via sequential dealumination and titanation. Structural and spectroscopic characterizations confirmed that Ti was anchored in silanol nests, generating electron-deficient Ti-LAS while tuning the Al-BAS to establish synergetic dual-acid-site. It was demonstrated that Ti-modified Al-BAS facilitated the protonation of cyclohexene, and benzene was adsorbed and enriched on Ti-LAS via pi-complexation adjacent to BAS. The optimized H beta-at-50 catalyst achieved complete cyclohexene conversion, even under the currently lowest benzene-to-cyclohexene volume ratio of 7:1. Furthermore, it demonstrated a significantly enhanced CHB formation rate of 18.6 mmolCHB<middle dot>gcat -1<middle dot>h-1, surpassing that of the parent H beta catalyst (2.98 mmolCHB<middle dot>gcat -1<middle dot>h-1) and the H beta-at catalyst (2.29 mmolCHB<middle dot>gcat -1<middle dot>h-1). The superior activity was corroborated by a reduced apparent activation energy (Ea = 35.1 kJ<middle dot>mol-1) compared to the parent H beta catalyst (Ea = 57.3 kJ<middle dot>mol-1). The catalytic stability decreased during cycling due to the light olefin polymerization, and its activity could be fully restored via thermal treatment. This work provides mechanistic insight into dual-site catalysis and a design framework for high-performance zeolite catalysts for hydrocarbon upgrading.

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