Synergy of extraframework Al³⁺ cations and Brønsted acid sites on hierarchical ZSM-5 Zeolites for butanol-to-olefin conversion

ZSM-5 zeolite catalyzed dehydration of biobutanol has been widely used for the sustainable production of butene. Brønsted acid sites (BASs) in zeolites usually act as catalytically active sites. Lewis acid sites (LASs) are also active for alcohol dehydration. Therefore, extraframework aluminum species have been introduced to zeolites as LASs. Tricoordinated Al species are the strongest LASs, which can enhance the acidity of adjacent surface BASs. Here, we combined solid-state nuclear magnetic resonance (NMR) spectroscopy and in situ diffuse reflectance infrared spectroscopy (DRIFTS) to investigate the local structures and acidity of hierarchical ZSM-5 zeolites with/without the introduction of Al³⁺ cations as well as their correlations with the catalytic performance in biobutanol dehydration. ²⁷Al magic angle spinning (MAS) NMR, ²⁷Al multiple-quantum (MQ) MAS NMR, and ³¹P MAS NMR after loading trimethylphosphine oxide (TMPO) probe molecules showed that the tricoordinated Al species are dominant acid sites after introducing Al³⁺ cations into pure silica hierarchical ZSM-5 zeolites. For BASs-rich hierarchical ZSM-5 zeolites (SiO₂/Al₂O₃ = 50), the Al³⁺ cations tend to stay in proximity to the bridging Si–OH–Al and thereby strongly enhance the acidic strength of BASs via the synergy of LASs and BASs. Contributing from the synergy effect, the ultrastrong BAS has been formed, which gives an obvious improvement in the biobutanol conversion and butene selectivity. In situ DRIFTS showed that tricoordinated Al³⁺ cations cooperating with BASs can promote the formation of butoxy intermediates at a low reaction temperature, which further improves the butanol dehydration.

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