Coordinating silanol with a Single Al^V site for Brønsted acidity on mesoporous amorphous Al₂O₃

Silica-alumina materials with Brønsted acidity (BAS) play a dominant catalytic role in the petrochemical and biorefining industries. Beyond the classic BAS generated by tetracoordinated aluminum species (Al^IV), penta-coordinated aluminum species (Al^V) has recently been discovered to contribute strong BAS acidity enhancement up to the strength of zeolitic BAS. Thus, the development of a straightforward and cost-effective synthesis method for AlV-BAS-based silica-alumina is beneficial both economically and environmentally for wide application in chemical reactions and processes. In this study, for the first time, we successfully synthesized isolated silanol groups with single Al^V sites on mesoporous alumina for surface-enriched Al^V-based Brønsted acidity. The formed silanol linkage density and Al^V-BAS density can be regulated by stepwise addition cycles of Si species. Their local structures and acidities have been characterized by electron energy loss spectroscopy (EELS) spectra and multinuclear solid-state nuclear magnetic resonance (ssNMR) spectra. During the catalytic conversion test of biomass 1,2-propanediol with cofed steam (at 10% and 20% v/v), it was observed that Al^V-based Brønsted acid sites exhibited higher catalytic activity compared to their Al^IV-based BAS counterparts. This achievement presents promising opportunities for large-scale industrial implementation of mesoporous alumina with Al^V-BAS.

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