LAUSR

Abstract Entrapping the small NbOx clusters into MFI zeolite crystals was directly prepared via a facile hydrothermal method, assisting with the sodium citrate and EDTA-2Na to dissolve and chelate the niobium precursors. The existing states and configurations of Nb species in the zeolites were carefully characterized by a series of techniques including XRD, FTIR, SEM, TEM, UV–Vis, XPS and Raman. These results clearly indicated that the entrapped oligomeric NbOx clusters in MFI zeolite crystals contained Nb-OH, Nb=O and Nb-O-Si bonds. These characters were the origin of both Lewis and Bronsted acid sites for these Nb-MFI samples, which was vividly confirmed by FTIR spectroscopy with pyridine absorption and solid-state NMR spectroscopy with ammonia and TMPO as probing molecules techniques. The as-prepared Nb-MFI zeolites were employed as sustainable Lewis acids in several catalytic reactions. Especially, Nb-MFI zeolites exhibited remarkable catalytic performance in the cross-aldol condensation between furfural and acetone for biomass upgrading, surpassing traditional Lewis acidic zeolites Sn-MFI and Sn-BEA in terms of activity and selectivity, respectively. The 2.6%Nb-MFI also showed very good stability during reaction and it could be recycled for six times without loss in activity. Finally, 2.6%Nb-MFI was verified as a general catalyst for the cross-aldol condensation reaction between small-molecule aldehydes and ketones. All these results demonstrate the great potential of Nb-MFI as a robust catalyst for the shape-selective Lewis acid catalysis. The synthesis strategy developed herein could be readily extended to the construction of zeolite entrapped high-valence transition metal clusters for catalytic applications.