Multiple active sites for glucose conversion identified in Sn-Beta catalysts by CPMG MAS NMR and operando UV-Vis spectroscopy

Abstract

Sn-Beta is an emerging heterogeneous zeolite catalyst that displays high activity for the catalytic conversion of glucose to desirable bio-based chemicals. Due to its promising catalytic properties, the synthesis and characterisation of Sn-Beta catalysts of different composition and preparation method have been the subject of a great number of studies. However, development of structure-activity-lifetime relationships for glucose conversion chemistry has been obstructed by several challenges, making the tailored preparation of high performance Sn-Beta catalysts harder to achieve. In this manuscript, we undertake continuous kinetic studies of four different Sn-Beta catalysts for glucose conversion, and pair the kinetic insights obtained with operando UV-Vis spectroscopy and quantitative 119Sn CPMG MAS NMR. In doing so, we reveal that at least two types of Sn species that are active for glucose conversion can exist in Sn-Beta catalysts, but that the two Sn species exhibit different levels of activity, selectivity and stability. In particular, we find that a Sn species resonating at − 690 ppm is characterised by high activity and is primarily selective towards the isomerisation of glucose to fructose, whereas a second Sn species resonating at − 658 ppm is characterised by lower activity for glucose conversion, and preferential selectivity towards alkylation and β-dehydration. Our studies also demonstrate that both Sn sites also possess different levels of stability during continuous operation. Consequently, we reveal that it is the ratio amongst these species that governs the overall catalytic performances of Sn-Beta catalysts for biomass conversion processes.