Abstract

The synergistic interplay between Brønsted and Lewis acid sites in zeolites plays a key role in biomass conversion, yet direct assessment of such site cooperation remains challenging. Here, advanced two-dimensional solid-state NMR spectroscopy combined with probe-molecule analysis is employed to directly identify and semiquantitatively quantify proximate Brønsted/Lewis acid site pairs in Sn–Al-β zeolites for glucose conversion. By tuning the Sn/Al ratio, the abundance of framework Sn-derived Lewis acid sites is varied while maintaining comparable Brønsted acidity, enabling the effect of acid-site proximity to be disentangled. 1H–31P and 31P–31P correlation NMR experiments reveal the spatial proximity between Brønsted acid sites and framework Sn-derived Lewis acid sites and show that neighboring Lewis acid sites are associated with enhanced Bro̷nsted acidity. The estimated concentration of BAS-LAS pairs correlates strongly with the apparent initial methyl levulinate formation rate, whereas the total BAS or LAS concentrations alone do not capture the same trend.

Link:https://pubs.acs.org/doi/full/10.1021/acs.jpclett.6c01617