Counting Sites in Lewis Acid Zeolite Sn-Beta: Connecting Site Quantification Experiments and Spectroscopy To Investigate the Catalytic Activity for the Alcohol Ring Opening of Epoxides

Sn-Beta is a promising catalyst for numerous reactions involved in biomass upgrading and fine chemical production, but it is complex with multiple types of active sites. The activity for Sn-Beta can be calculated on a per-site basis using site quantification experiments that involve adding a Lewis basic probe molecule, but it is not clear which types of sites are being titrated. Our work connects site quantification experiments with spectroscopic measurements to explain differences in the catalytic activity of materials crystallized for different amounts of time. For alcohol ring opening of epoxides, experiments reveal that Sn-Beta crystallized for 10 days (Sn-Beta-200-10d) is more active than Sn-Beta crystallized for 40 days (Sn-Beta-200-40d). These materials are investigated using site quantification experiments with three probes─triethylamine, pyridine, or 2,6-lutidine─to reveal the different fractions and types of sites. As the probe:Sn ratio is increased, these experiments result in two distinct slopes, indicating two distinct activities: high and low activity. The difference in activity between Sn-Beta-200-10d and Sn-Beta-200-40d can be attributed to the reduced fraction of high-activity sites. Although the two slopes have typically been assigned to open defect Sn sites for high activity and closed Sn sites for low activity, 15N NMR measurements of materials dosed with 15N-labeled pyridine contradict this assignment. Indeed, at low concentrations, pyridine adsorbs on both open defect and closed Sn sites whereas the low activity corresponds to pyridine binding to SnOH groups in addition to closed Sn sites. Overall, the identification of appropriate site quantification experiment parameters and the combination of these titrations with NMR techniques allows for the establishment of a synthesis–structure–activity relationship that has the potential to improve the performance of Sn-Beta.