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-Betais a promising catalyst for numerous reactions involvedin biomass upgrading and fine chemical production, but it is complexwith multiple types of active sites. The activity for Sn-Beta canbe calculated on a per-site basis using site quantification experimentsthat involve adding a Lewis basic probe molecule, but it is not clearwhich types of sites are being titrated. Our work connects site quantificationexperiments with spectroscopic measurements to explain differencesin the catalytic activity of materials crystallized for differentamounts of time. For alcohol ring opening of epoxides, experimentsreveal that Sn-Beta crystallized for 10 days (Sn-Beta-200-10d) ismore active than Sn-Beta crystallized for 40 days (Sn-Beta-200-40d).These materials are investigated using site quantification experimentswith three probes triethylamine, pyridine, or 2,6-lutidine toreveal the different fractions and types of sites. As the probe:Snratio is increased, these experiments result in two distinct slopes,indicating two distinct activities: high and low activity. The differencein activity between Sn-Beta-200-10d and Sn-Beta-200-40d can be attributedto the reduced fraction of high-activity sites. Although the two slopeshave typically been assigned to open defect Sn sites for high activityand closed Sn sites for low activity, N-15 NMR measurementsof materials dosed with N-15-labeled pyridine contradictthis assignment. Indeed, at low concentrations, pyridine adsorbs onboth open defect and closed Sn sites whereas the low activity correspondsto pyridine binding to SnOH groups in addition to closed Sn sites.Overall, the identification of appropriate site quantification experimentparameters and the combination of these titrations with NMR techniquesallows for the establishment of a synthesis-structure-activityrelationship that has the potential to improve the performance ofSn-Beta.

Automated summary

Sn-Beta is a complex catalyst with multiple types of active sites, and its activity can be calculated using site quantification experiments. This study connects site quantification experiments with spectroscopic measurements to explain the differences in catalytic activity between materials crystallized for different amounts of time. The activity difference between Sn-Beta-200-10d and Sn-Beta-200-40d can be attributed to the reduced fraction of high-activity sites. Combining site quantification experiments with NMR techniques allows for the establishment of a synthesis-structure-activity relationship that has the potential to improve the performance of Sn-Beta.