(690e) Determination of Catalytic Site Distributions and Contributions for Epoxide Ring Opening in Lewis Acidic Zeolite Sn-Beta

Zeolite Sn-Beta is an intriguing catalyst for numerous reactions involved in biomass upgrading and fine chemical production. Interestingly, Sn-Beta is proposed to have three distinct catalytic sites: defect-open, hydrolyzed-open, and closed sites. The quantification of different sites is crucial for establishing structure-reactivity relationships for Sn-Beta, as previous investigations suggest a correlation between specific types of sites and improved activity for certain reactions. To quantify and determine the catalytic contributions of different sites, “poisoning experiments” may be performed wherein a Lewis base is added to deactivate a certain fraction of Sn sites before catalytic testing. Though poisoning experiments are becoming more widespread, several challenges remain in that there is limited investigation of experimental parameters and inadequate evidence to support the assignment of catalytic contributions to specific sites. This work successfully addresses these challenges using NMR to complement poisoning experiment data.

The effect of poison (triethylamine, pyridine, 2,6-lutidine, or trimethylphosphine oxide) and solvent (hexane or dichloromethane) on poisoning experiment results is investigated. The poison affects resulting site distributions, and the solvent impacts poisoning mechanism (random vs. preferential). Pyridine in hexane is identified as an ideal poison/solvent combination for epoxide ring opening (ERO) using Sn-Beta. ¹⁵N NMR analyses suggest that pyridine (dosed in hexane) poisons sites in the following order: (1) all defect-open and a fraction of closed Sn sites, (2) hydroxyl groups coordinated to Sn at defect-open and hydrolyzed-open sites, and (3) remaining closed Sn sites. Additionally, results indicate that defect-open Sn sites have the highest activity, followed unexpectedly by hydroxyl groups then closed Sn sites. Ongoing work will resolve the role of hydroxyl groups in ERO. Overall, this work highlights poisoning experiments as a unique and insightful site quantification technique when combined harmoniously with NMR. Furthermore, results will help to establish valuable synthesis-structure-activity relationships for Sn-Beta materials.