Catalytic Site Dynamics in Sn-β Zeolites: A Time-Dependent Study of the Site Structure

Advancing the catalytic capabilities of zeolites like Sn-β requires a detailed understanding of the synthesis-structure–reactivity relationships. Sn-β is intriguingly complex, as it contains different types of catalytic sites that have been identified as the prominent active sites for different reactions. Previous work has indicated that the catalytic sites in Sn-β are affected by the composition and conditions during hydrothermal crystallization, forming a distribution of catalytic sites that are stable postsynthesis. However, our work reveals that postsynthetic transformations can occur at the catalytic sites, increasing activity in epoxide ring opening (ERO) of epichlorohydrin with methanol by a factor of 3. Specifically, when the calcination step is delayed, the catalytic sites transform. Standard characterization techniques demonstrate that bulk properties of the Sn-β are consistent for the batches calcined at different times, but 31P NMR measurements of materials dosed with trimethyl phosphine oxide reveal that the site distribution changes when calcination is delayed. After delaying calcination 14 or 21 days, Sn-β is determined to have 80% open-defect sites, a significant increase from the 50% observed for conventional materials. Sn-β with elevated quantities of open-defect sites then show an improvement in ERO activity over postcalcination time, suggesting a second transformation after calcination. Overall, the results demonstrate that Sn-β undergoes a dynamic process postsynthesis, and this work strengthens the connection between open-defect sites and ERO activity.