Deactivation and Reductive Regeneration of Sn-Beta for Liquid-Phase Biomass Conversion

Catalyst stability and deactivation remain significant hurdles, which hinder the realization of many promising chemical processes. This applies especially for biomass conversion over zeolitic materials, where the commonly applied solvothermal conditions adversely affect the stability of the catalysts. For example, tin-doped zeolite Beta, Sn-Beta, is one of the materials often used for a wide range of biomass reactions in the liquid phase. Herein, we present insights into the deactivation of Sn-Beta catalysts and assess different regeneration procedures. We identify tin and silicon leaching, along with tin restructuring into tin(IV) oxide, SnO2, as the primary deactivation mechanisms during the conversion of biobased-derived glycolaldehyde in methanol/water solvents. Concurrently, the spent catalysts have a range of mesopores over a highly ordered and poorly defective zeolitic framework. Furthermore, we highlight the critical impact of reaction medium compositions affecting the leaching of tin and silicon and provide levers to mitigate it (e.g., higher alcohols, low water concentrations). Through the implementation of an oxidation-reduction-oxidation regeneration procedure, the catalyst contains twice as high active site concentrations as the use of conventional thermal oxidation. The oxidation-reduction-oxidation procedure reverses some of the ongoing deactivation (tin restructuring into SnO2) with the transformation of the otherwise inactive SnO2 into active sites. Together, the generated understanding of Sn-Beta deactivation and the successful application of a superior regeneration method can bring this family of catalysts closer to industrial applications.