Xylose Acetals - a New Class of Sustainable Solvents and Their Application in Enzymatic Polycondensation

The use of organic solvents in academic research and industry applications is facing increasing regulatory pressure due to environmental and health concerns. Consequently, there is a growing demand for sustainable solvents, particularly in the enzymatic synthesis and processing of polyesters. Biocatalysts offer a sustainable method for producing these materials; however, achieving high molecular weights often necessitates use of solvents. In this work, we introduce a new class of alternative aprotic solvents with medium polarity produced directly from agricultural waste biomass in up to 83 mol % yield (on xylan basis). The new solvents have a largely unmodified xylose core and acetal functionality, yet they show no peroxide formation and provide reduced flammability risk. We also demonstrate their successful application in enzymatic polycondensation reactions with Candida antarctica lipase B (CaLB). In particular, the solvent dibutylxylose (DBX) outperformed the hazardous solvent diphenyl ether and facilitated polycondensation of the lignin-derived diester pyridine-2,4-dicarboxylate, yielding polyesters with a M_n of >15 kDa. Computational modelling studies provided further insight into the molecular structure and dynamics of CaLB in the presence of new solvents. Lastly, up to 98 wt % of the new xylose acetals were successfully recovered and recycled, further contributing to the sustainability of the overall process.