From Structure to Stability: A Comparative Evaluation of Ammonium‐ and Phosphonium‐Based Deep Eutectic Solvent Catalysts in Glycerolysis

Deep eutectic solvents (DESs) are efficient catalysts; however, their foundational hydrogen‐bonding framework is susceptible to water‐induced disruptions. In this study, a structure‐stability‐performance relationship is examined for two acidic DESs: tetrabutylammonium hydrogensulfate (N‐DES) and tetraphenylphosphonium bromide (P‐DES) with p‐toluenesulfonic acid monohydrate (PTSAM), in a glycerolysis reaction. Glycerolysis of free fatty acids (FFAs) generates water as by‐product, creating dynamic water‐rich reaction environment, providing a controlled basis for comparing two DESs against water‐ingression. N‐DES being more acidic achieved equilibrium conversion faster than P‐DES, under identical conditions, validated by lower apparent activation energy for N‐DES. Both DESs remained in the glycerol phase post‐reaction, and under recycle routine there was water build‐up in glycerol phase. The water ingress affected the catalytic stability of both DESs, while removing water via oven evaporation, resulted in improved catalytic conversion‐ N‐DES (40% to 62%) in comparison to P‐DES (46% to 81%), at the end of fifth cycle. TGA data confirms thermal stability of both DESs within operational ranges, eliminating DES degradation behind decreased catalytic ability. Insights from viscosity and molecular self‐diffusion data highlights the differences in component mobility and hydrogen‐bond network organization, as most probable cause of divergent resistance to water‐induced disruption during recycling.