Protonic ionic liquids as efficient phase-separation catalysts for esterification of trimethylolpropane and acrylic acid

Trimethylolpropane acrylate (TMPTA) is becoming an important organic chemistry intermediate, but it’s synthesis through the acid-catalyzed esterification of trimethylolpropane and acrylic acid is a challenge due to the poor activity and catalyst separation problems. In this work, a series of 1,3-propanesultone (PS)-functionalized imidazole ionic liquids (ILs) are prepared and evaluated as the acid catalysts for this reaction, including the catalytic performance comparison, optimization of reaction conditions, and the catalytic reusability. The obtained H2SO4-based IL ([IMPSH]HSO4) caused a phase-separation catalytic process (reacted as a homogeneous catalyst, but precipitated at the end of reaction like a heterogeneous catalyst), presenting the best trimethylolpropane conversion (100%) and TMPTA selectivity (86.4%). Such catalytic performance is even better than that of homogeneous H2SO4. Moreover, the catalyst can be steadily reused at least four times without obvious change in activity and its structure. The highly efficient and environmentally benign performance of [IMPSH]HSO4-catalyzed esterification of trimethylolpropane and acrylic acid make it possible to achieve industrial applications.