<p>In order to study the anion effect of piperazinium phosphate molten salts on catalytic activity, as a study of the structure–activity relationship (SAR), was investigated for preparing various pyrrolidinones. The solubility of these organic salts was unique, as they were soluble only in water. Although the catalytic preparation of pyrrolidinone derivatives is well-documented in the literature, water-soluble catalysts would be an attractive alternative for green chemistry principles due to easily separable water-miscible heterogeneous catalysts from the organic product phase, minimizing the use of hazardous solvents and waste generation, and promoting high recycling catalysis. This unique property ultimately lowers energy costs and creates a cleaner catalytic method, leading to a more environmentally friendly and sustainable process. Excellent catalytic activity in preparing pyrrolidinones was observed using both piperazinium phosphate molten salts; however, piperazinium dihydrogenphosphate was more active, probably due to more hydrogen bonding sites and acidity. Both organic molten salts exhibited high recyclability, and the selected recovered catalysts could be reused in successive runs with a negligible loss in their catalytic activity.</p> Graphical Abstract <p></p>