Physical properties of deep eutectic solvents based on p-toluene sulfonic acid and employment as catalyst

This work studied the physical properties of three deep eutectic solvents (DESs) and their catalytic activity in the esterification of palmitic acid and ethanol. Three types of deep eutectic solvents (DESs) were successfully prepared using p-toluene sulfonic acid monohydrate (PTSA) as hydrogen bond donor (HBD) with dodecyl trimethyl ammonium chloride (DTAC), tetradecyl trimethyl ammonium chloride (TTAC) and octadecyl trimethyl ammonium chloride (OTAC) as hydrogen bond acceptor (HBA). The freezing points of DESs were measured in the range of 50%–80% HBD mole percentage, and the composition of DTAC-PTSA, TTAC-PTSA and OTAC-PTSA at the eutectic point (the lowest freezing points) was determined respectively (DES1, DES2 and DES3). The physical properties (density, refractive index, dynamic viscosity, conductivity and pH value) of DES1∼DES3 were studied respectively in the temperature range of 298.15 K to 343.15 K. The results showed that dynamic viscosity, conductivity, density and refractive index all change approximately regularly with the increase of temperature, while pH value remains almost constant when the temperature exceeds 325 K. The catalytic ability of the three DESs in the esterification reaction of palmitic acid and ethanol was investigated, and DES2 was selected as the catalyst for subsequent experiments. The effects of temperature, acid-ethanol molar ratio, catalyst dosage and reaction time on esterification reaction were further studied, and DES' recyclability was also tested. The results obtained showed the great potential of DES in catalytic applications.