Highly active sulfonic ionic liquid modified heteropoly acid composite catalysts for efficient production of ethyl palmitate

Highly active inorganic-organic composite catalysts were synthesized by incorporating Keggin-type tungstophosphoric acid (H3PW12O40; HPW) with SO3H-functionalized ionic liquids (SFILs), which were prepared by integrating chloromethylated polystyrene (CP) resin, piperazine (PI) and 1.3-propane sultone, as primary sources. The [CPPI-SO3H]nH3−nPW12O40 (n = 1.0–3.0) catalysts were characterized by XPS, SEM, FT-IR, XRD, TG-DTA, TEM, and solid-state 31P MAS NMR. These HPW-SFIL composite catalysts were found to be highly stable and possess ultra-strong Brønsted acidity, desirable self-separation properties, and catalyst reuse for production of biodiesels through esterification of palmitic acid (PA) and ethanol (EtOH). On the basis of the Box-Behnken design (BBD), further process optimization using response surface methodology (RSM) over the [CPPI-SO3H]2.0H1.0PW12O40 catalyst led to the optimized experimental conditions: EtOH/PA = 7.7 mol/mol, catalyst amount = 4.4 wt%, reaction time = 2.5 h, reaction temperature = 363 K, accompanied by an ethyl palmitate yield of 97.2%, in excellent agreement with the predicted value. The catalyst was highly stable, retaining a 94.7% yield after six consecutive running cycles. Moreover, a kinetic study performed under optimal reaction conditions revealed an apparent reaction order of 1.40 and an active energy of 14.80 kJ mol−1.