Water-tolerant heteropolyacid on magnetic nanoparticles as efficient catalysts for esterification of free fatty acid

Heteropolyacids (HPAs) supported on magnetic nanoparticles (MNPs) have been prepared by a simple acid–base interaction between functionalized magnetic nanoparticles and HPAs. The surface of the catalyst had been modified by organic groups to protect the catalytic sites from destruction by water. The hybrid SiO2-MNP-HPW consists of a core of magnetic iron oxide MNPs, silica shell, amino-silica and organic groups on the surface, which catalyzed the esterification of palmitic acid with methanol to give 90.4% FFA conversion within 2 h and a high TOF of 4.95 × 102 h−1 under mild reaction conditions. The catalytic performance compared favourably to other solid acid catalysts, demonstrating the advantage of the small size of catalyst particles in reducing mass transfer limitations and providing the substrates with better access to the catalyst sites on the surface of carriers. The catalysts are easily separable from the reaction mixture under a magnetic field and showed high stability and recyclability, with no significant loss of productivity after five cycles of esterification.