Pyrazinium polyoxometalate tetrakaidecahedron-like crystals esterify oleic acid with equimolar methanol at room temperature

•New solid acid is synthesized from pyrazinium-ionic liquid and polyoxometalate.•The catalyst has sulfonate acid-functionalized uniform smooth crystal surface.•Uniform morphology translates into high density narrowly dispersed acid sites.•It converts 83% of oleic acid into biodiesel at 25 °C and equimolar condition.•Enhanced mass transfer on uniform external surface accounts for the high activity.

Ionic-liquid-modified polyoxometalate was prepared by self-assembly of Keggin-type heteropolyacids and sulfonated pyrazinium-IL, in which no template, surfactant, or organic solvent was used. The obtained ionic hybrid exhibits uniform morphology of cubic tetrakaidecahedron-like crystals and possesses a larger number of strong acid sites with narrower dispersion of acid strength than its analogues with similar chemical composition but different morphology. Catalytic activity tests reveal its high conversion, stable recycling, and good substrate compatibility for esterification of free fatty acids (FFA) with methanol into biodiesel at room temperature and over a wide range of molar ratios of methanol to FFA (10–50). Even under the ideal stoichiometric conditions (methanol:FFA = 1:1, a viscous solution), it converts 83% of oleic acid in 24 h. Morphology and performance are discussed, suggesting that the uniform crystals translate into high activity due to improved accessibility of the strong acid sites on the smooth external surface of the catalyst.

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