SO42−/ZrO2 as a solid acid for the esterification of palmitic acid with methanol: Effects of the calcination time and recycle method

Abstract

Two types of SO42−/ZrO2 solid acid catalysts with various calcination times were prepared via incipient wetness impregnation of (NH4)2SO4 to hydrothermally synthesized ZrO2 and subsequently employed to catalyze the esterification of palmitic acid with methanol. The resulting catalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and temperature-programmed oxidation (TPO) to elucidate their physicochemical properties, morphology, and deactivation mechanism. A calcination procedure is required to transform the amorphous ZrO2 into the crystal form. Both chelating and bridged bidentate SO42− coordinate with the ZrO2 surface. The calcination at 600 °C could well eliminate the water in the catalyst and a further higher temperature would accelerate the loss of SO42−. Long-time calcination also decreases the catalytic activity due to the transformation of monoclinic ZrO2 into tetragonal one and the slow leaching of SO42−. The catalytic activity increases with increasing catalyst loading amount, reaction temperature, and molar ratio of palmitic acid to methanol, while the heating temperature over 65 °C and excess methanol amount are unfavorable to the esterification reaction due to the low-boiling-point methanol and attenuation of the palmitic acid concentration. It appears that the reaction conditions of 65 °C, 6 wt % catalyst, 25:1 of methanol to palmitic acid, and 4 h reaction time are economically optimal under atmospheric pressure. The catalyst could not be well regenerated by the ultrasonic methanol washing method because of refractory organic residues. The catalyst activity could be well recovered without major activity loss by the calcination at 600 °C for 1 h. The catalyst deactivation is due to contamination by the refractory organic residues in the catalyst as well as by the leaching of SO42−, and thus both the calcination temperature and time should be strictly controlled to achieve a better catalyst lifetime.