Microwave-assisted preparation of coal-based heterogeneous acid catalyst and its catalytic performance in esterification

Powder coal is carbonized under nitrogen atmosphere and then treated with concentrate sulfuric acid with the assistance of microwave radiation for heterogeneous acid catalyst synthesis and the capability in catalyzing esterification of oleic acid with methanol for biodiesel production is subsequently studied. The catalysts are characterized by N2 adsorption-desorption, ultimate analysis, X-ray diffraction, Raman spectra, attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and acid amount tests to obtain the physicochemical property. Microwave irradiation is a faster and simpler process than the conventional heating, and the sulfonation duration can be shortened to be 5 min. Under the carbonization temperature of 250 °C for 30 min and sulfonation temperature of 75 °C for 5 min, the synthesized catalyst gains the acid amount of 1.73 mmol g−1 and esterification efficiency of 98.1% is achieved with the catalyst dosage mass percentage of 10 wt%, molar ratio of methanol to oleic acid of 12, esterification temperature of 65 °C and esterification duration of 180 min, where the commercial Amberlyst-15 catalyst only presents the efficiency of 71.5% under the same condition. Though the catalytic capability is crippled during the recycling reusage, it can be easily regenerated with its mostly original catalytic activity.