Catalytic activity of heterogeneous acid catalysts derived from corncob in the esterification of oleic acid with methanol

Sulfuric acid (H2SO4) pretreated corncob-derived residue was used as a starting material to prepare solid acid catalysts using different sulfonation chemicals (H2SO4, p-toluenesulfonic acid (TsOH) and H2SO4/TsOH mixtures) for biodiesel production from the esterification of oleic acid with methanol. Effects of the different sulfonation agents on the properties of the derived carbon-based materials were investigated using various characterization techniques. Lignin residues were obtained after the H2SO4 pretreatment step, and high lignin-containing carbon-based catalysts of ca. 69% (w/w) lignin were derived after sulfonation with H2SO4. Employing TsOH or H2SO4/TsOH mixtures for sulfonation gave materials with a higher carbon/hydrogen (C/H) ratio, indicating a relatively effective carbonization compared to that with H2SO4 sulfonation. The catalytic activity of the sulfonated corncob in the esterification of oleic acid with methanol was influenced by the acid density, acid strength and porous structure of the sulfonated materials. High methyl oleate yields (>80% after 8 h at 60 °C) were achieved using the acid catalyst obtained from either H2SO4 or TsOH sulfonation, whereas those from H2SO4/TsOH sulfonation gave slightly lower yields. Thus, the use of the non-volatile TsOH solid as a ‘greener’ sulfonating agent for the production of carbon-based solid acid catalysts with a high catalytic activity in the esterification reaction is supported.