Esterification of linoleic acid using HZSM-5 zeolites with different Si/Al ratios

Microporous HZSM-5 zeolite crystals of distinct Si/Al ratios (53, 33.3 and 14.4) were compared for linoleic acid esterification with methanol as a model reaction for the production of biodiesel. As-synthesized catalytic materials were characterized using XRD, SEM, TEM, N2 sorption, X-Ray fluorescence, FTIR, and 27Al MAS NMR. The results showed that HZSM-5 with higher acidity and lower Si/Al ratio achieved high linoleic acid conversions at earlier stages of the reaction, whereas more hydrophobic zeolites reached higher conversions as reaction time increased. A maximum methyl linoleate yield of 79.78% was achieved for the esterification of linoleic acid using HZSM-5 (Si/Al = 53) at 4 h reaction time, 6:1 methanol to linoleic acid molar ratio, 10 wt% catalyst loading and 180 °C. Linoleic acid is activated by being adsorbed on the acid sites of the zeolite surface with increased Si/Al ratios and reacts to form the corresponding methyl ester.