Biodiesel production from waste frying oil using heterogeneous transesterification process.

Abstract

Strategies to improve molecular diffusion limitations of HZSM-5 zeolites were investigated for the production of biodiesel. Zeolite crystals with short diffusion length and hierarchical porosity were compared with conventional coffin-shaped microcrystals for their catalytic activity in terms of acidic properties and pore structure. As-synthesized catalytic materials were characterized with instruments including XRD, SEM, TEM, BET-BJH porosimetry analysis, X-Ray fluorescence, and FTIR. Esterification of linoleic acid as a model reaction for biodiesel production and transesterification reactions were carried out at different reaction conditions (reaction time, reaction temperature, methanol to oil molar ratio, and catalyst loading) to investigate the effect of the reaction parameters on the behavior of the zeolite catalysts. The reactions were successfully catalyzed using hierarchical HZSM-5 zeolite catalysts produced in nanosheet and nanosponge morphologies as compared to highly acidic conventional big and nano-crystals of HZSM-5 zeolites. Improved accessibility and molecular transport of reactants from the outer mesoporous surface to the intrinsic active zeolitic framework resulted in achieving high conversions of 95.12 percent for linoleic acid transesterification and 48.29 percent for waste frying oil using HZSM-5 nanosheets at 4 h reaction time, 10 wtpercent catalyst loading, respective 6:1 and 12:1 methanol to linoleic acid-waste frying oil molar ratio and 180 °C. Although highly acidic, HZSM-5 nanosponges did not operate to their full potential as compared to HZSM-5 nanosheets given their higher hydrophilicity which favored water and glycerol adsorption to their surface and resulted in lower coverage of the less polar reactants. The correlation between the surface hydrophobicity and acidity of the zeolites with different Si-Al ratios and their catalytic performance was also assessed for biodiesel production. It was shown that Si-Al ratio of HZSM-5 zeolites determines their acidity and hydrophobic character with a combi