Study on the catalytic performance of different crystal morphologies of HZSM-5 zeolites for the production of biodiesel: a strategy to increase catalyst effectiveness

Abstract

Strategies to improve molecular transport and accessibility of ZSM-5 zeolites were investigated for the model reaction of esterification of linoleic acid with methanol for biodiesel production. Zeolite crystals with a 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 fully characterized with XRD, SEM, TEM, N₂ adsorption–desorption, X-ray fluorescence, and FTIR. The results showed that hierarchical zeolites with nanosheet and nanosponge morphologies achieved the highest catalytic performance due to improved accessibility and mass transfer of linoleic acid from the outer mesoporous surface to the intrinsic active zeolitic framework. A maximum conversion of 95.12% was reached for the esterification of linoleic acid using HZSM-5 nanosheets at 4 h reaction time, 10 wt% catalyst loading, 6 : 1 methanol to linoleic acid molar ratio and 180 °C. Despite the high conversions achieved with HZSM-5 nanosponges (86.40% (SD = 1.77)), these catalysts did not operate to their full acidic potential as compared to HZSM-5 nanosheets, due to their higher hydrophilicity which hindered linoleic acid adsorption onto their surface. HZSM-5 nanosheets' regenerability was tested under optimum reaction conditions and showed a high methyl ester conversion for 4 consecutive cycles.