Effective solid acid catalysts play a key role to produce high-quality biodiesel through triglyceride transesterification. The present work describes a facile technique for the synthesis of a high-performing sulfonated multi-walled carbon nanotube (S-MWCNTs) solid acid catalyst for fatty acid ethyl ester (biodiesel) production. The results indicated that S-MWCNTs possessed high acidity due to their polycyclic textural matrix. An overall conversion of 97.8% is achieved for triglycerides at 1 h, 150 °C using 3.7 wt% of catalyst in ethanol, outperforming its counterpart catalysts, such as hydrothermal carbonization synthesized sulfonated carbon and metal oxide catalyst WO₃/ZrO₂. This superior performance is a result of the high acidity that S-MWCNTs possess, stemming from their polycyclic textural matrix. The catalyst was fully characterized by BET, FT-IR, XPS and TEM measurements to understand and evaluate their physical and structural properties. Combining activity and characterization data enables the postulation of the following reaction mechanisms for triglyceride transesterification based on S-MWCNTs:SO₃H groups first absorb triglycerides and ethanol through the interaction between the acid sites and the atomic oxygen. The carbonyl carbon was then attacked by the nucleophilic ethanol to produce fatty acidethyl ester (CH₃CH₂COOR), the final product. Findings from this work provide useful insights on designing effective solid acid catalysts via facile synthesis and regeneration protocol for the transesterification of triglyceride to produce biodiesels.

The adsorption mechanism of carbon dioxide (CO ₂ ) on Li/UiO-66 was studied by an in situ DRIFTS study.