Use of HZSM-5 modified with citric acid as acid heterogeneous catalyst for biodiesel production via esterification of oleic acid

• Dealumination with citric acid caused a reduction of Brønsted and Lewis acid sites.
• External acidity was higher for samples modified with citric acid treatment.
• Samples treated at 60 °C presented the most appropriated acidic properties.
• Dealumination performed on HZSM-5 by citric acid was beneficial for esterification.

In this study, the efficiency of citric acid as dealumination agent was evaluated in the case of zeolite HZSM-5 by varying the experimental conditions, namely concentration (0.5, 1.0 and 2.0 mol L−1) and treatment temperature (60 and 80 °C). The effect of the CA treatment on structural, textural and acidity properties of the starting HZSM-5 material was monitored using XRD, N2 sorption and FTIR (pyridine and collidine) measurements. Heptane and methylcyclohexane cracking model reactions were used to evaluate the catalytic behavior of the modified materials, in terms of acid strength and active sites accessibility. The modified catalysts were tested in the esterification reaction, using oleic acid as reagent and different oleic acid/methanol molar ratios. All the results show the beneficial effect of the citric acid treatment on the physicochemical properties of the final materials, with an improvement of the external acidity, a crucial parameter for the reaction considered, i.e. the oleic acid esterification.

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