DRIFTS study of methanol adsorption on Mg–Al hydrotalcite catalysts for the transesterification of vegetable oils

Mg–Al hydrotalcites rehydrated after calcination are promising catalysts for the methanolysis of vegetable oils. To gain insight into the basis of their catalytic action, the adsorption of methanol over some commercial Mg–Al hydrotalcites was studied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Different species formed after methanol adsorption were identified, being the total quantity of methoxy species related to the basic character of the sample. A linear correlation between the amount of adsorbed monodentate methoxy species and the catalytic activity in the biodiesel production was found. Therefore, it is proposed that these species are the mainly involved in the transesterification reaction.

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► A correlation was found between monodentate methoxy species and methanolysis activity.
► Monodentate methoxy species are involved in methanolysis over Mg–Al hydrotalcites.
► Bidentate methoxy species are strongly adsorbed and less labile than monodentate ones.
► As the Al content increases the likelihood of forming bidentate species is higher.
► Mg/Al ratio and hydration state are key for the hydrotalcites methanolysis activity.