Polyoxometalate based mesoporous acidic catalyst from biogenic silica for vegetable oil methanolysis: Structure activity relationship study

- Mesoporous superacidic catalyst is synthesized from high purity biogenic silica.
- Amorphous polyoxotungstate phase formed with a specific tungsten impregnation.
- Higher tungsten loading leads to crystalline WO3 phase formation.
- Amorphous polyoxotungstate phase with W(VI) centres give strong acidic sites.
- These sites perform well in methanolysis of free fatty acid mixed soybean oil exhibiting good reusability of catalyst.

A superacidic mesoporous catalyst WOX/RHS-MCM-41 was synthesized using high purity biogenic silica extracted from an agro-waste, rice husk ash. Catalyst acidity was enhanced by dispersion of surface WOX groups on mesoporous support and was used for methanolysis of soybean oil and free fatty acid. Catalyst activity was found to depend upon the structure of WOX species formed over catalyst surface. Characterization with XRD, SEM, TEM, UV-vis DRS, Raman spectroscopy and acidity determination revealed that formation of highly dispersed amorphous polyoxotungstate phase on the catalyst surface developed with a specific tungsten loading and originated strong acidic sites showing high activity. Kinetic analysis revealed that pseudo-first order kinetics was followed up to the optimum reaction time with an activation energy 42.7 kJ mol−1. Catalyst showed high activity giving ∼90% FAME yield and reusability for multiple cycles without regeneration. The high catalytic activity was retained even in presence of free fatty acid also.

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