Efficient esterification reaction of palmitic acid catalyzed by WO_3-x/mesoporous silica

Abstract

Different solid catalysts were prepared by impregnation of tungsten species on commercial mesoporous silica. The resulting materials were characterized in order to elucidate their composition, properties and acidity. Moreover, the catalytic results were carefully studied, in an attempt to reach insights about the catalytic active species involved in the investigated reactions. The obtained solids were nominated Q15W-X, where X = 1 means an impregnation using ammonium paratungstate (APT), X = 2 sodium tungstate (Na₂WO₄) and X = 3 tungstic acid dihydrate (W-2) as tungsten source. They were evaluated as catalysts towards the esterification of palmitic acid by methanol, the average conversion was found to be between 80 and 90% and the catalysts were reused in four more reaction cycles, keeping similar conversion results as in the first use. The spectroscopic analyses suggested that the formation of polytungstate species over the silica with different tungsten species, mainly W⁵⁺ and W⁶⁺ is probably influence the esterification under mild conditions. Such species are not present in the solid resulting from the impregnation of W-2 on glass beads (GB) and the solid WO₃ obtained by the calcination of W-2 (WO₃-C), which presents poor catalytic activity and are mostly constituted by monoclinic WO₃.

Keywords:

• Esterification
• tungsten oxide
• mesoporous silica
• heterogenous catalysis
• biodiesel

Acknowledgements

This work was supported by CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, finance code 001), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and Fundação Araucária. T.P. and J.F.S. thank CAPES for PhD scholarships. The authors acknowledge the support from UFPR (Universidade Federal do Paraná), the Centro de Microscopia Eletrônica of UFPR for SEM and TEM analyses and to Departamento de Física of UFPR for XRD analyses.

Disclosure statement

No potential conflict of interest was reported by the authors.