Different ligand functionalized bimetallic (Zr/Ce)UiO-66 as a support for immobilization of phosphotungstic acid with enhanced activity for the esterification of fatty acids

Abstract

Different ligand functionalized bimetallic organic framework ((Zr/Ce)UiO-66-X (X = H, NO2, NH2 and (OH)2) solids-supported phosphotungstic acid (HPW) has been fabricated through a facile impregnation process, and used in the esterification of methanol with oleic acid (OA). The obtained composites were characterized by FTIR, XRD, NH3-TPD, SEM-EDS, N2 adsorption-desorption isotherm, TG, Py-FTIR, and XPS. The characterization analysis shows that the HPW/(Zr/Ce)UiO-66-NO2 composite possesses the proper catalytic features, such as the greatest number of both strong Brønsted and Lewis acid sites, wider pores, relatively high surface area, and the existence of synergistic catalytic effects between HPW and (Zr/Ce)UiO-66-NO2 framework. In the screening, the maximum OA conversion rate using (Zr/Ce)UiO-66-NO2, HPW/(Zr/Ce)UiO-66-NO2, HPW/(Zr/Ce)UiO-66, HPW/(Zr/Ce)UiO-66-NH2, HPW/(Zr/Ce)UiO-66-(OH)2 catalysts is 13.7%, 91.6%, 81.6%, 80.7%, and 72.6% at 110 °C for 3 h, respectively. Moreover, the HPW/(Zr/Ce)UiO-66-NO2 catalyst presents the better reusability over four cycles. Besides these, the HPW/(Zr/Ce)UiO-66-NO2 catalyzed esterification reaction is estimated to have an activation energy of 33.7 kJ/mol. This present study provides an effective route for realizing biofuel production on an industrial scale involving heteropolyacids/bimetallic MOFs synergistic catalysts.