Preparation of mesoporous polyoxometalate-tantalum pentoxide composite catalyst and its application for biodiesel production by esterification and transesterification

Mesoporous polyoxometalate-tantalum pentoxide composite catalyst, H3PW12O40/Ta2O5, with H3PW12O40 loading from 3.6 to 20.1% was prepared by a one-step sol-gel-hydrothermal route in the presence of a triblock copolymer surfactant. Bulk and surface sensitive probe testing results indicated that the primary Keggin structure remained intact after formation of the composite, and strong interaction between the Keggin unit and Ta2O5 framework existed in the composite. Additionally, the composite exhibited larger and well-distributed three-dimensionally interconnected pores (3.9-5.0 nm), larger BET surface area (106.0-126.9 m(2) g(-1)), high porosity (0.44-1.37 cm(3) g(-1)), and homogeneous dispersion of the Keggin unit throughout the composite. As an environmentally friendly solid acid catalyst, the catalytic performance of the H3PW12O40/Ta2O5 was evaluated in the esterification of lauric acid and myristic acid, the transesterification of tripalmitin as well as the direct use of soybean oil for biodiesel production. Regardless of the presence of free fatty acids, the H3PW12O40/Ta2O5 composite showed high reactivity and selectivity towards simultaneous esterification and transesterification under mild conditions. The catalyst can be recovered, reactivated and reused several times.