Zinc molybdate: A new catalyst for biodiesel synthesis by simultaneous esterification/transesterification reaction

The search for new solid catalysts for biodiesel production using high acidity raw materials is still a challenge, as it requires that simultaneous transesterification esterification reactions take place, with high efficiency. In this work, a new acid catalyst was evaluated, composed of zinc molybdate (ZnMoO4), synthesized by the modified Pechini method and characterized in relation to its structure, morphology and acidity. Characterization of ZnMoO4 by the pyridine adsorption method indicated a high amount of Lewis-type acid sites, classified as weak and moderated. The catalyst was used in the transesterification of soybean oil, in the esterification of oleic acid and in the simultaneous esterification transesterification reaction of soybean oil acidified with oleic acid, to simulate a residual oil. After the catalytic test, the reaction product was characterized by acidity index, gas chromatography and 1H and 13C nuclear magnetic resonance (NMR). According to the results of 1H NMR, the zinc molybdate was highly efficient in the esterification reaction, with 88 % of conversion at 160 °C for 120 min, while only 5.7 % of conversion was obtained in the transesterification at the same conditions. In spite of this, 69.5 % of conversion was obtained in the simultaneous reaction, indicating a strong synergic effect. This behavior seems to be due to the formation of new acid sites after esterification reaction, besides an acid-base character, as suggested by the formation of α-pyridone and pyridinium oxide on the material surface.