Biodiesel synthesis from microalgal lipids using tungstated zirconia as a heterogeneous acid catalyst and its comparison with homogeneous acid and enzyme catalysts

•Tungstated zirconia used for conversion of S. obliquus lipids.•Tungstated zirconia showed 94.58% FAME conversion and reuse up to 3 batches.•Heterogeneous catalyst showed comparable conversion with homogeneous catalyst.•Heterogeneous catalyst showed higher conversion than enzyme catalyst.•Time requirement for tungstated zirconia catalysis was lowest.

Downstream step of catalytic conversion is scarcely investigated area in microalgal biodiesel production process. In this study a heterogeneous acid catalyst, tungstated zirconia (WO3/ZrO2) is evaluated for conversion of S. obliquus lipids. Catalytic efficiency of tungstated zirconia catalyst was compared to the homogeneous acid catalyst and enzyme catalyst in terms of conversion efficiency, reaction parameters, energy consumption and reusability. Tungstated zirconia catalyst showed maximum biodiesel conversion of 94.58% at 100 °C temperature, 12:1 methanol to oil molar ratio and 15% of catalyst amount based on oil weight in 3 h. Tungstated zirconia showed comparable biodiesel conversion to homogeneous catalyst and higher conversion than the enzyme catalyst. The time requirement for heterogeneous catalyst was lowest, while, the energy consumption was highest among the selected catalysts. Most of the fuel properties of biodiesel synthesized by tungstated zirconia catalyzed conversion of S. obliquus lipids comply with the specifications set by ASTM and EN standards.