Walnut-shaped calcium oxide-cancrinite spheres for transesterification of waste frying oil

Abstract

Producing biodiesel via transesterification of waste oils or fats with short-chain alcohol is one of the possible ways to alleviate global energy and environmental issues, and further reducing the cost of transesterification catalyst is the essential strategy to enhance the competitiveness of biodiesel, such as applying industrial solid wastes as the source of catalyst active species. Thus in this work, via one-step pyrolysis of the calcium-rich residue, a byproduct from the hydrothermal treating (140–180 °C for 2–4 h) of paper mill sludge (PMS), a novel walnut-shaped calcium oxide-cancrinite based catalyst was prepared and applied to the transesterification of waste frying oil. The hydrothermal condition (changed from 140 oC-2 h to 180 oC-4 h) of PMS slightly influenced the yield of fatty acid methyl ester (YFAME), which ranged from 98.87% to 96.95% due to the slight decrease of the total basicity of strong basic sites (TSBβ+γ) from 149.6 to 102.9 μmol g−1. On the contrary, the pyrolysis temperature had a much greater effect on the transesterification performance of obtained catalysts. The optimum pyrolysis temperature was 750 °C and when it was increased to 850 °C, the increased bonding tendency of Si with Ca passivated the activity of calcium species and reduced the basic intensity and basicity of PSC-850, thus lowering its transesterification activity. Besides, the key process parameters were optimized and at the conditions of 70 °C, 3 h, 6.0 wt% of catalyst dosage, 10:1 of molar ratio of methanol to oil, over 95% of YFAME can be reached. The work herein not only provided a feasible way to recycle the inorganic solid residue of PMS but further lowered the cost of preparing biodiesel.