Various solid acid catalysts were evaluated for the production of biodiesel from low quality oil such as
waste cooking oil (WCO) containing 15 wt.% free fatty acids. The zinc stearate immobilized on silica gel
(ZS/Si) was the most effective catalyst in simultaneously catalyzing the transesterification of triglycerides
and esterification of free fatty acid (FFA) present in WCO to methyl esters. The optimization of reaction
parameters with the most active ZS/Si catalyst showed that at 200 8C, 1:18 oil to alcohol molar ratio and
3 wt.% catalysts loading, a maximum ester yield of 98 wt.% could be obtained. The catalysts were recycled
and reused many times without any loss in activity.