Esterification and transesterification using modified-zirconia catalysts

Abstract

In this paper, modified zirconias (WZ, SZ, and TiZ) have been extensively compared under the same reaction conditions as catalysts for both the transesterification of triglycerides (TGs) and the esterification of carboxylic acids (free fatty acids, FFAs) with ethanol. In addition, for the first time, the catalyzed rate of hydrolysis/esterification + transesterification of triglycerides has been quantified and compared to that of the rate of simultaneous esterification of FFAs present in the same reaction mixture. Tricaprylin and caprylic acid were used as representative compounds for the comparison of the catalyst for transesterification and esterification since they give reaction rates related to those of larger TGs and FFAs typically present in vegetable oils and animal fats. For the measurement of the kinetics for the simultaneous reaction of a TG-FFA mixture, the reaction of tricaprylin and oleic acid with ethanol was studied using the most promising of the Zr-based catalysts, WZ. Reaction was carried out in a well-mixed batch reactor under mild conditions (75–120 °C). TiZ, while more active than WZ for transesterification, had by far the lowest activity for esterification. SZ, while the most active catalyst (on a weight basis) for both transesterification and esterification reactions, exhibited significant sulfur loss, which greatly reduced its long term activity. As expected, esterification occurred at a much faster rate than transesterification. However, under simultaneous reaction conditions, by virtue of the water being produced in esterification and hydrolysis of the TG taking place, the conversion of the triglycerides to ester products was greatly increased. The resulting caprylic acid from hydrolysis did not accumulate in the reaction mixture due to its rapid esterification to ethyl caprylate. The activity results for WZ indicate how biodiesel can be efficiently produced from free-fatty-acid-containing feedstocks using a solid-acid-catalyzed pre-esterification step, and how a significant fraction of the triglycerides can also be converted at the same time assisted by initial hydrolysis with byproduct water from esterification.