Transesterification of triglycerides with methanol catalyzed by heterogeneous zinc hydroxy nitrate catalyst. Evaluation of variables affecting the activity and stability of catalyst.

Abstract

Zinc hydroxy nitrate Zn5(OH)8(NO3)2 × 2 H2O (Zn-5) was proved to be active heterogeneous catalyst in the transesterification of triglycerides with methanol and it retained its activity in the next catalytic cycles. Systematic studies were performed to characterize catalytic performance of Zn-5 in methanolysis which was carried out under mild conditions (temperature 50−60 °C and atmospheric pressure). Reaction was studied for triacetin (glycerol triacetate), a model triglyceride molecule and for vegetable oil, castor oil {(triglyceride of 12-hydroxy-9-octadecenoic acid (ricinoleic acid)}. The homogeneously catalyzed reaction by KOH was performed for comparison as well. The effects of methanol to triglyceride molar ratio, catalysts loading, reaction temperature, and precontacting Zn-5 catalyst with reactants (methanol or triglycerides) were investigated. The influence of experimental variables on the course of triacetin methanolysis and in particular on the content of partial glycerides (diacetin, monoacetin) was considered. The next investigated aspect referred to an influence of cosolvents. A correlation between polarity of cosolvents and the content of partial glycerides was found proving a change of adsorption properties of Zn-5 catalyst due to the presence of cosolvent. The Zn-5 catalyst underwent partial but reversible deactivation during the first reaction cycle due to the accumulation of glycerides/glycerol evidenced by the FTIR technique. By simple washing with THF/methanol, complete regeneration was achieved resulting in stable activity of Zn-5 catalyst in recycling use.