Properties of Immobilized Lipase as a catalyst for the transesterification of rapeseed

摘要

Immobilization of Saccharomyces cerevisiae lipase by physical adsorption on Mg-Al hydrotalcite wth a Mg/Al molar tatio of 4.0 led to a markedly improved performance of the enzyme. Immobilization conditions and characterization of the immobilized enzyme were investigated. Using an optimized immobilization protocol, a high activity of 725 U·g~(-1) immobilized lipase was obtained. The immobilized lipase retained activity over rwide ranges of temperature and pH than those of the free lipase. The optimum pH of the immobilized lipase was at pH 7.5-8.0, while the optimum pH of free lipase was at pH 7.5. The immobilized lipase retained more than 95% relative activity at 50°C, while the free lipase retained about 88% relative activity. The kinetic constants of the immobilized and free lipase were also determined. The Km and Vmax values of immobilized lipase were 5.4470 mg·mL~(-1) and 2.8952 mg·(mL·min)~(-1), respectively. The apparent activation energies (Ea) of the free and immobilized lipases were estimated to be 5.45 and 16.31 kJ·mol~(-1), while the apparent inactivation energies (Ed) of free and immobilized lipases were 20.28 and 29.02 respectively. So the stability of the immobilized lipase was higher than that of free lipase. In addition, the effects of water content, free fatty acid and methanol/oil molar ratio in the transesterification reaction were investigated. The optimum synthesis conditions giving 96.5% oil conversion were: reaction time 4.3 h, temperature 45°C, enzyme amount 1.5% (w/w of oil), methanol/oil molar ratio 4:1. The water content of the oil could be kept below 2.0 wt% and free fatty acid content of the oil could be kept below 3.5 mg KOH·g[oil]~(-1) in order to get the best conversion. This method was found to be satisfactory to produce stable and functioning biocatalyst and can maintain high reactivity for repeated 10 batches with oil conversion above 81.3%.