دانلود رایگان مقاله سنتز اوژنول استات با واکنش آنزیمی در دی اکسید کربن فوق بحرانی

Abstract

Supercritical carbon dioxide (SC-CO2) as reaction medium has gained attention in the production of terpenic esters catalyzed by lipases. Therefore, this work investigated the production of eugenyl acetate by esterification of eugenol and acetic anhydride in SC-CO2 using two commercial lipases (Lipozyme 435 and Novozym 435) as catalysts. The influence of enzyme concentration (1⿿10% weight/weight), substrates⿿ molar ratio (1:1 to 5:1), temperature (40⿿60 °C) and pressure of SC-CO2 (10⿿30 MPa) on the esterification rate (X; %) and specific productivity (SP; kg of product/kg of catalyst x hour) were evaluated. A home-made high-pressure stirred-batch reactor (100 ml) was used in the experiments. The use of Novozym 435 achieved higher conversion and specific productivity of eugenyl acetate than Lipozyme 435. An excess of acetic anhydride (5:1 M/M) and high enzyme concentration (10%) achieved higher esterification rates than the lowest conditions (1% and 1:1 M/M). The optimal temperatures and pressure for the synthesis of eugenyl acetate in SC-CO2 were 50 and 60 °C at 10 MPa, respectively. The phase behavior of the reaction system and the synthesis in organic medium were also studied. Kinetic experiments performed at 40, 50 and 60 °C indicated that the reaction follows the simple Ping-Pong Bi-Bi mechanism and the affinity of acetic anhydride to enzyme was larger than that of eugenol.

4. Conclusions

The synthesis of eugenyl acetate in SC-CO2 using the immobilized lipases Novozym 435 and Lipozyme 435 were investigated at several conditions. Emphasis was given to the differences between the enzymes, the number of the utilization cycles and the effect of temperature and pressure, and the esterification kinetics was also studied. The specific enzymatic activity (SA) obtained for Novozym 435 was about 18% higher than that achieved by Lipozyme 435. Differences were also observed in the particle size distribution and water content of the immobilized lipases. Novozym 435 achieved higher conversion and specific productivity of eugenyl acetate than Lipozyme 435. The influence of temperature and pressure was verified in reactions conducted with 1% of enzyme and substrate molar ratio of 5:1, and the optimal condition for the synthesis of eugenyl acetate in SC- CO2 was determined at 50 and 60 ◦C at 10 MPa, in which the study of phase behavior identified that the reaction mixture contained solid (enzyme), liquid and gas phases. The conversion and specific productivity of eugenyl acetate decreased with the increasing number of cycles, and the residual enzyme activity and the water content decreased with the increase of the number of depressurization/pressurization cycles. The results suggested that the reaction mechanism could be described to follow the simple Ping-Pong Bi-Bi mechanism within the range of studied concentrations. The affinity of acetic anhydride to the enzyme was larger than that of eugenol, and the affinity of both substrates to the lipase and the maximum reaction rate increased with temperature. The second step of the reaction kinetics required more energy that the first step, so it was identified as the rate-limiting reaction.