Lipase-mediated conversion of vegetable oils into biodiesel using ethyl acetate as acyl acceptor

Abstract

Ethyl acetate was explored as an acyl acceptor for immobilized lipase-catalyzed preparation of biodiesel from the crude oils of Jatropha curcas (jatropha), Pongamia pinnata (karanj) and Helianthus annuus (sunflower). The optimum reaction conditions for interesterification of the oils with ethyl acetate were 10% of Novozym-435 (immobilized Candida antarctica lipase B) based on oil weight, ethyl acetate to oil molar ratio of 11:1 and the reaction period of 12 h at 50 °C. The maximum yield of ethyl esters was 91.3%, 90% and 92.7% with crude jatropha, karanj and sunflower oils, respectively under the above optimum conditions. Reusability of the lipase over repeated cycles in interesterification and ethanolysis was also investigated under standard reaction conditions. The relative activity of lipase could be well maintained over twelve repeated cycles with ethyl acetate while it reached to zero by 6th cycle when ethanol was used as an acyl acceptor.

Introduction

Biodiesel offers an alternative fuel that is technically and environmentally acceptable and economically competitive. Different processes are currently available to achieve transesterification of oils for the production of biodiesel, which include chemical or enzyme catalysis or supercritical alcohol treatment (Fukuda et al., 2001, Kusdiana and Saka, 2004, Ma and Hanna, 1999, Shah et al., 2004, Shah et al., 2006, Warabi et al., 2004, Zhang et al., 2003). Although biodiesel can be successfully produced by chemical approach, there are several associated problems, such as glycerol recovery and removal of inorganic salts (Basri et al., 1997, Mittelbach, 1990). Use of biocatalysts (lipases) in transesterification of oils for biodiesel production addresses these problems (Iso et al., 2001, Kose et al., 2002, Nelson et al., 1996, Shimada et al., 2002) and offers an environmentally more attractive option to the conventional processes. However, high cost of the enzymes often presents the biggest obstacle. The key step in enzymatic processes lies in the successful immobilization of the enzyme, which will allow for its easy recovery and reuse (Balcao et al., 1996).

Immobilized lipase preparations have been found to be liable to be deactivated by lower linear alcohols such as methanol and ethanol conventionally used in transesterification for biodiesel production (Chen and Wu, 2003, Samukawa et al., 2000). The degree of deactivation has been found to be inversely proportional to the number of carbon atoms in the linear lower alcohols (Chen and Wu, 2003). Several measures have been adopted to overcome this problem. Nelson et al. (1996) used hexane as a diluent to prevent the deactivation by a lower alcohol. Samukawa et al. (2000) suggested maintaining a very low concentration of methanol (oil to methanol molar ratio of 1:0.33) during the reaction. However, diluent decreases the reaction rate and maintaining a very low concentration of methanol in the reaction mixture by a precise control is not a practical approach for industrial production of biodiesel.

The above-mentioned problems limit the viability of enzymatic approaches to biodiesel production. Recently the use of methyl acetate as an acyl acceptor for the interesterification of soybean oil has been reported (Du et al., 2004, Xu et al., 2005). The interesterification method eliminates the risk of deactivation of enzyme by glycerol, as no glycerol is produced in the reaction. Further, the triacetin, a by-product produced during interesterification, has no negative effect on the lipase activity (Du et al., 2004). Producing ethyl esters rather than methyl esters is of considerable interest because the extra carbon atom in the ethyl esters increases the heat content and cetane number (Encinar et al., 2002). Other desirable attributes of ethyl esters over methyl esters include lower pour and cloud points, higher flash and combustion points which improve cold starts and safety in handling (Encinar et al., 2002) and significantly lower smoke opacity and lower exhaust temperatures. The scope of the present study is to optimize the key process variables of interesterification employing ethyl acetate as an acyl acceptor for the preparation of biodiesel from crude jatropha, karanj and sunflower oils and to study the effect of ethyl acetate on operational stability of immobilized Candida antarctica lipase.