Researchers at Lund University (Sweden) have developed an optimized two-phase enzymatic (lipase) system for the conversion of plant oils to biodiesel. Applied to the solvent-free ethanolysis of rapeseed oil, the system delivered a yield of 96% under mild conditions. Under the mild conditions used, chemical catalysts were inefficient. An open access paper on their work is published in the journal Biotechnology for Biofuels.
The current predominant method for the transesterification of triglycerides (plant and animal oils and fats) to biodiesel (a mixture of esters) uses chemical catalysts (sodium or potassium hydroxides or alkoxides). Despite its predominance, there are several drawbacks with this approach, including the need to remove inorganic salt in the downstream process; the high temperature required; and undesirable side reactions. Further, these systems are inefficient when a high free fatty acid (FFA) content is present in the starting material, thus restricting the use of conventional chemical pathways to a highly pure feedstock. An alternative approach is the use of immobilized lipase-catalyzed transesterification in the presence of an organic solvent.
The possibility of reusing the enzyme and the simplicity of the downstream process makes this approach attractive. While the use of biocatalytic methods is well-established for the preparation of pharmaceutical products, much higher productivities are required for commodity chemicals due to the price of biocatalysts. Concerning biodiesel production, lipase immobilisation adds an extra cost to the process and partial inactivation may also occur. Therefore, the catalyst has to be reused many times in order to be economically feasible. For these reasons, the use of liquid lipase preparations has attracted interest, but the optimisation of these processes is needed in order to establish them as a realistic industrial alternative.
Two-phase systems are of particular interest for reactions with both non-polar and polar substrates and products. Lipases specifically evolved for the hydrolysis of fats and oils; they are activated in biphasic mixtures in a process called interfacial activation. Most lipases have a lid covering the lipophilic active site. Once the lipase comes into contact with a non-polar surface, the lid opens, exposing the lipophilic area where the active site is situated. This part is oriented towards the organic layer, such that the lipid can easily enter the active site and the hydrophilic products can diffuse into the aqueous layer. For that reason, two-phase reaction media seem to be the most suitable environment for these particular enzymes.—Mangas-Sánchez and Adlercreutz
In their study, the Lund researchers used a liquid formulation of T. lanuginosus lipase in an aqueous/organic two-phase system. This enzyme shows an excellent tolerance to ethanol, which not only made the process faster but also allowed for the total conversion of the free fatty acid into biodiesel with a short reaction time.
They optimized several different parameters in the system: biocatalyst composition, ethanol concentration and the presence of additives.
The use of solid silica particles led to changes in the emulsion structure which permitted a larger surface area in the interphase, overcoming mass transfer limitations and thus increasing the rate of the process.
The results, the authors suggested in their paper, constitute a good starting point for efficient and cheap biodiesel production. The next step would be to adapt the methodology to readily available cheap waste oils.
Juan Mangas-Sánchez and Patrick Adlercreutz (2015) “Highly efficient enzymatic biodiesel production promoted by particle-induced emulsification” Biotechnology for Biofuels 8:58 doi: 10.1186/s13068-015-0247-6