Lipase catalysed synthesis of speciality chemicals: technical, economical & environmental aspects
Abstract: Most people agree that the total ecological impact of society has become more than this planet can endure. This is because we consume our resources (such as oil and water) faster than they can regenerate and because we release more substances into the environment than can be assimilated (e.g. CO2). In other words: our society is not sustainable on a long term basis. Biocatalysis is being promoted as a clean, environmentally friendly technology because it is natural, inherently works at very mild conditions, and predominantly utilises raw material that comes from renewable resources. Biocatalysis is the use of whole cells or enzymes for the catalysis of chemical reactions. In this work two different model reactions have been studied: (1) production of an epoxide coating component (used e.g. in the painting of cars), through chemo-enzymatic epoxidation of allyl ethers, and (2) the production of alkanolamide surfactants through the amidation of fatty acid with ethanolamine. For the synthesis of the epoxide (glycidyl ether) the aim was to find an alternative route to avoid the use of the toxic reagent epichlorohydrin, conventionally used in its manufacturing. Lipase B from Candida antarctica (CALB) was used to catalyse the formation of peracid from a fatty acid and hydrogen peroxide, which was utilised in situ for the epoxidation of the terminal unsaturated carbon-carbon bond of an allyl ether to form the desired epoxide. The reaction was found to be feasible in small scale and through optimisation a 75% yield of the product could be obtained from a reaction mixture containing up to one molar of the starting material. However, the conditions under which the epoxidation reaction was optimal was found to be too harsh for the enzyme. The need for a cheaper or more stable enzyme was identified as a major hurdle for industrial application of the technology. The synthesis of alkanolamide surfactant was also catalysed by CALB; the reaction between dodecanoic acid and ethanolamine was used as a starting point. The reaction was found to work very well under solvent free conditions if the amine was added in a step wise manner to avoid the formation of the viscous amine-acid salt. The yield of the reaction was pushed near to completion by removal of water, and only a minor amount of the amide-ester by-product was detected. The stability of the preparation was fairly good even at 90ºC when the system was kept dry. For large scale production using immobilised enzymes, the preferred reactor set-up is the packed-bed. This is due to easier handling of the catalyst as well as to avoid degradation of the preparation from prolonged stirring and repeated filtrations etc. For this reason a reactor system where the packed bed was connected to a reservoir holding the reactants via a loop was developed and characterised. As the economics of the process was evaluated, it was found that both the stability and the cost of the biocatalyst had a great impact of the final cost of the product and at the current cost and deactivation rate of the enzyme, the enzymatic process would be approximately 30-40% more costly than a process catalysed by sodium methoxide. In order to decrease the cost of the biocatalyst, the liquid lipase was absorbed onto a macroporous polypropylene carrier. A relatively low loading of the lipase was used to minimise the cost of the preparation per unit catalytic activity. The preparation obtained was found to be around 1/4 to 1/2 of the price of the commercial preparation. When studying the reaction of a fatty acid mixture (olein fatty acid) with ethanolamine at 70-90ºC it was found that also the stability of the in-house preparation was superior. During repeated batches using a PBR-loop setup at 70ºC, no deactivation of the in-house preparation could be seen after five batches. By this approach the added cost of the enzymatic process could probably be cut to an acceptable 0.1-0.2 euro per kilogram product. The added quality of the product, reduced risks of manufacturing, improved working environment and decreased energy requirement in the process might well motivate this cost. Further decreases in biocatalyst cost are possible if the catalyst finds a major application, such as in biodiesel production, then a cut in cost by a factor of ten is not unthinkable. This would undoubtedly open the door for many other applications.
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