Enzymatic synthesis of acrylates. Catalyst properties and development of process and product
Abstract: The increasingly apparent environmental problems in our surroundings necessitate a quest for sustainable development in all areas of human endeavour. Within the field of chemistry and the production of chemicals, the focus is increasingly on the development of green chemistry. This discipline aims to reduce environmental impact by replacing petrochemical raw materials with renewable feedstock and also to increase the efficiency of chemical processes. The latter can be done by reducing energy use and by improving atom economy, i.e. to ensure that as much as possible of the used raw materials end up in the final product. Acrylates are highly reactive compounds, with applications in many areas. This work has focused on developing a process for the production of acrylates that can be used as UV-curing wood coatings, where these products have been shown to be highly efficient from an environmental point of view. The traditional process for making acrylates is relatively complex and requires lower reaction temperatures than are ideal for chemical catalysis. Enzymes, on the other hand, are ideally suited for such conditions. The use of lipases for acrylation reactions has previously been the subject of relatively little scientific research and a large part of this thesis has therefore been devoted to the catalytic function of the enzyme used in these reactions, Candida antarctica lipase B (CalB). It was found that because the chemical structure of acrylic compounds is different from that of ordinary fatty acids, they were relatively poor substrates for CalB. The results presented in this work indicate that the binding of acrylic substrates in the active site of the enzyme is inhibited both by linear alcohols (octanol) and by water present in the system. Furthermore, acrylic acid is shown to be a potent inhibitor of CalB, which makes this enzyme more suitable for transesterification reactions. Very high catalytic rates were achieved at dry, solvent-free conditions with ethyl acrylate in excess. CalB is commercially available as an immobilised enzyme preparation, Novozym® 435 (N435), which has been used successfully for making chiral intermediates in the production of pharmaceuticals. One problem with N435 is that it is a relatively expensive catalyst, which limits its use in more mundane applications such as the production of bulk chemicals. It was found that CalB was considerably more active and also more stable when immobilised onto porous polypropylene (Accurel® MP1000), making this type of preparation considerably more cost effective. Immobilisation on MP1000 also distributes the enzyme over a larger volume for a given protein loading, which allows these preparations to catalyse reactions at higher rates without being limited by substrate diffusion. The second part of the thesis is devoted to the development of a process for the preparative synthesis of polymer-based acrylates and to the formulation of a product that is able to compete with existing acrylates for wood coatings that are available today. The resulting process incorporates continuous distillation for by-product removal coupled with continuous addition of ethyl acrylate to limit product degradation and to increase the product yield. It is shown that the enzymatic activity in this process is high compared to previously published studies. The enzymatic stability in the process was also quite high, with half-lives between 180 and 320 h for the tested enzyme preparations. The combined high activity and high stability led to the conclusion that catalyst cost should not be a limiting factor in the implementation of this process, especially for the preparation based on MP1000. The products synthesised with the enzymatic acrylation process were evaluated as wood coatings and compared to commercial alternatives. All of the products (including the commercial acrylates) displayed some weaknesses, but it turned out that a mixture of two enzymatically acrylated products, one based on a polyester and the other on a polyether, performed well in the tests. This mixture cured at an adequate rate and produced a film with good hardness and excellent chemical resistance against coffee, alcohol, water and grease. The work presented in this thesis illustrates that enzymes can be useful tools for the production of bulk chemicals. The developed process is advantageous in that it allows the use of low reaction temperatures in the synthesis of products that are competitive with commercially available alternatives. Due to the high activity and stability of the enzyme preparation used, the presented method is deemed an economically feasible alternative to the traditional chemical process.
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