A Green Approach for Producing Solvent-free Anisyl Acetate by Enzymecatalyzed Direct Esterification in Sponge-like Ionic Liquids Under Conventional and Microwave Heating

Abstract

The biocatalytic synthesis of anisyl acetate fragrance was carried out by direct esterification of acetic acid with anisyl alcohol in sponge-like ionic liquids (SLILs), e.g. N,N,N,N-hexadecyltrimethylammonium bis(trifluoromethylsulfonyl) imide ([C16tma][NTf2], N,N,N,N-octadecyltrimethylammonium bis(trifluoromethylsulfonyl)imide ([C18tma] [NTf2]), etc. as reaction/separation media under conventional and microwave (MW) heating. These SLILs are temperatureswitchable ionic liquid/solid phases that behave as sponges. As liquid phases, they are excellent monophasic reaction media for the lipase-catalyzed synthesis of anisyl acetate, the product yield being improved up to 100% for 2 hour reaction under the appropriate reaction conditions (i.e. SLIL concentration, alcohol: acid molar ratio, enzyme amount, dehydrating molecular sieves, temperature and MW heating). As a function of the phase behaviour of different SLIL/anisyl acetate mixtures, a new clean separation protocol based on the centrifugation of the solid IL/flavour ester through nylon membranes was proposed, which provided a nearly full separation of the solid SLIL and the easy recovery of the reaction mixture. The enzymatic synthesis of anisyl acetate in [C16tma] [NTf2] under MW assistance, followed by the separation step of the solid SLIL, provided a nearly solvent-free fragrance product with up to 0.89 g/mL concentration. The catalytic activity of the enzyme / SLIL system remained unchanged for ten consecutive operational cycles. This work reports a straightforward and sustainable approach for producing anisyl acetate as a natural flavour and demonstrates to be suitable for scaling-up, providing a high potential for practical application. Keywords: Anisyl acetate, anisyl alcohol, biocatalysis, biocatalytic process, clean process, enzymatic synthesis, esterification, flavours, fragrances, green chemistry, ionic liquids, lipase, neoteric solvents, pure products, solvent-free, sponge-like ionic liquids, sustainable chemistry, switchable ionic liquids.