engleski

Enzymatic esterification in ionic liquids

It is well known that organic esters are important products or intermediates in chemical industry. Among all the catalyst of esterification, inorganic acids are most important. However, many drawbacks of this traditional method have shown up: Firstly, removal of water or/and use of excess amounts of reactants is needed to attain satisfactory conversion. Furthermore, the removal of the inorganic acid or the separation of the product from the catalyst also calls for a large effort, and pollution may be produced if a large number of volatile organic solvents and liquids are used. Water as a by-product of the esterification reaction can shift the equilibrium towards the direction of hydrolysis ; therefore various methods were applied for removal of the water produced. However, catalytic activity in non-conventional media strongly depends on the amount of water associated with the enzyme forming its native conformation. Ionic liquids (ILs) that are composed entirely of ions and are liquids at ambient or far below ambient temperature, have been extensively used as a potential alternative to toxic, hazardous, flammable and highly volatile organic solvents. Their unique and attractive physicochemical properties, including negligible vapour pressure, multiple salvation interactions with organic and inorganic compounds, excellent chemical and thermal stability make ILs great candidates for volatile organic compounds replacements. In addition, the physicochemical properties such as the viscosity, hydrophobicity, density, and solubility of ILs can be tuned by simply selecting different combinations of cations and anions as well as attached substituents to customize ILs for many specific demands. This is why ILs have been recognized as “designer solvents”. This work has shown that a variety of enzymes, particularly those that tolerate conventional organic solvents, are eminently capable of performing in ionic liquids. Activities are generally comparable with or higher that those observed in conventional organic solvents. Furthermore enhanced thermal and operational stabilities have been observed. Ionic liquids may be a key technology to enable such reactions to work efficiently. The efficient reuse and purification of ionic liquids, the possibility of using other ionic liquids bearing “greener” anions and a reduction in the cost factor of the ionic liquids are further important issues to be considered for their industrial applications in future. The results reported here clearly demonstrate that enzyme catalysis in ionic liquids containing systems in an exciting and burgeoning research area, which holds tremendous potential of opening up a new field of nonaqueous enzymology. The methodology developed is very promising for the sustainable synthetic biocatalysis in non-conventional media. The operational stability of the enzymes in an appropriate ionic liquid medium is much higher in many cases than in organic solvents. Continuous operation can be maintained by applying continuous reagent feeding and product removal. During the enzymatic manufacture of ethyl acetate from ethanol and acetic acid the activity of enzyme did not change in the 72 hors reaction time. Moreover the products (ethyl acetate and water) were possible to remove by an environmental-safe membrane process having small energy demand. This pervaporation assisted continuous process can be suitable to apply in other similar procedures. More compound, product can be selectively separated by using special membranes, while controlling the hydratation level of the enzyme can be assured by water removal. The enzymatic synthesis of ethyl lactate was successfully achieved in Cyphos 104 and Cyphos 202 ILs. These media can also be applied to extract LA from the fermentation broth therefore a separation step can be avoided. Further improvement of the esterification reaction is possible microwave heating instead of conventional one. During the esterfication of lactic acid, when the free lactic acid content decreased, the conversion of the dimer to monomer became the dominant reaction, which reaction is a water reducing process. The monomeric lactic acid produced converts to ester, that is the reason why higher than 100 % yield was achieved during some reaction.

ionic liquid; lipase; esterification

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