Solid-Gas Biocatalysis Using PVDF Enzyme-Loaded Membrane

Enzymes have been generally employed in aqueous solution, organic solvent systems, two phase water/ organic mixtures or microemulsions either for homogeneous or heterogeneous catalysis. Anyway, biocatalysis can also be applied to vapour phase systems, in which substrates and products are in gaseous phase and enzymes are in solid phase. Solid-gas biocatalysis is a special type of reaction system that exploits the ability of some enzymes, being in the solid state, to catalyze reactions of substrates in the gas phase. This approach has been proposed with the aim to develop new technologies for applications in production, waste treatment, biosensing and so on [1]. Solid-gas biocatalysis with respect to traditional solid-liquid biocatalysis has many advantages such as an higher thermostability of the dehydrated enzyme, absence of leakage, reduction of microbial contamination, improvements in mass transfer and product recovery. Several enzymes, such as lipases, esterases, cutinases, alcohol oxidases, alcohol dehydrogenases, that traditionally converts substrates in aqueous solutions has been tested in solid-gas systems showing significant activity [2]. Optimal supports for solid-gas biocatalysis are porous membrane materials, which allow to obtaining high flow rate and low pressure drop. The objective of this work was to develop a biocatalytic membrane reactor for hydrolysis of substrates in gaseous streams. As model reaction has been selected the hydrolysis of ethyl acetate (eq. 1) since it is relatively less toxic compared to other solvents. CH3COOCH2CH3 + H2O = CH3CH2OH + CH3COOH (eq. 1) Lipase from candida rugosa (LCR) immobilized onto functionalized polyvinylidene fluoride (PVDF) membrane was used as model enzyme. The functionalization strategy consisted in grafting reactive amino groups on the hydrophobic PVDF [3]. The PVDF hydrophobicity is a valuable property because their interaction with water remains very low even at high water activities. In order to investigate the best immobilization strategy LCR was immobilized either by ionic adsorption or by covalent bond (in this case using glutaraldehyde as crosslinker). In the developed biocatalytic membrane reactor, process parameters such as reaction temperature, flow rates of water and ethyl acetate were investigated. In addition, under the optimized condition the amount of LCR immobilized on the membrane was varied between 13 and 72 ?g cm-2. The ethyl acetate hydrolysis, which end up in ethanol and acetic acid production, was monitored following the ethanol production by gas chromatography analysis. Interestingly, the dehydrated-immobilized lipase showed higher thermostability and productivity when compared with the free-hydrated one at certain operating temperature. In particular, in experiments carried out at temperature of 50°C only the dehydrated LCR showed biocatalytic activity. In the biocatalytic membrane reactor, the best performance in terms of ethanol production were obtained at 40°C feeding 0.4 mL h-1 of water and 1.3 mL h-1 of ethyl acetate, for a membrane loaded with 72 ?g cm-2 of LCR attached by ionic adsorption. Under these conditions about 5 ?mol h-1 mgLCR-1 of ethanol (one of the two reaction products) during 12 h of reaction were produced. A first comparison with the literature data shows that the gas phase reaction in our biocatalytic membrane system has an enzyme activity about 50% higher. This strategy, in which the enzyme immobilized on membrane has been used to catalyze a reaction in gaseous phase, can be also promising for other enzymes and substrates of interest (e.g. phosphotriesterase).