Advances in gas-phase biocatalyst with membrane reactors

Introduction The health risks associated with certain harmful substances which can be released in air and the continuous production of various waste gases and odorous substances by industrial operations pushes research towards new efficient and environmentally sustainable processing strategies for their disposal. Enzymatic reactions owing to their specificity, efficiency and low energy consumption are among the most promising candidates to reach this goal. Membrane processes and in particular, biocatalytic membrane reactors (BMRs), have the potential to play a significant role in preventing pollution allowing process intensification by integrating biocatalysis and separation in a single unit. Solid-gas biocatalysis with respect to traditional solid-liquid biocatalysis has many advantages such as higher thermostability of the dehydrated enzyme, absence of leakage, reduction of microbial contamination, improvements in mass transfer. In addition, solid-gas systems offer very high production rates for minimal plant sizes, allow important reduction of treated volumes and permit simplified downstream processes. Therefore, it appears as a promising technology for new cleaner decontamination processes development as well as for biosensing of target substances in air. The development of BMRs highly specialized for the success of this technology is strongly required. From a nanoengineering point of view, a deeply understanding and management of key aspects governing the performance of immobilized enzymes is crucial. In this work, the parameters that affect immobilized enzyme performance in a solid-gas BMR will be presented as well as strategies to improve bioreactor productivity. Experimental Lipase from candida rugosa (LCR) and vaporized ethyl acetate were selected as model enzyme and gaseous substrate, respectively. LCR was immobilized onto polyvinylidene fluoride (PVDF) membranes functionalized in three different ways. In the first method, the negatively charged LCR was immobilized by ionic interaction on PVDF membrane functionalized with positively charged amino. In the second method, aldehyde groups were introduced on PVDF and LCR was immobilized by covalent bond. In the third method, LCR was immobilized employing as carrier polyacrylamide (PAAm) microgels synthesized in the desired size range and purpose-functionalized. The biocatalytic membranes were tested in the BMR working in gas phase, by studying the influence of LCR loading, ethyl acetate and water flow rates on the catalytic performance. Results and discussion Results demonstrate that all the three biocatalytic systems tested evidenced a better efficiency in terms of thermostability and productivity when the water vapour was supplied in controlled manner respect to the enzyme in water solution. In particular, during experiments carried out at temperature of 50 °C LCR immobilized in the water restricted system showed a better biocatalytic activity and long term stability (at least 5 months) whereas LCR in water solution resulted completely inactive. The best performance in terms of ethanol productivity were obtained feeding 1.3 mL h-1 of ethyl acetate and 0.3 mL h-1 of water, for the membrane containing 84 µg cm-2 of LCR immobilized by the microgels mediation. These results can be attributed both to the type of interaction enzyme-support and to the nature of the support itself. Anyway, the most important aspect influencing the performance of enzymes catalyzing hydrolysis reactions in solid-gas systems is the water availability in the enzyme microenvironment. The hydrophilic microgels improved the interaction between the LCR and the water required for the hydrolysis reaction by creating an hydrated microenvironment, which enhanced the LCR performance without significantly