Biocatalytic micromixer coated with enzyme-MOF thin film for CO2 conversion to formic acid

In this study, a novel micromixer with a 3D helical, threaded channel was fabricated via 3D printing. The micromixer can enhance the mass transfer of reactants and product in an enzymatic cascade reaction converting CO2 to formic acid. Two enzymes, including carbonic anhydrase (CA) and formate dehydrogenase (FDH), were biomineralised in a zeolitic imidazolate framework-8 composite thin film on the micromixer channel that has been modified with polydopamine/polyethyleneimine. The biocatalytic performance of the micromixer was evaluated by testing at various liquid flow rates, and an optimum liquid flow rate at 1 mL/min (Rel = 8, Del = 3) was observed as the two-phase flow pattern in the micromixer channel transitioned from slug flow to bubbly flow. A comparison of the micromixer performance with and without threaded channels revealed ~ 170% enhancement in formic acid yield, indicating improved mixing with the presence of threads. In addition, the formic acid production rate for the micromixer with threaded channel was three folds higher than a conventional bubble column, demonstrating the superior performance of the proposed micromixer. The ease of assembling multiple micromixer units in series also enabled the immobilisation of different enzymes in separate units to carry out sequential reactions in a modular system. As a proof of concept, the solution product collected from long term biocatalysis was also tested in a direct formic acid fuel cell, which showed a promising prospect of integrating these two systems for a closed-loop energy generation system.