In this study, the conversion of carbon dioxide to methanol was realized through a novel biochemical approach that was catalyzed by three dehydrogenases:  formate dehydrogenase (FateDH), formaldehyde dehydrogenase (FaldDH), and alcohol dehydrogenase (ADH). The dehydrogenases were encapsulated in an alginate−silica (ALG−SiO2) hybrid gel, which was prepared through in situ growth of the silica precursor within an alginate solution, which was followed by Ca2+ cross-linking. Methanol yields that were catalyzed by free dehydrogenases, and by dehydrogenases immobilized in pure alginate (ALG) gel and in ALG−SiO2 hybrid gel, were 98.8%, 71.3%, and 98.1%, respectively. Furthermore, methanol yield that was catalyzed by dehydrogenases in an ALG−SiO2 composite could be retained as high as 76.2% after 60 days storage and as high as 78.5% after 10 times recycling. The significantly improved catalytic properties of the dehydrogenases in the ALG−SiO2 composite were attributed to the creation of the appropriate immobilizing microenvironment:  high hydrophilicity, moderate rigidity and flexibility, ideal diffusion characteristics, and optimized cage confinement effect.

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In this study, two kinds of novel hybrid biocomposite beads were prepared using alginate as polymer moiety, silica as inorganic moiety, and yeast alcohol dehydrogenase (YADH) as enzyme moiety. Silica was incorporated into the biocomposites through two approaches:  in situ hydrolysis and polymerization of tetramethoxysilane in alginate solution, followed by Ca2+ cross-linking (this biocomposite was designated as ALG−SiO2−YADH); physical incorporation of silica gel (SG) into alginate solution, followed by Ca2+ cross-linking (this biocomposite was designated as ALG−SG−YADH). The porous network structure of the biocomposite beads ensured the facile accessibility of enzyme for NADH and formaldehyde. Loading efficiency of YADH in alginate−silica−enzyme hybrid biocomposite beads was higher than that in pure ALG−YADH biocomposite due to the more compact structure and less water loss. The catalytic activity of YADH in alginate−silica−enzyme hybrid biocomposite beads was higher than that in ALG−YADH biocomposite due to less enzyme leakage and improved carrier microenvironment. Furthermore, the storage stability and operational stability of YADH in alginate−silica−enzyme hybrid biocomposite beads were significantly improved due to the increase of the mechanical strength and swelling resistance. Compared to ALG−SG−YADH biocomposite, ALG−SiO2−YADH biocomposite showed much better performance due to the more homogeneous distribution of silica particles in the composite matrix.