G-G₃N₄-based Nanocomposites With Excellent Visible-light-driven NADH Regeneration

Biocatalysis has been widely applied in chemical and pharmaceutical industry.As an important biological catalyst,the market proportion of oxidordeuctase is about 30%in industrial enzymes.Coenzyme/cofactor（NAD（P）H）is necessary to supply the hydrogen and electron for the catalytic procedure of most of oxidoreductase.In the present study,two new-type g-C₃N₄-based photocatalysts were synthesized,which are termed as 2D quantum dots@flake carbon nitride isotype heterojunction（QDs@Flake g-C₃N₄）and red phosphorus quantum dots@nanotube nitride carbon（P-QDs@g-C₃N₄）,respectively.Their morphology,structure,optical properties,separation and transfer of photo-generated electron-hole pairs,and photocatalytic efficiency of NADH regeneration were investigated.Furthermore,the photocatalytic NADH regeneration is coupled with enzymatic hydrogenation of formaldehyde to prepare methanol catalyzed by yeast alcohol dehydrogenase（YADH）.The details are summarized as follows:First,inspired by the functional and structural properties of natural photoreaction system（system PS I and PS II）,a 2D g-C₃N₄ isotype heterojunction photocatalyst,termed as QDs@Flake g-C₃N₄,was synthesized by one-step calcination of cyanamide-treated CM for visible-light-driven NADH regeneration.The morphology,electron band structure,directional transfer properties of charge carriers of QDs@Flake g-C₃N₄and their photocatalytic activity for NADH regeneration were then investigated.Compared with bulk g-C₃N₄,QDs@Flake g-C₃N₄ shows a larger specific surface area and higher separation/migration rate of photo-generated electrons-hole pairs,thus leading to enhanced NADH regeneration efficiency（40%）.In addition,the photocatalytic NADH regeneration can provide hydrogen and electron for enzymatic hydrogenation of formaldehyde to prepare methanol,and the methanol yield reaches2.48 mmol/L for 2 h.Second,after the thermal condensation of CM,the obtained g-C₃N₄ nanotube was employed to prepare the high-performance P-QDs@g-C₃N₄.Red phosphorus was deposited onto the surface of g-C₃N₄ nanotubes through the vapor deposition method under inert argon gas,resulting in the formation of P-QDs@g-C₃N₄.The structure and photochemical properties of P-QDs@g-C₃N₄ can promote the separation and transfer of photo-generated electron-hole pairs,shorten the transmission path of photo-generated electrons,and thus improve the regeneration efficiency of NADH under visible light/blue LED irradiation.The regeneration efficiency of NADH reached nearly60%and 91.6%under visible light and blue LED irradiation,respectively.It is found that the charge carriers produced from P-QDs continuously inject into the g-C₃N₄surface via analyzing the energy band stuture of P-QDs@g-C₃N₄,which enhances the photocatalytic activity of NADH regeneration.The photocatalytic NADH regeneration was optimized with enzymatic hydrogenation of formaldehyde to methanol,and the methanol yield reached 22.54 mmol/L for 4 h.Finally,the calcium alginate capsule with good transparency is applied to encapsulate P-QDs@g-C₃N₄ and alcohol dehydrogenase（YADH）for the preparation of high-performance compact reactor（YADH-g-C₃N₄@Ca-alginate capsule）.The mechanical strength,permeability and the kinetics of photocatalytic NADH regeneration and enzymatic NADH consumption of YADH-g-C₃N₄@Ca-alginate capsule are systematically studied.Then,the two processes of photocatalytic NADH regeneration and enzymatic hydrogenation of formaldehyde were coupled and optimized in YADH-g-C₃N₄@Ca-alginate capsule.