Modulating Covalent Organic Framework Nanochannel Microenvironments for Sustainable Hydrogen Peroxide Photosynthesis

Photocatalytic hydrogen peroxide (H₂O₂) synthesis from water and oxygen is a promising alternative to the anthraquinone process, but its efficiency depends on charge separation and reactant transport. Herein, we report a side chain strategy to regulate nanochannel microenvironments of hydrazone-linked covalent organic frameworks (COFs). Four COFs bearing H, OMe, OEt, or OEtOMe groups were prepared using the same benzotrithiophene node and hydrazone-linked framework. The alkoxy side chains tune pore polarity, accessible porosity, electronic properties, and water and oxygen adsorption. Among them, COF-Hz-OEtOMe exhibits the strongest water and oxygen affinity and favorable charge-separation behavior. It achieves an H₂O₂ production rate of 7240 µmol g^-1 h^-1 at the optimized catalyst loading in pure water and 22910 µmol g^-1 h^-1 with benzyl alcohol. A 1 L reactor using tap water and air accumulates 1.42 mM H₂O₂ after 4 h under natural sunlight. Experiments and theoretical calculations suggest that the conjugated framework promotes charge separation, while the alkoxy-functionalized nanochannels improve reactant adsorption and oxygen activation. Life cycle assessment (LCA) highlights that natural sunlight and larger-scale catalyst synthesis can reduce the calculated climate impact. These results demonstrate that pore-wall side chain engineering provides an effective strategy for regulating charge behavior and reactant transport in porous photocatalysts.