Hierarchical Porous Covalent Organic Frameworks: The Influence of Additional Macropores on Photocatalytic Hydrogen Evolution and Hydrogen Peroxide Production

Covalent organic frameworks (COFs), an emerging class of crystalline porous materials, have garnered significant interest due to their low density, tunable chemical and physical properties, porous structure, and high surface area. COFs typically exhibit microporosity, i.e., pores below 2 nm influenced by their specific linkers and binding patterns. In addition, some COFs are reported to exhibit small mesopores, especially when extended linkers are used. In applications such as catalysis, where rapid mass transport is a crucial factor, hierarchical pore structures of catalysts are beneficial. This involves the presence of small micropores to generate a large surface area and additional macropores to facilitate the transport of reactants to catalytic sites and to promote product diffusion. This study describes the application of such a hierarchical porous COF (macro-TpBpy) for photocatalysis. The macro-TpBpy architecture seamlessly integrates intrinsic microporosity with additional macropores, thereby yielding a substantial increase in the surface area. The hierarchical porous material demonstrates a promising performance in photocatalytic hydrogen evolution reaction, exhibiting a rate of 4.88 mmol g(-1) h(-1). Notably, this is a fourfold improvement compared to the COF analogue featuring micropores only. Furthermore, the introduction of macropores proved to be beneficial for the photocatalytic production of hydrogen peroxide (H2O2). Specifically, macro-TpBpy exhibited a production rate of 2716 mu mol g(-1) h(-1), in water without sacrificial hole scavengers, whereas pristine TpBpy had a rate of 2134 mu mol g(-1) h(-1). This work thus contributes to the further development of COFs in photocatalysis and shows that, in addition to suitable band structure and surface functionality, the pore size and substrate transport influenced by it must also be considered as important factors.