Hierarchical Porous Covalent Organic Frameworks: The\nInfluence of Additional Macropores on Photocatalytic Hydrogen Evolution\nand Hydrogen Peroxide Production

Covalent organic\nframeworks (COFs), an emerging class of crystalline\nporous materials, have garnered significant interest due to their\nlow density, tunable chemical and physical properties, porous structure,\nand high surface area. COFs typically exhibit microporosity, i.e.,\npores below 2 nm influenced by their specific linkers and binding\npatterns. In addition, some COFs are reported to exhibit small mesopores,\nespecially when extended linkers are used. In applications such as\ncatalysis, where rapid mass transport is a crucial factor, hierarchical\npore structures of catalysts are beneficial. This involves the presence\nof small micropores to generate a large surface area and additional\n macropores to facilitate the transport of reactants to catalytic\nsites and to promote product diffusion. This study describes the application\nof such a hierarchical porous COF (macro-TpBpy) for photocatalysis.\nThe macro-TpBpy architecture seamlessly integrates intrinsic microporosity\nwith additional macropores, thereby yielding a substantial increase\nin the surface area. The hierarchical porous material demonstrates\na promising performance in photocatalytic hydrogen evolution reaction,\nexhibiting a rate of 4.88 mmol g<sup>–1</sup> h<sup>–1</sup>. Notably, this is a fourfold improvement compared to the COF analogue\nfeaturing micropores only. Furthermore, the introduction of macropores\nproved to be beneficial for the photocatalytic production of hydrogen\nperoxide (H<sub>2</sub>O<sub>2</sub>). Specifically, macro-TpBpy exhibited\na production rate of 2716 μmol g<sup>–1</sup> h<sup>–1</sup>, in water without sacrificial hole scavengers, whereas pristine\nTpBpy had a rate of 2134 μmol g<sup>–1</sup> h<sup>–1</sup>. This work thus contributes to the further development of COFs in\nphotocatalysis and shows that, in addition to suitable band structure\nand surface functionality, the pore size and substrate transport influenced\nby it must also be considered as important factors.