Theoretical Study on Design and Hydrogen Storage Properties of High-Valence Boron-Phosphorous Based COFs.

At present, the valence state (connectivity) of most covalent organic frameworks (COFs) building units is low, mostly trivalent and tetravalent, making only 18 types of topological structures of COFs materials. On the contrary, the valence states of MOFs building blocks vary from 3 to 24, making them have more than 2 000 topological structure types. If a higher valence COFs building block can be designed, it will greatly enrich its topological structure type and material number. To this end, a hexahedral boron-phosphorus unit (-B4 P4 O12-) and five linear organic linkers (phenyl, fluorophenyl, toluene, naphthalene, biphenyl) were selected, and five high-valent boron-phosphorus covalent organic frameworks (BP-COFs) with lta topology were theoretically designed by molecular mechanics methods. The results show that the five materials have structural characteristics that are conducive to hydrogen storage applications, such as low density (0.52 ~ 1.17 g⋅cm-3), large specific surface area (1 274.12 ~ 4 033. 95 m² ⋅g-1) and high porosity (0. 55% ~ 0.78%). The Grand Canonical Monte Carlo (GCMC) method was used to predict the physical adsorption properties of hydrogen molecules on five materials at 298 and 77 K, and the structure-activity relationship between material structure and hydrogen adsorption properties was analyzed. The results show that the five BP-COFs materials have strong hydrogen adsorption capacity. Especially at 77 K, BP-COF-10 and BP-COF-11 have the best performance in hydrogen storage performance. BP-COF-10 has the highest volumetric hydrogen storage capacity at 77 K, reaching 52.86 g⋅L-1. BP-COF-11 has a large accessible specific surface area and high porosity. It has a good hydrogen storage capacity at 77 K and a gravimetric hydrogen storage capacity of 9.90% at 5 000 kPa. With the increase of pressure, the hydrogen storage capacity of BP-COF-11 still maintains a good upward trend. BP-COF-7 has the largest hydrogen storage capacity at 298 K, and the hydrogen storage capacity reaches 4.83 g⋅L-1 at 5 000 kPa. This indicates that the designed material has good hydrogen storage potential. This study will provide a theoretical reference for the experimental development of new high-performance hydrogen storage materials. [ABSTRACT FROM AUTHOR]