Hexaazaphenalene-based pore-space-partitioned metal–organic frameworks for enhanced CO2 capture and photocatalytic fixation

Abstract

Hexaazaphenalene (HAP), a well-known large-ring conjugated system that endows its derivatives with abundant Lewis basic N-sites and excellent photoresponsive property, provides an ideal molecular design platform for CO₂ capture and conversion. However, the strong π–π conjugation between the HAP rings usually blocks the crystallization of porous crystalline materials. Herein, 2,5,8-tri(4-pyridyl)-1,3,4,6,7,9-hexaazaphenalene (TPHAP) is selected as an auxiliary ligand to combine with different tritopic organic linkers to fabricate two new pore-space-partitioned metal–organic frameworks (MOFs, SNNU-384 and SNNU-385). The TPHAP ligand acts as a pore partitioner through the insertion of pyridyl groups into the 1D hexagonal channels of parent frameworks to generate suitable pore spaces and helps in stabilizing the MOF architectures. Notably, SNNU-384 achieves a synergistic framework partition through three partitioner units, featuring higher HAP density, larger pore surface area and more Lewis basic N active sites than SNNU-385. Accordingly, SNNU-384 exhibits excellent room-temperature CO₂ adsorption capacity of 56.0 and 308.6 cm³ g⁻¹ under 1 bar and 30 bar, respectively, which are nearly three times that exhibited by SNNU-385. Furthermore, under visible-light irradiation, SNNU-384 can efficiently convert CO₂ molecules and various epoxides into cyclic carbonates (CCs) with a top-level turnover frequency (TOF) of 104.18 h⁻¹, maintaining nearly unchanged catalytic activity after six cycles. GCMC simulations verify dense CO₂ distribution in the partitioned MOF cages via multiple intermolecular interactions, including π–π stacking, hydrogen bonding, and C–N dipole–dipole interactions. At the same time, HAP-directed MOF frameworks with dual band gaps and additional absorption response at 700 nm (confirmed by UV-vis spectroscopy) enhance light harvesting. Electronic structure and photo-response analyses as well as in situ FT-IR spectra further confirm the efficient photocatalytic fixation of CO₂ into carbonate intermediates and cyclic carbonate products using hexaazaphenalene-based pore-space-partitioned MOFs.