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

Abstract

Hexaazaphenalene (HAP), a well-known conjugated large ring endowing its derivatives with abundant Lewis basic N-sites and excellent photoresponsive property, provides an ideal molecular design platform for CO2 capture and conversation. However, the strong π-π conjugation between HAP rings usually block the crystallization of porous crystalline materials.Herein, 2,5,8-tri(4-pyridyl)-1,3,4,6,7,9-hexaazaphenalene (TPHAP) was selected as 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). TPHAP ligands both act as pore partitioners through pyridyl groups inserting into 1D hexagonal channels of parent frameworks to generated suitable pore space and also help to stabilize the MOF architectures. Notably, SNNU-384 achieves synergistic framework partition through three partitioner units, featuring higher HAP density, larger pore surface area and more Lewis basic N-sites active sites than SNNU-385. Accordingly, SNNU-384 exhibits excellent room temperature CO2 adsorption capacity of 56.0 and 308.6 cm 3 g -1 under 1 bar and 30 bar, respectively, which are nearly three times of those for 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 the light harvesting. Electronic structure and photo-response analysis as well as in-situ FT-IR spectra further confirm the efficient photocatalytic fixation of CO₂ into carbonate intermediates and cyclic carbonate products by hexaazaphenalene-based pore-space-partitioned MOFs.