Integration of Open Metal Sites and Lewis Basic Sites for Construction of a Cu MOF with a Rare Chiral Oh -type cage for high performance in methane purification.

Abstract

A Cu metal-organic framework (MOF), [Cu4 (PMTD)2 (H2 O)3 ]⋅20 H2 O, 1, (where PMTD is 1,4-phenylenebis(5-methyl-4H-1,2,4-triazole-3,4-diyl)bis(5-carboxylato-3,1-phenylene)bis(hydroperoxymethanide)), with a rare chiral Oh -type cage, and dual functionalities of open metal sites and Lewis basic sites, based on a designed U-shaped ligand, was synthesized by hydrothermal methods. It exhibits high CO2 , C2 , and C3 hydrocarbon storage capacity under atmospheric pressure, as well as high H2 (1.96 wt.%) adsorption capacity at 77 K. Methane purification capacity was tested and verified step by step. Isosteric heats (Qst ) studies reveal that CH4 has the weakest van der Waals host-guest interactions among the seven gases at 298 K. Ideal adsorbed solution theory (IAST) calculation reveals that compound 1 is more selective toward CO2 , C2 H6 , and C3 H8 over CH4 in further calculating its separation capacity, as exemplified for CO2 /CH4 (50:50, 5:95), C2 H6 /CH4 (50:50, 5:95), or C3 H8 /CH4 (50:50, 5:95) binary gas mixtures. Breakthrough experiments show that 1 has a significantly higher adsorption capacity for CO2 , C2 H6 , and C3 H8 than CH4 . The selective adsorption properties of 1 make it a promising candidate for methane purification.