Merging open metal sites and Lewis basic sites in a NbO-type metal–organic framework for improved C2H2/CH4 and CO2/CH4 separation

Abstract

A new three-dimensional NbO-type porous metal–organic framework ZJNU-47 was synthesized via a solvothermal reaction of Cu(NO₃)₂·3H₂O and a Lewis basic nitrogen donor site-rich tetracarboxylate, namely, 5,5′-(pyridazine-3,6-diyl)-diisophthalate, and the structure was characterized by single-crystal X-ray diffraction to be isostructural with NOTT-101. With the synergistic effect of open metal sites, Lewis basic sites and a suitable pore space, the MOF material ZJNU-47a after activation can take up a large amount of C₂H₂ and CO₂. The gravimetric C₂H₂ uptake of 214 cm³ (STP) g⁻¹ at room temperature and 1 atm is the highest among all reported MOFs to date, and the gravimetric CO₂ uptake of 108 cm³ (STP) g⁻¹ is also among the highest reported for MOFs. Compared to the isostructural MOF NOTT-101a, ZJNU-47a exhibits a significant increase in C₂H₂ and CO₂ uptake and thus improved C₂H₂/CH₄ and CO₂/CH₄ separations. Significantly, comprehensive DFT studies of C₂H₂ and CO₂ adsorption have revealed that the open nitrogen donor sites are comparable and even superior to open metal sites regarding the adsorption sites. This work demonstrated that the simultaneous introduction of Lewis basic nitrogen donor sites and Lewis acidic metal sites into the framework is a promising approach to improve the gas sorption toward CO₂ and C₂H₂ and thus to produce materials possessing enhanced C₂H₂/CH₄ and CO₂/CH₄ separation performance.