The Weakening Effect of the Coordinate Modification of Amino Ligands on the Porosity and NO Adsorptive Separation Performance of Ni-MOF-74

In this paper, di(pyridyl-4-yl) amine (DPA) containing one -NH amino group and 2,3-diaminopyrazine (DAP) containing two -NH₂ amino groups were used for the direct modification of Ni-MOF-74, and the effect of the introduction of auxiliary ligands with different numbers of amines on the NO adsorption and separation performance of Ni-MOF-74 were systematically explored. The results showed that the new MOFs with a dual-ligand structure could not be synthesized successfully but the framework structure of the product remained unchanged with only a slight reduction in pore size. After the introduction of DPA and DAP, the NO adsorption separation performance of Ni-MOF-74 were significantly reduced with the decrease of NO adsorption capacity from 164.4 cc·g^-1 to 61.5 cc·g^-1 and 75.8 cc·g^-1, respectively, and NO/CO₂ adsorption selectivity from 247.7 to 42.4 and 200.3, respectively. It can be inferred that the amino functional groups on DPA and DAP both coordinated with the metal center Ni, leading to a loss of their NO adsorption function and weakening the chemisorption of Ni-MOF-74. Meanwhile, DPA and DAP occupied the two-dimensional pores of Ni-MOF-74, causing severe pore blocking and hindering the diffusion of gas adsorption molecules within the pores, especially when larger-molecule DPA was introduced. Under the dual effects, the number of available and accessible NO adsorption sites on Ni-MOF-74 was reduced after the introduction of DPA and DAP. Therefore, the functionalization modification method after the adsorptive coordination of amino ligands was not conducive to improving the NO adsorption and separation performance of MOF materials. This study offers new perspectives on synthesizing and modifying MOFs for NO adsorption and separation in flue gas mixtures.