Structural diversities and gas adsorption properties of a family of rod-packing lanthanide–organic frameworks based on cyclotriphosphazene-functionalized hexacarboxylate derivatives

Abstract

Solvothermal reactions of cyclotriphosphazene-functionalized hexacarboxylate derivatives bearing different substituents (methoxy, chloro, bromo and methyl) with Ho(NO₃)₃·6H₂O gave rise to four new three-dimensional lanthanide metal–organic frameworks (Ln-MOFs). Single-crystal X-ray diffraction analyses reveal that they all feature rod-shaped Ho-carboxylate chains as inorganic secondary building units but display substituent-driven structural diversities instead of the isoreticular structures. N₂ adsorption and desorption studies show that among these Ln-MOFs, only the methoxy-modified Ln-MOF ZJNU-63 after activation exhibits permanent porosity. Furthermore, the gas adsorption properties of ZJNU-63 with respect to C₂H₂, C₂H₄, C₂H₆, CO₂, and CH₄ were systematically investigated, revealing its possible potential for natural gas purification. Although the separation performance is not very impressive compared to the other reported MOFs, ZJNU-63 presents a rare example of permanently porous Ln-MOFs constructed from flexible organic ligands displaying selective gas adsorption. Besides, ZJNU-63 can adsorb a large amount of C₄ hydrocarbons with uptake capacities ranging from 58.8 to 75.7 cm³ (STP) g⁻¹ at 298 K and 1 atm.