Engineering the Pore Size and Environment of Pillar-Layered Zinc-Azolate-Carboxylate Frameworks for Efficient CH4 Purification and C2H2/CO2 Separation

Abstract

It is widely believed that engineering the pore size and environment of metal–organic frameworks (MOFs) is an effective method to improve their gas adsorption and separation performance. Herein, the pore size and environment of two pillar-layered zinc-azolate-carboxylate frameworks, {[Me₂NH₂]₂[Zn(H₂O)₆][Zn₈(PzC)₈(IM)₄]}_n (compound 1) and {[Me₂NH₂][Zn₂(PzC)₂(ATAZ)]}_n (compound 2) (where Me₂NH₂⁺ is the dimethylammonium cation produced by the decomposition of DMF molecules, PzC = 4-pyrazolatecarboxylate, IM = imidazolate, and ATAZ = 5-amino-1H-tetrazolate), were effectively regulated by using different azole ligands. Single-crystal X-ray diffraction data indicated that both compound 1 and compound 2 are 3D pillar-layered zinc-azolate-carboxylate frameworks constructed by connecting adjacent 2D grid-like zinc-azolate-carboxylate layers through IM or ATAZ ligands. The angle between the two adjacent PzC ligands in the 2D grid-like zinc-azolate-carboxylate layers of those two compounds is different, leading to large differences in the pore size and shape of those two compounds. Additionally, uncoordinate N atoms and the -NH₂ functional groups in the ATAZ ligand make compound 2 pores have more abundant Lewis N sites and −NH₂ functional groups. Gas adsorption and separation results indicated that compound 2 demonstrates preferential adsorption of C2 hydrocarbons and CO₂ gases over CH₄ with superior CO₂–CH₄, C₂H₂–CH₄, C₂H₄–CH₄ C₂H₆–CH₄, and C₂H₂–CO₂ binary mixture breakthrough time of 33, 57, 55, 60, and 17 min g^–1 at 298 K. GCMC simulation is further used to explore the interaction between the MOFs and gas molecules.