Selective CO2 or CH4 adsorption of two anionic bcu-MOFs with two different counterions: experimental and simulation studies

Abstract

A new bcu-MOF based upon a C_2v-symmetrical [Cu₄Cl(COO)₄N₈] cluster, [Cu₄(Cl⁻)(L)₄·4H₂O]·(DMA⁺)·xDMF·yMeOH (SNNU-Bai66, SNNU-Bai = Shaanxi Normal University Bai's group), has been sucessfully synthesized. By post-synthetic ion exchange experiments for the preparation of phases to be activated successfully, two new anionic derivatives, [Cu₄(Cl⁻)(L)₄·4H₂O]·[Li(H₂O)₄]⁺·xDMK (Li⁺-SNNU-Bai66, DMK = dimethyl ketone) and [Cu₄(Cl⁻)(L)₄·4H₂O]·(DMA⁺)·xDMK (DMA⁺-SNNU-Bai66, DMA⁺ = Me₂NH₂), have been achieved and their selective CO₂ or CH₄ adsorptions over N₂ gas have been systematically investigated in depth by both experimental and simulation studies. With the ionic pore surface and counterions cooperating with Lewis base N or O sites of the organic ligands in both Li⁺-SNNU-Bai66 and DMA⁺-SNNU-Bai66, the two MOFs exhibit highly selective CO₂ and CH₄ adsorption, especially for CH₄ gas. Moreover, with the counterion being tuned from Li(H₂O)₄⁺ to DMA⁺, selective CO₂ adsorption properties are slightly optimized due to the higher polarizability of DMA⁺ and the preferred adsorption site changing from site I [the opposite side of Cl⁻ toward Li(H₂O)₄⁺] in Li⁺-SNNU-Bai66 to site II (the position between the H_c atom of the L₁ ligand and DMA⁺) in DMA⁺-SNNU-Bai66, whereas those of CH₄ of both MOFs are similar due to their having the same preferred adsorption site, namely, site I [the opposite side of Cl⁻ toward DMA⁺ or Li(H₂O)₄⁺], in their frameworks. This work may further enrich the investigation of anionic MOFs in the field of selective gas adsorption with their inherent natures being more definitely displayed.