An adsorbate biased dynamic 3D porous framework for inverse CO2 sieving over C2H2

Abstract

Separating carbon dioxide (CO₂) from acetylene (C₂H₂) is one of the most critical and complex industrial separations due to similarities in physicochemical properties and molecular dimensions. Herein, we report a novel Ni-based three-dimensional framework {[Ni₄(μ₃-OH)₂(μ₂-OH₂)₂(1,4-ndc)₃](3H₂O)}_n (1,4-ndc = 1,4-naphthalenedicarboxylate) with a one-dimensional pore channel (3.05 × 3.57 Ų), that perfectly matches with the molecular size of CO₂ and C₂H₂. The dehydrated framework shows structural transformation, decorated with an unsaturated Ni(ii) centre and pendant oxygen atoms. The dynamic nature of the framework is evident by displaying a multistep gate opening type CO₂ adsorption at 195, 273, and 298 K, but not for C₂H₂. The real time breakthrough gas separation experiments reveal a rarely attempted inverse CO₂ selectivity over C₂H₂, attributed to open metal sites with a perfect pore aperture. This is supported by crystallographic analysis, in situ spectroscopic inspection, and selectivity approximations. In situ DRIFTS measurements and DFT-based theoretical calculations confirm CO₂ binding sites are coordinatively unsaturated Ni(ii) and carboxylate oxygen atoms, and highlight the influence of multiple adsorption sites.

Scheme 1. Schematic representation of dynamicity in a Ni-MOF and its preferential CO₂ sieving over C₂H₂.

Fig. 1. (a) Asymmetric unit of 1 executing different Ni(ii) environments. (b) Three-dimensional view along the a direction. (c) View of the bilayer {Ni₂-(μ-OH)(μ-OH₂)} chains supported by 1,4-ndc showing the bridging water and hydroxyl groups. (d) Pore view of 1 along the a direction. (e) Structural transformation of the as-synthesized → desolvated → rehydrated MOF as evident from PXRD analysis. (f) A close-up view of (e) reveals the differences in Bragg's reflection between the different phases.

Fig. 2. (a) N₂ and CO₂ adsorption isotherms of 1′ at 77 and 195 K (inset; log curve for a clear understanding of the close-to-open phase transition). (b) Adsorption isotherms of CO₂ and C₂H₂ at 298 K. (c) Isosteric heat of adsorption of CO₂ for 1′ at different loading concentrations with virial fitting in the inset. (d) Vapor adsorption isotherms of H₂O (298 K) and MeOH (293 K). In the adsorption isotherms, at points P and Q, the adsorbed samples were subjected to PXRD measurement which revealed a similar pattern to the as-synthesized framework after adsorbing water and methanol molecules as shown in Fig. S7.

Fig. 3. In situ DRIFTS of CO₂ adsorption in 1′ studied under ambient conditions. The X is the flow rate of CO₂ dosing inside the chamber to develop increased concentration inside the sample chamber. X = 10 L per hour. The schematic of CO₂–MOF interaction as anticipated from the adsorption, PXRD and DRIFTS study.

Fig. 4. Density functional theory (DFT) results showing (a) interactions between CO₂ and unsaturated Ni(ii) sites along with carboxylate oxygen atoms and (b) no such interactions between C₂H₂ and the framework.

Fig. 5. IAST selectivity approximation studied from a 298 K adsorption isotherm by fitting it in a dual-site Langmuir–Freundlich (DSLF) equation.

Fig. 6. (a) The bi-cyclic dynamic breakthrough separation of equimolar C₂H₂/CO₂ for 1′ and (b) respective concentration of C₂H₂ in the outlet (plotting cycle 2 with 0 s start time) considering the flow rate through the bed of 2.8–2.2 mL min⁻¹.