Thiophene-Functionalized Cadmium(II)-Based Metal-Organic Frameworks for CO2 Adsorption with Gate-Opening Effect, Separation, and Catalytic Conversion

Abstract

Two new porous three-dimensional cadmium(II) metal-organic frameworks (MOFs) containing thiophene-appended carboxylate acid ligands, formulated as [Cd(L1)(4,4'-Bipy)]_n.2n(DMF) (1) and [Cd(L2)(4,4'-Bipy)]_n.2n(DMF) (2) [where L1 = 5-{(thiophen-2-ylmethyl)amino}isophthalate, L2 = 5-{(thiophen-3-ylmethyl)amino}isophthalate, 4,4'-Bipy = 4,4'-bipyridine, and DMF = N,N'-dimethylformamide] have been synthesized and structurally characterized. The gas adsorption analysis of the activated MOFs shows that they specifically capture CO₂ (uptake amount 4.36 mmol/g under 1 bar at 195 K) over N₂ and CH₄. Moreover, both MOFs show a gate-opening-closing phenomenon, which features the S-shaped isotherms with impressive hysteretic desorption during the CO₂ adsorption-desorption process at 195 K. Ideal adsorbed solution theory (IAST) calculations of these MOFs displayed that the obtained selectivity values for CO₂/CH₄ (50:50) and CO₂/N₂ (15:85) are approximately 8.6-23 and 93-565, respectively. Configurational bias Monte Carlo simulation was performed to understand the mechanism behind the better CO₂ adsorption by these MOFs. Catalytic activity of the MOFs for the CO₂ fixation reactions with different epoxides to form cyclic carbonates were tested. These MOFs demonstrated a significantly high conversion (94-99%) of epichlorohydrin to the corresponding cyclic carbonate within 8 h of reaction time at 1 bar of CO₂ pressure, at 70 °C, and they can be reused up to five cycles without losing considerably their activity.