Halogen-Decorated Metal–Organic Frameworks for Efficient and Selective CO2 Capture, Separation, and Chemical Fixation with Epoxides under Mild Conditions

In the present work, three novel halogen-appended cadmium(II) metal-organic frameworks [Cd₂(L1)₂(4,4'-Bipy)₂]_n·4n(DMF) (1), [Cd₂(L2)₂(4,4'-Bipy)₂]_n·3n(DMF) (2), and [Cd(L3)(4,4'-Bipy)]_n·2n(DMF) (3) [where L1 = 5-{(4-bromobenzyl)amino}isophthalate; L2 = 5-{(4-chlorobenzyl)amino}isophthalate; L3 = 5-{(4-fluorobenzyl)amino}isophthalate; 4,4'-Bipy = 4,4'-bipyridine; and DMF = N,N'-dimethylformamide] have been synthesized under solvothermal conditions and characterized by various analytical techniques. The single-crystal X-ray diffraction analysis demonstrated that all the MOFs feature a similar type of three-dimensional structure having a binuclear [Cd₂(COO)₄(N)₄] secondary building block unit. Moreover, MOFs 1 and 2 contain one-dimensional channels along the b-axis, whereas MOF 3 possesses a 1D channel along the a-axis. In these MOFs, the pores are decorated with multifunctional groups, i.e., halogen and amine. The gas adsorption analysis of these MOFs demonstrate that they display high uptake of CO₂ (up to 5.34 mmol/g) over N₂ and CH₄. The isosteric heat of adsorption (Q_st) value for CO₂ at zero loadings is in the range of 18-26 kJ mol^-1. In order to understand the mechanism behind the better adsorption of CO₂ by our MOFs, we have also performed configurational bias Monte Carlo simulation studies, which confirm that the interaction between our MOFs and CO₂ is stronger compared to those with N₂ and CH₄. Various noncovalent interactions, e.g., halogen (X)···O, Cd···O, and O···O, between CO₂ and the halogen atom, the Cd(II) metal center, and the carboxylate group from the MOFs are observed, respectively, which may be a reason for the higher carbon dioxide adsorption. Ideal adsorbed solution theory (IAST) calculations of MOF 1 demonstrate that the obtained selectivity values for CO₂/CH₄ (50:50) and CO₂/N₂ (15:85) are ca. 28 and 193 at 273 K, respectively. However, upon increasing the temperature to 298 K, the selectivity value (S = 34) decreases significantly for the CO₂/N₂ mixture. We have also calculated the breakthrough analysis curves for all the MOFs using mixtures of CO₂/CH₄ (50:50) and CO₂/N₂ (50:50 and 15:85) at different entering gas velocities and observed larger retention times for CO₂ in comparison with other gases, which also signifies the stronger interaction between our MOFs and CO₂. Moreover, due to the presence of Lewis acidic metal centers, these MOFs act as heterogeneous catalysts for the CO₂ fixation reactions with different epoxides in the presence of tetrabutyl ammonium bromide (TBAB), for conversion into industrially valuable cyclic carbonates. These MOFs exhibit a high conversion (96-99%) of epichlorohydrin (ECH) to the corresponding cyclic carbonate 4-(chloromethyl)-1,3-dioxolan-2-one after 12 h of reaction time at 1 bar of CO₂ pressure, at 65 °C. The MOFs can be reused up to four cycles without compromising their structural integrity as well as without losing their activity significantly.