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

In the present work, three novel halogen-appended cadmium(II) metal-organic frameworks [Cd(L1)(4,4'-Bipy)]·4(DMF) (), [Cd(L2)(4,4'-Bipy)]·3(DMF) (), and [Cd(L3)(4,4'-Bipy)]·2(DMF) () [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 = ,'-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 [Cd(COO)(N)] secondary building block unit. Moreover, MOFs and contain one-dimensional channels along the -axis, whereas MOF possesses a 1D channel along the -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 CO (up to 5.34 mmol/g) over N and CH. The isosteric heat of adsorption () value for CO at zero loadings is in the range of 18-26 kJ mol. In order to understand the mechanism behind the better adsorption of CO by our MOFs, we have also performed configurational bias Monte Carlo simulation studies, which confirm that the interaction between our MOFs and CO is stronger compared to those with N and CH. Various noncovalent interactions, e.g., halogen (X)···O, Cd···O, and O···O, between CO 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 demonstrate that the obtained selectivity values for CO/CH (50:50) and CO/N (15:85) are ca. 28 and 193 at 273 K, respectively. However, upon increasing the temperature to 298 K, the selectivity value ( = 34) decreases significantly for the CO/N mixture. We have also calculated the breakthrough analysis curves for all the MOFs using mixtures of CO/CH (50:50) and CO/N (50:50 and 15:85) at different entering gas velocities and observed larger retention times for CO in comparison with other gases, which also signifies the stronger interaction between our MOFs and CO. Moreover, due to the presence of Lewis acidic metal centers, these MOFs act as heterogeneous catalysts for the CO 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 CO 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.