Enantiomeric Adsorption of Lactic Acid Mixtures in Achiral Zeolites

We studied the adsorption of chiral mixtures of lactic acid in several zeolites. All zeolite systems showed either no selectivity or heteroselectivity in which the minority enantiomer is adsorbed by a higher fraction than its fraction in the reservoir. Analysis of the mechanism showed that none of the previously identified origins of enantioselective adsorption of scalemic mixtures apply to lactic acid. However, on the basis of the lack of any ordered distribution in the adsorbed phase, we postulate a new mechanism that is likely to be very generic for chiral adsorption processes that proceed via chaotic packing of the adsorbate molecules. The new mechanism can explain several characteristics of the adsorption data and hints at new prospective separation methods with a high potential for pharmaceutical applications

Effect of Light Gases in the Ethane/Ethylene Separation Using Zeolitic Imidazolate Frameworks

We use molecular simulations to study the adsorption of ethane and ethylene in zeolitic imidazolate frameworks. The separation of these two compounds is a crucial step in many industrial processes, most of them related to production of ethylene. Separation methods such as fractional cryogenic distillation require large energy consumption that increases the costs of ethylene production. Here, we analyze the suitability of zeolitic imidazolate frameworks for the separation of these gases on the basis of structural and chemical features. We pay special attention to the effect exerted by other gases on the adsorption and diffusion of ethane and ethylene in the structures. We found that nitrogen has an important role in the separation process and depending on the structure, it can enhance or hinder the adsorption selectivity for ethane. The presence of gases other than nitrogen also causes an effect on the ethane/ethylene separation. A mixture containing hydrogen, oxygen, methane, carbon monoxide, carbon dioxide, ethane, and ethylene in zeolitic imidazolate frameworks is also investigated. Our results identify ZIFs with <i>rho</i>, <i>crg</i>, and <i>lta</i> topologies as good candidates for the separation of ethane and ethylene

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 (Qst) 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