RESUMO
This paper reports the newly measured experimental data for CO2 solubility in a blended aqueous solution of monoethanolamine (MEA) and 2-amino-2-methyl-propanol (AMP) at different amine mixing ratios (MEA/AMP/H2O = 9:21:70, 15:15:70, and 21:9:70 wt %) and working temperatures (323.15, 373.15, and 383.15 K). The successive substitution method was used for calculating the mole fractions of all molecules (four molecules) and electrolytes (three cations and four anions) from the equilibrium along with the material and charge balance equations (11 equations). The electrolyte nonrandom two-liquid (e-NRTL) model was used to investigate nonideality in the liquid phase. Using the abovementioned thermodynamic models, the partial pressures of CO2 in the gas phase, mole fractions of all components in the liquid phase, pH variations, heats of absorption, and cyclic capacities of CO2 according to the absorption/desorption temperature and the blending ratio of MEA/AMP were estimated.
RESUMO
Isostructural [M2(DOBDC)(EG)2] (M = Mg, Co, Ni) frameworks are first synthesized by controlling the pH* in the reaction medium. Coordinated ethylene glycols form a hexagonal OH cluster, which works as a template to grow single crystals with high crystallinity. After the liberation of solvated molecules, [M2(DOBDC)] shows notably higher surface areas than the reported values and completely different CO2 and CO separation properties depending on the kinds of unsaturated metal. Therefore, breakthrough experiments using a CO2/CO mixed gas show that Mg-MOF has a longer breakthrough time for CO2 than for CO, whereas Co/Ni-MOFs have longer breakthrough times for CO than for CO2. Apart from CO2 and CO, other gases such as CH4, H2, and N2 were almost not adsorbed at all in these materials at 298 K. To reveal the role of unsaturated metal sites, CO2 and CO adsorption sites are unequivocally determined by single-crystal X-ray diffraction analysis. One of very interesting discoveries is that there are two CO2 and CO adsorption positions (sites A and B) in the hexagonal channels. Site A is the unsaturated metal center working as Lewis acidic sites, and site B is the secondary adsorption site located between two A sites. A close inspection of crystal structures reveals that unsaturated Co(II) and Ni(II) sites adsorb both CO2 and CO, whereas the unsaturated Mg(II) sites strongly capture only CO2, not CO. Density functional theory calculations elucidate the discrepancy in CO affinity: Co(II) and Ni(II) form strong π-back-donating bonds with CO via electron transfer from the d orbitals of the transition metals to the antibonding molecular orbitals of CO, whereas Mg(II) does not participate in electron transfer or orbital overlap with CO. This observation provides new insight into the synthesis of novel functional materials with high CO2/CO separation performance.
RESUMO
Defect-free mixed-matrix membranes (MMMs) were prepared by incorporating hydrophilic metal-organic polyhedra (MOPs) into cross-linked polyethylene oxide (XLPEO) for efficient CO2 separation. Hydrophilic MOPs with triethylene glycol pendant groups, which were assembled by 5-tri(ethylene glycol) monomethyl ether isophthalic acid and CuII ions, were uniformly dispersed in XLPEO without particle agglomeration. Compared to conventional neat XLPEO, the homogenous dispersion of EG3 -MOPs in XLPEO enhanced CO2 permeability of MMMs. Upon increasing the amount of EG3 -MOPs, the membrane performance such as CO2 /N2 selectivity was steadily improved because of unsaturated CuII sites at paddle-wheel units, which was confirmed by Cu K-edge XANES and TPD analysis. Therefore, such defect-free MMMs with unsaturated metal sites would contribute to enhance CO2 separation performance.
