A microporous MOF Zn(BDC)(4,4'-Bipy)0.5 (MOF-508b, BDC = 1,4-benzenedicarboxylate, 4,4'-Bipy = 4,4'-bipyridine) was examined for the separation and removal of CO2 from its binary CO2/N-2 and CO2/CH4 and ternary CO2/CH4/N-2 mixtures by fixed-bed adsorption. With one-dimensional pores of about 4.0 x 4.0 angstrom to induce their differential interactions with the three components, MOF-508b exhibits highly selective adsorption to CO2 with the,adsorption capacity of 26.0 wt % at 303 K and 4.5 bar. This is the first example of microporous MOFs for the separation and removal of CO2 from its binary and ternary mixtures by fixed-bed adsorption, establishing the feasibility of the emerging microporous MOFs for their potential. applications in this very important industrial and environmental process.

a b s t r a c tAn adsorption study of hexane and xylene isomers mixtures was addressed in a rigid zirconium terephthalate UiO-66 (UiO for University of Oslo) with octahedral and tetrahedral cavities of free diameter close to 1.1 nm and 0.8 nm, respectively. Multicomponent equimolar breakthrough experiments show that the adsorption hierarchy of structural isomers in UiO-66 is opposite to the one observed in conventional adsorbents. For hexane isomers, it was found that the amount adsorbed increases with the degree of branching, being 2,2-dimethylbutane (22DMB) and 2,3-dimethylbutane (23DMB) the more retained molecules. Regarding the xylene isomers, the results show that the adsorption of the bulkier ortho-xylene (oX) is favoured compared to its homologues. The structural similarity between MOF UiO-66 and zeolite MCM-22 suggests that the reverse shape selectivity observed in the adsorption of hexane and xylene isomers might be attributed to the rotational freedom of the molecules inside the small cavities.

A three-dimensional microporous metal-organic framework Zn(BDC)(Dabco)(0.5) (BDC = 1,4-benzenedicarboxylate, Dabco = 1,4-diazabicyclo [2,2,2]octane), having two types of intersecting pores to encapsulate linear hexane and to block branched hexanes, and thus exhibiting highly selective sorption with respect to n-hexane, has been successfully applied to the kinetic separation of hexane isomers by fixed-bed adsorption. Document Type: Article Language: English

Sorption equilibrium and kinetics of hexane isomers: n-hexane (nHEX), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB) were studied in commercial pellets of zeolite BETA in the form HBEA with a SUM ratio of 150, between temperatures of 423 and 523 K and partial pressures up to 0.3 bar. Four different models were used to interpret the equilibrium data, named: Langmuir, multi-site Langmuir, dual-site Langmuir and Toth. The affinity to the adsorbent measured by the Henry's constants decreases with the degree of branching, with selectivities that can reach a value of 8.8 between nHEX and 22DMB. The heats of adsorption at zero coverage decrease with the degree of branching, being: 63.4 kJ/mol for nHEX, 59.7 kJ/ mol for 3MP, 57.1 kJ/mol for 23DMB and 53.6 kJ/mol for 22DMB. However, the isosteric heat of sorption changes with coverage with a different behavior for the three isomers. Sorption kinetics studied by the Zero Length Chromatography (ZLC) technique allowed us to find the nature of controlling the diffusion mechanism (macropore or micropore); for nHEX and 3MP macropore diffusion is controlling with activation energies similar to the heats of sorption at zero coverage. For 23DMB and 22DMB, the controlling mechanism changes, being the system governed apparently by both macropore and micropore diffusion. Data from this work are also compared with those reported in literature for both zeolite BETA and silicalite. (c) 2004 Elsevier Inc. All rights reserved. Document

Vapor-phase adsorption of the C8 alkylaromatic components p-xylene (p-x), m-xylene (m-x), o-xylene (o-x), and ethylbenzene (eb) on the three-dimensional microporous metal−organic framework (MOF) Zn(BDC)(Dabco)0.5 (BDC = 1,4-benzenedicarboxylate, Dabco = 1,4-diazabicyclo[2.2.2]octane) was investigated. Single- and multicomponent fixed-bed experiments were carried out at temperatures ranging from 125 to 175 °C and total hydrocarbon pressures up to 0.10 bar. At high pressure, the adsorption capacity for all the components varies from 35 to 26 g/100 gads at 125 and 175 °C. Henry’s constants are slightly different for all C8 alkylaromatics, except for o-xylene, which is significantly higher. The adsorption enthalpies at zero coverage for the different isomers ranges from 77.40 (eb) to 79.84 kJ/mol (o-x), indicating that the C8 alkylaromatics have comparable interactions with the framework at the low coverage. On the basis of binary and quaternary breakthrough experiments performed at different hydrocarbon pressures and temperatures, MOF Zn(BDC)(Dabco)0.5 was realized for the efficient and feasible separation of o-xylene from other C8 alkylaromatic components with the selectivity up to 1.88 because of the stronger interactions between o-xylene molecules and the framework and their differential pore-filling and molecular-packing effects confined within nanopores of MOFs.

An experimental study of single and binary fixed-bed adsorptions of hexane isomers n-hexane (nHEX), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB) was performed in commercial pellets of zeolite BETA, covering the temperature range between 423 and 523 K and partial pressures up to 0.3 bar. The effect of partial pressure and temperature on the shape of the breakthrough curves was addressed. From these data, single and binary adsorption equilibrium isotherms were collected. On the basis of the analysis of sorption events at the molecular level, two different models were used to interpret, with good accuracy, the equilibrium data: dual-site Langmuir (DSL) for nHEX and 3MP and multisite Langmuir (MSL) for 23DMB and 22DMB. Thereafter, a dynamic adsorption model was developed and tested, predicting with good accuracy the behavior of the fixed-bed experiments. At the partial pressures studied, it was found that the affinity of the isomers to the zeolite is nHEX > 3MP > 23DMB > 22DMB. The selectivity between the isomers is higher at low partial pressures, decreasing as the amount adsorbed increases. The Ideal Adsorbed Solution Theory using the DSL model to describe the pure component equilibrium of nHEX and 3MP and the MSL model for the dibranched isomers 22DMB and 23DMB gives a good prediction of the mixture adsorption data.

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Single and multicomponent fixed-bed adsorption of CO 2 , N 2 , and CH 4 on crystals of MOF-508b has been studied in this work. Adsorption equilibrium was measured at temperatures ranging from 303 to 343 K and partial pressures up to 4.5 bar. MOF-508b is very selective for CO 2 and the loadings of CH 4 and N 2 are practically temperature independent. The Langmuir isotherm model provides a good representation of the equilibrium data. A dynamic model based on the LDF approximation for the mass transfer has been used to describe with good accuracy the adsorption kinetics of single, binary and ternary breakthrough curves. It was found that the intra-crystalline diffusivity for CO 2 is one order of magnitude faster than for CH 4 and N 2 .