[Show abstract][Hide abstract] ABSTRACT: Mixed matrix membranes (MMMs) have a potential to improve the separation performance of polymeric membranes while maintaining their advantages of easy processing and lower costs. In this work, series of MMMs were developed via solution casting by adding porous carbon-silica nanocomposite (CSM) fillers to a readily available Matrimid® membrane. CSMs were prepared by a hard template synthesis technique to get a tuneable porosity and surface chemistry which is controlled by the optimization of the filler porosity using carbon deposition, the pyrolysis conditions, and the maximization of polarity via oxygen functional groups. SEM images of the synthesized MMMs confirmed the good adhesion and dispersion of the fillers within the polymer matrix. The separation results demonstrate that the overall separation efficiency is increased by the addition of a carbon phase, providing an increased affinity for the CO2 gas molecules next to the creation of extra porosity and free volume. It was showed that significantly improved CO2 mixed gas selectivity and permeability for CO2:N2 and CO2:CH4 gas mixtures at 9bar and 308K was achieved. For gas mixtures with a 50:50 (CO2:N2) feed composition, a 2-fold and 6-fold increase of the mixed gas selectivity (up to 42.5) and permeability (up to 27 Barrer) compared to unfilled PI was achieved, respectively. The performance of the membranes was compared to the existing literature data.
[Show abstract][Hide abstract] ABSTRACT: The catalytic activity of a new catalytic filter of combined design consisting of a catalytic filter candle with an integrated catalytic ceramic foam was calculated under model gas conditions at 800 and 850 °C in the absence and presence of 100 ppmv H2S on the basis of separate and combined measurements of appropriate catalytic filter element and ceramic foam disk samples. Real gas validation of the determined model gas activity of the catalytic filter of combined design in a bench-scale gasifier was performed to check-up, if model gas conversions can be used as bases for the prediction of real tar conversions. A calculation of the model gas activity of the catalytic filter at a superficial velocity of 2 cm/s based on the disk measurements results to 99% naphthalene conversion at 850 °C in the absence of H2S and 94% in the presence of 100 ppmv H2S. At 800 °C, the calculated conversion is 95% in the absence and 70% in the presence of 100 ppmv H2S resulting in an estimated conversion of 85% at an H2S content of 40 ppmv. This value is comparable with the measured real tar conversion of 81% at 790 °C and a superficial velocity of 2.5 cm/s.
[Show abstract][Hide abstract] ABSTRACT: A sulfone functionalized DUT-5 Metal Organic Framework ‘SO2-DUT-5’ was synthesized using 4,4′-bibenzoic acid-2,2′-sulfone linkers. Its adsorption properties were studied and compared to those of the pristine DUT-5 material. The inverse gas chromatographic method was used to study the adsorption of C5–C7 linear, branched and cyclic alkanes, alkenes and aromatic molecules. SO2-DUT-5 shows shape-selective behaviour in the adsorption of linear, branched and iso-alkanes, whereas DUT-5 is non-selective. The presence of sulfone groups results in both a reduced pore size and more specific interaction sites, leading to the shape-selective behaviour towards linear alkanes and the slightly elevated preference for aromatic compounds in the low coverage area.
[Show abstract][Hide abstract] ABSTRACT: COMOC-2, a flexible vanadium containing metal-organic framework, was investigated for its adsorption and separation properties of light hydrocarbons. COMOC-2 is an extended version of the MIL-47 framework with 4,4'-biphenyldicarboxylic acid linkers instead of terephthalic acid. Adsorption isotherms of methane to propane, ethylene and propylene were determined with a gravimetric uptake technique at temperatures between 281 K and 303 K. A pronounced breathing effect was observed (in contrast to the more rigid MIL-47 framework), in which the adsorption capacity increases by more than a factor two at a given breathing pressure. The breathing pressure decreases with increasing hydrocarbon molecular weight. The typical two-step isotherms are nearly identical for alkanes and alkenes, in accordance with the non-polar nature of the material. Binary isotherms of ethane and propane were also measured with the gravimetric uptake technique at different temperatures and total pressures. The mixture isotherms and breathing transition pressures were predicted relying on the osmotic framework adsorbed solution theory (OFAST). Finally, the separation potential of COMOC-2 for ethane/propane mixtures was looked into using breakthrough experiments for different compositions and different pressures.
[Show abstract][Hide abstract] ABSTRACT: Low coverage adsorptive properties of linear and branched alkanes, 1-alkenes, cycloalkanes, aromatics, and small polar adsorbates on the Zeolitic Imidazolate Framework ZIF-68 were explored by pulse gas chromatographic experiments at temperatures between 413 and 513 K. ZIF-68 has a complex structure with amphiphilic properties due to the presence of two different imidazole linkers (2-nitroimidazolate and benzimidazolate), resulting in small polar and large nonpolar channels. Shape selective properties are absent for linear and branched alkanes (C5-C12), which is not surprising given the occurrence of large channels in the structure. A specific mechanism for the adsorption of polar adsorbates is proposed via strong interactions with the nitro groups, pointing inward in the small channel. Further, n-alkanes, 1-alkenes, cycloalkanes, and aromatics were used as probe molecules to determine whether or not specific interactions were involved in the framework for nonpolar adsorbates. These experimentally obtained adsorption energy values were linked to a simple model, which relates the adsorption enthalpy to the degree of branching and to the contribution of nonspecific van der Waals (number of hydrogen atoms) and electrostatic (number of double bonds) interactions.
The Journal of Physical Chemistry C 01/2015; 119(4):150113160012002. DOI:10.1021/jp509840u · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Eight-membered ring (8 MR) zeolites hold large potential for industrial CO2 separation such as biogas separations. They offer large selectivity due to the constrained environment for adsorption, especially when large cations are present in the interconnecting windows [1-5]. The relatively small cages and windows of these zeolites increase the interaction strength between the adsorbent and CO2. At the same time, the diffusion of the slightly larger adsorbates CH4 and N2 through the 8 MR windows can be hindered. As a result, simulations predicted the highest CO2/CH4 selectivities in 8 MR structures among the different types of zeolites.  Experimental studies have mainly focused on the RHO, LTA, CHA and KFI structures up to now [7-12].
In the present work, the Rb- and Cs-exchanged ZK-5 zeolites (8 MR KFI type zeolites) were studied for kinetic CO2/CH4 separation. ZK-5 zeolites have the KFI structure, with a slightly higher Si/Al ratio (Si/Al = 3.6). It consists of a three-dimensional network of larger α- cages (11.6 Å in diameter) and smaller γ-cages (6.6 Å x 10.8 Å). The α- and γ-cages are connected through flat eight- membered rings with a diameter of 3.9 Å. A puckered eight-membered ring with a smallest diameter of 3.0 Å connects the γ-cages with each other.
Rb-ZK-5 and Cs-ZK-5 were thoroughly characterized via chemical analysis, argon porosimetry, X-ray diffraction and Rietveld refinements. These refinements showed that a majority of the 8 MR sites were filled with cations. Afterwards, the CO2/CH4 separation potential of both adsorbents was assessed via the measurement of kinetic and equilibrium data (T = 261.15 - 323 K), adsorption and desorption breakthrough measurements at 303 K (P = 1 - 8 bar), and simulations of their performance at lab-scale and in a model pressure swing adsorption (PSA) process. The high occupation of the central 8 MR sites with large cations causes strong diffusional limitations for CH4 on Rb-ZK-5 and Cs-ZK-5. As a result, both zeolites effectively separate CH4 from CO2 with very high selectivities (α = 17). A disadvantage for Cs-ZK-5 is the occurrence of mass transfer limitations for CO2, yielding lower mass transfer coefficients on Cs-ZK-5 compared to Rb-ZK-5 and a large part of the bed being unused for separation. The global performance of both adsorbents will be compared to a benchmark 13X zeolite.
Tom Rmy and Leen Van Tendeloo acknowledge FWO- Vlaanderen for financial support. Joeri F.M. Denayer acknowledges FWO-Vlaanderen for the 1.5.280.11N research grant. Elena Gobechiya and Christine E.A. Kirschhock acknowledge the Belgian Prodex Office and ESA for financial support. Johan A. Martens and Christine E.A. Kirschhock acknowledge the Flemish Government for long-term structural funding, Methusalem.
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[Show abstract][Hide abstract] ABSTRACT: Eight-membered ring (8 MR) zeolites hold large potential for industrial CO2 separations such as biogas separation. They offer large selectivity due to the constrained environment for adsorption, especially when large cations are present in the interconnecting windows. The Rb- and Cs-exchanged ZK-5 zeolites (8 MR KFI type zeolites) were studied for kinetic CO2/CH4 separation. First, Rb-ZK-5 and Cs-ZK-5 were thoroughly characterized via chemical analysis, argon porosimetry, X-ray diffraction and Rietveld refinements. Afterwards, the CO2/CH4 separation potential of both adsorbents was assessed via the measurement of kinetic and equilibrium data (T = 261.15 - 323 K), breakthrough measurements at 303 K (P = 1 - 8 bar), and simulations of their performance. The high occupation of the central 8 MR sites with large cations causes strong diffusional limitations for CH4 on Rb-ZK-5 and Cs-ZK-5. As a result, both zeolites effectively separate CH4 from CO2 with very high selectivities (α = 17 at 1 bar and 303 K). Despite their very large CO2 selectivities, the performance of Rb-ZK-5 and Cs- ZK-5 was still lower than for the benchmark 13X zeolite on a larger scale. Future research needs to further unravel the adsorption mechanism on low-silica 8 MR zeolites and their corresponding potential in separation processes such as biogas purification.
[Show abstract][Hide abstract] ABSTRACT: In this experimental study, the adsorption behavior of the ZIF-68 heterolinked zeolitic imidazolate framework has been explored. Vapor phase adsorption isotherms of linear C1-C6 alcohols, C6 alkane isomers, aromatics (benzene, toluene, xylene isomers, 1,3,5-trimethylbenzene and 1,3,5-triisopropylbenzene) and polar adsorbates (water, acetonitrile and acetone) are reported and discussed. The complex pore structure of ZIF-68, with two one-dimensional channels, each with a different polarity, displays an overall hydrophobic character. Its two-pore system results in S-shaped isotherms for small polar adsorbates (small alcohols, acetone and acetonitrile), while longer alcohols and nonpolar molecules, such as aromatics and C6 alkane isomers, lead to type I adsorption isotherms. Bulky molecules, with a kinetic diameter significantly larger than the pore windows, are adsorbed in large amounts, which gave reason to think that this ZIF-68 material has a certain degree of framework flexibility to enlarge the free aperture of the channels. Besides, diffusion coefficients from vapor phase uptake and infrared experiments point to a different adsorption mechanism for polar and nonpolar adsorbates. Liquid phase adsorption experiments demonstrated the separation of alcohol mixtures (ethanol/1-butanol) at low concentration from water, with a clear preference for 1-butanol.
[Show abstract][Hide abstract] ABSTRACT: Due to the combination of metal ions and organic linkers and the presence of different types of cages and channels, Metal-Organic Frameworks (MOF) often possess a large structural and chemical heterogeneity, complicating their adsorption behavior, especially for polar-apolar adsorbate mixtures. By allocating isotherms to individual subunits in the structure, the ideal adsorbed solution theory (IAST) can be adjusted to cope with this heterogeneity. The binary adsorption of methanol and n-hexane on HKUST-1 is analyzed using this segregated IAST (SIAST) approach and offers a significant improvement over the standard IAST model predictions. It identifies the various HKUST-1 cages to have a pronounced polar or apolar adsorptive behavior.
[Show abstract][Hide abstract] ABSTRACT: Adsorption and separation of n-hexane and cyclohexane on the UiO-66 metal organic framework was studied by pulse gas chromatography and vapour phase breakthrough experiments. Vapour phase breakthrough experiments were performed with mixtures of both components, diluted in helium. Experiments were carried out at temperatures between 50 and 200 degrees C, at partial pressures between 0.0024 and 0.05 bar and varying mixture composition. In all conditions, separation between both components is observed, with cyclohexane being the most strongly retained component. This unusual preference for adsorbing the more bulky cyclic alkane isomer was further studied by pulse GC measurements at low coverage, at temperatures between 200 and 300 degrees C. These experiments confirm the inverse shape selective behaviour of UiO-66 in the adsorption of linear and cyclic alkanes. Cyclohexane fits better in the cages of UiO-66, leading to a more negative adsorption enthalpy, together with a lower loss of entropy during adsorption as compared to n-hexane.
[Show abstract][Hide abstract] ABSTRACT: Zeolite crystals with cations present, such as ZSM-5, are widely used for gas sequestration, separations, and catalysis. One possible application is as an adsorbent to separate CO2 from N2 in flue gas mixtures. Typically, the zeolite framework is of a SiO2 composition, but tetravalent Si atoms can be replaced with trivalent Al atoms. This change in valence creates a charge deficit, requiring cations to maintain the charge balance. Experimental studies have demonstrated that cations enhance adsorption of polar molecules due to strong electrostatic interactions. While numerous adsorption studies have been performed for silicalite-1, the all-silica form of ZSM-5, fewer studies on ZSM-5 have been performed. Grand Canonical Monte Carlo simulations were used to study adsorption of CO2 and N2 in Na–ZSM-5 at T = 308 K, which is ZSM-5 with Na+ counter-ions present. The simulations suggest that a lower Si/Al ratio (or higher Na+ and Al content) substantially increases adsorption at low pressures. At high pressures, however, the effect of the Al substitutions is minor, because the Al−/Na+ sites are saturated with guest molecules. Similarly, a lower Si/Al ratio also increases the isosteric heat of adsorption at low loading, but the isosteric heats approach the silicalite-1 reference values at higher loadings. Comparison of simulations and experimental measurements of the adsorption isotherms and isosteric heats points to the importance of carefully considering the role of charge on the Na+ cations, and suggest that the balancing cations in ZSM-5, here Na+, only have partial charges.
[Show abstract][Hide abstract] ABSTRACT: Dynamic adsorption–desorption measurements of CO2 and CH4 in amino-MIL-53(Al) were carried out in an adsorption breakthrough setup at different temperatures (303, 318, and 333 K) and pressures (1, 5, and 30 bar) to study the desorption dynamics of CO2 in amino-MIL(Al) as it plays an important role in the design of pressure swing adsorption (PSA) process for the upgrading of biogas. 13X zeolite was used as a reference material. The dynamic adsorption selectivity as well as the desorption efficiency of CO2 in both amino-MIL-53(Al) and 13X zeolite were calculated to evaluate the potential of amino-MIL-53(Al) for the upgrading of biogas by PSA process.
[Show abstract][Hide abstract] ABSTRACT: Zeolitic Imidazolate Frameworks (ZIFs) are crystalline microporous materials, consisting of transition metals (M = Co, Cu, Zn, etc.) linked by imidazolate (Im) or functionalized Im ligands. This subfamily of the Metal-Organic Frameworks (MOFs) displays good chemical and structural stability, which make them attractive as separation media in liquid phase separations. ZIF-68 is an interesting member of this family, with a complex pore structure consisting of two one-dimensional channels formed by three different types of cages. The small channel is composed of alternating small and medium cages, while the large channel comprises the large cages .
In the present work, the adsorption behavior of ZIF-68 is studied via vapor phase kinetic and equilibrium experiments with C1-C6 alcohols and other organic compounds (acetone, acetonitrile, C6-isomers, tri-isopropylbenzene) and kinetic batch experiments of alcohol mixtures in water. S-shape isotherms are obtained for polar adsorbates (fig.1), which is related to (1) the presence of two chemically different channels in the ZIF-68 framework and (2) the rather apolar nature of the material, which results in unfavorable adsorption of polar compounds at low concentration. Depending on the component, the adsorption capacity varies between 0.25 and 0.30 g/g. A comparison to the Argon isotherm indicated that both types of channels of ZIF-68 are densely filled with the different organic compounds.
Uptake of the different alcohols occurs slowly on the large crystals (around 70 m) used in this study. Moreover, two different diffusion regimes can be observed, in which molecular transport in the two-pore systems occurs at different rates. Besides, it was observed that bulky molecules, with a kinetic diameter significantly larger than the pore windows, are adsorbed in large amounts, which gave reasons to think that also this ZIF material, just as ZIF-7  and ZIF-8 [3-5], has a certain degree of structural flexibility to enlarge the free aperture of the channels, enhancing the diffusion process. Liquid phase experiments indicate the potential of ZIF-68 in the selective uptake and separation of alcohol/water mixtures.
Fig.1: Adsorption isotherms of C1-C6 alcohols, acetone and acetonitrile at 50 C.
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The authors are grateful to the Agency for Innovation by Science and Technology in Flanders (IWT) for support in the SBO MOFshape project.
[Show abstract][Hide abstract] ABSTRACT: Porous materials such as metal-organic frameworks offer great potential for separation technologies. Over the past decade the synthesis of a large number of new structures has been reported but relatively little attention has been given to the application of these novel adsorbents as stationary phases in gas or liquid chromatography. In this work, we report on the potential of the metal-organic framework UiO-66 for the separation of cyclic isomers. The extremely high preferential retention of cyclic hydrocarbons of the aromatic’s and functionalized cycloalkane’s type is exploited to achieve difficult separations. Confinement effects play a crucial role in fine-tuning the adsorbent’s properties.
[Show abstract][Hide abstract] ABSTRACT: This work discusses the adsorption of polar and apolar molecules on the copper?benzene-1,3,5-tricarboxylate (Cu?BTC) metal?organic framework. Vapor phase adsorption isotherms of various polar adsorbates such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 1-hexanol, water, acetone, acetonitrile, tetrahydrofuran, and N,N-dimethylformamide, as well as some apolar adsorbates such as n-hexane, n-heptane, m-xylene, and cyclohexane, on the Cu?BTC framework are presented. We show that water exposure of the Cu?BTC framework has an adverse effect on the uptake capacity. However, with minimized water exposure, we find high adsorption capacities, exceeding 0.65 cm3/g for all adsorbates with the exception of water, and we show that small polar adsorbates exhibit a two-step adsorption behavior. This behavior is further studied using molecular simulation and proposed to occur due to the presence of the various Cu?BTC cages. The cages containing the exposed coordinatively unsaturated copper sites have a more
The Journal of Physical Chemistry C 08/2013; 117(35-35):18100-18111. DOI:10.1021/jp405509m · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Liquid-phase diffusion of 1-alcohols in SAPO-34 was explored by batch experimentation. The uptake of pure and binary mixtures of 1-alcohols, dissolved in tert-butanol, was obtained for C1–C8 1-alcohols at temperatures between 25 and 80 °C, concentrations varying between 0.5 and 10 wt %, and crystal sizes between 7.5 and 20 μm. The experimental uptake data were fitted with an intracrystalline diffusion model and a linear driving force model. The intracrystalline diffusion coefficient showed a nonuniform stepwise decrease with chain length, ranging from 10–12 m2/s for methanol to 10–20 m2/s for 1-pentanol. No effect of the external concentration on the intracrystalline diffusion coefficient was observed. Variation of the crystal size showed that the intracrystalline diffusion is the rate-limiting step. On the basis of the Arrhenius equation, the activation energies of diffusion of ethanol, 1-propanol, and 1-butanol were determined, being, respectively, 27.8, 47.8, and 47.2 kJ/mol. Co-diffusion occurred in the uptake of binary mixtures of methanol/ethanol, methanol/1-propanol, and ethanol/1-propanol, where mutual effects could be noticed. From this experimental work, it could be concluded that the small dimensions of the SAPO-34 framework generate a very sterically hindered diffusion of 1-alcohols into the crystals, resulting in a chain-length-dependent behavior, interesting to obtain efficient kinetic-based separations.
The Journal of Physical Chemistry C 05/2013; 117(19):9758-9765. DOI:10.1021/jp312287k · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study investigates the potential of the well-known metal-organic framework (MOF) Mg-MOF-74 for low-pressure CO2 separations under dynamic conditions. We developed a technique to make pellets of Mg-MOP-74 without large capacity loss and measured CO2 and CH4 isotherms on these pellets up to 40 bar in the temperature range of 293-338 K. Dynamic breakthrough separation experiments with mixtures of CO2 and CH4 on a column packed with Mg-MOF-74 pellets at 308 K and 1 bar indicated a higher CO2 capacity and separation efficiency for the MOP with respect to the benchmark 13X zeolite. The MOP regains 81% of its original capacity when purging with helium at 308 K during 10 mm and can be fully regenerated at 353 K, which indicates the facile desorption of CO2 on this material. However, the performance of Mg-MOF-74 severely deteriorates upon long-term exposure to relevant impurities in CO2 separations at low pressures such as water and oxygen.
The Journal of Physical Chemistry C 05/2013; 117(18):9301-9310. DOI:10.1021/jp401923v · 4.77 Impact Factor