Joeri F. M. Denayer

An Amine-Functionalized MIL-53 Metal−Organic Framework with Large Separation Power for CO2 and CH4

Functionalizing the well-known MIL-53(Al) metal-organic framework with amino groups increases its selectivity in CO(2)/CH(4) separations by orders of magnitude while maintaining a very high capacity for CO(2) capture.

Selective adsorption and separation of ortho-substituted alkylaromatics with the microporous aluminum terephthalate MIL-53.

The metal-organic framework MIL-53(Al) was tested for selective adsorption and separation of xylenes and ethylbenzene, ethyltoluenes, and cymenes using batch, pulse chromatographic, and breakthrough experiments. In all conditions tested, MIL-53 has the largest affinity for the ortho-isomer among each group of alkylaromatic compounds. Separations of the ortho-compounds from the other isomers can be realized using a column packed with MIL-53 crystallites. As evidenced by Rietveld refinements, specific interactions of the xylenes with the pore walls of MIL-53 determine selectivity. In comparison with the structurally similar metal-organic framework MIL-47, the selectivities among alkylaromatics found for MIL-53 are different. Separation of ethyltoluene and cymene isomers is more effective on MIL-53 than on MIL-47; the pores of MIL-53 seem to be a more suitable environment for hosting the larger ethyltoluene and cymene isomers than those of MIL-47.

Pore-filling-dependent selectivity effects in the vapor-phase separation of xylene isomers on the metal-organic framework MIL-47.

Vapor-phase adsorption and separation of the C8 alkylaromatic components p-xylene, m-xylene, o-xylene, and ethylbenzene on the metal-organic framework MIL-47 have been studied. Low coverage Henry adsorption constants and adsorption enthalpies were determined using the pulse chromatographic technique at temperatures between 230 and 290 degrees C. The four C8 alkylaromatic components have comparable Henry constants and adsorption enthalpies. Adsorption isotherms of the pure components were determined using the gravimetric technique at 70, 110, and 150 degrees C. The adsorption capacity and steepness of the isotherms differs among the components and are strongly temperature dependent. Breakthrough experiments with several binary mixtures were performed at 70-150 degrees C and varying total hydrocarbon pressure from 0.0004 to 0.05 bar. Separation of the different isomers could be achieved. In general, it was found that the adsorption selectivity increases with increasing partial pressure or degree of pore filling. The separation at a high degree of pore filling in the vapor phase is a result of differences in packing modes of the C8 alkylaromatic components in the pores of MIL-47.

Separation of Styrene and Ethylbenzene on Metal−Organic Frameworks: Analogous Structures with Different Adsorption Mechanisms

The metal-organic frameworks MIL-47 (V(IV)O{O(2)C-C(6)H(4)-CO(2)}) and MIL-53(Al) (Al(III)(OH)·{O(2)C-C(6)H(4)-CO(2)}) are capable of separating ethylbenzene and styrene. Both materials adsorb up to 20-24 wt % of both compounds. Despite the fact that they have identical building schemes, the reason for preferential adsorption of styrene compared to ethylbenzene is very different for the two frameworks. For MIL-47, diffraction experiments reveal that styrene is packed inside the pores in a unique, pairwise fashion, resulting in separation factors as high as 4 in favor of styrene. These separation factors are independent of the total amount of adsorbate offered. This is due to co-adsorption of ethylbenzene in the space left available between the packed styrene pairs. The separation is of a non-enthalpic nature. On MIL-53, the origin of the preferential adsorption of styrene is related to differences in enthalpy of adsorption, which are based on different degrees of framework relaxation. The proposed adsorption mechanisms are in line with the influence of temperature on the separation factors derived from pulse chromatography: separation factors are independent of temperature for MIL-47 but vary with temperature for MIL-53. Finally, MIL-53 is also capable of removing typical impurities like o-xylene or toluene from styrene-ethylbenzene mixtures.

Separation of C5-Hydrocarbons on Microporous Materials: Complementary Performance of MOFs and Zeolites

This work studies the liquid-phase separation of the aliphatic C(5)-diolefins, mono-olefins, and paraffins, a typical feed produced by a steam cracker, with a focus on the seldomly studied separation of the C(5)-diolefin isomers isoprene, trans-piperylene, and cis-piperylene. Three adsorbents are compared: the metal-organic framework MIL-96, which is an aluminum 1,3,5-benzenetricarboxylate, and two zeolites with CHA and LTA topology. All three materials have spacious cages that are accessible via narrow cage windows with a diameter of less than 0.5 nm. The mechanisms determining adsorption selectivities on the various materials are investigated. Within the diolefin fraction, MIL-96 and chabazite preferentially adsorb trans-piperylene from a mixture containing all three C(5)-diolefin isomers with high separation factors and a higher capacity compared to the reference zeolite 5A due to a more efficient packing of the trans isomer in the pores. Additionally, chabazite is able to separate cis-piperylene and isoprene based on size exclusion of the branched isomer. This makes chabazite suitable for separating all three diolefin isomers. Its use in separating linear from branched mono-olefins and paraffins is addressed as well. Furthermore, MIL-96 is the only material capable of separating all three diolefin isomers from C(5)-mono-olefins and paraffins. Finally, the MOF [Cu(3)(BTC)(2)] (BTC = benzene-1,3,5-tricarboxylate) is shown to be able to separate C(5)-olefins from paraffins. On the basis of these observations, a flow scheme can be devised in which the C(5)-fraction can be completely separated using a combination of MOFs and zeolites.

Biobutanol Separation with the Metal–Organic Framework ZIF‐8

Bioalcohols, such as bioethanol and biobutanol, are a promising alternative to petroleum-based chemicals. As a fuel, biobutanol has superior properties compared to bioethanol, including a higher energy density and a lower volatility. A major challenge in the economical production of biobutanol as chemical or fuel is its separation from the aqueous medium in which it is produced by the fermentation of biomass. Given the low concentration of the alcohols in the fermentation broth, separation of the butanol fraction via distillation would be energyand cost-intensive. Among alternative separation methods to recover butanol from fermentation broth, adsorption has been identified as the most energy-efficient technique. This requires adsorbents that, besides a high adsorption capacity and stability, have a high affinity towards alcohols (typically, the final butanol concentration is at most 20 g L ) and a low affinity for water. Typical adsorbents (i.e. , most zeolites, silica, and alumina) have a high preference to water and so are not suitable for this particular application. Oudshoorn et al. reported that among the commercially available hydrophobic zeolites, silicalitetype zeolites are the most selective for alcohols, but their adsorption capacity remains low. Although active carbon selectively adsorbs alcohols from water, the recovery of adsorbed alcohols is problematic. Metal–organic frameworks (MOFs) offer new opportunities in adsorption technology, with unprecedented capacities and chemical and structural tunability. Herein, it is demonstrated that the MOF ZIF-8, a member of the zeolitic imidazolate framework (ZIF) family, has promising features for the production of pure biobutanol from its fermentation medium. ZIFs contain tetrahedral Zn atoms linked by imidazolate ligands. A large variety of zeolite-like structures can be obtained by modification of the ligands. ZIFs offer high hydrothermal, chemical, and thermal stabilities. ZIF-8, discovered by Huang et al. , crystallizes into the zeolite sodalite topology, generating a resistant structure with cages of 12.5 connected via hexagonal windows of 3.3 . (Figure S2). Adsorption isotherms on ZIF-8 have been reported for Ar, CO2, CH4, N2, C2H6, C2H4, and H2, and also for longer alkanes, alkenes, and organic compounds. b, 7] Molecular simulations have been used to identify the adsorption sites of H2, N2, and CH4. [8] Selective ZIF-8-membranes have been designed, and their permeability for light gasses has been investigated. 9] ZIF-8 shows an only very low [a] J. Cousin Saint Remi, T. R my, V. Van Hunskerken, S. van de Perre, T. Duerinck, Prof. Dr. G. V. Baron, Prof. Dr. J. F. M. Denayer Department of Chemical Engineering Vrije Universiteit Brussel Pleinlaan 2, 1050 Brussel (Belgium) Fax: (+ 32) 2 629 17 98 E-mail : joeri.denayer@vub.ac.be [b] Dr. M. Maes, Prof. Dr. D. De Vos, Dr. E. Gobechiya, Prof. Dr. C. E. A. Kirschhock Centre for Surface Chemistry and Catalysis Katholieke Universiteit Leuven Kasteelpark Arenberg 23, 3001 Heverlee (Belgium) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201100261. Figure 1. Vapor-phase adsorption on ZIF-8. Full symbols: adsorption; open symbols: desorption. a) Adsorption isotherms at 50 8C. b) Adsorption capacity at 50 8C. c) Butanol isotherms at varying temperature. d) Isosteric heat of adsorption as a function of pore filling.

Evidences for pore mouth and key–lock catalysis in hydroisomerization of long n-alkanes over 10-ring tubular pore bifunctional zeolites

Abstract The evidence for the pore mouth catalysis model for n-alkane methylbranching on Pt/H-ZSM-22 hydroisomerization catalyst is reviewed. It is based on adsorption equilibria at catalytic temperatures determined using tracer and perturbation chromatography, reaction product distributions obtained with nC8–nC24 n-alkanes and rival model screening for catalytic conversions. In the Henry regime, methylbranched isomers have lower adsorption entropy and enthalpy compared to n-alkanes explained by the enhanced rotational and translational freedom of methyl and methylene groups positioned outside the pore interacting with the external surface. Adsorption isotherms for isoalkanes are in agreement with dual site adsorption in pore mouths and on external surfaces, respectively. The hydroisomerization can be modeled with a bifunctional reaction scheme and adsorption on the external crystal surfaces and pore mouths. The selectivity for 2-methylbranching reflects the optimum van der Waals interaction of the n-alkane with the zeolite pore and methylbranching in that part of the chain that is located outside the first 10-ring of the zeolite pore to facilitate desorption. Very long n-alkanes (C12+) exhibit key–lock adsorptions and penetrate simultaneously with their two ends into two different pores. Key–lock physisorption leads to branching at more central C atom positions.

Activation of the metal–organic framework MIL-47 for selective adsorption of xylenes and other difunctionalized aromatics

The capacity and selectivity of the metal-organic framework MIL-47 for liquid phase adsorption are shown to heavily depend on the pretreatment of the material, as illustrated in detail by the particular case of selective xylene adsorption. By totally removing the uncoordinated terephthalic acid from the pores and simultaneously avoiding oxidation to nonporous V(2)O(5), pore volume and uptake of xylenes can be maximized. The presence of uncoordinated terephthalic acid in the pores improves the selectivity between p- and m-xylene. Calcination bed thickness and oven geometry influence the optimal calcination procedure. The physicochemical modifications of MIL-47 during its activation are investigated in detail with XRD, SEM, nitrogen physisorption, TGA and diffuse reflectance UV-Vis spectroscopy. Using optimally pretreated MIL-47 as adsorbent for xylene, ethyltoluene, dichlorobenzene, toluidine or cresol isomers, the para-isomer is in each case preferred over the meta-isomer in pulse chromatographic and batch experiments. The role of stacking in the selective adsorption of these isomers is discussed. In the case of the dichlorobenzenes, the meta- and para-isomers can be separated in a breakthrough experiment with a selectivity of 5.0.

Adsorption and Separation of Light Gases on an Amino-Functionalized Metal–Organic Framework: An Adsorption and In Situ XRD Study

The NH(2)-MIL-53(Al) metal-organic framework was studied for its use in the separation of CO(2) from CH(4), H(2), N(2)C(2)H(6) and C(3)H(8) mixtures. Isotherms of methane, ethane, propane, hydrogen, nitrogen, and CO(2) were measured. The atypical shape of these isotherms is attributed to the breathing properties of the material, in which a transition from a very narrow pore form to a narrow pore form and from a narrow pore form to a large pore form occurs, depending on the total pressure and the nature of the adsorbate, as demonstrated by in situ XRD patterns measured during adsorption. Apart from CO(2), all tested gases interacted weakly with the adsorbent. As a result, they are excluded from adsorption in the narrow pore form of the material at low pressure. CO(2) interacted much more strongly and was adsorbed in significant amounts at low pressure. This gives the material excellent properties to separate CO(2) from other gases. The separation of CO(2) from methane, nitrogen, hydrogen, or a combination of these gases has been demonstrated by breakthrough experiments using pellets of NH(2)-MIL-53(Al). The effect of total pressure (1-30 bar), gas composition, temperature (303-403 K) and contact time has been examined. In all cases, CO(2) was selectively adsorbed, whereas methane, nitrogen, and hydrogen nearly did not adsorb at all. Regeneration of the adsorbent by thermal treatment, inert purge gas stripping, and pressure swing has been demonstrated. The NH(2)-MIL-53(Al) pellets retained their selectivity and capacity for more than two years.

Adsorption competition effects in hydroconversion of alkane mixtures on zeolites

In the present work, molecular competition effects in the hydroconversion of alkane mixtures in vapor and liquid phase were studied. The influence of the pore size was investigated by performing catalytic experiments with equimolar heptane/nonane mixtures on a series of bifunctional zeolite catalysts (Pt/H-Y, Pt/H-USY, Pt/H-Beta, Pt/H-MCM-22). Vapor phase catalytic experiments were performed at a total pressure of 4.5 bar, while a total pressure of 100 bar was applied in the liquid phase experiments. The experimental results were analyzed using a lumped adsorption-reaction model. In vapor phase, the longest chain is preferentially converted on all studied catalysts. In liquid phase, the differences in conversion rate were less pronounced. On Pt/H-MCM-22, with active pockets on the surface, and Pt/H-USY having large mesopores, the competition between short and long alkanes in liquid phase reflect the intrinsic reactivities of the reacting molecules. In zeolites with smaller pores (Pt/H-Y, Pt/H-Beta), an inversion of the reactivity order of alkanes of different chain length was observed when increasing the pressure from 4.5 bar and vapor phase to 100 bar and liquid phase. The inversion of apparent reactivity orders is due to changes in physisorption at high pressure, favoring uptake of the smallest molecules.