Luc Alaerts

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.

Direct Patterning of Oriented Metal–Organic Framework Crystals via Control over Crystallization Kinetics in Clear Precursor Solutions

[*] Prof. D. E. De Vos, R. Ameloot, Dr. E. Gobechiya, Prof. J. A. Martens, Dr. L. Alaerts, Prof. B. F. Sels Department of Microbial and Molecular Systems Center for Surface Chemistry and Catalysis Katholieke Universiteit Leuven Kasteelpark Arenberg 23, B-3001 Leuven (Belgium) E-mail: Dirk.DeVos@biw.kuleuven.be Dr. H. Uji-i, Prof. J. Hofkens Department of Chemistry Katholieke Universiteit Leuven Celestijnenlaan 200F, B-3001 Leuven (Belgium)

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.

Selective Removal of N‐Heterocyclic Aromatic Contaminants from Fuels by Lewis Acidic Metal–Organic Frameworks

Fossil fuels, such as diesel or gasoline, are blends of aromatic and aliphatic compounds that contain significant levels of heterocyclic aromatic contaminants. These contaminants have to be removed for environmental reasons. One of the most important issues is the presence of sulfur compounds, such as thiophene (TPH), benzothiophene (BT), and dibenzothiophene (DBT) in fuel feeds, which lead to the formation of SOx exhaust gases and eventually to acid rain. As environmental legislation becomes more stringent on SOx exhaust levels, it is imperative to keep lowering the sulfur concentrations to currently 10 ppmw S (parts per million by weight of sulfur) or less. The main industrial process is hydrodesulfurization (HDS) in which sulfur compounds are hydrogenated to hydrocarbons and H2S over typically a CoMo catalyst. However, nitrogen compounds, such as (substituted) indoles and carbazoles, which are also present in fossil fuels, compete for the active sites on these HDS catalysts, preventing a deep HDS. In the absence of nitrogen compounds, deep HDS can easily produce fuels with sulfur levels well below 10 ppmw, for instance by using the newest generations of materials based on Mo-W-Ni, which can lower sulfur levels to 5 ppmw. As the eventual aim is to have sulfur-free fuel, even these low concentrations will have to be removed. A promising way to selectively remove nitrogen contaminants would be adsorption on a microporous material. Efficient purification can be performed by adsorption as long as the interaction between the adsorbate and the adsorbent is relatively strong. A CuY zeolite has been described as a potential adsorbent for the removal of nitrogen compounds by p complexation, but the maximal capacity at saturation only amounted to 3 mg N per gram of adsorbent, and moreover sulfur compounds are adsorbed as well. An ideal adsorbent for such application should be easy to synthesize, stable in the given feed compositions, possess pores that are large enough to accommodate bulky organic molecules, such as carbazoles, have a sufficient capacity, and be highly selective for nitrogen over sulfur compounds. Metal–organic frameworks (MOFs) are an emerging class of highly porous materials, formed of inorganic subunits and organic linkers that bear multiple complexing functions (for example, carboxylates, phosphonates, and others), which enables a unique variety of potential interactions inside the pores. To date, they have been successfully used as adsorbents for the capture of greenhouse gases, such as CO2 and CH4, and in liquid-phase separations such as those of alkylaromatics and styrene, olefins and paraffins, and for fuel and water purification by adsorption of organic pollutants. Herein, we propose the use of mesoporous metal carboxylates with different topologies and compositions for the selective adsorption of nitrogen contaminants. These heterocyclic contaminants are found in fuel feeds that are typically aliphatic with a minor aromatic fraction. This system is simulated herein by using a solvent composed of heptane/toluene in a volumetric ratio of 80:20 (labeled hereafter as H/T). Specifically, the adsorptive removal of indole (IND), 2-methylindole (2MI), 1,2-dimethylindole (1,2DMI), carbazole (CBZ), and N-methylcarbazole (NMC) as well as of TPH, BT, and DBT has been studied. These molecules are the most important heterocyclic contaminants in fuel feeds. To study the influence of the toluenecontaining solvent on the adsorption and on the interaction strength between the host and the adsorbate, the adsorption of the contaminants has also been studied using a toluene/ [*] M. Maes, S. Schouteden, F. Vermoortele, Dr. L. Alaerts, Prof. Dr. D. E. De Vos Centre for Surface Chemistry and Catalysis Katholieke Universiteit Leuven Kasteelpark Arenberg 23, 3001 Leuven (Belgium) Fax: (+ 32)16-321-998 E-mail: dirk.devos@biw.kuleuven.be

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.

Tuning the catalytic performance of metal–organic frameworks in fine chemistry by active site engineering

The effect of a post-synthetic acid treatment on the catalytic performance of MOFs is evaluated for MIL-100(Fe), an iron-benzenetricarboxylate. The acid-treated frameworks are structurally robust as no differences have been found in XRD patterns after treatment. Porosity of the acid-treated MOFs gradually decreases, most probably as a consequence of anions remaining in the charged frameworks. Monitoring the modification of the MOFs by reactions of which the outcome depends on the acid properties of the catalyst suggests the presence of two types of active sites, with weak Bronsted acid sites in close vicinity to the Lewis acid open metal sites. This is supported by CO-chemisorption experiments which indicate a large increase of both Lewis and Bronsted acidity. In Diels–Alder reactions of oxygenated dienophiles with 1,3-cyclohexadiene, a strong increase of the activity is found for the acid-treated MOFs. This is explained by the enhanced activation of the dienophiles on the modified active sites.

Recent progress in the immobilization of catalysts for selective oxidation in the liquid phase

Reliable strategies are presented for the immobilization of molecular catalysts for selective oxidation in the liquid phase. Besides classical strategies such as ion exchange or covalent anchoring, new approaches are emerging, e.g. based on supported ionic-liquid phases or on incorporation of the active centre in a coordination polymer or a metal-organic framework.

Extracting organic contaminants from water using the metal–organic framework CrIII(OH)·{O2C–C6H4–CO2}

The water-stable metal-organic framework MIL-53(Cr) is able to adsorb phenol and p-cresol from contaminated water as well as the monomeric sugar D-(-)-fructose. Based on the isotherm for phenol uptake from the liquid phase, it is proposed that the framework breathes to maximize the uptake.