Frederik Vermoortele

Synthesis Modulation as a Tool To Increase the Catalytic Activity of Metal–Organic Frameworks: The Unique Case of UiO-66(Zr)

The catalytic activity of the zirconium terephthalate UiO-66(Zr) can be drastically increased by using a modulation approach. The combined use of trifluoroacetic acid and HCl during the synthesis results in a highly crystalline material, with partial substitution of terephthalates by trifluoroacetate. Thermal activation of the material leads not only to dehydroxylation of the hexanuclear Zr cluster but also to post-synthetic removal of the trifluoroacetate groups, resulting in a more open framework with a large number of open sites. Consequently, the material is a highly active catalyst for several Lewis acid catalyzed reactions.

Interfacial synthesis of hollow metal–organic framework capsules demonstrating selective permeability

Metal–organic frameworks (MOFs) are a class of crystalline materials that consist of metal ions and organic ligands linked together by coordination bonds. Because of their porosity and the possibility of combining large surface areas with pore characteristics that can be tailored, these solids show great promise for a wide range of applications. Although most applications currently under investigation are based on powdered solids, developing synthetic methods to prepare defect-free MOF layers will also enable applications based on selective permeation. Here, we demonstrate how the intrinsically hybrid nature of MOFs enables the self-completing growth of thin MOF layers. Moreover, these layers can be shaped as hollow capsules that demonstrate selective permeability directly related to the micropore size of the MOF crystallites forming the capsule wall. Such capsules effectively entrap guest species, and, in the future, could be applied in the development of selective microreactors containing molecular catalysts. The intrinsically hybrid nature of metal–organic frameworks (MOFs) — microporous crystalline solids composed of metal ions and organic ligands — has been exploited to grow thin MOF films at the aqueous–organic interface of a biphasic reaction mixture. These materials exhibit selective permeability and can also be obtained as hollow capsules that have potential as microreactors.

Electronic effects of linker substitution on Lewis acid catalysis with metal-organic frameworks.

Functionalized linkers can greatly increase the activity of metal-organic framework (MOF) catalysts with coordinatively unsaturated sites. A clear linear free-energy relationship (LFER) was found between Hammett σ(m) values of the linker substituents X and the rate k(X) of a carbonyl-ene reaction. This is the first LFER ever observed for MOF catalysts. A 56-fold increase in rate was found when the substituent is a nitro group.

Iron(III)-Based Metal–Organic Frameworks As Visible Light Photocatalysts

Herein, a new group of visible light photocatalysts is described. Iron(III) oxides could be promising visible light photocatalysts because of their small band gap enabling visible light excitation. However, the high electron-hole recombination rate limits the yield of highly oxidizing species. This can be overcome by reducing the particle dimensions. In this study, metal-organic frameworks (MOFs), containing Fe3-μ3-oxo clusters, are proposed as visible light photocatalysts. Their photocatalytic performance is tested and proven via the degradation of Rhodamine 6G in aqueous solution. For the first time, the remarkable photocatalytic efficiency of such Fe(III)-based MOFs under visible light illumination (350 up to 850 nm) is shown.

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.

Metal–organic frameworks as catalysts: the role of metal active sites

In this perspective we first critically compare the use of metal–organic frameworks (MOFs) as supports for catalytic functions with the possibilities offered by other classes of porous materials. We then discuss the incidental or deliberate formation of active sites in MOF lattices, and review some strategies to control the number and activity of these sites, ultimately resulting in MOF catalysts with improved performance.

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

p-Xylene-Selective Metal-Organic Frameworks: A Case of Topology-Directed Selectivity

Para-disubstituted alkylaromatics such as p-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal-organic frameworks: MIL-125(Ti) ([Ti(8)O(8)(OH)(4)(BDC)(6)]), MIL-125(Ti)-NH(2) ([Ti(8)O(8)(OH)(4)(BDC-NH(2))(6)]), and CAU-1(Al)-NH(2) ([Al(8)(OH)(4)(OCH(3))(8)(BDC-NH(2))(6)]) (BDC = 1,4-benzenedicarboxylate). Their unique structure contains octahedral cages, which can separate molecules on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating molecules by molecular sieving. These experimental data are in line with predictions by molecular simulations. Additional adsorption and microcalorimetric experiments provide insight in the complementary role of the two cage types in providing the para selectivity.

Active site engineering in UiO-66 type metal–organic frameworks by intentional creation of defects: a theoretical rationalization

The catalytic activity of the Zr-benzenedicarboxylate (Zr-BDC) UiO-66 can be drastically increased if some BDC linkers are missing, as this removes the full coordination of the framework metal ions. As a result, metal centers become more accessible and thus more active for Lewis acid catalysed reactions. Addition of modulators (MDL) to the synthesis mixture can create more linker deficiencies (Vermoortele et al., J. Am. Chem. Soc., 2013, 135, 11465) and leads to a significant increase in the catalytic activity due to the creation of a larger number of open sites. In this paper, we rationalize the function of the modulators under real synthesis conditions by the construction of free energy diagrams. The UiO-66 type materials form a very appropriate test case as the effect of addition of modulators hydrochloric acid (HCl) and trifluoroacetate (TFA) has been intensively investigated experimentally for the synthesis process and post-synthetic thermal activation. Under synthesis conditions, direct removal of BDC linkers requires a high free energy, but replacement of such linker by one or more TFA species might occur especially at high TFA : BDC ratios in the reaction mixture. Post-synthesis activation procedures at higher temperatures lead to substantial removal of the species coordinated to the Zr bricks, creating open metal sites. A mechanistic pathway is presented for the dehydroxylation process of the hexanuclear Zr cluster. For the citronellal cyclization, we show that the presence of some residual TFA in the structure may lead to faster reactions in complete agreement with the experiment. Hirshfeld-e partial charges for the Zr ions have been computed to investigate their sensitivity to substituent effects; a strong correlation with the experimental Hammett parameters and with the rates of the citronellal cyclization is found. The theoretical rationalization may serve as a basis for detailed active site engineering studies.