Lik Hong Wee

Copper Benzene Tricarboxylate Metal–Organic Framework with Wide Permanent Mesopores Stabilized by Keggin Polyoxometallate Ions

Porous solids with organized multiple porosity are of scientific and technological importance for broadening the application range from traditional areas of catalysis and adsorption/separation to drug release and biomedical imaging. Synthesis of crystalline porous materials offering a network of uniform micro- and mesopores remains a major scientific challenge. One strategy is based on variation of synthesis parameters of microporous networks, such as, for example, zeolites or metal-organic frameworks (MOFs). Here, we show the rational development of an hierarchical variant of the microporous cubic Cu(3)(BTC)(2) (BTC = 1,3,5-benzenetricarboxylate) HKUST-1 MOF having strictly repetitive 5 nm wide mesopores separated by uniform microporous walls in a single crystal structure. This new material coined COK-15 (COK = Centrum voor Oppervlaktechemie en Katalyse) was synthesized via a dual-templating approach. Stability was enhanced by Keggin type phosphotungstate (HPW) systematically occluded in the cavities constituting the walls between the mesopores.

Cr-MIL-101 encapsulated Keggin phosphotungstic acid as active nanomaterial for catalysing the alcoholysis of styrene oxide

Mesoporous chromium-based terephthalate metal–organic framework (MIL-101) encapsulated Keggin phosphotungstic acid (HPW) [MIL-101(HPW)] was demonstrated to be an active heterogeneous catalyst for selective catalysis of the ring opening reaction of styrene oxide with methanol, achieving 99% yield of 2-methoxy-2-phenylethanol in 20 minutes at 40 °C. Similar MIL-101 samples prepared using one-pot microwave synthesis in the absence of HPW or in the presence of hydrofluoric acid (HF) were less active. The impact of fluoride and HPW polyanion incorporation on the acidity of MIL-101 was investigated by the in situ infrared spectroscopy technique using CO as a probe molecule. Additional hydroxyl groups and Lewis acid sites are present in MIL-101(HPW) explaining the observed superior catalytic performance in styrene oxide methanolysis.

ZIF-71 as a potential filler to prepare pervaporation membranes for bio-alcohol recovery

Metal–organic frameworks (MOFs) are an emerging class of materials for potential use in separation. ZIF-71 has super-hydrophobic pore channels and a very flexible pore architecture, which make ZIF-71 a potential candidate for recovering bio-alcohols from fermentation broths. In the present study, mixed matrix membranes (MMMs) based on PDMS and ZIF-71 were prepared for separation of alcohols (methanol, ethanol, isopropanol (IPA) or sec-butanol) from aqueous solutions. Experimental results reveal that the pervaporation performance of the PDMS membrane was improved in both flux and separation factors upon embedment of ZIF-71. The pervaporation separation factors for alcohols of ZIF-71 filled the PDMS membrane (PDMS : ZIF-71 = 10 : 2) nearly double compared to those of unfilled PDMS membranes, suggesting that ZIF-71 is an excellent filler to prepare organophilic pervaporation membranes for bio-alcohol recovery.

Hierarchical Zeolitic Imidazolate Framework-8 Catalyst for Monoglyceride Synthesis

Metal–organic frameworks (MOFs) are heterogeneous porous catalysts with unprecedented catalytic functionality via coordinatively unsaturated metal nodes, provoked defects, organic functions of linker molecules, and intrinsic chirality. These materials are amenable to catalytic modification by means of encapsulation of nanoparticles, polyoxometalates, and porphyrins. MOFs have been demonstrated as active heterogeneous catalysts for various organic reactions, including acid–base and redox reactions, such as esterification, transesterification, epoxidation, alcoholysis, Knoevenagel condensation, 6] among others. The introduction of a secondary pore system, demonstrated to be advantageous in zeolite catalysis, is still in its infancy in the world of MOFs and might further lift the potential of MOF materials in catalysis. A very challenging area of heterogeneous catalysis is the synthesis of monoglycerides. Monoglycerides are produced according to two major routes: 1) direct esterification of fatty acid with glycerol and 2) transesterification of triglycerides with glycerol. Monoglycerides are popular natural emulsifiers in food and beverages, personal care products, and pharmaceuticals. According to a recent prediction, the global emulsifier market is primed to reach volume sales of 2.6 million metric tons by the year 2017. Industrial monoglyceride production is currently performed at high temperature (493–523 K) using a basic homogeneous catalyst with limited monoglyceride selectivity, owing to formation of diand triglyceride side products and soap. The high reaction temperature bears the risk of deterioration of taste, aroma, and color of the product. Developing a heterogeneous catalytic process at lower temperature for selective monoglyceride production is a major scientific challenge. We discovered nanoparticles of zeolitic imidazolate framework-8 (ZIF-8) transformed into hierarchical material through reaction with fatty acid to be a promising truly heterogeneous catalyst for monoglyceride synthesis. ZIFs possess excellent thermal stability up to 693 K and a tunable pore architecture. The ZIF-8 structure resembles sodalite with 1.16 nm wide cavities connected through 0.34 nm wide windows formed by four-ring and six-ring ZnN4 clusters. ZIF-8 is a promising material for gas separation, as well as for catalytic applications. ZIF-8 exhibits a unique pressure-induced change of pore size and amorphization. It has attractive tribological properties, and guest molecule trapping behavior. Immobilization of catalytically active nanoparticles, such as Pt, Ni, Au, and Co3O4, is a recent avenue in ZIF catalysis. ZIF-8 originally was synthesized via the solvothermal route from inorganic zinc compound and 2-methylimidazole in dimethylformamide (DMF) solvent and temperatures of 358–423 K. Alternative synthesis methods are precipitation from methanol or water solution at room temperature, steam-assisted synthesis, mechanochemical, and ultrasound treatment. Here we report another facile synthesis of ZIF-8 nanomaterial with large specific surface area and micropore volume. Nanosized ZIF-8 was prepared from a synthesis solution 2.2 times more concentrated than usual (see the Supporting Information for Experimental Details). The reproducibility was excellent. Phase purity was confirmed by XRD (Figure 1 a). The XRD pattern can be indexed with cubic unit cell parameter a = 1.710(5) nm, in agreement with literature. IR spectra further confirmed sample purity (Figure S1). SEM revealed a particle size of about 150 nm qualifying as nanomaterial (Figure 1 b). TGA in nitrogen (Figure S2) revealed this ZIF-8 nanopowder is stable till 723 K. The microporous nature of ZIF-8 is apparent from the Type 1 nitrogen adsorption isotherm (Figure 1 c). The BET and Langmuir specific surface area and micropore volume are 1388 m g , 2110 m g 1 and 0.78 cm g , respectively. These are high values for a nanosized ZIF-8 sample given that there were no chemical additives nor excess solvent used in the synthesis. [a] Dr. L. H. Wee, Prof. J. A. Martens Centre for Surface Science and Catalysis KU Leuven Kasteelpark Arenberg 23, Heverlee, B3001 (Belgium) Fax: (+ 32) 163-21998 E-mail : likhong.wee@biw.kuleuven.be johan.martens@biw.kuleuven.be [b] T. Lescouet, Dr. D. Farrusseng Institut de Recherche sur la Catalyse et l’Environnement de Lyon (IRCELYON), University Lyon 1, CNRS 2 Av. Albert Einstein, Villeurbanne, 69626 (France) [c] J. Ethiraj, Dr. F. Bonino, Prof. S. Bordiga NIS Centre of Excellence and INSTM Reference Centre Department of Chemistry, University of Turin Via Quarello 15, Turin, 10135 (Italy) [d] Dr. R. Vidruk, Prof. M. Herskowitz Blechner Center for Industrial Catalysis and Process Development Ben-Gurion University of the Negev Beer-Sheva (Israel) [e] Dr. E. Garrier, Dr. D. Packet Novance Rue les rives de l’Oise, Venette, B.P. 20609 Compiegne, Cedex, 60206 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201300581.

Recovery and reuse of heteropolyacid catalyst in liquid reaction medium through reversible encapsulation in Cu3(BTC)2 metal–organic framework

The solvent dependent solubility of HKUST-1 metal–organic framework (MOF) with encapsulated Keggin 12-tungstophosphoric acid (HPW@Cu3(BTC)2) was exploited to liberate homogeneous heteropolyacid catalyst in a reaction medium and to recover the catalyst by encapsulation in the MOF upon addition of hexane. The application of the new Keggin type heteropolyacid catalyst recovery concept was demonstrated in the esterification of acetic acid with 1-propanol and salicylic acid with ethanol.

1D-2D-3D Transformation Synthesis of Hierarchical Metal–Organic Framework Adsorbent for Multicomponent Alkane Separation

A new hierarchical MOF consisting of Cu(II) centers connected by benzene-tricarboxylates (BTC) is prepared by thermoinduced solid transformation of a dense CuBTC precursor phase. The mechanism of the material formation has been thoroughly elucidated and revealed a transformation of a ribbon-like 1D building unit into 2D layers and finally a 3D network. The new phase contains excess copper, charge compensated by systematic hydroxyl groups, which leads to an open microporous framework with tunable permanent mesoporosity. The new phase is particularly attractive for molecular separation. Energy consumption of adsorptive separation processes can be lowered by using adsorbents that discriminate molecules based on adsorption entropy rather than enthalpy differences. In separation of a 11-component mixture of C1-C6 alkanes, the hierarchical phase outperforms the structurally related microporous HKUST-1 as well as silicate-based hierarchical materials. Grand canonical Monte Carlo (GCMC) simulation provides microscopic insight into the structural host-guest interaction, confirming low adsorption enthalpies and significant entropic contributions to the molecular separation. The unique three-dimensional hierarchical structure as well as the systematic presence of Cu(II) unsaturated coordination sites cause this exceptional behavior.

Highly selective gas separation membrane using in situ amorphised metal–organic frameworks

Conventional carbon dioxide (CO2) separation in the petrochemical industry via cryogenic distillation is energy intensive and environmentally unfriendly. Alternatively, polymer membrane-based separations are of significant interest owing to low production cost, low-energy consumption and ease of upscaling. However, the implementation of commercial polymeric membranes is limited by their permeability and selectivity trade-off and the insufficient thermal and chemical stability. Herein, a novel type of amorphous mixed matrix membrane (MMM) able to separate CO2/CH4 mixtures with the highest selectivities ever reported for MOF based MMMs is presented. The MMM consists of an amorphised metal–organic framework (MOF) dispersed in an oxidatively cross-linked matrix achieved by fine tuning of the thermal treatment temperature in air up to 350 °C which drastically boosts the separation properties of the MMM. Thanks to the protection of the surrounding polymer, full oxidation of this MOF (i.e. ZIF-8) is prevented, and amorphisation of the MOF is realized instead, thus in situ creating a molecular sieve network. In addition, the treatment also improves the filler-polymer adhesion and induces an oxidative cross-linking of the polyimide matrix, resulting in MMMs with increased stability or plasticization resistance at high pressure up to 40 bar, marking a new milestone as new molecular sieve MOF MMMs for challenging natural gas purification applications. A new field for the use of amorphised MOFs and a variety of separation opportunities for such MMMs are thus opened.

PDMS membranes containing ZIF-coated mesoporous silica spheres for efficient ethanol recovery via pervaporation

The design of functional micro- and mesostructured composite materials is significantly important for separation processes. Mesoporous silica is an attractive material for fast diffusion, while microporous zeolitic imidazolate frameworks (ZIFs) are beneficial for selective adsorption and diffusion. In this work, ZIF-71 and ZIF-8 nanocrystals were grown on the surface of mesoporous silica spheres (MSS) via the seeding and regrowth approach in order to obtain monodispersed MSS–ZIF-71 and MSS–ZIF-8 spheres with a particle size of 2–3 μm. These MSS–ZIF spheres were uniformly dispersed into a polydimethylsiloxane (PDMS) matrix to prepare mixed matrix membranes (MMMs). These MMMs were evaluated for the separation of ethanol from water via pervaporation. The pervaporation results reveal that the MSS–ZIF filled MMMs substantially improve the ethanol recovery in both aspects viz. flux and separation factor. These MMMs outperforms the unfilled PDMS membranes and the conventional carbon and zeolite filled MMMs. As expected, the mesoporous silica core allows very fast flow of the permeating compound, while the hydrophobic ZIF coating enhances the ethanol selectivity through its specific pore structure, hydrophobicity and surface chemistry. It can be seen that ZIF-8 mainly has a positive impact on the selectivity, while ZIF-71 enhances fluxes more significantly.

Stable TiO2–USY zeolite composite coatings for efficient adsorptive and photocatalytic elimination of geosmin from water

Geosmin is an unpleasant tasting germacranoid sesquiterpene to which human taste buds are sensitive even at the parts per trillion (ppt) level. The elimination of this molecule from water is a great scientific challenge due to the extremely low concentration needing to be removed. In this paper, we report simple fabrication of stable titania (TiO2) and Y zeolite composite coatings via a layer-by-layer dipcoating approach for efficient adsorptive and photocatalytic elimination of geosmin up to 99% from 100 ppt geosmin aqueous solution within 120 minutes at 35 � C under UV light. Zeolite coatings are also found to be highly efficient for adsorptive elimination of geosmin at a minute concentration of 4 ppt, the typical concentration level found in aquaculture water. Geosmin concentrations in the ppt range were determined by the headspace solid phase microextraction (HS-SPME) technique in combination with gas chromatography-mass spectrometry (GC-MS). This study demonstrates the potential of a composite coating comprising zeolite Y for concentrating, and TiO2 photocatalyst for eliminating geosmin in drinking water production and aquaculture water purification.

Indirect Observation of Structured Incipient Zeolite Nanoparticles in Clear Precursor Solutions

The structure of the zeolite precursor particles present in clear synthesis solutions yielding colloidal silicalite-1 (purely siliceous zeolite with MFI-type structure) is one of the most controversial issues in modern zeolite science. Owing to the nature of the synthesis and the absence of Al, these systems are widely used as models to study the mechanism of zeolite crystallization in general. Ever since the “nanoslab” hypothesis was proposed byMartens et al. it has been the subject of a lot of criticism. The major disagreement is about the structure of the subcolloidal precursor particles present in the initial clear mixtures. Whereas Martens et al. suggest that these particles have MFI structural features, an amorphous nature of the particles has been proposed in numerous other studies. In some cases, the nanoparticles were extracted from the synthesis solutions, and concerns have been raised about possible changes in their structure or interference from the extraction procedure. 5] Other authors base their arguments against the crystalline nature of the precursor particles on the results of solid-state NMR analysis after Si enrichment. A recent NMR study on aqueous silicate solutions revealed the presence of structural units typical of the MFI structure. Other complementary techniques such as dynamic light scattering have also been used to further characterize the species present in the precursor solutions. Thus, despite the fact that a remarkable number of papers discussing the structure of the silicalite-1 precursor particles have already been published, the discussion is still ongoing and new publications regularly appear. Infrared spectroscopy has also extensively been used to study the structure of the silicalite-1 precursor particles. 7,12, 13] Based on the presence of a band near 550 cm , which is widely accepted to be characteristic of the MFI structure, either crystalline or amorphous character of the precursor particles has been suggested. These IR studies were performed on freeze-dried samples, extracted precursor particles, and original sols. However, the IR spectra published so far showed substantial differences in the 1000–1300 cm 1 spectral range which indicate that specific sampling affects the structural features of the nanoparticles. Recently, IR spectroscopy with synchrotron radiation was used for the first time to study in situ catalytic reactions in zeolite crystals. On the basis of synchrotron-based grazing-incidence reflection-absorption infrared (RAIR) spectra of ultrathin zeolite films on Au surfaces, we give unambiguous evidence for the existence of incipient zeolite nanoparticles in clear precursor solutions: these are subcolloidal particles having all the structural features of the zeolite framework but a size of only a few unit cells which is insufficient to generate a diffraction pattern. The novelty in our study is twofold: sample preparation and method of characterization. First, to separate the subnanoparticles from the solution, we used the Langmuir–Blodgett (LB) method to deposit them on Au substrates. The LB method allows concentration of the subcolloidal particles at the air/water interface as floating films and transfer of these films to the solid supports, which ensures preservation of the native particle structure. Second, we prepared the LB films on Au surfaces in order to characterize the samples by RAIR spectroscopy, which is the most efficient spectroscopic method for structural analysis of ultrathin films. Vibrational spectroscopy has great advantages over diffraction methods in detecting crystalline particles with a size of just 2–3 unit cells, because such particles are large enough to generate phonon modes of the corresponding crystal lattice but too small to give rise to detectible Bragg reflections even when synchrotron X-ray radiation is used. In addition, the vibrational spectra are easy to interpret and can unambiguously distinguish between amorphous and crystalline features, which is sometimes a problem for other surface-sensitive spectroscopic methods. Enhancement of the IR absorption due to the presence of a strongly reflecting noble metal surface allows the detection of even monomolecular layers when grazing-incident RAIR spectroscopy is [*] Dr. L. Tosheva, L. H. Wee, Dr. A. M. Doyle Division of Chemistry and Materials Manchester Metropolitan University Chester St., Manchester M15GD (UK) Fax: (+44)161-247-6357 E-mail: l.tosheva@mmu.ac.uk a.m.doyle@mmu.ac.uk