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Thin films of crystalline and porous metal-organic frameworks (MOFs) have great potential in membranes, sensors, and microelectronic chips. While the morphology and crystallinity of MOF films can be evaluated using widely available techniques, characterizing their pore size, pore volume, and specific surface area is challenging due to the low amount of material and substrate effects. Positron annihilation lifetime spectroscopy (PALS) is introduced as a powerful method to obtain pore size information and depth profiling in MOF films. The complementarity of this approach to established physisorption-based methods such as quartz crystal microbalance (QCM) gravimetry, ellipsometric porosimetry (EP), and Kr physisorption (KrP) is illustrated. This comprehensive discussion on MOF thin film porosimetry is supported by experimental data for thin films of ZIF-8. ispartof: ADVANCED MATERIALS vol:33 issue:17 ispartof: location:Germany status: published

Energy‐efficient indoors temperature and humidity control can be realised by using the reversible adsorption and desorption of water in porous materials. Stable microporous aluminium‐based metal–organic frameworks (MOFs) present promising water sorption properties for this goal. The development of synthesis routes that make use of available and affordable building blocks and avoid the use of organic solvents is crucial to advance this field. In this work, two scalable synthesis routes under mild reaction conditions were developed for aluminium‐based MOFs: (1) in aqueous solutions using a continuous‐flow reactor and (2) through the vapour‐assisted conversion of solid precursors. Fumaric acid, its methylated analogue mesaconic acid, as well as mixtures of the two were used as linkers to obtain polymorph materials with tuneable water sorption properties. The synthesis conditions determine the crystal structure and either the MIL‐53 or MIL‐68 type structure with square‐grid or kagome‐grid topology, respectively, is formed. Fine‐tuning resulted in new MOF materials thus far inaccessible through conventional synthesis routes. Furthermore, by varying the linker ratio, the water sorption properties can be continuously adjusted while retaining the sigmoidal isotherm shape advantageous for heat transformation and room climatisation applications., Going with the flow: A collection of microporous aluminium fumarate‐mesaconate metal–organic frameworks (MOFs) are prepared by using an aqueous flow reactor and vapour‐assisted synthesis. The materials have a variable and, for some, previously unreported crystal structure, and their tuneable water sorption properties depend on the preparation method and linker content.

The chlorine resistance of nanofiltration and reverse osmosis membranes is of high importance in the water treatment industry. Elastic recoil detection (ERD) is now presented as a powerful tool to uniquely provide elemental depth profiles, including hydrogen, of NaOCl-treated polyamide-based thin-film composite (TFC) membranes. The influence of pressure, pH, and chlorine feed concentration on the volume-averaged Cl uptake, the location of chlorine throughout the membrane, and the z-gradient in the Cl/N ratio is demonstrated. The results suggest that (i) higher volume-averaged Cl uptakes are achieved at higher chlorine doses and at acidic pH; (ii) chlorination is mostly restricted to the top layer; (iii) a gradient in the Cl/N ratio exists along the membrane depth; and (iv) the shape of this gradient is influenced by the chlorination pH and the applied pressure. Conclusions on the chlorination mechanisms could also be deduced. Conversely, no conclusive relationships between H fractions and Cl uptake could be drawn, even though changes in the H content after chlorination were observed. To corroborate these results and fully exploit the potential of ERD, the exact microstructure of the (chlorinated) TFC membranes should be better understood.

© 2019 Elsevier B.V. Membrane technology is currently still excluded from separations in more aggressive feeds due to limited chemical robustness. To extent its applicability, a novel thin-film composite (TFC)membrane was synthesized via the homopolymerization of epoxide monomers, resulting in robust poly(epoxyether)top-layers with >90% rose bengal (MW = 1017 Da)and 70% methyl orange (MW = 327 Da)retention with reasonable water fluxes (>2 L m -2 h −1 bar −1 ). The superior chemical stability of this novel nanofiltration membrane type was proven via treatments in pH 1 and 500 ppm NaOCl (pH 4)for, respectively, 48 h and 2.5 h, after which an unchanged or even improved membrane performance was observed. Additionally, the synthesis of the thin top-layer occurred via an interfacial initiation of the polymerization (IIP), rather than via state-of-the-art interfacial polymerization (IP). This IIP approach allowed to convert well-known monophasic bulk epoxide polymerization (commonly used in e.g. the automotive and coating industry), into the synthesis of thin, yet cross-linked top-layers. ispartof: JOURNAL OF MEMBRANE SCIENCE vol:582 pages:442-453 status: published

© 2019 Elsevier Ltd A set of new stiff and contorted polyimides were prepared to systematically link polymer structure with gas separation properties. Seven novel bridged binaphthalene polyimides were synthesized for this purpose using commercial dianhydrides pyromellitic dianhydride (PMDA), biphenyl-tetracarboxylic acid dianhydride (BPDA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA). From these polyimides, freestanding membranes were prepared and tested on mixed gas CO 2 /CH 4 and CO 2 /N 2 separation performance. They were further characterized using WAXS and PALS and through the simulation of their free volume elements (FVEs) and gas separation properties. The binaphthalene polyimides had CO 2 /CH 4 and CO 2 /N 2 mixed gas selectivities of respectively 40 and 29 and mixed gas permeabilities of about 20 barrer for both gas mixtures. ispartof: EUROPEAN POLYMER JOURNAL vol:114 pages:134-143 status: published

The landscape of possible polymorphs for some metal-organic frameworks (MOFs) can pose a challenge for controlling the outcome of their syntheses. Demonstrated here is the use of a template to control in the vapor-assisted formation of zeolitic imidazolate framework (ZIF) powders and thin films. Introducing a small amount of either ethanol or dimethylformamide vapor during the reaction between ZnO and 4,5-dichloroimidazole vapor results in the formation of the porous ZIF-71 phase, whereas other conditions lead to the formation of the dense ZIF-72 phase or amorphous materials. Time-resolved in situ small-angle X-ray scattering reveals that the porous phase is metastable and can be transformed into its dense polymorph. This transformation is avoided through the introduction of template vapor. The porosity of the resulting ZIF powders and films was studied by N2 and Kr physisorption, as well as positron annihilation lifetime spectroscopy. The templating principle was demonstrated for other members of the ZIF family as well, including the ZIF-7 series, ZIF-8_Cl, and ZIF-8_Br. ispartof: ANGEWANDTE CHEMIE-INTERNATIONAL EDITION vol:60 issue:14 pages:7553-7558 ispartof: location:Germany status: published

International audience; State-of-the-art desalination and water purification processes use reverse osmosis and nanofiltration membranes. Their thin polyamide (PA) top-layers ensure concurrent high water permeances and salt rejections, but are also intrinsically sensitive to chlorine, originating from disinfectant added upstream. The chlorine resistance of PA-based membranes has been thoroughly studied at lab-scale, as opposed to industrial-scale membrane modules, where fundamental studies are lacking. Therefore, to better understand chlorine-induced changes in membrane performance and physicochemical properties at industrial scale, chlorination of commercial 8” elements was conducted at different pH (4-7-10) in pressurized modules with low chlorine concentrations (0, 1, 20, 50 ppm NaOCl) during 2.5 h. After 50 ppm acidic chlorination, water permeability decreased (−40%) but salt rejection increased significantly (+0.4%, i.e., salt passage decreased with −78.8%). Boron (+27%) and isopropanol (+8%) rejection also increased. Chlorination with 20 ppm NaOCl at pH 7 and with 50 ppm NaOCl at pH 10 caused boron rejection to drop with −17% and −33%, respectively, but had negligible influence on isopropanol rejection. However, neutral and alkaline chlorination drastically improved water permeability with +40% and salt rejection with +0.6% (i.e., salt passage decreased with −66.9%), approaching and in some cases even slightly exceeding the salt/water permselectivity limit. It can thus be concluded that, under controlled conditions, chlorination can boost the performance of membrane modules. Significant changes in the membrane physicochemical properties were observed at pH 4. At pH 7 and pH 10, a low chlorine-uptake in the PA network was observed, although no significant PA deterioration was observed with XPS and ATR-FTIR. This study is the first to fundamentally investigate chlorination of PA-based real-scale membrane modules as a function of feed pH. Furthermore, it provides a promising strategy to boost membrane performance at real scale and highlights the importance of chlorination conditions.

© 2019 Elsevier B.V. Bipyridine-based UiO-67 Metal-Organic Frameworks (MOFs) are for the first time successfully applied as filler material in Matrimid mixed-matrix membranes (MMMs). Extensive characterization of fillers (via XRD, NMR, SEM, ATR-FTIR, CO 2 and N 2 physisorption) and membranes (via XRD, SEM, ATR-FTIR, DSC, TGA and PALS) were performed to gain insight in the MMM separation behavior. Lewis basic sites in the MMMs function as CO 2 -carriers, doubling CO 2 /CH 4 selectivity (from 38 for pristine Matrimid to 75 for the MMM with 10 wt% UiO-67-33), while the CO 2 permeability was simultaneously improved from 16 Barrer (Matrimid) up till 26 Barrer for the MMM with 10 wt% UiO-67-33 (+63%). Gas permeation experiments suggested a facilitated transport mechanism to be responsible for these high mixed-gas selectivities. The concentration of the filler inside the membrane could be increased to 30 wt%, resulting in a permeability increase of 350% without losing selectivity compared to the Matrimid membrane. Use of N-heterocyclic ligands proved to be an interesting strategy and good alternative for functionalization of MOF surfaces to obtain improved membrane performance. ispartof: JOURNAL OF MEMBRANE SCIENCE vol:576 pages:78-87 status: published

Ion-exchange membrane (IEM)-based processes are used in the industry or in the drinking water production to achieve selective separation. The transport mechanisms of organic solutes/micropollutants (i.e., paracetamol, clofibric acid, and atenolol) at a single-membrane level in diffusion cells were similar to that of salts (i.e., diffusion, convection, and electromigration). The presence of an equal concentration of salts at both sides of the membrane slightly decreased the transport of organics due to lower diffusion coefficients of organics in salts and the increase of hindrance and/or decrease of partitioning in the membrane phase. In the presence of a salt gradient, diffusion was the main transport mechanism for non-charged organics, while the counter-transport of salts promoted the transport of charged organics through electromigration (electroneutrality). Conversely, the co-transport of salts hindered the transport of charged organics, where diffusion was the main transport mechanism of the latter. Although convection played a role in the transport of non-charged organics, its influence on the charged solutes was minimal due to the dominant electromigration. Positron annihilation lifetime spectroscopy showed a bimodal size distribution of free-volume elements of IEMs, with both classes of free-volume elements contributing to salt transport, while larger organics can only transport through the larger class. ispartof: WATER RESEARCH vol:190 ispartof: location:England status: published

The applicability of state-of-the-art water purification membranes in harsh feed streams is limited due to their insufficient chemical robustness. Epoxide chemistry has been recently introduced to achieve pH- and chlorine-stable nanofiltration (NF). This study further investigated the influence of interfacial polymerization (IP) synthesis parameters on the resulting epoxide-based thin-film composite (TFC) membrane structure and performance. Epoxide polymerization could be initiated by N,N,N′,N′-tetramethyl-1,6-hexanediamine and NaOH. Surprisingly, neither the type of initiator, nor the initiator concentration used during IP, influenced membrane rejection of rose bengal (RB) dye (1017 g mol-1), which was constant at ~ 90%. This consistent RB rejection was primarily determined by annealing of the cross-linked polyimide support by toluene, which is the solvent used during IP. In contrast, the poly(epoxyether) top-layer determined membrane selectivity for methyl orange, a smaller dye of 327 g mol-1. The effect of solvent annealing of the membrane support by diethyl carbonate, dimethyl sulfoxide and m-xylene was also investigated and revealed that the changes induced by solvent contact are physical rather than chemical in nature. This study shows, for the first time, the substantial direct impact of the solvents used during IP to influence support properties and the resulting membrane performance. Solvent annealing can therefore be considered as a tool in membrane fabrication—during IP or as a post-treatment step—to further tune the separation performance for specific applications.

Membrane deformation under an applied hydraulic pressure, often termed compaction, is observed in almost all pressure-driven membrane processes. Most notably, compaction decreases water permeability in conventional reverse osmosis (RO) and is expected to critically hinder high-pressure reverse osmosis (HPRO) for hypersaline brine desalination. In this work, we demonstrated that compaction decreases the water permeability of commercial RO membranes from 2.0 L m−2 h−1 bar−1 at 70 bar applied hydraulic pressure to 1.3 L m−2 h−1 bar−1 at 150 bar. The morphological effects of compaction were primarily associated with changes in the support layer, where a ~60% decrease in cross-sectional thickness is observed following compaction at 150 bar hydraulic pressure. In contrast, positron annihilation lifetime spectroscopy demonstrates that the selective layer does not compact irreversibly. The mechanism that drives compaction was found to be the difference in hydraulic pressure across the interface of the selective and support layers. We further found that compaction can reduce the support layer surface porosity by up to ~95%. This decreased porosity is identified as the cause for compaction-induced water permeability decline, while the intrinsic permeability of the selective layer is not influenced by compaction. As such, we conclude that compaction of the support layer has an inextricable impact on composite membrane performance. Finally, we propose recommendations for developing compaction-resistant membranes that can maintain high water permeability, and thus good desalination performance, in high-pressure membrane applications, such as HPRO.

Chemistry of materials XX(XX), acs.chemmater.9b03435 (2019). doi:10.1021/acs.chemmater.9b03435, Robust and scalable thin film deposition methods are key to realize the potential of metal-organic frameworks (MOFs) in electronic devices. Here, we report the first integration of the chemical vapor deposition (CVD) of MOF coatings in a custom reactor within a cleanroom setting. As a test case, the MOF-CVD conditions for ZIF-8 are optimized to enable smooth, pinhole-free, and uniform thin films on full 200 mm wafers under mild conditions. The single-chamber MOF-CVD process and the impact of the deposition parameters are elucidated via a combination of in situ monitoring and ex situ characterization. The resulting process guidelines will pave the way for new MOF-CVD formulations and a plethora of MOF-based devices., Published by American Chemical Society, Washington, DC

Metal-organic frameworks (MOFs) enable the design of host-guest systems with specific properties. In this work, we show how the confinement of anthracene in a well-chosen MOF host leads to reversible yellow-to-purple photoswitching of the fluorescence emission. This behavior has not been observed before for anthracene, either in pure form or adsorbed in other porous hosts. The photoresponse of the host-guest system is caused by the photodimerization of anthracene, which is greatly facilitated by the pore geometry, connectivity, and volume as well as the structural flexibility of the MOF host. The photoswitching behavior was used to fabricate photopatternable and erasable surfaces that, in combination with data encryption and decryption, hold promise in product authentication and secure communication applications. ispartof: ANGEWANDTE CHEMIE-INTERNATIONAL EDITION vol:58 issue:8 pages:2423-2427 ispartof: location:Germany status: published

© 2018 American Chemical Society. It is generally accepted in membrane technology that crystalline zones in polymeric membranes do not contribute to transport of liquids or gases. In the current study, poly(3-alkylthiophene)s (P3ATs), i.e., homopolymers and random copolymers, were synthesized to study the influence of the supramolecular organization on membrane gas separations. The monomers were polymerized via KCTCP, and GPC analysis shows that the polymers have a narrow dispersity. DSC analysis of the polymers reveals that the homopolymers, in contrast to the copolymers, crystallized, confirming their higher degree of supramolecular organization. This was supported by UV-vis absorption spectra of the polymer films, where a red-shift and a characteristic shoulder absorption peak around 600 nm were observed for the homopolymers, while absent for the copolymers. More surprisingly, the homopolymers were found to be 2 orders of magnitude more permeable to CO2 than the copolymers and also more selective. ispartof: MACROMOLECULES vol:51 issue:23 pages:9943-9950 status: published

Chlorination remains a big hurdle in membrane technology as the most commonly used membranes for water purification consist of a polyamide top-layer, which is not fully resistant towards chlorine-induced oxidation. In this work, DOW FILMTECTM BW30 membrane elements were systematically chlorinated with NaOCl at pilot-scale under acidic conditions (pH4) at 10 bar for 2.5 h. Variations in membrane performance and their physico- chemical properties were determined by ATR-FTIR, XPS, WD-XRF, SEM, AFM and zeta-potential measurements. With increasing bleaching concentration, both membrane roughness and chlorine incorporation via N- and ring- chlorination increased, while surface charge remained quasi unaltered. Both water flux and salt passage de- creased proportionally over the whole concentration range. Accordingly, positron annihilation lifetime spec- troscopy (PALS) revealed a decrease in the size of the top-layer free-volume elements as chlorine concentration increased, confirming, for the first time in a quantitative manner, the so-called tightening effect. The obtained results also show that thin-film composite (TFC) membranes are altered differently when chlorinated under pressure than via simple immersion, as conventionally performed in literature. ispartof: Journal of Membrane Science vol:551 pages:123-135 status: published

An advanced synthesis protocol for the preparation of Thin-Film Nanocomposite (TFN) membranes is presented. The Evaporation-controlled Filler Positioning (EFP) method carefully pre-positions the filler (ZIF-8 nanoparticles) exactly at the water/solvent interface before the initiation of the interfacial polymerization. A permeance increase of 220% compared to the counterpart without filler is observed, corresponding to the highest ever reported permeance increase upon filler incorporation for MOF-based TFN membranes without rejection loss. Moreover, optimal membrane performance is already obtained at a MOF concentration as low as 0.005 w/v%, which represents an 80 times reduction in the required amount of filler compared to other reported ZIF-8 based TFN membranes. This explicitly proves the importance of pre-positioning the filler right where it is needed, i.e. at the organic/water interface, before the interfacial polymerization. ZIF-8 based TFN membranes are prepared via both the EFP and the conventional synthesis method and a comparison between both protocols was made. Additionally, the influence of the size of the filler is investigated. All prepared membranes are further characterized using SEM, TEM, ATR-FTIR and XPS. The EFP-method is found to lead to better and more reproducible filtration performances.

Incorporation of MOFs in interfacially polymerized Thin-Film Nanocomposite (TFN) membranes has widely been shown to result in increased membrane performance. However, the exact functioning of these membranes is poorly understood as large variability in permeance increase, filler incorporation and rejection changes can be observed in literature. The synthesis and functioning of TFN membranes (herein exemplified by ZIF-8 filled polyamide (PA) membranes prepared via the EFP method) was investigated via targeted membrane synthesis and thorough characterization via STEM-EDX, XRD and PALS. It is hypothesized that the acid generated during the interfacial polymerization (IP) at least partially degrades the crystalline, acid-sensitive ZIF-8 and that this influences the membrane formation (through so-called secondary effects, i.e. not strictly linked to the pore morphology of the MOF). Nanoscale HAADF-STEM imaging and STEM-EDX Zn-mapping revealed no ZIF-8 particles but rather the presence of randomly shaped regions with elevated Zn-content. Also XRD failed to show the presence of crystalline areas in the composite PA films. As the addition of the acid-quenching TEA led to an increase in the diffraction signal observed in XRD, the role of the acid was confirmed. The separate addition of dissolved Zn2+ to the synthesis of regular TFC membranes showed an increase in permeance while losing some salt retention, similar to observations regularly made for TFN membranes. While the addition of a porous material to a TFC membrane is a straightforward concept, all obtained results indicate that the synthesis and performance of such composite membranes is often more complex than commonly accepted.

Following current strong demands from, among others, paper, food and mining industries, a novel type of nanofiltration membrane was developed, which displays excellent performance in terms of selectivity/flux with a unique combination of chemical stability over the full (0-14) pH-range and thermal stability up to 120 °C. The membrane consists of polyvinylidene fluoride grafted with polystyrene sulfonic acid. The optimum membrane showed water permeances of 2.4 L h-1 m-2 bar-1 while retaining NaCl, MgSO4 and Rhodamine B (479 Da) for respectively ≈60%, ≈80% and >96%. ispartof: RSC ADVANCES vol:8 issue:16 pages:8813-8827 ispartof: location:England status: published

A simple solvent- and catalyst-free method is presented for the synthesis of the large-pore metal-organic framework (MOF) MAF-6 (RHO-Zn(eIm)2) based on the reaction of ZnO with 2-ethylimidazole vapor at temperatures ≤100 °C. By translating this method to a chemical vapor deposition (CVD) protocol, crystalline films of a large-pore material could be deposited for the first time entirely from the vapor phase. A combination of positron annihilation lifetime spectroscopy (PALS) and Kr physisorption measurements confirmed the porosity of these MOF-CVD films and the size of the MAF-6 supercages (diameter ∼2 nm), in close agreement with powder data and calculations. MAF-6 powders and films were further characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), pair distribution function (PDF), and extended X-ray absorption fine structure (EXAFS). The exceptional uptake capacity of MAF-6 in comparison to ZIF-8 is demonstrated by vapor-phase loading of a molecule larger than the ZIF-8 windows.