Ivo Stassen

Chemical vapour deposition of zeolitic imidazolate framework thin films

Integrating metal-organic frameworks (MOFs) in microelectronics has disruptive potential because of the unique properties of these microporous crystalline materials. Suitable film deposition methods are crucial to leverage MOFs in this field. Conventional solvent-based procedures, typically adapted from powder preparation routes, are incompatible with nanofabrication because of corrosion and contamination risks. We demonstrate a chemical vapour deposition process (MOF-CVD) that enables high-quality films of ZIF-8, a prototypical MOF material, with a uniform and controlled thickness, even on high-aspect-ratio features. Furthermore, we demonstrate how MOF-CVD enables previously inaccessible routes such as lift-off patterning and depositing MOF films on fragile features. The compatibility of MOF-CVD with existing infrastructure, both in research and production facilities, will greatly facilitate MOF integration in microelectronics. MOF-CVD is the first vapour-phase deposition method for any type of microporous crystalline network solid and marks a milestone in processing such materials.

Solvent-free synthesis of supported ZIF-8 films and patterns through transformation of deposited zinc oxide precursors

Film processing and patterning techniques are a prerequisite to fully exploit the potential of metal–organic frameworks (MOFs) in integrated applications. We report a solvent-free approach for the synthesis of ZIF-8 thin films and patterns through the reaction of ZnO films with melted 2-methylimidazole.

Improving the mechanical stability of zirconium-based metal–organic frameworks by incorporation of acidic modulators

The ability to retain structural integrity under processing conditions which involve mechanical stress, is essential if metal–organic frameworks (MOFs) are to fulfil their potential as serious candidates for use in gas sorption, separation, catalysis and energy conversion applications. A series of zirconium dicarboxylates, predicted to be amongst the more mechanically robust MOFs, have been found to undergo rapid collapse upon ball-milling, resulting in catastrophic losses of porosity. An inverse relationship between collapse time and framework porosity has been found. Addition of acidic modulator ligands (e.g. trifluoroacetic acid) to UiO-66 provided a striking increase in mechanical robustness, the degree of which is inversely related to modulator pKa. This effect, caused by an increased strength of the zirconium–carboxylate bond, provides an important concept to design microporous hybrid frameworks capable of retaining their structure under harsh processing conditions.

Green synthesis of zirconium-MOFs

The synthesis of Zr-MOFs under green, industrially feasible conditions was investigated. Two new compounds with bcu-topology and the fluorinated analogue of UiO-66 exhibiting fcu-topology were obtained and characterised. All products exhibit permanent porosity. In the bcu-frameworks the interaction with sulfate anions apparently induces an unusual eightfold connectivity of the Zr cluster.

Mechanical properties of electrochemically synthesised metal–organic framework thin films

We investigated the mechanical properties of metal–organic framework thin-film coatings grown by an electrochemical method, which allows fast deposition in environmentally friendly solvents. For the first time, Cu(CHDA) and Cu(INA)2 are electrochemically synthesised as dense coatings on Cu-electrodes, alongside the well-known Cu3(BTC)2 (CHDA = trans-cyclohexane-1,4-dicarboxylate; INA = isonicotinate; BTC = benzene-1,3,5-tricarboxylate). In order to probe the mechanical behaviour of the MOF coatings, both nanoindentation and nanoscratch experiments are performed. The indentation of a polycrystalline film allows the determination of average Youngs moduli and hardness of the coatings. Cu(CHDA) exhibits the highest stiffness and hardness, with values of 10.9 GPa and 0.46 GPa, respectively. Intermediate values are obtained for the well-known Cu3(BTC)2 and the smallest values for Cu(INA)2. A close inspection of the crystal lattice of the MOF materials under investigation allows for correlating the mechanical properties and structural building units of these materials. Finally, the effect of the fundamental mechanical properties of MOF films on their scratch and wear resistance is illustrated.

Vapor-Phase Deposition and Modification of Metal–Organic Frameworks: State-of-the-Art and Future Directions

Materials processing, and thin-film deposition in particular, is decisive in the implementation of functional materials in industry and real-world applications. Vapor processing of materials plays a central role in manufacturing, especially in electronics. Metal-organic frameworks (MOFs) are a class of nanoporous crystalline materials on the brink of breakthrough in many application areas. Vapor deposition of MOF thin films will facilitate their implementation in micro- and nanofabrication research and industries. In addition, vapor-solid modification can be used for postsynthetic tailoring of MOF properties. In this context, we review the recent progress in vapor processing of MOFs, summarize the underpinning chemistry and principles, and highlight promising directions for future research.

First examples of aliphatic zirconium MOFs and the influence of inorganic anions on their crystal structures

Utilizing the aliphatic linker molecule adipic acid (1,6-hexanedioic acid, HO2C–C4H8–CO2H) or 3-methyladipic acid (racemic mixture, HO2C–C4H7CH3–CO2H), the first crystalline zirconium adipates were synthesized under aqueous conditions. Their structures were deduced from powder X-ray diffraction data and were confirmed by Rietveld refinements. For all three compounds, the inorganic nodes are related to the well-known Zr6O4(OH)4 cluster frequently observed in aromatic zirconium MOFs. Employing ZrOCl2·8H2O and 3-methyladipic acid, a framework with bcu topology was obtained. Starting from adipic acid and Zr(SO4)2·4H2O, we observed the incorporation of sulfate into the crystal structure. Four sulfate anions are coordinated to each Zr–oxo cluster in a bidentate fashion. In this complex structure, square grids formed by Zr–oxo clusters and adipate anions and furthermore a hydrogen-bonded inorganic dia net can be observed. The third compound presented here is structurally related to the zirconium methyladipate. Using adipic acid and adding CrO42− under strongly acidic conditions leads to the incorporation of Cr2O72− into the bcu net. The dichromate anions are coordinated twofold to two different Zr–oxo clusters in a monodentate fashion and thus serve as inorganic connectors between the frameworks nodes.

Bio‐Based Nitriles from the Heterogeneously Catalyzed Oxidative Decarboxylation of Amino Acids

The oxidative decarboxylation of amino acids to nitriles was achieved in aqueous solution by in situ halide oxidation using catalytic amounts of tungstate exchanged on a [Ni,Al] layered double hydroxide (LDH), NH4 Br, and H2 O2 as the terminal oxidant. Both halide oxidation and oxidative decarboxylation were facilitated by proximity effects between the reactants and the LDH catalyst. A wide range of amino acids was converted with high yields, often >90 %. The nitrile selectivity was excellent, and the system is compatible with amide, alcohol, and in particular carboxylic acid, amine, and guanidine functional groups after appropriate neutralization. This heterogeneous catalytic system was applied successfully to convert a protein-rich byproduct from the starch industry into useful bio-based N-containing chemicals.

Resolving Interparticle Heterogeneities in Composition and Hydrogenation Performance between Individual Supported Silver on Silica Catalysts.

Supported metal nanoparticle catalysts are commonly obtained through deposition of metal precursors onto the support using incipient wetness impregnation. Typically, empirical relations between metal nanoparticle structure and catalytic performance are inferred from ensemble averaged data in combination with high-resolution electron microscopy. This approach clearly underestimates the importance of heterogeneities present in a supported metal catalyst batch. Here we show for the first time how incipient wetness impregnation leads to 10-fold variations in silver loading between individual submillimeter-sized silica support granules. This heterogeneity has a profound impact on the catalytic performance, with 100-fold variations in hydrogenation performance at the same level. In a straightforward fashion, optical microscopy interlinks single support particle level catalytic measurements to structural and compositional information. These detailed correlations reveal the optimal silver loading. A thorough consideration of catalyst heterogeneity and the impact thereof on the catalytic performance is indispensable in the development of catalysts.

Waste PET (bottles) as a resource or substrate for MOF synthesis

PET contains up to 85 wt% of terephthalic acid (BDC), but has never directly been used as a source of organic linker for MOF synthesis. By combining metal salts and PET under hydrothermal conditions in a microwave oven, PET hydrolysis and MOF synthesis occur simultaneously. With this one-pot reaction, MIL-53(Al) and MIL-47(V) have been successfully synthesized. Optimization of the reaction and activation conditions for MIL-53(Al) results in a phase-pure MOF with a BET surface of 1481 m2 g−1. When the hydrolysis is carried out as a separate first step, less stable MOFs like MIL-88B(Fe) can be synthesized by adding the metal salt and methanol to the hydrolyzed mixture in the second step. By partially depolymerizing the surface of PET bottles it is possible to grow MOF coatings of MIL-53(Al) and UiO-66(Zr) on the polymer surface, using the bottle itself as the synthesis reactor.