Ben Van de Voorde

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.

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.

N/S-heterocyclic contaminant removal from fuels by the mesoporous metal-organic framework MIL-100: the role of the metal ion.

The influence of the metal ion in the mesoporous metal trimesate MIL-100(Al(3+), Cr(3+), Fe(3+), V(3+)) on the adsorptive removal of N/S-heterocyclic molecules from fuels has been investigated by combining isotherms for adsorption from a model fuel solution with microcalorimetric and IR spectroscopic characterizations. The results show a clear influence of the different metals (Al, Fe, Cr, V) on the affinity for the heterocyclic compounds, on the integral adsorption enthalpies, and on the uptake capacities. Among several factors, the availability of coordinatively unsaturated sites and the presence of basic sites next to the coordinative vacancies are important factors contributing to the observed affinity differences for N-heterocyclic compounds. These trends were deduced from IR spectroscopic observation of adsorbed indole molecules, which can be chemisorbed coordinatively or by formation of hydrogen bonded species. On the basis of our results we are able to propose an optimized adsorbent for the deep and selective removal of nitrogen contaminants out of fuel feeds, namely MIL-100(V).

The Structure of the Aluminum Fumarate Metal–Organic Framework A520

The synthesis of the commercially available aluminum fumarate sample A520 has been optimized and its structure analyzed through a combination of powder diffraction, solid-state NMR spectroscopy, molecular simulation, IR spectroscopy, and thermal analysis. A520 is an analogue of the MIL-53(Al)-BDC solid, but with a more rigid behavior. The differences between the commercial and the optimized samples in terms of defects have been investigated by in situ IR spectroscopy and correlated to their catalytic activity for ethanol dehydration.

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.

Adsorption of N/S heterocycles in the flexible metal–organic framework MIL-53(FeIII) studied by in situ energy dispersive X-ray diffraction

The adsorption of N/S-containing heterocyclic organic molecules in the flexible iron(III) terephthalate MIL-53, Fe(III)(OH)(0.6)F(0.4)(O2C-C6H4-CO2)·(H2O), from the liquid phase was studied with in situ energy dispersive X-ray diffraction (EDXRD), in order to follow the adsorption-induced expansion of the structure. For comparison with the diffraction data, liquid phase adsorption isotherms were recorded for uptake of benzothiophene, benzothiazole and indole in isopropanol and in heptane. The solvent not only influences pore opening but is also a competing guest. The in situ EDXRD experiments allow the kinetics of guest uptake and the competition with solvent to be monitored directly. Indole uptake is limited; this adsorbate is barely capable of opening the closed, either hydrated or dehydrated, MIL-53(Fe) structure, or of penetrating the isopropanol-containing material in the concentration range under study. When isopropanol is used as a solvent, the guest molecules benzothiophene and benzothiazole must be present at a certain threshold concentration before substantial adsorption into the metal-organic framework takes place, eventually resulting in full opening of the structure. The fully expanded structures of benzothiophene or benzothiazole loaded MIL-53(Fe) materials have Imcm symmetry and a unit cell volume of ca. 1600 Å(3), and upon uptake of the guest molecules by the closed form (unit cell volume ~1000 Å(3)) no intermediate crystalline phases are seen. Successful uptake by MIL-53(Fe) requires that the adsorbate is primarily a good hydrogen bond acceptor; additionally, based on UV-visible spectroscopy, a charge-transfer interaction between the S atoms of benzothiophene and the aromatic rings in the MOF pore wall is proposed.

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.

Isolation of Renewable Phenolics by Adsorption on Ultrastable Hydrophobic MIL-140 Metal-Organic Frameworks.

The isolation and separation of phenolic compounds from aqueous backgrounds is challenging and will gain in importance as we become more dependent on phenolics from lignocellulose-derived bio-oil to meet our needs for aromatic compounds. Herein, we show that highly stable and hydrophobic Zr metal-organic frameworks of the MIL-140 type are effective adsorbent materials for the separation of different phenolics and far outperform other classes of porous solids (silica, zeolites, carbons). The mechanism of the hydroquinone-catechol separation on MIL-140C was studied in detail by combining experimental results with computational techniques. Although the differences in adsorption enthalpy between catechol and hydroquinone are negligible, the selective uptake of catechol in MIL-140C is explained by its dense π-π stacking in the pores. The interplay of enthalpic and entropic effects allowed separation of a complex, five-compound phenol mixture through breakthrough over a MIL-140C column. Unlike many other metal-organic frameworks, MIL-140C is remarkably stable and maintained structure, porosity and performance after five adsorption-desorption cycles.

Adsorption and Reactive Desorption on Metal-Organic Frameworks: A Direct Strategy for Lactic Acid Recovery

Biomass-derived lactic acid (LA) is an important platform chemical towards the sustainable production of numerous materials. However, the fermentation process currently in use is limited by the difficult recovery of the LA product from the fermentation broth and results in the generation of stoichiometric amounts of gypsum waste. Herein, we show that metal-organic frameworks (MOFs) of the UiO-66(Zr) type are effective adsorbents for the separation of LA from aqueous (buffer) solutions. These frameworks based on zirconium clusters and terephthalic acid derivatives display a tremendous uptake (up to 42 wt %) and a high affinity for LA. The latter can further be tuned by changing the hydrogen-bonding properties of the functional groups present on the organic ligand. A Rietveld refinement disclosed the specific interaction of LA with the clusters of UiO-66(Zr) and a preferential adsorption on open zirconium sites. Taking advantage of the catalytic activity of UiO-66(Zr), desorption of LA was performed in alcohols to recover up to 73 % as ester. Applied to the recovery of LA, adsorption and reactive desorption offer a direct and gypsum-free strategy as an alternative for the current multi-step process.

Adsorption and Selective Recovery of Citric Acid with Poly(4-vinylpyridine)

Citric acid (CA) is an important organic acid that is produced on a large scale by fermentation. Current methods to recover CA from the fermentation broth require large amounts of chemicals and produce considerable amounts of waste, while not all CA can be recovered. The use of adsorbents can increase the degree of product recovery and reduce chemical consumption and waste generation. In this work, poly(4-vinylpyridine) (PVP) is evaluated as an adsorbent for CA recovery. It has a high adsorption capacity (>30 wt %) at low pH and a high selectivity for CA at moderate pH in the presence of sulfate anions, two conditions that are frequently encountered during CA recovery. PVP could be efficiently regenerated after adsorption using simple alcohols like methanol and ethanol. Considering selectivity and regeneration, PVP distinctly outperforms more common adsorbents for organic acids, including commercial strongly and weakly basic anion exchangers. The desirable adsorptive features of PVP for CA can be attributed to the low basicity of the pyridine group.