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A Cu/PMA–MIL-101(Cr) metal–organic-framework-coated microreactor has been applied in the 1,3-dipolar cycloaddition of benzyl azide and phenylactetylene (click chemistry). The Cu/PMA–MIL-101(Cr) catalyst was incorporated by using a washcoating method. The use of tetraethylorthosilicate (TEOS) and a copolymer pluronic F127 as binders resulted in a stable and uniform coating of 6 μm. The application of the Cu/PMA–MIL-101(Cr) capillary microreactor in the click-chemistry reaction resulted in a similar intrinsic activity as in the batch reactor, and a continuous production for more than 150 h time-on-stream could be achieved. The presence of water in the reagent feed led to reversible catalyst deactivation and was necessary to be removed to obtain a stable catalyst operation.

Functionalization of metal–organic frameworks results in higher hydrogen uptakes owing to stronger hydrogen–host interactions. However, it has not been studied whether a given functional group acts on existing adsorption sites (linker or metal) or introduces new ones. In this work, the effect of two types of functional groups on MIL-101 (Cr) is analyzed. Thermal-desorption spectroscopy reveals that the −Br ligand increases the secondary building unit’s hydrogen affinity, while the −NH2 functional group introduces new hydrogen adsorption sites. In addition, a subsequent introduction of −Br and −NH2 ligands on the linker results in the highest hydrogen-store interaction energy on the cationic nodes. The latter is attributed to a push-and-pull effect of the linkers.

A series of amino-functionalized MIL-53 with different metals as nodes has been synthesized. By determining adsorption properties and spectroscopic characterization, we unequivocally show that the interaction between the amines of the organic linker and bridging μ(2)-OH of the inorganic scaffold modulates metal organic framework (MOF) flexibility. The strength of the interaction has been found to correlate with the electropositivity of the metal.

Several archetypical metal organic frameworks (MOFs), namely, HKUST-1, ZIF-8, MIL-100(Al), MIL-53(Al), and NH2-MIL-53(Al), were synthesized via anodic dissolution in an electrochemical cell. The influence of different reaction parameters such as solvent, electrolyte, voltage–current density, and temperature on the synthesis yield and textural properties of the MOFs obtained was investigated. The characterization of the samples involved X-ray diffraction, gas adsorption, atomic force microscopy, diffuse reflectance infrared Fourier transform spectroscopy, and scanning electron microscopy. In the present article, we demonstrate that electrochemical synthesis is a robust method offering additional degrees of freedom in the synthesis of these porous materials. The main advantages are the shorter synthesis time, the milder conditions, the facile synthesis of MOF nanoparticles, the morphology tuning and the high Faraday efficiencies. The synthesized MIL-53 and NH2-MIL-53 samples exhibit suppressed framework fle...

A new post-functionalization method for metal–organic frameworks (MOFs) has been developed to introduce acidity for catalysis. Upon treatment with a mixture of triflic anhydride and sulfuric acid, chemically stable MOF structures MIL-101(Cr) and MIL-53(Al) can be sulfated, resulting in a Bronsted sulfoxy acid group attached to up to 50% of the aromatic terephthalate linkers of the structure. The sulfated samples have been extensively characterized by solid-state NMR, XANES, and FTIR spectroscopy. The functionalized acidic frameworks show catalytic activity similar to that of acidic polymers like Nafion® display in the esterification of n-butanol with acetic acid (TOF ∼ 1 min−1 @ 343 K). Water adsorbs strongly up to 4 molecules per sulfoxy acid group, and an additional 2 molecules are taken up at lower temperatures in the 1-D pore channels of S-MIL-53(Al). The high water content and Bronsted acidity provide the structure S-MIL-53(Al) a high proton conductivity up to moderate temperatures.

Structuring heterogeneous catalysts from the micro- to the macro-level is essential for efficient catalyst utilization. The optimized synthesis and the catalytic performance of a Metal-Organic Framework, MIL-101(Cr), immobilized on a monolithic structure is presented. Secondary seeded growth is the optimal procedure to obtain uniform coatings of ∼9 wt.% inside the monolith channels. A monolithic stirrer reactor has been used as playground for exploring reactivation procedures of MIL-101 for the selective oxidation of tetralin in the liquid phase. The presented results confirm the long-term stability of the catalyst, and the absence of any transport limitations in this reaction. The easy recovery of the catalyst allows performing as many reuses as necessary, something impossible to realize when working in the often-practised slurry operation mode. The reported results demonstrate that MIL-101(Cr) undergoes reversible deactivation attributed to the strong adsorption of products on the Cr sites. Applying the proper regeneration procedure (washing in chlorobenzene and air treatment @ 523 K) the initial performance is fully recovered.

A new strategy has been developed for the direct encapsulation of polyoxometalates (POMs) into MIL-101(Cr). The addition of phosphotungstic acid (PTA) to the synthesis mixture of MIL-101 yields the direct encapsulation of chromium-containing polyoxometalates (POMs) inside the MOF structure, with a good distribution over the MIL-101 crystals. Vibrational Spectroscopy (DRIFT, Raman, UV–Vis) reveals the partial substitution of tungsten by Cr 3+ resulting in the so-called lacunary structures, which are highly active in catalysis. The medium-sized cavities of MIL-101 are occupied by POM units bigger than their pentagonal windows when this one-pot approach is followed, and no leaching is observed. These new catalysts show the highest activities reported to date at 313 K for the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate when using apolar toluene as solvent as well as when using polar DMF and ethanol, with TOFs exceeding 600 h −1 . In addition, they exhibit a remarkable activity in two acid-catalyzed reactions, the esterification of n -butanol with acetic acid in liquid phase slurry operation and the dimethyl ether production from methanol in a fixed bed gas phase operation, in contrast to the poor or absent activity of the catalysts prepared via the impregnation of the polyoxometalate in MIL-101, where the strong interaction between POM and support deteriorates the catalytic performance.

We report a series of powder X-ray diffraction experiments performed on the soft porous crystals MIL-53(Al) and NH2-MIL-53(Al) in a diamond anvil cell under different pressurization media. Systematic refinements of the obtained powder patterns demonstrate that these materials expand along a specific direction while undergoing total volume reduction under an increasing hydrostatic pressure. The results confirm for the first time the negative linear compressibility behaviour of this family of materials, recently predicted from quantum chemical calculations.

A new consecutive post-functionalization method has been developed for the inclusion of additional metal functionalities in Metal Organic Frameworks (MOFs) through oxamate as chelating agent. This may result in catalytic centers of metal–organic complexes or in controlled formation of metal nanoparticles, demonstrated for Cu, Pd and Au, in the highly stable MIL-101(Cr) framework. In a first post-synthesis step, reduction of the NO 2 -MIL-101(Cr) leads to the formation of NH 2 -MIL-101(Cr). The second functionalization consists of a straightforward condensation of the amino groups of the ligand with ethyl chloro-oxoacetate resulting in the formation of free oxamates attached to the linker. Two different copper complexes have been coordinated through the oxamate ligand, attaining loadings as high as 9 wt.% and showing similar performance as the best reported copper MOFs (HKUST-1, [Cu(2-pymo) 2 ] and [Cu(im) 2 ]) in “click chemistry” catalysis (1,3-dipolar cycloaddition reactions) and multicomponent coupling reactions (propargylamine formation). Highly dispersed palladium and gold nanoparticles can be incorporated and stabilized in the MOF. The resultant Pd-catalysts are very active and selective in Suzuki–Miyaura condensation, although palladium leaching is observed. After reduction of the Au-catalysts, immobilized gold nanoparticles are formed, which maintain a good performance in BA oxidation to benzaldehyde for at least 4 consecutive runs. No side reactions (further oxidation and esterification) are observed, which are attributed to the absence of any cationic gold species. TEM analysis and DRIFTS analysis during CO adsorption show the very uniform size (∼2 nm) and metallic character of these gold particles encapsulated in the MIL-101 cages.

The use of metal organic frameworks (MOFs) as catalytic nanoreactors is thoroughly reviewed. Two approaches can be followed for the encapsulation of catalytically active species into the scaffold of a MOF: (i) ship in a bottle and (ii) bottle around a ship. In the first case, formation of metallic nanoparticles or metal oxides are among the most studied systems, and metal precursor impregnation followed by reduction/oxidation is the widely used synthetic strategy. Also worth mentioning are the few examples of enzyme encapsulation. On the other hand, bottle around a ship has been used when the active phase is added to the MOF synthesis and in situ encapsulated. The most studied example has been described for heteropolyacids, where templating effects have been discovered. The encapsulation of other macromolecules, such as porphyrins, illustrate the great opportunities that MOFs offer for direct encapsulation. The confinement of the active sites affects their catalytic behaviour when compared with their homogeneous counterparts, in most cases enhancing both conversion and selectivity to the desired products. In addition, confined active sites are protected from deactivation by leaching or aggregation, thus facilitating catalyst reusability.

The use of metal–organic frameworks (MOFs) for the encapsulation of different active entities is thoroughly reviewed. Either by following ship in a bottle or bottle around a ship approaches, active species can be encapsulated in the porous framework of different MOFs. Encapsulated species vary from polymers to organometallics and from polyoxometalates to metal nanoparticles and metal oxides. The main advantages and limitations of the use of MOFs together with the synthetic approaches followed are evaluated.

The NH{sub 2}-MIL-53(Al) metal-organic framework was studied for its use in the separation of CO{sub 2} from CH{sub 4}, H{sub 2}, N{sub 2} C{sub 2}H{sub 6} and C{sub 3}H{sub 8} mixtures. Isotherms of methane, ethane, propane, hydrogen, nitrogen, and CO{sub 2} were measured. The atypical shape of these isotherms is attributed to the breathing properties of the material, in which a transition from a very narrow pore form to a narrow pore form and from a narrow pore form to a large pore form occurs, depending on the total pressure and the nature of the adsorbate, as demonstrated by in-situ XRD patterns measured during adsorption. Apart from CO{sub 2}, all tested gases interacted weakly with the adsorbent. As a result, they are excluded from adsorption in the narrow pore form of the material at low pressure. CO{sub 2} interacted much more strongly and was adsorbed in significant amounts at low pressure. This gives the material excellent properties to separate CO{sub 2} from other gases. The separation of CO{sub 2} from methane, nitrogen, hydrogen, or a combination of these gases has been demonstrated by breakthrough experiments using pellets of NH{sub 2}-MIL-53(Al). The effect of total pressure (1-30 bar), gas composition, temperaturemore » (303-403 K) and contact time has been examined. In all cases, CO{sub 2} was selectively adsorbed, whereas methane, nitrogen, and hydrogen nearly did not adsorb at all. Regeneration of the adsorbent by thermal treatment, inert purge gas stripping, and pressure swing has been demonstrated. The NH{sub 2}-MIL-53(Al) pellets retained their selectivity and capacity for more than two years.« less

The synthesis, characterization (FT-IR, XPS, NMR, UV–Vis), and catalytic performance of Pt supported on phosphotungstic acid (PTA) encapsulated in a metal organic framework (MOF) are reported. The highly dispersed Keggin units of PTA in NH 2 -MIL-101(Al), synthesized in one step, act as anchoring sites for the Pt precursor species. After different post-treatments, the resulting catalysts have been tested in the oxidation of CO, the preferential oxidation of CO in the presence of H 2 , and the hydrogenation of toluene. Reduction at 473 K results in the formation of small Pt 0 clusters and Pt 2+ species. Reduction at 573 K induces the formation of intermetallic Pt–W 5+ species, which exhibit the best CO oxidation activity and a higher selectivity toward CO 2 than alumina supported Pt, resembling the combination of a noble metal on a reducible support. In toluene hydrogenation, the MOF catalysts perform worse than Pt on alumina, ascribed to the too small size of the Pt clusters in the MOF catalysts.

In this work, the inclusion of free sulfonic acid groups in highly stable MOFs is explored. The synthesized catalysts have been applied in a model esterification reaction. Two metal organic frameworks bearing sulfonic acid moieties are investigated: HSO3-MIL-101(Cr) synthesized following different approaches and a new structure based on HSO3-bdc and Zr. The acidic properties, catalytic performance, deactivation and stability of the different structures are critically evaluated. In the case of MIL-101(Cr), deactivation of the sulfonic groups via formation of butanol sulfonic esters has been observed. Due to the strong interaction between –SO3 _ and the Cr open metal site where usually fluorine (F_) is located in the structure, the HSO3-MIL-101(Cr) catalysts are not stable under acidic regeneration conditions. When using Zr as a metal node, a new and stable sulfonic acid containing porous structure was synthesized. This structure showed high activity and full re-usability in the esterification of n-butanol with acetic acid. In this case, deactivation of the catalyst due to sulfonic ester formation could be reversed by reactivation under acidic conditions.

The one-pot encapsulation of phosphotungstic acid in the metal–organic framework MIL-100(Cr) has been studied under different synthesis conditions. Both conventional and microwave heating methods have been explored for three different solvent systems: pure aqueous or organic (DMF) phase and biphasic mixtures (water/2-pentanol). Biphasic systems yielded crystals with similar textural properties as those formed in water. The use of DMF as solvent promotes the formation of gel-like solids with dual porosity and enhanced accessibility. The addition of phosphotungstic acid (PTA, H3PW12O40.xH2O) to the MIL-100(Cr) synthesis mixture results in its direct encapsulation. 31P MAS NMR, elemental analysis, N2 adsorption and FT-IR spectroscopy confirm the incorporation of PTA in the sample. The highest PTA encapsulation loading (30 wt%) was obtained by synthesis with microwave heating in biphasic solvent systems (W/Cr molar ratio range between 0.5 and 0.25). Microwave irradiation decreases the time of synthesis (from 4 days to 3 hours) while the use of biphasic media preserves the PTA integrity without affecting the formation of the MOF. The interaction of PTA with the MIL-100(Cr) structure results in some loss of the Lewis acidity, while the Bronsted acidity is hardly affected.

The use of metal–organic frameworks (MOFs) for the encapsulation of different active entities is thoroughly reviewed. Either by following ship in a bottle or bottle around a ship approaches, active species can be encapsulated in the porous framework of different MOFs. Encapsulated species vary from polymers to organometallics and from polyoxometalates to metal nanoparticles and metal oxides. The main advantages and limitations of the use of MOFs together with the synthetic approaches followed are evaluated.