Jana Juan-Alcañiz

Metal–organic frameworks as scaffolds for the encapsulation of active species: state of the art and future perspectives

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.

Kinetic Control of Metal–Organic Framework Crystallization Investigated by Time‐Resolved In Situ X‐Ray Scattering

Metal–organic frameworks (MOFs) are among the most sophisticated nanostructured solids: they often possess high surface areas and pore volumes, with the possibility of finetuning their chemical environment by either selecting the appropriate building blocks or by postsynthetic functionalization. For many frameworks, flexibility of the lattice allows them to undergo a significant transformation in solid state.[1] All these features make MOFs a special class of solids with the potential of transcending many common limitations in different technological disciplines, such as ferromagnetism,[2] semiconductivity, gas separation,[3] storage,[4] sensing,[5] catalysis,[ 6] drug delivery,[7] or proton conductivity.[8] However, the crystallization mechanism of these complex structures is far from understood. Notwithstanding the plethora of publications that present new MOFs,[9] and the effectiveness of the high-throughput approach,[10] serendipity still governs the synthesis of new structures.

Adsorption and Separation of Light Gases on an Amino-Functionalized Metal–Organic Framework: An Adsorption and In Situ XRD Study

The NH(2)-MIL-53(Al) metal-organic framework was studied for its use in the separation of CO(2) from CH(4), H(2), N(2)C(2)H(6) and C(3)H(8) mixtures. Isotherms of methane, ethane, propane, hydrogen, nitrogen, and CO(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(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(2) interacted much more strongly and was adsorbed in significant amounts at low pressure. This gives the material excellent properties to separate CO(2) from other gases. The separation of CO(2) from methane, nitrogen, hydrogen, or a combination of these gases has been demonstrated by breakthrough experiments using pellets of NH(2)-MIL-53(Al). The effect of total pressure (1-30 bar), gas composition, temperature (303-403 K) and contact time has been examined. In all cases, CO(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(2)-MIL-53(Al) pellets retained their selectivity and capacity for more than two years.

Interplay of metal node and amine functionality in NH2-MIL-53: modulating breathing behavior through intra-framework interactions.

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.

Experimental evidence of negative linear compressibility in the MIL-53 metal–organic framework family

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 increase in 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.

Live encapsulation of a Keggin polyanion in NH2-MIL-101(Al) observed by in situ time resolved X-ray scattering

The templating effect of the Keggin polyanion derived from phosphotungstic acid (PTA) during the synthesis of NH(2)-MIL-101(Al) has been investigated by means of in situ SAXS/WAXS. Kinetic analysis and structural observations demonstrate that PTA acts as a nucleation site and that it stabilizes the precursor phase NH(2)-MOF-235(Al). Surprisingly kinetics of formation are little changed.

Towards efficient polyoxometalate encapsulation in MIL-100(Cr): influence of synthesis 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.

Metal-organic framework capillary microreactor for application in click chemistry

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.

CHAPTER 10:MOFs as Nano‐reactors

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.