The flexibility of modified-linker MIL-53 materials.

The flexibility of eight aluminium hydroxo terephthalates [Al(OH)(BDC-X)]·n(guest) (BDC = 1,4-benzene-dicarboxylate; X = -H, -CH3, -Cl, -Br, -NH2, -NO2, -(OH)2, -CO2H) crystallising in the MIL-53-type structure was investigated upon thermal dehydration of as-made samples, superhydration and methanol adsorption/desorption using in situ powder X-ray diffraction (PXRD). Profile fitting was used to determine lattice parameters as a function of time and/or temperature to describe their structural evolution. It has thus been shown that while methanol vapour adsorption induces an opening of all the modified frameworks, except the -NH2 material, superhydration only leads to open structures for Al-MIL-53-NO2, -Br and -(OH)2. All the MIL-53 solids, except Al-MIL-53-(OH)2 are present in the open structures upon thermal dehydration. In addition to the exploration of the breathing behavior of this MIL-53 series, the issue of disorder in the distribution of the functional groups between the organic linkers was explored. As a typical illustration, density functional theory calculations were carried out on different structures of Al-MIL-53-Cl, in which the distribution of -Cl within two adjacent BDC linkers is varied. The results show that the most energetically stable configuration leads to the best agreement with the experimental PXRD pattern. This observation supports that the distribution of the selected linker substituent in the functionalised solid is governed by energetics and that there is a preference for an ordering of this arrangement.

Continuous-flow hydrothermal synthesis for the production of inorganic nanomaterials.

As nanotechnology becomes increasingly important and ubiquitous, new and scalable synthetic approaches are needed to meet the growing demand for industrially viable routes to nanomaterial production. Continuous-flow hydrothermal synthesis or supercritical water hydrothermal synthesis (scWHS) is emerging as a versatile solution to this problem. The process was initially developed to take advantage of the tunable chemical and physical properties of superheated water to produce metal oxide nanoparticles by rapid nucleation and precipitation. The development of new mixing regimes and reactor designs has been facilitated by the modelling of reactor systems. These new reactor designs further exploit the properties of supercritical water to promote faster and more uniform mixing of reagent streams. The synthetic approach has been expanded beyond the metal oxide systems for which it was conceived, and now encompasses metal sulfides, metal phosphates, metal nanoparticles and metal-organic frameworks. In many of these cases, some degree of size and shape control can be achieved through careful consideration of both chemistry and reactor design. This review briefly considers the development of scWHS reactor technology, before highlighting some of our recent work in expanding the scope of this synthetic method to include a wide range of materials.

The sequential continuous-flow hydrothermal synthesis of molybdenum disulphide.

Molybdenum disulphide (MoS2) has been widely used as a catalyst and high temperature lubricant. It has been heavily researched recently as a graphene analogue and member of the so-called inorganic fullerenes. Here we report the first continuous flow hydrothermal synthesis of MoS2. With fast reaction times and flexibility the continuous flow hydrothermal system allowed MoS2 to be produced in a stepwise fashion, offering an insight into the mechanism involved. It has been found that the synthesis of MoS2 proceeded via the sulphidation of molybdate anions to thiomolybdate species, which are transformed to amorphous MoS3 by acidification in flow, before further hydrothermal treatment decomposes this amorphous precursor to tangled MoS2 nanosheets.

Conformation-controlled sorption properties and breathing of the aliphatic Al-MOF [Al(OH)(CDC)].

The Al-MOF CAU-13 ([Al(OH)(trans-CDC)]; trans-H2CDC = trans-1,4-cyclohexanedicarboxylic acid) is structurally related to the MIL-53 compounds that are well-known for their "breathing" behavior, i.e., the framework flexibility upon external stimuli such as the presence of adsorbate molecules. The adsorption properties of CAU-13 were investigated in detail. The sorption isotherms of N2, H2, CH4, CO, CO2, and water were recorded, and the adsorption enthalpies for the gases were determined by microcalorimetry. The structural changes upon adsorption of CO2 were followed with in situ synchrotron powder X-ray diffraction (PXRD). The patterns were analyzed by parametric unit cell refinement, and the preferential arrangement of the CO2 molecules was modeled by density functional theory calculations. The adsorption and separation of mixtures of o-, m-, and p-xylene from mesitylene showed a preferred adsorption of o-xylene. The structures of o/m/p-xylene-loaded CAU-13 were determined from PXRD data. The adsorption of xylene isomers induces a larger pore opening than that in the thermal activation of CAU-13. In the crystal structure of the activated sample CAU-13(empty pore), half of the linkers adopt the a,a confirmation and the other half the e,e conformation, and the presence of a,a-CDC(2-) ions hampers the structural flexibility of CAU-13. However, after the adsorption of xylene, all linkers are present in the e,e conformation, allowing for a wider pore opening by this new type of "breathing".

Distortions of a flexible metal-organic framework from substituted pendant ligands.

Four new variants of the 1,4-benzenedicarboxylate MIL-53 structure have been prepared for Co(II) under solvothermal conditions and their structures solved and refined from single-crystal X-ray data. All materials contain pendant pyridine-N-oxide ligands that bridge pairs of Co(II) atoms in the inorganic backbone of the structure via O. By the use of the ligands 3-bromopyridine-N-oxide, 4-methoxypyridine-N-oxide, isoquinoline-N-oxide and 4-phenylpyridine-N-oxide, materials are prepared with the same topology but distinct structures. These illustrate how the MIL-53 structure is able to distort to accommodate the bulk of the various substituents on the pyridine ring. The bulkiest pendant ligand, 4-phenylpyridine-N-oxide, results in a distortion of the diamond-shaped channels in an opposite sense to that seen previously in expanded forms of the parent MIL-53 structure. By comparison with published crystal structures for MIL-53 with various occluded guests, the structural distortions that take place to accommodate the pendant ligands are quantified and it is shown how a twisting of the 1,4-benzenedicarboxylate ligand, instead of a hinging about the µ(2)-carboxylate-metal connection, allows the new structures that are observed.

Adsorption of N/S heterocycles in the flexible metal-organic framework MIL-53(Fe(III)) 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.

Instant MOFs: continuous synthesis of metal-organic frameworks by rapid solvent mixing.

A continuous flow reactor allows the preparation of porous metal-organic framework materials with crystallisation induced by rapid mixing of streams of preheated water and solutions of reagents in organic solvent: this gives high volume production (132 g h(-1)) with crystallite size of the products from nanoscale to micron.

Uptake of liquid alcohols by the flexible Fe(III) metal-organic framework MIL-53 observed by time-resolved in situ X-ray diffraction.

A comprehensive, time-resolved, energy-dispersive X-ray diffraction study of the uptake of liquid alcohols (methanol, ethanol, propan-1-ol and propan-2-ol) by the flexible metal-organic framework solid MIL-53(Fe)[H(2)O] is reported. In the case of the primary alcohols, a fluorinated version of the MIL-53(Fe) host (C2/c symmetry V ca. 1000 Å(3)), in which a fraction of framework hydroxides are replaced by fluoride, shows uptake of alcohols to give initially a partially expanded phase (C2/c symmetry, V ca. 1200 Å(3)) followed by an expanded form of the material (either Imcm or Pnam symmetry, V ca. 1600 Å(3)). In the case of methanol-water mixtures, the EDXRD data show that the partially open intermediate phase undergoes volume expansion during its existence, before switching to a fully open structure if concentrated methanol is used; analogous behaviour is seen if the initial guest is propan-2-ol, which then is replaced by pyridine, where a continuous shift of Bragg peaks within C2/c symmetry is observed. In contrast to the partially fluorinated materials, the purely hydroxylated host materials show little tendency to stabilise partially open forms of MIL-53(Fe) with primary alcohols and the kinetics of guest introduction are markedly slower without the framework fluorination: this is exemplified by the exchange of water by propan-2-ol, where a partially open C2/c phase is formed in a step-wise manner. Our study defines the various possible pathways of liquid-phase uptake of molecular guests by flexible solid MIL-53(Fe).