4. RESULTADOS Y DISCUSIONES
4.5. Determinación del efecto en el porcentaje de humedad y temperatura del sustrato de
Structure determination is key to understanding important properties of MOFs, such as their pore sizes, the nature of the interactions with potential guest molecules and changes occurring to the framework upon the variation of experimental conditions. All this information proves of fundamental importance when investigating the potential applications of these materials. Solid-state characterisation of these compounds usually combines local and space-averaged structure determination by spectroscopy and XRD, respectively. When dealing with disordered guest molecules present in loaded or as-made structures, in particular, the interpretation of data from XRD, a technique highly reliant on the presence of long- range order, can be problematic and requires additional spectroscopic investigation of atomic-level dynamics and disorder. Electron microscopy can be employed to acquire information on the crystallite morphology, potentially affected by the very low amounts of solvent used during the optimised enrichment syntheses, and cation distribution in mixed metal compounds. Moreover, to experimentally determine the amount of 17O incorporated in the structures as a result of the DGC synthesis procedure, secondary ion mass spectrometry (SIMS) experiments have been carried out for some of the 17O-enriched samples.
2.2.1 Solid-state NMR spectroscopy
A multinuclear solid-state NMR investigation is the typical approach for the characterisation of the local structure of a MOF. 13C, in direct acquisition or by 1H- 13C cross-polarisation experiments, and 1H NMR spectra are acquired to obtain information on the nature of linker, possible decomposition of the structure, the presence of guest molecules in general and any hydration in particular. These investigations have been shown to help the determination of the behaviour of the framework as a function of different levels of hydration, the location of water molecules in hydrated structures and the presence of dynamics by deuteration studies.33-35 Spectroscopy investigation of the metal centers, where possible (i.e., for non-paramagnetic compounds or those with favourable NMR properties), enables
interaction with guest molecules.36-37 For the reasons outlined in Section 1.1.3, 17O is a much less investigated nucleus notwithstanding its fundamental structural role. Indeed, oxygen atoms, in terephthalate-based and many other MOFs, directly connect the linker molecules to the metal centers potentially providing a wealth of information on the structural changes occurring when, for example, a substitution occurs at the metal center or the pore conformation changes as a result of an interaction with different types of guest molecules or heat treatments. For such 17O NMR investigations to be possible within a reasonable timeframe, enrichment is necessary and has been shown to be possible38 with the identification of chemically and crystallographically distinct spectral signatures of various oxygen species in representative MOFs, such as CPO-27, MIL-53 and UiO-66. DNP NMR experiments have also been shown to provide enhancements focusing on indirect polarisation transfer to 13C or 15N in the linker molecules39 and to the metal centers, e.g., 27Al, also allowing the acquisition of heteronuclear correlation experiments precluded under standard conditions.40 In these cases, the choice of the size of the radical and resting time of the impregnated solid is of crucial importance to determine the possible diffusion of the radical species into the pores of the framework. Furthermore, the choice of the radical solution used is more critical than with other types of solids as MOFs can interact with solvent guest molecules, opening up the possibility of changes occurring to the framework conformation as a result of the type of solvent used.
The magnetic fields involved in this investigation range from 14.1 to 20.0 T and
magic angle spinning (MAS) was always applied to average the orientational dependence of the anisotropic components of the dipolar coupling, chemical
shielding, J-coupling and first-order quadrupolar interactions. A combination of17O,
1H, 27Al and 45Sc single-pulse, 17O and 1H spin echo, 1H-17O and 1H-13C cross- polarisation, 17O and 27Al MQMAS and 17O double rotation (DOR)experiments have been carried out on the range of MOFs synthesised. The theory of NMR and DNP NMR, along withdetailed descriptions of the experimental conditions used, will be
2.2.2 Powder X-ray diffraction
All the investigated MOFs have been obtained in the form of microcrystalline powders and PXRD patterns have been mainly used to identify, by comparison to literature patterns, the terephthalate MOFs synthesised and the breathing behaviour of as-made, calcined, hydrated and dehydrated forms. Rietveld refinements and Pawley fits have been carried out in collaboration with Dr Samuel Morris to evaluate unit cell variations when the larger Ga3+ cation was introduced and to determine the actual Al:Ga ratios in the mixed-metal MIL-53 frameworks synthesised.
PXRD data were collected on a STOE STADIP instrument operated in capillary Debye-Scherrer mode equipped with a Cu X-ray tube, a primary beam monochromator (CuKα1) and a scintillation position-sensitive linear detector. Typically, 3-50° ranges were investigated overnight. For calcined or dehydrated samples capillaries have been sealed to avoid exposure to air during acquisition. The theory behind XRD and the experimental geometries used will be separately introduced in Section 2.3.
2.2.3 SEM-EDX analysis
SEM analysis, as introduced in Section 2.1.4, enabled insight into the morphology of the MIL-53 samples obtained from the syntheses carried out in DGC conditions. Moreover, compositional analysis by EDX of single crystallites from mixed-metal MIL-53 samples has been carried out to better understand the cation distribution in these materials. For these latter experiments, powders have been sieved prior to analysis to obtain a better dispersion of the materials on the sample holders. Experimental conditions are the same as described in Section 2.1.4. The SEM-EDX characterisation has been carried out in collaboration with Samantha Russell.
2.2.4 Ion microprobe SIMS
Ion microprobe SIMS has been used for the determination of oxygen (16O, 17O and 18O) isotopic ratios in some of the 17O-enriched MOFs synthesised. This
oxygen isotopic ratios in geological samples41 and has been therefore investigated for its application to MOFs.
An ion microprobe consists of a primary column where the primary ion beam is generated, filtered, focused, shaped, positioned and rastered before reaching the sample. Typically the primary ion beam is composed of oxygen or cesium ions, with electropositive elements best analysed with an oxygen primary beam, either O+ or O−, and electronegative elements, such as oxygen, best analysed with cesium, only used as Cs+. Secondary ions are formed as a result of erosion, or sputtering, of the sample surface from the primary beam and are extracted by holding the sample at a high potential usually of opposite polarity to the primary beam. To keep a constant secondary ion beam current major voltage changes have to be avoided and, for insulating materials, this can be achieved, at least in part, by gold coating the sample. The generated secondary ions, characterised by a wide range of energies, are transferred into the mass spectrometer and passed into an electrostatic energy analyser where they are dispersed with deflected paths dependent on their energies. A movable slit allows the selection of the desired portion of secondary ions that are passed into the magnetic analyser where ions are refocused as a function of their mass and energy and subsequently detected. In such a double-focusing mass spectrometer, the combined use of energy and mass analysers allows high mass resolution to be achieved. For insulating specimens charge build-up can occur at the sputter site as a result of the impact of Cs+ ions and the emission of secondary electrons from the sample with insufficient electrons reaching the site from the conductive coating. In this case, the sample is bombarded with additional electrons from an electron flood gun. The instrument, schematically shown in Figure 2.4, has to be kept under ultra high vacuum for operation with the analysis chamber at 5×10−10 Torr to completely minimise the probability of a secondary ion hitting a gas molecule in its path.42
The oxygen isotope data were acquired at the University of Edinburgh (in collaboration with Dr John Craven) using a Cameca IMS 1270 with a 133Cs+ focused primary ion beam of ∼4 nA rastered over an area of 25 µm2. Secondary ions were extracted at 10 kV and 16O, 17O and 18O were monitored at a mass resolution of ~6,000 using either a Faraday cup or an electron multiplier depending on the count rates of the isotope. Background, relative detector yield and dead-time corrections were applied to the count rates recorded. Each analysis involved a pre-sputtering time of 60 s, followed by automatic centering of the secondary ion beam into the field aperture (3,000 µm) and entrance slits (30 µm). Isotopic data was acquired in two blocks of ten cycles, amounting to a total count time of 80 seconds per isotope. Results were obtained with a standard error of the mean in the range 0.3-6% depending on the uniformity of the sample surface. Instrument calibration and alignment was checked by measuring the isotopic ratios of a reference mineral standard (Ilmenite) at natural abundance. For powdered samples, such as MOFs in their as-made and calcined forms, the packing stage, in order to obtain a flat and polished surface stable in vacuum for analysis, proved to be extremely challenging. Figure 2.4 Simplified scheme of a secondary ion double-focusing mass spectrometer with the inset showing in detail the generation of secondary ions and charge compensation occurring at the sputter site on the sample surface.
level) mounts using a hydraulic press (approx. 5 tons) and then covered with a gold coat about 30 nm thick.