Philip Wormald

Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues

The challenges associated with synthesizing porous materials mean that new classes of zeolites (zeotypes)—such as aluminosilicate zeolites and zeolite analogues—together with new methods of preparing known zeotypes, continue to be of great importance. Normally these materials are prepared hydrothermally with water as the solvent in a sealed autoclave under autogenous pressure. The reaction mixture usually includes an organic template or ‘structure-directing agent’ that guides the synthesis pathway towards particular structures. Here we report the preparation of aluminophosphate zeolite analogues by using ionic liquids and eutectic mixtures. An imidazolium-based ionic liquid acts as both solvent and template, leading to four zeotype frameworks under different experimental conditions. The structural characteristics of the materials can be traced back to the solvent chemistry used. Because of the vanishingly low vapour pressure of ionic liquids, synthesis takes place at ambient pressure, eliminating safety concerns associated with high hydrothermal pressures. The ionic liquid can also be recycled for further use. A choline chloride/urea eutectic mixture is also used in the preparation of a new zeotype framework.

Inversion of particle-stabilized emulsions of partially miscible liquids by mild drying of modified silica particles

Using a system of modified silica particles and mixtures of water and 2,6-lutidine to form particle-stabilized emulsions, we show that subtle alterations to the hydration of the particle surface can cause major shifts in emulsion structure. We use fluorescence confocal microscopy, solid state nuclear magnetic resonance (NMR) and thermo-gravimetric analysis (TGA) to explore this sensitivity, along with other shifts caused by modifications to the silica surface chemistry. The silica particles are prepared by a variant of the Stöber procedure and are modified by the inclusion of 3-(aminopropyl)triethoxysilane and the dye fluorescein isothiocyanate. Treatment prior to emulsification consists of gently drying the particles under carefully controlled conditions. In mixtures of water and 2,6-lutidine of critical composition, the particles stabilize droplet emulsions and bijels. Decreasing particle hydration yields an inversion of the emulsions from lutidine-in-water (L/W) to water-in-lutidine (W/L), with bijels forming around inversion. So dependent is the emulsion behavior on particle hydration that microscopic differences in drying within a particle sample can cause differences in the wetting behavior of that sample, which helps to stabilize multiple emulsions. The formation of bijels at emulsion inversion is also crucially dependent on the surface modification of the silica.

Frequency-swept pulse sequences for 19F heteronuclear spin decoupling in solid-state NMR.

Heteronuclear spin decoupling pulse sequences in solid-state NMR have mostly been designed and applied for irradiating 1H as the abundant nucleus. Here, a systematic comparison of different methods for decoupling 19F in rigid organic solids is presented, with a special emphasis on the recently introduced frequency-swept sequences. An extensive series of NMR experiments at different MAS frequencies was conducted on fluorinated model compounds, in combination with large sets of numerical simulations. From both experiments and simulations it can be concluded that the frequency-swept sequences SWf-TPPM and SWf-SPINAL deliver better and more robust spin decoupling than the original sequences SPINAL and TPPM. Whereas the existence of a large chemical shift anisotropy and isotropic shift dispersion for 19F does compromise the decoupling efficiency, the relative performance hierarchy of the sequences remains unaffected. Therefore, in the context of rigid organic solids under moderate MAS frequencies, the performance trends observed for 19F decoupling are very similar to those observed for 1H decoupling.

A new methodology for zeolite analogue synthesis using ionic liquids as solvent and template

Here we report a new method of preparing aluminophosphate zeolite analogues using ionic liquids [1] as both solvent and template [2]. An imidazolium-based ionic liquid leads to four zeotype frameworks under different conditions. One material has a novel, structure type with unusual structural characteristics that can be traced back to the solvent chemistry. Because of the vanishingly low vapour pressure of ionic liquids this synthesis, takes place at ambient pressure, eliminating any safety concerns associated with high hydrothermal pressures. The ionic liquid can also be recycled for further use.

Variable temperature high resolution 29Si MAS NMR of siliceous zeolite ferrierite

Variable temperature high resolution magic angle spinning 29Si NMR spectroscopy is used to study the phase transition between the low and high temperature versions of pure silica ferrierite. Spectra collected at 133–400 K indicate that the low temperature phase is probably of lower symmetry than that previously reported using single crystal X-ray diffraction results. The NMR spectra around the phase transition are consistent with the description of the high temperature phase as a dynamic average of possible low temperature structures. A comparison of the NMR chemical shifts with the results of previous X-ray diffraction experiments confirms the validity of proposed correlations and indicates how they can be of predictive value in certain cases.

The Direct DIVAM Experiment : A Spin Dynamics Analysis

Domain selection in polymer NMR is limited to experiments specifically suited to each structural domain owing to its particular spin dynamics and relaxation properties. The DIVAM experiment can be tuned to select for signal from the domain of interest, making it possible to obtain signals specific to different domains using only one experiment. An early description of this sequence explains this tunability using a simple one-spin-relaxation model, thereby limiting the selection mechanism to incoherent processes and thus ignoring the coherent terms such as chemical shift anisotropy (CSA), dipolar coupling and offset terms. Experiments have shown that when the DIVAM sequence is applied directly to the nucleus of interest, referred to as direct DIVAM (DD), transient behavior is observed in the signal intensity on the sample spinning time scale. This indicates that the coherent terms are involved in the selection process; the exact role of these terms is explored in this work. SIMPSON simulations illustrate that the CSA and offset terms can play a dominant role in domain selection; however, the dipole term was relatively ineffective and required large values before substantial selection was predicted. Using a one-spin-relaxation model, which now includes a chemical shift evolution term, an analytical expression for the signal intensity was provided as a function of interpulse delay (tau), excitation angle (theta), relaxation time (T2), and offset frequency (Deltanu). These indicate that the selection behavior varies substantially with differing time scales and excitation angles. For small angles and long delay times DD behaves primarily as a relaxation filter, whereas for larger angles and short delay times the coherent terms take over dominated by the CSA interactions. The DD sequence can therefore be set to select on the basis of the transverse relaxation rate or the strength of the CSA interaction, depending on the excitation angle used.

Studies on the crystal structure, magnetic and conductivity properties of titanium oxycarbide solid solution (TiO1−xCx)

Titanium oxides and carbides are often considered as electrode materials in energy conversion and storage devices due to their high potential conductivity and good stability. Titanium monoxide and titanium carbide have structures that can both be described as rocksalt with the same cubic close packed titanium sublattice with oxygen and carbon, respectively, occupying the octahedral interstices; however, the oxide is characterised by extensive defects on both sublattices whilst the carbide is stoichiometric and might be considered as an interstitial metal. Despite the anticipated very different natures of the oxide and carbide sublattices, these two phases actually form a complete solid solution. In the present investigation, we carefully characterise this titanium oxycarbide solid solution, reporting on the crystal structure, magnetic and electronic conduction properties. Titanium oxycarbide powders (TiO1−xCx with x = 0 ≤ x ≤ 1) have been prepared by solid state reactions of TiO and TiC powder under controlled environments at elevated temperatures. X-ray diffraction and pycnometric density measurements illustrate the gradual transition of the crystal structure of titanium oxycarbides from a vacancy containing rock-salt structure of TiO to fully occupied TiC with increase in carbon content in the oxycarbide lattice. The variation of the lattice parameter of the oxycarbide crystal as a function of the carbon content has been found to be non-linear which can be attributed to variations in the level of vacancies present in metal as well as non-metal sub-lattices. The existence of a short-range ordering of anion vacancies in oxycarbide with a nominal composition of TiO0.5C0.5 where half of oxygen of TiO is replaced by carbon has been confirmed by selected-area electron diffraction studies. Low temperature magnetic and conductivity measurements confirm that all oxycarbide compositions are Pauli paramagnetic and good metallic conductors.