Mick D. Mantle

Molecular motion and ion diffusion in choline chloride based deep eutectic solvents studied by 1H pulsed field gradient NMR spectroscopy.

Deep Eutectic Solvents (DESs) are a novel class of solvents with potential industrial applications in separation processes, chemical reactions, metal recovery and metal finishing processes such as electrodeposition and electropolishing. Macroscopic physical properties such as viscosity, conductivity, eutectic composition and surface tension are already available for several DESs, but the microscopic transport properties for this class of compounds are not well understood and the literature lacks experimental data that could give a better insight into the understanding of such properties. This paper presents the first pulsed field gradient nuclear magnetic resonance (PFG-NMR) study of DESs. Several choline chloride based DESs were chosen as experimental samples, each of them with a different associated hydrogen bond donor. The molecular equilibrium self-diffusion coefficient of both the choline cation and hydrogen bond donor was probed using a standard stimulated echo PFG-NMR pulse sequence. It is shown that the increasing temperature leads to a weaker interaction between the choline cation and the correspondent hydrogen bond donor. The self-diffusion coefficients of the samples obey an Arrhenius law temperature-dependence, with values of self-diffusivity in the range of [10(-10)-10(-13) m(2) s(-1)]. In addition, the results also highlight that the molecular structure of the hydrogen bond donor can greatly affect the mobility of the whole system. While for ethaline, glyceline and reline the choline cation diffuses slower than the associated hydrogen bond donor, reflecting the trend of molecular size and molecular weight, the opposite behaviour is observed for maline, in which the hydrogen bond donor, i.e. malonic acid, diffuses slower than the choline cation, with self-diffusion coefficients values of the order of 10(-13) m(2) s(-1) at room temperature, which are remarkably low values for a liquid. This is believed to be due to the formation of extensive dimer chains between malonic acid molecules, which restricts the mobility of the whole system at low temperature (<30 °C), with malonic acid and choline chloride having almost identical diffusivity values. Diffusion and viscosity data were combined together to gain insights into the diffusion mechanism, which was found to be the same as for ionic liquids with discrete anions.

Single- and two-phase flow in fixed-bed reactors: MRI flow visualisation and lattice-Boltzmann simulations

Abstract Three-dimensional structural magnetic resonance imaging (MRI) and MRI velocimetry have been used to fully characterise the structure of the interparticle pore space and the single-phase flow field in a packed bed of alumina catalyst particles. Three orthogonal components of the velocity ( V x , V y and V z ) are acquired such that the fluid velocity vector is determined at a pore-scale resolution of 156 μm . The pore space has been analysed by unambiguously partitioning the pore space into individual pores. Characteristics of the individual pores are combined with the MRI velocity data to determine quantitative statistical information concerning flow through these pores. The ability of the lattice-Boltzmann simulation technique to predict the flow field visualised by MRI is also demonstrated by performing the simulation on a lattice derived directly from the MRI experimental three-dimensional image of the structure of the packed bed. A direct comparison of the MRI and lattice-Boltzmann results shows there is good agreement between the two methods. Using the pore analysis in conjunction with the velocity information, the flow field through the pore space is shown to be highly heterogeneous with 40% of the fluid flowing through only 10% of the pores. We also show that the lattice-Boltzmann data may be used to calculate average molecular displacement propagators similar to those acquired experimentally for such systems. The effect of the wall on the fluid velocity and porosity is calculated as a function of distance from the wall. Some difference between the MRI and lattice-Boltzmann results are observed close to the wall because of inertial effects in the high velocity channels which are not simulated by the lattice-Boltzmann method. Finally, we present initial results from the extension of this work to two-phase flow in packed beds. A case study of the visualisation of the extent of wetting of the packing as a function of time following start-up is presented.

Magnetic resonance imaging and X-ray microtomography studies of a gel-forming tablet formulation

The capabilities of two methods for investigating tablet swelling are investigated, based on a study of a model gel-forming system. Results from magnetic resonance imaging (MRI) were compared with results from a novel application of X-ray microtomography (XmicroT) to track the movements of embedded glass microsphere tracers as the model tablets swelled. MRI provided information concerning the movement of hydration fronts into the tablets and the composition of the swollen gel layer, which formed at the tablet surface and progressively thickened with time. Conversely, XmicroT revealed significant axial expansion within the tablet core, at short times and ahead of the hydration fronts, where there was insufficient water to be observed by MRI (estimated to be around 15% by weight for the system used here). Thus, MRI and XmicroT may be regarded as complementary methods for studying the hydration and swelling behaviour of tablets.

Investigation of the evaporation of embedded liquid droplets from porous surfaces using magnetic resonance imaging

Abstract For the first time, results are presented from studies using magnetic resonance imaging techniques to follow the behaviour of single water droplets evaporating from porous surfaces. The droplets are initially embedded in the porous substrate by impingement, and are then evaporated over a period of several hours, the surface of the substrate being ventilated by a controlled airflow. The configuration is intended to mimic the behaviour of droplets evaporating into atmospheric flows from surfaces such as sand, or concrete. The method produces several types of data, including images of impinged droplets inside the porous substrate and their development with time during the evaporation episode, one-dimensional concentration profiles through the substrates, and corresponding estimates of the mass fraction of liquid remaining, evaporation rate and mass flux per unit area. The results obtained show that the impinged droplet resides in the porous medium in a shape similar to a semi-spheroid. The results also indicate that the transport of liquid by capillary diffusion has a very strong influence upon the evaporation process, providing a challenge to the simple receding evaporation-front assumption that is utilised in many modelling procedures.

Bifurcation phenomena in the flow through a sudden expansion in a circular pipe

We report the results of an experimental investigation into bifurcation phenomena in laminar and time-dependent flows through a sudden expansion in a circular pipe. The flow state was investigated using high resolution magnetic resonance imaging techniques which have enabled us to identify important qualitative features in the flow field and make quantitative estimates of the velocity distributions. Using these techniques we have uncovered evidence for a steady symmetry breaking bifurcation at a critical value of the Reynolds number. The asymmetric steady state which arises at the bifurcation point eventually gives way to time dependence in the form of bursts of periodic motion with further increase in flow rate.

Single excitation multiple image RARE (SEMI-RARE): ultra-fast imaging of static and flowing systems.

This paper describes the development and application of a new rapid, full k-space acquisition imaging pulse sequence based on the rapid acquisition with relaxation enhancement (RARE) methodology. We have termed this pulse sequence single excitation multiple image RARE (SEMI-RARE). We demonstrate the application of SEMI-RARE to the visualisation of a static liquid phantom and it is shown that up to 120 images can be acquired from a single excitation. By exploiting the inherent relaxation and diffusion contrast within the series of images, the SEMI-RARE provides an ultra-fast method for characterising the spatial distribution of chemical species and phases within complex systems. The pulse sequence is then applied to the study of single- and two-phase flow in a single narrow tube of inner diameter 2.9mm. In particular, it is shown that 8 two-dimensional slice-selective images for two-phase bubble-train flow in a single tube can be acquired from a single excitation at effective echo-times of 37, 109, 181, 253, 325, 397, 469, and 541ms. The visualisation enables the determination of gas/liquid bubble sizes and velocities during two-phase flow. We also report the first direct evidence, obtained from magnetic resonance measurements, of liquid re-circulation zones associated with bubble-train flow. The robustness of the SEMI-RARE technique makes it an attractive fast imaging technique for the study of multi-phase flow phenomena, which are often characterised by large variations in magnetic susceptibility, and are of widespread interest in chemical engineering.

In situ magnetic resonance measurement of conversion, hydrodynamics and mass transfer during single- and two-phase flow in fixed-bed reactors

In recent years there has been increasing interest in applying magnetic resonance (MR) techniques in areas of engineering and chemical technology. The science that underpins many of these applications is the physics and chemistry of transport and reaction processes in porous materials. Key to the exploitation of MR methods will be our ability to demonstrate that MR yields information that cannot be obtained using conventional measurement techniques in engineering research. This article describes two case studies that highlight the power of MR to give new insights to chemical engineers. First, we demonstrate the application of MR techniques to explore both mass transfer and chemical conversion in situ within a fixed bed of catalyst, and we then use these data to identify the rate-controlling step of the chemical conversion. Second, we implement a rapid imaging technique to study the stability of the gas-liquid distribution in the low- and high-interaction two-phase flow regimes in a trickle-bed reactor.

Determination of bed voidage using water substitution and 3D magnetic resonance imaging, bed density and pressure drop in packed-bed reactors

Abstract Using a water substitution method to determine bed voidage, an independent relationship between bed height and bed voidage was observed for the trilobe (virgin and crushed) and cylindrical alumina supports. Typical bed voidage values of 0.49–0.51 (virgin trilobe). 0.46–0.52 (crushed trilobe) and 0.28–0.31 (cylindrical) were observed within 0.1 and 0.19 m i.d. columns. However, bed voidage values were approximately 6% larger in the 0.05 m i.d. column and could be attributed to a greater extent of wall zone voidage. Dense packing of the columns in all cases resulted in a decrease in bed voidage which had significant effects on bed density and column pressure drop. In addition to the water substitution measurements of bed voidage, three-dimensional magnetic resonance imaging (MRI) data were used in conjunction with digital image analysis techniques to obtain one-dimensional radial profiles of voidage from comparable alumina catalyst support material. Similar results and trends in voidage values between the water substitution method and those obtained from MRI data are evident. In all cases, the analysis of the MRI data yields voidage values that are consistently higher than those obtained from water substitution measurements.

Rapid distinction of intracellular and extracellular proteins using NMR diffusion measurements.

In-cell NMR spectroscopy offers a unique opportunity to begin to investigate the structures, dynamics, and interactions of molecules within their functional environments. An essential aspect of this technique is to define whether observed signals are attributable to intracellular species rather than to components of the extracellular medium. We report here the results of NMR measurements of the diffusion behavior of proteins expressed within bacterial cells, and find that these experiments provide a rapid and nondestructive probe of localization within cells and can be used to determine the size of the confining compartment. We show that diffusion can also be exploited as an editing method to eliminate extracellular species from high-resolution multidimensional spectra, and should be applicable to a wide range of problems. This approach is demonstrated here for a number of protein systems, using both (15)N and (13)C (methyl-TROSY) based acquisition.

Solvent Effect and Reactivity Trend in the Aerobic Oxidation of 1,3‐Propanediols over Gold Supported on Titania: NMR Diffusion and Relaxation Studies

In recent work, it was reported that changes in solvent composition, precisely the addition of water, significantly inhibits the catalytic activity of Au/TiO2 catalyst in the aerobic oxidation of 1,4-butanediol in methanol due to changes in diffusion and adsorption properties of the reactant. In order to understand whether the inhibition mechanism of water on diol oxidation in methanol is generally valid, the solvent effect on the aerobic catalytic oxidation of 1,3-propanediol and its two methyl-substituted homologues, 2-methyl-1,3-propanediol and 2,2-dimethyl-1,3-propanediol, over a Au/TiO2 catalyst has been studied here using conventional catalytic reaction monitoring in combination with pulsed-field gradient nuclear magnetic resonance (PFG-NMR) diffusion and NMR relaxation time measurements. Diol conversion is significantly lower when water is present in the initial diol/methanol mixture. A reactivity trend within the group of diols was also observed. Combined NMR diffusion and relaxation time measurements suggest that molecular diffusion and, in particular, the relative strength of diol adsorption, are important factors in determining the conversion. These results highlight NMR diffusion and relaxation techniques as novel, non-invasive characterisation tools for catalytic materials, which complement conventional reaction data.