In-situ microwave-assisted catalytic upgrading of heavy oil: Experimental validation and effect of catalyst pore structure on activity.

In-situ combustion alone may not provide sufficient heating for downhole, catalytic upgrading of heavy oil in the Toe-to-Heel Air Injection (THAI) process. In this study, a new microwave heating technique has been proposed as a strategy to provide the requisite heating. Microwave technology is alone able to provide rapid heating which can be targeted at the catalyst packing and/or the incoming oil in its immediate vicinity. It was demonstrated, contrary to previous assertions, that heavy oil can be heated directly with microwaves to 425 °C, which is the temperature needed for successful catalytic upgrading, without the need for an additional microwave susceptor. Upgrading of >3.2° API points, a reduction in viscosity to less than 100 cP, and >12% reduction in sulfur content was achieved using commercially available hydrodesulfurization (HDS) catalyst. The HDS catalyst induced dehydrogenation, with nearly 20% hydrogen detected in the gas product. Hence, in THAI field settings, part of the oil-in-place could be sacrificed for dehydrogenation, with the produced hydrogen directed to aid hydrodesulfurization and improve upgrading. Further, this could provide a route for downhole hydrogen production, which can contribute to the efforts towards the hydrogen economy. A single, unified model of evolving catalyst structure was developed. The model incorporated the unusual gas sorption data, computerized x-ray tomography and electron microprobe characterization, as well as the reaction behavior. The proposed model also highlighted the significant impact of the particular catalyst fabrication process on the catalytic activity.

Molecular hydrogen and catalytic combustion in the production of hyperpolarized 83Kr and 129Xe MRI contrast agents.

Hyperpolarized (hp) (83)Kr is a promising MRI contrast agent for the diagnosis of pulmonary diseases affecting the surface of the respiratory zone. However, the distinct physical properties of (83)Kr that enable unique MRI contrast also complicate the production of hp (83)Kr. This work presents a previously unexplored approach in the generation of hp (83)Kr that can likewise be used for the production of hp (129)Xe. Molecular nitrogen, typically used as buffer gas in spin-exchange optical pumping (SEOP), was replaced by molecular hydrogen without penalty for the achievable hyperpolarization. In this particular study, the highest obtained nuclear spin polarizations were P =29% for(83)Kr and P= 63% for (129)Xe. The results were reproduced over many SEOP cycles despite the laser-induced on-resonance formation of rubidium hydride (RbH). Following SEOP, the H2 was reactively removed via catalytic combustion without measurable losses in hyperpolarized spin state of either (83)Kr or (129)Xe. Highly spin-polarized (83)Kr can now be purified for the first time, to our knowledge, to provide high signal intensity for the advancement of in vivo hp (83)Kr MRI. More generally, a chemical reaction appears as a viable alternative to the cryogenic separation process, the primary purification method of hp(129)Xe for the past 2 1/2 decades. The inherent simplicity of the combustion process will facilitate hp (129)Xe production and should allow for on-demand continuous flow of purified and highly spin-polarized (129)Xe.

The Anatomy of Amorphous, Heterogeneous Catalyst Pellets.

This review focuses on disordered, or amorphous, porous heterogeneous catalysts, especially those in the forms of pellets and monoliths. It considers the structural characterisation and representation of the void space of these porous media. It discusses the latest developments in the determination of key void space descriptors, such as porosity, pore size, and tortuosity. In particular, it discusses the contributions that can be made by various imaging modalities in both direct and indirect characterisations and their limitations. The second part of the review considers the various types of representations of the void space of porous catalysts. It was found that these come in three main types, which are dependent on the level of idealisation of the representation and the final purpose of the model. It was found that the limitations on the resolution and field of view for direct imaging methods mean that hybrid methods, combined with indirect porosimetry methods that can bridge the many length scales of structural heterogeneity and provide more statistically representative parameters, deliver the best basis for model construction for understanding mass transport in highly heterogeneous media.

MF-DFT and experimental investigations of the origins of hysteresis in mercury porosimetry of silica materials.

In order to be able to make a proper interpretation of mercury porosimetry data, to obtain a structural characterization of a porous solid, a full understanding of the causes of hysteresis in mercury porosimetry is required. Several different theories have previously been proposed, but it is still difficult to make a priori predictions of the level of hysteresis anticipated. In this work, the effect of the degree of smaller scale surface roughness on the hysteresis width has been studied using mean-field density functional simulations and the results obtained confirmed by experiments on silica materials. It has been found that the hysteresis width decreases with increased degree of surface roughness, as characterized experimentally by the surface fractal dimension.

NMR studies of cooperative effects in adsorption.

The conversion of gas adsorption isotherms into pore size distributions generally relies upon the assumption of thermodynamically independent pores. Hence, pore-pore cooperative adsorption effects, which might result in a significantly skewed pore size distribution, are neglected. In this work, cooperative adsorption effects in water adsorption on a real, amorphous, mesoporous silica material have been studied using magnetic resonance imaging (MRI) and pulsed-gradient stimulated-echo (PGSE) NMR techniques. Evidence for advanced adsorption can be seen directly using relaxation time weighted MRI. The number and spatial distributions of pixels containing pores of different sizes filled with condensate have been analyzed. The spatial distribution of filled pores has been found to be highly nonrandom. Pixels containing the largest pores present in the material have been observed to fill in conjunction with pixels containing much smaller pores. PGSE NMR has confirmed the spatially extensive nature of the adsorbed ganglia. Thus, long-range (≥40 µm) cooperative adsorption effects, between larger pores associated with smaller pores, occur within mesoporous materials. The NMR findings have also suggested particular types of pore filling mechanisms occur within the porous solid studied.

Techniques for direct experimental evaluation of structure-transport relationships in disordered porous solids.

Determining structure-transport relationships is critical to optimising the activity and selectivity performance of porous pellets acting as heterogeneous catalysts for diffusion-limited reactions. For amorphous porous systems determining the impact of particular aspects of the void space on mass transport often requires complex characterization and modelling steps to deconvolve the specific influence of the feature in question. These characterization and modelling steps often have limited accuracy and precision. It is the purpose of this work to present a case-study demonstrating the use of a more direct experimental evaluation of the impact of pore network features on mass transport. The case study evaluated the efficacy of the macropores of a bidisperse porous foam structure on improving mass transport over a purely mesoporous system. The method presented involved extending the novel integrated gas sorption and mercury porosimetry method to include uptake kinetics. Results for the new method were compared with those obtained by the alternative NMR cryodiffusometry technique, and found to lead to similar conclusions. It was found that the experimentally-determined degree of influence of the foam macropores was in line with expectations from a simple resistance model for a disconnected macropore network.

Modeling the fractal growth of templated, mesoporous silica films.

Brewster angle micrographs have been obtained of the development of a silica film grown at an air-water interface with use of the surfactant template cetylpyridinium chloride. The micrograph images showed that the growing silica film exhibited complex, fractal-like patterns. These images have been analyzed to determine the values of the fractal dimension and lacunarity, and the forms of the autocorrelation function and Euclidean crossover behavior of the silica clusters have been observed. These statistical descriptors have been compared with the equivalent properties of simulated images of model structures generated by computer using a particular variant of the cluster-cluster aggregation (CCA) algorithm. Good agreement was found between the characteristic properties of typical experimental images and the simulated images. It was, therefore, suggested that the CCA process is a good model for the growth of the silica films.

NMR and confocal microscopy studies of the mechanisms of burst drug release from PLGA microspheres.

Pulsed-field gradient (PFG) NMR and confocal microscopy techniques have been used to study the structural evolution and drug release profile of poly(d,l-lactide-co-glycolide) (PLGA) microspheres over time during immersion in an aqueous phase. Variation of the drying process used in the synthesis of the PLGA microspheres has been found to significantly influence the degree of permeability of the spheres to water. PFG NMR has been used to study the change in the cavity sizes within the pore structure of the microspheres over time following initial immersion. In these studies, the temperature of the secondary emulsion, used in the sphere synthesis, has been found to significantly change the temporal evolution of the pore structure. Confocal microscopy studies of the release of a model drug from within the microspheres suggest that the rate-limiting step in drug release is the swelling rate of the polymer matrix, and that the mechanism may be a percolation process. These studies also showed that the local rate of drug release is heterogeneously distributed across a microsphere, and thus, strictly, cannot be modelled as purely a simple diffusive release process from a sphere.

Insights into the influence of the cooling profile on the reconstitution times of amorphous lyophilized protein formulations.

Lyophilized protein formulations must be reconstituted back into solution prior to patient administration and in this regard long reconstitution times are not ideal. The factors that govern reconstitution time remain poorly understood. The aim of this research was to understand the influence of the lyophilization cooling profile (including annealing) on the resulting cake structure and reconstitution time. Three protein formulations (BSA 50mg/ml, BSA 200mg/ml and IgG1 40mg/ml, all in 7% w/v sucrose) were investigated after cooling at either 0.5°C/min, or quench cooling with liquid nitrogen with/without annealing. Significantly longer reconstitution times were observed for the lower protein concentration formulations following quench cool. Porosity measurements found concomitant increases in the surface area of the porous cake structure but a reduction in total pore volume. We propose that slow reconstitution results from either closed pores or small pores impeding the penetration of water into the lyophilized cake.

Determining drug spatial distribution within controlled delivery tablets using MFX imaging.

In this study, the potential of micro-focus X-ray (MFX) imaging as a tool in the design and quality control of drug delivery systems has been demonstrated. MFX imaging has been used to map the spatial distribution of a drug down to length-scales of approximately 10-100 microm and has shown that the spatial distribution of the drug amiodarone within a hydroxypropylmethylcellulose (HPMC) controlled release tablet (diameter 10 mm) is not homogeneous. MFX imaging allows an assessment to be made of the degree of mixing of the original powders. Despite vigorous mixing during the manufacture of the tablets, the drug particles, originally of size 180-420 microm, were found to be concentrated within isolated but spatially extended patches of sizes up to 1 mm that were scattered across the tablet.

A statistical model for the heterogeneous structure of porous catalyst pellets.

The complex structures of the void space of porous media are often characterised by parameters such as pore network connectivity and lattice size. This paper presents a comparison of the estimates of these parameters obtained from two previous methods based on nitrogen sorption and mercury porosimetry, and also from a new, completely independent approach based on pulsed-gradient spin-echo nuclear magnetic resonance (PGSE NMR). It was found that the new PGSE NMR technique obtains estimates of connectivity and lattice size in agreement with nitrogen sorption but different to mercury porosimetry. This difference was attributed to the various physical processes involved actually probing different aspects of the pore space geometry. It was further suggested that the representation of the pore structure derived from either nitrogen sorption or PGSE NMR is really a mapping of the real pore space onto an equivalent abstract, random pore bond network. However, it has been shown that this mapping does capture some of the characteristic properties of the pore space that control transport over mesoscopic ( < 10 microm) length scales. For materials which additionally possessed macroscopic (> 10 microm) structural heterogeneity, it was found that the model could also be adapted to predict the macroscopic transport properties of the porous medium.

The influence of mercury contact angle, surface tension, and retraction mechanism on the interpretation of mercury porosimetry data.

The use of a semi-empirical alternative to the standard Washburn equation for the interpretation of raw mercury porosimetry data has been advocated. The alternative expression takes account of variations in both mercury contact angle and surface tension with pore size, for both advancing and retreating mercury meniscii. The semi-empirical equation presented was ultimately derived from electron microscopy data, obtained for controlled pore glasses by previous workers. It has been found that this equation is also suitable for the interpretation of raw data for sol-gel silica spheres. Interpretation of mercury porosimetry data using the alternative to the standard Washburn equation was found to give rise to pore sizes similar to those obtained from corresponding SAXS data. The interpretation of porosimetry data, for both whole and finely powdered silica spheres, using the alternative expression has demonstrated that the hysteresis and mercury entrapment observed for whole samples does not occur for fragmented samples. Therefore, for these materials, the structural hysteresis and overall level of mercury entrapment is caused by the macroscopic (> approximately 30 microm), and not the microscopic (< approximately 30 microm), properties of the porous medium. This finding suggested that mercury porosimetry may be used to obtain a statistical characterization of sample macroscopic structure similar to that obtained using MRI. In addition, from a comparison of the pore size distribution from porosimetry with that obtained using complementary nitrogen sorption data, it was found that, even in the absence of hysteresis and mercury entrapment, pore shielding effects were still present. This observation suggested that the mercury extrusion process does not occur by a piston-type retraction mechanism and, therefore, the usual method for the application of percolation concepts to mercury retraction is flawed.

Combining mercury thermoporometry with integrated gas sorption and mercury porosimetry to improve accuracy of pore-size distributions for disordered solids.

The typical approach to analysing raw data, from common pore characterization methods such as gas sorption and mercury porosimetry, to obtain pore size distributions for disordered porous solids generally makes several critical assumptions that impact the accuracy of the void space descriptors thereby obtained. These assumptions can lead to errors in pore size of as much as 500%. In this work, we eliminated these assumptions by employing novel experiments involving fully integrated gas sorption, mercury porosimetry and mercury thermoporometry techniques. The entrapment of mercury following porosimetry allowed the isolation (for study) of a particular subset of pores within a much larger interconnected network. Hence, a degree of specificity of findings to particular pores, more commonly associated with use of templated, model porous solids, can also be achieved for disordered materials. Gas sorption experiments were conducted in series, both before and after mercury porosimetry, on the same sample, and the mercury entrapped following porosimetry was used as the probe fluid for theromporometry. Hence, even if one technique, on its own, is indirect, requiring unsubstantiated assumptions, the fully integrated combination of techniques described here permits the validation of assumptions used in one technique by another. Using controlled-pore glasses as model materials, mercury porosimetry scanning curves were used to establish the correct correspondence between the appropriate Gibbs-Thomson parameter, and the nature of the meniscus geometry in melting, for thermoporometry measurements on entrapped mercury. Mercury thermoporometry has been used to validate the pore sizes, for a series of sol-gel silica materials, obtained from mercury porosimetry data using the independently-calibrated Kloubek correlations. The pore sizes obtained for sol-gel silicas from porosimetry and thermoporometry have been shown to differ substantially from those obtained via gas sorption and NLDFT analysis. DRIFTS data for the samples studied has suggested that the cause of this discrepancy may arise from significant differences in the surface chemistries between the samples studied here and that used to calibrate the NLDFT potentials.

Improving sensitivity and accuracy of pore structural characterisation using scanning curves in integrated gas sorption and mercury porosimetry experiments.

Gas sorption scanning curves are increasingly used as a means to supplement the pore structural information implicit in boundary adsorption and desorption isotherms to obtain more detailed pore space descriptors for disordered solids. However, co-operative adsorption phenomena set fundamental limits to the level of information that conventional scanning curve experiments can deliver. In this work, we use the novel integrated gas sorption and mercury porosimetry technique to show that crossing scanning curves are obtained for some through ink-bottle pores within a disordered solid, thence demonstrating that their shielded pore bodies are undetectable using conventional scanning experiments. While gas sorption alone was not sensitive enough to detect these pore features, the integrated technique was, and, thence, this synergistic method is more powerful than the two individual techniques applied separately. The integrated method also showed how the appropriate filling mechanism equation (e.g. meniscus geometry for capillary condensation equations), to use to convert filling pressure to pore size, varied with position along the adsorption branch, thereby enabling avoidance of the further systematic error introduced into PSDs by assuming a single filling mechanism for disordered solids.

NMR cryoporometry characterisation studies of the relation between drug release profile and pore structural evolution of polymeric nanoparticles.

PLGA/PLA polymeric nanoparticles could potentially enhance the effectiveness of convective delivery of drugs, such as carboplatin, to the brain, by enabling a more sustained dosage over a longer time than otherwise possible. However, the link between the controlled release nanoparticle synthesis route, and the subsequent drug release profile obtained, is not well-understood, which hinders design of synthesis routes and availability of suitable nanoparticles. In particular, despite pore structure evolution often forming a key aspect of past theories of the physical mechanism by which a particular drug release profile is obtained, these theories have not been independently tested and validated against pore structural information. Such validation is required for intelligent synthesis design, and NMR cryoporometry can supply the requisite information. Unlike conventional pore characterisation techniques, NMR cryoporometry permits the investigation of porous particles in the wet state. NMR cryoporometry has thus enabled the detailed study of the evolving, nanoscale structure of nanoparticles during drug release, and thus related pore structure to drug release profile in a way not done previously for nanoparticles. Nanoparticles with different types of carboplatin drug release profiles were compared, including burst release, and various forms of delayed release. ESEM and TEM images of these nanoparticles also provided supporting data showing the rapid initial evolution of some nanoparticles. Different stages, within a complex, varying drug release profile, were found to be associated with particular types of changes in the nanostructure which could be distinguished by NMR. For a core-coat nanoparticle formulation, the development of smaller nanopores, following an extended induction period with no structural change, was associated with the onset of substantial drug release. This information could be used to independently validate the rationale for a particular synthesis method. Hence, the specific reasons for the effectiveness of the synthesis route, for obtaining core-coat nanoparticles with delayed release, have been elucidated.

Investigation of the problems with using gas adsorption to probe catalyst pore structure evolution during coking.

A common approach to try to understand the mechanism of coking in heterogeneous catalysts is to monitor the evolution of the pore structure using gas adsorption analysis of discharged pellets. However, the standard methods of analysis of gas adsorption data, to obtain pore-size distributions, make the key assumption of thermodynamically-independent pores. This assumption neglects the possibility of co-operative adsorption phenomena, which will shown to be a critical problem when looking at coking catalysts. In this work the serial adsorption technique has been used to detect and assess the extent of co-operative effects in adsorption within coking catalysts. The reaction of decane over a hydroprocessing catalyst was used as a case study. It has been shown that the conventional analysis method would lead to a flawed picture of the pore structure changes during the coking process. For the case-study considered in this work, it was found that co-operative adsorption effects meant that 26% of the measured adsorption was occurring in pores up to three times larger than the size conventional analysis would presume. The serial adsorption technique was thus shown to provide important additional information on pore structure evolution during coking. A study of the kinetics of adsorption has been used to infer information about the general spatial location of the coking process within a pellet.

Probing hysteresis during sorption of cyclohexane within mesoporous silica using NMR cryoporometry and relaxometry.

The conventional data analysis methods for obtaining a pore size distribution (PSD) from gas sorption data make several critical assumptions that impact significantly on the accuracy of the PSD thereby obtained. In particular, assumptions must be made concerning the nature of the pore-filling or emptying process in adsorption, or desorption, respectively. The possibility of pore-pore interactions is also generally neglected. In this work, NMR cryoporometry and relaxometry have been used to study the adsorption and desorption of cyclohexane within a mesoporous, sol-gel silica catalyst support pellet with the aim of assessing the impact of the aforementioned problems for gas sorption PSDs and developing solutions. The advanced melting effect makes cryoporometry a particularly sensitive probe of adsorbate ganglia spatial distribution. It has been demonstrated that utilising gas sorption scanning curves provided insufficient additional information to alleviate the aforementioned problems with interpreting gas sorption data. The NMR data has shown how the nature of the sorption hysteresis changed with amount adsorbed, due to detectable variations in the mechanisms of pore-filling and emptying along the isotherm. Hence, relating a particular condensation or evaporation pressure to a specific characteristic pore size is not as straightforward as assumed in typical pore size analysis software. However, the NMR techniques reveal the additional information required to improve pore size estimates from gas sorption for disordered solids.

Determination of the location of coke in catalysts by a novel NMR-based, liquid-porosimetry approach.

In this work, a new technique, suitable for chemically-heterogeneous materials, has been used to characterise the structural properties of porous heterogeneous catalysts. A liquid-liquid exchange (LLE) process within nanoporous catalysts has been followed using NMR relaxometry and NMR diffusometry. In order to validate the new technique, two model materials were used. First, a chemically-pure, sol-gel silica, with a simple, mono-disperse pore-space, was studied. The second model material was a bidisperse, eggshell Pt-alumina catalyst. The Pt-alumina catalyst was studied both fresh, and coked following chemical reaction. The degree of structural and chemical complexity added by coking was restricted by the localisation of the coke deposition to the Pt-eggshell layer. Under so-called 'metered' supply conditions, when a high affinity liquid (water) displaced a low affinity liquid (cyclohexane) from the sol-gel silica, entrapment of the low affinity liquid was observed which was similar to that observed in mercury porosimetry. In a similar experiment, comparing LLE in fresh and coked samples of the Pt-alumina catalyst pellets, it was found, for the fresh sample, that water initially displaced cyclohexane from a sub-set of the most accessible, smallest pores, as might expected under metered conditions, but this did not occur for coked catalysts. This finding suggested coking had removed some smaller pores located close to the surface of the pellet, in agreement with where the Pt-metal was preferentially located and coking was known to have occurred.

Probing the impact of advanced melting and advanced adsorption phenomena on the accuracy of pore size distributions from cryoporometry and adsorption using NMR relaxometry and diffusometry.

The accuracy of pore size distributions (PSD) obtained from gas adsorption and cryoporometry is compromised by the presence of advanced adsorption and advanced melting effects, respectively. In order to improve PSD accuracy, it is necessary to know the extent of such effects. In this work cryoporometry and adsorption have been combined to study the onset of advanced melting effects in a sample partially-saturated with different volumes of condensate, in turn, by pre-equilibration with different vapour pressures of adsorbate. NMR relaxometry and diffusometry have been used to independently study the size and connectivity of adsorbed liquid ganglia at different molten fractions. It has been found that the onset of significant advanced melting coincided with abrupt changes in levels of individual pore-filling and ganglia inter-connections determined by NMR. These findings also highlighted where significant advanced adsorption processes were occurring, where larger pores were being filled with condensate before smaller pores. These studies have enabled the critical pores governing the advanced processes to be identified, and the likely errors in PSDs arising from advanced effects to be quantified.

Selective incorporation of functional dicarboxylates into zinc metal-organic frameworks.

Zinc(II) nitrate reacts with different ratios of 1,4-benzenedicarboxylic acid (H(2)bdc) and 2-halo-1,4-benzenedicarboxylic acid (H(2)bdc-X, X = Br or I) to give [Zn(4)O(bdc)(3-x)(bdc-X)(x)], in which preferential incorporation of bdc is observed. The selective incorporation is related to crystal growth rates, and the proportion of incorporated bdc-X rises with increasing reaction time.