Papers by Sharon Mitchell
Journal of Materials Chemistry A, 2022
Layer-by-layer redispersion of high-loading carbon-supported metal nanoparticles into small clust... more Layer-by-layer redispersion of high-loading carbon-supported metal nanoparticles into small clusters and single atoms via cyclic alternating exposure to C2H2 and HCl atmospheres.
Green Chemistry, 2008
Focused microwave irradiation (MI) and freeze drying (FD) techniques have been used to study the ... more Focused microwave irradiation (MI) and freeze drying (FD) techniques have been used to study the generation of layered double hydroxides prepared from MgO-Al 2 O 3 -H 2 O systems. Reactions were undertaken at temperatures of between 100 and 180 • C in order to study the formation of the 3R 1 and 3R 2 polytypes, respectively. MI was found to enhance the rate of formation of both polytypes. FD provided an effective means of quenching the reaction, enabling effective ex situ analysis at intermediate stages. The phase selectivity of the reaction was shown to vary with temperature, promoting the formation of an impurity Mg(OH) 2 at elevated temperatures.
Applied Catalysis B: Environmental, 2014
A key target to reduce current hydrocarbon emissions from vehicular exhaust is to improve their a... more A key target to reduce current hydrocarbon emissions from vehicular exhaust is to improve their abatement under cold-start conditions. Here, we demonstrate the exceptional properties of a hierarchical Cu/H-ZSM-5 zeolite-based system as a catalytic trap. The largely increased external surface area coupled with the preserved intrinsic properties attainable upon alkaline treatment and subsequent acid washing of a conventional ZSM-5 zeolite enable an improved dispersion of copper species in the hierarchical counterpart. The impact of post-synthetic modification on the preparation of copper-containing ZSM-5 is clearly revealed by XRD, N 2 sorption, FTIR, NH 3 -TPD, TEM, SEM, and HAADF-STEM. By providing stronger adsorption sites, the introduction of copper species improves the retention of propene and toluene with respect to the protonic zeolites, and enables their further conversion to carbon dioxide and water on reaching its desorption temperature. The equivalent adsorption capacity and improved oxidation activity and lower susceptibility to deactivation exhibited by the hierarchical Cu/H-ZSM-5 catalyst vis-a-vis its conventional counterpart yield a superior bifunctional catalyst able of reach full hydrocarbon conversion under realistic operating conditions.
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Layered double hydroxides (LDHs) are an important class of layered materials which have attracted... more Layered double hydroxides (LDHs) are an important class of layered materials which have attracted much industrial and academic attention due to the diversity of potential compositions and corresponding range of physical and chemical properties which may result including a number of ...
ACS Catalysis, 2020
We report the promoting effect of graphitic carbon nitride in Cu-catalyzed N-arylation. The abund... more We report the promoting effect of graphitic carbon nitride in Cu-catalyzed N-arylation. The abundance of pyridinic coordination sites in this host permits the adsorption of copper iodide from the reaction medium. The key to achieving high activity is to confine active Cu species on the surface, which is accomplished by introducing atomically-dispersed metal dopants to block diffusion into the bulk. The alternative route of incorporating metal during the synthesis of graphitic carbon nitride is ineffective as Cu is thermodynamically more stable in inactive subsurface positions. A combination of X-ray absorption, X-ray photoelectron, and electron paramagnetic resonance spectroscopy, density functional theory, and Kinetic Monte Carlo simulations is employed to determine the location and associated geometry as well as the electronic structure of metal centers. N-arylation activity correlates to the surface coverage by copper, which varies during the reaction due to an interplay between site formation via adsorption from the reaction medium and deactivation by diffusion into the bulk of the material, and is highest when an Fe dopant is used that hinders movement through the lattice.
Angewandte Chemie (International ed. in English), Jan 11, 2017
Indium oxide catalyzes acetylene hydrogenation with high selectivity to ethylene (>85 %); even... more Indium oxide catalyzes acetylene hydrogenation with high selectivity to ethylene (>85 %); even with a large excess of the alkene. In situ characterization reveals the formation of oxygen vacancies under reaction conditions, while an in depth theoretical analysis links the surface reduction with the creation of well-defined vacancies and surrounding In3 O5 ensembles, which are considered responsible for this outstanding catalytic function. This behavior, which differs from that of other common reducible oxides, originates from the presence of four crystallographically inequivalent oxygen sites in the indium oxide surface. These resulting ensembles are 1) stable against deactivation, 2) homogeneously and densely distributed, and 3) spatially isolated and confined against transport; thereby broadening the scope of oxides in hydrogenation catalysis.
Chemistry – A European Journal, 2015
Al iterature survey reveals ap rominentr eduction in the concentrationo fB r ønsted acid sites in... more Al iterature survey reveals ap rominentr eduction in the concentrationo fB r ønsted acid sites in hierarchically organized zeolites with increasing mesoporous or external surface area independent of the framework type or synthesis route;t his suggestsacommonf undamentale xplanation. To determine the cause, nature,a nd impact of the underlying changes in aluminums peciation, this study combines am ultitechnique analysist hat integrates basic characterization, ad etailed synchrotron XRD and multiple-quantum NMR spectroscopy assessment, and catalytic tests to correlate evolution of the properties with performance during successive steps in the preparation of hierarchicalM FI-typez eolites by desilication. Thef indings,s ubsequently generalized to FAU-and BEA-type materials, identify the crucial impact of calcination on the protonic form, which is an integral step in the synthesis and regeneration of zeolitec atalysts;o n aluminumcoordination;and the associated acidity trends.
Green Chem., 2014
Hydroxyapatites displaying high surface concentrations of calcium exhibit exceptional performance... more Hydroxyapatites displaying high surface concentrations of calcium exhibit exceptional performance in the gas-phase condensation of propanal, a model substrate for the intermediate deoxygenation of biocrude.
Microporous and Mesoporous Materials, 2015
ABSTRACT The predominantly insulating nature of zeolites, as many classes of porous catalysts, ca... more ABSTRACT The predominantly insulating nature of zeolites, as many classes of porous catalysts, can severely impair heat transfer and hence their performance in industrial processes. Strategies developed to engineer the thermophysical properties of technical zeolites for fixed-bed applications comprise the use of conductive secondary phases as structured catalyst supports or as inert diluents. However, the impact of integrating conductive additives into composite zeolite bodies (pellets, extrudates, or granules) has not been widely explored. Here, using a transient hot-plate technique to decouple the distinct contributions of porosity, sample hydration, and temperature, we quantify the impact of metallic (copper), ceramic (silicon carbide, aluminum nitride, boron nitride), and carbonaceous (graphite, carbon nanotubes) phases on the thermal conductivity of shaped zeolites at the body and packed-bed scales. The decisive role of particle morphology, dominating over the intrinsic conductivity of an additive, is corroborated through the three-dimensional reconstruction of data acquired by focused ion beam-scanning electron microscopy and X-ray microtomography coupled with in-situ thermographic studies. In particular, the order-of-magnitude improvement evidenced on application of graphite sheets stems from the extended paths of low thermal resistance created in the millimeter-sized catalyst ensemble. Through the identification of structure-property relations, our approach provides new insights into the rational design of composite porous materials with enhanced heat-transfer properties.
Angewandte Chemie International Edition, 2014
Angewandte Chemie International Edition, 2014
Without sensitive techniques to complex pore architectures, synthetic efforts to enhance molecula... more Without sensitive techniques to complex pore architectures, synthetic efforts to enhance molecular transport in zeolite and other porous materials through hierarchical structuring, lack descriptors for their rational design. Here, we demonstrate the power of positron annihilation lifetime spectroscopy (PALS) to characterize the pore connectivity in hierarchical MFI zeolites, establishing a direct link with the enhanced catalyst lifetime in the conversion of methanol to valuable hydrocarbons. The unique ability to capture subtleties of the hierarchical structure originates from the dynamic nature of the ortho-positronium response to the pore network. The findings reveal the strong dependence on the way in which the hierarchical zeolites are manufactured, having direct implications for the practical realization of these advanced catalysts.
ACS Catalysis, 2014
Additional porosity, such as meso-and macropores, was introduced in zeolite extrudates with the i... more Additional porosity, such as meso-and macropores, was introduced in zeolite extrudates with the intention intuit of improving the effective diffusivity of the catalysts. The samples were characterized in depth by nitrogen adsorption-desorption, mercury intrusion porosimetry, ammonia temperature programmed desorption and adsorption of pyridine followed by infrared spectroscopy. The results revealed no significant change in the acidity but an increase of the pore volume. According to significant improvement in the effective diffusivity, the samples were tested in the methanol-to-hydrocarbons reaction. The catalytic stability was greatly enhanced with an increase in the pore volume, demonstrating a relation between effective diffusivity and resistance to deactivation by coke formation. Further experiments also revealed a higher toluene adsorption capacity and a raise in the breakthrough time over the most porous samples due to better accessibility of toluene molecules into the active sites of the zeolite.
Catalysis Science & Technology, 2011
... Danny Verboekenda, Karine Thomasb, Maria Milinaa, Sharon Mitchella, Javier Pérez-Ramírez* a a... more ... Danny Verboekenda, Karine Thomasb, Maria Milinaa, Sharon Mitchella, Javier Pérez-Ramírez* a and Jean-Pierre Gilson*b. a Institute for Chemical and Bioengineering ... Transmission electron microscopy (TEM) was performed with a Phillips CM12 instrument operated at 100 kV. ...
Catalysis Science & Technology, 2012
Desilication of conventional zeolites in alkaline medium generates intracrystalline mesoporosity,... more Desilication of conventional zeolites in alkaline medium generates intracrystalline mesoporosity, but inevitably changes other properties such as the Si/Al ratio and aluminium distribution. Assessing the individual effects of porosity, composition, and acidity on the catalytic performance of desilicated zeolites is relevant for their optimal design. Herein, we decouple the respective impacts in the acid-catalysed alkylation of toluene (or cyclohexylbenzene) with benzyl alcohol. These reactions experience strong accessibility constraints to the micropores, providing high sensitivity to the properties of the developed mesopore surface. Through strategic comparison of alkaline-treated ZSM-5 zeolites prepared with and without subsequent acid treatment, we show that while acidity is important, the alkylation activity is dominated by the mesoporous surface area. The selectivity to (methylbenzyl)benzene does not depend on the available external surface. Large mesopore volumes offer no catalytic benefit, and variation in micropore volume has a minimal effect. Acid-treated mesoporous zeolites exhibit higher catalytic activities primarily due to textural enhancements by removal of aluminium-rich amorphous debris. The catalytic results are rationalised on the basis of extensive characterisation (AAS, N2 sorption, XRD, TEM, 27 Al MAS NMR, FTIR, NH3-TPD) and adsorption of toluene and cyclohexylbenzene. 65 2. Experimental 2.1. Post-synthetic treatments Various commercial ZSM-5 zeolites were obtained in the ammonium form:
The Journal of Physical Chemistry C, 2012
ABSTRACT Advanced physicochemical characterization of the pore structure of hierarchical zeolites... more ABSTRACT Advanced physicochemical characterization of the pore structure of hierarchical zeolites, including the precise knowledge of pore size, interconnectivity, and surface properties, is crucial in order to interpret their superior performance in catalytic and adsorption applications. Postsynthetic mesopore formation by alkaline treatment of zeolites leads to simultaneous compositional changes due to the selective dissolution of silicon. By careful tuning of these effects through subsequent acid treatment, the Si/Al ratio can be restored to that of the parent zeolite. We evaluate the application of argon (87.3 K) and water (298.5 K) adsorption to assess the porous properties of mesoporous ZSM-5 zeolites with equivalent porosity, but differing composition. An accurate and combined micro/mesopore size analysis is obtained by applying NLDFT (nonlocal density functional theory) analysis on the argon isotherms. The argon adsorption data clearly reveal the two different relative pressure regions of micro- and mesopore filling of hierarchical zeolites. In contrast, the two filling regions overlap upon adsorption of water due to the hydrophobic nature of the ZSM-5 micropores and the much more hydrophilic nature of the mesopores. This indicates that water adsorption is sensitive to the Si/Al ratio, the distribution of aluminum species in the zeolite, and to the presence of polar groups on the mesopore surface. Thus, further insights into the surface and structural properties of the pores in hierarchical zeolites prepared by desilication can be gained. Based on our results, we put forward a hydrophilicity index capable of identifying differences in surface chemistry between distinct porous materials, and also between the micro- and mesopores present within hierarchically structured nanoporous materials.
Nature Chemistry, 2012
A major challenge in the implementation of laboratory-designed catalysts is the scale-up into tec... more A major challenge in the implementation of laboratory-designed catalysts is the scale-up into technically relevant forms. Advanced characterization is essential to understand and optimize catalyst assembly and function in industrial reactors. This Article presents an integrated approach to visualizing millimetre-sized extrudates and granules of a hierarchical MFItype zeolite, displaying trimodal networks of micropores (0.56 nm), intracrystalline mesopores (∼10 nm) and macropores (∼200-300 nm). As exemplified for the conversion of methanol to olefins, the hierarchical zeolite yields a superior performance compared to its conventional analogue. The combination of dedicated specimen preparation with state-of-theart optical, X-ray and electron-based microscopic and tomographic techniques proves a powerful methodology to reveal otherwise inaccessible information regarding structural organization over the whole range of length scales. It is expected that these tools will play a crucial role in the rationalization of scale-up principles in catalyst development.
Journal of Materials Chemistry, 2010
Green Chemistry, 2007
The hydrothermal reaction of slurries containing MgO and Cp3 alumina has been investigated in sit... more The hydrothermal reaction of slurries containing MgO and Cp3 alumina has been investigated in situ using energy dispersive X-ray diffraction (EDXRD). A range of temperatures (100, 150, 180 and 240 uC) were studied. Kinetic data for the formation of the hydrotalcite-like ...
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Papers by Sharon Mitchell