Sharon Mitchell | Swiss Federal Institute of Technology (ETH) (original) (raw)

Papers by Sharon Mitchell

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

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 ...

Crystal Growth & Design, 2013

The manifestation of zeolite recrystallization and the formation of amorphous aluminosilicate spe... more The manifestation of zeolite recrystallization and the formation of amorphous aluminosilicate species during desilication are examined to better understand the properties of alkaline-treated hierarchical zeolites and their catalytic performance. This is achieved using a systematic experimental strategy, starting from treating the filtrate of alkaline-treated silicalite-1 at different temperatures in the presence of various external additives. No recrystallization is evidenced upon addition of tetrapropylammonium (TPA +) and/or aluminum hydroxide ions (Al(OH) 4-), confirming the low probability of zeolite nucleation and/or growth during desilication. Conversely, ordered mesoporous materials (OMMs) form on addition of cetyltrimethyl ammonium (CTA +) to the filtrate. By using other silicon sources, i.e. tetramethyl orthosilicate or the organosilane dimethyloctadecyl-[3-(trimethoxysilyl)propyl]-ammonium, we verify the facile formation of amorphous materials during alkaline treatment of USY zeolites in the presence of hydrophobic micelle-forming alkyl moieties. A systematic characterization by XRD, TEM, N 2 and Ar adsorption, ICP-OES, and FTIR spectroscopy of pyridine adsorbed demonstrates that zeolites exposed to base solutions containing CTA + display less zeolitic properties, compared to those prepared using TPA + , and should be considered as hierarchical zeolite/OMM composites. Catalytic tests in the alkylation of toluene with isopropyl alcohol or benzyl alcohol demonstrate that CTA +-derived composites do not outperform the conventional USY zeolite. Only the hierarchical USY zeolite prepared by alkaline treatment in the presence of TPA + yielded a superior catalytic performance.

CHIMIA International Journal for Chemistry, 2013

This review emphasizes key recent accomplishments towards the industrial exploitation of hierarch... more This review emphasizes key recent accomplishments towards the industrial exploitation of hierarchically structured zeolites in catalytic processes. A major milestone comprises the demonstration that affordable post-synthetic modifications enable the transformation of any conventional zeolite into hierarchical analogues with tunable porosity and functionality. Through specific examples, belonging to the transformation of fossil fuel and renewable feedstocks, we quantitatively illustrate the spectacular benefits attained upon application of hierarchical zeolite catalysts due to improved accessibility or modification of the type and distribution of active sites. A crucial step for these exciting lab-designed materials to be implemented in industrial processes is to shape them into technical forms. Accordingly, we studied the synthesis, characterization, and catalytic evaluation of millimeter-sized hierarchical zeolite bodies, enriching the fundamental understanding on scale-up and repr...

Chemistry - A European Journal, 2014

Our inspiration was the increasing complexity in the design of alkyne hydrogenation catalysts, fr... more Our inspiration was the increasing complexity in the design of alkyne hydrogenation catalysts, from clean surfaces (1930s), to the archetypal lead-poisoned Lindlar catalyst (1950s), and, more recently, to the newest generation of ligand-modified palladium nanoparticles (2010s). The ongoing pursuit of more selective and sustainable catalytic materials has also been reflected in the reactor operation, shifting from stirred tanks to continuous-flow devices. The cover metaphorically expresses the progress in the understanding of these materials over the years.

Chemical Society Reviews, 2013

Progress in catalysis has been, is, and will always be motivated by societal needs (e.g. environm... more Progress in catalysis has been, is, and will always be motivated by societal needs (e.g. environment, energy, chemicals, fuels), with the ultimate aim of improving process efficiency on a technical scale. Technical catalysts are often complex multicomponent millimetre-sized bodies consisting of active phases, supports, and numerous additives in shaped forms suitable for their commercial application. They can differ strongly in composition, structure, porosity, and performance from research catalysts, i.e. laboratory-developed materials constituted by a single bulk or supported active phase in powder form, which are the predominant focus of academic investigations. The industrial manufacture of heterogeneous catalysts, encompassing the upscaled preparation, formulation, and structuring, is encircled by secrecy and is decisive for the overall process viability. Yet despite the tremendous relevance, understanding the added complexity of these multicomponent systems and the consequences for the respective structure-property-function relationships has been largely neglected. Accordingly, our review examines the intricacies of the scale up of heterogeneous catalysts. While emphasising the lack of fundamental knowledge we point out the multiple functions that additives could provide by enhancing the mass and heat transfer properties, acting as co-catalysts, or imparting improved chemical, mechanical, or thermal stability. Recent exemplary studies developing rational approaches to prepare, characterise, and evaluate technical catalysts are analysed in detail and new directions for research in this field are put forward.