Spatiotemporal Spectroscopy of Molecular Sieves : Interplay between Hydrothermal Treatments and the Methanol-To-Hydrocarbons reaction

Unraveling the influence that heterogeneities within a catalytic material may have on its overall performance is far from trivial and still represents a scientific challenge. It requires the development and use of advanced in-situ characterization techniques, capable of bridging the different length scales, ranging from the level of the reactor down to the catalytically active phase. Moreover, such a selection of techniques should be applied under relevant reaction conditions, being able to characterize the inorganic as well as the organic phases present in the catalyst material and desirably causing the least possible interference with its working principles. Within this context, an array of bulk and micro-spectroscopic characterization techniques has been used in this PhD thesis to increase our understanding of the effect of a hydrothermal treatment on the physicochemical properties and related reactivity of the archetypal molecular sieves, namely ZSM-5 and SAPO-34, for the Methanol-to-Hydrocarbons (MTH) reaction. For this purpose, two different catalyst systems have been investigated, namely large zeolite ZSM-5 crystals and commercially available ZSM-5 and SAPO-34 catalyst powders. The former materials have been used as model systems to elucidate the effect of steaming on the porosity, pore accessibility, acidity and MTH reactivity of these catalytic solids. On the other hand, commercially available ZSM-5 and SAPO-34 powders have been used as practical catalyst systems to elucidate the effect of a hydrothermal treatment on both the physicochemical properties and reactivity at the level of a single catalyst particle. In this manner, new insights in the origin and impact of spatiotemporal heterogeneities on MTH catalysis have been obtained

Spatiotemporal Spectroscopy of Molecular Sieves : Interplay between Hydrothermal Treatments and the Methanol-To-Hydrocarbons reaction

Unraveling the influence that heterogeneities within a catalytic material may have on its overall performance is far from trivial and still represents a scientific challenge. It requires the development and use of advanced in-situ characterization techniques, capable of bridging the different length scales, ranging from the level of the reactor down to the catalytically active phase. Moreover, such a selection of techniques should be applied under relevant reaction conditions, being able to characterize the inorganic as well as the organic phases present in the catalyst material and desirably causing the least possible interference with its working principles. Within this context, an array of bulk and micro-spectroscopic characterization techniques has been used in this PhD thesis to increase our understanding of the effect of a hydrothermal treatment on the physicochemical properties and related reactivity of the archetypal molecular sieves, namely ZSM-5 and SAPO-34, for the Methanol-to-Hydrocarbons (MTH) reaction. For this purpose, two different catalyst systems have been investigated, namely large zeolite ZSM-5 crystals and commercially available ZSM-5 and SAPO-34 catalyst powders. The former materials have been used as model systems to elucidate the effect of steaming on the porosity, pore accessibility, acidity and MTH reactivity of these catalytic solids. On the other hand, commercially available ZSM-5 and SAPO-34 powders have been used as practical catalyst systems to elucidate the effect of a hydrothermal treatment on both the physicochemical properties and reactivity at the level of a single catalyst particle. In this manner, new insights in the origin and impact of spatiotemporal heterogeneities on MTH catalysis have been obtained

Imaging the Effect of a Hydrothermal Treatment on the Pore Accessibility and Acidity of Large Zeolite ZSM-5 crystals by selective staining

Confocal fluorescence microscopy has been used in combination with bulky non-reactive dyes (i.e. proflavine, stilbene and nile blue A) and two staining reactions (i.e. fluorescein synthesis and 4-fluorostyrene oligomerisation) to study the effect of steaming on pore accessibility and acidity of large ZSM-5 zeolite crystals. This approach enabled the 3-D visualization of cracks and mesopores connected to the outer zeolite surface as well as mesoporous ‘‘cavities’’ within steamed ZSM-5 zeolite crystals. It has been found that besides the generation of mesoporosity steaming makes the boundaries between the different crystal sub-units accessible for bulky molecules. Additionally, the fluorescein staining reaction reveals prominent formation of structural defects that are connected to the surface of the crystal via the microporous ZSM-5 system and which contain either Brønsted or Lewis acid sites. On the other hand, the 4-fluorostyrene staining reaction shows how mild steaming conditions increase the accessibility towards the Brønsted acid sites, while under severe steaming conditions the Brønsted acidity contained in the internal crystal sub-units is more accessible, although it is preferentially removed close to the surface of the lateral sub-units of ZSM-5 zeolite crystals

Architecture-dependent distribution of Mesopores in steamed Zeolite crystals as visualized by FIB-SEM Tomography

Break on through: Steaming-induced mesopores of individual ZSM-5 crystals were studied by a combination of focused ion beam (FIB) and scanning electron microscopy (SEM) tomography (see picture). In this manner, quantitative insight into the width, length, morphology, and distribution of mesopores generated within zeolite crystals has been obtained. Keywords:crystal intergrowth;scanning probe microscopy;mesoporosity;tomography;zeolite

Large Zeolite H-ZSM-5 Crystals as Models for the Methanol-to-Hydrocarbons Process: Bridging the Gap between Single-Particle Examination and Bulk Catalyst Analysis

The catalytic, deactivation, and regeneration characteristics of large coffin-shaped H-ZSM-5 crystals were investigated during the methanolto- hydrocarbons (MTH) reaction at 350 and 500 8C. Online gas-phase effluent analysis and examination of retained material thereof were used to explore the bulk properties of large coffin-shaped zeolite H-ZSM-5 crystals in a fixed-bed reactor to introduce them as model catalysts for the MTH reaction. These findings were related to observations made at the individual particle level by using polarization-dependent UV-visible microspectroscopy and mass spectrometric techniques after reaction in an in situ microspectroscopy reaction cell. Excellent agreement between the spectroscopic mea- ACHTUNGTRENUNGsurements and the analysis of hydrocarbon deposits by means of retained hydrocarbon analysis and time-of-flight secondary-ion mass spectrometry of spent catalyst materials was observed. The obtained data reveal a shift towards more condensed coke deposits on the outer zeolite surface at higher reaction temperatures. Zeolites in the fixed-bed reactor setup underwent more coke deposition than those reacted in the in situ microspectroscopy reaction cell. Regeneration studies of the large zeolite crystals were performed by oxidation in O2/inert gas mixtures at 550 8C. UV-visible microspectroscopic measurements using the oligomerization of styrene derivatives as probe reaction indicated that the fraction of strong acid sites decreased during regeneration. This change was accompanied by a slight decrease in the initial conversion obtained after regeneration. H-ZSM-5 deactivated more rapidly at higher reaction temperature