Luis R. Aramburo

In‐situ Scanning Transmission X‐Ray Microscopy of Catalytic Solids and Related Nanomaterials

The present status of in-situ scanning transmission X-ray microscopy (STXM) is reviewed, with an emphasis on the abilities of the STXM technique in comparison with electron microscopy. The experimental aspects and interpretation of X-ray absorption spectroscopy (XAS) are briefly introduced and the experimental boundary conditions that determine the potential applications for in-situ XAS and in-situ STXM studies are discussed. Nanoscale chemical imaging of catalysts under working conditions is outlined using cobalt and iron Fischer-Tropsch catalysts as showcases. In the discussion, we critically compare STXM-XAS and STEM-EELS (scanning transmission electron microscopy-electron energy loss spectroscopy) measurements and indicate some future directions of in-situ nanoscale imaging of catalytic solids and related nanomaterials.

The Porosity, Acidity, and Reactivity of Dealuminated Zeolite ZSM-5 at the Single Particle Level: The Influence of the Zeolite Architecture

A combination of atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), focused-ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron-based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM-5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol-to-olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM-5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM-5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5-50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.

X-ray Imaging of Zeolite Particles at the Nanoscale : Influence of Steaming on the State of Aluminum and the Methanol-To-Olefin Reaction

In view of the limited oil reserves the methanol-to-olefin (MTO) process is an interesting catalytic route to provide raw materials for chemical industries. In the last decades, a vast number of studies have been devoted to increase our understanding of this important catalytic reaction leading to a consensus concerning the mechanism.[1–4] Accordingly, MTO is thought to proceed through the so-called “hydrocarbon pool” (HCP) mechanism,[5, 6] in which methanol is added to an organic scaffold present within the zeolite framework. This is followed by elimination of olefinic species in a closed catalytic cycle. Microporous silicoaluminophosphates and aluminosilicates, such as SAPO-34 and ZSM-5, are often used as MTO catalysts because of their unique acidic and structural properties. In the case of ZSM-5 the formation of ethene and propene is governed by two different catalytic routes,[7,8] allowing in principle to control the ethene/propene ratio. Unfortunately, throughout the MTO reaction undesired carbon deposits are formed in the narrow micropore system of ZSM-5, leading to severely restricted diffusion and therefore limited catalytic activity.[9] To overcome these limitations efforts have been made to improve the pore accessibility during synthesis,[10–12] and/or in post-synthetic steps,[13, 14] resulting in significant improvements in the diffusion properties of ZSM-5. In this work, two commercial ZSM-5 zeolites with dimensions of approximately 200–800 nm have been studied by scanning transmission X-ray microscopy (STXM). The first sample, denoted as ZSM-5-C, was calcined for 6 h at 5508C, whereas the second sample, further labeled as ZSM-5-S, was steamed for 3 h at 7008C. Details on the preparation and characteristics of ZSM-5-C and ZSM-5-S can be found in the Supporting Information (Figures S1–S13, Tables S1–S6). We will show how STXM, in combination with bulk characterization techniques, allows investigating the physicochemical properties of ZSM-5 zeolites in a novel way at the nanoscale.[ 15, 16] More specifically, detailed chemical maps, with a spatial resolution of 70 nm, have been obtained of aluminum, oxygen, and carbon, even under realistic reaction conditions.[17–19] In this manner, the influence of steaming on the state of aluminum, that is, the coordination and spatial distribution, as well as on the MTO performance, has been unraveled.

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;zeolites

3D Nanoscale Chemical Imaging of the Distribution of Aluminum Coordination Environments in Zeolites with Soft X‐Ray Microscopy

Which side are you on? Scanning transmission X-ray microscopy is used for the first time to elucidate the coordination and distribution of aluminum in industrial-relevant zeolites at the single-particle level. Extended regions of a few hundred nanometers, rich in higher aluminum coordination environments, are heterogeneously embedded within the zeolite particle, before and after a steaming post-treatment.

X-Ray Imaging of SAPO-34 Molecular Sieves at the Nanoscale: Influence of Steaming on the Methanol-to-Hydrocarbons Reaction

The effect of a severe steaming treatment on the physicochemical properties and catalytic performance of H‐SAPO‐34 molecular sieves during the methanol‐to‐hydrocarbons (MTH) reaction has been investigated with a combination of scanning transmission X‐ray microscopy (STXM), catalytic testing, and bulk characterization techniques, including ammonia temperature programmed desorption and 27Al and 29Si magic angle spinning nuclear magnetic resonance. For this purpose, two samples, namely a calcined and a steamed H‐SAPO‐34 catalyst powder, have been compared. It has been found that calcined H‐SAPO‐34 displays a high selectivity towards light olefins, yet shows a poor stability as compared to a zeolite H‐ZSM‐5 catalyst. Moreover, in situ STXM at the carbon K‐edge during the MTH reaction allows construction of nanoscale chemical maps of the hydrocarbon species formed within the H‐SAPO‐34 aggregates as a function of reaction time and steam post‐treatment. It was found that there is an initial preferential formation of coke precursor species within the core of the H‐SAPO‐34 aggregates. For longer times on stream the formation of the coke precursor species is extended to the outer regions, progressively filling the entire H‐SAPO‐34 catalyst particle. In contrast, the hydrothermally treated H‐SAPO‐34 showed similar reaction selectivity, but decreased activity and catalyst stability with respect to its calcined counterpart. These variations in MTH performance are related to a faster and more homogeneous formation of coke precursor species filling up the entire steamed H‐SAPO‐34 catalyst particle. Finally, the chemical imaging capabilities of the STXM method at the Al and Si K‐edge are illustrated by visualizing the silicon islands at the nanoscale before and after steaming H‐SAPO‐34.

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 methanol-to-hydrocarbons (MTH) reaction at 350 and 500 °C. 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 measurements 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 °C. 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.

Imaging the effect of a hydrothermal treatment on the pore accessibility and acidity of large ZSM-5 zeolite 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 Bronsted or Lewis acid sites. On the other hand, the 4-fluorostyrene staining reaction shows how mild steaming conditions increase the accessibility towards the Bronsted acid sites, while under severe steaming conditions the Bronsted 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.

Phosphatation of Zeolite H‐ZSM‐5: A Combined Microscopy and Spectroscopy Study

A variety of phosphated zeolite H-ZSM-5 samples are investigated by using a combination of Fourier transfer infrared (FTIR) spectroscopy, single pulse (27)Al, (29)Si, (31)P, (1)H-(31)P cross polarization (CP), (27)Al-(31)P CP, and (27)Al 3Q magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, scanning transmission X-ray microscopy (STXM) and N2 physisorption. This approach leads to insights into the physicochemical processes that take place during phosphatation. Direct phosphatation of H-ZSM-5 promotes zeolite aggregation, as phosphorus does not penetrate deep into the zeolite material and is mostly found on and close to the outer surface of the zeolite, acting as a glue. Phosphatation of pre-steamed H-ZSM-5 gives rise to the formation of a crystalline tridymite AlPO4 phase, which is found in the mesopores of dealuminated H-ZSM-5. Framework aluminum species interacting with phosphorus are not affected by hydrothermal treatment. Dealuminated H-ZSM-5, containing AlPO4 , retains relatively more framework Al atoms and acid sites during hydrothermal treatment than directly phosphated H-ZSM-5.