Search

To gain insight into the underlying mechanisms of catalyst durability for the selective catalytic reduction (SCR) of NOx with an ammonia reductant, we employed scanning transmission X‐ray microscopy (STXM) to study Cu‐exchanged zeolites with the CHA and MFI framework structures before and after simulated 135 000‐mile aging. X‐ray absorption near‐edge structure (XANES) measurements were performed at the Al K‐ and Cu L‐edges. The local environment of framework Al, the oxidation state of Cu, and geometric changes were analyzed, showing a multi‐factor‐induced catalytic deactivation. In Cu‐exchanged MFI, a transformation of CuII to CuI and CuxOy was observed. We also found a spatial correlation between extra‐framework Al and deactivated Cu species near the surface of the zeolite as well as a weak positive correlation between the amount of CuI and tri‐coordinated Al. By inspecting both Al and Cu in fresh and aged Cu‐exchanged zeolites, we conclude that the importance of the preservation of isolated CuII sites trumps that of Brønsted acid sites for NH3‐SCR activity. [ABSTRACT FROM AUTHOR]

The high‐silica zeolite SSZ‐27 was synthesized using one of the isomers of the organic structure‐directing agent that is known to produce the large‐pore zeolite SSZ‐26 (CON). The structure of the as‐synthesized form was solved using multi‐crystal electron diffraction data. Data were collected on eighteen crystals, and to obtain a high‐quality and complete data set for structure refinement, hierarchical cluster analysis was employed to select the data sets most suitable for merging. The framework structure of SSZ‐27 can be described as a combination of two types of cavities, one of which is shaped like a heart. The cavities are connected through shared 8‐ring windows to create straight channels that are linked together in pairs to form a one‐dimensional channel system. Once the framework structure was known, molecular modelling was used to find the best fitting isomer, and this, in turn, was isolated to improve the synthesis conditions for SSZ‐27. [ABSTRACT FROM AUTHOR]

Abstract: Facile fabrication of well‐intergrown, oriented zeolite membranes with tunable chemical properties on commercially proven substrates is crucial to broadening their applications for separation and catalysis. Rationally determined electrostatic adsorption can enable the direct attachment of a b‐oriented silicalite‐1 monolayer on a commercial porous ceramic substrate. Homoepitaxially oriented, well‐intergrown zeolite ZSM‐5 membranes with a tunable composition of Si/Al=25–∞ were obtained by secondary growth of the monolayer. Intercrystallite defects can be eliminated by using Na+ as the mineralizer to promote lateral crystal growth and suppress surface nucleation in the direction of the straight channels, as evidenced by atomic force microscopy measurements. Water permeation testing shows tunable wettability from hydrophobic to highly hydrophilic, giving the potential for a wide range of applications. [ABSTRACT FROM AUTHOR]

Abstract: Struktur‐Zusammensetzungs‐Eigenschafts‐Beziehungen für zeolithbasierte Materialien sind entscheidend, um verbesserte Feststoffkatalysatoren zu entwickeln. Die Realisierung kann sich jedoch als schwierig erweisen, da sogar einzelne Zeolithkristalle Heterogenitäten aufweisen können, die sich auf Reaktivität, Diffusion und Stabilität auswirken können. Mithilfe von tomographischen Verfahren wurden Fortschritte bei der Charakterisierung dieser porösen Feststoffe erreicht, allerdings haben alle Methoden eine begrenzte räumliche Auflösung. Nur die Atomsondentomographie (APT) ist bisher in der Lage, 3D‐Rekonstruktionen mit Subnanometer‐Auflösung zu erzeugen. Hier behandeln wir die Untersuchung von Zeolithen mit APT: Hierzu gehören Aspekte der Probenvorbereitung, die Datenerfassung, die Signalzuordnung in Massenspektren, die kristallographische Auflösungsgrenze und eine genaue Datenanalyse. Sämtliche Abschnitte werden mit Beispielen erläutert. Zudem sollen bisher unveröffentlichte Daten und Analysen praktische Strategien aufzeigen, die mögliche Probleme des Einsatzes der APT bei Zeolithen lösen. [ABSTRACT FROM AUTHOR]

Highly b-oriented zeolite ZSM-5 films are critical for applications in catalysis and separations and may serve as models to study diffusion and catalytic properties in single zeolite channels. However, the introduction of catalytically active Al3+ usually disrupts the orientation of zeolite films. Herein, using structure-directing agents with hydroxy groups, we demonstrate a new method to prepare highly b-oriented zeolite ZSM-5 films with a broad range of Si/Al ratios (Si/Al=45 to ∞). Fluorescence micro-(spectro)scopy was used to monitor misoriented microstructures, which are invisible to X-ray diffraction, and show Al3+ framework incorporation and illustrate the differences between misoriented and b-oriented films. The methanol-to-hydrocarbons process was studied by operando UV/Vis diffuse reflectance micro-spectroscopy with on-line mass spectrometry, showing that the b-oriented zeolite ZSM-5 films are active and stable under realistic process conditions. [ABSTRACT FROM AUTHOR]

Zeolites, having widespread applications in chemical industries, are often synthesized using organic templates. These can be cost-prohibitive, motivating investigations into their role in promoting crystallization. Herein, the relationship between framework structure, chemical composition, synthesis conditions and the conformation of the occluded, economical template tetraethylammonium (TEA+) has been systematically examined by experimental and computational means. The results show two distinct regimes of occluded conformer tendencies: 1) In frameworks with a large stabilization energy difference, only a single conformer was found (BEA, LTA and MFI). 2) In the frameworks with small stabilization energy differences (AEI, AFI, CHA and MOR), less than the interconversion of TEA+ in solution, a heteroatom-dependent (Al, B, Co, Mn, Ti, Zn) distribution of conformers was observed. These findings demonstrate that host-guest chemistry principles, including electrostatic interactions and coordination chemistry, are as important as ideal pore-filling. [ABSTRACT FROM AUTHOR]

Understanding the formation of carbon deposits in zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, atom probe tomography (APT) has been used to spatially resolve the 3D compositional changes at the sub-nm length scale in a single zeolite ZSM-5 crystal, which has been partially deactivated by the methanol-to-hydrocarbons reaction using 13C-labeled methanol. The results reveal the formation of coke in agglomerates that span length scales from tens of nanometers to atomic clusters with a median size of 30-60 13C atoms. These clusters correlate with local increases in Brønsted acid site density, demonstrating that the formation of the first deactivating coke precursor molecules occurs in nanoscopic regions enriched in aluminum. This nanoscale correlation underscores the importance of carefully engineering materials to suppress detrimental coke formation. [ABSTRACT FROM AUTHOR]

Crystalline molecular sieves are used in numerous applications, where the properties exploited for each technology are the direct consequence of structural features. New materials are typically discovered by trial and error, and in many cases, organic structure-directing agents (OSDAs) are used to direct their formation. Here, we report the first successful synthesis of a specified molecular sieve through the use of an OSDA that was predicted from a recently developed computational method that constructs chemically synthesizable OSDAs. Pentamethylimidazolium is computationally predicted to have the largest stabilization energy in the STW framework, and is experimentally shown to strongly direct the synthesis of pure-silica STW. Other OSDAs with lower stabilization energies did not form STW. The general method demonstrated here to create STW may lead to new, simpler OSDAs for existing frameworks and provide a way to predict OSDAs for desired, theoretical frameworks. [ABSTRACT FROM AUTHOR]

Katalytisch aktives Aluminium kann unter Einsatz kostengünstiger Alkohole in perfekt orientierte Zeolithfilme eingebaut werden. In ihrer Zuschrift auf S. 11369 demonstrieren B. M. Weckhuysen und Mitarbeiter diesen Prozess, der durch die Windmühlen veranschaulicht wird, die Al in die Kanäle pumpen. Der homogene Al‐Einbau wird durch polarisierte Fluoreszenzmikroskopie mit reaktiven fluoreszierenden Sondenmolekülen bestätigt, symbolisiert durch die Sonne, die die “Fluoreszenz des Kanals” verursacht. [ABSTRACT FROM AUTHOR]