Jan Kornatowski

Space- and time-resolved in-situ spectroscopy on the coke formation in molecular sieves: methanol-to-olefin conversion over H-ZSM-5 and H-SAPO-34.

Formation of coke in large H-ZSM-5 and H-SAPO-34 crystals during the methanol-to-olefin (MTO) reaction has been studied in a space- and time-resolved manner. This has been made possible by applying a high-temperature in-situ cell in combination with micro-spectroscopic techniques. The buildup of optically active carbonaceous species allows detection with UV/Vis microscopy, while a confocal fluorescence microscope in an upright configuration visualises the formation of coke molecules and their precursors inside the catalyst grains. In H-ZSM-5, coke is initially formed at the triangular crystal edges, in which straight channel openings reach directly the external crystal surface. At reaction temperatures ranging from 530 to 745 K, two absorption bands at around 415 and 550 nm were detected due to coke or its precursors. Confocal fluorescence microscopy reveals fluorescent carbonaceous species that initially form in the near-surface area and gradually diffuse inwards the crystal in which internal intergrowth boundaries hinder a facile penetration for the more bulky aromatic compounds. In the case of H-SAPO-34 crystals, an absorption band at around 400 nm arises during the reaction. This band grows in intensity with time and then decreases if the reaction is carried out between 530 and 575 K, whereas at higher temperatures its intensity remains steady with time on stream. Formation of the fluorescent species during the course of the reaction is limited to the near-surface region of the H-SAPO-34 crystals, thereby creating diffusion limitations for the coke front moving towards the middle of the crystal during the MTO reaction. The two applied micro-spectroscopic techniques introduced allow us to distinguish between graphite-like coke deposited on the external crystal surface and aromatic species formed inside the zeolite channels. The use of the methods can be extended to a wide variety of catalytic reactions and materials in which carbonaceous deposits are formed.

NMR Studies of Single-File Diffusion in Unidimensional Channel Zeolites

Single-file diffusion is the restricted propagation of particles that cannot pass each other. The occurrence of this phenomenon should be reflected by a change in the time dependence of the mean particle displacement in comparison with ordinary diffusion. Although this process is considered to be the rate-controlling mechanism in a large variety of processes, so far no direct evidence of this phenomenon has been provided. Diffusion measurements made with pulsed field gradient nuclear magnetic resonance (NMR) in unidimensional pore systems (zeolites AlPO4-5 and Theta-1) reflect the expected time dependence of single-file diffusion.

Morphology-dependent zeolite intergrowth structures leading to distinct internal and outer-surface molecular diffusion barriers

Zeolites play a crucial part in acid-base heterogeneous catalysis. Fundamental insight into their internal architecture is of great importance for understanding their structure-function relationships. Here, we report on a new approach correlating confocal fluorescence microscopy with focused ion beam-electron backscatter diffraction, transmission electron microscopy lamelling and diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to study a wide range of coffin-shaped MFI-type zeolite crystals differing in their morphology and chemical composition. This powerful combination demonstrates a unified view on the morphology-dependent MFI-type intergrowth structures and provides evidence for the presence and nature of internal and outer-surface barriers for molecular diffusion. It has been found that internal-surface barriers originate not only from a 90 degrees mismatch in structure and pore alignment but also from small angle differences of 0.5 degrees-2 degrees for particular crystal morphologies. Furthermore, outer-surface barriers seem to be composed of a silicalite outer crust with a thickness varying from 10 to 200 nm.

Coke Formation during the Methanol‐to‐Olefin Conversion: In Situ Microspectroscopy on Individual H‐ZSM‐5 Crystals with Different Brønsted Acidity

Coke formation during the methanol-to-olefin (MTO) conversion has been studied at the single-particle level with in situ UV/Vis and confocal fluorescence microscopy. For this purpose, large H-ZSM-5 crystals differing in their Si/Al molar ratio have been investigated. During MTO, performed at 623 and 773 K, three major UV/Vis bands assigned to different carbonaceous deposits and their precursors are observed. The absorption at 420 nm, assigned to methyl-substituted aromatic compounds, initiates the buildup of the optically active coke species. With time-on-stream, these carbonaceous compounds expand in size, resulting in the gradual development of a second absorption band at around 500 nm. An additional broad absorption band in the 600 nm region indicates the enhanced formation of extended carbonaceous compounds that form as the reaction temperature is raised. Overall, the rate of coke formation decreases with decreasing aluminum content. Analysis of the reaction kinetics indicates that an increased Brønsted acid site density facilitates the formation of larger coke species and enhances their formation rate. The use of multiple excitation wavelengths in confocal fluorescence microscopy enables the localization of coke compounds with different molecular dimensions in an individual H-ZSM-5 crystal. It demonstrates that small coke species evenly spread throughout the entire H-ZSM-5 crystal, whereas extended coke deposits primarily form near the crystal edges and surfaces. Polarization-dependent UV/Vis spectroscopy measurements illustrate that extended coke species are predominantly formed in the straight channels of H-ZSM-5. In addition, at higher temperatures, fast deactivation leads to the formation of large aromatic compounds within channel intersections and at the external zeolite surface, where the lack of spatial restrictions allows the formation of graphite-like coke.

Optical second harmonic generation (SHG) on p-nitroaniline in large cyrstals of AIPO4-5 and ZSM-5

Abstract Molecular sieves are suitable host matrices to orient molecules of high hyperpolarizability for optical second harmonic generation (SHG). The intensity of the SHG effect of p-nitroaniline (PNA) adsorbed in large AIPO4-5 and ZSM-5 crystals was analyzed as a function of the direction of the plane of polarization of the incident laser light. The SHG effect was found to be strongly direction-dependent. For both AIPO4-5 and ZSM-5, the maximum SHG is obtained if the electric field vector of the incident laser lights oscillations in the direction of the straight channels, thus exciting the pearl-stringlike-incorporated PNA molecules. To have a hihg second harmonic intensity for any technical application requires that a majority of crystals in the sample should be aligned and not used as a disordered powder assemblage. A suitable method to align molecular sieve crystals by means of an electric field is presented.

Investigation of sorption and transport of sorbate molecules in crystals of MFI structure type by iodine indicator technique

Abstract Iodine indicator technique (IIT) involving light microscopy has been introduced in our laboratory to investigate sorption and mass transport phenomena in zeolites and peculiarities of crystal morphology via coloring of zeolite crystals. The coloring of silicalite-1 90°-intergrowths was performed using pure iodine vapors and binary solutions of iodine in organic solvents (benzene, cyclohexane, toluene, ethylbenzene, p -xylene, and decahydronaphthalene). Coloring from the liquid phase was carried out either under co- or counter-diffusion conditions. The rate of coloring from the vapor phase, and under co-diffusion conditions from the liquid phase, was found to be limited by external mass transport and the coloring was uniform. Under counter-diffusion conditions, the rate of coloring was to about 2 to 3 orders of magnitude lower than in the latter case. Except for decahydronaphthalene, the coloring patterns were non-uniform, and they visualized at least in the beginning stage of the coloring process the interfaces of crystal sections. Also, the cracks were visualized being decorated with iodine. There is a marked difference between the development of coloring patterns of benzene and cyclohexane on one side and their linear alkyl derivatives (toluene, ethylbenzene and p -ethyltoluene) on the other side. In the former case, the iodine enters the bulk of crystal sections via section interfaces. In the latter case, the bulk of the sections appears to be inaccessible for iodine molecules. Other information on channel system accessibility provides IIT for decahydronaphthalene, where the molecular sieve effect of 10-membrane oxygen rings with respect to decahydronaphthalene is indicated.

Spectroscopic studies of vanadium-substituted zeolitic silicates of MFI topology

Samples of silicalite-1 with vanadium ions incorporated in stable framework positions (KVS-5) have been synthesized using several different compounds of V. The materials were investigated by XRD, chemical analysis and SEM–EPM techniques and UV–VIS, EPR, NMR, IR and Raman spectroscopy. One group of the samples contained up to 0.1 wt. % of V only, predominantly in a tetrahedral coordination and in two oxidation states V4+(violet) and V5+(colourless). The other group consisted of materials with about 1 wt. % of V, coordinated mostly in square pyramids with vanadyl groups (dirty green to blue–grey). The pyramids occur in two frequent configurations and one that is rare. The different shapes are probably due to various distributions of stronger and weaker V—O bonds. After calcination, vanadium is fully oxidized to V5+ without leaving its framework position. The tetrahedral coordination remains stable and the pyramids transform to strongly distorted octahedra complemented by water molecules (yellow). All these transformations, as well as the redox reaction V4+↔ V5+, are reversible. In all coordinations, the V centres remain neutral and contain, in addition to the framework V—O bonds, either a type of non-acidic OH group or, more likely, ONa groups with non-exchangeable Na ions. A model is proposed for transformations of V complexes.

Decomposition of template in SAPO-5 and AlPO4-5 molecular sieves studied by IR and Raman spectroscopy

Abstract The state of the triethylamine (TEA) template in AlPO 4 -5 and SAPO-5 and the characteristics of the framework structure of the molecular sieves were studied by means of IR and Raman spectroscopy. Decomposition of TEA and the formation of OH groups were investigated by DRIFT measurements carried out within the temperature range from 300 to 633 K. It has been found that TEA exists as triethylammonium ions in the as-synthesized samples, and its decomposition occurs via abstraction of ethylene and the stepwise formation of diethyl- and ethylammonium cations. The decomposition mode under a nitrogen atmosphere is different from that under air and requires another temperature. A thermal treatment of both molecular sieves at above 970 K results in a transformation of the structure.

Growth of large crystals of ZSM-5 zeolite

The hydrothermal growth of large monocrystals of TPAZSM-5 zeolite up to 420 μm is reported. The samples consist of fully crystalline and pure zeolitic phases with good homogeneity of the crystal sizes.

New Molecular Sieve - Vanadium Silicalite Kvs - 5

Summary Vanadium silicalite KVS-5 has been grown under conditions of hydrothermal synthesis. Its structure corresponds to the MFI type. The product can be synthesized with a yield of 100% in form of slightly coloured large crystals up to 300 μm. The incorporation of V5+ ions on T-sites in the zeolitic framework has been investigated by a variety of methods: XRD, MAS NMR, SSIMS, ESR, TG/DTA, IR, and XRF.