Bodo Zibrowius

Multinuclear MAS n.m.r, and i.r. spectroscopic study of silicon incorporation into SAPO-5, SAPO-31, and SAPO-34 molecular sieves

27Al, 31P, 29Si, and 1H MAS n.m.r. as well as diffuse reflectance i.r. spectroscopy were used to investigate the isomorphous substitution of silicon into the framework of aluminophosphate molecular sieves with different pore sizes. 29Si MAS n.m.r. was found to be the only direct method to study this type of silicon incorporation. In all SAPOs investigated, the incorporation of silicon generates two different kinds of Bronsted acidic sites irrespective of the number of nonequivalent T sites in the framework. The results of both the i.r. spectroscopy and the 1H MAS n.m.r. spectroscopy in the presence of sorbed molecules corroborate the commonly accepted assignment of these two different species to undisturbed hydroxyls and hydroxyls interacting with further framework oxygen, respectively.

Spectroscopic and physicochemical characterization of P-Modified H-ZSM-5

Acidic properties of P-modified H-ZSM-5 have been characterized by means of ammonium exchange, temperature-programmed desorption of ammonia (NH3 TPD), as well as IR, 27Al MAS NMR, and 31P MAS NMR spectroscopy. The introduction of phosphorus by impregnation of the zeolite with an aqueous solution of orthophosphoric acid decreases the number of Bronsted acid sites. Bronsted acidity can be restored completely by elution of the orthophosphoric acid with hot water. Only after calcination or steaming at elevated temperatures of the H3PO4-impregnated sample is an irreversible decrease of Bronsted acidity caused by dealumination observed. The formation of aluminum phosphates in nonframework positions proceeds simultaneously. Thermal treatment of H3PO4-impregnated H-ZSM-5 results in degrees of dealumination lower than those with non-impregnated samples, indicating that phosphorus partially protects aluminum ions from being removed from their substitutional positions in the silica framework.

NMR and IR studies of zeolite H-ZSM-5 modified with orthophosphoric acid

By combined use of solid-state NMR, NMR self-diffusion techniques, and diffuse reflectance IR spectroscopy, important information on the nature of the interaction of orthophosphoric acid with zeolite H-ZSM-5 could be obtained. By aH NMR self-diffusion experiments it was found that the phosphoric acid enters the ZSM-5 channel system. However, with increasing amounts of H3PO 4 deposited, there is an enrichment of phosphorus species near the external surface of the zeolite crystals, forming a transport barrier for diffusing molecules. 27A1 MAS NMR shows that the decreased concentration of BrCnsted acid sites in H-ZSM-5 after impregnation with orthophosphoric acid and a subsequent thermal treatment at 500°C is a direct result of a framework dealumination. Combined 27A1, 29Si, 3ip, and 1H MAS NMR studies reveal that this dealumination process is promoted by the presence of increasing amounts of H3PO4 and results in the formation of occluded aluminum phosphate species. Furthermore, IH MAS NMR shows that H3PO 4 impregnation causes a drastic reduction in the concentration of nonacidic silanol groups. Both findings of 1H MAS NMR have been verified by diffuse reflectance IR spectroscopy. Consequently, treatment of ZSM-5 with orthophosphoric acid modifies acidity and diffusivity in the sample series under study. These effects can combine to give a useful catalyst of enhanced paraselectivity.

Investigation of internal silanol groups as structural defects in ZSM-5-type zeolites

A 1H Magic-angle spinning nuclear magnetic resonance investigation of large and uniform crystals of HZSM-5 type zeolites, synthesized either by means of TPA or without organic templates, shows that using TPA leads to a high concentration of defect centres within the ZSM-5 structure. Owing to non-intact Si—O—Si bonds, up to 8% of the lattice Si were found to be present as Si—OH groups. A comparison of the 1H—29Si cross-polarization spectra with the variation of the 1H n.m.r. chemical shift of the Si—OH groups as well as with the results of ion-exchange experiments suggests a vicinal arrangement of the silanol groups. High concentrations of such structural defects within the crystals lead to a slight decrease of crystal sorption capacity for n-hexane and significantly reduce crystal hydrophobicity. Both the intracrystalline self-diffusion and the molecular reorientation of sorbed molecules as measured by means of the 1H n.m.r. pulsed field gradient technique and 13C n.m.r. lineshape analysis, respectively, show an enhanced molecular mobility of guest molecules within the defective ZSM-5 structure

Deuterium nuclear magnetic resonance studies of the molecular dynamics of benzene in zeolites

Deuterium n.m.r. spectroscopy has been used to investigate the microdynamics of benzene molecules sorbed on HZSM-5, NaX and NaY zeolites. As in the liquid and solid benzene phases, the sorbed benzene molecules perform fast reorientations about their hexad axes. Superimposed on this C6 reorientation are jumps which the benzene molecules perform between the sorption sites. The mean residence time between two succeeding jumps depends significantly on the zeolite framework, the temperature and the loading. From this finding, diffusivities of sorbed benzene molecules can be estimated.

Synthesis and properties of the silicoaluminophosphate molecular sieve SAPO-31

The silicoaluminophosphate molecular sieve SAPO-31 has been synthesized and characterized by XRD, 27Al, 29Si, and 31P MASNMR, IR spectroscopy, electron microscopy, DTA-TGA, chemical analysis, and the catalytic conversion of methanol to light olefins. The results from XRD and NMR investigations show that hydration causes structural changes of the framework.

VPI-5 and related aluminophosphates: Preparation and thermal stability

Abstract The results of investigations on the crystallization conditions of VPI-5 and other AIPO4 phases are described. Four different aluminophosphates occurred and have been characterized by XRD, SEM, and t.g.a. The thermal stability of VPI-5 has been studied by n.m.r. and FTi.r. techniques and its micropore volume has been evaluated by nitrogen adsorption.

Molecular Mobility Measurement of Hydrocarbons in Zeolites by NMR Techniques

Publisher Summary This chapter reviews the most relevant experimental methods to follow molecular translations and/or reorientations of guest molecules in zeolite pores. The benefit of combining these techniques is illustrated by a series of diffusion studies of hydrocarbons adsorbed in zeolite catalysts. Because of the key role of structure-related diffusion processes in shape-selective catalysis and the unique possibilities of the nuclear magnetic resonance (NMR) self-diffusion techniques to investigate such processes, the fundamentals of the NMR self-diffusion techniques are presented in the chapter and examples are given of their application to characterize zeolite catalysts. The NMR results (self-diffusion coefficients, anisotropic diffusivities, jump lengths, and residence times) can be correlated with corresponding neutron scattering data and sorption kinetics as well as molecular dynamics calculations, thus giving a comprehensive picture of molecular motions in porous solids. Analyzing the self-diffusion behavior of guest molecules in a microporous catalyst by the combined application of pulsed-field gradient NMR self-diffusion techniques reveals the spatial distribution of transport resistances over the catalyst particles.

Na2C2 und K2C2: Synthese, Kristallstruktur und spektroskopische Eigenschaften

Durch Umsetzung von in flussigem Ammoniak gelostem Alkalimetall mit Acetylen und durch anschliesendes Erhitzen der erhaltenen Hydrogenacetylide im Hochvakuum wurden Na2C2 und K2C2 phasenrein dargestellt. Die bereits von Foppl beschriebenen Raumtemperatur-Modifikationen konnten rontgenographisch und im Falle des Dikaliumacetylids uber Neutronenbeugungsexperimente an pulverformigen Proben bestatigt werden. Beide Verbindungen kristallisieren tetragonal in der Raumgruppe I41/acd (Nr. 142, Z = 8) und konnen als verzerrte Varianten der Antifluorit-Struktur aufgefast werden. Bei Temperaturen oberhalb Raumtemperatur (Na2C2: 580 K, K2C2: 420 K) tritt in Analogie zu den Erdalkalimetallacetyliden eine reversible Phasenumwandlung (1. Ordnung) in eine kubische Hochtemperatur-Modifikation auf (Fm 3 m, Nr. 225, Z = 4), die einer unverzerrten Antifluorit-Struktur mit fehlgeordneten C22–-Hanteln entspricht. Die Ergebnisse raman- und 13C-MAS-NMR-spektroskopischer Untersuchungen sind in Ubereinstimmung mit dem Vorliegen von C22–-Hanteln in den Titelverbindungen und erlauben einen Vergleich mit den entsprechenden Monoalkalimetall- bzw. den Erdalkalimetallacetyliden. Na2C2 and K2C2: Synthesis, Crystal Structure, and Spectroscopic Properties By the reaction of sodium or potassium solved in liquid ammonia with acetylene and subsequent heating in high vacuum Na2C2 and K2C2 could be synthesised as single phase products. The crystal structures described by Foppl could be confirmed by X-ray and neutron diffraction experiments (K2C2) on powdered samples. Both compounds crystallise in a tetragonal structure (I41/acd, no. 142, Z = 8) which can be described as a distorted variant of the antifluorite-structure type. At temperatures above room temperature (Na2C2: 580 K, K2C2: 420 K) a reversible phase transition (1st order transition) to a cubic modification (Fm 3 m, no. 225, Z = 4) has been observed, analogous to the alkaline earth metal acetylides. This high temperature modification represents an undistorted antifluorite structure with disordered C22– dumbbells. The results of raman- and 13C-MAS-NMR-spectroscopic investigations are in agreement with acetylide dumbbells in the title compounds and allow a comparison to the respective monoalkalimetal and alkaline earth metal acetylides.

Microdynamic behaviour of benzene molecules adsorbed on silicalite as studied by 13C NMR

13C NMR spectra have been measured for benzene molecules adsorbed on silicalite. The observed lineshapes can be explained by thermal reorientation of the benzene molecules, jumping between distinct orientation in the silicalite lattice.