G. T. Kokotailo

Demonstration of contact induced ion exchange in zeolites.

In this paper, it is demonstrated that exchange of the cationic species Na/sup +/, Li/sup +/, K/sup +/, and Ca/sup 2 +/, can occur between different zeolite crystals based only on a simple physical contact between the crystallites. The structural changes accompanying the exchange have been monitored by powder XRD and /sup 29/Si MAS NMR/sup 3/ studies which, because they are sensitive to long- and short-range ordering effects, respectively, complement each other and give a complete description of the structures. Zeolite A has been chosen for this work as it has a well-defined structure and XRD pattern and gives a single sharp absorption in its /sup 29/Si MAS NMR spectrum. 4 references, 1 figure.

Investigation of the three-dimensional SiOSi connectivities in the monoclinic form of zeolite ZSM-5 by two-dimensional 29Si INADEQUATE experiments

Abstract Two-dimensional 29 Si MAS NMR INADEQUATE experiments have been used to investigate the three-dimensional silicon bonding network in the monoclinic form of zeolite ZSM-5. Thirty-eight of the expected forty-eight SiOSi connectivities are clearly observed, giving rise to two possible assignments of the 29 Si MAS NMR spectrum, reflecting the unit cell symmetry. An unambiguous choice of the correct assignment is made by correlation of the 29 Si chemical shifts with local geometric parameters from X-ray diffraction data.

The combination of synchrotron powder diffraction and high-resolution solid-state NMR experiments

Abstract Structural investigations on three zeolites, ZSM-11, ZSM-12 and ZSM-5, have been carried out by a combination of high-resolution solid-state 29Si NMR experiments and Rietveld refinement of synchrotron X-ray powder data. These techniques, being sensitive to short- and long-range order, respectively, gave a most complete picture of the structure of the solid state. Variable temperature NMR experiments of ZSM-11 revealed a displacive phase transition with a change in symmetry at about 300 K. A Rietveld refinement of a data set recorded at 373 K confirmed the proposed structure model. The structure of ZSM-12 was refined from room temperature synchrotron powder data. Space group ambiguities because of psuedo-symmetry of the silica framework of the zeolite were resolved with the aid of 29Si NMR experiments leading to C2/c as the true space group symmetry. 29Si NMR and X-ray studies of ZSM-5 showed that the symmetry of the crystal structure depends on the temperature, the concentration of the guest molecule, and on the number of Al atoms in the framework. A full pattern refinement of silica-ZSM-5 loaded with two molecules p-dichlorobenzene per unit cell showed that the site occupied by the sorbed molecule is not at the intersection of the channels but shifted by 1 4 along b0.

The structure of zeolite ZSM-23 (MTT) refined from synchrotron X-ray powder data

The structure of zeolite ZSM-23 (structure-type code MTT) loaded with NH 4 F [a o =11.129 (1), b o =5.025 (1) and c o =21.519 (1) A and β=89.85 (4) o ] was refined from high-resolution synchrotron X-ray powder data. The Rietveld refinement converged in space group P12 1 1 (No. 4) to R F =0.090 and R wp =0.085, confirming the proposed model of the silicate framework. The use of complementary methods (synchrotron X-ray powder diffraction, 29 Si magic-angle-spinning nuclear-magnetic-resonance spectroscopy, and distance and angle least-squares refinements) was essential for the determination of the true space-group symmetry, a prerequisite for the successful Rietveld structure refinement

Sorbate-induced structural changes in ZSM-5 (silicalite)

The high-field (9.4 T)29Si m.a.s. n.m.r. spectrum (m.a.s. = magic angle spinning) of highly siliceous ZSM-5 (silicalite) shows that changes in the zeolite structure occur when sorbate molecules are present; these changes occur at low loading levels and are characteristic of the nature of the sorbed molecules, and X-ray diffraction confirms that the effect is a perturbation of the zeolite structure itself.

Perspective: Zeolites; Unique materials for research and industry

Abstract Zeolites are an important class of materials which have found extensive use as catalysts, sorbents, ion exchangers and catalyst supports. They are framework structures with uniform pore systems and their unique feature is their selective accessibility to sorbates and reaction products. The catalytic properties are characteristic of the zeolite and are related to its composition pore geometry, distribution of T-atoms and the nature and mobility of the sorbed species. Cations also play an important part in catalysis and sorption. The application of newly developed instrumentation generates important insights into the relationships between structure, composition catalytic performance and synthesis mechanisms.

Isomorphous substitution of silicon by boron or aluminum in layered silicates

In order to systematically change the properties of layered silicate materials, we haveinvestigated the possibility of isomorphous substitution of silicon by aluminum or boron. It is demonstrated that addition of boron and/or aluminum compounds to the reaction mixture leads directly to boron and aluminum containing layered materials in a hydrothermal crystallization process without further treatment. The layered materials obtained were identified as hectorite types, magadiite and kenyaite-like types. The isomorphous substitutions were proven by 11B, 27Al, and 29Si solid state NMR spectroscopy.

Low Temperature Syntheses of Highly‐ordered Framework Materials: The Aluminium Phosphate Ammonium Taranakite

The formation of a highly crystalline ammonium analogue of the aluminium phosphate mineral taranakite at ambient temperature and above is described. The high crystallinity and ordering of the products obtained when ammonium is used as a templating agent suggests that it is intimately involved in the structure. At long crystallization times and at higher temperatures, transformation to the framework structure of AlPO4-15 occurs. Results from powder x-ray diffraction, solid state NMR, electron microscopy, thermal and element analyses are presented and discussed. They demonstrate the high degree of crystallinity and ordering of the taranakite samples synthesized by this method. These results – in particular the very mild synthesis conditions – suggest the possibility that there may be sequestering of ammonium phosphate fertilizers in form of NH4-taranakite from their interaction with clay materials in soils. Die Synthese von hoch-kristallinen Materialien bei niederen Temperaturen – Das Aluminiumphosphat Ammoniumtaranakit Die Bildung einer hochkristallinen Ammoniumform des Aluminiumphosphatminerals Taranakit bei Temperaturen zwischen Raumtemperatur und 70 °C wird beschrieben. Die hohe Kristallinitat und Ordnung der Produkte, die bei Verwendung von Ammoniumionen als Templatreagenz erhalten werden, legt nahe, das Ammoniumionen wesentlich zur Strukturbildung beitragen. Nach langeren Kristallisationszeiten und bei hoheren Temperaturen erfolgt die Umwandlung zur Geruststruktur von AlPO4-15. Ergebnisse aus Pulverrontgenbeugungs-Experimenten, Festkorper-NMR-Spektroskopie, Elektronenmikroskopie und Elementaranalyse werden prasentiert und diskutiert. Sie zeigen den hohen Grad an Kristallinitat und Ordnung der nach dieser Methode synthetisierten Taranakitproben. Diese Ergebnisse – insbesondere die sehr milden Synthesebedingungen – lassen vermuten, das Ammoniumphosphatdunger durch Reaktion mit in Boden enthaltenen Tonmineralien in der Form von NH4-Taranakit fixiert werden konnten.

Powder X-ray diffraction and solid state NMR techniques forzeolite structure determination

The synthesis of zeolites, porous aluminosilicates, had a large impact on the developmentof catalyst materials. The framework structures of these systems consist of Si and Al atoms, referred to as T-atoms, tetrahedrally coordinated to oxygen atoms to form topologies with well defined and regular channel systems. Cations are necessary to satisfy the charge on the Al atoms and are exchangeable. The T-atoms may also consist of any atom capable of isomorphorus substitutions for Si. Since good quality large zeolite crystals are not available, powder X-ray diffractionis used to derive structural information,and with improvements in powder diffraction instrumentation and the availability of synchrotron and rotating anode high intensity X-ray sources, much higher quality structure data is obtained. The combined application of solid state NMR which probes short-range order, X-ray diffraction which is a long-range order technique and Rietveld refinement has greatly increased both the quantity and quality of structural information available.

Characterization of a high-loaded intercalate of p-xylene with a highly siliceous form of ZSM-5 by high resolution 29Si solid-state NMR spectroscopy

High-resolution solid-state 29Si MAS NMR spectra of zeolite ZSM-5 with 4–8 molecules of sorbed p-xylene per unit cell reveal the presence of a ‘high-loaded’ phase for this system in which there are 24 T sites in the asymmetric unit; proton decoupling must be used to obtain good resolution in this phase and this is an indication for considerable dipolar interaction between the p-xylene protons and the 29Si nuclei in the zeolite lattice, in contrast to the case of the ‘low-loaded’ form with two molecules per unit cell previously observed and described; thus, cross-polarization may be used to discriminate in favour of the ‘high-loaded’ phase.