ZEOLITE STRUCTURE ANALYSIS WITH POWDER X …

1 Vol. 12 No. 1 1995 3 THE RIGAKU JOURNAL VOL. 12 / / 1995 ZEOLITE STRUCTURE ANALYSIS with POWDER X-RAY DIFFRACTION AND SOLID-STATE NMR TECHNIQUES G. T. KOKOTAILO AND C. A. FYFE Department of Chemistry, University of British Columbia, Vancouver, BC, CANADA Large advances in the development of catalytic materials were made with the synthesis of zeolites, which are porous crystalline materials that can be used as catalysts, catalyst supports, sorbents and ion exchangers. ZEOLITE structures consist of T-atoms tetrahedrally coordinated to oxygen atoms, where T can be AI, Si, or any other element capable of isomorphous substitution for Si.

2 The unavailability of good quality, large single crystals makes the POWDER diffraction technique essential. Originally the POWDER diffraction patterns of these materials were used as fingerprints, but on careful ANALYSIS and with improved diffraction techniques a wealth of information may be derived from these data. The availability of rotating anode and synchrotron X-ray and neutron sources and the application of the Rietveld refinement method results in considerable improvement in the quality of structural information that can be derived.

3 Further, the application of solid-state NMR techniques in conjunction with the X-ray diffrac-tion method greatly increases the short-range-order information obtainable from these systems. Introduction The importance of zeolites in the petroleum and chemical industries can scarcely be overesti-mated. The first application was the use of rare-earth exchanged synthetic faujasite as cracking catalysts [1-3]. ZSM-5 catalysts selectively con-vert methanol to gasoline, and are used in distil-late dewaxing, ethylbenzene synthesis, xylene isomerization, toluene disproportionation, as well as a host of other processes.

4 The size and shape of the channel system and the nature and location of cations determine the absorption and rate of diffusion into the ZEOLITE . The stability and catalytic properties are also functions of the structural characteristics. The understanding of the physical and catalytic properties of these materials is dependent on our knowledge of the structural features of the frame-works. These are also affected by crystallite size, faulting, twinning, and the nature of the cation sites.

5 Solid-state NMR is a very sensitive probe of the local environment of a particular atom in the STRUCTURE , while X-ray diffraction is sensitive to long-range order or the periodic STRUCTURE of the framework. The two techniques complement each other and together provide more detailed informa-tion on the STRUCTURE . There have been some very novel and exciting applications in these two areas and in the development of the solid-state NMR method. Factors Affecting Characterization Amorphous and less stable components can be removed from ZEOLITE samples by NaOH treatment [4].

6 In ZEOLITE systems, perturbations in the framework STRUCTURE , crystal morphology, extra-framework material, phase purity, crystallite size, and the setting and occupation of cation sites can produce differences in the X-ray diffraction pat-terns. The first requirement is good, clean crystal-line material that will yield very high-resolution patterns. This can be accomplished by the judi-cious choice of pH, temperature, recipe compo-nents, and their mixing and synthesis times.

7 This results in a considerable improvement of the X-ray diffraction pattern, intensity and resolution, which also increases the ability to characterize the ZEOLITE . The pH used is dependent on the caustic stability of the sample for varying periods. This reaction is time-temperature related. The im-provement in crystallinity of high-silica ZEOLITE A sample treated for five minutes with NaOH solution at room temperature is noted in Fig. 1. Similarly the X-ray diffraction pattern of a mor-denite sample indicated the presence of ZEOLITE Beta.

8 A 15-minute treatment with a NaOH solution at room temperature removed the Beta component completely as shown in Fig. 2. Some templates are difficult to remove from zeolites requiring high-temperature treatments that may lead to disorganization of the framework. The or-ganic template can be decomposed at a low tem-The Rigaku Journal 4perature (-300 C) and the decomposition products expelled by high-temperature hydrothermal treat-ment. Cation exchangers may be added to the wa-ter.

9 The ZEOLITE is then calcined at a high tempera-ture, preferably 500 C, to burn off any residue that has not been previously expelled [5]. Thus the organic material is removed without disrupting the framework with a corresponding improvement in the adsorptive and thermal properties. The X-ray diffraction pattern of a synthetic ferrierite shows considerable improvement in crystallinity from this treatment (Fig. 3) as well as improved adsorp-tive properties [5]. Cation Effects Erionite and offretite are natural zeolites that can be converted to active catalysts by ion ex-change.

10 Most of the Na can be removed, but the K content cannot be readily decreased. The struc-tures of erionite [6] and offretite [7] are related by Fig. 1 X-ray diffraction patterns of high silica zeiolite A. (A) As synthesized. (B) Treated for five minutes with NaOH solution at room temperature. Fig. 2. X-ray diffraction patterns of synthetic mordenite (A) As synthesized with beta ZEOLITE impurity. (B) Treated for 15 minutes with NaOH solution at room temperature. Fig. 3. X-ray diffraction patterns of ferrierite, as synthe-sized with template (A) Calcined four hours at 500 C in air.