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. 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.

2 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. 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.

3 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. 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 .

4 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]. 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. This results in a considerable improvement of the X-ray diffraction pattern, intensity and resolution, which also increases the ability to characterize the ZEOLITE .

5 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. 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.

6 Cation exchangers may be added to the wa-ter. 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. 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.

7 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. (B) Calcined two hours at 350 C in air. Treated with water for two hours at 100 C Calcined four hours at 500 C in air. Vol. 12 No. 1 1995 5a variation in stacking order, which also blocks off the twelve-membered ring (12MR) channel in offretite as seen in Fig. 4. Singlecrystal ANALYSIS showed that the K+ ions were in the cancrinite cages which have only 6MR exit openings.

8 In or-der to remove the K from these cages, a driving force was necessary that was provided by calcin-ing the calcium-exchanged from at a temperature of at least 400 C. Although the calcination is time/temperature related, it is possible to cause the required migration of calcium into the cancrinite cages at a lower temperature. The calcium ions migrated into the cancrinite cages by forcing the K out into the large cages and were then removed by subsequent exchange [8]. This migration of ions may be tracked with X-ray diffraction . The diffraction pattern of a natu-rally occurring erionite ZEOLITE is shown in Fig. 5a. This same sample was ion-exchanged with cal-cium nitrate and calcined at 500 C. This process was repeated, resulting in a decrease in K content from to while the calcium content increased from to The K content can be reduced to below [8].

9 The normal hex-ane absorption was not affected, indicating that the STRUCTURE had not been adversely affected. The X-ray diffraction pattern showed a shift in the lines (Fig. 5b) indicating a reduction of , in the C parameter which can be accounted for by the difference in ionic radius of calcium, compared to for K. If the calcined calcium exchanged erionite is Fig. 4 Views of erionite and offretite frameworks (A) Erionite. (B) Offretite. The Rigaku Journal 6heated in water at 100 C for two hours, the X-ray pattern reverts to essentially its original form, as seen in Fig. 5c. X-ray diffraction can, thus, be used to track the interchange of K and Ca cations in the cancrinite cages. Similar cation migration to stable positions was noted on calcination of La exchanged zeolites X and Y (synthetic faujasite) [9].

10 In this case the La migrates into the sodalite cages, forming a sta-ble complex, and contributes greatly to the hydro-thermal stability of the catalyst. Simulation of POWDER Patterns An old and useful approach to determining the framework topology of a ZEOLITE is that of model building. Trial model building is not a random process: All available data, lattice parameters and symmetry information from diffraction studies, size of channels from diffusion studies, density, ring ellipticity from absorption and diffusion rates, nature of the channel system from sorption and diffusion, transmission electron microscopy lattice imaging, and MAS NMR are used together to build a model consistent with this information. Good crystalline samples that yield high-resolution diffraction patterns and high quality MAS NMR are primary requirements.