Zeolites thermal analysis

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

Pal-Borbley, G. (2007) Thermal analysis of zeolites. Molec. Sieves Sci. Technol., 5, 67-101. 

The chemical analyses were done by a combination of wet chemical, atomic absorption (Hitachi Z-800) and ICP (JY-38 VHR) methods. The crystalline phase identification was carried out by XRD (Philips PW-1710 Cu K scanning electron microscopy (Cambridge, Stereoscan 400), thermal analysis (Netsch, Model STA 490), ESR... 

Thermal Analysis. The water contents of CPA and zeolite were measured by thermogravimetric analysis (TGA). Differential thermal analyses (DTA) were done for comparison. The instrument used was a Du Pont 900 with a TGA attachment. 

Thermal analysis is an appropriate technique to investigate the precise nature of the organic molecules occluded in zeolite frameworics (41). For a series of zeolite Beta samples synthesized under various conditions (Table VII) DTA provides evidence for presence of both TEA+ ionic species (DTA sharp peak near 460°C) and TEAOH ionic pairs (weak broader DTA peak recorded near 345°C) (61). Similar conclusions were proposed by Perez-Pariente et al. (31) for a number of Beta samples prepared under slightly different conditions TEA+ ions undergo decomposition above 350°C while the neutral TEAOH species are released between 220 and 350°C. Our TG-DTA combined system allowed a quantitative determination of both species (Table VII). 

With the aid of thermal analysis, X-ray diffraction and diffuse reflectance FTIR, it was possible to deduce that the removal of the zeolitic water brought about a partial structural transformation of the VPI-5 to AlP04-8. Schmidt etal. (1992) have studied this phase transition in some detail and have shown that it can be avoided if the sample is heated slowly under reduced pressure (10-5 hPa). In this manner, VPI-5 can be heated to 450-500°C without any detectable change of structure. 

The next stage of characterization focuses upon the different phases present within the catalyst particle and their nature. Bulk, component structural information is determined principally by x-ray powder diffraction (XRD). In FCC catalysts, for example, XRD is used to determine the unit cell size of the zeolite component within the catalyst particle. The zeolite unit cell size is a function of the number of aluminum atoms in the framework and has been related to the coke selectivity and octane performance of the catalyst in commercial operations. Scanning electron microscopy (SEM) can provide information about the distribution of crystalline and chemical phases greater than lOOnm within the catalyst particle. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) can be used to obtain information on crystal transformations, decomposition, or chemical reactions within the particles. Cotterman, et al describe how the generation of this information can be used to understand an FCC catalyst system. 

Bouvier F. and Weber G., Adsorption of a polar or a non-polar chloroalkene on a ZSM-5 zeolite at 298 K, Journal of Thermal Analysis 54 (1998) pp. 881-889. Brosillon S., Manero M.-H. and Foussard J.-N., Adsorption of acetone/heptane gaseous mixtures on zeolite co-adsorption equilibria and selectivities. Environmental Technology 21 (2000) pp. 457-465. 

Information about this energy has been obtained from isotherm measurements over a temperature range (3, 7, 8, 9), by calorimetry 18, 19), and by differential thermal analysis (JJ). From the isotherms and by direct calorimetry, the isosteric heats, q t, may be found as functions of the amount of water sorbed. However, some disadvantages may be associated with each procedure. Such is the affinity between water and zeolites that to determine q t for small uptakes may require isotherm measurements at temperatures above 200°C. At these temperatures, lattice breakdown can take place by side reactions involving the water. 

Secondly, calorimetric measurements from the vapor phase may refer to nonequilibrium distributions of water vv ithin the crystals and through the zeolite bed. The very energetic vv ater-zeolite bond, especially for smaller water uptakes, means that water molecules may stick on sites vv here they first land. Subsequent redistribution can be very slow on the time scale of the experiment, particularly at the low temperatures employed 19, 21), 23° and 44°C. Finally, the information derived from differential thermal analysis is qualitative or at best only semiquantitative. 

J. W. Ward It was previously shown (Ward, /. Catalysis 1968, 11, 251, Nature of Active Sites on Zeolites VI) that alkaline earth ions stabilized the hydrogen-Y zeolite. Examination of the infrared spectra, unit cell constant, ion exchange capacity, thermal analysis data, and other properties indicates that this material is not the so-called ultra-stable zeolite. 

The limit of stability of the crystal framework at different extents of Ni ion exchange of type A molecular sieve is shown by means of electron microscopy, differential thermal analysis, and x-ray diffraction. The data obtained from catalytic studies are in accord with the results of physical methods, showing preservation of the molecular sieve properties after reduction of the Ni ions. Metallic Ni aggregates on the external surface of the zeolite. In the dehydrogenation of cyclohexane and the hydrogenolysis of n-hexane, type A molecular sieve shows the properties of metallic Ni on an inert support. When NiNaA is mixed mechanically with CaY, a typical bifunctional catalyst is obtained. 

Small amounts of beryllium can be substituted into the tetrahedral framework sites of ZSM-5 zeolite by treatment with ammonium tetrafluoroberyllate. Although the amounts of Be substituted were too small for detection by thermal analysis and desorption measurements, the presence of Be in framework sites was detected by Be MAS NMR, which showed a resonance at — 5.0 ppm with respect to aqueous BeS04... 

More recently the commercial zeolite MAP , also with the GIS framework, has attracted interest as a detergent builder. Allen et al. [93] have examined the way that the incursion of different cations into this zeolite causes structural changes. They present reasoned arguments, supported by PXRD, MASMR and thermal analysis, to explain how they affect the observed selectivity series, based upon isotherm data, viz Ba>Sr>Ca Na>K>Rb>Cs... 

To explain this competition we hypothesized that Ti could play a role similar to that of A1 in the crystallization of zeolite Beta, that is, the creation of negative charges in the framework and thus its stabilization by interaction with the TEA+ templating cations. This hypothesis was also supported by the fact that the amount of TEA+ cations decomposing at T>620K in air (as determined by thermal analysis) was dependent on the total amount of A1 + Ti, rather than only on A1 (ref. 14). This hypothesis required the ability of zeolitic Ti to change its coordination number, something which obtained substantial support from XANES and EXAFS measurements (ref. 15). 

The identification of the solid phases and the determination of their crystallinities were carried out by X-ray powder diffraction. The alkali and A1 contents were determinated by PIGE [23], while the amount of orgcinic and water molecules was evaluated by thermal analysis. The amount of defect groups in the zeolite framework was calculated from solid state MAS 9Si-NMR spectra [21]. Finally, the identification of the decomposition products of hexamethonium ions were performed by combining TG-DTA analysis and MAS 13C-NMR [14]. 

The barium form of zeolite A is of moderate stability, unlike other alkali metal and alkaline earth forms which can be stable to about 1000°C. This work uses the techniques of differential thermal analysis (DTA) and thermo gravimetric analysis (TGA) in conjunction with isotopic labelling and x-ray powder photography to investigate the thermal stability of heteroionic forms of zeolite A. Reasons for the instability of BaA and Na/BaA zeolites are suggested and comparisons made with A zeolites containing Na and Sr cations. 

Differential Thermal Analysis. Details revealed in the differential thermograms of the 4 MA-Y zeolites (Figure 2) are consistent with the TGA results. They all exhibited an endotherm (122°-190°C) caused by loss of adsorbed water. Over the region where slow decomposition occurred, there were weak endotherms or inflections in the curve, and the rapid decomposition was confirmed by a sharp endotherm (516°-577°C). Dehydroxylation was clearly revealed by the endotherm at 785°C in MMA-Y, but was not as well resolved in the others. The exotherm (977°-1026°C) was owing to mullite transformation (I). However, all the samples were found to be amorphous (by x-ray) after 1 hour at 900°C. 

Thermal analysis techniques have been applied to almost every science area, from archaeology to zoology, and to every type of substance, from alabaster to zeolites. Indeed, it is difficult to find an area of science and technology in which the techniques have not been applied. This truly universal use of thermal analysis is consistent with its early history in. for example, clays, mineralogy, metallurgy, and inorganic substances. 

Z. Gabelica, B. Nagy, E. Derouane and J. Gilson, "The Use of Combined Thermal Analysis to Study Crystallization, Pore Structure, Catalytic Activity and Deactivation of Synthetic Zeolites", Clay Minerals. 1984,12, 803-824. 

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