Chapter 6 Instrumental Methods of FCC Catalyst Characterization

J.S. Magee and M.M. Mitchell, Jr. Fluid Catalytic Cracking: Science and Technology

Studies in Surface Science and Catalysis, Val. 76 0 1993 Elsevier Science Publishers B.V. All rights reserved.

183

CHAPTER 6 INSTRUMENTAL METHODS OF FCC CATALYST CHARACTERIZATION ALAN W. PETERS

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W. R. Grace & Co. Conn. Columbia, MD 21044 1. INTRODUCTION The FCC Catalyst and the FCC Process The FCC (Fluid Catalytic Cracking) catalyst is a coarse powder consisting of particles in the 40 to 100 micron size range with an average size of about 65 microns. It contains 20% to 50% of an active zeolite, usually faujasite. with a surface area of 900 meters2/gram. In some cases the catalyst may also contain a moderately high surface area alumina or silica alumina matrix of -- 150 meters2/gram. The balance is a low surface area inert filler, typically clay, and a low surface area amorphous silica or silica alumina binder (1). The filler and binder provide the physical properties such as attrition resistance and density that are required for operation in a commercial fluid catalytic cracking unit. The properties and preparation of the catalyst are described in more detail elsewhere in this book. Instrumental methods are used to characterize the FCC catalyst particle both fresh and after deactivation. The catalyst properties, such as zeolite content, the unit cell size of the zeolite, and the content and location of the active components determine the activity and selectivity of the catalyst. Catalyst stability is measured by comparing the fresh properties of the catalyst with the properties of the operating or equilibrium catalyst. In most advanced technology catalysts the zeolite is designed to hydrothermally dealuminate in a controlled and stable way to the intended unit cell size and surface area. The hydrothermal environment of the unit is in a sense part of the catalyst preparation. Also during use contaminant metals contained in the oil at the part-per-million level may deposit on the catalyst and cause changes in activity and selectivity. Control of these contaminants is one of the most important issues in catalytic cracking. It is important for the catalyst user, the manufacturer, and the researcher to be able to follow the chemical and physical changes that take place during the operation of the catalyst in the FCCU (FCC Unit). The operation of the FCCU is briefly described below. A more detailed discussion of the FCCU process is given by Venuto and Habib (2) and in other chapters of this book. During operation the catalyst alternately passes between the reactor and regenerator, spending a few seconds in the reactor and about 15 minutes in the regenerator during each cycle. The reaction occurs when the hot regenerated catalyst is mixed with the relatively cooler oil at the bottom of the riser at a mix temperature of about 500°C - 550°C (930°F 1020°F). The oil expands as it heats up and converts to lighter products. Both the oil and the catalyst move up the riser into a reaction vessel where the catalyst and hydrocarbon are separated. The products include gasoline, light gases, some unconverted oil, and coke embedded on the catalyst. The activity of the catalyst refers to the total amount of conversion to all light products including gasoline, lighter gases, and coke. The selectivity refers to the distribution of products. Desirable selectivities are for less coke, light gas, hydrogen, and

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high boiling heavy slurry oil, for more gasoline with a higher octane, and more light cycle oil with a higher cetane index. Requirements for a more environmentally acceptable gasoline involve selectivities for less aromatics, less sulfur, more light olefins and less heavy olefins in the gasoline. Activity and selectivity are unfavorably affected by metal contaminants present in the oil, especially vanadium and nickel, and including iron, copper, and sodium. These metals form a residue on the catalyst along with the coke. Unlike coke, these residues cannot be removed during operation. Instrumental methods of analysis are used to determine the location and chemistry of these contaminants. Before it is sent to the regenerator the catalyst is separated from the reaction products in the cyclones and passes through a stripping section where steam removes most of the remaining hydrocarbon from the catalyst. Stripping efficiency can depend to an extent on the pore structure of the catalyst. The catalyst discharges into the regenerator along with injected air. The coke on the catalyst is burned to CO, COz, water, and trace amounts of sulfur and nitrogen oxides. The heat of this reaction is enough to increase the temperature of the catalyst in the regenerator to the 720OC- 800°C (1320'F - 1450'F) range. About 20% of the regenerator off gas is water. It is the presence of water at these temperatures in the regenerator in combination with the metal contaminants on the catalyst that cause loss of activity and changes in the selectivity of the catalyst. Both the zeolite and the matrix component can undergo significant changes, some desirable and some undesirable. The zeolite can dealuminate resulting in lower activity but improved (lower) selectivity for coke and improved selectivity for the production of higher octane olefinic gasoline. Both the matrix component and the zeolite can deactivate by losing crystallinity and/or surface area, resulting in generally lower activity and poorer selectivity. Also, both the matrix and the zeolite can undergo changes in the pore size distribution. The zeolite in a well designed catalyst will dealuminate to the desired unit cell size without loss of structure. It wjll form a system of mesopores interconnected to the zeolitic micropores. The steam and temperature in the regenerator also has an effect on the vanadium on the catalyst. In this environment the vanadium destroys zeolite structure and surface area, reducing activity as well as causing changes in selectivity. The presence of nickel primarily causes increases in coke and hydrogen. The effects of nickel can be alleviated by adding antimony or bismuth compounds to the feed oil. A desirable catalyst will survive relatively high amounts of vanadium and nickel with acceptable losses in activity and selectivity. Currently vanadium levels of 7000 ppm and nickel levels of 4000 ppm are reasonable practical limits. As work continues in controlling the location and chemistry of these contaminants, the metal tolerance of catalysts will continue to increase.

Summary of the Use of Instrumental Methods of Analysis Instrumental methods are used to characterize the changes that occur in the catalyst during the FCC process. These changes are then related to desirable or undesirable changes in the selectivity and activity of the catalyst. Consequently analytical information concerning both the fresh catalyst and the operating catalyst is important to the refinerhser as well as to the manufacturer and researcher. The various instrumental methods of analysis are divided into five somewhat arbitrary groups. The techniques within each group tend to provide similar or complementary information, make use of similar instrumentation, and involve similar sample preparation techniques. The five groups are Diffractive Analysis (XRD, High Energy XRD, SAXS, Neutron Diffraction, EXAFS) Diffractive analysis is used to identify the structure, composition and amounts of crystalline materials such as zeolite and aluminas. In the case of EXAFS the local arrangements of atoms can be identified in the absence of a long range crystalline structure.

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Pore Structure Analysis, Adsorption (Nitrogen, Mercury, Water, He, Argon, and Organic Materials) Adsorption methods are used to determine the pore structure, surface area, pore volume, and the distribution of small and larger pore structures in the catalyst. Surface Analysis and Imaging (SEM,TEM, STEM, EDX, XPS (ESCA), Auger,SIMS, WDS (EPMA), STM,AFM ) Surface analysis provides information characterizing the morphology of the sample, provides element mapping and elemental associations, and can provide an estimate of the oxidation state of the elemental components. SpectroscopicAnalysis (lnfra red, Fl'IR, Raman, DRIFTS, Mossbauer, MASNMR, uv-visible) Spectroscopic analysis is used to estimate the strength, number, and type of acid sites (Lewis or protonic), describe aluminum coordination and acidity, and provide descriptions of silicon and aluminum distributions. Thermal Analyses (TPD, TGA, DSC,TPR, TPO, Microcalorimetry) Thermal analysis is also used to estimate the number and strength of acid sites, more generally the strength of adsorption, thermal and hydrothermal stability, and stability to oxidationheduction. 2. DIFFRACTIVE ANALYSIS (XRD,High Energy XRD, SAXS, Neutron Diffraction, EXAFS) Except for EXAFS and neutron diffraction, this group of techniques involves the diffraction of high energy radiation (x-rays) from the atomic lattice of crystalline compounds present in the sample. In the case of XAFS there is not necessarily a lattice, but there is a local symmetry or arrangement that gives a similar result., the presence of peaks at certain positions characteristic of the atomic distances and symmetry. In the case of neutron diffraction there is a lattice, but high energy neutrons are used rather than x-rays. These techniques are used to identify and to measure the relative amounts of crystalline materials in the catalyst. XRD techniques are commonly used to estimate the aluminum content in the zeolite by measuring the unit cell size.

Powder XRD (X-ray Diffraction) FCC catalysts may currently contain either or both of two different zeolites, faujasite and ZSM-5. and may contain a crystalline alumina as a matrix component. Further, the catalyst may contain clay, and the clay can transform to spinel or mullite. The zeolite may also transform into mullite and crystobalite. All of these phases are crystalline and can be identified and quantified by XRD. An explanation of the principle of the technique is illustrated in Figure 1. The crystalline compound forms a series of repeating planes with a spacing of a few tenths of a nanometer. The radiation is scattered by each atom in the plane. Part of the wave is reflected, but most goes through to the next plane where part is reflected, etc. The wave front interacts with the electrons associated with each atom on the plane. Each atom re-emits a spherical wavelet. The re-emmitted waves will be in phase and will give a peak at the detector for a particular angle, 26, such that the distances traveled by the wavelets differ by an integral wave length. At any other angles, even ones only slightly different, the amplitude of the wave reflected from neighboring atoms will be slightly out of phase, the wave from second neighbors will be

186 twice as much out of phase, and finally the negative amplitude of the reflected wave from some nth neighbor will cancel the reflection from the first and so on until there is a net cancellation. Consequently, the angle 26 is a measure of the distance between planes, and the intensity of the signal is a measure of the scattering power (number of electrons) and the density of the atoms in the diffracting plane. For a given material, the intensity is a measure of how much material there is and of how perfectly the planes are ordered.

Figure 1. Pictorial representation of X-ray scattering intensity reinforcements responsible for the distinctive X-ray patterns of crystalline materials. After H. P. Mug and L. E. Alexander, X-Ray Diffraction Procedures, John Wiley & Sons, 1974, p. 121. Identification of c The basis of h e XRD technique is that crystalline materials have peaks at values of 26 such that both the values of 26 and the intensities are characteristic of the structure of the material. Figure 2 shows the powder patterns of two zeolites, faujasite and ZSM-5, used in cracking catalysts as well as a pattern characteristic of a clay (kaolin) often used as a filler. Crystalline or amorphous aluminas may also form a part of the matrix of the cracking catalyst. The XRD scan of a cracking catalyst may show peaks due to all or several of these components. Amorphous materials, of course, do not have an X-ray pattern and cannot be identified by XRD. A major feature of current XRD systems is the existence of search routines capable of identifying zeolites, clays, or other catalytic components from an XRD scan. The JCPDS (Joint Commission for Powder Diffraction Standards) files include XRD patterns for about 20,000 inorganic materials available in a computer searchable format (6) and include materials that may occur in FCC catalysts such as zeolites, clay, and aluminas. Although currently only two zeolites, faujasite and ZSM-5, are being used commercially, other zeolites will almost certainly be used in the future. Several collections of zeolite XRD scans are available in the literature (7-9). A collection of XRD scans for aluminas is published by Alcoa (lo), and collections of scans for various clays have also been published (11). An improved search procedure using the full XRD scan rather than just a few major peaks has been described (12) and is commercially available through the authors from Pennsylvania State University along with an extensive and current zeolite data base. Nearly 300 programs for the analysis of powder diffraction data have been recently reviewed (13). Many of these

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programs are included in the Penn State package. Using these and other files it is possible to obtain an identification of any crystalline material provided its XRD pattern is available in a computer or hand searchable form.

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XRD intensities can be used to estimate the amount of a material present in a catalyst. The XRD intensities are compared to some standard defined as 100% crystallinity. There is a procedure defined as a standard test, ASTM D 3906, for the measurement of crystallinities of faujasite containing material (14). This technique is often used to measure the relative amounts of zeolite such as faujasite in either a fresh catalyst or in commercial or lab deactivated catalysts. The stability of the zeolite is the per cent zeolite retention. The presence of varying amounts of exchange cations such as sodium or rare earth can significantly alter intensity relationships. Further, the recently observed presence of an extensive system of internal surface defect structures (mesopores) in the framework of hydrothermally dealuminated zeolites is expected to cause an intensity loss. Atoms near the internal surface will be slightly displaced from their ideal lattice positions and so there will be some interference of amplitudes. In zeolites with a high mesopore surface area of -100m2/g, 10% or more of the atoms may be at a surface. These atoms may not be adequately counted even though they are a part of the zeolite structure. For these reasons quantitative analysis by XRD can be inaccurate and should only be used to compare samples of similar types of zeolite. Qystallite size Verv small crvstallites will have fewer diffracting Dlanes and so the angle over which amplitldes add Gill be larger. This is known as l i k broadening, and can' be used as a measure of crystallite size in the 0.01 p to 1 p size range (15) . Since line broadening is associated with a loss in peak height, intensity measurements for analytical purposes should be obtained from peak areas. Realistic estimates of crystallite size require that the optics of the instrument be adequate to eliminate instrumental broadening. Usually this involves the use of a primary monochromater as discussed in more detail in the section concerning high resolution XRD. Since most samples of zeolite are agglomerates with a rock pile morphology, crystallite size is not the same as particle size. Particle size and crystallite size are the same only if one has a collection of single crystals without intergrowths.

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u n i t Cell Size Measurement in F a u i w In a zeolite framework consisting of silicon and aluminum, the aluminum is the catalytically active ingredient. The activity, selectivity, and stability of the zeolite are all related to the framework aluminum content of the zeolite. Chemical analysis will give the aluminum content of the framework in a pure zeolite sample if there is no other source of alumina. However, the aluminum removed from the framework by dealumination, either during manufacture or during deactivation, remains within the structure in some form as nonframework alumina. Further the catalyst will contain other sources of alumina including clay and matrix components. It is desirable to be able to measure the amount of framework alumina in a zeolite independently of the occurrence of other forms of alumina in the catalyst or the zeolite. A number of instrumental methods of measuring framework aluminum content have been developed. It is possible to estimate the aluminum content of the framework by measuring the frequency shift of the 800 cm-' and 1050 cm-I i. r. absorption bands (16, 17). and also by 29Si MASNMR (18) as well as by the XRD methods discussed below. These other procedures are discussed in the section on spectroscopic analysis. The most common method of measuring the aluminum content of the framework and the only one directly applicable to the catalyst is based on the measurement of the unit cell by XRD. Since the Si-0-A1 bond is longer than the Si-0-Si bond, the unit cell increases slightly with aluminum content. Consequently the unit cell size is a very important parameter. The measurement of unit cell size by XRD has been standardized as ASTM Test D-3942 (14). In the case of faujasite there are several correlations of unit cell size and alumina content in current use, one developed by Breck and Flannigan (19), and another more recently by Fichtner-Schmittler (20) and by Sohn (17). The correlations of Breck and of Sohn are shown

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graphically in Figure 3. The earlier correlation of Breck and Flannigan was developed for low SUAl ratio (1.5-2.5) as-synthesized samples of sodium exchanged faujasite using direct chemical analysis as the reference, while the two latter correlations were developed from dealuminated and decationized samples of faujasite using 29Si MASNMR results as the Si/Al reference. Using these correlations there are simple but useful relationships between the unit cell size determined by XRD, the silicodaluminum atom ratio of the framework, R, and the number of aluminum atoms per unit cell. Provided there is no nonframework alumina present, it is possible also to calculate the number of aluminum atoms per unit cell from the chemical analysis, below, and to compare the results with an estimation of the aluminum content in the framework, Table 1. By Chemical Analysis R=Si/AI = (%SiO2~51)/(%Al203~60) Si02/Al203 = 2R # AYunit cell = 19U(R+1) Table 1. The measurement of aluminum content per unit cell in faujasite by XRD analysis using the published unit cell correlations, where a is the unit cell in nanometers.

# AVunit cell =

rn

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There is no single c o m t correlation since cation exchange and even the degree of hydration can significantly affect the unit cell size. As-synthesized sodium faujasites will have a unit cell of about 2.460 nm to 2.475 nm. After stabilization and dealumination the unit cell will be 2.440 to 2.460 nm. and after dealumination in the FCCU the unit cell will typically be 2.422 to 2.432 nm. The unit cell size of an equilibrium catalyst can be lower than the zero aluminum limit of the Sohn and the F-S correlations. A unit cell size of 2.417 nm, lower even than the minimum predicted by the Breck correlation, has been reported in the literature (21). One reason for lower than expected unit cell sizes is that as the catalyst dealuminates, some of the aluminum occupies exchange sites (22,23). Cation exchange can reduce the unit cell size by as much as 0.004 nm compared to the same decationated zeolite. Removal of cations such as sodium from as-synthesized faujasite or nonframework alumina from steam dealuminated faujasite results in an increase in unit cell by 0.002 to 0.004 nm. The older Breck relationship may be more useful for deactivated FCC catalysts, while the more recent correlations may be more appropriate for experimental or otherwise decationated zeolites. For other zeolites having a higher as-synthesized silicon to aluminum ratio this kind of relationship has not so far been useful. Zeolites such as ZSM-5 contain small amounts of aluminum, and small variations in aluminum content do not produce enough of a change in the unit cell size to be easily and quantitatively measurable. Resolution XRD A limiting factor in the accuracy of the unit cell size measurement is the resolution of the XRD unit. The X-ray source, the Cu K alpha emission line, is a doublet, and is close to the beta emission line. Most commercial X-ray units have a secondary filter placed after the sample that eliminates the beta line, but not the K alpha doublet. A primary filter will eliminate the doublet but requires a higher intensity source to compensate for the intensity

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loss associated with the primary filter. The use of an intense monochromatic beam available from a synchrotron source at one of the storage ring facilities, for example The National Synchrotron Light Source at Brookhaven National Laboratory, allows an order of magnitude improvement in the accuracy of the unit cell size measurement (24). A monochromatic and intense X-ray beam is useful in other contexts as well. It is possible to detect small amounts of crystalline impurity phases or to deduce crystal structures from high resolution powder data or from micrometer size single crystal data (25,26).

80

1

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60 ._...______________._ Breck, Ref. 19 Sohn, Ref. 17 40

20

0 2.42

2.43

2.44

2.45 2.46 Unit Cell, nm

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2.48

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Figure 3. Correlations between unit cell size in nanometers (10 = 1 nm) and aluminum content, aluminum atoms per unit cell, one unit cell contains 192 total atoms, excluding oxygen. From references 17 and 19.

SAXS (Small Angle X-ray Scatterinel Small angle XRD scattering has been used in the past to evaluate the distribution of relatively large structures such as particle size distributions or pore size distributions. In the case of pore size distribution the pores are fiiled with liquid containing a heavy atom. Since scattering power increases with the number of electrons per volume, a heavy atom will contribute a large scattering cross section (27). Recent experiments with light scattering have shown that the fractal dimensionality of a particle can be determined by scattering experiments (28). The fractal dimensionality is related to the particle shape. Neutron Diffraction The physics of neutron diffraction is very similar to XRD, Figure 1. The neutron beam is obtained from a nuclear reactor. The flux or number of particles per unit of time is lower than for the X-ray beam, so the analysis of complex structures can be difficult. Unlike the X-ray beam the neutron beam is strongly scattered by certain light elements including deuterium. It is possible to obtain structural details using neutron diffraction not normally obtainable by ordinary XRD methods. A recent study used neutron diffraction to locate the protons in the

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faujasite structure (29). Neutron diffraction experiments have also been used to show that the aluminum distribution is disordered in faujasite (30). A review of neutron scattering experiments has recently appeared in Science (31).

EXAFS (Extended X-ray Absorption Fine Structure Spectroscopy) XANES (X-ray Absorption Near Edge Spectroscopy) These techniques use sychrotron radiation as an X-ray source, available at some of the national laboratories such as Brookhaven. At the energy required to remove a core electron from an atom there is a sharp increase in X-ray absorption, called the absorption edge. Near the absorption edge there are intensity peaks and on the edge there are oscillations that result from an interference of amplitudes between the ejected electron and the electrons associated with the local surrounding atomic distribution, Figure 4a. The frequency spectrum of the oscillations can be Fourier analyzed to give interatomic distance information shown as a peak or series of peaks, Figure 4b. This technique is primarily applicable to catalysts containing deposited or impregnated metals where one is interested in the local coordination or bond distances around the catalytic metal atom. The technique has been applied to an analysis of vanadium chemistry on FCC catalysts. The results show that vanadium is not present as VzOs on the catalyst after calcination or steaming, but instead is present as a highly dispersed oxide species such as VOd3- (32). XANES results have further shown that vanadium interacts very strongly with proposed passivators such as magnesium (33).

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4a Vanadium edee X-Ray Absoiption &fIi clrlll

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5

10

15

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20

25

4h. Vanadium Radial Structure Function

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Y

0

2

4

6

8

Radius. i

Figure 4. a) X-Ray absorption coefficient at the vanadium edge for a vanadium impregnated FCC catalyst as a function of energy. b) The waves on the top of the edge are Fourier analyzed to give the Radial structure function as a function of distance from the vanadium atom. The peaks occur at distances where coordinating atoms occur, in this case oxygen, and are characteristic of the particular compound V04, D. J. Sajkowski, S. A. Roth, L. E. Iton, B. L. Meyers, C. L. Marshall, T. H. Fleisch, and W. N. Delgass, Appl. Catal., 1989,51,255.

3. PORE STRUCTURE ANALYSIS BY ADSORPTION AND ABSORPTION (Nitrogen, Mercury, Water, He, Argon, other Gases and Organic Materials) Adsorption methods are used to characterize the pore structure of the catalyst surface and also the thermodynamics of the interaction between the surface and the adsorbent. Besides a determination of the surface area, pore volume, and an indication of the distribution of pore sizes, it is possible to use adsorption methods to estimate the amount of zeolite in either the fresh catalyst or the deactivated catalyst. Another application, referred to as pore gauging,

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permits an estimation of the size of the pore opening in experimental zeolites. This method involves adsorption experiments with organic molecules . Gregg and Sing (34) have given a comprehensive review of the determination of the surface area and pore size distributions in catalysts as have Lowell and Shields (35). The Proceedings of recent IUPAC Symposia provides an update on many topics of current interest including those discussed here (36.37).

Nitrogen Nitrogen adsorption is used to determine the total surface area of a catalyst, and can also be used to estimate the small pore zeolitic component and a larger pore component. A procedure for the determination of the total surface area using the BET (Brunauer, Emmett, Teller) method is described as ASTM test D 3663 (14). The sample is calcined to remove moisture, exposed to a measured volume of nitrogen at some constant pressure P, and the volume adsorbed by the catalyst is measured. An adsorption curve, referred to as the adsorption isotherm, is generated by starting at some low pressure and measuring the volume adsorbed. The pressure is increased slightly, and the new adsorbed volume is measured. The adsorption data can provide an estimation of the relative amounts of small zeolitic pores and larger pores and is widely used in the analysis of FCC catalysts to separate the zeolite and matrix contributions to the surface area (38). This method is referred to as the t-plot method and has been developed as an ASTM standard test, D 4365 (14). The basis for these methods is the nitrogen isotherm. Microporous systems have what is called a type I isotherm. Catalysts containing both zeolitic micropores as well as mesopores have a combination of a type I isotherm with a type IV isotherm, Figure 5a. Adsorption in the small pores of the zeolite mostly occurs at a low pressure p relative to atmospheric po, p/po < 0.1, while adsorption by the larger pores of the matrix or the mesopore system occurs in the region p/po > 0.1. The adsorption data can be recalculated and plotted in such a way that the amount of nitrogen adsorbed is plotted against the thickness t of the adsorbed layer. For thin layers, both micropores and mesopores contribute, but for thicker layers, the micropores are filled up and only the mesopores contribute. Figure 5b shows the conversion of a conventional plot of the amount adsorbed with partial pressure to a t-plot of the amount adsorbed with the thickness of the layer. The t-plot method is frequently used to estimate the amount of zeolite present in a cracking catalyst. In the case of a faujasite dealuminated to a low unit cell size the method can be misleading since the zeolite can form a varying degree of mesoporosity during dealumination depending on the preparation and method of dealumination, e.g. with SiF,' or with steam (39). Other work has demonstrated that catalytic activity, especially for bottoms cracking, is associated with the development of mesoporosity (40). In the case of catalysts containing significant mesoporosity the t-plot method will count the mesoporosity as matrix surface area, when in fact it is zeolitic. As much as 20% and as little as 5% of the zeolite surface area at low unit cell size can appear as mesopores. The t-plot method has also been used to obtain an estimate of the particle size of as-synthesized zeolites (41). Since there is no mesoporosity in this case, the mesopore surface area measures the external surface area of the zeolite, typically 2 to 20 m2/g, depending on the particle size. Differences in particle sizes can be independently if only qualitatively confirmed by SEM pictures. Zeolite particle size can have important selectivity and stability consequences (41). The nitrogen desorption curve is sometimes used to determine a pore size distribution. Although this procedure has also been standardized as as ASTM test D 4641 (14). the results are not entirely reliable. It has been noted that 4.0 nm diameter pores are frequently observed as peaks in the desorption curve of a variety of materials. This is due to hysteresis in the adsorptiorddesorption isotherm at a P/PO of 0.4. probably due to the surface tension of the nitrogen film, Figure 5a. This discontinuity in the isotherm is reflected in the desorption pore size distribution (42). The nitrogen adsorption methods tends to be unreliable for very large pores over 60 nm in diameter where adsorption occurs at pressures near atmospheric. In cases of very low surface

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193

area catalysts krypton adsorption is used, ASTM standard D 4780. This test is not normally applicable to the typically high surface area FCC catalysts.

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Figure 5. a) The desorption isotherm of a typical cracking catalyst containing a region characteristic of microporous zeolites, and a region, p/po > 0.1, dominated by mesoporosity. The isotherm is of type IV, characteristic of a sample such as a cracking catalyst containing both mesopores and micropores. after S. J. Gregg and K. S. W. Sing, Adsorption, Surface area, and Porosity, Academic Press, New York, 1982. p. 4. Also shown as marked is an example of nitrogen adsorption/desorption hysteresis showing the ubiquitous anomaly at p/po of about 0.4 on the desorption branch due to the surface tension of the nitrogen film. The anomaly can produce artifacts in pore size distributions measured from desorption data, S. J. Gregg and K. S. W. Sing, Ibid., p.161. b) A t-plot of the isotherm in Figure 5a. The thickness of the layer of nitrogen being adsorbed is plotted against the amount adsorbed. The slope at a given p/po (or thickness) is the amount adsorbed per layer and is a measure of the surface area at that point. S. J. Gregg and K. S. W. Sing, Ibid., p. 97.

Mercury Mercury intrusion is especially useful for determining the volume and surface area of larger pores, typically over 20 nm to whatever size is appropriate. Powders will give spurious intrusion peaks in the 10oO nm range caused simply by the spaces between particles. The size and shape of the particles in the sample will set the upper limit of usefulness of this method. For this test also there is an ASTM standard, D 4284 (14). An analysis of the hysteresis can

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supply additional important information about the shape of the pores, ink bottles, cylinders, etc. (43). In using the mercury method, an important parameter used in the calculation of the pore size distribution is the wetting angle between the mercury and the pore wall. In practice, the wetting angle is set to a value that gives good agreement with the nitrogen pore size distribution over a range of 20 nm to 60 nm where both techniques are reliable. For silicdalumina an estimate of the wetting angle is about 140'.

Water The incipient wetness method measures the total pore volume by measuring the amount of water absorbed in the pores. Water is added to the sample until the particles just begin to stick together and form a cake. Helium Helium pycnometry is a helium adsorption technique for determining the skeletal density of porous solids (44).The helium atom is sufficiently small that it will enter into the smallest pores. The result is a measurement of the skeletal volume. If the weight of the sample is known then the skeletal density can be calculated. In the case of zeolites this result gives the framework density. Furthermore. from the skeletal density and the surface area it is possible to estimate the average pore size. This is a result frequently used in reaction engineering estimates of diffusivities (45). Low Pressure Adsorption Methods involving the use of argon at very low pressures have recently become available. These methods are useful in the estimation of pore size distributions in the small pore range of 2 nm or less pore diameter. Zeolites, for example, have pore sizes in the 0.5 to 1.5 nm range and some zeolites may have several types and sizes of pores. In these cases low pressure isotherms may be used to characterize zeolites or mixtures of zeolites. For example, using this technique M. Davis was able to show that the molecular sieve MCM-9 is a mixture of VPI-5 and SAPO-11 (46). Other catalytic materials such as pillared clays and carbon can also have distributions of very small pores. The application of these techniques to carbon has been recently described by Carrott and Sing (47).

Organic compounds Adsorption isotherms of organic compounds have been used to characterize the pore size of zeolites. A discussion of the use of different size molecules to characterize the dimetisions of the zeolite pores has been given by Szostak (48). The pores of zeolites are formed by circumscribed rings of -0-T-0-T- where T is either silicon or aluminum. In a small pore zeolite the rings contain eight T atoms, in a medium pore zeolite such as ZSM-5 the rings contain ten T atoms, and in a large pore zeolite such as faujasite or mordenite the rings contain twelve T atoms. The method is based on the idea, for example, that a small pore zeolite with eight membered rings and a pore diameter of about 0.4 nm will absorb n-hexane, but not a bulkier cyclohexane. Table 2 gives the approximate kinetic diameter of selected molecules used as adsorbents as well as approximate pore sizes of small, medium, and large pore zeolites. Recently M. Davis prepared a new zeolite, VPI-5, containing 18 membered rings with 1.2 nm pores. This material will absorb triisopropylbenzene with an estimated kinetic diameter of 0.85 nm (49). Organic compounds can also be used to measure surface areas, and isotherms generated using organic compounds can have catalytic significance. The original observation of the existence of a mesopore system in a dealuminated faujasite was based on mercury intrusion results and n-hexane isotherms (5031). Subsequent work with a wide range of organic absorbents showed that part of the pore volume of dealuminated faujasite is in the mesopore range. In this work the thermodynamics of adsorption of a wide range of organic materials on variously dealuminated zeolites was determined. General relationships were developed that will allow the development of (P,T) absorption isotherms from a limited amount of data (52).

195

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Table 2. Pore gauging. Molecular sieve pore size estimation from adsorption properties of a series of different sized molecules at P P o 0.4. Ring size Pore size -Pore diameter (nm)

Adsorbate

H20 n-hexane cyclohexane neopentane

8-Ring small 0.40

sizdml 0.26

0.4

0.6 0.62

Yes Yes no no

10-Ring medium 0.53-0.6 Does it dssxb? Yes Yes Yes no

12-ring large 0.6-0.75

Yes Yes Yes Yes

4. SURFACE ANALYSIS AND IMAGING (SEM,TEM, STEM, EDX, XPS (ESCA), Auger, SIMS,WDS (EPMA), STM,AFM )

The electron microscope with attachments for elemental analysis (EDS. WDS)is the most commonly used instrument for surface analysis. This technique involves bombarding the surface with an electron beam and detecting the backscattered electrons. This technique can be used to observe the surface topology of the catalyst including the occurrence and location of zeolite and matrix materials embedded in the catalyst particle. Since the beam penetrates. the microprobe provides an analysis of a small volume of the sample, 1 pm3. Consequently the location and distribution of contaminant metals and other elements is determined in a surface layer with a thickness of about 1 micron. In the case of the transmission electron microscope ( E M ) the electrons are transmitted through a thin section of the sample. This technique has been used to image zeolite lattice structures and can be used to identify very low zeolite levels in a catalyst particle. The other techniques use either an X-ray beam (XPS), an ionic or an atom beam (SIMS, FAB-MS) or do not use a beam (STM,AFM) to bombard the sample. The idea is to bombard the surface and to observe a reflected or an emitted particle. In this case the observed surface is only a few atoms thick. The energy that is lost or emitted will be characteristic of the type of atoms on the surface and will depend on the oxidation state of the atoms. XPS and Auger spectroscopy provide information concerning the location and sometimes the oxidation state of metals on the catalyst particle. These and other methods have been extensively reviewed by Briggs and Seah (53) and more recently by Fiem (54). In using most of the surface techniques the sample is typically embedded in a matrix, often epoxy, and is then cut and polished in such a way as to expose a section through the sample. Elemental migration from one part of the sample to another or between sample and matrix due to high pressures or to abrasion and high temperatures during polishing can be a problem unless special care is taken. The sample may also be cut in a thin section (microtomed) for transmission electron microscope studies. What is exposed and analyzed may be the surface of the particle or may be the interior of the catalyst particle exposed by the sectioning procedure. Frequently argon etching of the exposed surface is used to expose a fresh layer of catalyst. This procedure is referred to as depth profiling. Surface techniques are therefore not restricted to the surface, but rather refer to the fact that an analysis can be obtained for a small and very precise region of the sample rather than for the bulk. If an electron beam is used or if electrons are emitted, the development of an electric charge by the sample is an important consideration. Excessive charging of the sample lowers resolution and can divert the electron beam. Nonconducting samples such as FCC catalysts are coated with a conductive material such as carbon or a metal in an attempt to prevent the

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196

buildup of charge during bombardment with the electron beam. Frequently charge will build up no matter what, so electron bombardment methods can be difficult with nonconducting samples. In some cases the sample can acquire a positive charge. The SIMS technique uses a positive ion beam, and in the case of XPS the ejected photoelectron leaves a positive hole. A solution in this case is to flood the sample with electrons using the electron flood gun to neutralize the charge build up as it occurs. The ability to neutralize positive charge build-up on non conducting materials like FCC catalysts is an advantage in applying XPS and SIMS. '

THE ELECTRON MICROSCOPE AND MICROPROBE

and Elemental analysis

-

High Resolution Catalyst Topology

Electron microscope (SEM, STEM, TEM) There is a group of methods used for surface analysis that involve the bombardment of the surface with an electron. The type of information obtained and the resolution or the spot size depend on the energy and resolution of the beam and on the detection system, and the depth of penetration depends on the energy. The electron microscope is typically used to observe the structure and topography of catalytic materials at high resolution. The images occur as light and dark regions. Dark regions can be holes or may indicate the presence of lighter elements, while light regions may represent a clump of heavier elements. Under the best conditions the electron microscope (STEM or TEM) can resolve features well below one nanometer in size. The electron microscope can be used to observe the pore openings of zeolites such as faujasite, beta, and ZSM-5 as well as other zeolites. A commonly used instrument is the SEM, the Scanning Electron Microscope. It typically operates at a magnification of 20,O)lO or more and can resolve structures of 0.1 p or less. Since FCC particles are about 70 pm in size and are composed of O.1pm to l p m size particles of zeolite, clay, and matrix materials, the SEM can be used to observe the distribution of the zeolite, clay and matrix particles in the catalyst particle where the clay and zeolite particles can sometimes be identified by their shape (1). The SEM is also commonly used to identify the morphology of catalytic components such as zeolites. Zeolites can occur in a variety of sizes and shapes. These parameters are known to have effects on selectivity, for example as observed in the case on ZSM-5 (55). In preparing zeolites frequently small amounts of another phase may occur as an impurity. The SEM may be used to identify impurities by differences in morphology compared to the main component. The TEM (Transmission Electron Microscope) and the Scanning Transmission Electron Microscope (STEM) operate at a higher magnification, about 100,OOO or more, and can resolve features as small as 0.2 nm, about the size of an atom. These machines are frequently used to observe structures at the molecular level in both zeolites and in catalysts. TEM micrographs are used to observe the existence of intergrowths and faulting in pure zeolite samples. Intergrowths involving several structurally similar materials are commonly observed in ZSM-5 crystals and more recently in beta zeolite. The occurrence of significant mesoporosity (1 1) has been subsequently observed directly in TEM micrographs (56-58). Figure 6. The sensitivity of TEM is illustrated by recent work by Beyerlein (59) in carrying out an analysis of a FCCU deactivated catalyst. He was able to separate various fractions of the catalyst by age using a sinWfloat technique. He found that the oldest fractions having spent the longest time in the unit were the most severely deactivated. These fractions did not contain zeolite by XRD,but small zeolite fragments were clearly visible by TEM. The TEM or STEM can also be used to observe the growth and size of metal crystallites. Activity is proportional to the dispersion of the metal particles which is assumed to be inversely related to the size of the crystallites. The TEM has been used to observe nickel crystallites on metal contaminated FCC catalysts (60).

197

Figure 6. A TEM micrograph showing both the zeolite lattice and larger mesoporous holes resulting from hydrothermal dealumination, F. Mauge, A. Auroux, J. C. Courcelle, Ph. Engelhard, P. Gallezot, J. Grosmangin, Studies in Surface Science and Catalysis, Vol. 20, Catalysis by Acids and Bases, Elsevier, New York, 1985, p. 94. Both the STEM and the SEM are frequently equipped with elemental analysis capability, EDS or Microprobe. High resolution elemental analysis capability has permitted the use of the STEM to observe, for example, the aluminum profiles across a faujasite particle resulting from the various dealumination techniques. Low temperature solution phase dealumination by diammonium silicon hexafluoride creates an aluminum gradient such that the framework of the outside of the zeolite is silicon rich with additional silicon deposits occurring on the zeolite (61). In this case the external surface of the zeolite is more strongly dealuminated than the interior. As discussed in the appropriate section, XPS has been used to show that during hydrothermal dealumination the aluminum from the interior migrates to the external surface. Elemental Analysis (Electron Microprobe (EPMA, WDS), EDS, XES) It is often desirable to be able to analyze the chemical composition of the structures observed using the electron microscope. For example, one may want to know the locations of the zeolitic and matrix components, or the location of various contaminants or poisons such as nickel, vanadium, and iron. For this purpose the SEM often has an attachment capable of providing elemental analyses of a portion of the surface. The bombarding electron beam produces a characteristic X-ray emission spectrum from the inner electronic shells of the elements in the sample. The technique of measuring the X-ray emission spectrum of a sample

198

is called XES, or X-ray Emission Spectroscopy. The instrument that measures the the x-ray emission spectrum using electron bombardment is called an electron microprobe, and the technique of analysis using the electron microprobe is called EPMA (electron probe microanalysis). There are two different detector systems in use for measuring the X-ray spectrum, EDS or WDS. The EDS or energy dispersive spectrometer attachment typically has poor energy resolution so that peaks of some elements will overlap with others. If one is interested in the elemental composition of some region of the sample, the electron beam will focus on that spot, typically about 1 p in size and 1 p deep. X-ray emission intensities will be acquired in enough channels to provide the qualitative amounts of a relatively large number of elements over a few minutes. The WDS (wavelength dispersive spectrometer) has much better energy resolution and can resolve each element in the presence of others. Typically the WDS is used in a scanning mode where an FCC particle of 50-80 p is scanned and the profile of specific elements are obtained. For example, this technique has been used to identify the location of zeolite (regions of high silicon) and to associate the poison vanadium with various catalytic components such as rare earth (62) and alumina (63). The observation that in the presence of steam vanadium migrates both within a particle and between particles was based in part on WDS results (63). A draw back of the WDS and EDS methods is that sensitivities are different for different elements and therefore a calibration procedure is required. XPS (X-ray Photoelectron Spectroscopy) ESCA (Electron Spectroscopy for Chemical Analysis) UPS (Ultra violet Photoelectron Spectroscopy) XPS, sometimes identified as ESCA, involves bombarding a sample with X-ray radiation and measuring the energies of the emitted photoelectron. These energies are closely related to the binding energy of the core electrons in the particular compound. Each element has a characteristic spectrum of photoelectron emission energies. In a compound or in a solid, these binding energies are modified by the effective charge on the atom and to an extent by the surrounding atoms or ions. A positive charge will shift the binding energies higher, while a negative charge will produce a shift to lower binding energy. The XPS spectrum allows one to identify the major elements in a sample, and more importantly, from the position of the peak to estimate the oxidation state of the element. There are two different kinds of XPS instruments. XPS with a high spatial resolution may include a high intensity synchrotron radiation source. More commonly, the XPS instrument is used to observe the surface without spatial resolution and to provide information concerning the chemistry of the observed elements. XPS spectra has been used to identify the oxidation state of vanadium (+5) and nickel (+2,+3) on cracking catalysts (64). Recent XPS studies have experimentally confirmed that the acidity of zeolites is a property of the lattice associated with oxygen polarizability rather than a local phenomenon (65,66) and support previously proposed relationships between acidity, lattice electronegativity, and aluminum content (67,68). Steam dealumination produces a uniform framework dealumination profile in that an .equal amount of aluminum is removed from the surface and the interior of the zeolite particle. The XPS results further show that the alumina formed as a result of dealumination migrates to the outside of the zeolite during the steaming and does not remain in the pores (69-70). XPS has also shown that low temperature solution phase dealumination gives a silicon rich zeolite surface (71). as previously discussed (61). Taken together, these results have considerably clarified the events that occur both during catalyst preparation and during zeolite hydrothermal dealumination and deactivation in a FCC regenerator. UPS is similar to XPS except the lower energy of the photon in the ultraviolet region produces photoelectrons from the valence shells. Consequently the results are more sensitive to information concerning the details of the chemical bonding. UPS is not commonly used in the analysis of FCC catalysts.

199

Auger Electron Spectroscopy (AES) Auger Spectroscopy involves bombardment with electrons of a sufficiently high energy to ionize core electrons and an observation of the consequent relaxation process, the process responsible for removing the energy involved in creating the ionized state. Part of the energy is removed by a transition of an outer shell electron to the lower energy core shell. The excess energy is lost either by the emission of an electron, the Auger electron, or by the emission of a photon. The standard X-ray fluorescence technique detects the photon. The Auger spectrometer analyzes the energy of the electron, and based on the analysis can identify the element and, sometimes, the oxidation state. Cracking catalysts are nonconducting, so the electron beam from the Auger spectrometer produces significant charging of the sample. Since Auger and XPS can provide similar information, XPS is more commonly used. Auger in combination with Argon etching has been used to show that at 725 OC nickel antimony alloys even with a low antimony content are enriched in antimony at the surface (72). This result provides some insight into the mechanism for antimony passivation of nickel on FCC catalysts.

Figure 7. Cross sections of equilibrium catalysts containing zeolite, clay and alumina particles analyzed by the SIMS technique. An elemental mapping of b) silicon, c) lanthanum, d) vanadium, and e) nickel show that both nickel and vanadium appear on the surface of the particles, nickel more so than vanadium, from D. P. Leta, W. A. Lambetti, M. M. Disko, E. L. Kugler, and W. A. Varady, Fluid Catalytic Cracking II, M. L. Occelli, ed., ACS Symposium Series 452, American Chemical Society, Washington D. C., 1991. p. 276.

200

SIMS (Secondary Ion Mass Spectroscopy)

SIMS is an ion sputtering technique. A beam of positive ions (eg. argon, oxygen, cesium) ionize the surface and knock surface ions free. These ions are collected and analyzed in a mass spectrometer. The mass spectrometer gives the atomic weight of each ion observed. Since the detector normally used is sensitive to single ion events, the method provides a complete and a very sensitive elemental identification of all elements present. In a scanning ~ resolution. Since mode it can provide elemental maps of the catalyst surface with 0 . 1 spatial some elements sputter much more easily than others, one drawback of the technique is that the surface composition can change during the analysis. For those elements that sputter most easily the technique can be very sensitive, orders of magnitude more sensitive than the electron microprobe. Recent SIMS results on equilibrium FCC catalysts show the association of vanadium with alumina, with zeolite, and with rare earth (73) in a commercially deactivated equilibrium catalyst. Nickel is associated with alumina in the catalyst particle. Both vanadium and nickel appear to be deposited on the outside of the catalyst, Figure 7. Vanadium is more mobile and penetrates more easily (74,75). A study of laboratory impregnated and deactivated catalysts using SIMS gave similar but not exactly the same results (76). Both nickel and vanadium concentrated to an extent on the surface. This work is especially important in attempting to understand and to simulate catalyst poisoning by nickel and vanadium in the laboratory. The metals initially lay down on the outside of the particle and migrate to the interior. Nickel migrates much more slowly than vanadium, and both react strongly with aluminum. FAB-MS (Fast Atom Bombardment Mass Spectroscopy) In this technique a beam of energetic atoms knocks ions from the sample. This technique has been used to make the first observation of the enrichment of aluminum at the surface of hydrothermally dealuminated zeolites (77, 78). Since the sample is bombarded with neutral species, sample charging is less of a problem. STM (Scanning Tunneling Microscopy) AFM (Atomic Force Microscopy) In both techniques the tip of the spectrometer is placed very close to the sample and can follow the contour of the sample at an atomic level. In the case of STM the tip distance is maintained by resistance to electron tunneling from the tip to the sample. In the case of AFM the tip contains an optical balance that is very sensitive to changes in force. It is known that as neutral molecules approach each other there is an initial attractive force followed by repulsion. These forces are the Van der Waals or Leonard -Jones forces. The tip approaches the surface so closely that it is sensitive to these forces and scans the surface in such a way that this force is maintained constant. Since the surface is essentially defined by the Van der Waals distance, the path of the tip defines the surface with atomic accuracy. STM is applicable to conducting materials such as metals with a reasonably smooth surface. Since FCC catalyst materials have a rough surface and are non conducting powders, this method is has not been thought to be applicable. Recent results have shown that in the presence of ambient air an image of a silicalite surface can be obtained. Silicalite is a high silica zeolite with the structure of ZSM-5. The resolution is sufficient to show individual silicon atoms in six membered rings (79). AFM has also been used to obtain high resolution images of zeolite surfaces, showing pore openings and individual tetrahedra as well as the location and orientation of adsorbed organic molecules (80). The Usefulness of the Surface Techniques The morphology and structure of the FCC catalyst is important, especially the location of the catalytic components, the pore structure, and the location and chemistry of coke and metallic poisons. The instruments of most value so far have been the SEM (morphology), the electron microprobe (location and analysis of the chemical components), and XPS for a determination of oxidation states and chemistry of various elements on the catalyst.

201

Auger spectroscopy has been less useful primarily because the catalysts are insulating and charging effects tend to be relatively severe. The information obtainable by Auger spectroscopy (elemental analysis, oxidation states) is more readily available by XPS where charging can be compensated using the electron flood gun. SIMS equipment is expensive and it is difficult to provide quantitative elemental analyses at this time, so its use has been limited. It can be especially destructive of the surface, depending on the mode of operation. However the potential for new information is so great that one can expect an increasing number of applications especially in research connected with catalyst deactivation. STM and AFM techniques are just beginning to find use. The roughness of the FCC catalyst particle has so far ma& these techniques difficult to apply. 5. SPECTROSCOPIC ANALYSIS (Infrared, FTir, Raman, Diffuse Reflectance, MASNMR, uv-visible, EPR)

Spectroscopic analysis deals primarily with the acidic properties of the catalyst including the zeolite and matrix components. The results provide information concerning the number, the strength, and the types (Br0nsted (protonic) or Lewis) of acidic sites present. Infrared and uv-visible techniques are used either to directly observe the properties of the acidic -OH group or to observe interactions between the acidic surface and an adsorbed probe molecule. Probe molecules include ammonia, pyridine, benzene, hydrogen, and other more or less basic molecules. In silica alumina materials proton donor sites are usually associated with an aluminum atom connected to a silicon atom through an oxygen bridge as in the following structure, Figure 8.

si

H

'

0

\

Figure 8. Schematic illustration of the acidic bridged OH function believed to be responsible for the acid cracking activity of zeolites. The proton bonded to the oxygen can be characterized by an OH stretching frequency in the infrared and is distinguishable from other non-acidic or less acidic OH groups. Centers of both Brgnsted and Lewis acidity can be indirectly characterized by spectroscopically monitoring the interaction of the solid acid with various probe molecules. Frequently proton donor acids associated with an -OH group are called Bransted acids. In this discussion they will be referred to as protonic acids. MASNMR (Magic Angle Spinning Nuclear Magnetic Resonance) also provides information concerning acidity, but from a different point of view. The chemical shifts of silicon, aluminum, and protons are observed. Since zeolites and cracking catalysts are solid acids one would expect the 'H spectrum to be the most useful. However the acidic properties of zeolites are determined by the lattice and by the environment of the proton rather than by

202

the properties of the proton itself. Consequently MASNMR is most often used to describe the silicon and aluminum containing structures that appear to determine the density and strength of the acid. . Fierro (81) has provided a recent review of the various spectroscopic techniques used for measuring acidity, The measurement and nature of acidity in solid acids such as zeolites has been discussed by Tanabe (82), by Rabo and Gajda (83), by Vedrine (84) and by C o m a (85). With the exception of MASNMR, spectroscopic methods are generally applicable to cracking catalysts. Thermal methods discussed in the next section are also used to characterize acidity, especially in zeolites. The division into thermal and spectroscopic methods is somewhat artilkid since these methods are frequently combined.

Infrared; FTIR; Diffuse Reflectance, DRIFTS Infrared, FTir (Fourier Transform Infrared), and DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) techniques all involve spectroscopic observations in the infrared region using slightly different instruments. Both infrared and FTir operate in the transmission / adsorption mode. The sample is prepared, the beam is transmitted through the sample, and wavelengths where absorption occurs are observed as absorption peaks (or transmission valleys). An infrared spectrometer will scan over the region of interest using a diffraction grating and a series of slits and mirrors to control the wavelength. The FTir instrument splits the beam into two components. By slightly changing the path length of one of the beams and then recombining the two beams an interference pattern is created. The interference pattern contains a spectrum of frequencies for which absorption is measured. A large number of scans are acquired over a period of time to build up the spectrum. Fourier transform techniques are used to transform the frequency spectrum into the conventional series of peaks. If the sample is spread on a reflection ball and the intensities of the reflected infrared are detected, the technique is DRIFTS.

.

..

otonic Aciditv Absorption bands due to the stretching vibration of the OH bond can be observed directly in the 3500 cm-I to 3750 cm-' region of the infrared (86.87). Some assignments are listed in Table 3. Assignments of strong or weak acidic character are based on combined infra redhase titration experiments. An intensity reduction is correlated with titration by some strong base such as ammonia or pyridine. The strongly interacting or acidic OH bands will show up as peaks in the difference spectrum. Typically there are weakly or nonacidic silanol OH groups that appear near or above about 3700 cm-'. These are considered to be lattice termination groups. Strong acidity is associated with OH groups in the 3610 cm-' to 3650 cm-Lregion. Weaker acidity is associated with groups in the 3550 cm-I range. An example of a typical spectra is shown in Figure 9. Steam dealuminated faujasite contains a strong protonic acid with a band at -3610 cm-I (88) that has been associated with the formation of a nonframework species in severely dealuminated faujasite (89-91). A band in the same region has been associated with strong acidity in ZSM-5 and other high silica zeolites (92). Infrared spectra have been used to confirm the suggestion by Kuhl (22) that some of the nonframework aluminum can occupy exchange sites in the zeolite (23). Infrared spectra have been used to characterize the occurrence of hydroxyl nests, a lattice defect formed as a result of dealumination without silicon insertion or lattice rearrangement. The hydrolysis of the silicon aluminum bonds and the removal of the aluminum leaves four OH groups in the hole previously occupied by the aluminum atom. Defects of this type have been characterized by a broad infrared absorption in the 3000 cm-1 to 3750 cm-1 range (93). Hydroxyl nests are observed by comparing absorbances at a wave number such as 3710 cm-1 without overlapping contributions from other structures.

203

I

b

a

A

LPV

LPV

B

A 3610 3560

3630

a c

C

n .c L

C

U

s

3610

L

K

J-

n

1400

L

3610

23900

3500

I

I

1700

1500

1500

1600

1700

Frequency (crn-')

Figure 10. Pyridine adsorbed on silica alumina at 20O0C in successive doses 1-4 and (top curve) after desorption at 400OC. The large peak at 1450 cm-I is associated with Lewis sites and a much smaller peak at 1540 cm" is associated with protonic or Bronsted acidity. The figures and explanations are from N. C-Martinez and J. Dumesic, J. Catal., 1990,125,427.

c m-1

Figure 9. Infrared difference spectra of dealuminated faujasite before and after pyridine adsorption and evacuation 1) at 25OoC, 2) 35OoC, and 3) 400°C in two different infrared regions including a) the OH stretch region showing the occurrence of conventionally observed bands at 3630 cm-' and 3560 cm-1 and a strong protonic band at 3610 cm-I and b) the lower frequency spectrum of pyridine showing a large peak associated with protonic sites at 1540 cm-I and a smaller peak at 1450 cm" associated with Lewis sites, from G. Garralon, A. Coma, and V. Fomes, Zeolites, 1989,9,84.

204 J .ewis and Rot-

.

..

Direct observation of the OH stretch can only provide information concerning the protonic acidity. Breck and Skeels have shown that dehydroxylation can occur at about 115O0F (62OOC) (94). The result of dehydroxylation is the generation of Lewis sites. The infrared spectrum of absorbed pyridine, an aromatic organic base, provides a method of distinguishing between protonic and Lewis types of sites. The technique has been used to study sites on matrix materials as well as zeolites. The pyridine is adsorbed, the sample evacuated to desorb excess pyridine, and the infrared spectrum is obtained. Pyridine adsorbed on a Lewis site gives a peak at about 1450 cm-'. A band at 1545 cm-' occurs in the case of the pyridinium ion and is characteristic of a protonic acid. A non-diagnostic band occurs at 1485 cm-' as a result of adsorption on either Lewis and protonic sites (95,96). Figure 9 shows an infrared difference spectrum of pyridine absorbed on a silica alumina catalyst both in the OH region and in the pyridine region. The peak assignments are given on the figure. Dealuminated zeolites contain significant protonic activity while a silica alumina catalyst contains primarily Lewis sites (97), Figures 9 and 10.

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One objection to infrared studies involving the absorption of strong bases like pyridine and ammonia is that these compounds are strongly basic as well as highly polar and will absorb on nearly anything including polar as well as weakly or strongly acidic sites. Consequently the acidity that is observed may not be the acidity that is catalytically important. An alternative is to use much weaker bases which will form a complex only with very strong acids. Titration of the surface with a weaker base such as benzene shifts the band due to the interacting -OH group to a lower frequency. Since the shift is expected to be more or less proportional to the degree of interaction, the strength of the acid is correlated with the frequency shift of the 3610 cm-' hydroxyl band in the infrared (98). Shifts to lower wave numbers by 300 cm-I to 350 cm-' are typical of strong acids, while weaker acids give smaller shifts in wave numbers. Similar experiments using hydrogen as the probe molecule are useful for probing the strength of Lewis sites. The adsorption of hydrogen on a strong Lewis site is associated with peaks in the 4000 cm-I to 4080 cm-' region. Lewis sites are observed on dealuminated zeolites by this method (99).

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gv-visjble: weaklv basic dyes and aromahc Drobes One proposal is to use as a probe a series of molecules such as benzene, toluene, and xylene (100). A sufficiently strong acid will form a sigma complex observable in the near uv at a wavelength of about 335 nm. Benzene requires a stronger acid than does toluene, and toluene requires a stronger acid than xylene. This method has been used to experimentally rank zeolites by their strong acidity. The ranking is the same as that obtained by catalytic methods such as a measurement of the turnover number for the cracking of n-hexane. Dealuminated faujasite, the zeolite used in cracking catalysts, has only intermediate strength sites and no strong sites. The number of intermediate strength sites increase with dealumination. Mordenite and ZSM-5 contain strong sites. It may be that high gasoline selectivity is associated with sites of moderate strength. An approach suggested by Benesi (101) is to mix a weakly basic dye used as a Hammett indicator with the solid acid catalyst. If the acid protonates or interacts with the dye a visible color change will occur and one may infer the existence of acidic sites in the appropriate range of strengths. A series of dyes has been developed and has been used to characterize the acidity of various cracking materials. Recent work uses some of the same dyes but uses a uv spectrophotometer in place of the eye to measure the occurrence or non occurrence of the appropriate peaks in the uv region due to protonation (102). The results have shown that this is a reliable approach. The transitions associated with the protonic form of the dye occur in

205

the uv region. Since the eye is not sensitive in the uv region, visual color observation is misleading and spectroscopic observation is required. Acid strength is given in terms of the Hammett function Ho where 100% sulfuric acid has an Ho of -12. Super acids are stronger than 100% sulfuric and have Ho < -12. The results using the spectrophotometer show that mordenite is a moderate super acid. It contains strong acid sites marginally stronger than 100% sulfuric acid with an Ho value of -13. Faujasite has an acid strength equivalent to about 95% sulfuric acid, Ho -9 to -1 1. Silica alumina (13% alumina) is equivalent to about 70% sulfuric acid, Ho -4 to -8. One drawback of this method of characterizing FCC catalysts is the observation that the regenerator of an FCCU typically operates above 1250'F (68OoC), while Skeels and Breck have shown that zeolites dehydroxylate at about 1150'F (620'C). On the other hand, steam is present and hydrogens are available for rehydration. Consequently the sites observed by room temperature measurements may not be the active sites present in the operating catalyst. Infrared measurements in the OH stretching region during the cracking of cyclohexene made at operating temperatures show the presence of strong proton acidity in situ on a dealuminated zeolite catalyst and suggest the involvement of these protons in coke formation (103).

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-

-

Raman Raman spectra have been used primarily to observe vibrations characteristic of the silica alumina framework. There are peaks than can be related to the silicdalumina ratio of faujasite (16, 17). Raman spectroscopy can be used to identify zeolite framework structures, and also the structure of templates that may be present. MASNMR (Magic Angle Spinning Nuclear Magnetic Resonance) NMR has become a favorite tool of organic and physical chemists for structure determination and identification of organic compounds. Any nucleus with spin one half produces a very sharp resonance in solution. 13Carbon and 'H have been especially useful since these are the major components of organic compounds. Solid state catalytic applications are more recent. For these applications the nuclei studied include 27Al,29Si, 'H, and 129Xe. Since FCC catalysts are solid acids composed predominately of silica and alumina, the applicability of 27Al, 29Si, and 'H MASNMR is obvious. The acidic properties of these materials are a result of an aluminum atom tetrahedrally connected by means of an oxygen bridge to four silicon atoms. In this situation the aluminum has a formal negative charge balanced by a positively charged proton located on or near one of the bridging oxygen atoms, Figure 8. 27Al MASNMR has been used to characterize the Occurrence of least four types of aluminum structures in zeolites and other catalytic materials including three types of nonframework alumina and two types of classically acidic tetrahedral forms, one framework and one probably nonframework. 29Si MASNMR has been used to quantify the Occurrence of silicon with zero, one, two, three and four aluminum neighbors and to indirectly infer the degree of aluminum site isolation. Since 29Si has a relatively low (4.7%) natural abundance, the acquisition of a spectrum may require several hours and thousands of scans. More recently 12%e, 15N, and 31P NMR techniques have become useful. Nitrogen containing compounds are frequently used either as templates, adsorbents, or as exchange cations, and 129Xeis used to investigate the pore geometry of zeolites as well as catalysts. Phosphorus appears in the framework of a new class of molecular sieves containing phosphorus as well as aluminum and silicon. The term zeolites is normally restricted to silica alumina containing structures, while the term molecular sieve is used more broadly where the framework may contain significant amounts of other elements, eg. phosphorus. MASNMR is used to characterize the chemical bond involving a particular element, where the chemical shift is characteristic of the number and identity of the bonding atoms.

206

Table 4 gives correlations between chemical shifts and the bonding environment for silicon, aluminum, and phosphorus bonded to each other. SiOP bonds are normally not observed. While the chemical shifts are characteristic of the number and chemical identity of nearest neighbors, for a given chemical environment small variations in the chemical shift are observed that are a result of variations in the bond angle and other bonding parameters. For each environment the range of shifts observed is listed in Table 4. Table 4. and 31P. 29s

MASNMR chemical shifts (ppm) associated with structures containing 29Si, 27Al,

,,(OAI) -tive

n=

to TMS (Tr-

Q

1

2

3

4

105 to 107

95 to 105

88 to 96

86 to 92

80 to 86

to ~13+

27-tirelative

~

Tetrahedral

Qsi

50 to 70

m

35 to 45

. .

Coordumn

Octahedral

mi -5tolO

m -20

-oordina&

l2S.i

20 to 35

Octahedral

u

-15 to 0

The basic interaction is between the magnetic moment of the spinning nucleus and the magnetic field of the instrument. In the case of spin one half there are only two orientations, parallel or anti-parallel to the field. The sample is placed in the field of the probe, a radiofrequency oscillator. When the frequency of the probe oscillator corresponds to the energy of transition from parallel to anti parallel, the nucleus starts to precess and to change its orientation. This is associated with a change in magnetization measured in the probe. The position of the NMR signal of a bare nucleus would depend only on the nuclear magnetic dipole moment. However, in a chemical compound the nucleus is surrounded by moving electrons which also respond to the magnetic field in such a way as to produce shielding or deshielding effects. Depending upon the electronic environment, the local magnetic field at the nucleus will be slightly changed. The transition frequency will be shifted by an amount depending on the electronic environment of the particular nucleus. It is the magnitude of this shift, called the chemical shift, that provides useful chemical information. In a powder there are several sources of line broadening. Molecular orientation varies in a solid powder, and since the chemical shift depends on orientation, the signal will be broad, a result of chemical shift anisotropy. A second source of broadening is the dipolar interaction. Each nucleus interacts with other nearby nuclei. Since the nucleus is a small magnet, nearby nuclei contribute to changes in the local magnetic field. The degree of interaction depends on the relative orientations of the interacting nuclei to the field. The dipolar interaction is only a factor for neighboring nuclei. It is not a factor in the case of low natural abundance isotopes

207 such as 29Si. A third source of broadening is a result of the quadrupolar interaction between a nucleus of spin greater than 1/2 and a non spherically symmetric distribution of valence electrons. The interaction results in a line broadening due to unresolved multiplets. In solution the average orientation for each molecule is zero over the time required to obtain the signal. For spin 1/2 the peaks are sharp and well resolved and the position of the peak relative to some standard gives the chemical shift information. In a solid the orientation is fixed, leading to a broad signal that obscures the chemical shift information. However, if the sample is spun at an angle of 54.7Oto the imposed magnetic field. the orientation effects will average to zero. Such an experiment is called Magic Angle Spinning Nuclear Magnetic Resonance (MASNMR). The application of MASNMR gives relatively sharp resonance lines containing the desired chemical shift information in a variety of solids, including catalysts and zeolites. Comprehensive reviews have been provided by Fyfe (104) and more recently by Engelhardt and Michel(lO5). sof 2 9 w ~ ~ The silicon chemical shift in alumino-silicates depends on the number of neighboring aluminum atoms. There are five types of silicon that can be distinguished by their chemical shift in the 29Si MASNMR of faujasite. They correspond to silicons bonded through oxygen linkages to 0, 1, 2, 3, or 4 aluminum atoms in the structure of the faujasite schematically shown in Figure 11. The chemical shift depends to a lesser extent on structural details such as the bond angle and bond length associated with the particular structure. In favorable circumstances the variation in chemical shift can allow an estimate of the bond angles in a particular zeolite structure (106). Consequently there is a fairly broad range of chemical shifts that can be associated with each of the five types of silicon chemical environments, The chemical shift range appropriate to different silicon environments is provided in Table 4 (107). Amorphous non-zeolitic silica can often be further identified at a higher shift if present. The usefulness of the 29Si NMR spectra depends on certain characteristics of the zeolite structure. The structure consists of silicon and aluminum tetrahedra linked through oxygen. The tetrahedral aluminurn has a formal negative charge neutralized by a cation, typically sodium or hydrogen. The hydrogen exchanged form is the acidic form. It has been proposed that aluminum tetrahedra will not link together because of the repulsion of the two adjacent negative charges. This rule, known as Lowenstein’s rule, is based on generalized bonding considerations suggested by L. Pauling and is obeyed for zeolite structures (108). Consequently an aluminum will always be surrounded by four silicon tetrahedra. The total number of aluminum atoms per unit cell, #Al/uc, in the structure is therefore one fourth of the total number of silicon - aluminum bonds. Since the number of silicon atoms in various possible environments is proportional to the MASNMR intensities, #Al/silicon atom = Si(lAl)/4 + Si(2Al)/2 + 3Si(3Al)/4 + Si(4Al). where the numbers of silicon in the various environments are given by the respective MASNMR intensities normalized to a total intensity of one. The silicon to aluminum ratio R of the framework is the inverse. The number of aluminurn atoms per unit cell is given by #Al/unit cell = 192/(R+1). This :lationship has been used to define the relationship between the unit cell size and the alum urn content of zeolites, Table 1. Unfortunately cracking catalysts contain sources of sili, n other than the zeolite. These other sources such as clay and the silicon components of tl binder broaden the peaks and interfere with the application of 29SiMASNMR directly to cr; dng catalysts. As a result of Lowenstein’s rule the distribution of aluminum atoms can be indirectly inferred from the distribution ol the silicon intensities. The distribution of aluminum is important since issues such as coke selectivity, hydrogen transfer activity, and acid site

208

strength may be related to the degree of site isolation. As the SUAI ratio of faujasite increases there is a tendency towards site isolation reflected in the loss of intensity in peaks associated with Si(4Al) and Si(3Al) relative to the intensities expected of a random distribution (111 113). At higher SUAI ratios persistent intensity associated with the Si(2Al) peak shows that a degree of site pairing exists even at low unit cell sizes (110). It has been proposed that hydrogen transfer, coke selectivity, and octane selectivity can be correlated with the relative amounts of paired and isolated sites (114). A difficulty with this analysis is that the chemical shift due to an SiOH bond is similar to that due to SiOAl (115). Since dealuminated zeolites contain a large internal surface terminated by SiOH groups, the quantitative estimation of the number of silicons linked to zero, one or two aluminum atoms is uncertain. Si(2AI) -94

i--, , ,

, ,

,

,

-80

,

r

,

,

,

, , ,

,

,

,

,

,

,

.

-90

.

-100

-110

PPm Figure 11. 29Si MASNMR spectrum of faujasite, SUAl and structure identifications.

- 2.5, taken at 104.3 MHz with peak

Cross polarization techniques are often used to indicate the existence of contributions to the 29Si NMR spectrum from surface or defect structures where any surface terminated by SiOH is a defect structure. In this experiment the intensity of the SiOH group is enhanced relative to the intensity of the Si-0-Si and Si-0-Al groups because of the transfer of magnetization from the 'H nucleus to 29Si. Cross polarization experiments have shown that hydrothermally dealuminated zeolites contain relatively large amounts of terminal SiOH groups, presumably on the internal surface. These results are consistent with the observation by other methods of the occurrence of significant mesoporosity in zeolites.

209

Applications of 27AlMASNMR 27AlMASNMR has been used to characterize at least four types of aluminum structures in zeolites including three types of nonframework alumina and two types of classically acidic tetrahedral forms. It has also been used to characterize the occurrence of aluminum species in silica alumina materials other than zeolites. The chemical shift range appropriate to different silicon environments is provided in Table 4.

32.3

3.9

I

13.5 KHz

7.8 KHz

, 1

300

200

100

0 PPm

-100

-200

-300

Figure 12. 27AlMASNMR spectra of hydrothermally treated faujasite taken at 104.3 MHz and at different spinning rates showing the variations in intensity for the various alumina species. From A. W. Peters, W. C. Cheng, M. Shatlock, R. F. Wormsbecher, and E. T. Habib, Jr., in Guidelines for Ma&xbg the Properties of ’ D. Barthomeuf, E. G. Derouane, and W. Holderich, eds., Plenum Press, New York, 1990, p 365.

210

Tetrahedral aluminum as it occurs in the faujasite framework has a chemical shift of about 60 ppm relative to the octahedrally coordinated aqueous aluminurn cation used as a standard (1 16). This aluminum species can occur in other zeolite framework structures such as beta and ZSM-5 at a slightly lower shift of 52-57 ppm (1 17,118). Alumina, silica alumina, and

hydrothermally treated zeolites may contain octahedral (six-fold) aluminum characterized by a peak with a chemical shift of about 0 ppm (1 19, 120). Pentacoordinated alumina occurs at a shift of about 30 ppm in the mineral andalusite. in fine particle alumina, in severely hydrothermally dealuminated zeolites, and in other hydrothermally treated materials such as clay and silica alumina gels (121, 122). The spectra in Figure 12 are characteristic of hydrothermally treated faujasite. The relative intensities of the three peaks can depend strongly on the spinning rate (1 14). More than one aluminum species with a shift in the tetrahedral region has been observed, although with difficulty. Since the 27Alnucleus has a 512 spin, the spectrum is broadened by quadrupolar effects. More than one resonance with about the same chemical shift is difficult to observe under normal conditions. However, it is possible to set up the MASNMR instrument in such a way as to distinguish resonances from non-equivalent atoms that happen to have nearly the same chemical shift. In a two dimensional experiment the chemical shift spectrum is observed under a variety of pulse times. Atoms in different chemical environments will have different quadrupole coupling characteristics and will have different intensities under different pulse conditions. The line shape due to the different components of the resonance will change with pulse time leading to a two dimensional plot (123). Quantitative spectra are usually obtained at shorter pulse times (- 15' angle). Recently double rotation (DOR) experiments have been described that reduce quadrupolar broadening as well as ansiotropic effects ( 124).

60.1

120

80

LO

0

-LO PPfl

120

80

LO

0

-LO

PPH

Figure 13. 27AlMASNMR spectra of faujasite dealuminated to a) 2.422 uc and b) 2.420 uc showing a doublet in the tetrahedral region at 60.7 ppm and at 54.3 ppm for sample a). From A. Corma, V. Fornes, A. Martin&, and J. Sanz, Fluid Catalytic Cracking, M. Occelli, ed., ACS Sym. Ser. 375,1988, p. 22.

21 1

In severely dealuminated faujasite a non framework tetrahedral species has been observed by 2 dimensional MASNMR as well as conventionally as a slight split in the spectrum of specially prepared samples, Figure 13 (125, 126). In these materials tetrahedral aluminum occurs with both a 60 and a 55 ppm shift. The occurrence of this peak is independent of the dealumination method (1 15).

. .

4 l l h a W a 3 I -

Applications of 31P MASNMR are relatively recent and are used to characterize the coordination of phosphorous in catalysts and in zeolites. Incorporation of phosphorous into S A P 0 or ALP0 types of zeolites has become common. Phosphorous is believed to form bonds only to aluminum (not silicon) in these structures. The presence of phosphorous shifts both the octahedral and tetrahedral 27Al MASNMR signals about 20 ppm in the negative direction, Table 4. The tetrahedral 31P peak occurs at a shift of about -20 to -30 ppm compared to phosphoric acid. As multi-nuclear probe cross polarization techniques become more generally available it will be possible to identify bonds between a phosphorous with a particular shift and another element, say aluminum, also with an identifying shift.

s-

. .

of 1

2 9 NMR ~ ~

129Xe NMR, recently reviewed by Dybowski, Bansall and Duncan (127), is also an adsorption method in that Xe is adsorbed on the surface. The observed chemical shift relative to free Xe is related to the size and the shape of the pore in which the Xe is confined. Recent work has shown that molecules in very small zeolitic pores spend most of the time close to the surface of the pore as a result of conventional attractive Lennard-Jones type attractive forces. This interaction is specific to small pores and is known as the confinement effect (128). J. P Fraissard and coworkers have shown that the chemical shift observed for 129Xeis sensitive to interaction with the pore wall (129). I2%e NMR has been applied to the characterization of the pore structure of zeolites and other materials. An application to Table 5. Analysis of the pore structure of fresh and equilibrium FCC catalysts by 129Xe MASNMR. The micropore size is given in the number of atoms of Xe per micropore and as the volume A3 per micropore. The micropore capacity is the number of micropores per gram X volume per micropore as mmoles Xdgrarn (cm3/grarn). From Reference 132. Number of Micropores,

Micropore Capacity

% Zeolite

walk

Micropore size # Xe and ( A3/micropore)

Catalyst A Fresh Equilibrium

169 108

9.1 (406) 8.4 (375)

1.54 (0.041) 0.91 (0.025)

26 16

Catalyst B Fresh Equilibrium

133 81

9.1 (406) 8.2 (366)

1.20 (0.032) 0.67 (0.018)

20 12

Reprinted from: T. T. P. Cheung, J. Catal., 1990,124,5 11.

212

dealuminated faujasite showed that the 129Xe NMR results could be used to provide information concerning changes in the size of the microporous zeolite cages during dealumination as well as to observe the development of mesoporosity (130, 131). The change in size as measured by a change in the chemical shift is presumably a result of alumina deposits formed during dealumination or partial framework collapse. Subsequent results obtained on a series of fresh and equilibrium FCC catalysts showed that 12'Xe NMR could be used to describe catalyst stability by counting the number of surviving microporous cages (132), Table 5. In addition an estimate of the change in the size of the cage is obtained from an estimate of the number of '29Xe in each cage. Changes in the pore structure of the zeolite in a coked FCC catalyst coupled with low pressure argon adsorption showed a change in the chemical shift that correlated with the chemistry of the coke. The adsorption results showed some diminution in cage size associated with the coke deposits (133). Applications of 'HMASNMR While MASNMR techniques were applied to obtain proton ('H)magnetic resonance spectra very early in the development of the MASNMR technique, the interpretation of these spectra is not yet entirely settled. Peaks observed in the 1.5 to 2.5 ppm shift region relative to a TMS (trimethylsilane) standard have been attributed to surface silanol groups (134, 135). and a peak at 6-7 ppm has been attributed to a hydrated Lewis site (136). Peaks in the 4-5 ppm chemical shift range have been identified as being associated with protonic acidity and seem to require the presence of at least some water. Current peak assignments have been reviewed along with a discussion of correlations between protonic acidity and chemical shift (137). The 'H NMR technique is difficult and the spectra are often not well resolved. Some of the experimental difficulties are discussed in a review by Freude (138).

6. THERMAL ANALYSES (TPD, TGA, DSC, TPR, TPO, Microcalorimetry) With the exception of microcalorimetry, the thermal analysis methods involving temperature programming usually give kinetic information rather than equilibrium information. This kinetic information concerns rates of adsorptioddesorption (TGA, TPD), rates of oxidationheduction (TPO, TPR), and rates of phase changes and sintering (DSC). Consequently the analysis of the results is an important issue. Often the results are analyzed as though they gave equilibrium information. Ammonia TPD may be interpreted as though the temperature of desorption were related to the strength of the ammonium bond to the assumed acid site. The experiment gives this result only indirectly by answering the question, how fast does ammonia desorb from the sample as we increase temperature? Attempts to measure the stability of catalysts or zeolites using DSC results have the same problem. The sample is rapidly heated and one observes a peak or a valley at the temperature of sintering or of a phase transition. However, it is the activation energy and the rate of sintering that has physical and chemical meaning, not the temperature. The rate parameters are obtained by observing changes in the peak shape and in the apparent temperature of sintering as the heating rate is varied. Consequently the kinetic parameters must be extracted from the DSC curve to make a meaningful comparison (139). The same is true of TPD experiments (140). This is normally the situation for methods involving temperature programing. Only recently has this kind of an analysis been done for ammonia TPD experiments on zeolites where the result is a desorption activation energy. The recent review by Bhatia, et. al. emphasizes kinetic analysis (141). The monograph by Turi reviews thermal methods, but does not discuss applications to catalysis (142). Modern commercial thermal analysis equipment often includes software for the analysis of the desorption kinetics or the kinetics of phase transformations.

213 Microcalorimetric methods involve equilibrium measurements and give heats of adsorption directly. The interpretation is straight-forward. The strength of the interaction between the solid acid and the adsorbate is measured by the heat of adsorption. However, data collection is much more time consuming. The measurement of the heat of adsorption for a single system requires many thermal measurements. The heat generated is measured for each incremental amount of adsorbate added to the system, and each measurement may require hours in order to achieve thermal equilibrium.

TPD (Temperature Programed Desorption) In this experiment a base such as ammonia is adsorbed on the solid acid and the system is evacuated to remove excess ammonia. The sample is heated and the thermally desorbed ammonia is detected as it is removed either by a mass spectrometer or by some other detector. In the case of zeolites such as faujasite or USY the result is usually two peaks, Figure 14. These two peaks are ascribed to the occurrence of strong and weak acidity corresponding to the high and lower temperature peaks respectively. Sometimes these peaks are further identified with (or confused with) protonic and Lewis acidity.

Figure 14. Ammonia TPD of a) dealuminated faujasite with high (<350"C) and lower temperature peaks, A. Corma, V. Fornes, F. V. Melo, and J. Herrero, Zeolites, 1987.7.559 and b) ZSM-5showing both the low temperature alpha peak at about loO°C, the low temperature beta peak at about 200 to 250°C and the well separated high temperature gamma peak at > 400°C characteristic of the stronger acidity of the ZSM-5, from N.-Y.Tops@, K. Pedersen and E. G. Derouane, J. Catal., 1981,70,41. The intent of the test is to measure the number of acid sites and to estimate the strength of the interaction with the adsorbate. In measuring the number of sites it is assumed that each adsorbed ammonia accounts for one site. The strength measurement assumes that the heat of adsorption of the ammonia on the acid catalyst is proportional to the strength of the acid, and that the temperature of desorption is proportional in some way to the heat of adsorption. The test is normally used in a semi quantitative way to count the numbers of strong and weak sites, where the site strength is qualitatively given by the peak temperature of desorption. The interpretation of the comparison curves in Figure 14 is that ZSM-5 contains stronger acid sites than faujasite.

214

Ammonia TPD experiments suffer from most of the problems generally associated with temperature programming methods. as well as additional ones associated specifically with ammonia TPD. The analysis is difficult since there are a number of experimental issues to be dealt with. There is an obvious diffusion issue both through the bed and within the catalyst or zeolite particle. During the experiment it is likely that the ammonia will be desorbed and readsorbed many times depending on the bed thickness, particle size, the partial pressure of ammonia in the bed, and the residence time of the ammonia in the bed. The identity and the origin of the observed high and low temperature peaks is not clear. In the case of faujasite, the area under both peaks, the total amount of ammonia absorbed, is used to count the total number of sites per gram (143). It correlates with the total alumina content. In the case of ZSM-5 there are also two peaks with better separation (144). However, in this case the aluminum content is measured only if one counts the ammonia desorbed at the higher temperature. The origin of the lower temperature peak is unclear. While the low temperature peak occurs at about the same temperature for both zeolites. the high temperature peak in the case of ZSM-5occurs at a higher temperature, consistent with the proposed stronger acidity of ZSM-5.The somewhat arbitrary nature of the counting rules suggest that ammonia TPD is not a reliable method of counting the acid sites involved in catalytic cracking, especially on unknown materials. The problems with ammonia TPD are related to uncertainty in the assumptions underlying the method. It is assumed that since ammonia is a base the strength of the interaction is related to the strength of the acid, and that the sites measured are related in some simple way to cracking activity or selectivity. These assumptions are not necessarily reliable. Ammonia, besides being a strong base, has a significant polarity and can interact strongly with other polar oxides (145). Zeolites also contain significant polarity as well as acidity, so there is no guarantee that the interaction with the ammonia is simply an acid base interaction. The pore size of the zeolite as well as the polarity of the framework may contribute to the apparent strength of the interaction. In spite of the difficulties recent work has provided reliable values of about 100 W h o l e (24 Kcdmole) to 120 KJhnole (29 K c a h o l e ) for the activation energy of the desorption of ammonia on dealuminated Y zeolites (146). A second recent measurement gave 116 W h o l e (27 K c a h o l e ) and 122 KJ/mole (30 KcaVmole) for the desorption activation energy of ammonia on two samples of dealuminated faujasite (147, 148). Both studies contain detailed discussions of the experimental and theoretical difficulties associated with ammonia TPD on zeolites. The kinetics of desorption of a variety of aromatic, olefiiic, and saturated hydrocarbons have been obtained on catalysts and on zeolites (149,150). A recently developed method, isopropyl amine TPD, avoids most of these difficulties and may successfully count acid sites of some certain strength. The method involves the adsorption of isopropyl amine on the solid acid, removal of excess adsorbate by evacuation, and a temperature ramp up to about 500OC using preferably a mass spectrometer as a detector. The desorption of isopropyl amine is first observed at some low temperature, followed by the desorption of cracked products. The number of strong acid sites capable of cracking/deamination is measured by the amount of products, propene and ammonia, desorbed. Results with steamed cracking catalysts have shown that the activity of the steamed catalyst for gas oil cracking is proportional to the number of cracking sites measured by the method (15 1). Surface migration and reabsorption are possible sources of error. However, the initial removal of the isopropyl amine from the surface and the destruction of the adsorbed species during the cracking reaction should minimize these difficulties. While this method counts the number of sites capable of forming the required carboniudcarbenium ion intermediate, it does not provide an estimate of relative site strength. This method avoids at least in principle some of the objections to the use of ammonia as a TPD probe for acidity as it relates to catalytic cracking.

215

Microcalorimetry Both DSC and TPD techniques attempt to measure acidity by measuring the heat of adsorption of a base on a solid acid. Direct calorimetric methods are probably more accurate for this measurement as well as somewhat more complicated than TPD or DSC methods. A recent application is the measurement of the heat of adsorption of pyridine on a silica alumina catalyst. The results combined with infrared results showed that the strongest adsorption occurred on Lewis sites (97). TPR (Temperature Programmed Reduction) TPO (Temperature Programmed Oxidation) TPO or TPR measures the oxidative or reductive stability of a sample. The sample is heated in an oxidative or reductive environment and the rate of disappearance of the oxidant or reductant, e.g. 0 2 or H2, is observed. Cracking catalysts are not intrinsically oxidative or reductive, so the technique has limited usefulness. However, contaminants such as nickel and vanadium have oxidation-reduction cycles. In the case of nickel contaminant, the observation of differences in the ease (temperature) of reduction have been related to the degree of interaction between the nickel and the support (152, 153). The implication is that less easily reduced nickel will produce less hydrogen in the FCCU. Since the TPR experiment, like the other experiments involving temperature programming, measures kinetics, the conjecture is a reasonable one. It has been suggested that the TPR profile of metals on a cracking catalyst is characteristic of the activity of the contaminant metal, and that it may be possible to use this profile as a criterion for laboratory deactivation procedures (154). The lab impregnated and steamed catalyst should have the same TPR profile as the unit deactivated or equilibrium catalyst. TGA (Thermogravimetric Analysis) In using this technique, the sample is placed in a very sensitive microbalance. The sample may be exposed to various gases and the weight change noted. The sample may also be heated and the weight loss of the sample is detected. If the desorbing gas is detected, then TPD results can be obtained at the same time. TGA results have been used by Breck and Skeels to show the dehydroxylation of zeolites at about 600OC (1 150OF) (94). TGA experiments have been used to demonstrate the pick-up and release of sulfur oxides in the development of additives for SOX control in the FCCU. The additive picks up SOX as So3 in the regenerator and subsequently reduces and releases sulfur as HIS on the reactor side of the FCCU. This sequence of events forms the technical basis for the operation of the SOX removal additive. The TGA follows the pick up of SO3 as a weight gain. As hydrogen is added to the catalyst containing the bound sulfate the sulfate is reduced to sulfide and is released as H2S. The reduction and release is observed as a weight loss (155). DSC (Differential Scanning Calorimetry) DTA (Differential Thermal Analysis) In differential scanning calorimetry the sample and a reference are heated sufficiently to rapidly increase the temperature of the sample at a rate of between 10 and 100 degrees C per minute. The occurrence of an exothermic or an endothermic phase change in the sample will either release or absorb heat. The amount of heat required during the phase change to maintain the temperature ramp will be different for the sample than for the standard. This difference is measured as a function of temperature. During the phase change there will be either a positive or an inverted peak. This type of experiment is frequently done on cracking catalysts as well as the catalyst components. Zeolite will undergo an exothermic transition to either an amorphous phase or to christolbalite and mullite. Clay, a common inert ingredient of cracking catalysts, is known to go through several changes, first to metakaolin, to a spinel structure, and to mullite, all with well defined phase transitions. The information that one gets is the kinetic parameters of the transition. The temperature is ramped at several rates. As

216

the rate of temperature ramping increases the DSC peak moves to a higher temperature and changes its shape, usually becoming narrower. Kinetic parameters can be obtained either by measuring the change in peak position or in peak shape or both as the temperature ramp rate is changed (138). It is also possible to directly measure the heat released during a transition (156) or the heat capacity of a zeolite or catalytic material (157). There have been several studies of the stability of various forms of dealuminated Y zeolite (158 - 160). In both cases temperatures of sintering were reported rather than the more meaningful activation energy associated with sintering. A study by Pompe and others used essentially all of the thermal methods including DTA. TPR and TGA as well as XRD to show that vanadium forms a complex with rare earth in the form REV04. The study involved lab Ni, V impregnated commercial catalysts (161). 7. SUMMARY OF CURRENT TRENDS

One area of current interest is the development of high temperature controlled atmosphere in-situ analysis systems. Catalysts are observed at ambient or some other convenient temperature, in an oxidizing or neutral chemical environment, and at ambient or sub-ambient pressures. The FCC catalyst operates under very different high temperature conditions in a strongly reducing environment at pressures of 1-2 atmospheres. There is an increasing interest in observing catalysts under operating conditions. This involves developing in-situ methods of analysis. These methods may involve combining thermal and spectroscopic methods in a single experiment where the acidity or some other property of the active catalyst is spectroscopally observed under higher temperature conditions more representative of the operating environment (103). Other examples of in-situ methods of catalyst characterization, not necessarily related to FCC, have been recently collected (162). While a discussion of the methods available and in use for the analysis of FCC catalysts tends to deal with individual topics and methods, in practice a variety of methods are typically combined to give as complete a picture as possible of the catalyst. This kind of an approach is emphasized in discussions of the philosophy and guidelines for catalyst testing (163, 164). The interaction between performance evaluation and characterization of FCC catalysts (165) is discussed in another article from the same symposium appropriately entitled and W y s t Development. An Interactive Appim&, One example of an interactive approach (among many) is the work of Beyerlein and others in characterizing a spent FCC catalyst from an operating unit (59). Many of these methods are involved in catalyst process and quality control. This is especially true of the diffractive and adsorption methods discussed in sections 2 and 3. X-ray diffractive equipment and nitrogen adsorption equipment are commonly operated in the plant along with the usual methods of elemental analysis. As spectroscopic equipment becomes less expensive and more easily operated, it is also expected to find process and quality control applications.

8. ACKNOWLEDGEMENTS I would like to thank R. Kumar and N. D. Spencer of W. R. Grace for their helpful suggestions and comments. 9. REFERENCES 1

J. M. Maselli and A. W. Peters, in Catalysis and Surface Science, Marcel Dekker, Inc.. 1985, H. Heineman and G. A. Somorjai, eds., p 223.

217

2 3 4 5

6 7 8 9 10 11

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

P. B. Venuto and E. T. Habib, Fluid Catalytic Cracking with Zeolite Catalysts, Marcel Dekker, New York, 1979. S. Bhatia, Zeolite Catalysis: Principles and Applications, CRC Press, Boca Raton, Florida, 1990, p. 171-209. J. Scherzer, Octane Enhancing Zeolitic FCC Catalysts, Marcel Dekker, Inc.. New York . 1990. D. E. W. Vaughan, M. M. J. Treacy, and J. M. Newsam. Guidelines for Mastering the Properties of Molecular Sieves, Plenum Press, New York, 1990, p. 99. Powder Diffraction File, Published by the JCPDS International Centre for Diffraction Data, 1601 Park Lane, Swarthmore, Pennsylvania 19081, USA. R. von Balllmoos and J. B. Higgins, Zeolites, 1990, 10,313-520. P. A. Jacobs and J. A. Martens, Synthesis of High-Silica aluminosilicate Zeolites, Elsevier, New York. 1987. Tables of XRD data for a variety of newer zeolite structures are scattered throughout the book. R. Szostak, Molecular Sieves, Van Nostrand Reinhold, New York, 1989. XRD data for 300 zeolites and molecular sieves is listed in the appendix. K. Wefers and C. Misra, Oxides and Hydroxides of Aluminum, Alcoa Technical Paper No. 19, Revised, Aluminum Company of America, 1987. D. M. Moore and R. C. Reynolds, Jr., X-Ray Diffraction and the Identification and Analysis of Clay Minerals, Oxford University Press, New York, 1989. D. K. Smith, G. G.Johnson Jr., A. Scheible, A. M. Wims, J. L. Johnson, and G. Ulmann, Powder Diffraction, 1987,2,73. D. K. Smith and S. Gorter, J. Appl. Cryst., 1991,24,369. ASTM Book of ASTM Standards, 1916 Race Street, Philadelphia, PA 19103, USA, Volume 05.03. Tests relating to catalysts have been collected, ASTM Standards on Catalysts, Third Edition, 1988. R. D. Bonetto, H. R. Viturro, and A. G. Alverez. J. Appl. Cryst., 1990,23,136. P. Pichat, R. Beaumont, and D. Barrthomeuf, J. Chem. Soc.Faraday Trans. 1,1975, 70, 1402. J. R. Sohn, S. J. DeCanio, J. H. Lunsford, andD. J. O’Donnell, Zeolites, 1986,6,225. J. Klinowski, S. Ramdas, J. Thomas, C. A. Fyfe, and J. S. Hartman, J. Chem. Soc., Faraday Trans. 2, 1982,78,1025 . D. W. Breck and E. M. Flanigan, in Molecular Sieves, SOC.Chem Ind.. London, 1968, p. 47. Fichtner-Schmittler, U. Lohse, G.Engelhardt, and V. Patzelova, Cryst. Res. Tech., 1984, K1, 19. V. Bosacek, V Patzelova, Z. Tvaruzkova, D. Freude, U. Lhose, W. Schirmer, H. Stach. and H. Thamm, J. Catal., 1980,61,435. G. H. Kuhl, J. Phys. Chem. Solids, 1977,38, 1259. A. Corma, V. Fornes, and F. Rey, Applied Catal., 1990,59,267. M. Hart,R. J. Cernik, W. Parrish and H. Toraya, J. Appl. Cryst., 1990,23,286. J. M. Bennett, in Perspectives in Molecular Sieve Science, W. H. Flank and T. E. Whyte, Jr., eds., ACS Symposium Series 386. American Chemical Society, Washington, D. C., 1988, p. 162. D. E. Cox, Handbook on Synchrotron Radiation, Vol. 3, G. Brown and D. E. Moncton. ed., Elsevier Science Publishers B. V., 1991, p 156. A. Guinier and G. Fournet, Small Angle Scattering of X-rays, John Wiley and Sons, New York, 1955. J. Teixeira, J. Appl. Cryst., 1988.21.781. M. Czjzek, H. Jobic, A. N. Fitch, and T. Vogt, J. Phys. Chem., 1992,%, 1535. A. K. Cheetham, M. M. Eddy, J. Klinowski, and I. M. Thomas, J. Chem. Soc., Chem. Commun., 1983. p. 23. J. M. Newsam, Science, 1986,231,1093.

218

32 33 34 35 36 37 38 39 40 41 42 43

44 45 46 47 48 49 50 51 52 53 54

55 56 57 58 59 60 61 62 63 64 65

D. J. Sajkowski, S. A. Roth, L. E. Iton, B. L. Meyers, C. L. Marshall, T. H. Fleisch, and W. N. Delgass, Appl. Catal., 1989,51,255. G. L. Woolery, A. A. Chin, G. W. Kirker, and A. Huss, in Fluid Catalytic Cracking, ACS Symp. Ser. 375, M. Occelli, ed.. 1988, p. 215. S. J. Gregg and K. S. W. Sing, Adsorption, Surface area, and Porosity, Academic Press, New York, 1982. S. Lowell and J. E. Shields, Powder Surface Area and Porosity, Chapman and Hall, New York. 1984 (second edition). K. K. Unger. J. Rouquerol, and K. S. W. Sing, Ed., Studies in Surface Science and Catalysis, Vol. 39, Characterization of Porous Solids, Elsevier, New York, 1988. F. Rodrigiez-Reinoso, J. Rouquerol, K. S. W. Sing, and K. K. Unger. Ed., Studies in Surface Science and Catalysis, Vol. 62, Characterization of Porous Solids II, Elsevier, New York, 1988. Marvin F. L.. Johnson, J. Calal., 1978,52.425. J. Lynch, F. Raatz, and Ch. Delalande, Reference 25. p. 547. A. W. Peters, K. Rajagopalan, and E. T. Habib, Jr., 12' Simposio Ibero americano de Catalise, Rio de Janeiro, 1990, IBPKAT, p 421. K. Rajagopalan, A. W. Peters, and G. C. Edwards, Appl. Catal., 1986,23,69. Reference 34, p. 154 W. C. Conner Jr., C. Blanco, K.Coyne, J. Neill, S. Mendioroz, and J. Pajares, Reference 36, p. 273. Reference 35, p 217. J. M. Smith, Chemical Engineering Kinetics, McGraw Hill, New York, 1981, p. 334. M. E. Davis, P. E. Hathaway, and C. Montes, Zeolites, 1989,9,436. P. J. M. Carrott and K. S. W. Sing, Reference 36. p. 77. R. Szostak, Molecular Sieves, Van Nostrand Reinhold, New York, 1989, p. 306. M. E. Davis, C. Montes, P. E. Hathaway, J. P. Arhancet, D. L. Hasha, J. M. Garces, J. Am. Chem. SOC.,1989,111,3919. U. Von Lohse and M. Mildebrath, Z. anorg. allg. Chem., 1981, 126,476. W. Schirmer, H. Stach, H. Thamm, M. M. Dubinin. A. A. Isirikjan, N. I. Regent, and E. Ch. Anaktschjan, Z. Phgys. Chem., 1980,261,1129. H. Stach, U. Lohse, H. Thamm, and W. Schirmer, Zeolites, 6,74 (1986). D. Briggs and M. P. Seah, Practical Surface Analysis, John Wiley & Sons, New York, 1983. J. L. G. Fierro, Ed., Studies in Surface Science and Catalysis, Vol. 57 Part A, Spectroscopic Characterization of Heterogeneous Catalysts, Elsevier, New York, 1990. C. R. Kennedy, R. B. LaPierre, and C. J. Pereira, Ind. Eng. Chem. Res. 1991.30.12. J. Lynch, F. Raatz, and P. Dufresene, Zeolites, 1987,7,333. F. Mauge, A. Auroux, J. C. Courcelle, Ph. Engelhard, P. Gallezot, J. Grosmangin, Studies in Surface Science and Catalysis, Vol. 20, Catalysis by Acids and Bases, Elsevier, New York, 1985, p. 91. V. Patzelova and N. I. Jaeger, Zeolites, 1987,7,240. R. A. Beyerlein, G. A. Tamborski, C. L. Marshall, B. L. Meyers, J. B. Hall, and B. J. Higgins, Fluid Catalytic Cracking 11, M. L. Occelli, ed., ACS Symposium Series 452, American Chemical Society, Washington D. C., 1991, p. 109. M. L. Occelli, D. Psaras, S. L. Suib, and J. M. Stencel, Appl. Catal., 1986,28, 143. Q. L. Wang, M. Torrealba, G. Giannetto, M. Guisnet, G. Perot, M. Cahoreau and J. Caisso, Zeolites, 1990,10,703 . T. Masuda, M. Ogata, T. Ida, K. Takakura, and Y. Nishimura, J. Japan. Pet. Inst., 1983,26,344. R. F. Wormsbecher. A. W. Peters and 1. M. Maselli, J. Catal., 1986,100,130 S. Lars, T. Anderson, S. T. Lundin, S. Jaras, and J-E. Otterstedt, Appl. Catal., 1984,9, 317. T. L. Barr, Zeolites, 1990,10,760.

219

66 67 68 69 70 71 72 13 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

Y. Okamoto, M. Ogawa, A. Maezawa, and T. Imanaka, J. Catal., 1988,112,427. W. J. Mortier, J. Catal., 1978.55, 138. D. Barthomeuf, J. Phys. Chem., 1974,83,249. M. B. Ward and J. B. Lunsford, J. Catal., 87.524 (1984). Th. Gross, U. Lohse, G. Engelhardt, K.-H. Richter, and V. Patzelova, Zeolites, 1984.4, 25. J. N. Ness, D. J. Joyner and A. P. Chapple, Zeolites, 1989,9,250. G. T. Parks, Appl. Surf. Sci., 1980,5,92. S. Jaras, Appl. Catal., 1982,2,207. E. L. Kugler and D. P. Leta, J. Catal., 1988,109,387. D. P. Leta, W. A. Lambetti, M. M. Disko, E. L. Kugler, and W. A. Varady, Fluid Catalytic Cracking 11, M. L. Occelli, ed., ACS Symposium Series 452, American Chemical Society, Washington D. C., 1991, p. 269. M. Larocca, H. Farag, S. Ng, and H. de Lasa, Ind. Eng. Chem. Res., 1990,29,2181. J. Dwyer, F. R. Fitch, F. Machodo, G.Quin, S.M. Smith, and J. C. Vickerman, J. Chem. Soc., Chem. Comm., 1981,422. J. Dwyer, F. R. Fitch, G. Quin, and J. C. Vickerman, J. Phys. Chem. 1982,86,4574. J. C. Jansen, J. Schoonman, and H. van Bekkum, and V. Pinet, Zeolites,1991.11,306. J. E. MacDougall, S. D. Cox, G. D. Stuckey, A. L. Weisenhorn, P. K. Hansma, and W. S. Wise, Zeolites, 1991,11,429. J. L. G. Fierro. Ed., Studies in Surface Science and Catalysis, Vol. 57 Part B, Spectroscopic Characterization of Heterogeneous Catalysts, Elsevier, New York, 1990. K. Tanabe, M. Misono, Y. Ono, and H. Hattori, New Solid Acids and Bases, Elsevier, New York, 1989. J. A. Rabo and G. J. Gajda, Catal. Rev.-Sci. Eng. 1990,31(4), 385. J. C Vedrine, in Guidelines for Mastering the Properties of Molecular Sieves, D. Barthomeuf, E. G. Derouane, and W. Holderich, eds., Plenum Press, New York, 1990, p 121. A Corma, in Guidelines for Mastering the Properties of Molecular Sieves, D. Barthomeuf, E. G. Derouane, and W. Holderich, eds., Plenum Press, New York, 1990, p 299. J. B. Uytterhoeven, R. Schoonheydt, B. V. Liengme, and W. K. Hall, J. Catal., 1969, 13,425. J. W. Ward, in Zeolite Chemistry and Catalysis, ACS Monograph 171, J. A. Rabo, ed., 1976, pp 118-287. J. Scherzer and J. L. Bass, J. Catal. 1973,28,101. G. Garralon, A. Corma, and V. Fomes, Zeolites, 1989,9,84. N-Y Topsoe, K. Pedersen, and E. Derouane, J. Catal., 1981,70,41. P. 0. Fritz and J. H. Lunsford, J. Catal., 1989,118, 85. S. G. Hedge, R. Kumar, R. N. Bhat, and P. Ratnasamy, Zeolites, 1989,9,231. G. W. Skeels and D. W. Breck, Proc. 6th Intl. Zeolite Conf., Reno, USA, July, 1983, Butterworths, p 87. D. W. Breck and G. W. Skeels, Proc. 6th Intl. Cong. Catalysis, London, July, 1976, p. 645. J. W. Ward, J. Catal., 1969,14,365. E. P. Parry, J. Catal., 1963,2,371. N. C-Martinez and J. Dumesic, J. Catal., 1990,125,427. P. A. Jacobs, J. A. Martens, J. Weitkamp, and H. K. Beyer, J. Chem. Soc.,Faraday Discussions, 1981,72,351. V. B. Kazansky, Studies in Surface Science and Catalysis, Vol. 65, Catalysis and Adsorbtion by Zeolites, G. Ohlman, H. Pfeifer, and R. Fricke, eds., Elsevier. New York, 1991, p. 117.

220 100 C. Naccache, C. F. Ren, and G. Coudurier, P. A. Jacobs and R. A. Santen, eds.., Ed., Studies in Surface Science and Catalysis, Vol. 49 A, Zeolites: Facts, Figures, Future, Elsevier, Amsterdam, 1989. p. 661. 101 H. A. Benesi, J. Am Chem. Soc., 1956,78,5490. 102 B. S. Umansky and W. K. Hall, J. Catal. 1990,124,97. 103 J. F. Joly. N. Zanier-Szydlowski, S. Colin, F. Raatz, J. Saussey, and J. C. Lavalley. Calalysis Today, 1991,9, No. 1-2,31. ' C. F. C. Press, P. 0. Box 1720, 1983, 104 C. A. Fyfe, &J&&& NMR for Chemists. Guelph, Ontario, Canada. .. 105 G. Engelhardt and D. Miche1-, NMR zeolites. John Wiley and Sons, New York, 1987. 106 G. Engelhardt, S. Luger, J. Ch. Buhl and J. Felsche. Zeolites, 1989,9, 182. 107 G. Engelhardt, in Recent Advances in Zeolite Science, J. Klinowski and P. J. Barrie. eds., Studies in Surface Science and Catalysis, Vol. 52, Elsevier. Amsterdam, 1989, 151. 108 Loewenstein. W.. Am. Mineral., 1954,39,92. 109 M. T. Melchior, D.E.W. Vaughan, and A.J. Jacobson, J. h e r . Chem. Soc., 1982, 104,4859. 110 J. Klinowski, S. Ramdas, J. M. Thomas, C. A. Fyfe, and J. S. Hartman, J. Chem. Soc., Faraday Trans. 2 1982,78, 1025 * Amer. Chem. Soc.Symposium 218.1983, p. 217. 111 Vega, A.J., 112 B. Beagley, J. Dwyer, F. R. Fitch, R. Man, and J. Walters, J. Phys. Chem. 1984.88, 1744. 113 A. W. Peters, J. Phys. Chem.. 1982,86,3489.; A. W. Peters, Catalytic Materials: Relationship Between Structure and Reactivity, Amer. Chem. SOC.Symposium 1984, 248, p. 201. 114 A. W. Peters, W. C. Cheng, M. Shatlock, R. F. Wormsbecher, and E. T. Habib, Jr., in ies of MolecD. Barthomeuf, E. G. Derouane, and W. Holderich, eds., Plenum Press, New York, 1990, p 365. 115 G. Engelhardt, U. Lohse, A. Samoson, M. Magi, M. Tarmak, and E. Lippmaa, Zeolites, 1982,2,59. 116 J. B. Nagy. Z. Gabelica, G. Debras, E. G. Derouane, J.-P. Gilson, P. A. Jacobs, Zeolites, 1984,4, 133. 117 J. Ptrez-Pariente, J. Sanz, V. Fornes and A. Corma, J. Catal. 1990,124,217. 118 C. A. Fyfe, C. G. Gobbi, J. S. Hartman, J. Klinowski, and M. Thomas, J. Phys. Chem., 1982, $6,1247. 119 J. Klinowski, J. M. Thomas, C. A. Fyfe, and G. C. Gobbi, Nature, 1982,296,533. 120 C. A. Fyfe, G. C. Gobbi, J. S. Hartman, J. Klinowski and J. M. Thomas, J. Phys. Chem.,1982,86,1247. 121 L. B. Alemany and G. W. Kirker, J. Amer. Chem. SOC.1986,108,6158. 122 J. P. Gilson, G.C.E. Edwards, A.W. Peters, K. Rajagopalan, R. F. Wormsbecker, T. G Roberie, and M. P. Shatlock, J. Chem. Soc., Chem. Commun., No. 2, 1987.91. 123 A. Samoson and E. Lippmaa, Chem. Phys. Lett, 1983,100,205. 124 Y. Wu, B. F. Chmelka, A. Pines, M. E. Davis, P. J. Grobet and P. A. Jacobs, Nature, 1990,346,550. 125 A. Samoson, E. Lippmaa, G. Engelhardt, U. Lohse, and H. G. Jerschkewitz, Chem. Phys. Lett., 1987, 134,589. 126 J. Sanz, V. Fomts, and A. Corma, J. Chem. SOC.,Faraday Trans., l., 1988,84,3113. 127 C. Dybowski, N. Bansal and T. M. Duncan, Annu. Rev. Phys. Chem., 1991,42,433. 128 E. G. Derouane, in Guidelines for Mastering the Properties of Molecular Sieves, D. Barthomeuf, E. G. Derouane, and W. Holderich, eds., Plenum Press, New York, 1990, p 373. 129 T. It0 and J. P. Fraissard, J. Chem. Soc.Faraday Trans. 1 1987,83,451. 130 T. T. P. Cheung and C. M. Fu, J. Phys. Chem., 1989,93,3740.

221

131 R. L. Cotterman, D. A. Hickson, S. Cartlidge, C. Dybowski, C. Tsiao, and A. F. Venero, Zeolites, 1991,11,27. 132 T. T. P. Cheung, J. Catal.. 1990,124,511. 133 J. T. Miller, B. L. Meyers, and G. J. Ray, J. Catal. 1991,128,436. 134 C. Dodmieux-Morin, C. Martin, J.-M. Brkgeault and J. Fraissard, Appl. Catal.. 1991, 77, 149. 135 M. Hunger, D. Freude, H. Pfeifer, H. Bremer, M. Jank, and K.-P. Wendlandt, J. Chem. Phys. Lett., 1983,100.29. 136 C. E. Bronnimann, 1.-S. Chuang, B. L. Hawkins, and G. E. Maciel, J. Am. Chem. Soc.,1987,109,1562. 137 H. Pfeifer, D. Freude, and J. Karger, Studies in Surface Science and Catalysis, Vol. 65, Catalysis and Adsorbtion by Zeolites, G. Ohlman, H. Pfeifer, and R. Fricke, eds., Elsevier, New York, 1991, p. 89. 138 D. Freude, Stud. Surf. Sci. Catal., 1989,52, 169. 139 J. H. Flynn, J. Thermal Analysis, 1991.37.293. 140 Y. Amenomiya, ChemTech, 1976,6,129. 141 S. Bhatia, J. Belrramini, and D. D. Do, Catalysis Today, 1990,7,309. 142 E. A. Turi, Thermal Characterization of Polymeric Materials, Academic Press, New York, 1981. 143 A. Corma, V. Fornes, F. V. Melo, and J. Herrero, Zeolites, 1987,7,559. 1 4 4 N.-Y. Tops@e, K. Pedersen and E. G. Derouane, I. Catal., 1981,70,41. 145 M. V. Juskelis. J. P. Slanga, T. G. Roberie, and A. W. Peters, J. Catal., 1992,138, 391. 146 E. Dima and L. V. C. Rees, Zeolites, 1990,10,8. 147 L. Fomi and E. Magni, J. Catal., 1988,112,437. 148 L. Fomi, E. Magni, E. Ortoleva, R. Monaci, and V. Solinas, J. Catal., 1988,112,444. 149 L. Chen and L. V. C. Rees, Zeolites, 1990,10,626. 150 V. R. Choudhary, K. R. Srinivasan, and A. P. Singh, Zeolites, 1990,lO. 16, and references therein. 151 A. I. Biaglow, C. Gittleman, R. J. Gorte. and R. J. Madon, J. Catal., 1991,129,88. 152 R. G. Meisenheimer, J. Catal., 1962,1,356. 153 D. F. Tatterson and R. L. Mieville, Ind. Eng. Chem. Res. 1988,27, 1595. 154 W. Swarez and G. W. Young, AIChE Sym. Ser., Los Angeles Meeting, 1991, in press. 155 A. A. Bhattacharyya, G. M. Woltermann, J. S. Yoo, J. A. Karch, and W. E. Cormier, Ind. Eng. Chem. Res. 1988,27, 1356. 156 D. Vucelic, V. Vucelic, and N. Juranic, J. Thermal Analysis, 1973,5459. 157 A. J. Chandwadkar and S. B. Kulkami, J. Thermal Analysis, 1980,19,313. 158 J. A. Rabo, R. J. Pellet, J. S. Magee, B. R. Mitchell, J. W. Moore, W. S. Letzsh, L. L. Upson, and I. E. Magnusson, NPRA Annual Mtg., March, 1986, Los Angeles, CA, AM-87-69. 159 G. Zi, T. Yi, and Z. Yugin, Appl. Catal., 1989,56,83. 160 H. Bremer, W. Morke, R. Schodel. and F. Vogt. ’ W. M. Meier and J. B. Uytterhoeven, Ed., Adv. Chem. Series 121, Amer. Chem. Soc.,1973, p. 249. 161 R. Pompe, S. Jaras, N.-G. Vannerberg, Appl. Catal., 1984.13.171. 162 R. Burch, ed., Calalysis Today, 1991,9, No. 1-2. 163 W. H. Flank, Characterization and Catalyst Development, An Interactive Approach, S. A. Bradley, M. J. Gattuso, and R. J. Bertolacini, Eds., Amer. Chem. Soc. Symposium 41 1, 1989, p. 92. 164 F. M. Dautzenberg, Characterization and Catalyst Development, An Interactive Approach, S. A. Bradley, M. J. Gattuso, and R. J. Bertolacini, Eds., Amer. Chem. Soc. Symposium 411,1989, p. 99. 165 E. L. Moorehead, M. J. Margolis, and J. B. McLean, Characterization and Catalyst Development, An Interactive Approach, S. A. Bradley, M. J. Gattuso. and R. J. Bertolacini, Eds., Amer. Chem. SOC.Symposium 411, 1989, p. 92.