CHEMCATCHEM FULL PAPERS DOI: 10.1002/cctc.201300974

Characterization of a Fluidized Catalytic Cracking Catalyst on Ensemble and Individual Particle Level by X-ray Microand Nanotomography, Micro-X-ray Fluorescence, and Micro-X-ray Diffraction Simon R. Bare,*[a] Meghan E. Charochak,[a] Shelly D. Kelly,[a] Barry Lai,[b] Jun Wang,[c] and Yu-chen Karen Chen-Wiegart[c] A combination of advanced characterization techniques: synchrotron X-ray micro- and nanotomography, micro-X-ray fluorescence, and micro-XRD have been used to characterize a commercial spent equilibrium fluid catalytic cracking catalyst (ECAT) at both the ensemble and individual particle level. At the ensemble level, X-ray microtomography was used to determine the average size, shape, and respective distributions of over 1200 individual catalyst particles. This information is important to determine performance in commercial operation. It is shown that a large fraction of the particles contained large internal voids (5–80 mm diameter), and these voids likely aid the accessibility for large hydrocarbon molecules. At the individual particle level, by using X-ray nanotomography, these

voids were visualized at a much smaller scale (  100 nm– 12 mm in diameter). In addition, the individual phases that are present in the particle, for example, TiO2 and clay, are readily visualized in 3 D. Micro-X-ray fluorescence (XRF) was used to map, and semiquantitatively determine, both the contaminant (Ni, V, Fe) and inherent (La) catalyst elemental distributions. The distribution of zeolite Y in the ECAT particle was inferred from the La XRF map. Micro-XRD determined the lattice constant of the zeolite Y at the individual catalyst particle level. This in-depth characterization study at the ensemble and individual ECAT particle level presents a robust methodology that provides an understanding of the ECAT at both the micro- and nanometer scales.

Introduction The characterization of fresh and spent technical catalysts is of the utmost importance as it has the potential to provide insight into some of the critical issues that affect the performance of the catalyst. However, such characterization poses some challenging issues as information is desired on the formed catalyst itself, and the physical attributes of the catalyst such as the formulation, shape, size, and the like are dictated by the technical process and are not variables. Additionally, it is also useful to obtain information both on an ensemble of catalyst particles to ensure that the physical parameters that are measured are statistically valid and on individual catalyst particles to obtain more detailed structural and chemical information. In this paper, we discuss an approach by which such information is obtained by using advanced synchrotron-based [a] Dr. S. R. Bare, M. E. Charochak, Dr. S. D. Kelly UOP LLC, a Honeywell Company Des Plaines, IL 60016 (USA) [b] Dr. B. Lai Advanced Photon Source Argonne National Laboratory Argonne, IL, 60439 (USA) [c] Dr. J. Wang, Dr. Y.-c. K. Chen-Wiegart Photon Sciences Directorate Brookhaven National Laboratory Upton, NY, 11973 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201300974.

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characterization techniques, which provides a new understanding of the technical catalyst. The prototypical catalyst used in this study is a technical spent fluid catalytic cracking catalyst known as an equilibrium fluid catalytic cracking catalyst (ECAT), although the methodology we discuss could be used for many technical catalysts. The fluid catalytic cracking (FCC) process is the principle gasoline upgrading technology used in refineries worldwide. It involves the conversion of atmospheric gas oils, vacuum gas oils, certain atmospheric residues, and heavy stocks recovered from other refinery operations into high-octane gasoline, light fuel oils, and olefin-rich light gases such as propylene for petrochemical use. Today’s FCC units are required to process heavier molecular weight feeds that contain a higher concentration of metal impurities than ever before, and thus improved methods of characterization are warranted. The FCC process involves the circulation of the catalyst continuously between a reactor and a regenerator. During the cracking of the heavy hydrocarbon feed, coke is deposited onto the catalyst, which is burned off in the regenerator after first stripping with steam. This cyclic process, which is under quite severe conditions of temperature and moisture, is known to affect the structure and activity of the catalyst.[1] Commercial ECAT has the consistency of a coarse powder with roughly spherical particles in the 40–150 mm range and is manufactured by spray drying. It contains 20–50 % of an active ChemCatChem 0000, 00, 1 – 12

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CHEMCATCHEM FULL PAPERS zeolite, usually zeolite Y (faujasite, FAU), and the balance is clay and silica-sol binder. The zeolite Y is stabilized by a rare earth cation, for example, La, which provides the cracking activity. The clay, typically kaolin, provides the physical structure of the catalyst, such as strength, porosity, and attrition resistance, which are required for the specific commercial unit. The binder glues all the components together. In addition to the FAU Ycatalyst particles just described, many FCC ECATs include zeolite ZSM-5 additive particles to enhance liquefied petroleum gas, especially propylene, yields and improve naphtha octane. The ZSM-5-containing particles contain 20–50 wt % of ZSM-5 zeolite stabilized with P2O5 and held together with an aluminaor clay-based matrix. For both of these ECATs, it is well known that during commercial operation metals, primarily Ni and V that originate as metal porphyrins in the crude, are deposited on the catalyst and accumulate over time.[1b, 2] There are many diverse aspects of the ECAT that need to be characterized and understood. These include: 1) the fluidization characteristics of the catalyst, which are affected by the particle shape, hardness, and density; 2) the cracking ability, which is impacted by both the distribution and structure of the zeolite inside the catalyst particle; and 3) the deactivation of the catalyst, which is impacted by the distribution of the tramp metals that are deposited on the catalyst. Characterization methods that address each of these points will be discussed in this manuscript. To provide a more robust understanding of the transport properties of the catalyst (as much as 4000 t h 1 of hot catalyst is transported at up to 20–30 m s 1) better measurements of the particle shape and shape distribution are required. Also, given that the catalyst particles collide with both the reactor walls and with other particles, there will be abrasion of the catalyst, and this abrasion will affect the shape and shape distribution. Particle shape analysis is a relatively difficult problem in powder technology, and methods that use optical and electron microscopy have been devised to determine statistically relevant shape factors in the powder FCC catalyst.[3] Alternatively, laser light scattering can provide a particle size distribution for such ECAT particles.[4] The disadvantages of these methods include the time required for sample preparation and that shape information is obtained from a 2 D projection of the 3 D shape. The internal structure of FCC catalyst particles has also been a topic of study as the internal pore structure of the catalyst is one of the most significant parameters that affects the process kinetics, both in terms of reactivity and selectivity.[5] It is understood that a range of pore sizes from micropores to large macropores are needed for optimal performance and that it would be ideally advantageous to be able to define, control, and assemble this architecture. Although we are currently far from this capability, the ability to accurately measure and define what is present in the current generation of FCC catalysts is needed as a foundation. The method reported in the literature to determine the internal pore structure of giant macropores is a porosimetry method based on the pressurized intrusion of a low-melting-point alloy followed by sectioning and electron microscopy analysis.[5c] This is a cumbersome and time-con 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemcatchem.org suming procedure, which only provides information in 2 D for the 3 D network of pores. As will be shown, the ensemble and individual particle X-ray tomography methods reported here provide distinct advantages to directly obtain quantitative information in 3 D. Given the continued importance of the FCC process in refinery operations, it is not surprising that the characterization of FCC catalysts has been an ongoing exercise for at least 50 years.[6] In that time, a plethora of characterization techniques has been performed on these catalysts.[2a] In particular, there are many studies that focus on the characterization of Ni and V.[1b, 2] The use of soft and hard X-ray microscopy and tomography to probe the 2 D and 3 D structure and chemistry of heterogeneous catalysts has been a recent topic of interest, which have been reviewed by Grunwaldt and Schroer.[7] A more expansive review, focused on imaging catalysts at different scales in situ and under operando conditions, has been published recently.[8] The state-of-the-art in the application of hard X-ray spectroscopic nanoimaging of catalysts under operating conditions has recently been documented by Andrews and Weckhuysen.[9] Recently, elegant studies have focused on the detailed characterization of the FCC catalyst at the individual particle level and then combining this knowledge with catalyst activity.[10] Of particular relevance to this work is a recent article on the correlation of metal poisoning by Ni and V with zeolite Y deactivation at the individual particle level.[11] In this work the authors use micro-XRD computed tomography (CT) to determine the 2 D distribution of zeolite Y in an individual ECAT particle and concluded that there is both redistribution of the zeolite in an ECAT compared to a fresh catalyst and a concomitant decrease in the Si/Al ratio, which is correlated with a decrease in activity. In this manuscript, we show that with the development of synchrotron-based methods, it is now feasible to determine quantitative information on both the external shape and size distribution and quantitative information on the internal giant macroporosity of an ensemble of FCC catalyst particles. Then, by using X-ray tomography both at the micro- and nanometer scales, micro-X-ray fluorescence, and micro-XRD, we develop an unprecedented understanding of the structure and chemistry at an individual ECAT catalyst particle. These methods are widely applicable to a range of catalyst formulations.

Results and Discussion A secondary electron SEM image of a typical individual ECAT particle taken at 1250  magnification is shown in Figure 1. This catalyst particle is roughly spherical in shape and has a diameter of 50 mm. The surface of the particle has an irregular surface morphology. SEM images such as this are helpful to determine the overall characteristics of an individual ECAT particle, but information on an ensemble average is difficult and tedious to obtain by using SEM. In comparison X-ray microtomography reveals both the internal and external 3 D morphology of thousands of FCC catalyst particles simultaneously, as opposed to just the external surface image of a single particle in SEM. A single slice from ChemCatChem 0000, 00, 1 – 12

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www.chemcatchem.org Table 1. ECAT particle properties. Property

Value

Standard deviation

Mean Mean Mean Mean Mean Mean

0.67 0.47 0.67 204 mm 0.179 mm2 6.36  10 3 mm3

0.27 0.28 0.32

anisotropy elongation flatness equivalent spherical diameter external surface area volume

Figure 1. SEM image of a typical ECAT particle.

the tomogram collected from the ECAT particles inside a polyimide tube is shown in Figure 2. The video of the full tomogram can be found in the Supporting Information (Movie S1).

Figure 2. Single slice of the tomogram of the FCC particles inside a polyimide tube (circle around the image). The ECAT particles have a range of shapes and sizes and some are hollow (several are indicated by red circles).

There is a distribution of particle sizes, shapes, and morphologies (Figure 2 and Movie S1), and some of the particles contain internal voids to appear hollow (indicted by the red circles in Figure 2). At this magnification, the individual components of the ECAT (e.g., zeolite, binder, clay) inside each particle are not well resolved, although there are clear differences in greyscale. The differences in greyscale are directly proportional to the density/atomic number of the component/element that absorbs the X-rays in the sample (high absorption is bright, low absorption is black) as the tomographs were collected in absorption contrast mode. The data contained in the full tomogram (Movie S1) were used to determine information on the shape, size, and internal voids from a statistically relevant number (in this example, 1295 particles) of ECAT particles captured in the single tomogram. First, the raw tomogram was segmented to allow identification of the individual particles (Movie S2), and then the data were analyzed to extract quantitative information. The resulting mean anisotropy, elongation, flatness, average spherical diameter, average individual particle external surface area, and average volume were calculated and are summarized in Table 1. The equivalent spherical diameter distribution, the individual catalyst particle external surface area distribution, the individu 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

al catalyst particle volume distribution, and the particle roundness distribution are shown in Figure 3. These distributions, obtained nondestructively from a large number of ECAT particles, provide a quantitative description of the morphology of the catalyst. Each of these distributions indicates that there is a broad distribution of particle size and a roundness distribution that is far from spherical. The equivalent spherical diameter distribution (Figure 3 a) shows particles that range from 50–500 mm with most particles in the range of 130–250 mm, and an average equivalent spherical size of 204 mm. The corresponding anisotropy (0.67), elongation (0.47), and flatness (0.67) are far from the expected spherical values of 1, 1, and 0, respectively. The volume and external surface area distributions are strongly skewed to low volume and low surface area with a maximum in the external surface area at 0.125 mm2 and an average of 0.179 mm2. A measure of the sphericity of the particles is given by the roundness, the distribution of which is given in Figure 3 d. A value of roundness of 1 would indicate a perfect sphere. It is clear that as a result of using this methodology, a more complete determination of the size and shape distributions can be readily determined from a statistically relevant number of ECAT particles than from techniques such as SEM or laser light scattering. Such broad particle size distribution and imperfect sphericity have been reported previously for FCC catalysts, and it was discussed that the absolute values likely vary depending on the catalyst supplier.[4] It is readily apparent from the X-ray microtomography that there are many catalyst particles with internal voids that can be visualized at the micrometer resolution (Figure 2). Indeed, quantification of the imaged particles reveals that the majority of the particles have internal voids (dark areas inside the catalyst particle as shown in Figure 2), and some particles have several voids of different sizes. The distribution of the equivalent diameter of the internal voids is shown in Figure 4. The average equivalent spherical diameter of the voids was calculated to be 14.3 mm, and the average volume of the voids was calculated to be 1.07  10 5 mm3 (10 700 mm3). The percentage of voids (as a percentage of the total volume of the pills) was 2.04 %. The detailed quantitative information presented here is of direct commercial relevance as in operation the catalyst is fluidized and circulates through the riser section of the FCC unit. The parameters that control the fluidization of the catalyst are related to the shape and size distributions and the density of the catalyst. For example, the minimum fluidization velocity, ChemCatChem 0000, 00, 1 – 12

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Figure 3. a) Equivalent spherical diameter distribution, b) individual particle surface area distribution, c) individual particle volume, and d) roundness distribution of the analyzed ECAT particles.

Figure 4. Equivalent diameter of the internal voids in the ECAT sample as determined by X-ray microtomography.

below which the gas will flow through channels between the particles, is directly related to the average particle size (and particle density).[12] Although other techniques, for example, laser light scattering, are available to determine some limited information on the external shape, X-ray microtomography provides directly additional new information on both the ex 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ternal and internal structure from a large ensemble of ECAT catalyst particles. These data can then be analyzed to provide the necessary quantification that is required. It is well established that in an FCC catalyst there needs to be a distribution of pore sizes for optimum performance. In the “staged” cracking model, the presence of micropores (< 2 nm diameter) promotes the cracking of the smallest hydrocarbon fragments, mesopores (2–50 nm diameter) accommodate the larger molecules and allow precracking. Huge pores (> 100 nm diameter) trap the liquid and provide sites for the preliminary cracking of larger asphaltenes and similar moieties. A detailed understanding of an FCC catalyst requires quantification and knowledge of the different pore size regimes. This was highlighted by Mann[5c] who developed a 3 D stochastic model of the FCC pore structure. In an attempt to visualize the experimental pore space to correlate with the stochastic modeling, a pressurized penetration of a low-melting-point alloy (LMPA) was performed followed by sectioning and visualization by using SEM. Mann noted that there was a large variation in the penetration of the alloy into the particles, with some particles that exhibited no penetration and some with giant-sized pore voids. A drawback of the LMPA method is that it only reveals information in 2 D, and it is difficult to use these data to construct information on the 3 D network of pores. ChemCatChem 0000, 00, 1 – 12

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of diameters from approximately 100 nm to 12 mm. Thus, given the overlap in the size range from both the micro- and nanotomography, it is likely that there is a hierarchical distribution of such voids in the ECAT particles. It is hypothesized that the presence of these voids is a direct result of the spraydrying process used to manufacture the catalyst. For example, Walker and Reed[14] studied the effect of different slurry parameters on the structure of spray-dried particles and showed a relationship between the volume fraction of large pores and the ratio of solids/deflocculant/binder. However, it is clear that the use of both X-ray micro- and nanotomography enables the visualization and quantification of pores in 3 D. The presence of such voids will affect the apparent density of the particle and potentially also its strength and resistance to attrition, which are all critical properties of the commercial catalyst. It would also be interesting to determine the relative connectivity of the voids, as direct determination of this connectivity could be used to provide information on the flow properties of the reactants and products in an ECAT. Such work is outside the scope of this paper but is planned for future work. Recently, Tariq et al.[15] used similar multiscale 3 D imaging data to model flow at the individual catalyst pellet scale. In addition, at a significantly smaller scale, de Jong et al. demonstrated the applicability of 3 D electron tomography to provide information on the mesoporosity of zeolites and other materials of interest in catalysis.[16] From the data presented here, it is not readily apparent if these hierarchical voids have a direct relationship to the performance of the catalyst, for example, by aiding in the diffusion of reactant and products, as discussed above, which will be the focus of future studies. However, product literature from an FCC cataFigure 5. Rendering of the 3 D structure from a local region of the ECAT catalyst SM2. The catalyst particle is larger than the TXM field of view (40 mm), so the tomogram was collected from only part of the particle. The rounded lyst manufacturer states: “To edge at the top of the image is the edge of the particle. ensure high primary cracking activity while limiting secondary cracking of middle distillate products, catalysts must have the right combination of activity and accessibility for reactant molecules” (see, for example, product literature from Albemarle, available on the web). Further analysis of the TXM data of the four ECAT catalysts reveals the presence of other morphological features of interest. For example, a slice in which platelet-like features are clearly observed is shown in Figure 7, Figure 6. Individual sections from the TXM of ECAT LA1 (left) and SM1 (right) that show the internal voids (dark reand the tomogram of this partigions inside the image). Several ECAT particles were imaged by X-ray nanotomography by using a light-source-based transmission X-ray microscope (TXM) to obtain more detailed information on the internal structure of individual ECAT particles at a significantly shorter length scale than that obtained from X-ray microtomography. TXM data were collected from several catalysts, designated SM1, SM2, LA1, and LA2. A rendering of the 3 D structure from a local region of SM2 is shown in Figure 5. The 3 D movie of this tomogram can be viewed in Movie S3. In this image, the striking features are the bright (i.e., dense) particulates that are 200 nm–2.4 mm in diameter. These particulates are identified as Ti-rich by the micro-XRF data (see below) and are thus assigned as TiO2, which is known to be a common component of the clay matrix used in the catalyst, which depends on the source of the clay.[13] From the X-ray microtomography (Figure 3), it was seen that many FCC particles have huge internal voids (5–95 mm diameter, for which 5 mm is the detection limit to determine the size of the voids). Individual cross-section slices from the 3 D reconstructions of LA1 and SM1 (Figure 6), show that there are also voids present at a significantly smaller scale. Indeed, from the analysis of the TXM data, these smaller voids fall into a range

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Figure 7. Individual section of the TXM of ECAT SM2 that shows the plateletlike features (red arrow) attributed to the clay. A TiO2 particle is also visible as the bright region in the image (circled).

cle is shown in Movie S4 in 3 D. The platelet-like morphology of these features is consistent with that expected from the kaolin component of the FCC catalyst. These features are readily apparent in the images and vary in size and frequency of observation. The clay is added to the FCC catalyst as an inert densifier, which affects the apparent bulk density without affecting the activity and selectivity.[17] It has been reported that the average particle size of the added kaolin is 0.3–0.4 mm and that this small particle size is necessary for the catalyst to have good attrition resistance.[13] The observation of the larger platelet clay particles in the TXM images is rare and may result be a result of agglomeration caused by poor mixing prior to spray drying. Such TXM data could be used, for example, to provide feedback on the mixing and spray-drying parameters to ensure that optimum conditions are used to result in FCC particles that give the optimum performance. The presence of such zeolite domains inside an FCC catalyst can also be inferred from recent elegant work by Buurmans et al.[10b, c] who described staining methods combined with fluorescence microscopy from cross-sections of FCC beads. In their studies, individual zeolite crystallites cannot be visualized directly at the spatial resolution they employed, but it is inferred that zeolite aggregates must be present. To provide direct elemental identification and elemental 2 D mapping information, both complementary to and to aid in the interpretation of the absorption contrast X-ray micro- and nanotomography data, micro-XRF data were collected from three of the individual ECAT particles (LA1, LA2, and SM1). The resulting micro-XRF elemental maps are shown in Figure 8. The XRF data shown in Figure 8 are naturally 2 D projections of the 3 D ECAT particle. Thus, the middle of the particle is thicker than the edge and should yield a higher XRF signal simply because of the greater thickness. In addition to the matrix elements of Si and Al, the following elements were detected: P, Cl, K, Ca (these maps not shown), Ti, V, Fe, Ni, Zn, and La. These elemental maps reveal that in all three catalyst particles imaged the Ni concentration is localized on the sur 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemcatchem.org face of the ECAT and the V is distributed throughout the bulk of the catalyst. The effect of Ni and V on FCC catalysts has been much studied in the literature, and it is now accepted that the Ni preferentially deposits on the external surface of the ECAT catalyst particle, whereas V is uniformly distributed throughout the ECAT particle because of the enhanced mobility of V species during the commercial regeneration procedure.[1b, 2a–d, f] Our data are in agreement with these prior studies but serve to illustrate that these elements can now be readily mapped on ECAT samples without any involved sample preparation (as used previously for, for example, secondary ion mass spectrometry (SIMS) mapping), which is more sensitive than mapping by SEM and, moreover, is at least semiquantitative. As La is only present in the zeolite Y, the La map can be viewed as a map of the distribution of the zeolite Y in the ECAT particle. From the La maps shown in Figure 8, it appears that the zeolite is relatively uniformly distributed throughout the three ECAT particles examined, although there are regions of enhanced La intensity in LA1 and SM1, which suggests larger agglomerates of zeolite Y in these particles. The distribution of zeolite Y in a fresh and spent (ECAT) FCC particle has been determined previously by using micro-XRD CT.[11] In this work, the authors concluded that in a fresh FCC catalyst the zeolite Y is randomly distributed, whereas in an ECAT particle there is a significant change in the macroscopic distribution with the zeolite that becomes preferentially present in the periphery of the particle. However, Ruiz-Martnez et al.[11] do not offer any explanation or mechanism of the gross redistribution of the zeolite in the ECAT. In our work, the zeolite Y distribution in the ECAT is relatively randomly distributed and there is no indication that the zeolite is preferentially present in the periphery of the particle. Unfortunately, we did not collect data from a fresh FCC catalyst particle, so we cannot compare our La XRF map data from the ECAT to that of a fresh catalyst particle. However, in both our study and that of Ruiz-Martnez et al.[11] the statistics for these analyses are limited. We examined three particles and Ruiz-Martnez et al. investigated a single particle, so it raises some questions on the statistical validity of the conclusions drawn. Notably, without examination of exactly the same particle both fresh and spent, there will always be some doubt over the conclusions. Quantification from the XRF maps and the bulk measurement from inductively coupled mass spectrometry (ICP-MS) are compared in Table 2, in which the quantification is expressed as an elemental ratio. (A ratio is used as the quantification from XRF gives the concentration in mg cm 2, whereas the ICP-

Table 2. Comparison of elemental ratios obtained from ICP-MS and XRF measurements. XRF data were averaged over the LA1, LA2, and SM1 particles. Elemental ratio

ICP-MS ratio

XRF ratio

XRF standard deviation

Ti/V Fe/V Ni/V Zn/V La/V

0.86 1.00 0.63 0.04 1.27

0.77 1.19 0.82 0.07 1.50

0.16 0.20 0.33 0.02 0.67

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Figure 8. XRF maps of a) LA1, b) LA2, and c) SM1. The color of each element is autoscaled from the minimum (black) to the maximum (red) [mg cm 2] as follows: LA1 Ni 0–55, Ti 0–99, V 0–31, Fe 0–102, Zn 0–4, La 0–65; LA2 Ni 0–52, Ti 0–202, V 0–45, Fe 0–66, Zn 0–5, La 0–86; SM1 Ni 0–56, Ti 0–328, V 0–43, Fe 0– 99, Zn 0–6, La 0–173.

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CHEMCATCHEM FULL PAPERS MS measurements result in wt % of a given element). The agreement is reasonable given that the XRF was obtained on a particle-by-particle basis and the ICP-MS measurements are from a significantly larger sample size. Notably, the ratio determined by ICP-MS falls within one standard deviation of the value determined by micro-XRF. These data also illustrate that any self-absorption effects in the XRF measurements are minimal for these particular elements. The distribution of Ti is localized (Figure 8), which is interpreted as a confirmation of the presence of TiO2 particles in the catalyst, as hypothesized from the X-ray tomography. This assignment is supported by the equivalence of the size distributions of the bright particles in the nanotomography and the TiO2 particles in the micro-XRF, as shown in Figure 9 for the diameter range of 0.35–1.45 mm, which is the size range in which the micro-XRF and nanotomography overlap. Ideally the images from the two methods should be registered and overlaid, but it was not possible to achieve this in this study. Many bright particles with a size smaller than 0.35 mm in diameter, which is below the detection limit of micro-XRF, were also observed in the nanotomography data. The ability to focus X-ray beams to the micro(nano)meter dimension is now a relatively routine capability at today’s synchrotron radiation sources. As a result, the use of such focused beamlines has found applicability in a broad spectrum of science. Here we have demonstrated the ability to identify, spatially map, and quantify the trace elements present within individual ECAT particles. Although the information on the Ni (preferentially on the surface) and V (distributed through the bulk) is not new per se, it is important to consider the specific methodology used here compared to prior studies. Prior analysis has included methods that require the use of involved sample preparation (e.g., SEM) and/or is a vacuum-based tech-

www.chemcatchem.org nique (SIMS, X-ray photoelectron spectroscopy, SEM). The approach taken here involves no sample preparation and the data are collected in air. Clearly, a natural next step would be to conduct these measurements in situ, in which the effect of process conditions could be followed in real time on an individual catalyst particle. The beamline utilized for the XRF mapping measurements was also utilized to collect XRD patterns from the individual ECAT particles. An example diffractogram, obtained from LA2, is shown in Figure 10. The crystalline phases identified in the diffractogram from the individual LA2 particle (Figure 10) are zeolite Y and anatase

Figure 10. X-ray diffractogram from ECAT particle LA2. The peak marked with an asterisk is attributed to anatase (TiO2). All others peaks are assigned to FAU.

(TiO2). The presence of anatase was inferred previously from the Ti particles identified in the XRF maps and the bright particles in the X-ray nanotomography. There are also peaks that can be assigned to g-alumina (not shown). The XRD data are of sufficient quality to allow peakprofile fitting to extract the average unit cell site of the zeolite Y from each catalyst particle, the results of which are summarized in Table 3. The lattice parameter can be used to determine the Si/Al ratio in faujasite.[18] However, there is no unique correlation as the degree of cation exchange and the degree of hydration can affect the unit cell size significantly. It has been reported that the size of the unit cell of an ECAT is typically 24.22–24.32 , but values lower than this (even lower than the minimum predicted by the Breck calculation) have been reported.[19] The values reported in Table 3 are at the low end of the Breck correlation and would predict total dealumination. However, without knowing the extent of cation exchange, such results should be used with caution. Nevertheless, it is clear from the data presented here that if the cation in the zeolite was known, these data could be used to determine the average Si/Al ratio of the zeolite Y at an individual Figure 9. Equivalent diameter distribution of the TiO2 particles in LA1 determined from catalyst particle level. X-ray nanotomography (&) and micro-XRF (&) for the size range in which the two methods overlap.

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Table 3. Peak-profile fitting of the XRD data to extract the average unit cell site of zeolite Y from each catalyst particle. Sample[a]

Lattice parameter a []

SM 1 SM 2 LA 1 LA 2

24.147  0.04 24.159  0.023 24.203  0.031 24.168  0.022

[a] For the individual particle analysis four particles were chosen at random, two larger particles designated LA 1 and LA 2, and two smaller ones, designated SM 1 and SM 2.

Summary A combination of advanced characterization techniques, synchrotron X-ray micro- and nanotomography, micro-X-ray fluorescence (XRF), and micro-XRD have been used to characterize a commercial spent equilibrium fluid catalytic cracking catalyst (ECAT) at the individual particle level. X-ray microtomography was used to determine the average size and shape, and their respective distributions, of over 1200 individual ECAT particles. A large fraction of the particles contained large (5–85 mm diameter) internal voids, which will affect the density of the particle and its catalytic performance. As the commercial operation of a fluid catalytic cracking (FCC) unit involves fluidization of the catalyst in the reactor, knowledge of these properties is clearly important. X-ray nanotomography revealed richness in the local speciation of the components present in an ECAT particle. Although microtomography was able to resolve voids in the > 5 mm diameter size range, there were also voids in the sub-micrometer range, as small as approximately 100 nm in diameter, observed by nanotomography. It is likely that there is a hierarchical distribution of voids in the catalyst particles. It is not known at this stage whether these macropores are interconnected and aid in the diffusion of reactants and products. This will be the focus of future work. Nanotomography was also able to spatially resolve different phases in the catalyst, for example, aggregates of clay, TiO2 particles, and a volume of high X-ray absorption, attributed to an area of La-stabilized zeolite. The XRF measurements complemented the tomography data by providing spatial information on the quantification of the elements present. In agreement with previous work, it was shown that Ni is preferentially deposited on the exterior of the particle and V is deposited throughout. Both of these metals are related to the degradation of the performance of the catalyst. The distribution of the zeolite Y in the particle was inferred from the La XRF map and was shown to be randomly distributed. The micro-XRD patterns from single FCC catalyst particles allowed the identification of the zeolite present in the catalyst and, more importantly, allowed the lattice parameter of the faujasite to be determined. This value provides information on the dealumination of the zeolite, which is directly related to the acidity and the resulting activity.

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Experimental Section Materials The commercial spent ECAT was studied at RT under air for all of the analyses. The catalyst is a complex micrometer-sized spherical particle (  20–200 mm diameter) that contains mostly zeolite (faujasite and/or ZSM-5) in addition to matrix components such as clay, silica, and alumina. Although details of the manufacture of the catalyst are proprietary and the observations are specific to the particular ECAT studied, the characterization methodology presented here is illustrative of the type of characterization that can be performed on such a catalyst. For the individual particle analysis four particles were chosen at random, two larger particles designated LA 1 and LA 2, and two smaller ones, designated SM 1 and SM 2.

X-ray microtomography The X-ray microtomography work was conducted at beamline 2BM at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). Details of the tomography system at this beamline have been discussed.[20] Briefly, the bending magnet beamline at the APS uses a multilayer monochromator with an X-ray beam of approximately 1 mm  1 mm. The X-ray beam on the sample produced a contrast image on a scintillator that was captured by a 2048  2048 pixel CCD camera for each rotation of the sample around the vertical axis. Each tomogram was acquired by a rotation of 0.1258 to produce 1400 projections. The typical time to collect a dataset was 20 min. Once the data set was complete, preprocessing removed the white field, subtracted for the dark field, and centered the rotation axis on the CCD. The X-ray projections were reconstructed into the volumetric representation (slices) of the sample by using a cluster at ANL. A short piece of 1 mm diameter polyimide tube was filled with the FCC catalyst, which was held on a custom kinematic mount that was used by the robotic sample changer at the beamline. The tomograms were acquired by using an X-ray energy of 15 keV and at a voxel size resolution of 0.75 mm. A total of 1295 individual particles were segmented and analyzed. This number is somewhat arbitrary and resulted from a combination of the number of particles in the field of view and the computational power of the workstation used for the analysis. A feature was segmented as a void if there were 5 voxels present with the same intensity. This defines the lower limit for the size that is defined as a void. The tomogram analysis was performed by using Amira. The anisotropy is defined as 1 minus the ratio of the smallest to the largest eigenvalue of the covariance matrix and measures a region’s deviation from a spherical shape. The elongation is defined as the ratio of the medium and the largest eigenvalue of the covariance matrix. Elongated objects will have small values close to 0. Flatness is defined as the ratio of the smallest and the medium eigenvalue of the covariance matrix. Flat objects have small values close to 0. The equivalent diameter is defined as the equivalent spherical diameter for a given volume. The equivalent spherical diameter is given by (6 V/p)1/3, and the roundness is given by (26 V2)/(surface area)3.

Transmission X-ray microscopy (nanotomography) To characterize small features in the particle precisely, 3 D tomography data were further collected at nanometer scale. The nanotomography data were collected at the X8C beamline at the National Synchrotron Light Source (NSLS) at Brookhaven National LaboratoChemCatChem 0000, 00, 1 – 12

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CHEMCATCHEM FULL PAPERS ry (BNL).[21] A newly developed TXM with a few unique capabilities was used to facilitate full-field hard X-ray imaging with a resolution down to 25 nm. The data presented in this paper were acquired at 8 keV with a field of view of 40 mm (diameter and height) at a voxel size of 38.9 nm with a 2k  2k CCD but binned 2  2 pixels. A total of 1441 projections were collected at an angular resolution of 0.1258 for 3 D reconstruction. With such fine angular resolution, both the signal-to-noise ratio and the spatial resolution in 3 D were improved significantly. A standard filtered backprojection reconstruction algorithm was used to reconstruct the 3 D images.[22] The typical time to collect a dataset was 4–6 h. Data were collected from four different FCC particles, two smaller in diameter (  50–70 mm) SM1 and SM2, and two with larger diameters (  250–390 mm) LA1 and LA2.

Micro-X-ray fluorescence and micro-XRD Scanning X-ray fluorescence microscopy was performed at the 2ID-D beamline of the Advanced Photon Source at ANL.[23] The source was a 3.3 cm period undulator, and X-rays were monochromatized by a double-crystal Si(111) monochromator (Kohzu). A Fresnel zone plate focused the X-ray beam on the sample to a spot size of 0.3(h)  0.2(v) mm2.[24] The flux at the focal spot was measured to be  4  109 photon s 1 by using an ionization chamber in place of the sample. The specimen was placed in a He enclosure to minimize air absorption and was scanned by using a X-Y translation stage. An energy-dispersive silicon drift detector (Vortex-EM, Hitachi High-Technologies Science America, Northridge, CA) was placed at 908 from the incident beam, and X-ray fluorescence spectra were collected from the sample with a 0.5 s dwell time per pixel. In this way, spatial maps of different elements were acquired simultaneously. The same microfocus X-ray beam was used to measure XRD from the sample. An area detector was placed downstream of the sample in transmission geometry. After the angles were carefully calibrated, the data shown in Figure 10 (exposure time = 300 s) were obtained by integrating the powder-like diffraction rings in the azimuthal direction. Similar to conventional XRD, the width of the diffraction peaks were dominated by the size of the catalytic nanoparticles. Spectral Analysis: By using the program MAPS,[25] the spectrum at each pixel was individually fitted to remove overlaps between adjacent Ka emission lines to achieve more accurate quantification. Conversion of elemental fluorescence intensities to areal densities [mg cm 2] was performed by comparing X-ray fluorescence intensities with those from thin-film standards SRM-1832 and SRM-1833 (NIST, Gaithersburg, MD). Although the fluorescent X-rays are partially absorbed by the sample, this is only severe for low-energy fluorescence such as those from Si as illustrated in Figure S1 in which Si fluorescence far from the detector is absorbed. However, for elements of interest in this study, which include V and La that have the lowest energy fluorescent lines, no obvious self-absorption effect was observed in the data shown in Figure 8.

SEM Individual ECAT FCC particles were attached to an aluminum stub with carbon tape. Before examination in the scanning electron microscope, the mount was given a light coating of gold to promote  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemcatchem.org conductivity. Secondary electron images were obtained by using a Zeiss Evo 50 scanning electron microscope at 10 kV.

Acknowledgements Maryann Vanek and Bob Broach are thanked for their help with the XRD, Amanda Stolarski for the SEM, and Zhonghou Cai for the micro-XRD. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. DOE Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. Keywords: cracking · fluorescence spectroscopy heterogeneous catalysis · lanthanum · surface analysis

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FULL PAPERS S. R. Bare,* M. E. Charochak, S. D. Kelly, B. Lai, J. Wang, Y.-c. K. Chen-Wiegart && – && Characterization of a Fluidized Catalytic Cracking Catalyst on Ensemble and Individual Particle Level by X-ray Micro- and Nanotomography, Micro-X-ray Fluorescence, and MicroX-ray Diffraction

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Mini techniques, bigger picture: A combination of advanced characterization techniques, synchrotron X-ray micro- and nanotomography, micro-Xray fluorescence, and micro-XRD, are used to characterize a commercial spent equilibrium fluid catalytic cracking catalyst at both the ensemble and individual particle level.

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