DOI: 10.1002/chem.201303549

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& Zeolites

Dispersion and Orientation of Zeolite ZSM-5 Crystallites within a Fluid Catalytic Cracking Catalyst Particle Christoph Sprung and Bert M. Weckhuysen*[a]

Abstract: Confocal fluorescence microscopy was employed to selectively visualize the dispersion and orientation of zeolite ZSM-5 domains inside a single industrially applied fluid catalytic cracking (FCC) catalyst particle. Large ZSM-5 crystals served as a model system together with the acid-catalyzed fluorostyrene oligomerization reaction to study the interaction of plane-polarized light with these anisotropic zeolite crystals. The distinction between zeolite and binder material, such as alumina, silica, and clay, within an individual FCC particle was achieved by utilizing the anisotropic nature of emitted fluorescence light arising from the entrapped fluorostyrene-derived carbocations inside the zeolite channels. This characterization approach provides a non-invasive way for post-synthesis characterization of an individual FCC catalyst particle in which the size, distribution, orientation, and

Introduction Zeolites are an important group of solid catalysts, which find widespread use in petrochemical industry, for example, for cracking and isomerization reactions. The advantage of these zeolite catalyst materials is their highly ordered internal structure with cages, channels, and pores of different sizes to drive both reactant and product selectivity. Fluid catalytic cracking (FCC) is a central process in crude oil refineries to upgrade heavy oil fractions into mainly gasoline and light hydrocarbons, such as propene.[1] Both zeolite Y and ZSM-5 are employed in FCC catalysis accounting for the control of product selectivities, that is, zeolite Y for gasoline production, while ZSM-5 is known to boost the selectivity towards propene.[2, 3] Large, coffin-shaped ZSM-5 crystals are well-investigated model systems for zeolite research.[4–10]Although, these crystals are industrially less relevant, they have proven to be ideal for detailed micro-spectroscopic investigations due to their micrometer size. These zeolite crystals have a complex internal struc[a] Dr. C. Sprung, Prof. Dr. B. M. Weckhuysen Inorganic Chemistry and Catalysis Debye Institute for Nanomaterials Science Utrecht University, Universiteitsweg 99 3584 CG Utrecht (The Netherlands) Fax: (+ 31) 302511027 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303549. Chem. Eur. J. 2014, 20, 3667 – 3677

amount of zeolite ZSM-5 aggregates can be determined. It was found that the amount of detected fluorescence light originating from the stained ZSM-5 aggregates corresponds to about 15 wt %. Furthermore, a statistical analysis of the emitted fluorescence light indicated that a large number of the ZSM-5 domains appeared in small sizes of about 0.015– 0.25 mm2, representing single zeolite crystallites or small aggregates thereof. This observation illustrated a fairly high degree of zeolite dispersion within the FCC binder material. However, the highest amount of crystalline material was aggregated into larger domains (ca. 1–5 mm2) with more or less similarly oriented zeolite crystallites. It is clear that this visualization approach may serve as a post-synthesis quality control on the dispersion of zeolite ZSM-5 crystallites within FCC particles.

ture composed of several subunits, which has been visualized by polarized light microscopy[10] and also confocal fluorescence microscopy;[11] in total there are six subunits, four on each long side and two entering the bulk from either tip. The relation in the crystallographic orientation of these subunits to each other was determined by combining the results of X-ray diffraction and polarized light microscopy.[10] The ZSM-5 crystal has a monoclinic and orthorhombic structure before and after, respectively, addition of, for example, styrene.[12] The crystallographic c-axis of all subunits is parallel. The subunits, which are next to each other on the long sites, have their crystallographic a- and b-axis perpendicular to each other. Hence, their crystallographic orientation is rotated by 908. Various styrene derivatives have been employed as model staining reactions within ZSM-5 crystals.[4, 7] The dimerization reaction was therefore of interest; two main reaction products may result, that is, the linear and cyclic dimer.[13] These two species have different absorption and emission characteristics. The cyclic dimer absorbs light of shorter wavelength compared to the linear dimer.[13] A third absorption band was additionally observed in between these two species in a number of investigations,[4, 6] the origin of which has yet to be determined. The ZSM-5 zeolite has two types of micropores of about 5  in diameter; that is, straight and sinusoidal pores. If one compares the steric demand of the previously mentioned reaction products, it is logical, that the linear molecules are oriented along the straight pores, whereas the cyclic dimers would find space at points at which straight and sinusoidal pores inter-

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Full Paper sect. Some of the investigations interpret higher oligomers (i.e., trimers[4]) to be incorporated inside the pore system of ZSM-5. Proof that such species indeed do fit inside these pores, however, has not yet been provided and their spectral assignment therefore remains elusive. Light absorption and emission are polarization-dependent. ZSM-5 crystals consist of micropores oriented in the same direction, thus, molecules inside these pores are also aligned. The absorption of light is determined by the energy levels of occupied and unoccupied molecular orbitals relative to each other. A second condition for light absorption is the orientation of the transition dipole moment vector, which describes the transition of an electron between an occupied and unoccupied molecular orbital. The absorption is highest, when the orientation of the transition dipole moment vector and the polarization plane of the incoming light are parallel, and, hence, zero when these vectors are perpendicular to each other. A proof of principle was given by Kox et al.,[6] who analyzed the polarization plane of absorption in the visible range of oriented molecules inside a ZSM-5 crystal. In a follow-up work, this polarization dependency could be distinguished for different regions of the crystal.[4] Stavitski et al. provided experimental results and a theoretical description of the observed absorption of both visible and infrared light.[7] This contribution adds a detailed description of the relative orientation of large ZSM-5 crystals with respect to the polarization plane of the incoming light, i.e., the interaction of polarized light with an anisotropic crystal. The fact that the investigation was carried out in reflection mode was considered during the interpretation of the results. The gained knowledge with the large ZSM-5-crystal model system was transferred to investigations on industrial FCC catalyst particles incorporating ZSM-5 as a propene-boosting active phase. The size, distribution, and orientation of ZSM-5 crystals within the matrix of a single FCC catalyst particle were determined in terms of the characteristic interaction of the zeolite component with polarized light. Furthermore, the amount of zeolite ZSM-5 aggregates within such an FCC catalyst particle has been estimated with a fairly high accuracy.

Results Micro-spectroscopy of a large zeolite ZSM-5 crystal Large coffin-shaped ZSM-5 crystals are suitable model systems for micro-spectroscopic investigations due to their size. Styrene derivatives (preferably ortho-substituted) do fit in to the narrow micropore system of ZSM-5 crystals[4] and are therefore, suitable model reactants. In this study, 4-fluorostyrene was employed for staining the crystals. When these reactants dimerize inside the zeolite crystal, elongated molecules result. The cyclic or linear dimer[13] is aligned inside the pore system of the zeolite. This provides an anisotropic behavior of light absorption/ emission, which has to have a preferred polarization in order to interact with these molecules. In preliminary experiments, shown in Figure S1 of the Supporting Information, the coloration of the crystals was achChem. Eur. J. 2014, 20, 3667 – 3677

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ieved by placing iodine crystals next to the zeolite crystals, as inspired by the work of Kocirk et al.[17] The crystals were uniformly colored by iodine vapor. In both orientations for extinction, that is, 0 and 908 with respect to the vibrational path of the polarizer, the crystal appeared transparent. Hence, the results to be shown in the following were based on the fact that the preferred absorption (emission) of plane-polarized light was due to the alignment of the dimerized fluorostyrene products, which were trapped within the pore system of the zeolite. Figure 1 shows the large ZSM-5 crystals after oligomerization of 4-fluorostyrene at 453 K. The crystals were oriented to positions of 0, 45, and 908, with respect to the horizontal plane. The first and last positions are called extinction positions, that is, their long axes (crystallographic c-axis[10]) were parallel and perpendicular, respectively, to the vibrational path of the polarizer. Investigations by polarized light in the absence of interference may only be performed in this orientation. A sequence of results at an orientation of 458 with respect to the vibrational path of the polarizer is presented in Figure 1(b, e) and the appearance of interference colors substantiate the previous statement. These colors occurred due to double-refraction/bi-refringence. The incoming light was split into two mutually perpendicular polarized rays of light. These wave fronts recombined with constructive and destructive interference by passing through the analyzer. In Figure 1 the crystals were observed under non-polarized and polarized light, and the reflected polarization plane was analyzed. The top most images of all image series were recorded under non-polarized light. The crystals’ color changed slightly depending on their orientation under the microscope. The difference was more pronounced when the incoming light was plane polarized (always horizontal); images appeared lighter and with more intense color for orientations at 0 and 908, respectively. The observation was exactly opposite when the analyzer was inserted in crossed orientation, with and without the polarizer. Following the sequence of images with inserted polarizer: the crystals were translucent/faintly colored when oriented horizontally and intensely colored when oriented vertically, respectively (for more details see Figure 1a, c, d, f), except for images with the analyzer in crossed orientation. Absorption spectra in the 400–700 nm spectral region were taken at various orientations between 0 and 908 for large ZSM-5 crystals. The spectra are given in Figure 2. The “gable” position is that when the observer looks at the triangular tips at the short edges, whereas the “roof” position is a crystal rotated 908 around its long axis. The absorption spectra of the crystals in the gable position show absorption maxima at 520 and 560 nm. For crystals in the roof position an additional maximum at 600 nm is observed. These absorption bands were in line with previous observations by Buurmans et al.[18] The orientation series in Figure 2 shows a decrease of absorption intensity when the zeolite crystals were rotated from vertical to horizontal position. Even though unpolarized light was used for this experiment, the absorption spectra change significantly. The qualitative information in the spectra, however, remained the same.

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Figure 1. Color images of large coffin-shaped ZSM-5 crystals measured at 453 K after 4-fluorostyrene oligomerization, incoming light polarization was horizontal (east–west), the angular values represent the orientation of the vibrational path of the analyzer with respect to the crossed orientation; a)–c) crystal in gable position in 0, 45, and 908 orientation, respectively; d)–f) crystal in roof position in 0, 45, and 908 orientation, respectively.

Previous observations on the dependency of light absorption on the orientation of the crystal with respect to the polarization plane of incoming light, or more general with respect to the microscopic system, were inferred from investigations under the fluorescence microscope. Figure 3 and Figures S2 and S3 of the Supporting information illustrate how the intensity of fluorescence light changed when the zeolite ZSM-5 crystal was oriented differently with respect to the (fixed) vibrational path of the analyzer. The spectral intensity clearly reflected this dependency. When the crystal’s short axis was aligned with the vibrational path of the analyzer (which was fixed vertically, that is, north–south), the spectral intensity was the highest. Thus, the polarization of the emission light was polarized perpendicular to the crystal’s long axis. Confocal fluorescence microscopy of an industrial FCC catalyst particle In a next step, we employed this polarization-dependent micro-spectroscopy approach to evaluate the 3D location and orientation of ZSM-5 crystals within FCC catalyst particles. As Chem. Eur. J. 2014, 20, 3667 – 3677

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mentioned above, such a catalyst is a complex mixture of active components for the reaction (i.e., zeolite) and components to ensure the performance and mechanical strength under reaction conditions (various amorphous materials, including silica and alumina). The images in Figure 4 are part of the rotation sequence, recorded at orientations of 0, 45, 90, and 1358 of the FCC catalyst particle, with respect to the vibrational path of the analyzer. The highlighted pixels were of intensity higher than 0.5. The full set of rotation sequence images at a pixel intensity higher than 0.5 is presented in Figures S5 and S6, and the movie file (the arrow indicates the orientation of the particle under the microscope and the scale bar represents 10 mm) of the Supporting information. The indicators/arrows in Figure 4a–d show exemplified areas of recorded signal, observed in one but not in another image. The histograms in Figure 4e and f illustrate the difference in signal size distribution of the images Figure 4a–d. Figure 5 gives a more differentiated view on the selection of fluorescence signal. The 36 images of the rotation sequence were aligned (i.e., rotated back to 08 position) and superim-

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Figure 2. Absorption spectra in the visible region of coffin-shaped ZSM-5 crystals measured at 453 K after 4-fluorostyrene oligomerization, the indicated angle defines the orientation of the crystal’s long axis with respect to the horizontal line; a) the roof position with absorption bands at 520, 560, and 600 nm; and b) the gable position with absorption bands at 520 and 560 nm.

posed. In the following, the color intensity of the recorded pixels shall be looked into in more detail. The intensity scale ranges from zero to one, as indicated in the center of Figure 5. A certain threshold might arbitrarily be set. By treating the image as described in the Experimental Section, pixels that fall below this intensity threshold at least once during the se-

Figure 4. Principle of the appearance of polarized fluorescence signal: Images of an individual FCC catalyst particle containing ZSM-5 at 453 K after 4-fluorostyrene oligomerization, lex = 561 nm, emission range recorded between 580–750 nm. Images a)–d) were taken at orientations of 0, 45, 90, and 1358 of the FCC particle with respect to the vibrational path of the polarizer. The scale bar represents 10 mm. The selected fluorescence signal had an intensity higher than 0.5 (i.e., 50 % of the total). The images were rotated back to the initial position for better comparison. The arrows/pointer highlight exemplified areas where fluorescence signal appears/disappears. The white dashed lines were inserted for guidance. e) and f) compare the size of the recorded fluorescence signals, that is, its surface area at lower and higher sizes, respectively, for the signals in images a)–d). The histograms were based on a) 143, b) 139, c) 116, and d) 127 grouped signals. The amount of detected to overall detected fluorescence light (i.e., fluorescence signal, when all images of the rotation sequence are superimposed) was: a) 0.35, b) 0.30, c) 0.27, and d) 0.23.

quence appear black in Figure 5a. Hence, these pixels do disappear when the fluorescence signal was recorded under different orientations of the particle. On the right panel, Figure 5b pixels are highlighted that appeared at least once above the given threshold intensity during the sequence. A summary of images, as described here, with a threshold rage between 0.1– 0.9 is presented in Figure S4 of the Supporting information.

Figure 3. Color and fluorescence spectra in the visible region of a ZSM-5 crystal measured at 453 K after 4-fluorostyrene oligomerization at orientations of a) 0, b) 45, and c) 908 with respect to the vibrational path of the analyzer; excitation with non-polarized light from the Xe arc-lamp guided through the Nikon G-2 A filter cube, the blue-framed images and blue spectra were recorded with the analyzer inserted after the long pass emission filter. Chem. Eur. J. 2014, 20, 3667 – 3677

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Figure 5. Procedure of selecting the fluorescence signal by evaluating all 36 images of the rotation sequence: (middle) intensity gray scale, the pixels in the images on its left and right are either of zero or one intensity; a) all pixels, which fall at least once below the threshold (here 0.4) appear black; b) all pixels, which appear at least once with an intensity above the threshold (here 0.6) appear white.

Figure 6 illustrates the particle size distribution determined by these experiments. Figure 6a shows an image summarizing all fluorescence signals that were recorded during the rotation sequence, i.e., superimposing all images (e.g., as if all four fluorescence images in Figure 4 would be superimposed; Figure 6a, however, incorporates all 36). A quantitative description of the size distribution is given in Figure 6b. Most of the detected domains have a very small size, that is, below 0.06 mm2. The quantity of domains decreases with increasing domain size. A second view on the domain size distribution is given in a pie chart in Figure 6c. Here, the contribution of a certain size fraction to the total amount of detected fluorescence signal is illustrated. The smallest size fraction has the least contribution on the fluorescence signal, whereas the contribution increases with increasing size of the domain size fraction. The images in Figure 7 represent the focal planes, which were combined to a Z-stack, shown in Figure 8. The domains, which appear red, were selected according to the procedure mentioned above. The comparison between signal and dark areas gives a quantitative estimation of the amount of active material in the FCC particle. The density of zeolite was determined to be 1.79 g cm 3, and that of the binder to 2.76 gcm 3, as described in the Experimental Section. Two approaches were used to estimate the amount of zeolite; a circle in the images in Figure 7a–g approximates the shape of the focal plane slice. FCC particles are, to a good approximation, spherical, thus, the focal planes are shaped like a disk. The circles shape and size were a compromise between the roundness of the circle and the incorporation of the fluorescence signal. The best compromise was achieved in Figure 7d and e, in which most of the signal was incorporated in the circle. The given area ratios in the figure gave an amount of zeolite as 9.8 and 9.2 wt % in Figure 7d and e, respectively. In a second approach, low fluorescence light was employed to assume the shape of the particle, which may to a large extent be considered as stray light of the incident laser. Hence, the FCC particle shape may be represented more accurately. Since the images in Figure 7 belong to one orientation of the particle under the microscope, the correction to the overall detected fluorescence light has to be made. For an orientation of 08, 35 % of the overall fluorescence light was detected (compare the single images of Figure 4 with the overall image in Figure 6a). The fluorescence light of the single images in Figure 7a–g was Chem. Eur. J. 2014, 20, 3667 – 3677

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Figure 6. Domain size distribution: a) superimposed fluorescence signal of all 36 recorded orientations, scale bar represents 10 mm; b) histograms of domain size distribution; c) contribution of the various binned size fractions to the total size of detected fluorescence signal, the fractions are for ascending size: 0.7, 1.6, 6.5, 9.8, 24.4, and 57.0 %, respectively.

21.2 % of the total area, determined from the low intensity fluorescence light in Figure 7a’—g’ (remember, we are looking at focal planes), and, hence, 78.8 % were attributed to the binder material. Assuming a 1:1:1 mixture of boehmite, kaolinite, and silica as binder material, the amount of active component, that is, zeolite ZSM-5, was 14.9 wt %. A 3D reconstruction of the focal planes of Figure 7 is shown in Figure 8. The reconstruction clearly shows the variation in size and dimension from which the fluorescence signal originated.

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Figure 7. Series of slices of focal planes (h = 0.78 mm) recorded with confocal fluorescence microscopy for a single FCC particle containing ZSM-5 at the 08 orientation. A quantification approach was carried out by comparing the signal area to the assumed area of the FCC particle in that focal plane, indicated by the white ring. The area ratios of signal to total particle are included in the figure. The lower part, containing images a’)–g’) are the same focal planes as a)–g), however, the signals represent low fluorescence light. The scale bars are 10 mm.

Figure 8. 3D reconstruction of the fluorescence signal of the focal planes presented in Figure 7 recorded at 08 orientation with respect to the vibrational path of the polarizer, the height of the frame is 5 mm, which was the depth of investigation.

Discussion Evaluation of the polarization-dependent micro-spectroscopy The large coffin-shaped ZSM-5 crystals were investigated under polarized light. Anisotropic crystalline materials have two or more refractive indices, which split the incoming ray of light into two or more rays that travel with different speed through the crystal (birefringence). When these rays are observed through an analyzer, interference colors appear due to recombination. For large ZSM-5 crystals this was nicely shown Chem. Eur. J. 2014, 20, 3667 – 3677

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by Weidenthaler et al.,[10] who determined the optical properties of such crystals with respect to their crystallographic orientations. Anisotropic crystals do not show birefringent behavior when they are oriented in extinction, that is, for our large coffin-shaped ZSM-5 crystals their long and short axes have to be parallel to the vibrational path of the polarizer, respectively. At the extinction position, that is, in the absence of birefringence, the color of the oligomerization products inside the ZSM-5 crystals was uniform when the analyzer was in different positions (see Figure 1a, c, d, f). In comparison to that, the images in Figure 1b, e were taken of ZSM-5 crystals in 458 orientation with respect of the crystals’ long axis to the vibrational path of the polarizer. The birefringence of the crystals was clearly visible by the alteration of the observed colors depending on the orientation of the analyzer. Hence, different amounts of doubly-refracted light rays interfered, which produced the various colors for the observer. Several reports in the literature on the color of ZSM-5 crystals may, therefore, not be straightforward comparable, since the orientation of the crystal has not been specified.[4, 6, 9] In Figure 1a, c, d, f the polarization plane of absorbed light was unambiguously determined. The images labeled “only analyzer” clearly show the appearance and disappearance of color. This trend of changing color intensity for the image sequences in these two orientations resembles earlier descriptions.[4, 6] The light, which was absorbed from the molecules within the crystal, was polarized parallel to the short axis of the crystal. From crystallographic investigations it could be shown that the straight pores of the ZSM-5 crystal are aligned in this direction.[19, 20] Due to steric constraints of the elongated linear dimers they can only be aligned along these straight pores, which may be linked to the orientation of the absorption of light. However, this conclusion requires further verification. At least three different species may be recognized by their characteristics in the absorption spectra, as given in Figure 4, that is, linear and cyclic species. The interpretation of the absorption spectra has been supported by DFT-based theoretical calculations.[7] The band at 520 nm was assigned to cyclic species, which are supposed to be relatively scarce in ZSM-5 due to the narrow pore dimensions. Following the trend that these absorption bands shift bathochromic due to the substituent effect of fluorine versus methoxy,[21, 18] the band at 600 nm can be assigned to the linear dimer, that is, the 1,3bis(4-fluorophenyl)buten-1-ylium cation. This still leaves the question as to what is the origin of the species dominating the spectrum with a maximum absorption at 560 nm, although this observation is beyond the scope of this article. The crystals imaged under polarized light (Figure 1a, d, “only polarizer”) appeared transparent or invisible. This is due to the fact, that the polarization of incoming light was polarized perpendicular to the orientation of the short crystal axis, that is, the straight pores. The observation was opposite in Figure 1c, f, in which the ZSM-5 crystals were oriented 908 from the situation described above and appeared intensely colored. The situation was slightly more complex when the analyzer was inserted in a crossed orientation (i.e., 08). For both conditions of illumination by non-polarized (“only analyzer”) and polarized

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Full Paper (“incoming light polarized”) light, the crystals appeared colored and colorless for crystal orientations of 0 and 908, respectively. Although the conditions for light absorption in 08 orientation was unfavorable (incoming light was polarized along the long axis of the crystal in Figure 1a, d), the crystals were observed colored. A part of the incoming light changed its polarization direction to be exactly parallel to the crystal’s short axis. This is characteristic for anisotropic crystal systems. No absorption of light could be observed for other analyzer orientations. When the crystals were oriented at 908 (Figure 1c, f) they appeared colorless under crossed polarizers. The plane of absorbed light was blocked out by the analyzer. When the analyzer was then rotated away from the crossed orientation, the color intensity rose to a maximum when polarizer and analyzer were parallel. The absorption spectra in Figure 2 illustrate the dependency of the crystal orientation of zeolite ZSM-5 under the microscope. The incoming light was not polarized; however, the intensity of the spectra strongly depended on the orientation of the crystal. It is fair to state that it is not completely true that the incoming light was completely non-polarized, and thus an explanation of the present result would not be reasonable. This result, however, shows that the incoming light was (most probably) partially polarized due to the optics of the microscope. The typical absorption bands expected at 520, 560, and 600 nm were present for all orientations, only the intensity was significantly altered. The transfer of the orientation-dependency described above for the absorption of light was transferred to the combination of light absorption and emission. The polarization plane of emission light was analyzed and illustrated in Figure 3 for the 458 and both extinction orientations. The crystal was illuminated by non-polarized light, confirmed by the similar intensities of the emission in all three orientations (black spectra in Figure 3). After the analyzer was inserted, light with a vertical polarization plane (i.e., north–south) was analyzed. In accordance with the previous results, the emission light of highest intensity was recorded for the crystal oriented with its long axis perpendicular to the vibrational path of the analyzer (Figure 1a, d). Thus, both the absorption and emission light show polarization dependency.

Three-dimensional location of zeolite ZSM-5 crystallites within a FCC catalyst particle An FCC particle is composed of a complex mixture of zeolite crystals and other material; the former being crystalline, whereas the latter is mainly amorphous, with the exception of clay minerals. The most active sites for the oligomerization of probe molecules were expected to be (mainly) inside the zeolite crystals. However, binder materials also possess sufficient acidity to initiate the oligomerization under the given experimental conditions. The reaction products, which are inside the zeolite crystals, are densely grouped together in comparison to those that derive from a reaction on the surface of the various types of binder material. Thus, spots of higher intensity were supposed to originate from zeolite crystals, due to the denser Chem. Eur. J. 2014, 20, 3667 – 3677

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grouping of molecules, that is, more light was absorbed and therefore emitted. Another more reliable way to distinguish the zeolite component from other material in the FCC particle was to make use of the anisotropic nature of the ZSM-5 crystals. Hence, molecules trapped/aligned within the pores of ZSM-5 material would show a polarization-dependency of light absorption and emission, whereas the amorphous binder material, which may also be covered with oligomerization products, would not show such behavior, since the orientation of product molecules are random. The images in Figure 4 show such polarization-dependency of the fluorescence signal recorded from the same FCC particle. Due to the circumstances of the microscope settings the polarization plane of incoming and outgoing light were the same; hence, the conditions may be compared to the images labeled with “908” in Figure 1 for large ZSM-5 crystals. Several domains of fluorescence light in Figure 4 were found in one, but not in another image. This result is supported by the altered particle distribution in the histograms. Furthermore, a more general view was given in Figure 5 and Figure S4 of the Supporting information. The signal of higher intensity was presumed to originate from the zeolite component and, thus, has to show anisotropic behavior during the rotation sequence. The combination of both expressions, that is, a pixel intensity falls at least once below and above the threshold provides a reliable bases to distinguish between anisotropic (zeolite) material and other components inside the FCC particle. Figure S4 of the Supporting information presents a comprehensive intensity signal selection. Even at the lower end of the scale, that is, a threshold of 0.1 intensity value, not all signal falls (at least once) below it, which is a large fraction compared to the counterpart image of pixels which once exceed this threshold. Thus, there is a kind of background fluorescence. The anisotropic behavior of the signal becomes clearer at higher threshold settings, for example, 0.5. Most of the pixel intensities were at least once below this value and a high amount of pixels appear at least once with a higher intensity. It should be kept in mind, that each of the 36 single images is a 3D stack of focal planes viewed from the top. Pixels may not disappear due to the fact that fluorescence signal at the same spot originates from different focal planes or are an agglomeration of anisotropic material (in various orientations). An intensity threshold of 0.5 was found to be a good compromise to suppress background fluorescence and ensure anisotropic behavior of the recorded signal. Hence, all images were edited to show pixels that were recorded with an intensity higher than 0.5. The smallest recordable signal size is evidently limited by the size of the detector, that is, the amount of pixels and the diffraction limit of light. Furthermore, the size that one pixel represents depends on the magnification during the experiment; in these 410 experiments the pixel size was 0.12 mm per pixel. The lowest category in the histogram in Figure S7b of the Supporting Information corresponds to exactly this value, that is, 0.014 mm2. These were also the smallest domain sizes recorded for the fluorescence images in Figure 4. Here, all

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Full Paper values starting at 0 correspond to domain sizes below 0.0216 mm2 (0.147 mm). We cannot state that this smallest detected size of 0.014 mm2 represents a single zeolite crystallite. This value is lower than the diffraction limit of light for this system. A laser was employed with a wavelength of 561 nm, hence, roughly half that length might be distinguishable with the microscope system. The fact, that single pixel intensities were reported here can be explained by the selection procedure of only the most intense pixels. Secondly, a distinction between adjacent pixels was not attempted and, thus, not part of the interpretation. Fluorescence domains might appear also in sizes up to several mm2, which represents an agglomeration/ accumulation of a large number of crystallites. It can be concluded that the distribution of crystalline material within the investigated FCC catalyst particle was shifted towards agglomerates rather than small (presumably single) ZSM-5 crystallites. An assumption on the amount of zeolite material was carried out in Figure 7. It was most clear for Figure 7d and e, in which a reasonable circle could be drawn around the fluorescence domains. In the focal planes below (Figure 7f and g), part of the fluorescence signal, especially from the center of the FCC particle, was missing due to the depth. The weak fluorescence signal from the middle of the focal planes did not reach the detector anymore, due to the amount of material above it. From the outer parts, however, where less material was on top of it, the fluorescence signals were still recorded. A depth of 5–6 mm was, therefore, considered as a maximum for reliable data recording. For the focal planes shown in Figure 7a–c, the size of the FCC particle, that is, the estimation of a circle, was rather uncertain. Here, only a small amount of fluorescence domains appear which do not support the assumption of a perfect circular shape of the focal plane. FCC particles may have small particles attached to the outer surface. Therefore, the focal planes shown in Figure 7d and e were considered most reliable in terms of their plane shape. The amount of zeolite inside the FCC particle was determined to be about 10 wt %. Furthermore, the second approach, by employing the low intensity, stray light, to assume the shape of the catalyst particle and the correction for the not-recorded fluorescence light due to the polarization dependency, gave a relative weight of about 15 wt % of zeolite. For, such an industrial FCC catalyst, the amount of zeolite ranges between 5– 60 wt %, although most often the value is considered to be close to 20-25 wt %,[22–24] thus, our determined values are in fairly good agreement with these estimations. A schematic drawing in Figure 9 summarizes of what has been achieved by the experiments described and discussed in this contribution. A single industrial FCC catalyst particle has been investigated in a non-invasive manner by confocal fluorescence microscopy up to a depth of 5–6 mm. The zeolite component in this volume was stained by a selective acid-catalyzed reaction, namely the oligomerization of 4-fluorostyrene and could, therefore, be observed by detecting the related fluorescence light. In preliminary experiments on large ZSM-5 crystals the polarization-dependency of the fluorescence light originating from the reaction products could be validated, and was applied on the FCC catalyst particle. Inside the zoomed Chem. Eur. J. 2014, 20, 3667 – 3677

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Figure 9. Schematic drawing of the investigated volume within a single industrial FCC particle: The outer part of the particle was investigated down to a depth of about 5–6 mm. Within this volume we schematically illustrate a box (about 4  4  4 mm3) highlighting solely the zeolite component. These zeolite domains appear in a random orientation, indicated by their color. The domains may be agglomerated to large or small clusters with more or less similarly oriented domains. The confocal planes visualize areas of fluorescence in the investigated volume.

out box, with a size of about 4  4  4 mm3, only the zeolite component (for the sake of clarity) is illustrated. The colors indicate their relative orientation. The investigations above revealed, that a large variety of fluorescence domains were observed; the smallest sized signals were on the order of one/ several pixels (resolution limit determined by the set-up). Agglomerates, which could range up to several mm in all dimensions, were observed for a large portion of the present zeolite ZSM-5. Among these agglomerates are also large areas, which have a preferred orientation of zeolite crystals, that are named oriented domains. Fluorescence microscopy allowed us to take images (depending on the optical settings) at about every 0.8 mm, which are focal planes and illustrated in the lower left of the figure. From these images, the area distribution was determined in Figure 4. A large number of domains appear in small sizes, thus illustrating a high degree of dispersion, as illustrated in Figure 6. The highest amount of crystalline material is, however, aggregated into larger domains. Therefore, the developed approach allowed us to provide valuable insight into a single FCC particle to evaluate the dispersion of its active zeolite component.

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Full Paper Conclusion ZSM-5 crystals are anisotropic in nature and, thus, respective measurements of light absorption should only be carried out when the crystal is oriented in an extinction position. Otherwise the color, hence, the absorption spectra are biased by interference of unequally refracted light. Due to the polarization dependency of light absorption and emission, as validated for large ZSM-5 crystals as model systems, this knowledge has been transferred to an industrial FCC catalyst particle to selectively map the zeolite material inside the complex matrix mixture. The observed anisotropic high intensity fluorescence light was considered to originate from dispersed zeolite ZSM-5 material, that is, this signal appeared and disappeared while observing different polarization planes. Only 20–40 % of the total fluorescence signal was observed for one polarization. A large amount of these recorded fluorescence domains appear in small sizes, that is, less than 0.036 mm2, representing, however, only 0.7 % of the total detected fluorescence signal. The amount of detected fluorescence light corresponds to about 15 wt % of zeolite ZSM-5 inside the investigated volume of the FCC matrix material, which is a reasonable value for such a FCC particle. The developed method is non-destructive to the catalyst particle and could record reliably fluorescence signal from a probe reaction down to a depth of 5–6 mm. It is clear that this selective visualization may serve as a post-synthesis quality control on the dispersion of zeolite ZSM-5 crystallites inside a single FCC particle.

Experimental Section Materials and methods Large coffin-shaped ZSM-5 crystals of about 20  20  100 mm were provided by ExxonMobil and used in its acid form. FCC catalysts with ZSM-5 were provided by Albemarle Catalysts Company BV and calcined at 873 K before use. Details of both samples can be found in recent articles from our group.[25, 26] The samples were placed on a borosilicate glass in an open Linkam cell (FTIR 600, Linkam Scientific Instruments). At a temperature of 303 K, the samples were impregnated with excess of 4-fluorostyrene (Aldrich, used as received). After the reactant had evaporated, the sample was rinsed twice with mesitylene for cleaning purposes. Since it can be expected, that mesitylene does not diffuse into the pores of ZSM-5, only excess reactant (i.e., fluorostyrene) from the outside of the ZSM-5 crystals and other matrix components (in case of FCC catalyst) was rinsed off during this procedure. Hence, the background fluorescence signal was reduced. The reaction was initiated by increasing the temperature rapidly to 453 K, at which it was kept during all investigations.

Instrumentation and measurement protocols An upright optical microscope from Olympus (BX 41) was employed in reflectance mode, equipped with a 50  0.5 NA dry lens and a 30 W halogen lamp. The incoming light was plane-polarized in the E–W plane (i.e., east–west, horizontally) through a U-90 PO3 polarizer from Olympus. Reflected light was recorded with a CCD video camera (ColorView IIIu, Soft Imaging System GmbH) and observed through a 50/50 double view-port tube. The reflected light Chem. Eur. J. 2014, 20, 3667 – 3677

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was analyzed with a rotatable analyzer U-AN360 from Olympus. This device was completed by a home-made angular scale to increase precision of adjusting the allowed vibrational path through the analyzer. The sample was oriented manually under the objective to positions of 158 steps and the analyzer position was adjusted to steps of 308 with respect to the polarization plane of the incoming light. Hence, at each crystal orientation (0, 15, 30, .., 1658), six analyzer positions (0, 30, 60, .., 1508) were recorded. Fluorescence microscopy investigations were performed with a Nikon ECLIPS 90i confocal microscope in reflectance mode; its essential optical parts are illustrated in Figure 10. Excitation light was provided by a Xe arc lamp and a 561 nm laser, respectively, and focused through a 100  0.73 NA dry lens on the sample. Optical fibers carry excitation light and fluorescence light from the sources

Figure 10. Scheme of the essential parts of the optics provided by the Nikon ECLIPS 90i. Set-up A: excitation light from the Xe arc lamp was used, the filter cube containing the dichroic mirror 1 was inserted (the dichroic mirror 2 was not used in this set-up), fluorescent light was recorded with and without the analyzer inserted. Set-up B: excitation by laser light with lambda = 561 nm, the position of the above described filter cube was empty and the dichroic mirror 2 was inserted together with the polarizer/analyzer, thus, plane polarized light reaches the sample and fluorescence light of the same polarization plane was recorded.

to the microscope and to the spectral analyzing unit, respectively. A polarizer/analyzer was inserted between the dichroic mirror and the lens, hence, the polarization plane of excitation light was parallel to the polarization plane of the recorded emission light. The polarization plane of excitation/emission light was N–S (i.e., north– south, vertically oriented). The microscope was equipped with a Nikon A1 scan head, which accommodated the optics to couple fiber optics for excitation and emission light with the microscope. A spectral analyzer in the Nikon A1 system was equipped with 32 photomultiplier tubes (PMTs), which were set to collect emission light in the range between 580 and 750 nm with a resolution of 6 nm (i.e., 28 PMTs). The Nikon G-2A filter cube consisted of an excitation pass band 510–560 nm, dichroic mirror 575 nm, and a long pass emission filter cut-on 590 nm. The Linkam cell mounted on the sample stage of the ECLIPS 90i was additionally equipped with an angular scale (0–3558) with marks every 58. A home-made rotatable sample holder (stainless steel) was employed to orient the sample with respect to the incoming light polarization plane. FCC particles were investigated by a rotation sequence. The impregnation and initiation of the probe reaction was as described above. A suitable particle was chosen by observing the particles through the binoculars. The focus was set manually to this particle and the fluorescence images were collected in a series of focal

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Full Paper planes starting from the top of the particle until a depth of 5– 6 mm. The number of focal planes was the same for all investigations, that is, 7. The zoom was also kept constant to ensure equal pixel size. Before the next Z-series was recorded, the sample holder was turned 58 and the focus was set manually. The principle of polarization-dependent collection of fluorescence light is schematically illustrated in Figure 11. The sample was excited by plane-polarized laser light; however, more importantly, it was only fluorescence light of one polarization direction that was collected. Due to the anisotropy of the zeolite crystals under investigation and the alignment of long fluorostyrene-derived molecules

Figure 11. Principle of polarization dependent collection of fluorescence light: A sample contains zeolite crystals randomly oriented (here three orientations for simplification) indicated by their color. Molecules in these crystals are aligned due to the pore system of the zeolite, thus, the emitted fluorescence light is polarized. When this sample is observed through an analyzer, different fluorescence images appear depending on the relative orientation of sample and analyzer to each other. Note that in the real experiment in this manuscript, the sample was rotated relative to a static analyzer.

formed inside the pore system of the ZSM-5 crystals, the emitted light had a preferred polarization direction. When the sample and the analyzer were rotated relative to each other, different areas of the sample showed fluorescence. This is schematically shown by the blue, red, and yellow crystals and fluorescence images. It is important to note here that in the experiment the sample was rotated, and not the analyzer. What matters in the experiments is the relative position between these two. The form of illustration was chosen to facilitate the introduction to the employed principle.

Data treatment The fluorescence microscopy images were all taken as a Z-series. Their focal planes were combined into a 3D image and viewed from the top. Firstly, the focal planes of the separate images of the rotation sequence appear different (as presented later). Secondly, the focus before recording the images was set manually, hence, there were arbitrary mistakes in height, which were small. Thus, a 3D view catches the fluorescence signal in the volume of interest. These images were than rotated back to the 08 orientation and its position was adjusted manually to superimposition with the preceding image and other images. The whole sequence of 36 images was furthermore included into a movie (in the Supporting information) in which the rotating arrow indicates the orientation of the FCC particle under the microscope (the scale bar represents 10 mm). The low intensity fluorescence in the images was distinguished from the higher intensity/anisotropic signal by the following procedure: The colored images were separated into their three matrices according to the HSV color space. The intensity matrix was normalChem. Eur. J. 2014, 20, 3667 – 3677

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ized with respect to the highest value (set to be one). A threshold was set between 0–1. All 36 images of the rotation sequence were evaluated whether their signal was above or below the given threshold. After the selection of signal domains, these were colored by a color code (rainbow color) according to their orientation, which is summarized for all FCC particle orientations in Figure S5 and S6 of the Supporting information. Superimposing the images reveals fluorescent domains, which appear only once, and others, which were recorded for several orientations of the FCC particle. Their intensity matrices were added, hence, all pixels that appear more than once have an intensity value of greater than one. These values were set back to zero and a color, according to their orientation of appearance, was given to those pixels. The amount of active, that is, zeolite, material in an FCC particle was estimated by the weight ratio of the components. The density of zeolite ZSM-5 is 1.79 g cm 3.[14, 15] The binder material consists of aluminium oxide, that is, boehmite (density of AlO(OH) = 3.0 g cm 3[16]), kaolinite (density of kaolinite = 2.6 g cm 3[16]), and silica (density of SiO2 = 2.65 g cm 3[16]). Their exact ratio in the FCC catalyst particles provided is confidential; but we assume a 1:1:1 mixture, thus a density for the binder material of the density binder = 2.76 g cm 3.

Acknowledgements We acknowledge Dr. Machteld Mertens (ExxonMobil, Machelen, Belgium) and Albemarle Catalysts Company BV for providing the ZSM-5 crystals and the FCC catalyst particles, respectively. Dr. Andrew M. Beale is acknowledged for fruitful discussions. Keywords: fluid catalytic polarization · zeolites

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Received: September 8, 2014 Published online on February 24, 2014

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