Structural Changes in Deactivated Fluid Catalytic Cracking Catalysts

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Structural Changes in Deactivated Fluid Catalytic Cracking Catalysts Determined by Electron Microscopy Frank Krumeich, Johannes Ihli, Yuying Shu, Wu-Cheng Cheng, and Jeroen Anton van Bokhoven ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00649 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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Structural Changes in Deactivated Fluid Catalytic Cracking Catalysts Determined by Electron Microscopy Frank Krumeich*†, Johannes Ihli‡, YuYing Shu§, Wu-Cheng Cheng§, Jeroen A. van Bokhoven*†‡ †

Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland ‡

§

Paul Scherrer Institut, 5232 Villigen, Switzerland

W. R. Grace Refining Technologies, Columbia MD 21044, USA

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ABSTRACT: Fluid catalytic cracking, an important process in the chemical industry, uses porous composite particles to convert the heavy fractions of crude oil into transportation fuels and chemical feedstocks. The employed particles, a spherical composite of zeolite and clay, decrease in catalytic activity during long-term operation, which demands their continuous replacement. To extend the lifetime of these catalysts, it is essential to understand the structural changes that cause the observed decrease in activity. By using a range of electron microscopy and elemental mapping techniques, the structural and chemical makeup of pristine and progressively deactivated catalyst particles were characterized from the micro- to the nanometer scale. Independent of the deactivation degree, the catalyst particles are found to maintain a porous interior structure similar to that of the pristine catalyst. An increasingly dense amorphous silica-alumina envelope enwrapping individual catalyst particles is formed during operation. Regions that contain calcium compounds and spinel-type iron oxide particles are present. While the porosity of this envelope apparently decreases with deactivation, its thickness reaches a plateau at about two micrometers. This process happens not only independently of the detected impurity levels, but moreover, the uptake of impurities appears to be physically halted by the forming envelope.

KEYWORDS: catalysis, microstructure, porosity, deactivation, electron microscopy, elemental mapping.


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INTRODUCTION Providing the majority of the world's gasoline, fluid catalytic cracking (FCC), the conversion of the heavy fractions of crude oil into transportation fuels and chemical feedstocks, represents one of the most important industrial catalytic reactions.1 The catalyst employed in this process is a porous composite composed of La2O3-exchanged Y zeolite, a crystalline aluminosilicate, a functional matrix of calcined kaolin clay containing minor quantities of impurities such as TiO2, an amorphous aluminosilicate, and a binder composed of alumina and/or silica. The continuous operation of industrial units changes the structure of the FCC catalysts over time. The FCC catalyst cycles between catalytic conversion and catalyst regeneration several times an hour at elevated temperatures (500-750°C). The induced structural changes lead to a progressing decrease in catalytic activity and thus continuous replacement of a fraction of the spent catalysts with fresh ones is needed. To understand and eventually rectify this deactivation, it is essential to derive a chemical and structural comprehension of these catalysts across all length scales. To gain this insight it is required to monitor alterations in morphology as well as to characterize changes of interconnectivity in the occluded hierarchical pore network (macro- to micropores) and of the structure of individual clay and zeolite domains with progressing catalyst deactivation. X-ray nanotomography studies2,3,4,5,6 already yielded significant insights into the fundamental changes of FCC catalysts down to the mesoscale. These studies identified the harsh reaction environment in the FCC unit as well as the interaction with and the subsequent occlusion of feed contaminants, including Fe, Ca, Ni and V compounds,7,8,9 in the composite as the main reasons behind permanent catalyst deactivation.3,10,11 Both factors are suggested to lead to a progressing amorphization of zeolites and to the removal or clogging of pores that connect the active sites within the composite to the particle exterior.4 The latter process has received a lot of attention in recent years, leading to the proposition of two different but


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potentially interlinked deactivation mechanisms.3,4,9,10 While Yaluris et al.9 suggested this loss of pores to be a result of impurity-induced melting of the outer particle layer leading to the formation of an isolating amorphous silica-alumina (ASA) envelope,4 Meirer et. al.5,10,12 referred to the “clogging” of such pores by accumulating impurity deposits. To obtain a deeper insight into the structural changes associated with catalyst deactivation, we performed a comparative electron microscopy (EM) study of pristine and several deactivated FCC catalysts.13 Deactivated catalysts were retrieved from four industrial FCC units operating at increasing severity of catalyst deactivation, i.e. processing feedstocks of increasing impurity amount. Considering the continuous nature of FCC unit operation in industry, requiring tons of pristine catalysts on a daily basis, the particles extracted are neither pure nor completely homogeneous (present is an age and catalytic activity distribution). Therefore, we investigated a number of particles in each sample on different levels of magnification as outlined in Figure S1. The results presented are representative of the perceived sample average.

RESULTS AND DISCUSSION Samples and bulk characterization: Pristine and commercially deactivated catalysts (ECAT1-4), manufactured under identical conditions, were provided by W. R. Grace Refining Technologies. Deactivated catalysts were extracted from four FCC units operating at increasing levels of catalyst deactivation. The extracted particles were subjected to a final calcination at 866 K for 2 hours. The obtained catalyst particles were first analysed with several bulk characterisation techniques to derive elemental composition, surface area and catalytic activity of the average particle (Figure 1; for details, see Experimental Section in the Supporting Information). The particle’s composition remains constant independent of the deactivation degree in respect of the Al, La, and Ti concentrations. Furthermore, after an


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initial uptake, the levels of feedstock impurities such as V, Ni and Na appear to plateau. The exceptions to this observation are Fe and Ca (to a minor extent). The Fe content given on the basis of Fe oxide increases from 0.53 wt.% in the pristine sample to 2.43 wt.% in the most deactivated sample ECAT4. Parallel to this Fe increase, reductions in microporous surface area (down by 60-70%) and catalytic activity are registered. However, the decrease in activity only amounts to a decrease of 30-40 % in the most deactivated samples (ECAT3 and ECAT4) due to the partial replacement with fresh catalyst. This deviation between zeolite amorphization, i.e. pore structure collapse and loss in active sites, and observed decrease in catalytic activity suggests the existence of an inhomogeneous utilization of active sites during operation. Particle morphology and surface: To explore the catalyst structure in detail, we characterized pristine and deactivated particles by electron microscopy and associated elemental mapping methods, extracting information about structure and elemental distribution from the micron- to the nanoscale (cf. Figures S1 and S2). Special attention was paid to the outermost particle layer, the ASA envelope. SEM images of the as-sourced catalysts show that both the pristine and used catalysts consist of almost spherical particles on average 100 µm in diameter (Figure 2a,b). Particle size distributions range from ~20 to 200 µm independent of deactivation degree. Catalyst particles are frequently found to be intergrown, most likely a result of the spray drying synthesis process. Fractured particles provide a first view of the catalyst interior (Figure 2ch). Clearly visible are the individual catalyst components: zeolite crystals and lamellar clay. The interior pore network is preserved in all samples. The morphological appearance of the clay and zeolite elements changes significantly with deactivation degree. This change is especially visible in the vicinity of the particle exterior, where these entities appear to merge, forming a particle isolating envelope (Figure 2g,h).


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Figure 1. Composition and selected properties of pristine and deactivated fluid catalytic cracking catalysts. (a) and (b) elemental composition of catalyst particles extracted from industrial FCC units with the level of deactivation increasing from ECAT1 to ECAT4. Composition determined by inductively coupled plasma emission spectrometry. (c) Specific surface areas of catalyst particles (top) and relative changes in micro and mesoporosity (bottom). Surface areas were determined by physisorption. (d) Relative change in catalytic activity. Catalytic activity was determined using an Advanced Cracking Evaluation (ACE) unit.14,15


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Figure 2. SEM images obtained with secondary electrons (SE). (a,b) Overview images highlight the spherical morphology of pristine and deactivated catalysts. (c-h) Images of the catalyst interior taken from fractured particles. (c) The pristine sample possesses a thin particle envelope (indicated by an arrow). (d) A closer view of this envelope reveals cracks (arrows) and pores (encircled). (e,f) ECAT1 and ECAT2 are coated by a more substantial envelope while ECAT3 and ECAT4 (g,h) possess a dense envelope. Images of fractured particles further reveal that most pristine as well as all deactivated particles possess an enwrapping envelope. In pristine particles, there is a rather thin envelope (thickness < 100 nm) that contains significant amounts of exterior connecting macro- and


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mesopores and appears to be composed of various small zeolite and clay entities (size ≤ 1

µm) (Figures 2c and d). Typical micrographs of deactivated particles (Figure 2e-h) indicate that this layer progressively increases in thickness from ECAT1 to ECAT4 before plateauing at an average thickness of 1-2 µm. Particle cross-sections: To achieve a more detailed comprehension of the interior structure of the FCC catalyst, cross-sections of fresh and used particles were prepared (Figures S1 and S2) and investigated by SEM and EDXS. Typical cross-sections of pristine and deactivated catalysts alongside elemental maps are shown in Figure 3. In the pristine FCC catalyst, two types of particles were differentiated with respect to their morphology by a thorough inspection of the surrounding surface. Some particles have a clean surface that is terminated by the presence of the same material that builds up the core so that the porous characteristic of the internal structure is preserved at the surface (Figures 3a). However, more than half of the investigated FCC particles show a thin coating that completely or partially enwraps them (Figure 3b). Elemental distribution maps of both variants reveal a homogeneous mixture of Si- and Al-rich components (Figures 3a and b, center). Green areas are Si-rich and mark the presence of the catalytically active zeolite, red areas are Al2O3 or Al-rich metakaolin which both act as binding material. Mixed colors (yellow and orange) indicate the presence of intermediate Al:Si ratios reflecting either the actual local composition or resulting from a superposition of zeolite and clay crystals. These mixture and overlap effects occur in all maps and make a reliable quantification of the EDX spectra impossible. All types of phases appear at the rim of the pristine FCC particles as well (Figure 3a), and they apparently are present there as well even if the thin surface layer covers the particle (Figure 3b). TiO2, a common inpurity in caolin clay, is present in the catalyst material with an amount of ca. 1 wt.% (Figure 1a). It is concentrated in the form of nanoparticles with diameters covering a wide size range from a few nm up to 100 nm. These particles are uniformly


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distributed all over the catalyst and are closely embedded between the larger components and sometimes even present at the surface (Figure 3, right column). Impurities such as Ca- and Fe-rich deposits are rarely present (Figure 3b).16 Note that local enrichments of an element, e.g. in the form of a particle, are immediately visible in its EDXS map. This is for example the case for the Ti distribution: particles with a size of some 10 nm appear in the BSE-SEMcoupled EDXS maps (Figure 3). It is therefore likely that most of the Fe content of the pristine sample (ca. 0.5 wt.%, Figure 1) is evenly spread all over the sample in small amounts. The size and distribution of the TiO2 particles in the deactivated catalysts is similar to that in the pristine sample (Figure 3, right). The common characteristic of the used catalysts (ECAT1 to ECAT4) is the presence of a particle envelope that appears bright in the BSE-SEM images of their cross-sections (Figures 3c-g and 4b to e). Surprisingly, the average thickness of this bright zone is similar in all samples, namely ca. 1.5 µm, but varying in a range of 0.5 – 2 µm. However, the underlying surface structure is strikingly different in ECAT1 and ECAT2 (Figures 4b and c) on the one hand and in ECAT3 and ECAT4 (Figures 4d and e) on the other. In ECAT1 and ECAT2, there already is an increase of the image intensity at the rim of the catalyst particle. The green color of the envelope in the AlSi maps indicates a lower ratio Al:Si (Figures 3c and d). The layers in ECAT1 and ECAT2 frequently show pores and openings (arrows in Figures 4b and c) revealing that the particle’s pores are not completely blocked. In ECAT 3 and 4, however, a complete layer has been formed. Typical for this newly formed envelope are nodules sticking out of the surface by about 0.5 µm (Figures 4d and e).


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Figure 3. SEM images of particle cross-sections of pristine FCC catalyst (a,b) and of used ones (ECAT1-4 (c-g)) recorded with back-scattered electrons (BSE). Composite EDXS mappings corresponding to the SEM images above show distribution maps of different elements. A ubiquitously present additive are TiO2 particles (images on the right). Ca- and Fe-containing impurities are rarely observed in the pristine sample (c). The Si-rich coatings of the used samples ECAT3 and ECAT4 contain iron and calcium (c-e, right columns). The SEM images of the particle envelopes at increased magnification demonstrate the transition from individual isolated dense islands (ECAT1 and ECAT2) to a dense particle envelope (ECAT3 and ECAT4) and reveal that this transition is accompanied by compositional and structural changes (Figure 4). This is evident from the increasing brightness of the envelope in the SEM images recorded with BSEs. The thereby implied increase in scattering potential can find its origin in a transformation of present material, i.e. zeolite amorphization, ASA formation,4 incorporation of additional material inside the pores or the substantial deposition of material with a higher scattering potential,17 e.g. feedstock impurities as well as reactor and particle debris. According to EDXS analysis, this layer is mainly Al-containing SiO2 with a rather high iron content (Figure 3 and vide infra). Furthermore, the envelopes contain Ca (Figures 3e -g).


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Figure 4. SEM images of cross-sections of particles obtained with BSEs: (a) pristine FCC catalyst; (b-e) deactivated catalysts ECAT1 − ECAT4. Arrows point to pores in the surface layer. The thickness of the envelope is marked by a bar at selected sites.


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Particle isolating envelope of deactivated catalysts: In the following, we focus on the examination of these particle envelopes, comparing structure and composition of pristine and the two most deactivated particles, ECAT3 and ECAT4, at a further increased spatial resolution. Scanning transmission electron micrographs acquired with a high-angle annular dark field detector (HAADF-STEM) providing atomic-number contrast (Z contrast18) in combination with STEM-coupled EDXS mapping confirm the homogeneous mixture of the different phases in the pristine FCC catalysts up to the particle edge (Figure 5a). Visible are lamellar clay entities (Al:Si ratio > 2 (red)) embedded between well-defined lanthanum-containing zeolites (Al:Si ratio < 1 (green)) that typically have a diameter in the range of 200–1000 nm. Selected EDX spectra and images from the core of the particle are provided in Figures S3 and S4. In agreement with SEM-coupled EDX maps the porous envelope itself, ~100 nm in thickness, is composed of an intermediate mixture of Si-rich as well as Al-rich phases, a fraction of zeolites are located on the surface. The Si-rich dense envelopes of ECAT3 (Figures 5c and d) and ECAT4 (Figures 5e and f) examined at this magnification appear in most parts homogeneous in respect of both structure and composition. In particular, there aren’t any recognizable zeolite entities. Yet, feedstock introduced impurities of calcium and iron show a complex incorporation pattern in hindsight of the possibility of impurity-induced melting of the outer particle layer.9 While the layers formed on ECAT3 and ECAT4 are homogeneous according to SEM-coupled EDXS mappings (see Figure 3), the investigation at higher resolution unravels a more complex structure. Figures 5c and 5e reveal that the envelope (right side) appears bright, dense and has a varying thickness in the range between a half and two micrometers. Provided in Figures 5g and h are qualitative line scans, retracing the concentration profiles of Ca, Fe, Al and Si across a part of the particle envelopes of ECAT3 and ECAT4. Observable is a rather low


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concentration of Ca and Fe across the majority of the envelope diameter that suddenly increases in the outermost ~200 nm. This increase is mirrored by a decrease in Al and increase in Fe concentration in selected areas. Concurrent concentration changes can be suggested to be a result of localized dealumination initiated by the hydrothermal unit environment and/or feed impurities. Selected EDX spectra and images from the core of the used particles are provided in Figures S5 to S8. The amount and ratio of occluded impurities (Fe:Ca) is strongly dependent on the actual feed composition during the reaction which provides a possible origin of the inhomogeneous Ca and Fe distribution in the envelope. Several TiO2 particles are readily recognizable in the HAADF-STEM images as bright patches because of their higher scattering power compared to that of the mixed Al,Si oxides (Z contrast). They have a diameter of ca. 20-100 nm and are incorporated between the Si,Al oxides but some are located in the surface layer (Figures 5c and d). The HAADF-STEM image of the used catalyst ECAT4 (Figure 5e) has been taken at the rim of a particle but at a quite thick area of the specimen to achieve a strong EDXS signal. Thus, structural details are not recognizable in the inner part of the particle due to the large thickness and the resulting overlap of the different components in projection. The envelope mainly consists of SiO2 with some Fe and Ca (Figure S6). Besides these elements and Al, the EDX spectrum reveals the presence of traces of Mg, P, S, V and Ni in this layer (Figure S6e). The formation of this dense phase at the outermost layer during the catalytic operation is apparently the main reason for the loss of surface porosity and cause of catalytic deactivation.


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Figure 5. High-angle annular dark field STEM (HAADF-STEM) images of pristine FCC catalyst (a), ECAT3 (c) and ECAT4 (e), each recorded of an area located at the rim of a particle. The corresponding distribution maps of Al, (Ti) and Si as obtained from EDXS mappings of the individual elements are shown on the right side (b,d,f). Qualitative line scans of the framed areas in (d) and (f) are reproduced in (g) and (h) with the intensity maxima of ACS Paragon Plus Environment

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the individual elements normalized to 100%. EDX spectra for areas selected in image (a) are shown in Figure S3. The HAADF-STEM image reproduced in Figure 6a shows a part of the envelope that is located in a thin specimen area so that details of the structure and elemental distribution are well recognizable (cf. Figure S8). It is evident that the elemental distribution is not homogeneous. Fe is present in the form of nanocrystals with diameters mostly between 5 and 20 nm (Figure 6b) which are embedded in the Si-rich region. In contrast, Ca-containing particles are seldom recognizable due to the intimate mixture of Ca with the main components Al and Si oxide (Figure 6b, c). HRTEM images reveal the high crystallinity of the Fe nanoparticles that are coherently embedded inside amorphous material (Figure 6d). The nanoparticles are FeOx according to EDXS analysis (Figure 6f). Fourier transforms demonstrate their single crystalline characteristics (Figure 6e). The spot patterns can be indexed on the basis of spinel-type Fe3O4 (‫݀ܨ‬3ത݉, a = 0.8394 nm (ICSD 26410)). That iron oxide is present in form of spinel has been corroborated by a recent electron diffraction investigation.19 This finding agrees with the results of a tomographic X-ray absorption spectroscopy study that found Fe3+ to be mainly located at the rim of the catalyst particles.20


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Figure 6. Detailed characterization of the envelope in a thin area of ECAT4. (a) HAADFSTEM image and composite maps of (b) Ca and Fe and (c) Al, Si, Ca and Fe. (d) HRTEM image of crystals embedded in the amorphous material. (e) Fourier transforms (sections) of two crystals marked in (d) with some indices of the spinel-type given below the corresponding spots and (f) EDX spectrum of crystal 1.


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Structure of zeolite: Details of the blend of the different phases are recognizable in TEM images (Figure 7). In the area of the pristine catalyst shown in Figure 7a, zeolite crystals are completely embedded between plate-like clay materials. The crystalline structure of zeolite is identified in the other images (Figures 7b and c). The lattice planes observed there show a typical distance of ca. 1.38 nm corresponding to the {111} planes of cubic zeolite type Y (Figure 7b). The channels inside the zeolite are visible in the crystalline area in Figure 7c. The Fourier transform of this crystal (inset) reveals its single-crystalline characteristics and that it is orientated by chance along the direction [1ത14]. Frequently, the zeolite crystals are closely wrapped by clay (Figures 7a and c) and in contact with each in other sites (Figure 7b). TEM images of the core of an ECAT4 particle show a structure similar to that in the pristine sample (Figures 7d and f). In particular, the zeolite crystals are enclosed by clay. The crystal structure of the zeolite type Y is still well preserved and intact without any recognizable defects (Figure 7f). The lattice fringes observed with a distance of ca. 0.86 nm and 1.38 nm correspond to the {220} and {111} planes of zeolite type Y. This observation of lattice fringes gives a strong indication that the structure of at least a part of the zeolite crystals inside the core of the FCC particles has withstood the harsh conditions during the catalysis. In contrast, well-preserved crystalline zeolite particles close to the envelope could not be detected. Although several zeolite crystals were found in EDXS maps located close to the envelope and identified as such by their composition (i.e. La-containing Al2O3-SiO2, see Figure S8), hardly any lattice fringes could be detected in HRTEM images. This might be due to the fact that these zeolite are located in rather thick areas and are embedded in amorphous material. Nevertheless, this observation points to a reduced crystallinity as compared to the well-preserved zeolite crystals in the core of the particles.4


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Figure 7: TEM images of a pristine FFC catalyst (a,b,c) and of ECAT4 (d,e,f). Fourier transforms (section) of some crystalline areas are shown as an inset with indices of the cubic Y-type zeolite given below the corresponding spots. The approximate Al-content with respect to the Si-content in the zeolite crystals was determined by a quantitative evaluation of their EDX spectra. The atomic ratio Al:Si varies strongly in the range 20:80 – 35:65 in both pristine and deactivated samples. This might on one hand be due to different Al-contents in the zeolite crystals but on the other Al-rich


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material might be present above or below the measured area falsifying the measurement. Consequently, no clear conclusions can be drawn concerning a possible Al-depletion of the zeolite during reaction.

Envelope growth model: On the basis of the electron microscopy analysis, a possible growth mechanism of the ASA envelope can be suggested. In the pristine catalysts, we observe a surface porosity that equals the porosity in the core of the catalysts. Hence the penetration of reactants into the particle towards the occluded active zeolite crystals and the diffusion of the products out of the particle can occur without hindrance. The detected thin, porous envelope around pristine catalysts, having undergone an initial firing step at 400 °C, suggests a thermal instability of selected catalyst components. Exposure to the FCC unit environment initiates this surface passivation by restructuration of the outermost particle layer either though material deposition and or phase transformation processes. These processes gradually remove particle exterior connecting pores. Initially this process is potentially governed by the clogging of selected pores as it appears to be the case in ECAT1. Operation at more severe levels of catalyst deactivation increases the amorphization of zeolites and the potentially continued deposition of reactor and particle debris and impurities resulting in the formation of a dense particle envelope. With the beginning of this envelope formation, the enrichment of Fe in this newly formed layer starts as well (ECAT2). The envelope is partly developed in ECAT3 and fully in ECAT4. In the deactivated particles, characteristic nodules stick out of the surface. These envelopes are Si-enriched and contain Ca and Fe. Fe is present in the form of Fe3O4 dominantly restricted to the outermost parts of the ASA envelope. At this stage, the catalyst surface consists fully of a ca. 2 micrometer thick ASA envelope and the catalytic activity is at its minimum (Figure 1d). Remarkably, the loss of catalytic activity is 27% in ECAT1 and 30% in ECAT 2 and increases further to ca. 35%


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in ECAT3 and ECAT4. Considering the drastic difference of the envelope structure in ECAT1 and ECAT2 on one hand and ECAT3 and ECAT4 on the other, this increase is relatively small. The continuous addition of pristine catalyst during operation substituting a part of the deactivated catalyst obviously diminishes the loss of activity. The clogging of the pores and progressing zeolite amorphization occurring at the start of the reaction are the main causes behind the catalyst deactivation. The formation of a dense layer as observed for ECAT3 and ECAT4 might catch most of the insoluble impurities present in the feed stream. Limited catalyst residence time in the FCC unit, due to continuous catalyst addition and withdrawal, limits the amount of metals deposited on the catalyst. Furthermore, impurityinduced low-temperature eutectics and local surface hotspots could setup an environment that limits further feed penetration and thus metals deposition on the catalyst. 9,21 Such restrictions would thusly prohibit a continued growth of the envelope radially inwards once a certain envelope thickness is reached.22,23,24 This inward directed growth could explain the observed thickness limit as well as the restricted intrusion depth of selected impurities.6 The Sienrichment of the envelope itself can be a result of progressing zeolite and clay amorphization, i.e. increase in packing density due to loss of microporosity, Si relocation from the core of the particle to the edge and impurity/ steam induced dealumination.4,25,26 In the outlined process focusing on an inward directed growth, we are neglecting a substantial outward directed growth driven by the deposition of reactor, feed and or particle debris. Observed is a particle diameter independent thickness limit, meaning that catalyst particles seem not to grow in diameter with deactivation degree. The loss of surface porosity is further supported by the fact that no substantial impurity deposits i.e. neither calcium nor iron oxide areas were detected deep within the enveloped catalyst particles.6

CONCLUSIONS


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The examination of pristine and deactivated catalysts provides insight into the structural changes

that

parallel

the

deactivation

of

FCC

catalysts.

Compared

to

X-ray

nanotomography,3,4,10,20 this examination allows the investigation of multiple particles in 2D at similar or better spatial resolution. Electron microscopy confirms the persistence of the pore network with progressing deactivation4 and the existence of two types of pristine catalysts. These particles differ in the existence of a thin porous envelope. The absence of such an envelope around a certain fraction of pristine particles can be related to inhomogeneity in the spray-drying synthesis process and subsequent calcination step.27 The absence of such envelope-free particles in the deactivated samples can find origins in particle abrasion/fracture or envelope formation during FCC unit operation.22 The comparison of pristine and increasingly deactivated FCC particles clearly shows that the structure of the catalyst particle’s surface changes with deactivation as evidenced by the development and growth of the ASA envelope. While this envelope continues to compact with deactivation degree, its thickness seems to plateau at ~1-2 µm. This envelope does not only block the access to the catalytically active site within the particle but apparently acts also as membrane that absorbs selected poisonous components of the feedstock such as major iron oxide particles (here magnetite). The interior of the particles changes well beyond this layer as evidenced by the rarity of well-crystalline zeolite crystals close to the envelope. In contrast, crystalline zeolite can still be found in the inner core of even the most deactivated samples.


22

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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: acscatal.XXX. Experimental section; overview of the electron microscopic investigations; scheme explaining the TEM preparation procedure; HAADF-STEM and TEM images with corresponding elemental mappings and EDX spectra (PDF).

AUTHOR INFORMATION Corresponding Authors * Email: [email protected] * Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ORCID Frank Krumeich: 0000-0001-5625-1536 Johannes Ihli: 0000-0002-9541-5748 Jeroen A. van Bokhoven: 0000-0002-4166-2284 Notes The authors declare no competing financial interest.


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Funding Sources Swiss National Science Foundation grants 200021 and 153556. ACKNOWLEDGMENT Electron microscopy was performed at the Scientific Center for Optical and Electron Microscopy (ScopeM) of ETH Zurich. We thank the Swiss National Science Foundation (SNF) for the support of the work (grants 200021 and 153556). Grace Refining Technologies is acknowledged for the provision of samples and material characterization expertise. REFERENCES

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TABLE OF CONTENTS FIGURE


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Structural Changes in Deactivated Fluid Catalytic Cracking Catalysts

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