Coke distribution determines the lifespan of a hollow Mo/HZSM-5 capsule catalyst in CH4 dehydroaromatization

Xin Huang a, Xi Jiao c, Minggui Lin *a, Kai Wang *b, Litao Jia a, Bo Hou a and Debao Li a
aState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, PR China. E-mail:
bCollege of Chemical and Environmental Engineering, Anyang Institute of Technology, Anyang 455000, Henan, PR China. E-mail:
cCollege of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, PR China

Received 4th July 2018 , Accepted 24th September 2018

First published on 25th September 2018

The effect of coke distribution (external versus internal coke) on the deactivation behavior was investigated in methane dehydroaromatization (MDA) using benchmark metal-modified HZSM-5 catalysts. A hollow HZSM-5 capsule zeolite (HC) was facilely prepared by a one-step hydrothermal strategy with the assistance of Na2EDTA and n-butylamine, and a commercial HZSM-5 (CZ) with a similar Si/Al molar ratio was used as a reference support. Four catalysts (Mo/HC, W/HC, Mo/CZ, and W/CZ) were synthesized via an incipient wetness impregnation method, and tested in the MDA reaction. The properties of coke species were characterized by means of the TG, TPO, HRTEM, and UV-Raman techniques, and different locations of coking over the spent catalysts were obtained. The results showed that external coke can give rise to a more severe deactivation than internal coke owing to the blockage of the pore mouths with external coke that decreased the diffusion length of feed gas in the zeolite channels. This phenomenon could be explained by the excellent catalytic stability over the hollow Mo/HZSM-5 capsule catalyst during the MDA reaction, which is associated with the lower external coke formation rate. Finally, the evolution of Mo species and coke species over the coked Mo/HC catalyst was determined by in situ UV-Raman spectroscopy during the TPO process, and the regeneration by O2 was conducted for the hollow Mo/HZSM-5 capsule catalyst.

1. Introduction

With increasing concerns about environmental problems and energy security, the direct, nonoxidative, and atom-economical conversion of methane with zero CO2 emissions into high-value aromatic chemicals has attracted academic and industrial attention in recent decades.1–3 In 1993, Wang et al. firstly proposed the methane dehydroaromatization (MDA) under nonoxidative conditions over a shape-selective Mo/HZSM-5 catalyst.4 Afterwards, a variety of catalysts based on metal ions (including Mo, W, Cr, Zn, Re, Cu, Mn, Fe, V, Ga, etc.) dispersed on various zeolites (HZSM-5, HMCM-22, HZSM-11, HMCM-36, TUN-9, ITQ-2, HZRP-1, etc.) were extensively investigated.5–7 Among these, the benchmark Mo/HZSM-5 is the most studied catalyst system for the MDA process, in which partially reduced single-atom Mo sites stabilized by the zeolite framework activate methane to form carbon chains while concurrently restricting the carbon chain length.8 In comparison, many authors reported that the activity and stability of W/HZSM-5 with heat-resistance for the MDA reaction increased at a relatively high temperature.9–11 Nonetheless, the greatest challenge of the MDA process is the short lifetime for the Mo-/W-based catalysts due to excessive coke formation, which poisons the active sites and clogs the pore mouths of micro-zeolites.12

Several investigations have attempted to explain the reasons for coke formation over the zeolite catalysts during the MDA reaction. Bao et al. suggested at least three types of carbon species are formed during the MDA reaction: carbidic carbon in Mo2C, molybdenum-associated coke, and aromatic-type coke on acid sites.1,7 It is generally accepted that catalytic pyrolysis of CH4 on the active Mo2C/MoOxCy sites leads to amorphous coke deposits (soft coke), while oligomerization and/or cracking of the intermediates (C2H4), as well as polycondensation of formed aromatics on the Brønsted acid sites of the zeolite yield polyaromatic carbonaceous deposits (hard coke).13–15 For instance, Ismagilov and co-workers found that fullerene encapsulated β-Mo2C nanoparticles are formed over the external surface of HZSM-5 zeolite during the MDA reaction, and this fullerene-based coking leads to the encapsulated inner core of β-Mo2C nanoparticles being inactive to MDA owing to the Mo atoms not being accessible to the CH4 molecules.16–18 Tempelman et al. concluded that the main reason for catalyst deactivation is the formation of a carbonaceous layer (polyaromatic hydrocarbons) at the external zeolite surface.19 Liu et al. suggested that polyaromatic type carbon on the Brønsted acid sites is likely the main reason for catalyst deactivation.20 Recently, Kosinov and colleagues reported that the MDA reaction mechanism possess some similarity to the well-established methanol-to-hydrocarbon (MTH) mechanism in which benzene is derived from secondary reactions of confined carbon with the initial products of methane activation, and confined carbon (linear acenes) in the zeolite pores is necessary for the production of aromatics.8 Ryoo et al. suggested that coke deposition within micropores causes more effective catalyst deactivation than external coke formation during the MTH reaction.21,22 However, as for the MDA process, further research on establishing correlation between coke accumulation and deactivation behavior, and clarifying the types of coke that dominates the MDA catalyst deactivation, is still needed.23

Ways of improving the resistant-coking of Mo-based zeolite catalysts in MDA have been reported, including (1) adding second metal additives (such as Fe, Co, Cu, Zn, Ga, W, Ru, and Re), (2) adding small amounts of CO, CO2, O2, or H2O to the feed gas, (3) optimizing the zeolite support by dealumination or silylation, and (4) constructing a hierarchical pore structure in microporous HZSM-5/HMCM-22.24–27 Shu et al. showed that appropriate addition of Ru enhances the catalytic activity, selectivity to aromatics, and stability by modification of the acid sites of HZSM-5 and reduction of Mo species.28 Adding a few percent of CO/CO2 to methane feed gas exerts a significant promoting effect on the catalytic performances for the MDA reaction to enhance the benzene formation and catalyst stability, owing to the effective suppression of coke formation on the catalysts.29 The hierarchical Mo/HZSM-5, Mo/HMCM-22, Mo/TUN-9, and Mo/IM-5 catalysts exhibit remarkably enhanced CH4 conversion and aromatic yield during the MDA reaction, but the catalytic stability has not been apparently improved for the hierarchical Mo/zeolite catalysts.30–33

Hollow capsule zeolites are innovative materials in the hierarchical zeolite family due to the tunable and uniform shell thickness and large internal cavity serving as a nanoreactor.34,35 Chu et al. firstly reported the exceptional MDA performances of the nestlike hollow hierarchical Mo/HMCM-22 microsphere catalyst due to the hollow and hierarchical structure.36 Similarly, Tsubaki et al. also demonstrated that the hollow Mo/HZSM-5 zeolite capsule catalyst shows significantly improved methane conversion, formation rate of the benzene product, and catalytic stability, and inhibited the carbon deposition, due to the accelerated mass-transfer rate in the hollow structure.37 In our previous study, the zeolite capsule catalysts exhibited remarkable catalytic performances for both the MTA and MDA reactions.38,39 However, the relationship between coke distribution and catalyst durability over the hollow capsule catalysts in the MDA reaction still need to be clarified in further research.

In this work, a hollow capsule HZSM-5 and a commercial HZSM-5 were used as MDA supports, and Mo and W were employed as active metals. Hence, the different coke amounts distributed over four catalysts were obtained after the MDA reaction to investigate the effect of coke distribution on the deactivation behavior. The main reason for the excellent MDA properties over the hollow Mo/HZSM-5 capsule catalyst was clarified. Furthermore, the stepwise burn-off of coke species in the spent Mo/HZSM-5 hollow capsule catalyst was investigated by the in situ UV-Raman technique during the TPO process. Finally, the regeneration performance of the hollow Mo/HZSM-5 capsule catalyst was studied.

2. Experimental details

2.1. Catalyst preparation

A commercial HZSM-5 zeolite with a Si/Al molar ratio of approximately 32 was supplied by Nankai University, and denoted as CZ (commercial zeolite). A hollow HZSM-5 capsule zeolite was simply synthesized via a one-step hydrothermal method according to the literature,38,39 and denoted as HC (hollow capsule). Mo/CZ, W/CZ, Mo/HC, and W/HC catalysts were prepared by an incipient wetness impregnation method with an aqueous solution of (NH4)6Mo7O24·4H2O or (NH4)2WO4 to achieve 6 wt% loading. After impregnation, the as-obtained samples were dried at 110 °C overnight, and then calcined in air at 500 °C for 5 h.

2.2. Catalyst characterization

The bulk chemical composition was measured using a Thermo iCAP 7000 spectrometer. XRD patterns were recorded using a PANalytical Empyrean X'pert power diffractometer system. Cu Kα radiation was used in a 2θ range of 5–40°. The surface morphology and elemental composition of the sample was investigated using a Hitachi-S-4800 scanning electron microscope (SEM) with an energy-diffusive X-ray spectroscope (EDS). NH3-TPD and TPO were conducted on a GAM 200 mass spectrometer. The number of Lewis and Brønsted acid sites was characterized using Py-IR using a Nicolet 6700 spectrometer. HRTEM images were obtained using a JEOL JEM-2100F instrument. Textural properties were determined by N2 physisorption, which was conducted at liquid N2 temperature using a Micromeritics ASAP-2460 apparatus. The total amount of coke species of the spent catalyst was determined using a Shimadzu DTG-60 instrument. UV-Raman spectra was obtained using a Dilor T LabRAM II spectrometer equipped with a 325 nm excitation source. For more details of the sample characterization see the ESI.

2.3. Catalyst testing

The MDA reaction was conducted under ambient pressure in a fixed-bed quartz reactor (i.d. = 8 mm) with 600 mg of 20–40 mesh-sized catalyst in down-flow mode. The catalyst was first heated from room temperature to a given temperature (t = 670, 700 and 730 °C) with a ramp rate of 10 °C min−1 in a pure Ar flow (30 mL min−1). After flushing with Ar flow at a given temperature for 0.5 h, the MDA reaction was started by switching the reactor feed to a 90 vol% CH4/N2 (N2 as an internal standard) mixture with a flow rate of 15 mL min−1. Products were monitored by online analysis using two gas chromatographs (Shimadzu GC-2014 equipped with an FID detector and an HP-INNOWax column; Shimadzu GC-2014 equipped with a TCD detector and a Porapak Q column).40,41 After the reaction, the catalyst was cooled to room temperature in a pure Ar stream (30 mL min−1). Taking the Mo/CZ catalyst as an example, the used catalysts were collected and denoted as Mo/CZ-t (t = 670, 700, and 730).

3. Results and discussion

3.1. Physico-chemical properties of the hollow capsule catalyst

Fig. 1 depicts the characteristic diffraction peaks belonging to the ZSM-5 zeolite appearing at 2θ = 7.9, 8.8, 23.0, 23.7, 23.9, and 24.4° for all the samples, suggesting highly crystalline ZSM-5 zeolite. In fact, no diffraction peaks of Mo and W species were visible for the fresh catalysts, demonstrating a well-dispersed state of Mo-/W-bearing species.
image file: c8cy01391h-f1.tif
Fig. 1 XRD patterns of the zeolites and fresh catalysts.

Fig. 2a, b and S1a–c show a beautiful ellipsoidal capsule with an external diameter of approximately 20–30 μm that was assembled by many coffin-shaped nanocrystals for the HC support. Fig. 2c clearly shows that the radial distribution of Si Kα decreased sharply in the interface region between the shell and the cavity, suggesting the hollow capsule structure with a thickness of approximately 2 μm. In addition, Fig. 2i shows the lattice fringe with an interplanar spacing of 1.102 and 0.985 nm for the HC zeolite, matching the (101) and (020) planes of ZSM-5, respectively, underscoring the high crystallinity of the ZSM-5 structure.38 In contrast, Fig. S1d–f show the well-defined coffin-shaped crystals with an average particle size of approximately 700 nm for the CZ zeolite. In addition, Fig. S2c depicts a sharp increase in a relatively high-pressure zone (P/P0 = 0.8–0.95) for all the HC-derived samples, indicating the presence of macropores, which is reasonably ascribed to the inter-crystalline void between the coffin-shaped nanocrystals.42–44 Furthermore, Fig. S2d shows that the hierarchical HC-derived samples exhibited a wide pore size distribution of the distinct macropores and mesopores with the BJH diameters of 20–100 nm.

image file: c8cy01391h-f2.tif
Fig. 2 SEM images of the (a) HC, (b) HC with collapse-fissure and (c) associated EDS line scanning analysis of HC, (d) fresh Mo/HC catalyst, (e) Mo/HC-700 used for 44 h, EDX mapping of Mo/HC-700 used for 44 h with (f) Mo, (g) Si and (h) C elements, and (i) the TEM image of the HC zeolite.

To determine the textural and acidic properties of the samples, we performed characterization techniques including N2 physisorption, NH3-TPD, and Py-IR (Fig. S2–S4). As listed in Tables S1–S3, the CZ and HC zeolites had a similar Si/Al molar ratio, SBET, and number of strong and Brønsted acid sites. During the Mo/W impregnation and calcination processes, a portion of Mo-/W-bearing species migrated into zeolite channels and anchored to the Brønsted acid sites, leading to an obvious decrease in the BET surface area, micropore volume, and number of strong/Brønsted acid sites; consequently, the interacting Mo-/W-Brønsted acid nanoclusters are responsible for the catalytic MDA reaction.45–47 NH3-TPD and Py-IR results show that the metal phase (for both Mo and W) is significantly better dispersed on hollow capsule catalysts, and more Mo/W species migrated into the zeolite to form active species under intense interaction with Brønsted acid sites for both Mo/HC and W/HC when compared with Mo/CZ and W/CZ. This phenomenon may be the reason for better MDA activity of the hollow capsule zeolite catalyst. Table S2 shows that the external surface area of the HC zeolite was about two times larger than that of CZ (128 vs. 69 m2 g−1). This implies that the HC has more channels available for the Mo/W species migration than CZ. The Py-IR results are well consistent with the BET and NH3-TPD results (Fig. 3).

image file: c8cy01391h-f3.tif
Fig. 3 (a) NH3-TPD and (b) Py-IR results of the supports and fresh catalysts.

3.2. Catalyst performance evaluation

As anticipated, the CH4 conversion and aromatic yield over all the investigated catalysts showed the typical trend of a continuous decrease with elapsed reaction time, due to the buildup of coke deposits over the catalysts.48 The induction periods of the W-based catalysts were longer than those of the Mo-based catalysts owing to the higher carbonizing temperature for the W-bearing catalysts, which is confirmed by the CH4 TPSR (Fig. S5). The hollow Mo/HC capsule catalyst exhibited significantly improved CH4 initial conversion, aromatic yield, and catalytic durability with a relatively low deactivation rate when compared with the Mo/CZ catalyst at certain reaction temperatures. Interestingly, the W/HC catalyst guaranteed higher CH4 initial conversion and aromatic yield than the W/CZ catalyst, but the deactivation rate of the W/HC catalyst was similar to that of the W/CZ catalyst at reaction temperatures in the range of 670–730 °C.

More remarkably, the deactivation rate of CH4 conversion (0.27% h−1) for the Mo/HC catalyst is competitively lower than those of other hierarchical catalysts reported in the previous papers at 1 atm, GHSV = 1500 mL h−1 g−1 and 700 °C during the MDA reaction. For example, these values for the hierarchical Mo/HZSM-5 microsphere, hollow Mo/HZSM-5 zeolite capsule, and mesoporous Mo/HZSM-5 were 0.38, 0.55, and 0.63% h−1 in the MDA reaction, which was reported by Chu,31 Tsubaki,37 and Liu,27 respectively. It should be noted that the hollow capsule structure of the Mo/HC catalyst was well retained after both Mo impregnation and long-term MDA reaction, as shown in Fig. 2d and e. Fig. 2f–i verify that Mo, Si, and C were homogeneously distributed over the external surface of the capsule zeolite for the used Mo/HC-700 catalyst. Notably, the C content (18.2 wt%) measured by EDS in the selected area as shown in Fig. 4e was larger than the total amount of carbon deposits (10.3 wt%) obtained by TG analysis, revealing that more coke species are deposited on the external surface of the zeolite, which is shown by the TPO results below.

image file: c8cy01391h-f4.tif
Fig. 4 MDA performances of the catalysts at different temperature: (a, c, e) CH4 conversion, (b, d, f) aromatic yield as a function of the reaction time (reaction conditions: 1 atm, 1500 mL h−1 g−1, and 670/700/730 °C).

3.3. Coke formation and deactivation behavior

To gain insight into the coke distribution of the coked catalysts, we used the TPO and TG techniques. Fig. 5 and S6 show that four combustion temperature regions were divided, in which four peaks were identified corresponding to different coke types: 440–480 °C (Cα), 480–530 °C (Cβ), 530–610 °C (Cγ) and 610–660 °C (Cδ). In addition, obvious discriminations were seen in the coke types deposited depending on the metal type and zeolite structure during the MDA reaction. It is widely recognized that Cα, Cβ, Cγ, and Cδ are related to oxidation of the fullerene encapsulated β-Mo2C/WC, graphite-like C located on the external surface, aromatic type coke inside the zeolite channels, and carbon deposits on the free Brønsted acid sites deep inside the channels, respectively.13,14
image file: c8cy01391h-f5.tif
Fig. 5 Gaussian deconvolution of the TPO profiles of the spent catalysts.

Fig. 6 and S7 show that the fullerene encapsulated β-Mo2C nanoparticles were easy to find for all spent Mo-based catalysts. In contrast, the fullerene encapsulated WC nanoparticles and naked WC nanoparticles without fullerene encapsulation co-existed for all coked W-based catalysts. Fig. 6a and d clearly show the lattice fringe with an interplanar spacing of 0.23 nm, corresponding to the (101) planes of β-Mo2C.49Fig. 6e and f show the lattice fringe with an interplanar spacing of 0.24 and 0.19 nm, matching the (100) and (101) planes of the WC phase, respectively.50 The measured spacing of the graphene layers was approximately 0.34 nm and indexed to the (002) plane of graphite for all the fullerene encapsulated samples.51 The TPO results suggest that the average Cα formation rate over the Mo-based catalyst was much faster than that over the W-based catalyst owing to the high carbonizing temperature of the W-based catalysts during the MDA reaction. This result agrees well with the direct TEM observation. However, the fullerene-based coking deactivates the inner core of β-Mo2C or WC nanoparticles because the Mo/W atoms are not accessible to the CH4 molecules. Recently, Kosinov et al. found that “hard” and “soft” coke distinctions are mainly related to the location of coke species inside the pores and on the external surface, respectively. And MoO3 species act as active oxidation catalysts, reducing the combustion temperature of a certain fraction of coke.52–54 Similarly, it is suggested that the burn-off of fullerene-based coke is related to the oxidation of the inner core of the Mo2C nanoparticles during the TPO process.

image file: c8cy01391h-f6.tif
Fig. 6 TEM images of the used (a–c) Mo/HC-700, (d) Mo/CZ-700, and (e and f) W/HC-700.

Fig. 6b and c show that the graphite-like C on the external surface of the zeolite was readily detected by HRTEM for the used Mo/HC-700 catalyst. Zaikovshii et al. suggested that the graphite-like C deactivates the catalyst, which plugs up the outlets of the zeolite channels on the outer microcrystal surfaces.16 In addition, the influence of Cβ on the catalyst deactivation during the MDA reaction is well demonstrated in Zhang's and Hensen's reports,15,19 namely, the graphite-like C decreases the accessibility of the active Mo centers and Brønsted acid sites in the micropores. Recently, Hensen's group proposed a hydrocarbon pool mechanism in which confined polyaromatic carbon species (Cγ and/or Cδ), formed in the induction period, do not only participate in the MDA catalytic cycle but are necessary for the production of aromatics.8 However, in the gradual deactivation period, fast Cγ/Cδ accumulation also reduces the accessibility of the active centers inside the zeolite pores once the formation rate is higher than the consumption rate for the confined polyaromatic carbon species. In summary, the four types of coke species mentioned above could lead to the decrease of CH4 conversion during the MDA reaction, but their contribution to the deactivation rate are different, and yet unknown.

In terms of coke location, both Cα and Cβ were located on the external surface of zeolites, which can be referred to as external coke (Cex). Meanwhile Cγ and Cδ were situated inside the zeolite channels, which can be referred to as internal coke (Cin). As listed in Table S4, the Cex dominated the total amount of carbon for the used Mo-based catalysts. However, the Cin was the major carbonaceous species for the used W-based catalysts based on the value of the Cex/Cin ratio. Fig. 7 shows a linear relationship between the deactivation rate of CH4 conversion and average Cex formation rate with a slope of 0.22 for the Mo-bearing catalysts. On the other hand, the deactivation rate also had a linear relationship with the average Cin formation rate with a slope of 0.10 for the W-based catalysts. For this reason, it may be concluded that the external coke can give rise to a more severe deactivation than internal coke during the MDA reaction.

image file: c8cy01391h-f7.tif
Fig. 7 Relationship between coke distribution and deactivation behavior in the (a) Mo-based MDA system and (b) W-based MDA system.

As far as our information goes, the hollow Mo/HMCM-22 capsule catalyst, reported by Yang's group, shows the highest benzene yield and catalyst stability among the reported Mo-based zeolite catalysts in the MDA reaction.36 It is believed that the excellent catalytic lifespan is associated with the hollow capsule structure, accelerating the mass-transfer rate. Analogically, the Mo/HZSM-5 catalysts with a hollow capsule architecture exhibit a superior lifetime during the MDA reaction. In this study, as listed in Table S4, we found that the hollow capsule structure endowed the Mo/HC catalyst with a better capability to suppress the external coke formation, particularly graphite-like C(Cβ), thus leading to excellent durability in the MDA process.

3.4. In situ UV-Raman and catalyst regeneration by O2

Fig. S8 shows that two UV-Raman bands at ∼1600 and ∼1350 cm−1 were seen for all spent catalysts, strongly verifying the formation of significant amounts of graphite-like C and aromatic type coke in the used MDA catalysts.19,55 To determine the evolution of Mo species and coke species during the TPO process on the used Mo/HC-700 catalyst, we performed in situ UV-Raman spectroscopy measurements. As shown in Fig. 8, no new bands appeared when the temperature was below 400 °C. At 450 °C, new bands at 367, 820, and 993 cm−1, belonging to α-MoO3, emerged, revealing that β-Mo2C begins to be oxidized to α-MoO3. Hereafter, the intensity of α-MoO3 gradually increased with the increase in temperature. At 650 °C, the G and D bands completely disappeared as a result of the entire elimination of the coke species; this result is well consistent with the TPO results.
image file: c8cy01391h-f8.tif
Fig. 8 In situ Raman spectra of the used Mo/HC-700 catalyst under the TPO process.

As coke formation and deactivation seem to be inherent properties of zeolite-catalyzed MDA, treating a spent catalyst under an oxidative atmosphere at a higher temperature is the most conventional way to regenerate a coked catalyst.56,57 Based on the aforementioned TPO and in situ UV-Raman results, we adopted the O2 regeneration protocol at 650 °C. Fig. 9 shows that the regenerated Mo/HC exhibited a slightly lower CH4 conversion and aromatic yield than the fresh one over the whole test period, demonstrating a very strong ability of regeneration and repeatability for the hollow Mo/HZSM-5 capsule catalyst.39

image file: c8cy01391h-f9.tif
Fig. 9 Regeneration performance of the hollow Mo/HZSM-5 capsule catalyst: (a) CH4 conversion and (b) aromatic yield as a function of the reaction time (reaction conditions: 1 atm, 700 °C and 1500 mL g−1 h−1).

4. Conclusions

Coking leads to the rapid deterioration of Mo-/W-based zeolites during the MDA reaction. In the present work, the cooperative effects of the metal type (Mo and W) and zeolite structure on coke distribution and its consequence in deactivation during the MDA reaction were systematically studied. The external coke dominated the total coke content in the Mo-based catalysts, while the W-based catalysts exhibited heavier formation of internal coke than external coke. Results of the extent and location of coke species during the MDA reaction suggest that the hollow capsule structure was beneficial for suppressing external coke formation, which deactivates the catalysts faster than internal coke, and exerted little influence on internal coke formation. Consequently, the hollow Mo/HZSM-5 capsule catalyst showed superior lifespan, and could be considered a potential candidate for industrial application of the MDA reaction.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the National Natural Science Foundation of China (grant 21805300) and the Coal Base Key Technologies R&D Program of Shanxi province (grant MH2014-13).


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Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cy01391h
These authors contributed equally.

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