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Investigating the Coke Formation Mechanism of H-ZSM-5 during Methanol Dehydration Using Operando UV-Raman Spectroscopy Hongyu An, Fei Zhang, Zaihong Guan, Xuebin Liu, Fengtao Fan, and Can Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00928 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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Investigating the Coke Formation Mechanism of H-ZSM-5 during Methanol Dehydration Using Operando UV-Raman Spectroscopy Hongyu An a, b, Fei Zhang a, b Zaihong Guan c, Xuebin Liu c, Fengtao Fan a, *, Can Li a, *

a.

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean

Energy, The Collaborative Innovation Centre of Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China b.

University of Chinese Academy of Sciences, Beijing 100049, China

c.

Energy Innovation Laboratory, BP Dalian Branch.

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ABSTRACT Methanol dehydration on solid acid catalysts is a fundamental step in many industrial chemical processes, such as methanol-to-dimethyl-ether (MTD) and methanol-to-olefin (MTO). The performance of catalysts often encounters the detrimental effect of coke deposition. However, the heterogeneous distribution of feedstock and product in a fixed-bed reactor usually brings in difficulties in the study of coking mechanism. In this work, the coking progress of H-ZSM-5 in a fixed-bed reactor under MTD condition is investigated using operando UV-Raman spectroscopy. Methylbenzenium carbenium ions (MB+), a key precursor for the coke formation, was identified by UV resonance Raman spectroscopy and isotope exchange experiment. At higher temperature (473 K), MB+ rapidly transforms into “hard coke” at the beginning of the catalyst bed. The relative intensity of the 1605 cm-1 peak can serve as an indicator for the catalyst deactivation. Moreover, water formed during MTD can suppress the transformation of MB+ into “hard coke” at the later parts of the bed. These results provide the important information for the key steps and intermediates about coke formation on solid acid catalysts during methanol conversion, and the findings will contribute to the improved catalytic performance in the related catalytic reaction. KEYWORDS operando, Raman spectroscopy, coke, H-ZSM-5, methanol dehydration ACS Paragon Plus Environment

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1. INTRODUCTION The dehydration of methanol functions as a fundamental step in many catalytical processes, such as methanol/syngas to dimethyl ether (DME, known as the MTD or STD processes) at lower temperatures1-4 and methanol to hydrocarbons (known as MTH processes)5-7 at elevated temperatures. This step is usually catalyzed by solid acid catalysts such as heteropoly acids8, γ-Al2O39-10 or zeolites2,

11-15

. H-ZSM-5 has been proposed as a good candidate for its

stronger acid strength16-17, however the deactivation of H-ZSM-5 due to coke formation often becomes a drawback in its practical application4, 18-21. Although many attempts have been made to resolve this problem13,

22

, the coking

mechanism under working conditions is still not very well understood. The types and roles of carbon-containing species still need further clarification. Besides, the heterogeneous distribution of feedstock and product at different positions in a real fixed-bed reactor often results in complex deactivation behaviours21, leading to more difficulties for mechanistic study. Meanwhile, the carbonaceous species generated on zeolites during methanol dehydration is closely related to

the

carbon-pool

mechanism during MTH23.

The

transformation of the species in the carbon-pool, mainly olefins and arenes, have great effect on activity, selectivity and deactivation behavior24-27. Such investigations require characterization techniques with time and space resolution21, 28-31. The advent of operando technique, which is defined as real-time ACS Paragon Plus Environment

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characterization correlated with simultaneous online activity data, is able to get into the insights of catalytical mechanism32-33. Among various spectroscopic tools, UV-Raman spectroscopy stands out as an efficient technique for the characterization of carbon-containing species on zeolites thanks to its high sensitivity arising from the resonance-enhancement effect and resistance against fluorescence interruption34-36. However, due to the reduced signal intensity brought by the light absorption of working catalyst samples, the relatively long signal collection time sometimes limits the operando application of UV Raman spectroscopy. Recently, we further improved the sensitivity of UV-Raman system to enable faster scanning rate (down to 30 seconds, with very low laser power of ca. 1 mW), thus making the operando UV-Raman study of the methanol dehydration process experimentally feasible. In this study, the coking process of H-ZSM-5 during methanol dehydration reaction at different parts of a fixed-bed reactor is investigated using operando UV-Raman technique. The combination of UV-Raman spectroscopy and isotope exchange experiment enables the identification of an intermediate for coke formation (methylbenzenium carbenium ion, MB+). We also studied the role of water in the suppression of coke formation.

2. EXPERIMENTAL DETAILS


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UV-Raman spectra were collected using a home-built spectrometer. The system was composed of a 325 nm constant-wave laser (Kimmon Co.), a 25 mm diameter off-axis parabolic mirror (Edmund optics co.) as light collecting element, an edge filter (Semrock Co.) to filter Rayleigh scattered light, a spectrograph (Shamrock 500) and UV-CCD camera (Newton 920) produced by Andor. All spectra were calibrated by placing the main Raman peak of monocrystalline Si at 520 cm-1. To ensure optical throughput, the slit width was set at 150 um, resulting a spectral resolution of ~7 cm-1. For most experiments, the laser power at sample was kept below 1 mW to prevent burning effects. Typical accumulation time per spectrum was ~30 sec. H-ZSM-5 samples (with a Si/Al ratio of 11.5) was purchased from ZEOLYST Company (batch number CBV-2314). Methanol (99.9%, HPLC grade), 13C-labelled methanol (99 atom %) and hexamethylbenzene (99%) were purchased from Sigma-Aldrich. Sulfuric acid (98 wt. %) was purchased from Sinopharm Co. For operando UV-Raman experiments, 10 mg catalyst sample was calcined in air at 773 K before reaction. Then the feed gas (He, 99.99%) passed through a home-built methanol bubbler at 294 K. A reaction cell purchased from Xiamen Tops Co. was used to connect with GC and MS to constitute the operando system. The methanol partial pressure (0.137 bar) and the methanol GHSV (Gas hourly space velocity, 3696 h-1) were calibrated by GC. For the water-doping


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experiment, 1:1 (volume ratio) water/methanol mixture was used as feedstock, and bubbler temperature was kept at 318 K. Under this condition, the partial pressure of methanol in the feed gas was measured to be 0.0756 bar by GC. For the operando isotope exchange experiment using 13C-labelled methanol, 0.0756 bar methanol partial pressure was used by diluting the methanol vapor with helium. In this way, the flow speed could be increased to minimize the effect of reactor backmixing. The exchange was performed after 30 min of reaction with normal methanol. Online spectrometry was conducted using an Omnistar GSD320 quadruple mass spectrometer. The activity data measured with mass spectrometer was calibrated using GC data. MS was chosen over GC in this case because the sampling of GC causes frequent causes turbulence to the system.

3. RESULTS AND DISCUSSIONS 3.1 Growth of “soft coke” and “hard coke” in different regions of a fixed catalyst bed. To ensure optical throughput and remove the Raman signal of window materials, many conventional UV-Raman operando systems based on fixed-bed reactor design usually only allows the detection of Raman signal near the inlet of the reactor. To provide more information about the coking process, we


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changed the design of our gas routing system to allow feedstock introduction from either end of the reactor. In this manner, UV-Raman spectra can be collected at either the beginning or the end of the catalyst bed (the part of bed that first contacts with feedstock is defined as “beginning”, see the insets of Fig. 1).

Figure 1. UV-Raman spectra collected at the beginning of the catalyst bed during the first 15 mins of MTD reaction over H-ZSM-5. Reaction increases from bottom to top. Time interval between two adjacent spectra is 1.5 min. Reaction temperature: 423 K. Methanol partial pressure: 0.137 bar. GHSV: 5887 h-1.

At the beginning of the catalyst bed, many noticeable Raman features related to carbonaceous species are observed upon contact with methanol at 423 K as shown in Fig. 1. The Raman peaks around 1005 cm-1 can be attributed to


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C-O single bond vibration37. Methanol exhibits C-O Raman peak at 1040 cm-1, and shifts towards 990 cm-1 when mixed with sulfuric acid (see Fig. S1 of supporting information). The C-O peak position for DME is at 919 cm-1, much lower than the observed value38. Since the C-O peak of DME adsorbed on acid sites are expected to shift further towards lower wavenumbers, we propose that the observed 1005 cm-1 most likely originates from the adsorbed H-bonded methanol, in congruence with earlier reports11. The peak at 1440 cm-1 is assigned to the –CH3 bending vibrations39-40. The most prominent peak is found at 1605 cm-1. This peak is usually assigned to the G-band of coke or large polyaromatic hydrocarbons (PAH) in previous publications39, 41-43, which is usually linked with deactivation (referred to as “hard coke” in the following text). However, in this case, the related 1390 cm-1 peak which is usually found in such large conjugated systems39 (also known as the D-band in graphite-related systems44), is not observed. The D-band of heavily coked sample and activated carbon material can be detected using our instruments (see Fig. S10 of supporting info), suggesting that the absence of ~1390 cm-1 peak is caused by the different structure of this carbonaceous species from “hard coke” rather than caused by the 325 nm excitation laser. Under this condition the deactivation is very slight (no more than 5% of the initial activity is lost in first 24 hours, see Fig. S8 in supporting info), so we assign this peak to “soft coke” species that does not seriously affect the catalyst activity. Other carbon materials with no detectable D-bands


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(highly-ordered graphite44, diamond-like amorphous carbon45, carbon nanotube, etc.) do not meet the 1605 cm-1 band position. The intensity of this peak keeps increasing at longer time-on-stream, suggesting that the species are being accumulated over H-ZSM-5. Inversion of the direction of the feed gas allows us to investigate the evolution of the species at the end of catalyst bed, and the results are presented in Fig. 2. The species observed in this scenario are close to those in Fig. 1, suggesting that similar chemical processes are taking place. Yet one noticeable phenomenon is that the amount of “soft coke” at the end of the catalyst bed is significantly lower than at the beginning, implying that the formation of these species are inhibited at the end of the reactor bed.

Figure 2. UV-Raman spectra collected at the end of catalyst bed during the first 15 mins time-on-stream during methanol dehydration over H-ZSM-5. Time-on-stream increases from bottom to top. Time interval between two ACS Paragon Plus Environment

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adjacent spectra is 1.5 min. Reaction temperature: 423 K. Methanol partial pressure: 0.137 bar. GHSV: 5887 h-1

Upon increasing the reaction temperature to 473 K, the difference between the coke formation behaviors at the beginning and the end of the catalyst bed become more eminent, as depicted in Fig. 3. At the beginning of the reactor bed, the 1390 cm-1 coke shoulder is already visible after 3 hours of reaction, and becomes even stronger after 17 hours of reaction. Meanwhile, the intensity of 1605 cm-1 peak has also increased, implying that PAH-like “hard coke” has formed following the accumulation of “soft coke”. On the other hand, the situation at the end of the reactor bed is rather stable. No radical increase of the 1605 cm-1 peak is detected, and no clear 1390 cm-1 peak shoulder can be spotted even after 41 hours of time-on-stream, implying that only slow accumulation of “soft coke” takes place near the end of the catalyst bed, without detectable building up of “hard coke”.


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Figure 3. UV-Raman spectra collected at the beginning (left panel) and the end (right panel) of the catalyst bed during methanol dehydration over H-ZSM-5. Increasing reaction time from bottom to top. Reaction temperature: 473 K. Methanol partial pressure: 0.137 bar. GHSV: 5887 h-1

3.2 Identification of MB+ as “soft coke”: model compounds and isotope switching. In this section, we will demonstrate that the “soft coke” species with UV-Raman peak at 1605 cm-1 can be assigned to MB+. We choose hexamethylbenzene as model compound due to its simple substituent composition. It is mixed with 98 wt. % sulfuric acid under room temperature to produce hexmethylbenzenium carbenium ion (HMB+), and the resulting UV-Raman spectrum is shown in Fig. 4, along with the spectra of sulfuric acid and pure HMB. The UV-Raman spectrum of pure HMB is dominated by fluorescence, with one distinguishable feature at 1295 cm-1. Upon mixing with sulfuric acid, an attenuation of fluorescence background is spotted, and the HMB sample turns from white to very pale yellow. The resulting UV-Raman spectrum is then dominated by a clear peak at 1605 cm-1. The HMB molecule cannot undergo condensation or polymerization under room temperature due to the space hindrance of the methyl groups, thus the most possible origin of this peak is from the formation of HMB+. The UV-Raman spectrum of HMB+ is very similar to that of the “soft coke”


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observed in the previous section, since both exhibit main peak at 1605 cm-1 and neither contains discernible Raman features near 1390 cm-1. The appearance of MB+ in zeolites under MTO conditions has also been reported by Liu et al. using NMR spectroscopy46, Weckhuysen et al. using UV-Vis spectroscopy47 and theoretical calculation48. The fact that MB+ can function as an active intermediate during MTO and that other “soft coke” species with similar size have been found in TS-149 suggests that the size of HMB+ is not big enough to totally block the pore of H-ZSM-5, thus it is not expected to result in immediate catalyst deactivation, in accordance with the expected behavior of “soft coke”. Therefore, we assign the new Raman peak at 1605 cm-1 during this process without accompanying shoulder near 1390 cm-1 to the resonance enhancement of the C=C stretching vibrations in MB+, which is the major content of “soft coke”. Taken into account the close structure resemblance between MB+ and PAH, the transformation of MB+ to PAH and further to “hard coke” is a very likely route of coke formation.


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Figure 4. UV-Raman spectra of pure HMB (purple, inset figure), sulfuric acid (orange) and mixture of the two (blue).

A

13

C-labelled methanol exchange experiment is conducted under

operando conditions in order to further clarify the chemical nature of the MB+, and the results are shown in Fig. 5. After the feedstock is switched from 12

CH3OH to

13

CH3OH, the C-O vibration peak position quickly shifted from

1005 cm-1 to 981 cm-1. This -22 cm-1 shift is very close to the expected value (based on the simple oscillator model, see supporting information for more details). The shift completes in less than 2 min, very close to the isotope switching time scale of the DME product (approx. 100 sec, see Fig. S2, S3 in supporting information). On the other hand, the shifting behavior of C=C stretching of MB+ is quite sluggish. The position of this peak gradually switched from 1605 cm-1 to a new position of 1577 cm-1. The -28 cm-1 shift is less than half of the predicted value, 63 cm-1. To further verify this result, we also ACS Paragon Plus Environment

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conducted a control experiment using 13C methanol as the only feedstock (Fig. S4 in supporting information). In this experiment, the MB+ peak is spotted at 1556 cm-1, and the -49 cm-1 shift is closer to the calculated value. This suggests that the oscillator model suffices as a first-order approximation, and that the small isotopic shift observed during operando isotope switching indeed originates from the chemical nature of the MB+ formed before the exchange, such as different exchange activities of the carbon atoms.

Figure 5. UV-Raman spectra collected during operando isotope switching experiment. Increasing time-on-stream from bottom to top. Reaction temperature: 453 K. Methanol partial pressure: 0.086 bar. GHSV: 5887 h-1

MB+ has been detected by Bjørgen et al. via adsorption of HMB and tetra-MB on H-beta50-51, giving characteristic IR bands near 1604 cm-1. As an important group of intermediates in the carbon pool mechanism and key source ACS Paragon Plus Environment

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of alkene formation, the evolution of MB+ during MTH reactions has also been investigated using UV-Vis and NMR spectroscopy, but such works are usually carried out at higher reaction temperatures (above 623 K)52-53 than in the MTD scenario (around 473 K). We suggest that while the formation of MB+ is already detectable at 423 K, the temperature is not sufficient to trigger side-chain growth and cleavage reactions, so it mainly serves as a bystander at lower temperature and coking precursor above 473 K. Nevertheless, the methyl groups on the benzene ring can slowly exchange with methanol feedstock. For HMB+, if all 12 carbon atoms are equally involved in vibration and only the 6 methyl carbon atoms are exchanged to 13C, the expected shift would become -31 cm-1, very close to our experimental value. These are in congruence with the chemical nature of MB+, and further supports our assignment of the 1605 cm-1 peak to MB+. It is still difficult to unambiguously define the number and types of substitutes on the aromatic rings of these intermediates, or their interaction with zeolite, at least at current stage. However, we believe that the above results support the assignment of MB+ as an important species causing the 1605 cm-1 peak, which plays important roles during formation of “hard coke” and deactivation.

3.3 Relation between coke formation and the deactivation process A long time-on-stream experiment (156 hours) is conducted to build a correlation between the state of carbon species and simultaneous catalyst


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activity. The Raman features collected at the end of the catalyst bed are presented in Fig. 6. During the initial 108 hours of reaction, the spectroscopic features at the end of the bed do not undergo very drastic changes. While the intensity of the 1605 cm-1 carbenium peak slightly increases, the “hard coke” shoulder peak at ~1390 cm-1 is not prominent, indicating that the dominating process is still the accumulation of MB+ rather than rapid formation of “hard coke”. After 156 hours of reaction, “hard coke” was finally detected at the end of the bed, evidenced by the high intensity of the 1605 cm-1 peak and a shoulder at 1390 cm-1. Meanwhile the intensity of the 378 cm-1 ZSM-5 peak decreases due to the light absorption effect of “hard coke” species.

Figure 6. UV-Raman spectra collected at the end of the catalyst bed during methanol dehydration over H-ZSM-5 between 40 to 156 hours of


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reaction time. Increasing time-on-stream from bottom to top. Reaction temperature: 473 K. Methanol partial pressure: 0.086 bar. GHSV: 5887 h-1

To give a more detailed description of this process, the activity data obtained using online mass spectrometry is plotted along with UV-Raman results at both the beginning and the end of the catalyst bed in Fig. 7. The amount of carbonaceous species is reflected using the relative intensity of the 1605 cm-1 peak, which is normalized by the 378 cm-1 H-ZSM-5 peak as an internal reference, denoted as irel1605 in the following text (it should be mentioned that this procedure does not seek rigorous quantification of carbonaceous species, but instead serves as an indicator for emphasizing the trend of the accumulation of such species). For most of spectra with irel1605 > 10, the 1390 cm-1 “hard coke” shoulder can be well distinguished (similar to the 156 hour spectrum in Fig. 6), and those with irel1605 < 5 usually exhibit little 1390 cm-1 shoulder. In this way, the type and extent of coking can be estimated. The formation of carbonaceous species and coke is faster at the beginning of the catalyst bed, with some irel1605 exceeding 10 after only 24 hours of reaction. The result confirms the fast “hard coke” formation at the beginning of the catalyst bed. Meanwhile, the activity data starts to decrease, suggesting correlation between the start of “hard coke” formation and the commencing of the deactivation process. On the contrary, the coke formation at the end of the bed is very slow during the initial 120 hours, indicated by irel1605 below 5 in this


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phase. The fact that the reactor bed is still active suggests that when irel1605 < 5, the H-ZSM-5 catalyst can maintain its working condition with a fair amount of MB+. After 156 hours of reaction, considerable amount of “hard coke” is finally spotted at the end of the bed as irel1605 rises above 10. At this stage, both the activity and the deactivation rate become very slow, indicating that the deactivation process of the overall catalyst bed. Note that even at this stage, H-ZSM-5 can still show a residual low MTD activity even under heavily coked conditions due to the three-dimensional linkage between the channels, as has been reported by some earlier publications4. The above results support the notion that irel1605 can serve as an indicator for deactivation, and that “hard coke” grows from the beginning of the bed towards the end. The detection of “hard coke” at the end of the bed indicates the final stage of deactivation.


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Figure 7. Conversion (blue dots), normalized 1605 cm-1 peak intensity at the beginning of the bed (red dots) and normalized 1605 cm-1 peak intensity at the end of the bed (purple dots) with respect to the reaction time. Dashed curves are manually added to indicate the trend of coke growth. Reaction temperature: 473 K. Methanol partial pressure: 0.086 bar. GHSV: 5887 h-1

During the whole reaction process no hydrocarbon product can be detected by GC or MS. We attribute this to the lower reaction temperature in MTD process (~ 473 K) compared to MTH. In this scenario, the formation and evolution of “hydrocarbon pool” is not as fast as the dehydration process, reflected by the stability of the MB+ in the lower part of the reactor bed. This is in contrast to the situation of MTH, in which the rate of methylation may be comparable to the rate of methanol dehydration54. The carbonaceous species detected in this phase are more closely related to the coke formation during the methanol dehydration step, rather than deciding the hydrocarbon product selectivity. Further investigation of the evolution of carbon pool and its correlation with the selectivity towards hydrocarbon products under MTH conditions is already underway.

3.4 Effect of water on the suppression of coking


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Figure 8. UV-Raman spectra of H-ZSM-5 during methanol dehydration process using methanol (orange curves) and H2O-doped methanol (blue curves) as feedstock at 473 K. Methanol partial pressure: 0.086 bar. GHSV: 5887 h-1.

The delayed MB+ growth and coking behavior at the end of the reactor may originate from many factors. To clarify the role of water and methanol concentration, water is doped into the methanol feedstock and its effect on the coke formation can be found in Fig. 8 (both spectra collected at the beginning of the bed). At 473 K reaction temperature, “hard coke” still starts to form when dry methanol is used as feedstock as indicated by the higher intensity of the 1605 cm-1 peak and appearance of 1390 cm-1 shoulder. We have also reduced the partial pressure of methanol to 0.0137 bar without doping water into the feedstock, and the resulting spectra showed little difference from this result. On the contrary, the formation of “hard coke” is negligible when water is doped ACS Paragon Plus Environment

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into the feedstock, and the major spectroscopic features remain almost the same as at 423 K. This comparison demonstrates that water has greater effect on inhibiting the formation of “hard coke” compared to reduced methanol concentration. Water does not seriously affect the formation of MB+, but is able to suppress the further condensation and polymerization from MB+ to larger conjugated compounds. In this way, the water released at the beginning of the bed can protect the catalysts at lower parts of the bed from serious coking. These results are corroborated by the deactivation data (shown in Fig. S7). The diluted feedstock does result in less deactivation in congruence with the results of Hwang et al. and Martinez-Espin et al.55-56 , but the water-doped feedstock can preserve even more activity.

NH3-TPD and pyridine adsorption IR experiments were conducted to investigate the property of the acid sites of the H-ZSM-5 sample, and are shown in Fig. S5 and S6 of supporting information. The TPD curve exhibits a wide distribution with a stretched tail above 923 K, suggesting the existence of strong acid sites57. The Pyridine adsorption IR spectrum implies that both Brønsted and Lewis acid sites are present in the catalyst. A considerable amount of adsorbed pyridine can still be detected after evacuation at 673 K with peaks exhibiting redshifts of 1~3 cm-1, further confirming the existence of strong acid sites58. We propose that such sites are responsible for MB+ formation, because only acid sites with enough strength may possess the ability of triggering hydrogen


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transfer and methylation processes at the low reaction temperature, which are indispensible steps during MB+ formation. However, investigating the details on the carbon deposit formation requires more delicate control over the sample and environment. In the following work we wish to further investigate this effect using methods such as selective poisoning or partial ion exchange.

Based on the results from the above sections, we proposed the route of coke formation on H-ZSM-5 during methanol dehydration to DME as depicted in Scheme 1. The methanol first adsorbs on the acid sites of the zeolite. Then the majority of these adsorbed species would react with another methanol molecule to form DME, which is the desired reaction. However, some methanol would interact with stronger acid sites and form small fractions of carbenium ions via hydrogen transfer process59 and carbon-carbon bond formation60. These species most likely exist in small amounts and lack the necessary resonance requirement, thus cannot be directly detected by UV-Raman. With the prolonged reaction time, they would further polymerize into MB+ as probed by the 1605 cm-1 peak. It should be mentioned that the existence of these MB+ (and other possible coking precursors of similar chemical nature) does not immediately result in deactivation because they do not totally block the channel of H-ZSM-5. At the beginning of the catalyst bed, these precursors may rapidly form “hard coke” through further condensation, but this process can be effectively suppressed by water produced by the reaction, thus the “hard coke” grows


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ACS Catalysis

gradually from the beginning of the bed towards the end, as proved by the operando experiments.

Scheme 1. Coke formation on H-ZSM-5 during MTD process.

4. CONCLUSIONS The coke formation mechanism on H-ZSM-5 during the catalysis of methanol dehydration to dimethyl ether is investigated using operando-UV Raman technique. It is found that MB+, a key coking precursor, is detected based UV Resonance Raman during the reaction at 423 K, as confirmed by model compounds and isotope switching experiment. At 473 K, the transformation of MB+ to “hard coke” is rapid at the beginning of the catalyst bed, yet sluggish at the end of the bed. This is caused by the water released by


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dehydration, which can prevent MB+ from growing into “hard coke”. The relative intensity of the 1605 cm-1 peak can serve as an indicator for the deactivation process. The results prove that operando UV-Raman can serve as an effective probe for characterizing the coking process in a fixed-bed reactor under working conditions, and the obtained understanding provide the possibility to improve the performance of methanol dehydration catalysts.

AUTHOR INFORMATION CORRESPONDING AUTHOR * E-Mail: [email protected]; [email protected] ORCID Can Li: 0000-0002-9301-7850 NOTES The authors declare no competing financial issues.

ASSOCIATED CONTENT SUPPORTING INFORMATION


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Details abount UV-Raman spectra of methanol and methanol-H2SO4 mixture, calculation method of the oscillator model, isotope exchange experiment of C-O peak in the first 1.5 mins, product exchange profile during isotope exchange experiment, UV-Raman spectra collected using pure 13CH3OH, NH3 TPD, pyridine adsorption IR, comparison between lowering methanol partial pressure and water doping on deactivation, deactivation under 423 K, operando isotope exchange result of coked catalyst, 325 nm excited Raman spectra of activated carbon and coked ZSM-5.

ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21373212), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB06040400, XDB17020000), DMTO project of Dalian Institute of Chemical Physics (Grant No. DICP DMTO201407).

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BRIEFS Operando UV-Raman observation of coke formation in a fixed-bed reactor.

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