Research Article Cite This: ACS Catal. 2018, 8, 9207−9215
pubs.acs.org/acscatalysis
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*,† †
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, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Energy Innovation Laboratory, BP Dalian Branch, Dalian, People’s Republic of China ACS Catal. Downloaded from pubs.acs.org by DURHAM UNIV on 09/06/18. For personal use only.
S Supporting Information *
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 the coking mechanism. In this work, the coking progress of H-ZSM-5 in a fixed-bed reactor under MTD conditions is investigated using operando UV-Raman spectroscopy. Methylbenzenium carbenium ions (MB+), a key precursor for coke formation, was identified by UV resonance Raman spectroscopy and isotope exchange experiments. 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 important information for the key steps and intermediates about coke formation on solid acid catalysts during methanol conversion, and the findings will contribute to improved catalytic performance in the related catalytic reaction. KEYWORDS: operando, Raman spectroscopy, coke, H-ZSM-5, methanol dehydration and deactivation behavior.24−27 Such investigations require characterization techniques with time and space resolution.21,28−31 The advent of the operando technique, which is defined as real-time characterization correlated with simultaneous online activity data, has enabled insights into the catalytic mechanism.32,33 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 resonanceenhancement effect and resistance to fluorescence interruption.34−36 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 the UV− Raman system to enable a faster scanning rate (down to 30 s, with a very low laser power of ca. 2 mW), thus making the operando UV−Raman study of the methanol dehydration
1. INTRODUCTION The dehydration of methanol functions as a fundamental step in many catalytic 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 acids,8 γ-Al2O3,9,10 or zeolites.2,11−15 H-ZSM-5 has been proposed as a good candidate due to its stronger acid strength;16,17 however, the deactivation of H-ZSM-5 due to coke formation often becomes a drawback in its practical application.4,18−21 Although many attempts have been made to resolve this problem,13,22 the coking mechanism under working conditions is still not very well understood. The types and roles of carbon-containing species still need further clarification. In addition, the heterogeneous distribution of feedstock and product at different positions in a real fixed-bed reactor often results in complex deactivation behaviors,21 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 MTH.23 The transformations of the species in the carbon pool, mainly olefins and arenes, have great effect on the activity, selectivity, © XXXX American Chemical Society
Received: March 8, 2018 Revised: August 3, 2018 Published: August 7, 2018 9207
DOI: 10.1021/acscatal.8b00928 ACS Catal. 2018, 8, 9207−9215
Research Article
ACS Catalysis
provide more information about the coking process, we 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 the bed that first contacts with the feedstock is defined as “beginning”; see the insets of Figure 1).
process experimentally feasible. In this study, the coking process of H-ZSM-5 during the 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 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 offaxis parabolic mirror (Edmund Optics Co.) as the lightcollecting element, an edge filter (Semrock Co.) to filter Rayleigh scattered light, a spectrograph (Shamrock 500), and a 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 in a spectral resolution of ∼7 cm−1. For most experiments, the laser power at the sample was kept below 2 mW to prevent burning effects. The typical accumulation time per spectrum was ∼30 s. H-ZSM-5 samples (with a Si/Al ratio of 11.5) was purchased from ZEOLYST Co. (batch number CBV-2314). Methanol (99.9%, HPLC grade), 13C-labeled 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 of the catalyst sample was calcined in air at 773 K before the reaction. Then the feed gas (He, 99.99%) was passed through a homebuilt 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 experiment, a 1/1 (volume ratio) water/methanol mixture was used as feedstock, and the 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-labeled methanol, 0.0756 bar of 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 back-mixing. The exchange was performed after 30 min of reaction with normal methanol. Online spectrometry was conducted using an Omnistar GSD320 quadrupole mass spectrometer. The activity data measured with a mass spectrometer were calibrated using GC data. MS was chosen over GC in this case because the sampling of GC causes frequently causes turbulence to the system.
Figure 1. UV−Raman spectra collected at the beginning of the catalyst bed during the first 15 min of MTD reaction over H-ZSM-5. The reaction increases from bottom to top. The time interval between two adjacent spectra is 1.5 min. Conditions: 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 Figure 1. The Raman peaks around 1005 cm−1 can be attributed to C−O single-bond vibrations.37 Methanol exhibits a C−O Raman peak at 1040 cm−1 and shifts toward 990 cm−1 when it is mixed with sulfuric acid (see Figure S1 in the Supporting Information). The C−O peak position for DME is at 919 cm−1, much lower than the observed value.38 Since the C−O peaks of DME adsorbed on acid sites are expected to shift further toward lower wavenumbers, we propose that the observed 1005 cm−1 peak most likely originates from the adsorbed H-bonded methanol, in congruence with earlier reports.11 The peak at 1440 cm−1 is assigned to the −CH3 bending vibrations.39,40 The most prominent peak is found at 1605 cm−1. This peak has usually been assigned in previous publications to the G band of coke or large polyaromatic hydrocarbons (PAH),39,41−43 which are 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 the heavily coked sample and activated carbon material can be detected using our instruments (see Figure S10 in the Supporting Information), suggesting that the absence of an ∼1390 cm−1 peak is caused by the structure of this carbonaceous species being different ftom that of “hard coke” rather than caused by the 325 nm excitation laser.
3. RESULTS AND DISCUSSION 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 allow the detection of a Raman signal near the inlet of the reactor. To 9208
DOI: 10.1021/acscatal.8b00928 ACS Catal. 2018, 8, 9207−9215
Research Article
ACS Catalysis Under this condition the deactivation is very slight (no more than 5% of the initial activity is lost in first 24 h, see Figure S8 in the Supporting Information), and so we assign this peak to a “soft coke” species that does not seriously affect the catalyst activity. Other carbon materials with no detectable D bands (highly ordered graphite,44 diamond-like amorphous carbon,45 carbon nanotubes, etc.) do not feature 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 the catalyst bed, and the results are presented in Figure 2. The
Figure 2. UV−Raman spectra collected at the end of the catalyst bed during the first 15 min of time on stream during methanol dehydration over H-ZSM-5. The time on stream increases from bottom to top. The time interval between two adjacent spectra is 1.5 min. Conditions: reaction temperature, 423 K; methanol partial pressure, 0.137 bar; GHSV, 5887 h−1. Figure 3. UV−Raman spectra collected at the beginning (top panel) and the end (bottom panel) of the catalyst bed during methanol dehydration over H-ZSM-5. Increasing reaction time is shown from bottom to top. Conditions: reaction temperature, 473 K; methanol partial pressure, 0.137 bar; GHSV, 5887 h−1.
species observed in this scenario are close to those in Figure 1, suggesting that similar chemical processes are taking place. However, one noticeable phenomenon is that the amount of “soft coke” at the end of the catalyst bed is significantly lower than that at the beginning, implying that the formation of these species is inhibited at the end of the reactor bed. When the reaction temperature is increased 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 Figure 3. At the beginning of the reactor bed, the 1390 cm−1 coke shoulder is already visible after 3 h of reaction and becomes even stronger after 17 h 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 in the 1605 cm−1 peak is detected, and no clear 1390 cm−1 peak shoulder can be spotted even after 41 h of time on stream, implying that only slow accumulation of “soft coke” takes place near the end of the catalyst bed, without a detectable buildup of “hard coke”.
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 a UV− Raman peak at 1605 cm−1 can be assigned to MB+. We choose hexamethylbenzene as the model compound due to its simple substituent composition. It is mixed with 98 wt % sulfuric acid at room temperature to produce hexmethylbenzenium carbenium ion (HMB+), and the resulting UV−Raman spectrum is shown in Figure 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. When HMB is mixed with sulfuric acid, an attenuation of the 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 at room temperature due to 9209
DOI: 10.1021/acscatal.8b00928 ACS Catal. 2018, 8, 9207−9215
Research Article
ACS Catalysis
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 an accompanying shoulder near 1390 cm−1 to the resonance enhancement of the CC stretching vibrations in MB+, which is the major content of “soft coke”. When the close structure resemblance between MB+ and PAH is taken into account, the transformation of MB+ to PAH and further to “hard coke” is a very likely route of coke formation. A 13C-labeled 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 Figure 5. After the feedstock is switched from 12CH3OH to 13 CH3OH, the C−O vibration peak position quickly shifts from 1005 to 981 cm−1. This −22 cm−1 shift is very close to the expected value (based on the simple oscillator model, see the Supporting Information for more details). The shift is completed in less than 2 min, very close to the isotope switching time scale of the DME product (approximately 100 s; see Figures S2 and S3 in the Supporting Information). On the other hand, the shifting behavior of the CC 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 conducted a control experiment using 13C-methanol as the only feedstock (Figure S4 in the 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. MB+ has been detected by Bjørgen et al. via adsorption of HMB and tetra-MB on H-beta,50,51 giving characteristic IR
Figure 4. UV−-Raman spectra of pure HMB (purple, inset figure), sulfuric acid (orange), and mixture of the two (blue).
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” observed in the previous section, since both exhibit a 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 spectroscopy,46 Weckhuysen et al. using UV−vis spectroscopy,47 and theoretical calculations.48 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
Figure 5. UV−Raman spectra collected during an operando isotope switching experiment. Increasing time on stream is shown from bottom to top. Conditions: reaction temperature, 453 K; methanol partial pressure, 0.086 bar; GHSV, 5887 h−1. 9210
DOI: 10.1021/acscatal.8b00928 ACS Catal. 2018, 8, 9207−9215
Research Article
ACS Catalysis bands near 1604 cm−1. The evolution of MB+ during MTH reactions as an important group of intermediates in the carbon pool mechanism and key source of alkene formation 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 in comparison to those 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; thus, 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 with 13C, the expected shift would become −31 cm−1, very close to our experimental value. This is 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 the 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 the formation of “hard coke” and deactivation. 3.3. Relation between Coke Formation and the Deactivation Process. A long time on stream experiment (156 h) is conducted to build a correlation between the state of carbon species and simultaneous catalyst activity. The Raman features collected at the end of the catalyst bed are presented in Figure 6. During the initial 108 h 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 h of reaction,
“hard coke” is 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. 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 Figure 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 HZSM-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 the spectra with irel1605 > 10, the 1390 cm−1 “hard coke” shoulder can be well distinguished (similar to the 156 h spectrum in Figure 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 values exceeding 10 after only 24 h of reaction. The result confirms the fast “hard coke” formation at the beginning of the catalyst bed. Meanwhile, the activity data start to decrease, suggesting correlation between the start of “hard coke” formation and the commencing of the deactivation process. In contrast, the coke formation at the end of the bed is very slow during the initial 120 h, as indicated by irel1605 below 5 in this 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 h of reaction, a 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 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 in some earlier publications.4 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 toward the end. The detection of “hard coke” at the end of the bed indicates the final stage of deactivation. During the whole reaction process no hydrocarbon product can be detected by GC or MS. We attribute this to the lower reaction temperature in the MTD process (∼473 K) in comparison to MTH. In this scenario, the formation and evolution of a “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 dehydration.54 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 toward hydrocarbon products under MTH conditions is already underway. 3.4. Effect of Water on the Suppression of Coking. The delayed MB+ growth and coking behavior at the end of the reactor may originate from many factors. To clarify the role of
Figure 6. UV−Raman spectra collected at the end of the catalyst bed during methanol dehydration over H-ZSM-5 between 40 and 156 h of reaction time. Increasing time on stream is shown from bottom to top. Conditions: reaction temperature, 473 K; methanol partial pressure, 0.086 bar; GHSV, 5887 h−1. 9211
DOI: 10.1021/acscatal.8b00928 ACS Catal. 2018, 8, 9207−9215
Research Article
ACS Catalysis
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. Conditions: reaction temperature, 473 K; methanol partial pressure, 0.086 bar; GHSV, 5887 h−1.
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 the lower parts of the bed from serious coking. These results are corroborated by the deactivation data (shown in Figure 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 properties of the acid sites of the H-ZSM-5 sample, and the results are shown in Figures S5 and S6 in the Supporting Information. The TPD curve exhibits a wide distribution with a stretched tail above 923 K, suggesting the existence of strong acid sites.57 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 red shifts of 1−3 cm−1, further confirming the existence of strong acid sites.58 We propose that such sites are responsible for MB+ formation, because only acid sites with enough strength may possess the ability to trigger hydrogen transfer and methylation processes at the low reaction temperature, which are indispensible steps during MB + formation. However, investigating the details of 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. On the basis of the results from the above sections, we propose 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 a hydrogen transfer process59 and carbon−carbon bond formation.60 These species most likely exist in small amounts and lack the necessary resonance requirement and thus cannot be directly detected by UV−Raman. With the
water and methanol concentration, water is doped into the methanol feedstock and its effect on the coke formation can be found in Figure 8 (both spectra collected at the beginning of
Figure 8. UV−Raman spectra of H-ZSM-5 during the methanol dehydration process using methanol (orange curves) and H2O-doped methanol (blue curves) as feedstock at 473 K. Conditions: methanol partial pressure, 0.086 bar; GHSV, 5887 h−1.
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 the appearance of a 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. In contrast, the formation of “hard coke” is negligible when water is doped into the feedstock, and the major spectroscopic features remain almost the same as at 423 K. This comparison demonstrates that water has a greater effect on inhibiting the formation of “hard coke” in comparison to reduced methanol concentration. Water does not seriously affect the formation of MB+ but is able to suppress the further 9212
DOI: 10.1021/acscatal.8b00928 ACS Catal. 2018, 8, 9207−9215
Research Article
ACS Catalysis Scheme 1. Coke Formation on H-ZSM-5 during MTD Process
understanding provides the possibility to improve the performance of methanol dehydration catalysts.
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 MB+ species (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 gradually from the beginning of the bed toward the end, as proved by the operando experiments.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b00928.
4. CONCLUSIONS The coke formation mechanism on H-ZSM-5 during the catalysis of methanol dehydration to dimethyl ether has been investigated using operando UV−Raman techniques. It is found that MB+, a key coking precursor, is detected on the basis of UV−resonance Raman during the reaction at 423 K, as confirmed by model compounds and isotope switching experiments. At 473 K, the transformation of MB+ to “hard coke” is rapid at the beginning of the catalyst bed yet is sluggish at the end of the bed. This is caused by the water released by 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 fixedbed reactor under working conditions, and the obtained
■
(Details abount UV−Raman spectra of methanol and methanol/H2SO4 mixtures, calculation method of the oscillator model, isotope exchange experiment of the C− O peak in the first 1.5 min, product exchange profile during isotope exchange experiments, UV−Raman spectra collected using pure 13CH3OH, NH3 TPD, pyridine adsorption IR, comparison between lowering methanol partial pressure and water doping on deactivation, deactivation at 423 K, operando isotope exchange result of the coked catalyst, and 325 nm excited Raman spectra of activated carbon and coked ZSM-5 PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for F.F.:
[email protected]. *E-mail for C.L.;
[email protected]. ORCID
Can Li: 0000-0002-9301-7850 9213
DOI: 10.1021/acscatal.8b00928 ACS Catal. 2018, 8, 9207−9215
Research Article
ACS Catalysis Notes
Influence of Calcination Temperature. Chem. Eng. Res. Des. 2016, 111, 100−108. (17) Yang, Q.; Kong, M.; Fan, Z.; Meng, X.; Fei, J.; Xiao, F. S. Aluminum Fluoride Modified HZSM-5 Zeolite with Superior Performance in Synthesis of Dimethyl Ether from Methanol. Energy Fuels 2012, 26, 4475−4480. (18) Nordvang, E. C.; Borodina, E.; Ruiz-Martinez, J.; Fehrmann, R.; Weckhuysen, B. M. Effects of Coke Deposits on the Catalytic Performance of Large Zeolite H-ZSM-5 Crystals During Alcohol-toHydrocarbon Reactions as Investigated by a Combination of Optical Spectroscopy and Microscopy. Chem. - Eur. J. 2015, 21, 17324− 17335. (19) Laugel, G.; Nitsch, X.; Ocampo, F.; Louis, B. Methanol Dehydration into Dimethylether over ZSM-5 Type Zeolites: Raise in the Operational Temperature Range. Appl. Catal., A 2011, 402, 139− 145. (20) Schulz, H. ″Coking″ of Zeolites During Methanol Conversion: Basic Reactions of the MTO-, MTP- and MTG Processes. Catal. Today 2010, 154, 183−194. (21) Bleken, F. L.; Barbera, K.; Bonino, F.; Olsbye, U.; Lillerud, K. P.; Bordiga, S.; Beato, P.; Janssens, T. V. W.; Svelle, S. Catalyst Deactivation by Coke Formation in Microporous and Desilicated Zeolite H-ZSM-5 During the Conversion of Methanol to Hydrocarbons. J. Catal. 2013, 307, 62−73. (22) Rutkowska, M.; Macina, D.; Mirocha-Kubien, N.; Piwowarska, Z.; Chmielarz, L. Hierarchically Structured ZSM-5 Obtained by Desilication as New Catalyst for DME Synthesis from Methanol. Appl. Catal., B 2015, 174-175, 336−343. (23) Olsbye, U.; Svelle, S.; Bjørgen, M.; Beato, P.; Janssens, T. V. W.; Joensen, F.; Bordiga, S.; Lillerud, K. P. Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and Pore Size Controls Product Selectivity. Angew. Chem., Int. Ed. 2012, 51, 5810−5831. (24) Schulz, H. Coking” of Zeolites During Methanol Conversion: Basic Reactions of the MTO-, MTP- and MTG Processes. Catal. Today 2010, 154, 183−194. (25) Dai, W. L.; Wang, C. M.; Dyballa, M.; Wu, G. J.; Guan, N. J.; Li, L. D.; Xie, Z. K.; Hunger, M. Understanding the Early Stages of the Methanol-to-Olefin Conversion on H-SAPO-34. ACS Catal. 2015, 5, 317−326. (26) Dai, W. L.; Dyballa, M.; Wu, G. J.; Li, L. D.; Guan, N. J.; Hunger, M. Intermediates and Dominating Reaction Mechanism During the Early Period of the Methanol-to-Olefin Conversion on SAPO-41. J. Phys. Chem. C 2015, 119, 2637−2645. (27) Sun, X. Y.; Mueller, S.; Liu, Y.; Shi, H.; Haller, G. L.; SanchezSanchez, M.; van Veen, A. C.; Lercher, J. A. On Reaction Pathways in the Conversion of Methanol to Hydrocarbons on HZSM-5. J. Catal. 2014, 317, 185−197. (28) Rojo-Gama, D.; Signorile, M.; Bonino, F.; Bordiga, S.; Olsbye, U.; Lillerud, K. P.; Beato, P.; Svelle, S. Structure-Deactivation Relationships in Zeolites During the Methanol-to-Hydrocarbons Reaction: Complementary Assessments of the Coke Content. J. Catal. 2017, 351, 33−48. (29) del Campo, P.; Slawinski, W. A.; Henry, R.; Erichsen, M. W.; Svelle, S.; Beato, P.; Wragg, D.; Olsbye, U. Time- and Space-Resolved High Energy Operando X-Ray Diffraction for Monitoring the Methanol to Hydrocarbons Reaction over H-ZSM-22 Zeolite Catalyst in Different Conditions. Surf. Sci. 2016, 648, 141−149. (30) Vogt, C.; Weckhuysen, B. M.; Ruiz-Martinez, J. Effect of Feedstock and Catalyst Impurities on the Methanol-to-Olefin Reaction over H-SAPO-34. ChemCatChem 2017, 9, 183−194. (31) Rojo-Gama, D.; Etemadi, S.; Kirby, E.; Lillerud, K. P.; Beato, P.; Svelle, S.; Olsbye, U. Time- and Space-Resolved Study of the Methanol to Hydrocarbons (MTH) Reaction - Influence of Zeolite Topology on Axial Deactivation Patterns. Faraday Discuss. 2017, 197, 421−446. (32) Chakrabarti, A.; Ford, M. E.; Gregory, D.; Hu, R.; Keturakis, C. J.; Lwin, S.; Tang, Y.; Yang, Z.; Zhu, M.; Banares, M. A.; Wachs, I. E. A Decade Plus of Operando Spectroscopy Studies. Catal. Today 2017, 283, 27−53.
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21373212), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB06040400, XDB17020000), and the DMTO project of the Dalian Institute of Chemical Physics (Grant No. DICP DMTO201407).
■
REFERENCES
(1) Sun, J.; Yang, G.; Yoneyama, Y.; Tsubaki, N. Catalysis Chemistry of Dimethyl Ether Synthesis. ACS Catal. 2014, 4, 3346−3356. (2) Masih, D.; Rohani, S.; Kondo, J. N.; Tatsumi, T. LowTemperature Methanol Dehydration to Dimethyl Ether over Various Small-Pore Zeolites. Appl. Catal., B 2017, 217, 247−255. (3) Saravanan, K.; Ham, H.; Tsubaki, N.; Bae, J. W. Recent Progress for Direct Synthesis of Dimethyl Ether from Syngas on the Heterogeneous Bifunctional Hybrid Catalysts. Appl. Catal., B 2017, 217, 494−522. (4) Kim, S.; Kim, Y. T.; Zhang, C.; Kwak, G.; Jun, K. W. Effect of Reaction Conditions on the Catalytic Dehydration of Methanol to Dimethyl Ether over a K-Modified HZSM-5 Catalyst. Catal. Lett. 2017, 147, 792−801. (5) Lefevere, J.; Mullens, S.; Meynen, V.; van Noyen, J. Structured Catalysts for Methanol-to-Olefins Conversion: A Review. Chem. Papers 2014, 68, 1143−1153. (6) Hemelsoet, K.; van der Mynsbrugge, J.; De Wispelaere, K.; Waroquier, M.; van Speybroeck, V. Unraveling the Reaction Mechanisms Governing Methanol-to-Olefins Catalysis by Theory and Experiment. ChemPhysChem 2013, 14, 1526−1545. (7) Tian, P.; Wei, Y. X.; Ye, M.; Liu, Z. M. Methanol to Olefins (MTO): From Fundamentals to Commercialization. ACS Catal. 2015, 5, 1922−1938. (8) Ladera, R. M.; Ojeda, M.; Fierro, J. L. G.; Rojas, S. TiO2Supported Heteropoly Acid Catalysts for Dehydration of Methanol to Dimethyl Ether: Relevance of Dispersion and Support Interaction. Catal. Sci. Technol. 2015, 5, 484−491. (9) Takeishi, K.; Wagatsurna, Y.; Ariga, H.; Kon, K.; Shimizu, K.-i. Promotional Effect of Water on Direct Dimethyl Ether Synthesis from Carbon Monoxide and Hydrogen Catalyzed by Cu-Zn/Al2O3. ACS Sustainable Chem. Eng. 2017, 5, 3675−3680. (10) Akarmazyan, S. S.; Panagiotopoulou, P.; Kambolis, A.; Papadopoulou, C.; Kondarides, D. I. Methanol Dehydration to Dimethylether over Al2O3 Catalysts. Appl. Catal., B 2014, 145, 136− 148. (11) Jones, A. J.; Iglesia, E. Kinetic, Spectroscopic, and Theoretical Assessment of Associative and Dissociative Methanol Dehydration Routes in Zeolites. Angew. Chem., Int. Ed. 2014, 53, 12177−12181. (12) Li, H.; He, S.; Ma, K.; Wu, Q.; Jiao, Q.; Sun, K. MicroMesoporous Composite Molecular Sieves H-ZSM-5/MCM-41 for Methanol Dehydration to Dimethyl Ether: Effect of SiO2/Al2O3 Ratio in H-ZSM-5. Appl. Catal., A 2013, 450, 152−159. (13) Rownaghi, A. A.; Rezaei, F.; Stante, M.; Hedlund, J. Selective Dehydration of Methanol to Dimethyl Ether on ZSM-5 Nanocrystals. Appl. Catal., B 2012, 119−120, 56−61. (14) Ha, K. S.; Lee, Y. J.; Bae, J. W.; Kim, Y. W.; Woo, M. H.; Kim, H. S.; Park, M. J.; Jun, K. W. New Reaction Pathways and Kinetic Parameter Estimation for Methanol Dehydration over Modified ZSM5 Catalysts. Appl. Catal., A 2011, 395, 95−106. (15) Yang, Q.; Zhang, H.; Kong, M.; Bao, X.; Fei, J.; Zheng, X. Hierarchical Mesoporous ZSM-5 for the Dehydration of Methanol to Dimethyl Ether. Chin. J. Catal. 2013, 34, 1576−1582. (16) Li, L. Y.; Mao, D. S.; Xiao, J.; Li, L.; Guo, X. M.; Yu, J. Facile Preparation of Highly Efficient CuO-ZnO-ZrO2/HZSM-5 Bifunctional Catalyst for One-Step CO2 Hydrogenation to Dimethyl Ether: 9214
DOI: 10.1021/acscatal.8b00928 ACS Catal. 2018, 8, 9207−9215
Research Article
ACS Catalysis (33) Rasmussen, S. B.; Banares, M. A.; Bazin, P.; Due-Hansen, J.; Avila, P.; Daturi, M. Monitoring Catalysts at Work in Their Final Form: Spectroscopic Investigations on a Monolithic Catalyst. Phys. Chem. Chem. Phys. 2012, 14, 2171−2177. (34) Fan, F.; Feng, Z.; Li, C. UV Raman Spectroscopic Studies on Active Sites and Synthesis Mechanisms of Transition MetalContaining Microporous and Mesoporous Materials. Acc. Chem. Res. 2010, 43, 378−387. (35) Bordiga, S.; Lamberti, C.; Bonino, F.; Travert, A.; ThibaultStarzyk, F. Probing Zeolites by Vibrational Spectroscopies. Chem. Soc. Rev. 2015, 44, 7262−7341. (36) Beato, P.; Schachtl, E.; Barbera, K.; Bonino, F.; Bordiga, S. Operando Raman Spectroscopy Applying Novel Fluidized Bed MicroReactor Technology. Catal. Today 2013, 205, 128−133. (37) Mammone, J. F.; Sharma, S. K.; Nicol, M. Raman Spectra of Methanol and Ethanol at Pressures up to 100 kbar. J. Phys. Chem. 1980, 84, 3130−3134. (38) Huang, P.; Liu, X.; Wada, Y.; Katoh, K.; Arai, M.; Tamura, M. Decomposition and Raman Spectrum of Dimethyl Ether Hydrate. Fuel 2013, 105, 364−367. (39) Allotta, P. M.; Stair, P. C. Time-Resolved Studies of Ethylene and Propylene Reactions in Zeolite H-MFI by in-Situ Fast IR Heating and UV Raman Spectroscopy. ACS Catal. 2012, 2, 2424−2432. (40) Miller, F. A.; Mayo, D. W.; Hannah, R. W. Course Notes on the Interpretation of Infrared and Raman Spectra, 1st ed.; WileyInterscience: Hoboken, NJ, 2004; p 50. (41) Chua, Y. T.; Stair, P. C. An Ultraviolet Raman Spectroscopic Study of Coke Formation in Methanol to Hydrocarbons Conversion over Zeolite H-MFI. J. Catal. 2003, 213, 39−46. (42) Rumelfanger, R.; Asher, S. A.; Perry, M. B. UV Resonance Raman Characterization of Polycyclic Aromatic-Hydrocarbons in Coal Liquid Distillates. Appl. Spectrosc. 1988, 42, 267−272. (43) Signorile, M.; Bonino, F.; Damin, A.; Bordiga, S. In-Situ Resonant UV-Raman Spectroscopy of Polycyclic Aromatic Hydrocarbons. J. Phys. Chem. C 2015, 119, 11694−11698. (44) Ferrari, A. C. Raman Spectroscopy of Graphene and Graphite: Disorder, Electron-Phonon Coupling, Doping and Nonadiabatic Effects. Solid State Commun. 2007, 143, 47−57. (45) Ferrari, A. C.; Rodil, S. E.; Robertson, J. Interpretation of Infrared and Raman Spectra of Amorphous Carbon Nitrides. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 14095−14107. (46) Li, J. Z.; Wei, Y. X.; Chen, J. R.; Xu, S. T.; Tian, P.; Yang, X. F.; Li, B.; Wang, J. B.; Liu, Z. M. Cavity Controls the Selectivity: Insights of Confinement Effects on MTO Reaction. ACS Catal. 2015, 5, 661− 665. (47) Qian, Q.; Vogt, C.; Mokhtar, M.; Asiri, A. M.; Al-Thabaiti, S. A.; Basahel, S. N.; Ruiz-Martinez, J.; Weckhuysen, B. M. Combined Operando UV/Vis/IR Spectroscopy Reveals the Role of Methoxy and Aromatic Species During the Methanol-to-Olefins Reaction over HSAPO-34. ChemCatChem 2014, 6, 3396−3408. (48) Hemelsoet, K.; Qian, Q.; De Meyer, T.; De Wispelaere, K.; De Sterck, B.; Weckhuysen, B. M.; Waroquier, M.; Van Speybroeck, V. Identification of Intermediates in Zeolite-Catalyzed Reactions by in Situ UV/Vis Microspectroscopy and a Complementary Set of Molecular Simulations. Chem. - Eur. J. 2013, 19, 16595−16606. (49) Zhang, X.; Wang, Y.; Xin, F. Coke Deposition and Characterization on Titanium Silicalite-1 Catalyst in Cyclohexanone Ammoximation. Appl. Catal., A 2006, 307, 222−230. (50) Bjørgen, M.; Bonino, F.; Arstad, B.; Kolboe, S.; Lillerud, K. P.; Zecchina, A.; Bordiga, S. Persistent Methylbenzenium Ions in Protonated Zeolites: The Required Proton Affinity of the Guest Hydrocarbon. ChemPhysChem 2005, 6, 232−235. (51) Bjørgen, M.; Bonino, F.; Kolboe, S.; Lillerud, K. P.; Zecchina, A.; Bordiga, S. Spectroscopic Evidence for a Persistent Benzenium Cation in Zeolite H-Beta. J. Am. Chem. Soc. 2003, 125, 15863−15868. (52) Zhang, M. Z.; Xu, S. T.; Wei, Y. X.; Li, J. Z.; Wang, J. B.; Zhang, W. N.; Gao, S. S.; Liu, Z. M. Changing the Balance of the MTO Reaction Dual-Cycle Mechanism: Reactions over ZSM-5 with Varying Contact Times. Chin. J. Catal. 2016, 37, 1413−1422.
(53) Olsbye, U.; Svelle, S.; Lillerud, K. P.; Wei, Z. H.; Chen, Y. Y.; Li, J. F.; Wang, J. G.; Fan, W. B. The Formation and Degradation of Active Species During Methanol Conversion over Protonated Zeotype Catalysts. Chem. Soc. Rev. 2015, 44, 7155−7176. (54) Martinez-Espin, J. S.; Morten, M.; Janssens, T. V. W.; Svelle, S.; Beato, P.; Olsbye, U. New Insights into Catalyst Deactivation and Product Distribution of Zeolites in the Methanol-to-Hydrocarbons (MTH) Reaction with Methanol and Dimethyl Ether Feeds. Catal. Sci. Technol. 2017, 7, 2700−2716. (55) Hwang, A.; Kumar, M.; Rimer, J. D.; Bhan, A. Implications of Methanol Disproportionation on Catalyst Lifetime for Methanol-toOlefins Conversion by HSSZ-13. J. Catal. 2017, 346, 154−160. (56) Martinez-Espin, J. S.; De Wispelaere, K.; Westgård Erichsen, M.; Svelle, S.; Janssens, T. V. W.; Van Speybroeck, V.; Beato, P.; Olsbye, U. Benzene Co-Reaction with Methanol and Dimethyl Ether over Zeolite and Zeotype Catalysts: Evidence of Parallel Reaction Paths to Toluene and Diphenylmethane. J. Catal. 2017, 349, 136− 148. (57) Zheng, S.; Heydenrych, H. R.; Jentys, A.; Lercher, J. A. Influence of Surface Modification on the Acid Site Distribution of HZSM-5. J. Phys. Chem. B 2002, 106, 9552−9558. (58) Emeis, C. A. Determination of Integrated Molar Extinction Coefficients for Infrared Absorption Bands of Pyridine Adsorbed on Solid Acid Catalysts. J. Catal. 1993, 141, 347−354. (59) Muller, S.; Liu, Y.; Kirchberger, F. M.; Tonigold, M.; SanchezSanchez, M.; Lercher, J. A. Hydrogen Transfer Pathways During Zeolite Catalyzed Methanol Conversion to Hydrocarbons. J. Am. Chem. Soc. 2016, 138, 15994−16003. (60) Chowdhury, A. D.; Houben, K.; Whiting, G. T.; Mokhtar, M.; Asiri, A. M.; Al-Thabaiti, S. A.; Basahel, S. N.; Baldus, M.; Weckhuysen, B. M. Initial Carbon-Carbon Bond Formation During the Early Stages of the Methanol-to-Olefin Process Proven by ZeoliteTrapped Acetate and Methyl Acetate. Angew. Chem., Int. Ed. 2016, 55, 15840−15845.
9215
DOI: 10.1021/acscatal.8b00928 ACS Catal. 2018, 8, 9207−9215
