Efficient Conversion of Methane to Aromatics by Coupling Methylation

Subscriber access provided by the University of Exeter

Letter

Efficient Conversion of Methane to Aromatics by Coupling Methylation Reaction Yi Liu, Defu Li, Tianyun Wang, Yang Liu, Teng Xu, and Yi Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01362 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Efficient Conversion of Methane to Aromatics by Coupling Methylation Reaction Yi Liu, † Defu Li, † Tianyun Wang, † Yang Liu, † Teng Xu, *, ‡ Yi Zhang *, † †

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

Technology, Beijing 100029, China. ‡

Global Chemical Research, ExxonMobil Chemical, 4500 Bayway Drive, Baytown, Texas 77520,

USA.

ABSTRACT: We report that the coupling of methane dehydroaromatization (MDA) and methanol methylation over a Mo/HZSM-5 catalyst can realize the direct conversion of methane to benzene, toluene and xylene (BTX) with long-time steady state (60 h), higher activity (26.4%) and selectivity of BTX (>90%) at atmospheric pressure and 973 K. Based on characterization, it was confirmed that the formed benzene can be effectively methylated by methanol leading to high activity and stability, which proves that the coke from polycondensation of the formed benzene results in rapid deactivation of MDA.

KEYWORDS:

heterogeneous

catalysis;

methane;

methanol

methylation;

zeolites;

dehydroaromatization Methane is widely distributed around the globe. 1 Direct conversion of CH4 into chemicals has been a long-standing challenge because CH4 exhibits high C-H bond strength (434 kJ/mol), negligible electron affinity, and low polarizability.

2

Since Wang et al.

3

reported the direct

methane dehydroaromatization (MDA) to aromatics over Mo/HZSM-5 catalysts at non-oxidative conditions in 1993, Mo containing zeolites are still considered as one of the most adequate

1

ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 14

catalysts. However, the major problem of this process is the extremely low equilibrium conversion (~12% at 973K) 4 and severe deactivation due to the rapid carbon deposits on the catalysts. 5 Various approaches have been attempted to minimize carbon formation, including the addition of hydrogen or oxidants to reaction system, mesoporous structure,

6b

6a

use of HZSM-5 crystals with hierarchical or

modification of HZSM-5,

6c

and doping of a promoter to catalyst.

6d

However, up to now, almost all reported MDA reactions can only be conducted for a very short time with continuous deactivation. It is believed that deactivation of catalysts is mainly due to the coke originated from the methane pyrolysis on MoCx or MoCxOy species and polyaromatic hydrocarbons (PAHs) hard coke on Brønsted acidic sites (BAS).

7

Therefore, suppression of

polycondensation of formed benzene would be a useful factor to prohibit catalyst deactivation. Methanol is a key platform chemical 8 and can readily react with aromatic compounds such as benzene and toluene on acidic zeolite catalysts, leading to ring or side-chain methylations.

9a

In

addition, methanol can be used as a co-reactant for the simultaneous conversion of methane to hydrocarbons, such as gasoline, light olefins and aromatics, in which the methanol mainly acts as a reactant of methanol to hydrocarbon reaction.

9b, c, d

Herein, we report an effective method to

enhance MDA activity and stability by coupling MDA and methylation of formed benzene in sequence on Mo/HZSM-5. A key result obtained is that a co-feed of small amount of CH3OH with CH4 led to improved CH4 conversion and stability of MDA reaction at extended times on stream. This reaction was postulated to involve a 2-step mechanism involving intermediate benzene formation by MDA followed by benzene methylation with methanol in the second step. This coupled consecutive reaction yield benzene, toluene and xylene as the main products (>90%), and exhibit excellent catalytic stability. 2


Page 3 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Mo-containing catalysts (Mo wt% = 2-8 wt. %, XRD patterns are exhibited in Figure S1) were prepared by incipient wetness impregnation of HZSM-5 (Si/Al = 38, supplied by Nankai University) with an aqueous solution of ammonium molybdate ((NH4)6Mo7O24•4H2O).

3

Details

of the synthesis procedures of Mo/HZSM-5 catalysts and MDA reactions are presented in the Supporting Information (SI). CH4 dehydroaromatization reactions were conducted with 0.3 g of catalyst in a fixed bed, tubular quartz reactor (8 mm id) at atmospheric pressure and 973 K. Methanol were co-fed with CH4 (CH4/CH3OH=30:1) once the catalyst was fully carburized by CH4. As reported, the optimum Mo loading for MDA ranges from 2 to 6%. 10 Figure S2 shows the methane conversion of various Mo/HZSM-5 catalysts for MDA without methanol co-feed. All the catalysts deactivate very rapidly with time on stream (TOS) due to the rapid carbon deposits on the catalysts (Figure S3b), and this is in good agreement with the literature data.

10

For

6Mo/HZSM-5 catalyst, which showed the best catalytic behavior, methane was converted mainly to benzene (53.7%), naphthalene (14.8%) and coke (21.9%), while selectivity to other aromatics was only 4.9% (Figure 1a, Table S1). However, when methanol was co-fed with methane, the reaction performance was extremely stable, and no any deactivation was observed during a 60-hour test (Figure 1b). Moreover, methane and methanol conversion remained at >26% and >95% throughout this long run, respectively. Meanwhile, the selectivity of C6-C8 aromatics significantly increased to >90% (Table S1), and the selectivities were constant with TOS. The unprecedented efficiency of this new MDA process should be attributed to a couple of MDA and methylation reaction. To obtain unambiguous evidence for the effects of co-feeding CH3OH, we chose 13C-labeled CH3OH and 13C-labeled CH4 for the selective labeling experiments. Figure 2a shows the 13C NMR spectra of liquid products from MDA over 6Mo/HZSM-5 catalyst. 3


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 14

The signals at 129.1 ppm and 18.7 ppm belong to the 13C-labeled carbon atoms of aromatic ring and methyl group attached to the aromatic ring, respectively.

11

When we used

13

C-labeled CH4

and unlabeled CH3OH for the reaction, it indicates that the 13C-labeled carbon atom of the initial 13

CH4 mainly insert into the aromatic ring, little converted into the methyl group of aromatics.

Hence, it can be deduced that the methyl group of aromatics in the products was mainly from CH3OH. This deduction was further confirmed by the experiment with 13

12

C-methane and

C-methanol co-feed. As shown in Figure 2a, the resulting 13C NMR spectrum is dominated by

the signals of methyl group of alkyl-substituted aromatics (18.7 ppm), implying methylation by methanol is responsible for the enhancement of toluene and xylene generation in the coupled reaction system. Our 1H NMR spectroscopic studies also confirmed the methylation by methanol (Figure 2b) led to a noticeable growth of the 1H signals at 7.00-7.25 ppm and 2.25-2.35 ppm from alkyl-substituted ring and methyl group of toluene and xylene, respectively. 12 Many studies found that methanol can be converted to hydrocarbons (MTH) over acidic zeolites. 13

In order to determine the intrinsic functional mechanism of the methanol in MDA reaction, a

comparison experiment with methanol feed only was also performed on 6Mo/HZSM-5 catalyst under the same conditions as that of methane-methanol co-feed. In this case, nitrogen was used to maintain methane partial pressure. The results showed a CH3OH conversion of 14.8%, and the main products are light hydrocarbons, aromatics and coke (Figure 1a, Table S1). The amount of coke formed in MTH reaction (12.1%) was much higher than that of methane-methanol co-feed reaction (6.2%), due to the formation of the HCP (hydrocarbon pool) species. 13b In contrast, methanol was almost all converted into methyl group of aromatics by methylation of benzene in methane-methanol co-feed reaction, as illustrated in previous discussion. These results suggest 4


Page 5 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

that methanol was more likely to react with benzene to produce methyl-substituted aromatics rather than directly convert to hydrocarbons in this reaction condition,

14a

due to the much lower

apparent activation barriers for the methylation of benzene (~58 kJ/mol and ~153 kJ/mol for benzene methylation and MTH reaction for first C-C bond formation, respectively). 14 Moreover, HZSM-5 appears to be a more active catalyst than other zeolites in the methylation of benzene by methanol, 15 and adding Mo2C to the zeolite greatly promotes this process. 14a On the other hand, it is considered that lower partial pressure of co-fed methanol would also decrease the reaction rate of MTH, resulting in prior methylation by methanol in this coupling reaction system. Hence, the reaction may occur via Scheme 1. During the induction period, the carbonization of Mo-O-Al species occurs to form molybdenum carbide, 4 followed by the activation of CH4 to C2 species over MoCx sites and cyclization of these C2 intermediates to aromatics inside the zeolite channels.

16a

Meanwhile, methanol is likely to be strongly adsorbed on the Brønsted acid sites to

form surface methoxy species (SMS) intermediate. 8 SMS will then react with a benzene molecule loosely adsorbed in the close vicinity to form toluene and xylene through the cleavage of C-O bond on acidic zeolite catalysts, and the carbon-deficient site on the MoCx surface also promote the rupture of the C-O bond. 14a, 16 In the conventional MDA reaction, the yields of benzene have almost reached the thermodynamic equilibrium under reaction conditions, 4 as illustrated in Figure S6. Hence, a careful design of coupled chemical reaction to convert the products is needed to break the thermodynamic equilibrium to obtain a higher product yield. If benzene is selectively removed, equilibrium in MDA reaction would be shifted towards the products. On the other hand, because the MDA is an endothermic reaction and methylation is an exothermic reaction, coupling MDA and methylation would improve the reaction rate of both two reactions. Therefore, as we 5


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 14

found in the present work, the coupling with methylation of benzene significantly improved the conversion of methane and thermodynamically tended to form methyl-substituted aromatics at 973 K (Figure S7) because of the kinetics and thermodynamics driving force. TGA-DSC was employed to characterize the carbonaceous deposits on the spent catalysts (Figure 3a-c). Two types of carbon were distinguished during TGA in artificial air. These include relatively light carbonaceous species associated with soft coke (711~718 K) and hard coke (818~838 K). 7 The addition of methanol suppresses the production of hard coke as can be seen from the shift of peak position to the lower temperatures with respect to the conventional MDA. In addition, the results of TGA suggests that the coke formation was suppressed in the presence of CH3OH, although the catalyst after 10 h MDA reaction with CH3OH co-feed contained similar hard coke content (7.4%) with conventional MDA (7.5%), considering the CH4 conversion was quadrupled more than that of conventional MDA after 10 h reaction. Most notably, longer reaction times (60 h) led to a similar hard coke content (7.0%), implied that the formation of hard coke from PAHs was successfully suppressed due to the CH3OH co-feed, which shift the reaction from benzene polymerization to methylation. Moreover, the 13C and 1H solid state MAS NMR results (Figure 2c-d) further demonstrated that more carbon from aromatics (~129.1 ppm and ~7.51 ppm for 13C and 1H, respectively) deposit on the active sites after conventional MDA reaction, 17 which will deactivate the catalyst. In addition, TEM images of used catalysts showed that many carbon nanofibers and amorphous carbons layers were formed on the 6Mo/HZSM-5 catalyst after 10 h conventional MDA reaction (Figure S3b) in line with earlier findings, 7, 18 which covers the active sites and blocks the channels of the zeolites, leading to significant catalyst deactivation. In contrast, for the images of used catalyst for MDA with CH3OH co-feed (Figure. S3c), severe heavy 6


Page 7 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

carbonaceous deposition such as carbon nanofibers was not found at the surface and channels of the zeolites after 60 h reaction. Meanwhile, acidic properties of the catalysts were compared by NH3-TPD (Figure S8 and S9). All samples exhibit two NH3 desorption peaks centered at ~530 and ~768 K, respectively. As Ma 19a and Zhang 19b, 20 et al. reported, the former can be assigned to the desorption peak of physisorbed ammonia or ammonia adsorbed on the weak acid sites, while the high-temperature desorption peak is the Brønsted acid sites.

19, 20

The relative content of the

Brønsted acid sites of the catalysts is calculated by Gaussian fitting of the peak area with the assumption of one NH3 molecule per acid site, as listed in Figure 3d and Table S2. The large decreases in the acidity with TOS for conventional MDA reaction suggest that the active Mo2C species were fully covered by non-acidic carbon deposits.

20

However, for MDA reaction with

methanol co-feed, the number of Brønsted acid sites remained the similar amount even after 60 h reaction to ensure the aromatization activity. On the other hand, the 1H solid state MAS NMR spectrum (Figure 2d) confirmed that the peak at 3.90 ppm which assigned to the hydrogen atoms at Brönsted acid sites 8, 17d, 21 maintain the initial intensity even after 60 h reaction. In summary, based on various characterizations, we have discovered that the coupling of MDA and methanol methylation reactions with the Mo/HZSM-5 catalyst realized the direct conversion of methane to BTX. The selectivity of BTX can reach ~91% with a CH4 conversion of ~26% and exceptionally long-time stability (60 h) at 973 K. This research suggests that co-conversion with methanol may indeed work as an alternative approach to CH4 efficient conversion.

ASSOCIATED CONTENT Supporting Information Supporting Information available: Experimental details, Thermodynamics analyses, XRD, 7


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 14

NH3-TPD, 13C and 1H NMR original spectra of the liquid products, TEM micrographs for the used catalysts, and reaction data of MDA. This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Authors *

Email: [email protected] (Y. Zhang)

*

Email: [email protected] (T. Xu)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of P. R. China (No. 91334206), Ministry of Education of P. R. China (NCET-13-0653), National “863” program of P. R. China (No. 2013AA031702), Innovation and Promotion Project of BUCT (No. JC1505), BUCT Fund for Disciplines Construction and Development (No. XK1505), and China Postdoctoral Science Foundation (2016M591051).

REFERENCES (1) Spivey, J. J.; Hutchings, G. Chem. Soc. Rev. 2014, 43, 792-803. (2) Guo, X. G.; Fang, G. Z.; Li, G.; Ma, H.; Fan, H. J.; Yu, L.; Ma, C.; Wu, X.; Deng, D. H.; Wei, M. M.; Tan, D. L.; Si, R.; Zhang, S.; Li, J. Q.; Sun, L. T.; Tang, Z. C.; Pan X. L.; Bao, X. H. Science, 2014, 344 , 616-619. (3) Wang, L.; Tao, L.; Xie, M.; Xu, G.; Huang, J.; Xu, Y. Catal. Lett. 1993, 21, 35-41. (4) Tang, P.; Zhu, Q.; Wu, Z.; Ma, D. Energy Environ. Sci. 2014, 7, 2580-2591. (5) Ma, D.; Wang, D.; Su, L.; Shu, Y.; Xu Y.; Bao, X. J. Catal. 2002, 208 , 260-269. (6) (a) Liu, Z.; Nutt, M. A.; Iglesia, E. Catal. Lett. 2002, 81, 271-279. (b) Chu, N. B.; Yang, J. H.; Wang, J. Q.; Yu, S. X.; Lu, J. M.; Zhang, Y.; Yin, D. H.; Catal. Commun. 2010, 11, 513-517. (c) Ding, W. P.; Meitzner, G. D.; Iglesia, E. J. Catal. 2002, 206, 14-22. (d) Ohnishi, R.; Liu, S. T.; Dong, Q.; Wang, L. S.; Ichikawa, M. J. Catal. 1999, 182, 92-103. 8


Page 9 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(7) (a) Tempelman, C. H. L.; Hensen, E. J. M. Appl. Catal. B 2015, 176-177, 731-739. (b) Song, Y.; Xu, Y.; Suzuki, Y.; Nakagome, H.; Zhang, Z. G. Appl. Catal. A 2014, 482, 387-396. (c) Lezcano-Gonzlez, I.; Oord, R.; Rovezzi, M.; Glatzel, P.; Botchway, S. W.; Weckhuysen, B. M.; Beale, A. M. Angew. Chem. Int. Ed. 2016, 55, 5215-5219. (8) Yu, S. M.; Wu, J. F.; Liu, C.; Liu, W.; Bai, S.; Huang, J.; Wang, W. Angew. Chem. Int. Ed. 2015, 54, 7363-7366. (9) (a) Wang, W.; Buchholz, A.; Seiler, M.; Hunger, M. J. Am. Chem. Soc. 2003, 125, 15260-15267. (b) Majhi, S.; Dalai, A. K.; Pant, K. K. J. Mol. Catal. A: Chem. 2015, 398, 368-375. (c) Majhi, S.; Mohanty, P.; Dalai, A. K.; Pant, K. K. Energy Technol. 2013, 1, 157-165. (d) Choudhary, V. R.; Mondal, K. C.; Mulla, S. A. R. Angew. Chem. Int. Ed. 2005, 44, 4381-4385. (10) (a) Honda K.; Chen, X.; Zhang, Z. G. Appl. Catal. A 2008, 351, 122-130. (b) Xu, Y.; Bao, X.; Lin, L. J. Catal. 2003, 216 , 386-395. (11) Gabrienko, A. A.; Arzumanov, S. S.; Moroz, I. B.; Prosvirin I. P.; Toktarev, A. V.; Wang, W.; Stepanov, A. G. J. Phys. Chem. C 2014, 118, 8034-8043. (12) (a) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62, 7512-7515. (b) Fergoug, T.; Bouhadda, Y. Fuel 2014, 115, 521-526. (c) Adebajo, M. O. Green Chem. 2007, 9, 526-539. (13) (a) Olsbye, U.; Svelle, S.; Bjørgen, M.; Beato, P.; Janssens, T. V. W.; Joensen, F.; Bordiga, S.; Lillerud, K. P. Angew. Chem. Int. Ed. 2012, 51, 5810-5831. (b) Tian, P.; Wei, Y.; Ye, M.; Liu, Z. ACS Catal. 2015, 5, 1922-1938. (14) (a) Barthos, R.; Bánsági, T.; Süli Zakar, T.; Solymosi, F. J. Catal. 2007, 247, 368-378. (b) Hill, I.; Malek, A.; Bhan, A. ACS Catal. 2013, 3, 1992-2001. (c) Qi, L.; Wei, Y.; Xu, L.; Liu, Z. ACS Catal. 2015, 5, 3973-3982. (15) (a) Van der Mynsbrugge, J.; Visur, M.; Olsbye, U.; Beato, P.; Bjørgen, M.; Van Speybroeck, V.; Svelle, S. J. Catal. 2012, 292, 201-212. (b) Svelle, S.; Visur, M.; Olsbye, U.; Saepurahman, S.; Bjørgen, M. Top. Catal. 2011, 54, 897-906. (c) Vos, A. M.; Nulens, K. H. L.; Proft, F. D.; Schoonheydt, R. A.; Geerlings, P. J. Phys. Chem. B 2002, 106, 2026-2034. (16) (a) Song, Y.; Xu, Y.; Suzuki, Y.; Nakagome, H.; Ma, X.; Zhang, Z. G. J. Catal. 2015, 330, 261-272. (b) Wang, W.; Hunger, M. Acc. Chem. Res. 2008, 41, 895-904. (c) Svelle, S.; Tuma, C.; Rozanska, X.; Kerber, T.; Sauer, J. J. Am. Chem. Soc. 2009, 131, 816-825. (d) Van Speybroeck, V.; Van der Mynsbrugge, J.; Vandichel, M.; Hemelsoet, K.; Lesthaeghe, D.; Ghysels, A.; Marin, G. B.; Waroquier, M. J. Am. Chem. Soc. 2011, 133, 888-899. (e) Svelle, S.; Arstad, B.; Kolboe, S.; Swang, O. J. Phys. Chem. B 2003, 107, 9281-9289. (17) (a) Jiang, H.; Wang, L.; Cui, W.; Xu, Y. Catal. Lett. 1999, 57, 95-102. (b) Bonardet, J. L.; Barrage, M. C.; Fraissard, J. J. Mol. Catal. A: Chem. 1995, 96, 123-143. (c) Zhang, W.; Xu, S.; Han, X.; Bao, X. Chem. Soc. Rev. 2012, 41, 192-210. (d) Ma, D.; Shu, Y.; Zhang, W.; Han, X.; Xu, Y.; Bao, X. Angew. Chem. Int. Ed. 2000, 39, 2928-2931. 9


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 14

(18) Solymosi, F.; Szöke, A.; Cserényi, J. Catal. Lett. 1996, 39, 157-161. (19) (a) Ma, D.; Lu, Y.; Su, L. L.; Xu, Z. S.; Tian, Z. J.; Xu, Y. D.; Lin, L. W.; Bao, X. H. J. Phys. Chem. B 2002, 106, 8524-8530. (b) Cui, Y.; Xu, Y.; Lu, J.; Suzuki, Y.; Zhang, Z. G. Appl. Catal. A 2011, 393, 348-358. (20) Xu, Y. B.; Lu, J. Y.; Wang, J. D.; Suzuki, Y.; Zhang, Z. G. Chem. Eng. J. 2011, 168, 390-402. (21) (a) Ma, D.; Shu, Y.; Han, X.; Liu, X.; Xu, Y.; Bao, X. J. Phys. Chem. B 2001, 105, 1786-1793. (b) Deng, F.; Du, Y.; Ye, C.; Wang, J.; Ding, T.; Li, H. J. Phys. Chem. 1995, 99, 15208-15214.

10


Page 11 of 14

Tables and Figures

100

80 Benzene Toulene Xylene Naphthalene coke

Products selectivity (%)

70 60

CH4 Conv. CH3OH Conv.

95.2

95

30 50

26.4

25

40 20 30

15

14.8 11.8

20

Conversion (%)

(a)

10

10

5

0

0

CH4

CH3OH

CH4+CH3OH MDA Reaction

(b)

30

70

Conversion

50

Xylene

20

40 15

Toluene

30

Conversion without CH3OH co-feed

10

20 5

Products selectivity(%)

60

25

CH4 Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

10

Benzene 0 0

5

10

15

20

25

30

35

40

45

50

55

0 60

Time on stream (h)

Figure 1. a) Reaction performance of 6Mo/HZSM-5 catalyst for different reactants at atmospheric pressure and 973 K. The GHSV of methane was 2000 ml/gcat h, and the mole ratio of CH4/CH3OH was 30:1. The data were taken at 1 h of time on stream, calculated based on the carbon mole percent (c-mol %). b) Long-term stability test of 6Mo/HZSM-5 catalyst at atmospheric pressure, 973 K and 2000 ml (CH4)/gcat h for MDA reaction with CH3OH co-feed.

11


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 12 of 14

(c) 129.1

CH4+13CH3OH

18.7

140

120

CH4+CH3OH

*

*

CH4+CH3OH 129.1

CH4+CH3OH

100

80

60

13

40 20

0

CH4

*

240

200

*

160

120

80

40

0

δ13C (ppm)

δ13C (ppm)

(b)

*

*

13

13

51.0

CH4+13CH3OH

(d)

C6H5-CH3

CH4+CH3OH

C6H4-(CH3)2

7.51 6.50

3.90 1.73 60 h

7.31

6h 3h C6H4-(CH3)2 C6H5-CH3

CH4

10 h 6h

CH4

3h

7.5 7.4 7.3 7.2 7.1 7.0 6.9

2.4 2.3 2.2

12

10

8

δ1H (ppm)

6

4

δ1H (ppm)

2

0

-2

Figure 2. a, b) 13C (a) and 1H (b) NMR spectra of liquid products from MDA with or without methanol co-feed over 6Mo/HZSM-5 catalyst. The original NMR data was shown in Figure S4-S5, in which solvent C3D6O signals (δ 206.7 and 29.8) in 13C and residual solvent C3D6O peak (δ 2.05) in 1H were listed. c, d) Solid state 13C CP/MAS (c) and 1H MAS (d) NMR of the Mo/HZSM-5 catalysts after 10 h MDA reaction with different feed. Asterisks denote the spinning side-bands. 13 C NMR signals at 129.1 ppm are assigned to the carbon atoms from residual polyaromatics adsorbed on the surface or channel of zeolite. 17a, b The peak at 51.0 ppm is ascribed to terminal methoxy species or strongly-bonded methanol on catalysts. 8 1H NMR signals at 3.90 and 1.73 ppm are assigned to the hydrogen atoms at Brönsted acid sites and Si-OH groups, respectively. 8, 17d, 21 The peak at ca. 6.50 ppm is ascribed to water adsorbed on Lewis sites, 21b while that at 7.51 ppm, which is very intense in the spectra, can be assigned to the hydrogen atoms of the aromatic species that are produced on the aromatization centers-the Brönsted acid sites-and are then adsorbed on the catalyst. 17d Reaction conditions: T = 973 K, P = 0.1 MPa and GHSV=2000 ml (CH4)/gcat h.

12


Page 13 of 14

TG (%) 100 98 96 94

7.5 %

92 90 88

(a)

300

DSC (uV/mg) TG (%) 0.5 100 exc↓ 0.0 713 K 838 K 98 -0.5 96 -1.0 94 -1.5 92 -2.0 90 -2.5 88 -3.0 86 -3.5 84 -4.0 300 400 700 800 900 1000

400

500

600

TG (%) 818 K

100 711 K

98

-1.5 -2.0 -2.5

7.4 %

-3.0

DSC (uV/mg) 0.5 exc↓ 0.0 -1.0

94

-1.5

92

-2.0

90

-2.5

7.0 %

88

-3.0

(c) 400

-3.5 500

600

700

800

-3.5 500

600

700

800

-4.0 900 1000

Temperature (K)

CH4 MDA CH4+CH3OH MDA

-0.5

96

84 300

-1.0

(b)

Temperature (K)

86

DSC (uV/mg) 0.5 exc↓ 0.0 718 K 818 K -0.5

-4.0 900 1000

Peak area (i.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(d) 0h

3h

6 h 10h & 60 h

Temperature (K)

Time on stream Figure 3. TG/DSC curves (a-c) and relative amounts of Brønsted acid sites from NH3-TPD (d) of the 6Mo/HZSM-5 catalyst after MDA reaction at atmospheric pressure, 973 K and 2000 ml/gcat h. a) TOS for 10 h with only CH4 feed; b) TOS for 10 h with methanol co-feed; c) TOS for 60 h with methanol co-feed.

Scheme 1. Proposed Reaction Route for CH4 Dehydroaromatization by Coupling Methylation with Methanol.

13


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 14

Graphic abstract

14


Efficient Conversion of Methane to Aromatics by Coupling Methylation

Recommend Documents