Progress in Nonoxidative Dehydroaromatization of Methane in the

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Progress in Non-oxidative Dehydroaromatization of Methane in Last Six Years Kaidi Sun, Daniel M. Ginosar, Ting He, Yulong Zhang, Maohong Fan, and Ruiping Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04707 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Industrial & Engineering Chemistry Research

Progress in Non-oxidative Dehydroaromatization of Methane in Last Six Years Kaidi Suna, Daniel M Ginosarb, Ting Heb, Yulong Zhangg, Maohong Fana,c,d,e*, Ruiping Chena,f** a

Department of Chemical Engineering, University of Wyoming, Laramie, WY 82072, USA

b

Idaho National Laboratory, Idaho Falls, ID 83402, USA

c

Department of Petroleum Engineering, University of Wyoming, Laramie, WY 82072, USA

d

School of Energy Resources, University of Wyoming, Laramie, WY 82071, USA

e

School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

f

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,

Chinese Academy of Sciences, Fuzhou 350002, China g

Chemistry & Physics Center, National Institute of Clean-and-Low-Carbon Energy, P.O. Box 001 Shenhua,

NICE, Beijing, China [email protected] (M. Fan), Tel.: +1 307 766 5633; fax: +1 307 766 6777 [email protected] (R. Chen), Tel.: +86 0591 6317 3012

1 ACS Paragon Plus Environment

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Abstract: Revolutionary shale gas production has resulted in increasing interest in the use of methane for producing various types of high-value chemicals and among them is aromatics via methane dehydroaromatization (MDA). Progresses achieved in the area of MDA during the last six years are significant. However, a review in this area is lacking. This review is designed to fill the gap. The review not only discusses he development of MDA catalysts, including conventional molybdenum (Mo) based catalyst and non-Mo based catalysts and novel catalysts. Also, the insights for the associated reaction mechanisms and catalyst regeneration methods and applications of various types of reactors are provided. Key words: methane dehydroaromatization, Mo based catalysts, novel catalysts, mechanism, catalyst regeneration


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1 Introduction Methane is a very abundant resource around the globe. An effective process of converting lower price methane to higher value products is required.1,2,3,4,5,6,7 Since 19938, non-oxidative methane aromatization to produce benzene over Molybdenum loaded catalysts have been widely studied. The work showed high selectivity for the formation of aromatic hydrocarbons as follows: 6‫ܪܥ‬ସ → ‫ ଺ܪ ଺ܥ‬+ 9‫ܪ‬ଶ Thermodynamic calculations demonstrated that the possible direct non-oxidative conversion rate of CH4 to aromatics to be about 14% at 973 K.9 The main goal of the recent research is to develop new catalysts for improved performance and inhibiting excessive coke formation. Among the development of catalysts for MDA, the metal/zeolite system has attracted much attention due to their structure and acidic sites for the catalytic properties of the bifunctional systems. To achieve high methane conversion and product selectivity, various methods were applied to modify the supports, Mo and/or other metals loaded on zeolites catalysts were suggested, and catalyst regeneration methods for the MDA reaction were developed10,11,12,13,14,15,16,17,18. A novel catalyst with iron (Fe) on silicon dioxide (SiO2), Fe©SiO2, was synthesized and proved to have a high methane conversion ratio as well as good selectivity to aromatics19. Uniquely, neither coke nor carbon dioxide (CO2) was detected during the reaction running over the Fe©SiO2 catalyst, despite the relatively high reaction temperature. Semiconductor material gallium nitride (GaN) was first discovered to show the photocatalytic performance of methane conversion to benzene at low temperature and thermal catalytic activity of methane aromatization at high temperature20. Several possible MDA mechanisms have been proposed in the last few years,



including an elementary step based kinetic model and a microscopically reversible 54-step detailed reaction mechanism21 model over Mo-based zeolite catalysts15. Fixed bed reactors were applied by most researchers for the MDA reaction; however, in order to overcome thermodynamic limitation and rapid decreasing activity attributed to the accumulation of coke on external surface of the zeolite, several researchers have explored fluidized reactor22 and membrane reactor23,24. This review focuses on the development and regeneration of catalysts, recently raised reaction mechanism and utilization of new types of reactors. 2 Development of catalysts for MDA 2.1 Modification of molybdenum loaded catalysts Since the first effective catalyst (Mo/HZSM-5) for MDA reaction was found in the last century8, molybdenum-based ZSM-5 zeolites have been extensively investigated by industry and academia. It was suggested that ZSM-5 zeolite was one of the most suitable supports, and molybdenum was the proper metal component25. A variety of synthesis techniques and promoters were applied to improve the overall catalytic performance in conventional fixed-bed reactors.26,27,28,29,30 Various catalysts have been developed through modification of the structural feature of the zeolite supports31,32,33, adjusting metal species and acidity with the intention of enhancing the catalytic activity, stability, and selectivity to aromatics34,35,36. 2.1.1 Modification and optimizing Mo/ZSM-5 catalysts To achieve high conversion with minimum coke formation, some authors assessed the influence of catalytic supports in molybdenum loaded ZSM-5 zeolites for methane aromatization reaction. Extensive studies are reviewed and discussed below. Modification methods for Mo/HZSM-5 and catalytic performances were summarized and included in S1.


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Recently, a novel Mo/HZSM-5 catalyst with a capsule structure was developed and evaluated.37 Activated carbon was used as the hard template to synthesize the ZSM-5 zeolite through the method of self-assembly combined hydrothermal crystallization. Catalytic performances especially benene formation and stability were significantly improved. These improvements could attribute to accelerated mass-transfer rate that induced by the hollow structure of this new kind of HZSM-5 zeolite. Besides developing the hollow ZSM-5 zeolite with capsule structure, ZSM-5 with multilamellar structure was also synthesized and evaluated. Jin et al.38 synthesized an MFItype zeolite, HZ5@S1, with different core/shell ratios. These supports were used to prepare corresponding Mo/HZ5@S1 catalysts (6wt% Mo, Si/Al=15) via impregnated incipient wetness. The catalysts showed high aromatic selectivity as well as the stability (shown in Figure 1). Experimental results revealed that catalytic performance was related to the ratio of core/shell, which is because the overgrowth of the Silicalite-1 shell would severely inhibite Mo species to migrate into the zeolite channels.

Figure 1. (A) The CH4 conversions over catalysts, (B)Yields of total aromatics of catalysts. Reaction condition: T =993K, P = 1atm and GHSV = 1500 h-1.38 Reproduced with permission from reference 38. Copyright 2013 Elsevier.



Liu’s group39 impregnated Mo into multilamellar support materials MFI (zeolite structure code) to synthesize meso-/microporous zeolite catalysts (4.7wt% Mo, Si/Al=30) and systematically investigated the textural and acidity properties of the catalysts. Figure 23 showed the catalytic performances and the textural properties of Mo/MFI, Mo/lamellar MFI. The results demonstrated that Mo loaded lamellar supported catalysts showed higher methane conversion, aromatic production and coke formation than those of Mo loaded microporous MFI (crystal size of 1.4µm named as Mo/MFI-1.4, and crystal size of 0.2 µm noted as Mo/MFI-0.2). These results could be due to the presence of mesopores in zeolites. With the presence of these mesopores, the reactants could access active sites easier and the products could diffuse faster.


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Figure 2. The catalytic performance of catalysts. Reaction conditions: T = 983 K, P = 1atm and GHSV = 1600 h-1.39 Reproduced with permission from reference 39. Copyright 2014 Elsevier.

Figure 3. Scanning electron microscope (SEM) images of MFI-1.4 (a), MFI-0.2 (b), lamellar MFI (c).39 Reproduced with permission from reference 39. Copyright 2014 Elsevier.



Further, due to the outstanding catalytic performance achieved over Mo/lamellar MFI catalyst in MDA reaction, Liu’s40 group applied dual template synthesis strategy in the preparation of lamellar MFI zeolite The spatial distribution of tunable meso-/microporosity and the catalytic performance of the catalysts were investigated. Experiment results indicated that external surface area of the zeolite directly influences the formation of external and internal coke on the catalyst. Moreover, it was suggested that synthesis dual template could optimize the ratio of meso-/microporosity, which would result in better catalyst performance. A new 3 wt%Mo/HZSM-5-BP (BP, model of carbon template, Si/Al=26) catalyst was evaluated and compared to a reference Mo/HZSM-5 catalyst33. This new catalyst was comprised of carbon-templated nanoparticles zeolites with intracrystalline mesopores. Better resistance to carbonaceous deposition on the catalyst could be concluded from its performance, which was due to the existence of the intracrystalline mesopores which act as a trap for carbon deposition. This allowed a larger amount of carbon deposition to be formed in the 10-ring channels structure. Liu et al.

41

added two kinds of carbon as meso-

templates in the synthesis gel, then burned away the carbon templates to generate mesoporous ZSM-5 for preparing Mo modified mesoporous catalysts. Reaction results are summarized in Table 1. Catalytic performances of Mo modified mesoporous ZSM-5 catalysts exhibited similar CH4 conversion rates, but showed better stabilities and aromatics yields than the conventional ZSM-5 catalyst in the evaluation tests. The excellent catalytic performance achieved by this Mo/ZSM-5 catalyst (mesoporous) could be because of the formation of secondary mesoporous systems in the zeolite. It might make the reactants


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more accessible to active sites during the reaction, and allow large molecule products to diffuse from microporous channels. Table 1. Reaction results of methane aromatization.41 Reproduced with permission from reference 41. Copyright 2012 Elsevier. Catalysts

Mo-ZSM-5-C Mo-ZSM-5-M Mo-ZSM-5-S

Reaction time (min) 60 300 60 300 60 300

Conversion of CH4 (%)

C6H6

9.8 6.7 9.8 7.3 9.7 7.4

45 44 48 57 50 57

Selectivity (%) C7H8 C10H8 1.8 2.2 1.5 3.0 1.8 3.0

12 6 15 9 14 8

Coke

Aromatics yields (%)

41 47 35 31 34 32

5.8 3.5 6.4 5.0 6.4 5.2

Reaction condition: 1 atm, T = 973 K, GHSV = 1500 h-1. Catalyst synthesized from a conventional ZSM-5 zeolite was noted as Mo-ZSM-5-C. The sample prepared from CMK-3 was denoted as Mo-ZSM-5-S. The sample prepared from CMCM-44 was denoted as Mo-ZSM-5-M.

Cui and co-workers42,31 used several HZSM-5 zeolites with various average particle sizes, from 4.0, 1.9, 1.0, 0.5 and 0.2 µm on highly crystalline parent HZSM-5 (Si/Al = 20) to prepare 5wt% Mo/HZSM-5 catalysts, and carried out MDA reactions on these catalysts at 1073 K with different space velocities. Characterizations of the catalysts with different particle size supports were reported in Table 2. The NH3-TPD (Ammonia-Temperature Programmed Desorption) measurements (shown in Figure 4) showed that both Brønsted acidity and crystallinity of catalysts became lower with the decreasing of average particle size, which led to their lower activities. Catalytic performance indicated that the catalyst with an average size of 1.9 µm achieved higher outlet benzene concentrations and showed little reduction under the space velocities of 5000 - 20000 ml/g•h. These results could be due to the zeolites with smaller particle sizes made it easier for the formed C6H6 to migrate out of the zeolite channels. Experiments also confirmed that instead of external surface of



zeolite, the formation of C6H6 under high space velocities took place in the channels of zeolite.

Table 2. Characterizations of the catalysts.31 Reproduced with permission from reference 31. Copyright 2011 Elsevier. Particle size of Micropore surface Total pore Coke amount Relative HZSM-5 (µm) area (m2/g)a volume (cm3/g)b (wt%)c acidity (%)d 4.0 462 0.14 6.2 58 1.9 350 0.10 5.4 45 1.0 300 0.09 6.6 40 0.5 277 0.10 5.8 34 0.2 236 0.10 7.0 25 a Estimated by Brunauer-Emmett-Teller (BET) method in the range up to P/P0 = 0.2 by Ar adsorption (87 K). b Estimated in the range up to P/P0 = 0.8 by Ar adsorption (87 K). c Determined by thermal gravimetric analysis (TGA) for the spent catalyst samples from the tests at 10,000 ml/h/g and 1073K. d Estimated based on the NH3-TPD results.

Figure 4. (a) NH3-TPD results of used zeolites,(b) NH3-TPD results of 5%Mo/HZSM-5 catalysts.31 Reproduced with permission from reference 31. Copyright 2011 Elsevier.

Velebna and co-workers

9

loaded molybdenum into the parent ZSM-5 zeolite via

different methods: wet impregnation (WIM), wet impregnation with treatment of ultrasonic


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(US), wet impregnation with treatment in a rotavapour (RV), wet impregnation combined with microwave irradiations (MW) and mechanical mixing (MM). SEM images of the parent ZSM-5 zeolite and all prepared catalyst samples are showed in Figure 5. Catalyst characteristics are listed in Table 3. Catalytic performances and the physicochemical characteristics of the catalysts showed that the catalyst prepared with microwaves exhibited the highest CH4 conversion rate. The catalyst prepared with the assistance of mechanical mixing showed the lowest activity for methane aromatization reaction. That result could be due to the intrusion of molybdenum species into zeolite pores and their association with the Brønsted acid sites.

Figure 5. SEM pictures of zeolites.9 Reproduced with permission from reference 9. Copyright 2015 Elsevier. Table 3. Characteristics of catalyst samples.9 Reproduced with permission from reference 9. Copyright 2015 Elsevier. Catalyst ZSM-5

Mo (wt.%) -

SBET (m2/g) 343

Vmicro (cm3/g) 0.138


Acidity (mmol/g) 1.06

Bronsted acid sites 0.99


Mo/ZSM-5 (RV) Mo/ZSM-5 (US) Mo/ZSM-5 (MW) Mo/ZSM-5 (WIM) Mo/ZSM-5 (MM)

4.61 4.65 4.67 4.64 4.68

308 302 301 336 323

0.104 0.108 0.100 0.134 0.140

0.79 0.86 0.82 0.95 1.03

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0.75 0.79 0.78 0.91 0.96

Xu et al.43 added triethoxyphenylsilane for the modification of ZSM-5 zeolites, and synthesized several Mo-based zeolite catalysts by mixing molybdenum trioxide (MoO3) and HZSM-5 with a MoO3 content of 6wt%, that was then calcined in air. They drew a conclusion from the performances of these catalysts that proper crystal size as well as mesoporous characteristic resulted in high aromatics selectivity and better stability for MDA reaction. Tempelman and co-workers

44

introduced mesoporosity and silylation onto

the HZSM-5 for the preparation of molybdenum loaded (4wt% Mo, Si/Al=20) catalysts. The catalysts were prepared by physical mixing method with MoO3, or prepared by impregnation method with a solution of (NH3)6Mo7O24·4H2O. The results demonstrated that mesoporosity did not improve catalytic performance of Mo/HZSM-5, but the silylation modified catalyst showed improved benzene selectivity. This may because silylation could help the dispersion of Mo species, which could lead to higher CH4 conversion rates as well as lower carbon deposition. Kan et al.45 loaded three types of molybdenum oxide to modify commercial ZSM-5 zeolites (Si/Al=25) with different molybdenum weight (2-8wt% Mo) for investigating catalytic performances in methane non-oxidative aromatization reaction. Catalytic performances comparison of catalysts carried out under the same experimental conditions was shown in Figure 6. Results indicated that nano MoO3 loaded ZSM-5 catalyst achieved higher aromatics yield (9.5%), CH4 conversion rate (14.1%), and better catalytic stability


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than those of micron MoO3 modified ZSM-5 zeolites. Those results could be due to MoO3 particles with smaller sizes could easily sublimate and migrate in the channels of ZSM-5. These Mo species then interacted with Brønsted acid sites and form Mo-O-Al species. This resulted in more actives sites for methane activation. In this case, better catalytic performance was achieved.

Figure 6. Catalytic performance over different catalysts in MDA. Reaction conditions: P = 1 atm, T = 773K and GHSV = 1500 h-1. Catalyst prepared by physically mixing HZSM-5 and commercial MoO3 was abbreviated as 6Mo(C)-ZSM-5. The samples synthesized from micron MoO3 and nano MoO3 were named as 6Mo(I)-ZSM-5 and Mo(II)-ZSM-5, respectively.45 Reproduced with permission from reference 45. Copyright 2015 Royal Society of Chemistry.

Kucherov46 loaded Mo on ZSM-5 to prepare Mo/ZSM-5 (4wt% Mo, Si/Al=30) by mechanical mixing method. The catalysts then treated with the aqueous solution of NaOH, and then dealuminated with Al(NO3)3 solutions to modify the support materials for



mesoporous structure formation. Catalytic studies indicated that catalysts prepared from zeolites etched with sodium hydroxide solutions increased the methane conversion at the initial moment of operation and the total yield of aromatic hydrocarbons in the MDA reaction, but with no notable increase in the selectivity to aromatics. This might be due to the high acidity of the zeolite after reducing the SiO2/Al2O3 module. Catalytic performances also revealed that an increase of the alkali concentration at the stage of etching the initial zeolites was meaningless. This would lower the catalyst activity sharply, resulting in low yield and selectivity to aromatics, and rapid catalyst coking. Zhao and Wang

47

applied steam-treatment method to modify the HZSM-5 zeolite and

Mo/HZSM-5 catalyst (6wt% Mo, Si/Al=25) during catalysts preparation. The catalytic performance of Mo/HZSM-5 with different steaming treatments suggested that steaming at 813 K for 1 hour was most effective on reducing the deactivation rate of the catalysts. Hydrofluoric acid (HF) has also been used to pretreat the HZSM-5 support zeolite. Mo was then loaded to prepare the Mo/HZSM-5 (HF) (6wt% Mo, Si/Al=25) catalyst48. Catalytic performances displayed that the hydrofluorinated Mo/HZSM-5 catalyst had better coke tolerance than untreated Mo/HZSM-5, but exhibited lower activity for aromatics formation. This result could be attributed to the HF treatment that both strength and amount of Brønsted acid sites were decreased. All the modification methods for Mo/HZSM-5 and catalytic performances for modified catalysts were summarized and included in S1. Most of these modification methods have a promotion effect on methane conversion and benzene selectivity. These methods include zeolite structure reconstruction, catalyst particle size adjustment, molybdenum loading


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methods adjustment, acidity adjustment and steam treatment. However, modifying Mo/HZSM-5 by HF treatment could decrease methane conversion and selectivity to benzene. 2.1.2 Modification and optimizing Mo loaded catalysts supported by other framework materials To improve the activities, selectivities and stabilities of the catalysts, a good deal of research was achieved concentrating on synthesizing new support materials, which were modified with Mo to prepare catalysts for the MDA reaction. Mo loaded catalysts that supported by other framework materials are summarized. Catalytic performances of these catalysts are compared to conventional Mo/HZSM-5 catalyst. These contents are summarized in S2. A new high-silica ITQ-13 zeolite was developed, modified with Mo and tested in MDA32. Its catalytic performances were compared with Mo/HZSM-5. The structures of ZSM-5 and ITQ-13 are shown in Figure 7, which clearly shows differences in shapes. Mo/ITQ-13 showed lower activity and stability compared to Mo/HZSM-5, but it is better in selectivity to benzene. These results could be attributed to the unique three-dimensional structure in ITQ-13 supports constructed via 9-member-ring (4.0 × 4.9 Å), straight 10-member-ring (4.7 × 5.1 Å), and sinusoidal 10-member-ring (4.8 × 5.7 Å) down the a, b and c axes. This structure favored coke formation during reaction.



Figure 7. SEM images of (a) ITQ-13 and (b) ZSM-5.32 Reproduced with permission from reference 32. Copyright 2009 Springer Nature.

Kan and his co-workers49,50 synthesized a series of Mo modified catalyst using a new support material, IM-5, and MoO3 loading for the non-oxidative MDA reaction. Reaction results for different catalysts at 60 min are shown in Table 4. The catalytic performance of Mo/IM-5 catalyst achieved better CH4 conversion rate (shown in Figure 8) and benzene selectivity (shown in Figure 9), and better stability than that of Mo/ZSM-5, which could be because of its unique 10-member-ring channel structure and its three-dimensional cavities. It was concluded that the most suitable catalyst was Mo/IM-5 (Si/Al=50) with 6-8wt% MoO3 loading. The best results were obtained at 1 atm with a temperature range of 9731023K. Based on those preliminary results, they developed synthesis methods for the Mo/IM-5 catalysts preparation. Catalytic performances of Mo/IM-5-R catalyst exhibited better CH4 conversion rate and higher benzene selectivity than that of Mo/IM-5-S catalyst. The experiment results also revealed that when compared to catalyst synthesized by impregnation method, physical mixing method prepared catalysts had better stability and lower initial activity. They also investigated and suggested that acidity, textural properties as well as the dispersion of Mo species could be the key factors which considerably


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influence the catalytic performance and lead to different CH4 conversion rate, benzene selectivity and stability of the catalysts. Table 4. The numerical reaction results on the methane aromatization reactions of different catalysts at 60 min.49, 50 Reproduced with permission from reference 49. Copyright 2011, Elsevier. Reproduced with permission from reference 50. Copyright 2013 Springer Nature. Catalyst 2Mo-IM-5(50) 4Mo-IM-5(50) 6Mo-IM-5(50) 8Mo-IM-5(50) 6Mo-IM-5(40) 6Mo-IM-5(60) Mo-IM-5-R Mo-IM-5-S Mo-IM-5-R(i)

Conversion of CH4, % 7.3 10.2 11.5 12.5 11.5 10.2 11.7 9.6 12.4

Benzene 34.7 36.2 39.3 32.3 35.7 39.9 74.2 66.0 73.4

Selectivity % Toluene 1.4 1.1 1.3 1.2 1.4 1.4 3.2 2.1 1.6

Naphthalene 12.2 13.7 14.6 15.3 15.9 14.7 22.6 31.9 25

Yields of coke % 3.8 49 5.049 5.249 6.449 5.449 4.549 5.550 4.950 6.050

Reaction condition: P=1 atm, T = 973 K, GHSV = 1500 h-1. Mo-IM-5-R synthesized under rotating condition, Mo-IM-5-S synthesized under static condition, and Mo-IM-5-R(i) prepared by the impregnation method with Si/Al = 50 and 6wt% MoO3 loading. 2Mo-IM-5(50) prepared with IM-5 support having Si/Al ratios = 50 and 2wt% MoO3 loading. 4Mo-IM-5(50) with Si/Al = 50 and 4wt% MoO3 loading, 6MoIM-5(50) with Si/Al = 50 and 6wt% MoO3 loading, 8Mo-IM-5(50) with Si/Al = 50 and 8wt% MoO3 loading, 6Mo-IM5(40) with Si/Al = 40 and 6wt% MoO3 loading, and 6Mo-IM-5(60) with Si/Al = 60 and 6wt% MoO3 loading.

Figure 8. CH4 conversion over Mo-ZSM-5 (■) and Mo-IM-5 (●) without considering coke. Reaction condition: P=1 atm, T = 973 K, , GHSV = 1500 h1 49 . Reproduced with permission from reference 49. Copyright 2011 Elsevier.

Figure 9. C6H6 selectivity (■) and C10H8 selectivity (●) over Mo-ZSM-5(open symbols) and Mo-IM-5 (filled symbols) without considering coke. Reaction condition: P=1atm, T = 973 K, GHSV = 1500 h-1.50 Reproduced with permission from reference 50. Copyright 2013 Springer Nature.

Kan and his co-workers, Liu et al.51 also prepared a new support material TNU-9 (TNU, framework type) and synthesized Mo-modified catalysts (SEM images shown in Figure 10)



for the MDA reaction. The catalytic performance of Mo/TNU-9 exhibited higher CH4 conversion rate, higher C6H6 selectivity as well as better carbon resistance ability than that of Mo/ZSM-5. This could be due to its 3D 10 member ring (MR) structure as well as the existence of large 12MR cavities. In further research, they also carried out hydrothermal reaction methods to prepare several micro-mesoporous TNU-9-x molecular sieves with different contents of phenyltriethoxysilane (PTEOS) introduced as mesoporous maker.52 Conclusions of catalytic performances (shown in Figure 11) revealed that Mo/TNU-9-20 (6wt% Mo, Si/Al=25) showed better conversion of methane (14.9%), yield of aromatics (9.9%) and catalytic stability than those of Mo/TNU-9 (microporous) catalyst. This could due to the formation of secondary mesoporous systems in the zeolite.

Figure 10. The SEM images of TNU-9 support (a), ZSM-5 zeolite (b).51 Reproduced with permission from reference 51. Copyright 2011 Elsevier.


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Figure 11. Methane conversion, aromatization yield, benzene yield and naphthalene yield over different catalysts. Reaction condition: P = 1atm, T = 973K and GHSV = 1500 h-1.52 Reproduced with permission from reference 52. Copyright 2014 Royal Society of Chemistry.

Liu’s group39 impregnated Mo into multilamellar support material MWW (zeolite structure code) to synthesize meso-/microporous zeolite catalysts (4.7wt% Mo, Si/Al=30). The textural and acidity properties of these catalysts were studied. The catalytic performances and the textural properties of Mo/MWW and Mo/lamellar MWW are presented in Figure 12-13. It was showed that Mo/lamellar MWW catalysts exhibited higher methane conversion, lower benzene and toluene selectivity and higher naphthalene selectivity than those of Mo loaded microporous MWW. These results could be due to the



presence of mesopores in zeolites. The reactants could access active sites easier, and the products could diffuse faster.

Figure 12. The catalytic performance of catalysts. Reaction condition: T = 983 K, P = 1atm and GHSV = 1600 h-1.39 Reproduced with permission from reference 39. Copyright 2014 Elsevier.


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Figure 13. SEM images of MWW (a) and lamellar MWW (b).39 Reproduced with permission from reference 39. Copyright 2014 Elsevier.

Yang and co-workers53 used carbon particles and fluoride ions to prepare hierarchical MCM-22 zeolite aggregates (MCM-22-FC) for Mo loaded catalysts (6 wt% Mo, Si/Al=15) by a one-pot hydrothermal synthesis. They found that the Mo/MCM-22-FC catalyst exhibited improved stability, aromatic selectivity and benzene yield in the MDA reaction. This result could be owned to intergrown and stacked thin MCM-22 lamellas and mesopores and micropores constructed in the hierarchical MCM-22-FC aggregate zeolite, which led to a reduction in carbonaceous deposits and improving aromatic selectivity. The addition of fluoride ions reinforced the acidic strength and more active centers in the Mo/MCM-22-FC catalyst were formed. This was important for improved catalytic performance and controlling carbonaceous deposits. Tempelman et al.54 prepared nanocrystalline MCM-22 zeolite. During zeolite preparation, organosilane was applied as a crystal inhibitor, which led to a higher accessibility to active sites for reactants and better catalytic performance. Yin et al.55 prepared nanosized MCM-22 zeolites with a crystal size of about 40 nm by combined in-situ crystallization and self-assemble. The catalytic activity of Mo/MCM-22-



NZ (6wt% Mo, Si/Al=15) performed with considerably more stability, had higher methane conversion and benzene yield in comparison with microsized Mo/MCM-22. This was thought to be because of the addition of cationic polymer that avoided the synthesis colloids self-aggregation and intergrowth, which lowered the tendency to form coke. Hu and coworkers56 aimed at a simple method to prepare Mo/HMCM-22 by modifying commercial HMCM-22 zeolites with nano size MoO3. Catalytic performances demonstrated that the nano-MoO3 modified HMCM-22 zeolites (6wt% Mo, Si/Al=25) had better stability, higher CH4 conversion and yield to aromatics than commercial MoO3 modified HMCM-22 zeolites. The better catalytic performance may because nano size MoO3 was easily sublimated and formed molybdenum species at atomic or molecular level then interacted with Brønsted acid sites. Large-pore zeolites like SBA-15 and zeolite beta are also identified as promising supports in catalyst preparation because of their high surface areas.57,58 These large-pore zeolites lack the unique 10MR structure, which provides shape selectivity to aromatics in MDA reaction.38 In this case these large-pore zeolites are not currently applied in MDA reaction. However, large-pore zeolite such as zeolite beta can be applied in coaromatization of methane and propane.58 For co-aromatization of methane and propane, Luzgin et al. 58 used tetraethylammonium hydroxide as a template to synthesize zeolite beta (H-BEA) to prepare molybdenum-containing zeolite catalyst (2.6wt% Mo, Mo/H-BEA). Both gas chromatography–mass spectrometry (GC-MS) and solid-state nuclear magnetic resonance (NMR) spectroscopy were applied to analyze the pathway (shown in Figure 14) of the reaction. The results revealed that Mo-n-propyl species could probably be the intermediates of propane aromatization were detected in the co-aromatization reaction.


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Figure 14. The possible pathway for CH4 and C3H8 co-aromatization.58 Reproduced with permission from reference 58. Copyright 2013 American Chemical Society.

Table 5. The catalytic performances in the methane aromatization reaction of various catalysts at 60 min.

Catalyst

Mo/ZSM-5 Mo/ITQ-13 Mo/IM-5-R Mo/TNU-9 Mo/TNU-9-20a Mo(N)-MCM-22b Mo/HZ5@S1c

Conversion of methane (%) 9.5 ~3 11.7 11.3 14.9 13.1 8.6

Selectivity (%) Benzene 67.4 74.2 81.2 38.9 61.1 63.1

Toluene 2.4 3.2 2.2 1.7 3.1

Naphthalene 30.2 22.6 16.6 25.9 4.2 16.9

Yields of aromatics (%) 5.6 ~2.6 6.2 6.2 9.9 8.9 7.6

Yields of coke (%)

Ref. 49 51

/

32

5.5

50 51

5.0 4.1 11.3d

52 56 38

-1

Reaction condition: P=1atm, T = 973 K, GHSV = 1500 h . a Data collected at 80 min b Data collected at 130 min c Data collected at 20 hours. d It was the yield of non-aromatics for the reaction running 20 hours.

The catalytic performances of methane conversion, aromatic selectivity and coke yield collected for various catalysts with different supports at 60 min are summarized in Table 5. The results demonstrate that most of Mo loaded catalysts prepared form newly developed supports showed higher methane conversion than Mo modified conventional ZSM-5 zeolite support catalyst. Catalysts synthesized of TNU-9 supports with different content of PTEOS (phenyltriethoxysilane) showed the highest methane conversion or benzene selectivity,



which might be due to the 3D 10MR channel structure within the hierarchical pore materials. All Mo loaded catalysts that supported by other framework materials discussed in this section are listed in form S2. Catalytic performances of these catalysts are also summarized. Besides ZSM-5 zeolite, other supporting materials such as TNU-9, IM-5 and MCM-22 (nanosized) are promising candidates as catalyst supporting materials in MDA research. 2.1.3 Modification of molybdenum loaded catalysts by introducing other metal elements Early researches showed that Mo/HZSM-5 catalyst was the most suitable catalyst for methane conversion to aromatics. A number of investigations of MDA over the past few years have been carried out focusing on improving the catalytic activity and stability, as well as selectivity to aromatics of catalysts by introducing other metals to the Mo loaded zeolite for catalyst preparation. Xu and co-workers59 doped several metal species (0.5wt% M, M = Pd, Ru, Zn, Cu, Co, Fe, Mn and Cr) on 5 wt% Mo/HZSM-5 (Si/Al=15) catalyst by the co-impregnation method. The effects of these metals on the catalytic performance of Mo/HZSM-5 catalyst were investaged. Catalytic performances indicated that only Fe had a promotion effect on activity and stability due to the generation of carbon nanotubes induced by iron species. Table 6 shows the coke amount in spent all the spent catalysts. In further investigations, temperature programmed oxidation (TPO), TGA, SEM and BET were applied to study the promotion mechanism of the iron additive60. It was suggested that the promotional effect of iron species is attributed to carbon nanotubes induced by iron species. These carbon


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nanotubes could suppress the formation of surface coke that can be removed at low temperature. This remarkably improved the catalytic activity, stability and CH4 conversion. Table 6. Comparation of coke amount in spent catalysts.60 Reproduced with permission from reference 60. Copyright 2011 Elsevier.

Catalyst Time on Stream (min) Coke amount (wt%)a a

5%Mo/ HZSM-5 200 10.3

0.5%M–5%Mo/HZSM-5

Fe

Cu

Cr

Co

Ru

Pd

Zn

Mn

225 12.9

185 9.6

165 10.2

185 10.6

185 9.4

185 12.2

155 11.9

185 12.5

Determined by TGA.

From Xu’s research59, Cu and Zn didn’t show a promotion effect on the Mo/HZSM-5 catalyst. However, a different story was found when higher content of Cu and Zn was loaded on the Mo/HZSM-5 catalyst. In the research, Cu and Zn were applied to substitute half of Mo content in Mo/HZSM-5 catalyst (6wt% Mo, Si/Al=25)35. The performances of these catalysts suggested that the extra metal (Cu or Zn) decreased the catalytic activity, but enhanced yield and selectivity to C6H6 compared to the Mo/HZSM-5. The relationship between benzene yield and time on stream (TOS) and that between selectivity and TOS are shown in Figure 15.

Figure 15. Results of (a) C6H6 yield and (b) selectivity of catalysts. Reaction condition: P=1atm, T = 973 K,



GHSV = 1500 h-1.35 Reproduced with permission from reference 35. Copyright 2011 Elsevier.

Rodrigues et al. introduced Niobium (Nb) carbide to molybdenum loaded HMCM-22 zeolite for the methane dehydro-aromatization reaction61. The catalytic behavior of niobium containing catalyst showed lower activity and coke deposition than conventional Mo/MCM-22 catalyst. Benzene was the major product observed for conventional Mo containing zeolite, but naphthalene was predominant for the Nb loaded sample. Scurrell and co-workers62 doped platinum (Pt) and/or tin (Sn) on Mo/HZSM-5 zeolite catalyst (2wt% Mo, Si/Al=70) for methane aromatization reaction using incipient wetness co-impregnation method. Catalytic results of Pt-Mo/HZSM-5 catalyst (0.5wt% Pt) showed an increase of methane conversion from 4% to 7% and a 50% reduction of coke deposition with a 65% aromatics selectivity. However, the increase in the tin loading (0.05% - 0.2wt%) resulted in a decrease in methane conversion (7.2% - 4.3%). However, selectivity to aromatics reached 64%-83%, and benzene selectivity was ~55% when the loading of tin was at 0.1 wt%. Moreover, the coke deposit reduced from 30% to 15% at 0.05 wt% of Sn. Furthermore, they synthesized platinum and tin species modified Mo/HZSM-5 zeolite catalysts for MDA

29

. It was concluded that the catalytic performances of catalysts with

additional tin led to lower methane conversion but higher aromatic products selectivity. Coimpregnation prepared tin-platinum catalyst and sequentially impregnation prepared platinum/tin catalyst exhibited higher selectivity to aromatic and lower selectivity to carbon deposition than sequentially impregnated tin/platinum catalyst, which could be due to the better platinum dispersion within the zeolite. The order of platinum and tin species impregnated sequentially might also affect the structural and electronic properties of


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platinum in Mo/HZSM-5, where the availability of platinum sites could affect the coke deposition in the methane aromatization reaction. Table 7 showed the conversion and selectivity of those catalysts.

Table 7. The conversion and selectivity of catalysts prepared in different methods.29 Reproduced with permission from reference 29. Copyright 2015 Elsevier. Selectivity (%) Conversion Yield of Yield of Catalysts (%) Aromatics (%) Coke (%) C2s Benzene Toluene Naphthalene Pt-Sn 5.5 5.7 70.3 5.5 0.0 4.2 1.0 Pt/Mo 7.9 6.3 51.7 12.2 4.2 5.4 2.0 Sn/Pt 6.4 8.8 48.8 8.3 4.2 3.9 1.9 Pt/Sn 4.9 6.1 65.4 9.1 6.2 3.6 0.9 Reaction condition: P=1 atm, T = 973 K, space velocity = 1620 h-1, TOS =305min. The Pt-Sn catalyst was prepared by co-impregnation method. The Pt/Mo, Sn/Pt and Pt/Sn catalysts were prepared by sequential impregnation with different order of metal addition.

Fila et al.

63

introduced various amounts of transition metals (Co, Mn and Ce) into

Mo/ZSM-5 (4wt% Mo, Si/Al=30), and optimized the content of these metals. Experimental results (shown in Figure 16) indicated that Co modified Mo/ZSM-5 (0.8wt% Co) was optimal for benzene selectivity. Moreover, Co-Mo/ZSM-5 achieved better stability during reaction compared to Mo/ZSM-5, Mn-Mo/ZSM-5 as well as Ce-Mo/ZSM-5. Also, Cedoped catalyst had higher selectivity to toluene at elevated pressures.



Figure 16. The CH4 conversion (a) and molar fraction of C6H6 (b) over catalysts. Reaction condition: P = 150kPa, T = 973 K, GHSV = 1500 h-1.63 Reproduced with permission from reference 63. Copyright 2015 Elsevier.

Abdelsayed

64

introduced Fe and/or Zn to modify conventional Mo loaded HZSM-5

zeolites (4wt% Mo, Si/Al=55) using impregnation method. They investigated catalytic behaviors of these catalysts in MDA reaction. The experimental data demonstrated that FeMo/HZSM-5 displayed high stability and selectivity to aromatics. An addition of Zn slightly improved the C6H6 formation rate. However, the presence of both iron and zinc resulted in a significant decrease in CH4 conversion and C6H6 selectivity. Awadallah et al.

30

examined the impact of group VIII metals (3wt% Fe, Co and Ni) on

Mo/HZSM-5 (3wt% Mo, Si/Al=25) in MDA reactions. These catalysts were prepared using mechanical mixing method. The results (shown in Figure 17) suggested that the bimetallic catalysts showed faster decomposition of methane to carbon and hydrogen and lower aromatization activity when compared to that of monometallic 6 wt%Mo/HZSM-5. This may be due to the mechanical mixing method which kept the zeolite pores free of metal incorporation, and reduced interaction between acid site and metal species.


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Figure 17. Total aromatics (a) and benzene selectivity (b) vs. TOS over different catalysts. Reaction condition: P=1 atm, T = 973 K, GHSV = 1500 h-1.30 Reproduced with permission from reference 30. Copyright 2012 Elsevier.

Sun et al.65 introduced Fe nanoparticles to Mo/HZSM-5 zeolites (5wt% Mo, Si/Al=30) by mechanical mixing method. The results showed that an addition of nano-sized Fe could enhance the catalytic stability and increase production rate of aromatics in the MDA reaction. It was also revealed that carbon nanotubes were found on spent Mo/HZSM-5 catalyst even without the existence of nano Fe. In addition, it was also concluded that Fe can promote the formation process of carbon nanotubes and a certain amount of carbon nanotubes could enhance benzene yield and catalytic stability. Cheung et al.

66

prepared several Zn-based (1-2wt% Zn) catalysts by co-impregnation

with Mo (2wt% Mo) and/or Ga (1wt% Ga) over HZSM-5 (Si/Al=25) zeolite. The evaluation tests were carried out under atmospheric pressure and supersonic jet expansion. Catalytic activities (showed in Figure 18) indicated that an addition of Mo significantly increased methane conversion and aromatic selectivity under atmospheric pressure. However, Zn/HZSM-5 showed the highest catalytic performance under supersonic jet expansion condition. Two znic species, H3C-OZn+ and HZnO-, were detected on spent Zn/HZSM-5 catalyst. They also prepared Mo (1wt% Mo), Ga (1wt% Ga) and/or Zn (2wt% Zn) modified HZSM-5 (Si/Al=25) catalysts and investigated their catalytic activities on coconversion of methane and propane to aromatics

67

. The reactions were carried out under

supersonic jet expansion condition at 673K. C6H6, C7H8, C2H4, C9H8, C10H8, and C11H10 were detected from the obtained products by time of flight mass spectrometer. It was



indicated that CH4 was activated and converted on Lewis acid sites, and the carbon atom from CH4 entered the methyl group of toluene and methylnaphthalene.

Figure 18. CH4 conversion and selectivity to aromatics over the catalysts. Reaction condition: P=1

atm, T = 973 K, GHSV = 1680 h-1.66 Reproduced with permission from reference 66. Copyright 2011 American Chemical Society.

Martinez et al.68 partially exchanged H+ in HZSM-5 with alkali (Na+, Cs+) and alkalineearth (Ca+, Mg+) cations to modulate both strength and density of Brønsted acid sites. Coke-forming tendency and stability of Mo/ZSM-5 catalyst were also studied. They reported that partial exchange H+ with Na+ and Cs+ could reduce the density of Brønsted acid sites. Catalyst with a Na/Al atomic ratio of 0.1 exhibited lower coke formation and higher C6H6 selectivity compared to Mo/HZSM-5. However, catalysts exchanged with Ca+ and Mg+ cations did not show an enhanced stability. Zhang

69

investigated the effect of indium (In) on Mo/HZSM-5 in MDA. In-Mo/ZSM-5

was synthesized using impregnation method. It was concluded that an addition of indium could decrease methane conversion. But an addition of 1wt% indium can reduce coke


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selectivity to 50% that of Mo/ZSM-5. The results also showed that selectivity to C2 hydrocarbons and C6H6 over In-Mo/ZSM-5 remains the same level to that of Mo/ZSM-5. All the above-mentioned metal elements and their promotion effect are included and summarized in S3. From this table, the effect of a certain metal element varies with its addition amount are noticeable. Take Fe as an example, when 0.3 wt% or 0.5 wt% addition of Fe was added to Mo/HZSM-5, the catalyst showed higher CH4 conversion rate and C6H6 selectivity than non-Fe added catalyst. However, selectivity to benzene dropped when Fe content was raised to 3 wt%. In this case, different contents of metal should be added and tested to determine whether this metal has a promotion effect in MDA reaction. 2.2 Development of non-Mo loaded catalysts To improve catalytic performance and coke resistance ability, several researchers focused on modifying the zeolite supports, adding promoters, periodic methane/H2 switching treatment and shifting the reaction equilibrium by using membrane reactors to remove hydrogen from the reaction, et al. Nevertheless, many researchers also developed new catalysts without Mo species loaded in order to find a more robust catalyst. 2.2.1 Zeolite based non-Mo loaded catalysts Abdelsayed and co-workers 70 loaded different amount of Zn (1-8wt% Zn) over HZSM-5 (Si/Al=55) to prepare catalysts by incipient wetness impregnation method. Catalytic performances of MDA reaction (shown in Figure 19) exhibited obvious low stability (less than 12 hours), methane conversion (less than 2%) and benzene yield. This poor catalytic activity was due to coke formation during reaction. The results also showed that two kinds of Zn species, ZnO and [Zn(OH)]+, were presented during the reaction. The ZnO particles



were loosely bounded and easily reduced. Another Zn species, the anchored [Zn(OH)]+ was acted as strong Lewis acid and responsible for CH4 activation.

Figure 19. CH4 conversion (a) and C6H6 yield (b) over Zn loaded catalysts. Reaction condition: 1 atm, T = 973 K, GHSV = 3000 h-1.70 Reproduced with permission from reference 70. Copyright 2015 Elsevier.

Shan et al.71 applied a series of rare earth metal species (0.1wt% Eu, Dy, La, Pr, Ce, Gd, Nd, Tb, Sm and Ho) to modify Zn-based HZSM-5 zeolites (1.35wt% Zn, Si/Al=32) by conventional incipient wetness impregnation method. A mixture of methane and ethylene was used as a reactant gas to produce aromatics. Evaluation tests were carried out at low temperatures (723-823 K). The results (shown in S4) demonstrated that catalysts modified by rare-earth metal species exhibited high methane conversion and aromatic yields, especially the methane conversion reached 37.3% over Gd-Zn/HZSM-5. Stepanov and co-workers72 applied both X-ray photoelectron spectroscopy (XPS) and magic angle spinning (MAS) NMR in the research of methane-ethylene aromatization. The state of indium and quantity of Brønsted acid sites in In/HZSM-5 catalyst were investigated in the research. They demonstrated the possible pathway of methane transformation to surface species (shown in Figure 20). Isolated In+ or InO+ was the main state of In in the catalyst.

In+ZO- species was formed after the zeolite was reduced by hydrogen, and


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InO+ZO- species was formed by oxidative treatment. Methane was exclusively activated on the InO+ZO- sites to form oxyindium-methyl species and Brønsted acid sites, followed by the formation of oxyindium-methoxy, C2H4, formate and acetaldehyde.

Figure 20. The possible pathway of methane transformation to surface species.72 Reproduced with permission from reference 72. Copyright 2014 American Chemical Society.

Zn-ZSM-11 (Si/Al=17) catalysts were prepared for the co-conversion of methane (C1) and ethane (C2) to higher hydrocarbons73. Experiment design-response surface methodology (RSM) was applied to study reaction parameters. These parameters include reaction temperature, C1 molar fraction, and Zinc-loading factors. The optimal reaction parameters were decided based on C1 conversion and the yields of aromatics. The results obtained by RSM achieved a C1 conversion of 48.6 mol% C and aromatic yields of 47.2 mol% C. This optimal result led to the best operation conditions: XC1 should locate between 0.2-0.4; reaction temperature scope is 823-853K and Zn2+/Zn2+H=0.86. Gao et al.74 applied spectroscopies and computational method to study the structure of Cr species and anchoring sites (shown in Figure 21) on Cr/HZSM-5 catalysts in MDA reaction. Some catalysts were prepared by loading Cr species (from 0.5 to 2.6 wt%) over HZSM-5 (Si/Al=15), and others were prepared by loading 1 wt% Cr on different HZSM-5 zeolites



(Si/Al=15, 25, 40 and 140). Density functional theory calculation and multiple spectroscopic techniques were applied to characterize the Cr-oxide structures and anchoring sites in the catalysts. Spectroscopic techniques include ultraviolet visible spectroscopy, infrared spectroscopy (IR) and Raman spectroscopy with online mass spectrometry. Cr/ZSM-5 showed lower methane conversion and benzene yield, but higher stability than that of Mo/ZSM-5. The results also indicated that isolated Cr(VI) dioxo and Cr(III) monooxo structure on framework could migrate between framework Al anchoring sites and external surface Si sites at 773K in the absence of oxygen. This study is helpful in understanding the mechanism of methane aromatization reaction over Cr/ZSM-5.

Figure 21. Raman spectra of catalyst and anchoring sites of Cr species.74 Reproduced with permission from reference 74. Copyright 2015 American Chemical Society.

Tshabalala et al.75 prepared Mn/HZSM-5 (2wt% Mn), W-Mn/HZSM-5 (0.5-1.5wt%W, 2wt% Mn), Pt-W-Mn/HZSM-5 (Pt 0.5wt% Pt, 1.5wt% W, 2wt% Mn) and Ru-WMn/HZSM-5 (0.5wt% Ru, 1.5 wt% W, 2wt% Mn) catalysts by a sequential impregnation method. Catalysts were evaluated in a fixed-bed reactor at 1073K. It was concluded that adding tungsten to Mn/HZSM-5 decreased methane conversion, but increased catalyst stability. Further, an addition of platinum or ruthenium to W-Mn/HZSM-5 increased


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selectivity to aromatics. Thus, the addition of a noble metal improved the yield of aromatics and lowered the yield of coke. Catalysts with both Pt and Sn nanoparticles loaded on HZSM-5 zeolite were first synthesized and studied by Gerceker et al.

76

Among all samples tested in this research,

PtSn (1:2)/ZSM-50 showed highest yield of aromatics. The total production rate of aromatics was comparable to that of Mo/HZSM-5. Ag-Ga/ZSM-5 catalyst was recently synthesized and evaluated by He for coaromatization of CH4 and C3H6.27 The reaction could happen at a low temperature of 673K. The hydrogen from CH4 occupied aromatic hydrogen sites in the product of benzene and other aromatics, which has been verified by 1H, 2D and

13

C NMR tests. Compared to

methane aromatization reaction, co-aromatization of propylene and methane has a more extensive product distribution. The liquid products include C6H6, C7H8, and C8-C12 aromatics. C8 aromatics shows highest selectivity. However, Benzene is not the major product, and shows a relatively low component distribution among all aromatic products. Fe/zeolite catalysts were proved as promising catalysts for MDA process77. Recently, increasing research has been focusing on Fe loaded zeolite catalysts. Lai et al.77 prepared Fe-HZSM-5 through three synthetic methods, which include core-shell synthesis, isomorphous substitution and wet ion exchange. They studied the inference of the synthetic methods on the distribution of Fe species. The results showed that coke deposition reduced with an increase of Fe dispersion. Tan 78 also investigated catalytic performance of 2-6 wt% Fe/HZSM-5 or 2-6 wt% Fe/HMCM-22 catalysts in a fixed bed flow quartz reactor.. Three reaction periods were found through the reaction, including oxidization, decomposition and



Page 36 of 72

aromatization of CH4. The results of X-ray diffraction (XRD) and XPS indicated that the metallic iron was formed in the decomposition period. Iron carbide was formed in the aromatization period. The results also exhibited that carburized iron was the active phase for the conversion of CH4 to C2 hydrocarbons as well as aromatics. 2.2.2 Novel catalysts Two kinds of novel catalysts were developed for MDA reaction and drew lots of attention

recently.

These

catalysts

are

Fe©SiO2

19

and

GaN

nanoparticles/nanowires79.These two kinds of novel catalysts are discussed thoroughly in the following paragraphs. Fe©SiO2 was obtained by the following procedure. Ferrous metasilicate was fused with SiO2 from commercial quartz at 1973K in air atmosphere. The obtained material was then leached with aqueous HNO3 and dried at 353K. Fe©SiO2 was obtained and © represented a lattice-confined single Fe sites embedded in a silica matrix in the catalyst19. Fe©SiO2 contains 0.5wt % of Fe and its BET surface area is smaller than 1m2/g. The transmission electron microscopy (TEM) result of fresh Fe©SiO2 catalyst indicated that iron oxide nanoparticles (3-4 nm) were distributed homogeneously throughout the SiO2 matrix. scanning transmission electron microscope high-angle annular dark-field(STEM-HAADF) image of the spent catalyst is shown in Figure 22. It was revealed that Iron oxide species were embedded within the silica matrix by bonding with C and Si atoms. This result was also confirmed by Density functional theory (DFT) calculations


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Figure 22. STEM-HAADF image of spent catalyst (0.5% Fe©SiO2).19 Reproduced with permission from reference 19. Copyright 2014 The American Association for the Advancement of Science.

Online vacuum ultraviolet soft photoionization molecular-beam mass spectrometry (VUV-SPI-MBMS) was employed to investigate the reaction mechanism. Methyl radicals (•CH3) were clearly observed at 1193K in the reaction. Signals for ethylene, propyne, propylene, benzene, toluene and naphthalene were verified as well. A mechanism was hypothesized based on these results. •CH3 was generated in the beginning of reaction. C2H6 was then formed by combining of two •CH3 through an exothermic process. Then C2H6 was dehydrogenated to give H atoms and C2H4. The •C2H3 radical, resulted by abstraction of H from C2H4, tended to react with other C2H4 molecules. This process would have led to the formation of benzene by further dehydrogenation and cyclization. Since naphthalene is a more thermodynamically stable product, benzene could also be dehydrogenated readily by •H to form naphthalene. Fe©SiO2 catalyst showed excellent stability, methane conversion and aromatic selectively in MDA reaction. Neither coking nor deactivation was happened in a test lasted



for 60h. Conversion rate of methane kept around 32% during a long test at 1293 K, and exceeded 48.1% at 1363 K. Only ethylene (selectivity: 52.7%), benzene (selectivity: 21.5%) and naphthalene (selectivity: 25.8%) were produced, and selectivity to all the products kept >99% throughout the reaction. The selectivity to ethylene on catalyst Fe©SiO2 is much higher than that of conventional Mo/HZSM-5, which is lower than 10%80. All these new findings provide new chances for MDA fundamental research. GaN nanowires/nanoparticles catalysts are another series novel catalyst. Convert CH4 to benzene by photocatalysts over Si-GaN nanowires under ultraviolet (UV) illumination was reported for the first time

20

. Plasma-assisted molecular beam epitaxy (MBE) was used to

prepare GaN nanowires under nitrogen rich condition. However, synthesis of nanostructured GaN catalysts require a special technique, which is molecular beam epitaxy under high temperatures (1073 - 1273K). It was the first time that GaN has a wurtzite crystal structure was discovered to have thermal catalytic activity to convert light alkanes to benzene79. Photocatalytic performance evaluation tests of Si-doped GaN nanowires were conducted in an air-tight quartz reactor at 278K under UV irradiation. It was revealed that the methane conversion rate was proportional to the square of light intensity. The quantum efficiency was increased linearly as a function of light intensity. Minimum photon energy needed to drive methane aromatization reaction over GaN can be provided by UV light with a wavelength of 360 nm. The reactor was vacuumed first and pure methane was then introduced. Experimental results indicated that the selectivity to benzene achieved 96.5%. The GaN materials also demonstrated highly stable. No noticeable deactivation showed


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after several reaction cycles, and only a small amount of carbon deposits (polyaromatic and graphitic species) were found on the surface of GaN nanowires after a long reaction time. These carbon deposits could be completely stripped by the oxygen plasma treatment. GaN also exhibited superior stability, reactivity and high selectivity to benzene at an elevated temperature. In a 4-hour reaction at 723K, the maximum methane conversion was 0.56% and benzene selectivity was 89.8%. No noticeable deactivation was detected after ten repeat catalytic runs were performed over the GaN catalyst. The results also indicated that the conversion of methane would be enhanced from 0.56% to 0.9% when the reaction temperature was increased from 723 to 823K. The selectivity to C6H6 would decrease from 89.9% to 80%, because further dehydrogenation of C6H6 would form polycyclic aromatics as well as coke. Mechanism (shown in Figure 2320) was proposed and calculated using DFT theory 79. Under UV irradiation, the adsorbed methane and GaN lattice (consists Ga3+ and N3-) turned into CH3- and H+. The photo generated electron from GaN semiconductor migrated to the surface. An electron reduced the proton into a H atom, then, two H atoms combined to release H2 gas. On the other hand, the photo generated hole would oxidize CH3- to form a methyl radical. Then, ethane was formed by the C-C coupling reaction. The dehydrogenation of ethane could produce ethylene. Benzene could be formed from ethylene through the cyclization and dehydrogenation processes. The GaN nanoparticles possessed a regular wurtzite crystal structure. This structure could be revealed by their electron diffraction patterns and enlarged TEM images (shown in Figure 24). The morphology of these powders was not quite uniform, and the BET surface



area of the GaN powders was 6.73 m2/g. Further optimization of the MDA reaction over novel catalysts are in progress to achieve higher CH4 conversion, C6H6 selectivity, and minimizing carbon deposits.

Figure 23. Schematic diagram for methane C-H bond polarization on the surface of GaN m-plane. 20 Reproduced with permission from reference 20. Copyright 2014 American Chemical Society.


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Figure 24. TEM image (a), schematic structure (b) and high resolutionTEM images (c and d) of GaN nanoparticles.79 Reproduced with permission from reference 79. Copyright 2014 John Wiley and Sons.

2.3 Reaction mechanism Several parts are included in this section, including characterization methods that used to study the reaction mechanism and catalyst deactivation mechanism. 2.3.1 Characterization methods and reaction mechanism Various of characterization methods and tools have been applied to investigate MDA reaction mechanism. Diffuse reflectance infrared spectroscopy (DRIRS), XPS and electron spin resonance (ESR) spectroscopy were applied to investigate the state of molybdenum in Mo/HZSM-5 (4wt%, Si/Al=30) catalyst for methane aromatization reaction

81

. Results

assumed that two oxidation states of Mo (6+ and 5+) existed in catalysts. Existence of (MoO2)2+ and (Mo2O5)2+ intermediates was also possible.



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NMR and Electron paramagnetic resonance (EPR) were applied to study the location of Mo5+ in the Mo/HZSM-5 during reaction 82. The results suggested that Mo5+ existed in the form of aluminum molybdate phase. However, another interesting view point was first raised by Kosinov et al.

83

They claimed that

MDA reaction is happened in a

monofunctional manner on conventional Mo loaded zeolite based catalysts. Mo carbide species are the active sites for CH4 conversion in MDA reaction. The specific 10MR structure of ZSM-5 provides shape selectivity to aromatics. The Brønsted acid sites only have the function to disperse Mo species into micropores. Ultrahigh field

95

Mo NMR

spectroscopy was also applied to identify the active center for MDA reaction.84 The results obtained from ultrahigh field

95

Mo NMR spectroscopy showed that neither bulk nor small

crystallites of MoO3 is the main Mo species during reaction. It was concluded that the active centers for MDA reaction are the carburized molybdenum species that derived from the exchanged Mo species. To investigate the functions of Mo carbide nanoparticles, IR vibrational spectra and DFT calculation were conducted

85,86

. The IR spectroscopic results demonstrated that Mo oxide

species mainly anchored on Al sites in framework and partly anchored on Si sites on the external surface of the zeolite. Results of DFT calculations suggested that Mo carbide species prefer to bind within two molybdenum atoms. Moreover, Mo carbide nanoparticles (C/Mo>1.5) were more stable on external Si sites. This is the reason why these Mo carbide nanoparticles were tending to migrate from pores to the zeolite external surface. The possible reaction mechanism was also obtained by present periodic DFT calculations, which was concisely described as CH4 → C2H6 → C2H4 → C6H8→ C6H6 86.


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Operando time-resolved combined XRD and high energy resolution fluorescence detection X-ray absorption near-edge spectroscopy (HERFD-XANES) were used in the research. These techniques were applied under operando conditions to investigate the nature of Mo species in zeolite. The results are demonstrated in Figure 25. Along with the result from X-ray emission spectroscopy (XES), indicatig that isolated Mo-oxo species were converted by CH4 into metastable MoCxOy species. These Mo-oxo species were responsible for the formation of C2Hx/C3Hx. Then MoC3 started to form during further carburization, and its presence coincides with benzene formation. the possible reasons for the decrease in catalytic performance were also revealed by fluorescence-lifetime imaging microscopy. The main reason was that MoC3 was sintered and large hydrocarbons were accumulated on external surface of the catalyst 87.

Figure 25. Evolution of Mo species during MDA reaction. Copyright 2016 John Wiley and Sons.

87

Reproduced with permission from reference 87.

Different atmospheres (artificial air, He, CH4/He mixture) were applied during activation period of MDA. The effect of different atmospheres on Mo/ZSM was studied88. Catalyst carburized in CH4 showed higher selectivity to aromatics and lower deactivation rate phase was formed during heating in methane. Then less Mo oxide species could have migrated in the micropores to make the Brønsted acid sites inaccessible.



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Gao et al.89 applied density functional theory calculation and multiple spectroscopic techniques to determine the anchoring sites and identity of Mo species in catalysts for the methane aromatization reaction. Computational results and experiment observations (Figure 26) indicated that the Mo oxide initially exhibited as a single Mo atom, which would then agglomerate and form carbided Mo nanoparticles. Gas phase oxygen treatment was also employed to adjust the distribution of Mo nanostructures and restore catalytic ability. The investigation proposed that catalytic performance of the regenerated catalyst could be improved and higher than the fresh catalyst by repeat regeneration cycles.

Figure 26. Operando Raman spectra for Mo/ZSM-5. (A) fresh catalyst, (B) catalyst during reaction, and (C) catalyst after regeneration.89 Reproduced with permission from reference 89. Copyright 2015 The American Association for the Advancement of Science.

Several detailed reaction mechanisms for MDA reaction have also been raised in the past several years. In 2012, an elementary step based kinetic model of MDA mechanism over Mo-based zeolite catalysts had been constructed (shown in Figure 27)

15

: CH4 was first

dimerized into ethane on molybdenum sites, and then ethane was generated into C6H6 on acid

sites.

The

mechanism

includes

several

steps:


chemisorption,

desorption,

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oligomerization, β-scission, hydride transfer, protolyti dehydrogenation and hydrogenation, protolysis, alkylation and dealkylation of C7H8 and C10H8. From the observed results, differences in catalytic performances between Mo/MCM-22 and Mo/HZSM-5 were noticed. It was assumed that these differences mainly resulted from topological effects instead of differences in acidity. After a few years in 2015, based on this mechanism, Karakaya et al.18 developed an improved chemical kinetics reaction mechanism. Four methane activation reactions on Mo2C sites and reactions of hydrogen and ethylene happened on Brønsted acid sites (46 reaction steps) were included in this mechanism. In 2016, this same group 21 further modified this mechanism. They developed and validated a microscopically reversible 54-step detailed reaction mechanism for MDA chemistry through the measured packed-bed performance. This detailed mechanism satisfies entropic and enthalpic consistency.

Figure 27. Reaction mechanism for MDA over Mo/HMCM-22.15 Reproduced with permission from reference 15. Copyright 2012 Elsevier.

2.3.2 Catalyst deactivation mechanism



TGA, XPS and TPO were applied to explore the pathway of coke formation on the catalyst.90 There are three deactivation stages undergone during the during the reaction. These stages are the first 10 min, 10-100 min and 100-160 min of reaction. In the second stage, the selectivity to benzene remained high and constant, then decreased rapidly in the last stage. Coke formation in the third stage was observably higher than the second stage, resulting from the formation of C2H4 in the narrowed zeolite channels and cracked to coke. Three layers of Mo/HZSM-5 catalyst were loaded in fixed bed reactor to investigate the coking pathway for MDA reaction.91 The reaction was carried out at 1073K and an additional H2 feed was induced in the system to restrain coke formation. The results obtained from TPO, XPS and Raman tests confirmed that two kinds of coke were formed during the reaction. They are aromatic-type coke and graphite-like C. An additional H2 feed effectively suppressed the formation of aromatic-type coke. Three possible coke formation pathways were proposed and shown in figure 28. The coke formation on the three layers was nonuniform, and C2H4 was the main coke source. The cracking of C2H4 (shown as route ②) was confirmed as the dominant pathway for coke formation. The results also confirmed that cracking or/and polycondensation of the formed aromatics (shown as route ③) was not the major route for coke formation. The authors further proposed that it is possible that the free radical chain reactions (shown as route ①) could contribute to coke formation. However, further research need to be done to confirm this assumption.


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Figure 28. Schematic diagram of proposed coking pathways.91 Reproduced with permission from reference 91. Copyright 2015 Elsevier.

UV Raman spectroscopy, TGA, STEM-HAADF and TEM were applied to study the deactivation effect on Mo/HZSM-5 and silylated Mo/HZSM-5 in the MDA reaction

92

.

Catalytic results demonstrated that carbonaceous layer formed on the zeolite surface caused the deactivation of catalysts. The carbonaceous layer was formed by polyaromatic hydrocarbons. In addition, the carbonaceous layer would decrease the accessibility of the Brønsted acid sites. This may have resulted in the decrease of methane conversion rates and shifted the selectivity from C6H6 to olefinic intermediates. Moreover, silylation treatment would led to lower coke formation rate at the external surface of catalyst and slowed down deactivation rate. 2.4 Catalyst regeneration Severe carbon deposition on catalyst is the major defects in MDA reaction. Researchers have developed several approaches to extend catalyst life. Treating a spent catalyst under oxidative atmosphere at a higher temperature is the most conventional way to regenerate a coked catalyst. Some other researchers proposed to use H2 to remove the deposited coke. This method has been demonstrated to be effective. Catalyst can maintain a high activity under periodic CH4-H2 switching operation modes93.



2.4.1 Catalyst regeneration under oxidative atmosphere Martinez and co-workers94 proposed an effective procedure to prolong the lifespan of Mo/ZSM-5 catalyst. For this procedure, they combined reaction period and regeneration period (10 or 21 vol. % of O2 in N2) in a continuous cyclic mode. Figure 29 showed CH4 conversion and selectivity to aromatics with generation steps. This procedure could increase the benzene yield and selectivity. For instance, yield of benzene could increase from 33g/h•kgcat (for a single 18 h test) to 97g/h•kgcat in a reaction with 12 cycles.

Figure 29. Methane conversion (A) and selectivity to aromatics (B) for Mo/ZSM-5 vs. TOS for six reaction– regeneration cycles.94 Reproduced with permission from reference 94. Copyright 2015 Royal Society of Chemistry.

Kosinov95 raised a novel isothermal (973K) reaction – air regeneration protocol. Research of Mo/HZSM-5 catalyst regeneration at high reaction temperature by air has been reported. Structure and textural stability of Mo/HZSM-5 (Mo 1-8 wt.%) in air at elevated temperatures (773-973K) was studied. The results showed that Mo/HZSM-5 with a low Mo loading (Mo 1-2% wt.%) exhibited high oxidative stability. This feature makes low Mo loading catalysts suitable candidates for this novel catalyst regeneration method. Further,


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optimized Mo/HZSM-5 catalyst can retain more than 50% of its initial activity after 100 reaction-regeneration cycles, which is as long as one week on stream. Kosinov and co-authors proposed a catalyst regeneration method, which is periodically supply short pulses of O2 to oxidize coke formed on Mo/HZSM-5 catalyst.96 By optimizing the frequency of oxygen pulse, benzene yield achieved two times higher compared to the control test without oxygen pulse in methane feed. The coke formation rate was also decreased four times compared to control test. The results further demonstrated that O2 mainly reacts with molybdenum carbide species, and does not cause framework damage and loss of molybdenum. 2.4.2 Catalyst regeneration under CH4-H2 switch operation mode Zhang and co-workers investigated the effect of periodic CH4-H2 switch operation mode in MDA reaction. Their research was conducted on various catalysts and in different kinds of reactors

91,97,98,99,100,101

. In 2011, Zhang et al. carried out a research

100

to study catalytic

behaviors of regeneration (CH4-H2 switch) cycles over Mo/HZSM-5 catalyst (5wt% Mo, Si/Al=30) in a two-bed circulating fluidized bed system. The reaction occurred at 1073 K with space velocities of 21080 and 40000 mL/g•h. The results revealed that more than 50 wt% of the coke formed in the initial 2 minutes of the reaction. The results also indicated that 20 minutes of H2 exposures should be necessary to eliminate most coke formed during the 5-10 min methane exposure. In 2012, binder-added and binder-free Mo/HZSM-5 catalysts were prepared

99

. These

catalysts were tested under a CH4-H2 switch mode at 1073 K and 10000 mL/g•h. The



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results indicated that binder added Mo/HZSM-5 showed higher mechanical strength. However, it could also lead to an obvious decrease in C6H6 selectivity. This could because that an addition of binder accelerated the polyaromatization of aromatics to coke on the surface of catalyst. Later in 2013, they prepared a series of Fe-Mo/HZSM-5 (0.1-2.0wt% Fe, 5wt% Mo) with co-impregnated modification method. Three kinds of HZSM-5 zeolites with different sizes were applied 98. Catalytic evaluation tests were carried out in a CH4-H2 switch mode. It was suggested that 0.5 wt%Fe-5 wt%Mo/HZSM-5 (nano-sized HZSM-5) showed better activity and stability. They also studied effect of different CH4/ H2 regeneration cycle periods: 5 min CH4-5 min H2, 5 min CH4-10 min H2 and 5 min CH4-20 min H2

97

.

Experimental data (shown in Figure 30) revealed that Fe species could remarkably increase catalyst stability under a 5 min CH4-20 min H2 regeneration cycle. Moreover, results indicated that the reactivation of Fe nanoparticles required at least 20 min H2 exposure to eliminate most of the coke on the catalyst surface formed during a 5 min CH4 exposure.

Figure 30. Time-dependence of CH4 conversion (a), aromatics formation rates (b) and benzene selectivity (c) obtained over catalysts at 1073 K and GHSV = 10000 h-1 under periodic CH4-H2 switching modes. 97 Reproduced with permission from reference 97. Copyright 2014 Royal Society of Chemistry.


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In the same year, binder-free spherical-shaped, 17wt% binder-added cylinder-shaped and self-prepared Mo/HZSM-5 catalysts (6wt% Mo, Si/Al=15) were prepared. Catalysts were evaluated in fixed-bed and fluidized bed reactors. The reactions were carried out at 10731173 K with the space velocity of 10000 mL/g•h under CH4-H2 switch regeneration mode 101

. For reactions conducted in the fixed bed reactor, indiciating that both binder-free

spherical-shaped catalyst and self-prepared Mo/HZSM-5 exhibited good catalytic performance. Bao and co-workers 102 also applied the periodic switch procedure of CH4 (15 min, 1500 mL/g•h) and H2 (45 min, 2400 mL/g•h) in the stability test for MDA catalyst at 1033-1073 K (shown in Figure 31). Molybdenum modified nano-sized HZSM-5 zeolite (5.5wt% Mo, Si/Al=20) was used in this research. The catalyst showed long-term stability, higher CH4 conversion (16%) and aromatic yield (exceeded 10%) during a 1000 hours’ reaction. The results showed that treat the catalyst with periodic hydrogen input could suppress coke formation effectively by eliminating the aromatic-type coke.



Figure 31. Stability test (1000h, CH4/H2 switch mode, 1099-1073K) of Mo/HZSM-5 catalyst in MDA reaction (1) CH4 conversion, (2) C6H6 yield, (3) aromatic yield, (4) C2H4 selectivity, (5) C6H6 selectivity, (6) C10H8 selectivity, (7) coke selectivity.102 Reproduced with permission from reference 102. Copyright 2015 Elsevier.

Recently, song et al93 studied the deactivation mechanism for Mo/ZSM-5 catalyst in MDA process. The periodic switch mode of CH4 (5min)-H2 (5min) were carried out under 1073 K and a space velocity of 12000 mL/g•h. Besides the already known conclusion that periodic mode of CH4-H2 could significantly improve the catalyst stability and lifespan, coke accumulation and deactivation behavior were also studied. Two stages of deactivation were revealed and the low-temperature burning coke was confirmed as the major type of coke during the reaction and located in the channel of zeolite. Another kind of coke, graphitelike C was confirmed accumulated on the external surface and channel openings of the zeolite. 2.4.3 Catalyst regeneration with H2 co-fed method Besides using CH4-H2 switching operation modes to regenerate catalysts, H2 co-fed in MDA reaction is also an effective way to suppress coke formation. Zhang 91 synthesized a Mo/HZSM-5 micro zeolite catalyst by wet impregnation method. Three layers of catalyst were packed in a fixed-bed reactor. TG and TPO techniques were applied to investigate the distribution of coke formation in the reaction with H2 co-fed with CH4. Experimental data indicated that coke distribution in catalyst layers was non-uniform. The coke content in inlet layer was higher than outlet layer. An increasing of H2 concentration in feed could suppress the aromatic coke formation, as well as reduce the formation rate of graphite-like C. Catalytic performances also revealed that the outlet layer of catalyst showed highest


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aromatic concentration. Increasing the height of catalysts in reactor from the first layer to the third layers would result in an evidently decrease in the C2H4 concentration. More results demonstrated that the intermediate C2H4 could be the main coke source during the reaction. Most of the C2H4 consumed was converted to coke instead of aromatics. 3 Other reactors applied in MDA reaction Most of researches focus on fixed bed reactor in MDA reaction. However, the process conducted by fixed bed reactor met two obstacles that challenge its industrial application. Firstly, per-pass conversion rate of CH4 was limited by thermodynamics. Secondly, activity of catalyst decreased rapidly due to the accumulation of coke 23. In this case, other kinds of reactors, including fluidized bed reactor and membrane reactor started to be applied in MDA researches in order to overcome these barriers. Although fixed bed reactor shows disadvantages in industrial utilization. It is still the most widely applied reactor for MDA studies. Thermodynamic limitation could be improved by the method that coupling MDA with chemical looping. Based on a recent research, H2 could be separated from the products by regeneration on the Fe3O4/FeO redox pair. As a result, the yield of aromatics was significantly increased compared to current single-pass reaction

103

. In this section, other kinds of reactors including fluidized bed

reactor, plasma reactor as well as membrane reactor applied in MDA reaction are discussed. 3.1 Fluidized bed reactor and plasma reactor applied in MDA reaction



Due to rapid deactivation of catalysts caused by coke formation, some researchers believed that MDA reaction should be conducted under continuous catalyst regeneration mode. In this case, fluidized bed reactor was introduced in the research 22,104. Xu et al.

22

applied a circulating fluidized bed reactor (CFBR) system (shown in Figure

32) and fluidizable Mo/HZSM-5 catalyst in the research. This CFBR system allows easy and continuous regeneration of catalyst in the reaction. This system has proven to maintain a 4-day test with little fragmentation on the catalyst. The same research group also applied the CFBR system to study stability of Mo/HZSM-5 catalyst under CH4-H2 mode.100 The catalytic performances of catalyst in this fluidized bed system were discussed and included in the chapter 2.4.2. In another research, a triple-bed CFBR system (shown in Figure 33) was proposed to achieve two goals, which are continuous regenerating Mo/HZSM-5 and converting methane to benzene.

Figure 32. Schematic diagram of CFBR: (1)reactor; (2) catalyst regenerator. 22 Reproduced with permission from reference 22. Copyright 2017 Elsevier.


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Figure 33. Schematic diagram of a triple-bed CFBR system: (1) CH4 converter; (2-3) catalyst regenerator.100 Reproduced with permission from reference 100. Copyright 2011 Elsevier.

Since the C-H bond in CH4 molecule could be broken by high energy free electrons. Plasma reactor has been attracted more attention in the last few years in MDA research. A packed bed dielectric barrier discharge (DBD) plasma–catalyst hybrid system was applied by Park to test Mo/HZSM-5 catalyst in MDA reaction105. The results revealed that CH4 could be activated by plasma at 773K, which is much lower than conventional reaction temperature (973K). For MDA reaction conducted in DBD plasma-catalyst hybrid system at 773K, methane conversion rate was 4.9%, benzene selectivity was 8.7%. Both methane conversion rate and selectivity to benzene obtained by DBD plasma-catalyst hybrid system were lower compared to results obtained by fixed bed reactor (methane conversion: 7.8%, benzene selectivity: 53.9%). A two-stage PFC (plasma-followed-by-catalyst) reactor was also used to study MDA process.106 Ni/HZSM-5 catalyst was evaluated in this plasma reactor at 623K under atmospheric pressure. In the first stage, CH4 was converted to C2H2 by plasma. C2H2 was then trimerized on Ni/HZSM-5 in the second stage. Selectivity to



aromatics could achieve 47% over 1 wt%Ni/HZSM-5 catalyst, and CH4 conversion was about 35%. This study showed a promising opportunity to overcome the thermodynamic barrier in MDA reaction by utilizing plasma technology. 3.2 Membrane reactor applied in MDA reaction Per-pass conversion is limited by thermodynamics, and conversion of CH4 is a major obstacle in MDA process. To overcome this limit, it is necessary to selectively remove H2 in products from the reactor during reaction. Both experimental and simulation studies have been conducted by several groups 23, 24, 107, 108. A current controlled co-ionic membrane reactor integrated with an electrochemical BaZrO3-based membrane (shown in Figure 34) was applied to remove coproduct H2 during the reaction

23

. This membrane exhibits oxide ion and proton conductivity. better catalyst

stability and higher aromatic yield were achieved. The promotion effects were due to the removal of H2 and injection of oxide ions along the whole reactor. This co-ionic reactor also showed high carbon efficiencies (80%), thus improved viability of the technoeconomic process. In the co-ionic membrane reactor, the coke content in spent Mo/MCM-22 catalyst at TOS=45 h was about 0.5gcoke/gcatalyst. The coke deposition is much lower than that of fixed bed reactor, which is about 0.9gcoke/gcatalyst.


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Figure 34. Current controlled co-ionic membrane reactor. 23 Reproduced with permission from reference 23. Copyright 2016 The American Association for the Advancement of Science.

Xue et al. 109 developed a dense ceramic hydrogen-permeable membrane reactor (shown in Figure 35) to study MDA performance. The 6 wt%Mo/HZSM-5 catalyst was coated around outside of the fiber membrane. The fiber membrane (La5.5W0.6Mo0.4O11.25−δ, LWM0.4) served to remove H2 to the core side of the fiber. H2 was swept away by CO2 or Ar flowing in the interior of the reactor. The results showed that 40-60% of the H2 were extracted during reaction. The yield of aromatics could be increased at the beginning of the reaction by ~50-70% when compared to reaction conducted in the fixed-bed reactor. However, due to the removal of hydrogen, 10% more coke were formed than fixed-bed reactor.



Figure 35. Schematic diagram of membrane reactor. Copyright 2016 American Chemical Society.

109

Reproduced with permission from reference 109.

Hydrogen-permselective palladium membrane reactor (shown in Figure 36) was developed and applied in MDA research.110 The inner tube of the reactor was the retentate side, and the feed gas flowed through it. The outer tube of this reactor was the permeate side, and Ar sweep gas was used to recover hydrogen permeated through the membrane. At TOS=15h, the yield of benzene obtained by membrane reactor (2.05%) was much higher than that of fixed bed reactor (0.57%). Furthermore, the selectivity to naphthalene was also significantly suppressed in the membrane reactor compared to the results obtained by using fixed bed reactor (from 13.4% to 3.0%). During the reaction, an addition amount of H2 was generated in the retentate side of the reactor. In this case coke accumulated on the catalysts was inhibited.


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Figure 36. Membrane and catalyst testing system (GC: gas chromatograph, MFC: mass flow controller, PCV: 110 pressure control valve, PT: pressure transducer, TC: thermocouple) Reproduced with permission from reference 110. Copyright 2015 Royal Society of Chemistry

Besides Mo/HZSM-5 catalyst, Fe©SiO219 catalyst developed by Bao and co-authors was tested in the hydrogen-permeable tubular membrane reactor (shown in Figure 37) for the first time.111 The hydrogen-permeable membrane (SrCe0.7Zr0.2Eu0.1O3-δ) was supported on the surface of the SrCe0.8Zr0.2O3-δ tube. The results showed that removing H2 from nonoxidative methane conversion reaction could result in a remarkable improve in CH4 conversion. During the reaction, a 30% methane conversion rate was achieved, and the reaction showed 99% selectivity to C2, benzene and naphthalene.



Figure 37. H2 permeable tubular membrane reactor and experimental setup for non-oxidative CH4 conversion reaction. (A) As-prepared SrCe0.8Zr0.2O3-δ membrane tube, (B) SEM image showing the crosssectional image of membrane tube reactor comprised of SrCe0.7Zr0.2Eu0.1O3-δ thin film on the porous SrCe0.8Zr0.2O3-δ tubular support, and (C) assembly of H2 permeable membrane reactor for non-oxidative CH4 conversion reaction over Fe©SiO2 catalyst111 Reproduced with permission from reference 111. Copyright 2016 John Wiley and Sons

Removal hydrogen by hydrogen permeable membrane could result in accelerated coking on catalyst, and a well distributed and continuously feed of oxygen could lead to a better coking resistance ability of the catalyst.112,113 In this case oxygen-permeable membrane reactor (Figure 38) was suggested for MDA reaction.113 The material of the oxygen transporting membrane is Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) perovskite. Higher yield of aromatics (after 200 min, 30% higher) were obtained in the membrane reactor than that of fixed-bed reactor. Besides higher yield of aromatics, catalyst was more durable in the membrane reactor. The selectivity to coke in the fixed-bed reactor was higher than 60%. However, in the membrane reactor, it was only around 10%.


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Figure 38. MDA in a fixed bed reactor and a membrane reactor. reference 113. Copyright 2013 John Wiley and Sons.

113

Reproduced with permission from

Besides experimental research, simulation study was also carried out. Corredor et al.107 published a simulation study of MDA reaction in a plug-flow isothermal membrane reactor model. This simulation was conducted by Aspen Plus using CAPE-OPEN to CAPE-OPEN simulator. Kee et al.108 computational simulated MDA process that carried out in H2 removal membrane-coupled packed-bed reactors. Influence of operating conditions including H2 removal and operating pressure were studied. The simulation results showed that, when the pressure increases, CH4 conversion rate and C6H6 selectivity decrease, and the production rate of C6H6 increases. The research also revealed that when membrane was coupled with the packed bed reactor, conversion rate of methane significantly increased. However, selectivity to benzene decreased since higher yield of other undesired aromatics formed during the reaction.



4 Conclusions In MDA reaction, catalytic performance of Mo-based catalysts are influenced by several factors. These factors include the state of the Mo species, structure and acidity of zeolite supports. The main drawback of Mo-based catalysts was the severe coke and/or polyaromatics formed on the catalysts. Coke deposition could block zeolite channels and lead to the relatively low stability. Since the structure of HZSM-5 zeolite played a shapeselective role to aromatics,Mo-based zeolite catalyst system is still considered as one of the most active catalysts in MDA research so far. In the meantime, novel catalysts such as Fe©SiO2 catalyst and GaN nanoparticles showed promising activity and excellent stability. These novel catalysts may lead to a new research direction of catalyst preparation. Areas for further research should focus on increasing life-span of catalysts, development of novel catalysts, in-depth study of MDA reaction using fluidized and membrane reactors, and conduct more computational analysis to guide the development of reaction mechanism. Acknowledgement Authors thank Dr. Lucia M. Petkovicfor her valuable comments. Supporting Information Summaries of catalytic performance of catalysts in MDA reaction References (1)

Tang, M.; Xu, L.; Fan, M. Progress in Oxygen Carrier Development of MethaneBased Chemical-Looping Reforming: A Review. Appl. Energy 2015, 151, 143.

(2)

Xu, L.; Liu, Y.; Li, Y.; Lin, Z.; Ma, X.; Zhang, Y.; Argyle, M. D.; Fan, M. Catalytic CH4 Reforming with CO2 over Activated Carbon Based Catalysts. Appl. Catal. A Gen. 2014, 469, 387.


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Liu, H.; Wang, B.; Fan, M.; Henson, N.; Zhang, Y.; Towler, B. F.; Gordon Harris, H. Study on Carbon Deposition Associated with Catalytic CH4 Reforming by Using Density Functional Theory. Fuel 2013, 113, 712.

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Schwach, P.; Pan, X.; Bao, X. Direct Conversion of Methane to Value-Added Chemicals over Heterogeneous Catalysts: Challenges and Prospects. Chem. Rev. 2017, 117, 8497.

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Karakaya, C.; Kee, R. J. Progress in the Direct Catalytic Conversion of Methane to Fuels and Chemicals. Prog. Energy Combust. Sci. 2016, 55, 60.

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Ismagilov, Z. R.; Matus, E. V.; Tsikoza, L. T. Direct Conversion of Methane on Mo/ZSM-5 Catalysts to Produce Benzene and Hydrogen: Achievements and Perspectives. Energy Environ. Sci. 2008, 1, 526.

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Tian, B.; Qiao, Y.; Fan, J.; Bai, L.; Tian, Y. Coupling Pyrolysis and Gasification Processes for Methane-Rich Syngas Production: Fundamental Studies on Pyrolysis Behavior and Kinetics of a Calcium-Rich High-Volatile Bituminous Coal. Energy & Fuels 2017, 31, 10665.

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Wang, L.; Tao, L.; Xie, M.; Xu, G.; Huang, J.; Xu, Y. Dehydrogenation and Aromatization of Methane under Non-Oxidizing Conditions. Catal. Letters 1993, 21, 35.

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Velebná, K.; Horňáček, M.; Jorík, V.; Hudec, P.; Čaplovičová, M.; Čaplovič, L. The Influence of Molybdenum Loading on Activity of ZSM-5 Zeolite in Dehydroaromatization of Methane. Microporous Mesoporous Mater. 2015, 212, 146.

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Progress in Nonoxidative Dehydroaromatization of Methane in the

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