Catalytic Methane Dehydroaromatization with Stable Nano Fe Doped

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Catalytic methane dehydroaromatization with stable nano Fe doped on Mo/HZSM-5 synthesized with a simple and environmentally friendly method and clarification of a perplexing catalysis mechanism dilemma in this field for a period of time Kaidi Sun, Weibo Gong, Khaled Gasem, Hertanto Adidharma, Maohong Fan, and Rui-Ping Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02213 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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Industrial & Engineering Chemistry Research

Catalytic methane dehydroaromatization with stable nano Fe doped on Mo/HZSM-5 synthesized with a simple and environmentally friendly method and clarification of a perplexing catalysis mechanism dilemma in this field for a period of time Kaidi Suna, Weibo Gonga, Khaled Gasema,b, Hertanto Adidharmaa,b, Maohong Fana,b,c *, Ruiping Chena,d ** a

Department of Chemical Engineering, University of Wyoming, Laramie, WY, 82071, USA

b c

Department of Petroleum Engineering, University of Wyoming, Laramie, WY, 82071, USA

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

d

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China

Corresponding Author Email: [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: Dehydroaromatization of methane to aromatics provides a promising approach for cost-effective conversion of methane. This work was designed to study the promoting effect of nano-Fe on Mo/HZSM-5 catalyst in methane dehydroaromatization(MDA). Mo/HZSM-5 and nano-Fe modified catalysts were synthesized, then evaluated in a fixed-bed reactor along with an integrated on-line gas chromatography and a mass spectrometry system. The fresh or/and spent catalysts were characterized by ICP-MS, H2-TPR, CH4-TPSR, NH3-TPD, XRD, SEM, TEM, N2 adsorption/desorption, TGA, and DRIFT Spectroscopy. Nano Fe doped Mo/HZSM-5 catalysts prepared with an innovative method shows enhanced MDA performances especially stability. Moreover, research demonstrates that carbon nanotubes can not only form on Mo/HZSM-5 catalyst without the addition of Fe but also exist for a long time, which clarifies carbon nanotubes formation mechanism. It was concluded that nano Fe play an important role in promoting the growth of carbon nanotubes, and thus the activity and stability of the catalysts. Keywords: methane dehydroaromatization, catalysis, Mo/HZSM-5, nano-Fe addition, carbon nanotube


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1. Introduction The conventional and unconventional sources of natural gas, including shale gas, tight gas, coal bed methane and methane hydrates are abundant and offer a more environmental friendly alternative to crude oil. Therefore, they are suited for conversion into liquid fuels or other value-added chemicals of industrial interest1. Natural gas, which is about 90% methane, has been widely investigated as raw material for producing fuels and chemicals. In this regard, indirect utilization of methane is the most intensively applied method of conversion, where natural gas is converted to synthesis gas (CO+H2) through steam reforming, dry reforming, autothermal reforming or partial reforming. The resulting syngas can be then used to produce value-added chemicals like methanol and dimethyl ether (DME), or it can be converted to liquid fuels using Fischer-Tropsch reactions. However, since methane is quite stable, severe reaction conditions are necessary to activate the C-H bond, which requires high-energy consumption. Moreover, the preparation and compression of syngas accounts for up to 60%-70% of the capital cost2. As such, researchers are paying more attention to direct methane conversion, which has an economic advantage over indirect methods. Methane can be directly oxidized to produce methanol and formaldehyde or form ethylene by oxidative coupling (OCM). These processes are thermodynamically favorable with the assistance of oxidants. However, the hydrocarbon products are much easier to be further oxidized to CO2 and H2O in the presence of oxygen, which significantly reduces the selectivity and production. The low yield of products is the very reason why these methods have not been commercialized. As an alternative, methane dehydroaromatization is a promising route for converting methane into aromatics. Here, methane is converted to aromatics in the absence of O2, where further oxidation is avoided to ensure a higher selectivity and production. Since the pioneering work of Wang et al.3 in 1993, non-oxidative methane aromatization over molybdenum-loaded catalysts have been intensively studied. This approach has yielded high selectivity for the formation of aromatic hydrocarbons. Further, the specific structure of the metal/zeolite systems and the acid sites responsible for the catalytic activity in methane dehydroaromatization have attracted much attention. Early research has showed that the HZSM-5 zeolite was the best support, and Molybdenum was the best loaded ion component for methane nonoxidative direct conversion to aromatics. Mo-modified ZSM3 ACS Paragon Plus Environment


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54,5,6, and MCM-227,8 have provided the best activity toward methane dehydroaromatization. However, the catalysts were rapidly deactivated by coke formation and loss of Mo at high temperature (700°C). Accordingly, development of catalysts for methane dehydroaromatization with improved catalytic performance and stability remains a significant need and a challenge. Some novel catalysts have been developed such as the semiconductor GaN, which converts photocatalytically methane to benzene at low temperatures or facilitates methane aromatization at high temperatures using thermal catalysis9. Beyond that, modification of the catalyst supports10 and metals co-loading on zeolites were suggested11. In the latter, a second metal component was introduced to Mo-loaded zeolite to improve the activity, selectivity and stability of the catalysts, according to previous investigations. The second component was mainly a member of the first transition metals, including Zn, W, Re, Cu, Mn, Ni, Cr, V, Ga, Fe and so on2. As an abundant, stable, and environmentally friendly metal, Fe presents a great potential for industrial application12. A novel catalyst with Fe on SiO2 (Fe©SiO2) was synthesized and proved to have high methane conversion ratio to aromatics and better selectivity for them13. It is noteworthy that neither coke nor CO2 was detected during the reaction running over the Fe©SiO2 catalyst, despite the relatively high reaction temperature. Further, doping Mo/zeolite catalysts with iron as an additional metal have been extensively reported

14,15,16

.

Previous investigation documented that adding a small amount of Fe enables nano-zeolite based Mo/HZSM-5 catalysts to maximize their activity and stability 17. Contradictory results were also found, which indicated that an addition of Fe to Mo/HZSM-5 catalyst exhibited higher carbon deposition and lower activity18, and a 2.5 wt% or 3 wt% of Fe addition could contribute to the lower dehydroaromatization activity to benzene15,18. What’s more, the effect of doping nano-sized iron directly on Mo/HZSM-5 catalysts have not been reported yet. In this case, this work focuses on the promotion effect of nano-sized iron (˂ 2 wt%) on Mo/HZSM-5 catalyst on catalytic performance and mechanism of carbon nanotubes formation during the methane dehydroaromatizaiton process. In this research, 5%Mo/HZSM-5 and nano-sized Fe doped 5%Mo/HZSM-5 catalysts were synthized. For the first time, nano Fe was directly doped on Mo/HZSM-5 for preparing catalysts, which is not only different from conventional corresponding catalyst preparation method but also simple and 4 ACS Paragon Plus Environment

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environmentally friendly due to the elimination of the increasingly concerned NO3- needed for conventional methane dehydroaromatization catalyst preparation method in form of Fe(NO3)3. Although promising progress has been made, further research on the formation and effects of carbon nanotubes on methane conversion, benzene selectivity and catalytic stability is still in progress and alternative catalysts are being explored. 2. Experimental 2.1 Catalyst preparation 2.1.1 Preparation of Mo/HZSM-5 catalysts NH4ZSM-5 was prepared starting with a commercial ZSM-5 zeolite (Zeolyst International, SiO2/Al2O3 ratio of 30, CBV 3024E, 0.2µm) using an ultrasonic method (UM). Fully exchanged NH4ZSM-5 was obtained by ion exchange of 5g ZSM-5 zeolite with 30ml aqueous solution of ammonium nitrate (NH4NO3, 1mol/L) under ultrasonic processing (QSONICA Q700 Sonicator with microtip probe supplied) at room temperature for 60 minutes, followed by drying at 353K overnight. Mo/NH4ZSM-5 catalyst was prepared according to the impregnation method (IM). Each 5g of NH4ZSM-5 zeolite was impregnated with 30ml of aqueous solution of ammonium molybdate tetrahydrate (Sigma-Aldrich, BioUltra) at room temperature. The resulting material was dried at 383K and then calcined in air for 4 hours using a furnace (Thermo Scientific BF51866A-1) at 773K. The freshly prepared Mo/HZSM-5 samples were then crushed and sorted into sizes of 40~60 mesh size for further investigation. 2.1.2 Preparation of Fe(nano)-Mo/HZSM-5 catalysts The appropriate amounts of nano-sized Fe powder (25nm avg. 99.5%, Sigma-Aldrich) and the freshly prepared Mo/HZSM-5 zeolite was physically mixed to prepare Fe(nano)-Mo/HZSM-5 catalysts using a mechanical ball mill, followed by drying in air at 383K for six hours. Finally, the Fe(nano)-Mo/HZSM-5 catalysts were sieved to sizes of 40~60 mesh for further evaluation. Chemical composition of all catalysts prepared for this study are determined by ICP-MS and listed in Table 1.



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Table 1. Chemical composition of catalysts used in this study. Mo (wt%)

Nano-Fe (wt.%)

(ICP-MS result)

(ICP-MS result)

5%Mo

4.83

0

0.25%Fe(Nano)-5%Mo

4.75

0.24

0.5%Fe(Nano)-5%Mo

4.72

0.51

1.0%Fe(Nano)-5%Mo

5.01

0.85

1.5%Fe(Nano)-5%Mo

5.15

1.48

Catalysts supported on HZSM-5

2.2 Catalyst evaluation Reactions were carried out for methane dehydroaromatization in a flow system shown as in the schematic drawing (Figure 1). Typically, 1.0g of catalyst was loaded in the fixed bed reactor center; thermocouple was installed at the inlet of the reactor and in the center of the catalyst bed for temperature control. And then a continuous down-flow of 25 cm3/min of Ar (UHP grade) at atmospheric pressure was established through the reactor. The temperature was increased to 700°C and kept at this temperature for 30 min. Afterwards, the Ar flow was stopped and a gas mixture of CH4 and Ar (used as internal standard for GC analyses) in a volumetric ratio of CH4: Ar=10:1 was introduced to the reactor at a flow rate of 27.5 cm3/min, corresponding to a space velocity of 1650 cm3/ (gcat·h). The flow rate of the reactor effluent was measured by SUPELCO Humonic Optiflow 520 Digital Volumetric Flowmeter. The composition of reactor exhaust was analyzed by Mass Spectrometer (MS, HIDEN HPR20) and an online gas chromatograph (SRI 8610C) which equipped with both thermal conductivity detector (TCD) and frame ionization detector (FID). H2, CH4, CO and CO2 were separated by a molecular sieve column (6' MOL SIEVE 13X) and analyzed by TCD. Mixtures including CH4, C2 species (C2H4 and C2H6), C6H6, C7H8 and C10H8 were separated by two parallel columns, a packed column (6' HAYESEP D) and a capillary column (MXT-1 60m), and analyzed by FID.


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Figure 1. Catalyst evaluation system for Methane Dehydroaromatization reaction. [1–Fixed bed reactor, 2–Gas chromatograph, 3– Mass spectrometer, 4–Computer 5–Digital Volumetric (Bubble), Flowmeter 6–Ar cylinder, 7–CH4 cylinder, 8–H2 cylinder, 9-N2 cylinder, 10–13 Mass flow controller, 14–Mass flow controller power supply/control module, 15–Temperature controller]

2.3 Catalyst characterization 2.3.1 Chemical properties characterization of fresh catalysts Chemical compositions of catalysts were determined by inductively coupled plasma mass spectrometry (ICPMS, PerkinElmer NexIon 350). For each catalyst sample, 0.03 g catalyst was mixed with lithium metaborate (LiBO2) and fused by heating at 1100℃ for 5 minutes in air atmosphere. The glass-like mixture was then stirred with nitric acid (5 wt%) for 20 mins and repeated 5 times to dissolve the mixture. The dissolved solution was then diluted with deionized water to 100 g, followed by diluting the resulting solution again with 10 ml deionized water to obtain testable sample for ICP-MS measurement. Concentrations of Mo and Fe of all catalysts determined by ICP-MS are listed in Table 1. 2.3.2 Physical properties characterization of fresh and/or spent catalysts Textural properties were determined from the nitrogen adsorption/desorption at -196°C using a Quantachrome Autosorb-iQ unit. Prior to measurements, catalyst samples were degassed under vacuum at 7 ACS Paragon Plus Environment


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350°C for 6 hours to remove surface humidity and pre-adsorbed gases before exposure to the nitrogen (N2) gas. The surface area was calculated from N2 isotherm data using the BET model, and the micropore volumes and mesopore volumes were evaluated using t-plot analysis and BJH analysis, respectively. Powder X-ray diffraction analyses of fresh and spent catalysts were performed on a Rigaku X-ray diffraction system utilizing Cu Kα1 radiation at 40 kV and 40mA. Samples were placed on a zero diffraction Si holder and were scanned from 5° to 50° (2θ) with a 5°/min scanning rate at room temperature. In addition, the morphologies of fresh and spent catalysts were studied on a scanning electron microscope (SEM, FEI, Quanta 250) and a transmission electron microscopy (TEM, FEI, Tecnai G2 F20 S-TWIN). 2.3.3 Temperature-programmed reduction (H2-TPR, CH4-TPSR) Temperature-programmed reduction in H2 (H2-TPR) tests were carried out using an automated gas sorption analyzer (Autosorb iQ) to study the reducibility of the Mo phases in the catalysts. About 0.1 g catalyst was loaded in a U-shaped quartz tube and heated under helium (He) atmosphere with a heating rate of 10 ℃/min to 500°C and kept for 30 min, then cooled to 30°C under the same atmosphere. The H2-TPR analysis was then performed under a hydrogen-nitrogen mixture (5 vol.% H2) environment at a flow rate of 60 ml/min and ramped from 30°C to 1000°C with the heating rate of 5°C /min. Along with H2-TPR, temperature-programmed surface reduction in CH4 (CH4-TPSR) tests were also carried out to study the reducibility of the Mo phases in the catalysts. The tests were conducted in a TGA-MS system. For each test, about 0.02 g catalyst was loaded. The system was first heated to 100°C under Argon atmosphere for 1 h. Then, CH4-TPSR test was performed under methane-argon mixture atmosphere (20 vol.% CH4) at a flow rate of 87.5 ml/min and ramped from room temperature to 900°C with the heating rate of 2°C/min. The signal of carbon monoxide (CO) was detected and recorded by a mass spectrometer (MS, HIDEN HPR20) during the reaction. 2.3.4 Thermo-gravimetric analysis (TGA) Thermo-gravimetric analysis was conducted using the SDT Q600 apparatus to evaluate the amount of coke deposits formed during the methane dehydroaromatization reaction. Around 10 mg spent catalyst sample was 8 ACS Paragon Plus Environment

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loaded onto an uncovered alumina sample holder, heated from 25℃ to 700°C at a rate of 5°C /min in O2/N2 mixed flow (100 ml/min, O2:N2=1:5), and then kept at 700°C for 30 min to obtain the TGA curve. 2.3.5 Acidity evaluation of fresh catalysts The acidity of catalysts was determined via DRIFT spectroscopy. DRIFT spectra were recorded using Nicolet iS50 FT-IR spectrometer with a resolution of 4 cm-1. Catalyst samples were loaded in the sample cell of the Harrick Praying Mantis diffuse reflection environmental chamber (HVC-DRM-5), degassed under vacuum and maintained at 450°C for 6 hours. Then the temperature was decreased to 150°C and a background spectrum was collected under vacuum. The degassed sample was exposed to pyridine vapor at 150°C for two hours. After fully absorbing pyridine, the sample was degassed again under vacuum at 150°C for two hours to physically remove the absorbed pyridine. After degassing the catalyst sample spectrum was collected. The density of Lewis and Brønsted acid sites were evaluated from the integrated areas of the bands at 1450 and 1540 cm-1, respectively 1. Temperature-programmed desorption of ammonium (NH3-TPD) tests were also applied to determine the acidity of catalysts. NH3-TPD tests were conducted using an automated gas sorption analyzer (Autosorb iQ) equipped with a thermal conductivity detector (TCD). For each test, about 12-13mg catalyst was loaded in a Ushaped quartz tube. It was heated under helium atmosphere with a heating rate of 10°C/min to 500°C and kept for 60 min. The sample was cooled to 100°C under He atmosphere. It was then flushed by ammonium-nitrogen mixture (10 vol.% NH3) at a flow rate of 60 ml/min for 60 min. The sample was then flushed by helium at a flow rate of 60 ml/min for 60 min to remove the physisorbed NH3. The temperature was ramped from 100°C to 650°C with the heating rate of 15 °C /min. 3. Results and discussion The conversion of methane, formation rate of aromatics, selectivities of all hydrocarbon products ( SCx H y ; x=2, y=4 or 6; x =6, y =6; x =7, y =8; x =10, y =8), coke content in the spent catalyst and the coke formation were calculated according to the following equations



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% = = " % =

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(1)

× 100

(2)

× !

(3)

× 100

(4)

"#$%& % = 1 − ∑ " ) +,-& . #/./01,. % = 2

2344

× 100

(5)

56476 8965 78

0$,, = ,-& . #/./01,. :&;$?/.>$ − ?@& /;.&< $=>?/.>$

(6)

#$%& = AB > − C AB $@. + 2 × F + 6 × H AH + 7 × J AK + 10 × LM AK N (7) where fCx H y (nmol/GC peak area) represents a calibration factor for each aromatic product, and this factor was determined by using an external calibration method; ACx H y represents GC peak area for each aromatic product. 3.1 Effect of nano-Fe additive on the catalytic performance of 5%Mo/HZSM-5 catalysts Conversion rates of methane for different catalysts are shown in Figure 2. It is quite clear that all the nano-Fe modified catalysts showed higher CH4 conversion rates than that of traditional 5%Mo/HZSM-5 catalyst in the methane dehydroaromatization reaction. For each catalyst sample, the high CH4 conversion rate (around 40%) achieved at the beginning of the reaction is attributed to the induction period of the reaction. During the induction period, CH4 was consumed and MoO3 transform into Mo2C species. In the first 5 hour of reaction, all nano-Fe modified catalysts showed similar methane conversions. At the fifth hour of reaction, the CH4 conversion rate increased with increase in the concentration of nano-Fe from 0.25 wt % to 1.0 wt %, and decreased when 1.5 wt % of nano-Fe was added. As shown in the Figures 3-6, the time dependences of benzene, toluene and naphthalene formation rates have been observed for the nano-Fe modified 5%Mo/HZSM-5 catalyst in methane dehydroaromatization reactions. For comparison, the time dependence of aromatic products of the unmodified 5%Mo/HZSM-5 catalyst was shown in the figure as well. The nano-Fe modified 5%Mo/HZSM-5 catalysts provided a positive effect on the stability and the production of aromatics in the activity evaluation of 10 ACS Paragon Plus Environment

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catalysts. All the catalysts provided very similar and close variations of benzene concentration with time; specifically, they showed rapid increases to their respective maxima in the first two hours followed by the slow decreases until the end of the tests after 50 hours. which is much longer than 10 hrs reported by other groups6,19. Comparatively, all nano-Fe modified 5%Mo/HZSM-5 showed higher yield of benzene than the unmodified catalyst in the first ten hours of the reaction. As seen in Figure 3, the 0.25%Fe(nano) -5%Mo/HZSM-5 catalyst obtained its highest benzene formation rate of 109.9nmol/ (gcat•s) when the methane dehydroaromatization reaction has been in progress for about 1 hour. This benzene yield was about 40% higher than that of the unmodified 5%Mo/HZSM-5. This may indicate that a small amount addition of nano-sized Fe species could significantly increase the formation rate of benzene and improve the activity of Mo/HZSM-5 catalyst. Meanwhile, in the case of the naphthalene formation rate in Figure 5, it is clear that all the catalysts had similar trend in naphthalene formation: conversions reached their maxima at about 60 min of reaction, and then they decrease with increasing time on stream (TOS) until the end of the testing time. Furthermore, all iron-containing catalysts showed lower naphthalene yield than 5%Mo/HZSM-5 catalyst in methane aromatization reaction. However, the results in Figure 4 indicated that as more nano Fe species were introduced to the 5%Mo/HZSM-5 catalysts, the maximum of toluene formation rate slowed down and decrease with TOS in the evaluation reaction. Figure 6 indicates that among all evaluated catalysts, the 0.25%Fe(nano) -5%Mo/HZSM-5 catalyst had the highest formation rate of aromatic products in the first ten hours of reaction. Its formation rate of aromatics reached a maximum value of 175.3 nmol/ (gcat•s) at 1.5h after the reaction started. Further, after about 10 hours, a formation rate of aromatics of 100 nmol/(gcat•s) could still be obtained. The 1.0%Fe(nano)-5%Mo/HZSM-5 catalyst showed a higher methane conversion ratio than the other evaluated catalysts in the methane dehydroaromatization reaction that lasted for 50 hours; nevertheless, the 1.5%Fe(nano)-5%Mo/HZSM-5 catalyst had a better formation rate of aromatics until the end of the reaction. The catalytic activity and formation of aromatics observed in this work is comparable to the result reported in the literature 16. Herein, the highest benzene formation rate was about 277 nmol/(gcat•s) with a methane space velocity of 50 cm3/ (gcat·min) 11 ACS Paragon Plus Environment


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for 0.25%Fe-5%Mo/HZSM-5 catalyst. In the meantime, the formation rate of benzene in this research is higher than that published previously20. In this previous research, the average formation rate of benzene over CoMo/ZSM-5 catalyst between 100 and 400 min of time on stream was 41.3 nmol/ (gcat•s)20. Moreover, both the energy consumption and cost of catalyst were higher than that experienced in this research. Formation rates of C2 species during the catalytic evaluation tests are shown in Figure 7. All catalysts showed similar C2 formation in the first 10 h of reaction. Later, the 1.5% Fe(nano)-5%Mo/HZSM-5 catalyst showed highest C2 formation rate and 5%Mo/HZSM-5 catalyst exhibited lowest C2 formation rate till the end of the evaluation test. Formation rates of coke for all the tests are shown in Figure 8. 5Mo/HZSM-5 catalyst exhibited lowest coke formation rate and 1.5% (nano)Fe-5%Mo/HZSM-5 catalyst showed highest coke formation rate. As such, addition of nano-Fe species to Mo/HZSM-5 catalysts significantly improve both the activity and stability of catalyst performance in the methane dehydroaromatizaiton reaction. These performance enhancements could be attributed to the carbon nanotube/nanofiber detected by SEM/TEM on the spent catalysts21.

Figure 2. Conversion rate of CH4 over Fe(nano)-5%Mo/HZSM-5 catalysts.


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Figure 3. Formation rate 5%Mo/HZSM-5 catalysts.


of

benzene

over

Fe(nano)-

Figure 4. Formation rate of toluene over Fe(nano)-5%Mo/HZSM5 catalysts.

Figure 5. Formation rate of naphthalene over Fe(nano)5%Mo/HZSM-5 catalysts.

Figure 6. Formation rate 5%Mo/HZSM-5 catalysts.

Figure 7. Formation rate of C2 over Fe(nano)-5%Mo/HZSM-5 catalyst

Figure 8. Formation rate of coke over Fe(nano)-5%Mo/HZSM-5 catalyst


of aromatics over

Fe(nano)-


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The selectivity of all hydrocarbon products (benzene, toluene, naphthalene, C2 species) and coke was calculated and shown in Figure 9. Since all catalytic evaluation tests lasted for 50 hours and selectivity of aromatics, C2 species and coke change with increasing TOS, several TOS points including 1, 5, 10, 20, 30 and 40 hours were chosen to analyze the change of the selectivity with TOS. As shown in Figure 9 at the beginning of the reaction (TOS=1), 0.25%nano-Fe added catalyst showed the highest benzene selectivity among all catalysts, which is slightly higher than that of 5%Mo/HZSM-5 catalyst. Later (TOS=5, TOS=10), 5%Mo/HZSM-5 catalyst exhibited the highest selectivity to benzene and aromatics. After about 20 hours, 1.5%nano-Fe added catalyst showed the highest selectivity to benzene as well as other aromatics till the end of reaction. If the reaction continues, the catalytic activity of all catalysts will continue decrease and the performance of all catalysts would be similar. 5%Mo/HZSM-5 catalyst exhibited highest selectivity to C2 species throughout the reaction. 5%Mo/HZSM-5 catalyst also showed lowest selectivity to coke during the reaction.


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Figure 9. Selectivity of aromatics at TOS of 1h, 5h, 10h, 20h, 30h and 40h over 5%Mo/HZSM-5 and Nano-Fe modified 5%Mo/HZSM-5 catalysts.

The micropore size of HZSM-5 has been understood to be responsible for the activation of methane to form aromatic products because it was close to the dynamic diameter of benzene. It is commonly recognized that benzene was predominantly formed on the active sites inside the channels of zeolite catalysts, and naphthalene is evidently formed on the active sites near the external surface of the zeolites. The improvement in the benzene selectivity in the methane aromatization reaction might be due to the contribution of the nanotubes/nanofiber that were promoted by the Fe species introduced to the Mo/HZSM-5 catalyst. It was assumed that the nanotubes/ nanofiber could construct appropriated cavities with the metal species, which have kinetic diameter of about 10.0 Å. Activation of methane would take place over Brønsted sites inside the carbon nanotubes as



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well, thus could lead to the formation of benzene and other aromatic products, and allow diffusing out of the cavity. By comparison, some control experiments under the same conditions have been conducted. Experiments were carried out for (1) pure nano Fe, (2) 1.5% nano Fe/HZSM-5, (3) 5% Mo/ZSM-5 on top bed and nano Fe on bottom bed, (4) nano Fe on top bed and 5%Mo/ZSM-5 on bottom bed. For the reactions performed with pure nano-Fe and 1.5% nano Fe/HZSM-5, only coke, H2 and C2 productions were observed and no aromatic product was monitored. The reaction with separated Mo/HZSM-5 on top and nano-Fe on bottom has similar results in aromatic yield and coke formation when compared to that of Mo/HZSM-5 catalyst. The reaction with separated nano-Fe on top and Mo/HZSM-5 on bottom showed lower aromatic yield comparing to that of Mo/HZSM-5 catalyst. The reason is that a portion of CH4 has reacted with nano-Fe before reached the bottom Mo/HZSM-5 bed. The results of these control tests clearly show that pure nano-Fe and nano-Fe/HZSM-5 can only produce H2, C2 and coke in the reaction at 700°C under atmospheric pressure, but nano Fe does have positive effect when mixed with Mo/HZSM-5 catalyst in MDA process 3.2 Effect of nano-Fe additive on physical properties of 5%Mo/HZSM-5 3.2.1 N2-sorption analysis The BET surface area ("OPQ ), microporous surface areas ("2>#####

Catalytic Methane Dehydroaromatization with Stable Nano Fe Doped

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