Article pubs.acs.org/IECR
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 Ruiping Chen*,†,∥ †
Department of Chemical Engineering, ‡Department of Petroleum Engineering, and §School of Energy Resources, University of Wyoming, Laramie, Wyoming 82071, United States ∥ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China 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 the Mo/HZSM-5 catalyst in methane dehydroaromatization (MDA). Mo/HZSM-5 and nano-Fe modified catalysts were synthesized and then evaluated in a fixed-bed reactor along with an integrated online 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 show enhanced MDA performances especially stability. Moreover, research demonstrates that carbon nanotubes can not only form on the Mo/HZSM-5 catalyst without the addition of Fe but also exist for a long time, which clarifies the carbon nanotubes formation mechanism. It was concluded that nano Fe plays an important role in promoting the growth of carbon nanotubes and thus the activity and stability of the catalysts.
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 environmentally friendly alternative to crude oil. Therefore, they are suited for conversion into liquid fuels or other value-added chemicals of industrial interest.1 Natural gas, which is about 90% methane, has been widely investigated as a 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 account for up to 60%−70% of the capital cost.2 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 © 2017 American Chemical Society
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, nonoxidative methane aromatization over molybdenum-loaded catalysts has 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 Received: Revised: Accepted: Published: 11398
May 29, 2017 August 11, 2017 September 18, 2017 September 18, 2017 DOI: 10.1021/acs.iecr.7b02213 Ind. Eng. Chem. Res. 2017, 56, 11398−11412
Article
Industrial & Engineering Chemistry Research
of 5 g of the ZSM-5 zeolite with 30 mL of aqueous solution of ammonium nitrate (NH4NO3, 1 mol/L) under ultrasonic processing (QSONICA Q700 Sonicator with microtip probe supplied) at room temperature for 60 min, followed by drying at 353 K overnight. The Mo/NH4ZSM-5 catalyst was prepared according to the impregnation method (IM). Each 5 g of the NH4ZSM-5 zeolite was impregnated with 30 mL of aqueous solution of ammonium molybdate tetrahydrate (Sigma-Aldrich, BioUltra) at room temperature. The resulting material was dried at 383 K and then calcined in air for 4 h using a furnace (Thermo Scientific BF51866A-1) at 773 K. 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 nanosized Fe powder (25 nm avg. 99.5%, Sigma-Aldrich) and the freshly prepared Mo/HZSM-5 zeolite were physically mixed to prepare Fe(nano)-Mo/HZSM-5 catalysts using a mechanical ball mill, followed by drying in air at 383 K for 6 h. Finally, the Fe(nano)-Mo/HZSM-5 catalysts were sieved to sizes of 40−60 mesh for further evaluation. The chemical composition of all catalysts prepared for this study is determined by ICP-MS and listed in Table 1.
methane dehydroaromatization have attracted much attention. Early research has shown 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 ZSM-54−6 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 catalysis.9 Beyond that, modification of the catalyst supports10 and metals coloading on zeolites were suggested.11 In the latter, a second metal component was introduced to the 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 on.2 As an abundant, stable, and environmentally friendly metal, Fe presents a great potential for industrial application.12 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 them.13 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.14−16 A previous investigation documented that adding a small amount of Fe enables nanozeolite 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 the Mo/HZSM-5 catalyst exhibited higher carbon deposition and lower activity,18 and a 2.5 or 3 wt % of Fe addition could contribute to the lower dehydroaromatization activity to benzene.15,18 What is more, the effect of doping nanosized iron directly on Mo/HZSM-5 catalysts has not been reported yet. In this case, this work focuses on the promotion effect of nanosized iron (
