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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 40404-40411
Critical Surface Parameters for the Oxidative Coupling of Methane over the Mn−Na−W/SiO2 Catalyst Naseem S. Hayek, Nishita S. Lucas, Christine Warwar Damouny, and Oz M. Gazit* The Wolfson Faculty of Chemical Engineering, TechnionIsrael Institute of Technology, Haifa 3200003, Israel S Supporting Information *
ABSTRACT: The work here presents a thorough evaluation of the effect of Mn−Na−W/SiO2 catalyst surface parameters on its performance in the oxidative coupling of methane (OCM). To do so, we used microporous dealuminated βzeolite (Zeo), or mesoporous SBA-15 (SBA), or macroporous fumed silica (Fum) as precursors for catalyst preparation, together with Mn nitrate, Mn acetate and Na2WO4. Characterizing the catalysts by inductively coupled plasma−optical emission spectroscopy, N2 physisorption, X-ray diffraction, high-resolution scanning electron microscopy−energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and catalytic testing enabled us to identify critical surface parameters that govern the activity and C2 selectivity of the Mn−Na−W/SiO2 catalyst. Although the current paradigm views the phase transition of silica to α-cristobalite as the critical step in obtaining dispersed and stable metal sites, we show that the choice of precursors is equally or even more important with respect to tailoring the right surface properties. Specifically, the SBA-based catalyst, characterized by relatively closed surface porosity, demonstrated low activity and low C2 selectivity. By contrast, for the same composition, the Zeo-based catalyst showed an open surface pore structure, which translated up to fourfold higher activity and enhanced selectivity. By varying the overall composition of the Zeo catalysts, we show that reducing the overall W concentration reduces the size of the Na2WO4 species and increases the catalytic activity linearly as much as fivefold higher than the SBA catalyst. This linear dependence correlates well to the number of interfaces between the Na2WO4 and Mn2O3 species. Our results combined with prior studies lead us to single out the interface between Na2WO4 and Mn2O3 as the most probable active site for OCM using this catalyst. Synergistic interactions between the various precursors used and the phase transition are discussed in detail, and the conclusions are correlated to surface properties and catalysis. KEYWORDS: silica phase transition, cristobalite, methane, ethylene, oxidative coupling, sodium tungstate, manganese dispersion, β-zeolite
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INTRODUCTION Recent reports show a rapid increase in the proven reserves of natural gas over the years.1 However, to date, there are no economically feasible and scalable ways to convert large amounts of methane, the main component of natural gas, into value-added chemicals.2 Oxidative coupling of methane (OCM) is one of the most attractive routes to utilize natural gas because it is a direct route for synthesizing ethylene (reaction 1),3,4 a vital building block in the chemical industry with an expected increase in demand.5 Moreover, the reaction is exothermic and not limited thermodynamically.
(ethane and ethylene) under harsh reaction conditions (∼800 °C). The major challenge for OCM is the more favorable oxidation of methane to undesired carbon oxides (COx) in the presence of gas-phase oxygen at high temperatures (reaction 2). Previous theoretical and experimental work on OCM assigned a mechanistic upper limit of ∼30% C2 yield for conventional single-pass packed-bed reactors.6−8 A promising solution to overcome this limit is the combination of both catalyst and process engineering.9,10 For example, by using a membrane reactor with the Bi−Y−Sm-based catalyst, 39% C2 yield was achieved recently.11 However, no information regarding the stability of the catalyst or membrane was provided. Hence, the development of an active, selective, and stable catalyst for OCM is still of significant importance. Mn−Na−W/SiO2 is one of the few promising catalysts for OCM, currently showing 20−40% CH4 conversion with over 60−80% C2 product selectivity for more than 500 h time on
CH4 + 0.5O2 → 0.5C2H4 + H 2O 0 ΔH800 ° C = − 139 kJ/mol
(1)
CH4 + 2O2 → CO2 + 2H 2O 0 ΔH800 ° C = − 801 kJ/mol
(2)
Received: October 1, 2017 Accepted: October 25, 2017 Published: October 25, 2017
Despite intensive research, OCM still lacks an active and stable catalyst, which gives high selectivity toward C2 products © 2017 American Chemical Society
40404
DOI: 10.1021/acsami.7b14941 ACS Appl. Mater. Interfaces 2017, 9, 40404−40411
Research Article
ACS Applied Materials & Interfaces stream (TOS).12−15 A critical part of the high performance of this catalyst is the phase transition of amorphous silica to αcristobalite, which occurs following impregnation with Na2WO4 and calcination above 750 °C.16,17 It is believed that this step governs the high catalytic performance of the catalyst by dispersing and stabilizing the active metal oxides on the silica support.16−19 Despite intensive research, there is a clear disagreement with respect to the identity of the active site, where several candidates were suggested in the literature, such as Mn−O−Si, W−O−Si, Na−O−Mn, and Na−O− W.16,17,19−22 Moreover, the governing structural factors that enable this complex catalyst to demonstrate enhanced catalytic performance are still not completely understood.12 In this work, we investigated important design criteria related to the activity and selectivity of the Mn−Na−W/SiO2 catalyst. Specifically, we studied the effects of Mn dispersion, surface morphology, and surface composition on catalytic performance. To do so, we synthesized a set of Mn−Na−W/SiO2 catalysts with different properties by using nonporous (fumed), mesoporous (SBA-15), and microporous (dealuminated βzeolite) silica supports in conjunction with the use of two Mn precursors that behave differently (Mn nitrate and Mn acetate). It is known that Mn nitrate (MnN) generates poorly dispersed Mn oxide on supports, whereas Mn acetate (MnA) produces highly dispersed Mn oxide.23−26 Our results show distinctly that the role of phase transition is not only to disperse and stabilize surface metal oxide species, the currently perceived paradigm, but also to shape the appropriate surface texture and composition for OCM. We show that the catalytic activity of the catalyst is highly dependent on the silica and Mn precursors used for the preparation. The governing factors for enhancing the catalytic performance of Mn−Na−W/SiO2 are addressed and discussed in detail.
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Bulk compositions of the catalysts were determined by inductively coupled plasma−optical emission spectroscopy (ICP−OES). The analysis was performed on an iCAP 6000 (Thermo Scientific) spectrometer. For the analysis, samples were dissolved in HF aqueous solution and diluted with deionized water appropriately. The catalyst morphology and the distribution of metals on the surface were evaluated using high-resolution scanning electron microscopy (HR-SEM) and energy-dispersive spectroscopy (EDS), respectively. The analysis was performed on an Ultraplus (Zeiss) microscope. Samples were sprinkled on a carbon film, coated with a thin carbon layer and imaged at 1 kV acceleration voltage. SEM−EDS mapping of manganese and tungsten were performed at 15 kV. The surface composition of the catalysts was obtained by X-ray photoelectron spectroscopy (XPS). Measurements were performed in ultrahigh vacuum (2.5 × 10−10 Torr base pressure) using a 5600 MultiTechnique System (PHI, USA). The sample was irradiated with an Al Kα monochromated source (1486.6 eV), and the outcome electrons were analyzed by a spherical capacitor. C 1s at 285.0 eV was taken as an energy reference for all peaks. Catalytic Testing. All reactions were performed in a quartz tubular fixed-bed reactor of 7 mm inner diameter. For the reaction, the catalyst bed was prepared by taking 50 mg of the respective catalyst (95%. CH4 conversion and C2 selectivity were calculated according to eqs e3 and e4 in the Supporting Information, respectively.
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RESULTS AND DISCUSSION All catalysts were synthesized using conventional two-step incipient-wetness impregnation.29 However, to evaluate the effect of MnxOy dispersion on the different SiO2 precursors, each support was first impregnated with the Mn precursor and calcined at 500 °C prior to the impregnation with Na2WO4 and the second calcination at 800 °C. MnxOy/SiO2 Materials. The three different silica supports were impregnated with 2 wt % of Mn, either using MnN or MnA, followed by calcination to obtain MnxOy/SiO2. PXRD patterns and N2-physisorption data for the different parent SiO2 supports and the MnxOy/SiO2 materials are provided in Figure S1 and Table S1 in the Supporting Information. The PXRD patterns and the N2-physisorption data of the MnxOy/SiO2 materials showed no change in the structure of the silica precursors, retaining the amorphous nonporous structure of fumed silica, the amorphous mesoporous structure of SBA-15 (5.4 nm pore dia.), and the crystalline microporous structure of dealuminated β-zeolite (
