SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell-derived catalyst for oxidative coupling of methane

Abstract

SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were prepared and their fabrication was confirmed using transmission electron microscopy. The formation of Mn-based nanosheets on the silica spheres is important for the deposition of nanoscopic Na₂WO₄. The SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were used for the oxidative coupling of methane at a temperature of 700–800 °C at which the nanostructures were completely destroyed. Although the core–shell structures did not survive the high-temperature oxidative coupling of methane, the selective production of olefins and paraffins can be attributed to highly dispersed Na₂WO₄ derived from confined core–shell structures.

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1. Introduction

Methane is a major component of natural gas and potential feedstock for the production of chemicals and fuels.^1,2 While it is a building block for the preparation of many complex organic chemicals, the conversion of stable C–H bonds is difficult. Methane can be indirectly transformed into larger organic molecules via synthesis gas.³ It can also be directly converted into functionalized building blocks including methanol,⁴ methyl halides,⁵ and ethylene.⁶ Oxidative coupling of methane (OCM) to paraffins and olefins has attracted much interest because of its high productivity.^3,7,8 Because catalysts are important for the determination of the efficiency of OCM, many catalysts have been suggested for the development of economically feasible processes.^7,9,10

Several metal oxide catalysts including perovskites,^11,12 metal oxide nanoparticles,¹³ and other mixed metal oxides¹⁴ have been proposed for the active and selective OCM process. One of the most efficient OCM catalysts,¹⁵ Na₂WO₄/Mn/SiO₂ can be fabricated by depositing Na₂WO₄ and Mn on an SiO₂ support. Although the reaction mechanism is still controversial,⁹ it has been suggested that Na₂WO₄ and Mn act as active component and promoter, respectively.^9,16,17 Scientists attempted to improve the Na₂WO₄/Mn/SiO₂ catalyst by adding promoters^16,18,19 and modifying the support,²⁰ but nanostructured Na₂WO₄/Mn/SiO₂ has not been used because of the high OCM reaction temperature.

In this study, core–shell structures composed of Na₂WO₄–Mn–SiO₂ were prepared as precursors of OCM catalysts (Scheme 1). Based on the active or selective catalysis using core–shell structures reported in the literature,^21,22 the objectives of building a core–shell structure are the (i) dispersion of Na₂WO₄ and Mn on the silica surface, (ii) stabilization of Na₂WO₄ and Mn components at the high reaction temperatures of 700–900 °C, and (iii) formation of possible Na₂WO₄, Mn, and SiO₂ complexes to achieve an improved catalytic activity. Particularly, an outer shell of SiO₂ was added to immobilize Na₂WO₄ and MnO_x shells at the high reaction temperature and to potentially prepare a highly dispersed mixture of Na₂WO₄, MnO_x, and silica which could improve the catalytic activity. By using the highly dispersed Na₂WO₄ of the core–shell precursors, the selective formation of ethane and ethylene is favored compared with the bulk Na₂WO₄/Mn/SiO₂ catalyst, which allows the deep oxidation to CO₂. While well-defined core–shell structures cannot survive at high OCM reaction temperatures, they control the final annealed structures and thus potentially the OCM activity and selectivity. Active OCM components that are nano-confined by the initial core–shell structures can be prepared. The step-by-step preparation of core–shell particles and improvement of the OCM activity are discussed in this paper.

Scheme 1 Methane activation using core–shell catalysts.

2. Methods

2.1. Materials

All chemicals were used without further purification. Sodium tungstate dihydrate (Na₂WO₄·2H₂O) was purchased from Yakuri Pure Chemicals (Kyoto, Japan). Potassium permanganate (KMnO₄, 99.3%) was purchased from Daejung Co., Ltd. (Siheung, Korea). Manganese acetate tetrahydrate [Mn(CH₃CO)₂·4H₂O, 99.99%] was purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Silica gel (SiO₂, 70–230 mesh, 99.5%) was purchased from Alfa Aesar (Haverhill, Massachusetts, USA). The TEOS [Si(OC₂H₅)₄] was purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Aqueous ammonia solution (NH₃ (aq), 28.0–30.0%) was purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Methane (99.5%), oxygen (99.95%), and nitrogen (99.9%) were purchased from Shinyang Sanso (Seoul, Korea). Deionized (DI) water (18.2 MΩ m) was prepared using an aquaMAX-Ultra 370 series water purification system (Young Lin Instruments, Anyang, Korea).

2.2. Preparation of supported powder catalysts

The mixed slurry method was used to prepare Na₂WO₄/Mn/SiO₂ catalysts with 5 wt% Na₂WO₄ and 2 wt% Mn supported by SiO₂.²³ A silica gel support (28 g) was mixed with DI water (150 mL) and stirred at 105 °C for 1 h. The Mn(NO₃)₂·6H₂O (1.8 mL) dissolved in DI water (10 mL) and Na₂WO₄·2H₂O (1.684 g) dissolved in DI water (10 mL) were drop-wise added to the boiling mixture for 15 min using a syringe pump. The generated thick paste was dried in air at 105 °C for 16 h and then calcined in air at 800 °C for 5 h. Prior to the reaction, the catalyst powder was sieved using a 150–250 mesh. Using the same procedure, additional catalysts were prepared by adding KMnO₄ or Mn(CH₃CO)₂·4H₂O.

2.3. Preparation of SiO₂ spheres

The silica spheres used as cores were prepared using the Stöber method.^24–26 For this purpose, TEOS (30 mL) was hydrolysed in ethanol (170 mL) in the presence of aqueous ammonia solution (190 mL) and DI water (10 mL). The solvent was uniformly mixed using an impeller and added using a separatory funnel to hydrolyse the TEOS at a constant rate. The obtained silica spheres were washed with DI water and ethanol, centrifuged in a suspension, and dried in an oven at 80 °C for 24 h.

2.4. Preparation of SiO₂@MnO_x

The Mn species were deposited on the silica spheres (SiO₂@MnO_x) using a hydrothermal method. To prepare SiO₂@Mn_xO_y, the silica spheres (0.3 g) were dispersed in DI water (20 mL) at 30 °C for 1 h using ultrasonication,²⁷ followed by the addition of KMnO₄ (0.015–1.8 g) or Mn(CH₃CO)₂·4H₂O (or Mn-acetate, 0.15–1.8 g). Subsequently, the suspension was transferred to a Teflon-lined stainless-steel autoclave-type reactor and heated to 150 °C at a heating rate of 10 °C min⁻¹; the temperature was maintained at 150 °C for 48 h.^28–30 The autoclave-type reactor was then cooled to room temperature under ambient conditions. Subsequently, the SiO₂@MnO_x(KMnO₄) and SiO₂@MnO_x(Mn-acetate) core–shell structures were collected by centrifugation and washed five times or more with ethanol and DI water. Potassium ions were removed by washing the core–shell structures.

2.5. Preparation of SiO₂@MnO_x@Na₂WO₄

The Na₂WO₄ was deposited on SiO₂@MnO_x by drying an aqueous Na₂WO₄ solution in an oil bath. The Na₂WO₄·2H₂O (0.012 g) dissolved in DI water (15 mL) was stirred for 30 min at room temperature. The SiO₂@MnO_x (0.2 g) was added to the mixture, which then was stirred for an hour. The suspension was transferred to an oil bath preheated to 105 °C and stirred until the water solvent was completely evaporated. The prepared powder was dried in air at 80 °C for 6 h to completely remove any moisture.

2.6. Preparation of SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell structures

Nanostructured core–shell catalysts with three shell layers composed of the active components of the OCM catalyst were synthesized. First, the TEOS was hydrolysed using the Stöber method to fabricate silica spheres with sizes between 600–800 nm, which were used as cores. Subsequently, two manganese precursors were selected, and the silica spheres were coated with the MnO_x shells using hydrothermal synthesis in a Teflon-lined stainless-steel autoclave. To attach Na₂WO₄ to the surface of the prepared SiO₂@MnO_x, DI water, Na₂WO₄·2H₂O, and SiO₂@MnO_x were added, followed by heating and stirring. Finally, the same method was used to fabricate silica spheres for the silica coating on the outermost layer of the catalyst.

2.7. Characterization of nanomaterials

Scanning electron microscopy (SEM) images of the catalysts were obtained using a Nova Nano SEM 200 instrument (FEI, Hillsboro, Oregon, USA). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the catalysts were obtained using a Talos F200X instrument (FEI, Hillsboro, Oregon, USA). Both analyses were performed at the Korea Institute of Science and Technology (KIST) Advanced Analysis Center (Seoul, Korea). XRD results were collected using a Shimadzu XRD-6000 device equipped with a CuKα₁ (λ = 0.15406 nm) source.

2.8. Catalytic activity measurement

The catalytic reaction was performed in a fixed bed reactor. The outlets and inlets of the quartz reactor had the same size (internal diameter = 6 mm, external diameter = 8 mm). The catalyst bed was in the center of the reactor and the quartz wool was placed in the upper and lower layers of the catalyst bed. To minimize any homogeneous reactions, ZrSiO₄ beads (Cenotec, Co., Ltd.) were added to the quartz reactor. The CH₄ and O₂ reactants diluted with N₂, flowed to the catalyst bed with each gas flow was controlled by a mass flow controller (MFC). The catalyst bed was primed with a N₂ flow for 30 min prior to the reaction. Water produced during the OCM reaction was removed using a cold trap connected to a −2 °C chiller (Jeio Tech, VTRC-620) and the CH₄, O₂, N₂, CO, CO₂, and hydrocarbons were analysed using the methanizer-equipped-flame ionization detector (FID), thermal conductivity detector (TCD) and FID of online gas chromatograph (GC, Agilent 7890A with ShinCarbon ST GC Columns (micropacked, RESTEK, 100/120 mesh, 2 m, 80 psi) was used to detect hydrocarbons. The catalysis results were calculated as follows:

The standard deviations of measured CH₄ conversion and C₂₊ selectivity were less than 5 and 3%, respectively.

3. Results and discussion

3.1. Preparation of SiO₂@MnO_x particles

Prior to the fabrication of core–shell particles, silica spheres with diameters of 600–800 nm (Fig. 1) were prepared using the Stöber method.^24–26 The SiO₂@MnO_x (Mn oxide shell deposited on SiO₂ spheres) was prepared by the deposition of MnO_x on the cores of the silica spheres. The MnO_x was deposited with a hydrothermal method utilizing KMnO₄ and Mn(CH₃CO)₂·H₂O (Mn-acetate) as Mn species precursors.^28,29 The morphologies of the Mn shells differ depending on the Mn precursors. When KMnO₄ was used, plate-like shells formed on the surfaces of silica spheres. The morphology depends on the concentration of KMnO₄; the (KMnO₄)/(SiO₂) ratio ranged from 0.02 to 2.22 (mol/mol). At a (KMnO₄)/(SiO₂) ratio of 0.02–0.76 (mol/mol), SiO₂, the spheres were not completely covered with Mn species. However, they were completely covered at a (KMnO₄)/(SiO₂) ratio of 1.52 (mol/mol) or higher. The Mn species, which completely cover the SiO₂ spheres, form nanoplate-like structures with a complex morphology that are stacked on top of each other (Fig. 2(f–h)). The transmission electron microscopy (TEM) images and energy dispersive X-ray spectroscopy (EDS) maps of SiO₂@MnO_x(KMnO₄) confirm the deposition of nanoplate-like Mn species on the SiO₂ spheres and formation of Mn shells on SiO₂ spheres (Fig. 2(i–l)), respectively.

Fig. 1 SEM images of silica spheres prepared using the Stöber method.

Fig. 2 SEM images of SiO₂@MnO_x(KMnO₄) with (KMnO₄)/(SiO₂) ratios of (a) 0.02, (b) 0.04, (c) 0.38, (d) 0.57, (e) 0.76, (f) 1.52, (g) 1.90, and (h) 2.22 (mol/mol). (i and j) HAADF-STEM images of SiO₂@MnO_x(KMnO₄) with a (KMnO₄)/(SiO₂) ratio of 2.22 (mol/mol). (k) HAADF-STEM images and (l) EDS maps of SiO₂@MnO_x(KMnO₄) with a (KMnO₄)/(SiO₂) ratio of 0.19 (mol/mol).

In addition to KMnO4, Mn-acetate was added to the silica spheres. The Mn-acetate forms shells on the silica spheres and Mn oxide nanowires (not deposited on the silica surface; Fig. 3(a–f)). Interestingly, the EDS maps indicate the formation of yolk–shell-like structures, creating empty space between the MnO_x shells and silica spheres (Fig. 3(g and h)).

Fig. 3 SEM images of SiO₂@MnO_x(Mn-acetate) with (Mn-acetate)/(SiO₂) ratios of (a) 0.12, (b) 0.24, (c) 0.49, (d) 0.73, (e) 0.98, and (f) 1.47 (mol/mol). (g) TEM images and (h) EDS maps of SiO₂@MnO_x(Mn-acetate) with a (Mn-acetate)/(SiO₂) ratio of 0.73 (mol/mol).

3.2. Preparation of SiO₂@MnO_x@Na₂WO₄

The Na₂WO₄ was added to SiO₂@MnO_x(KMnO₄) and SiO₂@MnO_x(Mn-acetate). The deposition of Na₂WO₄ was analysed using EDS maps (Fig. 4). The morphology of SiO₂@MnO_x@Na₂WO₄ insignificantly differs from that of SiO₂@MnO_x. These observations indicate the Na₂WO₄ deposition in the empty space between the MnO_x nanosheets and stabilization of Na₂WO₄ by MnO_x shells. The roughness of the MnO_x shells improves the deposition of Na₂WO₄ (Fig. 2 and 3). Note that Na₂WO₄ could not be deposited on the silica spheres without Mn species when a mixture of aqueous Na₂WO₄ solution and silica spheres was used, which indicates that it is difficult to deposit Na₂WO₄ on the silica surface and that an interaction between Na₂WO₄ and MnO_x is required for the formation of a Na₂WO₄ layer.

Fig. 4 HAADF-STEM images and EDS maps of (a and b) SiO₂@MnO_x(KMnO₄)@Na₂WO₄ and (c and d) SiO₂@MnO_x(Mn-acetate)@Na₂WO₄.

3.3. Preparation of SiO₂@MnO_x@Na₂WO₄@SiO₂

The SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalyst was prepared by forming a silica shell on SiO₂@MnO_x@Na₂WO₄ (Fig. 5 and S1†). The thickness of the top SiO₂ shell was controlled by changing the quantity of tetraethyl orthosilicate (TEOS) precursors that were added to SiO₂@MnO_x@Na₂WO₄. Regarding the SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ catalyst, the top shell of SiO₂ forms a rough surface, mimicking the surface morphology (Na₂WO₄ layer) of SiO₂@MnO_x(KMnO₄)@Na₂WO₄. With increasing concentration of top-shell SiO₂, small SiO₂ particles (∼100 nm) form, covering the surface of SiO₂@MnO_x(KMnO₄)@Na₂WO₄. The EDS maps and TEM images indicate the formation of thin SiO₂ layers on the surface. Top-shell SiO₂ also forms on the surface of the SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂ catalyst. During the Mn-acetate deposition, isolated Mn rods form, which are also coated with SiO₂. The EDS maps and TEM images indicate that the deposition of SiO₂ insignificantly changes the core–shell structure of SiO₂@MnO_x(Mn-acetate)@Na₂WO₄.

Fig. 5 SEM images of SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ with (SiO₂ core)/(KMnO₄)/(Na₂WO₄)/(top-shell SiO₂) ratios of (a) 1/2.28/0.01/0.03, (b) 1/2.28/0.01/0.07, (c) 1/2.28/0.01/0.14, and (d) 1/2.28/0.01/0.29 (in mol) and SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂ with (SiO₂ core)/(Mn-acetate)/(Na₂WO₄)/(top-shell SiO₂) ratios of (g) 1/0.24/0.01/0.03, (h) 1/0.24/0.01/0.07, (i) 1/0.73/0.01/0.14, and (j) 1/1.47/0.01/0.29 (in mol). HAADF-STEM images and EDS maps of (e and f) SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ and (k and l) SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂.

3.4. OCM using core–shell structures

Core–shell catalysts prepared were used for the OCM (Fig. 6 and Table S1†). The OCM activity of SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ was measured and compared with those of SiO₂@MnO_x(KMnO₄)@Na₂WO₄ without outer SiO₂ shell, SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂, and conventional Na₂WO₄/Mn/SiO₂. Although SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ was covered by the outer SiO₂ shell, it exhibited the slightly higher C₂₊ selectivity compared to Na₂WO₄/Mn/SiO₂ at 740–800 °C. SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂ exhibited poor OCM activity with lower CH₄ conversion and the lower C₂₊ selectivity which was confirmed by the EDS-STEM images (Fig. 7). SiO₂@MnO_x(KMnO₄)@Na₂WO₄ without the outer SiO₂ shell also exhibited lower CH₄ conversion (4.3–17.0%) compared to SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ (10.8–25.4%). The higher OCM activity of SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ compared to that of SiO₂@MnO_x(KMnO₄)@Na₂WO₄ without the outer SiO₂ shell can be attributed to the better dispersed Na₂WO₄ and MnO_x mixed with the outer SiO₂ shell as depicted in the EDS-STEM images (Fig. 7). With increasing gas hourly space velocity (GHSV) or decreasing contact time, CH₄ conversion decreased, and the C₂₊ selectivity slightly increased (Table S2†). The long term stability of SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ was also observed, as a stable C₂₊ yield was observed for up to 100 h. That said, the C₂₊ selectivity decreased slightly up to 20 h and remained stable thereafter (Fig. S2†).

Fig. 6 Results of oxidative coupling of methane using core–shell catalysts. GHSV = 20 000 h⁻¹, CH₄/O₂/N₂ = 3/1/1 mol/mol/mol.

Fig. 7 HAADF-STEM images and EDS maps of (a) Na₂WO₄/Mn/SiO₂, (b) SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂, and (c) SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂ after the OCM reaction.

The TEM results confirm that core–shell SiO₂@MnO_x@Na₂WO₄@SiO₂ catalysts were successfully fabricated (Fig. 1–5). While the use of Mn as an oxygen supplier for the OCM reaction has been suggested,¹⁶ Mn is also required for the production of well-dispersed Na₂WO₄ on Mn-based nanosheets in the core–shell structures. The formation of Mn-based nanosheets on the silica core spheres could be clearly observed when the KMnO₄ precursor was used. The better catalytic activity of SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ can be attributed to the deposition of well-dispersed of Na₂WO₄ on Mn-based nanosheets. At high OCM reaction temperatures, the core–shell structure of SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ collapses into a complex SiO₂, MnO_x, and Na₂WO₄ composite. The successful OCM activity of collapsed SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ can be attributed to the formation of well-dispersed Na₂WO₄ particles on the Mn-bearing SiO₂ surface, which was derived from the initial core–shell structure. Core–shell structures and highly disperse catalysts are more selective with respect to the formation of C₂₊ compounds because of the confined Na₂WO₄ structure. Interestingly, SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂ exhibited spherical core–shell structures at high reaction temperature, despite being fully covered by the outer SiO₂ shell which suppressed the OCM reaction.

The XRD measurements indicate that the formation of α-cristobalite SiO₂ was observed for SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ and Na₂WO₄/Mn/SiO₂ but not for SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂ (Fig. 8). Because of the stabilizing effects of α-cristobalite SiO₂,¹⁷ the poor OCM activity of SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂ can be attributed to the absence of α-cristobalite SiO₂. The formation of α-cristobalite SiO₂ for SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ indicated the formation of a complex mixture of Na₂WO₄ and the outer SiO₂ shell because the permeation of Na into SiO₂ nucleated the α-cristobalite SiO₂.

Fig. 8 XRD results of (a) Na₂WO₄/Mn/SiO₂, (b) SiO₂@MnO_x(KMnO₄), (c) SiO₂@MnO_x(KMnO₄)@Na₂WO₄, (d) SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂, (e) SiO₂@MnO_x(Mn-acetate), (f) SiO₂@MnO_x(Mn-acetate)@Na₂WO₄, and (g) SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂. All catalysts were fresh prior to the reaction. (1: tridymite, 2: α-cristobalite, 3: Mn₂O₃, 4: MnO₂, 5: Na₂WO₄, 6: NaWO₃).

The XPS exhibited distinct peaks of tungsten (W 4f) and sodium (Na 1s) for SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂, but not for SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂ (Fig. 9). Because the OCM-active tungsten oxide was not clearly observed on the surface of SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂,^9,16,31,32 the OCM activity of SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂ was lower than those of other catalysts. These observations can be attributed to the outer SiO₂ shell blocking the active tungsten oxide for SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂ as depicted in the EDS-STEM images. SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ exhibited appreciable W 4f peaks indicating exposed active sites for the OCM.

Fig. 9 Na 1s and W 4f XPS results of (a) Na₂WO₄/Mn/SiO₂, (b) SiO₂@MnO_x(KMnO₄)@Na₂WO₄, (c) SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂, (d) SiO₂@MnO_x(Mn-acetate)@Na₂WO₄, and (e) SiO₂@MnO_x(Mn-acetate)@Na₂WO₄@SiO₂. All catalysts were fresh prior to the reaction.

3.5. Reaction mechanism on the core–shell catalysts

SiO₂@MnO_x(KMnO₄)@Na₂WO₄@SiO₂ exhibited slightly higher C₂₊ selectivity compared to the conventional Na₂WO₄/Mn/SiO₂ catalyst although it was covered by the outer SiO₂ shell. The selective OCM activity can be attributed to the formation of a complex of Na₂WO₄, MnO_x, and SiO₂ as observed on the TEM, XRD, and Raman results. The presence of complex mixed oxides improved the dispersion of Na₂WO₄ in the mixture of MnO_x and SiO₂ selectively catalysing OCM.

4. Conclusion

In this study, SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell structures were prepared. Nanostructured shells of Mn oxide and Na₂WO₄ consecutively formed and were protected using SiO₂ sols. Although the complex core–shell structures were destroyed at a temperature of 700–800 °C at which the OCM to olefins and paraffins was performed, highly dispersed Na₂WO₄ on the MnO_x layer is derived from nanoscopic core–shell structures. The Na₂WO₄ may be confined to the complex structures of MnO_x and SiO₂. The catalyst derived from the core–shell Na₂WO₄–Mn–SiO₂ complex exhibits an improved production of C₂₊ compounds and the further oxidation of C₂₊ compounds to CO and CO₂ may be suppressed by highly dispersed Na₂WO₄.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the C1 Gas Refinery Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2015M3D3A1A01064900).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra05081d

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