Synthesis of Na2WO4-MnxOy supported on SiO2 or La2O3 as fiber catalysts by electrospinning for oxidative coupling of methane

Kanticha Jaroenpanon; Worapinit Tiyatha; Thanaphat Chukeaw; Sarannuch Sringam; Thongthai Witoon; Chularat Wattanakit; Metta Chareonpanich; Kajornsak Faungnawakij; Anusorn Seubsai

doi:10.1016/j.arabjc.2021.103577

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Original article

02 2021

:15;

103577

doi:

10.1016/j.arabjc.2021.103577

Synthesis of Na₂WO₄-Mn_xO_y supported on SiO₂ or La₂O₃ as fiber catalysts by electrospinning for oxidative coupling of methane

Synthesis of Na₂WO₄-Mn_xO_y supported on SiO₂ or La₂O₃ as fiber catalysts

Kanticha Jaroenpanon^{^a}, Worapinit Tiyatha^{^a,^b}, Thanaphat Chukeaw^{^a,^b}, Sarannuch Sringam^{^a,^b}, Thongthai Witoon^{^a,^b,^c}, Chularat Wattanakit^{^d}, Metta Chareonpanich^{^a,^b,^c}, Kajornsak Faungnawakij^{^e}, Anusorn Seubsai^{^a,^b,^c,⁎}

a Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand

b Center of Excellence on Petrochemical and Materials Technology, Kasetsart University, Bangkok 10900, Thailand

c Research Network of NANOTEC–KU on NanoCatalysts and NanoMaterials for Sustainable Energy and Environment, Kasetsart University, Bangkok 10900, Thailand

d Department of Chemical and Biomolecular Engineering, School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand

e National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, Thailand Science Park, Khlong Luang, Pathum Thani 12120, Thailand

⁎Corresponding author at: Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand. fengasn@ku.ac.th (Anusorn Seubsai)

Received: 2021-9-22, Accepted: 2021-11-15,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The oxidative coupling of methane (OCM) is an attractive route to convert natural gas directly into value-added chemical products (C₂₊). This work comparatively investigated SiO₂- or La₂O₃-supported Na₂WO₄-Mn_xO_y (denoted as NWM) catalysts in powder and fiber forms. The powder catalysts were prepared using a co-impregnation method and the fiber catalysts were prepared successfully using an electrospinning technique. The NWM/La₂O₃ fiber catalysts were activated at low temperature (500 °C) and had a 4.7% C₂₊ yield, with the maximum C₂₊ yield of 9.6% at 650 °C, while the NWM/SiO₂ fiber catalyst was activated at 650 °C and had a maximum C₂₊ yield of 20.4% at 700 °C. The XPS results in the O 1s region indicated that NWM/La₂O₃ had a lower binding energy than NWM/SiO₂, suggesting that the lattice oxygen species is easily released from the catalyst surface and creates vacancy sites that enhance performance. The stability test of the catalysts indicated that the La₂O₃-containing catalysts had excellent activity and high thermal stability, while the SiO₂-containing catalysts had a higher C₂₊ yield when the prepared catalysts were compared at 700 °C. Considering the same component catalysts, the fiber catalysts achieved higher performance because their heat and mass transfer properties were enhanced.

Keywords

Fiber catalyst

La2O3

Na2WO4

Oxidative coupling of methane

SiO2

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1 Introduction

Natural gas is a type of hydrocarbon compound, which is formed by the accumulation of fossil microbes from plants and animals for hundreds of millions of years in layers on the earth’s surface (Faramawy et al., 2016). Over time, the fossils can transform into petroleum (oil and natural gas) due to pressure and heat. Natural gas consists of many components with the greatest proportion being methane (CH₄). CH₄ is one of the six types of greenhouse gases that cause global warming or the greenhouse effect, with a lifetime in the atmosphere of around 10 years (Liu et al., 2019). It causes global warming and generates climate change at levels approximately 25 times greater than carbon dioxide (CO₂) when the same amounts of these two gases are released into the atmosphere (Sringam et al., 2020). CH₄ is created naturally from biological decomposition processes but also a variety of human activities such as agricultural livestock farming, industry, and waste management. In the USA, it is estimated that human activities are responsible for approximately 50–65% of CH₄ emissions (Ciais et al., 2013).

To date, many researchers have tried to convert methane to value-added chemicals such as hydrogen, ammonia, methanol, and ethylene. Various routes of methane conversion to value-added chemicals have been developed recently. The most well-known route that is commercially viable is the indirect conversion of CH₄ to synthesis gas or syngas (a mixture of CO and H₂), which then can be converted to methanol and olefins via a methanol-to-olefin reaction (called methanol-to-olefins) (Huang et al., 2018). The non-oxidative coupling of CH₄ to C₂H₆ using photo-catalysts has also recently become an attractive route (Chen et al., 2021; Levin et al., 2020). The oxidative coupling of methane (OCM) is another direct route to convert methane to higher hydrocarbons such as ethylene and ethane (Gambo et al., 2018; Sringam et al., 2020). It involves a direct reaction between CH₄ and O₂ over a catalyst to form C₂₊ hydrocarbons (C₂H₄, C₂H₆, C₃H₆, C₃H₈, etc.), water, and heat. The OCM reactions for the formation of C₂H₄ and C₂H₆ are shown in equations (1) and (2), respectively (Hayek et al., 2019) :

(1)

{CH}_{4} + \frac{1}{2} O_{2} \to \frac{1}{2} C_{2} H_{4} + H_{2} O Δ H_{800^{°} C} = - 139 k J / m o l {CH}_{4}

(2)

{CH}_{4} + \frac{1}{4} O_{2} \to \frac{1}{2} C_{2} H_{6} + \frac{1}{2} H_{2} O Δ H_{800^{°} C} = - 87 k J / m o l {CH}_{4}

The mechanism of the OCM reaction can be generally explained as follows. First, CH₄ molecules interact with an adsorbed oxygen on a catalyst surface and generate *CH₃ radicals. Subsequently, two *CH₃ radicals combine into ethane (C₂H₆) in the gas phase and then, ethylene (C₂H₄) can be generated by dehydrogenation. The adsorbed oxygen species with the abstracted hydrogen atom then transform to H₂O. Afterward, O₂ molecules from the gas phase replace the vacancy sites and create active oxygen species, which subsequently proceed to the next cycle (Fleischer et al., 2016; Gambo, et al., 2018). However, the OCM reaction can create CO and CO₂ as side products. Alternatively, O₂ can be replaced by other, more mildly oxidant gaseous reagents, such as N₂O, CO₂, and S₂ (Arinaga et al., 2021). This is also an effective method for selectively producing C₂₊. Currently, the OCM process has yet to be made viable for industrial uses because of the lack of highly efficient catalysts (Fleischer, et al., 2016; Gambo, et al., 2018). Recent studies have shown that the economic feasibility of the OCM reaction using heterogeneous catalysts requires a product yield of high-value products (such as C₂H₄ and C₃H₆ = C₂₊) of over 30% (Karakaya et al., 2018) or a C₂₊ selectivity of at least 80% with CH₄ conversion of at least 30% (Chukeaw et al., 2021; Hayek, et al., 2019).

Over the past decades, many researchers have investigated various metal oxides as catalysts for the OCM reaction, including i) alkaline earth oxides (Elkins et al., 2016; Lim et al., 2020), ii) transition metal oxides (Chukeaw et al., 2019; Yunarti et al., 2014), and iii) alkaline earth, transition, and rare earth oxide-based catalysts doped with another alkali, alkaline earth, and rare earth cations (Gu et al., 2018; Jones et al., 2020; Kidamorn et al., 2020; Liu et al., 2017; Ortiz-Bravo et al., 2021a; Sringam et al., 2020 ). Among the active catalysts, Mn/Na₂WO₄/SiO₂-containing (denoted as NWMSi) catalysts are promising for industrial use. They provide high C₂₊ yields at approximately 1.3–29.0 % with 24–80% C₂₊ selectivity in a reaction temperature range of 800–825 °C (see Table S1). However, some reports showed that the NWMSi catalysts still deactivate with time on stream due to sintering and a loss of active Na₂WO₄ content at high temperatures (Hayek et al., 2019; Kidamorn, et al., 2020; Sringam et al., 2020 ).

Various catalyst supports have been investigated in the OCM reaction, such as La₂O₃, CaO, Al₂O₃, ZrO₂, SiO₂, SiC, TiO₂, and Fe₂O₃. The SiO₂ support is widely used in OCM because it has a high specific surface area that can provide well-dispersed active components (Hayek et al., 2019). In the OCM reaction, the SiO₂ support transforms to the α-cristobalite phase when adding Na, W, and Mn oxides as the active components. Although the NWMSi catalysts exhibit high performance, they have a low surface area (<10 m² g⁻¹) (Chukeaw et al., 2019; Kidamorn et al., 2020) and poor thermal stability with time-on-stream experiments (Chukeaw et al., 2019), while also requiring a high temperature to be activated (greater than700 °C) (Xu et al., 2018). Bare La₂O₃ is active for an OCM reaction by itself at a temperature around 600 °C (Jiang et al., 2016), which is more advantageous compared to the SiO₂ support. Furthermore, La₂O₃ has a large capacity for oxygen and various lattice oxygens that benefit the CH₄ activation for the formation of ethane and ethylene. In addition, La₂O₃ contains moderately basic sites that can improve performance (Huang et al., 2013; Jiang et al., 2016; Wang et al., 2019; Xu et al., 2019 ). Several La₂O₃-based catalysts are shown in Table S2. Other supports such as CaO, Al₂O₃, and ZrO₂ that were investigated with Na₂WO₄-Mn_xO_y had similar C₂₊ yields to the SiO₂ support, but those supports provided a much lower C₂₊ selectivity (Yildiz et al., 2014). Thus, catalysts using SiO₂ and La₂O₃ as catalyst supports are still excellent candidates in the OCM reaction.

Most catalysts in the OCM reaction are in the form of powder and are used in a fixed bed reactor, which has a high-pressure drop, poor heat and mass transfer, and causes catalyst sintering. Recently, nanofibers have gained much attention worldwide (Barhoum et al., 2019). There have been interesting studies based on their unique properties such as high tensile strength and high surface area and porosity (Barhoum et al., 2019; Thenmozhi et al., 2017). Nanofibers can be used for various applications in many fields such as electronics, energy storage cells, fuel cells (Kallem et al., 2018), catalysis (Noon et al., 2013; Qin et al., 2014; Sollier et al., 2020 ), and filtering membranes (Lee et al., 2019). The nanofiber catalysts based on CeO₂-La₂O₃ and Sr-La-Ce have been investigated in an OCM reaction (Noon et al., 2013; Sollier et al., 2020). The results indicated that the nanofibers performed better than the powder catalysts at low temperatures (600 °C) in the OCM reaction because the nanofibers provided benefits from their high heat and mass transfer properties.

Previous reports showed that nanofiber catalysts could be promising in OCM reactions. Additionally, catalysts containing Na₂WO₄-Mn_xO_y/SiO₂ provide high C₂₊ formation relative to other metal oxides (Chukeaw et al., 2019; Kiani et al., 2019) and La₂O₃-containing catalysts exhibit low temperature (approximately 500 °C) for CH₄ activation relative to Na₂WO₄-Mn_xO_y-containing catalysts (Song et al., 2015). Nevertheless, the performance of Na₂WO₄-Mn_xO_y supported on SiO₂ or La₂O₃ in the forms of powder and fiber has never been investigated comparatively. Therefore, the current work prepared and tested Na₂WO₄-Mn_xO_y supported on either SiO₂ or La₂O₃ in both powder and fiber forms for the OCM reaction. This study aimed to show the effect of catalyst structure on the OCM performance and whether the activity of Na₂WO₄-Mn_xO_y/La₂O₃ could compete with Na₂WO₄-Mn_xO_y/SiO₂. The catalysts in the forms of powder and fiber were prepared using co-impregnation and electrospinning techniques. Various characterization techniques were carried out to analyze the physical and chemical properties of the prepared catalyst and then the information was correlated with the activity results.

2 Materials and methods

2.1

2.1 Preparation of catalysts

2.1.1

2.1.1 Precursor materials

The following materials were used: sodium tungsten dihydrate (Na₂WO₄·2H₂O, 98.0–101.0%, Daejung), manganese (II) nitrate tetrahydrate (Mn(NO₃)₂·4H₂O, Chem-supply), lanthanum (III) nitrate hexahydrate (La(NO₃)₃·6H₂O, HIMEDIA), amorphous fumed SiO₂ (surface area 85–115 m²/g, Alfa Aesar), ethanol (C₂H₅OH, 99.9%, QREC), tetraethyl orthosilicate (Si(OC₂H₅)₄, TEOS, 99.0%, Aldrich), hydrochloric acid (HCl, QREC), acetic acid glacial (CH₃COOH, QREC), polyvinylpyrrolidone (PVP, MW of 1,300,000, Aldrich), and deionized (DI) water. All chemicals were used without purification.

2.1.2

2.1.2 Preparation of Na₂WO₄-Mn_xO_y/SiO₂ fibers and powder

The Na₂WO₄-Mn_xO_y/SiO₂ fibers were prepared using an electrospinning method (Zong et al., 2015). PVP (3.0 g), TEOS (3.0 g), Na₂WO₄·2H₂O (65.35 mg), and Mn (NO₃)₂·4H₂O (175.69 mg) were mixed and dissolved in ethanol (38 mL), followed by stirring for 2 h at room temperature. A small amount of HCl solution (4.11 M, 2.0 mL) was added dropwise into the solution for the hydrolysis of TEOS. The solution was stirred for 12 h at room temperature and then transferred into a 20 mL plastic syringe fitted with a blunt-ended stainless-steel needle. A high voltage power supply was used for the electrospinning process and the fiber collector was covered with aluminum foil. The distance between the collector and the tip of the needle was 15 cm. The voltage (20 kV) was applied to the needle and the solution feeding rate was 2.0 mL h⁻¹, which was controlled by a syringe pump (NE-300, Era Pump Systems). The fibers were calcined at 800 °C for 4 h using an air furnace with a heating rate of 0.5 °C min⁻¹ to completely remove the organic components in the fibers. This slow heating rate (0.5 °C min⁻¹) was used because the catalyst might not retain its fiber if the decomposition of PVP was too fast. Finally, the air furnace was naturally cooled to room temperature. The Na₂WO₄-Mn_xO_y/SiO₂ fibers were obtained and used in the activity test.

The Na₂WO₄-Mn_xO_y/SiO₂ powder catalyst was prepared using a co-impregnation method. Following our previous work’s our optimum ratio of Na₂WO₄:Mn_xO_y on SiO₂ (Chukeaw et al., 2019), the weight percentages (wt%) of Na₂WO₄ and Mn_xO_y were selected for inclusion in the final catalyst using the fume SiO₂ as the support at 6% and 4%, respectively. For example, the preparation of 100 mg of 6 wt% Na₂WO₄ + 4 wt% Mn_xO_y on the SiO₂ support was as follows. The Na₂WO₄·2H₂O and Mn (NO₃)₂·4H₂O precursors were dissolved in deionized water to make concentrations of 0.64 and 1.44 M, respectively. The Na₂WO₄·2H₂O solution at 34.8 µL and the Mn (NO₃)₂·4H₂O at 50.7 µL were pipetted onto the SiO₂ support (90 mg). The mixture was stirred at room temperature for 2 h and dried at 100 °C for 2 h or until dry. The dried powder was calcined using the same conditions as the Na₂WO₄-Mn_xO_y/SiO₂ fibers, except that the heating rate used was 5 °C min⁻¹. The Na₂WO₄-Mn_xO_y/SiO₂ powder catalyst was obtained and used in the activity test. A SiO₂ bare support was prepared in parallel with the Na₂WO₄-Mn_xO_y/SiO₂ powder using the fumed silica without the addition of active metals.

2.1.3

2.1.3 Preparation of Na₂WO₄-Mn_xO_y/La₂O₃ fibers and powder

The Na₂WO₄-Mn_xO_y/La₂O₃ fibers were prepared using the electrospinning method, with a slightly different procedure for that in section 2.1.2. The weight percentages of Na₂WO₄ at 6% and Mn_xO_y at 4% supported on La₂O₃ were selected for inclusion in the final catalyst. First, a mixture of ethanol (1 mL) and acetic acid (1 mL) was mixed and stirred for 1 h, denoted as Solution (A). Meanwhile, PVP (1.32 g) was mixed with ethanol (5 mL) and stirred for 2 h at room temperature until the solution was well mixed, denoted as Solution (B). Then, Solution (A) was added dropwise to Solution (B) with stirring for 6 h until a sol–gel precursor solution was obtained. After that, the Na₂WO₄-Mn_xO_y/La₂O₃ fiber was prepared using the parameters: solution feeding rate of 0.4 mL h⁻¹, setting voltage of 16 kV, calcination temperature of 625 °C, and heating rate of 2 °C min⁻¹.

The Na₂WO₄-Mn_xO_y/La₂O₃ powder was also prepared using the co-impregnation method and based on a similar preparation to that of the Na₂WO₄-Mn_xO_y/SiO₂ powder in section 2.1.2, except (La(NO₃)₃·6H₂O (0.28 g) was used instead of the fumed SiO₂, the calcination temperature was 625 °C, and the heating rate was 5 °C min⁻¹. Notably, according to the thermogravimetric analysis (TGA) of the prepared catalysts (see Figure S1), all the PVP and impurities were completely decomposed at a temperature below 625 °C. In parallel, a La₂O₃ bare support was prepared, as was the previously mentioned SiO₂ bare support, using (La(NO₃)₃·6H₂O without the addition of active metals.

2.2

2.2 Testing activity of catalysts

All the prepared catalysts were tested for the OCM reaction using a plug flow reactor at atmospheric pressure. Each sample (50 mg) was packed in the middle of a quartz tube reactor (0.5 cm inner diameter and 40 cm length) and sandwiched between two layers of quartz wool. The feed gas consisted of CH₄ (99.999%, Praxair), O₂ (99.999% Linde), and N₂ (99.999%, LABGAZ) with a gas ratio of CH₄: O₂: N₂ equal to 3:1:4—CH₄ and O₂ are the reactant gases and N₂ is the carrier gas—and a total feed flow rate of 35 mL min⁻¹. All the feed flow rates were controlled using mass flow controllers (Aalborg GFC17S). The reactor temperatures were in a range of 450–750 °C. The effluent gases were analyzed using online gas chromatography (GC, Shimadzu, GC-14A). An Unibead C column connected to a thermal conductivity detector (TCD) was used to determine CO, CO_2, and CH₄. A Porapak Q column connected to a flame ionization detector (FID) was used to analyze C₂H₄, C₂H₆, C₃H₆, C₃H₈, C₄H₈, and C₄H₁₀. An example of a GC chromatogram is shown in Figure S2. The activity of each catalyst was analyzed after the reactor system had reached the set point for 1 h. To quantify each gaseous product, a standard calibration curve of the gas with a coefficient of determination (R²) greater than 0.99 was made using five points of the gas concentration. The catalyst activity was reported in terms of % CH₄ conversion, % C₂₊ selectivity, % CO_x selectivity, % C₂₊ yield, and olefin-to-paraffin ratio, which were calculated using equations (3)–(7), respectively:

(3)

% {CH}_{4} c o n v e r s i o n = \frac{2 (n_{C_{2} H_{4}} + n_{C_{2} H_{6}}) + 3 (n_{C_{3} H_{6}} + n_{C_{3} H_{8}}) + 4 (n_{C_{4} H_{10}}) + n_{CO} + n_{{CO}_{2}}}{2 (n_{C_{2} H_{4}} + n_{C_{2} H_{6}}) + 3 (n_{C_{3} H_{6}} + n_{C_{3} H_{8}}) + 4 (n_{C_{4} H_{10}}) + n_{CO} + n_{{CO}_{2}} + n_{{CH}_{4, o u t}}} \times 100

(4)

% C_{2 +} s e l e c t i v i t y = \frac{2 (n_{C_{2} H_{4}} + n_{C_{2} H_{6}}) + 3 (n_{C_{3} H_{6}} + n_{C_{3} H_{8}}) + 4 (n_{C_{4} H_{10}})}{2 (n_{C_{2} H_{4}} + n_{C_{2} H_{6}}) + 3 (n_{C_{3} H_{6}} + n_{C_{3} H_{8}}) + 4 (n_{C_{4} H_{10}}) + n_{CO} + n_{{CO}_{2}}} \times 100

(5)

% {CO}_{x} s e l e c t i v i t y = \frac{n_{CO} + n_{{CO}_{2}}}{2 (n_{C_{2} H_{4}} + n_{C_{2} H_{6}}) + 3 (n_{C_{3} H_{6}} + n_{C_{3} H_{8}}) + 4 (n_{C_{4} H_{10}}) + n_{CO} + n_{{CO}_{2}}} \times 100

(6)

% C_{2 +} y i e l d = \frac{% {CH}_{4} c o n v e r s i o n \times % C_{2 +} s e l e c t i v i t y}{100}

(7)

\frac{Olefin}{Paraffin} = \frac{2 (n_{C_{2} H_{4}}) + 3 (n_{C_{3} H_{6}})}{2 (n_{C_{2} H_{6}}) + 3 (n_{C_{3} H_{8}})}

Thus, the data were reported as an average value with an error value of each catalyst, based on at least three repeats of each experiment. An example of carbon balance checks is shown in Figure S3 and Table S3. Note that the activity of each catalyst was presented in terms of the values obtained from equations (3)–(7). The amounts of H₂O, H₂, N₂, and O₂ were, therefore, not collected in every test.

2.3

2.3 Characterization of catalysts

TGA was used to evaluate the thermostability of each sample using a thermogravimetric analyzer (model TGA/SDTA 851e, Mettler Toledo, USA). To determine the content of moisture and organic compounds, the measurement for the thermal analysis was carried out over a range from 30 to 900 °C at a heating rate of 10 °C min⁻¹ in N₂ gas (50 mL min⁻¹).

An X-ray diffractometer (XRD; JEOL JDX-3530 and Philips X-Pert, using Cu- $K_{Î±}$ radiation at 45 kV and 40 mA with a step size of 0.02° and a step time of 0.5 s) was used to identify the crystalline phases of the prepared catalysts.

A nitrogen adsorption–desorption analyzer (3Flex Physisorption Micrometrics) was used to determine the surface areas, pore sizes, and pore volumes of the catalysts at −196 °C. The average specific surface area of each catalyst was calculated using the Brunauer-Emmett-Teller (BET) method in a range of P/P₀ values of 0.05–0.30. The pore size distribution of each catalyst was determined using the Barrett-Joyner-Halenda method. The total pore volume was estimated at a relative pressure (P/P₀) of 0.995.

A scanning electron microscope with an energy dispersive X-ray spectrometer (SEM/EDS; JEOL, JSM7600 F) was used to investigate the surface morphology and the distribution of metals on the surface of the catalysts.

A high angle annular dark-field scanning transmission electron microscope and energy dispersive X-ray spectrometer (HAADF-STEM/EDS; JEOL, JEM-2010) were used to investigate element dispersion at an atomic scale. The transmission electron microscopy imaging was operated at 200 kV. The sample for measurement was prepared by dispersing the catalyst in ethanol solution for 30 min, and then dropping the dispersant onto a copper grid and drying it in air at room temperature.

An X-ray photoelectron spectrometer (XPS; Kratos Axis Ultra DLD, using Al K_α for the X-ray source) was used to determine the elemental composition of the catalysts and the binding energy of each element in the catalysts. The binding energy of C 1s was used to standardize all binding energies.

3 Results and discussion

3.1

3.1 XRD analysis of catalysts

The Na₂WO₄-Mn_xO_y/SiO₂ and Na₂WO₄-Mn_xO_y/La₂O₃ fibers (denoted as NWMSi-F and NWMLa-F, respectively) and powders (denoted as NWMSi-P and NWMLa-P, respectively) were synthesized and tested for OCM reaction under the same conditions. The XRD patterns of the NWMSi and NWMLa are presented in Fig. 1 and their peak positions of each catalyst are shown in Table S4. In Fig. 1(a), the crystallinity of NWMSi-F was virtually the same as NWMSi-P. Furthermore, the characteristic peak of α-cristobalite (ICDD No. 00–001-0438)) appeared in both NWMSi-P and NWMSi-F. This confirmed that the α-cristobalite phase occurs at low calcination temperature (800 °C) due to the presence of Na (Kiani et al., 2019; Kidamorn et al., 2020) so that the amorphous SiO₂ phase can transform to the α-cristobalite phase at a lower calcination temperature than the usual temperature (∼1500 °C) (Kim and McIntyre, 2002; Yildiz et al., 2014). Additionally, small peaks of α-tridymite and Mn₂O₃ were observed in both NWMSi-P and NWMSi-F. Note that it was speculated that the XRD peak at 2θ of approximately 33° could be identified as Mn₂O₃ and Mn₇SiO₁₂. But in accordance with data concerning similar catalysts in the literature, its Raman spectrum indicated that this XRD peak should be assigned to the crystalline Mn₂O₃ phase (Ortiz-Bravo et al., 2021b). The characteristic peak of Na₂WO₄ also appeared in both NWMSi-P and NWMSi-F. In Fig. 1(b), the characteristic XRD peaks of La₂O₃, La(MnO₃), and La(OH)₃ are apparent in both NWMnLa-P and NWMnLa-F. The characteristic XRD peaks of Na₂WO₄ and Mn₂O₃ could not be observed, probably due to the high dispersion of small Na₂WO₄ particles in the fibers. The crystallinity of NWMLa-F seemed to be lower than that of NWMLa-P, which could be the effect of the high dispersion of active metals in the fiber catalysts (see more detail in HAADF-STEM images in Fig. 4. Notably, the small peaks of the La(OH)₃ phase were also observed because the La₂O₃ phase could interact with moisture in the air and transform into the La(OH)₃ phase (Nguyen et al., 2014) before the XRD measurement.

XRD patterns of fresh (a) NWMSi and (b) NWMLa in powder and fiber forms.

3.2

3.2 N₂ adsorption–desorption analysis of catalysts

The NWMSi and NWMLa in both powder and fiber forms were determined for their BET surface area, pore size, and pore volume using the N₂ adsorption–desorption analyzer. The results are shown in Table 1 and Fig. 2 and the plots of their pore size distribution are displayed in Figure S4. The pure Si and La in the forms of both powder and fiber were characterized using the same technique to compare the results with those catalysts. As seen in Table 1, the addition of NWM on either the Si or La support resulted in a decreased BET surface area. However, the addition of NWM on the Si support produced a much greater reduction of the BET surface area relative to the La support probably because the amorphous SiO₂ phase had converted to the α-cristobalite phase that normally has a low BET surface area (<10 m² g⁻¹). All the catalysts in both powder and fiber forms had a BET surface area in the range 3.9–8.5 m² g⁻¹. Additionally, the pore volume of every sample was relatively low, indicating that every sample had a small number of intraparticle pores. Consequently, the pore size of every sample could have been the size between particles (the interparticle pore size).

Table 1 BET surface area, pore-volume, and pore size of Si- and La-based catalysts in the powder and fiber forms.

Catalyst	BET Surface area (m² g⁻¹)	Pore-volume (cm³ g⁻¹)	Pore size (nm)
Si-P	79.7	0.23	1.4
Si-F	20.6	0.01	1.4
NWMSi-P	4.5	0.01	1.5
NWMSi-F	3.9	0.01	2.7
La-P	9.7	0.05	1.4
La-F	10.4	0.06	1.2
NWMLa-P	7.0	0.04	1.2
NWMLa-F	8.5	0.04	8.6

N2 adsorption–desorption isotherm of fresh (a) Si-based and (b) La-based catalysts.

The adsorption–desorption behavior of N₂ adsorbed onto the surface of the fresh Si-based and La-based catalysts are displayed in Fig. 2(a) and 2(b), respectively, in terms of the plots between the quantity of N₂ adsorbed and the relative pressure (P/P⁰). In both Fig. 2(a) and 2(b), the adsorption–desorption isotherms of all the catalysts could be classified into Type IV with an H3 hysteresis loop according to the International Union of Pure and Applied Chemistry classification (Alothman, 2012), indicating that the catalytic materials had a mesoporous structure and a wide pore size distribution. However, among the catalysts in Fig. 2(a), Si-P had a greater amount of N₂ adsorbed onto its surface relative to the others perhaps because it had a much greater BET surface area than the others. Furthermore, the hysteresis loops of Si-F, NWMSi-P, and NWMSi-F were barely observed, implying that these catalysts had been almost transformed into nonporous material.

3.3

3.3 Surface morphology and elemental distribution of catalysts

The surface characteristics of each catalyst analyzed using SEM are shown in Fig. 3. Fig. 3(a)–(d) show the surface morphology of powder catalysts that were prepared using the co-impregnation method. Comparing the Si-P and NWMSi-P catalysts as shown in Fig. 3(a)–(b), respectively, it was clear that the addition of NWM into the SiO₂ support increased the particle size of catalysts, and thus the BET surface area of NWMSi-P was largely reduced, as previously shown in Table 1. For La-P and NWMLa-P as shown in Fig. 3(c) and 3(d), respectively, the addition of NWM into the pure La powder resulted in a small change in the surface appearance of NWMLa-P; there seemed to be more agglomeration of particles in NWMLa-P than in La-P. Thus, the BET surface area of NWMLa-P was less than for La-P, consistent with the results in Table 1.

SEM micrographs of (a) Si-P, (b) NWMSi-P, (c) La-P, d) NWMLa-P, (e) Si-F, (f) NWMSi-F, (g) La-F, and (h) NWMLa-F.

Elemental distribution analysis of NWMSi-P, NWMSi-F, NWMLa-P, and NWMLa-F recorded by HAADF-STEM with EDS.

Fig. 3(e)–(h) show the surface morphology of fiber catalysts prepared using the electrospinning method. As observed, the catalysts in the fiber form were successfully prepared. As shown in Fig. 3(e), Si-F has long rod fibers (approximately 6.8 µm in diameter) with a smooth surface. However, NWMSi-F, as shown in Fig. 3(f), has long fibers with a much smaller diameter (approximately 0.9 µm in diameter) and a rough surface. This rough surface could be due to the presence of Na and the transformation of amorphous SiO₂ to α-cristobalite. Fig. 3(g) and 3(h) illustrate the fibers of pure La (La-F) and the addition of NWM into La (NWMLa-F), respectively, with a lot of cracks on the surface and body of La-F, with no cracking on the surface of NWMLa-F that had a non-smooth surface. This led to the overall length of La-F being shorter than for NWMLa-F and thus, the BET surface area of La-F was slightly larger than for NWMLa-F, as previously shown in Table 1. Nevertheless, the average diameter sizes of La-F and NWMLa-F were approximately the same at 1.5 µm, suggesting that the addition of NWM into Si had a greater effect on the diameter size of the fibers relative to adding NWM into La.

The elemental distribution of selected catalysts was investigated using HAADF-STEM with EDX, as shown in Fig. 4. Much lower color spots (i.e. low concentrations) of the active elements (Na, W, and M) relative to those for the elements (La and Si) of the supports are observed in every catalyst since the total metal loading is only 10 wt% on each support. Nonetheless, Uniform dispersions of Na and W can be observed in every catalyst. The Mn is also well-distributed in every catalyst except for the NWMSi-P, in which some large particle clusters of Mn are observed. This may suggest that the overall distribution of the elements in the catalysts prepared in the fiber form (NWMSi-F and NWMLa-F) is better than those in the powder form.

3.4

3.4 Performance of catalysts

All the prepared catalysts were tested for the OCM reaction in the temperature range 400–750 °C and the results are shown in Fig. 5. The catalytic performances of Si-F and Si-P (see Fig. 5(a)) were much lower relatively compared to the catalysts with the presence of the active metals (Na₂WO₄ and Mn_xO_y). The highest C₂₊ yields of Si-F and Si-P were obtained at 4.0% (32.8% C₂₊ selectivity with 12.2% CH₄ conversion) and 2.7% (27.6% C₂₊ selectivity with 9.7% CH₄ conversion) at 750 °C, respectively. NWMSi-P had the highest C₂₊ yield at 17.9% with 33.8% CH₄ conversion and 53.0 % C₂₊ selectivity at 750 °C, as shown in Fig. 5(b), indicating that the addition of Na, W, or Mn on SiO₂ improved the overall activity for C₂₊ formation. When the same catalyst was in the form of fiber (NWMSi-F), the C₂₊ yield increased even more from 2.1 to 7.6% and 17.3 to 20.4% at 650 and 700 °C, respectively. Notably, the NWMSi catalysts in either form did not show any product when the reactor temperature was below 600 °C.

OCM reaction performance of different catalysts in fiber and powder forms: (a) Si, (b) NWMSi, (c) La, (d) NWMLa. Reaction conditions: 50 mg catalysts under 35 mL min−1 and feed gas ratio of CH4:O2:N2 of 3:1:4.

The performance of La-F and La-P is shown in Fig. 5(c). The highest C₂₊ yields of La-F and La-P were 9.2% (34.4% C₂₊ selectivity with 26.7% CH₄ conversion) at 650 °C and 10.5% (40.8% C₂₊ selectivity 25.7% CH₄ conversion) at 700 °C, respectively. The addition of NWM into the La support provided the highest C₂₊ yield of 8.5% (36.3% C₂₊ selectivity with 23.4% CH₄ conversion) at 700 °C when the catalyst was in the form of powder, and 9.6% (38.9% C₂₊ selectivity with 24.1% CH₄ conversion) at 650 °C when the catalyst was in the form of fiber, as shown in Fig. 5(d). In fact, these C₂₊ yields for NWMLa-P and NWMLa-F at 700 °C were slightly lower than for La-P and La-F at 700 °C, indicating that the addition of NWM did not improve the overall activity for C₂₊ formation. Nevertheless, the activation temperatures for both NWMLa-P and NWMLa-F were lower than those for La-F and La-P at approximately 100 °C, indicating that the addition of NWM into La benefited by reducing the activation temperature of CH₄. In Fig. 5(d), the activation temperature of NWMLa-F started at 500 °C, the same as for the powder form. Nonetheless, comparing the results for the same testing conditions, the production of C₂₊ by NWMLa-F was greater than that of NWMLa-P, indicating that the activity of the catalyst in the fiber form was advantageous relative to the powder form. This excellent performance of the fiber catalysts could be ascribed to the excellent heat and mass transfer properties of the catalytic fibers compared to the powder ones (Sollier et al., 2020). In other words, the heat removal in the fiber catalysts is better than that in the powder catalysts (Chai et al., 2017; Zhao et al., 2011) and the products may diffuse out from the fiber catalyst bed easier than the powder one (Matatov-Meytal and Sheintuch, 2002). Notably, the activities (C₂₊ selectivity, CH₄ conversion, and C₂₊ yield) of the catalysts in Fig. 5 (b), (c), and (d) gradually decreased or minimally changed after passing the highest CH₄ conversion (at approximately 30%). That is because the highest CH₄ conversion obtained is when the oxygen in the feed is completely consumed, as indicated by the GC chromatograms. At that point, the activities cannot further increase, but they may decrease if the C₂₊ product can react with some active species (e.g. *OH radical) and produce undesirable CO_x (Karakaya et al., 2018; Kidamorn et al., 2020).

3.5

3.5 Olefin-to-paraffin ratios of catalysts

Based on the OCM mechanism, it is generally known that the activation of CH₄ creates *CH₃ radicals before proceeding to form the C₂₊ products. Therefore, one factor in an effective catalyst in OCM reaction is the catalyst ability for dehydrogenation, which can be specified using the olefins-to-paraffins ratio. The results from Fig. 5 were re-plotted in terms of the olefins-to-paraffins ratio, as shown in Fig. 6 and Table S5. In Fig. 6(a), NWMSi-F provided the highest ratio at every temperature above 550 °C, followed by NWMSi-P, Si-F, and Si-P, respectively. In Fig. 5(b), NWMLa-F had the highest C₂₊ yield among the La-containing catalysts with olefins-to-paraffins ratios in the range 0.7–1.2, similar to the other La-containing catalysts in the temperature range 600–700 °C. At 750 °C, NWMLa-P and La-P achieved relatively high olefins-to-paraffins ratios (1.8–4.2) compared to the other La-containing catalysts in the fiber form. Nevertheless, the highest olefins-to-paraffins ratio among all the prepared catalysts was for NWMSi-F at 6.4 at 750 °C; this catalyst also had excellent C₂₊ activity relative to the others at 700–750 °C. However, its C₂₊ activity was not apparent at low temperatures (500–600 °C), unlike the La-containing catalysts.

Olefin-to-paraffin ratio versus reactor temperature of (a) Si-based and (b) La-based catalysts.

3.6

3.6 Surface oxygen property analysis using XPS

In the OCM reaction mechanism, it is known that the CH₄ molecule is activated by the surface lattice oxygen species. Thus, the type of oxygen species on the catalyst surface is essential for the activation of CH₄ (Kwon et al., 2021; Sim et al., 2020; Xu et al., 2019 ). Therefore, the surface oxygen species of the catalysts (see Fig. 7) were investigated using XPS. The binding energies of specific species are summarized in Table S6. It is important to note that the y-axes between Fig. 7 (a) and (b) are plotted on a different scale (see the relative XPS peak area of each species for clarification in Table S7). The XPS spectra of Na, W, and Mn are shown in Figure S5. As shown in Fig. 7(a), Si-F had three main peaks at 535.0, 533.3, and 531.3 eV, corresponding to the superoxide oxygen ( $O_{2}^{-}$ ) (Sollier et al., 2020; Xu et al., 2019), Si—O (C.D. Wagner et al., 1982; Larina et al., 2015), and lattice oxygen (O^2–) (Ramana et al., 2011), respectively. NWMSi-F had five main peaks at 534.7, 533.8, 532.8, 531.3, and 529.7 eV, corresponding to Na—O—W (Dadachova et al., 1995), $O_{2}^{-}$ , Si—O, O^2–, and Mn—O (Yang et al., 2015), respectively. The binding energy of Si—O in NWMSi-F was considerably changed to a lower energy (532.8 eV) compared to that of Si-F because the SiO₂ component in Si-F is amorphous, while the SiO₂ component in NWMSi-F is α-cristobalite. In Fig. 7(b), La-F presented three main peaks, including the approximated peaks at 532.6 eV assigned to $O_{2}^{-}$ (Huang et al., 2013), 531.5 related to oxygen atoms of hydroxyl and carbonate groups (Huang et al., 2013; Ramana et al., 2011), and 529.1 eV assigned to the lattice oxygen (O^2–) in La₂O₃ (Huang et al., 2013; Li et al., 2019; Ramana et al., 2011 ). NWMLa-F had five main peaks at 534.5, 532.9, 531.7, 530.7, and 529.2 eV, indicating the presence of oxygen species of Na—O—W, $O_{2}^{-}$ , O^2–, Mn—O, and La—O, respectively. The surface lattice oxygen (O^2–) led to the formation of CO_x by the complete oxidation of methane (Huang et al., 2013; Jiang et al., 2016; Song et al., 2015; Xu et al., 2018; Xu et al., 2019 ). In comparison, the peak area of O^2– in La-F was greater than that of NWMLa-F, corresponding to the CO_x selectivity of La-F that was greater than for NWMLa-F, as shown in Fig. 5(c) and Fig. 5(d). The surface-adsorbed oxygen species ( $O_{2}^{-}, superoxide oxygen)$ have been claimed as active sites promoting C—H activation (i.e. increase CH₄ conversion) in the OCM reaction (Schwach et al., 2017; Sollier et al., 2020; Xu et al., 2019 ). Based on the XPS data in Fig. 7 and the XPS peak area in Table S7, the peak area of $O_{2}^{-}$ in NWMLa-F was lower than for the NWMSi-F catalyst, corresponding to the CH₄ conversion of NWMLa-F being lower than for the NWMSi-F catalyst. Notably, the peak area of the oxygen of Na—O—W in NWMLa-F was lower than for NWMSi-F; this was directly related to the amount of CH₄ conversion, confirming that the oxygen in Na—O—W was important for the activation of CH₄.

XPS O 1s spectra of (a) Si-F and NWMSi-F and (b) La-F and NWMSi-F catalysts.

3.7

3.7 Stability test of catalysts

A time-on-stream experiment over 24 h of each catalyst at its optimal conditions was carried out and the activity results (CH₄ conversion, C₂₊ selectivity, and C₂₊ yield) are shown in Fig. 8. The performance of NWMSi-P constantly increased to a maximum of 18.4% C₂₊ yield with 53.3% C₂₊ selectivity and 34.5% CH₄ conversion after 4 h of testing. After that, it slightly decreased to 17.5 % C₂₊ yield with 55.7% C₂₊ selectivity and 31.4% CH₄ conversion until the end of testing, decreasing by 28.7% in terms of C₂₊ yield. In contrast, the activity of NWMSi-F was good with an almost constant C₂₊ yield of 18.2–19.7 % (55.0–55.2% C₂₊ selectivity and 32.2–35.6 % CH₄ conversion), which was a reduction in the C₂₊ yield of approximately 8.6% over the testing period. The C₂₊ yields for NWMLa-P and NWMLa-F were similar, being maintained at approximately 7.7 and 8.7%, respectively, while their C₂₊ yield reductions were relatively low at approximately 12 and 11.5%, respectively, compared to the activity of NWMSi-P.

Catalyst stability tests for NWMSi-P, NWMSi-F, NWMLa-P, and NWMLa-F: (a) C2+ yield, (b) C2+ selectivity, and (c) CH4 conversion. Reaction conditions: 50 mg catalysts under 35 mL min−1 for CH4:O2:N2 flow rate of 3:1:4, reactor temperature of 700 °C for NWMSi-P and NWMSi-F and 650 °C for NWMLa-P and NWMLa-F.

To identify the causes of catalyst deactivation, the surface and chemical properties of the catalysts used for 24 h were examined using SEM and XRD, as shown in Fig. 9 and Fig. 10, respectively. The particle sizes of the used NWMSi-P and used NWMSi-F catalysts (Fig. 8(a) and (b), respectively) were much larger than for their fresh catalysts, with one possible cause of the deactivation of the catalyst being the aggregation of particles. Nevertheless, the cylindrical-shaped fibers could still be observed, as shown in Fig. 9(b). Fig. 9(c) shows there was no considerable change in the physical structure of NWMLa-P after use for 24 h. Fig. 9(d) shows that NWMLa-F had changed to cracked particles with various shapes and sizes probably because the NWMLa-F fibers were very fragile as the mechanical properties of NWMLa-F are not strong as for the Si-based fiber. However, there was no observed aggregation of NWMLa-F particles, suggesting that the catalyst deactivation caused by the aggregation was only applicable to the Si-based catalysts.

SEM images of used catalysts: (a) NWMSi-P, (b) NWMSi-F, (c) NWMLa-P, and (d) NWMLa-F.

XRD of fresh and used (a) NWMSi and (b) NWMLa in powder and fiber forms.

The differences in the XRD patterns of the fresh and used catalysts are shown in Fig. 9. The Na₂WO₄ peaks at 2θ = 16.8° and 27.6° of the used NWMSi-P had disappeared relative to the fresh NWMSi-P, as shown in Fig. 9(a). The Mn₂O₃ peak intensities of the used NWMSi-P at 2θ = 32.9°, 38.3°, and 55.3° indicated a slight reduction, suggesting that one of the causes of the deactivation of NWMSi-P was the loss of active phases, especially for Na₂WO₄ that could evaporate at 700 °C as part of the operating conditions for the long-term testing period (Hayek et al., 2019). However, the Na₂WO₄ peaks of the used NWMSi-F were still present with a slight reduction of the peak intensity relative to those of the fresh NWMSi-F, suggesting that the performance of NWMSi-F in the stability test was excellent for the time-on-stream testing due to the small loss of the Na₂WO₄ phase. The loss of the Na₂WO₄ phase in NWMSi-F was lower compared to NWMSi-P probably because during the preparation of NWMSi-F, all components (Na, W, Mn, and Si) were homogeneous and thus, the Na₂WO₄ phase was well distributed throughout the catalyst. In contrast, for NWMSi-P the Na, W, and Mn components were only located on the surface of the SiO₂ particles, leading to the Na₂WO₄ phase being easily lost during the reaction. Fig. 9(b) shows that the peaks of Na₂WO₄ for both NWMLa-P and NWMLa-F had no considerable change after testing. The peak intensities of La₂O₃ for both used NWMLa-P and used NWMLa-F slightly reduced relative to the fresh ones, while the peaks for La(MnO₃) completely disappeared. Nevertheless, these XRD changes caused no considerable changes in the performance of the NWMLa-P and NWMLa-F catalysts.

3.8

3.8 Current catalysts compared to others

A comparison plot between C₂₊ selectivity and CH₄ conversion of the current catalysts from this study with various NWM-based and La-based catalysts is displayed in Fig. 11. The details of each catalyst are described in Tables S1 and S2. A viable OCM catalyst should have a CH₄ conversion of more than 30% and a C₂₊ selectivity of more than 80% (Zavyalova et al., 2011). Many catalysts with over 30% CH₄ conversion have been identified, but their C₂₊ selectivity is, unfortunately, <80%. Since the majority of these catalysts were used at a reaction temperature around 700–800 °C, superior catalysts at relatively low activation temperatures (around 500 °C), like our NWMLa-Si, are attractive.

Survey of various NWM-based and La-based catalysts reported in the literature, including the catalysts in this work.

4 Conclusion

The performance of Na₂WO₄-Mn_xO_y supported on SiO₂ or La₂O₃ catalysts in the forms of powder and fiber for the OCM reaction was investigated. The XRD results and the SEM images confirmed that the desired catalysts were successfully prepared. The catalytic performance results showed that the fiber catalysts enhanced the CH₄ conversion and the C₂₊ selectivity because of the improvement of heat and mass diffusion in the catalyst bed. NWMSi in both powder and fiber forms activated the reaction at approximately 650 °C, with greater activation levels than for NWMLa in both powder and fiber forms that activated the reaction at 500 °C. The activation of CH₄ at low temperature was strongly related to the presence of the oxygen species at a relatively lower binding energy allowing the oxygen species to be easily allocated during the reaction. Nevertheless, the C₂₊ yield of the NWMSi catalysts, especially for NWMSi-F, was greater than for NWMLa under the same conditions, because of the synergistic catalytic effect of the active phases, including Na₂WO₄, Mn_xO_y, and α-cristobalite. The maximum C₂₊ yield of 20.4% with 34.0% CH₄ conversion and 60.1% C₂₊ selectivity was achieved from NWMSi-F at 700 °C. The stability test of the catalysts revealed that all catalysts had excellent stability over the 24 h period of testing, except for NWMSi-P. The loss of the active Na₂WO₄ phase was suspected as the main cause of catalyst deactivation. The fiber catalysts performed better than the powder forms in the OCM reaction and thus, future studies of catalyst development in the OCM reaction based on fibrous catalysts are strongly recommended.

CRediT authorship contribution statement

Kanticha Jaroenpanon: Data curation, Formal analysis, Investigation, Methodology, Writing – original draft. Worapinit Tiyatha: Data curation, Investigation, Methodology. Thanaphat Chukeaw: Data curation, Investigation, Methodology. Sarannuch Sringam: Data curation, Investigation. Thongthai Witoon: Resources, Writing – review & editing. Chularat Wattanakit: Resources, Writing – review & editing. Metta Chareonpanich: Resources, Writing – review & editing. Kajornsak Faungnawakij: Resources, Writing – review & editing. Anusorn Seubsai: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review & editing.

Acknowledgments

This research work was funded by: the National Research Council of Thailand (NRCT5-RSA63002-11) Thailand; the Kasetsart University Research and Development Institute Thailand; the Center of Excellence on Petrochemical and Materials Technology, Thailand; and the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its Research Network program NANOTEC (RNN).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Appendix A

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2021.103577.

Appendix A

Supplementary material

The following are the Supplementary data to this article:

Supplementary data 1

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Synthesis of Na2WO4-MnxOy supported on SiO2 or La2O3 as fiber catalysts by electrospinning for oxidative coupling of methane

Abstract

Keywords

Fiber catalyst

La2O3

Na2WO4

Oxidative coupling of methane

SiO2

1 Introduction

2 Materials and methods

2.1 Preparation of catalysts

2.1.1 Precursor materials

2.1.2 Preparation of Na2WO4-MnxOy/SiO2 fibers and powder

2.1.3 Preparation of Na2WO4-MnxOy/La2O3 fibers and powder

2.2 Testing activity of catalysts

2.3 Characterization of catalysts

3 Results and discussion

3.1 XRD analysis of catalysts

3.2 N2 adsorption–desorption analysis of catalysts

3.3 Surface morphology and elemental distribution of catalysts

3.4 Performance of catalysts

3.5 Olefin-to-paraffin ratios of catalysts

3.6 Surface oxygen property analysis using XPS

3.7 Stability test of catalysts

3.8 Current catalysts compared to others

4 Conclusion

CRediT authorship contribution statement

Acknowledgments

Declaration of Competing Interest

References

Appendix A

Supplementary material

Appendix A

Supplementary material

Supplementary data 1

Suggested read for related articles:

Synthesis of Na₂WO₄-Mn_xO_y supported on SiO₂ or La₂O₃ as fiber catalysts by electrospinning for oxidative coupling of methane

2.1.2 Preparation of Na₂WO₄-Mn_xO_y/SiO₂ fibers and powder

2.1.3 Preparation of Na₂WO₄-Mn_xO_y/La₂O₃ fibers and powder

3.2 N₂ adsorption–desorption analysis of catalysts