Ti-Doped SiO2-Supported

Research Article pubs.acs.org/journal/ascecg

Oxidative Coupling of Methane Using Mg/Ti-Doped SiO2‑Supported Na2WO4/Mn Catalysts Rika Tri Yunarti,†,‡,∇ Sangseo Gu,†,§ Jae-Wook Choi,† Jungho Jae,†,‡ Dong Jin Suh,†,‡,∥ and Jeong-Myeong Ha*,†,‡,∥ †

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea Korea University of Science and Technology, Daejeon 34113, Republic of Korea ∇ Department of Chemistry, Faculty of Mathematics and Natural Sciences, University of Indonesia, Depok 16424, Indonesia § Department of Chemical and Biological Engineering and ∥Green School (Graduate School of Energy and Environment), Korea University, Seoul 02841, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The oxidative coupling of methane (OCM) was performed using Na2WO4/Mn catalysts supported on Mg/Ti/Si mixed oxides. Na2WO4/Mn/Mg0.05Ti0.05Si0.90On exhibited a 23.1% C2 yield at 800 °C, which is a 35% higher C2 yield than that obtained using conventional Na2WO4/Mn/SiO2 in this study. The formation of Mg/Ti-doped α-cristobalite SiO2 was observed for supported Na2WO4/Mn catalysts, and their improved catalytic activity could be attributed to the suppressed incorporation of Na/ W/Mn compounds into the α-cristobalite SiO2 structure and the existence of OCM-active Na/W/Mn compounds exposed on the support surface. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and O2-temperature-programmed desorption were performed in an effort to understand the improved catalytic OCM activity of Mg/Ti-doped SiO2-supported Na2WO4/ Mn catalysts. KEYWORDS: Oxidative coupling of methane, Mg/Ti/Si mixed oxide, Catalyst, Methane upgrading

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INTRODUCTION

The OCM reaction occurs through a heterogeneous− homogeneous mechanism. The production of methyl radicals proceeds through methane activation on the catalyst surface; this is followed by the gas-phase coupling of methyl radicals, leading to diverse reaction products and determining the selectivity.17,20−22 A good catalyst to achieve desirable OCM performance must initiate the formation of methyl radicals at a lower temperature and be selective upon surface oxidation to avoid deep oxidation to CO and CO2. Research has focused on increasing the OCM activity and decreasing the oxidation rate to CO and CO2 products. The OCM performance is influenced considerably by the relationships between the gas reactants and the catalyst surfaces. The selectivity of the reaction can be adjusted by changing the design of the catalyst. Na2WO4/Mn/SiO2 is one of the most effective catalysts for OCM, with the optimum

Methane, the major component of natural gas, is a potential raw material that can be used as a feedstock for the production of hydrocarbons; its upgrading to valuable chemical building blocks may create sustainable value chains using abundant natural gas.1 While the exploration of shale gas increases the potential applications of methane,2−4 the thermochemical conversion or biological anaerobic digestion of biomass can also produce methane from renewable sources.5−8 Methanerich biogas from domestic waste, residue, and energy crops is replacing natural gas.9−12 Electrochemical methods to produce methane from water and carbon dioxide have also been reported.13,14 The direct conversion of methane through the oxidative coupling of methane (OCM) to ethane and ethylene at a high temperature has been studied for many years as an alternative means of producing valuable hydrocarbons from natural gas,15−17 and several possible mechanisms of methane coupling were proposed.18−20 In order to facilitate industrial applications, numerous catalytic materials have been studied to enable the OCM reaction to achieve high selectivity of C2 and higher carbon-number products at a high methane conversion. © 2017 American Chemical Society

Special Issue: Asia-Pacific Congress on Catalysis: Advances in Catalysis for Sustainable Development Received: December 1, 2016 Revised: February 27, 2017 Published: March 2, 2017 3667

DOI: 10.1021/acssuschemeng.6b02914 ACS Sustainable Chem. Eng. 2017, 5, 3667−3674

Research Article

ACS Sustainable Chemistry & Engineering Table 1. OCM Reaction Results of Na2WO4/Mn Catalystsa

a

C2 selectivity (%)

C2 yield (%)

C2H4/C2H6 (mol/mol)

CH4 conversion (%)

O2 conversion (%)

catalyst

775 °C

800 °C

775 °C

800 °C

775 °C

800 °C

775 °C

800 °C

775 °C

800 °C

Na2WO4/Mn/SiO2 (silica gel) Na2WO4/Mn/SiO2 (fumed silica) NWM/Si(100)

33.4 − 30.9

33.5 33.3 33.4

100 − 100.0

100.0 98.1 100.0

50.8 − 49.3

51.0 52.7 51.1

16.9 − 15.3

17.1 16.5 17.1

2.0 − 1.8

2.3 2.4 2.3

NWMn/TiO2 NWM/Ti(1)Si(99) NWM/Ti(3)Si(97) NWM/Ti(5)Si(95) NWM/Ti(10)Si(90)

28.2 15.0 16.6 24.3 28.4

30.1 25.2 24.8 35.8 32.0

100.0 100.0 100.0 70.0 100.0

100.0 100.0 100.0 97.3 100.0

43.7 40.9 47.5 55.0 44.7

46.7 46.0 49.5 58.5 47.2

12.3 6.1 7.9 13.4 12.7

14.1 11.6 12.3 20.9 15.1

1.5 0.8 1.0 1.5 2.0

1.8 1.6 2.0 2.2 2.3

NWM/Mg(5)Si(95) NWM/Mg(1)Ti(5)Si(94) NWM/Mg(3)Ti(5)Si(92) NWM/Mg(5)Ti(1)Si(94) NWM/Mg(5)Ti(3)Si(92) NWM/Mg(5)Ti(5)Si(90) NWM/Mg(5)Ti(10)Si(85) NWM/Mg(7)Ti(5)Si(88) NWM/Mg(10)Ti(5)Si(85)

5.6 26.1 32.3 24.0 25.8 30.9 14.41 25.6 18.5

19.5 28.0 35.2 28.9 29.5 38.3 27.4 29.1 25.5

17.1 89.8 84.6 100.0 100.0 76.7 100.0 84.2 54.1

65.3 99.8 100.0 100.0 100.0 98.7 100.0 97.3 78.8

46.0 40.3 58.7 44.9 43.7 62.3 33.6 42.6 49.9

40.4 37.3 53.6 44.1 43.8 60.3 43.9 41.2 46.5

2.6 10.5 19.0 10.8 11.3 19.3 4.8 10.9 9.3

7.8 10.4 18.9 12.8 12.9 23.1 12.0 12.0 11.9

0.4 2.0 2.0 1.5 1.7 1.7 1.1 2.1 0.7

1.5 2.4 2.6 2.2 2.5 2.4 1.9 2.7 2.0

Loading of 5 wt % of Na2WO4 and 2 wt % of Mn. Reaction condition: CH4:O2 = 3:1 (mol/mol); GHSV = 10,000 h−1.

concentration being 5 wt % Na2WO4 and 2 wt % of Mn.23,24 This catalyst has been studied by several researchers or research groups, including Li,25−31 Lambert,32,33 and Lunsford,34−36 and has been found to achieve appropriate performance with good long-term stability. With regard to this catalyst, sodium plays two important roles in the transformation of amorphous silica, with transformation into the crystalline α-cristobalite structure of the support as a favorable structure and the stabilization and dispersion of surface WOx species.37−40 It has been suggested that sodium polarizes the metal−oxygen bonds of tungsten and manganese oxides, increasing the surface mobility, and that W− O−Si species are responsible for the OCM reaction producing lattice oxygen atoms.41,42 The addition of manganese promotes oxygen mobility and the exchange between surface-adsorbed and lattice oxygen atoms.41,43 In this study, in order to develop favorable oxide composites that can achieve the optimum OCM activity, we prepared Na2WO4/Mn/Mgx/100Tiy/100Siz/100On (or NWM/Mg(x)Ti(y)Si(z), x + y + z = 100 atom %) catalysts through a one-pot sol− gel synthesis method. The addition of Ti to the silica support was selected based on our previous studies,44,45 which observed improved catalytic activity of TiO2-mixed catalyst pellets with SiO244 and the thermal stability of TiO2 nanowires catalysts.45 The OCM performance of Na2WO4/Mn/MgO was studied for the enrichment of Mn on the catalyst surface, although Na2WO4/Mn/MgO was found to exhibit poor activity.37,46 In addition, we previously performed OCM using Mg−Ti mixed oxide-supported Na2WO4/Mn catalysts with an adjusted composition of Mg/Ti.46 The properties of the prepared catalysts were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and O2-temperature-programmed desorption (O2-TPD).

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titanium(IV) n-butoxide (97%, Ti(OCH2CH2CH2CH3)4) were purchased from Aldrich (Milwaukee, WI, U.S.A.). Silica gel (99.5%, 60−325 mesh) was purchased from Alfa (Ward Hill, MA, U.S.A.). Ethanol (99.9%, anhydrous) was purchased from Daejung Chemicals & Metals (Siheung, Korea). The aqueous nitric acid solution used here (69.0−70.0%) was purchased from J.T. Baker (Pennsylvania, PA, U.S.A.). Manganese nitrate hexahydrate (98%, Mn(NO3)2·6H2O) was purchased from Kanto Chemicals (Tokyo, Japan). Sodium tungstate dihydrate (Na2WO4·2H2O) was purchased from Yakuri Pure Chemical (Kyoto, Japan). DI water (18.2 MΩ·cm) was prepared using an aquaMAX-Ultra 370 series water purification system (Young Lin Instrument, Anyang, Korea). Methane (CH4, 99.97%), oxygen (O2, 99.5%), nitrogen (N2, 99.99%), helium (He, 99.999%), and a mixture of gas products for GC calibration were purchased from Shinyang Sanso (Seoul, Korea). Catalyst Preparation. The Mg−Ti−Si mixed oxide-supported catalysts containing 5 wt % of Na2WO4 and 2 wt % of Mn were prepared by a one-pot sol−gel method. Tetraethyl orthosilicate (TEOS) was added to ethanol (100 mL) and stirred for 30 min. Subsequently, magnesium ethoxide and titanium(IV) n-butoxide were added to prepare mixed oxides with appropriate mixing ratios (Table S1). The aqueous nitric acid solution (5−10 mL) was slowly added to the mixture of catalyst precursors to obtain a pH of 3, and the mixture was further stirred for 1 h. Manganese nitrate hexahydrate (0.33 mL of 0.59 g, 2 wt % of Mn) was added dropwise to the solution, and this mixture was stirred for 30 min. Sodium tungstate dihydrate (0.34 g, 5 wt % of Na2WO4) dissolved in 15 mL of DI water was added dropwise to the solution. The final solution was heated for 48 h at 65 °C for gelation. The formed gel was air-dried at 105 °C for 16 h. The dried catalyst was calcined in air at 800 °C for 5 h. A silica-gel-supported Na2WO4/Mn catalyst, or Na2WO4/Mn/SiO2 (silica gel), was prepared by a slurry method.44 Silica gel (1.0 g) was mixed with DI water (50 mL) and stirred for 30 min at room temperature. Sodium tungstate dihydrate (0.0599 g, for 5 wt % of Na2WO4, dissolved in 10 mL of DI water) and manganese nitrate hexahydrate (0.0624 mL, for 2 wt % of Mn) were slowly added to the silica gel mixture using a syringe for 10 min. The mixture was air-dried at 105 °C for 16 h. The dry catalyst was then calcined at 800 °C in air for 5 h. A fumed-silica-supported Na2WO4/Mn catalyst, or Na2WO4/Mn/SiO2 (fumed silica), was also prepared using a slurry method. Fumed silica (2.0 g) was dispersed in DI water (70 mL) and stirred for 30 min at room temperature. Sodium

EXPERIMENTAL SECTION

Materials. All materials were used without further purification unless otherwise indicated. Magnesium ethoxide (Mg(OCH2CH3)2), tetraethyl orthosilicate (98%, Si(OCH2CH3)4), fumed silica, and 3668


Research Article

ACS Sustainable Chemistry & Engineering tungstate dihydrate (0.12 g) and manganese nitrate hexahydrate (0.125 mL) were added to the fumed silica mixture and stirred for 3 h. This mixture was then dried in air at 105 °C for 16 h and calcined in air at 800 °C for 5 h. Catalyst Characterizations. The XRD results were obtained using a Shimadzu (Tokyo, Japan) XRD-6000 instrument equipped with a CuKα1 source (λ = 0.15406 nm). XPS results were obtained using a PHI 5000 VersaProbe system (Chanhassen, Mn, U.S.A.); the spectra were calibrated using C 1s at 284.6 eV. The Raman spectra of the catalysts were obtained using a Renishaw-InVia Raman microscope (U.K.). O2-TPD was performed using a BELCAT device (Bel Japan, Osaka, Japan).46 Catalytic Activity Measurements. The catalytic activity was measured using a packed-bed quartz I-tube reactor, with a 350 mm straight cylinder tube with an internal diameter of 4 mm. The catalyst (0.18 mL, 60−325 mesh) was placed in the reactor between quartz wool plugs with a gas hourly space velocity (GHSV) of 10,000 h−1. Zirconia-silicate beads (0.8−1.0 mm size) were used to fill the remainder of the reactor volume. The reactor was placed in an electric furnace, and the reaction temperature was monitored by a thermocouple on the side of the catalyst bed. Prior to the reaction, a pretreatment was performed under a N2 flow (30 mL/min) at 700 °C for 1 h. A total flow rate of 30 mL/min of the mixed reactants, methane and oxygen ((CH4)/(O2) = 3 (mol/mol)), with a N2 flow at a constant rate of 6 mL/min (as a GC internal standard) was used for the catalyst bed at reaction temperatures of 750, 775, 800, 825, and 850 °C at a heating rate of 2.5 °C/min. The reaction was isothermally controlled at the desired temperature in each case for 30 min. The water vapor produced during the reaction was removed by passing the mixture through at condenser at −2 °C, and the gas products were analyzed using an online gas chromatography (GC System 7980A, Agilent Technologies) flame ionization detector (GC-FID) and a thermal conductivity detector (GC-TCD). The conversion of methane (%), the C2 hydrocarbon selectivity (%), the C2 hydrocarbon yield (%), and the ethylene/ethane ratio (mol/mol) were calculated according to the following equations:

methane conversion(%) =

C2 selectivity(%) =

moles of CH4 consumed × 100 moles of CH4 in the feed

2 × moles of C2 hydrocarbons × 100 moles of CH4 consumed

C2 yield(%) = methane conversion × C2 selectivity/100 C2H4 /C2H6(mol/mol) =

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moles of ethylene produced moles of ethane produced

Figure 1. OCM activity of catalysts.

reaction temperature from 800 to 850 °C, the C2 yield decreased, and deep oxidation to CO and CO2 was favored. In addition, the formation of ethylene from ethane by dehydrogenation was improved. The formation of ethylene from ethane appeared to be suppressed with the large quantity of Mg compared to Ti as observed for NWM/Mg(5)Si(95) and NWM/Mg(10)Ti(5)Si(85), which exhibited results of (C2H4)/ (C2H6) = 0.4−0.7 (mol/mol) at 775 °C and 1.5−2.0 (mol/ mol) at 800 °C. Note that the maximum conversion of methane did not exceed 40% because the reactant O2 is limited in its ability to improve the selectivity to C2 or larger hydrocarbons. For the conversion of O2, the calculated rate of O2 consumption based on the yields of the products was frequently larger than the quantity of the actually consumed O2 (Table S2), suggesting a possible water−gas shift reaction of CO to CO2 in the presence of H2O without O2 on the catalyst surface. Further works are underway regarding this in our lab. With the addition of Ti, the C2 yield of NWM/Ti(5)Si(95) became 20.9% at 800 °C, which is larger than that of Ti-free NWM/Si(100) (17.1% C2 yield). While the addition of Mg decreased the catalytic activity, the highest catalytic activity (38.3% methane conversion, 60.3% C2 selectivity, and 23.1% C2 yield at 800 °C) was observed for NWM/Mg(5)Ti(5)Si(90) with the addition of Mg and Ti. This higher activity compared to those of other catalysts was distinct at the lower reaction temperature (775 °C), where the C2 yield (19.3%) of NWM/ Mg(5)Ti(5)Si(90) was 7.4 and 1.4 times higher than those of NWM/Mg(5)Si(95) and NWM/Ti(5)Si(95), respectively. It must be noted that the addition of Mg without Ti for NWM/ Mg(5)Si(95) significantly decreased the C2 yield, providing evidence of the synergistic effects of Ti and Mg. Long-term stability of NWM/Mg(5)Ti(5)Si(90) was observed at 800 °C (Figure 2). The conversion of methane was 36% for the first 20 h and then reached 38% up to 100 h. The C2 selectivity was 60−65% for the first 30 h and then reached 60% for the last 10 h. The C2 selectivity exhibited a larger signal-to-noise ratio compared to methane conversion because the C2 selectivity was determined using the measured GC peak areas of the products and the methane reactant, while the methane conversion was determined using the measured GC peak areas of the methane reactant. Crystal Structures of the Catalysts. Because of the high calcination temperature (800 °C), the catalysts used in this study exhibited a negligible BET surface area (600 °C) were studied to observe the desorption of lattice oxygen atoms. While NWM/Mg(5)Ti(5)Si(90), the most active catalyst in this study, exhibited a desorption peak at a lower temperature of approximately 760 °C, the poor catalysts of NWM/Mg(5)Si(95) and NWM/ Mg(10)Ti(5)Si(85) did not exhibit distinct peaks up to 850 °C. These observations indicate that the lattice oxygen atoms in NWM/Mg(5)Ti(5)Si(90) are more easily desorbed to activate adsorbed methane molecules to produce C2 or larger hydrocarbons. Although the O2 desorption peak was observed for NWM/Mg(5)Ti(5)Si(90) at a lower temperature, the dependence of the catalytic activity on the O2-TPD results

Figure 4. Raman spectra of (a) NWM/SiO2 (silica gel), (b) NWM/ Ti(5)Si(95), (c) NWM/Mg(5)Si(95), (d) NWM/Mg(1)Ti(5)Si(94), (e) NWM/Mg(3)Ti(5)Si(92), (f) NWM/Mg(5)Ti(5)Si(90), (g) NWM/Mg(7)Ti(5)Si(88), and (h) NWM/Mg(10)Ti(5)Si(85). (---) α-Cristobalite SiO2, (□) anatase TiO2, (◆) rutile TiO2, (◇) MnTiO3, (●) MnWO4, and (▽) Mn2O3.

cannot be fully demonstrated because of poor quality of the O2 desorption results. Effects of the Addition of Mg and Ti on the OCM Activity. Based on the characterization results, the addition of Mg and Ti to silica appears to form Mg/Ti-doped α-cristobalite SiO2. The increasing surface concentrations of Na/W/Mn atoms in the XPS results and the shift of the broadened αcristobalite peaks in the XRD results confirm that Mg/Tidoped α-cristobalite SiO2 is less complexed with Na/W/Mn compounds, which were less incorporated into the lattices of the α-cristobalite SiO2 to form Na/W/Mn compounds on the silica surface. From the XRD and Raman spectroscopy results, which exhibit TiO2 particles but no Mg oxides, Mg appears to be more easily incorporated into α-cristobalite SiO2. Although 3671


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ACS Sustainable Chemistry & Engineering

60.3% C2 selectivity and a 23.1% C2 yield at 800 °C, values which represent 18% higher C2 selectivity and a 35% higher C2 yield compared to those of conventional Na2WO4/Mn/SiO2 as prepared in this study. As suggested by the XRD, XPS, and Raman spectroscopy results, the addition of Mg and Ti appears to form Mg- and Ti-doped α-cristobalite SiO2, suppressing the complexation of Na/W/Mn compounds and α-cristobalite SiO2 and improving the formation of Na/W/Mn compounds exposed on the silica surface.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02914. Tables S1−S4; Figures S1−S4; preparation methods of catalysts; detailed results of catalytic reactions; XPS results of fresh and spent catalysts; reaction results using selected catalysts (PDF)

Figure 5. O2-TPD results.

both Ti and Mg can be added to silica lattices, the roles of each dopant were not fully clarified in this study; however, it is clear that a greater quantity of Na/W/Mn compounds, most likely Na2WO4 and Mn oxides, on the external surface of silica improves the OCM activity because of the effects of the Mg and Ti dopants (Figure 6). The formation of α-cristobalite SiO2 has been suggested to be nucleated by Na 2 WO 4 stabilizing OCM-active Na2WO4.37−40 Although Na2WO4 can be stabilized by αcristobalite SiO2, Mn oxides and liquid Na2WO4 can be incorporated into the SiO2 during the transition of amorphous to α-cristobalite SiO2 at high temperature; moreover, Na/W/ Mn compounds are not completely exposed to the reactants during the reaction. The loss of Na2WO4 and Mn oxides may decrease the OCM activity. Thus, the doping of Mg and Ti to SiO 2 may suppress the loss of accessible Na/W/Mn compounds and increase the quantity of Na/W/Mn compounds on the external surface of α-cristobalite SiO2, thus improving the OCM activity by increasing the number of active sites on the support surface.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jeong-Myeong Ha: 0000-0002-0761-6356 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M3D3A1A01064900).

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CONCLUSION Na2WO4/Mn catalysts supported on Mg- and Ti-doped αcristobalite SiO2 exhibited improved OCM activity, achieving

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Figure 6. Effects of adding Mg and Ti to silica (red spheres represent oxygen atoms). 3672


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