Surface coupling of methyl radicals for efficient low-temperature oxidative coupling of methane

doi:10.1016/S1872-2067(20)63756-1

Abstract

Abstract:

Selective coupling of methyl radicals to produce C₂ species (C₂H₄ and C₂H₆) is a key challenge for oxidative coupling of methane (OCM). In traditional OCM reaction systems, homogeneous transformation of methyl radicals in O₂-containing gases are uncontrollable, resulting in limited C₂ selectivity and yield. Herein, we demonstrate that methyl radicals generated by La₂O₃ at low reaction temperature can selectively couple on the surface of 5 wt% Na₂WO₄/SiO₂. The controllable surface coupling against overoxidation barely changes the activity of La₂O₃ but boosts the C₂ selectivity by three times and achieves a C₂ yield as high as 10.9% at bed temperature of only 570 °C. Structure-property studies suggest that Na₂WO₄ nanoclusters are the active sites for methyl radical coupling. The strong CH₃· affinity of these sites can even endow some methane combustion catalysts with OCM activity. The findings of the surface coupling of methyl radicals open a new direction to develop OCM catalyst. The bifunctional OCM catalyst system, which composes of a methane activation center and a CH₃· coupling center, may deliver promising OCM performance at reaction temperatures below the ignition temperature of C₂H₆ and C₂H₄ (~600 °C) and is therefore more controllable, safer, and certainly more attractive as an actual process.

Key words: Oxidative coupling of methane, Bifunctional catalysis, Methyl radicals, Surface coupling, La₂O₃, Na₂WO₄/SiO₂

Shihui Zou, Zhinian Li, Qiuyue Zhou, Yang Pan, Wentao Yuan, Lei He, Shenliang Wang, Wu Wen, Juanjuan Liu, Yong Wang, Yonghua Du, Jiuzhong Yang, Liping Xiao, Hisayoshi Kobayashi, Jie Fan. Surface coupling of methyl radicals for efficient low-temperature oxidative coupling of methane[J]. Chinese Journal of Catalysis, 2021, 42(7): 1117-1125.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(20)63756-1

https://www.cjcatal.com/EN/Y2021/V42/I7/1117

Figures/Tables 4

Fig. 1. Catalytic performances of La2O3-5NaWSi and reference catalysts in OCM reaction. (a) C2 yield as a function of reaction temperature; (b) CH4 conversion, C2 selectivity and C2 yield at 571 ± 6 °C; (c) A stability test of La2O3-5NaWSi at 570 °C, CH4:O2:N2 = 3:1:2.6, total gas flow rate 66 mL min-1; (d) Integrated SVUV-PIMS peak intensity of gas-phase methyl radicals, ethylene and CO2 during OCM reaction catalyzed by various catalysts at 600 °C; (e) C2 yield as a function of reaction temperature over various catalysts (GHSV 20,000 mL h-1 gcat-1); (f) Schematic illustration of reaction process of OCM over La2O3-5NaWSi.

Fig. 2. The structure and property of 5NaWSi. (a) The difference in C2 yield (ΔC2 yield) between composite catalysts and pure La2O3 at 570 °C; (b) XRD patterns for various catalysts; (c) The W L3-edge XANES spectra; (d) EXAFS-derived Fourier transforms of Na2WO4/SiO2 with different mass loading; Catalytic performance of deep oxidation of (e) ethane and (f) ethylene (C2Hx:O2:N2 = 1:6:5, x = 4 or 6, gas flow rate 66 mL min-1). The red dash lines indicate the ignition temperature at 50% conversion. HAADF-STEM images of (g) 2NaWSi, (h) 5NaWSi and (i) 10NaWSi.

Fig. 3. DFT calculations for CH3· coupling over various catalyst models. (a) The energy profiles; (b) the corresponding configurations for CH3· coupling over (WO3)8 via L-H mechanism and E-R mechanisms; (c) The energy profiles for CH3· coupling; (d) the configurations for CH3? adsorption over various catalyst models. Other configurations are shown in Fig. S9. (e) The energy profile and (f) the corresponding configurations for the β-H elimination of W-O-CH3 over (WO3)8. Yellow: Si, red: O, blue: W, purple: Na, gray: C, and white: H. (WO3)2, (WO3)6, and (WO3)8 abbreviate Si48O112H30Na2(WO3)2, Si48O112H26Na6(WO3)6, and [Si12O28H6Na2(WO3)2]4, respectively.

Fig. 4. Catalytic performance of transition metal oxides and their mixture with 5NaWSi at 755 ± 15 °C for OCM reaction. CH4:O2:N2 = 3:1:2.6, total gas flow rate 66 mL min-1.

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