Performance of oxidative coupling of methane on LiMn2O4/TiO2 catalysts
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摘要: 采用固相反应法制备了具有尖晶石结构的LiMn2O4/TiO2系列催化剂, 探讨了TiO2、Li/TiO2、Mn/TiO2、LiMn2O4及LiMn2O4/TiO2等不同组成催化剂的甲烷氧化偶联反应性能, 采用XRD、XPS、CO2-TPD和H2-TPR等表征方法对该系列催化剂进行了分析。结果表明, 具有尖晶石结构的LiMn2O4化合物具有较高的甲烷氧化偶联催化活性, 在775℃、0.1MPa、7200mL/(h·g), CH4:O2(体积比) 为2.5的条件下, 甲烷转化率可达25.8%, C2选择性可达43.2%。TiO2的存在不仅进一步提高了甲烷转化率和C2选择性, 还有效抑制了甲烷完全氧化形成CO2的过程。负载8% LiMn2O4的LiMn2O4/TiO2催化剂性能达到最优, 此时甲烷转化率达到31.6%, C2选择性为52.4%, CO2选择性降低到26.3%。考察了不同焙烧温度对催化剂活性的影响, 850℃为LiMn2O4/TiO2催化剂的最佳焙烧温度。Abstract: A series of LiMn2O4/TiO2 catalysts with spinel crystal structure were prepared by solid state reaction method, and the catalytic performance of oxidative coupling of methane on the different catalysts, such as TiO2, Li/TiO2, Mn/TiO2, LiMn2O4 as well as LiMn2O4/TiO2, was evaluated. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, CO2 temperature programmed desorption and H2 temperature programmed reduction. It is found that LiMn2O4 with spinel structure has high catalytic activity in oxidative coupling of methane reaction. 25.8% of CH4 conversion, 43.2% of C2 selectivity was obtained under the reaction conditions of 775℃, 0.1MPa, 7200mL·h-1·g-1, CH4:O2(volume ratio)=2.5. The introduction of TiO2 support can not only improve CH4 conversion and C2 selectivity, but also restrain the deep oxidation of methane to CO2. The LiMn2O4/TiO2 with 8% loading amount showed the best activity, on which 31.6% of CH4 conversion, 52.4% of C2 selectivity were obtained and CO2 selectivity was decreased to 26.3%. The effect of calcination temperature on the activity of LiMn2O4/TiO2 catalysts was investigated. 850℃ is the optimal calcination temperature.
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Key words:
- oxidative coupling of methane /
- spinel /
- LiMn2O4 /
- TiO2 /
- synergistic effect
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表 1 不同组成催化剂的催化性能
Table 1 Catalytic performance of different catalysts
Catalyst Conversion of CH4 x/% Selectivity s/% Yield w/% C2H4/C2H6 C2 CO CO2 C2 TiO2 22.3 1.6 76.1 22.3 0.4 1.0 0.3%Li/TiO2 25.1 10.9 81.9 7.2 2.7 0.9 5%Mn/TiO2 23.1 21.1 20.8 58.1 4.9 1.0 LiMn2O4 25.8 43.2 8.1 48.7 11.1 1.7 LiMn2O4/TiO2 30.5 49.7 21.1 29.2 15.2 2.1 Li-Mn/TiO2 31.4 51.5 19.8 28.7 16.2 2.0 notes: C2 includes ethylene and ethane; the same content of Li and Mn are included in the LiMn2O4/TiO2 and Li-Mn/TiO2 catalysts, respectively 0.3%, 5%(mass fraction); reaction conditions: t=775℃,CH4:O2 (volume ratio)=2.5,GHSV=7200mL/(h·g) 表 2 不同负载量LiMn2O4/TiO2催化剂的催化性能
Table 2 Catalytic activity of LiMn2O4/TiO2 catalysts with different loadings
Catalyst Conversion of CH4 x/% Selectivity s/% Yield w/% C2H4/C2H6 C2 CO CO2 C2 4%LiMn2O4/TiO2 28.9 46.2 18.7 35.1 13.4 2.1 6%LiMn2O4/TiO2 29.7 47.6 25.4 27.0 14.2 2.1 8%LiMn2O4/TiO2 31.6 52.4 21.3 26.3 16.6 2.3 10%LiMn2O4/TiO2 30.8 50.2 19.9 29.9 15.5 1.9 12%LiMn2O4/TiO2 28.1 47.9 15.5 36.6 13.5 1.8 表 3 不同焙烧温度对LiMn2O4/TiO2催化剂性能的影响
Table 3 Influence of calcination temperature on performance of LiMn2O4/TiO2catalysts
Catalyst Conversion of CH4 x/% Selectivity s/% Yield w/% C2H4/C2H6 C2 CO CO2 C2 LiMn2O4/TiO2(500℃) 29.7 30.5 55.4 14.1 9.1 2.1 LiMn2O4/TiO2(850℃) 30.5 49.7 21.1 29.2 15.2 2.1 LiMn2O4/TiO2 (1000℃) 27.9 31.9 33.0 35.1 8.9 1.8 表 4 通过O 1s谱图得LiMn2O4/TiO2与Li-Mn/TiO2催化剂表面活性氧物种的组成
Table 4 Surface composition of LiMn2O4/TiO2 and Li-Mn/TiO2 catalysts obtained using O 1s photoelectron spectroscopy
Catalyst O22- O- O2- $\frac{\text{O}_{2}^{2-}+{{\text{O}}^{-}}}{{{\text{O}}^{2-}}}$ E/eV watom/% E/eV watom/% E/eV watom/% LiMn2O4/TiO2 530.7 41.3 530.1 6.1 528.9 52.6 0.90 Li-Mn/TiO2 531.1 43.0 530.1 9.9 528.9 47.1 1.12 LiMn2O4 530.6 32.9 532.1(CO32-) 8.8 528.9 58.3 0.56 表 5 H2-TPR谱图半定量分析
Table 5 Semi-quantitative analysis of H2-TPR profiles
Catalyst Area of peaks H2 consumption
/(mmol·g-1)LiMn2O4 36214.01 5.80 Li/TiO2 514.48 0.08 Mn/TiO2 1642.04 0.26 LiMn2O4/TiO2 1710.78 0.27 Li-Mn/TiO2 1763.71 0.28 -
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