LiMn_2O_4/TiO_2催化剂上甲烷氧化偶联反应性能的研究

采用固相反应法制备了具有尖晶石结构的LiMn_2O_4/TiO_2系列催化剂,探讨了TiO_2、Li/TiO_2、Mn/TiO_2、LiMn_2O_4及LiMn_2O_4/TiO_2等不同组成催化剂的甲烷氧化偶联反应性能,采用XRD、XPS、CO_2-TPD和H_2-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LiMn_2O_4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),CH_4∶O_2(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%。TiO_2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CO_2的过程。负载8%LiMn_2O_4的LiMn_2O_4/TiO_2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%,CO_2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LiMn_2O_4/TiO_2催化剂的最佳焙烧温度。

Performance of oxidative coupling of methane on LiMn2O4/TiO2 catalysts

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.