Acta Physico-Chimica Sinica ›› 2019, Vol. 35 ›› Issue (9): 1027-1036.doi: 10.3866/PKU.WHXB201902004

Special Issue: 碳氢键活化

• Article • Previous Articles    

Influence of the Composition/Texture of Solid Acid WO3/TiO2-Supported Lithium-Manganese Catalysts on the Oxidative Coupling of Methane

Fei CHENG1,2,Jian YANG1,*(),Liang YAN1,Jun ZHAO1,Huahua ZHAO1,Huanling SONG1,Lingjun CHOU1,3,*()   

  1. 1 State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China
    2 University of Chinese Academy of Sciences, Beijing 100049, P. R. China
    3 Suzhou Research Institute of LICP, Chinese Academy of Sciences, Suzhou 215123, Jiangsu Province, P. R. China
  • Received:2019-02-01 Accepted:2019-03-06 Published:2019-03-08
  • Contact: Jian YANG,Lingjun CHOU E-mail:yjian@licp.cas.cn;ljchou@licp.cas.cn
  • Supported by:
    The project was supported by the Petro China Innovation Foundation(2016D-5007-0506);the "Strategic Priority Research Program" of the Chinese Academy of Sciences(XDA09030101)

Abstract:

The selective oxidation of methane to basic petrochemicals (ethylene and ethane) is desirable and has attracted extensive research attention. The oxidative coupling of methane (OCM) is considered a promising one-step route for the production of C2 compounds (ethylene and ethane) from methane, and has been the focus of industrial and fundamental studies. It is widely accepted that the composition is a crucial factor governing the activity of a catalyst system. It was found that the phase structures, basicity, existing status and distribution of the active components, oxygen species, and chemical states of the catalyst were influenced by the composition and ratio, resulting in different catalytic performances for the OCM. In this study, a series of solid acid WO3/TiO2-supported lithium-manganese oxide catalysts for OCM were synthesized via the impregnation method. The impacts of diverse compositions, such as the individual contents (Li and Mn) and dual contents (Li-Mn), on the OCM were investigated in detail, using inductively coupled plasma optical emission spectrometry, X-ray diffraction, high-resolution transmission electron microscopy, CO2-temperature-programmed desorption, O2-temperature-programmed desorption, H2-temperature-programmed reduction, Raman spectroscopy, X-ray photoelectron spectroscopy, and CH4-temperature-programmed surface reaction. The addition of Li content to the catalyst not only led to the anatase-to-rutile crystal structure transformation of TiO2, and the reduction of the high-valence-state Mn species to low-valence-state Mn, but also increased the content of surface lattice oxygen and decreased the surface basicity. The observed effects on the structures and catalytic performance suggest that the Li content is helpful in suppressing the formation of completely oxidized CO2, and increases the C2 selectivity. Moreover, increasing the Li content of the catalyst facilitated the mobility of the lattice oxygen, which triggered the promotion of CH4 activation, thereby enhancing the OCM catalytic performance. The Mn content acted as the active sites for OCM; therefore, the performance of the catalyst was closely related to the Mn concentration and valence state. However, the WO3/TiO2-supported catalyst with excessive Mn content exhibited a high surface basicity, high valence state of Mn, and low abundant lattice oxygen, which was unfavorable for C2 selectivity. The Raman spectroscopy results revealed that MnTiO3 was formed due to the co-existence of Li and Mn on WO3/TiO2, and played an essential role in improving the low-temperature OCM performance. There was a synergic effect of the Li and Mn components on the OCM. The optimal performance (16.3% C2 yield) was achieved over the WO3/TiO2-supported lithium-manganese catalyst with n(Li) : n(Mn) = 2 : 1 at 750 ℃.

Key words: OCM, Mn species, WO3/TiO2, Lattice oxygen, MnTiO3

MSC2000: 

  • O643