Studies On The Direct Methane Conversion Over Supported Mo, V, And Ag Catalysts

Methane activation is of great industrial and scientific importance towards understanding the nature of scission of C-H of CH4, which is the most stable molecular in alkane family. The present work was focused on the study of methane direct conversion and four parts were included: (1) By combining the characterization results of the fresh and used Mo/MCM-22 catalysts and the time-on-stream reaction results for methane selective oxidation to C1-oxygenates, the following conclusions were drawn: the isolated tetrahedral molybdenum oxo-species is responsible for HCHO formation, while the octahedral polyoxomolybdate species will give rise to the total oxidation of methane. (2) VOx/SiO₂ catalysts with different kinds of vanadia species were prepared with different methods and characterized in many respects and the fresh and used VOx/SiO₂ catalysts were also compared. It was found that vanadia species were self-dispersed and partially reduced during methane selective oxidation. We also concluded that CO₂ was produced via two mechanisms: One by the sequence reaction of CH4→HCHO→CO→CO₂ over the monomeric vanadia species, and the other by the direct oxidation of methane to CO₂ over the oligomeric vanadia species. (3) The different amounts of Ag+ were supported on zeolite with different methods and the nonoxidative coupling of methane was tested. It was found that isolated Ag⁺ ions play a crucial role in the catalytic coupling of methane under nonoxidative conditions, which was also confirmed by the result of DFT calculation.