Synthesis of structured silica supported transition metal catalysts for C-1 hydrocarbon oxidation reactions.

摘要

Structured silica mesoporous materials have been given much attention in both scientific investigations and practical applications. These materials, such as MCM-41 and SBA-15, in addition to their large surface area and pore volume, have superior mechanical, thermal, and chemical stability, making them outstanding choices as catalyst supports. By using various synthesis techniques, such as hydrothermal incorporation and grafting, transition metal ions have been highly dispersed by either substituting for Si in the silica framework, or immobilizing through the reaction with surface silanol groups. Various characterization techniques, e.g., N2 physisorption, X-ray diffraction, transition electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, diffuse reflectance UV-visible spectroscopy, X-ray absorption spectroscopy, hydrogen temperature programmed reduction, and temperature programmed desorption, etc., have not only revealed their structural features, reducibility, and stability, but also gathered information on the nature of transition metal species and their interaction with the mesoporous support. The catalytic properties of these catalysts have been investigated through various C-1 reactions, such as methanol oxidation and methane partial oxidation.;As a chemical probe reaction sensitive to the nature of active sites, methanol oxidation has been carried out on V-MCM-41, providing fundamental information about the nature of surface active sites and the turnover numbers at different reaction conditions. Our refinement of the synthesis process using statistical experimental design allows pore variation at constant composition and structure, which makes the measurement of radius of curvature effect possible. The post-synthesis surface incorporation of vanadium cations on SBA-15 allows larger concentration of surface active sites, increased transport and thermal stability, thus showing a superb activity and selectivity.;Single step partial oxidation of methane to formaldehyde has also been investigated on V-MCM-41 catalyst. The process variables, i.e., temperature, V-loading, reactant ratio (CH4:O2), gas hourly space velocity, and pressure, were shown to strongly influence the reaction rate and the selectivity of products. By experimental design, we have developed quantitative models that allow us to understand the synergetic effect of theses process variables and obtain the optimum reaction conditions. Direct comparisons between Mo- and V-MCM-41 catalysts with similar site dispersion have been made in terms of structure and reactivity. The high formaldehyde selectivity of Mo-MCM-41 and high activity of V-MCM-41 is closely correlated with their difference in reducibility, which might vary the C-H bond cleavage barrier energy.