Thermogravimetry And Differential Thermogravimetry Research Articles

Ag–SiO 2–Al 2O 3 composite as highly active catalyst for the formation of formaldehyde from the partial oxidation of methanol

Catalytic oxidative dehydrogenation of methanol to formaldehyde was carried out over Ag–SiO 2–Al 2O 3 catalysts prepared by a sol–gel method. Detailed preparation conditions were investigated and the optimal preparation parameters were determined as the Si/Al molar ratio of 8.5 / 1.5 – 9 / 1 , silver loading of 20 wt%, and calcination temperature of 900–1000 °C with ethanol as the solvent. Under optimum reaction conditions, i.e., the reaction temperature of 640 °C, O 2/CH 3OH molar ratio of 0.39 with space velocity (GHSV) of 1.2 × 10 5 h − 1 , the as-prepared catalyst exhibits excellent activity and selectivity. The yield of formaldehyde reaches 91%, much higher than that obtained over the pumice-supported silver catalyst (75%) and even higher than that over the commercial electrolytic silver catalyst (85%). Based on combined characterizations, such as X-ray photoelectron spectroscopy and X-ray-excited Auger electron spectroscopy (XPS and XAES), nitrogen adsorption at low temperature, thermogravimetry and differential thermogravimetry (TG-DTG), differential thermal analysis (DTA), diffuse reflectance ultraviolet visible spectroscopy (UV–vis DRS), temperature-programmed reduction (TPR), scanning electron micrograph (SEM), X-ray diffraction (XRD), etc., the correlation of the catalytic performance to the structural properties of the Ag–SiO 2–Al 2O 3 catalyst is discussed in detail. It is found that almost all the catalysts are glass-like and nonporous with a surface area of ∼1 m 2/g. In the catalysts with silver loading lower than 20 wt%, all the silver species are present as Ag + ions before the reaction and the catalyst is ultrathermally stable even under elevated temperatures at 1100 °C owing to the stabilizing effect of the [AlO 4] group. Higher Ag loading in the catalysts leads to the presence of metallic silver species over the surface of the catalyst. During the catalytic reaction, Ag + ions are partially reduced to metallic Ag. These nano-sized Ag particles act as the active centers and the superior catalytic performance of the Ag–SiO 2–Al 2O 3 catalyst is attributed to its unique surface structure and the strong interactions between the support and the active phase.