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

Abstract

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.