Catalytic oxidative dehydrogenation of methanol to formaldehyde was carried out over Ag-SiO2-Al2O3 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 degreesC with ethanol as the solvent. Under optimum reaction conditions, i.e., the reaction temperature of 640 degreesC, O-2/CH3OH molar ratio of 0.39 with space velocity (GHSV) of 1.2 x 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-SiO2-Al2O3 catalyst is discussed in detail. It is found that almost all the catalysts are glass-like and nonporous with a surface area of similar to 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 degreesC owing to the stabilizing effect of the [AlO4] 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-SiO2-Al2O3 catalyst is attributed to its unique surface structure and the strong interactions between the support and the active phase. (C) 2004 Elsevier Inc. All rights reserved.