Copper nitrate redispersion to arrive at highly active silica-supported copper catalysts

Abstract

In order to obtain copper catalysts with high dispersions at high copper loadings, the gas flow rate and gas composition was varied during calcination of silica gel impregnated with copper nitrate to a loading of 18 wt % of copper. Analysis by X-ray diffraction (XRD), N2O chemisorption, and transmission electron microscopy (TEM) showed that calcination in stagnant air resulted in very large copper crystallites and a low copper surface area (12 m2·gCu–1). A moderate flow of air was sufficient to greatly enhance the copper surface area (90 m2·gCu–1) based on a bimodal particle size distribution of few large crystallites and a highly dispersed phase. Changing to an N2 flow resulted in similar copper surface areas compared to samples calcined in air at the same space velocity, while calcination in a 2% NO/N2 flow resulted in a relatively narrow particle size distribution peaking around 8 nm and a slightly lower copper surface area (84 m2·gCu–1). By use of SBA-15 supported samples, in situ XRD and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) showed that the decomposition of copper nitrate in NO occurred via a highly dispersed copper hydroxynitrate phase, while decomposition in N2 or air occurred partly via copper nitrate anhydrate and partly via poorly dispersed copper hydroxynitrate. The high CuO dispersions after calcination in an N2 or air flow were ascribed to the redispersion of copper nitrate anhydrate by interaction with the OH groups of the silica support. By utilization of this redispersion, high copper dispersions on silica gel with concomitant high activities were obtained for the gas-phase hydrogenation of butanal.