Abstract
Dehydrogenation promoters greatly enhance the performance of SiO2-MgO catalysts in the Lebedev process. Here, the effect of preparation method and order of addition of Cu on the structure and performance of Cu-promoted SiO2-MgO materials is detailed. Addition of Cu to MgO via incipient wetness impregnation (IWI) or coprecipitation (CP) prior to wet-kneading with SiO2 gave similar butadiene yields (similar to 40%) as when Cu was added to the already wet-kneaded catalyst. In contrast, the catalyst prepared by impregnation of Cu on SiO2 first proved to be the worst catalyst of the series. TEM, XRD, and XPS analyses suggested that, for all catalyst materials, Cu2+ forms a solid solution with MgO. This was confirmed by UV-vis, XANES, and EXAFS data, with Cu being found in a distorted octahedral geometry. As a result, the acid base properties, as determined by Pyridine- and CDCl3-IR as well as NH3-TPD, are modified, contributing to the improved performance. Operando XANES and EXAFS studies of the evolution of the copper species showed that Cu2+, the only species initially present, is extensively reduced to a mixture of Cu-0 and Cu+, leaving only a limited amount of unreduced Cu2+. This formation of Cu-0 is the result of the reducing environment of the Lebedev process and is thought to be mainly responsible for the improved performance of the Cu-promoted catalysts.