Speciation of Copper in Spherical Mesoporous Silicates: From the Microscale to Angstrom

MCM-41 type mesoporous silicate particles with spherical morphology containing copper nanospecies were synthesized by conventional hydrothermal synthesis. A narrow distribution of silicate particle sizes between 2 and 3 mu m and a well-defined mesoporosity were evidenced by electronic microscopy, even for loadings of copper of about 10 wt %. Moreover, CuO nanoparticles similar to 30 nm in diameter occluded in the amorphous silica, possibly filling some secondary mesoporosity, were also detected. By Fourier transform infrared spectroscopy with pyridine adsorption and UV-vis diffuse reflectance spectroscopy measurements together with reduction experiments under H-2 flow, a detailed study about the relationship between copper loading degree and the acid properties with the local isolated Cu delta+ species surroundings was performed. The incorporation of copper into the silica framework generated surface Lewis acid sites which increase with copper content. The origin of these acid sites could be attributed to the presence of isolated Cu delta+ species possibly coordinated with the framework oxygen atoms. It is noteworthy that Bronsted acid sites were not detected in our materials. By X-ray absorption near edge structure spectroscopy, we determine the coexistence of copper +1 and +2 states. The average coordination numbers calculated from extended X-ray absorption fine structure experiments are consistent with a mixture of nanosized species of copper: CuO nanoparticles, besides isolated Cu delta+ cations located in framework positions with oligonuclear clusters like [Cu delta+center dot center dot center dot O delta-center dot center dot center dot Cu delta+](n) in its surroundings, confined within the pores.