Physicochemical properties of Cu loaded onto core–shell Al-MCM-41: Effect of loading methods

Copper species in the structure of Cu/core–shell Al-MCM-41 catalysts prepared by different techniques of Cu loading including substitution, ion-exchange, and impregnation methods were characterized by using temperature-programmed reduction, surface area and porosity analyzer, diffuse reflectance UV–vis-near-infrared spectroscopy, Fourier transforms infrared spectroscopy. Cu2+ in tetrahedral coordination was mostly found from the substitution method. However, from this method CuO was formed from the excess solution of copper nitrate trapped within pores of Al-MCM-41. For the ion-exchange method, only Cu2+ ions were detected. On the other hand, for the impregnation method with high copper loading, the observed copper species was CuO crystals, while with low copper loading both CuO and Cu2+ ions were detected. The acidity of catalysts was also studied by NH3-TPD. It was found that the distinction of copper species in the structure had an effect on Brønsted acid of pre- and post-reduction catalysts, especially in the impregnation method. This study reveals a perspective in the fundamental understanding of copper species-based catalysis.