Reducibility and oxidation activity of Cu ions in zeolites: Effect of Cu ion coordination and zeolite framework composition

Abstract

Reducibility and catalytic activity in propane oxidation was studied for the Cu ions exchanged in zeolites of structural types ZSM-5 (MFI), ferrierite (FER) and erionite (ERI) with different Cu/Al/Si compositions, and Na+ or H+ ions present as co-cations. The Cu ions were exchanged at conditions of various Cu concentrations in salt solutions, anion type and pH to receive samples with different Cu ion structures. Reducibility of the Cu ions in zeolites has been investigated by means of temperature-programmed reduction by hydrogen, and zeolite evacuation and treatment in CO atmosphere with subsequent detection of the Cu+–(CO)n complexes by IR spectroscopy. Redox properties of the Cu ions, represented by their reducibility and catalytic activity in propane oxidation to CO and CO2, dramatically differed depending on Cu loading in the zeolite, content of aluminum in the framework and presence of co-cations (Na+ versus H+ ions), as well as on the state of the Cu ions, i.e. if atomically dispersed as exchanged cations or present in CuO oxidic-like species. These results have been compared with our previous studies revealing several defined Cu ion sites characteristic for high-silica zeolites, differing in coordination-siting, positive charges, and population depending on Cu loading, and content of Al in the framework. It has been shown that, well-dispersed CuO species exhibit much higher oxidation activity and reducibility compared to the exchanged Cu ions. The redox properties of the exchanged Cu ions have been found to be highly controlled, besides the local geometry of the ligand field, by the total delocalized negative framework charge, given by the Al content in the framework, and the local negative charge adjacent to the cation, controlled by the Si–Al sequences. The Cu ions balanced by a single AlO2− entity, being most populated in zeolites with high Cu loading and in zeolites with low concentration of aluminum in the framework are suggested to be those easily reducible, and exhibiting high oxidation activity.