Control of metal ion species in zeolites by distribution of aluminium in the framework: From structural analysis to performance under real conditions of SCR-NOx and NO, N2O decomposition Original Research Article

The structure, properties and activity of the counter Co, Cu and Fe ion species depending on the distribution of aluminium atoms in the framework between Al pairs of Al–O–(Si–O)2–Al sequences in one ring and isolated Al atoms (Al–O–(Si–O)n>2–Al sequences) in two rings are analyzed. The effect of the Al distribution on the activity of metal ion species is investigated for reactions of technology importance, i.e. SCR-NOx by propane, and decomposition of NO and N2O under real conditions of NOx streams. Zeolite samples of BEA, MFI and FER structures, of similar composition for each topology, but with basically different distribution of Al atoms in the framework, were either selected from commercial samples or prepared by the developed synthesis. The concentration of Al pairs and isolated Al atoms in the framework was determined by the combination of 29Si MAS NMR and quantitative analysis of the Vis spectra of dehydrated Co-zeolites with Co(II) ions exchanged to a maximum degree. The structure and reactivity of the metal sites was described by UV–Vis, FTIR of skeletal vibrations and adsorbed NO, TPR-H2 and reactions of C3H8-SCR-NOx/H2O, and NO and N2O decomposition. It is concluded that the local negative framework charge, originated from Al pairs and isolated Al atoms, constitutes a decisive parameter controlling the structure and reactivity of the counter metal ion species. The bare Co(II), Cu(II) and Fe(II) ions are balanced by Al pairs, while Co-oxo and Cu-oxo or Cu(I) species are adjacent to isolated Al atoms. The Cu(I) ions in vicinity of isolated Al atoms in Cu-MFI zeolites are the most active sites in NO decomposition. Co-oxo species attached to isolated Al atoms in Co-BEA zeolites, in contrast to bare Co(II) ions, exhibit high activity in C3H8-SCR-NOx/H2O in the presence of water vapor. N2O decomposition to elements over Fe-FER requires two Fe ions at a defined distance of β cationic sites, with each Fe(II) cation charge-balanced by an Al pair.