Critical evaluation of the role of the distribution of Al atoms in the framework for the activity of metallo-zeolites in redox N2O/NOx reactions

•Ratio of Al pairs and single Al atoms in zeolites can be tuned in a wide range.•Corresponding local negative charges balance bare divalent cations or metal-oxo species.•These cation/metal-oxo species control the activity in N2O and NOx related reactions.

Both the laboratory synthesized zeolites with tailored distribution of framework Al atoms and commercial ZSM-5 zeolites of various proveniences were investigated in a broad region of Si/Al ratio with respect to the distribution of Al atoms in the framework. The analysis of the distribution of framework Al atoms was done by using strictly defined conditions of Co(II) ion exchange and UV–Vis, 27Al and 29Si (CP) MAS NMR experiments. Evidence has been found that the distribution of framework Al atoms in ZSM-5, and in Si-rich zeolites in general, is not random and is not controlled by statistics, but occurs between the most populated Al pairs (Al–Si–Si–Al sequences in 6MRs) and single far distant Al atoms (Al–Sin>2–Al sequences) located in different rings. Depending on the type of Si- and Al-reactants and presence of TPA+/Na+ in gels, a broad distribution of Al atoms (from 5 to 85% of Al atoms in Al pairs) for Si/Al 12–20 can be achieved. At Si/Al > 20, the population of Al pairs decreases, but 90% of single Al atoms can be obtained in the whole range of Si/Al compositions. The local negative charge of the framework governed by the distribution of framework Al atoms inevitably controls the structure of the counter metal ion species, which is reflected in the catalytic properties of metallo-zeolites in redox reactions requiring different types of counter ion species. A high population of Al pairs in the framework is required for stabilization of divalent bare Cu(II), Co(II) and Fe(II) ions both in ZSM-5 and the beta zeolite, representing the active sites for C10H22-SCR-NOx, CH4- and C3H8-SCR-NOx, and for N2O decomposition, respectively. In contrast, single Al atoms charge-balance monovalent cations, metal-oxo or bridged metal-oxo(hydroxo) species, the active sites for NO decomposition (Cu(I) in ZSM-5 and BEA*), for NH3-SCR-NOx to N2 and NO-NO2 reaction (Fe(III)-oxo species in ZSM-5) and C3H8-SCR-NOx/H2O (Co(II)-oxo in BEA*) in the presence of a large excess of water existing in real exhaust gases. It is concluded that control of the distribution of Al atoms in the framework by hydrothermal synthesis of Si-rich zeolites could achieve the high concentration of an appropriate active structure of counter metal-ion species required for a specified reaction and conditions. Thus, in addition to the usually reported total concentration of Al atoms, the distribution of Al atoms in the framework, i.e. the local negative framework charge, is a parameter which should be included in the characteristics of Si-rich zeolites.

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