Alexei A. Slinkin

Solid-state reaction as a way to transition metal cation introduction into high-silica zeolites

Abstract A detailed review is given of transition metal cation incorporation into zeolites by a solid-state reaction. The procedures and techniques suitable for investigation of solid-state ion exchange in zeolites are described. It is shown that a solid-state reaction is a promising way for introduction of one or several transition metal ions into cationic positions of high-silica zeolites. The majority of these polyvalent cations cannot be introduced into these matrices by other methods. The influence of different factors (temperature and atmosphere of calcination, Al3+ content in zeolitic lattice, presence of either co-cations or anionic species) on cationic species stabilization is analyzed. It is shown that pentasil-based systems with discrete types of isolated transition metal cations are unique objects for studying the relationship between catalytic properties and local topography of isolated redox sites. A solid state modification of pentasils permits the preparation of active and stable catalysts both for complete oxidation of alkanes and for NOx decomposition.

New strategy of creation of local catalytic sites in definite electronic and coordination states

Abstract Atomic-scale engineering of catalytic functions of isolated redox sites in confined environments of zeolitic channels is proposed as a new approach to the investigation of structure-properties relationship in heterogeneous ‘biomimetic’ catalysis. It is shown that the design of such catalysts, on the base of high-silica zeolites, containing isolated transition-metal cations as active redox sites, may present promising opportunities for creation of new types of contacts from a practical point of view. A detailed analysis of transition-metal cation incorporation into high-silica zeolites by either conventional or solid-state exchange is given. Stabilization of one or several transition-metal ions by matrices of high-silica zeolites (mainly H-ZSM-5) is discussed. The influence of different factors on the processes of cationic-species stabilization is analyzed. These data are correlated with the results of catalytic testing in reactions of total oxidation of hydrocarbons (HC), direct decomposition of NOx and SCR of NOx by HC. The relationship between catalytic activity and selectivity of cationic sites and their coordination and electronic state, regulated by either thermal treatment or introduction of different anionic ligands, is analyzed.

Transformations of Cu/ZSM-5 system upon high-temperature reductive and oxidative treatments

Abstract Interaction of O2 with Cu/ZSM-5 samples (0.5-2.8 wt.% Cu) precalcined in air at 500 or 800°C and then reduced under different conditions was studied. The treatment of the samples precalcined at 500°C by H2 at the same temperature leads to a quantative reduction of copper with formation of large (30–60 nm) Cu0 crystals. A milder treatment of CuH/ZSM-5/500 (300°C, C3H6) also results in a total reduction of Cu2+, but a noticeable portion of the copper (about 20%) is retained inside the zeolitic channels either as disperse Cu0 crystals or as Cu+ ions. For the CuH/ZSM-5 samples precalcined above 800°C, the treatment by H2 or C3H6 at 500°C results in the reduction of only about 3 4 of the Cu2+ to Cu0, and about 1 4 of the cupric ions preserve the valent state Cu2+. The reducibility of isolated cupric cations is related to the coordination state of Cu2+ in ZSM-5, which in turn, depends on the temperature of the oxidative precalcination.

Redox and photo‐redox properties of isolated Mo5+ ions in MoH‐ZSM‐5 and MoH‐beta zeolites: in situ ESR study

Redox and photo‐redox properties of isolated Mo5+ ions stabilized in H‐ZSM‐5 and H‐beta zeolites are studied by in situ ESR in flowing O2, NO, H2, and C3H6. Upon oxidation of pre‐reduced samples at 20 °C, NO demonstrates a higher oxidative ability, as compared with O2. Interaction of Mo5+ ions with propene at 20 °C results in formation of a chemisorption complex with enhanced reactivity of Mo(V) toward NO. Illumination of the Mo5+/HZSM‐5 sample with UV‐visible light causes measurable acceleration of Mo(V) oxidation by NO at 20 °C. Therefore, photochemical activation of the oxidation step could be realized, in principle, for Mo/zeolite catalysts. At 500 °C in the reaction mixture NO + H2, the step of the catalytic site reduction is fast, and the dynamic equilibrium of the redox reaction Mo(VI) ↔ Mo(V) for MoH‐ZSM‐5 and MoH‐beta seems to be strongly shifted to Mo5+.