A. V. Kucherov

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.

Alkane oxidation on isolated Cu2+ ions in zeolitic matrix : a relation between catalytic activity and Cu2+-site local topography

Catalytic ethane oxidation, with O2, using H-ZSM-5 and H-mordenite containing different discrete types of isolated Cu(II) cations was studied. An ESR spectroscopic study was carried out on copper ion coordination in zeolites. It was shown that the intrinsic activity of Cu(II) ion depends strongly on the Cu2+-site local topography. Covalent-bonded Cu(II) cations in the most coordinatively unsaturated square-planar coordination in H-ZSM-5 demonstrated the highest activity in C2H6 complete oxidation.

Stabilization of the ethane oxidation catalytic activity of Cu-ZSM-5

Abstract The state of isolated copper ions in Cu-ZSM-5 containing additions of La, Ce, and Co was monitored in-situ by ESR under flow conditions. Treatment by steam at 630°C for 17 h or high-temperature dry calcination at 850°C induce an irreversible change in coordination for practically all square-planar Cu2+ ions in mono-cationic Cu-ZSM-5 without agglomeration or encapsulation of the isolated ions. All Cu2+ ions remain accessible to gas-phase molecules, but the catalytic reactivity of these altered copper sites decreases drastically. A stabilizing effect is noted for samples modified by a relatively large amount, ca. 5.0 wt.-%, of multivalent rare-earth ions La or Ce. Here a part of the copper ions (20–30%) preserves the parent square-planar Cu2+ state even after calcination at 850°C for 0.5 h. The effect of ca. 1% La or Ce is much less pronounced. The catalytic activity in the complete oxidation of ethane correlates well with the number of square-planar cupric cations retained by the samples after different treatments. The introduction of cobalt sharply increases the ethane oxidation activity of samples calcined at 500–650°C.

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.

Modification of zeolites by multi-charged cations by the use of in-situ formed “active gas-phase species”

Abstract A new way of the modification of zeolites is discussed which is based on the use of active gas-phase species formed in situ upon thermal treatment of a [H-zeolite+modifier oxide] mixture with an air flow containing CCl 4 . The chemical transport reaction, with the formation of reactive and mobile oxychloride fragments in the zeolitic bed, provides effective dissipation of the oxide phase and migration of active species into the zeolitic channels at temperatures as low as 150–300°C. ESR is used as an informative and sensitive method to monitor transformations in systems containing paramagnetic ions. The peculiarities of the introduction of Mo 5+ ions into the H-forms of several zeolites (ZSM-5, beta, ferrierite, USY) differing in channel size and structure are discussed. Some transformations of H-ZSM-5 containing Fe 3+ and Gd 3+ ions are also studied.

Adsorptive-catalytic removal of CH3OH, CH3SH, and CH3SSCH3 from air over the bifunctional system noble metals/HZSM-5

Abstract Nowadays, improvements are needed to achieve the highest purification level of odorous volatile organic compounds. An attractive possibility is to combine adsorption and oxidation. The bifunctional system [noble metals/HZSM-5] is tested in oxidative removal of individual or mixed CH 3 OH, CH 3 SH, and (CH 3 S) 2 in order to find an adsorptive-oxidative window of operation. Rh/HZSM-5 and (Au+Rh)/HZSM-5 catalysts demonstrate high activity and stability (sulfur-resistance) at temperatures as low as 320 o C. A less active mono-metallic Au/HZSM-5 also provides an effective and stable oxidative removal of [VOC + S-VOC] at 500 o C. A nonisothermal, microwave-assisted catalytic process on Au 0 -containing samples seems to be attractive for a further study.

Modification and stabilization of Cu-ZSM-5 by introduction of a second cation

Publisher Summary Cu-ZSM-5 has been extensively investigated in the low-temperature decomposition of NO to dements, in the selective catalytic reduction (SCR) of NOX in excess oxygen by organic compounds, and in the total oxidation of alkanes. In the past few years, Cu-ZSM-5 has been examined by a multitude of physical methods but the characterization often yields contradictory results about the changes in the coordinative and valence states of the copper after treatment in different gas mixtures. Hence, the nature of the active sites in Cu-ZSM-5 catalysts has not been completely elucidated. Little is also known about the mechanism of Cu-ZSM-5 deactivation and the stabilizing effect of promoters. This chapter summarizes the results of investigation of H-ZSM-5 containing copper and one other cation (Ba, Ga, La, Ce, Co, Nb). The results in the chapter provide evidence of a noticeable influence of different additives on treatment-induced bonding change between the isolated Cu2+ cations and tile framework oxygens of H-ZSM-5 linked, in turn, to the Al3+ ions.

Effect of the conditions of preparing mixed oxide catalyst of Mo-V-Te-Nb-O composition on its activity in the oxidative dehydrogenation of ethane

It is shown that catalytic activity of mixed oxide catalyst of Mo-V-Te-Nb-O composition in oxidative dehydrogenation (OD) of ethane is determined to a substantial degree by the Nb-to-(C2O4)2− ratio in niobium-containing precursors. A pH value of 2.8 to 3.0 for a mixture is optimal when conducting the hydrothermal synthesis of a mixed oxide catalyst; this is achieved by using oxaloniobic acid as a niobium-containing precursor. It is determined that substituting antimony for tellurium results in a loss of catalyst activity during the OD of ethane. The optimum Te content in a catalyst is 0.17 mol %.

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+.