Zdeněk Sobalík

Siting and Distribution of Framework Aluminium Atoms in Silicon-Rich Zeolites and Impact on Catalysis

Siting of Al atoms in the framework T sites, in zeolite rings and channel/cavity system, and the distribution of Al atoms between single Al atoms and close Al atoms in various Al-O-(Si-O)n-Al sequences in Si-rich zeolites represent key parameters controlling properties of counter ion species. Framework Al siting and distribution is not random or controlled by simple rules and depends on the conditions of the zeolite synthesis. Al in Al-O-(Si-O)2-Al in one 6-MR and single Al atoms predominate in Si-rich zeolites and their population can be varied to a large extent. The siting and distribution of framework Al atoms dramatically affect catalytic activity/selectivity both of protonic and transition metal ion-containing zeolite catalysts.

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

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.

State and coordination of metal ions in high silica zeolites incorporation, development and rearrangement during preparation and catalysis

Abstract Cu and Co ions in Na- and H-forms of ZSM-5 and ferrierite were used to elucidate siting-coordination-bonding of bare divalent cations and cation-extra-framework ligand complexes (with NO, CO, NO2, H2O and NH3) in high silica zeolite matrices. By using a multi-spectroscopic approach involving VIS-NIR diffuse reflectance spectra of the Cu2+ and Co2+ ions, Cu+ luminescence, FTIR spectra of skeletal T-O-T vibrations and vibrations of adsorbed molecules (ligands), information on the bonding of the bare cations and cation-guest-ligand complexes to the framework oxygen ligands was obtained. Based on our previous results on characteristic spectral features of the Cu siting in dehydrated ZSM-5, a correlation between coordination of the Cu ions in hydrated and dehydrated ZSM-5 zeolites at various Cu/Al/Si ratios was established, and the importance of Al distribution in the framework (so called ‘Al pairs’ and single Al atoms) for the Cu ion exchange and siting in zeolites was evidenced. Owing to the stable divalency of Co2+ in zeolites, this cation was used to monitor coordination-bonding of bare divalent cations and cation-ligand complexes in ferrierite. It has been found that the Co2+ ions induce local perturbations of the T-O-T bonds adjacent to the cation reflected in three characteristic (‘deformation’) shifts of the T-O-T framework vibrations. Simultaneously measured d-d transition spectra of the Co2+ ions, providing information on the symmetry of the framework oxygen atoms bonding a bare cation, indicated that the changes in T-O-T vibrations are accompanied by changes of VIS spectra. Depending on the strength and number of the extra-framework ligands, this local framework deformation induced by the Co2+ ion is partly. or eventually completely, removed (T-O-T vibration ‘relaxation’ shift) upon adsorption of molecules with formation of cation-guest-ligand complexes (also detected via ligand vibration itself).

Geochemistry of Miocene lacustrine sediments from the Sokolov Coal Basin (Czech Republic)

Abstract Sediments of the upper part of the Sokolov Formation of the Sokolov Brown Coal Basin consist mostly of brown lacustrine clays and claystones enriched in organic matter (2–18% TOC). The lower portion of the stratigraphic profile is formed mostly by kaolinite–illite clays and claystones, while the montmorillonite content increases in the upper portion. The change in the lithology of sediments is caused by the gradual erosion of the weathering crust in the source areas of the clastic material. Inversion of the weathering profile during erosion is manifested geochemically in a decrease in the Al 2 O 3 /Na 2 O ratio and an increase in the SiO 2 /Al 2 O 3 , Na 2 O/K 2 O and K 2 O/TiO 2 ratios from the base to the top of the studied part of the Sokolov Formation. The amounts of trace elements (La, Ce, Nb, Zr, Cr, V, Sn) also decrease in the same direction. On the other hand, the amounts of Rb and Sr increase. The organic matter of the upper part of the Sokolov Formation consists primarily of Types I and II kerogen. Only near the base of the studied part of the formation, i.e., in the coal stringer, kerogen of Type III prevails. The extremely low degree of thermal maturity permits determination of the source of the organic matter and characterization of its accumulation environment. The organic material in the sediments is predominantly of algae origin. Relatively high amount of pentacyclic triterpanes of the hopane series indicates either the occurrence of cyanobacteria as primary producers or bacterial reworking during sedimentation and diagenesis. Organic matter accumulated under the conditions of a fresh-water lake or lake with slightly elevated salinity, in a dysoxic aqueous environment. The alternation of organic carbon-rich layers with layers low in organic matter is explained in terms of changes in the degree of dysoxia of the aqueous environment and a variation in the intensity of oxidation and mineralization of the organic material.

Exchange of Co(II) ions in H-BEA zeolites: identification of aluminum pairs in the zeolite framework

Abstract The Co(II) ion exchange in NH 4 -, HNa- and H-BEA zeolites (Zeolite beta, BEA) was employed to monitor framework Al pairs in these zeolites balancing the charge of the divalent cations. The character and concentration of unperturbed bridging OH groups or those perturbed via hydrogen bonding, and Lewis sites (defective and tricoordinated framework Al) of NH 4 -, HNa- and H-BEA zeolites, and their changes induced by Co(II) ion exchange were investigated by quantitative analysis of IR spectra in the region of OH groups and vibrations of CN groups of adsorbed d 3 -acetonitrile. It is shown that the Co(II) ions in dehydrated BEA zeolites, requiring two AlO 2 − framework groups for their charge balance, are at cationic sites which were occupied in the parent zeolites by Na ions or by mutually interacting protons or by those exhibiting a high tendency to be dehydroxylated with the formation of Lewis sites. A part of the unperturbed bridging OH groups is not exchanged by divalent Co(II) ions, indicating the far distances of the respective framework aluminum atoms. This indicates that the framework local structures bearing close AlO 2 − groups in BEA represent an unstable environment with weakened framework Al–O bonds. Metal cations, both Na(I) and Co(II) ions greatly prefer occupation of cationic sites containing two close framework AlO 2 − groups. Thus metal ion exchange brings about a high stabilization of the BEA framework, and protects the local environment of the aluminum framework against substantial rearrangements leading to the formation of Lewis sites.

Cu ion siting in high silica zeolites. Spectroscopy and redox properties

Abstract It has been shown that two main Cu sites present in ion exchanged Cu-ZSM-5 which differ in positive charges on the Cu 2+ ion, exhibit substantially different redox properties and reactivity towards NO molecules investigated by IR spectra of adsorbed NO and CO, and Cu + emission spectra. The Cu ion with low positive charge (tested by electron density transfer from NO molecule to the cation) is assumed to be balanced by a low negative charge, i.e., a single framework AI atom. This Cu site preferably forms Cu + dinitrosyl complexes and exhibits high stability of Cu + even in oxygen at high temperature (720 K) compared to the other Cu ions with higher positive charges, which are suggested to be balanced by two framework AI atoms. A decisive role of local negative framework charge adjacent to the cation as well as total negative framework charge given by Si/Al ratio for the redox properties and reactivity of the Cu ions has been indicated.

Monitoring of skeletal T-O-T vibrations of metal ion exchanged zeolites : An attempt at quantitative evaluation

Abstract The ratio of the intensity of the T–O–T band perturbed by metal ion and the total antisymmetric T–O–T band measured by FTIR spectroscopy is evaluated for cobalt exchanged ferrierites. The limits of the method for detection of exchanged metal cations in zeolites with low content of Al (Si/Al>6), as related to published data, are given.

Differences in the structure of copper active sites for decomposition and selective reduction of nitric oxide with hydrocarbons and ammonia

Abstract Cu ion co-ordination-location in zeolites of MFI, erionite, mordenite matrices has been determined and the activity of the individual Cu sites compared for NO decomposition and its selective reduction by hydrocarbons or ammonia. It appears that Cu ions in the vicinity of one framework Al (site II), able to form stable Cu + -dinitrosyl complexes, and abundant in MFI structure, are responsible for high activity in NO decomposition. The Cu ions neighbouring two framework Al atoms (site I), and forming mostly mononitrosyl complexes, which dominate in erionite structure, provide a high activity in selective reduction of NO.

Comparison of 18O Isotopic Exchange After and During the Interaction of N2O, NO, and NO2 with Fe Ions in Ferrierites

Nitrous oxide deposits its oxygens on Fe-ferrierites at 200–250 °C in contrast with nitric oxide and nitrogen dioxide. This oxygen is readily exchangeable for 18O2 at room temperature and the reaction proceeds via a single-step exchange mechanism. All three nitrogen oxides in a mixture with 18O labeled dioxygen undergo isotopic exchange (IE) at 200–250 °C, N2O via the same single-step mechanism, while NO and NO2 react via a multiple-step mechanism. Zeolitic oxygens participate in IE above 250 °C during temperature-programmed desorption of surface species formed in the reaction of nitrogen oxides with 18O2.

Decomposition of nitrous oxide over Fe-ferrierites. Effect of deposited oxygen on the framework oxygens studied by 18O isotopic exchange

About 50-75% of oxygen captured during decomposition of N2O at 200 °C over Fe-FER (Fe/Al 0.03-0.6) was exchanged by 18O at room temperature. Complete desorption of captured oxygen containing mostly 16O isotope was reached at higher temperature. The 18O deficiency was rationalized by assuming labilization of the framework oxygen in Fe-FER.