Jiří Dědeček

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.

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).

Co2+ ions as probes of Al distribution in the framework of zeolites. ZSM-5 study

The occurrence of “Al pairs” in the framework of ZSM-5 and their spatial distribution in zeolites was estimated by using exchanged Co2+ ions as probes for “Al pairs”. The visible spectra of Co2+ ions were employed to monitor the distribution of Co2+ ions, and thus “Al pairs”, among the individual cationic sites, i.e. local framework arrangements at defined positions in the zeolite channels. The effects of concentration of aluminum in the framework and conditions of zeolite synthesis on the presence of “Al pairs” and their spatial distribution in the ZSM-5 framework were investigated. It has been shown that the distribution of Al atoms in the framework is not controlled by statistic rules, but depends on the conditions of zeolite synthesis and aluminum concentration in the framework.

Iron oxide mineralogy in late Miocene red beds from La Gloria, Spain: rock-magnetic, voltammetric and Vis spectroscopy analyses

Abstract Free ferric oxides of a red bed series were analyzed by rock-magnetic techniques (IRM component analysis) and by two less traditional methods: visible spectroscopy and voltammetry. All three methods have low limits of detection, making them suited for this type of analysis. The red bed samples studied contained clay minerals, quartz, and calcium carbonate as major constituents. Free Fe oxides occurred at a concentration of 0.3–2.1%, i.e. in the majority of the samples below the detection limit of X-ray powder diffraction. The combination of the employed analytical techniques enabled to characterize the mineralogy of free ferric oxides and to estimate the ratio of goethite and to hematite. This ratio changes substantially within the section that probably indicates paleoclimatic changes.

Coordination and properties of cobalt in the molecular sieves CoAPO-5 and -11

Abstract Diffuse reflectance electronic spectra have been utilized to investigate the local geometry of cobalt centers substituted in the three-dimensional framework of CoAPO-5 and -11 molecular sieves. Structural changes induced by calcination in an oxidative atmosphere, evacuation and hydrogen treatment, as well as by adsorption of amines and water are discussed in terms of the steric conditions of cobalt incorporation in the aluminophosphate matrix. We suggest a template–framework interaction between the framework cobalt centers and neutral amine-type templates as an alternative stabilizing effect in the as-synthesized samples. Spectral changes observed after calcination in oxygen are interpreted as distortion-induced charge transfer effects without oxidation of the divalent cobalt ions. Formation of oxygen vacancies in the close vicinity of the cobalt sites is proposed to explain the adsorption properties of the framework cobalt centers.

Geometry of the Cu+ 540 nm luminescence centres in zeolites

Cu+ ions, with characteristic emission at 540 nm, have been considered to be responsible for the high activity of Cu-ZSM-5 in NO decomposition. However, two types of Cu+ ions located in different structures, i.e. in ZSM-5 and Y, exhibit the 540 nm emission, but the Cu ions in Y zeolite do not exhibit such a unique activity in NO decomposition. Therefore, Cu+ emission spectra and luminescence decay of the Cu2+ ion exchanged and reduced in CuH-ZSM-5, CuH-, CuRb-, CuCs- and CuBa-mordenites and Cu-Y zeolites were used to identify cationic sites corresponding to the Cu+ luminescence centres in these zeolites emitting at 540 nm. The Cu+ ions with a single luminescence decay correspond to those with C3v symmetry in Y zeolite, while the Cu+ ions possessing a double-exponential decay represent Cu+ cationic sites in ZSM-5 and mordenite, different from that described for Cu-Y, but the local arrangement of these Cu+ sites is similar in Cu-ZSM-5 and Cu-mordenite. The effect of the presence of Rb, Cs and Ba co-cations and pyridine adsorption in CuH-mordenites on the luminescence spectra was used to identify the cationic site represented by the 540 nm emission in mordenite. These Cu+ ions are located in site E of the mordenite main channel. The elongated six-ring of this site is composed of two five-rings sharing two oxygens, and the Cu+ ions are suggested to be coordinated close to the rectangle of the main channel wall. The 540 nm Cu+ emission of the Cu-ZSM-5 was attributed to the Cu+ ion in a site with a local arrangement similar to that of mordenite. The Cu+ ions are located in the main (straight) channels of the ZSM-5 matrix and are coordinated to the oxygen oblong of the channel wall. The cationic site is formed, as in mordenite, by an elongated six-ring composed of two five-rings. Thus, the structure of the Cu active site in Cu-ZSM-5 was suggested.

Experimental study of the effect of Si/Al composition on the aluminum distribution in (Al)MCM-41

Abstract The effect of framework Al content on the number of framework Al atoms on the surface of (Al)MCM-41 channels and the distribution of these Al atoms among single Al atoms and various Al–Si–Al and Al–Si–Si–Al pairs was investigated. The number of framework Al atoms on the wall surface was characterized by the ion exchange capacity for Na+ ions, number of Al pairs on the surface by exchange capacity for Co2+ ions. Two different geometrical arrangements of Al pairs were characterized by VIS spectroscopy of bare Co2+ ions. At low framework Al content, Al atoms occupy only T sites on the surface of the channels. In aluminum rich (Al)MCM-41, Al atoms are incorporated also into the inner volume of the channel walls and at Si/Al∼11 only about 50% of Al atoms are on the channels surface. Al atoms on the wall surface preferentially form Al–Si1,2–Al pairs, enabling siting of divalent cations. Number of single Al atoms increases with framework Al content and overcome 50% of surface Al atoms for Si/Al∼11. The Al–Si1,2–Al pairs are present in two cationic sites and also the distribution of Al pairs into these sites depends on the framework Al content.

Alkylation and disproportionation of aromatic hydrocarbons over mesoporous molecular sieves

Abstract Toluene alkylation with propylene and trimethylbenzene (TMB) disproportionation were investigated over mesoporous molecular sieves of MCM-41 type of different pore dimensions, possessing various Si/Al ratios or modified with heteropoly acids (HPA) of Keggin type. It was shown that the acid strength of (Al)MCM-41 was sufficient to activate propylene in the alkylation of toluene providing cymene selectivity over 96% while the rate of consecutive reactions was negligible. In contrast, TMB disproportionation proceeded at a significantly lower extent than over zeolites. Siliceous MCM-41 modified by HPA (HPA-MCM-41) exhibited a lower rate of cymene isomerization compared to (Al)MCM-41. Moreover, m -cymene was mainly formed at the expense of o -cymene. In TMB disproportionation over HPA-MCM-41 (HPA=H 6 PV 3 Mo 9 O 12 ) significantly higher concentrations of xylenes were reached in comparison with (Al)MCM-41. m - and o -xylenes as primary products of TMBs disproportionation were preferentially formed.

Does Density of Cationic Sites Affect Catalytic Activity of Co Zeolites in Selective Catalytic Reduction of NO with Methane

Selective catalytic reduction of NO with methane (CH4-SCR) in an excess of oxygen over Co ions located in ZSM-5 of various Si/Al composition and in ferrierite, mordenite, chabazite and beta zeolite was investigated. From the comparison of the dependence of the TOF values per Co ion for NO conversion to N2 and Co ions distribution among the cationic sites on total Co ion concentration, the catalytic activity of the individual Co ions was estimated. The α-type Co2+ ions, located in the main channel of mordenite and ferrierite and coordinated above the rectangle of four framework oxygens of the channel wall exhibit the highest activity in these zeolites. On the other hand, the β-type Co2+ ions coordinated in the plane of four oxygens of the deformed six-member ring located in the channel intersection of ZSM-5 and in channels of beta zeolite control the activity of these Co zeolites. The sequence of activity of Co2+ ions in CH4-SCR of NO was FER Coα>ZSM-5 Coβ>BEA Coβ≈ZSM-5 Coα≫FER Coβ≈MOR Coα≫CHA Co≈MOR Coβ. A correlation between the activity of the individual Co ions in CH4-SCR of NO and a distance between the cationic sites was observed.

Location of Framework Al Atoms in the Channels of ZSM‐5: Effect of the (Hydrothermal) Synthesis

(27) Al 3Q MAS NMR and UV/Vis spectroscopy with bare Co(II) ions as probes of Al pairs in the zeolite framework were employed to analyze the location of framework Al atoms in the channel system of zeolite ZSM-5. Furthermore, the effect of Na(+) ions together with tetrapropylammonium cation (TPA(+)) in the ZSM-5 synthesis gel on the location of Al in the channel system was investigated. Zeolites prepared using exclusively TPA(+) as a structure-directing agent (i.e., in the absence of Na(+) ions) led to 55-90% of Al atoms located at the channel intersection, regardless the presence or absence of Al pairs [Al-O-(Si-O)2 -Al sequences in one ring] in the zeolite framework. The presence of Na(+) ions in the synthesis gel did not modify the Al location at the channel intersection (55-95% of Al atoms) and led only to changes in i) the distribution of framework Al atoms between Al pairs (decrease) and single isolated Al atoms (increase), and ii) the siting of Al in distinguishable framework tetrahedral sites.