Blanka Wichterlová

Acid-catalyzed synthesis of mono- and dialkyl benzenes over zeolites: Active sites, zeolite topology, and reaction mechanisms

This review describes mechanisms of catalytic reactions leading to mono- and dialkyl benzenes over zeolites, which represent an important part of petrochemical products. The emphasis is given to the effect of the type of acid sites, zeolite structure, and reaction conditions on the activity and selectivity of these complex reactions, and particularly to the individual products with respect to their iso- vs. n- and ortho-, meta-, and para-isomers. The first part of the paper deals with the description and analysis of the structure and properties of the acid sites, and zeolite pore inner structure applied in synthesis of alkyl benzenes. Individual reactions leading to the synthesis of propyl benzene, xylenes, ethyl toluenes, diethyl benzenes, and transformation of trimethyl benzenes to xylenes from the view point of the reaction mechanism, function of the acid sites, and inner pore geometry of zeolites are described in the second part.

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.

Determination of the number and acid strength of acid sites in zeolites by ammonia adsorption: Comparison of calorimetry and temperature-programmed desorption of ammonia

Abstract Calorimetry and the temperature-programmed desorption (TPD) of ammonia were used to determine the number and strength of acid sites in HZSM-5 zeolites and in mordenites with different Si:AI ratios. As the temperature peak maxima were found to be influenced not only by the acid site strength but also by the number of acid sites and the zeolites structure, they cannot be used for comparison of the acid site strengths of different zeolites. The TPD heats of ammonia were calculated by varying the carrier gas flow-rate or the amount of the zeolite sample. It is shown that the heats of adsorption of ammonia for individual zeolites obtained by TPD and calorimetry are in very good agreement. Similarly, the numbers of strong acid sites found by these methods are comparable and are lower than those calculated from the chemical analysis of zeolites.

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.

Effect of Broensted and Lewis sites in ferrierites on skeletal isomerization of n-butenes

Skeletal isomerization of n-butenes to isobutene at 620 K was followed over NaH-ferrierites (FER) with different concentration of Broensted sites, prepared by NH4+ vs. Na+ ion exchange, and over H-FER containing both, the Broensted and Lewis sites formed by zeolite dehydroxylation at temperatures ranging from 670 to 990 K. Quantitative analysis of IR spectra of OH groups and adsorbed d3-acetonitrile on ferrierite enabled determination of the concentration of Broensted and Lewis sites, and detected also those present in 8-ring ferrierite channels. With H-FER, the yield of isobutene is linearly proportional to the concentration of acidic OH groups. Formation of C3 and C5 olefins, C2–C5 paraffins and higher molecular weight products, resulting from di-, oligomerization of butenes followed by cracking/hydrogen transfer reactions, is enhanced by the presence of Lewis sites, if simultaneously present with the Broensted sites, that is in a substantial change of the selectivity of the reaction. On the other hand, both the conversion and yield of isobutene are nearly constant at decreasing concentrations of OH groups in NaH-FER, and only at low-concentration range of acidic hydroxyls a decrease in conversion was found. This indicates that the exchanged Na+ ions are placed preferably in small ferrierite channels, as also evidenced by 23Na MAS NMR, which size is not large enough to accommodate isobutene molecules. Thus, besides the well-known positive effect of the inner geometry of ferrierite channels on isobutene selectivity, it has been shown that the reaction of n-butenes to isobutene is proportional to the number of hydroxyls located in large channels of ferrierite, and simultaneous presence of Lewis sites enhances di-,oligomerization/cracking reactions leading to higher yields of by-products.

Decisive role of transport rate of products for zeolite para-selectivity: Effect of coke deposition and external surface silylation on activity and selectivity of HZSM-5 in alkylation of toluene

Toluene alkylation with methanol and ethylene over HZSM-5 zeolites has been investigated in order to understand the effects of deposited ?coke? and postsynthesis surface silylation on the toluene conversion and product selectivity. Long time-on-stream kinetic runs, performed under conditions in which total toluene conversion did not decrease but concentration of coke continuously increased were used to follow the changes in the individual xylene selectivity, the activity of the external surface, and the transport rates of individual xylene isomers. A very small increase in ortho- and para-xylene selectivity, no changes in xylene diffusivities, but a substantial decrease in the activity of the external surface were observed with increasing coke deposition. Under severe reaction conditions when coking caused a substantial decrease in toluene conversion, nearly no increase in para-xylene or para-ethyltoluene selectivity was found. The silylation of ZSM-5 zeolite with tetraethylorthosilicate blocked active sites on the external zeolite surface and decreased considerably the diffusivities of xylenes and ethyltoluenes. This modification also increased substantially the para-selectivity at high toluene conversions. Comparing the effect of coke deposition and surface silylation on the para-selectivity it is concluded that a decrease in the transport rates of xylenes is decisive for an enhancement of the para-selectivity, whereas secondary isomerization of para-isomers on the external surface plays a minor role.

Spectroscopic studies of vanadium-substituted zeolitic silicates of MFI topology

Samples of silicalite-1 with vanadium ions incorporated in stable framework positions (KVS-5) have been synthesized using several different compounds of V. The materials were investigated by XRD, chemical analysis and SEM–EPM techniques and UV–VIS, EPR, NMR, IR and Raman spectroscopy. One group of the samples contained up to 0.1 wt. % of V only, predominantly in a tetrahedral coordination and in two oxidation states V4+(violet) and V5+(colourless). The other group consisted of materials with about 1 wt. % of V, coordinated mostly in square pyramids with vanadyl groups (dirty green to blue–grey). The pyramids occur in two frequent configurations and one that is rare. The different shapes are probably due to various distributions of stronger and weaker V—O bonds. After calcination, vanadium is fully oxidized to V5+ without leaving its framework position. The tetrahedral coordination remains stable and the pyramids transform to strongly distorted octahedra complemented by water molecules (yellow). All these transformations, as well as the redox reaction V4+↔ V5+, are reversible. In all coordinations, the V centres remain neutral and contain, in addition to the framework V—O bonds, either a type of non-acidic OH group or, more likely, ONa groups with non-exchangeable Na ions. A model is proposed for transformations of V complexes.

Redox behaviour of Fe3+ impurities in Y zeolites. E.s.r. study

The redox behaviour of Fe 3+ impurities located in zeolite skeleton, in cationic sites and in the form of occluding hydroxy-oxidic compounds is investigated by means of e.s.r. spectroscopy. While the skeletal Fe 3+ are very resistant to reduction, the Fe 3+ in cationic sites are very easily reduced. Dehydrated and dehydroxylated zeolites with Fe 3+ valence state can be prepared only by heat treatment in oxygen atmosphere. This must be taken into account when evaluating the effect of Fe 3+ ions on the properties, especially the catalytic activity, of zeolites.