S. Beran

Determination of proton affinity of zeolites and zeolite-like solids by low-temperature adsorption of carbon monoxide

The i.r. spectra of HNaY, HAIY, and HZSM-5 zeolites and AlPO4-5 and SAPO-5 phosphates interacting with CO are used to characterize hydroxyl acid strength. The interpretation of observed effects is based on the quantum-chemical calculation of model clusters. It was found that the shift in the stretching vibration frequency of OH groups caused by the adsorption of CO varies between 90 and 332 cm−1 (corresponding to the proton affinity values of 1139–1390 kj mol−1) and increases in the sequence SiOH < AlOH ⩽ POH < SiOHAl. The acid-strength heterogeneity of bridging SiOHAl groups causes mainly the variation of their vibrational shift with increasing CO coverage. The influence of neutral alumina-like nonskeletal species on the acid strength of bridging hydroxyls is also discussed, and method of CO adsorption is compared with thermal decomposition of pyridinium ions.

Acidity of modified Y zeolites: Effect of nonskeletal Al, formed by hydrothermal treatment, dealumination with SiCl4, and cationic exchange with Al

Abstract The acidity of modified Y zeolites was studied using adsorption and thermal desorption of pyridine (i.r.s. technique). It was found that at least some nonskeletal Al species always take part in the compensation of lattice charge, thus affecting the number of skeletal hydroxyls. Oxidic clusters prevailing on hydrothermally treated zeolites yield a much lower fraction of strong Al electron-accepting sites than of cationic species, which are typical for AIHY and are also formed in a considerable amount in zeolites dealuminated with SiCl4. Both types of nonskeletal Al species decrease the Bronsted acid strength of the zeolite. The activity of modified zeolites in the oligomerization of ethylene is discussed taking into account the action of Al electron-accepting sites.

Quantum chemical calculations of the physical characteristics of the hydroxyl groups of zeolites

Abstract The CNDO/2 method was applied in the study of the electronic structure of zeolite HY using models employing (HO)3Si.OH.Al(OH)3 and (HO)3Si.O.Al(OH)−3 clusters for various types of skeletal oxygens. It has been shown that the OH groups on the O1 and O3 types of oxygen are the energetically most favourable ones of all studied groups. The calculated vibration of the O1H group lies at a higher frequency than the O3H group, even though their values are somewhat higher than their difference is smaller than experimental data.

Solid State Interactions of Mn or Fe Cations with ZSM-5 Zeolites

Abstract The state of Mn and Fe ions in ion-exchanged or ion-supported zeolites and in physical mixtures of Mn and Fe salts (or oxides) with H − or Na − ZSM-5 zeolites is investigated by using ESR, XPS, TPDA and transformations of methanol and toluene. It is shown that a strong, solid-state interaction between the metal ions and the HZSM-5 zeolite takes place at elevated temperatures, in contrast to NaZSM-5. The metal ions are found to be homogeneously distributed within the zeolite structure after this interaction and exhibit properties of metal ions coordinated in the zeolite cationic sites. The introduction of metal ions into the zeolites, reflected also in the lower number of strong acid sites, is manifested in modified catalytic activity and selectivity of the zeolites.

Quantum chemical study of the interaction of ethylene with hydroxyl groups of HZSM-5 zeolites

Abstract The CNDO/2 method has been used to study the interaction of ethylene with the hydroxyl group of HZSM-5 zeolites. It is shown that this interaction (which leads to the formation of a stable π-complex) is, as a result of the higher acid strength of the OH group of the HZSM-5 zeolite, energetically more favourable than the same interaction with faujasites. Similarly, the transfer of electron density from ethylene to the zeolite, as well as the perturbation of the ethylene molecule, is more pronounced for the ethylene complex with the HZSM-5 zeolite than with faujasites.

Quantum chemical study of the interaction of ethylene with the Lewis sites of zeolites

Abstract The CNDO/2 method is used to study the interaction of ethylene with the Lewis site of faujasites modelled by clusters Si 3 Al 3 O 6 (OH) 12 Al. It is shown that the interaction of ethylene leading to the formation of a stable π-complex is energetically more favourable than similar interactions with the zeolite hydroxyl groups. The π-complex may be transformed into a σ-complex which is energetically slightly more favourable than the π-complex. A weakening of the ethylene molecule bonds, which occurs as the result of the donation of electrons from ethylene to the Lewis site, is more pronounced than the weakening found for the interaction of ethylene with the O—H groups of the zeolite. The results of calculations are related to the observed oligomerization of ethylene over zeolites containing Lewis sites.

Preparation of Nihzsm-5 Catalyst for Isomerization of C8 Aromatics. Solid-State Incorporation of Nickel

Abstract A high temperature interaction between HZSM-5 and solid NiCl 2 , NiSO 4 , Ni(CH 3 COO) 2 and NiO was investigated. For NiCl 2 , disaggregation of the salt is complete at 770 K with evolution of gaseous hydrochlorid acid, resulting in solid–solid ion exchange of Ni ions into the zeolite cationic sites at the expense of the strong acid skeletal OH groups. The heat treatment of the zeolite with NiSO 4 above 670 K leads to a decrease in the number of skeletal OH groups to approximately half the original number. Moreover, no evolution of acid or its decomposition products is observed. Ni(CH 3 COO) 2 behaves differently. No ion–change (even in a wet paste) occurs up to the salt decomposition temperature (520 K). NiO formed at higher temperatures also does not interact with the skeletal OH groups. The described high–temperature solid–solid ion exchange between HZSM-5 and NiCl 2 was used successfully for the incorporation of Ni 2+ into the zeolite cationic sites. Subsequent reduction of these ions by hydrogen yields well–dispersed metallic nickel which is active in the isomerization of C 8 aromatic hydrocarbons.

Model calculations of the electrostatic field in ZSM-5 zeolites and its effect on molecules

Abstract The electrostatic field in the pore system of ZSM-5 zeolites was studied in the point charge approximation. It is demonstrated that this field can attain values of up to 1010 V m−1 in the zeolite channels. It then follows from model quantum chemical calculations that this field produces considerable changes in the physico-chemical properties of molecules.

Quantum chemical study of interactions of ketones with zeolites

Abstract The CNDO/2 method is employed to study the interactions of acetone, methyl ethyl ketone and diethyl ketone with OH groups in zeolites modelled by the Si3AlO4(OH)8 H clusters. It is demonstrated that this interaction, accompanied by the formation of the H-complexes of ketones, leads to the transfer of electron density from the ketone to the zeolite and polarization of the molecule. Both these effects then cause weakening of the CO, C-H and O-H bonds and strengthening of the C-C bonds. Similar perturbation of the acetone molecule is indicated by the results of model electrostatic calculations on the interactions of acetone with the Na cationic sites in zeolites. The calculated changes in the strengths of the individual bonds of the ketones interacting with the zeolite active sites correlate well with the shifts in the corresponding vibrational bands observed for ketones interacting with these centres.

Reactions of aliphatic ketones on zeolites

Reactions of some ketones (acetone, methyl ethyl ketone and diethyl ketone) on the H-form of ZSM-5, ZSM-11, mordenite and erionite have been studied in an integral reactor. At a reaction temperature of 300 or 350°C acetone is selectively transformed on all these zeolites into isobutene, while with higher ketones substantial amounts of aromatics are formed.AbstractРеакции некоторых кетонов (ацетона, метилэтилкетона и диэтилкетона) на Н-форме морденита ZSM-5, ZSM-11 и эрионита исследовали в интегральном реакторе. Показано, что при температуре реакции 300 или 350°C ацетон селективно превращается на всех этих формах цеолитов в изобутен, в то время как для остальных кетонов наблюдалось образование ароматических соединений в значительных количествах.