Pavel Jírů

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.

Reaction of Small Amounts of Methanol on Hzsm-5, Hy and Modified Y Zeolites

Abstract The interaction of methanol with HY, AlHY, dealuminated Y and HZSM-5 zeolites containing various amounts of extralattice Al and acidic OH groups was studied. Surface species with increasing thermal stability in the sequence MetOH ads, Al-OMet and O(zeol)-Met were identified from IR spectra. Under low-pressure flow conditions (1–3 Pa) at 670 K the products of methanol transformation consisted mainly of lower olefins and aromatics over Y zeolites; HZSM-5 yielded predominantly formaldehyde and methane, however, pressure increase to 100 Pa resulted in the appearance of hydrocarbons. Possible reaction mechanism involving mentioned surface species is discussed.

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.

Properties of Y-type zeolites with various silicon/aluminium ratios obtained by dealumination with silicon tetrachloride. Distribution of aluminium and hydroxyl groups and interaction with ethanol

Y-type zeolites with Si/Al ratios of 3.4–20, prepared by the dealumination of NaY (Si/Al = 2.5) with SiCl4, have been characterized using X-ray diffraction analysis, sorption-capacity data and by obtaining infrared spectra of the framework, OH groups and adsorbed pyridine. Dealuminated zeolites exhibit good crystallinity and some aluminium released from the lattice remains in extralattice positions in the zeolite cavities or in an amorphous phase to provide strong electron-acceptor sites. The zeolites were obtained directly in the H form; further exchange with NH+4 affected their proton-donor properties only slightly. In addition to weakly acidic or non-acidic hydroxyls (bands at 3740 and 3620 cm–1), strongly acidic framework hydroxyls were present (bands at 3630 and 3560 cm–1) in amounts dependent on the amount and distribution of aluminium. An analysis of substances released during temperature-programmed desorption from zeolites with preadsorbed ethanol revealed the influence of dealumination on the catalytic activity: the maximum of evolved ethylene as a main product was reached on moderately dealuminated zeolites (Si/Al = 3.4–6) with an appreciable number of both strong electron-acceptor and strong proton-donor sites.

Behaviour of Fe Species in Zeolite Structure

Abstract The changes of the form and reducibility of Fe(III) ion complexes within the zeolite structure resulting from zeolite hydrothermal treatment and treatment with NaCl solution were studied by means of ESR spectra of Fe(III) ion, TPR by hydrogen, IR spectra of adsorbed CO and XPS spectra of zeolite surface layers. The parent FeNH4Y and FeNaY zeolites contained Fe(III) in cationic sites mainly, where they were easily self-reduced by vacuo and reduced by hydrogen to Fe(II). Along with the reorganization of zeolite structure the hydrothermal treatment causes the formation of 1d coordinated Fe(III) in Fe-O and Al-O clusters which are not selfreduced and are with difficulty reduced by hydrogen to Fe(II). NaCl solution treatment does not lead to a simple exchange of Fe(III) ions from cationic sites into the solution. However, their rearrangement into new, most likely hydrolytic complexes within the zeolite takes place. In contrary to Fe(III) in cationic sites, these complexes are not self-reduced by vacuo and are partially reduced by hydrogen to metallic Fe.

Adsorption of acetonitrile as a tool for the characterization of acid-base properties of zeolite catalysts

It has been shown that in the IR spectrum of d3-acetonitrile adsorbed on various zeolite catalysts, the values characterizing the absorption bands in the ν(C≡N) range, namely their absorbances (together with their positions and thermal stabilities) may represent a relative measure for the concentration and the strength of their proton donor and electron acceptor centers.AbstractПоказано, что в ИК-спектрах ацетонитрила-d3, адсорбированного на различных цеолитных катализаторах, величины, характеризующие абсорбционные полосы в области ν(C≡N), а именно их абсорбции (вместе с их расположением и термической стабильностью) могут служить относительной мерой концентраций и сил их протоно-донорных и электроноакцепторных центров.

Study of the interaction of methanol with silica gel and Ag/SiO2 at elevated temperatures by infrared spectrometry

Abstract The influence of methanol at temperatures up to 420 °C on the surface of silica gel and Ag/SiO2 was investigated by infrared spectroscopy in a vacuum apparatus. Experimental data (spectra and tables) are reported. The participation of the different types of hydroxyl group and siloxane bridge of silica gel (pretreated at 450 °C in vacuum) in the interaction with methanol is discussed and the stability of the resulting methoxy groups towards oxidation at elevated temperatures (100–420°C) is investigated. Moreover, the properties of silica gel used as a support for silver are compared with those of pure silica gel by differential spectral measurements. In addition to firmly bound surface complexes, weakly adsorbed compounds were also investigated after interaction at 100 and 400°C, in order to obtain better insight into interaction of methanol with Ag/SiO2. The results were compared with literature data on the methanol-Fe, Co, Ni system, which has also been investigated by infrared spectroscopy.

A quantum chemical study of the influence of an electrostatic field on the reactivity of methanol over zeolites

The INDO method with an electrostatic potential term in its Hamiltonian has been used to estimate the changes in the electronic structure of methanol and methanol dimer due to the electrostatic field of zeolites. It has been shown that fields of the strength F=2–8×1010V/m change significantly the electronic structure as well as the geometry of both molecules and can cause their decomposition leading to the species H2O, CH3, CH3O, CH2O, and CH2.AbstractМетод INDO с термом электростатического потенциала в операторе гамильтона был использован для определения изменения электронной структуры метанола и димера метанола вследствие электростатического поля цеолитов. Было показано, что поля с силой F=2÷8·1010 в/м значительно изменяют как электронную структуру, так и геометрию обеих молекул и могут приводить к их разложению, давая частицы H2O, CH3, CH3O, CH2O и CH2.

Chromium ions within zeolites. Part 2.—A quantum-chemical study of the properties of chromium ions in faujasites

The CNDO (complete neglect of differential overlap) method is used for calculation of the physico-chemical characteristics of faujasites with CrII, CrIII, CrV and CrVI ions in the SII cation position, modelled by Si3Al3O6(OH)12Cr clusters. It is demonstrated that the Cr ions are bonded to the zeolite skeleton by electron donor–acceptor bonds whose formation leads to electron donation from the skeleton to the cation, thus stabilizing the valence states of the Cr ions. The most probable valence state of the Cr ions in the zeolites was found to be CrIII; however, it can readily be oxidized to CrV. In contrast, CrVI and CrV ions were found to exhibit high electron-acceptor ability and thus are readily reduced.

Study of the adsorption of n-butylamine on NaHY zeolites by microcalorimetry

The adsorption, of n-butylamine has been studied at 405 K on NaHY zeolites by microcalorimetry. The curves of the variation in the heat of adsorption as a function of the coverage change characteristically with the temperature of activation of the zeolite. From the course of these changes it is possible to establish which part of the curve corresponds to the adsorption of n-butylamine on Brönsted or Lewis acid centers.AbstractИсследована ацсорбция н-бутиламина при 405 К на цеолитах NaHY с помощью микрокалориметрии. Кривые зависимости теплоты адсорбции от заполненип характерно измехяются с температурой активации цеолита. На основе этих изменений можно определить, какая часть кривых отвечает адсорбции н-бутиламина на центрах Брёнстэда и какая на центрах Льюоиса.