Jürgen Seebode

Sorption and isomerization of cyclopropane in A-type zeolites studied by infrared spectroscopy

Abstract Cation sensitive red shifts of the CC band demonstrate the formation of π-complexes between propene and the cations. Cation dependent isomerization activity and i.r. spectra prove the ‘face-on’ arrangement of cyclopropane in front of S1 cations. Coadsorption of sulphur dioxide greatly enhances the rate of isomerization. In connection with the absence of Bronsted sites a concerted reaction mechanism is concluded, i.e. cations and sulphur dioxide act as Lewis acids polarizing the π-electron system of cyclopropane, thus activating the sorbed molecules for isomerization.

Formation of carbocations from C3 compounds in zeolites of different acidities

Generation of enylic carbenium ions from compounds possessing C3 carbon skeletons has been investigated on mordenites of different acidities by ultraviolet–visible spectroscopy. The formation of similar carbocations from allene, propene, cyclopropane, isopropanol and acetone has been identified. The rate of formation of allylic and polyenylic carbocations markedly depends on the number of acidic centres in the zeolite. The order of capability for carbenium ion formation found for organic compounds of similar structures was in good agreement with that observed previously in sulphuric acid solutions.

Transmission ultraviolet–visible–near-infrared spectroscopic investigations of the formation of carbocations in zeolites. Re-examination of propene/CoNaY and extension to trans-butene/CoNaY, propene/H-mordenite and cyclopropane/H-mordenite

The applicability of the u.v.–vis.–n.i.r. transmission technique in surface studies has been explored using the sorption of propene, trans-butene and cyclopropane in activated zeolites CoNaY and HM. On CoNaY, after dehydration, near-tetrahedral Co2+ complexes remain, which are approached by the hydrocarbons without change of coordination number or symmetry. In HM at room temperature and in CoNaY at slightly elevated temperatures the appearance of new bands proves that allylic and dienylic carbenium ions are formed in an isomerization–oligomerization reaction. Mechanisms for the formation of these carbocations via Lewis or Bronsted acid sites are discussed.

Isomerization of cyclopropane over A-type zeolites. Kinetic studies

Abstract Kinetic investigations augmented with i.r. spectroscopic studies show that cyclopropane isomerization on Lewis-type cationic sites in A—type zeolites follows first order kinetics and is clean in the sense that no oligomeric species are detectable in the gas phase under the reaction conditions applied. First order rate constants and activation energy have been determined.

CARBOCATION FORMATION IN ZEOLITES: UV-VIS-NIR SPECTROSCOPIC INVESTIGATIONS ON MORDENITES.

Abstract Generation of similar enylic carbenium ions from allene, cyclopropane, propene, isopropanol and acetone in H- and NaH-mordenite was proved by u.v.-vis.-spectroscopy. The rate of formation and consecutive oligomerization is markedly enhanced by the zeolite acidity.

UV-VIS and IR Spectroscopic Investigations on the Generation of Alkenyl Carbocations from Propene in Zeolites of Different Acidity

Abstract Generation and reaction of alkenyl carbocations from propene in mordenites and HNaY zeolites of different acidity have been investigated by means of UV-VIS and IR spectroscopy. The rate of carbenium ion formation runs parallel with the number of acid sites. The bands observed upon adsorption of propene at 1535 and 1505 cm −1 can be attributed to the C-C-C stretching vibration of the allyl system of these carbocations. The shift of the UV-VIS absorptions with increasing Bronsted acidity is characteristic of the higher degree of substitution of the same type of carbocations; the alkenyl ions, due to their strong bond to the surface, can be one of the components of the carbonaceous deposits formed by reactions of olefins at low temperature.

Generation of alkenyl-type carbocations from neopentane in HNaY zeolite

In the transformation of neopentane in HNaY zeolite, the presence of alkenyl-type carbocations could be detected by UV-VIS spectroscopy. Generation of these ions was not accompanied by the formation of methane which could be the primary cracking product of neopentane.AbstractВ превращениях неопентана на цеолите HNaY, присутствие карбокатионов алкенильного типа детектируют уф-видимой спектроскопией. Генерирование этих ионов не сопровождается образованием метана, который может быть первичным продуктом крекинга неопентана.

Skeletal isomerization of cyclopropane over CoNaA zeolites. A comparison of kinetic data obtained by i.r. spectroscopic and reactor measurements

Abstract Two different techniques have been applied to the study of cyclopropane isomerization in zeolite CoNaA: time-resolved i.r. spectroscopy at ambient temperatures and g.c. analysis using a recirculatory flow reactor at elevated temperatures. For the surface reaction reasonable rate constants could be obtained from both methods in spite of the different temperatures, indicating the reaction mechanism remains unchanged in this temperature range.

Frequencies, anharmonicities, and dissociation energies of OH groups in H-mordenite

Harmonic frequencies, anharmonicities and dissociation energies of surface hydroxy groups in H-mordenite have been determined. The application of the Lippincott-Schroeder potential function for similar problems is proposed.AbstractБыли определены гармонические частоты, негармоничность и энергии диссоциации поверхностных гидроксильных групп на Н-мордените. Предлагается применение потенциальных функций Липпинкотаа-Шрёдера для подобных проблем.

Normal coordinate analysis of molecules adsorbed on zeolite surfaces. Part 1.—Cyclopropane adsorbed on sodium faujasites and mordenites

Vibrational analysis of adsorbed polyatoms provides valuable information on the degree of participation of different bonds in the interaction with the adsorbent. As an example, normal coordinate analysis has been applied to cyclopropane in the free and the adsorbed state in sodium faujasites and mordenites of different Si/Al ratios using a valence force field. The alterations of the force constants can be interpreted in terms of changes of the molecular geometry and electronic structure as well as by the hindrance of the bending motion due to the interaction with the internal surface. The lack of influence of the lattice module supports the location of the cyclopropane molecule near the cations.