H. Förster

Investigations of coke deposits formed during deep oxidation of benzene over Pd and Cu exchanged Y-type zeolites

Abstract Carbonaceous deposits (coke) formed during the catalytic deep oxidation of benzene over copper and palladium zeolites Y have been analyzed by means of temperature programmed oxidation, FTIR spectroscopy and HRGC/MS techniques. The coke fractions insoluble in methylene chloride consist of polyaromatic compounds with different oxygen contents. The fact that the main constituent 1,4-naphthoquinone and five more identical components are found in the soluble coke fractions of PdY as well as CuY points to a similar reaction mechanism on both catalysts. Corresponding to the results a polymerization-oxidation mechanism is discussed. Starting from radical cations, polyaromatic compounds are formed which are decomposed via oxidic intermediates under formation of H2O and CO2. At elevated temperatures, the direct participation of oxygen from the gas phase seems probable. The lack of a soluble coke fraction and the comparatively low oxygen content in the insoluble coke fraction of HPdY supports the suggestion that the relatively low activity of this catalyst might be traced back to its restricted capability for coke decomposition.

Infrared spectroscopic study of the adsorption of hydrogen sulfide on zeolites NaA and NaCaA

Abstract The adsorption of hydrogen sulfide on zeolites Na A and NaCa A was studied in an UHV infrared cell. From the large frequency shifts and band shapes it is concluded that the H 2 S molecules prefer the S 2 A sites in the zeolite Na A and the S 1 A sites in the NaCa A according to the difference in their potential. The influence of the exchange cation seems to be less important. A chemisorbed species was detected in the case of Na A and interpreted as a hydrosulfide group attached to the cage wall. An observation of the H 2 S bending mode failed due to the broad vibrational band of the zeolite skeleton near 1000 cm −1 .

Sorption states of isoelectronic linear molecules in zeolites

Abstract Sorption states of di- and triatomics in zeolites A containing mono- and divalent ions were investigated, applying an electrostatic interaction model, and compared with experimental data, i.e. heats of sorption, IR band shifts, intensities and interatomic distances obtained from X-ray diffraction. In all cases zeolite cations have to be considered as centers of interaction, the isoelectronic pair CO and N 2 being attached with two possible orientations of CO. Due to their size the isoelectroNic triatomics CO 2 and N 2 O interact with two cations simultaneously. The calculated heats of adsorption turned out to be in good agreement with the experiment. Theoretical frequency shifts of CO and N 2 support the assignment of bands. The calculated electric field strength at the sorption site corresponds satisfactorily with values derived from the induced IR intensities of nitrogen. It is concluded that isoelectronic molecules reveal strong similarities in their sorptional behaviour.

Infrared spectroscopic studies on carbon dioxide adsorption in alkali-metal and alkaline-earth-metal ion-exchanged A-type zeolites. Part 1.—General features of CO2 interaction with A-type zeolites

The influence of different alkali-metal and alkaline-earth-metal ions on the formation of physisorbed and chemisorbed carbon dioxide in A-type zeolites has been studied by F.t.i.r. spectroscopy. Various species of both kinds can be distinguished, the mutual correlations of which are determined by the exchange cation. In the case of physisorbed CO2 the ν2 band and the 2ν2/ν1 Fermi diads, which become i.r. active upon adsorption, can be observed, and possible sites and geometries of the sorption complex are discussed. Compared to X- and Y-type zeolites the ν3 frequency shift shows a poorer correlation with the charge:radius ratio of the cation, so rendering the detection of sorption sites difficult. The general loss of rotational fine structure points to a strong hindrance to rotation. Satellite bands of the ν3 fundamental band are tentatively assigned to external motions.

IR and XAS investigations on the interaction of butadiene with zeolite CuY

Abstract The interaction of zeolite CuNaY with 1,3-butadiene was studied by the joint application of IR, EXAFS and XANES spectroscopy. Autoreduction to Cu(I) was essentially absent and could be not significantly enhanced by CO pretreatment under “mild” conditions. The formation of Bronsted acid sites was observed after dehydration, giving rise to undesired oligomerization of butadiene via carbocation intermediates. EXAFS and XANES measurements reveal that the copper ions are directly involved in the interaction with butadiene, thereby being partially reduced to monovalent copper.

Exploring cation siting in zeolite ZSM-5 by infrared spectroscopy, EXAFS and computer simulations

Abstract A new approach for retrieving information on cation siting in silicon-rich zeolites like ZSM-5 is presented. It is based on combined investigations using the cation sensitive methods, far-IR spectroscopy and EXAFS together with computer simulations. This combination proved to be successful for alkali and even for alkaline earth ions. It was possible to obtain cation locations for Cs + as well as for Ba 2+ ions in ZSM-5. In this context the series of far-infrared spectra of alkali exchanged ZSM-5 is shown for the first time.

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.

The thermal behaviour of sodium azide in zeolites

Abstract Upon thermal treatment, NaN3-doped zeolite transforms stepwise. After endothermic dehydration, the NaN3 decomposes exothermically in air and under N2 flow. The temperature of decomposition is higher than that of the pure azide. Part of the sodium azide remains intact up to 1000 K. The process was also monitored by IR spectroscopy. 23Na NMR measurements revealed that metallic sodium clusters were formed.

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.

Induced overtones of homonuclear diatomics in zeolitic matrices

Abstract For the first time the overtones of oxygen, nitrogen and hydrogen induced by the internal surface field of zeolites are observed in the infrared spectrum. Their frequency shifts, full widths at half height and intensities are compared to the induced fundamentals, and changes in the molecular potential upon adsorption are discussed in terms of a weak interaction due to physisorption.