B. Hunger

Adsorption of water on zeolites of different types

We have investigated the interaction of water with Na+-ion exchanged zeolites of different structures (LTA, FAU, ERI, MOR and MFI) by means of temperature-programmed desorption (TPD). The non-isothermal desorption of water shows, depending on the zeolite type, differently structured desorption profiles. In every case the profiles have, however, two main ranges. Using a regularization method, desorption energy distribution functions have been calculated. The desorption energy distributions between 42–60 kJ mol−1, which can be attributed to a non-specific interaction of water, show two clearly distinguished energy ranges. The water desorption behaviour of this range correlates with the electronegativity of the zeolites and the average charge of the lattice oxygen atoms calculated by means of the electronegativity equalization method (EEM). The part of the desorption energy distributions in the range of 60–90 kJ mol−1, reflecting interactions of water with Na+ cations, shows two more or less pronounced maxima. In agreement with vibrational spectroscopic studies in the far infrared region, it may be concluded that all samples under study possess at least two different cation sites.

Structure of adsorption complexes of water in zeolites of different types studied by infrared spectroscopy and inelastic neutron scattering

We combined the results of diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) measurements (4000–1500 cm−1) and inelastic neutron scattering (INS) (5–100 meV) to study the structure of adsorption complexes of water in zeolites of types FAU and MFI exchanged with alkali metal cations. In the case of faujasites, at relatively high water loadings we observe correlation between the position of the broad band at about 3400–3500 cm−1 arising from OH-stretching of hydrogen bonded water molecules and the basicity of framework oxygen atoms of the zeolite. This result indicates that at these water loadings a significant number of water molecules forms hydrogen bonds to framework oxygen. Temperature-programmed DRIFT studies show that depending on the type of exchanged cation (Li-LSX, NaX) the position of the band at about 3400–3500 cm−1 changes suddenly in the temperature region 400–450 K and then remains stable upon further increase of the temperature. The observed behavior is caused by the formation of smaller, more strongly bonded water-cation clusters from the large web-like structures as a significant number of water molecules is removed. Bands due to hydrogen bond stretchings (O–H⋯O) and up to three librations of water molecules are observed in the INS spectra in the regions 15–44 meV and 44–89 meV respectively. The observed shifts of the band positions for different water loadings are in agreement with the results of DRIFT studies.

Adsorption of water on ZSM-5 zeolites☆

The non-isothermal desorption of water adsorbed on ZSM-5 zeolites at room temperature shows, depending on the cation type (H+, Na+), differently structured desorption curves. A kinetic analysis based on various methods shows that the course of desorption is well described by a first order rate equation considering a distribution function for the desorption energy. In addition, adsorption isotherms were measured at room temperature. The adsorption energy distribution functions calculated with a regularization method can be correlated with the desorption energy distributions. FTIR investigations of water-loaded zeolites enables the assignment of desorption steps to different adsorbate complexes.

Heterogeneity of Si–OH–Al groups in HNaY zeolites

Using time-resolved FTIR spectroscopy it has been shown for NH3 TPD on an HNaY zeolite that ammonia is bound to the Si–OH–Al groups in two clearly distinguishable adsorption states. Adsorbed ammonia causing the band at 1450 cm–1 in the NH bending region desorbs at temperatures above 473 K. The course of desorption can be described by a unimodal desorption energy distribution function in the energy range of 100–135 kJ mol–1. More weakly adsorbed ammonia, characterised by NH bands at 1680, 1500 and partly 1390 cm–1, is desorbed below 473 K with a desorption energy distribution of 90–120 kJ mol–1. This assignment is confirmed by evaluation of the OH stretching region [high frequency (HF) and low frequency (LF) stretching vibrations]. At both NH4+ adsorption complexes there are contributions from both the HF and the LF bands. The stronger acid sites are characterised by OH sub-bands at 3635 cm–1(HF) and 3565 cm–1(LF), i.e. at the same wavenumbers as found for the OH groups in highly dealuminated faujasites. They are therefore assigned to isolated Bronsted acid sites. The acidity distribution seems to be mainly determined by the number of Al atoms in the vicinity of the Si–OH–Al groups, rather than by the different position of the corresponding framework oxygen.

Characterization of Acidic OH Groups in Y-Type Zeolites by Means of Different Methods of Temperature-Programmed Desorption (TPD) of Ammonia

The acidic properties of HNaY zeolites with different degrees of exchange have been studied by means of two complementary techniques of temperature-programmed desorption (TPD) of ammonia, namely, FTIR-TPD and conventional TPD. In HNaY zeolites with low exchange levels only a single type of OH group is observed. The analysis of the spectra in the region of NH bending modes during NH3-FTIR-TPD on zeolites with higher proton content (40% or higher) shows that ammonia is bound in two different adsorption states. This assessment is further confirmed by evaluation of the OH stretching region. Furthermore, desorption energy distribution functions for the different types of OH groups have been calculated using a numerical regularization method.

Inelastic neutron scattering and infrared spectroscopic study of furan adsorption on alkali-metal cation-exchanged faujasites

Inelastic neutron scattering (INS) as well as infrared (IR) transmission and diffuse reflection infrared Fourier transform (DRIFT) spectra of furan adsorbed on Li-LSX, NaY, NaX, K-LSX, and CsNaX zeolites have been measured in the range 2000-200 and 4000-1300 cm(-1), respectively. On the basis of an assignment of normal modes of furan taken from the literature and our own quantum chemical calculations of vibrational frequencies, the observed frequency shifts between bulk furan and furan adsorbed on the zeolites mentioned above have been interpreted in view of the interactions between furan and zeolite. For an explanation of frequency shifts of CH out-of-plane bendings, CH stretchings and some ring vibrations, it has to be assumed that in addition to the interaction between furan and the corresponding cation of the zeolite, a further interaction between the CH bonds and lattice oxygen atoms exists.

Adsorption of Water and Methanol on a NaZSM-5 Zeolite. A temperature-programmed desorption (TPD) study

Using temperature-programmed desorption (TPD), we have investigated the desorption behavior after subsequent co-adsorption of methanol and water and after adsorption of their mixtures on a NaZSM-5 zeolite. The course of desorption indicates that a strong mutual displacement of both components occurs. However, on the strongest adsorption sites methanol is preferentially adsorbed, and already the addition of small amounts of methanol leads to a displacement of water. Our results support the idea of a subdivision of the pore space for adsorption of water/methanol mixtures. Above all, the experiments show that in the part of the pore space where both components are adsorbed, different sites are of importance which vary significantly in their interaction strength.

Characterization of Extra-framework Cations in Zeolites. A temperature-programmed desorption (TPD) study

We have investigated the interaction of a few 5-ring organic compounds (cyclopentane, cyclopentene, furan, 2-methylfuran, 2,5-dihydrofuran and tetrahydrofuran) with alkali-metal cation-exchanged faujasites (LSX, X and Y types) by means of temperature-programmed desorption (TPD). The desorption behavior at higher temperatures of all probe molecules on the sodium ion containing faujasites with different Si/Al ratios reflects the higher cation content of zeolites with greater aluminum content. Only the desorption profiles of tetrahydrofuran and 2,5-dihydrofuran show, depending on the kind of cation, additional desorption features at higher temperatures. Using a regularization method, desorption energy distribution functions for furan and tetrahydrofuran were calculated. The calculated desorption energy distributions clearly illustrate the very different adsorption behavior of furan and tetrahydrofuran which leads to large differences in the binding energies between the corresponding adsorption complexes.

Investigation of the non-isothermal water desorption on alkali-metal cation-exchanged X-type zeolites: a temperature-programmed diffuse reflection infrared Fourier transform spectroscopic (TP-DRIFTS) study

The non-isothermal desorption of water on alkali-metal cation-exchanged X-type zeolites was investigated by means of temperature-programmed diffuse reflection infrared Fourier transform spectroscopy (TP-DRIFTS). Based on the areas under the DRIFT spectra recorded at temperature intervals of 10 K, the desorption profiles could be calculated. Their shape is principally identical to the desorption curves obtained by conventional TPD (EGD/EGA). Using a numerical regularisation method, desorption energy distribution functions have been calculated. The assignment of several characteristic bands to differently bonded water molecules and explanations for certain observed trends have been given.

Temperature-programmed surface reactions (TPSR) on heterogeneous surfaces

On the basis of calculations using a simple model of the energetic heterogeneity of a solid surface (assuming linear dependence of activation energy of desorption of the reactant on the degree of coverage), it is shown that both the degree of conversion and the course of desorption of the reactants are strongly influenced by the degree of heterogeneity assuming non-isothermal conditions. In contrast to a homogeneous solid surface, the degree of conversion for a heterogeneous surface depends strongly on the initial coverage of a catalyst by reactant. Possibilities for kinetic evaluation are indicated from the modelling calculations.ZusammenfassungFür eine irreversible Oberflächenreaktion 1. Ordnung wird anhand von Rechnungen mit einem einfachen Modell für die energetische Heterogenität einer Festkörperoberfläche (lineare Abhängigkeit der Aktivierungsenergie der Desorption des Ausgangsstoffes vom Bedeckungsgrad) gezeigt, daß der Grad der Heterogenität den Umsatz und den Desorptionsverlauf der Reaktanten unter nichtisothermen Bedingungen entscheidend beeinflußt. Im Gegensatz zu einer homogenen Festkörperoberfläche ist der Umsatz bei Annahme einer heterogenen Oberfläche relativ stark von der Anfangsbelegung des Katalysators mit dem Ausgangsstoff abhängig. Aus den Modellrechnungen werden Möglichkeiten für eine kinetische Auswertung abgeleitet.