The interaction of methanol with a NaZSM-5 and a HZSM-5 zeolite was investigated by means of temperature-programmed desorption (TPD) and adsorption measurements. For both zeolites the desorption curves are structured, indicating different adsorption sites. In the case of HZSM-5 the desorption at higher temperatures is accompanied with a partial conversion of methanol. It was investigated how the course of the reaction depends on the initial adsorbed amount of methanol. Using a regularization method, desorption energy distribution functions have been calculated. The energy range of these distributions correlates well with the heats of adsorption in the literature. The desorption energy distributions between 50 and 65 kJ mol-1, which can be attributed to a nonspecific interaction, reflect the formation of stronger hydrogen bonds in the methanol clusters at higher loading on HZSM-5 than on NaZSM-5. The shape of the desorption energy distribution of NaZSM-5 in the range 65−100 kJ mol-1, specific for the interaction of methanol with Na+ cations, shows two clearly distinguished maxima, indicating two kinds of Na+ cations. The obtained results for the HZSM-5 zeolite reflect the energetic heterogeneity of the bridging Si−OH−Al groups. Both, the shape and wideness of adsorption energy distributions extracted from measured adsorption isotherms agree well with the determined desorption energy distributions.
Diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) measurements (4000−1500 cm-1) and the results of neutron powder diffraction have been combined to study the structure of adsorption complexes of water in a NaX zeolite at different water loadings (25, 48, 72, and 120 water molecules per unit cell, respectively). Sharp bands corresponding to non-hydrogen-bonded OH groups of water molecules and broad associate bands due to hydrogen-bonded molecules are observed in the DRIFT spectra. We observe a remarkable downshift of the high-frequency associate band in a narrow temperature interval when the water amount decreases from 120 to 72 molecules per unit cell, which could signify some kind of "phase transition" for the water inside the zeolite cavities. Neutron powder diffraction results show that water molecules are predominantly localized in or near the 12-ring windows. Water molecules with hydrogen-bonded and non-hydrogen-bonded OH groups were found, in agreement with the observation of sharp and broad bands in the DRIFT spectra. We find strong evidence for the formation of cyclic hexamers of water molecules localized in the 12-ring windows, which are further stabilized by hydrogen bonds to framework oxygen atoms.
Ammonia TPD experiments have been carried out on protonated forms of the zeolites FAU, FER, MFI, and MOR to investigate the acid strength dependence on OH group location and Al content. Effective adsorption energy distributions are derived using a regularization method and corrected to achieve a truer measure of acid strength using nonspecific interaction energies from atomistic Monte Carlo simulations. The correction energies include all interactions not associated with the OH groups and range from 18 to 37 kJ/mol, depending on the region and the zeolite structure. In the FAU structure, we find a bimodal distribution of adsorption energies in both the supercages and the sodalite cages, with the stronger acid sites being more common in the supercages. In contrast, essentially the same acid strength for different regions in MOR is seen, though the acid strength for those sites in the side pockets may be slightly lower. For FER and MFI, the TPD gives unimodal distributions and therefore prohibits distinction between different regions. In agreement with the model of Barthomeuf et al., we see an apparently linear decline (60 kJ/mol) in acid strength beyond a certain aluminum framework density. However, we see a smaller but consistent increase (16 kJ/mol) in acidity before this threshold, despite model predictions that it should be constant.
The nature and strength of interaction of pyrrole adsorbed in alkali metal cation-exchanged faujasites have been investigated by the combined use of different surface vibrational techniques, X-ray diffraction and temperature-programmed desorption accompanied by theoretical studies. The results obtained depend on the kind of cation, the degree of ion exchange and the Si/Al ratio of the zeolites, indicating that the effective strength of interaction depends on both the Lewis acidic properties of the extraframework cations and the Lewis basic properties of the lattice oxygen atoms. It was found that pyrrole is adsorbed due to these two different kinds of interactions which occur parallel and influence each other: pyrrole is bound via the NH group to the lattice oxygen atoms by hydrogen bonding and, simultaneously, via the aromatic system to the cations, yielding strong host–guest interactions. The experimentally obtained results were verified by quantum mechanical calculations and force field simulations.
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.
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.
By means of model calculations it could be shown for an irreversible surface reaction of 1st order that the determination of the activation energy of the desorption of the reactant or, respectively, of the surface reaction is possible by application of the method of variation of the heating rate to the desorption curve of the reactant, according to circumstances whether the ratio of the activation energy of the surface reaction and of the desorption of the reactant is greater or smaller than one. The possibilities of the kinetic evaluation are applied to the isomerization of cyclopropane on a NaX-zeolite catalyst. The resulting heat of adsorption of cyclopropane and the activation energy of the reaction agree well with the values of literature obtained by isothermal measurements in a pulse reactor.
