Authors:B. Hunger, J. Hoffmann, O. Heitzsch, and M. Hunger
The temperature-programmed desorption (tpd) of the amount of ammonia which is preadsorbed at about 373 K at HZSM-5 zeolites yields a complex desorption curve consisting of two overlapped peaks (Β andγ peak). Parts of the ammonia desorbed can be attributed to SiOHAl groups considering also1H-MAS NMR measurements.
Authors:B. Hunger, S. Matysik, M. Heuchel, and W.-D. Einicke
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.
Authors:B. Hunger, M. v. Szombathely, J. Hoffmann, and P. Bräuer
Desorption energy distributions were calculated for temperature-programmed desorption (TPD) of ammonia from H zeolites of
different type by means of regularization. This method does not require any limiting assumptions about the distribution function.
It could be shown that the desorption energy distributions obtained are nearly independent of the experimental conditions
and therefore they should represent a suitable measure for the distribution of the strength of acidic sites. The calculated
desorption energy distributions for the ammonia desorption from the isolated bridging SiOHAl groups of H zeolites of different
type significantly differ from each other in shape. The increase of the desorption energy of the main range of the distribution
functions correlates well with the increase of the average acid strength of the SiOHAl groups with decreasing Al content of
Authors:B. Hunger, S. Matysik, M. Heuchel, E. Geidel, and H. Toufar
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.