Curves of the temperature-programmed desorption (TPD) of ammonia from zeolites were evaluated with several kinetic models. An approximately linear correlation was found between the activation energy of desorption or the heat of adsorption of H zeolites with various Si/Al ratios and the intermediate electronegativity of the zeolites, the latter representing a measure of the acid 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, 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.
We have investigated the temperature-programmed desorption (TPD) of ammonia during the activation of NH4Na-mordenites of different
exchange degrees. Using a regularization method, desorption energy distribution functions have been calculated. The obtained
results indicate the heterogeneity of the bridging Si-OH-Al groups in HNa-mordenites. This was concluded from the width of
the distribution functions and from the presence of submaxima. For HNa-mordenites of exchange degrees below 50%, containing
only hydroxyls in the broad channels, two distinct submaxima are present, thus suggesting the presence of at least two kinds
of bridging hydroxyls of various acid strengths. In HNa-mordenites of exchange degrees above 50%, the hydroxyls appear in
narrow channels and the distribution of ammonia desorption energy broadens on the side of higher energies. This may be related
to a strong stabilization of ammonium ions inside narrow channels. The maximum concentrations of hydroxyls of desorption energies
between 95 and 135 kJ mol-1 and between 135 and 165 kJ mol-1 calculated from TPD data were 3.9 and 3.3 OH per unit cell (u.c.). These values agree well with our previous IR results of
concentrations of hydroxyls in broad and in narrow channels (3.7 and 2.8 OH per u.c.). The TPD data obtained for the heterogeneity
of OH groups in HNa-mordenites are in accordance with the IR data concerning ammonia desorption. The IR band of OH groups
restoring upon saturation of all the hydroxyls with ammonia and subsequent step-by-step desorption at increasing temperatures
shifts to lower frequencies indicating that there are hydroxyls of various acid strengths and the less acidic hydroxyls restore
first at lower desorption temperatures.
, B , Szombathely , MV , Hoffmann , J , Brauer , P . 1995 Characterization of the acidic properties of zeolites by means of temperature-programmeddesorption (TPD) ofammonia. Calculation of distribution function of the desorption energy . J
acidity values. Fig. 4 gives a comparison of the percentage conversion and selectivity towards dealkylation and dehydrogenation with the acidity data. The total acidity of the catalysts as determined by the temperatureprogrammeddesorption (TPD) of
with a heating rate of 10 K min −1 . In order to gain an understanding of the acidity of the catalyst, the raw and modified clinoptilolites were analyzed by the temperature-programmeddesorption (TPD) ofammonia. The zeolite samples were pretreated in