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Abstract  

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.

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Abstract  

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.

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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.

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Summary Using temperature-programmed desorption (TPD), we have investigated the interaction of carbon dioxide with alkali-metal cation-exchanged faujasite type zeolites (LSX, X and Y). TPD in the temperature range between 300 and 500 K results in desorption profiles of different intensities depending on the kind of cation and the aluminium content of zeolites. For NaX the desorbed amount corresponds to about one percent of the saturation capacity at 298 K. In case of NaX and X type zeolites exchanged with Cs+ ions an additional desorption peak above 500 K could be observed. Taking into account desorption curves of different heating rates, desorption energy distribution functions were calculated by using an extended integral equation. Initial adsorbed CO2 could be assigned to carbonate species in different environments by DRIFT spectroscopy.

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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.

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Journal of Thermal Analysis and Calorimetry
Authors: B. Hunger, M. v. Szombathely, J. Hoffmann, and P. Bräuer

Abstract  

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 the zeolites.

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On the examples of the temperature programmed desorption of water from a NaA and a 0.45 NiNaX zeolite, it is shown that, through the use of non-linear temperature programmes with increasing heating rate, the individual desorption steps at higher temperatures of a complex desorption process are better recognizable, or can be distinguished almost completely from desorption at lower temperatures. This type of temperature processing therefore offers a suitable means of improving the finding on complex desorption processes relating to porous catalysts and other systems.

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TPD studies on the kinetics of deammoniation of an NH4NaY zeolite showed that the use of a hyperbolic temperature programme led to kinetic parameters agreeing with those resulting from a linear heating process. Because of the progressive increase of the heating rate in the case of hyperbolic heating schedules, the parameters can be considered as independent of the heating rate within certain limits. The better resolution of complex desorption spectra with hyperbolic programmes is an additional reason for their use.

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Through simulation of TPD model curves for desorption from adsorption sites of different strength could be shown that the use of non-linear temperature programmes influences the complex desorption process in such a manner that the appearing desorption maxima are better visible.

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