Thermal activation of ammonium forms of Y zeolites. I. T.p.d., i.r., and catalytic investigations of the deammoniation of NH4NaY zeolites

1989

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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.Temperature programmed desorption (TPD) is a method frequently used to characterize the surface-chemical properties of porous catalysts [1,2]. For the most part, the thermal desorption of suitable probe molecules preadsorbed in a stream of carrier gas is studied during linear heating of the sample, using for example NH3, H 2 or CO. However, this method can also be used for the study of surface processes occurring during the thermal activation of catalysts (e.g. dehydration, deammoniation and dehydroxylation). In both cases, however, typical structured desorption curves are obtained because of the overlapping of several desorption processes, and single desorption maxima are not often observed. A quantitative evaluation with regard to the number of different desorption processes, to desorbed amounts of single steps and to kinetic calculations is often very difficult, because of the low amounts of desorbed compounds at higher temperatures, frequently with strong overlapping with the desorption of greater amounts at lower temperatures. An improvement of the resolution of structured desorption curves is possible if low linear heating rates are applied, but this often leads to detector responses which are not evaluable because of the accompanying decrease of the desorption rate. When larger amounts of sample are used additional problems appear (e.g. heat transfer, back-mixing, chromatographic regime); this is therefore not a suitable way to obtain evaluable desorption curves at low linear heating rates.

mentioning

confidence: 99%

Temperature programmed desorption (TPD) on zeolites by means of non-linear temperature programmes

1989

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“…This is realistic only if the same model can be used for the description of the kinetics of the desorption process. However, in the case of HY and HZSM-5 zeolites, it could be shown that a satisfactory description of the course of desorption is possible only by means of different kinetic models [27,28]. Because of the distinct difference between the shapes of the desorption curves of H mordenite and of HY and HZSM-5 zeolites [7][8][9][10], the higher peak maximum temperature in the case of H mordenite under comparable experimental conditions cannot be valid evidence of a higher acidity of the mordenite.…”

Section: Resultsmentioning

confidence: 93%

Temperature-programmed desorption (TPD) of ammonia from the H form of a mordenite after different pretreatment temperatures

Hoffmȧnn

Mothsche

1987

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Temperature-programmed desorption of ammonia from the H form of a modernite was carried out after pretreatment at different temperatures. It could be shown that a quantitative registration of the acidic OH groups is possible. Moreover, from the desorption curves, information is obtained about the alteration of the acidic properties as a function of the temperature of pretreatment.H mordenites are frequently used as catalysts because of their porous structure, acidity'and thermal stability [1,2]: In order to characterize the number, nature and strength of the acidic sites ofzeolites, NH3 can be used as a suitable probe molecule. For this, temperature-programmed desorption (TPD) is a suitable method [2-13], besides the microcalorimetric estimation of adsorption heats [14][15][16][17][18][19] and other methods [20][21][22][23]. In this connection, many papers report the change in NH 3 desorption depending on the Si/A1 ratio, and also the dealumination of H mordenites [2,[8][9][10]. The acidic properties ofH mordenites after exchange with K + ions [8], or after treatment with Si(OCH3) 4 [11], are also characterized by this method. However, only limited information is available about the changes in the TPD curves of ammonia after pretreatment at different temperatures. Thus, in the present paper the influence of the temperature of pretreatment of H mordenite on the TPD of ammonia is investigated systematically, because it is known that the adsorption behaviour Of ammonia is changed significantly by reason of the partial dehydroxylation of the zeolite.

“…The activation energies of the desorption process on zeolites with different ammonium contents are very suitable for characterizing the acidity of the appropriate H forms [3], provided, however, that the activation energies calculated are widely independent of the experimental conditions and of the method of evaluation.…”

Section: Nh4naymentioning

confidence: 99%

“…Desorption of the high-temperature form of the sorbed NH 3 can be well described over a relatively large range of coverage by a rate equation of first order without readsorption [3]. The activation energies of the desorption process on zeolites with different ammonium contents are very suitable for characterizing the acidity of the appropriate H forms [3], provided, however, that the activation energies calculated are widely independent of the experimental conditions and of the method of evaluation.…”

Section: Nh4naymentioning

confidence: 99%

Temperature-programmed desorption (TPD) studies of the deammoniation kinetics of NH4NaY zeolites using a hyperbolic temperature programme

Hoffmȧnn

1983

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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 Js an additional reason for their use.The thermal activation of NH4NaY zeolites is connected with the desorption of water and ammonia, the latter occurring in the temperature interval from 473 to 673 K and resulting in two overlapping peaks [1--3]: NH4NaY> HNaY + NH 3 ?Desorption of the high-temperature form of the sorbed NH 3 can be well described over a relatively large range of coverage by a rate equation of first order without readsorption [3]. The activation energies of the desorption process on zeolites with different ammonium contents are very suitable for characterizing the acidity of the appropriate H forms [3], provided, however, that the activation energies calculated are widely independent of the experimental conditions and of the method of evaluation.On the other hand, besides other influences, the literature reports on the dependence of the kinetic parameters on the heating rate in thermoanalytical studies on other desorption processes by means of linear heating schedules [4][5][6].The application of non-linear, especially hyperbolic, temperature programmes could be very useful to obtain information about the influence of the heating rate:Furthermore, this kind of temperature guidance would permit a correction-free integral evaluation [7,8]. In comparison with linear heating schedules, the shape index would advantageously be a function of reaction order only or of the mechanism of the desorption process in the case of hyperbolic temperature programmes [9,10],