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

Abstract

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.

A Study into the γ-Al2O3 Binder Influence on Nano-H-ZSM-5 via Scaled-Up Laboratory Methanol-to-Hydrocarbon Reaction

“…HTNDP changes are possibly caused by the partial neutralization of zeolite Brønsted acid sites by the introduced alumina. A similar phenomenon has been reported in previous studies on methanol-butene cracking and paraffin conversion, which also leads to an improved coke tolerance [4,18,25,26].…”