High‐Temperature Produced Catalytic Sites Selective for n‐Alkane Dehydrogenation in Acid Zeolites: The Case of HZSM‐5

Abstract

The effect of high‐temperature treatment on the catalytic activity and selectivity of two HZSM5 samples has been investigated by using propane activation as the test reaction. The samples have been characterized by using X‐ray powder diffraction, nitrogen adsorption, FTIR spectroscopy, and ammonia temperature‐programmed desorption (TPD). Before high‐temperature treatment, the samples have activation energies and selectivity toward propane cracking versus dehydrogenation comparable with similar samples reported in the literature. After high‐temperature treatment (825 °C in N2), the sample’s microporous and crystalline structure remains nearly intact, but a clear decrease in the number of Brønsted‐acid sites is detected by using FTIR and ammonia TPD. Despite the decrease (≈70 %), the rate of reaction is about the same before and after heating. The selectivity toward dehydrogenation, however, increases by at least a factor of 2, whereas the activation energy for dehydrogenation decreases considerably. This is evidence that new catalytic sites are formed upon dehydroxylation. By using naphthalene as a probe molecule, it is shown that the new sites have the ability to form stable radical cations out of the neutral naphthalene, which suggests that the new sites activate propane by a redox mechanism. The kinetic isotope effect on the rate of dehydrogenation is also consistent with a redox process. The effect of heating in an oxygen atmosphere at lower temperatures was also investigated and only minor effects on reaction rates and selectivity were observed. Naphthalene adsorption experiments on oxygen‐treated samples, however, produce more radical cations than on the samples treated at high temperatures, a result that does not correlate with the dehydrogenation activity of oxygen‐treated samples.

Effects of Zeolite Structural Confinement on Adsorption Thermodynamics and Reaction Kinetics for Monomolecular Cracking and Dehydrogenation of n-Butane

“…For most samples, dehydrogenation rates decayed significantly with time on stream (TOS). For the reasons given in ref 14 and by other authors who have observed similar transient behavior for monomolecular dehydrogenation, 31,32 we suggest that a Lewis acid site is the cause of the initial dehydrogenation activity since cracking rates and selectivities do not change significantly with TOS. The somewhat greater loss of cracking activity for TON indicates that some of the one-dimensional pores (see Figure 1) become blocked by carbonaceous species on the crystal surface.…”

Endothermic catalytic cracking of liquid hydrocarbons for thermal management of high-speed flight vehicles

“…The chemical structure of the fuel and the nature of the catalyst both contribute to determining the type of specific endothermic reaction that occurs in the cracking process. Dehydrogenation of alkanes, aromatics and cycloalkenes generally requires very high temperatures (beyond 500 °C), 33 due to the stable nature of these molecules. Dehydrocyclization of normal alkanes to form saturated cyclic compounds is not highly endothermic, therefore this reaction proceeds slowly on known catalysts and not considered as effective heat sink.…”

Pt-Sn Supported on Beta Zeolite with Enhanced Activity and Stability for Propane Dehydrogenation

“…These acid sites can strengthen the metal-support interaction and promote metal dispersion by strongly anchoring metal ions, reinforcing anti-sintering capability. In terms of catalytic activity, Lewis acid sites enhance the adsorption of reactants and increase the conversion of reaction, but do not activate alkanes to significantly change the selectivity of reaction [11]. On the other hand, Brønsted acid sites can promote cracking, oligomerization, and aromatization, resulting in coke formation [12,13].…”