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

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 N 2 ), 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.