Formation and evolution of naphthalene radical cations in thermally treated H-ZSM-5 zeolites

The effect of zeolite pre-treatment conditions on the ionization of naphthalene, to form naphthalene radical cations, is investigated on H-ZSM-5 with Si/Al ratios of 12 and 18. The ZSM-5 samples were prepared using an all-inorganic route to avoid heating the sample prior to naphthalene adsorption. Naphthalene is adsorbed into ZSM-5 by mixing solid naphthalene with the pre-treated ZSM-5 sample. The samples are characterized using X-ray powder diffraction, nitrogen adsorption isotherms, UV/Vis diffuse-reflectance spectroscopy and ammonia temperature-programmed desorption (TPD). The samples are treated in Ar at 780 °C or in oxygen at 500 °C. Radical cation formation is observed in both cases with a higher ionization yield observed for the oxygen-treated samples and also for the sample with the lower aluminum content (Si/Al ∼ 18). Control experiments using silicalite-1 or Na-ZSM-5 show that framework Al and Brønsted acid sites are both needed to form electron-abstracting sites. The time-evolution of the UV/Vis spectra after naphthalene adsorption indicates that after a few hours, the naphthalene radical cation captures an electron from the zeolite framework forming long-lived electron–hole pairs. The rate of formation of the electron–hole pairs as well as details of the electron–hole electronic transitions observed by UV/Vis spectroscopy are different, indicating that the sites generated by the Ar and O2 treatments are different as well. Oxygen gas treatment at temperatures as low as 200 °C lead to the formation of naphthalene radical cations, suggesting that the electron-abstracting sites can be formed during calcination procedures used prior to catalyst testing. These electron-abstracting sites appear to be responsible for the changes in the selectivity of the propane cracking vs. dehydrogenation reaction at high temperatures, as was recently reported.

Figure optionsDownload as PowerPoint slideHighlights
► Electron abstracting sites generated in H-ZSM-5 in oxygen above 200 °C.
► Naphthalene radical cations formed after adsorption in pre-treated H-ZSM-5.
► Different electron abstracting sites formed depending on the pre-treatment.
► Electron abstracting sites may explain differences in propane cracking selectivity of H-ZSM-5.
► Typical zeolite catalysts pre-treatments may generate these sites inadvertently.