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

Abstract

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.