PNNL

Abstract

Low-temperature gas-phase methylcyclohexane dehydrogenation to toluene was carried out over catalysts of 5 wt% nominal Pt on Al2O3 (Pt/Al2O3) and on SiO2 (Pt/SiO2) at 220 ?C. Pt/Al2O3 exhibited higher activity and better stability. The influence of supports and support acidity on catalyst stability was examined in detail, utilizing positron annihilation lifetime spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy, temperature-programmed oxidation, H2 pulse chemisorption, and scanning transmission electron microscopy. Deactivation of both catalysts resulted from the formation of soft coke. Coke deposition on Pt/Al2O3 occurred on the Brønsted and Lewis acid sites of the support (~80%), with the remainder at the Pt-support interface. Pt/SiO2 experienced coking of the Pt. To highlight the role of acid sites in the superior stability of Pt/Al2O3, acid site blocking experiments were performed with NH3 pre-treatment. Compared to the untreated Pt/Al2O3, although not all the acid sites of Pt/Al2O3 were masked at the reaction temperature, the NH3-treated sample accumulated about 15% more coke on the metal-support interface (due to limited removal of coke precursors by the acid sites) and doubled the total amount of coke, thereby accelerating deactivation of the catalyst. This confirms that acid sites on Al2O3 contribute to the catalyst’s high stability.