Infrared Study of Coke on Alumina and Zeolite

Abstract

In situ infrared studies of coke deposited on alumina or on platinum/alumina, by exposure to acetylene at 250°C, show a carboxylate species in the coke. This carboxylate produces a pair of bands at 1580 and 1460 cm −1. This observation is contrary to literature reports of infrared spectra of coke deposits on zeolites, which are equivalent to conventional spectra of carbonaceous materials and do not indicate the presence of an oxidized species in the coke. Study of the hydroxyl bands of alumina during coke deposition shows that only the higher frequency bands are consumed. These high-frequency hydroxyls are considered to be the more basic. This suggests that carboxylates are not formed on zeolites because none of the hydroxyls in zeolites have a sufficiently basic character. Previous work has shown that the addition of tin to alumina lowers both the production of carboxylate and total coke. This is consistent with the decrease in hydroxyls which tin produces. After severe sulfiding, only small quantities of carboxylates and conventional coke are observed on alumina. Conventional coke, but not carboxylate, is observed on sulfided platinum/alumina.