Selective steam reforming of aromatic hydrocarbons: V. Steam conversion and hydroconversion of selected monoalkyl- and dialkylbenzenes on Pt and Ni catalysts

Abstract

Steam and hydrogen conversions of a series of mono- and dialkylbenzenes were studied at 713 K on variously supported platinum and nickel catalysts and compared with previous results obtained on rhodium catalysts (Duprez, D., Miloudi, A., Delahay, G., and Maurel, R., J. Catal. 90, 292, 1984). Three main types of reaction were observed: dealkylation (CC bond splitting in the side chains), dehydrogenation (into alkenylbenzenes) and degradation (i.e., ring opening). The latter reaction primarily yielded C 1 molecules and hydrogen. In the presence of steam, the monoalkylbenzenes mainly dealkylate into benzene on Pt and Ni (as on Rh). Nevertheless, Pt is characterized by a very high relative activity in dehydrogenation and by the absence of ring opening. The inverse behavior is observed on Ni. For a given metal, the activities in degradation are constant across the entire series of hydrocarbons, which suggests that ring opening occurs on specific sites. The proportion of these sites at the metal surface invariably decreases with the electronegativity of the metal. In the presence of hydrogen, the product spectra are deeply modified. Ring opening is considerably inhibited, whereas the primary CC bond ruptures shift toward the middle and the end of the chain. Secondary reactions of realkylation, leading to toluene from benzene and CH x fragments, are less pronounced on Ni than on Pt and Rh, which suggests that the hydrocarbon species are deeply dehydrogenated on Ni, indistinguishably from carbonaceous deposits.