Selective steam reforming of aromatic hydrocarbons IV. Steam conversion and hydroconversion of selected monoalkyl- and dialkyl-benzenes on Rh catalysts

Steam conversion and hydroconversion of a series of monoalkylbenzenes (C6H5R, R = C2H5, n-C3H7, i-C3H7, tert-C4H9) and of dialkylbenzenes (o- and p-xylenes) are studied at 713 K and atmospheric pressure on supported rhodium catalysts (RhAl2O3, RhSiO2 and RhTiO2), and compared to the toluene steam dealkylation previously studied on the same catalysts. Three types of reaction, namely dealkylation (CC rupture on the side chain), dehydrogenation (on the side chain), and degradation (i.e., ring opening) account for virtually the whole product spectrum. Isomerization, transalkylation, and dehydrocyclization reactions may, in general, be discounted. In the presence of steam, the main initial product of monoalkylbenzene dealkylation is invariably benzene, but the splitting of the CC bonds in the middle or end of the side chain always increases with conversion. As a rule, the specific activities (per metal site) in dealkylation decrease with the degree of substitution in the alkyl group (primary > secondary > tertiary > quaternary carbon). On the other hand, the specific activities in ring opening remain constant for all the hydrocarbons and even for the benzene. In the presence of hydrogen, multiple CC bond splittings are invariably observed and benzene is no longer, in general, the principal initial product. The activities in ring opening are equally constant, but at a lower level than in steam conversion. These results are in overall agreement with the model of the dual active sites: sites I appear operative for dealkylation and dehydrogenation, whereas ring opening takes place at sites II with a high probability, independent of the alkyl group size. Possible adsorbed species on each type of site are described. An attempt is made to rationalize the effects of assorted selectivity-determining factors (metal particle size, support effects, selective poisons such as S and CO) in terms of electronic or geometric effects.