Competitive hydrogenation of poly-aromatic hydrocarbons on sulfur-resistant bimetallic Pt-Pd catalysts

The hydrogenation of tetralin, naphthalene, and phenanthrene as individual feed components and in diesel model feeds has been studied over a series of mono- and bimetallic Pt and Pd supported on gamma-alumina at 300degreesC and 500 psig in excess of H-2. The influence of fluorine on activity level and sulfur resistance was evaluated in the presence of controlled concentrations (0--1000 ppm) of dibenzothiophene (DBT). Among the most significant results, a strong inhibition of the tetralin conversion to decalin has been observed when either phenanthrene or naphthalene is present in the feed. Reversible drops in tetralin conversion of almost two orders of magnitude were observed when the 2- and 3-ring aromatics were present in the reactor, even in minute concentrations. That is, tetralin conversion could only be achieved after essentially all the 2- and 3-ring aromatics were converted. The addition of F has a positive impact on the activity under sulfur. Significantly higher activity was observed on the F-promoted catalyst than on the unpromoted Pt-Pd catalyst when the feed contained 1000 ppm S. Interestingly, the inhibition by sulfur compounds strongly depended on the space velocity. At high space velocities and low DBT conversions, the inhibition by sulfur was reversible. At low space velocities and high DBT conversions, the poisoning was irreversible. Under those conditions of low space velocity, EXAFS analysis showed further evidence of the beneficial role of F. No Pt-S bonds were formed on the F-containing catalyst, contrasting with the unpromoted catalysts, on which Pt-S bonds were clearly observed. (C) 2003 Elsevier B.V. All rights reserved.