Hydrogenation of aromatics over supported Pt-Pd catalysts

Abstract

Silica–alumina (SA)- and β-zeolite (βZ)-supported Pt, Pd, and Pt-Pd catalysts were compared for the simultaneous hydrogenation (HYD) of toluene (T) and naphthalene (NP) in the presence of dibenzothiophene (DBT). Under the selected conditions ( P=50 bar, T=448 K, weight hourly space velocity (WHSV)=25.8 h −1), all the catalysts were resistant to poisoning with 113 ppm of S (as DBT). The turnover frequency (TOF) calculated at zero time decreased in the following order: T>NP>DBT. On the basis of TOF values for the HYD of toluene, the monometallic Pt/βZ showed larger activity than the bimetallic Pt-Pd H/βZ at short time on-stream, but this order was reversed under steady-state conditions. From FTIR of adsorbed CO and XPS measurements, the formation of the Pt–Pd alloy on this catalyst was excluded. The enhancement in activity observed for Pt-Pd H/βZ as compared with Pt/βZ is related to the synergic effect of the two metals, which led to a lower deactivation and larger S-tolerance. On the other hand, the high activity of the βZ-as compared with SA-supported samples was attributed to the higher acidity of the former. The involvement of acid sites in activity was studied by comparing the performances of fresh and pyridine-neutralized Pt-Pd H/βZ and Pt/βZ catalysts. For HYD of toluene, the value of TOF t=0 for the two zeolite catalysts was much lower on pyridine-neutralized samples, but still higher than on the non-neutralized Pt-Pd L/SA and Pt-Pd H/SA. It is proposed that Pt-Pd phases deposited on acidic βZ develop stronger activity, which does not change after neutralization.