New Pd/Pt on Mg/Al basic mixed oxides for the hydrogenation and hydrogenolysis of naphthalene

New Pd/Pt catalysts supported on a basic Mg/Al mixed oxide obtained by calcination of a commercial hydrotalcite (SASOL, D) have been fully characterized by XRD, SEM, and FTIR and investigated in the vapor-phase hydrogenation of naphthalene in order to put in evidence the role of the Pd/Pt active phase and the acidity of the support on the hydrogenolysis/ring-opening reaction as well as the thio-tolerance of the catalysts. After calcination the hydrotalcite support (HT) gave rise to a Mg/Al mixed oxide with high surface area and a "brain-like" surface morphology. The IR spectra after adsorption of CO over the reduced samples showed the interaction between bimetallic particles and Mg-Al(O) basic support. The main band in the range 2080-2070 cm(-1) is due to on-top CO adsorbed over Pd-0 or Pt-0, while the complex absorption below 2000 cm-1 is due to CO species bridging over Pd-0. All the samples showed mainly a hydrogenation activity, highlighting the role of the support acidity in the ring-opening reactions to high molecular weight (HMW) products, having a boosting effect on the cetane number of the fraction obtained. A significant increase in the amount of HMW products was obtained by decreasing the Pd/Pt ratio, showing also the role of hydrogenolysis reactions attributable mainly to Pt, thus suggesting a hydrogenolysis/ring-opening mechanism. Furthermore, the Pd/Pt on basic oxide catalysts did not give rise to useless low molecular weight (LMW) cracking compounds, which thus formed only on highly acid sites. Finally, these catalysts, investigated by feeding increasing amounts of dibenzothiophene (DBT), showed an almost constant activity up to 3000 wt ppm of DBT. This surprising result pointed out the intrinsic thio-resistance of the Pd/Pt pair, regardless of a possible contribution of the acid sites of the support, and is mainly attributed to its high hydrodesulfurization (HDS) activity. (C) 2004 Elsevier Inc. All rights reserved.