Consecutive hydrogenation of benzaldehyde over Pd catalysts: Influence of supports and sulfur poisoning
Introduction
Supported Pd catalysts find widespread use in both petrochemical and fine chemical industries for many chemical reactions, most of them having problems of selectivity. In particular, Pd is suggested as a very effective metal for catalyzing hydrogenation of aromatic aldehydes to alcohols and of the latter ones to hydrocarbons [1]. Previous work in this area regarded both gaseous and, mostly, liquid phases. An exhaustive discussion of the whole work performed before 1997 is given by Vannice and Poondi [2], to whom we will refer throughout the present paper. More recent work concerns supported-Cu [3] and oxidic [4] catalysts.
The problem of directing selectivity towards alcohol or hydrocarbon is of primary importance, as alcohol is usually the desired product. We have chosen benzaldehyde as reactant and the liquid phase, the latter because allows better temperature control during the reaction. On the basis of previous studies about solvent effect [2], showing that aliphatic alcohols give good performances, ethanol was chosen as solvent. As no investigation appears to have been performed until now about the influence of the carrier and of sulfur poisoning on benzaldehyde hydrogenation in liquid phase over supported Pd, we have focused our study on such items. So we have investigated catalysts based on Pd supported on active carbon, silica and alumina, as they are the main carriers used in industrial catalysis. Both physical characterizations (TPR, XRD, chemisorption), and kinetic tests were performed to get as extensive as possible knowledge of this catalytic system. In particular, recently developed procedures [5], [6], [7] were used for the measurement of real Pd dispersion, thus allowing to determine correct turnover frequencies (TOF) [8].
Section snippets
Experimental
0.5% Pd/C catalyst was prepared by wet impregnation of a commercial granular coconut active carbon (1200 m2/g) with an H2PdCl4 solution, followed by reduction with sodium formate at 80°C. Another sample was prepared with a Pd(NO3)2 solution. All the Pd/C catalysts were ground before testing.
5% Pd/Al2O3 and 5% Pd/SiO2 catalysts were prepared by incipient wetness impregnation, with an H2PdCl4 solution, of γ-Al2O3 (Condea, 257 m2/g) and SiO2 (Akzo, 329 m2/g), previously impregnated with a NaOH
TPR
In Fig. 1 TPR data are reported for Pd/Al2O3 catalysts prepared from chloride and nitrate. Curve ‘a’ refers to the as-prepared sample from chloride, the corresponding curve of sample from nitrate being very similar, and points to an extensive interaction between Pd oxide species and alumina surface, evidenced by the reduction peak in the range 300–450°C.
As the ex-nitrate sample shows the same behavior, it can be excluded that such high-temperature reduction peak is due to oxychlorides. A small
Conclusions
Physical characterizations of the Pd catalysts we have prepared over active carbon, silica and alumina showed that a strong metal–support interaction, favored by the presence of chloride species, occurs with alumina, while it is negligible with silica and active carbon. As a consequence, very high metal dispersion was obtained for Pd/Al2O3 treated at 500°C.
The good agreement between advanced XRD and standard CO chemisorption techniques (the Pd/CO=2 stoichiometric ratio was confirmed) allowed to
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