Benzaldehyde hydrogenation over supported nickel catalysts

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Abstract

The hydrogenation of benzaldehyde over nickel catalysts supported on Al2O3, SiO2, TiO2 and CeO2 was studied at atmospheric pressure in the range temperature of 70–140 °C under H2 flow. They were characterized by their BET surface area, XRD spectra, reducibility properties under H2. The obtained products were benzylalcohol, toluene, benzene with yields depending on the nature of the support and reaction temperature. Methylcyclohexane was also produced, mainly at low temperature and on Ni/α-Al2O3 as catalyst. The products of benzaldehyde hydrogenation (benzylalcohol, toluene and methylcyclohexane) and hydrogenolysis (benzene) were preferentially formed at low/ middle and high reaction temperature respectively. The observed selectivities also suggested the existence of bifunctional sites for both the hydrogenation and hydrogenolysis reactions. These sites were believed to be involved as acidic/cationic centers for the adsorption of the oxygenate molecules and metal center for the hydrogenation or hydrogenolysis step.

Introduction

One class of reactions important to produce fine chemical compounds is the hydrogenation of aldehydes and ketones containing unsaturated Cdouble bondC bonds [1], [2], [3], [4], [5], [6]. The selective hydrogenation of crotonaldehyde and butyraldehyde [7], and most recently, phenylacetaldehyde [8], benzaldehyde and O-tolualdehyde [9] have received a growing interest since the production of fine chemicals by selective catalytic activation of carbonyl because it is the challenge for reduction of energy consumption. It was found that nickel, platinum and copper based catalysts preferentially hydrogenate the carbonyl group [9], [10], [11], [12], [13], [14]. For the aromatic compounds, the benzene ring and the carbonyl group should be hydrogenated but hydrogenolysis should also compete [9], [15], [17]. The obtained selectivities mainly depended on the nature of the catalyst [2], [3], [4], [9], [15].

Heterogeneous catalytic studies of benzaldehyde hydrogenation have been largely devoted to the liquid phase medium [5], [6], [18], [19] whereas relatively few studies have been reported for the gas phase medium [9], [15], [16], [20]. The later system were studied over Pt [15] or Ni [9] supported catalysts or metal oxides catalysts [16]. For the metal supported catalysts it was suggested that the observed performances were related to new active sites created in the metal–support interfacial region [15], [21] whereas for metal oxides catalysts it was found that they strongly depended on both the reducibility and surface acid–base properties [16]. We recently confirmed the role of metal and acid–base properties in the same reaction conducted over copper supported catalysts [20].

The aim of the present paper is to report the study of the hydrogenation properties of 10% nickel catalysts impregnated on α-Al2O3, SiO2, TiO2, and CeO2 supports: α-Al2O3 and SiO2 are both unreducible oxides and, also, amphoteric and acidic respectively; TiO2 and CeO2 are reducible oxides and, in addition, behave as basic oxides [16]. The catalysts were characterized by their BET surface area, XRD spectra and reducibility under H2 flow. The reaction was performed at atmospheric pressure in the reaction temperature range of 70–140 °C.

Section snippets

Catalysts preparation and characterization

The catalysts were prepared by impregnation of an aqueous suspension of the support by nickel nitrate, then evaporating the solvent at 80 °C and drying at the same temperature for 12 h. The obtained precursors were calcined at 350 °C in air for 3 h. All the supports were commercial materials (Table 1) and calcined before impregnation in air at 500 °C for 2 h.

The contents of nickel were determined by atomic absorption. The specific area measurements were performed by the classical BET method on a

Catalysts characterization

The specific area of the catalysts increased with that of the corresponding supports and were in the range 4–127 m2 g−1 (Table 1). However, this range of specific area is lower than that of the supports (10–200 m2 g−1). During the impregnation stage of the preparation, surface hydroxyl groups of the support were consumed by reaction with the active phase precursor. Such a surface reaction may have caused the decrease of available surface area of the support, probably by closure of the pores.

In

Conclusions

The hydrogenation of benzaldehyde over nickel catalysts supported on Al2O3, SiO2, TiO2 and CeO2 at atmospheric pressure and 70–140 °C produced competitively benzylalcohol, toluene, benzene and methylcyclohexane with yields depending on the nature of the support and the reaction temperature. The order of activity was attributed to crystallite size and degree of dispersion of nickel on the supports. The acid–base properties can operate and induced adsorption phenomena which explained a gap in

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