Hydrogenation of aromatics on modified platinum–alumina catalysts
Introduction
Hydrogenation of benzene to cyclohexane is an important reaction from the industrial point of view. Cyclohexane is used in the production of caprolactam and as a starting material for nylon synthesis. Cyclohexane is the only reaction product and is mainly obtained by platinum and palladium catalysts in the temperature range 300–350°C and under 1.01–2.02 MPa hydrogen pressure 1, 2, 3. Above 130–200°C normal paraffins of C1–C6 are also formed over ruthenium, technetium and rhenium catalysts [3]. Benzene ring cannot be hydrogenated in the presence of rhenium up to 250°C, since above this temperature it decomposes to methane.
Hydrogenation of adsorbed toluene has many features in common with the hydrogenation of adsorbed benzene [4]. Lindfors and Salmi [5]found that methyl cyclohexane was the only product detected through toluene hydrogenation, partially hydrogenated intermediates are not formed.
The adsorption of benzene and toluene on metallic surfaces has been assumed to occur via the interaction of π-electrons of the aromatic ring with d-orbitals of the metal 6, 7, 8. Thus, these molecules are adsorbed with the aromatic ring parallel to the surface 9, 10.
The present work aims to promote a 0.35 Pt/Al2O3 catalyst to improve its aromatics hydrogenation activity.
Section snippets
Catalyst preparation
10 g of Al2O3 produced by “Rhone Poulenc-Chimie Fine” with the following specifications: white extrudates (2.5×3 to 5 mm); surface area 216 m2/g, grain density 1.24 g/cm2, structural density 3.0 g/cm3 and total pore volume 0.47 cm3/g, were impregnated in an aqueous solution of hexachloroplatinic acid, 0.019 M, such as to obtain a catalyst containing 0.35% Pt/Al2O3. Citric acid was added to the platinum precursor solution to improve the penetration and dispersion of Pt into the catalyst pores 11, 12.
Results and discussion
Benzene and toluene are ideal probe molecules for studying the interaction of aromatics with transition metal surfaces. Furthermore, they are involved in many catalytic processes in the petrochemical industry as intermediates or final products.
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