Influence of modifiers on the performance of Ru-supported catalysts on the stereoselective hydrogenation of 4-acetamidophenol

Ruthenium-supported catalysts modified by the addition of Mg, Na or Ce were studied in the selective hydrogenation of paracetamol (4- acetamidophenol) to cis- and trans-4-acetamidocyclohexanol. In both series, catalysts supported on carbon or on alumina, the modified catalysts were more selective to the trans isomer than the corresponding monometallic catalysts. However, the modified catalysts, particularly the supported on carbon, were less active. Moreover, there is a reduction in the hydrogenolysis reaction of the OH group on carbon-supported catalysts. The enhancement observed in the stereoselectivity for the modified catalysts may be explained by an alteration in the acid–base properties of the catalyst surface, which eventually control the mode of adsorption of the molecule of paracetamol and probably the tautomeric equilibrium of the 4- acetamidocyclohexene-1-ol intermediate. The optimum amount of promoter to improve the stereoselectivity seems to depend on the inherent acid– base characteristics of both the modifier and the support. Therefore, the proper adjustment or fine tuning of these parameters is crucial for obtaining the best results in terms of stereoselectivity.