Influence of water on fast hydrogenation reactions with monolithic and slurry catalysts

Abstract

The influence of water on the hydrogenation of α-methyl styrene (AMS) with Ni–Al2O3 slurry and monolithic catalysts was investigated. Addition of water to this liquid-phase hydrogenation results in a four-phase system, as applied industrially for the partial hydrogenation of benzene. Addition of already a limited amount of water to AMS reaction mixture resulted in a strong decrease in activity. This could not be explained by a poisoning effect of water.It has been made plausible that the reaction changes from diffusion limited in hydrogen to diffusion limited in AMS in the presence of free water. The formation of a water layer around the catalyst introduces an external mass transfer resistance for AMS. Two mechanisms are proposed to explain the trend of decreasing activity upon increasing the amount of water. Both the slurry and the monolithic catalyst show the same behaviour. The water layer is formed on monoliths as easily as on slurry catalysts. As the formation of a water layer is a prerequisite for high selectivity in the partial hydrogenation of benzene, the application of monoliths in this reaction is promising.