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

The influence of water on the hydrogenation of alpha -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, a s applied industrially for the partial hydrogenation of benzene. Addition o f already a limited amount of water to AMS reaction mixture resulted in a s trong decrease in activity. This could not be explained by a poisoning effe ct 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 tr end of decreasing activity upon increasing the amount of water. Both the sl urry and the monolithic catalyst show the same behaviour. The water layer i s 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 hydrog enation of benzene, the application of monoliths in this reaction is promis ing. (C) 2001 Elsevier Science B.V. All rights reserved.