MODEL REACTION FOR THE IN-SITU CHARACTERIZATION OF THE HYDROGENATING AND ACID PROPERTIES OF INDUSTRIAL HYDROCRACKING CATALYSTS

A kinetic study of o-xylene transformation was carried out on a sulfided NiMo on Y zeolite catalyst during the hydrocracking of a model compound (n-heptane) under the following conditions of the process: high hydrogen pressure (5.7 MPa), presence of nitrogen and sulfur-containing compounds, o-Xylene inhibits n-heptane transformation, which can be explained by a competition for the adsorption on the acid sites between o-xylene and the olefinic intermediates of hydrocracking. The products of o-xylene transformation are the following: p- and m-xylenes (isomerization), toluene and trimethylbenzenes (disproportionation), and saturated C-8 naphthenes (dimethylcyclohexanes and trimethylcyclopentanes). It is shown that 1,3- and 1,4- dimethylcyclohexanes (and trimethylcyclopentanes) result from the isomerization of 1,2-dimethylcyclohexane and not from the hydrogenation of m- and p-xylenes. Therefore, the hydrogenating activity of hydrocracking catalysts can be characterized by the formation of dimethylcyclohexanes and trimethylcyclopentanes. Furthermore, the isomerization of xylenes, which occurs through an acid mechanism, can be used for characterizing the acid activity of hydrocracking catalysts. This is not the case for disproportionation whose rate depends on hydrogen pressure. The validity of o-xylene transformation for characterizing the acid and hydrogenating activities of bifunctional hydrocracking catalysts was confirmed by the use of a series of catalysts having either the same content of NiMo and different contents of zeolite or the same content of zeolite and different contents of NiMo. While the isomerization activity is strictly proportional to the zeolite content and independent of the NiMo content, the hydrogenating activity is proportional to the NiMo content and independent of the zeolite content. (C) 1995 Academic Press, Inc.