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 sulfid ed NiMo on Y zeolite catalyst during the hydrocracking of a model comp ound (n-heptane) under the following conditions of the process: high h ydrogen pressure (5.7 MPa), presence of nitrogen and sulfur-containing compounds, o-Xylene inhibits n-heptane transformation, which can be e xplained by a competition for the adsorption on the acid sites between o-xylene and the olefinic intermediates of hydrocracking. The product s of o-xylene transformation are the following: p- and m-xylenes (isom erization), toluene and trimethylbenzenes (disproportionation), and sa turated C-8 naphthenes (dimethylcyclohexanes and trimethylcyclopentane s). It is shown that 1,3- and 1,4- dimethylcyclohexanes (and trimethyl cyclopentanes) result from the isomerization of 1,2-dimethylcyclohexan e and not from the hydrogenation of m- and p-xylenes. Therefore, the h ydrogenating activity of hydrocracking catalysts can be characterized by the formation of dimethylcyclohexanes and trimethylcyclopentanes. F urthermore, the isomerization of xylenes, which occurs through an acid mechanism, can be used for characterizing the acid activity of hydroc racking catalysts. This is not the case for disproportionation whose r ate depends on hydrogen pressure. The validity of o-xylene transformat ion for characterizing the acid and hydrogenating activities of bifunc tional hydrocracking catalysts was confirmed by the use of a series of catalysts having either the same content of NiMo and different conten ts of zeolite or the same content of zeolite and different contents of NiMo. While the isomerization activity is strictly proportional to th e zeolite content and independent of the NiMo content, the hydrogenati ng activity is proportional to the NiMo content and independent of the zeolite content. (C) 1995 Academic Press, Inc.