FORMATION OF COKE ON HYDROTREATING CATALYSTS AND ITS EFFECT ON ACTIVITY

Combined use of HPLC and GC/MS methods to separate oils into distinct aromatic and aliphatic fractions has shown that coke precursors are found among polyaromatic compounds including those containing nitrogen. The initial adsorption of the molecules in the polyaromatic fraction takes place either via heteroaromatic nitrogen or via the pi-relectrons in the aromatic compounds. The use of MAS C-13-NMR and other techniques has enabled characterization of the aromatic and aliphatic fraction of the carbonaceous deposits and led to a detailed model for the coke formation. It is shown that coke consists of condensed aromatic clusters with short terminal or bridging aliphatics. In situ studies using model compounds show that the coke does not act as a poison but causes deactivation by physically blocking the inorganic sulfur phase. Finally, spent catalyst studies have separated deactivation by metals from coke deactivation.