CATALYST ACTIVITY MAINTENANCE OR DECAY - A MODEL FOR FORMATION AND DESORPTION OF COKE

A new kinetic model provides a theoretical framework to describe how coke formation, adsorption, and desorption can lead to catalyst activity maintenance or decay in reaction mixtures. The system consists of the reactions of 1-hexene, its product isomers, and hexene oligomers on a Pt/gamma-Al2O3 catalyst in sub- or supercritical reaction mixtures. The formation of coke compounds on the catalyst proceeds via the production of hexene oligomers in the bulk fluid phase, followed by their transformation to mobile polynuclear aromatic coke compounds on the catalyst. A kinetic scheme is presented for describing the formation of these coke compounds followed by their reversible adsorption/desorption between the catalyst phase and the fluid phase. Based on this kinetic scheme a mathematical model is developed to describe catalyst activity dynamics when Pt/gamma-Al2O3 catalyzed 1-hexene isomerization occurs in a continuous stirred tank reactor. The model predictions relating to temporal conversion and activity profiles, activity maintenance or decay, and the formation of oligomers and coke compounds are consistent with reported experimental observations. In spite of increased coke formation at supercritical pressures, activity maintenance is predicted due to enhanced desorption of coke-forming compounds in the dense supercritical reaction mixture.