COKING, AGING AND REGENERATION OF ZEOLITES .14. KINETIC-STUDY OF THE FORMATION OF COKE FROM PROPENE OVER USHY AND H-ZSM-5

The formation of irreversibly blocked compounds (coke) on USHY and H-Z SM-5 zeolites during Propene transformation was investigated using a m icrobalance for propene pressures varying from 0.33 to 13 kPa and temp eratures ranging from 120-degrees-C to 450-degrees-C. At 120-degrees-C coking is initially much faster with H-ZSM-5 than with USHY, the cont rary being observed at 450-degrees-C. However with both zeolites the g reater the propene pressure and the lower the temperature the faster t he coke formation. The very pronounced effect of the temperature can b e related to an increase with the temperature of the difficulty of for ming molecules irreversibly blocked in the pores. At low temperature o ligomers which are easily formed remain blocked in the pores because o f their low volatility and their physical adsorption. At high temperat ure the formation of aromatic molecules larger than the pore apertures , hence trapped in the cavities or at channel intersections, is partic ularly difficult, as it requires numerous successive steps, most of th em being sterically inhibited. Low-temperature coke can occupy the tot al pore volume while, because of pore blockage, only part of the pore volume can be occupied by high-temperature coke. Low-temperature coke can be totally eliminated by an adequate thermal treatment in vacuum, the temperature programme being adjusted so as to obtain a very high c racking/aromatization ratio.