B. Dimon et al., COKING, AGING AND REGENERATION OF ZEOLITES .14. KINETIC-STUDY OF THE FORMATION OF COKE FROM PROPENE OVER USHY AND H-ZSM-5, Applied catalysis. A, General, 101(2), 1993, pp. 351-369
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.