Catalytic cracking of vacuum gas oil was carried out at 500 degrees C
over two series of mordenites treated by either steam/HCl or HF. Secon
dary mesopores were formed in the mordenite by these treatments and th
eir volumes increased with the degree of dealumination, whereas the mi
cropore volumes decreased proportionately. The conversion of gas oil a
nd the yield of gasoline and kerosene + diesel were correlated with th
e acid-amount/mesopore-volume ratio. A maximum conversion and yield we
re observed at a point of acid-amount/mesopore-volume ratio. This maxi
mum point was interpreted as an optimum condition for the cracking act
ivity between two inverse tendencies. The one is the decreasing tenden
cy of cracking activity with reducing acid density. The other is the o
ne increasing in activity with an increasing volume of the mesopore wi
th silica/alumina ratio for both conversion of VGO and yields of gasol
ine and kerosene + diesel. On the treated mordenite of higher silica/
alumina ratio, more olefins were formed due to a low rate of hydrogen
transfer reaction. Aromatics are found to be formed via cyclization of
olefins which can occur inside an enlarged mesopore of dealuminated m
ordenite. The largest content as an alkyl-aromatic compound in the pro
duced gasoline was toluene, which was possibly interpreted as the diff
usion limitation in the formation of higher alkyl-branched aromatics.
(C) 1998 Academic Press.