Process for producing light olefins by catalytic conversion of hydrocarbons

A process for the catalytically cracking a petroleum hydrocarbon feedstock to selectively produce light olefins comprising ethylene, propylene, isobutylene and isoamylene comprising:

• catalytically converting said preheated hydrocarbons to selectively produce ethylene, propylene, isobutylene and isoamylene by contacting said preheated hydrocarbons with phosphorus and rare earth containing high silica zeolite having a structure of pentasil catalysts in a riser or downflow reactor at a temperature of 530.degree. to 680.degree. C. and a pressure of 1.2.times.10.sup.5 to 4.times.10.sup.5 Pa, with a contact time of 0.1 to 6 seconds, a weight ratio of catalyst to feedstock of 5:1 to 15:1 and a weight ratio of steam to feedstock of 0.01:1 to 0.5:1 to selectively produce light olefins comprising ethylene, propylene, isobutylene and isoamylene
• wherein said phosphorus and rare earth containing high silica zeolite having a structure of pentasil catalysts comprises a rare earth containing high silica zeolite having a structure of pentasil which has been pre-exchanged with ammonium ion to decrease its sodium content to a level of not more than 0.1% by weight, calculated as Na.sub.2 O, and monogeneously mixed with an aluminum phosphate sol having a composition of Al.sub.2 :P.sub.2 O.sub.5 =1:1-3 according to a weight ratio of P.sub.2 O.sub.5 :zeolite (anhydrous basis)=1:5-99, followed by calcination at 300.degree.-600.degree. C. for 0.5-6 hours in the presence of 10-100% steam.