La、Ce改性对ZSM-5分子筛上烯烃裂解制丙烯反应的影响及其作用机理

在550 ℃、常压、加入水蒸气的条件下，研究稀土La和Ce改性ZSM-5分子筛上FCC汽油的催化裂解反应。结果表明，稀土La和Ce改性可以提高ZSM-5分子筛的总酸量和强酸量，从而使FCC汽油转化率，特别是烯烃裂解反应的转化率明显提高，烯烃反应的选择性和气相产物乙烯、丙烯、丁烯，特别是丙烯的选择性显著增加。分子模拟计算结果表明，La3+和Ce4+位于ZSM-5分子筛Z型孔道的拐弯处，距离孔壁的距离为0.3～0.4 nm，使得弯道处的体积明显减小，导致烯烃裂解反应能垒、环化反应能垒、叠合反应能垒均有不同程度的增加，但裂解反应能垒增加的幅度最小，从而提高了烯烃裂解反应的选择性。

EFFECTS AND MECHANISM OF La OR Ce MODIFIED ZSM-5 ZEOLITE ON OLEFIN CRACKING FOR PROPYLENE

The catalytic cracking reaction of FCC naphtha was studied on La or Ce modified ZSM-5 zeolite under 550℃, atmospheric pressure and water vapor conditions. Experimental results suggest that the modification of rare earth La3+ and Ce4+ can increase the total acid amount of ZSM-5 molecular sieve, especially strong acid acidity, so FCC gasoline conversion rate and in particular the olefin cracking reactions obtain marked improvement. In addition, after the modification, the selectivity of the olefin cracking reaction and the yield of ethylene, propylene, butylene, especially propylene are also significantly increased. Molecular simulation results show that La3+ and Ce4+ locate at the corner in bent Z type channel of ZSM - 5 molecular sieve, 3~4? away from the hole center, making the volume of the bend decrease obviously. This leads to the increase of energy barrier for all reactions including cracking, cyclization, aromatization and polymerization, but the cracking barrier increases minimum which improves the selectivity of cracking reaction of olefins.