The demand for propylene has been growing recently. The concentration of olefins in the gasoline is strictly limited by the related environmental regulations. The olefins contained in the gasoline used as the feed could be cracked into light olefins to slash the olefin concentration in the gasoline to yield more propylene at the same time. The monolithic catalyst washcoated on the modified ZSM-5 zeolite was used in the experiments. The effect of the temperature, the Si/Al ratio in ZSM-5 and the addition of the rare earth elements on the selectivities and the yields of the light olefins were studied. The high yields of propylene and butene could be obtained under the experimental conditions of a higher temperature and Si/Al ratio with the addition of rare earth elements.
根据FCC汽油的PONA(Paraffins,Olefins,Naphthenes and Aromatics)分析结果,确定了不同结构的FCC汽油模型化合物。采用半经验量子力学方法计算了所选模型化合物分子的尺寸,用分子动力学方法计算了模型化合物分子在Y、ZSM-5和β分子筛孔道中的扩散能垒。计算结果表明,模型化合物分子在分子筛中的扩散能垒与其自身的尺寸和分子筛孔径大小密切相关。Y和β分子筛孔径较大,不同结构模型化合物分子在这两种分子筛中的扩散能垒差别不大;ZSM-5分子筛的孔径较小,与汽油模型化合物分子尺寸相近,不同结构模型化合物分子在ZSM-5分子筛中的扩散能垒具有明显的差别。实验结果证明,ZSM-5对FCC汽油具有优良的裂化选择性,能够使汽油中的烯烃选择性地裂化为C3、C4烯烃。在降低FCC汽油烯烃含量的同时,实现了增产丙烯的目的。
