n(硅)∶n(铝)对HZSM-5分子筛膜催化裂解性能的影响

采用二次生长法在不锈钢管内壁上制备了不同n(硅)∶n(铝)的HZSM-5分子筛膜.采用XRD、SEM和原位吡啶吸附红外光谱对样品的结构和酸性质进行了表征,以超临界正十二烷的催化裂解为模型反应对样品的催化性能进行了评价.随着n(硅)∶n(铝)的增加,HZSM-5交织生长程度变大并且Brφnsted酸量逐渐减少.低n(硅)∶n(铝)的分子筛膜表面具有较高的Brφnsted酸量,在反应的初始阶段存在活性的快速下降行为.适当增加分子筛膜的n(硅)∶n(铝),可以减缓反应初始阶段的活性下降速度,从而明显提高HZSM-5分子筛膜的催化活性.但过高的n(硅)∶n(铝)使分子筛膜表面的酸量大幅减少、致密性增加,不利于裂解反应.n(硅)/n(铝)为125的HZSM-5分子筛膜具有最高的反应活性和初始活性稳定性.

Influence of Si/Al Ratio on Catalytic Cracking of n-Dodecane over HZSM-5 Membranes

A series of HZSM-5 membranes on the inner surface of stainless steel tubes with different Si/Al ratio were synthesized by secondary growth technique and used for catalytic cracking of supercritical n-dodecane.The surface properties of membranes were characterized by X-ray diffractometer(XRD),scanning electron microscope(SEM) and Fourier transform infrared spectroscopy(in-situ FT-IR).With the increase of Si/Al ratio,the HZSM-5 crystals were gradually cross-linked and the amount of Brnsted sites decreased.The sample with low Si/Al ratio gave a rapid decrease in the conversion of n-dodecane at the beginning of the reaction because of the high density of acid sites.An increase in the Si/Al ratio can delay the deactivation of the membranes,resulting in the significant improvement of the catalytic activity.But,the excess increase in the Si/Al ratio caused the decrease in the amount of acid sites and the increase in the density of the zeolite layer on the supports,which is unfavorable for the activity of membranes.The sample with Si/Al ratio of 125 gave the highest catalytic activity and initial stability.