ZSM-5分子筛催化剂的积碳及其稳定性

COKE DEPOSITION ON ZSM-5 ZEOLITE CATALYST AND ITS STABILITY

分子筛催化剂上的积碳是导致失活的主要原因.本文以甲苯-乙醇在固定床积分反应器中的乙基化反应作探针,研究ZSM-5分子筛上的积碳情况.结果表明,对于HZSM-5,反应时间t<4.5h时,积碳较快,催化剂活性(C_t)、乙基甲苯产率(Y_(ET))和对乙基甲苯选择性(S_(P-ET))均有较大幅度的变化;t>4.5h时,积碳减慢,催化剂性能的变化也较小.对MgHZSM-5,各种催化性能的变化则较小.进一步分析表明,HZSM-5分子筛的积碳主要在外表面(占95%),而MgHZSM-5的积碳主要在孔内.阐明了保持积碳C/H比较低的条件下,采取加入添加剂抑制积碳生成的办法,可望收到延长催化剂寿命的效果.对含有适量添加剂A和B的HZSM-5催化剂,进料200h后催化剂仍有较高活性和选择性.

The coke deposition,leading to the deactivation of ZSM-5 zeolite catalyst,has been investigated by the alkylation reaction of toluene with ethanol in a fix-bed integral reactor.For zeolite HZSM-5,it is observed that coke is formed on the external surface of the crystallites at a faster rate when the time on stream is less than 4.5 hours and leads to greater changes of catalytic performance such as catalyst activity,ethyltoluene yield and p-ethyltoluene selectivity.Beyond 4.5 hours,the coke is mainly formed in the internal pores at a lower rate and leads to less changes in catalytic performance because of the intimate nature of the channel network.For magnesium modified zeolite,MgHZSM-5,coke deposition takes place on the internal surface.During 15 hours on stream,95% coke is on the external surface for HZSM-5,whereas for MgHZSM-5,nearing 100% coke is on the internal surface.The experiment shows that the stability of catalyst can be enhenced by constraining coke deposition at lower C/H ratio.For HZBZ-5(MgHZSM-5 containing appropriate amount of additives A and B),both toluene conversion and p-ethyltoluene selectivity are still maintained at higher levels after 200 hours on stream.