杂原子分子筛Ga-ZSM-5晶化过程考察

INVESTIGATION ON THE CRYSTALLIZATION OF MFI TYPE GALLOSILICATES

用多种有机胺合成了不同SiO_2/Ga_2O_3比的Ga-ZSM-5分子筛。以SEM、XRD技术对其晶化过程进行了详细的考察。发现Ga-ZSM-5的水热晶化过程经历了诱导期、晶体生长期及转晶期三个阶段。增大原料SiO_2/Ga_2O_3比导致诱导期及晶体生长期缩短,转晶加快;在相同条件下,Ga-ZSM-5的诱导期及晶体生长期均比ZSM-5长,且较易转晶。形貌变化及晶相分析表明,Ga-ZSM-5的转晶过程很可能首先在外表面发生,逐步进入分子筛晶粒体相。采用分子筛合成的液相传质机理对Ga-ZSM-5的晶化过程进行了描述。

MFI type gallosilicates(Ga-ZSM-5), which have been found to be excellent for the aromatization of lower hydrocarbons and methanol were synthesized by using four kinds of organic amine as templates. The crystallization process was investigated by means of SEM and XRD.It seems that the process involves three stages:induction, crystal growth and phase transformation. During the induction period, crystal nuclei were formed, which can be detected by IR but not by SEM and XRD.During the crystal growth period, the crystal phase of Ga-ZSM-5 increased with increasing time, gradually reaching a maximum.In the final stage, the crystal surface of Ga-ZSM-5 became rough, and α-SiO_2 appeared, suggesting that the phase transformation of Ga-ZSM-5 starts from the external surface. With increasing SiO_2/Ga_2O_3 ratio, the induction and crystal growth periods became shorter and phase transformation was accelerated.Under identical conditions, the induction period and the crystal growth period of Ga-ZSM-5 were both longer than those of ZSM-5. For different amines, the sequence is diethanol amine>hexadiamine>n-butylamine>ethyldiamine.A liquid phase mechanism was proposed to describe the crystallization process of Ga-ZSM-5.