摘要
采用ＸＲＤ，ＳＥＭ，ＩＲ和ＣＰ／ＭＡＳ及ＭＡＳＮＭＲ等手段，对快速水热合成ＳＡＰＯ３４分子筛的全过程进行了监测，研究了由原始凝胶到ＳＡＰＯ３４结晶体过程中固相物质的组成结构随晶化时间的变化规律．结果表明，ＳＡＰＯ３４晶核的形成过程既是一个硅氧、磷氧和铝氧四面体由无序排列的胶团到有序排列的晶格骨架的重排过程，同时又是羟基缔合脱水环化的过程．晶化分为晶化前期和晶化后期两个阶段．晶化前期，硅原子直接参与晶核的形成和晶粒的长大过程，生成Ｓｉ（４Ａｌ）结构；晶化后期，少量硅以同时取代一对磷铝原子的方式进入分子筛骨架形成Ｓｉ（３Ａｌ），Ｓｉ（２Ａｌ），Ｓｉ（１Ａｌ），Ｓｉ（０Ａｌ）等多种硅结构．但在整个晶化过程中Ｓｉ（４Ａｌ）结构的生成并不排除硅单独取代磷机理的共同作用．
For rapid syntheses of SAPO 34 molecular sieves, XRD, SEM, IR and NMR techniques were applied to monitor the structure and composition changes of the solid samples in the whole crystallization process in order to get the evidence for crystallization mechanism. XRD and SEM determination of the as synthesized samples revealed that high crystallinity (～80%) can be achieved in 2 5 h, indicating a characteristic feature of fast crystallization. IR spectra showed that the formation of crystal nucleus of SAPO 34 arose from the structure arrangement of the initial gel and the condensation of the hydroxies. According to 27 Al, 31 P, 29 Si MAS and 29 Si CP/MAS NMR studies, in the initial stage (<2 5 h) of the crystallization, only Si(4Al) was found in the structure, and the increase of the relative content of Si, P and Al in the framework of the as synthesized samples showed a similar trend as that of the crystallinity, which was different from those in the later crystallization stage (>2 5 h). This gives evidence that Si atoms directly take part in the formation of the crystal nucleus as well as in the growth of the crystal grains in the initial stage (<2 5 h) of the crystallization. It has been found that the relative content of Si increased slightly with slight decrease of Al and P after 2 5 h. The appearance of the Si(3Al), Si(2Al), Si(1Al) and Si(0Al) suggested that the substitution of Si atoms for the phosphorus and aluminum pair took place in the later stage of the crystallization. There is another possibility that can not be ruled out for the increase of the Si content, which is the replacement of the P atoms by Si.
出处
《催化学报》
SCIE
CAS
CSCD
北大核心
1998年第5期436-441,共6页
