Zeolite molecular sieve is an inorganic crystalline material with a wide range of applications in adsorption,catalysis and separation due to its structural and performance properties.Among many types of zeolite molecular sieves,the silica-aluminaphosphate SAPO-34 zeolite has a unique octet ring pore and cage structure,as well as a suitable acid strength and acid density,which enables it to have a high low-carbon olefin selectivity in the methanol to olefin(MTO)reaction.And the MTO reaction,as an important method in the preparation of low carbon olefins by non-petroleum route,has been a hot research topic in the energy field.In addition to its lamellar structure,the two-dimensional layered silicate magadiite contains a large number of silica hydroxyl groups on the surface of its lamellar structure,which can produce strong adsorption,and these unique structures have attracted the attention of many researchers.In this thesis,pure-phase SAPO-34 zeolite were prepared by hydrothermal synthesis using magadiite as the silica source,and the products were characterized and the effects of different factors on the synthesis were discussed.A systematic characterization method was used to investigate the change process of the two-dimensional layered silicate skeleton to SAPO-34 zeolite skeleton during the reaction process,and the growth mechanism of SAPO-34 zeolite was reasonably inferred from the experimental results.The main findings are as follows:(1)SAPO-34 zeolite were prepared in a hydrothermal system with tetraethylammonium hydroxide as a templating agent using magadiite as the silica source,and the obtained SAPO-34 zeolite have a regular cubic morphology,which is typical of microporous materials and contain a small amount of mesopores.The effects of the crystallization temperature and the initial gel ratios on the synthesis results were discussed,and the synthesis method was summarized.(2)The crystallization process of SAPO-34 zeolite synthesized hydrothermally using magadiite as the silica source was investigated,and the mechanism of crystallization was speculated.The lamellar wheat magadiite structure dissociated in the initial gel produced a certain adsorption effect due to the enrichment of silica hydroxyl groups on the surface,and the lamellae were surrounded by a higher concentration of material than the liquid phase,which provided an environment for the formation of nuclei.At the same time,the six-membered ring structure in magadiite is more stable than the five-membered ring,which can form secondary structural units to stay in the solution system.As the crystallization process continues,a large number of double six-membered rings(D6Rs)and quaternary rings are formed,which are connected to form a cha-cage structure under the structural guidance of the templating agent,and self-assembly can continue to occur between these structural units,forming the nuclei of SAPO-34 zeolite,which continue to grow and finally form SAPO-34 zeolite grains with good crystallinity.(3)Based on the synthesis of SAPO-34 zeolite,the SAPO-34/ZSM-5 composite molecular sieve was obtained by adding the dry powder of ZSM-5 molecular sieve synthesized with magadiite as the silica source to the initial gel of SAPO-34 molecular sieve without additional silica source and secondary crystallization.The ZSM-5 zeolite synthesized by using magadiite as the silica source has a lamellar structure similar to magadiite,and the SAPO-34 zeolite can grow on the surface of these lamellar structures to finally obtain the composite molecular sieve.It was shown that the catalytic lifetime of the synthesized composite molecular sieve samples reached 48 h,and the average selectivity of ethylene and propylene also reached 65.3%,which showed good catalytic performance. |