Study On The Hydrothermal Methods Of Synthesis Of SSZ-13 Zeolites And Their MTO Reaction

SSZ-13 molecular sieve is composed of T-O?Si-O,or Al-O?tetrahedron connected by oxygen atom head and tail,cha cage connected by octahedral ring window?0.38 nm×0.38 nm?and d6R unit connected by 4-membered ring.It belongs to the same CHA topology as SAPO-34 molecular sieve.Because of its unique structure and physicochemical properties,SSZ-13zeolite exhibits excellent catalytic performance in methanol-to-olefin?MTO?,removal of NO_x from automobile exhaust,and adsorption and separation of CO₂.At present,SSZ-13 molecular sieves are synthesized with organic templates,which have some disadvantages,such as narrow synthetic phase region,large amount of templates,low yield,high preparation cost,and high water consumption of subsequent ion exchange.It is of great theoretical and practical value to explore new environmentally friendly synthetic methods such as low template dosage,low cost and sodium-free system synthesis.Based on the above background,this paper explored three new methods to synthesize SSZ-13 molecular sieves:hydrothermal seed route,ammonia-water route and tetraethyl ammonium hydroxide route.Various factors affecting the synthesis process were investigated.The products were characterized by XRD,SEM,FT-IR,NH₃-TPD,elemental analysis,TG-DTG,BET and their catalysis performance was evaluated by methanol-to-olefin reaction.The main work is as follows:1.The synthesis time of SSZ-13 zeolite is long,the template is expensive and large.The addition of homogeneous seed reduces the amount of template,inhibits the formation of heterogeneous crystals,improves the crystallization rate and reduces the preparation cost.In this study,the traditional synthesis conditions were optimized,and the products were characterized by XRD,SEM and NH₃-TPD.Finally,the optimum ratio of SSZ-13 molecular sieve synthesized with seed addition was obtained:n?SiO₂?/n?A1₂O₃?=200,n?Na₂O?/n?SiO₂?=0.12,n?R?/n?SiO₂?=0.11,n?H₂O?/n?SiO₂?=10.3.The crystallization temperature affects the crystallinity of the product.With the increase of temperature,the crystallization rate increases and the crystallization time decreases accordingly.The methanol conversion,diene selectivity and lifetime of the samples synthesized at 165?and 160?tend to be the same,which are higher than those of crystal seed?standard sample,BY?.The new method shortens the crystallization time of SSZ-13 zeolite synthesized by hydrothermal method.The product particles are obviously smaller than those synthesized by traditional method,and have longer catalytic life.2.In ammonia synthesis route,ammonia-water is used to replace NaOH,ammonium ion is used to replace sodium and other alkali metal ions to promote the skeleton charge matching of molecular sieves.Silicon-aluminium ratio SSZ-13 ammonium molecular sieves with 25,50 and 75 are synthesized by high temperature crystallization at 170 C.It can be directly used for catalytic reaction of methanol-to-olefins without ion exchange after calcination.It was found that the addition of ammonia water could also reduce the amount of template,the size of product was micron,the particle size was larger and the crystallinity was higher.With the increase of Si/Al ratio and the decrease of acid content,the product showed better catalytic performance in MTO catalytic reaction.The catalytic life of samples with Si/Al ratio of 25,50 and 75was 49 min,92 min and 177 min,respectively.3.Within a certain range,the increase of the ratio of silicon-to-aluminium can increase the ratio of silicon to aluminium.Generally,the ratio of silicon-to-aluminium of SSZ-13 molecular sieve is less than 100.Simply increasing the ratio of silicon to aluminium can not break through this limitation,and the yield will be greatly reduced.In this study,high-silicon molecular sieve SSZ-13 with CHA topology was synthesized by adding tetraethyl ammonium hydroxide?TEAOH?organic base instead of part NaOH inorganic base under hydrothermal conditions.For the first time,the ratio of silicon-to-aluminium is over 300.high-silicon SSZ-13molecular sieve was synthesized in fluoride-free system,the crystallinity is high and the morphology is regular.