Effect Of Additives On The Synthesis Of SAPO-56 Molecular Sieve

Methanol to olefins（MTO）as one of the most promising non-petroleum routes to produce olefins is widely recognized by researchers in petrochemical field.The key aspect of MTO reaction is to screen for appropriate catalysts.Some catalysts such as ZSM-5,SAPO-34 molecular sieves have been applied in the industry in large-scale.However,these catalysts have some drawbacks such as low selectivity,carbon deposit,and short life et al.Therefore,the focus of this work is to develop some MTO catalysts with better performance.In this article,SAPO-56 is selected as the research object to study the effects on the purity and morphology of SAPO-56 in the system of alcohol–water and inorganic salts as the additivies.Based on the original composite template synthesis,the amount of N,N,N’,N’-tetramethyl-1,6-diamine（TMHD）is decreased to 0.2 to explore the best ratio of synthesis of SAPO-56.And the morphology,acidity and catalytic properties of MTO were also studied under different synthesized conditions.Firstly,methanol,ethanol,propanol,butanol,isopropanol,ethylene glycol is fed into system to replace the water of reaction system,respectively.Then,the impacts on the purity,morphology,acidity and MTO catalytic properties of the product are investigated for different types of alcohol and various amount of alcohol.The results showed that the methanol strongly influences the synthesis of SAPO-56 molecular sieve.When the ratio of water replaced by methanol is increased to 12.5%,nearly no SAPO-56 can be synthesized.The main product is the pure SAPO-20.For propanol,when the ratio of is increased to 12.5%,a considerable amount of SAPO-17 miscellaneous crystals is generated.For other alcohol,such as ethanol,isopropanol,n-butanol and ethylene glycol,when the ratio is reached at 12.5%,the production of the reaction is mainly relatively pure SAPO-56.With the increase of the amount of alcohol in the reaction system,the system gradually generates SAPO-17,SAPO-41,SAPO-11 and SAPO-20 and other miscellaneous grain.On the other hand,alcohol-water system also has a great impact on the morphology of SAPO-56.When the ratio of water substituted by alcohol is 12.5%,the morphology of the product is changed from the hexagonal thicker toppers to six double cone or hexagonal bipyramid.Furthermore,when the ratio of ethanol increases to 12.5%,the production of reaction has a higher content of acid than that of hydrothermal synthesis in pure water system.The yield ratio of MTO reaction reaches at 51.84%,which is slightly higher than the hydrothermal synthesis system.Secondly,various inorganic salts are also investigated to explore their impacts on the purity,acidity and catalytic properties of SAPO-56.The results showed that Na Cl and Na F is useful to improve the crystallinity of SAPO-56,while Na NO₃ has no obvious impact on the purity and crystallinity of SAPO-56.The inorganic salts such as KCl,KF,KNO₃ strongly inhibit the growth of SAPO-56 molecular sieve,resulting to the production of SAPO-17 miscellaneous crystals.On the other hand,the addition of inorganic salts such as NH₄Cl,NH₄F,NH₄NO₃ can make SAPO-17 miscellaneous crystals,but they does not affect the synthesis of SAPO-56 molecular sieve.The addictive of Na⁺have a great impact on the morphology of SAPO-56 molecular sieve.When the amount of Na⁺is 0.001 and 0.02,there are some patterns on the surface of the product.When the amount of Na⁺is 0.05,the product will become thinner and the surface of SAPO-56 molecular sieve will generation some amorphous substance.When the amount of Na⁺is 0.1,the shape of samples has positive hexagon,toppers hexagonal pyramid and spherical.The surface of the samples will become very rough.When different amounts of Na⁺are fed into the system,it has different impacts on the performance of synthetic products.The results indicated that when the amount Na⁺is 0.001,the synthetic product of MTO reaction has almost no catalytic activity.When the content of Na⁺is increasing to 0.02,0.05,and 0.1,respectively,it has the same performance for the catalytic samples of MTO reaction to the situation where Na⁺is not added into the system.Finally,the ratio of N,N,N’,N’-tetramethyl-1,6-hexanediamine（TMHD）is decreased to 0.2.Then,SAPO-56 is synthesized using the composite template additive,the best synthesis condition is identified as follows:1.8CH₃CH₂CH₂NH₂:0.2 TMHD:0.7 Si O₂:0.8 Al₂O₃:1 P₂O₅:30 H₂O.The acid content of molecular sieve has been improved greatly,as well as the yield of light olefins in the MTO reaction,about 75.05%.