Synthesis And Characterization Of LTA Molecular Sieves

LTA (Linde Type A) molecular sieve is widely used in industrial adsorption, catalysis, ion-exchange, separation, and high-tech fields, because of their unique crystal structure and physico-chemical performances. The present study focuses on the transformation of molecular sieve from kaolin based on its rich reserves.In this thesis, aimed at the key issues such as activation of kaolin, preparation of zeolite precursor, and crystallization of zeolite and so on, the main technical conditions and parameters were investigated intensively.On the basis of a comprehensive review of the emerging achievements concerning kaolin converted to zeolites, some basic concepts, trend of research and problems existing in this field are discussed.Kaolin was pretreated by physical and chemical methods. The content of SiO2 and Al2O3 was enriched to 93.15wt%from 78.28wt%, which facilitates much more the formation of LTA molecular sieve.The phase transition law for kaolin to be activated by the calcination and the alkaline-fusion method was explored by TG-DTA and XRD techniques, respectively. Two approaches to molecular sieves from kaolin are explored, which are the calcination hydrothermal treatment and the alkali-fusion hydrothermal synthsis.Suitable conditions of the calcination hydrothermal treatment method were obtained by experiment, which include three stages. Firstly, metakaolin is transferred from kaolin by calcining at 600℃for 2 hours; Then, the mixture of metakaolin and NaOH solution in a molar ratio of n(Na2O):n(SiO2)=2.0 and n(H2O):n(Na2O)=60 is aged at 60℃for 2 hours; Lastly, NaA molecular sieve is synthesized from the aged composition by hydrothermal reaction at 90℃for 4 hours. However, this method is not suitable for the development because of its high consumption of energy.The alkali-fusion hydrothermal synthsis method was stressed. A comparative study of kaolin activation effect by NaOH and Na2CO3 was carried out. It is shown that NaOH is a better fit reagent for kaolin-activation than Na2CO3. SiO2 and Al2O3 can be activated efficiently, if the mixture of kaolin and NaOH in a molar ratio of n(Na2O):n(SiO2)=2.0 was once calcined under 400℃for 2 hours. This laid a foundation for the preparation of molecular sieve precursors. The aging process before the hydrothermal synthesis is investigated, and the optimized technique parameters are gotten. The mixture of water and kaolin-activated by alkali-fusion in a molar ratio of n(H2O):n(Na2O)=55 was heated under 40℃for 2 hours, precursors of molecular sieves were synthesized. Based on the above mentioned, precursors were converted into LTA zeolites under 85℃for 5 hours in company with 150-200r/min stiring hydrothermally.The composition, crystal phases and microstructure of synthesized products were characterized by XRF, XRD and SEM, and the absorption performance of molecular sieve was measured. It is shown that the cnversion rate of molecular sieve was 90.40wt%, and its static water absorption reached 23.27wt%. Especially, a temperature-rising test method was established, by which the effective content of molecular sieve in product can be analyzed qualitatively.In order to probe the preparation of zeolites with different functions from kaolin, KA molecular sieve was gained successfully from NaA molecular sieve by ion exchange.