Synthesis, Ion Exchange And Adsorption Performance Of X Zeolites With Low Si/Al Ratio

Low Silica X type (LSX) zeolite having good ion exchange, nitrogen and oxygenadsorption as well as separation ability is widely used in drying and gas separationindustry, because the LSX type molecular sieve has both important theoretical andpractical guiding significance. Sodium low silica X type zeolite (Na-LSX) wasdirectly synthesized using low temperature hydrothermal synthesis one-step method.In order to improve the molecular sieve characteristics of LSX type zeolite andachieve high oxygen and nitrogen adsorption and separation performance, Li+, Ca2+(Li-LSX)(Li, Na)-LSX,(Ca, Na)-LSX, and (Li, Ca, Na)-LSX zeolites were preparedusing aqueous ion exchange procedure. Since many decades the research have beendone on the water adsorption and cation (Na+, Li+, and Ca2+) distribution in LSXmolecular sieve framework and important role it plays in the nitrogen and oxygenseparation. Although it is evident that the water and the cation have key influence onits thermal stability and nitrogen oxygen separation performance, but the thermalstability and adsorption/separation mechanisms of LSX zeolite is not clear yet.Main conclusions obtained this study are as follows:1. Na-LSX was synthesized via hydrothermal synthesis method using sodiummeta-aluminate and sodium silicate as raw materials, and investigated the influence ofcrystallization temperature, crystallization time, pH on the process of synthesis.Moreover, on inspection of the above mentioned aspects of Na-LSX zeolites synthesis,it is observed that the crystallization temperature and crystallization time greatlyeffect in the form of changing LSX type molecular sieve in to sodalite molecular sieve.During the synthesis if the alkalinity get increased then Si/Al ratio would be increasedand speed up the nucleation time and crystallization process. To determine theoptimum parameters of Na-LSX hydrothermal synthesis for: Al2O3：2SiO2：6Na2O：96H2O, the best crystallization temperature chosen was90C, and time was6h.2. The aqueous solution exchange method was used to prepare different degrees(Li, Na)-LSX,(Ca, Na)-LSX,(Li, Ca, Na)-LSX zeolites by changing the switchingfrequency, which were then characterized by ICP-AES, XRD, FT-IR and SEMtechniques. The results showed that at same concentrations the obtained ion exchangeselectivity order is as follows：Ca2+> Li+> Na+, and with the increase in the degree ofLi+exchange (Li, Na)-LSX the cell constant increases, while with the increase of Ca2+exchange degree,(Ca, Na)-LSX cell constants get reduced, but the morphology andskeleton structure were not influenced. In (Li, Ca, Na)-LSX molecular sieve, someparts of skeleton were replaced by Ca2+Li+, which further improved nitrogen oxygenseparation selectivity and reduced the amount of lithium. 3. Using in situ XRD and TG-DSC characterization methods and combining withthe thermodynamic calculation, the detailed investigations were carried out oninfluence of different cations on their thermal stability, water adsorption ability ofLSX zeolites and it is clearly evident that the stability of Na-LSX is higher than thatof Li-LSX, but the order of water adsorption ability is as follows:Ca-LSX>Li-LSX>Na-LSX, and further it is found that water were mainlyphysisorbed, chemisorbed and in the form of residual water. The calculated appearantactivation energies for three types of water were increased as follows:50-70kJ/mol,160-190kJ/mol and200-270kJ/mol.4. The nitrogen/oxygen adsorption experiment was carried out at roomtemperature and the results showed that adsorption capacity of N2/O2gases onNa-LSX zeolite was8.71cm3/g and3.41cm3/g, Li-LSX was28.31cm3/g and4.51cm3/g, and Ca-LSX was31.4cm3/g and7.50cm3/g. Their nitrogen oxygen separationcoefficients were2.55,6.27and4.18. Herein we conclude that the oxygen nitrogenseparation mainly depends on the nature of cation, as one can see from the highercoefficient values that Li is the best one can be used for adsorption and then Ca morebetter as compared to Na.