Study On The Synthesis Of Low-Silica X Zeolite

Pressure swing adsorption?PSA?is the most important technology of nitrogen and oxygen separation,but the development of PSA technology mainly depends on the adsorbents in the long run.The zeolite adsorbent is widely used in the gas separation industry.Low-silica X?LSX?zeolite [ n?SiO₂?/n?Al₂O₃?= 2.0-2.2 ] with maximum content of exchangeable cations and large pore size diameter is more suitable for selective adsorption of N₂ than zeolite A.Furthermore,Li-LSX zeolites prepared by Li+ exchange with LSX zeolite have higher N₂ capacities as well as higher N₂/O2 selectivity.However,the pure LSX could be obtained only with in a very narrow phase zone range.Therefore,it is difficult to synthesize pure LSX zeolite on a large scale.Based on the conventional hydrothermal synthesis of LSX zeolite,single sodium source method,dual sodium source method and microwave heating method were used to synthesize LSX zeolite.The main conclusions basically consist of the following three sections: 1.The preparation of LSX zeolite using single sodium source methodLSX zeolite synthesized by hydrothermal method usually needs forming crystal nucleus at a low temperature,and then crystal growing at the high temperature.The influence factors were investigated,such as crystallization conditions,aging conditions,n?H₂O?/n?NaK?ratios,etc.The obtained samples were characterized by XRD,FT-IR,SEM,Raman spectrum,N₂ physisorption and chemical analysis techniques.The results showed that the presence of K+ has a crucial role in the synthesis of LSX zeolite.In addition,the aging conditions,crystallization conditions and the addition of seed solutions have important influence on phase purity and crystallinity of LSX zeolite.However,pure LSX could be obtained using single sodium source method only within a rather narrow synthetic range,especially a narrow n?H₂O?/n?NaK?ratio?16¹7?.2.The preparation of LSX zeolite using dual sodium source methodLSX zeolites were hydrothermally synthesized using Na₂Co₃ as an auxiliary sodium source with the aim of the fine adjustment of the ratios of n?Na?/n?NaK?and n?H₂O?/n?NaK?in sodium-potassium system.The results showed that the pure LSX zeolites could be successfully obtained using dual sodium source with the ratios of n?Na₂Co₃?/n?Na2O?=0.5 and n?H₂O?/n?NaK?= 11-16.Compared to the single sodium source method,the dual sodium source strategy could maintain a stable pH value in a broader composition range due to the weak alkalinity of sodium carbonate,and can broaden obviously synthetic phase region,therefore make it easier to synthesize the pure LSX zeolites.And it can provide reference for the industrial production.3.The preparation of LSX zeolite using microwave heating methodLSX zeolites were synthesized via microwave and conventional hydrothermal methods.The three important factors were also investigated,such as crystallization conditions,aging conditions and n?H₂O?/n?NaK?ratios.The results showed that LSX can be prepared under the lower temperature by microwave heating and the products exhibit the smaller zeolite crystal with relatively narrower particle size distribution using the microwave heating method.Compared with the conventional heating method,shorter heating time were required for the synthesis of pure LSX zeolite by microwave heating.