Synthesis And Performance Of Rare Earth Ionexchanged NaA Zeolite Under Microwave Condition

The development and production of many new high-tech products need a low humidity environment for deep dehumidification. The rotary dehumidification system with NaA zeolite as adsorbent is especially suitable for deep dehumidification because of its unique structural design and excellent functional features. However, high desorption temperature(250°C)does not favor energy saving of dehumidification system. In this paper, in order to improve desorption performance, we carried out the research of rare earth(RE) modification to NaA zeolite. The research has a profound significance in theory research and practical application.Firstly, using lanthanum(La) and neodymium(Nd) as modifying agents, the RE ionexchanged NaA zeolites were synthesized by ion exchange method and microwave technology. The influences of synthetic conditions including RE ion concentration, pH, the mole ratio between rear earth ion(La3+, Nd3+) and sodium ion, and microwave reaction temperature on the exchange degree and the adsorption/desorption performance of the modified NaA zeolites were systematically discussed. Secondly, the structural and the composition of the modified products were characterized using X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FT-IR) and energy dispersive X-ray spectroscopy(EDS), and their adsorption/desorption performances were also tested by static adsorption, differential scanning calorimetry(DSC), thermogravimetry(TG) and temperature programmed desorption(TPD).The experimential results showed that, La3+ modification to NaA zeolite, it obtained lower exchanged degree(30.62%), retained the adsorption performance of adsorbent(slight increase), while its desorption performance improved a little. Nd3+ modification, it had higher exchanged degree(63.25 %) and an obvious improvement to the desorption of NaA zeolite. The complex modification of La3+ and Nd3+ would obtain a comprehensive adsorption and desorption performance.XRD and FT-IR results showed that, by RE modification, the crystallinity of NaA zeolite had a little decline because of lower diffraction peaks intensity, while there no fundamental change in the NaA zeolite framework. EDS analysis results indicated that the proportion between silicon, aluminum and oxygen in RE-modified NaA zeolite had no obvious change, suggesting no effect on NaA crystal cell. The existence of rear earth atom(La, Nd) and the significant reduction of Na indicated that there existed ion-exchange between sodium and rare earth elements.Static adsorption results showed that the early adsorption rate of RE ion-exchanged NaA zeolites were both higher than that of NaA zeolite. The saturated adsorption ratio was 26.57, 27.33, 17.53 and 19.46% for no-modified, La3+-exchanged, Nd3+-exchanged and La3+/Nd3+-exchanged NaA zeolite, respectively. TG results demonstrated that the desorption temperature of the RE-modified Na A zeolites decreased, especially one of La3+/Nd3+-exchanged NaA zeolite. TPD results revealed that the desorption activation energy was 84.94, 66.17, 62.11, 54.13 kJ·mol-1 for no-modified, La3+-exchanged, Nd3+-exchanged and La3+/Nd3+-exchanged NaA zeolite, respectively, which indicating that the desorption performance of RE-modified NaA zeolites improved greatly.