Microwave Hydrothermal Synthesis And Performance Of NaA Zeolite Monolithic Adsorbent

Humidity control has very important significance for improving living conditions, developing the production technology and controlling the quality of products. Molecular sieve desiccant wheel,namely monolithic adsorbent, as the representative of the third generation of desiccant technology, has a broad application prospects. In this paper, in order to improve the saturated adsorption ratio and the early adsorption rate of NaA zeolite monolithic adsorbent, a study on synthesis and performance of NaA zeolite monolithic adsorbent obtained by microwave hydrothermal method was carried out.Firstly, NaA zeolite was synthesized by microwave hydrothermal method with sodium silicate, sodium aluminate and sodium hydroxide as the basic materials. Then, with the same raw materials and synthesis technology, and the synthetic NaA zeolite as seed crystal, NaA zeolite monolithic adsorbent was in-situ synthesized on the surface of honeycomb ceramic matrix. Effects of Si/Al mole ratio, crystallization technology including the crystallization temperature and time, the seed coating and the honeycomb matrix molding process on performances of NaA zeolite and its monolithic adsorbent were systematically discussed. The synthetic products were characterized using X-ray diffraction(XRD), and scanning electron microscopy and energy dispersive X-ray spectroscopy(SEM-EDS), and their adsorption and desorption performances were evaluated by static adsorption, thermogravimetry(TG) and temperature programmed desorption(TPD).The experimental results indicated that the honeycomb ceramic matrix could retain its shape well(no hole-blocking, no falling apart and keeping white) after sequentially impregnated with mass fraction of 20% silica sol and 2% calcium chloride solution. NaA zeolite monolithic adsorbent could be successfully synthesized with seed coating which could inhibit crystal transformation; Under microwave conditions, in the range of temperature of 120~180℃, Si/Al ratio of 0.482 to 0.482 and reaction time of 25~55min, NaA zeolite monolithic adsorbent could be successfully obtained. Low temperature and short crystallization time resulted in incomplete crystallization, high temperature and long crystallization time leaded to crystal transformation. The optimum preparation conditions were the crystallization temperature of 160℃, Si/Al ratio of 0.964, and the crystallization time of 45 min.XRD spectra showed that NaA zeolite and its monolithic adsorbent could be successfully and rapidly synthesized by microwave hydrothermal process within 25 and 45 min, respectively. SEM images indicated that, in comparison with industrial-grade NaA zeolite(1.5-3 μm), the particle size of synthetic NaA zeolite dramatically decreased to about 0.9 μm. Similarly, compared with industrial-grade NaA zeolite coated by impregnation method, the NaA zeolite synthesized by in-situ method could be evenly and densely dispersed in the honeycomb ceramic matrix. Static adsorption curves showed that the saturated adsorption honeycomb ceramic matrix. Static adsorption curves showed that the saturated adsorption ratio and the early adsorption rate of NaA zeolite monolithic adsorbent synthesized by microwave hydrothermal process were both much higher than that by impregnation method. TG and DTG curves demonstrated that the desorption performances of synthetic NaA zeolite and its monolithic adsorbent were slightly better than that of industrial-grade one, which was embodied in a lower peak temperature under the maximum weight loss rate. TPD curves revealed that the desorption activation energies of synthetic NaA zeolite and its monolithic adsorbent were lower than that of industrial-grade one.