Synthesis Parameters And Crystallization Mechanism Study Of NaA Zeolite Prepared With Different Aluminum Sources

NaA zeolite, with the general formula Na96[(AlO2)96(SiO2)96]-216H2O, is one of earliest artificial molecular sieves. Due to its ordered molecular-sized pores and excellent ion-exchange performance, it is widely used as detergent builders, adsorbents and metal ion carriers. Recently, NaA zeolite membranes have been developed and applied in ethanol dehydration, which showed broad application prospects.Previous studies revealed that the adsorption and separation capability of NaA zeolite depended, to a large extent, on its size distribution and crystallinity. Hence, it is vital to find appropriate synthesis parameters for preparing NaA zeolite crystals with uniform size and high crystallinity. On the basis of other’s researches, we synthesized NaA zeolite with different aluminum sources and investigated the influences of different factors, such as alkalinity, crystallization temperature, crystallization time, on the crystallization rate and end products morphology in respective reaction systems. Further, we discussed the growth process and phase transformation pathway of NaA zeolite crystals.To avoid the interferences of impurities and increase reaction activity, the Al2O3-2SiO2dry gel powder was prepared by means of sol-gel method with tetraethyl orthosilicate (TEOS) as silicon source and aluminum chloride, aluminum sulfate and aluminum nitrate as aluminum sources, respectively. Then the NaA zeolite crystals with uniform size distribution were synthesized by the dry-gel conversion method (DGC). The end crystalline products were characterized by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR) and scanning electron microscopy (SEM) and as a result, the optimal parameters were determined, correspondingly. The results showed that when aluminum chloride was used as the aluminum source, NaA zeolite, which had high purity and uniform crystal morphology, could be synthesized in a wide range of alkalinity, crystallization temperature and crystallization time. Based on the experiments, the optimal parameters were determined as follows: crystallization temperature=90℃, crystallization time=4h, n(H2O)/n(Na2O)=30, n(Na2O)/n(SiO2)=3.0. Under this condition, NaA zeolite with a uniform size of2μm was obtained. However, when aluminum sulfate was used as the aluminum source, the end crystalline products were sensitive to the synthesis conditions. And NaA zeolite often precipitated together with sodalite. Based on the experiments, the optimal parameters were determined as follows: crystallization temperature=90℃, crystallization time=2h, n(H2O)/n(Na2O)=35,n(Na2O)/n(SiO2)=2.0. When aluminum nitrate was used as the aluminum source, well-crystallized NaA zeolites could only be synthesized with relatively low alkalinity and crystallization temperature. Similarly, its optimal parameters were finally determined as follows:crystallization temperature=85℃, crystallization time=2h, n(H2O)/n(Na2O)=40, n(Na2O)/n(SiO2)=2.0.By contrast, the crystallization rate was fast when using aluminum sulfate and aluminum nitrate as the aluminum source and the NaA zeolite in final products was prone to transform to more stable phases. While, when using aluminum chloride as the aluminum source, the crystallization rate was relatively moderate. It was proposed that the oxyanions (SO42-、NO3-) in the reaction system could promote the formation of certain secondary building units (SBU) by their electrostatic interactions with the zeolite framework and thus accelerate the crystallization process.