Study On Synthesis Of Low-Silica X Zeolite And Its Secondary Crystallization

Low-silica X-type molecular sieves ?LSX? are widely used for gas separation because of their unique porous structure, excellent properties for ion exchange and adsorption separation. Pressure swing adsorption is an important separation technology for air separation and has been widely emploited in industry. To improve the efficiency of separation, one of the key issues is choosing one kind of excellent molecular sieve adsorbent. Presently, Li+ exchanged LSX molecular sieves are the best adsorbent because it has the highest separation efficiency and performance. Therefore, synthesis of LSX molecular sieves with high purity and high exchange capability is of great importance in industry. Moreover, with the development of the industry, the amount of molecular sieves used is largly growing. This accordingly produces abundant waste molecular sieves, which, if not properly treated, will cause severe environmental problem. Therefore, to use the waste molecular sieves again is also an important issue of research.The research aims of this thesis are to synthesize LSX molecular sieves with improved crystallinity and to reuse the wast molecular sieves. To these ends, this thesis systemetically optimizes the two-step, hydrothermal route used for their synthesis using sodium silicate and sodium aluminate as raw materials. The main research contents and conclusions were outlined as below:1?LSX molecular sieves were synthesized in a Na-K system via a two-step hydrothermal process, that is, aging at a low temperature and crystallization at a high temperature. This thesis detaily investigated the effects of ?Na₂O+K₂O?/SiO₂ ratio, H₂O/?Na₂O+K₂O? ratio, aging temperature, crystallization temperature and time on the crystallinity of the molecular sieves. The synthesized samples were characterized using X-ray diffraction analysis ?XRD?, scanning electron microscope ?SEM?, Fourier-transform infrared spectroscopy ?FT-IR?, and static, saturated adsorption of water. These characterizations indicates that the optimum conditions for the synthesis of LSX molecular sieves with high crystallinity are as below:SiC/Al₂O₃=2, ?Na₂O +K₂O?/SiO₂=3.75, Na/NaK=0.77, H₂O/?Na₂O+K₂O?=18, aging temperature 70 ?, aging time 24 h, crystallization temperature 100?, and crystallization time 5 h. The products synthesized under these optimun conditions have a uniform size and clear outline with no impurity phase produced. These results achieved pave the way for large-scale synthesis of LSX molecular sieve that are essential for industrial applications.2?From the viewpoint of reutilization of waste molecular sieve, this thesis further invetigated the secondary crystallization of LSX molecular sieve. To this end, industrial LSX sieve was first dissolved in a hydrochloric acid solution, forming SiO₂ colloids, and then sodalite and NaA were hydrothermally synthesized from the colloids by adjusting their alkalinity ?uisng NaOH solution? to a suitable range. The phases, skeleton structure, and morphology of products were characterized using XRD, SEM, and FT-IR specscopy. Experimental results show that under stuitable alkalinity condition, different kinds of molecular sieves can be synthesized if using the colloids of acid ?HCl?-acidified LSX as the precursors. When the Na₂O/SiO₂ and H₂O/Na₂O ratios were in the range of 3.75?4.10 and less than 22,respetively, the resulting products were sodalite manily. When the Na₂O/SiO₂ ratio and the H₂O/Na₂O ratio were in the range of 3.61? 2.16 and 25?41.8, respectively, the main products were NaA zeolite. When the H₂O/Na₂O ratio is less than 14 or greater than 41.8, the resulting products are amorphous product. This is because high basicity may dissolve crystal nucleus formed, and in contrast, crystal nucleus can not form in low alkalinity. These results achieved may be used to transform waste, unusable LSX into other valuable products.