Synthesis And Characterization Of FAU Zeolites And Membrane

Zeolite materials are a kind of crystalline inorganic microporous solids of aluminumsilicates. These materials have tunable uniform pores or cages, as well as variable chemicalcompositions, which have be widely used as catalyst and adsorbent in the fields of oil processing,petrochemical products and fine chemicals.FAU zeolites have natural mineral faujasite structure, their pore size is about0.73nm,belonging to large-pore zeolites. Such zeolites can adsorb small molecules, such as CO2, N2andCH4. It is noted that the high aluminum content in the framework of FAU (Si/Al ratio from1-1.5is NaX, from1.5-3is NaY) favors the adsorption CO2, due to the electrostatic interaction of thisquadrupolar molecule with the charge-compensating cations (usually Na+). Thus, FAU zeolitescan preferentially adsorb CO2.The particle size change from micron to nanometer causes changes in the nature of thematerial properties. Compared with micron size of zeolite crystals, nanocrystalline zeolites havemore large surface area, higher catalytic activity, faster diffusion rate, and more holes exposedon the outer surface. Therefore, the nanocrystalline zeolites attract more attendance in thetraditional application fields such as catalysis and separation. In addition, such zeolites can beprepared to film, fiber, self-supporting zeolite membrane, and used in photochemistry,electrochemistry and light electrical fields.Over the past20years, a variety of zeolite membrane materials have been prepared byusing inorganic porous substrates, which greatly expand the applications of the molecular sieveson separation of the mixture of gas and liquid. FAU-type zeolite membranes have been appliedfor CO2separation, the separation of CO2from exhaust gases in combustion process,theseparation of CO2from synthesis gas mixture, etc. In recent years, the syntheses and reproducibility study of defect-free FAU zeolitemembranes have aroused more interest. The synthesis of most FAU membranes involves usingNaY or NaX zeolite crystals coated in the basal surface, soaking in the silicon aluminate gel orsolution, hydrothermal treatment of the secondary growth making seed layer more dense, andgrowing into a membrane. Usually, in the synthesis of zeolite membrane, the seed layer decidesthe crystal structure of zeolite membrane, the secondary growth controls the density andthickness of the membrane, as well as the purity of crystal phase. In order to increase the qualityof FAU membrane and the synthesis repeatability, the researchers have made more efforts toimprove the seed coating quality and control the secondary growth. For examples, the use ofnano-sized crystal seeds and electrophoretic deposition method to improve the coverage of theseed, and reduce initial intergranular pores of the seed layer; the microwave heating technologyusing clarify synthesis solution to complete more unified crystallization process. However, thereare many problems for the preparation of FAU membranes, such as the repeatability of syntheticresult, the control of micro defects and zeolite purity aspects, etc.In this thesis, we have rapidly synthesized a series of FAU nanocrystalls with differentSi/Al ratios and without organic template agent under the hydrothermal system. The influence ofvarious synthetic factors, such as the crystallization temperature and time, the raw material,theSi/Al ratio, the alkali content and water content, on the synthesis of FAU zeolites has beendiscussed in order to optimize the best synthetic conditions of FAU nanocrystals. The researchresults show that aluminum sheet and silicon sol are the best materials for the synthesis of purephase of FAU nanocrystals, other aluminum (such as NaAlO2) and silicon sources (fumed silica,sodium silicate) favor the formation of NaP zeolite or mixture phases. The pure phase of FAUnanocrystals can be synthesized from the reaction mixture of12SiO2:Al2O31:17Na2O:675H2O,crystallized under100℃for12hours, and the minimum particle size of the product is about100nm. The higher temperature (>100℃) and more crystallization time (more than12hours) resultthe synthesis of NaP zeolites. In the synthesis, the increasing Si/Al ratio and water content willcause the increase of crystal size while the increasing alkaline content will decrease the crystalsize of the product.Employing the as-synthesized FAU nanocrystals as seeds and alumina tube as the basal,FAU molecular sieve membranes have been prepared by using dip-coating and the secondarygrowth methods prepared (The influence of various synthetic factors, such as the Si/Al ratio, thealkali content, the water content, on the growth of FAU membranes have been studied in order tofind the optimum synthetic condition for FAU membranes. The results show that the increase of Si/Al ratio and water content, the decrease of alkali amount the increase of will enhance thesmoothness and continuity of FAU membrane. The preparation of FAU membranes with highquality will provide the opportunity to further explore the gas separation of FAU zeolitemembraness.