Preparation And Application Of Silica-aluminum Zeolites Strengthened By High-gravity Technology

Silica-alumina zeolites are widely used as the common catalyst,adsorbent and ion-exchange material.In industry,silica-alumina zeolites are usually prepared by hydrothermal method,and the synthesis system is mostly sodium-containing viscous hydrogel system.In the traditional stirring tank reactor（STR）,the low interphase（liquid phase and gel phase）mass transfer efficiency of this synthesis system and the slow intrinsic nucleation rates of zeolites could lead to the long synthesis period,abundant usage of organic amine template,unsatisfactory performance of zeolites and high production cost.To solve these issues,high-gravity technology is adopted in this paper.The purpose of this paper is to optimize the dispersion effect of gel phase in the liquid phase,enhance the interphase mass transfer efficiency and the nucleation rate of zeolites through high-gravity technology,reveal the strengthening mechanism of high-gravity technology and its influence law on the dispersion-transport-crystallization of multiphase viscous system,study the influence of microenvironment on the growth process and performance of silica-alumina zeolites,and finally invent a novel green synthesis technology of high-performance silica-alumina zeolites with independent intellectual property rights.The above research provides technical support for the industrial preparation of silica-aluminum zeolites in China.The main research contents and conclusions of this paper are as follows:1.The microenvironment in the synthesis process and its influence on the growth process of silicon-aluminum zeolite were revealed.It was found that in the hydrogel system,the sizes of gel particles（silicoaluminate aggregates）dispersed in the liquid phase reached micrometer ranges,and thier distributions were very inhomogeneous.Results indicated that the gel-liquid interfaces were the main nucleation sites of zeolites.In addition,it was found that hydrated sodium ions could transport from liquid to gel phase at the initial period of nucleation,and the content at the gel-liquid interfaces was the most.Simulation results indicated that hydrated sodium ions can induce the dehydrogenation of Si-OH to produce Si-O·,which can significantly reduce the nucleation barrier of zeolites.2.The influence mechanism of the microenvironment of multiphase viscous system strengthened by high gravity on nucleation and crystallization was revealed.The rotating packed bed（RPB）was adopted to treat the synthetic system of silicon-aluminum zeolites.The dispersion laws of gel particles in liquid phase under different RPB operating conditions were studied,and the empirical correlation was established.The optimized operating conditions of RPB used in this experiment were determined,including the rotating speed of 2500 rpm,the premixing time of 30 min and the circulation rate of 300 m L/min.On this basis,it was found that RPB-treatment can significantly increase the gel-liquid interfaces and hence improve the transfer rate and amount of hydrated sodium ions from liquid to gel phase,which significantly improved the growth rate of silicon-aluminum zeolites.Typical silicon-aluminum zeolites（Y,ZSM-5 and A zeolites）prepared by RPB had larger specific surface area and pore volume compared with traditional stirred tanks.On the other hand,it was proved that RPB can enhance the generation of nano/micro-bubbles by light scattering technology.It can be deduced that the main reason for the generation of nano/micro-bubbles in RPB was the air entrainment phenomenon caused by the impact of droplets or liquid columns on the wall of RPB or the surface of wire mesh packing.Furthermore,RPB was applied to the benzoic acid cooling crystallization as the model system.It was found that nano/micro-bubbles generated in RPB could be used as crystal seeds to nucleate and crystallize benzoic acid at low supersaturation,which indicates nano/micro-bubbles in the RPB can promote the nucleation and crystallization process.3.The synthesis process of high-gravity was developed.ZSM-5 zeolites were prepared by different operating processes,including RPB premix-static crystallization,RPB premix-dynamic crystallization,STR premix-static crystallization,and STR premix-dynamic crystallization.The effects of enhanced mass and heat transfer efficiency on the physicochemical properties（particle size distribution,crystallinity,pore structure,specific surface area and acidity）of ZSM-5 zeolites were studied.Results showed that enhancing mass and heat transfer efficiency in the premixing and crystallization process both can optimize the physicochemical properties of zeolites.Compared with other products,the product synthesized by RPB premix-dynamic crystallization exhibited the smallest average particle size（～300 nm）,the largest specific surface area（520.9 m²/g）,hierarchical structure and the most acid content（weak acid content:0.33 mmol/g;strong acid content:0.46mmol/g）.While adopting it for catalyzing C4-olefin cracking reaction,the conversion of C4 olefins was 65%and the yield of propylene was 30%at 44 h.Furthermore,results showed that enhancing mass and heat transfer efficiency can improve the utilization rate of organic amine template,which can effectively reduce the production cost of silicon-aluminum zeolites.4.The structure-activity relationship and application of zeolite-based catalysts were studied.ZSM-5@Mn O_xcatalyst was designed and prepared for the first time based on the characteristics of silicon-aluminum zeolites,and it was applied to catalyze the selective oxidation of benzyl alcohol to benzaldehyde.Results showed that the active site of this catalyst was the oxygen vacancy on Mn O_x.The catalytic activities of ZSM-5@Mn O_xcatalysts were regulated by adjusting the relative content of sodium ion and acidic hydrogen proton on ZSM-5 supports.It was found that sodium ion was favourable to the coating of Mn O_xon the ZSM-5 supports but unfavourable to the generation of more oxygen vacancies on Mn O_x,while acidic hydrogen proton was unfavourable to the coating of Mn O_xon the ZSM-5 supports but favourable to the generation of oxygen vacancies.Results showed that oxygen vacancy can be generated by the dehydration between hydroxyls on Mn O_xand hydrogen protons on ZSM-5.In order to further improve the catalytic oxidation activity,ZSM-5@Mn O_xcatalyst was then treated at 270℃in N₂for the further dehydration of Si-OH group on the ZSM-5 support with the residual hydroxyl group on Mn O_x.Compared with other reprorted Mn-based catalysts,the catalyst prepared in this paper showed excellent catalytic performance.