Study On The Green Controlled Synthesis And Formation Mechanism Of The Nano-aggregated Zeolites

After the crystal size of the nano-aggregated zeolites reduced to only a few unit cell,the corresponding zeolites will possess unique properties of many nano-materials.The remarkable characteristics of nanometer zeolites have strongly promoted the application of zeolites in heterogeneous catalysis,ion exchange,adsorption,separation,sensing,biomedicine and other fields.Recently,special attention has been paid to nanometer-sized zeolites due to their exclusive properties,such as large external surface area and activities,more accessible active sites as well as shorter diffusion pathways,compared with micrometer-sized zeolite crystals.To date,only a few approaches for the fabrication of nanometer-sized zeolite are developed.These include the clear alumino-silicate solutions method with the assistance of an organic template,space-confined synthesis and the organic-template-free synthetic route in an alkali metal-containing or zeolite crystal seeds system respectively.However,the organic-template and space-confined synthesis approaches,due to the using of organic templates and polymer additives,which are environmental unfriendly,non-recyclable,and expensive,are highly complex in preparation routes and also expensive in producing nanosized zeolites.The most important is the formation mechanism of nanosized zeolites is uncertain.Aiming at solving the above problems in the synthesis method of nanosized zeolites,in this paper,a novel organic template-free green synthetic nano zeolites method is established:in-situ-post-treatment method.This method divides the synthesis of nano zeolites into two separated stages which are nucleation and crystal growth.Embryonic nuclei precursors are post-treated to prepare nano-sized zeolites,then the performance of the synthetic materials is characterized and the reaction mechanism is tracked,and then fed back to the cycle of the synthesis scheme design,until the nano-zeolites with uniform phase morphology and uniform size are obtained.Nano-aggregated FAU and nano-SOD zeolites are prepared by in-situ-post-treatment method.Firstly,embryonic precursor with high silicon-aluminum ratio and uniformly dispersed morphology was in-situ synthesized through controlling the mixing way?ice-water bath?of the silicon source and the aluminum source,and controlling aging conditions?temperature,time,and drying method?.Secondly,the nano-aggregated FAU and nano-SOD zeolites were synthesized by post-treating.In this process,the post-treatment solution concentration,crystallization time,and crystallization temperature were investigated.The physical and chemical properties of the products were investigated by XRD,NMR,SEM,TEM,IR,and N₂adsorption and the synthesis mechanism was explored.It is interesting to realize the mutual transformation between the non-porous SOD and the porous FAU zeolites by simply controlling the concentration of the post-treatment agent and the post-treatment reaction conditions.The discovery of this phenomenon provides an in-depth understanding of the zeolites pore formation process and crystal growth process.The study of the mechanism depends on the theoretical foundation and experimental data,and has important academic and practical significance.Nano X-type and aggregated nano Y-type zeolites were synthesized by in-situ-post-treatment method.The FAU zeolites are divided into two phases:X-type and Y-type.By adjusting the silicon-to-alumina ratio,pH,concentration,and aging conditions?temperature,time,and drying method?of the reaction solution,a uniformly dispersed,embryonic nucleus precursor with a high silica-alumina ratio was prepared.With different post-treatment fluids?concentrations,types?,pure phase aggregated nano X-type and Y-type zeolites were successfully synthesized under different post-treatment conditions.The physical and chemical properties of the products were investigated by XRD,NMR,SEM,TEM,IR and N₂ adsorption and the synthesis mechanism was explored.The study finds that NaOH solution is an excellent post-treatment fluid,and NaOH concentration and crystallization temperature are important parameters to control the zeolites phase and size morphology.NaOH solution has two functions in the zeolites formation process:one is as green template and pore-forming agent for pore formation;another one is as a dissolving agent and inhibitor which inhibits the formation of nano-zeolites further aging and re-growth.The dealkylation performance of the Y-type zeolites was measured using 1,3,5-TiPBz as a model compound.Compared with the microsized Y-type zeolites,the aggregated nano-zeolites synthesized in this paper showed a better catalystic performance in the macromolecular dealkylation.The reaction result showed an up to 99%conversion rate and good stability.It is proved that the nanometer zeolites synthesized in this paper have a large external surface area,many active sites,and are not easy to deactivate coking,and have a long service life.The in-situ-post-treatment method in this paper could also expand the preparation method of other zeolites.It has a high academic value and significance for exploring the new synthesis method and clarifying the synthesis mechanism of nano-zeolites.