Seed-assisted Template-free Synthesis Of Nano-sized ZSM-5 With Related Investigation Of Mechanism

The unique pore structure of ZSM-5 molecular sieve（0.56×0.53 nm straight channel and0.53×0.51 nm zigzag channel）and abundant acidic sites make it widely used in the field of industrial catalysis.However,the strong acidity and long channels of the micron ZSM-5molecular sieve which limit the diffusion of reactants and products generate by-products blocking the pores and causing the catalyst to quickly deactivate.The nano ZSM-5 molecular sieve,due to its large surface area,more active sites and shorter diffusion channels,can improve its catalytic activity and stability,so it has attracted more and more researchers’attention.At present,in the synthesis process of nano-ZSM-5 molecular sieve,by adding organic ammonium cations（TPA⁺is more common）,products with high yield and high crystallinity can be obtained.Due to the high cost of the template and the subsequent treatment of the molecular sieve to remove the template in the pores,the process was regarded as not only consuming energy but also polluting the environment,so the template-free synthesis of nano-ZSM-5 molecular sieve has become the focus.However,relying only on a very limited amount of Na⁺as a structure-directing agent makes the molecular sieve nucleation and growth activation energy relatively high,and it is difficult to form a large number of crystal nuclei in a short time.The method introducing an appropriate amount of seed crystals into the synthesis system to provide existing crystal nuclei is the most effective and fastest solution to existing problems.In this paper,nano-silicalite-1 seed crystals were self-made firstly,through XRD and SEM characterization methods,the influence of water content,temperature,alkalinity and template content on seed crystal size were explored,and finally optimal seed was screened out.Then the ZSM-5 molecular sieve was prepared by the seed-assisted one-stage crystallization method.On the one hand,a suitable range was determined for the crystallization time of the two-stage method,and on the other hand,it was compared with the product synthesized by the seed-assisted two-stage method.Finally,through a series of characterizations,the best product of the two-stage synthesis process was selected,and an in-depth study of its mechanism was also conducted.On the basis of this crystallization process,a comparative experiment of calcined seed crystals with uncalcined seed and widening the silicon to aluminum ratio were carried out.The main conclusions are as follows:1)Under the molar ratio of 2Si O₂:0.2～0.5TPAOH:8Et OH:20～120H₂O:0～0.2Na₂O,by adjusting the content of the template,the content of water,adding a certain amount of inorganic alkali Na OH and changing the crystallization process,a series of different sizes of silicalite-1 seed crystal were made.The effects of various factors on the size of the seed crystal are as follows:Water content is the most important factor affecting the silicalite-1 seed crystal.The crystal size of the seed is positively related to the water content.When the water to silicon ratio is 10,the seed crystal of 60～90 nm was prepared successfully.The Na OH can affect the crystallinity of the seed crystal.When the water content is high（water to silicon ratio is 60）,as the alkalinity increases,the size of the seed crystal will increase（the crystallinity first increases and then decreases）;When the water content is low（water to silicon ratio is 10）,the size of the seed crystal will not increase,but the surface of the crystal is smooth and accompanied by an increase on crystallinity.The two-stage crystallization method（first low-temperature pre-crystallization for a period of time）can also reduce the grain size,and as the pre-crystallization temperature rises,the size of the seed crystal also increases.When synthesizing nano-silicalite-1 seed crystals,inorganic bases cannot replace the role of the template TPAOH.The size of the seed crystals depends on the content or concentration of the template.When a certain amount of inorganic alkali Na OH was added,the more templating agent content in the system,the more uniform and smaller size of the seed crystal.2)A series of ZSM-5 products were prepared under the condition of molar composition of 8～14Na₂O:80Si O₂:2Al₂O₃:1600～3500H₂O,using nano-silicalite-1 seed assisted one-stage crystallization method at two different crystallization temperatures of 220℃and 180℃,and exploring the crystallization rate of crystals at different temperatures.The results showed that the crystallization rate of crystals at different temperatures is completely different.When the crystallization temperature is 180℃,the product still has been amorphous after 7 hours of crystallization and reaching the crystallinity about 90%as the crystallization time was extended to 12 hours,moreover,with the extension of the crystallization time,the agglomeration phenomenon became smaller;when the crystallization temperature is 220℃,the product after 1 h of crystallization is amorphous,while the crystallization time is extended to 2 h,the crystallinity of the synthesized product can reach 90%,and the crystal grown into submicron agglomerates about 500 nm,indicating that the crystallization rate is very fast at this temperature and serious agglomeration will occur.By adding a certain amount of seed crystals in the system and adopting a one-stage crystallization method,the crystal size of the obtained product is obviously reduced,but they are all agglomerates of nano-ZSM-5molecular sieve.It shows that the one-stage method which only changes the crystallization time cannot obtain the nano ZSM-5 molecular sieve with high dispersion and high crystallinity at the same time.However,compared with commercial ZSM-5 molecular sieve,the results of N₂adsorption and desorption of nano-aggregates showed that the total pore volume of aggregates increased,indicating that nano-ZSM-5 molecular sieve has abundant pores.3)The seed-assisted two-stage method（high temperature 220℃-low temperature180℃）was used to synthesize nano-ZSM-5 molecular sieve under the condition of molar composition of 8～14Na₂O:80Si O₂:2Al₂O₃:1600～3500H₂O.Two control experiments were set up at 220℃corresponding to 1h and 4h.The ZSM-5 molecular sieve with different morphology was obtained then crystallized at 180℃for different time.After the crystal is crystallized in the high temperature section,the crystallization rate and the crystallinity change in the low temperature section are investigated.In addition,the mechanism involved in the seed-assisted two-stage crystallization process has been deeply studied,and related experiments have been done on the synthesized products of roasted seed crystals.Finally,by adjusting the crystallization time and alkalinity under this synthesis system,an attempt was made to synthesize a ZSM-5 molecular sieve with a higher silicon-to-aluminum ratio.The specific conclusions are as follows:The seed-assisted two-stage method of high-low temperature successfully prepared nano-ZSM-5 molecular sieve Z-1-6（size of 70～90 nm）with high crystallinity,good dispersion and uniform crystal size.Compared with the commercial micron ZSM-5 molecular sieve,the two-stage process product has an increase in surface defects and significantly reduces the B/L acid ratio.If calcined seed crystals were added to the system,the inducing effect will be weakened due to the lack of structure guidance of the template at the initial stage of the two-stage crystallization.When the crystallization time was extended to 4h at low temperature 180℃,high crystallinity and highly dispersed submicron 200×500nm crystals can be obtained.High-temperature crystallization product Z-2-0 by adding uncalcined seed was aggregates,but the product obtained by adding the calcined seed was a hexagonal prism crystal about 300×700 nm.Compared with the commercial micron ZSM-5,the total acid content did not decrease,but due to the reduction of its crystal size,the L acid content increased significantly,and the B/L acid ratio decreased obviously.In the template-free system,the ZSM-5 molecular sieve with high crystallinity and silicon-to-aluminum ratio≥100 could not be prepared by adjusting the crystallization time,temperature and alkalinity,and the obtained products all contained a large amount of amorphous or heterocrystalline phases.