Template-free Synthesis Of Coal Fly Ash Based ZSM-5 Molecular Sieve With Related Investigation Of Mechanism

Fly ash as a solid waste is emitted in China at an annual rate of over 500 million tonnes（the highest in the world）.The main components of fly ash are SiO₂ and Al₂O₃,which account for about 80% of the fly ash mass and can be used as raw materials for the synthesis of zeolites.Among them,ZSM-5 molecular sieve is widely used in industrial production due to its special pore structure.Traditional hydrothermal synthesis method requires organic amine as template agents,but organic template agents are expensive and need to be removed,which causes energy consumption and environmental pollution and is not conducive to production.In this paper,fly ash was first activated by calcinating with alkali,and then the activated fly ash was heated with alkali in a water bath to extract silica-aluminium elements,and different low silica-aluminium ratio molecular sieves were successfully synthesized using purified fly ash.The ZSM-5 molecular sieves were synthesized by transforming different low-silica zeolites without organic amine templates and without crystalline species,and the effects of different additives on ZSM-5 were investigated.Finally,the mechanism of interzeolite transformation was investigated by a series of characterizations and comparative tests.The main results are as follows.（1）The optimum conditions for the activation of raw fly ash by high temperature alkali fusion were a calcinating temperature of 850 °C,a calcinating time of 2 h,an alkali to ash ratio of 1.25 and an activator of Na OH.The optimum conditions for low temperature alkali dissolution purification of activated fly ash are at a alkali concentration of 6 mol/L,a water bath temperature and time of 60°C and 40 min.The mechanism of fly ash removal is to make use of the difference in solubility of silica-aluminate and other impurities in the concentrated alkali solution,a solution rich in silica-aluminate can be obtained by filtration,and then silicaaluminate is precipitated by adjusting the p H of the filtrate,and high purity silica-aluminate can be obtained after centrifuging and drying,followed by the preparation of the low silica zeolites.（2）A series of ZSM-5 molecular sieves were synthesized by using fly ash based low silica zeolite Na A,hydrosodalite（HS）and Na A+HS hybrid zeolite as the parent phase zeolites.The effects of crystallization time,crystallization temperature,alkalinity,water content and additives on the preparation of ZSM-5 molecular sieves by transformation of different parent phase zeolites were investigated.The results showed that under the conditions of Na OH/SiO₂=0.3,SiO₂/Al₂O₃=30 and H₂O/SiO₂=1470,when HS and Na A+HS were used as the parent phase zeolites,pure phase ZSM-5 could be obtained at 160 ℃ for 48 h and 60 h,respectively;when Na A was used as the parent phase zeolite,the introduction of Na Cl（Na+/SiO₂= 0.4）or ethanol（Et OH/SiO₂=0.5）for 36 h at 160 °C to obtain pure phase ZSM-5with high crystallinity and high specific surface area.（3）A comparison of the synthesis of ZSM-5 from different parent-phase zeolites by interzeolite transformation showed that the synthesis time was the lowest（36 h）and the product had the highest crystallinity（106%）and acidity when Na A was used as the parent zeolite,due to the fact that Na A was thermodynamically easier to depolymerize to form more structural units than the other two parent zeolites,which led to the synthesis of ZSM-5.The particle size of the ZSM-5 synthesized using Na A and Na A+HS was similar（2-3 μm）,while the particle size of the ZSM-5 synthesized using HS was 1 μm.Furthermore,the products developed with the addition of Na Cl were both more acidic than those developed with the addition of Et OH.In addition,the mechanism of ZSM-5 preparation by transforming Na A was revealed by design kinetic experiments,which showed that:（1）a synergistically structural guiding effects of parent zeolite Na A and four-coordinate complexes of sodium ion with water and/or ethanol in the assembly of MFI framework,where 4,6-membered rings as common building units of Na A and ZSM-5 made the interzeolite transformation of LTA to MFI kinetically possible and actually feasible.（2）The parent zeolite is partially depolymerized in the alkaline synthesis solution and the depolymerized structural units are directed to produce ZSM-5,which facilitates the depolymerization of the parent zeolite（pulling the structural units from the parent zeolite into the solution）,then the final pure phase ZSM-5 could be formed.