Synthesis And Structure Control Of Silicoaluminophosphate Molecular Sieves In Low Dosage Of Solvent And Their Catalytic Performance

Silicoaluminophosphate?SAPO?molecular sieves are one of the important member of zeolite material,they have been widely used as a heterogeneous catalyst in the chemical industry.The ionothermal and solvent-free synthesis method possess many interesting features and potential advantages over the traditional hydrothermal/solvothermal synthetic route of zeolites.For example,owing to the low dosage of solvent in the reaction system,zeolite synthesis can take place at ambient pressure,eliminating safety concerns associated with high hydrothermal pressure.More importantly,this synthesis route can reduce waste generation and increase zeolite yield.In addition,molecular sieve materials with nano-sized and hierarchical structure have gradually become a research hotspot in recent years due to their larger exposed surface and faster mass transfer efficiency than conventional microporous zeolite crystals.In this paper,the ionothermal/solvent-free method combined with the seed-assisted and mechanochemical effect was used to study the synthesis,structure control and catalytic performance of SAPO molecular sieves with various topologies in low dosage of solvent.The main research contents are as follows:1.The SAPO-5 molecular sieve nanosheets with AFI topology was efficiently synthesized by microwave radiation ionothermal method with low-dosage eutectic mixture as solvent and structure directing agent.The synthesized samples were characterized by XRD,SEM,N₂ physical adsorption/desorption and NH₃-TPD techniques.The effects of different hydrogen bond donors,microwave heating power,temperature,time,seed crystal and mechanochemical pretreatment on the synthesis of SAPO-5 zeolite were thoroughly discussed.The results show that the different hydrogen bond donors,microwave heating power and time have a great impact on the morphology of SAPO-5 zeolite.The typical hexagonal petal-like SAPO-5 crystals assembled by nanosheets were synthesized by induction of special seed crystals.The results of N₂ physical adsorption/desorption showed that the synthesized AFI molecular sieves belong to hierarchical SAPO-5 molecular sieve containing microporous-mesopore-macroporous structure.Under optimized synthesis conditions,the utilization efficiency of inorganic atoms of product is up to 93.4%.Most importantly,the SAPO-5 nanosheets catalyst prepared by improved ionothermal mothod exhibits superior catalytic performance in alkylation of benzene with benzyl alcohol than the traditional hydrothermally synthesized counterpart.This simple and efficient method provides a new avenue for the preparation of other high performance hierarchical nanostructured molecular sieve catalysts.2.The nano-sized SAPO-34 molecular sieves with high crystallinity and yeild were synthesized using a low-dosage morpholine as structure-directing agent under solvent-free reaction system assisted with seed crystals activated by physical or chemical treatment methods.XRD,SEM,N₂ physical adsorption/desorption and NH₃-TPD techniques were used to characterize the synthetic samples.The effects of seed and mechanochemical pretreatment on the synthesis,morphology and texture properties of SAPO-34 zeolite were discussed in detail.The results show that in a solvent-free system,the introduction of activated seeds can significantly reduce the size of zeolite crystals,increase the surface area of materials,which is good for preparation of hierarchical SAPO-34 molecular sieves with nano-sized structure.On the other hand,mechanochemical pretreatment not only affect the morphology and size of zeolite crystals,but also significantly improve the crystallinity of products.In addition,nano-sized SAPO-34 zeolites exhibit superior MTO performance with about three-times prolonged catalytic lifetime compared to the micron-sized SAPO-34 catalysts.3.The hierarchical SAPO-11 zeolites with nano-sized structure have been synthesized with the assistance of activated seeds under solvent-free conditions by using diisopropylamine as the organic template,and the optimum synthetic conditions of this material were refined.XRD,SEM,TEM and N₂ physical adsorption/desorption characterization were used to investigate the effects of seed crystals and mechanochemical pretreatment on the synthesis,morphology and texture properties of SAPO-11 zeolite.The results indicate that in a solvent-free system,the introduction of seeds activated by acid or milling treatment methods has a positive effect on the crystallinity,surface area and mesoporous volume of SAPO-11zeolite,which is contribute to synthesizing nano-sized SAPO-11 zeolite with hierarchical structure.On the other hand,mechanochemical pretreatment has an effect on the morphology and crystal size of zeolite,the surface of crystal changes from rough to smooth,and the crystal size distribution is more uniform.In addition,the combined effect of seed crystals and mechanochemical pretreatment not only significantly improves the crystallinity of zeolite,but also facilitates the formation of mesoporous structure,which provides a reference for the synthesis of hierarchical molecular sieve catalysts.