The Synthesis And Catalytic Performance Of New Core-shell Structured ZSM-5@MCM-41

Core-shell structured composite molecular sieves with mesoporous coated microporous are the new functional materials.It has zeolite molecular sieves’ performances of high crystallinity and high stability,and the same time it has mesoporous molecular sieves’ developed pore structure,hence it can accelerate the transfer rate of matters.The key factor of synthesis of core-shell structure composite molecular sieves is the design of core phase structure unit.The surface of the core phase structure unit determines the success of synthesis of core-shell structure composite molecular sieves and the performance of the stability of the core-shell structure composite molecular sieves.Therefore design and synthesis the core unit with silicon surface hydroxyl structure and suitable for growth of mesoporous shell are the key factors of synthesis of core-shell structure composite molecular sieves with high hydrothermal stability.In this paper,we used four different alkali solutions as the zeolite core unit’s processing solution in the early period,the surfactant of CTAB as mesoporous shell’s template agent,High modulus sodium silicate Na₂O·（3.3）SiO₂Na₂O as the inorganic silicon source,successfully synthesized the core-shell structure composite molecular sieves with the performance of high hydrothermal stability by sol-gel and hydrothermal synthesis.The materials’ composition and structure and the morphology are analyzed by using X-ray powder diffraction（XRD）,N₂ adsorption-desorption（BET）,fourier transform infrared spectroscopy（FT-IR）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,and other characterization methods,while we also studied and analyzed the material’s performances of hydrothermal stability and catalytic activity.The main research contents are as follows,（1）.the synthesize of four different core-shell structured composite molecular sieves.Using NaOH strong alkaline solution as the core unit’s processing solution,sodium metasilicate Na₂O·（3.3）SiO₂Na₂O as the inorganic silicon source of shell,surfactant CTAB as the mesoporous structure’s guide agent,successfully explored the core-shell structure composite molecular sieves’ synthesis experiment conditions,NaOH alkaline solution’s concentration is 0.6mol/L,alkali treatment time is 30 min,alkali treatment temperature is 80 ℃,The optimal crystallization pH value of mesoporous shell is 9.5,the optimal crystallization temperature is 110℃,the crystallization time is 36 h,calcination temperature is 550℃,calcination time is 5.5h.In the same way,using Na₂CO₃ weak alkaline solution as the core unit’s processing solution,successfully synthesis the core-shell structure composite molecular sieves ZT@MCM-41,the optimum concentration of Na₂CO₃ is 3.0 mol/L,alkali treatment time is 30 min,alkali treatment temperature is 80℃.Using NH₃H₂O weak alkaline solution as the core unit’s processing solution,successfully synthesis the core-shell structure composite molecular sieves ZA@MCM-41,the optimum concentration of NH₃H₂O is 10.9mol/L,alkali treatment time is 30 min,alkali treatment temperature is 80℃.Using Na2O（3.3）SiO2 weak alkaline solution as the core unit’s processing solution,successfully synthesis the core-shell structure composite molecular sieves ZG@MCM-41,the optimum concentration of Na2O（3.3）SiO2 is 0.1mol/L,alkali treatment time is 30 min,alkali treatment temperature is 80℃.For the four kinds of system,the same conditions are the alkali treatment time and temperature,the different condition is the concentration of the alkaline solution,because the purpose of alkali treatment to the zeolite is changing the surface structure of zeolite molecular sieve to generate more hydroxyl silicon on the surface of zeolite structure,therefore,the effect of alkali treatment time and temperature on the zeolite’s surface structure is relatively weak,the concentration of alkali solution is the key factor.（2）.The characterization of four different core-shell structured composite molecular sieves.Using NaOH strong alkaline solution as the core unit’s processing solution,the treated zeolite molecular sieve was denoted by ZQ,XRD characterization indicates that the main organizational structure of the unit ZQ has not changed,the main peak of ZSM-5 still exists,FT-IR characterization indicates that the surface acidity of ZQ is stronger than ZSM-5,further characterization of NH3-TPD,indicates that the amount of strong surface acid of ZQ was improved,while the amount of strong surface acid in proportion to the amount of surface hydroxyl silicon.The core-shell structured composite molecular sieves ZQ@MCM-41 was characterizated,XRD indicates that it has combinated characteristic of microporous and mesoporous diffraction peak,BET indicates that the composite molecular sieves has microporous and mesoporous adsorption volume,SEM and TEM was used to observe its surface morphology and internal structure,the results showed that composite molecular sieves has core-shell structured morphology,mesoporous molecular sieve successfully coated on the surface of zeolite molecular sieve,and mesoporous shell have ordered hexagonal structure.By the same method,using Na₂CO₃,NH₃H₂O,Na2O（3.3）SiO2 weak alkaline solution as the core unit’s processing solution respectively,the samples were characterized by XRD and NH3-TPD,results show that the main structure of zeolite after processing has not changed,the main peak of ZSM-5 still exists,the amount of strong acid in surface compared with ZSM-5 has improved.The core-shell structured composite molecular sieves were characterized by XRD and BET,the results show that the mesoporous shell was successfully coated on the surface of zeolite molecular sieve,composite molecular sieves has microporous structure,and also has the characteristic of the mesoporous structure.（3）.Hydrothermal stability evaluation of four different core-shell structured composite molecular sieves.Using NaOH strong alkaline solution and three kind of weak alkaline solution as the core unit’s processing solution,core-shell structured composite molecular sieves was successfully synthesized.Using Na2O（3.3）SiO2 weak alkaline solution as the core unit’s processing solution,the treated zeolite molecular sieve was denoted by ZG,it was coated by mesoporous shell,the core-shell structured composite molecular sieves was denoted by ZG@MCM-41,the experimental samples were treated by boiling water with4,6,8,10 days,samples were characterized by XRD and BET,XRD shows that the samples dealed with4,6,8 days,the main structure of composite material has not changed,the main peak still exists,BET suggests that the samples dealed with 4,8,10 days,their specific surface area were 90.1%,87.4% and 80.0% respectively,the sample treated with 8d,its miroporous volume remain at a rate of 91.8%.The data proved that composite molecular sieves ZG@MCM-41 has good hydrothermal stability.Using NaOH,Na₂CO₃ and NH₃H₂O as the core unit’s processing solution,by the same method,successfully synthesize core-shell structured composite molecular sieves.Using the same experimental method to evaluated their hydrothermal stability,the result showed that the hydrothermal stability from high to low is Na₂O（3.3）,SiO₂ > NH₃H₂O > Na₂CO₃ > NaOH.（4）.Catalytic performance evaluation of four different core-shell structured composite molecular sieve.Using the four different core-shell structured composite molecular sieves as the catalyst support,using the NOx selective catalytic reduction as evaluation,the activity temperature of the reaction is 400-500℃.Using the sample of ZG@MCM-41 as catalyst support,loading 15% Cu as catalyst active component,the NO conversion was 40%,besides using the sample of ZG@MCM-41 which was treated by boiling water for 8d as the catalyst support,the NO conversion was also 40%,it shows the composite molecular sieve has good hydrothermal stability at high temperature,Using NaOH,Na₂CO₃,and NH₃H₂O solution as the core unit’s processing solution,by the same method get the core-shell structured composited molecular sieves,the experimental results show that the catalytic performance from high to low is NH₃H₂O> Na₂O·（3.3）SiO₂>Na₂CO₃>NaOH.Hydrothermal stability at high temperature of composite molecular sieves ordered from high to low is: Na₂O·（3.3）SiO₂>NH₃H₂O>Na₂CO₃>NaOH.（5）.Molecular simulation studies of core-shell structure composite molecular sieves.The mechanism of interaction between CTAB and silicon source was studied by dissipative particle dynamics DPD simulation method which was used as mesoscopic simulation approach.It can provide a theoretical basis for the synthesis process of the core shell composite molecular sieves.It was found by simulating the assembly process that the hydrophobic end of the positively charged surfactant,namely,the long alkyl chain,formed a spherical micelle,and then formed a nucleus with the core zeolite molecular sieve.And the hydrophilic end of the negatively charged surfactant,namely,the quaternary ammonium salt,formed a shell with a two or more polymeric unit of the silicon source.