Modification And Crystallization Study Of ZSM-35 Zeolites Using Hard Template

The industrial use of ZSM-35 zeolites generally has common problems such as unsatisfied catalyst activity and stability as well as easy coking and deactivation due to the poor diffusion of microporous pores.For this reason,this thesis intends to introduce hard templates to improve the diffusion of zeolites,thereby improving its catalytic performance.The effects of hard template including carbon nanotubes(CNT),graphene oxide(GO)and activated carbon(AC)on the morphology,pore structure,acidity and catalytic performance of ZSM-35 zeolites were systematically explored this thesis.Also the crystallization of ZSM-35 zeolites and the influence of hard templates on the crystallization of ZSM-35 zeolites were preliminary investigated.The effect of hard templates on the structure of ZSM-35 zeolite was discovered via characterization using X-ray diffraction(XRD),scanning electron microscopy(SEM),high-resolution transmission electron microscopy(HRTEM),micro-mesoporous total static adsorption(BET),pyridine infrared(Py-IR),ammonia desorption by programmed temperature(NH3-TPD),solid nuclear magnetic(NMR),thermogravimetry(TG)etc.The structure-activity relationship was studied through the skeletal isomerization of n-butene reaction.The main conclusions are as follows:(1)The introduction of hard templates in the crystallization will form thin zeolites and can reduce the degree of zeolites stacking.Such a zeolite can accelerate the reaction products diffusion thus easier to leave the pore of the zeolite resulting in improving the catalytic stability of zeolites.Among them,CNT can be used as an interlayer during the crystallization process to block the stacking of zeolites.Moreover,the zeolites can grow around CNT and GO,when they are removed by calcination,they will leave mesoporous pores,thereby improving the pore connectivity of zeolites.But AC cannot improve the connectivity of zeolites channels.(2)Taking butene isomerization reaction as probe reaction,the ZSM-35 zeolites synthesized with hard templates have much better catalytic activity and stability.The best type of hard template is CNT,and the best CNT addition is 40%(CNT/Al2O3=40%).(3)Using CNT as the hard template will increase the Br?nsted acid(B acid)content of the zeolites,and using GO as the hard template will reduce the B acid content of the zeolites.(4)The introduction of hard templates can shorten the crystallization time of zeolites.Among them,the zeolites with 40%CNT as the hard templates have the fastest crystallization rate,followed by the zeolites with 40%AC,and the zeolites with 40%GO.Compared with the conventional ZSM-35 zeolites,the zeolites synthesized with the hard templates have a new dandelion-like morphology during initial crystallization.This new morphology is a key step for the transformation of the amorphous silica-alumina gel state into the FER zeolites,and it is also a prerequisite for the formation of non-stacked zeolites.(5)The growth process of ZSM-35 zeolites is presented.In the induction period,the zeolites are in the state of amorphous silica-alumina gel.When the induction period turns to crystallization region,aluminum enters the framework of FER zeolites,and the coordination environment is mainly Si(4Al),Si(3Al)and Si(2Al).At this moment,the zeolites begin to have a certain amount of acid and a large number of mesopores.When the crystallization region turns to stabilization period,five-coordinate aluminum and six-coordinate non-framework aluminum were produced.The coordination environment is Si(1Al)and Si(0Al),and the acid content of zeolites is gradually increased.At the same time,the content of mesopores is reduced and a large number of micropores are formed.At this point,the FER framework structure is formed.The zeolites are remodeled after entering the stabilization period.The type of hard template will affect the restructuring process.The zeolites with GO as hard templates appears the process of Si(1Al)partial transformation to Si(0Al),five-coordination aluminum partial transformation to four-coordination skeleton aluminum and six-coordination aluminum.The zeolites with CNT as hard templates appear the process of five-coordination and six-coordination aluminum partial transforming to fourcoordination skeleton aluminum.The process of Si(1Al)partial transformation to Si(0Al)and fivecoordination aluminum partial transformation to four-coordination framework aluminum was observed in blank FER zeolites.The zeolites with AC as hard template do not undergo the reconstruction process.During the reconstruction,the contents of B acid,L acid and mesoporous of the zeolite were greatly increased.