The Preparation And Characterization Of Two-dimensional Zeolite Nanosheets

Two-dimensional materials,as a new type of materials,exhibit superior advantages in the fabrication of membrane materials due to their high aspect ratio and specific surface areas.Particularly,two-dimensional zeolite nanosheets are attractive building blocks for the fabrication of zeolite nanosheet membranes.However,the preparation of two-dimensional zeolite nanosheets is a great challenge.Up to now,only a few layered zeolites,such as MFI and MWW,have been exfoliated to obtain two-dimensional zeolite nanosheets.In this thesis,a novel wet oxidation method was used to detemplate the layer-structured multilamellar MFI(ML-MFI)and MCM-22P,followed by ultrasonic treatment to obtain the two-dimensional zeolite nanosheets in aqueous solution.The research content mainly consists of two parts as follows:1)MFI zeolite nanosheets:ML-MFI zeolite was synthesized by using an organic structure directing agent(OSDA)composed of long-chain alkyl groups and two quaternary ammonium head groups spaced by a C6 alkyl linkage(C22-6-6).XRD and SEM techniques showed that ML-MFI zeolite particles were composed of MFI lamella assembles.Then the ML-MFI was treated with piranha solution(3H2SO4:1H2O2).Characterizations of XRD,TG,IR,N2 adsorption and desorption showed that the OSDA was completely removed and the micropore were completely opened,after the ML-MFI was repeatedly treated by piranha solution for 4 times.Afterwards,the OSDA-free zeolite was redispersed in water,followed by the ultrasonic treatment for exfoliation to prepare open-pore MFI nanosheet aqueous suspension.The effects of ultrasonication frequency and time on MFI nanosheets as well as the sedimentation time on the purification of MFI nanosheet suspension were investigated.The results indicated that MFI zeolite nanosheet suspension with negligible amount of nonexfoliated MFI zeolite particles were obtained after the piranha solution treated ML-MFI was sonicated at a frequency of 45 kHz for 10 hours followed by sedimentation for more than 7 days.SEM and TEM techniques confirmed that the prepared MFI nanosheets were in single-layer with dimensions up to 1 ?m.2)MWW zeolite nanosheets:MCM-22P was synthesized by using piperidine(PI)as template and boric acid(H3BO3)as the crystallization promoter.XRD,SEM and TEM techniques showed that MCM-22P had a typical MWW lamellar structure.The as-synthesized MCM-22P was treated with piranha solution for 3 times,finding that the template was completely removed and the micropores in zeolite were opened,confirmed by various techniques,such as XRD,TG,IR,N2 adsorption.The as-obtained detemplated MWW samples were redispersed in water and then ultrasonicated.After optimizing the sonication and sedimentation conditions,it was found that MWW nanosheets in aqueous solution were obtained from the supernatant of the suspension,which was sonicated at a frequency of 45 kHz for 3 hours followed by sedimentation for at least 2 days.The obtained MWW nanosheets were characterized by SEM and TEM,confirming that the lateral size is in the range of 100 to 300 nm.