Preparation Of Ultrathin Oriented MFI Membranes

The MFI zeolite consists of straight channels parallel to the b axis(5.6×5.3 A)and "Z" shaped channels parallel to the a axis(5.5×5.1 A),which are widely used in the separation field.The preparation of ultra-thin oriented MFI zeolite membranes is a hot topic in the field of membrane materials.The preparation of b-oriented zeolite nanosheets has been proved to be an effective method to shorten the diffusion path.At present,the preparation of continuous and ultra-thin b-oriented MFI zeolite membranes based on zeolite nanosheets is a hot and difficult topic in the research field of zeolite membranes.This paper focuses on the preparation of MFI zeolite nanosheets,the ordered assembly of MFI zeolite nanosheets by Langmuir-Blodgett(L-B)technology,and the preparation of ultra-thin oriented MFI zeolite membranes by template-free secondary growth method.The main research work includes:1.Benzene ring-like silicalite-1 crystals with thickness of 400 nm were prepared.After calcination to remove the template agent,the L-B technology was used to realize the monolayer ordered assembly of the crystals on the porous support,and in-plane growth of the zeolite crystals was realized by templatefree secondary growth method.By exploring the effect of crystallization time on the membrane morphology,the MFI zeolite membrane with the thickness of 500 nm was obtained.The use of template-free secondary growth method effectively avoids the key problem of defects in the high temperature calcination process of zeolite membrane.The gas separation test shows that the separation factor of the prepared MFI zeolite membrane for H2/CO2 mixed gas is 18.2.Top-down method was used to prepare silicalite-1 nanosheets,The calcined benzene ring-like silicalite-1 crystals were etched by alkali source.The concentration of tetrapropylammonium hydroxide(TPAOH)was changed to 0.217 M,0.220 M,0.223 M,0.225 M,0.227 M and 0.230 M,respectively,and the nanosheet silicalite-1 crystals with different etching degrees were synthesized by controlling theconcentration of TPAOH.The results showed that the best concentration of TPAOH was 0.225 M,and the thickness of the nanosheets is 35 nm.The ordered assembly of MFI zeolite nanosheets on the support was realized by L-B technology,and a monolayer zeolite crystal layer was obtained.The MFI zeolite membrane with the thickness of 903 nm was synthesized by template-free secondary growth method.The gas separation test shows that the separation factor of the prepared MFI zeolite membrane for H2/CO2 is about 10,and the permeance of H2 is about 20 gas permeance units(GPU,1 GPU=3.35×10-10 mol-1·m-2·s-1·Pa-1).3.Bottom-up method was used to prepare fluorine-containing silicalite-1 nanosheets.Ammonium fluoride(NH4F)was introduced as the mineralizer.The morphology and b-axis thickness of the zeolite nanosheets were controlled by adjusting the aging time and crystallization time.And at a low temperature of 90℃,the fluorine-containing silicalite-1 nanosheets with the b-axis thickness of 110 nm were obtained.Then,the b-oriented crystal layer was prepared on the aluminium oxide(Al2O3)support by L-B method.The template-free secondary growth method was used to realize the in-plane growth of zeolite nanosheets,and obtained the MFI zeolite membrane with the thickness of about 1.4 μm.The single-component H2 and CO2 test shows the ideal selectivity of H2/CO2 can reach 17 at 0.05 MPa,while the H2 permeance is 331 GPU.The separation test of bicomponent H2/CO2 mixed gas shows that after 10 times of testing at room temperature,the separation factor of H2/CO2 is stable at about 7,and the H2 permeance can reach 310 GPU,with high H2 permeability,and changing the test temperature(25-100℃)do not affect the separation factor.