Preparation Of Ultra-thin MFI-type Molecular Sieve Membrane And Its Gas Separation Performance

As the demand for hydrocarbon compounds in chemical production processes has gradually increased,the refining and ethylene-related industries have grown by leaps and bounds.The ethylene cracking and petroleum cracking processes produce large amounts of C4 fractions,of which the C4 fractions from FCC contain up to 50%of butane mixtures（n-butane and isobutane）.N-butane can be used directly as a fuel,as well as a solvent for subcritical biotechnology extraction,as a refrigerant,and as a raw material for organic synthesis.Isobutane can be used as aerosol promoter,gasoline octane improver and refrigerant,etc.Since n-butane and isobutane are isomers,their boiling points are similar,which makes the traditional low-temperature distillation separation technology suffer from high cost and low product purity.In recent years,membrane technology has received a lot of attention from researchers at home and abroad for its advantages of high efficiency,energy saving,environmental protection and simple operation.Among them,MFI zeolite molecular sieve membrane is one of many membrane materials,which shows good application prospects in gas separation because of its regular pore structure and unique adsorption properties.The MFI molecular sieve topology contains sinusoidal channels parallel to the a-axis（0.51×0.55 nm）and through channels parallel to the b-axis（0.53×0.56 nm）.Due to the different sizes and shapes of MFI molecular sieve channels,MFI molecular sieve membranes can be prepared with different orientations to obtain high separation selectivity depending on the molecular characteristics of the system to be separated.h0h/a oriented sinusoidal pore size of MFI molecular sieve membranes is between the molecular dynamics sizes of n-butane（0.47 nm）and isobutane（0.53 nm）,which is an ideal membrane for the separation of butane isomers.material.Meanwhile,the separation performance of MFI-type zeolite membrane depends on the quality of the membrane,and the ultra-thin zeolite membrane can reduce the gas transfer resistance and maximize the permeation rate.In this work,the microstructural tuning of MFI zeolite membranes was carried out by optimizing the membrane preparation parameters in order to prepare high-performance molecular sieve membranes and apply them to the study of n-/isobutane separation process.The main study consists of three parts as follows:（1）Preparation of small-size MFI-type molecular sieve crystals.MFI-type molecular sieve crystals were prepared by hydrothermal synthesis.First,the effects of reaction gel aging time and synthesis temperature on the size and morphology of MFI-type molecular sieves were investigated.It was found by XRD and SEM characterization that spherical molecular sieve crystal seeds of 100 nm in size with uniform crystal size and high crystalline strength were prepared at the reaction gel aging time of 48 h and the synthesis temperature of 95°C.（2）Preparation of MFI zeolite membranes.Firstly,the MFI zeolite membranes were hydrothermally synthesized by secondary growth method after loading the crystalline seed layer on the outer surface of the tubular support body by dip-coating method.The effects of the size/shape of crystalline seeds,synthesis temperature and synthesis time on the microscopic morphology and separation performance of MFI zeolite membranes were investigated.The experiments showed that the crystalline layer prepared by using small-sized spherical crystalline seeds has high stacking density and can significantly reduce the intergranular defects in the crystallization synthesis,which can significantly improve the separation selectivity of the membrane.In contrast,the stacking density of the crystalline layer prepared with large-sized columnar seeds is lower,and the defect density of the prepared membrane layer is higher.In addition,the crystalline strength of MFI zeolite membranes with high synthesis of small-sized spherical crystal seeds was higher than that of MFI zeolite membranes prepared by large-sized columnar crystal seeds.In summary,the ultra-thin MFI zeolite membranes with high h0h orientation of 1.9μm thickness were prepared by hydrothermal synthesis of 100 nm small-size spherical crystal seeds at 110°C for 20 h through optimization of preparation parameters.The H₂ permeation rate of the membrane was as high as3.65×10^-6 mol（m² s Pa）^-1,and the ideal selectivity of n-/i-C₄H₁₀ was 270 and that of H₂/i-C₄H₁₀ was as high as 24.8,indicating that the membrane layer was free of macroporous defects.（3）Study on the performance of ultra-thin MFI zeolite membrane for n-/isobutane separation.Firstly,the effects of temperature,pressure and purge gas on the single gas permeation performance of the MFI zeolite membrane were investigated;secondly,the effects of operating conditions such as test temperature,pressure,feed flow rate and feed concentration on the separation performance of the mixed n/isobutane system were examined.The results showed that the separation factor of n-butane/isobutane gas mixture was always higher than the ideal separation factor,which was due to the permeation of n-butane effectively suppressing the permeation of isobutane.By optimizing the operating conditions,the n-C₄ permeation rate of the membrane was2.34×10^-7 mol（m² s Pa）^-1 for equimolar n-/i-C₄ gas mixture at a temperature of 60°C,a feed pressure of 151 k Pa（absolute）,a feed flow rate of 200 ml/min and a N₂ purge gas flow rate of 100 ml/min^-1,the separation factor of n/i-C₄ was as high as 42,and the separation performance of the membrane was higher than the results reported in the literature so far.