Research On High Performance Mixed Matrix Membranes For CH₄/N₂ Separation Application

At present,developing clean energy has become an important strategic choice for the sustainable development of countries in the world.As a clean and high-quality energy,natural gas can effectively reduce CO₂,SO₂,and other greenhouse gas emissions,attracting more and more attention.However,the separation of complex systems such as CH₄/N₂in unconventional gas restricts its further development.Compared with traditional separation technology,membrane separation technology has the advantages of high selectivity,easy expansion of modular design,simple operation,low energy consumption and low cost,showing unique advantages in the field of CH₄/N₂gas separation.Mixed Matrix Membranes（MMMs）combining the machinability,easy film-forming of polymer with the high permeability and selectivity of the filler,has showed a broad application prospect in gas separation.However,the poor acid-base stability of porous filler,the interface compatibility between filler and polymer matrix and the difficulty to enlarge the membrane area or improve the permeability and selectivity simultaneously limit the further application of MMMs in CH₄/N₂gas separation.In this paper,the acid-base stability of metal-organic framework（MOF）materials and CH₄/N₂gas separation performance of MOF based MMMs were greatly improved by the surface functional group regulation of MOF and the creation of metal active sites.The main research contents are as follows:（1）Considering the acidic environment of unconventional natural gas（CH₄/N₂system）affecting the stability of membrane performance,a high vacuum resistance calcination（HVRC）method was proposed to construct a series of ZIF-8@VR nanoparticles with surface carbonization and stiffening structure,which provides reference for expanding the application range of membrane materials.Due to the excellent chemical stability of carbon materials,ZIF-8@VR still maintains a complete crystal structure in acidic p H=2.8.The adsorption capacity for CH₄of ZIF-8@VR is increased by 63%and the adsorption selectivity of CH₄/N₂is increased by 1.4 times due to the carbonization structure with Zn-rich sites on the surface.Further,dense PAA/ZIF-8@VR MMMs were prepared by using ZIF-8@VR as filler and weak acid polyacrylic acid（PAA）as polymer matrix.The results show that the introduction of ZIF-8@VR filler can promote the interaction between gas molecules and nanoparticles,increase the activation energy of large gas molecules（CH₄）,and improve the performance for CH₄/N₂separation of MMMs.PAA/ZIF-8@VR MMM shows high CH₄permeance（1763 GPU）and acceptable CH₄/N₂selectivity（3.12）at66.8 wt%filler loading.（2）To further expand suitable MOFs filler for CH₄/N₂separation system,amorphous Ui O-66 materials（ACU）with good alkaline stability and film-forming was synthesized by HVRC,and the effect of different currents on the structure and properties of ACU was studied.The results show that the Zr⁴⁺-rich active sites formed by HVRC can improve the adsorption capacity for CH₄of ACU,and ACU-120A has the best CH₄/N₂adsorption selectivity when the current is 120A.Further,using ACU-120A as filler and polyvinyl alcohol（PVA）as polymer matrix,a series of amorphous MOF based MMMs with excellent CH₄/N₂separation performance were obtained.Specifically,the CH₄permeance of PVA/ACU-120/MPSf MMM is 5584GPU and the CH₄/N₂selectivity is 3.43 at 80 wt%ACU-120A loading.（3）To further improve the CH₄/N₂separation performance,the effects of different vacuum calcination currents on the structural characteristics and gas adsorption performance of MOF materials were investigated.It is found that the MOF material（CSZ）with abundant metal active sites and functional groups can be uniformly carbonized by adjusting the current.Among them,CSZ-95A has the highest CH₄/N₂adsorption selectivity and good alkali stability.Further,using CSZ-95A as filler,alkaline polyvinylamine（PVAm）as polymer matrix and modified MPSf as support,PVAm/CSZ-95A/MPSf MMMs with an area of 1120 cm²（ca.2 pieces of A4-size paper）were prepared.Due to the synergistic strengthening effect of Zn-rich sites and-NH₂functional groups,the CH₄/N₂selectivity of MMM can reach 3.89 and the CH₄permeance is 6701 GPU when CSZ-95A is loaded at 85 wt%.In addition,spiral-wound membrane modules were prepared with the effective area up to 2000cm².The CH₄/N₂separation performance of the spiral-wound membrane modules is not decreased,which confirmed the good practicability of MMMs based on CSZ-95A.This work provides a new CH₄/N₂separation MMMs,which is conducive to promoting the large-scale production and application of MMMs.（4）To overcome the adverse effect of increased MOF loading on the selectivity of gas separation,the influence of different vacuum calcination time on the structural characteristics of CSZ-H_xmaterial was analyzed on the basis of the previous work.It is found that the calcination time has an effect on the mesoporous properties of CSZ-H_x.When the time is 3 h,CSZ-H₃not only retains the high adsorption and separation performance for CH₄/N₂,but also has high mesoporous specific surface area and pore volume,which promoted the interface contact with the polymer,thus improving the compatibility with the polymer.Further,using CSZ-H₃as filler and PVA as matrix,PVA/CSZ-H₃/MPSf MMMs with high loading capacity and high separation performance were prepared.When the CSZ-H₃loading is 95 wt%,MMM has the highest CH₄/N₂selectivity（4.44）and ultra-high CH₄permeance（7283 GPU）.This work provides a reference for the construction of MMMs with high filler loading and performance.