Alkali-stable MOFs Materials And Their Mixed Matrix Membranes For CH₄/N₂ Separatio

Environment protecting,economic space occupation and simple operation are the advantages of membrane separation technology.With the development of global industrialization,unconventional natural gas has been widely concerned by researchers.It contains high concentrations of N₂and CH₄,and CH₄emissions into the air can cause a huge greenhouse effect.As a consequence,the separation of CH₄/N₂has became a hot issue in modern industrial purification for unconventional natural gas.Mixed matrix membranes（MMMs）have advantages of either porous materials or polymer materials owning to corporate porous materials and polymer materials.It has attracted extensive attention of researchers for its permeability,selectivity and membrane formation.MOFs provide promising opportunities of developing in gas separation area due to their portability,structural diversity,controllable porosity and high adsorption affinity.However,the chemical stability of MOFs are the key factors to restrict the development in the field of membrane separation.Therefore,this paper improved the alkaline stability of MOF materials as a starting point and prepared the MMMs corporate the alkali-stable MOFs material and the polymer with strong alkali properties,and further studied the MMMs gas separation performance.The main research contents are as follows:（1）we proposed a strategy which is transforming 3D CAU-10-H into MOF materials with high base stability.We changed the ligand of CAU-10-H（Isophthalic acid）into 4-chloroisophthalic acid which has halogen atoms and prepared the materials by solvothermal method.This work indicates that halogen atom might improve the stability of materials.And morphology of the materials is also changed with the improvement of the reaction temperature:it is from 3D spherical CACl-10（130）to CACl-10（150）,then,converted to 2D rodlike CACl-10（180）.Its structure is also transformed into an amorphous structure,and part of Al³⁺in the framework is converted into Al OOH.Eventually,the CACl-10（180）alkaline stability is improved.As a result,CACl-10（180）can stay stable in p H=14 sodium hydroxide solution for24h.（2）On the basis of the above work,MMMs were prepared by alkali resisting 2D CACl-10（180）and basic polymer PVAm.2D CACl-10（180）can absorb CH₄by itself and the polar polymer PVAm can also adsorb the non-polar molecule CH₄with polar bonds.These contribute to the separation of PVAm-CAU-10（180）/MPSf MMMs for CH₄.Hence,the CH₄/N₂separation performance has a breakthrough improvement:the CH₄/N₂selectivity is from 2.28 to 3.10 and CH₄permeance is from447.56 GPU to 1647.99 GPU.（3）In order to further improve the performance of CH₄/N₂,2D CACl-10（180）with moderate alkali stability and good crystallization property was compounded with polyethylene imine（PEI）to prepare CACl-10（150）@PEI.Compared with CACl-10（180）,the CH₄adsorption capacity of CACl-10（150）is increased,because the high reaction temperature will destroy the adsorption vacancy of the material surface,resulting that CACl-10（180）has fewer active sites for CH₄adsorption.CACl-10（150）@PEI exhibits both adsorption capacity for CH₄and alkali stability.Meanwhile,the best loading of PEI in CACl-10（150）@PEI is 20wt%,because when the loading is high or low,free polymer ends block the pores resulting in structural instability of the materials.Then,PEI/CACl-10（150）@PEI/MPSf MMMs were prepared by combining CACl-10（150）@PEI with PEI.As a result,CACl-10（150）@PEI has a good compatibility with PEI,and it has an excellent separation performance for CH₄/N₂:the permeability of CH₄is 2592GPU,and the selectivity of CH₄/N₂is 4.2.In conclusion,the permeability of PEI/CACl-10（150）@PEI/MPSf MMMs to CH₄and the selectivity of CH₄/N₂have been improved,which provide a favorable support for the application of CH₄/N₂separation membrane.