Structure Design Of MOF/polyimide Composite Membranes And Separation Properties Of CO₂/CH₄ In Biogas

Biogas is a clean energy produced by anaerobic digestion and exhibited good application prospects in power generation,heating,natural gas substitution and vehicle gas.However,the presence of CO₂ impurities reduces the combustion calorific value of biogas and corrodes the transportation and storage equipment.Therefore,the application of separation technology to achieve the capture of CO₂ and the upgrading of CH₄ is not only conducive the interoperability of biogas,natural gas and vehicle fuel gas,but also significant for the resource utilization of CO₂.Compared with traditional separation technology,membrane technology has advantages in cost,energy efficiency and environment protection.The trade-off between permeability and selectivity of polymer membrane has greatly limited the application of membrane technology in biogas upgrading.In this paper,the internal porous structure and properties of metal organic framework（MOF）fillers were designed based on the difference in physical and chemical properties of CO₂ and CH₄ to regulate the chemical and water environment of polyimide membrane.This study focused on the effect of fillers modification on the CO₂/CH₄ separation performance,plasticization resistance and gas impurity resistance of polyimide membranes.Finally,the transfer mechanism of gas in the membrane was explored and provide a theoretical basis for the efficient separation of CO₂/CH₄ from biogas.The UiO-66 filler was modified with amino groups and filled into the polyimide matrix to improve the gas separation performance of the polyimide membrane by taking advantage of the strong affinity between amino groups and CO₂ molecules.Characterization and gas permeability test results show that polyethyleneimine（PEI）is successfully loaded on the surface of UiO-66 and the filler can still maintain a complete framework structure.The porous filler is easier to absorb CO₂ molecules with high polarizability and quadrupole mement.And the porous filler with abundant affinity CO₂ sites constructs a gas selective transport channel in the polyimide membrane.When the filler loading was 15 wt%,feed pressure was 0.1 MPa and temperature was 35℃,the CO₂ permeability coefficient was 28.24 Barrer(1Barrer=1×10^-10 cm³（STP）cm cm^-2 s^-1cm Hg)and the CO₂/CH₄ separation factor was56.49.In order to improve the affinity of the UiO-66-PEI and the polyimide matrix,and avoid the apperarance of non-selective voids between the dispersion phase and the continuous phase,the ionic liquid（IL）of[bmim][Tf₂N]was loaded in UiO-66-PEI fillers.The IL coated on the surface of MOF acted as a“wet agent”to optimize the interface morphology of the filler and the polymer.Furthermore,the IL have high CO₂ solubility,which can improve the CO₂/CH₄ separation performance of the polyimide membrane.FTIR,SEM and N₂ adsorption-desorption isotherms results confirmed that the IL was successfully loaded on the surface and the internal pores of UiO-66-PEI.SEM images confirmed the uniform dispersion of IL@UiO-66-PEI in the polyimide matrix.When the feed pressure was 0.1 MPa and the temperature was35℃,the CO₂/CH₄ selectivity of PI/IL@UiO-66-PEI-15 MMM was increased by 6.2%and the CO₂ permeability coefficient decreased by 8.43%compared with PI/UiO-66-PEI-15 MMM.Although the loading of IL can improve the gas separation performance of polyimide membranes,the decrease of the gas permeability coefficient and the loss of IL driven by the pressure differences would limited the application of PI membrane.In order to improve the gas permeability coefficient in the membrane,the poly（sulfobetaine methacrylate）（pSBMA）was grafted on the surface of UiO-66-PEI to improve the gas separation performance.Polyzwitterionic can form a tight hydration layer through electrostatic interaction with water molecules.Therefore,the UiO-66-PEI-pSBMA filler can regulate the water environment of polyimide membrane.The difference in the solubility of CO₂ and CH₄ in water is used to design and construct gas selective transport channels in the membrane.The test results of the water environment and the arrangement of polymer chains in the MMM showed that the UiO-66-PEI-pSBMA can increase the water content and cause a rigidification of polymer chains around fillers.At the same time,the increase of d-spacing in polymer chains can result in an increase of CO₂ permeance and inhibit the plasticization of polyimide.Permeation test results showed that the CO₂ permeability and CO₂/CH₄selectivity of the PI/UiO-66-PEI-pSBMA-15 MMM in wet state was 4.48 times and4%higher than the test result in dry state.Under the optimum test condition of feed pressure was 0.1 MPa,the operating temperature was 35℃,the CO₂ permeability and CO₂/CH₄ separation factor of PI/UiO-66-PEI-pSBMA-15 MMM reached 185.12Barrer and 60.32,overcoming the trade-off between permeability and selectivity of polymer membranes.In order to achieve the ultra-high permeance of CO₂ and strong mechanical strength of membrane materials,this study used PI/UiO-66-PEI-pSBMA membrane as the selective layer and the polydimethylsiloxane（PDMS）as the gutter layer to construct polyimide composite membranes.The optimized synthesis condition are that gutter layer concentration is 2 wt%,the polyimide concentration is 2 wt%and the UiO-66-PEI-pSBMA filler content is 8 wt%.Under the feed pressure of 0.6 MPa,the operating temperature of 35℃,the addition of UiO-66-PEI-pSBMA filler can increase the CO₂ permeance rate of polyimide membrane by 129.34%and the CO₂/CH₄ separation factor by 55.58%.Meanwhile,UiO-66-PEI-pSBMA fillers can improve the plasticization resistance and H₂S impurity resistance of the polyimide membrane.Finally,the transfer mechanism of gas molecules in the composite membrane was investigated by comparing the the gas permeance and CO₂/CH₄separation factor of the PI and the PI/UiO-66-PEI-pSBMA membrane.The cross-flow model was used to calculate the membrane area which meet the requirements of the natural gas pipeline network,further confirming that the improvement of separation performance is conducive to reduce the investment costs of actual separation process.