Preparation Of MOFs Composite/Pebax Mixed Matrix Membranes And Gas Separation Performances

With the rapid development of industry,the use of fossil fuels has increased dramatically,resulting in increasing CO₂ emissions in the atmosphere.The emission of CO₂ exceeds the capacity of the atmosphere,leading to a greenhouse effect.And our government has committed to achieve carbon peaking by 2030 and carbon neutrality by 2060.Therefore,carbon capture and storage technology is very important.Compared with other separation methods,membrane separation technology has the advantages of high efficiency,low energy consumption,easy operation,and easy coupling,so it has very great development prospects and application prospects.Mixed matrix membranes（MMMs）prepared by combining polymer matrix with fillers can overcome the trade-off effects of permeability and selectivity and enhance separation performance.Although traditional inorganic fillers can improve the gas separation performance,when the loading increases,the separation performance will decrease due to the weak interfacial compatibility with the polymer.The mixed matrix membranes based on MOFs have more advantages than MMM with traditional inorganic fillers,but at high loadings,the agglomeration phenomenon will severely affect the gas separation performance.Therefore,in this thesis,the one-dimensional inorganic filler halloysite nanotubes（HNT）were first selected to combine with Ui O-66,and a composite material Ui O-66@HNT with a continuous long-range one-dimensional structure was designed and prepared by a solvothermal method,and dispersed into Pebax-1657 matrix to prepare MMM for CO₂/N₂ separation.HNT has a hollow nanotube structure with an open end,and the resistance to gas in the closed tube is smaller than that outside the tube,making HNT an excellent molecular transport channel.When the materials are aligned vertically,the diffusion of molecules is facilitated and the permeability of the MMM is improved.In addition,Ui O-66 has a high affinity for CO₂.Ui O-66 was coated on HNT,which accelerated CO₂ into the HNT lumen under the attraction of Ui O-66.Furthermore,the Ui O-66 on the outer surface of the HNT forms a continuous layer,which provides a continuous channel for gas transport when the composite is aligned horizontally.Thereby the permeability of CO₂ is enhanced as well as CO₂/N₂ selectivity At25°C and 20 wt%loading,the CO₂ permeability of the mixed matrix membrane reached 119.06Barrer,and the CO₂/N₂ selectivity reached 76.26,performing beyond the Robeson upper bound（2008）.Furthermore,the presence of Ui O-66@HNT endowed the MMM with good long-term stability and excellent interfacial compatibility.On the other hand,transport resistance for membrane separation is generally minimized due to the high aspect ratio of nano-thickness in 2D nanomaterials,but the filler content is very low,and permeability is generally reduced because of transport resistance due to high aspect ratios.In order to improve the intra-membrane loading and gas separation performance,boron nitride nanosheets（BNNS）were selected as the template and combined with ZIF-8 to construct the composite ZIF-8@BNNS.The amino group of BNNS binds to Zn²⁺and in situ grows ZIF-8 on the surface.On the one hand,BNNS act like a barrier to intercept the gas transport path.Gas molecules are forced to diffuse through the nanosheets through the surface of the 2D BNNS.On the other hand,the continuous ZIF-8 layer with high porosity is like a fast channel around BNNS,and the pore size of ZIF-8 can permit the diffusion of CO₂ selectively over N₂,not only can effectively sieveing CO₂ and N₂,Moreover,the mass transfer resistance is effectively reduced,far less than the resistance caused by the barrier effect of BNNS in the mixed matrix membrane,so the gas separation performance of the mixed matrix membrane is improved.At25°C and 20 wt%loading,the mixed matrix membrane exhibited a CO₂ permeability of 106.5Barrer and a CO₂/N₂ selectivity of 83.2,surpassing the Robeson upper bound（2008）.The results show that the continuous MOF layer can effectively reduce the mass transfer resistance and improve the gas separation performance.