In-situ Fabrication And Compatibility Enhancement Of MOF-Based Mixed Matrix Membranes

Since the 21st century,the global consumption of fossil fuels has been increasing continuously.The reduction of greenhouse gas emission and the purification of energy gases are important reasons for CO₂ capture.The representative separation systems are flue gas?CO₂/N₂?and natural gas?CO₂/CH₄?.Membrane separation,a new separation technology,has been used to capture CO₂ for years,due to efficiency,low cost and small carbon footprint.Mixed matrix membrane?MMM?is composed of continuous polymer matrix and dispersed inorganic filler.It combines the advantages of easy processing of polymer membrane and high separation performance inorganic membrane and is considered as an promising direction in gas separation membranes.Metal organic framework?MOF?is a new kind of crystal material with regular pore structure,composed of transition metal ion and organic ligand bridged by covalent or coordination bond.Compared with traditional inorganic fillers,MOF can be synthesized in mild conditions and be functionalized easily.Hence,MOF-based MMMs have become a hotspot in current research.However,few studies have been done on the MOF-based MMMs with thin selective layers.In addition,there are issues about MOF/polymer interf'acial defects and flexible MOF framework,which could weaken the selectivity of MMMs.In this paper,based on the special synthesis routes of three MOFs?ZIF-7,ZIF-8 and Cu-BTC?,we prepared MMMs with thin selective layers by in-situ synthesis of MOFs and enhanced the MOF/polymer interfacial compatibility by ionic liquid preloading into MOFs.Firstly,ZIF-7/PES and MW-ZIF-7/PES asynmmetric membrane were prepared by direct blending and microwave-assisted method,respectively,based on the traditional preparation process?nonsolvent-induced phase separation,NIPS?.The addition of ZIF-7 can increase the permeation rate and selectivity to H2 and CO₂ simultaneously.MW-ZIF-7/PES membrane had higher H2 and CO₂ permeation rate than that of PES membrane after PDMS coating,but the selectivity decreased significantly.This is because the NIPS method has strict requirement for the casting solution.Secondly,in order to broaden the condition of in-situ MOF synthesis in polymer solution,a compatible system of Cu-BTC precursor and PES in solvent DMSO was developed.Under high temperature,the thermal-induced phase separation?TIPS?of polymer and Cu-BTC synthesis occur at the same time,leading to an asyrmmetric Cu-BTC/PES membrane.For the first time,the asymmetric MOF-based MMM was fabricated via one step for the first time.The Cu-BTC/PES membrane has a typical asymmetric structure and consists of a dense skin layer and a porous support layer.The Cu-BTC/PES membrane can be successfully activated by the replacement of ethanol with DMSO.The content of Cu-BTC in MMM can be adjusted by the dosage of Cu?NO₃?₂.With the increase of the amount of Cu?NO₃?₂,the thickness of the dense layer decreases and the permeation rate of the MMM increases drasticly.When the content of Cu-BTC was increased from 7.5 wt%to 21.0 wt%,the permeation rate of H2 and CO₂ increased by nearly 20 times and the selectivity of CO₂/N₂ was maintained at about 20.Furthermore,the in-situ room-temperture preparation of Cu-BTC/EC MMM was realized with the.help of the complexation of cellulose ether?EC?and Cu?NO₃?₂.Cu-BTC significantly increased the permeability of the membrane to each gas.With the Cu-BTC content of 26 wt%9 the CO₂ permeation coefficient was 69%higher than that of the pure EC,while the CO₂/CH₄ selectivity decreased from 10.7 to 9.9.It was found that Cu-BTC and EC had good interfacial compatibility when.the content of Cu-BTC was low,and the Cu-BTC/EC MMMs have better thermal,mechanical and gas permeation properties.The resistance to CO₂ plasticizing was also enhanced by the Cu-BTC loading.Finally,ionic liquid[bmim][Tf2N]with high CO₂ selectivity and polymer affinity was introduced into the ZIF-8 channel to enhance the interfacial compatibility between the MOF and the polymer?Pebax?in the MMMs.IL@ZIF-8 and IL@ZIF-8/Pebax membrane were prepared.The effect of IL@ZIF-8 on the diffusion of IL in the membrane was investigated.The mechanical properties and gas separation performance of IL@ZIF-8/Pebax membranes were tested.The results showed that IL@ZIF-8 enhanced the mechanical properties and CO₂ separation performance of pure Pebax membranes.When the fiiler loading is 15 wt%,the CO₂ permeation coefficient is 104.0 Barrer,while the CO₂/N₂ selectivity 83.9,and CO₂/CH₄ selectivity 34.8,exceeding the Robeson limit of 2008,The SEM and DSC characterization of the membrane showed that the enhancement of the mechanical properties and gas separation performance of Pebax membrane was mainly due to the enhancement of ionic compatibility of ZIF-8/Pebax interface.