Preparation Of Composite Membrane By Interfacial Ploymerization Incorporated With Functionalized Carbon Nanomaterials For Gas Separation

In recent years,in order to solve the air pollution problem caused by the using of large amount energy,various processes have been developed to separate and recover carbon dioxide（CO₂）in industrial exhaust gas.In these methods,the absorption,adsorption method require long cycle and high cost.The low-temperature distillation method consumes a large amount of energy,it is not suitable for the separation and recovery of CO₂although it has advantages in special gas separation.Therefore,membrane separation is considered as the best means to separate CO₂ because of its green,high efficiency,low energy consumption and low cost.The interfacial polymerization method has the advantages of wide variety of monomers,simple process and controllable structure,which has been widely used in the preparation of nanofiltration and reverse osmosis membranes.At present,this method is rarely used in the preparation of the gas separation membrane.Therefore,it is a very meaningful work to develop a gas separation membrane by interfacial polymerization process and apply in the field of CO₂ separation.Presently,in the study of CO₂ separation by membrane method,the trade-off phenomenon in which the permeation flux and selectivity are mutually restricted and this phenomenon is severely constrained the application and performance of this method.In order to improve the comprehensive performance of the separation membrane,many researches have been found to achieve this goal.The main methods can be divided into three types:1.Ultrathinization of the separation layer;2.Controllable modification of the separation layer medium;3.The introduction of carbon nanomaterials.The interfacial polymerization process used in this thesis is an effective method widely used in the preparation of ultrathin composite membranes in recent years.Here,the water phase monomer-β-cyclodextrin with special structure and the new carbon nanomaterial carbon quantum dots（CQDs）as a dopant for the aqueous phase monomer.The performance of CO₂ separation was discussed.The influence of nanomaterials on the morphology of the composite membrane was further investigated.The relationship between the microstructure of the composite membrane and the gas separation performance was discussed.According to the gas separation performance of the composite membrane,the cyclodextrin and acid chloride monomer were firatly selected,then the various process parameters,such as monomer concentration and reaction time,were optimized.The gas separation composite membrane with excellent performance was fabricated at the optimal condition.The chemical composition of the membranes was determined by Fourier transform infrared spectrometer,and the morphology of the membrane was characterized by scanning electron microscopy and atomic force microscopy.The results showed thatβ-cyclodextrin and trimesoyl chloride（TMC）are used as the water phase and oil phase monomer respectively,and the morphology of the film with special"cyclic porous"structure can be obtained.The optimal condition of membrane fabrication is 2wt.%β-cyclodextrin and 1wt.%Na OH as the aqueous phase solution,1wt.%TMC in hexane as the oil phase solution.The reaction time of aqueous phase and oil phase is 10 min.The heat treatment of the membrane is 50°C for 10min.The gas separation performance of the composite membrane prepared under the optimal condition has a permeation flux of CO₂ of 200 GPU and CO₂/N₂ selectivity of 10.53.In order to further improve the gas separation performance of the composite membrane,CQDs were prepared by pyrolytic citric acid method.The chemical structure was determined by Fourier transform infrared spectrometer.The morphology and particle size of CQDs were determined by transmission electron microscopy,X-ray photoelectron spectroscopy and dynamic light scattering.The results indicated that the particle size of CQDs was 2.8nm and uniform distribution.Furthermore,the amino functionalization of CQDs was carried out by hydrothermal reaction.The corresponding structural characterization confirmed that amino group with stronger water solubility,reactivity and selectivity to CO₂ was successfully grafted on the surface of CQDs.The functionalized CQDs were introduced into the aqueous phase monomer,and the composite membranes with different content of CQDs doping was prepared by interfacial polymerization.Scanning electron microscopy（SEM）results showed that the introduction of CQDs had significant influence on the surface morphology of the composite membrane.When the doping amount of the unmodified carbon quantum dots was 0.1 wt.%,the CO₂permeation flux of the composite membrane was 157 GPU,and the separation selectivity of CO_2/N₂ was 15.9.When the doping amount of CQDs after amination was 0.1wt.%,the CO₂permeation flux of the composite membrane was 148 GPU,and the CO₂/N₂selectivity was further increased to 19.7.It can be seen that the introduction of carbon quantum dots was very obvious for the comprehensive performance of the composite membrane.In order to compare the effect of zero-dimensional carbon nanomaterials CQDs on the separation performance of composite membranes,multi-walled carbon nanotubes（MWCNTs）were also introduced into the composite membranes.The surface morphology was observed by scanning electron microscopy.The SEM image and the gas separation performance were used to investigate the effects of different latitude carbon nanomaterials on the morphology and properties of the composite membrane.The separation mechanism of carbon nanomaterials on the composite membranes was discussed.