Fabrication And Performance Of Polyoxyethlene Based Membrane For CO₂ Separation

Fossil fuels provide energy and materials for society,but they also emit large numbers of greenhouse gases,like CO₂,leading to the greenhouse effect,which results in serious climate and ecological environment issues.The amount of CO₂ emitted by coal-fired power generation takes a higher portion in the overall carbon-emitting industry,thus,capture and utilization of CO₂ in flue gas are quite essential.Compared with the traditional separation process,membrane separation technology is one of the most effective technologies for CO₂ capture due to its distinctive features,such as small footprint,low energy consumption,and environmental friendliness.High-performance membrane materials are the core that determines of the final separation performance of CO₂ separation membranes.Polyethylene oxide（PEO）-base membrane commonly shows high CO₂/N₂ separation selectivity,owning to the dipole-quadrupole interaction between the ether oxygen groups of PEO and CO₂ molecules promoting the solution of CO₂ in the membrane.However,the low free volume of PEO membrane renders CO₂permeability,which hampers the application of PEO membranes.To solve this problem,this study proposes to construct a high-speed mass transfer channel in the membrane to enhance the mass transfer of CO₂ and thus improve CO₂ permeability.1)Tailoring the flexibility of the PEO chain to increase the free volume of the membrane,enhancing the CO₂ permeability;2)in-situ growth of porous additives in the PEO membrane to create more mass transfer channels for CO₂ transport,boosting the CO₂ permeability.Firstly,a“high-temperature activation coupled dipole-locking”strategy was used to tailor the microstructure of the crosslinked PEO membrane,which is designed to enhance the membrane free volume.Polyethylene glycol diacrylate（PEGDA）and polyethylene glycol methyl ether acrylate（PEGMEA）were used as prepolymers for reparation of crosslinked PEO membranes via UV-induced radical polymerization.The microstructures of crosslinked PEO membranes were tailored by solvothermal annealing.The resultant membrane microstructures were characterized by Fourier transform infrared spectroscopy（FT-IR）,X-ray diffraction（XRD）,density measurement,electrochemical impedance,and mechanical test.Moreover,the mechanism of the flexibility increases for the PEO chain via solvent-thermal annealing was elucidated.The dependence of the separation performance on the annealing temperature,reaction time,and solvent types were detailed investigated.The results showed that,under the optimal condition（methanol as the solvent,annealing at 85^oC for 20 hours）,the CO₂ permeability of the PEO membrane was enhanced to 1294 Barrer,2-fold enhancement compared with the pristine crosslinked PEO membrane,with the CO₂/N₂ selectivity（51）unchanged.Subsequently,a“confined swelling coupled solvent-controlled crystallization”（CSSC）strategy was employed for in-situ growth of porous zeolite imidazolate skeleton material（ZIF-8）nanocrystals and construction of freeway channels for CO₂transportation in ZIF-8/PEO mixed matrix membranes（MMMs）.The crosslinked PEO membrane was prepared via UV-induced radical polymerization,which offered the matrix for in situ crystallization of porous ZIF-8 with the assistance of the synergistic effect of ammonia（NH₃·H₂O）and methanol（Me OH）to promote the coordination reaction between the zinc ion and 2-methylimidazole.The microstructure of the ZIF-8/PEO MMMs was characterized by FT-IR,XRD,scanning electron microscopy（SEM）,N₂ isothermal adsorption,X-ray photoelectron spectroscopy（XPS）,and inductively coupled plasma（ICP-MS）.The results showed that porous ZIF-8nanoparticles were closely stacked in the membrane and formed good interface with PEO matrix,which thus constructed the gas mass transfer channels for gas molecules.The effects of NH₃·H₂O concentration,reaction time,and the amount of precursor in the casting solution on the structure and properties of the membranes were investigated systematically.The results showed that the CO₂ permeability was significantly boosted from 588 to 2490 Barrer with ZIF-8 content increasing to 51.8 vol%,a 4-fold increase,with the CO₂/N₂ selectivity was 37.In addition,the incorporation of ZIF-8 remarkably improved the anti-plasticization capability of PEO-based separation membranes.Finally,a“ligand-assisted interface strengthening”strategy was proposed to optimize the interface structure of ZIF-8/PEO MMMs and augment the ZIF-8 loadings in the membrane.Taking advantage of the strong coordination interaction between ligands and metal ions,we increased the content of 2-methylimidazole in the casting solution to weaken the interaction between metal ions and PEO.Resultantly,the flexibility of PEO chains and the reactivity of zinc ions were improved,the ZIF-8crystal size was reduced,and the ZIF-8 loading in the membrane was increased.FT-IR,XRD,ICP-MS,low-field nuclear magnetic resonance（LF-NMR）,and SEM were employed to characterize the membrane microstructures,which demonstrated that the proposed strategy could significantly improve the ZIF-8/PEO interactions and increase the loading of ZIF-8 in the membrane（up to 61 vol%）.The effects of ZIF-8 precursor ratio,ammonia content,and the amount of precursor in the casting solution on the membrane microstructure and gas separation performance were investigated.The results revealed that the CO₂ permeability coefficient reached 2868 Barrer when the loading of ZIF-8 was 61 vol%,which was 5 times higher than that of the PEO cross-linked membrane,and the CO₂/N₂ selectivity was 35.In this thesis,the rapid CO₂ mass transfer channels were constructed by tailoring the flexibility of PEO chains or in-situ crystallization of porous ZIF-8 nanocrystals in PEO matrix,which significantly enhanced the CO₂ permeability of PEO-based membranes.The work provides novel ideas for the design and preparation of high-performance PEO-based gas separation membranes,which promotes the potential application of the PEO-based membranes for CO₂ capture.