Synthesis Of Quaternary Ammonium Functionalized Polymers And Their Application In Direct Air Capture

A series of problems caused by the continuous acceleration of global warming have aroused great concern of the whole society about anthropogenic CO₂emissions,and promoted the development of carbon capture,utilization,and storage(CCUS)technologies.Controlling temperature rise below 1.5°C by the end of this century requires the participation of negative emission technologies(NETs),including direct air capture(DAC)with the lowest land footprints and flexible deployments.The development of medium or high-temperature swing technologies,e.g.,alkali metal-based absorption and solid amine adsorption,is constrained by high energy penalty and high cost.By introducing a novel regeneration mechanism,moisture swing adsorption has the potential to significantly reduce cyclic energy consumption and operational cost,but there still exist problems such as low CO₂capacity and kinetics,weak tolerance towards moisture,and excessive water consumption.In this paper,we explored the influences of interfacial hydrophilicity and micro/mesoporosity on the reaction-diffusion process of ultra-dilute CO₂sorption under humid atmosphere.The rational designs of new materials with targeted regulation of pore structure and hydrophobicity were achieved to improve CO₂capture performance.The theoretical fundamentals for coupling oriented external electric field(OEEFs)to enhance the adaptability towards moisture was built.In this paper,quaternary ammonium(QA)-functionalized polymeric adsorbents were synthesized using lignocellulose,porous polystyrene(PS),and vinylbenzylchloride-fluorostyrene block copolymer as the matrix,respectively.Factors affecting the degree of QA substitution(DS)and utilizing efficiency were investigated,and results showed that the DS in heterogeneous systems was governed by the density and activity of modification sites,as well as the pore structure.With optimized synthesis parameters,the QA contents of cellulose and porous PS-based adsorbents were 0.77 and 1.41 wt.%,respectively.The QA content of block copolymers in homogeneous systems was only related to the ratio of active/inert chain segments and was as high as 4.6?6.6 wt.%.The utilization of QA groups for direct air capture correlated with DS,with 69%,61%,and35%for QA functionalized cellulose,porous PS,and block copolymers,respectively.The uniform distribution of QA groups on the microfiber surface or within regular mesopores is the key factor to efficient modification.The evolutions of microscopic morphology,pore structure,and hydrophilicity were revealed during the screening of PS supports and the optimization of synthesis,through N₂adsorption and isothermal H₂O vapor sorption.The pore structure analysis showed that the filling of meso-macropores and generation of micropores in chloroacetylation were the main causes of morphological and structural changes.The consistency of total and incremental H₂O vapor sorption capacity with humidity reflected the uniform QA substitution in regular mesopores.The positive correlation between capacity and DS was found by isothermal CO₂adsorption tests,and was influenced by the differences in ion-exchange efficiency within complex pore structures.The 8.5%advantage of substitution and 21.0%advantage of ion-exchange efficiency for the mesopore-dominant QMPR-2 over micropore-dominant QMPR-1 resulted in 27%and 83%gains in the DAC and full capacities,respectively.The 22.4%deficiency of ion-exchange efficiency for the meso/macropore-dominant QMPR-3 over QMPR-2 at similar DS was the main reason for 21%?34%reduction in CO₂capacity.The correlations between adsorption kinetics and pore characteristics,e.g.,specific surface area,pore pattern,and distribution,were examined using a mixed 1,2-order kinetic model.The results indicated that the local partial pressure of water vapor is prone to accumulate in narrow channels of irregular pore(ink-bottle or slit),and the combined benefits of cylindrical pores and high specific surface area could shortened the half time to 2.9 minutes,which is the highest kinetics reported so far.The hydrophilicity evaluation based on H₂O vapor sorption was proposed for hydroxyl-rich cellulose matrix and sorption capacity of 0.55?5.85 mol/mol FG at20%?90%RH was obtained.For the first time,the non-monotonic response of CO₂binding energy of QA adsorbents was found to be initially enhanced and then weakened with the humidity(60%?70%RH as the node).The molecule-scale simulation and GAB sorption model revealed the intrinsic mechanisms,as the microscopic anisotropy in cellulose hydrophilicity under hydrated states and the low heat of sorption(47 k J/mol FG)reflecting the activity difference of QA groups under macroscopic weak hydrophilicity.Controllable radical polymerization technique was used to explore the hydrophilicity-hydrophobicity modulation for QA-functionalized polymers.The directional and continuous modulation from strong hydrophilicity to hydrophobicity(3.69?0.93 mmol H₂O/g)was achieved by adjusting the type and number of fluorine-containing chain segments.The D-W hydration model was applied to reveal the interfacial water distribution of“strong/weak/multi-layer sorption”layer by layer and to determine the amount of water sorbed in each layer.The kinetic analysis showed that micro-phase separation and ion transport within the hydrophilic/hydrophobic chain segments were the key factors influencing the CO₂adsorption performance,and the half time was reduced from 11.3 to 4.3 minutes with the acceleration of CO₃^2–/HCO₃^–diffusion.CO₂capacity was decided by multi trade-offs including the decreased active sites and enhanced OH^–generation by hydrophobic blocks,the enhanced ion transport by phase separation and inhibited transport due to over-assembly.Tolerance towards high moisture could be improved by controlling the hydrophobicity at moderate levels with?2 mmol CO₂/g capacity achieved.DFT simulations of oriented external electric fields(OEEFs)-enhanced CO₂capture were developed in a finite field framework.Results showed that the field strength can precisely regulate the direct interaction energy between QA ion pair and CO₂,the proton transfer can be driven by an electric field and influenced by field strength,and the enhancement is more significant by increasing the field strength component in the H-bond axis while the electric field exerted in negative reaction axis weakens the ionic bond strength.The electric field can amplify the hydrophilicity of reactants and hydrophobicity of products simultaneously.The radii of hydrated CO₃^2–/H₂O or HCO₃^–/H₂O system could increase by 7.2%or decrease by 9.4%,respectively.The electron density data reflected the opposite trends of the H-bonding strength and interfacial interactions within the water molecular clusters of both.The free energy calculations at highly-hydrated states revealed that rational modulation of field strength can make adsorption proceed spontaneously,and electric field can be used as a special catalyst to reduce the activation energy barrier while avoiding the weakening of binding strength.