Design,Synthesis And Performance Of Ionic Liquids Or Porous Carbons-based Absorbents/Adsorbents For Acidic Gas Capture

The emission of acidic gas?e.g.,CO₂,SO₂ and H₂S?from utilization of fossil fuels has brought huge threat to environment,eco-system and human health.It is of scientific and practical significance to develop technologies for acidic gas capture.Capture medium is the heart of an acidic gas capture system,however,there exist various defects in the capture media extensively used in industry,which results in many environmental and energetic problems with the acidic gas capture technologies.Ionic liquids are a class of efficient,green and energy-saving media for acidic gas capture,however,their performance and applicability still remain to be improved for practical applications.In this work,we designed and fabricated various ionic liquid-based media for acidic gas capture with the purpose of "high-performance" and"applicable",and systematically investigated their performance in acidic gas capture.First,we prepared supported liquid membranes impregnated with fully deprotonated dicarboxylate-based ionic liquids which exhibit good reversibility for the chemical absorption of CO₂.Their performance in the facilitated separation of CO₂ was determined.In comparison with the supported liquid membranes impregnated with normal ionic liquids,amine-functionalized ionic liquids and carboxylate-based ionic liquids,fully deprotonated dicarboxylate-based ionic liquids have superior performance in the selective separation of low-pressure CO₂.For example,the permeabilities of CO₂ with saturated humidity in[N2224]2[malonate]and[N2224]2[maleate]are 2147 and 2840 barrers at 40°Cand 0.1 bar,the permselectivities of CO₂/N2 are 178 and 265,and the permselectivities of CO₂/CH4 are 198 and 221.It is proved that fully deprotonated dicarboxylate-based ionic liquids are a class of highly efficient media for membrane separation of CO₂.Second,we designed and synthesized a series of acid salt ionic liquids?ASILs?with not only high absorption capacity of SO₂ but also low enthalpy change.A Reaction Equilibrium Thermodynamic Model?RETM?was derived to correlate the solubility of SO₂ in ASILs to calculate the thermodynamic properties of SO₂ absorption.Results reveal that ASILs are a class of highly efficient and energy-saving media for SO₂ capture.Due to the high cost and viscosity of pure functionalized ionic liquids,we designed and fabricated two ionic liquid-based mixed absorbents:ionic liquid buffers and ASIL/sulfolane binary mixture.They can be easily prepared from readily available materials and absorb large amount of SO₂ with fast rate,making them highly suitable for industrial applications.By making use of ASILs,ability of capturing SO₂ with good reversibility for the chemical absorption,we prepared supported liquid membranes impregnated with ASILs for the facilitated separation of SO₂-[N2224][dimanolate]display a SO₂ permeability of 7208 barrers at 40°C and 0.05 bar under saturated humidified condition,and the permselectivities of SO₂/N2,SO₂/CH4 and SO₂/CO₂ are 585,271 and 18,respectively.It is proved that ASILs are a class of highly efficient media for membrane separation of SO₂.To explore alternatives to high-cost and highly viscous ionic liquids for SO₂ capture,we investigated the absorption of SO₂ in N-functionalized imidazoles and low-volatile organic solvents.Due to the presence of weak alkaline heterocyclic tertiary amine in N-functionalized imidazoles,they exhibit not only high absorption capacity of SO₂ but also high absorption selectivity of SO₂/CO₂,making them highly efficient media for the selective capture of SO₂.Due to the small molecular weight of low-volatile organic solvents,their absolute solubility?in the scale of molality?for SO₂ is very high,even higher than most high-performance functionalized ionic liquids.Then,considering the low absorption capacity of H₂S in normal ionic liquids,we proposed to use weak alkaline imidazolium carboxylates as functionalized ionic liquids forH₂S absorption.The solubility of H₂S in imidazolium carboxylates is much higher than that in normal ionic liquids.We extended the RETM equation to correlate the solubility of H₂S in imidazolium carboxylates to calculate the thermodynamic properties of H₂S absorption.With assistance of RETM modeling,it is illustrated that one H₂S molecule complexes with two imidazolium carboxylate molecules during absorption.In order to obtain functionalized ionic liquids with not only high absorption capacity of H₂S but also high absorption selectivity of H₂S/CO₂,we designed a series of dual-Lewis base functionalized ionic liquids?DLB-ILs?.Due to the presence of active protons in H₂S molecule,it can interact with the tertiary amine group in DLB-ILs through chemical reaction or strong acid-base interaction.However,the absence of active proton in CO₂ molecule results in a very weak interaction between CO₂ and tertiary amine group.Furthermore,the electron-withdrawing effect of tertiary amine group can weaken the alkalinity of adjacent carboxylate group so that it can not interact with CO₂ strongly like free carboxylate anions any more,but this does not affect its interaction with H₂S.As a result,DLB-ILs are a class of highly efficient media for the selective capture of H₂S.It is also found in this work that in comparison with other normal ionic liquids,hydroxylammonium carboxylate-based protic ionic liquids have higher physical absorption capacity of H₂S and absorption selectivity of H₂S/CO₂.Protic ionic liquids can be easily prepared from readily available materials,making them highly suitable physical absorbents for the selective capture of H₂S.To explore alternatives to high-cost and highly viscous ionic liquids for H₂S capture,we investigated the absorption of H₂S in N-functionalized imidazoles.Due to the presence of weak alkaline heterocyclic tertiary amine in N-functionalized imidazoles,they exhibit not only high physical absorption capacity of H₂S but also high absorption selectivity of H₂S/CO₂,making them highly efficient physical absorbents for the selective capture of H₂S.Traditional absorption process for H₂S capture is energy-intensive,and the storage and transportation of highly toxic H₂S is very risky.We investigated the possibility of using ionic liquids as the media for liquid-phase Claus reaction.It is found that the reaction of H₂S with SO₂ in ionic liquids can proceed very fast and completely to result in sulfur under mild conditions.Therefore,ionic liquids are effective media for the integrated capture-conversion of H₂S.The absorption performance of acidic gas in ionic liquids is mostly determined by the alkalinity of ionic liquids.In order to demonstrate the effect of alkalinity of ionic liquids on their selective absorption of SO₂/CO₂ and H₂S/CO₂,we designed and synthesized a series of substituted benzoate-based ionic liquids?SBILs?with tunable alkalinity.We correlated the solubility of SO₂ and H₂S,absorption selectivity of SO₂/CO₂ and H₂S/CO₂,and thermodynamic properties of SO₂,H₂S and CO₂ absorption in SBILs with the alkalinity of SBILs.Through systematically analyzing,we figured out the proper alkalinity of ionic liquids with optimized absorption selectivity of SO₂/CO₂ and H₂S/CO₂.We also correlated the solubility of SO₂ or H₂S with absorption selectivity of SO₂/CO₂ or H₂S/CO₂ in SBILs to provide a guidance for designing functionalized ionic liquids with both high absorption capacity and high absorption selectivity for the selective absorption of SO₂ or H₂S.Highly efficient and energy-saving chemical absorbents for acidic gas capture should have not only high absorption capacity but also low enthalpy change.By combining RETM equation with van't Hoff equation,we derived equations that can directly correlate gas solubility with enthalpy change of chemical absorption of acidic gas.From the variation curves of gas solubility vs.enthalpy change,it is found that the relationship between them is a distinctive Sigmoid function.With this result,we proposed a criteria for judging whether a chemical absorbent is highly efficient and energy saving.Finally,since traditional methods for the activation and N-doping of porous carbons highly rely on high-temperature?>600°C?reactions,we developed a novel strategy for the activation and N-doping of porous carbons under relatively low temperatures?230³80??by utilizing the strong substitution reaction of NaNH₂ with oxygen species.The low-temperature reactions can effectively avoid the etching of carbon,results in the high yield?>90%?of products.We investigated the adsorption performance of CO₂ on the prepared N-doped porous carbons and results reveal that the adsorption capacity of CO₂ and adsorption selectivity of CO₂/N2 on N-doped porous carbons are significantly enhanced in comparison with pristine porous carbons.Our work results in several new species of ionic liquid-based media for acidic capture,providing various alternative choices for the improvement of acidic gas capture technologies.Furthermore,we supplemented the database of physio-chemical properties of ionic liquids and thermodynamics of acidic gas absorption in ionic liquids.In addition,the work on the relationship between selective absorption performance of acidic gas and the alkalinity of ionic liquids,and the relationship between gas solubility and enthalpy change of chemical absorption of acidic gas,provides a guidance for designing absorbents for acidic gas capture.