Design And Preparation Of Nitrogen-Containing Functional Materials And Their Performance On CO₂ Capture And Conversion

As a major greenhouse gas,carbon dioxide（CO₂）is one of the main causes of global climate warming,which generally comes from fossil energy combustion and factory exhaust.In order to alleviate related environmental problems,the concept of"carbon neutrality"came into being,including the core technology of CO₂ capture and conversion.The traditional materials for CO₂ capture or adsorption always specialize in pure CO₂ gas,but the CO₂content in industrial waste gas only accounts for about 15%actually,which makes it difficult for these materials to capture diluted CO₂ effectively and selectively.Moreover,the molecular inertness of CO₂ further complicates its conversion process.Taking the cycloaddition reaction of CO₂ with epoxides as an example,highly active metal sites are usually required to convert it into cyclic carbonates under harsh conditions（high temperature and pressure）.It is worth emphasizing that for the diluted CO₂ cycloaddition reaction,even if the reaction system contains metal active sites,it is extremely difficult to be converted efficiently.Therefore,a variety of advanced materials have been developed by many researches and successfully used in-the applications of CO₂ capture and conversion in recent years.Among them,N-containing functional materials have attracted extensive attention due to their structural and molecular properties,including N-doped porous carbon materials and N-containing functionalized ionic liquids（ILs,ionic liquids）.On the one hand,N-doped porous carbon materials exhibit remarkable selective CO₂ adsorption capacity in CO₂ capture.On the other hand,N-containing functionalized ILs perform excellent catalytic activity in the field of CO₂conversion.This work focused on the capture and conversion of diluted CO₂ and developed a series of low-cost and high-performance materials and catalysts.Firstly,the one-step preparation of ultra-microporous nitrogen-doped porous carbon materials was studied,and the influence of synergistic effect between material structure and heteroatoms on performance of diluted CO₂ capture and conversion was discussed.Secondly,a series of functionalized ILs were synthesized by regulating the anion and cation structures of ILs,and then the role of ionic structures in diluted CO₂ cycloaddition reactions was explored.Thirdly,hierarchical porous ionic carbon composites were successfully constructed by introducing functionalized ILs into ultra-microporous nitrogen-doped porous carbon materials.On this basis,the in-depth mechanism of composite material to efficiently catalyze the diluted CO₂cycloaddition reaction was thoroughly studied.This paper not only enriches the methods of diluted CO₂ capture and conversion,but also is expected to promote the development of the flue gas treatment industry,which will effectively improve the operational efficiency of the industrial chain including carbon emission and carbon utilization.The main research contents are summarized as follows:（1）To solve the difficulty in selectively capturing diluted CO₂ in exhaust stream,we employ cheap and abundant biomass glucose as the carbon source,and urea as the nitrogen source.Then,a one-step carbonization was performed using green and nontoxic potassium acetate（KAc）as the chemical activator to prepare N/O-rich ultramicroporous activated carbon（PGNC）with a variety of pore size distributions,When the amount of KAc was 0.5 g（The amount of glucose and urea are both 1 g）,the PGNC-0.5 material had optimal porosity and heteroatom content.Moreover,relying on the synergistic effect between the ultra-microporous structure and double heteroatoms,PGNC-0.5 could achieve high-selectivity and high-capacity capture of diluted CO₂（0.15 bar and 0.05 bar）.In addition,PGNC-0.5 also exhibited excellent effective retention time in the breakthrough experiment of CO₂/N₂ binary mixture.（2）Furthermore,PGNC-0.5 was used in the cycloaddition reaction of atmospheric pressure CO₂（1 bar）with epoxides,and the characterization of CO₂-TPD and FT-IR revealed that the PGNC-0.5 catalyst has the ability to activate CO₂ at multiple sites.As a metal-free catalyst,PGNC-0.5 could effectively catalyze the cycloaddition reaction of CO₂ and different epoxides under the condition of 1 bar CO₂,with the yield between 96-99%and the highest conversion frequency（TOF）of 17.74 h^-1.The thermal filtration experiments showed that the PGNC-0.5 catalyst has good stability and the cyclic catalytic results showed that PGNC-0.5can maintain the initial catalytic activity in multiple catalytic reactions.These results demonstrate that the PGNC catalysts snot only possess excellent catalytic efficiency but also outstanding recyclability.Additionally,based on the reported literature and the experimental results of this work,the possible catalytic mechanism of PGNC+TBAB catalyst was proposed in the cycloaddition reaction of CO₂ and epoxides at atmospheric pressure.（3）Then,a variety of functionalized ILs were synthesize and the effects of structural parameters on the diluted CO₂（0.15 bar）cycloaddition reaction were explored,including hydrogen bonding,cation structure,anion species,functionalized monomers,and steric hindrance.These studies provide theoretical support for the rational design of catalysts to efficiently catalyze diluted CO₂ cycloaddition reactions.Specifically speaking,the study found that the hydrogen bond donor ability of the catalyst is the key to catalyze the conversion of diluted CO₂ efficiently,and the cations of ILs also affect the electrophilic and nucleophilic efficiency of the cycloaddition reaction to a certain extent.Moreover,the positive charge delocalization of ILs cations can effectively enhance the ability to activate epoxides and CO₂.For example,the catalyst of[Al TMG]Br and[TMGH]Br both have hydrogen bond donor ability,but the catalytic efficiency of positively charged delocalized[Al TMG]Br is 19%higher than that of[TMGH]Br under normal pressure CO₂（1 bar）.Under the diluted CO₂ condition,the catalytic efficiency of[Al TMG]Br is 87%higher than that of[TMGH]Br.In addition,the DFT theoretical calculation results also show that the interaction between[Al TMG]Br and the reaction substrates（epoxide and CO₂）is optimal,which enables the[Al TMG]Br catalyst to efficiently activate the reaction substrates even under diluted CO₂.（4）The functionalized ILs were employed as ionic sites to improve the catalytic performance of PGNCs and endows the material with functionality.Firstly,functionalized ILs were introduced into the PGNC framework by free radical polymerization,and various hierarchical porous ionic carbon composites（PGNC-ILs）were successfully prepared.The introduction of high catalytic sites and the formation of hierarchical pores improved the catalytic conversion of dilution CO₂ and the mass transfer efficiency,which broke through the dilemma that heterogeneous catalysts are difficult to complete the conversion of diluted CO₂without metal additives.In addition,this series of composite materials can realize the regulation of catalytic activity by adjusting the functionalized ILs structure.The composite of PGNC-0.5-[Al TMG]Br-2 easily completed the cycloaddition reactions between CO₂（1 bar and 0.15 bar）and different epoxides without any additives or co-catalysts with excellent product yields.Furthermore,DFT calculation results show that the electrostatic potential value of the ionic site of the PGNC-0.5-[Al TMG]Br-2 is higher（59.92 kcal/mol）,and its TS1and TS2 transition state energies are also the lowest.The above factors endow PGNC-0.5-[Al TMG]Br-2 better substrate activation ability,which is beneficial to diluted CO₂cycloaddition reaction.