Preparation Of Nitrogen-doped Porous Carbon Fibers And Their Carbon Dioxide Adsorption Properties

Carbon dioxide emissions lead to global warming,which has a great impact on the environment where animals and plants live.The CO₂ capture is an effective technology which can mitigate climate change caused by massive CO₂ emitted into the atmosphere.Porous carbon fiber has a good application prospect in post-combustion CO₂ capture by virtue of high specific surface area,controllable surface chemical structure,high physicochemical stability,and good mechanical properties.In order to further enhance the CO₂ adsorption capacity and selectivity of carbon materials,polyacrylonitrile(PAN)-based and polyimide(PI)-based porous carbon fibers were prepared using fibers with high nitrogen content as precursors.The textural properties and nitrogen-containing functional groups of nitrogen-doped porous carbon fibers were adjusted,and the mechanism of the influence of the structural characteristics of the adsorbent materials on CO₂adsorption and selectivity was explored.This paper first used nitrogen-enriched oxidized PAN fibers as precursors,nitrogen-doped porous carbon fibers were obtained via heat treatment in air.In the process,the precursor fibers were carbonized and activated simultaneously.After air heat treatment,oxidized PAN fibers formed graphite-like microcrystallines and produced abundant ultramicropores whose volume was0.108?0.122 cm³/g.In addition,the nitrogen-doped porous carbon fibers had a nitrogen content that was higher than 20.8 wt%,and most of the nitrogen was in the type of pyrrolic-/pyridonic-N,which was benefical for CO₂ capture.The nitrogen-doped porous carbon fibers exhibited a CO₂adsorbed amount of 99 mg/g and an excellent CO₂/N₂ selectivity of 183,which is mainly due to the plentiful nitrogen-containing functional groups.What's more,the sample had a low isosteric heat of adsorption,between 26.65 k J/mol and 30.84 k J/mol,and owned good renewability and reusability.PI fibers were used to precursors,Zn Cl₂,and K₂CO₃ were selected as activators.Nitrogen-doped porous carbon fibers were prepared by heat-treating the mixtures of PI fibers and activator in nitrogen atmosphere.Compared with the porous carbon fibers prepared by direct carbonization,the activator significantly increased the specific surface area and pore volume of porous carbon fibers.PCF-K₂CO₃ had the largest ultramicropore volume,reaching 0.309 cm³/g.Furthermore,PCF-Zn Cl₂ showed the highest nitrogen content(7.33 wt%)and yield.The results of CO₂isothermal adsorption and desorption experiments showed that the adsorption capacity of porous carbon fibers prepared by activation method had also been improved.At 25 ^oC and 1 bar,the CO₂adsorbed amount of PCF-Zn Cl₂ and PCF-K₂CO₃ were 144 mg/g and 202 mg/g,respectively.However,while the CO₂ adsorbed amount of porous carbon fibers prepared by direct carbonization was 128 mg/g.K₂CO₃ was selected as the activator,the textural properties and chemical structure of PI-based porous carbon fibers are further regulated by heat treatment temperature.After synchronous carbonization and activation treatment,PI fibers exhibited a disordered graphite structure.The ultramicropore volume first increased and then decreased with the increase of heat treatment temperature,and showed the highest ultramicropore volume at 800 ^oC.It was found that the enhancement of ultramicropore volume of the PCFs facilitate CO₂ capture even if the reduction of nitrogen content diminished the strength of interaction between adsorbent and CO₂.Besides,PI-based porous carbon fiber showed an acceptable Henry's CO₂/N₂ selectivity of 24,displayed a low isosteric heat of adsorption,between 24.69 k J/mol and 25.63 k J/mol,and had good renewability and reusability.