Study On Adsorption And Separation Of Carbon Dioxide Based On Porous Carbon

The extensive use of fossil fuels has led to the emission of a large amount of carbon dioxide（CO₂）,which has caused many environmental problems.In 2020,China officially put forward the carbon peaking and carbon neutrality goals to strictly control CO₂ emissions.Therefore,how to reduce CO₂ emissions is an important research topic.Porous carbon is one of the research hotspots in the field in recent years because of its extensive sources,easy adjustment of pore structure and surface properties.However,low adsorption capacity and poor adsorption selectivity seriously limit its application.This paper aims to explain the adsorption mechanism of CO₂ on the porous carbon,reveal the influence of pore structure and surface functional groups on CO₂ adsorption and separation,and guide the development of porous carbon adsorbent with excellent performance.The research contents mainly include the following four aspects:（1）Through multi-scale simulation,the influence mechanism of various functional group doping and pore size distribution on CO₂adsorption and separation was systematically studied.Based on the Density Functional Theory and the giant canonical Monte Carlo calculation method,the slit adsorption models of oxygen-containing functional group doping,nitrogen-oxygen co-doping and alkali metal doping was constructed.The results show that when the adsorption pressure is 1 bar,the rules of oxygen functional group doping and nitrogen and oxygen co-doping are basically the same,that is,when the pore size is 0.35 or more than 0.9 nm,the doping of functional group can not improve the adsorption capacity of CO₂.This is because when the pore size is too small,the main form of absorption is pore-filling process.When the pore size is too large,the interaction intensity between functional groups and CO₂molecules is insufficient to overcome the interface resistance and enter the adsorption site;The most suitable pore size range of functional group doping is 0.35-0.9 nm.When the adsorption pressure is 0.15 bar,the promotion effect of nitrogen,oxygen and sulfur functional groups on adsorption is limited to a certain extent,and alkali metals show better adsorption promotion effect,among which the effect of Na doping is the best,and the optimal range of CO₂adsorption pore size is reduced to 0.35 to 0.6 nm;For the selective separation process of CO₂,alkali metal doping has a wider effective pore size,which is because on the one hand,it enhances the electrostatic interaction between the surface and CO₂ molecules,on the other hand,it inhibits the adsorption of N₂.（2）By improving the synthesis strategy,nitrogen-oxygen doped narrow microporous carbon with high CO₂ adsorption capacity was prepared.The doping of functional groups often leads to the deformation of carbon skeleton,resulting in the change of pore size and the consumption of carbon atoms.Through the uniform introduction of potassium ions in the resin precursor,in situ activation produces abundant narrow micropores.This method has high efficiency and can avoid the enlargement of pore size and the loss of carbon atom as much as possible while introducing nitrogen and oxygen element.The as-prepared porous carbon has a yield of 30-40 wt%,a fixed shape,and the maximum nitrogen and oxygen content and narrow pore volume are 17.73 wt%and 0.30 cm³/g,respectively.When the adsorption temperatures were 0,25 and 40°C,respectively,the maximum saturated adsorption capacity reached 7.06,5.00 and 3.70 mmol/g,which was higher than most of the reported similar adsorbents（3）Porous carbon with narrow microporous-mesoporous cross-linked structures was prepared by in-situ bimetallic activation,which promoted the CO₂ adsorption at low partial pressure.The adsorption process of CO₂in the actual flue gas atmosphere is seriously hindered due to the low fraction.Therefore,alkali metals that can prepare narrow micropores and alkaline earth metals that can prepare mesoporous structures were introduced into resin precursors.By controlling the reaction ratio,it was found that the BET surface areas of Li@Mg-1 and Na@Mg-1 were 289and 442 m²/g,respectively,much smaller than the 1020 m²/g shown by K@Mg-1.It shows that K is more suitable for joint activation with other metals.Binary dynamic penetration experiments showed that the CO₂adsorption capacities of K@Mg-1,K@Ca-1 and K@Zn-1 were 1.36,1.48and 1.54 mmol/g,respectively,which were significantly higher than those of KC samples.It is further proved that they have faster mass transfer rate by Pseudo-first-order model.（4）The alkali metal modified porous carbon was prepared by impregnating the porous carbon rich in acid COOH group in alkali solution.The results of theoretical studies have proved the catalytic effect of alkali metals,but reports of alkali metal doping only focus on MOFs adsorbents.Therefore,firstly,porous carbon with COOH group accounting for 33.67%was prepared,and then the porous carbon modified by alkali metal was prepared by dipping in alkali solution.It was found that due to the existence of water washing process,the use of Li₂CO₃,Na₂CO₃ and K₂CO₃ solution impregnation had no effect on the CO₂ adsorption performance of porous carbon.The use of Li OH,Na OH and KOH solution impregnation not only did not change the pore size distribution of porous carbon,but also significantly improved the CO₂ adsorption and separation performance.The CO₂/N₂ selecitivity calculated by IAST method were 47,41 and 39,respectively.However,the water and thermal stability of the modified sample is poor,and the adsorption performance decreases significantly after a long time of water treatment or heating treatment.