Preparation And CO₂ Adsorption Properties Of Phenolic Resin-derived Nitrogen-doped Porous Carbons

Carbon dioxide is one of the main greenhouse gases.Excessive emissions of greenhouse gases can cause a series of environmental problems,such as melting glaciers,rising sea levels,and global warming.To mitigate the CO₂ emission,CO₂capture,storage and utilization have been regarded as the most effective and feasible methods.Among various CO₂ separation and capture techniques,adsorption via solid adsorbents has the advantages of low equipment investment cost,simple operation,low energy consumption,easy regeneration of adsorbent,and no equipment corrosion problem.The key of this technology is to find sorbents with superior CO₂ adsorption properties.Among various solid porous sorbents such as zeolites,silica,alkali metal carbonates,metal organic frameworks?MOFs?and carbons,porous carbonaceous sorbents are considered as the most promising solid adsorbents due to their simple preparation,low production cost,high stability,easy regeneration,adjustable pore structure and hydrophobicity.In order to obtain low-cost and high-effective porous carbonaceous adsorbents,a commercial phenolic resin was selected as the carbon precursor.Three different synthesis strategies were employed to synthesize nitrogen-doped porous carbons.The as-synthesized adsorbents were characterized using different techniques.Their CO₂ adsorption properties were also carefully investigated.The CO₂ adsorption mechanism for these sorbents was further explored in combination with various characterization results.The summaries of the results are as follows:1.Using commercial phenolic resin as carbon precursors,carbonaceous sorbents have been synthesized through a carbonization-urea modification-KOH activation strategy.Nitrogen-doped porous carbon adsorbents with different porous structure and surface chemistry were obtained by changing the ratio of KOH to carbon?2:1,3:1,4:1?and activation temperature?600oC,650oC,700oC?.The as-prepared porous carbon materials were characterized by SEM,TEM,XRD and XPS etc,and their CO₂adsorption capacities were measured at 1bar,25°C and 0°C,respectively.It was found that these nitrogen-doped porous carbons exhibit CO₂ adsorption capacity of 3.30⁴.61mmol/g at 25oC and 5.32⁷.13mmol/g at 0oC under 1bar.The sample?RUK-600-3?prepared under activation temperature of 600oC and mass KOH/precursor of 3,demonstrated the maximum CO₂ uptake capacity among this series of samples under ambient conditions.A systematic study shows that the synergetic effects of narrow microporosity and nitrogen content determine the sorbents'capability to capture CO₂.In addition,the pore size and the narrow micropores'distribution affect the CO₂adsorption capacity of this series of porous carbons.Moreover,these resin derived carbons show other superior CO₂ capture properties such as fast sorption kinetics,stable recyclability,high CO₂/N₂ selectivity,moderate heat of adsorption,and high dynamic CO₂ capture capacity.These excellent properties indicate that the phenolic resin-based nitrogen-doped porous carbons have application prospects in CO₂ capture and separation.2.In order to further simplify the synthesis process and thus to reduce the preparation cost of the adsorbent,in this section,the above-mentioned two steps i.e.carbonization and urea nitridation are combined into a one-step reaction.The mixture of the resin and the urea is heated under the nitrogen atmosphere to achieve nitrogen-doped carbon precursor.Following KOH activation under various conditions,a series of porous carbonaceous adsorbents with different porous structure and surface chemistry were achieved.It was found that the adsorbent synthesized by this method have a developed porous structure and high nitrogen content.More importantly,the maximum CO₂ adsorption capacity of these adsorbents are 5.01mmol/g at 25°C and7.47mmol/g at 0°C,at 1 bar respectively,which are higher than those prepared by the conventional carbonization-nitridation-KOH activation strategy.It was found that the CO₂ uptake of these sorbents was determined by the synthetic effects of pore volume of narrow microporosity,pore size,pore size distribution and nitrogen content.The more facile preparation method and better CO₂ adsorption properties make these adsorbents more attractive in the application of CO₂ capture.3.Despite their high CO₂ adsorption capacity of the above-mentioned nitrogen-doped porous carbons,they were achieved by KOH activation.KOH is extremely corrosive and its corrosivity increases with increasing temperature,which often damages the reactor and hinders its industrial application.Therefore,it is urgent to find other effective activators to replace it.In this section,sodium amide was chosen as the activator.Sodium amide can effectively activate the carbon precursor under relatively low temperatures?400⁵00oC?due to its strong nucleophilicity and basicity.Another specific feature for NaNH₂ is that it can also act as a nitridation reagent.The nitrogen-doped porous carbons with highly developed narrow microstructures can be obtained by a single-step reaction of NaNH₂ and carbon precursor.In this section,a series of nitrogen-doped porous carbons with samples different porous structures and nitrogen contents were achieved by changing the amount of sodium amide?mass ratio of sodium amide to carbonized resin is 1:1,2:1,3:1,4:1?and activation temperature?400°C,450°C,500°C?.The as-obtained samples were carefully characterized and their CO₂adsorption capacities were also measured.The results show that the sample obtained at the activation temperature of 450°C and mass ratio of sodium amide to carbonized resin of 3?RN-450-3?has the maximum CO₂ adsorption capacity among these samples,which are 4.64mmol/g at 25°C and 7.13mmol/g at 0°C under 1bar.To the best of our knowledge,this value is higher than any porous carbons obtained without using KOH as the activator and at activation temperature lower than 500°C.In addition,these porous carbons demonstrate rapid adsorption kinetic,excellent recyclability,stability,and dynamic CO₂ capture capacity,as well as moderate heat of adsorption and high CO₂/N₂ selectivity.The multiple merits of these carbons reveal that they have comprehensive application prospect in the CO₂ capture and separation.