Preparation Of Cellulose-based Microporous Carbon By Solvent-free Method And Its Adsorption And Separation Of CH₄/N₂

Guizhou Province has a large amount of coalbed bed methane（CBM）resources,but the concentration of CH₄ in the mined CBM is low and cannot be used directly because of the special topography.Therefore,the effective separation and enrichment of CH₄ in CBM is critical to improve the utilization rate of CBM.The pressure swing adsorption method is suitable for the separation and enrichment of CH₄ in low-concentration CBM,and the adsorbent is the key to the realization of this technology.The carbon materials are widely used because of the low preparation cost,simple preparation method,and excellent structure with high specific surface area and pore volume.However,there are still some problems in the preparation of carbon materials,such as the uneven distribution of pores and the low pore volume of micropores,which leads to low adsorption capacity of carbon materials for CH₄ and the inability to effectively separate and enrich CH₄/N₂.KOH activation method based on K intercalation is an effective method to increase the ultra-micro pore volume of carbon materials.The carboxyl group（-COOH）on the surface of carbon precursor reacts with K⁺to generate-COOK,which can realize the introduction of K into the carbon precursor to improve the ultra-micro pore volume of carbon materials.Cellulose acetate has a higher O/C ratio than cellulose,and the content of acetyl groups in cellulose acetate has a linear correlation with oxygen-containing functional groups,especially carboxyl groups（-COOH）.At the same time,the functional groups on the surface of the material are increased by doping heteroatoms,and the surface properties of the adsorbent are changed to improve its catalysis and adsorption performance.Based on this,a series of carbon materials with high micropore volume and uniform micropore distribution were prepared with cellulose derivatives as raw materials and strong alkali or organic alkali of potassium as an activator in the absence of solvent,which were applied to the separation and enrichment of CH₄/N₂ in low-concentration Cbm.The process steps are as follows:1.The carbon materials are prepared under solvent-free conditions by using cellulose acetate after hydrothermal carbonization as the carbon source and KOH as the activator.The surface carboxyl group content is regulated by adjusting the acetyl content of cellulose acetate,thereby affecting the microscopic volume of carbon materials.The results show that the cellulose acetate precursor with higher acetyl content has more oxygen-containing functional groups on the surface,which can increase K⁺interaction,and the prepared material has higher ultra-micro pore volume and uniform pore size distribution after high temperature activation,thus increasing the adsorption capacity of CH₄.The micropore volume of the material prepared under the activation temperature of 700°C and a low ratio of hydrothermal carbon:KOH=1:2 is 0.49 m³/g.and it shows excellent adsorption effect.At 273k and1bar,the adsorption capacity of CH₄ is 3.18 mmol/g（the same below）,the selectivity of CH₄/N₂ is 21.6,and it has good regeneration performance.2.In order to further improve the micropore volume of the materials and reduce the strong corrosiveness caused by KOH,the carbon materials were prepared by the co-activation method of KOH/KAc/CH₄N₂O using cellulose acetate with an acetyl content of 54.5%as the raw material.The amount of KOH is reduced by adding KAc to reduce the corrosivity,and CH₄N₂O is added to introduce N element into the carbon structure to change the surface properties of the material.The results show that the addition of KAc and CH₄N₂O promotes the development of the microporous structure in the carbon structure,which makes the materials have higher microporous pore volume.The materials synthesized under the optimal preparation conditions（hydrothermal carbon:KOH:KAc:CH₄N₂O=1:1:1:1,activation temperature of 800°C）have large micropore specific surface area（1780 m²/g）and micropores pore volume（0.88 m³/g）.The adsorption capacity of CH₄ was 2.76 mmol/g,and the selectivity of CH₄/N₂ was 18.0,which shows good adsorption and separation performance.3.In order to increase the N element content in the carbon material to change the surface properties of the material and increase the micropore volume of the material,the carbon material was prepared by using the polyionic liquid produced by the reaction of carboxymethyl cellulose and nitrogen-containing organic base1-methylimidazole as the precursor,and then mixed with KOH after carbonization.The carbon material with the addition of 1-methylimidazole has better stability,higher N content and higher micropore specific surface area（2489 m²/g）.The doping of N element increases the active sites of the carbon material,thereby increasing the defect structure of the material and obtaining a higher micropore volume.The material prepared under the condition（carbonized carbon:KOH=1:2,activation temperature of800°C）has the best performance.The adsorption capacity of CH₄ is 3.04 mmol/g,and the selectivity of CH₄/N₂ is 20.5.It has the potential to be applied to the separation and purification of CH₄ in low-concentration CBM.