Synthesis and Gas Adsorption of Pitch-based N-Doped Porous Carbon

Author:Song Xin Zuo

Supervisor:gao jin sen ning guo qing


Degree Year:2018





The energy consumption of our country is very large all over the world,the exhaust gas from refinery,power plant and other large factories contains a variety of harmful gases.The acid gas SO2 and greenhouse gas CO2 are the main exhaust components which cause serious environmental problems and need to be urgently governanced.As the environmental problems have become increasingly severe,the greenhouse gases like SO2and CO2 require urgent governance.Porous carbon materials present promising application prospect due to the abundant porous structure,favorable adsorption ability,heat resistance and corrosion resistance.The dry desulfurization technologies using carbon materials in power plants or refinery plants usually suffer from the problems such as frequent regeneration,carbon corrosion aroused by the high temperature(higher than350oC)regeneration,and the carbon loss due to the inevitably moving and abrasion of carbon particles.The most commonly used technology for CO2 capture is the solvent adsorption method,which often uses organic amine as adsorption solvents.However,organic amines are corrosive,and are easily decomposed and deactivated.Therefore,looking for an adsorbent with high mechanical strength,high adsorption capacity and which is convinent for the desorption is necessary for the gas adsorption industry.This project aims to synthesis porous carbons with satisfactory SO2 and CO2 adsorption performance in an industrial scale.Using pitch as carbon precursor,via pore structure construction and surface modification,the thesis includes the following aspects:(1)The porous carbon MPC and N-doped porous carbon NMPC are synthesized by a template method,using pitch as carbon precursor,MgO as template and melamine as N precursor.The morphology is perfectly duplicated which is presented by scanning electron microscopy.The regular mesoporous structure of the MPC and NMPC are observed by the transmission electron microscopy.The N content in the NMPCs increases with the higher melamine usage,which is demonstrated by XPS and elemental analysis,and the doped N elements mainly exist as pyridine N.With the increase of N content,the specific surface area(SSA)show a decrease.The desulfurization ability of NMPC greatly enhanced attribute to N doping as compared to the undoped MPC.With the melamine to pitch mass ratio of 1:1,the as-prepared NMPC shows a sulfur capacity(SC)of 13.7 mg/g,twice that of the commercial active carbon.The materials show excellent cycle ability,and no obvious decrease of SC appears after heating regeneration for 10 cycles at 120oC.The existence of water vapor shows a negative effect on the adsorption process due to the competitive adsorption between water and SO2 on the surface active sites.(2)The collision and friction of the carbon particles are unavoidable during the industrial application,which usually causes carbon thinning and carbon loss.N-doped carbon nanotubes(NCNTs)are engaged to build homogeneous 3D networks to enhance the mechanical strength of the carbon materials.Due to the good dispersion ability and stability of NCNTs which is attributed to the existence of N,the networks that built by NCNTs are even-distributed.The NCNTs shows much higher reaction activity as compared to the undoped CNTs due to the exstence of N atoms,which greatly contributes to the stronger chemical bonding between NCNTs and carbon matrix during the carbonization.Therefore,the as-obtained NCNTs/carbon composite shows fewer defects with a much regular arrangement.The NCNTs are added into the mixture of the starting materials,then followed by briquetting and carbonization,thus realizing the preparation of carbon block by a one-step synthesis process.Compared to the ordinary molding method by binders,the carbon blocks prepared by the one-step synthesis method present better integrity,thus enhancing the mechanical strength of the porous carbon.The optimal sample 0.1NCNT-NMPC is derived at the NCNTs amount of 0.1 wt%,with the fracturing strength of 7.35 MPa and attrition resistance index of 2.91%,which are 57%higher and80.1%lower than the original MPC,respectively.Moreover,the SC of 0.1NCNT-NMPC is increased by 84%as compared to MPC.The abundant of the N surface groups provides more active sites for the SO2 adsorption on the one hand.On the other hand,the NCNTs networks effectively helps inhibit the restack of the carbon pieces,thereby enhanced the inter-diffusion of the SO2 in the bulk phase.(3)The carbon materials with hierarchical porous structure(MPCKx)which present favorable CO2 adsorption abilities are prepared by calcination of the powder mixture of MPC and KOH,followed by water purification.The optimal material MPCK2 delivers the CO2 uptake of 8.3 mmol/g at 25oC,under the pressure of 5 bar.Isotherm and kinetic investigations are taken out to study the adsorption behaviors.The conclusion is drawn that the CO2 molecules are adsorbed in the micropores with monolayer distribution preferentially,and then come to capillary condensations in mesopores.The physical adsorption mechanism provides the convenient and fast desorption pathway,which makes it appropriate to the CO2 capture and transport applications.(4)The performance of electrochemical energy storage for pitch-based hierarchical porous carbon are studied.The hierarchical porous carbon(HPC)is obtained after graphitization of the KOH activated porous carbon.Supercapacitor and lithium ion capacitor tests are carried out for the as-preapred HPC.Results show that the HPC has superior electrochemical performance in the aspects of high energy densitiy(341 Wh/kg at 722 W/kg),high power density(14431 W/kg at 202 Wh/kg)and ultralong cyclability(91.3%retention after 10000 cycle).The excellent electeochemical performance is due to the good electric conductivity of the highly graphitized 3D carbon framework and the abundant pore structure which act as the ion diffusion channels.In conclusion,this project uses pitch as carbon precursor which is a cheap and available heavy oil residue,and prepares hierarchical porous carbon materials by a template method with the help of activation.This method is costless and simple in design,moreover,it can accomplish the large-scale industrial production.The porous carbon materials with specific adsorption abilities on CO2 or SO2 can be synthesized by the control of surface properties and pore structures.The method is expected to be applied to the gas purification,gas capture and electrochemical aspects on an industrial scale.