Formation Mechanism Of Pore Structure Of Stabilized Soda Lignin-based Activated Carbon

In order to clarify the evolution and formation reaction path of pore structure,electrochemistry and adsorption properties of soda lignin-based activated carbon(SL-AC),in this study,soda lignin isolated from black liquor produced by pulp and paper making processes was utilized as raw materials.The themal stability of soda lignin after oxidative thermostabilization and chemical curing treatment,respectively,were investigated comprehensively.The influence of different thermostabilization treatment methods to the pore structure of SL-AC were examined.Then the morphology,pore structure,crystal structure and chemical structure of SL-AC during carbonization and water steam activation processes were discussed.Futhermore,the effect of pore structure and surface functional groups to the electrochemistry and adsorption properties of SL-AC were also explored.The main conclusions of the current research were as follows:1.No much differences among the thermal stability of soda lignin treated by oxidative thennalstability with various heating rates were found,while the thermal stability of soda lignin after chemical curing treatment were firstly improved and then poor with the increasing concentration of hexamethylenetetramine(Hexamine).Soda lignin after oxidative thermalstabilization showed the best thermal stability,followed by which treated with chemical curing method,and the untreated soda lignin exhibited the poorest thermal stability.2.The specific surface area(SBET)and total pore volume(Vtotal)of SL-AC derived from oxidative thennalstability treatment were only 64 m2/g and 0.078 cm3/g,respectively,and almost no micropore was obtained.Whileas,SL-AC derived from chemical curing treatment showed higher SBET and Vtotal.Especially for the soda lignin treated with 10%of hexamine,the corresponding AC showed the highest SBET of 1814 m2/g and Vtotal of 1.275 cm3/g,respectively.Moreover,with the increase of concentration of hexamine,the specific surface area and pore volume of micropores of the SL-AC increased and then decreased,while those of mesopores decreased.3.The chemical reaction paths of soda lignin after chemical curing during carbonization were as follows:when the carbonization temperature was below 300?,the breaking of ether bondings of ?-O-4 and C3-O-CH3 and dehydration between carboxyl and hydroxyl groups mainly occurred.Between the temperatures of 300?-500?,radicals including hydroxyl groups and protons from unsaturated carbon of benzene rings of soda lignin reacted to form the small gas,such as hydrogen,methane,methanol and water steam.Meanwhile,benzene rings were combined to form the fused benzene rings structures,and gradually transformed into hexagonal carbon structure which were enlarged during 500?-700?,especially for the size of La deriction.When the temperature of 900?,the size and layer distance of La deriction in hexagonal carbon increased and those of Lc deriction decreased.4.The evolution of pore structure of SL-AC during water steam activation were as follows:ultramicropores were firstly produced after activation,and partially enlarged into small amount of micropores and mesopores with the pore size distribution of 2 nm-4 nm,inducing that the specific surface area and pore volume of micropores of SL-AC rapidly increased and them slowly increased with the increasing of activation time,while those of mesopores started to increase until the appearance of pore enlargement,and the increasing amount of mesopores were less than that of micropores.5.During the activation process,the disorder carbon were mainly comsumed by water steam,resulting in the decrease of microcrystal in analogously graphtic carbon and increase of layer distance,althougth the relative amount of disorder carbon were almost no change.The change of carbon also accelerated to the pores formation.Besides,the water steam was also reacted with oxygen groups existed in surface of SL-AC,leading to the decrease in content of hydroxyl groups and carbonyl groups and element oxygen and increase in element carbon and carboxyl groups.6.Specific capacitance of SL-AC were affected with their specific surface area of mesopore and pore volume of micropore,and increased with the increase of relative content of quinone.SL-AC derived from chemical curing showed higher specific surface area of mesopore and pore volume of micropore and amount of quinone than those of SL-AC derived from oxidative thermostabilization and untreated soda lignin,respectively,resulting in the better electrochemistry.When the current density of 0.5 A/g,the specific surface area of AC was 106 F/g.The specific capacitance ratio reached 0.86 when the current density of 10 A/g.Besides,it showed excellent recycle stability after 2000 times of charging-discharging when the current density of 3 A/g.7.The adsoption property of methylene blue of SL-AC were mainly affected with the quinone and carboxyl functional groups on their surface,while the micropore and mesopore volume showed little effection to the adsorption property,contrasting inversely to the iodine adsorption property of SL-AC.Both adsorption propertis of SL-AC derived from chemical curing were better than those of SL-AC derived from oxidative thermostabilization and untreated soda lignin,contributing to the more amount of quinone and carboxyl functional groups and microstructure.The highest adsorption value of methylene blue and iodine were 586 mg/g and 1474 mg/g,respectively.