The Study On Preparation And Applications Of Biomass-based Colloidal Carbons

With the accelerated process of global industrialization, a large number of fossilresources are consumed, and the emission of CO₂is increasing, which cause theglobal "greenhouse effect" and bring a range of adverse effects to human livingenvironment. So how to reduce CO₂emissions becomes the urgent task for humanbeings to protect themselves and save the planet, which attracted the worldwideattention. Biomass is a kind of renewable resource. Atmosphere, water and sunshinecombined affect the growth of biomass. Part of the CO₂in the atmosphere is reducedin the process of biomass formation. Therefore, how to obtain the fixed carbon frombiomass is the only way to reduce carbon dioxide emissions and achieve negativegrowth of carbon dioxide in atmospheric, which is an effective way to reduce thecontent of carbon dioxide in the atmosphere.At the same time, with the consumption of fossil resources, the non-renewableresource for human survival is exhausted. So, it is the urgent task to look for a newcarbon source ensured the human survival and the sustainable development ofhuman society. Biomass is the preferred raw material for preparation of series ofchemical products instead of products prepared from fossil resources, as they arerenewable, high performance, and the available carbon source, which become theurgent task to be completed for scientists around the world. Biomass is the onlycarbon source, so how the fixed biomass carbon, and its series of chemical productsas a raw material preparation to replace the products of fossil resources is the task ofscientists around the world urgently needs to be done. In this dissertation, rice husk as renewable resource was the raw materials forpreparation of biomass-based colloidal carbons by acid catalyzed hydrolysis ofhemicellulose and cellulose into sugars, and the hydrolyzate was polymerized andcarbonized in situ. The colloidal carbons were used as the raw material to prepareporous carbons with different characteristics by the techniques of selective reactionand directional etching; the biomass-based carbon blacks were prepared fromcolloidal carbons by thermo-chemical conversion treatment; and finally, thebiomass-based nano-silica was prepared from the hydrolysis residue of rice husk.The study provided the scientific basis for high value comprehensive utilization ofrice husk, and a series of innovations were obtained.1. The study on optimum technological parameters of acid-catalyzed hydrolysisSulfuric acid was the hydrolysis catalysts as its cheap and good catalyticefficiency. The hydrolysis conditions such as the concentrarion of sulfuric acid, theratio of rice husk to acid, hydrolysis temperature and time on hydrolysis efficiencyof cellulose and hemicellulose were investigated in detail. The optimal condition forrice husk hydrolysis is72%sulfuric acid, hydrolysis temperature of50oC,hydrolysis time of10min and the ratio of solid to liquid was1:10(g/mL). After thereaction, the hydrolysis ratio of rice husk was up to53%and the yield of sugar wasabout42%. When the cellulose of rice husk (residue of rice husk, hemicellulose,lignin and silica was removed from rice husk) was hydrolyzed, the hydrolysis ratiowas up to91%, and the sugar yield was about78%. The advance of this techniquewas high hydrolysis ratio and sugar yield were obtained under low temperature andatmospheric pressure in short time. This route resolved the problem of thehydrolysis efficiency of biomass and achieved the technique of continuousproduction.2. The study on optimum technological parameters of polycondensationcarbonization in situThe hydrolyzate contained sugar derived from hydrolysis of rice husk andcellulose of rice husk was the raw material. The effects of acid concentration,carbonization temperature and time on the morphology, particle size and yield of colloidal carbons were investigated in detail. The optimum condition was as follows:(1) the sphere-like colloidal carbon materials with regular size of about500nm wereobtained at95oC for carbonization of6h under low-temperature and atmospheric inwater bath;(2) the colloidal carbon spheres with regular size of about500～1000nm were obtained at160～180oC for2～6h under different acid concentrations(10%～30%) in hydrothermal reaction conditions. The advance of this techniquewas the solid colloidal carbons were obtained after the polycondensation andcarbonization of hydrolysis solution in situ, sulfuric acid can be recycled afterfiltration of colloidal carbons and acid solution. This route resolved the problem forseparating sugar from acid, so the cost of production was reduced and theenvironmental pollution was avoided simultaneously.The morphology of prepared carbon spheres was rounder, the particle size waslarger, and the surface was smoother when the hydrolysis solution reacted underhigh pressure. The morphology of carbon spheres prepared from cellulose of ricehusk was rounder and the particle size was more uniform than rice husk as rawmaterial. The results indicated that single component of sugar derived from celluloseof rice husk hydrolysis was more propitious to prepare colloidal carbons withuniform particle size. The results indicated that the carbon spheres prepared bydifferent methods has similar characteristics, there were a large number of organicfunctional groups (O-H, C-H, C=O, C-C and C-O) in their surface, and localizedgraphitization structure was manifested by X-ray diffraction. The presence of thesefunctional groups offered the possibility of further functionalization and made thematerials more hydrophilic and highly dispersible in water.3. The study on optimum technological parameters for preparation of porouscarbonThe colloidal carbons were used as the raw material to prepare high-performanceporous carbons by the techniques of selective reaction and directional etching. Theeffects of activation conditions (activation temperature, time and the ratio ofactivated reagents to carbon materials) on the characteristics of porous carbon wereinvestigated, and the properties and applications of porous carbon were further studied. The optimum conditions was as follows:(1) the porous carbon with BETsurface area of2700m²/g, iodine adsorption capacity of2131mg/g and the specificcapacitance of130F/g was prepared by phosphoric acid activation, when thecolloidal carbons prepared under42%sulfuric acid concentration were activated at500oC for1h;(2) after the colloidal carbons prepared under42%sulfuric acid wereheated to500oC for1h under nitrogen gas, it was mixed with potassium hydroxide atfixed weight ratio of1:5, then the mixture were heated to400oC for0.5h, and thenheated to800oC for1h, finally, the high-performance porous carbon with BETsurface area of3362m²/g, pore volume of2.2cm3/g, iodine adsorption capacity of2386mg/g and the specific capacitance of332F/g was obtained by potassiumhydroxide activation. The results indicated that the porous carbon has local graphitestructure, larger iodine adsorption capacity and good electrochemical performance.The X-ray diffraction indicated that the porous carbon has local graphite structure.4. The study on optimum technological parameters for preparation ofbiomass-based nano-carbon blackThe biomass-based nano-carbon black were prepared from colloidal carbons bypyrolysis at different temperature. The effects of different temperatures onmicrostructure, surface functional groups, and other physical and chemicalproperties of carbon black were investigated. After investigation of experimentalconditions, the carbon black pyrolyzed at different temperatures shows aggregatesconstituted of sphere-like particles300～500nm in diameter. As the pyrolysistemperature was increased from400to800oC, the carbon content increased from83.41%to94.66%, the number of O-H, C-H, C=O, and C=C surface functionalgroups decreased, and based on Brunauer-Emmett-Teller (BET) results, the specificsurface area and pore volume of carbon black increased from389to1034m²/g andfrom0.258to0.487cm3/g, respectively. X-ray diffraction pattern and Ramanspectroscopy analyses of samples pyrolyzed at400～800oC showed a localizedgraphitic structure. It is possible that the route used in this study could also beapplicable to the preparation of carbon black from other types of biomass. Themethod replaced the traditional process of burning two tons tar to produce1ton carbon black, biomass as raw materials and low energy consumption of theproduction process were introduced for the development of nano-carbon blackindustry.5. The study on optimum technological parameters for preparation ofbiomass-based nano-silicaThe biomass-based nano-silica was obtained by heat treatment when thehydrolysis residues of rice husk were raw material. The effects of pyrolysistemperature and time on the performance of silica were investigated. Finally, themorphology of silica was sphere-like with the particle diameter of100nm, the whitedegree of silica was93.3%, the grade of purity was higher than99%, and thespecific surface area was about136.4m²/g, the pore size distribution was mainlyconcentrated in10nm, which was mainly mesoporous, such product was obtainedwhen the rice husk residua was pyrolyzed at optimum condition of800oC for1.5h.There were a certain amount of organic functional groups in silica surface, whichprovided a basis for further modification.A new route for comprehensive utilization of rice husk resources was studied inthis dissertation. A variety of high value-added materials included colloidal carbons,high-performance porous carbon, biomass-based nano-carbon black and ultrafinesilica products were obtained by series of physical and chemical methods ofprocessing. The greatest value of renewable resources was exploited. This study wasthe foundation for comprehensive utilization of biomass resources in the future.