Synthesis And Characterization Of Chitosan Oligosaccharide Derived In-situ N-doped Carbon Materials

Nitrogen-doped porous carbon materials are widely used in catalysis,gas adsorption,supercapacitors and drug carriers due to their unique advantages such as large specific surface area,large pore capacity,narrow pore size distribution,good hydrophilicity and specific adsorption sites.Among various related methods,hydrothermal curing is easy to operate and good for control of sample properties with short experiment periods.Therefore,this thesis chose easily accessible chitosan oligosaccharides with high nitrogen content and a low degree of polymerization as the carbon and nitrogen source.However,hydrothermal temperature of biomass is generally above 180 C.F127(triblock copolymer)micelle and TEOS(tetraethoxysilane)polymers are unstable under such a temperature,and there will be issues like reunion and micelles being partly damaged.Thus,crosslinking agent(glyoxal or glyoxalic acid)was added to lower the hydrothermal temperature to explore pore structure properties of porous carbon materials.In view of this,the thesis,aiming to synthesize N-doped porous carbon materials with developed pores and concentrated pore size distribution,used chitosan oligosaccharide,a nitrogen-containing biomass material,as the precursor,and selected low temperature hydrothermal method to synthesize N-doped porous carbon materials with abundant pores and a narrow pore size distribution.Influence that synthesis methods have on pores of N-doped porous carbon materials was explored.The research was carried out as following:Studied the synthesis of glyoxal crosslinked chitosan oligosaccharide derived N-doped porous carbon materials by using F127 as template and XRD,FITR,XPS and Raman features of one chosen sample.The results showed that the N-doped porous carbon material CGF-A-B-C-D was successfully synthesized with F127 being the template and glyoxal the crosslinking agent.It was found under relatively stable and suitable pH value and pressure,polymerization carried out most orderly in the hydrothermal process,and the pore diameter distribution of porous carbon materials obtained was narrower.With increasing pH value and volume ratio of hydrothermal solution,the pore size distribution first tended to concentrate and then dispersed,indicating the system is more sensitive to reaction pH value,hydrothermal pressure and hydrothermal solution concentration.The polymerization process was significantly influenced.The appropriate synthesis conditions:used drying oven as the hydrothermal equipment,added glyoxal when preparing the chitosan oligosaccharide solution with a hydrothermal volume ratio(reaction liquid to distilled water)being 40:20.When the pH value was 3.5,the pore size distribution was best,mostly at 18 nm.Specific surface area,total pore volume,and average pore diameter were 325.1205 m~2/g,0.5121 cm~3/g and 6.3004 nm respectively.The material was amorphous carbon which had less oxygen groups on the surface and a 5.87wt%surface nitrogen content.With a low degree of graphitization,it was mainly doped with pyridine nitrogen and quaternary ammonium nitrogen.Some defect sites were spotted on the sample surface.The pore structure is honeycomb pore with particle accumulation.Studied the synthesis of acetaldehyde crosslinked chitosan oligosaccharide derived N-doped porous carbon materials by using F127 as template and XRD,FITR,XPS and Raman features of one chosen sample.The results showed that the N-doped porous carbon material CAF-A-B-C-D was successfully synthesized through using F127 as the template and acetaldehyde,which has better adhesiveness than glyoxal in theory,as the crosslinking agent.It was found that the addition of reaming agent seriously reduced the stability of F127 micelle and resulted in easy self-agglomerate.At the same time,the system is sensitive to pH value,dilution and carbonization temperature.With increasing the pH value,pore diameter distribution tended to be concentrated at the beginning and then dispersed,while a greater degree of dilution and higher carbonization temperature brought a more concentrated pore diameter distribution.The appropriate synthesis conditions:the most concentrated pore size distribution(mainly at 1.3 nm)was achieved with the conditions of 1:0 template reaming agent molar ratio,pH 2.5,50m L dilution,700 C carbonization temperature.Specific surface area,total pore volume and average pore size were 391.6940 m~2/g,0.2231 cm~3/g and 2.2788 nm respectively.The material was amorphous carbon which had less oxygen groups on the surface and a 5.32wt%surface nitrogen content.With a low degree of graphitization,it was mainly doped with pyridine nitrogen and quaternary ammonium nitrogen.Some defect sites were spotted on the sample surface.The pore structure is honeycomb pore with spherical particle accumulation.Studied the synthesis of acetaldehyde crosslinked chitosan oligosaccharide derived N-doped porous carbon materials by using TEOS as template and XRD,FITR,XPS and Raman features of one chosen sample.The results showed that the N-doped porous carbon material CGT-A-B-C was successfully synthesized by choosing TEOS,which is more stable in hydrothermal carbonization process,to replace F127 as the template agent and glyoxal as the crosslinking agent.It was found with enough stirring time,the lower the hydrothermal temperature is,the narrower the pore diameter distribution of the synthesized material is.It indicated that lowering hydrothermal temperature could result in a more stable polymerization process and polymerize more regular intermediates.Meanwhile,the reduction of Si/C ratio could improve thebimodal distribution of porous carbon materials,largely because there's less silicon.The decreasing amount of free silicon led to the sharp reduction in the number of 1.5nm holes and the obtainment of porous carbon materials with a unimodal distribution.The appropriate synthesis conditions:the narrowest pore diameter distribution was generated mostly around 2.8nm when hydrothermal temperature was 95 C,stirring time8 h,and silicon-carbon ratio 1.Specific surface area,total pore volume and average pore size were 382.0799 m~2/g,0.3506 cm~3/g and 4.2705 nm respectively.The material was amorphous carbon which had less oxygen groups on the surface and a 6.68 wt%surface nitrogen content.With a low degree of graphitization,it was mainly doped with pyridine nitrogen and quaternary ammonium nitrogen.Some defect sites were spotted on the sample surface.The pore structure is a honeycomb hole in the blocky carbon.