Preparation And Formation Mechanism Of Hydrogen Bond-ion Bond Double-driven Chitosan-based Mesoporous Carbons

Mesoporous carbon materials can be widely used in industry,energy,medicine and other fields as the lots of advantages,such as large specific surface area,tunable pore size,tailorable surface properties and good chemical stability.Chitosan has attracted much attention for its good biocompatibility,and friendly environment in many carbon resources to prepare mesoporous carbon,in which hydroxyl group,amino group,glycoside bond and acetylamino group contains.The amino group can be protonated and the positive charge is carried in dilute acid solution system,which can participate in ionic cross-linking reaction with substances of carrying with the negative charge.Therefore,the chitosan based mesoporous carbon prepared by soft template method,which is easy to synthesize and environment-friendly,has become a research hot spot in recent years.However,some chitosan based mesoporous carbons still possesses problems such as smaller the total pore volume and mesoporous volume,lower specific surface area,uneven pore size distribution and poor tunnability,of which is related to the poor order and stability of the formed mesoscopic structure that is directly controlled by the forces originated from the components forming the mesoscopic structure.Therefore,in this paper,on the basis of conventional raw material using chitosan as carbon source and triblock amphiphilic Pluronic F127 as template to prepare the hydrogen bond driven mesoporous carbon,sodium tripolyphosphate was introduced to form ionic bond between the protonated amino group in chitosan and phosphate ion.Hydrogen bond and ionic bond are used to strengthen the force in each component,and increase the order and stability of mesoscopic structure of mesoporous carbon further to reveal theoretically the synthesis mechanism of chitosan based mesoporous carbon material,which prepared by driving from the hydrogen bond together with hydrogen bond and ionic bond.The influence of different preparation methods such as sol hydrothermal method,sol-gel method,sol-spray method,and different process parameters on mesoporous carbon pore structure were investigated,and a series chitosan based mesoporous carbon with large total pore volume and mesoporous pore volume,and high specific surface area,uniform pore size were prepared driven by hydrogen bond and ionic bond.The main research contents and conclusions are as follows:(1)The hydrogen bond driven mesoporous carbon was prepared by sol-hydrothermal method using chitosan as carbon source and F127 as template.After hydrothermal polymerization between chitosan and F127,the prepolymer was carbonized to some extent by characterization and analyzing the pore structure,functional groups(FTIR,Raman)and the variation of element binding energy(XPS)of prepolymer and mesoporous carbon,the characteristic peak of hydroxyl group,amino group and acetylamino group,glycoside bond in chitosan,as well as the ether group and hydroxyl group of PEO chain in F127 were red shiftted or blue shifted,which testified there is a strong hydrogen bond between CS and F127.The nitrogen from amino group and acetylamino group,and the oxygen from hydroxyl group,acetylamino group,and glycoside bond in chitosan,together with ether group,and hydroxyl group in the hydrophilic segment of PEO in F127 are the main active sites for hydrogen bond formation.In mesoporous carbon,the bonding among atoms or molecules is mainly relying on the hydrogen bond.The properties of hydrogen bond driven mesoporous carbon prepared by sol-hydrothermal method is as follows:the total pore volume is0.4623 cm³ g^-1,the mesoporous pore volume is 0.4010 cm³ g^-1,the specific surface area is 163 m² g^-1,and the pore size is 2.121 nm.(2)The hydrogen bond and ionic bond double driven chitosan based mesoporous carbon was prepared by introducing sodium tripolyphosphate as the cross-linking agent to strengthen the force among each component.The results show that:(1)the double bonds driven mesoporous carbon is formed under the synergistic effect of hydrogen bond and ionic bond.Among them,the hydrogen bond comes from the bond ing between the amino group,acetylamino group,hydroxyl group,and glycoside bond in CS together with the ether and hydroxyl group on PEO of F127,and the ionic bond originates from the bonding between the protonated amino group on the CS molecular chain and the phosphate ion in TPP.(2)During the carbonization process at high temperature,the hydrothermal prepolymer re leased CO₂,CO,NH₃,CH₄,tetraglycerin dimethyl ether,and 2,6-diisopropylphenyl isocyanate to form micropore of the mesoporous carbon material,and the mesopore was formed from pyrolysis of molecular chain PPO in F127.(3)The double bonds driven mesoporous carbon possesses the better unimodal pore size compared with the hydrogen bond driven mesoporous carbon,and the total pore volume is 0.5517 cm³ g^-1,the mesoporous pore volume is 0.4414 cm³g^-1,the specific surface area is 262 m² g^-1,and the pore size is 2.195 nm.(3)The hydrogen bond and ionic bond double driven mesoporous carbon was prepared by sol-gel method under the mild reaction conditions to avoid the carbonation of prepolymers during the hydrothermal polymerization process,and better and more effectively protect the active sites on the chain segments of chitosan and F127 via strengthening the interaction force in different molecules.The degassing conditions for mesoporous carbon,the carbonation conditions for prepolymer,and the influences of sol-gel process parameters on the structure characteristics of mesoporous carbon were discussed.The related characterization and mechanism of preparation of prepolymer and mesoporous carbon were also deduced.The results show that:(1)CS,F127 and TPP were participated in the reaction according to the mass ratio of 4:2:1,the reaction temperature was 40?,stirring rate was 300 rpm,the time of sol polymerization was 24hours,and the carbonization conditions were as follows:the temperature was raised from room temperature to 300?and 410?at the speed of 2?min^-1 and 1?min^-1,respectively,and when it was up to 410?,held for 4 hours,and the temperature was raised next from 410?to 900?at the speed of 5?min^-1,and the temperature was kept at 900?for 2 hours.The degassing temperature and degassing time for mesoporous carbon was 300?and 6 hours,respectively.The hydrogen bond and ionic bond double driven mesoporous carbon prepared by sol-gel method had the more ideal single peak distribution on the pore distribution curve,and the total pore volume,mesoporous volume and specific surface area is 0.4127 cm³ g^-1,0.3134 cm³ g^-1 and 237m² g^-1,respectively,and the pore size is 5.012 nm.(2)During the carbonization of prepolymer,CO₂,CO,CH₄,NH₃,acetic acid,water,thioacetic acid,1-(butyloxyethoxy)-2-propanol and ether are released to form micropore of the mesoporous carbon,and the decomposition of PPO in F127 is still the main source of mesopore.(4)The double bond driven mesoporous carbon was prepared by sol-spray method to avoid uneven heating of mesoscopic structure during gel process and meanwhile shorten the time of gel process.The effects of feed rate,inlet temperature of dry air and prepolymerization time on mesoporous carbon pore structure were discussed,and the preparation mechanism was deduced.The results show that:(1)when the inlet temperature of dry air is 200?,and feed rate is 4.0 m L min^-1,and the prepolymerization time is 24 hours,the mesoporous carbon prepared by sol-spray method possesses the largest pore structure parameters values and a wonderful ideal single peak distribution,and the total pore volume,mesoporous volume and specific surface area are 0.5900 cm³ g^-1,0.3665 cm³ g^-1 and 540 m² g^-1,respectively,and the pore size is 5.012 nm.(2)During the carbonization of prepolymer,CO₂,CO,CH₄,NH₃,acetic acid,water,thioacetic acid and diethylene glycol dibutylene ether are released to form the micropore of the mesoporous carbon materials,the decomposition of PPO in F127 is still the main source of mesopore.(3)Compared with sol-hydrothermal method and sol-gel method,the hydrogen bond and ionic bond coordination force in each component of the prepolymers prepared by sol-spray method is the strongest.(5)The double bonds driven mesoporous carbon prepared by sol-hydrothermal method and sol-gel method was used to adsorb tannic acid,respectively.The maximum adsorption capacity was 93.2 mg g^-1 and 160.7 mg g^-1 at 40?,respectively.The results show that the tannic acid can easily adsorb on mesoporous carbon with uniform distribution of pores.The adsorption process was a spontaneous physical adsorption.In this paper double bonds driven mesoporous carbon with large total pore volume,high specific surface area and uniform pore size was prepared through the synergistic effect of hydrogen bond and ionic bond using chitosan as carbon source,F127 as template and sodium tripolyphosphate as cross-linking agent,and the synthesis mechanism was discussed in detail to provide theoretical support for the following research.Secondly,the preparation process of mesoporous carbon was simple,safe and friendly environmental.Mesoporous carbon can be used to adsorb tannic acid with ideal adsorption effect,which is testified as a kind of chitosan based mesoporous carbon material with great application prospect.