CO₂ Enhanced Hydrothermal Conversion Of Biomass And Nanonization Of Hydrochar

Hydrothermal treatment,a thermochemical technology for biomass conversion,is a simple and green process that requires mild reaction conditions in an aqueous reaction medium.This method converts biomass into chemicals and advanced carbon materials,meeting the current demands for sustainable energy production and advanced materials.Through the design of the catalysis system,biomass can be efficiently converted into high value-added platform chemicals including lactic acid,levulinic acid（Le A）,formic acid（FA）,furfural and 5-hydroxymethylfurfural（HMF）,which play an important role in synthesizing bio-based materials and fuel additives.Furthermore,hydrothermal treatment is an important method to convert biomass into carbon dots（CDs）,which can be applied in optical imaging and sensing.However,it is still a big challenge to highly effectively convert biomass into platform chemicals and CDs with high yield through hydrothermal treatment.This thesis focuses on biomass hydrothermal conversion technology.Biomass can be converted into high value-added chemical and novel carbon nanomaterials by constructing new catalytic systems and green synthesis methods,which can achieve the total conversion of biomass based on hydrothermal treatment,as well as provides theoretical and technical supports for efficient and high-value conversion of biomass.1.This study proposed a new and highly effective catalysis system to convert cellulose into formic acid and levulinic acid by using in-situ carbonic acid from CO₂ as a green acid in the presence of Cr Cl₃.The synergy effect of in-situ carbonic acid and Cr Cl₃ could highly effectively hydrolyze cellulose to glucose,isomerize glucose to fructose,dehydrate fructose to HMF,and rehydrate HMF to FA and Le A in a one-pot way.Here,49%FA and 32%Le A could be obtained from cellulose at 180℃,90 min.Our results demonstrated that this new catalysis system is comparable to other catalysis systems,and in-situ carbonic acid can be used as a low-cost acid to replace mineral acids such as H₂SO₄,HCl,and H₃PO₄ and organic acids such as C₆H₆O₃S,H₂C₂O₄,and Cl₃CCOOH to constitute novel,highly effective and less environmental impact catalysis systems for producing formic acid and levulinic acid from cellulose.2.This work presented the utilization of in-situ carbonic acid from CO₂ as a green acid to enhance the conversion of cellulose to HMF in the presence of Zr O₂ and Ti O₂.The combination of in-situ carbonic acid,Zr O₂ and Ti O₂ exhibited distinctive catalysis in the hydrolysis of cellulose and monosaccharide,isomerization of glucose and monosaccharide conversion in the THF/H₂O/Na Cl system.Especially,in-situ carbonic acid played a significant role in cellulose degradation and accelerated the transformation of insoluble polysaccharides into soluble oligosaccharides or monosaccharides,thus promoting the cellulose conversion catalyzed by metal oxides.A high yield of HMF from cellulose（48.4%）was achieved through physically mixing commercial Ti O₂,Zr O₂,and CO₂.Due to the significant effects on cellulose degradation and HMF selectivity improvement,in-situ carbonic acid offered a green,low-cost and environmentally friendly co-catalyst for highly effective converting cellulose into HMF.3.The yield of CDs synthesized from the hydrothermal conversion of biomass is generally low,which limits the application of this method in the large-scale production of fluorescent carbon nanomaterials.The mechanisms of producing CDs and hydrochar through hydrothermal treatment of biomass both follow the La Mer model.However,correlations among biomass CDs yield,hydrothermal coke yield and hydrothermal reaction conditions are not clear enough.Based on the mechanism of biomass hydrothermal carbonization,the formation of hydrochar can be suppressed by decreasing the concentration of reactant and increasing the p H of reactant,thereby increasing the yield of biomass CDs.In addition,the alkaline condition of the reactant significantly increased the concentration of CDs in the outcome,and therefore,increased the yield of CDs by 5 times when the p H of the reactant was 13.The synthesized CDs with sizes ranging from 2 to 5 nm,showed excellent photoluminescence properties,which made them applicable in fluorescent imaging through mixing with PVP and screen printing.4.This work presented a novel,facile,and effective method for large-scale synthesis of CDs from biomass-derived carbon including hydrochar and carbonized biomass through mild oxidation（Na OH/H₂O₂ solution）.The abundant carbonyl groups and quinoid structures in hydrochar are oxidized during the processing,resulting in the release of CDs with abundant carboxyl and hydroxyl groups on the surface.An ultrahigh carbon dots yield of 76.9 wt%can be obtained,which is much higher than those obtained from traditional hydrothermal and strong acid oxidation processes.Furthermore,the CDs have an excellent quantum yield that is higher than（or comparable to）those from other methods.In addition,the CDs have uniform sizes（～2.4 nm）and their surface states can be regulated to significantly improve the quantum yield by adjusting the concentration of oxidants.The carbon dots displayed excellent sensitivity for Pb²⁺detection along with good linear correlation ranging from 1.3 to 106.7μM.