Study On Hydrothermal Reduction Of CO₂ By Biomass

The greenhouse effect caused by excessive CO₂ emissions and the depletion of fossil energy have become huge challenges for humanity.CO₂ is a rich,cheap and clean carbon resource.If green,fast and efficient conversion of CO₂ into high value-added chemical products/fuels could be realized,an effective way to fundamentally solve CO₂problems and also a viable way to alleviate energy crisis could be established.The conversion of CO₂ to chemicals calls for energy or hydrogen sources.Although the solar/electrical driven CO₂ reduction methods solve the problem of energy demand,the current research still faces the problems such as low efficiency,complication in electrode material preparation,and difficulty in industrialization.While the catalytic hydrogenation of CO₂ is expected to achieve industrialization,the consumption of fossil energy in the production,transportation and storage of hydrogen cannot be avoided.Therefore,how to achieve low energy consumption and highly efficient CO₂ reduction is still a challenge.It has been reported that the in-situ hydrogen produced under hydrothermal conditions has high activity for CO₂ reduction,and biomass can produce hydrogen by splitting water or self-cracking under hydrothermal conditions,while itself can be simultaneously converted into high value-added chemicals.These results indicate that it is possible to achieve efficient and rapid conversion of CO₂ by utilizing the in-situ hydrogen released from biomass under hydrothermal conditions.On this account,the main research content of this thesis is that using biomass as an inexpensive hydrogen source input under hydrothermal conditions,to achieve efficient CO₂ reduction on the one hand,and promote the coordinated conversion of biomass to high value-added chemicals on the other hand,thereby achieving the synergetic conversion of biomass and CO₂.The research mainly focuses on the hydrothermal reduction of CO₂ with two mainstream biomass:carbohydrate and N-containing biomass.Considering CO₂ is generally captured with alkaline adsorbents and stored in the state of HCO₃^-or CO₃^2-,the main research was conducted with HCO₃^-as the CO₂ source.The specific research contents and results are as follows:Firstly（Chapter 2）,the hydrothermal reduction of NaHCO₃ by methanol,a carbohydrate model compound,was studied.The results show that under hydrothermal conditions,the highly selective reduction of NaHCO₃ to formic acid can be achieved with methanol,and methanol is simultaneously oxidized to formaldehyde and formic acid.To distinguish the NaHCO₃ and methanol source formic acid,a ¹³C-NMR qualitative/quantitative method to determine formic acid was established.Further,by adjusting the reaction temperature,time and water filling,the efficiency of methanol reduction of NaHCO₃ was increased to 42.3%.The intermediates were captured by a series of time gradient experiments to determine the reaction pathway,in which methanol was sequentially oxidized to formaldehyde and formic acid to reduce NaHCO₃.Furthermore,to study the reaction mechanism,a high temperature/pressure in-situ infrared equipment was first established,and accompanied with H/²H-NMR and DFT calculation,the detailed mechanism for methanol reducing NaHCO₃ was revealed,which consists of two pathways:hydrogen from water catalyzed methanol reforming and direct hydrogen transfer of methanol to reduce NaHCO₃.Secondly（Chapter 3）,the ability of glucose,a representative of carbohydrate,reducing NaHCO₃ under hydrothermal conditions was studied.Through the regulation of the amount of glucose and NaHCO₃,glucose reduction of NaHCO₃ to formic acid was successfully achieved.Further,the yield of formic acid was optimized to 33.6%by changing the reaction time,temperature and water filling.Through the study of the product distribution after the reaction of glucose and NaHCO₃,it was found that glucose was converted into formic,acetic,and lactic acid simultaneously.CO₂ could also be reduced with glucose after presoak in alkaline solution,and cellulose was also proved to be able to reduce NaHCO₃.Through the intermediates study,the reaction mechanism of glucose reduction NaHCO₃ was confirmed,mainly through the aldehyde group.The main pathway was glucose breaking to form small molecular sugars such as glyceraldehyde and glycolaldehyde,and then the small molecular sugars completed NaHCO₃ reduction.However,due to the special structure of glyceraldehyde（adjacent dihydroxy group）,the dehydration of glyceraldehyde can not be avoided,which leads to the decreased reduction efficiency of NaHCO₃.Therefore,a two-step method was proposed to promote the reduction efficiency,with which a yield of 55%formic acid was obtained.Further,the efficiency of cellulose reduction of NaHCO₃ was optimized with the two-step reaction,and a 76.2%formic acid yield was obtained.Thirdly（Chapter 4）,the hydrothermal reduction of NaHCO₃ by N-containing biomass was studied.Due to the advantages of wide distribution,low cost and easy availability,microalgae were selected as the representative of N-containing biomass to study the feasibility and reaction mechanism of hydrothermal reduction of NaHCO₃.Through optimization of experimental conditions,it was found that when the microalgae reacted with NaHCO₃ at 300°C for 2 h,NaHCO₃ could be reduced to formic acid with a yield of 15.6%.While reducing NaHCO₃,the microalgae were converted into high value-added chemicals such as organic acids and N-substituted amides.Further,by comparing the reaction of microalgae components such as protein,sugar and fat with NaHCO₃,it was confirmed that the microalgae mainly reduced NaHCO₃ through its protein component,and sugar and fat almost had no effect on the process.The intermediate products were captured by a series of time gradient experiments,and it was found that the protein in the microalgae was converted to lactam to reduce NaHCO₃.Finally,CO₂ was directly used in the reaction,and after pre-soak in alkaline solution,it was reduced to formic acid too.Chlorella and its residue after oil extraction were also tested,and NaHCO₃ was reduced either,indicating the reaction was available for any N-containing biomass.Finally（Chapter 5）,to investigate the reaction mechanism of hydrothermal reduction of NaHCO₃ with N-containing biomass,the reaction characteristics and mechanism of hydrothermal reduction of NaHCO₃ by lactam were studied.Through the control experiments and reaction intermediates,it was found that the lactam formed an amino group by hydrolysis,and amino group was sequentially oxidized to hydroxylamine,hydrazine,and nitroso to complete the reduction of NaHCO₃.Thereafter,the nitroso reacted with the large amount of ammonium ions present in the solution to form nitrogen,and ammonium ions were derived from the process by which lactam formed lactone.The formed lactone was further hydrolyzed and decarboxylated to form alcohols,which were subsequently oxidized to acids or added to lactam to form N-substituted lactams.Finally,to improve the reaction efficiency of hydrothermal reduction of NaHCO₃ with N-containing biomass,the catalyst for NaHCO₃ reduction was explored with lactam as the reducing agent.It was found that Pd/C had good catalytic activity and could improve the yield of formic acid to 30%.At the same time,Pd/C had good cycle stability in the reaction,and no dissolution or morphological damage was found.To sum up,this paper confirmed the feasibility of hydrothermal reduction of CO₂with carbohydrate or N-containing biomass,and simultaneously realized the in-situ conversion of biomass to value-added chemicals.The hydroxyl/aldehyde and amino groups rich in biomass molecule were confirmed to be the reducing agents for CO₂reduction.This study provides an important theoretical basis for research of CO₂reduction with biomass.