Preparation Of Lactic Acid And 5-Hydroxymethylfurfural By Hydrothermal Catalysis Of Sugar-Rich Microalgae Based On Bifunctional Catalyst

Energy and resources have always been an important cornerstone of human society.Since the mid-20th century,with the gradual decline of fossil resources,the demand for renewable alternative resources has become increasingly urgent and severe.As a renewable resource,biomass has once again entered people's vision.Due to its wide range of sources and low cost,it has great potential in the preparation of high value-added chemicals,fuels and materials.Compared with terrestrial biomass,microalgae attract researchers'attention because of its high photosynthetic efficiency,lignin-free,and oriented culture.According to the characteristics of microalgae and the advantages of hydrothermal catalysis,this paper firstly proposed the research on the preparation of important small molecule oxygenated chemicals like lactic acid and5-hydroxymethylfurfural?HMF?by hydrothermal conversion of self-cultivated Scenedesmus?carbohydrate 50.6 wt%?over bifunctional catalysts such as Sn-Beta/formic acid and H-ZSM-5.The influence of hydrothermal catalytic reaction factors was analyzed.The influence of the structure and composition of sugar-rich microalgae was clarified,and the reaction mechanism of lactic acid and HMF prepared by hydrothermal catalysis of sugar-rich microalgae was further investigated,which provided a theoretical basis for the preparation of oxygenated chemicals by hydrothermal catalytic conversion of microalgae.Firstly,based on the self-designed Sn-Beta/formic acid bifunctional catalyst,sugar-rich microalgae were hydrothermally catalyzed to prepare lactic acid.At the reaction temperature of483 K,the initial concentration of sugar-rich microalgae was 1.0 wt%,and the reaction was carried out for 2 h under the initial 4.0 MPa helium pressure.The yield of lactic acid was up to83.0%.Under the same reaction conditions,the yield of lactic acid using glucose as a raw material was only 64.7%.The yield was increased by 18.3%.By comparing the two catalysts of Sn-Beta and formic acid,it was found that the yield of lactic acid from microalgae catalytic conversion over Sn-Beta was very low,only 7.4%,and the addition of a small amount of formic acid could realize the obvious upgrade of lactic acid yield.The cycle experiment results of the catalyst showed that the lactic acid yield remained relatively stable and the concentration of formic acid did not change significantly before and after the reaction.The reaction solution was analyzed by ICP,and no Sn element was detected.The XRD results showed that the structure of the Sn-Beta molecular sieve did not change significantly.Secondly,the mechanism of“controlled-release”hydrolysis reaction of sugar-rich microalgae under the induction of formic acid.The TEM analysis of the sugar-rich microalgae showed that the outer layer of the cell was a thick cell wall,and the carbohydrate was in the form of starch granules in the cytoplasm.When the sugar-rich microalgae were used as the raw material,the solution was catalyzed by formic acid.During the reaction,glucose concentration was relatively low and stable,which indicated that the addition of formic acid could gradually disintegrate the cell wall which had cellulose as the main component.Hence,the glucose in the sugar-rich microalgae was slowly released and maintained at a proper concentration,which is beneficial to reduce the agglomeration of glucose,which resulted in the upgrade of lactic acid yield.The whole reaction network is that formic acid promoted"controlled release"of glucose by attacking the cell wall of sugar-rich microalgae.Glucose was converted to fructose by isomerization under the catalysis of Sn-Beta,and fructose was reversed by retro-aldol condensation to form C₃ compound under the catalysis of Sn-Beta.The C₃ compound underwent isomerization and rehydration to finally form lactic acid.The reaction kinetics model of lactic acid prepared by Sn-Beta catalyzed glucose was preliminarily studied.The apparent activation energy of lactic acid prepared by glucose over Sn-Beta catalysis was about 20.30 kJ·mol-¹.Third,HMF was prepared by hydrothermal dehydration of rich sugar microalgae over bifunctional catalyst H-ZSM-5.At 473 K,1 wt%initial microalgae concentration,4.0 MPa helium pressure for 2 h,the highest yield of HMF is 40.6%.Through the ball-milled cell wall-breaking experiment of sugar-rich microalgae,it was found that the HMF yield had no obviously change before and after the ball-milling,which indicated that H-ZSM-5 could effectively destroy the cell wall.hydrothermal catalysis of glucose over H-ZSM-5 mainly obtained the decarboxylation product levulinic acid?LA?,which indicated that the composition of the sugar-rich microalgae affected the hydrothermal catalytic process.The effect of triglyceride and amino acid on hydrothermal catalytic process was studied by using glucose as a model compound.The results showed that the addition of triglyceride could increase the yield of HMF and reduce the yield of LA.The cycle experiment of the catalyst showed that the structure of the catalyst after the reaction was basically unchanged,while the yield of HMF was slightly decreased,but the yield of HMF was again increased after calcination reduction.In summary,the self-designed Sn-Beta/formic acid and commercial H-ZSM-5 highly selective hydrothermally catalyzed sugar-rich microalgae to prepare platform compounds such as lactic acid and HMF.Organic acid-induced microalgae"controlled release"hydrolysis mechanism and the mechanism of the action of components in microalgae will provide reference for the preparation of important oxygen-containing chemicals by hydrothermal conversion of sugar-rich microalgae.