Reaction characteristics of catalyzed Chlorella hydrothermal carbonization to co-produce functional carbon materials
Microalgae carbonization technology has attracted wide attention because of no requirement for drying high moisture feedstock,green reaction solvent,and relatively mild reaction conditions,which can co-produce functionalized carbon materials such as carbon dots(CDs)and hydrochar(HC).It is necessary to add suitable catalysts in the hydrothermal process,as the carbon materials prepared by microalgae hydrothermal carbonization currently show low yield and mediocre performance.Therefore,the effects of H2SO4,CH3COOH,NaOH,and Na2CO3 as catalysts on the preparation of CDs and HC from the hydrothermal carbonization of Chlorella pyrenoidosa(CP)were explored,and a kinetic model for the generation of multiphase products during hydrothermal carbonization was estab-lished by lumped parameter method.The results showed that all four catalysts could increase the CDs yield,and the addition of NaOH could increase the CDs yield by about 73.3％.The CDs size became smaller,and the average particle size was only 1 nm with the addition of an acid catalyst,while the CDs size did not change significantly,and the particle size was concentrated at 3.0～4.0 nm under the condition of a basic catalyst.The CH3COOH catalyzed system produced CDs with a fluorescence quan-tum yield of 7.88％when CP was hydrothermally treated at 230 ℃ for 100 min.The strongest emission fluorescence with a wavelength of 420 nm was emitted under the excitation of 340 nm.Elemental analy-sis showed that all four catalysts decreased the N content in CDs,and all of them increased the HHV(High Heating Value)of hydrochars.The improvement effect of HHV from strong to weak was H2SO4＞CH3COOH＞NaOH＞Na2CO3.Thermogravimetric analysis showed that the conversion of microalgal carbohydrates was faster than that of proteins during the hydrothermal carbonization process.The addi-tion of CH3COOH promoted the conversion of microalgal proteins,while the alkaline catalyst was not conducive to the conversion of carbohydrates,and these four catalysts had no noticeable effect on the conversion of microalgal lipids.The kinetic analysis showed that the main reactions in the CH3COOH catalyzed microalgae hydrothermal carbonization process included the production of hydrochars via CDs polymerization,the precipitation of bio-oil via microalgae degradation,and the generating of other phase products via microalgae degradation.The microalgal CDs were mainly generated by the polymeri-zation of small molecules in other phase products and the direct cracking of macromolecules in microal-gae.The hydrolysis of CDs was challenging to occur,and it was difficult to reverse the hydrolyzed mi-croalgal bio-oil at 230 ℃ in the presence of CH3COOH.
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