小球藻水热催化联产功能化碳材料过程反应特性研究

Reaction characteristics of catalyzed Chlorella hydrothermal carbonization to co-produce functional carbon materials

微藻水热碳化技术具有无需干燥高含水率微藻、反应溶剂绿色无污染、反应条件相对温和等优势,可用于制备碳点和水热炭等功能化碳材料,受到广泛研究关注。然而,目前微藻水热碳化制备的碳材料存在质量产率较低、性能较差的问题,需通过添加适宜的催化剂以提高碳材料的产率与品质。因此,研究了H_(2)SO_(4)、CH_(3)COOH、NaOH、Na_(2)CO_(3)等催化剂对蛋白核小球藻水热碳化制备碳点与水热炭的影响规律,并采用集总参数法建立了水热碳化过程中多相产物的生成反应动力学模型。研究结果表明,4种催化剂均能提高微藻碳点质量产率,其中NaOH可使碳点产率提高约73.3%。添加酸性催化剂使碳点尺寸变小,碳点平均粒径最小仅为1 nm;而在碱性条件下获得的碳点尺寸变化不明显,粒径主要集中在3.0~4.0 nm之间。在230℃和100 min条件下,CH_(3)COOH作为水热催化剂制备的微藻碳点的荧光量子产率最高可达7.88%,并在340 nm的吸收波长激发下发射出波长为420 nm的最强发射荧光。元素分析表明,4种催化剂均降低碳点中的N元素含量,且均能提高水热炭的高位热值。催化剂对水热炭高位热值的提高效果由强到弱为H_(2)SO_(4)>CH_(3)COOH>NaOH>Na_(2)CO_(3)。热重分析发现,在水热碳化过程中,微藻碳水化合物的水解转化比蛋白质更快;添加CH_(3)COOH可以促进蛋白质水解转化,而碱性催化剂不利于碳水化合物的水解转化,这4种催化剂对微藻脂质降解过程的影响均不明显。动力学分析发现,水热碳化过程中主要反应包括碳点聚合生成水热炭、微藻生物油的析出以及微藻降解生成其他相产物。碳点主要是通过其他相产物中的小分子聚合和微藻大分子裂解直接生成;在230℃、CH_(3)COOH催化条件下,微藻碳点较难发生水解,且微藻生物油水解后难以发生逆向反应。

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 H_(2)SO_(4),CH_(3)COOH,NaOH,and Na_(2)CO_(3)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 established 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 CH_(3)COOH catalyzed system produced CDs with a fluorescence quantum 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 analysis 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 H_(2)SO_(4)>CH_(3)COOH>NaOH>Na_(2)CO_(3).Thermogravimetric analysis showed that the conversion of microalgal carbohydrates was faster than that of proteins during the hydrothermal carbonization process.The addition of CH_(3)COOH 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 CH_(3)COOH 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 polymerization of small molecules in other phase products and the direct cracking of macromolecules in microalgae.The hydrolysis of CDs was challenging to occur,and it was difficult to reverse the hydrolyzed microalgal bio-oil at 230℃in the presence of CH_(3)COOH.