Synthesis of carbon microspheres by hydrothermal process of native starch

Abstract

Synthesis of carbon microspheres (CMSs) with controlled particle size distributions using hydrothermal process of native and modified corn starch has been investigated systematically in a batch reactor. Formation mechanisms of CMSs were also proposed. Hydrolyzed rates of native corn starch were demonstrated to have strong effects on morphology, particle size distributions, uniformity and yields of the synthesized CMSs. By varying the reaction time between 0 and 24 hours at 180ºC, CMSs were irregular shape because the CMSs development still was not complete with the small amount of glucose from the hydrolysis reaction in the system at the beginning of the reaction. Nevertheless, the CMSs became uniform and large size with primary particles of 8.0 µm when the reaction time increased to 12 hours. On the other hand, primary particles of 2.0 µm were obtained at high reaction temperature (at 220ºC) because of the rapid hydrolyzed rate but the CMSs were likely to aggregate secondary particles. In addition, the smallest primary particles of 0.12 µm were obtained from hydrothermal process of glucose because high quantity of glucose in the system simultaneously formed many nuclei of CMSs. Therefore, the nuclei formation plays an important role than the growth mechanism and the CMSs were the smallest size than others. Proposed reactions during hydrothermal process composed of hydrolysis of native corn starch to glucose, subsequently dehydration of glucose, finally polymerization and aromatization to form the CMSs which were predicted from van Krevenlen chart and FT-IR results. Moreover, the results from TEM observations and FT-IR analysis revealed that the structure of the synthesized CMSs mainly consisted of condensed aromatic-carbon-ring compounds as a core and reactive hydrophilic compounds as a shell. To gain an insight into the CMS formation mechanisms, concentrations of intermediates formed during the hydrothermal process were determined at different times for the determination of reaction kinetic parameters. It was found that the experimental data from hydrothermal process of glucose, could be fitted with the assuming pseudo-first-order kinetic model. The proposed reaction pathway could also be verified with the experimental results. Nevertheless, the hydrothermal treatment results of native corn starch deviated from the first-order kinetic model because of its difficult to be hydrolyzed.