Process time variation and critical growth onset analysis for nanofoam formation in sucrose-based hydrothermal carbonization

The paper presents a systematic study of the formation of carbon nanofoam from sucrose by hydrothermal carbonization. It is shown that for the process temperature of 150 °C, carbonization is not a gradual process but rather occurs suddenly at a specific threshold time of 4.5 h. pH value and electrical conductivity (EC) of the sucrose solution were monitored during carbonization. In the first 4.5 h prior to carbonization, the sucrose solution shows a sharp drop from pH 7.8 to pH 2 and sharp increase of EC. From this point on the values of pH and EC remain approximately constant. After the 4.5 h threshold, we examined the evolution of mass yield, density and morphology of the resulting carbon nanofoam. The yield first shows a steep increase around 4.5 h and then a further gradual increase up to 38% at 54.6 h. The mass density just after the 4.5 h threshold is 0.28 g/cm 3 and decreases with process time to reach a constant value of 0.14 g/cm 3 , between 20 and 54.6 h. This shows that desired conditions, such as low density and high yield, are obtained with sufficient process time below 5 h. Due to the release of intermediates of the conversion reaction into the sucrose solution, both pH and EC were found to be excellent indicators for the progression of hydrothermal carbonization.