Carbonaceous Nanostructures Obtained by Hydrothermal Conversion of Biomass ^†

By thermal treatment of biomass, different types of biochar can be produced, like carbon nanotubes (CNTs) and graphitic nanostructures, graphene, graphene oxide, or activated carbons, to name a few [1]. Their properties depend on the reaction conditions (temperature, pressure, reaction time, pH, and with or without catalysts), in addition to the type of biomass used [2,3]. Also, in the liquid phase, valuable chemicals like leuvulinic acid, furfural, 5-hydroxymethylfurfural can be obtained.

Lignocellulosic biomass (LCB) from corn stalks was mechanically grinded and subjected to hydrothermal conversion in water, under thermally generated pressure in a hermetic reactor, at different temperatures (140 °C, 180 °C, 200 °C, and 220 °C), and for different reaction times (2, 4, 6, 8, and 20 h). At the end of the hydrothermal reaction, solid and liquid phases were separated by filtration, and further analytically characterized using Fourier-Transform Infra-Red spectroscopy (FTIR), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Thermo-Gravimetrical Analysis (TGA), respectively Gas-Chromatography coupled with Quadrupole Mass Spectrometry (GC-MS/MS), High_Performance Liquid Chromatography coupled with Time-of-Flight Mass Spectrometry (HPLC-TOF/MS), and Ultra-Violet Visible (UV-Vis) spectroscopy for the liquid phase.

Mild hydrothermal carbonization (140 °C, 180 °C, at 2, 4, and 6 h) led to a brownish, amorphous, lignin-rich solid phase, and a liquid phase containing aldehydes, organic acids, and polyphenolic compounds, while high temperature conditions led to a high content of black hydrothermal carbon with increased crystallinity (Figure 1), respectively furans, furfurals, and levulinic acid in the liquid phase.

Hydrothermal carbonization of LCB from corn stalks at higher temperatures and reaction times leads to nanostructured hydrothermal carbon, nanographitic, and nanowhiskers structures, with improved adsorption properties so this material can be recommended for use in the depollution of waste waters, for the abatement of volatile organic compounds (VOCs) from indoor spaces, and even for photovoltaics and nanoelectronics.