Conversion of the hydrochar recovered after levulinic acid production into activated carbon adsorbents

Abstract

Levulinic acid production by acid-catalyzed hydrothermal conversion of (ligno)cellulosic biomass generates significant amounts of carbonaceous hydrochar, which is currently considered a final waste. In this work, the hydrochar recovered after the levulinic acid production, was subjected to cascade pyrolysis and chemical activation treatments (by H₃PO₄ or KOH), to synthesize activated carbons. The pyrolysis post-treatment was already effective in improving the surface properties of the raw hydrochar (Specific Surface Area: 388 m²/g, V_P: 0.22 cm³/g, V_MESO: 0.07 cm³/g, V_MICRO: 0.14 cm³/g), by removing volatile compounds. KOH activation resulted as the most appropriate for further improving the surface properties of the pyrolyzed hydrochar, showing the best surface properties (Specific Surface Area: 1421 m²/g, V_P: 0.63 cm³/g, V_MESO: 0.10 cm³/g, V_MICRO: 0.52 cm³/g), which synergistically makes it a promising system towards adsorption of CO₂ (∼90 mg/g) and methylene blue (∼248 mg/g). In addition, promising surface properties can be achieved after direct chemical activation of the raw hazelnut shells, preferably by H₃PO₄ (Specific Surface Area: 1918 m²/g, V_P: 1.34 cm³/g, V_MESO: 0.82 cm³/g, V_MICRO: 0.50 cm³/g), but this choice is not the smartest, as it does not allow the valorization of the cellulose fraction to levulinic acid. Our approach paves the way for possible uses of these hydrochars originating from the levulinic acid chain for new environmental applications, thus smartly closing the biorefinery loop of the hazelnut shells.

Keywords: Activated carbons; Hazelnut shells; Hydrochar; Hydrothermal processing; Levulinic acid.