Kraft lignin as a precursor for microporous activated carbons prepared by impregnation with ortho-phosphoric acid: Synthesis and textural characterisation

Activated carbons were prepared by activation of Kraft lignin with ortho-phosphoric acid at various temperatures (400–650 °C), weight ratios of ortho-phosphoric acid to lignin (P/L = 0.7–1.75) and impregnation times (1–48 h). The resulting carbons were characterised by elemental analysis, N2 adsorption at 77 K and SEM. The results indicate that the pyrolysis of lignin impregnated with ortho-phosphoric acid produces essentially microporous carbons, with a percentage of the total micropore volume approximately constant (80%), whatever the carbonisation temperature. The fraction of ultramicropores decreases with increasing temperature whereas that of supermicropores reaches a maximum at 600 °C. The maximum surface area (1305 m2/g) and pore volume (0.67 cm3/g) are reached at 600 °C, while pyrolysis of acid-impregnated lignin at temperatures higher than 600 °C produces a significant reduction of both pore volume and BET surface area due to: (i) the shrinkage of the material caused by the degradation of the phosphate and polyphosphate bridges and (ii) the oxidation of the carbon caused by the loss of the protecting P2O5. Increasing the P/L ratio increases the carbon yield and involves changes in the total volume of pores, the pore size distribution and the BET surface area. There exists an optimum P/L ratio which probably depends on the activation temperature, an excess of ortho-phosphoric acid beyond this optimum value reducing the surface area and the pore volume of the resultant activated carbon. Increasing the impregnation time, the surface area and the pore volume are lowered, and such an effect is more severe at higher activation temperatures.