Synthesis of selective CO2 sorbents for post-combustion capture: The key role of the intrinsic basicity originated from oak wood

Selective CO2 sorbents were successfully synthesized through the physical (CO2) activation of oak wood-based chars. Raw material was previously carbonized either by pyrolysis at 800 °C and by hydrothermal synthesis (HTC) at 250 °C. A more dramatic development of texture was attained for the hydrothermally carbonized biomass, therefore promoting HTC as a cost-effective route for the preparation of porous activated carbons (ACs). ACs thus synthesized along with a commercial carbon included for comparison purposes were tested for CO2 capture by using a thermogravimetric analyser (TGA). Under pure CO2 and 35 °C, although having much lower surface areas (highest SBET = 627 m2/g), oak wood-derived carbons exhibited uptakes as large as those achieved by the commercial AC (SBET = 1231 m2/g). This finding was attributed to the comparable ultramicropore volume (d<0.7 nm) measured for all the samples. This suggests that carbon dioxide is mostly adsorbed onto the narrowest pores. On the other hand, upon changing to post-combustion conditions (ca. 53 °C, 15 % CO2/85 % N2), oak wood-based sorbents exhibited similar (OW250PA) or even greater (OW800PA) sorption capacity than the commercial AC. The higher selectivity shown by oak wood derivatives was associated with their substantial amount of Ca-based inorganic fraction. This was revealed by EDX and was evidently related to the outstanding basicity measured by Boehm’s titrations on the synthesized sorbents’ surface (up to 93 %). Conversely, commercial AC is characterised by a far poorer inorganic content, thus showing lower basicity (ca. 67 %). Accordingly, it was proved that under post-combustion conditions the contribution of a more favoured (basic) surface chemistry outweighs the texture effect. Basic functionalities ensured stronger interactions with the carbon dioxide molecule, therefore implying a more selective sorption at lower gas concentration. Therefore, it was demonstrated how selective CO2 sorbents could be prepared exploiting the advantageous properties of the raw biomass rather than applying expensive and environmentally unsustainable chemical treatments.