Bridging lignin-polysaccharide-zeolite gel precursors rheology and their activated porous structures properties

The use of lignin, as a residue, in the synthesis of materials can be important from an environmental point of view. Based on this fact, this study develops lignin-based activated composites by embedding calcium lignosulphonate into the polymeric network of hydrogels made from two natural polysaccharides (gellan gum and alginate). Initially, composites with null specific surface areas (<5 m² m².g^-1) were obtained via freeze-drying. However, an activation with KOH (weight loss lower than 50%) increased the SSAs up to 420 m²·g^-1 for gellan gum composites and 520 m².g^-1 for alginate composites. Moreover, a successful embedding of ZSM-5 (according to FTIR and EDX analysis) into the polymeric network further increased the SSA of the composites of gellan gum (762.78 m²·g^-1) and of alginate (790.68 m²·g^-1) without modifying the structure of the pre-activated composite. Oscillatory tests revealed the formation of strong hydrogels, observing a maximum G' of 57,080 Pa for the gellan gum composites and 33,970 Pa for the alginate composites. Moreover, these results showed a relationship between the storage modulus of the precursor hydrogel and the SSAs depending on the crosslinking methodology. The SSA of the alginate composites (ionotropic gelation with CaCl₂) increased with the storage modulus, whereas this relationship was only found for certain compositions of gellan gum composites (thermal induced gelation). These results demonstrated the feasibility of this technique to quickly synthesize activated lignin-based activated carbons and how the viscoelastic properties of the precursor hydrogels can be used as a predictive tool of the textural properties of these materials.