Influence of acids and alkalis on transglycosylation and β-elimination pathway kinetics during cellulose pyrolysis

The primary/initial thermal degradation pathways for the thermal degradation of cellulose are via intermolecular transglycosylation reactions within the glucose monomers of cellulose and via β-elimination (acid catalysed, heterolytic, ring-opening) reactions. Using the model-free isoconversional approach, the apparent activation energy of pure cellulose and cellulose that have been infused with acids (H3PO4, H3BO3) and alkalis (Ba(OH)2, Ca(OH)2) can be found. We find that acids influence the thermal degradation by promoting the β-elimination pathway leading to more cellulose degrading via that route at the expense of transglycosylation. This manifests itself as an increase in the apparent activation energy of the overall cellulose pyrolysis. On the other hand, alkalis have a suppressing effect on the acid-catalysed β-elimination pathway which then allows more cellulose to degrade via transglycosylation. This seems to lower the apparent activation energy. Overall, we can see that the magnitude of the apparent activation energies are ordered as Ea,acid-cellulose > Ea,pure-cellulose > Ea,alkali-cellulose.