Catalytic mechanism of sulfuric acid in cellulose pyrolysis: A combined experimental and computational investigation

Sulfuric acid (H2SO4) is widely used as a strong acid catalyst in biomass pyrolysis process, and exhibits prominent catalytic effects on the pyrolytic reactions and product distribution. In this study, the fundamental catalytic mechanism of H2SO4 on cellulose pyrolysis process was investigated via combined experimental and computational methods. Both thermographic analysis (TGA) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) experiments were performed to reveal the pyrolytic characteristics and product distribution in the H2SO4-catalyzed pyrolysis of cellulose. In addition, quantum chemistry methods were employed to build the reaction models and investigate the major H2SO4-assisted reactions in initial cellulose pyrolysis process, i.e., depolymerization of the cellulose chain, bridged dehydration, ring-opening, ring-contraction and dehydration of the free hydroxyl groups. The results indicate that the H2SO4-assisted depolymerization via C1-O1 bond scission and bridged dehydration reactions take place in a two-step mechanism involving a sulfate ester intermediate. While other reactions occur via a one-step mechanism involving only hydroxyl group or both hydroxyl and sulfonic groups of H2SO4. The activation energies of above reactions are all decreased by adding H2SO4, and thus lowering the degradation temperature of cellulose. Among all initial pyrolytic reactions, dehydration reactions which are inconspicuous in the non-catalytic process, become very important in the catalytic process, because their activation energies are decreased dramatically by adding H2SO4 (from 322.7 to 372.1 to 152.2-237.7 kJ/mol). The promoting effect on the dehydration reactions plays a vital role in the increased char yield and significant change of organic volatile product distribution. Through the promoted dehydration reactions, unsaturated structures of cellulose are formed, which is favorable for the formation of certain dehydrated products at the expense of depolymerized and ring scission products.