Chemical structure evolution of char during the pyrolysis of cellulose

•The initial temperature for water evolved chemically is around 200 °C.•The intramolecular dehydration predominated for pyrolysis below 300 °C.•The resultant char was a disordered three-dimension network at 300–430 °C.•The smallest aromatic clusters undergoes deoxygenation at 430–650 °C.•Large aromatic structure was formed as temperature exceeded 650 °C.

The formation and evolution of chemical structure of char during cellulose (ash-free) pyrolysis were investigated using two-dimensional infrared correlation spectroscopy combined with Raman spectroscopy. The initial temperature at which water evolved chemically from cellulose pyrolysis is around 200 °C. The cleavage of intra- and inter-molecular hydrogen bonds (H-bonds) and the subsequent dehydration were the primary reactions as temperature below 300 °C, among which intra-molecular dehydration was the predominated reaction. However, dehydration occurred principally inter-molecularly along with decarbonylation, ring-opening and aromatization at temperature over 300 °C. The concentration of carbonyl, conjugated olefin, ether and aromatic structure increased significantly at the expense of glycosidic bond, pyran ring and hydroxyl groups diminishing in the residual char. The oligosaccharides, aliphatic hydrocarbons and aromatics were bonded through the ether linkage to form a disordered three-dimensional network. Dehydration was almost accomplished at 430 °C and smallest aromatic clusters grafted with oxygenated groups underwent significant deoxygenation and condensation at 430–650 °C. The predominant reaction shifted from deoxygenation toward dehydrogenation as the temperature exceeded 650 °C and the char was highly aromatic with large aromatic systems composed of over six fused ring structures. The study provides insightful details into the initial stage of cellulose degradation.