Mechanism of cellulose fast pyrolysis: The role of characteristic chain ends and dehydrated units

Understanding the fundamental reactions and mechanisms during biomass fast pyrolysis is essential for the development of efficient pyrolysis techniques. In this work, quantum chemistry calculation, kinetic analysis and fast pyrolysis experiment were combined to reveal the cellulose pyrolysis mechanism. During cellulose pyrolysis, the indigenous interior units, reducing end (RE end) and non-reducing end (NR end) initially form various characteristic chain ends and dehydrated units which then evolve into different pyrolytic products. As the rising of the degree of polymerization (DP), reactions occurring at the interior unit and NR end are more competitive than those taking place at the RE end, resulting in distinct pyrolytic product distribution for cellulose and glucose-based carbohydrates. The reactions occurring at the three indigenous units of cellulose chain all favor the formation of levoglucosan-terminated end (LG end) and/or NR end, which then generate levoglucosan (LG). The acyclic d-glucose end (AG end), which mainly derives from the RE end, is essential for the formation of 1,6-anhydro-β-d-glucofuranose (AGF), 1,4:3,6-dianhydro-α-d-glucopyranose (DGP), furfural (FF), 5-hydroxymethyl furfural (5-HMF) and hydroxyacetaldehyde (HAA). Compared with the chain ends, the dehydrated units are not feasible to be generated, and their decomposition favors the formation of HAA.