302 / 2018-07-30 04:03:01

Formation Mechanism of Levoglucosenone in Sulphuric Acid Catalyzed Pyrolysis of Cellulose

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Levoglucosenone (LGO) is an important anhydrosugar product in cellulose pyrolysis. Its yield is very low in non-catalytic pyrolysis process, but can be significantly increased in certain acid catalyzed pyrolysis process such as sulphuric acid, which allows selective production of LGO from cellulose/biomass. In the present work, quantum chemistry computational approach has been employed to give a deep insight into the formation mechanism of LGO in sulphuric acid catalyzed cellulose pyrolysis. During the cellulose pyrolysis process, LGO can derive from the primary decomposition of the glucose unit in cellulose chain or the secondary decomposition of levoglucosan (LG) and 1,4:3,6-dianhydro-α-D-glucopyranose (DGP). Computation results indicate that the possible formation pathways of LGO are almost the same in the non-catalytic and catalytic pyrolysis processes, whereas, the competiveness of different pathways has altered greatly under the catalysis of sulphuric acid. In these pathways, sulphuric acid changes the structures of the transition states of elementary reactions and decreases the activation free energies dramatically. In non-catalytic LGO formation process, the favorable pathways involve the dehydrated LG intermediate which forms LGO through successive tautomerization, carbonyl transfer and dehydration at 4-OH + 3-H site. The final dehydration is the rate-determining step, with an activation free energy of 291.9 kJ/mol (350°C). In the catalytic process, the primary decomposition of the glucose unit is the most favourable for LGO formation. The glucose unit successively undergoes dehydration at 3-OH + 2-H site, tautomerization and depolymerization to produce LGO. The rate-determining step is the first step of dehydration, with an activation free energy of 218.4 kJ/mol (350°C). The secondary decomposition of LG is also a vital source for LGO formation, with two major patterns. The first pattern involves the dehydration at 3-OH + 4-H site limiting the rate of the pathway (△G350°C = 242.2 kJ/mol). In the second pattern, LG decomposes into LGO through successive dehydration at 3-OH + 2-H site, tautomerization and dehydration at 4-OH + 3-H site. The first step of dehydration is the rate-determining step (△G350°C = 244.0 kJ/mol). Whereas, the secondary decomposition of DGP can only result in trace of LGO in both non-catalytic and catalytic processes. In addition, temperature has a manifest influence to the pathways in the catalytic process. The activation free energies of the rate-determining steps at 500°C are about 20 kJ/mol higher than those at 350°C. Therefore, the yield of LGO at 350°C is higher than that at 500°C in sulphuric acid catalyzed cellulose pyrolysis.