Mechanism Research On The Conventional And The Inorganic A Cid-catalyzed Pyrolysis Of Holocellulose

Biomass is one kind of important renewable resources.Its clean and efficient utilization is very meaningful to alleviating energy crisis and environmental pollution.Fast pyrolysis is one of the green and efficient way to convert biomass materials into fuels and various chemicals.Holocellulose(cellulose and hemicellulose),is the vital component of lignocellulosic biomass.The adding of appropriate acidic catalysts,such as sulphuric acid(H2SO4),phosphoric acid(H3PO4)and other inorganic acids,can efficeiently control the pyrolysis process,resulting in the formation of levoglucosenone(LGO),furfural(FF)and other high value-added chemicals.However,the current studies cannot give a clear and systematic understanding of the holocellulose conventional pyrolysis mechanisms,and the inorganic acids-catalyzed pyrolysis mechanisms are rarely reported,which limits the development and renovation of the pyrolysis techniques to a great degree.Hence,quantum chemistry calculation and fast pyrolysis experiment were combined in this work to study the mechanisms of the holocellulose conventional pyrolysis and inorganic acid-catalyzed pyrolysis.Firstly,cellobiose and cellotriose were selected as the model compounds to study the conventional pyrolysis mechanism of cellulose.The results indicate that the indigenous interior units,reducing end and non-reducing end in cellulose chain initially convert into various characteristic chain ends and dehydrated intermediates during the pyrolysis process.Afterwards,these chains and dehydrated intermediates then evolve into different final pyrolytic products.reactions occurring at the interior unit and non-reducing end are more competitive than those taking place at the reducing end,along with the rising of the degree of polymerization.Hence,cellulose and glucose-based carbohydrates pyrolysis resulted in distinct pyrolytic product distribution.The reactions occurring at the three indigenous units of cellulose chain all favor the formation of levoglucosan-terminated end(LG end)and/or non-reducing end,which then generate levoglucosan(LG).The acyclic D-glucose end,which mainly derives from the ring-opening of the reducing end,is essential for the formation of 1,4:3,6-dianhydro-?-D-glucopyranose(DGP),1,6-anhydro-?-D-glucofuranose(AGF),5-hydroxymethyl furfural(5-HMF),FF and hydroxyacetaldehyde(HAA).By comparison,the formation of the dehydrated intermediates is not as feasible as the chain ends.The decomposition of the dehydrated intermediates favors the formation of HAA.Secondly,xylose,xylobiose and xylan were selected as the model compounds to study the conventional pyrolysis mechanism of hemicellulose.The results indicate that acyclic D-xylose is an important intermediate for the formation of 1,4-anhydro-D-xylopyranose(ADX),FF and HAA which are the major pyrolytic products in xylose pyrolysis.The successive cyclization of D-xylose is the only source for ADX formation.3-Deoxy-xylose,which derives from the dehydration of D-xylose,is the most important intermediate for FF formation.HAA results from the C-C bond scission of D-xylose by retro-aldol and cyclic Grob fragment.In addition to the above pathways,the ring-contraction reaction results in the formation of the five-membered intermediate which can transform into FF with high selectivity.The dehydrated xylose is also another source for HAA formation.The fast pyrolysis mechanism of xylobiose is similar to that of xylose,and thus similar pyrolytic product distribution can be obtained from them.However,the pyrolytic products of xylan differ significantly from those of xylose and xylobiose,which is ascribed to its high degree of polymerization and the branches in it.Thirdly,the H2SO4-catalyzed reaction model was established based on the results of the studies on the holocellulose convention pyrolysis mechanism to 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 Cl?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 the participation of 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 conventional pyrolysis process,become very important in the catalytic process,because their activation energies are decreased dramatically by adding H2SO4(from 322.7?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.Finally,the formation mechanism of LGO,which is the major product in acid-catalyzed pyrolysis of cellulose,was further studied based on the result of the last chapter.The results indicate that H3PO4 significantly decreased the energy barriers of the pyrolytic reactions and altered the competitiveness of the LGO formation pathways,promoting LGO formation.Through different pathways in the non-catalytic and H3PO4-catalyzed conditions,LGO is mainly produced from the primary decomposition of lucose units of cellulose and secondary conversion of levoglucosan.The major catalytic formation pathways of LGO comprise similar reactions,with dehydration at the 3-OH+2-H site as the rate-determining step.Importantly,secondary conversion of DGP is not feasible for LGO formation,in contrast to previous reports.In addition,a high degree of polymerization is beneficial for the selectivity of LGO formation in the catalytic process,because the glycosidic bond is important for the formation of the bicyclic structure(1,5-and 1,6-acetal rings).