The Formation Mechanisms Of Furfural And Anhydrosugar Derivatives During The Pyrolysis Of Holocellulose-based Monosaccharides

Biomass pyrolysis liquefaction technology is one of the effective means to solve the fossil fuels depletion and environmental pollution problems.Cellulose and hemicellulose,also called holocellulose,are two major components of biomass.Their pyrolysis products contain a variety of organic compounds with high value,such as 5-hydroxymethyl-furfural(5-HMF),furfural(FF),levoglucosenone(LGO),1,6-anhydroglucofuranose(AGF),1,4:3,6-dianhydro-?-D-glucopyranose(DGP),1,4-anhydro-Dxylopyranose(ADX),1,5-anhydro-4-deoxy-D-glycero-hex-1-en-3-ulose(APP),1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one(LAC)and so on.To obtain more valuable products from biomass using selective pyrolysis technology,studys on the formation mechanisms of these products are essential.In this study,density function theory(DFT)method was employed to research the formation mechanisms of 5-HMF,FF,AGF,DGP,APP,LAC and ADX from fast pyrolysis of four holocellulose-based monosaccharides,i.e.,?-D-glucopyranose,?-D-mannopyranose,?-D-xylopyranose and O-acetyl-?-Dxylopyranose.(1)The formation mechanisms of 5-HMF from ?-D-glucopyranose and ?-Dmannopyranose were researched in detail and the results indicate that ?-Dglucopyranose through the following pathway to generate 5-HMF is the most feasible.?-D-glucopyranose firstly undergoes the ring-opening reaction to form D-glucose,which then converts to D-fructose by the isomerization.D-fructose is prone to form fructofuranose.After that,fructofuranose undergoes three dehydration reactions sequentially to produce 5-HMF.As for ?-D-mannopyranose,it also undergoes the ringopening and isomerization reactions to form fructofuranose.And then three dehydration reactions occur to produce 5-HMF.(2)The most feasible formation pathways of FF from ?-D-glucopyranose,?-Dmannopyranose and ?-D-xylopyranose were found by the calculation.?-Dglucopyranose firstly undergoes the ring-opening reaction to form D-glucose.Then,Dglucose undergoes the dehydration(3-OH + 2-H)and keto-enol tautomerization to form 3-deoxy-glucosone.Afterwards,3-deoxy-glucosone undergoes the ring-formation reaction(5-O(OH)+ C2),the breakage of the C5–C6 bond which accompanies with the dehydration between 4-OH and 6-H(OH)and the dehydration between 2-OH and 3-H to form FF.Two favorable formation pathways of FF from ?-D-mannopyranose were obtained.One of them is described below: ?-D-mannopyranose firstly undergoes the ring-opening reaction to form D-mannose.Then the breakage of C5–C6 bond of the D-mannose with the dehydration between 4-OH and 6-H(OH),the ring-formation reaction which accompanies with the dehydration(2-OH + 5-H(OH))and the dehydration between 3-OH and 2-H occur successively to produce FF.In another favorable pathway,the important intermediate 3-deoxy-glucosone is obtained by the successive dehydration(3-OH + 2-H)and keto-enol tautomerization.Then FF is generated from 3-deoxyglucosone.For ?-D-xylopyranose,the first step is the ring-opening reaction to form D-xylose.The D-xylose then undergoes the dehydration between 3-OH and 2-H,the ringformation which accompanies with the dehydration between 2-OH and 5-H(OH)and the dehydration between 4-OH and 5-H to form FF.While,for O-acetyl-?-Dxylopyranose,it hardly produces FF because of the O-acetyl group at C2 position.(3)The formation mechanisms of AGF,DGP,APP and LAC from ?-Dglucopyranose were revealed.As for AGF,?-D-glucopyranose firstly undergoes the ring-opening reaction to form D-glucose,and then the ring formation reaction(6-O(OH)+ C1)and the dehydration between 4-OH and 1-H(OH)which accompanies with the ring-formation reaction(1-O(OH)+ C4)occur to produce AGF.The most feasible formation pathway of DGP from ?-D-glucopyranose is as follows.?-D-glucopyranose firstly undergoes the dehydration between 1-OH and 2-H.Then,the breakage of C1=C2 bond,the dehydration between 3-OH and 6-H(OH)and the ring-formation reaction(6-O(OH)+ C1)occur simultaneously.Afterwards,the new C1–O(C6)bond and C2=C3 bond break,and the ring-formation between 6-O(OH)and C3 occurs to form the intermediate.At last,the C1=C2 bond of the intermedaite cracks which accompanies with ring-formation reaction(4-O(OH)+ C1)to produce DGP.There are two favorable formation pathways of APP from ?-D-glucopyranose.In one of them,?-D-glucopyranose undergoes the dehydration between 4-OH and 3-H,keto-enol tautomerization and the dehydration between 1-OH and 2-H successively to form APP.In another favorable pathway,LG is formed as an important intermediate.LG undergoes the dehydration between 4-OH and 3-H,keto-enol tautomerization and breakage of the 1,6-glycoside bond to produce APP.As for LAC,?-D-glucopyranose undergoes the dehydration between 1-OH and 2-H,keto-enol tautomerization,dehydration between 4-OH and 3-H,keto-enol tautomerization and the ring-formation between 6-O(OH)and C2 successively to produce the intermediate IA.Then,the C1-C2 bond of IA cracks and the bond between C1 and C3 formed to produce LAC.(4)The main feasible formation pathways of DGP,APP and LAC from ?-Dmannopyranose and the formation pathway of ADX from ?-D-xylopyranose were found.For DGP,?-D-mannopyranose firstly undergoes the dehydration between 3-OH and 2-H.Then,the ring-formation(6-O(OH)+ C1)and dehydration between 1-OH and 6-H(OH)occur.Afterwards,the new C1–O(C6)bond and C2=C3 bond break,and the ring-formation between 6-O(OH)and C3 occurs.At last,the C1=C2 bond cracks which accompanies with ring-formation reaction(4-O(OH)+ C1)to produce DGP.The most feasible formation pathway of APP from ?-D-mannopyranose is as follows.?-D-mannopyranose undergoes the dehydration between 1-OH and 2-H,dehydration between 4-OH and 3-H and keto-enol tautomerization successively to form APP.The most feasible formation pathway of LAC from ?-D-mannopyranose is almost the same as that from ?-D-glucopyranose.?-D-mannopyranose undergoes the dehydration between 1-OH and 2-H,keto-enol tautomerization,dehydration between 4-OH and 3-H,keto-enol tautomerization and the ring-formation between 6-O(OH)and C2 successively to produce the intermediate IA.Then LAC is generated from IA.ADX is generated from ?-D-xylopyranose through the following pathway.?-Dxylopyranose undergoes the dehydration between 1-OH and 2-H and the C1=C2 bond breaks which accompanies with ring-formation reaction(4-O(OH)+ C1)to produce ADX.