Fundamental Investigation On The Directional Production Of Platform Chemicals From Low-Temperature Pyrolysis Of Biomass Based On Controllable Cleavage Of Glycosidic Bonds

The efficient conversion and utilization of biomass is one of the most significant technical routes to achieve the carbon neutral goal of Chinese government before 2060.Biomass fast pyrolysis can produce bio-oil with high-efficiency and low-cost.However,fast pyrolysis of biomass can drastically promote uncontrolled C-C/C-O bond cleavages and polycondensation reactions,resulting in the complex compostions of bio-oil and the formation of considerable amounts of C1-C4 light oxygenates,permanent gas,and char,which reduces the efficiency of whole pyrolysis system and limits the commercial application of bio-oil.A selective sequential fractionation protocol for biomass is developed to quantitatively clarify the influence of individual component on the product distribution from biomass fast pyrolysis.It is found that the catalytic activity of inherent alkali/alkaline earth metals（AAEMs）within biomass is the most critical factors determining the product distribution,especially the yields of anhydrosugars from biomass fast pyrolysis.A small amount of H₂SO₄loading for passivating the inherent AAEMs wtihin biomass may be one of the most ideal pretreatment methods.Our studies show that pyrolysis of 2.75%H₂SO₄-loaded corncobs at300°C can selectively break the C-O bonds within holocellulose and almost completely inhibit the cleavage of C-C bonds,including decarboxylation,decarbonylation and ring opening reactions,in order to maximize the yield of anhydrosugar such as levoglucosan and1,4-anhydro-α-D-xylopyranose,while minimizing or even completely inhibiting the degradation of lignin and the formation of permanent gas,C1-C4 small oxygenates and char.Low-temperature controllable pyrolysis of 2.75%H₂SO₄-loaded orncobs at 300°C can significantly increase the yields of levoglucosan and 1,4-anhydro-α-D-xylopyranose from0.75 and 0%to 39.0 and 13.4%,respectively.The feedstock flexiblity and energy consumption of low-temperature controllable pyrolysis of biomass are discussed.It is confirmed that this novel process is succesfully applied to different types of biomass,including herbaceous agricultural biomass waste（corncobs）,hardwood biomass（eucalyptuses）,softwood biomass（pines）,and herbaceous industrial biomass waste（bagasses）.The optimal H₂SO₄loading of different types of biomass to maximize the yields of anhydrosugars is related to their AAEMs content,which is between0.4%and 2.75%.The results prove that low-temperature controllable pyrolysis of H₂SO₄-loaded biomass may be a versatile and feedstock flexible method to realize directional pyrolysis of biomass to produce value-added platform chemicals.In addition,the heat required for low-temperature controllable pyrolysis of biomass was calculated,and it is found that,compared with untreated biomass at 500°C,the reduction in heat requirement reach78.5%,which proves that H₂SO₄loading can not only achieve the directional pyrolysis of biomass into anhydrosuagars at low-temperature,but also achieve the efficient energy-saving.The type of acid is the key to achieve low-temperature controllable pyrolysis of biomass.By comparing the product distribution from low-temperature pyrolysis of pines loaded with different inorganic acids（H₂SO₄,H₃PO₄,HNO₃and HCl）and organic acids（HCOOH and CH3COOH）,it is found that organic acids cannot achieve low-temperature controllable pyrolysis of biomass to produce high yields of platform chemicals.However,inorganic acid loading can realize low-temperature controllable pyrolysis of biomass to produce high yield of platform chemicals such as levoglucosan,and the anion of inorganic acid is also very important.SO4^2-and PO₄^3-as anions of inorganic acids can achieve low-temperature controllable pyrolysis,but NO₃^-and Cl^-can not achieve it.This may be due to that H₂SO₄or H₃PO₄participates in the low-temperature pyrolysis reaction as a whole.And the acidicity of H₂SO₄or H₃PO₄combining with the inherent AAEMs within biomass is a sufficient condition for achieving low-temperature directional pyrolysis of biomass.Low-temperature controlled pyrolysis of biomass mainly acts on its cellulose and hemicellulose components.Using cellulose and xylan as model compounds,the reaction mechanism of their low-temperature controllable pyrolysis is studied.By comparing the product distribution of H₂SO₄-loaded raw,demineralized biomass,cellulose,and xylan,it is speculated that the key for achieving the low-temperature controllable pyrolysis of biomass is the“acid catalysis and subsequent AAEMs inhibition”formed by the H₂SO₄loading combing with the inherent AAEMs within biomass.H₂SO₄can realize low-temperature activation of the C-O bonds（glycosidic bonds）within cellulose and xylan to generate levoglucosan and1,4-anhydro-α-D-xylopyranose,respectively.The presence of AAEMs can inhibit the dehydration reactions of cellulose and xylan to produce levoglucosenone and furfural.Moreover,the low-temperature activation methods for cellulose and xylan are different.H₂SO₄loading and HCl deminerilization can respectively realize the directional pyrolysis of cellulose and xylan into anhydrosugars at low-temperature.HCl deminerilization cannot achieve the low-temperature activation of cellulose.In-situ DRIFT experiment combined with2D-PCIS analysis shows that H₂SO₄can promote the condensation of the hydroxyl groups at the C6 and C1 positions of the glucopyranose ring within cellulose and the cleavage of glycosidic bonds to form levoglucosan.AAEMs can inhibit the intramolecular dehydration of the C3 hydroxyl groups and C2 hydrogen atoms at the levoglucosan and levoglucosan-end to form levoglucosone.It is found that the optimal H₂SO₄loading of biomass to maximize the yields of anhydrosugars is equal to the total AAEMs content within the biomass divided by the cellulose content of the biomass.Low-temperature controlled pyrolysis of biomass can only achieve the low-temperature activation of cellulose and hemicellulose components,while inhibiting the low-temperature pyrolysis of lignin.It is difficult to achieve complete conversion of all biomass components,especially lignin components,by single step of low-temperature controllable pyrolysis.Two-staged controllable pyrolysis of biomass is thus proposed,which combines H₂SO₄loading and two-staged pyrolysis,to realize the directional and staged conversion of holocellulose and lignin within biomass.First,the biomass with the optimal H₂SO₄loading is subjected to low-temperature controllable pyrolysis at 260-320°C.The lignin-rich solid residue is then fast pyrolyzed at 500-550°C to produce phenols and char.It is found that the temperature and reaction time of low-temperature controllable pyrolysis exert significant impacts on the yields of platform chemicals produced by the two-staged controllable pyrolysis of 0.5%H₂SO₄-loaded eucalyptuses.The highest total yield of levoglucosan（82.4%）was obtained by the low-temperature pyrolysis of 0.5%H₂SO₄-loaded eucalyptuses at 300°C with240 s followed by fast pyrolysis at 550°C,while the highest total yield of1,4-anhydro-α-D-xylopyranose（17.0%）was obtained by low-temperature pyrolysis of0.5%H₂SO₄-loaded eucalyptuses at 280°C with 20 s followed by fast pyrolysis at 550°C.By controlling the reaction time of low-temperature controllable pyrolysis at 300°C,the two-staged controllable pyrolysis can convert holocellulose and lignin into corresponding anhydrosugars and phenols in a staged and directional mode.Agricultural wastes,such as corn stalks,usually contain a high content of AAEMs,which leads to high consumption of H₂SO₄during low-temperature controllable pyrolysis.Based on the principle of“acid catalysis and subsequent AAEMs inhibition",the demineralization of corn stalks combing with H₂SO₄loading is proposed.It is found that the demineralization of corn stalks can evidently reduce the optimal H₂SO₄loading from 5%to0.2%.The yields of levoglucosan and 1,4-anhydro-α-D-xylopyranose from two staged controllable pyrolysis of demineralized corn stalks with 0.2%H₂SO₄-loading are significantly higher than those of 5%H₂SO₄-loaded raw corn stalks.And their yields of are respectively enhanced by 119 and 354%.