生物质基酸性低共熔溶剂用于高效溶解木质素及溶解机理

Biomass-based Acidic Deep Eutectic Solvents for Efficient Dissolution of Lignin:Towards Performance and Mechanism Elucidation

设计能高效溶解木质素的溶剂对木质素的高值化利用具有重要意义。本文中设计了基于氯化胆碱、甜菜碱和左旋肉碱作为氢键受体(HBA)和四种氢键供体(HBD)的生物质衍生的酸性低共熔溶剂(DESs),可以溶解包括碱木质素(AL)、脱碱木质素(DAL)、酶解木质素(EHL)和硫酸盐木质素(KL)在内的不同类型的木质素。在大多数所设计的DESs中,EHL比AL、KL和DAL更容易溶解,而不同木质素中羟基的含量对木质素的溶解有显著影响,但是并非在所有DESs中木质素的溶解情况都符合上述规则。氯化胆碱是构建DESs的首选HBA,具有良好的性能并适应于不同类型木质素的溶解,而合适的酸度使苯甲酸和没食子酸乙酯成为对木质素溶解有利的HBDs。研究表明能有效溶解木质素的DESs应具有强的氢键酸度(α值>0.95)以及与溶解的木质素匹配的合适极性。此外,HBD的pKa值和DESs的酸度也是评价酸性DESs溶解木质素性能的有效指标。通常具有适中pKa值的HBDs能够用于构建具有高效的木质素溶解性能的DESs。DESs的粘度对木质素溶解也有一定影响,较低的粘度有助于木质素溶解。

Lignocellulose utilization has the unique feature of net-zero carbon emissions.Thus,the utilization of lignocellulose as a renewable alternative to fossil-based carbon resources is one of the most promising strategies to achieve“carbon neutrality”.Lignin has been recognized as the most abundant aromatic biopolymer on Earth;hence,it could be the most promising alternative to fossil-based aromatics.The efficient dissolution of lignin is crucial for lignin upgrading,which relies on the design of innovative and robust solvents.Herein,we designed several biomass-derived acidic deep eutectic solvents(DESs)using choline chloride,betaine,and L-carnitine as hydrogen bond acceptors(HBAs),and four protic compounds as hydrogen bond donors(HBDs),namely,oxalic acid,benzoic acid,ethyl gallate,and 5-methoxysalicylic acid.The designed DESs can dissolve different types of lignin,including alkali lignin(AL),dealkaline lignin(DAL),enzymatic hydrolysis lignin(EHL),and Kraft lignin(KL).Lignin dissolution was found to be affected by the relative contents of three phenylpropanoid monomers in lignin:syringyl(S),guaiacyl(G),and p-hydroxyphenyl(H)units.More S and fewer H units in lignin could result in higher solubility.G-,S-,and H-type structural units were found in EHL,while AL,KL,and DAL had only G-type structural units.Therefore,EHL could be more easily dissolved than AL,KL,and DAL in the most developed DESs.The hydroxyl group content of the four lignin samples had a significant impact on lignin dissolution.AL(1.98 mmol·g−1)and EHL(1.93 mmol·g−1)had much higher contents of phenolic hydroxyl groups than DAL(0.62 mmol·g−1),implying that AL and EHL had higher polarity than DAL.This resulted in different dissolution behaviors in different DESs with varying polarities.However,the sulfonate groups afforded KL with much higher polarity,thus resulting in the special dissolution behavior of KL.It is to be noted that not all cases of dissolving lignin in the developed DESs conformed to the above rules.Therefore,it is necessary to further explore the effect of the properties of the DESs on the dissolution of different lignins.Choline chloride was the preferred HBA to construct DESs with good performance and adaptability to lignin dissolution,whereas suitable acidity enabled benzoic acid and ethyl gallate to be favorable HBDs.Systematic investigation revealed that an efficient DES for lignin dissolution should possess stronger hydrogen-bonding acidity(α>0.95)and appropriate polarity matching with the dissolved lignin.In addition,the pKa value of the HBD and the acidity of the DESs were also efficient indices for estimating the performance of an acidic DES in dissolving lignin,and the pKa value and acidity could be well correlated with the polarity.Generally,HBDs(e.g.,BA and EG in this study)with moderate pKa values can be employed to construct robust DESs to dissolve lignin with satisfactory solubility.Additionally,the viscosity of the DESs should have an impact on lignin dissolution,and a lower viscosity is helpful for dissolving lignin.Therefore,the better performance of the developed ChCl-based DESs than the Betaine-and L-Carnitine-based DESs was partially because of the lower viscosity of the ChCl-based DESs.