Effect Of Lignin Fractionation On The Performance Of Lignin-based Catalyst

As the most abundant aromatic biopolymer in nature,lignin presents great potential to replace petroleum products to produce valuable materials and chemicals because of its unique benzene ring structure.However,the majority of industrially obtained lignins are commonly burned directly as waste and only a small amount of them are converted to commercial products.The heterogeneity,including molecular weight polydispersity and structural complexity,causes unstable properties of lignin in down-stream applications,which severely limit the high-value utilization of lignin.In order to obtain the lignin fractions with reduced molecular weight polydispersity and structural complexity and improve the application value of lignin,lignin with reduced heterogeneity was obtained by fractionation process,and the effect of lignin heterogeneity on the catalytic performance of lignin-based nanospheres supported metal catalyst was investigated.The main research contents of this work are as follows:（1）In view of the problems of complex process,high cost,and dependence on toxic reagents in the existing fractionation methods,we developed a novel“one-step-three fractions”fractionation strategy to reduce lignin heterogeneity using aqueous two-phase system（ATPS）composed of（NH₄）₂SO₄ and ethanol.In contrast to conventional step-wise fractionation processes,the proposed process subdivided heterogeneous lignin into three homogeneous fractions in only one step:the first fraction（F1）dissolved in the ethanol-rich top layer;the second fraction（F2）dissolved in the salt-rich bottom layer and the last fraction（F3）insoluble in both two layers.F2 presented the lowest molecular weight（7493 Da）followed by F1（9625 Da）while F3 showed the highest molecular weight（13915 Da）.With the increase of molecular weight,the contents of guaiacyl unit andβ-O-4 linkage increased while the content of hydrophilic groups（carboxyl and aromatic hydroxyl）decreased significantly.Moreover,the ATPS exhibited satisfactory recyclability and the fractionation approach could be applied to different types/sources of lignin.Consequently,the work indicates that ATPS is a simple and effective way to fractionate lignin and reduce its molecular weight polydispersity and structural heterogeneity in one step.（2）In this regard,the current work focuses on the effect of lignin heterogeneity on the green synthesis of lignin nanosphere（LNS）supported Pd nanoparticles（Pd@LNS）and their catalytic performance.Three lignin fractions with reduced heterogeneity（F1,F2,and F3）obtained by fractionation process were used for LNS fabrication and the as-prepared LNSs were further applied as carriers for Pd nanoparticles（Pd NPs）synthesized using lignin as an in-situ reducer.The LNS from F1 had a large size about 500 nm and the size decreased noticeably to 350 nm for F2 LNS,while the LNS from F3 was only about 60–80 nm in size.Owing to the superior reducing power of F1,the LNS prepared from F1 presented the highest loading amount of Pd NPs among the three LNSs.Moreover,these Pd NPs had the smallest particle size and they were dispersed in a much more uniform way than those on the LNSs from F2 and F3.Therefore,the Pd@LNS prepared from F1 exhibited significantly increased catalytic activity in the Cr（VI）reduction（3min）compared with those from F2 and F3,as well as the initial lignin.Consequently,the work implies that lignin heterogeneity has considerable influences on the green formation of Pd NPs on LNS,and the catalytic performance of Pd@LNS can be substantially improved using low molecular weight lignin fraction as the feedstock.（3）In this study,we proposed a novel method to prepare lignin-based carbon nanospheres（LCNS）supported Pd nanoparticles catalyst（Pd@LCNS）,and the effect of lignin heterogeneity on the catalytic performance of Pd@LCNS was investigated.Three lignin fractions with reduced heterogeneity（F1,F2,and F3）obtained by fractionation process were used for LNS fabrication.Then the as-prepared lignin nanospheres were used to adsorb the Pd²⁺,which was reduced to Pd NPs during the process of lignin carbonization.The prepared Pd@LCNS was used to catalyze vanillin hydrodeoxygenation to produce 2-methoxy-4-methylphenol.Due to the high adsorption capacity of F3,the LCNS prepared from F3 presented the highest loading amount of Pd NPs among the three LCNSs.Therefore,the Pd@LCNS prepared from F3 exhibited significantly increased catalytic activity in the process of catalyzing vanillin hydrodeoxygenation reaction compared with those from F1and initial lignin,and the conversion rate was higher than 99%.Consequently,the work implies that lignin heterogeneity has considerable influences on the catalytic performance of Pd@LCNS,and it can be substantially improved using high molecular weight lignin fraction as the feedstock.