| Lignin has a great potential for producing value-added chemicals.Liquid phase depolymerization technology has been widely used in lignin’s conversion.However,most of reports about depolymerization so far focused on the technological parameters such as reaction time,temperature and catalysts.Little was about the solvents effect on depolymerization.Therefore,in this study,the theory of solvent effects was proposed to develop models for predicting the conversion of lignin,and even for the estimation of the products distribution in the process of lignin depolymerization.Experiments were performed on lignin from the residues of enzymatic corn stalks,and 10 different kinds of organic solvents were tried for the depolymerization process.According to the theories of Reichardt and Kamlet-Taft,solvent property parameters such as polarity(ET),polarity/polarizibility(π*),H-bonding properties(α,β),acceptor number(AN)and donor number(DN)can be used to describe the depolymerization results.It was shown that hydrogen-bond donor(a)had the most significant effect on the increase of lignin conversion while hydrogen-bond acceptor(β)had a negative effect.Four different models were established by solvent parameters of AN、DN、ET、andσ separately.The values of R2 were 0.979、0.898、0.979、0.919 respectively for each model.The ET model was the best among all because ET was a parameter corresponding to an average of H-bond donor and H-bond acceptor.Because of that,ET model may suit for more solvents.In addition,the ET model showed a good relevance to the results.And among 10 solvents studied,7 had the deviations of below 2%between the experimental and predicted values.It was found that the solubility in solvents has a great impact on the depolymerization of lignin.The relationship between solubility parameter and lignin depolymerization rate was confirmed by methanol/water mixed solvent system.The effects of solubility on the depolymerization were analyzed by n-hexane and different volumes of methanol/water mixed involved as solvents.When n-hexane was used as the solvent,the solubility of lignin was poor thus the conversion rate was low.When methanol/water with different volume fraction was used as the reaction solvent,the yields of liquid products from lignin were observed increased first and then decreased.When the volume ratio was 80:20,the yield of the liquid products was the highest.According to the calculation,the σ value of lignin was 13.98(cal/cm3)1/2,which was very closest to the measured one of 15.15(cal/cm3)1/2 for methanol/water volume ratio of 80:20.Also,with the increase in the volume fraction of methanol,the amount of 2,6-dimethoxy-4-propenylphenol and 2-methoxy-4-ethylphenol were also increased in the products,and their trends were about the same as that of the yields for the overall liquid products.The reaction mechanism of lignin depolymerization was studied by H-type and G-type model compounds.The main depolymerization products of the H-type model compounds were phenols,accompanying the long chain alkane formed.But the depolymerization of H-type model compound in the methanol system did not performed very well.A large number of ether bond structures appeared in the main depolymerization products of the G-type model compound.NMR results proved that solvent was directly involved in the reaction during depolymerization.The reaction mechanism of G-type model compound was analyzed,and it was proved that the chemical activity of ether bond on β-O-4 was the highest during the depolymerization of G-type model compound,and the fracture usually occurred first on it in the process of lignin depolymerization. |
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