Study On Preparation Of Lignin-based Polyols For Polyurethane By Biomass Pretreatment

As the second largest biomass resource in the world,lignin,together with hemicellulose and cellulose,constitutes lignocellulose,which is stored in most plants.Lignin has attracted extensive attention due to its characteristics of polyhydroxyl content.Lignin is an aromatic macromolecular polymer polymerized by phenylpropanyl groups,which is linked with cellulose and hemicellulose by chemical bonds.However,due to the large molecular weight and complex and heterogeneous structure of natural lignin,its application is hindered.The natural lignin is pretreated by the method of biorefinery to obtain lignin oligomers with smaller molecular weight and more uniform distribution,which replace the polyether polyol in the polyurethane rigid foam system to prepare lignin-based polyurethane rigid foam,which can improve the mechanical properties of the foam.Depolymerization of natural lignin into stable monomers is an important complementary method to the active protection strategy of lignin,and no repolymerization occurs after depolymerization of lignin into monomers.In the depolymerization of natural lignin,the most studied method is Reductive catalytic fractionation（RCF）or catalytic upstream biorefinery.In this method,metal catalysts（such as Ru/C,Pd/C,Ni Mo,etc.）are mostly used.In this study,isopropanol（2-Pr OH）was used as the hydrogen donor,the ratio of isopropanol to water was used as the variable research condition,Raney-Ni was used as the catalyst to catalyze the hydrogen transfer reaction,and natural lignocellulose was used to prepare lignin oil,that is,small Molecular lignin oligomers.The lignin oil yield（%）obtained after RCF was up to 20.68%when 2-Pr OH/H2O was 7/3.The compressive strength of the polyurethane foam prepared by using 30 wt%lignin oil instead of polyether polyol can reach 0.27±0.01 MPa,and the apparent density is 106.59±1.66 kg·m^-3.Deep eutectic solvent（DES）pretreatment of biomass also belongs to the category of biorefinery.Through the strong hydrogen bond interaction in the DES solution prepared by the hydrogen bond donor and the hydrogen bond acceptor,the lignin and cellulose can be effectively separated after treatment.In DES pretreatment,the effects of different DES solutions composed of different types of hydrogen bond donors and hydrogen bond acceptors on biomass pretreatment were studied.The pretreated DES lignin was modified by oxypropylation and used in the preparation of rigid polyurethane foam.The resulting foams have higher compressive strength（0.51±0.01 MPa）and lower apparent density(92.71±0.62 kg·m^-3).The addition of lignin oil and DES lignin both effectively improved the performance of rigid polyurethane foam and made the preparation process of polyurethane more environmentally friendly.