Preparation Of High-value Chemicals By Selective Depolymerization Of Lignin With Metal-based Ionic Liquids

Living with limited resources on the planet represents a tremendous challenge due to our increasing global population.The growing demand for fuels and chemicals and the society's dependence on nonrenewable petroleum should be addressed simultaneously through the development of sustainable technologies that would enable the efficient utilization of renewable resources.Among various renewable resources?e.g.,solar energy and wind,etc.?,biomass is the only renewable organic carbon resource in nature,which endows it with unique advantage in producing value-added products.Lignin is the largest renewable source of aromatic building blocks in nature and has significant potential to serve as starting material for the production of small molecule or functionalized aromatic compounds to offer suitable alternatives to the universally used.In this regard,the catalytic conversion of lignin was found extremely challenging,mainly due to the robustness and complexity of its structure.It became clear,that the development of efficient catalytic methods for lignin selective depolymerization will play a central role in lignin valorization.Currently,there are some scientific problems in the catalytic conversion of lignin,such as low conversion efficiency and poor product selectivity.In this dissertation,a series of metal-based ionic liquids catalysts and novel catalytic systems for lignin selective depolymerization have been developed.The possible mechanism of lignin selective production of high value-added chemicals under the catalysis of metal-based ionic liquids are proposed.The detail research contents are as follows:?1?An efficient and reusable metal-based ionic liquid?MBIL?was developed for the selective tailoring of p-coumaric acid ester?pCA?,a typical p-hydroxyphenyl?H?unit,into methyl p-hydroxycinnamate?MPC?,a versatile chemical for the petrochemical industry.Under optimized conditions?0.25 g lignin,1.0 mmol catalyst,10 mL CH₃OH,180°C,6.0 h?and in the presence of catalyst[Bmim][FeCl₄],a volatile aromatic product of 10.5 wt%was obtained,of which,70.5%separated as pure MPC with an isolated yield of 71.1 mg g^-1.FT-IR,¹³C NMR,ANO and 2D HSQC demonstrated that this effective MPC production process can be classified as a highly selective molecular tailoring approach.In which,H unit is dissociated through the selective cleavage of the ester bond,leaving G and S units without notable structural change in Re-lignin,of which,86.0 wt%of H structure unit is cutting off from lignin,with 70.6%being selectively converted to MPC.While DFT calculations combined with model compound experiments illustrated that superior catalytic performance exhibited by[Bmim][FeCl₄]can be ascribed to the relatively narrow energy gap between the lignin ester bond and[FeCl₄]^-anion.A possible mechanism is proposed in this chapter.?2?For further improve the MPC yield,an efficient strategy to selective methanolysis herbaceous lignin into MPC through metal-based deep eutectic solvents?M-DES?as catalyst is presented.The relationship between catalyst dosage,parameters of reaction conditions,as well as the structural evolution of regenerated lignin?Re-lignin?using 2D HSQC,quantitative ¹³C NMR,GPC etc.were investigated.The results demonstrated that Fe-based DES?ChCl[FeCl₃]₂?exhibited excellent catalytic performance in selective depolymerization bagasse lignin,yielding105.8 mg g^-1 of MPC with 74.1%selectivity.Compared with above?1?,the MPC yield in this process increased by 36%.In addition,this catalysis system is also suitable to other herbaceous lignin.What's more,the catalyst is simple to prepare,atomically economical,and biodegradable.?3?At last,an iron-based ionic liquid catalyst[Bmim][Fe₂Cl₇]for the oxidative ring-opening of lignin to produce C2 and C4 esters has been investigated.The experimental results show that under optimized conditions?0.2 g lignin,0.4 mmol catalyst,15 mL ethanol,200°C,4.0 h?,lignin conversion is 69.4%,and produce C2 ester(ethyl acetate:643.9 mg g^-1)and C4ester(ethyl maleate:143.3 mg g^-1)as the two major products.In the total of molecular compounds,the selectivity of ethyl glycolate is 63.3%.The experiment further expanded the lignin substrate and found that this method also has a good catalytic effect on other herbaceous lignin as well as wood lignin.Through characterization of the structure changes in lignin before and after the reaction,it was found that in the lignin depolymerization process,this method preferentially depolymerized S and G units,which containing methoxy groups,while the oxidative ring opening effect of H structural unit was insufficient.To this end,combining the experimental results of model compounds,a possible reaction mechanism was also proposed.