Reductive Depolymerization Of Lignin And Separation And Upgrading Of The Products

As the world’s second most abundant biomass,lignin represents a potential source of renewable value-added chemicals.Diversified chemicals,especially phenolics,are available from depolymerization of lignin due to its special structure of phenolic polymers,making it potential to partially substitute petroleum-derived products and establish a sustainable society.However,in the field of lignin depolymerization and utilization,many problems,including unclear mechanism for depolymerization,low production efficiency,harsh operating conditions,product separation and application,remain to be resolved at the present stage.Aiming to solve the problems and develop the techniques for lignin valorization,in this thesis,we studied key scientific and technique issues within the field,revealed the mechanisms of depolymerization,and established new techniques and methods on depolymerization,product separation and upgrading.Chapter 1: Depolymerization techniques and products were reviewed.Mainstream depolymerization techniques were reviewed.Based on yield,complexity,the value of products,and other factors,we selected and proposed that five types of products,including aromatic aldehydes and acids,fatty acids,C3-phenols and fuels,and related conversion processes were more promising products and techniques for lignin valorization at industrial scale.Chapter 2: Catalyst deactivation in one-pot Deep reductive depolymerization of lignin to hydrocarbonic fuel and process improvement.In this work,lignin-to-fuel process was studied using real lignin as feedstock.Unlike model compounds,we found real lignin poisons the catalyst,causing rapid deactivation.We found that heavy carbonaceous deposits on the inner surface of the catalyst were responsible for the fast deactivation encountered in the conventional onestep reaction process.Moreover,the ester structures in lignin,which have previously been neglected in research,were confirmed to considerably worsen catalyst deactivation because they induce easier polymerization and hinder lignin conversion.Based on these points,we developed a three-step method that eliminates serious catalyst deactivation in lignin-to-fuel conversion process,involving depolymerization of lignin,extraction of lignin monomers,and hydrodeoxygenation of lignin monomers.The fuel production of three-step method was 8.1–17.6-times that of the one-step batch reaction.Chapter 3: A new technique for mild reductive depolymerization of lignin.Reductive catalytic fractionation(RCF)is an efficient and selective way to produce phenolic monomers from lignin.However,this strategy is difficult to scale up due to its high operating pressure.In this work,we investigated and developed RCF reaction under atmospheric air and without the use of hydrogen.The atmospheric RCF(ARCF)was conducted in acidified ethylene glycol in glass vessels at 185–195 °C catalyzed by 5%Ru/C.The products mainly include propylguaiacol and propylsyringyl(up to 95.6% among the lignin monomers)and do not contain propanolguaiacol,propanolsyringyl,or H monomers.Although the total yield of lignin monomers in ARCF is about one-quarter less than that of RCF,the operation of ARCF is much easier,milder,safer,and cheaper due to the atmospheric condition and the feasibility of semicontinuous operation.Chapter 4: A new and easy protocol for the separation of propylguaiacol and propylsyringol from RCF and ARCF products.RCF and ARCF are emerging techniques for lignin depolymerization and produce value-added phenolic monomers.In this work,we established a new and easy protocol,mainly including preparation of monomer-enriched oil from RCF and ARCF products,extraction of the monomer-enriched oil with KOH solution,concentration,crystallization,and reverse-extraction,which successfully separated the two main monomeric products,propylguaiacol and propylsyringol,from RCF and ARCF products.The GC purity of isolated propylguaiacol and propylsyringol reached 88.4%and 96.4%,respectively,and could be increased to 93.1% and 98.3%,respectively,by separate recrystallization.The separation is based on(1)the huge difference in solubility between propylguaiacol and propylsyringol in KOH solution,and(2)the suitable acidity of the two monomers.Chapter 5: A new technique that produces phenazine and its derivatives from lignin-derived catechols.In this work,we report a simple strategy to convert catechol to phenazine,a useful N-heterocycle three-aromatic-ring compound,whose current synthetic procedure is complex via petroleum-derived feedstock.The reaction applies catechol as the sole carbon source,and aqueous ammonia as reaction media and nitrogen source.Without additional solvent,phenazine is obtained in 67% yield in the form of high purity crystals(>97%)over Pd/C catalyst after a one-pot-two-stage reaction.If cyclohexane is used as a co-solvent in the first step,higher yield(81%)and purity(>99%)can be achieved.Mechanistic investigations involving control experiments and isotope labeling study reveal that hydrogenation,amination,coupling,and dehydrogenation reactions are the key steps leading to phenazine formation.The conversion of other lignin-derived catechols highlights the protocol is extendable to produce substituted phenazines.