Synthesis And Characterization Of Lignin Model Compounds Containing Different Bond Types

Lignin is one of the main components of cell wall in plants. It is considered to be themost abundant renewable resources and the biggest natural polymers containing aromaticcompounds, accounting for almost15~30%of plant biomass and40%of the energy.Approximately50million tons of industrial lignin is produced in the pulp and paper industriesper year. However, the structure of lignin is complex and diverse, which results in a difficultyin the high valued utilization of lignin. In the process of the efficient utilization of lignin,lignin model compounds play an important role in the study of the reaction mechanism oflignin. A series of lignin model compounds containing different bonds were synthesized andcharacterized by using Gas Chromatography-Mass Spectrometry (GC-MS), High ResolutionMass Spectrometer (HRMS), Fourier Transform Infrared Spectroscopy (FTIR), ElementalAnalysis (EA) and Nuclear Magnetic Resonance (NMR) in this work.The dimeric lignin model compound containing-O-4bond3-methoxy-4-benzyloxyace-tophenone (I) was synthesized assisted by Microwave Irradiation (MI). The results indicatedthat the MI could greatly shorten the reaction time, and the yield was higher than that in thewater base heating (WB) under the same conditions. The yield of3-methoxy-4-benzyloxyace-tophenone was up to95.91%under the optimal reaction conditions of80℃for15min.A lignin model compound composed of-O-4and β-5linkages (II) was synthesized bythe radical polymerization with isoeugenol as the raw material and horseradish peroxidase(HRP)/H2O2as the catalyst. The process variables of the synthesis were investigated. It wasfound that a more than90%of yield was achieved under the optimal reaction conditions of1:1molar ratio of isoeugenol to H2O2,3:4of volume ratio of acetone to phosphate buffersolution (pH=5.83), the total volume35mL，1.5mL of10g/L HRP,50℃of the reactiontemperature and2h reaction time.A dimeric lignin model compound containing5-5’ linkage (III) was synthesized，usingacetovanillone as raw material and potassium persulfate and ferrous sulfate as redox initiator.The effects of reaction temperature, reaction time and the concentration of the reactant on theyield were explored. The result showed that the yield was up to85%under the optimal conditions, where the concentration of the acetovanillone was25g/L, the concentration of thepotassium was20g/L and the concentration was2g/L at the reaction temperature80℃for10min.Two trimeric lignin model compounds (V) and (VI) composed of-O-4and β-O-4linkages and a tetrameric lignin model compound (VII) composed of-O-4, β-O-4and β-5w-ere synthesized. The intermediate3-methoxy-4-benzyloxy--bromoacetophenone (IV) wassynthesized using3-methoxy-4-benzyloxyacetophenone and copper bromide as raw materials.Secondly,3-methoxy-4-benzyloxy--bromoacetophenone was selected as a raw material toreact with guaiacol, isoeugenol and lignin model compound (II) and3-methoxy-4-benzyloxy--(2-methoxyphenoxy)-acetophenone (V),(E)-1-(4-(benzyloxy)-3-methoxyphenyl)-2-(2-me-thoxy-4-(prop-1-en-1-yl) phenoxy)ethanone (VI) and a tetrameric lignin model compound(VII) were prepared successively.The molecular weight and molecular formula of the lignin model compounds weremeasured by GC-MS, HRMS and EA. The structures of the products were characterized byFTIR,1H-NMR and13C-NMR.