Depolymerization features of lignin under supercritical ethanol state in the presence of metal catalysts

다국어 초록 (Multilingual Abstract)

Catalytic depolymerization of lignin to monomeric phenols as an alternative to the petrochemical industry is the most promising approach to realize sustainable utilization of lignin. However, reaction pathways and chemistry behind this technology are very complex due to structural recalcitrance of lignin. This research focuses on an external effect on physicochemical properties of lignin depolymerized products to provide an insight into the lignin decomposition behavior.
First, the effects of various parameters, such as temperature, reaction time, solvent amount, and initial hydrogen gas pressure on the distribution of lignin depolymerized products formed during solvolytical depolymerization of organosolv lignin (OL) was investigated. During this process, concurrent reactions involving depolymerization and recondensation as well as secondary decomposition were significantly accelerated with increasing temperature, leading to increase in both lignin derived phenols in the phenol-rich oil fraction (lignin-oil) and undesirable products (char and gas).
Second, soda lignin (SL) was directly depolymerized over Pt/C, Pd/C, Ru/C, and Ni/C under supercritical alcohols (methanol, ethanol, 2-propanol, and t-butanol) to investigate the effect of catalyst and solvent types on the physicochemical properties of lignin-oil. 1H-/2D-HSQC-NMR and GPC analysis revealed that the Mw of lignin-oil remarkably lower than that of SL, which is clear evidence of β-O-4 and β-β bond cleavages. The top four main monomeric phenols in lignin-oil were 4-ethylphenol, guaiacol, 4-ethylguaiacol, and syringol, for which the sum was mostly produced in E-Pt (4.2 wt%). It was observed that excessive catalyst dosage caused side reactions, resulting in decreasing monomeric phenols yield.
Third, SL was sequentially fractionated by organic solvents (ethyl acetate: F1, methanol: F2, acetone: F3, dioxane/water: F4, and insoluble fraction: F5) for homogeneous preparations of lignin. The Mw of SL, F1, F2, F3, and F4 were 2800, 1120, 2860, 5850, 7200 Da. 2D-HSQC-NMR analysis revealed that lignin fraction with lower Mw had highly condensed structure (poor β-O-4 linkage). Each fraction was efficiently depolymerized into lignin-oil under the combination of supercritical ethanol (350 °C) and 5 wt% Ru/C to compare depolymerization feature of lignin with different molecular distribution. The yield of lignin-oil, mixture of monomeric phenols as well as high molecular phenolics, ranged from 62.5 to 81.4 wt% and was inversely proportional with the Mw values of lignin fraction. The selectivity of monomeric phenols produced from lignin depolymerization process was clearly affected by the Mw value of lignin. Especially, relative highly condensed fraction yielded higher amount of non-alkylated phenols, methylated-, and ethylated phenols more than other lignin fractions.
Lastly, phenol was selectively produced from SL via a RuNi/SBA-15-catalyzed supercritical treatment. The bimetallic catalysts used in this work were prepared by a wetness impregnation method with different molar ratios of Ru and Ni (RuxNi1-x, x = 0.2, 0.4, 0.6, 0.8, and 1.0). This study revealed that the chemical properties of lignin-oil were clearly affected by the physicochemical properties of bimetallic catalysts. By increasing the amount of desorbed H2 and the acidity of the catalyst, the yields of lignin-oil, total phenols, and phenol increased while that of char decreased. The yields of lignin-oil, total monomeric phenols, and phenol were largest with the Ru0.6Ni0.4 catalyst (77.5 wt%, 12.7 wt%, and 4.7 wt%, respectively).

목차 (Table of Contents)

• Chapter1 Introduction 1
• 1. Background 2
• 1.1. Lignin structure & application 2
• 1.2. Residual lignin from pulping & sugar based biorefinery 6
• 1.3. Solvolytic lignin depolymerization process 10
• 1.4. Catalysts in lignin depolymerization process 14
• 1.5. Lignin fractionation 15
• 2. Objectives 17
• 3. Literature review 19
• 3.1. Structural characterization of lignin macromolecules 19
• 3.1.1. Wet chemistry & spectroscopic analyses 19
• 3.1.2. Thermal degradation characteristics 24
• 3.1.3. Pyrolysis- Gas Chromatography/Mass spectrophotometry 26
• 3.2. Solvolytic lignin depolymerization process 28
• 3.2.1. Model compounds study 28
• 3.2.2. Lignin macromolecules study 32
• 3.3. Bimetallic catalyst in lignin depolymerization process 36
• 3.4. Lignin fractionation 39
• 3.4.1. Solvent fractionation 39
• 3.4.2. Ultra- and nano-filtration 41
• Chapter2 Effects of reaction parameters on solvolytical depolymerization of lignin in sub- and supercritical ethanol 43
• 1. Introduction 44
• 2. Materials and methods 46
• 2.1. Materials 46
• 2.2. Solvolysis of lignin with ethanol 49
• 2.3. Characterization of lignin depolymerized products 53
• 2.3.1. Gas Chromatography/Mass Spectroscopy (GC/MS) analysis 53
• 2.3.2. Gel Permeation Chromatography (GPC) analysis 54
• 2.3.3. Elemental analysis 54
• 3. Results and discussion 55
• 3.1. Effects of reaction conditions on yields of essential products 55
• 3.1.1. Effect of temperature 55
• 3.1.2. Effect of reaction time 56
• 3.1.3. Effect of solvent amount 57
• 3.1.4. Effect of initial hydrogen gas pressure 58
• 3.2. Chemical properties of lignin-oils and chars 60
• 3.2.1. Average molecular weights of lignin-oils 60
• 3.2.2. Elemental compositions of lignin-oils 63
• 3.2.3. Characterization of monomeric phenols 66
• 3.3. Chemophysical properties of chars 72
• 4. Conclusions 75
• Chapter3 Catalytic depolymerization of lignin to alkylated phenols over various metal catalysts in supercritical alcohols 77
• 1. Introduction 78
• 2. Materials and methods 80
• 2.1. Materials 80
• 2.1.1. Lignin preparation 80
• 2.1.2. Catalyst preparation 83
• 2.2. Depolymerization of lignin to lignin-oil in supercritical alcohol state 83
• 2.3. Characterization of lignin depolymerized products 86
• 2.3.1. Nuclear Magnetic Resonance (NMR) analysis 86
• 2.3.2. Gel Permeation Chromatography (GPC) analysis 86
• 2.3.3. Elemental analysis 86
• 2.3.4. Gas Chromatography/Mass Spectroscopy (GC/MS) analysis 87
• 3. Results and discussion 88
• 3.1. Yield of lignin depolymerized products 88
• 3.2. NMR analysis of lignin-oil 91
• 3.3. Chemical properties of lignin-oil 94
• 3.4. GC/MS analysis of lignin-oil 98
• 3.4.1. Yield of monomeric phenols in lignin-oil 101
• 3.4.2. Catalyst selectivity to top four phenols 105
• 3.5. Effect of catalyst dosage on lignin-oil properties 107
• 4. Conclusions 111
• Chapter4 Catalytic ethanolysis features of lignins having different molecular weight distribution 113
• 1. Introduction 114
• 2. Materials and methods 116
• 2.1. Sequential solvent fractionation 116
• 2.2. Lignin characterization 116
• 2.3. Lignin depolymerization in supercritical ethanol state 117
• 2.4. Chemical properties of lignin-oil 118
• 2.4.1. Gas Chromatography/Mass Spectroscopy (GC/MS) analysis 118
• 2.4.2. Gel Permeation Chromatography (GPC) 118
• 2.4.3. Elemental analysis 119
• 3. Results and discussion 120
• 3.1. Chemical and structural characteristics of lignin fractions 120
• 3.2. 2D-HSQC-NMR analysis of lignin fractions 124
• 3.3. Yield of lignin depolymerized products 128
• 3.4. Chemical properties of lignin-oils 130
• 3.4.1. Yield of monomeric phenols in lignin-oils 130
• 3.4.2. The average molecular weight of lignin-oils 137
• 3.4.3. Elemental composition of lignin-oils 140
• 4. Conclusions 142
• Chapter5 Selective production of phenol from lignin macromolecule over RuNi/SBA-15 bimetallic catalyst 143
• 1. Introduction 144
• 2. Materials and methods 146
• 2.1. Lignin 146
• 2.2. Catalyst preparation 146
• 2.3. Catalyst characterization 147
• 2.4. Lignin depolymerization in supercritical ethanol state 148
• 2.5. Chemical properties of lignin-oil 149
• 2.5.1. Gas Chromatography/Mass Spectroscopy (GC/MS) analysis 149
• 2.5.2. Gel Permeation Chromatography (GPC) 149
• 3. Results and discussion 150
• 3.1. Characterization of catalysts 150
• 3.2. Bimetal catalyzed lignin depolymerization process 160
• 3.2.1. Yield of lignin depolymerized products 160
• 3.2.2. Yield of monomeric phenols in lignin-oils 162
• 3.2.3. The molecular weight distribution of lignin-oils 166
• 3.3. Correlation between catalyst characteristics and lignin-oil properties 169
• 4. Conclusions 172
• Chapter 6 Concluding remarks 173
• References 177
• 초록 199