Research paperUsing 2D NMR to characterize the structure of the low and high molecular weight fractions of bio-oil obtained from LignoBoost™ kraft lignin depolymerized in subcritical water
Graphical abstract
Introduction
Lignin is one of the three major components that form the structure of wood: approximately 20–30% of wood is composed of lignin. Lignin is a heterogeneous, random, aromatic macromolecule that is cross-linked by carbon-oxygen (CO) and carbon-carbon (CC) bond networks [1]. Although lignin structures vary, the main types are softwood, hardwood and annual plant lignin. Whilst the general characteristics of lignins are similar, there are variations in the frequencies of the different bonds and side groups. This paper deals with softwood lignin. Lignin, as mentioned above, is a heterogeneous, random, aromatic macromolecule, which means that its exact structure is unknown. However, a vast amount of work has already been carried out with the aim of defining its various building elements and type of bonds. The structure of native lignin has still to be clarified; the closest structure is currently regarded as being milled wood lignin (MWL) [2], [3]. The MWL lignin from softwood has been found to be comprised of different phenyl-propane units, i.e. coniferyl alcohol (>95%) and para-coumaryl alcohol (<5%, Fig. 1) connected by aliphatic/aromatic CC and CO bonds (ether bonds, Fig. 1). The aliphatic inter-unit linkages found in softwood lignin are (Fig. 1) β-O-4′ (β-ether, 45–50%), α-O-4′ (α-ether, 6–8%) and β-β′ (pinoresinol, secoisolariciresinol, 3%) and the aliphatic-aromatic are: β-1′ (diphenyl ethane, 7–10%) and β-5′ (phenylcoumaran, 9–12%), whilst the aromatic connecting linkages are 4-O-5′ (diphenyl ether, 4–8%) and 5-5′ [biphenyl and dibenzodioxocine (5-5′-α, β-O-4′), 18–25%] [2], [4]. During the kraft process lignin is solubilized by the cleavage of ether bonds, yielding an introduction of phenolic groups. The most important structural change that occurs is the formation of phenolic hydroxyl groups, which enhance the water solubility of lignin in an aqueous solution. It is known that bonds such as β-O-4′, α-O-4'and β-1′ are hydrolyzed during the kraft process, and that 4-O-5′, 5-5′, β-β′ and β-5′ structures are usually more resistant to the alkaline environment of the kraft process and are therefore often found in kraft lignin (Fig. 1) [4], [5], [6]. The rather harsh conditions also cause other typical structural changes to the kraft lignin, such as new CC bonds from condensation reactions, unsaturation (i.e. stilbene and vinyl ether structures) and a reduction in the lignin side chains of aliphatic structures [6], [7], [8], [9]. The lignin obtained after a kraft cook has a rather high molecular weight (Mw, 3–17 kDa) and, due to the formation of CC bonds, it is rather stable [10].
Today, large quantities of lignin are dissolved from wood in the production of paper pulp. This lignin is mostly combusted at present, and the heat used to produce steam. However, lignin is a natural source of aromatic rings, i.e. phenols, guaiacols and syringols, making it a potential renewable source for generating fuel and chemicals that could replace a part of the current demand for fossil fuels. A new, green, process technology for the separation and purification of kraft lignin from the black liquor generated from the kraft pulping process currently exists, and is known as the LignoBoost™ process [11]. LignoBoost™ kraft lignin is precipitated from alkaline black liquor with carbon dioxide, thereby generating a solid LignoBoost™ kraft lignin. This may be used in future biorefineries to develop new, high value, products from lignin. However, additional processes need to be developed if the kraft lignin is to be transformed into new, high value, products.
Recent years a huge effort for find a new thermochemical process technology for depolymerization of lignin to small aromatic structures i.e. pyrolysis, gasification, hydrogenolysis, chemical oxidation, catalysis and hydrolysis under sub and supercritical conditions have been studied. All these thermochemical process struggle with understanding the complex depolymerization mechanism, low yield and complex product mixtures of small aromatics as well as lignin repolymerization reactions to higher molecular weight (Mw) byproducts (i.e. residual lignin, char and coke) [12], [13], [14], [15].
One promising technique for the depolymerization of lignin involves procedures based on subcritical water to generate bio-oil [12], [14], [15], [16], [17], [18]. Water gains unique properties when it is close to its critical point (372.9 °C, 22 MPa): it resembles an organic solvent and can therefore be used in the solubilization of organic structures such as the phenolic lignin network. Water at these temperatures acts as reactant, catalyst and enhances solubility of both organic structures, inorganic ions and gases which yields exceptional opportunities for promoting lignin depolymerization with an environmental friendly solvent [19], [20], [21], [22]. Another advantage is that in the reactor it is subcritical conditions and the reactions proceeds in “one phase”, but after the reactor, when the pressure and temperature has been lowered (<350 °C, <25 MPa) the system separates into two phase system i.e. bio-oil and water phase. By this means byproducts like: water, inorganic salts and more hydrophilic and reactive intermediates (carboxylic acids, aldehydes and alcohols) transports to the water phase This phase separation have been shown to be very beneficial for the bio-oil storage stability [23]. This opens up for the sustainable depolymerization of lignin by means of “green chemistry”.
Several studies of subcritical water have shown that the reaction rates for breaking the various bonds in lignin are very high: that of aryl-alkyl ethers (i.e. β-O-4 and α-O-4) is very high at 270–290 °C already [24], [25]. Catalysts and/or higher temperatures are necessary for bonds with a bond strength, such as CC bonds [21], [26]. Numerous studies of the sub/supercritical water depolymerization of lignin have been carried out at various temperatures and pressures [12], [14], [15], [17], [18], [27], [28]. The reaction pathway is thought to start with lignin being depolymerized by hydrolysis and dealkylation reactions to form aromatic monomers (i.e. phenol, guaiacol, catechol and cresol), larger oligomeric structures and smaller aliphatic reactive intermediates (i.e. aldehydes and alcohols). However, these reactive functional groups may also be located on aromatic monomers as well as on the larger oligomeric structures. The existence of many reactive groups/fragments mean that unfavorable repolymerization reactions accompany the depolymerization process, causing cross-linking to fragments of higher Mw (e.g. residual lignin, heavy oil, char, solids, residual solids, phenolic char and polyaromatic char) [29], [30], [31], [32]. Recent investigations into continuous reactors have shown that depolymerization and repolymerization are fast competing reactions that start immediately in sub/supercritical water [27], [28]. Depolymerization is often promoted by using a base catalyst, i.e. BCD [18], [25], [33], [34]. Other common additives include capping agents/cosolvents, i.e. phenol, para-cresol [31], [35], [36], [37], [38], [39], [40], [41], [42], [43], ethanol, methanol, 2-propanol and 1-butanol [43], [44], [45], [46], [47], [48], [49], [50], to diminish the extent of the repolymerization reactions and thus enhance the yield of monomers.
The Bio-oil formed from depolymerization of lignin is a complex mixture of low and high Mw structures. Today the most common analysis of the bio-oil is by extraction with organic solvents (DEE, acetone, methanol and ethyl acetate) and analyzed with Gas Chromatography-Mass Spectrometry (GC-MS), however, this yields only structural characterization of volatile organic compounds in the bio-oil (Mw 40–400 Da) which represents approximately 1/3 of the total composition of the bio-oil. Typically, the low Mw fraction of bio-oil is composed of phenolic compounds like: guaiacol, catechol, phenol, alkyl phenols and vanillin [51], [52], [53].
The remaining part of the bio-oil the higher Mw fractions could be unreacted lignin or repolymerized structures formed during the process or, indeed, a combination of both [54]. It is currently believed that the high Mw fractions (i.e. char) developed in sub/supercritical water are composed of low levels of phenolic or alcoholic groups, aliphatic groups and aromatic structures as well as large clusters formed from the recombination of aromatic rings (polycyclic aromatic hydrocarbons, PAH) [32], [55], [56], [57], [58]. A few studies reported from various subcritical conditions point towards the fact that higher Mw fractions are composed of structural motifs other than lignin. They show, for example: a higher carbon and lower oxygen content than those present in lignin; a simplified monomeric compound pattern with pyrolysis-GC-MS and a residue (solids) that could not be used for the formation of new bio-oil [25] [57] [58].
Today only a few publications can be found that cover both structural analysis of low and high Mw compounds from depolymerization of lignin in subcritical water with a multi-technique analytical approach i.e. GC-MS, Gel Permeation Chromatography (GPC), 13C solid-state NMR (cross-polarization/magic angle spinning nuclear magnetic resonance, 13C CP/MAS NMR) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/MS) [57], [59]. However, by utilizing one dimensional solid state NMR together with high resolution FT-ICR/MS is it not possible to resolve any organic structural network in the high Mw structures. Knowledge about the chemical structural composition of the higher Mw fractions from subcritical liquefaction processes is crucial and this can be obtained by use of the high resolution two-dimensional 13C,1H-correlated Heteronuclear Single Quantum Coherence NMR technique (2D HSQC NMR).
Our group has previously performed BCD on LignoBoost™ kraft lignin in subcritical water (K2CO3, 350 °C, 25 MPa) with the addition of a capping agent i.e. phenol. The depolymerization reaction of kraft lignin was performed in a continuous, high-pressure, bench-scale catalytic reactor filled with ZrO2 pellets and yielded bio-oil, water soluble organics and char (deposits on the ZrO2 catalyst, Fig. 2) [49], [60], [61]. In addition, it is known from GC-MS analysis of the bio-oil that the major phenolic monomeric/dimeric structures formed are: alkylphenol (methyl and ethyl) > catechol > anisole > guaiacol > phenolic dimer (ArCH2Ar, ArCH2CH2Ar). However, for a better understanding of the depolymerization process in subcritical water, an additional structural analysis of the various bio-oil fractions is necessary.
This work is therefore focused on elucidating the structural composition of the different fractions (Fig. 2), i.e. light oil (DEE soluble fraction), heavy oil (THF soluble fraction) and suspended solids (insoluble THF residue) that are obtained from LignoBoost™ kraft lignin BCD in subcritical water (350 °C, 25 MPa). In this work we have used the unique ability of the high resolution 2D HSQC NMR (18.8 T), combined with GC-MS, along with Mw distribution measurements made by GPC and the determination of the elemental composition of the bio-oil fractions. By using a multilevel analysis approach it's possible to gain new insight in both low and high Mw structural composition of the bio-oil fractions (light oil, heavy oil and suspended solids).
Section snippets
Materials
All reagents used in this study were purchased from Sigma–Aldrich (Stockholm, Sweden) and used as received. The LignoBoost™ kraft lignin used for subcritical water BCD to bio-oil originates from softwood and was produced at the Bäckhammar Mill (Sweden).
Reaction conditions for subcritical based catalyzed water depolymerization
The bio-oil and solids used in this study were produced in BCD subcritical water (350 °C, 25 MPa) in a continuous, high-pressure, bench-scale catalytic reactor (500 cm3) filled with ZrO2 pellets. The conditions used were: LignoBoost™ kraft lignin
Elemental analysis of the bio-oil fractions and LignoBoost™ kraft lignin
Investigation of the elemental composition of the different bio-oil fractions allowed the H/C atomic ratio to be estimated (Table 1). The order of the H/C atomic ratio in the different fractions was found to be: light oil (1.21) > heavy oil (0.98) > suspended solids (0.84). This indicates that the carbon component increases within the series, and more aromatic and condensed structures are therefore to be expected in the fractions with lower values i.e. suspended solids [64], [65].
Determination of the molecular weight of the bio-oil fractions and LignoBoost™ kraft lignin using GPC
When
Conclusions
This work has demonstrated that LignoBoost™ kraft lignin depolymerizes/repolymerizes in a continuous reactor with subcritical water (350 °C, 25 MPa, phenol, K2CO3, KOH). After fractionation of the bio-oil into low Mw (light oil) and high Mw fractions (heavy oil and suspended solids), 2D NMR showed that all of the aliphatic inter-unit linkages in the lignin, i.e. β-O-4′, β-β′, β-1′ and β-5′, had disappeared in each bio-oil fraction: light oil, heavy oil and suspended solids. Using 2D NMR and 13C
Acknowledgements
This work is supported by grants from Chalmers Energy Initiative – LignoFuel Project, Valmet AB and The Swedish Energy Agency. Our thanks go to Ms. Cecilia Persson for her technical support with the NMR analysis at the Swedish NMR Centre atThe University of Gothenburg, Mr. Tommy Friberg for his analytical contributions to the experiments.
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