Lignin Valorization via Acidolysis with Ethylene Glycol Stabilization
De Santi, Alessandra
DOI:
10.33612/diss.169171380
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Publication date: 2021
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De Santi, A. (2021). Lignin Valorization via Acidolysis with Ethylene Glycol Stabilization. University of Groningen. https://doi.org/10.33612/diss.169171380
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The biorefinery concept and
lignocellulose valorization strategies
This chapter was published as part of:
Sun, Z., Fridrich, B., De Santi, A., Elangovan, S., Barta, K., Chem. Rev. 2018, 118, 614-678
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1.1. The biorefinery concept
Embracing the concept of “bioeconomy”, the necessity of finding renewable alternatives to limited and depleting fossil resources used today became broadly recognized1, together with improving the production processes aiming for minimum waste and low greenhouse gas emissions.2 Biomass has been identified as the main available renewable carbon source, lignocellulosic materials being the most promising since they don’t compete with food production.3 Lignocellulosic biomass consists of three main polymers: cellulose (30-60%), hemicellulose (20-40%), and lignin (10-30%), as illustrated in Figure 1.1.
Cellulose is a linear polymer consisting of glucose units linked together via β-1,4-glycosidic bonds with different degree of polymerization depending on the source (up to 10000 units).4 Through extensive hydrogen bonding networks, the cellulose chains assemble into rigid, semi-crystalline fibrils which are insoluble in most conventional solvents, including water.
Hemicellulose is characterized by a highly branched structure consisting of several types of sugars including pentoses (e.g. xylose, rhamnose, arabinose) and hexoses (e.g. glucose, mannose, galactose). Usually, uronic acids and acetyl moieties are present as side-chain groups.4 The composition is heavily dependent on the biomass source and the degree of polymerization is generally lower than cellulose (50-300 units). Hemicellulose is an amorphous polymer, and it is easier to solubilize and more susceptible to chemical attack compared to cellulose.
Lignin is a complex and recalcitrant polymer which is randomly held together by strong C−C and C−O bonds.5 Its main building blocks are three monolignols, namely p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol (Figure 1.1), which undergo oxidative radical polymerization resulting in several types of linkages (Figure 1.1), ratio of which dependents on the plant source.6 Softwood (e.g. pine, cedar, spruce) is mainly constituted by guaiacyl units (G, from coniferyl alcohol), while mainly G, and syringyl units (S, from sinapyl alcohol) are present in hardwood (e.g. birch, poplar). The most recurring type is the β-O-4 linkage that typically makes up about 50% of all linkages and therefore has been the focus of most depolymerization strategies.
Figure 1.1. Schematic representation of lignocellulose structure and its three main components (cellulose, hemicellulose, and lignin).
Given the different chemical nature of the cellulose and hemicellulose (sugar origin) compared to lignin (aromatic origin), lignocellulose offers the possibility of obtaining several kinds of chemical intermediates. However, this also means that the complexity of the material needs to be addressed. Similarly to oil-based refineries, the raw material needs to be processed into a range of useful products. Thus, we can define a biorefinery as a facility where biomass is processed efficiently to produce fuels, power, chemicals and value-added products, maximizing the value of each component and minimizing waste (NREL).7
Although large-scale biomass processing units exist already (e.g., paper production), they mainly focus on the utilization of the carbohydrate portion of lignocellulose, leaving lignin behind and employing it as source of energy. However, the money equivalent of lignin used as fuel is estimated to be 0.18 US $/kg while 1.08 US $/kg in case it is converted to useful chemicals.8 Thus, it is clear that to guarantee the economic feasibility of biorefinery processes, every lignocellulose component should be fully valorized and therefore it is of paramount importance to find novel ways of lignin valorization. With this aim, strategies have been developed following two approaches. On the one hand, lignin can be first isolated (Section 1.2) and then depolymerized (Section 1.3). On the
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other hand, lignin can be extracted and depolymerized in-situ in a one step process (Section 1.4).
1.2. Lignin isolation strategies
Several methods are available for the isolation of lignin from lignocellulosic biomass. The existing strategies can be divided into two main categories related to the extent of structural modification induced in lignin by the fractionation conditions, an important aspect when considering catalytic valorization of any lignin feed (Figure 1.2).
Figure 1.2. A summary of procedures for isolation of lignin from lignocellulose. 1.2.1. Methods resulting in significant structural modification of lignin
Pulping methods such as the Kraft,9,10 the Sulfite,11 the Alkaline,11 and the Klason12,13 process (Figure 1.2) generally focus on obtaining high quality cellulose from lignocellulose and result in structurally heavily modified lignins under relatively harsh processing conditions such as the extensive use of inorganic salts, base, or acid.14 For instance, Kraft lignin is modified by cleavage of most β-aryl ether bonds10 and in addition to recondensation reactions, the system is attacked by strongly nucleophilic hydrogen sulfide ions, leading to a sulfur-enriched structure (1.5−3% sulfur). The presence of sulfur poses an additional difficulty to catalyst development since it frequently leads to
decreased activity of, especially, noble metal catalysts. Due to its abundance, there is increasing interest to develop novel catalytic methods for the efficient valorization methods of Kraft lignin to valuable chemicals15,16,17 and fuels.18,19 Similarly to Kraft, sulfite lignin is also characterized by the incorporation of about 4−8% of sulfur, albeit in the form of sulfonate groups, providing water-soluble lignin. As a result, catalytic depolymerization of lignosulfonate to aromatics without it being negatively affected by sulfur is a challenging task. A promising solution is the catalytic removal of sulfur from the reaction system in the form of H2S gas, as proposed by Song and co-workers with heterogeneous Ni catalysts.20 The Klason process that employs 72% sulfuric acid causes a significant damage to the native lignin structure, while the soda pulping process presents structural modification to a lesser extent compared to the other technical lignins allowing for catalytic conversion, as reviewed recently.21
1.2.2. Methods resulting in mild structural modification of lignin
The Björkman Process
The Björkman process involves extensive grinding followed by extraction of lignin with an organic/aqueous solvent (usually dioxane/water 96/4 v/v for 24 h) to produce the so-called milled-wood lignin (MWL). This lignin has a more similar structure to native lignin due to the pH neutral and mild conditions used during extraction. However, the milling process can cause structural modification such as the presence of additional carbonyl and hydroxyl groups, especially in hardwood.22 The obtained lignin yields, typically 20−40%, depend on the raw material used.11
Cellulolytic Enzyme Lignin (CEL) and Enzymatic Mild Acidolysis Lignin (EMAL) The procedure to obtain CEL involves the treatment of the finely ground wood with cellulolytic enzymes, which cause the partial hydrolysis of cellulose and hemicellulose. Afterward, the residue is extracted with a solvent (typically dioxane/water), the solution is concentrated, and lignin precipitated in water. This procedure typically requires a few days and leads to protein and carbohydrates impurities. An improvement is represented by enzymatic mild acidolysis lignin (EMAL), where the enzymatic treatment is combined with mild acidolysis of biomass.23 This procedure is reported to offer gravimetric lignin yields 2−5 times greater than those of the corresponding MWL and CEL. Interestingly, Guerra et al.24 then investigated the differences in the lignins obtained by MWL, CEL, and EMAL treatment, employing several raw materials and showed that EMAL is characterized by a highest molecular weight (Mn ∼ 30000− 63000 g/mol) followed by
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CEL (Mn ∼ 17000−30000 g/mol) and MWL being the lowest value (Mn ∼ 6000−16000 g/mol).
Ionic Liquid Treatment
Ionic liquids (IL) have also been proposed as solvents for lignocellulose fractionation due to their special and highly tunable solvent properties.25–28 Limitations exist related to the cost of IL as well as ease of product separation and solvent recyclability. Interestingly, George et al.25 reported the synthesis of several ethylammonium sulfate ILs resulting in efficient delignification without significant reduction of cellulose crystallinity. In addition, a production cost close to conventional organic solvents was shown. Furthermore, ILs can act as reaction media for lignin dissolution and depolymerization due to the incorporation of acidic functionality or other catalytic properties.29–31
The Organosolv Process
Organosolv lignin originates from treatment of lignocellulose with organic solvents such as ethanol, acetic acid, methanol alone, or mixed with water at 140−220 °C.32 This delignification method is known to be more environmentally friendly compared to Kraft or sulphite lignin, especially when performed without added acid. The most well-known example is the Alcell process where a mixture of EtOH/water (1:1) is employed as the cooking medium at 175−195 °C for 1 h, enabling the dissolution of lignin and producing furfural as a byproduct.33 Several variations have been reported, mainly in order to improve the yield of lignin, involving different solvent mixtures (including glycerol,33 THF,34,35 MeTHF,36 and GVL37) and several catalysts (oxalic acid,36 HCl,38 Lewis acids,39 metal chlorides,40 and ammonia41), indicating varying efficiencies. Typically, cleavage of the β-O-4 linkages occurs to a lesser extent compared to technical lignins, leading to a more nativelike structure, and the recovered materials are characterized by molecular weights (Mn) typically between 500 and 5000 g/mol42 and good solubility in polar organic solvents.43 However, organosolv processing does lead to partial degradation of the native structure and some decrease in the fraction of β-O-4 linkages, the extent of which depends on the plant source and specific reaction parameters used.44 The review of Rinaldi et al.45 discusses several aspects of the influence of pulping conditions on the structure of resulting lignins in great detail. For instance, the gradual structural changes observable by HSQC NMR spectroscopy in acetolysis lignin upon low-, mild- and high severity conditions are described. It was concluded that mild processing conditions (110 °C, no H2SO4, 15 min) produce native-like lignin structure, while more severe conditions (160 °C, 0.6% H2SO4, 45 min) result in completely modified lignin
structure, in which, besides the −OCH3 and general aromatic signals, no beta-ethers are left. Another excellent way to provide a more quantitative description of the extent of structural modifications is to compare lignins originating from the same lignocellulose source but via a different treatment method by subjecting them to the same catalytic treatment conditions. The yield of aromatic monomers thus obtained would correlate with the content of cleavable β-O-4 linkages (assuming that the catalytic methodology targets these linkages). The influence of processing conditions on the aromatic monomer yields obtained via acidolysis in conjunction with stabilization of reactive intermediates with ethylene glycol to produce C2-acetals (see also section 1.3) was also studied by Deuss, Barta, and coworkers.42 It was confirmed that the highest monomer yields were obtained from lignins that were obtained by mild organosolv methods, being the ones with highest β-O-4 linkage retention. The addition of formaldehyde46 or other aldehydes47 during organosolv processing was reported by Luterbacher and co-workers is an excellent approach to avoid repolymerization and promote high-selectivity monomer production. In fact, an acetal-stabilized lignin structure is obtained, maintaining the original native-like high β-O-4 content (Figure 1.3). Similarly, Deuss et al. could access elevated β-O-4 containing lignins stabilized in a α-ether form using alcohol-rich solvent extraction mixtures44,48,49 (Figure 1.3).
Thus far, specific methods of general applicability have not been developed, and the groups working on the development of novel catalytic methods typically reported on specific organosolv procedures prior to catalytic treatment (Figure 1.2). Indeed, structural modification during pretreatment is one of the greatest challenges in catalytic lignin valorization since the β-O-4 linkages are required for efficient depolymerization with the catalytic methods currently available. Therefore, the formation of more robust C−C bonds causes low efficiency of depolymerization and decreases monomer yields.
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Figure 1.3. Acetal, alcohol stabilized and condensed lignin structures.
1.3. Lignin valorization to aromatic monomers
Lignin depolymerization is an intriguing task that is challenged by the structural complexity and recalcitrance of this aromatic biopolymer. Any sustainable methodology aiming for lignin depolymerization should deliver specific (preferably) aromatic compounds in high enough yield and selectivity to allow separation and subsequent valorization to well-defined products. Several strategies have been developed. Traditionally, thermochemical methods such as gasification or pyrolysis were employed to break lignin down to lower molecular weight species which are widely reviewed elsewhere.50–52 Even though these methodologies are relatively simple in practice, they often produce a wide variety of products and are not selective to specific aromatic monomers.
As mentioned in section 1.2, the cleavage of ether bonds (typically β-O-4 moiety) have been identified as the key to high aromatic monomers yield and selectivity. Interestingly, it is possible to calculate a maximum theoretical monomer yield based on the assumption that lignin is an infinite linear polymer in which C9 monomers are randomly connected via C-O and C-C bonds via eq.1 (one monolignol has to be surrounded by two ether linkages to produce one monomer after ether bond cleavage).45,53–56
Here we provide an overview of catalytic methodologies which are tuned to cleave specific bonds in the lignin structure to produce aromatic monomers in high yield/selectivity (Figure 1.4). Emphasis will be put on acid-catalyzed depolymerization which is the focus of this thesis.
Figure 1.4. A summary of lignin catalytic depolymerization strategies and commonly obtained monomers yield: every methodology with references is explained in detail in the following paragraphs.
Oxidative depolymerization
Oxidative strategies mainly produce vanillin, which has historically been used as flavoring agent in food, beverages and pharmaceuticals57 as well as for polymers
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production.58 The vanillin production process is one of the oldest in the field.51,59 Nowadays, 85% of vanillin originates from non-renewable resources via synthetic routes, especially using guaiacol as starting material, while only 15% of vanillin is produced from lignin.57 More exactly, the Norwegian company Borregaard uses the sulfite pulping of wood and process the lignosulfonate-rich sulfite liquor obtained as by-product to finally obtain vanillin in about 15% yield60.
Even though lignin from the sulfite pulping process is the only one industrially employed to produce vanillin so far,57 more and more work has been done to efficiently depolymerize all kind of lignins embracing the biorefinery concept. Usually, the process involves the use of an aqueous solution of lignin treated in alkaline , acid or pH-neutral conditions in the presence of oxidants such as oxygen, hydrogen peroxide, nitrobenzene or peroxyacids resulting in aromatic aldehydes, acids, ketones, or benzoquinones.61,62 Interesting systems have been developed for lignin depolymerization involving La-based catalysts (commercial organosolv63 and non-commercial lignin64), ionic liquids (organosolv commercial lignins)65,66, tetrabutylammonium hydroxide 30-hydrate (Japanese cedar non-commercial lignin)67, diluted inorganic acid (Kraft lignin)68–70, peracetic acid/Nb2O5 (diluted acid corn stover lignin and steam explosion spruce lignin)71, reaching combined aromatic monomers yield of 5-50% being the main products vanillin and syringaldehyde. In some cases, the aromatic ring can be broken leading to the formation of non-phenolic carboxylic acids such as oxalic acid, succinic acid, formic and acetic acid.72–76 In order to achieve that, the instability of the first oxidation phenolic aromatics products is exploited to fully convert them and total carboxylic acid yields are reported to be up to 56%.77
Overall, oxidative pathways have the possibility of using conditions, which are already widely used in the pulping industry62,78 and that products such as vanillin are already existing in the market. Additionally, the functionality that these molecules carry (aldehydes, acids) offer wide opportunity for further functionalization. However, new processes should be developed to allow safe and sustainable lignin conversion to specific chemicals with sufficient selectivity. In fact, limiting the over-oxidation is challenging and processes involving radicals may lead to poor selectivity due to uncontrolled repolymerization causing severe purification issues.79
Reductive depolymerization
Reductive depolymerization has been widely studied since is the methods that usually delivers the highest aromatic monomers yield (up to 80%). It can be performed at
different temperatures (130-450 °C), solvents (mainly alcohols or ethers) and in the presence of hydrogen or various hydrogen-donors, together with a metal-catalyst. The usually obtained products are substituted methoxyphenols (common substituents are propyl, ethyl, methyl, and propanol) when the temperature is below 300 °C. However, harsher conditions (T> 300 °C) can lead to the hydrogenation of the aromatic ring resulting in cycloalkanes mixtures.
Among the broad variety of available examples, some reported particularly high monomers yield and/or selectivity. For example, Luterbacher et al.46 reported Ru/C-catalyzed hydrogenolysis (THF, 250 °C, 40 bar H2) of formaldehyde-stabilized lignins from spruce, beech and high-syringyl transgenic poplar (F5H-poplar) with 21, 47 and 78% total monomers yield respectively. Importantly, when the lignin was extracted with no formaldehyde stabilization, the monomers yield dropped to 7% for beech-lignin and 24% for F5H-poplar lignin, proving the importance of the stabilization during lignin isolation. Even though the yield from F5H-poplar lignin was extremely high, the monomers selectivity was not excellent (22% to 4-propylsyringol, 29% to 3-methyl-4-propylsyringol, 16% to syringol and 15% to 4-(3-hydroxypropyl)-3-methyl syringol). In a follow-up work47, the same group applied Pd/C as catalyst for the reductive depolymerization of F5H-poplar lignin to enhance the selectivity to 4-(3-hydroxypropyl)-guaiacol and 4-(3-hydroxypropyl)-syringol, reaching 70% monomer yield with 80% selectivity to hydroxypropyl)-syringol and 9% to 4-(3-hydroxypropyl)-guaiacol. Anastas et al.80 used Cu-PMO (MeOH, 140 °C, 40 bar H2) to depolymerize candlenut lignin obtaining 64% monomers yield (43% selectivity to 4-(3-hydroxypropyl)benzene-1,2-diol) – here the high yield clearly resulted from the structure of the starting material. Similarly, catechyl lignin (C-lignin, benzodioxane homopolymer without condensed units) was subjected to hydrogenolysis (Pd/C) by Luterbacher and Ralph81 reaching up to 90% selectivity in catechylpropanol monomer. Westwood and Bugg82 reported an overall monomer yield of 54% from oak lignin when treating it with Pt/Al2O3 (MeOH/water 50/50, 300 °C, 20 bar H2) even though the highest selectivity towards a single products was 17% to 3-methylguaiacol. An interesting option was provided by Cantat et al.83: formacell lignins were tested at room temperature with an excess of Et3SiH (CH2Cl2, B(C6F5)3 as a Lewis acid catalyst) and yield of 11-41 wt% was reported from hardwood isolated lignin, being the major products the silylated version of 4-(3-hydroxypropyl)-2,6-syringol and 4-(3-hydroxypropyl)-2-guaiacol. More recently, Gu et al.84 reported the use of the nonprecious metal catalyst NiCu/C to convert organosolv poplar lignin to monophenols in ethanol/isopropanol solvent mixture with a maximum yield of63.4% (61% selectivity to 4-propylsyringol/4-propylguaiacol) at 270
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°C for 4 hours without external H2. Ralph and Dumesic85 performed a kinetic and mechanistic study on lignin hydrogenolysis to monomers in a continuous flow reactor reaching 29% monomers yield (main products: hydroxypropyl)-syringol and 4-(3-hydroxypropyl)-guaiacol) with Pd/C, near-theoretical yields for this specific lignin. Li and Wang86 investigated poplar lignin hydrogenolysis over a series of Nbm−Nin/ZnO−Al2O3 catalysts in methanol solvent showing that Nb2−Ni1/ZnO−Al2O3 as the most promising for lignin depolymerization, with an 87.1 wt% bio-oil yield and 22.4 wt% phenolic monomer yield.
Depolymerization via pre-oxidized lignins
Important strategies have been developed involving first a selective lignin oxidation step in the α-position, followed by a reductive C−O ether bond rupture. In fact, this allows to decrease the bond dissociation energy of the C−O bond and therefore makes the β-O-4 linkage more labile. More importantly, oxidation of the benzylic position can also be viewed as a ‘stabilization strategy’. The first example was reported by Stahl et al.87 where the secondary alcohol in the β-O-4 linkage was oxidized to ketone using a TEMPO mediated procedure and the following cleavage was performed using sodium formate in aqueous formic acid at 110 °C reaching about 60% aromatic monomers yield. Westwood and co-workers88 reported a similar approach using 2,3-dichloro-5,6-dicyano- 1,4-benzoquinone (DDQ) and tert-butyl nitrite (tBuONO) in the oxidation step followed by cleavage with Zn/NH4Cl at 80 °C. Interestingly, an α-oxidized lignin was electrochemically oxidized and depolymerized via photocatalysis in mild conditions by Stephenson et al.89 Overall, this approach allows to use milder conditions in the reductive depolymerization step compared to the previous discussed examples. Also, the delivered aromatic monomers are still ketones which can be easier to functionalize compared to the usual relatively unreactive monomers obtained via reductive depolymerization. However, an extra step is added which can be energy and time consuming.
Biochemical depolymerization
In nature, lignin is broken down via oxidative enzymes present in fungi and bacteria.90During cleavage, aromatic radicals are produced resulting in a huge range of aromatics which are subsequently utilized to generate few key intermediates such as catechol or protocatechuate.91 Chemical lignin depolymerization method usually delivers a variety of products which can be hard to separate. Therefore, getting inspiration from the natural pathways, which funnel aromatic mixtures to selected chemicals, opened the
opportunity to a biological lignin valorization. Both bacteria92 and fungi93 have been tested as potential candidates for converting lignin-derived aromatics to specific compounds such as cis,cis-muconate or vanillin.50,92 In this perspective, pioneering work on ‘biological funneling’ of complex lignin-derived mixtures was reported by Beckham and co-workers 94,95 to obtain cis,cis-muconate and polyhydroxyalkanoates using Pseudomonas putida KT2440. In the context of biochemical depolymerization, major challenges are found in how to match lignin solubilization strategies for biological conversion with an ad-hoc designed biocatalyst optimized for both the feed stream and targeted final product.96,97
Base-catalyzed depolymerization
Base-catalyzed lignin depolymerization is performed in harsh conditions (240– 340 °C)4,98 in the presence of a homogeneous (mostly NaOH)99–106 or heterogeneous base107– 110 as catalyst and water or aqueous organic solvents. The predominant products at temperatures below 300°C are methoxyphenols, while at higher temperature the selectivity shifts mainly to catechol and alkylcatechols. The monomer yield does not exceed 20%, usually due to char formation.4,98In these procedures, the solvent was found to have a major impact on the product structure and yield.109,110
Acid-catalyzed depolymerization
Lignin acidolysis has been investigated since early 1900. Initially, studies were mainly focused on elucidating the lignin structure rather than obtaining high aromatic monomers yield. In fact, this method targets the cleavage of ether bonds, in particular the β-O-4 moiety. Importantly, research conducted by Hibbert111–113 and Lundquist114– 121 elucidated the presence of two reaction pathways (namely C2 and C3, Figure 1.5) derived from the β-O-4 bond acidolysis, as well as the formation of dimeric species. The C2 pathway delivers C2-aldehydes (4, Figure 1.5), while the C3 yields a group of compounds known as Hibbert’s ketones (5, Figure 1.5). It also appeared clear that condensation reactions could occur (3, Figure 1.5) and that the monomers could undergo repolymerization reactions resulting in high molecular weight products. Later on, Yokoyama and Matsumoto conducted extensive model compound studies122–127 on the kinetic aspects of the β-O-4 cleavage using 0.2 mol·L-1 HBr in 82% aqueous 1,4-dioxane at 85 °C. Interestingly, when comparing with other acids (HCl, H2SO4),123 it was found that the C2 pathway was more favored in the case of H2SO4 while the C3 one was preferred when HBr or HCl were employed. It was suggested that Cl− being smaller, it can better abstract the β-proton from the carbocation intermediate leading mainly to
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C3 route (Figure 1.5, purple). On the other hand, HSO4- is sterically large and hence preferentially abstracts the proton from the γ-hydroxyl group of the carbocation intermediate, channeling to C2 route (Figure 1.5, green). However, they claim that the solvent can also perform the proton abstraction and so the size of the counter anion should not matter.
Figure 1.5. β-O-4 linkages acidolysis: C2 and C3 reaction pathways.
The achievement of high aromatic monomers yield from lignin hence depends on minimizing lignin condensation reaction as well as monomers repolymerization. In this context, Barta and de Vries128 established that triflic acid was able to cleave simple β-O-4 model compounds and that the derived C2-aldehyde (β-O-4, Figure 1.5) was very prone to recondensation reactions leading to high molecular weight side products. Thus, stabilization strategies for the C2-aldehyde were developed via C2-acetals formation with ethylene glycol (namely dioxolan-2-yl)methyl)-2-methoxyphenol and 4-((1,3-dioxolan-2-yl)methyl)-2,6-dimethoxyphenol, G-C2-acetal and S-C2-acetal respectively), hydrogenation or decarbonylation (Figure 1.6). In particular, the formation of C2-acetals with ethylene glycol was found to be extremely efficient giving a 3-fold increase in monomer yields relative to the control experiments. Aiming to gain a deeper and more realistic understanding of the lignin system, Barta and Westwood129 investigated the use of (β-O-4)-(β-5) dilinkage models in acidolysis conditions in conjunction with stabilization via acetals formation elucidating that ethylene glycol also plays a role in trapping the intermediates from β-5 units. Then, the use of metal triflates
was investigated to replace triflic acid where Fe(OTf)3 and Bi(OTf)3 were shown to be as efficient in cleaving the β-O-4 moiety.130 The direct lignin depolymerization was performed on 28 different lignins using Fe(OTf)3 as catalyst reaching a maximum of 35% combined acetals yield from walnut lignin.42 A very relevant founding was that the yield of targeted C2-acetals correlated with the β-O-4 content in the starting lignin, highlighting the importance of the isolation procedure. This method was also applied by Barta and Westwood44 to butanol or ethanol extracted yielding 18% combined C2-acetals yield.
Figure 1.6. C2-aldehyde stabilization via acetal formation, hydrogenation and decarbonylation.128
Focusing on the C3 pathway instead, Westwood131 reported the synthesis of Hibbert’s ketones proving that these structures were present in dioxasolv lignins in different amount depending on the acid concentration used during isolation (HCl). Additionally, they demonstrated the presence of Hibbert’s ketones ethylene glycol derivatives when lignin was used in acidolysis conditions (catalyst: Sc(OTf)3). However, the C2-acetal was still the main product. Next, the same group reported a model study were the acidolysis selectivity was switched from C2 to C3 pathway protecting the OH groups as ester in α and γ position.132 The methodology was applied to dioxasolv Douglas fir lignin obtaining less than 1% monomer yield.
Acidolysis in combination with in-situ decarbonylation was performed by Bruijnincx et al.133 A water-tolerant Lewis acids was used to promote depolymerization and a homogeneous Rh complex to enable decarbonylation of the C2-aldehyde formed in the first step. The method was established using model compounds and subsequently applied to dioxasolv lignin, isolated from brewer’s spent grain. Poplar dioxasolv lignin was successfully depolymerized using Yb(OTf)3 to obtain an overall monomer yield of 12.4% at 175 °C in dioxane/H2O and the major products were 4-(1-propenyl)- phenols.
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1.4. Lignocellulose valorization via “lignin-first” strategies
As discussed in section 1.2, lignin isolation can lead to structural modifications in different extent depending on the method used. Therefore, the success of subsequent depolymerization depends on the outcome of the isolation step, which is one of the reasons why comparing lignin depolymerization methods is not straightforward.
To overcome this issue, the so-called “lignin-first” strategy has been explored and considered very promising.134 Such approach involves the solubilization and depolymerization of lignin in a one-step process and aims to a full valorization of the lignocellulose material. In fact, the process conditions would be tuned so that the removal of lignin is immediately followed by catalytic conversion to aromatic monomers, resulting in high aromatic monomers yield thanks to the extensive presence of cleavable C-O linkages and not C-C linkages. Additionally, the sugar fraction of lignocellulose should remain intact to be further valorized.7,135 In this field, reductive catalytic fractionation (RCF) is the dominant methodology.4,45,136,137 RCF involves the extraction of lignin reactive intermediates (coniferyl alcohol, sinapyl alcohol and oligomers), which are immediately stabilized via catalytic hydrogenation/hydrogenolysis over a heterogeneous metal-catalyst in the presence of hydrogen or a hydrogen donor.138 The typical obtained monomers are alkyl-methoxyphenols or propanol-methoxyphenols depending on conditions and catalyst used. At the end of the process, a lignin oil is obtained together with a solid fraction composed by (hemi)cellulosic pulp and solid catalyst (Figure 1.7).
Figure 1.7. A schematic representation of reductive catalytic fractionation (RCF): dark red hexagons represent native lignin monomer units, orange hexagons correspond to reactive units, and blue hexagons represent stabilized units. Inspired by ref [136].
This strategy dates back to 1940s, when Hibbert139,140 and later on Pepper141 (1960) were performing structural analysis of native lignin. About 50 years later, this approach regained attention in the biorefinery context since it was delivering nearly theoretical monomers yield. Since then, several catalytic systems have been developed and elegant solutions were found demonstrating high aromatic monomers yield (up to 50%) and high sugars retention (up to 90%). Usually, the process system involves the use of a solvent (mainly alcohols) in combination or not with water and a heterogeneous metal catalyst.79,142The solvent has the role of solubilizing and partially depolymerize lignin to unsaturated reactive intermediates which are hydrogenated further by the metal catalyst resulting in a stable lignin oil composed by monomers, dimers and oligomers.136 One of the issues faced by this methodology is the catalyst recycling/separation and so the cellulose valorization. In this regard, elegant solutions have been developed in order to have an easy catalyst recovery including the use of magnetic catalysts143,144, liquid-liquid extraction145,146, or a microporous catalyst cage.147,148
Remarkably, some groups focused on a full lignocellulose valorization, including the sugar fraction. Using a microporous catalyst cage, Sels et al.147 were able to easily recover the carbohydrates fraction separately from the catalyst (Ni/Al2O3) and perform a semi-simultaneous saccharification-fermentation resulting in bioethanol (73% compared to theoretical yield) after obtaining about 40% yield in aromatic monomers from birch. Using a different approach, Barta et al.149 did not separate Cu-PMO from the cellulose after RCF treatment (36% aromatic monomers yield from poplar, 10% from pinewood) converted it directly to a mixture of aliphatic alcohols which were turned into fuel-range alkanes. A one-pot lignocellulose conversion to gasoline alkanes and monophenols was reported by Ma and coworkers150 treating several biomass sources with Ru/C+LiTaMoO
6 catalysts with phosphoric acid obtaining up to 82% yield (based on cellulose and hemicellulose) while the lignin fraction was converted to monophenols. Gasoline and kerosene/diesel drop in fuels production was studied by Rinaldi et al.151 treating the lignin oil after RCF treatment (Raney nickel) with Ni2P/SiO2 catalyst. Remarkably, the delignified holocellulose was used as hydrogen source making the process hydrogen self-sufficient. Wang et al.152 performed the one-pot catalytic conversion of cornstalk into liquid alkylcyclohexanes (from lignin fraction) and polyols (from cellulose and hemicellulose) over Ru/C reaching 97% alkylcyclohexanes yield (based on lignin monomers in cornstalk) and 53% polyols. The same group developed another methodology153 where Pd/C is used for birch RCF treatment obtaining 83% lignin monomers yield which are then hydrodeoxygenated to arenes over Ru/Nb2O5 catalyst with 86% selectivity to arenes based on lignin oil. The sugar fraction located in the Pd/C
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containing solid residue was then converted to HMF and furfural (24% combined mass yield) in a THF/concentrated seawater system.
Song et al.148 reached 50% aromatic monomers yield using Pd/C catalyst and valorized the residual cellulose by FeCl3 treatment to levulinic acid (56% yield) and furfural (74% yield) simultaneously.
The quality of the pulp was proven by Kim and coworkers via enzymatic hydrolysis to glucose (about 70% glucose yield)149 after obtaining 24% lignin derived monomers yield via Nix–Al/AC catalyzed RCF treatment. Samec et al.154 used Pd/C in a hydrogen-free system to produce up to 36% aromatics yield together with about 90% glucose yield after pulp enzymatic hydrolysis. Interestingly, the yield in aromatic monomers was found to grow with the β-O-4 content. The same cellulose valorization method was used by Abu-Omar et al.155 with 95% glucose yield after a Zn/Pd/C catalyst for poplar RCF treatment reaching 54% aromatic monomers yield. On the same line, Román-Leshkov et al.156 investigated RCF of corn stover with supported Ni and Ru catalysts (co-catalyst: homogeneous or heterogeneous acid) obtaining up to 38% aromatic monomers and >90% glucose yield.
Interestingly, semi-continuous processes were also developed157,158 decoupling the lignin solvolysis and the depolymerization steps. This approach opens the possibility of using different conditions in the two steps giving the opportunity to perfectly tune both. Additionally, the catalyst separation problem is inherently solved, and the pulp can be readily used. Monitoring the system is also easy, and an in-depth investigation of structural differences of lignin released at different times can be performed. As a drawback, these systems are characterized by a higher solvent consumption compared to the batch. Also, the fact that the system is intrinsically not fully continuous can limit the applicability.136
Defunctionalization approaches in order to access high-volume bulk chemicals have been a constant focus of the field, and method development regarding deoxygenation/C-C bond scission is considered a highly active research area.159 These methods are important for the conversion of mixtures originating from the “lignin first” biorefinery approach. Since the latter usually results in mixtures of G, S and H aromatics, with frequent variations also encountered on the aliphatic chain (propyl, ethyl, propanol…), these may directly undergo “catalytic funneling” to simpler important aromatics such as phenol or BTX, as investigated by several groups.160 The “catalytic funneling” approach usually relies on hydrodeoxygenation procedures (HDO) aiming to narrow down and homogenize monomers mixtures from depolymerization strategies, resulting
consequently in higher yield of desired product and thus making the method more attractive for actual industrial application.
A particular recent example has been reported by Sels and Liao161 on the conversion of as high as 78% of birch lignocellulose into xylochemicals. Importantly, the work aims at producing a variety of important chemicals, with in-depth techno-economic analysis supporting the feasibility of the proposed lignin-first biorefinery. The reported approach relies on a RCF step with Ru/C resulting in aromatic monomers, which were further catalytically funneled into phenol (20% yield on original lignin) and propylene (7% yield on original lignin) via gas-phase hydroprocessing and dealkylation. The cellulose pulp was amenable for bioethanol production and the residual phenolic oligomers (30 wt%) were used in printing ink. The techno-economic analysis revealed that feedstock and product pricing had the largest economic effect, while the catalyst price was negligible as far as it was recyclable. Phenol was also produced from pinewood by Hensen and coworkers.162 In this case, pinewood was first treated with Pt/C to obtain aromatic monomers, which were subsequently transformed into phenol via MoP/SiO2-catalyzed demethoxylation and zeolite-catalyzed transalkylation in 9.6 mol% yield based on initial lignin content. An important early example was shown by Yan and co-workers163 who proposed a full reaction sequence from RCF derived phenol to terephtalic acid. Specifically, this methodology involved lignin-oil demethoxylation to 4-alkylphenols, carbonylation to 4-alkylbenzoic acids and oxidation to terephthalic acid (15.5 wt% yield to lignin content in corn stover). Birch derived lignin oil was employed by Wang et al.160 to produce indane and derivatives through an intramolecular cyclization reaction followed by a HDO procedure (about 20 % from birch lignin oil).
Some other examples of “lignin-first” strategies not involving any reductive environment have also been reported (Figure 1.7). In fact, even though the aromatic monomers yield/selectivity are high and the holocellulose can be valorized, RCF utilizes metal catalysts (often expensive), harsh conditions and hydrogen, which all represent limitations for further development of this strategy. Additionally, the obtained monomers are often characterized by not functionalized alkyl chains, which can make a further upgrading problematic.
Several metal-free approaches have also been developed (Figure 1.8). As one of the first, Watanabe et al.164 studied direct acid-catalyzed depolymerization of Japanese cedar and Eucalyptus globulus in a system composed by toluene as a solvent, methanol as a trapping agent, and H2SO4 as a catalyst to produce non-cyclic G-C2-acetal (approximately 5% and 10% yield respectively from J. cedar and E.globulus). Later, Corma and Samec165 reported zeolite-mediated birch wood fractionation of
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lignocellulose obtaining a range of 15 aromatic monomers in a total yield up to 20% (from lignin), furfural/ethylfurfural in 52% yield (from hemicellulose) and ethyl-levulinate, levulinic acid, and the ethyl ether of 5-HMF in 21% yield (from cellulose). Bruijnincx and coworkers133 explored Rh-catalyzed decarbonylation in combination with Lewis-acid catalyzed depolymerization of sawdust reaching up to 10% aromatic monomers yield.
Ma and Sels166 employed alkaline oxidative methodology (NaOH, O2), to obtain vanillin (21% yield) and valorize cellulose as formic acid (up to 15% yield) and levulinic acid (up to 51% yield). An oxidative approach (NaOH, copper (II) sulfatepentahydrate, O2) was also adopted by Djakovitch et al.167 to produce vanillin (up to 19% yield) together with 65% of reducing sugars from cellulose after enzymatic hydrolysis. Minami and Saka168 studied the solvolysis of beech wood in water-added supercritical methanol (270 °C) reaching 45% total aromatic monomers yield with 62% selectivity to sinapyl alcohol γ-methyl ether. Additionally, about 15% γ-methyl-glucoside yield was obtained.
As mentioned earlier, metal and hydrogen-free lignin-first strategies can be advantageous since the characteristic limitations of RCF methods can be circumvented (expensive catalyst, use of hydrogen, harsh conditions). In this thesis, Chapter 2 and 5 will show a metal-free lignin first strategy developed relying on lignin acidolysis.
1.5. Overview on lignin-derived monomers applications
As described in section 1.3 and 1.4, various different monomers can be derived from lignin depending on the depolymerization method used. In contrast to the field of carbohydrates where several platform chemicals were defined earlier, that is not the case for lignin. A series of ‘Top value-added’ platform chemicals obtained from carbohydrates was already listed in 2004169 and then updated in 2010.170Additionally, the production of bio-based ethanol, furfural or levulinic acid is an industrial reality.135 Except for some “traditional” products, such as vanillin, a gap exists between obtaining well-defined products from lignin and their direct use.171Thus, effort was made in two directions: on the one hand, defunctionalization strategies were applied to produce bulk chemicals and fuels172 such as phenol161,162 or BTX. This approach has the advantage of providing existing chemicals, even though the production cost should be somehow competitive with traditional routes. On the other hand, the functionalization of the monomers through C-C or C-N bond formation to obtain fine chemicals (such as indane160 and aromatic amines173,174), polymer building blocks and materials was investigated.95,175–182 Examples of the possible lignin-derived monomers applications are reported in Figure 1.9.
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Figure 1.9. Examples of lignin-derived monomers applications.
A substantial challenge is the purification of the lignin derived monomers obtained after depolymerization.171 Several approaches have been investigated including liquid-liquid extraction183,184, distillation185, membrane separation186, or column chromatography149. Since distillation and membrane separation are established industrial separation methodologies, they are considered more relevant for industrial application, while column chromatography and liquid-liquid extraction are more convenient for laboratory scale, especially if a monomer of analytical purity is required in the case of chromatography.171
As mentioned earlier, attempts have been made to use lignin depolymerization mixtures directly with no purification by ‘catalytic funneling’ approaches, mainly when adopting
defunctionalization strategies (section 1.4, Sels and Liao161, Hensen et al.162 , Rinaldi et al.151 , Yan et al.163 and Wang et al.160) .
Purified aromatic monomers are typically used in the polymer synthesis field.187 However, some examples of applying the lignin depolymerization oil directly to produce polymers are also reported. For instance, Watanabe et al.188 employed low-molecular-mass lignin containing a C2-acetal structure (from acidolysis procedure) to obtain epoxy resins with controlled glass-transition temperature. Epoxy resins were synthetized by van de Pas and Torr189 as well from pinewood lignin oil derived after mild hydrogenolysis procedure.
1.6. Thesis outline
Finding sustainable methodologies to valorize all lignocellulose components became of outmost importance in the context of biorefinery development. This thesis explores the development of a “lignin-first” strategy based on acidolysis with ethylene glycol stabilization to produce specific aromatic monomers (C2-acetals). The aim of this work is also to demonstrate the value of these products with possible applications. Additionally, the different reactivity of G- and S- units in lignin was investigated through model compounds studies. Mechanistic insights were provided thanks to a very fruitful collaboration with Susanna Monti and Giovanni Barcaro which performed the computational studies in Chapter 6.
Chapter 2 focuses on the development of a “lignin-first” strategy based on acidolysis with ethylene glycol stabilization to produce specific aromatic monomers (G-C2-acetal). This project is based on previous lignin acidolysis strategy developed in the group and aims to translate the method directly to lignocellulose, avoiding the lignin isolation step. After optimization, a system of dimethyl carbonate as solvent, ethylene glycol as stabilization agent and H2SO4 as catalyst was found to be working in mild conditions (140 °C, 40 minutes reaction). Softwood (pine, spruce, cedar, Douglas fir) was successfully processed reaching a maximum of 8.8 wt% G-C2-acetal yield from pinewood (98% compared to theoretical maximum determined via Derivatization Followed by Reductive Cleavage).
As a follow up of Chapter 2, Chapter 3 aims to provide a deep characterization of the pinewood streams after processing, together with lignocellulose mass balance evaluation. Lignin-derived dimers structures were suggested and EG and MeOH modified sugars dissolved in the liquor were identified. The leftover (hemi)cellulose was treated via
1
enzymatic hydrolysis and 85% glucose yield was reached, showing that the cellulose quality is preserved. 56% of the initial lignocellulose was valorized.
In Chapter 4, the conversion of G-C2-acetal to homovanillyl alcohol was performed, which is an important intermediate in medicinal chemistry. After screening solvents, heterogenous catalysts and temperature a system using Ru/Al2O3 in water at 90 °C was found to provide 92% homovanillyl alcohol isolated yield. This monomer was then used in a follow-up work by Dr. Anastasiia Afanasenko to synthetize a library of tetrahydroisoquinolines, quinazolin-4(3H)-ones, and 3-arylindones to test their biological activity.
Chapter 5 concentrates on applying the catalytic strategy developed for softwood in Chapter 2 to hardwood, which was expected to deliver higher monomers yield (about 30-50%) thanks to a higher β-O-4 content. However, the hardwood reactivity was found different than softwood, since syringyl Hibbert’s ketone derivatives were found in similar yield as S-C2-acetal, which was not the case for softwood. Furthermore, the combined monomers yield was somewhat lower than expected (15-30 wt%). Thus, we synthetized a library of 10 model compounds (phenolic and not-phenolic) representing G- and S- units to study the reactivity in our system. Interestingly, we found that phenolic S-model compounds are more prone to uncontrolled reactions compared to their G- equivalents. Additionally, a strong dependence on acid concentration was observed, which was different between S- and G- units. These studies suggested that the two units would likely behave differently under processing conditions and thus fine-tuning the conditions will be necessary to target one specific unit.
Chapter 6 is the result of a productive collaboration with Susanna Monti, PhD and Giovanni Barcaro, PhD (CNR-Pisa). Here, experimental results and theoretical calculations were combined to gain mechanistic insight in our acidolysis system (dimethyl carbonate as solvent, ethylene glycol as stabilization agent and sulfuric acid as catalyst). The unique role of sulfuric acid as proton acceptor-donor was elucidated, together with reaction pathways for ethylene glycol stabilization and different reactivity of compounds with different methoxy substituents. An alternative pathway leading to acetal formation, different from the classical aldehyde stabilization, was suggested.
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