Base-catalyzed Depolymerization Research Articles

Base-Catalyzed Depolymerization of Organosolv Lignin into Monoaromatic Phenolic Compounds

Base-Catalyzed Depolymerization of Organosolv Lignin into Monoaromatic Phenolic Compounds

Hexaniobate as a Recyclable Solid Base Catalyst to Activate C–H Bonds in Lignin Linkage Boosting the Production of Aromatic Monomers

The targeting cleavage of lignin into value-added aromatic monomers has attracted increasing attention. Although the base-catalyzed depolymerization of lignin has been developed, the use of excess corrosive bases and their poor recyclability limit their industrial implementation. Herein, a catalytic amount of solid base K7HNb6O19 (KNb6) coupled with copper-modified graphitic carbon nitride (Cu/C3N4) shows enhanced catalytic performance for the oxidative cleavage of β-O-4 lignin linkages using molecular oxygen. Due to the synergetic effect between KNb6 and Cu/C3N4, 96% of the β-O-4 ketone model was converted under relatively mild conditions, and phenol (yield: 90%) and other aromatic monomers (yield: 90%) were obtained. Moreover, KNb6–Cu/C3N4 is robust, and its catalytic activity is basically maintained after five cycles. Experimental and theoretical studies (spectroscopic analysis, control experiments, kinetic study, density functional theory calculations, etc.) reveal that the surface basic O atoms of KNb6 can simultaneously activate the Cβ–H bond of β-O-4 ketone to promote the insertion of molecular oxygen, and subsequently, Cu/C3N4 catalyzes the selective cleavage of the Cα–Cβ bond. The catalysts were also active for the oxidative cleavages of other lignin models and oxidized organosolv lignin.

New insights into the base catalyzed depolymerization of technical lignins: a systematic comparison.

A first systematic approach on the base catalyzed depolymerization (BCD) of five technical lignins derived from various botanical origins (herbaceous, hardwood and softwood) and covering the main three industrial pulping methods (soda, kraft and organosolv) is reported. This study provides a first of its kind in-depth quantification and structural characterization of two main BCD fractions namely lignin oil and lignin residue, describing the influence of the BCD process conditions. Depolymerization is evaluated in terms of lignin conversion, lignin oil yield, phenolic monomer selectivity and the production of lignin residue and char. Lignin oils were extensively characterized by size exclusion chromatography (SEC), GC-MS, GC-FID, 13C-NMR, HSQC NMR and elemental analysis. GC × GC-FID was used to identify and quantify distinct groups of monomeric compounds (methoxy phenols, phenols, dihydroxy-benzenes) in the lignin oil. The lignin oil yields (w/w) ranged from 20-31% with total monomer contents ranging from 48 to 57% w/w. SEC analysis indicated the presence of dimers/oligomers in the lignin oil, which through HSQC NMR analysis were confirmed to contain new, non-native interunit linkages. 13C NMR analyses of the lignin oils suggest the presence of diaryl type linkages (i.e. aryl-aryl, aryl C-O) evidencing deconstruction and recombination of lignin fragments during BCD. Irrespective of the lignin source, a residue, often regarded as 'unreacted' residual lignin was the main product of BCD (43 to 70% w/w). Our study highlights that this residue has different structural properties and should not be considered as unreacted lignin, but rather as an alkali soluble condensed aromatic material. HSQC, DEPT-135, 13C, and 31P NMR and SEC analyses confirm that the BCD residues are indeed more condensed, with increased phenolic hydroxyl content and lower molecular weights compared to all feed lignins. Subsequent BCD of solid residual fractions produced only low oil yields (6-9% w/w) with lower phenolic monomer yields (4% w/w) compared to original lignin, confirming the significantly more recalcitrant structure. Our study improves the overall understanding of the BCD process, highlights important feedstock-dependent outcomes and ultimately contributes to the complete valorization of BCD-derived lignin streams.

Lignin oligomers from mild base-catalyzed depolymerization for potential application in aqueous soy adhesive as phenolic blends

This study has transformed kraft lignin into lignin oligomers using base-catalyzed depolymerization at mild temperatures. The lignin oligomers, used as a phenolic blend, improved the bonding strength of the soy protein adhesive.

The Catabolic System of Acetovanillone and Acetosyringone in Sphingobium sp. Strain SYK-6 Useful for Upgrading Aromatic Compounds Obtained through Chemical Lignin Depolymerization.

Acetovanillone is a major aromatic monomer produced in oxidative/base-catalyzed lignin depolymerization. However, the production of chemical products from acetovanillone has not been explored due to the lack of information on the microbial acetovanillone catabolic system. Here, the acvABCDEF genes were identified as specifically induced genes during the growth of Sphingobium sp. strain SYK-6 cells with acetovanillone and these genes were essential for SYK-6 growth on acetovanillone and acetosyringone (a syringyl-type acetophenone derivative). AcvAB and AcvF produced in Escherichia coli phosphorylated acetovanillone/acetosyringone and dephosphorylated the phosphorylated acetovanillone/acetosyringone, respectively. AcvCDE produced in Sphingobium japonicum UT26S carboxylated the reaction products generated from acetovanillone/acetosyringone by AcvAB and AcvF into vanilloyl acetic acid/3-(4-hydroxy-3,5-dimethoxyphenyl)-3-oxopropanoic acid. To demonstrate the feasibility of producing cis,cis-muconic acid from acetovanillone, a metabolic modification on a mutant of Pseudomonas sp. strain NGC7 that accumulates cis,cis-muconic acid from catechol was performed. The resulting strain expressing vceA and vceB required for converting vanilloyl acetic acid to vanillic acid and aroY encoding protocatechuic acid decarboxylase in addition to acvABCDEF successfully converted 1.2 mM acetovanillone to approximately equimolar cis,cis-muconic acid. Our results are expected to help improve the yield and purity of value-added chemical production from lignin through biological funneling. IMPORTANCE In the alkaline oxidation of lignin, aromatic aldehydes (vanillin, syringaldehyde, and p-hydroxybenzaldehyde), aromatic acids (vanillic acid, syringic acid, and p-hydroxybenzoic acid), and acetophenone-related compounds (acetovanillone, acetosyringone, and 4'-hydroxyacetophenone) are produced as major aromatic monomers. Also, base-catalyzed depolymerization of guaiacyl lignin resulted in vanillin, vanillic acid, guaiacol, and acetovanillone as primary aromatic monomers. To date, microbial catabolic systems of vanillin, vanillic acid, and guaiacol have been well characterized, and the production of value-added chemicals from them has also been explored. However, due to the lack of information on the microbial acetovanillone and acetosyringone catabolic system, chemical production from acetovanillone and acetosyringone has not been achieved. This study elucidated the acetovanillone/acetosyringone catabolic system and demonstrates the potential of using these genes for the production of value-added chemicals from these compounds.

Highly Ordered Supramolecular Assembled Networks Tailored by Bioinspired H-Bonding Confinement for Recyclable Ion-Transport Materials

Highly Ordered Supramolecular Assembled Networks Tailored by Bioinspired H-Bonding Confinement for Recyclable Ion-Transport Materials

Chemobiocatalytic Synthesis of a Low-Molecular-Weight Heparin

Heparin products are widely used clinical anticoagulants essential in the practice of modern medicine. Low-molecular-weight heparins (LMWHs) are currently prepared by the controlled chemical or enzymatic depolymerization of unfractionated heparins (UFHs) that are extracted from animal tissues. In many clinical applications, LMWHs have displaced UFHs and currently comprise over 60% of the heparin market. In the past, our laboratory has made extensive efforts to prepare bioengineered UFHs relying on a chemoenzymatic process to address concerns about animal-sourced UFHs. The current study describes the use of a novel chemoenzymatic process to prepare a chemobiosynthetic LMWH from a low-molecular-weight heparosan. The resulting chemobiocatalytic LMWH matches most of the United States pharmacopeial specifications for enoxaparin, a LMWH prepared through the base-catalyzed depolymerization of animal-derived UFH.

A DFT Mechanistic Study on Base-Catalyzed Cleavage of the β-O-4 Ether Linkage in Lignin: Implications for Selective Lignin Depolymerization.

The detailed mechanism of the base-catalyzed C-C and C-O bond cleavage of a model compound representing the β-O-4 linkage in lignin is elucidated using DFT calculations at the M06/6-31G* level of theory. Two types of this linkage have been studied, a C2 type which contains no γ-carbinol group and a C3 type which contains a γ-carbinol. Cleavage of the C2 substrate is seen to proceed via a 6-membered transition structure involving the cation of the base, the hydroxide ion and the α-carbon adjacent to the ether bond. The reaction with KOH has the lowest activation barrier of 6.1 kcal mol−1 with a calculated rate constant of 2.1 × 108 s−1. Cleavage of the C3 substrate is found to proceed via two pathways: an enol-formation pathway and an epoxide-formation pathway. The first path is the thermodynamically favored pathway which is similar to the pathway for the C2 substrate and is the preferred pathway for the isolation of an enol-containing monomer. The second path is the kinetically favored pathway, which proceeds via an 8-membered transition state involving a hydrogen hopping event, and is the preferred pathway for the isolation of an epoxide-containing monomer. The KOH-catalyzed reaction also has the lowest activation barrier of 10.1 kcal mol−1 along the first path and 3.9 kcal mol−1 along the second path, with calculated rate constants of 2.4 × 105s−1 and 8.6 × 109s−1 respectively. Overall, the results provide clarity on the mechanism for the base-catalyzed depolymerization of lignin to phenolic monomers. The results also suggest both NaOH and KOH to be the preferred catalysts for the cleavage of the β-O-4 linkage in lignin.

Base-catalyzed depolymerization of lignin into phenols: methoxy groups’ secondary reactions triggered phenol regulation and repolymerization

Base-catalyzed depolymerization of lignin into phenols: methoxy groups’ secondary reactions triggered phenol regulation and repolymerization

Two-step separation for enriching phenols and esters from base-catalyzed depolymerization of sorghum stalk

Base-catalyzed depolymerization of sorghum stalk using Na2CO3, NaOH, and CaO was performed to afford methanol-soluble portion, and then extraction together with column chromatography was employed for enriching phenols and esters. The maximum total yield (ca. 53 wt%) of extract solutions 1–5 (ES1–ES5) was acquired at 280 °C with the equal mass of CaO and sorghum stalk. According to gas chromatograph/mass spectrometer analysis, phenols and esters are the predominate products. The phenols obtained from Na2CO3-catalyzed depolymerization of sorghum stalk mainly are methoxylphenols, while those obtained from NaOH and CaO-catalyzed depolymerization are dominated by alkylphenols, suggesting that NaOH or CaO plays an important role in demethoxylation during biomass depolymerization process. Phenols, methyl alkanoates/benzenecarboxylates, and dimethyl alkanedioates were roughly enriched into ES1, ES3, and ES5via sequential extraction with petroleum ether (PE), PE, and ethoxyethane, respectively. By the following column chromatographic separation of ES1 and ES3, alkylphenols and guaiacols were mainly enriched into PE/CS2 (4:3)-eluted fractions with maximum concentration of 92.2% and 92.1%, respectively. Saturated and unsaturated methyl alkanoates were isolated in concentration of near 100% by elution with PE/CS2 (9:1) and PE/CS2 (4:3), respectively. Extraction combined with column chromatography as two-step separation provides a promising approach for enriching phenols and esters from biomass-derived liquids.

From SiO2 to Alkoxysilanes for the Synthesis of Useful Chemicals.

The transformation of silica (SiO2) to useful chemicals is difficult to explore because of the strength of the Si–O bond and thermodynamic stability of the SiO2 structure. The direct formation of alkoxysilanes from SiO2 has been explored as an alternative to the carbothermal reduction (1900 °C) of SiO2 to metallic silicon (Simet) followed by treatment with alcohols. The base-catalyzed depolymerization of SiO2 with diols and monoalcohols afforded cyclic silicon alkoxides and tetraalkoxysilanes, respectively. SiO2 can also be converted to alkoxysilanes in the presence of organic carbonates, such as dimethyl carbonate. Alkoxysilanes can be further converted to useful chemicals, such as carbamates, organic carbonates, and chlorosilanes. An interesting and highly efficient pathway to the direct conversion of SiO2 to alkoxysilanes has been discussed in detail along with the corresponding economic and environmental implications. The thermodynamic and kinetic aspects of SiO2 transformations in the presence of alcohols are also discussed.

Techno‐economic and life‐cycle assessment of a wood chips‐based organosolv biorefinery concept for production of lignin monomers and oligomers by base‐catalyzed depolymerization

AbstractMore wood for technical valorization is to be expected in Europe over the coming years, due to climate change and the bark beetle. To further independence from fossil‐fuel resources a novel approach to the base‐catalyzed depolymerization (BCD) of organosolv lignin into monomers and oligomers, while maximizing yields and minimizing coke formation, was scaled up in a theoretical wood‐based biorefinery with a yearly input of 40 000 t of wood chips. Other process steps were modeled from the literature. A novel work‐up approach was evaluated under laboratory conditions, implemented, and compared with a simulation‐based pervaporation technique. The life‐cycle assessment (LCA) showed that the biorefinery provided a significantly lower global warming potential (GWP) (excluding biogenic carbon) than its fossil counterparts. Moreover, the majority of impacts on the other midpoint categories was also smaller than for the fossil reference. However, after allocating the GWPs, it was evident that subsequent conversion of the C6 fraction to value‐added products is necessary for optimal results. The discounted cash flow analyses for the biorefinery setups (40 to 400 kt year–1) showed that they were not profitable with prices based on fossil references. However, when using higher prices/t from the literature, such as €1615 for monomers, €2000 for oligomers, and €510 for C6 sugars, a positive net present value could be reached at approximately 100 000–120 000 t processed annually. Hence, the conclusion was that a biorefinery utilizing wood not only brings environmental benefits and a reduction in the use of fossil resources but can also be financially beneficial. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd

Three-step conversion of Indulin AT to muconic acid under mild conditions

The conversion of technical lignin into valuable chemicals is important for the complete utilization of lignocellulose. The kraft process dominates the pulp industry, and large amounts of kraft lignin are produced worldwide. However, the complexity of lignin limits its valorization. In this study, a kraft lignin, Indulin AT, was depolymerized under base conditions, and the effects of substrate loading, NaOH concentration, temperature, and hydrogen peroxide on depolymerization were investigated. The production of aromatic monomers is favored by a continuous flow process, even with a very short reaction time (2 min vs. 30 min in a batch reactor). Higher temperature and NaOH concentration led to a higher degree of depolymerization, and thus more guaiacol and vanillin. A guaiacol yield of 5.1% was obtained after depolymerization of 5 wt% of lignin and 2 wt% of NaOH at 200 °C with a residence time of 2 min. When taking the whole operation, including the cost and depolymerization efficiency, into consideration, the concentration, lignin loading of 4 wt% of lignin, NaOH concentration of 2 wt%, and the temperature of 210 °C were chosen as the optimal conditions for the production of a guaiacol-rich fraction. The permeate obtained by ultrafiltration of the depolymerized Indulin AT sample contained a higher amount of guaiacol, which was quantitatively converted to muconic acid using an engineered strain of Pseudomonas putida KT2440. The three-step process in this work involves base-catalyzed depolymerization, ultrafiltration, and biological conversion represent an efficient approach to convert Indulin AT lignin into muconic acid.

Muconic Acid Production Using Engineered Pseudomonas putida KT2440 and a Guaiacol-Rich Fraction Derived from Kraft Lignin

Industrial lignin such as kraft lignin is an abundant feedstock for renewable chemicals and materials. In this study, a process was developed for depolymerization of kraft lignin followed by an upgrading separation step and further bioconversion of the obtained monoaromatic compounds to muconic acid. First, industrial kraft lignin, Indulin AT, was processed into a guaiacol-rich stream using base-catalyzed depolymerization. This stream was subsequently upgraded using liquid–liquid extraction and evaporation to yield a more concentrated and less inhibitory stream, adapted for bioconversion. Finally, guaiacol was quantitatively converted to muconic acid through bioconversion using an engineered Pseudomonas putida strain containing cytochrome P450 and ferredoxin reductase for guaiacol assimilation and deletion of the native catBC genes for muconic acid production. Isomerization of muconic acid in a fermentation medium depending on pH was also studied.

Investigating Lignin-Derived Monomers and Oligomers in Low-Molecular-Weight Fractions Separated from Depolymerized Black Liquor Retentate by Membrane Filtration.

Base-catalyzed depolymerization of black liquor retentate (BLR) from the kraft pulping process, followed by ultrafiltration, has been suggested as a means of obtaining low-molecular-weight (LMW) compounds. The chemical complexity of BLR, which consists of a mixture of softwood and hardwood lignin that has undergone several kinds of treatment, leads to a complex mixture of LMW compounds, making the separation of components for the formation of value-added chemicals more difficult. Identifying the phenolic compounds in the LMW fractions obtained under different depolymerization conditions is essential for the upgrading process. In this study, a state-of-the-art nontargeted analysis method using ultra-high-performance supercritical fluid chromatography coupled to high-resolution multiple-stage tandem mass spectrometry (UHPSFC/HRMSn) combined with a Kendrick mass defect-based classification model was applied to analyze the monomers and oligomers in the LMW fractions separated from BLR samples depolymerized at 170–210 °C. The most common phenolic compound types were dimers, followed by monomers. A second round of depolymerization yielded low amounts of monomers and dimers, while a high number of trimers were formed, thought to be the result of repolymerization.

Formation of p-Unsubstituted Phenols in Base-catalyzed Lignin Depolymerization

Lignin valorization is one of the biggest challenges for developing biomass-based chemical industry. Recent innovative works has enabled depolymerization of lignin to the monomers with the yield close to theoretical maximum. The main product is a mixture of phenols derived from the cleavage of β-O-4 linkages between phenylpropanoid units. However, the obtained highly functionalized phenols are not common in current industry and, therefore, needs further processing for the utilization in intended applications. The present study shows that simple phenols without functional groups at para-position form selectively in base-catalyzed depolymerization of lignin. The yield is not sufficiently high, up to 6.9% on a carbon basis under the examined conditions, but the selectivity among released lignin monomers reaches 80%. The study also discusses the mechanism of p-unsubstituted phenols formation.

Production of Aromatic Compounds by Catalytic Depolymerization of Technical and Downstream Biorefinery Lignins.

Lignocellulosic materials are promising alternatives to non-renewable fossil sources when producing aromatic compounds. Lignins from Populus salicaceae. Pinus radiata and Pinus pinaster from industrial wastes and biorefinery effluents were isolated and characterized. Lignin was depolymerized using homogenous (NaOH) and heterogeneous (Ni-, Cu- or Ni-Cu-hydrotalcites) base catalysis and catalytic hydrogenolysis using Ru/C. When homogeneous base catalyzed depolymerization (BCD) and Ru/C hydrogenolysis were combined on poplar lignin, the aromatics amount was ca. 11 wt.%. Monomer distributions changed depending on the feedstock and the reaction conditions. Aqueous NaOH produced cleavage of the alkyl side chain that was preserved when using modified hydrotalcite catalysts or Ru/C-catalyzed hydrogenolysis in ethanol. Depolymerization using hydrotalcite catalysts in ethanol produced monomers bearing carbonyl groups on the alkyl side chain. The analysis of the reaction mixtures was done by size exclusion chromatography (SEC) and diffusion ordered nuclear magnetic resonance spectroscopy (DOSY NMR). 31P NMR and heteronuclear single quantum coherence spectroscopy (HSQC) were also used in this study. The content in poly-(hydroxy)-aromatic ethers in the reaction mixtures decreased upon thermal treatments in ethanol. It was concluded that thermo-solvolysis is key in lignin depolymerization, and that the synergistic effect of Ni and Cu provided monomers with oxidized alkyl side chains.

Study of a renewable capping agent addition in lignin base catalyzed depolymerization process

The aim of this work was to study the influence of a renewable capping agent (CA) in the production of monomeric compounds from the depolymerization of an organosolv lignin (Pinus radiata) by a base catalyzed depolymerization (BCD) process. After the delignification process of the Pinus radiata sawdust, the lignin contained in the black liquor was subjected to a first mild depolymerization process, without using any catalyst, in order to obtain two different products: a lignin which was the feed in the subsequent BCD process and a phenolic oil which was used as CA in the same BCD process. This process was carried out at 230 °C in three consecutive cycles of 20 min using different amounts of capping agent dissolved in a 4% wt. NaOH solution with a lignin:solvent ratio of 1:20. The use of a renewable capping agent derived from lignin was effective not only in terms of phenolic oil production increasing but also in maximizing the monomeric phenolic compounds concentration. The use of CA: lignin ratio of 1:1 resulted the most effective with 10.64% of oil yield and almost 44% of monomeric phenolic compounds concentration while minimizing de undesired by-products yield.

Systematic Diffusion-Ordered Spectroscopy for the Selective Determination of Molecular Weight in Real Lignins and Fractions Arising from Base-Catalyzed Depolymerization Reaction Mixtures

The valorization of biorefinery downstream lignin fractions is a key issue in increasing the sustainability of second-generation biofuels. The development of reliable methodologies for the selective determination of the apparent masses of the poly(hydroxy)-aromatic ethers arising from lignin depolymerization reaction is crucial. Diffusion-ordered spectroscopy (DOSY) has been tested to estimate the molecular weight in downstream biorefinery lignins and base-catalyzed depolymerization reaction mixtures. Excellent correlation was found in the calibration of molecular weight and diffusion coefficients with standards. DOSY permitted the selective estimation of the apparent masses of different fractions in the lignin and in the depolymerization reaction mixtures, providing a more profound knowledge of the reaction mixture composition than that obtained with traditional size-exclusion chromatography (SEC). Excellent correlations have been achieved in the estimation of the apparent masses of poly(hydroxy)-aromatic ethers between SEC and DOSY. This permits a reliable estimation of the molecular weight of different fractions in the lignin and in the depolymerization product, which is essential for their further applications.

Membrane Separation of the Base-Catalyzed Depolymerization of Black Liquor Retentate for Low-Molecular-Mass Compound Production.

One way of valorizing the lignin waste stream from the pulp and paper industries is depolymerizing it into low-molecular-mass compounds (LMMC). However, a common problem in the depolymerization of Kraft lignin is the low yields of small aromatic molecules obtained. In the present work, the combination of the repeated depolymerization of lignin and the separation of LMMC from depolymerized lignin to upgrade them into value-added chemicals was studied. In so doing, we investigated the possibility of depolymerizing black liquor retentate (BLR). The base-catalyzed depolymerization of BLR was performed using a continuous flow reactor at 170–210 °C, with a 2 min residence time. The results obtained indicate that BLR can be depolymerized effectively under the experimental conditions. Depolymerized lignin LMMC can be successfully separated by a GR95PP membrane, and thus be protected from repolymerization. Through combining membrane filtration with base-catalyzed depolymerization, more than half of the lignin could be depolymerized into LMMC. Around 46 mg/g of lignin monomers (guaiacol, vanillin, acetovanillone, and acetosyringone), which can potentially be upgraded to high-valued chemicals, were produced. On the basis of our results, we suggest use of a recycling Kraft lignin depolymerization and filtration process for maximizing the production of LMMC under mild alkaline conditions.