OrganoCat is a method for the pretreatment and fractionation of lignocellulose under mild conditions into lignin, fermentable sugars, and cellulose pulp. In a biogenic, biphasic solvent system of water and 2-methyltetrahydrofuran with 2,5-furancarboxylic acid as catalyst, the OrganoCat products are separated in situ for straightforward product recovery.
The shift from a petroleum-based to a more sustainable and bio-based economy requires the development of new refinery concepts to maintain the supply of raw materials and energy. For these novel and sustainable biorefinery concepts, it is important to use catalysts and solvents that are aligned with the principles of Green Chemistry. Therefore, the implementation of biogenic alternatives can be a promising solution. The lignocellulose pretreatment and fractionation process presented herein-OrganoCat-is an integrated fractionation of lignocellulose into its main components using biogenic acids such as 2,5-furandicarboxylic acid as catalyst. Hemicelluloses and other non-cellulosic polysaccharides are selectively depolymerized by the diluted acid and dissolved, while the crystalline cellulose remains in the solid pulp. In the presence of a second organic phase consisting of biogenic 2-methyltetrahydrofuran, disentangled lignin is extracted in situ. The process allows for the efficient fractionation of the three main components-lignin, cellulose, and non-cellulosic sugars. This helps to focus on the quality of the lignin, the improvement of enzymatic hydrolysis of the cellulose-enriched pulp, and the mild non-cellulosic sugar extraction with low degradation.
The use of fossil resources has brought great technological advances as they form the basis for numerous products that are essential for everyday life. However, the limitation of resources such as oil and gas on earth and the environmental damages connected with their exploitation create an urgent need for alternatives. Lignocellulosic biomass is a promising source for carbon-based chemicals, as it is renewable, versatile, and carbon neutral1. Lignocellulose basically consists of three main fractions to make use of: hemicelluloses, cellulose, and lignin. Its industrial processing has a long history. However, established and widespread processes, such as the sulfite and Kraft processes from paper industry, mainly focus on cellulose for utilization in the pulp and paper industry2. A full valorization of all three lignocellulosic fractions is needed to make lignocellulose processing towards chemicals more profitable from economic and environmental perspectives.
In many lignocellulose valorization strategies, lignin is a mere byproduct that is often burned for energy recovery. Currently, only 1-2% of the industrially produced lignin is used to produce value-added products such as concrete additives, surfactants, and vanillin3. Nevertheless, it is the largest renewable source of aromatics and therefore has promising properties for application as a basis for polymers4, carbon fibers5, and fuel2. The challenges in the valorization of lignin lie in its complex structure and diversity, depending on the source material and extraction conditions. Moreover, due to their process conditions, the most prevalent lignocellulose fractionation processes deliver sulfonated lignin with a high number of C-C linkages between the monomer units. Therefore, commercially available lignin is challenging to depolymerize.
A range of different approaches, which focus on the holistic utilization of all three fractions, have been developed for lignocellulose fractionation. Most processes rely on the hydrolysis of hemicellulose, either with diluted acids and bases or by utilizing the autoprotolysis of water at elevated temperatures. As one of the most explored options, organosolv processes use low-boiling organic solvents, usually in combination with water. Well-known variants of this process include the Alcell process, which utilizes 50% ethanol, and the Organocell process, which uses methanol in the first step and adds NaOH in the second step. Acid organosolv processes that use formic or acetic acid are also described2. Owing to the recent focus on the valorization of lignin as a major biorefinery product, new approaches have been developed, which combine lignin extraction with subsequent or integrated conversion steps to yield smaller lignin compounds and more stable and valuable products6,7,8.
The OrganoCat lignocellulose fractionation process (OrganoCat) is based on a two-phase system of water and 2-methyltetrahydrofuran (2-MTHF)9. Additionally, a recyclable organic acid is used as catalyst, which selectively hydrolyzes hemicelluloses at mild temperatures. All process chemicals can be produced in a relatively inexpensive and biogenic manner, which lowers the environmental impact of the process in accordance to the principles of Green Chemistry10. The process delivers three separate product streams with lignin in the organic phase, depolymerized hemicellulose sugars in the aqueous phase, and cellulose-enriched pulp as a solid residue. As the product streams can be easily separated, downstream steps, energy demand, and material costs can be reduced significantly compared to, for example, monophasic approaches. The lignin has a relatively low molecular weight and a high number of β-O-4 linkages11. The depolymerized hemicellulose sugars can be used for fermentation or conversion into fine chemicals12. The cellulose pulp is highly accessible for enzymatic depolymerization9.
The original OrganoCat process uses oxalic acid as catalyst to fractionate lignocellulose. Oxalic acid can then be recovered by crystallization9. However, this increases the process costs for cooling the reaction and the partial evaporation of water. The partial decomposition of oxalic acid would diminish the revenues further13. For this reason, the OrganoCat process was improved by introducing 2,5-furandicarboxylic acid (FDCA) as catalyst11. FDCA is not only sufficiently acidic to catalyze the reaction, but can also be derived from glucose via dehydration to 5-hydroxymethylfurfural and subsequent oxidation with metal-based catalysts or biocatalysts14,15,16,17. Although the acidity of FDCA is slightly lower, it has a higher thermal stability than oxalic acid. FDCA has a low solubility in water at room temperature, which allows its straightforward recovery from the aqueous phase after the reaction.
A scale-up of the OrganoCat process was successfully developed to a 3 L reactor18. Additional studies on OrganoCat lignin found that antisolvent precipitation with n-hexane or n-pentane allow an energy efficient lignin recovery19. It was possible to get lignin fractions with different molecular weights20. This paper presents the full preparative method for a scalable, one-step fractionation process of lignocellulosic biomass using FDCA as catalyst. This process yields extracted lignin, depolymerized hemicelluloses, and cellulose pulp in three easily separable product streams.
NOTE: The process may be paused at any point by leaving the samples at room temperature (for a few days) or in the refrigerator (for longer periods). See the Table of Materials for details on the materials used in this protocol.
1. Beech wood particles
2. Lignocellulosic fractionation and workup
3. Analysis
A typical set of conditions for the lignocellulose pretreatment and fractionation process OrganoCat (OrganoCat) uses 0.1 M FDCA as a catalyst, a biomass loading of 100 g L-1 (beech wood, compared to the aqueous phase), 1 h of reaction time and 160 °C as the reaction temperature. The composition of beech wood has been published elsewhere21 (~48% cellulose, 27% hemicellulose, 26% lignin). Figure 1 shows the extracted hemicellulose hydrolysate with this set of conditions as well as longer reaction time (3 h) and lower temperature (140 °C).
Using harsher conditions, e.g., higher temperature and longer reaction time, might lead to higher extraction yields, but also leads to more degradation of the products-furfural is a degradation product of xylose, whereas 5-(hydroxymethyl)furfural (5-HMF) is the corresponding degradation product of glucose. A higher amount of furfural was noted in the products (distributed between the aqueous and the organic phases) with a reaction time of 3 h at 160 °C. As the sugar degradation products are highly reactive and tend to form humins-oligomers of furans and sugars-the shorter reaction time at higher temperature might be considered a good compromise between high extraction efficiency and low sugar degradation.
The amount of extracted lignin is directly related to the reaction temperature and time as well. Figure 2 displays the amount of lignin extracted, the β-O-4-linkage content, and the mass-average molar masses of the extracted lignins. While the extracted lignin yield rises with longer reaction time, the number of intact β-O-4-linkages diminishes by approximately half when reacting for 3 h instead of 1 h. Lowering the reaction temperature from 160 °C to 140 °C has a much lower impact on lignin, resulting in slightly less yield, smaller mass-average molar mass, and higher β-O-4-content.
As enzymatic hydrolysis of (lingo-)cellulose is a common indicator for pulping efficiency, a commercial cellulose cocktail was applied to the different OrganoCat pulps resulting from the above mentioned OrganoCat reaction condition sets (Figure 3). As the cellulase is not optimized for the substrates, the overall cellulose conversion is not comparable to state-of-the-art performance; however, it allows the comparison of the different pulps to each other. The longer reaction time exhibits a significant impact on the initial reaction time and the glucose yield after 72 h, improving by a factor of ~2.5. Lowering the temperature appears to show a much smaller impact, implying that the main factor causing the differences in enzymatic digestibility within this treatment is the degree of delignification.
Figure 1: Sugar extraction and furfural production in OrganoCat process with 0.1 M 2,5-furandicarboxylic acid as catalyst and 100 g L-1 beech wood (compared to the aqueous phase) at different reaction temperatures and times as indicated on the x-axis11. All experiments have been performed in triplicate. The average is shown with the standard deviation. Please click here to view a larger version of this figure.
Figure 2: Amount and analysis of lignin extracted by OrganoCat process with 0.1 M 2,5-furandicarboxylic acid as catalyst and 100 g L-1 beech wood (compared to the aqueous phase) at different reaction temperatures and times as indicated on the x-axis11. Lignin yields have been calculated in triplicate. The average is shown with the standard deviation. Molecular mass and linkages were derived from representative single experiments. Please click here to view a larger version of this figure.
Figure 3: Enzymatic hydrolysis of pulps. Glucose yields after 72 h (blue bars) and reaction rates within the first hour (gray bars) from the hydrolysis of untreated beech wood and cellulose-enriched pulps obtained from OrganoCat with 0.1 M 2,5-furandicarboxylic acid as catalyst and 100 g L-1 beech wood (compared to the aqueous phase) at different reaction temperatures and times as indicated on the x-axis. Cellulase was applied to the different substrates at 50 °C for up to 72 h in contrite buffer (pH 4.5)11. All experiments have been performed in triplicate. The average is shown with the standard deviation. Please click here to view a larger version of this figure.
Unit | Shift δ (1H)(13C) | Linkage | Shift δ (1H)(13C) |
[ppm] | [ppm] | ||
S2,6 | (6.95–6.46)(106.8–101.9) | α β-O-4 | (5.08–4.69)(75.8–69.9) |
G2 | (7.12–6.72)(113.4–108.7) | α β-β | (4.72–4.58)(87.46–84.0) |
G5 | (7.04–6.51)(117.8–113.4) | β β-β | (3.35–3.11)(62.0–57.9) |
H2,6 | (7.01–6.8)(129.1–123.2) | γ β-β 1 | (4.26–4.09)(73.0–70.0) |
γ β-β 2 | (3.87–3.71)(73.0–70.0) | ||
α β-5 | (5.51–5.41)(88.8–86.6) | ||
β β-5 | (3.52–3.42)(54.0–52.1) | ||
γ β-5 | (3.80–3.67)(64.1–62.1) |
Table 1: Chemical shifts determined by 1H-13C heteronuclear single quantum correlation nuclear magnetic resonance (1H-13C-HSQC NMR) for different linkages in lignin. Abbreviations: S = syringyl unit, G = guaiacyl unit, H = p-hydroxyphenyl unit.
The described fractionation of lignocellulose shows a tradeoff between hemicellulose hydrolysis efficiency and selectivity to avoid sugar degradation to furans, depending on reaction time and temperature (Figure 1). Lignin extraction was similarly affected by the harsher conditions. Especially the reduction of β-O-4-linkages and enhancement of mass average molecular weight due to recondensation at higher temperature and reaction time underlines this compromise that must be made. The selection of reaction time and temperature is therefore a critical step in this lignocellulose fractionation process. As the efficiency of enzymatic hydrolysis appears to be mostly determined by delignification in the FDCA-catalyzed OrganoCat process, the harshest processing conditions afford the most accessible pulp. Other variations of the process9,11,18,22, e.g., using different catalysts, show that the strength of the catalyst and the final pH in the reactive solution have the strongest effect on process efficiency. Modifications of the procedure, e.g., preswelling with phosphoric acid, have been shown to have a beneficial effect as well22. Due to the variety in composition, however, the process needs optimization, depending on the different feedstocks21. Considering the overall process performance, downstream purification of the separated fractions must be considered, which is why selectivity plays a major role. Compared to other organosolv-like processes, OrganoCat uses a biphasic water/2-MTHF system, which affords the major components in three relatively straightforward, separate streams. This way, further downstream and resulting energy and equipment costs can be reduced13,18.
The authors have nothing to disclose.
This work was carried out as part of the Cluster of Excellence "Tailor-Made Fuels from Biomass" and "Fuel Science Center", which are funded by the Excellence Initiative of the German Research Foundation to promote science and research at German universities, as well as part of the Bioeconomy Science Center (BioSC), supported in the project AP³ Focus Lab. The scientific activities of the Bioeconomy Science Center were financially supported by the Ministry of Innovation, Science, and Research within the framework of the NRW Strategieprojekt BioSC (no. 313/323-400-002 13).
1200 HPLC system | Agilent | n.a. | was used for size exclusion chomatogaphy |
2,5-furandicarboxylic acid | TCI Deutschland GmbH | F0710 | Purity: >98.0%(T)(HPLC) |
2-methyltetrahydrofuran | Carl Roth GmbH | 6845.4 | SOLVAGREEN ≥99 %, extra pure |
Accellerase 1500 | Provided by Genencor (60 FPU mL-1 and 82 CBU mL-1; 2300 AE Leiden, Netherlands) | n.a. | cellulase for pulp hydrolysis |
beech wood (Fagus sp.) | local supplier | n.a. | |
BioTek Power Wave HT UV-Vis Spectrometer | BioTek Germany, 74177 Bad Friedrichshall, Germany | BT-RPRWI | |
Bruker AS400 (400 MHz) Spectrometer | Bruker, Billerica, MA 01821, USA | n.a. | HSQC-NMR analysis |
CarboPac PA20 column | Dionex | 302747 | monosaccharide separator column for high-performance anion-exchange chromatography |
centrifuge 5430 R | Eppendorf | 5428000610 | |
Focus GC | Thermo Fischer | n.a. | gas chromatograph |
Glucose (hexokinase) assay kit | Sigma-Aldrich | GAHK20-1KT | |
GPC- precolumn PSS PolarSil in DMAc | PSS Polymer Strandards Service GmbH | PSA080505 | precolumn with polar silica (8 x 50 mm) |
HP-INNOwax column 30 m | Agilent J & W | 19091N-213IE | GC column with a polar polyethylene glycol stationary phase |
PSS MCX | PSS Polymer Strandards Service GmbH | MCA0830051E3 | gel columns (8 x 300 mm, particle diameter: 5 µm, nominal pore width: 1000 Å |
ThermoMixer | Eppendorf | n.a. | mixing and heating block |
tinyclave steel Typ 3 / 25 mL | Büchi | 49,33,45,10,000 | 100 bar, 200 °C |