Research Highlight 8 – Polyesters Directly Produced from Biomass for a Circular Economy

To deal with current plastic pollution and our material dependency on finite resources, a shift from current linear economy (take–make–use–dispose) to a circular economy (take–make–use–recycle) has now become necessary and unavoidable [1]. Lignocellulosic biomass is one of the most abundant renewable resources available on the earth, that can be utilized for this purpose – for designing of novel recyclable polymers. However, the research in this field is majorly focused on exploiting either cellulosic components (cellulose and hemicellulose) for producing high quality cellulose for pulp and paper industry, carbohydrates for second generation bioethanol and platform chemical production {5-hydroxymethylfurfural (5-HMF), furfural – and it’s further upgrading}, which results in a highly condensed lignin due to the harsh conditions employed during fractionation, and thus it’s use is limited for producing energy [2-4]. Otherwise, the research is devoted in the direction of fractionation of high quality lignin under mild conditions to avoid condensation reaction and improve the yield of aromatics that can be further utilized for the production of drop-in-chemicals or as a polymer building block with further upgrading process [4]. Here, Prof. Jeremy S. Luterbacher and colleagues at École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, came up with a plot twist in the story with their strategy “Aldehyde Assisted Fractionation (AAF) of Lignocellulosic Biomass” to produce recyclable and degradable biopolymers – polyesters named poly(alkylene xylosediglyoxylates) (PAX). The work is recently published in “Nature Chemistry” (see Main Figure) [5].

In their AAF strategy, the utilization of an aldehyde with a secondary functionality such as ‘glyoxylic acid’ allowed efficient fractionation of all major three components from the lignocellulosic biomass (birch wood) in the presence of an acid catalyst {such as sulfuric acid (H2SO4) or hydrochloric acid (HCl) – a simple filtration of the reaction mixture to obtain cellulose rich solids and followed by precipitation of the concentrated solution by addition of water to obtain glyoxylic acid stabilized lignin. The remaining liquor – consisting of xylose produced via depolymerization of hemicellulose (xylan), unreacted glyoxylic acid and the acid catalyst employed – is utilized to produce a novel monomer ‘Dimethylglyoxylate Xylose’ (DMGX) that can provide degradability/recyclability in the polymer (see Figure 1). This process involves a simple evaporation of water from the AAF lignocellulosic liquor on a laboratory evaporator (at 45-90 °C), which allows acetalization of xylose with glyoxylic acid and forms a diacid monomer ‘Diglyoxylic Acid Xylose’ (DGAX) and subsequent esterification of carboxylic acid functionality in DGAX in methanol resulted in DMGX. Polyesters (that is PAXs) are produced via melt-polycondensation (at 190-200 °C) of DMGX with various aliphatic diols in the presence of a Lewis acid catalyst, for example, poly(pentylene xylosediglyoxylate) (PPTX) is produced by reaction of DMGX with 1,5-pentanediol. PAXs are the first sugar-based polyesters directly produced from lignocellulosic biomass by taking advantage of its fractionation method! Currently, 2,5-furandicarboxylic acid (FDCA) monomer is used to produce bio-based polyester such as poly(ethylene furanoate) (PEF) as an alternative for petro-based poly(ethylene terephthalate) (PET). However, FDCA production from lignocellulosic biomass involves many steps – enzymatic hydrolysis of cellulose into glucose, isomerization of glucose to fructose, then dehydration of fructose to produce platform chemical 5-HMF, and finally catalytic oxidation of 5-HMF to FDCA [6]. Thus, DMGX produced from AAF liquor has the potential to replace both FDCA and petro-based monomer terephthalic acid (TPA) in industrial PET production, in terms of its simplified process and renewability.

Prof. Luterbacher has introduced this AAF strategy in 2016 with the aim of protecting benzylic alcohols (α-OH) on the lignin backbone (see Figure 1, lignin β-O-4 motif) from undergoing dehydration during fractionation – which forms reactive benzylic carbocation – that can undergo electrophilic substitution reaction with aromatic rings on the lignin to form C-C bonds [7]. This generally results in poor quality condensed lignin with low β-O-4 content and thereby a low yield for aromatic monomers after depolymerization. His AAF strategy, that is, protection of α,γ-OH diols with aldehyde such as formaldehyde or propionaldehyde stabilized lignin during lignocellulosic fractionation via acetal formation (1,3-dioxane type, see Figure 1) – which resulted in a near theoretical yield of aromatic monomers after hydrogenolysis {mainly 4-propanolguaiacol/syringol & 4-ethylguaiacol/syringol} [7,8]. This study also revealed in situ acetal protection of C5/C6-sugars produced from hydrolysis of cellulose/hemicellulose during fractionation – for example formation of diformylxylose via in situ acetalization reaction of formaldehyde with diols in xylose – which prevented these sugars from undergoing dehydration. Prof. Luterbacher has now able to practically utilize those previous research insights for making recyclable and degradable polymers that have the real potential to mitigate ‘plastic pollution’. In order to proceed in situ acetal protection, water content in the fractionation medium should be minimal [9] and a secondary functionality in glyoxylic acid reduced it’s reactivity as compared to formaldehyde – thus, xylose remained unreactive during fractionation in the presence of glyoxylic acid. This also simplified the isolation of the other two components (cellulose rich solids and glyoxylic acid stabilized lignin), at the same time provided an easy accessible liquor, which can be directly used for DMGX production without the need for any additional reagents! In the article, further enzymatic hydrolysis of cellulose to produce glucose is demonstrated. Also, hydrogenolysis of glyoxylic acid stabilized lignin to produce high value aromatics (see Figure 1). The glyoxylic acid stabilized lignin can also be utilized as it is – without any further modification/depolymerization/upgrading. In a separate study [10], Prof. Luterbacher and his team have demonstrated it’s application as a surfactant in cosmetic industry (hand creams and lotions). Thus, the whole strategy is highly profitable. This is a perfect example of an ideal biorefinery where all components of lignocellulosic biomass are utilized for high-value applications.

Very few reports are available on a complete biomass utilization [11,12]. Examples are: (1) fractionation of lignocellulosic biomass using γ-valerolactone/water and sulfuric acid – in which high purity cellulose pulp was extracted for textile fiber production, hemicellulose fraction was utilized to produce platform chemical ‘furfural’ and lignin fraction was utilized to produce high value carbon based products (carbon foam) [11]; and (2) Reductive catalytic fractionation (that is, lignin-first approach) of lignocellulosic biomass in the presence of a metal catalyst (Ru/C) and hydrogen – which produces two fractions – a carbohydrate pulp and a lignin depolymerized oil (monomer & oligomers) {via solvolysis–hydrogenation–hydrogenolysis processes}. The carbohydrate pulp was utilized for bioethanol production (via semi-simultaneous saccharification-fermentation), monomer fraction from the lignin oil was utillized for producing drop-in chemicals ‘phenol and propylene’ (via catalytic funneling – hydroprocessing + dealkylation) and oligomers for replacing fossil-based ‘para-nonylphenol’ in lithographic printing ink [12]. Prof. Luterbacher’s strategy is simple and stands out from these works because it focuses on the direct development of circular polymers from lignocellulosic biomass.

Large scale synthesis of DMGX using commercial xylose is also demonstrated in the article by reacting with glyoxylic acid in the presence of sulfuric acid using a one-pot and two-stage acetalization + esterification (in methanol) process. DMGX obtained from both AAF lignocellulosic liquor and commercial xylose are structurally identical and consists of a mixture of 4-stereoisomers – obtained as a transparent oil after distillation from the crude mixture at 180 °C, followed by activated carbon treatment (to remove yellow impurities). The Luterbacher team also has a more simplified process for DMGX purification (instead of neutralization–extraction–distillation steps) by precipitation of crude mixture in ice, which yielded slight yellowish white powder (62%). Currently, they are exploring a different antisolvent that could enhace the yield of the precipitate and also allow the direct recycling of other components in the crude reaction mixture: glyoxylic acid, sulfuric acid and methanol. DMGX oil can also be crystallized, either as crystals of 4-stereoisomers by dissolving in hot ethanol (77%; crystallization at -20 °C) or these 4-stereoisomers can be resolved by crystallization using cyclopentyl methyl ether as 34% of a single isomer (crystallization at 4 °C) and 38% of 1:1 mixtures of two isomers (crystallization at -20 °C). The oil (4-stereoisomers) is used for most of the PAXs synthesis, by reacting with diols of different aliphatic chain length (m= 2-6, see Main Figure), except in case of PPTX and PHX – in which a single isomer of DMGX (obtained via AAF lignocellulosic liquor) was also reacted with 1,5-pentanediol/1,6-hexanediol for structure-property correlation. The properties of PAXs produced via AAF liquor and commercial xylose method are same.

The novel polyesters, PAXs are amorphous in nature (including those produced from a single isomer – 1SPAXs) because of the asymmetry in DMGX structure. PAXs {that is, poly(butylene xylosediglyoxylate) (PBX), poly(pentylene xylosediglyoxylate) (PPTX), poly(hexylene xylosediglyoxylate) (PHX) produced from DMGX/1,4-butanediol, DMGX/1,5-pentanediol and DMGX/1,6-hexanediol respectively} exhibited a unique combination of properties such as high glass transition temperature (Tg): 72-100 °C {PEF: ~85 °C; PET: ~75 °C; polylactide (PLA): ~65 °C}, good mechanical properties with tensile moduli (ET) of 2000-2500 MPa, ultimate tensile strength (UTS) of 63-70 MPa, and elongation at break of 50-80% and strong gas barrier properties with oxygen transmission rates (OTR) of 11-24 cc/m2.day.bar (PET: 11.59; PLA: 157) and water vapour transmission rates (WVTR) of 25-36 g/m2.day (PET: 6.2; PLA: 54). These properties are similar to petro-based PET. The tricyclic rigid DMGX structure on the backbone is responsible for these excellent properties.

These PAXs can be easily processed (within the temperature range of 140-200 °C) by using various industrial techniques such as compression moulding, injection moulding, vacuum-forming and twin-screw extrusion + 3D printing. Because of PAXs high plasticity and strain hardening behaviour (especially in PHX), the Luterbacher team believes that PAXs are also suitable for blow moulding process (to produce bottles). They have made various objects (that we use in our day–to–day life) from PAXs by using these techniques to demonstrate their real potential for industrial commercialization and competence with PET and PLA. The Luterbacher team was very grateful to provide some of the pictures of the produced products, see Figure 2 – (a) USB drive packaging using PHX via vacuum forming – in this case, antioxidant such as TPP (triphenyl phosphite) and an optimized catalyst such as tin(II) 2-ethylhexanoate (Sn(Oct)2) {instead of zinc acetate/dibutyltin oxide} were used during melt-polycondensation of DMGX {crystals of 4-stereoisomers produced via ethanol recrystallization} and 1,6-hexanediol in order to prevent colouration in the final product; (b) Cup and EPFL logo made using PBX via twin-screw extrusion and subsequent 3D printing – here, PBX is produced using DMGX oil (4-stereoisomers), which is why yellow colour for the product; (c) Fiber produced using PPTX made from a single isomer of DMGX – in this case no antioxidants were used since no colouration was observed when using a single isomer crystals for the production of thin products {using dibutyltin oxide as catalyst for melt-polycondensation}. These examples thus indicate that PAXs can be mold into any shape using existing industrial infrastructure and plenty of options are available to tailor them (by incorporation of additives) for commercial applications (especially for packaging industry). In their practical utility test, PBX thin cup could withstand boiling water thanks to its high Tg (100 °C) whereas PLA cup became soft and eventually lost its shape.

Chemical Recycling of PAXs is demonstrated by alcoholysis (methanolysis) – by refluxing PAX in methanol for 4 hours – which depolymerized PAXs back into it’s starting monomers: DMGX (diester) and Diols (for example 1,6-hexanediol from PHX). The monomers can be isolated via simple liquid-liquid extraction (using dichloromethane and water) and further separate distillation of these fractions. These monomers are then utilized for re-polymerization, and dynamic mechanical analysis of chemically recycled PAXs showed properties similar to the virgin PAXs – hence, ‘closing–the–loop’ of a circular economy. The Luterbacher team has stated in the article that the PAX samples used for chemical recycling contained antioxidants (Irganox 1010) – which is common in plastic industry, and it had no negative impact on PAXs recycling efficiency. Selective depolymerization of PAX from a mixed plastic waste stream containing PET bottle pieces, high density polyethylene (HDPE) bottle caps, Zytel polyamide-6,10, polypropylene (PP) and polycarbonate (PC) from safety glasses, is also demonstrated with zero contamination in the depolymerized monomers from these plastic wastes (while enabling 100% mass recovery of plastic wastes). An important point worth mentioning here is that, all these plastics are susceptible to solvolysis except HDPE and PP, but need much high energy intensive process for their depolymerization (in terms of catalyst reactivity, high reaction temperature and pressure) than the strategy used here for PAXs depolymerization. Thus, Prof. Luterbacher’s PAXs are suitable for real-world applications not only in terms of their unique combination of properties, but also because of their selective recyclability from a mixed plastic waste stream.

The Luterbacher team also envisages ‘closing–the–loop’ via mechanical recycling (at the very least for once to produce same quality product, for example, “bottle to bottle”) because of PAXs stability towards thermal degradation (onset of degradation temperature: 319-344 °C) during processing – only a slight decrease in molecular weight was observed (12.6%) after twin-screw extrusion and subsequent 3D printing at elevated temperature (at 200 °C) – and no changes in chemical structure.

PAXs are also biodegradable – these can be hydrolysed in the presence of water to DGAX (diacid) and diol (see Main Figure), and under accelerated conditions (reflux) DGAX is further breakdown into xylose and glyoxylic acid – all these breakdown products can uptake by microorganisms and return nutrients back to the ecosystem – thus, this could be the fate of PAXs if they end up in the open environment. PAXs are highly degradable than PLA, a polymer known for its biodegradability. In a study conducted by the Luterbacher team, ~55% decrease in molecular weight of PAXs was observed after 77 days of exposure in acidic (pH 2) and neutral conditions (pH 7) at room temperature (RT), but there was no change in molecular weight of PLA. This hydrolytic degradability of PAXs has no negative impact on their service life since thermo-mechanical properties were found intact with 100% mass retention in the degraded polymer (after 77 days in pH 2 & 7 at RT). Also, at ambient air exposure, PAXs are more stable over a period of 18 months.

The Luterbacher team has 3 different lignocellulose fractionation scenarios for introducing DMGX and thereby PAXs into a flexible integrated biorefinery – that can be exploited depending upon the market demand. Scenario 1 utilized glyoxylic acid, H2SO4 as acid catalyst and dioxane as solvent for the fractionation of lignocellulosic biomass which maximized the yields of all three components: glucose (84%), DMGX (70%) and aromatic monomers produced from the stabilized lignin (18%). Scenario 2 utilized neat glyoxylic acid and H2SO4 as acid catalyst (solventless strategy), which enhanced the yield of DMGX (83%) {glucose (51%) and aromatic monomers (2%)}. Scenario 3 utilized glyoxylic acid, HCl as acid catalyst and dioxane as solvent, which maximized the yields of glucose (99%) and aromatic monomers (31%) with DMGX yield of 47%. DMGX can also be produced from other biomass resources such as raw corn cobs with high hemicellulose content.

According to their techno-economic and life-cycle assessment (cradle to gate), the estimated minimum selling price (MSP) for DMGX is US$1,543/t and the GWP (global warming potential) is 2.33 kg CO2e/kg. The MSP of DMGX is similar to terephthalic acid (TPA) (US$800-1,500/t) – one of the starting monomers for fossil-based PET production and the GWP is 20% less than TPA (2.88 kg CO2e/kg). These values can be further reduced when commercial xylose (US$1/kg) is replaced with AAF liquor for DMGX production and also if commercial glyoxylic acid (US$1/kg) is produced via a more sustainable route such as electrochemical reduction of oxalic acid produced from CO2.

Overall, PAXs are unique biopolymers derived from lignocellulosic biomass with good thermal, mechanical and barrier properties, at the same time they impart inherent recyclability and degradability – achieving all these properties in one polymer in a balanced way is rather difficult – such combination of properties are generally achieved via copolymerization. An example is polyesters based on poly(γ-butyrolactone) (PγBL) and poly(trans-hexahydrophthalide) (PT6HP) – both are recyclable and degradable. PγBL lacks barrier properties, but exhibits excellent ductile behaviour (elongation at break ~380%), whereas PT6HP has good barrier properties, but lacks ductility (elongation at break ~5%). Thus, the individual homopolymers cannot be utilized for packaging applications – for that copolymers of PγBL and PT6HP are explored [13]. Here, Prof. Luterbacher is successful in making polyesters that possess balanced qualities, which are suitable for packaging applications, and every potential to replace fossil-derived PET and bio-based PEF. Their competence is also in terms of cost and low carbon footprint of DMGX as compared to TPA.

The whole work is a joint effort of various experts from multi-disciplinary fields at EPFL – notable names (who joined forces with Prof. Luterbacher to make this work happen) include Prof. Harm-Anton Klok (Polymer Chemistry), Prof. Véronique Michaud and Dr. Yves Leterrier  (Polymer Composite Manufacturing), Prof. François Maréchal (Life Cycle Assessment and Techno-Economic Analysis), and also, Prof. Antje Potthast (Wood Chemistry) at the University of Natural Resources and Life Sciences, Austria.

Prof. Luterbacher and his group next plans are focused in the following directions: (1) Poly(ethylene xylosediglyoxylate) (PEX) and poly(propylene xylosediglyoxylate) (PPX) produced in this study {from DMGX/ethylene glycol and DMGX/1,3-propanediol respectively} – which are structurally most resemble to PET – exhibited low molecular weight (3-10 kDa) as compared to other PAXs (m= 4, 5 & 6) (31-43 kDa) which hindered their processability, but they have the highest Tg values, 137 °C and 117 °C . Thus, the Luterbacher team has plans to take advantage of high Tg of PEX and PPX for high-temperature applications – by using an improvised polymerization method (for example, using a more reactive catalyst during melt-polycondensation) to achieve high molecular weight polymer and thereby processability. (2) Another plan is focused on further improving thermo–mechanical–barrier properties of PAXs by taking advantage of 4-stereiolsomers in DMGX and their easy separation via crystallization. Because in this study, PAX produced from a single isomer of DMGX (that is 1SPPTX) showed much high ultimate tensile strength (77 MPa) than those produced from a mixture of 4-stereoisomers (UTS of PPTX: 66 MPa). PAXs produced from a single isomer also imparted high hydrolytic stability {>99% mass retention was observed for 1SPHX in pH 7 at 37 °C for 15 days, whereas ~25% for PHX produced from 4-stereoisomers} – which make them suitable for moisture sensitive applications. Thus, the Luterbacher team hopes that 1:1 mixtures of two stereoisomers of DMGX could be utilized for stereo-specific polymerization that may provide PAX block copolymers with interesting properties. (3) Lastly, they also have plans to look into DGAX (diacid monomer) toxicity and biodegradability (uptake by microorganisms) since it is one of the primary hydrolyzed products of PAXs – in order to gain more in depth understanding of PAXs fate in the open environment.

References:

[1]. The new plastics economy: Rethinking the future of plastics & catalysing action, Ellen MacArthur Foundation 2017, http://www.ellenmacarthurfoundation.org/publications.

[2]. Schutyser, W.; Renders, T.; Van den Bosch, S.; Koelewijn, S.-F.; Beckham, G. T.; Sels, B. F. Chemicals from lignin: An interplay of lignocellulose fractionation, depolymerisation, and upgrading. Chem. Soc. Rev. 2018,47, 852-908.

[3]. Aditiya, H. B; Mahlia, T. M. I.; Chong, W. T.; Nur, H.; Sebayang, A. H. Second generation bioethanol production: A critical review. Renew. Sustain. Energy Rev. 2016, 66, 631-653.

[4]. Luterbacher, J. S.; Alonso, D. M.; Dumesic, J. A. Targeted chemical upgrading of lignocellulosic biomass to platform molecules. Green Chem., 2014, 16, 4816-4838.

[5]. Manker , L. P.; Dick , G. R.; Demongeot , A.; Hedou, M. A.; Rayroud, C.; Rambert, T.; Jones , M. J.; Sulaeva, I.; Vieli, M.; Leterrier , Y.; Potthast, A.; Maréchal, F.; Michaud , V.; Klok, H.-A.; Luterbacher , J. S. Sustainable polyesters via direct functionalization of lignocellulosic sugars. Nat. Chem. 2022, https://doi.org/10.1038/s41557-022-00974-5 (Featured Article!).

[6]. Sajid, M.; Zhao, X.; Liu, D. Production of 2,5-furandicarboxylic acid (FDCA) from 5-hydroxymethylfurfural (HMF): Recent progress focusing on the chemical-catalytic routes. Green Chem. 2018, 20, 5427-5453.

[7]. Shuai, L.; Amiri, M. T.; Questell-Santiago, Y. M.; Héroguel, F.; Li, Y.; Kim, H.; Meilan, R.; Chapple, C.; Ralph, J.; Luterbacher, J. S. Formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization. Science 2016, 354, 329-333.

[8]. Amiri, M. T.; Dick, G. R.; Questell-Santiago, Y. M.; Luterbacher, J. S. Fractionation of lignocellulosic biomass to produce uncondensed aldehyde-stabilized lignin. Nat. Protoc. 2019, 14, 921-954.

[9]. Luo, X.; Li, Y.; Gupta, N. K.; Sels, B. F.; Ralph, J.; Shuai, L. Protection strategies enable selective conversion of biomass. Angew. Chem. Int. Ed. 2020, 59, 11704.

[10]. Bertella, S.; Figueirêdo, M. B.; De Angelis, G.; Mourez, M.; Bourmaud, C.; Amstad, E.; Luterbacher, J. S. Extraction and surfactant properties of glyoxylic acid-functionalized lignin. ChemSusChem 2022, e202200270.

[11]. Alonso, D. M.; Hakim, S. H.; Zhou, S.; Won, W.; Hosseinaei, O.; Tao, J.; Garcia-Negron, V.; Motagamwala, A. J.; Mellmer, M. A.; Huang, K.; Houtman, C. J.; Labbé, N.; Harper, D. P.; Maravelias, C. T.; Runge, T.; Dumesic, J. A. Increasing the revenue from lignocellulosic biomass: Maximizing feedstock utilization. Sci. Adv. 2017, 3, e1603301.

[12]. Liao, Y.; Koelewijn, S.-F.; Van den Bossche, G.; Van Aelst, J.; Van den Bosch, S.; Renders, T.; Navare, K.; Nicolaï, T.; Van Aelst, K.; Maesen, M.; Matsushima, H.; Thevelein, J. M.; Van Acker, K.; Lagrain, B.; Verboekend, D.; Sels, B. F. A sustainable wood biorefinery for low–carbon footprint chemicals production. Science 2020, 367, 1385-1390.

[13]. Sangroniz, A.; Zhu, J. B.; Tang, X.; Etxeberria, A.; Chen, E. Y.-X.; Sardon, H. Packaging materials with desired mechanical and barrier properties and full chemical recyclability. Nat. Commun. 2019, 10, 3559.

[14]. All references cited in the highlighted article [5].

Created by: Sini Nalakathu Kolanadiyil