Fully lignocellulose-based PET analogues for the circular economy

The study addresses the urgent need for fully bio-based, recyclable alternatives to polyethylene terephthalate (PET), given PET’s massive production and accumulation in landfills and oceans. Conventional PET is predominantly fossil-derived from ethylene glycol and terephthalic acid. The research aims to develop robust catalytic and integrated biorefinery strategies to produce PET analogues entirely from lignocellulosic biomass, aligning with circular economy principles. The central hypothesis is that lignin can be funneled catalytically to a single aliphatic diol suitable for polyester synthesis, while cellulose-derived streams can provide aromatic diacids, enabling competitive polymer properties and chemical recyclability.

Emerging industrial strategies include replacing fossil terephthalic acid (TPA) with furan dicarboxylic acid (FDCA) derived from 5-hydroxymethylfurfural, and biomass pathways to ethylene glycol and TPA have been reported. Lignin-derived monomers such as ferulic and syringic acids have enabled PET mimics and reinforced plastics. Reductive catalytic fractionation (RCF) has become a powerful method to generate aromatic monomers for polymers (including TPA, 4-propylcyclohexanol, and lignin-based bisphenols). Upcycling strategies also include modifying PET with bio-based comonomers to yield higher-value materials. Prior catalytic defunctionalization studies typically achieved extensive hydrogenolysis; selective conversion to defined aliphatic diols while preserving functional handles has been challenging. This work builds on RCF for selective generation of 4-n-propanolguaiacol/syringol and introduces a non-noble metal funneling route to a single diol for PET analogues.

• Biomass fractionation and lignin funneling: Lignocellulose (beech, pine, poplar) is subjected to reductive catalytic fractionation over Cu2O-PMO (e.g., 2 g biomass, 0.4 g catalyst, methanol solvent, 40 bar H2, 180 °C, 18 h) to generate crude lignin oil enriched in 4-n-propanolguaiacol (1G) and 4-n-propanolsyringol (1S), alongside dimers/oligomers and sugars. For scale-up, 10 g beech with 2 g Cu2O-PMO and 120 mL methanol under identical pressure and temperature was used.
• Crude oil workup: Crude lignin oil is extracted/fractionated with ethyl acetate and washed with saturated NaHCO3 and brine to remove oligomers, sugars, and acids that deactivate catalysts.
• Catalytic funneling: The EtOAc extract is converted over Raney Ni in isopropanol under H2 to the aliphatic diol 4-(3-hydroxypropyl)cyclohexan-1-ol (PC). Optimized conditions for model 1G/1S: typically 120–150 °C, 10–30 bar H2, isopropanol solvent, 2–12 h. Kinetic analysis indicates a pathway via demethoxylation of 1G to 1H (rate-limiting) followed by rapid aromatic ring hydrogenation to PC; hydrogenation to 1 is a slower parallel pathway. Raney Ni’s transfer hydrogenation activity and lower aromatic saturation tendency versus noble metals enhance selectivity.
• Product fractionation: The crude aliphatic oil from funneling is fractionally distilled (Kugelrohr, 1 mPa) to obtain three cuts: A (100–105 °C, 4-alkylcyclohexanols), B (115–120 °C, PC and 1), and C (>120 °C, aliphatic dimers/oligomers). PC can be isolated by chromatography or obtained directly in Fraction B at high purity.
• Polymer synthesis: PC is copolymerized with methyl esters of TPA (dimethyl terephthalate, DMTA) using Zn(OAc)2 or with methyl ester of FDCA (dimethyl FDCA, DMFD) using titanium(IV) butoxide (TBT). Typical conditions: equimolar diol and diester, 1 mol% catalyst, 190 °C under N2 for 1 h, then 230 °C under high vacuum (≈1 mPa) for 1–3 h. Purification by dissolution-precipitation can increase Mw.
• Hydrodeoxygenation to fuels: Fractions A and C are hydrodeoxygenated over Raney Ni with HZSM-5 co-catalyst in cyclohexane (e.g., 220 °C, 30 bar H2, 4–6 h) to generate gasoline-range C7–C9 cyclic alkanes (from A) and higher-density C14–C17 bicyclic/tricyclic alkanes (from C).
• Carbohydrate valorization: Catalyst-free cellulose-rich residues are converted to FDCA via hydrolysis to glucose, dehydration to HMF, and oxidation to FDCA (overall up to ~32.7 wt% yield on cellulose basis). Surplus carbohydrates can be processed to ethanol or ethylene glycol per literature methods.
• Characterization: Polymers characterized by 1H/13C NMR, 2D NMR (HSQC/HMBC), FT-IR, GPC (Mw/Mn), TGA (T5%, T90%), DSC (Tg). Polymer microstructure (dyads H-H, H-T/T-H, T-T) assigned from 13C NMR carbonyl signals.
• Recycling: Chemical recycling of poly(PC/TPA) via methanolysis (e.g., 190 °C, 4 h, no additives) to recover PC and DMTA at high yields; repolymerization regenerates polymer with comparable properties.
• Techno-economic assessment (TEA): Preliminary TEA includes RCF, fractionation, funneling, polymerization, and solvent recovery. Assumes high solvent recovery (isopropanol/methanol) and current product yields; sensitivity to FDCA prices and solvent recovery assessed.

• A two-step, non-noble metal catalytic sequence converts lignin to a single aliphatic diol, 4-(3-hydroxypropyl)cyclohexan-1-ol (PC), in high yield and selectivity from complex RCF oils.
• From beech RCF oil, PC is obtained at 11.7 wt% isolated yield on lignin basis; EtOAc extract funneling gives PC selectivity ~74–86% for model and real feeds. Pine and poplar give PC yields of 4.6 wt% and 15.5 wt% on lignin basis (selectivity ~75–77%).
• Kinetic analysis shows demethoxylation of 1G to 1H is rate-limiting; the rate order is k2 (hydrogenation of 1H) > k1 (demethoxylation of 1G) > k3 (hydrogenation of 1G to 1) >> k4 (demethoxylation of 1).
• Fractional distillation affords Fraction B containing only PC and 1 at >99% purity, enabling direct copolymerization without extensive purification.
• Fully wood-derived PET analogues poly(PC/TPA) and poly(PC/FDCA) exhibit Tg values between ~70–90 °C, comparable to or better than commercial PET (67–80 °C). Representative Mw values reach ~46.3 kg/mol for poly(PC/TPA) and ~50.6 kg/mol for poly(PC/FDCA) after purification. T5% thermal stability under N2 is around 310–329 °C.
• Methanolysis enables chemical recycling: at 190 °C, isolated recovery yields of 90% PC and 92% DMTA; repolymerized material shows Mw ≈16 kg/mol, Tg ≈72 °C, T5% ≈319 °C, similar to virgin polymer.
• Co-processing of other fractions to fuels: Fraction A yields gasoline-range C7–C9 alkanes; Fraction C yields predominantly bicyclic C14–C17 alkanes after HDO.
• Overall lignin valorization achieves a total carbon yield of 29.5% (isolated products).
• Preliminary TEA indicates a positive balance with ~6.4% rate of return at 99% solvent recovery; economics are sensitive to solvent recovery and FDCA market price, with FDCA and furfural as key revenue streams.

The work demonstrates that lignocellulose can be converted into fully bio-based PET analogues by combining RCF-derived lignin monomers and cellulose-derived aromatic diacids. The catalytic funneling strategy selectively transforms heterogeneous phenolic mixtures into a single aliphatic diol (PC), thereby simplifying downstream polymer synthesis and overcoming costly purification challenges. The resulting polyesters match or exceed PET’s glass transition temperatures and are readily chemically recyclable in methanol, satisfying circular economy objectives. Integrating fuel production from non-polymerizable fractions improves carbon utilization and product portfolio. TEA suggests feasibility at early scale, particularly if solvent recovery is optimized and FDCA yields are increased, indicating that the approach can be competitive as bio-based chemical markets mature.

This study introduces a noble-metal-free, integrated biorefinery approach to produce PET analogues entirely from wood. A Cu2O-PMO-based RCF followed by Raney Ni catalytic funneling delivers a single diol (PC) in high yield from complex lignin streams, enabling efficient copolymerization with cellulose-derived DMFD or DMTA to yield polyesters with competitive thermal and molecular weight properties. The polymers are amenable to high-yield methanolysis and repolymerization, supporting circular use. Additional lignin-derived fractions are upgraded to gasoline and jet-range alkanes, achieving 29.5% carbon yield from lignin. Future work should improve RCF monomer yields (e.g., continuous flow), further optimize solvent usage and recovery, increase FDCA yields (target ~80%), and enhance polymer molecular weights and mechanical properties via comonomer selection and PC purification, thereby strengthening techno-economic viability and scalability.

• Lab-scale process: Yields and selectivities are demonstrated at gram-scale with batch operations; scale-up may reveal challenges in mass/heat transfer and catalyst handling.
• Solvent dependence: Process economics are sensitive to high solvent recovery (isopropanol and methanol); losses impact returns significantly.
• Catalyst deactivation: Direct processing of crude RCF oil led to deactivation, necessitating EtOAc extraction; robustness toward impurities must be improved.
• Polymer properties: Although Tg and Mw are competitive, T5% is lower than commercial PET; mechanical properties require further optimization and may depend on comonomer composition and PC purity.
• Market dependency: Profitability hinges on FDCA pricing in the absence of a mature market; TEA outcomes are sensitive to assumed FDCA values.
• Product mixture: Biomass-derived diol mixtures (PC with diol 1) can reduce polymer Mw; additional purification or process control may be necessary.