Selective lignin arylation for biomass fractionation and benign bisphenols

a,b, HPLC analysis of purified S-VGs and a S-VG mixture from the α-arylation (syringolation) of VG. The purified mS-V′G and pS-V′G were obtained by flash-column separation of the S-VG mixture from the syringolation reaction. The purified mS-VG and pS-VG were obtained by deformylation of the purified mS-V′G and pS-V′G. The syringolation reaction condition: VG (0.25 mmol); syringol (0.75 mmol, 3 equiv.) in 5 mL of 88 wt% formic acid; reaction time, 1 h; reaction temperature, 80 °C. c, ¹H–¹³C HSQC NMR spectra of VG, purified S-VG isomers, and the crude mixture of S-VGs after the syringolation reaction under the typical condition.

Exemplified by the mS-VG model, the major pathway 1 is initiated by protonation of the β-ether’s to cleave the β-aryl ether bond, resulting in a β-carbocation. The electron-rich aromatic ring (aided by the methoxy at the ortho- or para-position) then migrates to the β-carbocation in a classical anchimerically assisted (neighboring group participation) fashion to produce a spiro-phenonium intermediate. The γ-hydroxymethyl group undergoes a formaldehyde elimination reaction that cleaves the C_β–C_γ bond, liberating formaldehyde and producing stilbene (P1). The ethylene bridge is then subject to hydrogenation, resulting in the formation of a more stable 1,2-diarylethane product (P2). The formaldehyde elimination plays a role in facilitating aryl migration but is not strictly essential, since evidence confirms a deprotonation pathway to P4. In the minor pathway 2, dearylation occurs predominantly when acid-catalyzed aryl migration is impeded. The syringyl group was protonated, resulting in rupture of C_α–C_aryl bonds to release syringol. In the minor pathway 3, demethylation is initiated by the protonation of the syringyl group and is supposed to go through a water addition pathway to yield the demethylated products (P9 and its isomers). B^– denotes CH₃C₆H₄SO₃^– or conjugate bases of other strong acids; R¹ denotes H or OH.

a,b, TCF-bleached pulp from the CLAF of pulp in 88 wt% FA at 120 °C for 30 min. a, Images of pulp isolated by the formic acid fractionation (FFP), pulp isolated by the lignin-syringolated fractionation (SFP), and bleached SFP (BSFP). b, Pulp compositions determined by the Klason method. Error bar represents s.d. based on two replicates. The bleached pulp (BSFP, 95.3% bleaching yield) reached up to 90% ISO brightness with negligible loss of cellulose chain length (DP 1065). The BSFP composition included cellulose (94.0%) and xylan (2.9%) together with only traces of Klason lignin which was confirmed by the pulp’s low Kappa number (0.6). c, Cellulase digestibility of SFPs. SFP (1 g, o.d.) was first pretreated by acidic deformylation (120 °C in 10 mL of 1 wt% sulfuric acid for 60 min) prior to cellulase-based saccharification. Enzymatic saccharification parameters: 50 °C, 10 mL of 0.05 M acetic acid/sodium acetate buffer, 20 FPU g^–1_substrate of Novozymes Cellic Ctec 2 loading. Yields of glucose and xylose were calculated based on the initial weight of SFP. d, Yields of xylose, furfural, and total soluble xylose (total sol. xylose), and syringol recovery in the hydrolysate from the CLAF of poplar. Note: Total sol. xylose was determined by post-hydrolysis of the fractionation hydrolysate in 4 wt% H₂SO₄ at 121 °C for 1 h. Increasing the fractionation temperature increased xylose yields from 8.1% (80 °C for 1 h) to 64.5% (120 °C for 0.5 h). The total sol. xylose yield increased to 90.5%, taking other forms of xylose (e.g., oligomers and xylose formylates) into account. At 120 °C, xylose yields were relatively independent of syringol addition, indicating that syringol condensation with xylose was unlikely to occur. Recovery of unreacted syringol decreased at higher temperatures, in accordance with the increased S-lignin yields, indicating that syringol was favorably coupled into the S-lignin.

a, Yield of monomers and bisphenols referenced to the β–O–4-aryl ether content in native lignin, m/p denotes the yield of [BP2(mSG)+ BP10(mSS)] divided by the yield of [BP5(pSG)+ BP12(pSS)]. b, Mass yields of syringol, monophenols, and bisphenols based on the Klason lignin content in poplar. S-lignin was isolated by the CLAF of poplar at 120 °C for 30 min in 88% FA. The SG and SS bisphenols represent the major bisphenols (accounting for up to 70.3 wt% of the total bisphenols). c,d, A typical GC-MS spectrum and peak assignment of monophenols (c) and bisphenols (d) that were generated in the tandem catalytic depolymerization of S-lignin. Effective carbon number (ECN) of the corresponding trimethylsilyl (TMS)-derivatized compound was used to calibrate the flame ionization response for quantitative analysis by GC-FID. Empirical ECN contribution for carbon atom, oxygen atom, and TMS was +1, –1 (–0.5 for primary alcohol), and 3, respectively. The four major bisphenols (mSG, pSG, mSS, and pSS) were isolated as pure crystals and quantified using authentic purified compounds as external standards.

a, Quantitative calibration of bisphenol crystals by GC-FID using dodecane as the internal standard. The peak area of the silylated bisphenol was divided by the peak area of dodecane for calibration of the area ratio. b, ¹H–¹³C HSQC NMR spectra of the four major bisphenols from S-lignin. c, Mass spectra of the four bisphenols isolated from tandem catalytic depolymerization of the syringolated lignin. The spectra were collected from GC-MS and the values were recorded from high-resolution mass spectrometry.

a, Polyarylates were synthesized using mSG and mSS for BPA replacement. b, poly(aryletherketone) was synthesized using mSG for BPA replacement. c, Epoxy resins were synthesized using mSG and S/P-lignin oligophenols for BPA replacement. S-lignin oligophenols (SLO) were harvested from the reflux extraction of S-lignin oil (RF3), and the P-lignin oil mixture (PLOM) was collected from the tandem catalysis of P-lignin without further separation/purification. Note: The chemical structures of SLO and PLOM are used for illustration, and may not be representative of all the moieties in the mixtures.

a, Fractionation of native birch lignin to bisphenols by selective phenolation and subsequent tandem catalytic depolymerization. b, Comparison of aliphatic regions in HSQC spectra. Ball-milled raw birch was dispersed in DMSO-d₆/pyridine-d₅ (4:1). Phenolated lignin (P-lignin) and products from tandem catalytic depolymerization (P-lignin oil) were dissolved in DMSO-d₆ and CDCl₃, respectively. P-lignin was isolated at 100 °C for 2 h in 88 wt% FA with 80 wt% phenol loading. The tandem catalytic depolymerization of P-lignin was conducted at 200 °C for 6 h catalyzed by 2 g l^–1 pTSA and Pd/C in MeOH:H₂O (7:3). c, Mass yields of bisphenols and monophenols from the tandem catalytic depolymerization of P-lignin. Yields of lignin oils were calculated gravimetrically to be 70.5 wt% (0.5 h), 63.3 wt% (2 h), 68.6 wt% (6 h), 71.5 wt% (12 h), 73.7 wt% (L/S: 10), 73.4 wt% (L/S: 8), 69.6 wt% (L/S: 5), 79.8 wt% (L/S: 5^a), and 76.4 wt% (L/S: 5^b). The tandem catalysis at varied times was loaded with the P-lignin feedstock isolated from birch using the 1-L high-pressure reactor with an impeller. The fractionation process, involving varied liquid-to-solid (L/S) ratios, was conducted using a rotary digester, enabling high birch loading. Subsequently, the tandem catalysis was performed at 200 °C for 2 h. Note: ^a and ^b denote 60 wt% and 40 wt% phenol loadings (based on the loading weight of birch) in the CLAF.

a, A GC-MS spectrum and peak assignment of monophenols and bisphenols in the tandem catalytic depolymerization of P-lignin (pPG and pPS in the main text refer to peaks of BP26 and BP30, respectively). b, GC calibration curves of pPG and pPS, both of which were referenced by the internal standard (dodecane). c, HSQC NMR spectra of pPG and pPS in CDCl₃. Note: the pPG and pPS crystals were isolated by reflux extraction followed by flash-column separation, and then purified by recrystallization from CHCl₃ and heptane.

The range of bisphenol exposure concentrations in the RLA test was refined to keep a minimum of 80% cell viability as referenced to the blank. Error bar represents s.d. based on 3 replicates.

Note: Bisphenols in Ref. A and B were synthesized by crosslinking lignin RCF monomers with a formaldehyde^16,35; bisphenols in Ref. C were synthesized by a Friedel-Crafts-type alkylation between vanillyl alcohol and guaiacol¹⁹; bisphenols in Ref. D were synthesized by a Friedel-Crafts-type alkylation between isoeugenol and guaiacol²⁰. To validate the comparison, all the EC₅₀ and RLA data were collected in Ref. A–D based on the results from the in vitro MELN study, which are comparable to the in vitro MVLN assay in this study. Bisphenols (same with those in Ref. C) were investigated using the in vitro VM7Luc4E2 cells^36,37, and the results showed similar benign properties, but were not included here in part because of the differences in the evaluation system.