Phenolation to Improve Lignin Reactivity toward Thermosets Application

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Phenolation to Improve Lignin Reactivity towards Thermosets Application Xiao Jiang, Jie Liu, Xueyu Du, Zhoujian Hu, Hou-Min Chang, and Hasan Jameel ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00369 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Sustainable Chemistry & Engineering

Phenolation to Improve Lignin Reactivity towards Thermosets Application Xiao Jiang, Jie Liu, Xueyu Du, Zhoujian Hu, Hou-min Chang* and Hasan Jameel Department of Forest Biomaterials, North Carolina State University, Campus Box 8005, Raleigh, NC 27695, USA *Corresponding author: E-mail: [email protected]

ABSTRACT Phenolation can be used to improve the reactivity and decrease the molecular weight of lignin, thereby making it more useful for various applications. We repot an effective phenolation process with only a catalytic amount of sulfuric acid and using phenol as solvent. The optimum phenolation conditions for pine kraft lignin and sweetgum biorefinery lignin were determined to be Lignin/Phenol (L/P, wt/wt) of 3/5, 5% acid charge at 90 °C for 2 hours and L/P of 2/5, 5% acid charge at 110 °C for 2 hours, respectively. Phenolation resulted in introducing 30 wt% of phenol onto pine kraft lignin and 60 wt% of phenol onto sweetgum biorefinery lignin and significantly decreasing in the molecular weight of lignin. Phenol was incorporated onto both the sidechains and aromatic nuclei of lignin. All lignin substructures of β-O-4’, β-5’/α-O-4’, β-β’, αcarbonyl etc. were reacted, resulting in significant decrease in aliphatic hydroxyl groups and increase in the phenolic hydroxyl groups. The comprehensive characterization revealed that most of ethers linkages were cleaved during phenolation. The β-elimination of the γ-hydroxymethyl group as formaldehyde was the main reaction of sidechains. The released formaldehyde reacted 1 ACS Paragon Plus Environment

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with phenol and lignin to form diphenylmethanes. Plausible mechanisms for lignin phenolation are also discussed. KEYWORDS Phenolation, Softwood kraft lignin, Hardwood biorefinery lignin, β-elimination INTRODUCTION Lignin is a cross-linked natural phenolic polymer that is composed of phenylpropane (C9) units. Because of the structural similarity and its availability at low cost, technical lignin has been an attractive phenol substituent in the synthesis of phenol-formaldehyde (PF) resin.1-3 Technical lignins are produced as byproduct from pulp and paper industry and biorefinery processes for the production of biofuels and biochemicals, and are mainly burnt to recover chemicals and energy. Their structure could undergo drastic modification during the chemical processing, resulting in very low reactivity for various applications. Compared to phenol, lignin has lower number of active sites in the aromatic nuclei for cross-linking. Many efforts have been made to improve the reactivity of lignin, such as methylolation (hydroxymethylation),4-5 demethylation,6-7 amination,8-9 and phenolation.10-16 Among these methods, phenolation is one of the most promising modifications. Lignin is treated with phenol under acidic conditions, leading to the condensation of phenol with lignin side chains. The reactivity is improved by directly increasing the phenolic hydroxyl (Ph-OH) groups and readily available reactive sites. Phenolation has been studied under various conditions. Ono et al. reported phenolation of steam explosion lignin conducted at 170 °C for 4 hours with a lignin/phenol (L/P, wt/wt) of 2/1.16 Funaoka et al. developed a phase separation reaction system where lignin was selectively phenolated at Cα position without severely cleaving original interunit linkages.10 Their

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phenolation reactions were performed at room temperature and it required a large amount of phenol derivative and sulfuric acid (10 and 32 times weight of the wood meal, respectively). Matsushita et al. reported that sulfuric acid lignin (condensed lignin) also can be phenolated and syringyl unit has higher reactivity than guaiacyl unit during phenolation.11 However, they used a very high sulfuric acid (72%) charge at 25 times the weight of lignin and reacted at 60 °C for 6 hours. Recently, Podschun et al. reported phenolation of organosolv lignin with L/P (wt/wt) of 1/2, a sulfuric acid (98%) charge of 20 wt% based on lignin at 110 °C for 20 minutes.12 They concluded based on the

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P NMR data that phenol was introduced into lignin not only at Cα

position but also at Cγ position. In all these studies, the use of massive amounts of concentrated sulfuric acid makes it economically unattractive, as the acid needs to be neutralized and disposed. Moreover, most of the lignins used in these previous studies are not commercially available. Thus, it is worthwhile to study and develop a more commercially feasible phenolation process based on commercially available lignins. In the present study, a more economical and industrially feasible phenolation process was developed using only a catalytic amount of sulfuric acid. Two different lignins were used for comparison, namely a commercial softwood kraft lignin and a hardwood lignin isolated from biorefinery process involving autohydrolysis and enzymatic hydrolysis. Different factors (such as L/P ratio, acid charge and reaction temperature) were examined to determine optimum phenolation conditions. Thereafter, the phenolated lignins were fully characterized by various analytical methods to elaborate on plausible phenolation pathways. EXPERIMENTAL Materials



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BioChioceTM lignin (BCL), a pine (softwood) kraft lignin produced by the LignoBoost process at the Plymouth pulp mill (NC), was kindly provided by Domtar. BCL was thoroughly washed with deionized water and dried prior to use. The other lignin was a hardwood (sweetgum, Liquidambar) lignin isolated as an enzymatic hydrolysis residue by our biorefinery process involving autohydrolysis followed by mechanical refining and enzymatic hydrolysis.17-19 The composition analysis was performed using the standard protocol provided by the National Renewable Energy Laboratory.20 The autohydrolysis-enzymatic hydrolysis residue (AER) contains 82.4% lignin, 11.6% polysaccharides and 6.0% others. When AER was extracted with methanol/acetone (1:6 v:v, a good lignin solvent), only 30% was soluble (AER-S), the rest 70% was insoluble (AER-I). BCL is totally soluble and contains 97% lignin, 2% polysaccharides and 1% ash.21 Lignin phenolation Phenolation was performed as shown in Scheme S1, and the L/P ratio, acid charge and reaction temperature were investigated (Table S1). A mixture containing 1.2 g lignin (BCL or AER), a corresponding amount of phenol and 98% (72% for AER) sulfuric acid was charged into a three-neck flask equipped with a reflux condenser and a mechanical stirrer. The flask was heated by submerging it in a temperature-controlled oil bath. After stirring at defined temperature for 2 hours, the flask was quenched by cold tap water and 20 ml of ethyl acetate was added to dissolve (or partially dissolve in the case of AER) the whole mixture. The solution (or suspension) was precipitate into 200 ml petroleum ether, the insoluble fraction was filtered out and washed thoroughly by petroleum ether to remove excess phenol. Then, the insoluble solid was washed with DI water till the eluate became neutral and dried as phenolated lignin (P-BCL or P-AER). For phenolation of AER, since the P-AER was not fully soluble in ethyl acetate, 4 ACS Paragon Plus Environment

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another extraction was performed. P-AER was extracted with methanol/acetone mixture, the soluble fraction was collected and evaporated to dry as P-AER-S, and the insoluble fraction was dried as P-AER-I. Gel permeation chromatography (GPC) for molecular weight determination Prior to analysis, acetylation was accomplished to enhance lignin’s solubility.22 GPC analyses were performed using a Shimadzu instrument equipped with a UV detector (set at 280 nm) using tetrahydrofuran (THF) as the eluent at a flow rate of 0.7 ml/min at 35 °C. A sample concentration of 1 mg/ml and an injection volume of 50 µL were used. Two ultra styragel linear columns linked in series (Styragel HR 1 7.8 x 300mm and Styragel HR 5E 7.8 x 300mm) were used. A series of monodispersed polystyrene standards were used as calibration standards. α-Carbonyl, phenolic hydroxyl, and stilbene contents by UV spectrophotometry The UV-Vis spectroscopy was used to determine the α-carbonyl, Ph-OH and stilbene contents.23 The detailed procedure was exactly the same as described in our previous work.21 The α-carbonyl content was calculated based on the difference in absorbance at 305 nm between neutral and sodium borohydride reduced lignin samples. The Ph-OH and stilbene contents were calculated based on ionization difference spectra at 300 and 378 nm respectively. Because of stilbene structure’s negative effect in the ionization spectra, the Ph-OH content was corrected to give a range. Since only 30% of AER is soluble, this method was not used to characterize AER and P-AER. The UV spectra were recorded on a HP8453E UV-Vis spectrophotometer (Hewlett Packard Company, Palo Alto City, CA, USA). NMR analyses



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P NMR analysis was conducted to determine the hydroxyl content following a reported

method.24-25 The 31P NMR spectrum was acquired using a Bruker 300 MHz spectrometer with a Quad probe. The 2D HSQC NMR spectra were recorded using a Bruker 500 MHz spectrometer according to the procedures described by Capanema, et al.26 The 2D HSQC NMR spectra were acquired on the spectrometer equipped with a 5 mm double resonance broadband BBI inverse probe using a coupling constant J1 C-H of 147 Hz. The experimental parameters used were 160 transients (scans per block) acquired using 1K data points in F2 (1H) dimension for an acquisition time of 151 ms and 256 data points in F1 (13C) for an acquisition time of 7.68 ms for a total of 16.5 h. RESULTS AND DISCUSSION Yields and molecular weights of phenolated lignins In our phenolation process, phenol is used both as a reactant and as a solvent for lignin. The objectives are to achieve maximum phenol incorporation and lignin fragmentation with the minimum amount of acid catalyst and maximum L/P ratio. For phenolation of BCL (Table S2), at a given acid charge, the L/P ratios have only a small effect on yield but a strong effect on lignin fragmentation as indicated by weight average molecular weight. Acid charge had significant effects on both yield and molecular weight. The highest yields were found to be at 5% sulfuric acid charge (130% at the L/P of 1/5 and 3/5). Higher acid charge (10% and 15%) did not give higher yield than 5% sulfuric acid (data not shown). P-BCL reached a maximum level under L/P of 3/5 and 5% acid charge at 90 °C. The yield of 130% indicated that 33 g of phenol was incorporated based on 100 g of BCL (calculation S1). There exist two general pathways for incorporation of phenol. Phenol may add directly to a carbocation without replacing a hydroxyl group, which would result in the addition of 94 g/mol of phenol added. Phenol may also add by 6 ACS Paragon Plus Environment

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replacing a hydroxyl group on the lignin sidechain, which would add 76 g/mol of phenol added. Both scenarios probably exist in the actual situation. Thus, the incorporation of 33g of phenol was equivalent to 0.65-0.81 mole of phenol per C9 unit of BCL, or 3.6-4.5 mmol/g of lignin, assuming the C9 unit weight of 180 g for BCL27. Accordingly, a range will be given for phenol incorporation onto lignin throughout this paper. As for phenolation of AER, the situation is quite different from BCL. Only 30% of AER (AER-S) was soluble, its molecular weight (MW) was higher than that of BCL (Table S3). It can be safely assumed that the AER-I had much higher molecular weight than that of AER-S. In addition, AER was obtained from a hardwood under a milder condition (autohydrolysis vs kraft pulping). Thus, lignin in AER is expected to be structurally different from BCL, i. e. less condensed and less altered, albeit higher molecular weight. Since AER has a higher molecular weight than BCL, higher acid charge, lower L/P ratio and higher temperature than those employed for BCL were investigated. The maximum yield of P-AER-S was monitored as an additional criterion for lignin fragmentation (Table S3). The L/P ratio had opposite effect on PAER-S yield compared to P-BCL where higher L/P ratio resulted in lower yield. This discrepancy was unexpected and needs further investigation. One possible explanation could be that at low L/P ratio, there was not enough acid to hydrolyze and phenolate the AER as acid charges are based on the amount of AER. The effects of acid charge and reaction temperature can be concluded as follows: (1) at the same reaction temperature, high acid charge gave high PAER-S yield; (2) with the same acid charge, the higher reaction temperature, the higher yield. However, there was one exception. When phenolation was undertaken with at L/P of 2/5, 10% acid charge at 110 °C, both the soluble and the total yields are lower than those obtained either at 10% acid charge at 90 °C or at 5% acid charge at 110 °C. It might be because that the reaction



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conditions were too severe that considerable degradation occurred and some phenolated lignin became soluble in petroleum ether. The yield of P-AER-I seems only affected by acid charge and reaction temperature, the higher acid charge and/or reaction temperature, the lower P-AER-I yield. Taken data in Table S3 into consideration, P-AER-S was isolated, under the conditions: L/P of 2/5, 5% acid charge and 110 °C for further structural study. The total yield of P-AER under the above conditions was 145% with 129% of P-AER-S and 16% of P-AER-I. It is equivalent to the incorporation of 1.28-1.60 mole of phenol per C9 unit of lignin in AER or 6.48.0 mmol/g of lignin, assuming the C9 unit weight of 200 g and assuming no phenolated lignin became soluble in petroleum ether (Calculation S2). The molecular weight of P-BCL increased as the L/P ratio increased (Table S2). Even though the P-BCL yields were almost same when the L/P ratio were 1/5 and 3/5 (at same acid charge), the number average molecular weights were lower at L/P of 1/5. This implied that more fragmentation reactions occurred when L/P ratio was 1/5. The effect of acid charge was obvious in that the higher the acid charge, the lower the P-BCL molecular weight. As for P-AER-S (Table S3), the L/P ratio did not have a significant effect, whereas the effects of acid charge and reaction temperature were significant. The phenolation of AER tended to generate lower molecular weight P-AER-S at higher reaction temperature with more acid. Despite the incorporation of a large amount of phenol, the molecular weight of phenolated lignin decrease substantially. This is especially true for AER, as the molecular weight of AER-S (30% of AER); the rest is presumably of even higher molecular weight. Functional groups estimated by UV spectroscopy The α-carbonyl, Ph-OH and stilbene contents in BCL and P-BCL were estimated by UV spectroscopy (Table S4). As expected, the α-carbonyl and stilbene contents decreased, and Ph8 ACS Paragon Plus Environment

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OH content increased after phenolation. However, the Ph-OH contents determined by UV spectroscopy for BCL could be underestimated due to the high pKa of second proton in biphenyl and catechol substructures, which may not be fully ionized. For the Ph-OH content of P-BCL, an additional factor came into play as increase in Ph-OH content in phenolated BCL resulted from both the incorporation of phenol and the cleavage of aryl ethers (such as β-O-4’, α-O-4’, vinyl ether and dibenzodioxocin (DBDO)). The molar extinction coefficients of ionized p-hydroxyl phenyl units (2,500 L-cm/mol) at 300 nm are lower than that of the guaiacyl units (4,100 Lcm/mol). The latter was used for the Ph-OH content, resulting in further underestimation. Despite these inaccuracies, the trend in the increase of Ph-OH contents on phenolation is obvious. More accurate characterization by 31P NMR will be discussed later. The net changes of α-carbonyl, Ph-OH and stilbene contents (Table S4) were calculated (Equation S1). There was no significant difference between L/P of 1/5 and 3/5, whereas the αcarbonyl, Ph-OH and stilbene contents showed a smaller change when L/P ratio was 4/5. The effect of acid charge on α-carbonyl and stilbene change was insignificant among all L/P ratios. But it exhibited significant impact on increase of Ph-OH content. These results suggest that αcarbonyl and stilbene substructures react readily with phenol even at low acid charge whereas hydrolysis of aryl ethers, which creates Ph-OH groups, is strongly dependent on the acid charge. The highest Ph-OH increase was observed when L/P of 3/5 and 5% acid charge. Thus, phenolation of α-carbonyl and stilbene structures does not require high acid charge. On the contrary, acid charge is vital to reach high degree phenolation of other lignin substructures, i.e. β-O-4’, β-β’, β-5’ etc. Phenolation of BCL at L/P of 3/5 with 5% sulfuric acid showed the best result considering phenolated lignin yield and molecular weight as well as process feasibility. Thus, P-BCL from these phenolation conditions was selected for further characterization.



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Contents of various hydroxyl groups by 31P NMR The aliphatic and phenolic hydroxyls contents in raw and phenolated lignins were determined by

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P NMR (Table S5). The obvious increase of p-hydroxyphenyl (H-OH) and

decrease of aliphatic hydroxyl (aliph-OH) groups were observed, indicating that significant amount of phenol was incorporated onto lignin and substantial structural changes occurred on the sidechain. The aliph-OH content of BCL was 2.11 mmol/g, which is in good agreement with our previous study,21, 27 and decreased to 0.95 mmol/g upon phenolation. The decreased aliph-OH content in the phenolated lignin has been reported in the literatures.9, 12, 28-32 Comparing the H-OH in BCL and P-BCL, there was an increase of 3.14 mmol/g (calculated using Equation S1), which was lower than the phenolation yield (3.6-4.5 mmol/g). The discrepancy may be due, at least partly, to the fact that phenol may incorporated through both the para- and ortho-positions. In 31P NMR, only para-substituted Ph-OH group is determined as HOH, while the ortho-substituted Ph-OH group is determined as G-OH.33-34 The small increase in G-OH in P-BCL is consistent with this argument. Another source of error could come from the suggested integration range for H-OH.35 The H-OH peaks were very large and had tails running above 138.5 ppm (Figure S1), resulting in underestimation. The 5-substituted-OH was also increased. This increase is due to the cleavage of ethers in some substructures (such as β-5’ and DBDO) and to the elimination of γ-hydroxymethyl group as formaldehyde and the subsequent incorporation of phenol/formaldehyde onto the uncondensed G-unit of BCL, the evidence for the latter will be discussed along with data from the 2D HSQC NMR. The AER-S contains aliph-OH of 4.30 mmol/g (0.86 mol/C9) and total Ph-OH of 2.43 mmol/g (0.49 mol/C9). The P-AER-S has a low aliph-OH content and a much higher total Ph-OH content along with a high newly created H-OH content. The decrease in aliph-OH content and 10 ACS Paragon Plus Environment

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the increase in total Ph-OH content and H-OH were much larger for AER than BCL, which are consistent with the larger yield increase of P-AER. Again, the amount of phenol introduced into AER estimated by increase of H-OH (4.5 mmol/g, Table S5) did not match the lignin yield increase of P-AER (6.4-8.0 mmol/g). Probable reasons of the discrepancy were the same as PBCL discussed previously. The 5-substituted-OH and the G-OH were also increased, same explanations were discussed before. 2D HSQC NMR The original lignin substructures were assigned by comparison with the published literature (Figure 1 and Table S6).27, 36-41 The prominent signals observed in side chain region (δC/δH 50–105/2.5–6.0 ppm) were β-O-4’ (A, A’), β-5’/α-O-4’ (B), and β-β’ (C) linkages, which were present in the spectra of both BCL and AER-S. For BCL, the substructures of secoisolariciresinol (E), G-CH(OH)-COOH (F), coniferyl alcohol (I) were also observed. The signals of polysaccharide were detected in both BCL and AER-S, such as β-D-(1→4) linked xylosyl units (X) in BCL and β-O-4’/ α-O-alkyl (BE) units. These signals may be attributed to the residual carbohydrates present in the form of lignin-carbohydrate complexes (LCC). In addition, the stilbene (L) and vinyl ether (K) signals are present in the aromatic region (δC/δH 105-130/6.0-8.0 ppm) of BCL but are absent in ARE-S as expected.



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Figure 1. 2D HSQC NMR spectra of BCL, P-BCL, AER-S and P-AER-S

The 2D HSQC spectra of phenolated lignins showed enormous difference from those of the 12 ACS Paragon Plus Environment

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starting lignins, indicating that substantial structural changes occurred during phenolation process. All the signals of lignin substructures (A, B, C, E, F, I, X, BE) quantitatively vanished from their original chemical shifts along with three new signals appears in the same region; a new methoxyl signal (δC/δH 60.1/3.44 ppm), a new Cγ-hydroxyl signal (δC/δH 64.3/3.48 ppm) and a pair of signals (δC/δH 72.3/3.40 and 3.77 ppm). More importantly, many new signals appeared (δC/δH 30–55/2.0–5.5 ppm) in the spectra of both P-BCL and P-AER-S. The new signals were assigned by comparison with published literature and the predictions (with DMSO-d6 as solvent at 500 MHz) from Modgraph NMR Predict Desktop (version 4.983) running within MestReNova software (version 9.0.1). The plausible substructures present in phenolated lignin are shown in Figure 2 and the corresponding chemical shifts are listed in Table S7. The substructures of 1, 2, 3, 4, 5 and 6 were originated from substructures of β-O-4’ (A), β-5’/α-O-4’ (B), and β-β’ (C), theirs signals were both detected in P-BCL and P-AER-S. All substructures have only twocarbon sidechains except for 3. These results indicated that lignin underwent significant amount of β-elimination reactions after the formation of carbocation at Cα, resulting in releasing of formaldehyde. The 3 was the only one with Cγ-hydroxyl remain in phenolated lignin (3γ, δC/δH 64.3/3.48 ppm). These results are in good agreement with low aliph-OH contents obtained by 31P NMR for phenolated lignins. These results are also consistent with the disappearance of original Cγ signal in 13C NMR spectra after phenolation of spruce kraft lignin at 60 °C using 72% sulfuric acid reported Du et al.9 Low aliph-OH content in phenolated organosolv lignin were also reported by Podschun et al., they attributed this to the phenolation of Cγ-hydroxyl groups.12 However, with the exception of 11, we could not find any signal resulting from the phenolation of Cγ-hydroxyl group. The signals found for 8-10 clearly suggest that the β-elimination was favoured over phenolation of Cγ-hydroxyl group, as these diphenylmethanes can only be formed



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by acid catalysed condensation of phenol/formaldehyde with lignin. The β-elimination of γhydroxymethyl group is well-known and occurred in both lignin acidolysis and liquefaction (in the presence of phenol), especially when sulfuric acid was used as catalyst.28-32

Figure 2. Plausible substructures present in phenolated lignin

The 10 was formed by incorporation of formaldehyde and phenol at G2 (in biphenyl or other condensed structures) and/or S2/6. Interestingly, condensation at C2 position of biphenyl substructure and at either C2 or C6 position of the syringyl nuclei resulted in the down shift of the methoxyl signal from δC/δH 56/3.6 ppm to δC/δH 60/3.6 ppm. Thus, two methoxyl signals were observed for the phenolated lignins. It is reasonable that the signal intensity of 10 is higher in PAER-S than in P-BCL because of the presence of S units in hardwood lignin. The signals of 7 which is originated from the substructures of secoisolariciresinol (E), were only detected in PBCL. The signals of 11 were only observed in P-BCL and were assigned based on Lin et al.29 This structure was formed by the cleavage of β-O-4’ with the assistance of G6, resulting in a four-member ring, and further addition of phenol at Cγ. It is important to point out that the 14 ACS Paragon Plus Environment

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interunit linkages assigned above only described partial of the phenolated lignin structures, since the signals highly overlap with each other and the intensity of signals of Cα, Cβ from same substructure are different. The aromatic region (δC/δH 100–135/6.0–8.0 ppm) highlighted the differences in guaiacyl/syringyl/p-hydroxyphenyl (G/S/H) distributions. For BCL, the signals intensity of G2, G5 and G6 in guaiacyl nuclei decreased slightly after phenolation. This was because that the guaiacyl nuclei concentration decreased owing to introducing the p-hydroxyphenyl to lignin. Moreover, the signals assigned to guaiacyl units with α-carbonyls (G’2, δC/δH 111.5/7.51 ppm; G’6, δC/δH 122.9/7.56 ppm) decreased as well, indicating α-carbonyls reacted with phenol. There was a significant intensity increase of aromatic carbon signals ranging from 125 to 132 ppm, corresponding to Cα/β of stilbene moieties and C2/6 in p-hydroxyphenyl (H) units. The intensity increases mainly resulted from the increased p-hydroxyphenol units since the stilbene structure was reactive during phenolation process. As for AER-S, signals of syringyl (S2/6, δC/δH 104.0/6.74 ppm; S’2/6, δC/δH 106.3/7.35 ppm) units were observed as well as guaiacyl units (G2, δC/δH 110.4/6.95 ppm; G5 + G6, δC/δH 115.1/6.78 and 118.7/6.79 ppm). The signal intensities of syringyl units with α-carbonyls (S’2/6) disappeared after phenolation, indicating the reaction of α-carbonyls with phenol. The signals of G2 were shifted downfield (from δC/δH 110.4/6.94 ppm to 112.1/6.67 ppm)29,

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and the signal intensity also decreased

significantly. There was no signal of p-hydroxyphenyl (H) units detected in AER-S, whereas strong signals (H2/6 at δC/δH 125-132/6.5-7.5 ppm) appeared in P-AER-S spectra, suggesting considerable amount of phenol was introduced onto lignin. Comparing the 2D HSQC spectra before and after phenolation, it can be concluded that lignin substructures of β-O-4’, β-5’/α-O-4’, β-β’, α-carbonyl etc. are reactive during phenolation 15 ACS Paragon Plus Environment


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process. The introducing of phenol onto lignin results in disappearance of original side chain structures and emerging of p-hydroxyphenol units. Proposed phenolation mechanisms

Scheme 1 Phenolation mechanism of β-O-4’ substructure 16 ACS Paragon Plus Environment

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Based on our data and pertinent data reported in the literature, plausible phenolation mechanisms are proposed. For the phenolation of a β-O-4’ substructure (A) (Scheme 1). The first step of reactions under the acidic conditions is the formation of the benzylic carbocation a.10, 28, 32 From a, three reaction routes are involved, namely elimination of formaldehyde from the γcarbon to an enol ether K (route 1), addition of phenol through para position to b (route 2) and addition of phenol through ortho position to c (route 3). All the initial products K, b and c are intermediates and will undergo further reactions. The enol ether K reacts with phenol to form d, followed by elimination of the β-ether to form the intermediate e, which can react with another phenol to form 1 or 2. From b, elimination of β-ether affords the intermediate f (route 2a), which further undergo elimination of formaldehyde from the γ-carbon gives stilbene L. Protonation of the double bond in L and subsequent incorporation of a phenol at either side of the double bond give 1 or 2. The elimination of β-ether in b also can be assisted by the C2 and C6 carbons of G or S aromatic nucleus, resulting in a four-member ring in the side chain (route 2b, g). From g, the incorporation of the second phenol gives 11. From c, the elimination of β-ether is assisted by G unit to form the intermediate h, which further adds a phenol to form 3. There are two key points that are especially noteworthy, Firstly, incorporation of phenol onto the benzylic carbocation a may occur via either para (b and d) or ortho (c) position of phenol. The chemical shifts of the resulting benzylic C/H

are different between the para-substituted (1 and 2) and ortho-

substituted (3) structures (Table S7). Secondly, after the initial addition of the first phenol, the cleavage of β-aryl ether is assisted by the neighbouring phenyl groups to form relatively stable carbocation intermediates (c, f, and h). The neighbouring phenyl groups can be either the incorporated phenol (c and f) or the aromatic nuclei of lignin (G or S, h). The detail mechanism



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of the neighbouring group assisted cleavage of β-aryl ether, starting from b is shown in Scheme S2. Protonation of the β-aryl ether in b results in three intermediate carbocations, f, i and j. βelimination of γ-hydroxymethyl group in both f and i results in the formation of stilbene L, which is further phenolated to form 1 and 2. Rearomatization of j affords g, which is further phenolated at γ-hydroxyl group to form 11. The β-5’ (B) and β-β’ (C) react similarly (Scheme 2). The β-5’ (B) undergoes elimination of γ-hydroxymethyl after formation of benzylic carbocation to give stilbene (L’), which further react with phenol to form 4 or 5, depending on the phenol incorporation position (ortho or para). For the β-β’ (C), elimination of two formaldehydes from the γ-hydroxymethyl group gives highly conjugate diphenyl-butadiene intermediate, which incorporates a phenol to each double bond sequentially to form 6.

Scheme 2 Phenolation mechanism of β-5’ and β-β’ pinoresinol substructures As aforementioned, lignin undergoes a significant amount of β-elimination of γhydroxymethyl group as formaldehyde. The fate of the liberated formaldehyde is of great interest. The liberated formaldehyde can further react with lignin and phenol to form 18 ACS Paragon Plus Environment

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diphenylmethane linkages (Scheme 3). It can occur at G5 and G6 in the uncondensed G, G2 in biphenyl or condensed G structures and S2 or S6 in S structures to form 8, 9 and 10. Judging from the very strong new methoxyl signal mentioned above and the strong signals of 8 and 9 (Figure 1), we presume that most of the liberated formaldehyde reacts with the readily available phenol and then condensed with lignin.

Scheme 3 Mechanism of the formation of diphenylmethane substructures in phenolation The β-β’ secoisolariciresinol (E) form a cyclic ether 7 by elimination of water under reaction conditions (Scheme 4). 7 was found only in P-BCL since substructure E has only been report in softwood lignin.36, 42 BCL also contains two structures, stilbene (L) and enol ether (K), formed during the kraft pulping process. These structures will be phenolated according to the mechanism described in Scheme 1.



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Scheme 4 Fate of β-β’ secoisolariciresinol substructure in BCL during phenolation Conclusion Two lignin preparations, a pine kraft lignin (BCL) and a sweetgum biorefinery lignin (AER), were phenolated with low acid charge and in phenol as solvent aiming at improving technical lignin reactivity. The optimum phenolation conditions for BCL and AER were L/P of 3/5, 5% acid charge at 90 °C for 2 hours and L/P of 2/5, 5% acid charge at 110 °C for 2 hours, respectively. Phenolation results in introducing 30 wt% of phenol onto BCL and 60 wt% of phenol onto AER and a large decreased in the molecular weight of lignin. All lignin substructures of β-O-4’, β-5’/α-O-4’, β-β’, α-carbonyl etc. were reacted, resulting in decrease of aliph-OH and increase of Ph-OH groups. The β-elimination of the γ-hydroxymethyl group as formaldehyde was the main reaction of sidechains. The released formaldehyde reacted with phenol and lignin to form diphenylmethanes. The sophisticated comparison in lignin structures before and after phenolation allows insight into the phenolation mechanisms. Phenolated lignin contains more Ph-OH groups and free reactive sites, thereby improving it reactivity towards thermosets application as phenol substitute. Supporting information

Schematic phenolation process, neighboring group assistant mechanism, 31P NMR spectra, tables of phenolation conditions, phenolated lignin yield and molecular weight, UV,

31

P NMR data,

chemical shift assignments, detailed calculations and equation. 20 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research project was supported financially in part by a USDA grant through Domtar and in part by the Biomass to Biochemicals and Biomaterials Research Consortium. The authors are grateful to USDA, Domtar and members of the research Consortium. REFERENCES

1. Sen, S.; Patil, S.; Argyropoulos, D. S., Thermal properties of lignin in copolymers, blends, and composites: a review. Green Chemistry 2015, 17 (11), 4862-4887. 2. Liu, W.-J.; Jiang, H.; Yu, H.-Q., Thermochemical conversion of lignin to functional materials: a review and future directions. Green Chemistry 2015, 17 (11), 4888-4907. 3. Kai, D.; Tan, M. J.; Chee, P. L.; Chua, Y. K.; Yap, Y. L.; Loh, X. J., Towards lignin-based functional materials in a sustainable world. Green Chemistry 2016, 18 (5), 1175-1200. 4. Alonso, M. V.; Rodriguez, J. J.; Oliet, M.; Rodriguez, F.; Garcia, J.; Gilarranz, M. A., Characterization and structural modification of ammonic lignosulfonate by methylolation. J. Appl. Polym. Sci. 2001, 82 (11), 2661-2668. 5. Zhao, L.-w.; Griggs, B. F.; Chen, C.-L.; Gratzl, J. S.; Hse, C.-Y., Utilization of softwood kraft lignin as adhesive for the manufacture of reconstituted wood. J. Wood Chem. Technol. 1994, 14 (1), 127-145.



Page 22 of 27

6. Wu, S. B.; Zhan, H. Y., Characteristics of demethylated wheat straw soda lignin and its utilization in lignin-based phenolic formaldehyde resins. Cellul. Chem. Technol. 2001, 35 (3-4), 253-262. 7. Campion, S.; Suckling, I., Effect of lignin demethylation on oxygen delignification. Appita J. 1998, 51 (3), 209-212. 8. Matsushita, Y.; Yasuda, S., Reactivity of a condensed–type lignin model compound in the Mannich reaction and preparation of cationic surfactant from sulfuric acid lignin. J. Wood Sci. 2003, 49 (2), 166-171. 9. Du, X.; Li, J.; Lindström, M. E., Modification of industrial softwood kraft lignin using Mannich reaction with and without phenolation pretreatment. Ind. Crops Prod. 2014, 52, 729735. 10. Funaoka, M.; Matsubara, M.; Seki, N.; Fukatsu, S., Conversion of native lignin to a highly phenolic functional polymer and its separation from lignocellulosics. Biotechnol. Bioeng. 1995, 46 (6), 545-552. 11. Matsushita, Y.; Sano, H.; Imai, M.; Imai, T.; Fukushima, K., Phenolization of hardwood sulfuric acid lignin and comparison of the behavior of the syringyl and guaiacyl units in lignin. J. Wood Sci. 2007, 53 (1), 67-70. 12. Podschun, J.; Saake, B.; Lehnen, R., Reactivity enhancement of organosolv lignin by phenolation for improved bio-based thermosets. Eur. Polym. J. 2015, 67, 1-11. 13. Abarro, G. J.; Podschun, J.; Diaz, L. J.; Ohashi, S.; Saake, B.; Lehnen, R.; Ishida, H., Benzoxazines with enhanced thermal stability from phenolated organosolv lignin. RSC Advances 2016, 6 (109), 107689-107698.


Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14. Podschun, J.; Stücker, A.; Buchholz, R. I.; Heitmann, M.; Schreiber, A.; Saake, B.; Lehnen, R., Phenolated lignins as reactive precursors in wood veneer and particleboard adhesion. Ind. Eng. Chem. Res. 2016, 55 (18), 5231-5237. 15. Podschun, J.; Stücker, A.; Saake, B.; Lehnen, R., Structure–function relationships in the phenolation of lignins from different sources. ACS Sustainable Chem. Eng. 2015, 3 (10), 25262532. 16. Ono, H.-K.; Sudo, K., Wood Adhesives from Phenolysis Lignin. In Lignin: Properties and Materials, Glasser, W. G.; Sarkanen, S., Eds. American Chemical Society: Washington, DC, 1989; Vol. 397, pp 334-345. 17. Batalha, L. A. R.; Han, Q.; Jameel, H.; Chang, H.; Colodette, J. L.; Gomes, F. J. B., Production of fermentable sugars from sugarcane bagasse by enzymatic hydrolysis after autohydrolysis and mechanical refining. Bioresour. Technol. 2015, 180, 97-105. 18. Huang, C.; Jeuck, B.; Du, J.; Yong, Q.; Chang, H.; Jameel, H.; Phillips, R., Novel process for the coproduction of xylo-oligosaccharides, fermentable sugars, and lignosulfonates from hardwood. Bioresour. Technol. 2016, 219, 600-607. 19. Narron, R. H.; Han, Q.; Park, S.; Chang, H.; Jameel, H., Lignocentric analysis of a carbohydrate-producing lignocellulosic biorefinery process. Bioresour. Technol. 2017. 20. Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D.; Crocker, D., Determination of structural carbohydrates and lignin in biomass. Laboratory analytical procedure 2008, 1617, 1-16. 21. Hu, Z.; Du, X.; Liu, J.; Chang, H.; Jameel, H., Structural characterization of pine kraft lignin: BioChoice lignin vs Indulin AT. J. Wood Chem. Technol. 2016, 36 (6), 432-446.



Page 24 of 27

22. Glasser, W. G.; Davé, V.; Frazier, C. E., Molecular weight distribution of (semi-) commercial lignin derivatives. Journal of wood chemistry and technology 1993, 13 (4), 545-559. 23. Castellan, A.; Nourmamode, A.; De Violet, P. F.; Colombo, N.; Jaeger, C., Photoyellowing of

milled

wood

lignin

and

peroxide-bleached

milled

wood

lignin

in

solid

2-

hydroxypropylcellulose films after sodium borohydride reduction and catalytic hydrogenation in solution: An UV/VIS absorption spectroscopic study. J. Wood Chem. Technol. 1992, 12 (1), 1-18. 24. Granata, A.; Argyropoulos, D. S., 2-Chloro-4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaphospholane, a reagent for the accurate determination of the uncondensed and condensed phenolic moieties in lignins. J. Agric. Food. Chem. 1995, 43 (6), 1538-1544. 25. Cui, C.; Sun, R.; Argyropoulos, D. S., Fractional Precipitation of Softwood Kraft Lignin: Isolation of Narrow Fractions Common to a Variety of Lignins. ACS Sustainable Chem. Eng. 2014, 2 (4), 959-968. 26. Capanema, E. A.; Balakshin, M. Y.; Kadla, J. F., A comprehensive approach for quantitative lignin characterization by NMR spectroscopy. J. Agric. Food. Chem. 2004, 52 (7), 1850-1860. 27. Jiang, X.; Savithri, D.; Du, X.; Pawar, S. N.; Jameel, H.; Chang, H.; Zhou, X., Fractionation and Characterization of Kraft Lignin by Sequential Precipitation with Various Organic Solvents. ACS Sustainable Chem. Eng. 2016. 28. Ito, H.; Imai, T.; Lundquist, K.; Yokoyama, T.; Matsumoto, Y., Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin. Part 3: Search for the rate-determining step of a non-phenolic C6-C3 type model compound. J. Wood Chem. Technol. 2011, 31 (2), 172-182. 29. Lin, L.; Yao, Y.-g.; Shiraishi, N., Liquefaction mechanism of β-O-4 lignin model compound in the presence of phenol under acid catalysis. Part 1. Identification of the reaction products. Holzforschung 2001, 55 (6), 617-624.


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30. Lin, L.; Nakagame, S.; Yao, Y.; Yoshioka, M.; Shiraishi, N., Liquefaction mechanism of-O-4 lignin model compound in the presence of phenol under acid catalysts. Part 2. Reaction behavior and pathways. Holzforschung 2001, 55 (6), 625-630. 31. Cox, B. J.; Jia, S.; Zhang, Z. C.; Ekerdt, J. G., Catalytic degradation of lignin model compounds in acidic imidazolium based ionic liquids: Hammett acidity and anion effects. Polym. Degrad. Stab. 2011, 96 (4), 426-431. 32. Imai, T.; Yokoyama, T.; Matsumoto, Y., Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin IV: Dependence of acidolysis reaction on the type of acid. J. Wood Sci. 2011, 57 (3), 219-225. 33. Jiang, Z.-H.; Argyropoulos, D. S., Coupling P-31 NMR with the Mannich reaction for the quantitative analysis of lignin. Can. J. Chem. 1998, 76 (5), 612-622. 34. Jiang, Z. H.; Argyropoulos, D. S.; Granata, A., Correlation analysis of 31P NMR chemical shifts with substituent effects of phenols. Magn. Reson. Chem. 1995, 33 (5), 375-382. 35. Balakshin, M.; Capanema, E., On the quantification of lignin hydroxyl groups with 31P and 13C NMR spectroscopy. J. Wood Chem. Technol. 2015, 35 (3), 220-237. 36. Balakshin, M. Y.; Capanema, E. A.; Chen, C.-L.; Gracz, H. S., Elucidation of the structures of residual and dissolved pine kraft lignins using an HMQC NMR technique. J. Agric. Food. Chem. 2003, 51 (21), 6116-6127. 37. Capanema, E. A.; Balakshin, M. Y.; Chen, C.-L.; Gratzl, J. S.; Gracz, H., Structural analysis of residual and technical lignins by 1H-13C correlation 2D NMR-spectroscopy. Holzforschung 2001, 55 (3), 302-308. 38. Kim, H.; Ralph, J.; Akiyama, T., Solution-state 2D NMR of ball-milled plant cell wall gels in DMSO-d 6. BioEnergy Research 2008, 1 (1), 56-66.



Page 26 of 27

39. Yuan, T.-Q.; Sun, S.-N.; Xu, F.; Sun, R.-C., Characterization of lignin structures and lignin– carbohydrate complex (LCC) linkages by quantitative 13C and 2D HSQC NMR spectroscopy. J. Agric. Food. Chem. 2011, 59 (19), 10604-10614. 40. Huang, C.; He, J.; Narron, R.; Wang, Y.; Yong, Q., Characterization of Kraft Lignin Fractions Obtained by Sequential Ultrafiltration and Their Potential Application as a Biobased Component in Blends with Polyethylene. ACS Sustainable Chem. Eng. 2017, 5 (12), 1177011779. 41. Huang, C.; He, J.; Du, L.; Min, D.; Yong, Q., Structural characterization of the lignins from the green and yellow bamboo of bamboo culm (Phyllostachys pubescens). J. Wood Chem. Technol. 2016, 36 (3), 157-172. 42. Lundquist, K.; Rolf, S., On the Occurrence of Structural Elements of the Lignan Type (ß—ß Structures) in Lignins-The Crystal Structures of (+)-Pinoresinol and (±)-trans-3, 4Divanillyltetrahydrofuran. Holzforschung 1988, 42 (6), 375-384.


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For Table of Contents Use Only

Synopsis This work describes an effective phenolation process to improve lignin reactivity and plausible phenolation mechanism.


Phenolation to Improve Lignin Reactivity toward Thermosets Application

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