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. 2018 Sep 4;3(9):10433–10441. doi: 10.1021/acsomega.8b01978

CuSO4/H2O2-Catalyzed Lignin Depolymerization under the Irradiation of Microwaves

Jinhuo Dai 1, Gavin N Styles 1, Antonio F Patti 1,*, Kei Saito 1,*
PMCID: PMC6645013  PMID: 31459170

Abstract

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The increasing demand for renewable materials in the world has resulted in sustained efforts to utilize biomass in a better way. Lignin, a natural and abundant polymer in plants, has provided an ongoing challenge for many researchers seeking ways to better utilize this abundant resource. Here, we report a very efficient lignin depolymerization strategy with the assistant of microwave radiation. Copper sulphate (CuSO4) and hydrogen peroxide (H2O2) were used to generate hydroxyl radicals to depolymerize lignin under the irradiation of microwaves. Three different types of lignin, organosolv lignin, kraft lignin, and alkali lignin, were all successfully depolymerized using microwave irradiation at a temperature of 110 °C for 7 min. The use of 1H/13C two-dimensional nuclear magnetic resonance spectroscopy enabled the confirmation of structural changes, comparing before and after depolymerization. Liquid chromatography–mass spectrometry was used to characterize the products. Both monomers and oligomers were detected after depolymerization.

Introduction

The increasing concern regarding the shortage of energy and the environmental pollution has led to the sustained research effort to investigate abundant, renewable, and cleaner materials to produce energy, fuels, or chemicals.1−3 Biomass, a key building block of biofuels and composed largely of oxygen, carbon, and hydrogen, is one of the most competitive candidates.4−6 Lignin, a well-known renewable material, is more popular because of the potential to be used as an abundant resource for the production of aromatics and related phenolic fine chemicals.7−9 Lignin is a three-dimensional natural polymer with aromatic rings linked by several typical bonds, including β-O-4, α-O-4, 4-O-5, β–β, β-5, and β-1 bond (Scheme 1).4,7,9,11

Scheme 1. Proposed Lignin Structure with Typical Bonds.

Scheme 1

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Several strategies have been established to isolate lignin from biomass, including the kraft process, sulfur-free alkali (soda) pulping, and organosolv processes, generally associated with paper manufacturing.11−14 Kraft pulping is the dominant pulping process over the world accounting for about 90% of the entire production capacity.9,15 Paper mills use this method to isolate cellulose fibers by dissolving lignin and hemicellulose in a solution of sodium sulfite and sodium hydroxide.7 Sulfur-free alkali (soda) pulping process takes up about 5% of entire pulp production. In this method, 10–15% NaOH by weight, based on the raw material, is often used for the isolation of lower lignin content biomass from agricultural wastes.9,13 The lignin removal rate compared to the kraft process is lower, however the lignin produced is essentially sulfur-free.8 The presence of sulfur in lignin from the kraft process is potentially a barrier against the application of catalytical lignin depolymerization approaches because of the poisoning of the catalysts.9,16 The organosolv process commonly involves using organic solvents such as alcohols (e.g., methanol and ethanol) and organic acids (e.g. formic acid and acetic acid).4,17

Different lignin isolation processes normally produce lignin with various structures and also different contents of the typical linkages, as previously described. However, numerous papers have been published to establish pathways to degrade different types of lignin.7,11 Lignin catalytic cracking, hydrolysis, gasification, lignin reduction, and lignin oxidation are the most popular strategies to depolymerize lignin.7,18 Compared to other methods, lignin reduction and oxidation methods utilize milder conditions to degrade lignin, which can thus be suitable for the industry.19,20 Our group has previously reported several oxidative lignin depolymerization strategies21,22 and also reviewed the recent developments in the chemical degradation of lignin involving catalytic oxidation.23 Lignin catalytic oxidation methods normally show high selectivity in the products formed. It is an approach that is very promising for generating highly functionalized oligomeric and monomeric chemicals via lignin catalytic oxidation.10,23

For most of the lignin oxidation strategies, conventional heating is required, however, using microwave irradiation is another option.26 Gedye et al.24 compared oxidation results under conventional heating conditions with those under microwave irradiation and concluded several advantages from using microwave irradiation. They concluded that microwave heating is a highly efficient and potentially reaction-selective heating method. In some cases, the activation energies of reactions are more readily achievable under microwave irradiation. Polar materials can generate more free radicals compared to conventional heating.24,25 Liu et al.27 developed a method to depolymerize lignin with the assistance of microwave irradiation in isopropyl alcohol without catalysts. About 45% of low-molecular-weight products were obtained after the reaction, which contains ethanone, 1-(4-hydroxy-3-methoxyphenyl) (1), and ethanone, 1-(4-hydroxy-3, 5-dimethoxy phenyl) (2) as the monomeric products (Scheme 2).

Scheme 2. Two Phenolic Products from Lignin Depolymerization under Microwave Irradiation.

Scheme 2

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Zhu et al.28 also reported the depolymerization of lignin with selective Cα–Cβ bond cleavage with a catalyst ferric sulfate and the application of microwave irradiation. Lignin model compounds were utilized in these conditions. Their work described microwave-assisted methylation of benzyl alcohols in the lignin structure, which leads to lignin degradation and production of phenolic compounds from lignin.29 Wang et al.29 showed a pathway to depolymerize lignin using polyoxometalate as a catalyst in 1-ethyl-3-methylimidazolium acetate under microwave irradiation. Delignification was confirmed with the generation of lower-molecular-weight phenolic products.

It is well-known that copper catalyst is an excellent recipient of quantitative oxidation.25,30,31 Widespread interest has led to the development of copper catalyst systems for lignin degradation. Zhu et al.32 proposed a two-step lignin depolymerization strategy using a series of metal chlorides and metal sulphate salts, including CuCl2, CuSO4, FeCl3, MgSO4, and so forth. These salts were used as a catalyst for lignin methylation with methanol under the irradiation of microwave first, and Pd/C was used for methylated lignin hydrogenolysis for the cleavage of β-O-4 bonds afterward. In this case, the microwave and the copper catalysts were only used for the methylation step and not for the lignin degradation step. Pan et al.25 reported a microwave-assisted oxidation reaction of lignin model compounds with metal salts, including CuCl2, CrCl3, and MnCl2. In their study, these metal chloride salts were used together with H2O2 to oxidize lignin model compounds and not the real lignin. Goñi and Montgomery30 presented a microwave digest system, with CuO as a catalyst, to depolymerize lignin at a temperature of 150 °C for 90 min, which generated a large quantity of phenolic compound from geochemical lignin samples. However, this process required 90 min to degrade lignin, where the reaction time was longer than other microwave reactions. A novel microwave-assisted copper catalyzed lignin degradation method which can proceed readily warrants investigation.

Compared to other copper complexes, copper salts, such as CuSO4 and CuCl2, turn out to be an excellent competitor because of their good solubility in water, enabling water to be used as a reaction solvent.32−34 Additionally, these catalysts can be separated from the low molecular products and can be reused by a simple extraction after the depolymerization reaction. Herein, we report the use of CuSO4 in combination with H2O2 as a catalyst to depolymerize lignin under microwave irradiation. The whole process was completed within 7 min at a temperature of 110 °C. Lignin was depolymerized with the production of oligomeric and monomeric products. The method was shown to be widely applicable with three different types of lignin, including organosolv lignin, alkali lignin, and kraft lignin, all being successfully depolymerized. Two-dimensional nuclear magnetic resonance (2D-NMR) spectroscopy was used to confirm the cleavage of β-O-4 bonds, β–β bonds, and β-5 bonds after the reaction. Monomeric products, including vanillic acid, 4-hydroxybenzaldehyde, vanillin, syringaldehyde, 2-hydroxy-3-(4-hydroxyphenyl)propanal, 4-hydroxybenzoic acid, and 2-hydroxy-3-(4-hydroxy-3-methoxyphenyl)propanal were detected and characterized by the liquid chromatography–mass spectrometry (LC–MS) technique.

Results and Discussion

Model Microwave-Assisted Reactions with 1-Phenylethanol

In the beginning, 1-phenylethanol was used as a model compound for this reaction to test the oxidation ability of this microwave system. CuSO4 and H2O2 were chosen as the catalyst in this system. The percentage conversion of the secondary alcohol of 1-phenylethanol to a ketone was calculated based on the 1H NMR results, which indicated the ratio of acetophenone to the products. The signal of the terminal methyl group was used as a reference and comparison, which appears as a doublet at 1.41 ppm for 1-phenylethanol and as a singlet at 2.59 ppm for acetophenone.35 From the result, it was shown that the oxidation reaction only occurred if H2O2 was present, and the conversion was enhanced by the presence of CuSO4 (Supporting Information Table S1). The conversion was only 10%, when an autoclave with heat was used instead of microwave (Table S1, entry 5 in the Supporting Information). This also indicated that microwave is necessary for accelerating the reaction (Figure 1).

Figure 1.

Figure 1

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Oxidation reaction of 1-phenylethanol.

The optimization of reaction conditions was carried out at the same time. For each part, only one parameter was changed compared to the initial conditions. First, the effect of the dosages of CuSO4 and H2O2 were investigated. Figure 2 shows the results of oxidation with different amounts of CuSO4 and H2O2. Figure 2a shows that with the increase of CuSO4 dosage, the conversion increased from about 26 to 66%. When the amount of CuSO4 increased beyond 13% of the molar amount of 1-phenylethanol, the conversion remained steady at around 65%. This indicated that the dosage of CuSO4 could positively promote the oxidation reaction to generate more acetophenone.

Figure 2.

Figure 2

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Effect of CuSO4 dosage on the oxidation of 1-phenylethanol.

The effect of the quantity of H2O2 used in the reaction was also investigated. As the results show in Figure 2b, with the increase of H2O2 dosage, the conversion increased from about 13 to 66%. When the amount of H2O2 increased beyond 2.0 × 10–3 mL/mg to 1-phenylethanol, the conversion slightly decreased to 60%. This indicated that the dosage of H2O2 could also positively promote the oxidation reaction to generate more acetophenone.

In addition, the temperature and the reaction time were also investigated. However, the temperature did not significantly affect the oxidation conversion. When the temperature was increased from 80 to 110 °C with other conditions kept constant, the conversion showed no great difference (about 56 and 58%, respectively). Similarly, the conversion to the product did not show a significant shift when the reaction time was increased from 10 to 30 min.

Microwave Reaction with Lignin

The optimized reaction conditions established with the model compound were then applied to lignin. In this study, organosolv lignin, kraft lignin, and alkali lignin were selected to perform the lignin degradation reaction under microwave irradiation. These three lignin samples were treated with methanol to obtain a methanol-soluble part and a methanol-insoluble part to form the lignin with narrow polydispersity to follow the depolymerization easily and effectively. Six lignin samples including methanol-insoluble organosolv lignin, methanol-soluble organosolv lignin, methanol-insoluble kraft lignin, methanol-soluble kraft lignin, methanol-insoluble alkali lignin, and methanol-soluble alkali lignin were obtained; the molecular weights and polydispersity (D̵) of the six lignin samples are listed in Table 1, entry 1.

Table 1. Molecular Weights of Lignin Samples (Methanol-Soluble and Methanol-Insoluble) before and after the Reaction.

entry   MIO MSO MIK MSK MIA MSA
1 Mn//D̵ before the reaction 5600/3.6 1000/3.7 9000/5.2 5300/5.3 12 000/5.3 1000/3.8
2 Mn/D̵ after the reaction 400/34.6 400/4.3 600/5.6 400/5.5 600/6.2 500/4.4
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All six types of lignin samples were subjected to microwave treatments to effect depolymerization. For each reaction, the initial reaction condition, 0.1 mass ratio of CuSO4 to lignin (5 mg of CuSO4 to 50 mg of lignin) and 1 × 10–3 mL/mg of H2O2 to lignin (0.5 mL of H2O2 to 50 mg of lignin) were used to depolymerize lignin under microwave irradiation at the temperature of 80 °C for 10 min. After reactions, the solvents were removed to give the depolymerized lignin products, which were characterized by size exclusion chromatography (SEC) to give the molecular weights (Figure S1–S3 in the Supporting Information).

The results in Table 1, entry 2 suggest that this method can apply and degrade different lignin types with ranges of different molecular weights and that the methanol pretreatment is not necessary and does not require narrow polydispersity samples as in other depolymerization techniques. Consequently, organosolv lignin, kraft lignin, and alkali lignin were subjected directly without any pretreatment to the microwave depolymerization method to investigate the depolymerization ability of the microwave system to original lignin samples. These three original lignin samples were analyzed by SEC and also by 2D-NMR spectroscopy to show the differences in the structure before and after microwave treatment (2D-NMR results in Figure S7 in the Supporting Information). These three types of lignin have overall molecular weights around Mn = 1500 with quite a high polydispersity.

Table 2, entries 1 and 2, summarizes the molecular weight and polydispersity of organosolv lignin, kraft lignin, and alkali lignin before and after the reactions using the initial reaction condition as described above. The SEC chromatograms of the molecular weights of organosolv lignin, kraft lignin, and alkali lignin before and after the initial reactions are shown in Figure S4 in the Supporting Information.

Table 2. Molecular Weights and Polydispersity of Lignin under Different Conditions.

entry condition alkali lignin (Mn/D̵) organosolv lignin (Mn/D̵) kraft lignin (Mn/D̵)
1 original lignin 1500/5.9 1400/12.9 1500/7.7
2 after initial condition 600/2.6 600/2.6 650/2.4
3 after optimized condition 120/3.2 150/2.8 180/3.6
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As the results show in Table 2, in entries 1 and 2, compared to the original lignin samples, there was a significant molecular weight decrease and polydispersity increase of the three original lignin samples observed after the initial reaction. After the reaction, ethyl acetate was used to extract the low-molecular-weight fractions from the product. Table 3 summarizes the yield of ethyl acetate-soluble fraction before and after the initial reactions. The yield of the ethyl acetate-soluble fraction of the three samples all increased from less than 16% to more than 30% (Table 3, entries 1 and 2). This indicated that the all these three lignin samples were depolymerized with the generation of low-molecular-weight fractions.

Table 3. Yield of Ethyl Acetate-Soluble Fraction for Lignin Samples.

entry condition alkali lignin (%) organosolv lignin (%) kraft lignin (%)
1 original lignin 16 6.7 15
2 after initial condition 38 31 37
3 after optimised condition 52 54 48
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Condition Optimization of Lignin Depolymerization

Following the confirmation of lignin depolymerization, further work to optimize the reaction conditions was undertaken to maximize the yields of lower-molecular-weight fractions. In this case, alkali lignin, with higher molecular weight than the other two lignin samples, was used for optimizing the reaction conditions. As the reaction with 1-phenylethanol demonstrated, the dosages of CuSO4 and H2O2 can positively affect the microwave reaction. On this basis, different dosages of CuSO4 and H2O2 were used to degrade alkali lignin. After the reaction, ethyl acetate was used to extract the low-molecular-weight fractions from the reaction mixture. The SEC chromatograms are listed in Figure 3.

Figure 3.

Figure 3

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SEC chromatograms of microwave degradation of alkali lignin under different conditions: (a) different CuSO4 dosages; (b) different H2O2 dosages; and (c) different reaction times.

The effect of different levels of CuSO4 and H2O2 were investigated separately. For each part, only one parameter was investigated, compared to initial conditions. The dosage of CuSO4 was calculated based on the mass ratio with lignin (Figure 3a). With the increasing dosage of CuSO4, the signal from SEC shown by the red dash line at lower retention time decreased in intensity. This indicated that a higher molecular weight fraction was depolymerized with the increasing mass ratio of CuSO4 to lignin in the reaction mixture. Meanwhile, the intensity at a high retention time, which is marked by a black dash line at 17–19 min, only shifted slightly. When the mass ratio of CuSO4 increased from 0.14 to 0.18, the SEC intensity marked by the red dash line decreased. When the mass ratio of CuSO4 was increased more than 0.18, similar results were obtained; hence the mass ratio of 0.18 CuSO4 to lignin was considered as the most efficient mass ratio for lignin degradation, for the conditions that were tested.

The effect of varying the amount of H2O2 was also investigated, with the results shown in Figure 3b. All reactions were carried out with 0.1 mass ratio of CuSO4 to lignin at 80 °C for 10 min. The dosage of H2O2 was calculated based on the mass ratio with lignin. When the ratio of H2O2 (30 wt %) to lignin was increased from 1.0 × 10–3 to 1.4 × 10–3 mL/mg, a shift in the molecular-weight distribution was observed, evidenced by the decrease in the intensity of the SEC signal in the retention time region of 12.5–15 min. This provides strong evidence of the loss of high-molecular-weight material, and the corresponding increase in the intensity of the signal in the 16–19 min region of the chromatogram was consistent with this. However, when the ratio of H2O2 to lignin increased from 1.4 × 10–3 to 1.8 × 10–3 mL/mg, a shift in the molecular-weight distribution to higher molecular weights was observed. The intensity of the signal at the retention times marked by the red dash line increased while the signal intensity at the retention time marked by the black dash line increased. Interestingly, the peak that appeared at 17.5–18 min with the ratio of 1.0 × 10–3 mL/mg H2O2 (marked with the black dashed line) shifted to lower retention times at higher H2O2 dosage. This indicated that the increasing ratio of H2O2 could certainly promote the degradation reaction, however, when an excess amount of H2O2 was added into the reaction, repolymerization of the smaller fractions could occur. The reason can be that with the increasing dosage of H2O2, more hydroxide radicals can be generated, which could lead to repolymerization. Competition between depolymerization and repolymerization has been previously reported when radical reaction mechanisms are involved.21

The effect of time on the reaction of this system was also studied. All reactions were carried out with a 0.1 mass ratio of CuSO4 to lignin and 1.0 × 10–3 mL/mg of H2O2 to lignin at 80 °C. The depolymerized lignin products were extracted by ethyl acetate to give about 40–50 wt % of products and were analyzed by SEC to give the results shown in Figure 3c. As the curves showed, the SEC signal intensity marked by the red dash line, which is the depolymerized oligomer, increased with the increase in the reaction time. Again, the striking appearance of a signal intensity at ∼18 min retention time and after 1 min of reaction time suggests rapid formation of a low-molecular-weight material. This steadily increased in intensity up to 7 min, but at 9 min, repolymerization was again evident as the intensity of this peak decreased significantly. Thus, it was decided to stop the reaction at 7 min to have the best depolymerization.

The effect of temperature was also tested with 0.1 mass ratio of CuSO4 to lignin and 1.0 × 10–3 mL/mg of H2O2 to lignin for 10 min. The results show that higher temperature is better for depolymerization (Supporting Information, Figure S5), however, to keep the reaction mild, a reaction temperature of 110 °C was set.

In conclusion, the optimum conditions based on these investigations for microwave-assisted lignin degradation were chosen as 0.18 mass ratio of CuSO4 to lignin and 1.4 × 10–3 mL/mg of H2O2 to lignin at the temperature of 110 °C for 7 min. This condition was then applied for all three original lignin samples (Table 2). The SEC results of ethyl acetate-extracted products were compared with the initial conditions, which is shown in the Supporting Information (Figure S6).

As the results show in Table 2, entry 3, all these three lignin samples showed better depolymerization under the optimized conditions chosen, compared to the initial conditions. The overall molecular weight of the ethyl acetate-extracted fraction decreased from 600 g/mol to less than 200 g/mol. Moreover, the low-molecular-weight peak, which appeared at higher retention time, increased in intensity with the peaks at lower retention time decreasing (Figure S6 in the Supporting Information).

The percentage of ethyl acetate-soluble fractions in different lignin samples is listed in Table 3. After reaction, the yield of ethyl acetate-soluble fractions increased by more than 2 times. The reason is that the amount of small-molecular-weight fractions increased after microwave treatment because of the depolymerization of lignin.

Control reactions of this catalyst system for lignin degradation were carried out with alkali lignin as the starting material (summarized in Table S3 in the Supporting Information). The yield of ethyl acetate fractions from the reaction without CuSO4 and H2O2 and also with only CuSO4 or H2O2 was 15–18%, and there were no significant increase from the original lignin (15%). This indicated that both CuSO4 and H2O2 are necessary for this reaction, which is also consistent with our optimization condition results. A reaction using an autoclave with heat was also used for this reaction instead of the microwave, which only gave 20% yield of the ethyl acetate-soluble fraction that was significant lower than the yield (52%) using microwave irradiation. The hypothesis is that CuSO4 plays an important role as a catalyst to generate hydroxyl radicals from H2O2, and the irradiation of microwave accelerated the reaction, and these radicals degrade the lignin as previously reported.32

The ethyl acetate-soluble fractions, which yield about 50%, were analyzed by 2D-NMR spectroscopy to characterize the monomeric and oligomeric products. Figure 4 shows three typical bonds detected by 2D-NMR spectroscopy. The 2D-NMR spectra of the alkali lignin and depolymerized alkali lignin are shown in Figure 5.

Figure 4.

Figure 4

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Different bonds in lignin detected by 2D-NMR spectroscopy.

Figure 5.

Figure 5

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2D-NMR spectra of alkali lignin and depolymerized alkali lignin.

Comparing Figure 5a,b, the 2D-NMR spectra after microwave-assisted lignin depolymerization indicate that the β-O-4 bonds, β–β bonds, and β-5 bonds were cleaved. The signals from each bond, which existed in alkali lignin, disappeared after depolymerization (the dash circle in Figure 5b). This indicated that this microwave-assisted strategy could efficiently degrade lignin via cleaving these particular three bond types, which leads to the generation of oligomeric and monomeric products. Similar results were also obtained from kraft lignin and organosolv lignin (Supporting Information, Figure S7).

LC–MS was used to analyze the extracted low molecular products. The compounds detected are shown in Figure 6 and the LC–MS chromatogram results are shown in Figure 7. From the LC–MS results, seven monomers have been detected. These are vanillic acid (compound 1), 4-hydroxybenzaldehyde (compound 2), vanillin (compound 3), syringaldehyde (compound 4), 2-hydroxy-3-(4-hydroxyphenyl)propanal (compound 5), 4-hydroxybenzoic acid (compound 6), and 2-hydroxy-3-(4-hydroxy-3-methoxyphenyl)propanal (compound 7).

Figure 6.

Figure 6

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LC–MS detected monomers from depolymerized lignin products.

Figure 7.

Figure 7

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Liquid chromatogram spectra of the extracted low-molecular-weight products from (a) depolymerized alkali lignin, (b) depolymerized kraft lignin, and (c) depolymerized organosolv lignin.

Figure 7 shows the LC chromatograms of three depolymerized lignin products. From these three lignin samples, vanillic acid (compound 1), 4-hydroxybenzaldehyde (compound 2), and vanillin (compound 3) were all generated in common. This demonstrated that microwave irradiation was very effective in the production of these three monomers with multiple types of lignin as feedstock. Moreover, depolymerized kraft lignin and depolymerized organosolv lignin also showed the presence of other aromatic monomers compared to the depolymerized alkali lignin. These observations are consistent with differences in the original structures of each lignin sample with different contents of G units and S units. The wide broad peaks after 36 min in all these three LC chromatograms were assigned to dimeric or oligomeric products. The mass spectra are shown in the Supporting Information (Figures S8–S10).

This one-pot catalyst system can degrade lignin into monomeric and oligomeric products in 7 min with CuSO4/H2O2 as catalysts. MnCl2 and CuO, which have been reported28,30 for lignin model compound degradation under the irradiation of microwave, have been attempted and compared with our catalyst under this condition. The yield of the ethyl acetate-soluble fraction from alkali lignin using MnCl2 and CuO were 41 and 43%, respectively, which were less than our yield of 52%. These results indicate that our optimized condition has a potential to be used for other catalyst systems. In addition, a milder reaction condition was used in our system compared to other lignin degradation systems reported using different copper catalysts.30,36

Conclusions

In this work, we presented a very efficient approach to depolymerize lignin with CuSO4 and H2O2 as catalysts under microwave irradiation conditions. These conditions can depolymerize different lignin samples into low-molecular-weight monomeric and oligomeric products at a temperature of 110 °C for only 7 min with power of 80 W. Monomers, such as vanillin, 4-hydroxybenzaldehyde, and vanillic acid were detected and characterized in the products. The high efficiency of this method showed the potential to utilize lignin for the production of valuable low-molecular-weight aromatics in industrial applications.

Experimental Section (Materials and Methods)

Materials

Organosolv lignin was purchased from Chemical Point. Kraft lignin was isolated from black liquor supplied by Maryvale Paper mill, by the method reported previously.22 Alkali lignin, CuSO4, and 30 wt % H2O2 solution were purchased from Sigma-Aldrich Australia. Ethyl acetate, acetonitrile (ACN), dimethyl formamide (DMF), chloroform (CDCl3), dimethyl sulfoxide (DMSO-d6), and methanol were purchased from Merck Australia. All reagents were used as purchased from the supplier without any further purification.

Instruments and Measurements

CEM Discover SP-microwave synthesizer was used to perform all microwave reactions. 1H NMR spectra were recorded on a Bruker DRK-600 spectrometer operating at 600 MHz for solutions in CDCl3 and DMSO-d6. The molecular weight of isolated lignin and depolymerized samples were measured by SEC performed on a Tosoh EcosHLC-8320 system equipped with both refractive index and ultraviolet (UV) detectors (UV detection, λ = 280 nm) using Tosoh alpha 4000 and 2000 columns. DMF (with 10 mM LiBr) was used as a solvent to dissolve all lignin samples as well as the mobile phase with a flow rate of 1.0 mL/min. UV detector in SEC chromatograms set at 280 nm was used to analyze all depolymerized samples and isolated lignin. Calibration curves were obtained using polystyrene standards. LC–UV/MS was performed using an Agilent 1260 Infinity liquid chromatograph system coupled with a 6120 series quadrupole mass spectrometer. A column of Kinetex 5 μm C18 100 Å Column (250 mm × 4.6 mm i.d.; particle size 5 μm) with two mobile phases A and B was applied. Mobile phase A consisted of water with 0.2% formic acid and mobile phase B consisted of 0.2% formic acid in ACN. The gradient program selected used a flow rate of 0.8 mL/min and an ambient temperature. The gradient for analysis was as follows: 0 min, 0% ACN; 30 min, 30% ACN; 45 min, 100% ACN; 51 min, 100% ACN; 52 min, 0% ACN; and 56 min, 0% ACN. UV wavelengths: 210, 254, 260, 280, 320, and 378 nm; data acquisition rate, 5 Hz. Data acquisition and processing were performed using Mass IQ (PerkinElmer), MS parameters: drying gas flow of 9.8 L/min, nebulizer pressure of 28 psi, drying gas temperature of 300 °C, capillary voltage of 3000 V, and mass scan range of 100–900 Da with a cycle time of 1.57 s per cycle.

Isolation of kraft lignin from black liquor was undertaken according to the method we have previously reported.22 Elemental analysis of carbon, hydrogen, nitrogen, and sulfur gave the following result (average over three measurements): carbon 65.30%, hydrogen 6.07%, nitrogen <0.3% (detection limit), and sulfur 2.57%.

Microwave-mediated oxidation of 1-phenylethanol with copper sulfate involved using a 10 mL microwave reactor vial charged with 1-phenylethanol (0.05 mL, d = 1.013 g/mL, 50 mg, 0.41 mmol), 2.5 mL of water, 2.5 mL of ACN, 5 mg (0.03 mmol) of copper sulfate, and 0.5 mL (30 wt % in H2O, 0.15 g, 4.41 mmol) of H2O2, which was placed in the microwave reactor and heated to 80 °C for 10 min with a maximum microwave power of 80 W. The resulting mixture was then partitioned between water (30 mL) and ethyl acetate (2 × 20 mL). The combined organic layers were washed with saturated brine (1 × 30 mL) and dried over anhydrous magnesium sulfate. The solvent was removed in vacuo, and the product was then analyzed by 1H NMR spectroscopy to determine the percentage conversion of secondary alcohol to ketone.

Isolation of the high-molecular-weight fraction of lignin was undertaken using a solubility method reported before.21 The molecular-weight distribution of the methanol-insoluble fraction of lignin (M-lignin) was determined by SEC.

Microwave-Assisted Reaction with 1-Phenylethanol

Several reactions have been tested to optimize the reaction conditions. The typical reaction procedure is as follows: 1-phenylethanol (0.05 mL, d = 1.013 g/mL, 50 mg, 0.41 mmol), 2.5 mL of water, 2.5 mL of ACN, 5 mg (0.03 mmol) of copper sulfate, and 0.5 mL (30 wt % in H2O, 0.15 g, 4.41 mmol) of H2O2 were placed in the 10 mL vessel of a microwave reactor and heated to 80 °C for 10 min with a maximum microwave power of 80 W. For each reaction, 0.41 mmol (50 mg, 0.05 mL) 1-phenylethanol was used as the substrate, and the yield of total recovered material was always around 90–95%.

Procedure for Microwave Oxidation of Lignin with Copper Sulfate

Several reactions have been tested to optimize the reaction conditions. The typical reaction procedure is as follows: A 10 mL microwave reactor vial was charged with 50 mg of lignin, 2.5 mL of water, 2.5 mL of ACN, 5 mg (0.03 mmol) of copper sulfate, and 0.5 mL (30 wt % in H2O, 0.15 g, 4.41 mmol) of H2O2. The vial was then placed in the microwave reactor and heated to 80 °C for 10 min with a maximum microwave power of 80 W. The solvent was then removed; the remaining solid was dissolved in a 10 mmol/L solution of LiBr in DMF to give a concentration of 2 mg/mL of oxidation products; and the solution was analyzed by SEC.

Acknowledgments

The financial support of the ARC Industrial Transformation Research Hub—Bioprocessing Advanced Manufacturing Initiative (BAMI), Monash University, PRESTO JST, JPMJPR1515, Japan, and the China Scholarship Council (CSC) is gratefully acknowledged.

Glossary

Abbreviations

SEC

Size exclusion chromatography

LC–MS

liquid chromatography–mass spectrometry

2D-NMR

two-dimensional nuclear magnetic resonance spectroscopy

MIA

methanol-insoluble alkali lignin

MSA

methanol-soluble alkali lignin

MIO

methanol-insoluble organosolv lignin

MSO

methanol-soluble organosolv lignin

MIK

methanol-insoluble kraft lignin

MSK

methanol-soluble kraft lignin

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01978.

  • Results from the control reaction of 1-phenylethanol, ethyl acetate-soluble fraction yields of different lignin samples, SEC and 2D-NMR analysis of original lignins, and LC–MS mass spectra of depolymerized products (PDF)

Author Contributions

‡ These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao8b01978_si_001.pdf (1.1MB, pdf)

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