Two-step hydrogen transfer catalysis conversion of lignin to valuable small molecular compounds

2.1 Material

Industrial lignin was purchased from Lanxu Biotechnology Co. Ltd. (Hefei, China) and washed with deionized water three times. Fresh lignin was obtained in the laboratory by acidic hydrolysis and removal of the polysaccharides of biomass. Deionized water with resistivity of 18 MΩ cm was produced by Milli-Q equipment (Millipore, USA) and was used for solution preparation. Other chemicals, including 2-propanol, ethyl acetate, and phenol, were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). In addition, H₂SO₄ (98%) was purchased from Shanghai Chemical Reagents Company and was used directly without further purification. The gas chromatography (GC) standard samples, such as 4-ethylphenol, 4-ethyl-2-methoxyphenol, 2,6-dimethoxyphenol, guaiacol, 2-methoxy-4-propylphenol, and 2,6-di-tert-butylphenol were purchased from Aladdin Chemical Reagent Co., Ltd (Shanghai, China).

2.2 Characterization of lignin and product

Lignin was characterized by means of Fourier transform infrared spectroscopy (FT-IR), gel permeation chromatography (GPC), elemental analysis, thermogravimetric analysis-differential scanning calorimetry (TGA-DSC), and UV-Vis spectroscopy. Reaction products were analyzed using GPC (dimethyl formamide, DMF), gas chromatography-flame ionization detector (GC-FID), gas chromatography-mass spectrometer (GC-MS), transmission electron microscopy, elemental analysis, UV-Vis, and FT-IR. FT-IR spectra were recorded on a Bruker spectrometer (EQUINOX 55, Bruker, UK) applying the potassium bromide (KBr) assay method. GPC, (LC-20AD, Shimadzu, Japan) with a Shimadzu Shodex GPC KD-804 column and a refractive index detector (RID-10A) was used to compare the molecular weights of the samples. The measurement was achieved at 60°C using DMF as the mobile phase at a determined sample concentration (5 mg/ml). UV-Vis spectra were record on a Shimadzu UV-2401 PC ultraviolet instrument. GC was conducted on a Thermo Fisher UV1800 instrument for quantitative analysis of the product.

2.3 Synthesis of Raney Ni catalyst

Raney Ni catalyst was prepared using the W-2-type preparation method [16]. A total of 10 g of sodium hydroxide was dissolved in 50 ml of distilled water cooled to 10°C in an ice bath. Twelve grams of Ni-Al alloy was added in small portions to the liquid with stirring to ensure that the solution temperature was maintained between 10°C and 15°C. After the addition was complete, the flask was removed from the ice bath, and the reaction was allowed to warm to room temperature. When hydrogen generation was slow, it could be heated slowly in an oil bath, and the heating was continued until the occurrence of bubbles became slower again. This process lasted for approximately 2 or 3 h. Then, the flask was put aside, after the formation of the nickel precipitates, and the supernatant was removed. Distilled water was added to the original volume, and then the solution was stirred to suspend the nickel powder. Again, the nickel powder precipitated, and the supernatant was discarded. Thisstep was repeated three times. Next, the Raney Ni was washed three times with 2-propanol. The prepared Raney Ni was stored in 2-propanol avoiding contact with air.

2.4 Depolymerization of lignin over catalyst

Conversions of lignin were performed in a 50-ml polytetrafluoroethylene autoclave. When Raney Ni was used as the catalyst, 800 mg of the substrate and 800 mg of the catalyst were uniformly stirred and dispersed in 24 ml of isopropanol (isopropanol: H₂O=7:3). Nitrogen gas was then introduced at ambient temperature for 2 h to remove oxygen. The vessel was closed, and the reactor was heated from room temperature to 180°C at a heating rate of 5°C/min for 180 min on a magnetic stirrer. After reaction, 20 ml of isopropanol was added to the liquid, and the obtained mixture was filtered, and the filtrate was washed with anhydrous sodium sulfate and then diluted with isopropanol to 50 ml for analysis. Next the product obtained by Ni catalysis was used for the Pd/C catalysis, and 900 mg of substrate, 150 mg of catalyst, and 1.5 ml of NaBH₄ solution (0.092 m) were homogeneously stirred and dispersed in 7.5 ml of ethyl acetate solution. Nitrogen gas was then introduced at ambient temperature for 2 h to remove oxygen. The vessel was closed, and the reactor was heated from room temperature to 180°C at a heating frequency of 5°C/min for 180 min on a magnetic stirrer. Then, it was allowed to stand at room temperature. After reaction, the liquid was subjected to suction filtration, and the filtrate was washed with a fixed amount of anhydrous sodium sulfate in 50 ml of ethyl acetate to carry out the analysis.

3.1 Analysis of lignin

It is well known that the composition of lignin internal elements, functional groups, and various aromatic units is determined by the types of biomass. To further analyze the composition and structure of lignin and compare them with those of the products after the reaction, the lignin was analyzed by a wide variety of techniques, such as FT-IR, GPC, UV, calcinations, and CHNO elemental analysis. The CHNO elemental analysis and calcinations experimental results are listed in Table 1.

3.2 Hydrogen transfer reaction of lignin over Raney Ni catalysis

Raney Ni catalyst has a very wide range of applications in industry [17]. When lignin polymer is catalyzed by Raney Ni by the hydrogen transfer reaction, the conversion from solid to liquid is efficient. Table 2 lists the results of the conversion of lignin at 180°C for 18 h. It was found that alkali lignin has a better conversion than that of Klason lignin because of different preparation processes. The role of high concentrations of acid in the Klason lignin preparation process resulted in condensation reaction. However, the occurrence of alkaline hydrolysis caused reduction molecular weight in the preparation process of alkali lignin. Lignin solids can be successfully converted to liquefied products, which still have high molecular weight. In other words, the reaction product of Raney Ni catalysis has a higher conversion but lower selectivity for small molecules. Conversion (C%), Total Yield (TY%), and Yield (Yi%) are defined as Eqs. (1–3), respectively.

3.3 Hydrogen transfer reaction of lignin over Pd/C catalysis

Like the Raney Ni catalyst, Pd/C catalyst plays an important role in industrial applications such as pharmaceutical production [18], [19]. It can be also used as a catalyst for hydrogen transfer reaction. Here, lignin was directly used as feedstock for hydrogen transfer reaction over Pd/C, and the reaction has a low conversion and liquefaction rate as shown in Figure 2, indicating Pd/C is not suitable as catalyst for direct conversion of lignin. In addition, the hydrogen provided from NaBH₄ still caused the lignin to degrade by hydrogenation reaction in the absence of catalyst.

Figure 2:

Conversion and total yield of the 12 compounds of alkali lignin degradation over Pd/C catalyst.

3.4 Hydrogen transfer reaction of lignin by the two-step catalysis

From the earlier studies, it was found that the two types of catalysts exhibited different characteristics in the process of lignin depolymerization. When Raney Ni catalyst was used, the conversion from solid to liquid was higher, but the total yield was low. When Pd/C catalyst was used, the conversion of lignin to liquid was low, but most of the degradation products had low molecular weights (Figure 3). The results are similar to the previous results of Ferrini and Galkin, who successfully liquefy unprocessed lignin through Raney Ni, while Galkin used Pd/C to convert low molecular weight model compound to monomer (~30% in selectivity). Therefore, the use of two-step catalytic (TSC) process is proposed. First, the Raney catalyst was used for liquefaction, and then the Pd/C catalyst was used to degrade high molecular weight liquid lignin to monomer. Figure 3 shows the GPC curves of products obtained from alkali lignin under different catalysts. For comparison, two catalysts were utilized separately or in combination. It can be clearly seen that the original lignin has a relatively large molecular weight. When Pd/C is used as the catalyst, two peaks occur, in which the unreacted lignin peak is higher than the peak of the small molecular weight product. When using Raney Ni as the catalyst, the molecular weight distribution of the lignin depolymerization product is average and does not exhibit obvious peaks. However, the degradation peak with low molecular weight is very obvious after using both catalysts, while the height of the high molecular weight peak is very low. These results clearly demonstrate that the two catalysts used in combination are significantly better than using a single catalyst alone (Supplemental Figure S1).

Figure 3:

Gel permeation chromatograms obtained from alkali lignin. The products are degraded using two different catalysts separately or degraded using TSC.

Degradation of lignin can also be verified by the FT-IR studies. Figure 4 shows the FT-IR spectra of the lignin and the liquid phase of products for different catalytic processes. A strong, broad band is seen at 3400 cm⁻¹, indicating the presence of hydroxyl groups in large quantities in the liquid phase, as large amounts of hydroxyl groups are produced by degradation [20]. The main differences are observed for the C–O and C=O bonds. The increase of carbonyl group and the decrease of carbon-oxygen single bond actually indicate the degradation of lignin because of the breaking of the carbon-oxygen bond during catalysis, and the results also support the result that two catalysts system is efficient for lignin degradation.

Figure 4:

FT-IR spectra of alkali lignin and the degradation products.

The mass fraction of products after each stage of the reaction is shown in Figure 5. The solvent of the products was completely removed in the rotary evaporator. It can be seen that the content of hydrogen is lower than that of the hydrogen in the lignin when using a single hydrogen transfer catalyst, such as Raney Ni or Pd/C. However, when using the two-step method, the hydrogen content is much higher, indicating that as the reaction proceeds, hydrogen ions enter the lignin and break its chemical bonds, which improves the hydrogen mass ratio and promotes the degradation of lignin. As the reaction continues, the mass ratio of C element increases, indicating that the content of O element in the reaction product decreases.

Figure 5:

Elemental analysis of alkali lignin degradation products over different catalysts.

Thermo gravimetric (TG) and deferential thermo gravimetric analysis (DrTGA) curves of the alkali lignin and the solid residue after reaction at each stage of the reaction are shown in Figure 6. The degree of charring of the reactants during the reaction can be deduced by analyzing the reaction residue. It can be seen that the weight loss peak of lignin is approximately 350°C, and when the temperature reaches 400°C, the pyrolysis is complete. Both catalysts lead to partial carbonization of the lignin, causing the weight losing peak to move to a high temperature. However, when the two catalysts are used sequentially, the degradation of the residue after the reaction is sufficient, the degree of carbonization is low, also indicating the efficient degradation of lignin.

Figure 6:

TG (A) and DrTGA (B) curves of alkali lignin and the degradation products.

The GC-MS was used to analyze the components of the liquid products, as shown in Figure 7. The major products include 4-ethylphenol (1), acetosyringone (2), 4-ethyl-2-methoxyphenol (3), 3,4,5-trimethoxytoluene (4), 2,6-dimethoxyphenol (5), guaiacol (6), 4-ethyltoluene (7), ethylbenzene (8), 2-methoxy-4-propylphenol (9), phenol (10), 2,6-di-tert-butylphenol (11), and p-tolualdehyde (12), and contents of compounds 1–6 are relatively high.

Figure 7:

GC-FID of the final products and compounds found in products: 4-ethylphenol (1), acetosyringone (2), 4-ethyl-2-methoxyphenol (3), 3,4,5-trimethoxytoluene (4), 2,6-dimethoxyphenol (5), guaiacol (6), 4-ethyltoluene (7), ethylbenzene (8), 2-methoxy-4-propylphenol (9), phenol (10), 2,6-di-tert-butylphenol (11), and p-tolualdehyde (12).

Figures 8 and 9 show the quantitative analysis of the products. It can be seen that among the degradation products, compounds 1, 7, 8, 10, and 12 are derived from the p-hydroxy-phenyl lignin structure. In addition, compounds 6, 3, 9 and other degradation products are derived from the guaiacyl lignin structure. Compounds 2, 4, 5, and 11 are derived from the syringyl lignin structure. The three structural units of lignin are all effectively degraded [21]. The representative alkali lignin structure is shown in Figure 10. Blue color shows the syringyl unit, red color and green color display the p-hydroxy-phenyl unit and guaiacyl unit.

Figure 8:

Quantitative analysis of the products over different catalysts.

Figure 9:

Conversion and total yield of the 12 compounds in the degradation product.

Figure 10:

Representative alkali lignin structure and possible degradation mechanism.

The main pathway for the lignin hydrogenation reaction is the cleavage of ether linkages connecting α, β, and γ carbon atoms of a side chain and position 4 of a phenolic ring unit. Depending on reaction conditions, carbon-carbon linkages may also be disrupted during hydrogenolysis, especially at α, β, and γ sites. As shown in Figure 10, compounds 1, 2, 3, 6, 9, and 10 seem to have been generated from ether linkage bond cleavage, followed by carbon-carbon breakdown reactions that lead to compounds 4, 5, 7 and 8, GC-MS studies for degradation products also support the mechanism (Supplemental Figure S2). Hydrogenation reactions can cause the disappearance of methoxy groups in compound 11 (particularly at positions 2 and 6 of the phenolic ring), which leads to the production of methyl groups. Compounds 2 and 12 exhibit a carbonyl group at a position (ketone or aldehyde) that could have originated from further reactions of the intermediates formed during the ether linkage cleavage between an aliphatic carbon and oxygen from the ether bond (generally in position 4) [20].