Catalytic Depolymerization of Alkaline Lignin into ... - ACS Publications

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Cite This: Ind. Eng. Chem. Res. 2019, 58, 13041−13052

Catalytic Depolymerization of Alkaline Lignin into Phenolic-Based Compounds over Metal-Free Carbon-Based Catalysts Sansanee Totong,† Pornlada Daorattanachai,† Armando T. Quitain,‡ Tetsuya Kida,‡ and Navadol Laosiripojana*,† †

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The Joint Graduate School for Energy and Environment (JGSEE), King Mongkut’s University of Technology Thonburi, Prachauthit Road, Bangmod, Bangkok, 10140, Thailand ‡ Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555, Japan S Supporting Information *

ABSTRACT: Low-cost alkaline lignin, a major waste from pulp manufacturers, is currently a promising renewable feedstock for converting pulp to high-value phenolic compounds via a catalytic depolymerization reaction. Among potential catalysts for this reaction, metal-free carbon-based catalysts have been widely developing due to several favorable characteristics such as high catalytic activity and selectivity, long catalyst life, and ease in recovery. In this study, five types of synthesized metal-free carbon-based catalysts, e.g., graphene oxide (GO), nitrogen-doped GO (N-GO), solvothermal carbon (STC), functionalized STC with H2SO4 (SO3-STC), and nitrogen-doped STC (N-STC), were tested for alkaline lignin depolymerization. It was found that, in the presence of catalysts, a significantly higher phenolic monomers yield and a lesser amount of char formation were observed from the reaction. Under the optimized reaction conditions (at 250 °C for 3 h), NSTC enhanced the highest phenolic monomers yield (7.52%). Various techniques, such as gas chromatography-mass spectrometry (GC-MS), gas chromatography-flame ionization detector (GC-FID), pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), Fourier transform-infrared spectroscopy (FT-IR), proton nuclear magnetic resonance with respect to hydrogen-1 nuclei (1H NMR), and gel permeation chromatography (GPC), were used to analyze for complete identification and quantification of raw lignin, lignin product, and residual lignin. pyrocatechol, hydroxyacetophenone, and their derivatives.5−10 Hydrolysis of lignin is the simple method to convert lignin to high value-added products because this method operates under milder conditions than other methods. Man et al. have been studied the depolymerization of lignin in supercritical water. They reported that lignin was hydrolyzed to low-molecular weight fragments, then the intermediates reacted and dealkylated to other compounds such as formaldehyde and phenolic-based chemicals.11 Moreover, research studies have been carried out under the acid or alkaline conditions such as formic acid, hydrochloric acid, sulfuric acid, sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, and potassium carbonate. Under these conditions, results in the literature show that the acid-catalyzed or base-catalyzed depolymerization of lignin using acids or alkalis can effectively convert lignin into phenolic monomers.12−18 The catalysts used in lignin depolymerization can be either homogeneous or heterogeneous catalysts. Homogeneous catalysts have been widely employed due to the highly selective for bond cleavage.

1. INTRODUCTION Lignin is one of the most important polymeric organic substances in wood or biomass.1 The structure of lignin is an amorphous heteropolymer material consisting of three different phenylpropane units, which are coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol, held together by several kinds of linkages.2,3 Lignin has a unique and very complex structure because of the various linkages between the phenylpropane units and functional groups. Typically, this compound is the major waste from the biomass pretreatment process, from which a huge amount of lignin has been generated from the pulp and paper industries. Nowadays, lignin is used as a lowgrade fuel in a boiler to generate heat and power, while less than 5% of lignin is utilized for other processes.4 Based on its structure, lignin has high potential for converting to several bulk chemicals because it has a complex structure (polymer structure), high molecular weight, various reactive functional groups, and high-energy content. Recently, several conversion processes have been studied such as gasification, pyrolysis (thermolysis), hydrolysis, chemical oxidation, hydrogenolysis (hydrocracking), and depolymerization.2 The potential products from lignin depolymerization are a diversity of phenolic monomeric and aromatic compounds such as phenol, syringol, guaiacol, vanillin, catechols, © 2019 American Chemical Society

Received: Revised: Accepted: Published: 13041

April 11, 2019 June 26, 2019 July 1, 2019 July 1, 2019 DOI: 10.1021/acs.iecr.9b01973 Ind. Eng. Chem. Res. 2019, 58, 13041−13052

Article

Industrial & Engineering Chemistry Research

products were identified and determined by gas chromatography (GC) with a flame ionization detector (FID) and a mass spectrometer (MS). Additionally, the Klason lignin method, elemental analysis, pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), Fourier transform-infrared spectroscopy (FT-IR), and gel permeation chromatography (GPC) were also intensively investigated for understanding structural changes of lignin before and after the depolymerization process. Moreover, the characterizations of the synthesized catalysts were evaluated using FT-IR, X-ray diffraction (XRD), and scanning electron microscopy (SEM) to study the relationship between their physical-chemical properties of the catalyst and the catalytic activity on the depolymerization reaction.

However, heterogeneous or solid catalysts have several advantages over homogeneous catalysts such as lower environmental impacts, easy of separation, and reusability.19 Typically, heterogeneous transition metal catalysts such as vanadium,20−23 rhenium,24 and cobalt25,26 are used to depolymerize lignin. Currently, one of key challenges in catalysis research is the development of metal-free catalysts.27−29 Among them, metalfree carbon-based catalyst is one of the promising catalysts due to its high activity, low cost, nontoxic, and environmentally friendly. Recently, carbon-based catalysts have been demonstrated to be efficient for many applications such as fuel cell, pollutant reduction and biosensing, clean energy generation and storage, biomass conversion, bio-oil production, and valueadded chemical production.30−35 Recently, many carbon-based catalysts such as graphene oxide (GO), carbonaceous material, and functionalized carbonaceous catalysts have been widely used for biomass conversion. For instance, Zhu et al. reviewed graphene and its derivatives for catalytic conversion of biomass to biochemicals and biofuels. Graphene-based materials showed great potential for biomass conversion and were used as catalysts in an acid-catalyzed reaction or an oxidation reaction or as a catalyst support.36 Blandez and co-workers investigated the catalytic properties of graphene oxide, reduced graphene oxide, and B-doped graphene oxide for oxidative depolymerization of lignin. They found that graphene oxide and its derivatives could promote the oxidative degradation of lignin and exhibited a high product yield.37 Moreover, Li et al. have successfully developed nitrogen-doped carbon materialsupported iron catalysts for the selective cleavage of the C−O bond of lignin without hydrogenation of the aromatic ring.38 In our previous research of carbon-based catalysts, their synthesis, characterization, and applications were investigated. For example, we have studied the synergizing of graphene oxide with microwave irradiation for cellulose depolymerization into glucose.39 It was found that the glucose yield obtained from graphene oxide was higher than using other solid acid catalysts such as Amberlyst 15, sulfated zirconia, and phosphotungstic acid owing to the variety of functionalities on the surface and high microwave absorptivity. This study aims to prepare five promising carbon-based catalysts and test lignin decomposition to produce phenolic monomers. From a review of the literature, several researchers have revealed that preparation methods, modification, and functionalization processes influence the catalytic performance of carbon-based catalysts for lignin depolymerization.37,40 In detail, synthesized solvothermal carbon (STC) and graphene oxide (GO) were applied as catalysts for lignin depolymerization in this work because of their low-cost and natural abundance, and they are a metal-free alternative catalyst, environmental friendly, and a simple technique for preparation. Sulfonated solvothermal carbon (functionalized STC with H2SO4, SO3-STC) prepared from organosolv lignin was prepared and investigated toward the depolymerization of lignin. Moreover, the nitrogen-doped GO (N-GO) and nitrogen-doped STC (N-STC) were also synthesized and investigated for comparing. It is expected that doping nitrogen with an ammonia precursor into a carbon-based material can improve the catalytic reactivity due to the presence of a basic active site on the surface of the catalyst. Then, the depolymerization conditions (i.e., reaction temperature and residence time) of the selected catalyst on the product yield were determined and discussed. The obtained phenolic

2. MATERIALS AND METHODS 2.1. Raw Materials and Chemicals. The commercial alkaline lignin was applied as a raw material compound for this work and provided by Aldrich (low sulfonate content ∼4% and pH ∼10.5). Organosolv lignin from Chemical Point UG (Germany) was used as a carbon source for synthesizing STC. Graphite, sodium nitrate (NaNO3), potassium permanganate (KMnO4), sulfuric acid (H2SO4, >96%), hydrochloric acid (HCl), hydrogen peroxide (H2O2) and ammonia solution (30% NH3) were used in catalyst synthesis. Ethyl acetate (EtOAc) and tetrahydrofuran (THF) were used in the separation of the product fraction. The phenolic compounds: phenol, p-cresol (4-methoxy phenol), o-guaiacol (2-methyl phenol), 4-ethyl phenol, catechol (1,2-benzenediol), 4-ethyl guaiacol, vanillin (4-hydroxy-3-methoxybenzaldehyde), and 3,4-dimethoxy benzaldehyde were used as analytical standards. All chemicals were purchased from Wako Pure Chemical Industries Ltd. (Japan). 2.2. Catalyst Synthesis. Graphene oxide (GO) was synthesized from graphite by the modified Hummer’s method.41 Concentrated H2SO4 (92 mL) and NaNO3 (2 g) were added to graphite powder (2 g) contained in a 500 mL glass beaker and stirred for 30 min by magnetic stirrer. Then, 10 g of KMnO4 was added slowly to the solution in an ice bath to prevent the rapid increase temperature from oxidation reaction. The solution was moved to an oil bath at 35 °C and continuously stirred for 40 min to complete an oxidation step. Subsequently, the solution was slowly dropped with distilled water (92 mL) and transferred to an oil bath at 95 °C with stirring for 15 min for the hydrolysis reaction. Next, 200 mL of water were added into the solution and stirred. After that, 20 mL of H2O2 were added into the solution to stop the reaction by removing the excessive KMnO4. The solution was centrifuged by 3 000 rpm for 5 min to obtain precipitated graphene oxide. The precipitated mixture was washed with 5% HCl and centrifuged at 3 000 rpm three times for 5 min each. Subsequently, the mixture was washed with DI water and centrifuged at 4 000 rpm for three times for 30 min each. The next step was exfoliated by sonication. The temperature was held and did not exceed 60 °C for 4−6 h. Then, supernatant GO resulted after centrifugation at high-speed at 10 000 rpm for 30 min. Finally, the material was completely dried in a hot air oven at 60 °C for 72−96 h. Organosolv lignin obtained from Chemical Point UG (Germany) and methanol (solid loading = 10% w/v) were loaded into the pressurized reactor. Then, the air inside the reactor was removed by purging nitrogen gas several times. The STC was carbonized at constant temperature of 200 °C for 4 h. After that, the STC 13042

DOI: 10.1021/acs.iecr.9b01973 Ind. Eng. Chem. Res. 2019, 58, 13041−13052

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Figure 1. Schematic diagram of product separation process after depolymerization reactions.

placed in the incubator shaker at 30 °C for 2 h with a constant speed at 200 rpm. Then, distilled water was added to adjust the concentration of sample solution reaching to 4%. After that the sample was autoclaved at 121 °C for 1 h. Next, the sample was filtered to separate the solid and liquid fractions. The solid fraction composed of acid insoluble lignin (AIL or Klason lignin) and acid insoluble ash (AIA) was air-dried at 105 °C before placing in the muffle furnace at 575 °C for 3 h. The percentage of each fraction was calculated based on dry weight matter. A LECO 628 series CHNS elemental analyzer from LECO Corporation was used to measure the elemental compositions, carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) content, in commercial alkaline lignin while the oxygen (O) content was calculated by difference. The results were used to calculate atomic ratios and empirical formula of the macromolecule of lignin given as a hypothetical hydroxyphenyl structural unit. The FT-IR analysis was used to interpret functional groups of lignin before and after the depolymerization process using the same method and operation with the part of catalyst characterization. Proton nuclear magnetic resonance with respect to hydrogen-1 nuclei (1H NMR) was also analyzed to determine the structure of lignin. Alkaline lignin was dissolved in deuterated dimethyl sulfoxide (DMSO-d6) and then was analyzed with 1H NMR spectroscopy using a Bruker Avance III HD 400 MHz spectrometer (Bruker, Germany). The Py-GC-MS analysis was performed using a multishot pyrolyzer EGA/PY-3030 D (Frontier, Japan) hyphenated with a gas chromatograph-mass spectrometer model 7890 B GC-5977A MSD (Agilent) operated by the single shot method at 500 °C. The scan range of the mass spectrum was 45 to 500 m/z, and the mass spectra were identified with NIST 14 and Polymer libraries. 2.4. Catalytic Depolymerization of Lignin. The catalytic depolymerization of lignin was conducted in a 10 mL tubular batch reactor (stainless steel type 316, 1.27 cm o.d., and 10 cm length) connected with a thermocouple for measuring actual temperature inside the reactor. The system contained 0.1 g of alkaline lignin, 0.01 g of catalyst (equal to 10 wt %/wt referred to initial lignin), and 5 mL of DI water.

was filtered and washed with methanol and dried at 105 °C overnight. For the synthesis of sulfonated solvothermal carbon (SO3-STC), concentrated H2SO4 and STC were mixed in a 3neck round-bottom flask. Then, the mixture was heated up to a temperature of 150 °C and kept constant and continuously stirred for 6 h. Next, functionalized STC with H2SO4 (SO3STC) was filtered and washed with hot DI water until the pH was equal to 5 and dried at 105 °C overnight. For nitrogendoped GO (N-GO) and nitrogen-doped STC (N-STC), 0.127 g of GO or STC and 5 g of 30% NH3 solution were mixed inside the 8.8 mL Inconel batch reactor. Then, the reaction temperature and reaction time were set at 250 °C for 3 h. Finally, the synthesized N-GO and N-STC were washed with ethanol and dried in an oven at 60 °C overnight. 2.3. Characterizations. The synthesized catalysts were carried out to identify properties of catalysts by XRD, FT-IR, and SEM techniques. The specific area, pore volume, and pore size diameter of synthesized catalysts were determined by the N2-physisorption technique and calculated with a the Brunauer-Emmet-Teller (BET) method using a BELSORPmini analyzer (BEL Japan, Tokyo, Japan). All catalysts were degassed under vacuum at 150 °C for 3 h to remove moisture and air residual before the measurement. The XRD technique was carried out using a Bruker model D8 Discover with GADDS with Cu Kα radiation of a wavelength of 1.54006 Å from 10 to 80° of 2θ at a rate of 0.05° s−1. Infrared spectra of all synthesized carbon-based catalysts were obtained by a FTIR spectrophotometer (PerkinElmer Spectrum One FT-IR spectrometer) using the potassium bromide (KBr) pellet method. The measurement resolution was set at 4 cm−1 with 64 coadded scans in the range of infrared spectrum 4000 to 500 cm−1 with TGS detector. Additionally, the morphologies of the synthesized carbon-based catalysts were observed by SEM (Hitachi S-3400, Japan) operated at an accelerating voltage of 15 kV. The alkaline lignin prior to the experimental run was analyzed by the Klason lignin method, elemental analysis, FT-IR, 1H NMR, and Py-GC-MS. For the Klason lignin method, lignin sample (0.3 g) was mixed with 3 mL of 72% H2SO4 in 100 mL-cylinder tubes. The sample tubes were 13043


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Figure 2. FT-IR spectrum of (a) GO (graphene oxide), (b) N-GO (nitrogen-doped graphene oxide), (c) STC (solvothermal carbon), (d) SO3STC (sulfonated solvothermal carbon), and (e) N-STC (nitrogen-doped solvothermal carbon).

Then, nitrogen gas was flowed several times into the reactor for leak testing and removing the air in the reactor. After that the reactor was placed in a preheated zone of an electrical furnace for heating to 250 °C with a heating rate of 10 °C min−1. A constant heating rate was fixed in every experiment for eliminating the effect of the complication of the intermediate reaction that would occur before the actual depolymerization was achieved. The heating rate is the one of the strong influences on the degradation of lignin.42 Next, the reactor was moved into reaction zone of the furnace and kept constant at a temperature of 250 °C until a complete reaction. From the start to the end of the depolymerization reaction, the reactor was continuously shaken for obtaining a well-mixed system. After the depolymerization reaction was completed, the reaction was immediately stopped at room temperature by submerging the reactor into a water bath. The procedure for separation of products after the depolymerization step is shown in Figure 1. First, the reaction product mixture solution was acidified with 1 M of H2SO4 until reaching pH < 2 and centrifuged to separate the liquid fraction (phenolic products) and solid fraction (unconverted lignin). Next, the liquid fraction was extracted by using the liquid−liquid extraction technique with ethyl acetate in order to extract phenolic monomer products. The solid fraction was a mixture of unconverted lignin, high-molecular weight degraded lignin, char, and catalyst. The solid fraction was dissolved in THF and filtrated. Unconverted lignin and highmolecular weight product that are represented in terms of residual lignin were dissolved in the THF-soluble fraction, whereas the char and catalyst remained in the THF-insoluble fraction. The phenolic monomer products from lignin depolymerization contained in the organic phase were identified by GC-MS and quantified by GC-FID with the internal standard calibration method using 3,4-dimethoxybenzaldehyde as an internal standard compound. The monomer products were identified by GC-MS (Agilent Technologies Inc.) equipped

with a HP-5MS capillary column (5% phenyl methyl siloxane, 60 m × 250 μm × 0.25 μm, Agilent Technologies Inc.). The temperature program was as follows: oven temperature start at 40 °C followed by heating to 300 °C for 5 min (at 5 °C min−1). The injector and detector were set at 200 and 325 °C, respectively. Quantification of the main monomeric compounds in the sample was carried out with a GC-2014 (Shimadzu, Japan) with FID using the same column and conditions as the GC-MS experiments. All the main phenolic products (i.e., phenol, p-cresol (4-methoxyphenol), o-guaiacol (2-methylphenol), 4-ethylphenol, catechol (1,2-benzenediol), 4-ethyl guaiacol, and vanillin (4-hydroxy-3-methoxybenzaldehyde)) were used as standard compounds. The conversion of lignin, the yield of residual lignin (unconverted lignin as well as high-molecular weight product), and char were calculated according to eqs 1−3, respectively. Additionally, the yields of the main phenolic monomer products; phenol, p-cresol, o-guaiacol, 4-ethyl phenol, catechol, 4-ethyl guaiacol, and vanillin were evaluated according to eq 4. ji weight of initial lignin (g) − weight of residual lignin (g) zyz zz = jjj j z weight of initial lignin (g) k { (1) × 100%

lignin conversion (%)

ij weight of residual lignin (g) yz zz × 100% = jjj j weight of initial lignin (g) zz k {

residual lignin (%)

(2)

ji weight of THF‐insoluble fraction − weight of catalyst (g) zyz zz = jjj j z weight of initial lignin (g) k { (3) × 100%

char yield (%)

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Figure 3. XRD patterns of (a) GO (graphene oxide), (b) N-GO (nitrogen-doped graphene oxide), (c) STC (solvothermal carbon), (d) SO3-STC (sulfonated solvothermal carbon), and (e) N-STC (nitrogen-doped solvothermal carbon).

Figure 4. Morphology of (a) GO (graphene oxide), (b) N-GO (nitrogen-doped graphene oxide), (c) STC (solvothermal carbon), (d) SO3-STC (sulfonated solvothermal carbon), and (e) N-STC (nitrogen-doped solvothermal carbon) analyzed by the SEM technique.

ij weight of phenolic monomer i (g) yz zz × 100% = jjj zz j weight of initial lignin (g) k {

The molecular weights of lignin samples before and after depolymerization were determined by the gel permeation chromatography (GPC) technique. GPC analyses were performed by using a Waters e2695 separation module (Waters Corporation) equipped with two PLgel 10 μm

phenolic monomer yield of i (%)

(4)

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the initial alkaline lignin are summarized in Table S1. The Klason lignin method was applied for the determination of lignin content and purity in lignin samples. It was found that the Klason lignin of alkaline lignin exhibited 81.30% and low sugar content in the lignin sample. Moreover, the CHNS elemental analysis resulted that the alkaline lignin composed of 51.58% C, 4.72% H, 4.47% S, and a trace amount of N. The sulfur and sodium content was found in alkaline lignin due to the use of NaOH and Na2S in the isolation process, while the nitrogen content was obtained from cell proteins in the original wood. The Py-GC-MS analysis was determined to classify the main pyrolysis lignin-derived products of alkaline lignin based on the relative p-hydroxyphenyl (H)-, guaiacyl (G)-, and syringyl (S)type compounds, which are summarized in Table S2. Guaiacyland p-hydroxyphenyl-type compounds were the predominant product from pyrolysis of alkaline lignin. Trace amounts of syringyl type compounds were identified in the alkaline lignin pyrolysis product. The abundance of guaiacyl- and phydroxyphenyl-type compounds can conclude that the alkaline lignin sample belongs to characteristics of softwood lignin.47 1 H NMR spectra of the raw alkaline lignin were presented in Figure S1. The spectra results were analyzed according to the literature for identifying the lignin structure.48,49 The chemical signals between 6.0 and 8.0 ppm can be described as aromatic protons in the guaiacyl and syringyl units. The chemical shift of the methoxyl protons (−OCH3) illustrated sharp signals between 3.55 and 3.95 ppm. The chemical shift of the signal at 2.5 ppm corresponded to DMSO-d6 while those signals between 2.0 and 0.8 ppm are from the aliphatic protons or the hydrocarbon protons of lignin. 3.2. Catalytic Activities toward Lignin Depolymerization. The synthesized carbon-based catalysts (i.e., GO, N-GO, STC, SO3-STC, and N-STC) were tested toward the depolymerization of lignin at 250 °C for 3 h with the catalyst loading of 10 wt %. It is noted that the reaction without catalyst was also carried out for comparison. Their performance in terms of lignin conversion, residual lignin, and char formation were evaluated and presented in Table 1. In the

MIXED-B columns (MW resolving range 500−10 000 000) and a refractive index (RI) detector model 3580 (Viscotek). Analyses were carried out at 35 °C using THF as the eluent at a flow rate of 1.0 mL min−1. Samples were dissolved in THF (2 mg mL−1) overnight and then filtered using a nylon-66 membrane (0.45 μm pore size) before injection. The GPC system was calibrated with polystyrene standards in a molecular weight range from 1 220 to 1 214 000 g mol−1.

3. RESULTS AND DISCUSSION 3.1. Catalyst and Lignin Characterizations. The FT-IR spectra of all synthesized carbon-based catalysts (i.e., GO, NGO, STC, SO3-STC, and N-STC) were presented in Figure 2. It was found that all synthesized carbon-based catalysts show a broad band at 3100−3600 cm−1 attributed to stretching of the hydroxyl group (−OH). The synthesized GO demonstrated a peak at 1594 and 1735 cm−1 related to the CC stretching vibration and CO stretching of the carbonyl group (−COOH), respectively. The C−O epoxide group had observed peaks at 1062 and 1232 cm−1. Similar spectra were observed for N-GO. However, the infrared intensities of N-GO provided much lower IR intensities as compared to GO due to loss of an oxygen-containing functional group on the GO structure. In the case of STC-based catalysts, we found that SO3-STC presented broad vibration bands at 1050 and 1495 cm−1 indicating the SO3− stretching and OSO stretching in SO3H, respectively. Meanwhile, the N-STC revealed a spectral band at 1641 cm−1 that is attributed to the N−H bending vibration of the primary amide group. XRD patterns of GO, N-GO, STC, and SO3-STC catalysts exhibited a strong, broad diffraction peak at 2θ = 18−30° and 43.5°, as shown in Figure 3. These XRD patterns were a typical characteristic of amorphous carbonaceous materials oriented in a random arrangement or graphitic structure.43,44 The graphitic peak of GO shifted from the characteristic peak of graphite at 2θ = 26.6° to 2θ = 10° after chemical oxidation of graphite powder due to successful integration by oxygen species in the graphite layers.45 The peak at 2θ = 10° disappeared in the XRD pattern of N-GO because of the decrease of the oxygen functionalities in the GO structure during the nitrogen functionalization process. After the sulfonation process, the SO3-STC catalyst presented a similar XRD pattern to the bare STC catalyst. In contrast, the diffraction peak of the amorphous carbonaceous disappeared in the N-STC catalyst and a new superstructural peak at 2θ ∼ 12° was presented in the XRD pattern, indicating extreme defect and disorder in the graphic structure of STC supports.46 From SEM images (Figure 4), the surface of GO showed layered smooth sheets with a large thickness. In addition, highly wrinkled flakes were observed on the N-GO surface because morphology of GO was change from GO into reduced GO on reduction reaction by losing of oxygen containing group from the GO structure. For SEM images of synthesized STC and SO3-STC, it was clear that both catalysts showed a similar shape and particle sizes. There is no doubt that the sulfonation process did not effect the structure and size of the STC material. Nonetheless, the structure of N-STC was reformed to a uniform microsphere after functionalizing STC with nitrogen. Before catalytic depolymerization testing, the characteristics of alkaline lignin as a raw material in this work were determined for understanding their structure properties and nature of lignin. The chemical and elemental compositions of

Table 1. Lignin Conversion, Residual Lignin, and Char Yield (%, w/w) Referred to Initial Lignin Weight Obtained from Depolymerization of Alkaline Lignin over Different Carbon-Based Catalystsa catalyst

lignin conversion (%)

residual lignin (%)

char (%)

without catalyst GO SO3-GO N-GO STC SO3-STC N-STC

39.30 52.50 46.60 49.20 48.10 50.60 55.20

60.70 47.50 53.40 50.80 51.90 49.40 44.80

25.80 18.00 19.60 13.30 2.80 8.50 7.30

Depolymerization reaction conditions: T = 250 °C, time = 3 h, lignin substrate concentration ∼2% (w/v), and catalyst loading ∼10% (w/ w) compared to initial lignin weight. a

presence of catalysts, the lignin conversion increased considerably, while less char was observed. Among carbonbased catalysts, STC, SO3-STC, and N-STC showed the best catalytic performance on suppressing char formation during lignin depolymerization. 13046


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rization over each synthesized carbon-base in the current work was proposed in Figure S2. These proposed mechanism through adsorption and interaction of the lignin molecule over surface functional groups of carbon-based catalyst was detailed. The GO catalyst has various oxygen functional groups including carboxyl (−COOH), hydroxyl (−OH), carbonyl (−CO), epoxy (−C−O−C−), and ketone (−CO) groups presented on the surface that acted as active sites for catalytic reaction. These functional groups induce acidic along with oxidizing properties to the GO catalyst. The acidic properties of GO can promote the acid-catalyzed reaction of the lignin molecule. The adsorption of lignin molecule on GO via an ether bond resulting in the formation of a hydrogenbond and followed by acid-catalyzed hydrolysis into phenolic products (Figure S2a). However, the GO showed the smallest yield, suggesting that the GO have a lower thermal stability than other catalysts. The active sites of GO (oxygen-containing functional groups) were eliminated during depolymerization of lignin at high temperature. Furthermore, the GO, N-GO, and SO3-STC gave the lower phenolic yield when compared with STC and N-STC. As shown in the FT-IR spectra (Figure 2), the GO, N-GO, and SO3-STC contained fewer −OH groups, which could decrease the ability of hydrophilicity in aqueous medium.50 The −OH functional group is widely known to be a highly hydrophilic functional group, with the hydrophilicity properties directly related to the catalytic performance. In this case, the hydrophilic surface of the catalyst facilitates the adsorption between the catalyst surface and hydrophilic reactants (lignin water-soluble) resulting in the enhancement of the bonding interactions of lignin accessing the active sites of the catalyst. The doping with nitrogen atom can increase hydrophilic property of N-GO and N-STC, conceptually, and the addition of the N atom containing an additional electron changed the electron density and increased the basicity of the carbon catalyst. These improvements can enhance the dispersion of catalyst in water, more adsorption of molecules of water, and increase the interaction of catalysts and substrates (e.g., lignin or lignin-intermediates). Moreover, nitrogen doped on carbon catalyst has strong electronegativity and a lone pair electrons resulting in Lewis base sites provided on the catalyst surface, resulting in improving a higher dispersion in water.51 Meanwhile, the introduction of nitrogen atom (N) into the carbon skeleton of both carbon-based materials (GO and STC) led to an increase of catalyst structure defects. The structure defect was clearly found in the N-STC catalyst structure corresponding to a superstructural peak in the XRD pattern (see Figure 3). These defects acted as an anchor resulting in an increase of the adsorption efficiency of lignin molecule substrates on the catalyst surface that could enhance the depolymerization activity. The reaction mechanism with N-doped carbon catalysts: N-GO and N-STC are proposed in Figure S2b,e, respectively. The reaction started with adsorption of the lignin molecule on the Lewis-base active sites followed by breaking of the ether bond to desired products using electron transfer/charge transfer interactions between the lignin substrate and the catalyst. The sulfonic groups (−SO3H) on the surface of the SO3-GO and SO3-STC catalysts (see Figure S2d) have similar roles with H2SO4, resulting in it being a good acid catalyst in the acid-catalyzed reaction. In contrast, the SO3-GO and SO3-STC catalyst still exhibited a low phenolic yield because the acidity of the sulfonated carbon catalyst is too strong, which leads to a further undesired side reaction of phenolic compound to

The phenolic products yield (wt %) referred to as initial lignin weight and product distribution over different carbonbased catalysts compared with the blank test are presented in Figure 5. It was found that the phenolic compounds, phenol, o-

Figure 5. Effect of carbon-based catalysts on phenolic product yield after depolymerization of alkaline lignin at 250 °C for 3 h.

guaiacol, catechol, 4-ethyl guaiacol, and vanillin, were observed as the main products in all cases. These compounds corresponded with lignin-derived products from Py-GC-MS analysis. The presence of catalyst for lignin depolymerization reaction can promote a higher phenolic product yield. Compared with all metal-free carbon-based catalysts, STC and N-STC exhibited best activity on total phenolic products yield. Although N-STC showed the highest surface area (18.70 m2 g−1) compared to another catalysts according to the surface area and pore size analysis (Table 2), the N-STC gave a lower Table 2. Textural Properties of the Different Catalysts catalyst

BET surface area (m2 g−1)

total pore volume (cm3 g−1)

mean pore diameter (nm)

GO SO3-GO N-GO STC SO3-STC N-STC

5.40 16.80 17.20 6.30 14.70 18.70

0.03 0.09 0.14 0.05 0.11 0.17

24.80 20.50 19.50 24.20 22.60 19.10

total yield of phenolic products (7.52%) than STC (7.61%) with a smaller surface area. Therefore, it seems to be not only the surface area; the pore volume and pore size were important parameters for the lignin depolymerization process. The catalytic activity should be additionally dependent on the unique characteristics of catalysts such as acidity/basicity, acid/basic strength, surface structure, and especially in type of active sites on catalyst.18 Importantly, the interaction between lignin with the active sites of the catalyst having an effect on bond cleavage in lignin were an interesting factor on the catalytic activity of lignin depolymerization. However, the accurate reaction mechanism of lignin depolymerization is not clearly understand at this stage, and we assume such a catalyst plays a role based on the review. The possible reaction mechanism of lignin depolyme13047


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Industrial & Engineering Chemistry Research another compounds. Moreover, it can indicate that the −SO3H groups in the sulfonated carbon catalyst were decomposed during the depolymerization process, resulting in the lack of active sites on the catalyst surface.52 Nevertheless, it was confirmed that intense studies are required to obtain some evidence for supporting this proposed reaction mechanism. A comparison was done of the catalyst performance of the metal-free carbon-based catalyst with the conventional metalbased catalysts in term of activity and selectivity. Recently, Hita et al. has evaluated using four different noble metals (Ru, Pt, Pd, Rh) on activated carbon and Al2O3 support for depolymerizing Kraft lignin at 450 °C for 4 h with a high H2 pressure of 100 bar. The best catalytic activity was obtained when using Rh/Al2O3 as a catalyst, which gives a total monomer yield of 30 wt % composed of alkylphenolics, aromatics, oxygenated, and alkane compounds. Afterward, the catalyst deactivation was tested by its activity of the reusability of the Rh/Al2O3 catalyst. It was found that the total monomer yield dropped to 20.70 wt % when using regenerated Rh/Al2O3 catalyst compared with 30 wt % obtained from fresh catalyst (calculated to ∼31% deactivation) within only one cycle of reaction.53 Meanwhile, previous studies from our group have reported on the catalytic depolymerization of organosolv lignin using carbonaceous solid acids (CSA) derived from the hydrothermal process of lignocellulosic compounds (glucose, cellulose, and lignin) at 350 °C for 3 h using MIBK as the solvent. We found that the CSA catalyst provided a low deactivation percentage of only 8.14% after testing in five consecutive cycles of the depolymerization reaction.54 3.3. Effect of Operating Conditions. N-STC demonstrated the best catalyst activity giving the highest lignin conversion of 55.20%, phenolic products yield, and small char formation. Moreover, the N-STC can enhance a higher yield of catechol and vanillin in lignin phenolic product that is a valuable phenolic product. Consequently, N-STC catalyst was selected for optimizing the catalytic activities of the depolymerization of lignin. The depolymerization of lignin was performed at a reaction temperature of 250 °C with different reaction times of 1−9 h in the presence of N-STC for examining the effects of reaction time on lignin conversion, residual lignin, and char formation. The results shown in Table S3 demonstrated that the reaction time was a significant parameter. The conversion of lignin and char formation had a significant increase, whereas residual lignin declined with prolonged reaction times. According to previous studies, lignin was converted to char by repolymerization.55 Figure 6 shows the effect of reaction time on phenolic product yield and product distribution in the presence of N-STC. It was found that the phenolic yield increased with increasing the reaction time from 1 to 3 h. Guaiacol achieved the highest concentration for 3 h before decreasing with a longer time. At a longer reaction time, the phenolic yield had destructive effects. Cracking reactions, condensation, and/or repolymerization occurred simultaneously during lignin decomposition. This is confirmed from the proportions and components of phenolic compounds in liquid product. Therefore, it can be concluded that the phenolic compounds were easily reacted and rapidly degraded into other compounds. The molecular weight of original lignin and residual lignin were measured for investigating the changes in lignin structure and elucidating the interaction between depolymerization and repolymerization reactions. The weight-average molecular weight (Mw), number-average molecular weight (Mn), and

Figure 6. Effect of reaction time on lignin-derived product and phenolic product yield after depolymerization of alkaline lignin over N-STC catalyst at 250 °C.

polydispersity index (PDI) of the alkaline lignin and residual lignin after depolymerization at different reaction times are presented in Table 3. The average molecular weight (Mw) Table 3. Molecular Weight of Raw and Residual Lignin at Different Reaction Conditions catalyst N-STC

a

temperature (°C)

time (h)

Mn (g/mol)

Mw (g/mol)

PDI ( Mw/Mn)

alkaline lignina 250 250 250 250 250

1 3 5 7 9

N/A 1 011 950 981 887 976

∼10 000 1 213 1 123 1 162 1 010 1 191

N/A 1.201 1.183 1.185 1.139 1.220

Data from Sigma-Aldrich company.

clearly exhibited that it was continuously decreased with the increasing retention time of depolymerization from 1 to 7 h. The results indicated that the alkaline lignin was degraded to phenolic monomers or smaller molecular weight compounds. At a longer reaction time (9 h), Mw of residual lignin increased, and this could be explained that lignin was repolymerized to form a larger molecule. These results corresponded to the previous studies, e.g., Brandt et al.56 and Huang et al.57 They have suggested that lignin was depolymerized in the early state of reaction resulting in the molecular weight of lignin decreasing, then the molecular weight increased because repolymerization and condensation reaction took place at prolonged times. The comparison of the lignin structure and the interfunctional group in raw alkaline lignin and residual lignin were examined by FT-IR analysis for studying structural changes occurring in lignin samples during depolymerization at different reaction times, and the FT-IR spectra are shown in Figure 7. The spectrum of the alkaline lignin presented a broad absorption band at 3432 cm−1 that is attributed to hydroxyl groups (−OH) in the phenolic and aliphatic structures 13048


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Figure 7. FT-IR spectra of (a) alkaline lignin and residual lignin after depolymerization at 250 °C over the N-STC catalyst for (b) 1 h, (c) 3 h, (d) 5 h, (e) 7 h, and (f) 9 h.

followed by a band at 2938 and 2846 cm−1 resulting from C− H asymmetric stretching of methyl and methylene groups in the aromatic structure, respectively. The band at 1634 cm−1 corresponded to the CO stretching in the conjugated carboxylic acid and ketone groups. Additionally, the bands at 1596 and 1508 cm−1 related to aromatic skeleton vibrations. The band of the asymmetric aliphatic C−H deformations of methoxyl and methylene groups appeared at 1459 and 1420 cm−1. The bands at 1326, 1266, and 1138 cm−1 correlated with C−H deformation and CO stretching vibrations of guaiacyl and syringyl lignin. In addition, the bands of 1216, 1042, and 851 cm−1 are attributed to the C−C or C−O stretching, aromatic C−H deformation vibrations in-plane, and the C−H stretching vibrations out-of plane. When considering raw alkaline lignin and residual lignin after depolymerization at different reaction times, the lignin structure were not obviously changed, resulting in similar spectra. However, the band intensity especially in the −OH group in phenolic and aliphatic structures was dramatically decreased. Moreover, the band intensity of the asymmetric aliphatic C−H deformations was also reduced, suggesting that the demethoxylation and demethylation reactions of lignin possibly arise during the depolymerization process.58 The effect of reaction temperature on lignin depolymerization activity was varied from 250 to 400 °C at a constant reaction time of 3 h (Table S3). The lignin conversion was dramatically increased from 55.20 to 92.90% with increasing reaction temperatures from 250 to 400 °C, and char formation increased from 7.30 to 27.90%. The repolymerization of degraded lignin, unsaturated oligomer, and smaller molecular weight compounds into char occurred remarkably at high reaction temperatures.55 The unsaturated bond in lignin molecule can be repolymerized when the temperature was higher than 240 °C.59 Considering the phenolic product yield, it was found that the reaction temperature is an important parameter for lignin depolymerization in terms of product yield and product

distribution (Figure 8). The highest yield of guaiacol and vanillin were obtained at the lowest reaction temperature (250

Figure 8. Effect of reaction temperature on lignin-derived product and phenolic product yield after depolymerization of alkaline lignin over N-STC catalyst for 3 h.

°C) and dramatically decreased with increased reaction temperature. Moreover, increasing the reaction temperature led to a higher concentration of phenol, p-cresol, 4-ethyl phenol, and 4-ethyl guaiacol. Phenol can be formed directly by demethoxylation of guaiacol or demethylation of guaiacol followed by dehydration or hydrogenolysis.60 While, p-cresol appeared only after 250 °C and increased with an increase in reaction temperature, it could be a result of transalkylation of guaiacol followed by hydrodeoxygenation delivery to p13049


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Figure 9. GC-MS chromatograms of the phenolic products from depolymerization of alkaline lignin over N-STC catalyst at different reaction temperatures for 3 h: (A) 250 °C, (B) 300 °C, (C) 350 °C, and (D) 400 °C.

cresol.61 Otherwise, p-cresol can be produced by conversion of vanillin, resulting in higher vanillin conversion into p-cresol was shown at higher temperature that has been reported by Kayalvizhi and Pandurangan.62 Catechol could be formed by demethylation of guaiacol, which is in agreement with Forchheim et al.63 They found that the amount of catechol would be increased with increasing reaction temperature and retention time. From these results that are mentioned above, it was interpreted that the decreasing concentration of guaiacol due to the reactive intermediate further converting to other compounds. The possible reaction pathways of the degradation of phenolic compounds affected the change in the phenolic

product distribution at various operating conditions as suggested in Figure S3. Additionally, product distributions were clearly observed in GC-MS chromatograms as shown in Figure 9. It was found that the compositions and concentrations of phenolic compounds were obtained in dissimilar chromatograms. This observation provided clear support for confirmation of intermediate decomposition and further reactions resulting from increasing the reaction temperature. 13050


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(2) Zakzeski, J.; Bruijnincx, P. C.; Jongerius, A. L.; Weckhuysen, B. M. The catalytic valorization of lignin for the production of renewable chemicals. Chem. Rev. 2010, 110 (6), 3552−3599. (3) Kleinert, M.; Barth, T. Phenols from lignin. Chem. Eng. Technol. 2008, 31 (5), 736−745. (4) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem. Rev. 2006, 106 (9), 4044−4098. (5) Gardner, D. J.; Schultz, T. P.; McGinnis, G. D. The pyrolytic behavior of selected lignin preparations. J. Wood Chem. Technol. 1985, 5 (1), 85−110. (6) Windt, M.; Meier, D.; Marsman, J. H.; Heeres, H. J.; de Koning, S. Micro-pyrolysis of technical lignins in a new modular rig and product analysis by GC−MS/FID and GC× GC−TOFMS/FID. J. Anal. Appl. Pyrolysis 2009, 85 (1−2), 38−46. (7) Saisu, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Conversion of lignin with supercritical water-phenol mixtures. Energy Fuels 2003, 17 (4), 922−928. (8) Sricharoenchaikul, V. Assessment of black liquor gasification in supercritical water. Bioresour. Technol. 2009, 100 (2), 638−643. (9) Elliott, D. C.; Hallen, R. T.; Sealock, L. J., Jr Alkali catalysis in biomass gasification. J. Anal. Appl. Pyrolysis 1984, 6 (3), 299−316. (10) Villar, J. C.; Caperos, A.; Garcia-Ochoa, F. Oxidation of hardwood kraft-lignin to phenolic derivatives with oxygen as oxidant. Wood Sci. Technol. 2001, 35 (3), 245−255. (11) Man, X.; Okuda, K.; Ohara, S.; UMETSU, M.; TAKAMI, S.; ADSCHIRI, T. Disassembly of Organosolv Lignin in Supercritical Fluid. Nippon Enerugi Gakkaishi 2005, 84 (6), 486−490. (12) Lavoie, J. M.; Baré, W.; Bilodeau, M. Depolymerization of steam-treated lignin for the production of green chemicals. Bioresour. Technol. 2011, 102 (7), 4917−4920. (13) Roberts, V.; Stein, V.; Reiner, T.; Lemonidou, A.; Li, X.; Lercher, J. A. Towards quantitative catalytic lignin depolymerization. Chem. - Eur. J. 2011, 17 (21), 5939−5948. (14) Beauchet, R.; Monteil-Rivera, F.; Lavoie, J. M. Conversion of lignin to aromatic-based chemicals (L-chems) and biofuels (L-fuels). Bioresour. Technol. 2012, 121, 328−334. (15) Toledano, A.; Serrano, L.; Labidi, J. Organosolv lignin depolymerization with different base catalysts. J. Chem. Technol. Biotechnol. 2012, 87 (11), 1593−1599. (16) Gasson, J. R.; Forchheim, D.; Sutter, T.; Hornung, U.; Kruse, A.; Barth, T. Modeling the lignin degradation kinetics in an ethanol/ formic acid solvolysis approach. Part 1. Kinetic model development. Ind. Eng. Chem. Res. 2012, 51 (32), 10595−10606. (17) Forchheim, D.; Gasson, J. R.; Hornung, U.; Kruse, A.; Barth, T. Modeling the lignin degradation kinetics in a ethanol/formic acid solvolysis approach. Part 2. validation and transfer to variable conditions. Ind. Eng. Chem. Res. 2012, 51 (46), 15053−15063. (18) Deepa, A. K.; Dhepe, P. L. Lignin depolymerization into aromatic monomers over solid acid catalysts. ACS Catal. 2015, 5 (1), 365−379. (19) Daorattanachai, P.; Khemthong, P.; Viriya-empikul, N.; Laosiripojana, N.; Faungnawakij, K. Conversion of fructose, glucose, and cellulose to 5-hydroxymethylfurfural by alkaline earth phosphate catalysts in hot compressed water. Carbohydr. Res. 2012, 363, 58−61. (20) Hanson, S. K.; Wu, R.; Silks, L. A. C-C or C-O Bond Cleavage in a Phenolic Lignin Model Compound: Selectivity Depends on Vanadium Catalyst. Angew. Chem., Int. Ed. 2012, 51 (14), 3410−3413. (21) Mottweiler, J.; Puche, M.; Räuber, C.; Schmidt, T.; Concepción, P.; Corma, A.; Bolm, C. Copper-and VanadiumCatalyzed Oxidative Cleavage of Lignin using Dioxygen. ChemSusChem 2015, 8 (12), 2106−2113. (22) Son, S.; Toste, F. D. Non-Oxidative Vanadium-Catalyzed C-O Bond Cleavage: Application to Degradation of Lignin Model Compounds. Angew. Chem., Int. Ed. 2010, 49 (22), 3791−3794. (23) Hanson, S. K.; Baker, R. T.; Gordon, J. C.; Scott, B. L.; Thorn, D. L. Aerobic oxidation of lignin models using a base metal vanadium catalyst. Inorg. Chem. 2010, 49 (12), 5611−5618.

4. CONCLUSION The alkaline lignin was depolymerized to phenolic monomers and/or low-molecular weight products using synthesized metal-free carbon-based catalysts including GO, N-GO, STC, SO3-STC, and N-STC. The major compounds were phenol, pcresol, 4-ethyl phenol, 4-ethyl guaiacol, and vanillin. Among the metal-free carbon-based catalysts studied, N-STC exhibited the most effective catalyst presented in terms of lignin conversion up to 55.20% and monomer products yield of 7.52%. Moreover, the highest yield of catechol and vanillin were produced in the presence of N-STC catalyst. It was also revealed that the reaction time and reaction temperature were significant parameters influencing catalytic activities in term of lignin conversion, phenolic product yield, and product distribution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01973.

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Figure S1, 1H NMR spectra of alkaline lignin measured in DMSO-d6; Figure S2, proposed mechanism scheme through adsorption and interaction of lignin molecule over surface functional groups of carbon-based catalyst: GO, N-GO, STC, SO3-STC, and N-STC; Figure S3, possible reaction pathways on degradation of phenolic compounds; Table S1, chemical and elemental compositions of alkaline lignin sample; Table S2, lignin-derived compound analysis identified by Py-GC-MS from pyrolysis lignin; Table S3, effect of reaction time and reaction temperature on lignin conversion, residual lignin, and char yield (%, w/w) referring to initial lignin weight obtained from depolymerization of alkaline lignin with N-STC (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +662-872-9014. Fax: +662-872-6736. E-mail: [email protected]. ORCID

Pornlada Daorattanachai: 0000-0001-5071-1036 Armando T. Quitain: 0000-0003-1051-3726 Tetsuya Kida: 0000-0001-9357-9557 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge The Joint Graduate School of Energy and Environment (JGSEE), Petchra Pra Jom Klao Doctoral Scholarship, King Mongkut’s University of Technology Thonburi (KMUTT) for financially supported this project. The Graduate School of Science and Technology, Kumamoto University, Japan, is acknowledged for supporting the experimental facilities. The authors also express their sincere appreciation to the Thailand Research Fund (TRF, Grant Number RTA5980006) for supporting the study financially.

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REFERENCES

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