Abstract
Lignin depolymerization through an oxidation method could provide value-added products, but it is challenging in terms of recovering catalysts or separating products in time to avoid over-oxidation. In this study, a process of selectively oxidative degradation of lignin model compounds was operated in a two-phase reaction system. Lignin model compounds of 4-benzyloxyphenol (PBP) or guaiacylglycerol-β-guaiacyl ether (GGE) in a bottom phase of 1-butyl-3-methylimidazole chloride ([BMIM]Cl) ionic liquid were selectively oxidized by H2O2 in the presence of a solid acid (SO42-/Fe2O3-ZrO2), and the degradation products immediately diffused into the upper organic solvent phase (butyl acetate). In this kind of reaction system, the yield of the products was improved due to the prolonged life of ∙OH in ionic liquid, and the product selectivity was maintained due to the timely product separation, and the ionic liquid and the catalyst were easily recycled.
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A Two-phase Reaction System for Selective Oxidative Degradation of Lignin Model Compounds
Yueying Chen, Aiguo Xue, Haomin Jiang, Yujuan Cheng, Yuan Ren, Yanzhi Sun, and Yongmei Chen *
Lignin depolymerization through an oxidation method could provide value-added products, but it is challenging in terms of recovering catalysts or separating products in time to avoid over-oxidation. In this study, a process of selectively oxidative degradation of lignin model compounds was operated in a two-phase reaction system. Lignin model compounds of 4-benzyloxyphenol (PBP) or guaiacylglycerol-β-guaiacyl ether (GGE) in a bottom phase of 1-butyl-3-methylimidazole chloride ([BMIM]Cl) ionic liquid were selectively oxidized by H2O2 in the presence of a solid acid (SO42-/Fe2O3-ZrO2), and the degradation products immediately diffused into the upper organic solvent phase (butyl acetate). In this kind of reaction system, the yield of the products was improved due to the prolonged life of ∙OH in ionic liquid, and the product selectivity was maintained due to the timely product separation, and the ionic liquid and the catalyst were easily recycled.
Keywords: Lignin model compound; Products selectivity; Ionic liquid; Solid acid
Contact information: Institute of Applied Electrochemistry, College of Chemistry, Beijing University of Chemical Technology, No.15 North Third Ring Road, Chaoyang District, Beijing 100029, China;
* Corresponding author: chenym@mail.buct.edu.cn
INTRODUCTION
Lignin makes up a significant portion of the mass of dry wood and, after cellulose, it is the second most abundant form of organic carbon in the biosphere. Lignin is also regarded as a unique renewable aromatic resource in nature (Amidon and Liu 2009), because it is composed of a large number of phenylpropyl (C9) units, such as guaiacyl, syringyl, or p-hydroxypropyl units and the propyl side-chains. Since these units are randomly linked with each other by the C-O-C bonds (such as α-O-4, β-O-4, and 4-O-5) or C-C bonds (such as 5-5, β-5, and β-β) (Xu et al. 2014) and the linkage bonds are usually weak, the small-weight aromatic compounds could be obtained if these linkage bonds could be selectively cleaved by chemical methods. These selective depolymerization methods are the important ways of lignin valorization (Li and Song 2019).
Several chemical methods, such as pyrolysis (Yang et al. 2010; Persson et al. 2018), hydrolysis (Tarabanko et al. 2000; Yuan et al. 2010; Li et al. 2015), and hydrogenolysis (Xu et al. 2012; Song et al. 2013) are used to depolymerize lignin, and the researchers are trying to optimize the operation conditions under mild temperature and pressure with more selective productions. The cleavage of an aryl-alkyl ether bond occurs, catalyzed by a Lewis acid (Deepa and Dhepe 2015; Wang et al. 2018; Guan et al. 2020). Lignin or lignin model compounds could be depolymerized by H2O2 oxidative degradation, in which the generated reactive oxygen species (ROS) act as oxidizing reagent (Kang et al. 2019). Certain products are generated by selective cleavage of linkage bonds (Lei et al. 2017; Jiang et al. 2019). However, the product selectivity is reduced if a liquor having strong acidity (such as H2SO4) is used as the co-catalyst because over-oxidation of the products can occur, making product separation difficult.
Reactive oxygen species are stabilized in ionic liquids (ILs) because the large ion volume, steric hindrance, and solvation effect of ILs can slow the self-quenching process of radicals (Zigah et al. 2009). Some ILs, for example ILs with imidazolium cations, possess good dissolvability for lignin or lignin model compounds because of the strong π-π conjugation between their aromatic cations and the benzene rings in lignin molecules (Prado et al. 2016). If the depolymerized products could be separated immediately from the reaction system, ILs could be an ideal solvent for lignin depolymerization by oxidation. Otherwise, parts of phenol groups in the degraded products might be over-oxidized to aldehyde or carboxylic groups.
To avoid the over-oxidization of the depolymerization, an IL ([BMIM]Cl) and an organic solvent (butyl acetate), which are undissolved with each other, were chosen to compose a two-phase reaction system in this study. 4-Benzyloxyphenol (PBP) and guaiacylglycerol-β-guaiacyl ether (GGE) (Fig. 1) as lignin model compounds were dissolved in IL, and they were supposed to react with the ROS generated through the decomposition of H2O2 catalyzed by a solid acid (SO42-/Fe2O3-ZrO2), while the degraded products (compounds with single benzene ring) might transfer into the organic solvent due to the distribution coefficient. The cleavage mechanism of linkage bonds was considered in this study based on the products analysis and degradation rates.
Fig. 1. Molecular structures of (a) 4-benzyloxyphenol (PBP); (b) guaiacylglycerol-β-guaiacyl ether (GGE)
EXPERIMENTAL
Materials
Sulfuric acid, hydrogen peroxide (30%), potassium permanganate, iron nitrate nonahydrate, and ammonium hydroxide were purchased from Beijing Chemical Reagents Company (Beijing, China). Sodium oxalate was obtained from Guangfu Technology Development Limited Company (Tianjin, China). Zirconium oxychloride octahydrate was supplied by Jinke Chemical Research Institute (Tianjin, China). Methanol was purchased from Thermo Fisher (Shanghai, China), with chromatographically purity. The 4-benzyloxylphenol (PBP) and guaiacylglycerol-β-guaiacyl ether (GGE) were purchased from J&K Scientific (Beijing, China) and 1-butyl-3-methyl imidazole chloride ([BMIM]Cl) was supplied by Linzhou Branch Material Technology Co., Ltd (Linzhou, China).
Preparation of Solid Acid
First, 4.8 g of ZrOCl2·8H2O and 6.0 g Fe(NO3)3∙9H2O were dissolved in deionized water, and ammonia (30 wt%) was added dropwise to adjust the pH to 9 to 10 under vigorous stirring. The solution was stored at room temperature for 24 h. The filter residue was washed by deionized water until no Cl– was detected. After drying at 110 °C for 12 h, the solid was ground and sieved to obtain powder with particle size under 300 μm. The powder was immersed in 1.0 M sulfuric acid for 24 h. The filter residue was dried at 110 °C for 12 h and calcined at 650 °C for 4 h in a muffle furnace. After cooling to room temperature, the solid acid SO42-/Fe2O3-ZrO2 was obtained.
Characterization of Solid Acid
The X-ray powder diffraction (XRD) patterns were tested on a diffractometer with Cu Kα radiation (40 kV, 150 mA) and of wavelength 1.5406 Å (D-8 Advanced, Bruker, Karlsruhe, Germany). X-ray photoelectron spectroscopy (XPS) measurements were performed by an X-ray photoelectron spectrometer using Al as the exciting source (Escalab 250Xi, Thermo Fisher). The acidity of the solid acid was tested by a pyridine-thermal desorption-infrared (Py-IR) spectrograph, using a Fourier transform infrared spectrometer (Nicolet Is50, Thermo Fisher).
Degradation of Lignin Model Compounds
A total of 10.0 mg PBP was dissolved in 10.0 mL of [BMIM]Cl solution containing 15 wt.% water, which was used as the stock solution. Next, 1.0 mL of stock solution was pipetted for each degradation experiment, in which 2.0 to 10.0 mg of solid acid and 0.16 to 0.80 mol/L hydrogen peroxide were added. The degradation reactions were kept at different temperatures in a water bath under stirring for 2 h. GGE was degraded as the same process as described above.
Evaluation of Degradation Rate and Products Selectivity
For evaluating the degradation rate, 100 μL of the reaction solution was sampled before and after the degradation reaction. The amount of PBP or GGE before and after degradation were determined by a high-performance liquid chromatography (HPLC, Thermo Fisher Ultimate 3000) based on standard curve method with the linear regression coefficient of 0.9992. The degradation rate (DR) is shown as Eq. 1,
DR (%)=V/V0 × 100% (1)
where V0 and V (mAU∙min) are the peak areas of lignin model compounds before and after degradation, respectively.
Since the signals of the degradation products are covered by the signal of IL, the relative amounts of the degradation products in the reaction solution were determined by GC-MS after the extracted by organic solvent. The reaction solution after degradation was extracted twice with butyl acetate, and the butyl acetate phase was combined together. The compounds in it were identified by a gas chromatography-mass spectrometer (GC-MS, Shimadzu QP 2010 Ultra, Tokyo, Japan). The operation details of both HPLC and GC-MS are described in the Supplementary Information.
RESULTS AND DISCUSSION
Structure and Acidity Characterization of the Solid Acid
The XRD patterns of the prepared solid acid SO42-/Fe2O3-ZrO2 (Fig. 2a) disclosed that SO42-/Fe2O3-ZrO2 is amorphous, while ZrO2 and SO42-/ZrO2 prepared through the same process displayed perfect crystallinity. The phenomenon indicated that there were defect sites introduced in SO42-/Fe2O3-ZrO2 by doping of Fe.
Fig. 2. (a) XRD patterns of ZrO2, SO42-/ZrO2 and SO42-/Fe2O3-ZrO2; (b) Py-IR spectra of SO42-/Fe2O3-ZrO2 at 150 C, 250 C, and 350 C ; (c) O1s XPS spectra of SO42-/ZrO2 and SO42-/Fe2O3-ZrO2; (d) diagram of L-acid and B-acid sites formed in SO42-/Fe2O3-ZrO2
The Py-IR spectra of SO42-/Fe2O3-ZrO2 at different temperatures is shown in Fig. 2b. According to Yue et al. (2019), the peak at 1445 cm-1 was assigned to the pyridine molecule adsorbed on the Lewis acid (L-acid) site of the solid acid, while the peak at 1540 cm-1 originated from the Brønsted acid (B-acid) site, and the peak at 1490 cm-1 was caused by both L-acid and B-acid sites. Although the intensity of these peaks decreased as the temperature increased, it is still obvious at 350 °C, meaning these L-acid and B-acid sites are strong enough.
The relative content of the oxygen vacancies was determined by XPS spectra shown in Fig. 2c. The O1s peaks in the spectra disclose two kinds of O atoms with different chemical atmosphere existed in the sample. The peaks at 530.01 and 531.68 eV are assigned to the oxygen atoms in lattice and the oxygen atoms in suspending hydroxyl groups, respectively (Ismail et al. 2019), and the peak originated from oxygen atoms in adsorbed SO42- might be too weak to be detected. The relative content of lattice oxygen in SO42-/Fe2O3-ZrO2 was 48.5%, while in SO42-/ZrO2 was 73.7%. The less lattice oxygen implied there were more oxygen vacancies introduced in SO42-/Fe2O3-ZrO2.
The schematic diagram of the acidic sites formed on SO42-/Fe2O3-ZrO2 is shown in Fig. 2d. The oxygen vacancies (Ovs) caused by the defects in lattice lead to the formation of L-acid sites on SO42-/Fe2O3-ZrO2, and the suspending –OH groups on the surface of the solid are related to the B-acid sites.
H2O2 Decomposition Catalyzed by Solid Acid
The bromocresol green (BG) method was used to confirm the kind of ROS generated from H2O2 decomposition catalyzed by solid acids. The detailed experimental section is described in the Supplementary Information in the Appendix. The results showed that the absorption intensity of BG solution decreased in the presence of H2O2 and the solid acid, but the absorption intensity was almost unchanged if only H2O2 or solid acid were present. As an extremely stable molecule, BG is reported to be only degraded by hydroxyl radicals (·OH) attacking the electronegative -Br group followed by degradation (Fassi et al. 2014). The results confirmed that hydroxyl radical (·OH) was generated through H2O2 decomposition catalyzed by the solid acids. Thus, the oxygen vacancies in the solid acids promoted the amounts of ·OH generated by decomposition of H2O2(Li et al. 2017), while ZrO2 itself demonstrates a positive effect on Fenton-like reaction (Gao et al. 2019).
Optimization of Degradation Conditions
Based on the degradation rate of PBP, the operating conditions were screened. The results showed that ionic liquid as the solvent was more favorable than methanol or acetonitrile due to the stabilization effect of ROS in IL. Very little PBP was degraded when just the solid acid was present, and only about 20% degradation rate could be achieved only with H2O2. Over 70% PBP was degraded when both of H2O2 and solid acid were present. It seems to be necessary to set up the reaction system using IL as the solvent with presence of H2O2 and solid acid.
The parameters of reaction conditions, such as catalyst dosage, H2O2 concentration, reaction time, and temperature on the degradation efficiency were systemically studied. The results are shown in Fig. 3. As shown in Fig. 3a, the degradation rate (DR) increased from 20.4% to 38.5% when 2.0 mg solid acid was added in 2 h, and the DR increased gradually up to 73.7% as the amount of catalyst was increased to 10.0 mg. More PBP were degraded as the H2O2 concentration increased (Fig. 3b), and over 90% PBP was degraded when 0.32 mol/L H2O2 was used. The remaining amount of PBP was decreased as the reaction time increased.
As shown in Fig. 3c, the degradation rate was about 57.8% in 0.5 h, and 94.3% in 2 h. When the temperature elevated, the degradation rate increased greatly (Fig. 3d). About 85.7% of PBP was degraded in 2 h at 60 °C, while less than 5% was degraded at 20C in 2 h.
Fig. 3. The degradation rate of PBP under different operation conditions, (a) with different dosage of catalyst; (b) with different initial concentration of H2O2; (c) as the reaction time under the initial H2O2 concentration of 0.80 mol/L;(d) at different temperature. The other operation conditions were the same as follows if not mentioned: the solid acid was SO42-/Fe2O3-ZrO2, the initial concentration of H2O2 was 0.80 mol/L, catalyst dosage was 10.0 mg, the temperature was 60 °C, and the duration was 2 h.
The above phenomena indicate that the DR of PBP strongly depends on the amount of hydroxyl radicals (·OH) that are produced through catalyzed decomposition of H2O2. The decomposition of H2O2 follows the first-order kinetic reaction with reaction rate constants of 0.0121 min-1 at 20 °C, 0.0396 min-1 at 40 °C, and 0.0965 min-1 at 60 °C, respectively (Lei et al. 2017). The amount of ·OH increased when the initial concentration of H2O2, the dosage of catalyst, and the reaction temperature were increased.
The degradation of another lignin model compound, GGE, in this reaction system was also performed. GGE has a β-O-4 bond, while PBP possesses an α-O-4 bond. As shown in Fig. 4d, 97.3% of GGE was degraded in presence of 0.80 mol/L H2O2 and 10.0 mg SO42-/Fe2O3-ZrO2, after reacting at 60 °C for 2 h.