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A Study of the Mechanisms of Guaiacol Pyrolysis Based on Free Radicals Detection Technology

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A Study of the Mechanisms of Guaiacol Pyrolysis Based on Free Radicals Detection Technology catalysts Article A Study of the Mechanisms of Guaiacol Pyrolysis Based on Free Radicals Detection Technology Guoxiang Li, Zhongyang Luo * , Wenbo Wang and Jianmeng Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China; [email protected] (G.L.); [email protected] (W.W.); [email protected] (J.C.) * Correspondence: [email protected]; Tel.: +86-5718-795-2440 Received: 6 February 2020; Accepted: 2 March 2020; Published: 5 March 2020 Abstract: In order to understand the reaction mechanism of lignin pyrolysis, the pyrolysis process of guaiacol (o-methoxyphenol) as a lignin model compound was studied by free radical detection technology (electron paramagnetic resonance, EPR) in this paper. It was proven that the pyrolysis reaction of guaiacol is a free radical reaction, and the free radicals which can be detected mainly by EPR are methyl radicals. This paper proposes a process in which four free radicals (radicals 1- C6H4(OH)O*, radicals 5- C6H4(OCH3)O*, methyl radicals, and hydrogen radicals) are continuously rearranged during the pyrolysis of guaiacol. Keywords: guaiacol; methyl; EPR; reaction pathway 1. Introduction Energy is an important material basis for human survival and development. Due to environmental problems and energy shortages caused by excessive use of fossil energy, biomass as a renewable energy source has received more and more attention [1–3]. Lignin is one of the three main components of lignocellulosic biomass, and the other two components are cellulose and hemicellulose. It is mainly located between cellulose fibers, and plays a role in resisting pressure. Among woody plants, lignin is the second largest organic substance in the world, accounting for 25% [4]. It is a major by-product of second-generation bioethanol production, and is the main impurity for separating cellulose from pulp and wood for papermaking. Because of its aromatic qualities, it has potential as biofuel and chemical aromatic hydrocarbon feedstock [5]. Lignin is a complex phenolic polymer formed from three alcohol monomers (p-coumarol alcohol, coniferyl alcohol, and sinapyl alcohol, see Figure1a). This polymerization is a chemically controlled random free-radical coupling reaction [6]. Due to different monomers, lignin can be divided into 3 types: syringin lignin (S-lignin), guaiacyl lignin (G-lignin), and p-hydroxyphenyl lignin (H-lignin) (Figure1b). They are generally connected by β-O-4, α-O-4 and 5-5 bonds, of which β-O-4 bonds are dominant and account for more than half of the lignin bond structure [7,8]. A large number of studies on lignin pyrolysis have been conducted in recent years. The pyrolysis of lignin and model compounds has been frequently studied by many researchers using methods like thermogravimetric analysis (TGA), Fourier Transform infrared spectroscopy (FTIR), gas chromatography (GC), mass spectrometry (MS), and others [9–14]. These extensive studies have reached a generally accepted hypothesis that pyrolysis follows complex reaction paths and that each reaction path is dominated by free radical reactions. Some of these radicals may be the cause of char formation [15–17]. The study of free radicals involved in lignin pyrolysis is mainly limited to ex situ measurements and inference based on reactants and products. Only recently, researchers have started to detect the generation of free radicals during lignin pyrolysis by using electron paramagnetic resonance (EPR) [18–22]. In order to further explore the reaction mechanism of the pyrolysis of lignin, Catalysts 2020, 10, 295; doi:10.3390/catal10030295 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 295 2 of 9 Catalysts 2020, 10, x FOR PEER REVIEW 2 of 10 the pyrolysis process of guaiacol (o-methoxyphenol) as a lignin model compound was studied by free radical detection technology (electron paramagnetic resonance) in this paper. (a) (b) Figure 1. StructureStructure of of (a ()a the) the three three alcohol alcohol monomers; monomers; (b) (theb) thehydroxyphenyl hydroxyphenyl (H), (H),guaiacyl guaiacyl (G), (G),and andsyringyl syringyl (S) units (S) units in the in lignin the lignin polymer. polymer. 2. ResultsA large number of studies on lignin pyrolysis have been conducted in recent years. The pyrolysis of lignin and model compounds has been frequently studied by many researchers using methods like 2.1. Py-GC/MS Experiments thermogravimetric analysis (TGA), Fourier Transform infrared spectroscopy (FTIR), gas chromatographyThe data seen (GC), in Figure mass2 spectrometrywas obtained (MS), by normalizing and others the [9–14]. results These of Py-GC extensive/MS experiments.studies have Asreached is shown a generally in Figure accepted2a, the total hypothesis amount ofthat guaiacol pyrolysis pyrolysis follows products complex reached reaction a maximumpaths and atthat 500 each◦C, whilereaction the path yield is at dominated 400 ◦C was by substantially free radical similarreactions. to thatSome at of 600 these◦C. Itradicals can be seenmay inbe Figurethe cause2a that of char the yieldsformation of both [15–17]. naphthalene The study and2-methyl-phenol of free radicals involved decreased in lignin with increasingpyrolysis is temperature, mainly limited and to the ex same situ appliesmeasurements to phenol, and although inference no formationbased on ofreactants phenol wasand detectedproducts. at Only 400 ◦ C.recently, Moreover, researchers the generation have ofstarted 2-hydroxy-benzaldehyde to detect the generation was only of detected free radica at 500ls◦ C.during The yield lignin of 1,2-dimethoxy-benzene pyrolysis by using electron reached theparamagnetic maximum atresonance 500 ◦C. According (EPR) [18–22]. to the productIn order analysis,to further at 400explore◦C, the the pyrolysis reactionreaction mechanism of guaiacol of the maypyrolysis only of be lignin, carried the out pyrolysis by simple process functional of guaiacol group (o-methoxyphenol) replacement, including as a lignin hydrogen model radical compound and methylwas studied substitution. by free radical And as detection the temperature technology increases, (electron demethoxylation paramagnetic resonance) becomes easier. in this Whenpaper. the temperature reaches 600 ◦C, the reduction in liquid phase product may be due to an increase in the product2. Results of the gas phase. A discussion of this mechanism will be mentioned later. 2.2.2.1. InPy-GC/MS Situ Observation Experiments of Pyrolysis ToThe further data seen investigate in Figure the e2ff wasect of obtained temperature by no onrmalizing free radical the formation, results of wePy-GC/MS performed experiments. isothermal pyrolysisAs is shown of guaiacolin Figure at2a, 400 the◦ totalC and amount 600 ◦C. of Andguaiacol the pyrolysispyrolysis processproducts and reached radical a maximum formation at were 500 observed°C, while bythe in yield situ at EPR 400 experiments. °C was substantially We observed similar every to that 30 sat after 600 the°C. It pyrolysis can be seen reaction in Figure reached 2a that the setthe temperature.yields of both The naphthalene result of in and2-methyl-phenol situ EPR experiments decreased can be seen with in increasing Figure3. temperature, and the same applies to phenol, although no formation of phenol was detected at 400 °C. Moreover, the generation of 2-hydroxy-benzaldehyde was only detected at 500 °C. The yield of 1,2-dimethoxy- benzene reached the maximum at 500 °C. According to the product analysis, at 400 °C, the pyrolysis reaction of guaiacol may only be carried out by simple functional group replacement, including hydrogen radical and methyl substitution. And as the temperature increases, demethoxylation becomes easier. When the temperature reaches 600 °C, the reduction in liquid phase product may be due to an increase in the product of the gas phase. A discussion of this mechanism will be mentioned later. Catalysts 2020, 10, 295 3 of 9 Catalysts 2020, 10, x FOR PEER REVIEW 3 of 10 Benzene,1,2-dimethoxy- Naphthalene Phenol,2-methyl- Benzaldehyde,2-hydroxy- 2.4E8 Phenol 1.5E8 1.4E8 Peak Area 5.4E7 4.4 444℃ 544℃ 644℃ (a) 444℃ 74 544℃ 644℃ 64 54 44 34 24 Content percentage (%) 14 4 ABCDE (b) (c) FigureFigure 2. 2. GuaiacolGuaiacol pyrolysis pyrolysis product product distribution. distribution. (a) product (a distribution) product distribution(b) proportion (b of) pyrolysis proportion productsof pyrolysis (c) A. products 1,2-dimethoxy-benzene; (c) A. 1,2-dimethoxy-benzene; B. naphthalene; B.C. naphthalene;2-methyl-phenol; C. 2-methyl-phenol;D. 2-hydroxy- benzaldehyde;D. 2-hydroxy-benzaldehyde; E. phenol. E. phenol. 2.2. In Situ Observation of Pyrolysis Catalysts 2020, 10, x FOR PEER REVIEW 4 of 10 To further investigate the effect of temperature on free radical formation, we performed isothermal pyrolysis of guaiacol at 400 °C and 600 °C. And the pyrolysis process and radical formation were observed by in situ EPR experiments. We observed every 30 s after the pyrolysis Catalystsreaction2020, 10reached, 295 the set temperature. The result of in situ EPR experiments can be seen in Figure4 of 9 3. 673K_4s 673K_34s -4.75 -4.75 -4.84 -4.84 -4.85 -4.85 -4.94 -4.94 Intensity Intensity -4.95 -4.95 -1.44 -1.44 -1.45 -1.45 3364 3384 3444 3424 3444 3464 3484 3364 3384 3444 3424 3444 3464 3484 Field (G) Field (G) 673K_64s 673K_124s -4.74 -4.75 -4.75 -4.84 -4.84 -4.85 -4.85 -4.94 -4.94 Intensity Intensity -4.95 -4.95 -1.44 -1.44 -1.45 -1.45 3344 3364 3384 3444 3424 3444 3464 3344 3364 3384 3444 3424 3444 3464 Field (G) Field (G) (a) 873K_4s 873K_34s -4.85 -4.85 -4.94 -4.94 Intensity Intensity -4.95 -4.95 -1.44 -1.44 3324 3364 3444 3444 3484 3524 3324 3364 3444 3444 3484 3524 Field (G) Field (G) 873K_64s 873K_124s -4.85 -4.85 -4.94 -4.94 Intensity Intensity -4.95 -4.95 -1.44 -1.44 3284 3324 3364 3444 3444 3484 3524 3284 3324 3364 3444 3444 3484 3524 Field (G) Field (G) (b) FigureFigure 3.
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