Article pubs.acs.org/EF
Pyrolysis Mechanism Study of Lignin Model Compounds by Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry Tao He,*,† Yimeng Zhang,†,‡ Yanan Zhu,§ Wu Wen,§ Yang Pan,§ Jingli Wu,†,‡ and Jinhu Wu*,† †
Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, Shandong 266101, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China ABSTRACT: To investigate the lignin pyrolysis mechanism, two α-O-4 and one completely substituted β-O-4 lignin dimeric model compounds were studied using in situ synchrotron vacuum ultraviolet photoionization time-of-flight mass spectrometry (SVUV PIMS) at 350−500 °C. The collision-reduced vacuum condition, in situ characteristic, and “soft” ionization technique of this reactor system allowed for the direct detection of thermolysis radicals and high-boiling-point compounds. For the α-O-4 compound 4-(benzyloxy)phenol (BOP) pyrolysis, benzyl radical, p-semiquinone radical, toluene, and bibenzyl were confirmed, supplying firm evidence for the free radical dominant mechanism. Experiments of the p-methoxy substituent on the hydroxyl position of BOP show that the p-methoxy substituent can lower the Caromatic−Cα and Oether−Caromatic bond dissociation energies. For β-O-4 compound guaiacylglycerol-β-guaiacyl ether (GGGE), it is inferred that the C−O homolysis mechanism was minor and the concerted reaction dominated in the experimental temperature range. Also, parallel experiments were conducted to find the temperature effect on the product distribution; it reveals that a high temperature leads to the further decomposition of the primary products and an increase of the aldehyde and ketone components.
1. INTRODUCTION Biomass is a sustainable source for the production of fuels and chemicals. Lignin is the second most abundant organic compound, next to cellulose, accounting for 30 wt % of dry wood. Lignin is an irregular macromolecule built and derived from polymerization of coniferyl, sinapyl, and p-coumaryl alcohol by over eight types of linkages, of which the β-O-4 and α-O-4 linkages occupied more than half of the linkage structures of lignin.1 Fast pyrolysis is a promising technology to convert biomass into fuels and chemicals.2 Through extensive work on pyrolysis of lignin over the past few decades, the weight-loss behavior, pyrolysis products, kinetic parameters, and apparent activation energies have been investigated yet, as a result of the complexity of the lignin structure and its high molecular weight, the detailed mechanism of thermolysis is still inadequate; an advanced research method and analytical facility are an imperative necessity.3−6 To know the intricate reaction of lignin conversion well, model compounds representing structural features present in lignin are studied instead. A detailed description of the previous study on pyrolysis of some lignin and model compounds has been reviewed and suggested that the radical mechanism is a main route during the early lignin degradation.7 Because β-O-4 is the most abundant structure in lignin, a mass of work corresponding to β-O-4-type model compounds had been dedicated to it. Literature8−10 reported pyrolytic decomposition of phenolic and non-phenolic β-ether dimers to investigate the effects of side-chain hydroxyl groups in pyrolysis. The results showed that Cγ−OH played an important role in the formation of quinone methide via the generation of hydrogen bonds between Cα− and Cγ−hydroxyl groups, and quinone methide © XXXX American Chemical Society
and a radical mechanism were proposed. Chu et al. employed a β-O-4-type oligomeric lignin model compound to study its pyrolysis behavior at a temperature range from 250 to 550 °C and proposed a free-radical reaction pathway to explain the product chemistry.11 Using photoionization mass spectrometry (PIMS), the presence of both homolytic bond breaking and concerted decomposition reactions was revealed and the concerted retro-ene and Maccoll reactions were dominant at low temperatures (below 1000 °C).12 Thermolysis of PhCD2CH2OPh and PhCH2CD2OPh indicated that there was no significant contribution of a concerted retro-ene pathway to the thermolysis of phenethyl phenyl ether.13 In recent years, computational studies were made to calculate carbon−oxygen and carbon−carbon bond dissociation enthalpies and concerted reaction barriers, transition states, intermediates, and products.14,15 Huang et al. reported that the concerted reactions would dominate over free-radical homolytic reactions at lower temperatures for the β-O-4 model compound and free-radical reactions dominated at lower temperatures for the α-O-4 model compound.16 However, Chen et al. showed that the lignin β-O-4 dimer would undergo the C−O bond homolysis at low pyrolysis temperatures.17 Through density functional theory (DFT) calculations, Beste et al. reported that the initial step in the decomposition of the phenethyl phenyl ether was the homolytic cleavage of the oxygen−carbon bond; also, results showed that activation Received: November 9, 2015 Revised: January 26, 2016
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DOI: 10.1021/acs.energyfuels.5b02635 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
sample using a fast pyrolysis analyzer (CDS5200) coupled to an Agilent 7890A GC with an Agilent 5975C MS system. Approximately 0.5 mg of sample was loaded in an quartz tube (2.0 mm inner diameter and 2.5 mm outer diameter) between quartz wool. The sample was pyrolyzed at a set point temperature at a ramp rate of 20 K/ms with the final dwell time of 15 s using helium as the carrier gas. The chromatographic peaks were identified according to the NIST08 mass spectrum library. All reactions have been at least performed in duplicate and reproduced within 95%.
energies for the concerted reactions were somewhat lower than the bond dissociation energies.18,19 Whereas the mechanism mentioned above was still controversial because of the shortage of supporting evidence, new advanced analytical techniques have been developed and employed to dramatically improve our knowledge of the degradation mechanisms as well as the composition of the liquid products.20,21 Here, we employ a recently developed technique called synchrotron vacuum ultraviolet photoionization equipped with a time-of-flight (TOF) mass spectrometry (SVUV PIMS) to detect isolated radicals as well as stable products during pyrolysis of α-O-4 and β-O-4 lignin model compounds. The synchrotron vacuum ultraviolet (SVUV) light of the Combustion Beamline at National Synchrotron Radiation Laboratory (NSRL) in Hefei has the merits of a broad photon energy range of 7.0−20.0 eV, energy-resolving power of 2000, and relatively high photon flux of 1 × 1013 photons/s. More importantly, SVUV photoionization is a “soft” ionization technique; the mass spectra with few or no fragment ions obtained by the ionization method is easily interpretable. The detailed description of SVUV PIMS can be found elsewhere.22 The pyrolytic behavior under flash heating and identification of products were also conducted by pyrolysis−gas chromatography/mass spectrometry (Py−GC/MS) to study the effect of the temperature on product distribution in parallel.
3. RESULTS AND DISCUSSION Pyrolysis−SVUV PIMS and Py−GC/MS were employed to investigate the generation of radicals and stable hydrocarbons produced during pyrolysis. The main compounds were quantified by the area normalization, and the explanation of thermolysis results of each model compound is discussed as follows. 3.1. Pyrolysis of BOP. To investigate the pyrolysis mechanism, pyrolysis−SVUV PIMS was used to detect the possible radicals and intermediates. Figure 2 shows the product
2. EXPERIMENTAL SECTION 2.1. Materials. The model compounds tested in this experiment are 4-(benzyloxy)phenol (BOP), CAS Registry Number 103-16-2 (>99.5%, J&K), 4-(benzyloxy)anisole (BOA), CAS Registry Number 6630-18-8 (>99%, J&K), and guaiacylglycerol-β-guaiacyl ether (GGGE), CAS Registry Number 7382-59-4 (>99%, J&K). No further purification was carried out in this work. 2.2. In Situ Pyrolysis−SVUV PIMS Experiment. The pyrolysis− SVUV PIMS experimental work was performed at NSRL in Hefei, China. The pyrolysis setup is shown in Figure 1, and the detailed
Figure 2. Photoionization mass spectrum of the pyrolysis products of BOP at 500 °C.
mass spectra of the BOP pyrolysis product under 500 °C and 143.5 Pa, and the photon energy was tuned from 10.5 to 9.5 eV to avoid the secondary fragments. The peak at m/z 91 was emerging together with m/z 92, 108, 110, and 182, which were identified as benzyl radical, toluene, p-benzoquinone, phydroquinone, and 1,2-diphenylethane (bibenzyl), respectively. There are small signals for m/z 109 (representing the psemiquinone radical), apart from the 13/12C isotopic ratio of m/ z 108. The small signals of the p-semiquinone radical may be due to the polar effects in the transition state or the short lifetime at a high temperature [p-semiquinone radical was detected by low-temperature matrix isolation−electron paramagnetic resonance (LTMI−EPR) at 77 K].13,25 The presence of the benzyl radical, p-semiquinone radical, toluene, and bibenzyl prove the theory that pyrolysis is initiated via the homolytic cleavage of the Cα−O bond, which is the weakest bond in the molecule, with a bond dissociation energy (BDE) of 56.4 kcal/mol.15 Homolytic bond cleavage of the Cα−O bond forming the benzyl and p-semiquinone radicals is illustrated in Figure 3. After Cα−O bond cleavage, the benzyl and p-semiquinone radicals can abstract hydrogen to form toluene and p-hydroquinone, respectively, as shown in Figure 3. Benzyl radicals tend to favor the termination reaction to
Figure 1. Photo diagram of the SVUV PIMS pyrolysis setup. description of the pyrolysis−SVUV PIMS apparatus, which is only briefly described here, has been reported elsewhere.23 An analytical balance was used to make sure that 20−40 mg of sample was used for each measurement. Pyrolysis occurred in vacuum conditions; the products in situ passed and entered the photoionization region; and then the ions produced by SVUV were guided into a TOF mass spectrometer.24 The pyrolysis experiments were performed at 350− 500 °C, and the mass spectra were collected at the photon energy of 9.5−10.5 eV. The peaks were identified according to the m/z value and the Py−GC/MS results in comparison. The background signal was gained by measuring a blank sample at the same conditions and deducted. 2.3. Py−GC/MS. The Py−GC/MS system was employed to separate and identify the pyrolysis volatiles of model compounds under flash heating mode. Py−GC/MS tests were carried out on each B
DOI: 10.1021/acs.energyfuels.5b02635 Energy Fuels XXXX, XXX, XXX−XXX
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below 500 °C. The temperature-course changes of the phydroquinone and p-benzoquinone peak areas in the range of 350−500 °C are generally in good agreement with other studies. 3.2. Pyrolysis of BOA. Section 3.1 has revealed the homolytic cleavage of the Cα−O bond. Methoxy groups are an important structural feature of lignin, and researchers27,28 have reported the effects of methoxy groups on the aromatic ring and found that the ortho-methoxy substituent enhances C−O homolysis in the α,β-ether structure as a result of steric hindrance induced by the ortho-methoxy groups. This section will focus on the impact of para-position methoxy substituents on the reaction pathways of the α-O-4 model compound, including Caromatic−Cα, Oether−Caromatic, and Cα−O bonds. BOA is a model compound with the p-methoxy substituent on the hydroxyl position of BOP. Figure 5 shows the main products of BOA from Py−GC/MS under 350, 400, and 500 °C pyrolysis conditions. According to Figure 3. Homolytic Cα−O bond cleavage mechanism of BOP.
produce bibenzyl as a consequence of their fast termination rate constant. (E)-1,2-Diphenylethene was formed by β-scission of hydrogen of the bibenzyl radical, which rarely occurred in the vacuum condition. Two pathways are considered for the psemiquinone radical, involving both abstracting hydrogen to form p-hydroquinone and the direct dissociation of a phenoxyl−hydrogen bond of the p-semiquinone radical (ΔH = 80.8 kcal/mol)26 to form p-benzoquinone. To investigate the temperature effect on the pyrolysis reaction and product distribution, three temperature levels were performed on Py−GC/MS for BOP pyrolysis. Figure 4
Figure 5. Product distribution of BOA pyrolysis in Py−GC/MS at three temperatures.
the above research, mequinol and bibenzyl are the main products of Cα−O bond homolysis; the higher GC/MS peak of mequinol under 350 °C indicates that methoxy substitution promotes the C−O homolysis under a low-temperature range. Second, substituted structures produced many different types of pyrolysis products in the higher temperature of 500 °C, such as 4-methoxybenzaldehyde, anisole, benzene, benzaldehyde, and 1,4-dimethoxybenzene, obviously resulting from different radical species and reaction pathways. According to the product analysis and DFT calculation,16 anisole and benzaldehyde should be the main products of Oether−Caromatic bond cleavage. 1,4-Dimethoxybenzene and benzene were the direct products of Caromatic−Cα bond cleavage, and 4-methoxybenzaldehyde was formed typically by the 1,2-phenyl shift reaction after Caromatic− C bond homolysis. The bond cleavage reaction of Caromatic−Cα and Oether−Caromatic occurred hardly in the BOP pyrolysis because of the higher bond dissociation energy than that of Cα−O. It is concluded that the p-methoxy substituent can lower the Caromatic−Cα and Oether−Caromatic bond dissociation energies and result in complex reaction pathways and pyrolysis products. 3.3. Pyrolysis of GGGE. The DFT study has shown that the order of BDE is Cβ−O < Cα−Cβ < Cα−OH < Cβ−Cγ < Caromatic−C < O−Caromatic, and therefore, the cleavage of the
Figure 4. Product distribution of BOP pyrolysis in Py−GC/MS at three temperatures.
shows the main products from Py−GC/MS under 350, 400, and 500 °C pyrolysis conditions. The main products from pyrolysis of BOP were 1,2-diphenylethane, p-hydroquinone, pbenzoquinone, and toluene. With an increasing temperature, the selectivity to 1,2-diphenylethane and p-benzoquinone increases, while the selectivity to p-hydroquinone decreases, revealing that a high temperature favors the H-abstraction reaction. Phenol was detected in trace amounts, because the Caro−O bond with the BDE of 85.1 kcal/mol is relatively stable C
DOI: 10.1021/acs.energyfuels.5b02635 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Cβ−O and Cα−Cβ bonds should be the initial step in the thermal decomposition of the β-O-4-type lignin dimer model compound.14 Figure 6 shows the normalized mass spectra of
difficult to further distinguish between retro-ene and Maccoll reactions. On the basis of product analysis and prior DFT calculations,14,17,19 the GGGE reaction pathways are depicted in Figure 8.
Figure 6. Photoionization mass spectrum of the pyrolysis products of GGGE at 500 °C.
the GGGE pyrolysis protuct under 500 °C and 143.5 Pa using the pyrolysis−SVUV PIMS system, and the photon energy was tuned from 9.5 to 10.5 eV. Compounds with m/z 196 and 180 emerged together with m/z 178, 168, 166, 152, 150, 136, and 124, which were identified as 3-hydroxy-1-(4-hydroxy-3methoxyphenyl)-1-propanone, coniferyl alcohol, coniferyl aldehyde, 4-(1-hydroxyethyl)-2-methoxyphenol, acetovanillone, vanillin, 2-methoxy-4-vinylphenol, 2-methoxybenzaldehyde, and guaiacol, respectively. The pyrolysis temperatures at 350, 400, and 600 °C were also tested, and the compounds at m/z 124, 166, 178, 180, and 196 were dominant in the testing temperature range; they were all typical compounds from the Cβ−O bond rupture, homolytic cleavage reaction, or concerted reaction.12 However, there was no radical signal in the mass spectra at m/z 123 and 197 that represents the homolytic cleavage mechanism of the Cβ−O bond, as shown in Figure 7.
Figure 8. Pyrolysis pathways of GGGE.
As shown in Figure 8, routes 1 and 2 are the concerted mechanism, route 3 is the homolysis mechanism, and guaiacol a is a common product of the different pathways. Phenylallyl alcohols b and c can be formed directly via a four- or sixmember ring intermediate, without radicals. The enol structure of compounds b and c is unstable and would tautomerize to phenyl acetone alcohol compounds e and f, respectively. Compound e can further decompose to compound h and formaldehyde. Compound f may decarbonylate to form compound i and carbon monoxide. Vanillin k is the further decomposition product. Phenyl propanediol compound d is most likely formed from homolysis radical m/z 197, which was not detected because of the unstable structure and rapid conversion. Via the dehydration reaction, compound d converts to compound g. The H atom is abstracted from the hydroxyl group on Cγ from compound g and forms coniferyl aldehyde j. Compound m is formed by decarbonylation from compound j. The C−O bond of the methoxy group is also weak, but on the basis of the average Arrhenius parameters, the cleavage of the C−O bond of the methoxy group is expected to be approximately 4 times slower than that of the Cβ−O bond;28 therefore, methoxy substituents on the aromatic ring were wellpreserved. Figure 9 shows the temperature effect on the product distribution of GGGE pyrolysis using the Py−GC/MS system. To capture the high-boiling-point compound, the temperatures of desorbtion, oven, and detector in GC/MS were adjusted to the upper limit value. As shown, the major products detected by Py−GC/MS are consistent with the SVUV PIMS results. Among the products, 2-methoxybenzaldehyde, vanillin, acetovanillone, and 3-hydroxy-1-(4-hydroxy-3-methoxyphenyl)-1propanone increase with the temperature and guaiacol decreases dramatically as the temperature increases to 500
Figure 7. Homolytic Cβ−O cleavage mechanism of GGGE.
For pyrolysis of the phenethyl phenyl ether, Jarvis et al.17 have captured the phenoxy and benzyl radicals in the hightemperature range of 900−1100 °C using PIMS, and in the medium- and low-temperature ranges, there was no radical signal, concluding that, under typical pyrolytic conditions (T < 600 °C), the concerted reactions will dominate over homolytic pathways. The theoretical calculation has shown that concerted reactions are more facile than the Cβ−O homolysis reaction as a result of the lower energy barrier.18 According to the above details, it is inferred that the C−O homolysis mechanism was minor for GGGE pyrolysis in the experimental temperature range and the concerted reaction dominated. However, it is D
DOI: 10.1021/acs.energyfuels.5b02635 Energy Fuels XXXX, XXX, XXX−XXX
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NOMENCLATURE SVUV = synchrotron vacuum ultraviolet PIMS = photoionization mass spectrometry TOF = time of flight BOP = 4-(benzyloxy)phenol BOA = 4-(benzyloxy)anisole GGGE = guaiacylglycerol-β-guaiacyl ether GC/MS = gas chromatography/mass spectrometry REFERENCES
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Figure 9. Product distribution of GGGE pyrolysis in Py−GC/MS at three temperatures.
°C. Guaiacol is the major product of the primary pyrolysis reaction, and aldehydes and ketones are further and secondary reaction products. This indicates that a low temperature favors the phenol product and a high temperature favors the dehydrogenation reaction to form aldehydes and ketones.
4. CONCLUSION The lignin α-O-4 model compounds BOP and BOA and the completely substituded β-O-4 lignin model compound GGGE have been studied by the pyrolysis−SVUV PIMS and Py−GC/ MS systems. Detection of the benzyl radical, p-semiquinone radical, toluene, hydroquinone, and bibenzyl indicates that the pyrolysis reaction of α-O-4 model compounds mainly occurs by homolytic cleavage of the lowest BDE bond Cα−O. This is consistent with the theoretical calculations. BOA is a model compound with the p-methoxy substituent on the hydroxyl position of BOP. Experiments show that the p-methoxy substituent can lower the Caromatic−Cα and Oether−Caromatic bond dissociation energies and result in complex reaction pathways and pyrolysis products. For β-O-4 compound GGGE, there was no radical signal in the mass spectra at m/z 123 and 197 that represent the homolytic cleavage mechanism of the Cβ−O bond. On the basis of product analysis and prior DFT calculations, it is inferred that the C−O homolysis mechanism was minor for GGGE pyrolysis and the concerted reaction dominated in the experimental temperature range. However, it is difficult to further distinguish between retro-ene and Maccoll reactions; therefore, a further study is necessary to understand the details of the mechanism.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant 11375249) is greatly acknowledged. The authors thank the National Synchrotron Radiation Laboratory of China for their kind help in the part of the SVUV PIMS experiment. E
DOI: 10.1021/acs.energyfuels.5b02635 Energy Fuels XXXX, XXX, XXX−XXX
