Peculiarities of Pyrolysis of Hydrolytic Lignin in Dispersed Gas Phase

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Article Cite This: Energy Fuels 2017, 31, 12156-12167

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Peculiarities of Pyrolysis of Hydrolytic Lignin in Dispersed Gas Phase and in Solid State Mohamad Barekati-Goudarzi,† Dorin Boldor,*,† Cosmin Marculescu,‡ and Lavrent Khachatryan*,§ †

Department of Biological & Agricultural Engineering, Louisiana State University and LSU AgCenter, Baton Rouge, Louisiana 70803, United States ‡ Faculty of Power Engineering, University Politehnica of Bucharest, Bucharest 060042, Romania § Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States S Supporting Information *

ABSTRACT: The unique decomposition pathways of hydrolytic lignin (HL) dissolved in an acetone/water mixture (9:1) and dispersed by a droplet evaporation technique under nitrogen gas flow has been investigated in a conventional reactor at atmospheric condition, a temperature region of 400−550 °C, and a residence time of 0.12 s. The results validate the fact that dispersion of the lignin into the gas phase by decreasing the sample size (as well as “minimizing the char area to avoid catalytic contact” of molecular products/radicals with the surface) may open new perspectives in understanding the chemistry of the depolymerization of lignin. Using Laser Desorption Ionization-Time of Flight-Mass Spectrometry (LDI-TOF-MS) the intrinsic ion m/z = 550, as the major MS peak from fresh HL dissolved in an acetone/water mixture before pyrolysis, was detected. Surprisingly, the expected phenolic compounds after pyrolysis were in trace amounts at less than 15% conversion of lignin. Instead, oligomeric intermediate substances with low (550 Da) containing ligninsubstructures (trapped on quartz wool located at the end of the reactor at ∼300 °C) were detected as major products using LDITOF-MS. The hypothesis about a largely disputed key question on lignin pyrolysis as to whether the phenolic compounds or oligomers (dimers, trimers, etc.) are the primary products is discussed. Additionally, a focus on the free-radical mechanism of depolymerization of solid lignin by formation of free intermediate radicals from initial lignin macromolecules as well as from inherent, low molecular weight oligomer molecules is developed based on the Low Temperature Matrix Isolation (LTMI) EPR technique.

1. INTRODUCTION Lignin has been described as a random, three-dimensional network polymer comprised of variously linked hydroxyphenylpropane units such as trans-p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol connected with ether and carbon− carbon bonds.1 Although the study on lignin pyrolysis dates back to 1965,2 detailed mechanisms still remain unclear, largely due to its complex structure and random composition.1,3 This is further complicated by the inevitable modification of the chemical structure of naturally occurring lignin during its separation from lignocellulosic materials.4 The identification of the reactive intermediates (radicals, molecular complexes, oligomers) from lignin pyrolysis becomes essential in terms of regulation of the pyrolysis process.5 New peculiarities of hydrolytic lignin (HL) pyrolysis will be presented depending on specific experimental conditions concerning pyrolysis of dispersed HL in atmospheric N2 gas flow, intermediate radicals from depolymerization of lignin in both vacuum and atmospheric N2 gas environments, and finally radicals on the residue-biochar. Lignin Depolymerization through the Radical Mechanism. It is commonly believed that lignin is thermally decomposed via three different mechanisms depending on the conditions: concerted molecular, ionic, and free-radical mechanisms.6−10 Some literature data will be briefly presented to indicate the importance of the radical mechanism of lignin pyrolysis. © 2017 American Chemical Society

Although the concerted mechanism, i.e., formation of products without intermediacy of free radicals, has been proposed by several researchers as a dominant degradation pathway of lignin (or model compounds), especially at lower temperatures,9,11 the free-radical mechanism is also generally considered both in earlier6,8 and more recent studies,12−16 including ours.10,17−19 The free-radical reactions leading to the formation of monomeric phenolic compounds during pyrolysis have been well reviewed by Amen-Chen et al.20 and Zakzeski et al.21 Laser pyrolysis molecular beam mass spectroscopy (LPMBMS) was used to characterize reactive intermediates from charring the biomass components22 as well as the organic compounds released upon rapid heating of organosolv lignin.23 The volatilized products observed in LPMBMS were primary products − virtually no secondary cracking or condensation of the evolved species occurred.22,23 Additional spin trapping experiments revealed formation of intermediate radicals of phenyl, benzyl, phenoxy, and methyl-naphthyl radicals from biomass samples. While not certainly conclusive, the authors present evidence23 for radical production as M-1 ion (suggesting that the entity is being ablated from the char surface as a radical); for instance m/z 167 as 4-methyl-2,6Received: June 27, 2017 Revised: September 22, 2017 Published: September 26, 2017 12156

DOI: 10.1021/acs.energyfuels.7b01842 Energy Fuels 2017, 31, 12156−12167

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Energy & Fuels dimethoxyphenol, m/z 151 as p-vinylguaiacol, and m/z 137 as 4-methylguaiacol radicals. Even formation of propargyl radical is evidenced during cellulose char formation at high temperatures under CO2 laser rapid heating. Recently, we have developed molecular-radical decomposition mechanisms for lignin model compounds such as sinnamyl, p-coumaryl alcohols validated experimentally and theoretically.17−19 We also introduced the important role of gas-phase, phenoxyl type radicals during lignin pyrolysis.10 Note that in some studies substituted phenoxyl type radicals were abbreviated as neutral semiquinone (SQ•) radicals24−27 in contrast to a semiquinone anion radical (SQ•−).28,29 The transient participation of phenoxyl type radicals is consistent with the structures of lignols isolated from the enzymatic polymerization of the lignin precursors, for instance, the participation of p-coumaryl alcohols has also been inferred by the detection of a weak, unresolved EPR signal during the initial stages of polymerization.24 The observation of phenoxyl radicals formation in other studies has led to the general conclusions that the free-radical mechanism involves the thermolysis of the alkyl-aryl ether linkages in lignin.7,30 It has been also demonstrated that prior to pyrolysis, lignin isolated from biomass contains a significant amount of stable substituted o-semiquinone (o-SQ•) radicals, with a concentration in the range of ∼1 × 1017 to ∼5 × 1017 spins·g−1.14,27 Compared with raw material, bio-oil produced from pyrolysis of lignin exhibited a higher concentration of free radicals, i.e., 7.5 × 1020 spin·g−1, which was 375 and 138 times higher than that from cellulose and corn stover, thus evidencing a free-radical mechanism of lignin pyrolysis.16 The free radicals have been detected in various media associated with pyrolysis of lignin or lignin model compounds, e.g., gas phase,10 bio-oil,15,16 and biochar.31 Bährle et al. identified the disproportionation of semiquinone (SQ•) radicals to quinone and hydroquinone (HQ) species from lignin pyrolysis at 550 °C using EPR.14 Custodis et al. detected phenyl, phenoxy, cyclopentadienyl (CPD), and propargyls radicals from lignin model compound, diphenylether (DPE) pyrolysis and o-hydroxyphenoxy and hydroxycyclopentadienyl radicals from guaiacol pyrolysis in the respective temperature ranges of 650−900 °C and 550−850 °C using imaging photoelectron photoion coincidence (iPEPICO) with the vacuum ultraviolet (VUV) synchrotron radiation technique.32 As for bio-oil, based on EPR analysis, a mixture of a stable carbon-centered benzyl radical and an oxygen-centered phenoxy radical was proposed to be present from three methoxy substituted α-O-4 lignin dimeric model compounds pyrolyzed at the temperature of 500 °C15 as well as lignin pyrolysis at the temperature of 600 °C.16 We are uncertain about identification of carbon-centered benzyl and oxygencentered phenoxy radicals claimed in ref 15. One of the mentioned phenoxy radicals can be only trapped under vacuum condition as shown in our recent publication33 and other publications;32 they easily may escape at high pressures (1 atm) by mutual annihilation or during radical-molecule reactions. Moreover, trapping these types of monomeric radicals, being carried by atmospheric He gas, is challenging due to the high conductivity of He. The heated effluent gas exiting the reactor will affect the trapping temperature to be established at much higher values than 77 K in a U-tube (immersed in liquid nitrogen15) which is consequently not favorable for condensation of light radicals.

In relation to biochar, it was suggested from EPR measurements that oxygen-centered radicals dominate at low charring temperatures (200−300 °C), while a mixture of oxygen and carbon-centered radicals coexists at elevated charring temperatures (400−500 °C).31 The Primary Processes in Lignin Pyrolysis: Phenolics or Oligomers? The mechanistic understanding of lignin pyrolysis primary pathways has remained unclear. The question whether phenolic compounds or oligomers are primary products is under dispute, and a detailed summary is presented in a recent publication.34 Unfortunately, literature data about direct measurements of oligomers in the hot zone in the gas phase, as well as evolving them from a solid matrix (or melted lignin), are scarce.35,36 Generally, limited information is obtained on the oligomeric fraction of the pyrolyzate. The high molecular weight constituents up to 450 Da, mostly molecules containing β-5, 5-5, β-1, β-β, and 5-5 (biphenyl linkages), were detected from MWL pyrolysis (with small sample sizes - 5−20 μg) using in-source pyrolysis-mass spectrometry (PyMS) under electron impact conditions37 and Curie-point Py-GC-MS38 especially with dimeric structures in the last case. A precaution to distinguish a monomer−oligomer formation sequence using time-resolved mass spectrometry has been demonstrated in ref 39 in the case of pyrolysis of a polystyrene polymer. It was shown that the time-resolved mass spectra have entirely different characteristics depending on the heating method used (regular quartz tube or filament “in-source” technique). Reliable time-resolved mass spectra are achievable only when filament techniques and thin samples are employed even for such small size samples as 5 μg.39 False results were detected from complementary quartz tube experiments containing a 5 μg sample. In fact, the mechanisms by which lignin-derived oligomers are formed during pyrolysis are not well-known,36,40 but here is considerable evidence that a significant fraction of oligomers is indeed formed inside the pyrolysis reactors.40 Kawamoto et al. have proven that, in addition to the formation of monomers, several competitive pathways leading to the formation of dimers were also identified during the pyrolysis of lignin.35 Bai et al. recently confirmed that a significant fraction of primary products during pyrolysis of lignin is monomers as well as dimers that survived in the hot zone and can be reoligomerized easily.34 A molar mass of oligomers up to 504 Da was detected in fresh bio-oil derived from pyrolytic lignin (0.5 mg) pyrolysis using a high resolution technique.34 Note the fact that reoligomerization seems to continue even in bio-oil during aging time.16 The monomeric compounds were considered the primary pyrolysis products of lignin reported recently.41 It was shown that these phenolic products recombine after primary pyrolysis to produce oligomeric compounds due to further gas-phase secondary reactions of phenolics. On the other hand, the research shows a high content of radicals as in the gas phase as well as in the biochar presented above. These radical species may recombine especially in the colder areas of the reactor during condensation of the pyrolysis vapor. The radicals formed directly from depolymerization of lignin may form either from the nascent gas-phase, simple phenolic type molecules containing one aromatic ring or from oligomers containing multiple aromatic rings. It is pertinent to state that in some modeling calculations lignin was considered by analogy to coal pyrolysis as a substrate with a distribution of 12157


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and low molecular oligomers as a group of peaks in the m/z range of 400−550. The majority of oligomers in HL correspond to dimers and trimers containing 4 oxygen atoms (C18H18O4) and 6 oxygen atoms (C27H26O6), respectively; the dimer (C18H18O4) is a major fraction in hydrolytic lignin.43 Lignin was fractionated by molecular sieves, and the fraction of ≤120 μm was used. The hydrolytic lignin was well dissolved in an acetone:water (9:1) mixture.47 This solution was then dispersed in the gas phase by droplet evaporation using a syringe pump described below. 2.1.1. EPR Measurements. A technique referred to as lowtemperature matrix isolation EPR (LTMI-EPR) in the literature was used in this study.48 LTMI-EPR allows accumulation and detection of trace quantities of radicals during the gas-phase thermal degradation of many classes of organics. An electrically heated reactor interfaced to a liquid nitrogen-cooled dewar located in the cavity of an EPR spectrometer was used for the pyrolysis of lignin (Figure S1). 10 mg of HL was plugged from both sides by quartz wool and placed in the center of the reactor. The products released after pyrolysis were pumped by a rotary pump (at 10−3 Torr) and collected on a coldfinger maintained at 77 K. The EPR spectra were recorded on a Bruker EMX-20/2.7 EPR spectrometer (Bruker Instruments, Billerica MA) with dual cavities, Xband (100 kHz) and microwave frequency at 9.516 GHz. The typical parameters were as follows: sweep width 200 G, EPR microwave power from 0.5 to 64 mW, modulation amplitude ≤4 G, and time constant and sweep time in most cases were 10.24 ms and 167.77 s, respectively. The dimensionless magnetic moment of a radical, g-factor (a characteristic value which measures how the magnetic environment of unpaired electrons differ from that of a free, gas-phase electron, g = 2.0023) was calculated using Bruker’s WINEPR program, which is a comprehensive line of software, allowing control of the EPR spectrometer, data-acquisition, automation routines, tuning, and calibration on a Windows-based PC.49 The exact g-values for key spectra were determined by comparison with a 2,2-diphenyl-1picrylhydrazyl (DPPH) standard. In some experiments, gradual warming of the Dewar was employed to allow annealing of the matrix and annihilation of mobile or reactive radicals. This resulted in cleaner, sharper spectra of single radicals under environmentally isolated conditions. 2.1.2. Intermediate Radicals Trapped from Lignin Vacuum Pyrolysis. To investigate the radical characteristic of depolymerization of lignin in solid state, pyrolysis of lignin was performed in a simple tubular quartz reactor located near the EPR cavity under vacuum, Figure S1. A detailed description of the cryogenic trapping method, the LTMI EPR technique, is described elsewhere.48,50 Briefly, for each experiment 20 mg of lignin was weighted in a quartz basket and placed inside the reactor. The reaction was carried out at different temperatures ranging from 425 to 525 °C to investigate the effect of temperature on the nature and intensity of the EPR signal. The effluent of the reactor was pumped at vacuum of ∼10−3 Torr; the timeof-flight of released molecules from the lignin matrix to the coldfinger at 77 K, placed within the microwave cavity of the EPR spectrometer (Figure S1), was in the range of milliseconds.51 2.1.3. Laser Desorption Ionization-Time of Flight-Mass Spectrometry (LDI-TOF-MS). LDI-TOF MS is a tool for characterizing high molecular weight substances such as biomolecules (proteins, oligonucleotides), synthetic polymers, lignocellulosic materials, etc. LDI-TOF MS measurements were performed on a commercial instrument (Ultraflextreme, Bruker Daltonics, Billerica, MA, USA). Spectra were recorded in positive ion reflectron mode with an accelerating voltage of 25 kV and analyzed in the mass range of 100− 1500 Da. The spectra were acquired after calibration of the instrument with a peptide standard (Peptide Calibration Standard II, Bruker Daltonics, MA, USA). A 1 μL dilute solution of HL dissolved in acetone/water solvent (9:1) was deposited on the NALDI target and dried under pure nitrogen flow (no matrix was used). A minimum of 500 laser shots per sample was used to generate each mass spectrum. A computer-controlled random raster was used in the acquisition.

oligomers; a theory has been developed that combines random cleavage of weak bonds in the lignin-like polymer (to produce Metaplast) with transport of depolymerization fragments by vaporization and diffusion.42 Even though the X band EPR technique shows the presence of oxygen- or carbon-centered radicals, it does not confirm how many aromatic rings exist there. Oligomer radicals released from the lignin matrix (or from higher molecular weight oligomer molecules present initially) are relatively stable and detectable by EPR and other techniques. For instance, an unusual stability of the cation-radical C16H16O4+ (m/z 272) is reported in ref 43. Depending on the residence time in the hot zone area, the intermediate radicals either may convert to low molecular weight chemicals or leave the hot zone and condense on the cold areas. Some of them may even remain on the char surfaces due to the interaction of the radical group with the matrix as suggested in ref 27. The question whether the phenolic compounds are primary and oligomers secondary (by polymerization of phenolics) or vice versa is thus unanswered. It is hard to distinguish whether primary products are oligomers or phenolic compounds as the lignin physical form changes during heating because of melting and swelling which affects the rate of products evaluation (oligomers, phenolics, their radicals). Another alternative worthy of attention is that phenolic compounds and oligomers are both primary products, i.e. their processes of formation are effectively parallel reactions suggested, for instance in the process of styrene pyrolysis.39 To avoid a misleading interpretation of pyrolysis processes of lignin, the following conditions are crucial to perform an experiment: small sample sizes, fast heating rates, and low partial pressures which enable a collisionless and fast mass transfer of primary pyrolysis fragments from the dissociating sample matrix.37,39 Therefore, using a qualified, state-of-the-art technique to follow kinetic behavior of intermediates such as oligomers becomes critical. In this study, we have dispersed lignin in the solvent and delivered it into the conventional reactor by droplet evaporation with the goal to avoid any contact of released products with the solid residue and to eliminate mass transfer limitation on the surface of the solid lignin. The smallest sample molecule that is possible in the gas phase is the lignin macromolecule with an aerodynamic diameter of around a few hundred nm (assuming the lignin polymer has a modular structure).44 Except that lignin dissolved in an acetone:water (9:1) mixture was passed through the 0.45 μm syringe filter providing macromolecules in the gas phase with a size less than 450 nm (assuming no further association of lignin macromolecules). The hydrolytic lignin was used in the current work with the mean molecular weight reported between 1000 and 1700 g/mol.45,46,43 To minimize secondary processes, the pyrolysis was carried out at a shorter residence time, 0.12 s in 1 atm of carrier gas N2. Additionally, a vacuum pyrolysis of lignin was performed to follow the released paramagnetic radicals in conjunction with the LTMI-EPR technique to support the depolymerization mechanism for lignin on a molecular level.

2. EXPERIMENTAL SECTION 2.1. Materials. Hydrolytic lignin was supplied from Sigma-Aldrich (catalog number 37-107-6). The detailed analysis of this type of lignin is reported in ref 43 as a compound containing a high number of methoxy groups, the weight percent ratio of C/H/O equal to 76:6:18, 12158


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Energy & Fuels MALDI flex Analysis 3.3 software (Bruker) was used for the mass spectra analysis. 2.1.4. Lignin Gas-Phase Continuous Delivery: Droplets Evaporation. The continuous gas-phase lignin pyrolysis was performed using a syringe pump to inject the solution of lignin dissolved in an acetone/ water solvent (9:1) in a preferably high temperature area (≈250 °C) in a quartz reactor with i.d. of 12 mm and length of 14 cm (see Figure 1).

The pyrolysis products were trapped by dichloromethane (DCM) in an impinger (Figure 1) at iced water temperature. The need for a second impinger was ruled out due to the fact that only trace amounts of compounds were detected. A quartz wool was plugged at the end of the reactor at a distance of ∼10 mm (the immediate temperature measured at 10 mm was ∼300 °C) to condense heavy intermediates or the unreacted lignin.

3. RESULTS AND DISCUSSION 3.1. Intermediate, Primary Radicals Released from Lignin Vacuum Pyrolysis. The results of the EPR measurements for intermediate radicals released from HL pyrolysis are summarized in Figure 2 and Figure 3. These types of EPR spectra, Figure 2(a), identified as O-centered radicals from pyrolysis of lignin and lignin model compounds p-coumaryl and cinnamyl alcohols have been reported in our earlier and recent publications.10,17,18 They form, most probably, by rupture of weak β-Ο-4 bonds in, most probably, lignin macrolmolecules (or from oligomer molecules) and then are released as fragments into the gas phase. A free-radical analysis by cleavage of specific interunit linkages (β-O-4, α-O-4, β-5, β-β) and their further free-radical reactions has been developed in ref 12. On the other hand, these types of radicals may also easily form from nascent molecules in the gas phase. However, due to the low yields of phenolic compounds from vacuum pyrolysis of lignin and the fact that the yields of radicals from neat compounds pyrolysis, for instance from phenol,52 catechol (CT), or hydroquinone (HQ),26 are negligible below 500 °C, we can state that the observed radicals are produced mainly from lignin macromolecules pyrolysis. Temperature dependence of the radicals’ yield (detected at different microwave powers) was produced from lignin pyrolysis in vacuum and trapped cryogenically at 77 K. All spectra detected in the temperature region of 425−525 °C, Figure 2a, indicate high gvalue (characteristic for O-centered radicals) and large ΔHp‑p, a shallow minimum around 450 °C, and a maximum around 490−500 °C, Figure 3a. A complex behavior of EPR spectra saturation dependence of radicals intensity versus squared root

Figure 1. A schematic diagram for droplet evaporation. Pure nitrogen gas with a flow rate of 1000 mL/min was used as a carrier gas to carry the evaporated solvent with lignin dispersed particles through the reactor chamber which was heated using a conventional furnace. The temperature was set and studied in a range of 475−575 °C at a syringe flow rate of 7.5 mL/h which totals 5 mL of solution injected into the reactor during the reaction time of 40 min. The blank experiments were performed without lignin dissolved in the solvent’s mixure. The residence time was kept at 0.12 s for the isothermal zone in the middle of the reactor to minimize the occurrence of secondary processes.

Figure 2. a) Temperature dependence of the EPR spectra profiles of the radicals produced from lignin pyrolysis in vacuum and trapped cryogenically at 77 K (3 scans). b) Temperature dependence of the radicals’ yield (detected at different microwave power) produced from lignin pyrolysis in vacuum and trapped cryogenically at 77 K. 12159


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Figure 3. a) Temperature dependence of the g-value and ΔHp‑p of radicals produced from lignin pyrolysis in vacuum and trapped cryogenically at 77 K. b) Power dependence of the radicals produced from lignin pyrolysis at different temperatures.

Figure 4. a) The yield of radicals on the biochar after ampule pyrolysis of lignin at 400 °C in 1 atm of N2 gas. (The inset figure from the top - a singlet line EPR spectra of intrinsic radicals (g-value = 2.0043 and ΔHp‑p = 5.80 G) of initial lignin and biochar radicals from ampule pyrolysis of lignin in N2 gas at 400 °C (g-value = 2.0034 and ΔHp‑p = 5.51 G) and 450 °C (g-value = 2.0032 and ΔHp‑p = 5.72 G)). b) Time dependence of the EPR characteristic parameters (g-value and ΔHp‑p) of radicals on the biochar at 400 and 450 °C from ampule pyrolysis of lignin in N2 gas.

and 450 °C for up to 10 min pyrolysis time. The intensity of EPR spectra (Figure 4a) grows constantly with increasing pyrolysis time. The detected EPR spectra present a simple singlet line (Figure 4a, inset spectra) with low g-values (from 2.0031 to 2.0034) and a relatively narrow spectrum with ΔHp‑p in the range of 4.9−5.8 G (Figure 4b) characteristic most probably to carbon centered radicals. Similar radicals (identified as soot radicals) were reported in our earlier publication.53 Formation of soot is typical during lignin low temperature and longtime pyrolysis and is reported as coke formation.54 In fact, the secondary processes (condensation/polymerization) occur in the gas phase16 as well as on the surface of residue from lignin pyrolysis producing coke/soot particles with characteristic EPR spectra, Figure 4a (inset spectra). Note that the nature of soot radicals was also verified by microwave power dependence experiments when near linear dependence was detected between intensity of radicals and squared microwave power at temperatures of 400 and 450 °C (at lower than 20 mW microwave power) and almost linear dependence at 500 °C in a wide range of the microwave power (see Figure 5). This phenomenon, namely a linear dependence,

of microwave power intensity, Figure 3b, at each temperature indicates a mixture of different radicals being detected.18,29 3.1.1. Radicals on the Biochar after Pyrolysis of Lignin in Vacuum. A question naturally arises about the nature of radicals remaining on the residue (or on biochar). Lignin pyrolysis was performed under vacuum in a regular EPR tube at 10 min pyrolysis time. The volatile components were removed quickly by a rotary pump to avoid any contact of evolved compounds with the residue matrix. A simple singlet line shape for EPR spectra was detected (as of the one obtained for the radicals on the biochar from ampule pyrolysis of lignin in N2 gas at 400 or 450 °C, Figure 4a, inset spectra, section 3.3). These radicals on the residue are most likely a counterpart of radicals after scission of aryl-ether β-O-4 bonds when Ocentered volatile radicals were trapped cryogenically under vacuum described in subsection 3.1, Figure 2a. 3.1.2. Radicals on the Biochar from Ampule Pyrolysis of Lignin in 1 atm of N2 Gas. Additional pyrolysis experiments were performed similarly to that described in section 3.1.1, however, in 1 atm of N2 gas. The pyrolysis experiments of 20 mg of lignin in an EPR tube (ampule) were carried out at 400 12160


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the substituted o-SQ intrinsic radical (as substituted phenoxy radical) was recently identified in the initial lignin depending on extraction procedures in lignin manufacturing.27 The ease of saturation of semiquinone radicals has been demonstrated in an earlier publication55 (refer also to the curve with asterisks in Figure 5). On the other hand, the g-values below 2.0040 can to some extent mean that the soot radical (or coke radical) is oxygenated. The less oxygenated soot radicals are the less the g-value is. For instance, if an oxygen atom is adjacent to a free electron, the g-value may be as high as 2.0036.56 If there is not oxygen in close proximity to the free electron, as in the case of PAH radicals, the g-value drops from 2.0040 to 2.0026(8).57 3.3. Lignin Depolymerization in the Gas Phase. By droplet evaporation presented above, Figure 1, the HL was almost quantitatively transported through the hot zone at a high flow rate of the carrier gas, 1000 mL/min and a short residence time (0.12 s). Around 85−92% of HL was recovered at the end of the reaction zone almost in all temperature regions investigated. The traces of phenolics were detected (vide inf ra). With this scenario (i.e., the formation pathways of phenolics were suspended) we could follow the transformation of the initial lignin into intermediates (radicals, oligomers) by following the process of pyrolysis using LTMI EPR and LDITOF-MS spectroscopy. The GC-MS data of lignin gas-phase pyrolysis and blank experiments (pure acetone and pyrolyzed pure acetone both condensed down to ∼100 μL from an initial volume of 20 mL) are summarized in Figure 6, Table 1, and Table S1. Trace amounts of organics were detected for condensed and pyrolyzed pure acetone up to an elution time of 35 min, Figure 6 (red and blue lines, respectively), while a large peak (tentatively identified as the O4 dimer, − C17H16O4, Table 1) was detected at ∼26.42 min along with trace amounts of a few phenolics namely phenol, 2-methoxy; 2-methoxy-4-vinylphenol; phenol, 2,6-dimethoxy; and benzofuran, 2,3-dihydro. The results of the temperature dependence of the major products (Figure 7) from lignin dispersed in the gas-phase pyrolysis experiment obviously show that the detectable products reach a maximum at ∼490 °C. 2-Methoxy-4-vinylphenol and benzofur-

Figure 5. Power dependence of radicals on the biochar from ampule pyrolysis of 20 mg of lignin at 400 °C, 450 °C, and 500 °C in atmospheric N2 gas. The curve with asterisks represents power dependence of intrinsic radicals in pure lignin.

provides evidence of high delocalization of a free electron over a soot matrix and indicates that sooting tendency is increased with temperature in lignin ampule pyrolysis leading to formation of much less oxygenated soot particles at T > 500 °C with a g-value of 2.0030. Therefore, in accordance with the literature data the soot radicals are not saturated in a large region of the microwave power (Figure 5, the line with squares), while the intrinsic radicals in lignin are saturated (Figure 5, the line with asterisks). It is interesting to compare the EPR spectra of lignin intrinsic radicals and radicals after pyrolysis in ampule experiments (Figure 4a inset spectra). Even with the apparent similarity of the spectra, they differ in g-values (lignin intrinsic radicals have a g-value of 2.0043 while g-values of the residue after pyrolysis are 2.0034, and 2.0032 at 400 and 450 °C, respectively), while there is not a large difference in ΔHp‑p (5.8 G for lignin intrinsic radicals; radicals on the residue after pyrolysis at 400 and 450 °C have ΔHp‑p of 5.51 and 5.72 G, respectively). In addition,

Figure 6. GC-MS spectra of lignin gas-phase pyrolysis, pure acetone pyrolysis (blank experiment), i.e. condensed down to ∼100 μL, and initial acetone condensed the same way (red spectrum). 12161


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the absence of major phenolic compounds. Primarily, phenoxy radicals must form through the breaking of the C−O bonds in the β-O-4 unit which are the weakest ones in the lignin macromolecule; for instance, the bond energy for some simple model compounds is as low as 63 to 69 kcal/mol.12,58−60 High activation energy is needed to break that interunit unit. Similar breakage was considered for a number of lignin molecular model compounds with various decomposition rate constants6,61−63 presented in Table 2. The discrepancies in global

Table 1. Products (Abbreviated Li) from the Pyrolysis of HL Dissolved in an Acetone:Water (9:1) Mixturea label L1 L2 L3 L4 L5 L6 L7 a

compound name benzene, 1,3,5trimethyl2,5hexanedione phenol, 2methoxybenzofuran, 2,3-dihydro2-methoxy-4vinylphenol phenol, 2,6dimethoxydimerb

RT (min)

height

area

C9H12

7.83

3.46 × 1006

1.41 × 1007

C6H10O2

8.09

6.70 × 1006

1.61 × 1007

C7H8O2

10.07

8.78 × 1005

4.38 × 1006

C8H8O

11.52

1.79 × 1006

1.96 × 1007

C9H10O2

12.42

3.99 × 1006

1.21 × 1007

C8H10O3

12.81

5.49 × 1005

1.98 × 1006

26.42

1.66 × 10

6.68 × 10

formula

O4 dimer

07

Table 2. Rate Constantsa for Decomposition of Lignin Molecular Model Compounds6,59,61−63 model compound phenethyl phenyl ether eugenol 4-vinylguaiacol 4-propylguaiacol anisole

07

b

Dispersed in the gas phase at 0.12 s residence time. Tentatively assigned to the O4 dimer as C17H16O4 with a molecular mass of 284 Da.

pre-exponential factor

activation energy (kcal/mol)

ref

1 × 10+11.1

45.0

6

50.7 45.3 41.4 58.0

61 59 62 63

1 1 1 1

× × × ×

10+14.0 10+13.0 10+11.0 10+13.7

a

All rate constants are presented in the form of the Arrhenius equation, K = A exp(−Ea/RT), where A, Ea, R, and T are the preexponential factor, the activation energy, the gas constant, and the temperature, respectively.

kinetic parameters for overall decomposition in model compounds are most probably due to the influence of different structural environment. High activation energy was reported for the anisole decomposition compared to 4-propylguaiacol, Table 2. It could be due to the effects of the substituted neighboring groups on the C−O ether bond decreasing dissociation energy from 58 kcal/mol for anisole (as a reference) down to 41.4 kcal/mol for 4-propylguaiacol.62 For simplification, let assume that lignin macromolecules may be decomposed (as a pseudo-first-order decomposition) with an average rate constant of 1.0 × 10+13 exp(−45.3/RT) s−1 close to the value presented in Table 2 for 4-vinylguaiacol,59 1.0 × 10+13 exp(−45.3 kcal/mol/RT) s−1. The 1/e lifetime of lignin macromolecules (the time when the concentration of lignin molecule drops by a factor of e) can be approximated as ∼1/k which will give at 500 °C about 0.5 s. The residence time for lignin pyrolysis in the current work is 0.12 s; therefore, lignin macromolecules cannot be significantly influenced at a residence time of 0.12 s and 500 °C in our experimental conditions. Lignin was transported through the hot zone and collected on quartz wool, Figure 1. EPR analyses were subjected to fresh as well as aged samples. The fresh samples of residue have a small difference in EPR parameters as the initial lignin (g = 2.0043, ΔHp‑p = 5.8 G). For instance, g-value = 2.0039, ΔHp‑p = 6.7 G and g-value = 2.0037, ΔHp‑p = 6.3 G for the samples after the passing of HL through the pyrolysis zone at 490 °C and 550 °C, respectively). The intensity of the EPR signals before (initial lignin) and after reaction (on quartz wool) was raised largely by a factor of 7 and 10 at 490 and 550 °C, respectively. Detailed experimental evidence is depicted in Figure 8 as a relationship between lignin derived radicals intensity versus EPR microwave power (saturation phenomenon of radicals18 vide inf ra). All curves show a similar behavior and have the same trend of saturation typical for the organic radical as for the initial intrinsic radicals in the lignin27 (red curve), as well as for the lignin residue trapped on quartz wool at the end of the

Figure 7. Temperature dependence of the relative yields of major products from pyrolysis of lignin dispersed in the gas phase at a residence time of 0.12 s.

an, 2,3-dihydro reach a little more distinctive second maximum at ∼560 °C, Figure 7. A notable experimental fact is that the well-known depolymerization reactions of lignin leading usually to formation of the mainly phenolic compounds were suspended in this case. The quantified amounts were as follows: 6.86 × 10−02 μg/μL benzofuran, 2,3-dihydro; 4.99 × 10−01 μg/μL phenol, 2-methoxy; 1.87 μg/μL 2-methoxy-4-vinylphenol; and 3.06 × 10−02 μg/μL phenol, 2,6-dimethoxy. The total yields of three phenolics and benzofuran, 2,3-dihydro reach less than 5% of initial lignin mass. The reasons of the low yields of phenolic compounds as a competition of radical−radical (formation of most abundant dimers) vs radical-molecule (formation of phenolics) reactions are discussed below in section 3.3.1. 3.3.1. Formation of Phenolics and Condensation Reactions. EPR Measurements. The short residence time (0.12 s), practically absent of any contact of released products with the sample surface and low conversion of lignin in the dispersed gas phase, can be explained by difficulties encountered during the rupture of aromatic-aryl ether bonds in the lignin and, hence, 12162


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linkages, for instance β-O-4, and can further decompose as lignin macromolecules.42 The formation of “intermediate lignin” or oligomers was recently suggested in our presentation to explain the profile of decomposition of lignin by mathematical modeling;71 a subsequent decomposition of “intermediate lignin” (along with initial lignin decomposition) into tar components was suggested. If experimental conditions favor further secondary reactions (for instance, long residence time or any contact with the lignin matrix), the oligomers (or their radicals) as intermediate “lignin molecules” along with initial lignin macromolecules may generate the basic fractions of lignin pyrolysis − phenolics. The results are in coincidence with the conclusions developed by Zhou et al.,70 that the primary products of lignin pyrolysis are oligomers, while monomer phenols evolve from further secondary reactions of these oligomers in the gas phase. The radicals collected on quartz wool are persistent which follows from Figure 9; after 3 days aging being in a closed EPR Figure 8. Power dependence of the intrinsic radicals from pure lignin and radicals trapped by quartz wool (located at the end of the reactor) from the pyrolysis of lignin (5 mg of lignin transported through the reactor) dispersed in the gas phase at reaction temperatures of 490 and 550 °C and a residence time of 0.12 s.

reactor at reaction temperatures of 490 °C (blue curve) and 550 °C (black curve). These experimental facts evidence that the intermediate paramagnetic species were formed in the gas phase and trapped successfully on quartz wool. These most probably oxygen and carbon centered radicals would rather anneal through selfcondensation reactions due to high reactivity of these radicals and short residence time. The favorite reactions of the phenoxy type of radicals toward self-condensation in comparison with phenoxy radical-molecule reactions (formation of phenolics) were discussed thoroughly in the literature;64−68 the phenol formation may only govern at high concentrations of H-donor compounds and phenoxy radicals.66 Depending on the reaction temperature, some relatively stable radicals may cross all the way through the reactor by increasing the radical pool intensity at the end of the reactor (vide supra). These O-(and carbon) centered radicals may arise from either inherent dimers and trimers in the hydrolytic lignin69 or from initial macromolecules of lignin. Note that the color of the initial, dry lignin (brown) is changed to black (as a char) after the reaction with slightly changed g and ΔHp‑p values, indicating that the char consists of mainly lignin initial, probably dehydrated initial macromolecules, dimers (vide inf ra), and intermediate radicals. Whether formation of lignin intermediate macromolecules/ radicals trapped on quartz wool at 300 °C is due to secondary repolymerization reactions of condensed phenolics during the long accumulation time is a question we pose. However, the experiments with and without quartz wool evidenced formation of the same trace amounts of phenolics and the intermediate lignin macromolecules at a residence time of 0.12 s. The high volatility of detected phenolics and 2,3-dihydrobenzofuran at 300 °C ruled out the condensation and then secondary cracking and repolymerization of phenolics. Partially cracked lignin fragments have been described in publications by Kim et al.15,16 and in refs 36 and 70. These fragments are probably paramagnetic intermediates and, in fact, are present as relatively stable macromolecular radicals; they still contain lignin type

Figure 9. Aging experiments for the samples collected on quartz wool (5 mg of lignin was transferred through the pyrolysis zone). Aging was performed in a closed EPR tube under ambient air environment.

tube under ambient air environment the intensity of the EPR signal dropped only by 40%. This may happen because of partial condensation of carbon-centered lignin radicals discussed above. The nonsignificant change of g and ΔHp‑p values for radicals (inset spectrum in Figure 9) during the long exposure time indicates the dominance of carbon centered radicals with low g-values around 2.0028 and ΔHp‑p of ∼9 G. Further aging of radicals up to one month leads to doubling of the intensity of the EPR spectrum which is similar to the changes of tobacco radicals in the tar in typical circumstances reported from our lab as an effect of the adsorbed water.72 Laser Desorption Ionization-Time of Flight-Mass Spectrometry (LDI-TOF-MS) Measurements. Other experimental data concerning the important role of intermediate lignin macromolecules during gas-phase pyrolysis of HL were obtained by using LDI-TOF-MS. The experimental conditions for HL pyrolysis were kept unchanged when trace amounts of phenolics, similar to the products shown in Figure 6, were generated. The residue from HL pyrolysis was collected on the quartz wool and extracted into acetone for LDI-TOF-MS analysis. 12163


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The intensification of another lignin substructure at m/z 358 after pyrolysis, Figure 10, might be by dimerization of coniferyl alcohol73 or dehydrogenation of coniferyl alcohol via radical intermediates suggested in ref 78, Figure S3. This is excellent evidence for existence of the radicals and self-condensation of them78 by formation of such dimers at m/z 358 and prevailing of reactions of condensation (radical−radical reactions) over the formation of phenolics due to radical-molecule reactions (no coniferyl alcohol was detected by GC-MS). The dimer at m/z 358 resembles substituted phenylcoumarans, Figure S3.78 The class of compound-like derivatives of phenylcoumarans with m/z from 342 to 358 was identified in ref 74 from some fractions of pyrolytic lignin. Therefore, the existence of the intrinsic dimer, probably trimer lignin substructures (refer to Figure 10, dimer peaks at m/z 284 and 368, and the peak at m/z 522.79, 550.81 which might be some trimers), provides evidence that the oligomers may have a potential role in the formation of phenolics in corresponding experimental conditions. However, the possibility of parallel formation of these compounds, phenolics and oligomers in the processes of lignin pyrolysis, remains open for further discussion. 3.4. Lignin Promotes Acetone Pyrolysis and vice versa − A Synergic Effect. The fact that lignin quantitatively is transported through the hot zone (85−92% recovered almost in all temperature regions investigated) is evidence that less than ∼15% consumed lignin may accelerate the conversion of acetone. Lignin seemed to act as a hydrogen donor in the gas phase (while being a hydrogen deficient macromolecule7) forming a complex between the end γ carbon, −CH2OH group with acetone allowing smooth transformation of hydrogen into the acetone molecule because of hydrogen bonding. In fact, lignin may catalyze the conversion of acetone. Formation of 2,5-hexanedione during HL:acetone mixture pyrolysis can be simply explained by dimerization of acetonyl radicals (•CH2− CO−CH3) formed by abstraction of hydrogen from the acetone methyl group by H atoms originated from lignin. As a source of alternative-transferable hydrogen, catalyzing further conversion of acetone, one could also mention formaldehyde. It is one of the intermediate products released in the early stages of pyrolysis of different types of lignins due to the splitting of the end −CH2OH group,7 with a maximum yield of the formaldehyde peak between 300 and 450 °C.79 Another possibility for generation of hydrogen atoms from lignin end groups might be cinnamyl alcohol catalytic dehydrogenation over the dehydrogenation catalysts (such as Pd/Al2O380). The content of the trace metals in lignin shown in refs 31 and 81 could be a sign of catalytic generation of hydrogen. As a result, the pyrolysis reaction of acetone may be accelerated due to high activity of H atoms released during pyrolysis of HL; such occurrence has been recently shown in our publications during pyrolysis of some model compounds of lignin such as cinnamyl and p-coumaryl alcohols.17,19 Note that the side chain transformation/fragmentation prior to homolytic cleavage of the β-aryl ether linkage at 600 °C was recently suggested in ref 82. Sequentially, lignin pyrolysis can also be additionally initiated by CH3 radicals released from acetone, a major source of CH3 radicals in photochemical83 as well thermal pyrolysis conversion of acetone.84 The synergic effect for formation of new products in mixtures is attractively shown in ref 85. The pyrolysis of single model compounds (hydroxyacetone/cyclopentanone for cellulose and hemicellulose and vinyl-guaiacol for lignin) did

The LDI-TOF mass spectrometry results are demonstrated in Figure 10. Those data shown with green color arrows (m/z

Figure 10. LDI-TOF spectra for fresh initial HL and the residues (collected on quartz wool in close proximity to the hot zone at ∼300 °C) after gas-phase pyrolysis of lignin.

358.39; 568.72; 596.76; 624.79; 668.80; 836.03) are the ions after lignin pyrolysis. However, they do not appear in the current initial spectrum as they are not appreciably being detected for analysis. The major difference in the spectrum can be seen at m/z 550, 522, 368 (a dimer with the structure of C17O6H20 suggested in ref 73) and m/z 284. The peaks at m/z 550 (detected also in ref 74 from one of the lignin fractions by SEC-size exclusion chromatography), m/z 522 and 368 almost completely disappeared during the reaction, and the m/z 284 peak grew several times more than the initial lignin phase. This is due to the fact that while quantitative transport of HL occurred through the hot zone, those peaks converted into the other detected ions at m/z 284 (the major peak after pyrolysis), m/z 358.39 and higher molecular weight intermediates appeared, after the initial HL major peak (m/z 550.81), at m/z 568.72, 596.76, 624.79, 668.80, 836.03, and 863.03. All LDI-TOF analysis was done immediately after experiments with a fresh sample because the associations of lignin macromolecules can also happen even at room temperatures. Note that a group of peaks in the m/z range of 400−550 has been shown in chemical composition analysis of bio-oil components from fast pyrolysis of biomass.43 Therefore, formation of lignin intermediate macromolecules during gasphase pyrolysis is an experimental fact and worthy of further attention. Another intriguing fact is the dominant formation of the peak at m/z 284 (Figure 10). The GC-MS results also show the major peak at 26.42 min, Figure 6, with molecular weight of 284 Da. However, the GC-MS mass-spectral database in this case is not conclusive. The literature data are similarly controversial: it was suggested that the peak at m/z 284 is C18 fatty acid75 from wood decay by white-rot and brown-rot fungi or derivatives of methoxyhydroxyphenanthrene.37 However, we are biased to an explanation developed in refs 76 and 77 stating a transformation of the protonated phenylcoumaran derivative 4 [C19H19O6]+ at m/z 343.08 to protonated (M+H+) moieties with m/z 285 (C17H16O4 + H+), Figure S2. The m/z 284 (O4 dimer C17H16O4) has been also clearly shown in ref 43 during fast pyrolysis of biomass. 12164


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not result in the aliphatic hydrocarbons; however, pyrolysis of a mixture of these model compounds yielded aliphatic hydrocarbons validating the importance of the intermolecular interactions.85 Note that the solvent swelling pretreatment of some coals using acetone can improve the coal conversion and oil yields.86

CONCLUSIONS These results validate the fact that dispersion of the lignin into the gas phase by decreasing the sample size (“minimizing the char area to avoid catalytic contact” of molecular products/ radicals with the surface) may open new perspectives to understand the chemistry of depolymerization of lignin. This is an important question to recognize the undelaying chemistry and elucidation of the primary decomposition pathways of lignin which may be a key approach to regulate the rates of formation of phenols/oligomers from lignin pyrolysis. Moreover, any additional validation of this hypothesis could directly answer to the largely disputed key question on lignin pyrolysis as to whether the phenolic compounds or oligomers (dimers, trimers, etc.) are the primary products. On the other hand, a free-radical mechanism of depolymerization of lignin can be plausible on the way of these transformations of lignin macromolecules. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01842. Cold finger assembly with the pyrolysis reactor; products identification from the pyrolysis of HL dissolved in an acetone:water (9:1) mixture (Table S1); transformation of the protonated phenylcoumaran derivative 4; formation of a dimer with m/z = 284 and 358 (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +1-225-578-4417. E-mail: [email protected]. *Phone: +1-225 578 7762. E-mail: [email protected]. ORCID

Lavrent Khachatryan: 0000-0002-8067-7964 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by a National Science Foundation grant (#1330311). The authors acknowledge partial support from NSF EPSCoR (OIA #1632854), NSF CBET (award# 1437810), USDA NIFA via Hatch funding mechanisms (project LAB #94146), and the Romanian Ministry of Research and Innovation, Priority Axis 1: Research, Technological Development and Innovation (RD&I) to Support Economic Competitiveness and Business Development, Action 1.1.4: Attracting high-level personnel from abroad in order to enhance the RD capacity (ID/My SMIS Code: P_37_768/ 103651; Contract # 39/02.09.2016). Published with the approval of the Director of the Louisiana Agricultural Experiment Station as manuscript 2017-232-31345. 12165


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Peculiarities of Pyrolysis of Hydrolytic Lignin in Dispersed Gas Phase

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