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Pyrolysis reactions of coniferyl alcohol as a model of the primary structure formed during lignin pyrolysis
Journal of Analytical and Applied Pyrolysis 104 (2013) 573–584
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Pyrolysis reactions of coniferyl alcohol as a model of the primary structure formed during lignin pyrolysis Takeo Kotake, Haruo Kawamoto ∗ , Shiro Saka Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan
a r t i c l e
i n f o
Article history: Received 9 April 2013 Accepted 11 May 2013 Available online 21 May 2013 Keywords: Lignin Pyrolysis Coniferyl alcohol Reaction mechanism Volatilization Degradation
a b s t r a c t It has been suggested that cinnamyl alcohol-type structures are formed during lignin primary pyrolysis using model dimers. In this article, the pyrolysis reactions of trans-coniferyl alcohol (CA) bearing a guaiacyl moiety were studied under N2 at temperatures in the range of 200–350 ◦ C, with particular emphasis on the evaporation/degradation processes. Some (less than 15%) of the CA evaporated without undergoing any degradation reactions, whereas large portions of the CA were converted to polymerization products together with monomers (up to ∼15% in total) with various side-chains. The cis-isomer of CA and 4-vinylguaiacol with a C2 side-chain were also identified. Methylation of the phenolic OH group of CA substantially reduced the formation of polymerization products, whereas the influence of the methylation on the side-chain-converted monomers was limited. Since the methylated CA was not effective for quinine methide formation, quinine methide and radical pathways were indicated as more important reaction mechanisms for the polymerization and side-chain-conversion processes, respectively. These results suggest that CA, if it was formed through pyrolytic cleavage of lignin -ether linkages, tended to be degraded before it could be recovered through evaporation. Furthermore, the recovery of different monomers suggested the process was greatly dependent on the relative evaporation/degradation efficiencies. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Pyrolysis is the underlying principle of the thermochemical conversion processes of lignocellulosic biomass, which include gasification, fast pyrolysis (bio-oil production), and carbonization. With this in mind, the development of a deeper understanding of the chemical reactions that occur during these pyrolysis processes could allow for these pyrolysis-based conversion processes to be improved. Lignin, which represents about 20–30 wt% of the content of wood, is an aromatic polymer composed of phenylpropane units that are connected through ether and condensed (C C) linkages. Model compound studies using a variety of different ether [1–7] and condensed [4] linkages have revealed that the ether linkages are cleaved in the temperature range of 200–400 ◦ C, which is close to the temperature range (300–400 ◦ C) [8–11] where major weight-loss typically occurs during the thermogravimetric analysis of lignin. Under such conditions, the condensed linkages of the model compounds have been reported to be stable [4]. Accordingly, the cleavage of the ether linkages, especially the -ether types,
∗ Corresponding author. Tel.: +81 75 753 4737; fax: +81 75 753 4737. E-mail address: [email protected] (H. Kawamoto). 0165-2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2013.05.011
which are the most abundant of the ether linkages, has been considered to play an important role in the depolymerization of lignin and the weight-loss behavior that occurs during pyrolysis. Consideration of the pyrolysis reactions that occur during the pyrolytic cleavage of the -ether linkages is particularly important for understanding the mechanisms responsible for the formation of the pyrolysis products (i.e., the monomers and oligomers) from lignin. Kawamoto et al. [4–7] have reported that cinnamyl alcohols such as coniferyl alcohol were formed in substantial yields from the -ether cleavage reactions of model dimer compounds. Based on structure-reactivity relationship studies involving a variety of p-substituted model compounds [12], a homolytic C O bond cleavage mechanism has been proposed that proceeds via a quinone methide intermediate. Radical chain mechanisms initiated by the abstraction of hydrogen atoms from the side-chain [1–3,7] and the phenolic hydroxyl group [7] have also been proposed for the -ether cleavage reactions. The results from these types of model compound studies have indicated that coniferyl alcohol is formed as a particularly important primary product during the pyrolysis of guaiacol (G)-type lignins. Various monomeric products have been identified from the pyrolysis and fast pyrolysis oils obtained from wood and isolated lignins [13–18]. The substitution patterns on the aromatic rings of the G-type lignins changed during the pyrolytic conversion
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process from guaiacol-type structures to catechol-, o-cresol- and phenol-type structures [19–23]. These changes are attributed to the homolytic cleavage of the O-CH3 bond of the guaiacol-type structures and the subsequent radical induced rearrangement (ipso substitution) of the OCH3 groups on the lignin aromatic rings, with both of these processes becoming effective at temperatures greater than 400 ◦ C [23]. The structures of the side-chains also changed from double bonds (i.e., >C C< and >C O) to saturated alkyl groups and hydrogen [21]. With these results in mind, the primary pyrolysis products formed through the -ether cleavage of lignin would be recovered below 400 ◦ C as a variety of different guaiacol-based derivatives. Contrary to expectations derived from the model compound study, coniferyl alcohol was not reported as a major product from the pyrolysis of G-lignin [13–18]. Coniferyl aldehyde, isoeugenol, dihydroconiferyl alcohol, 4-vinylguaiacol, and vanillin were reported instead as more important products. Arias et al. [18] also reported that the signal of coniferyl aldehyde appeared as the major pyrolysis product by gas chromatography-mass spectroscopy (GC–MS) from the pyrolysis of pine wood at 250 ◦ C. The occurrence of these discrepancies between the data for lignin/wood and those derived from the model dimers encouraged us to study the reactivity of coniferyl alcohol. To the best of our knowledge, there have only been a few papers published to date describing the pyrolysis of coniferyl alcohol [24–26]. Masuku [24] studied the thermal decomposition of coniferyl alcohol in a glass ampoule under argon at temperatures of 200 and 275 ◦ C, and found that the condensation product was one of the major products. He also identified dihydroconiferyl alcohol, coniferyl aldehyde, isoeugenol and dihydroconiferyl aldehyde as the monomeric products. However, no further systematic studies have been conducted into the relative evaporation/degradation efficiency or the role of the phenolic hydroxyl group in the pyrolysis process. Herein, we have used an open-top type of reactor to evaluate the relative efficiencies of the evaporation and degradation processes of coniferyl alcohol and other monomeric products. The role of the phenolic hydroxyl group was also evaluated using 4-O-methylconiferyl alcohol to develop a better understanding of the condensation and side-chain-conversion mechanisms.
2. Experimental 2.1. Materials trans-Coniferyl alcohol (trans-CA, 1) was prepared by the reduction of trans-coniferyl aldehyde with sodium borohydride. cis-Coniferyl alcohol (cis-CA, 2) was separated from the pyrolysis mixture using preparative thin-layer chromatography (TLC) on a silica gel plate (Kieselgel 60 F254 , Merck). Coniferyl aldehyde (3, Sigma-Aldrich Co. LCC., MO, USA), dihydroconiferyl alcohol (4, Tokyo Chemical Industry Co. Ltd., Tokyo, Japan) and isoeugenol (5) and 4-vinylguaiacol (6) (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were commercially available as the guaranteed grades and used without further purification. 4-O-Methylconiferyl alcohol was prepared by the methylation of trans-coniferyl aldehyde with methyl iodide and potassium carbonate in N,N-dimethylformamide, followed by reduction with sodium borohydride. The chemical structures of all of the compounds prepared in this report were confirmed based on their 1 H NMR spectra. All of the 1 H NMR spectra were measured on a Varian AC-400 (400 MHz) spectrometer. The chemical shifts and coupling constants (J) are shown as ı and Hz, respectively.
Fig. 1. Reactor following the pyrolysis of coniferyl alcohol (N2 /350 ◦ C/5 min) and the two fractions obtained following the cutting of the reactor and subsequent extraction with methanol.
2.2. Pyrolysis and separation of volatile and non-volatile products/CA An open-top reactor system similar to that used in our previous paper [27] was used for the current work. trans-Coniferyl alcohol (5.0 mg) was placed on the glass wall up to around 1.5 cm from the bottom of a Pyrex glass tube (internal diameter 8.0 mm, length 300 mm, wall thickness 1.0 mm) through evaporation from a methanol (MeOH) solution. Then, a nitrogen bag was attached to the top of the tube reactor via a three-way tap, and the air inside the reactor was purged with N2 using an aspirator connecting through the three-way tap. The bottom two thirds of the reactor were then inserted into a muffle furnace, which had been preheated at 200–350 ◦ C, through a small hole in the top of the furnace. After heating for 5 min, the reactor was immediately cooled by flowing air over the reactor for 1 min and subsequently by cold water for 1 min. An example of the reactor after heat treatment is shown in Fig. 1. The reactor wall was cut off into two parts at approximately 1.5 cm from the bottom of the reactor, and both parts were rinsed with MeOH (1.0 mL) to recover the MeOH-soluble products together with the remaining coniferyl alcohol. The MeOH-soluble fractions obtained from the upper and lower parts of the reactor wall will be denoted throughout the rest of this paper as FUpper and FLower , respectively. Given that the coniferyl alcohol (starting compound) was placed at the bottom of the reactor prior to the heat treatment, the product recovered in the FUpper was a volatile product that had evaporated from the bottom part of the reactor. Accordingly, we were able to analyze the volatile products (or CA) separately from those remaining in the melt/solid phase at the bottom of the reactor. 2.3. Product analysis and quantification The products from the pyrolysis mixtures were separated by preparative TLC to isolate the pyrolysis products. The chemical structures of the isolated products were determined based on a comparison of their 1 H NMR spectra (measured in CDCl3 ) and Rf values with those of the authentic compounds. The quantification of these products and the remaining CA was mainly conducted by analyzing the MeOH-soluble portions by highperformance liquid chromatography (HPLC) on a Shimadzu LC-10A system under the following conditions: column: Cadenza CD C-18; flow rate: 0.7 mL/min; eluent: binary gradient MeOH/H2 O = 30/70 (0 min) → 45/55 (0 min → 5 min), 45/55 (5 min → 25 min) → 100/0 (25 min → 55 min), 100/0 (55 min → 70 min); detector: UV280 nm ; temperature: 40 ◦ C. The product yields and CA recoveries were determined from their peak areas compared with that of 1,2,3trimethoxybenzene, which was used as an internal standard. For the dihydroconiferyl alcohol, the UV absorption at 280 nm was relatively low, which could lead to a reduction in the accuracy of
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the quantification by HPLC analysis. With this in mind, the dihydroconiferyl alcohol yield was also determined based on the peak area of one of its 1 H NMR signals (ı 2.65 ppm, t, J = 7.4, C␣ -H). The identification and characterization of the products from 4-Omethylconiferyl alcohol were also conducted in a similar manor. Gel-permeation chromatography (GPC) was also used to obtain molecular weight (MW) distribution information for the MeOHsoluble products using a Shimadzu LC-10A system under the following conditions: column: Shodex KF-801 [exclusion limit: 1500 (polystyrene)]; flow rate: 0.6 mL/min; eluent: THF; detector: UV280 nm ; temperature: 40 ◦ C. Under some of the pyrolysis conditions, a black-colored MeOHinsoluble portion remained following the extraction process, which is considered as an intermediate for formation of coke. This fraction adhering to the reactor wall was dried in an oven at 105 ◦ C for 24 h, then closed from the air, and incinerated at 600 ◦ C for 2 h to give CO and CO2 , with both of these gases being quantified by Micro GC with a Varian CP-4900 under the following conditions: channel 1 – column: MS5A 10 m; carrier gas: argon; temperature: 100 ◦ C; pressure: 170 kPa; detector: thermal conductivity detector (TCD); channel 2 – column: PoraPLOT Q 10 m; carrier gas: helium; temperature: 80 ◦ C; pressure: 190 kPa; detector: TCD. The yield the MeOH-insoluble products was determined as a C-based % of the initial amount of coniferyl alcohol.
FUpper
FLower
2.4. Bond dissociation energy calculation The density functional theory (DFT) calculation was conducted under AM1 at the B3LYP/6-311+G** level with “Spartan’08” (Wavefunction Inc.) to obtain a bond dissociation energy (BDE). A zero-point energy correction was not made. 3. Results and discussion 3.1. Evaporation and degradation behavior of coniferyl alcohol Fig. 2 illustrates the results of the GPC analysis of the MeOHsoluble portions, i.e., the FUpper and FLower portions that were recovered from the upper and lower parts of the reactor wall, respectively. At 200 ◦ C, the signal corresponding to CA (12.8 min) remained relatively unchanged in the FLower from that obtained under the assumption of a 100% recovery, whereas the corresponding CA signal observed in the FUpper was only very small. As the pyrolysis temperature increased, the CA signals in the FLower became smaller and almost disappeared at temperatures of 300 and 350 ◦ C. These observations indicated that the evaporation/degradation of coniferyl alcohol started at temperatures around 200–250 ◦ C under the current pyrolysis conditions. For the evaporation of CA, the CA signal in the FUpper became larger following an increase in the pyrolysis temperature from 200 to 300 ◦ C, although these signals were much smaller than that of the CA obtained by assuming 100% evaporation. In contrast, some major signals were observed in the FLower together with some broad signals that occurred in the high MW region (shorter retention times). Thus, the volatilization of CA was not effective under the current pyrolysis conditions, with the CA being converted to the polymerization products as reported by Masuku [24]. The appearance of shoulders in the FUpper at slightly longer retention times (13.5–14.5 min) than that of CA indicated the formation of some monomeric products. Several monomeric guaiacols were identified as the low MW products from the CA, as illustrated in the HPLC chromatogram shown in the figure (350 ◦ C, Fig. 3). The side-chain structures of these products existed in different oxidation levels to that of CA. For example, the coniferyl aldehyde (3) represents an oxidation
8
10
12
14
16
Retention time (min) Fig. 2. Gel permeation chromatograms of the methanol-soluble portions obtained from the pyrolysis of coniferyl alcohol (CA) at various temperatures (N2 /5 min). a: retention time of CA; –·–·–·: 200 ◦ C; – – –: 250 ◦ C; ----: 300 ◦ C, —: 350 ◦ C; : chromatogram assuming that CA is completely recovered from the FLower or FUpper fractions without decomposition.
product, whereas dihydroconiferyl alcohol (4) and isoeugenol (5) represent reduction products. Based on these results, it was clear that some redox reactions were taking place during the pyrolysis of CA. The cis-isomer (2) of CA and 4-vinylguaiacol (6) with a C2 side-chain were also identified. Fig. 4 shows the influence of the pyrolysis temperature on the yield of the monomeric guaiacols in the FLower and the FUpper . The total yields (FLower + FUpper ) increased with increasing pyrolysis temperature for most of the products. These products tended to be recovered from the FLower at 200 and 250 ◦ C, although the recoveries from the FUpper become rather more important at higher temperatures. These observations could be explained by the formation and evaporation processes, in that these monomeric guaiacols formed in the liquid/solid phase and tended to be evaporated at higher temperatures. This hypothesis appeared to be reasonable, because the boiling points of these products were expected to be within a similar temperature range, including CA (332.2 ± 0.0 ◦ C [28]), dihydroconiferyl alcohol (339.8 ± 27 ◦ C [28]), isoeugenol (266 ◦ C), coniferyl aldehyde (338.8 ± 27 ◦ C [28]), and 4vinylguaiacol (245 ◦ C ± 20 ◦ C [28]). Fig. 5 provides a summary of the recovery levels of CA, as well as the yields of the monomeric guaiacols and the MeOH-insoluble products at each pyrolysis temperature. A small amount of the
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Fig. 3. HPLC chromatogram of the methanol-soluble portion (FUpper ) obtained by the pyrolysis of coniferyl alcohol (N2 /350 ◦ C/5 min) with identification of some signals.
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2 CA ( cis ) (2)
Coniferyl aldehyde (3 3)
Dihydroconiferyl alcohol (4) 4
Isoeugenol ( 5 5)
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Pyrolysis temperature (°C)
4-Vinyl guaiacol (6 6) 4.0
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Pyrolysis temperature (°C) Fig. 4. Yields of monomeric guaiacols from the pyrolysis of coniferyl alcohol (CA) (N2 /200–350 ◦ C/5 min). : FLower; : FUpper ; : total.
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Fig. 5. Recovery of coniferyl alcohol (CA) and yields of the monomeric guaiacols and the MeOH-insoluble products from the FUpper and FLower fractions obtained after pyrolysis of CA at various temperatures (N2 /5 min).
: CA recovery (FLower );
:
CA recovery (FUpper ); : monomeric guaiacol yield (FUpper + FLower ); : yield of the MeOH-insoluble; : higher MW products and unknown; : total.
MeOH-insoluble products was obtained only at 350 ◦ C. Based on the proposal of Hosoya et al. [22], which states that the o-quinone methide formed through ipso substitution at temperatures greater than 400 ◦ C is an important precursor of lignin coke formation, we believe that the lower pyrolysis temperatures employed in this paper would not be high enough to facilitate coke formation.
The total values for the recovery and the yields of monomeric guaiacols decreased when the pyrolysis temperature was increased from 200 to 300 ◦ C. In contrast, however, they increased slightly when the temperature was increased to 350 ◦ C. For the CA recovery, the total recovery (FLower + FUpper ) was reduced significantly to around 15% when the pyrolysis temperature was increased to 300 and 350 ◦ C. Although the monomeric guaiacol yields also increased up to around 15 wt% at 350 ◦ C, the amounts were not so large. Consequently, coniferyl alcohol was found to polymerize prior to evaporation and the formation of a variety of different monomeric guaiacols. Given that the depolymerization and weight-loss processes of lignin occur predominantly at temperatures in the range of 300–400 ◦ C, these results suggested that the coniferyl alcohol yield would be low even if it was formed during the primary pyrolysis of G-type lignin because of secondary polymerization events. The influences of the pyrolysis temperature on the recoveries of coniferyl aldehyde (3), dihydroconiferyl alcohol (4), isoeugenol (5) and 4-vinylguaiacol (6) from the FLower and the FUpper are shown in Fig. 6. A comparison of the data presented in Figs. 5 and 6 provided some additional information regarding the role of the relative evaporation/degradation efficiency in the formation of the monomeric products. The total recoveries (FLower + FUpper ) of the compounds 3–6 were much higher than those of the CA. Given that the boiling point of CA (332.2 ± 0.0 ◦ C, [28]) is similar to those of coniferyl aldehyde (3) and dihydroconiferyl alcohol (4), it is possible that the lower recovery of CA could have originated from its higher degradation reactivity, especially for the condensation reaction, than those of compounds 3 and 4. Dihydroconiferyl alcohol, which does not contain a conjugated C C double bond, exhibited relatively high recovery values. The high recoveries encountered in this case were probably caused by the lower formation reactivity
Fig. 6. Recoveries of dihydroconiferyl alcohol, isoeugenol, coniferyl aldehyde and 4-vinylguaiacol from their heat treatment at various temperatures (N2 /5 min). FUpper;
: FLower ; : total.
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of the quinone methide (an intermediate in the condensation). This will be discussed in greater detail later. 4-Vinylguaiacol (6) showed a different temperature dependency, in that the total recovery increased directly with increasing pyrolysis temperature. Thus, the relative evaporation/degradation efficiency would effectively encourage the evaporation of 4-vinylguaiacol at higher temperatures, based on the high condensation reactivity of compound 6 reported at 250 ◦ C and its relatively low boiling point (245 ◦ C ± 20 ◦ C [28]).
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3.2. Reactivity of 4-O-methylconiferyl alcohol Nakamura et al. [26] obtained the two condensation products of CA with creosol (4-methylguaiacol) at 250 ◦ C in 12 and 8.3 mol% yields (based on CA), respectively. They went on to propose a quinone methide-based mechanism as the condensation mechanism, which included the nucleophilic addition of the aromatic ring carbon or hydroxyl oxygen to the C␣ - or C␥ -carbons of the quinone methide intermediates. The quinone methide mechanism has also been confirmed by the reduction in the polymerization reactivity of CA caused by 4-O-methylation. Based on these preliminary results, we studied the pyrolytic reactivity of 4-O-methtylconiferyl alcohol (4-O-Me CA) with a view to developing a better understanding of the molecular mechanisms of the condensation and side-chain conversion reactions. Methylation inhibits the quinone methide formation. Figs. 7 and 8 show the recoveries and the GPC results, respectively, obtained for the pyrolysis of 4-O-Me CA and a mixture of 4-O-Me CA and CA (1:1, w/w) at temperatures in the range of 250–350 ◦ C. Relatively high recoveries (60–90%) of 4-O-Me CA were obtained compared with those of CA, which indicated that the quinone methide mechanism plays an important role in the
50
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0 250
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Pyrolysis temperature (°C) Fig. 7. Recoveries (FUpper + FLower ) of coniferyl alcohol (CA) and 4-O-methylconiferyl alcohol (4-O-Me CA) from their individual pyrolysis reactions and the pyrolysis of their mixture at various temperatures (N2 /5 min). : CA; : 4-O-Me-CA; —: individual, -----: mixture (CA:4-O-Me-CA = 1:1, w/w).
condensation of CA. This was also confirmed by the GPC results, where only the signals corresponding to 4-O-Me CA were observed in the chromatograms (FLower ). Even 4-O-Me CA became reactive in the presence of CA. The recovery of 4-O-Me CA was dramatically reduced (Fig. 7) and the 4-O-Me CA signals almost disappeared in the FLower from the pyrolysis of the mixture (Fig. 8). The evaporation of 4-O-Me CA was also suppressed as indicated by the reduction of the signals in the FUpper . Thus, even the non-phenolic structure condensed with CA (phenolic structure). This would be
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4-O-Me CA + CA (1 : 1, wt/wt)
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Retention time (min) Fig. 8. Gel permeation chromatograms of the methanol-soluble portions obtained from the pyrolysis of 4-O-methyl coniferyl alcohol (4-O-Me CA) and its mixture with coniferyl alcohol (CA) (CA:4-O-Me-CA = 1:1, w/w) at various temperatures (N2 /5 min). – – –: 250 ◦ C; ----: 300 ◦ C; —: 350 ◦ C; a: retention time of CA; b: retention time of 4-O-Me CA.
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Fig. 9. Proposed condensation mechanisms of coniferyl alcohol and 4-O-methylconiferyl alcohol.
important for considering the pyrolysis of lignin, in that coniferyl alcohol-type structures (phenolic forms) would add to the repeating phenylpropane units (non-phenolic forms). Although further studies would be necessary to identify the condensation sites of 4-O-Me CA, the aromatic ring carbons or side-chain double bond could be involved in the condensation with the quinone methide intermediates A and B, as shown in Fig. 9. Interestingly, the side-chain conversion products were also obtained from the 4-O-Me CA in the yields summarized in Fig. 10, although their yields were slightly lower than those encountered with CA. Furthermore, their formation was enhanced in the presence of CA to levels similar to those of CA pyrolysis. These results suggested that the formation of these products involved some radical chain reactions, in that the radical species formed from the CA would activate the conversion of 4-O-Me CA. 4-O-Methylvinylguaiacol in the pyrolysis mixture could not be quantified because of the occurrence of overlapping peaks in the HPLC analysis. This problem could not be solved even by changing
the chromatographic conditions including the binary gradient conditions and flow rates. Consequently, the quinone methide and radical pathways were found to play more important roles in the condensation and sidechain conversions of CA, respectively. This information would be useful for the discussion of the molecular-based reaction mechanisms.
3.3. Molecular mechanisms for the pyrolytic conversion of coniferyl alcohol The mechanisms proposed for the formation of cis-coniferyl alcohol (2), coniferyl aldehyde (3), dihydroconiferyl alcohol (4), isoeugenol (5), and 4-vinylguaiacol (6) are illustrated in Figs. 11–14. These mechanisms include the quinone methide and radical chain pathways, although further studies would be necessary to confirm the mechanisms and completely understand their contributions.
Fig. 10. Yields (FUpper + FLower ) of monomeric 4-O-methyl guaiacols from the pyrolysis of 4-O-methylconiferyl alcohol (4-O-Me CA) and its mixture with coniferyl alcohol (CA) (CA: 4-O-Me-CA = 1:1, w/w) at various temperatures (N2 /5 min). : 4-O-methylvinylguaiacol
: 4-O-methyldihydroconiferyl alcohol;
: O-methylisoeugenol;
: 4-O-methylconiferyl aldehyde;
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Fig. 11. Proposed pyrolytic formation mechanisms of cis-coniferyl alcohol and coniferyl aldehyde from trans-coniferyl alcohol.
Some of the bond dissociation energies obtained from the DFT calculation have also be included for discussion. The isomerization from trans to cis CA is possible via both the quinone methide (Fig. 11A) and C␣ - and C␥ -radicals (Figs. 12 and 13). Higher trans/cis ratios were usually observed during pyrolysis, with the equilibrium tending toward the more stable trans isomer for steric reasons. The coniferyl aldehyde would be formed via the C␥ -radical intermediate (Fig. 11B). Two pathways were considered for the formation of this radical intermediate, involving either homolytic cleavage of the C␥ H bond or hydrogen abstraction from the C␥ H bond by some radical species. The relatively large calculated BDE (79.9 kcal/mol) suggest that the latter H-abstraction pathway is more probable at the pyrolysis temperatures used in the current study. The -scission type reaction from the C␥ -radical would provide a coniferyl aldehyde together with a hydrogen radical. This hydrogen radical could be used for addition to a double bond and H-donation to stabilize other radical species. Dihydroconiferyl alcohol is a hydrogenation product of CA. The direct addition of a hydrogen radical to the double bond of CA and the hydrogenation to the quinone methide intermediate are both
plausible reaction pathways (Fig. 12). As a similar mechanism to the latter pathway, the formation of o-cresol (2-methylphenol) from guaiacol has been considered to proceed via the hydrogenation of the o-quinone methide intermediate [19,20]. The 4-O-Me CA also gave a dihydroconiferyl alcohol type derivative, which indicated that the direct hydrogen radical addition pathway was involved in this transformation. The formation of a hydrogen radical would be necessary for both pathways. The formation of this type of compound could therefore be used as a probe for the hydrogen radical in the pyrolysis environment, together with isoeugenol, although this will be discussed in our following paper [29]. Both the quinone methide and radical mechanisms were considered for the formation of isoeugenol from CA (Fig. 13). The quinone methide intermediate formed by the elimination of the hydroxyl group from C␥ of the CA would be hydrogenated. In the radical mechanism, the elimination of the C␥ -hydroxyl group could proceed in one of two different ways, in that it could occur via a -scission type elimination of the OH radical from the C radical intermediate (pathway a) or via the direct homolysis of the C␥ OH bond (pathway b). The calculated BDE (72.2 kcal/mol) would, however, be too large for the homolysis pathway at a
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Fig. 12. Proposed pyrolytic formation mechanisms of dihydroconiferyl alcohol from coniferyl alcohol.
pyrolysis temperature less than 350 ◦ C. The C -radical pathway would therefore be more probable. In a manner analogous to the dihydroconiferyl alcohol formation mechanism, the formation of the isoeugenol type structure from 4-O-Me CA suggested that pathway was involved in this conversion process. Two sites were possible for the addition of hydrogen to the double bond of CA (Fig. 15). For pathway a (Fig. 13), the hydrogen radical would add to the C␣ position. Thermodynamically, the formation of the more stable C␣ -radical obtained by the addition of hydrogen to the C would be more favorable because of the resonance stabilization of the C␣ -radical with the aromatic ring, as shown in Fig. 15a. In contrast, the stereoelectronic features are known to be an important factor affecting the addition reaction of the radial to the double bond [30]. In this case, the reaction proceeds under kinetic control. Generally, the singly occupied orbital of the radical attacks the alkene at an angle close to 107◦ to maximize overlap with the alkene * orbital. Under such conditions, the less hindered C␣ would be preferred to the C , which would be sterically hindered to a greater extent by the C␥ hydroxyl group and hydrogens (Fig. 15 b). Further study would be necessary to identify the relative reactivities of the C␣ and C positions. 4-Vinylguaiacol would be formed via C␣ -radical (Fig. 12) and quinone methides (Fig. 14). The homolytic C C␥ bond cleavage of the quinone methides would occur, which are obtained from CA and coniferyl aldehyde (Fig. 14). The electron-withdrawing quinone methide moieties attaching to the C positions would
reduce the BDEs of the C C␥ bonds to 51.8 and 50.1 kcal/mol, which were calculated for the CA- and coniferyl aldehyde-derived quinone methides, respectively. 4-Vinylguaiacol was obtained also from the pyrolysis of coniferyl aldehyde [yield: 1.1 wt% (350 ◦ C), 0.4 wt% (300 ◦ C) and 0.1 wt% (250 ◦ C)]. Formation of this type of products from 4-O-Me CA suggested that the radical pathway was involved in the formation.
3.4. Implications for lignin pyrolysis The current results provided some insights into the pyrolytic formation of monomeric guaiacols during the primary pyrolysis of G-type lignins. The reduced stability of coniferyl alcohol would be attributed to the conjugated C C double bond and the C␥ OH type structures proposed for the formation of the quinone methides A and B in Fig. 9. Nakamura et al. [26] reported that 4-vinylguaiacol derivatives with conjugated C␣ C double bonds are much more labile than the corresponding C␣ OH derivative at 250 ◦ C. The results from this report suggest that the primary pyrolysis of lignin would provide products that were much less stable (such as coniferyl alcohol) than the original lignins containing the C␣ OH derivatives as the main structure. This was also supported by the observation that the temperature range (300–400 ◦ C) where the primary pyrolysis of the lignin occurred to form volatile products is much higher than the degradation temperature (250–300 ◦ C) of the coniferyl alcohol.
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Fig. 13. Proposed pyrolytic formation mechanisms of isoeugenol from coniferyl alcohol.
Similar discussions have been made for cellulose pyrolysis [31], where levoglucosan (1,6-anhydro--d-glucose), which is an important primary pyrolysis product, decomposes (polymerizes) at temperatures higher than 250 ◦ C, representing a temperature range that was much lower than the temperature range (300–350 ◦ C) where cellulose primary pyrolysis takes place. Although the reason for lignin pyrolysis currently remains unknown, the acid-catalysis mechanism [32] involving proton-donation through intermolecular hydrogen bonding has been proposed as a mechanism to explain the unique reactivity of levoglucosan, which is much more stable in the gas phase than the liquid phase [33]. Such interactions are possible only in the liquid (melt) phase, and not in the gas phase. Consequently, the pyrolysis temperature would be high enough to cause the secondary reaction of coniferyl alcohol, following its formation during the primary pyrolysis of lignin at 300–400 ◦ C. The expected boiling point of coniferyl alcohol (332.2 ± 0.0 ◦ C, [28])
suggested that the evaporation and secondary degradation process are competitive under such conditions, as summarized in Fig. 16. As indicated by Masuku’s paper [24] and the results of this study, the most important reaction in the process is the condensation reaction, followed by the side-chain conversion reactions. The relative efficiencies of the evaporation/polymerization/sidechain conversion processes would determine the yield of coniferyl alcohol. For the condensation of coniferyl alcohol, the quinone methide intermediate was suggested to be an important intermediate. Generally, the electro-positive carbons of quinone methides tend to react with the electro-negative aromatic and double bond carbons rather than the oxygen atoms of the side-chain and the phenolic groups. This reactivity can be explained by the “hard and soft acid and base” rule. Given that the condensed (C C) linkages have been reported to be much more stable than the ether linkages during the lignin pyrolysis process [19], re-depolymerization would not
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Fig. 14. Proposed pyrolytic formation mechanisms of 4-vinylguaiacol from coniferyl alcohol.
Fig. 15. The ␣/ regioselectivity of the hydrogen addition to the coniferyl alcohol C␣ C double bond.
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Coniferyl alcohol Other monomeric guaiacols Evaporation
Lignin
Coniferyl alcohol
Side-chain conversion
Monomeric guaiacols
Heating zone (Liquid /solid)
Condensation products
Char
More C-C linkage
Fig. 16. Role of evaporation/condensation/side-chain conversion processes in the formation of coniferyl alcohol and other monomeric guaiacols from lignin pyrolysis.
be effective for the condensation products. Together with the high condensation reactivity of the primary products, this would be a good reason why lignin pyrolysis would tend to form solid product (char) more preferentially. 4. Conclusions The following pyrolysis reactivities of trans-coniferyl alcohol were clarified as a lignin primary pyrolyzate under the pyrolysis conditions of 200–350 ◦ C/N2 /5 min, with particular focus on the evaporation/degradation processes. This information could be useful for understanding and controlling the pyrolysis-based biomass conversion processes. 1. The evaporation/degradation of coniferyl alcohol started at 200–250 ◦ C. 2. The polymerization was a more important process than the evaporation and side-chain conversion processes. 3. Coniferyl aldehyde (an oxidation product), and isoeugenol and dihydroconiferyl alcohol (reduction products) were obtained as side-chain-conversion products together with cis-coniferyl alcohol and 4-vinylguaiacol. 4. Coniferyl alcohol was much more reactive than most of the other side-chain conversion products. 5. 4-O-Methylation of the coniferyl alcohol inhibited the polymerization, but did not inhibit the side-chain conversion processes. 6. 4-O-Methyl coniferyl alcohol condensed in the presence of coniferyl alcohol. 7. A quinone methide mechanism was proposed for the polymerization of coniferyl alcohol. 8. Both quinone methide and radical mechanisms were proposed for the side-chain conversion processes. 9. Lignin pyrolysis was discussed at the molecular level using the present results. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research (B)(2) (No. 24380095, 2012.4-2016.3) and the Kyoto University Global COE program for “Energy Science in the Age of Global Warming”. References ´ V. Mihálov, V. Kováˇcik, Low temperature thermolysis of lignins. I. [1] R. Breˇzny, Reactions of -O-4 model compounds, Holzforschung 37 (1983) 199–204. [2] S.T. Autrey, M.S. Alnajjar, D.A. Nelson, J.A. Franz, Absolute rate constants for the beta-scission reaction of the 1-phenyl-2-phenoxypropyl radical—a model for radical reactions of lignin, Journal of Organic Chemistry 56 (1991) 2197–2202.
[3] P.F. Britt, A.C. Buchanan III, K.B. Thomas, S.-K. Lee, Pyrolysis mechanisms of lignin: surface-immobilized model compound investigation of acid-catalyzed and free-radical reaction pathways, Journal of Analytical and Applied Pyrolysis 33 (1995) 1–19. [4] H. Kawamoto, S. Horigoshi, S. Saka, Pyrolysis reactions of various lignin model dimers, Journal of Wood Science 53 (2007) 168–174. [5] T. Nakamura, H. Kawamoto, S. Saka, Pyrolysis behavior of Japanese cedar wood lignin studied with various model dimers, Journal of Analytical and Applied Pyrolysis 81 (2008) 173–182. [6] H. Kawamoto, M. Ryoritani, S. Saka, Different pyrolytic cleavage mechanisms of -ether bond depending on the side-chain structure of lignin dimers, Journal of Analytical and Applied Pyrolysis 81 (2008) 88–94. [7] T. Watanabe, H. Kawamoto, S. Saka, Radical chain reactions in pyrolytic cleavage of the ether linkages of lignin model dimers and a trimer, Holzforschung 63 (2009) 424–430. [8] O. Faix, E. Jakab, F. Till, T. Székely, Study on low mass thermal-degradation products of milled wood lignins by thermogravimetry-mass-spectrometry, Wood Science and Technology 22 (1988) 323–334. [9] D.J. Gardner, T.P. Schultz, G.D. McGinnis, The pyrolytic behavior of selected lignin preparations, Journal of Wood Chemistry and Technology 5 (1985) 85–110. [10] E. Jakab, O. Faix, F. Till, Thermal decomposition of milled wood lignins studied by thermogravimetry/mass spectrometry, Journal of Analytical and Applied Pyrolysis 40–41 (1997) 171–186. [11] M. Asmadi, H. Kawamoto, S. Saka, Gas- and solid/liquid-phase reactions during pyrolysis of softwood and hardwood lignins, Journal of Analytical and Applied Pyrolysis 92 (2011) 417–425. [12] H. Kawamoto, T. Nakamura, S. Saka, Pyrolytic cleavage mechanisms of lignin–ether linkages: a study on p-substituted dimers and trimers, Holzforschung 62 (2008) 50–56. [13] R.J. Evans, T.A. Milne, Molecular characterization of the pyrolysis of biomass. 1. Fundamentals, Energy and Fuels 1 (1987) 123–137. [14] W. Genuit, J.J. Boon, O. Faix, Characterization of beech milled wood lignin by pyrolysis-gas chromatography–photoionization mass spectrometry, Analytical Chemistry 59 (1987) 508–513. [15] J. Ralph, R.D. Hatfield, Pyrolysis-GC–MS characterization of forage materials, Journal of Agricultural and Food Chemistry 39 (1991) 1426–1437. [16] K. Sipilä, E. Kuoppala, L. Fagernäs, A. Oasmaa, Characterization of biomassbased flash pyrolysis oils, Biomass and Bioenergy 14 (1998) 103–113. [17] B. Scholze, D. Meier, Characterization of the water-insoluble fraction from pyrolysis oil (pyrolytic lignin). Part I. PY-GC–MS, FTIR, and functional groups, Journal of Analytical and Applied Pyrolysis 60 (2001) 41–54. [18] M.E. Arias, O. Polvillo, J. Rodríguez, M. Hernández, J.A. González-Pérez, F.J. González-Vila, Thermal transformations of pine wood components under pyrolysis/gas chromatography/mass spectrometry conditions, Journal of Analytical and Applied Pyrolysis 77 (2006) 63–67. [19] A. Vuori, Pyrolysis studies of some simple coal related aromatic methyl ethers, Fuel 65 (1986) 1575–1583. [20] E. Dorrestijn, P. Mulder, The radical-induced decomposition of 2methoxyphenol, Journal of the Chemical Society, Perkin Transactions 2 (1999) 777–780. [21] T. Hosoya, H. Kawamoto, S. Saka, Secondary reactions of lignin-derived primary tar components, Journal of Analytical and Applied Pyrolysis 83 (2008) 78–87. [22] T. Hosoya, H. Kawamoto, S. Saka, Role of methoxyl group in char formation from lignin-related compounds, Journal of Analytical and Applied Pyrolysis 84 (2009) 79–83. [23] M. Asmadi, H. Kawamoto, S. Saka, Thermal reactions of guaiacol and syringol as lignin model aromatic nuclei, Journal of Analytical and Applied Pyrolysis 92 (2011) 88–98. [24] C.P. Masuku, Thermolytic decomposition of coniferyl alcohol, Journal of Analytical and Applied Pyrolysis 23 (1992) 195–208. [25] A.E. Marman-Ware, M. Crocker, A.P. Kaur, M.S. Meier, D. Kato, B. Lynn, PyrolysisGC/MS of sinapyl and coniferyl alcohol, Journal of Analytical and Applied Pyrolysis 99 (2013) 161–169. [26] T. Nakamura, H. Kawamoto, S. Saka, Condensation reactions of some lignin related compounds at relatively low pyrolysis temperature, Journal of Wood Chemistry and Technology 27 (2007) 121–133. [27] N. Shimada, H. Kawamoto, S. Saka, Different action of alkali/alkaline earth metal chlorides on cellulose pyrolysis, Journal of Analytical and Applied Pyrolysis 81 (2008) 80–87. [28] pKa; Calculated using Advanced Chemistry Development (ACD/Labs) Software V11.02, Scifinder, Chemical Abstracts Service: Columbus, OH, 1994–2013, https://scifinder.cas.org (accessed 11.03.13). [29] T. Kotake, H. Kawamoto, S. Saka, unpublished. [30] A.F. Parsons, An Introduction to Free Radical Chemistry, Blackwell Science, Oxford, 2000, pp. 87–90. [31] T. Hosoya, H. Kawamoto, S. Saka, Pyrolysis behavior of wood and its constituent polymers at gasification temperature, Journal of Analytical and Applied Pyrolysis 78 (2007) 328–336. [32] H. Kawamoto, Y. Ueno, S. Saka, Thermal reactivities of non-reducing sugars in polyether—role of intermolecular hydrogen bonding in pyrolysis, Journal of Analytical and Applied Pyrolysis (2012), http://dx.doi.org/10.1016/j.jaap. 2012.08.009 (in press). [33] T. Hosoya, H. Kawamoto, S. Saka, Different pyrolytic pathways of levoglucosan in vapor- and liquid/solid-phases, Journal of Analytical and Applied Pyrolysis 83 (2008) 64–70.
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Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Pyrolysis reactions of coniferyl alcohol as a model of the primary structure formed during lignin pyrolysis Takeo Kotake, Haruo Kawamoto ∗ , Shiro Saka Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan
a r t i c l e
i n f o
Article history: Received 9 April 2013 Accepted 11 May 2013 Available online 21 May 2013 Keywords: Lignin Pyrolysis Coniferyl alcohol Reaction mechanism Volatilization Degradation
a b s t r a c t It has been suggested that cinnamyl alcohol-type structures are formed during lignin primary pyrolysis using model dimers. In this article, the pyrolysis reactions of trans-coniferyl alcohol (CA) bearing a guaiacyl moiety were studied under N2 at temperatures in the range of 200–350 ◦ C, with particular emphasis on the evaporation/degradation processes. Some (less than 15%) of the CA evaporated without undergoing any degradation reactions, whereas large portions of the CA were converted to polymerization products together with monomers (up to ∼15% in total) with various side-chains. The cis-isomer of CA and 4-vinylguaiacol with a C2 side-chain were also identified. Methylation of the phenolic OH group of CA substantially reduced the formation of polymerization products, whereas the influence of the methylation on the side-chain-converted monomers was limited. Since the methylated CA was not effective for quinine methide formation, quinine methide and radical pathways were indicated as more important reaction mechanisms for the polymerization and side-chain-conversion processes, respectively. These results suggest that CA, if it was formed through pyrolytic cleavage of lignin -ether linkages, tended to be degraded before it could be recovered through evaporation. Furthermore, the recovery of different monomers suggested the process was greatly dependent on the relative evaporation/degradation efficiencies. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Pyrolysis is the underlying principle of the thermochemical conversion processes of lignocellulosic biomass, which include gasification, fast pyrolysis (bio-oil production), and carbonization. With this in mind, the development of a deeper understanding of the chemical reactions that occur during these pyrolysis processes could allow for these pyrolysis-based conversion processes to be improved. Lignin, which represents about 20–30 wt% of the content of wood, is an aromatic polymer composed of phenylpropane units that are connected through ether and condensed (C C) linkages. Model compound studies using a variety of different ether [1–7] and condensed [4] linkages have revealed that the ether linkages are cleaved in the temperature range of 200–400 ◦ C, which is close to the temperature range (300–400 ◦ C) [8–11] where major weight-loss typically occurs during the thermogravimetric analysis of lignin. Under such conditions, the condensed linkages of the model compounds have been reported to be stable [4]. Accordingly, the cleavage of the ether linkages, especially the -ether types,
∗ Corresponding author. Tel.: +81 75 753 4737; fax: +81 75 753 4737. E-mail address: [email protected] (H. Kawamoto). 0165-2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2013.05.011
which are the most abundant of the ether linkages, has been considered to play an important role in the depolymerization of lignin and the weight-loss behavior that occurs during pyrolysis. Consideration of the pyrolysis reactions that occur during the pyrolytic cleavage of the -ether linkages is particularly important for understanding the mechanisms responsible for the formation of the pyrolysis products (i.e., the monomers and oligomers) from lignin. Kawamoto et al. [4–7] have reported that cinnamyl alcohols such as coniferyl alcohol were formed in substantial yields from the -ether cleavage reactions of model dimer compounds. Based on structure-reactivity relationship studies involving a variety of p-substituted model compounds [12], a homolytic C O bond cleavage mechanism has been proposed that proceeds via a quinone methide intermediate. Radical chain mechanisms initiated by the abstraction of hydrogen atoms from the side-chain [1–3,7] and the phenolic hydroxyl group [7] have also been proposed for the -ether cleavage reactions. The results from these types of model compound studies have indicated that coniferyl alcohol is formed as a particularly important primary product during the pyrolysis of guaiacol (G)-type lignins. Various monomeric products have been identified from the pyrolysis and fast pyrolysis oils obtained from wood and isolated lignins [13–18]. The substitution patterns on the aromatic rings of the G-type lignins changed during the pyrolytic conversion
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process from guaiacol-type structures to catechol-, o-cresol- and phenol-type structures [19–23]. These changes are attributed to the homolytic cleavage of the O-CH3 bond of the guaiacol-type structures and the subsequent radical induced rearrangement (ipso substitution) of the OCH3 groups on the lignin aromatic rings, with both of these processes becoming effective at temperatures greater than 400 ◦ C [23]. The structures of the side-chains also changed from double bonds (i.e., >C C< and >C O) to saturated alkyl groups and hydrogen [21]. With these results in mind, the primary pyrolysis products formed through the -ether cleavage of lignin would be recovered below 400 ◦ C as a variety of different guaiacol-based derivatives. Contrary to expectations derived from the model compound study, coniferyl alcohol was not reported as a major product from the pyrolysis of G-lignin [13–18]. Coniferyl aldehyde, isoeugenol, dihydroconiferyl alcohol, 4-vinylguaiacol, and vanillin were reported instead as more important products. Arias et al. [18] also reported that the signal of coniferyl aldehyde appeared as the major pyrolysis product by gas chromatography-mass spectroscopy (GC–MS) from the pyrolysis of pine wood at 250 ◦ C. The occurrence of these discrepancies between the data for lignin/wood and those derived from the model dimers encouraged us to study the reactivity of coniferyl alcohol. To the best of our knowledge, there have only been a few papers published to date describing the pyrolysis of coniferyl alcohol [24–26]. Masuku [24] studied the thermal decomposition of coniferyl alcohol in a glass ampoule under argon at temperatures of 200 and 275 ◦ C, and found that the condensation product was one of the major products. He also identified dihydroconiferyl alcohol, coniferyl aldehyde, isoeugenol and dihydroconiferyl aldehyde as the monomeric products. However, no further systematic studies have been conducted into the relative evaporation/degradation efficiency or the role of the phenolic hydroxyl group in the pyrolysis process. Herein, we have used an open-top type of reactor to evaluate the relative efficiencies of the evaporation and degradation processes of coniferyl alcohol and other monomeric products. The role of the phenolic hydroxyl group was also evaluated using 4-O-methylconiferyl alcohol to develop a better understanding of the condensation and side-chain-conversion mechanisms.
2. Experimental 2.1. Materials trans-Coniferyl alcohol (trans-CA, 1) was prepared by the reduction of trans-coniferyl aldehyde with sodium borohydride. cis-Coniferyl alcohol (cis-CA, 2) was separated from the pyrolysis mixture using preparative thin-layer chromatography (TLC) on a silica gel plate (Kieselgel 60 F254 , Merck). Coniferyl aldehyde (3, Sigma-Aldrich Co. LCC., MO, USA), dihydroconiferyl alcohol (4, Tokyo Chemical Industry Co. Ltd., Tokyo, Japan) and isoeugenol (5) and 4-vinylguaiacol (6) (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were commercially available as the guaranteed grades and used without further purification. 4-O-Methylconiferyl alcohol was prepared by the methylation of trans-coniferyl aldehyde with methyl iodide and potassium carbonate in N,N-dimethylformamide, followed by reduction with sodium borohydride. The chemical structures of all of the compounds prepared in this report were confirmed based on their 1 H NMR spectra. All of the 1 H NMR spectra were measured on a Varian AC-400 (400 MHz) spectrometer. The chemical shifts and coupling constants (J) are shown as ı and Hz, respectively.
Fig. 1. Reactor following the pyrolysis of coniferyl alcohol (N2 /350 ◦ C/5 min) and the two fractions obtained following the cutting of the reactor and subsequent extraction with methanol.
2.2. Pyrolysis and separation of volatile and non-volatile products/CA An open-top reactor system similar to that used in our previous paper [27] was used for the current work. trans-Coniferyl alcohol (5.0 mg) was placed on the glass wall up to around 1.5 cm from the bottom of a Pyrex glass tube (internal diameter 8.0 mm, length 300 mm, wall thickness 1.0 mm) through evaporation from a methanol (MeOH) solution. Then, a nitrogen bag was attached to the top of the tube reactor via a three-way tap, and the air inside the reactor was purged with N2 using an aspirator connecting through the three-way tap. The bottom two thirds of the reactor were then inserted into a muffle furnace, which had been preheated at 200–350 ◦ C, through a small hole in the top of the furnace. After heating for 5 min, the reactor was immediately cooled by flowing air over the reactor for 1 min and subsequently by cold water for 1 min. An example of the reactor after heat treatment is shown in Fig. 1. The reactor wall was cut off into two parts at approximately 1.5 cm from the bottom of the reactor, and both parts were rinsed with MeOH (1.0 mL) to recover the MeOH-soluble products together with the remaining coniferyl alcohol. The MeOH-soluble fractions obtained from the upper and lower parts of the reactor wall will be denoted throughout the rest of this paper as FUpper and FLower , respectively. Given that the coniferyl alcohol (starting compound) was placed at the bottom of the reactor prior to the heat treatment, the product recovered in the FUpper was a volatile product that had evaporated from the bottom part of the reactor. Accordingly, we were able to analyze the volatile products (or CA) separately from those remaining in the melt/solid phase at the bottom of the reactor. 2.3. Product analysis and quantification The products from the pyrolysis mixtures were separated by preparative TLC to isolate the pyrolysis products. The chemical structures of the isolated products were determined based on a comparison of their 1 H NMR spectra (measured in CDCl3 ) and Rf values with those of the authentic compounds. The quantification of these products and the remaining CA was mainly conducted by analyzing the MeOH-soluble portions by highperformance liquid chromatography (HPLC) on a Shimadzu LC-10A system under the following conditions: column: Cadenza CD C-18; flow rate: 0.7 mL/min; eluent: binary gradient MeOH/H2 O = 30/70 (0 min) → 45/55 (0 min → 5 min), 45/55 (5 min → 25 min) → 100/0 (25 min → 55 min), 100/0 (55 min → 70 min); detector: UV280 nm ; temperature: 40 ◦ C. The product yields and CA recoveries were determined from their peak areas compared with that of 1,2,3trimethoxybenzene, which was used as an internal standard. For the dihydroconiferyl alcohol, the UV absorption at 280 nm was relatively low, which could lead to a reduction in the accuracy of
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a
the quantification by HPLC analysis. With this in mind, the dihydroconiferyl alcohol yield was also determined based on the peak area of one of its 1 H NMR signals (ı 2.65 ppm, t, J = 7.4, C␣ -H). The identification and characterization of the products from 4-Omethylconiferyl alcohol were also conducted in a similar manor. Gel-permeation chromatography (GPC) was also used to obtain molecular weight (MW) distribution information for the MeOHsoluble products using a Shimadzu LC-10A system under the following conditions: column: Shodex KF-801 [exclusion limit: 1500 (polystyrene)]; flow rate: 0.6 mL/min; eluent: THF; detector: UV280 nm ; temperature: 40 ◦ C. Under some of the pyrolysis conditions, a black-colored MeOHinsoluble portion remained following the extraction process, which is considered as an intermediate for formation of coke. This fraction adhering to the reactor wall was dried in an oven at 105 ◦ C for 24 h, then closed from the air, and incinerated at 600 ◦ C for 2 h to give CO and CO2 , with both of these gases being quantified by Micro GC with a Varian CP-4900 under the following conditions: channel 1 – column: MS5A 10 m; carrier gas: argon; temperature: 100 ◦ C; pressure: 170 kPa; detector: thermal conductivity detector (TCD); channel 2 – column: PoraPLOT Q 10 m; carrier gas: helium; temperature: 80 ◦ C; pressure: 190 kPa; detector: TCD. The yield the MeOH-insoluble products was determined as a C-based % of the initial amount of coniferyl alcohol.
FUpper
FLower
2.4. Bond dissociation energy calculation The density functional theory (DFT) calculation was conducted under AM1 at the B3LYP/6-311+G** level with “Spartan’08” (Wavefunction Inc.) to obtain a bond dissociation energy (BDE). A zero-point energy correction was not made. 3. Results and discussion 3.1. Evaporation and degradation behavior of coniferyl alcohol Fig. 2 illustrates the results of the GPC analysis of the MeOHsoluble portions, i.e., the FUpper and FLower portions that were recovered from the upper and lower parts of the reactor wall, respectively. At 200 ◦ C, the signal corresponding to CA (12.8 min) remained relatively unchanged in the FLower from that obtained under the assumption of a 100% recovery, whereas the corresponding CA signal observed in the FUpper was only very small. As the pyrolysis temperature increased, the CA signals in the FLower became smaller and almost disappeared at temperatures of 300 and 350 ◦ C. These observations indicated that the evaporation/degradation of coniferyl alcohol started at temperatures around 200–250 ◦ C under the current pyrolysis conditions. For the evaporation of CA, the CA signal in the FUpper became larger following an increase in the pyrolysis temperature from 200 to 300 ◦ C, although these signals were much smaller than that of the CA obtained by assuming 100% evaporation. In contrast, some major signals were observed in the FLower together with some broad signals that occurred in the high MW region (shorter retention times). Thus, the volatilization of CA was not effective under the current pyrolysis conditions, with the CA being converted to the polymerization products as reported by Masuku [24]. The appearance of shoulders in the FUpper at slightly longer retention times (13.5–14.5 min) than that of CA indicated the formation of some monomeric products. Several monomeric guaiacols were identified as the low MW products from the CA, as illustrated in the HPLC chromatogram shown in the figure (350 ◦ C, Fig. 3). The side-chain structures of these products existed in different oxidation levels to that of CA. For example, the coniferyl aldehyde (3) represents an oxidation
8
10
12
14
16
Retention time (min) Fig. 2. Gel permeation chromatograms of the methanol-soluble portions obtained from the pyrolysis of coniferyl alcohol (CA) at various temperatures (N2 /5 min). a: retention time of CA; –·–·–·: 200 ◦ C; – – –: 250 ◦ C; ----: 300 ◦ C, —: 350 ◦ C; : chromatogram assuming that CA is completely recovered from the FLower or FUpper fractions without decomposition.
product, whereas dihydroconiferyl alcohol (4) and isoeugenol (5) represent reduction products. Based on these results, it was clear that some redox reactions were taking place during the pyrolysis of CA. The cis-isomer (2) of CA and 4-vinylguaiacol (6) with a C2 side-chain were also identified. Fig. 4 shows the influence of the pyrolysis temperature on the yield of the monomeric guaiacols in the FLower and the FUpper . The total yields (FLower + FUpper ) increased with increasing pyrolysis temperature for most of the products. These products tended to be recovered from the FLower at 200 and 250 ◦ C, although the recoveries from the FUpper become rather more important at higher temperatures. These observations could be explained by the formation and evaporation processes, in that these monomeric guaiacols formed in the liquid/solid phase and tended to be evaporated at higher temperatures. This hypothesis appeared to be reasonable, because the boiling points of these products were expected to be within a similar temperature range, including CA (332.2 ± 0.0 ◦ C [28]), dihydroconiferyl alcohol (339.8 ± 27 ◦ C [28]), isoeugenol (266 ◦ C), coniferyl aldehyde (338.8 ± 27 ◦ C [28]), and 4vinylguaiacol (245 ◦ C ± 20 ◦ C [28]). Fig. 5 provides a summary of the recovery levels of CA, as well as the yields of the monomeric guaiacols and the MeOH-insoluble products at each pyrolysis temperature. A small amount of the
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Fig. 3. HPLC chromatogram of the methanol-soluble portion (FUpper ) obtained by the pyrolysis of coniferyl alcohol (N2 /350 ◦ C/5 min) with identification of some signals.
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Coniferyl aldehyde (3 3)
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300
350
Pyrolysis temperature (°C) Fig. 4. Yields of monomeric guaiacols from the pyrolysis of coniferyl alcohol (CA) (N2 /200–350 ◦ C/5 min). : FLower; : FUpper ; : total.
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Fig. 5. Recovery of coniferyl alcohol (CA) and yields of the monomeric guaiacols and the MeOH-insoluble products from the FUpper and FLower fractions obtained after pyrolysis of CA at various temperatures (N2 /5 min).
: CA recovery (FLower );
:
CA recovery (FUpper ); : monomeric guaiacol yield (FUpper + FLower ); : yield of the MeOH-insoluble; : higher MW products and unknown; : total.
MeOH-insoluble products was obtained only at 350 ◦ C. Based on the proposal of Hosoya et al. [22], which states that the o-quinone methide formed through ipso substitution at temperatures greater than 400 ◦ C is an important precursor of lignin coke formation, we believe that the lower pyrolysis temperatures employed in this paper would not be high enough to facilitate coke formation.
The total values for the recovery and the yields of monomeric guaiacols decreased when the pyrolysis temperature was increased from 200 to 300 ◦ C. In contrast, however, they increased slightly when the temperature was increased to 350 ◦ C. For the CA recovery, the total recovery (FLower + FUpper ) was reduced significantly to around 15% when the pyrolysis temperature was increased to 300 and 350 ◦ C. Although the monomeric guaiacol yields also increased up to around 15 wt% at 350 ◦ C, the amounts were not so large. Consequently, coniferyl alcohol was found to polymerize prior to evaporation and the formation of a variety of different monomeric guaiacols. Given that the depolymerization and weight-loss processes of lignin occur predominantly at temperatures in the range of 300–400 ◦ C, these results suggested that the coniferyl alcohol yield would be low even if it was formed during the primary pyrolysis of G-type lignin because of secondary polymerization events. The influences of the pyrolysis temperature on the recoveries of coniferyl aldehyde (3), dihydroconiferyl alcohol (4), isoeugenol (5) and 4-vinylguaiacol (6) from the FLower and the FUpper are shown in Fig. 6. A comparison of the data presented in Figs. 5 and 6 provided some additional information regarding the role of the relative evaporation/degradation efficiency in the formation of the monomeric products. The total recoveries (FLower + FUpper ) of the compounds 3–6 were much higher than those of the CA. Given that the boiling point of CA (332.2 ± 0.0 ◦ C, [28]) is similar to those of coniferyl aldehyde (3) and dihydroconiferyl alcohol (4), it is possible that the lower recovery of CA could have originated from its higher degradation reactivity, especially for the condensation reaction, than those of compounds 3 and 4. Dihydroconiferyl alcohol, which does not contain a conjugated C C double bond, exhibited relatively high recovery values. The high recoveries encountered in this case were probably caused by the lower formation reactivity
Fig. 6. Recoveries of dihydroconiferyl alcohol, isoeugenol, coniferyl aldehyde and 4-vinylguaiacol from their heat treatment at various temperatures (N2 /5 min). FUpper;
: FLower ; : total.
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:
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of the quinone methide (an intermediate in the condensation). This will be discussed in greater detail later. 4-Vinylguaiacol (6) showed a different temperature dependency, in that the total recovery increased directly with increasing pyrolysis temperature. Thus, the relative evaporation/degradation efficiency would effectively encourage the evaporation of 4-vinylguaiacol at higher temperatures, based on the high condensation reactivity of compound 6 reported at 250 ◦ C and its relatively low boiling point (245 ◦ C ± 20 ◦ C [28]).
100
Recovery (wt%)
75
3.2. Reactivity of 4-O-methylconiferyl alcohol Nakamura et al. [26] obtained the two condensation products of CA with creosol (4-methylguaiacol) at 250 ◦ C in 12 and 8.3 mol% yields (based on CA), respectively. They went on to propose a quinone methide-based mechanism as the condensation mechanism, which included the nucleophilic addition of the aromatic ring carbon or hydroxyl oxygen to the C␣ - or C␥ -carbons of the quinone methide intermediates. The quinone methide mechanism has also been confirmed by the reduction in the polymerization reactivity of CA caused by 4-O-methylation. Based on these preliminary results, we studied the pyrolytic reactivity of 4-O-methtylconiferyl alcohol (4-O-Me CA) with a view to developing a better understanding of the molecular mechanisms of the condensation and side-chain conversion reactions. Methylation inhibits the quinone methide formation. Figs. 7 and 8 show the recoveries and the GPC results, respectively, obtained for the pyrolysis of 4-O-Me CA and a mixture of 4-O-Me CA and CA (1:1, w/w) at temperatures in the range of 250–350 ◦ C. Relatively high recoveries (60–90%) of 4-O-Me CA were obtained compared with those of CA, which indicated that the quinone methide mechanism plays an important role in the
50
25
0 250
300
350
Pyrolysis temperature (°C) Fig. 7. Recoveries (FUpper + FLower ) of coniferyl alcohol (CA) and 4-O-methylconiferyl alcohol (4-O-Me CA) from their individual pyrolysis reactions and the pyrolysis of their mixture at various temperatures (N2 /5 min). : CA; : 4-O-Me-CA; —: individual, -----: mixture (CA:4-O-Me-CA = 1:1, w/w).
condensation of CA. This was also confirmed by the GPC results, where only the signals corresponding to 4-O-Me CA were observed in the chromatograms (FLower ). Even 4-O-Me CA became reactive in the presence of CA. The recovery of 4-O-Me CA was dramatically reduced (Fig. 7) and the 4-O-Me CA signals almost disappeared in the FLower from the pyrolysis of the mixture (Fig. 8). The evaporation of 4-O-Me CA was also suppressed as indicated by the reduction of the signals in the FUpper . Thus, even the non-phenolic structure condensed with CA (phenolic structure). This would be
FLower
FUpper
4-O-Me CA
8
10
8
10
12
14
16
4-O-Me CA + CA (1 : 1, wt/wt)
12
14
16
8
10
12
14
16
Retention time (min) Fig. 8. Gel permeation chromatograms of the methanol-soluble portions obtained from the pyrolysis of 4-O-methyl coniferyl alcohol (4-O-Me CA) and its mixture with coniferyl alcohol (CA) (CA:4-O-Me-CA = 1:1, w/w) at various temperatures (N2 /5 min). – – –: 250 ◦ C; ----: 300 ◦ C; —: 350 ◦ C; a: retention time of CA; b: retention time of 4-O-Me CA.
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Fig. 9. Proposed condensation mechanisms of coniferyl alcohol and 4-O-methylconiferyl alcohol.
important for considering the pyrolysis of lignin, in that coniferyl alcohol-type structures (phenolic forms) would add to the repeating phenylpropane units (non-phenolic forms). Although further studies would be necessary to identify the condensation sites of 4-O-Me CA, the aromatic ring carbons or side-chain double bond could be involved in the condensation with the quinone methide intermediates A and B, as shown in Fig. 9. Interestingly, the side-chain conversion products were also obtained from the 4-O-Me CA in the yields summarized in Fig. 10, although their yields were slightly lower than those encountered with CA. Furthermore, their formation was enhanced in the presence of CA to levels similar to those of CA pyrolysis. These results suggested that the formation of these products involved some radical chain reactions, in that the radical species formed from the CA would activate the conversion of 4-O-Me CA. 4-O-Methylvinylguaiacol in the pyrolysis mixture could not be quantified because of the occurrence of overlapping peaks in the HPLC analysis. This problem could not be solved even by changing
the chromatographic conditions including the binary gradient conditions and flow rates. Consequently, the quinone methide and radical pathways were found to play more important roles in the condensation and sidechain conversions of CA, respectively. This information would be useful for the discussion of the molecular-based reaction mechanisms.
3.3. Molecular mechanisms for the pyrolytic conversion of coniferyl alcohol The mechanisms proposed for the formation of cis-coniferyl alcohol (2), coniferyl aldehyde (3), dihydroconiferyl alcohol (4), isoeugenol (5), and 4-vinylguaiacol (6) are illustrated in Figs. 11–14. These mechanisms include the quinone methide and radical chain pathways, although further studies would be necessary to confirm the mechanisms and completely understand their contributions.
Fig. 10. Yields (FUpper + FLower ) of monomeric 4-O-methyl guaiacols from the pyrolysis of 4-O-methylconiferyl alcohol (4-O-Me CA) and its mixture with coniferyl alcohol (CA) (CA: 4-O-Me-CA = 1:1, w/w) at various temperatures (N2 /5 min). : 4-O-methylvinylguaiacol
: 4-O-methyldihydroconiferyl alcohol;
: O-methylisoeugenol;
: 4-O-methylconiferyl aldehyde;
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Fig. 11. Proposed pyrolytic formation mechanisms of cis-coniferyl alcohol and coniferyl aldehyde from trans-coniferyl alcohol.
Some of the bond dissociation energies obtained from the DFT calculation have also be included for discussion. The isomerization from trans to cis CA is possible via both the quinone methide (Fig. 11A) and C␣ - and C␥ -radicals (Figs. 12 and 13). Higher trans/cis ratios were usually observed during pyrolysis, with the equilibrium tending toward the more stable trans isomer for steric reasons. The coniferyl aldehyde would be formed via the C␥ -radical intermediate (Fig. 11B). Two pathways were considered for the formation of this radical intermediate, involving either homolytic cleavage of the C␥ H bond or hydrogen abstraction from the C␥ H bond by some radical species. The relatively large calculated BDE (79.9 kcal/mol) suggest that the latter H-abstraction pathway is more probable at the pyrolysis temperatures used in the current study. The -scission type reaction from the C␥ -radical would provide a coniferyl aldehyde together with a hydrogen radical. This hydrogen radical could be used for addition to a double bond and H-donation to stabilize other radical species. Dihydroconiferyl alcohol is a hydrogenation product of CA. The direct addition of a hydrogen radical to the double bond of CA and the hydrogenation to the quinone methide intermediate are both
plausible reaction pathways (Fig. 12). As a similar mechanism to the latter pathway, the formation of o-cresol (2-methylphenol) from guaiacol has been considered to proceed via the hydrogenation of the o-quinone methide intermediate [19,20]. The 4-O-Me CA also gave a dihydroconiferyl alcohol type derivative, which indicated that the direct hydrogen radical addition pathway was involved in this transformation. The formation of a hydrogen radical would be necessary for both pathways. The formation of this type of compound could therefore be used as a probe for the hydrogen radical in the pyrolysis environment, together with isoeugenol, although this will be discussed in our following paper [29]. Both the quinone methide and radical mechanisms were considered for the formation of isoeugenol from CA (Fig. 13). The quinone methide intermediate formed by the elimination of the hydroxyl group from C␥ of the CA would be hydrogenated. In the radical mechanism, the elimination of the C␥ -hydroxyl group could proceed in one of two different ways, in that it could occur via a -scission type elimination of the OH radical from the C radical intermediate (pathway a) or via the direct homolysis of the C␥ OH bond (pathway b). The calculated BDE (72.2 kcal/mol) would, however, be too large for the homolysis pathway at a
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Fig. 12. Proposed pyrolytic formation mechanisms of dihydroconiferyl alcohol from coniferyl alcohol.
pyrolysis temperature less than 350 ◦ C. The C -radical pathway would therefore be more probable. In a manner analogous to the dihydroconiferyl alcohol formation mechanism, the formation of the isoeugenol type structure from 4-O-Me CA suggested that pathway was involved in this conversion process. Two sites were possible for the addition of hydrogen to the double bond of CA (Fig. 15). For pathway a (Fig. 13), the hydrogen radical would add to the C␣ position. Thermodynamically, the formation of the more stable C␣ -radical obtained by the addition of hydrogen to the C would be more favorable because of the resonance stabilization of the C␣ -radical with the aromatic ring, as shown in Fig. 15a. In contrast, the stereoelectronic features are known to be an important factor affecting the addition reaction of the radial to the double bond [30]. In this case, the reaction proceeds under kinetic control. Generally, the singly occupied orbital of the radical attacks the alkene at an angle close to 107◦ to maximize overlap with the alkene * orbital. Under such conditions, the less hindered C␣ would be preferred to the C , which would be sterically hindered to a greater extent by the C␥ hydroxyl group and hydrogens (Fig. 15 b). Further study would be necessary to identify the relative reactivities of the C␣ and C positions. 4-Vinylguaiacol would be formed via C␣ -radical (Fig. 12) and quinone methides (Fig. 14). The homolytic C C␥ bond cleavage of the quinone methides would occur, which are obtained from CA and coniferyl aldehyde (Fig. 14). The electron-withdrawing quinone methide moieties attaching to the C positions would
reduce the BDEs of the C C␥ bonds to 51.8 and 50.1 kcal/mol, which were calculated for the CA- and coniferyl aldehyde-derived quinone methides, respectively. 4-Vinylguaiacol was obtained also from the pyrolysis of coniferyl aldehyde [yield: 1.1 wt% (350 ◦ C), 0.4 wt% (300 ◦ C) and 0.1 wt% (250 ◦ C)]. Formation of this type of products from 4-O-Me CA suggested that the radical pathway was involved in the formation.
3.4. Implications for lignin pyrolysis The current results provided some insights into the pyrolytic formation of monomeric guaiacols during the primary pyrolysis of G-type lignins. The reduced stability of coniferyl alcohol would be attributed to the conjugated C C double bond and the C␥ OH type structures proposed for the formation of the quinone methides A and B in Fig. 9. Nakamura et al. [26] reported that 4-vinylguaiacol derivatives with conjugated C␣ C double bonds are much more labile than the corresponding C␣ OH derivative at 250 ◦ C. The results from this report suggest that the primary pyrolysis of lignin would provide products that were much less stable (such as coniferyl alcohol) than the original lignins containing the C␣ OH derivatives as the main structure. This was also supported by the observation that the temperature range (300–400 ◦ C) where the primary pyrolysis of the lignin occurred to form volatile products is much higher than the degradation temperature (250–300 ◦ C) of the coniferyl alcohol.
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Fig. 13. Proposed pyrolytic formation mechanisms of isoeugenol from coniferyl alcohol.
Similar discussions have been made for cellulose pyrolysis [31], where levoglucosan (1,6-anhydro--d-glucose), which is an important primary pyrolysis product, decomposes (polymerizes) at temperatures higher than 250 ◦ C, representing a temperature range that was much lower than the temperature range (300–350 ◦ C) where cellulose primary pyrolysis takes place. Although the reason for lignin pyrolysis currently remains unknown, the acid-catalysis mechanism [32] involving proton-donation through intermolecular hydrogen bonding has been proposed as a mechanism to explain the unique reactivity of levoglucosan, which is much more stable in the gas phase than the liquid phase [33]. Such interactions are possible only in the liquid (melt) phase, and not in the gas phase. Consequently, the pyrolysis temperature would be high enough to cause the secondary reaction of coniferyl alcohol, following its formation during the primary pyrolysis of lignin at 300–400 ◦ C. The expected boiling point of coniferyl alcohol (332.2 ± 0.0 ◦ C, [28])
suggested that the evaporation and secondary degradation process are competitive under such conditions, as summarized in Fig. 16. As indicated by Masuku’s paper [24] and the results of this study, the most important reaction in the process is the condensation reaction, followed by the side-chain conversion reactions. The relative efficiencies of the evaporation/polymerization/sidechain conversion processes would determine the yield of coniferyl alcohol. For the condensation of coniferyl alcohol, the quinone methide intermediate was suggested to be an important intermediate. Generally, the electro-positive carbons of quinone methides tend to react with the electro-negative aromatic and double bond carbons rather than the oxygen atoms of the side-chain and the phenolic groups. This reactivity can be explained by the “hard and soft acid and base” rule. Given that the condensed (C C) linkages have been reported to be much more stable than the ether linkages during the lignin pyrolysis process [19], re-depolymerization would not
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Fig. 14. Proposed pyrolytic formation mechanisms of 4-vinylguaiacol from coniferyl alcohol.
Fig. 15. The ␣/ regioselectivity of the hydrogen addition to the coniferyl alcohol C␣ C double bond.
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Coniferyl alcohol Other monomeric guaiacols Evaporation
Lignin
Coniferyl alcohol
Side-chain conversion
Monomeric guaiacols
Heating zone (Liquid /solid)
Condensation products
Char
More C-C linkage
Fig. 16. Role of evaporation/condensation/side-chain conversion processes in the formation of coniferyl alcohol and other monomeric guaiacols from lignin pyrolysis.
be effective for the condensation products. Together with the high condensation reactivity of the primary products, this would be a good reason why lignin pyrolysis would tend to form solid product (char) more preferentially. 4. Conclusions The following pyrolysis reactivities of trans-coniferyl alcohol were clarified as a lignin primary pyrolyzate under the pyrolysis conditions of 200–350 ◦ C/N2 /5 min, with particular focus on the evaporation/degradation processes. This information could be useful for understanding and controlling the pyrolysis-based biomass conversion processes. 1. The evaporation/degradation of coniferyl alcohol started at 200–250 ◦ C. 2. The polymerization was a more important process than the evaporation and side-chain conversion processes. 3. Coniferyl aldehyde (an oxidation product), and isoeugenol and dihydroconiferyl alcohol (reduction products) were obtained as side-chain-conversion products together with cis-coniferyl alcohol and 4-vinylguaiacol. 4. Coniferyl alcohol was much more reactive than most of the other side-chain conversion products. 5. 4-O-Methylation of the coniferyl alcohol inhibited the polymerization, but did not inhibit the side-chain conversion processes. 6. 4-O-Methyl coniferyl alcohol condensed in the presence of coniferyl alcohol. 7. A quinone methide mechanism was proposed for the polymerization of coniferyl alcohol. 8. Both quinone methide and radical mechanisms were proposed for the side-chain conversion processes. 9. Lignin pyrolysis was discussed at the molecular level using the present results. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research (B)(2) (No. 24380095, 2012.4-2016.3) and the Kyoto University Global COE program for “Energy Science in the Age of Global Warming”. References ´ V. Mihálov, V. Kováˇcik, Low temperature thermolysis of lignins. I. [1] R. Breˇzny, Reactions of -O-4 model compounds, Holzforschung 37 (1983) 199–204. [2] S.T. Autrey, M.S. Alnajjar, D.A. Nelson, J.A. Franz, Absolute rate constants for the beta-scission reaction of the 1-phenyl-2-phenoxypropyl radical—a model for radical reactions of lignin, Journal of Organic Chemistry 56 (1991) 2197–2202.
[3] P.F. Britt, A.C. Buchanan III, K.B. Thomas, S.-K. Lee, Pyrolysis mechanisms of lignin: surface-immobilized model compound investigation of acid-catalyzed and free-radical reaction pathways, Journal of Analytical and Applied Pyrolysis 33 (1995) 1–19. [4] H. Kawamoto, S. Horigoshi, S. Saka, Pyrolysis reactions of various lignin model dimers, Journal of Wood Science 53 (2007) 168–174. [5] T. Nakamura, H. Kawamoto, S. Saka, Pyrolysis behavior of Japanese cedar wood lignin studied with various model dimers, Journal of Analytical and Applied Pyrolysis 81 (2008) 173–182. [6] H. Kawamoto, M. Ryoritani, S. Saka, Different pyrolytic cleavage mechanisms of -ether bond depending on the side-chain structure of lignin dimers, Journal of Analytical and Applied Pyrolysis 81 (2008) 88–94. [7] T. Watanabe, H. Kawamoto, S. Saka, Radical chain reactions in pyrolytic cleavage of the ether linkages of lignin model dimers and a trimer, Holzforschung 63 (2009) 424–430. [8] O. Faix, E. Jakab, F. Till, T. Székely, Study on low mass thermal-degradation products of milled wood lignins by thermogravimetry-mass-spectrometry, Wood Science and Technology 22 (1988) 323–334. [9] D.J. Gardner, T.P. Schultz, G.D. McGinnis, The pyrolytic behavior of selected lignin preparations, Journal of Wood Chemistry and Technology 5 (1985) 85–110. [10] E. Jakab, O. Faix, F. Till, Thermal decomposition of milled wood lignins studied by thermogravimetry/mass spectrometry, Journal of Analytical and Applied Pyrolysis 40–41 (1997) 171–186. [11] M. Asmadi, H. Kawamoto, S. Saka, Gas- and solid/liquid-phase reactions during pyrolysis of softwood and hardwood lignins, Journal of Analytical and Applied Pyrolysis 92 (2011) 417–425. [12] H. Kawamoto, T. Nakamura, S. Saka, Pyrolytic cleavage mechanisms of lignin–ether linkages: a study on p-substituted dimers and trimers, Holzforschung 62 (2008) 50–56. [13] R.J. Evans, T.A. Milne, Molecular characterization of the pyrolysis of biomass. 1. Fundamentals, Energy and Fuels 1 (1987) 123–137. [14] W. Genuit, J.J. Boon, O. Faix, Characterization of beech milled wood lignin by pyrolysis-gas chromatography–photoionization mass spectrometry, Analytical Chemistry 59 (1987) 508–513. [15] J. Ralph, R.D. Hatfield, Pyrolysis-GC–MS characterization of forage materials, Journal of Agricultural and Food Chemistry 39 (1991) 1426–1437. [16] K. Sipilä, E. Kuoppala, L. Fagernäs, A. Oasmaa, Characterization of biomassbased flash pyrolysis oils, Biomass and Bioenergy 14 (1998) 103–113. [17] B. Scholze, D. Meier, Characterization of the water-insoluble fraction from pyrolysis oil (pyrolytic lignin). Part I. PY-GC–MS, FTIR, and functional groups, Journal of Analytical and Applied Pyrolysis 60 (2001) 41–54. [18] M.E. Arias, O. Polvillo, J. Rodríguez, M. Hernández, J.A. González-Pérez, F.J. González-Vila, Thermal transformations of pine wood components under pyrolysis/gas chromatography/mass spectrometry conditions, Journal of Analytical and Applied Pyrolysis 77 (2006) 63–67. [19] A. Vuori, Pyrolysis studies of some simple coal related aromatic methyl ethers, Fuel 65 (1986) 1575–1583. [20] E. Dorrestijn, P. Mulder, The radical-induced decomposition of 2methoxyphenol, Journal of the Chemical Society, Perkin Transactions 2 (1999) 777–780. [21] T. Hosoya, H. Kawamoto, S. Saka, Secondary reactions of lignin-derived primary tar components, Journal of Analytical and Applied Pyrolysis 83 (2008) 78–87. [22] T. Hosoya, H. Kawamoto, S. Saka, Role of methoxyl group in char formation from lignin-related compounds, Journal of Analytical and Applied Pyrolysis 84 (2009) 79–83. [23] M. Asmadi, H. Kawamoto, S. Saka, Thermal reactions of guaiacol and syringol as lignin model aromatic nuclei, Journal of Analytical and Applied Pyrolysis 92 (2011) 88–98. [24] C.P. Masuku, Thermolytic decomposition of coniferyl alcohol, Journal of Analytical and Applied Pyrolysis 23 (1992) 195–208. [25] A.E. Marman-Ware, M. Crocker, A.P. Kaur, M.S. Meier, D. Kato, B. Lynn, PyrolysisGC/MS of sinapyl and coniferyl alcohol, Journal of Analytical and Applied Pyrolysis 99 (2013) 161–169. [26] T. Nakamura, H. Kawamoto, S. Saka, Condensation reactions of some lignin related compounds at relatively low pyrolysis temperature, Journal of Wood Chemistry and Technology 27 (2007) 121–133. [27] N. Shimada, H. Kawamoto, S. Saka, Different action of alkali/alkaline earth metal chlorides on cellulose pyrolysis, Journal of Analytical and Applied Pyrolysis 81 (2008) 80–87. [28] pKa; Calculated using Advanced Chemistry Development (ACD/Labs) Software V11.02, Scifinder, Chemical Abstracts Service: Columbus, OH, 1994–2013, https://scifinder.cas.org (accessed 11.03.13). [29] T. Kotake, H. Kawamoto, S. Saka, unpublished. [30] A.F. Parsons, An Introduction to Free Radical Chemistry, Blackwell Science, Oxford, 2000, pp. 87–90. [31] T. Hosoya, H. Kawamoto, S. Saka, Pyrolysis behavior of wood and its constituent polymers at gasification temperature, Journal of Analytical and Applied Pyrolysis 78 (2007) 328–336. [32] H. Kawamoto, Y. Ueno, S. Saka, Thermal reactivities of non-reducing sugars in polyether—role of intermolecular hydrogen bonding in pyrolysis, Journal of Analytical and Applied Pyrolysis (2012), http://dx.doi.org/10.1016/j.jaap. 2012.08.009 (in press). [33] T. Hosoya, H. Kawamoto, S. Saka, Different pyrolytic pathways of levoglucosan in vapor- and liquid/solid-phases, Journal of Analytical and Applied Pyrolysis 83 (2008) 64–70.