Selective production of bio-based aromatics by aerobic oxidation of native soft wood lignin in tetrabutylammonium hydroxide

Article abstract of DOI:10.1039/d0ra03420g

Aerobic oxidation of native soft wood lignin in an aqueous solution of Bu₄NOH facilitates efficient production of vanillin (4-hydroxy-3-methoxybenzaldehyde), which is one of the platform chemicals in industry. Oxidation of Japanese cedar (Cryptomeria japonica) wood flour at 120 °C for 4 h under O₂in Bu₄NOH-based aqueous solutions produced vanillin in 23.2 wt% yield based on the Klason lignin content of the starting material. This yield was comparable to that in alkaline nitrobenzene oxidation of the same material (27.2%), which indicated that our aerobic oxidation exploited the full potential of the wood flour for vanillin production. Further mechanical investigation with lignin model compounds suggested that the vanillin formation occurred mainly through following successive reactions: alkaline-catalyzed degradation of ?-ether linkages in middle units of lignin polymer to form a glycerol end group, oxidation of the glycerol end group by O₂to a HC_a?O moiety, and release of vanillin from the HC_a?O end. One of the reasons for the high performance of Bu₄NOH for the vanillin production was explained by the general understanding in organic chemistry that Bu₄OH is a stronger base than simple alkali,e.g.NaOH. The other more fundamental mechanical aspect was that Bu₄N⁺suppressed disproportionation of the vanillin precursor (the C_aHO end group) probably due to strong interaction between the cation and the HC_a?O end group.

Full text of DOI:10.1039/d0ra03420g

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Selective production of bio-based aromatics by
aerobic oxidation of native soft wood lignin in
tetrabutylammonium hydroxide†
Cite this: RSC Adv., 2020, 10, 19199
a
a
a
Kohei Yamamoto, Hisashi Miyafuji and Tatsuhiko Yamadab
*
Takashi Hosoya,
Aerobic oxidation of native soft wood lignin in an aqueous solution of Bu4NOH facilitates efficient
production of vanillin (4-hydroxy-3-methoxybenzaldehyde), which is one of the platform chemicals in
industry. Oxidation of Japanese cedar (Cryptomeria japonica) wood flour at 120 ꢀC for 4 h under O2 in
Bu4NOH-based aqueous solutions produced vanillin in 23.2 wt% yield based on the Klason lignin content
of the starting material. This yield was comparable to that in alkaline nitrobenzene oxidation of the same
material (27.2%), which indicated that our aerobic oxidation exploited the full potential of the wood flour
for vanillin production. Further mechanical investigation with lignin model compounds suggested that
the vanillin formation occurred mainly through following successive reactions: alkaline-catalyzed
degradation of b-ether linkages in middle units of lignin polymer to form a glycerol end group, oxidation
of the glycerol end group by O2 to a HCa]O moiety, and release of vanillin from the HCa]O end. One
of the reasons for the high performance of Bu4NOH for the vanillin production was explained by the
general understanding in organic chemistry that Bu4OH is a stronger base than simple alkali, e.g. NaOH.
The other more fundamental mechanical aspect was that Bu4N+ suppressed disproportionation of the
vanillin precursor (the CaHO end group) probably due to strong interaction between the cation and the
HCa]O end group.
Received 17th April 2020
Accepted 11th May 2020
DOI: 10.1039/d0ra03420g
rsc.li/rsc-advances
biomass-based vanillin production process. Due to this draw-
back, recent vanillin production is almost completely
Introduction
substituted with phenol-based ones, where phenol is manu-
factured from fossil resources. Considering recent environ-
mental requirements, however, the biomass-based vanillin
production is worth being re-investigated and re-developed.
Our previous studies have reported that selective vanillin
production can be made by aerobic oxidation of several so
wood lignin samples, e.g. so wood our, milled wood lignin,
and several types of technical lignins including lignosulfonate,
at 120 ꢀC in the presence of tetrabutylammonium ion Bu4N+
and OHꢂ.5,8 When Japanese cedar (Cryptomeria japoniꢀca) wood
our is degraded in 1.25 mol Lꢂ1 Bu4NOH aq. at 120 C under
air, vanillin and vanillic acid are produced in 15.4 and 3.9%,
respectively, based on the Klason lignin content of the wood.8
These yields are considerably higher than those obtained in
NaOH aq. with the same OHꢂ concertation,5,8 indicating that
the Bu4NOH solution exhibits much better performance for
vanillin production than the corresponding NaOH aq.
Vanillin (3-methoxy-4-hydroxybenzaldehyde) is a major target
compound to be produced from so wood lignin. Quite a few
lignin depolymerization methods such as degradation with
various oxidants,1–14 catalytic degradation,6,12–15 and pyrolytic
processes16 make it clear that vanillin is one of the major
minima in the potential energy map in those lignin degradation
reactions. Also, vanillin is a platform compound in the chemical
industry, and is a source of several medicines, polymer mate-
rials, etc.17,18
Until the 1990s industrial vanillin production was based
mainly on aerobic oxidation of lignosulfonate,
a major
component of waste water from sulte pulping.3,6,19–21 This
vanillin production was carried out in alkaline media under
compressed air with the yield of vanillin being ꢁ5% based on
the original lignosulfonate.3,6,7 However, management of waste
water containing sulfur compounds is a major problem of this
The Bu4NOH-based oxidation method has several advan-
tages over the process with lignosulfonate: this method employs
reaction temperature (120 ꢀC) signicantly lower than that of
the lignosulfonate process (ꢁ170 ꢀC);3 the process can utilize
non-sulfur containing low materials such as wood our and
soda-lignin as well as lignosulfonate. Also, the use of molecular
oxygen as an ideal oxidation reagent in this process should be
aGraduate School of Life and Environmental Sciences, Kyoto Prefectural University, 1-5
Shimogamo-hangi-cho, Sakyo-ku, Kyoto 606-8522, Japan. E-mail: miyafuji@kpu.ac.jp
bForestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaragi
305-8687, Japan
† Electronic supplementary information (ESI) available: Total ion chromatogram
in GC/MS analysis of the acetylated reaction mixture obtained aer the
degradation of LM1. Values in Fig. 1, 3, and 5. See DOI: 10.1039/d0ra03420g
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highly emphasized.4b On the other hand, there are several issues with the corresponding gas and immediately sealed. The tube
to be further managed: considerably long reaction times (up to was then heated at 120 ꢀC in an oil bath with stirring. At
72 h) and unclear mechanical aspects of the reaction involved in a certain reaction time, the tube was cooled with cold water and
the aerobic oxidation, especially the roles of Bu4N+ in overall then
1,5-dihydroxy-1,2,3,4-tetrahydronaphthalene/ethanol
reactions. It is also noted that the highest yield of the target solution (2.0 g Lꢂ1, 800 mL) was added as an internal stan-
compound obtained with the Bu4NOH-based method is still dard. A sample (50 mL) of the resulting reaction mixture was
lower than those obtained in the alkaline nitrobenzene (AN) taken and added to acetonitrile (450 mL) containing 1.5% acetic
oxidation,8 which is used as a benchmark to evaluate the acid. The solution was ltered and used directly for high-
performance of lignin oxidation processes.
performance liquid chromatography (HPLC) analysis. The
The rst half of this article handles the above issues of conditions for the HPLC analysis were described in our previous
reaction time and the yields of vanillin and vanillic acid. We will studies.5,8
report the reaction time required to reach the maximum
Gas chromatography/mass spectrometric (GC/MS) analysis
vanillin and vanillic acid yields is signicantly shortened along was also carried out for the reaction mixture. Prior to the GC/MS
with improved yields of the compounds, when the reaction was analysis, the pH of the reaction solution (1.0 mL) aer the
carried out with the introduction of pure O2 to the reaction degradation experiment was adjusted to ꢁ5 by the addition of
system. In the second ha of this article, we will report results saturated citric acid aq. The solution was then extracted with
obtained with lignin model compounds (see Scheme 1 for their ethyl acetate (2.0 mL) and the organic layer was dried with
structures) and discuss the reasons for the high performance of anhydrous Na2SO4 and evaporated under reduced pressure. The
Bu4NOH, including the roles of Bu4+ in the oxidation process.
residue was acetylated in 1.0 mL of acetic anhydride/pyridine (1/
1, v/v) and directly used for the GC/MS analysis. The GC/MS
analysis was carried out with Shimadzu GCMS-QP2010 ultra
apparatus under the following conditions: column, Agilent J&W
Experimental
Materials
ꢀ
DB-5ms; injector temperature, 250 C; prole of column oven
temperature, 60 ꢀC (1 min), 60 / 300 ꢀC (1 / 48 min), 300 ꢀC
(10 min); carrier gas, helium; ow rate, 1.5 mL minꢂ1; emission
current, 20 mA; ionization time, 2.0 ms.
1.5 mol Lꢂ1 Bu4NOH aq. was purchased from the Sigma-Aldrich
Co. The solution was diluted to 1.25 mol Lꢂ1 with distilled water
and used as reaction solution. Vanillin, vanillic acid, vera-
traldehyde, veratric acid, veratryl alcohol, and sodium
hydroxide were purchased from FUJIFILM Wako Pure Chemical
Co. The particle size of Japanese cedar wood our was 90–180
mm. Lignin model compounds, veratryl glycerol-b-guaiacyl ether
(LM1) and guaiacyl glycerol-b-guaiacyl ether (LM2), were
purchased from Tokyo-Kasei Co. and used without purication
(see Scheme 1 for the structures of LM1 and LM2).
Identication of enol ethers, 2-methoxy-4-[2-(2-
methoxyphenoxy)vinyl]phenol
Model compound LM2 (20 mg) was degraded in 1.25 mol Lꢂ1
Bu4NOH aq. (2.0 mL) at 120 ꢀC for 1 h according to similar
manner described above. Aer cooling the tube with cold water,
saturated citric acid aq. was added to the reaction solution to
adjust the pH to ꢁ5. The reaction mixture was then extracted
with ethyl acetate (6 mL) and the organic layer was dried with
anhydrous Na2SO4. Evaporation of ethyl acetate under reduced
pressure gave crude target compounds. The products were
puried with silica gel column with the eluent being ethyl
acetate/n-hexane (1/1, v/v) to obtain cis/trans-2-methoxy-4-[2-(2-
methoxyphenoxy)vinyl]phenol (EEs, 8.0 mg). trans-2-Methoxy-
4-[2-(2-methoxyphenoxy)vinyl]phenol: 1H NMR (400 MHz,
1.25 mol Lꢂ1 NaOD/D2O): d (ppm) ¼ 3.50 (s, 3H), 3.62 (s, 3H),
6.00 (d, J ¼ 11.8 Hz, 1H), 6.33 (d, J ¼ 9.5 Hz, 1H), 6.54 (dd, J1 ¼
8.0 Hz, J2 ¼ 2.1 Hz, 1H), 6.65 (d, J ¼ 1.5 Hz, 1H), 6.78 (dt, J1 ¼
7.1 Hz, J2 ¼ 2.1 Hz, 1H), 6.83 (d, J ¼ 12.0, 1H), 6.90 (m, 3H). cis-2-
Methoxy-4-[2-(2-methoxyphenoxy)vinyl]phenol: 1H NMR (400
MHz, 1.25 mol Lꢂ1 NaOD/D2O): d (ppm) ¼ 3.48 (s, 3H), 3.64 (s,
3H), 5.47 (d, J ¼ 6.8 Hz, 1H), 6.22 (d, J ¼ 6.8 Hz, 1H), 6.34 (d, J ¼
8.3 Hz, 1H), 7.16 (d, J ¼ 1.5 Hz, 1H), the other protons were not
discernable due to their overlapping with the signals from the
trans-isomer.
Degradation of samples and analysis of reaction mixture
Staring material (the wood our or the lignin model compound,
3.0–20 mg) was placed in a 10 mL glass tube. Aer adding the
reaction solution (1.25 mol Lꢂ1 Bu4NOH aq. or 1.25 mol Lꢂ1
NaOH aq., 1.0–2.0 mL), the tube was tightly sealed. In several
experiments, the concentration of OHꢂ of the Bu4NOH aq. was
increased to 3.75 mol Lꢂ1 by the addition of solid NaOH (200 mg
in the case of 2.0 mL of the reaction solution). In the experi-
ments that were performed under O2 or N2, the tube was ushed
The HPLC analysis was performed for isolated EEs under the
analytical conditions identical to those presented above to the
same internal standard being employed. The obtained calibra-
tion curves were used for quantication of EEs formed by the
degradation of LM2.
Scheme 1 b-O-4 type dimeric lignin model compounds employed in
this study.
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thus stated that O2 introduction made the reaction more
vanillin selective. One of the possible reasons for this selectivity
change is that vanillic acid is more reactive toward O2 than
vanillin and readily release CO2. As shown in Fig. 1A, the
maximum yield of vanillic acid was achieved at 4 h in the
Bu4NOH aq. under O2 and then the yield gradually decreased.
Note that increase in the yield of vanillic acid was not observed
during the degradation of vanillin from 8 to 20 h, which sug-
gested oxidation of vanillin to vanillic acid was not important
under the employed conditions.
Results and discussion
Vanillin production from wood our with introduction of O2
Our previous study reported that aerobic oxidation of the wood
our (14 mg) in 1.25 mol Lꢂ1 Bu4NOH aq. (2.0 mL) under air for
43 h produced vanillin and vanillic acid in 15.4 and 3.9 wt%
yields, respectively, based on the Klason lignin amount of the
wood our.8 The 1.25 mol Lꢂ1 aqueous solution of Bu4NOH is
the molten form of the Bu4NOH$30H2O crystal. To further
enhance the degradation, we carried out similar degradation
experiments aer ushing the reaction system (sealed 10 mL
test tube) with pure O2. We will hereaer call this “degradation
under O2” for convenience, although the atmosphere of the
reaction system is not fully substituted with O2. Note that
1.25 mol Lꢂ1 is the best concentration of Bu4N+ in terms of
vanillin production according to our previous study:5 increase
in [Bu4N+] to more than 1.25 mol Lꢂ1 scarcely boosts the vanillin
yield and also results in degradation of Bu4N+ probably into
BuOH and Bu3N.
Fig. 1A shows changes in the yield of vanillin during the
degradation in the Bu4NOH aq. under O2 along with our
previous results obtained under air in the same reaction
medium (Fig. 1C). It becomes clear that the vanillin formation is
signicantly faster under O2: the time required for reaching the
maximum vanillin yield was 8 h under O2 whereas the reaction
under air needed 43 h. The maximum yield of vanillin was also
increased to 19.8 wt% (8 h) from 15.4 wt% (43 h) by the O2
substitution. Aer reaching the maximum yield in 8 h, the yield
of vanillin was decreased to 17.0 wt% at 20 h and this degra-
dation of vanillin was more signicant under O2 than under air.
This indicates that vanillin is somewhat reactive toward O2 and
reaction time should be carefully controlled when the vanillin
production is carried out under O2 instead of air.
We then carried out the degradation of the wood our (14.0
mg) in 1.25 mol Lꢂ1 Bu4NOH aq. (2.0 mL) under O2 with the
addition of NaOH(s) (200 mg) to increase the OHꢂ concentra-
tion of the reaction solution to 3.75 mol Lꢂ1, as vanillin
formation is signicantly enhanced in stronger alkaline media.5
In this alkaline-enhanced solution, as shown in Fig. 1B, the
reaction time required for the maximum yield of vanillin was
further shortened to 4 h. The yield of vanillin was also improved
to 23.2 wt% by the addition of NaOH. We thus tried further
addition of NaOH to the solution, but when the concentration
of OHꢂ was set at 5.0 mol Lꢂ1 by the addition of 300 mg of
NaOH, signicant decrease in the vanillin yield was observed.
Table 1 summarizes the maximum yield of vanillin obtained
from the wood our in the above Bu4NOH-based reaction
solutions along with that obtained in alkaline nitrobenzene
(AN) oxidation of the same wood our. AN oxidation is one of
the most selective methods to convert various lignin samples to
the corresponding monomeric benzaldehydes (vanillin in the
case of so wood lignins) and used as a benchmark to measure
the performance of lignin conversion methods. The maximum
vanillin yield (23.0 wt%) and the shortest reaction time (4 h)
were both achieved when the reaction was carried out under O2
in the Bu4NOH aq. with the addition of NaOH. Considering that
our previous study about the reaction under air required 43 h
reaction time to achieve much lower maximum vanillin yield
(15.4 wt%), the O2 introduction investigated in this study had
drastic effects for improving the performance of our vanillin
production method. Also, the 23.0 wt% vanillin yield obtained
Fig. 1 also shows the yield of vanillic acid (3-methoxy-4-
hydroxybenzoic acid) along with that of vanillin, as vanillic
acid is the second major product, as reported previously.5,8 The
reaction under O2 tends to form more vanillin than vanillic acid,
when it is compared to the one under air (Fig. 1A and C). It is
Fig. 1 Changes with time in yields of vanillin (C), vanillic acid (-), and their total yields (O) during degradation of Japanese cedar wood flour (14
mg) in 1.25 mol Lꢂ1 Bu4NOH aq. (2.0 mL) under O2 (A), under O2 with the addition of NaOH(s) (200 mg) (B), and under air8 (C) at 120 ꢀC. The total
amount of Klason lignin and acid-soluble lignin of the wood flour was 34.3 wt%.
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Table 1 Yields (wt%, based on the Klason lignin amount) of vanillin and vanillic acid from the Japanese cedar wood flour (14.0 mg) degraded in
the Bu4NOH aq.-based reaction media (2.0 mL) at 120 ꢀC and their yields from alkaline nitrobenzene (AN) oxidation
Yield (wt%)
Medium
Atmosphere
Reaction timea (h)
Vanillin
Vanillic acid
Total
Bu4NOH aq. + NaOH(s)b
Bu4NOH aq.8c
O2
Air
Air
4.0
43
2.5
23.2
15.4
27.2
1.2
3.9
1.2
24.4
19.3
28.4
AN oxidation8
a
The reaction times that result in the maximum vanillin yield are selected for each column. For AN oxidation, the reaction time was 2.5 h according
to the standard procedure. b Reaction solution was prepared byꢂt1he addition of NaOH(s) (200 mg) to 1.25 mol Lꢂ1 Bu4NOH aq. (2.0 mL). c These are
the results presented in our previous study,8 where 1.25 mol L Bu4NOH aq. (2.0 mL) was used without the NaOH addition.
in the NaOH-added Bu4NOH aq. under O2 was comparable to probably due to its very high polarity, but the presence of
those in AN oxidation (Table 1). This also indicates that our signicant amount of the compound in the reaction mixture
Bu4NOH-based reaction system well exploits vanillin forming was strongly suggested from the GC/MS analysis of the acety-
potential of the wood our.
lated reaction mixture (see Fig. S1 in ESI†). The yields of the A-
ring-derived compounds were gradually increased at prolonged
reaction times (Fig. 3A). Veratraldehyde was predominant in the
A-ring derived products until 6 h and vanillin then became the
major product with almost full consumption of LM1. At 43 h
vanillin and veratraldehyde were nally formed in 15.7 mol%
and 2.2 mol% yields, respectively, with the LM1 recovery being
3.7%. Note that the required 43 h reaction time is in good
agreement with the experimental fact that vanillin yield became
almost constant aer 43 h in the degradation of the wood our
in the Bu4NOH aq. under air (see Fig. 1C).8
The above results suggest that, as presented in Scheme 2,
veratryl glycerol formed by the b-ether cleavage is gradually
oxidized into veratraldehyde by O2 via pathway B. However, our
attempts to detect any intermediates in this aerobic oxidation
step were not successful. Veratraldehyde then undergoes
a demethylation reaction on the 4-OMe group to nally form
vanillin via pathway C. The detailed mechanism of this deme-
thylation is not clear at the moment. One of the possibilities is
a simple SN2 reaction on the methyl group by OHꢂ, but
a mechanism involving a nucleophilic addition of OHꢂ on the 4-
position of the benzene ring followed by the elimination of
MeOꢂ cannot be ruled out due to the presence of a strong
electron-withdrawing CHO group at the 1-position. It is also
noted that the demethylation of the 3-OMe group is disfavored
because of the lack of the inuence of the CHO group at the 3-
position.
Fig. 2 and 3 indicate the reaction mixture also contains
considerable amount of veratric acid and veratryl alcohol. It is
likely that these two compounds are produced through
disproportionation of veratraldehyde via Cannizzaro reaction
shown in pathway D in Scheme 2. This consideration is sup-
ported by the fact that the amount of these three compounds
increased with the gradual decrease in the amount of vera-
traldehyde (Fig. 3A). There also might be another pathway
where LM1 directly gave veratraldehyde, pathway E in Scheme 2,
as suggested from the result that the sum of the guaiacol yield
and the LM1 recovery was less than 100% at 43 h reaction time.
However, this pathway must be minor compared to the
Mechanical investigation with lignin model compounds
We have so far shown that vanillin is effectively produced by
aerobic oxidation of the wood our in the Bu4NOH-based
solutions. Since such effective vanillin production cannot be
achieved in a simple alkali solution (e.g. NaOH aq.) with the
same OHꢂ concentration as that of the Bu4N+-based one,5,8 it is
clear that chemical nature specic for Bu4NOH plays pivotal
roles in the vanillin formation. To investigate the mechanisms
underlying the vanillin production in the Bu4NOH solution,
degradation of several types of lignin model compounds
(mainly LM1 and LM2, see Scheme 1) was carried out in
1.25 mol Lꢂ1 Bu4NOH and NaOH solutions and the reaction
behaviors were carefully compared. In these model experi-
ments, we carried out the degradation under air (not under O2)
to minimize degradation of the products.
Fig. 2 presents a HPLC chromatogram of the reaction
mixture obtained from the degradation of LM1 (20 mg) in the
Bu4NOH aq. (2.0 mL) under air, where b-O-4 is the most abun-
dant linkage type in native lignin. LM1 formed vanillin along
with veratraldehyde (3,4-dimethoxybenzaldehyde), veratryl
alcohol (3,4-dimethoxybenzyl alcohol), veratric acid (3,4-dime-
thoxybenzoic acid), and guaiacol. The compounds with the
veratryl moiety and vanillin are most likely to be formed from
the A ring of LM1 and guaiacol is derived from the B ring (see
Scheme 1 for the denition of the A- and B-rings).
As shown in Fig. 3A, degradation of LM1 for short reaction
times gave guaiacol and small amount of the A-ring-derived
compounds. At the 1 h reaction time (the second plot from
the le), for instance, guaiacol was formed in 12.3 mol% yield
and the A-ring-derived compounds, mainly veratraldehyde,
were produced in 7.2 mol% yield, with 86.4% of LM2 remaining
intact. This suggests that the initial step of the degradation is
alkaline-catalyzed b-ether cleavage shown as pathway A in
Scheme 2, as reported in many studies.22–25 This b-ether cleavage
results in the formation of veratryl glycerol from the A-ring
along with the releasement of guaiacol from the B-ring.
Veratryl glycerol was not detected in the HPLC analysis
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Fig. 2 HPLC chromatogram of the reaction mixture obtained after the degradation of LM1 (20 mg) in 1.25 mol Lꢂ1 Bu4NOH aq. (2.0 mL) under air
for 4 h.
veratraldehyde formation via the pathways A and B, since the compounds was 33.7 mol% (35.1 mol% based on degraded
guaiacol yield is quantitative at least at the initial stage of the LM1) and the rest (64.9 mol%) undetected products probably
reaction, as discussed above. It is also noted that guaiacyl involve considerable amount of veratryl glycerol (see above
moieties including guaiacol degrade in the presence of O2 (ref. discussion and Fig. S1 in ESI†).
26–30) and this is probably one of the reasons for the small yield
The degradation of LM1 in the NaOH aq., on the other hand,
of the compound at the prolonged reaction times.
formed relatively small amounts of the four A-ring-derived
We then performed the degradation of LM1 in 1.25 mol Lꢂ1 products (the total yield: 14.9 mol% aer 43 h degradation),
NaOH aq., a simple alkaline solution with the same OHꢂ as summarized in Table 2. This is because complete degrada-
concentration as that of the Bu4NOH aq. As shown in Fig. 3B, tion of LM1 was not achieved in the NaOH aq. even aer 43 h
the degradation velocity of LM1 was lower in the NaOH aq. than degradation (see Fig. 3B). The total yield in the NaOH aq.
in the Bu4NOH aq. The maximum yield of vanillin in the NaOH eventually became comparable to that in Bu4NOH when it was
aq. (0.8 mol%, 6 h) was also much lower in the NaOH solution. calculated based on the degraded LM1: 35.1 mol% in the
One of the plausible reasons for this is that the activity of OHꢂ is Bu4NOH aq. and 34.7 mol% in the NaOH aq. It is therefore
greater in the Bu4NOH aq. than in the NaOH aq. even when likely that reactivity of veratryl glycerol toward O2 to form
their OHꢂ concentrations are the same. In other words, OHꢂ in veratraldehyde – in other words the velocity of the reactions
the Bu4NOH aq. is less inuenced by the counter cation Bu4N+, involved in the pathway B – is almost constant no matter
which renders the Bu4NOH solution more basic over the NaOH whether the reaction was carried out in the Bu4NOH aq. or the
aq. The fast degradation of LM1 will result in smooth formation NaOH aq. However, as shown in Table 2, the total yield of the
of the veratryl glycerol, which is the major origin of vanillin aldehyde products in NaOH aq. based on degraded LM1
(Scheme 2).
(13.0 mol%) was smaller than that in Bu4NOH (18.6 mol%) with
There is another mechanical aspect that accounts for the the total yields of the non-aldehyde products in NaOH aq.
better performance of Bu4NOH for vanillin production. Table 2 (21.7 mol%) being greater than that in Bu4NOH (16.5 mol%).
summarizes the yields of the major products quantied aer These different yields of the aldehyde and the non-aldehyde
the degradation of LM1 for 43 h. In the Bu4NOH aq. LM1 products suggest that the pathway C is more favored over the
formed the aldehyde products (veratraldehyde and vanillin) in pathway D in the Bu4NOH aq., which results in the increased
17.9 mol% (18.6 mol%) yield, where the yield in the parenthesis yield of vanillin.
is the one based on degraded LM1. The other non-aldehyde A-
To check this idea, we degraded veratraldehyde (5.0 mg) at
ring derived products (veratric acid and veratryl alcohol) were 120 ꢀC for 4 h under N2 either in the Bu4NOH aq. or the NaOH
formed in 15.8 mol% yield (16.5 mol%) in total, which was aq. (1.0 mL) and compared the product distribution between
similar to that of the aldehyde products. These results suggest the degradations in the two reaction media. As summarized in
that the reactions via the pathways C and D almost evenly occur Table 3, the degradation of veratraldehyde in Bu4NOH resulted
in the Bu4NOH aq. It is also noted that total yield of the four in the formation of vanillin, veratryl alcohol, and veratric acid in
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Fig. 3 Changes with time in recovery of LM1 ( ), yields of guaiacol ( ), veratraldehyde ( ), vanillin ( ), veratric acid ( ), veratryl alcohol ( ),
and the total yield of the A-ring-derived products ( ) during degradation of LM1 (20 mg) at 120 ꢀC under air in 1.25 mol Lꢂ1 Bu4NOH aq. (2.0 mL)
(A) and in 1.25 mol Lꢂ1 NaOH aq. (2.0 mL) (B).
Scheme 2 Reaction pathways from LM1 to major products detected in this study.
very similar yields. On the other hand, the degradation in NaOH Bu4NCl-added NaOH aq., where the 1.25 mol Lꢂ1 Bu4NOH aq.
aq. gave much smaller amount of vanillin and more amount of was mimicked by the addition of the corresponding amount of
veratryl alcohol and veratric acid. This is in complete agreement Bu4NCl to 1.25 mol Lꢂ1 NaOH aq. The addition of Bu4NCl also
with the above consideration that the demethylation pathway C increased the selectivity toward vanillin as indicated from the
(Scheme 2) is more favored over the disproportionation pathway higher vanillin yield (20.2 mol%) based on the degraded vera-
D in the Bu4NOH aq. We also carried out similar degradation in traldehyde and the lower yield of veratryl alcohol and veratric
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Table 2 Yields (mol%) of major A-ring-derived products after the degradation of LM1 (20 mg) at 120 ꢀC for 43 h under air in 1.25 mol Lꢂ1
Bu4NOH aq. and 1.25 mol Lꢂ1 NaOH aq. (2.0 mL)
Aldehydes
Vanillin
Non-aldehydes
Veratryl alcohol
Veratraldehyde
Total
Veratric acid
Total
Total
Bu4NOH aq.
NaOH aq.
15.7 (16.3)a
0.4 (0.9)
2.2 (2.3)
5.2 (12.1)
17.9 (18.6)
5.6 (13.0)
9.1 (9.5)
3.9 (9.1)
6.7 (7.0)
5.4 (12.6)
15.8 (16.5)
9.3 (21.7)
33.7 (35.1)
14.9 (34.7)
a
The value in the parenthesis shows the yield based on the amount of degraded LM1: 96.3% and 42.9% in the Bu4NOH aq. and the NaOH aq. based
on the initial amount of LM1, respectively.
Table 3 Yields (mol%) of vanillin, veratryl alcohol, and veratric acid along with recovery (%) of veratraldehyde after degradation of veratraldehyde
(5.0 mg) at 120 ꢀC for 4 h under N2 in 1.25 mol Lꢂ1 Bu4NOH aq. and NaOH aq. (1.0 mL)
Yield
Vanillin
Veratryl alcohol
Veratric acid
Recovery
Bu4NOH aq.
30.6 (33.5)a
5.9 (6.7)
8.7 (20.2)
28.1 (30.7)
38.3 (43.3)
12.7 (29.5)
31.2 (34.1)
41.6 (47.0)
13.9 (32.3)
8.6
11.5
57.0
NaOH aq.
NaOH aq. + Bu4NCl(s)b
a
The value in the parenthesis shows the yield based on the amount of degraded veratraldehyde. b Solid Bu4NCl was added to the NaOH solution so
that the concentration of Bu4N+ in the reaction solution became 1.25 mol Lꢂ1
.
acid (29.5 and 32.3 mol%, respectively) than those obtained in D2O-based solutions. As summarized in Table 4, the signal of all
the simple NaOH aq. Interestingly, in this Bu4NCl-added protons of veratraldehyde considerably shied to lower
system, the degradation of veratraldehyde was considerably magnetic elds by 20 to 112 Hz, when Bu4NCl was added to the
suppressed: much more starting material (57.0%) was recovered NaOD/D2O. This shi scarcely occurred when the same molar
in the Bu4NCl-added NaOH aq. These results suggest that Bu4N+ amount of NaCl was added to the solution, which strongly
slows down both pathways C and D, and along with that, makes suggested that the shi was caused by Bu4N+ and not by Clꢂ. It
the degradation more vanillin-oriented. Similar recoveries of is therefore likely that veratraldehyde is in a close contact with
the starting material obtained between the Bu4NOH and the Bu4N+ in the solutions containing Bu4N+. The largest shi
NaOH aq. – they are 11.5 and 8.6%, respectively – would be observed for the aldehyde proton (112 Hz) suggests strong
explained by the idea that the slowing-down effect by Bu4N+ was interaction between Bu4N+ and the aldehyde group. The
compensated by the strong basicity of Bu4NOH (see above).
disproportionation reaction is eventually suppressed when both
To obtain deeper insights into the effect of Bu4N+, we carried reactants are in the “cage” of Bu4N+. On the other hand, in the
out 1H NMR analyses of veratraldehyde in 1.25 mol Lꢂ1 NaOD/ case of the demethylation (pathway C in Scheme 2), one of the
Table 4 Chemical shifts (ppm) in 1H NMR analyses of veratraldehyde (0.5 mg) in 1.25 mol Lꢂ1 NaOD/D2O solution (0.75 mL) and those with the
additionsa of Bu4NCl and NaCl
C2–H (d)
C5–H (d)
C6–H (dd)
Ca–H (s)
C3–OCH3 (s)
C4–OCH3 (s)
1.25 mol Lꢂ1 NaOD/D2O
7.16
6.89
7.09 (+80)
6.93 (+40)
7.32
7.50 (+72)
7.36 (+16)
9.33
9.61 (+112)
9.38 (+20)
3.64
3.69 (+20)
3.67 (+12)
3.69
3.75 (+24)
3.73 (+16)
1.25 mol Lꢂ1 NaOD/D2O + Bu4NCl
1.25 mol Lꢂ1 NaOD/D2O + NaCl
7.29 (+52)b
7.18 (+8)
a
b
Bu4NCl and NaCl were added to the NaOD solution with the concentration of the additives being set at 1.25 mol Lꢂ1
.
Changes in the chemical
shi (Hz) caused by the additive were shown in the parenthesis.
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Fig. 4 HPLC chromatogram of the reaction mixture obtained after the
degradation of LM2 (5 mg) in 1.25 mol Lꢂ1 Bu4NOH aq. (1.0 mL) under
air for 1 h.
reactants is OHꢂ, which is a small molecule and hence can
more easily squeeze itself into the “cage”. Although further
investigation is necessary to elucidate more detailed mecha-
nism, this idea can reasonably explain the above experimental
facts. According to these mechanisms, it can be stated that
Bu4N+ does not have a specic catalytic site, but the interaction
between the aldehyde group and Bu4N+ is a major origin of the
good performance of Bu4NOH for the vanillin production. Also,
as shown in Fig. 3, the yield of guaiacol in the degradation in the
Bu4NOH aq. is extremely higher than that in the NaOH aq. and
this result may be also explained with the idea that guaiacol was
protected from the O2 attack by the cage effect caused by the
presence of Bu4N+.
Fig. 5 Changes with time in recovery of LM2 ( ), yields of guaiacol
(
), vanillin ( ), vanillic acid ( ), and enol ethers EEsa
(
) during
degradation of LM2 (5.0 mg) at 120 ꢀC under air in 1.25 mol Lꢂ1
Bu4NOH aq. (1.0 mL) (A) and in 1.25 mol Lꢂ1 NaOH aq. (1.0 mL) (B).
a The yields of EEs are shown as the sum of the cis- and trans-isomers.
respectively (see also Table 5). Vanillic acid started to form
aer 15 min and the yield reached 12.7 mol% at 4 h, although
the compound was not detected in the reaction mixture
before 15 min. These results suggest that LM2 is rst
degraded to EE as presented in pathway F in Scheme 3 and EE
is then degraded into vanillin and vanillic acid along with the
releasement of guaiacol (pathway G). The pathway F is
a simple alkaline degradation process via quinone methide
formation followed by succeeding elimination of Cg as
formaldehyde, as extensively investigated previously.22,31–33
The pathway G requires O2 according to the previous studies
reporting that EE is substantially stable under N2 even at
elevated temperatures (ꢁ170 ꢀC) but readily to be oxidized
into vanillin and guaiacol in the presence of O2.26,28 Note that,
according to comparison between the results in Fig. 3 and 5,
the degradation of LM2 was much faster than that of LM1
and this will be discussed in the next section.
We also carried out degradation of cis/trans mixture of iso-
lated EE (3.0 mg) in the Bu4NOH aq. (1.0 mL) under air. As
summarized in Table 5, the degradation for 4 h resulted in the
formation of 43.6, 22.2, and 41.4 mol% of vanillin, vanillic acid,
and guaiacol, respectively. Thus, the yields of vanillin and
guaiacol from EE are moderately smaller than those from LM2
obtained under the same reaction conditions (vanillin:
59.5 mol% and guaiacol: 61.9 mol%, see Fig. 5 and Table 5).
Fig. 4 shows a typical HPLC chromatogram of the reaction
mixture obtained aer the degradation of the phenolic lignin
model LM2 (5.0 mg) in 1.25 mol Lꢂ1 Bu4NOH aq. (1.0 mL) at
ꢀ
120 C under air. In the case of LM2, vanillin and vanillic acid
were detected as major A-ring-derived products along with
guaiacol as B-ring-derived one. We also found two large peaks at
37.6 and 39.0 min, which were assigned as cis- and trans-
1
isomers of the enol ether EE with H NMR analysis aer their
isolation from the reaction mixture with column chromatog-
raphy (see experimental section for details). The formation of
the trans-isomer was favored in the degradation. Note that EE
are reported as major products in degradation of phenolic
lignin models under alkaline conditions in many previous
studies.22,31–33
In the degradation of LM2, as shown in Fig. 5A, vanillin,
guaiacol, and EEs were produced from the initial stage of the
degradation in the Bu4NOH aq. Aer 2 h reaction time, the
yield of EEs started to decrease with increase in the yield of
vanillin and guaiacol. At 4 h reaction time, vanillin and
guaiacol were produced in 59.5 and 61.9 mol% yields,
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Table 5 Yields (mol%) of vanillin, vanillic acid, and guaiacol and the recovery (%) of the starting material after the degradation of cis/tarns mixture
of EE (3.0 mg) at 120 ꢀC for 4 h under air in 1.25 mol Lꢂ1 Bu4NOH aq. and 1.25 mol Lꢂ1 NaOH aq. (2.0 mL)
Yield (mol%)
Staring material
Reaction medium
Vanillin
Vanillic acid
Guaiacol
EE
Recovery (%)
LM2
LM2
EE
Bu4NOH aq.
NaOH aq.
Bu4NOH aq.
NaOH aq.
59.5
83.8
43.6
51.8
12.7
5.9
22.2
6.5
61.9
65.5
41.4
49.7
7.0
6.0
—
1.0
7.4
29.2
17.7
EE
—
Scheme 3 Reaction pathways from LM2 to the products detected in this study. The formation of vanillic acid via pathway H is less significant than
that via pathway G.
These results mildly suggest that there is a reaction pathway NMR analyses of trans-EE in the 1.25 mol Lꢂ1 NaOD/D2O with
forming vanillin and guaiacol directly from LM2 (pathway H in and without the addition of Bu4NCl indicated that C2–H and C6–
Scheme 3). For vanillic acid, on the other hand, the yield from H on the benzene ring and the protons on the Ca]Cb group
EE (22.2 mol%, Table 5) was greater than that from LM2 signicantly shied to upper magnetic elds when Bu4NCl was
(12.7 mol%, Fig. 5 and Table 5), which suggests that vanillic added to the NaOD/D2O solution, whereas the shi observed for
acid is formed mostly via EE in the degradation of LM2.
C5–H was relatively small. The addition of the same molar
The degradation of LM2 in the NaOH aq. was then carried amount of NaCl again did not cause those shis. These results
out under the same reaction conditions. As shown in Fig. 5B, we suggest that Bu4N+ is located mainly around the Ca]Cb bond of
observed reaction behaviors qualitatively the same as those EE. It has been reported that the initial step of the aerobic
observed in the case of the Bu4NOH aq. This indicates that the oxidation of EE to vanillin is addition of O2 to the Ca]Cb
degradation follows the pathways in Scheme 3 both in the group.26,28 Binding of the Ca]Cb group to Bu4N+ will render the
Bu4NOH and the NaOH aq. Through quantitative comparison of O2 addition more demanding, by which the degradation of EE
the reaction behaviors between the Bu4NOH and the NaOH aq. to vanillin is inhibited. Note that the changes in the chemical
(Fig. 5), the degradation in the NaOH aq. exhibited (1) slower shis of EE by the addition of Bu4N+ (Table 6) were opposite
degradation of LM2, (2) lower yields and faster degradation of from those observed for the veratraldehyde–NaOD/D2O system
EE, (3) larger yields of vanillin and guaiacol, and (4) lower yields (Table 4), that is, upper-magnetic eld shi and downer one
of vanillic acid. We will discuss these four differences below.
were observed in the EE and in the veratraldehyde solutions,
The relatively slow degradation of the starting material in the respectively. However, the reasons for this difference are not
NaOH aq., the rst difference mentioned above, can be clear at the moment.
explained again with the stronger basicity of Bu4OH over NaOH.
The reasons for the low yield of vanillic acid in the NaOH aq.
The reactions in the pathway G (Scheme 3) requires the (the fourth difference between the Bu4NOH aq. and the NaOH
deprotonation of the phenolic OH of LM2 and Cg–OH of the aq. mentioned above) are not clear at the moment. As presented
intermediate quinone methide. These deprotonation steps are in Table 5, this vanillic acid-oriented degradation behavior was
considered to be less feasible in the NaOH aq., ending up the re-produced also in the degradation of EE in the Bu4NOH aq.:
slow degradation of LM2.
EE formed vanillic acid in 22.2 mol% on the basis of degraded
The second and third differences are closely linked together, starting material in the Bu4NOH aq., whereas the yield of the
as the faster degradation of EE is the greater the yields of the compound was much smaller in the NaOH aq. (6.5 mol%). This
products (vanillin and guaiacol) should become. It is possible suggests that the degradation of EE (pathway G in Scheme 3) is
that, as an analogy to the stabilization of veratraldehyde by modied by the presence of Bu4N+ to be more vanillic acid-
Bu4N+ (see previous section), EE is stabilized by the presence of selective. The interaction between EE and Bu4N+ discussed
Bu4N+. The larger recovery of EE (29.2%) aer the degradation above would play some roles in this selectivity change, but more
for 4 h in the Bu4NOH aq. than that in the NaOH aq. (17.7%, systematic investigation is necessary to elucidate the underlying
1
Table 5) also supports this idea. As summarized in Table 6, H mechanism.
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Table 6 Chemical shifts (ppm) of several protons of trans-EEa (1.0 mg) in its 1H NMR analyses in 1.25 mol Lꢂ1 NaOD/D2O (0.75 mL) and those with
the additionsb of Bu4NCl and NaCl
C2–H (d)
C5–H (d)
C6–H (dd)
Ca–H (s)
Cb–H (s)
1.25 mol Lꢂ1 NaOD/D2O
6.65
6.33
6.54
6.83
6.00
1.25 mol Lꢂ1 NaOD/D2O + Bu4NCl
1.25 mol Lꢂ1 NaOD/D2O + NaCl
6.46 (ꢂ76)c
6.65 (ꢃ0)
6.27 (ꢂ24)
6.38 (ꢂ64)
6.71 (ꢂ48)
5.89 (ꢂ44)
6.36 (+12)
6.56 (+8)
6.85 (+8)
6.02 (+8)
a
b
The results only for the trans-isomer are shown as most of the cis-isomer signals were overlapped with those of the trans-isomer. Bu4NCl and
NaCl were added to the NaOD solution with the concentration of the additives being set at 1.25 mol Lꢂ1
caused by the additive were shown in the parenthesis.
. Changes in the chemical shi (Hz)
c
Effects of Bu4N+ on aerobic oxidation of lignin polymer
selectivity to vanillic acid-oriented. However, these negative
effects of Bu4N+ are not very serious in the vanillin production
from native lignin in Bu4NOH, since the vanillin production
from the phenolic end is fundamentally of little importance in
entire vanillin production reactions.
We have so far discussed the degradation behavior of the lignin
model compounds processing b-O-4 linkages, which consist
around 50% of the inter-unit linkages of native lignin polymer.
In this section we wish to discuss aerobic oxidation of native
lignin polymer focusing mainly on vanillin production, on the
basis of the results obtained above. In the discussion below, we
assume that the non-phenolic model LM1 represents the reac-
tivity of the monomeric unit in the middle of the polymer and
the phenolic model LM2 corresponds to that of the phenolic
end of the polymer.
Besides a series of the reactions on the phenolic end, alka-
line degradation of non-phenolic middle units gradually takes
place. As shown in Scheme 4, the initial step of the degradation
on a non-phenolic b-O-4 type middle unit is alkaline-catalyzed
ether link cleavage to form a glycerol end group, which corre-
sponds to the pathway A in Scheme 2. This step is signicantly
enhanced in Bu4NOH due to its strong basicity and this is one of
the benets in the utilization of Bu4NOH instead of NaOH.
The oxidation of the glycerol group by O2, pathway B in Scheme 2
in the case of the model compound LM1, then occurs to form
a benzaldehyde end group (vanillin end group when the lignin
sample is derived from so wood lignin). There are two possibilities
for the fate of the benzaldehyde end group. One is the free benz-
aldehyde formation by the cleavage of the ether link connecting the
benzaldehyde end to the adjacent monomeric unit, which corre-
sponds to the pathway C in Scheme 2. The other is the dispropor-
tionation of the aldehyde group resulting in the formation of
benzoic acid and benzyl alcohol ends (pathway D in Scheme 2). The
degradation in Bu4NOH facilitates the former reaction pathway to
increase the benzaldehyde yield.
These proposed reactions suggest that the reaction mixture
from the wood our should involve some amount of vanillyl
alcohol. However, the compound was not detected in the
degradation of the wood our under any conditions, although
vanillic acid was detected as a major product (Fig. 1). This is
probably because vanillyl alcohol end group is unstable due to
the presence of benzyl hydroxy group which is readily subjected
to condensation reactions.22 It is also noted that vanillic acid,
unlike vanillyl alcohol, can also be formed from the oxidation of
the enol ether end, as indicated from Fig. 5, and some of the
origins of vanillic acid produced from the wood our is
considered to be phenolic end groups of lignin polymer.
The initial step of the degradation of lignin polymer is
simple alkaline degradation of the phenolic end, which is the
fastest reaction observed in this study (Fig. 5). When the
phenolic end has a b-O-4 linkage, the degradation results in the
formation of an enol ether end group. The enol ether end
formed – if it is in contact with O2 – is further oxidized into
vanillin or vanillic acid and forms a new phenolic end in the
adjacent monomeric unit. When these stepwise reactions
continued, lignin polymers could degrade completely into the
low molecular weight compounds. However, this type of
“unzipping” reaction seems not to play major roles in the
degradation. The reaction time required for completing the
degradation of LM2 was ꢁ8 h, which was far from sufficient for
vanillin production from the wood our under the same
conditions: at least 43 h is necessary to fully produce vanillin
from the wood our at 120 C under air,8 although the intro-
ꢀ
duction of pure O2 shortens the reaction time (see above). On
one hand, the unzipping vanillin/vanillic acid production may
be stopped when the reaction “bumps” into a less reactive
linkage such as b–5, b–b, 4-O-5 and 5–50 linkages. On the other
hand, other types of reactions such as condensations are
possible to prevent the vanillin/vanillic acid production from
the phenolic end.
Our results obtained for LM2 revealed that Bu4N+ did no
good for the vanillin production aer all: Bu4N+ stabilizes the
enol ether to make the vanillin formation slow and changes the
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Scheme 4 Vanillin formation from a b-O-4 middle unit of lignin polymer. The pathways with bold arrows are positively influenced by Bu4N+.
Promotion of Science. The GC/MS analysis was carried out with
the apparatus in Kyoto Research Park Corp.
The degradation of Japanese cedar wood our under O2
Conclusions
produced vanillin and vanillic acid in 23.2 and 1.2% yields on
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the Technologies for Creating Next-
Generation Agriculture, Forestry, and Fisheries under the Cross-
Ministerial Strategic Innovation Promotion Program (SIP)
administered by the Council for Science, Technology, and
Innovation (CSTI), Japan, and a Grant-in-Aid for Young Scien-
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