Original Article

LigninLignin 2, 9-18 (2021)
DOI: https://doi.org/10.62840/lignin.2.0_9

The Lignin Society

Production of Vanillin and Vanillic Acid by Aerobic Oxidation ofPolyethylene Glycol (PEG)-modified Glycol Lignin
in Tetrabutylammonium Hydroxide
Takashi Hosoya,a Daiki Okamoto,a Hisashi Miyafuji,a,* and Tatsuhiko Yamada baGraduate School of Life and Environmental Sciences, Kyoto Prefectural University,
Kyoto 606-8522, Japan;bForestry and Forest Products Research Institute, Tsukuba 305-8687, Japan;*Corresponding author: miyafuji@kpu.ac.jp
Received: 22 January 2021; revised in form: 22 April 2021; accepted: 5 May 2021

Abstract
Aerobic oxidation of lignin in reaction media containing Bu4NOH facilitates efficient production of vanillin(4-hydroxy-3-methoxybenzaldehyde) and vanillic acid (4-hydroxy-3-methoxybenzoic acid). This study
presents production of these compounds from polyethylene glycol (PEG)-modified glycol lignin (PEGlignin) from Japanese cedar (Cryptomeria japonica). A Bu4NOH-based reaction medium was prepared by
the addition of NaOH(s) to 1.25 mol/L Bu4NOH aq. Degradation of the PEG-lignin (14 mg) in 2.0 mL of themedium at 120 oC under O2 in a sealed test tube produced vanillin and vanillic acid with 9.1 and 2.6 wt%
yields, respectively, based on the Klason lignin amount of the PEG-lignin. The total yield of vanillin andvanillic acid reached 79.6 % of that in the corresponding alkaline nitrobenzene oxidation (14.7 wt%),
indicating high performance of the medium. Bench-scale aerobic oxidation of the PEG-lignin (100 g /1.0 L)
under O2 pressurized at 0.7 MPa gave the products with their maximum yields similar to those in the above
lab-scale experiment. Further increase in the O2 pressure to 2.5 MPa significantly shortened the reaction timeto achieve the maximum product yield. This pressure increase did not affect the vanillin yield, but suppressed
the formation of O2-sensitive vanillic acid, by which selective vanillin production was achieved.
Keywords: Alkali; Conversion; Depolymerization; Ionic liquid; Quaternary ammonium

INTRODUCTION
Vanillin (3-methoxy-4-hydroxybenzaldehyde) is one of the target compounds to be produced
from softwood lignin. The compound is a platform chemical in industry, which is a source of several
medicine, polymer material, etc 1-4). Aerobic oxidation of lignosulfonate, a major component of waste
water from sulfite pulping, is a way to produce lignin-based vanillin 3-8). This vanillin production isusually carried out in alkaline media such as NaOH under compressed air at 170-180 oC in the presenceof Cu(II)-based catalysts. However, management of waste water containing sulfur compounds is a major
draw-back of this biomass-based vanillin production process 3). Worldwide decrease in the sulfite pulingprocess also makes this type of vanillin production less commercially attractive.
Our previous studies have reported that selective vanillin production can be made by aerobic
oxidation of several softwood lignin samples at 120 oC in the presence of tetrabutylammonium ion
Bu4N+ and OH¯ 9-11), where the employed medium Bu4NOH is a precursor of various Bu4N+-based ionicliquids. For example, when Japanese cedar (Cryptomeria japonica) wood flour is oxidized in 1.25 mol/L
Bu4NOH aq. at 120 oC under O2, vanillin is produced in 23.2 wt%, based on the Klason lignin amountof the wood 11). This vanillin yield is comparable to that obtained in the alkaline nitrobenzene (AN)oxidation (27.2 wt%), indicating high performance of the aerobic oxidation in Bu4NOH for vanillin
production. This high efficiency achieved with the Bu4NOH-based medium has been attributed tostabilization of a vanillin precursor formed at the end of lignin polymer by positive interaction between
Bu4N+ and the aldehyde group of the precursor 11). The aerobic oxidation also produces vanillic acid (3-9Original Article

LigninLignin 2, 9-18 (2021)
DOI: https://doi.org/10.62840/lignin.2.0_9

The Lignin Societyhydroxy-4-hydroxybenzoic acid), the oxidized form of vanillin, as the second major product, where
vanillic acid is also expected as a source of biomass-based polymer materials 12).
The Bu4NOH-based oxidation method has several advantages over the process withlignosulfonate and simple alkali: this method employs reaction temperature (120 oC) that is significantly
lower than that of the lignosulfonate process (~170 oC); the process can utilize non-sulfur containing
row materials such as wood flour and soda-lignin as well as lignosulfonate. Also, the use of molecularoxygen as an ideal non-toxic oxidation reagent in this process should be emphasized.
Polyethylene glycol (PEG)-modified glycol lignin (PEG-lignin hereafter) is produced bysolvolysis of native lignin in PEG in the presence of a catalytic amount of sulfuric acid 13-16). PEG-lignin
from Japanese cedar wood shows viscous thermal flow properties, which enables its application as
various lignin-derived plastic materials 17-19). The present paper shows another option of using PEGlignin as useful monomer chemicals. Our idea in this article is to produce vanillin from PEG-lignin with
the Bu4NOH-based method being employed. The PEG-lignin production process - as it goes for any
material production processes - cannot avoid formation of some amount of the products that do not meet
quality standard from the view point of their material use, but almost chemically identical to the required
PEG-lignin. The production of vanillin, a compound with significant usefulness, from such by-produced
PEG-lignin is beneficial to make overall PEG-lignin production process more commercially efficient.
On the basis of the above considerations, this article presents production of vanillin and vanillicacid by aerobic oxidation of PEG-lignin in our Bu4NOH-based medium, first in a small lab-scale (14mg of the sample/2.0 mL of the medium) system that has been previously applied to several lignin
samples 9-11). We will discuss difference in the vanillin and vanillic acid formation behaviors between
PEG-lignin and native lignin in Japanese cedar and clarify several properties specific for PEG-lignin asa raw material for the aerobic oxidation. Next issue to be handled in this paper is scaling-up of thereaction system, looking ahead at industrial application of the process. In this topic, we carry out the
aerobic oxidation of PEG-lignin in a 5.0 L bench-scale reactor under pressured O2 atmosphere, withincreased amounts of the starting material and the medium (100 g and 1.0 L, respectively).
EXPERIMENTAL
Materials
1.5 mol/L Bu4NOH aq. was purchased from the Sigma-Aldrich Co. The solution was diluted
to 1.25 mol/L with distilled water and used as the reaction solution. PEG-lignin was prepared bysolvolysis of Japanese cedar wood with PEG-200 in the presence of sulfuric acid, accroding to theprocedure presented in the literature 16). The Klason lignin content and the PEG/lignin ratio of the PEGlignin was 81.7 and 22.3 wt%, respectively 16).
Lab-scale Experiment
The PEG-lignin (14-200 mg) was placed in a 10 mL glass tube. After adding the reaction
medium (2.0 mL of 1.25 mol/L Bu4NOH aq. containing 200 mg of NaOH), the tube was flushed with
O2 and immediately sealed. This medium will be called Bu4NOH-based reaction medium, hereafter.
The PEG-lignin was completely solubilized into the Bu4NOH-based reaction medium with a darkcolored homogeneous solution being eventually formed. The tube was then heated at 120 °C in an oilbath with stirring. These reaction conditions were similar to those optimized for the aerobic oxidationof Japanese cedar wood 11). After the reaction solution was stirred for a certain time, the tube was cooledwith cold water and an ethanol solution of an internal standard (1,5-dihydroxy-1,2,3,4tetrahydronaphthalene, 2.0 g/L, 800 µL) was added to the resulting solution. A sample (50 L) of thereaction mixture was taken and added to acetonitrile (450 L) containing 1.5% acetic acid. The solution
was filtered and used directly for high-performance liquid chromatography (HPLC) analysis.
The HPLC analysis was carried out with HPLC system (Shimadzu, Ltd., Kyoto, Japan)
equipped with pump (LC10AD), column oven (CTO-10A), and ultraviolet–visible detector (SPD-10A)
set at 280 nm. Analytical conditions were a Cadenza CD-C18 (Imtakt, Co., Kyoto, Japan) column, aflow rate of 0.8 mL/min, a 1.5% acetic acid aq/acetonitrile eluent (90/10 → 45/55 0–30 min, 45/55 →10

Original Article

LigninLignin 2, 9-18 (2021)
DOI: https://doi.org/10.62840/lignin.2.0_9

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0/100 30–35 min, 0/100 35–40 min, 0/100 → 90/10 40–45 min, 90/10 45–60 min) after passing throughthe degasser (DGU-14A, Shimadzu, Ltd., Kyoto, Japan), and a column temperature of 30 °C.
Bench-scale Experiment
The PEG-lignin (100 g) was mixed with 1.0 L of 1.25 mol/L Bu4NOH aq. containing 100 g of
NaOH (the Bu4NOH-based medium). The resulting mixture was then put in a high pressure reaction
vessel illustrated in Fig. 1. After closing the vessel, O2 was introduced via the valve A in Fig. 1 and theinside was pressurized at 0.7–2.5 MPa. The reaction solution was stirred at 300 rpm with the mixer andthe inside temperature was elevated to 80–120 oC. We defined the reaction time zero as the time theinside temperature reached the target temperature; it usually took 20 to 30 min after the heating wasstarted. During the reaction around 10 mL of the reaction solution was taken by carefully opening thevalve B in Fig. 1 at certain reaction times. The ethanol solution (800 L) of the internal standard wasthen added to 1.0 mL of the sample and the resulting solution was analyzed with HPLC with the samemanner presented above.

Fig. 1. High pressure 5.0 L reactor for the bench-scale experiments.

The pressure in the vessel was reverted back to atmospheric by opening the valve A. After thereaction solution was cooled to room temperature, all solution in the reaction vessel was taken awayfrom the drain in Fig. 1. We usually observed two separated phases for the resulting solution probably
due to the insufficient solubility of NaOH to the Bu4NOH aq.: one of the phases was dark-colored withthe other being nearly colorless and their volume ratio was around 9:1. We sampled 1.0 mL of bothphases and analyzed them with HPLC, finding that the minor colorless phase contained only trace of
HPLC-detectable products. Thus, we do not take the products in the minor phase into consideration for
overall discussion in this article.

11Original Article

LigninLignin 2, 9-18 (2021)
DOI: https://doi.org/10.62840/lignin.2.0_9

The Lignin Society

RESULTS AND DISCUSSION
Property of PEG-lignin as a Source for Aerobic Oxidation
To investigate the performance of the Bu4NOH-based reaction medium for vanillin production
from the PEG-lignin, we first carried out the aerobic oxidation of a small amount (14 mg) of the PEGlignin in the lab-scale reactor (10 mL test tube) under the conditions similar to those previously
employed for Japanese cedar wood: 2.0 mL of the Bu4NOH-based reaction medium/ 120 oC/ O2 11). Fig.
2 shows a typical HPLC chromatogram of a resulting reaction mixture. Vanillin and vanillic acid were
produced from the PEG-lignin as the major low molecular weight products from the PEG-lignin alongwith relatively minor amounts of p-hydroxybenzaldehyde, acetoguaiacone, and guaiacol; vanillic acidappears as a seemingly small peak in Fig. 2 due to its low absorbance at 280 nm.

p-Hydroxybenzaldehyde

Internal
standard

Vanillin

Acetoguaiacone

GuaiacolVanillic acid

*

*
5

10

15

20

25

*
30

Retention time (min)Fig. 2. HPLC chromatogram of the reaction mixture obtained after the aerobic oxidation of the PEGlignin (14 mg) in the Bu4NOH-based medium (2.0 mL) at 120 oC under air (detector: UV280nm).
Unidentified peaks are indicated with asterisk.
As shown in Fig. 3A, the yields of the major products, vanillin and vanillic acid, reachedmaximum at 6 h and their yields were 9.1 and 2.6 wt%, respectively, on the basis of the Klason lignin
amount of the PEG-lignin. Table 1 compares these yields to those obtained in the AN oxidation of thesame sample (entry 1 and 4). It becomes clear that the total yield of vanillin and vanillic acid (11.7 wt%)
obtained after the aerobic oxidation in the Bu4NOH-based medium reached 79.6 % of that after thecorresponding AN oxidation (14.7 wt%). We therefore concluded that the aerobic oxidation in the
Bu4NOH-based medium produced these compounds, especially vanillin, with significant efficiency
from the PEG-lignin.
Table 1 also compares the yields of vanillin and vanillic acid from the PEG-lignin (14 mg) tothose from Japanese cedar (14 mg) reported previously (entry 1 and 5) 11). The aerobic oxidation of the
PEG-lignin obviously formed vanillin with lower yields than that of Japanese cedar. For vanillic acid,on the other hand, the maximum yield (2.6 wt%, entry 1) was higher in the PEG-lignin than in Japanesecedar (1.2 wt%, entry 5). We have previously suggested that vanillin and vanillic acid are produced via
different pathways: oxidation of vanillin into vanillic acid is not a major pathway at least under theconditions employed in our studies 11). It would be thus stated that some modification of the reaction
pathway has been made by the changes of the sample from Japanese cedar to the PEG-lignin, althoughdetailed mechanistic background is unclear at present.

12Original Article

LigninLignin 2, 9-18 (2021)
DOI: https://doi.org/10.62840/lignin.2.0_9

Yield (wt%, Klason lignin basis)

15

The Lignin Society

3

A

B

12
2
9

6

1
3

0

0
0

4

8

12

Reaction time (h)

0

4

8

12

Reaction time (h)Fig. 3. Changes in the yields of vanillin (◆), vanillc acid (■), and their total yield () during the lab-scale
aerobic oxidation of 14 mg (A) and 200 mg (B) of the PEG-lignin in 2.0 mL of the Bu4NOH-based reaction
medium at 120 oC under O2.
Table 1. The yields (wt%, Klason lignin basis a)) of vanillin and vanillic acid after the aerobic oxidation of
the PEG-lignin and Japanese cedar in the Bu4NOH-based medium (2.0 mL) under O2 at 120 oC, along withthe results of alkaline nitrobenzene (AN) oxidation of the PEG-lignin.
SampleYield (wt%)(mg)
Oxidant
Entry
SampleVanillic/medium/reaction time b)
Vanillin
Total
acid
(mL)
1
PEG-Lignin
14/ 2.0
O2/ 6 h
9.1
2.6
11.7
2
PEG-Lignin
200/ 2.0
O2/ 8 h
1.0
0.54
1.5
3
PEG-Lignin
20/ 0.2
O2/ 8 h
1.5
0.50
2.0
c)
4
PEG-Lignin
14/ 2.0
Nitrobenzene/ 2.5 h
12.1
2.6
14.7
11)
5
Japanese cedar
14/ 2.0
O2/ 4 h
23.2
1.2
24.4
6
Japanese cedar 11)
14/ 2.0
Nitrobenzene/ 2.5 h
27.2
1.2
29.4
a) The Klason lignin contents were 81.7 and 32.4 wt% for the PEG-lignin 16) and Japanese cedar11), respectively. b) The results of the aerobic oxidation in the Bu NOH-based medium are those4at the reaction time the total yield of vanillin and vanillic acid became maximum. c) AN oxidation
was carried out under the conditions of the sample (14 mg)/ 2.0 M NaOH aq. (2.0 mL)/
nitrobenzene (50 L)/ 170 oC.

Solvolysis of native lignin in wood by PEG leads to the introduction of PEG chains mainly tothe -positions of the lignin polymer 16). In fact, 2D HSQC NMR analyses of the PEG-lignin suggested
that -O-4 units in the PEG-lignin were derivatized with a PEG chain at the -position and do notpossess free -OH groups. The presence of such PEG moiety would be a reason for the above
differences. The PEG moieties introduced to the non-phenolic units of lignin are especially persistingunder alkaline conditions and expected to be a major reason for the above modification of the reactivity.

13Original Article

LigninLignin 2, 9-18 (2021)
DOI: https://doi.org/10.62840/lignin.2.0_9

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Additional aerobic oxidation experiments with -substituted lignin model compounds, especially -O4 models, will provide mechanical supports for the role of the -PEG groups in the PEG-lignin. In the
AN oxidation substitution of C-OH with an alkoxy group usually ends up in decrease in the yield of
the benzaldehydes 20). This explains the relatively low yield of vanillin in the AN oxidation of the PEGlignin (12.1 wt%, entry 4 in Table 1). The above lower yield of vanillin from the PEG-lignin in the
aerobic oxidation could also be explained from the same reason, but further systematic investigation on
the mechanisms in the AN oxidation and the aerobic oxidation is necessary to clarify the details. It isalso noted that the PEG-lignin employed in this study have not undertaken intensive acid condensations
during its production from the raw material 16), and hence the low yield of vanillin is not attributed tothe presence of the condensed structures in the PEG-lignin.
In addition to the different yields, the PEG-lignin took longer reaction time (6 h) than Japanesecedar (4 h) to achieve the maximum total yields of vanillin and vanillic acid, according to Table 1 (entry
1 and 5). The observations about the yields and the reaction times suggest that the PEG-lignin is a moredifficult starting material than native lignin in the original wood, at least when vanillin is set as the target
compound. However, it should be emphasized that the product yield from the PEG-lignin is still
significantly higher than that in the current vanillin production from lignosulfonate, which is usuallyaround 5 % 3,6-8). Also, our method employs a much lower temperature (120 oC) and does not necessitate
metal catalysts. It is thus concluded that our Bu4NOH-based process is worth being applied to thevanillin and vanillic acid production from the PEG-lignin.
To render the reaction system more practical from industrial perspective, we then carried out
the aerobic oxidation of the PEG-lignin with the concentration of the sample being increased from 14
mg/ 2.0 mL to 200 mg/ 2.0 mL. As shown in Fig. 3B, however, the increase in the concentration ended
up in drastic decrease in the product yields (see also entry 1 and 2 in Table 1). One of the possible
reasons for this decrease is that in such concentrated solution reaction pathways other than those forming
the desired products became more significant. For instance, bimolecular alkaline condensation, i.e.
formation of diphenylethane structures 21), would be enhanced with the increase in the sample
concentration, although the HPLC analysis of the reaction mixture did not indicate the presence of suchcondensed products. Products derived from the condensations, supposedly dimers and trimers, are notusually detected as major products even under the conditions bimolecular reactions are enhanced. Thisis probably because condensations generally occur among aromatic units in the lignin polymer, resulting
in the formation of HPLC-undetectable compounds with rather high molecular weights.
It is also possible that the O2 amount relative to the PEG-lignin became insufficient with theincrease in the concentration, which led to the low product yield; note that we here employed tightly
sealed reaction system (see experimental section). To check the possibility of the O2 shortage, we carriedout the aerobic oxidation of 20 mg of PEG-lignin in 0.2 mL of the reaction solution, where the volumeof the reaction solution was decreased from 2.0 mL with the sample concentration and the volume of
the reactor (10 mL) being fixed. As listed in entry 3 in Table 1, the aerobic oxidation under O2 in thissmaller reaction scale gave vanillin and vanillic acid in 1.5 and 0.50 wt%, respectively, which weremoderately greater than those obtained in the 200 mg/ 2.0 mL system. As one of the major differences
between the 200 mg/ 2.0 mL and the 20 mg/ 0.2 mL systems is the amount of oxygen relative to PEGlignin, we concluded that the decrease in the product yields caused by the increase in the concentration
was partially due to the depravation of O2 to fully oxidize the PEG-lignin to the final products.
Bench-scale Aerobic Oxidation of PEG-lignin
The considerations in the previous section led us to the idea that vanillin was expected to be
produced in significant yields even in the concentrated system, when the reaction was carried out underpressurized O2. To this end, we performed the aerobic oxidation of the PEG-lignin (100 g) in the
Bu4NOH-based medium (1.0 L) under O2 pressurized at 0.7 MPa with the bench-scale reaction system
in Fig. 1 being employed. As shown in Fig. 4A, the maximum product yield was obtained at the reaction
time of 3 h. This reaction time was significantly shorter than that in the case of the lab-scale experiment(6 h, Fig. 3A). Furthermore, the maximum vanillin yield achieved in the bench-scale experiment (8.7
wt% as listed in entry 1 in Table 2) was different by only 0.4 wt% from that obtained in the lab-scale
reaction system (9.1 wt%, entry 1 in Table 1) with the 14 mg/ 2.0 mL sample concertation. Hence, the14Original Article

LigninLignin 2, 9-18 (2021)
DOI: https://doi.org/10.62840/lignin.2.0_9

The Lignin Society

Yield (wt%, Klason lignin basis)

introduction of the sufficient amount of O2 to the system by the pressurization drastically improved thevanillin yields and the scaling-up of the reaction system has little influence on the performance of ourreaction system at least in terms of vanillin production. We also carried out the experiments at lower
temperatures of 80 and 100 oC, expecting that the pressurization may cause the decrease in the optimum
reaction temperature. However, as listed in entry 2 and 3 in Table 2, the decrease in the temperature
resulted in the decrease in the product yields as long as a realistic reaction time of 3 h was employed.

10

10

10

A

8

8

8

6

6

6

4

4

4

2

2

2

0

0

0
0

1

2

3

4

C

B

5

0

Reaction time (h)

1

2

3

4

5

Reaction time (h)

0

1

2

3

4

5

Reaction time (h)Fig. 4. Changes in the yields of vanillin (◆), vanillc acid (■), and their total yield () during the benchscale aerobic oxidation of 100 g of the PEG-lignin in 1.0 L of the Bu4NOH-based reaction medium at 120o
C under O2 pressurized at 0.7 MPa (A), 1.5MPa (B), and 2.5 MPa (C).
Table 2. The yields (wt%, Klason-lignin a) basis) of vanillin and vanillic acid after the bench-scale aerobic
oxidation of the PEG-lignin (100 g) in the Bu4NOH-based medium (1.0 L) under pressurized O2.
Entry

Temperature(oC)

Reaction time(h)Pressure(MPa)

Yield (wt%)
Vanillin

Vanillic acid

Total

1
120
3.0
0.7
8.7
0.54
9.2
2
100
3.0
0.7
4.5
0.93
5.4
3
80
3.0
0.7
2.5
0.81
3.3
b)
4
120
2.0
1.5
8.6
0.41
9.0
b)
c)
5
120
2.0
2.5
8.4
ND
8.4
a) The Klason lignin contents of the PEG-lignin was 81.7 wt% 16). b) The maximum vanillin yields
are shown along with the corresponding conditions in the case of the experiments at 1.5 and 2.5
MPa. c) ND means “not detected”.

The formation of vanillic acid, as compared to that of vanillin, became much faster in the caseof the bench-scale experiment under pressurized O2: the maximum yield (1.5 wt%) was obtained at 1 h
and 0.7 MPa while 3 h was needed to achieve the maximum vanillin yield (Fig. 4A). As the lab-scale
experiment showed almost simultaneous formation of vanillin and vanillic acid (Fig. 3), the formationof vanillic acid was more significantly influenced by the pressurization of O2 than that of vanillin. Thisobservation further supports our previous suggestion that these two compounds are produced in different
reaction pathways 11). At reaction times longer than 1 h, gradual degradation of vanillic acid wasobserved and the yield became almost zero at 4 h (Fig. 4A). Vanillic acid is more sensitive towards O2
than vanillin and degrades into guaiacol by decarboxylation. This is probably one of the reasons for thisdisappearance of vanillic acid. These results suggest that it is not easy for the aerobic oxidation of the15

Original Article

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DOI: https://doi.org/10.62840/lignin.2.0_9

The Lignin Society

PEG-lignin under pressurized O2 to produce both vanillin and vanillic acid with their potential
maximum yields. This seems to be a drawback of the O2 pressurization, but when viewed from the
opposite side, selective production of these two compounds would be possible by careful selection of
the reaction conditions, especially the O2 pressure.
According to this idea, we made systematic investigations about the influence of the O2 pressure
with the reaction temperature being fixed at 120 oC. At 0.7 MPa, as discussed above and clearly shownin Fig. 4A, the product yield got maximum at 3 h. This reaction time became shorter with the increasein the O2 pressure to 2.5 MPa: the reaction times required for the maximum total product yield were 2
h at 1.5 MPa and 1 h at 2.5 MPa, according to Fig. 4. It is also indicated that the maximum yields ofvanillin were almost constant no matter which pressure was employed: they were 8.7, 8.6, and 8.4 wt%
at 0.7, 1.5, and 2.5 MPa, respectively (entry 1, 4, and 5 in Table 2). It is thus stated that, as long asvanillin is focused, increase in the O2 pressure effectively shortens the reaction time without decreasein the yield.
For vanillic acid, on the other hand, the increase in the O2 pressure from 0.7 to 1.5 MPa
moderately lifted up the yield from 1.5 to 2.0 wt% with the required reaction time being unchanged, but
further increase in the pressure to 2.5 MPa resulted in the decreased yield of 1.1 % (Fig. 4). Also, thedegradation of vanillic acid was enhanced by the increase in the pressure. These results are consistent
with the above O2-sensitive chemical nature of vanillic acid. This difficult nature of vanillic acid,however, renders selective vanillin production feasible. The aerobic oxidation at 2.5 MPa for 2 h gave
vanillin in 8.4 wt% yield without forming detectable amount of vanillic acid (Fig. 4C and entry 5 inTable 2). In other words, after the almost complete degradation of vanillic acid in 1 h, vanillin, which
was more tolerant to further degradation, survived until 2 h. It is thus stated that vanillin is a more
realistic target compound in our process. This consideration would be also supported by theexperimental fact that our aerobic oxidation always produces more vanillin than vanillic acid by nature.
CONCLUSIONS
1. The aerobic oxidation of the PEG-lignin in the Bu4NOH-based medium gave vanillin and vanillicacid with the yields comparable to those in the corresponding AN oxidation, indicating highperformance of the reaction medium.
2. The PEG-lignin produced vanillin in lower yields than Japanese cedar in both alkaline aerobic andnitrobenzene oxidations, probably due to the presence of the PEG moiety at the -position. This
indicates that, when being seen from the side of the vanillin production as a sub-process, moderate(or not very intensive) introduction of PEG to the native lignin is recommended during the chief
PEG-lignin production process. Solvolysis under mild conditions will produce PEG-lignin that issuitable for vanillin production.
3. As long as sufficient amount of O2 is supplied to the system, the lab-scale (2 mL) and bench-scale(1.0 L) experiments gave the products with similar yields. Pressurization of O2 was very effective
when the oxidation is carried out in large scale.
4. Selective vanillin production can be achieved by taking advantage of the faster degradation ofvanillic acid than vanillin at high O2 pressures.

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-inAid for Young Scientists (B) (No. 17K18008) from the Japan Society for the Promotion of Science.

16

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Original Article

LigninLignin 2, 9-18 (2021)
DOI: https://doi.org/10.62840/lignin.2.0_9

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