Efficient production of adipic acid from 2-methoxycyclohexanone by aerobic oxidation with a phosphotungstic acid catalyst

Article abstract of DOI:10.1039/d0gc01277g

Oxidative cleavage reaction of 2-methoxycyclohexanone (2-MCO) to adipic acid (AA) and methanol with O2 in water solvent was investigated. 2-MCO and AA are one of the lignin-based compounds produced via hydrogenation of guaiacol and an important monomer in industry, respectively. Various vanadium compounds and heteropolyacids were tested as homogeneous catalysts because vanadium compounds, especially phosphomolybdovanadic acids, have been known to be active in various oxidative cleavage reactions with O2. Simple vanadium-free phosphotungstic acid (H3PW12O40), which has not been regarded as an oxidation catalyst using O2 as the oxidant, showed good catalytic activity and excellent selectivity to AA. The carbon-based AA yield reached 74% (86% in molar basis) and this value was higher than those obtained with vanadium-based catalysts. A reuse test and 31P NMR confirmed that the H3PW12O40 catalyst was stable and reusable. Kinetic studies and the reaction test using a radical inhibitor suggested that the reaction mechanism is not auto-oxidation involving free radicals. Instead, the substrate was first activated by one-electron oxidation by H3PW12O40 catalyst and then reacted with O2.

Full text of DOI:10.1039/d0gc01277g

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Page 1 of 41
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DOI: 10.1039/D0GC01277G
Efficient production of adipic acid from 2-
methoxycyclohexanone by aerobic oxidation with
phosphotungstic acid catalyst
Kosuke Hatakeyama, Yoshinao Nakagawa,a,b,* Masazumi Tamura,a,b,† and
a
Keiichi Tomishigea,b,*
a Department of Applied Chemistry, School of Engineering, Tohoku University, Aoba 6-6-07,
Aramaki, Aoba-ku, Sendai, 980–8579, Japan
b Research Center for Rare Metal and Green Innovation, Tohoku University, Aoba 468-1, Aramaki,
Aoba-ku, Sendai, 980–0845, Japan
*
Corresponding authors.
Yoshinao Nakagawa: yoshinao@erec.che.tohoku.ac.jp; tel&fax: +81-22-795-7215.
Keiichi Tomishige: tomi@erec.che.tohoku.ac.jp; tel&fax: +81-22-795-7214.
†
Current address: Research Center for Artificial Photosynthesis, The Advanced Research Institute
for Natural Science and Technology, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi, Osaka,
58-8585, Japan
5
1

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DOI: 10.1039/D0GC01277G
Abstract
Oxidative cleavage reaction of 2-methoxycyclohexanone (2-MCO) to adipic acid (AA) and
methanol with O in water solvent was investigated. 2-MCO and AA are one of lignin-based
2
compounds produced via hydrogenation of guaiacol and an important monomer in industry,
respectively. Various vanadium compounds and heteropolyacids were tested as homogeneous
catalysts because vanadium compounds, especially phosphomolybdovanadic acids, have been known
to be active in various oxidative cleavage reactions with O . Simple vanadium-free phosphotungstic
2
acid (H PW O ), which has not been regarded as an oxidation catalyst using O as the oxidant,
3
12 40
2
showed good catalytic activity and excellent selectivity to AA. The carbon-based AA yield reached
4% (86% in molar basis) and this value was higher than those obtained with vanadium-based
7
3
1
catalysts. Reuse test and P NMR confirmed that the H PW O catalyst was stable and reusable.
3
12 40
Kinetic studies and the reaction test using a radical inhibitor suggested that the reaction mechanism
is not auto-oxidation involving free radicals. Instead, the substrate was first activated by one-electron
oxidation by H PW O catalyst and then reacted with O .
3
12 40
2
Keywords
Oxidative cleavage; Heteropolyacid; Biomass; Adipic acid
2

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DOI: 10.1039/D0GC01277G
1. Introduction
Amid the concern of global warming and depletion of petroleum, which is indispensable in
modern society as energy and source of chemicals, alternative resources for petroleum are being
looked for. Biomass is attracting attention as one of alternative renewable resources to petroleum
1
‒7
because biomass can be used for not only energy but also production of chemicals . Among various
types of biomass, woody biomass (lignocellulose) is valuable because it does not compete with human
food production and exists in large quantity. Woody biomass mainly consists of cellulose,
hemicellulose and lignin. While cellulose and hemicellulose are composed of sugar units and can be
converted to similar compounds to those derived from edible biomass,1-4 conversion of lignin has
much different chemistry from that of cellulose, hemicellulose and edible biomass.5-7 Lignin is
expected as renewable raw material of cyclic compounds because lignin is a complex polyether of
aromatics. In fact, fast pyrolysis of lignin (or woody biomass itself) can produce liquid compounds
(
bio-oil) which are available as source of energy and production of chemicals at high yield.5,6
However, direct use of bio-oil is difficult because of the too much oxygen amount, instability and
complex composition. So, researches on effective upgrading of bio-oil have been actively conducted.7
Research on guaiacol conversion to useful compounds is one of them, such as phenol production
using CoMo-,8,9 Fe-,10,11 Ni-,12-15 Mo-16-18 and ceria- based catalysts and cyclohexanol production
19
using Ru-,20-23 Ni-,24-27 Co-, and Pt- based catalysts. It is because guaiacol has the smallest
28
29
molecule with three major types of C-O bonds in aromatic component of bio-oil (C -OH, C -OR
aryl
aryl
and CH -O) and because guaiacol itself is sometimes one of major component of bio-oil.30,31
6,7
3
Synthesis of guaiacol by decarbonylation of vanillin, which is also a major component of bio-oil, has
been reported.32
In this research, we focused on adipic acid, one of the most important monomers for
3

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3
3
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DOI: 10.1039/D0GC01277G
plastics , as a target product of guaiacol conversion. Adipic acid is currently produced from
petroleum-based cyclohexane via the aerobic oxidation to K/A oil and further oxidation with nitric
acid. Production of cyclohexane from renewable resources is not easy: typical method is isolation
from total hydrodeoxygenation product of bio-oil which requires large amount of H and difficulty in
2
keeping the activity of hydrodeoxygenation catalyst. Guaiacol can be directly isolated from bio-oil,
although the yield is not large, and it is an alternative source of renewable adipic acid. Conversion of
guaiacol to adipic acid involves oxidation of 1 and 2 positions of carbon atoms in the phenol ring and
hydrogen addition to 3, 4, 5 and 6 positions (Scheme 1). Therefore, the combination of hydrogenation
and oxidation is necessary in this conversion. We select 2-methoxycyclohexanone (2-MCO) as the
intermediate compound because 2-MCO is the simple product of hydrogenation of guaiacol at 3, 4, 5
and 6 positions (eq. (1)).
OH
OH
O
O
O
O
+
2H2
2-MCO
(1)
3
4
Pd catalysts are known to be effective in selective hydrogenation of phenols to cyclohexanones, and
3
5
that of guaiacol to 2-MCO has been actually reported with Pd/HAP catalyst. Catalytic reduction of
3
6
guaiacol to 2-MCO with formate as reducing agent has been also reported. On the other hand, there
are a number of studies on guaiacol hydrodeoxygenation to phenol, cyclohexanone or
cyclohexanol.8‒29 Cyclohexanone and cyclohexanol can be oxidized to adipic acid with conventional
nitric acid-based oxidation system.37 Phenol can be easily hydrogenated to cyclohexanone or
cyclohexanol. However, the production route of adipic acid via these monooxygenates from guaiacol
consumes more amount of hydrogen (3 and 4 equiv. of H for cyclohexanone and cyclohexanol
2
4

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production, respectively), and in addition, it usually converts the methoxy group to less valuable
DOI: 10.1039/D0GC01277G
methane, which requires further consumption of hydrogen. Other production routes of adipic acid
from biomass typically use sugar-derived intermediates such as 2,5-furandicarboxylic acid38-42 and
sugar acids;43-48 however, these routes require difficult and H -consuming (4 equiv. of H )
2
2
hydrodeoxygenation to produce the tetramethylene group of adipic acid. Biological production of
muconic acid or even adipic acid itself is possible,49,50 and muconic acid can be converted to adipic
acid by hydrogenation; however, these biological processes have not been established at a practical
level. Other chemical production methods of adipic acid from biomass include extension of carbon
chain from C5 intermediate compounds by (hydro)formylation,51-54 carboxylation55 and self-
metathesis.56 As an oxidative cleavage reaction of 2-MCO, Aakel et al. reported that
phosphomolybdovanadic acid H PV Mo O catalyzes the oxidation of 2-MCO with O in methanol
6
3
9
40
2
solvent into dimethyl adipate (eq. (2)) in 52% mol-based yield.57 On the other hand, use of
nonflammable water solvent (eq. (3)) is more desirable from the viewpoint of safety, and in addition,
free adipic acid can be easily crystallized or extracted from water solvent.
O
O
O
O
+
O2 + MeOH
+
H O
2
O
O
2
-MCO
O
Dimethyl adipate
(
2)
3)
O
O
HO
+
O2
+
+ MeOH
OH
H2O
O
2-MCO
Adipic acid (AA)
(
However, oxidation of 2-MCO with O to free adipic acid has not been reported in the literature, to
2
the best of our knowledge. Here, we develop catalytic systems for eq. (3). First, we tested vanadium-
5

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based catalysts because they are known to be effective in oxidative cleavage reaction of a related
DOI: 10.1039/D0GC01277G
compound 2-hydroxycyclohexanone (2-HCO).57-62 We also explored heteropolyacids as catalysts
because phosphomolybdovanadic acid is one of heteropolyacids and the redox properties of
heteropolyacids and their salts (polyoxometalates) can be controlled at molecular level.63,64
Surprisingly, we found that vanadium-free H PW O is an effective catalyst in this reaction, in
3
12 40
contrast to the general knowledge that H PW O is not an oxidation catalyst with O as an oxidant.
3
12 40
2
2
. Experimental
2
.1. Catalyst
ꢀ
V O was purchased from Wako. H PV Mo O ·25H O was purchased from Nippon Inorganic
2 5 5 2 12 40 2
Colour & Chemical Co., Ltd. H PMo O ·nH O (n=21 as determined by thermogravimetric and
3
12 40
2
differential thermal analysis (TG-DTA)) was purchased from Tokyo Chemical Industry Co., Ltd.
H PW O ·nH O (n=10 as determined by TG-DTA) was purchased from Yoneyama Yakuhin Kogyo
3
12 40
2
Co., Ltd. H SiW O ·26H O and Na PW O ·nH O (n=13 as determined by TG-DTA) were
4
12 40
2
3
12 40
2
purchased from Nakalai Tesque, Inc. H WO was purchased from Strem Chemicals, Inc.
2
4
H PVW O was synthesized by the following method based on the literature:65 5 g of
4
11 40
NaVO (dissolved in 50 ml of 0.91 M aqueous oxalic acid) and 3 g of Na HPO (dissolved in 50 ml
3
2
4
of distilled water) were mixed. 50 g of Na WO ·2H O (dissolved in 150 ml of distilled water) was
2
4
2
added, then the mixed solution was heated to 353 K and then 30 ml of concentrated H SO was slowly
2
4
added to the solution. The solution was stirred for 8 h. After cooling the solution to room temperature,
H PVW O was extracted with diethyl ether from the solution. The solvent was evaporated to obtain
4
11 40
3
1
51
a solid product which was washed with distilled water and dried. P and V NMR was measured
with Bruker AV400 instrument. Phosphoric acid and SiVW O405-aq were used as external standards
1
1
6

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3
1
51
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DOI: 10.1039/D0GC01277G
(
0 ppm ( P) and -550 ppm ( V), respectively). Aqueous solution of reduced phosphotungstic acid
(
H PW O ) was prepared by the following method: H PW O (220 mol) was dissolved in water
4
.3
12 40
3
12 40
(
20 g). Pt/C (5 wt% Pt, Wako, 67 mg) was added, and the mixture was treated with 1 MPa of H at
2
room temperature in an autoclave for 1 h. The solution of reduced phosphotungstic acid was collected
by filtering out the Pt/C catalyst under N . The reduction degree was determined by titration with
2
4
+
aqueous Ce(NH ) (SO ) until the deep blue color of the reduced heteropolyanion disappeared (xCe
4
4
4 4
3
+
+
+
H PW O → xCe + H PW O + xH ).
3+x 12 40 3 12 40
2
.2. Activity tests
The oxidation reaction was performed in a 190 mL stainless-steel autoclave with an inserted glass
vessel. The catalyst, substrate, and water (10 mL) were put into the autoclave together with a spinner.
All the catalysts in this study are soluble in water (homogeneous system). The pH was measured by
using a pH meter if necessary. After sealing, the reactor was filled with 0.8 MPa O (at r. t.). The
2
autoclave was heated to 353 K, which took about 40 min to reach, and the temperature was monitored
by using a thermocouple inserted in the autoclave. The stirring rate was fixed at 500 rpm (magnetic
stirring). After an appropriate reaction time, the reactor was cooled by a cold water bath, and the gases
were collected in a gas bag. The autoclave contents, sometimes containing adipic acid crystals, were
diluted/dissolved with 2-propanol and transferred to a vial. If catalyst-derived solid was formed it
was separated by filtration through a membrane filter. The liquid phase was analyzed by using HPLC
(Shimadzu Prominence; Aminex HPX-87 H column (Bio-Rad), 0.01 M sulfuric acid as eluent, UV
detector (210 nm) and RID). 1,6-Hexanediol was used as internal standard which was added after
reaction. The gas phase was analyzed by using GC with flame ionization detection (FID) and
methanator (Shimadzu GC-2014, Porapak N packed column). External standard method was used for
7

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gas phase analysis, and the analysis was repeated to exclude the failure in sampling. The formation
DOI: 10.1039/D0GC01277G
1
of adipic acid was also confirmed by H NMR of the reaction mixture using D O as solvent (Fig. S1).
2
The conversion, selectivity, yield and carbon balance were calculated from the following formulas:
Conversion [%]=(mol of consumed substrate)/(mol of charged substrate)×100; Selectivity [%-
C]=[(mol of product)×(number of carbon atoms in the product)]/[(mol of consumed
substrate)×(number of carbon atoms in the substrate)]×100; Yield [%-C]=[(mol of product)×(number
of carbon atoms in the product)]/[(mol of charged substrate)×(number of carbon atoms in the
substrate)]×100; Carbon balance [%]=[(mol of compound)×(number of carbon atoms in the
compound)]/[(mol of charged substrate)×(number of carbon atoms in the substrate)]×100. We used
carbon-based selectivity and yield because the oxidative dissociation including CO formation
2
increases the number of molecules and the sum of mol-based selectivities does not become 100%.
Mol-based yield of adipic acid [%], defined as [mol of adipic acid]/[mol of charged 2-MCO]×100,
can be calculated by the following formula: (carbon-based yield of adipic acid [%-C])×7/6. Similarly,
mol-based yield of methanol [%] can be calculated by (carbon-based yield of methanol [%-C])×7.
3
. Result and discussion
3
.1. Catalyst survey
Table 1 lists the results of the catalyst screening for the oxidation of 2-
methoxycyclohexanone (2-MCO). Theoretical maximum yield of adipic acid (AA) was 86% because
product yield was calculated on carbon basis. First, we tested vanadium catalysts which have been
reported to be active in oxidative cleavage reactions of various substrates such as hydroxyketones,57-62
carbohydrates,66-74 catechols,75-77 lignin and lignin-model compounds,78-83 malic acid84 and 5-
hydroxymethylfurfural.85 H PV Mo O system, which is a typical vanadium-based oxidation
5
2
10 40
8

Page 9 of 41
Green Chemistry
catalyst63,64 and showed good performance in the previous report for 2-MCO oxidation to dimethyl
View Article Online
DOI: 10.1039/D0GC01277G
5
7
adipate , showed high conversion; however, AA yield was low (entry 2) and in addition the reaction
stopped before complete conversion of 2-MCO (Fig. S2). V O catalyst, which is the simplest
2
5
vanadium catalyst, showed low AA yield even at high conversion level (entries 3 and 4). Various by-
products including shorter-chain dicarboxylic acids, formic acid and CO were observed with these
2
two vanadium catalysts. Next, we explored the catalysts of the combination of vanadium species and
heteropolyacids. Addition of H PMo O to V O greatly increased the conversion and AA
3
12 40
2
5
selectivity, especially by suppression of shorter-chain dicarboxylic acid and formic acid (entry 5).
Addition of H PW O to V O also greatly increased the conversion and slightly increased the AA
3
12 40
2
5
selectivity (entry 6). Addition of H SiW O to V O greatly increased the conversion, while the AA
4
12 40
2
5
selectivity decreased (entry 7). Although addition of H PMo O showed the highest AA yield, it is
3
12 40
known that various types of vanadium species and polyoxometalates are formed in the mixed solution
6
3,64
of vanadium and H PMo O ,
making it difficult to discuss the true active species. We are
3
12 40
interested in the good result of V O + H PW O system because H PW O has been hardly used
2
5
3
12 40
3
12 40
8
6
as oxidation catalyst using O as an oxidant. In fact, H PW O was reported to be inactive in
2
3
12 40
8
4
85
oxidation of malic acid and 5-hydroxymethylfurfural with O in organic solvent in the conditions
2
where H PV Mo O showed good activity. We tested H PVW O and H PW O as catalysts
5
2
10 40
4
11 40
3
12 40
(
entries 8 and 9) because we considered that the formation of H PVW O might be a cause of
4 11 40
increase of AA yield in V O + H PW O system. H PVW O catalyst system showed similar
2
5
3
12 40
4
11 40
conversion to that in V O + H PW O system and much higher AA selectivity than that in V O +
2
5
3
12 40
2
5
H PW O system (entry 8). However, interestingly, H PW O alone system showed the highest
3
12 40
3
12 40
AA selectivity and relatively high conversion (entry 9). The reactions with other typical
heteropolyacids were also carried out. H SiW O alone system also showed activity (entry 10).
4
12 40
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DOI: 10.1039/D0GC01277G
However, the conversion and AA selectivity were lower than that of H PW O . H PMo O and
3
12 40
3
12 40
H SiMo O alone systems showed similar activity of H PW O system (Table S1, entries 10 and
4
12 40
3
12 40
1
1). The color of the reaction solution with H PMo O or H SiMo O became black or green (Fig.
3 12 40 4 12 40
S3 (b) and (c)), which indicated the reduction of heteropolyacid, while in the case of H PW O the
3
12 40
reaction solution remained colorless (Fig. S3 (a)). Therefore, we further investigated the H PW O
3
12 40
system because of the simplicity and easy characterization of the catalyst. In addition, elution effect
of metal species from the wall of autoclave (stainless steel) in H PW O system was checked
3
12 40
because in our autoclave a stainless steel sheath for thermocouple contacted with the reaction solution.
When the sheath was covered with a teflon tube to avoid the direct contact between the reaction
solution and stainless steel, the reaction result was not changed (Table S1, entry 8). The reaction with
H WO as catalyst showed almost no activity (entry 11). The reaction with H SO showed that small
2
4
2
4
amount of AA was formed with a simple acid catalyst (entry 12). However, the activity and selectivity
were much lower than the case of H PW O catalyst. The results of these control experiments
3
12 40
suggest that H PW O was actually an active catalyst in the 2-MCO oxidation to AA with O (eq.
3
12 40
2
(3)). Methanol was formed almost stoichiometrically (carbon-based yield with about 1/7 of the
conversion value), and the formation of CO was negligible in the H PW O catalyst system. The
2
3
12 40
small formation of CO is characteristic to the H PW O catalyst system, which contrasts with the
2
3
12 40
6
1,62,66-
vanadium-based C‒C cleavage reaction systems where significant amount of CO was formed.
2
7
4
The higher selectivity of H PW O catalyst than other catalysts in Table 1 is further confirmed
3
12 40
by comparing the conversion-AA yield dependency of H PW O catalyst obtained in later sections
3
12 40
with each entry in Table 1 data (Fig. S4). All the other catalysts in Table 1 showed lower adipic acid
yield than H PW O catalyst at the same conversion level.
3
12 40
1
0

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3
3
.2. Optimization of reaction conditions
.2.1. Effect of the oxygen pressure
First, the results of the reactions under different oxygen pressures are shown in Table 2. In
the range of O pressure of 0.1‒0.8 MPa, the conversion and AA selectivity were almost unchanged
2
(entries 1‒4). These results indicated the zero-order dependence on oxygen pressure in this range,
suggesting that the steps involving O molecule were fast. Nevertheless, the AA yield was very
2
slightly larger under higher O pressure. We selected 0.8 MPa O as the standard condition.
2
2
When the reaction was carried out without O (at zero O pressure), the conversion of 2-
2
2
MCO was much lower than the cases with O and a small amount of 2-HCO was detected as the
2
hydrolysis product (entry 5; eq. (4)). The color of reaction solution was changed to black, which
indicates the reduction of H PW O (Fig. S3 (d)). The reduction degree of the heteropolyacid (x in
3
12 40
4
+
H PW O ) after the 24 h reaction without O was determined by titration with Ce . The
3
+x
12 40
2
determined x value was 0.37, which means that more than 60% of the heteropolyacid remained in the
fully oxidized state (H PW O ). At shorter reaction time (6 h), the reduction degree of the
3
12 40
heteropolyacid was x=0.21, which was lower than that at 24 h. These data indicated that the reduction
of H PW O to free H PW O proceeded very slowly under O -free conditions. The role of 2-
3
12 40
4
12 40
2
HCO and reduced heteropolyacid in the reaction mechanism will be discussed later.
O
O
O
OH
+
H O
+
MeOH
2
2-MCO
2-HCO
(4)
3
.2.2. Effect of the reaction temperature
The results of the reactions at different reaction temperature (323‒383 K) are shown in Table
1
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3
. The reaction proceeded very slowly at 323 K and the conversion was low even after long reaction
DOI: 10.1039/D0GC01277G
time (entries 1 and 2). At 353 K, the conversion increased to almost 100% at longer reaction time
with keeping high AA selectivity (entries 3 and 4). At 383 K, the activity was further increased;
however, the AA selectivity was decreased (entry 5). We selected 353 K as the standard reaction
temperature.
.2.3. Effect of the ratio of [H ] and [PW O403-]
+
3
1
2
The reactions in different ratio of [H ] and [PW O403-] were conducted to investigate the
+
1
2
+
3-
effect of their concentrations because H PW O was ionized to H and PW O in water solution.
3
12 40
12 40
+
3-
+
The ratio of [H ] and [PW O ] was adjusted by using H SO as H component and Na PW O
12 40
1
2
40
2
4
3
3
-
as PW O component. The results are shown in Fig. 1. The conversion was improved by addition
1
2
40
+
+
of H to Na PW O and was increased with the increase of H amount. The conversion was also
3
12 40
3
-
3-
improved by addition of PW O and was increased with the increase of PW O amount. These
1
2
40
12 40
+
3-
results indicated that both H and PW O were involved in the catalysis. The reaction results using
1
2
40
different amount of H PW O are shown in Fig. S5. The conversion and AA yield were increased
3
12 40
linearly with the increase of catalyst amount, which suggests the first-order dependence on
-1
H PW O amount. The initial TOF of AA production calculated by the slope was 0.8 h .
3
12 40
3
.2.4. Time course of oxidation with H PW O
3
12 40
Time course of the 2-MCO oxidation with H PW O as catalyst is shown in Fig. 2. The
3
12 40
conversion at 0 h means the reaction during the heating from room temperature to target one which
took about 1 h. The conversion increased with time and reached almost 100% at 120 h. The highest
yield of AA (74%-C) was obtained at this time (same data of Table 3, entry 4). The mol-based yield
1
2

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of AA was 86% and this value is much higher than the diester yield in the previous report (52%).
DOI: 10.1039/D0GC01277G
The TON of AA production at the condition with the highest yield was 34.
The relationship between ln(C2-MCO/C ) (C2-MCO: concentration of 2-MCO after reaction, C0:
0
initial concentration of 2-MCO) and time (Fig. S6) showed a straight line through the origin. This
result suggests the first-order reaction with respect to 2-MCO concentration.
3
.2.5. Reusability of H PW O
3
12 40
The reuse experiment of H PW O catalyst was carried out. Because H PW O is a
3
12 40
3
12 40
homogeneous catalyst and the solubility of AA is not large in cold water, the following method was
applied to the reuse test: The solution after reaction was cooled to 273 K, the precipitated AA was
collected by filtration (the collected AA yield was about 30%-C), 2-MCO was added in the consumed
amount to the solution to the original concentration level, and the next run was conducted. The yield
and selectivity were calculated based on the increase of the amount of each product. The results are
shown in Fig. 3. The conversion and AA selectivity hardly changed during the reuse experiment. The
data also indicate no formation of insoluble solid derived from catalyst such as tungsten bronze. In
3
1
addition, P NMR of the reaction solution after the reuse experiment was measured to investigate
3
1
whether H PW O maintained the polyoxometalate structure. As shown in Fig. 4, the P NMR
3
12 40
3
-
signal of PW O was not changed, indicating that decomposition of Keggin structure or formation
1
2
40
of lacunary polyoxometalates did not proceed during the catalysis. These data indicated that
H PW O was a stable and reusable catalyst. The reusability of the H PW O solution suggests
3
12 40
3
12 40
the potential of semi-batch process with continuous feeding of 2-MCO and removal of adipic acid.
3
.3. Reaction mechanism
1
3

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DOI: 10.1039/D0GC01277G
3
.3.1. Kinetics
As described above, the reaction order with respect to O and 2-MCO concentration were
2
determined to be zero and suggested to be one, respectively. To confirm the first-order dependence
on 2-MCO concentration, the reactions in different substrate amount were carried out. As shown in
Fig. 5, the conversion values were similar in the reactions in up to 0.43 mol/L 2-MCO concentration
range at the same reaction time. These results indicated that the converted amount was proportional
to the 2-MCO concentration. However, at 0.86 mol/L 2-MCO the conversion value became lower.
These data showed that at lower 2-MCO concentrations (≤ 0.43 mol/L) the reaction order with respect
to 2-MCO concentration was one, while at higher concentration the reaction order was lower.
3
.3.2. Application to related substrates
The H PW O catalyst system was applied to various related substrates, and the results are
3
12 40
shown in Tables 4 and 5. The reaction of guaiacol hardly proceeded (Table 4, entry 2). Next, the
reaction of 2-HCO was conducted because 2-HCO may be an intermediate in the reaction of 2-MCO
(Table 4, entries 3‒5). The dependence of 2-HCO concentration was also investigated because 2-
HCO concentration in the reaction of 2-MCO should be very low. Although the conversion of 2-HCO
was very high, the AA selectivity was low and did not depend on the concentration. These results
indicated that 2-HCO was not an intermediate and AA was produced by direct cleavage of 2-MCO.
The reactions of trans-1,2-cyclohexanediol, cyclohexanone and methoxycyclohexane hardly
proceeded (Table 4, entries 6‒8). The presence of both ketone and methoxy/hydroxyl groups was
essential in the reactivity. In addition, the reactions of 3-hydroxy-2-butanone, hydroxyacetone,
methoxyacetone and methoxyacetic acid which have both of ketone and hydroxyl/methoxy groups
were tested (Table 5). The reaction of 3-hydroxy-2-butanone proceeded to some extent and 48%-C
1
4

Page 15 of 41
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yield of acetic acid was obtained (Table 5, entry 1). While the reaction of hydroxyacetone also
DOI: 10.1039/D0GC01277G
proceeded to some extent, the conversion was low and selectivities to target products (formic acid
and acetic acid) were not high (Table 5, entry 2). The reactivity of methoxyacetone was lower than
that of hydroxyacetone (Table 5, entry 3 and 4); while the selectivities to target products (formic acid
and acetic acid as well as methanol derived from the methoxy group) were higher than the case of the
reaction of hydroxyacetone. These lower reactivities and higher selectivities of methoxyacetone than
hydroxyacetone were similar to the case of 2-MCO/2-HCO. The reaction of methoxyacetic acid did
not proceed at all (Table 5, entry 5). These results indicated that H PW O catalyst system is
3
12 40
effective to the oxidative C‒C cleavage of -methoxyketones and less effective to that of -
hydroxyketones.
3
.3.3. Effect of the addition of radical inhibitor
The addition of 2,6-di-tert-butyl-p-cresol as a radical inhibitor to 2-MCO oxidation with
H PW O was tested to determine whether or not this oxidation proceeds by auto-oxidation
3
12 40
mechanism where free organic radicals unbound from catalyst react with another substrate molecule
and/or O biradical. As shown in Table 6, addition of radical inhibitor did not affect both conversion
2
and selectivity (entries 2 and 3). This result suggested another mechanism than auto-oxidation
involving free radicals.
3
.3.4. Reactivity of reduced heteropolyacid
The reduced form of aqueous H PW O (H PW O ) was synthesized by reduction of
3
12 40
4.3
12 40
H PW O solution with Pt/C catalyst and H . The solution was used for oxidation of 2-MCO under
3
12 40
2
standard reaction conditions (0.8 MPa O ). The “heteropoly blue” color disappeared even when the
2
1
5

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solution was heated to the reaction temperature (353 K), and the reaction result after 24 h was
DOI: 10.1039/D0GC01277G
3
1
essentially the same to the standard run with H PW O catalyst (Table 6, entry 4). The P NMR of
3
12 40
the reaction solution of reduced heteropolyacid catalyst showed preservation of the structure of
3
-
PW O . These data mean that the H PW O catalyst can be easily reoxidized with O even when
1
2
40
3
12 40
2
reduced. On the other hand, as described in section 3.2.1, the amount of reduced heteropolyacid was
increased very slowly under O -free conditions: x=0.21 and 0.37 as H PW O at 6 h and 24 h,
2
3+x
12 40
respectively. This means that only 1-3% of H PW O was reduced to H PW O per 1 h reaction
3
12 40
4
12 40
-1
with substrate. The reaction rate was much lower than the TOF value (0.8 h ) of AA formation in the
standard reaction conditions. This difference indicates that direct involvement of free reduced
heteropolyacid (H PW O ) in the reaction cycle is not plausible. The formation of free H PW O
4
12 40
4
12 40
is a side reaction in the oxidation of 2-MCO to AA; however, this reaction is not a large problem
because it is much slower than the main reaction and the formed reduced H PW O is rapidly
4
12 40
reoxidized with O in the standard reaction conditions.
2
3
.3.5. Proposed reaction mechanism
The reaction kinetics showed that the reaction orders with respect to H PW O catalyst, 2-
3
12 40
MCO and O were one, one and zero, respectively (section 3.3.1). These kinetics suggest that the 2-
2
MCO is first activated by the reaction with H PW O catalyst, and this is the rate-determining step.
3
12 40
The activated substrate then reacts with O . Considering the high reactivity of the intermediate with
2
O , we think that the radical formed by one-electron oxidation of 2-MCO with H PW O is the
2
3
12 40
intermediate. However, as discussed in section 3.2.1 and 3.3.4, the formation of dissolved reduced
heteropolyanion (PW O404-) as a free form is very slow. We think that the activated substrate radical
1
2
4
-
is attached to PW O formed by the one-electron reduction. However, there is no strong interaction
1
2
40
1
6

Page 17 of 41
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between heteropolyanion and neutral radical. The softness of heteropolyacids can be a key;
DOI: 10.1039/D0GC01277G
heteropolyanions have general ability to stabilize organic cations, which is assumed to play an
6
3
important role in the organic reactions. We think that the radical cation which is first formed by
electron abstraction from the substrate is stabilized and involved in the catalysis. The proposed
reaction mechanism is shown in Fig. 6. First, 2-MCO is oxidized by one-electron by H PW O to
3
12 40
the radical cation of the enol form of 2-MCO (step (i)). This is the rate-determining step. The organic
4
-
cation is paired with PW O based on the softness of heteropolyanions. The acidity of the reaction
1
2
40
4
-
media is necessary to form the organic cation. The pairing with reduced PW O can decrease the
1
2
40
reactivity of the radical with radical inhibitor which is a one-electron reducing agent. On the other
hand, the radical cation can have high reactivity with one-electron oxidant. Then, O is added to the
2
carbon atom bonded with the methoxide group (step (ii)). The formed superoxide radical rapidly
reacts with the reduced heteropolyacid (H PW O ), which has been attached to the activated
4
12 40
substrate, to form the hydroperoxide and oxidized H PW O (step (iii)). This step is a kind of
3
12 40
intramolecular process, and it is faster than the reaction of the superoxide radical with an externally
added radical inhibitor, leading to the absence of addition effect of radical inhibitor (section 3.3.3).
The C‒C bond in vicinal hydroperoxo keto structure is cleaved by Grob-type fragmentation (step (v))
8
7
after addition of water to keto group (step (iv)). The dissociation of C-C bond concerted with O-O
cleavage of connected peroxo group via Grob-type fragmentation has been reported for Fe- and Cu-
catalyzed systems.88-90 Monomethyl adipate is formed by the fragmentation, and it is easily
hydrolyzed in acidic water to form final product adipic acid (step (vi)). In this mechanism, the key
step is clearly the formation of radical cation (step (i)). In the case of 2-HCO oxidation, the product
of step (iii) is 2-hydroxy-2-hydroperoxycyclohexanone which can be decomposed to 1,2-
cyclohexanedione and H O . The large amount of by-products in 2-HCO oxidation can be derived
2
2
1
7

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from oxidation of 1,2-cyclohexanedione. We carried out DFT calculation of the formation energy of
DOI: 10.1039/D0GC01277G
radical cation of various molecules, i.e. energy change by one-electron oxidation (Table 7). The
radical cation of the enol form of 2-MCO can be favorably formed, according to the low ΔE (+694
-1
-1
kJ mol ). This value is even lower than that of 2,6-di-tert-butyl-p-cresol (+706 kJ mol ) which is a
well-known one-electron reducing agent and works as a radical inhibitor. The calculated low
formation energy of radical cation supported the involvement of the radical cation in the oxidation
catalysis. The calculated structure of the radical cation of 2-methoxycyclohexanone is shown in Fig.
7
. The cation is stabilized by the electron-donating ‒OH and ‒OCH groups. The unpaired electron
3
is mainly present as πC‒C bond (Fig. 7, (b)), and O will attack the unpaired π electron to form a C‒O
2
bond.
4
. Conclusions
H PW O can selectively convert 2-methoxycyclohexanone (2-MCO) to adipic acid (AA)
3
12 40
and methanol with O as an oxidant in water solvent. The carbon-based yield of AA reached 74%-C.
2
The mol-based yield of AA was 86% and this value is much higher than the diester yield in a previous
report47 using phosphomolybdovanadic acid catalyst and methanol solvent (52%). Vanadium
catalysts such as V O and phosphomolybdovanadic acid in our conditions using water solvent
2
5
showed lower selectivity to AA. Significant amount of CO was produced with both vanadium-based
2
catalysts because of the high activity of vanadium as oxidation catalysts. In addition, catalyst
deactivation was observed for phosphomolybdovanadic acid. In contrast, the H PW O catalyst was
3
12 40
stable during the reuse experiment. The produced AA was easily separated from the reaction solution
by crystallization. This catalytic system is also active in the oxidative cleavage of -hydroxyketones;
however, the selectivity to the corresponding acids is lower than the case of -methoxyketones.
1
8

Page 19 of 41
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Cyclohexanone (a simple ketone), methoxycyclohexane (a simple methoxide), trans-1,2-
DOI: 10.1039/D0GC01277G
cyclohexanediol (a vicinal diol), guaiacol and methoxyacetic acid (an -methoxycarboxylic acid) are
not reactive. The catalysis followed the kinetics of first orders on substrate (2-MCO) and catalyst
(
H PW O ) concentrations and zero order on O pressure. These kinetics suggest that the substrate
3 12 40 2
is first activated on the catalyst as the rate-determining step and then reacts with O molecule. The
2
mechanism which is different from typical auto-oxidation involving free radicals is also supported by
the negligible effect of the addition of radical inhibitor (2,6-di-tert-butyl-p-cresol) to the system. We
3
-
propose that the first step is the one-electron oxidation of 2-MCO with PW O to form the ion pair
1
2
40
of radical cation of the enol form of 2-MCO and PW O404-, considering that 2-
1
2
4
-
hydroxycyclohexanone (2-HCO; hydrolysis product of 2-MCO) and free PW O are suggested not
1
2
40
to be intermediates. This new reaction not only enables efficient and safe production of AA with
minimum amount of H from biomass but also enlarges the catalytic chemistry using heteropolyacids.
2
Conflicts of interest
ꢀ
There are no conflicts of interest to declare．
Acknowledgement
This work is supported by JSPS KAKENHI 18H05247.
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6. I.A. Weinstock, R.E. Schreiber and R. Neumann, Chem. Rev. 2018, 118, 2680−2717.
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Green Chemistry
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2
5

Green Chemistry
Page 26 of 41
OH
1
O H
H H H
View Article Online
DOI: 10.1039/D0GC01277G
H 6
O
2
OH
5
HO
H
3 H
H H H H
4
O
H
2
3 4 5 6 1
Guaiacol
Adipic acid
Scheme 1. Conversion of guaiacol to adipic acid.
2
6

Page 27 of 41
Green Chemistry
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DOI: 10.1039/D0GC01277G
Table 1. 2-MCO oxidation with various catalysts.
Entry Catalyst
Amount/ µmol)
Reaction Conv. Yield /%-C
time /h /%
AA Carbon
sel. balance
(
AA GA SA FA MeOH Others CO
2
/
%-C /%
1
2
3
4
5
Blank
24
24
24
72
24
7
71
31
93
77
1.9 0.2 <0.1 0.1
1.2
3.9
3.7
13
7.0
16
0.1 26
6.2 44
3.6 37
96
97
H
V
V
V
5
2
2
2
PV
2
W
10
O
40 (55)
31
2.6
1.4
5.0
0.8
3.2 3.9 11
O
O
O
5
5
5
(55)
(55)
11
30
46
1.9 1.8
105
92
9.3 6.0 11
0.5 1.2 11
17
32
(55) + H
(55) + H
(55) + H
3
3
4
PMo12
O
40
14
3.1 60
100
(110)
6
7
V
2
O
5
PW12
O
40
24
24
79
84
36
23
1.9
3.7
1.6 2.7 11
9.6 8.1 12
1.2 1.3 10
21
5.8 45
98
90
(110)
V
2
O
5
SiW12
O
40
8.7 19
28
(110)
8
9
1
1
1
H
H
H
H
H
4
3
4
2
2
PVW11
PW12
SiW12
O
40 (110)
40 (110)
40 (110)
24
24
24
24
24
75
60
38
6
46
45
21
1.5
1.1
1.0
12
2.7 61
0.2 74
0.8 56
0.3 19
0.2 46
96
O
0.2 0.4
0.3 0.7
9.7
3.8
97
0
1
2
O
7.9
1.3
2.9
5.9
2.8
5.4
108
WO
4
(1320)
1.2 0.1 <0.1 0.7
8.2 0.6 0.1 0.2
98
96
SO
4
(165)
18
Reaction conditions: 2-Methoxycyclohexanone (2-MCO) 4.3 mmol, catalyst 0-1320 µmol, water 10 g, O
t.), 353 K, 24 or 72 h. AA: adipic acid, GA: glutaric acid, SA: succinic acid, FA: formic acid.
2
0.8 MPa (at r.
2
7

Green Chemistry
Page 28 of 41
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DOI: 10.1039/D0GC01277G
Table 2. Effect of O pressure in 2-MCO oxidation with H PW O catalyst.
2
3
12 40
Entry O pressure Conv. Yield /%-C
AA sel. Carbon
/%-C balance
2
/
MPa
/%
2
-
AA
GA
SA
FA
MeOH Others CO2
/
%
HCO
1
2
3
4
5
0.1
55
56
57
60
6
<0.1 40
<0.1 38
<0.1 41
<0.1 45
0.4
0.7
1.1
1.1
0.1
0.2 6.8
0.2 7.9
0.4 8.1
0.4 9.7
6.9
8.9
6.1
3.8
1.8
0.1
0.3
0.4
0.2
0.1
73
67
72
74
95
0.3
0.1
0.2
0.2
108
103
97
0.5
0.8
0 (N2)
2.3
0.5 <0.1 <0.1 <0.1 1.2
8.5
96
Reaction conditions: 2-methoxycyclohexanone (2-MCO) 4.3 mmol, H PW O 110 µmol, water 10 g, O 0.1-
3
12 40
2
0
.8 MPa (7-60 mmol) or N 0.8 MPa (at r. t.), 353 K, 24 h. 2-HCO: 2-hydroxycyclohexanone, AA: adipic acid,
2
GA: glutaric acid, SA: succinic acid, FA: formic acid.
2
8

Page 29 of 41
Green Chemistry
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DOI: 10.1039/D0GC01277G
Table 3. Effect of reaction temperature in 2-MCO oxidation with H PW O catalyst.
3
12 40
Entry Temp. /K Reaction Conv. Yield /%-C
AA sel. Carbon
/%-C balance
time /h
/%
AA
GA
SA
FA
MeOH Others CO2
/
%
1
2
3
4
5
323
323
353
353
383
24
72
7
3.2 <0.1 <0.1 <0.1
1.1 2.3
2.5 2.4
9.7 3.8
<0.1 48
96
96
97
94
92
17
60
12
45
74
65
0.1 <0.1
0.1
0.4
0.1 70
0.2 74
1.2 75
3.7 65
24
1.1
2.8
4.8
0.2
0.5
1.3
120
24
98
1.0 13
1.9 14
6.1
9.3
>99
Reaction conditions: 2-methoxycyclohexanone (2-MCO) 4.3 mmol, H PW O 110 µmol, water 10 g, O 0.8
3
12 40
2
MPa. AA: adipic acid, GA: glutaric acid, SA: succinic acid, FA: formic acid.
2
9