Solvolysis of benzyl phenyl ether in high-temperature aqueous methanol solution under high-pressure carbon dioxide

Article abstract of DOI:10.1039/d0gc04008h

Alcoholysis of benzyl phenyl ether to various aromatic compounds was studied in high-temperature aqueous methanol solution under high-pressure carbon dioxide conditions. The products formed included benzyl methyl ether, benzyl alcohol, phenol and toluene. As high as 70.7 ± 0.2% yield of monocyclic aromatic hydrocarbon with 16.1 ± 0.3% yield of benzyl methyl ether was obtained by treating benzyl phenyl ether with an aqueous solution with 0.2 molar fraction of methanol, under 17.7 MPa carbon dioxide for 1 h at 573 K.

Full text of DOI:10.1039/d0gc04008h

Showcasing research from Professor Shirai's laboratory,
School of Chemistry, Iwate University, Morioka, Japan.
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Volume 23
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21 February 2021
Pages 1391-1884
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Solvolysis of benzyl phenyl ether in high-temperature
aqueous methanol solution under high-pressure
carbon dioxide
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Monocyclic aromatic hydrocarbons (benzyl methyl ether,
benzyl alcohol and phenol) were obtained by treatment
of benzyl phenyl ether in aqueous methanol solution. The
highest yield of monocyclic aromatic hydrocarbons was
obtained in aqueous methanol solution with 0.2 methanol
molar fraction under 17.7 MPa of carbon dioxide at 573 K. The
alcoholysis to benzyl methyl ether did not proceed in pure
methanol. High-temperature carbonated water is effective
for the alcoholysis.
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Solvolysis of benzyl phenyl ether in high-
temperature aqueous methanol solution
under high-pressure carbon dioxide
Cite this: Green Chem., 2021, 23,
1658
Kenkichi Taniguchi,a Hidetaka Nanao,a Osamu Sato,b Aritomo Yamaguchi b and
a,b
Masayuki Shirai
*
Alcoholysis of benzyl phenyl ether to various aromatic compounds was studied in high-temperature
aqueous methanol solution under high-pressure carbon dioxide conditions. The products formed
included benzyl methyl ether, benzyl alcohol, phenol and toluene. As high as 70.7 0.2% yield of mono-
Received 25th November 2020,
Accepted 12th January 2021
cyclic aromatic hydrocarbon with 16.1
0.3% yield of benzyl methyl ether was obtained by treating
DOI: 10.1039/d0gc04008h
benzyl phenyl ether with an aqueous solution with 0.2 molar fraction of methanol, under 17.7 MPa carbon
dioxide for 1 h at 573 K.
rsc.li/greenchem
However, acid catalysts are needed for alcoholysis. Güvenatam
et al. showed that Lewis acid catalysts such as iron, copper,
Introduction
Lignin is a major component of lignocellulosic biomass and nickel and aluminium chloride were active in ethanol for the
has a structure in which aromatic monomers are randomly depolymerization of lignin model compounds to obtain mono-
bonded in various types of linkages. There are many reports on aromatic hydrocarbons; however, the alcoholysis reaction did
the catalytic decomposition of lignin to aromatic compounds by not proceed in ethanol solvent with Lewis acid catalysts.15 It has
the cleavage of ether linkages in the lignin structure, using been reported that the alcohol–water co-solvent (aqueous
heterogeneous and homogeneous catalysts.1–10 Because ligno- (methanol or ethanol) solution) was effective for the biomass
cellulosic biomass contains a lot of water, the partial decompo- direct liquefaction to fuel at a high temperature around 573 K
sition of lignin in water is desirable for saving energy. (ref. 16) but it has not been reported for the production of
Hydrolysis of the ether linkage of lignin proceeds in high-temp- chemicals without using heterogeneous catalysts.
erature water; however, the subsequent dehydration of these
High-temperature carbonated water, which could be obtained
products having several hydroxyl groups immediately occurs to using high-temperature water under high-pressure carbon
produce char due to repolymerization.11 Not only the develop- dioxide, is a green acidic solvent, because the neutralization of
ment of active catalysts, but also the optimization of reaction the system can be achieved merely by cooling and depressuriza-
conditions such as solvents should be done.6,12 To obtain aro- tion, without a base.17–25 The types of ether linkages in lignin
matic compounds with high yields from lignin in water, chemi- depend on the variety of wood and are classified in terms of
cal transformation of lignin to small and stable molecules is β-O-4 (43–50%), α-O-4 (6–8%), β-5 (9–12%), 5-5 (10–25%), 4-O-5
needed. Hydrolytic cleavage of aromatic C–O bonds in aromatic (4%), β-1 (3–7%) and β–β (2–4%).1 Among these linkages, α-O-4
ether molecules proceeded at around 473 K with gaseous hydro- bonds could be dissociated with the lowest energy.26
gen13 and 2-propanol;14 however, supported noble metal cata-
Here, we studied alcoholysis of benzyl phenyl ether, which
lysts are dispensable. Alcoholysis of lignin, in which lower is a simple model compound having a lignin α-O-4 bond, in
alcohol molecules react with the ether linkage in lignin to form water, methanol and aqueous methanol solution. We found
the corresponding valuable aryl ethers, is a feasible way to that the alcoholysis to benzyl methyl ether in addition to
obtain stable aromatic compounds in an aqueous solution. hydrolysis proceeded in aqueous methanol solution and that
the yield of benzyl methyl ether was enhanced in the presence
of high-pressure carbon dioxide.
aChemistry Course, Department of Chemistry and Biological Sciences, Faculty of
Science and Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551,
Experimental
Japan. E-mail: mshirai@iwate-u.ac.jp
bResearch Institute for Chemical Process Technology, National Institute of Advanced
Benzyl phenyl ether (Tokyo Chemical Industry Co., Ltd), benzyl
methyl ether (Wako Pure Chemical Industries), benzyl alcohol
Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino, Sendai, Miyagi
983-8551, Japan
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(Wako Pure Chemical Industries), phenol (Wako Pure
Chemical Industries), toluene (Wako Pure Chemical
Industries), anisole (NACALAI TESQUE, INC.), and n-butanol
(Wako Pure Chemical Industries) were purchased and used
without any further purification. The solvolysis reaction of
benzyl phenyl ether was carried out in a batch reactor (inner
volume: 6.0 cm3) made of a SUS316 tube.27,28 After benzyl
phenyl ether (0.10 mmol), water (3.0–0 mL (0.2–0 mol)), and
methanol (0–3.0 mL (0–0.07 mol)) were loaded into the
reactor, the air inside was removed with nitrogen gas. Carbon
dioxide (8–12 MPa) was fed to the reactor at 323 K. The partial
pressure of carbon dioxide in the reactor at 573 K was esti-
mated to be 14.2, 17.7 and 21.3 MPa, based on the equation of
Charles’s law, corresponding to the initial pressure of 8, 10
and 12 MPa at 323 K, respectively. The reactor was then sub-
merged in a sand bath at 573 K for a given reaction time and
then submerged in a water bath for cooling quickly to ambient
temperature after the reaction. The carbon dioxide gas in the
reactor was removed by depressurization at room temperature.
A mixture of the reactant and liquid products after the reaction
was taken out from the reactor using n-butanol. The quantitat-
ive analysis of the reactant and liquid products was conducted
by gas chromatography with a flame ionization detector
(GC-FID) equipped with a DB-WAX capillary column (Agilent
Technologies) using tetrahydrofurfuryl alcohol (Wako Pure
Chemical Industries) as an internal standard. The products
were identified by their retention time in the GC-FID analysis,
compared with those for known materials: benzyl phenyl
ether, benzyl methyl ether, benzyl alcohol, phenol, toluene
and anisole. The conversion and yield of products were calcu-
lated as follows:
Yieldð%Þ ¼ ððMol of products formedÞꢁ
ðThe number of benzene ring in productsÞÞ=
ððMol of benzyl phenyl ether introducedÞ ꢁ 2Þ ꢁ 100:
The recovery was calculated based on the number of
benzene rings in the products as follows:
Recoveryð%Þ ¼ ðAmount of benzene rings in productsÞþ
2 ꢁ ðUnreacted benzyl phenyl etherÞ=
2 ꢁ ðThe initial amount of benzyl phenyl etherÞ:
The experimental error was obtained by reproducing the
same experiment two times.
Results and discussion
Solvolysis reaction of benzyl phenyl ether in aqueous
methanol solution
Benzyl phenyl ether was treated in aqueous methanol solution
for 30 min at 573 K in a batch reactor. The conversion and
product yields as a function of the molar fraction of methanol
are shown in Fig. 1. In water, benzyl phenyl ether was decom-
posed, and the conversion was as high as 98.3 0.6%. Benzyl
alcohol and phenol were obtained; however, their yields were
5.9 1.1 and 6.5 1.0%, respectively. Güvenatam et al. studied
the decomposition behaviour of benzyl phenyl ether in high-
temperature liquid water and showed that benzyl phenyl ether
underwent the acid-catalyzed hydrolysis of the ether bond to
form benzyl alcohol and phenol, which are the main monocyc-
lic aromatic products obtained at 573 K; however, the yields
were very low.15 He et al. also reported that the mono-aromatic
oxygenated products formed were proved to undergo self- and
cross-condensation reactions to form di-aromatic and high-
molecular weight compounds.29 We analyzed total organic
compounds (TOC) in the recovered solution after the treatment
Conversionð%Þ ¼
ð1 ꢀ ðThe amount of unreacted benzyl phenyl etherÞ=
ðThe initial amount of benzyl phenyl etherÞÞ ꢁ 100;
Fig. 1 Effect of the molar fraction of methanol on the conversion and the yields of aromatic products for the solvolysis reaction of benzyl phenyl
●
ether. Benzyl phenyl ether conversion (a), aromatic product yield (b and c), conversion ( ), benzyl methyl ether ( ), benzyl alcohol ( ), phenol ( ),
▼
toluene ( ), benzyl methyl ether + benzyl alcohol + toluene ( ), temperature 573 K, reaction time 30 min, water : methanol = 3.0 : 0.0–0.0 : 3.0
(mL : mL), 0.2–0 : 0–0.07 (mol : mol).
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of benzyl phenyl ether in pure water for 30 min at 573 K,
which is the subcritical condition of water : methanol = 3 : 0 in
Fig. 1. 20% carbon yield was obtained in the solution, indicat-
ing that the decomposition of benzyl phenyl ether proceeded
in liquid water at 573 K, followed by polymerization of the
decomposition products to water-soluble compounds. We
could not identify the water-soluble compounds in detail.
Decomposition of benzyl phenyl ether proceeded in high-
temperature liquid water; however, the subsequent repolymeri-
zation resulted in low yields of aromatic compounds.
In methanol at 573 K (supercritical conditions), the conver-
sion was as low as 10.0 4.7% with the formation of several
aromatic products such as benzyl methyl ether, benzyl alcohol,
phenol, and toluene; however, the yields were also as low as
0.4 0.2, 0.3 0.0, 4.7 0.0 and 3.4 0.1%, respectively, while
anisole was not detected. Similar results were reported by
Yokoyama et al. who studied the reaction behaviour of benzyl
phenyl ether in supercritical methanol and found that metha-
nolysis of benzyl phenyl ether hardly proceeded and toluene
and phenol were formed by thermolysis.30 Güvenatam et al.
studied the decomposition behaviour of benzyl phenyl ether in
methanol at 573 K (under supercritical conditions) and
reported that toluene and phenol were obtained as monoaro-
matic compounds.15 Toluene would be formed through the
hydrogenolysis of benzyl phenyl ether by hydrogen derived
Scheme 1 Plausible reaction pathway for the decomposition of benzyl
phenyl ether.
however, benzyl methyl ether was formed in an aqueous
methanol solution. Reaction paths (a), (b) and (c) proceed in
aqueous methanol solution. Acid catalysts are needed for the
alcoholysis; then a probable explanation for the methanolysis
in aqueous methanol solution is that protons derived from
water provided Brønsted acid sites. We discussed later the role
of protons in the section of high-pressured carbon dioxide
(Scheme 2).
Benzyl alcohol was formed but anisole was not formed in
aqueous methanol solution at 573 K. The benzyl cation is
more stable than the phenyl cation; hence, anisole would not
be formed. Also, the benzyl cation could react with methanol
to produce benzyl methyl ether in aqueous methanol solution.
The yield of phenol was almost the same as the sum of
benzyl methyl ether + benzyl alcohol + toluene (Fig. 1(c)), indi-
cating that hydrolysis (Scheme 1(a)), methanolysis
(Scheme 1(b)), and hydrogenolysis (Scheme 1(c)) reactions
proceed under the conditions of the present work. In a control
experiment, the effect of sulfuric acid on the solvolysis of
benzyl phenyl ether in aqueous methanol solution was
assessed and it was found that the methanolysis reaction of
benzyl phenyl ether and its hydrogenolysis to toluene did not
proceed at room temperature under acidic solution conditions
(Table 1, entry 7).
from methanol, which is
a
hydrogen donor solvent.
Mauriello’s group studied the catalytic conversion of aromatic
ethers over Pd/C and Ru/C in hydrogen donor solvents (metha-
nol, ethanol and 2-propanol) at 493 K and showed that hydro-
genolysis to the corresponding aromatic products proceeded
by hydrogen from the solvent in the first step of the
conversion.14
In aqueous methanol solution, the conversion decreased
with an increase in methanol molar fraction (Fig. 1(a)). Benzyl
methyl ether, benzyl alcohol, phenol, and toluene were
formed; however, anisole was not formed. Saturated cycloalk-
anes such as cyclohexanol were not formed, indicating that the
reductive cleavage of benzyl phenyl ether did not proceed
without supported metal catalysts. The yields of monocyclic
aromatic hydrocarbons depended on the methanol molar frac-
tion. The yields of benzyl alcohol, phenol and benzyl methyl
ether increased with an increase in methanol molar fraction
and became maximum at 0.2 molar fraction (2.0 mL water and
1.0 mL methanol at room temperature) and then decreased to
zero at 0.5 molar fraction concentration of methanol. Fig. 1
shows that both methanol and water are needed for the metha-
nolysis reaction to produce benzyl methyl ether. Toluene was
not formed in water; however, it formed in aqueous methanol
solution and the yield increased with an increase in the molar
fraction of methanol, indicating that methanol worked as a
hydrogen donor.
Scheme 1 shows the proposed reaction paths for solvolysis
and hydrogenolysis of benzyl phenyl ether in an aqueous
methanol solution. Hydrolysis of benzyl phenyl ether to benzyl
alcohol and phenol proceeded in water (Scheme 1, path (a)).
Scheme 2 Plausible reaction scheme for methanolysis of benzyl
phenyl ether in aqueous methanol solution in the presence of carbon
Benzyl methyl ether was not formed in the methanol solvent; dioxide.
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An aqueous methanol solution with 0.2 molar fraction of
methanol was effective for the solvolysis of benzyl phenyl ether
at 573 K, as shown in Fig. 1. We tested the temperature effect
on the solvolysis yield in an aqueous methanol solution with
0.2 molar fraction of methanol. The reaction time shown in
Table 1 is 15 min to study the initial reaction rate. The conver-
sion increased; however, the recovery decreased with an
increase in temperature, which would be caused by the
decomposition of benzyl phenyl ether and the following
polymerization. The solvolysis yield (benzyl methyl ether +
benzyl alcohol) was 4% at 523 K and increased to 17% at
573 K; however, it decreased to 13% at 623 K, and then we
focused on the study of benzyl phenyl ether solvolysis in
aqueous methanol solution at a methanol molar fraction of
0.2, at 573 K. The reaction profile in aqueous methanol solu-
tion (Fig. 2 open key and dotted line) shows that the conver-
sion increased from 59.0 0.0 to 87.7 3.9% with an increase
in reaction time and became constant at 90%. Various aro-
matic products formed were benzyl methyl ether, benzyl
alcohol, phenol, and toluene but anisole was not detected. The
yields of benzyl methyl ether, benzyl alcohol, phenol, and
toluene also increased with an increase in reaction time and
were 8.4 0.9, 14.8 1.1, 27.0 2.2, and 2.9 0.2% at 60 min,
respectively. The amount of phenol was almost the same as
the sum of benzyl methyl ether + benzyl alcohol + toluene
during the reaction (Fig. 2(c)), indicating that methanolysis
(path (b) in Scheme 1) and hydrolysis (a), which are catalyzed
by protons derived from high-temperature water, and hydroge-
nolysis (c) proceeded.
Effect of high-pressure carbon dioxide on solvolysis
The addition of carbon dioxide is highly beneficial for acid
catalyzed reactions in high-temperature liquid water.17–25
Benzyl phenyl ether was treated in aqueous methanol solution
under 17.7 MPa of carbon dioxide at 573 K (Fig. 2, filled keys
and solid line). The conversion of benzyl phenyl ether
increased with an increase in reaction time and the final con-
version at 180 min in the presence of 17.7 MPa of carbon
dioxide was 93.4 0.5%, which was almost the same as 87.7
3.9% in the absence of carbon dioxide (Fig. 2(a)). Benzyl
methyl ether, benzyl alcohol, phenol and toluene were formed
as aromatic hydrocarbons, but anisole was not observed,
which is the same result obtained without the addition of
carbon dioxide. The amount of phenol was almost the same as
the sum of benzyl methyl ether + benzyl alcohol + toluene
(Fig. 2(c)), indicating that methanolysis, hydrolysis and hydro-
genolysis proceeded in aqueous methanol solution under 17.7
MPa of carbon dioxide. Also, it is noteworthy that the aromatic
compound yield at 60 min under 17.7 MPa of carbon dioxide
(70.7 0.2%) was higher than that in the absence of carbon
dioxide (53.1 4.4%), indicating that the side reactions such
as self- or cross-condensation were suppressed in the presence
of high-pressure carbon dioxide. The addition of carbon
dioxide was effective for the solvolysis of benzyl phenyl ether
to aromatic compounds. Under high-pressure carbon dioxide,
carbonic acid is formed, and the number of protons increased
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Fig. 2 Effect of high-pressure carbon dioxide on the solvolysis reaction of benzyl phenyl ether. 0 MPa (open symbol), 10 MPa (closed symbol),
● ○
), phenol ( ,
benzyl phenyl ether conversion (a), aromatic product yield (b and c), conversion (
,
), benzyl methyl ether (
,
), benzyl alcohol (
,
▼
▽
), benzyl methyl ether + benzyl alcohol + toluene (
), toluene (
(mol : mol).
,
,
), temperature 573 K, water : methanol = 2.0 : 1.0 (mL : mL), 0.11 : 0.02
by the dissociation of carbonic acid. Benzyl methyl ether was The reactions were tested for 60 min to study the stability of
formed in high-temperature aqueous methanol and the yield substrate in aqueous methanol solution in the absence and
was enhanced by the addition of high-pressure carbon dioxide, presence of carbon dioxide. The reaction time in Table 2 is
indicating that the methanolysis proceeded by protons. The 60 min and is different from that in Table 1 (15 min). The reac-
plausible mechanism to produce benzyl methyl ether from tion was tested two times and the average values are shown.
benzyl phenyl ether in high-temperature aqueous methanol
When benzyl methyl ether was treated in an aqueous
solution and/or in the presence of high-pressure carbon methanol solution, 12.4 3.5% of benzyl alcohol was obtained
dioxide is shown in Scheme 2. Protons formed by the dis- and the recovered monomer yield (benzyl methyl ether, benzyl
sociation of water and by dissociation of carbonic acid would alcohol, phenol, and toluene) was as high as 88.7
2.2%
attack the oxygen atom in benzyl phenyl ether to produce a (Table 2, entry 1). The hydrolysis was found to be promoted
protonated intermediate followed by the formation of phenol and 25.2 0.6% of benzyl alcohol was obtained with a recov-
and the benzyl cation. A methanol molecule could react with ered monomer yield of 86.9 0.6% when 17.7 MPa of carbon
the cation to produce benzyl methyl ether and protons dioxide was added in aqueous methanol solution (Table 2,
through a protonated intermediate.
entry 2). When benzyl alcohol was treated in aqueous metha-
The effect of carbon dioxide pressure on aromatic yields nol solution, dehydration between benzyl alcohol and metha-
was also studied. Table 1 shows that the yields of aromatic nol proceeded in aqueous methanol solution and 9.5 0.6%
compounds increased with an increase in carbon dioxide of benzyl methyl ether was obtained (Table 2, entry 3). The de-
pressure (entries 4–6). It is reported that carbon dioxide dis- hydration yield was improved to 43.0 0.4% by the addition of
solved in water produces carbonic acid and the amount of dis- carbon dioxide (Table 2, entry 4). More than 95% of phenol
solved carbon dioxide and protons from carbonic acid was recovered in aqueous methanol solution both with and
increased with an increase in carbon dioxide pressure.21,22 The without 17.7 MPa of carbon dioxide (Table 2, entries 5 and 6),
number of protons increased with an increase in carbon indicating that phenol is stable in high-temperature aqueous
dioxide pressure and the alcoholysis reaction of benzyl phenyl methanol solution.
ether to benzyl methyl ether would be enhanced by the
The stability of the product mixture was also studied. The
increased number of protons with an increase in carbon value of the recovered yield of mixtures of benzyl methyl ether
dioxide pressure.
+ phenol and benzyl alcohol + phenol was high and benzyl
phenyl ether was not formed (Table 2, entries 7–10). Scheme 3
shows the reaction pathways for hydrogenolysis, hydrolysis and
Reaction pathway in aqueous methanol solution
The benzyl phenyl ether solvolysis reaction proceeded in methanolysis of benzyl phenyl ether in aqueous methanol
aqueous methanol solution achieving more than 90% of con- solution under high-pressure carbon dioxide. Accordingly,
version; however, the highest aromatic compound yield (sum benzyl alcohol and phenol are produced from benzyl phenyl
of benzyl methyl ether, benzyl alcohol, phenol, and toluene) ether in the presence of water due to hydrolysis. Benzyl methyl
was 70.7 0.2% at 573 K. The stability of products during the ether is produced by the reaction of benzyl phenyl ether and
solvolysis reaction was studied to understand the reaction methanol (methanolysis) and the reaction pathway is pro-
pathways. Table 2 shows the results of aromatic compounds by moted by the addition of carbon dioxide. Toluene and phenol
the treatment with 0.2 molar fraction of methanol in aqueous are produced by hydrogenolysis, while the introduction of
solution under 17.7 MPa carbon dioxide for 60 min at 573 K. carbon dioxide promotes the methanolysis reaction and the
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Scheme 3 Plausible reaction pathways for hydrogenolysis, methanoly-
sis and hydrolysis of benzyl phenyl ether.
hydrolysis reaction, but it does not promote hydrogenolysis.
Furthermore, benzyl alcohol reacts with methanol in the pres-
ence of carbonic acid to form benzyl methyl ether.
Conclusions
Solvolysis of benzyl phenyl ether as a model compound having
the lignin α-O-4 bond was studied in water, methanol and
aqueous methanol solution at 573 K, under high-pressure
carbon dioxide in a batch reaction system. The following con-
clusions were obtained.
(1) The hydrolysis of benzyl phenyl ether proceeded in
water at 573 K; however, the yield of benzyl alcohol and phenol
was low.
(2) The methanolysis of benzyl phenyl ether did not
proceed but hydrogenolysis to toluene and phenol occurred in
methanol.
(3) Benzyl methyl ether, benzyl alcohol and phenol were
obtained by the treatment of benzyl phenyl ether in an
aqueous methanol solution. The highest yield was obtained in
an aqueous methanol solution with 0.2 molar fraction of
methanol at 573 K.
(4) The total yield of monocyclic aromatic hydrocarbons
(benzyl methyl ether, benzyl alcohol and phenol) in aqueous
methanol solution was enhanced by the addition of carbon
dioxide under high pressure.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was supported by the Japan Society for the
Promotion of Science (JSPS) KAKENHI (grant number
18H03421).
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14 E. Paone, A. Beneduci, G. A. Corrente, A. Malara and
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