Selective production of cyclohexanol and methanol from guaiacol over Ru catalyst combined with MgO

Article abstract of DOI:10.1039/c3gc42322k

Selective demethoxylation from aqueous guaiacol proceeded over Ru catalysts at relatively lower temperatures (≤433 K). Addition of MgO to the reaction media suppressed the unselective C-O dissociation. Cyclohexanol and methanol were obtained in high yield (>80%). A reaction route is proposed where partially hydrogenated guaiacol is decomposed into methanol and phenol, which is further hydrogenated to cyclohexanol. the Partner Organisations 2014.

Full text of DOI:10.1039/c3gc42322k

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Selective production of cyclohexanol and
methanol from guaiacol over Ru catalyst
combined with MgO
Cite this: Green Chem., 2014, 16,
197
2
Yoshinao Nakagawa,* Momoko Ishikawa, Masazumi Tamura and Keiichi Tomishige*
Selective demethoxylation from aqueous guaiacol proceeded over Ru catalysts at relatively lower temp-
eratures (≤433 K). Addition of MgO to the reaction media suppressed the unselective C–O dissociation.
Cyclohexanol and methanol were obtained in high yield (>80%). A reaction route is proposed where
partially hydrogenated guaiacol is decomposed into methanol and phenol, which is further hydrogenated
to cyclohexanol.
Received 11th November 2013,
Accepted 6th January 2014
DOI: 10.1039/c3gc42322k
www.rsc.org/greenchem
Introduction
Much attention has been paid to the concept of biorefinery,
where the renewable biomass-related raw material is converted
1
–8
to valuable chemicals and fuel. Since the oxygen content of
biomass is usually much higher than that for common chemi-
cals and transportation fuel, removal of oxygen with hydrogen
(hydrodeoxygenation) is a key reaction in biorefinery. Further-
more, oxygen-containing chemicals are more attractive targets
than hydrocarbon fuels because a lower amount of hydrogen is
necessary and the market price of products is higher, while
selective partial hydrodeoxygenation to an oxygen-containing
chemical is usually more difficult than total hydrodeoxygena-
3
,6
tion to hydrocarbons.
Guaiacol (2-methoxyphenol) is an
important bio-related compound as one of the components of
bio-oil produced by fast pyrolysis of lignocellulose and as a
1
,9–11
model compound of lignin-derived monomers.
Hydro-
deoxygenation of guaiacol has been studied using convention-
al hydrotreating catalysts such as CoMoS and NiMoS under
1
2–14
15,16
17–19
severe conditions (typically ≥573 K).
Iron,
nickel,
2
0
21
22,23
23,24
25
cobalt, rhenium, tungsten,
molybdenum,
have been also tested by
copper
1
7,24,26–33
and noble metal catalysts
several research groups using a similar high reaction temp-
erature. Reduction of guaiacol produces various compounds
(
(
Fig. 1). Total hydrodeoxygenation gives hydrocarbons
benzene and cyclohexane). Partial hydrodeoxygenation gives
various oxygen-containing compounds: phenol, cyclohexanol,
anisole and methoxycyclohexane. Demethylation reaction pro-
duces catechol and the hydrogenated compound 1,2-cyclo-
hexanediol. The removed methoxy group is converted into
Fig. 1 Typical products of guaiacol reduction.
methanol or methane. Simple hydrogenation of guaiacol can
also proceed, producing 2-methoxycyclohexanol. Although
there are some catalytic systems for the production of hydro-
Department of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07
Aoba, Aramaki, Aoba-ku, Sendai, Japan. E-mail: yoshinao@erec.che.tohoku.ac.jp,
tomi@erec.che.tohoku.ac.jp; Fax: +81 22 795 7214; Tel: +81 22 795 7214
1
7,27
carbons and methanol
or for the production of phenol
2
3–25,33
or cyclohexanol with good yields (<80%),
selective
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formation of both a partial hydrodeoxygenation product and Table 2 pH values of the reaction mixture
1
4
methanol has not been well established. In this study, selec-
tive conversion of guaiacol to cyclohexanol and methanol was
achieved by using Ru catalyst under basic conditions at a rela-
Components of the media
pH
Ru/C
6.0
4.1
10.8
13.0
10.7
8.9
tively lower temperature. The selective removal of methanol Ru/C + H-ZSM-5
Ru/C + MgO
from substrates with both hydroxy and methoxy groups may
Ru/C + NaOH
become a new route for the production of chemicals from
lignocellulosic resources.
Ru/MgO
Ru/C + MgO + guaiacol
Ru/C + MgO + guaiacol (after reaction)
9.9
The amount of each compound was the same as that shown in
Table 1.
Results and discussion
Optimization of catalysts
First, various carbon-supported noble metal catalysts were the methane formation (entries 6 and 7). The pH values before
tested for the reduction of aqueous guaiacol with H
at 433 K and after the reaction (Table 2) confirmed that the reaction
2
in a batch reactor (Table 1, entries 1–4). Both hydrogenation proceeded under basic conditions, while without a base the
and hydrodeoxygenation products were formed such as reaction media was slightly acidic. The amount of C1 products
2
-methoxycyclohexanol, cyclohexanol and cyclohexane. The + methoxy groups in the reaction mixture was almost the same
methoxy group that was removed during the formation of as that of loaded guaiacol, indicating that addition of a base
cyclohexanol was mainly observed as methanol. The highest suppressed the cracking reaction. We also prepared the Ru/
yield of cyclohexanol was obtained over Ru/C catalyst (entry 1), MgO catalyst by the impregnation method and conducted the
although the formation of methane was significantly large. activity test. The dispersion of Ru determined by CO adsorp-
The total amount of methanol + methane + methoxy groups tion (CO/Ru) was similar for both Ru/C and Ru/MgO (0.44 and
(
guaiacol + 2-methoxycyclohexanol) in the reaction mixture 0.36, respectively). Ru/MgO gave product distributions similar
over Ru/C was larger than that of the loaded guaiacol (27% to those obtained by the Ru/C + MgO system (entries 6 and 8).
excess), and this is probably due to the fact that methane was However, the Ru/MgO crumbled during the catalysis into mud,
also formed by a cracking reaction. Rh/C and Pt/C showed and the Ru particles were detached from the support and
some activity in cyclohexanol formation; however, the selectiv- formed Ru black. We selected Ru/C + MgO as an optimum
ities to cyclohexanol were lower than that of Ru/C. Pd/C system because of the high performance and easy handling. It
showed almost no activity in hydrodeoxygenation and the should be noted that the use of a base in hydrodeoxygenation
main products were hydrogenation products (2-methoxycyclo- reactions is relatively rare, in comparison with the use of an
hexanone and 2-methoxycyclohexanol). The Ru/C-catalyzed acid. One of the rare examples of hydrodeoxygenation under
process was modified by addition of co-catalysts, and the basic conditions is selective formation of 1,2-propanediol from
3
4
results are also shown in Table 1 (entries 5–7). The pH values glycerol over metal catalysts. However, the mechanism pro-
of the reaction mixture are shown in Table 2. The addition of posed for this reaction can be only applied to 1,3-disubstituted
acid promotes the total hydrodeoxygenation (entry 5), similarly substrates: dehydrogenation to aldehyde (glyceraldehyde),
2
7
to the previous studies. The addition of a base (MgO or removal of α-hydrogen and β-OH group as dehydration to con-
NaOH) increased the yields of cyclohexanol and suppressed jugated aldehyde (2-hydroxyacrolein), and hydrogenation to
Table 1 Reduction of guaiacol over various catalysts
a
b
Yield (%)
Yield of C1 product (%)
Conv.
(%)
2-Methoxy-
Cyclohexanol cyclohexanol Cyclohexane Others
Entry Catalyst
Methanol
Methane
1
2
3
4
5
6
7
8
Ru/C
Rh/C
Pd/C
Pt/C
>99
>99
82
66
36
1
45
<1
79
77
78
19
26
43
41
6
12
18
14
10
12
1
5
46
7
5
70
66
5
50
69
85
76
83
38
12
<1
2
40
3
c
Others 6, undetected 18
2-Methoxycyclohexanone 33, others 4
4
95
c
Ru/C + H-ZSM-5 >99
Others 5, undetected 44
Ru/C + MgO
Ru/C + NaOH
Ru/MgO
98
>99
98
1
2
2
4
3
2
<1
2
Reaction conditions: guaiacol, 0.5 g; catalyst, 50 mg (5 wt% metal); additive, 50 mg (MgO or H-ZSM-5) or 1 mmol (NaOH); water, 10 g; H ,
a
b
1
(
.5 MPa; 433 K; 2 h. Calculated based on the number of C6 rings.c Calculated based on the number of methoxy groups in the loaded substrate
i.e. [produced methanol or methane]/[loaded guaiacol] × 100 (%)). Conversion (%) − (total yield of detected ≥C6 products (%)).
2198 | Green Chem., 2014, 16, 2197–2203
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product (1,2-propanediol). The mechanism of our system (24 h). A good yield of cyclohexanol and methanol was
should be different from that of glycerol hydrodeoxygenation obtained at 2 h. Phenol and cyclohexanone were detected at
over metal + base catalysts.
short reaction time, suggesting that these compounds were the
intermediates of cyclohexanol formation. The time course was
more complex in the case without MgO (Fig. 2b). In the initial
stage, the main products were cyclohexanol, 2-methoxycyclo-
Effect of reaction conditions
The time courses of reduction of guaiacol over Ru/C with or hexanol and phenol, similarly to the case in the presence of
without MgO are shown in Fig. 2. The reaction proceeded to MgO, although the selectivity to cyclohexanol was lower. Selec-
some extent during the heating to the reaction temperature. tivities to cyclohexanol and 2-methoxycyclohexanol were
Over RuC + MgO (Fig. 2a), yields of cyclohexanol, 2-methoxy- decreased after the guaiacol conversion reached 100%. Large
cyclohexanol and cyclohexane were not changed after the guaia- amounts of cyclohexane and methane were formed at long
col conversion reached 100%. Hydrogenolysis of methanol to reaction time, and methanol was almost completely consumed
methane gradually proceeded, and the selectivity to methanol after 24 h. Formation of 1,2-cyclohexanediol was also charac-
based on the converted methoxy group (= conversion − yield of teristic of the case without MgO. The selectivity to 1,2-cyclohex-
2
-methoxycyclohexanol) decreased from >95% (2 h) to 46% anediol increased at first, indicating that 1,2-cyclohexanediol
was the product of consecutive reactions. Among the products
observed in the initial stage, 2-methoxycyclohexanol can be
the direct source of 1,2-cyclohexanediol. The yield of 1,2-cyclo-
hexanediol decreased at long reaction time, probably because
of the hydrodeoxygenation to cyclohexanol and eventually
cyclohexane. Anyway, without MgO the hydrodeoxygenation of
saturated compounds can proceed over Ru/C. The yield of
cyclohexanol over Ru/C alone was always lower than that over
Ru/C + MgO.
We investigated the dependences of selectivity of the Ru/C +
MgO system on the reaction parameters (Fig. 3). Although the
conversion of guaiacol was very high in these data, the selecti-
vity patterns can be directly compared because reactions of
cyclohexanol, 2-methoxycyclohexanol or cyclohexane are very
slow over Ru/C + MgO (Fig. 2a). Higher selectivity to cyclohexa-
nol was obtained at higher reaction temperature under lower
hydrogen pressure. 2-Methoxycyclohexanol formation was sup-
pressed by these conditions, while cyclohexane formation was
slightly enhanced by higher reaction temperature.
Fig. 2 Time courses of reduction of guaiacol over Ru/C + MgO (a) and
only Ru/C (b). Conditions: guaiacol, 0.5 g; Ru/C, 50 mg; MgO, 0 or
Fig. 3 Effect of reaction conditions on the reduction of guaiacol over
Ru/C + MgO. Conditions: guaiacol, 0.5 g; Ru/C, 50 mg; MgO, 50 mg;
water, 10 g; 2 h.
2
50 mg; water, 10 g; H , 1.5 MPa; 433 K. Selectivities to products not
shown in the graph were always lower than 1%.
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is also an important motif in the bio-oil-based or lignin-based
1
1
monomers: both methoxy groups were removed and a good
cyclohexanol yield was obtained. For 1,2-dimethoxybenzene
and anisole, which have only methoxy groups and no OH
group, hydrogenation was the main reaction and the yields of
hydrodeoxygenation products were low. Therefore, the Ru/C +
MgO system is effective in selective removal of the methoxy
group from phenolic substrates.
Reaction mechanism
In the literature several reaction routes have been proposed for
the hydrodeoxygenation of guaiacol over noble metal catalysts to
mono-functionalized products (phenol, cyclohexanone and
cyclohexanol): demethylation of guaiacol to catechol followed by
24,28,33
hydrogenolysis (eqn (1)),
hydrogenation of guaiacol to
2
-methoxycyclohexanol followed by hydrogenolysis to cyclohexanol
3
1
(
eqn (2)) and hydrogenation to 2-methoxycyclohexanone fol-
2
6,27,29
lowed by hydrogenolysis to cyclohexanone (eqn (3)).
In
Fig. 4 Reuse experiments of Ru/C + MgO. Conditions: guaiacol, 1 g;
the cases of non-noble-metal based catalysts, the mechanism
involving demethoxylation of guaiacol has been proposed (eqn
Ru/C, 100 mg; MgO, 100 mg; water, 10 g; H
2
, 1.5 MPa; 433 K; 2 h.
“Undetected” means the loss of material balance: [(Loaded guaiacol
(
(
mol)) − (Total detected ≥C6 products (mol)) − (Unreacted guaiacol (4)), while the reaction route represented as eqn (1) is reported
1
4,15,21,24,25
mol))]/(Loaded guaiacol (mol)) × 100 (%).
to be also involved in some systems.
Nimmanwudi-
pong et al. reported that the Pt/MgO catalyst can convert guaia-
col to a mixture of phenol, catechol, cyclopentanone and other
products at 573 K via various reaction routes including eqn (1)
The reuse experiment was conducted to see the stability of
the catalyst (Fig. 4). The used Ru/C and MgO were collected by
filtration, washed with ethanol, dried, and reused for another
catalytic run. The used catalysts showed a good yield of cyclo-
hexanol and methanol. However, the activity was slightly
decreased by reuses, and a significant amount of guaiacol (or
products) disappeared from the reaction solution over the
used catalysts. During the catalysis, MgO powder was trans-
3
0
and (4).
ð1Þ
2
formed into muddy material, probably Mg(OH) . The decreases
of catalytic activity and material balance might be due to the
ð2Þ
ð3Þ
ð4Þ
reaction of guaiacol with Mg(OH) . With enough reaction time,
2
the used catalyst can give almost the same yield of cyclohexa-
nol and methanol as those obtained by a fresh one, suggesting
that trapping of guaiacol by Mg(OH) is weak and reversible.
2
Application to other methoxyphenols
The Ru/C + MgO system was applied to various related
substrates (Table 3). Guaiacol and 3-methoxyphenol were
converted to cyclohexanol in good yield. 4-Methoxyphenol was
also converted to cyclohexanol although the yield was lower.
The system can also be applied to 2,6-dimethoxyphenol, which
Table 3 Reduction of aromatics with methoxy groups over Ru/C + MgO
a
b
Substrate
Guaiacol
Conv. (%) Product (yield (%))
C1 product (yield (%))
98
>99
>99
>99
>99
Cyclohexanol (79), 2-methoxycyclohexanol (12), cyclohexane (7), others (1)
Cyclohexanol (85), 3-methoxycyclohexanol (10), cyclohexane (4), others (1)
Cyclohexanol (55), 4-methoxycyclohexanol (41), cyclohexane (4), others (1)
Cyclohexanol (69), 2-methoxycyclohexanol (16), cyclohexane (4), others (10) Methanol (84), methane (5)
1,2-Dimethoxycyclohexane (47), methoxycyclohexane (19), cyclohexane (16), Methanol (43), methane (9)
cyclohexanone (9), cyclohexanol (7), 2-methoxycyclohexanol (3), others (<1)
Methanol (85), methane (3)
Methanol (85), methane (7)
Methanol (52), methane (8)
3
4
2
1
-Methoxyphenol
-Methoxyphenol
,6-Dimethoxyphenol
,2-Dimethoxybenzene
Anisole
>99
Methoxycyclohexane (69), cyclohexanol (21), cyclohexane (9)
Methanol (25), methane (14)
a
Reaction conditions: substrate, 0.5 g; Ru/C, 50 mg; MgO, 50 mg; water, 10 g; H
rings. Calculated based on the number of methoxy groups in the loaded substrate.
2
, 1.5 MPa; 433 K; 2 h. Calculated based on the number of C6
b
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Table 4 Reduction of various substrates over Ru/C + MgO or Ru/C alone
Time Conv.
a
b
Entry Substrate
Catalyst
Ru/C + MgO
Ru/C
(h)
(%)
Product (yield (%))
C1 product (yield (%))
1
2
3
4
Catechol
Catechol
2
57
Cyclohexanol (14), 1,2-cyclohexanediol (40),
cyclohexane (2), others (1)
Cyclohexanol (39), 1,2-cyclohexanediol (39),
cyclohexane (7), others (2)
1,2-Cyclohexanediol (4), cyclohexanol (2),
cyclohexane (2)
1,2-Cyclohexanediol (29), cyclohexanol (5),
cyclohexane (1)
Cyclohexanol (89), cyclohexanone (4), others (2)
Cyclohexanol (95), cyclohexane (5), others (1)
Cyclohexane (1), cracked products (1)
Cyclohexane (5), cracked products (6)
—
—
2
86
—
c
2-Methoxycyclohexanol Ru/C + MgO
2-Methoxycyclohexanol Ru/C
2
8
36
96
Methanol (1), methane (14)
Methanol (3), methane (46)
c
2
5
6
7
8
9
1
Phenol
Phenol
Cyclohexanol
Cyclohexanol
Methanol
Ru/C + MgO 0.5
—
—
—
—
Ru/C
Ru/C + MgO
Ru/C
Ru/C + MgO
Ru/C
0.5
2
2
2
2
>99
2
11
16
85
d
Methane (16)
Methane (85)
d
0
Methanol
—
a
Reaction conditions: substrate, 0.5 g; Ru/C, 50 mg; MgO, 0 or 50 mg; water, 10 g; H
2
, 1.5 MPa; 433 K. Calculated based on the number of C6
b
c
rings. Calculated based on the number of methoxy groups in the loaded substrate. Trans–cis isomerization was excluded for calculation.
d
Methanol, 4 mmol.
We investigate the reactivity of various substrates including numbers). The activity of Ru/C alone was much higher in C–O
possible intermediates to determine the reaction mechanism dissociation of saturated alcohols such as cyclohexanol,
of the Ru/C + MgO system (Table 4). Catechol, which is an 2-methoxycyclohexanol and methanol than Ru/C + MgO. There-
intermediate in the reaction route of eqn (1), was less reactive fore one role of MgO in the catalysis of Ru/C + MgO is the sup-
than guaiacol over Ru/C + MgO (entry 1), and the main pression of unselective C–O dissociation. Ru/C showed very
product was 1,2-cyclohexanediol. Considering that catechol high activity in phenol hydrogenation, as well as Ru/C + MgO.
and 1,2-cyclohexanediol were not observed in the time course Phenol was detected at very short reaction time in guaiacol
of guaiacol reduction over Ru/C + MgO (Fig. 2a), the reaction reduction over Ru/C (Fig. 2b), suggesting that the reaction
route of eqn (1) is not plausible. It should be also noted that route of eqn (4) was present in the system of Ru/C alone, while
methane was hardly formed at shorter reaction time. The reac- the route where 2-methoxycyclohexanol and 1,2-cyclohexane-
tivity of 2-methoxycyclohexanol was even lower (entry 3). The diol act as intermediates can proceed to some extent.
time course shown in Fig. 2a also shows the low reactivity of
Demethoxylation of guaiacol to phenol and hydrogenation
2
-methoxycyclohexanol, indicating that 2-methoxycyclohexanol of guaiacol to 2-methoxycyclohexanol can simultaneously
is not an intermediate of cyclohexanol formation; the reaction proceed, and the relative rate between these reactions affects
route of eqn (2) is not plausible. The reactivity of 2-methoxy- the selectivity. Considering the higher selectivity to cyclohexa-
cyclohexanol was slightly higher when used alone as a substrate nol and the absence of the reaction route from 2-methoxycyclo-
than during guaiacol reduction. This phenomenon can be hexanol to cyclohexanol in the presence of MgO, another
explained by the competitive reaction of methanol which can role of MgO is to increase the relative rate of demethoxyl-
be adsorbed more strongly on the catalyst surface than 2-meth- ation to phenol in comparison with hydrogenation to
oxycyclohexanol. Hydrogenation of phenol, which was 2-methoxycyclohexanol.
observed at short reaction time in the reduction of guaiacol,
Now we propose the reaction network of guaiacol reduction
was fast and produced cyclohexanol and a small amount of over Ru/C with or without MgO (Fig. 5). Demethoxylation of
cyclohexanone (entry 5). Cyclohexanone was also observed at guaiacol to phenol consists of one hydrogen molecule and
low conversion in the reduction of guaiacol (Fig. 2 and 4). removal of one methanol. First guaiacol is partially hydrogen-
These data suggest that phenol is an intermediate in the for- ated to dihydroguaiacol. Two reactions can proceed from dihy-
mation of cyclohexanol, agreeing with the reaction route of droguaiacol: further hydrogenation to 2-methoxycyclohexanol
eqn (4). Phenol is a more frequently observed product than and removal of methanol to phenol. The latter reaction consti-
cyclohexanol in the literature for guaiacol demethoxylation. tutes demethoxylation of guaiacol (eqn (4)). Acidic phenolic
The difference in the main product between our system and compounds are stabilized by basic conditions, while saturated
the literature ones can be due to the difference in reaction compounds such as 2-methoxycyclohexanol are not. This
temperature (433 K and ∼573 K) and/or the difference in difference might increase the relative rate of demethoxylation
hydrogenation activity of catalysts. The reaction route of eqn to hydrogenation. Higher temperature can also increase the
(3) does not involve phenol as an intermediate and is not relative rate of methanol removal, which increases the number
plausible in our case. of molecules, to hydrogenation, which decreases the number
We also conducted the reactivity tests of the same sub- of molecules. Higher H pressure promotes the hydrogenation
2
strates over the Ru/C catalyst alone (Table 4, entries with even of dihydroguaiacol, decreasing the relative rate of methanol
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Fig. 5 Proposed reaction pathways of guaiacol reduction over Ru/C + MgO catalysts.
removal. Phenol is readily hydrogenated to cyclohexanol over to phenol followed by hydrogenation to cyclohexanol. The
Ru/C regardless of whether MgO is added or not. Without presence of a base suppresses the unselective C–O dissociation
MgO, further C–O dissociations can proceed over Ru/C. There by Ru catalyst, and may also promote the demethoxylation step
is another route for cyclohexanol production over Ru/C: C–O via stabilizing the produced phenol.
dissociation of 2-methoxycyclohexanol. However, C–O dis-
sociation of cyclohexanol in the absence of base decreases
much the cyclohexanol yield. As a result, higher cyclohexanol Experimental section
yield can be obtained in the presence of MgO. In addition,
Carbon-supported noble metal catalysts (5 wt% loading) were
methanol yield is enhanced by addition of MgO that sup-
purchased from Wako. MgO-supported Ru catalyst (5 wt% Ru)
was prepared by impregnation of MgO (UBE 500 A, calcined at
presses C–O dissociation of methanol to methane.
The mechanism of C–O dissociation of saturated com-
3 3
973 K for 1 h) with aqueous Ru(NO)(NO ) and subsequent
pounds is not well known. The reaction route composed of
acid-catalyzed dehydration to alkenes and subsequent hydro-
genation has been proposed for hydrodeoxygenation of alco-
hols such as cyclohexanol over metal catalysts under acidic
heating under N2 at 573 K for 1 h. XRD patterns were
measured using a Rigaku MiniFlex600 diffractometer. The
amount of CO chemisorption was measured in a high-vacuum
system by a volumetric method at room temperature. The
3,35–37
conditions (eqn (5)).
The dehydration + hydrogenation
2
samples for CO adsorption were treated with H at 473 K just
mechanism may be involved in this system and inhibited by
the presence of a base. However, the dehydration + hydrogen-
ation mechanism cannot be applied to the formation of
methane from the methoxy group or methanol (Table 4,
entries 4 and 10). Ruthenium catalysts are known to be very
effective in methanation reaction (hydrogenation of carbon
before measurement. Substrates were commercially available
and used as received. The hydrodeoxygenation of guaiacol was
performed in a 190 ml stainless-steel autoclave with an
inserted glass vessel. The noble-metal catalyst, co-catalyst
(
MgO; H-ZSM-5: JRC-Z5-90H(1), Süd-Chemie Catalysts and Cata-
3
8
lysis Society of Japan, Si/Al2 = 90), guaiacol (substrate), and
water (solvent) were put into the autoclave together with a
spinner. After sealing the reactor, the air content was purged
by flushing three times with 1 MPa hydrogen (99.99%; Nippon
Peroxide Co., Ltd). The autoclave was then heated to the reac-
tion temperature, and the temperature was monitored using a
thermocouple inserted in the autoclave. The heating required
about 40 min. The catalyst was reduced during the heating.
Then the hydrogen pressure was increased to the intended
value. After an appropriate reaction time, the reactor was
cooled down and the gases were collected in a gas bag. The
oxides to methane), which property may be related to the
methane formation in this system.
ð5Þ
Conclusions
The Ru catalyst combined with MgO can selectively convert autoclave contents were diluted with ethanol and transferred
methoxyphenols such as guaiacol into cyclohexanol and to a vial, while the catalyst was separated by centrifugation and
methanol with H under relatively mild conditions. This reac- filtration. Analyses were conducted by GC and GC-MS with
2
tion competes with hydrogenation to methoxycyclohexanols, TC-WAX, InertCap 5MS/Sil, or Rtx-1-PONA capillary column.
and higher reaction temperature and lower hydrogen pressure Yields were calculated based on the number of C6 rings in the
conditions are favorable for the production of cyclohexanol. reactant and products except for C1 products (methanol and
The reaction is suggested to proceed via the demethoxylation methane). Yields of methanol and methane were calculated
2202 | Green Chem., 2014, 16, 2197–2203
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from the number of methoxy groups in the substrate. Yields of 18 M. V. Bykova, D. Y. Ermakov, V. V. Kaichev, O. A. Bulavchenko,
cracked products were calculated on carbon basis.
A. A. Saraev, M. Y. Lebedev and V. A. Yakovlev, Appl. Catal.,
B, 2012, 113–114, 296.
1
2
2
2
2
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Acknowledgements
This research is supported by the JSPS KAKENHI (23760737)
and the Cabinet Office, Government of Japan through its
“Funding Program for Next Generation World-Leading
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