Selective hydrodeoxygenation of lignin-derived phenols to alkyl cyclohexanols over a Ru-solid base bifunctional catalyst

Article abstract of DOI:10.1039/c6gc01097k

Cyclohexanol and alkyl cyclohexanol are important chemical intermediates. It is meaningful to prepare cyclohexanols from non-fossil-based biomass. Here we report Ru/ZrO₂-La(OH)₃, a metal-solid base bifunctional catalyst, to show its excellent performance on the partial hydrodeoxygenation of lignin-derived phenols. Guaiacol could be converted to cyclohexanol with a 91.6% yield in water. Alkyl phenols with one or two methoxy groups were converted into alkyl cyclohexanols with yields over 86.9%. The catalyst had good activity of removing a methoxy group and retaining a hydroxyl group. In this catalyst, Zr and La interacted with each other to form a mixed (hydr)oxide, thus making ZrO₂-La(OH)₃ a stable support. Ru was highly dispersed on the ZrLa support. The pathway from guaiacol to cyclohexanol was investigated and proposed as two parallel ways, demethoxylation followed by hydrogenation (I), the saturation of the aromatic ring through hydrogenation and then demethoxylation through direct hydrogenolysis (II).

Full text of DOI:10.1039/c6gc01097k

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Selective Hydrodeoxygenation of Lignin-Derived Phenols to Alkyl
Cyclohexanols over Ru-Solid Base Bifunctional Catalyst
†
†
Guang-Yue Xu, Jian-Hua Guo, Yan-Chao Qu, Ying Zhang*, Yao Fu and Qing-Xiang Guo
Received 00th January 20xx,
Accepted 00th January 20xx
Cyclohexanol and alkyl cyclohexanol are important chemical intermediates. It is meaningful to
prepare cyclohexanols from non-fossil-based biomass. Here we report Ru/ZrO
solid base bifunctional catalyst, to show its excellent performance on the partial
hydrodeoxygenation of lignin-derived phenols. Guaiacol could be converted to cyclohexanol with
1.6% yield in water. Alkyl phenols with one or two methoxy groups were converted into alkyl
2 3
-La(OH) , a metal-
DOI: 10.1039/x0xx00000x
www.rsc.org/
9
cyclohexanols with yields over 86.9%. The catalyst had good activity of removing methoxy group
and keeping hydroxyl group. In this catalyst, Zr and La interacted with each other to form a mixed
(
2 3
hydr)oxide, thus made ZrO -La(OH) a stable support. Ru was highly dispersed on ZrLa support.
The pathway from guaiacol to cyclohexanol was investigated and proposed as two parallel ways,
demethoxylation followed by hydrogenation (I), the saturation of the aromatic ring through
hydrogenation and then demethoxylation through direct hydrogenolysis (II).
methoxyphenol), eugenol (2-methoxy-4-allylphenol), etc.
Simple hydrogenation can not lead to the selective formation
Introduction
Lignin, one of the main constituents of lignocellulosic biomass of cyclohexanol or alkyl cyclohexanols. Selective removal of
(
15-30% by weight and 40% by energy), is a natural amorphous methoxy group is necessary in the conversion. Traditional
1
-4
polymer consisting of methoxylated phenylpropane units.
method to produce alkyl cyclohexanols is catalytic
Lignin is also recognized as a potential resource for a wide hydrogenation of alkyl phenols without methoxy groups. Few
variety of bulk and fine chemicals, particularly aromatic studies have been focused on the selective conversion of lignin
5
,6
compounds. Through different methods, such as pyrolysis, derived methoxyphenols to alkylcyclohexanols. Wang et al.
hydrogenolysis, biological methods, etc., lignin can be converted phenol compounds and bio-oil derived phenolic
7
-12
converted into phenolic compounds.
These phenolic compounds to cyclohexanols and less functionalized alkyl
2
3
compounds can be hydrogenated to alkane fuels, phenols with Raney Ni and isopropanol (hydrogen donor).
1
aromatic hydrocarbon or used as value-added chemicals.
3-20
Nakagawa et al. applied Ru/C combined MgO to convert
Different from fossil-based chemicals, biomass derived guaiacol to cyclohexanol and methanol with 80% yield at 160
2
4
chemicals have extensive functional groups. It is obligatory to °C and 1.5 MPa H
2
.
The presence of base promotes the
find effective methods to obtain useful chemicals from demethoxylation step and suppresses the unselective C-O
biomass through selective catalytic process to tailor the dissociation. However, MgO is not stable in aqueous solution.
molecules.
Recently, Long et al. used Ni/MgO to produce cyclohexanol
Cyclohexanol is an important feedstock in industry, which from guaiacol in decahydronaphalene with 97.74% yield at 160
2
5
can be converted into cyclohexanone, polymerized to °C and 3 MPa H
2
.
2
Ni/CeO was employed by Sels et al. to
plasticizer, or used as solvent. Alkyl cyclohexanols are also convert alkylmethoxylphenols into alkylcyclohexanols at 250-
2
6
important intermediates in the preparation of spices, 300 °C.
Ru-Mn catalyst was used by Tomishige et al. to
Alkyl cyclohexanols can be selectively cut the methoxyl group on substituted phenols to
obtained from the hydrogenation of corresponding alkyl produce cyclohexanol with 81% yield at 160 °C and 1.5 MPa H
2
1,22
medicines, and polymers.
2
.
2
7
phenols. Nevertheless, the phenolic compounds obtained from
Although there is some promising progress, the
lignin always have methoxy groups, such as guaiacol (2- development of new efficient catalytic systems is still
necessary to convert methoxyphenols to cyclohexanols with
high selectivity and stability.
Collaborative Innovation Center of Chemistry for Energy Materials,
In our previous work, we found that 5 wt% Ru/SBA-15
Anhui Province Key Laboratory of Biomass Clean Energy and
Department of Chemistry, University of Science and Technology of
China, Hefei, 230026, China. Fax: (+) 86-551-63606689; Tel:+86-
catalysts had a good activity to convert phenols separated
2
8
from bio-oils into C
6
-C10 alcohols at 170 °C in isopropanol.
551-63603463; E-mail: zhzhying@ustc.edu.cn

Besides, ZrO -La O was reported as a good solid base with
2
2 3
These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: the catalyst preparation and high stability. Based on the fact that Ru catalysts always
characterization. See DOI: 10.1039/x0xx00000x
2
9
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display good performance in phenolic compounds
XRD analysis was conducted on an X-ray diffractometer
hydrogenation and the promotion effect of base in the (TTR-III, Rigaku Corp., Japan) using Cu Kα radiation (λ= 1.54056
conversion of methoxy phenols, we develop a new catalytic Å). The data were recorded over 2θ ranges of 10-70°.
system to selectively transform methoxyl phenols to
The pH value of the catalyst-water mixtures was measured
cyclohexanol and alkyl cyclohexanols over bifunctional by Mettler Toledo 320-S pH Electrodes. The catalyst-water
catalysts in water.
mixtures were prepared as follow: 0.10 g catalyst was added in
2
0 mL H O and stirred for 30 min.
2
XPS analysis was conducted on an X-ray photoelectron
Experimental
spectrometer (ESCALAB250, Thermo-VG Scientific, USA) at
-
8
-9
room temperature under a vacuum of 10 -10 torr using
monochromatized Al Kα radiation (1486.92 eV).
La(NO3)3 ·6H O (99.99% metals basis), RuCl ·xH O (Ru%
2
2
3
≥37.5 %) were purchased from Aladdin Reagents (Shanghai)
H2-TPD analysis was carried out on a self-built chemical
adsorption instrument. 80 mg of catalyst was loaded in a
quartz tube and was reduced at preparation condition. After
cooling down to room temperature, the adsorption process
Co., LTD. Zr(NO ) ·5H O (AR) was purchased from Sinopharm
3
4
2
Chemical Reagent Co., Ltd. ZrO2 was obtained from Saint-
Gobain Norpro. All chemicals were obtained commercially and
used without further treatment.
was conducted in a 5% H /Ar flow (60 mL/min) for 1 h followed
2
by sweeping in Ar flow (40 mL/min) for 1 h to remove physical
adsorbed species. During the desorption process, the sample
Preparation of the catalysts
ZrO was obtained from Saint-Gobain Norpro. Since La O is cell was heated up at a ramping rate of 10 °C/min to 450 °C in
2 2 3
easy to be converted to corresponding hydroxides in water, Ar flow (40 mL/min), and the TPD data was recorded. An ice-
the catalysts were directly prepared as Ru/hydroxide. The salt trap with KOH filling was used to remove water and CO2
La(OH) and ZrO -La(OH) with different Zr:La molar ratios (4, (absorbed on La species when the catalyst was exposed to air)
3
2
3
2, 1) were prepared by precipitation method. First, certain formed during the tests.
amounts of La(NO ) ·6H O and Zr(NO ) ·5H O were HAADF-STEM images and EDS mapping analysis were taken
2
3
3
2
3 4
dissolved in 200 mL of water and stirred for 2 h. Then 2 M with a JEOL-2100F field emission transmission electron
ammonia hydroxide was added into the mixture to adjust the microscopy.
solution pH to 10 and stirred at room temperature for another
h. Then the solid was filtrated, washed and dried over night atmosphere.
at 105 °C. After calcined at 700 °C for 4 h, the solid was stirred
for 2 h in water. The La(OH) and ZrO -La(OH) with different Experimental Procedure
DSC-TGA was taken by an SDT Q600 V20.9 Build 20 in air
2
3
2
3
Zr:La ratios were obtained after filtrated and dried at 40 °C for
2 h.
The 5 wt % Ru/ZrO , 5 wt % Ru/La(OH) , and 5 wt %
Typical procedure for hydrogenation of phenolic compounds:
guaiacol (5 mmol) and 5% Ru–based catalyst (0.10 g), solvent
1
2
3
(
20 mL H O) were mixed in a Parr reactor (316L stainless steel,
2
Ru/ZrO -La(OH)3 with different Zr:La ratio (4, 2, 1) were
2
5
0 mL reactor volume with a quartz lining). After purging the
prepared by deposition-precipitation method. RuCl ·3H O was
3
2
reactor with H , the autoclave was pressurized with 4 MPa H2
2
dissolved in water, and a calculated amount of carrier was
added to this solution and stirred at room temperature for 2 h.
Then ammonia hydroxide was added into the mixture to adjust
the solution pH to 10 and stirred at room temperature for
another 2 h. After filtering, washing and drying over night at 40
at room temperature and then heated at 170 °C for 4 h at a
stirring speed of 1000 rpm. The heating rate was 10 °C/min.
After the reaction was halted, the reactor was put into cold
water until cooled down to room temperature. The liquid
product was extracted by ethyl acetate, and the aqueous
phase was also collected. The aqueous and organic layers were
both analyzed by GC (Kexiao 1690) and GC-MS (Agilent 7890A).
n-Hexanol was used as the internal standard to determine the
product amount.
°
C, the catalysts were reduced in a H and N atmosphere at
2 2
280 °C for 3 h. The flow rate of hydrogen and nitrogen was 10
and 100 mL/min, respectively. The catalysts prepared above
were labelled as RuZr, RuZrLa-4 RuZrLa-2, RuZrLa-1, and RuLa.
The conversion and yield were calculated by mol%.
Catalysts Characterization
Nitrogen adsorption measurements were performed using a
Coulter SA 3100 adsorption analyzer which reports adsorption
isotherm, specific surface area and pore volume automatically.
The Brunauer-Emmett-Teller (BET) equation was used to
calculate the surface area in the range of relative pressures
between 0.05 and 0.20. The pore sizes were calculated from
the adsorption branch of the isotherms using the
thermodynamic based Barrett-Joyner-Halenda (BJH) method.
The metal content in catalysts and liquid residue were
analyzed by an Optima 7300 DV ICP-OES.
molaramountof guaiacolafterreaction
Conv.(%) = (1−
Yield (%) =
)×100%
×100%
molaramountof guaiacolin thestartingmaterial
molar amount of each product
molar amount of guaiacol in the starting materia
The separated catalyst was dried at 40 °C after filtration
and acetone sequential washing. During the catalyst stability
test, the catalyst was reused without any further treatment.
Results and discussion
Catalyst Characterization
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Table 1. Characteristics of the Ru-based catalysts.
Ru
S
BET
-1
V
pore
3
d
pore
Code
2
-1
(
wt%)
4.85
4.92
4.90
4.93
4.99
(m g ) (cm g )
(nm)
15.3
14.2
14.6
14.5
12.3
RuZr
58.3
43.6
40.4
38.4
17.7
0.26
0.20
0.22
0.17
0.07
RuZrLa-4
RuZrLa-2
RuZrLa-1
RuLa
In this work, a series of Ru-based catalysts were prepared. The
main characteristics of the Ru-catalysts are shown in Table 1.
2 3 2 3
The Ru/ZrO , Ru/La(OH) , and Ru/ZrO -La(OH) with different
Zr:La ratio were labelled as RuZr, RuLa, RuZrLa-X (X=4, 2, and
2
). The RuZr catalyst had the highest surface area (58.3 m g ),
-1
1
3
-1
the largest pore volume (0.26 cm g ), and the largest pore
diameter (15.3 nm). The surface area of the catalysts
decreased with the rise of La content. The pore volume and
the average pore diameter were similar among the RuZrLa
catalysts, while they are lowest with RuLa catalyst.
Figure 1 shows the powder XRD patterns of Ru-catalysts
with different supports. No peaks of Ru were observed in the
XRD patterns, which indicated that Ru existed in an amorphous
state or well dispersed in catalysts. The peaks in the RuZr
Figure 1. XRD patterns of the Ru catalysts.
recycle alkaline solution for the environmental and economic
concerns, so Ru-solid base bifunctional catalyst should be a
good choice for the selective hydrodeoxygenation of
methoxyphenols. Ru/MgO (RuMg) catalyst was prepared for
MgO is a typical solid base. The mixture of RuMg and water
showed alkaline (Table S1, entry 2). 66.8% cyclohexanol was
obtained by using RuMg as catalyst (Table 2, entry 4).
However, RuMg was not stable in hot water since it became
muddy after reaction because of the hydration of MgO. The
yield of cyclohexanol was barely satisfactory, which might be
attributed to the instability of RuMg. The mixture of RuLa and
water was alkalescent (Table S1, entry 3)and 75.2%
cyclohexanol was obtained by using RuLa as catalyst (Table 2,
entry 5). The solid base ZrO -La(OH) could be an appropriate
catalyst corresponded to the diffraction peaks of ZrO
Card No. 37-1484). The peak intensity of ZrO
2
(JCPDS
planes
2
decreased with the decrease of Zr content. The peaks in RuLa
catalyst corresponded to the diffraction peaks of La(OH)
3
(
JCPDS Card No. 36-1481) and La
2
O
2
CO
804). In RuZrLa-1 and RuZrLa-2 catalysts, the peaks of
could be observed obviously. As more Zr adding, the
RuZrLa-4 revealed mainly ZrO with weak diffraction peaks of
La(OH)
3
(JCPDS Card No. 37-
0
La(OH)
3
2
3
.
Catalysis
2
3
As a model compound, guaiacol was chosen to investigate the carrier of Ru-based catalyst. According to the data of Table S1,
effect of the catalysts on converting methoxyphenols into the the mixtures of RuZrLa catalysts with different Zr/La ratios and
corresponding alcohols. Table 2 shows the hydrogenation of water were alkaline and had higher pH value than that of RuLa
guaiacol in water with Ru catalysts supported on different and RuZr. The yield of cyclohexanol was 81.6% when RuZrLa-2
carriers. The main products were cyclohexanol, 2- was used as catalyst. When the reaction time doubled to 8 h,
methoxycyclohexanol and cyclohexane (Scheme 1). Trace the yield increased slightly to 85.1%. Temperature is a
other intermediates were also detected, such as significant factor in the reaction. As the temperature increased
methoxycyclohexane and 1,2-cyclohexanediol. Methane and to 200 °C, the guaiacol could be converted into cyclohexanol
methanol were both detected C1 products. When the reaction with an excellent yield of 91.6%. Higher temperature was
2
was conducted for 4 h at 170 °C, 4 MPa H in the presence of appropriate for the demethoxylation reaction. While the
RuZr (Table 2, entry 1), the primary products were temperature was further increased to 230 °C, cyclohexanol
cyclohexanol (58.1%), 2-methoxycyclohexanol (36.2%), and further converted to cyclohexane with a certain yield of
cyclohexane (4.3%). Previous study indicated that the presence methane as well as ethane and propane via the C-C bond
of base can promote the demethoxylation step and suppress cleavage over Ru. Comparing the structures of guaiacol and
24
the unselective C-O dissociation. When the reaction took cyclohexanol, RuZrLa-2 catalyst had a good activity of
place in 0.5 wt% KOH solution instead of pure water with removing methoxy group and keeping hydroxyl group under
2
Ru/ZrO as catalyst (Table 2, entry 2), the yield of cyclohexanol relative mild conditions. Then the catalyst was reused for
increased from 58.1% to 81.8%, while the yield of 2- another run at 170 °C. The yield of cyclohexanol decreased
methoxycyclohexanol decreased from 36.2% to 15.5% from 81.6% to 73.3%. While, the yield of cyclohexanol
simultaneously. KOH without Ru catalysts revealed no catalytic decreased from 75.2% to 58.1% by using RuLa. Therefore,
activity (Table 2, entry 3). Usually, it requires extra effort to
RuZrLa-2 catalyst was more stable than pure RuLa catalyst. The
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a
Table 3. Hydrodeoxygenation of lignin-derived methoxyphenols.
Phenols Conv./% Products and Yields/%
Scheme 1. The hydrogenation of guaiacol.
1
1
1
00
00
00
8
8.7
8.6
6.8
8.5
1.1
2.5
2.2
a
Table 2. The effect of catalysts in the hydrogenation of guaiacol.
Yield/%
Entr
y
T/
°C
Conv
./%
89.5
Catalyst
Meth Meth
a
b
c
-
anol
24
22
0
-ane
33
50
0
1
RuZr
RuZr+KOH
KOH
170
170
170
170
170
170
170
170
99.1 36.2 58.1
99.4 15.5 81.8
4.3
0.2
0
8
9.1
b
2
O
b
3
0
0
0
HO
HO
O
HO
O
4
5
6
7
8
RuMg
100
100
100
100
100
32.7 66.8
23.8 75.2
19.4 77.3
15.4 81.6
17.7 78.6
0.3
0.5
1.1
0.2
0.2
17
19
14
17
15
44
40
55
61
56
b
RuLa
100
9
0.8
3.4
4.1
1.6
RuZrLa-4
RuZrLa-2
RuZrLa-1
RuZrLa-2
b
1
00
b
86.9
7.3
3.8
2.3
9
170
100
6.6
89.9
0.1
15
69
+KOH
a
Reaction conditions: 5 mmol reactant, 0.10 g RuZrLa-2, 20 ml H
MPa H , 200 °C, 3 h, reaction mixture stirred at 1000 rpm. Reaction
2
O, 4
c
b
1
0
RuZrLa-2
RuZrLa-2
RuZrLa-2
RuZrLa-2
RuLa
170
200
230
170
170
100
100
100
100
100
11.1 85.1
0.5
1.3
11
4
63
93
2
1
1
2
5.8
0
91.6
2.2
time: 8 h.
1
42.8
2.3
2
350
56
d
1
3
4
20.2 73.3
31.7 58.1
13
15
d
1
0.7
39
Discussion
a
Reaction conditions: 5 mmol guaiacol, 0.10 g 5 wt% Ru-based catalyst, 20
b
ml H
2 2
O, 4 MPa H
, 4 h, reaction mixture stirred at 1000 rpm. Solvent: 20 mL To explain why RuZrLa catalysts showed better
c
d
0
.5 wt% KOH solution. Reaction time 8h. The catalyst used for the second performance on selectively converting methoxyphenols to
run.
allylphenol, a series of characterazation and analyses were
carried out. The XPS spectra showed the chemical
properties of Ru catalysts, as shown in Figure 2. The straight
dotted lines showed the binding energy of each species in
RuZrLa-2 catalyst. The Zr and La species were not simple
mixed, but interacting with each other. In the ZrLa support,
Zr donated electron to La to make a relative electronegative
metal losses based on the original catalyst in the liquid residue
were shown in Table S2. In RuLa catalyst, the La loss (0.903
wt%) was much higher than that in RuZrLa catalysts (between
0
.061 to 0.125%). The losses of Ru (between 0.041 to 0.052
wt%) and Zr (between 0.007 to 0.018 wt%) were similar (Table
S2). It indicated that the ZrO -La(OH) supports were more
stable than pure La(OH) support. As the amount of leached
4-δ
3+δ
Zr and electropositive La, like Zr
2 3
O -La (OH) mixed
2
3
(
hydr)oxide species. As Zr content increasing, the binding
3
energy of Zr 3d shifted to high value while that of La 3d
shifted oppositely to low value. This tight interaction effect
metals on RuZrLa-2 catalyst was quite small, the activity
loss may caused by coke deposition on the catalyst. Both
fresh and used catalyst were characterized by TGA under air
atmosphere. From TGA curves shown in Figure S1, the
remarkable weight loss of the used catalyst at about 336 °C
proved the existence of coke.
2 3
made ZrO -La(OH) a stable material, which could explain
the much lower La loss in RuZrLa catalysts. The chemical
shifts of Ru 3p were also in keeping with the experimental
results. The binding energies of Ru 3p on RuZrLa catalysts
were higher than that on RuLa or RuZr catalysts. As is
known, the binding energy of nanoclusters increased with
Apart from guaiacol, the RuZrLa-2 was also tested in the
hydrodeoxygenation of more complex methoxyphenols, which
with more alkyl or methoxy groups, in water at 200 °C. A
similar selectivity for alkyl cyclohexanols (88.7-89.5%) was
obtained with the phenols which had one methoxy group,
including 2-methoxy-4-ethylphenol, 2-methoxy-4-propyl-
phenol, and 2-methoxy-4-allylphenol. However, the selectivity
for alkyl cyclohexanols decreased when 2,6-dimethoxyphenol
and 2,6-dimethoxy-4-allylphenol were used as substrates. It
indicated that extra methoxy group on the benzene ring would
increase the difficulty of demethoxylation reaction. When the
reaction time was extended to 8 h, a higher yield could also be
achieved.
30
the size decreasing. A smaller metal particle size would
lead to higher surface area of metal and thus gave rise to
better catalytic activity. In RuZrLa-2 catalyst, the binding
energy of Ru 3p was the highest among the catalysts, which
could lead to the prominent catalyst activity of RuZrLa-2
catalyst. To compare the Ru dispersion of the catalysts, H
TPD was carried out, as shown in Table S3 and Figure S2.
RuZrLa-2 showed the highest H uptaken amount among
2
-
2
the catalysts, and it matched well with the variation
tendency of binding energy and catalytic activity.
According to the electronic image by HADDF-STEM, as
shown in Figure 3, the RuZrLa-2 catalyst was random
spontaneous stacked clusters. EDS mapping results were
4
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Figure 3. EDS mapping of RuZrLa-2 catalyst.
Figure 4. Product distribution after the hydrogenation of guaiacol at
00 °C with RuZrLa-2. It took 15 min to heat up to 200 °C. Time -0.25 h
is the start of heating up while time 0 h means temperature reaches
00 °C.
2
2
Figure 2. XPS spectra of (top) Zr 3d, (middle) La 3d and (bottom) Ru 3p
in Ru catalysts. The binding energies were calibrated to the carbon
with C 1s band.
guaiacol was hydrotreated at 200 °C over RuZrLa-2. The
change of the main products distribution with time is
shown in Figure 4. The conversion of guaiacol reached 94%
at the beiginning of the reaction, and no unsaturated
products were detected after 0.5 hours. The primary
products were cyclohexanol (42%) and 2-methoxycyclohexanol
employed to show the distribution of each element. There
was little distinction between Zr and La distribution, which
(
50%) at 0 h (The reaction was terminated immediately once
further proved the interaction between ZrO
Considering of the XPS results, the ZrLa support was a
mixed (hydr)oxide rather than isolated or simple mixed ZrO
and La(OH) species. Because of the similar contrast with La,
2 3
and La(OH) .
the temperature reaches 200 °C by cold water). The yield of
cyclohexanol increased with the extension of reaction time
and reached the highest yield (91.6%) at 4 h, while the yield
of 2-methoxycyclohexanol decreased in the same time
period. Only small amount of cyclohexane (<2.5%) was formed
via the hydrodeoxygenation during the conversion. Trace
methoxycyclohexane (< 0.1%) and 1,2-cyclohexanediol (<
2
3
the Ru nanoparticles could hardly be directly observed in
STEM image. However, it can be observed that Ru was
highly dispersed on ZrLa support from the EDS result.
0
.1%) were also detected by the GC-MS.
Reaction Pathways
To further study the reaction pathway, the main
intermediates detected in guaiacol hydrogenation were used
as reactants (Table 4). Methoxycyclohexanol was the most
important intermediate detected in the guaiacol conversion
To further investigate the process of the selective
hydrodeoxygenation of lignin-derived methoxyphenols,
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process. The conversion of 2-methoxycyclohexanol was 87%
and the main product was cyclohexanol under the same
operating conditions. Three ways could exist to achieve
Table 4. Hydrogenation of main products detected in guaiacol
a
hydrogenation.
Reactants t/h Conv./%
Selectivity/%
cyclohexanol
from
methoxycyclohexanol:
directly
demethoxylation to cyclohexanol, hydrogenolysis of methoxy
group to 1,2-cyclohexanediol, and dehydroxylation of hydroxy
2
4-26
group to methoxycyclohexane.
Both 1,2-cyclohexanediol
4
0
87
8
91
0.06
0.03
0.02
0
1.1
0.1
and methoxycyclohexane were employed as feedstock under
the same operating conditions. The conversion of 1,2-
cyclohexanediol was 63% and the main product was
cyclohexanol. Suppose that hydrogenolysis of methoxy group
to 1,2-cyclohexanediol was a main route, there should be a
certain amount of 1,2-cyclohexanediol detected due to the
relatively slow reaction rate of 1,2-cyclohexanediol. However,
only trace 1,2-cyclohexanediol was detected in the reaction. It
revealed that the hydrogenolysis was not a main route.
Likewise, the methoxycyclohexane showed a lower conversion
with 35% cyclohexane selectivity while only trace amount of
methoxycyclohexane was detected as intermediate, so the
dehydroxylation was not a main route, either. Meanwhile,
when cyclohexanol was employed as feedstock, it was hard to
transform to cyclohexane. The selectivity of cyclohexane was
low from guaiacol but high from methoxycyclohexane, which
further denied the dehydroxylation route. Considering that the
hydrogenation process was very quick under this condition,
the reaction was terminated during increasing temperature to
catch the intermediate products. The reactor was put into cold
water just when the temperature reached 150 °C. The
conversion of guaiacol was 67%, and the selectivity of
cyclohexanol and 2-methoxycyclohexanol were 47.2% and
9
6
4
63
9
7
0.7
35
4
4
35
6
3
1.7
100
1
00
1
9
8
0.8
4
100
1
2
53
32
a
Reaction conditions: 5 mmol reactant, 0.10 g RuZrLa-2, 20 ml H
2
O, 4
MPa H
2
, 200 °C, reaction mixture stirred at 1000 rpm.
OCH3
H2
H2
3
-
H2O
OH
49.8%, respectively. Trace phenol (1.2%) and anisole (0.2%),
Demethoxylation
H2
3H2
which came from the demethoxylation and dehydroxylation of
guaiacol, could be detected in GC-MS. Experiments with less
catalyst (0.01 g) lead to similar result (0.1% phenol was
detected). When anisole was tested under same operating
conditions, the selectivity of cyclohexane was 32%. If the
majority of guaiacol was converted to anisole, more
cyclohexane could be found in the products. Comparing with
the product distribution of guaiacol, this is not the main route.
Phenol can be hydrogenated to cyclohexanol with high
conversion and selectivity. According to the conversion of 2-
methoxycyclohexanol was 8%, and the yield of cyclohexanol
was 42%, the cyclohexanol could come from the
demethoxylation of guaiacol followed by hydrogenation to
cyclohexanol. Phenol was not detected at 200 °C could be due
to its high reaction speed. Furthermore, as shown in Table 2,
methanol was detected after reaction as the side product of
-
CH3OH
OH
OH
OH
OCH3
OCH3
3H2
H2
H2
-H2O
-
CH3OH
OH
OH
H2
-
CH4
OCH3
H2
-
H2O
Scheme 2. The main pathway for the conversion of guaiacol in the
presence of RuZrLa-2.
Accordingly, the main pathway for the conversion of
demethoxylation. While methane was also detected, which guaiacol is given in Scheme 2. There are two main routes in the
could be produced by the hydrogenolysis of methanol. hydrogenation of guaiacol. Guaiacol was converted to phenol
Comparing entries 7, and 10 in Table 2, the ratio of methane to via demethoxylation process, and then converted to
methanol increased with reaction time extending. Since during cyclohexanol via hydrogenation (I). Guaiacol was converted to
the extending time, the main reaction pathway could not 2-methoxycyclohexanol through the saturation of aromatic
change, methane should come from methanol. Further ring followed by the demethoxylation to cyclohexanol (II).
comparing entries 7, 11 and 12 in Table 2, the ratio of Therefore this catalyst was benefit for the demethoxylation
methane increased with temperature increasing, which was reaction.
because that high temperature was in favor of methanol
hydrogenolysis.
6
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PAPER
1
1
7 P. Gallezot, Catal. Today 2007, 121, 76-91.
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Conclusions
In conclusion, a bifunctional RuZrLa catalyst was used in the
selective hydrodeoxygenation of lignin-derived phenolic
compounds to cyclohexanol and alkyl cyclohexanols, which can
be used as chemical feedstock. Over 88% yield of
cyclohexanols can be obtained from monomethoxy-
substituted phenols hydrotreated at 200 °C for 4 h. The extra
methoxy group would decrease the selectivity for
cyclohexanols. Longer reaction time can increase the
selectivity and yield effectively. In RuZrLa catalyst, Zr and La
interacted with each other to form a mixed (hydr)oxide, thus
made ZrLa a stable material. Ru was highly dispersed on ZrLa
support. RuZrLa-2, with a Zr/La ratio of 2, showed the highest
activity in the reaction. The main pathway to form the
cyclohexanols from lignin-derived phenolic compounds
including two ways simultaneously.
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The authors are also grateful to NSFC (21572213, 21325208),
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the National Basic Research Program of China 29 H. Sun, Y. Q. Ding, J. Z. Duan, Q. J. Zhang, Z. Y. Wang, H. Lou,
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Innovative Research Team in University of the Ministry of
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support.
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