An investigation on the aqueous-phase hydrodeoxygenation of various methoxy-substituted lignin monomers on Pd/C and HZSM-5 catalysts

Article abstract of DOI:10.1039/c6ra22492j

Aqueous phase catalytic upgrading of lignin monomers to hydrocarbons via hydrodeoxygenation (HDO) has been explored using a combination of Pd/C and HZSM-5 catalysts under 2 MPa of H₂ (ambient temperature). Model monomers with varying numbers of methoxy groups, including phenol, anisole, guaiacol and 2,6-dimethoxy-phenol, were chosen as lignin model compounds. Mechanistic studies revealed cascade and parallel reaction pathways via hydrogenation and dehydration (hydrolysis) processes, which were catalyzed by Pd/C and HZSM-5, respectively. Hydrogenation was preferred at lower temperature, whereas higher temperature was favorable for the removal of oxygen-containing functional groups. The effect of methoxy groups on the HDO of these monomers was also investigated systematically. Basically, the conversion of multi-substituted monomers was tougher than that of mono-substituted ones, due to steric constraint and the inhibition of the electron-donating hydroxyl group. The selectivities to cyclohexane from phenol and anisole were improved significantly by increasing the temperature to 413 K. However, cyclohexanone was preferably produced over cyclohexane when using model compounds with multi-substituents (guaiacol and 2,6-dimethoxy-phenol), even at temperatures above 513 K. Comparative experiments were also conducted on the HDO of 1,2-cyclohexanediol with or without the presence of phenol, which clearly suggested that the further HDO of guaiacol and 2,6-dimethoxy-phenol was inhibited, probably due to the strong interactions between cyclohexanediol molecules and Br?nsted acid sites.

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An Investigation on the Aqueous-phase Hydrodeoxygenation of
Various Methoxy-substituted Lignin Monomers on Pd/C and
HZSM-5 Catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
‡
Cong Zhang , Jingbo Qi , Jing Xinga,b, Si-Fu Tanga, Liang Songa,*, Yuanyuan Suna, Chuanhui
‡
DOI: 10.1039/x0xx00000x
Zhanga, Hongchuan Xina, Xuebing Lia,*
www.rsc.org/
Aqueous phase catalytic upgrading of lignin monomers to hydrocarbons via hydrodeoxygenation (HDO) has been explored
using a combination of Pd/C and HZSM-5 under 2 MPa of H2 (ambient temperature). Model monomers with varying
numbers of methoxy groups, including phenol, anisole, guaiacol and 2, 6-dimethoxy-phenol, were chosen as lignin model
compounds. The mechanism studies revealed the cascade and parallel reaction pathways via hydrogenation and
dehydration (hydrolysis) processes which were catalyzed by Pd/C and HZSM-5, respectively. Hydrogenation was preferred
at lower temperature, whereas higher temperature was favorable for the removal of oxygen-containing functional groups.
The effect of methoxy groups on the HDO of these monomers was also investigated systematically. Basically, the
conversion of multi-substituted monomers is tougher than mono-substituted ones due to steric constraint and the
inhibition of the electron-donating hydroxyl group. The selectivities to cyclohexane from phenol and anisole could be
improved significantly by increasing the temperature to 413 K, however, cyclohexanone was preferably produced rather
than cyclohexane for model compound with multi-substituents (guaiacol and 2,6-dimethoxy-phenol) even at temperature
above 513 K. Comparative experiments were also conducted on the HDO of 1,2-cyclohexanediol with or without the
presence of phenol, which clearly suggested that further HDO of guaiacol and 2, 6-dimethoxy-phenol was resisted
probably due to the strong interactions between cyclohexanediol molecules and the Brønsted acid sites.
improved12. Appropriate and careful design of catalyst is
required in order to remove oxygen at a maximum degree with
the minimum amount of hydrogen consumed (i.e., low H2
1. Introduction
Currently, plant biomass uniquely provides sustainable organic
carbon source1-3, and biofuels derived from which are the
unique sustainable source of liquid fuels by far. Compared to
fossil fuels, greenhouse gas released by biofuels is significantly
decreased, and can even be neutral if biofuels are produced in
efficient methods4-6. However, the direct application of bio-oil
was limited by its corrosivity, low vapor pressure, high
viscosity and instability7-9. Because biomass-derived platform
molecules are generally highly oxygenated compounds (15-40
wt% of its dry mass)10, the upgrading process before
application is necessary to overcome the deleterious
properties of biomass pyrolysis oil11. Hydrodeoxygenation
(HDO) processes have recently received considerable attention
as the most effective method for bio-oil upgrading into liquid
transportation fuels, by which the effective H/C ratio is
pressure), which is the key challenge of HDO13.
Phenolic compounds (phenol, guaiacol and other
substituted phenol compositions) derived from the
decomposition of lignin8 are the lowest reactive chemical
compounds in biomass pyrolysis oils14 and represent
significant fraction of the total bio-oils15 and other biomass- or
lignin-derived product streams16,17
Therefore, it is very
a
.
important to identify the reaction pathways that phenols may
undergo18-22. It has been proposed that the aqueous-phase
dehydration is the key step in the overall HDO reaction, which
is consistent with the high apparent activation energy10,23. C
−O
bond cleavage is the rate-determining step in the conversion
of β-O-4 and 4-O-5 ether bonds and can be catalyzed by
hydrogenolysis on metal catalysts24. While parallel
hydrogenolysis and hydrolysis accounts for the cleavage of C
−
O bond of the 4-O-5 linkage25. As reported, high selectivity of
HDO of lignin-derived phenolic monomers and dimers to
cycloalkanes was achieved on Pd/C and HZSM-5 catalysts in
aqueous phase at 473 K26. The reactivity of mono- and dimeric
lignin model compounds has been investigated on metal and
acid co-catalysts. Dimeric oxygen-bridged model compounds
such as benzylphenyl ether and diphenyl ether can be readily
converted to monomeric species, suggesting that the reaction
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mechanism of monomeric species is the basis to investigate NH4+-form were calcined at 823 K for 4 h (heating rate: 1 K
HDO of lignin model compounds10. The reaction pathways and min-1) to convert them into H+-form.
product distribution can be affected by the acidity of the 2.3 Catalytic measurements
zeolite27, size effect of nanoparticles28, reaction temperature29,
Catalytic HDO reaction was performed in a stainless steel
Lewis basicity of solvent30. Additionally, the substitution of the
autoclave with an internal volume of 80 mL. Typically, 0.0025
phenyl group can also affect the HDO of the monomeric
mol of phenolic model compound, 0.05 g of Pd/C and 0.5 g of
phenol compounds. It has been reported the HDO of diverse
HZSM-5 and 20 mL of water were added into the autoclave
para-substituted phenolic monomers could be achieved
effectively with satisfying conversion rates31-33. Whereas the
pressurized with H2 to a total pressure of about 2 MPa at room
deoxygenation and ring hydrogenation of guaiacol on Pt-
reactor. Then it was flushed with H2 for three times and
temperature. The autoclave was then heated to a designated
modified zeolites were significantly suppressed by the
temperature for 2 h under stirring at 680 rpm. After the
methoxy group in comparison with phenol (no substitution)
reaction, the reactor was cooled down to room temperature in
and o-cresol (methyl substitution), because the adsorption
an ice bath. The organic phase was extracted with ethylacetate
modes and transport parameters were affected by their
molecular dimensions34.
(3×10 mL). The gas phase, organic phase and aqueous phase
were separated and analyzed by GC-MS (Agilent 7890A/5975C)
Considering the ubiquity of methoxy group in bio-oil, it is
equipped with a capillary column (HP-5; 30 m×250 μm×0.25
very important to carry out a systematic study on the effect of
μm) and a flame ionization detector (FID). It was found that
substituent methoxy groups on the HDO of lignin-derived
only hydrogen and trace amount of CO2 and methanol existed
monomeric phenols. Very few researches have been focused
in the gas phase. As a result, only the changes in the liquid
on this although there is increasing interest in biofuel
production by bio-oil upgrading35-39. Therefore, we decided to
The conversion and selectivity were calculated based on
investigate the reaction pathways of various methoxy-
phase were considered.
carbon balance equations. Conversion = 100% - (C atoms in
substituted phenolic monomers. Phenol, anisole, guaiacol and
residual reactants/total C atoms in the products besides
2,6-dimethoxy-phenol were selected as starting model
residual reactants)*100%. Selectivity = (C atoms in each
compounds based on the following considerations: 1) phenol is
product/total C atoms in the products)*100%. Oxygen removal
an attractive model compound to investigate the principal
= 100%-∑i(N0*Si*ai/Ni*a0)*100%, where N0 is the number of C
hydrogenation and deoxygenation routes; 2) anisole (or
methoxy-benzene) can be used to explore the relative
reactivity of the methoxy group; 3) guaiacol and 2,6-
dimethoxy-phenol possessing two types of C–O bonds, Csp2–
OH (hydroxyl) and Csp2–OCH3 (methoxy) are good model
compounds to probe the catalyst behavior with respect to
these different organic functions.
atoms in a reactant molecule, Ni is the number of C atoms in a
i-product molecule, Si is the selectivity of the i-product, a0 is
the number of O atoms in a reactant molecule, ai is the
number of O atoms in a i-product molecule.
2.4 Catalyst characterization
The X-ray diffraction (XRD) pattern was collected on a Bruker
D8 Advance diffractometer (Cu Ka = 1.54056 Å) at 40 kV/100
mA on a spinner in the range of 2θ = 5–50° with a step size of
0.01°. Scanning electron microscopy (SEM) was performed on
a Hitachi S-4800 SEM-microscope with an accelerating voltage
5 kV. Prior to scanning, the dry samples were platinum-coated.
Energy-dispersive X-ray (EDX) analysis was conducted on an
Oxford Inca X-ray energy detector. Transmission electron
micrographs (TEM) were recorded on a field emission H-7600
electron microscope operated at 120 kV. Before the
measurement, the samples were prepared by depositing a
drop of an ultrasonicated methanol suspension of the solid
material onto a carbon-coated Cu grid for TEM measurement.
Nitrogen adsorption-desorption was carried out at 77 K using a
Micromerities ASAP 2000-M+C apparatus. Prior to adsorption,
the sample was degassed in a vacuum at 523 K for 24 h. The
specific surface area was determined according to the
Brunauer-Emmett-Teller (BET) method in the linear range
(p/p0 = 0.01-0.25). The t-plot was used to evaluate the external
surface area and the micropore volume. The mesoporous
volume was then conducted by BJH method. As shown in Table
S1, Si/Al ratio, BET surface areas, amounts and properties of
acid sites on zeolite catalyst were characterized and
calculated. It was evidenced by the BJH mesopore size
2. Experimental section
2.1 Chemicals and commercial catalysts
The chemicals were obtained from commercial suppliers:
phenol (Aladdin, ≥99.5% standard for GC), anisole (Aladdin,
≥99.5% standard for GC), cyclohexanol (Aladdin, ≥99.0%
standard for GC), methoxy-cyclohexane (J&K, 98% GR assay),
guaiacol (Aladdin, 99.0% AR assay), 2,6-dimethoxy-phenol
(Aldrich, 99%), ethyl acetate (Aladdin, 99.5% GR assay), ZSM-5
(Catalyst Plant of Nankai University, NKF-5; Si/Al=12.5), Pd/C
(Aladdin, loading 5 wt.% Pd).
2.2 Catalyst preparation
The purchased catalysts were washed with deionized water at
ambient temperature. After drying at 393 K overnight, the
sample was calcined in air (flow rate: 30 mL min-1g cat-1) at 823
K for 4 h with a heating rate of 3 K min-1 in order to remove the
template. The catalysts were tested after the Na-ZSM-5 was
converted to acidic form (H+) by refluxing three times in 1 mol
L-1 NH4NO3 at 313 K for 6 h. The solid was filtered, washed,
dried at 393 K overnight and calcined at 823 K for 4 h (heating
rate: 1 K min-1) in flowing air (60 mL min-1). The samples in
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distributions that the sample ZSM-5 contained almost no
The acidity property of HZSM-5 was characterized by NMR
mesopore volume (0.04 cm3 g-1) and relatively small external and NH3-TPD. NMR experiment was recorded on a Bruker
surface area (ca. 13 m2g-1).
Ascend-500 spectrometer at resonance frequencies of 130.24
Table 1 Product distributions of hydrodeoxygenation of phenol over Pd/C and HZSM-5 as a function of reaction temperature. a
Selectivity of products (C %)
Conversion of
(C %)
Reaction Temperature (K)
Entry
1
2
3
4
5
383
413
443
473
413b
0.0
49.2
99.6
99.9
0.0
99.8
50.8
0.0
0.2
0.0
0.0
0.0
0.1
0.0
0.0
0.4
0.1
0.0
100.0
100.0
100.0
100.0
100.0
0.0
99.9
a Typical reaction conditions: phenol (0.005 mol), HZSM-5 (Si/Al=15,0.5 g), Pd/C (0.05 g), H2O (20 mL), 2 MPa H2, 2 h, stirred at 680 rpm.
b Only Pd/C as the catalyst.
MHz for 27Al. 27Al NMR experiments were recorded on a 4
mm triple-resonance MAS probe at a spinning rate of 10 kHz.
Pulse MHz for 27Al. 27Al NMR experiments were recorded on a
4 mm triple-resonance MAS probe at a spinning rate of 10
kHz. Pulse width (p/2) for 27Al was measured to be 1.7 μs. 27Al
MAS NMR spectra were recorded using a small-flip-angle
technique with a pulse length of 0.28 l s (p/12) and a recycle
delay of 1 s. Temperature programmed desorption of
NH3(NH3-TPD) over HZSM-5 catalysts was performed on a
Micromeritics 2920TR chemisorption analyzer. Firstly, around
15 mg of sample was activated by heating at 773 K for 1 h in
Ar with a heating rate of 10 K min-1. After cooling to 373 K,
NH3 adsorption was carried out. NH3 was adsorbed for 2 h by
adding 10 vol.% to the Ar carrier gas with total flow 40
mL/min at 373 K. Physically adsorbed NH3 was removed by
degassing at 373 K for 2 h with Ar (20 ml/min) until no further
weight loss was observed. The NH3-TPD of the sample was
carried out by heating the sample in flowing Ar at a rate of 10
K min-1 from 373 to 873 K and the species desorbing was
detected by TCD (Thermal conductivity detector). For
calibration of the method, a standard calibration experiment
was performed.
Pd/C are shown in Figures S1 and S2. The HZSM-5 catalyst
was irregularly shaped built up by laminated structure but
uniform crystallite size of 1.5-2 μm. Pd particles were
uniformly supported on carbon base, but the formation of
aggregates was observed as well.
27Al MAS NMR was used to detect the coordination state of
aluminum species in the HZSM-5 catalyst (Fig. S1).
Apparently, the sample exhibited a strong peak centered at
53 ppm and a small peak centered at 1 ppm, which was
attributed to tetrahedrally coordinated aluminum
(framework Al, FAl) and octahedrally coordinated aluminum
(extra-framework Al, EFAl), respectively. It was indicated that
Al in the sample was mainly tetrahedrally coordinated.
a
OH
O
OH
2H2
H2
-H2O
H+
H2
Pd/C
Pd/C
Pd/C
b
O
O
OH
2H2
-CH3OH
H+
-H2O
H+
H2
Pd/C
Pd/C
In addition, the numbers of weak and strong acid sites
were obtained by analyzing the amount of ammonia
desorbed via integrating the TPD curves with the Gauss curve
fitting method (Fig. S3). It is reported that the increase of acid
quantity and the decrease of the activation energy of
ammonia desorption on strong acid sites of HZSM-5 followed
by a Si/Al ratio decrease27. Consequently, Pd/C and HZSM-5
with Si/Al ratio of 15 were jointly used as catalyst in this work
O
Scheme 1 Reaction pathways for phenolic compounds to cyclohexane over Pd/C and
HZSM-5 in the liquid phase: (a) phenol; (b) anisole.
according
to
its
excellent
performance
in
3.2 HDO of mono-substituted lignin monomers
hydrodeoxygenation previously27.
HDO of phenol was conducted under acid-free condition to
clarify the role of Pd/C (Table 1). It was found that the
aromatic ring was completely hydrogenated to cyclohexanol
at 413 K, with 0.1 % cyclohexanone as intermediate and no
cyclohexane detected. It is obvious that Pd/C is much more
3. Results and Discussion
3.1 Characterization of the catalyst
efficient in hydrogenation than in deoxygenation10,40,41
.
The powder XRD pattern of HZSM-5 exhibit well-resolved
diffraction peaks (see Fig. S1 in the Supplementary
Information), which are characteristic of the MFI framework
structure. SEM images of HZSM-5 catalyst and TEM images of
As proposed in Refs.37,42, the mechanism for gas-phase
phenol conversion to cyclohexane with heterogeneous
sulfide catalysts under 523 K and 100 bar H2 pressure
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undergoes direct hydrogenolysis to benzene followed by
benzene hydrogenation or firstly hydrogenation to
cyclohexanol then followed by cyclohexanol hydrogenolysis.
However, in this contribution with Pd/C and HZSM-5 in the
aqueous-phase, the HDO of phenol and cyclohexanol
observed did not follow the classical hydrogenolysis route
under lower temperature and H2 pressure. The mechanism
for the aqueous-phase HDO of phenol to cyclohexane with
Pd/C and acid catalysts proceeds via an initial metal-catalyzed
aromatic
ring
hydrogenation,
naphthenic
alcohol
dehydration, and subsequently metal-catalyzed cycloalkane
hydrogenation (Scheme 1a), which is similar to the overall
reaction pathway proposed previously with Pd/C and
phosphoric acid as bi-functional catalysts43.
Table 2 Product distributions of hydrodeoxygenation of anisole over Pd/C and HZSM-5 as a function of reaction temperature. a
Selectivity of products (C %)
Conversion of
(C %)
Reaction Temperature (K)
Entry
CH3OH
1
2
3
4
383
413
443
10.1
13.5
14.3
0.3
29.0
3.0
25.9
3.5
34.6
0.0
0.0
2.2
0.2
92.4
98.0
98.8
77.8
85.3
0.2
0.0
0.0
473
14.5
81.0
0.2
0.2
4.3
0.0
97.0
a Typical reaction condition: anisole (0.005 mol), HZSM-5 (Si/Al=15, 0.5 g), Pd/C (0.05 g), H2O (20 mL), 2 MPa H2, 2 h, stirred at 680 rpm.
Table 3 Hydrodeoxygenation of anisole over Pd/C and HZSM-5 with different amounts of reactant and catalysts added. a
Amount of reactant and catalysts
Selectivity of products (C %)
Conversion of
Entry
Anisole
HZSM-5
(g)
(C %)
Pd/C (g)
CH3OH
(mol)
1
2
3
4
5
6
0.005
0.005
0.03
0.03
0.03
0.03
0.50
0.10
0.10
0.02
0.02
0.00
0.05
0.05
0.05
0.10
0.20
0.20
13.5
9.1
8.3
5.6
4.3
5.3
77.8
0.7
1.1
0.2
0.5
0.3
3.0
3.5
0.0
0.2
2.2
0.3
0.0
0.0
0.0
0.0
98.0
99.3
55.9
65.8
96.6
87.1
36.0
42.0
61.1
69.7
62.9
53.7
24.4
18.8
20.0
20.3
24.2
14.2
5.5
11.1
a Typical reaction condition: the amount of anisole, HZSM-5 (Si/Al=15) and Pd/C was added as displayed above, H2O (20 mL), 2 MPa H2, 413 K, 2 h, stirred at 680 rpm.
The conversion and selectivity for phenol hydrogenation
over Pd/C and HZSM-5 as a function of reaction temperature
are shown in Table 1. With reaction temperature as low as
383 K, the amount of cyclohexane was below detection
limitation and the product distribution is similar to that under
acid-free conditions. The selectivity of cyclohexane via
formation of cyclohexanol from phenol was improved to
49.2% when the temperature increased from 383 K to 413 K,
and even significantly higher at 443 K and 473 K. The
deoxygenation of the cyclohexanol intermediate was
inhibited without the addition of HZSM-5 or under low
temperature (383 K). In line with previous studies10, the
combination of acidic condition and elevated reaction
temperature resulted in almost complete HDO of phenol to
cyclohexane with approximately 100% of selectivity.
same time on the dual-functional catalyst system Pd/C and
H3PO4 under the selected conditions, while hydrolysis of
anisole to phenol is the dominating reaction24. In our work,
according to the conversion and product distribution of
anisole over Pd/C and an HZSM-5 zeolite displayed in Table 2
at varying reaction temperatures, the mechanism varies
significantly. Selectivity of methoxy-cyclohexane and
cyclohexanone (intermediate) reaches 29.0 and 34.6% at 383
K, respectively. When the reaction temperature exceeded
413 K, yield of cyclohexane was improved extremely to
approximately 80 % with only trace amount of methoxy-
cyclohexane and cyclohexanone detected (Table 2). During
anisole conversion, there is no phenol detected when
temperature varied, indicating that hydrogenation of the
aromatic ring is the main reaction pathway (Scheme 1b).
Anisole could not be converted when HZSM-5 was applied
alone as a catalyst under 2 MPa of H2. However, when Pd/C
and HZSM-5 were used jointly, the rate of C–O bond cleavage
was enhanced steeply. Moreover, when only Pd/C was used
as catalyst, the selectivity of cyclohexane was only 0.6% and
the value increased to 1.5% in the combined catalyst system
(Table 3). These results suggest that the presence of dual
catalytic functions is favorable for the overall HDO. In
addition, the acid sites of HZSM-5 is critical to break down the
As a typical component of bio-oil, anisole (methyl phenyl
ether or methoxy-benzene) contains a methoxy group that
provides interesting chemistry. Bimolecular reactions
including the following transalkylation reactions were
observed by Zhu et al44: (a) anisoles to phenol and methyl-
anisole; (b) phenol and methyl-anisole to cresols; (c) phenol
and anisole to cresol and phenol; (d) methyl-anisole and
cresol to phenol and xylenol. However, the hydrogenation of
the aromatic ring and methoxy hydrolysis proceeded at the
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ether bond between saturated carbon ring-and methyl, which
could be observed by improving the amount of HZSM-5 from
0.1 g to 0.5 g (Table 3). The selectivity of cyclohexane
increased drastically from 0.7 to 77.8%, accompanied by
decreasing of methoxy-cyclohexane and cyclohexanol. When
the amount of reactant was increased six times,
hydrogenation was incomplete leaving 44.1% of anisole
unconverted. Large amount of cyclohexanone was detected.
The selectivity of cyclohexane follows the same trend as a
function of temperature (Fig. 1). It can be found that when
the
reaction
Table 4 Product distributions of hydrodeoxygenation of guaiacol over Pd/C and HZSM-5 as function of reaction temperature. a
Selectivity of products (C %)
Conversion of
(C %)
Entry
Reaction Temperature (K)
products with one
oxygen-containing
functional group
products with two
oxygen-containing
functional groups
CH3OH
1
2
3
4
5
383
413
443
473
513
0.9
3.0
0.0
0.0
0.0
0.0
5.9
5.8
7.9
93.2
89.1
80.1
70.1
25.7
39.4
100.0
86.4
83.0
77.4
9.6
10.4
18.3
54.0
11.5
14.3
a Typical reaction conditions: guaiacol (0.005 mol), HZSM-5 (Si/Al=15,0.5 g), Pd/C (0.05 g), H2O (20 mL), 2 MPa H2, 2 h, stirred at 680 rpm.
Fig. 2 Selectivities of main products over Pd/C and HZSM-5 as function of reaction
temperature using multi-substituted phenolic compounds as reactants: (a) guaiacol,
(b) 2, 6-dimethoxy-phenol. Typical reaction conditions: reactant (0.005 mol), HZSM-5
(Si/Al=15, 0.5 g), Pd/C (0.05 g), H2O (20 mL), 2 MPa H2, 2 h, stirred at 680 rpm.
temperature was below 413 K, the amount of cyclohexane
was under detection limitation. While with reaction
temperature further increased above 413 K, the selectivity of
cyclohexane improved significantly, especially hydrocarbons
derived from anisole (79.4%). Moreover, as the temperature
was further increased to 443 K, the HDO of phenol and
anisole to cyclohexane proceeded quantitatively at high rates
with intermediates hardly detected.
3.3 HDO of multi-substituted phenolics
Depending on the biomass source and the pyrolysis process
condition, the phenolic compounds represent a significant
fraction of the total bio-oil. Among them, the methoxy
phenols (guaiacol, syringol and their derivatives) are
particularly abundant. Therefore, it is important to identify
the reaction pathways that methoxy phenols may undergo on
zeolites. According to the previous considerations, guaiacol
possessing two types of C–O bonds, namely Csp2–OH
(hydroxyl) and Csp2–OCH3 (methoxy), was chosen as a model
compound to probe the catalyst behavior with respect to
these two organic functions. Similar to phenol conversion,
the main products were due to fast hydrogenation of the
aromatic ring, such as 2-methoxy-cyclohexanol and 2-
methoxy-cyclohexanone. In all cases, the conversion of
guaiacol was lower than that of phenol. 1,2-Cylohexanediol
and 2-hydroxyl-cyclohexanone derived from the hydrolysis of
methoxy group were not detected even at 443 K (Figures 2a),
pointing to the lower reactivity of guaiacol, which can be
attributed to steric constraint for the active metal centre to
approach the C-O bonds46 and the electron-donating
character of hydroxyl group that would stabilize the
transition state carbocation and decrease the hydrolysis rate
of the methoxy group. In addition, high temperature could
Fig. 1 Selectivities of main products over Pd/C and HZSM-5 as function of reaction
temperature using mono-substituted monomer as reactants: (a) phenol, (b) anisole.
Typical reaction conditions: reactant (0.005 mol), HZSM-5 (Si/Al=15, 0.5 g), Pd/C (0.05
g), H2O (20 mL), 2 MPa H2, 2 h, stirred at 680 rpm.
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facilitate the hydrolysis of methoxy group, and 1, 2-
cylohexanediol and 2-hydroxyl-cyclohexanone could be
obtained. However, the conversion of these products
originated from the hydrolysis of methoxy group was
hindered.
The selectivity of cyclohexane was only 5.9% at 513 K (Table 4
and Table S2) although the selectivity of products containing
one oxygen functional group reached 54.0% with
cyclohexanone being the largest percentage. The high
Table 5 Product distributions of hydrodeoxygenation of 2,6-dimethoxy-phenol over Pd/C and HZSM-5 as a function of reaction temperature. a
Selectivity of products (C %)
Conversion of
(C %)
Reaction
products with one
oxygen-containing
functional group
products with two
oxygen-containing
functional groups
products with three
oxygen-containing
functional groups
Entry
Temperature (K)
CH3OH
1
2
3
4
5
6
383
413
443
473
513
18.8
21.9
23.2
23.0
24.4
0.0
5.6
9.6
19.4
65.9
57.1
62.6
61.0
51.1
9.6
24.0
10.0
6.2
6.4
0.0
6.5
89.2
53.1
66.9
73.1
413 K with Pd/C and
Beta（Si/Al=12.5）
19.0
3.9
70.4
6.9
92.3
a Typical reaction conditions: 2,6-dimethoxy-phenol (0.005 mol), HZSM-5 (Si/Al=15, 0.5 g), Pd/C (0.05 g), H2O (20 mL), 2 MPa H2, 2 h, stirred at 680 rpm.
OH
OH
O
O
O
O
3H2
Pd/C
-CH3OH
H2
Pd/C
H+
-CH3OH
O
O
O
2H2
Pd/C
H2
Pd/C
HO
O
HO
H+ -CH3OH
H+
OH
-CH3OH
HO
HO
H2
Pd/C
O
H+ -H2O
H2 Pd/C
H+
-H2O
O
OH
Fig. 3 Oxygen removal over Pd/C and HZSM-5 as function of reaction temperature
using different reactants: (a) phenol, (b) guaiacol, (c) 2, 6- dimethoxy-phenol. Typical
reaction conditions: reactant (0.005 mol), HZSM-5 (Si/Al=15, 0.5 g), Pd/C (0.05 g), H2O
(20 mL), 2 MPa H2, 2 h, stirred at 680 rpm.
H2
Pd/C
Scheme 2 Reaction pathways for 2, 6-dimethoxy-phenol to cyclohexane over Pd/C
and HZSM-5 in the liquid phase.
selectivity to cyclohexanone indicates a higher adsorption
constant of the aromatic rings compared to the saturated
cyclic compounds.
It is indicated in Table S3 that the HDO of 2, 6-dimethoxy-
phenol with one hydroxyl group and two adjacent methoxy
groups is much more difficult. Cyclohexane was not observed
even at 513 K (see Supplementary Information). In addition,
the conversion of 2, 6-dimethoxy-phenol was the lowest
comparing with phenol, and guaiacol. The main products
included 2-hydroxyl-cyclohexanone, 1, 2-cyclohexandione, 1,
2-cylohexanediol and 2, 6-dimethoxy-cyclohexanol. From the
product distributions, it is obvious that the selectivity of
products with one oxygen-containing functional group
increased with the temperature, showing a contrary trend to
those of products with two and three oxygen-containing
functional groups (Table 5). The oxygen removal ratio of
three phenolic model compounds (phenol, guaiacol and 2, 6-
dimethoxy-phenol) increased as the temperature increased
Guaiacol was hydrodeoxygenated through the reaction
pathway shown in Scheme 2. The aromatic ring of guaiacol
was fully hydrogenated above 383
K
and further
deoxygenated to cyclohexanol, cyclohexanone, and
cyclohexane when heated to 513 K. Because hydrogenation
of the aromatic ring of guaiacol was preferred at lower
temperature, the formation of phenol was not observed
despite that phenol could be formed by demethylation and
dehydration of guaiacol7. Because of the excellent
hydrogenation ability of Pd/C, the reaction pathway (guaiacol
to 2-methoxycyclohexanol to hydrodeoxygenation products)
proposed in Scheme 2 is similar with that of Lercher et al43
and quite differs from some other reported catalytic systems
(guaiacol to phenol to hydrodeoxygenation products)45-48
.
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(Fig. 3), which suggested that higher temperature was
favorable for the removal of oxygen-containing functional
groups. At 513 K, 2, 6-dimethoxy-cyclohexanol disappeared,
2-hydroxyl-cyclohexanone and cyclohexanone were the
primary products, coinciding with those of guaiacol. The
selectivity of products with one oxygen-containing functional
group reached 54.0% at 513 K.
Table 6 Product distributions of hydrodeoxygenation trans-1, 2-cyclohexanediol over Pd/C and HZSM-5. a
Selectivity of products (C %)
Conversion of
(C %)
Reaction Temperature (K)
413
0.0
2.3
0.0
0.0
0.0
0.0
0.0
0.0
0.0
93.6
14.3
0.0
0.0
0.0
1.2
0.0
0.0
4.1
100.0
0.0
1.3
100.0
38.3
0.0
513
513 K without Pd/C
513 K without HZSM-5
81.8
0.0
2.6
0.0
a Typical reaction conditions: trans-1,2-cyclohexanediol (0.005 mol), HZSM-5 (Si/Al=15,0.5 g), Pd/C (0.05 g), H2O (20 mL), 2 MPa H2, 2 h, stirred at 680 rpm.
Table 7 Product distributions of hydrodeoxygenation of phenol and trans-1, 2-cyclohexanediol over Pd/C and HZSM-5. a
Selectivity of products (C %)
Conversion of
and
Reaction Temperature (K)
(C %)
413
443
473
513
0.0
0.0
33.0
41.0
6.7
0
66.9
58.5
49.8
49.8
0.1
0.5
0.0
0.0
75.2
75.6
54.0
99.9
43.5
50.2
a Typical reaction conditions: phenol (0.005 mol), trans-1,2-cyclohexanediol (0.0025 mol), HZSM-5 (Si/Al=15, 0.5 g), Pd/C (0.05 g), H2O (20 mL), 2 MPa H2, 2 h, stirred at
680 rpm.
presence of cyclohexanediol with or without double bond. To
validate this hypothesis, a group of comparative experiments
were performed and the results were summarized in Table 6
and Table 7.
Firstly, the HDO of 1, 2-cylohexanediol was conducted over
Pd/C and HZSM-5 (Table 6). At 413 K, no cyclohexane could
be detected in the product. 1, 2-cylohexanediol was
completely converted to 2-hydroxyl-cyclohexanone, implying
that it was rather difficult to cleave the C-O bonds of
saturated alcohol. With the temperature increased to 513 K,
cyclohexane could be detected (about 2.3%), but the product
was dominated by cyclohexanone (about 93.6%), suggesting
that dehydration was favorable at higher temperature. This
result is very similar to those of guaiacol and 2, 6-dimethoxy-
phenol. Most probably the further hydrogenation of
cyclohexanone in the conversions of guaiacol and 2, 6-
dimethoxy-phenol was inhibited. To test the respective
functions of the combined catalyst, the HDO performance of
Pd/C and HZSM-5 on 1,2-cylohexanediol was tested
separately. It was found that 1,2-cylohexanediol could not be
converted without HZSM-5 even at 513 K, but could be
converted to cyclopentanecarbaldehyde with a selectivity of
81.8% without Pd/C, verifying that HZSM-5 mainly played a
dehydration role in the current reaction condition and HZSM-
5 contributed to the isomerization. The reaction pathway
could be proposed that the hydroxyl group of 1,2-
cylohexanediol was first protonated by the Brønsted acid
Fig.
4 Conversion and selectivities of main products over Pd/C and HZSM-5 as
function of reaction temperature using different reactants: (a) only phenol (0.005
mol) as reactant, (b) with 1, 2-cyclohexandiol (0.0025 mol) added into phenol (0.005
mol). Typical reaction conditions: reactant (0.005 mol), HZSM-5 (Si/Al=15, 0.5 g), Pd/C
(0.05 g), H2O (20 mL), 2 MPa H2, 2 h, stirred at 680 rpm.
3.4 The influence of 1, 2-cylohexanediol on further HDO of multi-
substituted phenolic compounds
A very large fraction of the products hydrodeoxygenated
from guaiacol and 2, 6-dimethoxy-phenol were dominated by
2-hydroxyl-cyclohexanone and 1, 2-cylohexanediol. The
conversion of 2-hydroxyl-cyclohexanone to 1, 2-
cylohexanediol with double bond could be accomplished by
ketoenol tautomerism. Therefore, the removal of hydroxyl
groups of cylohexanediol should be the key reaction and we
can hypothesize that the reaction was retarded by the
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sites and then underwent dehydration, isomerization,
deprotonation and hydrogenation to form cyclopentyl-
methanol, which is similar to that of 1,2-cylohexanediol on
Ni/HZSM-546.
The effects of temperature and substitution of the benzene
ring on the HDO were also investigated systematically. For
simple phenolic monomers like phenol and anisole, the HDO
can be accomplished with very high selectivity to generate
cyclohexane at elevated temperature. While for multi-
substituted monomers, only trace of hydrocarbon can be
obtained even above 513 K. The low conversion of guaiacol
and 2, 6-dimethoxy-phenol may be due to steric constraint
for the active metal ceter to approach the C-O bonds and the
inhibition of the electron-donating hydroxyl group that can
stabilize the transition state carbocation. There is a large
fraction of byproducts, including cyclohexanone, 2-hydroxyl-
cyclohexanone and 1, 2-cylohexanediol, in the HDO products
of guaiacol and 2, 6-dimethoxy-phenol. The comparative
experiments imparted on 1, 2-cylohexanediol with or without
the presence of phenol clearly demonstrated that the HDO of
guaiacol and 2, 6-dimethoxy-phenol were retarded by the
presence of cyclohexanediol which can form unreactive
protonated species and inhibit the further HDO of guaiacol
and 2, 6-dimethoxy-phenol.
Secondly, the HDO of phenol and trans-1, 2-
cyclohexanediol (molar ratio: 2:1) was conducted using the
same reaction condition (Table 7, Fig. 4). As mentioned
above, catalyst system composed of Pd/C and HZSM-5
exhibited excellent reactivity on phenol individually (Table 1).
While in the presence of 1, 2-cylohexanediol, almost no
cyclohexane was detected at 413
K and 443 K (the
selectivities of cyclohexane from phenol at 413 K and 443 K
are 49.2 and 99.6%, respectively), confirming the catalytic
conversion of saturated cyclic alcohol was difficult and
implying that the catalysts could be deactivated in the
presence of 1, 2-cylohexanediol. According to the reaction
mechanism (Scheme 1a), the benzene ring of phenol was first
partially hydrogenated to form cyclohexanone, which could
be further hydrogenated to form cyclohexanol and then
dehydrated to cyclohexane. The hydrogenation of phenol to
cyclohexanol was so fast that hardly any cyclohexanone as
intermediate was observed (Table 1). However, the selectivity
of cyclohexanone among products of phenol was the highest
when 1, 2-cylohexanediol was added into the reactant. Thus,
primary conclusion can be made that under the likely
operation of this two-step mechanism, the Pd/C–ZSM-5
system was very active for the first step but not active for the
second step under the experimental conditions. Probably, the
catalysts were deactivated by 1, 2-cylohexanediol via forming
unreactive protonated species between 1, 2-cylohexanediol
Acknowledgement
This work is supported by National Natural Science
Foundation of China (Grant No. 21276263, 21306214,
21171173) and “100 Talents” program of Chinese Academy of
Sciences (Grant No. KJCX2-EW-H05).
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