Selective Hydrodeoxygenation of Guaiacol to Cyclohexanol Catalyzed by Nanoporous Nickel

Article abstract of DOI:10.1007/s10562-019-02967-5

Abstract: Cyclohexanol is an important feedstock in the chemical industry and the selective hydrodeoxygenation of lignin-derived guaiacol to cyclohexanol have gained increasing research attention in recent years. In this work, a series of nanoporous metal catalysts were employed for the hydrodeoxygenation of guaiacol and nanoporous Ni (NP-Ni) exhibited high catalytic performance for the preparation of cyclohexanol. With water as solvent, 100% conversion of guaiacol and over 90% selectivity to cyclohexanol were achieved at 180?°C and 2?MPa for 4?h. In order to further promote the stability of NP-Ni, induction melting, vacuum arc melting and mechanical alloying were separately employed for the preparation of NiAl precursor alloy. Mechanical alloying seemed to be an effective method for the alloying process and the as-prepared NP-Ni could keep almost stable after 10 times recycling. Furthermore, the reaction mechanism was investigated with NP-Ni for guaiacol hydrodeoxygenation. Scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron micrographs (TEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) were employed for the characterization of NiAl alloy and the optimal preparation methods of NP-Ni were acquired according to the characterization results. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02967-5

Catalysis Letters
https://doi.org/10.1007/s10562-019-02967-5
Selective Hydrodeoxygenation of Guaiacol to Cyclohexanol Catalyzed
by Nanoporous Nickel
Jiqing Lu1 · Xing Liu1 · Guanqun Yu1 · Jinkun Lv1 · Zeming Rong1 · Mei Wang1 · Yue Wang1
Received: 31 July 2019 / Accepted: 18 September 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Cyclohexanol is an important feedstock in the chemical industry and the selective hydrodeoxygenation of lignin-derived
guaiacol to cyclohexanol have gained increasing research attention in recent years. In this work, a series of nanoporous metal
catalysts were employed for the hydrodeoxygenation of guaiacol and nanoporous Ni (NP-Ni) exhibited high catalytic per-
formance for the preparation of cyclohexanol. With water as solvent, 100% conversion of guaiacol and over 90% selectivity
to cyclohexanol were achieved at 180 °C and 2 MPa for 4 h. In order to further promote the stability of NP-Ni, induction
melting, vacuum arc melting and mechanical alloying were separately employed for the preparation of NiAl precursor alloy.
Mechanical alloying seemed to be an efective method for the alloying process and the as-prepared NP-Ni could keep almost
stable after 10 times recycling. Furthermore, the reaction mechanism was investigated with NP-Ni for guaiacol hydrodeoxy-
genation. Scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, X-ray photoelectron
spectroscopy (XPS), high-resolution transmission electron micrographs (TEM), energy dispersive spectrometer (EDS) and
X-ray difraction (XRD) were employed for the characterization of NiAl alloy and the optimal preparation methods of NP-Ni
were acquired according to the characterization results.
Graphic Abstract
Keywords Guaiacol · Cyclohexanol · Hydrodeoxygenation · Nanoporous nickel · Mechanical alloying
Electronic supplementary material The online version of this
1 Introduction
article (https://doi.org/10.1007/s10562-019-02967-5) contains
supplementary material, which is available to authorized users.
With the rapid depletion of fossil energy, biomass has
received significant attention as a sustainable resource,
releasing the dependence on fossil resources and reduc-
ing the emissions of greenhouse gases [1–4]. Lignin is a
key component of lignocellulosic biomass with natural
Jiqing Lu and Xing Liu have contributed equally.
* Zeming Rong
zeming@dlut.edu.cn
Extended author information available on the last page of the article
Vol.:(0123456789)
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J. Lu et al.
amorphous polymer structure [5, 6] and is mainly com-
posed of three structural units of p-hydroxyphenyl propane,
guaiacyl propane, and syringylpropane [7, 8]. Considering
the chemical structure complexity of lignin, guaiacol is
usually chosen as a highly representative model compound
for downstream transformation processes of lignin [9–13],
because it contains the most abundant functional groups in
the lignin structure (phenolic and methoxy groups) [14–16].
Cyclohexanol is an important feedstock in the chemical
industry and it can be dehydrogenated into cyclohexanone,
polymerized to plasticizer, or used as solvent [17–19].
According to the reported reaction mechanism [20],
cyclohexanol could be prepared by the selective hydrode-
oxygenation (HDO) of guaiacol, but it is generally regarded
to be a challenging task because guaiacol contained two
types of O-containing functional groups (Caryl–O–H and
Caryl–O–CH3) [21]. Besides, it also involves the complete
HDO to give hydrocarbon products such as benzene [22, 23]
and cyclohexane [24–26], or the partial removal of oxygen to
give the as-required cyclohexanol [27]. Hence, the selective
HDO of guaiacol has gained increasing research attention
in recent years [28–31].
In this work, nanoporous nickel (NP-Ni) was employed
for the conversion of guaiacol to cyclohexanol through
HDO process using water as solvent and it exhibited high
catalytic performance under comparatively mild reaction
conditions (180 °C and 2 MPa H2). Furthermore, several
methods, such as induction melting, vacuum arc melting
and mechanical alloying, were separately employed for the
preparation of NiAl precursor alloy. Mechanical alloying
method seems to be more efcient and the corresponding
NP-Ni showed almost permanent catalytic performance in
the recycling experiments under the condition of aqueous
medium, indicating a worthy antiacid-corrosion ability and
better stability.
2 Experimental
2.1 Materials
All chemicals were analytical grade and used without fur-
ther purifcation: Guaiacol (Sinopharm Chemical Reagent
Co., Ltd.), Cyclohexanol (Guangdong Xilong Chemical Co.,
Ltd.), 2-Methoxycyclohexanol (Shanghai Aladdin Biochem-
ical Technology Co., Ltd.), Phenol (Sinopharm Chemical
Reagent Co.,Ltd), Ethanol (Guangdong Xilong Chemical
Co., Ltd), Aluminum powder (Dalian King Choice Non-Fer-
rous Metals Products Co., Ltd), Nickel powder (Shenyang
Zhanyu Technology Development Co., Ltd.), Aluminum foil
and Nickel foil (Dalian King Choice Non-Ferrous Metals
Products Co., Ltd.). And commercial nanoporous Cu, Fe,
Co, Ni were purchased from Dalian Tongyong Chemicals
Co., Ltd.
Noble metal Ru-based catalysts, i.g. Ru/MgO [32],
Ru–MnOx/C [10] and Ru/ZrO2–La(OH)3 [33], are known
to possess excellent catalytic activities for selective HDO
of guaiacol to cyclohexanol, but the high cost limits their
broad industrial application in biomass conversion. Ni and
Co based catalysts were reported as those very few active
non-noble metal catalysts and exhibited good catalytic activ-
ity for the preparation of cyclohexanol. For example, Long
et al. studied supported Ni-based catalyst for HDO of guai-
acol and high yield of cyclohexanol was obtained [34]. MgO
as the basic carrier played an important role in promoting
efcient production of cyclohexanol. Jiang et al. employed
NiCo/γ-Al2O3 catalyst and high yield of cyclohexanol was
obtained as the main product, mainly due to the proper acid-
ity and interaction between metal particles and support [35].
However, the above-mentioned non-noble metal catalysts
might possess low resistance to water in an acid medium and
are prone to deactivating [36] because phenol generated as a
main intermediate during HDO of guaiacol, while environ-
mentally benign water-based systems generally are consid-
ered to be advantageous for the separation of the products,
such as water-insoluble alkanes, from the aqueous phase.
Nanoporous metal is one kind of classical catalytic mate-
rials and often employed in HDO reactions due to its unique
catalytic performance [37]. Rinaldi [38, 39] employed com-
mercial nanoporous Ni (RANEY® Ni) for the conversion of
guaiacol to phenol through H-transfer reactions. It revealed
that the parallel adsorption between guaiacol and RANEY®
Ni was benefcial to the removal of methoxy C-O bond.
However, RANEY® Ni was merely investigated under the
condition of organic solvent, but not aqueous solution.
2.2 Catalyst Characterization
X-ray powder difraction (XRD) was carried out by Lab
XRD-7000S, using Cu-Kα (40 kV, 100 mA) radiation to
determine the crystalline structure. The sample was meas-
ured with a scanning rate of 8° min−1 in the range from 20
to 100°.
X-ray photoelectron spectroscopy (XPS) analysis was
performed using an Escalab 250Xi spectrometer of Thermo
Scientific with Al–Kα as the photon source. The bind-
ing energy reference was taken by using the C1 s peak at
284.8 eV.
The surface composition distribution of the sample was
observed using a low angle backscatter mode (LA-BSE)
of a Hitachi SU8220 cold feld emission scanning electron
microscope (SEM).
High-resolution transmission electron micrographs
(HRTEM) of NP-Ni was taken using a Tecnai G2 F30. At
the same time, the surface composition of the catalyst was
analyzed by the energy dispersive spectrometer (EDS).
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Selective Hydrodeoxygenation of Guaiacol to Cyclohexanol Catalyzed by Nanoporous Nickel
The elementary composition of the solid sample and the
loss of metal element in the reaction solution was meas-
ured by inductively coupled plasma atomic emission spec-
trometry (ICP-AES). The sample was measured using an
Optimi2000DV.
and the alloy powder is weighed according to the ratio of the
alloy powder to the NaOH mass ratio of 2:7. The alloy pow-
der was added slowly and repeatedly, and the temperature
was set to 90 °C for 1 h. After the end of the activation, the
solution was cooled to room temperature and the acquired
mixture was washed ten times with unionized water until
neutral and kept it in water.
The physical adsorption experiment was conducted by
using a 3H-2000PS1 sorption system (Beishide Instrument-
ST Co., Ltd). The specifc surface areas of catalysts were
calculated by the Brunauer–Emmett–Teller method. The
pore distribution of the sample was estimated by the Bar-
rett–Joyner–Halenda method.
2.4 Hydrogenation of Guaiacol
The catalytic hydrogenation reaction was conducted in a
stainless-steel autoclave (75 ml capacity). Typically, guai-
acol (2 mmol), water (10 mL) and NP-Ni (0.025 g) were
directly added into the reactor, then it was sealed and purged
with N2 and H2 for three times respectively. The reactor was
flled with 0.3 MPa H2 at room temperature and heated to the
required temperature. Then the magnetic stirring was turned
on and kept for 4 h. After that, the autoclave was cooled to
room temperature and the catalyst was separated from the
product by fltration.
2.3 Catalyst Preparation
2.3.1 Preparation of Precursor Alloy
The high-frequency induction melting (IM) method com-
bined with the quenching method to produce the NiAl pre-
cursor alloy, denoted as NiAlx-IM. Under the atmosphere
of argon, 5 g Ni foil and 5 g Al foil were melted at a high
temperature of 1873 K, and the melted mixtures were cooled
down by the rapid quenching method with the high-speed
rotating copper drum. The NiAl alloy ribbon is subsequently
ground to a fne powder of about 200 mesh size.
2.5 Product Analysis
The sample was analyzed using a Tianmei GC7890F
gas chromatograph with a capillary column DP-FFAP
(30 m×320 μm×0.50 μm). The specifc test conditions are
as follows: the gasifcation chamber temperature was set
at 260 °C and the detector temperature was set at 280 °C.
The column temperature was initially set at 60 °C for
3 min, then the temperature was raised to 220 °C with a
rate of 20 °C min−1. 2-Amyl alcohol was used as an internal
standard.
Vacuum arc melting (AM) method was employed for the
preparation of NiAl alloy, which is denoted as NiAlx–AM.
Typically, high-purity nickel foil and aluminum foil with a
mass ratio of 1:1 were put into a crucible of the arc melt-
ing furnace. It should be pointed out that the high melting
point nickel foil was placed above the low melting point
aluminum foil to prevent the aluminum metal from splashing
out during the dissolution process. The furnace was pumped
to 6×10−3 Pa and then replaced with an argon atmosphere of
0.5 MPa. The arc gun produced high-intensity arc at a high-
intensity current of 100–300 A to smelt the metal foil. Then
the precursor alloy was ground to 200 mesh powder for use.
Mechanical alloying (MA) was employed for the prepara-
tion of NiAl precursor alloy (denoted as NiAlx–MA) with
a planetary ball mill machine (QM-3SP04, Nanjing Nanda
Instrument Co., Ltd.). 2 g Ni powder and 2 g Al powder were
added into a ball mill tank equipped with 60 g grinding ball,
and a certain amount of ethanol. The ball mill machine then
runs at 500 rad/min, during which the equipment adjusts the
direction of rotation every 2 h, until the required length of
time. The ball mill tank was transferred to a vacuum glove
box, and the alloy powder was taken out in a nitrogen atmos-
phere. Then the removed alloy powder was heat-treated in a
tube furnace for 2 h in a nitrogen atmosphere.
3 Results and Discussion
3.1 Performance of Diferent Commercial
Nanoporous Catalysts
Firstly, commercial nanoporous metal catalysts were
employed for guaiacol hydrogenation under the same reac-
tion conditions, and results were shown in Table 1. Similar
to the reference reported elsewhere [24, 40], the product’s
mixtures mostly included cyclohexanol, 2-methoxycy-
clohexanol and phenol. Cyclohexanol was the main product.
2-Methoxylcyclohexanol and phenol were the intermediates,
which could be furthermore transferred into cyclohexanol
through the removal of methoxy group and hydrogenated
saturation of benzene ring [10, 41], respectively. Under the
reaction conditions, the nanoporous Cu, Fe and Co showed
low activity to the catalytic hydrogenation of guaiacol.
The conversion was less than 20%, and the selectivity of
cyclohexanol was less than 50%. Comparatively speaking,
2.3.2 Activation Process of Precursor Alloy
The preparing process of NP-Ni described as follows: a 17%
NaOH aqueous solution is placed in a water bath at 50 °C,
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J. Lu et al.
Table 1 Catalytic activity of diferent nanoporous metal catalysts
area and pore size (Table 2). For the nanoporous metal cata-
lysts, the active sites might be relevant with catalytic per-
formance at a certain extent because their inner and outer
surface was fully comprised of the metal element, which
corresponding to the catalytic active site. According to the
C/A data in Table 2, the catalytic action sequence per spe-
cifc surface area listed as followed: Cu>Co>Ni>Fe. Since
NP-Ni has a specifc surface area of 55.9 m2 g−1, which was
higher than that of other three catalysts, NP-Ni showed the
best catalytic performance.
Cat. Conv. (%) Sel. (%)
Cyclohexanol 2-Methoxy
cyclohexanol
Phenol Others
Cu
Fe
Co
Ni
14.6
2.5
16.1
60.3
34.9
40.0
48.4
62.2
24.7
36.0
13.0
21.0
15.1
12.0
34.2
10.1
25.3
12.0
4.4
6.7
Reaction conditions: 2 mmol guaiacol, 10 ml H2O, 0.025 g catalyst,
180 °C, 4 MPa, 2 h
The effect of reaction temperature was investigated
for guaiacol hydrogenation with NP-Ni catalysts, and
results were shown in Fig. 1a. As the reaction temperature
increased, the conversion of guaiacol also promoted, but the
selectivity of cyclohexanol as the main product appeared to
increase frst and then decrease. When the temperature was
raised to 180 °C, the conversion of guaiacol was 94.3%, and
the selectivity of cyclohexanol was 89.4%. As the tempera-
ture continues to rise, the conversion and selectivity are not
dramatically improved.
Table 2 Physicochemical characters of diferent nanoporous catalysts
Cat.
Surface area
Pore volume
Pore diameter C/A*
(m2 g−1
)
(ml g−1
)
(nm)
Cu
Fe
Co
Ni
10.5
25.7
13.2
55.9
0.09
0.10
0.07
0.15
20.8
12.2
12.4
6.6
0.0139
0.00097
0.0122
0.0108
Generally, guaiacol was attributed to one kind of phenolic
compounds and its phenolic hydroxy group might react with
active metal Ni or Al, which would destroy the structure of
NP-Ni. Therefore, ICP-AES was used for the detection of
product’s solution and results were shown in Table 3. Obvi-
ously, the higher the reaction temperature became, the more
the catalyst metal element dissolved in the solution, meaning
the serious activity loss of NP-Ni. Balancing the catalytic
activity and stability, 180 °C was chosen as the optimum
reaction temperature.
*C/A was defned as the ratio of conversion of guaiacol in Table 1
and specifc surface area
NP-Ni can efectively catalyze the transformation of guai-
acol, and the selectivity to cyclohexanol was 62.2%, indicat-
ing its best performance for the hydrogenation of benzene
ring and cleavage of the C-O bond among several nanopo-
rous catalysts.
Furthermore, N2 adsorption–desorption of several cata-
lysts was carried out for the analysis of the specifc surface
The efect of reaction pressure on guaiacol hydrogenation
was investigated at 180 °C. As shown in Fig. 1b, when the
Fig. 1 a efect of reaction temperature on guaiacol hydrogenation; b efect of reaction pressure on guaiacol hydrogenation. Reaction conditions:
2 mmol guaiacol, 10 ml H2O, 0.025 g NP-Ni, 4 h
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Selective Hydrodeoxygenation of Guaiacol to Cyclohexanol Catalyzed by Nanoporous Nickel
Table 3 Efect of temperature on the amounts of leached elements
to be lower and wider than that of Ni–IM and Ni–AM, indi-
cating its lower crystallization degree or smaller average
particle size for Ni–MA catalyst.
Element
Temperature
180 °C
200 °C
220 °C
The physical properties such as specifc surface area, pore
volume and pore size generally have an important infuence
on the activity and selectivity of the catalyst. So N2 physi-
cal adsorption–desorption experiments were conducted to
examine the physical properties and the results were summa-
rized in Table 4. It can be observed that three kinds of NP-Ni
catalysts have a rich pore structure. The pore volume and
average particle diameter of the three catalysts are similar,
and the specifc surface area is more than 35 m2 g−1, which
can provide a large number of active sites for catalysis reac-
tion. Ni–MA has moderate specifc surface area and pore
size compared to other catalysts.
Al
Ni
0.0006%
0.0682%
0.0018%
0.7245%
0.0073%
1.6531%
Reaction conditions: 2 mmol guaiacol, 10 ml H2O, 0.025 g NP-Ni,
4 MPa, 2 h
reaction pressure was increased from 1 to 2 MPa, the yield of
guaiacol increased from 73.4 to 86.9%, and the selectivity of
cyclohexanol increased from 82.8 to 87.7%. As the pressure
continued to increase, the conversion of guaiacol increased
slightly, but the selectivity decreased signifcantly. The main
by-product was 2-methoxycyclohexanol intermediate, which
is hardly transferred into cyclohexanol through the C-O bond
cleavage. Obviously, the lower hydrogen pressure might be
benefcial to suppress the generation of 2-methoxycyclohex-
anol, so 2 MPa H2 pressure was chosen as the optimum reac-
tion pressure.
Three kinds of NP-Ni were employed for guaiacol hydro-
genation to examine their catalytic performance and results
were shown in Table 5. Under the same reaction conditions,
Ni–AM and Ni–MA have better activity, and the conver-
sion are 99.1% and 99.7%, respectively. This indicates that
the specifc surface area does not completely determine the
activity of the catalyst. According to the XRD results, the
amorphization of metal may has an important infuence on
the catalytic performance. Ni–MA has an obvious amor-
phous structure compared with Ni–IM and Ni–AM accord-
ing to the analysis of difraction peak intensity in Fig. 2b. In
summary, it could be concluded that the preparation method
has a signifcant efect on the catalytic performance.
For further to compare three catalyst’s stability, the recy-
cling experiments were investigated. As showed in Fig. 2d,
the catalytic activity of Ni-IM showed the best catalytic per-
formance in the frst kettle with 94.3% conversion of guai-
acol and 89.4% selectivity of cyclohexanol. The catalytic
activity of Ni–AM also signifcantly decreased in recycling
experiments. It may be the case that the higher reaction tem-
perature and the acidity of intermediate phenol result in the
decrease of catalyst activity. And it was found that the NP-Ni
prepared by MA showed excellent performance in reactivity
and stability, the conversion of substrate and main product
yield remains relatively stable (the average conversion rate
is 99.0%, and the selectivity of cyclohexanol exceeds 85%).
Until the ninth recycling test, the catalyst activity showed a
signifcant decrease (the conversion rate is 86.6%, and the
selectivity is 74.6%). Therefore, the stability of the NP-Ni
prepared by the MA is best.
3.2 Catalytic Performance Comparison of NP‑Ni
Prepared with Diferent Method
According to Table 3, Ni and Al element of commercial
NP-Ni could be etched in the reaction solution, so three
methods (IM, AM and MA) were employed for the prepa-
ration of NiAl precursor alloy to promote the stability of
NP-Ni catalyst.
Firstly, NiAl precursor alloy prepared with three meth-
ods was characterized with XRD and results were shown
in Fig. 2a. It can be found that the precursors have obvious
alloy phases with NiAl3 (JCPDS Card No. 02-0416) and
Ni2Al3 (JCPDS Card No. 14-0642) as the main components.
Generally speaking, the stability of Ni2Al3 is higher than that
of NiAl3 phase, thus Ni2Al3 phase mainly provides a stable
nanoporous structure, while NiAl3 phase is etched during
the activation process. It is worth noting that the NiAlx–MA
precursor alloy phase showed the lowest difraction peak,
indicating the highest degree of amorphization among three
alloys.
Furthermore, the NiAl alloy would be activated with
NaOH and most of the metal aluminum in the alloy precur-
sor would be etched to form a skeleton catalyst with the
main components of nickel. XRD characterization of NP-Ni
catalyst obtained by three diferent precursor alloys is shown
in Fig. 2b, in which 2θ=44.5°, 51.8°, 76.4° and 92.9° was
corresponding to difraction peak of Ni (111), (200), (220),
(311) (JCPDS card No. 04-0850). At the same time, a small
amount of Ni2Al3 was detected because the intermetal-
lic compound is more difcult to be etched by NaOH and
removed from the alloy phase. The peak of Ni–MA seemed
In order to investigate the diference in the stability of
the NP-Ni prepared by three methods, we measured the
loss amount of NP-Ni with the analysis of ICP-AES. As
shown in Table 6, NP-Ni prepared by MA has the least
amount of loss in the frst tank, and the loss of nickel
is only 0.1891%, which is far lower than that of Ni–IM
(1.1545%) and Ni–AM (0.6348%). It can be noted that
the elements of the NP-Ni prepared by MA are not easily
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J. Lu et al.
1 3

Selective Hydrodeoxygenation of Guaiacol to Cyclohexanol Catalyzed by Nanoporous Nickel
Table 4 Physicochemical characters of diferent NP-Ni catalysts
initial catalytic activity (Table 5) for the reaction that the
metallic state Ni generally played a key role in HDO process.
Catalysts
Surface area
Pore volume
Pore
(m2 g−1
)
(ml g−1
)
diameter
(nm)
3.3 Study of NP‑Ni Prepared with MA Method
Ni-IM
Ni-AM
Ni-MA
58.1
37.0
41.7
0.18
0.11
0.11
7.5
11.8
10.9
3.3.1 Efect of Milling Time
The ball milling process includes multiple steps, such as
metal powder’s refnement, fracture, cold welding, etc., so
the ball milling time often has a critical infuence on the
alloy structure and corresponding performance. Four kinds
of NiAl alloy were prepared under diferent ball milling
time from 10 to 40 h, and the alloy composition and micro-
structures were characterized by the XRD (Fig. 3a). As the
ball milling time was lengthened, the characteristic peak of
single metal nickel or aluminum shows a tendency to be
weakened. Among them, the difraction peak of aluminum
is the most obvious, and almost completely disappears after
40 h of ball milling. This is explained by the fact that a
long period of MA made the nickel and aluminum metal
achieve to interatomic fusion, thus the crystal lattice gradu-
ally changed to be the solid solution state.
Table 5 Catalytic performance of NP-Ni prepared with diferent
methods
Catalysts Conv. (%) Sel. (%)
Cyclohexanol 2-Methoxy Phenol Others
cyclohex-
anol
Ni-IM
Ni-AM
Ni-MA
94.3
99.1
99.7
89.4
92.7
90.1
9.1
5.5
8.8
0.8
0.3
0.1
0.6
1.4
1.0
Reaction conditions: 2 mmol guaiacol, 10 ml H2O, 0.025 g NP-Ni,
180 °C, 2 MPa, 4 h
Furthermore, the surface morphology of the alloy was
observed by SEM. It can be viewed in Fig. 3d that the alloy
particles become smaller and uniform as the milling time
was extended, indicating the gradual alloying process of the
nickel and aluminum metal powder. The above phenomenon
is in agreement with the XRD results. The metal powders
tend to exist as smaller particles under the action of mechan-
ical force with the grinding ball.
Table 6 The loss amount of NP-Ni after one recycling
Element
Catalysts
Ni-IM
Ni-AM
Ni-MA
Al
Ni
0.0009%
1.1545%
0.0009%
0.6348%
0.0065%
0.1891%
The catalytic performance of as-prepared NP-Ni was
investigated for guaiacol hydrogenation, and results were
shown in Table 7. With the prolongation of milling time, the
conversion of guaiacol increased gradually, and the yield of
cyclohexanol increased at the same time. It indicated that
the longer milling time was favorable for MA between two
kinds of metal powder, thus improving the catalytic activity
of NP-Ni.
Reaction conditions: 2 mmol guaiacol, 10 ml H2O, 0.025 g NP-Ni,
180 °C, 2 MPa, 4 h
corroded and have higher stability. Therefore, the catalyst
prepared by MA maintains stable activity and selectivity
in the recycling experiments, while the catalyst prepared
by the other two methods performs poorly in the contrast
experiment.
3.3.2 Efect of Heating Treatment Temperature
XPS was used to characterize the chemical state of nickel
for the as-prepared catalysts. As shown in Fig. 2c, the peaks
at 852.5 eV, 852.4 eV and 852.2 eV originated from the
metallic Ni 2p3/2 element. Peaks with higher binding ener-
gies at 856.5 eV, 855.8 eV and 856.1 eV were considered
as the oxidative state Ni2+ 2p3/2 spectra, which might be
derived from the air oxidation of NP-Ni, during the etch-
ing process of NiAl alloy or the preparation process of test-
ing sample. According to the peak area of deconvolution,
the relative concentration of metallic state Ni and oxida-
tive state Ni were calculated to be 0.7, 11 and 3.8, respec-
tively for Ni–IM, Ni–AM and Ni–MA. It indicated that the
anti-oxidation ability for three catalysts listed as followed:
Ni–IM<Ni–MA<Ni–AM, which is coincident with their
Furthermore, NiAl alloy was heating treated under N2
atmosphere at 500 °C, 700 °C and 900 °C, respectively.
XRD was used for the characterization of as-prepared alloy
and results were shown in Fig. 3b. It seemed that the heat-
ing treatment temperature had an obvious efect on the for-
mation of the metal crystal phase. After heat treatment at
500 °C, the alloy undergoes high-temperature transforma-
tion to form Ni2Al3. It is very similar for the heat treatment
temperature between 700 °C and 500 °C, and the difraction
peak of Ni2Al3 becomes sharper compared to that of the
original alloy. When the temperature is raised to 900 °C, the
peak shape of the difraction peak becomes sharper and the
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J. Lu et al.
Fig. 3 a XRD patterns of NiAl alloy with diferent milling time; b XRD patterns of alloy with diferent heating treatment temperature; c XRD
patterns of alloy with diferent control agent; d SEM images of alloy with diferent milling time: (1) 10 h, (2) 20 h, (3) 30 h, (4) 40 h
Table 7 Efect of milled time on the catalytic performance of NP-Ni
shows a tendency to rise frst and then decrease. After the
heating treatment at 700 °C, the conversion increased from
81.8 to 97.4%, and the selectivity of cyclohexanol increased
from 83.9 to 85.3%. It can be understood as the fact that the
metallic elements in the alloy after ball milling can be mutu-
ally difused by heating treatment, thus the alloy structures
become more uniform to form the intermetallic compound.
However, when the heating treatment temperature reaches
900 °C, the catalytic activity of NP-Ni decreases. It might
be due to the decreasing of defective active sites, caused by
the increase of crystallization degree of alloy.
Time Conv. (%) Sel. (%)
Cyclohexanol 2-Methoxy
cyclohexanol
Phenol Others
10
20
40
26.3
57.6
81.8
45.6
76.6
83.9
6.5
7.5
11.1
36.5
13.9
3.2
11.4
2.1
1.8
Reaction conditions: 2 mmol guaiacol, 10 ml H2O, 0.025 g NP-Ni,
180 °C, 2 MPa, 4 h
Figure 4a shows the HRTEM of the catalyst obtained
from activation of NiAl alloy annealed at 700 °C. The inter-
planar spacing of the lattice fringes in the particles was
0.20 nm, which matched well with the (111) crystal plane
of fcc Ni. Further EDS characterization results show that the
Al element is evenly distributed in the structure of the cata-
lyst (Fig. 4b). According to the characterization information
peak width becomes narrower, indicating that the degree of
crystallization is higher or the particles size increases.
The catalytic performance of the heat-treated catalysts
was evaluated for guaiacol hydrogenation and the results
were shown in Table 8. As the heating treatment temperature
increases, the catalytic activity of the corresponding catalysts
1 3

Selective Hydrodeoxygenation of Guaiacol to Cyclohexanol Catalyzed by Nanoporous Nickel
Table 8 Efect of heating
Temperature (°C)
Conv. (%)
Sel. (%)
treatment temperature
Cyclohexanol
2-Methoxy
Phenol
Others
cyclohexanol
Untreated
500
700
81.8
91.0
97.4
73.8
83.9
81.3
85.3
69.9
11.1
16.4
14.1
16.7
3.2
0.4
0.1
3.9
1.8
1.9
0.5
9.5
900
Reaction conditions: 2 mmol guaiacol, 10 ml H2O, 0.025 g NP-Ni, 180 °C, 2 MPa, 4 h
Fig. 4 a HRTEM patterns of NP-Ni; b EDS elemental mapping of NP-Ni
of XRD, the Al element in the catalyst exists in the form of
alloy phase and it was generally considered to play important
roles in stabilizing the nanoporous structure as an indispen-
sable ingredient of alloy ligament.
Table 9 The relationship between alloy yield and process control
agent dosage
Process control agent dosage (ml)
Alloy yield (%)
0.1
31.5
0.2
76.5
0.4
85.6
3.3.3 Efect of Process Control Agent Dosage
The amount of control agent can signifcantly afect the
structure of the alloy powder. Besides, it also can prevent
cold welding between the metal powder and the ball mill,
thus prompting the alloy yield. As shown in Fig. 3c, XRD
patterns show that the difraction peak intensity of NiAl
alloy is higher when the ethanol dosage is 0.1 ml and 0.2 ml.
When the amount of the control agent is increased to 0.4 ml,
the excess control agent increases the fuidity of the alloy
powder and reduces the cold welding, so the speed of MA
gradually becomes slow. On the other hand, the amount of
the control agent seriously afects the yield of the alloy pow-
der, as shown in Table 9. When the dosage is 0.1 ml, cold
welding is very serious, causing less yield of metal powder,
only 31.5% of the initial mass. Considering the speed of MA
and the yield of powder, 0.2 mL of ethanol was selected as
the process control agent dosage.
3.4 Proposed Reaction Mechanism
In order to explore the reaction mechanism of guaiacol
hydrogenation, the content of each substance was traced
with analysis of gas chromatography during the reaction pro-
cess and the concentration–time curve are shown in Fig. 5a.
Obviously, the yield of cyclohexanol as the main product
gradually accumulates with the reaction time, reaching a
maximum value (81.5%) at 12 h, and then keeping basically
stable. The content of phenol as a frst intermediate increases
and then decreases with a maximum accumulation of 6.3%.
1 3

J. Lu et al.
Fig. 5 a The concentration–time curve for guaiacol hydrogenation with NP-Ni; b proposed reaction pathways for guaiacol over NP-Ni. Reaction
conditions: 2 mmol guaiacol, 10 ml H2O, 0.025 g NP-Ni, 180 °C, 2 MPa
2-Methoxycyclohexanol as another intermediate rapidly
accumulated and reached a maximum value of 9.3% at 4 h.
After that, 2-methoxycyclohexanol slowly decreased with
the reaction time, indicating that it is difcult for the conver-
sion of 2-methoxycyclohexanol to cyclohexanol through the
scissor of the C–O bond.
rate of phenol is 20 times of that of guaiacol under the
same reaction conditions. It indicates that phenol was eas-
ily converted into cyclohexanol, which is consistent with
the results of Fig. 5a, in which only small amount’s phenol
were detected because of the quick transformation of phenol
to cyclohexanol. On the other hand, it is difcult to transfer
another intermediate 2-methoxycyclohexanol to cyclohex-
anol through the removal of methoxy group under the same
conditions when using 2-methoxycyclohexanol as a start-
ing material. As a result, 2-methoxycyclohexanol accumu-
lated in Fig. 5a because it could not be transferred into fnal
products. Thus, it can be concluded that Path A is the main
pathway for the conversion of guaiacol to cyclohexanol and
the rate-control step is the demethoxylation of guaiacol to
phenol (Fig. 5b).
According to the result mentioned above and references
ever reported [14, 42, 43], a proposed mechanism is con-
cluded that there are three reaction paths for the catalytic
hydrogenation of guaiacol to cyclohexanol. Path A, guaiacol
was converted to phenol intermediate through the removal
of methoxy group, and phenol is further hydrogenated to
cyclohexanol. Path B, guaiacol is first hydrogenated to
2-methoxycyclohexanol by the saturation of benzene ring,
then 2-methoxycyclohexanol is demethoxylated to cyclohex-
anol. Path C, guaiacol is demethanized to 1, 2-cyclohex-
anediol, which is then further dehydroxylated to form
cyclohexanol.
4 Conclusion
In order to investigate the rate-limited process of guaiacol
conversion, the control experiments were carried out under
the same reaction conditions with phenol and 2-methoxycy-
clohexanol intermediates as starting materials, respectively.
The results are presented in Table 10. When phenol and
guaiacol are used as the raw materials, the productivity ratio
of cyclohexanol is 0.35/0.017=20, that is, the conversion
In this contribution, the catalytic hydrogenation of guaiacol
is investigated with several kinds of nanoporous catalysts
and NP-Ni exhibited the highest catalytic performance under
the same reaction conditions. In order to solve the problem
of NP-Ni’s low stability caused by the etching of acidic solu-
tion due to the generation of phenol intermediate, several
Table 10 Control experiments
Substrate
Time (h)
Con. (%)
Sel. of cyclohexanol (%)
Productivity
(molcyclohexanol
gcatalyst −1 h−1
)
Guaiacol
Phenol
4
0.2
94.3
90.8
5.0
89.4
95.9
24
0.017
0.35
0.00024
2-Methoxy cyclohex- 4
anol
Reaction conditions: 2 mmol guaiacol, 10 ml H2O, 0.025 g NP-Ni, 180 °C, 2 MPa, 4 h
1 3

Selective Hydrodeoxygenation of Guaiacol to Cyclohexanol Catalyzed by Nanoporous Nickel
18. Pal N, Pramanik M, Bhaumik A, Ali M (2014) J Mol Catal A
methods were employed for the preparation of NiAl pre-
cursor alloy. MA seemed to be an efective method for the
alloying process and the as-prepared NP-Ni showed the best
anti-corrosion ability and stability. Under the optimal condi-
tion of 40 h milling time, 700 °C heat treatment temperature,
0.2 ml control agent dosages for the preparation of NiAl
alloy, the acquired NP-Ni could be recycled for 10 times.
Furthermore, the reaction mechanism was investigated with
NP-Ni for guaiacol hydrodeoxygenation. The main reaction
route is concluded that guaiacol was converted to phenol
intermediate through the removal of methoxy group, and
phenol is further hydrogenated to cyclohexanol. The rate-
control step is considered to be the demethoxylation of guai-
acol to phenol.
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1 3

J. Lu et al.
Afliations
Jiqing Lu1 · Xing Liu1 · Guanqun Yu1 · Jinkun Lv1 · Zeming Rong1 · Mei Wang1 · Yue Wang1
1
State Key Laboratory of Fine Chemicals, School
of Chemical Engineering, Dalian University
of Technology, Dalian, Liaoning Province 116024,
People’s Republic of China
1 3