Selective Preparation of 4-Alkylphenol from Lignin-Derived Phenols and Raw Biomass over Magnetic Co–Fe?N-Doped Carbon Catalysts

Article abstract of DOI:10.1002/cssc.201901578

Lignin valorization to produce high-value chemicals selectively is an enormous challenge in biorefinery. In this study, 4-alkylphenol, formed by breaking the robust C_aryl?OCH₃ bonds solely with the retention of other structures in lignin-derived methoxylalkylphenols, was produced selectively over a Co₁–Fe_0.1?NC catalyst from real lignin oil as feedstock, which was obtained by a “lignin-first” strategy from either birch or cornstalk. A yield of 64.7 or 88.3 mol % of 4-propylphenol was obtained if birch lignin oil or eugenol was used as the substrate, respectively. The catalysts were characterized by using methods that include Brunauer–Emmett–Teller measurements, XRD, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and temperature-programmed desorption with synchrotron vacuum ultraviolet photoionization mass spectrometry. The results of catalyst characterization and comparison experiments indicated that CoN_x was the main active phase for demethoxylation and hydrogenation, and the incorporation of Fe weakens the adsorption of 4-propylphenol to the catalyst, which inhibits the excessive hydrogenation of 4-propylphenol. This work shows the potential to produce high-value-added 4-alkylphenol from renewable raw biomass.

Full text of DOI:10.1002/cssc.201901578

Accepted Article
Title: Selective Preparation of 4-alkylphenol from Lignin-derived
Phenols and Raw Biomass over magnetic Co-Fe@N-doped
Carbon Catalysts
Authors: Xiaohao Liu, Chenguang Wang, Ying Zhang, Yan Qiao, Yang
Pan, and Longlong Ma
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To be cited as: ChemSusChem 10.1002/cssc.201901578
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10.1002/cssc.201901578
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Selective Preparation of 4-alkylphenol from Lignin-derived
Phenols and Raw Biomass over magnetic Co-Fe@N-doped
Carbon Catalysts
Xiaohao Liu1,3, Chenguang Wang1, Ying Zhang2 *,Yan Qiao4, Yang Pan5, Longlong Ma1,3*
Abstract: Lignin valorization to selectively produce high-valued
chemicals is an enormous challenge in biorefinery. In this study, 4-
alkylphenol, formed by solely breaking the robust Caryl-OCH3 bonds
while retaining other structures in lignin-derived methoxyl-
alkylphenols, was selectively produced over Co1-Fe0.1@NC catalyst
using real lignin oil as feedstock, which was obtained by ‘lignin-first’
strategy from either birch or cornstalk. 64.7 mol% or 88.3 mol% yield
of 4-propylphenol was obtained when birch lignin oil or eugenol was
used as substrate, respectively. The catalysts were characterized by
a series of methods including BET, XRD, XPS, HRTEM, HAADF-
STEM, EDS, and TPD-SVUV-PIMS. The results of catalyst
characterization and comparison experiments indicated that CoNx
proposed in recent years to effectively depolymerize lignin prior to
carbohydrate valorization. Compared with traditional biorefinery
process, this method can effectively utilize all components of
lignocellulose, and lignin oil with nearly theoretical yield of
monophenols has been reported (the maximum theoretical
monomer yield for birch is 45–58%, and 52 % yield of monomer
was obtained by using Ru/C+methanol system) 4,5. However, a
large amount of methoxy groups and other oxygen-containing
groups are present in these phenols, which making lignin oil
cannot be utilized directly6. Therefore, hydrodeoxygenation
(HDO) was carried out to remove these groups (Figure 1).
Although many works have been done to selectively produce
was the main active phase for the demethoxylation and hydrogenation, arenes, cycloalkanes or cyclitols from lignin or its derived
and the incorporation of Fe weakens the adsorption of 4-propylphenol
to the catalyst, thereby inhibiting the excessive hydrogenation of 4-
propylphenol. This work shows the potential to produce high value-
added 4-alkylphenol from renewable raw biomass.
methoxylphenols7-13, the reports on the selective preparation of 4-
alkylphenol are rare, mainly because it is difficult to avoid other
side reactions when breaking the robust Caryl-OCH3 bond of the
methoxyl-alkylphenols (Scheme 1). The 4-alkylphenol is
important feedstock in industry and widely used as the
intermediates in preparation of phenolic resins, nonionic
surfactants, and agrochemicals14. In recent studies, Ru-Ni/HZSM-
5, Au/TiO2, Ag/TiO2 and Pt/C showed good activity in converting
lignin-derived methoxylphenols to (4-alkyl)phenol15-18. When
using guaiacol or 4-propylguaiacol (dihydroeugenol) as substrate,
50-70% yields of phenol or 4-propylphenol were obtained under
optimal conditions. However, expensive noble metals or harsh
reaction conditions (300-400 °C) were required in the reaction.
MoCx/C also showed a good activity in converting guaiacol to
phenol (75% yield)19. However, when complex pyrolysis oil was
used as the reactant, arenes were the major product while only
2.5 wt% yield of alkylphenols was obtained. It should be noted
that the components of lignin oils obtained by different
depolymerization methods are different, so the results of
upgrading these lignin oils may also be different20. Most recently,
Song et al. used MoOx/C as catalyst to catalyze 4-propylguaiacol
to 4-propylphenol with 71% yield at 320 °C and 3 MPa H2. Using
corn stover lignin oil as feedstock, the monophenols in the lignin
oil were efficiently converted to 4-alkylphenol21.
Introduction
Lignocellulosic biomass is the most abundant renewable
carbon resource on earth. As an important part of lignocellulose,
lignin has the potential to substitute the traditional aromatic
resources because it is composed by various methoxylated
phenylpropanoid units1, 2. Nevertheless, lignin valorization has
always been a challenge due to its complex and recalcitrant three-
dimensional amorphous structure3. ‘Lignin-first’ method was
[1]
[2]
X. Liu, Prof. C. Wang, Prof. L. Ma
CAS Key Laboratory of Renewable Energy, Guangdong Key
Laboratory of New and Renewable Energy Research and
Development, Guangzhou Institute of Energy Conversion, Chinese
Academy of Sciences, Guangzhou 510640, China
E-mail: mall@ms.giec.ac.cn
Prof. Y. Zhang
Department of Chemistry, University of Science and Technology of
China, Hefei, 230026, China.
E-mail: zhzhying@ustc.edu.cn
In our previous study8, Co@NC, which was obtained by
pyrolysis of Co supported on cellulose in ammonia, showed a full
conversion of eugenol to 4-propylcyclohexanol under mild
conditions. Different from other hydrogenation metals such as Ru,
[3]
[4]
X. Liu, Prof. L. Ma
University of Chinese Academy of Sciences, Beijing 100049, China
Prof. Y. Qiao
Shanxi Engineering Research Center of Biorefinery, Institute of Coal
Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China
Prof. Y. Pan
National Synchrotron Radiation Laboratory, University of Science
and Technology of China, Hefei, 230029, China
Pd, and Ni, cobalt-based catalysts can selectively cleave the Caryl
-
OCH3 bond before the hydrogenation of benzene ring, which is
the most advantageous route for the 4-alkylphenol production
(Scheme 1). However, the 4-propylphenol was easily further
hydrogenated to 4-propylcyclohexanol over monometallic cobalt-
[5]
Supporting information for this article is given via a link at the end of the
document.
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Figure 1. Two strategies for lignocellulose refining. The green dashed box represents a two-step process for preparing 4-alkylphenol and 4-alkylcyclohexanols from
the lignocellulosic biomass in this work. The HDO of lignin oil was carried out in an autoclave reactor using dodecane as reaction solvent. The yield of lignin oil
obtained from birch wood was about 25 wt%, that is, 1 g of birch wood could obtain 250 mg of lignin oil. The monophenols content in the lignin oil is 36.5wt%. The
mass yield of 4-alkylphenol from birch lignin oil was 18.1 wt%. That is, 18.1 mg of 4-alkylphenol can be obtained from 100 mg of lignin oil. The yield in parentheses
is the yield of 4-alkylphenol from raw birch wood. That is, about 4.5 mg of 4-alkylphenol can be obtained from 100 mg of birch wood.
Results and discussion
Preparation of 4-alkylphenol from raw biomass
To prepare 4-alkylphenol from raw biomass, a two-step process
was developed (Figure 1). First, lignin oil was obtained by
catalyzing the depolymerization of raw biomass in Ru/C +
methanol system (lignin-first process); then the obtained lignin oil
was further hydrodeoxygenated to produce 4-alkylphenol. Ru/C
was chosen as the catalyst to depolymerize lignin due to its high
hydrogenation activity to rapidly stabilize the intermediates,
preventing them from polymerizing again to form oligomers which
Scheme1. Reaction pathway for selective HDO of lignin-derived phenols to 4-
are difficult to depolymerize, thereby obtaining more
monophenols.4, 22 Two different types of biomass, birch wood and
alkylphenol
based catalyst7,8. Therefore, catalyst with high hydrogenation
activity is detrimental to selectively obtaining 4-
cornstalk were selected to prepare 4-alkylphenol. Birch wood
lignin rich in S-units was efficiently depolymerized to
dihydroeugenol (4-propylguaiacol) (11.8 wt%) and 2,6-dimethoxy-
4-propylphenol (22.6 wt%) with high selectivity in phenol-oil, while
other monophenols such as 4-methylguaiacol and 4-ethylguaiacol
were extremely low (Table S1). Unlike birch wood, the phenol oil
derived from cornstalk was very complex. A viscous and dark
phenolic oil with low monophenolic content (5.8 wt%) was
obtained due to the low content and different structure of cornstalk
lignin. After obtaining the phenol oil, the next and also a critical
step is to convert the phenols to 4-alkylphenol while avoiding side
reactions (Scheme 1). As shown in Figure 2a, when using Co1-
Fe0.1@NC (Fe/Co mass ratio was 0.1/1) as catalyst, the
monophenols in birch lignin oil were converted, and 64.7 mol%
yield of 4-propylphenol and small amount of 4-propylcyclohexanol
were obtained under 250 °C, 0.6 MPa H2. The total yield of 4-
alkylphenol reached 67.8 mol% (Table 1). The mass yield of 4-
alkylphenol from birch lignin oil was 18.1 wt%, and the yield from
alkylphenol,especially when complex bio-oil containing different
phenols (one or two methoxy groups and different para-
substituents) are used as feedstock. The conversion rates of
different phenols in bio-oil are different. If the catalyst cannot
inhibit the hydrogenation of the 4-alkylphenol effectively, it is
difficult to obtain such a single kind of product from bio-oil with
high selectivity. In this work, Fe, with a weak hydrogenation
activity, was employed as a second-metal to modulate the
Co@NC catalyst. 4-alkylphenol were selectively obtained when
real lignin oil was used as feedstock, which was obtained by
‘lignin-first’ strategy from either birch or cornstalk. Using eugenol
as a model compound, a high yield of 4-propylphenol was
achieved under mild conditions. A series of Co-Fe@NC catalysts
with different Fe/Co ratios were prepared and characterized to
explore the effects of Fe. The catalytic mechanisms were
proposed and the stability of the catalyst was investigated.
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Table 2. HDO of eugenol over Co-Fe@NC catalystsa
Yield (%)
3
Entry
Catalyst
Conversion (%)
1
2
4
1b
2b
3b
4b
5b
Co@NC
100
100
100
100
100
0.6
-
-
56.6
22.7
21.9
1.2
Co1-Fe0.05@NC
Co1-Fe0.1@NC
Co1-Fe0.15@NC
Co1-Fe0.2@NC
57.9
88.3
52.5
43.5
5.8
5.4
4.8
4.1
trace
trace
trace
41.5
47.3
6b
7c
Co1-Fe0.4@NC
Fe@NC
100
19.9
--
75.9
15.5
trace
--
trace
--
37.8
a
Reaction conditions: 164 mg of eugenol, ca. 40 mg of catalyst, 10 mL of
dodecane at 250 °C , 0.6 MPa H2 for 8 h. b Other products that not listed in the
table are low in content, including 4-propylcyclohexene, 2-methyl-4-propylphenol
and other alkylated 4-propylphenol, propylbenzene.c Other products are mainly
isoeugenol.
corresponding demethoxyphenols such as phenol, 4-
methylphenol, 4-ethylphenol and 4-propylphenol, while the
content of over-hydrogenation products (cyclohexanols) was
low, indicating that the Co1-Fe0.1@NC catalyst had high
catalytic activity even in complex chemical environment.
Additionally, 93.5 mol% yield of propylcyclohexanol was
selectively obtained from birch lignin oil using Co@NC as the
catalyst (Figure S1).
Co-Fe@NC catalytic activity test and discussion
To investigate the effect of Fe, a series of catalysts with
different Fe/Co ratios were prepared. Eugenol was used as a
model compound of lignin-derived methoxyl-alkylphenols to
explore the activity of the catalysts. As shown in Table 2, Co@NC
converted eugenol completely and the main products were over-
hydrogenated products including 56.6% 4-propylcyclohexanol
and 21.9% 4-propylcyclohexane, while the yield of 4-propylphenol
was only 0.7% (entry 1). However, the incorporation of a very
small amount of Fe (Fe/Co=0.05）significantly increased the yield
of 4-propylphenol to 57.9% (entry 2). 88.3% yield of 4-
propylphenol was achieved when Co1-Fe0.1@NC was used as
catalyst, which should be the highest yield that reported for 4-
alkylphenol production from lignin-derived phenols, and only 5.8%
of propylcyclohexanol and trace amount of alkanes were formed.
Further increasing the Fe content, the yield of 4-propylphenol
decreased with dihydroeugenol increased. Especially, only 19.9%
yield of 4-propylphenol was obtained over Co1-Fe0.4@NC. Other
products were mainly dihydroeugenol (75.9%), and almost no
excessive hydrogenation product was formed. For Fe@NC (entry
7), the activity of the catalyst was extremely low and no 4-
propylphenol was produced.
Figure 2. Gas chromatograms of (a) lignin oil from birch wood and its HDO
products (b) lignin-oil from cornstalk and its HDO products over Co1-Fe0.1@NC
Table 1. HDO of lignin oil from birch wood over Co1-Fe0.1@NC
Products
Yield wt%
Yield mol%
4-propylphenol
4-ethylphenol
17.4 (4.4)
0.4
64.7
1.8
4-methyphenol
phenol
0.3
1.3
0.6
3.4
4-propylcyclohexanol
4-propylguaiacol
Total 4-alkylphenol
3.0
10.7
5.8
1.9
18.1(4.5)
67.8
Reaction conditions: 120 mg of lignin oil, 40 mg of Co1-Fe0.1@NC,10mL of
dodecane, 0.6 MPa H2, 250 °C. The yield in parentheses is the yield of
products from raw birch wood.
raw birch wood was about 4.5 wt%. Higher concentration of birch
lignin oil was also investigated (Table S3, Table S4). 52.6 mol%
yield of 4-alkylphenol was obtained when 2.0 g of lignin oil with 8
mL of solvent was added to the reactor. The HDO of corn stalk
lignin oil over Co1-Fe0.1@NC was also studied (Figure 2b, Table
S2). All of methoxyphenols were efficiently converted to the
A series of characterizations were carried out to explore why a
small amount of Fe can significantly change the catalytic activity.
The morphologies and metal dispersions of the catalysts are
shown in Figure S2. Surface area, pore structure and elements
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Table 3. The properties of Co1-Fex@NC catalysts
Content wt%
O
SBET
（m2/g)a
Pore
volume(cm3/g)a
Pore size
(nm)a
Catalysts
Cb
Nb
Hb
Coc
Fec
Fe/Co
Co@NC
51.1
84.7
115.1
82.1
136
0.12
0.18
0.27
0.23
0.37
0.24
0.2
1.51
1.63
1.63
1.63
1.63
1.63
1.1
59.56
57.3
52.33
48.9
52
8.97
9.18
8.03
8.19
8.1
2.63
2.54
2.5
5.61
6.42
4.93
8.19
7.43
9.63
6.41
23.23
23.31
29.52
28.30
25.23
18.77
--
--
--
Co1-Fe0.05@NC
Co1-Fe0.1@NC
Co1-Fe0.15@NC
Co1-Fe0.2@NC
Co1-Fe0.4@NC
Fe@NC
1.25
2.69
4.12
4.78
7.28
22.45
0.05
0.09
0.15
0.19
0.39
--
2.29
2.45
2.46
2.54
123.9
79.9
53.27
57.56
8.59
11.04
a measured by nitrogen adsorption/desorption, b measured by Elementar, c measured by AAS.
metallic cobalt (JCPDS No. 01-1255) and Co4N (JCPDS No. 41-
0943)23. However, as shown in the partial XRD patterns (Figure
3b), the diffraction peak of Co@NC was between the standard
XRD patterns of the two cobalt species. Compared with the
standard data of metallic cobalt, the peaks of Co@NC shifted
toward smaller angle, which could be due to the incorporation of
N into the lattice of cobalt metal to form cobalt nitride8. When Fe
was added, the peaks of cobalt nitride further shifted to smaller
angle and which was fully consistent with the standard XRD
pattern of cobalt nitride when Fe/Co ≥ 0.15. The shift of the peaks
could be due to a slight change in the N content of the cobalt
nitride. Herein, we represent these cobalt species as CoNx. The
diffraction peaks at 45.2° and 65.7° are attributed to (110) and
(200) planes of cubic Co7Fe324. For Co1-Fe0.1@NC, the peak of
Co7Fe3 (110) is very small, revealing that the Co7Fe3 is well
dispersed in the catalyst. As the Fe content increases, the
intensity of these peaks increases. When the Co/Fe ratio is 1/0.4,
there are almost only Co7Fe3 diffraction peaks, indicating that the
Co7Fe3 alloy is the main component in Co1-Fe0.4@NC. When only
Fe is present, the Fe mainly exists in the form of Fe2N25. In all of
the catalysts, no diffraction peaks of the Fe metal or Fe oxides
were observed
Figure 3. XRD patterns of (a) the Co1-Fex@NC catalysts with different Co/Fe
ratio (b) partial XRD patterns of Co1-Fex@NC catalysts and standard XRD
patterns of cobalt species.
The chemical state of the catalysts surface was investigated by
ex-situ X-ray photoelectron spectroscopy (XPS) analyses (Figure
4). Co0 ， cobalt oxides (Co-O), cobalt nitride(Co-N) were
observed on the surface of the catalysts8, 25. The presence of a
large amount of cobalt oxides on the catalyst surface which was
not detected by XRD could result from the surface oxidation of
cobalt nitride or Co0 when the catalysts were treated by 1% O2/N2.
For Fe2p spectra, peak fitting was not carried out because of the
interference of the Co Auger peak (713.0 eV) which was
overlapped with Fe2p3/2 peak27. It should be noticed that the in-
situ characterization is better to reflect the chemical state of the
catalysts surface. However, the harsh conditions of catalysts
preparation (650 °C, NH3 atmosphere) limit the use of in-situ
characterization.
Figure 4. XPS spectra of Co1-Fex@NC catalysts (a) Co 2p3/2 and (b) Fe 2p of
Co1-Fex@NC catalysts
To further confirm the structure of the catalyst, HRTEM of Co1-
Fe0.1@NC was performed (Figure 5a and 5b). The well-resolved
fringes with an interplanar spacing of 0.207 nm was observed,
which is attributed to the (111) plane d-spacing of the cobalt
nitride. However, the lattice fringes of Co7Fe3 were not found.
Therefore, HAADF-STEM and EDS analyses of Co1-Fe0.1@NC
were carried out to explore how the elements distributed, and
whether the cobalt and iron species co-exist or exist
independently. As shown in Figure 5c, representative region (red
contents of the catalysts are shown in Table 3. Briefly, a large
amount of metal particles with average size of 20-30 nm are
distributed on the N-doped carbon supports which formed by
cellulose carbonization in an ammonia atmosphere. The Fe/Co
ratio is consistent with the addition ratio of metal precursors in the
catalyst preparation.
The ex-situ X-ray diffraction (XRD) analyses are shown in
Figure 3. The diffraction peaks of Co@NC at 44.1°, 51.3° and
75.5° were both close to the (111), (200), and (220) planes of
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Figure 6. Product distributions of 4-propylphenol HDO as a function of reaction
time. The solid line represents the catalysis by Co1-Fe0.1@NC; the dotted line
represents the catalysis by Co@NC. Reaction conditions: 136 mg of 4-
propylphenol, 40 mg of catalyst, 10 mL of dodecane, 0.4 MPa H2, 250 °C .
Figure 5. (a) HRTEM images of Co1-Fe0.1@NC, (b) enlarged image of the
selected red box area in Figure 1a. (c) HAADF image of Co1-Fe0.1@NC, (d)
EDS images of the selected red box area in Figure 4c, (e) EDS line scan
across two particles as directed by the orange line, (f) catalyst model of Co1-
Fex@NC with low Fe content (Fe/Co≤0.1).
efficiently catalyze eugenol to 4-propylcyclohexanol8. The
difference in the activity of the two catalysts may be caused by
the newly formed Co7Fe3. However, Co1-Fe0.4@NC catalyst with
Co7Fe3 as the main component only achieved 19.9% yield of 4-
propylphenol, and the main product was dihydroeugenol (Table 2,
entry 6). Therefore, Co7Fe3 alloy was synthesized (Figure S4) and
used for the HDO of eugenol to qualitatively determine the
catalytic activity of the Co7Fe3 alloy (Scheme 2). No 4-
propylphenol was formed, and the products were dihydroeugenol
and isoeugenol under 250 °C , 0.6 MPa H2. This indicates that the
Co7Fe3 alloy has no demethoxy activity and the hydrogenation
activity is very low under the reaction conditions. Based on the
above results, it seems that CoNx is still the main active phase for
HDO of eugenol and the incorporation of Fe inhibits the
hydrogenation of the 4-propylphenol.
HDO of 4-propylphenol over Co@NC and Co1-Fe0.1@NC at
different reaction times were carried out. As shown in Figure 6,
using Co@NC as catalyst, more than half of the 4-propylphenol
has been converted to propylcyclohexanol during the initial
heating process (0 h). After reaction for 15 minutes, the
conversion of 4-propylphenol has been >80%. However, for Co1-
Fe0.1@NC, the conversion of 4-propylphenol was still less than
16% even after reaction for 4 h. It further confirming that the
presence of a small amount of Fe significantly inhibited the
hydrogenation of 4-propylphenol. This inhibition is very important
in the selective preparation of 4-alkylphenol from complex lignin
oil because the conversion rates of different phenols are different.
The previously produced 4-alkylphenol may have been
hydrogenated while other phenols (such as containing two
methoxy groups) with a slower conversion rate have not been
completely converted when the catalyst is not effective in
inhibiting the hydrogenation of the 4-alkylphenol.
box) containing both small and large particles were selected for
EDS analysis. In the Co and Fe EDS images (Figure 5d), it can
be observed that Fe species is well coexisted with Co species.
There is no Co species exist alone, although the Co content
(29.52 wt%) is much higher than Fe content (2.69 wt%) and the
composition shown in the XRD is mainly the cobalt nitride
species(Table 3 and Figure 3). From the N and C mapping images,
N element is well distributed on the support to form N-doped
carbon material. Figure 5e shows the EDS line scan across two
nanoparticles with different sizes. In both cases, the signal
change of Fe is synchronous with the Co, indicating that Co and
Fe are well distributed in the particles, no matter they are large or
small. The reason why the lattice fringes of Co7Fe3 were not
observed could be because the Co7Fe3 was well dispersed in the
particles, which was consistent with the XRD analysis that only a
very weak diffraction peak of Co7Fe3 were detected (Figure 3b).
The catalysts with lowest and highest Fe/Co ratio (Co1-Fe0.05@NC
and Co1-Fe0.4@NC) demonstrate similar results (Figure S3).
Based on all the characterization results, the structure of the
catalysts with low Fe content (Fe/Co≤0.1) should be as follows.
The metal particles are supported on the N-doped carbon material.
Co and Fe are well dispersed in the metal particles. The main
component of the particles should be CoNx, and the Fe species
(Co7Fe3) is uniformly dispersed in the particles (Figure 5f).
Compared with the Co@NC catalyst, the main composition of
the Co1-Fe0.1@NC catalyst is CoNx with small amount of Co7Fe3
(Figure 3). Eugenol and dihydroeugenol intermediate were fully
converted, but the selectivity of the products was completely
different. High yield (88.3%) of 4-propylphenol rather than 4-
propylcyclohexanol was obtained by Co1-Fe0.1@NC (Table 2,
entry1 and 3). According to our previous research, CoNx can
If the active phases of the catalysts came from CoNx, the
incorporation of Fe may influence the adsorption of 4-
propylphenol to the catalyst and thus alter the product selectivity.
However, traditional characterization methods such as FT-IR are
difficult to detect the differences in adsorption when the adsorbed
species is at low content (Figure S5). The temperature
programmed desorption combined with online synchrotron
vacuum ultraviolet photoionization mass spectrum (TPD-SVUV-
Scheme 2. HDO of eugenol over Co7Fe3 alloy
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adsorption of 4-propylphenol is strong. When the dihydroeugenol
is demethoxylated to form 4- propylphenol, the 4-propylphenol will
further undergo hydrogenation reaction. The stability of the
catalyst was tested. After 6 runs, there was no significant change
in the yield of 4-propylphenol (Figure 8). Future stability tests
need to be performed in continuous flow reactors.
Conclusions
An efficient catalyst of Co1-Fe0.1@NC was prepared by
pyrolysis of Co-Fe precursors incorporated cellulose in ammonia
to selectively catalyze the lignin-derived phenols to 4-alkylphenol.
Birch and cornstalk were used as raw materials and lignins in both
types of biomass were efficiently converted to 4-alkylphenol by a
two-step process. 64.7 mol% yield of 4-propylphenol was
obtained when birch lignin oil was used as the substrate. Using
eugenol as the model compound, a high yield of 4-propylphenol
(88.3%) was achieved. A series of characterization methods such
as BET, XRD, XPS, HRTEM, HAADF-STEM, EDS, TPD-SVUV-
PIMS were utilized and comparative experiments were performed.
We found that the main active phase of the catalyst is CoNx
instead of the newly formed Co7Fe3 alloy. The presence of the Fe
species mainly serves to reduce the adsorption of 4-propylphenol
to the catalyst surface, thereby inhibiting the further conversion of
the 4-propylphenol. This work realizes the efficient preparation of
high value-added 4-alkylphenol from raw biomass.
Figure 7. The TPD-SVUV-PIMS curves of (a) 4-propylphenol adsorbed on
Co@NC, (b) 4-propylphenol adsorbed on Co1-Fe0.1@NC
Experimental section
Reagents
α-Cellulose (50 μm), eugenol, dihydroeugenol, 4-
propylcyclohexanol,
4-propylphenol,
4-ethylphenol,
4-
methyphenol, guaiacol, 4-ethylguaiacol, 2,6-dimethoxyphenol
and dodecane were purchased from Aladdin Chemistry Co., Ltd.;
phenol, methanol, Co(NO3)2·6H2O, Fe(NO3)3·9H2O, NH3·H 2O and
ethyl acetate were purchased from Sinopharm Chemical Reagent
Co., Ltd.
Figure 8. Recycle of Co1-Fe0.1@NC catalyst. 164 mg of eugenol, about 30 mg
Co1-Fe0.1@NC, 10 mL of dodecane, 0.6 MPa H2, 250 °C for 8 h.
Catalyst preparation
All the Co1-Fex@NC catalysts were prepared by the same
method except for the difference in metal content. The Co1-Fex
represents the mass ratio of Co/Fe is 1/x, and the original cobalt
loading for all catalysts was 5 wt% (relative to cellulose). For
Fe@NC, the original iron loading was 5 wt%. In a typical
preparation process, 2.60 g of Co(NO3)2·6H2O and a certain
amount of Fe(NO3)3·9H2O was dissolved in 20 g of deionized
water. Then the mixture solution was dripped into a round-bottom
flask which contained 10 g of cellulose and 400 mL deionized
water. The above suspension was stirred at 45 °C for 12 h. After
that, the pH of the solution was adjusted to 10 with aqueous
ammonia. After another 6 h, the solid was filtered, washed and
dried at 105 °C overnight. The dried powder was added to a feed
pipe and purged with NH3 (200 mL/min) for 30 min. Then, the
powder was added into a quartz tube slowly at 300 °C in NH3 flow.
After holding at 300 °C for 30 min, the temperature was raised to
650 °C at a heating rate of 1 °C/min and kept at 650 °C for 2 h in
NH3 flow. When the temperature was cooling down to room
temperature, the material was purged with nitrogen for 2 h and
then treated with 1% O2/N2 for another 0.5 h. All the catalysts were
stored in dodecane before use.
PIMS) study was proceeded to fulfill this goal. This equipment is
highly sensitive and energy tunability of SVUV light can ionize
molecules with few fragments, so as to better reflect the
information of substances28-30. Figures 7a showed the molecular
ion peak signals (m/z=136) of the 4-propylphenol desorbed from
Co@NC and Co1-Fe0.1@NC catalysts, respectively. As the
temperature increased, the 4-propylphenol gradually desorbed
from the surface of the catalysts. From Co@NC, the 4-
propylphenol was slowly desorbed until the temperature rose to
400 °C. In contrast, 4-propylphenol almost completely desorbed
below 250 °C from Co1-Fe0.1@NC (Figures 7b). The desorption
signals of the fragment peak (m/z=107) is consistent with the
molecular ion peak (Figure S6). Since a higher desorption
temperature indicates a stronger adsorption, the adsorption of 4-
propylphenol on Co1-Fe0.1@NC is significantly weaker than that
on Co@NC. The incorporation of Fe attenuates the adsorption of
4-propylphenol with the catalyst, thereby inhibiting further
hydrogenation of 4-propylphenol. As for the Co@NC catalyst, the
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10.1002/cssc.201901578
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Co7Fe3 alloy was prepared as follows, Co(NO3)2·6H2O (7 mmol)
and Fe(NO3)3·9H2O (3 mmol) were added into a round-bottom
flask which contained 110 mL of deionized water. 5 g of
polyvinylpyrrolidone (PVP) was dissolved in 30 g of water and
added into the above flask. After stirring at room temperature for
1.5 h, 0.05 g/mL of NaOH was added dropwise to adjust the
solution pH to 10. After stirring for another 6 h, the mixture was
centrifuged and washed with deionized water until the solution
was neutral. The solid was dried and then calcined in air at 550 °C
for 6 h. Then the solid powder was reduced at 600 °C for 2 h.
When the temperature was cooling down to room temperature,
the powder was purged with nitrogen for 2 h and then treated with
1% O2/N2 for another 0.5 h.
About 1.0 g raw biomass (20-40 mesh of birch sawdust
orcornstalk without any pretreatment), 250 mg Ru/C, 10 mL of
methanol were added into a 25 mL autoclave with mechanical
stirring (Anhui Kemi Machinery Technology Co., Ltd). After
evacuating with hydrogen for several times, the initial hydrogen
pressure of 3 MPa was charged. Then, the reaction was carried
at 250 °C for 8 h. The solid residue (carbohydrate + Ru/C) was
separated by filtration and methanol was removed by evaporation.
The obtained bio-oil was extracted with a two-phase ethyl
acetate/water system. The lignin-derived phenols were mainly
concentrated in the ethyl acetate phase. After removing ethyl
acetate, the obtained lignin-oil was used as a substrate for the
preparation of 4-alkylphenol. The contents of monophenols in
lignin oil were identified by a GC-MS (Agilent, Model 5975C) and
quantified by a GC (Kexiao, Model 1690) with a HP-INNOWAX
capillary column. Bicyclohexane was used as internal standard.
Catalyst characterization
The transmission electron microscopy (TEM) images were
taken on a JEOL Model JEM-2010 LaB6 TEM system. The high
resolution transmission electron microscopy (HRTEM), the high-
angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) images and the energy dispersive X-
ray spectroscopy (EDS) were obtained with a FEI Tecnai G2 F20
at 200 kV. Powder X-ray diffraction (XRD) patterns were collected
on a TTR-III X-ray diffractometer using Cu Kα radiation over 2θ
ranging from 20° to 80°. X-ray photoelectron spectroscopy (XPS)
were performed on an ESCALAB250 X-ray photoelectron
spectrometer. Fourier-transform infrared spectroscopy (FT-IR)
was performed by Nicolet 8700.
The nitrogen adsorption/desorption isotherms were obtained by
a Micromeritics TriStar II system. The surface area was calculated
by the Brunauer−Emmett−Teller (BET) method. The average
pore volume and pore size were measured by
Barrett−Joyner−Halenda (BJH) method.
Hydrodeoxygenation (HDO) of lignin oil or eugenol
All the reactions were conducted on a 25 mL autoclave with
mechanical stirring. 120 mg of birch lignin oil or 170 mg of
cornstalk lignin oil or 164 mg (1 mmol) of eugenol, about 40 mg
catalyst, 10 mL dodecane were added into the autoclave. After
evacuating with hydrogen for several times, the hydrogen was
charged to 0.6 MPa. Then, the reaction was carried out at 250 °C
with a stirring speed of 400 rpm. After reaction, the reaction liquid
was diluted with ethyl acetate and bicyclohexane was used as
internal standard. The products were identified by a GC-MS
(Agilent, Model 5975C) and quantified by a GC (Kexiao, Model
1690) with
a HP-INNOWAX capillary column. The initial
temperature of GC oven was 50 °C, and then heated to 250 °C at
a heating rate of 10 °C/min.
The calculation formulas of HDO of lignin oil are as follows:
Elemental analysis was measured using an Elementar vario EL
cube. Metal content detection was performed on an AA800 atomic
absorption spectrophotometer (AAS). The sample handling
process was as follows: 10 mg of catalyst was added into 4 mL of
aqua regia in a round-bottom flask and stirred at 80 °C overnight.
Then, the mixture was diluted to 100 mL in a volumetric flask.
The procedure for 4-propylphenol adsorption on catalyst was
as follows. About 400 mg of the catalyst was added to a solvent
of n-hexane containing 400 mg of 4-propylphenol. After stirring at
room temperature for 24 h, the mixture was centrifuged and
washed with n-hexane for 10 times. The 4-propylphenol adsorbed
catalyst was used in FT-IR and temperature programmed
desorption analyses.
The online temperature programmed desorption analyses were
performed under the undulator VUV beamline (BL03U) at the
National Synchrotron Radiation Laboratory (Hefei, China). The
details of BL03U and the tunable synchrotron vacuum ultraviolet
photoionization mass spectrometry (SVUV-PIMS) setup used in
this work has been described elsewhere28, 31. In simple terms, the
experimental setup includes a tubular furnace for temperature
programmed desorption, a transfer line for introducing desorption
species, and a homemade orthogonal acceleration time-of-flight
mass spectrometer (oa-TOF-MS). About 200 mg of the catalyst
adsorbed with 4-propylphenol was placed in the tubular furnace.
After purging with N2 for 15 minutes to ensure an inert atmosphere,
the temperature of the furnace was raised from 100 °C to 500 °C
at a heating rate of 5 °C/min. High purity nitrogen (200 mL/min)
carried the desorbed species through the transfer line into the
photoionization region. The transfer line was heated to 250 °C to
prevent desorbed species condensation. The desorbed species
were ionized by the crossed SVUV light (11.0 eV) and the ions
were captured by the oa-TOF-MS
the mass of product
Yield wt%=
the mass of lignin oil
number of moles of product
Yield mol%=
total number of moles of monophenols in lignin oil
The total number of moles of monophenols is the sum of the
moles of each monophenols detected by GC, including
dihydroeugenol, 2,6-dimethoxy-4-propylphenol, guaiacol, 4-
methylguaiacol, 4-ethylguaiacol, 2,6-dimethoxyphenol, 4-
ethylphenol (TableS1).
Catalyst recycle
After the reaction, the catalyst was separated by an external
magnet. After washing with dodecane for 2 times, the catalyst was
added into reactor for the next run.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Nos. 51536009 and 51876200).
Keywords: alkylphenol • biomass• cobalt • hydrodeoxygenation
• lignin
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
Xiaohao Liu, Chenguang Wang, Ying
Zhang*,Yan Qiao, Yang Pan, Longlong
Ma*
Page. – Page .
Selective Preparation of 4-alkylphenol
from Lignin-derived Phenols and Raw
Biomass over magnetic Co-Fe@N-
doped Carbon Catalysts
4-alkylphenol was selectively produced over magnetic Co1-Fe0.1@NC
catalyst using real lignin oil as feedstock, which was obtained by ‘lignin-
first’ strategy from raw biomass. The incorporation of Fe attenuates the
adsorption of 4-alkylphenol with the catalyst, thereby inhibiting further
hydrogenation of 4-alkylphenol to 4-alkylcyclohexanol
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