Phenol Catalytic Hydrogenation over Palladium Nanoparticles Supported on Metal-Organic Frameworks in the Aqueous Phase

Article abstract of DOI:10.1002/cctc.201800211

Metal-organic frameworks (MOFs) have been extensively applied as supports in hydrogenation catalysis owing to their topological structure and high hydrogen storage capabilities. Pd nanoparticles (NPs) supported on a hollow box-shaped MOF were prepared for phenol hydrogenation in the aqueous phase. The MOF preparation modulated by monocarboxylic acids grow into different structures. MOF140-AA, modulated by acetic acid at 140 °C, presents the structure of regular cube with a smooth surface. Compared to other supports, Pd NPs supported on MOF140-AA presents high phenol conversion due to the high hydrogen storage capability of MOF140-AA. Phenol is completely converted to cyclohexanol over Pd/MOF140-AA reacted at 260 °C for 2 h with high selectivity. The reaction mechanism of phenol hydrogenation is studied by density functional theory (DFT). The phenol hydrogenation mechanism is calculated on a Pd₃₈ cluster, which describes the reaction pathway consistent with experimental results.

Full text of DOI:10.1002/cctc.201800211

Accepted Article
Title: Phenol catalytic hydrogenation over palladium nanoparticles
supported on metal-organic frameworks in aqueous phase
Authors: Hao Chen, Yulian He, Lisa D. Pfefferle, Weihua Pu, Yulong
Wu, and Suitao Qi
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using the Digital Object Identifier (DOI) given below. The VoR will be
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To be cited as: ChemCatChem 10.1002/cctc.201800211
Link to VoR: http://dx.doi.org/10.1002/cctc.201800211
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ChemCatChem
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Phenol catalytic hydrogenation over palladium nanoparticles
supported on metal-organic frameworks in aqueous phase
[a]
[c]
[c]
[b]
[b]
[a]
Hao Chen , Yulian He , Lisa D. Pfefferle , Weihua Pu , Yulong Wu* and Suitao Qi*
Abstract: Metal-organic frameworks (MOFs) have been extensively
applied as supports in hydrogenation catalysis owing to their
topological structure and high hydrogen storage capabilities. Pd
nanoparticles (NPs) supported on a hollow box-shaped MOF were
prepared for phenol hydrogenation in the aqueous phase. The MOF
preparation modulated by monocarboxylic acids grow into different
structures. MOF140-AA, modulated by acetic acid at 140 °C,
presents the structure of regular cube with a smooth surface.
Compared to other supports, Pd NPs supported on MOF140-AA
presents high phenol conversion due to the high hydrogen storage
capability of MOF140-AA. Phenol is completely converted to
cyclohexanol over Pd/MOF140-AA reacted at 260 °C for 2h with
high selectivity. The reaction mechanism of phenol hydrogenation is
studied by density functional theory (DFT). The phenol
hydrogenation mechanism is calculated on a Pd38 cluster, which
describes the reaction pathway consistent with experimental results.
production of liquid fuels from these bio-derived chemicals is
significant and practical, although directly converting biomass
into chemicals is difficult and complex. Therefore, many
researchers focus on the development of potential approaches
to produce chemicals and renewable fuels from biomass .
Cyclohexanol, which can be produced via the hydrogenation of
phenol or the oxidation of cyclohexane, is a key chemical used
[
5]
in industry. Phenol hydrogenation is
a preferable green
technology for cyclohexanol production because the process of
cyclohexane oxidation, which is normally carried out at high
[
6]
pressure and temperature, has a large energy requirement . In
addition, phenol is one of the products from the decomposition
of the lignin fraction of the lignocellulosic biomass.
In recent studies, the catalytic hydrogenation of phenol to
cyclohexanol has obtained increasing attention owing to its
[
7]
environmental friendliness and low energy consumption . It is
commonly accepted that the reaction mainly occurs between
hydrogen stored in the support and the activated phenol on the
[
8]
metal nanoparticles (NPs) . Thus, supports with high hydrogen
storage capacities are desirable for phenol hydrogenation. Wide
Introduction
[
9]
2
ranges of materials have been applied in H gas storage , such
The contamination of the environment by fossil fuels has
triggered an increased research interest in the production of
as metal-organic frameworks (MOFs)[10]. In the past decade,
MOF have been widely investigated by global researchers. The
surface area, pore size and framework topology of MOFs can be
tuned by selecting appropriate precursors, most notably organic
ligands and metal ions. As reported by Teles et al., the effect of
[
1]
environmentally-friendly and sustainable fuels . Among these
alternative resources ， biomass has been regarded as
a
2]
[
potential resource to produce various liquid fuels or chemicals .
Because the formation of fossil fuels is nonrenewable and
generally takes millions of years, it is important that various
platform molecules and liquid fuels are produced from bio-based
chemicals instead of fossil resource. In the past decade, industry
and academia have paid more attention to bio-based chemicals
as they possess important functionalized properties which are
metal type for the phenol hydrogenation was investigated over
silica supported catalysts. Multiple catalysts, such as Pd[11], Pt[12]
,
[13]
[14]
[15]
Ru , Rh
and Ni , have been developed so far for this
[16]
reaction. Particularly, Pd
exhibits excellent selectivity and
activity for phenol hydrogenation in the aqueous phase. It was
found that the selectivity of phenol hydrogenation depends on
the reaction pathways over different metallic catalysts. The
reason is the correlation between hydrodeoxygenation activity
and the affinity of metals to bond with oxygen[17]. Noronha et al.
[
3]
suitable for meeting the needs of modern society . A feasible
way for the production of chemicals is upgrading the bio-source
[
4]
obtained from lignocellulosic biomass . More importantly, the
investigated the effect of the type of metal (Pt, Pd, Rh, Ru, Cu,
Ni, Co) on the performance of ZrO
2
-supported catalysts for
provided the best
phenol hydrodeoxygenation[11b]
.
Pd/ZrO
[a]
[b]
[c]
H.Chen, Prof. Dr. S. Qi
School of Chemical Engineering and Technology
Xi'an Jiaotong University
Xi'an, 710049, PR China
E-mail: suitaoqi@mail.xjtu.edu.cn
W. Pu, Prof. Dr. Y. Wu
Institute of Nuclear and New Energy Technology
Tsinghua University
Beijing, 100084, PR China
Email: wylong@tsinghua.edu.cn
Y. He, Prof. Dr. L. D. Pfefferle
Department of Chemical & Environmental Engineering
Yale University
2
catalytic activity among the prepared catalysts. Pd supported by
different supports exhibits different selectivity for phenol
hydrogenation. A series of supported Pd metal catalysts were
prepared by Noronha et. al and studied in phenol
[11c]
hydrogenation . While Pd supported on SiO
2 2 3
and Al O
exhibits high selectivity to cyclohexanone, when supported on
an oxophilic support such as ZrO , it favors the selectivity toward
2
benzene, reducing the formation of ring-hydrogenated products
like cyclohexanone and cyclohexanol. Pd also exhibits highly
efficient hydrodeoxygenation performance. In Dong’s work, 4-
nitrophenol was efficiently hydrogenated by Pd nanoparticles
supported by magnetic porous carbon composite synthesized
New Haven, Connecticut, 06520-8286, United States
Supporting information for this article is given via a link at the end of
the document.
from metal organic framework[11d]
.
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In the present work, Pd nanoparticle catalysts supported on
MOFs with high hydrogen storage has been applied for phenol
hydrogenation. The catalytic activity and selectivity of phenol
hydrogenation have been studied in the aqueous phase. The
reaction pathways of phenol conversion were also analyzed
using density functional theory (DFT), revealing the mechanism
of phenol hydrogenation.
Results and Discussion
Figure 1. SEM images of MOFs (a: MOF140; b: MOF140-FA; c: MOF110-AA;
d: MOF140-AA; e: MOF170-AA; f: MOF140-PA)
Modulation of MOF topological structure
The metal-organic frameworks were synthesized in order to
obtain coordination structure. To control the growth rate of the
crystallization process, the common approach is the addition of
monocarboxylic acids as modulators. During the nucleation
process, monocarboxylic acids compete with the organic linkers
for coordination to the metal clusters. Therefore, the crystal
prefers to grow into small particles. Aggregation of the single
crystals is prevented due to the addition of monocarboxylic acid
modulators[18]. A series of MOFs of varying morphology were
prepared at different temperatures with additions of different
monocarboxylic acids, which are shown in Figure 1. Figure 1a
Characterization of MOF140-AA
To further study MOF140-AA and analyze its components, other
characterization techniques have been applied. FT-IR
spectroscopy, thermogravimetric analysis (TGA) and powder X-
ray diffraction (PXRD) have been carried out to characterize the
prepared powder samples. FT-IR spectroscopy analysis of
MOF140-AA (Fig. S1) clearly shows the characteristic C-O
-1
stretching frequency at ~1205 cm and the peak of C=O
-1
stretching at ~1500 cm suggesting the presence of a squaric
2
-1
shows MOF140 with 77 m g specific surface area prepared at
40 °C without the addition of monocarboxylic acids in the
-1
acid molecule. The O-H stretching peak at ~3400 cm indicates
the presence of H O. TGA of MOF140-AA in an N environment
2 2
shows 12% weight loss at about 250 °C, which can be ascribed
to the dehydration of the two water molecules (Fig. S2). The
material is stable up to 340 °C and then about 46% weight loss
occurs resulting from the carbonization of squaric acid. The
1
preparation process. The morphology of MOF140 is that of an
irregular cube with about 2μm side length. Figure 1b is MOF140-
2
-1
FA with 93 m g specific surface area prepared at 140 °C with
the addition of formic acid (FA). Under FA modulation, the
morphology of MOF140-FA grows into strip shapes of various
sizes. Figure 1c~e are samples (MOF110-AA, MOF140-AA,
MOF170-AA) prepared at different temperatures with an addition
of acetic acid (AA). The MOF structure grows into a cube under
AA modulation. In Figure 1c, some MOF110-AA particles, with a
result is similar to that of Co(C
4
O
4
)(H
2
O)
2
prepared by
[19]
Eswaramoorthy’s group . In addition, the XRD pattern in Fig.
S3 is similar to that of Co(C
4
O
4
)(H
2 2
O) , which suggests
MOF140-AA and Co(C )(H O)
4
O
4
2
2
possess a similar structure.
The nitrogen adsorption–desorption isotherms of MOF140-AA in
Fig. S4a is representative of type IV curves with the sharp
capillary condensation steps suggesting that MOF140-AA is
mesoporous. As shown in Fig. S4b, the pore-size distributions of
2
-1
specific surface area of 104 m g , formed cubic structures.
However, a portion of the particles did not fully exhibit the cubic
morphology. MOF140-AA with 257 m2g-1 specific surface area in
Figure 1d grew into regular cubes with smooth surfaces.
However, the MOF170-AA particles, with 115 m2g-1 specific
surface area in Figure 1e, grow together when the preparation
temperature reaches 170 °C. With the addition of propionic acid
MOF140-AA are concentrated at about
1 nm to 6nm,
respectively. TEM was used to investigate morphologies of the
MOF140-AA. Figure 2a shows a TEM image of MOF140-AA,
which exhibits a hollow box morphology. The sizes of the cavum
in MOF140-AA have a side length of about 500 nm. It has been
(
PA), MOF140-PA has a morphology resembling a polyhedron
2
-1
and has a specific surface area of 179 m g (Figure 1f). As seen
in Figure 1, the morphology and specific surface area of the
MOFs are influenced by temperature and the choice of
modulator. This indicates that preparation temperature and
modulator addition are important factors that can be used to
control the growth of MOF crystals. MOF140-AA with a regular
cubic morphology presents the biggest surface area among the
prepared MOFs.
-
reported that OH ions present in the reaction medium play a
crucial role in etching the faces of the growing cubes leading to
the formation of cages[19]. The molecular structure of MOF140-
2+
AA simulated by DFT in Figure 2b shows that each Ni center is
2-
connected to two different C
framework.
4
O
4
2
and four H O to form a 3D
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Pd/MOF140-AA ranged from 1nm to 19nm, with most of the Pd
particles having a size of 3nm (Figure 3(a2)). Pd particles on
Pd/SBA-15 disperse in the channel of SBA-15 and the size of Pd
particles concentrates at 10nm. The Pd particles size of Pd/ZrO
2
,
Pd/Al and Pd/SiO mainly concentrate at about 20nm, 10nm
2
O
3
2
and 12nm, respectively. Pd dispersion increases with the
decrease of Pd nanometer diameter, which indicates more Pd
sites are exposed to react.
2
It has been found that the H storage performance of the
support plays an important role in the hydrogenation process[20].
The specific surface area of MOF140-AA is not the biggest
among the prepared catalysts but its hydrogen storage capacity
is the highest in prepared catalysts. The correlation between
pore sizes and hydrogen storage of MOFs reported so far shows
Figure 2. (a) TEM image of MOF140-AA;(b) the 3D coordination framework of
MOF140-AA.
that MOFs with a certain pore size have better hydrogen storage
capacity[
20].
Pd/MOF140-AA exhibits remarkably enhanced
catalytic activity in phenol hydrogenation, which should be
ascribed to the hollow feature with a high specific surface area
The effect of H
hydrogenation
2
storage performance on phenol catalytic
and the porous structure of MOF[21]. The achievement of high H
2
uptake at room temperature is required to synthesize the
materials with small pore size, since the small pore enables the
overlap of energy potentials between the opposing walls, leading
Firstly, the catalyst activities with different Pd loadings have
been studied. The results, detailed in Table 1, show that
MOF140-AA without Pd loading possesses very low catalytic
activity. However, the catalytic activity remarkably increases with
the increase of Pd loading content, indicating that phenol
hydrogenation may be strongly correlated with Pd loading in the
catalyst. The performance of 2wt.% Pd/MOF140-AA is the best
among the tested catalysts, and was therefore chosen for further
study.
to higher interaction energy with the H
2
molecules[22]. Note that
the kinetic diameter of hydrogen is 2.9 Å. Zhou et al. found that
a 6 Å width pore size exhibits the highest hydrogen uptake at
very low pressures because of the strongest interaction potential.
A larger pore with 9 Å width is suitable for the maximal capacity
of H
2
storage at high pressure[ . Therefore, MOF140-AA with
23]
2
about 1nm pore size possesses high H storage capacity. In
addition, the metal species inside the MOF pores activate
dihydrogen molecules more efficiently than the MOFs solely,
reducing the activation energy of the hydrogenolysis reaction
Table 1. Conversion of phenol and yield of products over Pd/MOF140-AA with
different Pd loading.
and generating higher concentrations of reactive H-species[24]
.
Products yield (%)
Excluding the impacts of particles size and distribution on
different supports, turnover frequency (TOF) of the prepared
catalysts is shown in Figure 4 in order to study the effect of H2
storage performance on catalytic activity. Pd/MOF140-AA with
the highest TOF value exhibits the best catalytic activity for
phenol conversion. It is a nearly linear trend in TOF versus the
Pd
loading
wt.%)
Conversion
b
(%)
Cyclo-
hexanol
Cyclo-
Cyclo-
[
a]
(
Benzene
hexanone
hexane
0
7.6
2.3
2.1
9.3
1.3
0
0.5
0.9
2.0
3.8
84.3
93.1
98.3
96.6
32.1
36.9
40.1
38.7
21.2
25.3
29.3
30.6
1.7
2.2
3.1
2.9
H
2
storage performance. The catalytic activity of the prepared
catalysts increases with the increase of catalysts H storage
performance except for Pd/ZrO . The H storage performance of
Pd/SBA-15 is better than that of Pd/ZrO while the catalytic
activity of Pd/ZrO is better than that of Pd/SBA-15. In order to
study the reason Pd/ZrO has a better TOF than Pd/SBA-15
despite its lower H storage performance, Pd 3d XPS of the
2
10.8
14.3
11.7
2
2
2
2
2
2
[
(
a] Pd content was confirmed by ICP analysis. [b] Reaction condition
Temperature=260 °C; Time = 2 h; 2 MPa H pressure; 0.2 g catalyst; 1 g
phenol).
prepared samples was explored. The results are shown in
Figure 3(a3~e3). The Pd 3d5/2 and Pd 3d3/2 peaks at 334.9 eV
2
0
and 340.3 eV can be attributed to Pd , while the Pd3d5/2 peak at
2
+
3
36.7 eV and the Pd 3d3/2 peak at 341.8 eV are related to Pd .
0
In Figure 3 (a3), the characteristic peak of Pd is detected but
2
+
0
there is no characteristic peak of Pd , indicating only Pd
species exist in Pd/MOF140-AA. However, the characteristic
peaks of Pd0 and Pd2+ are observed in Pd/SBA-15, Pd/ZrO
Pd/Al and Pd/SiO
. The presence of Pd2+ can be attributed to
Pd nanoparticles dispersed on different supports have been
applied in phenol catalytic hydrogenation as well. The
characterizations of prepared catalysts are shown in Figure 3
and Table 2. TEM images and particles distribution of the
prepared catalysts have been shown in Figure 3(a1~e1) and
Figure 3(a2~e2), respectively. The Pd particle size in
2
,
2
O
3
2
the strong interactions between the Pd precursors and the
support that lead to the generation of a small amount of oxidized
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Pd species[5b]. The Pd ratio of Pd/ZrO
0
with 72.7% is the highest
2
Pd/ZrO
2
112
74
1.7
1.9
3.71
4.7
19.6
14.7
among the four catalysts. According to Chen’s research[11g], the
higher Pd0 ratio will increase the catalytic activity for phenol
Pd/SiO2
hydrogenation. Therefore, the catalytic activity of Pd/ZrO
higher than that of Pd/SBA-15 although the storage
performance of Pd/ZrO is lower than that of Pd/SBA-15. Taken
together, the high H storage performance of the supports is
beneficial for phenol hydrogenation. In addition, the Pd :Pd
2
is
[
a] Pd content was confirmed by ICP analysis. [b] Calculated according to CO
H
2
uptake.
2
2
0
2+
ratio in Pd particles is also a factor which impacts the catalytic
activity for phenol hydrogenation.
Pd/MOF140-AA
0
0
0
.6
.4
.2
Pd/ZrO2
Pd/SBA-15
Pd/Al O
2
3
Pd/SiO2
0.4
0.8
1.2
1.6
H gravimetic (wt.%)
2
Figure 4. TOF over different catalysts versus
H
2
storage capacity of
2
catalysts.(Hydrogen gravimetric storage was measured at 1 bar H pressure
and 77K; TOF calculation method is shown in Supporting Information (Fig.
S5).)
Catalytic activity, selectivity and stability of Pd/MOF140-AA
Temperature is an important parameter in the hydrogenation
reaction. As shown in Figure 5, the conversion of phenol over
Pd/MOF140-AA at different reaction temperatures dramatically
increases with the increase of temperature. Phenol completely
converts at 260 °C with the main products being cyclohexane,
cyclohexanol,
cyclohexanone
and
benzene.
Clearly,
Pd/MOF140-AA exhibits
a
high selectivity to cyclohexanol
Figure 3. The size(a1~e1), distribution(a2~e2) and Pd3d XPS(a3~e3) of Pd
nanoparticles on different supports: (a) Pd/MOF140-AA; (b) Pd/SBA-15; (c)
Pd/ZrO ; (d) Pd/Al O ; (e) Pd/SiO .
2 2 3 2
(45.1%) and cyclohexane (29.3%) at 260 °C. The selectivity to
cyclohexanol also increases with the increase of temperature
which indicates high temperature is beneficial for cyclohexanol
production.
2
To study the selectivity to products, the effect of H pressure
has been investigated at the 260 °C reaction temperature. As
seen in Figure 6, the selectivity to cyclohexane gradually
increases as the H pressure increases while the selectivity to
2
Table 2. Characterization of as-prepared catalysts.
Pd loadinga
(wt.%)
Pd dispersionb
cyclohexanol and cyclohexanone gradually decreases
correspondingly. The reason is a part of cyclohexanol and
S
BET
Pore size
(nm)
Catalysts
2
-1
(
m g )
(%)
cyclohexanone convert to cyclohexane as the H
2
pressure
Pd/MOF140-AA
Pd/SBA-15
257
392
93
2.0
1.6
1.7
1.0
5.5
2.9
27.9
17.3
11.1
increases. High H pressure is conducive to the increase of
2
selectivity to cyclohexane because of the successive
cyclohexanone and cyclohexanol hydrogenation to cyclohexane.
The hydrothermal stability of MOF and catalytic stability of the
active compound were studied in phenol hydrogenation. A
2 3
Pd/Al O
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number of MOF without good hydrothermal stability, they are
incompetent as supports in aqueous phase. The XRD patterns
100
(
Fig. S3) show the crystal structure of MOF140-AA has not
Conversion
changed after hydrothermal test under 300 °C and 2 MPa H
2
80
60
40
20
0
pressure. This indicates that MOF140-AA exhibits excellent
hydrothermal stability and can be used as a support for phenol
hydrogenation in aqueous phase. The catalytic stability of
Pd/MOF140-AA was demonstrated by 5 repetitions of the phenol
hydrogenation reaction. As shown in Figure 7, the conversion of
phenol and selectivity to yield are nearly constant after 5
repetitions of the reaction. This indicates that Pd/MOF140-AA
exhibits good stability for phenol hydrogenation in the aqueous
phase.
cyclohexane
cyclohexanol
cyclohexanone
benzene
1
2
3
4
5
cycles
1
00
cyclohexane
cyclohexanol
cyclohexanone
benzene
Conversion
Figure 7. Cycle tests of Pd/MOF140-AA catalyst (Reaction condition:
Temperature = 260 °C; 2 MPa H
cyclic time = 2 h)
2
pressure; 0.2 g catalyst; 1 g phenol; One
8
6
4
2
0
0
0
0
0
Pathway of phenol hydrogenation over Pd/MOF140-AA
Figure 8a shows phenol conversion and product yield at 260 °C
as a function of reaction time. The complete conversion of
phenol is observed at 260 °C for 120 min. As the reaction
proceeds, the yield of cyclohexanone firstly increases to 26.4%
and then gradually decreases after 40min. The yield of
cyclohexanol increases with the increase of reaction time. The
yield of cyclohexanol dramatically increases with the decrease of
cyclohexanone after 40 min.
170
200
230
260
290
o
Temperature( C)
Figure 5. Conversion of phenol and yield of products over Pd/MOF140-AA at
different reaction temperatures.( Reaction condition: Time = 2 h; 2 Mpa H
pressure; 0.2 g catalyst; 1 g phenol).
2
To
further
understand
phenol
hydrogenation,
the
representative intermediate, cyclohexanone and cyclohexanol,
was converted in an independent experiment on Pd/MOF140-AA
at conditions equivalent to phenol hydrogenation. According to
product distributions shown in Figure 8b, the main products of
cyclohexanone
cyclohexane. The yield of cyclohexanol gradually increased to
3.7% and then decreased gradually, accompanied by an
increase in cyclohexane selectivity as reaction time proceeded.
As seen in Figure 8c, the yield of cyclohexane increased almost
linearly with cyclohexanol conversion, attaining 50.3% after 2 h.
Based on the results, phenol initially prefers to produce
cyclohexanone and then cyclohexanone further hydrogenates to
form cyclohexanol. More details of reaction pathways are
investigated by density functional theory (DFT).
hydrogenation
are
cyclohexanol
and
1
00
Conversion
5
8
6
4
2
0
0
0
0
0
cyclohexane
cyclohexanol
cyclohexanone
benzene
(a)
phenol conversion
benzene
1
2
3
100
cyclohexanone
cyclohexanol
cyclohexane
H2 pressure(MPa)
80
60
40
20
0
Figure 6. Phenol conversion and yield of main products over Pd/MOF140-AA
as a function of H pressure (Reaction condition: Temperature = 260 °C; 0.2 g
catalyst; 1 g phenol; Reaction time = 2 h).
2
0
20
40
60
80
100
120
Time(min)
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(b)
100
The lattice distance of Pd nanoparticles shown in HRTEM image
is 0.225 nm, which is assigned to the (111) family planes of Pd
with cubic lattice spacing[28]. The surface of Pd38 cluster includes
eight Pd(111) surface and six Pd(100) surface which are shown
in Figure 9b. Several initial configurations of phenol adsorption
on Pd38 surfaces were constructed and optimized.
cyclohexanone conversion
cyclohexanol
cyclohexane
80
60
40
20
0
0
20
40
60
80
100
120
Time(min)
(c)
cyclohexanol conversion
cyclohexane
50
40
30
20
10
0
Figure 9. (a) HRTEM image of Pd nanoparticle on MOF140-AA; (b) Pd38
cluster model.
Four stable configurations of phenol adsorption shown in
Figure 10 adsorb on sites of Pd38 cluster and the corresponding
adsorption energies are listed in Table 3. Figure 10a shows
phenol adsorbs on the edge of two (111) surfaces. C2 and C3,
carbon atoms in the ring, co-adsorb on Pd atoms while C5 and
C6 co-adsorb on adjacent Pd atoms with an adsorption energy
of -0.613eV. Phenol also can adsorb on the edge of (111)
surface and (100) surface (Figure 10b). C3 and C6 respectively
adsorb on the top site of Pd atoms and the adsorption energy is
-0.341eV. The most stable configuration shown in Figure 10c is
that phenol flatly adsorbs on (111) surface of Pd38 cluster with
low adsorption energy of -1.126eV. C1, C3 and C5 respectively
adsorb on the tops site of Pd atoms. The configuration is
consistent with the reported configuration on Pd(111) surface of
periodic slab model[25b,29]. Figure 10d shows phenol flatly
adsorbs on (100) surface of Pd38 cluster with high adsorption
energy of -0.143eV. It indicates this configuration is not as stable
as the other configurations.
0
20
40
60
80
100
120
Time(min)
Figure 8. (a) Phenol conversion over Pd/MOF140-AA as a function of reaction
time; (b) Cyclohexanone conversion over Pd/MOF140-AA as a function of
reaction time; (c) Cyclohexanol conversion and yield of cyclohexane over
Pd/MOF140-AA as
2
Temperature = 260 °C; 2 MPa H pressure; 0.2 g catalyst; 1g phenol).
a function of reaction time (Reaction condition:
DFT calculation
In this section, the phenol hydrogenation mechanism on the Pd
nanoparticle was simulated by DFT calculations. The main
reaction route was calculated to compare with the experiment
results. According to our experimental results and previous
reports[ , phenol hydrogenation mainly occurs on the Pd
surface, so the mechanism of phenol hydrogenation is focused
on the Pd surface as well in this work.
Compared to periodic mode, the cluster model is similar to Pd
nanoparticles in catalyst. In addition, the reaction activation
energy would be would be lower on the cluster model than that
on the periodic model[26]. A Pd38 cluster as a stable model has
been applied in a wide range of DFT calculations[27]. The size of
the Pd38 cluster model with a diameter of 0.7 nm in Figure 9b is
close to the size of Pd nanoparticles supported on MOF140-AA
as shown in Figure 9a. In this section, the reaction mechanism
of phenol hydrogenation was investigated on the Pd38 cluster
model. To explore the phenol hydrogenation mechanism, we
first determined the adsorption of phenol on the Pd38 cluster.
Elementary reaction steps of phenol hydrogenation divided
into 18 steps are shown in Figure11~Figure 13 and the
corresponding potential energy profiles of elementary reactions
are shown in Figure 14. The phenol hydrogenation reaction
initially starts from two pathways: one is direct hydrogenation to
cyclohexanol; the other is hydrogenation to cyclohexanone and
25]
then hydrogenation to cyclohexanol. Finally,
a
part of
cyclohexanol and cyclohexanone was hydrogenated to
cyclohexane.
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ChemCatChem
10.1002/cctc.201800211
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cluster model than that of the slab model because the cluster is
a better electron donor[31]
.
Phenol hydrogenation to cyclohexanone: Phenoxy is an
important species on the Pd38 cluster. Phenoxy hydrogenation
may kinetically compete with the direct phenol hydrogenation.
The reaction of phenol hydrogenation to cyclohexanone also
includes six elementary steps, step7 to step 12 in Figure 12,
which also shows geometry optimizations and corresponding
energies of these elementary steps. The hydroxy hydrogen atom
of phenol firstly dissociates with a low activation energy of
0.178eV to form phenoxy adsorbing on Pd(111) surface of the
Pd38 cluster for further hydrogenation (step 7). To evaluate the
relative contribution of adsorbed phenol and phenoxy to the
hydrogenation product, the activation barriers of adding the first
hydrogen to the carbon in the phenyl ring of adsorbed phenol
and phenoxy were calculated and compared. The activation
energy of step 8, the first hydrogen atom adding to the carbon
ring of phenoxy, is also high because conjugated effect inhibits
first hydrogenation. Compared to direct hydrogenation of phenol
(step 1), it is evident that the energy barrier for phenoxy
hydrogenation (0.849 eV) is lower than that of phenol
hydrogenation (1.004 eV), indicating that the phenoxy
hydrogenation is kinetically preferred on the Pd38 cluster. After
the first hydrogenation step, consecutively adding four hydrogen
atoms to the phenyl ring results in surface adsorbed
cyclohexanone. Among five hydrogenation steps, the third
hydrogenation (step 10) has the highest activation energy of
Figure 10. Adsorption structure of phenol on Pd38 cluster.(Yellow number is
carbon atom; White number is palladium atom).
Table 3. Adsorption energy (Eads) and Bond length of phenol on Pd38 cluster.
Configurations
Figure 10a
Eads(eV)
-0.613
Bond length(Å)
C2-Pd1: 2.24; C3-Pd1: 2.24; C5-Pd2: 2.25;
C6-Pd2:2.26
Figure 10b
Figure 10c
-0.341
-1.126
C3-Pd2:2.35; C6-Pd1:2.31
C1-Pd1: 2.12; C3-Pd2: 2.14; C5-Pd4: 2.20
C1-Pd4: 2.44; C2-Pd1: 2.59; C4-Pd2: 2.54;
C5-Pd3: 2.68
Figure 10d
-0.143
0.889eV due to the high energy requirement of the carbon ring
transformation from the slab conformation to chair conformation.
The step 10 is the rate-determining step in phenol hydrogenation
to cyclohexanone.
Phenol hydrogenation to cyclohexanol: This reaction is divided
into six elementary reactions (step 1 to step 6 in Figure 11). The
geometry optimizations and corresponding energies are also
shown in Figure 11. The carbon atoms in the ring are added by
hydrogen atom step by step. The destruction of conjugation in
the phenol ring by the first hydrogenation step (step1)
possesses a high activation energy of 1.004 eV. Once the first
hydrogen is added, the following hydrogenation steps proceed
more easily with lower activation energies. Five other
intermediates from hydrogenating the carbon atoms have
activation energies in the range between 0.578 and 1.390eV.
Comparing the energies after the hydrogen atoms were added
to different carbon atoms of the phenyl ring on Pd38 cluster, the
fifth hydrogenation step (step 5) has the highest activation
energy of 1.390 eV because the ring intermediate transforms
from boat conformation to chair conformation, requiring more
energy. The fifth hydrogenation step with the highest activation
energy is the rate-determining step of hydrogenation of phenol
to cyclohexanol. These results indicate that direct hydrogenation
of phenol on Pd38 cluster is both thermodynamically and
Cyclohexanone hydrogenation to cyclohexane: Cyclohexane
is one of the products observed experimentally. Cyclohexanone
hydrogenation to cyclohexane is calculated and divided into
three elementary reactions (step13 to step 15 in Figure 13). In
step13, the oxygen atom first dissociates from the carbon ring
with a high activation energy of 1.122eV. Deoxygenation of
cyclohexanone is the rate-determining step in cyclohexanone
hydrogenation to cyclohexane and hard to react. Once the
oxygen atom dissociates from the carbon ring, the following two
hydrogen atom (step14 and step15) is easy to add on the
carbon ring with low energy barriers.
Cyclohexanone hydrogenation to cyclohexanol: As reaction
time increases, a part of cyclohexanone hydrogenation to
cyclohexanol is observed in experimental results. This reaction
includes two elementary reactions (step16 and step 17 in Figure
13). The energy barrier for adding hydrogen to the O of
cyclohexanone (step16) is 0.551 eV and the energy barrier for
adding hydrogen to C1 of cyclohexanone (step17) is 0.698 eV.
The energy barriers of two steps are not high so that the
experiment shows high selectivity to cyclohexanol. In step16, the
adsorption energy of the initial state is almost the same as the
final state, which indicates the hydrogenation of cyclohexanone
to cyclohexanol is a reversible reaction.
Cyclohexanol hydrogenation to cyclohexane: Hydroxyl directly
dissociates from cyclohexanol (step18 in Figure 13) and then the
6 10
intermediate (C H ) further hydrogenates to cyclohexane
kinetically favorable and is expected as
a key step for
hydrogenation of phenol. Li[30] et al. suggested that adding the
first hydrogen atom to the phenyl ring has the high reaction
barrier of 1.22eV for phenol hydrogenation on the periodic slab
model Pd(111), which is higher than that on the Pd38 cluster.
The result suggests the reactant is easier to activate on the
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ChemCatChem
10.1002/cctc.201800211
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following the step14 and step15. The energy barrier of step18 is
0.893 eV which is lower than that of step13. It indicates most of
cyclohexane comes from cyclohexanol hydrogenation.
Figure 12. Geometrical structures and corresponding energies of initial state
(
IS), transition state (TS) and final state (FS) of elementary reaction in phenol
hydrogenation to cyclohexanone.
Figure 11. Geometrical structures and corresponding energies of initial state
(
IS), transition state (TS) and final state (FS) of elementary reaction in phenol
hydrogenation to cyclohexanol.
Figure 13. Geometrical structures and corresponding energies of initial state
(
IS), transition state (TS) and final state (FS) of elementary reaction in
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10.1002/cctc.201800211
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cyclohexanone hydrogenation to cyclohexane; cyclohexanone hydrogenation
to cyclohexanol; cyclohexanol hydrogenation to cyclohexane.
subsequent hydrogenation to cyclohexanol. Finally, a part of
cyclohexanol converts to cyclohexane.
Phenol hydrogenation to cyclohexanol
Phenol hydrogenation to cyclohexanone
Cyclohexanone hydrogenation to cyclohexane
Cyclohexanone hydrogenation to cyclohexanol
Cyclohexanol hydrogenation to cyclohexane
2
1
1
0
0
.0
.5
.0
.5
.0
Step13
Step17
Step14
Step10
Step1
Step11
Step5
Step16
Step18
Step12
Step6
Step15
Step9
Step3
Step4
Step2
Step8
-0.5
-1.0
-1.5
-2.0
Step7
Figure 15. Proposed reaction pathway for phenol hydrogenation over
Pd/MOF140-AA. (Ea is the activation energy of rate-determining step).
Figure 14. Potential energy profiles of phenol hydrogenation on Pd38 cluster.
Conclusions
Pd nanoparticles supported on Ni-MOF were prepared for
phenol hydrogenation. Ni-MOF modulated by acetic acid
showed regular hollow cubic structure with smooth surfaces.
Pd/MOF140-AA with a high specific surface area and hydrogen
storage exhibited high phenol conversion. Compared to
catalysts of Pd supported on traditional supports with low
hydrogen storage, Phenol is completely converted with high
selectivity to cyclohexanol at 260 °C for 2h over Pd/MOF140-AA.
It suggests that supports with high hydrogen storage can
remarkably enhance the catalytic activity of phenol
hydrogenation. Complementary to the results of experiment, the
reaction pathway is described by density functional theory.
Phenol hydrogenation initially can be divided into two pathways,
hydrogenation to cyclohexanol and hydrogenation to
cyclohexanone, and then cyclohexanol and cyclohexanone are
further hydrogenated to cyclohexane. Based on DFT results, the
main pathway is phenol first hydrogenates to cyclohexanone;
then cyclohexanone hydrogenates to cyclohexanol; finally,
cyclohexanol hydrogenate to cyclohexane. The reaction
pathway described by DFT calculation is consistent with our
experiment results.
Starting from two phenol adsorption modes, two possible
pathways have been modeled according to the results of
potential energy profiles. The energy barrier of the rate-
determining step with 1.39eV in phenol hydrogenation to
cyclohexanol is higher than that in hydrogenation to
cyclohexanone of 0.889eV, which suggests the selectivity of
phenol hydrogenation to cyclohexanone is higher than that of
hydrogenation
to
cyclohexanol.
Cyclohexanol
and
cyclohexanone are not only the products of phenol
hydrogenation but also can be hydrogenated sequentially as the
intermediates. Cyclohexanone can be hydrogenated to
cyclohexanol and cyclohexane. In the pathway of
cyclohexanone to cyclohexane, the oxygen atom directly
dissociates from cyclohexanone with a high energy barrier of
1.122 eV and then hydrogenates to cyclohexane. Another
pathway of cyclohexanone hydrogenation to cyclohexanol has a
low energy barrier of 0.698eV. The hydroxy of cyclohexanol can
also be directly dissociated with a lower energy of 0.893eV than
that of the oxygen atom dissociation from cyclohexanone. In
summary, cyclohexanone preferentially hydrogenates to
cyclohexanol and further hydrogenate to cyclohexane. These
experimental and computational results allow us to formulate the
overall reaction pathway for phenol conversion. As seen in
Figure 15, the phenol hydrogenation prefers to follow the
pathway in red which is required to overcome lower activation
energies. Two reaction pathways proceed on Pd nanoparticles:
one pathway is phenol hydrogenation to cyclohexanol; the other
pathway is phenol hydrogenation to cyclohexanone and then
Experimental Section
Chemicals
Nickel(II) hydroxide (Ni(OH)
(
(
2
, 99.9% from Macklin); Polyvinylpyrrolidone
PVP, Macklin); Squaric acid (C , 98% from Macklin); Formic acid
HCOOH, 99% from Macklin); Acetic acid (CH COOH, 99.9% from
CH COOH, 99% from Macklin); Aluminum
, 99+% from Macklin); Zirconium dioxide (ZrO , 99% from
Macklin); Silicon dioxide (SiO2, Macklin); SBA-15 (XF NANO); Palladium
4 2 4
H O
3
Macklin); Propionic acid (C
2
5
oxide(Al
O
2 3
2
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ChemCatChem
10.1002/cctc.201800211
FULL PAPER
acetate (Pd(acac)
2
, 99.9% from Macklin); Palladium nitrate (Pd(NO
3
)
2
,
was cooled down to room temperature, the conversion of phenol and the
yield of reaction product were recorded by a gas chromatograph (Agilent
GC6820) equipping a GsBP-Inowax (30 m × 0.32 mm × 0.25 m) capillary
column with a flame ionization detector(FID). The equation of conversion
of phenol and yield of each product are as follows:
99%, from Macklin).
Preparation of Pd/MOF
MR
Support preparation: In a typical synthesis, Ni(OH)
2
(1 mmol) is dissolved
C  (1
)100%
MI
in water (10 ml) and mixed with polyvinylpyrrolidone (PVP, 0.163 g, Mw
24,000). To this solution, squaric acid (0.67 mmol) and mixed with
MP
=
Y 
100%
acetic acid (100 mmol) dissolved in deionized water (10 ml) is added in a
quick succession. The mixture is then transferred to a Teflon container
MI
where C is the conversion of phenol; M
is the moles of remaining phenol; Y is the yield of each product; M
the moles of each product.
I
is the mole of the starting phenol;
(50 ml capacity) and stirred for 30 min. The container is then put in a
M
R
P
is
steel autoclave and heated at 140°C for 48 h in a temperature-controlled
oven. After 48 h of reaction time, the solid obtained is washed thoroughly
with water and ethanol and then dried at room temperature for further
analysis.
Calculation of the turnover frequency (TOF):
M p
TOF=
MPd  DPd time
p
Where M is mol of phenol conversion; MPd is mol of Pd on catalyst; DPd
Pd loading: 1g prepared support was soaked in 5ml trichloromethane
is Pd dispersion.
2
dissolved with 0.06g Pd(acac) for 24h and then dried in air. Then the
sample and PVP (MW = 30000, 50 mg) were added into DMF (20 mL).
The resulting homogeneous solution was transferred to a 50 mL Teflon-
lined stainless-steel autoclave. The sealed vessel was then heated at
Computational Methods
150 °C for 4 h and subsequently cooled to room temperature naturally.
The solid obtained is washed thoroughly with water and ethanol and
dried at room temperature for further analysis.
All DFT calculations were performed using the program package DMol3
in the Materials Studio software (version 6.1). Generalized gradient
approximation (GGA) in the form of PBE exchange-correlation functional
developed was employed and the localized double-numerical basis
setting with polarization functions (DNP) was used. The core electrons of
the metal atoms were treated using the effective core potentials (ECP). In
configuration optimization, the parameter of the tolerance for self-
2 3 2 2
Preparation of Pd/SBA-15, Pd/Al O , Pd/SiO and Pd/ZrO
Pd(NO
support(SBA-15, Al
impregnation for 24h. After that, the sample was calcinated at 550 °C for
h with a heating rate of 10 °C/min. After calcination, the reduction of Pd
was carried out in a tube furnace in the presence of H . The temperature
3
)
2
with desired Pd loadings was loaded on the commercial
2 3
O ,
SiO and ZrO by incipient wetness
2
2
)
consistent field(SCF), energy, maximum force, and
a maximum
−
5
−5
displacement were set to 1.0e Hartree, 2.0e Hartree, 0.004 Hartree/Å
and 0.005 Å, respectively. Throughout calculations, a thermal smearing
of 0.005 Ha was applied to improve computational performance. All of
the calculation were performed with spin-polarization. The Brillouin-zone
integrations were optimized (Fig. S6) and set to 3 × 3 × 1 k-point grid.
The transition states of the hydrogenation mechanisms on Pd cluster
surfaces were determined by the complete LST/QST method, where LST
and QST stand for linear synchronous transit and quadratic synchronous
transit, respectively. The transition state structures were confirmed to
have only single imaginary frequency.
3
2
was kept at 300 °C for 3 h with a heating rate of 10 °C/min.
Catalyst characterization
The specific surface areas of prepared materials were calculated by the
Brunauer-Emmett-Teller
(BET) method. Transmission electron
microscope (TEM) images were collected by FEI tecnai F20 instrument
and scanning electron microscope (SEM) images were collected by
SU8010 instrument. By using KBr powder, FT-IR absorption spectra
were recorded on a Bruker model Tensor 27 equipment in the range from
The adsorption energy is calculated using the following formula:
Eads  EA/slab  Eslab  EA(g)
-
1
4
000 to 400cm . Pd loading content was obtained by inductively coupled
plasma (ICP) atomic emission spectrometry. Powder X-ray diffraction
XRD) patterns were carried out on Bruker D8 Advance X-ray
where EA/slab is the total energy of the slab with an adsorbate bound to it;
Eslab is the energy of the clean metal slab; EA(g) is the energy of the
(
a
adsorbate in gas-phase.
The energies of activation barrier(E
formula:
diffractometer using Cu Kα radiation (operation condition: 36 kV, λ=
a
) is calculated using the following
o
0
2
.1541 nm, 20 mA, scanning step = 2 min−1). Data were recorded in the
o
θ range from 5 to 80 . The surface composition of the samples was
E  E  E
a
TS
IS
characterized by X-ray photoemission spectroscopy (XPS, Kratos AXIS
Ultra). The TGA experiments were carried out on 10 mg powder sample.
where ETS is the energy of the transition state; EIS is the energy of the
initial state.
The results were acquired from
thermogravimetric analyzer, heating from room temperature to 900 °C at
0 °C/ min in 50 mL/min of nitrogen atmosphere.
a TA Instruments SDT Q600
1
Acknowledgements
Catalytic activity tests
This work was supported by the National Natural Science
Foundation of China (No. 21576155, No.21376140, No.
Phenol deoxygenation tests were carried out in a 100 mL autoclave in
batch mode. In a typical experiment, the catalyst Pd/MOF was highly
dispersed into 50 mL deionized water by ultrasound with 1 g phenol and
then the compound was moved into 100 mL stainless steel reactor. The
21376136 and U1463202), the authors thank Computer Network
Information Center in Chinese Academy of Science for access to
the software of Materials Studio.
2 2
reactor was purged three times with H and then pressurized with H to 2
MPa with constant stir rate (200 rpm). Then the reactor was heated to the
desired reaction temperature and maintained for 2h. After the autoclave
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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Phenol hydrogenation mechanism
was divided into two pathways. The
main pathway is phenol firstly
hydrogenate to cyclohexanone and
then continuously hydrogenate to
cyclohexanol. The side pathway is
Hao Chen, Yulian He, Lisa D. Pfefferle,
Weihua Pu, Yulong Wu* and Suitao Qi*
Page No. – Page No.
Phenol catalytic hydrogenation over
palladium nanoparticles supported
on metal-organic frameworks in
aqueous phase
phenol
cyclohexanol. Finally,
cyclohexanol converts to cyclohexane.
directly hydrogenate to
a
part of
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