An active, selective, and stable manganese oxide-supported atomic Pd catalyst for aerobic oxidation of 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c9gc01674k

Here we report for the first time a Pd-MnO₂ catalyst with a single-atom feature that can convert 5-hydroxylmethyfurfural (HMF) into the important bioplastic building block 2,5-furandicarboxylic acid (FDCA) with a high yield of 88% in aqueous solution using O₂ as an oxidant at ambient pressure. Pd-MnO₂ shows higher activity in the productivity of FDCA (100.91 mmol h^-1 g_Pd^-1) than its Pd nanoparticle counterpart (45.57 mmol h^-1 g_Pd^-1) and state-of-the-art Pd-based catalysts. Pd-MnO₂ displays promising recyclability with no degradation after five catalytic runs. Experimental and theoretical results suggest that the single-atom Pd sites that have enhanced binding affinity to HMF and their surrounding sites on the MnO₂ support work synergistically toward HMF oxidation.

Full text of DOI:10.1039/c9gc01674k

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An active, selective, and stable manganese
oxide-supported atomic Pd catalyst for aerobic
Cite this: DOI: 10.1039/c9gc01674k
oxidation of 5-hydroxymethylfurfural†
a
a
a
b,c
d
e
Xuemei Liao, Jindou Hou, Ya Wang, Hao Zhang, Yu Sun, Xiaopeng Li,
*
f
g
h
b,i
Siyang Tang,
Kenichi Kato, Miho Yamauchi
and Zheng Jiang
*
2
Here we report for the first time a Pd-MnO catalyst with a single-atom feature that can convert 5-hydroxyl-
methyfurfural (HMF) into the important bioplastic building block 2,5-furandicarboxylic acid (FDCA) with
a high yield of 88% in aqueous solution using O
higher activity in the productivity of FDCA (100.91 mmol h−
2
2
as an oxidant at ambient pressure. Pd-MnO shows
1
Pd−1) than its Pd nanoparticle counterpart
g
(45.57 mmol h− gPd−1) and state-of-the-art Pd-based catalysts. Pd-MnO displays promising recyclability
1
Received 20th May 2019,
Accepted 5th July 2019
2
with no degradation after five catalytic runs. Experimental and theoretical results suggest that the single-
DOI: 10.1039/c9gc01674k
atom Pd sites that have enhanced binding affinity to HMF and their surrounding sites on the MnO
2
rsc.li/greenchem
support work synergistically toward HMF oxidation.
mediate in the production of environmentally friendly bio-
based plastics (e.g. polyethylene furanoate).
Oxidation of HMF can produce several furan chemicals, for
1
. Introduction
1
1–13
Sustainable conversion of renewable biomass, such as ligno-
cellulose and rotten grains, into various high value-added pro- example, 2,5-diformylfuran (DFF), 5-hydroxymethyl-2-furancar-
ducts has attracted significant attention as a promising substi- boxylic acid (HMFCA), 5-formyl-2-furancarboxylic acid (FFCA),
tute for non-renewable resources for the manufacture of key and FDCA. Substantial progress has been made in converting
1
–8
building-block chemicals.
5-Hydroxymethyfurfural (HMF), HMF to FDCA using O2 as an oxidant and catalyzed by
1
2,14–17
as an ideal platform compound derived from cellulose or cell- supported metal nanoparticle catalysts, such as Pt,
ulose-derived carbohydrates, can be used for the synthesis of Pd,
various chemical products.
4
,11,13,18–20
5,21–24
25–28
29–32
Au,
Ru
and their bimetallics,
in
2
,9–11
2,5-Furandicarboxylic acid an aqueous medium. To obtain a superior yield of FDCA,
(FDCA) obtained from oxidation of HMF is an important inter- earlier researchers mostly focused on the composition and dis-
persion of noble metals and reaction parameters, such as elev-
ated pressures, temperatures, and properties and dosages of
a
the base, to assist in oxidation of the hydroxyl group of HMF.
School of Food and Biological Engineering, Xihua University, Chengdu 610039,
China
Very recently, the properties of catalysts, such as acid–alkali
b
5,12,15,16,33
Shanghai Institute of Applied Physics, Chinese Academy of Science,
catalysts derived from functionalized carbon
or
Shanghai 201800, China
4,22,24,31
doped hydroxide supports,
have been investigated in
c
University of Chinese Academy of Sciences, Beijing 100049, China
d
order to replace the homogeneous alkali-catalyzed aerobic oxi-
dation of HMF in water. However, high oxygen pressure (up to
2.4 MPa) is usually required to achieve good yields of FDCA in
these reaction systems. For example, Kim et al. reported that
MnCo O spinel-supported ruthenium catalysts showed high
Department of Chemistry, Faculty of Science, Hokkaido University, N10W8, Kita-ku,
Sapporo, Hokkaido 060-0810, Japan
e
CAS Key Laboratory of Low-Carbon Conversion Science and Engineering,
Shanghai Advanced Research Institute (SARI), Chinese Academy of Sciences (CAS),
No. 100, Haike Road, Shanghai 201210, China. E-mail: lixp@sari.ac.cn
f
2 4
Low-Carbon Technology and Chemical Reaction Engineering Laboratory,
catalytic activity for oxidation of HMF to FDCA, with a high
yield of 99.1% at 120 °C for 10 h under 2.4 MPa air pressure,
School of Chemical Engineering, Sichuan University, Chengdu 610065, China
g
RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
h
−1
−1 34
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER),
but low FDCA productivity (33 mmol h gRu ). In addition,
if metal nanoparticles (NPs) interacted weakly with the
support, aggregation or leaching of NPs often occurred and
Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan
i
Shanghai Synchrotron Radiation Facility, Zhangjiang Lab,
Shanghai Advanced Research Institute (SARI), Chinese Academy of Sciences (CAS),
Shanghai 201210, China. E-mail: jiangzheng@sinap.ac.cn
1
6,17
resulted in low catalytic recyclability for this reactions.
Acknowledging these important achievements, there is still a
need to search for high FDCA productivity under milder con-
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc01674k
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ditions for the oxidation of HMF to FDCA, and for recycling of MnO
2
(denoted as PdNP/MnO
2
) was prepared using the follow-
catalysts for repeated use.
Single-atom catalysts (SACs), viewed as a new frontier in cat- into Pd(NO
alysis, have attracted increasing attention.
ing procedure: 20 mg polyvinylpyrrolidone (PVP) was added
solution under stirring for 30 min. The α-MnO
Downsizing nanowire was added into the above solution with stirring for
3
)
2
2
3
5–41
metal nanoparticles into single atoms not only improves the another 30 min. Finally, NaBH solution was dropped slowly
4
atomic efficiency, but also gives rise to new and intriguing into the mixture. After stirring for 30 min, the black powder
catalytic properties owing to the quantum size effect and obtained was filtered, washed until pH = 7 and dried in the
4
2
enhanced electronic metal–support interactions.
Recent oven at 120 °C for 12 h.
reports have shown that SACs exhibit outstanding catalytic per-
4
3
formances in thermocatalysis (e.g. CO oxidation, water gas
2
.2. Characterization
44
45
shift,
hydrogenation of alkynes ), electrocatalysis (e.g.
4
6
47
Synchrotron X-ray diffraction (SXRD) measurements were per-
formed at the beamline BL44B2 of SPring-8 (Hyogo, Japan).
The Pd–MnO sample was sealed in borosilicate glass capil-
2
laries under vacuum and data collected using a Debye–
Scherrer camera with an imaging plate installed as an X-ray
detector. The incident wavelength was 0.5003 Å. Calibration
oxygen reduction reaction, oxygen evolution reaction ), and
4
8
photocatalysis. Despite these significant advances, the appli-
cation of SACs in oxidation of HMF is rare. It is both techno-
logically and fundamentally important to make a comparative
study of SACs and their NP counterparts for HMF oxidation,
and to probe the underlying mechanism.
2
was achieved by applying CeO as the standard. High-resolu-
To construct an ideal catalyst for HMF oxidation, we incor-
porated atomic Pd into the crystalline lattice of a metal oxide
support, thereby creating maximized interfacial sites and indu-
cing favorable electronic metal–support interactions for activat-
ing HMF. A reducible oxide with rich oxygen vacancies that
can effectively activate molecular oxygen is generally used as a
support. Comparing the commonly reported reducible oxide
tion transmission electron microscopy (HRTEM) was per-
formed using the FEI Tecnai G2, to investigate the microstruc-
ture. Sub-Ångström-resolution HAADF-STEM images were
recorded on a JEOL ARM200F equipped with a CEOS probe
corrector and with a guaranteed resolution of 0.08 nm. XPS
was performed on a Thermo Scientific K-Alpha XPS 33 spectro-
meter using an Al-Kα X-ray source (1486.6 eV). The C 1s photo-
electron line was used for spectral calibration using the refer-
ence position with a binding energy (C 1s) of 284.8 eV.
38,49,50
51
52
supports (e.g. CeO
2
,
x
CoO ,
x
and NiO ), MnO -based
materials have attracted growing interest due to their eco-friendly
and noncarcinogenic characteristics. Recent results indicate that
XAS measurements were carried out at the BL14W1 station
of the Shanghai Synchrotron Radiation Facility (SSRF, 3.5 GeV,
x
MnO possesses moderate activity for the oxidation of HMF into
53–58
FDCA.
system (0.8–2.0 MPa) with a long reaction period (≥12 h).
Here, we selected MnO as the support. Synergistic interaction
However, the reaction usually requires a pressurized
5
9
250 mA in maximum, Si (311) double crystals). XAS spectra
were recorded in fluorescence mode. IFEFFIT software
packages were applied to analyze the data. The extended X-ray
absorption fine structure (EXAFS) contributions were separated
2
between atomic Pd and the surrounding sites of the MnO2
support is expected to enhance the catalytic performance for
from different coordination shells using a Hanning window
(
2
HMF oxidation. Supported atomic Pd on MnO was found to be
dk = 1.0 Å− ). The quantitative curve fittings were performed
1
more active, more selective and more stable than its nanoparticle
counterpart. Extensive characterizations, including X-ray photo-
electron spectroscopy (XPS), aberration-corrected high-angle
annual dark-field scanning transmission electron microscopy
in R-space (1.0–3.2 Å) with a Fourier transform k-space range
of 3.0–12.0 Å− employing the ARTEMIS module of IFEFFIT.
1
All fits were performed in R-space with a k-weight of 2. The
2
overall amplitude reduction factor, S , was fixed at the best-fit
(HAADF-STEM), and X-ray adsorption spectroscopy (XAS), were
0
value of 0.73, which was determined from fitting the data for
metal Pd foil. The EXAFS R-factor for measurement of the per-
centage misfit of the theory to the data was used to evaluate
the goodness-of-fit. Specifically, the R-factor is defined as:
applied to probe the atomic structure and physicochemical pro-
perties of these catalysts. Quantum chemistry calculations were
conducted to understand the reaction mechanism.
N
P
2
ðdatai ꢀ fitiÞ
2
. Experimental
i¼1
R ¼
:
N
P
2
.1. Catalyst preparation
The Pd-MnO catalyst was prepared by mixing MnSO
NH , Pd(NO ·2H O and (NH SO in an aqueous
ðdatai2Þ
i¼1
2
4 2
·H O,
(
4
)
2
S
2
O
8
3
)
2
2
4
)
2
4
All raw data in k-space weighted by 0 were extracted from
solution. The solution was then transferred into a Teflon-lined Athena software, and during wavelet transform (WT) analysis
stainless steel autoclave and heated at 140 °C for 12 h, and, the data were weighted by 2.
finally, allowed to cool down to room temperature. The black
2 2 2
H -Temperature programmed reduction (H -TPR) and O -
precipitate was collected, filtered, washed several times with temperature programmed desorption (O -TPD) were carried
2
water and dried at 120 °C for 12 h. The undoped α-MnO
wire was prepared under identical conditions except with the with a thermal conductivity detector (TCD). Typically, for H
addition of Pd(NO ) ·2H O. The Pd nanoparticle deposited on TPR, 40 mg of sample was loaded and pretreated under He
2
nano- out on an AutoChem II 2920 adsorption apparatus equipped
2
-
3
2
2
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with a flow of 50 mL min− at 150 °C for 1 h. After cooling, the
1
E
(–O)-slab is the total energy of the Pd-MnO
slab losing one
2
−
1
gas was changed to 10% H /Ar with a flow of 50 mL min
oxygen atom, and Efree HMF and Efree O2 are the energies of free
2
and, meanwhile, the temperature was programmed to increase HMF and free O in vacuum.
2
−
1
2
to 400 °C at a rate of 10 °C min . For O -TPD, about 40 mg of
sample was pretreated by flowing gas of 20% O /He at 100 °C
2
for 2 h. Then, after cooling to 50 °C, the temperature was elev-
ated to 700 °C at a rate of 10 mL min in He.
3
. Results and discussion
−
1
3.1. Structure characterization of Pd-MnO and PdNP/MnO
2 2
2
.3. Catalytic tests
MnO
synthesized via hydrothermal treatment of a mixed aqueous
solution of Pd(NO , MnSO , (NH SO , and (NH at
40 °C for 12 h. Pd ions were expected to dope into the crys-
talline framework of the oxide support during the crystalliza-
tion growth of MnO . Fig. 1(a and b) shows the typical TEM
image. The as-prepared Pd-MnO shared identical nanowire
morphology with undoped α-MnO (Fig. S1†). The average dia-
2 2
-supported atomic Pd catalyst (denoted as Pd-MnO ) was
HMF oxidation was performed in a three-mouth bottle under
atmospheric pressure, with a flow rate of 25 mL min 10% O
in Ar. Typically, 100 mg of catalyst was suspended in 100 ml of
deionized water containing 0.6 mmol HMF and 2.4 mmol
2 3
K CO . Then, the reactor was heated to 100 °C under vigorous
stirring (900 rpm). Samples of the reaction mixture were with-
drawn, filtered with 0.2 μm PTFE filters and analyzed using
high-performance liquid chromatography on a Shimadzu LC-16
equipped with a UV detector and a C18 chromatographic column.
A mixture of acetonitrile and a 0.1 wt% aqueous acetic acid solu-
tion, with a volume ratio of 10/90, was used as the mobile phase,
and the flow rate was 1 ml min . Products were characterized
using the external standard method. The HMF conversion rate
was defined as the molar percentage of HMF converted in the
reaction, while the selectivity of each product was defined as the
molar percentage of each product in the converted HMF.
−
1
3
)
2
4
4
)
2
4
4 2 2 8
) S O
2
2
+
1
2
2
2
2
meter of the Pd-MnO nanowire was approximately 10 nm.
Energy dispersive spectroscopy (EDS) mapping confirmed the
co-existence of Pd, O and Mn (Fig. 1(d–f)). No Pd aggregates
were detected from Pd signals (Fig. 1(f)), suggesting that the
Pd was uniformly distributed throughout the nanowire struc-
ture. The HAADF-STEM images, with a guaranteed resolution
of 0.08 nm, show the presence of bright dots, which corres-
pond to Pd single atoms (Fig. 1(g and h)). The actual Pd
−1
loading of Pd-MnO was determined to be 1.31 wt% by induc-
2
tively coupled plasma optical emission spectrometry
(
ICP-OES).
2
.4. DFT calculations
All spin-polarized calculations were carried out using the
Vienna ab initio simulation package version 5.3.3, which is a
plane wave density functional code. The electron–electron
exchange and correlation interactions were described using
the generalized gradient approximation with the form of the
Perdew–Burke–Ernzerhof function. The projector-augmented
wave method was employed to describe the interaction
between the core and the valence electrons. The wave func-
tions were expanded in plane wave basis sets with a cutoff
energy of 400 eV. The convergence criteria for the electronic
self-consistent iteration and the ionic relaxation loop were set
−
5
−4
−1
to values of 1 × 10 eV and 1 × 10 eV Å , respectively.
Brillouin zone integration was performed with k-point
sampling using a Monkhorst–Pack (MP) grid of 3 × 3 × 1 and
5
× 5 × 1 for bulk and slab models, respectively, to ensure that
the calculations can be done in a reasonable time. The
Methfessel–Paxton method was used to determine the valence
electron occupancies, with a smearing width of 0.1 eV.
2 2
The slab models of Pd-MnO and Pd13/MnO surfaces were
constructed with 3 × 2 surface unit cells. The bottom Mn layer
and two O layers were fixed, while the rest of the atoms were
fully relaxed. The adsorption energy (Eads) and O vacuum-forming
energy (Evacu) were defined according to the following equations:
Eads ¼ EHMF-slab ꢀ Eslab ꢀ Efree HMF
2
Fig. 1 (a, b) TEM and (c–f) HAADF-STEM images of Pd-MnO . (d–f)
Evacu ¼ Eð–OÞ-slab þ 1=2Efree O ꢀ Eslab
EDS elemental mapping images showing the distribution of Mn, O, and
Pd, respectively. (g) Sub-Ångström-resolution HAADF-STEM image, Pd
single atoms are highlighted by red circles. (h) Intensity profile of the
2
where EHMF-slab is the total energy for HMF adsorption struc-
tures, Eslab is the total energy for the Pd/MnO slab structure, scan along the dashed frame marked in (g).
2
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2
The contrast sample PdNP/MnO was prepared using a con-
ventional deposition precipitation method. TEM characteriz-
ation (Fig. 2(a–c)) showed the presence of tiny Pd NPs with an
average size of ∼2 nm that were firmly attached to the MnO
2
surface. The HRTEM image shows clear lattice fringes with
interspacing of 0.22 nm (Fig. 2(c)), corresponding to the (111)
plane of metallic Pd. EDS mapping shows the intensified Pd
signal from the Pd NPs (Fig. 2(d–g)). The Pd loading measured
by ICP-OES was found to be 2.23 wt%.
Previous research has stated that for reducible oxide-sup-
ported SACs the metal single atoms may locate at several
different types of site in the crystalline support, including
4
3,44,60,61
being adsorbed onto the surface,
crystalline lattice or decorated at crystalline defects.
determine the bonding environment of Pd-MnO accurately,
embedded in the
3
8,62–65
To
2
we carried out SXRD and XAS measurements. Fig. 3(a) shows
the SXRD pattern of Pd-MnO and its fitted line. The Pd-MnO
2
2
sample shows increased lattice parameters of a = b = 9.895 Å,
3
c = 2.857 Å, and V = 279.735 Å , compared with a = b = 9.881 Å,
3
c = 2.855 Å, and V = 278.710 Å for undoped α-MnO , as deter-
2
mined by the Riveted refinement. This result suggests that Pd
atoms with large ionic radii may substitute Mn sites in MnO .
2
Such lattice doping of low valent ions usually leads to the
creation of oxygen vacancies to maintain charge neutrality, as
has been frequently reported in reducible oxide-supported
3
8,49,63,64,66
catalysts.
Fig. 3 XANES and EXAFS analysis: (a) SXRD data and refinement of Pd-
Fig. 3(b and c) shows the X-ray absorption near-edge spec- MnO . (b) The normalized XANES spectra at the Pd K-edge of Pd foil,
2
3
troscopy (XANES) and EXAFS spectra. The Pd K-edge absorp- PdO, and Pd-MnO
2
. (c) The k -weighted FT-EXAFS spectra of the Pd
K
III-edge without phase-shift correction. (d) WT contour plots of Pd-
tion of Pd-MnO is close to that of PdO, which indicates that
the Pd valence is around +2. Fourier transformation (FT) of
EXAFS was conducted. The FT-EXAFS curve of Pd-MnO shows
2
2
MnO and MnO . Note: The WT plot of Pd-MnO was separated into
2
2
2
two figures. Two peaks located in the range 2–4 Å were intentionally
enhanced for better comparison. The full WT plot is provided in Fig. S3.†
a prominent peak at ∼1.5 Å, which is in line with the Pd–O
coordination of PdO (Fig. 3(c)). Quantitative FT-EXAFS fitting
(Fig. S2 and Table S1†) yields an average Pd–O bond length of
dPd–O-1 ∼ 2.0 Å, and a coordination number (CN) of 4.5 in the
first nearest neighbor coordination shell. Two weak shoulder
peaks centered at 2.4 Å and 2.9 Å were also observed, which
can be attributed to overlapping contributions from Pd clus-
ters and high shell Pd–Mn coordination, respectively. The CN
of Pd–Pd was estimated to be 1.5, suggesting the presence of a
small amount of Pd clusters. Wavelet transformation of EXAFS
was further applied to distinguish back-scattering atoms.
2
Fig. 3(d) shows the WT plot of Pd-MnO with improved resolu-
tion in R-space using Morlet parameters of κ = 25 and η = 1.
The WT contour plot of Pd-MnO is similar to that of undoped
2
2 2
MnO (Fig. 3(e)), which suggests that Pd species in Pd-MnO
predominantly exist as single atoms and substitute Mn sites in
the crystalline lattice.
3.2. Catalytic performance
α-MnO nanowire supported, atomically dispersed and nano-
2
-
sized Pd species were evaluated in the aerobic oxidation of
HMF under mild reaction conditions (aqueous media under
ambient pressure). The profiles of reaction rate versus reaction
time are shown in Fig. 4 and the main product was FDCA.
Other side products were DFF, HMFCA and FFCA. The conver-
Fig. 2 (a, b) TEM, (c) HRTEM and (d–g) HAADF-STEM images of PdNP/
MnO . (e–g) Elemental mapping showing the distribution of Mn, O, and
Pd, respectively.
2
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Of the catalysts shown, Pd-MnO
2
shows an exceptionally high
FDCA productivity under mild conditions.
The influence of catalyst loading and HMF dosage on HMF
2
oxidation catalyzed by Pd-MnO was investigated (Fig. S4†).
The HMF conversion and FDCA yield increased linearly with
catalyst dosage (20–100 mg). This result can be attributed to
an increase in the number and availability of catalytically
active sites. Fig. S4(b)† shows the conversion of HMF and yield
of FDCA as a function of HMF dosage. When the HMF amount
increased from 60 mg to 80 mg, HMF conversion and FDCA
yield maintained the maximum values of 100% and 88.1%,
respectively. By further increasing HMF loading, to 140 mg,
the conversion of HMF declined to 78% and the yield of FDCA
dropped to 56.4%.
To elucidate the role of gaseous oxygen in the reaction, we
conducted the oxidation of HMF in an Ar atmosphere. The
conversion of HMF reached ∼10% at 2 h and was unable to
proceed afterwards. The reaction rate for HMF oxidation in Ar
2
Fig. 4 Catalytic performance and stabilities of the Pd-MnO (a, b) and
PdNP/MnO
2
catalysts (c, d). Reaction conditions: m(HMF)/m(catalyst) =
CO ) = 1 : 4, water as solvent with an O flow rate of
5 mL min , T = 100 °C, 0.1 MPa.
0
2
.8, n(HMF) : n(K
2
3
2
−
1
−1
was determined as 5.7 mol molPd , which is ten times lower
−
1
than that under an O atmosphere (50.8 mol mol
). Such a
2
Pd
large difference suggests that gaseous oxygen participated in
sion rates of HMF over both Pd-MnO
2
and PdNP/MnO
2
the oxidation of HMF. To further verify this, we investigated
samples increased with the reaction time. As the reaction the effect of O pressure on conversion of HMF. If the gaseous
2
exceeded 4 h, HMF was completely converted into FDCA, with oxygen acted as the main oxygen source for the product, a
8
8.1% and 67.8% yields for Pd-MnO
2
and PdNP/MnO
2
,
2
positive relationship between HMF conversion and O pressure
respectively (Table 1). Fig. 4(a) shows that the Pd-MnO sample should be observed. Indeed, HMF conversion rose from 62.3%
2
−
1
has an FDCA production rate of 50.80 mmol mmolPd , which to 100% when increasing the O2 pressure from 0.1 MPa to
is much higher than that of PdNP/MnO (18.01 mmol 1.0 MPa, as shown in Fig. S5.† The influence of O pressure
2
2
mmolPd− ). Meanwhile, from Table 1 it can be seen that became weak when the pressure was above 1.0 MPa, since the
1
−
1
−1
Pd-MnO
2
gives 100.91 mmol h
g
Pd
of FDCA productivity, O supply was in large excess.
2
which is higher than that for nanosized Pd species
(
Kinetic fitting of the reaction profiles (Fig. 4(a and b)) was
45.57 mmol h− gPd ). Comparing the activity of the carried out to estimate the reaction pathway and determine
1
−1
Pd-MnO
exposed Pd atom, Pd-MnO
2
and PdNP/MnO
2
2
catalysts by normalizing per the rate-limiting step. The fitting details are provided in the
is still more active in the pro- ESI.† Table S3† gives the estimated rate constants for Pd-MnO2
duction of FDCA (Table S3†). In Table 1, the catalytic perform- and PdNP/MnO2 and the kinetic model reveals that the rate
ance of Pd-MnO is compared with commercially available constants for oxidation of HMF to HMFCA (k ) and DFF (k )
Pd/C catalyst and recently reported Pd nanoparticle-based cata- are relatively small. Considering that k is higher than k , the
2
1
2
2
1
lysts. Although the reported 4%Ru/MnO catalyst has a high catalytic oxidation of HMF to FDCA proceeded preferentially
2
2
FDCA yield (99.1%), high O pressure (2.4 MPa) was required. through the pathway as HMF → DFF → FFCA → FDCA, in
2
Table 1 Comparison of catalytic performance of Pd-MnO , commercial Pd/C and the reported Pd-based catalysts
HMF/Pd
(mole ratio)
O
(MPa)
2
pressure
Conversion of HMF
(%)
Yield of FDCA
(%)
FDCA productivity
−1
−1
Pd
)
Catalyst
Pd-MnO
(mmol h
g
2
48.74
28.63
—
0.13
100
33.6
10
0.1
0.1
0.1
0.1
0.1
2.4
0.1
1.0
100
100
42.9
100
98.4
100
92.1
100
88.1
67.8
30.1
76.6
65.9
99.1
83.0
84.0
100.91 (this work)
45.57 (this work)
—
PdNP/MnO
MnO
Commercial Pd/C
2
2
22.98 (this test)
a
0.81Pd@PECN
4Ru/MnO
2.5Pd/γ-Fe
1.4Ru/Mn
27.87 (ref. 67)
b
2
32.92 (ref. 34)
c
2
O
Ce
3
27.67 (ref. 19)
d
4
1
O
Y
50
23.09 (ref. 68)
flow rate of 25 mL min− , reaction time =
1
Reaction conditions: m(HMF)/m(catalyst) = 0.8, n(HMF) : n(K
2
CO
3
) = 1 : 4, water as solvent with an O
2
a
−1
4
9
h, reaction temperature = 100 °C, 0.1 MPa. Reaction conditions: HMF = 0.1 mmol, O = 10 mL min , K CO = 0.4 mmol, water = 2 mL, T =
2 2 3
6
7
b
3
4
c
0 °C, t = 12 h.
Reaction conditions: HMF = 2 mmol, solvent (H
O) = 8 mL, K CO = 0.4 mmol, O balloon, T = 100 °C, t = 12 h.
O) = 20 mL; P(air) = 1 MPa, T = 120 °C; t = 18 h.
2
O) = 20 mL, t = 10 h, T = 120 °C, P(air) = 2.4 MPa.
Reaction conditions:
Reaction conditions: HMF = 2 mmol; solvent
1
9 d
HMF = 0.4 mmol, solvent (H
2
2
3
2
6
8
(H
2
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which HMF → DFF is the rate-limiting step for both catalysts. manganese, and palladium are clearly seen in Pd-MnO
2
and
The rate constants k and k of Pd-MnO are higher than those PdNP/MnO2 samples. Fig. 5(b) shows the high-resolution Pd
1
2
2
for PdNP/MnO
2
,
suggesting higher catalytic activity of 3d spectra with characteristic Pd 3d5/2 and 3d3/2 doublet peaks.
has a much higher k value than Pd- The binding energy (BE) of the Pd 3d5/2 peak position in
Pd-MnO . PdNP/MnO
2
2
4
2
+
MnO . This implies that the oxidation of DFF to FFCA for Pd-MnO is 337.5 eV, which is higher than the BE of Pd in
2
2
PdNP/MnO
2
was so fast that a tiny amount of DFF was bulk PdO (337.2 eV). Such a high positive BE shift has also
2
+
detected in the reaction process. Similar results were obtained been observed in the Pd
x
2+
Ce1−x
O
2−δ structure because the
6
9
by Besson et al.
The catalytic stability of atomic and nanosized Pd species from that in bulk PdO.
supported on α-MnO was tested. The spent catalyst was fil- main BE of Pd 3d5/2 was located at a low value of 336.3 eV,
ligand environment of Pd in the solid solution is different
7
0,71
2
For the PdNP/MnO sample, the
2
tered and washed with deionized water and ethanol and dried suggesting that the highly dispersed Pd nanoparticles with
at 60 °C overnight, and then used in the repeated test. Pd- positive charge resulted from electron transfer from Pd NPs to
MnO
FDCA yield after five catalytic cycles (Fig. 4(c)), while nanosized in Pd-MnO2.
Pd species supported on MnO revealed low catalytic re- The O 1s spectra were deconvoluted to three peaks corres-
usability (Fig. 4(d)). TEM characterization of Pd-MnO shows ponding to the lattice O (denoted as O
no microstructure changes after five cycles (Fig. S6†). The Pd binding energy of 529–530 eV, the adsorbed oxygen (denoted
content of used Pd-MnO catalyst was determined by ICP-OES as Oads) at 531–532 eV, and the adsorbed water (denoted as
and little Pd (≈ 0.1%) had been leached from the catalyst after H O) at 532–533 eV (Fig. 5(c)). Considering the fact that
2 2
catalyst showed no degradation of HMF conversion and the MnO surface. Such electron transfer is more pronounced
2
2
−
2
L
) at a relatively low
2
O
2
five runs. The Pd and Mn content in the reactant filtrate were oxygen activation is a key step in HMF oxidation, quantifi-
also measured, and the leaching amounts were negligible. The cation of adsorbed oxygen binding at oxygen vacancies is
7
2
above results suggest that atomic Pd supported on MnO
2
is important in order to understand the catalytic performance.
quite stable in HMF oxidation and could be recycled for five Therefore, the relative ratio of Oads/(Oads + O ) was used to
L
runs without loss of catalytic activity.
.3. Surface physicochemical analysis
comprehend the catalytic behavior of all the investigated cata-
lysts towards oxidation reactions. Among them, the Pd-MnO
catalyst has a much higher ratio of Oads/(Oads + O ) (27.1%)
than the PdNP/MnO (20.7%) and MnO (13.0%) samples.
2
3
L
2
2
To understand the enhanced catalytic performance of Pd-
Fig. 5(d) shows the Mn 3s XPS spectra and the average oxi-
dation state (AOS) of Mn was estimated using the following
MnO , we carried out multiple surface chemistry characteriz-
2
ations. XPS was applied to determine the surface elemental
composition and valence state of the catalysts. Fig. 5(a) shows
the XPS survey scans and peaks corresponding to oxygen,
7
3
formula: AOS = 8.956 − 1.126ΔE3s
,
where ΔE3s is the binding
energy difference between the doublet Mn 3s peaks. The calcu-
lated AOS of Pd-MnO2 is 3.57, which is lower than that of
2
MnO (3.82). This result validates the creation of rich oxygen
vacancies upon incorporation of atomic Pd.
H -TPR experiments were first performed to investigate the
2
redox properties and metal–support interactions of Pd-MnO
In Fig. 6(a), the PdNP/MnO catalyst shows two reduction
peaks. One broad peak at 110 °C can be attributed to reduction
of the PdO species interacting with MnO . The second peak at
33 °C is ascribed to the reduction of MnO to MnO. By con-
trast, only one prominent asymmetric reduction peak is
observed for the Pd-MnO catalyst, which can be assigned to
2
.
2
x
2
2
2
2
the reduction of Pd–O–Mn moieties, followed by reduction of
the MnO support. Compared with PdNP/MnO , the reduction
2
2
temperature of oxidized Pd species in Pd-MnO
2
shifts to a
higher value (157 °C), owing to the enhanced metal–support
7
4
interaction. Meanwhile, the reduction peak of the support
shifts to lower temperature, indicative of enhanced
reducibility.
O -TPD characterization was employed to further investi-
2
gate the oxygen species and the results are displayed in
Fig. 6(b). Generally, the O
three regions: low temperature (<230 °C), medium temperature
230–470 °C), and high temperature (>470 °C), which corres-
pond to desorption of oxygen species adsorbed on the surface,
2
-TPD spectra can be separated into
Fig. 5 (a) XPS survey spectra of Pd-MnO
Pd 3d spectra of Pd-MnO and PdNP/MnO
, and MnO ; (d) O 1s spectra of Pd-MnO
2
, PdNP/MnO
; (c) O 1s spectra of Pd-
, PdNP/
2 2
, and MnO ; (b)
(
2
2
MnO
MnO
2
, PdNP/MnO
, and MnO
2
2
2
7
5
2
2
.
surface lattice oxygen and bulk lattice oxygen, respectively.
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Fig. 7 Geometric structures of HMF adsorbed onto the activated Pd
2
site supported on the α-MnO (110) surface. (a) Free HMF in vacuum. (b)
HMF adsorbed on Pd-MnO . (c) HMF adsorbed on Pd13/MnO
2
2
.
Table 2 Adsorption energy (Eads) of HMF on the Pd-MnO
Pd13/MnO
2
(110) and
2
(110) surfaces, Evacu, bond lengths of Pd–O in the catalysts
and C–O in the HMF, charge distribution changes of Pd and adsorbed
HMF molecule. A negative value means losing the electrons and vice
versa
Fig. 6
catalysts.
2 2 2 2
H -TPR (a) and O -TPD (b) of Pd-MnO and PdNP/MnO
Pd-MnO
2
(110)
2
Pd13/MnO (110)
Compared with PdNP/MnO , the Pd-MnO sample released
larger amounts of surface-adsorbed oxygen species, as can be
seen from the desorption peak at 208 °C.
2
2
Eads(eV)
Evacu(eV)
Pd–O (Å)
C–O (Å)
−3.34
−0.21
2.05
1.27
−1.63
−0.29
−2.79
1.78
2.14
1.26
−0.14
−0.08
Therefore, combining the XPS results, it is reasonable to
Pd charge (e)
deduce that Pd-MnO
2
has larger oxygen vacancies for the HMF charge (e)
adsorption and activation of molecular oxygen. Furthermore,
desorption of surface lattice oxygen from the Pd-MnO sample
2
is greater, as seen in the region of medium temperature, indi-
cating the improved mobility of surface lattice oxygen.
In brief, the XPS, H -TPR, and O -TPD results demonstrated
tronic properties of the catalyst were also investigated. The
charge density distribution plot (Fig. S7†) reveals electron
transfer from atomic Pd to the support. Based on the Bader
charge analysis (Table 2), the Pd single atom (1.63e) lost more
electrons than Pd13 (0.14e), which is consistent with the XPS
result. In consequence, HMF received more electrons from Pd-
2
2
that there is a strong metal–support interaction in the Pd-
MnO and the presence of atomic Pd activates the lattice
2
oxygen of the support and facilitates the creation of rich
oxygen vacancies. These properties are believed to be favorable
for HMF oxidation.
2 2
MnO (0.29e) than from Pd13/MnO (0.08e), respectively. The
pronounced electron exchange in Pd-MnO
activation of the HMF molecule.
2
verifies the ease of
3
.3. DFT calculation for Pd-species-dependent HMF
On the other hand, the promoted HMF oxidation is
involved the activation of molecular oxygen dissolved in
oxidation
Density functional theory (DFT) calculations were performed aqueous solution. It has been shown that the reducible MnO
2
5
7
to elucidate the origin of the high catalytic activity of can effectively adsorb and dissociate O
2
,
since the oxygen
Pd-MnO
2
. Fig. 7 shows the stable adsorption configurations of vacancies can reduce the band gap and increase the Femi
and elongate the O–O band of the adsorbed
We preferentially explored the formation energies of
, due to the huge compu- the oxygen vacancy (Evacu) when single Pd or Pd13 were loaded
tational demand of simulating PdNP. It can be seen that the on MnO . Pd-MnO exhibits a notably lower oxygen vacancy
distance between the HMF and Pd single atom is 2.05 Å, while formation energy (−0.21 eV) compared with Pd13/MnO
the HMF molecule on Pd-MnO and Pd /MnO . It should be levels of MnO
2
13
2
2
5
7,76–79
noted that the Pd13 cluster supported by MnO
as an approximation for PdNP/MnO
2
was modeled
2
O .
2
2
2
2
the distance between the HMF and Pd13 is longer (2.14 Å). (1.78 eV), which indicates that the formation of oxygen
This suggests that the Pd single site has stronger binding vacancies is more favorable in the presence of atomic Pd. This
affinity for HMF, which can be further verified by the calcu- theoretical result agrees well with the increased oxygen
lated adsorption energies. As listed in Table 2, Pd-MnO
higher adsorption interaction energy of −3.34 eV compared TPD results.
with Pd13/MnO (−2.79 eV). In addition, the C–O bond in HMF
The characterization and theoretical results suggest that the
2
2 2
has a vacancy concentration in Pd-MnO as detected by XPS and O -
2
is stretched from 1.23 Å to 1.27 Å, indicative of activation of atomic dispersion of Pd not only increases the atomic
the HMF molecule after adsorption onto Pd-MnO . The elec- efficiency, but also maximizes the metal–support interactions
2
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and effectively tunes the electronic properties of the Pd atoms, Shanghai
Institute
of
Applied
Physics,
China
thereby enhancing their binding affinity to the HMF mole- (2017-SSRF-PT-00257, 16ssrf00787).
cules. Additionally, the presence of substitutional Pd atoms in
the MnO promotes formation of rich oxygen vacancies and
2
increases the mobility of surface lattice oxygen, which facili-
tates oxygen adsorption, dissociation and replenishment.
Considering the interface between Pd and MnO as active
2
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Green Chem.
This journal is © The Royal Society of Chemistry 2019