Effect of MnO2 Crystal Structure on Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

Article abstract of DOI:10.1021/jacs.8b09917

Aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastics monomer is efficiently promoted by a simple system based on a nonprecious-metal catalyst of MnO₂ and NaHCO₃. Kinetic studies indicate that the oxidation of 5-formyl-2-furancarboxylic acid (FFCA) to FDCA is the slowest step for the aerobic oxidation of HMF to FDCA over activated MnO₂. We demonstrate through combined computational and experimental studies that HMF oxidation to FDCA is largely dependent on the MnO₂ crystal structure. Density functional theory (DFT) calculations reveal that vacancy formation energies at the planar oxygen sites in α- and γ-MnO₂ are higher than those at the bent oxygen sites. β- and λ-MnO₂ consist of only planar and bent oxygen sites, respectively, with lower vacancy formation energies. Consequently, β- and λ-MnO₂ are likely to be good candidates as oxidation catalysts. On the other hand, experimental studies reveal that the reaction rates per surface area for the slowest step (FFCA oxidation to FDCA) decrease in the order of β-MnO₂ > λ-MnO₂ > γ-MnO₂ ≈ α-MnO₂ > δ-MnO₂ > ?-MnO₂; the catalytic activity of β-MnO₂ exceeds that of the previously reported activated MnO₂ by three times. The order is in good agreement not only with the DFT calculation results, but also with the reduction rates per surface area determined by the H₂-temperature-programmed reduction measurements for MnO₂ catalysts. The successful synthesis of high-surface-area β-MnO₂ significantly improves the catalytic activity for the aerobic oxidation of HMF to FDCA.

Full text of DOI:10.1021/jacs.8b09917

Article
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
pubs.acs.org/JACS
Effect of MnO Crystal Structure on Aerobic Oxidation of
2
5
‑Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid
† † †,§ † §,‡
Naoki Tsunoda, Yu Kumagai, Fumiyasu Oba,†
Eri Hayashi, Yui Yamaguchi, Keigo Kamata,
and Michikazu Hara*
,
†,⊥
†
‡
Laboratory for Materials and Structures, Institute of Innovative Research and Materials Research Center for Element Strategy,
Tokyo Institute of Technology, Yokohama 226-8503, Japan
§
⊥
Precursory Research for Embryonic Science and Technology (PRESTO) and Advanced Low Carbon Technology Research and
Development Program (ALCA), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi 332-0012, Japan
*
S Supporting Information
ABSTRACT: Aerobic oxidation of 5-hydroxymethylfurfural
HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastics
(
monomer is efficiently promoted by a simple system based on a
nonprecious-metal catalyst of MnO and NaHCO . Kinetic studies
2
3
indicate that the oxidation of 5-formyl-2-furancarboxylic acid
FFCA) to FDCA is the slowest step for the aerobic oxidation of
(
HMF to FDCA over activated MnO . We demonstrate through
2
combined computational and experimental studies that HMF
oxidation to FDCA is largely dependent on the MnO crystal
2
structure. Density functional theory (DFT) calculations reveal that
vacancy formation energies at the planar oxygen sites in α- and γ-
MnO are higher than those at the bent oxygen sites. β- and λ-
2
MnO consist of only planar and bent oxygen sites, respectively,
2
with lower vacancy formation energies. Consequently, β- and λ-MnO are likely to be good candidates as oxidation catalysts. On
2
the other hand, experimental studies reveal that the reaction rates per surface area for the slowest step (FFCA oxidation to
FDCA) decrease in the order of β-MnO > λ-MnO > γ-MnO ≈ α-MnO > δ-MnO > ε-MnO ; the catalytic activity of β-
2
2
2
2
2
2
MnO exceeds that of the previously reported activated MnO by three times. The order is in good agreement not only with the
2
2
DFT calculation results, but also with the reduction rates per surface area determined by the H -temperature-programmed
2
reduction measurements for MnO catalysts. The successful synthesis of high-surface-area β-MnO significantly improves the
2
2
catalytic activity for the aerobic oxidation of HMF to FDCA.
INTRODUCTION
and γ-MnO have been proposed as active phases based on
2
■
4
their unique properties such as chemisorption, electron
Manganese oxide-based materials have received much
attention in broad fields of catalysis, magnetism, chemical
sensing, and electrochemistry (lithium-ion batteries, super-
capacitors, etc.) because of their natural abundance, low cost,
environmental friendliness, and specific chemical/physical
5
5b,6
transfer,4 number of active species,
and chemical
b,c,6d,7
bonding.
However, systematic studies on liquid-phase
organic reactions with crystalline MnO are still limited; there
2
are only a few reports on the activity-structure relationship
among ≥3 crystal structures of MnO . In this context, the
2
properties including diverse crystal structures and oxidation
1
rational design and synthesis of active crystalline MnO
2
states. Among them, manganese dioxides (MnO ) have been
2
catalysts are attractive, yet challenging to establish the desired
extensively investigated as heterogeneous catalysts for liquid-
phase selective oxidation as well as gas-phase total oxidation
of hydrocarbons, alcohols, NO, and CO. Many efforts have
been made to improve the catalytic performance of MnO2
through variation of the specific surface area, crystal form,
oxidation state, morphology, and porosity. In particular, the
8
2
liquid-phase organic reaction.
3
Renewable and sustainable biomass feedstocks have received
significant attention as substitutes of nonrenewable fossil
resources such as crude oil, coal, and natural gas for the
manufacture of high value-added products. In particular, 5-
hydroxymethyl furfural (HMF) is a key platform compound
that is synthesized from cellulose because HMF can be
converted into various high value-added compounds such as
crystal structure of MnO plays a crucial role in determining
2
the catalytic properties, and some possibly active crystal
structures have been proposed depending on the target
reactions. Crystalline MnO2 materials consist of MnO6
octahedral units shared by corners or edges, which result in
various tunnel and layered structures (Figure 1). For the well-
studied electrochemical reactions and gas-phase reactions, α-
2,5-diformylfuran (DFF), dimethylfuran, levulinic acid, 2,5-
Received: September 12, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b09917
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
could act as an efficient and reusable precious-metal-free
heterogeneous catalyst for the oxidation of HMF to FDCA in
16
the presence of NaHCO with O as the sole oxidant.
3
2
However, the reaction mechanism of HMF oxidation,
including the structure−activity relationship of MnO , has
2
not yet been clarified. In this paper, we report the synthesis and
characterization of MnO -based materials with various crystal
2
structures and their application as catalysts for the aerobic
oxidation of HMF to FDCA. In addition, the reaction
mechanism including the catalyst effect, kinetics, and
experimental and computational studies on the reactivity of
constituent oxygen atoms is investigated.
EXPERIMENTAL SECTION
■
Instruments. Synthesized MnO samples were identified by X-ray
2
diffraction (XRD) using a Rigaku Ultima IV diffractometer with Cu
Kα radiation (λ = 1.5405 Å, 40 kV−40 mA) equipped with a high-
speed one-dimensional detector (D/teX Ultra). Data were collected
in the 2θ range 10−80° in 0.02° steps with a continuous scanning rate
−
1
of 20° min . The Rietveld refinements were carried out using Rigaku
PDXL2 software. The chemical composition was analyzed by
inductively coupled plasma-atomic emission spectroscopy (ICP−
AES) using a Shimadzu ICPS-8100 spectrometer. The specific surface
areas of samples were determined by nitrogen adsorption−desorption
isotherms measured at 77 K using a Quantachrome Nova-4200e
surface area and pre size analyzer, and the Brunauer−Emmett−Teller
Figure 1. Structures of (a) α-MnO , (b) β-MnO , (c) γ-MnO , (d) δ-
2
2
2
MnO , and (e) λ-MnO . Pink, green, and red spheres represent Mn,
2
2
K, and O atoms, respectively.
bisaminomethylfuran, dimethylfuran, and 5-(dimethoxymeth-
(
BET) surface areas were estimated over the relative pressure (P/P )
9
0
yl)-2-furanmethanol (Scheme 1). 2,5-Furandicarboxylic acid
range of 0.05−0.30. Physisorbed water of the samples was removed by
heating at 423 K for 1 h under vacuum prior to their measurements.
The morphologies were characterized by scanning electron micros-
copy (SEM; S-5500, Hitachi). Transmission electron microscopy
Scheme 1. Conversion of HMF into Various High Value-
Added Compounds
(
TEM) studies were carried out using a JEOL JEM2100F trans-
mission electron microscope operating at an accelerating voltage of
00 kV. After Cu grids were directly mixed with samples, the Cu grids
2
were collected and mounted on a stage. Raman spectroscopic analyses
were performed on a Jasco NRS-7600 spectrometer with laser
excitation wavelength of λ = 532 nm. Thermogravimetry-differential
thermal analysis (TG-DTA) was performed using a Rigaku TG8120
differential thermal analyzer, and the measurements were conducted
−
1
from r.t. to 1273 K at a heating rate of 10 K min in N flow (200 mL
2
−
1
min ). X-ray photoelectron spectroscopy (XPS) data were collected
on a Shimadzu ESCA-3400HSE spectrometer with a standard Mg Kα
source (1253.6 eV) working at 10 kV and 25 mA. Samples were
pressed into a pellet and fixed on a piece of double-sided carbon tape.
The binding energies were calibrated by assuming the biding energy
of the C 1s line to be 284.6 eV. The spectrum was fitted using the
XPS Peak 4.1 program after a Shirley type background subtraction.
(
FDCA) is also one of the most attractive raw materials for
polyethylene furanoate (PEF) because biopolyester PEF is not
only a substitute for polyethylene terephthalate, but also a
functional material with high gas barrier properties and
excellent heat resistance that can be easily processed.
Therefore, various types of effective heterogeneous catalysts
based on precious metals (Au, Pt, Ru, and Pd ) have
been developed for the direct oxidation of HMF to FDCA with
The deconvoluted Mn 2p spectrum of MnO shows three peaks with
2
binding energies of 640.8, 641.8, 642.8, and 644.5 eV, which ascribes
II
III
IV
17
1
0
to Mn , Mn , Mn , and a shakeup peak, respectively. The average
oxidation states of Mn species were determined by iodometric
titration using an autotitrator (Mettler Toledo, Easy Pro Titrator
11
12
13
14
System). A MnO sample (ca. 10 mg) was added to a mixed solution
2
of 0.5 M HCl aq. (12 mL) and 2 M KI aq. (5 mL), and the resulting
molecular oxygen (O ) as the sole oxidant in the presence of
18
2
solution was titrated with an aqueous solution of 0.01 M Na S O .
2 3 3
base additives, and several precious metal-based catalysts
efficiently promote HMF oxidation, even in the absence of
bases (Table S1). On the other hand, systems using
nonprecious metal catalysts have disadvantages such as low
yield and selectivity toward FDCA, requirement for severe
reaction conditions, use of organic oxidants and solvents, and
H2 temperature-programmed reduction (H -TPR) profiles were
measured on a BEL Japan BELCAT-A chemisorption analyzer
equipped with a thermal conductivity detector (TCD) to measure
2
changes in the H
decrease of the gas stream. The y-axis means the
2
TCD signal intensity and integrated area gives total H gas
2
19
consumed. Fifty milligrams of sample was placed in a quartz cell
and then heated from 323 to 923 K at a rate of 10 K min under 5%
H /Ar flow (50 mL min ). The initial reduction rates were estimated
−
1
15
poor recyclability. Very recently, several research groups have
−
1
2
developed effective catalyst systems based on mixed oxides
from 553 to 593 K, which correspond to the range, which is under
1
5a−e,k
such as Ni-, Ce-, Fe-, Cu-containing manganese oxides
1
0% of the lattice oxygen atoms reduced by H , and all the MnO
15f−h
2
2
Zn-, Ce-, Ru-containing iron oxides,
and Co-containing or
for aerobic oxidation of HMF to
FDCA. We have reported for the first time that MnO itself
catalysts were stable in this range. High performance liquid
chromatography (HPLC; LC-2000, Jasco) analyses were performed
using photodiode array and refractive index detectors with a Aminex
15i,j
metal-free carbon nitrides
2
B
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Journal of the American Chemical Society
Article
HPX-87H column (7.8 mm diameter × 300 mm, Bio-Rad
Laboratories, Inc. Co. Ltd.; eluent (0.5 mM H SO ), flow rate (0.5
mL/min), column temperature (308 K)). The retention times for
FDCA, 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 5-formyl-
irreversible transformation of MnO2 to MnOOH likely
occurred when the δ value in MnO2−δ was more than ∼0.23.
Approximately three oxygen atoms per surface Mn species
were estimated to be transferred to the substrate (see details
given in the Supporting Information). All these results indicate
that the oxygen atoms from the oxide surface layer are involved
in the oxidation reaction without the reductive activation of
2
4
2
4
-furancarboxylic acid (FFCA), HMF, and DFF were 20.9, 27.3, 29.4,
1.7, 52.1 min, respectively. The crystal structures were drawn using
the visualization for electronic and structural analysis (VESTA)
program.
20
27
Procedure for Catalytic Oxidation. The catalytic oxidation of
various substrates was carried out in a 30 mL glass vessel or in a 13
mL autoclave reactor with a Teflon vessel containing a magnetic
stirring bar. A typical procedure for the catalytic oxidation of HMF
was as follows. HMF (0.2 mmol), MnO (0.05 g), NaHCO (0.6
O . The role of O is essential for the rapid reoxidation of
2 2
MnO2−δ to MnO without the transformation into inactive
2
MnOOH, establishing the catalytic cycle. On the other hand,
the energy for oxygen vacancy formation has been accepted as
a good descriptor of the oxidizing power of an oxide; a lower
2
3
mmol), water (5 mL), and O2 (1 MPa) were charged into the
autoclave reactor. The reaction solution was heated at 373 K for 24 h.
After the reaction, the catalyst was separated by filtration and the
filtrate was diluted 10 times with water. The recovered catalyst was
washed with water (25 mL) and then dried at 353 K.
28
vacancy formation energy indicates a better oxidant.
Therefore, the DFT calculations were conducted to estimate
vacancy formation energies for MnO catalysts with various
2
crystalline structures. Although the oxygen vacancies have been
examined based on DFT studies correlating with experimental
Synthesis of High-Surface-Area β-MnO2 (β-MnO −HS). β-
2
29
MnO −HS was synthesized according to the following procedure. An
2
catalytic performance of β-MnO2, the polymorph depend-
ence on the oxygen vacancy formation energy and catalytic
performance has not been investigated.
aqueous solution (20 mL) of MnSO ·5H O (1.45 g, 6 mmol) was
4
2
added dropwise to an aqueous solution (20 mL) of NaMnO ·H O
4
2
(
0.63 g, 4 mmol) with vigorous stirring, and the resulting suspension
In this study, α-, β-, γ-, δ-, ε-, and λ-MnO were synthesized
2
was stirred for a further 30 min. The precipitates were collected by
filtration, washed with water (2 L), and dried at 353 K overnight. The
product was calcined at 673 K for 5 h to give a black powder of β-
by hydrothermal and sol−gel methods (see details in the
Supporting Information). The crystal structures of these MnO
2
MnO −HS. Yield: 0.85 g (98%).
catalysts are shown in Figure 1. Hollandite-type α-MnO and
2
2
Quantum Chemical Calculations. The density functional theory
pyrolusite-type β-MnO have one-dimensional (1 × 1) and (2
2
3
0
(
DFT) calculations of the oxygen vacancy formation were performed
×
2) tunnel structures. Nsutite-type γ-MnO and akhtenskite
21
2
using the projector augmented-wave (PAW) method as imple-
ε-MnO are composed of an intergrowth of (1 × 1) tunnels
22
2
mented in the Vienna Ab initio Simulation Package (VASP) code.
(
pyrolusite) and (1 × 2) tunnels (ramsdellite), while ε-MnO2
Very recently, Kitchaev et al. have reported that strongly constrained
and appropriately normed (SCAN) meta generalized gradient
exhibits more structural faults and microtwinning than γ-
31
23
MnO2. The removal of lithium cations from LiMn O spinel
results in λ-MnO with a cubic defective spinel structure.
approximation uniquely yields accurate formation energies and
2
4
1b
properties across all MnO polymorphs. Thus, we applied the SCAN
2
2
24
functional in this study. Plane-wave cutoff energies of 550 and 400
eV were employed for the perfect-crystal and point-defect models,
respectively. Spin polarization was allowed in all of the vacancy
calculations. PAW data sets with radial cutoffs of 1.22 and 0.80 Å for
Mn and O, respectively, were employed, and Mn-3d and 4s and O-2s
and 2p were described as valence electrons. The oxygen vacancies
were modeled using 72-, 96-, 96-, and 108-atom supercells for β-, α-,
Birnessite δ-MnO2 has a layered structure containing
intercalated water and cations between the MnO octahedral
2
32
sheets.
The calculated oxygen vacancy formation energies for α-, β-,
γ-, and λ-MnO are summarized in Table 1. The calculations
2
for δ- and ε-MnO were omitted here because their crystal
2
structures are not uniquely modeled. The resultant vacancy
λ-, and γ-MnO , respectively.
2
When calculating point defects in solids with band gaps, all relevant
charge states must be considered. Therefore, 0, 1+, and 2+ charge
states were considered for oxygen vacancies. Consequently, β-MnO2
showed only 1+ and 2+ charge states, whereas 0, 1+, and 2+ charge
states appeared in the other MnO phases. Thus, neutral oxygen
formation energies for MnO significantly depend on the
2
structure and type of constituent oxygen atoms (planar and
bent Mn (μ -O)). Among the MnO phases considered, only
3
3
2
β-MnO consists of planar μ -oxygen atoms with a low vacancy
2
3
2
vacancy formation energy in β-MnO2 was estimated with the
condition that the oxygen vacancy exists in the 1+ charge state and
one electron is located at the conduction band minimum. Corrections
for charged defects were performed with the extended Freysoldt−
Table 1. Calculated Energies for Oxygen Vacancy
Formation
a
25,26
Neugebauer−Van de Walle procedure.
See ref 25 for more details.
RESULTS AND DISCUSSION
■
Computational Studies on Structure−Reactivity Re-
lationship. MnO has various crystallographic structures, each
2
of which significantly influences the properties of the MnO as
2
a catalyst; thus, we investigate the oxidation reactivity of MnO2
with various crystal structures. On the basis of the catalyst
effect, the differences of reactivity in O and Ar atmospheres,
reaction
ΔE (eV)
2
and XRD, XPS, and TG-DTA results, it has been proposed
oxygen vacancy formation
that activated MnO -catalyzed oxidation proceeds via a Mars−
2
α-MnO
2
→ MnO2−x + x/2 O
2
3.95 (O
A
atom) 3.25 (O
B
atom)
atom)
van Krevelen mechanism and consists of the following two
β-MnO → MnO + x/2 O2
3.25 (OA atom)
3.84 (O atom) 3.15 (O
3.44 (O atom)
2
2−x
1
6
steps: (i) oxidation of substrates with MnO to form a
2
γ-MnO
2
→ MnO2−x + x/2 O
2
A
B
partially reduced manganese species, MnO2−δ, and (ii) rapid
λ-MnO → MnO + x/2 O2
2 2−x
B
reoxidation of MnO2−δ by O . According to the total yields for
2
a
the oxidation of HMF with MnO under Ar atmosphere, the
O and O represent planar and bent oxygen atoms, respectively.
A B
2
C
DOI: 10.1021/jacs.8b09917
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Journal of the American Chemical Society
Article
formation energy (3.25 eV). λ-MnO also has only bent μ -
oxygen atoms with a vacancy formation energy of 3.44 eV. In
example, the following lattice parameters (tetragonal, a = 4.40
Å, c = 2.87 Å) were obtained by the Rietveld refinements for
2
3
sharp contrast to the two MnO -based samples, α-MnO and
XRD data of β-MnO (Figure S7), and these values well agree
2
2
2
34
γ-MnO consist of both types of oxygen atoms with higher and
with the previously reported ones. The TEM image for β-
MnO showed that the clear lattice fringes were observed
through the particle, indicating the high crystallinity of β-
MnO particles (Figure S8). The distances between two fringes
are 0.31 and 0.24 nm, which are close to the d-spacings of the
2
lower vacancy formation energies (bent, 3.25 and 3.15 eV;
planar, 3.95 and 3.84 eV). As expected from a structural
viewpoint, the vacancy formation energies at the bent oxygen
sites are generally lower than those at the planar oxygen sites.
2
2
3
4
β-MnO is an exception, in which the planar oxygen site
(110) and (101) planes of β-MnO
2
, respectively. The Raman
2
spectrum of fresh β-MnO showed the bands around 650 and
exhibits a lower vacancy formation energy. This could be
attributed to the much smaller calculated band gap of 0.4 eV
2
−
1
5
30 cm assignable to A and E modes characteristic of β-
1g g
35
MnO , respectively.
than those in the other MnO phases (α, 1.7 eV; γ, 1.2 eV; λ,
2
2
The metal contents, amounts of physisorbed water, and
average oxidation states (AOSs) of the manganese species in
1
.9 eV). Taking into account the ratio of oxygen sites with
lower vacancy formation energies (β-MnO (3.25 eV, 100%),
2
the synthesized MnO were determined using ICP−AES, TG-
λ-MnO (3.44 eV, 100%), α-MnO (3.25 eV, 50%), and γ-
2
2
2
3
6
DTA,
and iodometry, respectively. The results are
MnO (3.15 eV, 50%)), it is expected that β- and λ-MnO
2
2
summarized in Table 2. The Mn contents of all MnO
would be good candidate oxidation catalysts.
2
33
catalysts were in the range of 47.86−62.39 wt %. α- and δ-
Effect of MnO Crystal Structure on the Oxidation of
2
MnO have some cations or water inside the tunnels or
HMF to FDCA. To confirm the possible superiority of the β-
and λ-MnO catalysts, MnO with various crystal structures
2
between the layers to stabilize the structures; thus, the Mn
2
2
contents of α- and δ-MnO were lower than those of β-, γ-, and
2
were synthesized, characterized, and the oxidation catalysis was
30,32
λ-MnO2.
Since ε-MnO was synthesized in the presence of
2
investigated. α-, β-, γ-, δ-, ε- and λ-MnO were synthesized
2
Fe species, low Mn content (47.46 wt %) was observed due to
small amounts of residual Fe species (6.10 wt %). The AOS for
according to previously reported procedures (see details in the
6f,32,33
Supporting Information).
The structure of each sample
β-, γ, and ε-MnO was approximately 4, while the AOSs for α-,
2
was characterized by powder XRD analysis. The XRD patterns
δ-, and λ-MnO were lower (3.70−3.81), which is in good
2
are shown in Figures 2 and S1−S6 and are in good agreement
+
agreement with the charge compensation by the presence of K
ions as previously reported (Table S2).
with those reported for tetragonal α-MnO (JCPDS 00−044−
2
37
0
141), tetragonal β-MnO (JCPDS 00−024−0735), ortho-
2
SEM images of the synthesized MnO samples are shown in
2
rhombic γ-MnO (JCPDS 00−014−0644), trigonal δ-MnO
2
2
Figure 3. The morphology and average particle size are
(
0
JCPDS 01−073−7867), hexagonal ε-MnO (JCPDS 00−
2
summarized in Table 2. α-, β-, and γ-MnO have rod-like
2
30−0820), and cubic λ-MnO (JCPDS 00−044−0992). For
2
particle morphology (Figure 3a−c), and the particle size was
dependent on the sample. δ- and λ-MnO have spherical-like
2
particle morphology (Figures 3d,f). ε-MnO has flower-like
2
spherical particles composed of nanoplates (Figure 3e). The
specific surface areas of the synthesized MnO samples were
2
calculated from Brunauer−Emmett−Teller (BET) plots of the
N adsorption isotherms (77 K). The specific surface area of ε-
2
2
−1
MnO (181 m g ) was significantly large, while those of α-,
2
β-, γ-, δ-, and λ-MnO were relatively low to moderate (14−67
2
2
−1
m g ). These results are in good agreement with the average
particle sizes determined from SEM observations.
The oxidation of HMF with O (1 MPa) in the presence of
2
NaHCO (3 equiv with respect to HMF) was investigated
3
using MnO catalysts with various crystal structures, and the
2
results are shown in Table 3. In the absence of catalysts, the
16
reaction did not proceed. In all cases, the HMF conversion
was in the range of 93−99%. Among the catalysts tested,
activated MnO , α-MnO , and ε-MnO exhibited moderate
2
2
2
activities to afford FDCA in 59−74% yield. Although β-, γ-, δ-,
and λ-MnO also exhibited high HMF conversion, FFCA was
2
obtained as the main product in 60−69% yield with low FDCA
yields (5−28%). To confirm the stability of the catalysts after
the oxidation of HMF, XRD patterns of the recovered MnO
2
catalysts were measured, and the amounts of metal species
leached into the reaction solution were determined by ICP-
AES analysis. In all catalysts except for ε-MnO , the leaching of
2
Mn species into the reaction solution was negligible (Table
S3). There was no significant difference in the XRD peak
positions of α-, β-, and ε-MnO2 between the fresh and
recovered catalysts, while the peak intensities of recovered α-
Figure 2. XRD patterns for (a) α-MnO , (b) β-MnO , (c) γ-MnO ,
and β-MnO were slightly increased. The SEM measurement
2
2
2
2
(
d) δ-MnO , (e) ε-MnO , and (f) λ-MnO .
for the recovered ε-MnO shows that the morphology changed
2
2
2
2
D
DOI: 10.1021/jacs.8b09917
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Journal of the American Chemical Society
Article
a
Table 2. Bulk Contents, Surface Area, Particle Morphology, and Particle Size of MnO Catalysts
2
specific surface area (m g−1)
2
particle morphology
particle size (nm)
d
catalyst
bulk contents (wt %)
a
a
a
α-MnO2
β-MnO2
γ-MnO2
δ-MnO2
ε-MnO2
λ-MnO2
Mn, 56.75; K, 7.12; water, 0.82
Mn, 60.81; water, 0.46
Mn, 62.03; water, 2.45
Mn, 52.43; K, 8.67; water, 6.86
Mn, 47.86; Fe, 6.1; water, 4.84
Mn, 62.39; water, 3.76
30 ± 1
14 ± 1
39 ± 1
35 ± 1
181 ± 1
nanorod
nanorod
nanorod
30−60 wide, 60−1000 long
50−70 wide, 150−1000 long
10−60 wide, 40−800 long
50−150
b
nanosphere
c
flower-like nanosphere (composed of nanoplates) 60−200 wide, 10−20 thick
nanosphere
b
67 ± 4
b
40−70
d
a
c
Particle size of nanorod is a diameter and length. Average particle diameter. Thickness and length of nanoplate. Particle size from SEM
observations.
selectivity did not much change for the oxidation of HMF
using recovered ε-MnO (Table S4); thus, the morphology of
2
ε-MnO does not significantly affect the catalytic activity, while
2
the effect of morphologies of other MnO on their catalytic
2
activity could not be ruled out.
There was no significant difference in XPS and Raman
spectra and morphology observed by SEM images between
fresh and recovered β-MnO catalysts, which also supports the
2
stability of β-MnO (Figures S10−S12). Some XRD peaks
2
(
e.g., (110), (310), and (221) peaks) for recovered γ-MnO
2
were shifted to lower angles after the reaction, which was likely
due to expansion of the unit cell caused by H-insertion into the
material. On the other hand, the peak intensities assignable
38
to the layered structure of δ-MnO ((003) and (006) peaks)
2
+
were decreased, and the significant leaching of K ions (55%
relative to the fresh sample) was confirmed by ICP-AES
analysis. In addition, the XRD peaks observed for fresh λ-
MnO almost disappeared after the oxidation reaction. These
2
results indicate that the structures of δ- and λ-MnO would not
2
be completely maintained under the reaction conditions
employed in contrast to other MnO . The SEM images of δ-
2
and λ-MnO after the oxidation of HMF showed that both
2
morphologies changed spherical-like particles to rod-shaped
particles (Figure S13). Despite the highest reaction rate per
Figure 3. SEM images of (a) α-MnO , (b) β-MnO , (c) γ-MnO , (d)
2
2
2
Mn of λ-MnO for oxidation of FFCA, the catalytic activity of
δ-MnO , (e) ε-MnO , and (f) λ-MnO .
2
2
2
2
λ-MnO for oxidation of HMF was low, which likely indicates
2
Table 3. Effect of MnO Catalysts on Oxidation of HMF to
FDCA
the poor stability of λ-MnO during the oxidation of HMF. On
2
2
a
the other hand, the recovered δ-MnO catalyst showed higher
2
FDCA yield than fresh one in the reuse experiments (Table
S5). The structure changes including the cation exchange or
morphology effect possibly affect the reactivity, while the
mechanism of increase in yield is unclear.
Reaction Mechanism for Oxidation of HMF to FDCA
Catalyzed by Activated MnO . It is difficult to estimate the
2
intrinsic reactivity of MnO from the oxidation of HMF to
2
FDCA because of the complicated sequential reactions
involved. Thus, kinetic analysis was conducted to determine
the key step for the aerobic oxidation of HMF with activated
yield (%)
catalyst
conv. (%)
HMFCA
DFF
FFCA
FDCA
MnO . Scheme 2 shows the possible reaction pathways from
b
b
2
activated MnO2
α-MnO2
β-MnO2
γ-MnO2
δ-MnO2
>99
>99
>99
93
96
>99
>99
−
−
−
−
3
1
−
−
15
36
66
60
69
27
66
74
59
28
5
10
63
12
HMF to FDCA. The hydroxyl and formyl groups of HMF are
oxidized to give DFF and HMFCA, respectively. The
successive oxidation of DFF and HMFCA gives FFCA, and
FFCA is finally oxidized to FDCA. The reaction pathway from
HMF to FFCA is dependent on the catalytic system. While the
b
1
1
2
b
b
−
b
ε-MnO2
1
−
11
12
b
b
pathway through HMFCA has been proposed for Au-, Pt-,
λ-MnO2
13 14
Ru-, and Pd-supported catalyst/NaOH/O2 systems, a
comparable pathway through DFF and HMFCA has been
a
Reaction conditions: catalyst (0.05 g), HMF (0.2 mmol), NaHCO3
b
(
0.6 mmol), water (5 mL), pO (1 MPa), 373 K, 24 h. Not detected.
12c
2
reported for the Pt/C catalyst/NaHCO /O system. On the
3
2
other hand, the possible reaction pathway and the most
from flower-like spherical particles composed of nanoplates to
rod-like particle (Figure S9). The catalytic activity and
important step in this MnO -based catalysis has not yet been
examined.
2
E
DOI: 10.1021/jacs.8b09917
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Scheme 2. Reaction Pathways for Oxidation of HMF to
FDCA, and Rate Constants for the Aerobic Oxidation of
HMF with Activated MnO2
Table 4. Effect of Synthesized MnO Catalysts on Oxidation
of FFCA to FDCA
2
a
a
R (μmol h− m−2
1
)
catalyst
conv. (%)
FDCA yield (%)
0
activated MnO2
α-MnO2
β-MnO2
γ-MnO2
δ-MnO2
30
8
7
10
6
34
12
12
14
9
5.6
7.6
16.4
7.4
5.3
ε-MnO2
λ-MnO2
16
47
21
50
2.3
12.2
a
Reaction conditions: catalyst (0.05 g), FFCA (0.2 mmol), NaHCO3
0.6 mmol), water (5 mL), pO (1 MPa), 373 K, 2 h.
(
2
a
structures. The Mn 2p and O 1s XPS spectra for the MnO2
catalysts are shown in Figures S15 and S16. The deconvolution
Reaction conditions are shown in Figure S14.
4
+
3+
results are summarized in Table 5. Surface Mn , Mn , and
2
+
Therefore, the rate-constants (k −k in Scheme 2) were
Mn species were observed in the Mn 2p XPS spectra, and the
surface Mn valent states were estimated to be 3.00−3.57 by
deconvolution of the Mn 2p spectra. The broad O 1s peaks can
be deconvoluted into the peaks around 529.2, 531.2, and 533.0
eV, which correspond to lattice oxygen, adsorbed oxygen, and
1
5
estimated from the time course for each product (Figure S14),
assuming first-order reactions with respect to organic
molecules for all steps. The kinetic simulation gave solid
−
3
−4
−2
lines with k = 2.4 × 10 , k = 2.0 × 10 , k = 4.2 × 10 , k =
1
2
3
4
−
3
−4 −1
42
1
.2 × 10 , and k = 4.6 × 10 s , and good fits were
adsorbed molecular water, respectively. It has been reported
5
observed. k was approximately 10-times smaller than k , which
that the states of metal or adsorbed oxygen species (rather than
lattice oxygen species) likely play an important role in aerobic
2
1
indicated that the conversion of HMF into FFCA through DFF
was the main pathway over activated MnO . It has been widely
5b,6
oxidation over metal oxide catalysts.
However, no good
2
accepted that MnO oxidizes benzylic and allylic alcohols to
the corresponding carbonyl compounds not only stoichio-
agreement between the reactivity and the surface as well as
bulk average Mn valences or amounts of adsorbed oxygen was
observed (Figure S17), which indicates that the reactivity
order cannot be simply explained by the two factors and that
the intrinsic reactivity of oxygen atoms likely due to the crystal
structure would be crucial for this oxidation.
2
metrically, but also catalytically without significant formation
39
of carboxylic acids. Therefore, the oxidation of alcohols with
MnO proceeds much faster than that of aldehydes, which is in
2
2
good agreement with the proposed pathway through DFF.
The significantly lower value for k than k , k , and k supports
the oxidation of FFCA to FDCA as the rate-determining step
H -TPR analysis was performed next, and the H -TPR
5
1
3
4
2
2
profiles for MnO are shown in Figure 4. All MnO catalysts
2
2
based on the dependence of the reaction rate on the O
pressure.
were reduced, starting from 410−470 K, and showed two main
2
1
6
reduction peaks attributed to transitions of (i) MnO to
2
Effect of MnO Crystal Structure on Oxidation of
FFCA to FDCA. On the basis of the kinetic results, the
Mn O (through Mn O ) and (ii) Mn O to MnO. The peak
2
3
4
2
3
3
4
top temperature was different for each crystal phase, and there
was no significant relation between the peak top temperature
and the reactivity for HMF oxidation. A decrease in the surface
area of the catalyst has been reported to cause a shift of the
reduction peaks toward higher temperatures due to the
structure−reactivity relationship for the MnO -catalyzed
2
oxidation was examined by comparison of the reaction rates
(
R0) for the oxidation of FFCA to FDCA, which is the slowest
step for the aerobic oxidation of HMF to FDCA (Table 4). To
43
investigate the effect of surface areas of MnO , the α-MnO
increased diffusion resistance, and that the reaction rate
2
2
catalysts, which were most frequently studied, were synthesized
by the solid-state and precipitation methods with the surface
areas of 215 and 96 m g , respectively. The catalytic
(e.g., the oxidation of alcohol and oxidative dehydration of
carbohydrate) is dependent on the initial reduction rate.
44
2
−1
40
Thus, the reactivity was not compared with the reduction peak
tops, but with the initial reduction rates estimated from the H2-
TPR profiles. The reduction rates per surface area decreased in
activity of α-MnO for the oxidation of HMF increased as the
2
surface area increased (Table S6), which indicates that surface
−
1
−2
areas of MnO are important. Therefore, we should compare
the order of β-MnO (174 H μmol h m ) > δ-MnO (161
2
2 2 2
−
1
−2
−1
−2
the catalytic performance per surface area to investigate the
intrinsic activity of oxygen atoms depending on the crystal
structures. The initial reaction rates per surface area decreased
H μmol h m ) > λ-MnO (95 H μmol h m ) > α-
2 2 2
−
1
−2
−1
MnO (89 H μmol h m ) > γ-MnO (69 H μmol h
2
2
2
2
−
2
−1
−2
m ) > ε-MnO (23 H μmol h m ). Plots of the reduction
2
2
−
1
−2
in the order of β-MnO (16.4 μmol h m ) > λ-MnO (12.2
rates against the FFCA oxidation rates are shown in Figure 5.
An almost linear correlation was observed for the various
MnO catalysts, except for unstable δ- and λ-MnO , which
2
2
−
1
−2
−1
−2
μmol h m ) > α-MnO (7.6 μmol h m ) ≈ γ-MnO (7.4
2
2
−
1
−2
−1
−2
μmol h m ) > δ-MnO (5.3 μmol h m ) > ε-MnO (2.3
2
2
2
2
−
1
−2
μmol h m ), and the order was in good agreement with that
supports the present reaction mechanism in which oxidation of
the substrate likely proceeds with oxygen supplied from MnO2.
41
expected from computational results.
XPS analysis was performed to clarify the superior FFCA-
Although synthesis for β-MnO using hydrothermal method
has been reported, their specific surface areas are generally low
2
oxidation ability of β-MnO over those of other MnO crystal
2
2
F
DOI: 10.1021/jacs.8b09917
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
a
Table 5. Binding Energy, Fraction and Average Oxidation State of Synthesized MnO Catalysts
2
binding energy of Mn 2p (eV)
binding energy of O 1s (eV)
catalyst
Mn (IV)
Mn (III)
lattice oxygen
adsorbed oxygen species
adsorbed molecular water
α-MnO2
β-MnO2
γ-MnO2
δ-MnO2
ε-MnO2
λ-MnO2
642.4 (54%)
642.2 (44%)
642.2 (54%)
642.5 (36%)
642.4 (32%)
642.2 (53%)
641.4 (46%)
641.2 (56%)
641.2 (46%)
641.5 (64%)
641.4 (38%)
529.5 (77%)
529.2 (74%)
529.4 (71%)
529.7 (73%)
529.5 (69%)
529.6 (71%)
531.2 (19%)
531.2 (23%)
531.2 (25%)
531.4 (21%)
531.4 (27%)
531.4 (25%)
533.0 (4%)
533.0 (3%)
533.0 (4%)
533.0 (6%)
533.0 (4%)
533.0 (4%)
b
641.2 (47%)
a
Values in parentheses are peak percentages. Mn2+ species were observed on the surface of ε-MnO2.
b
Figure 5. Relationship between FFCA oxidation rate (R ) and
0
reduction rate estimated from H -TPR (■, α-MnO ; ▲, β-MnO ; ●,
2
2
2
γ-MnO ; □, δ-MnO ; ◆, ε-MnO ; △, λ-MnO ).
2
2
2
2
Figure 4. H -TPR profiles for (a) α-MnO , (b) β-MnO , (c) γ-MnO ,
2
2
2
2
(
d) δ-MnO , (e) ε-MnO , and (f) λ-MnO .
2 2 2
(
Table S8), leading to a problem that often limits the catalytic
4a,6a,7a,d
Figure 6. (a) XRD patterns for β-MnO
2
−HS (upper) and β-MnO
2
performance.
We successfully developed a simple
(lower, JCPDS 00−024−0735) and (b) SEM image of β-MnO −HS.
2
synthetic method for β-MnO to improve the specific surface
area from 14 m /g (by the hydrothermal method in this work)
to 82 m /g (see experimental section). The calcination of
amorphous precursor prepared by Na[MnO ] and Mn(SO )
2
2
Scheme 3. Comparison of Catalytic Activities between β-
MnO and β-MnO −HS for Oxidation of HMF to FDCA
2
2
2
4
4
with the molar ratio of 2:3 gave analytically pure β-MnO (β-
2
MnO −HS). The XRD pattern for β-MnO −HS well agreed
2
2
with that for tetragonal β-MnO2 (JCPDS 00−024−0735,
Figure 6a), and the SEM image showed the formation of
flower-like spherical particles composed of small nanoplates
(
ca. 40−80 wide, 10−20 thick, Figure 6b). The grain sizes
estimated from (110) and (101) diffraction lines using
Scherrer’s equation were 11 and 15 nm, respectively, in
agreement with the SEM measurements. The catalytic
suggests that adsorbed organic compounds would be removed.
The weight loss (4.6 wt %) of β-MnO −HS was much larger
2
performance of β-MnO −HS was significantly improved in
than that (0.5 wt %) of β-MnO ; thus, the adsorption amounts
2
2
comparison with β-MnO2 synthesized by hydrothermal
method to give FDCA in 86% yield for 24 h (Scheme 3).
The TG-DTA curve of the recovered catalysts had exothermic
peaks with small weight losses at around rt−657 K, which
of reactants and intermediates would be increased in the case
of β-MnO −HS.
2
Figure 7 shows the time course for aerobic oxidation of
HMF to FDCA with β-MnO −HS. The formation rate of
2
G
DOI: 10.1021/jacs.8b09917
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
where the oxidation of FFCA to FDCA is the slowest step.
Investigation of the structure−activity relationship for six types
of crystalline MnO (α-, β-, γ-, δ-, ε-, and λ-MnO ) indicated
2
2
the highest intrinsic reactivity per surface area for β-MnO2.
Furthermore, the intrinsic reactivity was well correlated with
the calculated oxygen vacancy formation energy as well as the
experimental reduction rates. The combined computational
and experimental approaches can thus lead to the rational
design and synthesis of highly effective MnO catalysts for
2
aerobic oxidation. Actually, the potential of β-MnO as a highly
2
active oxidation catalyst for liquid phase aerobic oxidation was
evidenced by the successful synthesis of high-surface-area β-
MnO . Further functionalization of β-MnO will open up a
2
2
new avenue for the development of highly efficient
heterogeneous catalysts for the oxidation of various types of
substrates including biomass-derived compounds.
Figure 7. Time courses for the oxidation of HMF to FDCA with O2
over β-MnO −HS and activated MnO (■, β-MnO −HS; ○,
2
2
2
activated MnO ). Reaction conditions: catalyst (0.05 g), HMF (0.2
2
mmol), NaHCO (0.4 mmol), water (5 mL), pO (1 MPa), 373 K.
Experimental errors of FDCA yield were estimated to be ±2%.
3
2
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b09917.
■
*
S
FDCA with β-MnO −HS was much higher than that with
2
activated MnO in spite of the lower specific surface area (83
2
2
2
16,45
m /g) of β-MnO −HS than activated MnO (122 m /g).
2
2
Experimental details, tables, figures (PDF)
The time courses of all products for the oxidation of HMF to
FDCA are shown in Figure S18. There was no significant
difference in the reaction profiles of HMF, DFF, and HMFCA
between activated MnO and β-MnO −HS. On the other
AUTHOR INFORMATION
Corresponding Author
hara.m.ae@m.titech.ac.jp
■
*
2
2
hand, both the decrease in FFCA yield and increase in FDCA
yield with β-MnO −HS were much faster than that with
ORCID
2
activated MnO , which suggests that β-MnO −HS more
Keigo Kamata: 0000-0002-0624-8483
Fumiyasu Oba: 0000-0001-7178-5333
Michikazu Hara: 0000-0003-3450-5704
2
2
efficiently catalyzed the oxidation of FFCA to FDCA than
1
6
activated MnO2. All these results indicate that the synthesis
of high-surface-area β-MnO is a promising strategy for the
2
Notes
development of highly efficient oxidation of HMF with MnO2
catalysts and that planar oxygen site or related adsorbed
The authors declare no competing financial interest.
oxygen species in β-MnO likely play an important role in the
2
ACKNOWLEDGMENTS
■
present oxidation reaction (Figure S19). However, the reaction
−
1
−2
The authors thank M. Yumoto (JASCO Corporation) for the
measurement of Raman spectra. This work was supported in
part by the ALCA (JPMJAL1205) and PRESTO
rate per surface area of β-MnO −HS (6.2 μmol h m ) for
2
the oxidation of FFCA to FDCA was lower than that (16.4
μmol h m ) of β-MnO probably due to the low crystallinity
−
1
−2
2
(
JPMJPR15S3 and JPMJPR16N4) programs of the Japan
or different morphology. On the other hand, the effect of ε-
MnO2 morphology on its catalytic activity was hardly
observed, while the effect of morphologies of other MnO2
on their catalytic activity could not be ruled out (see the details
in the Supporting Information). Since the physicochemical
properties of materials (e.g., oxidation state of Mn species,
amount of oxygen vacancy, crystallinity, morphology, surface
Science and Technology Agency (JST) and Grants-in-Aid for
Japan Society for the Promotion of Science (JSPS) Fellows and
for Scientific Research from the Ministry of Education,
Culture, Science, Sports, and Technology (MEXT) of Japan.
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