Highly Efficient Ir-CoOX Hybrid Nanostructures for the Selective Hydrogenation of Furfural to Furfuryl Alcohol

Article abstract of DOI:10.1021/acs.langmuir.0c03367

Decoration of noble metals with transition-metal oxides has been intensively studied for heterogeneous catalysis. However, controllable syntheses of metal-metal oxide heterostructures are difficult, and elucidation of such interfaces is still challenging. In this work, supported IrCo alloy nanoparticles are transformed into supported Ir-CoOx close-contact nanostructures by in situ calcination and following selective reduction. Relative to Ir/Al2O3, Ir-CoOx/Al2O3 shows greatly enhanced activities for the hydrogenation of furfural derivatives to the corresponding furfuryl alcohol derivatives with more than 99% selectivity and demonstrates significantly improved activities and selectivity for hydrogenations of α,β-unsaturated aldehydes to α,β-unsaturated alcohols. The modification of Ir surfaces with CoOx prevents Ir nanoparticles from growing, achieving high thermal and catalytic stabilities. Theoretic calculation suggests that the better catalytic performance of Ir-CoOx/Al2O3 is ascribed to the Ir-CoOx interaction, which promotes the absorption of furfural as well as desorption of furfuryl alcohol, resulting in enhanced catalytic activities.

Full text of DOI:10.1021/acs.langmuir.0c03367

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Highly Efficient Ir−CoO Hybrid Nanostructures for the Selective
x
Hydrogenation of Furfural to Furfuryl Alcohol
Hongbo Yu, Jihao Zhao, Chunzheng Wu, Bo Yan, Shuangliang Zhao, Hongfeng Yin,*
and Shenghu Zhou*
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sı Supporting Information
ABSTRACT: Decoration of noble metals with transition-metal oxides has been
intensively studied for heterogeneous catalysis. However, controllable syntheses
of metal−metal oxide heterostructures are difficult, and elucidation of such
interfaces is still challenging. In this work, supported IrCo alloy nanoparticles
are transformed into supported Ir−CoO close-contact nanostructures by in situ
x
calcination and following selective reduction. Relative to Ir/Al O , Ir−CoO /
2
3
x
Al O shows greatly enhanced activities for the hydrogenation of furfural
2
3
derivatives to the corresponding furfuryl alcohol derivatives with more than 99%
selectivity and demonstrates significantly improved activities and selectivity for
hydrogenations of α,β-unsaturated aldehydes to α,β-unsaturated alcohols. The
modification of Ir surfaces with CoO prevents Ir nanoparticles from growing, achieving high thermal and catalytic stabilities.
x
Theoretic calculation suggests that the better catalytic performance of Ir−CoO /Al O is ascribed to the Ir−CoO interaction, which
x
2
3
x
promotes the absorption of furfural as well as desorption of furfuryl alcohol, resulting in enhanced catalytic activities.
INTRODUCTION
the H activation on Ir−ReO interfaces. Such a catalytic
2
x
■
enhancement by metal−metal oxide interaction has also been
Recently, selective hydrogenation of furfural (a renewable
chemical from biomass) to furfuryl alcohol has received great
attention due to wide applications of furfuryl alcohol in biofuel
2
5−30
observed in other noble metal systems,
either by
traditional impregnation/co-precipitation methods or by the
transformation of bimetallic nanoparticles (NPs) with proper
treatments. Despite the progress achieved in promoting their
catalytic performance, the highly efficient Ir-based catalysts for
selective hydrogenation of furfural to furfuryl alcohol are rarely
1
−4
and pharmaceutical industries.
However, serious side
reactions, such as furan ring hydrogenation, C−O hydro-
genolysis, and C−C dissociation, often occur during this
2
,5−8
process.
To economically obtain furfuryl alcohol, highly
18
reported.
efficient and selective catalysts are desired.
Typically, the Cu−Cr catalysts are industrially used for the
conversion of furfural to furfuryl alcohol under high H
pressures and elevated temperatures. However, the activities
and selectivity over the Cu−Cr catalysts are moderate, and the
chromium component is toxic and harmful to the environment.
Herein, we report Ir−MO /Al O (M = Co, Fe, and Ni)
x
2
3
hybrid nanocatalysts for the selective hydrogenation of furfural
to furfuryl alcohol under mild reaction conditions. Scheme 1
2
9
illustrates the syntheses of Ir−MO hybrid nanostructures and
x
hydrogenation of furfural to furfuryl alcohol over them. The
5
,10,11
3,12
13
Ir−MO close-contact nanostructures are obtained by the in
x
Alternatively, literature has reported Pt,
Ir,
Co, and
1
4
situ transformation of alumina-supported IrM alloy NPs
Cu for the hydrogenation of furfural. Among various systems,
Ir-based catalysts are recognized as efficient catalysts for the
hydrogenation of CO bonds relative to CC bonds due to
their larger band widths of d bands, but their hydrogenation
activities are relatively low under mild reaction condi-
through calcination and the following selective reduction.
The Ir−MO interaction effectively prevents the Ir NPs from
x
agglomeration with greatly enhanced thermal stabilities, and
Ir−CoO interfaces show much improved catalytic perform-
x
3
,15−17
18−20
ance relative to the monometallic Ir/Al O and other reference
2 3
tions.
Various supports,
alloys such as nickel−
21
catalysts. Under 45 °C and 0.8 MPa H , the alumina-supported
2
iridium alloy with highly active isolated Ni atom, and strong
metal−support interaction
the catalytic performance of Ir-based catalysts while introduc-
3
,16
have been employed to enhance
Received: November 23, 2020
Revised: January 12, 2021
Published: January 25, 2021
1
2,22−24
ing proper metal oxides
is one of the most efficient
strategies. For example, Ir−ReO /SiO catalysts show superior
selectivity to furfuryl alcohol (>99%) relative to the
monometallic Ir catalysts. In the above-mentioned study,
x
2
ReO modifies Ir to tailor the adsorption of reactants and assist
x
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2021 American Chemical Society
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Synthesis of Ir−MO /Al O and Ir/Al O3 Nanocatalysts. The
Scheme 1. Schematic Illustration for the Syntheses of
x
2
3
2
collected IrM (M = Ni, Co, and Fe) alloy NPs or individual Ir NPs
were redispersed in 60 mL of cyclohexane in a 250 mL three-necked
round-bottomed flask. A calculated amount of alumina was loaded
Supported Ir−MO Nanostructures and Selective
x
Hydrogenation of Furfural Over Them
into the flask. Then, the mixture was purged by N at 70 °C with
2
vigorous magnetic stirring to remove the cyclohexane to obtain the
IrM/Al O and Ir/Al O precursors. After that, the IrM/Al O and
2
3
2
3
2
3
Ir/Al O precursors were calcined at 500 °C for 2.0 h in a muffle
2
3
furnace, followed by H reduction at 250 °C for 3.0 h in a tube
2
furnace to give Ir−MO /Al O and Ir/Al O nanocatalysts,
x
2
3
2
3
respectively.
Catalyst Characterizations. X-ray diffraction (XRD) patterns of
various materials were collected on a Bruker D8 Advance X-ray
diffractometer with Cu Kα radiation in the range from 10 to 90 2θ
degree. Transmission electron microscopy (TEM) images with
energy-dispersive spectroscopy (EDS) were taken on a JEOL 2100
transmission electron microscope operated at 200 kV. The high-angle
annular dark-field scanning transition electron microscopy (HAADF-
2
STEM) images were obtained using a Titan G 80-200 ChemiSTEM,
Ir−CoO nanostructures show greatly enhanced performance
x
operated at 200 kV for the STEM model. X-ray photoelectron spectra
(XPS) were obtained on an AXIS ULTRA DLD multifunctional X-ray
photoelectron spectroscope with an Al source. The instrument was
calibrated by C 1s (284.8 eV), and the data were processed by the
Casa XPS software. The actual Ir loadings of the nanocatalysts were
determined using a PE Optima 2100DV inductively coupled plasma
optical emission spectrometer (ICP-OES). Fourier transform infrared
for hydrogenation of furfural derivatives to the corresponding
furfuryl alcohol derivatives with ≥99% of selectivity at ≥98%
conversions and exhibit excellent catalytic stability during
recycling experiments. For hydrogenations of α,β-unsaturated
aldehydes, Ir−CoO illustrates significantly enhanced activities
x
as well as selectivity to the corresponding α,β-unsaturated
alcohol derivatives. Theoretical calculation by density function
theory (DFT) suggests that the catalytic enhancement
(
FT-IR) spectra of the samples were collected on a Bruker Tensor 27
spectrophotometer. The H temperature-programmed reduction (H -
2
2
TPR) curves of the Ir-based nanocatalysts were obtained on an
AutoChemII 2920 instrument. A portion of 50 mg of the samples was
heated from room temperature to 700 °C with a ramping rate of 5
°C/min in a 10/90 (v/v) H /N flow (30 mL/min), and a thermal
originated from the Ir−CoO interaction, which increases the
x
adsorption of furfural and promotes the desorption of furfuryl
alcohol, resulting in enhanced activities.
2
2
conductivity detector was employed to monitor the hydrogen
EXPERIMENTAL SECTION
consumption. Diffuse reflectance Fourier transform infrared spectra
■
(
(
DRIFT-IR) with CO probes of the samples were recorded using a
Chemicals. Hexachloroiridium acid hydrate (Ir, ≥36%), nickel(II)
Nicolet-6700 Fourier transform infrared spectrometer. The samples
acetylacetonate (Ni(acac) , 95%), cobalt(II) acetylacetonate (Co-
2
were pretreated with H at 200 °C for 20 min and then cooled down
2
acac) , 97%), iron(III) chloride (AR), oleylamine (80−90%), 1-
2
to 30 °C and recorded as the DRIFT-IR background spectrum. After
that, the nanocatalysts were exposed to pure CO (99.99%) at a gas
flow rate of 20 mL/min at 30 °C for 30 min and subsequently purged
with Ar to remove the free CO before recording the DRIFT-IR
spectra.
octadecene (90%), n-butyllithium solution (2.7 M in cyclohexane),
furfural (AR, 99%), furfuryl alcohol (AR, 98%), 5-hydroxymethyl-2-
furaldehyde (99%), 2,5-furandimethanol (98%), 5-methylfurfural
(
(
(
98%), (5-methyl-2-furyl)methanol (97%), cinnamaldehyde
≥95%), cinnamyl alcohol (>99%), α-methyl-trans-cinnamaldehyde
95%), 2-methyl-3-phenyl-2-propen-1-ol (95%), and nonane (99%)
were purchased from Aladdin. Toluene (AR), acetone (AR), absolute
ethyl alcohol (AR), and cyclohexane (AR) were purchased from
Shanghai Chemical Reagent Company. Aluminum oxide powders
were obtained from Qingdao Haiyang Chemical Co., Ltd. and
calcined at 500 °C for 3.0 h before using. All of the reagents were used
as received.
Catalytic Hydrogenations. Hydrogenations of various substrates
were performed in an autoclave at 45 °C and 0.8 MPa H . In each
2
reaction, 0.0500 g of supported Ir-based nanocatalysts, 5.0 mL of
absolute ethanol, 1.0 mmol reactants, and 0.50 mmol nonane used as
internal standards were loaded into the autoclave. The system was
then purged with H to remove the air. After that, the system was
2
heated to and maintained at 45 °C under 0.8 MPa H with a vigorous
2
magnetic stirring (700 rpm) for the defined reaction time. After
reactions, the reactor was cooled down to room temperature, and the
hydrogenation products were collected by centrifugation and analyzed
by a gas chromatograph with a flame ionization detector.
Catalyst Preparation. Synthesis of IrNi, IrCo, and IrFe Alloy
NPs. IrM (M = Ni, Co, and Fe) alloy NPs were synthesized in a
31
standard Schlenk line under N by our previously reported method.
2
In a typical synthesis of IrNi alloy NPs, 25.0 mL of 1-octadecene were
charged into a 100 mL three-necked round-bottomed flask. The
system was heated to and kept at 80 °C for 2.0 h with vigorous
magnetic stirring. Then, 1.5 mL of n-butyllithium solution (2.7 M in
cyclohexane) was quickly injected into the above solution, followed
by the injection of the solution containing 0.15 mmol H IrCl ·xH O,
Cycle-to cycle experiments were conducted to measure the catalytic
stability of Ir−CoO /Al O for the hydrogenation of furfural. The
x
2
3
reaction conditions in the first cycle were the same as those
mentioned above. Because of the catalyst loss during the recovery
process after each cycle, the amounts of reactants, solvents, and
internal standards were proportional to the amount of the recovered
catalysts.
2
6
2
0
.15 mmol Ni(acac) , and 3.0 mL oleylamine in 2.0 mL of 1-
2
octadecene. The resultant mixture was maintained at 80 °C for 20
min to ensure the complete reduction of metal precursors. After that,
the system was further heated to and aged at 240 °C for 2.0 h. Upon
cooling to room temperature, the IrCo NPs were collected by
centrifugation and washed with methanol/acetone four times. Finally,
the collected IrCo NPs were redispersed in cyclohexane for further
use. The IrCo and IrFe NPs were obtained by the same procedures
except that Co(acac)2 and FeCl3 precursors are used instead of
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Ir−MO /Al O
x
2
3
Hybrid Nanostructures. The alumina-supported Ir−MO
x
hybrid nanocatalysts were prepared by the in situ trans-
formation of the supported IrM alloy NP precursors into Ir−
MO hybrid nanostructures through calcination at 500 °C,
Ni(acac) . For the synthesis of individual Ir NPs, only 0.15 mmol
x
2
H IrCl ·xH O were used, and other procedures were the same as
followed by reduction of Ir oxides with H at 250 °C. Figure 1
2
2
6
2
those of IrNi alloy NPs.
presents the XRD patterns of various materials during the
1
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clearly observed, indicating the Ir oxidation under these
conditions. However, no patterns of FeO species are observed,
x
suggesting that FeO is highly dispersed or amorphous. The
x
distinct Ir (111) peaks appear in Figure S2e when the calcined
sample is then reduced with H at 250 °C, suggesting the
2
formation of Ir−FeOx hybrid nanostructures. The same
phenomenon is also observed for IrNi/Al O (Figure
2
3
S2c,f,g), suggesting the transformation of IrNi alloy to Ir−
NiO hybrid nanostructures.
x
Figure 2 shows the TEM images of IrCo and Ir−CoO /
x
Al O . More TEM images of other Ir-based materials and their
2
3
particle size histograms are shown in Figures S3 and S4,
respectively. As shown in Figure 2a, IrCo NPs present irregular
shapes with an average particle size of 2.4 nm (Figure S4c).
Figure 2b shows the TEM images of Ir−CoO /Al O , where
x
2
3
Figure 1. XRD patterns showing (a) Ir NPs, (b) IrCo NPs, (c) IrO −
small NPs are observed on alumina. The size histogram in
Figure S4d gives an average particle size of 2.5 nm, which is
nearly identical to that of the as-synthesized IrCo NPs,
2
CoO NPs, and (d) IrO −CoO /Al O obtained by the calcination of
x
2
x
2
3
IrCo/Al O at 500 °C and (e) Ir−CoO /Al O obtained by the
2
3
x
2
3
reduction of IrO −CoO /Al O with H at 250 °C.
2
x
2
3
2
suggesting the inhibitory effect of the Ir−CoO interaction on
x
particle growth. The same effect is also observed in IrFe/Ir−
FeO and IrNi/Ir−NiO , as supported by the particle size
x
x
process. As evidenced by the appearance of the (111)
diffractions of fcc Ir (Figure 1a) and fcc IrCo (Figure 1b), Ir
and IrCo alloy NPs can be successfully synthesized by the
reduction of the corresponding precursors with butyllithium in
the presence of oleylamine. The high-angle diffractions of Ir
and IrCo are not observed due to the less crystallinity with
comparisons between the alloy NPs and Ir−MO hybrid
x
nanostructures in Figure S4e−h. In contrast, Ir NPs in Figure
S3a are extremely small with ∼0.8 nm particle size (Figure
S4a), while Ir NPs of the Ir/Al O nanocatalysts in Figure S3d
2
3
obviously grow with ∼5.8 nm particle size (Figure S4b).
Figure 2c shows the HRTEM image of Ir−CoO /Al O .
small NPs. After calcination of IrCo, IrO appears (Figure 1c),
x
2
3
2
Due to the residence of the Ir−CoO hybrid nanostructure just
while CoO diffractions are not observed due to the formation
x
x
on the periphery of alumina, the contrast between Ir and CoO
of amorphous CoO . The formation of CoO is confirmed by
x
x
x
is enough to differentiate the outside amorphous CoO from
XPS and EDS and are discussed later.
x
the inner Ir particle. Apparently, the CoO phase and the Ir
Figure 1d shows the XRD patterns of IrO −CoO /Al O .
x
2
x
2
3
phase are in close contact, which prevents Ir NPs from growing
and could induce new active sites for catalytic reactions. The
The characteristic diffractions of IrO and Al O are clearly
2
2
3
visible while CoO diffractions are still invisible. After further
x
EDS phase mappings in HAADF-STEM of Ir−CoO /Al O
reduction of IrO −CoO /Al O , the XRD patterns of Ir−
x
2
3
2
x
2
3
are shown in Figures S5 and S6, which further confirms the
CoO /Al O in Figure 1e show the characteristic (111)
x
2
3
close contact of the CoO phase and the Ir phase. The inset of
diffraction of Ir, confirming the reduction of IrO to Ir by H .
x
2
2
Figure 2c presents the DRIFT-IR spectra with CO probes of
Here, it is mentioned that alumina-supported Ir-based
nanocatalysts with higher metal loadings (3.3 times more
than those of the real catalysts for catalytic reactions) are used
for the XRD characterization in Figure 1d,e since the real Ir-
based nanocatalysts with ∼1.6 wt % of Ir do not show visible
XRD diffractions from Ir-based nanostructures due to the
predominant presence of alumina (Figure S1).
Ir/Al O and Ir−CoO /Al O . Two characteristic bands at
2
3
x
2
3
−
1
2077 and 2044 cm are clearly observed for Ir/Al O . The
2 3
former is assigned to gemdicarbonyl adsorbed CO on the Ir
sites while the latter is indexed to the linear adsorbed CO on
3
2,33
metallic Ir.
The Ir−CoO
/Al O exhibits a band at 2070
x 2 3
−
1
cm for gemdicarbonyl adsorbed on Ir and a shoulder band at
−
1
More XRD patterns of other Ir-based materials are shown in
Figure S2, where IrFe and IrNi NPs show their characteristic
2044 cm for linear CO adsorbed on Ir. We speculate that the
significantly weaker band for linear CO adsorbed on Ir−
CoO /Al O is possibly due to the less exposure of Ir surfaces
diffractions, and Ir diffractions are observed for Ir−FeO /
x
x
2
3
Al O and Ir−NiO/Al O with higher Ir loadings. As shown in
by partial coverage of CoOx.
2
3
2
3
Figure S2d, by calcining IrFe/Al O at 500 °C in air, well-
Figure 3a shows the H -TPR profiles of IrO /Al O and
2
3
2
2
2
3
defined IrO peaks at 2θ degrees of 34.7, 40.0, and 54.1° are
IrO −CoO /Al O . As shown in Figure 3a, a well-defined
2
2
x
2
3
Figure 2. TEM images showing (a) IrCo NPs and (b) Ir−CoO /Al O nanocatalysts. (c) HRTEM image of Ir−CoO /Al O . Inset showing the
x
2
3
x
2
3
DRIFT-IR spectra with CO probes of Ir/Al O and Ir−CoO /Al O nanocatalysts. Scale bars of (a), (b), and (c) are 20, 20, and 5 nm, respectively.
2
3
x
2
3
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4
a are assigned to 4d5/2 and 4d3/2 of the Ir0+ species,
respectively, while those at 299.0−300.0 and 314.2−315.2 eV
δ+
are indexed to the Ir species existing in Ir and Ir−CoO NPs
x
due to the surface oxidation. In terms of Co 2p in Figure 4b,
both Co and Co species may exist, but CoO should be the
dominant phase according to the two satellite peaks at 785.4
and 802.3 eV for CoO.
Ir−NiO NPs in Figure S8 show similar Ir 4f spectra, with the
presence of multivalent Fe species (i.e., Fe and Fe )
3
+
2+
41,42
The XPS spectra of Ir−FeO and
x
2
+
3+ 43,44
for
2
+
Ir−FeO NPs and the only the presence of Ni species for Ir−
x
37
NiO NPs.
The absence of metallic Fe, Co, and Ni species in Ir−MO
x
NPs further confirms the selective reduction of IrO in IrO -
2
2
Figure 3. (a) H -TPR profiles of IrO /Al O and IrO −CoO /Al O .
2
2
2
3
2
x
2
3
MO to Ir−MO and is in good agreement with XRD and H -
x
x
2
(
b) FT-IR spectra of IrCo/Al O precursors (without calcination)
2 3
TPR studies. Table 1 summarizes the valence states of Ir and
and Ir−CoO /Al O .
x
2
3
0
δ+
Table 1. Atomic Ratios of Ir /Ir and Ir/M of Ir−MO NPs
reduction peak was observed at around 218 °C for IrO /Al O ,
x
2
2
3
34
Obtained by XPS and EDS Measurements
which is related to the reduction of IrO to Ir. For IrO −
2
2
CoO /Al O , the reduction peak derived from the IrO species
0
δ+
x
2
3
2
samples
Ir /Ir
Ir/M (XPS)
Ir/M (EDS)
is shifted to a lower temperature of 162 °C, indicating the weak
Ir−NiO
Ir−CoOx
80.4/19.6
86.7/13.3
80.2/19.8
51.2/48.8
44.8/55.2
45.7/54.3
54.6/45.4
47.8/52.2
48.1/51.9
interaction of IrO and CoO or a strong interaction between
2
x
Ir and CoO . The reduction of CoO is not observed up to 600
x
x
35
Ir−FeO
x
°
C, which is consistent with the previous work. Figure S7
presents the H -TPR profiles of IrO −FeO /Al O and IrO −
2
2
x
2
3
2
NiO/Al O . IrO −FeO /Al O shows two Ir reduction peaks
M species in Ir−MO NPs and Ir/M ratios calculated from the
XPS and EDS measurements. As shown in Table 1, the
2
3
2
x
2
3
x
at 175 and 205 °C, and the former is possibly related to the
0
IrO species neighboring the FeO species. Moreover, Figure
percentages of Ir in Ir−FeO , Ir−CoO , and Ir−NiO NPs are
2
x
x
x
S7 suggests that the onset reduction temperature of the NiO
80.2, 86.7, and 80.4%, respectively, while the presence of a
δ+
species is ∼400 °C while that of the FeO species is ∼300
small amount of Ir species is due to the exposure to air. The
x
3
6,37
°C.
Therefore, the reduction with H at 250 °C transforms
Ir/M ratios calculated from XPS and EDS measurements
(shown in Figures S9−S11) are in good agreement and close
to the expected value of the 1/1 ratio.
2
IrO −MO /Al O into Ir−MO /Al O as expected. Figure 3b
2
x
2
3
x
2
3
shows the FT-IR spectra of IrCo/Al O precursors and the Ir−
2
3
CoO /Al O nanocatalysts. The two peaks at 2925 and 2853
Catalytic Performance of Ir−MO /Al O and Ir/Al O
x
2
3
x
2
3
2
3
−
1
cm of the uncalcined IrCo/Al O are related to the residual
Nanocatalysts. Hydrogenation of furfural at 45 °C and 0.8
2
3
3
8
oleylamine
while the above-mentioned peaks totally
MPa H was performed to evaluate the catalytic performance
2
disappear for Ir−CoO /Al O , confirming the complete
of Ir−MO /Al O and Ir/Al O . The theoretic Ir loadings are
x
2
3
x
2
3
2
3
removal of organic residues by calcination.
3.0 wt % for all catalysts while the actual Ir loadings of Ir−
FeO /Al O , Ir−NiO/Al O , Ir−CoO /Al O , and Ir/Al O
Figure 4 shows the XPS spectra for Ir and Ir−CoO NPs
x
x
2
3
2
3
x
2
3
2
3
while more XPS spectra for Ir−FeOx and Ir−NiO are
nanocatalysts detected by ICP-OES are 1.57, 1.68, 1.72, and
1.59 wt %, respectively, due to the nanoparticle loss during the
collection process. Table S1 shows the catalytic performance of
furfural over Ir−CoO /Al O with different Ir/Co ratios. As
x
2
3
shown in Table S1, Ir−CoO /Al O shows the best catalytic
x
2
3
performance. More or less, CoO will decrease the catalytic
x
activity. Especially for more CoO , Ir −(CoO ) /Al O shows
x
1
x 3
2
3
significantly lower activity with a furfural conversion of 15%,
possibly due to much less naked Ir surfaces. Therefore, the Ir−
MO /Al O nanocatalysts with a 1/1 ratio of Ir/M were
x
2
3
selected for further study.
Table 2 summarizes the catalytic results over various
catalysts. To keep the same moles of Ir atoms in each
experiment, different amounts of catalysts were used. Turnover
frequency (TOF, h ) is measured as moles of products per
total molar Ir atoms per hour at the reaction time of 2.0 h. As
shown in Table 2, furfuryl alcohol is the main product for all
Figure 4. (a) Ir 4d XPS spectra for individual Ir and Ir−CoO NPs.
−1
x
(
b) Co 2p XPS spectra for Ir−CoO NPs.
x
presented in Figure S8. Since the XPS signals of Ir−MO /
catalysts, but the selectivity of furfuryl alcohol over Ir−CoO /
x
x
Al O with real Ir loadings of ∼1.6 wt % are very weak, the
Al O is the highest at 99%. The byproducts are mainly
2
3
2
3
unsupported Ir−MO NPs prepared by calcination and
tetrahydrofurfural and tetrahydrofurfuryl alcohol from the
x
following the reduction of IrM NPs were used for the analyses.
Considering that the binding energies of Co 3p and Ir 4f are
hydrogenation of the CC bond and the complete hydro-
45,46
genation of furfural, respectively.
The furfural conversions
32
partially overlapped, we selected Co 2p and Ir 4d to evaluate
over Ir/Al O and Ir−FeO /Al O are 29 and 30%,
2
3
x
2
3
39,40
their chemical states. According to the literature,
the
respectively, suggesting no promoting effect of FeO . Ir−NiO
x
binding energies at 296.2−296.5 and 311.4−311.7 eV in Figure
shows an increased conversion of 49%, indicating the presence
1
897
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Table 2. Hydrogenations of Furfural over Ir-Based
Nanocatalysts
those over Ir/Al O , the conversions of 5-hydroxymethyl-2-
2 3
furaldehyde and 5-methylfurfural over Ir−CoO /Al O are
x
2
3
significantly higher than those over Ir/Al O . For the α,β-
a
2
3
selectivity (%)
unsaturated aldehydes shown in entries 3 and 4, Ir−CoO /
x
a
conversion (%) furfuryl alcohol others TOF (h−1
)
catalysts
Al O demonstrates remarkably enhanced activities and
2
3
Ir/Al O3
Ir−NiO/Al O3
29
49
98
30
96
98
99
97
4
2
1
3
36
58
109
40
selectivity, suggesting good adaptability of Ir−CoO /Al O
2
x
2
3
for hydrogenation reactions. Besides, hydrogenations of several
aliphatic acyclic α,β-unsaturated aldehydes and ketones over
Ir−CoO /Al O are also investigated. As shown in Table S4,
2
Ir−CoO /Al O
x
2
3
Ir−FeO /Al O
x
2
3
x
2
3
a
the selectivity to the corresponding α,β-unsaturated alcohols is
not as good as those of hydrogenations of furfural and its
derivatives due to the easier hydrogenations of the CC
bonds in these substrates.
Figure 5 presents the effects of reaction conditions on the
hydrogenation of furfural to furfural alcohol over Ir−CoOx/
Reaction conditions: furfural, 1.0 mmol; nonane (internal standard),
.50 mmol; EtOH, 5.0 mL; 0.80 MPa H ; reaction temperature, 45
0
2
°C; reaction time, 2.0 h; speed of agitation, 700 rpm. Ir−NiO/Al O ,
2
3
0
.0512 g (Ir loading, 1.68 wt %); Ir−CoO /Al O , 0.0500 g (Ir
x
2
3
loading, 1.72 wt %); Ir−FeO /Al O , 0.0548 g (Ir loading, 1.57 wt
%
x
2
3
); and Ir/Al O , 0.0541 g (Ir loading, 1.59 wt %).
2 3
of a synergistic effect between Ir and NiO . In contrast, the Ir−
x
CoOx nanostructures significantly increase the furfural
conversion to 98%, showing the beneficial effect of the Ir−
CoO interaction. Such a phenomenon has also been observed
x
47
in other catalytic hydrogenation systems. Noteworthy, Ir−
−
1
CoO /Al O achieves the highest TOF of 109 h , which is
x
2
3
comparable to the state-of-the-art catalysts for this reaction
under similar reaction conditions (shown in Table S2). In
addition, CoO /Al O catalysts with 1.0 wt % of Co loading
x
2
3
were prepared for hydrogenation of furfural. As shown in Table
S3, the CoO /Al O catalysts show no activity at the same
x
2
3
reaction conditions. Therefore, the significant catalytic
enhancement of Ir−CoO /Al O originated from the syner-
Figure 5. Effects of (a) reaction temperatures and (b) reaction
pressures on furfural hydrogenation over Ir−CoO /Al O . Reaction
x
2
3
x
2
3
conditions: furfural, 1.0 mmol; EtOH, 5.0 mL; H pressure for (a),
gistic effect between Ir and CoO species. Compared with
2
x
0
.80 MPa; reaction temperature for (b), 45 °C; reaction times for (a)
NiO and FeO , CoO could provide a suitable metal−metal
x
x
x
and (b) are 2.0 and 1.0 h, respectively; speed of agitation, 700 rpm;
oxide interaction, resulting in a significant catalytic enhance-
ment.
and Ir−CoO /Al O , 0.0500 g.
x
2
3
Table 3 presents hydrogenations of various unsaturated
aldehydes over Ir−CoO /Al O . The main reaction is the
Al O . As shown in Figure 5a, Ir−CoO /Al O exhibits a 44%
x
2
3
2
3
x
2
3
hydrogenation of the aldehyde group to the hydroxyl group
while the side reaction is the hydrogenation of the CC bond.
As shown in entries 1 and 2, although the selectivity to the
corresponding allyl alcohols over Ir−CoO /Al O is similar to
conversion and a 98% selectivity at 25 °C. Further increasing
the reaction temperatures greatly increases the furfural
conversion, and the furfural conversion at 45 °C reaches
98%. As for the H pressures (Figure 5b), at the reaction time
x
2
3
2
a
Table 3. Catalytic Hydrogenations of Various Unsaturated Aldehydes over Ir−CoO /Al O and Ir/Al O Nanocatalysts
x
2
3
2
3
a
Reaction conditions: reactants, 1.0 mmol; nonane (internal standard), 0.50 mmol; EtOH, 5.0 mL; H , 0.80 MPa; reaction temperature, 45 °C;
2
reaction time, 2.0 h; speed of agitation, 700 rpm. Ir−CoO /Al O , 0.0500 g (Ir loading, 1.72 wt %); Ir/Al O , 0.0541 g (Ir loading, 1.59 wt %).
x
2
3
2
3
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of 1.0 h, the furfural conversions increase from 17% at 0.1 MPa
to 98% at 3.0 MPa while the selectivity increases from 91 to
CoO species is beneficial to the furfural adsorption. Moreover,
the adsorption energy of furfuryl alcohol on the Ir (111)−CoO
surfaces is −1.52 eV and is obviously weaker than that on the
x
9
9%. Obviously, the increasing conversions with increasing H2
pressures are due to the increased dissolved hydrogen
Ir (111) surfaces (−1.69 eV), indicating that the Ir−CoO
x
concentrations at higher H pressures. The effect of reaction
hybrid nanostructures can effectively promote the release of
furfuryl alcohol molecules from the Ir (111)−CoO surfaces.
According to the calculation results, it is concluded that the
2
time on furfural hydrogenation is shown in Figure S12, where
the conversions reach the highest at 2.0 h with a slight change
in selectivity during the reaction process.
catalytic enhancement of Ir−CoO is originated from the
x
Catalytic Stability of the Ir−CoO /Al O Nanocata-
synergistic effect between Ir and CoO species, where the
x
2
3
x
lysts. The stability test of the Ir−CoO /Al O nanocatalysts
interaction between Ir and CoO facilitates the adsorption of
x
2
3
x
for the hydrogenation of furfural to furfuryl alcohol was
reactant molecules (furfural) and the desorption of product
molecules (furfuryl alcohol), resulting in a greatly enhanced
performance.
conducted at 45 °C and 0.8 MPa H , and the result is shown in
2
Figure 6. After each cycle, the catalysts were separated from the
CONCLUSIONS
In summary, the Ir−MO /Al O (M = Fe, Co, and Ni)
nanocatalysts were successfully prepared by the in situ
transformation of alumina-supported IrM alloy NPs through
calcination and following reduction. Studies of HRTEM, EDS
phase mapping, and DRIFT-IR with CO probes confirmed the
■
x
2
3
formation of Ir−CoO close-contact hybrid nanostructures,
x
where the Ir NPs are partially covered by a thin layer of CoO .
x
Compared to Ir/Al O , Ir−CoO /Al O exhibits a greatly
2
3
x
2
3
enhanced activity for the selective hydrogenation of furfural to
furfuryl alcohol and illustrates remarkably improved activity
and selectivity for the selective hydrogenation of α,β-
unsaturated aldehydes to α,β-unsaturated alcohols. Such a
catalytic enhancement is assigned to the formation of Ir−CoO
Figure 6. Cycle-to-cycle experiments of hydrogenations of furfural
over Ir−CoO /Al O . In the first cycle, furfural, 1.0 mmol; nonane
x
close-contact hybrid nanostructures, where the interaction
x
2
3
(
internal standard), 0.50 mmol; EtOH, 5.0 mL; H pressure, 0.80
between Ir and CoO promotes furfural adsorption and furfuryl
2
x
MPa; reaction temperature, 45 °C; reaction time, 2.0 h; speed of
alcohol desorption, resulting in enhanced catalytic activity.
Moreover, such an interaction prevents the Ir NPs from
growth, resulting in enhanced thermal and catalytic stabilities.
We believe that such a strategy of the transformation of
bimetallic alloy NPs to close-contact metal−metal oxide hybrid
nanostructures could be extended to other systems, and this
type of hybrid nanostructures could find more applications in
heterogeneous catalysis.
agitation, 700 rpm; and Ir−CoO /Al O , 0.0500 g.
x
2
3
reaction mixture by centrifugation, washed with ethanol and
then acetone, and dried in air for the next cycle. Because of a
slight loss of the catalysts during the recovery process, the
amounts of furfural and ethyl alcohol in the following cycle are
proportional to the weight of the recovered catalysts. As can be
seen from Figure 6, after five cycles, the selectivity to furfuryl
alcohol is basically unchanged while the furfural conversions
slightly decrease from 96 to 93%, indicating good catalytic
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.langmuir.0c03367.
■
*
sı
stability. Moreover, the TEM images of the used Ir−CoO /
x
Al O catalysts (Figure S13) show no obvious change in the
2
3
Ir−CoO particle morphology and their size analysis shows a
x
Further materials characterizations, size analysis, and
more catalytic experiments (PDF)
nearly identical average particle size, confirming that the Ir−
CoO interaction can anchor the particles onto the alumina
x
supports and thereby prevent the particles from agglomerating.
Origin of the Enhanced Catalytic Performance of the
AUTHOR INFORMATION
Corresponding Authors
■
Ir−CoO /Al O Nanocatalysts. The synthesized Ir−CoO /
x
2
3
x
Al O exhibits a greatly enhanced catalytic performance for
2
3
Hongfeng Yin − Ningbo Institute of Materials Technology and
Engineering, Chinese Academy of Sciences, Ningbo 315201,
Zhejiang, P. R. China; Email: yinhf@nimte.ac.cn
Shenghu Zhou − Shanghai Key Laboratory of Multiphase
Materials Chemical Engineering, School of Chemical
Engineering, East China University of Science and
Technology, Shanghai 200237, P. R. China; orcid.org/
selective hydrogenations of various unsaturated aldehydes to
their corresponding unsaturated alcohols, and the superior
catalytic performance of Ir−CoO should have originated from
x
their close-contact hybrid nanostructures. To understand the
effect of CoO species on the catalytic activity of Ir−CoO , the
x
x
adsorption behaviors of furfural and furfuryl alcohol molecules
on Ir (111) and Ir (111)-CoO were studied by DFT
calculation. Table S5 summarizes the adsorption energies of
furfural and furfuryl alcohol molecules, and the calculation
details are shown in the Supporting Information. As shown in
Table S5, the adsorption energy of furfural on the Ir (111)
surfaces is −0.97 eV, while that on the Ir (111)−CoO surfaces
is −1.10 eV, suggesting that the decoration of Ir NPs with
0000-0002-8203-6546; Email: zhoushenghu@
ecust.edu.cn; Fax: (+86) 21-64253159
Authors
Hongbo Yu − Shanghai Key Laboratory of Multiphase
Materials Chemical Engineering, School of Chemical
Engineering, East China University of Science and
1
899
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Technology, Shanghai 200237, P. R. China; Ningbo Institute
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of Materials Technology and Engineering, Chinese Academy
of Sciences, Ningbo 315201, Zhejiang, P. R. China;
orcid.org/0000-0002-6808-1244
Jihao Zhao − Shanghai Key Laboratory of Multiphase
Materials Chemical Engineering, School of Chemical
Engineering, East China University of Science and
Technology, Shanghai 200237, P. R. China
Chunzheng Wu − Institute of Fundamental and Frontier
Science, University of Electronic Science and Technology of
China, Chengdu 610054, P. R. China
Bo Yan − Ningbo Institute of Materials Technology and
Engineering, Chinese Academy of Sciences, Ningbo 315201,
Zhejiang, P. R. China
(
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1891.
(
14) Zhou, X.; Feng, Z.; Guo, W.; Liu, J.; Huang, J.; et al.
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Hydrogenation and Hydrolysis of Furfural to Furfuryl Alcohol,
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Improvement on the Selective Hydrogenation of Crotonaldehyde
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work is financially supported by the National Natural
Science Foundation of China (Grant Nos. 22078099,
■
over CrO - and FeO -Promoted Ir/SiO Catalysts. Catal. Sci. Technol.
x
x
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2
(
016, 6, 4294−4305.
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2072171, 21776090, 21571183, and 21902169) and the
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019B10041 and 2018B10077) from the Ningbo Science and
16) Xie, L.; Chen, T.; Chan, H. C.; Shu, Y.; Gao, Q. Hydrogen
Doping into MoO3 Supports toward Modulated Metal-Support
Interactions and Efficient Furfural Hydrogenation on Iridium
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Technology Bureau. H. Yu thanks the project funded by the
China Postdoctoral Science Foundation (Grant No.
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17) Tamura, M.; Tokonami, K.; Nakagawa, Y.; Tomishige, K.
2
020M671830) and the Natural Science Foundation of
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https://dx.doi.org/10.1021/acs.langmuir.0c03367
Langmuir 2021, 37, 1894−1901