Low Temperature Oxidation of Ethane to Oxygenates by Oxygen over Iridium-Cluster Catalysts

Article abstract of DOI:10.1021/jacs.9b06986

Direct selective oxidation of light alkanes, such as ethane, into value-added chemical products under mild reaction conditions remains a challenge in both industry and academia. Herein, the iridium cluster and atomically dispersed iridium catalysts have been successfully fabricated using nanodiamond as support. The obtained iridium cluster catalyst shows remarkable performance for selective oxidation of ethane under oxygen at 100 °C, with an initial activity as high as 7.5 mol/mol/h and a selectivity to acetic acid higher than 70% after five in situ recycles. The presence of CO in the reaction feed is pivotal for the excellent reaction performance. On the basis of X-ray photoelectron spectroscopy (XPS) analysis, the critical role of CO was revealed, which is to maintain the metallic state of reactive Ir species during the oxidation cycles.

Full text of DOI:10.1021/jacs.9b06986

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Communication
Low Temperature Oxidation of Ethane to Oxygenates
by Oxygen over Iridium-Cluster Catalysts
Renxi Jin, Mi Peng, Ang Li, Yuchen Deng, Zhimin Jia, Fei Huang, Yunjian Ling, Fan Yang,
Hui Fu, Jinglin Xie, Xiaodong Han, Dequan Xiao, Zheng Jiang, Hongyang Liu, and Ding Ma
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06986 • Publication Date (Web): 24 Oct 2019
Downloaded from pubs.acs.org on October 24, 2019
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Low Temperature Oxidation of Ethane to Oxygenates by Oxygen
over Iridium-Cluster Catalysts
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†,#
†,#
‡,#
†
§
§
ǁ
ǁ
Renxi Jin, Mi Peng, Ang Li, Yuchen Deng, Zhimin Jia, Fei Huang, Yunjian Ling, Fan Yang,
†
†
‡
¶
⊥, ∇,*
§,*
Hui Fu, Jinglin Xie, Xiaodong Han, Dequan Xiao , Zheng Jiang,
Hongyang Liu, and Ding
†
,*
Ma
†
Beijing National Laboratory for Molecular Engineering, College of Chemistry and Molecular Engineering and College of
Engineering, BIC-ESAT, Peking University, Beijing 100871, P. R. China
‡
Beijing Key Laboratory of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing,
China.
§
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang
1
10016, P. R. China.
ǁ
State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience, iChEM, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian 116023, China.
¶Center for Integrative Materials Discovery, Department of Chemistry and Chemical Engineering, University of New Haven, 300 Boston
Post Road, West Haven, CT 06516, the United States
⊥
Shanghai Institute of Applied Physics, Chinese Academy of Science, Shanghai 201800, China
∇
Shanghai Synchrotron Radiation Facility, Zhangjiang Lab, Shanghai Advanced Research Institute, Chinese Academy of
Science, Shanghai 201210, China
Supporting Information Placeholder,
reaction media that limits their applications.4 To pursue for
sustainable chemical processes, the use of molecular oxygen to
oxidize ethane under mild condition is preferred because of low
cost and environmentally friendly characteristics of oxygen. In
ABSTRACT: Directly selective oxidation of light alkanes, such
as ethane, into value-added chemical products under mild reaction
conditions remains a challenge in both industry and academia.
Herein, the iridium cluster and atomically dispersed iridium
catalysts have been successfully fabricated using nanodiamond as
support. The obtained iridium cluster catalyst shows remarkable
performance for selective oxidation of ethane under oxygen at
1
992, Sen group firstly reported an oxygen-based catalytic system
o
for direct oxidation of ethane to acetic acid at 100 C. Although an
acidic media was used, it was considered a breakthrough for low-
temperature oxidation of ethane. No further progress had been
made along this direction over the following two decades. Until
4
a
o
100 C, with an initial activity as high as 7.5 mol/mol/h and a
selectivity to acetic acid higher than 70% after five in-situ
recycles. The presence of CO in the reaction feed is pivotal for the
excellent reaction performance. Based upon XPS analysis, the
critical role of CO was revealed, which is to maintain the metallic
state of reactive Ir species during the oxidation cycles.
5
a
5b
recently, the groups of Flytzani-Stephanopoulos and Tao
reported independently that atomically dispersed rhodium species
on the zeolite support could convert methane to acetic acid using
o
O
2
and CO in aqueous solutions at 150 C by a CO insertion
mechanism. Methanol can be the main products via an O-insertion
mechanism, if the acidic site on the support was blocked or a non-
acidic support was used. Both two routes showed a relatively low
productivity to methanol. The TiO
2
supported Rh showed a 100%
With the continuous exploitation of petroleum resources, it is
critically important to find a non-crude oil dependent approach for
the fabrication of bulk and fine chemicals. Thus, the utilization of
natural gas as a chemical feedstock has attracted more and more
selectivity to methanol with a yield of 4.6 μmol in a three-hour
5a
test.
1
Herein, we report that the synthesis of nanodiamond (ND)
supported iridium cluster (Ir /ND) and atomically dispersed Ir
(Ir /ND) catalysts. We found that Ir /ND possessed high activity
and stability for partial oxidization of ethane to valuable C
attention. Ethane is the second most abundant component in
n
natural gas and is difficult to be activated to obtain value-added
2
1
n
chemicals. In its direct catalytic activation, high temperature is
2
always needed, and large amount of energy is required. Moreover,
the target products (e.g., ethanol, acetaldehyde) are easier to be
o
2
chemicals at 100 C in the aqueous phase by O in the presence of
CO. The structure of the catalysts was investigated by TEM,
XAFS and IR. Based on the in-situ XPS studies, it was confirmed
that the oxidation state of iridium is critical for the efficient
activation of ethane, and the role of CO added is to maintain the
metallic state of the reactive Ir species.
oxidized to CO
x
than ethane itself under high temperature, leading
to the difficulty in reaching high activity and high selectivity
simultaneously.
3
Direct selective oxidation of ethane at low temperatures is an
alternative approach. However, due to the low reactivity of ethane,
such a process usually requires expensive oxidants or corrosive
The ND is a type of diamond-structured nano-carbon materials
with a size of 5-10 nm (Figure S1). All ND supported metal
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catalysts were prepared by an impregnation method. By
controlling the loading amount of Ir, we could tune the dispersion
of Ir to get two types of catalysts: Ir /ND (0.05 wt%) and Ir /ND
3.8 wt%).
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Figure 2. (a, b) HAADF-STEM images of Ir
n
/ND; (c-g) HAADF-
STEM images of Ir /ND.
1
The structural nature of the two Ir/ND catalysts was further
studied by CO adsorption via infrared spectroscopy. Ir /ND
exhibited multiple overlapped IR peaks (Figure 3a) between 2100
n
-1
-1
and 1800 cm . The strongest absorption peak at 2037 cm is
assigned to the atop binding of CO on Ir clusters, and the broad
-1
band centered at 1935 cm is contributed by bridged CO species,
6
which indicate the existence of Ir clusters or nanoparticles. Two
Figure 1. (a) XRD patterns of bare ND and Ir/ND catalysts; (b) Ir
-edge EXAFS of Ir/ND catalysts; WT analysis of Ir /ND (c)
and Ir /ND (d).
-1
additional features are at 2065 and 1985 cm , which may be
L
3
n
7
attributed to dicarbonyl species (Ir(CO)
2
). Of note, dicarbonyl
5a, 8
1
species can only be formed on isolated species.
bridged CO species and dicarbonyl species exist in Ir
suggesting that Ir species have a relatively broad distribution. As
The atop and
n
/ND,
From XRD patterns (Figure 1a), no diffractions associated
with Ir were observed on the Ir /ND and Ir /ND catalysts, proving
1
n
-1
1
expected, for Ir /ND, the absorption peaks at 2065 and 1985 cm
that the Ir species were highly dispersed on the two catalysts. The
X-ray absorption fine structure (XAFS) results further provided
evidence to determine the dispersion of Ir species on ND. Fourier-
are dominated, which should be derived from the dicarbonyl
species. A small amount of atop CO was observed, which could
be contributed by small Ir nanoclusters in the catalyst. The
infrared spectra confirmed the structural feature of Ir species in
both catalysts, which is consistence of both TEM and XAFS
3
transformed k -weighted extended X-ray-absorption fine-structure
(EXAFS) analysis are shown in Figures 1b and S2. For Ir /ND,
the Ir-Ir scattering is the major peak, with the coordination
number of 9.7 (Table S1). For Ir /ND, Ir-O/C coordination peak at
.96 Å is the dominant one, while the Ir-Ir scattering is
significantly weakened. The Ir-Ir coordination number reduces to
.8, and the Ir-O/C coordination number is 4.9. These findings
imply that highly dispersed iridium species are the dominant
structure in Ir /ND catalyst. Wavelet transform (WT) of Ir L
n
n 1
results: Ir on Ir /ND are small clusters whereas on Ir /ND are
1
mainly atomically dispersed with very few clusters.
1
Different metal/ND catalysts were prepared for the oxidation of
1
o
ethane by oxygen at 100 C. The results are summarized in Table 1.
The nanodiamond support cannot catalyze the ethane activation
1
3
-
(Table 1, entry 1). Clearly, Ag/ND is nearly inactive for ethane
edge EXAFS further revealed the dispersion nature of Ir species
in the catalysts. Figure 1c shows a maximum of the WT plots at
oxidation (Table 1, entry 2), and so does Au/ND (Table 1, entry 3).
For Pd/ND, the productivities towards ethanol, acetaldehyde,
acetic acid and ethene increased simultaneously, which agrees
2
.5 Å that corresponds to the Ir-Ir scattering in Ir
a maximum at 1.6Å in Figure 1d is correlated with the Ir-O/C
contribution (WT analysis of Ir and IrO are shown in Figure S3
n 1
/ND. For Ir /ND,
4a
well with previous report. The activity of Pt/ND reaches to 2.6
2
mol/mol/h, which is even higher than Pd/ND. Among these
catalysts, iridium cluster catalyst (Ir /ND) exhibited the highest
n
activity (7.5 mol/mol/h, see Entry 6 of Table 1 and Figure S4).
This is the best activity that has been reached (Table S3) so far.
Significantly, only water was used as solvent in our case, in
for comparison). To investigate the dispersion of Ir, aberration-
corrected high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM) was employed (Figure 2).
For Ir
observed, along with a small amount of atomically dispersed Ir
species. Of note, for Ir /ND, Ir species were atomically dispersed
n
/ND, ultrafine Ir clusters with a size of 1-2 nm were clearly
contrast to the use of acid (HCl or CF
previous reports.
3
COOH) as co-catalyst in
Here, the main products are ethanol,
acetaldehyde, and acetic acid, accompanied by a small amount of
methanol, ethylene, and propionic acid (CO is not included).
4a, 9
1
on the support predominantly, together with few small
nanoclusters.
2
When the dispersion of Ir species was reduced from Ir clusters to
2
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Journal of the American Chemical Society
atomically dispersed Ir species (Ir
3.5 mol/mol/h and acetaldehyde is the only product(Table 1, Entry
). This finding indicates that iridium is indeed active for ethanol
oxidation, and the dispersion of Ir has significant impact on the
1
/ND), the activity still kept at
n
product distribution. Moreover, the Ir /ND catalyst is very stable,
no visible deactivation was observed even after five reaction
cycles (Figures 3b and S5).
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Table 1. Catalytic performance of ethane oxidation reaction with different catalysts
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Typical reaction conditions: 20 mg catalyst, 15 mL water, feeds (C
were normalized over metal dosage. Activities were calculated based on the amount of converted C
considered to originate from one C molecule).
2
H
6
/ CO / O
2
/ He / Ar / N
2
= 35/ 8 / 2.5 / 1 / 1 / 52.5), 3MPa, 100 C. The productivities
2
H
6
over metal dosage (two CH OH molecules were
3
2
H
6
Figure 3. (a) Infrared spectra of CO adsorbed on Ir/ND catalysts; (b) Stability of Ir
n
/ND catalyst in ethane oxidation reaction; (c) XPS
studies of Ir /ND catalyst under different reaction atmospheres.
n
Interestingly, we found that CO played a critical role in ethane
oxidation. When C is absent, no C2+ products were observed
see Table 1, Entry 8). Understandably, C2+ products are derived
from ethane. If the reactants are ethane and oxygen but without
CO, the formation of C2+ products is also prohibited (Table 1,
Entry 9).
could be easily over-oxidized to a high-valence state, which is
considerably unable to adsorb the alkane at low temperature and
consequently showed no activity for ethane oxidation (Table 1,
entry 9). After the sample was treated in a mixture of CO and
water vapor at 100 C for 3 hours, the binding energy of Ir 4f7/2
shifted to 61 eV. This demonstrated that the Ir cluster species
were reduced by CO to a metallic state. The role of CO is to
reduce Ir clusters and make it stay in a low-valance state (close to
metallic state). Indeed, for the catalyst after real catalytic reaction,
we can clearly see that Ir clusters was slightly oxidized, but could
still maintain its low-valence state, implying the importance of
CO in the catalytic reaction of Ir cluster catalyst.
2
H
6
(
1
2
º
Although it has been reported that the involvement of CO is
pivotal for this system, no solid evidence has been given
regarding the role of CO. Flytzani-Stephanopoulos has smartly
speculated that CO can regenerate the low valence state of Rh
+
5a
(
Rh ) in the Rh-ZSM-5 catalyst for methane oxidation.
Following this speculation, we designed in-situ XPS
experiments to analyze the critical role of CO in this process on
Ir /ND. The characteristic peaks of Ir /ND after preactivation in
(Figure 3c) are centered at approximately 60.7 eV (Ir 4f7/2) and
In summary, we discovered a highly active nanodiamond
supported iridium clusters to catalyze the direct oxidation of
ethane to oxygenates in the presence of oxygen and carbon
n
n
o
H
2
monoxide at 100 C in the aqueous phase. The activity of Ir
0
6
3.7 eV (Ir 4f5/2), which can be assigned to metallic Ir species.1
Subsequently, the sample was treated in the mixture of C , O
and water vapor at 100 C for 3h. Clearly, the binding energy of Ir
7/2 shifted to 62.0 eV, implying the formation of iridium oxide
clusters/nanoparticles is higher than atomically dispersed Ir. The
critical role of CO was clearly demonstrated by in-situ XPS
experiments, CO maintains the low-valance state (metallic state)
of Ir that is required for ethane adsorption during the reaction.
This work provides a basis for the investigation of new Ir catalysts
2
H
6
2
º
4
f
4
+
11
(Ir ) species. Thus, for the reaction without CO, Ir species
3
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AUTHOR INFORMATION
Corresponding Author
*jiangzheng@sinap.ac.cn
*
*
liuhy@imr.ac.cn
dma@pku.edu.cn
Mild Oxidation of Ethane to Acetic Acid by H
sSupported µ-Nitrido Diiron Phthalocyanines. J. Organomet. Chem., 2015,
93, 139-144. (g) Forde, M.M.; Armstrong, R.D.; McVicker, R.; Wells,
2 2
O Catalyzed by
Author Contributions
#These authors contributed equally to this work.
7
P.P.; Dimitratos, N.; He, Q.; Lu, L.; Jenkins, R.L.; Hammond, C.; Lopez-
Sanchez, J.A.; Kiely, C.J.; Hutchings G. J. Light Alkane Oxidation Using
Catalysts Prepared by Chemical Vapour Impregnation: Tuning Alcohol
Selectivity Through Catalyst Pre-treatment. Chem. Sci., 2014, 5, 3603-
Notes
The authors declare no competing financial interest.
3
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Taylor, S. H.; Hutchings, G. J. The Role of Copper Speciation in the Low
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ACKNOWLEDGMENT
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Catalysts. ChemPhysChem, 2018, 19, 469-478. (i) Li, Y.; Tang, Y.;
This work was supported by the National Natural Science
Foundation of China (21725301, 21932002, 91645115, 21821004,
U1732267, 51872008, 21573254, 91845201), the Ministry of
Science and Technology (2017YFB0602200), Beijing Natural
Science Foundation (No. 1182005), the Youth Innovation
Promotion Association, Chinese Academy of Science (CAS). The
XAS experiments were conducted in Shanghai Synchrotron
Radiation Facility (SSRF).
Nguyen, L.; Tao, F. F. Catalytic Oxidation of Ethane to Carboxylic Acids
in the Liquid Phase at Near Room Temperature at Ambient Pressure. ACS
Sustainable Chem. Eng., 2019, 75, 4707-4715.
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A highly active iridium catalyst is investigated for partially oxidizing ethane to valuable oxygenates by O
CO at 100 C in water.
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