Direct synthesis of ethanol via CO2 hydrogenation using supported gold catalysts

Article abstract of DOI:10.1039/c6cc08161d

An efficient titania supported Au nanocluster (NC) has been prepared for the direct synthesis of useful EtOH from CO₂ and H₂. The unique creation of an excellent synergistic effect between Au NCs and the underlying TiO₂ su

Full text of DOI:10.1039/c6cc08161d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Direct Synthesis of Ethanol via CO2 Hydrogenation Using
Supported Gold Catalysts†
Dong Wang,a,b,c,e Qingyuan Bi,*b Guoheng Yin,b Wenli Zhao,b Fuqiang Huang,*b,c,d Xiaoming Xiea,c
and Mianheng Jianga,c
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient titania supported Au nanoclusters (NCs) has been
Hydrogenation of CO2 to EtOH with homogeneous catalysts
made for the direct synthesis of useful EtOH from CO2 and H2. The has been widely investigated;18,19 however, the use of indispe-
unique creation of excellent synergistic effect between Au NCs nsable and air-sensitive organic ligands and/or additives tends
and underlying TiO2 support, especially the anatase crystal phase to limit their practical large-scale application. Some of the
with abundant oxygen vacancies can achieve the high performa- drawbacks associated with organic complexes are largely
nce for EtOH synthesis under moderate and practical conditions.
mitigated in heterogeneous catalysts. Among the solid cataly-
sts, many active components, such as Rh/SiO2, [Rh10Se]/TiO2,
CoMoS, and K/Cu-Zn-Fe have been employed to be effective
As an abundant, cheap, non-toxic, and renewable carbon
source, CO2 has been largely used for making value-added
chemicals and low-carbon fuels, such as methane, alcohols,
for EtOH synthesis from CO2 hydrogenation, due to simple
handling and significant stability.20 Whereas most reaction
25
−
ethers, esters, and formic acid.1 6 This essential transformation
−
o
processes were performed at high temperatures (> 250 C)
is usually identified as an ideal solution to global warming,
which can result in a variety of products, and the selectivity to
EtOH need further improvement.20 Based on the previous
25
−
ocean acidification, and shortages of fossil fuels, and attracted
ever-increasing attentions.7 However, the major challenge
10
−
work on selective hydrogenation of CO2 into HAs with homog-
eneous Ru-Rh-Li system,19 Han and co-workers have recently
reported an efficient Pt/Co3O4 with excellent synergistic effect
can give a good space-time-yield (STY) of alcohols and HAs
selectivity as well as robust stability employing high pressure
of 8 MPa.26 However, besides the inherent inconvenience ass-
ociated with the handling problems, this system requires large
amount of complicated 1,3-dimethyl-2-imidazolidinone (DMI)
as the indispensable solvent.26 Thus, the development of an
efficient catalytic system for direct synthesis of EtOH from CO2
hydrogenation (Scheme 1) without or with simple solvent
under moderate conditions is urgently required.
in CO2 utilization is the high thermodynamic stability of CO2
molecule and the high energy barrier in cleavage of the C=O
bond. One promising approach to overcoming the obstacle is
the chemical reduction activation of CO2 by catalytic
hydrogenation.11,12 The hydrogenation of CO2 to yield useful
alcohols (mainly methanol) is one of the most important ways
15
−
for CO2 conversion.13
Ethanol (EtOH) and higher alcohols
(HAs) have higher energy density than methanol, and show
broader applications in neat fuels, fuel additives, and raw
chemical materials.16,17 However, the direct synthesis of EtOH
via CO2 hydrogenation is much more difficult than methanol
generation due to the subsequent carbon chain growth step
being involved in the former process.
Cat., solvent
HO
CO2 + H2
+
△P, △T
HO
HO
a.State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of
Microsystem and Information Technology, Chinese Academy of Sciences,
Shanghai 200050, P. R. China
HO
b.State Key Laboratory of High Performance Ceramics and Superfine
Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences,
Shanghai 200050, P. R. China
OH
E-mail: biqingyuan@mail.sic.ac.cn; huangfq@mail.sic.ac.cn
c. School of Physical Science and Technology, ShanghaiTech University, Shanghai
200031, P. R. China
Scheme 1. Direct synthesis of ethanol via CO2 hydrogenation. Compounds
surrounded with dotted line are the potential products of primary HAs.
d.Beijing National Laboratory for Molecular Sciences and State Key Laboratory of
Rare Earth Materials Chemistry and Applications, College of Chemistry and
Molecular Engineering, Peking University, Beijing 100871, P. R. China
e. University of Chinese Academy of Sciences, Beijing 100049, P. R. China
† Electronic supplementary information (ESI) available: See DOI: 10.1039/
x0xx00000x
From a synthetic point of view, it turns out that an
engineered catalyst with excellent ability to activate both CO2
and H2 would be a better candidate for this particular transfor-
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mation. Interest in using supported gold nanoparticles (Au higher dispersion of active species can significantly improve
NPs) or nanoclusters (NCs) as a versatile catalyst with a the performance (entry 5 vs. 6). The Au NCs (Fig. S2†) display-
surprising activity and selectivity for green and sustainable ed a high STY of 635.4 mmol gMetal h−1 (entry 6). Already, this
1
−
oxidation/red-uction and fine chemical synthesis has attracted result represents the best catalytic activity ever reported for
26
−
considerable attention over the past decades.27 32 Moreover, it EtOH synthesis via CO2 hydrogenation.19
Compared with
−
is also dem-onstrated that the titania-supported gold (Au/TiO2) other metal-oxide supports, TiO2 is more suitable for loading
catalyst with average particle size of ca. 3 nm can efficiently Au NPs or NCs for CO2 reduction (entries 5–9). Note that the
achieve the formation of formic acid from CO2 and H2 in high- previously reported Pt/Co3O4 showed very low activity under
pressure conditions.33 Upon considering that the small Au NPs the same reaction conditions, and the methanol generation
can acti-vate CO2 as well as H2 molecules and the well CO- was also detected (entry 10).
tolerant ability,33 we were motivated to investigate the
possibility of using gold catalyst for improved EtOH synthesis
(b)
(a)
via CO2 hydrogenation under mild conditions. Herein, for the
first time, we report an efficient titania supported Au catalyst
for direct synthesis of EtOH from CO2 and H2 within simple
solvent under moderate and practical conditions. Key to the
successful application of the designed catalyst was the unique
creation of excellent synergistic interactions between Au NCs
and underly-ing TiO2, especially the anatase TiO2 with
abundant oxygen vacancies. These findings can make a
significant contribution, not only to provide an effective
catalytic system for the produ-ction of EtOH, but also to add
another important example of heterogeneous Au-catalyzed
reactions.
0.349 nm
anatase (101)
30
(d)
(c)
1.0
± 0.1 nm
25
20
15
10
5
4f5/2 87.6 eV (Au0)
4f7/2 83.8 eV (Au0)
At the start of this work we examined the direct synthesis of
EtOH from CO2 and H2 in the presence of simple solvent of
N,N-dimethylformamide (DMF). First of all, we have studied
several potential supported-metal catalysts prepared with a
conventional impregnation technique or deposition-precipitat-
ion (DP) method and some commercially available samples on
the conditions of 60 bar H2/CO2 (3/1) and 200 oC. Upon consid-
0
0.5
1.0
1.5
94
92
90
88
86
84
82
80
78
Binding energy (eV)
Particle size (nm)
Fig. 1 (a) HRTEM image (inset shows the SAED pattern) of a-TiO2, (b) HAADF-
STEM image, (c) Au particle size distribution, and (d) XPS data of Au/a-TiO2.
ering that Cu and other transition metals can more favorably
catalyze CO2 hydrogenation to methanol,14,34 it was approp-
36
−
Bearing in mind that the crystal phase has proven to be essential
for supported metal catalysts,37 we envisioned that the enhanced
metal-support interactions between Au NCs and underlying TiO2
material, especially the anatase phase with abundant oxygen vacan-
cies could be more effective for the direct synthesis of EtOH. To
investigate this possibility, four gold catalysts composed of different
titania polymorphs of anatase (a-), rutile (r-), brookite (b-), and
amorphous (am-) supported Au NCs were prepared and evaluated
(see ESI†).38 X-ray diffraction (XRD) patterns (Fig. S4†) clearly show
the well-defined crystals of anatase, rutile and brookite phase, and
the typical difference between these TiO2 polymorphs. There are no
structural changes after the Au loading, and the absence of any Au-
containing crystal phases indicates the low gold loading and high
dispersion of the gold species. Raman spectra in Fig. S5† exhibit the
characteristic Ramanactive modes for crystal phases, and the high
purity of the as-prepared TiO2 polymorphs. High-resolution
transmission electron microscopy (HRTEM) images and selected
area electron diffraction (SAED) patterns in Figs. 1a, S6a†, and c†
further confirm the morphologies and well-faceted crystals with
interplanar spacing of 0.349, 0.322, and 0.344 nm that matches the
anatase (101), rutile (110), and brookite (111) plane, respectively.
High-angle annular dark-field scanning transmission electron micro-
scopy (HAADF-STEM) images show that the uniform Au NCs with
the average size of 1.0 ± 0.1 nm had been formed and anchored on
the corresponding TiO2 supports (Figs. 1b, c, S6b†, d†, S7†, and
riate to focus our investigation on the noble-metal catalysts.
Prior to catalytic test, all catalysts were pretreated with H2 at
the indicated temperature to ensure all metal species were in
their metallic state (see ESI†). While Pd, Ir, and Rh (Fig. S1†)
exhibited very poor performance for EtOH synthesis with the
STY lower than 60 mmol gMetal h−1 (Table 1, entries 1–4), only
1
−
catalysts based on gold can deliver notable activity (entries 5–
9). Of the Au catalysts tested, the use of lower Au loading on
the same TiO2 support showing smaller metal particle size and
Table 1 Study of various solid catalysts for the synthesis of EtOH from CO2
hydrogenation.a
1
1
−
Entry Catalyst
nEtOH (mmol)
0.9
STY of EtOH (mmol gMetal h−
)
1
Pd/C
18.1
2
3
Pd/TiO2
Ir/TiO2
0.5
0.4
50.2
41.1
4
Rh/TiO2
Au/TiO2
Au/TiO2
Au/ZrO2
Au/Al2O3
Au/Co3O4
Pt/Co3O4
0.6
2.8
1.9
1.7
1.4
1.3 (0.8)
0.6 (0.3)
59.6
5b
6
282.5
635.4
213.5
232.3
128.8
60.3
7
8
9c
10c
a Reaction conditions: 100 mg catalyst, 5 mL DMF, 45 bar H2 and 15 bar CO2
at 25 oC, 200 oC, 10 h. b The Au/TiO2 sample was supplied by Mintek. c The
numbers in parentheses refer to the amount of methanol generated.
2 | J. Name., 2016, 00, 1-4
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1000
(b)900
1000
(a)900
100
90
80
70
60
50
S8†). Meanwhile, X-ray photoelectron spectroscopy (XPS) data
clearly demonstrate that there is only metallic Au species on the
surface of the Au-supported sample (Fig. 1d).
800
700
600
500
400
300
200
100
0
800
700
600
500
400
300
200
100
0
It is widely accepted that defects like oxygen vacancies can serve
as reactive perimeter sites at the solid−solid interface (e.g.
Au−TiO2), and they can also in turn afford large amount of highly
mobile oxygen species being able to react with CO2.39,40 Surface
NMP
DMF
THF
H2O
150
180
200
240
Cyclohexane
Reaction temperature (oC)
Solvent
1000
1000
(d)900
100
oxygen vacancies in a-TiO2 can strongly enhance the metal-support (c) 900
800
800
700
600
500
400
300
200
100
0
90
80
70
60
50
interac-tions and the surface adsorption properties of its supported
700
600
metal catalyst. The TiO2 with different polymorphs supported Au
500
400
NCs were tested for direct synthesis of EtOH from CO2 and H2 under
300
200
the conditions mentioned above. To our delight, Au/a-TiO2
100
exhibited excellent EtOH selectivity (> 99%) and unprecedented
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Ratio of H2/CO2
0.4
0.8
1.2
1.6
2.0
2.4
2.8
1
1
activity of 942.8 mmol gAu h− , as shown in Fig. 2, which was 32-
fold higher than that of the best catalyst reported in literature
(Pt/TiO2)26 and over three times of that on commercial Au/TiO2
catalyst (Table 1, entry 5). By contrast, Au/r-TiO2 and Au/b-TiO2
displayed lower activities and Au/am-TiO2 with the poor crystallinity
showed the lowest. To better understand the superior performance
of Au/a-TiO2, the catalysts were characterized with UV−vis diffuse
reflectan-ce spectra and diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS). The fact that Au/a-TiO2 exhibited
more oxygen vacancies than other two counterparts (Figs. S9†,
S10†, and Table S2†) indicates that the stronger metal-support
interaction can be obtained in Au−a-TiO2. And CO2 molecules can be
activated more easily on the surface of Au/a-TiO2. These data,
together with the catalytic results (Fig. 2), unambiguously suggest
that the oxygen vacancies as the defect in metal-oxide material for
gold catalysis is essential to tune the metal-support interactions and
thus significan-tly facilitate the performance for direct synthesis of
EtOH.
−
Au loading (wt%)
Fig. 3 STY of EtOH from CO2 hydrogenation over Au/a-TiO2 catalyst as a
function of (a) solvent. 5 mL solvent, 100 mg catalyst, 45 bar H2 and 15 bar
CO2 at 25 oC, 200 oC, 10 h. (b) reaction temperature. 5 mL DMF, 100 mg
catalyst, 45 bar H2 and 15 bar CO2 at 25 oC, 10 h. (c) ratio of H2/CO2. 5 mL
DMF, 100 mg catalyst, total pressure of 60 bar, 200 oC, 10 h. (d) Au loading.
5 mL DMF, 0.3 mg Au, 45 bar H2 and 15 bar CO2 at 25 oC, 200 oC, 10 h. The
1
1
−
unit of STY of EtOH is mmol gAu h−
.
between the TiO2 surface defects and Au surface atoms in the
hydrogenation process (Fig. 3a). The results in Fig. 3b revealed that
a temperature of 200 oC is optimal for this reaction. Higher
o
temperatures (e.g. 240 C) can lead to appreciable generation of
toxic CO gas deriving from the reverse water-gas shift (RWGS)
reaction, thus decreasing the EtOH selectivity and STY. The increase
of total pressure is beneficial to the reaction (not shown), and the
H2/CO2 molar ratio of 3/1 is most suitable (Fig. 3c). More H2 dosage
can result in the generation of other C2+ by-products and decrease
EtOH selectivity. For Au/a-TiO2, the particle size increases (Fig.
S12†) while the catalytic activity decreases (Fig. 3d) with the Au
loading increasing, indicating that the Au-catalyzed direct synthesis
of EtOH was a typical particle-size-dependent reaction.
Further investigation into the effects of reaction parameters on
the catalytic performance of Au/a-TiO2 was studied, as shown in Fig.
3. Solvent effect is remarkable for the direct synthesis of EtOH
catalyzed by Au NCs. Compared to N-methyl-2-pyrrolidone (NMP),
cyclohexane, tetrahydrofuran (THF) and water, DMF as the most
appropriate solvent exhibited its unique advantages for CO2 dissolu-
tion and acting as a “shuttle” for carrying the dissolved CO2 species
Based on the above results, Table S3†, and the relevant literatu-
re,20,25,26 a possible reaction pathway of EtOH synthesis via excellent
synergistic combination of Au NCs and a-TiO2 with abundant oxygen
1000
900
800
700
600
500
400
300
200
100
0
1000
100
900
800
90
700
600
500
400
300
200
100
0
80
70
60
50
1
2
3
4
5
6
Durability times
Au/a-TiO2
Au/am-TiO2
Au/r-TiO2
Au/b-TiO2
Fig.
4 Recycling of Au/a-TiO2 catalyst for EtOH synthesis from CO2
Fig. 2 Catalytic activity of TiO2 polymorphs supported Au catalysts for the
synthesis of EtOH from CO2 hydrogenation. Reaction conditions: 100 mg
catalyst, 5 mL DMF, 45 bar H2 and 15 bar CO2 at 25 oC, 200 oC, 10 h.
hydrogenation. Reaction conditions: 100 mg catalyst, 5 mL DMF, 45 bar H2
and 15 bar CO2 at 25 oC, 200 oC, 10 h.
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11 Q. Zhu, J. Ma, X. Kang, X. Sun, H. Liu, J. Hu, Z. Liu and B. Han,
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vacancies over Au/a-TiO2 catalyst was proposed (Scheme S1†). The
CO2 was first hydrogenated to –CH3 species, and then followed by
CO insertion to form –CH3CO intermediates which were further
hydrogenated into EtOH and subsequently desorbed from the solid
surface of Au/a-TiO2. To confirm whether the Au-catalyzed reaction
occurs on the surface of Au/a-TiO2 sample, it was removed with
filtration from the reaction system after 5 h. Further processing of
the resulting filtrate under the similar conditions for another 5 h did
not increase the conversion. In addition, inductively coupled plasma
atomic emission spectroscopy (ICP-AES) analysis of the filtrate
showed the content of Au or Ti in the solution was below the
detection limit of 0.1 ppm, revealing that no leaching occurred
during the reaction and the heterogeneous catalysis nature of Au/a-
TiO2. This catalyst remains excellent stability for EtOH synthesis, as
shown in Fig. 4; it can be reused up to six times without significant
loss of catalytic activity and EtOH selectivity. Compared with the
fresh Au/a-TiO2, there was no obvious change of morphology and
no aggregation of NCs for the used catalyst (Fig. S13†). These
results further demonstrate the effectiveness of Au/a-TiO2 on the
indicated conditions for direct synthesis of EtOH from CO2 and H2.
In summary, we have demonstrated, for the first time, that an
efficient titania supported Au NCs catalyst was made for direct
synthesis of useful EtOH from CO2 and H2 within simple solvent of
DMF under moderate and practical conditions. The unique creation
of excellent synergistic effect between Au NCs and underlying TiO2
support, especially the anatase crystal phase with abundant oxygen
vacancies, facilitated the successful application of the designed
catalyst. The engineered Au/a-TiO2 with strong metal-support
interactions exhibited unprecedented activity and high selectivity to
desired EtOH and showed excellent stability. The present findings
can make a significant contribution, not only to provide an effective
catalytic system for the production of EtOH, but also to add another
important example of heterogeneous Au-catalyzed reactions.
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ä
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