Selective hydrogenolysis of carbon-oxygen bonds with formic acid over a Au-Pt alloy catalyst

Article abstract of DOI:10.1039/C6CC09599B

We reported selective hydrogenolysis of carbon-oxygen species over a CeO₂-supported Au-Pt alloy catalyst (Au-Pt/CeO₂) using biomass-derived formic acid as the hydrogen source. The success of this reaction is reasonably attributed to the high efficiency of Au-Pt/CeO₂ in the tandem steps of formic acid dehydrogenation and carbon-oxygen species hydrodeoxygenation.

Full text of DOI:10.1039/C6CC09599B

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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DOI: 10.1039/C6CC09599B
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Selective Hydrogenolysis of Carbon-Oxygen Bond with Formic
Acid over a Au-Pt Alloy Catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Liang Wang,*a Jian Zhang,a Guoxiong Wang,a Wei Zhang,*b Chengtao Wang,a Chaoqun Bian,a and
Feng-Shou Xiao*a,c
DOI: 10.1039/x0xx00000x
www.rsc.org/
We reported selective hydrogenolysis of carbon-oxygen species methylphenol (MMP), which is one of potential biofuels in the
over a CeO2-supported Au-Pt alloy catalyst (Au-Pt/CeO2) using future,6 but by-product of 4-hydroxymethyl-2-methoxyphenol
biomass-derived formic acid as the hydrogen source. This success (HMP) usually formed by uncompleted hydrogenation of
is reasonably attributed to the high efficiency of Au-Pt/CeO2 in the vanillin. One possible strategy to achieve both high activity and
tandem steps of formic acid dehydrogenation and carbon-oxygen selectivity is employing hydrogen sources with higher activity
species hydrodeoxygenation.
than gaseous hydrogen, which might be favourable in the
hydrogenolysis reactions.
Biomass has attracted much attention in the production of
fuels and fine chemicals due to the shortage of fossil
resources.1-4 One efficient route for biomass utilization is the
fast pyrolysis of lignocellulosic feedstocks into phenolic bio-oil,
which has been deemed a promising alternative to fossil fuels.4
However, it is impossible to directly use these crude bio-oils,
because their high oxygen content causes low energy density,
high viscosity, and poor stability. Therefore, upgrading the
crude bio-oil by the selective cleavage of C-O/C=O bonds to
decrease oxygen content is important to obtain high-quality
alternative fuels.4,5 Because of the highly stable C-O/C=O
linkages, the conventional hydrodeoxygenation (HDO) routes
for the cleavage of C-O/C=O bonds require high temperature
Recently, various organic molecules have been used as
the hydrogen source in hydrogenation reactions via tandem
dehydrogenation and hydrogenation, and better performance
than with gaseous hydrogen was obtained in some cases.7
Formic acid (FA), which has high hydrogen density and is easily
obtained from the hydrogenolysis of cellulose, has been
considered one of the most promising hydrogen sources for
wide application via a dehydrogenation at a low temperature
of <100
temperature is higher than 150
HDO), FA is usually transformed via dehydration (HCOOH
°
C (HCOOH
→
CO2 + H2).8-10 However, when the
C (which is necessary for
°
→
CO + H2O), hindering the formation of H2 from FA
dehydrogenation. The design of an efficient tandem process
combining FA dehydrogenation and selective HDO is greatly
challenging.
In this work, we report Au-Pt alloy nanoparticles
supported on CeO2 (Au-Pt/CeO2) that exhibit not only good
selectivity for FA dehydrogenation but also excellent selectivity
for the hydrogenolysis of carbon-oxygen species in the HDO
(>200C) and gaseous hydrogen pressure, which lead to side
reactions of over-hydrogenation, cracking, and coke formation.
These features make the conventional HDO routes
unsatisfactory in some cases due to lack of the product
selectivity.
One solution to this issue is conducting the reaction under
mild conditions, such as relatively low temperature and
gaseous hydrogen pressure. However, the activities are very
low and unsatisfactory for the cleavage of C-O/C=O bonds
under the mild conditions. For example, vanillin (4-hydroxy-3-
methoxybenzaldehyde), a common component of pyrolysis oil,
is excepted to be hydrodeoxygenated to 2-methoxy-4-
processes at 150 °C. For the tandem reactions of vanillin HDO
with FA, the Au-Pt/CeO2 is highly active, selective, and
recyclable for the hydrogenolysis carbon-oxygen species in the
reactants.
aKey Laboratory of Applied Chemistry of Zhejiang Province, Department of
Chemistry, Zhejiang University, Hangzhou 310028, China
Email: liangwang@zju.edu.cn; fsxiao@zju.edu.cn
bDepartment of Materials Science and Key Laboratory of Mobile Materials MOE,
Jilin University, Changchun 130012, China.
Scheme 1. Synthetic procedure for Au-Pt/CeO2.
Scheme 1 shows the synthetic procedure for the Au-
Pt/CeO2 catalyst. The bimetallic Au-Pt colloid was synthesized
with HAuCl4 and H2PtCl6 as precursors and NaBH4 as a
Email: weizhang@jlu.edu.cn
cKey Laboratory of Biomass Chemical Engineering of Ministry of Education,
Zhejiang University, Hangzhou 310027, China.
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reductant under the protection by polyvinyl pyrrolidone (PVP), from the standard metallic Au0 and Pt0 (Figures 1D and E). This
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followed by dialysis to remove undesirable Na+ and Cl- ions. phenomenon should be attributed to thDeOinI:t1e0r.1a0c3t9io/Cn6CbCe0tw95e9e9nB
Then, the Au-Pt colloid was loaded on CeO2 support, and then Au-Pd alloy nanoparticles and Ce3+ sites of CeO2 support,
calcined for removing the PVP species to obtain Au-Pt/CeO2 which is well known as strong metal-support interaction and
(See the XRD patterns in Figure S1). Based on ICP analysis, the usually observed in the redox CeO2-supported catalysts.11b
Au and Pt loadings were 0.77 and 0.92 wt%, respectively.
A
B
A
CeO2
2.32 Å
100
80
60
40
20
0
B
C100
100
80
60
40
20
0
2.31 Å
c
80
60
40
20
0
C
a
b
1.6 2.0 2.4 2.8
Particle size (nm)
Au0
Pt0
d
E
D
Pt2+
0
1
2
3
4
(b)
Catalysts
(b)
Reaction time (h)
Figure 2. (A) Scheme for the conversion of vanillin to HMP and MMP;
(B) HDO of vanillin with FA over various catalysts; (C) Dependence of (a)
FA, (b) vanillin conversion, (c) MMP and (d) HMP selectivity on the
reaction time over the Au-Pt/CeO2 catalyst. Reaction conditions: 0.8
mmol of vanillin, 40 mg of catalyst, 8 mmol of FA, 10 mL of water, 15
(a)
(c)
92 90 88 86 84 82 80
Binding Energy (eV)
78 76 74 72 70 68
Binding Energy (eV)
Figure 1. (A-C) TEM images and metal nanoparticle size distribution of bar N2, 150 °C, and 4.5 h. The carbon balances are over 99% for all the
the Au-Pt/CeO2. (D) Au4f and (E) Pt4f XP spectra of the (a) Au/CeO2, (b) tests.
Au-Pt/CeO2 and (c) Pt/CeO2 samples.
The catalytic study start from the HDO of vanillin with FA
in water solvent over various catalysts (Figure 2). Figure 2B
shows the catalytic data of various catalysts. The CeO2-
supported monometallic Au and Pt catalysts (Au/CeO2 and
Pt/CeO2) exhibit low conversion (3.1-16.1%) and poor MMP
selectivity (2.2-2.9%). In contrast, most bimetallic Au-Pt
catalysts (Au-Pt/AC, Au-Pt/SiO2, and Au-Pt/CeO2) give
significantly higher conversions (>31.0%) and selectivities
(>70.0%) than the monometallic Au and Pt catalysts.
Particularly, when Au-Pt/CeO2 is used as the catalyst, the MMP
selectivity can reach 99.8% at full conversion of vanillin,
outperforming other Au-Pt catalysts with similar Au and Pt
loadings on other supports (Tables S1 and S2). Additionally,
when the sizes of Au-Pt nanoparticle on CeO2 support were
2.5-6.0 nm via precipitation synthesis (Figures S4 and S5), the
Au-Pt/CeO2 still exhibited full conversion of vanillin but
relatively low MMP selectivity at 80% (Table S3). These data
suggest that the superior catalytic performance of Au-Pt/CeO2
is owing to the contribution of the alloy metals, small
nanoparticles, and CeO2 support.
Figure 2C shows the dependence of FA and vanillin
conversion and MMP and HMP selectivity on the reaction time
over the Au-Pt/CeO2 catalyst. The pressure of the autoclave
reactor increased significantly in the beginning of the reaction,
indicating the formation of a large amount of gaseous product
(H2 and CO2, by GC analysis). The conversion of FA is always
higher than that of vanillin, which indicates that this reaction is
a tandem of FA dehydrogenation and vanilline HDO. When the
reaction time reaches 4.5 h, the vanillin is completely
converted, by-products, such as HMP, are nearly undetectable,
and MMP appears as the sole product during the reaction
process, indicating that MMP is from the direct hydrolysis of
Figure 1A shows a TEM image of Au-Pt/CeO2, in which the
metal nanoparticles are highly dispersed on the CeO2 support.
The high-resolution image of Au-Pt/CeO2 shows a lattice
spacing of 2.31-2.32 Å for the {111} plane (Figure 1B). Based
on measurements of more than 150 nanoparticles, the Au-Pt
bimetallic particle sizes are distributed in a narrow range (1.6-
3.0 nm, average of 2.3 nm, Figure 1C). In contrast, Au/CeO2
and Pt/CeO2 exhibit wider metal size distributions (2.0-6.0 and
2.0-7.0 nm, Figure S2). The high-resolution images of the
Au/CeO2 and Pt/CeO2 (Figures S2B and E) show that the lattice
spacing of {111} Au (2.36 Å) and {111} Pt (2.25 Å) are different
from that for the Au-Pt alloys (2.31-2.32 Å), suggesting the
successful formation of Au-Pt alloys on Au-Pt/CeO2 rather than
separate phases of Au or Pt. These results are consistent with
those reported previously that the formation of bimetallic Au-
Pt alloys might change the electronic structure of the Au and
Pt species, leading to variation in lattice spacing.11a Figure S3
shows the energy dispersive X-ray (EDX) analysis of randomly
selected regions in Au-Pt/CeO2, showing almost unchanged
Au/Pt ratios and confirming the successful formation of Au-Pt
alloys in the Au-Pt/CeO2.
Figures 1D and E show the Au4f and Pt4f XP spectra of
various samples. The Pt/CeO2 samples exhibits Pt4f7/2 electron-
binding energy at 72.2 eV, assigning to Pt2+ species. In contrast,
Au-Pt/CeO2 gives Pt4f7/2 binding energies of 70.8 eV, which is
assigned to metallic Pt. These results demonstrate that the Pt
should interact with Au on Au-Pt/CeO2 catalyst to hinder the
formation of Pt2+ species, forming Au-Pt alloys—all in good
agreement with the results of electron microscopy.
Additionally, it is worth noting that the Au4f7/2 and Pt4f7/2
electron-binding energies of AuPt/CeO2 catalyst are decreased
2 | J. Name., 2012, 00, 1-3
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reactions are normally performed at relatively high
temperatures.
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DOI: 10.1039/C6CC09599B
Table 1 Catalytic data in FA dehydrogenation and WGS over various
catalysts.
Figure 3A shows the Raman spectra of CeO2 and Au-
Pt/CeO2 before and after FA dehydrogenation, which are
denoted as fresh and FA-treated samples, respectively. For
CeO2, there is no change in the Raman peak at 464 cm-1 (F2g
mode) after treatment (Figure S8). However, the FA treatment
of Au-Pt/CeO2 leads to a shift of 4 cm-1 for the F2g peak, along
with the appearance of a broad peak at 655 cm-1 (insert in
Figure 3A). The broad peak is attributed to the defect sites in
the sample.14 Figure S9 shows the H2-TPR curves of CeO2 and
FA dehydrogenationa
Entry
Catalyst
TOF
(h-1)
Selectivity (%)
CO2 + H2
CO + H2O
1
Au/SiO2
Au/CeO2
Pt/CeO2
14
298
370
340
842
1637
95.0
94.2
5.0
5.8
2
3
4
5
6
32.9
67.1
80.0
--b
Pt/C
20.0
Au-Pt/CeO2. CeO2 gives a wide peak at 120-350
°C and a
Au-Pd/CeO2
Au-Pt/CeO2
>99.0
>99.0
narrow peak centralized at 443 C, which are related to the
°
--b
reduction of weakly and strongly bonded oxygen species on
CeO2. In contrast, the reduction of very weakly bonded oxygen
species occurs at 86 °C on Au-Pt/CeO2, which is due to the
a
Reaction conditions: 160 C, 15 bar of N2, 10 mg of catalyst, 2 mmol
of FA, 10 mL of water, and 25 min. b Trace.
vanillin. Even if the amount of hydrogen source (FA) was
significantly reduced, high selectivity to MMP at 99.0% was still
obtained (Figure S6). This phenomenon is different from the
HDO of vanillin with gaseous hydrogen, where HMP is
activation of the Ce-O bond associated with defects sites by
strongly bound Au-Pt nanoparticles.15
459
901.1
B
A
917.3
883.1
655
898.4
463
(c)
(a)
902.6
obtained as
a
by-product from the uncompleted
(a)
885.2
hydrogenolysis of vanillin.6
400
600
800
Raman Shift (cm-1
)
(b)
(c)
Based on the catalytic data for the HDO with FA, the
superior performance of Au-Pt/CeO2 might be related to its
high efficiency in the tandem catalysis of the individual steps,
including dehydrogenation of FA and hydrogenolysis of C=O
bond. Figure S7 shows the dependence of FA conversion on
time over various catalysts, including Au/SiO2, Au/CeO2,
Pt/CeO2, Pt/C, and Au-Pt/CeO2 at relatively high temperature
(c)
(a)
920 910 900 890 880
Binding Energy (eV)
400
600
800
Raman Shift (cm-1
)
529.4
D
1704
C
(a)
531.6
(b)
(c)
(a)
(b)
(160 C). The turnover frequencies (TOFs) were calculated
1669
based on the FA conversion of the starting reactions. As
presented in Table 1, the monometallic Au catalysts (Au/CeO2
and Au/SiO2) catalyze the dehydrogenation of FA with high
selectivity for CO2 and H2 (94.2-95.0%) but low TOFs (14-298 h-
1, entries 1 and 2 in Table 1). In contrast, the monometallic Pt
catalysts (Pt/CeO2 and Pt/C) exhibit higher activities (340-370
h-1) but lower selectivities (20.0-32.9%, entries 3 and 4 in Table
1) than those of the monometallic Au catalysts. The lower
selectivity is due to the side reaction of FA dehydration. When
the bimetallic Au-Pt/CeO2 is used as a catalyst, both high
activity (1637 h-1) and extraordinary selectivity (>99.0%, entry
6 in Table 1) are obtained. The bimetallic Au-Pd catalysts have
been reported to be highly efficient catalysts for the
conversion of FA to produce hydrogen;10,12 therefore, we also
studied the catalytic performances of CeO2-supported Au-Pd
(c)
1900 1800 1700 1600 1500
Wavenumber (cm-1
534
532
530
528
526
Binding Energy (eV)
)
Figure 3. (A) Raman spectra, (B) Ce3d and (C) O1s XP spectra of
the (a) as-synthesized Au-Pt/CeO2 and Au-Pt/CeO2 after FA
dehydrogenation treatment for (b) 0.5 h and (c) 1 h. (D)
Acetone-adsorption IR spectra of (a) CeO2, (b) Au-Pt/CeO2 and
(c) Au-Pt/CeO2 after FA dehydrogenation treatment for 1 h.
Figures 3B and C give Ce3d and O1s XP spectra of fresh
and FA-treated Au-Pt/CeO2. For the Ce3d spectra, the fresh
Au-Pt/CeO2 exhibits bands at 883.1, 898.4, 901.1, and 917.3 eV.
The doublets at 883.1 eV (Ce3d5/2) and 901.1 eV (Ce3d7/2) are
assigned to the primary photoemission from Ce(IV)-O, and the
doublets at 894.8 eV (Ce3d5/2) and 917.3 eV (Ce3d7/2) are from
the transfer of one or two electrons from a filled O2p orbital to
an empty Ce4f orbital. The treated Au-Pt/CeO2 has two
additional peaks at 885.2 and 902.6 eV, which are assigned to
the presence of Ce3+ species.16 This phenomenon indicates the
partial reduction of Ce4+ cations during FA dehydrogenation,
which is also evidenced by the O1s XP spectra of the sample.
The fresh Au-Pt/CeO2 gives two peaks at 529.4 and 531.6 eV in
the O1s XP spectra, which are associated with the lattice
oxygen and chemically adsorbed oxygen species on the
catalyst surface, respectively. After FA treatment, these peak
intensities are significantly decreased due to the reduction of
the catalyst surface, in good agreement with the results
obtained from the Ce3d XP and Raman spectra.
catalyst (Au-Pd/CeO2) in the dehydrogenation of FA at 160 °C.
As presented in Table 1 (entry 5), the Au-Pd/CeO2 gives good
selectivity to H2 with undetectable CO, but the TOF of Au-
Pd/CeO2 is much lower than that of Au-Pt/CeO2. These results
indicate the excellent performances of Au-Pt/CeO2 in
dehydrogenation of FA. Despite the development of
encouraging bimetallic catalysts for FA dehydrogenation in
recent years,10,12,13 it is still difficult to find an efficient
bimetallic catalyst for FA dehydrogenation at higher
temperature (e.g., 160
°C), where the relatively high
temperature is favourable to tandem HDO because these
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To recognize the interaction between C=O group with the
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In summary, an alternative process for the selective
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catalysts, FT-IR spectra of adsorbed molecules on the catalysts hydrogenolysis of C=O bonds using FA DaOsI:a10h.1y0d3r9o/Cge6CnCs0o9u59rc9eB
were studied using acetone as a probe molecule. Figure 3D over the bimetallic Au-Pt/CeO2 catalyst has been developed.
shows the C=O stretching band of the adsorbed acetone at The excellent performance of Au-Pt/CeO2 is related to the
1704 cm-1 for CeO2 and Au-Pt/CeO2. The treated Au-Pt/CeO2 catalyst synergy for both the dehydrogenation of FA to active
has a band at 1669 cm-1, with a shift of 35 cm-1 from the C=O hydrogen and the hydrogenolysis of C=O species. The strategy
stretching band on CeO2 (1704 cm-1). This red shift indicates in this work would be helpful for developing green processes
that the treated Au-Pt/CeO2 interacts more strongly with the for selective hydrogenolysis in the future.
C=O group than the fresh catalysts.17 Considering that the
This work is supported by the National Natural Science
reactions were conducted in water solvent, the adsorption of Foundation of China (91634201, 21403192 and 91645105).
acetone on various samples in the presence of water was also
studied (Figure S10), similar results were also obtained to the
water-free test. The strong interaction between C=O with the
Notes and references
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catalyst reasonably originates from the presence of oxygen
defects on the treated Au-Pt/CeO2,18 which facilitates the
cleavage of C=O bonds in HDO. After C=O cleavage, the oxygen
was removed by hydrogenation to form water, thus leading to
Au-Pd/CeO2 catalyst with oxygen defects, as proposed in
Scheme S1.
5
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We also tested the catalytic performance of Au-Pt/CeO2
for HDO using gaseous hydrogen instead of FA as a hydrogen
source, where more hydrogen molecule are used
(hydrogen/vanillin ratio at 36) than that using FA (FA/vanillin
ratio at 10). The vanillin conversion is 90%, which is obviously
lower than that (nearly 100%) using FA (Table S4). This result
indicates that FA has better capability than gaseous hydrogen
in the HDO reaction, which might be related to the higher
solubility of FA than gaseous hydrogen in a polar water solvent.
The hydrogen from FA dehydrogenation on the catalyst
surface could directly reduce vanillin, while the gaseous
hydrogen has to be dissolved into the catalytic system for the
reaction. Additionally, the MMP selectivity in the HDO with
gaseous hydrogen is only 41.1%, much lower than that with FA
(99.8%). Higher activities/selectivities in the HDO using FA than
gaseous hydrogen should are important for potential
application of FA.
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0
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4
Run
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8
Figure 4. (A) Catalytic data of Au-Pt/CeO2 for the HDO of
vanillin in the recyclability test.
Figure 4 shows the recyclability tests for the HDO of
vanillin with FA over Au-Pt/CeO2. The catalyst can be easily
recycled by filtration. Notably, the recyclable catalyst gives an
almost stable conversion of vanillin (65.5-68.4%) and a good
selectivity to MMP (>98.7%), indicating the excellent
recyclability of the Au-Pt/CeO2 catalyst. The carbon balances in
these reactions are greater than 98.5%. The leaching of Au and
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between the metal nanoparticles with CeO2.11b
4 | J. Name., 2012, 00, 1-3
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