Mechanistic Study of Hydrogen-Driven Deoxydehydration over Ceria-Supported Rhenium Catalyst Promoted by Au Nanoparticles

Article abstract of DOI:10.1021/acscatal.7b02879

The structure and activity of ReO_x-Au/CeO₂ catalysts for deoxydehydration (DODH) of polyols to alkenes with H₂ were investigated in detail. Based on X-ray diffraction (XRD) and transmission electron microscopy (TEM), the sizes of Au particles are in a similar scale to CeO₂ support particles and the number of Au particles is much smaller than that of CeO₂ support particles. Nevertheless, the catalytic activity and temperature-programmed reduction (TPR) data of the physical mixture of Re/CeO₂ and Au/CeO₂ indicate that Re species on all the CeO₂ particles can be reduced with H₂ and can work as a catalytic center. The H₂ activation ability of the catalyst with larger (～12 nm) Au particles (^impAu; imp means impregnation for loading) is lower than that with smaller (～3 nm) Au particles (^dpAu; dp means deposition-precipitation for loading), and the DODH reaction rate over ReO_x-^impAu/CeO₂ is limited by the H₂ activation rate. The CeO₂-supported ^dpAu particles have also higher activity in C=C hydrogenation, C=C migration, and cis/trans isomerization of diols. The C=C hydrogenation and C=C migration are side reactions in DODH, and in the DODH reaction of polyols, such as glycerol and erythritol, ReO_x-^impAu/CeO₂ shows higher yield of DODH products than ReO_x-^dpAu/CeO₂. In contrast, in the case of diols, the selectivity decrease by these side reactions is small, and ReO_x-^dpAu/CeO₂ is a better catalyst than ReO_x-^impAu/CeO₂ because of the higher activity. In addition, the cis/trans isomerization activity of ReO_x-^dpAu/CeO₂ enables the DODH of trans-1,2-cyclohexanediol, which is usually unreactive in DODH, into cyclohexene via isomerization to cis-1,2-cyclohexanediol and the subsequent DODH.

Full text of DOI:10.1021/acscatal.7b02879

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Article
Mechanistic Study of Hydrogen-driven Deoxydehydration over
Ceria-supported Rhenium Catalyst Promoted by Au Nanoparticles
Yoshinao Nakagawa, Shuhei Tazawa, Tianmiao Wang, Masazumi
Tamura, Norihito Hiyoshi, Kazu Okumura, and Keiichi Tomishige
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02879 • Publication Date (Web): 05 Dec 2017
Downloaded from http://pubs.acs.org on December 5, 2017
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Mechanistic Study of Hydrogen-driven Deoxydehydration over Ceria-supported Rhenium Catalyst
Promoted by Au Nanoparticles
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[a,b]
[a]
[a]
[a,b]
[c]
Yoshinao Nakagawa,* Shuhei Tazawa, Tianmiao Wang, Masazumi Tamura, Norihito Hiyoshi,
[d]
[a,b]
Kazu Okumura, Keiichi Tomishige*
[
a]
Department of Applied Chemistry, School of Engineering, Tohoku University, 6ꢀ6ꢀ07, Aoba,
Aramaki, Aobaꢀku, Sendai 980ꢀ8579, Japan
[b]
Research Center for Rare Metal and Green Innovation, Tohoku University, 468ꢀ1, Aoba, Aramaki,
Aobaꢀku, Sendai 980ꢀ0845, Japan
[c]
Research Institute for Chemical Process Technology, National Institute of Advanced Industrial
Science and Technology (AIST), 4ꢀ2ꢀ1 Nigatake, Miyaginoꢀku, Sendai 983ꢀ8551, Japan
[d]
Department of Applied Chemistry, Faculty of Engineering, Kogakuin University, 2665ꢀ1
Nakanoꢀmachi, Hachioji, Tokyo 192ꢀ0015, Japan
*Corresponding author: Yoshinao Nakagawa and Keiichi Tomishige
School of Engineering, Tohoku University,
6ꢀ6ꢀ07, Aoba, Aramaki, Aobaꢀku, Sendai, 980ꢀ8579, Japan
Eꢀmail: yoshinao@erec.che.tohoku.ac.jp, tomi@erec.che.tohoku.ac.jp
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Abstract
Structure and activity of ReO ꢀAu/CeO catalysts for deoxydehydration (DODH) of polyols to
x
2
alkenes with H were investigated in detail. Based on XRD and TEM, the size of Au particles is in a
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similar scale to CeO support particles, and the number of Au particles is much smaller than that of
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CeO support particles. Nevertheless, the catalytic activity and TPR data of the physical mixture of
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Re/CeO and Au/CeO indicate that Re species on all the CeO particles can be reduced with H and
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can work as a catalytic center. The H activation ability of the catalyst with larger (~12 nm) Au
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imp
particles ( Au; “imp” means impregnation for loading) is lower than that with smaller (~3 nm) Au
dp
particles ( Au; “dp” means depositionꢀprecipitation for loading), and the DODH reaction rate over
imp
dp
ReO ꢀ Au/CeO is limited by the H activation rate. The CeO ꢀsupported Au particles have also
x
2
2
2
higher activity in C=C hydrogenation, C=C migration and cis/trans isomerization of diols. The C=C
hydrogenation and C=C migration are side reactions in DODH, and in the DODH reaction of polyols,
imp
such as glycerol and erythritol, ReO ꢀ Au/CeO shows higher yield of DODH products than
x
2
dp
ReO ꢀ Au/CeO . In contrast, in the case of diols, the selectivity decrease by these side reactions is
x
2
dp
imp
small, and ReO ꢀ Au/CeO is better catalyst than ReO ꢀ Au/CeO because of the higher activity.
x
2
x
2
dp
In addition, the cis/trans isomerization activity of ReO ꢀ Au/CeO enables the DODH of
x
2
transꢀ1,2ꢀcyclohexanediol, which is usually unreactive in DODH, into cyclohexene via isomerization
to cisꢀ1,2ꢀcyclohexanediol and the subsequent DODH.
Keywords
Deoxydehydration, hydrogen spillover, heterogeneous catalysis, isomerization, rhenium
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Introduction
Applying a heterogeneous catalytic system to reactions that have been carried out with a
homogeneous catalytic system is important in view of industrialization.[1] One of the chemical
transformations that are desired to industrialize is the deoxygenation of polyꢀfunctionalized
compounds such as polyols to unsaturated alcohols and alkenes.[2] Polyols are typically derived
from sugars or vegetable oils in large scale, while unsaturated alcohols and alkenes are especially
important platform chemicals because of the reactivity toward versatile addition reactions.
Deoxygenation requires reductants, and the most inexpensive and greenest reductant in industry is
molecular hydrogen (H ); however, deoxygenation with H typically produces saturated compounds
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and selective production of unsaturated target compound is generally difficult.[3]
Deoxydehydration (DODH) is one kind of deoxygenation reactions and has received increasing
attention during the past few years (Scheme 1).[4] This reaction transforms cisꢀvicinal OH groups
into C=C bond accompanying the consumption of a reductant. Highꢀvalent Re catalysts have been
most investigated as an active center in DODH, while there are also Mo [5ꢀ7] and V [8,9] catalysts
for DODH. As variants of DODH, deepoxidation,[4b,10] reductive coupling of alcohols [11] and
monodeoxygenation of vicinal OH groups [12] have been also reported. The widely accepted
mechanism of DODH (Scheme 1) is that diolate complex with metal center reduced by two electrons
is transformed into product (alkene) and fully oxidized metal center. As reductants, alcohols have
been most used, while PPh ,[13,14] sulfite,[15] metal,[16] hydroaromatics,[17] and CO [8b] have
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been also used to give high yields of DODH products. One of the most typical DODH systems with
high selectivity to olefins and wide substrate scope is the combination of CH ReO catalyst and a
3
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secondary alcohol reductant reported by Toste et al.[18]
Development of heterogeneous catalysts using H reductant is one of the current topics in the
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research field of DODH. Jentoft, Nicholas and coꢀworkers reported a H ꢀdriven DODH over ReO ꢀC
2
x
as a heterogeneous catalyst in 2013.[19] However, there is room for improvement in the catalytic
performance, such as the low activity and the leaching of Re. After that, ReO /TiO was reported as a
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recyclable solid catalyst for DODH using 3ꢀoctanol as a reductant, but the yields of DODH products
were moderate (~50%).[20] Recently, HReO immobilized on polyꢀbenzylamine was reported to
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exhibit high selectivity in the DODH of tartaric acid, but the serious leaching of Re was
observed.[21] Moreover, the latter two DODH systems with solid catalysts were driven by a
secondary alcohol reductant. There are two issues in the use of H as reductant for DODH: (i) it is
2
difficult to keep high valence of supported Re species when H is used as reductant [22] and (ii) the
2
unsaturated product is easily hydrogenated.[14a,23]
We have recently discovered that CeO is an appropriate support for Re center for DODH with H2
2
as a reductant.[24ꢀ26] We first reported heterogeneous ReO ꢀPd/CeO catalyst for the simultaneous
x
2
removal of vicinal OH groups (DODH + hydrogenation) such as 1,4ꢀanhydroerythritol and glycerol
to tetrahydrofuran and 1ꢀpropanol, respectively.[24] ReO ꢀPd/CeO exhibited much higher activity
x
2
(TOF) than all the previously reported homogeneous and heterogeneous DODH catalysts, and very
high yields of the hydrogenated DODH products are obtained with H as a reductant. The supported
2
Re species are highly dispersed, and monomeric Re species with +4 or +6 valence, which is different
from the valence (+5 or +7) of homogeneous Re catalysts for DODH, are supposed to be the active
center.[25] Higher loading of Re decreases the number of monomeric Re species, decreasing TOF
and even weightꢀbased activity. The decrease of activity by excess loading, as well as the higher
activity than homogeneous catalysts, also supports the heterogeneous nature of ReO ꢀPd/CeO2
x
catalyst. The Pd species is present as metal clusters having low PdꢀPd coordination number (~3) on
the CeO surface, activating H for the promotion of the Re reduction and the hydrogenation of
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DODH products. To obtain DODH products without hydrogenation such as 2,5ꢀdihydrofuran and
allyl alcohol from 1,4ꢀanhydroerythritol and glycerol, respectively, we later found that Au particles
are very effective promoters, and ReO ꢀAu/CeO catalyst exhibited high yields of olefinic
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compounds in H ꢀdriven DODH.[26] We tested two preparation methods for Au loading:
2
depositionꢀprecipitation (dp) and impregnation (imp). Similarly to the numerous studies on Au
dp
imp
catalysts,[27] Au/CeO has smaller Au particles (~3 nm) than Au/CeO (~12 nm). Both the
2
2
Au/CeO catalysts have almost no activity in glycerol reduction with H . On the other hand,
2
2
ReO ꢀAu/CeO catalysts prepared by impregnating Au/CeO with Re (1 wt% Re, Au/Re = 0.3
x
2
2
mol/mol) have DODH activity of glycerol to allyl alcohol with H . Similarly to many Auꢀcatalyzed
2
dp
reactions including hydrogenation and H /D exchange,[27,28] ReO ꢀ Au/CeO with smaller Au
2
2
x
2
imp
particles showed higher activity than ReO ꢀ Au/CeO . However, because of the overreduction of
x
2
dp
allyl alcohol to 1ꢀpropanol over dispersed Au in ReO ꢀ Au/CeO , the final allyl alcohol yield (91%)
x
2
imp
was higher in the case of ReOxꢀ Au/CeO . In this system, the large size of Au particles, even larger
2
than CeO support particles, raises questions: how the activated H is transferred from Au (or
2
2
Auꢀsupport interface site)[29] to distant Re species? In this study, we discuss the details of
ReO ꢀAu/CeO catalysis especially in view of H activation. We found that the hydrogen species are
x
2
2
surely transferred from Au/CeO to Re species on different CeO particles. The difference in the
2
2
imp
dp
performance between ReO ꢀ Au/CeO and ReO ꢀ Au/CeO was investigated in detail, and in some
x
2
x
2
dp
cases ReO ꢀ Au/CeO is found to be better catalyst: DODH of reactive diol substrates such as
x
2
1,4ꢀanhydroerythritol and that of transꢀdiols which need isomerization before DODH.
Results and discussion
Structure model of the catalysts
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imp
dp
We used two ReO ꢀAu/CeO catalysts, ReO ꢀ Au/CeO and ReO ꢀ Au/CeO , the same catalysts
x
2
x
2
x
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in our previous report.[26] For both catalysts, Au/CeO was first prepared by impregnation
2
imp
dp
(
Au/CeO ) or depositionꢀprecipitation ( Au/CeO ), and then Re was loaded by impregnation. The
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loading amount of Au was set 0.32 wt%, since more amount of Au loading further increases the Au
imp
particle size in Au/CeO .[26] The Re amount was set at Au/Re=0.3 mol/mol, corresponding to Re
2
1
wt%, unless noted. The XRD patterns of ReO ꢀAu/CeO catalysts after use in glycerol DODH are
x 2
reprinted in Figure 1,[26] and now the particle sizes of the crystallites of each component including
imp
CeO are compared. As described in the Introduction, the Au particles in ReO ꢀ Au/CeO and
2
x
2
dp
ReO ꢀ Au/CeO have 12 and 3 nm sizes, respectively. The formation of Au particles with such sizes
x
2
has been also supported by Au L ꢀedge XAFS (coordination number of AuꢀAu shell = 11.6±1.7 and
2
imp
dp
8.9±1.5 in ReO ꢀ Au/CeO and ReO ꢀ Au/CeO , respectively),[26] and the presence of
x
2
x
2
XRDꢀinsensitive monomeric Au or small Au clusters is unlikely based on the coordination numbers.
imp
dp
The XRD patterns of Au/CeO and Au/CeO are shown in Figure S1, and the structures of Au
2
2
species are similar to those in Reꢀloaded catalysts. There are no peaks for Re species in both catalysts,
which supports the high dispersion of ReO . The CeO support has 8 nm size of crystallites in all
x
2
imp
samples. The Au particles in ReO ꢀ Au/CeO are clearly larger than CeO particles. The TEM
x
2
2
images and the EDX analysis results of ReO ꢀAu/CeO catalysts are shown in Figure 2.
x
2
dp
Unfortunately, very few Au particles were observed in both ReO ꢀ Au/CeO (Figure 2A) and
x
2
imp
ReO ꢀ Au/CeO (Figure 2B) due to the small contrast between CeO support and Au particles, and
x
2
2
it was not possible to count the Au particles with each particle size. Few large spherical Au particles
imp
were observed in ReO ꢀ Au/CeO (Figure 2B); however, the number of such large particles was
x
2
imp
much smaller than that in ReO ꢀ Au/CeO with higher Au loading amount (0.64 wt% Au; double
x
2
amount of standard catalyst) (Figure 2C). According to our previous paper, the catalyst with higher
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Au loading amount has large average Au particle size (dXRD = 25 nm),[26] which agreed with the
TEM image (Figure 2C). The much smaller number of large Au particles in standard
imp
ReO ꢀ Au/CeO (Figure 2B), even considering the lower loading amount, indicates that most Au
x
2
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species were not observed in the TEM image. The characterization of Re species was conducted with
XAFS. The Re L ꢀedge XANES spectra of ReO ꢀAu/CeO and reference compounds are shown in
3
x
2
Figure 3. The average Re valence was estimated by the relationship between the white line area and
the valences of reference compounds (the relationship is shown in Figure S2), and the estimated
imp
values are added to Figure 3. The average valence of the Re species in both ReO ꢀ Au/CeO and
x
2
dp
imp
dp
ReO ꢀ Au/CeO after calcination was +7.0, and that in ReO ꢀ Au/CeO and ReO ꢀ Au/CeO after
x
2
x
2
x
2
glycerol DODH was +5.9 and +5.3, respectively. These values are similar to the result of
ReO ꢀPd/CeO after 1,4ꢀanhydroerythritol reduction under similar conditions (average valence =
x
2
imp
5
.1).[25] The slightly higher valence of Re in ReOxꢀ Au/CeO may be related to the limited supply
2
of activated hydrogen, as discussed later. The curve fitting results of Re L ꢀedge EXAFS are
3
summarized in Table 1, and the details are shown in Figure S3 (spectra) and Table S1 (curve fitting
imp
dp
results). The results for ReO ꢀ Au/CeO and ReO ꢀ Au/CeO are similar to those for
x
2
x
2
ReO ꢀPd/CeO . These XAFS data suggest that the structure of Re species in ReO ꢀAu/CeO is
x
2
x
2
IV
VI
similar to that in ReO ꢀPd/CeO where Re and Re species are dispersed on CeO surface.[25]
x
2
2
Now we calculate the number of particles in the catalysts. Based on the weight ratio (CeO : Au =
2
ꢀ3
ꢀ3
99: 0.3), densities (
2
ρ(CeO ) = 7.22 g cm ; ρ(Au) = 19.3 g cm ) and particle sizes, the number ratios
of Au particles to CeO particles are calculated to be about 1 to 3000 and 1 to 40 in
2
imp
dp
ReO ꢀ Au/CeO and ReO ꢀ Au/CeO , respectively. Assuming that Re atoms in ReO ꢀAu/CeO are
x
2
x
2
x
2
randomly dispersed on the surface of CeO2 support, which is the proposed structure in
ReO ꢀPd/CeO ,[25] the average number of Re atoms on one CeO particle is calculated to be about
x
2
2
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
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3
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0. Some of Re species have monomeric structure with catalytic activity, and some of Re species
inevitably have polymeric structure because of the limitation of CeO surface sites (BET surface
2
2
ꢀ1
ꢀ1
n+
area: 84 m g ; 1.1 mmol g surface sites for Re adsorption, corresponding to 20 wt% loading of
Re).[25] The polymeric Re species are supposed to be inactive based on the stoichiometric DODH
reaction of reduced ReO ꢀPd/CeO .[25] According to our previous paper,[25] 75% of Re species in
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
x
2
ꢀ1
ReO (ꢀM)/CeO with 1 wt% Re (0.05 mmolꢀRe g ) are monomeric based on the randomlyꢀlocated
x
2
IV
model. The structure models of ReO ꢀAu/CeO catalysts in the reduced form (Re ) are shown in
x
2
IV
Figure 4. The active monomeric Re species is coordinated with substrate as diolate complex. This
VI
complex can release alkene product and the Re species becomes the oxidized form (Re ) without
move on the CeO surface. The catalytic cycle is completed by reꢀreduction of the oxidized form of
2
Re species and coordination of substrate (Scheme 1).
Most CeO support particles are located far from the Au particles in both ReO ꢀAu/CeO catalysts.
2
x
2
On the other hand, in ReO ꢀPd/CeO (Re 2 wt%, Pd/Re=0.25), the average number of Pd atoms on
x
2
one CeO particle is about 30. Considering the very low PdꢀPd coordination number (~3),[24,25] the
2
Pd particles are composed of few (<10) Pd atoms. Therefore, most CeO particles support Pd
2
particles as well as ReO species.
x
Reduction of Re catalyzed by distant Au particles: mixture of ReO /CeO and Au/CeO
2
x
2
It seems questionable that reduction of Re species with H is indeed catalyzed by distant Au
2
particles. Although EXAFS data do not suggest, is there the effect of very small XRDꢀinsensitive Au
species loaded on every Re/CeO particle? Although the leached amount of metal species from ICP
2
analysis (0.2% Re and Au; filtration was carried out after cooling)[26] was very small and can be
explained by limitation of filterable size, is there a possibility that dissolved Re species are involved
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in the reduction of Re species? To clarify the former point, we tested the catalysis of the mixture of
ReO /CeO + Au/CeO , and the results are shown in Table 2. The physical mixture of ReO /CeO +
x
2
2
x
2
imp
dp
Au/CeO (entry 5) and that of ReO /CeO + Au/CeO (entry 6) exhibited comparable activity to
2
x
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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4
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5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
imp
dp
ReO ꢀ Au/CeO (entry 2) and ReO ꢀ Au/CeO (entry 3), respectively. The effect of increasing
x
2
x
2
imp
CeO amount was checked by the mixture of ReO ꢀ Au/CeO + CeO (entry 4), and it is negligible.
2
x
2
2
The physical mixture of ReO /CeO + Pd/CeO also showed comparable activity to ReO ꢀPd/CeO
2
x
2
2
x
(entries 7 and 8). These data indicate that Re species located far from noble metal particles can work
as catalytic center.
We further carried out TPR measurements of the mixtures of ReO /CeO + Au/CeO to check that
x
2
2
reduction of Re species is indeed catalyzed by distant Au particles. The TPR experiments do not use
solvent, and therefore there is no dissolution and deposition of metal species during TPR. The results
are shown in Figure 5. Our previous paper already showed that ReO ꢀAu/CeO catalysts have
x
2
reduction peaks of Re species at lower temperature than ReO /CeO ,[26] and the data are also shown
x
2
for comparison. The TPR profiles of the physical mixtures of ReO /CeO + Au/CeO ((e) and (f) for
x
2
2
imp
dp
Au and Au, respectively) have lowꢀtemperature signals (420ꢀ710 K and 330ꢀ580 K, respectively)
at similar positions to those of the corresponding ReO ꢀAu/CeO ((c) and (d), respectively). These
x
2
lowꢀtemperature signals can be assigned to reduction of Re and some CeO . Only weak broad signal
2
was observed in the 600ꢀ800 K range profiles of ReO /CeO + Au/CeO , and the broad signal can be
x
2
2
assigned to CeO ꢀderived one because both CeO (a) and ReO /CeO (b) have such signal. In the
2
2
x
2
profile of ReO /CeO (b), sharper strong signal assignable to Re reduction (and some CeO ) was
x
2
2
observed in 600ꢀ680 K range overlapping the weak broad signal, while this sharp signal was absent
in the same range in ReO /CeO + Au/CeO and ReO ꢀAu/CeO ((c)~(f)). These data indicate that
x
2
2
x
2
Au/CeO can catalyze the reduction of almost all of the Re species in the physically mixed Re/CeO .
2
2
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The positions of the lowꢀtemperature signals are also similar to those of the corresponding Au/CeO2
imp
dp
((c), (e) and (g) for
Auꢀcontaining sample; (d), (f) and (h) for Auꢀcontaining sample). The
lowꢀtemperature signals in Au/CeO can be assigned to the reduction of surface Ce, and therefore Re
2
0
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0
and surface Ce are reduced almost simultaneously by hydrogen species supplied by Au.
Kinetic studies
The effect of H pressure in the DODH of 1,4ꢀanhydroerythritol over ReO ꢀAu/CeO catalysts was
2
x
2
investigated (Figure 6; the detailed data are shown in Figure S4). The reactivity of
,4ꢀanhydroerythritol in DODH is generally high,[14a,25,26] and therefore the difference in the rate
1
of Re reduction step can be more likely to affect the total reaction rate than the case of less reactive
substrates. In our previous report for ReO ꢀPd/CeO catalyst system,[25] the reaction order with
x
2
respect to H pressure was estimated to be 0 in the range of 0.5ꢀ8 MPa, indicating that Re reduction
2
step is faster than the latter step (release of alkene). On the other hand, the reaction rate over
imp
ReO ꢀ Au/CeO (Figure 6(a)) was increased with increasing H pressure in the range 1ꢀ8 MPa
x
2
2
dp
(
reaction order: +0.6). The reaction rate over ReO ꢀ Au/CeO showed saturation in the increase of
x 2
H pressure: The reaction rate was increased until P(H ) ~4 MPa and it became almost constant
2
2
above the pressure (Figure 6(b)). These results suggest that the rateꢀdetermination step under the
dp
standard conditions (H 8 MPa, 413 K) in ReO ꢀPd/CeO and ReO ꢀ Au/CeO system is the release
2
x
2
x
2
imp
of alkene, while that in the ReO ꢀ Au/CeO system is the reaction steps associated with hydrogen,
x
2
including the reduction of Re species. The promotional effect for the reduction of Re species
dp
imp
decreases in the order of Pd > Au/CeO > Au/CeO . The order between the two Au catalysts can
2
2
imp
be explained by the number of Au particles. The lower H activation ability of ReO ꢀ Au/CeO may
2
x
2
be related to the higher Re valence observed in Re L ꢀedge XANES analysis (Figure 3; average
3
1
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valence = 5.9).
The effect of substrate concentration in the DODH of 1,4ꢀanhydroerythritol over
imp
ReO ꢀ Au/CeO catalyst was also investigated (Figure 7; the detailed data are shown in Figure S5).
x
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
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9
0
1
2
3
4
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9
0
1
2
3
4
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
The ratio of substrate to catalyst was set constant (substrate/Re = 6×10 mol/mol) by only changing
the solvent amount. The increase of reaction rate was almost saturated around 10 wt% substrate
concentration. Therefore under standard conditions (20 wt% substrate) almost all the active Re
species are coordinated with the substrate molecule. The saturation curve is very similar to the
reduction of 1,4ꢀanhydroerythritol over ReO ꢀPd/CeO ,[25] which supports the idea that Re species
x
2
is the same between ReO ꢀPd/CeO and ReO ꢀAu/CeO .
x
2
x
2
Next, the reactivities of different substrates were compared (Figure 8; detailed data in Table S2).
The tested substrates were 1,4ꢀanhydroerythritol, glycerol and erythritol. The concentration was set
at 5.4 mmol in 4 g 1,4ꢀdioxane, which corresponds to 20 wt% for 1,4ꢀanhydroerythritol. Considering
the more number of vicinal OH groups in glycerol and erythritol molecules than
1,4ꢀanhydroerythritol, the substrate concentration is enough to reach saturation in the reaction rate.
The H pressure was set 8 MPa where the effect of H pressure is saturated for ReO ꢀPd/CeO and
2
2
x
2
dp
ReO ꢀ Au/CeO . The amount of Re was set constant for all runs. The conversions of glycerol,
x
2
imp
1,4ꢀanhydroerythritol and erythritol over ReO ꢀ Au/CeO were similar (Figure 8(I)), which agreed
x 2
with that the rateꢀdetermining step is the reduction of Re species as suggested by the reaction orders.
dp
On the other hand, the reaction rate over ReO ꢀ Au/CeO and ReO ꢀPd/CeO (Figure 8(II and III))
x
2
x
2
is dependent on the substrate, and the reactivity order is the same between these two catalysts:
1,4ꢀanhydroerythritol > erythritol > glycerol. These results agree with that the rateꢀdetermining step
dp
over ReO ꢀ Au/CeO catalyst is the release of alkene, and the relative rate represents the reactivity
x
2
dp
of substrate. Furthermore, the conversion over ReO ꢀ Au/CeO was about 1.5 times of the
x
2
1
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dp
conversion exhibited by ReO ꢀPd/CeO . The higher conversion over ReO ꢀ Au/CeO can be
x
2
x
2
explained by the higher ratio of monomeric Re species to total Re species caused by the lower
loading amount: 75% and 50% in 1 wt% and 2 wt% Re loading, respectively.[25] Thus, the reaction
rate per active Re is constant regardless of the promoter metal (Pd or Au) if the promoter metal can
sufficiently activates H2.
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0
Based on the results above, the remote promotion of Reꢀcatalyzed DODH on CeO support by Au
2
particles can be explained by the surface diffusion of active H species on CeO support. The H2
2
molecule is activated at some sites on Au particles such as defect sites and Au particleꢀCeO interface
2
sites.[29] The active H species is transferred onto CeO surface followed by the spillover onto the
2
surface of another CeO particle nonꢀcontacted with Au particle. The schematic representation is
2
shown in Figure 9. It is well known that spillover of H atoms actually occurs from metal particles to
various supports, such as reducible supports, carbon, and nonꢀreducible supports with defects.[30,31]
On reducible supports including Au/CeO , it is reported that the electron transfer process is most
2
reasonable where H atoms migrate from a metal particle to the support as the protons and electrons
on the surface and in the bulk of support, respectively.[32] Partial reduction of CeO in
2
ReO ꢀM/CeO with H is surely observed in TPR (Figure 5): the consumption amount of H in the
x
2
2
2
lowꢀtemperature peak is significantly larger than the reduction of Re (valence change: +7 → +4).
Water molecules generated by the reduction of CeO or the OH group in substrates may help the
2
move of proton on CeO surface. The formed Re species with +4 valance will reduce the coordinated
2
substrate, releasing alkene product and regenerating oxidized Re species with +6 valence. As
discussed above, the rateꢀdetermining step is different between catalysts: the reduction of Re species
dp
is faster than the release of alkene in the case of ReOxꢀ Au/CeO , while the order is opposite in the
2
imp
case of ReO ꢀ Au/CeO .
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Comparison of the performance between ReO -Au/CeO catalysts
x
2
imp
In our previous paper we described that ReO ꢀ Au/CeO gives higher yield of allyl alcohol by
x
2
1
1
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0
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0
dp
glycerol DODH than ReO ꢀ Au/CeO .[25] However, because of the limiting H activation ability of
x
2
2
imp
dp
Au/CeO , the activity is lower than that of ReO ꢀ Au/CeO . Now we extend the substrate scope
2
x
2
dp
and check whether or not there are substrates that can utilize the higher activity of ReO ꢀ Au/CeO
x
2
without loss of yield. The results are shown in Table 3. For polyol substrates with three or more
imp
number of OH groups (entries 1ꢀ6), ReO ꢀ Au/CeO catalyst clearly showed higher selectivity to
x
2
dp
the corresponding alkene. The byꢀproducts over ReO ꢀ Au/CeO catalyst are mainly DODH +
x
2
hydrogenation products such as 1ꢀpropanol from glycerol, butanediols from erythritol, and 1ꢀbutanol
from 1,2,4ꢀbutanetriol. In addition, isomerization of the DODH product via C=C migration also
dp
occurred over ReO ꢀ Au/CeO , giving 2ꢀbutenꢀ1ꢀol from 1,2,4ꢀbutanediol. On the other hand, in the
x
2
cases of diol substrates, the selectivities to the DODH products are similar between
imp
dp
dp
ReO ꢀ Au/CeO and ReO ꢀ Au/CeO . Considering the higher activity, ReO ꢀ Au/CeO is the
x
2
x
2
x
2
better catalyst for diol substrates. A noteworthy result was obtained for transꢀ1,2ꢀcyclohexanediol
reduction. In contrast to general DODH reactions that cannot convert transꢀdiols,[4]
dp
ReO ꢀ Au/CeO reduced transꢀ1,2ꢀcyclohexanediol to cyclohexene in good yield (90%).
x
2
Reactivity of DODH products
For further understanding of the selectivity, the reaction of DODH products themselves was
investigated for ReO ꢀAu/CeO and Au/CeO catalysts (Table 4). An important point is that there are
x
2
2
two paths of hydrogenation of unsaturated alcohols (Scheme 2): (a) direct hydrogenation of C=C
bond and (b) combination of C=C migration to enol, ketoꢀenol tautomerization and hydrogenation of
1
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C=O (indirect hydrogenation). In the case of allyl alcohol reduction (Table 4, entries 1ꢀ4), the
dp
dp
imp
imp
activity was in the order of ReO ꢀ Au/CeO ≃ Au/CeO > ReO ꢀ Au/CeO > Au/CeO . The
x
2
2
x
2
2
main product was 1ꢀpropanol for all cases. Significant amount of propanal, which is an intermediate
0
1
2
3
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9
0
1
2
3
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0
imp
of indirect hydrogenation of allyl alcohol, was formed over Auꢀcontaining catalysts. The aldol
condensation product of propanal, namely 2ꢀmethylꢀ2ꢀpentenal, was also detected. C3 hydrocarbons,
which are dehydration or overdeoxygenation products of 1ꢀpropanol, were also formed over
dp
dp
Reꢀcontaining catalysts. The similar high activity of ReO ꢀ Au/CeO and Au/CeO catalysts
x
2
2
dp
indicates that Au is active in hydrogenation of allyl alcohol. The activity in hydrogenation of allyl
dp
alcohol agrees with the low selectivity of glycerol DODH to allyl alcohol over ReO ꢀ Au/CeO . In
x
2
view of the reaction path of 1ꢀpropanol formation (direct, indirect or both), the indirect route is likely
to be involved because propanal was actually formed, although the involvement of the direct route
cannot be excluded. The hydrogenation of 1ꢀoctene was carried out (Table 4, entries 5 and 6), where
indirect hydrogenation involving C=O bond is impossible. Both the hydrogenation to nꢀoctane and
dp
imp
the C=C migration to 2ꢀoctene proceeded over ReO ꢀ Au/CeO , while ReO ꢀ Au/CeO showed
x
2
x
2
dp
almost no activity. Therefore both direct and indirect hydrogenations are catalyzed by Au/CeO2.
The reduction rates of unsaturated compounds in entries 2, 4 and 6 are relatively high, in comparison
with the DODH reactions (Table 3). The reduction of unsaturated compounds seems to be slowed
down during DODH. To confirm this point, reaction of the mixture of allyl alcohol and
1,4ꢀanhydroerythritol was carried out (Table 4, entries 7 and 8). The conversion of allyl alcohol was
dp
dp
decreased from the case without 1,4ꢀanhydroerythritol over both ReO ꢀ Au/CeO and Au/CeO2
x
2
dp
(
11% from 64% (entry 2), and 35% from 61% (entry 4), respectively). In Au/CeO ꢀcatalyzed
2
reaction (entry 8) where DODH of 1,4ꢀanhydroerythritol was negligible, the conversion decrease of
allyl alcohol (61% → 35%) is probably caused by the obstruction of the adsorption of allyl alcohol
1
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onto catalyst ( Au) by the competitive adsorption of 1,4ꢀanhydroerytheritol. In
dp
ReO ꢀ Au/CeO ꢀcatalyzed reaction, the conversion of allyl alcohol was more decreased (64% →
x
2
1
1%). This result can be interpreted by that a part of active hydrogen species were consumed for
1
1
1
1
1
1
1
1
1
1
2
2
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2
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0
1
2
3
4
5
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7
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9
0
1
2
3
4
5
6
7
8
9
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
,4ꢀanhydroerythritol DODH instead of allyl alcohol hydrogenation. As shown in Table 3, selectivity
dp
imp
of diol DODH is similar between ReO ꢀ Au/CeO and ReO ꢀ Au/CeO . Two factors may be
x
2
x
2
dp
involved in the suppression of side reactions in ReO ꢀ Au/CeO : competitive adsorption between
x
2
diol and alkene and competitive use of active hydrogen between DODH and side reactions.
Reductions of 3ꢀbutenꢀ1ꢀol and 2,5ꢀdihydrofuran were also carried out (Table 4, entries 9ꢀ12).
dp
Similarly to the case of allyl alcohol reduction, Au/CeO showed much higher activity in both
2
imp
hydrogenation and C=C migration than Au/CeO . The reaction of 2,5ꢀdihydrofuran was also tested
2
dp
with Au/CeO under Ar atmosphere (Table 4, entry 13). The C=C migration as well as
2
hydrogenation did not proceed at all, indicating that the C=C migration requires active hydrogen
species, probably via the mechanism that addition of active hydrogen species to C=C and subsequent
abstraction of hydrogen atom (Scheme 3).
The reaction of 1,2ꢀcyclohexanediol over Au/CeO catalysts was investigated in order to realize
2
the transꢀdiol DODH (Table 5). The trans/cis isomerization of 1,2ꢀcyclohexanediol readily
dp
proceeded over Au/CeO (Table 5, entries 1 and 2) and the equilibrium was easily reached (Table 5,
2
entries 3 and 4; the same trans/cis ratio from both trans and cis alone). The isomerization also
proceeded under Ar atmosphere (Table 5, entry 5), in contrast to the case of C=C migration of
2
1
,5ꢀdihydrofuran (Table 4, entry 13). 2ꢀHydroxycyclohexanone, the dehydrogenation product of
,2ꢀcyclohexanediol, was detected in the reaction under Ar atmosphere. Based on these data, the
reaction route of transꢀ1,2ꢀcyclohexanediol DODH can be shown in Scheme 4: Dehydrogenation and
dp
hydrogenation
of
transꢀ1,2ꢀcyclohexanediol
over
Au
site
reversibly
produces
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cisꢀ1,2ꢀcyclohexanediol, and then DODH of cisꢀ1,2ꢀcyclohexanediol over Auꢀpromoted ReO /CeO2
x
produces cyclohexene. The consumption of cisꢀ1,2ꢀcyclohexanediol by DODH shifts the trans/cis
equilibrium to the cis side, and finally transꢀ1,2ꢀcyclohexanediol is totally converted into
0
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imp
cyclohexene. The isomerization activity of Au/CeO was very low (Table 5, entry 1), which agrees
2
imp
with the low activity of ReO ꢀ Au/CeO in transꢀdiol DODH.
x
2
Conclusions
ReO ꢀAu/CeO catalysts which are active in deoxydehydration (DODH) with H as reductant
x
2
2
contain much fewer Au particles than CeO support particles. Au particles can promote the reduction
2
of Re species on CeO support even when there are many other CeO particles located between Au
2
2
and ReO /CeO , probably via diffusion of proton and electron on CeO particles. CeO ꢀsupported Au
x
2
2
2
dp
particles prepared by depositionꢀprecipitation ( Au/CeO ) have smaller size and higher activation
2
imp
ability of H2 than those prepared by impregnation ( Au/CeO ). The DODH activity of
2
dp
ReO ꢀ Au/CeO per active Re site is similar to that of ReO ꢀPd/CeO where Pd clusters as well as
x
2
x
2
imp
ReO are present on most CeO particles. The activity of ReO ꢀ Au/CeO in DODH is lower than
x
2
x
2
dp
ReO ꢀ Au/CeO and is limited by the rate of H activation. On the other hand, the rates of other
x
2
2
dp
reactions than DODH are higher in ReO ꢀ Au/CeO : C=C hydrogenation, C=C migration and
x
2
cis/trans isomerization. The former two reactions are sideꢀreactions in DODH, and the high activity
dp
of ReO ꢀ Au/CeO in these two reactions decreases the selectivity to the target alkene product in
x
2
polyol DODH. These two reactions of C=C compounds are suppressed to some extent in the
presence of diol/polyol substrate because of competitive adsorption and competitive consumption of
active hydrogen species. In diol DODH, these side reactions are suppressed enough to obtain similar
dp
imp
yield of the target alkene with ReO ꢀ Au/CeO to that with ReO ꢀ Au/CeO . The cis/trans
x
2
x
2
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isomerization can be useful: it enables transꢀdiol DODH which is difficult with literature systems of
Reꢀcatalyzed DODH such as those with homogeneous Re complex catalysts. Therefore,
dp
imp
ReO ꢀ Au/CeO is better DODH catalyst for simple diols, while ReO ꢀ Au/CeO is better for
x
2
x
2
1
1
1
1
1
1
1
1
1
1
2
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0
polyols with three or more OH groups and unsaturated diols.
Experimental section
Catalyst preparation. The catalysts were prepared with the same procedure as our previous
imp
works.[24ꢀ26] ReO /CeO , Au/CeO and Pd/CeO catalysts were prepared by impregnating CeO
2
x
2
2
2
2
ꢀ1
powder (Daiichi Kigenso Co., Ltd., HS, BET surface area: 84 m g , after calcination at 873 K for 3
imp
h) with an aqueous solution of NH ReO , HAuCl and Pd(NO ) . ReO ꢀ Au/CeO catalyst was
4
4
4
3 2
x
2
imp
prepared by impregnation of 383 Kꢀdried
Au/CeO with an aqueous solution of NH ReO .
2 4 4
dp
Au/CeO catalyst was prepared by slow addition of aqueous solutions of HAuCl for 4 h at 353 K
2
4
with pH adjustment to 8 by addition of aqueous ammonia. The catalysts were dried at 383 K for 12 h
dp
and then calcined in air at 673 K for 4 h. ReO ꢀ Au/CeO catalyst was prepared by impregnating
x
2
dp
calcined Au/CeO with aqueous solution of NH ReO , followed by further drying and calcination
2
4
4
at 673 K.
Activity test. Activity tests were performed in a 190 mL stainless steel autoclave. Substrate,
internal standard (dodecane) and solvent (1,4ꢀdioxane) was placed into the autoclave containing an
inner glass cylinder with a spinner and appropriate amount of catalysts. After sealing the reactor, the
air content was purged by flushing three times with H , and H pressure was increased to appropriate
2
2
value (typically 5.7 MPa, for 8.0 MPa at 413 K). The autoclave was then heated to the reaction
temperature. The temperature was monitored using a thermocouple inserted in the autoclave.
Counting the reaction time started when the temperature reached the set value. The heating took
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about 40 min. During the experiment, the stirring rate was fixed at 250 rpm (magnetic stirring). After
an appropriate reaction time, the autoclave was cooled down with running water. The gases were
collected in a gas bag. The autoclave contents and the washing solvent (methanol or 1,4ꢀdioxane, 15
mL) were transferred to vials, and the catalyst was separated by centrifugation and filtration.
The reaction conditions for the various spectroscopic measurements of used catalyst were as
follows: 0.5 g (5.4 mmol) glycerol, 300 mg catalyst, 0.1 g dodecane, 4 g 1,4ꢀdioxane, 413 K reaction
temperature, 8 MPa initial hydrogen pressure (at reaction temperature) and 2 h reaction time. The
reaction conditions of activity tests are described in each result. In the reaction with a physical
mixture of catalysts, the catalysts were sequentially placed into a glass cylinder without preꢀmixing
of catalysts.
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Product analysis. The products in both gas and liquid phases were analyzed using a gas
chromatograph (Shimadzu GCꢀ2025) equipped with FID and an HPLC (Shimadzu LCꢀ10A)
equipped with a refractive index detector. A TCꢀWAX capillary column (diameter 0.25 mm, 30 m)
and an Aminex HPXꢀ87C column (diameter 7.8 mm, 300 mm) were used for GC and HPLC,
respectively. For the reaction of erythritol, 1,2,4ꢀbutanetriol and 3ꢀbutenꢀ1ꢀol, a GC equipped with
®
Rtx ꢀ1 PONA capillary column (diameter 0.25 mm, 100 m) was used for the gas phase analysis.
Products were also identified using GCꢀMS (QP5050, Shimadzu). The conversion and the
selectivities were calculated on the carbonꢀbasis. The mass balance was also confirmed in each result
and the difference in mass balance was always in the range of the experimental error (±10% except
in the cases of erythritol at almost full conversion where the mass balance was lower (87% in the
worst case) probably because of the difficulty in capturing all the C4 hydrocarbons).
Catalyst characterization. The surface areas of the catalysts were measured using BET method
(
N adsorption) with an apparatus (Micromeritics Gemini). Powder Xꢀray diffraction (XRD) patterns
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were recorded by a diffractometer (Rigaku MiniFlex600) with Cu Kα radiation. When Scherrer’s
equation was applied to the Au peak, the XRD peaks of CeO support were removed using the
2
pattern of CeO powder after calcination at 873 K for 3 h, and then the Au (fcc) peak at 38.2° in the
2
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obtained patterns was fitted by a Gaussian function. Scanning transmission electron microscope
(STEM) images were taken by JEOLARMꢀ200F with Cu grids.
Temperatureꢀprogrammed reduction (H ꢀTPR) was carried out in a fixedꢀbed reactor equipped
2
with a thermal conductivity detector (TCD) using 5% H diluted with Ar (30 mL/min). The amount
2
of catalyst or physical mixture of catalysts was 100 mg, and temperature was increased from room
temperature to 1173 K at a heating rate of 10 K/min. The sample of physical mixture was prepared
by mixing catalysts in an agate mortar with an agate pestle for 15 min.
The Xꢀray absorption fine structure (EXAFS and XANES) spectra were measured at the BL01B1
station at SPringꢀ8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI;
Proposal No. 2016A1352). The storage ring was operated at 8 GeV, and a Si(111) single crystal was
used to obtain a monochromatic Xꢀray beam. The spectra of reference compounds (Re powder, ReO2,
ReO and NH ReO ) were obtained in a transmission mode. Ion chambers for I and I were filled
3
4
4
0
with 100 % N and 30% Ar + 70% N , respectively. The edge jumps were in the range of 0.1ꢀ1. The
2
2
catalyst samples were prepared after catalytic use by transferring the used catalyst powder to a
measurement cell after reaction and cooling in a gas bag filled N to avoid exposure to air. The data
2
were collected in a fluorescence mode using a 19ꢀelement Ge solidꢀstate detector (19ꢀSSD). An ion
chamber for I was filled with 100 % N . The edge jumps were in the range of 0.0022ꢀ0.0024. For
0
2
EXAFS analysis, the oscillation was first extracted from the EXAFS data using a spline method [33].
3
Fourier transformation of the k ꢀweighted EXAFS oscillation from the k space to r space was carried
out to obtain a radial distribution function. The inversely Fourier filtered data were analyzed using a
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usual curveꢀfitting method [34]. For curveꢀfitting analysis, the empirical phase shift and amplitude
functions for the Au–Au and Re–O bonds were extracted from data for Au metal and NH ReO ,
4
4
respectively. Analyses of EXAFS data were carried out using a computer program REX2000, ver. 2.6
Rigaku Corp.). Error bars for each parameter, while optimizing the others parameter, until the
0
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(
residual factor becomes 2 times its minimum value [35]. In the analysis of XANES spectra, the
normalized spectra were obtained by subtracting the preꢀedge background from the raw data with a
modified Victoreen equation and normalizing them by the edge height [36ꢀ38].
Associated content
Supporting information: XRD patterns of Au/CeO (Figure S1), detailed data of XANES (Figure
2
S2), EXAFS (Figure S3 and Table S1), kinetics (Figures S4 and S5) and activity comparison between
catalysts (Table S2).
Acknowledgements
This work is in part supported by “ALCA” program of Japan Science and Technology Agency.
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
(
K. Green Chem. 2013, 15, 2636ꢀ2654.
(29) (a) T. Fujitani, I. Nakamura, T. Akita, M. Okumura, M. Haruta, Angew. Chem. Int. Ed. 2009, 48,
9515ꢀ9518. (b) M. Pan, A. J. Brush, Z. D. Pozun, H. C. Ham, W.ꢀY. Yu, G. Henkelman, G. S. Hwang,
C. B. Mullins, Chem. Soc. Rev. 2013, 42, 5002ꢀ5013.
(30) (a) Conner, W. C.; Falconer, J. L. Chem. Rev. 1995, 95, 759ꢀ788. (b) Prins, R. Chem. Rev. 2012,
1
12, 2714ꢀ2738.
31) (a) Bando, K.; Ichikuni, N.; Soga, K.; Kunimori, K.; Arakawa, H.; Asakura, K. J. Catal. 2000,
94, 91ꢀ104. (b) Tomishige, K.; Okabe, A.; Fujimoto, K. Appl. Catal. A 2000, 194-195, 383ꢀ393. (c)
Yoshida, K.; Begum, N.; Ito, S.; Tomishige, K. Appl. Catal. A 2009, 358, 186ꢀ192.
32) (a) Panayotov, D. A.; Yates, J. T. J. Phys. Chem. C 2007, 111, 2959ꢀ2964. (b) Juárez, R.; Parker,
(
1
(
S. F.; Concepción, P.; Corma, A.; Garcia, H. Chem. Sci. 2010, 1, 731ꢀ738. (c) Panayotov, D. A.;
Burrows, S. P.; Yates, J. T.; Morris, J. R. J. Phys. Chem. C 2011, 115, 22400ꢀ22408.
(33) Cook, J. W.; Sayers, D. E. J. Appl. Phys. 1981, 52, 5024ꢀ5031.
(
34) (a) Okumura, K.; Amano, J.; Yasunobu, N.; Niwa, M. J. Phys. Chem. B 2000, 104, 1050ꢀ1057.
b) Okumura, K.; Matsumoto, S.; Nishiaki, N.; Niwa, M. Appl. Catal. B 2003, 40, 151ꢀ159.
(
(35) Tomishige, K.; Asakura, K.; Iwasawa, Y. J. Catal. 1994, 149, 70ꢀ80.
36) (a) Kubota, T.; Asakura, K.; Ichikuni, N.; Iwasawa, Y. Chem. Phys. Lett. 1996, 256, 445ꢀ448. (b)
(
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Kubota, T.; Asakura, K.; Iwasawa, Y. Catal. Lett. 1997, 46, 141ꢀ144.
(37) Mansour, A. N.; Cook, J. W., Jr.; Sayers, D. E. J. Phys. Chem. 1984, 88, 2330ꢀ2334.
(38) Horsley, J. A. J. Chem. Phys. 1982, 76, 1451ꢀ1458.
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Table 1. Curve fitting Results of Re L
3
x 2
ꢀedge EXAFS of ReO ꢀAu/CeO (1 wt% Re, Au/Re = 0.3) and
ReO
x
ꢀPd/CeO
2
(2 wt% Re, Pd/Re = 0.25)
Condition
a
ꢀ1
b
Sample
shells
CN
R / 10 nm
imp
dp
ReO
x
x
ꢀ
Au/CeO
2
Calcined
Calcined
Calcined
Re=O
Re=O
Re=O
Re=O
Re–O
Re=O
Re–O
Re=O
Re–O
Re–Pd
Re=O
4.0
1.74
1.74
1.74
1.74
1.96
1.74
1.96
1.74
1.94
2.66
1.73
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ReO
ꢀ Au/CeO
2
4.0
4.0
1.9
4.2
1.7
4.0
1.3
4.2
0.4
4
c
ReO ꢀPd/CeO2
x
imp
d
ReO
ReO
ReO
x
x
x
ꢀ
Au/CeO
2
Used
dp
d
ꢀ Au/CeO
2
Used
c
e
ꢀPd/CeO
2
Used
f
4 4
NH ReO
a
b
c
d
CN = Coordination number. R = Bond distance. Ref. 25. Reaction conditions: glycerol 0.5 g, catalyst 0.3
e
g, 1,4ꢀdioxane 4 g, H
,4ꢀdioxane 4 g, H
Kꢀedge EXAFS.
2
8 MPa, 413 K, 2 h. Reaction conditions: 1,4ꢀanhydroerythritol 1 g, catalyst 0.15 g,
f
1
2
8 MPa, 413 K, 4 h. This shell is added based on the PdꢀRe coordination number in Pd
2
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Table 2. Reduction of glycerol over the physical mixture of various catalysts
Entry Catalyst
Conv. Selectivity / %
Allyl
1ꢀPrOH 2ꢀPrOH 1,2ꢀPrD 1,3ꢀPrD Propanal Others
/ %
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
alcohol
1
2
3
4
ReO
x
/CeO
imp
2
6
57
72
54
90
2
3
<1
<1
<1
<1
<1
<1
4
<1
<1
<1
<1
3
1
1
3
3
1
2
2
ReOxꢀ Au/CeO2
95
62
91
dp
ReO
x
ꢀ Au/CeO
2
30
3
imp
ReOxꢀ Au/CeO + CeO2
1
2
imp
5
6
7
ReO
ReO
ReO
x
x
x
/CeO
2
2
+
Au/CeO
2
38
81
37
43
95
83
<1
<1
1
12
83
86
<1
<1
<1
<1
1
2
<1
<1
<1
<1
1
1
2
2
2
1
dp
/CeO
+ Au/CeO
2
b
ꢀPd/CeO
2
15
13
<1
<1
b
8
ReO /CeO + Pd/CeO
x 2 2
a
Glycerol (0.5 g), 1,4ꢀdioxane (4 g), Wcat = 0.3 g or 0.3 + 0.3 g (30 mg Re, Au/Re = 0.3), H (8 MPa), 413 K,
2
b
2
4 h. PrOH = propanol, PrD = propanediol. Wcat = 0.15 g or 0.15 + 0.15 g (30 mg Re, Pd/Re = 0.25).
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Table 3. Reduction of diols and polyols over ReO
x
ꢀAu/CeO
t / Conv. Products (Selectivity / %)
/ %
2
ꢀ Au/CeO 52 >99
2
catalysts
Entry Substrate
Catalyst
h
b
imp
1
ReO
ReO
ReO
x
x
x
(
91),
(5),
(1),
(1),
Glycerol
(
<1), hydrocarbons (1), others (2)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
dp
2
3
ꢀ Au/CeO
2
24
60
72
97
(62),
5), hydrocarbons (1), others (1)
83), butenes (3),
(30),
(
b
imp
ꢀ
Au/CeO
2
(
(7),
(1),
(
<1),
(1),
Erythritol
monoꢀols (1), C≤3 hydrocarbons (<1), others (3)
dp
4
ReO
x
ꢀ Au/CeO
2
16
94
(
57), butenes (5),
(9),
(8),
(9),
(5),
monoꢀols (3), C≤3 hydrocarbons (<1), others (3)
b
OH
imp
5
ReO
x
x
ꢀ
Au/CeO
2
72
24
80
65
OH
HO
(80),
6),
(7),
(4),
(7),
1,2,4ꢀButanetr
iol
(
(1), others (1)
dp
6
ReO
ꢀ Au/CeO
2
(
30),
(19),
(38),
(2), others (5)
b
imp
7
ReO
x
x
ꢀ
Au/CeO
2
24
2
93
(
(
86),
86),
(9),
(3), others (2)
(3), others (1)
dp
8
ReO
ꢀ Au/CeO
2
>99
1
,4ꢀAnhydroer
ythritol
(10),
imp
9
1,2ꢀOctanedio ReO
x
ꢀ
Au/CeO
2
48
4
42 1ꢀOctene (76), nꢀoctane (8), others (16)
dp
10
l
ReO ꢀ Au/CeO
74 1ꢀOctene (84), 2ꢀoctenes (9), nꢀoctane (7), others (<1)
x
2
b,c
imp
11
ReO
x
ꢀ
Au/CeO
2
8
91
(94),
(1),
(2), others (3)
(8), others (11)
cisꢀ1,2ꢀcycloh
c
dp
1
2
ReO
x
ꢀ Au/CeO
2
2
8
>99
9
exanediol
(
92),
(6), others (2)
c
c
imp
13
ReOxꢀ Au/CeO2
(
(
81),
92),
(<1),
transꢀ1,2ꢀcycl
dp
14
ReO ꢀ Au/CeO
8
98
x
2
ohexanediol
(7), others (1)
a
b
c
Substrate (0.5 g), 1,4ꢀdioxane (4 g), Wcat = 0.3 g (1 wt% Re, Au/Re = 0.3), H
2
(8 MPa), 413 K. Ref. 26.
Substrate (0.25 g), 443 K.
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Table 4. Reduction of unsaturated compounds over ReO ꢀAu/CeO and Au/CeO catalysts
x
2
2
Entry Substrate
Catalyst
Conv. Products (Selectivity / %)
/
%
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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8
9
0
1
2
3
4
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9
0
1
2
3
4
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6
7
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9
0
imp
1
ReO
x
ꢀ
Au/CeO
2
14
64
3
(
54),
(20),
(1),
(<1),
(2),
Allyl alcohol
C3 hydrocarbons (21), others (4)
dp
2
3
4
ReO
x
ꢀ Au/CeO
2
(73),
(1),
C3 hydrocarbons (24), others (2)
imp
Au/CeO
2
(
74),
(18),
C3 hydrocarbons (<1), others (6)
dp
Au/CeO
2
61
(
94),
(2),
(<1),
C3 hydrocarbons (1), others (3)
imp
dp
5
6
7
1ꢀOctene
Au/CeO
2
1
nꢀOctane (>60)
Au/CeO
2
47
11
nꢀOctane (21), transꢀ2ꢀoctene (52), cisꢀ2ꢀoctene (27)
dp
+
x 2
ReO ꢀ Au/CeO
(
52),
(42),
(2),
b
[
37]
b
C3 hydrocarbons (6), others (<1), [
(97)]
dp
8
9
Au/CeO
2
35
[1/3 equiv.]
(
57),
(29),
(7),
b
[
<3]
C3 hydrocarbons (1), others (6)
imp
Au/CeO
2
5
(73),
(16),
(6),
(6),
3
ꢀButenꢀ1ꢀol
(
(
2),
(2), others (<1)
dp
10
Au/CeO
2
59
(
70),
(<1), others (1)
(23),
<1),
imp
dp
1
1
2
3
Au/CeO
2
3
(
34),
(66), Others (<1)
(47), Others (4)
2,5ꢀDihydrofuran
1
Au/CeO
2
68
<1
(49),
c
dp
1
Au/CeO
2
ꢀ
a
2
Substrate (5.4 mmol), 1,4ꢀdioxane (4 g), Wcat = 0.3 g (0 or 1 wt% Re, 0.32 wt% Au (Au/Re = 0.3)), H (8
b
c
MPa), 413 K, 1 h. Based on 1,4ꢀanhydroerythritol. The catalyst was preꢀreduced in 1,4ꢀdioxane under 8
MPa H at 413 K for 0.5 h with stirring, and the reaction was conducted under Ar atmosphere (1 MPa).
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Table 5. Reaction of transꢀ1,2ꢀcyclohexanediol over Au/CeO2
Entry Catalyst
Wcat t / Conv. Selectivity / %
Others
/ g / %
h
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
imp
1
2
3
4
Au/CeO
2
0.15
1
1
2
2
1
3
19
33
62
21
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
90
7
<1
<1
<1
24
3
dp
dp
dp
dp
Au/CeO
Au/CeO
Au/CeO
2
2
2
0.15
0.3
99
99
1
1
1
1
b
c
0.3
99 (trans)
75
5
Au/CeO2
0.15
a
b
Substrate (0.25 g), 1,4ꢀdioxane (4 g), catalyst (0.32 wt% Au), H
2
(8 MPa), 443 K.
Substrate:
c
cisꢀ1,2ꢀcyclohexanediol. The catalyst was preꢀreduced in 1,4ꢀdioxane under 8 MPa H
2
at 413 K for 0.5 h
with stirring, and the reaction was conducted under Ar atmosphere (1 MPa).
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