Supported rhenium nanoparticle catalysts for acceptorless dehydrogenation of alcohols: Structure-activity relationship and mechanistic studies

Article abstract of DOI:10.1039/c6cy00252h

Al₂O₃-supported Re with different oxidation states and Re⁰ metal nanoparticles on various supports are prepared, characterized and tested for the dehydrogenation of 2-octanol. The activity of Re/Al₂O₃ increases with the fraction of metallic Re. The activity of metallic Re depends on the support oxides, and the support with moderate electronegativity (Al₂O₃) gives the highest turnover frequency (TOF) per surface Re⁰ site. Re/Al₂O₃ is effective for acceptorless dehydrogenation of various aliphatic secondary alcohols to ketones. The kinetic isotope effects on the dehydrogenation of 2-propanol show that dissociation of the α-C-H bond of 2-propanol is the rate-limiting step. The IR study of the reaction of gas phase 2-propanol over the Re/Al₂O₃ surface shows that the acid-base pair site of Al₂O₃ is responsible for the O-H dissociation of 2-propanol. The structural requirements are discussed on the basis of the mechanistic results.

Full text of DOI:10.1039/c6cy00252h

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Supported rhenium nanoparticle catalysts for acceptorless dehydrogenation of alcohols:
structure-activity relationship and mechanistic studies
Kenichi Kon a, Wataru Onoderaa, Takashi Toyao,a,b,* Kenꢀichi Shimizu a,b,
*
a Institute for Catalysis, Hokkaido University, Nꢀ21, Wꢀ10, Sapporo 001ꢀ0021, Japan
b
Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura, Kyoto
615ꢀ8520, Japan
*Corresponding author
Kenꢀichi Shimizu
Institute for Catalysis, Hokkaido University, Nꢀ21, Wꢀ10, Sapporo 001ꢀ0021, Japan, Eꢀmail:
kshimizu@cat.hokudai.ac.jp, Fax: +81ꢀ11ꢀ706ꢀ9163, Tel: +81ꢀ11ꢀ706ꢀ9164
Abstract
Al2O3ꢀsupported Re with different oxidation state and Re0 metal nanoparticles on various
supports are prepared, characterized, and tested for the dehydrogenation of 2ꢀoctanol. The
activity of Re/Al2O3 increases with the fraction of metallic Re. The activity of metallic Re
depends on the support oxides, and the support with moderate electronegativity (Al2O3) gives
the highest turnover frequency (TOF) per surface Re0 sites. Re/Al2O3 is effective for
acceptorless dehydrogenation of various aliphatic secondary alcohols to ketones. The kinetic
isotope effects on the dehydrogenation of 2ꢀpropanol show that dissociation of αꢀCꢀH bond of
2ꢀpropanol is the rate limiting step. IR study of the reaction of gas phase 2ꢀpropanol over
Re/Al2O3 surface shows that the acidꢀbase pair site of Al2O3 is responsible for the OꢀH
dissociation of 2ꢀpropanol. The structural requirements are discussed on the basis of the
mechanistic results.
Keywords: alcohols · dehydrogenation· nanoparticles · rhenium
Introduction
Catalytic acceptorless dehydrogenation of alcohols to carbonyl compounds and H2 has been
an attractive area of research in catalysis, sustainable energy and organic synthesis.1,2 Previous
attempts have been mainly focused on Ruꢀ or Ir ꢀbased homogeneous catalysts.1ꢀ7 However,
most of the homogeneous catalysts suffer from drawbacks such as needs of an acid or base
additive and expensive ligands and difficulties in separation and recycle of the catalyst. To meet
environmental and economical acceptability, efforts have been devoted to heterogeneous
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catalytic systems for acceptorless dehydrogenation of alcohols.8ꢀ20 However, rational
improvement of the catalytic efficiency of heterogeneous catalyst for this reaction is still
difficult due to a lack of fundamental information on the structure–activity relationship and
reaction mechanism. For example, controversial models are proposed on the oxidation states of
transition metal catalysts for this reaction; metallic species are proposed to be effective for the
supported Ag,17 Co,18 Ni19 and Pt20 catalysts, whereas Re0ꢀcore/ReO2ꢀshell model is proposed as
an active Re species of unsupported nanocrystalline catalyst.16
As our continuous studies on the structural and mechanistic aspects of supported transition
metal catalysts for this reaction,17ꢀ20 we report herein the effects of oxidation state of Re and
support materials on heterogeneous Re catalysis for the dehydrogenation of secondary alcohols.
It will be shown that metallic Re sites and acidꢀbase pair sites on the support are key elements in
this catalytic system. Spectroscopic and kinetic studies will suggest mechanistic roles of the
metallic Re site and the acidꢀbase sites.
Results and Discussion
Effect of Re Oxidation State on the Activity of Re/Al2O3
To study the dependence of the catalytic activity on the mean oxidation state of Re/Al2O3, first
we prepared Re/Al2O3 catalysts with different oxidation state by changing the H2ꢀreduction
temperature of the oxidic precursor, ReOx/Al2O3. Figure 1A shows Fourier transforms of Xꢀray
absorption fine structure (EXAFS) of ReOx/Al2O3 and the catalysts reduced at 200, 300, 400,
and 500 ˚C. The results of curve fitting analysis of the EXAFS are listed in Table 1, and the
coordination numbers of ReꢀO and ReꢀRe shells are plotted as a function of the H2ꢀreduction
temperature in Figure 2. The EXAFS of ReOx/Al2O3 consists of a ReꢀO shell at bond distance
(R) of 1.74 Å with coordination number (N) of 4.8, and these values are close to the reported
data of NH4ReO4 (R = 1.74 Å, N = 4).21a,22 This indicates that the Re species in ReOx/Al2O3
have perrhenateꢀlike structure with the oxidation state of Re7+. A recent EXAFS study of
ReOx/SiO2 also reported similar EXAFS results and proposed the same structural model.[21a]
The EXAFS of the sample after reduction at 200 ˚C also consists of the ReꢀO shell (1.74 Å)
with lower coordination number (N = 2.4). This suggests that Re species in the sample are Re
oxides with lower number of ReꢀO bonds, or in other words Re oxides with the lower valence
than those in ReOx/Al2O3 (Re7+). Figure 1B shows Re L3ꢀedge Xꢀray absorption nearꢀedge
structures (XANES). It is established that decrease in the average valence of Re species results
in decrease in the intensity of the 2p3/2ꢀ5d peak named white line, and hence the area intensity
of the white line is an index of the oxidation state of Re species.21 The XANES spectrum of the
sample reduced at 200 ˚C (spectrum b) shows smaller intensity of the 2p3/2ꢀ5d peak (10537
eV) than ReOx/Al2O3 (spectrum a). This shows that reduction of ReOx/Al2O3 at 200 ˚C
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decreases the average valence of Re, which is consistent with the EXAFS result. The EXAFS of
the samples after the reduction treatments at 300 and 400 ˚C consist of two shells (ReꢀO shell at
2.01ꢀ2.10 Å and ReꢀRe shell at 2.74 Å). The EXAFS of the samples after reduction at 500 ˚C
consists of a ReꢀRe shell at 2.74 Å, indicating that the sample consists of only metallic Re
species.[21] As plotted in Figure 2, the ReꢀO coordination number decreases with increasing
H2ꢀreduction temperature, while the ReꢀRe coordination number increases with the reduction
temperature. These results indicate that the perrhenate (Re7+)ꢀlike species in ReOx/Al2O3 are
gradually reduced to give the metallic Re by the reduction at higher temperature. This is
consistent with the XANES result that the intensity of the 2p3/2ꢀ5d peak decreases with the
reduction temperature (Figure 1B).
To quantify the average valence of Re species in the samples, we estimated the area of the
2p3/2ꢀ5d peak in the Re L3ꢀedge XANES spectra by the method adopted in our previous
studies of Pd and Rh L3ꢀedge XANES analyses.23,24 As illustrated in Figure 3, an arctangent
function shown as a dashed line, corresponding to the continuum absorption, was subtracted
from the raw spectrum (dots) to give the absorption due to the electron transition from 2p3/2 to
5d3/2 and 5d5/2 (solid line). The white line area intensities estimated are plotted in Figure 2 as a
function of the reduction temperature. The result quantitatively indicates that the white line
area, or in other words, the average valence of Re decreases with the reduction temperature.
This tendency is consistent with the tendency in the coordination numbers from EXAFS
analysis coꢀplotted in Figure 2; the metallic Re species became dominant at higher the
reduction temperature.
As evidenced by the structural results, we have prepared a series of Reꢀloaded Al2O3
catalysts with different average oxidation states. Using the catalysts reduced at 200, 300, 400,
and 500 ˚C, next we carried out a model dehydrogenation of 2ꢀoctanol to 2ꢀoctanone under the
same reaction conditions (under N2 in reflux conditions in oꢀxylene for 0.5 h). The initial rates
per total number of Re atom in the catalyst were determined under the conversion levels below
20% and were plotted as a function of the reduction temperature in Figure 2. The reaction rate
increases with the reduction temperature, and this tendency coincides with the decrease in the
valence of Re species (increase in the fraction of metallic Re species) with the reduction
temperature. The result clearly indicates that the metallic Re species are catalytically important
species in the acceptorless dehydrogenation of the alcohol by the heterogeneous Re catalysts.
This conclusion is consistent with our previous conclusions on the valence state of the
transition metals (Ag,17 Co,18 Ni,19 Pt20)ꢀloaded catalysts for the dehydrogenation of the alcohol.
Hence, it can be generally concluded that metallic species are the active species in the
acceptorless dehydrogenation of alcohols by the supported transition metal catalysts.
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Structure and Performance of the Standard Re/Al2O3 Catalyst
The above chapter shows that the Re/Al2O3 catalyst reduced at 500 ˚C, consisting of only
metallic Re species, is the most active catalyst in the Al2O3ꢀsupported Re catalysts tested. Based
on various characterization results, here we discuss the structure of this standard catalyst
hereafter designated as Re/Al2O3. The EXAFS result (Table 1) of this sample shows smaller
ReꢀRe coordination number (8.9) than that of bulk Re metal (12),21 which is characteristic to
metal nanoparticles with a small mean diameter. To estimate the particle size, we carried out
TEM analysis of Re/Al2O3. As shown in the representative picture (Figure 4), a few nm sized
semiꢀspherical Re metal nanoparticles were observed. The analysis of the particle size
distributions (Figure 4) showed the mean particle diameter of 4.2 ± 1.1 nm, corresponding to the
volumeꢀarea mean diameter25 of 5.2 ± 1.1 nm. The volumeꢀarea mean diameters (5.2 ± 1.1 nm)
estimated by TEM analysis is consistent with those estimated by the CO adsorption experiment
(4.6 nm) within the experimental error of the TEM analysis, which indicates that the mean Re
size from the CO adsorption is the accurate.
To demonstrate the catalytic performance of the standard Re/Al2O3 catalyst, dehydrogenation
of various alcohols is shown in Table 2. In the presence of 1 mol% of Re/Al2O3 powder,
containing 0.01 mmol of Re, 1 mmol of various aliphatic secondary alcohols were selectively
converted to the corresponding ketones with good yields (72ꢀ96%). For the reaction of
cyclooctanol (entry 1), turnover number (TON) with respect to the total Re atoms in the
catalysts is 84, which is higher than the TON (50) of the recently reported unsupported Re
nanoparticle catalyst at higher reaction temperature (180 ˚C).16
Effect of Supports on the Activity of Re Catalysts
Next, we prepared a series of 5 wt% Reꢀloaded catalysts with different support materials. The
mean particle size of the Re metal particles on each catalyst was estimated by the CO adsorption.
Table 3 lists the reduction temperature, mean size of Re metal particles, of the catalysts together
with the averaged electronegativity of metal oxide supports. Note that the catalyst was reduced
under H2 at different temperatures (TH2) in order to prepare the catalysts with the similar Re
particle size (3.9ꢀ6.6 nm) but with different support materials.
With the catalysts in Table 3, we studied influence of supports on the catalytic activity for
dehydrogenation of 2ꢀoctanol to 2ꢀoctanone under the same reaction conditions (under N2 in
reflux conditions for 0.5 h). Using the number of Re0 sites estimated by the CO adsorption
experiment, the initial rates were converted to the turnover frequency (TOF) per surface Re0
sites (Table 4). The activity (TOF) depended strongly on the support material, and Re/Al2O3 was
found to give the highest activity. Al2O3 itself was inactive (result not shown). To discuss the
support effect, Figure 5 plots TOF versus the electronegativity of the support metal oxide, which
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is generally used as a parameter of acidity of metal oxides.26 There is a volcanoꢀtype
relationship between TOF and the electronegativity of the support. Basic supports (MgO, La2O3,
La2O3) and acidic supports (Nb2O5, SnO2) show lower activity than amphoteric oxide supports
(CeO2, Al2O3, TiO2). Taking into account the result in Table 4 that Re nanoparticles supported
on nearly neutral supports (SiO2 and carbon) gave a order of lower TOF than those on the
amphoteric oxide supports (CeO2, Al2O3, TiO2), it is shown that the strong support effect in this
catalytic system is caused by the acidꢀbase characteristics of the supports and amphoteric
(acidꢀbase bifunctional) nature gives positive effect on the activity of the supported Re.
Reaction Mechanism
To investigate reaction mechanism, in situ IR experiments on the reaction of gas phase
2ꢀpropanol with the surface of Re/Al2O3 was carried out at 80 ˚C. Figure 6A shows the spectra
of adsorbed species formed by the pulse injection of 2ꢀpropanolꢀOꢀd, (CH3)2CHOD, over
Re/Al2O3 which was in situ preꢀreduced at 500 ˚C. Initially (t = 120 s), the bands due to
2ꢀpropoxide (2973, 1467, 1164, 1130 cm−1) and AlꢀOD (2530 cm−1) on the Al2O3 surface, were
observed in the IR spectrum. We have previously reported the same IR experiment for pure
Al2O3.20 The positions of the bands for Re/Al2O3 are essentially the same as those for Al2O3.20
This indicates that 2ꢀpropoxide and D+ ions are adsorbed not on Re but on the Al2O3 surface
(Eqn. 1).
2AlꢀO + (CH3)2CHOD → (CH3)2CHꢀOꢀAl + AlꢀOD
(1)
Figure 6B shows the height of the bands due to 2ꢀpropoxide (AlꢀOiPr), AlꢀOD and adsorbed
acetone as a function of time in a flow of He. With increase in the reaction time, intensity of the
bands due to the 2ꢀpropoxide species decreased and a band at 1698 cm−1 due to C=O stretching
of adsorbed acetone appeared. The same IR experiment with pure Al2O3 gave no bands due to
acetone under the same conditions (results not shown). These results suggest that Re metal is
responsible for CꢀH dissociation of the 2ꢀpropoxide to yield acetone (Eqn. 2). Figure 6 shows
that the formation of 2ꢀpropoxide finished within 120 s, while its decrease took more than 1000
s. This indicates that the formation of 2ꢀpropoxide is faster than the dehydrogenation of
2ꢀpropoxide to acetone. Therefore, the OꢀD dissociation of (CH3)2CHOD is faster than αꢀCꢀH
dissociations of the 2ꢀpropoxide.
(CH3)2CHꢀOꢀAl + Re0 → (CH3)2C=O + AlꢀO + ReꢀH
(2)
The intensity of the band due to AlꢀOD (2530 cm−1) decreased with time. This can be
explained by the reaction of ReꢀH with AlOD to yield dihydrogen (HD), accompanying the
regenerations of the Re0 and AlꢀO sites (Eqn. 3).
ReꢀH + AlꢀOD → HD + Re0 + AlꢀO
(3)
We also studied kinetic isotope effect (KIE) in the liquid phase 2ꢀpropanol dehydrogenation by
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Re/Al2O3 in toluene under refluxing conditions. Figure 7 shows the kinetic curves for the
formation of acetone from (CH3)2CHOH, (CH3)2CHOD and (CH3)2CDOH. Initially, the amount
of acetone linearly increased with the reaction time, and hence the initial slope corresponds to
the zero order rate constant. Using the rate constants for (CH3)2CHOH and (CH3)2CHOD, the
KIE (kH/kD) was estimated to be 1.9, which is classified as secondary KIE.27 The kinetic isotope
effect from the data of (CH3)2CHOH and (CH3)2CDOH showed the primary KIE27 of 7.6. The
results indicate that the αꢀCꢀH dissociation of 2ꢀpropanol is the rate limiting steps. Taking into
account the IR result that the formation of 2ꢀpropoxide is faster than its dehydrogenation, the
secondary KIE of 1.9 observed for (CH3)2CHOH and (CH3)2CHOD suggests that a step
involving D other than the dissociation of OꢀD in (CH3)2CHOD is relatively slow. Considering
the reaction mechanism in Eqn. (1)ꢀ(3), the most probable step that can show KIE of 1.9 is the
OꢀD dissociation step in the reaction of ReꢀH with AlOD (Eqn. 3). Therefore, the KIE of 1.9
observed for (CH3)2CHOH and (CH3)2CHOD is an indirect evidence of the cooperative function
of Al2O3 surface during the H2 formation step (Eqn. 3). The cooperative function of Al2O3
involves both deprotonation (Eqn. 1) and protonation (Eqn. 3), possibly caused by the basic and
acidic functions of Al2O3. This is a possible reason why the Reꢀcatalyzed dehydrogenation of
alcohols requires acidꢀbase bifunctional supports (Figure 5). It is important to note that the
cooperative mechanism in Eqn. (1)ꢀ(3) is similar to those proposed in our previous studies on
Agꢀ,17 Coꢀ,18 Niꢀ,19 and Pt20ꢀloaded Al2O3 catalysts.
4. Conclusion
The Al2O3ꢀsupported Re nanoparticle were found to be efficient heterogeneous catalyst for the
acceptorless dehydrogenation of secondary alcohols. Re metal particles supported on
amphoteric supports (CeO2, Al2O3, TiO2) showed higher TOF per surface Re sites than those on
acidic and basic supports. IR results show that fast dissociation of alcohol to alkoxide and
proton occurs on the Al2O3 surface, while Re metal is responsible for CꢀH dissociation of the
alkoxide. The primary KIE of 7.9 for the dehydrogenation of (CH3)2CHOH and (CH3)2CDOH
showed that dissociation of αꢀCꢀH bond of 2ꢀpropanol was the rate limiting step. Combined
with the IR results, the secondary KIE of 1.9 for (CH3)2CHOH and (CH3)2CHOD suggests the
reaction of ReꢀH with AlOD as a H2 formation step. The cooperative functions of Al2O3
(deprotonation and protonation) are the reason why the Reꢀcatalyzed dehydrogenation of
alcohols requires acidꢀbase bifunctional supports.
Acknowledgment
This work was supported by GrantꢀinꢀAids for Scientific Research B (26289299) from MEXT
(Japan), a MEXT program “Elements Strategy Initiative to Form Core Research Center” and a
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GrantꢀinꢀAid for Scientific Research on Innovative Areas “Nano Informatics” (25106010) from
JSPS.
Experimental
General
Commercially available organic compounds (Tokyo Chemical Industry) were used without
further purification. 2ꢀPropanolꢀOꢀd, (CH3)2CHOD with 98 atom % D, and 2ꢀpropanolꢀ2ꢀd1,
(CH3)2CDOH with 99.4 atom % D, were purchased from SigmaꢀAldrich. The GC (Shimadzu
GCꢀ14B) and GCMS (Shimadzu GCMSꢀQP2010) analyses were carried out with Ultra ALLOY
capillary column UA+ꢀ1 (Frontier Laboratories Ltd.) or CPꢀPoraBOND Q capillary column
(Varian) using nitrogen or He as the carrier gas.
Catalyst preparation
Al2O3 (mainly θꢀphase, surface area of 75m2 gꢀ1) was prepared by calcination of γꢀAlOOH
(Catapal B Alumina purchased from Sasol) for 1 h at 1100 °C. SiO2 (Qꢀ10, 300 m2 gꢀ1) was
supplied from Fuji Silysia Chemical Ltd. TiO2 (JRCꢀTIO4, 50 m2 gꢀ1), MgO (JRCꢀMGOꢀ3, 19
m2 gꢀ1), CeO2 (JRCꢀCEO3, 81 m2 gꢀ1), and TiO2 (JRCꢀTIOꢀ4, 50 m2 gꢀ1) were supplied from
Catalysis Society of Japan. Active carbon (296 m2 gꢀ1) was purchased from Kishida Chemical.
H2SnO3 (Kojundo Chemical Laboratory Co., Ltd.) and Nb2O5ꢁnH2O (supplied by CBMM) were
commercially supplied. Hydroxides of Y and La were prepared by hydrolysis of metal nitrates
in distilled water by gradually adding an aqueous NH4OH solution (1 mol dmꢀ3), followed by
filtration of precipitate, washing with distilled water three times, drying at 100 °C for 12 h.
Y2O3, La2O3, Nb2O5 and SnO2 were prepared by calcination of the hydroxides at 500 °C for 3 h.
Re/Al2O3 catalysts with Re content of 5 wt% were prepared by an impregnation method. A
mixture of Al2O3 and an aqueous solution of NH4ReO4 was evaporated at 50 °C, followed by
drying at 90 °C for 12 h, calcination in air at 300 °C for 1 h, and by in situ preꢀreduction in a
glass (pyrex or quartz) tube under a flow of H2 (20 cm3 minꢀ1) at different reduction
temperatures (TH2) for 0.5 h. Other 5 wt% Reꢀloaded catalysts on different supports were
prepared by a similar method.
Characterization
The number of surface metal atoms in Re catalysts, preꢀreduced in H2 at TH2 (in Table 3) was
estimated from the CO uptake of the samples at room temperature using the pulseꢀadsorption of
CO in a flow of He by BELCAT (MicrotracBELL Corp.). The CO adsorption experiments for
the Reꢀloaded basic oxides (MgO, Y2O3, La2O3, CeO2) were carried out at ꢀ78 °C.[28] The
average particle size in Table 3 was calculated from the CO uptake assuming that CO was
adsorbed on the surface of semispherical Re particles at CO/(surface Re atom) = 1/1
stoichiometry. Transmission electron microscopy (TEM) measurements were carried out by
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using a JEOL JEMꢀ2100F TEM operated at 200 kV.
Xꢀray absorption nearꢀedge structures (XANES) and Xꢀray absorption fine structure
(EXAFS) at Re L3ꢀedge were measured in transmission mode at the BLꢀ9C of KEKꢀPF
(Tsukuba, Japan) with a ring energy of 2.5 GeV and a stored current of 250–350 mA (Proposal
No. 2014P003). The storage ring was operated at 8 GeV. A Si(111) single crystal was used to
obtain a monochromatic Xꢀray beam. The catalysts preꢀreduced in a flow of 100% H2 (20 cm3
minꢀ1) for 0.5 h at different temperatures (200ꢀ500 °C) were cooled to room temperature in the
flow of H2 and were sealed in the cells made of polyethylene under N2. Then, the EXAFS
spectrum of the sealed sample was taken at room temperature. The EXAFS analysis was
performed using the REX version 2.5 program (RIGAKU). The parameters for the Re–O and
Re–Re shells were provided by the FEFF6.
In situ IR spectra were recorded on a JASCO FT/IRꢀ4200 equipped with a quartz IR cell
connected to a conventional flow reaction system. The sample was pressed into a 40 mg of
selfꢀsupporting wafer (φ = 2 cm) and mounted into the quartz IR cell with CaF2 windows.
Spectra were measured accumulating 15 scans at a resolution of 4 cmꢀ1. A reference spectrum of
the catalyst wafer in He taken at measurement temperature was subtracted from each spectrum.
Prior to each experiment the catalyst disk was heated in H2 flow (100 cm3 minꢀ1) at 500 °C for
0.5 h, followed by cooling to 80 °C and purging with He.
Condition of catalytic reactions
Re/Al2O3 reduced at 500 °C was used as the standard catalyst. After the preꢀreduction at 500 °C,
we carried out catalytic tests using a batchꢀtype reactor without exposing the catalyst to air as
follows. The mixture of oꢀxylene (1 mL), alcohol (1 mmol), and nꢀdodecane (0.2 mmol) was
injected to the preꢀreduced catalyst inside the reactor (cylindrical glass tube) through a septum
inlet, followed by filling N2. Then, the resulting mixture was stirred under reflux; typically, the
bath temperature was 155 °C and reaction temperature was ca 144 °C. Conversion and yields of
products were determined by GC using nꢀdodecane as an internal standard. Progress of the
reaction was monitored by GC analysis of aliquots (ca. 0.02 g). The initial rate of reaction was
measured under the conditions where the conversion was below 20%. The products were
identified by GCꢀMS equipped with the same column as GC and by comparison with
commercially pure products.
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Table 1. Curveꢀfitting analysis of Re L3ꢀedge EXAFS of the samples prepared by H2ꢀreduction of
ReOX/Al2O3 at different temperatures (TH2).
TH2/°C
ꢀe
Shell
O
N a
4.8
2.4
1.6
5.6
1.5
8.1
8.9
R /Å b
1.74
1.74
2.01
2.74
2.10
2.74
2.74
σ /Å c
0.049
0.045
0.050
0.085
0.020
0.075
0.069
Rf (%) d
1.1
200
300
O
1.3
O
5.7
Re
O
400
3.2
3.5
Re
Re
500 f
a Coordination number.
bBond distance.
c DebyeꢀWaller factor.
d Residual factor.
e ReOX/Al2O3.
f The standard Re/Al2O3 catalyst.
Table 2. Dehydrogenation of various alcohols by Re/Al2O3 (reduced at 500 oC)
.
OH
R2
O
1 mol% Re/Al2O3
+ H2
R1
1 mL o-xylene, reflux
R1
R2
under N2, 24 h
1 mmol
Entry
Alcohol
Cyclooctanol
Cyclododecanol
exoꢀNorborneol
4ꢀOctanol
Conv. (%)
Yield (%)
1
2
100
93
71
96
76
72
7
84
84
58
96
71
72
1
3
4
5
6a
2ꢀOctanol
2ꢀPropanol
7
1ꢀOctanol
a Reflux conditions in 1mL toluene.
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Table 3. List of 5 wt% Reꢀloaded catalysts.
Catalyst a
EN b
2.07
2.21
2.30
2.35
2.47
2.53
2.60
2.76
2.77
ꢀ
TH2 c / °C
D d / nm
5.4
Re/MgO
400
Re/Y2O3
300
5.7
Re/La2O3
Re/CeO2
400
6.6
600
4.1
4.6 (5.2) [e]
Re/Al2O3
Re/TiO2
500
500
3.7
Re/Nb2O5
Re/SnO2
300
4.5
300
6.3
Re/SiO2
500
4.2
Re/C
500
3.9
a Re loading is 5 wt%.
b The averaged electronegativity of metal oxides is calculated according to the concept of
Sanderson29 on the basis of electronegativity of the element defined in ref. 30.
c Reduction temperature of the catalysts under H2 flow.
d Average particle size (nm) of the supported Re estimated from the CO adsorption experiment.
e Average particle size (nm) of the supported Re estimated from the TEM.
Table 4. Dehydrogenation of 2ꢀoctanol by 5 wt% Reꢀloaded catalysts listed in Table 3.
O
OH
1 mol% Re/Al2O3
+ H2
n-C6H13
n-C6H13
1 mmol
1 mL o-xylene, reflux
under N2, 0.5 h
Catalyst
Conv.
(%)
2
Yield
(%)
Rate a
/mol molReꢀ1 hꢀ1 / hꢀ1
TOFb
Re/MgO
Re/Y2O3
Re/La2O3
Re/CeO2
Re/Al2O3
Re/TiO2
Re/Nb2O5
Re/SnO2
Re/SiO2
Re/C
2
5
4
10
6
36
10
5
94
66
3
22
21
21
12
8
21
21
20
10
4
42
42
40
20
8
290
314
250
150
85
1
1
2
14
3
2
4
26
a Initial rate defined as mole of 2ꢀoctanone hꢀ1 per mole of Re.
b TOF defined as mole of 2ꢀoctanone hꢀ1 per mole of surface Re (from CO adsorption data).
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5.0
e
d
e
d
c
c
b
a
b
a
0
1
2
R / Å
3
4
5
6 10520 10530 10540 10550
X-ray energy / eV
Fig. 1. Re L3ꢀedge XANES spectra (A) and EXAFS Fourier transforms (B) of Reꢀloaded Al2O3
samples: (a) unreduced ReOX/Al2O3 and 5 wt% Re/Al2O3 reduced at (b) 200 °C, (c) 300 °C, (d)
400 °C, (e) 500 °C.
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unreduced ReO X/Al2O3
30
28
26
24
22
10
8
6
4
2
0
40
30
20
10
0
200
300
400
500
o
Reduction temperature / C
Fig. 2. Effect of H2ꢀreduction temperature of 5 wt% Re/Al2O3 on () the reaction rate based on the
total number of Re atom for the dehydrogenation of 2ꢀoctanol, (ꢀ) area of the white line peak
(XANES analysis) and coordination number of (ꢁ) ReꢀO and (ꢂ) ReꢀRe bonds (EXAFS analysis).
XANES spectrum ( ꢀ)
White line
- a)
(ꢀ
arctangent
function (a)
10520
10530
10540
10550
X-ray Energy/ eV
Fig. 3. An example of the Re L3ꢀedge XANES analysis (5 wt% Re/Al2O3 reduced at 500 °C). An
arctangent function (dashed line) for the continuum absorption is subtracted from the raw spectrum
(●) to give a function corresponding to the white line peak (solid line).
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50
40
30
20
10
0
D = 4.2±1.1 nm
0
2
4
6
8
10
Particle size / nm
Fig. 4. A representative TEM image and Re metal particle size distribution of Re/Al2O3
(reduced at 500 oC). The mean diameter (nm) of Re particle was 4.2 ± 1.1 nm, corresponding to
the volumeꢀarea mean diameter of 5.2 ± 1.1 nm.
Al2O3
CeO2
300
TiO2
200
Nb2O5
La2O3
100
SnO2
MgO
Y2O3
0
2
2.2
2.4
2.6
2.8
Electronegativity of suport metal oxide
Fig. 5. Effect of the electronegativity of the metal oxide support on TOF per surface Re atom for
dehydrogenation of 2ꢀoctanol by 5 wt% Reꢀloaded metal oxides with similar Re particle size (Table
3).
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A
0.4B
Al-OiPr
(1130cm-1)
0.1
0.3
0.2
0.1
0
acetonead(1698cm-1)
Al-OD
(2530cm-1)
120 s
400 s
900 s
3000
2500
2000
1500
0
500
t / s
1000
Wavenumber / cm-1
o
Fig. 6. (A) IR spectra of adsorbed species on Re/Al2O3 (reduced at 500 C) and (B) time course of
the height of the bands due to () 2ꢀpropoxide on the support (1130 cmꢀ1), (●) adsorbed acetone
(1698 cmꢀ1), and (ꢁ) AlꢀOD group (2530 cmꢀ1) in a flow of He at 80 °C. At t = 0 s, 0.5 mmol gꢀ1 of
(CH3)2CHOD was introduced to the catalyst.
0.6
0.4
k[CH3CH(OH)CH3] / k[CH3CH(OD)CH3] = 1.9
0.2
k[CH3CH(OH)CH3] / k[CH3CD(OH)CH3] = 7.6
0
5
10
15
t / h
Fig. 7. Kinetic isotopic effects in the dehydrogenation of () 2ꢀpropanol, (ꢃ) 2ꢀpropanolꢀOꢀd,
(ꢀ) and 2ꢀpropanolꢀ2ꢀd1 by Re/Al2O3 (1 mol%): Conditions: 2ꢀpropanol (0.5 mmol), toluene (1
mL), 80 °C. Kinetic isotope effects estimated from the zero order rate constants (the slope of the
lines) are shown.
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R1
R2
R2
R1 R2
R1
H
H
O
H2
O
OH
Al
OH
Al
Re
O
Mechanistic and structure-activity relationship studies show cooperative catalysis of Re0 sites
on the supported Re metal nanoparticles and acid-base sites of alumina for the title reaction.