A reusable unsupported rhenium nanocrystalline catalyst for acceptorless dehydrogenation of alcohols through γ-C-H activation

Article abstract of DOI:10.1002/anie.201307665

Rhenium nanocrystalline particles (Re NPs), of 2 nm size, were prepared from NH₄ReO₄ under mild conditions in neat alcohol. The unsupported Re NPs convert secondary and benzylic alcohols to ketones and aldehydes, respectively, through catalytic acceptorless dehydrogenation (AD). The oxidant- and acceptor-free neat dehydrogenation of alcohols to obtain dihydrogen gas is a green and atom-economical process for making carbonyl compounds. Secondary aliphatic alcohols give quantitative conversion and yield. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Re K-edge X-ray absorption near-edge structure (XANES), and X-ray absorption fine structure (EXAFS) data confirmed the characterization of the Re NPs as metallic rhenium with surface oxidation to rhenium(IV) oxide (ReO₂). Isotope labeling experiments revealed a novel γ-CH activation mechanism for AD of alcohols. Active particles: A rhenium nanoparticle (Re NP) catalyst is generated from NH₄ReO₄ under mild solution conditions in neat 3-octanol at 180°C. The resulting Re NPs catalyze acceptorless dehydrogenation of alcohols through a novel C-H activation pathway, and are fully recyclable. Copyright

Full text of DOI:10.1002/anie.201307665

Angewandte
Chemie
DOI: 10.1002/anie.201307665
Heterogeneous Catalysis
A Reusable Unsupported Rhenium Nanocrystalline Catalyst for
Acceptorless Dehydrogenation of Alcohols through g-C–H
Activation**
Jing Yi, Jeffrey T. Miller, Dmitry Y. Zemlyanov, Ruihong Zhang, Paul J. Dietrich,
Fabio H. Ribeiro, Sergey Suslov, and Mahdi M. Abu-Omar*
Abstract: Rhenium nanocrystalline particles (Re NPs), of
2 nm size, were prepared from NH4ReO4 under mild condi-
tions in neat alcohol. The unsupported Re NPs convert
secondary and benzylic alcohols to ketones and aldehydes,
respectively, through catalytic acceptorless dehydrogenation
(AD). The oxidant- and acceptor-free neat dehydrogenation of
alcohols to obtain dihydrogen gas is a green and atom-
economical process for making carbonyl compounds. Secon-
dary aliphatic alcohols give quantitative conversion and yield.
Transmission electron microscopy (TEM), X-ray photoelec-
tron spectroscopy (XPS), Re K-edge X-ray absorption near-
edge structure (XANES), and X-ray absorption fine structure
(EXAFS) data confirmed the characterization of the Re NPs as
metallic rhenium with surface oxidation to rhenium(IV) oxide
(ReO2). Isotope labeling experiments revealed a novel g-CH
activation mechanism for AD of alcohols.
of discrete oxorhenium complexes as homogeneous cata-
lysts.[15–21] While there are many examples of homogeneous
rhenium deoxygenation catalysts, a limited number of rhe-
nium nanoparticle catalysts have been reported.[22–26] Herein,
we report on rhenium nanocrystalline particles (Re NPs), of
2 nm size, that catalyze AD of alcohols through a novel g-CH
activation mechanism.
Re NPs of 2 nm size without any support (Figure 1a) were
prepared from NH4ReO4 under mild conditions (1808C,
without the need to protect from air) in neat 3-octanol.
Figure 1b shows a HRTEM image of a Re nanocrystal aligned
to the [0001] zone axis. An interplanar spacing of 2.39 ꢀ,
which belongs to {1À100} crystallographic planes in a hexag-
onal structure, is measured based on the HRTEM image as
well as the fast Fourier transform (FFT) pattern (inset of
Figure 1b). Both TEM and powder XRD (Figure S7 in the
Supporting Information) are consistent with hexagonal Re
crystals (P63/mmc) for the structure of Re NPs. Approx-
imately 200 Re atoms make up the nanoclusters (2 nm). The
surface or outside Re atoms are irregular in shape because
they are susceptible to oxidation upon exposure to air to give
rhenium(IV) oxide.
In alcohol solution and under reflux, catalytic AD
reactions with Re NPs can be conducted without the need
for protection from the ambient atmosphere (air). The
addition of ten equivalents of mercury metal relative to Re
does not poison the catalyst. However, isolated Re NPs
contain surface Re atoms that are oxidized to ReIVO2, as
evident by X-ray absorption spectroscopy (XAS) and X-ray
photoelectron spectroscopy (XPS). According to the Re K-
Catalytic acceptorless dehydrogenation (AD) reactions of
alcohols, which generate dihydrogen and a more reactive
functional group (carbonyl), are attracting significant
research attention.[1] They eliminate production of organic
and inorganic waste side products and the only by-product,
hydrogen, is useful as a source of green energy. Until now,
chemists have focused on AD using homogeneous catalysts;
however, most homogeneous catalysts require an acid or base
additive, extensive synthesis of ligands, and are difficult to
separate and recycle.[1–6] Heterogeneous catalysts[7–14] for AD
reactions are also highly dependent on the support acting as
a base or acid. Our work and that of others on deoxydehy-
dration (DODH) of biomass-derived polyols illustrate the use
[*] J. Yi, Prof. Dr. M. M. Abu-Omar
Prof. Dr. J. T. Miller, Dr. P. J. Dietrich, Prof. Dr. F. H. Ribeiro,
Prof. Dr. M. M. Abu-Omar
School of Chemical Engineering, Purdue University
West Lafayette, IN 47907 (USA)
Department of Chemistry, Purdue University
West Lafayette, IN 47907 (USA)
E-mail: mabuomar@purdue.edu
Homepage: http://www.chem.purdue.edu/abu-omar/
[**] This work was supported by the US Department of Energy, Office of
Basic Energy Sciences (DE-FG02-06ER15794); and (DE-AC02-
06CH11357) for Advanced Photon Source use. MRCAT operations
were supported by the Department of Energy and the MRCAT
member institutions. Funding for J.T.M., P.J.D., and F.H.R. was
provided as part of the Institute of Atom-efficient Chemical
Transformations, an Energy Frontier Research Center funded by the
US Department of Energy, Office of Basic Energy Sciences. Dr. Bruce
R. Cooper is acknowledged for assistance with GC–MS; and Dr.
Kothanda R. Pichaandi, Dr. Benjamin Wegenhart, and Prof.
Christopher Uyeda for helpful discussions.
Prof. Dr. J. T. Miller
Chemical Science & Engineering Division, Argonne National
Laboratory, Argonne, IL 60429 (USA)
Dr. D. Y. Zemlyanov, Dr. S. Suslov
Birck Nanotechnology Center, Discovery Park, Purdue University
West Lafayette, IN 47907 (USA)
R. Zhang
School of Materials Engineering, Purdue University
West Lafayette, IN 47907 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307665.
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833

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Angewandte
Communications
particle size effect. In comparison, the Re NPs that have been
exposed to air do not show a Re 4f7/2 peak at 40.8 eVand only
rhenium(IV) oxide is observed on the surface (Figure S4). In
summary, Re NPs can be made by a solution method in
alcohol at 1808C. The surface atoms are easily oxidized to
ReIVO2.
Since the Re NPs are prepared in neat 3-octanol solution
starting from rhenium(VII), the alcohol must be sufficiently
reducing. Indeed under acidic and neutral conditions reduc-
tion of rhenium oxides to the metal is exergonic [ReO4À/Re =
0.375 and ReO2/Re = 0.276 V versus the normal hydrogen
electrode (NHE)]. Because of their small size, the Re NPs
disperse evenly in 3-octanol and do not aggregate, thereby
making their separation by filtration difficult. Therefore, the
Re NPs are isolated by evaporation of the 3-octanone product
under vacuum. Remarkably, the recovered Re NP catalyst
can be recycled several times without major loss in activity
and selectivity (Table 1). FTIR spectra of the recycled Re NP
=
catalyst (Figure S12) do not show Re O stretching frequen-
=
cies in the characteristic region for Re O multiple bonds 800–
1000 cmÀ1.
Figure 1. Identification of Re NPs. a) TEM image of Re NPs diluted in
acetone and evaporated over Cu grids without interference of other
supports. b) HRTEM image of hexagonal Re NPs, revealing the
{1À100} crystallographic planes with a d spacing of 2.39 ꢀ. The inset
in (b) shows the FFT pattern of a single Re nanoparticle. c) Re L3
XANES (10.50 to 10.57 keV) for Re0 (black c), ReO2 (blue c), and
NH4ReO4 (red c) references, and the Re NP sample (black b).
d) EXAFS Fourier transforms of Re NPs (Dk=2.7 to 10.4 AÀ1).
Table 1: Dehydrogenation of 3-octanol over recycled Re NP catalyst.[a]
Reuse
1st
2nd
100
3rd
97
4th
96
5th
90
6th
83
7th
82
8th
79
Conv. [%][b]
100
[a] Reaction conditions: NH4ReO4 (0.3 mmol) and 3-octanol (15 mmol)
were mixed and refluxed at 1808C for 10 h, followed by vacuum
distillation of 3-octanone and drying of Re NPs for the next catalytic cycle.
[b] Conversions are based on 1H NMR spectroscopy.
edge X-ray absorption near-edge structure (XANES) in
Figure 1c, the K-edge energy of the Re NPs is similar to
that of ReO2. However, the shape is slightly different, thus
illustrating additional types of Re. A fit of the XANES shows
45% of the sample being ReO2 and 55% rhenium(0). This
result is also consistent with Fourier transforms (FT) of k2-
weighted extended X-ray absorption fine structure (EXAFS)
data in Figure 1d and Table S1 in the Supporting Information.
The effect of temperature on the conversion rate for
dehydrogenation of 3-octanol into 3-octanone is shown in
Figure S11a. Temperatures of 1808C or higher give maximum
conversion rate while retaining product selectivity for 3-
octanone. Testing of several precursors for preparing the Re
NP catalyst showed NH4ReO4 to be the best precursor
(Figure S11b). Although MTO (CH3ReO3, methyltrioxorhe-
nium) also gave a high conversion rate, its slightly lower
selectivity (83% 3-octanone) is due to the known activity of
homogeneous MTO for alcohol dehydration. The major by-
product with MTO is 3-octene (17%). Similar behavior was
observed for (Ph3SiO)ReO3 as a precatalyst, which also gave
dehydration by-product. However, the conversion rate when
starting with (Ph3SiO)ReO3 was also significantly lower (28%
in 10 h). Other rhenium oxides such as Re2O7 and ReO3 are
suitable precursors for making the Re NP catalyst but at
a lower efficacy than NH4ReO4 or MTO. NH4ReO4 appears
to be the best starting precatalyst to produce Re NPs, which is
consistent with a previous report.[26]
Dehydrogenation of various alcohols was investigated and
the results are summarized in Table 2. Secondary aliphatic
alcohols (Table 2, entries 1–3) all afforded quantitative con-
version (100%) and high selectivity (> 99%). Primary
alcohols such as 1-heptanol (Table 2, entry 4a) did not convert
to aliphatic aldehydes. Using 2-heptanol as an additive (1:1
molar ratio to 1-heptanol, entry 4b) also gave low conversion
(5%) of 1-heptanal and even low conversion (35%) of 2-
À
À
EXAFS indicates besides Re O bonds (2.05 ꢀ) metallic Re
Re bonds (2.70 ꢀ). Assuming that the oxidation takes place at
À
the surface of the Re NPs, the Re O bond in the nano-
particles (2.05 ꢀ) is longer than that in bulk ReO2 (1.98 ꢀ).[27]
The fraction of oxidized Re in the nanoparticles can be
À
estimated from the number of Re O bonds, which are 6 in
bulk ReO2 and 2.8 in Re NPs. Thus the percentage of ReO2 in
Re NPs is 2.8/6 = 0.47 (i.e. 47%). In summary, Re NPs are
approximately 2 nm in size composed of a rhenium(0) core
with surface ReIVO2. Since many metallic nanoparticles are
susceptible to oxidation,[28] it is not surprising that our Re NPs
show oxidation of surface Re atoms to rhenium(IV) oxide.
X-ray photoelectron spectroscopy (XPS) provided further
characterization of the surface atoms in Re NPs. Two samples
were compared, Re NPs open to air and Re NPs transferred in
a glovebox under Ar atmosphere. Figure S3 compares Re NPs
transferred under Ar with Re polycrystalline foil as a refer-
ence. The curve fitting analysis shows the Re NPs exhibit a Re
4f7/2 peak at 40.8 eV, which is 0.5 eV higher than that observed
for Re foil (Figure S3). This difference is attributed to the
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Table 2: Dehydrogenation of alcohols by Re NPs.[a]
The Re NPs quantitatively convert secondary aliphatic
Product[b]
Conv. Sel.
alcohols to ketones through catalytic acceptorless dehydro-
genation (AD). To explore the mechanism of Re NP
catalyzed AD reactions of alcohols, we used deuterated 3-
octanol substrates (Scheme 1). Deuteration of the alcohol
Entry Substrate
Additives
[%][c] [%][c,d]
1
2
–
–
100
100
>99
>99
3a
3b
4a
4b
–
100
33
0
>99
33
0
1-heptanal[e]
–
–
2-heptanol[e]
5
5
[f]
5
C6Me6
45
44 (42)
[f]
6
C6Me6
40
40 (39)
Scheme 1. Deuterated alcohol substrates for KIE experiments. Reaction
conditions: NH4ReO4 (0.2 mmol) and alcohol or deuterated alcohol
(20 mmol) were mixed and refluxed at 1808C for 180 min. Results are
based on 1H and/or 2D NMR as the average of two runs.
7
–
100
99
8
–
–
–
100
100
25
99
9
80[g] (78)
2[h]
hydrogen atom (-OH versus -OD) did not result in a kinetic
isotope effect (KIE). Surprisingly, deuteration of the b-CH
also did not result in a KIE. However, when g-CH was
replaced with g-CD, an inverse KIE of 0.38 was observed
(Scheme 1). This result indicates that the g-CH is involved,
and since the KIE is < 1.0, it must be resulting from
a combination of a prior equilibrium and a kinetic C–H
cleavage step. Therefore, we hypothesize that an alkane
complex with g-CH is formed with rhenium NPs followed by
C–H activation (Scheme 2). This mechanistic proposal is
corroborated by the observation of C-H/D scrambling in the
3-octanone product as well as a previously suggested agostic
interaction in heterogeneous rhenium catalysts.[22] Further
support for C–H scrambling and involvement of the g-CH was
obtained by detecting HD and D2 products by mass spec-
trometry [see residual gas analysis (RGA) in the Supporting
Information]. In Scheme 2, a Re0 is used to represent the
active atoms on the Re nanocluster. The generated proton (in
gray circle) during the course of the reaction is most likely
accommodated on an oxo-Re site or picked up by rhenium
hydride to release dihydrogen.
10
61[i] (55)
[f]
11
C6Me6
100
[a] Reaction conditions: NH4ReO4 (0.1 mmol) and alcohol (5 mmol)
were mixed and refluxed at 1808C for 10 h. [b] Confirmed by GC, GC–MS,
and NMR (the Supporting Information). [c] Based on GC. [d] The values
in brackets correspond to those after isolation by silica column
chromatography. [e] Additive and substrate molar ratio is 1:1. [f] Additive
and substrate molar ratio is 2:1. [g] Side products: 6-methyl-2-heptanone
(15%) and 6-methylhept-6-en-2-one (5%). [h] Major product is 2,2,4-
trimethylhex-3-ene (22%). [i] Protected under Ar, reaction time is 3 h, and
the yield of toluene is 38%.
heptanol to 2-heptanone. This result indicates that primary
alcohols and aldehydes act as inhibitors of the dehydrogen-
ation reaction. Indeed mixing 1-heptanal and 2-heptanol in
a 1:1 molar ratio (Table 2, entry 3b), gave low conversion
(33%) to 2-heptanone. Dehydrogenation of cyclohexanol
(Table 2, entry 5) gave lower conversion (45%) than other
C7- or C8-cyclo-alcohols (entries 7 and 8, 100%). The reason
À
might be due to the difference in the C H bond strength
between these molecules. Dilution of cyclohexanol with
hexamethylbenzene as a solvent reduced the aldol side
product. While hexamethylbenzene is a solid at room temper-
ature, it is a liquid at the reaction temperature of 1808C. Upon
completion of the reaction and cooling, the mixture was
dissolved in CH2Cl2 for GC and GC–MS analyses, then
concentrated and run through a silica column to separate
cyclohexanone and recycle hexamethylbenzene. Dehydro-
genation of the unconjugated alkene alcohol 6-methyl-5-
hepten-2-ol (Table 2, entry 9) gave 100% conversion and
moderate selectivity for 6-methyl-5-hepten-2-one. Other side
products included 6-methyl-2-heptanone and 6-methylhept-6-
en-2-one. Conjugated systems like benzyl alcohol (entry 11)
must be proceeding through a different pathway/mechanism
because the reaction is not strictly AD. The reaction of the Re
NP catalyst with benzyl alcohol produces only approximately
20% H2 and a large amount of toluene (38%) from complete
deoxygenation of the alcohol.
Scheme 2. Proposed acceptorless dehydrogenation (AD) of 3-octanol
through g-CH activation.
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.
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[9] W. Fang, J. Chen, Q. Zhang, W. Deng, Y. Wang, Chem. Eur. J.
2011, 17, 1247 – 1256.
[10] K.-I. Shimizu, K. Sugino, K. Sawabe, A. Satsuma, Chem. Eur. J.
2009, 15, 2341 – 2351.
[11] T. Mitsudome, Y. Mikami, H. Funai, T. Mizugaki, K. Jitsukawa,
K. Kaneda, Angew. Chem. 2008, 120, 144 – 147; Angew. Chem.
Int. Ed. 2008, 47, 138 – 141.
[12] K.-I. Shimizu, K. Kon, M. Seto, K. Shimura, H. Yamazaki, J. N.
Kondo, Green Chem. 2013, 15, 418 – 424.
[13] K.-I. Shimizu, K. Kon, K. Shimura, S. S. M. A. Hakim, J. Catal.
2013, 300, 242 – 250.
[14] B. N. Wigington, M. L. Drummond, T. R. Cundari, D. L. Thorn,
S. K. Hanson, S. L. Scott, Chem. Eur. J. 2012, 18, 14981 – 14988.
[15] J. Yi, S. Liu, M. M. Abu-Omar, ChemSusChem 2012, 5, 1401 –
1404.
[16] S. Vkuturi, G. Chapman, I. Ahmad, K. M. Nicholas, Inorg.
Chem. 2010, 49, 4744 – 4746.
[17] I. Ahmad, G. Chapman, K. M. Nicholas, Organometallics 2011,
30, 2810 – 2818.
Additionally the g-abstraction mechanism was tested with
2,2,4,4-tetramethyl-3-pentanol, which has no g-CH (entry 10
in Table 2). The conversion was low with the corresponding
ketone product as the minor product (2%), and the major
product 2,2,4-trimethylhex-3-ene (22%), which must be
resulting from a sigmatropic methyl shift in association with
alcohol dehydration.
In conclusion, unsupported crystalline Re NPs (ca. 2 nm)
were generated under very mild conditions (1808C and neat
3-octanol). The resulting Re NPs are easily oxidized to ReO2
on the surface while maintaining some metallic rhenium(0) in
the core. The Re NPs are excellent and selective catalysts for
AD reactions of different alcohols. The dehydrogenation
mechanism is quite novel involving g-CH bond activation.
Further investigation of the Re NPs to broader green
chemistry applications are under way.
[18] C. Boucher-Jacobs, K. M. Nicholas, ChemSusChem 2013, 6, 597 –
599.
[19] E. Arceo, J. A. Ellman, R. G. Bergman, J. Am. Chem. Soc. 2010,
132, 11408 – 11409.
Received: August 31, 2013
Revised: October 21, 2013
Published online: November 26, 2013
[20] J. E. Ziegler, M. J. Zdilla, A. J. Evans, M. M. Abu-Omar, Inorg.
Chem. 2009, 48, 9998 – 10000.
[21] M. Shiramizu, F. D. Toste, Angew. Chem. 2012, 124, 8206 – 8210;
Angew. Chem. Int. Ed. 2012, 51, 8082 – 8086.
[22] A. Lesage, L. Emsley, M. Chabanas, C. Coperet, J.-M. Basset,
Angew. Chem. 2002, 114, 4717 – 4720; Angew. Chem. Int. Ed.
2002, 41, 4535 – 4538.
[23] Y. Y. Chong, W. Y. Chow, W. Y. Fan, J. Colloid Interface Sci.
2012, 369, 164 – 169.
[24] A. A. Zinn, U.S. Patent 7736414B1, June 15, 2010.
[25] K. M. Babu, M. R. Mucalo, J. Mater. Sci. Lett. 2003, 22, 1755 –
1757.
[26] J. W. Jurewicz, J. Guo, U.S. Patent 7494527B2, February 24, 2009.
[27] A. L. Ivanovskii, T. I. Chupakhina, V. G. Zubkov, A. P. Tyutyun-
nik, V. N. Krasilnikov, G. V. Bazuev, S. V. Okatov, A. I. Lichten-
stein, Phys. Lett. 2005, 348, 66 – 70.
Keywords: alcohols · dehydrogenation ·
heterogeneous catalysis · nanoparticles · rhenium
.
[1] C. Gunanathan, D. Milstein, Science 2013, 339, 341.
[2] G. E. Dobereiner, R. H. Crabtree, Chem. Rev. 2010, 110, 681 –
703.
[3] J. Choi, A. H. R. MacArthur, M. Brookhart, A. S. Goldman,
Chem. Rev. 2011, 111, 1761 – 1779.
[4] A. C. Marr, Catal. Sci. Technol. 2012, 2, 279 – 287.
[5] J. Zhang, M. Gandelman, L. J. W. Shimon, H. Rozenberg, D.
Milstein, Organometallics 2004, 23, 4026 – 4033.
[6] J. Zhang, E. Balaraman, G. Leitus, D. Milstein, Organometallics
2011, 30, 5716 – 5724.
[7] K. Mori, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am.
Chem. Soc. 2004, 126, 10657 – 10666.
[28] D. J. Commereuc, Chem. Soc. Chem. Commun. 1995, 791 – 792.
[8] K. Kon, S. M. A. Hakim Siddiki, K.-I. Shimizu, J. Catal. 2013,
304, 63 – 71.
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