Ceria in hydrogenation catalysis: High selectivity in the conversion of alkynes to olefins

Article abstract of DOI:10.1002/anie.201203675

Active and selective: Ceria shows a high activity and selectivity in the gas-phase hydrogenation of alkynes to olefins (see picture). This unprecedented behavior has direct impact on the purification of olefin streams and, more importantly, it opens new perspectives for exploring this fascinating oxide as a catalyst for the selective hydrogenation of other functional groups.

Full text of DOI:10.1002/anie.201203675

Angewandte
Chemie
DOI: 10.1002/anie.201203675
Heterogeneous Catalysis
Ceria in Hydrogenation Catalysis: High Selectivity in the Conversion of
Alkynes to Olefins**
Gianvito Vilꢀ, Blaise Bridier, Jonas Wichert, and Javier Pꢀrez-Ramꢁrez*
In recent years, ceria has attracted increasing interest in the
Herein, we report for the first time the catalytic perfor-
[1]
fields of medicine, electronics, and chemistry. Its success in
catalysis relates to the unique redox and structural properties
associated with oxygen diffusion and oxygen storage/release
mance of CeO for the partial hydrogenation of alkynes to
2
olefins. This type of reaction, widely exploited in steam
crackers for purification of olefin streams as well as in the
manufacture of fine chemicals, is conventionally carried out
[
2]
capacity. Because of these intrinsic features, the presence of
[
11]
CeO in combination with noble metals (e.g. Pt and Rh) and
over promoted palladium-based catalysts.
In this work,
2
other oxides (e.g. g-Al O ) leads to superior activity and/or
ceria was tested in the gas-phase hydrogenation of propyne
(ethyne) at ambient pressure, attaining an olefin selectivity of
91% (81%) at a degree of alkyne conversion of 96% (86%).
We show that the specific surface area and the degree of
reduction are key descriptors of the catalytic performance.
Operando infrared spectroscopic studies enable to derive
a reaction mechanism.
2
3
lifetime, like in the three-way catalyst for automotive
[
3]
emission control. Additional examples that boosted the
use of ceria as a catalyst carrier are 1) the water-gas shift
reaction for H generation over La O -doped Au/CeO , 2) the
2
2
3
2
preferential oxidation of CO in H -rich streams over Pt or Au/
2
CeO , 3) the combustion of alkanes over Cu-promoted CeO ,
2
2
and 4) the hydrogenation of different functional groups over
CeO was the sole crystalline phase in the X-ray diffrac-
2
[
4]
CeO -supported Ni, Pd, Pt, and Au catalysts. Doped with
tion pattern of the commercial ceria nanopowder (Figure S1
in the Supporting Information). Chemical composition anal-
yses by inductively coupled plasma optical emission spectros-
copy (ICP-OES) corroborated the high purity of the sample,
which contains no trace of any conventional hydrogenation
metal, such as Pd, Pt, Au, and Ni. The first step was to identify
2
Gd, Sm, or Au, ceria has been recently applied in the solar-
driven thermochemical dissociation of H O, while in the
2
pyrolysis of glucose, the addition of CeO2 to H-ZSM-5
catalysts reduces the formation of coke and enhances the
[
5]
selectivity to acetaldehyde.
Most of the times, ceria does not possess a stand-alone
catalytic function, but it magnifies the performance of the
main active phase acting as a promoter, stabilizer, or co-
catalyst. In fact, the use of pure ceria in heterogeneous
reaction conditions in which CeO , calcined at 673 K, was
active and selective for propyne hydrogenation. Figure 1a
shows the influence of temperature on the hydrogenation
2
performance, keeping the feed H /alkyne ratio and the
2
[
6]
catalysis is sporadic and exclusive for oxidations. In hydro-
genation catalysis, reduced metals are prototypically
contact time constant. The propyne conversion is maximal
(96%) at 523 K, with a selectivity to propene of 91%. The
lower conversion above 523 K relates to the detrimental
[
7]
applied. Exceptionally, early studies reported that ZnO,
ZnO-MnO , V O -Al O , and Cr O are active for the gas-
effect of CeO reduction, which starts at about 573 K (Fig-
2
2
5
2
3
2
3
2
phase hydrogenation of various substrates (e.g. acetonitrile,
nitrobenzene, 1-decanol, propene, 1-hexene, 1-octene, buta-
ure S2). This important point is elaborated below. The
propene selectivity strongly decreases with temperature,
too, from 91% at 523 K to 25% at 673 K. Instead, the
amount of propadiene formed (propyne isomerization prod-
uct) increases sharply. Despite the large hydrogen excess in
the feed (H /C H ratio of 30:1), propane formation was not
[8]
diene, and acetylene), although selectivity values were often
not provided. Density functional theory (DFT) simulations
have shown that H adsorbs dissociatively on CeO (111) with
2
2
a relatively low activation barrier (0.2 eV) and strong
2
3
4
[9]
exothermicity (À2.82 eV), providing strong hints of its
detected at any condition and the selectivity to oligomers was
remarkably low (2–10%). Figure 1b shows the influence of
the feed H /C H ratio on the activity and the product
potential catalytic function in hydrogenation reactions.
CeO and other reducible oxides catalyze the liquid-phase
2
2
3
4
[10]
hydrogenation of benzoic acid to benzaldehyde, but other
functional groups have not been investigated.
distribution of CeO . The conversion of propyne increases
2
quasi-linearly upon increasing the inlet partial pressure of
hydrogen. The selectivity to propene increases, too, while the
amount of propadiene progressively drops till zero at a H2/
[*] G. Vilꢀ, Dr. B. Bridier, J. Wichert, Prof. J. Pꢀrez-Ramꢁrez
C H4 ratio of 30:1. These results strongly suggest that
3
Institute for Chemical and Bioengineering
hydrogen activation on the ceria surface is the rate-limiting
step. To obtain a suitable H coverage for the reaction to
Department of Chemistry and Applied Biosciences, ETH Zurich
Wolfgang-Pauli-Strasse 10, 8093 Zurich (Switzerland)
E-mail: jpr@chem.ethz.ch
proceed selectively on CeO , a high hydrogen excess in the
2
feed mixture is required. Otherwise, the hydrogenation
activity vanishes and, favored by the high operating temper-
ature, the isomerization of the triple bond to the diene
becomes the main process. The selectivity to oligomers did
not exceed 10% even at a H /C H ratio of 5:1. The effect of
[
**] The ETH Zurich is acknowledged for financial support. Dr. C.
Mondelli is thanked for discussions on the infrared spectroscopy
results.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203675.
2
3
4
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Communications
Figure 1. Steady-state conversion of propyne (X ) and selectivity to propene (S ), propadiene (SPD), and oligomers (SOL) at 1 bar over CeO2
PY
PE
versus a) temperature (at a H /C H ratio of 30:1 and t=0.21 s), b) H /C H ratio (at T=523 K and t=0.21 s), and c) contact time (at a H /C H
4
2
3
4
2
3
4
2
3
ratio of 30:1 and T=523 K). Optimal conditions are emphasized.
the contact time is shown in Figure 1c.
The propyne conversion doubles when the
contact time increases from 0.08 to 0.21 s,
while the propene selectivity is practically
unchanged. However, a longer contact
time favors C–C coupling reactions (selec-
tivity to oligomers up to 27%) at the
expense of a lower olefin yield. As a con-
trol experiment, we assessed the sensitiv-
ity of the reaction selectivity to metal
impurities. For this purpose, we deposited
minute amounts of palladium (0.05 wt%
Pd) on CeO and evaluated the perfor-
2
Figure 2. Conversion of propyne (X ) and selectivity to propene (S ) and propadiene (SPD) at
PY
PE
mance in propyne hydrogenation, under
the optimal conditions identified for ceria.
As shown in Figure S3, the undesired fully
hydrogenated product (propane) was
1
bar as a function of a) the calcination temperature and b) the reduction temperature of
CeO . Reaction conditions: H /C H ratio of 30:1, T=523 K, and t=0.21 s. The influence of
2
2
3
4
the calcination temperature on the specific surface area of the solid (SBET; determined by N2
sorption) and the reduction temperature on the extent of surface vacancies (determined by
obtained with a selectivity exceeding infrared spectroscopy; A=absorbance) are plotted in the secondary y axis of (a) and (b),
5%.
respectively.
The unprecedented selective charac-
9
ter of CeO for partial alkyne hydrogena-
2
tion was corroborated using ethyne as the
substrate. In this case, a stable selectivity to ethene of 81% at
an ethyne conversion of 86% was achieved at 523 K, a H2/
C H ratio of 30:1, and t = 0.35 s. These results exceed the
perature of the sample. This relates well to the decreased
specific surface area of CeO at a higher calcination temper-
2
ature because of sintering. This was substantiated by H2
2
2
[
11]
performance of conventional metal-based catalysts, imply-
ing promising perspectives for application in hydrorefining of
C –C cuts in steam crackers. It should be stressed that all the
temperature-programmed reduction (H -TPR, Figure S4)
2
and TEM (Figure S5) analyses. Figure 2b displays the influ-
ence of the degree of surface reduction on the catalytic
performance. The highest propyne conversion and propene
selectivity were obtained with the oxidic catalyst. Upon pre-
reduction of CeO in 5 vol% H /He (see H -TPR in Fig-
2
3
catalytic data reported here were measured under steady-
state conditions, and that the performance was stable at every
condition for at least 4 h. Other reducible metal oxides,
specifically TiO , ZnO, and V O , were evaluated in propyne/
2
2
2
ure S2), the alkyne conversion and alkene selectivity
decreased drastically. The effect is more pronounced at
a higher reduction temperature, and has been correlated
2
2
5
ethyne hydrogenation at the above conditions. These oxides
were inactive, further emphasizing the outstanding properties
[
8b]
of CeO . In the literature,
V O was reported to hydro-
with a gradual increase of surface vacancies detected by
2
2
5
À1 [12c]
genate ethyne at 673 K with a selectivity to ethane of 93%,
although other conditions were not stated.
infrared spectroscopy (absorption at 3640 cm ).
This
result, consistent with the decreased propyne conversion
above 573 K in Figure 1a, points to the detrimental influence
of surface reduction (directly linked to the amount of
vacancies) on the catalytic efficiency. This contrasts with the
A high specific surface area and a controlled degree of
surface reduction are crucial features for CeO to perform as
2
a superior hydrogenation catalyst. Figure 2a illustrates that
the propyne conversion (strongly) and the propene selectivity
beneficial role of oxygen vacancies on CeO in oxidations and
2
(
moderately) decrease upon increasing the calcination tem-
2
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Chemie
bands of ceria was obtained. The evolution
with time of the diffuse reflectance infrared
Fourier transform (DRIFT) spectra is dis-
played in Figure S6. The above-mentioned
observations and assignments suggest that
surface oxygen species are crucial to stabi-
lize active hydrogen.
A milestone for future studies is to
establish the link between oxidation and
hydrogenation catalysis over ceria. In oxi-
dations, oxygen vacancies are essential for
the performance of ceria and are easier to
form on the (100) and (110) facets, while the
(
111) surface has the less mobile oxygen
[
4e]
atoms. In contrast, our data points to the
detrimental influence of an excessive degree
of surface reduction on the hydrogenation
performance of ceria. Attending to these
observations, it can be inferred that the
oxygen species involved in hydride transfer
might differ from the more mobile oxygen
species involved in oxidation reactions. Fur-
ther studies will focus on studying the gas-
phase hydrogenation of alkynes over ceria
particles with distinct morphology and thus
a variable ratio of exposed planes. In this
Figure 3. a) DRIFT spectra of CeO recorded at 523 K, after calcination (in He flow), during
2
hydrogenation of propyne (H /C H ratio of 30:1 and t=0.21 s), after hydrogenation of
2
3
4
propyne (in He flow), and during reduction of H (in 5 vol% H /He; A=absorbance).
2
2
b) Lateral and frontal views of the CeO (111) facet depicting the proposed reaction
mechanism.
respect, CeO nanooctrahedra and nanorods
with an increased relative amount of (111)
and (110) surfaces, respectively, have been
2
2
reported. Dedicated mechanistic studies on
different facets by DFT are also expected to be useful. These
studies could also explain the low degree of oligomerization
[
4e,6]
water-gas shift reactions,
pointing to differing criteria in
the design of ceria as oxidation or hydrogenation catalyst.
Further understanding of the reaction mechanism was
gathered by operando infrared spectroscopy, which is com-
plemented by published DFT simulations. Figure 3b sche-
matically shows the different steps involved in the reaction.
Hydrogen dissociatively adsorbs on surface oxygen, leading
to two OH groups (step 1). Comparing the spectra of the
calcined catalyst with the one recorded under reaction
conditions (Figure 3a), an increase in the OH stretching
region bands was observed. In particular, the bands at 3708
over CeO . We tentatively attribute it to the “site isolation” of
2
cerium atoms (each cerium atom is surrounded by four
surface oxygen atoms, where hydrogen is adsorbed) in
comparison to pure metals, preventing C–C coupling of
intermediates and maximizing the olefin production.
[9]
In conclusion, we have reported for the first time the
superior performance of bulk CeO in the gas-phase hydro-
2
genation of alkynes. With a propene (ethene) selectivity of
91% (81%) at a high degree of alkyne conversion and stable
behavior, pure ceria is one of the most efficient catalysts ever
reported for this industrially relevant reaction. The high
yields have been ascribed to the influence of both the surface
area and the degree of surface reduction. The latter, in
particular, impacts on the lattice structure through the
formation of oxygen vacancies, which are detrimental for
the hydrogenation performance. Recent results have con-
firmed that even the use of ceria supported on an inert carrier,
À1
and 3660 cm are assigned to mono-coordinated and doubly
[
12c]
bridging OH species.
3
The broad OH absorption at
À1
528 cm appeared only in the presence of C H and relates
3
4
to the hydrogen abstraction from propyne, which forms
[
12b]
another hydroxy group.
strong acidic/basic sites of ceria (cerium and oxygen atoms,
This abstraction is favored by the
[
12a]
respectively),
and leads to the formation of methylacety-
lide (CH ÀCꢀC) onto the cerium atom (step 2). A similar
3
behavior was reported for the interaction of other hydro-
like anatase TiO , leads to a high propyne conversion (86%)
2
[2a]
carbon molecules with ceria. Methylacetylide is partially
hydrogenated to produce propene (step 3), which finally
desorbs as shown in step 4. In fact, the bands at 3104 and
064 cm are due to gas-phase propene.
with the characteristic band at 3470 cm ,
and propene selectivity (93%), and that the amount of
oxygen vacancies of the support is closely connected to the
degree of vacancies on the CeO phase and, therefore, to the
2
À1
[12d]
3
Adsorbed water,
catalytic performance of the material. This contribution opens
exciting perspectives for exploring this oxide as a catalyst for
the selective hydrogenation of other functional groups.
À1 [12d]
was detected
during the reduction in H (proving the formation of oxygen
2
vacancies), but not under reaction conditions. After the
reaction, all the bands assigned to propyne and propene
vanished and a clean surface with the characteristic structural
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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.
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Communications
Sauer, Phys. Rev. Lett. 2009, 102, 026101; d) J. A. Farmer, C. T.
Campbell, Science 2010, 329, 933 – 936.
[3] a) A. Trovarelli in Catalysis by Ceria and Related Materials,
Imperial College Press, London, 2002; b) R. J. Gorte, AIChE J.
2010, 56, 1126 – 1134.
Experimental Section
Commercial CeO nanopowder (Aldrich, ref: 1001091023) with
2
2
À1
a total surface area of 70 m g in the as-received form was calcined
in static air at different temperatures in the range of 673–1173 K for
À1
5
h using a heating rate of 5 Kmin and eventually in situ pre-
[
4] a) Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Science
003, 301, 935 – 938; b) S. Carrettin, P. Concepciꢀn, A. Corma,
J. M. Lꢀpez Nieto, V. F. Puntes, Angew. Chem. 2004, 116, 2592 –
594; Angew. Chem. Int. Ed. 2004, 43, 2538 – 2540; c) P.
reduced in 5 vol% H /He at 523–773 K. The gas-phase hydrogenation
2
2
of ethyne and propyne was studied over these materials at 1 bar in
a continuous-flow fixed-bed microreactor (12 mm in diameter) using
2
0
.3 g of the catalyst (particle size = 0.2–0.4 mm) at variable temper-
Concepciꢀn, A. Corma, J. Silvestre-Albero, V. Franco, J. Y.
Chane-Ching, J. Am. Chem. Soc. 2004, 126, 5523 – 5532; d) J. A.
Rodriguez, P. Liu, J. Hrbek, J. Evans, M. Pꢁrez, Angew. Chem.
atures (423–673 K), contact times (t = 0.08–0.42 s), and feed H /C H
2
3
4
ratios (5–30). The inlet alkyne concentration was 2.5 vol% and the H2
concentration was varied using He as the balance gas. The compo-
sition of the gas at the reactor outlet was analyzed by an online gas
chromatograph (Agilent GC6890N), equipped with a GS-GasPro
column, and a flame ionization detector. The conversion of alkyne, X,
was determined as the amount of reacted alkyne divided by the
amount of alkyne at the reactor inlet. The selectivity of each
compound, S, was calculated as the quantity of product formed
divided by the amount of converted alkyne. The selectivity to
oligomers was obtained using the carbon balance, SOL = 1ÀS S , in
2007, 119, 1351 – 1354; Angew. Chem. Int. Ed. 2007, 46, 1329 –
1332; e) R. Si, M. Flytzani-Stephanopoulos, Angew. Chem. 2008,
120, 2926 – 2929; Angew. Chem. Int. Ed. 2008, 47, 2884 – 2887;
f) F. Yang, J. Graciani, J. Evans, P. Liu, J. Hrbek, J. F. Sanz, J. A.
Rodriguez, J. Am. Chem. Soc. 2011, 133, 3444 – 3451; g) M.
Makosch, J. Sa, C. Kartusch, G. Richner, J. A. van Bokhoven, K.
Hungerbꢂhler, ChemCatChem 2012, 4, 59 – 63.
[
5] a) W. C. Chueh, C. Falter, M. Abbott, D. Scipio, P. Furler, S. M.
Haile, A. Steinfeld, Science 2010, 330, 1797 – 1801; b) A. Primo,
T. Marino, A. Corma, R. Molinari, H. Garcꢃa, J. Am. Chem. Soc.
i
i
which i represents alkanes, alkenes, and alkadienes formed. Other
oxides evaluated in alkyne hydrogenation were ZnO (Carl Bittmann,
ref: 8849), anatase TiO (Aldrich, ref: 637254), rutile TiO (ABCR,
2
2
011, 133, 6930 – 6933; c) G. T. Neumann, J. C. Hicks, ACS Catal.
012, 2, 642 – 646.
2
2
ref: AB204459), and V O (Aldrich, ref: 223794). Operando infrared
2
5
[
6] A. P. Amrute, C. Mondelli, M. Moser, G. Novell-Leruth, N.
Lꢀpez, D. Rosenthal, R. Farra, M. E. Schuster, D. Teschner, T.
Schmidt, J. Pꢁrez-Ramꢃrez, J. Catal. 2012, 286, 287 – 297.
7] S. Nishimura in Handbook of Heterogeneous Catalytic Hydro-
genation for Organic Synthesis, Wiley, New York, 2001.
8] a) P. Sabatier, A. Fernandez, Compt. Rend. 1927, 185, 241 – 244;
b) V. I. Komarewsky, L. B. Bos, J. R. Coley, J. Am. Chem. Soc.
1948, 70, 428 – 430; c) V. I. Komarewsky, D. Miller, Adv. Catal.
1957, 9, 707 – 715.
spectroscopy in alkyne hydrogenation was carried out in a Thermo
Nicolet 5700 spectrometer equipped with a SpectraTech Collector II
diffuse reflectance infrared Fourier transform (DRIFT) accessory and
a high-temperature cell, ZnSe windows, and a mercury–cadmium–
telluride (MCT) detector. The cell was filled with powdered catalyst
and carefully leveled off to reduce reflections of the sample surface
and to ensure reproducible results. The spectra were automatically
recorded every 10 minutes, in the range of 650–4000 cm , by co-
addition of 200 scans at a nominal resolution of 4 cm . The catalysts
prior to and after the reaction were characterized by inductively
coupled plasma optical emission spectroscopy (Horiba Ultra 2), X-
ray diffraction (PANanalytical X’Pert PRO-MPD), N2 sorption at
[
[
À1
À1
[
9] a) K. Sohlberg, S. T. Pantelides, S. J. Pennycook, J. Am. Chem.
Soc. 2001, 123, 6609 – 6611; b) G. Vicario, G. Balducci, S. Fabris,
S. de Gironcoli, S. Baroni, J. Chem. Phys. B 2006, 110, 19380 –
1
9385; c) H. T. Chen, Y. M. Choi, M. Liu, M. C. Lin, Chem-
PhysChem 2007, 8, 849 – 855.
10] D. G. Cheng, M. Chong, F. Chen, X. Zhan, Catal. Lett. 2008, 120,
2 – 85.
11] a) A. Borodzi n´ ski, G. C. Bond, Catal. Rev. Sci. Eng. 2008, 50,
79 – 469; b) D. Teschner, J. Borsodi, A. Wootsch, Z. Rꢁvay, M.
Hꢄvecker, A. Knop-Gericke, S. D. Jackson, R. Schlçgl, Science
008, 320, 86 – 89; c) J. A. Anderson, J. Mellor, R. P. K. Wells, J.
7
7 K (Quantachrome Quadrasorb-SI), transmission electron micros-
copy (Phillips CM12), and temperature-programmed reduction of H2
Thermo TPDRO 1100).
[
[
(
8
Received: May 11, 2012
Revised: July 4, 2012
Published online: && &&, &&&&
3
2
Catal. 2009, 261, 208 – 216; d) B. Bridier, N. Lꢀpez, J. Pꢁrez-
Ramꢃrez, Dalton Trans. 2010, 39, 8412 – 8419; e) M. Garcꢃa-
Mota, J. Gꢀmez-Dꢃaz, G. Novell-Leruth, C. Vargas-Fuentes, L.
Bellarosa, B. Bridier, J. Pꢁrez-Ramꢃrez, N. Lꢀpez, Theor. Chem.
Acc. 2011, 128, 663 – 673.
Keywords: alkynes · ceria · heterogeneous catalysis ·
hydrogenation · olefins
.
[
1] a) E. P. Murray, T. Tsai, S. A. Barnett, Nature 1999, 400, 649 –
[12] a) J. M. Vohs, M. A. Barteau, J. Phys. Chem. 1990, 94, 882 – 885;
b) V. R. Choudhary, V. H. Rane, J. Catal. 1991, 130, 411 – 422;
c) A. Badri, C. Binet, J. C. Lavallay, J. Chem. Soc. Faraday Trans.
1996, 92, 4669 – 4673; d) B. H. Stuart in Infrared Spectroscopy—
Fundamentals and Applications, Wiley, Chichester, 2004.
651; b) J. Chen, S. Patil, S. Seal, J. F. McGinnis, Nat. Nanotechnol.
2006, 1, 142 – 150; c) W. C. Chueh, Y. Hao, W. Jung, S. M. Haile,
Nat. Mater. 2012, 11, 155 – 161.
[
2] a) A. Trovarelli, Catal. Rev. Sci. Eng. 1996, 38, 439 – 520; b) M. V.
Ganduglia-Pirovano, A. Hofmann, J. Sauer, Surf. Sci. Rep. 2007,
62, 219 – 270; c) M. V. Ganduglia-Pirovano, J. L. F. Da Silva, J.
4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Heterogeneous Catalysis
Active and selective: Ceria shows a high
activity and selectivity in the gas-phase
hydrogenation of alkynes to olefins (see
picture). This unprecedented behavior
has direct impact on the purification of
olefin streams and, more importantly, it
opens new perspectives for exploring this
fascinating oxide as a catalyst for the
selective hydrogenation of other func-
tional groups.
G. Vilꢁ, B. Bridier, J. Wichert,
J. Pꢁrez-Ramꢂrez*
&&&& — &&&&
Ceria in Hydrogenation Catalysis: High
Selectivity in the Conversion of Alkynes to
Olefins
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
These are not the final page numbers!