Oxidative coupling of alcohols and amines over bimetallic unsupported nanoporous gold: Tailored activity through mechanistic predictability

Article abstract of DOI:10.1002/cctc.201402843

An unsupported nanoporous gold catalyst is employed for the direct coupling of primary alcohols and amines to the corresponding amides in the liquid phase. Among others, the reaction of methanol and dimethylamine to the industrially relevant dimethylformamide proceeds smoothly at 40°C (turnover frequency≈40 h^-1). The activation of molecular oxygen is identified as a key parameter. Doping of the unsupported catalyst by an admetal (Ru, Ag) is used to increase the activity of the catalyst considerably for this reaction (turnover frequency ≈ 100 h^-1).

Full text of DOI:10.1002/cctc.201402843

CHEMCATCHEM
COMMUNICATIONS
DOI: 10.1002/cctc.201402843
Oxidative Coupling of Alcohols and Amines over Bimetallic
Unsupported Nanoporous Gold: Tailored Activity through
Mechanistic Predictability
[
a]
Andre Wichmann, Marcus Bꢀumer, and Arne Wittstock*
[
2]
[1,3]
An unsupported nanoporous gold catalyst is employed for the
direct coupling of primary alcohols and amines to the corre-
sponding amides in the liquid phase. Among others, the reac-
tion of methanol and dimethylamine to the industrially rele-
vant dimethylformamide proceeds smoothly at 408C (turnover
ety of catalytic reactions in the gas and liquid phases,
emerging as the prime representative of skeletal gold cata-
[4]
lysts. Key to the unique catalytic performance of this bulk
nanostructured material is its extended and strongly curved
gold surface providing an abundance of low-coordinated sur-
face atoms, otherwise only present in very small nanoparti-
À1
frequencyꢀ40 h ). The activation of molecular oxygen is
[5]
identified as a key parameter. Doping of the unsupported cata-
lyst by an admetal (Ru, Ag) is used to increase the activity of
cles. The absence of an oxidic support material simplifies the
understanding of the surface chemistry and enables transfer of
mechanistic knowledge from model experiments to reaction
the catalyst considerably for this reaction (turnover frequency
À1
[6]
ꢀ
100 h ).
conditions. This has been demonstrated for the oxidative
[2]
(
cross-)coupling of primary alkyl alcohols in the gas phase.
Demonstration of the generality of this concept for further and
more complex reactions and its application to tailoring catalyt-
ic reactivity based on the identification of mechanistic key
steps remain major challenges.
Unsupported nanoporous gold catalysts (npAu, Figure 1) were
first discovered to be highly active oxidation catalysts in 2006
for the aerobic oxidation of carbon monoxide in the gas
[
1]
[7]
phase. Since then this catalyst has been investigated in a vari-
Amides are one of the most important functional groups
as not only the key group in biological systems but also the
[8]
basis of numerous synthetic polymers and modern pharma-
[9]
ceutical molecules. The direct amidation of an alcohol is the
most economic pathway, involving starting materials that are
[10]
cheap, stable, and available in a sufficient quantity.
007, the direct coupling of alcohols and amines has been re-
ported, employing in particular ruthenium complexes in homo-
Since
2
[11]
genous phases. The development of heterogeneous catalysts
that function under green and environmentally friendly condi-
tions is required. Gold catalysts, known for their low-tempera-
ture oxidation potential, high selectivity, and non-toxic nature
[2b,12]
have proven ideal candidates.
Accordingly, the oxidative
coupling of alcohols and amines by colloidal gold systems
[
13]
(
gold nanoparticles stabilized by, e.g., polyvinylpyrrolidone
[14]
[15]
or DNA ), supported gold systems (on titania), and unsup-
[16]
ported monolithic gold has been investigated. Yet, mecha-
nistic understanding and, hence, directed catalyst design are
lacking.
In the present study, we aim to understand and optimize
nanoporous gold catalysts—that work under reaction condi-
tions—for the direct amine–alcohol coupling. We begin by in-
vestigating the oxidative coupling of the two most simple re-
actants, methanol and dimethylamine, to the industrially rele-
vant product dimethylformamide (DMF) at oxygen pressures of
up to 8 bar (1 bar=100 kPa). In a subsequent set of experi-
ments, the alcohol (by carbon chain length) and the amine (by
nucleophilicity) will be varied and the impact on activity and
selectivity investigated. By aligning mechanistic insights from
Figure 1. Scanning electron micrographs of a cross-section of a npAu disk
(top: entire cross-section, bottom: higher magnification).
[a] Dr. A. Wichmann, Prof. M. Bꢀumer, Dr. A. Wittstock
University Bremen
Centre for Environmental Research and Sustainable Technology
and Institute of Applied and Physical Chemistry
Leobener Strasse NW2, 28359 Bremen (Germany)
Fax: (+49)421-218-63188
E-mail: awittstock@uni-bremen.de
Homepage: http://iapc.chemie.uni-bremen.de
[17]
ultra-high vacuum (UHV) work with our results, we establish
a set of key parameters. Finally, tuning of the surface chemistry
of the npAu catalyst by small fractions of an admetal such as
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402843.
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 5
&
1
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
COMMUNICATIONS
www.chemcatchem.org
Ag or Ru shows whether such mechanistic insight can be used
for generating catalysts with improved catalytic performance.
The coupling of methanol and dimethylamine to DMF in the
liquid phase was investigated in an autoclave setup at oxygen
pressures of up to 8 bar (Figure 2) and in an ambient pressure
reflux setup typical for chemical synthesis (Figure S1). No cou-
pling agent (e.g., strong base) or organic solvent was added
(methanol acted as both solvent and reactant) and “green” re-
action conditions were used. As indicated by the model stud-
ies on single crystals, the presence of oxygen on the gold sur-
[
18]
face was crucial for the activity and selectivity. According to
the mechanism of the initial phase of the coupling reaction
(
Scheme 1), the reactant should only be activated in the pres-
[
17]
ence of this active surface oxygen. Indeed, no catalytic activ-
ity was detected in the absence of molecular oxygen in the re-
action chamber (nitrogen, atmospheric pressure). By introduc-
ing oxygen (ꢁ1 bar), the coupling product DMF was detected
at 408C with 100% selectivity. The generation of DMF was
found to be strictly connected to the oxygen pressure and
supply in both experimental setups. In addition to a fine dis-
persion of oxygen using the reflux setup, the oxygen pressure
in the autoclave system was found to directly relate to the ob-
served conversion of amine and alcohol (Figure 2). Despite the
complexity of the subject of oxygen supply (mass transport
inside the porous structure and transfer from the gas phase
into solution), both findings indicated that the concentration
of oxygen in solution determined the reaction rate. Notably,
in model reactions the selectivity of the reaction was found
to be dependent on the concentration of reactive surface
Figure 2. Reaction of dimethyl amine with primary alcohols catalyzed by
unsupported nanoporous gold (30 mmol dimethylamine, excess alcohol as
solvent): a) Generation of DMF as a function of oxygen pressure (at 408C) in
an autoclave setup after 30 min, b) Variation of alcohol (dimethylamine+
methanol/ethanol/propanol, 408C, continuous oxygen flow at 1 bar).
[
18–19]
oxygen,
implying that under experimental condi-
tions even with high oxygen pressures (e.g., 8 bar)
the steady-state concentration of reactive oxygen on
npAu is low (<0.1 monolayers).
The alcohol–amine coupling experiments showed
a sigmoidal curve shape, suggesting consecutive re-
action kinetics via an intermediate, which according
to the model studies was a surface-bonded methoxy
[
19]
or aldehyde species. To understand the parameters
regarding the formation of an intermediary aldehyde
and its impact on the subsequent coupling with the
amine, 1) liquid-phase oxidation of methanol in the
absence of dimethylamine and 2) a systematic varia-
tion of the alcohol source were performed. In the
liquid-phase oxidation of methanol, no formaldehyde
was found but instead the self-coupling product
methyl formate that can be formed via surface-
bonded aldehyde-like species as reported in the gas-
[
2b]
phase oxidation of methanol on npAu. In the pres-
ence of the amine, however, no methyl formate was
detected (notably, no direct reaction between methyl
formate and the amine was detected, excluding
a direct reaction pathway). A very similar reactivity
was observed for the coupling of aldehydes with al-
[
2b,20]
cohols over gold surfaces.
This confirmed that in
Scheme 1. Mechanism for the coupling of primary alcohols (e.g., methanol) and secon-
dary amines (e.g., dimethylamine) and the alternative pathway in the absence of the
amine component. The investigated parameters and the related reaction steps are
our experiments the formation rate of the aldehyde
by b elimination (Scheme 1, step 3) was rate-deter-
mining. In the presence of surface-bonded amine, marked by boxes (1: oxygen, 2: amine, 3: alcohol).
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 5
&
2
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
COMMUNICATIONS
www.chemcatchem.org
[18,22]
the aldehyde reacted immediately to form the amide product
nucleophile.
Ruthenium, on the other hand, has been the
(
cross-coupling).
most commonly used material for this reaction in the homoge-
neous phase owing to the generation of highly active rutheni-
um-bound methoxy species, which were even more active in
The selectivity for primary alcohol oxidation depended on
the alkoxy chain length, which was attributed to the varying
ease of b elimination of hydrogen. As gas-phase studies re-
vealed, the selectivity for aldehyde production increased with
increasing alkoxy chain length due to the greater tendency for
b elimination, meaning 1-butanol almost exclusively produced
[23]
coupling with amines than with the free aldehyde. Rutheni-
um and silver were deposited on npAu by surface-limited
[24]
redox replacement. Both the silver-doped nanoporous gold
samples (Ag-npAu, Figure 3) and the ruthenium-doped sam-
ples (Ru-npAu) were generated reproducibly with dopant con-
centrations below 3 at% for both sample types, as confirmed
by energy dispersive X-ray spectroscopy (EDX) mapping (for
detailed preparation and characterization, see the Experimental
Section).
[
2a,21]
the corresponding aldehyde butanal.
In the liquid-phase
reaction of methanol, ethanol, or n-propanol with dimethyla-
mine, we found a distinct trend of increasing conversion rates
with increasing alcohol chain length (Figure 2b). As compared
to the results of the gas-phase study, this was indirect evi-
dence for the formation of intermediary aldehyde species.
Aligning the experimental results in this study with the model
derived from experiments on a single crystalline surface under
[
17]
UHV conditions, we propose an extended reaction mecha-
nism of the alcohol–amine coupling including liquid-phase oxi-
dation of methanol on npAu (Scheme 1).
In the next set of experiments, differently substituted aro-
matic amines were coupled with methanol. The corresponding
amide was formed exclusively at temperatures as low as 408C
in satisfying yield (Table 1). The introduction of electron-donat-
Table 1. Oxidative coupling of aromatic amines with methanol by using
[
a]
npAu catalysts.
Entry
1
Amine
Product
Yield [%]
42
Selectivity [%]
100
Figure 3. Cross-sectional scanning electron micrographs of silver-doped
npAu with elemental mapping, revealing a homogenous deposition of silver
over the whole sample (2–3 at% Ag, determined by EDX).
The catalytic performance of the metal-doped nanoporous
gold samples (Ag-npAu, Ru-npAu) was tested by performing
the formylation reaction between methanol and dimethyla-
mine in the autoclave setup (Figure 4). By doping the npAu
with silver, the conversion to DMF could be increased by ap-
2
3
30
58
100
100
À1
proximately 33% (turnover frequency=52 h ). With the Ru-
[
a] Reaction conditions: amine (0.1 mmol), methanol (excess as solvent),
doped npAu samples, the activity could be increased by
À1
À1
oxygen flow (50 mLmin , ambient pressure), reflux setup, 408C, 20 h.
a factor of approximately 2.5 (turnover frequency=97 h ).
Even though the amount of deposited silver (2–3 at%) was
higher compared to that of ruthenium (0.25–2 at%), use of the
latter resulted in almost twice as much conversion (Figure 4)
indicating that the Ru-npAu combination was preferable for
amide bond formation. With respect to the ruthenium com-
plexes used in homogeneous catalysis, surface-bound methoxy
could only be formed on the metal if strong s donor and weak
p acceptor ligands were used to create an electron-rich ruthe-
ing or withdrawing groups in aromatic amines for altering nu-
cleophilicity of the amine appeared, however, not to have a no-
ticeable influence on the reactivity. These findings were in line
with the UHV results, which showed that neither activation of
the NÀH bond nor nucleophilic attack (Scheme 1, step 2) of
the amine were rate-determining steps.
[11d,25]
nium center.
An explanation for the higher catalytic activi-
An optimization of the catalytic activity according to the ob-
tained mechanistic insights must address either the oxygen
availability on the surface (step 1) or the conversion of the al-
cohol to the aldehyde (step 3). We chose silver and ruthenium
as reasonable dopants for npAu. Silver was demonstrated to
efficiently dissociate molecular oxygen, generating atomic
oxygen on the catalyst surface as a Brønsted base and strong
ty of the Ru-npAu-material would thus involve a transfer of
[26]
electron density from the gold substrate to the ruthenium,
similar to the action of ligands in homogeneous ruthenium
catalysts.
In summary, this unsupported nanoporous gold catalyst pro-
vides an ideal platform for studying the key parameters for the
coupling of alcohols and amines. The structural simplicity of
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 5
&
3
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
COMMUNICATIONS
www.chemcatchem.org
tion mixture was stirred with 500 rpm at 25–608C depending on
the experiment for 4 h if not specified otherwise in the text. The
autoclave experiments were performed in a 60 mL stainless steel
reactor with a feed valve for charging the autoclave with 4 bar
oxygen pressure, a depressurization valve, and a drill-hole for
a thermocouple to control the temperature inside the reactor.
Both reactors, the flask at ambient conditions, and the autoclave
were loaded with npAu (6–8 mg), condensed dimethyl amine
(
ꢀ2 mL; Linde, 2.0), methanol (10 mL, p.a., Sigma-Aldrich), and
deionized water (0.5 mL).
Figure 4. DMF production catalyzed by different npAu samples. Experiments
Acknowledgements
were performed at 408C over 4 h with either unsupported npAu with
À1
5
0 mLmin constant oxygen flow or unsupported npAu in comparison with
Financial support by the Func-Band Initiative and the State of
Bremen is acknowledged. We thank Petra Witte (Willems group,
Historical Geology—Palaeontology, Geology Department of the
University Bremen) for experimental support in the SEM and EDX
measurements; in addition Dr. Thomas Dꢁlcks and Dorit Kemken
silver or ruthenium-modified npAu in an autoclave (4 bar oxygen, 30 mmol
dimethylamine and excess methanol).
this unsupported catalyst material opens the door to transfer
mechanistic knowledge from ultra-high vacuum conditions to
applied reaction conditions and, hence, allows for a description
of the reaction on a molecular level. This can be used to fur-
ther develop and optimize the catalyst and its surface reactivi-
ty. Introduction of small quantities of an admetal such as silver,
which is known to provide activity for the bonding and dissoci-
ation of molecular oxygen, can be used to increase the activity
of the catalyst.
(
Spiteller group, Department of Chemistry, University of Bremen)
are thanked for performing GC–MS measurements. We also
thank the school for laboratory technicians of the University
Bremen for providing access to the gas chromatograph used in
almost all experiments in this work.
Keywords: bimetallic catalysts · heterogeneous catalysis ·
gold · nanoporous materials · oxidation
[
1] V. Zielasek, B. Jꢂrgens, C. Schulz, J. Biener, M. M. Biener, A. V. Hamza, M.
Bꢀumer, Angew. Chem. Int. Ed. 2006, 45, 8241–8244; Angew. Chem.
2006, 118, 8421–8425.
Experimental Section
All chemicals were purchased commercially and used without fur-
ther purification. The amide products were quantified by a Chrom-
pack CP 9001 gas chromatograph with a SE-54 column. For all
amides, dimethylacetamide was used as internal standard except
for the amine-variation experiment, in which 1,3,5-trimethylben-
zene was used (Table 1). The aromatic amides (Table 1) were also
characterized by GC–MS by using HP 5890 (CP Sil-8CB as column)
and MAT 95 (electron ionization, 70 eV) instruments. The retention
times in the GC analysis and the MS spectra were compared to
those of pure products obtained from commercial sources.
[
2] a) K. M. Kosuda, A. Wittstock, C. M. Friend, M. Bꢀumer, Angew. Chem. Int.
Ed. 2012, 51, 1698–1701; Angew. Chem. 2012, 124, 1730–1733; b) A.
Wittstock, V. Zielasek, J. Biener, C. M. Friend, M. Bꢀumer, Science 2010,
3
27, 319–322.
[
3] a) H. M. Yin, C. Q. Zhou, C. X. Xu, P. P. Liu, X. H. Xu, Y. Ding, J. Phys. Chem.
C 2008, 112, 9673–9678; b) Y. Yamamoto, Tetrahedron 2014, 70, 2305–
2317.
[4] a) A. Wittstock, M. Bꢀumer, Acc. Chem. Res. 2014, 47, 731–739; b) A.
Wittstock, J. Biener, J. Erlebacher, M. Bꢀumer, Nanoporous Gold: From an
Ancient Technology to a Novel Material, Vol. 1, 1st ed., RSC, 2012.
[
5] T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, T. Tokunaga,
S. Arai, Y. Yamamoto, N. Tanaka, Y. Ishikawa, N. Asao, Y. Yamamoto, J. Er-
lebacher, M. Chen, Nat. Mater. 2012, 11, 775–780.
Cross-sectional scanning micrographs were collected on a ZEISS
SUPRA 40 instrument with a SE2 detector and accelerating voltage
of 15 kV. EDX [with spatial resolution (mapping)] was performed by
using a Bruker XFlash 6130 detector.
[6] K. J. Stowers, R. J. Madix, C. M. Friend, J. Catal. 2013, 308, 131–141.
[7] C. L. Allen, J. M. J. Williams, Chem. Soc. Rev. 2011, 40, 3405–3415.
[
8] A. Greenberg, C. M. Breneman, J. F. Liebman, The Amide Linkage: Struc-
tural Significance in Chemistry, Biochemistry and Materials Science, Wiley-
VCH, 2002.
All npAu samples were fabricated by immersing a 100 mm-thick
silver–gold alloy (70 at% Ag, 30 at% Au) with a diameter of 6 mm
for 24 h in concentrated nitric acid (65%wt, 50 mL, Omnilab). After
dealloying, the samples were washed 3 times with deionized water
[9] J. S. Carey, D. Laffan, C. Thomson, M. T. Williams, Org. Biomol. Chem.
006, 4, 2337–2347.
10] G. E. Dobereiner, R. H. Crabtree, Chem. Rev. 2010, 110, 681–703.
11] a) C. Gunanathan, Y. Ben-David, D. Milstein, Science 2007, 317, 790–792;
b) C. Chen, Y. Zhang, S. H. Hong, J. Org. Chem. 2011, 76, 10005–10010;
c) I. S. Makarov, P. Fristrup, R. Madsen, Chem. Eur. J. 2012, 18, 15683–
2
[
[
(
ꢀ30 mL) and dried for 24 h in air. These samples were either used
as unmodified npAu samples for catalysis or doped with silver or
ruthenium by electrochemical deposition (see the Supporting In-
formation for details).
15692; d) J. Malineni, C. Merkens, H. Keul, M. Moller, Catal. Commun.
2
013, 40, 80–83.
Catalytic experiments under ambient conditions were performed
by using a three-necked 50 mL flask with a reflux condenser con-
nected with a Jꢂrgens Haake FK cryostat at À28C for reactant re-
[
12] A. Wittstock, B. Neumann, A. Schaefer, K. Dumbuya, C. Kubel, M. M.
Biener, V. Zielasek, H. P. Steinrꢂck, J. M. Gottfried, J. Biener, A. Hamza, M.
Bꢀumer, J. Phys. Chem. C 2009, 113, 5593–5600.
storation. In addition, a cooling trap (À408C, EtOH/liquid N mix-
2
[
13] P. Preedasuriyachai, H. Kitahara, W. Chavasiri, H. Sakurai, Chem. Lett.
ture) was installed behind the reflux condenser to collect potential
2
010, 39, 1174–1176.
reactants. The other two necks were charged with an oxygen feed
[14] Y. Wang, D. P. Zhu, L. Tang, S. J. Wang, Z. Y. Wang, Angew. Chem. Int. Ed.
2011, 50, 8917–8921; Angew. Chem. 2011, 123, 9079–9083.
À1
(
50 mLmin ; Linde AG, 4.5) and a septum for sampling. The reac-
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 5
&
4
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
COMMUNICATIONS
www.chemcatchem.org
[15] a) S. K. Klitgaard, K. Egeblad, U. V. Mentzel, A. G. Popov, T. Jensen, E.
[22] I. E. Wachs, R. J. Madix, Surf. Sci. 1978, 76, 531–558.
Taarning, I. S. Nielsen, C. H. Christensen, Green Chem. 2008, 10, 419–
[23] a) Y. Zhang, C. Chen, S. C. Ghosh, Y. X. Li, S. H. Hong, Organometallics
2010, 29, 1374–1378; b) D. Cho, K. C. Ko, J. Y. Lee, Organometallics
2013, 32, 4571–4576.
[24] R. E. Rettew, J. W. Guthrie, F. M. Alamgir, J. Electrochem. Soc. 2009, 156,
D513–D516.
423; b) S. Kegnæs, J. Mielby, U. V. Mentzel, C. H. Christensen, A. Riisager,
Green Chem. 2010, 12, 1437–1441; c) J. Mielby, A. Riisager, P. Fristrup, S.
Kegnaes, Catal. Today 2013, 203, 211–216; d) J. F. Soulꢃ, H. Miyamura, S.
Kobayashi, J. Am. Chem. Soc. 2011, 133, 18550–18553.
[
[
[
16] S. Tanaka, T. Minato, E. Ito, M. Hara, Y. Kim, Y. Yamamoto, N. Asao, Chem.
Eur. J. 2013, 19, 11832–11836.
17] B. Xu, L. Zhou, R. J. Madix, C. M. Friend, Angew. Chem. Int. Ed. 2010, 49,
[25] F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122–3172;
Angew. Chem. 2008, 120, 3166–3216.
[26] M. Kuhn, J. A. Rodriguez, J. Hrbek, A. Bzowski, T. K. Sham, Surf. Sci. 1995,
341, L1011–L1018.
3
94–398; Angew. Chem. 2010, 122, 404–408.
18] C. G. F. Siler, B. J. Xu, R. J. Madix, C. M. Friend, J. Am. Chem. Soc. 2012,
34, 12604–12610.
1
[19] B. J. Xu, R. J. Madix, C. M. Friend, Chem. Eur. J. 2012, 18, 2313–2318.
[20] B. J. Xu, X. Y. Liu, J. Haubrich, C. M. Friend, Nat. Chem. 2010, 2, 61–65.
[21] B. J. Xu, C. M. Friend, Faraday Discuss. 2011, 152, 307–320.
Received: October 21, 2014
Published online on && &&, 0000
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 5
&
5
&
ÞÞ
These are not the final page numbers!

COMMUNICATIONS
A. Wichmann, M. Bꢀumer, A. Wittstock*
&& – &&
Oxidative Coupling of Alcohols and
Amines over Bimetallic Unsupported
Nanoporous Gold: Tailored Activity
through Mechanistic Predictability
Green, gold, and ready for amidation:
An unsupported nanoporous gold cata-
lyst is employed for the direct coupling
of primary alcohols and amines to the
corresponding amides in the liquid
phase. The activation of molecular
oxygen is identified as a key parameter.
Doping of the unsupported catalyst by
an admetal (Ru, Ag) is used to consider-
ably increase the activity of the catalyst
for this reaction by a factor of 4.
DMF=Dimethylformamide.
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 5
&
6
&
ÞÞ
These are not the final page numbers!