Molecular Engineering of Trifunctional Supported Catalysts for the Aerobic Oxidation of Alcohols

Article abstract of DOI:10.1002/anie.201603673

We describe a simple and general method for the preparation and molecular engineering of supported trifunctional catalysts and their application in the representative Cu/TEMPO/NMI-catalyzed aerobic oxidation of benzyl alcohol. The methodology allows in one single step to immobilize, with precise control of surface composition, both pyta, Cu^I, TEMPO, and NMI sites on azide-functionalized silica particles. To optimize the performance of the heterogeneous trifunctional catalysts, synergistic interactions are finely engineered through modulating the degree of freedom of the imidazole site as well as tuning the relative surface composition, leading to catalysts with an activity significantly superior to the corresponding homogeneous catalytic system.

Full text of DOI:10.1002/anie.201603673

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201603673
German Edition: DOI: 10.1002/ange.201603673
Supported Catalysis
Molecular Engineering of Trifunctional Supported Catalysts for the
Aerobic Oxidation of Alcohols
Antony E. Fernandes,* Olivier Riant, Klavs F. Jensen, and Alain M. Jonas*
Abstract: We describe a simple and general method for the
preparation and molecular engineering of supported trifunc-
tional catalysts and their application in the representative Cu/
TEMPO/NMI-catalyzed aerobic oxidation of benzyl alcohol.
The methodology allows in one single step to immobilize, with
precise control of surface composition, both pyta, CuI,
TEMPO, and NMI sites on azide-functionalized silica par-
ticles. To optimize the performance of the heterogeneous
trifunctional catalysts, synergistic interactions are finely engi-
neered through modulating the degree of freedom of the
imidazole site as well as tuning the relative surface composi-
tion, leading to catalysts with an activity significantly superior
to the corresponding homogeneous catalytic system.
Herein, we demonstrate the easy and rational heteroge-
nization of a complex trifunctional catalytic system of
considerable practical importance,[9] leading to performances
significantly superior to the corresponding homogeneous case
while benefiting from the well-known advantages of sup-
ported catalysis. Most importantly, our methodology allows
not only to quantitatively tune the surface composition of the
catalyst, but also to adjust the degree of freedom of the
catalytic components for increased activity.
We recently reported[10] on a versatile methodology for
the preparation of bifunctional supported molecular catalysts
based on simultaneously reacting couples of alkynylated
partners to an azide-functionalized mesoporous silica plat-
form using the copper-catalyzed azide–alkyne cycloaddition
(CuAAC)[11] reaction. Notably, we demonstrated that the
surface composition faithfully reflected the relative propor-
tion of each ingredient in the grafting solution. This strategy
was initially applied to the CuI/2,2,6,6-tetramethyl-1-piper-
idinyloxyl (TEMPO)-catalyzed aerobic oxidation of alco-
hols,[9a,12] and allowed the precision preparation of bifunc-
tional supported catalysts displaying an activity superior to
that of their monofunctional equivalent as a result of binary
synergistic interactions.[10]
T
he development of multifunctional heterogeneous catalysts
based on homogeneous catalytic systems has recently
emerged as a promising field for advancing modern and
sustainable chemistry.[1] This family of catalysts is crucial for
reactions requiring multiple active components (such as
metal, ligand, base, and other cofactors), and can prove
particularly advantageous for performing cascade of reac-
tions[2] within a single and recyclable material incorporating
a complete cocktail of the required active ingredients.[3] The
careful positioning of these individual functions on a solid
support can also provide ways for improving activity[4] and
selectivity[5] by tailoring synergistic interactions, in a way
reminiscent of enzymatic catalysis. This requires precise
control of the relative concentrations[6] and the dynamic
interactions[7] between the immobilized partners.
From our previous study,[10] we envisioned that incorpo-
rating N-methylimidazole (NMI) sites (NMI is known to
accelerate the Cu/TEMPO-catalyzed aerobic oxidation of
alcohols by decreasing the CuII/CuI reduction potential)[12a,b]
would improve the catalytic performances of our binary
supported Cu/TEMPO catalysts. Accordingly, preliminary
results indeed suggested that adding free NMI to the binary
supported Cu/TEMPO catalyst could lead to a substantial
acceleration. This guided us to the preparation of supported
trifunctional catalysts that incorporate pyridyltriazol
(pyta),[5e,13] CuI, TEMPO, and NMI sites (Scheme 1).
Meanwhile, we surmised that another advantage could
result from using CuAAC chemistry by considering the CuI
catalyst (CuI) used during the grafting step as an inherent
component of the final catalytic system; thus, reducing the
immobilization procedure to a single step from readily
However, it remains challenging to transfer such complex
systems from homogeneous catalysis to the realm of hetero-
geneous catalysis, because this requires synthetic efforts
commensurate with the number of active molecules to
immobilize, often necessitating orthogonal and sequential
approaches.[8] Therefore, general and robust strategies for
rapidly accessing such multifunctional catalysts need to be
developed.
accessible
1 (Scheme 1).
azide-functionalized
mesoporous
silica
[*] Dr. A. E. Fernandes, Prof. O. Riant, Prof. A. M. Jonas
Institute of Condensed Matter and Nanosciences
Universitꢀ Catholique de Louvain
Based on the surface area and the amount of grafted azide
in 1 (270 m2 gÀ1, 0.26 mmolgÀ1), we calculated that an average
distance of ~ 1.3 nm separates each functionalizable site. To
tune the ability of the additional imidazole site to reach this
distance, and thus to act cooperatively, imidazole derivatives
2a–d (Scheme 1), bearing oligo(ethylene oxide) spacers of
various lengths, were synthesized (see the Supporting Infor-
mation). The statistical theory of chain molecules allowed us
to compute the probability to have the imidazole at a specific
Louvain-la-Neuve, 1348 (Belgium)
E-mail: antony.fernandes@uclouvain.be
alain.jonas@uclouvain.be
Dr. A. E. Fernandes, Prof. K. F. Jensen
Department of Chemical Engineering
Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201603673.
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Figure 1. a) Catalytic activity of 5a–d, 6, and 7 in the aerobic oxidation
of benzyl alcohol. b) Initial TOF plotted versus the calculated proba-
bility for the imidazole site to reach another site located 1.3 nm away
from the grafting point of the linker. c) Initial TOF as a function of the
number of ethylene oxide units n on the NMI site. Conditions: BnOH
(0.26 mmol) in toluene (0.2m), O2 bubbling (5.5 mLminÀ1), 808C.
Reactions were performed using a 5 mol% loading in Cu. Yields were
determined by GC analysis using dodecane as the internal standard.
Scheme 1. Preparation of supported bifunctional and trifunctional
catalysts in one step from azide-functionalized mesoporous silica 1.
Table 1: Values of initial turnover frequencies (TOFs).
Catalyst
TOF [ꢁ10À2 minÀ1 [a]
]
5a
5b
5c
5d
14.8
30.0
46.4
55.1
3.1
location depending on the spacer length (Supporting Infor-
mation); the maximal probability to be 1.3 nm away from the
grafting site being attained with an hexaethylene glycol spacer
(Figure S2).
6
7
1.3
Trifunctional catalysts 5a–d were thus prepared using
a 2:1:3 mixture of 2a–d/3/4[14] (NMI being generally used in
a 2-fold excess vs. Cu; 1:3 being the optimal pyta/TEMPO
proportions obtained with the bifunctional supported cata-
lysts)[10] in one step from silica platform 1, ensuring in each
case an equimolar CuI loading with respect to 2-ethynylpyr-
idine 3 (Scheme 1). The catalysts were characterized by FT-
IR, XPS, ICP-AES, and N2-physisorption (Supporting Infor-
mation). Namely, FT-IR showed in each case the disappear-
ance of the characteristic azide band at ~ 2100 cmÀ1, while
XPS analysis confirmed the appearance of CuI signals. As
expected, the Cu concentration, as determined by ICP AES
analysis, was found to be nearly the same for each catalyst
(~ 0.050 mmolgÀ1; Supporting Information). Similarly, bifunc-
tional supported catalysts 6 and 7 were prepared from a 1:3
mixture of 3/4 and a 2:1 mixture of 2a/3, respectively
(Scheme 1).
6 + NMI
7 + TEMPO
5a + NMI
25.4
22.7
56.9
68.0
52.4
94.4
74.0
90.7
102.0
80.4
102.9
11.7
11.9
6.8
5d + NMI
5d + TEMPO
5d[b]
8[b]
5d[b] + TEMPO
5d[b] + NMI
8[b] + TEMPO
8[b] + NMI
NMI/9-Cu/TEMPO (2:1:3)
NMI/9-Cu/TEMPO (2:1:1)
NMI/9-Cu/TEMPO (2:1:0)
NMI/9-Cu/TEMPO (0:1:1)
–
[a] Initial TOFs were obtained by fitting the corresponding experimental
data at low conversions with a linear function passing through the origin
(Supporting Information) and are calculated with respect to Cu
(5 mol%). [b] Catalyst prepared from a new batch of 1 (see in the text).
Catalysts 5a–d, 6, and 7 were evaluated in the benchmark
aerobic oxidation of benzyl alcohol to benzaldehyde in
toluene at 808C under O2 bubbling (5.5 mLminÀ1; Figure 1a).
The trend in catalytic activity with the trifunctional
catalysts linearly followed the probability to have the
imidazole end of the linker at a point located 1.3 nm away
from its grafting point on the substrate (Figure 1b). The
hexaethylene glycol spacer gives the best results among the
probed spacer lengths (as predicted in Figure S2), with a ~ 3.5-
fold improvement of the initial turnover frequency (TOF)
compared to 5a bearing no ethylene oxide spacer (Figure 1c,
Table 1). Interestingly, bifunctional catalysts 6 and 7 per-
formed particularly slowly, giving, respectively, 32% and
12% of the aldehyde product after 3 hours of reaction. In
parallel, when combining the binary supported systems 6 or 7
with homogeneous NMI or TEMPO, respectively, the activity
raised to the one of 5a–d (Figure 1); however, the corre-
sponding initial TOFs only reached less than 50% of those
obtained with catalyst 5d (Table 1).
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Altogether, this clearly demonstrates the importance of
the NMI site and the beneficial synergistic effects in trifunc-
tional catalysts 5a–d, which is in agreement with the recently
postulated mechanisms for the Cu/TEMPO/NMI-catalyzed
aerobic oxidation of alcohols.[9c,12b,15] Although the mecha-
nism is still debated, it was recently demonstrated that the
reaction path involves the formation of an initiating (bipy)-
(NMI)Cu adduct that combines with TEMPO species. There-
fore, both bipyridine, Cu, NMI, and TEMPO are essential for
the reaction to proceed efficiently.
To verify whether the degree of freedom of the oligo-
(ethylene oxide) spacer in 5d was indeed optimal, we
performed a series of tests adding homogeneous NMI to the
reaction mixture (Figure 2).
Figure 3. Catalytic activity of 5d and 8 in the aerobic oxidation of
benzyl alcohol and comparison with homogeneous controls. Condi-
tions: BnOH (0.26 mmol) in toluene (0.2m), O2 bubbling
(5.5 mLminÀ1), 808C. Reactions were performed using a 5 mol%
loading in Cu. Homogeneous control experiments correspond to CuI
(5 mol%), 9 (5 mol%), TEMPO, and NMI. Yields were determined by
GC analysis using dodecane as the internal standard.
Cu content (0.079 mmolgÀ1) compared to the new 5d
(0.054 mmolgÀ1), resulting in ~ 30% less catalyst being
needed to achieve a 5 mol% Cu loading vs. benzyl alcohol
in the catalytic tests (Figure 3).
Remarkably, 8 performed only slightly slower than 5d
(initial TOF being inferior by ~ 20%), with both catalysts
yielding complete transformation of benzyl alcohol to ben-
zaldehyde in less than 45 minutes (Figure 3 and Table 1).
Grafting density and surface area are important param-
eters because they directly impact the average distance
between each site. This explains the difference of activity
between 5d prepared from the two different batches of azide-
functionalized silica 1 (Table 1).
Figure 2. Catalytic activity of 5a and 5d in the aerobic oxidation of
benzyl alcohol in presence of additional free NMI and TEMPO.
Conditions: BnOH (0.26 mmol) in toluene (0.2m), O2 bubbling
(5.5 mLminÀ1), 808C. Reactions were performed using a 5 mol%
loading in Cu. Yields were determined by GC analysis using dodecane
as the internal standard.
While adding homogeneous TEMPO to 8 did not bring
significant improvements, adding free NMI led to a substantial
~ 40% increase in the initial TOF, whereas only a ~ 10%
increase is observed with 5d prepared from the same batch of
1 (Table 1 and Supporting Information). Altogether, this
suggests that the distribution of the active sites in 8 is less
optimal than in 5d, but still allows achieving better activity
per mass unit of catalyst (~ 15% based on initial TOFs). A
maximal turnover number (TON) of 228 and initial TOF of
0.95 minÀ1 could be obtained with 8 based on Cu (Supporting
Information).
Notably, our trifunctional catalysts all performed much
faster than homogeneous control experiments carried out
with pyridyl-triazole ligand 9[16] (Figure 3), CuI, NMI, and
TEMPO (Figure 3 and Table 1), emphasizing the beneficial
synergistic effects of concentrating each catalytic partner in
close proximity on the silica supports. Remarkably, it
appeared that when performing the homogeneous tests with
9 instead of the usual bipyridine ligand, the reaction works
with or without TEMPO (Figure 3 and Supporting Informa-
tion). This is obviously not the case with our heterogeneous
catalysts: the bifunctional pyta-Cu/NMI supported catalyst 7
is the least active, and good activity is restored upon adding
homogeneous TEMPO to 7 (Figure 1). Further studies are
Only a slight improvement could be observed when
combining 5d and free NMI (initial TOF increasing by
~ 20%), whereas when using 5a/NMI, the TOFs reached what
could be obtained with 5d only (Table 1). This confirmed that
the hexaethylene glycol spacer is of proper length to ensure
maximal freedom while keeping the imidazole site close to
the catalyst surface. Similarly, no difference could be noted
when using 5d or 5d/TEMPO (Figure 2 and Table 1).
Next, we were wondering whether it could be possible to
increase the specific activity of 5d, that is, the activity per
mass unit of catalyst. Following our experimental procedure
and earliest results,[10] the surface concentration of Cu in our
multifunctional catalysts is directly controlled by the fraction
of 3 used in the grafting solution. Thus, increasing this
proportion would result in having more Cu sites on the
surface and, in principle, would require less silica catalyst to
achieve similar activity.
Accordingly, 8 was prepared by reacting a 2:1:1 mixture of
2d/3/4 with a new batch of silica 1 (252 m2 gÀ1, 0.29 mmolgÀ1)
using our procedure (Scheme 1). To ensure reliable compar-
ison, 5d was also prepared from this same batch of silica.
Trifunctional catalyst 8 (see the Supporting Information for
full characterization details) showed a ~ 50% increase in its
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Acknowledgements
The authors acknowledge the F.R.S.-FNRS (A.E.F.), Wallo-
nie-Bruxelles International (A.E.F.), Belgian Federal Science
Policy (IAP P7/05), and Novartis for financial support. Anne
Iserentant, Cꢀcile D’Haese, and FranÅois Devred are
acknowledged for ICP-AES, XPS, and physisorption meas-
urements, respectively. A.E.F. is a Chargꢀ de Recherches of
the F.R.S.-FNRS.
Keywords: alcohol oxidation · CuAAC ·
multifunctional catalysts · supported catalysis ·
synergistic effects
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Received: April 15, 2016
Revised: May 30, 2016
Published online: && &&, &&&&
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Communications
Supported Catalysis
A simple and modular strategy is
reported for the preparation and engi-
neering of trifunctional supported cata-
lysts bearing Cu, TEMPO, and NMI sites
for application in the aerobic oxidation of
alcohols. Variation of the linker length on
the NMI component and optimization of
surface composition allowed synergistic
interactions to be tailored for optimal
activity and recycling.
A. E. Fernandes,* O. Riant, K. F. Jensen,
A. M. Jonas*
&&&& — &&&&
Molecular Engineering of Trifunctional
Supported Catalysts for the Aerobic
Oxidation of Alcohols
6
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