Aerobic Oxidative Dehydrogenation of Amines Catalyzed by a Recoverable Ruthenium Catalyst under Mild Reaction Conditions

Article abstract of DOI:10.1002/cctc.201701220

A highly active catalyst based on perruthenate ions supported inside the channels of periodic mesoporous organosilica with a bridged imidazolium ionic liquid framework (Ru@PMO-IL) was developed. The material was found to be an efficient, durable, and recoverable catalyst for the oxidative dehydrogenation of various types of amines such as benzylic, aliphatic, and cyclic aliphatic amines under mild reaction conditions. The products were obtained in excellent yields with excellent selectivities.

Full text of DOI:10.1002/cctc.201701220

Accepted Article
Title: Aerobic Oxidative Dehydrogenation of Amines Catalysed by a
Recoverable Ruthenium Catalyst under Mild Reaction Conditions
Authors: Babak - Karimi, Omolbanin Yari, Mojtaba Khorasani,
Hojatollah Vali, and Fariborz Mansouri
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ChemCatChem
COMMUNICATION
Aerobic Oxidative Dehydrogenation of Amines Catalysed by a
Recoverable Ruthenium Catalyst under Mild Reaction Conditions
Babak Karimi*,[a, b] Omolbanin Yari,[a] Mojtaba Khorasani,[a] Hojatollah Vali[c], and Fariborz Mansouri[a]
Abstract: A highly active catalyst based on perruthenate ions
supported inside the channels of the periodic mesoporous
organosilica with bridged imidazolium framework (Ru@PMO-IL) is
developed. The material was found to be an efficient, durable and
recoverable catalyst for oxidative dehydrogenation (ODH) of various
types of amines such as benzylic, aliphatic and cyclic aliphatic
amines in excellent yields and selectivities under mild reaction
conditions.
involved the immobilization of typically low-valent Ru species on
the various solid supports such as hydroxyapatite (HAP), Al2O3,
Fe3O4, CeO2, SiO2, TiO2, Co3O4 and MoS2. While by using these
systems, considerable improvements have been achieved in the
catalytic activity and selectivity at high loading of Ru, elevated
temperatures and unusual O2 pressure. In addition, high energy
barrier is generally needed for the re-oxidation of the reduced
form of ruthenium catalyst by molecular oxygen during reaction
progress.[7] Therefore, there are still major problems with
supported Ru based catalysts. It seems that an efficient and
successful way to circumvent the deactivation of Ru catalysts is
the use of high-valent ruthenium species such as ruthenium
tetroxide and perruthenate ions.[7] An important feature of these
active catalysts is their high tendency to oxidize various organic
substrates such as alkenes, alcohols, amines, and even none-
activated hydrocarbons under mild reaction conditions.[7] Despite
high activity in oxidation, to the best of our knowledge,
heterogenized perruthenate-based catalyst has not been
explored for direct oxidation of primary amines to the related
nitriles, yet.
We have already shown that supported perruthenate ions
inside the channels of the periodic mesoporous organosilicas
with imidazolium network (Ru@PMO-IL, Fig. S1) could serve as
an efficient catalyst for the aerobic oxidative dehydrogenation of
alcohols into the related carbonyl compounds under mild
reaction conditions.[8] In continuation of this study, we found that
Ru@PMO-IL could even more effectively catalyse aerobic
oxidation of amines under mild reaction conditions. To the best
of our knowledge, there are currently, no reports either of
employing PMOs as support or supported perruthenate catalyst
in the aerobic oxidation of amines to the corresponding nitriles.
At the first step, the PMO-IL scaffold was prepared according to
our literature procedure with slight modifications.[9] Perruthenate
ions were then immobilized into the PMO-IL channels through a
simple ion-exchange technique.[8] In brief, a desired amount of
PMO was first dispersed in water and resulting suspension was
sonicated for several minutes to obtain a mixture with very
uniform PMO particles. A solution of potassium perruthenate
(0.047 g, 0.22 mmol) in water was then added to the mentioned
mixture, for replacing some chloride ions with perruthenate ions.
Fig. 1 shows N2 sorption isotherm and the corresponding BJH
pore size distribution for both PMO-IL and Ru@PMO-IL. Both
samples exhibited type IV isotherm with sharp H1 hysteresis
loop which, implying the presence of uniform cylindrical
mesoporous channels in the materials. The BET surface area
from 560 to 490 m2g-1 and the pore volume from 0.99 to 0.92
cm3g-1 slightly decreased after perruthenate immobilization
(Table 1), probably due to fact that the perruthenate ions were
well immobilized in the interior of the mesochannels of the
pristine PMO-IL.
Nitriles are valuable synthetic building blocks in the synthesis of
valuable compounds.[1] Nitriles are traditionally prepared by the
ammoxidation of arenes or alkenes, hydrocyanation, Sandmeyer
and Schmidt reactions.[2] In recent years, a substantial amount
of researches has been directed toward the development of
direct oxidative dehydrogenation (ODH) of primary amines to
nitriles by molecular oxygen in the presence of transition metal
catalysts; a pathway which avoids both dangerous reagents and
severe reaction conditions.[3] However, the ODH of primary
amine to nitrile is a challenging task since it requires the removal
of 4 electrons and 4 protons and generally involves several
competing, dehydrogenation, coupling and transamination
pathways.[4] Therefore, this reaction could potentially give a
broader diversity of products, which include oximes, imines,
amides, nitriles, amine oxides, and azo compounds. For this
reason, designing new efficient catalytic system to afford each of
the described products in a highly selective pathway is still of
paramount importance.
Among metal-catalyzed aerobic amoxidation processes,3
ruthenium-based catalysts received a great deal of attentions to
achieve high efficiency and selectivity in the ODH reaction of
amines under aerobic conditions.[5] However, in contrast to
aerobic oxidation of alcohols, the oxidation of amines using
heterogeneous catalyst have not been widely explored. From
the viewpoint of practical applications, heterogeneous Ru-based
catalysts have been drawn particular attentions due to simple
recovery and superior performance of catalyst and more
importantly minimization of metal contamination in the final
products.[6] The majority of supported ruthenium catalysts are
[a]
Prof. Dr. Babak Karimi, Omolbanin Yari, Dr. Mojtaba Khorasani
Faculty of Chemistry, Institute for Advanced Studies in Basic
Sciences (IASBS)
PO-Box 45195-1159, Gava zang, Zanjan 45137-66731, Iran.
E-mail: karimi@iasbs.ac.ir
[b]
[c]
Research Center for Basic Sciences & Modern Technologies
(RBST), Institute for Advanced Studies in Basic Sciences (IASBS),
Zanjan 45137-66731, Iran
Prof. Hojatollah Vali
Department of Anatomy and Cell Biology and Facility for Electron
Microscopy Research, McGill University, Montreal, Quebec, H3A
2A7 (Canada)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201701220
ChemCatChem
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(Table 1). These findings were also in good agreement with
the results of FT-IR technique. Finally, the amount of
Table
1 Textural properties of the PMO-IL, Ru@PMO-IL and recovered
-
loaded RuO4 ion in the catalyst was found to be 0.4 mmol
g-1 using inductively coupled plasma atomic emission
Ru@PMO-IL.
[a]
[b]
[c]
Material
SBET
Vt
DBJH
F.G. [d]
(mmol.g-1)
spectroscopy (ICP-AES) for its acid washed solution.
(m2g-1)
After characterization of Ru@PMO-IL, its catalytic
performance was evaluated in the aerobic oxidation of
amines in the presence of molecular oxygen. To assign the
optimal experimental conditions, the influence of various
reaction parameters such as solvent, catalyst loading, and
reaction temperature was investigated in the aerobic
oxidation of benzyl amine as substrate model (Table 2). In
this regard, it was found that the oxidation of benzyl amine
in the presence of 2.5 mol% of Ru@PMO-IL in water as
PMO-IL
560
490
397
0.99
0.92
0.88
10.6
10.6
9.2
1.1 (IL)
Ru@PMO-IL
0.40 (Ru)
0.37 (Ru)
Recovered
Ru@PMO-IL
[a] SBET: specific surface area from linear part of adsorbed nitrogen in the
range from 0.05 to 0.3. [b] Vt: total pore volume derived from adsorbed
nitrogen from relative pressure ≈ 0.99. [c] DBJH: average pore size diameter
calculated from the adsorption branch using BJH method. [d] Loading of
functional groups (F.G) estimated by TGA, elemental analysis or ICP-AES.
solvent at 70 ̊C was not successful, giving 6% conversion
and 28% selectivity for benzonitrile within 10h (Table 2,
entry 1). Notably, by prolonging reaction time to 24h,
although starting amine was completely consumed, the
reaction selectivity to benzonitrile remarkably dropped and
a mixture of benzaldehyde, benzoic acid, banzamide, and
self-condensed benzyl amine was obtained, according to
the GC analysis (Table 2, entry 1). This observation is in
good agreement with the report by Mizuno and co-workers
in amine oxidation in aqueous medium.[12] Owing to the high
electrophilic character of intermediate imine in the first
oxidation step, the C=N unit is more likely prone to undergo
either hydrolysis or nucleophile attack by starting amines or
water to give related side-products, thus lowering selectivity
toward the formation of nitrile. However, a considerable
improvement was achieved when α,α,α-trifluorotoluene
(TFT) was employed as non-aqueous reaction solvent at
Figure 1. Nitrogen adsorption-desorption and BJH pore size distribution for
PMO-IL and Ru@PMO-IL (a) TEM images of PMO-IL (b) and Ru@PMO-IL (c);
scale bar: 100 nm
TEM images of the materials also provided further evidence
that the long range hexagonal P6mm ordering in both
materials was reserved after surfactant extraction and
immobilization of perruthenate ions (Figures 1a, 1b, S2 and
S3). Additionally, both PMO-IL and Ru@PMO-IL were
further characterized by EDX spectroscopy (Figures S4 and
S5). As can be clearly seen, after ion exchange process, a
new characteristic peak related to perruthenate was
developed, while the peak corresponding to chloride
disappeared in the spectrum of Ru@PMO-IL.
70
benzonitrile in good selectivity of 80% (Table 2, entry 2). In
this regard, increasing temperature to 90 had no
̊C, in which the reaction proceeded well to afford
̊
C
significant effects on the reaction selectivity, while the
reaction times proved to be decreased to some extent
(Table 2, entry 3). Although the oxidation of benzyl amine in
TFT led to complete conversion of benzyl amine to oxidized
product, it should be underlined that a more or less
comparable outcome was obtained in toluene under
otherwise identical reaction conditions (Table 2, entry 4).
However, it was found that water impurities in solvent
FT-IR spectrum of Ru@PMO-IL displayed peaks in the
range of 2930–3150 cm-1 and 1620 cm-1, which is attributed
to the asymmetric stretching vibration of aliphatic or
aromatic C–H bonds and the stretching vibration of C=N
bonds in imidazolium moieties, respectively (Figure
S5).[10,11] The 13C CP/MAS NMR spectrum of PMO-IL
exhibits distint characteristic signals at 9.1, 24.0, 52.1,
122.6, and 136.4, which are corresponding to carbon
species of the alkyl imidazolium bridging groups in the
material (Figure S7). These data imply that the imidazolium
groups were well incorporated into the materials. In
addition, no signal corresponding to methoxy groups was
observed in the 13C-NMR spectrum, which strongly confirms
a high degree of hydrolysis and efficient cross linking in the
material. TG pattern of catalyst was recorded in the range
of 25−800 °C (Figure S8). The catalyst exhibited high
thermal stability, with remarkable weight loss of organic
functional groups removed at temperatures >300 °C as
measured by TGA, which was in close agreement with
theoretical organic loadings and elemental analysis data
played
a crucial role in the decreasing of reaction
selectivity, possibly via either hydration of nitrile or
hydrolysis of intermediate imine. Therefore, we decided to
conduct the rest of experiments in the anhydrous toluene
(Table 2, entries 5-7). This has also a significant positive
impact on both selectivity and the rate of reaction, which
allowed for 98% conversion and 94% selectivity at 85 °C
after only 6 h (Table 2, entry 5). Neither increasing the
reaction temperature nor catalyst loading was found to be
beneficial for improvement in the conversion, selectivity,
and reaction times (Table 2, entries 6, 7). Therefore, the
best conditions were found to be, Ru@PMO-IL (2.5 mol%),
O2 (1 atm.) in anhydrous toluene (2 mL) at 85 °C (Table 2,
entry 5). The high catalytic activity of Ru@PMO-IL in
toluene may be attributed to the superior solubility of
molecular oxygen in this solvent. Under these conditions,
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no evidence for formation of self-condensed benzyl amine
was observed, while in many transition-metal catalytic
protocols aerobic oxidative homo-coupling of amines could
strongly compete the desired dehydrogenation of amines,
furnishing self-condensed imines as major product.[3a] To
collect more information on the decisive role of bridged
ionic liquid and catalyst structure, the aerobic oxidation of
benzyl amine was also tested using a number of selected
Ru-supported catalysts (with the same loading of Ru and/or
ionic liquid) such as Ru@SBA-15-IL, Ru@SBA-15, as well
as homogenous KRuO4 under the same reaction conditions
(Table 2, entries 8-10). The results clearly confirm the
beneficial role of the IL inserted inside the catalyst
framework in obtaining acceptable catalytic activities.
establish a general reaction trend based on the electron
properties of the substrates alone, but one might argue that
electron-withdrawing groups should slow down the reaction
as they would decrease the electron density on amino
groups to some extent and render them less susceptible to
undergo dehydrogenation. (Table 2, entries 7-8). Selective
oxidation of primary aliphatic amines into the corresponding
nitriles is also considered to be particularly challenging.
Although, somewhat higher catalyst loadings were needed,
3-phenyl propane amine was selectively oxidized to the
related nitrile within 24h at 85 ̊C in excellent yield and
selectivity (Table 3, entry 9). The oxidative aromatization of
heterocyclic secondary amines was selectively mediated
using the Ru@PMO-IL under the optimized reaction
conditions (Table 3, entries 10-11). Indoline was easily
converted into indole in excellent yield and selectivity (Table
3, entry 10). Interestingly, 1,2,3,4-tetrahydroquinoline
reacted significantly slower than Indoline and required
higher catalyst loading of 4 mol% and longer reaction time
of 36 h to selectively afford quinolone in 86% yields (Table
3, entry 11).
Table 2. Optimization conditions for aerobic oxidation of benzyl amine in the
presence of Ru@PMO-IL.[a]
Sel.
[b]
t
T
Con. [b]
(°C) (%)
Entry
1
Catalyst (mol%)
Solvent
H2O
Table 3. Aerobic oxidation of amines using perruthenate supported on PMO-IL
(Ru@PMO-IL).[a]
(h)
(%)
28
2[c]
80
84
78
94
96
95
94
99
-
Ru@PMO-IL (2.5)
10
24
16
12
14
6
70
6
Entry
Substrate
Product
t (h)
Yield (%)[b]
70 >99
70 >99
90 >99
C6H5-CH2NH2
C6H5-CN
2
3
Ru@PMO-IL (2.5)
Ru@PMO-IL (2.5)
Ru@PMO-IL (2.5)
Ru@PMO-IL (2.5)
Ru@PMO-IL (2.5)
Ru@PMO-IL (3.5)
Ru@SBA-15-IL (3.5)
Ru@SBA-15 (3.5)
KRuO4 (3.5)
TFT
1
2
6
6
94
94
96
96
84
80
86
80
70
4-MeO-C6H4-CH2NH2
4-Me-C6H4-CH2NH2
4-Cl-C6H4-CH2NH2
2-Cl-C6H4-CH2NH2
3-MeO-C6H4-CH2NH2
2,4-di-Cl-C6H4-CH2NH2
4-NO2-C6H4-CH2NH2
C6H5-CH2CH2CH2NH2
4-MeO-C6H4-CN
4-Me-C6H4-CN
4-Cl-C6H4-CN
2-Cl-C6H4-CN
3-MeO-C6H4-CN
2,4-di-Cl-C6H4-CN
4-NO2-C6H4-CN
C6H5-CH2CH2CN
TFT
4
Toluene
90
85
98
99
98
99
50
30
<2
3
7
5
Anhyd. Toluene
Anhyd. Toluene
Anhyd. Toluene
Anhyd. Toluene
Anhyd. Toluene
Anhyd. Toluene
4
8
6
6
100
85
5
16
16
24
24
24[c]
7
6
6
8
6
85
7
9
6
85
8
10
6
85
9
10
[a] Reaction conditions: benzyl amine (0.25 mmol, 27 mg), Ru@PMO-IL
(2.5 mol%, 16 mg), solvent (2 mL) and O2 (1 atm.) unless otherwise
mentioned. [b] GC yield using internal standard method. [c] The main by-
products are benzaldehyde, benzoic acid, banzamide, and self-
condensed benzyl amine, which have been observed only in water.
10
99
11
36[c]
24[c,d]
86
85
24[c]
17[c,d]
95
97
The much higher activity of Ru@PMO-IL, as compared to
other catalysts may be attributed to presence of bridged IL
units inside the channel walls of PMO, which would favour
activation and stabilization of Ru species and thus enhance
the catalytic activity of catalyst (Table 2, entry 7 Vs. 8 and
9). It should be noted that we checked the ODH of benzyl
amine under the optimized reaction conditions using air
instead of oxygen as the final oxidant. However, the
reaction was not proceeded effectively under atmospheric
pressure of air. We ascribed that the use of higher pressure
of air as oxidant may be required to achieve reasonable
results in this transformation.
12
13
24[c]
14[c,d]
89
95
[a] Reaction conditions: amine (0.25 mmol), Ru@PMO-IL (2.5 mol%, 16 mg),
anhydrous toluene (2mL) at 85 C unless otherwise stated. [b] GC yield using
internal standard addition. [c] 4 mol% catalyst was used.[d] at 90 C.
̊
̊
Secondary open-chain amines such as dibenzylamine
could be also converted to related imine under the
described oxidation conditions. Under optimized reaction
conditions, dibenzyl amine selectively oxidized to N-benzyl
phenyl imine in excellent yield and selectivity (Table 3, entry
12). Interestingly, benzophenone was obtained as major
product during oxidation of benzhydryl amine (Table 3,
entry 13).
With the optimal reaction conditions in hand, the
performance, selectivity and substrate scope of the
presented Ru@PMO-IL were investigated with various
either primary or secondary amines (Table 3). As shown in
Table 3, primary amines bearing either electron-
withdrawing or electron-donating groups converted to their
benzonitrile derivatives in good to excellent yields and
selectivities (Table 3, entries 1–8). Although it is difficult to
Recyclability of Ru@PMO-IL catalyst was also examined in
the aerobic oxidation of benzyl amine under described
reaction conditions (Table 3, entry 1) but in a large-scale
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experiment (using 2.5 mmol substrate). In this regard, after
each reaction run, catalyst was carefully separated using
simple centrifugation and reuse for four subsequent
reactions with only a slight decrease in the catalyst activity
and selectivity under the same reaction conditions (Figure
S9). The amounts of Ru species in the final recovered
catalyst were found to be 0.37 mmol g-1 which was very
close to the fresh catalyst (0.4 mmol g-1). Moreover, hot
filtration test was used to determine the amounts of
perruthenate species leached into the solution. This test
revealed that benzonitrile was obtained in only 5% yield in
24 h at 85 °C after Ru@PMO-IL catalyst was removed from
the reaction solution after 1h. This data was further
confirmed by ICP-AES of the filtrate solution that showed
any ruthenium content within the detection limit (less than 1
ppm). Therefore, it is believed that the observed monotonic
decrease in the yield of bezonitrile through the subsequent
runs in recycling experiment is likely due to the both slight
loss of recovered catalyst in each run as well as a little
leaching out the Ru species from the support channels. To
gain more information about the structural stability of
catalyst, a sample of recovered catalyst Ru@PMO-IL from
the last cycle of this transformation was also analyzed using
N2 sorption measurement, TEM images and TG analysis.
The nitrogen physisorption analysis of recovered catalyst
indicated a typical type IV isotherm with hysteresis loop H1
demonstrating that the recovered catalyst is still
characterized as ordered mesoporous materials (Figure
S10). Moreover, the sharp capillary condensation step
establishes meso-structural uniformity of the catalyst did not
change in multiple cycles (Table 1). By comparing structural
parameter of the fresh catalyst with those of recovered
catalyst, it is clear that no significant change in either
structural order or pore size distribution occurred during the
catalysis and recycling steps (Figure S11, Table 1).
Additionally, TG pattern of recovered catalyst indicated that
the catalyst compositions were stable during the reaction
conditions. Predictably, TEM images of the recovered
Ru@PMO-IL after the 4th reaction cycle indicates the
presence of a two-dimensional ordered structure with
estimated pore size 10 nm a finding which is in good
agreement with nitrogen sorption analysis (Figure S12).
The present system compares very well with previously
reported Ru-based catalysts in the aerobic oxidation of
benzylamine to the benzonitrile (Table S1). The collected
results show that under very close reaction conditions, our
protocol is superior to ruthenium catalysts supported on the
anatase (TiO2),[6h] molybdenum disulfide (MoS2),[6i]
hydroxyapatite (HAP),[6e] magnetic nanoparticles (Fe3O4),[6d]
alumina (Al2O3),[6c] and cerium oxide (CeO2)[6h] in terms of
TON values at lower catalyst loading and reaction
temperature (Table S1, entries 1-6). Although, our catalyst
system exhibits comparable performance to Ru@Co3O4,[3g]
it has the advantage of working in less expensive toluene at
lower reaction temperatures (Table S1, entries 5-8).
may be suggested that high activity and reusability of
catalyst could be attributed to the fact that imidazolium
groups in the size-restricted mesopores of PMO-IL is
effective for preventing both leaching and the
agglomeration of Ru species in Ru@PMO-IL. Further
studies on other applications of this catalyst system are
currently ongoing in our laboratories.
Experimental Section
Preparation of Ru@PMO-IL: PMO-IL scaffold was prepared according
to our literature procedure with slight modifications.[9a] In a typical
procedure, PMO-IL (0.5 g, 1.0 mmol.g-1 IL) was added to deionized water
(10 mL) and sonicated for at least 10 min. A solution of KRuO4 (0.047 g,
0.22 mmol) in deionized water (3 mL) was gradually added to said
suspension and stirred at room temperature for 5 h. The resulting system
was filtered and washed with deionized water (3×10 mL) and acetone
(2×10 mL), respectively. The resulting solid denoted as Ru@PMO-IL was
dried at room temperature in vacuum. The Ru contents were founded to
be 0.4 mmol.g-1 by ICP-AES.
General procedure for oxidation of amines with Ru@PMO-IL: The
related amine (0.25 mmol), Ru@PMO-IL (2.5-4 mol%, 16-25 mg) and
toluene (2 mL) were added to a two necked flask equipped with a
condenser, and the suspension was then stirred at 85 °C for requisite
time under an O2 atmosphere. The reaction progress was monitored by
gas chromatography using internal standard addition method. After
completion of the reaction, the mixture was allowed to cool down to the
room temperature and catalyst was successfully isolated with
centrifugation and washed with toluene (2×10 mL) and dried under the
vacuum for 12h. Then, the collected toluene phase was first washed with
water, dried over Na2SO4, and the solvent was concentrated with
evaporation under the reduced pressure to give the corresponding N-
containing products. To check the catalyst recovery, the same procedure
were conducted in repeated runs but in a large-scale experiment using
2.5 mmol of benzyl amine as substrate.
Acknowledgements
The authors acknowledge the Institute for Advanced Studies in
Basic Sciences (IASBS) Research Councils, the Iranian National
Science Foundation (INSF), NanoQuebec, and the Natural
Sciences and Engineering Research Council of Canada (H.V.)
for support of this work.
Keywords: Periodic Mesoporous Organosilica • Supported Ru •
Ionic Liquid • Aerobic Oxidation of Amines • Nitriles
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In summary, we showed that aerobic oxidation of various
types of amines could effectively catalyzed by perruthenate
ions supported inside the channels of periodic mesoporous
organosilica with ionic liquid framework (Ru@PMO-IL). It
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10.1002/cctc.201701220
ChemCatChem
COMMUNICATION
Entry for the Table of Contents
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COMMUNICATION
A highly stable perruthenate catalyst
supported inside the channels of the
periodic mesoporous organosilica with
bridged imidazolium framework
Babak Karimi*, Omolbanin Yari, Mojtaba
Khorasani, Hojatollah Vali, Fariborz
Mansouri
Page No. – Page No.
(Ru@PMO-IL) was found to be an
efficient, durable and recoverable
catalyst for oxidative dehydrogenation
(ODH) of various types of amines in
excellent yields and selectivities under
mild reaction conditions.
Aerobic Oxidative Dehydrogenation
of Amines Catalysed by a
Recoverable Ruthenium Catalyst
under Mild Reaction Conditions
This article is protected by copyright. All rights reserved.