Mesoporous cobalt/manganese oxide: A highly selective bifunctional catalyst for amine-imine transformations

Article abstract of DOI:10.1039/c8gc00862k

Herein, we discuss a heterogeneous catalytic protocol using cobalt doped mesoporous manganese oxide for amine-alcohol cross-coupling to selectively produce symmetric or asymmetric imines. Thorough investigations on the surface chemistry and physical properties of the material revealed its outstanding oxidation-reduction properties and reaction mechanism which was supported by quantum mechanical calculations done by using density functional theory (DFT).

Full text of DOI:10.1039/c8gc00862k

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Mesoporous Cobalt/ Manganese Oxide: A Highly Selective
Bifunctional Catalyst for Amine–Imine Transformations
th
Received 14 March 2018
a
a
a
b
b
a
Biswanath Dutta, Seth March, Laura Achola, Sanjubala Sahoo, Junkai He, Alireza Shirazi Amin,
b
a
b
a, b, *
Yang Wu, Shannon Poges, S. Pamir Alpay, and Steven L. Suib
www.rsc.org/
Herein, we discuss a heterogeneous catalytic protocol using cobalt catalyst is highly important. Numerous catalytic systems have been
doped mesoporous manganese oxide for amine-alcohol cross- reported to date to perform that coupling reaction efficiently.
coupling to selectively produce symmetric or asymmetric imines. Popular protocols can be categorized as, 1) precious metal-based
1
3
14
15
16
17
Thorough investigations on the surface chemistry and physical systems, such as Au, Ag, Pd, Pt, and Ru; 2) non-precious
1
2,18
19
20
21
properties of the material revealed its outstanding oxidation- metal-based systems, such as Mn,
Fe, Cu, and CeO
2
;
and 3)
Despite obtaining
supported by quantum mechanical calculations done by using good conversions from some systems, most of them suffered from
2
2
23
2 4 3 4
reduction properties and reaction mechanism which was photocatalysts such as TiO , and BiVO /g-C N .
1
3–17
8,20
density functional theory (DFT).
the use of i) precious metals,
ii) toxic catalysts,
iii)
2
4
12
complicated synthetic procedure, iv) prolonged reaction time, v)
2
5
Imines, also known as Schiff bases, are an essential class of
compounds for their versatile applications in the production of
heterocyclic chemicals, pharmaceutically and biologically active
harsh reaction conditions, such as high pressure,
or
1
6
23
temperature, vi) high energy light irradiation, and vii) oxidative
20
promoters. Hence, the development of a catalytic system is highly
important which can minimize all above-mentioned issues and
evolve as a cost-effective, atom economic, additive free, and highly
1
–3
compounds, and fine chemicals. The C=N bond in imines, being
chemically unstable, are widely used for various organic
1
,4
2
6
transformations such as reduction, addition, and cyclization.
reusable procedure, following the foundations of green chemistry.
Conventionally, imines are produced from the condensation
reaction of amines to activated aldehydes in the presence of
various dehydrating agents, bases and Lewis acid catalysts which
In the past two decades, the use of heterogeneous catalytic
procedures has rapidly evolved due to their i) excellent reusability,
2
7
ii) high cost-economy, and ii) minimum waste generation.
5
limit their industrial applications. Over the past decade, extensive
Therefore, we aimed to develop a highly efficient, heterogeneous
catalytic system for amine-alcohol cross-coupling reactions.
In recent years, mesoporous manganese oxide (meso-MnOx)
based systems have been extensively studied for their superior
research has been devoted to facilitating the acceptor-less process
of synthesizing imines. During this development various alternative
approaches have been marked to synthesize different imines, such
as i) the self-coupling of amines, ii) oxidation of secondary amines
heterogeneous
catalytic
activity
for
various
organic
6
2
8
and iii) coupling of alcohols to amines. Though the latter two are
transformations, such as a) alcohol oxidation, b) homo-coupling of
2
9
30
2
used for synthesizing asymmetric imines, the oxidation of
secondary amines is not a useful way due to the requirement of
amines to imines, c) anilines to aromatic azo compounds, d) sp -
2
31
32
C to sp -C coupling, and e) alkane oxidations. The superior
catalytic activity of meso-MnOx over the other systems, such as
meso-CoOx, meso-FeOx, and meso-NiOx, was recognized due to: i)
1
their prior access. Consequently, the alcohol-amine coupling is
considered to be the only effective tool for selectively synthesizing
asymmetric imines.
3
3
weakest metal-oxygen bond, ii) the excellent redox properties due
3
+
Several approaches have been employed for this transformation
to the prevalence of multi-valency, iii) abundant active sites (Mn ),
iv) enhanced lattice oxygen content, and v) high mobility of lattice
7
using stoichiometric oxidants such as various iridium-complexes,
8
3
1
and periodates. The generation of toxic waste and its complicated
oxygens. Despite such development of manganese oxide catalysis,
relatively higher reaction time was recorded for amine to alcohol
9
separation method make these processes industrially impractical.
1
2,18
Hence, aerobic oxidation methods that use air as the sole oxidant
are considered to be most economical and environmentally
coupling.
Controlling the selectivity for benzylic alcohol-amine
1
8,29
coupling was also found to be challenging.
These studies
1
0–12
benign.
Direct use of air being difficult, a mediator or co-
resulted in an increasing demand for enhancing catalytic reactivity
and selectivity of hetero coupled products. We have made an effort
to solve these issues by increasing the surface defects of the
catalyst. This is the first example of a heterogeneous system which
a
Department of Chemistry, University of Connecticut, Storrs, CT 06269 (USA).
Institute of Material Science, University of Connecticut, Storrs, CT 06269 (USA).
b
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exclusively produces cross-coupled products from benzylic alcohols revealed by the XPS analysis (Table S1). The surface area (Figure S2)
and amines besides enhancing the catalytic efficiency by ~1.5 fold. of a material, which is a direct measure of active site density, also
Additionally, the ability of this protocol to follow all laws of green displayed a proportional relation with reaction conversions (Table
chemistry, make this superior over other existing protocols. 1, S1 and S2). These suggest that the surface active lattice oxygens,
Detailed investigations of the reaction kinetics, surface chemistry, which proportionally varies with the surface area, are extremely
and quantum mechanics computations based on Density Functional critical to enhancing the reactivity of meso-Co-MnOx. However, this
Theory (DFT) were pursued to validate the feasibility of the reaction does not seem to be consistent with the poor catalytic activity of
protocol.
Inspired by the surface defect mediated enhancement of can be attributed to the higher metal-oxygen bond strength in CoOx
meso-CoOx, despite its high lattice oxygen content (Table S1). This
2
8
oxidizability of meso-MnOx, we selected a cationic dopant to as compared to MnOx. The impact of lattice oxygens was further
maximize this property without changing its crystal structure. noticed when reactivities of differently calcined meso-MnOx were
Therefore, the size of the metal dopant was supposed to be smaller compared. An increase in calcination temperature increased the
or similar to that of Mn. Among all options Co was an obvious lattice oxygen content (Table 1, entry 9-10 and Table S1), which
3
+
°
choice due to the i) ease of reducibility of Co species (E red = + enhanced the conversion.
0
2+
1
.81), and ii) excellent proton adsorption by both Co and Co
A continuous increase in product formation with increasing
3
4
2+
3+
sites. Interestingly, both Co and Co species are found to co- catalyst loading was obtained from the system (Figure S7), which
exist in mesoporous cobalt oxide which is known for its strong indicated the absence of mass transfer limitations or adsorption.
3
5
oxidizability. Hence, we prepared cobalt (Co) doped meso-MnOx The screening of solvents and their volumes showed that one mL of
where Co was present as mesoporous cobalt oxide. To verify the toluene was the best combination to achieve conversion of 80 %
presence of cobalt oxides on the surface of meso-MnOx, X-ray (Table S3 and S4). The increase in solvent volume probably lowered
photoelectron spectroscopic (XPS) techniques were used, where an the collision frequency between the substrate and catalyst, which
increasing peak intensity was observed with the increment of the resulted in the declining yield (Table S4). The ratio of substrates
percentage of the dopant (Figure S4, b). To estimate the impact of was also optimized, where an excess of the alcohol over amine
cobalt oxide modification on the meso-MnOx surface, further yielded the hetero-coupled product with better efficiency (Table S5,
catalytic experiments were performed.
entry 3). This can be rationalized by the slower rate of alcohol
oxidation than the homo-coupling of amine, as revealed by the
Table 1. Aerobic oxidative coupling of benzylamine and 4-methoxy benzylalcohol to obtained rate constants (Table S8). Moreover, the impact of
a
selectively synthesize unsymmetrical imines (I) in the presence of different catalysts.
reaction environment was understood as the reaction yield
increased gradually with increasing the oxygen content of the
atmosphere (Table S6). The yield increased from 66 % to > 99 % as
the nitrogen atmosphere was replaced with oxygen. This was
Conversion
Selectivity of I
(%)
c
Entry Catalyst
b
b
TOF
probably due to the facile replenishment of lattice oxygen vacancies
by atmospheric oxygen. Hence, one mL of toluene as the solvent at
100 °C under oxygen environment was chosen to be optimal for the
coupling reaction in the presence of the meso-1%Co-MnOx catalyst.
(%)
1.
2.
3.
No catalyst
Meso-CoOx
10
41
20
96
71
N/A
0.64
1.06
Meso-MnOx 68
Meso-1%Co-
4
.
.
.
> 99
98
97
97
1.55
1.38
1.25
MnOx
Meso-3%Co-
MnOx
Meso-5%Co-
MnOx
Scheme 1. Aerobic oxidative coupling of amines and alcohols to selectively synthesize
a
imines via cross-coupling (I).
5
88
80
6
7
d
e
.
.
Meso-MnOx 93
Meso-MnOx 98
78
70
1.45
1.53
8
a
Reaction conditions: benzylamine (0.5 mmol), 4-methoxy benzylalcohol (1.2 eqv.),
°
°
b
Toluene (1 mL), 25 mg of catalyst (calcined up to 250 C), 100 C, 2 hours. Conversions
c
and selectivities were determined by GC-MS. TOF = TON/ Time (h), TON = no of moles
d
°
e
°
of limiting reagent converted to product per mole of catalyst. 350 C, 450 C.
A coupling reaction between benzylamine and benzylalcohol
a
Reaction conditions: amines (0.5 mmol), alcohols (1.2 eqv.), Toluene (1 mL), 25 mg of
°
°
was considered as the model to verify the enhancement of activity catalyst (1% Co-MnOx-250 C),
2 h, 100 C. Conversions and selectivities were
determined by GC-MS. Selectivity of all cross-coupled imines were 98 %. Numbers
depicted within parenthesis and without parenthesis are conversions and isolated
upon cobalt doping. The cobalt oxide modified meso-MnOx surface
exhibited better reactivity than bare meso-CoOx or meso-MnOx yields.
Table 1, entry 1-3). To verify the role of Co in this process, its
loading amount was optimized. The 1 mol % CoOx on meso-MnOx
(
These optimized reaction conditions were used to explore the
was the most active with > 99 % of conversion (Table 1, entry 2-6). substrate scope of this protocol. Almost all benzylamines and
Beyond this, a gradual decrease of imine formation was observed benzylalcohols were efficiently coupled (mostly above 98 %) to yield
with increasing cobalt doping. This trend of conversion was found different asymmetric and symmetric imines (Scheme 1). Bulky
to be proportional to the content of surface active lattice oxygen, as reactants, such as 2-naphthylmethanol, suffered from a lower rate
2
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of reaction than others (Scheme 1). Whereas, metal poisoning by converted to their corresponding imines with excellent yield
heteroatoms caused inhibition of benzylamine to pyrrolidin-2- (mostly > 99 %) (Scheme 3). However, under high pressure of
ylmethanol coupling yielding only 54 % (Scheme 1). However, no hydrogen (300 PSI), all imines were completely (> 99 %) degraded to
impact of i) the electronic nature of substituents, and ii) the their corresponding amines and alcohols (Scheme 4). This superior
aromaticity of reagents, was observed in the protocol (Scheme 1).
oxidizability and efficient reducibility of the catalyst depicts its dual
catalytic behavior (Scheme 1-4).
*
Scheme 2. Other aerobic oxidative processes.
Most heterogeneous systems suffer from i) the leaching of
active metals in solution, and ii) the deactivation of the active sites
due to adsorption or aggregation of nanoparticles. To study the
leaching of active metals, a hot filtration test (Figure S7) and the
reusability of the catalyst (Figure S8) were evaluated. In the hot
filtration test, the catalyst was separated from the system after 30
minutes (at about 37 % conversion), and the filtrate was reinstated
99 %. under the same reaction conditions for another 30 minutes. No
significant improvement in imine formation after catalyst
separation confirmed the absence of active metal species in the
*
Reaction conditions: a) Diol (0.5 mmol), n-Butanol (2 mL), 25 mg of 1% Co-MnOx-250
°
°
C, 100 C. b) 1,2,3,4-tetrahydroisoquinoline (0.5 mmol), toluene (0.5 mL), 25 mg of 1%
°
°
Co-MnOx-250 C, 100 C. Selectivity of all cross-coupled imines were
Conversions and selectivities were determined by GC-MS.
>
The enhanced oxidizability of the meso-Co-MnOx catalyst was
further validated as the relatively difficult oxidation of benzene-1,2-
dimethanol to lactone, which is not possible by bare meso-MnOx,
was achieved feasibly (Scheme 2. a, Table S10). This oxidation
process is highly dependent on the distance between its aliphatic
branches, which causes difficulty in expanding the substrate scope
solution. Whereas, to determine reusability, the retrieved catalyst
was washed and reactivated at 250 °C for 1 hour, before reuse, to
remove adsorbed foreign molecules. These post-reaction
treatments were repeated with the catalyst after every cycle for the
next five cycles until the conversion decreased by ~20 % of its initial
activity (Figure S8, a). An X-ray diffraction study on the catalyst
th
(Scheme 2. a). The transformation of relatively inert secondary
after the 5 cycle was performed to confirm the preservation of its
amine (1,2,3,4-tetrahydroisoquinoline) to the corresponding imine,
also evidenced a ~4 times higher catalytic activity for meso-1%Co-
MnOx than the previously reported meso-MnOx catalyst (Scheme
amorphous nature (Figure S8, b). This categorized our catalytic
system to be genuinely heterogeneous, stable, and reusable.
The excellent catalytic activity of the system, motivated us to
investigate the reaction kinetics and surface chemistry. A time-
dependent study was performed by continuous sampling from the
29
2. b). Thereby, both of these systems support the enhanced
oxidizability of meso-1%Co-MnOx catalyst.
°
reaction mixture at 100 C over a period. This revealed a first-order
a
Scheme 3. Reductive transformation of imines to amines.
-1
rate equation, with a rate constant (k) of 0.02 min , as compared to
the benzylamine (Figure S11). To evaluate the apparent activation
energy of 31 KJ/mol from Arrhenius plot, several time-dependent
°
reactions were performed in a temperature range of 30-120 C
(
Figure S12).
a
Reaction conditions: imine (0.5 mmol), Toluene (2 mL), 25 mg of catalyst (5% Co-
°
°
4
MnOx-250 C), NaBH (1 eqv.), 5h, and 100 C. Selectivity of all cross-coupled imines
were > 99 %. Conversions and selectivities were determined by GC-MS.
Scheme 4. Reductive coupling of imines to amines and alcohols in the presence of 10%
a
Co-MnOx.
Scheme 5. The overall reaction mechanism for the coupling of amine and alcohols to
imines.
The above-mentioned experimental findings led us to propose
a mechanism to explain the present catalytic protocol. Control
experiments and characterization such as H -TPR (temperature
2
programmed reduction) and XPS were used to understand the
a
30,31
Reaction conditions: imine (0.5 mmol), Toluene (2 mL), 25 mg of catalyst (1% Co- surface chemistry. Learning from our previous findings,
a radical
pathway involving an electron transfer from the adsorbed amine to
the metal center is believed to initiate the reaction. Sequential
elimination of C−H and N−H protons is reported to cause the
°
°
MnOx-250 C), 300 psi, 6 h, 100 C. Selectivity of all cross-coupled imines were > 99 %.
Conversions and selectivities were determined by GC-MS.
To explore the reducibility of the system, transformation of
benzylidene phenyl imine to the corresponding secondary amine
was selected as a model reaction. By optimizing the reaction
conditions, meso-5%Co-MnOx was found to be the best catalyst in
29
formation of RCH=NH, which after hydrolysis forms RCH=O. These
aldehydes upon coupling with amines produced symmetric imines
(
Scheme 5). Whereas, the aldehydes generated from the alcohols
are coupled to amines to yield asymmetric imines (Scheme 5).
4
the presence of a reducing agent NaBH (Table S7, entry 1).
However, the challenges to understanding i) the enhanced activity
Moreover, using these optimized conditions, various amines were
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of meso-Co-MnOx over meso-MnOx, and ii) the dominance of gradient approximations (such as Perdew, Burke, and Ernzerhof
4
5
46
hetero-coupling during the reaction. Therefore, the two-step (PBE) functionals), and Hubbard U correction on the d-orbitals
reaction mechanism, consisting of dehydrogenation and of Mn and Co were used to estimate the total energies of all
dehydration, for imine formation requires more investigation.
configurations, from reactants to intermediates to products. Our
As discussed before, the correlation between the conversion primary intention was to determine the reaction mechanism and
and lattice oxygen content indicated the importance of lattice identify the source of the faster kinetics of meso-MnOx in the
oxygens during the reaction (Table 1 and S1). The excellent presence of Co doping (Table 1, Table S8).
3
+
reactivity of manganese (Mn ) oxides can be attributed to the
3
6
elongation of Mn-O bonds due to a Jahn–Teller (J–T) distortion,
which assists the formation of OH radicals. The presence of
2
+
additional Co ions in meso-MnOx, also facilitates this metal-O
bond elongation, contributing to the OH radical content. Therefore,
2
+
the presence of Co in meso-MnOx not only increases the lattice
oxygen content due to enhanced surface defects but also the
content of reactive oxygen species.
To understand the dominance of hetero-coupling over homo-
coupling, several control experiments were performed. This
revealed faster kinetics (by ~50 times) of homo-coupling of amines
-
1
by meso-MnOx (k= 0.02 min ) than with meso-CoOx (k= 0.0004
-1
min ) (Table S8). This slow reactivity in the presence of CoOx
impacted the result of Co doped meso-MnOx systems by decreasing
the reactivity about ~9-18 times than that of meso-MnOx (k=
-
1
Figure 1. The black and red curves represent the energy profiles of benzylamine to
benzyl alcohol coupling reaction on (100) plane of meso-MnOx and meso-Co-MnOx in
the gas phase, respectively. In this DFT computed comparison, optimized geometries of
0
.0011–0.0023 min ) (Table S8). This suppressed rate of homo-
coupling resulted in a better selectivity of hetero-coupling.
-1
Interestingly, the observed rate (k= 0.0198 min ) of benzylalcohol - the reactant, transition state, and product were used for both cases. ∆E: The energy
difference of the reactant, transition state, and product to the corresponding reactants
for MnO and Co-doped MnO surface. Green, purple, red, grey, blue and while balls
benzylamine coupling reaction was found to be ~3 times slower
than the benzaldehyde - benzylamine coupling reaction (Table S8, denote the Mn, Co, O, C, N and H atoms, respectively.
Figure S10). This indicates the presence of a different mechanistic
pathway which involves intermediates other than imines (RCH=NH)
and aldehydes (RCH=O) as proposed before.
According to these calculations, the reaction occurred in two
main steps. In the first step, a condensation reaction between
A detailed investigation was carried out to identify the PhCH NH2 and PhCH OH led to the formation of corresponding
2
2
reactive oxygen species (ROS) formed during the reaction. secondary amine, which lost a molecule of H2 in the rate
37
According to the possible ROS pathway, molecular oxygen is determining step (the second step) to produce the desired imine.
converted to water through the sequential formation of superoxide Therefore models were prepared to capture energetics of all
anion, hydrogen peroxide, and hydroxyl radical. Therefore, specific intermediates of this step. A comparison between the DFT energy
additives were used to generate characteristic fluorescence or UV- profiles of meso-MnOx and meso-1%Co-MnOx in the gas phase
vis peak from different ROS, to identify them. The superoxide anion show a lowering of the activation barrier upon introducing Co in
was detected by the characteristic UV-Vis peak in between 450-700 meso-MnOx (by 112 meV or 2.58 kcal/mol) (Figure 1). This lowering
nm in the presence of nitro tetrazolium blue chloride (NBT) additive of the activation barrier was assumed to be responsible for
3
8
(
Figure S5. a). Whereas, an increasingly intense characteristic UV- enhancing the reaction by ~1.5 times. Also, all intermediates were
Vis peak (at 596 nm, in the presence of leuco crystal violet) found to bind with the MnO surface through the nitrogen of
confirmed the presence of hydrogen peroxide (Figure S5. b) during benzylamines. These findings led us to propose a mechanism which
3
9
the reaction.
characteristic fluorescent peak at 430 nm in the presence of aldehydes (during the homo-coupling of amines), and the presence
The hydroxyl radical was detected by the supports the existence of hydroxyl radicals, the formation of
4
0
29
disodium terephthalate (DST) (Figure S5. c). The identification of of a negatively charged intermediate (Figure S13).
two ROS in a single reaction, required the detection of the terminal
We propose the formation of RCH=NH (for homo coupling)
ROS. Consequently, ROS quenchers curcumin, and butylated and RCH N=CHR (for hetero-coupling) can be done either by
4
1
2
4
2
hydroxytoluene (BHT) were introduced to terminate both abstracting H from corresponding amine using OH radical (path a
)
-
+
superoxide anion and hydroxyl radical, respectively. The quenching or by a sequential elimination of e and H (path b). Ideally, path b
of hydroxyl radicals inhibited the formation of imines (Figure S5. D, should be more time-consuming than path
due to the
a
entry 2). Whereas, a considerable amount (~15 %) of imine was involvement of multiple steps. This suggests the involvement of
obtained, despite quenching superoxide anions (Figure S5. d, entry path b in benzylamine -benzylalcohol coupling, as the observed rate
-
1
1
). This confirmed the hydroxyl radical to be the terminal ROS.
(k= 0.0198 min ) was found to be significantly lower than the
To understand this reaction mechanism and validate the role anticipated rate (from benzaldehyde - benzylamine coupling) (Table
of Co in meso-MnOx, computational calculations using density S8).
functional theory (DFT) were performed as implemented in the
In summary, we have successfully developed a cobalt doped
4
3,44
Vienna ab initio Simulation Package (VASP).
The generalized mesoporous manganese oxide material with enhanced oxidizability
4
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and selectivity. This mesoporous material with a surface area of 81
V. R. Gonçales, J. J. Gooding and B. A. Messerle, Green
Chem., 2017, 19, 3142–3151.
2
m /g and monomodal pore-size distribution (pore diameter 3.4 nm)
was found to have a high content of lattice oxygen and multi-
valency of both Co and Mn. The meso-1%Co-MnOx system
exhibited an enhanced activity (by ~1.5 times) as compared to the
8
9
K. C. Nicolaou, C. J. N. Mathison and T. Montagnon, Angew.
Chemie Int. Ed., 2003, 42, 4077–4082.
T.-L. Ho, Tandem organic reactions, Wiley, 2015.
M. Largeron and M.-B. Fleury, Angew. Chem. Int. Ed., 2012,
51, 1–5.
un-doped meso-MnOx material. This was also observed during the 10
relatively robust oxidation of diols and 1,2,3,4-
tetrahydroisoquinoline to their corresponding lactones and imine, 11
respectively (Scheme 2). The enhanced efficiency of this catalyst 12
was supported by the lowest reduction temperature as revealed
M. Largeron and M.-B. Fleury, Science 2013, 339, 43–4.
B. Chen, J. Li, W. Dai, L. Wang and S. Gao, Green Chem.,
2014, 3328–3334.
from H
2
-TPR (Figure S14). Most importantly, the unprecedented 13
H. Sun, F.-Z. Su, J. Ni, Y. Cao, H.-Y. He and K.-N. Fan, Angew.
Chem. Int. Ed., 2009, 48, 4390–4393.
result of cross-coupling reaction with > 99 % selectivity was truly
surprising. In contrast to this excellent activity, i) facile reduction of 14
J. Mielby, R. Poreddy, C. Engelbrekt and S. Kegnæs, Chinese
J. Catal., 2014, 35, 670–676.
4
imines to amines in the presence of NaBH ; and ii) transformation
of imines to its parent alcohols and amines, revealed the 15
applicability of this catalyst. Therefore in the absence of ligands,
additives, bases, and precious metals; this economical and 16
sustainable heterogeneous system is superior to the existing ones.
The combined effect of multivalent metal species and lattice oxygen 17
was found to be critical for the enhanced reactivity and excellent
selectivity. DFT computations were utilized to estimate the relative 18
total energies of the rate determining step and propose a reaction
mechanism which supports experimental findings. We intend to 19
continue our investigations to understand further details of the
surface chemistry and identify the role of other surface defects, 20
such as oxygen-vacancy.
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Graphical Abstract:
Cobalt doped mesoporous manganese oxide: An excellent
oxidative catalyst (for producing asymmetric imines with
>
98% selectivity) with significant reductive properties.