Direct synthesis of amides from amines using mesoporous Mn-SBA-12 and Mn-SBA-16 catalysts

Article abstract of DOI:10.1016/j.catcom.2013.03.020

Manganese incorporated SBA-12 and SBA-16 catalyze the tandem reaction of aliphatic primary amine, aerial oxygen and ammonia solution at moderate conditions producing amide in yields as high as 50 mol%. The Mn-SBA-12 and Mn-SBA-16 catalysts with Si/Mn outp

Full text of DOI:10.1016/j.catcom.2013.03.020

Catalysis Communications 37 (2013) 36–40
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Direct synthesis of amides from amines using mesoporous Mn-SBA-12
and Mn-SBA-16 catalysts
⁎
Anuj Kumar, Devadutta Nepak, Darbha Srinivas
Catalysis Division, CSIR-National Chemical Laboratory, Pune, 411008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Manganese incorporated SBA-12 and SBA-16 catalyze the tandem reaction of aliphatic primary amine, aerial
oxygen and ammonia solution at moderate conditions producing amide in yields as high as 50 mol%. The
Mn-SBA-12 and Mn-SBA-16 catalysts with Si/Mn output molar ratio in the range 230 to 748 were prepared
by the direct hydrothermal synthesis method and characterized. Weak acidity and Mn in +3 oxidation state
are the key factors enable the synthesis of product amide in high yields.
Received 21 February 2013
Received in revised form 18 March 2013
Accepted 20 March 2013
Available online 27 March 2013
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Manganese incorporated SBA-12 and SBA-16
Amide synthesis
Aerial oxidation
Benzyl amine
Mesoporous Mn-silica catalyst
1. Introduction
2. Experimental
Amides are valuable chemicals. Methacrylamide and caprolactum
are two industrially important amides used in polymer manufactur-
ing [1]. Amides find application also in the treatment of HIV and
cardiovascular diseases, hypertension and high blood pressure [2].
Conventionally, they are prepared by reacting amines with acid
chloride or anhydride [3]. This method is non-ecofriendly as equi-
molar amount of acid is generated as waste. Several elegant ap-
proaches have been reported which include the synthesis from
primary alcohols and amines using pincer-type ruthenium complex
[4], from various aldoximes in water over rhodium hydroxide on
alumina [5] and by direct oxygenation of primary amines in pres-
ence of water using ruthenium hydroxide supported on alumina
catalysts [6]. All these methods use expensive metal catalysts.
Development of a greener approach which doesn't generate waste
and uses cheaper metal catalysts is desired. Wang et al. [7] reported
that manganese oxide octahedral molecular sieves catalyze the
direct conversion of primary amines to amides. We report here, for
the first time, the application of manganese incorporated mesoporous
silica (Mn-SBA-12 and Mn-SBA-16) catalysts for preparing amides
from primary amines by direct reaction with aerial oxygen and ammo-
nia solution.
2.1. Catalyst preparation and characterization
Mn-containing SBA-12 and SBA-16 were prepared by direct hy-
drothermal synthesis method using Brij76 and Pluronic F127 struc-
ture directing agents and 0.1 M and 2 M HCl solutions, respectively.
Tetraethyl orthosilicate and Mn(NO3)2·4H2O were used as Si and
Mn sources, respectively. Detailed synthesis and characterization pro-
cedures are provided in Supplementary data, S1.
2.2. Reaction procedure
Benzylamine (5 mmol), 1,4-dioxane (15 mL), 25 wt.% aqueous
ammonia (1 mL) and catalyst (0.2 g) were charged into a 100 mL
stainless-steel reactor (Paar 4875 power controller and 4871 process
controller) which was then pressurized with air (6–10 bar). Tempera-
ture of the reactor was raised to 373–423 K and the reaction was
conducted for a desired period of time (2–10 h). After the completion
of reaction, the reactor was brought to 298 K and the air remained
was vented out. The catalyst was separated by filtration and the liquid
product was analyzed by gas chromatography and identified by GC–MS.
3. Results and discussion
3.1. Structural and textural characterization
Small-angle X-ray diffraction (XRD) patterns of Mn-SBA-12
⁎
Corresponding author. Tel.: +91 20 2590 2018; fax: +91 20 2590 2633.
E-mail address: d.srinivas@ncl.res.in (D. Srinivas).
showed a main peak at 1.67° due to (002) reflection and two less
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.03.020

A. Kumar et al. / Catalysis Communications 37 (2013) 36–40
37
manganese oxide phase were detected in the wide-angle region
(Fig. 1) revealing that manganese is highly dispersed.
a
0
23
1.67° (002)
Nitrogen-physisorption isotherms of Mn-SBA-12 were typical of
type-IV with H1 type hysteresis loop corresponding to hexagonal
mesopore structure. Mn-SBA-16 showed type-IV isotherms but
with a broad H2 type hysteresis loop revealing that the material
has cubic cage-like interconnected mesopore structure (Supple-
mentary data, S2). The specific surface area (SBET) of Mn-SBA-16
decreased (from 800 to 569 m2/g) with increasing Mn content.
Mn-SBA-12 (Si/Mn = 230) has SBET of 587 m2/g. All these materials
showed narrow pore size distribution with its average value in the
range of 2.7–3.5 nm. Unit cell parameters and textural properties
of these catalysts are listed in Table 1. ICP analysis revealed that
only a small portion of input Mn got incorporated into the structure.
While a large number of mesoporous solids (M41S type) are prepared
in alkaline conditions, SBA materials of this study are prepared in highly
acidic conditions. Under such conditions, Mn exists in its cationic form
than as oxy/hydroxy species. The former cannot be introduced easily
into the framework via condensation processes with silica species in acid-
ic conditions. Moreover, the ionic radius of Mn2+ in tetra-coordination is
higher (0.066 nm) than that of Si4+ (0.026 nm). In view of all these, only
a small portion of input Mn got introduced into SBA-12 and SBA-16 lat-
tices. High resolution transmission electron microscopic (HRTEM) images
of Mn-SBA-12 and Mn-SBA-16 confirmed long-range, three-dimensional
mesoporous ordering of these materials (Fig. 2). The electron diffraction
pattern (inset) shows that these materials are crystalline in nature. In
conformity with XRD data, no separate bulk manganese oxide-like parti-
cles were detected even in HRTEM.
1.27° (100)
10 20 30 40 50 60 70 80
2.67° (112)
3.0° (300)
1
2
3
4
5
2θ (degree)
b
0
23
0.92° (110)
10 20 30 40 50 60 70 80
1.7° (200)
2.1° (211)
Mn-SBA-12 and Mn-SBA-16 showed four overlapping absorption
bands at 215, 270, 345 and 500 nm, respectively (Fig. 3). The bands at
215 and 270 nm are assigned to O2− → Mn3+ charge-transfer transi-
tions of Mn ions in tetra-coordinated geometry [9]. The band at 345 nm
is associated with Mn3O4 type species with Mn in hexa-coordinated
geometry. A part of Mn is also present in +2 oxidation state which
1
2
3
4
5
2θ (degree)
showed crystal field transitions (6A1g
→
4T2g) at 500 nm [9]. As the Mn
Fig. 1. Small- and wide-angle (inset) XRD profiles of (a) Mn-SBA-12 (Si/Mn = 230) and
content increased, the absorption bands have broadened and red shifted.
UV–visible spectroscopy, thus, reveals that manganese ions in Mn-SBA-12
and Mn-SBA-16 are present in both +3 and +2 oxidation states in tetra-
hedral as well as in octahedral coordination geometries.
(b) Mn-SBA-16 (Si/Mn = 278).
resolved peaks at 2.67° and 3.0° corresponding to (112) and (300) re-
flections, respectively (Fig. 1). A weak shoulder to the main peak
arising from (100) reflection was also observed. This XRD pattern of
Mn-SBA-12 is consistent with an ordered, three-dimensional,
hexagonal mesoporous structure having p63/mmc space group [8].
Mn-SBA-16 (Si/Mn output molar ratio = 278) showed an intense
peak at 0.92° [(110) reflection] and two weak peaks at 1.28° and
1.72° [(200) and (211) reflections, respectively] which could be
indexed to a three-dimensional, mesoporous, cubic structure with
space group of Im3m [8]. Except for a weak, broad peak (at ~23°)
due to microporous silica, no other peaks due to crystalline
In general, all these samples are paramagnetic and showed a
sextet-line hyperfine pattern in the electron paramagnetic resonance
(EPR) spectrum centered at gav = 2.002 which is typical for Mn2+
ions (3d5, S = 5/2, I = 5/2) in an octahedral environment of oxygen
atoms [9–11]. However, intensity of normalized EPR signal increased
with increasing Mn content in the catalyst indicating that in catalysts
with higher Si/Mn ratio, Mn is mostly in a +3 oxidation state.
Increasing amounts of Mn2+ species were present as Si/Mn ratio de-
creased. Mn3+ are non-Kramers ions (3d4, S = 2) and they don't show
EPR signals at X-band frequency [12]. The EPR signals of Mn2+ became
Table 1
Chemical composition and textural properties.
Catalyst
Si/Mn molar ratio
SBET (m2/g)b
Pore volume (cm3/g)b
Average pore size (nm)b
Acidity (mmol/g)c
Input
Outputa
SBA-16
SBA-12
–
–
800
672
585
627
569
585
625
587
0.67
0.64
0.50
0.71
0.39
0.45
0.48
0.51
3.4
3.8
3.4
3.4
2.7
3.1
3.1
3.5
0.04
0.03
0.27
0.28
0.29
0.30
0.34
0.39
–
–
Mn-SBA-16 (748)
Mn-SBA-16 (736)
Mn-SBA-16 (649)
Mn-SBA-16 (348)
Mn-SBA-16 (278)
Mn-SBA-12 (230)
80
50
40
30
20
20
748
736
649
348
278
–
a
ICP-OES.
N2-physisorption.
NH3-TPD.
b
c

38
A. Kumar et al. / Catalysis Communications 37 (2013) 36–40
Fig. 2. HRTEM images: (a) Mn-SBA-12 (Si/Mn = 230) and (b) Mn-SBA-16 (Si/Mn = 278).
narrower at 77 K and showed forbidden signals in between the allowed
signals (Fig. 4). From the spectral study it can be concluded that the
framework substituted Mn is in a +3 oxidation state and the extra
framework Mn is in a +2 state.
Temperature-programmed desorption of ammonia (NH3-TPD) stud-
ies determining acidity of the catalysts showed three desorption peaks
with maximum at 456, 600 and 645 K (Supplementary data, S3).
While the first peak is attributed to ammonia desorbed from weakly
acidic silanol groups, the latter two are attributed to desorptions from
strong acidic sites (framework-substituted, tetrahedral manganese
ions). The overall acidity of Mn-SBA-12 was higher (0.39 mmol/g)
than that of Mn-SBA-16 (0.26–0.34 mmol/g) (Table 1). This agrees
well with the higher Mn content in Mn-SBA-12 than in Mn-SBA-16.
3.2. Catalytic activity
Benzylamine (A) can be converted directly into benzamide (B)
by simultaneously reacting with air and aqueous ammonia in pres-
ence of a catalyst. 1,4-Dioxane was the suitable solvent because
of high solubility of A, ammonia and water in it. Benzamide (B),
benzylidinebenzylamine (C), benzaldehyde (D) and benzonitrile
(E) are possible oxidation products in this reaction. Product E was
not detected while using Mn-SBA-12 and Mn-SBA-16. However, a
small amount of unidentified products (others) was observed at cer-
tain reaction conditions. Blank, controlled experiments revealed
that the reaction of A doesn't take place in the absence of catalyst.
Bulk MnO2 was active and 100 mol% conversion of A was observed
but benzamide selectivity was significantly low (4.7 mol%). Turn-
over frequency (TOF) of MnO2 was very low (b1 h−1). Both
Mn-SBA-12 and Mn-SBA-16 showed high catalytic oxidation activity
(conversion of A = 100 mol%) (Table 2). Mn-SBA-16 (Si/Mn output
molar ratio = 736) showed higher selectivity for benzamide
(40.3 mol%). TOF and benzamide yield increased with decreasing
Si/Mn ratio up to a value of 736 and thereafter, showed a decreasing
trend revealing that only the framework substituted Mn in +3 oxi-
dation state are more active and product B selective while the extra
framework Mn2+ preponderant in low Si/Mn catalysts are selective
for product C. Neat SBA-12 and SBA-16 were also active but conver-
sion of A was 70 mol% and benzamide selectivity was 20 mol% only,
212
a) Mn-SBA-12 (Si/Mn = 230)
270
345
500
200
300
400
500
600
700
800
Wavelength (nm)
Si/Mn = 230
215
Mn-SBA-12 at 77 K
b) Mn-SBA-16
270
345
500
Si/Mn = 230
Mn-SBA-12 at 298 K
Si/Mn = 278
Si/Mn = 348
Si/Mn = 348
Si/Mn = 649
Mn-SBA-16 at 298 K
Si/Mn = 736
Si/Mn = 748
2800
3200
3600
4000
4400
200
300
400
500
600
700
800
Wavelength (nm)
Magnetic Field (G)
Fig. 3. Diffuse reflectance UV–visible spectra.
Fig. 4. EPR spectra of Mn-SBA-12 and Mn-SBA-16.

A. Kumar et al. / Catalysis Communications 37 (2013) 36–40
39
80
Table 2
Catalytic activity of Mn-SBA-12 and Mn-SBA-16.a
Aldehyde + Alkylimine
70
60
50
40
30
20
Catalyst
TOF (h−1
)
Product yield (mol%)
B
C
D
Others
MnO2
b1
103
138
122
65
4.7
95.3
46.9
44.7
51.1
78.0
68.9
62.3
54.3
0
–
Mn-SBA-16 (748)
Mn-SBA-16 (736)
Mn-SBA-16 (649)
Mn-SBA-16 (348)
Mn-SBA-16 (278)
Mn-SBA-12 (230)
Mn–Al-SBA-16 (350)
36.0
40.3
34.0
19.3
25.6
30.9
24.5
11.6
15.0
14.9
1.7
5.5
–
Si/Mn = 736
748
–
–
649
43
52
6.4
6.8
9.7
–
–
Amide
–
11.5
348
278
a
Reaction conditions: Catalyst = 0.2 g, benzylamine (A) = 5 mmol, aqueous NH3
(25%) = 1 mL, 1,4 dioxane = 15 mL, air = 6 bar, reaction temperature = 423 K
and reaction time = 8 h. Turn over frequency (TOF) = moles of benzylamine
converted per mole of Mn (output) in the catalyst per hour. Conversion of A =
100 mol%. B = benzamide, C = benzylidinebenzylamine and D = benzaldehyde.
0.26
0.28
0.30
0.32
0.34
Acidity (mmol/g, NH3-TPD)
Fig. 5. Acidity verses product selectivity correlation over Mn-SBA-16.
Table 3
Influence of reaction temperature on the conversion of benzylamine to benzamidea.
transformation by two pathways [7]. Oxidative dehydrogenation which
is dependent on metal redox behavior and hydrolysis/amination which
are dependent on acidic character of the catalyst are the two competing
reactions controlling product selectivity. Complete conversion of A indi-
cates that the first step (amine to aldimine) is facile. Acidity verses
redox property of the catalyst determines whether aldimine undergoes
hydrolysis to benzaldehyde (D) which further reacts with A to form
benzylidinebenzylamine (C) or follows the other route. The backward
reaction of benzaldehyde (D) to aldimine can also take place in presence
of ammonia. But, benzylamine is more basic than ammonia [pKb = 4.66
(benzylamine) and 4.75 (ammonia)] and hence, formation of C is
preferred over acid catalysts to the reverse reaction of D to aldimine.
Moreover, aldimine could not be detected suggesting that it is an interme-
diate at our experimental conditions. Under mild acidic conditions and
Mn with a facile redox state, further oxidative dehydrogenation followed
by hydrolysis and amination converts aldimine into benzamide (B). Benz-
aldehyde (D) can get further oxidized to benzoic acid which upon reac-
tion with ammonia forms a salt that can settle on the catalyst. Analysis
of the spent catalyst by FTIR spectroscopy ruled out that possibility at
our experimental conditions (Supplementary data, S5).
As seen from Fig. 5, with increasing acidity of the catalyst higher
amounts of (C + D) have formed and the selectivity for B decreased.
As noted before with decreasing Si/Mn ratio, the concentration of Mn2+
(electronic band at 500 nm and sextet-line EPR pattern) has increased.
At higher Si/Mn ratio, most of the Mn is in +3 oxidation state. While
both the Mn species are active for the conversion of A to aldimine,
only Mn3+ facilitate amide formation while the extra framework
Mn2+ lead to (C + D). Thus, weak acidity and framework substituted
Mn3+ ions are the key features responsible for the high catalytic activity
of Mn-SBA-16 and Mn-SBA-12 in the direct oxidation of primary amines
to amides.
Run
no.
Reaction
temperature (K)
Conversion
of A (mol%)
TOF
Product selectivity
(mol%)
(h−1
)
B
C
D
1
2
3
373
403
423
10.2
90.0
100.0
14
124
138
–
100.0
73.8
44.7
–
26.2
40.3
–
15.0
a
Reaction conditions: Catalyst, Mn-SBA-16 (Si/Mn = 736) = 0.2 g, benzylamine
(A) = 5 mmol (0.53 g), aqueous NH3 (25%) = 1 mL, solvent = 1,4 dioxane (15 mL),
stirring speed (rpm) = 600, reaction pressure = 6 atm and reaction time = 8 h.
TOF (turn over frequency) = number of moles of benzylamine converted per
mole of Mn (output) present in the catalyst per hour. B = benzamide, C =
benzylidinebenzylamine and D = benzaldehyde.
suggesting that framework substituted Mn enhances the catalytic
activity and benzamide selectivity.
Temperature and pressure showed notable effects on conversion of
A and product B selectivity (Table 3; Supplementary data, S4). When
the reaction was performed at 373 K over Mn-SBA-16 (Si/Mn = 736),
conversion of A was 10.2 mol% and C was the only product. As the tem-
perature was increased to 403 K, conversion of A increased to 90 mol%
and selectivities for B and C were 14.5 and 73.8 mol%, respectively. At
423 K, complete conversion of A was observed and amide formed with
a selectivity of 40.3 mol% (Table 3). When the reaction was continued
for 10 h, selectivity of B increased to 47.1 mol% (Supplementary data,
S4). Also an increase in product B selectivity was observed when pressure
was raised from 6 to 10 bar. Incorporation of Al3+ in the catalyst led to
reduced selectivity of benzamide.
In this reaction, amine (A) is converted into an intermediate aldimine
through oxidative dehydrogenation (Scheme 1) which undergoes further
Scheme 1. Reaction mechanism for formation of amides from primary amines.

40
A. Kumar et al. / Catalysis Communications 37 (2013) 36–40
4. Conclusions
References
[1] U.S. Patent 5,626,836 (1997).
Catalytic application of Mn-SBA-12 and Mn-SBA-16 for the eco-
friendly synthesis of amide from primary amine (benzyl amine, for
example) was reported. The catalysts contained manganese in +2
and +3 oxidation states, concentration of the former increased with
decreasing Si/Mn ratio. Both acidity and oxidation state of Mn con-
trolled amide selectivity in the product.
[2] E. Valeur, M. Bradley, Chemical Society Reviews 38 (2009) 606–631.
[3] M.B. Smith, J. March, Advanced Organic Chemistry: Reactions, Mechanisms, and
Structure, 6th ed. Wiley, Hoboken, NJ, 2007.
[4] C. Gunanathan, Y.B. David, D. Milstein, Science 317 (2007) 790–792.
[5] H. Fujiwara, Y. Ogasawara, K. Yamaguchi, N. Mizuno, Angewandte Chemie Inter-
national Edition 46 (2007) 5202–5205.
[6] J.W. Kim, K. Yamaguchi, N. Mizuno, Angewandte Chemie International Edition 47
(2008) 9249–9251.
[7] Y. Wang, H. Kobayashi, K. Yamaguchi, N. Mizuno, Chemical Communications 48
(2012) 2642–2644.
[8] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, Journal of the American Chemical
Society 120 (1998) 6024–6036.
[9] K.M. Parida, S.S. Das, Journal of Molecular Catalysis A: Chemical 306 (2009)
54–61.
Acknowledgment
AK and DN acknowledge Council of Scientific and Industrial Research
(CSIR), New Delhi for the Research Fellowship.
[10] M. Selvaraj, T.G. Lee, The Journal of Physical Chemistry
B 110 (2006)
21793–21802.
[11] A. Ramanathan, T. Archipov, R. Maheswari, U. Henefeld, E. Roduner, R. Gläser,
Journal of Physical Chemistry Part C 112 (2008) 7468–7476.
Appendix A. Supplementary data
[12] D.P. Goldberg, J. Telser, J. Krzystek, A.G. Montalban, L.-C. Brunel, A.G.M. Barrett,
B.M. Hoffman, Journal of the American Chemical Society 119 (1997) 8722–8723.
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2013.03.020.