MnO2/graphene oxide: A highly active catalyst for amide synthesis from alcohols and ammonia in aqueous media

Article abstract of DOI:10.1039/c2jm34652d

Rod-like MnO₂ uniformly attached on both side of GO sheets (MnO₂/GO) is an efficient heterogeneous catalyst for the synthesis of primary amides from primary alcohols and ammonia as well as from aldehydes or nitriles. Water is the best solvent for these reactions, analytically pure crystals of product could be isolated by simply cooling in ice and this catalyst has excellent recyclability.

Full text of DOI:10.1039/c2jm34652d

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MnO2/graphene oxide: a highly active catalyst for amide synthesis from
alcohols and ammonia in aqueous media†
a
a
a
b
a
a
Renfeng Nie, Juanjuan Shi, Shuixin Xia, Lian Shen, Ping Chen, Zhaoyin Hou and Feng-Shou Xiaoa
*
Received 16th July 2012, Accepted 31st July 2012
DOI: 10.1039/c2jm34652d
amides from alcohols, aldehydes with NH3 or nitriles.16,29,30 More
Rod-like MnO2 uniformly attached on both side of GO sheets
recently, Yamaguchi et al. found that OMS-2 can also catalyze the
synthesis of primary amides from alcohols (or aldehydes) and
aqueous ammonia using 1,4-dioxane as a solvent.31,32
(MnO2/GO) is an efficient heterogeneous catalyst for the synthesis
of primary amides from primary alcohols and ammonia as well as
from aldehydes or nitriles. Water is the best solvent for these
reactions, analytically pure crystals of product could be isolated by
simply cooling in ice and this catalyst has excellent recyclability.
However, there still remains a challenge for the improvement of
these reported methods, because the preparations of catalysts are
time-consuming and precious metals have relatively high cost and
these reactions require the use of organic solvents. Therefore, amide
synthesis from alcohols and ammonia in a green solvent—water—
over a cheap and active heterogeneous catalyst is strongly desirable
(Scheme 1).
Amide is one of the most important functional groups and plays a
major role in the elaboration and composition of biological as well as
chemicals and polymer compounds,1–5 while the formation of amide
bonds remains a challenge in organic synthesis.2,6 The most tradi-
tional method for amide synthesis is from activated carboxylic acid
derivatives, such as acids,6 aldehydes,7,8 acyl halides,9 ketones or
carbinols,10 mixed anhydrides11 and esters which react with amines or
acyl azides,12 and/or from the reactions of acyl hydrazides with
reductants.13,14 Alternative procedures include the hydration of
nitriles to primary amides in the presence of acids,15 bases,16 or
transition metal catalysts including homogeneous17,18 and heteroge-
neous complexes.19 Other methods such as Beckmann rearrange-
ment, Aube–Schmidt rearrangement, and Staudinger reaction are
also commonly reported in published papers.20–23 However, limita-
tions in existing methods, such as the use of toxic inorganic cyanides
or production of vast amounts of wasteful inorganic salts, severely
hinder their application for amide synthesis at a large scale in
industrial processes.24–27
MnO2 is a cheap, low toxic, easily handled reagent,33 which has
been used as catalyst in alcohol oxidations under mild conditions for
a long time.31–37 However, MnO2 usually delivers a relatively low
surface area (10–80 m2 gꢀ1) and limited electrical conductivity,38
which strongly depresses its catalytic activities. It is well known
that graphene oxide (GO) possesses large surface area (up to 400–
1500 m2 gꢀ1 39,40 because both sides of the nanosheets are accessible.41
)
The functionalities of surface groups40,42,43 and the presence of holes,44
oxygen,45 carbon vacancies and defects, which are generated in partial
oxidation produced sheets from graphitic domains,46,47 may intro-
duce chemically active sites in catalysis and also act as anchoring sites
for deposition of metal nanoparticles. It has been suggested that GO
could be an ideal support for growth of functional nanoparticles and
would render them electrically conductive, highly dispersive, and
catalytically active.39
Recently, heterogeneous catalysts for direct synthesis of amides
from primary alcohols and amines have been paid much attention
due to their high efficiency, easy catalyst recycling, and high atomic
economy. Shimizu et al. reported an oxidant-free dehydrogenative
amide synthesis from primary alcohols and secondary amines over
Ag/Al2O3 catalyst, in which toluene was selected as solvent and
excessive base was added.27 Wang and co-workers disclosed an
Au/DNA catalyst for the formation of secondary amides from
alcohols and amines using LiOH$H2O as a base promoter.28
Ruthenium-based catalysts were also widely used for syntheses of
Herein, by combining the advantages of MnO2 nanocrystals
(active catalyst) with GO (large surface area, good electrical
conductivity and outstanding dispersion in water medium), an
effective and fast one-pot soft chemical route has been adopted to
directly produce a well-organized MnO2 nanorods/graphene oxide
composite (MnO2/GO). This hybrid material was first used as a new
efficient and recyclable heterogeneous catalyst in the direct synthesis
of amides from primary alcohols and aqueous ammonia in the
presence of a green solvent (water). Very importantly, catalyst/
product separation is easy because of the green solvent water, and this
catalyst is active compared with conventional manganese catalysts.
aInstitute of Catalysis, Department of Chemistry, Zhejiang University,
310028 Hangzhou, P.R. China. E-mail: zyhou@zju.edu.cn; Fax: +86 571
88273283; Tel: +86 571 88273283
bCollege of Chemical Engineering and Materials Science, Zhejiang
University of Technology, 310014 Hangzhou, P.R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm34652d
Scheme 1 Green and atom-economical synthesis of primary amides.
J. Mater. Chem., 2012, 22, 18115–18118 | 18115
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Fig. S1 (ESI†) shows X-ray diffraction (XRD) patterns of GO,
MnO2, and MnO2/GO. The (002) diffraction peak of GO in MnO2/
GO (Fig. S1a†) shifted slightly to a high angle around 11.6ꢁ with
nearly the same basal spacing of 0.87 nm but lower intensity than that
of pure GO (Fig. S1b†). These results suggest that the presence of
more disordered stacking, less agglomeration of GO sheets in the
composite and the regular stacks of GO are exfoliated continuously
due to the insert of MnO2 nanoparticles.48,49 The diffraction peaks of
a-MnO2 (JCPDS 44-0141) in MnO2/GO are faint, compared with
bare MnO2, which indicates that MnO2 nanoparticles deposited on
GO are in small sizes. On the other hand, free MnO2 prepared via a
similar approach shows strong diffraction peaks (Fig. S1c†), which
implies a good degree of crystallization or the presence of larger
nanocrystallites (Fig. S2a†).50 Fig. S3† shows N2 adsorption
isotherms of MnO2 and MnO2/GO. Obviously, MnO2/GO
(112 m2 gꢀ1, 0.613 cm3 gꢀ1) exhibits much higher surface area as well
as pore volume than MnO2 (79 m2 ꢀ1, 0.307 cm3 gꢀ1). Corre-
g
spondingly, MnO2/GO shows major mesopores centered at 35.0 nm,
much larger than that of MnO2 (14.9 nm average pore diameter).
These improvements in hierarchical porous structure (including
mesopores and macropores) and larger surface area can improve the
contact between active sites with the substrate in reaction solution.
Fig. S4† shows the Raman spectra of GO, MnO2, and MnO2/GO.
Notably, MnO2/GO exhibits characteristics of both GO and MnO2.51
The bands at 500–700 cmꢀ1 can be assigned to Ag spectroscopic
species that originate from breathing vibrations of MnO6 octahedra
within a tetragonal hollandite-type framework, whereas the low-
frequency bands at 200–400 cmꢀ1 are assigned to the bending modes
of Mn–O.52 The calculated ID/IG intensity ratio of MnO2/GO (1.06) is
higher than that of GO (0.91), indicating the growth of in situ formed
crystals may also in turn contribute to the destruction of regular
layered GO sheets and the formation of exfoliated graphene oxide.51
Fig. S5† shows XPS spectra of GO, MnO2, and MnO2/GO.
Deconvolution of the C1s peak (Fig. S5a†) reveals the presence of
38.6% heterocarbon, which is slightly lower than that of GO (48.2%)
(Fig. S5b, Table S1†), in agreement with XRD and Raman analyses.
The decreased oxygen-containing groups (C–O and C]O) are
expected to capture Mn2+ ions, partially reduced by Mn2+ ions and
anchored MnO2 nanoparticles on the surface of GO. The O1s
spectrum of MnO2/GO shows two peaks, in comparison with that of
GO. The dominant peak at 529.6 eV is assigned to the oxygen
bonded with manganese (Mn–O) in MnO2 (Fig. S5c†), and another
O1s peak at 532.1 eV might be attributed to oxygen-containing
groups of GO.45,53 The Mn2p spectrum exhibits two characteristic
peaks at 641.9 and 653.6 eV, corresponding to Mn2p3/2 and Mn2p1/2
spin orbit peaks of a-MnO2 (Fig. S5d†).45,54 These results indicate
that the in situ growth of MnO2 crystals can render MnO2/GO
conductive via electron transfer. At the same time, the functional
groups of GO sheets would improve its dispersion in aqueous media
(Fig. S6†).
Fig. 1 TEM images of MnO2/GO nanocomposites obtained at different
time intervals of (a) 15 (inset: HRTEM image), (b) 60 min, (c) 60 min
(HRTEM image, inset: SAED pattern), (d) 120 min (inset: HRTEM
image), and (e) STEM images and EDX of C, O, Mn of in sample d.
GO increased continuously with increasing refluxing time (from 15 to
60 min) via a dissolution–crystallization mechanism,51 which finally
gives rise to wider MnO2 nanoneedles on GO sheets (Fig. 1b). All
diffraction spots in the SAED pattern of MnO2/GO (Fig. 1c) could
be indexed as a body-centered tetragonal phase, indicating that
MnO2 nanoneedles are single crystals. The morphology of MnO2
changed gradually from needle-like to rod-like (5–20 nm in diameter)
through aggregating along the lateral faces when the refluxing time
was prolonged to 120 min (Fig. 1d). These rod-like MnO2 were
uniformly and densely attached on both sides of GO, leading to thick
and dense nanohybrid sheets. In addition, SEM images also suggest
MnO2 nanorods were highly dispersed on the GO sheets (Fig. S2†).
These results indicated that the size of MnO2 nanorods could be
controlled via manipulating the refluxing time. HRTEM images of
MnO2/GO obtained at a refluxing time of 120 min (Fig. 1d) clearly
show the lattice fringes with d spacings of 0.31, 0.49 and 0.69 nm,
corresponding to the (310), (200) and (110) planes of a-MnO2 crys-
tals, respectively. STEM image and EDX analysis (Fig. 1e) show that
there is a good spatial correspondence of C, O and Mn elemental
maps. It was found that the assembly of nanorods on the surface of
graphene sheets to some extent prevents the stacking of GO sheets
due to van der Waals interactions, leading to a large available surface
area and rich porous structure for transferring reactants and prod-
ucts. On the other hand, SEM and TEM revealed smaller diameters
in MnO2/GO (ꢂ5–20 nm in diameter, Fig. 1d) than bare MnO2
(ꢂ20–40 nm in diameter, Fig. S2a†), These results fit well with the
theoretical predictions that the graphene sheets could isolate the
growth of metal nanowires.55,56
Fig. 1 shows transmission electron microscopic (TEM) images of
MnO2/GO nanocomposites obtained from various time intervals.
Pure GO sheet is a transparent film (Fig. S2b†), crumpling and
agglomeration exists throughout the sheet, as a result of deformation
upon the exfoliation and restacking process. After addition of
manganese salts in the starting system, rare needle-like MnO2 with
diameters of 3–10 nm and lengths of 20–500 nm were dispersed
randomly on the surface of GO in short refluxing time (15 min,
Fig. 1a). The amount of MnO2 nanoneedles formed on the surface of
18116 | J. Mater. Chem., 2012, 22, 18115–18118
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Table 1 Conversion and yield of amides from primary alcohols (alde-
hydes) and ammoniaa
The catalytic performance of MnO2/GO for the transformation of
benzyl alcohol with molecular O2 and aqueous ammonia (28%) to
benzamide in various solvents is presented in Table S2.† Clearly, 1,4-
dioxane, DMF and DMSO are good solvents for this reaction, but
yields of benzamide were relatively low (<25%, entries 2–4). When
water was employed as a solvent, the yield of benzamide reached
58.5% at 1 h (entry 1), and further increased to 97% at 3 h (entry 11,
Table S2†). This yield is comparable to that of OMS-2 in 1,4-dioxane
solvent.31,32 Noble metal catalysts are active for the oxidation of
alcohols (aldehydes), but require assistance from supports or bases
for nitrile hydration step.16,29,30
Conversion
(%)
Yield
(%)
Entry
Substrate
Time (h)
1
2
3
3
100
100
97.0
96.5
3b
4
5
100
98.0
61.5
It seems that an excess amount of water is beneficial for the
hydration of nitriles to the corresponding amides, and a large amount
of oxygen-containing groups on GO sheets can improve the disper-
sion of the MnO2/GO catalyst in aqueous media (Fig. S6†), which
provides a sufficient contact between active sites of MnO2 nanorods
and the substrate. The MnO2/GO catalyst could be easily separated
from the reaction mixture by filtration (Fig. S7a and b†). After
separation of the catalyst, analytically pure white crystals of benza-
mide appeared when the filtrate was cooled in ice (Fig. S7c†). These
crystals could be easily isolated by filtration with high yield (>80%).
In contrast, this reaction does not work in the absence of catalysts
(entry 10, Table S2†); MnO2 (prepared via a similar procedure as
MnO2/GO, entry 7), MnCl2 (entry 8) and GO (entry 9) also show
relatively low activities (yield of benzamide <10%). These results
suggested that the size of MnO2 nanorods plays a crucial role in the
catalytic activity, and the enhanced activity of MnO2/GO could be
attributed to its high dispersion in water, high MnO2 dispersion on
GO and suitable functional groups on carbon materials.
30
93.5
5
6
7
8
3
3
3
3
100
100
100
98.7
97.9
97.4
97.2
93.8
9
5
5
100
100
96.3
98.4
10b
a
Reaction conditions: substrate (0.5 mmol), MnO2/GO (0.13 g), 28% aq.
ammonia (0.1 mL), water (4 mL), O2 (3 MPa), 150 ꢁC. b 130 ꢁC.
In order to verify whether the observed catalysis is truly hetero-
geneous, the reaction was stopped after 0.5 h by filtering MnO2/GO
from the reaction mixture, and then heated again for 3 h, but no
further production of benzamide was observed. This result suggested
that the observed catalysis is truly heterogeneous in nature. More-
over, after five recycles, MnO2/GO still shows high TOF mol
(molMn h)ꢀ1 relative to the initial TOF of 0.15 (Fig. S8†), indicating
that MnO2/GO nanohybrid is really recyclable.
the steadily increased yield of benzamide. On the other hand, ben-
zamide formed slowly with increasing reaction time when benzoni-
trile was used as a reactant (Fig. S9c†). These results suggest that the
dehydration of aldehyde goes faster than the hydration of nitrile to
amide.
Importantly, when various primary alcohols or aldehydes are used
as substrates, MnO2/GO nanohybrid catalyst still gives very high
activities and yields (Table 1). In the transformation of benzylic
alcohols (aldehydes) to the corresponding amides (entries 2, 6–9), the
catalyst has high yields (>93% yields) toward electron-donating as
well as electron-withdrawing substituents, whereas weak electroneg-
ativity such as Br has a negative influence on this reaction (entry 8).
Additionally, allylic alcohols or aldehydes proceeded efficiently to
afford the corresponding unsaturated amides without isomerization
and hydrogenation of the double bond (entries 3 and 10). Aliphatic
alcohol could also be converted to the corresponding amide in slightly
lower yield (entry 4).
According to these results, it could be deduced that a route from
alcohol to amide follows a sequential transformation: benzyl alcohol
/ benzyl aldehyde / benzonitrile / benzamide (Fig. 2). The rate
constant of each step could be calculated from the linear regression
between reaction time (t) and ln(n0/n0 ꢀ n) plot as the slope of plus
one. It was concluded that the rate constant of dehydration of
aldehyde (k2 ¼ 12.9 hꢀ1) is faster than the oxidative dehydrogenation
of alcohol (k1 ¼ 3.13 hꢀ1), and hydration of benzonitrile (k3 ¼
1.07 hꢀ1) goes slower than the previous two steps. That is, hydration
of benzonitrile is the rate-determining step in the transformation of
benzyl alcohol to benzamide in aqueous solution over MnO2/GO
Fig. S9† shows catalytic kinetics for the conversion of benzyl
alcohol, benzyl aldehyde, and benzonitrile to benzamide over MnO2/
GO. When benzyl alcohol was used as a reactant, it was observed that
benzyl aldehyde, benzonitrile, and benzamide formed consecutively
(Fig. S9a†). After reaction for 1 h, benzyl alcohol was almost
consumed, benzonitrile reached its maximum yield (>35% yield). The
yield of benzamide increased steadily with reaction time (0–3 h). For
benzyl aldehyde (Fig. S9b†), it was found that benzyl aldehyde was
converted completely within 15 min. The yield of benzonitrile reached
its maximum (78%) at 15 min, and then decreased continuously with
Fig. 2 Possible reaction pathway of amidation from primary alcohols to
primary amides catalyzed by MnO2/GO.
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In summary, well-organized MnO2 nanorods on graphene oxide
(MnO2/GO) were fabricated though a novel and easily controlled
chemical route. Rod-like MnO2 with 5–20 nm diameter and 100–
600 nm length were uniformly and densely attached on both side of
GO sheets. Compared with bare MnO2, the MnO2/GO nano-
composite is an efficient heterogeneous catalyst for a widely appli-
cable synthesis of primary amides from primary alcohols and
ammonia as well as for transformation of aldehydes or nitriles. More
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This work was financially supported by the National Natural Science
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