Controlled photocatalytic hydrolysis of nitriles to amides by mesoporous MnO2 nanoparticles fabricated by mixed surfactant mediated approach

Article abstract of DOI:10.1016/j.jphotochem.2017.04.013

The solid-phase MnO₂ nanoparticles fabricated by surfactant template method were exploited as the photocatalyst for the effective one-step synthesis of amides. Cationic or anionic surfactants and their combinations were used as porous templates to obtain the mesoporous MnO₂ nanoparticles with variable pore volume (0.23–1.95?cm³/g). The morphological and structural observation of the material confirms the uniform facet structure (37.68?nm) of MnO₂ nanoparticles. The surface elemental state was confirmed by XPS analysis confirming Mn 2p_3/2 (642.5?eV) and Mn2p_1/2 (654.7?eV) spin states, that are common for the tetravalent Mn ions. Presence of surfactant as stabilizer was also witnessed with a strong peak of C 1s (283–286?eV). The textural parameters obtained from XRD and Raman analysis depicted the β-phase and rutile type framework of MnO₂. The selective conversion of nitriles to amides was studied without any acid by products under visible light irradiation in the basic/neutral medium. Amides were obtained from various substrates (nitriles) with excellent yields (70–90%).

Full text of DOI:10.1016/j.jphotochem.2017.04.013

Accepted Manuscript
Title: Controlled Photocatalytic Hydrolysis of Nitriles to
Amides by Mesoporous MnO2 Nanoparticles Fabricated by
Mixed Surfactant Mediated Approach
Author: Akansha Mehta Soumen Basu
PII:
S1010-6030(17)30222-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2017.04.013
JPC 10602
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
21-2-2017
7-4-2017
11-4-2017
Please cite this article as: A. Mehta, S. Basu, Controlled Photocatalytic Hydrolysis of
Nitriles to Amides by Mesoporous MnO2 Nanoparticles Fabricated by Mixed Surfactant
Mediated Approach, Journal of Photochemistry and Photobiology A: Chemistry (2017),
http://dx.doi.org/10.1016/j.jphotochem.2017.04.013
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Controlled Photocatalytic Hydrolysis of Nitriles to Amides by Mesoporous MnO2
Nanoparticles Fabricated by Mixed Surfactant Mediated Approach
Akansha Mehta and Soumen Basu*
School of Chemistry and Biochemistry, Thapar University, Patiala-147004, India
*Corresponding author. E-mail address: soumen.basu@thapar.edu
Page 1 of 21

Graphical Abstract
Page 2 of 21

Highlights
 Synthesis of mesoporous MnO2 by using mixture of cationic and anionic surfactants
with variable pore volume.
 The cat-anionic surfactant yields 5 times higher surface area (195.32m2/g) than their
corresponding individual form (CTAB 29 m2/g and SDS 36 m2/g).
 Synergic effect, low critical aggregation concentration of cat-anionic surfactant is
endorsed for its high surface area.
 The mesoporous MnO2 as a novel catalyst for the hydrolysis of nitriles to amides.
 Higher yield (90%) in basic reaction medium compare to neutral medium (70%).
Page 3 of 21

Abstract
The solid-phase MnO2 nanoparticles fabricated by surfactant template method were exploited
as the photocatalyst for the effective one-step synthesis of amides. Cationic or anionic
surfactants and their combinations were used as porous templates to obtain the mesoporous
MnO2 nanoparticles with variable pore volume (0.23 to 1.95 cm3/g). The morphological and
structural observation of the material confirms the uniform facet structure (37.68 nm) of
MnO2 nanoparticles. The surface elemental state was confirmed by XPS analysis confirming
Mn 2p3/2 ( 642.5 eV) and Mn2p1/2 (654.7 eV) spin states, that are common for the tetravalent
Mn ions. Presence of surfactant as stabilizer was also witnessed with a strong peak of C 1s
(283-286 eV). The textural parameters obtained from XRD and Raman analysis depicted the
β-phase and rutile type framework of MnO2.The selective conversion of nitriles to amides
was studied without any acid by products under visible light irradiation in the basic/neutral
medium. Amides were obtained from various substrates (nitriles) with excellent yields (70-
90%).
Keywords: MnO2 nanoparticles; synergistic effect of CTAB and SDS; photocatalysis;
hydrolysis of nitriles;
Page 4 of 21

1. Introduction
One of the most important industrial reactions in the organic chemistry is amide bond
formation as it constitutes important moieties in many pharmaceutical and biologically active
ingredients[1]. Moreover, the flattering properties of amides like stability, conformational
diversity and high polarity make it one of the most reliable and accepted functional groups in
every branch of organic chemistry. Among all the methods reported for the fabrication of
amides, hydration of nitriles has become one of the most widely used methods to obtain
primary amides. But this is a challenging reaction as there is possibility of acid formation
from amides. The hydrolysis reaction is catalysed in acidic and basic medium, but many of
the methods requires harsh conditions with poor yields due to further conversion of amides to
acids[2, 3]. Improved methods for the selective and controlled synthesis of amides using
catalysts under stable conditions are of great demand [4(a)]. In this regard formation of
amides by hydrolysis of nitriles using transition metal oxides nanomaterials as a photocatalyst
is good choice to obtain better yields. It is a principal objective of the present study to provide
a new method for converting nitriles to amides.
Nanoporous transition metal oxides with controlled morphology, porous structures, tunable
nano pores and well ordered mesostructures make them ideal photocatalysts in the field of
catalysis. During recent times, of all the transition metal oxides, manganese dioxide (MnO2)
nanostructures have received keen attention due to its attractive physical and chemical
properties[4(b, c)]. The MnO2 is non-stoichiometric compound and can exist in different
crystallographic forms. The crystallographic forms are generally supposed to be responsible
for their variable properties and the controlled synthesis of MnO2 which make the controlled
design of a specific forma primary objective [5]. Previously, many processes have been
developed for the synthesis of MnO2 nanoparticles with different shapes and morphology.
Out of all the processes MnO2 nanostructures are commonly synthesized using hydrothermal
treatment[6, 7], sol-gel process[8, 9], refluxing[10, 11] and normal thermal approach[12] for
enormous catalytic application [13, 14] because of their different morphology and variable
oxidation states. Several attempts have been made to further improve the catalytic activity by
using some facile techniques [15, 16] but enhancing surface area is an excellent way to
improve its catalytic properties. As a remedial surfactants based synthesis are considered as
best route to develop a mesoporous structure. They have the exceptional property to self-
organize in the solution, which may modify the interfacial properties and increase the
Page 5 of 21

compatibility among the particles. Materials prepared without using surfactants or other
stabilizing agents lead to agglomeration as well as reduce the processing temperature of metal
oxide and several other materials [17]. Earlier Yan et al. have reported the synthesis of
MnO2 using the cationic surfactant (CTAB or CTAC) by direct electrodeposition method
with a specific surface area of 102 m2/g [18], Roy et al. have synthesized mesoporous
manganese oxides nanostructures by using a bi-template approach with the surface area of 65
m2/g [19], Zhi et al. have fabricated MnO2 nanoarrays using vacuum assisted nanocasting
with a surface area of 139 m2/g [20], Saputra et al. have synthesized one-dimensional MnO2
nanoparticles with a surface area of 194 m2/g [21]. But still the role of individual surfactants
as a porous template for the synthesis of high surface area mesoporous MnO2 nanoparticles
and their effect to convert nitriles to amides has not been fully explored.
In this regard current work signifies the single step selective and controlled hydrolysis of
nitriles to amides under visible light both in the basic and neutral medium using MnO2 as
photocatalyst. The mechanism for the selective fabrication of amides without any acid
moieties has also been fully explored.
2. Experimental
2.1 Chemicals and Reagents
Manganese (II) nitrate tetrahydrate (MnNO3.4H2O), Cetyl trimethylammonium bromide
(CTAB), Sodium dodecyl sulphate (SDS) and acetonitrile were received from Aldrich. An
aqueous solution of MnNO3.4H2O (1 M) with the mixture of CTAB (0.25 × 10-3 M) and SDS
(0.25 × 10-3 M) was stirred at normal room temperature. For complete conversion of Mn (II)
to Mn (IV), the solution was stirred for 20 min in the presence of NaOH and change in the
color of solution was observed from light pink to dark brown, which indicates the formation
of MnO2 nanoparticles denoted as m-MnO2 (CTAB+SDS) as shown in Fig. S1. The product
was recovered by filtration, washing with distilled water, drying and calcined at 623 K for 3 h
and used as a solid catalyst for the hydrolysis of nitriles. Similarly, other samples of MnO2
were also prepared using cationic, CTAB (0.25 × 10-3 M) as well as anionic, SDS (0.25 × 10-3
M) surfactant were denoted as c-MnO2 and s-MnO2 respectively.
2.2 Photocatalytic Activity
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The photocatalytic experiment was carried out in a test tube containing 5 mL of nitrile in 100
μL of solvent (Et3N or water) and 20 mg of the catalyst. The reaction mixture was illuminated
under visible light (40 W CFL lamp) for 5 h by keeping a distance of 1cm apart from the
surface of reaction mixture in the test tube. The reactions was also performed under dark and
without catalysts but no conversion of nitriles to amides are obtained which confirms that
reaction does take place without the presence of both visible light and catalyst. The product
was isolated from the reaction mixture by using nylon syringe filter (0.22µm) and the product
was quantitatively analysed by GC-MS and 13C NMR to confirm the formation of amides.
3. Results and Discussion
3.1 Characterization of Photocatalyst
The phase identification of crystalline MnO2 was analyzed through XRD analysis
(PANALYTICAL X’Pert PRO) with Cu Kα (λ=1.540 Å) as radiation source over the range
0°≤2θ≤90°.The diffraction pattern of as- synthesized different MnO2 samples exhibits intense
diffraction peaks of (110), (101), (211), (220) and (002) which is characteristics of β-MnO2
structure (JCPDS Card no. 24-0735) with lattice parameters a = 4.399 Å, c = 2.874 Å and
space group P42/mnm (Fig. 1a).Some other peaks are also highlighted in the Fig. 1 which are
due to an addition of surfactants. The crystallite size and lattice strain was calculated by
Scherrer equation (D = 0.9λ/βcosθ), the sample m-MnO2 has largest crystallite size and
smallest lattice strain (13nm, 0.0129) compared to c-MnO2 and s-MnO2, which is shown in
Table 1. This larger crystallite size is due to self- assembly in cat-anionic surfactant, which
occurs synergistically because of electrostatic attraction between the polar head groups of the
surfactants.
The chemical composition of m-MnO2 sample was studied by X-ray photoelectron
spectroscopy (XPS) shown in Fig. 2, the survey spectra reveals the presence of elements like
Mn, O, C. The peaks for β-MnO2 corresponding to Mn 2p3/2 and Mn 2p1/2 were observed at a
binding energy of 642.5 and 654.7 eV respectively, which clearly indicates the presence of
tetravalent Mn ions. Similarly the deconvulated peaks of O 1s at 528.7 eV and 531.3 eV are
due to the linkage of Mn-O and Mn-OH respectively. The C 1s peaks at 283.9 eV and 286.2
eV are associated with the presence of surfactant. The symmetric stretching vibration (Mn-O)
of [MnO6] groups is analyzed by Raman spectra (Fig. S2 in supporting information). The
sharp peaks were observed at 310 cm-1,475 cm-1, 537 cm-1 and 662 cm-1 are indicative of a
well-developed rutile type framework of MnO2 [22].
Page 7 of 21

The morphology and surface characteristics were confirmed by FESEM (JEOL JSM-
6510LV) and HRTEM (FEI Technai G2 F20 operating at 200KV) studies. The FESEM
studies (Fig.S3) displayed the discrete morphology of the m-MnO2 and the mean particle size
distribution calculated by TEM analysis has been observed to be of 30-40 nm (Fig. 3(d)),
measured by Axio Vision Rel. 4.9.1.0 software. The HRTEM images explore faceted
morphology with little aggregating nature, which may be due to the drying of the sample on
the TEM grid. Fig. 3(d) also shows the plot of the frequency of particles against the logarithm
of size. The particle size follows the following lognormal distribution function;
(
)
√
∑
∑
where
;and √
∑
∑
Where f (d) designates the lognormal distribution of particle size, di represents the size of an
th
∑
i particle of NbC, is the total number of particles in consideration, μ is mean diameter
and σ is the standard deviation of particle size. The variation in average particle size
calculated by different criteria is due to the few agglomerated nanoparticles. The structure,
dispersion and lattice fringes of MnO2 nanoparticles are shown in HRTEM micrograph (Fig.
2) which corresponds to (101) plane with interplanar distance of 0.39-0.80 nm that was
further supported by the SAED pattern (Fig. 3 Inset). The energy dispersive analysis (EDX)
also confirmed the purity of the MnO2 nanoparticles (Fig. S4).
The N2 adsorption- desorption isotherms and pore size distribution plot (Table 1) was
monitored with the help of BEL mini-II, Micro Trac Corp. Pvt. Ltd, Japan. The adsorption
results exhibit type IV classification which presents their mesoporous characteristics (Fig. 4).
The m-MnO2 shows highest surface area (195.3 m2/g) and smaller pore diameter (40.48 nm:
Fig S4) due to the charges present on the surface of surfactants as well as on metal ions
(positive charge) [23].
3.2 Mechanism for the Synthesis of Mesoporous MnO2
Mixed micelle formation in aqueous phase is usually achieved when two surfactants of the
different polarities are mixed. The hydrophilic polar head of cat-anionic surfactants were
actively participating in the reaction which facilitates by either the electrostatic attraction or
repulsion between the ions. Generally, rod-shaped micelles are formed in the case of CTAB
rich corner and spherical micelles are formed in SDS rich corner. The morphology suddenly
Page 8 of 21

turned upon addition of equal amount of SDS into CTAB results in wormlike micelles
because SDS reduces the surface potential by charge neutralization and also increment in the
ionic strength is observed by an asset of the released counter ions. Thus, oppositely charged
cat-anionic surfactants shows enrichment in a synergistic effect over that of either parent
surfactant, an effect which may be exploited in various catalytic applications[24].
3.2 Photocatalytic Activity
The aim of present work is to study the reactivity of m-MnO2 by converting nitriles into
amides under visible light in the weakly basic medium as well as in neutral medium. The
13
product was isolated from the reaction mixture and characterized by GC-MS and C NMR
(Fig. 5 (a)) to confirm the formation of amides. The photocatalytic activity is analyzed by
using all three photocatalysts (c-MnO2, s-MnO2, m-MnO2).The yield calculated from c-MnO2
and s-MnO2 is around 50 % which is very less as compare to m-MnO2 (70 - 90 %). The GC-
MS spectra of the product authenticate the formation of amides from the reaction of
acetonitrile. The fragmentation pattern shows, peak at m/z = 59 due to acetamide (Fig. 5(b)).
13
The C NMR spectrum shows carbonyl carbon peak at δ = 171.67 ppm and the methyl
carbon peak at 22.72 ppm which authenticates the formation of acetamide (Fig. 5(a)). A
similar strategy was followed for propionitrile and acrylonitrile compounds moreover the
product was analyzed by GC-MS (Fig. S6, S7). The % yield and reaction time for all the
nitriles are summarized in Table 2. The other importance of the catalyst is its reusability,
which remains constant (Fig. S5) even after 5 cycles. The photocatalyst is easily separable
from the reaction mixture by centrifugation process and has no permanent adsorption of
nitriles/amides over the catalyst. The % yield of the catalyst is calculated theoretically
(Actual yield/theoretical yield*100) which lies in the range of 70-90 % as presented in Table
2.
The reported mechanism suggests that the reaction of hydrolysis of nitriles took place in two
steps [25]. The first step is conversion of acetonitrile to acetamide and second step is rapid
hydrolysis to acetic acid. But, the brightness of m-MnO2 catalyst falls on this point that it
limits the reaction up to the formation of acetamide giving desired product. This happens
because MnO2 confines the water molecule droplets and there is no direct hydration of
amides. (hydration directly in the presence of water can undergo further hydrolysis to the
acid) [26,27]. The catalytic activity of m-MnO2 is attributed to its large surface area along
with additional active sites which is pictorially represented in Fig. 7. The proposed
mechanism for the hydrolysis of nitriles to amides in a basic/neutral medium under visible
Page 9 of 21

light is heterogeneously catalyzed hydrolysis involving water on the surface of the catalyst, as
m-MnO2 retains moisture on its surface. The reaction of nitriles only with water (neutral
medium) would be so slow (10 h) because of the poor nucleophilic nature of water and results
in fewer yields (70-80 %). So, the reaction is carried out in alkaline medium (base catalyzed
reaction) to activate the nitrile group in a short period of time to get good yields (80 - 90 %)
(Fig. 7). The reaction strategy for the synthesis of m-MnO2 is quite simple, cost effective and
is devoid of any harsh atmospheric conditions. By observing several advantages in the
present strategy it can be mentioned that commercially available MnO2 and other reported
MnO2 catalysts could not produce such effective products with good yields under the same
protocol [28, 29].
Conclusion
A pioneering synthetic approach is designed for the conversion of nitriles to amides
with excellent yields. The mechanism of the hydrolysis reaction under visible light is also
explored which shows that reaction of nitriles only with water (neutral medium) is slow due
to poor nucleophilic nature of water and gives fewer yields as compare to basic medium.
Cationic, anionic and cat-anionic surfactants were used as template for controlling the shape;
size and surface area of catalysts. The hydrophilic polar head of both cat-anionic surfactants
were found to participate actively by either electrostatic attraction or repulsion between ions
which leads to its higher synergistic effect and catalytic activity. Different oxidation states
and structural parameters of the m-MnO2 were confirmed by Raman and XPS studies. The
morphology and textural properties were discussed to determine the particle size and pore
composition of the catalyst. The present protocol is relevant for the selective formation of
amides thus have high potential application for the hydrolysis of different organic
compounds.
Conflict of interests:
The authors declare that there is no conflict of interests regarding the publication of
this.
Acknowledgements
Page 10 of 21

Authors are thankful to DST (Grant No: SB/FT/CS-178/2013), New Delhi, BRNS
(Grant No: 34/14/63/2014) for financial assistance and Dr. S. K. Pandey, School of
Chemistry & Biochemistry, Thapar University for discussion and suggestions.
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Figures
Fig. 1: XRD pattern for different MnO2 nanostructures.
Page 13 of 21

Fig. 2: XPS spectrum of m-MnO2.
Page 14 of 21

Fig. 3: HRTEM images with particle size distribution plot and SAED pattern of m-MnO2.
Page 15 of 21

Fig. 4: N2 adsorption-desorption isotherms for MnO2 nanostructures synthesized by using
CTAB (c-MnO2), SDS (s-MnO2) and CTAB+SDS (m-MnO2).
Page 16 of 21

13
Fig. 5: (a) C-NMR spectrum of isolated acetamide in CDCl3, (b) GC-MS spectra of the
product (acetamide, as the fragmentation pattern shows peak at m/z 59 due to acetamide).
Page 17 of 21

Fig. 6: Pictorial representation of the hydrolysis of acetonitrile to acetamide in basic/neutral
medium.
Page 18 of 21

Fig. 7: Proposed reaction mechanism for the hydrolysis of nitriles to amides in neutral/basic
medium.
Page 19 of 21

Table 1. Textural properties of different MnO2 nanostructures.
b
e
Samples
CTAB
da
εL
Sbetc (m2g)
DBJH
d
Vp (Cm3/g)
9.28
9.09
13
0.0126
0.0129
0.0090
29.0
0.23
0.30
1.95
46.79
46.85
40.48
SDS
36.0
CTAB+SDS
195.3
a Crystallite size determined from scherrer formula.
b Lattice strain determined from XRD analysis.
c BET surface area calculated from the linear part of the BET plot (p/p0 = 0.05-0.25).
d Total pore volume, taken from the volume of N2 adsorbed at p/p0 = 0.990.
e Mean pore diameter estimated using the adsorption isotherm.
Page 20 of 21

Table 2. Catalytic activity of m-MnO2 with different substrates and solvents.
Substrate
Product
Medium
Reaction Time Yield
(h)
(%)
O
Triethylamine
5
90
N
Acetonitrile
H3C
NH2
Acetamide
Water
10
4
75
88
O
Triethylamine
N
NH2
Propionitrile
Water
Triethylamine
Water
8
4
8
72
85
71
Propionamide
O
N
NH2
Acrylonitrile
Acrylamide
Page 21 of 21