Surface modification of polyhedral nanocrystalline MgO with imidazolium carboxylates for dehydration reactions: A new approach

Article abstract of DOI:10.1039/c6ra16358k

The surface modification of nanomaterials with organic molecules and the utilization of modified materials in various applications is equally important. Here we demonstrate, the surface modification of polyhedral nano MgO with imidazolium carboxylate, which generates the NHC stabilized material MgO-[NHC]. The resultant material was successfully utilized in the catalytic dehydration of glucose under heterogeneous conditions. Furthermore, it was used in the dehydration of nitro alcohol to olefins in high yields. In addition, the surface modified catalyst was characterized by using various techniques like XRD, FE-SEM, TEM, FT-IR, XPS and AES analysis.

Full text of DOI:10.1039/c6ra16358k

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DOI: 10.1039/C6RA16358K
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Surface modification of polyhedral nanocrystalline MgO with
imidazolium carboxylates for dehydration reactions: A new
approach
Received 00th January 20xx,
Accepted 00th January 20xx
Melad Shaikha#, Mahendra Sahua#, Santimoy Khilarib, A. Kiran Kumarc, Pathik Maji a and Kalluri V.S.
DOI: 10.1039/x0xx00000x
Ranganatha*
www.rsc.org/
The surface modification of nanomaterial with organic molecules and the utilization of modified
materials in various applications is equally important. Here we demonstrate, the surface modification
of polyhedral nano MgO with imidazolium carboxylate, which generates NHC stabilized material
MgO-[NHC]. The resultant material was successfully utilized in the catalytic dehydration of glucose
under heterogeneous conditions. Further, it was demonstraed in the dehydration of nitro alcohol to the
olefins in high yields. In addition, the surface modified catalyst was characterized by using various
techniques like XRD, FE-SEM, TEM, FT-IR, XPS and AES analysis.
The nanostructured materials possess a unique property like Since N,N’-disubsituted imidazolium carboxylates are readily
high surface area and accessibility of various active facets. available and moreover NHC-CO2 adducts are air and water
Their catalytic activity dependent on the structure and stable reagents, we have selected them as a choice of surface
composition of the surface, both can change by modifying the modifiers on nanomaterial. Due to high thermal stability in
surface.1-3 The utilization of nanomaterials in various directions nature, robustness and high accessibility, nano MgO is selected
is increasing rapidly after surface modification due to change in the present study as a promising nanomaterial to modify the
of their environment and hence properties at the surface.4 surface.12
Various methods have been employed for the surface Herein, we report the covalent surface modification of nano
modification of nanomaterials using silane, polymers primary MgO with Imidazolium carboxylates, (zwitterions) which leads
amines, long chain carboxylic acids and recently dicarboxylic to the formation of MgO-[NHC] complex (Scheme 1).
acids.5 There are large number of surface modified
nanomaterials, including metal oxides, metal nanoparticles
with various chiral and achiral moieties.6 To develop a new
material for catalytic application after surface modification of
nanomaterials is still quiet interesting.7
N-heterocyclic carbenes (NHCs) are rapidly expanding area of
research in transition metal catalysis as well as in organo
catalysis.8,9 The coordination of NHC ligands is not only limited
Scheme 1 Surface modification of nano MgO with L1 to generate MgO-[NHC]
to transition metals, but also to non-transition metals.10 It is
This method represents a novel and is having more advantage
worth noting that the manipulation of free carbenes are quite
difficult due to their highly reactive nature.11 Recently, the use
towards environmental concern compared with earlier
reported Mg-NHC complexes with organometallic reagents,
of CO2 adducts of NHCs attracted much attention in
which are highly expensive and lot of waste could be
organocatalysis as well as in the synthesis of NHC-metal
generated after the reaction.13,14 To the best of our
complexes.11
a Dept. Of Chemistry, Guru Ghasidas University, Bilaspur-495009, India
knowledge, non-transition metal-NHC complexes have not
been used under heterogeneous catalysis.15 The resultant
MgO-[NHC] complex was used as a versatile heterogeneous
catalyst in the dehydration of glucose to 5-
hydroxymethylfurfural (5-HMF) and also in the dehydration of
nitro alcohol to alkenes (Scheme 2).
b Materials Science Centre, IIT-Kharagpur, India
c Materials and Metallurgical Centre, RGUKT, Basar, India
Electronic supporting information is available. [XPS, AES, elemental mapping, FTIR
spectrum and HPLC are available.]
#both authors contributed equally.
This journal is © The Royal Society of Chemistry 20xx
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calculated using Debye-Scherer equation and it was observed
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that 8.2, 5.4 and 6.2 nm for freshly prepaDreOdI: M10.g10O3,9M/Cg6ROA-1[6N3H58CK]
and the recycled catalyst respectively.
Scheme 2 MgO-[NHC] complex catalyzed dehydration reactions.
The nano MgO was prepared by the so-called solvothermal
method as reported in the literature.16 To open the doors for
application of nano MgO, its surface was modified by
imidazolium carboxylate in toluene at 80
C. The catalytic
Fig. 1. XRD pattern of (a) nano MgO (b) MgO-[NHC] (c) after recycling of the catalyst.
properties of the surface modified nanomaterial was initially
evaluated in the dehydration of glucose in DMF at 100
transformation provides valuable compound,
hydroxymethylfurfural (HMF), which is

C. This
Remarkably, the particle size increases to 50.84% after three
recycles from 45.97% before recycling (Table 1.)
5-
a
well known
17-19
component of biofuel.
We were pleased to identify the Table 1. Relative crystalline size of nano MgO
conversion of glucose is 99% with 92% selectivity towards
HMF. To understand clearly, the role of surface modifier, we
performed the reaction separately in the presence of nano
MgO (without surface modification) and also using
imidazolium carboxylate (without MgO). Notably, neither
nano MgO nor organic ligand catalysed the conversion of
glucose. The dehydration of glucose has been catalyzed by
using various acid and base catalysts under homogeneous
conditions as well as under heterogeneous conditions.20
Later the dehydration reaction was performed in different
solvents like water and DMSO, which are commonly, used. The
dehydration reaction in aqueous medium furnished the full
conversion of glucose with expected product, HMF in 23%,
along with a significant number of by-products, one of which
was identified as levulinic acid (46%). Surprisingly, in DMSO
higher selectivity was observed upto 73% towards HMF, with
Parameter
MgO
MgO-[NHC-L1]
MgO-[NHC-L1]
(recycling)
β (HWFM)
0.464
0.92
1.01
0.42
0.914
0.47
Relative
Particle size
%
of particle
100 %
45.97 %
51.08 %
size
The field emission scanning electron microscopy (FE-SEM)
images of nano MgO and MgO-[NHC] shows distinct
differences of surface structure. The unmodified MgO
nanoparticles are self assembled to form 3D flower like
morphology. However the MgO-[NHC] results the
incorporation of organic moieties inside the 3D flowers (Fig. 2).
Further the elemental distribution was carried out from the
elemental mapping energy dispersive X-ray (EDX) analysis (Fig.
S1 and S2). The representative elemental mapping of the MgO-
[NHC] indicating the presence of carbon, nitrogen along with
magnesium and oxygen confirming the surface modification
with ligand.
less conversion upto 52% at 100

C.
The
heterogeneous
nature of this catalyst system was demonstrated by recycling
of the catalyst. The surface modified catalyst was used three
times without any significant decrease in activity and
selectivity. After completion of the first reaction, the catalyst
was washed several times with ether dried for 2 h. A new
reaction is then performed with fresh glucose using the
recovered catalyst under the same conditions.
The MgO-[NHC] complex was characterized by X-Ray
Diffraction (XRD). The chromatograms were comparable to
JCPDS (card number 45-0946). No significant change was
observed in the plane position of MgO-[NHC] and MgO (Fig. 1).
However, the decrease of particle size of MgO-[NHC] was
highly remarkable. This may be due to the interaction of Mg-C
than Mg-Mg interactions and prevents the agglomeration of
nano MgO. In addition, the XRD data of MgO-[NHC] indicated
the decrease in intensity of (220) and (200) planes due to the
interaction of ligand preferentially, whereas the intensity of
(111) and (110) planes remain same (Fig. 1). There was no
significant change in the XRD data of recycled catalyst in the
dehydration of glucose (Fig. 1c). The crystallite size was
Fig. 2. SEM of (a) nano MgO (b) nano MgO treated with L (c) after recycling of
the catalyst
The FE-SEM image of MgO-[NHC] after recycling shows well
dispersed flower-like structures in the range of 1-50 m and
open-flakes of flowers in less than 100 nm (Fig. 2).
Additionally, the transmission electron microscopy (TEM)
indicates that nanomaterials are of 20-50 nm in diameter for
MgO-[NHC] complex (Fig. S3). The distinct ring diffraction
pattern shown as insect of figure S3 further suggests that the
crystalline nature of the MgO-[NHC]. Moreover the chemical
2 | J. Name., 2012, 00, 1-3
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modification of nano MgO was confirmed from X-ray photo simultaneously the MgO of MgO-[NHC] abstractVtiheweAhrtyicdlerOonxliynel
DOI: 10.1039/C6RA16358K
electron spectroscopy (XPS) and Fourier transform infrared group of the sugar (like in reverse bias condition) (Scheme 4).
spectroscopy (FT-IR). The successful modification was
indicated by XPS from the most important characteristic of
heteroatom sites (nitrogen) of imidazolium moiety at 400 eV
(Fig. S4). The N1s binding energy near to 400 eV was earlier
reported by Glorius and co-workers for imidazolium salts
modified magnetite nanoparticles.4d Notably, binding energies
of Mg 2p, C1s are at 49.6 and 295 eV respectively (for
complete XPS of each element, see the supporting information
Fig. S4). In addition Auger electron spectroscopy (AES), further
confirms the presence of nitrogen and carbon along with
magnesium and oxygen in the surface modified MgO (Fig. S5)
and observed that 3.3% of nitrogen was present.
Scheme 4 nano MgO-[NHC] complex like in semiconductor.
Finally water is removed from biomass followed by the hydride
rearrangement to produce dehydrated product in high yields.
Later, the dehydration of nitrolacohol was performed using
MgO-[NHC-L1] complex and the results are shown in the table
S1. The dehydration nitro alcohol is very important since the
resultant compounds, nitro alkenes are used as key
intermediates in the synthesis of natural products.21 In
addition, nitro alkenes have been using as a receptors of
Michael addition reaction.22 In the present system, nano-MgO
itself catalyze the dehydration of β-nitro alcohol, gave nitro
The FTIR spectrum of surface modified MgO materials further
confirmed the presence of ligand on surface of nano MgO. Of
particular note was that the absence of stretching frequency of

(CO2) at 1615 cm-1 suggests that released carbon dioxide is
not adsorbed on nano MgO surface (Fig. S6). Interestingly, the
surface hydroxyl groups are not affected (3600 cm-1) even
after surface modification with imidazolium salts (MgO-[NHC]).
The stretching frequency at 772 cm-1 is may be due to the Mg-
C bond of surface modified MgO. Apart from these differences,
there is a good agreement with rest of the spectrum,
suggesting the remaining molecular identity is retained after
modification. These results further confirm that an organic
layer exists and the linkage between inorganic material and
organic nuclei is covalent linkage. The FT-IR of the recycled
catalyst MgO-[NHC] shows the stretching frequency at 3612
cm-1 is due to the adsorption of water molecules.
The reaction from glucose to HMF is generally assumed to
involve isomerisation of glucose to fructose followed by
dehydration to HMF. The plausible mechanism for the
synthesis of 5-HMF from glucose is shown in the Scheme 3.
Initially, the isomerisation of glucose to fructose may takes
place in the presence of MgO-[NHC] (Scheme 3A) and then
from fructose to 5-HMF (Scheme 3B) using the same catalyst.
In MgO-[NHC] complex the electron cloud is towards electron
deficient MgO from electron rich NHC (like in the semi
conductor in zero bias system).
alkene in 82% yield at 80C. The surface modified catalyst
increase the rate of reaction and nitro alkene was isolated in
85% yield after 2 h (Table S1). To the best of our knowledge,
our work is the first report of using NHC-CO2 adducts as
surface modifiers, and also as stabilizer for nano MgO.23
As a part of our ongoing research towards the synthesis of
enantiopure secondary alcohols under heterogeneous
conditions, our quest is finding an efficient, environmentally
friendly protocol for the synthesis of secondary alcohol.24
Therefore, the chiral imidazolium salts were used as modifiers
on the surface of nano MgO to generate chiral catalysts MgO-
[NHC-L2*] and MgO-[NHC-L3*]. These two L2 and L3 were
selected due to easily isolable and which play a major role in
asymmetric heterogeneous catalysis and moreover chiral NHCs
are known to stabilize nanoparticles (Fig. 3).25
Fig. 3. Chiral imidiazolium salts on nano MgO used for dehydration of nitro
alcohols.
The dehydration of nitro alcohol was then performed using
both chiral catalysts MgO-[NHC-L2*] and MgO-[NHC-L3*].
Notably, when the simple (
5mol% of chiral catalyst, MgO-[NHC-L2*], at 25

)-nitroalcohol was reacted with
C, the

dehydration took 48 h for giving 28% of nitroalkene in THF
solvent. In this asymmetric reaction, 61% of nitroalcohol was
recovered with 32% enantiomeric excess (ee) (Table 2). When
Scheme 3 Plausible mechanism for the dehydration of glucose using MgO-[NHC]
complex (A) glucose to fructose (B) fructose to HMF.
the same reaction was performed at 60 C, 81% of nitroalkene
When the glucose was added, it may be in the reverse direction
due to NHC abstract the acidic proton of sugar and
was isolated provided the recovered nitroalcohol in 4%
without ee. However, in the presence of MgO-[NHC-L3*], at
25

C, asymmetric reaction took 48 h and nitroalkene was
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Nolan, Ed.; Wiley-VCH: Weinheim, Germany, 2006; (c) D. Enders, O.
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isolated in 18% without ee in THF (Table 2). Moreover, when
the dehydration reaction was performed in water, no reaction
was observed either using MgO-[NHC-L2*] or MgO-[NHC-L3*].
This may be due to not proper dispersion of surface modified
nano MgO in the reaction medium.
Niemeier, A. Henseler, Chem. Rev., 2007, 107, 5606-5655.
DOI: 10.1039/C6RA16358K
9
For a Recent review on NHC and properties see: (a) M. N. Hopkinson, C.
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NHC-CO2 adducts: (a) A. M. Voutchkova, M. Feliz, E. Clot, O. Eisentein
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Table 2 Dehydration of nitro alcohol catalyzed by asymmetric catalysts MgO-
[NHC-L2*] and MgO-[NHC-L3*]
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Time (h)
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Ee(%)
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1
2
3
4
Mg-NHC-L2
Mg-NHC-L2
Mg-NHC-L3
Mg-NHC-L3
48
12
48
20
25
60
25
60
28
81
18
85
32
-
-
-
15
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In conclusion, the present study demonstrates the formation
of MgO-[NHC] complex using zwitterionic salts. This modified
material with its new ensemble of catalytically active entities
catalyzed dehydration of glucose to HMF. Further work will try
to elucidate the underlying active principle of this new class of
heterogeneous catalysts for surface modification and catalysis.
17
Recent Review on HMF synthesis and properties: A. A. Rosatella, S. P.
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We would like to thank department of science and technology
(DST) with a file number: SR/NM/NS-1169/2012 for financial
support.
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DOI: 10.1039/C6RA16358K
Surface modification of polyhedral nanocrystalline MgO with N-Heterocyclic Carbene
(NHC) for Dehydration Reactions: A New Approach
Melad Shaikh, Mahendra Sahu, Santimoy Khilari, Pathik Maji, A. Kiran Kumar and Kalluri V.S.
Ranganatha *