Heterostructured Hybrid rGO@α-MnO2/rGO@δ-MnO2 Nanoflower: An Efficient Catalyst for Aerobic Solvent-Free N-Alkylation Reactions and Energy Storage Material

Article abstract of DOI:10.1002/cctc.201902179

A new reduced graphene oxide (rGO) based bi-phasic crystal of MnO₂, namely α-MnO₂ nanorods and δ-MnO₂ nanoflakes containing heterostructured hybrid nanoflower rGO@α-MnO₂/rGO@δ-MnO₂ has been fabricated through a facile hydrothermal method followed by annealing treatment. The successful synthesis of the hybrid material was studied by XRD, Raman, BET, FESEM with EDX, FTIR and TEM analyses. An efficient N-alkylation reaction of substituted aromatic amines with aromatic alcohols was carried out under solvent-free aerobic conditions in the presence of catalytic amount of rGO@α-MnO₂/rGO@δ-MnO₂. The catalyst shows excellent activity in terms of high yields (up to 98 %), short reaction time (10 h) along with a simple work-up process. The spent material can be regenerated several times without causing any serious decrease in catalytic activity. Moreover, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and cyclic stability techniques were executed to evaluate the performances of rGO@α-MnO₂, rGO@δ-MnO₂, and rGO@α-MnO₂/rGO@δ-MnO₂ as energy storage materials. Among all those materials, rGO@α-MnO₂/rGO@δ-MnO₂ exhibited a proficient specific capacitance, C_S (267 F/g at 1 A/g) along with excellent cycling ability (～83 % retention up to 10000 cycles). The superb electrochemical performance of rGO@α-MnO₂/rGO@δ-MnO₂ might be ascribed to the combination of bi-phasic α-MnO₂ and δ-MnO₂ with rGO sheets, resulting in a flower-like structure.

Full text of DOI:10.1002/cctc.201902179

Accepted Article
Title: Heterostructured Hybrid rGO@α-MnO2/rGO@δ-MnO2
Nanoflower: An Efficient Catalyst for Aerobic Solvent-Free N-
Alkylation Reactions and Energy Storage Material
Authors: Haribandhu Chaudhuri and Niranjan Karak
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201902179
Link to VoR: http://dx.doi.org/10.1002/cctc.201902179
A Journal of
www.chemcatchem.org

ChemCatChem
10.1002/cctc.201902179
Heterostructured Hybrid rGO@α-MnO /rGO@δ-MnO Nanoflower: An Efficient
2
2
Catalyst for Aerobic Solvent-Free N-Alkylation Reactions and Energy Storage Material
Haribandhu Chaudhuri, and Niranjan Karak*
Dr. Haribandhu Chaudhuri, Prof. Niranjan Karak
Advanced Polymer and Nanomaterial Laboratory, Department of Chemical Sciences, Tezpur
University, Tezpur-784028, Assam, India.
E-mail: karakniranjan@gmail.com
Keywords: Heterostructured hybrid material, rGO@α-MnO /rGO@δ-MnO nanoflower, N-
2
2
alkylation reaction, energy storage material, cycling performance
Abstract: A new reduced graphene oxide (rGO) based bi-phasic crystal of MnO , namely α-
2
MnO nanorods and δ-MnO nanoflakes containing heterostructured hybrid nanoflower
2
2
rGO@α-MnO /rGO@δ-MnO has been fabricated through a facile hydrothermal method
2
2
followed by annealing treatment.The successful synthesis of the hybrid material was studied
by XRD, Raman, BET, FESEM with EDX, FTIR and TEM analyses. An efficient N-
alkylation reaction of substituted aromatic amines with aromatic alcohols was carried out
under solvent-free aerobic conditions in the presence of catalytic amount of rGO@α-
MnO /rGO@δ-MnO . The catalyst shows excellent activity in terms of high yields (upto
2
2
9
8 %), short reaction time (10 h) along with a simple work-up process. The spent material can
be regenerated several times without causing any serious decrease in catalytic activity.
Moreover, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and cyclic
stabilitytechniques were executed to evaluate the performances of rGO@α-MnO , rGO@δ-
2
MnO , and rGO@α-MnO /rGO@δ-MnO as energy storage materials. Among all those
2
2
2
materials, rGO@α-MnO /rGO@δ-MnO exhibited a proficient specific capacitance, C (267
2
2
S
F/g at 1 A/g) along with excellent cycling ability (~83% retention upto 10000 cycles). The
superb electrochemical performance of rGO@α-MnO /rGO@δ-MnO might be ascribed to
2
2
1
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
the combination of bi-phasic α-MnO and δ-MnO with rGO sheets, resulting in a flower-like
2
2
structure.
1
. Introduction
In recent years, manganese dioxide (MnO ) has drawn a great attention due to its unique
2
crystal phases, attractive morphologies and excellent properties, which are very useful for
[
1-8]
various applications including catalysis, sensors, and energy storage.
It is also often used
[
9]
as a mild oxidant in alcohol oxidations. Researchers have already developed synthetic
procedures to obtain imine and amine derivatives from alcohols and amines using MnO as
2
[
10]
the oxidant and NaBH as the reductant.
But, use of large amount of oxidants and
4
reductants limits the acceptability of those approaches from the view point of environment,
complexity, and cost. Furthermore, there are some reports on N-alkylation reactions, where
poor recyclability, high leaching problems, very long reaction time, and harsh reaction
[
11]
conditions were experienced.
In this connection, organonitrogen compounds including
amine derivatives are not only the source of nitrogen for synthesis of various biomolecules
but also important chemicals abundant in natural products as well as pharmaceutically
important molecules. The products from N-alkylation reaction have been greatly used as
intermediates for the synthesis of fine chemicals, surfactant, dye stuffs etc. Till now, transition
metal complexes catalyzed N-alkylation reaction of amines with alcohols through hydrogen
auto transfer or self-hydrogen transfer procedure and direct amination procedure were
considered as environmentally benign approach, with the former being one of the most
[
12]
versatile ways for the synthesis of amine derivative. But, the method needs to be improved
as these reactions demand costly metal complexes and require inert atmosphere along with
special equipment including glove box. Moreover, Xu and co-workers carried out MnO2-
catalyzed N-alkylation of amines with alcohols under air, which is relevant to the present
[
13]
work. But, the method suffered from very high temperature (135°C), very long time (24 h
2
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
to 35 h), and only moderate to good yield. Therefore, it is required to develop a budget
friendly efficient N-alkylation procedure for alcohols with amines that not only readily
performs under aerobic solvent free condition but also proceeds in low temperature range
producing high yields of the desired products.
Apart from the catalytic property, MnO acts as an excellent energy storage material and
2
exhibits eminent properties owing to its thermal stability, high theoretical pseudocapacitance
−
1
[4, 14]
(
1370 F g ) with high cycle stability, low cost, and environmental compatibility.
Because
of the presence of MnO octahedral basic unit, MnO can form different crystal phases
6
2
including tunnel type α-MnO , β-MnO , γ-MnO , spinel-type λ-MnO , and layered-type δ-
2
2
2
2
[
4]
MnO2. The presence of specific inorganic cations and the type of reducing agent mainly
affects the formation of various phases. Some previous and recent reports reveal that α-MnO2
and δ-MnO2 are promising and hence significantly studied for electrochemical
[
4, 15]
supercapacitors and batteries, in comparison to other phases.
Moreover, graphene oxide,
graphene, or reduced graphene oxide (rGO) or anchored MnO based nanomaterials have also
2
[
16]
been studied over the past few years for electrochemical applications. Cheng and co-
workers reported the synthesis and application of α-MnO nanowire/graphene and graphene
2
[
17]
asymmetric supercapacitors. Hu and co-workers synthesized rGO sheets anchored with 1D
[
18]
δ-MnO nanoscrolls for Li-ion batteries.
But, for all the above mentioned cases,
2
electrochemical performances in terms of oxidation-reduction potential, cell distortion under
high voltage along with cycle performance were not very convincing. This leads to a strong
need to improve the electrochemical performances of rGO anchored α-MnO and δ-MnO
2
2
materials.
Thus, from the viewpoint of the above studies and statements, we report the successful
synthesis of heterostructured hybrid nanoflower of rGO@α-MnO /rGO@δ-MnO . The
2
2
nanohybrid was synthesized by coassembly between rGO@α-MnO and rGO@δ-MnO
2
2
through a hydrothermal process followed by annealing treatment. The hybrid material was
3
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
used as a heterogeneous catalyst for efficient N-alkylation reactions of substituted aromatic
amines with aromatic alcohols under aerobic solvent-free conditions. It is well-known that
nano-sized metal has been immensely used as effective catalyst in organic synthesis and
electrocatalysis due to the proficient surface area and presence of active sites within it. Thus,
rGO sheets anchored bi-phasic α-MnO and δ-MnO based nanomaterial have high surface
2
2
2
area (211 m /g) along with active sites, resulting in excellent catalytic activity. Moreover, this
material has some sort of unreduced functional groups, which actively participates in the
reaction mechanism. It not only generates unpaired electrons during the self or auto hydrogen
transfer process, but also these species need to be transferred to the intermediate too. These
reactions were performed in the presence of small amount of catalyst at 100°C and produced
high yield of the desired products, with a maximum upto 98% within 10 h along with
negligible leaching problem, which appeared as much better results, in comparison to the
[
1, 10, 13]
other reported procedures.
In addition, such efficient unpaired electron transferring
heterogeneous nanocatalyst proved to be a promising energy storage material. The charge
storage behavior of rGO@α-MnO and rGO@δ-MnO was studied to compare their
2
2
electrochemical performance for use as energy storage material.
2
2
. Results and Discussion
.1. Preparation and Characterizations of Nanoflower Material
The synthesis of heterostructured hybrid nanoflower material was achieved through the
combination of rGO anchored 2D materials (bi-phasic crystal of MnO ). It can be assumed
2
that π-π stacking interaction and weak van-der Waals force play the key roles for the
formation of rGO@α-MnO /rGO@δ-MnO during the synthetic process (Scheme 1). Thus,
2
2
several instrumental analyses were carried out to inspect the successful formation of the
nanoflower material. We have first analyzed XRD results (Figure 1a). Peaks near at 2θ = 22°
and 42° appeared due to the presence of rGO and unreacted graphite, respectively, present in
4
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
all the synthesized materials (yellow highlighted, Figure 1a). It has been reported that both α-
[
3, 4]
MnO and δ-MnO phases have nearly similar interlayer spacing.
Thus, some of the
2
2
diffraction peaks appear at the same position, as seen in Figure 1a. Both rGO@α-MnO (110)
2
and rGO@δ-MnO (001) planes are located near at 2θ = 12°, while rGO@α-MnO (400) and
2
2
rGO@δ-MnO (110) planes are situated near at 2θ = 37°. Further, rGO@α-MnO (411) and
2
2
rGO@δ-MnO (112) planes are located near at 2θ = 50°, while rGO@α-MnO (600) and
2
2
rGO@δ-MnO (160) planes are placed near at 2θ = 57°. Moreover, both rGO@α-MnO (002)
2
2
and rGO@δ-MnO (312) planes are situated near at 2θ = 65° (α-MnO reference JCPDS #44-
2
2
0
141 and δ-MnO reference JCPDS#42-1317). But, there are some peaks which clearly
2
differentiates these two phases. For example, rGO@α-MnO (200) and (310) planes are
2
placed near at 2θ = 18° and 28°, respectively. On the otherhand, rGO@δ-MnO (002) plane is
2
situated near at 2θ = 26°. Thus, it may be presumed that both 2D materials including rGO@α-
MnO and rGO@δ-MnO were successfully prepared and the heterostructured hybrid
2
2
nanoflower material has contained both phases of MnO2.
Thus, after getting the XRD spectra of all the as-prepared materials, EDX analyses of
rGO@α-MnO -1.5, rGO@α-MnO -3.0, and rGO@α-MnO -4.5 were carried out to get an
2
2
2
insight about the maximum loading of α-MnO or δ-MnO on the surface of rGO. It was
2
2
observed (Figure S1-S3, Supporting Information, SI) that the loading capacity of α-MnO2
on the surface of rGO was increased to the maximum, as the concentration of KMnO was
4
increased from 1.5 mmol to 3.0 mmol. But, further increment in concentration of KMnO4
(from 3.0 mmol to 4.5 mmol) caused no such substantial increment in loading capacity of α-
MnO on the surface of rGO. It was also observed from the XRD spectra (Figure 1a) that the
2
peak intensities of both rGO@α-MnO -3.0 and rGO@α-MnO -4.5 remained about the same.
2
2
Similarly, same trend was followed for both rGO@δ-MnO -3.0 and rGO@δ-MnO -4.5
2
2
(
Figure 1a). Thus, rGO@α-MnO -3.0 and rGO@δ-MnO -3.0 were chosen as the optimized
2 2
materials and annealing were carried out at 250°C and 450°C to get the desired
5
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
heterostructured nanohybrid materials. Again, XRD analyses were carried out to check the
successful formation of the heterostructured nanohybrid materials (Figure 1b). As shown in
Figure 1b, all the signatures related to both rGO@α-MnO and rGO@δ-MnO were present
2
2
in the spectra of both the heterostructured hybrid nanoflower materials with some sorts of
shifting of peaks took place. Peaks are appearing more intense for the rGO@α-
MnO /rGO@δ-MnO material, annealed at 250°C, in comparison with the rGO@α-
2
2
MnO /rGO@δ-MnO material, annealed at 450°C.
2
2
We also seek affirmation from Raman spectroscopic study, considering that MnO is Raman
2
active. The Raman spectra of rGO@α-MnO , rGO@δ-MnO , and rGO@α-MnO /rGO@δ-
2
2
2
MnO are presented in Figure 1c. As shown in Figure 1c, all spectra have common typical
2
-1
-1
-1
-1
-1
bands in the region, 1750 cm - 1050 cm and 690 cm - 300 cm . In the region 1750cm -
-1
1
050 cm , two typical broad bands, namely G band and D band appeared which are ascribed
[
19]
to the presence of rGO in the as-prepared material. There are five bands which appear in
-1
-1 [4]
-1
-1
the region 690 cm - 300 cm . The band near 640 cm - 630 cm is observed because of the
existence of symmetric stretching vibration of Mn-O in MnO octahedral (Figure 1c, i). Band
6
associated to Mn-O stretching along the chains of the MnO framework has been observed
2
-1
-1
near 585 cm - 575 cm (Figure 1c, ii). Moreover, deformation mode of the metal-oxygen
-1
-1
chain Mn-O-Mn in the MnO octahedral is noticed near 525 cm - 515 cm (Figure 1c, iii).
6
-1
-1
The band near 400 cm - 390 cm is observed due to the existence of the stretching mode of
MnO octahedral (Figure 1c, iv). Infrared active mode of MnO octahedral is noticed near
6
6
-1
-1
3
35 cm - 325 cm (Figure 1c, v). The main difference between rGO@α-MnO and rGO@δ-
2
-1
-1 [20]
MnO is observed in the region 90 cm - 165 cm . For rGO@α-MnO , band associated to
2
2
the external vibration of translational motion ofthe MnO octahedral attached to the cations of
6
-1
-1
octahedral tunnel, is observed near 165 cm - 140 cm . For rGO@δ-MnO , band near 110
2
-1
-1
cm - 90 cm is observed, which is attributed to the presence of the external vibration of
translational motion of the MnO octahedral related to the water molecules in the MnO . It
6
2
6
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
-1
-1
-1
-1
can also be observed that both bands near 110 cm - 90 cm and 165 cm - 140 cm , are
present in rGO@α-MnO /rGO@δ-MnO , which implied the successful formation of the
2
2
heterostructured hybrid material.
EDX analysis (Figure 1d) of rGO@α-MnO /rGO@δ-MnO confirmed the presence of C, O,
2
2
and Mn with the corresponding wt% values of 20.75, 40.86, and 38.39, respectively.
The N adsorption-desorption isotherms of rGO@α-MnO , rGO@δ-MnO , and rGO@α-
2
2
2
MnO /rGO@δ-MnO annealed at 250°C were analyzed and are depicted in Figure 2. All the
2
2
materials show a vertiginous increment at P/P values after 0.5, 0.6, and 0.6 (for rGO@α-
0
MnO , rGO@δ-MnO , and rGO@α-MnO /rGO@δ-MnO , respectively) which can be
2
2
2
2
[
4]
ascribed to the occurrence of capillary condensation. The BET surface area of rGO@α-
2
MnO is 160 m /g and the overall pore volume is 0.570 cc/g with the average pore diameter of
2
2
6
.78 nm whereas BET surface area of rGO@δ-MnO is 183 m /g and the overall pore volume
2
is 0.599 cc/g with the average pore diameter of 6.90 nm. The rGO@α-MnO /rGO@δ-MnO
2
2
2
material shows higher BET surface area (211 m /g), greater overall pore volume (0.618 cc/g),
and bigger average pore diameter (7.03 nm) as compared to those individual materials.
The analyses of FESEM images were done to get an insight about the morphologies of
rGO@α-MnO , rGO@δ-MnO , and rGO@α-MnO /rGO@δ-MnO annealed at 250°C. From
2
2
2
2
Figure 3a, b, it can be observed that α-MnO nanorods are randomly grown on the rGO
2
sheets and small crumpled nature δ-MnO nanoflakes are developed on the rGO sheets. A
2
perusal of Figure 3c, d also shows that a nanoflower like morphology with an average
diameter of 1000-2000 nm, in which rGO intrinsically occupied on the hybrid structure,
having small sheets along with the mixture of nanorods like morphology of α-MnO and
2
nanoflakes like morphology of δ-MnO . A close view of the FESEM image further reveals
2
that several small nanosheets of rGO@α-MnO /rGO@δ-MnO varying thickness of ~40-200
2
2
nm along with very small adjacent distances were assembled to make a flowerlike
heterostructured nanohybrid, implying the presence of some synergistic interaction between
7
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
rGO@α-MnO and rGO@δ-MnO materials. Apart from this, a part of rGO@α-
2
2
MnO /rGO@δ-MnO nanoflower is developed by single nanosheets interconnecting to the
2
2
center fragment with genuine edges to develop a hierarchical heterostructure hybrid
morphology. This observation confirms that the present synthesis protocol for rGO@α-
MnO /rGO@δ-MnO nanoflower is ascribed to the neighbouring C atom of rGO and partially
2
2
reduced functional groups in the rGO matrices along with α-MnO and δ-MnO nanomaterials.
2
2
This may improve the catalytic performance and high durability of the hybrid material. In this
[15, 18, 21]
context, the morphologies of both α-MnO and δ-MnO samples were reported earlier.
2
2
So, the authors foresee that both rGO@α-MnO and rGO@δ-MnO materials were combined
2
2
to create a new heterogeneous hybrid material having nanoflower like morphology, during
annealing process.
TEM analyses were executed to further explore the intrinsic morphologies of rGO@α-MnO2,
rGO@δ-MnO , and rGO@α-MnO /rGO@δ-MnO annealed at 250°C (Figure 4). The SAED
2
2
2
pattern of rGO@α-MnO /rGO@δ-MnO is shown in inset (Figure 4). From Figure 4a, b, it
2
2
can be seen that α-MnO nanorod based rGO (rGO@α-MnO ) and δ-MnO nanoflake
2
2
2
mingled rGO (rGO@δ-MnO ) are formed successfully. A perusal of Figure 4c, d reveals the
2
formation of rGO@α-MnO /rGO@δ-MnO occurred by the combination of both α-MnO
2
2
2
nanorod integrated rGO and δ-MnO nanoflake coalesced rGO materials. From Figure 4d, it
2
can also be observed that (110) and (200) lattices of α-MnO (d ~0.69 nm, and 0.26 nm,
2
respectively) and (001) and (110) lattices of δ-MnO (d ~0.72 nm, and 0.25 nm, respectively)
2
are present, confirming not only the existence of both crystal phases in the hybrid material,
but also corresponds great with the XRD data (Figure 1a, b). The SAED pattern indicates the
existence of the amorphous nature of rGO@α-MnO /rGO@δ-MnO , due to the combination
2
2
of two single crystal MnO phases with rGO.
2
To check the presence of various functional groups in rGO and rGO@α-MnO /rGO@δ-MnO
2
2
material, FTIR analysis was done for both rGO and rGO@α-MnO /rGO@δ-MnO materials.
2
2
8
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
A perusal of Figure S4 (SI) reveals that several peaks including C-H stretching, C=O
stretching, C=C stretching, O-H deformation, epoxy C-O stretching, and alkoxy C-O
stretching are present in both the material. This also indicates that the rGO was not fully
reduced. Obviously, the peak intensities of all those peaks are decreased for rGO@α-
MnO /rGO@δ-MnO material in comparison to rGO. For rGO@α-MnO /rGO@δ-MnO
2
2
2
2
material, a new intense peak is observed which is related to the Mn-O in MnO . This also
2
[
22]
sanguine the successful grafting of MnO nanoparticles in between rGO.
2
2
.2. Catalytic Application of rGO@α-MnO /rGO@δ-MnO in N-alkylation of Aromatic
2 2
Amines with Aromatic Alcohols
2
.2.1. Catalyst Screening and Optimization of Reaction Conditions
The catalytic activity of rGO@α-MnO /rGO@δ-MnO was examined for N-alkylation of
2
2
different aromatic amines and different aromatic alcohols. To investigate the effect of catalyst
loading along with the screening of reaction conditions on the reaction rate and its efficiency,
the model reaction was used, and the results are summarized in Table 1. Initially, the reaction
was carried out in absence of catalyst at RT and 130°C, but no product was formed even after
4
1
8 h (Table 1, entries 1, 2). In presence of 5 wt% of catalyst and 0.3 mmol of K CO at
2 3
00°C, desired product (40% yield) was found after 16 h (Table 1, entry 3). Then, the yield
of the desired product increased (48%) with the increment in catalyst loading (10 wt%)
Table 1, entry 4). The product yield was slightly reduced to 45%, when rGO@α-
MnO /rGO@δ-MnO , annealed at 450°C, was used as the catalyst for the same reaction
(
2
2
(
Table 1, entry 5). In presence of 10 wt% of catalyst and 0.6 mmol of K CO at 100°C, yield
2 3
of the product was 50% after 16 h (Table 1, entry 6). But, for a further increment in base
concentration and catalyst loading in the reaction medium, no substantial increment was
observed in yield of the desired product (Table 1, entries 7, 8). Subsequently, KOH (strong
base) was used as the base in place of K CO , to enhance the product yield. From the results,
2
3
9
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
it was found that in the presence of 5 wt% of catalyst with 0.3 mmol and 0.6 mmol of KOH at
00°C, yields of the products raised to 63% and 78%, respectively, after 12 h of reaction
Table 1, entries 9, 10). While in the presence of 10 wt% of catalyst and 0.3 mmol of KOH at
00°C, yield of the product was 75% after 12 h (Table 1, entry 11). As shown in Table 1, the
1
(
1
yield of the desired product increased (80%) with the increment in catalyst loading (20 wt%)
in the presence of 0.3 mmol of KOH (entry 12). The highest yield (98%) of desired product
was found when the reaction was carried out in the presence of 10 wt% of catalyst and 0.6
mmol of KOH at 100°C within 10 h, shortest time in comparison with the other reported
[
1, 10, 13]
literatures (Table 1, entry 13).
The product yield was reduced to 90%, when rGO@α-
MnO /rGO@δ-MnO , annealed at 450°C, was used as the catalyst (10 wt% and 0.6 mmol of
2
2
KOH) in the reaction medium (Table 1, entry 14). But again, no further increase in the
desired product yield was observed with further increment in base concentration and catalyst
loading in the reaction medium (Table 1, entries 15, 16).
2
.2.2. N-alkylation of Different Substituted Aromatic Amines and Substituted Aromatic
Alcohols
To examine the advantage, limitation and applicability of the N-alkylation process, the
reaction scope was expanded to different substituted aromatic amines and substituted aromatic
alcohols using the optimized conditions. The results obtained are presented in Table 2. As
depicted in Table 2, different aromatic amines reacted with different benzyl alcohols in the
presence of rGO@α-MnO /rGO@δ-MnO and gave proficient to excellent yields of the
2
2
desired products. Aniline produced N-benzylaniline with 98% yield within 10 h (Table 2,
entry 1). In order to examine the effect of substitution on the aromatic ring of benzyl alcohol
on the reaction with aniline, we have carried out the reactions with different substitutions.
Substitution with moderately deactivating groups on benzyl alcohol slightly decreases the
reactivity and yielded 86% within 10 h (Table 2, entry 2) whereas substitution of highly
activating group on benzyl alcohol provided less product with 79% yield after 12 h (Table 2,
1
0
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
entry 3). Substitution of moderately activating group on benzyl alcohol gave excellent yields
97%) of the desired product within 10 h (Table 2, entry 4). Thereafter, in order to check the
(
effect of substitution on the aromatic ring of aniline on the reaction with benzyl alcohol, we
have executed the reactions with various substitutions. At first, highly activating group
present on aniline resulted only 74% yield of desired product after 14 h (Table 2, entry 5),
while moderately activating group substituted on the aromatic ring of aniline provided
excellent yield of 97% within 10 h (Table 2, entry 6). Moreover, moderately deactivating
group present on the aromatic ring of aniline gave 89% yield after 10 h (Table 2, entry 7),
whereas highly deactivating groups substituted on ortho-, meta-, and para- positions provided
low to good yields (56% within 16 h, 80% within 14 h, and 91% within 14 h, respectively) of
the desired products (Table 2, entries 8-10).
2
.2.3. Investigation of the Catalytic Reactions of the Components of rGO@α-MnO /rGO@δ-
2
MnO in the N-Alkylation of Substituted Aromatic Amines and Aromatic Alcohols
2
From the optimization of catalyst and substrate variation experiments, it was observed that the
N-alkylation reaction of different substituted aromatic amines and substituted aromatic
alcohols was greatly increased by 10 wt% of rGO@α-MnO /rGO@δ-MnO , annealed at
2
2
2
50°C, in the presence of 0.6 mmol KOH. To explain the reason of this increment, the
catalytic role of the components were examined with different variants of the catalyst like
rGO, rGO@α-MnO , rGO@δ-MnO , α-MnO @δ-MnO , and rGO@α-MnO /rGO@-δMnO
2
2
2
2
2
2
annealed at 250°C, and the results are summarized in the Table 3. For this purpose, the N-
alkylation reaction between aniline and benzyl alcohol was chosen to be the model reaction,
as the reaction resulted in excellent yield and completed in faster time as compared to other
reactions. In the presence of rGO, the reaction proceeded very slowly, with a very low yield
of 16%, as shown in Table 3 (entry 1). This can be ascribed to the fact that rGO was not fully
reduced, some sort of epoxy-, hydroxyl-, or carboxylic acid functionalities remained on the
surface. Those functionalities along with the unpaired electrons exist at the fringes of rGO and
1
1
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
took part in the reaction to get a very low yield of the desired product. The product yield was
increased to 81% within 15 h, when rGO@α-MnO was incorporated as catalyst in the
2
reaction medium (Table 3, entry 2). Similarly, rGO@δ-MnO was introduced as the catalyst
2
in the reaction medium, resulting in 80% yield of the product within 15 h (Table 3, entry 3).
For both instances (Table 3, entries 2, 3), high yield of the products as well as short reaction
times were observed, which can be attributed to the fact that partially reduced functionalities
as well as the unpaired electrons present at the fringe of rGO and Mn-mediated N-alkylation,
facilitated alcohol oxidation to generate aldehyde along with removal of water molecules
followed by the production of amine derivative. In presence of α-MnO @δ-MnO , the product
2
2
yield enhanced to 90%, within 13 h (Table 3, entry 4). So, from entries 1, 2, 3, and 4, it can
be ascribed that single crystal phase of MnO (α-MnO /δ-MnO ) not only facilitated the N-
2
2
2
alkylation reaction and increased the product yield, but also played the leading role in the N-
alkylation reaction, as compared to only rGO. Beside this, in the absence of rGO, the yield of
the desired product raised to 90% (Table 3, entry 4), which can be ascribed to the presence of
hybrid crystal phases of MnO (α-MnO @δ-MnO )that fully acted as the key factor in Mn-
2
2
2
mediated N-alkylation reaction. It is also necessary to mention that alcohol can be much
effectively activated in the presence of hybrid crystal phases of MnO , resulting in higher
2
yield (Table 3, entry 4) in comparison to others (Table 3, entries 1, 2, and3). Finally,
incorporation of rGO@α-MnO /rGO@δ-MnO , annealed at 250°C, resulted in highest yield
2
2
(98 %) within 10 h as compared to other reactions (Table 3, entry 5).
2
.2.4. Regeneration and Reusability of Catalyst
The regeneration and reusability of the catalyst are essential from the viewpoint of sustainable
chemistry. Thus, the catalyst was screened for regeneration and reusability by incorporating
the model reaction for successive runs and analyzed by GC (Figure S5 and Table S1, SI).
The catalyst was filtered, washed with dichloromethane and dilute HCl followed by distilled
water, and dried at 80°C in vacuum oven after completion of the reaction. As shown in
1
2
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
Figure S5 (SI), the catalyst was found to be reusable up to the fourth run without any loss of
the catalytic activity. Moreover, after fourth run, the catalyst showed slight losses in the yields
th
th
of the products (96% yield within 10.5 h for 5 run and 95% yield within 11.5 h for 6 run,
please see SI) with higher completion times of reactions, as compared to previous runs. Those
losses of catalytic activities can be ascribed to the agglomeration of rGO@α-MnO /rGO@δ-
2
MnO , leading to slight deactivation of the N-alkylation process after fourth runs of the
2
reaction. XRD patterns and EDX of spent materials (Figure S6, SI) were examined to check
the effectiveness after six consecutive cycles. It was observed from XRD patterns (Figure
th
S6a, SI) that peak intensities were reduced after 6 cycle, in comparison to the fresh catalyst.
The EDX study revealed about the same elemental compositions, as before (Figure S6b, SI).
Moreover, N adsorption-desorption isotherm of used hybrid material (Figure S7, SI) was
2
carried out to check the efficiency of the used material. The BET surface area of the used
2
material is 193 m /g and the overall pore volume is 0.545 cc/g along with the average pore
diameter of 5.59 nm.The results reveal that the BET surface area, the overall pore volume,
and the average pore diameter decreased a little as compared to the unused hybrid material,
but the isotherm nearly retains the same as virgin rGO@α-MnO /rGO@δ-MnO material after
2
2
th
6
cycle. So, it might be concluded that rGO@α-MnO /rGO@δ-MnO can act as a highly
2 2
active and reusable catalyst for the N-alkylation of substituted aromatic amines and aromatic
alcohols.
In addition, leaching of Mn in the reaction medium was also studied by ICPMS analysis (SI).
The study revealed that the leached Mn in the reaction medium was 0.272 ppm, which was
negligible.
-1
Turnover number (TON) and turnover frequency (TOF) were found to be 1166 and 117 h ,
respectively, for the N-alkylation reaction of different substituted aromatic amines and
aromatic alcohols. Detailed calculations have been provided in the SI.
2
.2.5. Plausible Catalytic Mechanism
1
3
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
[
23]
[24, 25]
It is well-known that direct amination
and self or auto hydrogen transfer processes
were established as environmentally feasible protocols for N-alkylation reaction of alcohols
with amines using transition metal or transition metal complexes. It has also been considered
that self or auto hydrogen transfer process is the most prominent method for the synthesis of
[
26]
[24]
amine or amide derivatives. This self or auto hydrogen transfer
methodology usually
occurred to allow those reactions to prosecute at low temperature through the path, where a
transition metal or transition metal complex influenced dehydrogenative alcohol activation to
st
produce aldehyde with the production of the hydridometal species (Figure 5, 1 step) that
nd
acts as the reducing agent in the reduction of the intermediate imine (Figure 5, 2 step) to
rd
get desired amine (Figure 5, 3 step), thus supplying the active or borrowed hydrogen and
repeating the catalytic cycle. As shown in Figure 5, in the initial stage of the reaction the
metal catalyst is deactivated by the aromatic amine so that the probability of hydrogen
abstraction from alcohol to produce aldehyde along with hydridometal species is reduced
considerably, making it the rate-limiting step. As a consequence, air or H-acceptors or
st
oxidants can ease the aldehyde generation step (Figure 5, 1 step) at lower temperature,
during which Mn (II) might be produced. Thereafter, Mn (II) catalyzes the condensation
nd
rd
(
Figure 5, 2 step) and transfer hydrogenation (Figure 5, 3 step) reactions are effective at
th
lower temperature and the aldehyde would be generated (Figure 5, 4 step) for the next
catalytic cycle. Herein, KOH is essential in borrowing hydrogen reactions and transfer
hydrogenation is the KOH-dependent step in those N-alkylation reactions. Moreover, we
propose that the unpaired electrons along with the partially reduced functional groups
occupied at the edge of rGO react with atmospheric O to evolve Ȱ which thereafter reacts
2
2
with the benzyl alcohol to assist not only the alcohol oxidation but also the conversion of Mn
IV) to Mn (II) along with the elimination of water molecule. The participation of atmospheric
(
O as oxidant is implied from the fact that when the reaction was executed under N
2
2
atmosphere, no product was found.
1
4
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
2
.3. Electrochemical Studies
Thus, it has been observed that rGO@α-MnO /rGO@δ-MnO can be efficiently used for N-
2
2
alkylation of aromatic amines with aromatic alcohols during the self or auto hydrogen transfer
process through the transfer of unpaired electrons to the intermediate for the conversion of
Mn (IV) to Mn (II) in presence of unreduced functional groups of rGO. This electron transfer
phenomenon influences us to study the energy storage performances of all as-prepared
materials. The energy storage or capacitive performances of all these nanomaterials were
[
6, 8, 14]
evaluated using CV and GCD techniques.
First of all, CV experiments were carried out
for rGO@α-MnO , rGO@δ-MnO , and rGO@α-MnO /rGO@δ-MnO at different scan rates
2
2
2
2
(1 to 50 mV/s). As illustrated in Figure 6, all those materials have symmetric and nearly
quasi-rectangular shaped curves at different scan rates, implying the perfect capacitive
performances. A perusal of Figure 6a-c reveals that the current density increases with the
+
increment in the scan rate from 1 mV/s to 50 mV/s. For all varied scan rates, the Na ions
migrate into almost all accessible sites into the hybrid materials, showing nearly perfect
[
14, 16, 21]
capacitive behavior.
From Figure 6a-c, it can be seen that rGO@α-MnO /rGO@δ-
2
MnO shows larger CV loop along with higher anodic-cathodic currents with respect to the
2
scan rates, as compared torGO@α-MnO and rGO@δ-MnO materials. Figure 6d clearly
2
2
shows that rGO@α-MnO /rGO@δ-MnO has greater capacitive performance as compared to
2
2
others, at a scan rate of 10 mV/s.
GCD technique was also implemented to get a detail scenario about the use of all those
[
8, 14]
materials as energy storage electrode materials.
As shown in Figure 7a-c, the charge
curves are nearly symmetric to the corresponding discharge curves with a little inflection,
[
5]
indicating a high reversibility of the as-prepared materials. As shown in Figure 7a-c, the
capacitance values along with the time of charging-discharging processes decrease with the
increment in current densities. This might be attributed to the availability of both inner and
+
outer surface pores of the as-prepared electrode materials, where the diffusion of Na ions
1
5
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
could easily occur, particularly at lower current density. But, at higher current density, the
+
inadequate accessibility of Na ions affects inauspicious to the formation of the intercalation
and deintercalation equilibrium approach, which is compulsory to achieve the absolute redox
[
8, 16]
reaction, resulting in decrement in the time of charging-discharging processes.
The
specific capacitances (C ) of all used materials were calculated according to C = (It)/(mV),
s
s
where t indicates discharge time, I specifies constant discharge current, m is the mass of the
[
16b, 27]
electrode material, and V indicates potential window.
C values of materials were found
s
to be (rGO@α-MnO /rGO@δ-MnO vs. rGO@δ-MnO vs. rGO@α-MnO ): 267 F/g vs. 158
2
2
2
2
F/g vs. 104 F/g, 236 F/g vs. 132 F/g vs. 87 F/g, 217 F/g vs. 114 F/g vs. 78 F/g, 185 F/g vs.
01 F/g vs. 72 F/g, and 141 F/g vs. 90 F/g vs. 69 F/g, respectively, at discharge current
1
densities of 1, 2, 3, 4, and 5 A/g, respectively. For all three materials, capacitance retentions
of 53%, 57%, and 66% (for rGO@α-MnO /rGO@δ-MnO , rGO@δ-MnO , and rGO@α-
2
2
2
MnO , respectively) of their initial values are achieved with the increment in the current
2
[
5]
densities from 1 to 5 A/g, implying excellent rate capability of those electrode materials.
Beside this, with the increment in the scan rate from 1 mV/s to 50 mV/s, C values of all
s
[
8, 28]
materials decrease,
as shown in Figure 7d. The promising cyclic performance of an
electrode material is very much important from the viewpoint of practical application as
energy storage devices. To evaluate the cyclic performance of all three electrode materials,
GCD measurements were carried out at 5 A/g over 10000 cycles and the results are shown in
Figure 7e and Figure S8-S10. The results of cyclic performances reveal that rGO@α-
MnO /rGO@δ-MnO exhibits proficient charge and discharge capacitances with huge
2
2
stability (retention ~ 83%, Figure 7e and Figure S8-S10). This retention is lower in
comparison with rGO@δ-MnO and rGO@α-MnO electrode materials (~ 92% and ~ 90%,
2
2
respectively), but the capacity value is notably higher (e.g., 141 F/g vs. 90 F/g vs. 69 F/g for
rGO@α-MnO /rGO@δ-MnO vs. rGO@δ-MnO vs. rGO@α-MnO at a current density of 5
2
2
2
2
A/g). In this connection, because of the good conductivity, rGO in hybrid material could be
1
6
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
involved in the efficient electron transfer through conductive channels to improve the
[
14]
electrochemical performance.
Thus, it is worthwhile to mention that for rGO@α-
MnO /rGO@δ-MnO , with well-dispersed bi-phasic MnO on the surface of rGO, can
2
2
2
+
effectively reduce the length of diffusion of Na ions during the charge-discharge process,
enhancing the electrochemical performance. Moreover, in Table S2 of SI, the authors have
summarized the results of as-prepared materials along with the previous reported values for
different MnO morphologies. It has been observed that rGO@α-MnO /rGO@δ-MnO
2
2
2
electrode material exhibited a proficient C with brilliant cyclic performance. All in all, the
S
above results envisage that the combination of both crystal phases can assure an enhanced
electrochemical performance.
3
. Conclusion
To sum up, the authors have demonstrated a hybrid rGO@α-MnO /rGO@δ-MnO
2
2
nanoflower material, which is proven to be efficient as a heterogeneous catalyst and energy
storage material. The hybrid material was developed in the combination of rGO anchored bi-
phasic MnO materials. An efficient N-alkylation method for different aromatic amines using
2
aromatic alcohols was developed by simply executed the reactions under aerobic solvent free
conditions using only catalytic amount of rGO@α-MnO /rGO@δ-MnO . This procedure
2
2
avoids the use of costly complex ligands, needs no inert atmosphere, needs only short reaction
time, has negligible leaching problem, and can simplify the workup processes. Furthermore,
the rGO based MnO crystal phases including α-MnO nanorod and the δ-MnO nanoflake
2
2
2
have different morphologies and capacitive performances, which play a vital role for the huge
capacitance improvement in rGO@α-MnO /rGO@δ-MnO .rGO@α-MnO /rGO@δ-MnO
2
2
2
2
shows proficient C value of 267 F/g at 1 A/g with very stable cyclic performance with ~83%
s
capacitance retention upto 10000 cycles. Finally, this work paves the way not only in the
development of an efficient N-alkylation method of different aromatic amines with aromatic
1
7
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
alcohols but also creating energy storage materials with huge capacitive performances using
bi-phase hybrid heterostructure.
4
. Experimental Section
Used Chemicals and Solvents:Graphite, potassium permanganate (KMnO ), hydrochloric acid
4
(
HCl, ~37%), hydrogen peroxide (H O , ~30%), concentrated sulphuric acid (H SO ), sodium
2 2 2 4
sulphate anhydrous (Na SO ), and all other solvents were purchased from Merck, India. Silica
2
4
(60-120 mesh size) was used for column chromatography. Aniline, benzyl alcohol and their
substrates were procured from Sigma Aldrich, Germany.Millipore distilled water was used
throughout the experiments.
Syntheses: First of all, GO was prepared from natural graphite flakes according to the known
[
29]
procedure. rGO was synthesized by reduction of different surface functional groups of GO
[
30]
using ascorbic acid.
Both, rGO anchored α-MnO nanorods, namely rGO@α-MnO , and
2 2
rGO anchored δ-MnO nanoflakes, namely rGO@δ-MnO were obtained by varying the mass
2
2
loading of different KMnO molar concentrations on the surface of rGO via hydrothermal
4
processes. In detail, for the synthesis of rGO@α-MnO , three 100 mL round bottom (RB)
2
flasks were taken, in each, 20 mg of rGO was dispersed in 60 ml of distilled water by
ultrasonic dispersion for 25 min. Then, 1.5 mmol (0.237 g), 3.0 mmol (0.474 g), and 4.5
mmol (0.711 g) of KMnO were dissolved in those above suspensions, respectively, and
4
stirred for about 30 min at room temperature (RT). After 30 min, 25 mmol concentrated HCl
was added in each of those resulting solutions and stirred for another 1 h. Thereafter, the
resulting solutions were sealed in three 100 mL Teflon autoclaves and hydrothermally treated
at 140°C for 1 h. Finally, the as-synthesized materials were centrifuged and washed with
ethanol-water mixtures, followed by drying at 80°C for 6 h. under vacuum. The as-prepared
materials are named as rGO@α-MnO -1.5, rGO@α-MnO -3.0, and rGO@α-MnO -4.5.
2
2
2
Similarly, for the synthesis of rGO@δ-MnO , the procedure was the same except the reaction
2
1
8
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
conditions (140°C for 10 h.). The as-synthesized materials are encoded as rGO@δ-MnO -1.5,
2
rGO@δ-MnO -3.0, and rGO@δ-MnO -4.5. Finally, both the optimized rGO@α-MnO and
2
2
2
rGO@δ-MnO materials were annealed at 250°C and 450°C, respectively for 3 h to get the
2
desired rGO@α-MnO /rGO@δ-MnO heterostructured hybrid materials.
2
2
General Procedure for Catalytic Experiments: A mixture of aniline (1 mmol, 0.093 g), benzyl
alcohol (1.3 mmol, 0.140 g), and KOH (0.6 mmol, 0.033 g) was taken in a 20 mL Schlenk
tube and the as-prepared catalyst (10 wt % of used aniline) was added into it. Then, the
mixture was allowed to react at 100°C for 10 h. The progress of the reaction was checked
using thin layer chromatography (TLC). After completion of the reaction, the reaction was
quenched with ethyl acetate. The catalyst was separated from the reaction mixture by
filtration, washed several times with dichloromethane (DCM), dilute HCl followed by
distilled water and dried under vacuum at 80°C. The catalyst was further used for next
catalytic reactions and regenerated six times. The isolation of the desired product was
performed by column chromatography using ethyl acetate and petroleum ether as the eluent.
The products were authenticated for NMR spectral analyses as provided in the SI.
Preparation of Electrodes and Electrochemical Experiment: All electrochemical experiments
including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and cyclic
stability were carried out using three-electrode system in 1 M Na SO aqueous solution at RT.
2
4
For making a working electrode, individual material including rGO@α-MnO -3.0, rGO@δ-
2
MnO -3.0, and rGO@α-MnO /rGO@δ-MnO (annealed at 250°C) were separately mixed in a
2
2
2
mixture of Nafion and N-methyl-2-pyrrolidone (NMP) with a ratio of 1 mg: 0.4 μL: 500 μL
and dispersed using ultrasonic instrument for 30 min. Thereafter, the well dispersed mixture
was pasted on the tip of a carbon electrode and dried in an air-oven at 120°C for 12 h. Before
the electrochemical test, the prepared electrode was soaked overnight in a 1 M Na SO
2
4
solution and that solution was used as electrolyte. A platinum electrode and a saturated
calomel electrode (SCE) were used as the counter and reference electrodes, respectively.
1
9
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
Characterizations and Instruments: The X-ray diffraction (XRD) spectra were measured
using P-XRD instrument (model: D8 FOCUS, Bruker AXS, Germany) with CuK as the
α
radiation source. Raman spectra were recorded using Renishaw Basic Series Raman
spectroscopy (Renishaw, UK) attached with 514 nm laser. FTIR analyses were carried out for
rGO and rGO@α-MnO /rGO@δ-MnO using KBr pellets by a Nicolet (Madison, USA,
2
2
Impact 410 spectrophometer). N sorption measurements of as-prepared material and spent
2
catalyst were carried out at 77 K with Nova 3200e (model: 1000E, Quantachrome, USA). The
elemental compositions and morphologies of the different materials were analyzed by Energy
dispersive X-ray (EDX) and Field emission scanning electron microscope (FESEM, model:
Supra 55, Zeiss) instruments. To obtain a brief idea about the inner morphology of the as-
prepared material, transmission electron microscopic (TEM, model: TECNAI G2 20 S-TWIN
(200KV);Resolution: 2.4A0) analysis was done. The leaching experiment of element was
carried out by inductively coupled plasma mass spectrometry (ICPMS, model: NexION 2000,
PerkinElmer, USA). For catalytic reactions, the products were collected and analysed using
1
H nuclear magnetic resonance instrument (NMR, model: ECS-400, JEOL, Japan) using
tetramethylsilane as the internal standard and CDCl as the solvent. The chemical shift (δ)
3
values are reported in parts per million (ppm) and coupling constants (J) in Hertz (Hz).To
evaluate the regeneration and reusability of the catalyst, the reaction products were collected
and analysed using gas chromatography (GC 1000, with an SE-30 capillary column).All
electrochemical measurements including CV, GCD, and cyclic stability were performed using
an electrochemical working station (Auto lab PGSTAT302N, Metrohm UK).
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
2
0
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
Authors would like to thank Council of Scientific and Industry Research (CSIR), New Delhi,
India, for financial assistance through granting the project No. 22(0759)/17/EMR-II. The
authors also express their gratitude to SAIC, Tezpur University, India, and STIC, Cochin,
India, for helping in instrumental analyses.
Conflict of Interest
The authors declare no conflict of interest.
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
References
[
[
1] C. Liu, S. Liao, Q. Li, S. Feng, Q. Sun, X. Yu, Q. Xu, J. Org. Chem., 2011, 76, 5759-5773.
2] M. M. Reddy, M. A. Kumar, P. Swamy, M. Naresh, K. Srujana, L. Satyanarayana, A.
Venugopal, N. Narender, Green Chem., 2013, 15, 3474-3483.
[
3] (a) D. Zheng, Z. Yin, W. Zhang, X. Tan, S. Sun, Cryst. Growth Des., 2006, 6, 1733-1735.
b) Z. Y. Leong, H. Y. Yang, ACS Appl. Mater. Interfaces, 2019, 11, 13176-13184. (c) Y.
(
Guo, L. Li, L. Song, M. Wu, Y. H. Gao, J. Chen, C. Mao, J. Song, H. Niu, J. Mater. Chem. A,
019, 7, 12661-12668.
4] C. Zhu, L. Yang, J. K. Seo, X. Zhang, S. Wang, J. W. Shin, D. Chao, H. Zhang, Y. S.
Meng, H. J. Fan, Mater. Horiz., 2017, 4, 415-422.
2
[
[
[
[
[
5] G. Zhang, X. Liu, Y. Wang, C. Liu, S. Xing, Chem. Eur. J., 2017, 23, 5557-5564.
6] D. Su, H. -J. Ahn, G. Wang, J. Mater. Chem. A., 2013, 1, 4845-4850.
7] J. Zhang, Y. Li, L. Wang, C. Zhang, H. He, Catal. Sci. Technol., 2015, 5, 2305-2313.
8] R. B. Rakhi, B. Ahmed, D. Anjum, H. N. Alshareef, ACS Appl. Mater. Interfaces, 2016, 8,
1
8806-18814.
9] (a) T. Mallat, A. Baiker, Chem. Rev., 2004, 104, 3037-3058. (b) R. J. K. Taylor, M. Reid,
J. Foot, S. A. Raw, Acc. Chem. Res., 2005, 38, 851-869.
10] (a) L. Blackburn, R. J. K. Taylor, Org. Lett., 2001, 3, 1637-1639. (b) C.Guérin, V.
Bellosta, G. Guillamot, J. Cossy, Org. Lett., 2011, 13, 3534-3537.
[
[
2
1
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
[
11] (a) R. Cano, D. J. Ramón, M. Yus, J. Org. Chem., 2011, 76, 5547-5557. (b) C. Gonzalez-
Arellano, K. Yoshida, R. Luque, P. L. Gai, Green Chem., 2010, 12, 1281-1287. (c) W. He, L.
Wang, C. Sun, K. Wu, S. He, J. Chen, P. Wu, Z. Yu, Chem. Eur. J., 2011, 17, 13308-13317.
[
12] (a) R. H. Crabtree, Organometallics, 2011, 30, 17-19. (b) G. Guillena, D. J. Ramón, M.
Yus, Chem. Rev., 2010, 110, 1611-1641. (c) A. J. A. Watson, J. M. J. Williams, Science, 2010,
29, 635-636.
3
[
[
[
[
13] X. Yu, C. Liu, L. Jiang, Q. Xu, Org. Lett., 2011, 13, 6184-6187.
14] M. Kim, Y. Hwang, K. Min, J. Kim, Phys. Chem. Chem. Phys., 2013, 15, 15602-15611.
15] S. Devaraj, N. Munichandraiah, J. Phys. Chem. C, 2008, 112, 4406-4417.
16] (a) S. Chen, J. Zhu, X. Wu, Q. Han, X. Wang, ACS Nano, 2010, 4, 2822-2830. (b) J. Zhu,
J. He, ACS Appl. Mater. Interfaces, 2012, 4, 1770-1776. (c) Y. Wang, W. Lai, N. Wang, Z.
Jiang, X. Wang, P. Zou, Z. Lin, H. J. Fan, F. Kang, C. -P. Wong, C. Yang, Energy Environ.
Sci., 2017, 10, 941-949.
[
17] Z. -S. Wu, W. Ren, D. -W. Wang, F. Li, B. Liu, H. -M. Cheng, ACS Nano, 2010, 4,
835-5842.
18] H. Liu, Z. Hu, L. Tian, Y. Su, H. Ruan, L. Zhang, R. Hu, Ceram. Int., 2016, 42, 13519-
3524.
5
[
1
[
[
[
19] X. Liu, N. Wen, X. Wang, Y. Zheng, ACS Sustainable Chem. Eng., 2015, 3, 475-482.
20] R. B. Hadjean, J. –P. Pereira-Ramos, Chem. Rev., 2009, 110, 1278-1319.
21] Y. -T. Wang, A. -H. Lu, H. -L. Zhang, W. -C. Li, J. Phys. Chem. C, 2011, 115, 5413-
5
421.
[
[
22] A. Kumar, Aathira M. S., U. Pal, S. L. Jain, ChemCatChem, 2018, 10, 1844-1852.
23] (a) Y. Zhao, S. W. Foo, S. Saito, Angew. Chem., Int. Ed., 2011, 50, 3006-3009. (b) M.
Roggen, E. M. Carreira, J. Am. Chem. Soc., 2010, 132, 11917-11919. (c) T. Ohshima, Y.
Miyamoto, J. Ipposhi, Y. Nakahara, M. Utsunomiya, K. Mashima, J. Am. Chem. Soc., 2009,
1
31, 14317-14328.
24] (a) G. E. Dobereiner, R. H. Crabtree, Chem. Rev., 2010, 110, 681-703. (b) G. Guillena, D.
J. Ramón, M. Yus, Angew. Chem. Int. Ed., 2007, 46, 2358-2364.
25] (a) S. Imm, S. Bähn, L. Neubert, H. Neumann, M. Beller, Angew. Chem. Int. Ed., 2010,
9, 8126-8129. (b) F. Shi, M. K. Tse, X. Cui, D. Gördes, D. Michalik, K. Thurow, Y. Deng,
[
[
4
M. Belle, Angew. Chem. Int. Ed., 2009, 48, 5912-5915. (c) X. Cui, Y. Zhang, F. Shi, Y. Deng,
Chem. Eur. J., 2011, 17, 1021-1028.
2
2
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
[
26] (a) K. A. DeKorver, H. Li, A. G. Lohse, R. Hayashi, Z. Lu, Y. Zhang, R. P. Hsung,
Chem. Rev., 2010, 110, 5064-5106. (b) R. Severin, S. Doye, Chem. Soc. Rev., 2007, 36, 1407-
420.
27] T. Deng, W. Zhang, O. Arcelus, J.-G. Kim, J. Carrasco, S. J. Yoo, W. Zheng, J. Wang, H.
Tian, H. Zhang, X. Cui, and T. Rojo, Nature Comm., 2017, 8, 15194.
28] X. Chu, T. Deng, W. Zhang, D. Wang, X. Liu, C. Zhang, T. Qin, L. Zhang, B. Zhang, C.
Chen, W. Zheng, J. Energy Chem., 2018, 27, 507-512.
1
[
[
[
[
29] W. S. Hummers, Jr., R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1939.
30] J. Zhang, H. Yang, G. Shen, P. Cheng, J. Zhang, S. Guo, Chem. Commun., 2010, 46,
1
112-1114.
2
3
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
Scheme 1. Proposed scheme for the synthesis of rGO@α-MnO /rGO@δ-MnO .
2
2
2
4
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
Figure 1. (a) XRD spectra of rGO (1), rGO@α-MnO -1.5 (2), rGO@α-MnO -3.0
2
2
(
3),rGO@α-MnO -4.5 (4), rGO@δ-MnO -1.5 (5), rGO@δ-MnO -3.0 (6), and rGO@δ-MnO -
2 2 2 2
4
2
.5 (7). (b) XRD spectra of annealed materials namely, rGO@α-MnO /rGO@δ-MnO at
2 2
50°C and 450°C. (c) Raman spectra of rGO@α-MnO -3.0, rGO@δ-MnO -3.0, and rGO@α-
2
2
MnO /rGO@δ-MnO annealed at 250°C. (d) EDX of rGO@α-MnO /rGO@δ-MnO annealed
2
2
2
2
at 250°C.
2
5
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
Figure 2: N adsorption-desorption isotherms and pore-size distribution curves of (a)
2
rGO@α-MnO , (b) rGO@δ-MnO , and (c, d) rGO@α-MnO /rGO@δ-MnO annealed at
2
2
2
2
2
50°C, respectively.
2
6
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
Figure 3: FESEM images of (a) rGO@α-MnO , (b) rGO@δ-MnO , and (c, d) rGO@α-
2
2
MnO /rGO@δ-MnO nanoflower material in different magnifications.
2
2
2
7
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
Figure 4: TEM images of (a) rGO@α-MnO , (b) rGO@δ-MnO , and (c, d) rGO@α-
2
2
MnO /rGO@δ-MnO nanoflower material in different magnifications. SAED pattern of
2
2
rGO@α-MnO /rGO@δ-MnO (d, inset image) shows the presence of different planes in the
2
2
hybrid material.
2
8
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201902179
Figure 5. Possible reaction mechanism of air-promoted rGO@α-MnO /rGO@δ-MnO
2
2
catalyzed N-alkylation of aromatic amines with aromatic alcohols.
2
9
This article is protected by copyright. All rights reserved.