Efficient bifunctional reactivity of K-doped CrO(OH) nanosheets: Exploiting the combined role of Cr(iii) and surface -OH groups in tandem catalysis

Article abstract of DOI:10.1039/c8cy02345j

This work reports the bifunctional catalytic activity of layered K-doped α-CrO(OH). The combined action of the redox active Cr(iii) and the surface hydroxyl groups was efficiently used to carry out 2-3 oxidation reactions in tandem followed by condensation/coupling reactions in one pot. Oxidation of benzyl alcohol followed by Knoevenagel condensation or coupling reactions forming C-C and C-N linkages in one pot is demonstrated. The catalyst has been characterized using XRD, IR, TGA, CO₂-TPD, cyclic voltammetry, XPS and microscopic techniques to gain insight into the nature of active sites. The role of O^- and O^2- on the CrO(OH) catalyst in the bifunctional activity was studied using analytical techniques. Recyclability and leaching tests confirmed that K-α-CrO(OH) is a stable and environmentally safe catalyst.

Full text of DOI:10.1039/c8cy02345j

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Catalytic mechanism of C–F bond cleavage: insights from QM/MM
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ARTICLE
Efficient bifunctional reactivity of K-doped CrO(OH) nanosheets:
Exploiting the combined role of Cr(III) and surface -OH groups in
tandem catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
ab
c
ab
d
DOI: 10.1039/x0xx00000x
Dnyanesh Vernekar , Satyajit Ratha , Chandrashekhar Rode and Dinesh Jagadeesan *
www.rsc.org/
The work reports the bifunctional catalytic activity of layered K-doped α-CrO(OH). The combined action of the redox active
Cr(III) and the surface hydroxyl groups was efficiently used to carry out 2 – 3 oxidation reactions in tandem followed by
condensation/coupling reactions in one pot. Oxidation of benzyl alcohol followed by Knoevenagel condensation or
coupling reactions forming C-C and C-N linkages in one pot is demonstrated. The catalyst has been characterized using
XRD, IR, TGA, CO
2
-TPD, Cyclic voltametry, XPS and microscopic techniques to gain insights into the nature of active sites.
-
2-
The role of O and O
on CrO(OH) catalyst for the bifunctional activity was studied using analytical techniques.
stable and environmentally safe catalyst.
Recyclability and leaching tests confirmed that K-α-CrO(OH) is
a
Introduction
Synthesis and applications of multifunctional heterogeneous
catalysts are of contemporary interest with promising
outcomes. Literature pertaining to the catalysis of sequential
Synthesis of complex organic molecules is usually carried out
over several steps of functional group transformations.
Reaction sequences in such synthesis strategies often involve
separation and purification of intermediate products and
change of catalysts. It is exciting to think of a possibility to
carry out the full sequence of synthesis in a single pot so that
the yield of the target molecule as well as the environmental
benevolence of the reaction are enhanced.
strategies to achieve this may require to carry out all the
catalyzed steps of a reaction sequence in a single pot by
8
oxidation - condensation reactions revealed that majority of
the catalysts employed the expensive noble metals and a
9
, 10
base.
In fact, one could classify the heterogeneous catalysts
1
for the sequential oxidation-coupling organic transformations
into two major types. The first type of catalysts are
1
1
predominantly noble metals on inert oxides such as Ag-Al
2
O
O
3
,
.
2, 3
One of the
1
2
13
14
Pd/AlOOH , Pd–Si–Pr–NiXantphos/SiO
These catalysts employ external bases such as K
and Cs CO to complete the reaction. The second type of
catalysts are noble metals on basic oxides or supports with
2
,
Pt-Sn/C-Al
2
3
3
PO
4
, LiOH,
4
2
3
introducing a common heterogeneous catalyst. For example,
a reaction sequence requiring a combination of two or more of
the active sites such as an acid, a base, or a reduction or an
oxidation site (for reaction steps like hydrolysis, condensation
or oxidation) may be tandemly carried out in a single pot in the
presence of a single catalyst possessing all such required sites
on its surface. One can envisage that such bifunctional
catalysts capable of catalyzing a sequence of oxidation and
condensation/coupling reactions in tandem shall be useful in
efficiently synthesizing biologically active molecules such as
1
5
basic functional groups such as amines eg Au@Cu(II)-MOF ,
1
6
17
18
x
Pd(0)@UiO-68-AP , Au-Pd/HT , Cu Ag1-x/HT , Pd-Au@Mn(II)-
1
9
MOF . Examples of non-noble metal based bifunctional
catalysts without using an external base are rare. Notable ones
2
0
21
22,
are Co on N doped carbon , Tris-LDH-X
.
4
(PW
)
9 2
and CeO
2
2
3
-
Our group reported that –OH and –O groups on the
surface of Iron oxyhydroxides could catalyze organic
transformation requiring sequential acid and base catalyzed
5
-7
chalcones, benzylidene malononitriles, and imines.
2
4
steps in a single pot. Importantly, we also showed that
oxyhydroxides are much simpler acid-base bifunctional
catalysts as it does not involve grafting of organic amines and
acids on its surface. We were curious to explore if a redox-
active metal oxyhydroxide could offer the advantage of the
redox property of the metal and basic properties of the surface
a. Chemical Engineering and Process Development Division, CSIR-National Chemical
Laboratory, Pune-411008, India.
b.
Academy of Scientific and Innovative Research (ACSIR), Ghaziabad-201002, India
School of Basic Sciences, Indian Institute of Technology, Bhubaneswar, Odisha,
c.
7
51013 India.
d.
Indian Institute of Technology Palakkad, Kozhippara P.O., Palakkad – 678 557,
-
India.
–O groups. In such a case, one could envisage non-noble
*Correspondence: Email – d.jagadeesan@iitpkd.ac.in
metal based bifunctional catalyst for sequential oxidation and
condensation or coupling reactions. Particularly, we were
interested to explore the 2D layered metal oxyhydroxide such
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Journal Name
as CrO(OH). Layered materials have a higher specific surface as
compared to their bulk counterparts or nanoparticles
(
View Article Online
DOI: 10.1039/C8CY02345J
spherical).25 This leads to an increase in the number of
accessible active sites (Redox and acid-base) on the surface of
D layered material, thus increasing the chance of efficient
2
catalysis. Besides, the electron mobility in 2D layered materials
is excellent leading to ultrafast electron transfer and hence
2
6-28
improving redox ability.
Thirdly, due to its layered nature,
these materials can act as excellent supports with improved
catalyst dispersion.29 In this work, we prepared K- doped
layered CrO(OH) and carried out a detailed study of its activity
in a single-pot oxidation followed by coupling reactions.
Concerned about the environmental impact of using CrO(OH),
we thoroughly verified if CrO(OH) is extremely stable under
the reaction conditions and found no traces of leaching.
Results and discussion
Synthesis and Characterization of CrO(OH)
Fig 2: a) Full scan XPS spectra b) deconvoluted XPS spectra of K
2
p signal c) deconvoluted Cr 2p3/2 signal (inset Cr Auger Signal)
The preparation method to obtain CrO(OH) is given in the d) deconvoluted O 1s signal of K-α-CrO(OH)
3
0
Supporting Information. According to a reported procedure,
one-step reduction of K CrO in H at 500 ºC resulted in the An intense peak in the low angle region (2Ɵ =10.5°) of the XRD
2
4
2
formation of K doped CrO(OH). X-ray diffraction pattern (XRD) pattern corresponded to a d spacing of 0.8 nm. This peak
of the sample as shown in Fig. 1a confirmed the presence of indicated the presence of ordered layers of K-α-CrO(OH)
crystalline grimaldiite (α phase), a mineral phase of chromium stabilized by an inter-layer hydrogen bonding (Scheme 1b) . In
oxyhydroxide α-CrO(OH) (JCPDS card no 09-0331) with a space Fig. 1b, FT-Infrared (IR) spectra of K-α-CrO(OH) showed
3
1
group R3m (Scheme 1a). The amount of K present in the as- characteristic absorption bands due to Cr-O-Cr and Cr-O
prepared sample K-α-CrO(OH), was determined to be 2.7 wt% stretching modes at 985 cm-1 and 1188 cm-1 respectively, the
using inductively coupled plasma atomic emission latter due to the dissociative chemisorption of oxygen.32 The
spectroscopic technique (ICP-AES).
broad band centered around 3400 cm-1 is due to the
stretching of surface hydroxyl groups of K-α-CrO(OH) along
3
0
with non-dissociated water molecules.
2
N sorption isotherm
showed type II hysteresis loop with a BET surface area of 61.66
2
m /g (Fig. S1). N
2
sorption isotherm showed type II hysteresis
2
loop with a BET surface area of 61.66 m /g (Fig. S1). The
microstructure of K-α-CrO(OH) was analyzed using
Transmission Electron Microscopy (TEM) (Fig. 1c) which
showed thin layers of sheet-like structures.
Fig. 1: a) X-Ray Diffraction pattern for K-α-CrO(OH), b) Infrared
Spectrum of K-α-CrO(OH) c) TEM image of K-α-CrO(OH) (inset
showing the stacks of K-α-CrO(OH) sheets) d) HRTEM image of
the K-α-CrO(OH) showing the lattice fringes of (101) plane
Scheme 1 : a) crystal structure of α-CrO(OH) with Cr as the
central atom of the polyhedra b) ordered layers of CrO(OH)
stabilized by hydrogen bonding forming the 2D nanostructure
2
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The stacks of K-α-CrO(OH) sheets can be observed in the inset Table 1: Different base catalyzed reactions tested by K-α-
View Article Online
DOI: 10.1039/C8CY02345J
of Fig. 1c clearly showing an inter-layer spacing of 0.8 nm.This CrO(OH) and other catalysts.
value of inter-layer separation correlated with the d spacing
obtained from the low angle peak in XRD at 2Ɵ =10.5° (Fig. 1a).
H
O
Atomic force microscopic studies of K-α-CrO(OH) under
tapping mode showed K-α-CrO(OH) having a flake-like
topology (Fig. S2). The height profile measurements of several
flakes showed that the average step height was around 50 nm
while the length of each flake varied from 0.3-1 micrometers
making it a quasi-two-dimensional nanosheet. HRTEM image
of K-α-CrO(OH) (Fig. 1d) showed lattice fringes with a d spacing
of 0.25 nm corresponding to (101) planes. Chemical analysis of
the surface of K-α-CrO(OH) was carried out using X-ray
Photoelectron Spectroscopy (XPS). All XPS spectra were charge
corrected and referenced to adventitious carbon (284.6eV). Fig
Catalyst
R2
R1
2
+
Solvent, Temperature
1
3
O
O
H2N
NC
NC
CN
CN
O
2
a
2b
2d
2c
O
O
O
N
CN
CN
3a
3
c
3d
3
b
2
a shows a full-scan XPS spectra for K-α-CrO(OH). Peaks due to
Entry
Catalyst
Reactant 2 Product 3 Solvent Time Conversion of GC Selectivity
Cr(2p), O(1s), K(2s, 2p) and Auger peaks due to Cr (LMM) were
observed. The signal of 2p3/2 at BE 576.4 eV is due to Cr(III)
(h)
1 (%)
(%)
Product 3
92
2
-
attached to O . Peaks at BE 577.6 eV and at 579.0 eV are due
to Cr(III) bonded to hydroxyls and Cr(III) coordinated to H
1
2
3
4
K-α-CrO(OH)
2a
2a
2a
2a
3a
3a
3a
3a
CH
3
CN
6
100
96
2
O
Cr
O
2 3
CH
3
CN
CN
CN
15
15
15
21
respectively (Fig. 2c). Inset of Fig 2c shows the Auger signals at
BE 972 eV corresponding to LMM transitions in Cr. LMM refers
to those processes with the initial vacancy in the L shell and
final double vacancy in the M shells. Also, the kinetic energies
of the Auger lines are independent of the ionizing radiations.
The oxygen O1s spectra showed peaks at BE 530.2 eV due to
γ-CrO(OH)
CH
CH
3
57
71
K- γ-CrO(OH)
3
3
3
72
83
5
SiO
2
-NH
2
2a
3a
CH
CH
CN
CN
15
71
100
6
7
8
K-α-CrO(OH)
K-α-CrO(OH)
K-α-CrO(OH)
2b
2c
2c
3b
3c
3c
12
24
24
100
100
100
90
100
95
CH CN
3
xylene
2
-
O and BE 531.4 eV due to surface OH. The peak at BE 532.4
9
K-α-CrO(OH)
2d
3d
xylene
12
100
100
3
1
eV is due to physiosorbed moisture (Fig 2d).
Reaction conditions: 1 (1 mmol), Entries 1-5 (1: Catalyst weight ratio=1.08) and for Entries 6-7
it was 0.27, 2 (2 mmol), Solvent (5 mL), Temperature of the reaction was 80 °C for acetonitrile
and 110 °C for xylene, Nitrogen atmosphere.
Catalytic studies of K-α-CrO(OH)
a) Base property of K-α-CrO(OH)
In another example, benzaldehyde (1) could be completely
coupled with acetophenone (2c) to form 1,3-diphenyl-2-
The basic property of K-α-CrO(OH) was demonstrated by its propene-1-one (3c) (Table 1, entry 6) in the presence of K-α-
catalytic activity on Knoevenagel condensation, C-C and C-N CrO(OH). The catalyst showed complete conversion in 24 h
coupling reactions for the first time. Table 1 briefly describes with 100% selectivity to (3c) (Table 1, Entry 7). In xylene, the
various base catalyzed reactions and the conditions under selectivity reduced to 95% (Table 1, Entry 8) due to non-polar
which they were performed. For condensation reactions, four nature of the solvent. It is important to highlight that the
different active methylene compounds containing aliphatic reaction is usually carried out using environmentally unfriendly
and aromatic substituents were chosen. For example, organic or organometallic coupling partners such as dppe and
1
3
Knoevenagel condensation reaction between benzaldehyde (1) BuNiXantphos which are air/moisture sensitive, difficult to
and malononitrile (2a) was carried out to give recycle and are used along with toxic bases such as K CO
benzylidinemalanonitrile (3a) as the product (Table 1, entry 1). Cs CO , LiOH. Though the time required is slightly higher, it is
K-α-CrO(OH) showed a great promise with 100% conversion in necessary to recognize the importance of avoiding the non-
h at 80 ºC in acetonitrile. The activity of K-α-CrO(OH) was usage of soluble bases. N-benzylidine aniline (3d), an
2
3
,
2
3
6
compared with control catalysts (Synthesis & characterization important intermediate for various pharmaceutical and
of the control catalysts are described in ESI, S1 and Fig S4, Fig biological compounds was also synthesized using K-α-CrO(OH)
S5). Cr
but more of benzoic acid. In comparison, the standard grafted 1, entry 9). Synthesis of 3d involves the use of strong organic
catalysts such as NH
- functionalized silica showed a high and inorganic bases such as DABCO, NaOH and KOH which
2 3
O
(Table 1, Entry 2) gave only 21% selectivity for 3a from benzaldehyde and aniline under similar conditions (Table
2
selectivity for 3a but the conversion was limited to 71% even leads to the formation of large amounts of undesirable salts. In
after 15 h (Entry 5). Similarly, ethylcyanoacetate (2b) could be contrast, K-α-CrO(OH) yielded 100% of 3d in 12 h without the
completely converted to ethyl-2-cyano-3-phenylacrylate (3b)
using K-α-CrO(OH) (Table 1, entry 6) with 90% selectivity after conversion could be achieved (in the synthesis of 3d) after 12h
2 h in acetonitrile.
in the presence of the K-α-CrO(OH).
external base (Table 1 Entry 9). Importantly, 70% higher
1
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Fig 4: In situ IR difference spectra of CO
CrO(OH) and (b) K-α-CrO(OH) Inset different carbonate &
bi)carbonate vibration modes
2
adsorbed on (a) α-
(
2 2 3
Fig 3 : CO TPD profiles of (i) Cr O (ii) γ-CrO(OH) and (iii) K- γ-
CrO(OH) iv) α-CrOOH v) K-α-CrO(OH)
K-free γ-CrO(OH) too yielded Knoevenagel condensation
product 3a(57%) but the selectivity for 3a was limited to only
Hot filtration test carried out using K-α-CrO(OH) after 1 h of
reaction showed no time-dependent conversion of 1 to 3a at
7
1% due to the formation of benzoic acid. Interestingly, K-γ-
CrO(OH) (with 3 wt% of K) showed a slight improvement in
conversion to 72% as compared to the K – free γ-CrO(OH) and
an increase in selectivity to 83% (Table 1, Entry 4). However,
the activity of K-γ-CrO(OH) was far less as compared to K-α-
CrO(OH) even though the loading in each case is comparable.
One of the reasons for this difference in activity between the
polymorphs could be the difference in the exposure of active
sites to the reactants.
8
0 ºC (Fig. S3) suggesting the role of K-α-CrO(OH) as a
heterogeneous catalyst.
Effect of K doping
In order to assess the effect of K doping on the activity of K-α-
CrO(OH), we attempted to prepare K-free α-CrO(OH) by
treating it with 0.01M HCl at 70°C for 48 h and then washing
several times with distilled water. The characterization of K-
leached sample (α-CrO(OH)) using X-ray Diffraction, Infrared
Spectroscopy, XPS, TEM & elemental analysis data confirmed
that the chemical and structural integrity of the sample was
preserved (Fig S4 and Fig S5). Thus K doping does not effect
the structure of parent CrO(OH). To understand the effect of K
on the reaction, the activity of K-α-CrO(OH) with 2.7 wt% K and
α-CrO(OH) were compared in the synthesis of 3a at the end of
Nature of Basic Sites
The nature and the strength of the basic sites in CrO(OH)
catalysts were further probed using Temperature Programmed
Desorption (TPD) of CO (Fig 3 and Table S7). The CO
2 2
desorption from room temperature to 400 °C provided
information about weak to moderate basic sites on the surface
3
h of reaction (Fig S6). The K-α-CrO(OH) with 2.7 wt% K
of K-α-CrO(OH). The CO
compared with those of α-CrO(OH), γ-CrO(OH), K- γ-CrO(OH)
and Cr 3. It can be observed that α-CrO(OH) gave a CO
2
2
-TPD profiles of K-α-CrO(OH) was
showed a better conversion (67%) and selectivity (95%) to 3a
in comparison to α-CrO(OH).
2
O
adsorption amount of 0.59 mmol/g of the material, which was
lower than that of K-α-CrO(OH) which adsorbed 0.93 mmol/g.
In CrO(OH), each Cr(III) ion is surrounded by three O ions and Interestingly, γ-CrO(OH) which is a different polymorph
Effect of Polymorph
2
-
-
three OH to give CrO
3
(OH)
3
octahedra which is the basic showed an overall basicity of 0.33 mmol/g, which was lesser
structural unit (Scheme 1). The specific arrangement of these than both α-CrO(OH) and K-α-CrO(OH). Although, doping of K
3
3
octahedra gives rise to different polymorphs. Both K-α- on γ-CrO(OH) improved its basicity to 0.56 mmol/g, it still
CrO(OH) and α-CrO(OH) have a unique layered structure in remained less in comparison to its layered counterparts. Cr O
2
3
which layers of CrO(OH) (with
d = 0.25 nm) are naturally showed a very poor basicity of 0.02 mmol/g of the
mesostructurally ordered with an inter-layer spacing of 0.8 nm catalyst. TPD profiles strongly supported the significantly
due to H bonding. Another polymorph of CrO(OH) namely the better basicity in layered α-CrO(OH) pointing out the role
γ-CrO(OH) was also prepared by hydrothermal synthesis (Table played by the morphology and the further enhancement in
1
, Entry 3). Interestingly, γ-CrO(OH) differed from α-CrO(OH) in basicity due to the presence of K (Table 1, Entry 1 & 4).
the crystal structure as well as the morphology (Fig S4 & S5). K-
free γ-CrO(OH) and 3wt % K- doped γ-CrO(OH) were prepared
and compared with the catalytic performance of α-CrO(OH)
counterparts to understand the effects of polymorphs.
4
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Basicity due to Potassium
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The role of potassium (K) in promoting the basicity by electron
donation has been well studied in metal oxide catalysts in the
context of Fischer-Tropsch reaction and side chain alkylation.
Potassium in K-α-CrO(OH) could be in the form of K O as the
3
4
2
XPS signal due to K2p (Fig 2b) closely matched with this
phase.35 Also, a small peak at BE 530.2 eV in the O1s spectra
of K-α-CrO(OH) is attributed to O in K+
2
-
O (Fig 2d). It is
O either exists in very small traces or probably
in poorly crystalline form thereby remaining undetected in
XRD. Another possible phase KCrO which may be formed
2-
35
2
possible that K
2
2
3
6
under the synthesis conditions of K-α-CrO(OH) is highly
moisture-sensitive37 and may react with water to give
CrO(OH) and KOH.
Fig.5: Thermogravimetric (TGA) profiles of (i) K-α-CrO(OH), (ii)
2 3
α-CrO(OH), (iii) K-γ-CrO(OH), (iv) γ-CrO(OH) and (v)Cr O
2 2
KCrO + H O↔CrO(OH) + KOH…………..(i)
Since KOH is completely removed by washing in water, its role
in the basicity can be completely ruled out. Analysis of the
3
0
The thermogravimetric profiles of the CrO(OH) polymorphs
were compared and semi-quantitatively correlated to the
presence of surface hydroxyl groups on the respective samples
O1s spectra of the K doped samples in both α & γ polymorphs
2
-
of CrO(OH) showed that the contribution from O (BE
30.2eV) to the XPS signal was 10 – 16% higher in K-doped
CrO(OH) compared to their K-free counterparts (Fig S9).
In situ IR analysis of K-α-CrO(OH) with CO adsorbed on it
was carried out to probe the basic sites on the surface. The Fig
shows the difference in FT-IR spectra of CO -adsorbed on K-
α-CrO(OH) and the bare K-α-CrO(OH). Similarly, the difference
FT-IR spectra of CO -adsorbed on α-CrO(OH) with the
(
Fig 5 & Table S8). Weight loss in the temperature region
5
o
below 150 C is assigned to be loss of physisorbed water.
Continued heating till 350 C leads to progressive dehydration
o
2
accompanied by the loss of hydroxyl groups eventually leading
to the formation of oxides. Hence, the weight loss in the
4
2
o
temperature region between 150 – 350 C was compared
between the samples to understand the presence of surface
2
4
2
hydroxyl groups. The fact that K-α-CrO(OH) has a large
weight loss in this thermal regime implies that dehydration of
surface hydroxyl groups is a major thermal event on it
corresponding bare sample is shown. The difference spectra
gave two strong absorption bands in the range 2000-1000 cm-
1
due to surface carbonates and (bi)carbonates. The bands in
-
1
-1
2 3
compared to other controls. As expected, Cr O showed the
the range of 1200-1300 cm and 1600 cm are due to
symmetric and antisymmetric O-C-O vibrations of the surface
poorest of weight loss i.e. 1.08wt% indicating least amount of
surface hydroxyl groups. Weight loss in K-α-CrO(OH) (8.48%)
and α-CrO(OH) (6.68%) are higher than that for γ-CrO(OH)
3
8
carbonates/(bi)carbonates bound to Cr(III), respectively. The
probable vibration modes are shown in the inset in Fig 4.
Earlier studies on Chromium oxides have shown that five-
(4.91%) and K- γ-CrO(OH) (3.71%) (Table S8) . Interestingly,
2
-
this also correlated well with the amount of CO desorbed
coordinate Cr(III) cations and three-coordinate O anions
provide strongest basic sites of interaction with CO to form bi-
The basicity of
2
from the surface of these samples in TPD experiments
suggesting that surface hydroxyl groups have a role in surface
2
3
8, 39
dentate carbonates at room temperature.
2
-
40 basicity.
O
sites is well documented in alkaline metal oxides.
Surface hydroxyl groups could be in equilibrium with O-
ions and exhibit basic property. To conclusively prove the
presence of anionic sites, we performed fluorescence studies
of dye adsorbed on K-α-CrO(OH) and α-CrO(OH). Rhodamine
6G (Rh-6G), a positively charged dye is routinely used to probe
Consistent with our results, studies on CO -adsorbed
2
intermediates on the surface of metal oxyhydroxides have
showed that O2- forms carbonate species whereas the O-
groups in the vicinity of surface hydroxyl groups may form
4
1
(
2 2
bi)carbonate species (-O-CO H or –(O) -COH). Thus, the
4
3
2
-
-
anionic molecules. Specifically, the adsorption of Rh-6G on
deprotonated hydroxyl groups of graphene may be detected
results strongly indicate CrO(OH) has O and O sites on the
surface. Quantifying the carbonate and (bi)carbonate species
based on IR absorption is challenging as they occur in the same
frequency region.
4
4
using fluorescence spectroscopy. The samples were stirred in
Rh-6G solution for 3h and then filtered using syringe filter
before analyzing the fluorescence of the dye (λex = 500nm and
λem = 568nm). The intensity of the fluorescence of the Rh-6G
Basicity due to surface hydroxyl groups
solution that had CrO(OH) samples in it was compared with
that of the standard Rh-6G solution. It was observed that the
intensity of fluorescence of Rh-6G decreased strongly
indicating the adsorption of the dye on the negatively charged
surface of the CrO(OH) samples due to electrostatic
In addition to the presence of O2- and O sites that were
-
probed using FT-IR, we used thermal analysis and fluorescence
techniques to study the surface hydroxyl groups and O sites.
The origin of O on the surface of CrO(OH) and its predominant
activity could be understood as in the equation (ii).
-
-
4
5, 46
interactions
(Fig 6c). The complex K-α-CrO(OH) with
-
CrO(OH) ↔CrO(O ) …………..(ii)
surface-adsorbed Rh-6G was completely dried on rotavap and
the Infrared spectrum was recorded (Fig S11).
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benzaldehyde (1) using K-α-CrO(OH) was studied and the
View Article Online
DOI: 10.1039/C8CY02345J
details are presented in Table S12. Xylene was chosen as the
preferred solvent for oxidation of alcohol due to its non-polar
nature. When 1 mmol of benzyl alcohol was used with 0.1 g of
K-α-CrO(OH) (benzyl alcohol to K-α-CrO(OH) weight ratio =
1
2
.08) we got 64% conversion under O atmosphere at 110 °C
(Entry 2) after 15h. When the catalyst amount was increased
to 0.4 g (benzyl alcohol to K-α-CrO(OH) weight ratio = 0.27) we
got 100% conversion and 95% selectivity for benzaldehyde in
1
5 h (Table S12, Entry 3). In comparison, α-CrO(OH) gave a
conversion of 83% in 15h due to the absence of K (Table S12,
Entry 4). Similar to the basicity trend, the promotion of
oxidation due to K is also more pronounced in α-CrO(OH) than
in γ-CrO(OH) (Table S12, Entry 5 & 6). The promotion of
Fig 6: a) Structure of Rhodamine 6G b) Interaction of Rh-6G
with CrO(OH) surface c) Floroscence spectroscopy for standard
Rh-6G sample and K-α-CrO(OH), α-CrO(OH) adsorbed solutions
2-
oxidation using K doping occurs via the creation of O centres
2-
on the surface. The role of O has been discussed in the
context of oxidation of toluene using K-doped Vanadia
On comparing the IR patterns of K-α-CrO(OH)-Rh-6G complex
with bare K-α-CrO(OH) and Rh-6G, we observed a shift in the -
OH stretching frequency of the surface -OH groups (non-
50 51 52
catalysts.
Most of the literature of tandem reactions
involving oxidation reaction of benzyl alcohol was shown to be
carried out using noble metals such as Pd and Au. It is
important to note that 100% conversion of benzyl alcohol was
achieved without the use of any expensive noble metal in our
case.
-1
-1
deprotonated) from 3247 cm to 3452 cm in the case of K-α-
CrO(OH)-Rh-6G complex (Fig S11a). Interestingly, a slight shift
-1
-1
in the C-N stretching of Rh-6G from 1347 cm to 1321 cm in
the fingerprint region of Rh-6G-K-α-CrO(OH) complex was also
Cyclic voltammetry studies were carried out on all the
samples in 0.5M aqueous solution of H SO electrolyte to
2 4
4
7
observed (Fig S11b). These interactions are graphically
represented in Fig 6b. These results were consistent with those
observed for Rh-6G-graphine oxide and strongly proving the
probe the surface-active oxidation sites (Fig S13). The
potential values at which the peaks were obtained suggests
2
-
-
44
presence of O and O sites.
2+
3+
the oxidation of Cr to Cr and subsequent reduction process
(
In the context of the present work, it must be considered
that in comparison to K-doped-CrO(OH), the K-free analogues
of the polymorphs have also shown high activity in the
condensation reactions. Therefore, K-free analogues of
3+
2+
from Cr to Cr ). As can be observed from the cyclic
voltammograms, prominent peaks for oxidation and reduction
are at ~0.32 V and ~0.43V, respectively in the case of α-
CrO(OH), K-α-CrO(OH) γ-CrO(OH) and K-γ-CrO(OH) signifying
their better reversible re-dox property. The redox peaks and
subsequent potential values in case of all the samples have
been found to be consistent and show little deviation which
suggests that only chromium species are involved in the
action. Importantly, smaller FWHM value may be correlated to
its faster charge transfer kinetics (i.e. better reduction-
oxidation property) in response to the applied potential sweep
rate. Though the catalytic property is explicitly shown by the
chromium species, the current response in terms of FWHM
value and reversibility are significantly influenced by the K-
doping. Here, it is expected that both α- and γ-CrO(OH) show
significant change in their electrochemical activities when
-
CrO(OH) has intrinsic O basic active sites, arising from the
deprotonation of surface -OH groups, necessary for catalyzing
the condensation reaction. Interestingly, deconvoluted Cr2p3/2
3
+
XPS spectra of K-α-CrO(OH) in Fig 2c clearly shows that Cr
-
bonded O species at BE 577.6 eV having a larger contribution
to the signal (52%) as compared to the contribution from Cr(III)
bonded to O at BE 576.4 eV (25%). The contributions of O
species in α-CrO(OH), γ-CrO(OH) and K-γ-CrO(OH) as compared
to O species (Fig S10) showed similar trends confirming that
the O species were more in density in all samples. Studies on
2
-
-
2
-
-
hydroxylated MgO catalysts have shown that the formation of
C-C and C-N linkages are catalyzed by both Brønsted basic sites
-
2-
48
(
O ) and Lewis basic (O ) sites. Importantly, Brønsted basic
2 4
doped with potassium. In our system, where 0.5 M H SO has
-
sites (O ) are more active than the Lewis basic counterparts
been taken as the electrolyte, a change in the redox property
of both the species (α- and γ-) can be observed. The improved
redox property of the K-doped α-species over its pristine
counterpart can be observed in terms of the prominent redox
couple of the doped species from the obtained cyclic
voltammetry data which signifies faster charge transfer rate. In
the pristine α-species however, such redox peaks are absent.
This might be due to slower kinetics at the catalysis surface
resulting in sluggish charge transfer (during oxidation) and
subsequent recovery (or reduction). Furthermore, if the cyclic
voltammetry data of both K-α-CrO(OH) and γ-CrO(OH) (Fig S13,
b & c) are compared, there are no visible differences between
2
- 49
(
O ). Based on the evidence from CO
2
-TPD, CO
2
-FT-IR, TGA
-
and XPS, one can conclude that Brønsted sites O are the major
active basic sites for catalyzing the condensation reaction in K-
free and K-doped analogues of CrO(OH). However, doping of K
increases the concentration of Lewis basic O which further
increases the basicity of the catalysts.
2
-
b) Oxidation property of K-α-CrO(OH)
Presence of Cr(III) from XPS studies prompted us to test the
oxidation ability of K-α-CrO(OH). Oxidation of benzyl alcohol to
6
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Catalysis Science & Technology
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ARTICLE
the curves if slower scan rates (i.e., 10 and 50 mV/s) are Table 2: Table showing the catalytic activity ofVKie-wαA-rCticrlOe O(OnlHine)
considered. However, at a scan rate of 100 mV/s, redox and other catalysts for one-pot DtOaI:n1d0e.1m039/CO8xCiYd0a2t3io4n5J-
couples generated from K-α-CrO(OH) are more prominent than Knoevenagel Condensation / Coupling reaction
those obtained from γ-CrO(OH), which suggests its faster
current response and better surface kinetics. From the
detailed analysis of the voltammetry curves, the redox activity
of the K-α-CrO(OH) was found to be at least 3 fold higher than
that of γ-CrO(OH) and K-γ-CrO(OH) . This was calculated by
taking into consideration, both the electrochemical accessible
surface area and the current response values at the redox
peaks for each sample. This can be correlated to the significant
impact of K-doping on the selectivity of a catalyst in the
reaction (primarily due to the chemical structure/composition)
in a given electrolytic medium (here acidic). The CrO(OH)
showed strikingly different current responses in our controlled
environment (in an acidic electrolytic medium), as α- and γ-
OH
O
H
C
Condensation/
Coupling
Oxidation
O2
R
2
1
3
benzyl alcohol
Entry
Catalyst
Time
Conversion
of benzyl
alcohol
GC Selectivity
(%)
tox + tcon
(
h)
3
1
(
%)
5
3
1
K-α-CrO(OH)
15+1
100
5
(3a)95(89)
CrO(OH) have contrasting physical and chemical properties.
Though γ-species was found to be electrochemically active in
.5 M H SO , the anomaly in the same after K-doping needs
2
3
α-CrO(OH)
γ-CrO(OH)
15+1
15+6
83
26
19
4
(3a)81
(3a)96
0
2
4
4
5
K-γ-CrO(OH)
K-α-CrO(OH)
15+6
15+1
38
3
8
(3a)99
further analyses in several pH controlled environments in
order to establish its selectivity. However from the cyclic
voltammetry studies, we can definitely establish better redox
behavior of K-α-CrO(OH) compared to other controls.
100
(3b)92(80)
6
7
8
9
K-α-CrO(OH) ‡
K-α-CrO(OH)*
K-α-CrO(OH)*
K-α-CrO(OH)
15
48
24
24
20
100
62
4
4
(3b)96
(3c)92(82)
(3c)78
22
c) Bifunctional activity of K-α-CrO(OH)
100
5
(3d)95(87)
Reaction conditions: Weight of the catalyst is 0.4 g, 1 mmol of benzyl alcohol, 2
mmol of 2, 5 mL of xylene, 110 °C ‡ethylacynoacetate (2b) was present from the
beginning of the reaction.*Reactant 2 was 1-phenylethanol. Oxidation carried out
in the presence of O balloon. For Entries 1-3, tox : time for oxidation reaction, tcon:
2
time for Knoevenagel condensation. For remaining entries it is the total time of
reaction. The products were isolated by column chromatography and
Having shown that K-α-CrO(OH) can independently catalyze
the oxidation and base catalyzed condensation/coupling
reactions and probing the active sites, we proceeded to couple
these two steps in a single pot. We carried out the sequential
conversion of benzyl alcohol to 3 over K-α-CrO(OH) without
separating any intermediates. The results are tabulated in
Table 2. The alcohol oxidation was allowed to take place for
1
13

characterized by H and C NMR (Supporting Information). Isolated yields
presented in parenthesis.
However, lowering the time to 24 h gave just 62% conversion
Table 2, Entry 8). The formation of N-benzylidine aniline (3d)
was 95% when benzyl alcohol and 2d were reacted for 24 h in
the presence of K-α-CrO(OH) as the catalyst. Thus, K-α-
CrO(OH) showed a great promise with high selectivity and
conversion for oxidative condensation/coupling reactions.
1
5h followed by the addition of malononitrile. This was done
(
to ensure malononitrile (2a) and ethylcyanoacetate (2b) does
not inhibit the oxidation of benzyl alcohol by blocking the
active sites. As seen in entry 6, the conversion was just 20%
when ethylcyanoacetate was present from the start of the
reaction. α-CrO(OH) naturally showed a lower selectivity of
8
1% of 3a (Table 2, Entry 2). Conversions by γ-CrO(OH) and K-
γ -CrO(OH) were lower than that observed for K-α-CrO(OH)
Table 2, Entry 3, 4). One of the reasons could be the nano-
OH
O
OH
H
O
O
(
Coupling
Oxidation
sheet morphology of α-CrO(OH) and K-α-CrO(OH) as compared
to γ-CrO(OH) and K- γ-CrO(OH), leading to difference in
accessibility to the active sites on the catalyst. Oxidative cross
+
+
benzyl
alcohol
3c
1
-phenyl
1
2c
ethanol
coupling between benzyl alcohol and 1-phenyl ethanol Scheme 2 : Two oxidations followed by a coupling reaction in
Scheme 2) required 48 h for 100% conversion of benzyl one pot to yield 3c
(
alcohol with 92% selectivity for (3c) (Table 2, Entry 7) with
both the reactants present from the start of the reaction.
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formation of 3c. In 18 h, 80% of benzyl alcohol was converted
View Article Online
DOI: 10.1039/C8CY02345J
with 40% selectivity to 3c. This indicated formation of 3c was
slow at the start of the reaction from 1. This is because
acetophenone (2c) was not readily available to react with
benzaldehyde and needed to be produced from oxidation of 1-
phenylethanol which was the other reactant along with benzyl
alcohol (Scheme 2). Thus, K-α-CrO(OH) was used as a catalyst
to carry out oxidation of two different substrates and their
condensation in a single pot. Despite three reactions occurring
in tandem, we achieved a high yield of 92% of 3c with
complete conversion of benzyl alcohol. Fig 7c shows the
formation of 3d from the benzyl alcohol and aniline. In 24h,
the reaction was completed with 95% yield of 3d.
Potential Toxicity and Leaching studies
Hexavalent Chromium Cr(VI) is proven to be hemotoxic,
5
4
genotoxic, and carcinogenic. Interestingly, Chromium (III) is
needed as a trace mineral for human nutrition for glucose
5
5
metabolism and is generally stable and not easily absorbed in
the body by any route. Hence, the toxicity of Chromium is
mainly attributable to Cr(VI). Our materials K-α-CrO(OH), α-
CrO(OH) γ-CrO(OH) and Cr
2 3
O are all in Cr(III) state. The
experimental reduction of Cr(VI) to lower oxidation states is
5
4
termed as a detoxification process which is actually utilized
in our synthesis of K-α-CrO(OH). Nevertheless, leaching of
Cr(III) species during the reaction into the solvent must be
avoided at any cost as it can get re-oxidized to Cr(VI) due to
natural processes. Hence, microwave plasma atomic emission
spectroscopy (MP-AES) analysis of the filtrate was done to test
any leaching of Cr(III) species. After 48h refluxing to yield 3c
from benzyl alcohol, the catalyst was removed from the filtrate
using a syringe filter and the resultant filtrate was analyzed
using MP-AES technique. The calibration plots and the MP-AES
data are given in the supporting information (S2).
Fig 7: Time dependent study for K-α-CrO(OH) to yield a) 3a (ii)
is an expanded area of (i) after addition of malononitrile b) to
yield 3c c) to yield 3d
A time-dependent study of the reaction to yield 3a, 3c and 3d
was carried out. (Fig 7a) shows a trace of concentration of the
reactants and products during the course of complete
reaction. In this case, malononitrile was added after 15h of the
start of the reaction. The time was determined based on prior Fig. 8: Recyclability of K-α-CrO(OH) for one pot oxidative
studies probing the completion of the oxidation reaction. Fig coupling reaction to give 3c
7
a (ii) shows the expanded portion of the dotted rectangle in
i) showing the time-dependent formation of 3a. Fig 7b shows
the trace of benzyl alcohol and benzaldehyde along with the
(
8
| J. Name., 2012, 00, 1-3
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The filtrate did not show any presence of Cr(III)/Cr(VI)
whatsoever within the detection limits of the instrument
View Article Online
DOI: 10.1039/C8CY02345J
(Sample 1, S2). This is a very important result indicating no
leaching of Cr(III) species into the solvent during the reaction.
However, the leaching of Cr(III) in the case of other solvents
needs further studies.
Recyclability and Comparison with state of art
catalysts
The catalyst was recycled over three cycles for the oxidation-
coupling reaction between benzyl alcohol and 1-phenylethanol
without appreciable loss in activity as seen in Fig. 8. XRD and
TEM of used catalyst after oxidation- coupling reaction did not
show any significant changes from the fresh catalysts as seen
from Fig. S14. Thus K-α-CrO(OH) acts as a stable catalyst under
the reaction conditions.
Table 3 shows various heterogenous catalysts used for one-
pot tandem oxidation-condensation/coupling reactions and
the conditions under which the reaction is performed to attain
maximum conversion. One can clearly observe that noble
metals are commonly preferred to carry out the oxidation step
Scheme 3: Proposed reaction mechanism occurring on K-α-
CrO(OH) Oxidation: A) deprotonation of Benzyl alcohol B)
oxygen activation on Cr(III) of K-α-CrO(OH) C) formation of
Benzaldehyde (1). Coupling/Condensation: D) proton
abstraction of active methylene group (2) R=RXH=
malononitrile, ethyl cyanoacetate, acetophenone, aniline E)
attack of nucleophile on carbonyl carbon of benzaldehyde F)
water loss to give the respective products (3)(3a, 3b, 3c, 3d).
(
Entries 1-3, 6-11, 13). The use of soluble bases is also
prevalent in most works (Entries 6, 9, 10, 12 &13).
Polyoxometalates (Entry 3) CeO and Co-N-C are rare examples
Entry 4, 5, 12). K-α-CrO(OH) is a promising noble metal free
2
Activation of O molecule takes place over Cr(III) sites in Step
2
B. Benzaldehyde (1) is formed in step C. The deprotonated
(
-
hydroxyl groups (O ) activate the methylene groups of the
and base free catalyst to attain maximum conversion under
similar reaction conditions. The combined role of the reactivity
of Cr(III) and surface -OH for tandem reactions is explored for
the first time with comparable activities with those reported in
literature.
reactants (2) by abstraction of proton in Step D and ably
2-
supported by O ions. Step E involves the attack of the
nucleophile to the carbonyl carbon of benzaldehyde and
subsequent formation of the desired product (3) by loss of
water molecule in Step F.
A proposed reaction mechanism on K-α-CrO(OH) is
depicted in Scheme 3. Step A involves the proton removal
from benzyl alcohol over deprotonated surface hydroxyl
-
groups (O ) of K-α-CrO(OH).
Conclusions
The design and utility of K-α-CrO(OH) as a bifunctional catalyst
was explored in sequential transformation of benzyl alcohol to
Table 3: Comparing the different catalysts and conditions used
in literature for one-pot oxidation-coupling reactions
a
variety of C-C and C-N coupling products. The
transformations were feasible because of the co-existence of
-
2-
redox Cr(III) and basic sites (O and O ) on the surface of the K-
Sr
No
1
Catalyst
Desired
Reaction Conditions
Reference
2-
α-CrO(OH). The presence of electron rich sites (O ) due to
product
doping of K and the surface hydroxyl groups were found to be
the basic sites on the catalysts using spectroscopy and thermal
probe studies. This being the first report on use of K-α-
CrO(OH) in multifunctional catalysis, there is a promising scope
to utilize this material for other catalytic transformations
useful in fine chemical and pharmaceutical industries where
multistep transformations are needed.
Au@Cu(II)-MOF
3a
toluene, 110 °C, 22h
toluene, 80 °C, 38h,
15
16
2
Pd(0)@UiO-68-AP
3a,3b
3
Tris–LDH–X
4
(PW
9
)
2
3b
acetonitrile, H
2
O
2
, 80 °C, 6h
21
4
CeO
2
3c
p-xylene, 150 °C, 12h
mesitylene, 60 °C, 48h
22
5
CeO
2
3d
23
6
Pd/AlOOH
Pd−Au@Mn(II)-MOF
Cu Ag1-x/HT
Pt–Sn/C-Al
3c
toluene, K
3
PO
4
, 80 °C, 20h
12
7
3d
toluene, 110 °C, 24h
xylene, 150 °C, 1h
19
8
x
3c
18
9
2
O
3
3c
xylene, K
3
PO
4
, 155 °C, 48h
14
Conflicts of interest
There are no conflicts to declare
1
1
1
1
1
0
1
2
3
4
Pd–Si–Pr–NiXantphos/SiO
Au-Pd/HT
2
3c
mesitylene, LiOH, 100 °C, 24h
xylene, 120 °C, 5h
13
3c
17
Co−N−C
3c
3c
H
2
O/t-BuOH, LiOH, 70 °C, 24h
toluene, Cs CO , 115 °C, 24h
xylene, 110 °C, 24-48h
20
Ag-Al
2
O
3
2
3
11
K-α-CrO(OH)
3a,3b,3c,3d
This work
Acknowledgements
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Journal Name
2
2
2
2
4.
5.
6.
7.
D. Vernekar and D. Jagadeesan, CatalysViisewSAcrtiieclnecOenlin&e
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DV acknowledges DST Inspire doctoral Fellowship (IF150027).
DJ acknowledges the financial support of Ramanujan
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1
12/2012), Government of India. The authors thank B.
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DOI: 10.1039/C8CY02345J
TOC
Layered K-α-CrO(OH) nanosheets as a non-noble
metal based tandem catalyst for sequential
oxidation and coupling/condensation reactions