Gold incorporated hematite nanocatalyst for solvent-free CO2fixation under atmospheric pressure

Article abstract of DOI:10.1039/d0nj01377c

We have developed an efficient one-pot synthesis strategy for the preparation of hematite (α-Fe2O3) and gold-integrated hematite (Au/α-Fe2O3) nanocatalysts. The nanostructured hematite along with tetrabutylammonium iodide (TBAI) has emerged as a proficient catalyst-combination for the reaction between carbon dioxide (CO2) and epoxide to form cyclic carbonate with 77% conversion under atmospheric CO2 pressure and solvent-free conditions. The catalytic activity of hematite further improved on the incorporation of a miniscule amount of gold (Au) into the hematite system. The as-prepared Au/α-Fe2O3 nanocatalyst with TBAI is found to be very efficient for the CO2 fixation reaction with 91% conversion under the optimized reaction conditions. The excellent catalytic activity of Au/α-Fe2O3 is attributed to the synergistic effect between α-Fe2O3 and Au species for the efficient fixation of CO2. Furthermore, the recyclability and post catalytic analysis imply the robust nature of the heterogeneous Au/α-Fe2O3 nanocatalyst.

Full text of DOI:10.1039/d0nj01377c

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Yellowish and blue luminescent graphene oxide quantum dots
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Gold Incorporated Hematite Nanocatalyst for Solvent-free CO2
Fixation under Atmospheric Pressure
Gouri Tudu, Sourav Ghosh,‡ Tanmoy Biswas‡ and Venkataramanan Mahalingam*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We have developed an efficient one-pot synthesis strategy for the preparation of hematite (-Fe2O3) and gold integrated
hematite (Au/-Fe2O3) nanocatalysts. The nanostructured hematite along with tetrabutylammonium iodide (TBAI) has
emerged as a proficient catalyst-combination for the reaction between carbon dioxide (CO2) and epoxide to form cyclic
carbonate with 77% conversion under atmospheric CO2 pressure and solvent-free conditions. The catalytic activity of
hematite further improved on incorporation of miniscule amount of gold (Au) into the hematite system. The as-prepared
Au/-Fe2O3 nanocatalyst with TBAI is found to be very efficient for the CO2 fixation reaction with 91% conversion under
the optimized reaction conditions. The excellent catalytic activity of Au/-Fe2O3 is attributed to the synergistic effect
between -Fe2O3 and Au species for efficient fixation of CO2. Further, the recyclability and post catalytic analyses imply the
robust nature of heterogeneous Au/-Fe2O3 nanocatalyst.
electrolytes for lithium ion battery and intermediates in the
synthesis of several drug molecules.14-18 Moreover, industrial
process of cyclic carbonate production involves toxic ingredients
Introduction
There is a growing global concern about climate change and the
resulting unpredictable weather conditions. The primary cause for
this is the increase in the greenhouse gases in the atmosphere,
particularly carbon dioxide (CO2).1 The major reasons for the
increase in the CO2 concentration in the atmosphere are
deforestation, burning fossil fuels, etc. Reducing the concentration
of CO2 is one of the approaches to control the climate change.
However, increasing population and the corresponding energy
demand lead to enhanced CO2 concentration in the atmosphere. To
cope with the situation, the best alternative is to find some suitable
methods to reduce the concentration of CO2 from the atmosphere.2
It would be doubly beneficial if the CO2 can be converted into value
added chemicals. Use of CO2 for chemical synthesis is advantageous
as it is non-toxic and economical C1 stock, which can be utilized for
several chemical conversions such as catalytic hydrogenation of CO2
to alternate fuels like CH4, CH3OH and HCOOH.3-8 However, such
processes are restricted due to limited hydrogen supply from
traditional industrial methods which use non-renewable energy
resources. Further, one of the alternative routes of converting CO2
to value added products is through synthesis of cyclic carbonates
from epoxides under ambient conditions, which has attracted huge
interest in recent times.9-13 Among different CO2 derived chemicals,
cyclic carbonates are interesting and have potential applications in
industry as they are well-known precursor for polycarbonates,
like phosgene.19 Therefore, solvent-free synthesis of cyclic
carbonate through a simple green process is extremely desirable.
However, the requirement of elevated temperature and high CO2
pressure is a major bottleneck for such conversion. This demands
the development of efficient catalysts for the conversion of CO2 to
value added chemicals under ambient conditions.
Several catalytic materials such as nitrogen based polymers,
ionic-liquids, zeolites, metal organic frameworks (MOFs), porous-
organic polymers (POPs), ion-exchange resins, metal oxides,
supported metal catalysts etc. have been employed for the
conversion of epoxides into cyclic carbonates utilizing CO2 till
date.20-36 However, nano-porous materials with definite
architecture are rarely reported for the conversion.37,38 Hence,
designing a catalyst which can work under ambient conditions
constituting earth abundant elements is advantageous. Our idea is
to use iron-based material to develop a catalyst, which will be able
to convert epoxides to cyclic carbonates under ambient CO2
pressure. Among iron-based oxide materials hematite (-Fe2O3) is
the most stable oxide form of iron, which is earth abundant, cheap,
non-toxic, and environment friendly. Hematite has been utilized for
photocatalysis, photo-electrochemical water splitting and
adsorption of metals.39,40 In addition, it has been reported to be
very efficient for CO2 adsorption. For example, Baltrusaitis et al.
have studied the adsorption of CO2 on oxide nanoparticle surfaces,
and have shown that Fe2O3 has better CO2 adsorption capability
than -Al2O3 and TiO2.41 Similarly, Mendoza et al. investigated CO2
capture capacity of Fe2O3/Fe and Fe3O4/Fe, and found that former
possess better activity.42 In another work, Nagaraju et al. gave
insights for activation of CO2 by hematite from theoretical
perspective.43 The above reports suggest that hematite can be a
a. Department of Chemical Sciences and Center for Advanced Functional Materials
(CAFM), Indian Institute of Science Education and Research (IISER) Kolkata,
Mohanpur, West Bengal, 741246, India.
*E-mail: mvenkataramanan@yahoo.com; Fax: 91-33-25873020;
Tel: +91(0)9007603474.
[‡] equal contribution.
See DOI: 10.1039/x0xx00000x
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promising candidate in CO2 fixation strategy. To the best of our wavenumber for Au/α-Fe2O3 (1629 cm-1) and α-Fe2O3 (1620 cm-1)
knowledge, there are no reports on hematite for the solvent-free suggesting the attachment of PVP tDoOI: t1h0e.103V9nie/awDn0AorNtpicJal0er1tO3icn7ll7ienCes
conversion of epoxide to cyclic carbonate using CO2 at atmospheric preferentially through carbonyl groups. The broad peak near 3426
cm-1 in all spectra is due to OH stretching vibration of adsorbed
water molecules on the surface of the materials. Raman
spectroscopic analysis further confirms the formation of -Fe2O3
(Fig. 1c). The Raman peaks observed at 223, 243, 290, 406, 606 cm-1
are well matched with the previously reported literature on -
Fe2O3.56 The peaks at 223 and 493 are attributed to the A1g modes,
and other peaks at 243, 290, 406 and 606 cm-1 are attributed to Eg
modes of -Fe2O3. Furthermore, signature Raman peak around 606
cm-1 correlates with phase-purity of disordered α-Fe2O3 without
appearance of any impurity such as magnetite matrix. Nitrogen
adsorption-desorption experiment further reveals the Type-IV
isotherm and H3 hysteresis loop indicating mesoporous nature and
slit-like pore geometry, respectively for both α-Fe2O3 and Au/α-
Fe2O3 samples (Fig. 1d).57,58 From BJH adsorption pore size
distribution, the pore diameter was found as 6.4 and 6.2 nm for -
Fe2O3 and Au/-Fe2O3 respectively, confirming mesoporous nature
of catalytic framework (Fig. S1). In addition, the reduction in both
surface area and pore diameter is also encountered because of the
incorporation of gold nanoparticles within the hematite matrix
(Table S1).59 The influence of nanoparticle incorporation is further
supported with the appearance of hysteresis at slightly elevated
relative pressure for Au/α-Fe2O3 sample (Fig. S2).
pressure. On the other hand, Au-based nanocatalysts are usually
employed for CO oxidation, H2 activation, hydrogenation of
unsaturated substrates, etc.44-46 However, very few works have
been reported utilizing gold based nanocatalysts for conversion of
CO2 into different value added chemicals.47-50 Our objective is to
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integrate both Au and -Fe2O3 to develop
a gold-hematite
composite nanocatalyst, and evaluate its epoxide to cyclic
carbonate conversion efficiency utilizing CO2 under ambient
conditions.
In this article, we demonstrate a one-pot autoclaving method
for the preparation of mesoporous gold-hematite integrated system
(Au/-Fe2O3) at 180 ℃ for 24 h in presence of dimethylformamide
(DMF), sodium acetate (NaOAc) and polyvinylpyrrolidone (PVP). The
catalytic activity of the prepared heterogeneous nanocatalysts was
investigated towards epoxide to cyclic carbonate conversion
utilizing CO2 at atmospheric pressure (1.02 atm) by using
tetrabutylammonium iodide (TBAI) as co-catalyst. The Au/-Fe2O3
nanocatalyst is able to convert a wide variety of epoxides to cyclic
carbonates with good conversion yield. The study reveals that the
strategy to design morphology-tuned gold-hematite nano-
framework is effectual for the CO2 fixation and the high catalytic
activity is the result of synergistic effect between Au and -Fe2O3,
which plays crucial role in the catalytic process.
Au/-Fe2O3
(a)
(b)
PVP
-Fe2O3
1290(CN)
1661
(C=O)
Au/-Fe2O3
-Fe2O3
Results and discussion
Au [004-0784]
1629
(C=O)
The -Fe2O3, Au/-Fe2O3 composite and other hematite-based
nanocatalysts were prepared by a generalized one-pot autoclaving
reaction at 180 ℃ for 24 h (see ESI for experimental details). Briefly,
PVP, a water-soluble polymer was used as a structure-directing
agent for the synthesis of nanoparticles. NaOAc was utilized to
control the pH of the reaction medium, which further initiates the
hydrolysis of DMF whereas, DMF plays dual role as a templating
agent as well as reducing agent.51-53
-Fe2O3 [033-0664]
1620
3426
(OH stretching)
(C=O)
563
(FeO stretching)
476
10
20
30
40
50
60
70
80
4000
3000
2000
1000
Wavenumber (cm-1)
2 (degree)
(c)
A1g
(d)
150
Au/-Fe2O3
Au/-Fe2O3
-Fe2O3
100
50
0
Eg
Eg
All the synthesized materials were characterized by powder X-
ray diffraction (PXRD) technique to obtain the information about
their crystal structure and phase purity (details of all
characterization techniques are mentioned in the ESI). The PXRD
pattern (Fig. 1a) of the synthesized -Fe2O3 sample perfectly
matches with standard pattern of α-Fe2O3 (ICDD #033-0664), which
reveals the formation of pure phase hematite with rhombohedral
crystal packing. In case of the synthesized Au/α-Fe2O3, the PXRD
pattern shows the presence of both metallic Au (ICDD #004-0784)
and α-Fe2O3 in the system as the peaks were found to exactly match
with corresponding standard patterns. This result suggests the
successful formation of phase pure metallic gold-hematite
composite system. For the functional group determination, Fourier-
transform infrared spectroscopy (FTIR) analysis was performed for
the synthesized materials (Fig. 1b). The sharp peaks observed at 563
and 476 cm-1 for both -Fe2O3 and Au/-Fe2O3 are attributed to the
FeO stretching vibrational modes in -Fe2O3.54 In addition, the
attachment of PVP to the Au/-Fe2O3 is confirmed by using FTIR
measurements. For pure PVP, the peaks appeared at 1661 cm-1 and
1290 cm-1 are assigned to C=O and CN stretching vibrational
modes, respectively.55 The peak corresponding to carbonyl group in
PVP moiety has slightly shifted from 1661 cm-1 to lower
Eg
Eg
-Fe2O3
0.0
0.2
0.4
0.6
0.8
1.0
200
300
400
500
600
700
Wavenumber (cm-1)
Relative Pressure (P/Po)
Fig. 1. (a) PXRD patterns, (b) FTIR spectra, (c) Raman spectra and (d) N2 gas
adsorption-desorption isotherms of -Fe2O3 and Au/-Fe2O3 nanocatalysts.
Morphological features of the as-synthesized materials have
been characterized by the electron microscopy techniques (Fig. 2).
Field emission scanning electron microscopic (FESEM) and
transmission electron microscopic (TEM) images of both α-Fe2O3
(Fig. 2a-d) and Au/α-Fe2O3 (Fig. 2e-h) indicate formation of
monodispersed parallelepiped nanostructures with smooth surface
topology and high crystallinity.60 The elemental composition of α-
Fe2O3 was verified with energy-dispersive X-ray spectroscopy (EDS)
and elemental mapping (Fig. S3). Further, the average particle size,
porosity and microstructural integrity were investigated by TEM
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(c)
(g)
(a)
(b)
(f)
(d)
d= 0.271 nm
(104)
-Fe2O3
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(h)
f)
(e)
d= 0.24 nm
(111)
Au
d= 0.183 nm
(024)
-Fe2O3
Au
Fe
O
(i)
(j)
Fig. 2. Morphology analysis of -Fe2O3 and Au/-Fe2O3 nanocatalysts: (a) FESEM image, (b, c) TEM images, (d) HRTEM image of -Fe2O3; (e) FESEM image, (f,
g) TEM images, (h) HRTEM image, (i) STEM and its elemental mapping of Au, Fe, and O and (j) corresponding EDS of Au/-Fe2O3.
analysis. The particle size distribution plots of α-Fe2O3 and Au/α- region, the peaks at 83.26 eV and 86.94 eV can be attributed to Au
4f7/2 and Au 4f5/2, respectively, confirming the presence of Au0
state.61 Fe (2p) peaks are obtained at 710.51, 719.07 (satellite),
Fe2O3 nanocatalysts show the average size of -Fe2O3 is ~35 nm
whereas for that of Au/-Fe2O3 composite is ~50 nm (Fig. S4 and
S5). TEM analysis also demonstrates the parallelepiped Au/α-Fe2O3
nanoparticles with mesoporous network within nano regime (Fig.
2f-h). The close inspection again reveals the existence of gold
nanoparticles (around 2-3 nm) on the surface of parallelepiped α-
Fe2O3 (Fig. S6). The HRTEM images reclaim the crystallinity of
nanoparticles with the coexistence of both Au and α-Fe2O3 lattice
fringes (Fig. 2h and Fig. S7). The HRTEM image depicts the
interplanar distances of 0.24 nm and 0.183 nm for Au (111) and α-
Fe2O3 (024) facets, respectively (Fig. 2h). Elemental mapping with
colour contrast implies the uniform distribution of gold
nanoparticles throughout the parallelepiped framework, whereas,
EDS analysis shows the existence of O, Fe, Au elements within the
mesoporous hematite matrix (Fig. 2i and j). Additionally, the results
of inductively coupled plasma atomic emission spectroscopy (ICP-
AES) indicate elemental ratio of Fe: Au is 1: 0.0217 in the bulk
system inferring that minimal amount of gold (~2%) was
incorporated into the system (Table S2).
4f7/2 (83.26)
Survey
Au 4f
(a)
(b)
Fe 2p
O 1s
4f 4/2(86.94)
Au 4f
C 1s
0
200
400
600
800 1000 1200 80
82
84
86
88
90
Binding Energy (eV)
Binding Energy (eV)
2p
3/2 (710.51)
Fe 2p
O 1s
529.53
(c)
(d)
2p1/2
(724.07)
satellite
(719.07)
satellite
(732.53)
531.63
532
The X-ray photoelectron spectroscopy (XPS) analysis has been
performed to examine the chemical states of the elements in Au/-
Fe2O3 nanoparticles (Fig. 3). From the survey spectrum, peaks
corresponding to Fe (2p), O (1s) and Au (4f) are prominent and
confirm their presence. In the elemental scanning of the Au (4f)
705
710
715
720
725
730
735
526
528
530
534
Binding Energy (eV)
Binding Energy (eV)
Fig. 3. XPS spectra of Au/-Fe2O3: (a) Survey scan, (b) Au 4f, (c) Fe 2p and (d)
O 1s.
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Table 1. Effect of reaction parameters on the coupling of CO2 and epichlorohydrin.a
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O
Au@?-Fe O + TBAI
Catalyst3+ heat
2
O
CO2
O
+
O
R
Cl
neat conditions
1.02 atm, 105 ƒC, 20 h
Cl
R
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Entry
Catalyst
Amount of Catalyst
Condition
Conversion
(%)
Temp. & Time
105 ℃, 20 h
105 ℃, 20 h
105 ℃, 20 h
105 ℃, 20 h
105 ℃, 20 h
105 ℃, 20 h
105 ℃, 20 h
105 ℃, 20 h
105 ℃, 20 h
105 ℃, 20 h
85 ℃, 20 h
1
2
3
4
5
6
7
8
9
No Catalyst
-Fe2O3
TBAI
-Fe2O3 + TBAI
0
trace
15
77
81
86
91
trace
17
60

50 mg
1.09 mol%
50 mg + 1.09 mol%
50 mg + 1.09 mol%
50 mg + 1.09 mol%
50 mg + 1.09 mol%
50 mg
50 mg + 1.09 mol%
50 mg + 1.09 mol%
50 mg + 1.09 mol%
Cu(4.8 mol%)/-Fe2O3 + TBAI
Ag(4.8 mol%)/-Fe2O3 + TBAI
Au(4.8 mol%)/-Fe2O3 + TBAI
Au(4.8 mol%)/-Fe2O3
Au(2.4 mol%)/-Fe2O3 + TBAI
Au(7.0 mol%)/-Fe2O3 + TBAI
Au(4.8 mol%)/-Fe2O3 + TBAI
10
11
20
1
a
Conditions: substrate, epichlorohydrin 23 mmol; a CO2 filled balloon (1.02 atm, 99.5% pure). Conversion was measured using H NMR
analysis (See Figure S11 and S12 for entry 4 and 7, respectively).
724.07 and 732.53 (satellite) eV. Peaks around 710.51 and 724.07 various other epoxides. Briefly, for a typical conversion set up, in a
eV signify Fe 2p3/2 and Fe 2p1/2 respectively. For both of these 50 mL Schlenk round bottom flask, 50 mg of the catalyst and 1.09
states, satellite peaks located at ~8 eV higher values are evident. mol% co-catalyst (ratio 92:1) was added to the epoxide and the
According to literature reports, these values and the relative reactions were performed at 105 ℃ for 20 hours under CO2 filled
position of the satellite peaks w.r.t. the primary peaks clearly balloon (see ESI for detail catalytic reaction procedure). All the CO2
indicate the presence of Fe3+ in the material.62,63 The peaks for O conversion experiments were performed under atmospheric
(1s) around 529.53 and 531.63 eV indeed suggest the presence of pressure and summarized in Table 1 and 2. The epoxide to cyclic
lattice oxygen and surface adsorbed water, respectively.63
carbonate conversion was calculated using the 1H NMR analysis
(Fig. S11 to S22) and 13C NMR spectra of the cyclic carbonates for
entry 1, 2, 3, 4 and 7 were recorded and given in ESI (Fig. S23 to
S27).
To evaluate the suitability of the developed catalysts for CO2
fixation through epoxide to cyclic carbonate conversion the
optimization of the reaction conditions was executed (Table 1, Fig.
S8). Initially, the catalytic performance of only the hematite was
investigated and no significant conversion was observed. Then the
Further, to understand the reactivity of the Au/-Fe2O3 catalyst
towards other functionalized epoxides, the cyclic carbonate
conversion was investigated under optimized reaction conditions
(Table 2, Fig. S28). Apart from epichlorohydrin, other epoxides like,
allyl glycidyl ether and 1,2-epoxyhexane displayed catalytic
conversions of 82% and 81%, respectively (Table 2, entry 2 and 3).
experiment
was
performed
in
the
presence
of
Tetrabutylammonium iodide (TBAI) and observed 77% catalytic
conversion (Table 1, entry 2 and 4, respectively).64 For further
enhancement in the activity of hematite, a miniscule amount of
coinage metals (M= Cu, Ag and Au) were incorporated into -Fe2O3 For 1,2-epoxy-9-decene the conversion was about 60% (Table 2,
entry 4). It is observed that the catalyst shows excellent reactivity
for carbon chain containing epoxides like 1,2-epoxyhexane and 1,2-
epoxy-9-decene even in the absence of any electronegative atom
(e.g., oxygen). Interestingly, in case of phenyl glycidyl ether, though
it was expected that the ether linkage will facilitate the three
membered ring opening due to the –I effect of oxygen, but
relatively lesser conversion was obtained (Table 2, entry 5). We
presume, this is due to the presence of electron rich phenyl moiety
which is undergoing a repulsive interaction with the electron rich
Au(0) species present on the surface of hematite.65 Similarly, the
observed less conversion for styrene oxide (Table 2, entry 11)
further corroborates the decisive role of the phenyl ring for such
low conversion. The effect of electronegative atom was further
verified by the observed 55% conversion in the case of 1,4-
butanediol diglycidyl ether (Table 2, entry 6). Furthermore, the
nanoparticles (i.e., M(4.8 mol%)/-Fe2O3) and their activity towards
CO2 fixation was investigated (Morphological analysis of Ag/-Fe2O3
and Cu/-Fe2O3 are given in Fig. S9 and S10, respectively). Upon
introduction of coinage metals (Cu, Ag and Au) into hematite, the
catalytic efficiency of hematite increased, and the highest
conversion was noted for Au/-Fe2O3/TBAI (Table 1, entry 5, 6 and
7). Hence, Au/-Fe2O3/TBAI catalyst combination was considered
for further studies. A series of reactions were carried out to
optimize the reaction conditions and to obtain the most effective
catalyst combination. The gold loading was also varied [Au(x
mol%)/-Fe2O3, x= 2.4, 4.8 and 7.0] to optimize the activity of the
catalyst (Table 1, entry 7, 9 and 10). The catalyst, Au( 4.8 mol%)/-
Fe2O3 was found to be the best combination which is evaluated for
further investigation. For our work, we have chosen epichlorohydrin
as the model substrate, and thereafter the scope was extended to
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catalytic ability of the Au/-Fe2O3 was evaluated for substrates
containing bulky groups. For this purpose, (3-
View Article Online
DOI: 10.1039/D0NJ01377C
5
6
Table 2. Conversion efficiency of Au/-Fe2O3 and TBAI combined catalyst for different substrates.a
7
8
9
O
O
Au@α-Fe O + TBAI
Au/-Fe O + TBAI
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
3
3
2
O
CO2
O
+
R
1.02 atm, 105 °C, 20 h
Product
R
Entry
Substrate
Conversion (%)
91
O
O
O
1
Cl
O
Cl
O
O
O
O
2
82
O
O
O
O
O
3
4
81
60
O
O
O
O
O
O
6
6
O
O
O
5
6
55
55
Ph
O
O
Ph
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Si(OCH3)3
7
8
O
53
50
O
O
Si(OCH3)3
O
O
O
O
O
O
O
O
O
O
O
O
O
O
9
46
37
33
O
O
O
O
O
O
O
O
10
11
O
O
O
O
O
O
O
O
Ph
Ph
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a Conditions: catalyst, Au(4.8 mol%)/-Fe2O3 50 mg; co-catalyst, TBAI 0.25 mmol; substrate, epoxide 23 mmol; CO2 filled balloon (1.02 atm,
View Article Online
1
13
DOI: 10.1039/D0NJ01377C
99.5% pure); temp. 105 ℃; and time 20 h. Conversion was measured using H NMR analysis (see Figure S12 to S22). C NMR spectra of the
cyclic carbonates for entry 1, 2, 3, 4 and 7 are shown in Fig. S23 to S27.
6
7
8
9
recycled Au/-Fe2O3
O
(i)
(c)
(a) (b)
91
91
O
90
87
86
R
100
O
Au/-Fe2O3
Au [004-0784]
(ii)
80
60
10
11
12
13
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20
21
22
23
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49
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53
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59
60
CO2
(iii)
-I
40
20
0
-Fe2O3 [033-0664]
Au/ Fe O

2
3
I
O
(iv)
O
O
1
2
3
4
5
Au
O
O
R
Au/ Fe O

10
20
30
40
50
60
70
80
2
3
Number of catalytic cycle
2 (degree)
O
(f)
(d)
(g)
(e)
R
O
O
Au/Fe2O3
O
O
d= 0.273 nm
(104)
-Fe2O3
Au/Fe2O3
O
R
O
d= 0.238 nm
(111)
Au
I
R
Bu4N
I
Fig. 4 (a) Schematic depicting the proposed mechanism for the epoxide to cyclic carbonate conversion, (b) Catalyst recycling efficiency evaluation with
epichlorohydrin, (c) PXRD patterns of (i) recycled catalyst and (ii) as prepared catalyst for comparison, and (d-g) TEM images of Au/-Fe2O3 nanocatalyst
after recyclability study.
glycidyloxypropyl)trimethoxysilane and tert-butyl glycidyl ether Whereas, gold nanoparticles act as electron donor to coordinate
were subjected to the cyclic carbonate conversion and observed with CO2 molecule. Therefore, in the initial step of the mechanism
53% and 50% conversion, respectively. This suggests that bulky the combination of gold nanoparticles with the hematite matrix
functional groups render steric hindrance and influence the activates CO2 molecule. Subsequently, coordination between gold
reactivity (Table 2, entry 7 and 8). Also, the relatively lesser nanoparticles and oxygen atom of epoxide takes place. Next, the
conversion of neopentyl glycol diglycidyl ether is likely due to the co-catalyst TBAI attacks the less hindered carbon center of epoxide
presence of sterically crowded, bulky quaternary carbon leading to epoxide ring opening to form oxyanion species. The
functionality. Additionally, the presence of two epoxide rings generated oxyanion then attacks the electrophilic carbon of CO2-
further results in lower conversion i.e., 46% (Table 2, entry 9). In nanocatalyst intermediate which finally results in cyclic carbonate
case of 1,2,7,8-diepoxyoctane only 37% conversion (Table 2, entry formation through ring closing step.37 It is to be mentioned that the
10) was observed. The lesser conversion might be due to the lack of tetrabutylammonium cations remained within the reaction medium
electronegative atom in the side chain and the presence of two to maintain the overall electrical neutrality through ion-pair
epoxide rings in the terminal positions. Further, to investigate the formation. Therefore, the proposed reaction mechanism suggests
reactivity of the catalyst towards 1,2-disubstituted epoxide the the active performance of catalyst is due to the synergistic role of
reaction between epoxycyclohexane and CO2 was performed. The mesoporous network and active site distribution throughout the
1
conversion calculated from the H NMR analysis of the product is
found to be very less (~5%). We presume this could be due to the
parallelepiped like microstructure in Au/-Fe2O3 composite.
1
steric hindrance induced by the bulky ring in the structure. The H Recyclability of Au/-Fe2O3
and 13C NMR spectra of the product are shown in Fig. S29 and S30,
respectively.20 In addition, we have isolated the corresponding
cyclic carbonates of both epichlorohydrin and allyl glycidyl ether by
column chromatography (Table 2, entry 1 and 2) and obtained an
yield of 85% and 74%, respectively (Table S3). The obtained yields
are only slightly less than the % conversion calculated from the
To investigate the stability and reusability of Au/-Fe2O3 catalyst,
the recycling experiment was performed with epichlorohydrin,
following the optimized reaction conditions. After completion of
the reaction in each catalytic cycle, the catalyst was separated from
the reaction mixture by centrifugation and washed with ethyl
acetate twice followed by vacuum drying and reused for next
cycles. The catalyst was reused for five consecutive cycles and
found to be consistently effective for the conversion (Fig. 4b). The
resulting catalyst after the 5th cycle was then characterized by
PXRD, FESEM, EDS, TEM and XPS analyses. The PXRD pattern of the
recovered Au/-Fe2O3 nanocatalyst matches well with that of the
as-prepared Au/-Fe2O3 catalyst (Fig. 4c). The absence of any peak
shift or new peak indicates robustness of the catalyst and retention
of the nanoframework. Furthermore, TEM, HRTEM and FESEM
analyses reveal monodispersed parallelepiped like microstructure
crude products. The results of both H and 13C NMR analyses are
shown in Fig. S31-S34.
1
Based on the observed results and support from literature, we
propose that both hematite and gold play crucial role in the
catalysis (Fig. 4a).37,43,49,66 The hematite acts as an efficient CO2
adsorbent and also as an epoxide molecule activator presumably
through electrostatic interactions between Fe3+ ions and oxygen
atoms of epoxide molecule. It further enhances the electrophilicity
of carbon centre within the three membered epoxide ring.
6 | J. Name., 2012, 00, 1-3
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with high crystallinity retained even after the catalysis (Fig. 4d-g and
S35). Further, ICP-AES analysis indicates no significant change in
elemental ratio of Fe: Au, which suggests strong binding between
Au and -Fe2O3 in the integrated system (Table S2). XPS analysis
supports that the chemical states of the elements remained
unaltered after the catalytic process (Fig. S36). All the results
collectively suggest that Au/-Fe2O3 is chemically stable, robust and
efficient heterogeneous nanocatalyst for epoxide to cyclic
carbonate conversion.
3
4
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W. Zhou, K. Cheng, J. Kang, C. Zhou, V. Subramanian, Q.
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Zhang and Y. Wang, Chem. Soc. Rev., 2019, 48, 3193–3228.
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O. Jacquet, X. Frogneux, C. D. N. Gomes and T. Cantat,
Chem. Sci., 2013, 4, 2127–2131.
S. Il In, D. D. Vaughn and R. E. Schaak, Angew. Chem. Int.
Ed., 2012, 51, 3915–3918.
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F. Arena, K. Barbera, G. Italiano, G. Bonura, L. Spadaro and
F. Frusteri, J. Catal., 2007, 249, 185–194.
Conclusions
Z. Zhang, Y. Xie, W. Li, S. Hu, J. Song, T. Jiang and B. Han,
Angew. Chem. Int. Ed., 2008, 47, 1127–1129.
This work demonstrates a generalized one-pot synthesis technique
to prepare M0/-Fe2O3, (M= Cu, Ag and Au) nanoparticles with
uniform size and high crystallinity without any calcination process.
To best of our knowledge, this work is the first report on Au/-
Fe2O3 heterogeneous catalyst for cycloaddition reaction of CO2 and
epoxide to form cyclic carbonate under atmospheric CO2 pressure
and solvent-free conditions. The hematite not only acts as the
catalytic support but also plays an important role in the CO2
conversion process. The synergic role between Au and -Fe2O3 10
results in excellent catalytic efficiency for the CO2 fixation reaction.
This study broadens the scope of earth abundant iron-based
nanocatalysts for the CO2 fixation reactions. The present method of
conversion from environmentally hazardous CO2 to value added
chemicals (cyclic carbonates) utilizing iron-based nanomaterials is
indeed economic as well as eco-friendly approach towards
sustainability.
Y. Y. Ohnishi, T. Matsunaga, Y. Nakao, H. Sato and S. Sakaki,
J. Am. Chem. Soc., 2005, 127, 4021–4032.
C. Martín, G. Fiorani and A. W. Kleij, ACS Catal., 2015, 5,
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R. R. Shaikh, S. Pornpraprom and V. D’Elia, ACS Catal.,
2018, 8, 419–450.
11
T. Sakakura and K. Kohno, Chem. Commun., 2009, 11,
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O. Sodpiban, S. D. Gobbo, S. Barman, V. Aomchad, P.
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13
C. Maquilón, B. Limburg, V. Laserna, D. G.Ruiz, J. G. Fabra,
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Conflicts of interest
The authors declare no conflict of interest.
Organometallics,
2020,
DOI:
10.1021/acs.organomet.9b00773.
14
15
J. H. Clements, Ind. Eng. Chem. Res., 2003, 42, 663–674.
Acknowledgements
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Verevkin, A. Preetz and A. Börner, Angew. Chem. Int. Ed.,
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The authors acknowledge DST, India, for funding the project
(DST/TMD/MES/2K17/70) and IISER Kolkata for instrumental
facilities. G.T. thanks the RGNF UGC for the fellowship and
CSIR-NIIST for the XPS measurements. G.T. thanks Dr. S.G.,
K.M. and K.S.P. for their valuable suggestions and her family
for their continuous support.
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ORCID ID:
A. A. G. Shaikh and S. Sivaram, Chem. Rev., 1996, 96, 951–
976.
Venkataramanan Mahalingam: 0000-0003-1414-805X
Gouri Tudu: 0000-0003-2634-8403
Sourav Ghosh: 0000-0003-2684-6795
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338x190mm (96 x 96 DPI)