Alumina as Solid-State Ligand in Enhancing the Redox Catalytic Property of Iron Oxide Grafted AlSBA-15 towards Arylation of Arene

Article abstract of DOI:10.1002/cctc.201800855

2–8 wt % iron has been grafted over SBA-15 and 10 wt % alumina grafted SBA-15 (AlSBA-15) through selective extraction deposition technique using LaFeO₃ as iron precursor in acidic solution. DRS-UV spectra confirmed the presence of iron as isolated Fe(III), Fe(III)-clusters and amorphous FeOx oligomers. SEM-EDX mapping, FT-IR, XPS, HR-TEM and ²⁷Al MAS NMR characterisation results confirmed that iron grafted over AlSBA-15 preferentially reside on the alumina. The donor-acceptor interaction of grafted iron ions with alumina atoms displaying Lewis acid properties stabilized iron at higher oxidation state Fe(III+δ). This increased the BE of Fe2p electrons (XPS) and shifted the temperature of iron reduction in 2–8 wt %Fe/AlSBA-15 materials to higher value (600 °C) compared with Fe/SBA-15 (H₂-TPR). In arylation of benzene, 8 wt %Fe/AlSBA-15 demonstrated excellent redox catalytic activity with TOF close to homogeneous catalyst and 82 % biphenyl yield. The catalyst demonstrated consistent catalytic performance for 7 cycles.

Full text of DOI:10.1002/cctc.201800855

Accepted Article
Title: Alumina as solid-state ligand in enhancing the redox catalytic
property of iron oxide grafted AlSBA-15 towards arylation of
arene
Authors: Rajendran Chandran, Thirumoorthy Kulandaivellu,
Govindaswamy Satishkumar, and Miron V Landau
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Alumina as solid-state ligand in enhancing the redox catalytic
property of iron oxide grafted AlSBA-15 towards arylation of
arene
[
a]
[a]
[a]
Chandran Rajendran , Kulandaivellu Thirumoorthy , Govindaswamy Satishkumar *, Miron V.
Landau[b]
Abstract: 2-8wt% iron has been grafted over SBA-15 and 10wt%
to its low price, high abundance, being environmentally benign
and exhibiting variety of oxidation states. However, designing of
alumina grafted SBA-15 (AlSBA-15) through selective extraction
3
deposition technique using LaFeO as iron precursor in acidic solution. such type of catalysts with abundance of grafted iron ions is highly
DRS-UV spectra confirmed the presence of iron as isolated Fe(III),
Fe(III)-clusters and amorphous FeOx oligomers. SEM-EDX mapping,
challenging since iron cations tend to agglomerate into bulk-oxide
phases over the support. [6-8]
27
FT-IR, XPS, HR-TEM and Al MAS NMR characterisation results
confirmed that iron grafted over AlSBA-15 preferentially reside on the
alumina. The donor-acceptor interaction of grafted iron ions with
alumina atoms displaying Lewis acid properties stabilized iron at
higher oxidation state Fe(III+δ). This increased the BE of Fe2p
electrons (XPS) and shifted the temperature of iron reduction in 2-
Recently, we devised an easy way to graft more isolated iron over
the silica support using LaFeO
3
as precursor by selective
extraction and deposition (SED) of iron ions in acidic
condition.[9,10] With the aim of designing an effective
heterogeneous iron catalyst for C-C coupling reaction, in our
recent report, by implementing the SED technique 8wt% of iron
was successfully grafted as isolated, cluster and oligomeric oxidic
8wt%Fe/AlSBA-15 materials to higher value (600°C) compared with
Fe/SBA-15 (H2-TPR). In arylation of benzene, 8wt%Fe/AlSBA-15
demonstrated excellent redox catalytic activity with TOF close to
homogeneous catalyst and 82% biphenyl yield. The catalyst
demonstrated consistent catalytic performance for 7 cycles.
iron species over the high surface area mesoporous silica support
(SBA-15) without forming the bulk iron oxide.[
11]
The
8wt%Fe/SBA-15 was employed as redox catalyst for activation of
3
2
C(sp )-H and C(sp )-H reactions like arylation of biphenyl
methane[12] and benzene[13]
respectively. The catalyst
demonstrated excellent activity like the homogenous iron salt
FeCl ) in case of arylation of biphenyl methane. On the other
hand, although arylation of benzene using aryl boronic acid with
wt%Fe/SBA-15 catalyst does not require additives and ligands
like homogeneous iron catalyst Fe(OTf) , the reaction time and
the required amount of oxidant were increased to 72 h and 3
equivalents, respectively, to get similar yield to that obtained with
homogeneous counterpart. The difference in catalytic activity of
,
Introduction
(
2
Despite the drawbacks remaining on homogenous transition-
metal catalysts such as separation problems, high consumption
and use of toxic additives, engaging them in organic
transformations is still prevalent.[1,2] Substituting with
heterogenized transition metal complex catalysts tethered on
insoluble supports is continue to be a difficult option as often it
shows low activity and stability due to leaching.[3,4] Grafting of
isolated transition metal cations through oxo-ligand on a suitable
high surface area support would be an interesting strategy to
heterogenize the transition metal ionic catalysts.[5] In this case,
higher amount of transition metal cations could be uniformly
distributed to increase the catalytic activity and direct bonding of
metal cation with the support through oxo-ligand would enhance
the stability. Iron is a good choice for grafting metal cations due
8
3
8
wt%Fe/SBA-15 towards mentioned two organic transformations
raises the question whether every type of oxide iron species
isolated ions, clusters, oligomeric species) existing at the surface
(
of mesoporous silica support are catalytically active or only
isolated iron ions. If only isolated iron ions are catalytically active
then the scope for this kind of heterogeneous catalysts in organic
transformations is limited by low-percentage iron materials as
grafting of more iron cations would generally result in the
formation of multiple oxide species. Meanwhile, restricting to
isolated catalytic site alone by loading of less amount of iron
(
2wt%) would substantially enhance the catalyst amount to carry
[
[
a]
b]
C. Rajendran, K. Thirumoorthy, Dr. G. Satishkumar
Advanced materials and catalysis Lab
Department of chemistry
out the desired reaction. Owing to the above circumstances
further improvement of the performance of Fe-silica catalysts in
organic transformations by increasing of iron loading is strongly
hindered.
School of Advanced Sciences
Vellore Institute of Technology, Vellore-632 014, Tamil Nadu, India.
E-mail: satishkumar.g@vit.ac.in, satishgsamy@gmail.com
Prof. Dr. Miron V. Landau
Blechner Center for Industrial Catalysis and Process Development,
Department of Chemical Engineering, Ben-Gurion University of the
Negev, Beer-Sheva 84105, Israel.
Alumina is known to be an excellent support to stabilize the iron
oxide particles with high dispersion. [4,14] Strong alkaline nature
and ion exchange capabilities of alumina assist in achieving high
dispersion of iron oxide particles. [15] However, hitherto, no special
grafting technique has been applied to graft the iron ions over the
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alumina support. It is anticipated that grafting of 8wt% of iron ions
over alumina using the SED technique will yield merely single site
grafted iron ions and/or activate all types of surface iron oxide
species (isolated, cluster, oligomeric) through its Lewis acid
character. The objective of this study is to investigate the ability of
alumina for dispersing (creation of single sites) and activating the
different oxide iron species grafted through SED technique. In the
present work, in order to attain better dispersion of iron ions
instead of alumina support, iron has been grafted over 10wt%
alumina coated SBA-15 with high surface area. 2, 4 and 8wt% of
iron has been successfully grafted over SBA-15 and alumina
coated SBA-15 (AlSBA-15) using the SED technique. The
SED technique in avoiding the iron oxide particles formation at
higher iron loadings as well. N
2
adsorption and desorption
isotherms and the corresponding pore size distributions of SBA-
15, AlSBA-15 and iron grafted (2, 4, 8wt%) over SBA-15 and
AlSBA-15 are shown in Figure S3. The structural parameters
derived from these isotherms are summarized in Table 1. Alike
the parent SBA-15, all the grafted materials exhibit type IV
isotherm which represents the characteristic of tubular ordered
mesopores. The amount of iron grafted over SBA-15 and AlSBA-
15 confirmed by EDAX measurements (Figure S4) is in good
agreement with the calculated values. As expected, total surface
area, pore volume and pore diameter of parent SBA-15 decreases
with increase of iron loadings (Table 1). In case of AlSBA-15,
loading of alumina decreases the mesopores and micropores
synthesised catalysts were characterized using XRD,
N
2
adsorption and desorption, SEM-EDX mapping, DRS-UV, FT-IR,
HR-TEM, XPS, 27Al MAS NMR and TPR to identify and
understand nature of oxide iron species existing over the support.
The synthesised catalysts have been tested in arylation of
benzene as model reaction using arylboronic acid with di-tert-
butyl peroxide (DTBP) as oxidant. It was expected that the iron
grafted AlSBA-15 catalyst would demonstrate high catalytic
activity without the support of ligands and additives in contrast to
homogeneous iron catalysts and high loading Fe/SBA-15.
2
surface area of SBA-15 from 793 to 679 m /g and from 109 to 42
2
m /g, respectively (Table 1). Further grafting of iron ions (2, 4,
8wt%) declines only the mesopore surface area while the
micropore surface area remains unaffected. This observation
suggests that the iron ions grafted in the mesopores of AlSBA-15.
Location and state of the grafted iron oxide over SBA-15 and
AlSBA-15
In order to gain insight about the dispersion of oxide iron species,
all the iron grafted samples have been characterized by DRS-UV
and the results are displayed in Figure S5. Irrespective of the iron
loading (2, 4, 8wt%) and kind of support (SBA-15, AlSBA-15),
each one of the samples contains a peak, intense band and
shoulder at about 250 nm, 320 nm and 470 nm, respectively. It
Results and Discussion
Mesostructure characterization
The efficient development of well-arranged ordered mesoporous
SBA-15 with two-dimensional hexagonal structure has been
confirmed by the appearance of necessary (100) peak besides
III
confirms the presence of isolated Fe in octahedral environment
III
(
270 nm), Fe -clusters (320 nm) and amorphous FeO
x
oligomers
(470nm). [
7,17-20]
The non-appearance of peaks above 500 nm
110) and (200) in the low-angle XRD [16] (Figure S1a). The above
(
indicates the absence of bulk iron oxide particles in all the
samples, which is in good agreement with the wide angle XRD
results. The similar DRS-UV patterns of 2, 4, 8wt%Fe/SBA-15 and
characteristic peaks of hexagonal structure are retained after
grafting of 10wt% alumina over the same SBA-15 (AlSBA-15)
(
Figure S1b). The wide-angle XRD patterns of SBA-15, AlSBA-15
2
, 4, 8wt%Fe/AlSBA-15 rises ambiguity on (i) effect of alumina in
and 2, 4, 8wt% iron grafted over SBA-15 and AlSBA-15 are shown
in Figure S2 (b) and (c). The AlSBA-15 exhibits at high angles a
similar amorphous XRD pattern of SBA-15 which indicates that
alumina has been uniformly coated over the pore walls of SBA-15
not forming a separate phase. The absence of representative
controlling the dispersion of various oxide iron species (ii) whether
iron has been grafted over alumina and/or remaining silanols.
Therefore, the materials were further characterized with SEM-
EDX mapping, FT-IR, XPS, 27Al MAS NMR, HR-TEM and TPR
techniques. Figure S6 displays the results of SEM-EDX mapping
of synthesized 8wt%Fe/SBA-15 and 8wt%Fe/AlSBA-15 particles.
peaks of LaFeO
3
and iron oxide particles after the grafting of
various amounts of iron ions highlights the accomplishment of
Table 1: Physicochemical properties of synthesized material
S. No.
Material
Iron
weight
Iron
weight
(%)
Total
Mesopore
Micropore
Pore
Pore
Diameter
(nm)
Surface
Surface area
Surface area
volume
a
b
2
2
2
3
(%)
area (m /g)
(m /g)
(m /g)
(cm /g)
1
2
SBA-15
2wt%Fe/SBA-15
-
2
-
902
793
793
705
109
88
1.69
1.57
7.35
6.72
2.03
3
4
4wt%Fe/SBA-15
8wt%Fe/SBA-15
4
8
3.82
8.05
736
610
689
557
47
53
1.21
1.10
6.60
6.56
5
6
7
AlSBA-15
2wt%Fe/AlSBA-15
4wt%Fe/AlSBA-15
-
2
4
-
720
635
571
678
591
529
42
44
42
1.03
0.80
0.79
6.02
5.52
5.46
1.85
4.12
8
9
8wt%Fe/AlSBA-15
γ-Al O
2 3
8
-
8
8.27
-
8.31
438
241
207
392
241
207
46
-
-
0.73
0.43
0.32
5.14
5.06
4.89
10
8wt%Fe/Al
O
2 3
a
Theoretical; b obtained from SEM-EDAX
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SEM-EDX mapping suggests that element Aluminium was
distributed uniformly throughout the SBA-15 particle. However, in
case of Iron element, similar distribution pattern in SBA-15 and
AlSBA-15 suggests that the influence of alumina on dispersing
the iron is not significant and this observation reinforce the DRS-
UV inference. FT-IR spectra of materials containing various
amounts of iron grafted on SBA-15 and AlSBA-15 are shown in
Figure S7. The reduction of Si-OH stretching peak intensity at 963
cm-1 for 2wt%Fe/SBA-15 when compared to parent SBA-15
indicates participation of silanols in the grafting procedure (Figure
S7a). [21] Nevertheless, further increasing of iron content does not
reduce the Si-OH peak intensity implying that successive iron
grafting occurred over earlier grafted oxide iron species. AlSBA-
Figure1 (c). The appearance of Al-O-Al lower binding energy
peak at 74.34 eV accompanied by Al-O-Si at 74.95 eV is due to
existence of grafted alumina in the mesoporous channels.[23] The
third peak at 73.49 eV is owing to the establishment of Al-O-Fe
bridges due to grafting of oxide iron species over grafted alumina
in the mesoporous channels.[24] The deconvoluted O1s spectra of
8wt%Fe/AlSBA-15, alike Al2p, exhibited three peaks at different
binding energy values which could be attributed to Si–O–Si
(532.56 eV) [25], Al-O-Al (531.69)[26] and Al-O-Fe (529.85)
species Figure 1(d). Finally, deconvoluted Fe2p3/2 spectra of
[27]
3+
8wt%Fe/SBA-15 displayed typical ground state Fe peak at
binding energy values at 710.4 eV and Fe3 peak in iron oxide
clusters or oligomer at 712.8 eV Figure1 (f).[28] Insertion of Al in
8wt%Fe/AlSBA-15 shifts the binding energy of Fe2p electrons to
higher values significantly to 711.0 eV and 713.3 eV Figure 1(e).
It is attributed to the strong donor-acceptor interaction between
grafted Fe-ions and Al-ions where electrons are shifted from Fe
to Al. Al acts as acceptor Lewis sites increasing the effective
positive charge on Fe ions on AlSBA-15 compared with Fe ions
on SBA-15.
+
-1
1
5 displayed three intense peaks at 3865, 3730, 3617 cm
-1
besides a shoulder at 3527 cm (Figure S7b). The high-frequency
bands at 3865 and 3730 cm−1 are related to stretching frequency
of terminal OH groups of tetrahedral Al sites (Al ) and octahedral
IV
Al sites (Al ), respectively.14 The peak at 3617 cm−1 with a
shoulder at 3527 cm−1 are assigned to bridged hydroxyl groups of
framework Al–OH–Si formed during grafting process. [22] It is
noteworthy that after iron grafting OH stretching peak of
octahedral aluminium and framework Al–OH–Si decreased
whereas, the OH stretching peak of tetrahedral Al site is not much
affected. The nature and coordination of the elements existing in
VI
It has been reported that using 27Al MAS NMR technique the
presence of iron in the close vicinity of aluminium could be reliably
identified according to paramagnetic broadening.[
29-32]
The
8
wt%Fe/AlSBA-15 were investigated by XPS in the binding
paramagnetic iron centre in the close proximity of aluminium ion
strongly broadens the aluminium nuclei peak and makes it out of
NMR detection. The 27Al MAS NMR spectra of the catalysts
AlSBA-15 and 8wt%Fe/AlSBA-15 are shown in Figure 2. In both
samples, two resonance signals approximately at chemical shifts
of 60 and 10 ppm were observed correspond to tetrahedrally
energy region of Si2p, Al2p, O1s and Fe2p (Figure 1). All the
spectra of Si2p, Al2p, O1s and Fe2p exhibit multiple chemical
states owing to primary silica support, silica-alumina and alumina-
iron interface. Deconvoluted Si2p spectra shown in Figure 1 (b)
exhibit two binding energies at 103.08 and 102.41 eV correspond
to Si-O-Si and Al-O-Si respectively.[23] Al2p spectra after
deconvolution exhibit peaks corresponding to three binding
energies of 74.95, 73.49 and 74.34 eV confirming three different
kinds of Al environment present in the 8wt%Fe/AlSBA-15 material
(framework) and octahedrally (extra-framework) coordinated
aluminium species like in γ-Alumina. [
33,34]
It is expected that in the
course of the alumina grafting procedure, thermal treatment at
550 °C transforms part of the grafted Al to framework Al (inserted
AlTd AlOh
8
7
6
5
4
3
2
1
000 (a) Survey spectrum
(b)
800
Si2p
000
000
000
000
000
000
000
0
59.19
9.21
600
400
200
102.41
(b)
Si-O-Al
Si-O-Si
03.08
1
1200
1000
800
600
400
200
0
110
100
90
Binding Energy (eV)
Binding Energy (eV)
10.31
(a)
(
c)
Al2p
(d)
O1s
(e)
Fe2p
3/2
3/2
(f)
Fe2p
Al-O-Si
531.69
850
6
0.37
300
250
200
150
100
4000
711.02
900
710.38
8
00 712.80
50
Al-O-Si
4.34
3
000
713.35
7
800
7
Al-O-Fe
3.49
Al-O-Fe
529.85
7
2000
74.95
532.56
Si-O-Si
7
00
00
700
650
Al-O-Al
1000
6
600
0
715
710
705
540
530
520
715
710
705
80
70
60
-100 -200
ppm
Binding Energy (eV)
Binding Energy (eV)
200
100
0
Binding Energy (eV)
Binding Energy (eV)
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Figure 3. Schematic illustration of (a) AlSBA-15 and (b) various oxide forms of iron present in 8wt%Fe/AlSBA-15.
in to the mesopore and micropore walls) [35] and the remaining Al
exist as alumina clusters in lateral and radial form as shown in
Figure 3(a). Subsequent to iron grafting (8wt%Fe/AlSBA-15) the
dominant extra-framework resonance peak intensity has
significantly decreased to half which endorses grafting of iron over
alumina clusters. However, due to the presence of inaccessible
Al species (adjacent to mesoporous silica wall) in alumina cluster,
extra-framework resonance peak was not completely
disappeared. On the other hand, tetrahedrally coordinated Al
framework resonance signal intensity remains unaltered before
and after iron grafting implying the location of grafted oxide iron
species only over the alumina cluster present in the mesopores of
AlSBA-15. From all the above investigations, it is understood that
alumina played a critical role in dispersing the various oxide iron
species.
studied materials, similar TPR profile with multiple reduction
events was observed for the SBA-15 support (Figure 4a).
Occurrence of reduction events at about 410-450°C, 564°C, 615-
680°C and above 800°C are attributed to two-dimensional
oligomeric FeOx species, Fe(III)-clusters, single site Fe(III) and
single site Fe(III) in the micropores of SBA-15, respectively.[ The
challenge in achieving isolated iron site is clearly evident from the
multiple reduction patterns of material grafted with lowest amount
of iron 2wt%Fe/SBA-15. Important that TPR profiles of none of
the iron grafted materials over the AlSBA-15 comprise multiple
reduction events. Further, irrespective of the iron content, all the
grafted Fe/AlSBA-15 materials exhibit single reduction event
centred about 600°C which correspond to isolated iron sites
(Figure 4b). The absence of peak above 700°C indicates that
grafting of iron in the micropores of AlSBA-15 was prevented as
part of the grafted aluminium clogged the micropores. This
11]
The influence of alumina on the redox properties of grafted
2
observation is in good agreement with the N adsorption (Table
oxide iron species can be certainly determined using H
studies. Based on the redoxibility of supported metal oxide
moieties, H -TPR studies provide vital information about the
degree of dispersion, nature and interaction with the support. The
2
-TPR
1) and 27Al MAS NMR results (Figure 2). According to DRS-UV
data, reflecting chemical bonding and environment of Fe-ions, the
Fe/AlSBA-15 catalysts contain multiple Fe-sites: isolated Fe(III)-
2
ions, Fe(III)-clusters and amorphous FeO
x
oligomers. On the
3
+
isolated Fe iron ions which interact strongly with the support are
reduced with hydrogen at temperatures between 600 and 700 °C.
other hand, the H reduction ability of all Fe-ions as measured by
2
TPR in Fe/AlSBA-15 is similar to that of isolated Fe-ions.
Contradictory results observed with DRS-UV and TPR for 2, 4,
8wt%Fe/AlSBA-15 led to following conclusions. Irrespective of
type of oxidic iron species, the electron-acceptor Lewis acid
property of the grafted alumina ions polarises the electrons
density around all Fe(III) oxide iron species bonded with them.[
As a result, the oxidation potential of various Fe(III) oxide species
became further increased which in turn increases the energy
demand for reduction of Fe-ions and shifting the positions of TPR
peaks to higher temperature in TPR studies.
[
7]
However, two-dimensional amorphous FeO
x
and three-
dimensional Fe clusters displaying weak interaction with the
2
O
3
support, undergo reduction around 500°C and below 400°C,
respectively. [7] Figure 4 shows the TPR profiles of iron grafted
over SBA-15 and AlSBA-15 at different amounts. It is important to
note that none of the materials prepared using SED technique
show typical two-step reduction TPR profile of the bulk particles
Fe O
2 3
to Fe0 [36] yet again confirms the absence of bulk iron oxide
crystallites that may be not visible by XRD due to low content and
high dispersion. Regardless of the content of grafted iron in
37]
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o
(
a)
564 C
8wt% Fe/SBA-15
o
8
4
wt% Fe/AlSBA-15
wt% Fe/AlSBA-15
(
b)
602 C
4wt% Fe/SBA-15
wt% Fe/SBA-15
o
o
2
4
10 C
o
815 C
2wt% Fe/AlSBA-15
AlSBA-15
6
78 C
o
4
52 C
o
5
92 C
o
o
6
79 C
616 C
o
o
6
16 C
4
47 C
T=Constant
T=Constant
o
o
(850 C)
(850 C)
0
200 400 600 800 1000 1200
Temperature (°C)
-
----Isothermal
200 400 600 800 1000 1200
-----Isothermal
Temperature (°C)
Figure 4. TPR profiles of (a) 2wt%Fe/SBA-15, 4wt%Fe/SBA-15, 8wt%Fe/SBA-15; (b) AlSBA-15, 2wt%Fe/AlSBA-15, 4wt%Fe/AlSBA-15, 8wt%Fe/AlSBA-15
Figure 5 (A and B) shows the HR-TEM images of 8wt%Fe/AlSBA-
5 particles recorded at different magnifications and combined
Catalytic performance of grafted iron ions in arylation of
benzene
1
with corresponding EDAX data. From these TEM images, it is
evident that both Al and Fe were successfully grafted inside the
mesopores of SBA-15 channels. It is also observed that
mesoscopic channels were well preserved after the grafting
procedure. Based on the all the above characterization results, it
is concluded that grafted iron in Fe/AlSBA-15 with 2, 4 and 8wt%
loading exists as isolated ions, clusters and oligomeric species
The homogeneous iron (Fe(III) trifluoromethanesulfonate)
catalyzed arylation of benzene by aryl boronic acid requires
suitable expensive additives (substituted 1,10 phenanthroline) to
achieve better yield of aryl-benzenes. In our recent work, we
experimented the same reaction over 8wt%Fe/SBA-15 with the
intention to prove the establishment of Fe(IV) ions in the course
of the reaction.[
11]
However, we obtained low yield of aryl-
(
Figure 3b) having the redox properties equivalent to that of
isolated iron sites.
benzenes in absence of additives at testing conditions
corresponding to that used for testing of the homogeneous iron
catalyst with additives. Therefore, it is coherent to experiment the
same reaction over iron grafted AlSBA-15 material in order to
prove their superior redox property. Hence, we carried out the
model reaction arylation of benzene using aryl boronic acid with
DTBP as oxidant over iron grafted SBA-15 and AlSBA-15 with 2,
A
B
4,8wt% iron content (Table 2). 2, 4, and 8wt%Fe/SBA-15
demonstrated low catalytic activity with maximum of 48% yield of
biphenyl owing to the presence of various oxide iron species
(
2
Table 2, Entry 1-3). It is fascinating to note that iron grafted
wt%Fe/AlSBA-15 catalyst with lowest iron content performed
with 52% biphenyl yield which is higher than yield obtained with
the Fe/SBA-15 catalyst having maximal iron content of 8wt%.
Further increase of iron loading in Fe/AlSBA-15 catalyst
significantly improved the biphenyl yield up to 82% and the
obtained isolated yield of 74% is equal to the yield obtained with
the homogeneous iron catalyst in the presence of additives.[
The effect of electron withdrawing groups attached to the
homogeneous iron catalyst and addition of substituted 1,10-
phenanthroline on enhancing coupled product yield has to be
taken into account to understand the role of grafted alumina in
13]
8
wt%Fe/AlSBA-15 as a solid-state ligand to boost biphenyl yield
[
13]
through its Lewis acid nature.
Figure 5. HR-TEM images of 8wt%Fe/AlSBA-15.
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Table 2 Catalytic performance of various catalyst towards arylation of benzene
Oxidant
Product
Selectivity
Product
yield
TOF
X10
S. No.
Catalyst (100mg)
Conversion(a)
TON(f)
(
b)
-5(g)
(Equiv.)
1
2wt%Fe/SBA-15
DTBP (3)
45%
80%
36%
6.28
7.27
2
3
4
5
6
7
8
9
1
1
1
4wt%Fe/SBA-15
8wt%Fe/SBA-15
2wt%Fe/AlSBA-15
4wt%Fe/AlSBA-15
8wt%Fe/AlSBA-15
8wt%Fe/AlSBA-15
AlSBA-15
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
TEMPO (3)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
51%
55%
60%
65%
85%
-
88%
87%
87%
92%
96%
-
45%
48%
52%
60%
3.56
1.92
8.38
4.54
2.97
-
4.12
2.22
9.70
5.25
3.43
-
(
c)
82% (74%)
-
-
-
-
40%
46%
-
-
-
8wt%Fe/AlSBA-15(500)
50%
55%
-
80%
84%
-
1.75
1.92
-
2.02
2.22
-
0
1
2
2 3
8wt%Fe/Al O
8wt%FeOx/AlSBA-15(PM)
-
(
(
d)
e)
-
-
-
-
-
Fe(OTf)
3
(0.020mmol), L3
1
3(h)
DTBP (2)
99%
73%
72 %(b)
9.9
11.4
(0.020 mmol)
(a) Based on phenylboronic acid conversion (b) Yield determined by GC using n-decane as the internal standard. (c) Yield of isolated product (d) Physical
mixture (e) Without catalyst (f) Calculated as dividing the moles of reactant converted at 24 h (86400 sec) by the moles of metal present in the catalyst. (g)
Calculated as dividing TON by reaction time. (h) Reference [
13]
We propose that in the present research work the alumina
species at the supports surface through its Lewis acid property
polarize the electron density around Fe(III) ions making them
easily oxidizable to Fe(IV). It also indicates that the reducibility
of Fe(III) oxide species grafted over alumina should be hindered
compared to that grafted on silica due to withdrawal of electrons
from Fe-ions by grafted alumina. This was observed in TPR
studies. All the iron species grafted over alumina are reduced
only at high temperature. The higher catalytic activity of iron
grafted alumina catalyst compared with silica grafted is due to
the superior redox property of iron. The donor- acceptor
interaction of Fe-ions with alumina anchoring ions (Lewis sites)
favours stabilization of iron in higher oxidation state. Therefore,
at the surface of Fe/AlSBA-15 catalyst, the Fe (III)  Fe(IV)
transition is easily happen.
The TON and TOF numbers calculated for testing of
Fe/SBA-15 and Fe/AlSBA-15 at period of 24 hours (Table 2)
show that they are maximal at lowest iron loading and increase
with raising of iron content. They are 1.3-1.5 times higher for iron
grafted on AlSBA-15 compared with Fe/SBA-15 at all iron
contents. It may be assumed that the maximal catalytic activity
is displayed by isolated iron ions at solids surface which content
increases after grafting of SBA-15 with alumina and decreases
with increasing of iron loading. The blank experiment and test
conducted with AlSBA-15 without iron gave zero yield of aryl-
benzene attributing the measured reagents conversions fully to
iron active species. The TON number measured with Fe/AlSBA-
Fe/AlSBA-15 catalyst decreases to 46 and then to 30% of the
value corresponding to homogeneous catalyst (Table 2). But
this decrease (by a factor of 1.85-2.83) is lower than the growth
of iron loading (by a factor of 2-4) bringing to high isolated
biphenyl product yield measured with 8wt%Fe/AlSBA-15 (74%)
compared to 72% reported for homogeneous catalyst.
The scope of the best catalyst 8wt%Fe/AlSBA-15 was
further explored with phenyl boronic acid bearing various
functional groups, and the results are listed in Figure 6. The
catalyst demonstrated excellent regio control with moderate to
good yield towards various biphenyl derivatives (3a-3o). The
coupling reaction proceeded neither over AlSBA-15 catalyst nor
with the addition of TEMPO (3 equiv) to the reaction mixture
(Table 1, entry 7, 8) suggesting the involvement of radical
species generated by the grafted iron in the coupling process.
Therefore, it is expected that reaction may proceed through iron-
assisted single electron transfer (SET), alike our earlier report
with 8wt%Fe/SBA-15.[11] The tentative detailed mechanism for
the above reaction is given in the supplementary information
(Scheme S1).
Based on the above findings we were interested to know
whether this increase in the redox catalytic property of the oxide
iron species over alumina would extend to iron oxide particles
as well. Moreover, is it necessary to choose alumina grafted
mesoporous support for iron grafting? Hence, 8wt% of iron has
been impregnated and grafted using SED technique over
15 at low iron loading of 2wt% is close to that displayed by
homogeneous catalyst at similar conditions comprising 85% of
its value. At higher iron loadings the TON measured with
AlSBA-15
(8wt%Fe/AlSBA-15(500))
and
γ-Alumina
(8wt%Fe/Al O ), respectively. Holding their parent support’s
2 3
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ChemCatChem
10.1002/cctc.201800855
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Figure 7. Recycle study of 8wt%Fe/AlSBA-15 catalyst under optimized testing
conditions.
8wt%Fe/AlSBA-15 is owing to the existence of higher number of
Al-O-Fe bonds facilitating the donor-acceptor interaction
between iron and aluminium ions which in turn increases the
redox catalytic activity of oxide iron species. The above
investigation indeed revealed the significance of application of
SED technique to deposition of iron on high surface area
alumina support.
The recognition of efficient heterogeneous catalyst is based on
its stability and ability to survive the performance in recyclable
study of several testing runs. Hence, the recyclable study was
evaluated for arylation of benzene with phenylboronic acid in
presence of 8wt%Fe/AlSBA-15 catalyst, and the results are
shown in Figure 7. Remarkably, catalyst performed without
significant loss in its activity even up to 7 cycles. After
completion of the reaction, the catalyst held removed by filtration
and the filtrate was extracted with distilled water then the
aqueous layer was tested by Atomic adsorption spectroscopy
(AAS) analysis. No Fe and Al ions were detected by AAS
analysis. Furthermore, no biphenyl product was detected when
reaction was carried out with the filtrate solution. SEM-EDAX
and mapping of spent catalyst shown in Figure S10 and S11
respectively confirm the presence of 7.8wt% of Fe and 9.5wt%
Al in the spent catalyst after seven runs. The low-angle XRD
pattern of spent 8wt%Fe/AlSBA-15 displayed (Figure S12) a
high intensity peak (100) with two less intensive reflections like
the fresh 8wt%Fe/AlSBA-15 confirms the retainment of
hexagonal structure of the catalyst. Amorphous wide angle XRD
pattern (Figure S13) and absence of iron oxide peak above 500
nm in DRS-UV of spent catalyst (Figure S14) assures the
grafted alumina and oxidic iron species are intact on the SBA-
15 support.
Figure 6. 8wt%Fe/AlSBA-15 catalyzed arylation of benzene with various
aryl boronic acids. All Yields are isolated product.
(
AlSBA-15 and γ-Alumina[38]) wide angle XRD pattern endorses
the dispersion ability of alumina in these materials (Figure S2d).
DRS-UV spectra of these samples shown in Figure S5c display
a shoulder peak at 510 nm besides intense peak at 360 nm and
a band at about 250 nm confirms the presence of iron oxide
particles.[
1
8]
Despite the high iron content, 8wt%Fe/AlSBA-
catalysts demonstrated low catalytic
5(500) and 8wt%Fe/Al
2
O
3
performance with low biphenyl yield (40%) as they fail to create
high contact interface with alumina (Table 2, entry 9, 10). In case
of 8wt%Fe/AlSBA-15(500), in spite of high surface area
impregnation technique results predominantly iron oxide
particles due to fast hydrolysis and condensation of iron
precursor, as a result it forms less Al-O-Fe bonding with alumina.
However, in SED technique-controlled release of iron ions from
3
LaFeO precursor prevented the formation of iron oxide and
thereby create strong contact with AlSBA-15 through more Al-
O-Fe linkage (8wt%Fe/AlSBA-15). On the other hand, less
surface area support γ-Alumina (241 m /g) offered limited Al
Conclusions
2
sites for iron grafting. Hence, even the reliable SED technique
could not avoid the formation of iron oxide particles.
Furthermore, physically mixed 8wt% iron oxide with AlSBA-15
catalyst does not yield the desired product (Table 2, entry 11).
Therefore, it is concluded that high catalytic activity of
8
wt%Fe/AlSBA-15 catalyst where iron ions were grafted at the
surface of SBA-15 silica partially covered with alumina has been
evolved as highly stable, efficient and sustainable
heterogeneous redox iron catalyst. In the benzene arylation, as
model reaction, it demonstrated 82% of biphenyl yield which is
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ChemCatChem
10.1002/cctc.201800855
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higher than the yield obtained with the homogeneous iron
catalyst in the presence of expensive additives. In this study, the
need of tricky preparation strategy yielding isolated active iron
sites has been overcome by enhancing the redox catalytic
activity of oxide iron species like cluster and oligomer using silica
support covered with alumina. Alumina played critical role as
solid-state ligand through its Lewis acid property, similar to the
electron withdrawing groups attached to homogeneous iron
catalysts to polarise the electrons density in iron ions of all Fe(III)
oxide species bonded with them. The single high temperature
2 3
Synthesis of 8wt%Fe/Al O
γ-Alumina was synthesized by following the procedure as described in
Synthesis of AlSBA-15 without addition of SBA-15 support. 8wt% of iron
has been grafted over the γ-Alumina support using the SED technique
as detailed above.
Synthesis of 8wt%Fe/AlSBA-15(500)
8
wt%Fe/AlSBA-15(500) catalyst was prepared by impregnation of
[
41]
required quantity of ferric nitrate over AlSBA-15. Concisely, 0.585 g of
Fe(NO .9H O in 10 mL of ethanol was stirred with 1 g of calcined
3
)
3
2
AlSBA-15 at 60 °C overnight and calcined at 500 °C.
2
reduction peak centred at 600 °C was observed in H -TPR for
all the iron grafted AlSBA-15 catalysts evidencing for strong
interaction between grafted iron ions and alumina. Poor catalytic
performance of iron oxide particles supported on AlSBA-15 and
γ-Alumina by impregnation revealed the importance of
application of SED technique and mesostructured alumina-
covered silica for creating more Al-O-Fe bonds. Stability of
Catalysts testing
For arylation of benzene with different arylboronic acids 100mg of
catalyst was added to the reaction mixture of 0.5 mmol aryl boronic acid
(1 equiv.), 50 mmol of benzene (100 equiv.) and 1.5 mmol of DTBP
oxidant (3 equiv.). The reaction mixture was stirred for 24 h under N
2
atmosphere at 80°C. After the reaction, the product was taken out of the
reaction system and analyzed by gas chromatography (GC-2014,
Shimadzu) using capillary column (30 meter, 0.25 mm ID) with FID
detector and n-decane as internal standard. The gas chromatographic
8wt%Fe/AlSBA-15 catalyst has been proven through its
consistent performance in 7 consecutive cycles. The results of
above study increase the scope of heterogeneous catalysts
application in chemical industries for demanding organic
transformations.
2
conditions employed were: carrier gas, N at 50 kPa; split ratio, 3;
injection volume, 1μL; injection temperature, 250°C; detector
temperature, 300°C; column temperature program, heating rate 10°C
min−1 from 150°C to 300°C, hold at 150°C for 2 minutes and 300°C for 6
minutes. The conversion of phenylboronic acids was calculate as M
C T
/M
Experimental Section
X 100, M = moles of phenylboronic acid converted and M = total moles
C
T
of phenylboronic acid taken. The product selectivity was calculated as
M /M X 100, MP = moles of biphenyl product formed. TON was
Synthesis of AlSBA-15 support
P
C
C
calculated by M /MFe, MFe = moles of catalyst employed. TOF was
Mesoporous silica SBA-15 was synthesized through hydrothermal
technique using P123 triblock copolymer structure directing agent as
reported in the literature.[16] The grafting of 10wt% alumina on
mesoporous silica was done by following the procedure proposed in the
literature.[39] In brief, 1.5 ml of trimethylamine and 1 g SBA-15 were
calculated by TON/ reaction time 24 h (86400 Seconds). The isolated
products of arylated benzene were obtained through column
chromatography using silica gel (100-200 mesh) with hexane/ethyl
acetate (95:5) solvent mixture to afford the desired products.
Recyclability of the catalyst was examined by separating the catalyst
from the reaction mixture by filtration and dried overnight then it was
employed for the next cycle.
3
introduced to the solution containing 14.5 g of Al(O-sec-Bu) and
anhydrous toluene (100 ml) at 85°C. The reaction mixture stirred
continuously for 6 h. The solid was separated by filtration, washed with
toluene, and dried. The obtained dry solid was added to 30ml of ethanol
and hydrolysed at 25°C for 24 h. Finally, the solid was washed with
ethanol, filtered and then dried in air at 120°C. Further, it was calcined
at 500 °C in air for 5 h at the heating rate of 1 °C/min.
Acknowledgements
The
authors
thank
DST-INDO-ISRAEL
PROJECT
(
DST/INT/ISR/P-09/2014) Government of India, (Grant #3-
11667) Israel Ministry of Science and Technology and VIT seed
Synthesis of 2, 4, 8wt%Fe/SBA-15, AlSBA-15
grant for the financial support.
The grafting of various amounts of iron over SBA-15 and AlSBA-15 was
carried out using LaFeO
LaFeO perovskite was prepared through citrate-based sol-gel method
by following the procedure reported in the literature.[40] Briefly, 0.5g of
calcined AlSBA-15 and 0.190g of LaFeO were mixed together and
as iron precursor through SED technique.[9,10]
3
Keywords: Mesoporous, Heterogeneous iron, Iron oxide,
3
Alumina, CH activation, C-C coupling
3
transfered to the 250ml 3 neck round bottom flask containing 150ml
distilled water. The mixture was heated up to 80⁰C and pH of the solution
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grafted over the AlSBA-15 support. The addition of acid to the solution
continued till entire LaFeO decomposes and the solution pH remains
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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[
a]
Chandran Rajendran , Kulandaivellu
[
a]
Thirumoorthy , Govindaswamy
Satishkumar *, Miron V. Landau
[
a]
[b]
Page No. – Page No.
Alumina as solid-state ligand in
enhancing the redox catalytic property
of iron oxide grafted AlSBA-15 towards
arylation of arene
Iron grafted over alumina containing SBA-15 (8wt%Fe/AlSBA-15) has been evolved as
highly stable, efficient and sustainable heterogeneous redox iron catalyst. In the
benzene arylation, it demonstrated high catalytic activity like the homogeneous iron
catalyst. Alumina played a critical role as solid-state ligand through its Lewis acid
property to enhance catalytic activity of 8wt%Fe/AlSBA-15.
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