Indium oxide nanoparticles embedded in TUD-1 as a highly selective catalyst for toluene to benzaldehyde oxidation using TBHP as oxidant

Article abstract of DOI:10.1007/s11696-020-01054-z

Abstract: C–H activation reaction is one of the most challenging reactions in the present scenario. The green synthesis of benzaldehyde from toluene using TBHP in a sustainable amount as an oxidant over indium-incorporated TUD-1 is performed in the presen

Full text of DOI:10.1007/s11696-020-01054-z

Chemical Papers
https://doi.org/10.1007/s11696-020-01054-z
ORIGINAL PAPER
Indium oxide nanoparticles embedded in TUD‑1 as a highly selective
catalyst for toluene to benzaldehyde oxidation using TBHP as oxidant
Prangya Paramita Das1 · Biswajit Chowdhury1
Received: 4 September 2019 / Accepted: 8 January 2020
© Institute of Chemistry, Slovak Academy of Sciences 2020
Abstract
C–H activation reaction is one of the most challenging reactions in the present scenario. The green synthesis of benzalde-
hyde from toluene using TBHP in a sustainable amount as an oxidant over indium-incorporated TUD-1 is performed in
the present study. In(1)-TUD-1 (In/Si=1/100 mol ratio) shows 48% toluene conversion and 83% benzaldehyde selectivity
using acetic acid as solvent. The catalyst was recyclable up to fve times. A plausible reaction mechanism is also proposed
based on the characterization results.
Graphic abstract
Keywords Benzaldehyde · Indium · Mesoporous silica · TBHP · Toluene oxidation
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-020-01054-z) contains
Introduction
supplementary material, which is available to authorized users.
* Prangya Paramita Das
The selective liquid phase oxidation of hydrocarbons to the
analogous oxygen-carrying compounds has the potential to
simplify the synthesis of complex molecules greatly and
has remained a most promising and challenging reaction in
prangyaparamita.das11@gmail.com
* Biswajit Chowdhury
biswajit_chem2003@yahoo.com
Extended author information available on the last page of the article
Vol.:(0123456789)
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Chemical Papers
both academic and industry circles (Luo et al. 2013). Selec-
tive oxidation of C–H bond is still an unsolved problem as
the intermediates such as alcohol and aldehydes are more
responsive to secondary oxidation to form carboxylic acid
which lowers the selectivity (Kantam et al. 2002). Oxygen-
ated products are the indispensable intermediates for agro-
chemicals, pharmaceuticals, fne chemicals, fragrances,
polymers and many chemical feedstocks (Saravanamurugan
et al. 2004; Rao et al. 2009). Many researchers have been
devoted to the design and synthesis of new reagents and
catalysts which can convert these unreactive C–H bonds
into C=O. Toluene has a great signifcance among various
hydrocarbons and which on oxidation produces benzyl alco-
hol, benzaldehyde and benzoic acid. Amidst the oxidized
products, benzaldehyde is the most prudent product but it is
very prone to oxidation to form benzoic acid. In most of the
oxidation reactions, higher oxidation state of metal reagents
like manganese, osmium and chromium are used as oxidants
in stoichiometric amounts (Visuvamithiran et al. 2013;
Brutchey et al. 2005). The above reagents produce huge vol-
umes of organic wastes which are environment unfriendly.
Transition metal catalysis has emerged as a powerful tool
for C–H activation, but the techniques are accompanied by
drawbacks such as low selectivity and use of large amount
of solvents and bromides (Zhao et al. 2004; Qian et al. 2005;
Maksimchuk et al. 2012). Supported Au–Pd nanoparticles
show quite a better result in this aspect (Saiman et al. 2012).
Many metals-incorporated metal oxides and zeolites have
also been used (Saravanamurugan et al. 2004). Despite nota-
ble recent eforts, the development of general and mild strat-
egies is very indispensable in the feld of C–H activation.
Silica materials have received enormous attention for
their distinctive physico-chemical properties like high ther-
mal stability, high specifc surface area, ready surface modi-
fcation, corrosion resistance and environment friendly (Lai
2013). It also enhances the stability of metal oxide nano-
particles. Mesoporous silica-supported metal catalysts like
Metal-TUD-1 (Co, Ti, Cr, Fe, Mn) (Anand et al. 2009; Wang
et al. 2018) catalyzed aerobic oxidation of cyclohexane using
radical initiators like TBHP and CHHP. Vanadium phos-
phate containing KIT-6 using H2O2 did not perform so well
(Rezaei et al. 2017). NDHPI on SBA-15 carrier was used as
a catalyst for toluene to benzaldehyde oxidation (Zhou et al.
2016). Indium-incorporated silica for C–H activation has not
been explored well till now.
We here for the frst time report the utilization of indium-
incorporated TUD-1 with two diferent indium loadings for
toluene to benzaldehyde using TBHP as single oxygen donor
under very mild reaction conditions. We have also studied
the efect of aging time and temperature on the catalytic
activity. The catalysts were characterized by N2 physisorp-
tion, HRTEM, UV–Vis, pyridine IR and O2 pulse chem-
isorption, EDX and elemental mapping techniques. Among
the indium-loaded catalysts In-TUD-1 (In=1 and 4 mol%),
In(1)-TUD-1 shows higher catalytic activity, i.e., 48% tolu-
ene conversion and 83% benzaldehyde selectivity using ace-
tic acid as solvent.
Experimental
Materials and methods
Catalyst synthesis procedure
Diferent indium-loaded In-TUD-1 catalysts were synthe-
sized by sol–gel procedure as reported in literature. Tetra-
ethyl orthosilicate (TEOS, 98%, Acros Organics) was added
to the aqueous solution of indium nitrate (99.9%, Sigma-
Aldrich) in deionized water with continuous stirring. Trieth-
anolamine (TEA, 99%, Acros Organics) was added dropwise
to the mixture. The entire mixture was stirred for 10 min
followed by the addition of tetraethylammonium hydrox-
ide (TEAOH, 20% aqueous solution), (Merck Germany).
The resulting gel composition of the mixture is TEOS:
In(NO3)3:TEA: H2O: TEAOH = 1: x: 2: 11: 1 (x = 0.01,
0.04). At room temperature, the complete mixture was
stirred for 24 h and the synthesized gel was dried at 110 °C
for 24 h in a static oven. In the end, the dried material was
calcined in a mufe furnace at 700 °C for 10 h with a tem-
perature ramp of 1 °C/min.
To study the efect of aging time and temperature on the
catalytic activity, we synthesized two other catalysts. The
frst one is by varying aging temperature, i.e., 80 °C for 24 h
and the latter one is by varying time of 14 h at 110 °C. In
both the cases, indium loading of 1 mol% was kept con-
stant and the catalysts are labeled as In(1)-TUD-1-80C and
In(1)-TUD-1-14H, respectively. The details of the synthe-
sized procedures are depicted in supporting information as
(catalyst preparation).
Our primary clear need is the development of an environ-
mentally benign catalyst which can produce benzaldehyde
selectively from toluene under very mild reaction conditions.
Instead of choosing any transition metal, we have chosen
post-transition metal like indium. Indium salts are very sta-
ble to water and air, less toxic, more abundant and cheaper
compared to transition metals, easy to handle and also very
well known for its Lewis acidity in its +3 oxidation state.
Catalyst characterizations
The nitrogen adsorption–desorption isotherms of the
prepared catalysts were measured at liquid nitrogen tem-
perature at − 196 °C with a Quantachrome NOVA 3200,
USA. Pretreatment of the samples was done at 200 °C for
3 h under high vacuum. The surface area was calculated
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by Brunauer–Emmett–Teller (BET) equation and pore size
distribution was calculated by the BJH method. The HRTEM
image of the prepared catalysts was obtained on JEOL JEM
2100 microscope (USA) operated at 200 kV acceleration
voltage using lacey carbon-coated Cu grid of 300 mesh size.
The UV–Visible (UV–Vis) was performed in Varian Cary
500 (Shimadzu) spectrophotometer in the wavelength range
of 200–800 nm. The pulse chemisorption was performed
using 5% oxygen in helium for 2 h during pretreatment fol-
lowed by fushing at 200 °C. During analysis, 5% O2/He
was dosed by 2750 loop of volume 0.5 cm3. The chemisorp-
tion was performed at room temperature. FTIR analysis of
the samples was performed using Agilent Cary 600 series
with praying mantis setup. Before pyridine treatment, the
catalysts were dried at 100 °C in a hot air oven. The cata-
lysts (50 mg each) were taken in sample cups and pyridine
(0.1 cc) was added to it. To remove the extra or physisorbed
pyridine, the sample cups were kept in a hot air oven at
100 °C. The samples were used at room temperature and IR
spectra were taken in the spectral range of 1700–1400 cm−1
with 64 scan and at a resolution of 8 cm−1 using KBr back-
ground. EDX and elemental mapping were carried out by
S-3400N model Hitachi instrument.
Fig. 1 UV-Vis spectra of a In(1)-TUD-1 and b In(4)-TUD-1
Catalytic activity studies
In a two-necked round-bottom fask, toluene (8 mmol) and
70% t-butyl hydroperoxide (20 mmol) and 10 ml of ace-
tic acid as solvent were added. The mixture was shaken in
molecular level followed by stirring in a thermostated oil
bath maintained at 80 °C and 100 mg of catalyst was added
to it. Before performing the reaction, the catalyst was dried
for 3 h at 100 °C in a hot air oven. The round-bottomed fask
was ftted with a condenser. Aliquots were taken at regular
intervals of time to check the progress of the reaction. After
completion of the reaction, the catalyst was separated by fl-
tration and the reaction mixture was extracted with ethyl ace-
tate and dried with anhydrous Na2SO4. The products were
analyzed with a gas chromatograph (Agilent 7890B) ftted
with a FFAP capillary column (30 m × 32 mm × 0.25 µm)
and an FID detector. Temperature used was from 120 °C
(2 min) to 230 °C at a heating rate of 20 °C/min.
Fig. 2 Infrared spectra of pyridine adsorbed at 298 K on fresh a In(1)-
TUD-1and, b In(4)-TUD-1. L Lewis acid site, B Bronsted acid site,
L+B Lewis+Bronsted acid site
higher surface area and larger pore diameter, better disper-
sion of metal nanoparticles over the surface of TUD-1, more
reducible In2O3, more oxygen uptake and lower surface acid-
ity compared to In(4)-TUD-1 (In=4 mol%).
The UV–Vis spectra of the synthesized catalysts have
been represented in Fig. 1. The absorption band at 250 nm
instead of at 330 nm indicates the presence of In2O3 nano-
particles which may be due to the weak quantum confne-
ment efect. With increasing indium loading, the peak is
intensifed and shifted to a higher wavelength (Kumar et al.
2016).
Results and discussion
Characterization results
BET surface area and porosity measurement, XRD,
HRTEM, EDX and STEM, FTIR, NH3-TPD, H2-TPR and
O2-TPO techniques have been depicted thoroughly in our
previous paper (Rahman et al. 2015). The characterization
techniques revealed that In(1)-TUD-1 (In=1 mol%) shows
Figure 2 depicts the pyridine-adsorbed FTIR spectra of
the fresh samples at 298 K. The absorption peaks at 1640,
1550 cm−1 correspond to the pyridine adsorbed in Bron-
sted acid sites and peaks at 1596, 1445 cm−1 correspond to
pyridine adsorbed in Lewis acid sites. Peak at 1485 infers to
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pyridine adsorbed in both the Bronsted and Lewis acid sites.
Bronsted acid sites are more populated for In(4)-TUD-1
(Reddy et al. 2009). The percentage of acidic and basic sites
is depicted in Table S1.
EDX result depicted in Table S3 reveals the presence of
a little more amount of indium in In(1)-TUD-1-80C than
In(1)-TUD-1-14H. Such a small increase in indium has been
found to have no such profound efect on catalytic activity.
O2 pulse chemisorption study depicted in Table 1 dis-
closes the dispersion of indium oxide nanoparticles over
TUD-1 matrix. The lower dispersion may be due to stronger
metal and support interaction. Better dispersion was found
for lower indium-loaded catalyst compared to higher indium-
loaded catalyst because in case of higher metal-loaded cata-
lyst, agglomeration of particles and larger crystallite size
was also supported by XRD and HRTEM studies.
Catalytic activity and discussion
Initially, oxidation of toluene was conducted using molecu-
lar oxygen, but no reaction occured. The reaction was also
carried out using the silica support TUD-1 and 70% TBHP
in water but a small amount of product was obtained. Again,
the reaction was conducted using indium-incorporated
TUD-1 silica using 70% TBHP in water as oxidant and it
showed quite an appreciable result. The catalytic activity
was optimized by varying oxidant, metal loading, tempera-
ture, time, toluene and TBHP ratio, solvent and represented
in Tables 2, 3, 4, 5, 6 and 7, respectively.
In(1)-TUD-1-80C and In(1)-TUD-1-14H were character-
ized by N2 physisorption study, HRTEM, elemental mapping
and EDX. N2 physisorption results are depicted in Table S2
and isotherms as Fig. S1, HRTEM, SAED and elemental
mapping are depicted as Figs. S2, S3 and S4, respectively
(Supporting Information). It was observed that In(1)-TUD-
1-14H shows better catalytic activity both in terms of tolu-
ene conversion and benzaldehyde selectivity compared to
In(1)-TUD-1-80C and such enhanced catalytic behavior
of In(1)-TUD-1-14H can be attributed to its larger surface
area and pore diameter. HRTEM micrograph confrms the
sponge-like morphology and SAED pattern confrms the
amorphous nature of silica. Agglomeration of indium oxide
nanoparticles and lower dispersion of indium on silica were
observed from HRTEM and elemental mapping, respec-
tively, which might be the reason behind inferior catalytic
activity of In(1)-TUD-1-80C compared to In(1)-TUD-1-14H.
Table 2 delineates the efect of oxidants on toluene oxi-
dation. Oxidation of toluene was conducted using molecu-
lar oxygen, but no reaction was observed. We performed
the reactions using both TBHP in decane (5.5 M) and 70%
TBHP in water as oxidant and found that in case of both
the oxidants, by increasing the metal loading, the catalytic
activity increased a little in terms of conversion along with
an appreciable decrease in benzaldehyde selectivity. So, we
performed the other reactions using 70% TBHP in water as
oxidant.
The catalytic activity studies varying the indium loading
are described in Table 3. We have performed the reactions
using both 70% TBHP in water as oxidant. By increasing
the metal loading, the catalytic activity increased a little in
terms of conversion along with an appreciable decrease in
benzadehyde selectivity. In(4)-TUD-1 exhibits higher sur-
face acidity than In(1)-TUD-1 found from NH3-TPD and
more Bronsted acid sites were observed from pyridine IR.
(Martin et al. 1999) also explained very precisely that both
Bronsted and Lewis acid sites are responsible for toluene
chemisorption and increase in basicity of the catalyst surface
by blocking the Bronsted acid site enhances the desorption
rate of aldehyde. Bronsted acid sites are very necessary for
Table 1 O2 pulse chemisorption study of diferent indium-loaded
TUD-1 catalysts
Catalyst
O2 uptake Dispersion
(μmol/g) (%)
Surface area Average
of indium
crystallite
(m2/g)
diameter (nm)
In(1)-TUD-1 1.02
In(4)-TUD-1 0.98
1.99
0.62
7.94
2.4
103.3
342
Indium amount was found from EDX described in our previous paper
(Rahman et al. 2015)
Table 2 Toluene oxidation activity using diferent oxidants
Catalyst
Oxidants
Toluene conver- Selectivity (%)
sion (%)
Benzyl alcohol
Benzaldehyde
Benzoic acid
–
Others
In(1)-TUD-1
In(1)-TUD-1
In(1)-TUD-1
In(4)-TUD-1
In(4)-TUD-1
O2
–
–
–
–
70% TBHP in water
TBHP in decane (5.5 M)
70% TBHP in water
TBHP in decane (5.5 M)
42
45
47
51
20.6
89.07
31.3
93.6
78.2
3.5
64.5
2.8
<2
7.4
3.2
<1
<1
Reaction conditions: toluene: 8 mmol, TBHP: 20 mmol, time: 5 h, temperature: 80 °C
Reaction performance in optimum condition are shown in bold
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Table 3 Toluene oxidation
activity using diferent indium-
loaded TUD-1
Catalyst
Indium load- Toluene
ing in (mol%) conversion
(%)
Selectivity (%)
Benzyl alcohol Benzaldehyde Benzoic acid Others
–
–
–
1
1
4
7
8
92.8
92.2
98.5
20.6
31.3
7.1
7.7
TUD-1
In(1)-TUD-1
In(1)-TUD-1
In(4)-TUD-1
2a
<1
42
47
78.2/0.53b
<2
<1
64.5/0.22b
Reaction conditions: toluene: 8 mmol, TBHP: 20 mmol, catalyst: 100 mg, temperature: 80 °C
Results in optimum conditions are represented in bold
aReaction in the absence of TBHP
bTOF, TOF=Moles of toluene converted/moles of indium present per hour. Indium amount was found
from EDX described in our previous paper (Rahman et al. 2015)
Table 4 Efect of reaction
temperature on selective
oxidation of toluene
Catalyst
Tempera- Toluene con- Selectivity (%)
ture (°C)
version (%)
Benzyl alcohol
Benzaldehyde
Benzoic acid
–
Others
In(1) TUD-1
In(1) TUD-1
In(1) TUD-1
30
80
100
29
42
45
32.6
20.6
17.1
67.2
78.2
62
–
<2
–
20.7
Reaction conditions: toluene: 8 mmol, TBHP: 20 mmol, catalyst: 100 mg, time: 5 h
Reaction performance in optimum condition are shown in bold
Table 5 Efect of reaction time
Catalyst
Time (hours) Toluene con- Selectivity (%)
version (%)
on selective oxidation of toluene
Benzyl alcohol Benzaldehyde Benzoic acid Others
In(1) TUD-1
In(1) TUD-1
In(1) TUD-1
In(1) TUD-1
In(1) TUD-1
2
5
6
12
26
21
42
51
58
59
21
79
–
–
<2
<1
20.6
17.2
9.1
78.2
76.5
73
6.1
17.6
31.9
4.5
63.1
Reaction conditions: toluene: 8 mmol, TBHP: 20 mmol, catalyst: 100 mg, solvent: 10 ml, time: 5 h, tem-
perature: 80 °C
Reaction performance in optimum condition are shown in bold
Table 6 Efect of toluene:
TBHP molar ration on toluene
oxidation
Catalyst
Toluene:
TBHP molar
ratio
Toluene
conversion
(%)
Selectivity (%)
Benzyl alcohol Benzaldehyde Benzoic acid Others
In(1)-TUD-1 1:1
In(1)-TUD-1 1:1.25
In(1)-TUD-1 1:2.5
In(1)-TUD-1 1:4
36
38
42
45
25.3
22.6
20.6
12.1
73
1.2
2
75.3
78.2
65.8
<1
<2
<1
21
Reaction conditions: time: 5 h, temperature: 80 °C, catalyst: 100 mg
Reaction performance in optimum condition are shown in bold
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Table 7 Efect of diferent
Catalyst
Solvent
–
Ethyl acetate 43
Acetonitrile
Toluene con- Selectivity (%)
version (%)
solvents on toluene oxidation
Benzyl alcohol Benzaldehyde Benzoic acid Others
In(1)TUD-1
In(1)TUD-1
In(1)TUD-1
In(1)TUD-1 Acetic acid
In(1)TUD-1 Ethanol
42
20.6
16.5
13.2
14.5
18.2
78.2
74.5
79.8
83.2
81.5
<2
8.2
6.5
45
48
47
<2
Reaction conditions: toluene: 8 mmol, TBHP: 20 mmol, catalyst: 100 mg, solvent: 10 ml, time: 5 h, tem-
perature: 80 °C
Reaction performance in optimum condition are shown in bold
toluene conversion but more Bronsted acid sites are associ-
ated with more coke deposition and catalyst deactivation
(Mendez-Roman and Cardona-Martinez 1998; Bulushev
et al. 2004). At higher metal loading of 4 mol%, toluene
conversion should be profoundly high because of its higher
acidity. As the catalyst in lower indium loading exhibits less
surface acidity, so, it should show lower toluene conversion
(Mal et al. 2018). Although the acidity of In(4)-TUD-1 is
high, no appreciable change in toluene conversion may be
due to poor metal dispersion compared to In(1)-TUD-1 was
observed from XRD, HRTEM, H2-TPR and O2 pulse chem-
isorption study as well as lower surface area observed from
N2 physisorption. The decrease in benzaldehyde selectivity
can be explained by the more populated Bronsted acid site
which leads to deactivation of the catalyst.
Table 4 describes the efect of temperature on toluene
oxidation. We started the reaction at 80 °C following the lit-
erature. By increasing the temperature from room tempera-
ture (30 °C) up to 80 °C, both the conversion and selectivity
increased, but above 80 °C, toluene conversion increased
but benzaldehyde selectivity decreased. Higher temperature
favors total oxidation instead of partial oxidation.
Fig. 3 Efect of reaction time on toluene conversion using In(1)-
TUD-1
To check the progress of the reaction, aliquots were taken
at some regular intervals of time and the catalytic activities
are depicted in Table 5. With the increase of time from 2 to
12 h, gradual increase in toluene conversion and variation
in product selectivity were observed. For prolonged reac-
tion time of 26 h, benzoic acid formation increased sub-
stantially meanwhile benzyl alcohol selectivity decreased.
This implies that with time, benzyl alcohol is getting further
oxidized to benzoic acid which is a very stable product. Fig-
ure 3 depicts the efect of time on toluene conversion.
Table 6 delineates the impact of toluene:TBHP ratio on
toluene oxidation. The molar ratio of toluene: TBHP var-
ied as (1:1, 1:1.25, 1:2.5 and 1:4) and we observed a better
result in 1:2.5 molar ratio which is 48% toluene conversion
and 83% benzaldehyde selectivity. No benzoic acid was
found, but a small amount of o-cresol was observed. A small
increase in toluene conversion and appreciable decline in
benzaldehyde selectivity were observed with toluene:TBHP
(1:4) ratio. In case of toluene:TBHP ratio of 1:1 and 1:2.5,
the amount of TBHP may not be sufcient for converting
benzyl alcohol to benzaldehyde for which a higher amount
of benzyl alcohol was found compared to benzaldehyde and
no acetic acid was found. The higher amount of TBHP in
the ratio of toluene: TBHP 1:4 may result in total oxidation
which produced benzoic acid by total oxidation instead of
partial oxidation as a result of which decrease in benzalde-
hyde selectivity was observed.
Again, the target of achieving optimum catalytic activ-
ity led to the use of solvents like acetic acid, acetonitrile,
ethyl acetate and ethanol. We found the optimum catalytic
activity, i.e., 48% toluene conversion and 83% benzaldehyde
in the case of acetic acid. Acidity of the solvents is in the
order acetic acid>ethanol>acetonitrile>ethyl acetate. The
solvent acidity has great signifcance in oxyfunctionaliza-
tion and with increasing solvent acidity, both the toluene
conversion and benzaldehyde selectivity increased in our
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Table 8 Catalyst regeneration and substrate variation
Catalyst
Substrate
Conversion Major product
(%)
selectivity (%)
In(1)-TUD-1
In(1)-TUD-1
In(1)-TUD-1
In(1)-TUD-1
In(1)-TUD-1
In(1)-TUD-1
In(1)-TUD-1
In(1)-TUD-1
In(1)-TUD-1
Toluene
48
47
44
40
36
28
8
83.2g
84.2g
83.3g
80.9g
80.5g
78.6g
63.5g
88h
Toluenea
Tolueneb
Toluenec
Toluened
Toluenee
Toluenef
Ethylbenzene
Cyclohexane
54
21
67i
Reaction conditions: reactant=toluene 8 mmol; TBHP=20 mmol;
catalyst=0.1 g; acetic acid: 10 ml, temperature=80 °C; time=5 h
Reaction performance in optimum condition are shown in bold
Fig. 4 N2 physisorption isotherm and pore size distribution curve of
In(1)-TUD-1 catalyst after ffth regeneration
aFirst recycle
bSecond recycle
cThird recycle
dFourth cycle
eFifth cycle
Table 9 N2 physisorption results of fresh and sixth cycle catalyst
Catalyst
Surface area
(m2/g)
Pore volume
(cc/g)
Pore
fSixth cycle
diameter
(nm)
gBenzaldehyde
hAcetophenone
iCyclohexanone
In(1)-TUD-1
In(1)-TUD-1
688.7a
500
1.094a
0.048
8.30a
6.36
aBET results reported by Rahman et al. 2015
study. The efect of the solvent acidity on catalytic reac-
tion is not clear (Bauer 2017; Velu et al. 1999; Zhang et al.
2005). Efect of solvent on toluene oxidation is represented
in Table 7.
Table 10 EDX results of ffth and sixth cycle In(1)-TUD-1 catalyst
Sample
O (at %) Si (at %) In (at %) Total (at %)
In(1)-TUD-1 (ffth
83
16.53
14.23
0.04
100
100
Catalyst regeneration and substrate variation
cycle)
In(1)-TUD-1 (sixth
85.73
0.004
To commercialize the catalyst, catalyst recyclability is one
of the essential parameters. Catalytic activities of the cata-
lysts did not vary to a greater extent up to fve cycles as
represented in Table 8. The gradual decrease in conversion
up to ffth cycle can be due to the leaching of metal and
loss of catalyst during fltration and drying. After the ffth
cycle, a sudden decline in catalytic activity was observed.
To investigate the reason behind the deactivation of cata-
lyst, the sixth cycle used catalyst was characterized by N2
physisorption and EDX and represented as shown in Fig. 4,
Tables 9 and 10, respectively. N2 physisorption result reveals
that the mesopore structure is still retained and decrease in
surface area, pore volume and pore diameter may be due to
the blocking of surface sites or pores of the catalyst by the
reactant molecules. Again, to clear the presence of indium,
EDX was performed. From the EDX results of ffth (indium
0.4 at %) and sixth cycle catalyst (indium 0.00 at 4%), we
observed than an acute change in indium amount. So, we
may conclude that the catalyst is stable and leaching of
cycle)
indium which is the active component loss may be the rea-
son behind deactivation of the catalyst. The catalyst also
induced oxyfunctionalization of ethylbenzene and cyclohex-
ane. Acetophenone and cyclohexanone were observed to be
the major products of ethylbenzene and cyclohexane oxida-
tion, respectively.
Proposed reaction mechanism
Metal nanoparticles can decompose oxidants like TBHP to
form .OH and .OOH radicals (Anand et al. 2006). Early tran-
sition metals can accelerate liquid phase oxidations in their
highest oxidation state, like Ti4+ or V5+, activate peroxides
for their Lewis acidity (Corma et al. 1995; Shylesh and Singh
2006). Indium in its (III) oxidation state is also well known
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Scheme 1 A plausible mecha-
nism for selective oxidation
of toluene over indium-loaded
TUD-1 silica (Rao et al. 2009)
for its Lewis acidity (Gandara et al. 2008). Herein, TBHP
successfully performed as an oxidant in our reaction, but
in the absence of metal, no appreciable amount of products
formed. TBHP undergoes radical chemistry mostly. In fact,
when we added a radical scavenger like quinhydrone, the
reaction stopped immediately. This suggests that In-TUD-1
catalyzed reaction not surprisingly proceeds through radi-
cal intermediates or via radical chain mechanism. It is also
worth mentioning that the reaction also produces t-BuOH
as byproduct.
Table 11 provides a comparison of the results achieved
from our present catalyst with the other reported catalysts
(Guo et al. 2005; Wang et al. 2005; Li et al. 2006). Our
present catalyst exhibits a very comparable conversion and
selectivity at moderate temperature with a shorter reaction
time.
Conclusion
Toluene reacting with TBHP over In-TUD-1 proceeds
with the generation of tertiary butyl peroxy (t-BuOO.) and
tertiary butoxy (t-BuO.) radicals. In the frst step, coordina-
tion of TBHP with In (III) occured. The tert-butyloxy radical
reacts with toluene to form the benzyl radical. The formation
of benzyl alcohol (I) proceeds via the formation of benzyl
cation by the transfer of electron from benzyl radical to the
catalyst. The benzyl cation reacts with hydroxyl anion to
produce benzyl alcohol. The reduced catalyst is oxidized by
TBHP. The benzyl radical combines with tert-butyl peroxy
radical to produce benzaldehyde. It may proceed through
two steps. As benzyl alcohol is hydrophilic, the presence
of water near the active site allows subsequent oxidation to
benzaldehyde. Benzyl alcohol rearranged to form benzalde-
hyde, whether by dehydration or by abstraction of hydrogen
followed by rearrangement to benzyldehyde (Scheme 1).
Selective oxidation of toluene to benzaldehyde over a
variety of heterogeneous catalysts has been well studied.
In-TUD-1 catalysts of diferent loading (1 and 4 mol%)
were prepared by sol–gel method. In(1)-TUD-1 shows bet-
ter selective oxidation of toluene to benzaldehyde using
acetic acid as solvent. Elevated surface acidity of the cata-
lyst, solvent acidity and better distribution of indium oxide
nanoparticles over silica are the key enablers for better
catalytic activity. Bronsted acid site is a must for toluene
conversion, but an optimum Bronsted/Lewis acid site is
required for selective benzaldehyde production. Interest-
ingly, 48% toluene conversion was found in case of In(1)-
TUD-1 with 83% selectivity for benzaldehyde. The present
catalyst has several advantages like environment friendly,
mild reaction condition, economic, simple work-up and
recyclability of the catalyst up to ffth cycle.
Acknowledgements PPD and BC gratefully acknowledge IIT (ISM),
Dhanbad for providing funding for carrying out the research.
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Chemical Papers
Table 11 A comparison table of toluene oxidation with other reported catalysts
Catalyst
Temperature/pressure
Oxidant
Conversion (%)
Selectivity (%)
References
Present catalyst
Co-SBA-15
Co-(II)TPP
MPAV2/Nb2O5
V-Mo-Fe-O
Cu–Fe/TiO2
80 °C/1 atm
80 °C/1 atm
150 °C/0.8 MPa
RT/1 atm
80 °C/1 atm
190 °C/1 MPa
190 °C/1 MPa
110 °C/10 kg/cm2
30 °C/1 atm
80 °C/1 atm
80 °C/1 atm
TBHP
TBHP
O2
TBHP
H2O2
O2
O2
O2
TBHP
TBHP
H2O2
41
7.97
8.9
22
40.3
1.8
17.2
38.31
22a
4.4
59.5
77
63.8
33
76
84.5
27
20
1.01
_
32
90
Present work
Brutchey et al. 2005
Guo et al. 2005
Rao et al. 2009
Reddy et al. 2009
Wang et al. 2005
Li et al. 2006
Kantam et al. 2002
Saravanamurugan et al. 2004
Saiman et al. 2012
Bulushev et al. 2004
Cu–Mn (1:1)
C0/Mn/Br−
Sil-Si-Ti(OiPr)
1%Au–Pd/TiO2
MnWO4 nanobars
aTBHP conversion
Lai CY (2013) Mesoporous silica nanomaterials applications in
catalysis. J Thermodyn Catal 5:1. https://doi.org/10.4172/2157-
7544.1000e124
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Afliations
Prangya Paramita Das1 · Biswajit Chowdhury1
1
Department of Chemistry, Indian Institute of Technology
(Indian School of Mines), Dhanbad, India
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