A novel iron(iii)-based heterogeneous catalyst for aqueous oxidation of alcohols using molecular oxygen

Article abstract of DOI:10.1039/c5ra16734e

A novel silica supported iron(iii)-based heterogeneous catalyst (FeCl₃-imine@SiO₂) has been synthesized and characterized by several techniques, such as FTIR, nitrogen adsorption analysis (BET), SEM-EDX, XRD, EPR, AAS and XPS analyses. The catalyst showed excellent activity for selective oxidation of alcohols to corresponding carbonyl compounds in water under atmospheric pressure of oxygen without using any additive. Notably, the heterogeneous catalyst showed remarkable improvement over its homogeneous counterpart, which might be attributable to the synergic effect between the metal and the supported ligand. The catalyst could be easily recovered by simple filtration and reused several times without compromising its activity. A wide range of primary and secondary alcohols including heterocyclic alcohols are tolerated as substrates.

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A novel iron(III)-based heterogeneous catalyst for aqueous oxidation of
alcohols using molecular oxygen
Debojeet Sahua, Ana Rosa Silvab and Pankaj Dasa*
A novel silica supported iron(III)ꢀbased heterogeneous catalyst (FeCl3ꢀimine@SiO2) has
been synthesized and characterized by several techniques, such as FTIR, Nitrogen adsorption
analysis (BET), SEMꢀEDX, XRD, EPR, AAS and XPS analyses. The catalyst showed excellent
activity for selective oxidation of alcohols to corresponding carbonyl compounds in water under
atmospheric pressure of oxygen without using any additive. Notably, the heterogeneous catalyst
showed remarkable improvement over its homogeneous counterpart, might be attributable to the
synergic effect between the metal and the supported ligand. The catalyst could be easily
recovered by simple filtration and reused several times without compromising with its activity. A
wide range of primary and secondary alcohols including heterocyclic alcohols are tolerated as
substrates.
Key words: heterogeneous catalyst; iron(III); alcohol oxidation; molecular oxygen; water
Introduction
Selective oxidation of alcohols to corresponding carbonyl compounds is one of the most
industrially significant reactions in organic synthesis. The use of transition metalꢀbased
homogeneous1 and heterogeneous2 catalysts are wellꢀprecedented for this reaction mainly
because of their ability to replace conventional oxidation methods of using stoichiometric
oxidants such as chromate or permanganate with molecular oxygen. From both economic and
green chemistry perspectives, molecular oxygen is an ideal oxidant as it is readily available and
produces water as the only byꢀproduct.1,3 Unfortunately, majority of the reported protocols for
efficient oxidation of alcohols using molecular oxygen require noble metals such as palladium,4
__________________________________________________________________________
aDepartment of Chemistry, Dibrugarh University, Dibrugarh, Assam, India. Fax: +91-373-
2370323; Tel: +91-373-2370210; E-mail: pankajd29@yahoo.com
.
bCICECO-Aveiro Institute of Materials, University of Aveiro, Aveiro, Portugal.
†Electronic Supplementary Information (ESI) available: Textural data of BET studies, FTIR
spectra, XPS survey spectra, reusability diagram and GCꢀMS spectra of the carbonyl products.
1

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rhodium,5 ruthenium,6 gold,7 platinum,8 etc. Apart from their costs, these noble metals also have
serious toxicity issues which make these catalytic systems impractical for industrial use.1d
Hence, over the years, considerable efforts have been made towards replacement of these
expensive precious metal catalysts with cheaper base metals like copper or iron.1a-c Although,
copperꢀbased systems already displayed tremendous potential to act as an alternate to platinum
metal catalysts in oxidation of alcohols,9 ironꢀbased systems remain relatively underexplored. It
is fair to say that from sustainability point of view iron is in much more advantageous position
over copper due to its to its lower price, and high environmental acceptability.10 It is surprising to
note that the first ironꢀcatalyzed oxidation of alcohols was reported only in 2002,11 while copperꢀ
catalyzed alcohol oxidation was first reported in 1966.12 Since the first report, ironꢀcatalyzed
systems have been increasingly investigated in alcohol oxidation studies.13,14. Typical examples
are Wang’s FeCl3ꢀTEMPOꢀNaNO2 system,13a Repo’s FeCl3ꢀthyamine system,13b Liu’s Fe(NO3)ꢀ
TEMPOꢀNaCl system,13c,13d etc. However, majority of these successful systems are
homogeneous in nature where separation and recovery of the catalyst is extremely difficult
which pose significant limitations to their industrial scale applications. Thus, in recent years,
strategies have been adopted to immobilize iron(III) salts/compounds into/onto different supports
such as aluminosilicates,14b carbon,14c mesoporous silica,14i grapheneꢀoxide14g, polymer,14e etc.
and applied in alcohol oxidation reaction. Although notable progress was achieved in some cases
with respect to development of stable and reusable catalysts of high efficiency, such catalysts
usually showed lower activities compared to their homogeneous counterparts. Moreover, except
one report of ironꢀoxide nanoparticles as catalyst,14b in all the cases the reactions were performed
in organic solvents that are not only expensive but also toxic in nature.
In recent years, because of economical and environmental issues, organic reactions in
water have gained much attention. However, to the best of our knowledge, there is no report on
the use of ironꢀbased heterogeneous catalysts for alcohol oxidation reaction in water with
molecular oxygen as terminal oxidant. In this work, we disclose a simple procedure for the
preparation of a new iron(III)ꢀbased heterogeneous catalyst supported on silica and its
application for alcohol oxidation reaction with molecular oxygen in neat water.
Experimental section
General information
2

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3ꢀAminopropylꢀfunctionalized silica gel (APTES@SiO2: 1 mmol/g of silica gel; 200–400 mesh)
was purchased from SigmaꢀAldrich. Acetamide and anhydrous FeCl3 was purchased from
Thermo Fisher scientific. The substrates used in the alcohol oxidation reactions were purchased
from SigmaꢀAldrich, Spectrochem and Rankem. All other chemicals and solvents were
purchased from different Indian firms. The solvents were distilled and dried prior to use.
The Fourier transform infrared (FTIR) spectra (KBr; 1800ꢀ1400 cmꢀ1) of the materials were
recorded on a Shimadzu IR spectrophotometer (prestigeꢀ21). Xꢀray powder diffraction (XRD)
was performed with a Bruker diffractometer (model AXS D8) with CuꢀKa (1.541 °A) radiation.
The scanning electron microscope (SEM) images were obtained from JEOL Model JSM ꢀ
6390LV and energy dispersive Xꢀray (EDX) peaks were recorded in JEOL Model JED – 2300.
The iron content was determined by atomic absorption spectroscopy (AAS) using a Thermo
Scientific spectro photometer (IEC 3000). Electron spin resonance (ESR) spectra was recorded
on a JEOL instrument (JESꢀFA200) at liquid N2 temperature. Nitrogen adsorption studies were
carried out at 77 K using a Quantachrome, ASIQA 2010 analyzer. Prior to analysis the samples
o
were degassed by heating at 200 C for 4 h under vacuum. Specific surface area was calculated
following the Brunauer–Emmett–Teller (BET) calculation and poreꢀsize distribution was
obtained by using the Barrett–Joyner–Halenda (BJH) method. Xꢀray photoelectron spectroscopy
(XPS) was performed at ‘‘Centro de Materiais da Universidade do Porto’’ (Portugal), in
a Kratos Axis Ultra HSA spectrometer using a nonꢀmonochromatized Mg Ka radiation (1253.6
eV). Gas chromatography–mass spectrometry (GC–MS) studies were done using Agilent 7820A
GC system with Agilent 5975 Series MSD Mass detector equipped with a 30 m × 0.25 mm × 0.5
µm HPꢀ5MS capillary column.
Synthesis of the silica based materials:
Synthesis of imine-functionalized silica gel: imine@SiO2
In a suspension of 3 g of APTES@SiO2 in 40 mL ethanol, 3 mmol of acetamide was added in
small portions under continuous stirring. The solution was then stirred under refluxing condition
for 6 hrs. The solids were separated from the solvent by filtration and washed repeatedly through
Soxhlet extraction with ethanol and acetone. The resulting acetamide grafted silica was dried at
120 oC for 24 h and designated as imine@SiO2.
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Immobilization of FeCl3 onto imine@SiO2: FeCl3-imine@SiO2
2.5 g of imine@SiO2 was added to a solution of 2.5 mmol of anhydrous FeCl3 in 40 mL of
o
acetonitrile and the reaction mixture was stirred at 60 C for 8 h. The resulting solution was
filtered off and the brown solids were washed several times through Soxhlet extraction with
acetonitrile (until the extracted solvent became colorless). The material was designated as FeCl3ꢀ
imine@SiO2.
General procedure for the alcohol oxidation reactions
A 50 mL two necked round bottomed flask fitted with a condenser and gas bubbler was charged
with alcohol (1 mmol), catalyst (46 mg, 2 mol% Fe) and water (6 mL). Then O2 (1 atm.) was
bubbled into the mixture for the required time and the reaction mixture was heated at 80 oC with
continuous stirring. On completion of the reaction, the catalyst was separated from the reaction
mixture by simple filtration. The filtrate was extracted with ether and dried over Na2SO4. After
evaporation of the solvent under reduced pressure the concentrated organic product is then
extracted with ether and analyzed by GCꢀMS. The product was isolated by column
chromatography technique using ethyl acetate/hexane (1:9) as eluent
Results and discussion
Synthesis and characterization of the catalyst
The catalyst, FeCl3ꢀimine@SiO2, has been prepared in two steps (Scheme 1). Firstly,
commercially available 3ꢀaminopropyltriethoxysilane functionalized silica gel (APTES@SiO2)
underwent a condensation reaction with acetamide to get an imineꢀfunctionalized silica gel
(imine@SiO2) which subsequently treated with anhydrous FeCl3 to get the ironꢀcatalyst. The
FTIR spectra (Fig. S1, ESI) of the silicaꢀbased material are in good agreement with the proposed
coordination environment. In particular, the bidentate bonding mode of the supported ligand in
FeCl3ꢀimine@SiO2 was confirmed by the shift of υCH=N band from 1646 cmꢀ1 to 1630 cmꢀ1 and
υNH band from 1531 cmꢀ1 to 1511 cmꢀ1. The nitrogen adsorptionꢀdesorption isotherms of the
2
silicaꢀbased materials displayed typical type IV patterns with a sharp hysteresis loop, which is
characteristics of mesoporous materials (Fig. 1). The surface area and pore size distribution of
the support (APTES@SiO2) studies reveals a high specific surface area of about 416 m2/g with
specific pore volume of 0.65 cm3/g (Table S1, ESI). These values upon metal immobilization
4

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(FeCl3ꢀimine@SiO2) decreases significantly to 283 m2/g and 0.43 cm3/g which is consistent with
successful immobilization of iron onto silica matrix.
The XRD patterns (Fig. 2) of the imine@SiO2 material exhibit a broad peak centered at
2θ = 22.480 for silica which upon immobilization of FeCl3 does not show any prominent shift (2θ
= 22.130), except a slight decrease in intensity. No crystalline phases related to metallic iron or
iron oxides were observed in the spectra of FeCl3ꢀimine@SiO2. However, a prominent peak at 2θ
= 35.90 was observed which matched well with the XRD patterns of FeCl3 or FeCl3 doped
polymer,15 suggesting the presence of FeCl3 crystallites within silica matrix. The SEM image of
the fresh catalyst material shows silica particles are of different dimensions (Fig. 3a). The
existence of iron in the catalyst was further confirmed by SEMꢀEDX analysis. The ESR spectra
of FeCl3ꢀimine@SiO2 displays six lines (Fig. 4) with an average g value of 2.01, typical for a
Fe+3 high spin (I = 5/2) system with hyperfine splitting. The iron content determined by AAS
analysis was found to be 0.44 mmol gꢀ1.
The XPS survey spectrum (Fig. S2, ESI) of the ironꢀbased material clearly shows the
presence of iron, chloride and nitrogen, besides of carbon, silica and oxygen. In the N 1s region
(Fig. 5a) the high resolution spectrum exhibits two peaks at 398.0 eV and 400.2 attributable to
amine and imine group respectively.16 The Fe(2p) band(Fig. 5b) splits into two peaks, 2p3/2 and
2p5/2, with corresponding binding energies at 711.1 eV and 724.8 eV respectively and the values
are very much close to the Fe2p binding energies of reported iron(III) compounds.14c,16
Catalytic activity
The catalytic activity of the material FeCl3ꢀimine@SiO2 was investigated for alcohol oxidation
reaction using water as solvent. Initial experiments for determining comparative efficacy and
optimization study (e.g. temperature, catalyst quantity) was performed by considering oxidation
of benzyl alcohol as model reaction (Table 1). Initially, oxidation reaction was performed under
aerobic condition by bubbling air into the reaction mixture at room temperature. After 24 h only
42% benzaldehyde formation was detected by GCꢀMS (entry 1). However, when the same
reaction was performed by bubbling molecular O2 (1 atm.) for 8 h moderate improvement in
o
benzaldehyde formation was noticed (entry 2). On increasing temperature to 80 C, significant
improvement in the product yield was observed (entry 4). Interestingly, under similar
experimental condition, homogeneous FeCl3 as catalyst gave only 36% benzaldehyde (entry 5).
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It may be worth to mention that usually immobilized catalysts show lesser activity compared to
their homogeneous counterparts, however in our case a significant improvement in yield was
observed by the catalyst FeCl3ꢀimine@SiO2 compared to homogeneous FeCl3 possibly
suggesting a cooperative effect between the metal and supported ligand which plays a crucial
role in promoting oxidation reaction.
Encouraged by our initial result, we extended the scope of our ironꢀbased catalyst for a
range of other representative primary and secondary alcohols including heterocyclic alcohol
o
(Table 2). Under the optimized condition (80 C, 2.0 mol% Fe), primary benzylic alcohols
bearing electronꢀdonating groups at para position were effectively converted to their
corresponding aldehydes in excellent yields (entries 2 and 4). However, conversions of benzylic
alcohol bearing electronꢀwithdrawing substituents at para position were found to be moderate
(entry 5). Our result also revealed that the positional isomers of the substituents have a
significant influence on the catalytic performances. For instance, oxidation of mꢀ
methylbenzylalcohol gave 73% product while under similar experimental condition pꢀ
methylbenzylalcohol gave 92% aldehyde (entries 3 and 2). The secondary benzyl alcohols like
benzhydrol (entry 10) or 1ꢀphenyl ethanol (entry 9) also gave excellent yields of corresponding
ketones. In fact, our protocol was also amenable to allylic alcohols such as cinnamyl alcohol
(entry 7) or heterocyclic alcohol like furfuryl alcohol (entry 8) which are usually considered to
be difficult substrates for alcohol oxidation reaction.4e In fact, our catalytic system was also
found to be effective for oxidations of aliphatic alcohols like cyclohexanol (entry 12) or 1ꢀ
octanol (entry 13), however only moderate yields were obtained.
Reusability and heterogeneity test
Our study (Fig. S3, ESI) reveals that the catalyst could be reused at least five cycles
without significant loss in activity. The AAS analysis of the five times used catalyst shows a
very negligible decrease in the Fe content (<2%). The SEM picture of the used catalyst (Fig. 4b)
is very much similar to that of the fresh catalyst (Fig. 4a) except slight decrease in silica particle
sizes, clearly indicating the surface morphology of the catalyst remained unaffected even after
five consecutive uses. The SEMꢀEDX spectra of the used catalyst also shows the presence of Fe
suggesting that the catalyst could be reused for further runs. In order to confirm the heterogeneity
of our system we have performed hot filtration test by taking benzyl alcohol as a model
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substrate. For this purpose the oxidation reaction was stopped after 2 h and the solid catalyst was
separated from the mixture by filtration under hot condition (conversion 46%) and the filtrate
was then allowed to continue stirring under identical conditions. After 4 h of stirring no further
change in the product concentration was noticed (Fig. 6), indicating that no leaching of catalyst
was occurred.
Conclusion
In conclusion, an iron(III)ꢀbased heterogeneous catalyst has been developed for the first time that
effectively oxidizes primary and secondary alcohols to corresponding carbonyl compounds in
water with molecular oxygen without using any additive or base. The low costs coupled with
environment friendly properties of the materials used in our protocol (e.g. iron, silica, O2, water)
will make this system a sustainable alternative to expensive platinum metal based catalysts.
Acknowledgement
The CSIR, New Delhi, is gratefully acknowledged for financial support (Grant no: 02/0081/12).
The UGC, New Delhi was acknowledged SAPꢀDRS programme and BSRꢀRFSMS fellowship.
DST, New Delhi has also been acknowledged for providing Surface Area Analyzer to the
Department under FIST programme.
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Table 1. Optimization of reaction conditions for the oxidation of benzyl alcohola
Entry Catalyst
Oxidant
Catalyst
Temp (oC) Time (h)
Yield (%)b
(mol%)
1
2
3
4
5
6
7
8
FeCl3@imineꢀSiO2 air
FeCl3@imineꢀSiO2 O2
FeCl3@imineꢀSiO2 O2
FeCl3@imineꢀSiO2 O2
3
3
3
3
3
2
1
2
30
30
60
80
80
80
80
80
24
8
8
8
8
8
8
6
42
64
85
96
36
96
84
96
FeCl3
O2
FeCl3@imineꢀSiO2 O2
FeCl3@imineꢀSiO2 O2
FeCl3@imineꢀSiO2 O2
aReaction conditions: benzylalcohol (1 mmol), O2 (1 atm., bubbling), ironꢀcatalyst, water (6
mL); bGC yield
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Table 2: Oxidation of various alcohols in aqueous media catalyzed by FeCl3ꢀimine@SiO2 using
molecular O2
Entry
1
Substrate
Product
Time(h)
6
Yield(%)a,b
96 (93)
OH
2
3
6
6
92 (88)
73 (68)
4
5
6
6
7
7
89 (86)
65 (63)
46 (44)
O
O2N
7
6
89 (87)
8
9
6
6
78
OH
90 (87)
10
11
6
7
91 (90)
73
12
13
7
8
51
62
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14
8
56
Reaction conditions: Alcohol (1 mmol), O2 (1 atm.), H2O (6 ml), catalyst (2 mol%), 80 oC,
aGC yield; bIsolated yield in the parenthesis.
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Legends: Scheme, Tables and Figures
Scheme 1: Synthesis of the silica supported iron catalyst, FeCl3ꢀimine@SiO2.
Fig. 1: N2 adsorptionꢀdesorption isotherms of silicaꢀbased materials.
Fig. 2: Powder XRD patterns of (a) APTES@SiO2 and (b) FeCl3ꢀimine@SiO2.
Fig. 3a SEMꢀEDX spectra of FeCl3ꢀimine@SiO2 (before catalysis).
Fig. 3b SEMꢀEDX spectra of FeCl3ꢀimine@SiO2 (after 5th use).
Fig. 4: ESR spectra of FeCl3ꢀimine@SiO2 recorded at liquid nitrogen temperature.
Fig. 5: XPS spectra of FeCl3ꢀimine@SiO2
Fig. 6 Hot filtration test for the benzyl alcohol oxidation using FeCl3ꢀimine@SiO2
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H3C
H3C
O
NH2
NH2
Cl
N
N
NH2
H3C
C
NH2
FeCl3
Fe
Ethanol
Reflux, 6h
CH3CN, 60 oC
8 h
Cl
Si
Cl
Si
Si
O O O
O O O
O O O
FeCl3-imine@SiO2
imine@SiO2
APTES@SiO2
Scheme 1
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7
6
5
4
3
2
1
0
APTES@SiO
2
imine@SiO
2
FeCl -imine@SiO
3
2
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure P/P
0
Fig. 1
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Fig. 2
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Fig. 3(a)
Fig. 3(b)
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600
400
200
0
g=2.18
g=2.11
g=2.05
g=1.98
-200
-400
-600
-800
g=1.92
g=1.86
200
250
300
350
400
450
Field (mT)
Fig. 4
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Fe 2p
N 1s
(b)
(a)
2p3/2
2p5/2
8
406 404 402 400 398 396 394 392 390
binding energy (eV)
740
735
730
725
720
715
710
705
binding energy (eV)
Fig. 5
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100
80
60
40
20
0
Non filtered
Hot filtered
0
1
2
3
4
5
6
Time (h)
Fig. 6
20