Aqueous oxidation of alcohols catalysed by recoverable iron oxide nanoparticles supported on aluminosilicates

Article abstract of DOI:10.1039/c3gc40110c

Supported iron oxide nanoparticles on aluminosilicate catalysts were found to be efficient and easily recoverable materials in the aqueous selective oxidation of alcohols to their corresponding carbonyl compounds using hydrogen peroxide under both conventional and microwave heating. The protocol features an easy work-up, simplicity and the utilisation of mild reaction conditions as well as high selectivity toward aldehydes is highly advantageous compared to alternatively reported methodologies. The supported iron oxide nanoparticles could be easily recovered from the reaction mixture and reused several times without any loss in activity. ICP-MS results proved that there is no metal leaching observed, demonstrating the stability of the catalyst under the investigated conditions.

Full text of DOI:10.1039/c3gc40110c

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Aqueous oxidation of alcohols catalysed by recoverable
iron oxide nanoparticles supported on aluminosilicates†
Cite this: DOI: 10.1039/c3gc40110c
a
b
a
b
Fatemeh Rajabi,* Antonio Pineda, Sareh Naserian, Alina Mariana Balu,*
b
b
Rafael Luque and Antonio A. Romero
Supported iron oxide nanoparticles on aluminosilicate catalysts were found to be efficient and easily
recoverable materials in the aqueous selective oxidation of alcohols to their corresponding carbonyl com-
pounds using hydrogen peroxide under both conventional and microwave heating. The protocol features
an easy work-up, simplicity and the utilisation of mild reaction conditions as well as high selectivity
toward aldehydes is highly advantageous compared to alternatively reported methodologies. The sup-
ported iron oxide nanoparticles could be easily recovered from the reaction mixture and reused several
times without any loss in activity. ICP-MS results proved that there is no metal leaching observed, demon-
strating the stability of the catalyst under the investigated conditions.
Received 15th January 2013,
Accepted 22nd February 2013
DOI: 10.1039/c3gc40110c
www.rsc.org/greenchem
involving environmentally benign oxidants such as H
2 2 2
O or O
Introduction
(
which generate as much as water as a unique reaction bypro-
Transition metal nanoparticles (NPs) have recently been of duct) is highly desirable from both environmental and econo-
scientific and technological interest to develop readily avail- mic viewpoints.
able, cheaper and more efficient catalysts as alternatives to tra-
A range of catalytic systems have been reported to be active
. Heterogeneous cata-
ditionally employed noble metal catalysts in several catalytic in the oxidation of alcohols with H
2 2
O
1
10–15
processes. Iron oxide NPs have been recently explored as cata- lysts based on transition metal oxides
and noble
1
–3
16–18
lysts for a range of processes including oxidations, C–C and metals
have been previously utilised as oxidation
4
5
C-heteroatom couplings, alkylations, and alkene production materials. Among this wide variety of materials, iron oxide-
6
from synthesis gas. Their stabilization onto supports via based materials emerged as green catalytic alternatives due to
alternative methodologies including microwave irradiation their low toxicity, abundance and interesting activities that
1
9
(
MWI), ultrasounds (US) and ball-mill (BM) are some of the allow the utilisation of mild reaction conditions. Low-loaded
most appealing approaches for the design of well dispersed, supported iron oxide-nanoparticles have also been successfully
small
applications.
The selective oxidation of alcohols to their corresponding
size
nanoparticles
with
enhanced
catalytic utilised in microwave-assisted protocols involving production
2
,7
3,20
of oxidation products from alcohols.
Furthermore, the combination of iron oxide nanoparticles
aldehydes or ketones is a relevant process in synthetic organic as catalysts in oxidation technologies performed in water
chemistry because the products obtained are valuable precur- offers an additional bonus designing safer and more environ-
sors or intermediates for the production of fine chemicals and mentally friendly processes. Indeed the proposed aqueous
8
,9
pharmaceuticals.
These processes have been traditionally approach facilitates the separation of aldehyde and/or ketone
carried out with stoichiometric amounts of inorganic oxidants products (typically with low solubility in water) that spon-
such as chromium(VI) and manganese(VII) reagents which are taneously separate out from the aqueous phase in contrast to
expensive, hazardous and generate great amounts of heavy- classical methodologies (which utilise organic solvents and
metal waste. The design of novel and innovative processes require distillation or tedious work-up protocols to separate
reaction products).
In continuation of our recent endeavors aimed at the devel-
a
opment of heterogeneously catalysed methodologies based on
Department of Science, Payame Noor University, P. O. Box: 19395-4697, Tehran,
Iran. E-mail: f_rajabi@pnu.ac.ir; Fax: +982813344081; Tel: +982813366366
3
,5,20
the use of supported iron oxide nanoparticles,
herein we
b
Departamento de Química Orgánica, Universidad de Córdoba, Campus de
describe a simple and efficient methodology for the mild
aqueous oxidation of alcohols to carbonyl compounds with a
Rabanales, Edif. Marie Curie, Ctra Nnal IV-A, Km 396, E14014, Córdoba, Spain.
E-mail: z82babaa@uco.es; Fax: +34957212066; Tel: +34957211050
†
green oxidant such as H O . In this work, we have compared
Electronic supplementary information (ESI)
0.1039/c3gc40110c
available. See DOI:
2 2
1
reactions carried out both under conventional and microwave
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heating as well as two different aluminosilicate supports for sample holder using double-sided adhesive tape and sub-
iron oxide nanoparticles, namely Al-MCM-41 and Al–SBA-15, sequently evacuated under vacuum (<10− Torr) overnight.
6
2
1
fully characterised in previous studies.
Eventually, the sample holder containing the degassed sample
was transferred to the analysis chamber for XPS studies.
Binding energies were referenced to the C1s line at 284.6 eV
from adventitious carbon. Deconvolution curves for the XPS
spectra were obtained using software supplied by the spec-
trometer manufacturer.
ICP-MS analysis was conducted at the Servicios Centrales
de Apoyo a la Investigación (SCAI) from Universidad de
Cordoba using an ICP-MS ELAN-DRC-e (Perkin Elmer). Fe was
Experimental
Preparation of catalysts
Al-MCM-41 and Al-SBA-15 supports were prepared as reported
Tetraethoxyorthosilicate
TEOS) and aluminum isopropoxide were utilised as silica and
2
1
by the group in previous work.
(
alumina sources, respectively. Fe oxide nanoparticles were sub-
sequently deposited on the pre-formed aluminosilicate sup-
ports by means of a simple microwave-assisted methodology
similarly reported for a series of metals and metal oxide nano-
3
extracted from materials upon treatment with an HNO –HCl–
HF mixture (5 mL, 2 : 2 : 1 ratio) and then subsequently ana-
lysed. A calibration curve was developed with several standards
for quantitative purposes.
3
,5,7,22
particles.
In a typical preparation, 2 g of aluminosilicate
was suspended in 20 mL water to which a solution of 1 g of
the iron precursor (FeCl ·4H O) in 2 mL ethanol was added.
Typical procedure for selective oxidations of alcohols to
carbonyl compounds (conventional heating)
2
2
The mixture was microwaved in a CEM-DISCOVER focalised
microwave reactor at 150 W (ca. 120 °C, maximum temperature
achieved) for 15 min. The final catalyst was thoroughly washed
with ethanol and acetone, dried overnight at 100 °C and finally
calcined (4 h) at 400 °C under air. The iron loading in both
materials was ca. 0.5 wt% (Table 1).
In a typical oxidation reaction, 50% of aqueous H O (0.3 mL)
2
2
was added to a solution of the corresponding alcohol
2 mmol) and Fe/aluminosilicate (0.05 g) in water. The mixture
(
was stirred at 80 °C for the various reaction times as indicated
in Table 1. The reaction was followed by GC and GC/MS as well
as by flash chromatography (isolated products). The catalyst
was separated by filtration, washed with ethyl acetate and
heated at 70 °C prior to its reuse in the next reaction. The com-
bined filtrate and ethyl acetate washing were then washed with
water and the organic layer separated and dried over mag-
nesium sulphate. The product was obtained after removal of
the solvent. The identity and purity of all final products were
Characterisation of materials
X-Ray diffraction patterns were recorded on a Siemens D-5000
(
α
40 kV, 30 mA) diffractometer with Cu K radiation (λ = 1.54 Å).
−
1
Diffractograms were collected at 0.5 min in the 10° < 2θ <
8
0° range.
XPS measurements were performed in a ultra-high vacuum
1
additionally verified by elemental analysis as well as by H and
(
UHV) multipurpose surface analysis system (SpecsTM model,
1
3
−
10
C NMR (see selected examples from ESI†). H
%) was calculated as products (mol) per consumed
H O (mol) × 100. The decomposition of hydrogen peroxide
2 2
O efficiency
Germany) operating at pressures <10
mbar using a conven-
(
tional X-ray source (XR-50, Specs, Mg–Kα, 1253.6 eV) in a
stop-and-go” mode to reduce potential damage due to sample
irradiation. The survey and detailed Fe high-resolution spectra
pass energy 25 and 10 eV, step size 1 and 0.1 eV, respectively)
2
2
“
was measured in the final reaction mixture by running it into
an acidified solution of potassium iodide, titrating the iodine
produced with a solution of thiosulphate using a starch
indicator.
(
were recorded at room temperature with a Phoibos 150-MCD
energy analyser. Powdered samples were deposited on a
Typical procedure for selective oxidations of alcohols to
Table 1 Textural properties [surface area (m g−1), pore size (nm) and pore carbonyl compounds (microwave irradiation)
2
−
1
volume (mL g )], metal content (wt% Fe) and surface acidity [measured by
In a typical microwave-heated reaction, 2 mmol of the corres-
and 0.05 g
using a gas chromatographic mode using pyridine (PY) and 2,6-dimethylpyri-
2 2
dine (DMPY) as probe molecules at 300 °C] of synthesized iron oxide materials ponding alcohol, 0.3 mL of 50% aqueous H O
compared to their parent supports
catalyst in 2 mL water or acetonitrile were microwaved for a
period of time between 1 and 15 min at 300 W (maximum
temperature reached 120 °C, average temperature 80 °C).
Samples were filtered off and subsequently analysed by GC
and GC/MS.
Surface acidity at
00 °C (μmol g−
1
3
)
Iron
Surface Pore size/
PY
(total
DMPY
(Bronsted
loading area
(wt%)
volume
(m g−1) (nm mL g−1
2
−1
)
acidity) acidity)
Catalyst
Reuse experiments (conventional heating)
Al-MCM-41
—
937
970
747
688
2.3/0.66
2.3/0.67
8.0/0.65
7.8/0.63
102
265
82
77
102
61
Upon completion of the first reaction to afford a quantitative
yield of the corresponding aldehyde or ketone, the catalysts
were recovered by filtration, washed and finally dried at
(Si/Al = 30)
Fe/Al-
MCM-41
Al-SBA-15
0.54
—
100 °C. A new reaction was then performed with fresh solvents
(Si/Al = 37)
Fe/Al-SBA-15 0.63
104
88
and reactants under identical conditions.
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compared to similar nanoentities supported on purely siliceous
mesoporous MCM-41 and SBA-15 (fact that could be in prin-
Results and discussion
21,22
Table 1 summarises the characterisation data of the synthesized ciple extended to activated alkenes; i.e. styrene).
A compre-
catalysts from the present work. Iron containing materials fea- hensive study on the observed potential Fe/Al synergy of these
2
−1
tured high surface areas (>650 m
(
g
) and pore volumes materials is currently under investigation by means of a range
>0.6 mL g− ), with pore sizes of ca. in the 2 to 8 nm range of techniques including Mossbauer spectroscopy, EXAFS and
1
depending on the type of support (Table 1). Iron content in the XAS. Preliminary results seem to point out the existence of elec-
synthesized materials as measured by ICP/MS was found to be tronic-like perturbances, with XAS spectra having significant
2
3
around 0.5–0.6 wt%, with average iron oxide nanoparticle sizes differences in the low energy domain.
in the 7–8 nm range. XRD of the materials confirmed the pres-
2
1
In view of these reported preliminary findings, the catalytic
ence of the hematite phase (Fe O , JCPDS card 39-0664) visible activity in the oxidation of alcohols could in principle be
2
3
even at low metal loadings for Fe/Al-SBA-15 (Fig. 1), which was favoured for iron oxide aluminosilicate materials due to the
3
+
also confirmed by XPS measurements (typical Fe bands at BE observed beneficial Fe/Al synergy. Results for the catalytic oxi-
21
7
14 (Fe2p3/2) and 725 eV (Fe2p1/2), Fig. 2). Full characterisation dation of a range of alcohols using Fe/Al-MCM-41 and Fe/
of the materials seemed to point out a synergetic effect Fe/Al Al-SBA-15 materials under microwave irradiation have been
which was found particularly useful in the microwave-assisted included in Table 2.
selective oxidation of benzyl alcohol to benzaldehyde catalysed
by iron oxide nanoparticles supported on aluminosilicates as
Table 2 Microwave-assisted oxidation of alcohols using Fe/aluminosilicate
a
catalysts
Sel.
Conversion Product
Entry Substrate
Catalyst
(mol%)
(mol%)
1
2
Fe/Al-MCM-41 69
Fe/Al-SBA-15 88
>99
>99
3
4
Fe/Al-MCM-41 41
Fe/Al-SBA-15 53
92
80
5
6
Fe/Al-MCM-41 56
Fe/Al-SBA-15 51
96
>99
7
8
Fe/Al-MCM-41 46
Fe/Al-SBA-15 45
93
90
9
Fe/Al-MCM-41 47
Fe/Al-SBA-15 45
94
97
10
1
1
1
2
Fe/Al-MCM-41 50
Fe/Al-SBA-15 52
90
87
1
1
3
4
Fe/Al-MCM-41 57
Fe/Al-SBA-15 53
94
93
1
1
5
6
Fe/Al-MCM-41 37
Fe/Al-SBA-15 37
>99
>99
Fig. 1 XRD pattern of Fe/Al-SBA-15. Bottom lines correspond to the JCPDS
9-0664 card of hematite phase Fe
b
1
1
1
2
7
8
9
0
Fe/Al-MCM-41 15
Fe/Al-SBA-15 13
Fe/Al-MCM-41 <15
Fe/Al-SBA-15 <15
Fe/Al-MCM-41 >95
68
63
65
63
3
2 3
O .
b
b
b
c
21
22
23
>99
d
d
Fe/Al-SBA-15
Fe/Al-SBA-15
>97
45
90
b
68
a
Reaction conditions: 2 mmol substrate, 0.3 mL H
mL acetonitrile, 300 W, 3 min reaction. The difference of 100 in
2
O
2
50%, 0.05 g cat.,
b
2
selectivity corresponds to the main by-products generated in the
c
reaction, dihexylether and di-isoctylether. 15 min microwave
irradiation. 30 min microwave irradiation.
d
Fig. 2 XP spectra of survey (left figure) and Fe2p (right figure) of Fe/Al-SBA-15.
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Table 3 Selective oxidation of a range of alcohols to the corresponding carbonyl compounds using Fe/Al-SBA-15 catalysts
a
a
Entry
1
Substrate
Product
Time (h)
Yield (%)
4 (0.25)
99 (98)
2
3
4 (0.25)
6 (0.25)
99 (95)
97 (97)
4
4 (0.5)
98 (99)
5
6
6 (0.5)
7 (0.5)
99 (96)
95 (98)
7
8
7 (0.5)
99 (96)
98 (90)
12 (0.5)
9
12 (1)
92 (88)
94 (98)
1
1
0
1
12 (0.5)
12 (0.5)
96 (90)
1
1
2
3
12 (0.5)
12 (0.5)
<30 (40)
<10 (25)
2 2
Reaction conditions: 2 mmol substrate, 0.3 mL H O 50%, 0.05 g cat., 2 mL water, heating at 80 °C or microwave irradiation (300 W, maximum
a
temperature reached 120 °C, average experiment temperature 80 °C). Numbers in brackets correspond to times of the reaction and yields under
microwave irradiation.
In principle, the reaction seemed to work under very mild conversions were observed for linear alcohols (Table 2, entries
conditions (80 °C, 3 min microwave irradiation in acetonitrile 17 to 20). Increasing the time of the reaction to 15–30 min,
as a solvent) to a reasonable extent with substituted benzyl almost quantitative conversion was obtained for most sub-
alcohols, providing moderate to good yields to target aldehyde/ strates (Table 2, entries 21 and 22) with the exception of linear
ketone products with very high selectivities.
alcohols that only provided moderate conversions (max.
Catalysts seemed to efficiently work both in the presence of 40–50%; Table 2, entry 23). In the case of linear alcohols, an
electron-donating or electron-withdrawing substituents as well interesting acid-catalysed intermolecular etherification
as in less sterically-favoured positions within the aromatic ring between 2 molecules of the linear alcohol takes place, compet-
(Table 2, entries 7 to 10). The protocol was also amenable to ing with the expected oxidation process under the investigated
cyclic alcohols (Table 2, entries 15 and 16), although low reaction conditions. This is an important reaction that is
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currently under consideration as alkylethers and derivatives that already proved the high stability and activity of similar
2
4
22
can have relevant industrial applications.
supported iron oxide nanoparticle systems. In this particular
Interestingly, the use of the Fe/Al-SBA-15 seemed to provide case, the iron oxide nanoparticle species seemed to be particu-
slightly improved conversions to target products for certain larly stabilised by Al from the mesoporous aluminosilicate
2
1
examples (particularly for electron-withdrawing substituents) network.
with respect to Fe/Al-MCM-41. The difference in acidity
between both materials, indication of the slightly different
interactions between Fe and Al in the systems, is believed to be
Conclusions
responsible for the different activities, which is currently
2
3
under deeper and thoughtful investigation.
We report a facile route for the synthesis of the carbonyl com-
Selecting Fe/Al-SBA-15 as a catalyst, the scope of the reac- pounds using supported iron oxide nanoparticles on alumino-
tion was extended to a series of substrates including mono- silicate materials as efficient, highly selective and active
and di-aromatic alcohols, furfuryl alcohol (an interesting and catalysts in aqueous hydrogen peroxide under both conven-
potentially derived biomass chemical) as well as cyclic alcohols tional and microwave heating. Materials could be easily recov-
such as cyclohexanol and cyclopentanol. The reaction medium ered from the reaction mixture and used five times without
was also switched to water and reactions were compared under any significant activity loss and no metal leaching was
both conventional and microwave heating. Results summar- observed during the course of the reuses. We envisage the
ised in Table 3 demonstrate the high oxidation activity of the extension of this highly efficient system to related aqueous
supported iron oxide nanoparticle system, which is able to phase transformations including oxidation of biomass-derived
provide isolated yields in excess of 90% for most of the sub- platform molecules (e.g. sugars and polyols) to high-added
strates studied.
value chemicals, currently under investigation in our
The only exception of the protocol was linear alcohols due laboratories.
to the low inherent conversion obtained for these systems as
well as to the competing acid-promoted chemistry observed in
such cases. In any case, the reaction system was fully compati-
Acknowledgements
ble with aqueous media under the investigated conditions
using the proposed system. In all cases, the hydrogen peroxide Rafael Luque gratefully acknowledges support from the
efficiency was over 90%, indicating that no significant Spanish MICINN via the concession of a RyC contract (ref.
decomposition of hydrogen peroxide in the systems takes RYC-2009-04199) and funding under projects P10-FQM-6711
2 2
place under the investigated conditions (H O is only con- (Consejeria de Ciencia e Innovacion, Junta de Andalucia) and
sumed as an oxidisiding agent), in good agreement with pre- CTQ2011 28954-C02-02 (MICINN). Fatemeh Rajabi thanks
vious results from the group in similar oxidation Payame Noor University and Iran National Science Foundation
2
0,22
chemistries.
(INSF) for financial support.
The utilised catalyst was found to be fully reusable under
aqueous conditions as clearly shown in Fig. 3, preserving over
9
0% of its initial activity after 5 cycles. ICP-MS analysis of the
reaction mixture indicated no detectable iron leaching
<0.05 ppm) under the investigated conditions, in good agree-
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