Highly selective and "green" alcohol oxidations in water using aqueous 10% H2O2 and iron-benzenetricarboxylate metal-organic gel

Article abstract of DOI:10.1016/j.ica.2012.05.007

A metal-organic-gel catalyst based on Fe(NO₃)₃ and benzene-1,3,5-tricarboxylic acid (H₃BTC) was synthesised and characterized by IR, elemental analysis, thermogravimetry/mass spectrometry (TG/MS), M?ssbauer spectroscopy an

Full text of DOI:10.1016/j.ica.2012.05.007

Inorganica Chimica Acta 391 (2012) 75–82
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Highly selective and ‘‘green’’ alcohol oxidations in water using aqueous 10% H2O2
and iron-benzenetricarboxylate metal–organic gel
Hassan Hosseini-Monfared a, ,1, Christian Näther b, Heiner Winkler c, Christoph Janiak a,
⇑
⇑
a Institut für Anorganische Chemie und Strukturchemie, Universität Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany
b Institut für Anorganische Chemie, Universität Kiel, Olshausenstr. 40, 24098 Kiel, Germany
c Universität zu Lübeck, Institut für Physik, Ratzeburger Allee 160, D-23538 Lübeck, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A metal–organic-gel catalyst based on Fe(NO3)3 and benzene-1,3,5-tricarboxylic acid (H3BTC) was syn-
thesised and characterized by IR, elemental analysis, thermogravimetry/mass spectrometry (TG/MS),
Mössbauer spectroscopy and environmental scanning/transmission electron microscopy (ESEM/TEM).
The Fe-BTC metal–organic gel activity was evaluated for the oxidation of primary and secondary alcohols
with dilute aqueous 10% H2O2 in water at 90 °C to the corresponding aldehydes or ketones with high che-
moselectivity and with absence of overoxidation to carboxylic acids. The Fe-BTC gel catalyst allows to use
only water as solvent and does not require any organic solvents. The inexpensive and non-toxic iron cat-
alyst works with environmentally friendly hydrogen peroxide at safe, low 10% aqueous H2O2 concentra-
tion without the use of phase transfer conditions. A progressive addition of hydrogen peroxide
significantly improved the conversion of benzyl alcohol.
Received 28 February 2012
Received in revised form 5 May 2012
Accepted 10 May 2012
Available online 22 May 2012
Keywords:
Metal–organic gel
Iron
Hydrogen peroxide
Oxidation catalysis
Alcohol
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
dation reactions [17]. It overcomes the disadvantage of the tradi-
tional toxic inorganic oxidants [18], like MnO4 [19]. Besides, it
ꢀ
Selective oxidation of alcohols to carbonyl compounds is a fun-
damental transformation in organic synthesis because the target
molecules can be obtained directly in one-pot sequences and they
are important precursors for various chemicals [1–3]. Many cata-
lytic methods have been developed for oxidation of alcohols based
on different transition metals such as Pd [4–6], Cu [7], Ru [8], Co [9]
and Au [10,11]. Photocatalytic oxidation of benzyl alcohol has been
reported by using O2/TiO2 and modified O2/TiO2 [12]. However,
most of these reported systems suffer from high reagent load,
stringent reaction conditions and high cost or toxicity of the metal.
Now there is a scientific challenge for chemists both in industry
and academia [13] to develop more efficient, cheaper, chemoselec-
tive, and greener oxidation processes [14].
provides a high content of active oxygen species with water as
the sole byproduct, and it is much cheaper and safer than organic
peroxides or peracids. The utilization of peroxides as surrogates of
dioxygen has been extensively investigated for the oxidation of
various organic substrates, including amines, sulfides, alkanes, al-
kenes or alcohols [20,21].
Iron-containing enzymes found in nature inspired the use of
iron in catalyst design [22]. The cytochrome P450 enzyme family
contains an iron porphyrin heme cofactor, which oxidizes a range
of organic substances in living organisms [23,24]. Non-heme en-
zymes such as methane monooxygenase catalyze similar oxidation
reactions [25]. Iron has a number of advantages over other
transition metals typically employed in catalysis; it is relatively
non-toxic, abundant, cheap and environmentally friendly [14].
Consequently during the last decades, iron has become broadly
used for a multitude of redox processes. Apart from reductions
[26], various iron systems have been reported for epoxidation
[14,27], sulfoxidation [28], hydroxylation [29], as well as oxidation
of arenes to phenols and quinines [30,31]. Although iron performs
well in nature, the difficulty to prevent non-selective radical reac-
tions makes its usage for organic synthesis more difficult [21,32].
Compared with enzyme catalysts, porous materials such as zeolites
[33,34], modified open-structure microporous aluminophosphates
(AlPOs) [35], metal–organic frameworks (MOFs) [36] and gels [37]
are interesting for catalytic oxidation, because these frameworks
A ‘‘green’’ oxidation process, using hydrogen peroxide or, ide-
ally, molecular dioxygen as the oxidant, is the most attractive oxi-
dation technology for selective organic syntheses [15,16].
Hydrogen peroxide is a favorable and clean oxidant for various oxi-
⇑
Corresponding authors. Address: Institut für Anorganische Chemie und Struk-
turchemie, Universität Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Ger-
many.
E-mail addresses: monfared@znu.ac.ir (H. Hosseini-Monfared), janiak@uni-
duesseldorf.de (C. Janiak).
1
Permanent address: Department of Chemistry, University of Zanjan 45195 313,
Zanjan, Iran
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.05.007

76
H. Hosseini-Monfared et al. / Inorganica Chimica Acta 391 (2012) 75–82
2. Experimental
OH
O
oxidant: 10% H2O2
solvent: H2O
2.1. General
OH
O
2.1.1. Materials
metal-organic gel catalyst
[Fe2(BTC)(NO3)3]n
Reagents and solvents were purchased from the given suppliers
and used without further purification: Fe(NO3)3ꢁ9H2O (99.8%), ben-
zene-1,3,5-tricarboxylic acid (95%) (H3BTC), benzaldehyde (98%),
benzyl alcohol (99%), cyclohexanol (99%), cyclohexanone (99%)
and ethanol (p.a.) from Merck; acetonitrile (99.5%) 2-phenylethyl
alcohol (99%), 3-buten-1-ol (96%) from Sigma–Aldrich; phenylacet-
aldehyde (98%), DL-sec-phenylethyl alcohol (1-phenylethanol)
(98%), from Acros and 2-cyclohexen-1-ol (95%), acetophenone
(99.5%) and 2-cyclohexen-1-one (98%) from Riedel-de-Haën. Aque-
ous H2O2 (35%) was from Solvay from which a 10% base solution
was prepared. Aqueous 10% w/w hydrogen peroxide was verified
to be 3.07 mol/l upon titration with potassium permanganate solu-
tion. For the titrations the H2O2 sample was diluted, if necessary,
its pH adjusted to about 0 by addition of a few ml of conc. H2SO4
(c = 18 mol/l). A 0.05 mol/l KMnO4 solution was used as the titrator
[53]. Synthetic and spectroscopic procedures were conducted un-
der aerobic conditions.
(gelator: BTC = benzene
-1,3,5-tricarboxylate)
selectivity 100%
yield quantitative
Scheme 1. The ‘‘green’’ catalysis route from combination of safe 10% H2O2 oxidant,
water as solvent and a simple Fe–gel catalyst for the selective and quantitative
alcohol oxidation as described in this work.
could prevent entrapped metal complexes from dimerizing and
degrading [38] and also allow for control of the reactivity and
selectivity of the system through the microenvironment created
by the frameworks [39].
In order to prevent the formation of unwanted waste and toxic
reagents, the use of inexpensive and less toxic iron complexes to-
gether with hydrogen peroxide represents an ideal combination
for such oxidations. In general, chemoselectivity is the key issue
for these reactions due to potential non-selective Fenton- or Gif-
type chemistry (H2O2–Fen+) [40–42].
Several systems have been described employing different kinds
of iron catalyst with H2O2 for the selective oxidation of alcohols
and olefins; namely Fe(III)-porphyrin [43], dinuclear Fe complexes
[44], Fe(III) salt [45], in situ generated thymine-1-acetate iron(III)
complex [46], supported Fe2O3 nanoparticles (300 W microwave-
assisted) [47], iron(II)-iminopyridine complexes [48], H2O2/nano-
2.1.2. Instrumentation
For the addition of hydrogen peroxide in the catalytic oxidation
experiments an automatic buret Metrohm 716 DMS Titrino was
used. IR spectra (KBr pellet) were measured on a Bruker Optik
IFS 25. CHN-Elemental analyses were obtained on a VarioEL from
Elementaranalysensysteme GmbH. Iron analyses were carried out
by atomic absorption spectrometry (AAS, Analytik Jena-Vario 6)
(see Supporting information), titration with titriplex(III) (EDTA)
(see Supporting information) or gravimetric analyses (see Support-
ing information) [54].
c
-Fe2O3 [49], H2O2/Fe(NO3)3 with pH-buffer to increase the
selectivity [50] and nanoparticulare Fe2O3 [51] However, they need
stringent reaction conditions and high cost of the ligand and mul-
tistep synthesis, or have the disadvantages like low oxidation rate
and low turn-over number. Therefore, the development of practi-
cal, inexpensive, simple and green chemical processes for oxidation
is still needed.
Here we report the use of a simple and inexpensive iron(III)-ben-
zenetricarboxylate (Fe-BTC) metal–organic gel, originally synthe-
sized by Wie and James [52], as a catalytic system for the
oxidation of alcohols with 10% H2O2 in water (Scheme 1). The BTC
ligand (cf. Fig. 1) features prominently in MOF compounds [36].
Environmental scanning electron microscope (ESEM) samples
were analyzed with an ESEM2020. The Fe-BTC gel emulsion/dis-
persion was placed on a polished brass metal substrate surface to
follow the drying process (see Supporting information). Alterna-
tively, the dried Fe-BTC sample were put on a C-pad as substrate,
sputtered with 24 nm gold coating or on a copper grid. The ESEM
measurements were typically performed at 5–7 Torr, 5 °C and at
23 kV voltage using an ETD detector.
Transition electron microscopy (TEM) photographs were taken
at room temperature from a carbon coated copper grid on a Zeiss
LEO 912 transmission electron microscope operating at an acceler-
ating voltage of 120 kV. The Fe-BTC gel emulsion sample was
loaded on holey, carbon coated copper grids and dried at room
temperature.
Electron spray ionization mass spectroscopy (ESI–MS) was car-
ried out using a Finnigan MAT LCQ Advantage. MeOH was used as
the solvent. Mass spectra were measured in positive and negative
mode in the range of m/z = 200–2000.
O
O–(H+)
Fe(NO3)3·9H2O +
57Fe Mössbauer spectra were recorded with a conventional
O
O
spectrometer in constant-acceleration mode with
source. The velocity calibration was performed with an
a
57Co[Rh]
-Fe foil
(H+)–O
O–(H+)
a
at room temperature, for which the magnetic hyperfine splitting
is known with high accuracy. The measured isomer shifts are re-
H3BTC, BTC3–
ferred to this a-Fe standard. The experimental spectra were fitted
benzene-1,3,5-tricarboxylic acid,
benzene-1,3,5-tricarboxylate
by a sum of Lorentzian lines by means of a least-squares procedure.
DTA–TG/MS instrumentation: STA-409CD with skimmer cou-
pling from Netzsch, quadrupole mass spectrometer QMA 400 from
Balzers. The MS measurements were performed in analog and
trend scan mode. All measurements were corrected for buoyancy
and current effects and were performed using heating rates of
4 °C minꢀ1. in Al2O3 crucibles in a dynamic helium atmosphere
(purity 4.6; flow-rate: 75 ml minꢀ1.). The thermobalance was cali-
brated using standard reference materials.
H2O / CH3CN
RT
[Fe2(BTC)(NO3)3]n
metal-organic gel
Fig. 1. Synthetic route to and appearance of Fe-BTC metal–organic gel, as
synthesised before separating its solvent by centrifugation. The gel remains in this
state without any precipitation even after 24 h.

H. Hosseini-Monfared et al. / Inorganica Chimica Acta 391 (2012) 75–82
77
2.2. Preparation of Fe-BTC gel
The preparation was carried out following a brief protocol in
Ref. [52] with slight solvent modification. Solutions of Fe
(NO3)3ꢁ9H2O (12.10 g, 30.0 mmol) in 75 ml of acetonitrile–water
1:1 (v/v) and H3BTC (2.10 g, 10.0 mmol) in 100 ml acetonitrile–
water 1:1 (v/v) were rapidly mixed together and instantly a brown
gel was formed. After 5 min of stirring the gel was collected by cen-
trifugation (10 min, 2000 rpm) and decantation of the supernatant
solvent. Gelation was confirmed by the inverted test-tube method
[52]. Gel formation was reproducibly repeated 10 times. Part of the
gel was dried at 50 °C under vacuum for 8.5 h. After grinding this
dried gel to a fine powder in a mortar, drying was continued under
vacuum at 100 °C for 5.5 h. Anal. calc. for [Fe2(C9H3O6)(NO3)3
(H2O)3] (558.88) calcd. C, 19.34: H, 1.62: N, 7.52: Fe, 19.99. Found:
C, 18.87: H, 1.56: N, 7.38: Fe, 20.1%. IR (KBr, cm–1): 3421 (m), 2923
(s), 2854 (m), 2362 (w), 1720 (m), 1634 (m), 1455 (w), 1384 (vs),
1307 (w), 1239 (s), 1153 (m), 983 (w), 754 (w), 711 (w), 621 (w),
419 (m).
Fig. 2. Solidified gel after drying at 100 °C for 5.5 h as it was used in the catalytic
oxidation reactions.
the three-dimensional covalent and/or hydrogen-bonded network
created by the benzenetricarboxylate or benzenetricarboxylic acid
metallogelator [55]. The gel does not dissolve in water at room
temperature.
2.3. Alcohol oxidation, general procedure
Special care was taken to conduct a series of experiments under
identical conditions. The oxidant H2O2 (5, 10 or 35% w/w, corre-
sponding to 1.83, 3.07 or 11.0 mol/l, respectively) was added
through a syringe pump with a chosen constant rate (between
0.06 and 0.20 ml minꢀ1) to an aqueous mixture (5 ml) containing
Various methods for preparing single crystals of Fe3+ and H3BTC
were examined but failed (see Supporting information). To allow
for a defined mass of the metal–organic gel to be used in catalysis,
the gel was dried at 50 °C under vacuum for 8.5 h. After grinding
this dried gel to a fine powder in a mortar, drying was continued
under vacuum at 100 °C for 5.5 h to give an orange-brown glassy
solid (Fig. 2). The dried gel was ground well for a high surface area
but not too small for a convenient filtration after catalytic use.
The drying process could also be followed by environmental
scanning electron microscopy (ESEM, see Fig. S1 in Supporting
information). A transmission electron micrograph (TEM) of a room
temperature dried Fe-BTC gel sample from the emulsion shows
isolated dispersion particles with very good resolution. The main
particle size is about 150 nm but smaller particles of less than
50 nm are also present (Fig. 3).
the dried Fe-BTC gel (10 mg, 36 lmol Fe, 18 lmol [Fe2(C9H3O6)
(NO3)3(H2O)3] catalyst) and benzyl alcohol (1.00 g, 9.26 mmol).
During H2O2 addition the mixture was stirred in a 50 ml three neck
round bottom flask at a rate of 1300 rpm while the temperature
(RT, 60 or 90 °C) was kept constant. Reaction conversion was mon-
itored by GC/MS analysis of 0.20 ml aliquots withdrawn periodi-
cally from the reaction mixture. Yields were based on GC/MS
analysis only. The products were not isolated. For the GC/MS anal-
ysis the sample was homogenized by adding 0.2 ml ethanol. All
catalytic reactions were run at least three times and analyzed by
GC/MS and the average conversion yields are presented. Conver-
sion was based on the consumption of the starting alcohol together
with the amount of the aldehyde or ketone product.
When dried under vacuum at 100 °C for 5.5 h the particles
aggregate. Yet, ESEM investigations of the dried samples reveal
the build-up of these aggregates from smaller particles (Fig. 4).
For the GC/MS analysis an HP 5890 Series II GC fitted with an
Ultra2 column (stationary phase Ph-Me-silicone, 45 m, outer diam-
eter 0.2 mm, inner diameter 0.11 mm, carrier gas He) and con-
nected to
a quadrupole mass spectrometer HP 5989A (EI
ionization) was employed. The area ratios of the alcohol starting
material and the aldehyde or ketone product were calibrated by
using 2:1, 1:1, 1:0.5 (mol/mol) mixtures of the genuine substances.
For example, at a 1:1 M ratio the area ratio was not 1, so the cali-
bration curve was needed to convert the experimental area ratio to
the starting material and product substance percentage. Calibra-
tion curve was obtained by drawing product/alcohol area ratio
against product percent in solution. Only with the calibration curve
a direct and linear correlation was obtained between area ratio and
the product or reactant percentage.
3. Results and discussion
3.1. Synthesis and characterization
Fe(III)-containing gels are easily prepared by mixing Fe(NO3)3
and H3BTC solutions in 1/1 (v:v) mixture of CH3CN/H2O instantly
within 2 min [52]. This gel is stable for months and its physical
state remains the same without any precipitate formation if excess
solvent is not removed by centrifugation (Fig. 1). Formation of the
gel is caused by entrapping solvent molecules into the spaces of
Fig. 3. TEM image of a Fe-BTC sample dried at room temperature on carbon-coated
copper grid; see Fig. S2 in Supporting information for additional TEM pictures.

78
H. Hosseini-Monfared et al. / Inorganica Chimica Acta 391 (2012) 75–82
Fig. 4. ESEM of a Fe-BTC sample after drying at 100 °C for 5.5 h. (a) Particles on C-pad as substrate and sputtered with 24 nm gold coating and (b) particles on a copper grid.
Additional ESEM pictures are given in Figs. S3 and S4 in Supporting information.
was used as solvent during the synthesis (Fig. 1). A first TG step de-
rives from the water (m/z = 18) loss of the sample. Further heating
leads to formation of NO (m/z = 30) and NO2 (m/z = 46, not shown)
due to nitrate decomposition. At 350 °C a plateau is reached with
D
m ꢃ ꢀ38% before further mass loss resumes with loss of CO2
(m/z = 44) and presumably benzene (m/z = 78) of
D
m ꢃ ꢀ33% to a
total mass loss of ꢀ71.5% at 600 °C. In agreement with elemental
analysis (see Section 2) an assumed sum formula of [Fe2(C9H3O6)(-
NO3)3(H2O)3] for the dried gel is largely in accordance with a mass
loss of H2O (9.6%) + NO3(33.3%) ꢀ xO (remaining as Fe2O3) to yield
the first step of
D
m ꢃ ꢀ38% until 350 °C. The loss of C9H3O6
(37%) ꢀ yO (remaining as Fe2O3) then accounts for the second step
between 375 and 600 °C. The residue amount is in accord to
Fig. 5. Mössbauer spectrum of dried Fe-BTC gel at T = 77 K. A sum of Lorentzians
(solid line, parameters in text) is used to fit the experimental data (dots).
remaining Fe2O3 (Dmexp: 28.5%, Dmtheor: 28.6%). The residue is
amorphous proven by X-ray powder diffraction.
ESI–MS spectra of the solution phase of the dried gel dispersed
in methanol gave in positive ion mode major signals at m/z = 327,
591 and 679 corresponding to [Fe(H2BTC)NO3]+, [Fe2(BTC)(HBTC)
(CH3OH)2]+ and [[Fe3(BTC)2(NO3)(OH)(H2O)]+, respectively, with
[Fe(H2BTC)NO3]+ as the most intense signal (Fig. S5 in Supporting
information).
3.2. Catalytic activity
We first examined the catalytic activities of the dried Fe-BTC gel
toward hydrogen peroxide oxidation of benzyl alcohol in H2O as a
A Mössbauer spectrum for the dried Fe-BTC gel is shown in
Fig. 5. At 77 K, the Mössbauer parameters were d = 0.53 mm/s
and DEQ = 0.56 mm/s indicating an octahedral high-spin Fe(III)
(S = 5/2) center, with the iron bound to O-atom donors [56]. For
a
high-spin Fe(III) the observed isomer shift (d -Fe < 0.6 mm sꢀ1
)
and small quadrupole splitting are typical [57,58]. Note that even
though there is a symmetric charge distribution of high-spin d5-
Fe(III), there is a contribution for a small quadrupole splitting from
the ligand field which is of lower than Oh symmetry (D4h for a
0
0
00
trans-FeO4O2 coordination and D2h for a FeO2O2 O2 coordination
if the differences between benzenetricarboxylato, nitrato and aqua
ligands are taken into account) [59]. The full width at half-height
(C = 0.61 mm/s) is only slightly broader than in defined molecular
Fe(III) complexes (theoretical width ꢂ0.35 mm/s) and still within
the range of widths observed for the high-spin component of Fe(III)
spincrossover complexes [57]. This indicates very similar pseudo-
octahedral Fe(III)–oxygen environments or a very uniform Fe-sam-
ple. We rule out the presence of different Fe(III) species, e.g. in the
form of Fe(NO3)3 aq starting material (see also below in Section
3.2).
Investigations using simultaneous differential thermoanalysis
and thermogravimetry coupled to mass spectroscopy (DTA–TG/
MS) of the dried Fe-BTC gel (Fig. 6) do not yield a well-resolved
TG curve which precludes a good quantitative analysis. Still, the
dried gel does not show any loss of acetonitrile (CH3CN) which
Fig. 6. DTG, TG, DTA and MS trend scan curves for m/z = 18 (H2O), 30 (NO), 44 (CO2)
and 78 (presumably C6H6) (from top to bottom) for dried Fe-BTC gel; values refer to
the peak temperature (Tp) in °C (DTG) and to the mass loss in % (TG) (Al2O3 crucible,
He atmosphere; 75 ml minꢀ1).

H. Hosseini-Monfared et al. / Inorganica Chimica Acta 391 (2012) 75–82
79
Table 1
Effect of hydrogen peroxide concentration and addition rate on the oxidation of benzyl alcohol by Fe-BTC gel.a
b
Entry
Temp. (°C)
Total H2O2-Vol./ml (conc.)
H2O2 addition rate (ml minꢀ1
)
Time (min)
TON
Conversion (%)
1
2
3
4a
4b
5a
5b
6
25
25
60
90
90
90
90
90
3.4 (35%)
20 (35%)
3 (35%)
10.8 (10%)
12.6 (10%)
at once
0.12
at once
0.06
0.06
0.12
0.12
0.10
0.20
at once
420
420
300
180
210
180
210
180
180
240
10
10
2
2
118
447
463
504
514
252
421
<10
23
87
90
98
quant.
49
82
<2
c
c
d
20 (10%)
d
20 (10%)
e
18 (5%)
20 (5%)
e
7
8
90
90, no cat.
3.4 (35%)
a
Conditions: dried Fe-BTC gel 0.010 g (2 mg Fe, 36 lmol Fe, 18 lmol [Fe2(C9H3O6)(NO3)3(H2O)3] catalyst); benzyl alcohol 9.26 mmol; molar ratio benzyl alcohol/Fe = 9.26/
0.036 = 257; aqueous 35% H2O2 c = 11 mmol/ml; aqueous 10% H2O2 c = 3.07 mmol/ml; aqueous 5% H2O2 c = 1.83 mmol/ml; H2O 5 ml.
b
TON = turnover number [mol product/mol catalyst], the turnover frequency TOF [mol product/(mol catalyst ꢄ time)] is obtained by dividing TON through time.
c
Molar ratio H2O2/benzyl alcohol = 3.3.
d
Molar ratio H2O2/benzyl alcohol = 6.6.
Molar ratio H2O2/benzyl alcohol = 3.9.
e
test reaction. Benzyl alcohol was taken as a model substrate for
oxidation activity studies of the Fe-BTC gel/H2O2 system because
of its solubility in water. In addition, benzaldehyde is an interest-
ing target molecule, since it is easily over-oxidized to benzoic acid,
therefore, the formation of benzoic acid is a direct measure of the
selectivity of the protocol. At room temperature oxidation of ben-
zyl alcohol with aqueous 35% H2O2 was not catalyzed (conversion
2%) by the gel even after 420 min (7 h, entry 1, Table 1). In general,
the Fe-BTC gel does not appear to activate H2O2 at room tempera-
ture but only at elevated temperature. Increasing the H2O2 amount
(entry 2) or the gel concentration did not improve the conversion
at room temperature. Therefore, we can also rule out the presence
of Fe(NO3)3 aq starting material as soluble Fe(III) species leading to
Fe3+(aq) ions decompose H2O2 instantly [60]. The dried Fe-BTC gel
was, however, found to effectively catalyze the oxidation of benzyl
alcohol to benzaldehyde by hydrogen peroxide above room tem-
perature at 60 or 90 °C.
By adding 3 ml of aqueous 35% H2O2 at once at 60 °C, the benzyl
alcohol oxidation was 23% after 300 min (entry 3, Table 1). KMnO4
tests proved the absence of H2O2 in the reaction mixture after this
time. In addition, when 3 ml of aqueous 35% H2O2 was added in
small portions at 60 °C the substrate conversion increased (not gi-
ven in Table 1). Both of these results indicate a catalase-like dismu-
tation of H2O2 by Fe-BTC gel. Consequently, two main reactions,
namely the substrate oxidation and the H2O2 dismutation, are
competing. Previously, iron(III) complexes with porphyrin/pheno-
late [61] and tetraaza[14]annulenes [62] ligands have been shown
to be catalase models. Because of the second reaction order in
hydrogen peroxide in catalase-type dismutation (two molecules
of H2O2 are required to produce one molecule of O2) a decrease
of the local H2O2 concentration should strongly disfavor the dismu-
tation reaction with respect to the catalytic oxidation. Therefore
we added the oxidant progressively with the help of a Metrohm
microburette a 90 °C. Quantification of H2O2 concentration with
KMnO4 confirmed that aqueous H2O2 is stable in the presence of
the reactant and the absence of dried Fe-BTC gel at room temper-
ature, 60 or 90 °C. Benzaldehyde and H2O2 in absence of the Fe-BTC
catalyst showed only 1% alcohol-to-aldehyde conversion in
240 min (entry 8, Table 1).
Similarly for 5% H2O2 at an addition rate of 0.10 ml min–1 con-
version proceeded slowly up to 49% after 180 min (entry 6,
Fig. 7c) but continuous without any further addition to 85% within
1020 min (17 h, not shown). By increasing the addition rate for 5%
H2O2 to 0.20 ml min–1 the oxidation rate increased to a conversion
of 82% after 180 min (entry 7). However, the conversion was
incomplete even after 240 min (Fig. 7d). Thus, a concentration of
5% H2O2 was too low for effective oxidation.
During the beginning of the oxidation reaction, the gel remains
solid-like and heterogeneous. However, after 15–20 min because of
vigorous stirring at a temperature of 90 °C the gel disperses and a
yellow colloidal solution resulted, with or without alcohol and
H2O2. This indicates that the catalytic action is not truly heteroge-
neous but that the gel serves as a reservoir for the slow release of
active ‘‘homogeneous’’ iron-species into solution at higher temper-
atures than room temperature (see above). Vigorous stirring was
necessary because the organic alcohol reactant and aldehyde prod-
uct are not fully miscible with water. At room temperature the or-
ganic benzyl alcohol phase separates at the bottom of the flask
(density = 1.05 g/cm3). Hence, sampling for the GC measurements
was carried out when the oxidation mixture was stirred vigorously.
After cooling the reaction mixture to room temperature, the dis-
persed gel precipitated again as an orange-brown solid. By freezing
the mixture, the gel precipitation increased. The Fe-BTC gel was
separated by decanting the supernatant solution.
The possibility of recycling the Fe-BTC gel and re-using it as a
catalyst was checked with benzyl alcohol as the reactant. The oxi-
dation of benzyl alcohol with the recycled gel decreased to 20%
conversion (20 ml 10% H2O2, 0.12 ml minꢀ1, 180 min). Recycling
the gel three times decreased the benzyl alcohol conversion to
10% even when increasing the reaction time to 360 min (6 h).
Over-oxidation was not seen in Fe-BTC gel catalyzed oxidation
of benzyl alcohol as the only product was benzaldehyde with
>99% selectivity (entry 1, Table 2). In some reported systems of
Fe(III)-Schiff base/H2O2 a large excess of substrate to H2O2 (molar
substrate:H2O2 ratio 1000:20) has been used to minimize over-oxi-
dation of oxidation products [63]. A slight drawback of the Fe-BTC
gel system is the need for a 3- to 6.6-fold excess of hydrogen per-
oxide relative to substrate. The excess consumption of H2O2 is
attributed to the competitive parallel reaction of hydrogen
peroxide decomposition promoted by the catalyst (catalase-like
activity). This situation has been also reported for Mn(III) and
Mo(IV)-salen complexes immobilized on mesoporous silica gel
[64] and oxidant requirement of the H2O2/cyclooctene ratio of 10
order in epoxidation reactions [65,66].
Various addition rates (0.06–0.20 ml min–1) and concentrations
of H2O2 (5%, 10% and 35%) were used. Conversion depended on
both the H2O2 concentration and its addition rate. An addition rate
for 10% H2O2 of 0.06 ml min–1 already gave a good conversion to
87% in 180 min (entry 4, Fig. 7a). Increasing the addition rate for
10% H2O2 to 0.12 ml min–1 gave near quantitative (98%) conversion
in 180 min and no more benzaldehyde could be seen by GC/MS
after 210 min (quantitative conversion) (entry 5, Fig. 7b).
After demonstrating the selective oxidation of benzyl alcohol,
we evaluated the Fe-BTC gel in the oxidation of other alcohols by

80
H. Hosseini-Monfared et al. / Inorganica Chimica Acta 391 (2012) 75–82
Fig. 7. Oxidation of benzyl alcohol by Fe-BTC and progressive addition of different H2O2 concentration at different rates; see Table 1 for further details.
Table 2
Fe-BTC gel catalyzed oxidation of various alcohols.a
b
Entry
1
Substrate
Product(s)
Selectivity (%)
>99
Time (min)
180
TON
504
Conversion (%)
98
OH
O
O
2
48
52
180
118
23
OH
O
3
4
5
30
180
288
514
56
100
OH
O
>99
>99
>99
60
120
150
200
339
375
39
66
73
OH
OH
O
O
60
120
175
267
34
52
a
Conditions: dried Fe-BTC gel 0.010 g (2 mg Fe, 36 lmol Fe, 18 lmol [Fe2(C9H3O6)(NO3)3(H2O)3] catalyst), alcohol 9.26 mmol, molar ratio alcohol/Fe = 9.26/0.036 = 257;
total H2O2-volume 20 ml of 10% H2O2 (c = 3.07 mmol/ml) at an addition rate of 0.12 ml minꢀ1; H2O 5 ml; molar ratio H2O2/alcohol = 6.6.
b
TON = turnover number [mol product/mol catalyst], the turnover frequency TOF [mol product/(mol catalyst x time)] is obtained by dividing TON through time.
using 20 ml 10% H2O2 at an addition rate of 0.12 ml minꢀ1 as stan-
dard procedure (Table 2). The oxidation of 2-phenyl ethanol re-
sulted in 23% conversion after 3 h with almost equal ratio of
benzaldehyde and 2-phenylacetaldehyde (entry 2). From 1-phen-
ylethanol and cyclohexanol the corresponding ketones were ob-
tained with an excellent selectivity of more than 99% for
acetophenone or cyclohexanone, respectively (entry 3 and 4, Table
3). Conversion of the benzylic alcohol 1-phenylethanol was quan-
titative within 180 min. Also the oxidation of the allylic alcohol
3-cyclohexenol (entry 5, Table 3) proceeded with excellent selec-
tivity of >99% to cyclohexenone but at moderate conversion of
52%. In all cases no other side-products than those stated could
be identified.
The aforementioned Fe-BTC gel catalytic accomplishments
have to be seen in comparison to other Fe-catalysts from the
literature. Various Fe(II)/Fe(III)-catalysts have been used for
the oxidation of cyclohexanol, 1-phenyl ethanol, benzyl alcohol
and cyclohex-2-enol with H2O2 and t-BuOOH (Table 3). None
of the catalysts shows a high activity and selectivity like the
Fe-BTC gel catalyst. An exception is the catalyst [FeCl2(Py2S2)]
(1,6-bis(20-pyridyl)-2,5-dithiahexane=Py2S2) which shows high
activity in the oxidation of 1-phenyl ethanol to acetophenone

H. Hosseini-Monfared et al. / Inorganica Chimica Acta 391 (2012) 75–82
81
Table 3
Oxidation characteristics of various alcohols by different Fe catalysts.
g
Entry
Substrate to product
Conversion (%)
Selectivitya (%)
Time (min)
Oxidant/catalyst
TOF (h–1
)
References
1
2
3
4
5
6
7
8
9
10
11
12
13
14
cyclohexanol to cyclohexanone
73
36
56
70
56
80
65
100
84
80
72
89
52
99
100
100
99
100
100
100
99
100
90
100
66
150
240
30
90
30
15
60
210
30
90
H2O2/Fe(III)-BTC gel
t-BuOOH/Fe(II)-Py2S2/MW
150
45
22
this work
[67]
[68]
[69]
this work
[67]
[69]
this work
[69]
[70]
[49]
b
b
c
H2O2/Fe(II)-L3-Na2CO3
H2O2/Fe(III)-Schiff based
H2O2/Fe(III)-BTC gel
12
1-phenyl ethanol to acetophenone
benzyl alcohol to benzaldehyde
576
1602
13
171
34
13
1
4
134
40
t-BuOOH/Fe(II)-Py2S2/MW
H2O2/Fe(II)-L3-Na2CO3
H2O2/Fe(III)-BTC gel
H2O2/Fe(II)-L3-Na2CO3
c
c
d
H2O2/Fe(III)-Schiff base
e
12 h
12 h
120
30
H2O2/nano-
c
-Fe2O3
f
89
100
76
H2O2/Fe(NO3)3/KH2PO4
H2O2/Fe(III)-BTC gel
H2O2/Fe(II)-L3- Na2CO3
[50]
this work
[69]
cyclohex-2-enol to cyclo-2-enone
c
a
b
c
d
e
f
Selectivity refers to the chemoselectivity of aldehyde or ketone from alcohol.
MW = microwave assisted oxidation, catalyst [FeCl2(Py2S2)] (1,6-bis(20-pyridyl)-2,5-dithiahexane = Py2S2).
L3 = 6-(N-phenylbenzimidazoyl)-2-pyridinecarboxylic acid, Na2CO3, CH2Cl2. 0.025 mmol iron salt, substrate.
Fe(III)-(triphenylphosphine)2(N-(2-mercaptophenyl)salicylideneimine).
Benzyl alcohol (10 mmol), H2O2 (10 mmol, 30 wt.% in water), 1 mol% of catalyst, 75 °C, 12 h. H2O2 was added continuously in 12 h.
Benzyl alcohol (10 mmol), Fe(NO3)3ꢁ9H2O (0.2 mmol), KH2PO4 (0–0.6 mmol), 75 °C. 15 mmol H2O2 (30 wt.% in water) was added continuously with a syringe pump in
12 h.
g
Turnover frequency [mol product/(moles metal catalyst ꢄ hour)] = TON/(time in hour).
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A novel and easily prepared metal–organic Fe-BTC gel catalyst is
described for the oxidation of benzylic and allylic alcohols and
cyclohexanol which works with 10% aqueous hydrogen peroxide
as oxidant under mild conditions. This catalyst is more efficient,
cheaper, more chemoselective, hence, ‘‘greener’’ than comparative
literature systems. So far, the Fe-BTC gel catalyst appears mainly to
be effective towards reactive benzyl alcohols. In future work we
will investigate the possibility to increase the Fe-BTC gel efficiency
towards less active alcohols.
Despite their power as a synthetic tool and abundant use in aca-
demic research, oxidation reactions as a whole comprise as little as
3% of the reactions used on a preparative scale in the pharmaceu-
tical industry [68]. Perhaps the greatest factor influencing the hes-
itation to employ oxidation reactions on a large scale is the safety
of these processes. Using diluted H2O2 (less than 30%) and water as
solvent improves oxidation process safety in large scale syntheses.
Dilute aqueous hydrogen peroxide solution is a safe, non-toxic and
low cost oxidant. It can readily be activated by Fe(III) complexes
such as the easily and rapidly prepared Fe-BTC gel used here.
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Acknowledgments
This work was in part supported by DFG Grant Ja466/23-1 (ini-
tiation of bilateral cooperation HHM-CJ). We thank Dr. Yi Thomann
and Dr. Ralf Thomann for the ESEM and TEM pictures, Mrs. Lydia
Walter (Institute of pharmacy) for the GC/MS analyses and the Uni-
versity of Zanjan for a partial fellowship to H. Hosseini-Monfared.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2012.05.007.
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