Nanorods of FeVO4: An efficient heterogeneous catalyst for chemoselective oxidation of benzylic alcohols

Article abstract of DOI:10.1080/15533174.2016.1186035

Controllable fabrication of iron vanadate (FeVO₄) was achieved by the hydrothermal approach. The excellent catalytic activity of the prepared nanorods of iron vanadate for the oxidation of benzylic alcohols as well as its potential for oxidation of the benzylic sp3 C-Hs in the presence of urea hydrogen peroxide (UHP) as oxidant was reported. The prepared nanorods and nanospheres were fully characterized by FT-IR, XRD, EDAX, ICP-AES, SEM, and TEM. In the presence of the catalyst, alcohols chemoselectively (100%) convert to the corresponding aldehydes/ketones, giving a total turnover number about 380 for 10 consecutive runs.

Full text of DOI:10.1080/15533174.2016.1186035

Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry
ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20
Nano-Rods of FeVO4: an Efficient Heterogeneous
Catalyst for Chemo-Selective Oxidation of Benzylic
Alcohols
Akbar Heydari, Mehdi Sheykhan, Masoud Sadeghi & Iman Radfar
To cite this article: Akbar Heydari, Mehdi Sheykhan, Masoud Sadeghi & Iman Radfar (2016):
Nano-Rods of FeVO4: an Efficient Heterogeneous Catalyst for Chemo-Selective Oxidation
of Benzylic Alcohols, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal
Chemistry
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Date: 27 July 2016, At: 14:20

Nano-Rods of FeVO4: an Efficient Heterogeneous Catalyst for Chemo-
Selective Oxidation of Benzylic Alcohols
Akbar Heydari,*1 Mehdi Sheykhan,2 Masoud Sadeghi1 and Iman Radfar1
1Chemistry Department, Tarbiat Modares University, P.O. Box14155-4838 Tehran, Iran
*Corresponding author. E-mail: heydar_a@modares.ac.ir
2Chemistry Department, University of Guilan, P.O. Box 41335-1914 Rasht, Iran
Abstract
Controllable fabrication of iron vanadate (FeVO4) was achieved by the hydrothermal approach.
The excellent catalytic activity of the prepared nano-rods of iron vanadate for the oxidation of
benzylic alcohols as well as its potential for oxidation of the benzylic sp3 C-Hs in the presence
of urea hydrogen peroxide (UHP) as oxidant was reported. The prepared nano-rods and nano-
spheres were fully characterized by FT-IR, XRD, EDAX, ICP-AES, SEM and TEM. In the
presence of the catalyst, alcohols chemo-selectively (100%) convert to the corresponding
aldehydes/ketones, giving a total turnover number about 380 for ten consecutive runs.
Keywords
Selective oxidation; Heterogeneous catalysis; Iron vanadate; UHP

Introduction
Obviously, chemo-selective oxidation of alcohols to aldehydes and ketones is a crucial
transformation in organic chemistry for the synthesis of chemicals of industrial interest [1-7].
Traditional methods for oxidation of alcohols largely consisted of the use of chlorinated solvents
and exploiting either Cr(VI) or Mn(VII) reagents affording harsh conditions that restricted using
other functional groups within the molecule [4,8] and generate large amounts of toxic waste
when used on an industrial scale [8a]. Hence, utilizing milder approaches are always in demand.
The other problem that should be tackled about oxidizing agents is the environmental issues. To
date, even with newer achievements in utilizing hypervalent iodine reagents, the environmental
concerns have not been satisfied yet [9]. These concerns motivated chemists to take advantage of
greener methods such as free- or green solvent systems and green or less poisonous reagents and
catalysts [10-31]. In this regard, use of environmentally-benign hydrogen peroxide, which leaves
water behind at the end of reaction as the only by-product, has taken up a special position as a
green oxidant [31-32].
Shi and co-workers reported a heterogeneous nano-Fe(III) oxide catalyzed oxidation of
naphthalene derivatives using hydrogen peroxide which benefited from fenton process.33 In
addition, it is known that Fenton-like transformations of V(V) to V(IV) participate in the vast
majority of the vanadium-induced intracellular events [34-35]. Metal orthovanadates (MVO4) are
an interesting class of compounds with diverse applications in various fields such as solar cells,
gas sensors, cathode in lithium batteries and more importantly as catalysts [36-41]. To our
knowledge, studies on Fe-V nanocatalysts in selective oxidation of alcohols have been rarely
reported. Because of the development of several catalysts through introduction of the transition
metals into the iron oxide structure in activation of hydrogen peroxide [42-43] as well as the

abilities of Fe(III) and V(V), two ingredients of the ‘iron vanadate; FeVO4’, in promotion of
Fenton process [44], we decided to use iron vanadate as the catalyst of choice. Hence,
nanoparticles of FeVO4 with different morphologies were prepared and the prepared catalysts
were tested in the oxidation of benzyl alcohol derivatives. It was found that the size of our
catalyst has a great effect on its catalytic activity but in the case of morphology it does not
matter.
Results and discussion
There are several well-established records for the synthesis of FeVO4 such as heating equimolar
o
of Fe2O3 and V2O5 at 600 C [45], wet chemical synthesis via ferric nitrate (Fe(NO3)3) and
ammonium methavanadate (NH4VO3) [46], co-precipitation method using Fe(NO3)3.9H2O and
NH4VO3 in the presence of NaOH [47] and porous FeVO4 nanorods and nanoparticles via
hydrothermal synthesis [48].
Nanospheres of FeVO4 1 and nanorods of FeVO4 2 were prepared according to the Ref [48]. The
prepared nano-materials were fully characterized by XRD (Figure 1), SEM (Figure 2), TEM
o
(Figure 3), EDAX (Figure 4a), ICP, and FTIR (Figure 4b). Diffraction peaks at 10.1o, 11.4 ,
13.8 o, 14.2 o, 16.6 o, 17.5 o, 19.9 o, 20.8 o, 22.9 o, 23.3 o, 25.2 o, 25.8 o, 26.8 o, 27.1 o, 31.3 o, 41.0
o
corresponding to the (001), (010), (0-11), (100), (011), (-111), (101), (-102), (020), (0-21),
(012), (-121), (021), (-201), (-122) and (-132) are readily recognized from the XRD pattern. The
observed diffraction peaks agree well with the triclinic structure of iron vanadate (JCPDS card
no: 38-1372). No other phase except the iron vanadate is detectable.
The morphologies of the catalysts 1 (Figure 2a-b) and 2 (2c-d) were proved by SEM. As it is
clear, the catalyst 1 has a sphere-like morphology but in the case of the catalyst 2 the little

nanorods are detectable. The obtained histogram (Figure 2e-f) confirmed the fact that the size of
distribution for 50 observed nanospheres and nanorods was a narrow normal one with an average
value of 81 nm and 305 nm × 2.5 µm, respectively.
Figure 3 shows the typical TEM images of nanospheres 1 (a-d) and nanorods 2 (e-h) prepared by
the hydrothermal reaction. As can be seen from the image, the average diameter of the as-
synthesized nanospheres and nanorods were about 70 nm and 150 nm × 1.3 µm, respectively.
TEM images prove the prepared nanoparticles have good dispersibility in size and shape.
To confirm the presence of iron and vanadium elements, the energy-dispersive X-ray
spectroscopy (EDAX) was used. As is illustrated in Figure 4a, elemental analysis of FeVO4
nanoparticles by means of EDAX spectroscopy shows Fe and V elements in the prepared
structures. The quantitative analysis by using EDAX proved the presence of Fe and V at 31.2 and
29.6 % as expected.
Verifying the results obtained by EDAX is carried by the Inductive Coupled Plasma-Atomic
Emmission Spectroscopy (ICP-AES). The analysis showed 32.5 % Fe and 29.3% V in the
structure of the prepared nanoparticles.
In FTIR (Figure 4b), characteristic absorption bands due to the V-O terminal stretching appeared
at 964, 925 and 887 cm-1 while bridging V-O---Fe stretching can be seen at 740 and 694 cm-1.
Additionally, V-O-V deformation was observed at 516 cm-1 [49].
The efficiency of iron vanadate nanorods was initially investigated in the oxidation of benzyl
alcohol as a model reaction. Reaction conditions such as solvent, temperature, oxidant, mole
percent and morphology of the catalyst were optimized as is shown in Table 1.

R
R
O
catalyst
oxidant, solvent, 50 oC
OH
G
G
It was found that starting from benzyl alcohol, 2 mol% of catalyst 2, 2.5 equivalent of UHP as
o
oxidant, 2 mL of acetonitrile as solvent at 50 C led to the formation of benzaldehyde in 82%
yield and 100% selectivity (entries 1-12). Compared to FeVO4 in bulk, nano forms had better
function (entries 13,14). The results showed no marked difference when FeVO4 was exploited in
the forms of either nanospheres 1 or nanorods 2. Diversity of the reaction was investigated by
using the catalyst 2 proving that under the similar reaction conditions it is a highly efficient and
selective heterogeneous catalyst for the oxidation of a wide range of primary and secondary
benzylic alcohols (Table 2).
The oxidation of hydrocarbons to more functional compounds is one of the most important
processes in petrochemical transformation which were previously operated with various catalytic
processes using catalysts such as Ru(III) and oxidants such as hypervalent iodine and oxone [50-
52]. However, these methods have some disadvantages such as leaching of iron ion, necessity of
separation of iron ions at the end of the reaction, limited pH range applicable (pH=2-3) and
deactivation of the catalyst due to the formation of a complex between iron ions and anions
present in the reaction media [53]. Recently, Zhang et al. have developed a novel heterogeneous
Pd catalyzed aerobic oxidation of hydrocarbons [54]. Although it was an outstanding
breakthrough, efforts to increase the reaction conversion is still of great importance. To highlight
the aptitude of the catalyst prepared, we tested the oxidation of sp3 C-H bond of indane using our
catalyst. Oxidation of indane provided good results (Scheme 1). In fact, it was selectively
oxidized to 1-indanone while oxidation to indan-1-ol, 1-H-indene or hydroperoxy compound did
not occur. In addition, the blank oxidation (in the absence of the catalyst) gave no conversions.

Noticeably, the result was promising indicating potential of the catalyst in these kinds of
reactions: selective oxidation of alkyl aromatic compounds to aromatic ketones.
The most liked mechanism of Fe(III) and V(V) participations in Fenton-like reaction is to
activate hydrogen peroxide as the following reactions [44, 55-56] (Scheme 2):
Hence the above processes were proved by different methods [44], the reaction mechanism
should be related to the production of free radical OH. We propose that after the production of
hydroxyl radical (as shown in scheme 2 attributed to either the catalytic mechanism involved in
the Fe(III)/Fe(II) or V(V)/V(IV) redox couples), it abstracts a hydrogen atom from the alcohol to
form an alcohol-radical species that subsequently undergoes a terminative reaction with another
hydroxyl radical and then an α-elimination to give aldehyde and water (Scheme 3).
Ruling out the deactivation (caused by the leaching/sintering) of a heterogeneous catalyst is
within issues that matters and must be scrutinized. The heterogeneous nature of the catalyst was
confirmed when the reaction mixture of entry 1 in Table 2 was filtered after 20 min (15%
conversion). The catalytic activity was suppressed in the filtrate during 8 h after filtration. To
show that no deactivation occurred in the oxidation of benzyl alcohol (Table 2, entry 1), the
recovered catalyst was successively used in a second run. Setting the reaction time to 2 h,
comparably good yields of benzaldehyde were obtained in 10 subsequent runs using the same
catalyst to give a total turnover number (TON) about 380.
Conclusion
In summary, a promising fenton-like catalyst for oxidation of benzylic alcohols as well as
benzylic C-H bonds was introduced by use of nanorods and nanospheres of FeVO4. Under the
foregoing conditions, the reactions of the alcohols chemo-selectively (>98% to 100%) led to the

corresponding aldehydes/ketones, mainly due to the mild reaction conditions. In addition to
compatibility with green chemistry, FeVO4-H2O2 revealed some advantages than does precedent
catalysts (like iron oxide) for benzylic alcohol oxidation including (i) being free from leaching,
(ii) having applicable pH in which the reaction can be conducted (pH=3-8 for FeVO4 in
comparison with pH=2-3 for iron oxide) and (iii) not forming a complex between iron ion and
available anions in solution.
Experimental
Preparation of nanorods and nanospheres of FeVO4 [48]
All of the chemical reagents were in analytical grade. For typical synthesis of FeVO4 nanorods,
10 mL of 0.3 mol.L-1 ammonium methavanadate (NH4VO3) was added to 10 mL 0.3 mol.L-1
ferric nitrate (Fe(NO3)3) under stirring. Orange precipitation formed was transferred to the
o
autoclave, heated at 180 C for 3 h and then allowed to cool down to room temperature. The
FeVO4.1.1H2O formed was collected by centrifuge and washed with deionized water and ethanol
o
several times. The product was then vacuum-dried at 60 C for 6 h. The FeVO4 nanorods were
obtained by calcination of FeVO4 prepared in a furnace at 550 oC for 24 h.
To prepare FeVO4 nanospheres, the same procedure was adopted using less concentration (0.01
mol.L-1) of precursors (iron vanadate and ammonium methavanadate).
General procedure for catalytic oxidation of benzylic alcohols
Alcohol (1 mmol) and 2 mol% of catalyst were added to a glass reactor and stirred at 50 °C for 3
h during which either hydrogen peroxide (30%, 0.2 mL, 2.5 equiv.) or urea hydrogen peroxide
(2.5 equiv.) was continuously added. The progress was monitored by TLC. Ultimately, the

catalyst was separated by filtration, washed with diethyl ether and dried to be reused in the next
run. All isolated products gave satisfactory spectral data [57].
General procedure for benzylic C-H oxidation
Substrate (1mmol) and 2 mol% of FeVO4 nanorods were added to a glass reactor containing
acetonitrile as solvent (2 mL). Hydrogen peroxide (30%, 0.2 mL, 2.5 equiv.) was added and the
above-mentioned mixture was allowed to be stirred at 80 °C for 12 h. The catalyst was separated
by filtration, washed with diethyl ether and dried to be reused in the next run. The isolated
product gave satisfactory spectral data. The reaction was also implemented using urea hydrogen
peroxide (2.5 equiv.) under the same conditions giving rise to the similar results detected by
TLC.
Acknowledgement
The authors would like to acknowledge the financial support provided by the Tarbiat Modares
University for carrying out this research.

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Table 1. Solvent, catalyst and oxidant screening for the oxidation of benzyl alcohola
entry
1
2
3
4
5
6
7
8
catalyst mol%
oxidant
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
UHP
UHP
H2O2
H2O2
UHP
solvent
DCE
DCE
DCE
ACN
DCM
Toluene
H2O
solvent free
ACN
t (h) yield (%)b selectivity (%)c
0.5
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
5
5
5
5
5
5
5
5
1
2
3
3
10
3
3
3
29
35
43
77
48
trace
44
69
48
65
74
82
29
72
56
80
77
72
80
94
85
n.d.d
74
89
9
n.d.d
n.d.d
96
10
11
12 e
13f
14g
15h
ACN
ACN
ACN
ACN
ACN
ACN
100
n.d.d
95
90
>98
16 i
ACN
a
Unless otherwise stated, reactions were performed on a 1 mmol scale of benzyl alcohol using 2.5 equiv. H2O2 in
o
b
c
the presence of catalyst 2 at 50 C in 2 mL solvent. Yields of isolated products. Determined by flash
d
e
chromatography as the ratio of (benzaldehyde)/(benzaldehyde+benzoic acid). not determined. 2.5 equiv. UHP
f
h
i
was used. 0.5 equiv. UHP was used. g Catalyst 1 was used. Bulk FeVO4 was used. The reaction was performed
on a scale of 10.0 mmol benzyl alcohol.

Table 2. Chemo-selective oxidation of benzylic alcohols via FeVO4 nanorods a
entrysubstrate
1d
product
Obs. m.p. Yield TONb
(oC)
TOFc
(h-1)
13.7
Selectivity
(%)
-
82 41(>380e)
100
CHO
OH
2
CHO
-
85
72
42
36
14
12
100
100
OH
MeO
MeO
3
OH
-
O
4
5
54-55
45-47
78
75
39
37
13
100
100
CHO
OH
Br
Br
12.3
OH
O
6
7
OH
-
80
77
40
38
13.3
12.7
100
100
O
NHMe
NHMe
CHO
57-60
OH
NO2
NO2
NO2
8
9
41-43
71-73
75
84
37
42
12.3
14
>98f
100
NO2
CHO
OH
H3CO
H3CO
H3CO
H3CO
CHO
OH
OCH3
OCH3
10
69-72
87
43
14.3
100
CHO
OH
(H3C)2N
(H3C)2N
a
b
c
As the procedure described in experimental section. [reacted moles of alcohol/total moles of catalyst].
[TON/reaction time (h)]. d The catalyst was reused 10 times. e Total turnover number of 10 experiment. f Determined
by flash chromatography as the ratio of (aldehyde)/(aldehyde+carboxylic acid).

Figure 1. X-ray diffraction spectroscopy of the catalyst 2

a
b
c
d
f
e
Figure 2. SEM photographs of catalyst 1 (a-b) and catalyst 2 (c-d), statistical analysis of catalyst 1 (e) and
catalyst 2 (f)

a
b
d
c
e
f

g
h
Figure 3. TEM images of catalyst 1 (a-d) and catalyst 2 (e-h)

Figure 4. Energy-dispersive X-ray spectroscopy (EDAX) (a) and FTIR (b) of catalyst 2

OH
O
OOH
2 (2 mol%)
or
or
H2O2 (2.5 equiv.)
80 oC, ACN, 12 h
70%
not detected
Scheme 1. Catalytic oxidation of sp3 C-H bond

FeII
FeIII
FeII
H2O2
H2O2
H
+
+
OOH
+
+
+
+
FeIII
HO
OH
+
VIV
VV
VV
H2O2
H2O2
H
OOH
+
+
+
+
VIV
HO
OH
+
H
H
O
O
O
O
H
O
O
O
H
O
O
O
O
O
O
HO
HO
OH
FeIII
+
+
V
H
FeIII
FeII
V
V
O
HO
O OH
HO
O
FeII
O
OH
FeIII
+
FeII
Scheme 2. Proposed simplified mechanism for decomposition of one hydrogen peroxide molecule by a
Fe-O-V moiety in iron vanadate

R
R
OH
H
OH
OH
H2O
+
G
G
R
R
R
OH
OH
OH
O
OH
H2O
+
G
G
G
Scheme 3. Proposed mechanism for the oxidation of alcohols by the FeVO4-H2O2 catalyst