A synergistic effect in the combination of H2O2, FeAPO-5 and NaBr for selective oxidation of benzyl alcohols

Article abstract of DOI:10.1016/j.catcom.2011.09.035

The H₂O₂/FeAPO-5/NaBr combination has been found to be active in the oxidation of benzyl alcohols. The remarkably different activity results obtained with the H₂O₂/FeAPO-5/NaBr, the H ₂O₂/FeAPO-5 and the H₂O₂/NaBr confirm the existence of a strongly synergistic effect in the ternary reaction system of H₂O₂, FeAPO-5 and NaBr, leading to a significantly enhanced conversion of benzyl alcohols under mild reaction conditions.

Full text of DOI:10.1016/j.catcom.2011.09.035

Catalysis Communications 16 (2011) 225–228
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
A synergistic effect in the combination of H2O2, FeAPO-5 and NaBr for selective
oxidation of benzyl alcohols
⁎
Lin Qi, Xingyi Qi , Jing Wang, Liwei Zheng
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, 37 Xueyuan Road,
Haidian District, Beijing 100191, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 August 2011
Received in revised form 25 September 2011
Accepted 28 September 2011
Available online 7 October 2011
The H2O2/FeAPO-5/NaBr combination has been found to be active in the oxidation of benzyl alcohols. The re-
markably different activity results obtained with the H2O2/FeAPO-5/NaBr, the H2O2/FeAPO-5 and the
H2O2/NaBr confirm the existence of a strongly synergistic effect in the ternary reaction system of H2O2,
FeAPO-5 and NaBr, leading to a significantly enhanced conversion of benzyl alcohols under mild reaction
conditions.
© 2011 Elsevier B.V. All rights reserved.
Keyword:
Selective oxidation
Benzyl alcohols
Hydrogen peroxide
FeAPO-5 molecular sieves
Sodium bromide
1. Introduction
pure AlPO4 materials has drawn considerable attention. Research has
given rise to the generation of the so-called redox molecular sieves
The selective oxidation of alcohols is of fundamental importance
in the synthetic organic chemistry because the obtained aldehyde
and ketone compounds constitute one valuable kind of precursors
or intermediates used in the fine chemical, pharmaceutical and agro-
chemical industries [1]. Traditionally, such alcohol transformations
are performed employing expensive, hazardous stoichiometric
metal oxidants, e.g., chromium(VI) and manganese(VII) reagents
[2,3], which inevitably results in large amounts of heavy-metal
waste and environmental pollutions. Catalytic alcohol oxidations
with eco-friendly oxidants such as O2 and H2O2 are then greatly de-
sired and preferred for both environmental and economical benefits.
So far most examples of metal-catalyzed alcohol oxidations involve
the use of transition metal elements, e.g., titanium [4], vanadium [5],
chromium [6], manganese [7], iron [8], cobalt [9], nickel [10], copper
[11], silver [12], gold [13], ruthenium [14], palladium [15], bimetallic
ruthenium-copper [16] and palladium-gold [17]. Most of efforts in
these studies and elsewhere have been made to immobilize or anchor
these metal species as active center on an appropriate support in
order to develop optimized heterogeneous catalysts for alcohol
oxidations.
(MeAPO-n) when transition metal cations (Me) are introduced into
the synthesis gel and Me substitution for Al and/or P in the AlPO4 frame-
work during hydrothermal crystallization occurs [19]. For years,
MeAPO-n molecular sieves have been found to be active, stable and se-
lective solid catalysts for catalytic oxidations of a variety of organic sub-
strates using O2 and H2O2 as oxidants. Some excellent work includes:
the aerobic oxidation of alkanes [20] and alkylaromatics [21], the
H2O2-expoxidation of olefins [22] as well as the H2O2-hydroxylation
of aromatics [23] and phenols [24]. As for the O2- or H2O2-oxidation of
alcohols catalyzed by MeAPO-n molecular sieves, little achievement
has been made now mainly owing to low activity observed for the reac-
tion systems although there are a few reports in this regard [22,25].
Herein, we present the informative results of the selective oxida-
tion of benzyl alcohols as model alcoholic substrates by the H2O2/
FeAPO-5/NaBr combination and show that as compared with the bi-
nary counterparts of the H2O2/FeAPO-5 and the H2O2/NaBr, a concert-
ed action arising from the reaction triplet of H2O2, FeAPO-5 and NaBr
leads to a significantly enhanced conversion of benzyl alcohols.
2. Experimental
Aluminophosphate (AlPO4) molecular sieves were originally disco-
vered in 1982 by Wilson et al. [18]. Since then, the incorporation of
other metal and/or non-metal elements into the framework of the
According to the modified synthesis procedure [26], the series of
FeAPO-5 molecular sieves with one-dimensional channel (pore size:
0.73 nm) was prepared by static hydrothermal crystallization of a re-
active gel with molar composition 2R:xFe2O3:(1−x)Al2O3:1.5P2O5:
240H2O, where R is triethylamine as an organic template agent for
the AFI topology, ammonium ferric sulfate (NH4Fe(SO4)2⋅12H2O) is
⁎
Corresponding author. Fax: +86 10 82313981.
E-mail address: qixy@buaa.edu.cn (X. Qi).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.09.035

226
L. Qi et al. / Catalysis Communications 16 (2011) 225–228
Fe source, aluminum isopropoxide Al source, phosphoric acid
(85 wt.%) P source and x was selected as 0 (a), 0.005 (b), 0.015 (c),
0.025 (d), 0.040 (e), 0.065 (f), 0.075 (g) and 0.100 (h), respectively.
The letters (a–h) here represent the synthesized samples with various
Fe contents (Fig. 1). The gel was sealed in a Teflon-lined stainless steel
autoclave and heated in an oven at 190 °C under autogeneous pres-
sure for 48 h. The crystal AFI products were isolated by filtration,
washed with deionized water and air dried at 120 °C for 6 h.
XRD patterns were collected at a scanning rate of 3°/min in the 2θ
range of 5–50° on a D/max 2200PC Rigaku diffractometer using Cu Ka
radiation (λ=0.1542 nm) and nickel filter and operating at 40 kV
and 25 mA. SEM images were obtained with a JSM-5800 scanning
electron microscope. ICP-AES analysis was carried out with a Varian
Vista-MPX spectrometer.
The oxidation experiments were performed under atmospheric
pressure in a stirred 25-mL batch glass reactor fitted with a water-
cooled reflux condenser The desired reaction temperature was
achieved and maintained by a temperature-controlled water circula-
tion system. In a typical run, 50 mg calcined FeAPO-5 catalyst was
fully dispersed in 5 mL ethanol as solvent followed by an addition of
5.3 mmol benzyl alcohol (or its derivatives). After stabilization of
the temperature, 0.5 mL H2O2 (30 wt.%, 10.6 mmol) was added in
one single portion and the benzyl alcohol oxidation process was
kept going with magnetic stirring for 8 h. After that, the spent solid
catalyst in the reaction system was separated and removed by centri-
fugation. The reaction products in the clear solution obtained were
analyzed by a Varian CP-3800 gas chromatograph equipped with a
25 m-long OV-17 capillary column and an FID detector.
Fig. 2. SEM image of as-synthesized FeAPO-5 (f) (Fe: 1.78 wt.%).
and P were determined for AlPO4-5 (a). The lattice stoichiometry
of this type reveals that the isomorphous heteroatom substitution
mechanism should be regarded as Fe substitution for Al in the AFI
framework (for more characterization results, see supplementary
material).
To the best of our knowledge, this is the first report until now re-
garding the synthesis of FeAPO-5 molecular sieves with a wide range
of Fe content and their applications in combination with H2O2 and
NaBr, in an attempt to devise an efficient, catalytic, FeAPO-5-based re-
action system for benzyl alcohol oxidations.
3. Results and discussion
As shown in Fig. 1, the well-crystallized FeAPO-5 (b–h) and
AlPO4-5 (a) with an AFI type structure have been synthesized from
the above gel composition; no XRD peaks indexed to non-AFI phase
impurity were detected. A representative SEM image of as-synthesized
FeAPO-5 (f) (Fe: 1.78 wt.%) further evidences high crystallinity and
purity of the series of FeAPO-5 molecular sieves which is characteristic
of hexagonal morphology (Fig. 2). Moreover, no changes in both the
crystal structure and habit were found for the calcined FeAPO-5 molec-
ular sieves (550 °C, 6 h); ICP-AES data show that for FeAPO-5 (b–h),
the sum of the molar fractions of Fe and Al nearly approaches the
molar fraction of P whereas almost the same molar fractions of Al
Comparisons of the catalytic performance of FeAPO-5 (b–h) and
AlPO4-5 (a) are summarized in Table 1. As shown in Table 1, only a
10% conversion of benzyl alcohol was obtained with ca. 85.0% selec-
tivity to benzaldehyde for the H2O2/FeAPO-5 (f) and the H2O2/NaBr
(Entries 1, 2). However, the mergence of these two binary systems
into the H2O2/FeAPO-5/NaBr combination results in a noticeable
increase in the benzyl alcohol conversion. An 82.6% conversion of
benzyl alcohol with 63% selectivity to benzaldehyde was achieved
with the H2O2/FeAPO-5 (f)/NaBr (Entry 8). Also as has been seen in
Table 1, the Fe content in FeAPO-5 molecular sieves exerts an impor-
tant influence on the oxidation of benzyl alcohol. As the Fe loading in
Table 1
a
Oxidation of benzyl alcohol carried out in various reaction systems
.
h
g
b
c
Entry
Reaction system
Fe (mg)
Conv. (mol%)
Sel. (mol%)
1
2
3
4
5
6
7
8
H2O2/FeAPO-5 (f)
H2O2/NaBr
0.89
nil
nil
10.6
11.1
12.4
40.2
63.9
70.6
75.1
82.6
74.7
70.0
15.8
88
83
85
70
62
61
62
63
56
52
61
H2O2/AlPO4-5 (a)/NaBr
H2O2/FeAPO-5 (b)/NaBr
H2O2/FeAPO-5 (c)/NaBr
H2O2/FeAPO-5 (d)/NaBr
H2O2/FeAPO-5 (e)/NaBr
H2O2/FeAPO-5 (f)/NaBr
H2O2/FeAPO-5 (g)/NaBr
H2O2/FeAPO-5 (h)/NaBr
f
0.11
0.24
0.40
0.65
0.89
1.17
1.43
0.89
e
d
c
b
a
9
10
11
d
H2O2/Fe(III)/NaBr
a
Reaction conditions: reaction temperature (70 °C), reaction time (8 h), FeAPO-5
catalyst (50.0 mg, calcined at 550 °C for 6 h), benzyl alcohol (5.3 mmol), benzyl
alcohol/H2O2 molar ratio (1 : 2), solvent (ethanol, 5 mL), [NaBr] (0.1 M). H2O2 was
added in one single portion and no oxidation of ethanol was observed in the blank
experiment.
b
10
20
30
)
40
50
The Fe loading values in the reaction systems were calculated by multiplying the
2θ/( o
amount of catalyst (50.0 mg) by the corresponding Fe content (wt.%) of catalyst.
c
The remaining product was benzoic acid.
The same amounts of H2O2, Fe(III) and NaBr and the same reaction conditions (as
d
Fig. 1. XRD patterns of as-synthesized AlPO4-5 (a) and FeAPO-5 (b–h): a Fe: 0.00 wt.%,
b Fe: 0.21 wt.%, c Fe: 0.48 wt.%, d Fe: 0.80 wt.%, e Fe: 1.30 wt.%, f Fe: 1.78 wt.%, g Fe:
2.34 wt.%, h Fe: 2.85 wt.% (determined by ICP-AES analysis).
those chosen for the H2O2/FeAPO-5 (f)/NaBr combination) were adopted in the homo-
geneous reaction system of H2O2, Fe(III) and NaBr.

L. Qi et al. / Catalysis Communications 16 (2011) 225–228
227
Table 2
the reaction systems was varied over the range of nil to 0.89 mg, a
a
Oxidations of other benzyl alcohols with the H2O2/FeAPO-5(f)/NaBr
.
sharp increase in the conversion of benzyl alcohol took place, yielding
a variation of benzyl alcohol conversion from 12.4% to 82.6% (Entries
3–8). Compared with the H2O2/AlPO4-5 (a)/NaBr (Fe loading: nil),
the superior catalytic behavior exhibited by the H2O2/FeAPO-
5/NaBr can be reasonably attributed to the presence of the redox
metal iron in the AFI framework of FeAPO-5 molecular sieves. Fur-
thermore, the H2O2/FeAPO-5 (g)/NaBr and the H2O2/FeAPO-5
(h)/NaBr with higher Fe loadings gave relatively lower conversions
of benzyl alcohol with lower selectivity to benzaldehyde (in com-
parison with the H2O2/FeAPO-5 (f)/NaBr, Entries 8–10) because of
the increased formation of benzoic acid and the more severe decom-
position of H2O2 under the chosen reaction conditions.
The Fe leaching experiment was performed to ascertain if the re-
action is truly heterogeneous: the conversion of benzyl alcohol
(35.3%) remained unchanged after filtering off the FeAPO-5 (f) cata-
lyst (70 °C, 2 h) and letting the reaction continue for the rest of reac-
tion period (6 h); ICP-AES analysis also shows no observable changes
of Fe contents in both fresh and spent catalysts. These phenomena in-
dicate that the oxidation of benzyl alcohol indeed goes through a het-
erogeneous catalytic reaction process. In addition, any iron which
leached into the liquid phase in the reaction system is not concluded
to be homogeneously active due to the fact that only a 15.8% conver-
sion of benzyl alcohol was obtained in the homogenous reaction sys-
tem composed of H2O2, Fe(III) and NaBr (Entry 11) containing the
same amounts of the above three reaction reagents as those used in
the H2O2/FeAPO-5 (f)/NaBr combination.
Moreover, some additional observations were made as follows:
(1) the presence of bromine (Br2) in the reaction system was demon-
strated by a light brown coloration which developed during the oxi-
dation of benzyl alcohol with the H2O2/FeAPO-5/NaBr; (2) addition
of equivalent amount of cyclohexene (5.32 mmol) as the competing
substrate into the reaction system deeply suppressed the oxidation
of benzyl alcohol and the formation of 1, 2-dibromocyclohexane
was detected by GC–MS analysis, implying that Br2 is a key interme-
diate species involved in the oxidation of benzyl alcohol; (3) no ap-
preciable, irreversible consumption of NaBr was found during the
oxidation of benzyl alcohol because post-reaction addition of aqueous
AgNO3 solution into the liquid phase reaction mixture free of the
spent FeAPO-5 catalyst prompted a quantitative precipitation of AgBr.
In summary, the concomitant presence of H2O2, FeAPO-5 and NaBr
seems clearly to be a prerequisite for the high efficiency of in the ox-
idation of benzyl alcohol. As is well known, H2O2 has a higher redox
b
Substrate
Product
Conv. (mol%)
Sel. (mol%)
OH
O
56.2
100
100
57c
O
OH
60.1
83.2
Cl
Cl
OH
O
OH
O
52.1
61.3
54c
58c
F
F
OH
O
Cl
NO
Cl
NO
2
2
40.8
88c
OH
O
a
The same reaction conditions as those of Table 1 were adopted in Table 2.
Around 5–10% conversions of benzyl alcohols were obtained with the binary reac-
b
tion systems of the H2O2/FeAPO-5 (f) and the H2O2/NaBr.
c
The remaining products were the corresponding benzoic acids.
potential (Eaθ, H O /H O =1.77 V) than HOBr (Eaθ,
=1.33 V) and
-
HBrO/Br
the latter reage2nt very easily reacts with Br- to afford Br2; Br2 added
directly or generated in situ is capable of oxidizing alcohols into
their corresponding carbonyl compounds; such alcohol oxidations
have been confirmed to proceed through a mechanism in which the
rate-determining step involves a hydride transfer by Br2 from the
OH-bearing carbon followed by a rapid proton removal from the oxy-
gen [27–29]. Also based on the observations here, the reaction steps
involved in this oxidative transformation can be diagrammed in
Scheme 1.
2
2
Therefore, the nature of the H2O2/FeAPO-5/NaBr for the oxidation
of benzyl alcohol can be described as subsequently: FeAPO-5 molecu-
lar sieves first catalyze the H2O2−oxidation of bromide ion to HBrO/
BrO−; the newly formed HBrO/BrO− then reacts with bromide ion
to yield bromine; as for the last step in the overall reaction scheme,
the formation of benzaldehyde involves the bromine-mediated hy-
dride ion abstraction as mentioned earlier, with being accompanied
by regeneration of bromide ion and initiation of another reaction
cycle.
The scope of the H2O2/FeAPO-5/NaBr can be partly illustrated by
Table 2 in which oxidations of several other benzyl alcohols were
also investigated under the same reaction conditions. As with benzyl
alcohol, the selected benzyl alcohols can readily undergo oxidation
into the corresponding aldehydes or ketones with moderate to high
conversions.
4. Conclusions
To summarize, we have found an interesting reaction feature of
the H2O2/FeAPO-5/NaBr combination which derives itself from the
synergistic effect among H2O2, FeAPO-5 and NaBr, and can be utilized
for the oxidation of benzyl alcohols. Although the limited reactivity
needs further improving, we wish that our present work can provide
an alternative design methodology for the development of new,
Scheme 1. Reaction steps involved in the oxidation of benzyl alcohol with the H2O2/
FeAPO-5/NaBr combination.

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