Oxidative coupling and hydroxylation of phenol over transition metal and acidic zeolites: Insights into catalyst function

Article abstract of DOI:10.1007/s10562-013-1142-z

Reaction of phenol with hydrogen peroxide over H-MFI, Fe-MFI, H-BEA, Fe-BEA and TS-1 zeolite catalysts was investigated. Over H-BEA, biphenyl product was observed. It is suggested, that the larger pore size of H-BEA facilitates coupling of two phenol molecules. Two distinct reaction mechanisms are proposed for acid and redox catalysts.

Full text of DOI:10.1007/s10562-013-1142-z

Catal Lett (2014) 144:9–15
DOI 10.1007/s10562-013-1142-z
Oxidative Coupling and Hydroxylation of Phenol over Transition
Metal and Acidic Zeolites: Insights into Catalyst Function
•
Jerry Pui Ho Li Eric Kennedy
•
Michael Stockenhuber
Received: 8 September 2013 / Accepted: 12 October 2013 / Published online: 6 November 2013
Ó Springer Science+Business Media New York 2013
Abstract Reaction of phenol with hydrogen peroxide
over H-MFI, Fe-MFI, H-BEA, Fe-BEA and TS-1 zeolite
catalysts was investigated. Over H-BEA, biphenyl product
was observed. It is suggested, that the larger pore size of
H-BEA facilitates coupling of two phenol molecules. Two
distinct reaction mechanisms are proposed for acid and
redox catalysts.
[7] and thus removal and conversion of phenols to valuable
products is highly desirable.
Apart from its toxicity, phenol creates significant prob-
lems through its reaction with other species present as
VOC’s in the flue gases streams of combustors. These
reactions can lead to oligomerization [8, 9], resulting in
significant and undesirable flue gas treatment challenges.
One potentially viable treatment strategy involves the
upgrading of phenol through a hydroxylation reaction,
using hydrogen peroxide, in order to produce p-benzoqui-
none and catechol (Scheme 1). These products are used as
valuable precursors for rubber manufacture [10] and as fine
chemicals in perfumes and pharmaceuticals [11].
Keywords Catalysis ꢀ Zeolites ꢀ Phenol ꢀ
Hydroxylation ꢀ Redox ꢀ Pore size
1 Introduction
Hydrogen peroxide (H2O2) treatment has emerged as a
viable approach for treatment of aromatic compounds found
in aqueous waste streams. Its use, in general, does not lead
to the formation of undesirable by-products and it is an
ecologically ‘clean’ and non-toxic chemical [12]. Studies of
the hydroxylation of phenol by hydrogen peroxide cata-
lyzed by protonic FAU zeolites has been undertaken [13]
and other studies indicate that a catalyzed process over
porous materials is possible, provided that the catalyst pore
sizes are sufficiently large. For zeolites, catalysts with too
small a pore size are not active, due to the large pore
diameter required to accommodate the reaction [14, 15].
It has been argued that the catalytic reaction of phenol
with H2O2 does not proceed through an electrophilic aro-
matic substitution reaction with a hydroxonium cation
(which would be generated from hydrogen peroxide at the
surface of the acid catalyst). Instead, it is generally con-
sidered that the catalytic process occurs via a redox
mechanism involving hydroquinone/p-benzoquinone [16,
17]. The goal of this research is to compare and contrast the
catalytic performance of zeolite catalysts in terms of their
pore size, redox properties, and acidity.
The use of biomass combustion for power generation is
regarded as an attractive greenhouse gas reduction strategy.
Such processes revolve around the pyrolysis of lignocel-
lulose that results in the formation of bio-oils. Other pro-
cesses include the blending of biomass with coal; a
practical and attractive approach to increase the renewable
energy input in traditional coal power plants [1, 2]. How-
ever as a consequence of co-combustion with coal,
numerous types of VOCs can be formed, including aro-
matic hydrocarbons [3, 4]. The aromatic hydrocarbons are
present in wood smoke that is a product of biomass com-
bustion [5, 6]. The toxicity of the aromatic hydrocarbons,
especially phenol, is known to cause symptoms like muscle
weakness, convulsions and coma if introduced to humans
through skin absorption, inhalation and vapour inhalation
J. P. H. Li ꢀ E. Kennedy ꢀ M. Stockenhuber (&)
PRCfE, Chemical Engineering, School of Engineering, Faculty
of Engineering & Built Environment, University of Newcastle,
Callaghan, NSW 2308, Australia
e-mail: michael.stockenhuber@newcastle.edu.au
123

10
J. P. H. Li et al.
Scheme 1 Hydroxylation of phenol and the observed products
2 Method
area analyzer. The samples were pre-treated in vacuum at a
pressure of 1.2 9 10-3 mbar at 115 °C for 24 h.
2.1 Catalyst Preparation
Temperature programmed desorption (TPD) measure-
ments were conducted in a compact, stainless steel appa-
ratus equipped with a turbo pump. The desorption cell was
connected through a leak valve to an independently
pumped, Pfeiffer Prisma quadrupole mass spectrometer.
The catalyst samples of known mass were introduced into
the desorption cell, evacuated and activated at 500 °C at
5 °C/min, followed by cooling to 115 °C in order to adsorb
probe molecules for acid site quantification. Adsorption
was performed at 115 °C in order to minimize multilayer
adsorption of the adsorbates. For acid site quantification,
ammonia was adsorbed on the catalyst at 1 mbar pressure.
Loaded catalysts were cooled to 30 °C and followed by
controlled desorption of the probe from 30 to 500 °C with a
heating rate of 5 °C/min.
Inductively coupled plasma-optical emissions spec-
trometry (ICP-OES) was performed on the Fe-MFI cata-
lysts in order to know more accurately the actual Fe
loading. A Varian Radial 715-ES series ICP-OES was used
for the elemental analysis. Samples were digested with acid
in a microwave, using an yttrium-based internal standard
for quantification.
The zeolite catalysts studied here (MFI and BEA) have a
three dimensional pore system that is rather similar (con-
sisting of intersecting straight and sinusoidal channels) and
mainly vary in the pore diameter. Thus structural depen-
dency of the catalytic reaction is restricted to pore size to
be able to pinpoint the origin of the different reactivities.
H-MFI and H-BEA were prepared by calcining
NH4-MFI (Zeolyst, Si/Al = 15) and NH4-BEA (Zeolyst,
Si/Al = 12.5) respectively, at 500 °C in air at a rate of
5 °C/min. This was held for 2 h before cooling to 300 °C.
Fe-MFI and Fe-BEA was prepared by ion-exchange. In
separate procedures, commercial NH4-MFI (Zeolyst, Si/
Al = 15) and NH4-BEA (Zeolyst, Si/Al = 12.5) were
impregnated with an aqueous solution of 0.01 M
Fe(NO3)3ꢀ9H2O, and maintained at a pH of 8 by dropwise
addition of ammonium hydroxide. The procedure was
repeated three times. Following the transition metal
exchange procedure, the catalysts were vacuum filtered and
dried at 115 °C overnight. The catalysts were then calcined
at 500 °C at a rate of 5 °C/min. This was held for 2 h
before cooling to 300 °C, before adding to the reaction
mixture. TS-1 (Zeolyst Si/Ti = 72) was activated by
heating to 500 °C in air at a rate of 5 °C/min. This was held
for 2 h before cooling to 300 °C, before adding the catalyst
to the reaction mixture.
2.3 Reaction
Experiments were carried out in a 10 mL stirred glass
reactor equipped with a condenser and a magnetic stirrer.
Blank tests were conducted with H2O2 and phenol in the
absence of catalysts.
2.2 Characterization
The reaction was performed at 100 °C with continuous
stirring and at atmospheric pressure. The reaction mixture
Surface area measurements were made by N2 adsorption/
desorption by using a Micrometrics Gemini II 2370 surface
123

Oxidative Coupling and Hydroxylation of Phenol
11
hydroquinone. Over H-BEA however, an additional reaction
pathway is evident, leading to the formation of
[1,10-biphenyl]-2,50-diol in addition to the hydroxylation
products. The catalytic activity of H-MFI, Fe-MFI and TS-1
is consistent with the results reported in other studies [19–
21]. The results for the catalytic activity and product selec-
tivity of the catalysts examined are summarized in Table 2.
In order to demonstrate that the catalytic activity is not
significantly affected by mass transport, Fe-MFI of varying
iron loading was prepared. With increasing iron loading on
the MFI structure, the Langmuir surface area and the total
pore volume decreases slightly. Results showed that
2.14 wt% Fe-MFI had the highest conversion compared to
the other Fe-MFI samples. 2.14 wt% Fe-MFI exhibited
reduced surface area and total pore volume, thus the cat-
alytic activity is mainly determined by the number of redox
sites and not mass transport limitations.
Table 1 Summary of catalyst properties Langmuir surface area, total
pore volume and acid concentration
Catalyst
Langmuir
Total
Acid
surface area pore volume concentrationb
(m2/g)
(mL/g)
(mmol/g)
TS-1
563
451
0.20
0.28
0.22
0.21
0.21
0.18
0.41
0.40
0.046
1.450
0.918
1.088
0.910
0.810
1.209
0.850
H-MFI
Fe-MFI (0.61 wt%)a 442
Fe-MFI (1.56 wt%)a 426
Fe-MFI (2.14 wt%)a 373
Fe-MFI (2.78 wt%)a 355
H-BEA
Fe-BEA
a
731
710
Fe content determined by ICP-OES
b
Acid concentration determined by NH3-TPD
contained 4.7 g of phenol and 0.45 g of H2O2 (30 wt%
aqueous solution). Toluene (140 mg) was added as an
internal standard. Catalyst (140 mg) was added to the
reaction mixture while hot, in order to minimize moisture
adsorption from the atmosphere.
4 Discussion
For TS-1 (a non-acidic catalyst), its redox properties (and
structure) are relevant and have a significant influence on
its catalytic activity. The catalytic activity of TS-1 mani-
fests its properties from the tetrahedral titanium present in
the TS-1 structure. The titanium in the TS-1 structure is
able to transition between oxidation states and can form
titanyl species on the surface [22]. The presence of tetra-
hedrally coordinated titanium species in TS-1 facilitates
redox reactions occurring in TS-1, and in turn it is likely to
result in the formation of radicals from H2O2. The radical
formation is key for the TS-1 catalyst because as shown by
the TPD experiments, TS-1 possesses the lowest acid
concentration in comparison to the other catalysts used, yet
exhibits the highest phenol conversion. Similar catalytic
activity is observed in other titanium impregnated catalysts
[23, 24], which can generate electron holes that oxidize
water to create hydroxyl radicals. The hydroxyl radicals
then react with the phenol (in the phenoxy radical state)
which generates the products p-benzoquinone, catechol,
and hydroquinone in a direct route. The surface of TS-1
also provides chemoselective control of product formation
which results in the formation of hydroquinone and cate-
chol, with very little p-benzoquinone being produced.
H-MFI exhibited higher concentration of acid sites and
in the group of Fe-MFI catalysts the 1.56 wt% loading
possessed the highest acid site concentration. The addition
of Fe results in ion exchange with the protonic acid sites.
Fe on the other hand introduces some Lewis acidity as well
as the redox function. With the 2.14 wt% loading, an
optimal balance of the acid sites generated by the Fe atoms,
the acidic protons and redox sites gives rise to a higher
phenol conversion compared to the other Fe-MFI zeolites.
The reaction components were quantified and identified
using gas chromatography mass spectrometry (GC/MS).
The GC used for the analysis was an Agilent 6890 equip-
ped with an Agilent 5973 mass selective detector (MSD),
and a 30 m Restek Rtx-200ms column with a crossbond
trifluoropropylmethyl polysiloxane stationary phase. Sep-
aration was carried out using a temperature program that
commences at 45 °C, held for 5 min, heated to 115 °C at a
rate of 10 °C/min and held for 3 min. This was then fol-
lowed by heating up to 285 °C at a rate of 10 °C/min and
held for 3 min. The detector was turned off at time periods
of 0.01–3.30 min and 4.40–8.00 min in order to avoid
saturating the detector with signal from the solvent peak.
3 Results
The results of the Langmuir surface areas, pore volumes
and acid site concentrations are summarized in Table 1 .
In the blank reaction using only H2O2 and in the absence
of any catalyst, trace amounts of p-benzoquinone and
catechol were detected, with a higher selectivity towards
p-benzoquinone. Using H2O2, it is generally recognized
that a phenolate anion is formed, which facilitates resonant
stabilization to hydroxylate phenol at the para and ortho
positions. The general mechanistic pathway for phenol
interaction with H2O2 is shown in Scheme 2 [18].
The catalysts studied in the investigation effectively cat-
alyzed the phenol hydroxylation reaction, and all catalysts
converted phenol to p-benzoquinone, catechol and
123

12
J. P. H. Li et al.
Scheme 2 Non-catalysed phenol oxidation reaction and major product formation mechanism [18]
Table 2 Summary of phenol conversion and product selectivity after 6 h
Phenol
conversion
after 6 h (%)
Product selectivity (%)
p-benzoquinone
Catechol
hydroquinone
[1,10-biphenyl]-2,50-diol
No catalyst
5
80
5
20
45
0
0
0
0
0
0
0
0
0
7
0
TS-1
30
2
50
0
H-MFI
100
100
91
38
35
3
Fe-MFI (0.61 wt%)
Fe-MFI (1.56 wt%)
Fe-MFI (2.14 wt%)
Fe-MFI (2.78 wt%)
H-BEA
8
0
0
8
8
0
20
10
23
8
35
34
62
33
27
31
28
33
Fe-BEA
34
At 2.14 and 2.78 wt% loading, the acidic protons are
replaced with the Lewis acidic Fe cations which result in
reduction in the acid site concentration. We observed an
increased selectivity towards catechol and hydroquinone.
The acid site concentration does not appear to have a
simple relationship with the catalytic activity. However,
it the data suggests that the nature and concentration of
acid sites do have an influence on the product selectivity
as H-MFI and 0.56 wt% Fe-MFI formed only p-benzo-
quinone. Further addition of Fe atoms reduces the
selectivity towards p-benzoquinone, forming catechol and
hydroquinone. A possible explanation is an acid cata-
lyzed conversion of catechol and hydroquinone to
benzoquinone or a direct route via stabilization of the
phenoxy radical. This is consistent with the data found
for 2.14 and 2.78 wt% Fe-MFI, as well as TS-1 catalyst.
For H-BEA, the acid sites are suggested to catalyse the
formation of the condensed product, because the pore
size in BEA is larger which enables the condensation
reaction. This also suggests that the redox properties of
the catalysts have more influence on the conversion than
the acidity.
123

Oxidative Coupling and Hydroxylation of Phenol
13
Scheme 3 Phenol hydroxylation reaction pathway involving H2O2 and Fe3?/Fe2? (fenton reaction) [25, 26]
It has been suggested that the catalytic decomposition of
hydrogen peroxide, in the presence of Fe2? ions (Fenton
reaction), takes place through a redox mechanism [25, 26]. A
representative pathway is shown in Scheme 3. The oxidation
potential of iron is not high enough to generate similar
electron holes in titanium dioxide. For iron catalysts to be
effective, it has been found that iron in a tetrahedral coor-
dination on a support results in more effective active sites
[27, 28]. The reactivity for both the Fe-MFI and Fe-BEA
suggest that the iron coordination on the respective zeolite
surfaces creates a sufficient concentration of Fe2? species
that enables a redox reaction between the zeolite and
hydrogen peroxide. In both the Fe-MFI and Fe-BEA cata-
lysts, the selectivity of p-benzoquinone, catechol, and
hydroquinone are virtually equal, suggesting that the Fe
provides little chemoselective control of product selectivity.
It is important to note that in many studies focusing on
the hydroxylation of phenol using H2O2 as an oxidant,
p-benzoquinone, catechol, and hydroquinone are the only
products to form. However in the present study, we observe
an additional product [1,10-biphenyl]-2,50-diol] formed in
during the reaction catalyzed by H-BEA (Scheme 1).
Although Atoguchi and colleagues observed similar con-
version levels and product selectivity as the present study,
in their work only catechol, and hydroquinone were
reported as major products [20]. It is likely that they had
produced the additional [1,10-biphenyl]-2,50-diol product,
but was not observed in their study as they had used a
biphenyl internal standard, which is expected to have very
similar retention time to [1,10-biphenyl]-2,50-diol on the
GC column and thus could have been missed in the ana-
lysis. H-BEA differs from the H-MFI, Fe-MFI and TS-1
catalysts in that its effective average pore diameter is sig-
nificantly larger than those of the other zeolites studied. It
also possesses significant mesoporosity and has accessible
surface acid sites in these mesopores.
Oxidative coupling of phenols has been thoroughly
investigated using biological catalysts, observing biphenyl
123

14
J. P. H. Li et al.
and triphenyl compounds in many cases [29, 30], and even
high molecular weight products formed from polymeriza-
tion [31]. The coupling of phenol moieties would suggest
that polymerization is more likely to occur in biological
system than over heterogeneous catalysts due to a combi-
nation of the enzyme activity and the lack of space
restriction. In some studies, functional groups were added
to phenol which facilitates the selective coupling to occur
[32, 33].
catalyst pores restricts further polymerization to occur
between the individual phenoxy radicals. Since hydroxylate
products and benzoquinone are also formed over acidic
H-BEA, a biphenyl compound is a possible intermediate for
the acid catalyzed reaction. Despite the high acid concen-
tration of H-MFI, the pores of acidic H-MFI are not suffi-
ciently large enough to enable the formation of the
condensed product, and result in very low selectivity for the
formation of the biphenyl species H-MFI possesses a pore
˚
diameter of 5.5 A, while a phenol molecule has a kinetic
˚
diameter of 2.5 A [34]. The diffusion in and out of the pores
The larger pore size of H-BEA in combination with the
high acid concentration generated from the surface protons,
enables two molecules of phenol to diffuse inside its pores
and coordinate two molecules of phenoxy radicals to a single
proton on the H-BEA surface in a hydride formation. The
proximity of the two molecules of phenoxy radicals enables
coupling at the ortho positions of the molecules. The high
product selectivity towards catechol would suggest that the
phenoxy intermediate is more susceptible to reaction from
the ortho positions of the molecules. As a result of the cou-
pling, one of the hydroxyl groups is removed and transferred
to the adjacent phenoxy intermediate, leading to a formation
of a diol in the para position in one of the phenoxy molecules.
This results in the formation of [1,10-biphenyl]-2,50-diol,
which is small enough to diffuse out of the pores. The sig-
nificance of this coupling reaction is that it occurs in mild and
solventless conditions. This holds great scientific interest as
it demonstrates the potential of forming other useful products
in addition to the commonly observed catechol, hydroqui-
none and benzoquinone. Biphenyl compounds are for
example useful precursor materials for valsartan: a drug used
in the treatment for Alzeimer’s disease.
would be restricted to single file diffusion of phenol and
˚
benzoquinone. H-BEA possesses a pore diameter of 7.8 A
which allows at least two molecules of phenol to diffuse,
form a transition state and accommodate diffusion of a
single molecule of the biphenyl compound to diffuse out of
the catalyst pores. Thus, for the acid catalyzed reaction, the
reaction intermediate is too bulky to be accommodated in
the MFI structure but can accommodate a single hydrox-
ylated species (or phenyl radical) which can further react to
benzoquinone. Over TS-1 (again an MFI structure), a
smaller intermediate is formed with the redox active site but
the reaction stops at the hydroquinone/catechol level,
because of the lack of acid sites.
Reaction using Fe-BEA is less effective than H-BEA, as
demonstrated by its reduced acid concentration and cata-
lytic activity. Fe-BEA is unable to catalyze the formation
of biphenyl compounds. This is intriguing, considering that
Fe-BEA possess similar pore size with H-BEA. This would
indicate that along with the pore size being a factor for
[1,10-biphenyl]-2,50-diol formation, the nature of the cata-
lyst surface is also important. While Fe ions on the zeolite
surface are able to facilitate a redox reaction with hydrogen
peroxide that lead to phenol hydroxylation, it is unable to
stabilize phenoxy radicals sufficiently for the coupling
reaction to occur. Over Fe modified MFI, similar to TS-1,
an intermediate species of smaller kinetic diameter is
formed that restricts to the formation of product species.
The pore sizes for all the zeolites investigated in this
study are of sufficient diameter to accommodate at least a
single ringed aromatic compound, and a biphenyl com-
pound in the case of reaction over H-BEA. The acidic
protons from H-BEA stabilize the phenoxy radicals, as
shown in Scheme 4. This facilitates their reaction, which
results in the formation of the condensed product. The
Scheme 4 Phenoxy radicals
stabilized on H-BEA surface
123

Oxidative Coupling and Hydroxylation of Phenol
15
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