Insight into contributions to phenol selectivity in the solution oxidation of benzene to phenol with H2O2

Article abstract of DOI:10.1016/j.catcom.2010.10.024

A high selectivity to mono-oxidation of benzene to phenol is a critical parameter for the evaluation and utilization of catalysts for the one-step oxidation process to synthesize phenol The results presented herein indicate that the uncatalyzed oxidation of phenol with H₂O₂ in solution is a key contributor to phenol oxidation (and, hence, phenol selectivity) in liquid phase oxidations of benzene with H₂O ₂. Additives (ex. sulfolane, acetonitrile, certain inorganic salts) are capable of inhibiting the uncatalyzed phenol oxidation, leading to their potential utility in improving phenol selectivities for benzene oxidation with H₂O₂.

Full text of DOI:10.1016/j.catcom.2010.10.024

Catalysis Communications 12 (2011) 480–484
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Insight into contributions to phenol selectivity in the solution oxidation of benzene
to phenol with H2O2
⁎
Michael L. Neidig, Kurt F. Hirsekorn
The Dow Chemical Company, Corporate R&D, Midland, MI, 48674, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2010
Received in revised form 22 October 2010
Accepted 27 October 2010
Available online 3 November 2010
A high selectivity to mono-oxidation of benzene to phenol is a critical parameter for the evaluation and
utilization of catalysts for the one-step oxidation process to synthesize phenol The results presented herein
indicate that the uncatalyzed oxidation of phenol with H2O2 in solution is a key contributor to phenol
oxidation (and, hence, phenol selectivity) in liquid phase oxidations of benzene with H2O2. Additives (ex.
sulfolane, acetonitrile, certain inorganic salts) are capable of inhibiting the uncatalyzed phenol oxidation,
leading to their potential utility in improving phenol selectivities for benzene oxidation with H2O2.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Phenol oxidation
TS-1
Benzene oxidation
H2O2
Zeolite
1. Introduction
catalysts for the oxidation of benzene with hydrogen peroxide. A
challenge in the mono-oxidation of benzene is the lack of kinetic
One of the most important intermediates in the chemical industry,
phenol is utilized in the fabrication of polycarbonates and epoxy
resins with more than 90% produced via the cumene process [1–4].
Significant efforts have been made towards more attractive routes to
phenol with most focusing on reducing hazardous waste and raw
material costs; the economic efficiency of the three-step cumene
process is strongly dependent on the market price of propylene (raw
material) and acetone (by-product). Towards this goal, gas-phase
oxidants, including N2O [5–7] and molecular oxygen [8–10], have
been explored for one-step routes, though these oxidants have
struggled to gain commercialization. Molecular oxygen leads to
selectivity problems as the high temperatures or strong catalysts
required to activate O2 lead to over-oxidation of benzene. While N2O
oxidation of benzene via the Solutia Alphox™ process leads to high
phenol selectivities (97–100%) at complete N2O conversion [11], on-
purpose production of N2O as a feedstock for this process is currently
not economical, limiting application of this technology to operations
with integrated N2O production processes such as the manufacture of
adipic acid [12].
control due to the higher reactivity of phenol towards oxidation than
for benzene, leading to substantial production of multiple-oxidation
products of benzene (i.e. hydroquinone, benzoquinone, catechol)
[16,17]. While numerous titanium-based materials have been inves-
tigated as liquid phase oxidation catalysts with hydrogen peroxide
including Ti-MCM-41, Ti-Beta and Ti-SBA-15 [18–20], MFI-type
zeolites have been the focus of significant research in benzene
oxidation catalysis due to the fact that the pore size of the MFI lattice
(5.5 Å) is similar to the diameter of the benzene molecule and may
impart selectivity to mono-oxidation. However, catalytic studies in
our lab of [Fe,Al]-MFI and TS-1 for the solution oxidation of benzene
with hydrogen peroxide under the same conditions, have shown very
different phenol selectivities (N98% for Fe and ≤90% for Ti) for similar
conversions under analogous reaction conditions [21]. Studies with
TS-1 have also shown that the addition of a dialkyl sulfone (e.g.
sulfolane) to batch reactions for oxidation of benzene with H2O2 can
dramatically improve phenol selectivity, limiting the formation of
multiple-oxidation products [22]. Due to this improvement in phenol
selectivity for TS-1 with an additive, we were interested in obtaining
further insight into the factors influencing phenol selectivity,
especially the selectivity difference between iron and titanium
based MFI catalysts in order to develop a viable catalytic process for
benzene oxidation with aqueous H2O2 and MFI-based materials.
While a recent study published during the revision of this
manuscript reports on factors contributing to TS-1 selectivities and
possible roles of sulfolane in affecting phenol selectivity [23], the
present study expands these experiments to iron-based materials and
additional additives which enhance selectivity, and provides insight
Due to the challenges in utilizing O2 and N2O as oxidants, the
development of a liquid-phase direct oxidation process with H2O2 also
stands as an attractive one-step route for the direct oxidation of
benzene [13–15]. High selectivity to mono-oxidation of benzene to
phenol is a critical parameter for the evaluation and utilization of
⁎
Corresponding author. Tel.: +1 989 638 4267.
E-mail address: kfhirsekorn@dow.com (K.F. Hirsekorn).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.10.024

M.L. Neidig, K.F. Hirsekorn / Catalysis Communications 12 (2011) 480–484
481
into the importance of the amount of H2O2 remaining in reaction to
phenol over-oxidation. The results herein provide insight on the
nature of the differences in phenol selectivity for TS-1 and [Fe,Al]-MFI
catalysts, and highlight the contribution of the uncatalyzed homoge-
neous reaction of phenol with hydrogen peroxide towards the
production of undesired higher-oxidation products even at low
reaction temperatures (i.e. 60 °C). The roles of potential additives
for improving phenol selectivity are also discussed.
analyzed by gas chromatography. Inclusion of the internal standard,
toluene allowed for quantitative analysis of the diluted product
mixture, and subsequent extraction of the product mixture concen-
trations pre-dilution. Diluted blanks, representative of the starting
reactant mixture, were included in each GC analysis to provide an
accurate measure of the initial phenol concentration in solution.
Phenol conversion was calculated as the concentration of the oxidation
products divided by the concentration of phenol in the original
reactant mixture. An analogous procedure was utilized for benzene
oxidation, where the total volume of the reactant mixture was
approximately 5 mL, comprising benzene (1.4 mL), H2O2 (0.6–3.6 mL
of 30 wt.% in water), cosolvent (0–2 mL), inorganic salt (0–1 mmol)
and catalyst (~80 mg). There was no evidence for the production of
tars in the reaction screens performed. In addition to GC analysis,
aliquots of the benzene oxidation diluted product mixture were
removed for titration by the ceric sulfate method to determine the
extent of H2O2 consumption [25].
2. Experimental
2.1. Catalyst preparation
Microporous material, TS-1 was prepared following literature
methods [24] by first mixing tetraethyl orthosilicate (302 g) and
titanium (IV) butoxide (6.1 g) in a Teflon beaker under a blanket of
nitrogen at room temperature. In a separate Teflon beaker tetra-
propylammonium hydroxide (118 g of 40 wt.% TPAOH in water) and
distilled water (287 g) were mixed. The TPAOH solution was then
slowly added to the first solution with stirring at room temperature.
An additional loading of distilled water (287 g) was added slowly
with stirring until the solution became clear. The resulting solution
was then transferred to a series of 125 mL Teflon liners which were
placed into 125 mL Parr reactors inside a static oven. The reactors
were heated at 160 °C for 96 h, cooled to ambient temperature, and
the contents were combined. The pH was adjusted to 8 using 1 M
nitric acid, and the mixture was centrifuged at 7500 rpm for 15 min.
The solids were isolated, washed with distilled water, and centrifuged
3 times. The resultant solid was dried at 100 °C for 24 h, and then
calcined first under nitrogen and then air. The calcination procedure
involved heating to 230 °C under nitrogen at a rate of 1 °C/min,
holding for 24 h, then heating to 540 °C at a rate of 1 °C/min, holding
for 4 h, and then switching the gas feed to air and holding at 540 °C for
8 h before cooling to ambient temperature. The resulting calcined
sample was analyzed by X-ray powder diffraction, with the spectrum
indicative of an MFI-type lattice, UV/Vis and Ti content analysis
(Supplementary data).
3. Results and discussion
3.1. Uncatalyzed phenol oxidation in solution
The mechanism of oxidation reactions with H2O2 over TS-1 have
been extensively studied, including phenol oxidation, with a hydro-
peroxo titanium species generally accepted to be the active species
involved in catalysis [26]. However, the potential contributions from
an uncatalyzed homogenous reaction between phenol and hydrogen
peroxide under the conditions employed for benzene oxidation with
H2O2 have not been as widely explored. Therefore, initial studies
focused on the investigation of uncatalyzed phenol oxidation. The
results of the batch reactor studies (60 °C, 3 h, phenol: H2O2 =2) are
given in Table 1. In the presence of only H2O2 and water, a phenol
conversion of ~13% is observed, where the predominant products
observed are hydroquinone and catechol in a ~2:1 ratio. This result is
key to understanding higher-oxidation of benzene in the presence of a
catalyst as it suggests that an uncatalyzed homogenous reaction
between phenol and H2O2 may contribute to further oxidation of
phenol product under these conditions. To determine the possible
effects of the catalyst support on this solution oxidation, analogous
phenol reactions were performed with the addition of ZSM-5 (1.25%
Al), silicalite and amorphous silica. In all cases, the observed phenol
conversion was largely unchanged (~12%) indicating that the support
surfaces did not inhibit or contribute further towards the uncatalyzed
oxidation of phenol.
Microporous material, [Fe,Al]-MFI (1.04 mol% Fe, 1.25 mol% Al),
was prepared by the method described above except iron(II) nitrate
was utilized in place of the titanium source and aluminum hydroxide
was included within the second addition of distilled water. The
resulting calcined sample was analyzed by X-ray powder diffraction,
with the spectrum indicative of an MFI-type lattice, UV/Vis and Ti
content analysis (Supplementary data). TS-1 and [Fe,Al]-MFI were
used for reaction testing without further treatment or activation.
3.2. Time course of phenol oxidation in solution
2.2. Oxidation reaction and products analysis
The uncatalyzed homogenous reaction of phenol with H2O2
indicates that this undesired side reaction may be a key component
of the overall phenol selectivity (i.e. extent of phenol oxidation)
observed in MFI-type zeolite catalyzed oxidations of benzene with
H2O2 at 60 °C. The relative rate of the solution (uncatalyzed) phenol
oxidation compared to the rate of catalyzed benzene oxidation to
phenol is important in this evaluation. Hence, the oxidation of phenol
in solution with H2O2 (uncatalyzed) was determined as a function of
time. From the results in Fig. 1A, phenol oxidation is minimal during
the initial 1–2 h of reaction then increases dramatically with further
A Biotage-Argonaut Endeavor® reactor was utilized for phenol and
benzene oxidation reaction screening. The Endeavor reactor utilizes
eight stainless-steel beds (~10 mL volume each) with independent
temperature and pressure control. These reactors permitted screening
with high stir rates (1000 rpm) under high N2 pressures (~300 psi), in
addition to the ability to perform facile repetitions to reduce
experimental error. In a typical phenol oxidation run, the total volume
of the reactant mixture was approximately 5 mL, comprising phenol
(0.95 g), H2O2 (0.574 mL of 30 wt.% in water), water (0–3.5 mL),
cosolvent (0–3.5 mL) and catalyst (0–80 mg). All reactions were run at
60 °C for 0.5–3 h. For reaction mechanism studies, catechol (5 mmol)
or 2,6-di-tert-butyl-4-methylphenol (BHT, 0.1 mmol) were also added
to the reaction mixture. Inhibition studies involved the use of
acetonitrile or sulfolane as cosolvent or the addition of an inorganic
salt (0–1 mmol) to the reaction mixture. Upon completion of a run, the
liquid phase was homogenized with acetonitrile (15 mL) containing
an internal standard (2 wt.% toluene), the solutions were filtered to
remove catalyst and a sample of the diluted product mixture was
Table 1
Phenol oxidation with H2O2 in a batch reactor.
Additive
Phenol conversion (%)
None
12.8
12.0
11.7
12.4
ZSM-5 (1.25 mol% Al)
Silicalite
Amorphous silica (PQ MS-1733)

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M.L. Neidig, K.F. Hirsekorn / Catalysis Communications 12 (2011) 480–484
Fig. 1. Time course of uncatalyzed phenol oxidation with H2O2. (A) phenol oxidation
) and with 5 mmol catechol ( ) or 0.1 mmol BHT ( ) added to the
reaction and (B) H2O2 consumption during phenol oxidation and with
0.1 mmol BHT added to the reaction ( ).
Fig. 2. Time course of benzene oxidation with H2O2 catalyzed by (A) TS-1 and
(B) [Fe,Al]-MFI.
(
(
)
same time period. While the benzene conversion and peroxide
consumption time profiles are interesting, a key observation relates
to phenol selectivities with time. The iron-based catalyst exhibits high
phenol selectivities (N98%) throughout the course of the reaction
while TS-1 gives a much lower phenol selectivity after 3 h (b80%).
However, the phenol selectivity for TS-1 is high over the initial hour of
reaction (N90%) and steadily decreases with further reaction time
even though overall benzene conversion only slowly increases. Thus,
it appears that elongated reaction times in TS-1 serve predominately
to oxidize phenol to undesired multiple-oxidized products and that, at
short reaction times, TS-1 exhibits much higher phenol selectivities.
Comparison of the relative rates of catalyzed benzene oxidation
and uncatalyzed homogeneous phenol oxidation yields an interesting
hypothesis. The fact that the uncatalyzed phenol oxidation profile
coincides with the decrease in phenol selectivity with time for TS-1
suggests that it is uncatalyzed homogeneous oxidation of phenol and
not catalyzed oxidation of product in the zeolite that is responsible for
the decreased phenol selectivity in TS-1. Furthermore, the lack of
solution oxidation of phenol in the iron-based catalyst compared to
that observed for the titanium-based catalysts can be explained in
terms of the amount of unreacted H2O2 present at longer reaction
times where the uncatalyzed phenol oxidation reaction becomes
significant. Titanium based catalyst consume relatively low amounts
of peroxide throughout the reaction (b30% total) whereas iron-based
catalysts consume 50% of the peroxide within the initial 30 min of
reaction. Since the uncatalyzed oxidation of phenol would be
expected to be dependent upon the concentration of H2O2, it appears
that the high phenol selectivity of [Fe,Al]-MFI derives from the fact
that this catalyst rapidly consumes a majority of the peroxide in
solution, thus, limiting the amount of H2O2 available for thermal
decomposition to generate the H2O2 decomposition oxidant for
uncatalyzed phenol oxidation.
time. Importantly, the rate of H2O2 consumption correlates to that of
phenol oxidation (Fig. 1B), with an initial lag phase and higher H2O2
consumption at longer reaction time. For this reaction, an autocata-
lytic mechanism could exist whereby the hydroxyphenol products
catalyze the reaction of H2O2 with phenol. To test this possibility,
phenol oxidation reactions were performed with the addition of
catechol to the reaction mixture. However, catechol addition
(5 mmol) does not change the rate of phenol oxidation (Fig. 1A)
indicating that an alternative mechanism must exist. Since some
thermal H2O2 decomposition occurs even at 60 °C over time in a batch
reaction, it is possible that the active oxidant is part of the H2O2
decomposition route – possibly a radical species. If a radical
mechanism is present, the addition of a radical trap (2,6-di-tert-
butyl-4-methylphenol, BHT) to the reaction should inhibit phenol
oxidation. From Fig. 1A, the addition of 0.1 mmol BHT to the reaction
substantially reduces phenol oxidation, in addition to reducing the
rate of H2O2 consumption (Fig. 1B). Therefore, these mechanistic
studies and the observed lag phase to the reaction are consistent with
the formation of a radical H2O2 decomposition product which oxidizes
phenol, where the rate of reaction of phenol with the H2O2
decomposition product is much faster than the formation of this
decomposition product.
The relative rates of benzene oxidation to phenol catalyzed by TS-1
and [Fe,Al]-MFI in batch reactions are given in Fig. 2. While the overall
rate of oxidation is slower for benzene relative to phenol, a greater
percentage (~50%) of the total oxidation occurs within the initial
30 min of reaction time. Hydrogen peroxide consumption is low for
the titanium based catalysts at short reaction times, in comparison to
the iron-based system which rapidly consumes over 50% of the total
peroxide within 30 min. As has previously been observed, significant
unreacted H2O2 remains in the reaction with TS-1 after three h while
[Fe,Al]-MFI consumes the majority of the peroxide in solution over the
To further test this hypothesis, phenol oxidation batch reactions
were performed with the addition of either TS-1 or [Fe,Al]-MFI to the

M.L. Neidig, K.F. Hirsekorn / Catalysis Communications 12 (2011) 480–484
483
reaction. If phenol oxidation is largely an uncatalyzed homogeneous
reaction dependent upon the concentration of H2O2, it would be
expected that addition of TS-1 would have little effect on oxidation
whereas [Fe,Al]-MFI would inhibit this reaction through its high H2O2
consumption rate. In fact, this is exactly what is observed. Whereas
TS-1 has essentially no effect on phenol oxidation compared to the
uncatalyzed reaction (13% phenol oxidation with TS-1), addition of
[Fe,Al]-MFI leads to a significant reduction in total phenol oxidation to
7%. Based upon these uncatalyzed phenol oxidation studies, a new
reaction scheme for the control of phenol selectivity in MFI-type
zeolites can be obtained (Fig. 3). While some metal catalyzed phenol
oxidation (k6) may be possible, uncatalyzed phenol oxidation (k5)
appears to be the dominate pathway determining phenol selectivity.
However, the fact that this oxidation requires H2O2 (or a thermal
decomposition product thereof) as a reactant leads to the fact that the
rate of metal catalyzed H2O2 consumption (which is predominately
uncoupled, k3) is key to the control of phenol selectivity. Thus, as
observed, those catalysts which rapidly consume hydrogen peroxide
(i.e. Fe) will exhibit higher phenol selectivities than those which only
slowly consume peroxide (i.e. Ti) unless a way to suppress
uncatalyzed homogeneous H2O2 decomposition leading to phenol
oxidation can be identified.
Fig. 4. Effects of additives on uncatalyzed oxidation of phenol with hydrogen peroxide
(1 mmol addition effects; solid black denotes use at 3.5 mL in a 5 mL total reaction
volume).
sulfolane in solution is an inhibitor of the uncatalyzed homogeneous
oxidation of phenol. Furthermore, if sulfolane is used in excess
(3.5 mL in a 5 mL total volume reaction) a decrease in phenol
oxidation of ~100-fold is observed. Thus, sulfolane can effectively
inhibit the uncatalyzed homogeneous oxidation of phenol under
these reaction conditions. These results suggest that the observed
improvements to phenol selectivities in the previously reported TS-1
study may have originated from inhibition of uncatalyzed phenol
oxidation by sulfolane in lieu of pathways involving the prevention of
phenol coordination to the active catalyst, consistent with a recent
study by Barbera et al. [23].
Another co-solvent to investigate as a potential inhibitor of the
uncatalyzed solution oxidation of phenol is acetonitrile. Previous
studies have shown acetonitrile to be the optimal co-solvent for use in
[Fe,Al]-MFI catalyzed benzene oxidations, providing a balance
between H2O2 consumption and benzene conversion. Based upon
previous literature data, it was speculated that acetonitrile may react
with hydrogen peroxide to form a peroxycarboximidic acid which is
slower to react with a catalyst than pure hydrogen peroxide [27].
Utilizing acetonitrile as an additive to the phenol oxidation reaction,
analogous results are observed as in the sulfolane addition experi-
ment whereby acetonitrile at 1 mmol addition reduces phenol
oxidation by half and excess acetonitrile leads to a ~100-fold decrease
3.3. Effects of additives on phenol oxidation
Since high H2O2 selectivities are required in addition to high
phenol selectivities for any commercial process, it is desirable to
investigate alterative methods to inhibit uncatalyzed oxidation of
phenol beyond utilizing catalysts with high H2O2 consumption rates
and, thus, inherently poor H2O2 selectivities. One approach that could
be employed, and has been reported in the literature, for improving
phenol selectivities is the use of co-solvents/additives to the reaction.
Bianchi and co-workers have previously reported that the use of
sulfolane as a co-solvent for the oxidation of benzene to phenol with
TS-1 leads to improvement in phenol selectivity [22]. This effect was
rationalized in terms of the ability of sulfolane to form a complex with
phenol, with the sterically hindered complex being unable to enter
the TS-1 pores to undergo further oxidation.
Based upon our previous findings in this study indicating that
uncatalyzed phenol oxidation is a key contributing factor to phenol
over-oxidation, it was interesting to consider whether sulfolane could
inhibit phenol oxidation in the absence of a catalyst. The addition of
1 mmol sulfolane to the batch reaction of phenol with H2O2 (Fig. 4)
results in a ~50% decrease in total phenol oxidation, indicating that
Fig. 3. Reaction scheme for benzene oxidation with H2O2 including uncatalyzed phenol oxidation.

484
M.L. Neidig, K.F. Hirsekorn / Catalysis Communications 12 (2011) 480–484
in phenol oxidation. Thus, in addition to high H2O2 consumption rates
for iron-based catalysts, the utilization of acetonitrile as a co-solvent
further aides in the high phenol selectivities attained. Unfortunately,
the negative impact of acetonitrile on catalytic activity in titanium-
based catalysts prevents its utilization as a co-solvent for TS-1.
From these additive studies, it is apparent that the addition of a co-
solvent can inhibit the uncatalyzed phenol oxidation reaction through
interaction with either reactant (phenol or H2O2), thus sequestering it
from the reaction. Inorganic salts, which are commonly used as
peroxide stabilizers, also have the potential to interact with H2O2
through acid–base reactions or hydrogen bonding in solution. While
testing the effects of inorganic salts on H2O2 stability and efficiency,
high phenol selectivities were observed with monobasic phosphate
salts, suggesting that these salts may also inhibit uncatalyzed phenol
oxidation. As shown in Fig. 4, the addition of 1 mmol (NH4)H2PO4 or
K2H2PO4 to the phenol oxidation reaction decreases oxidation to less
than 25% of the conversion in the absence of an additive. While these
salts likely interact with peroxide, sequestering it from the reaction,
the specific molecular level mechanism of inhibition is currently
unresolved (i.e. acid–base, H-bonding, etc.).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi:10.1016/j.catcom.2010.10.024.
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4. Conclusions
The results presented herein indicate that the uncatalyzed
homogeneous oxidation of phenol with H2O2 in solution is a key
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liquid phase oxidations of benzene with H2O2. A key contribution to
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