One-step oxidation of benzene to phenol over Cu/Ti/HZSM-5 catalysts

Article abstract of DOI:10.1007/s10562-009-0240-4

The gas-phase catalytic oxidation of benzene over copper supported on HZSM-5 added with titanium (Cu/Ti/HZSM-5) was carried out by using molecular oxygen as an oxidant. Phenol was effectively produced by the titanium addition to Cu/HZSM-5. The titanium addition to Cu/HZSM-5 induces the easy reduction of Cu²⁺ Species to Cu⁺ species in the catalysts, and the produced Cu⁺ species seems to produce the phenol effectively.

Full text of DOI:10.1007/s10562-009-0240-4

Catal Lett (2010) 134:324–329
DOI 10.1007/s10562-009-0240-4
One-Step Oxidation of Benzene to Phenol over Cu/Ti/HZSM-5
Catalysts
•
Yuichi Ichihashi Yo-hei Kamizaki Naoya Terai
•
•
•
Keita Taniya Shigeru Tsuruya Satoru Nishiyama
•
Received: 12 October 2009 / Accepted: 19 November 2009 / Published online: 11 December 2009
Ó Springer Science+Business Media, LLC 2009
Abstract The gas-phase catalytic oxidation of benzene
over copper supported on HZSM-5 added with titanium
(Cu/Ti/HZSM-5) was carried out by using molecular
oxygen as an oxidant. Phenol was effectively produced by
the titanium addition to Cu/HZSM-5. The titanium addition
to Cu/HZSM-5 induces the easy reduction of Cu2? species
to Cu? species in the catalysts, and the produced Cu?
species seems to produce the phenol effectively.
phenol from benzene has been focused on, not only from
the point of view of an organic synthesis, but also from a
chemical industrial point of view. Phenol production is
currently carried out through the Cumene process or tolu-
ene oxidation. Especially, former process which is a most
popular process of phenol production consists of three steps
and produces acetones as byproduct. The direct oxidation
of benzene is more desirable for the phenol production,
because of more economical and without byproducts.
The liquid-phase direct hydroxylation of benzene has
been reported by Dixon and Norman using ferrous sulfate-
H2O2 system (Fenton reagent) [6]. The partially exchanged
hetero-poly acid has been also reported as a catalyst for the
liquid-phase oxidation of benzene in the presence of H2O2
oxidation [7, 8]. The gas-phase oxidation of benzene to
phenol with N2O has been actively studied by many
authors [9–12]. Especially, Panov et al. have reported that
Fe/ZSM-5 effectively catalyze the benzene oxidation to
phenol with N2O [10–12]. However, N2O is expensive for
use as an oxidant on a large scale. Sasaki and co-workers
have demonstrated the benzene oxidation to phenol with
high selectivity in the presence of both O2 and H2 over a
supported Cu-Pd binary catalyst. However, the use of a
mixture of H2 and O2 has the risk of explosion [13, 14]. It
has been more recently studied that molecular oxygen
directly oxidizes benzene to phenol on various catalysts in
the presence of reducing regent [15–24]. We have previ-
ously reported that the supported copper catalysts effec-
tively oxidize the benzene in acetic acid solution to phenol
with molecular oxygen in the presence of ascorbic acid as a
reducing reagent [22–24]. In these cases, copper ions have
been indicated to have redox properties according to phy-
sic-chemical measurements [22–28]. However, the direct
oxidation of benzene to phenol without reducing reagents
is more desirable due to the industrialization.
Keywords Cu/Ti/HZSM-5 ꢀ
Partial oxidation of benzene ꢀ Phenol synthesis
1 Introduction
The partial oxidation of hydrocarbons into various deriv-
atives such as alcohols, aldehydes, ketones, carboxylic
acids and epoxides has been extensively investigated,
because such products are industrially significant [1–5].
However, the industrial oxidation process usually demands
high selectivities, high yields, energy efficiency or
economical efficiency. Phenol has been well known to be
one of the most important chemicals among the fields of
resin, fiber, and medicine. Recently, the direct synthesis of
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-009-0240-4) contains supplementary
material, which is available to authorized users.
Y. Ichihashi (&) ꢀ Y. Kamizaki ꢀ N. Terai ꢀ K. Taniya ꢀ
S. Tsuruya ꢀ S. Nishiyama
Department of Chemical Science and Engineering, Graduate
School of Engineering, Kobe University, Rokkodai, Nada,
Kobe 657-8501, Japan
e-mail: ichiy@kobe-u.ac.jp
123

One-Step Oxidation of Benzene to Phenol
325
We have investigated about the gas-phase direct oxi-
dation of benzene to phenol without reducing reagents. The
copper catalysts supported on HZSM-5 or NaZSM-5 have
been found to oxidize benzene with only molecular oxygen
in the gas-phase [29]. In this catalyst, the obtained yield
and selectivity of phenol have been quite low. It has been
also reported that the calcination of the Cu/HZSM-5 at the
higher temperatures leads to the improvement of phenol
formation and the isolated square-pyramidal Cu species is
suggested to be the active species for the phenol formation
[30]. In this study, we attempt the gas-phase direct oxida-
tion of benzene to phenol with molecular oxygen over
Cu/HZSM-5 unmodified and modified by titanium. The
effect of titanium addition is investigated in detail.
performed under atmospheric pressure using a conventional
continuous flow fixed-bed reactor (pyrex glass, i.d. =
18 mm). The catalyst was usually pre-treated at 773 K for
2 h in air flow. Before the reaction, the reactor including the
pre-treated catalyst was purged with N2 gas for 30 min at
773 K. The oxidation was started by supplying benzene to
the reactor using a micro-feeder. The standard reaction
conditions are as follows: W/F = 2.36 9 102 gcat min/mol;
W (catalyst weight), 0.5 g; F (total flow rate), 2.12 9
10-3 mol/min; reaction temperature, 673 K; benzene:
N2:O2 = 0.18:4:1. The liquid products which were trapped
at 223 K with a refrigerant (liquid N2 ? diethyl malonate)
were diluted with a mixed solvent of toluene and 2-propanol
(v/v, 1:1). The liquid products such as phenol, p-benzoqui-
none, and hydroquinone were analyzed with a liquid chro-
matograph (Jasco, LC-2000 Plus) using a C18 column
(0.15 m) under the mixed solution carrier of 0.1 vol% tri-
fluoroacetic acid aq. and acetonitrile (v/v, 4:6). The gaseous
products such as CO and CO2 were analyzed with a gas
chromatograph equipped with TCD using a stainless steel
columns containing active carbon (1 m), active carbon
(4 m) and molecular sieve 5A (1 m) at 413 K, room tem-
perature, and room temperature, respectively under H2 car-
rier. Yield of benzene, p-benzoquinone, hydroquinone, CO
or CO2 was defined as follows:
2 Experimental
HZSM-5 was obtained commercially (ZEOLYST:
Si/Al = 29). Ti supported on HZSM-5 (Ti/HZSM-5) was
obtained by an impregnation with an aqueous solution of
[TiO(C2O4)2]ꢀnH2O. After the impregnation, the material
was dried at 393 K overnight, then calcined at 773 K for
5 h in air flow. The supported Cu catalysts (Cu/HZSM-5 or
Cu/Ti/HZSM-5) were also prepared by impregnating either
yield of phenol; p-benzoquinone; or hydroquinone ð%Þ
ð1Þ
½mole number of phenol; p-benzoquinone; or hydroquinone producedꢂ
¼
ꢁ 100
½mole number of benzene fedꢂ
HZSM-5 or Ti/HZSM-5 with an aqueous solution of
yield of CO or CO2ð%Þ
Cu(CH3COO)2ꢀH2O. After the impregnation, the catalyst
was dried at 393 K overnight, then calcined at 1,273 K in
air flow for 5 h. The samples will be designated as n wt%
Cu/HZSM-5 or Cu/Ti/HZSM-5 [Ti/Cu = m] where n rep-
resents the content of copper (n = 0.7) and m represents
Ti/Cu atomic ratio (m = 0, 0.4, 0.5, 0.7, and 1.0).
ð1=6Þ ꢁ ½mole number of CO; or CO2 producedꢂ
¼
ꢁ 100
ð2Þ
½mole number of benzene fedꢂ
Yield of partial oxidation products was also defined as
follows:
yield of partial oxidation productsð%Þ
ð3Þ
½sum total of mole number of phenol; p-benzoquinone; or hydroquinone producedꢂ
¼
ꢁ 100
½mole number of benzene fedꢂ
Benzene (Nakarai Tesque, guaranteed reagent) was used
without further purification. The oxidation of benzene was
The amounts of CO adsorption on the catalysts were
measured as follows: Sample (0.05 g) was calcined at
123

326
Y. Ichihashi et al.
773 K in O2 (2.6 kPa) for 1 h in the quartz cell connected
to the vacuum systems. It was cooled to the reaction
temperature (673 K) with O2, and evacuated under vacuum
condition (10-2 Pa) for 2 h, and then the dead volume of
this sample was measured by using He gas. The sample
was re-evacuated under vacuum condition for 0.5 h at
673 K, then CO gas (2.6 kPa) was induced to the sample at
673 K. The amount of adsorbed CO was estimated by the
measurements of pressure change.
X-ray absorption fine structure (XAFS) spectra for the
samples were taken at room temperature in fluorescence
mode for K-edge of Cu at the beam-line BL01B1 of
SPring-8. After the calcination of samples at 773 K for 1 h,
the samples also were sealed with polyethylene films in
nitrogen atmosphere. Fourier transformations for Cu sam-
ples were performed on k3-weighted EXAFS oscillations in
the range of 3–12 A-1. Electron spin resonance spectra of
˚
Cu/HZSM-5 and Cu/Ti/HZSM-5 catalysts were measured
at room temperature with a JEOL JES-TE300 X band-
spectrometer. Prior to ESR measurements, samples were
calcined at 773 K for 1 h in O2, followed by evacuation at
room temperature or 773 K for 2 h at 10-2 Pa. The mag-
netic field was corrected by using a manganese maker.
Fig. 1 Time profile of phenol yield for the oxidation of benzene over
Cu/HZSM-5 (open square) and Cu/Ti/HZSM-5 (filled circle). Cata-
lyst, Cu/Ti/HZSM-5 (Cu = 0.7 wt%, Ti/Cu = 0.7, Si/Al = 29),
Cu/Ti/HZSM-5 (Cu = 0.7 wt%, Si/Al = 29)
3 Results and Discussion
Cu/HZSM-5 and Cu/Ti/HZSM-5 catalysts oxidized ben-
zene to phenol, hydroquinone, p-benzoquinone, CO, and
CO2. No formation of oxidation products was observed
over the catalysts unsupported with Cu, such as HZSM-5
and Ti/HZSM-5. The phenol yield during the oxidation of
benzene over Cu/HZSM-5 and Cu/Ti/HZSM-5 catalysts is
shown in Fig. 1. Titanium addition to Cu/HZSM-5 catalyst
is found to cause the increase of phenol yield. In this case,
the selectivity of phenol formation also improved by the Ti
addition to Cu/HZSM-5. The maximum phenol yield of
Cu/HZSM-5 is obtained at the time on stream of 100 min,
whereas Cu/Ti/HZSM-5 catalyst provides maximum that at
the reaction time of 80 min. It was previously reported that
the redox cycle of between Cu? and Cu2? was related with
the formation of oxidation products [29, 30]. The induction
period may be due to the generation of Cu? from Cu2?
species. Hence, it is decided to estimate the yield of the
every product in the point of time which is got a maximum
phenol yield. Figure 2a also shows the yield of every
product until 4 h of the time on stream. The yield of partial
oxidation products, such as phenol, hydroquinone, and
p-benzoquinone, gradually decrease with the time on
stream. On the other hand, yield of CO and CO2 continu-
ously increase. The re-calcination treatment of the used Cu/
Ti/HZSM-5 catalyst leads to the significant recovery of
partial oxidation yield (see Fig. 2b). The carbon deposit is
Fig. 2 Time profile of product yields for the oxidation of benzene
over (A) fresh and (B) re-calcined Cu/Ti/HZSM-5 catalysts. Re-
calcined catalyst was obtained by the calcinations of used catalyst for
5 h at 773 K. Catalyst, Cu/Ti/HZSM-5 (Cu = 0.7 wt%, Ti/Cu = 0.7,
Si/Al = 29); Yield of phenol (filled circle), hydroquinone (times),
p-benzoquinone (filled square), CO (filled diamond) and CO2 (filled
triangle)
found to suppress the formation of partial oxidation prod-
ucts, and the re-calcination treatment of used catalysts
causes desorption of carbon deposit on the catalysts. This
indicates that Cu/Ti/HZSM-5 catalysts are reusable by the
re-calcination treatment in air flow. Figure 3 also shows the
XAFS spectra of Cu/Ti/HZSM-5 catalysts before and after
123

One-Step Oxidation of Benzene to Phenol
327
Fig. 3 XANES [A, B, C] and
k3-weighted Fourier transforms
of EXAFS [a, b, c] of
Cu/HZSM-5 (Cu = 0.7 wt%,
Si/Al = 29) [A, a], and
Cu/Ti/HZSM-5
(Cu = 0.7 wt%, Ti/Cu = 0.7,
Si/Al = 29) before [B, b]
and after reaction [C,c]
(C)
(B)
(A)
(c)
(b)
(a)
8950
9000
9050
9100
0
2
4
6
Energy (eV)
Distance (Å)
reaction. XANES and Fourier transforms of EXAFS spectra
indicate that no significant change of the valency and
coordination geometry of Cu species is observed after the
reaction. Therefore, these results were demonstrated that the
carbon deposit led to the deactivation of phenol formation.
Figure 4 indicates the dependence of the amounts of
titanium addition to Cu/HZSM-5 catalysts. As mentioned
above, the yield of every product was estimated in the point
of time which got a maximum phenol yield. These results
indicate that the increase of Ti/Cu atomic ratio until 0.7
leads to the improvement of phenol yield and selectivity.
The yields of hydroquinone and p-benzoquinone were also
improved by the increase of Ti/Cu atomic ratio. The
maximum phenol yield is obtained at 0.7 of Ti/Cu atomic
ratio, and then the phenol yield gradually decreases with
the increase of Ti/Cu atomic ratio. Hence, phenol yield is
optimized at 0.7 of Ti/Cu atomic ratio. Although the yields
of hydroquinone and p-benzoquinone show the almost
same behavior as phenol yield, CO2 yield continuously
increases with the increase of Ti/Cu atomic ratio. It is
speculated that the active site of CO2 formation is different
from that of phenol, hydroquinone and p-benzoquinone.
It is investigated about the relationship between Cu
species and the titanium addition to Cu/HZSM-5 in detail.
XANES spectra of both Cu/HZSM-5 and Cu/Ti/HZSM-5
catalysts show that there are most as Cu2? species in these
catalysts [31] (see Fig. 3). No significant change of the
valency and coordination geometry of Cu species is
observed by Ti addition. UV–Vis measurements of both
catalysts also gave same results as XAFS measurements
(figure not depicted). Hence, the almost copper species
exists as CuO species in the Cu/HZSM-5 and Cu/Ti/
HZSM-5 catalysts. It is known that the evacuation of these
Fig. 4 Dipendence of the Ti/Cu atomic ratio of Cu/Ti/HZSM-5
catalysts on the benzene oxidation. Catalyst, Cu/Ti/HZSM-5
(Cu = 0.7 wt%, Ti/Cu = 0–1.0, Si/Al = 29); Yield of phenol (filled
circle), hydroquinone (times), p-benzoquinone (filled square), CO
(filled diamond) and CO2 (filled triangle): Selectivity of phenol (open
circle)
catalysts at higher temperatures under vacuum condition
leads to the formation of Cu? species by reduction of Cu2?
species. And, CO gas selectively adsorbs to the Cu? spe-
cies. In short, the amounts of adsorbed CO to the copper
catalysts can imply the amounts of Cu? species which is
generated by the reduction treatment under vacuum con-
dition. The relationship between the yield of partial
oxidation products and the amount of adsorbed CO to the
Cu/Ti/HZSM-5 catalysts for the different Ti/Cu atomic
123

328
Y. Ichihashi et al.
Fig. 5 Relationship between yield of partial oxidation products
(filled circle) and the amounts of adsorbed CO (open triangle) on
Cu/Ti/HZSM-5 catalysts. Catalyst, Cu/Ti/HZSM-5 (Cu = 0.7 wt%,
Ti/Cu = 0–1.0, Si/Al = 29)
Fig. 6 Parallel component of ESR spectra of Cu2? species for
Cu/HZSM-5 (a, b) and Cu/Ti/HZSM-5 (c, d). Both catalysts were
calcined at 773 K for 1 h in O2, followed by evacuation at room
temperature (a, c) or 773 K (b, d) for 2 h at 10-2 Pa. Catalyst, Cu/Ti/
HZSM-5 (Cu = 0.7 wt%, Ti/Cu = 0.7, Si/Al = 29), Cu/Ti/HZSM-5
(Cu = 0.7 wt%, Si/Al = 29)
ratio was shown in Fig. 5. The amounts of adsorbed CO
increase with increasing Ti/Cu atomic ratio, and the maxi-
mum amounts of adsorbed CO are obtained at 0.7 of Ti/Cu
atomic ratio. This result indicates that Cu species of the
Cu/Ti/HZSM-5 catalysts at 0.7 of Ti/Cu atomic ratio is
reduced most easily to Cu? species. Furthermore, the
maximum yield of partial oxidation products over Cu/Ti/
HZSM-5 is also obtained by 0.7 of Ti/Cu atomic ratio. It is
recognized that the yield of partial oxidation products shows
the almost same behavior as the amounts of adsorbed CO.
Hence, the yield of partial oxidation products seems to have
relation to the amounts of Cu? species.
The degree of the reduction of Cu2? species in the
Cu/Ti/HZSM-5 is investigated by ESR measurements
(Fig. 6). Two signals (g1k ¼ 2:33; Ak1 ¼ 157; g2k ¼ 2:31;
A2k ¼ 159) are detected in the ESR spectra of Cu2? species
for Cu/HZSM-5 and Cu/Ti/HZSM-5, indicating two indi-
vidual configurations of Cu2? species presence in the
Cu/HZSM-5 and Cu/Ti/HZSM-5. Kucherov et al. were
reported that these signals were assigned to two square-
pyramidal (g1k ¼ 2:33; Ak1 ¼ 158; gk2 ¼ 2:31; Ak2 ¼ 160)
and a square-planar (g3k ¼ 2:28; Ak3 ¼ 171) Cu2? species
[32, 33]. Hence, these signals of Cu/Ti/HZSM-5 are
assigned to the square-pyramidal. The intensities of these
spectra are decreased by the reduction treatment. In espe-
cial, it is obvious that the intensity of Cu/Ti/HZSM-5
significantly decreases by the reduction treatment. The
intensity of the reduced Cu/Ti/HZSM-5 was about 40% of
the untreated catalyst. As for the reduced Cu/HZSM-5, the
peak intensity was about 75% of untreated catalyst. It
means that the Cu/Ti/HZSM-5 can be easily reduced in
comparison with Cu/HZSM-5. We previously reported that
Cu? species on the zeolite played an important role for
phenol formation in the gas-phase direct oxidation of ben-
zene [29, 30]. In this report, it was considered that the redox
cycle of Cu? and Cu2? caused the partial oxidation of
benzene. Hence, the titanium addition to Cu/HZSM-5 seems
to induce the easy reduction of Cu2? species to Cu? and the
produced Cu? species may produce the phenol effectively.
The excessive addition of titanium to Cu/HZSM-5, how-
ever, may inhibit the access of reactant to active Cu sites or
may cause the further oxidation of partial oxidation products
to CO or CO2.
4 Conclusions
The gas-phase catalytic oxidation of benzene was per-
formed over Cu/HZSM-5 catalysts added with titanium
using gaseous oxygen as an oxidant. Phenol was effectively
produced by the titanium addition to Cu/HZSM-5.
Although the phenol yield decreased with passing the time
on stream, the re-calcination of the used Cu/Ti/HZSM-5
catalysts was found to cause the recovery of phenol for-
mation. Cu/HZSM-5 catalysts added with titanium were
also optimized for the formation of phenol by the Ti/Cu
atomic ratio. The measurements of adsorbed CO amounts
indicated that the amounts of Cu? species in the reduced
123

One-Step Oxidation of Benzene to Phenol
329
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