Layered silicate as an excellent partner of a TiO2 photocatalyst for efficient and selective green fine-chemical synthesis

Article abstract of DOI:10.1021/ja406855e

When the partial oxidation of benzene to phenol, which is one of the most important reactions in chemical industry, was conducted using TiO₂ in the presence of a phenol-philic adsorbent derived from a layered silicate, phenol was recovered in u

Full text of DOI:10.1021/ja406855e

Communication
pubs.acs.org/JACS
Layered Silicate as an Excellent Partner of a TiO2 Photocatalyst for
Efficient and Selective Green Fine-Chemical Synthesis
Yusuke Ide,*,† Masato Torii,‡ and Tsuneji Sano‡
†World Premier International (WPI) Research Center, International Center for Materials Nanoarchitectonics (MANA), National
Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan
‡Graduate School of Engineering, Department of Applied Chemistry, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima
739-8527, Japan
S
* Supporting Information
dioxide as a result of the presence of highly oxidizing radical
species such as hydroxyl radical.19 We expected that during the
ABSTRACT: When the partial oxidation of benzene to
phenol, which is one of the most important reactions in
chemical industry, was conducted using TiO2 in the
presence of a phenol-philic adsorbent derived from a
layered silicate, phenol was recovered in unprecedentedly
high yield and purity. This resulted from the fact that the
adsorbent captured the generated phenol promptly,
selectively, and effectively to prevent the overoxidation,
after which the captured phenol could be easily eluted.
photocatalysis, a coexisting adsorbent that can promptly and
selectively adsorb phenol from a mixture of water, benzene, and
phenol would separate the photocatalytically formed phenol
from TiO2 to prevent such overoxidation (Figure 1). Layered
ine-chemical synthesis by solar-driven photocatalysis,
Fespecially using environmentally benign reactants such as
molecular oxygen or water, is one of the most compelling
objectives in modern chemistry. Thanks to its abundance,
nontoxicity, and high photostability, TiO2 is a promising
photocatalyst for this purpose. However, organic substrates
tend to be overoxidized to undesired byproducts such as carbon
dioxide on TiO2 because of the presence of highly oxidizing
species. Accordingly, the design of novel photocatalysts has
been extensively investigated;1−5 however, photocatalytic
activities for desired partial oxidations are still insufficient for
practical applications. The use of titanosilicate zeolites
composed of a silica framework and highly dispersed TiO2 is
a green methodology for photocatalytic partial oxidations.6
However, the presence of micropores (with aperture diameters
below 1 nm) often goes hand-in hand with diffusion limitations
that adversely affect the photocatalytic activity and limit the
available organic substrates. In this communication, we describe
a conceptually new methodology to attain a high photocatalytic
activity for desired partial oxidations even using TiO2. As an
example partial oxidation, the oxidation of benzene to phenol
was conducted. Phenol, which is currently produced from
benzene by a three-step cumene process, is one of the most
important commodity chemicals in industry. Because the
cumene process is energy-consuming, has a low phenol yield,
and produces unwanted byproducts, extensive efforts have been
made to develop catalysts for direct benzene oxidation.7−18
One of the main difficulties in partial oxidations of organic
substrates on TiO2 is that the desired products are easily
overoxidized. During the photocatalytic oxidation of benzene
on TiO2 with water containing dissolved O2, once phenol is
generated, it is promptly overoxidized to hydroxyphenols (e.g.,
catechol and hydroquinone) and finally oxidized to carbon
Figure 1. Scheme for effective and selective recovery of phenol by a
phenol-philic adsorbent, layered silicic acid (H-mag), during the
photocatalytic oxidation of benzene in water over TiO2 under solar
irradiation.
inorganic solids, such as layered clay minerals, have been widely
investigated as adsorbents because of their large surface areas
derived from the nanometer-thick layers assembled naturally in
a layer-by-layer manner and their surface reactivities (ion
exchange, hydrogen bonding, etc.).20 Magadiite (Na2Si14O29,
denoted as Na-mag) is a naturally occurring layered silicate that
can be readily synthesized by a simple hydrothermal
reaction.21,22 The layered silicic acid derived from magadiite
via a cation-exchange reaction of the interlayer sodium ions
with protons (H2Si14O29, denoted as H-mag) has been known
to take up various polar organic molecules (e.g., alcohols) in the
interlayer space via hydrogen-bonding interactions with the
surface silanol groups.23,24 In this study, we prepared Na-mag
and H-mag and investigated their adsorption of phenol in the
presence of water and benzene (from an aqueous benzene
solution).
Received: July 5, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja406855e | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
The time-dependent adsorption of phenol on H-mag from
an aqueous mixture of benzene and phenol (Figure 2a) showed
Figure 2. (a) Time-dependent phenol adsorption from water
containing benzene by H-mag. (b) Adsorption isotherms of phenol
from water containing benzene for Na-mag (red circles) and H-mag
(blue circles). The initial concentrations of phenol and benzene were
0.05−3 and 600 ppm, respectively.
Figure 3. (a) HPLC chromatograms of aqueous benzene after
photocatalysis under solar-simulator irradiation using TiO2 in the
absence (black line) or the presence of Na-mag (red line) or H-mag
(blue line). Peaks B, P, and S are assigned to benzene, phenol, and the
products of phenol overoxidation, respectively. (b) HPLC chromato-
grams of eluates obtained after the recovered Na-mag (red line) or H-
mag (blue line) was washed with aqueous ethanol. (c) Time courses of
the amounts of benzene (black squares) and phenol (blue circles) in
aqueous benzene during the photocatalysis of TiO2 with H-mag.
that phenol adsorption occurred on a scale of minutes. The
adsorption isotherm of phenol on H-mag from the aqueous
mixture with benzene (Figure 2b) was an H-type isotherm
according to the Giles classification,25 indicating strong
adsorbent−adsorbate interactions. These results revealed that
H-mag could promptly and selectively adsorb phenol from
water even in the presence of benzene. On the other hand,
pristine Na-mag hardly adsorbed phenol from the aqueous
mixture with benzene (Figure 2b), presumably because of
competition with water, which strongly interacts with the
sodium ions in the interlayer space.
Photocatalytic benzene oxidation on TiO2 was conducted
using water as the solvent and oxidant and dissolved O2 as the
oxidant under simulated solar irradiation (λ > 320 nm) in the
absence or presence of Na-mag or H-mag. We used flaky TiO2
with a size of a few square millimeters rather than fine TiO2
particles, which allowed us to easily remove TiO2 from the
reaction mixture after the photocatalysis and thus to elute
possibly captured products only from the adsorbent. The
results of the photocatalytic oxidations were summarized in
Table 1. When TiO2 alone was used, significant amounts of
products of phenol overoxidation (e.g., catechol, hydroxyqui-
none, and 1,2,3-trihydroxybenzene) were detected in the
chromatogram of the supernatant of the reaction mixture
after the photocatalysis (Figure 3a), giving 4% phenol
selectivity at ca. 80% benzene conversion after photoirradiation
for 24 h (Table 1). However, when H-mag was present under
identical photoirradiation conditions, phenol and the over-
oxidized products were hardly detected in the supernatant
despite benzene conversion of ca. 80% (Figure 3a), and only
phenol was detected in the chromatogram of the eluate
obtained from H-mag using aqueous ethanol (Figure 3b). In
this case, phenol was recovered with 100% selectivity (purity)
at ca. 80% benzene conversion (Table 1). It should be noted
here that phenol was hardly detected in the reaction mixture at
any point during the course of the photocatalysis (Figure 3c).
These results showed that H-mag adsorbed the newly-formed
phenol so promptly and selectively that phenol was hardly
overoxidized, allowing it to be recovered selectively and
efficiently. The adsorbed phenol was stabilized as a result of
the separation of phenol from the TiO2 surface;26 it is probably
difficult for oxidizing radical species generated on TiO2 to
diffuse inside H-mag before they are deactivation. On the other
hand, phenol was considerably less effectively recovered when
Na-mag was used instead of H-mag (Table 1). On the basis of
the fact that phenol was detected in both the reaction mixture
and the eluate from Na-mag (Figure 3a,b), Na-mag adsorbed
phenol via relatively weak interactions and released it in a
repetitive manner during the photocatalysis. We cannot rule
out the possibility that variation in the nature of the reaction
medium (pure water, water/H-mag dispersion, water/Na-mag
dispersion), such as its hydrophobicity or hydrophilicity, affects
the photocatalytic performance of TiO2.27
Table 1. Photocatalytic Oxidations of Benzene on TiO2 with
a
or without Adsorbent
b
in supernatant after photocatalysis
amount of
in eluate
phenol
phenol
formed
(μmol)
phenol
benzene
recovery
selectivity conversion recovered selectivity
c
d
e
adsorbent
(%)
(%)
(μmol)
(%)
The presently observed phenol production (recovery)
activity was considerably higher than those reported for
catalytic and photocatalytic processes for direct benzene
oxidation,9−18 although the present process required only
abundant, safe materials and reagents (TiO2, silicate, water, O2,
and ethanol). The present activity was also much higher
compared with that on a titanosilicate zeolite (65% phenol
selectivity at <20% benzene conversion under UV irradiation
for 24 h),6 which can be explained by the fact that nonporous
TiO2 does not limit the diffusion of benzene and phenol. To
none
4.9
0.3
4.1
0.2
77.4
76.2
76.1
−
119.2
−
100
100
H-mag
Na-mag
26.6
22.7
13.0
a
Benzene, 154 μmol; water (saturated with O2), 20 mL; flaky TiO2, 60
mg; adsorbent (if present), 60 mg; simulated solar irradiation time, 24
b
h. Solution obtained after washing the adsorbent with 100 mL of
c
aqueous ethanol (1:1 v/v). [formed phenol]/[reacted benzene] ×
100%. [reacted benzene]/[added benzene] × 100%. Based on
HPLC results.
d
e
B
dx.doi.org/10.1021/ja406855e | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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incorporated (pillared) layered clays28 and TiO2 supported on
the particle surface of layered clays29] has been investigated for
the decomposition (complete oxidation) of organic substrates,
where the layered clays play a role in concentrating the organic
substrates near the catalyst surface. However, this is the first
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partner of TiO2 for the partial oxidation of organic substrates.
Adsorbents based on layered inorganic solids can be tailor-
made for a wide variety of organic molecules with varied size
and structure by intercalation reactions.20 In combination with
recent progress in the design of TiO2 with higher sunlight
absorption for photocatalysis,30 the large-scale production of
phenol and other fine chemicals in an environmentally and
economically benign fashion is expected upon optimization of
the reaction conditions (e.g., light intensity, the kind and
amount of the added adsorbents, reactors, etc.).
(20) Okada, T.; Ide, Y.; Ogawa, M. Chem.Asian J. 2012, 7, 1980.
(21) Eugster, H. P. Science 1967, 157, 1177.
(22) Kosuge, K.; Yamazaki, A.; Tsunashima, A.; Otsuka, R. J. Ceram.
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In summary, we have reported that phenol was recovered
with unprecedentedly high efficiency and selectivity (purity)
when the photocatalytic oxidation of benzene in water over
TiO2 under simulated solar light was conducted in the presence
of a layered silicic acid (protonated magadiite). We expect that
the combination of layered silicates with TiO2 will open a door
to green fine-chemical synthesis.
(26) Ogawa, M.; Sohmiya, M.; Watase, Y. Chem. Commun. 2011, 47,
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(27) Ide, Y.; Matsuoka, M.; Ogawa, M. ChemCatChem 2012, 4, 628.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
IDE.Yusuke@nims.go.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. M. Sadakane, N. Kagawa, N. Tsunoji, and H.
Hattori at Hiroshima University for discussions and support.
This work was partly supported by a Grant-in-Aid for Young
Scientists (B) (24750145) and a research grant from The
Mazda Foundation.
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dx.doi.org/10.1021/ja406855e | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX