Hydrogen-free ring hydrogenation of phenol to cyclohexanol over a rhodium-loaded titanium(IV) oxide photocatalyst

Article abstract of DOI:10.1016/j.apcata.2019.04.001

Since photocatalytic reactions are almost consistent with the concept of green chemistry, substance conversion using photocatalysts has recently attracted the attention of researchers in the fields of organic chemistry, physical chemistry and material chemistry. We investigated photoinduced ring hydrogenation of phenol over a metal-loaded titanium(IV) oxide (TiO₂) photocatalyst without the use of H₂ gas and we report here the effects of various parameters, including the type and amount of metal co-catalyst loaded on TiO₂ and the kinds of solvents and hole scavengers, on the ring hydrogenation. We found that the combination of an Rh co-catalyst, water and oxalic acid resulted in the highest yield of cyclohexanol. Detailed analyses revealed that phenol was first hydrogenated to cyclohexanone via keto-enol tautomerism of cyclohexenol followed by hydrogenation of cyclohexanone to cyclohexanol and that adsorption of phenol onto Rh-TiO₂ is a factor of great importance for the ring hydrogenation.

Full text of DOI:10.1016/j.apcata.2019.04.001

Accepted Manuscript
Title: Hydrogen-free ring hydrogenation of phenol to
cyclohexanol over a rhodium-loaded titanium(IV) oxide
photocatalyst
Authors: Atsufumi Kinoshita, Kousuke Nakanishi, Ryosuke
Yagi, Atsuhiro Tanaka, Keiji Hashimoto, Hiroshi Kominami
PII:
S0926-860X(19)30143-7
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2019.04.001
APCATA 17033
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
17 February 2019
28 March 2019
1 April 2019
Please cite this article as: Kinoshita A, Nakanishi K, Yagi R, Tanaka A, Hashimoto
K, Kominami H, Hydrogen-free ring hydrogenation of phenol to cyclohexanol over a
rhodium-loaded titanium(IV) oxide photocatalyst, Applied Catalysis A, General (2019),
https://doi.org/10.1016/j.apcata.2019.04.001
This is a PDF file of an unedited manuscript that has been accepted for publication.
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Hydrogen-free ring hydrogenation of phenol to cyclohexanol over a
rhodium-loaded titanium(IV) oxide photocatalyst
Atsufumi Kinoshita,a Kousuke Nakanishi,a Ryosuke Yagi,a Atsuhiro Tanaka,b,c Keiji
Hashimoto,b Hiroshi Kominamib*
a
Molecular and Material Engineering, Interdisciplinary Graduate School of Science and
Engineering, Kindai University, 3-4-1, Kowakae, Higashiosaka, Osaka 577-8502, Japan
b Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University,
3-4-1 Kowakae, Higashiosaka, Osaka 577-8502, Japan.
c Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and
Technology Agency (JST), 4-1-8 Honcho, Kawaguchi 332-0012, Japan.
E-mail: hiro@apch.kindai.ac.jp
Graphical Abstract

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Highlights
 Rh-TiO2 exhibited high performance in ring hydrogenation of phenol under H2-free
condition.
 Phenol was first hydrogenated to cyclohexanone via keto-enol tautomerism of
cyclohexenol.
 Cyclohexanone was then hydrogenated to cyclohexanol.
 Various phenols were converted to the corresponding cyclohexanols.
 Adsorption of phenols onto Rh-TiO2 is a factor of great importance for the ring
hydrogenation.

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Abstract
Since photocatalytic reactions are almost consistent with the concept of green chemistry,
substance conversion using photocatalysts has recently attracted the attention of researchers
in the fields of organic chemistry, physical chemistry and material chemistry. We investigated
photoinduced ring hydrogenation of phenol over a metal-loaded titanium(IV) oxide (TiO2)
photocatalyst without the use of H2 gas and we report here the effects of various parameters,
including the type and amount of metal co-catalyst loaded on TiO2 and the kinds of solvents
and hole scavengers, on the ring hydrogenation. We found that the combination of an Rh
co-catalyst, water and oxalic acid resulted in the highest yield of cyclohexanol. Detailed
analyses revealed that phenol was first hydrogenated to cyclohexanone via keto-enol
tautomerism of cyclohexenol followed by hydrogenation of cyclohexanone to cyclohexanol
and that adsorption of phenol onto Rh-TiO2 is a factor of great importance for the ring
hydrogenation.
Keywords: photocatalyst, TiO2, ring hydrogenation, phenol, cyclohexanol
1. Introduction
Recently, the number of reports on reductive conversion of organic compounds by
using photogenerated electrons has been increasing [1-5]. The most popular application of
photocatalytic reduction is conversion of nitrobenzenes to aminobenzenes [6-17]. Our

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research group has explored photocatalytic reductions other than nitrobenzene reductions and
has reported 1) deoxygenation of epoxides [18] and sulfoxides [19], 2) conversion of
aldehydes to alcohols [20, 21], and 3) hydrogenation of unsaturated carbon-carbon bonds [22,
23] and carbon-nitrogen bonds [24]. Since a reduction reaction occurs on a co-catalyst loaded
on TiO2, a suitable co-catalyst is different depending on the substrate to be reduced
(hydrogenated). For example, a copper co-catalyst works effectively for semihydrogenation
of alkynes [25, 26] and palladium acts as co-catalyst for hydrogenation of benzonitrile to
benzyl amine [24]. In other words, co-catalyst widens the possibility of photocatalytic
reduction. In addition, various alcohols and organic acids can be used as electron donors. This
means that hydrogenation of various compounds can be achieved without the use of hydrogen
gas (H2), which makes it possible to perform hydrogenation safely. Very recently, we
succeeded in H2-free ring hydrogenation of benzoic acid (BA), i.e., conversion of BA to
cyclohexane carboxylic acid in an aqueous suspension of a rhodium (Rh) co-catalyst loaded
on a titanium(IV) oxide (TiO2) photocatalyst in the presence of oxalic acid as the hole
scavenger (electron donor) [27]. Our results indicate that aromatics can also be regarded as
target compounds for photocatalytic hydrogenation without the use of H2. It is scientifically
important to make the expandability clear. Among the various aromatics, we have been
interested in phenols as the next target in the photocatalytic ring hydrogenation under an
H2-free condition. Catalytic ring hydrogenation of phenol is carried out in the chemical
industry to produce cyclohexanone and cyclohexanol, which are very important compounds
as intermediates for fabricating nylon 6,6 and nylon 6. Ring hydrogenation of phenol has
been performed over various metal catalysts such as rhodium (Rh), palladium (Pd) and
platinum (Pt) under a high pressure of H2 at an elevated temperature [28-32]. Recently, it has

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been reported that some catalysts work under an ambient pressure of H2 at room temperature;
however, the active catalysts do not produce fully hydrogenated cyclohexanol and an H2-free
process is preferable from the viewpoint of green chemistry [33]. In this study, we examined
H2-free ring hydrogenation of phenol in suspensions of metal-loaded TiO2 photocatalysts, and
we report here (1) the effects of co-catalysts and reaction conditions on the phenol
hydrogenation and (2) the expandability and results of a mechanistic study to understand this
reaction.
2. Experimental
2.1. Preparation of TiO2 having co-catalyst
Ishihara ST-01 was used as TiO2 photocatalyst. Loading of co-catalysts (Cu, Ag, Au,
Ru, Ir, Pd, Pt, Rh) on TiO2 was performed with the photodeposition method reported
previously [23, 27]. Details of the photodeposition were described in supplementary
information.
2.2. Photocatalytic ring hydrogenation over TiO2 having co-catalyst
Photocatalytic ring hydrogenation of phenol in water over TiO2 having co-catalyst
was carried out in a similar way reported previously [23, 27]. Details of the photocatalytic
reaction and product analysis were described in supplementary information.
2.3. Adsorption experiment
To understand the photocatalytic performance of TiO2 having co-catalyst, adsorption
properties of TiO2 having co-catalyst was evaluated. Details of the adsorption experiment

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were described in supplementary information.
2.4. Action spectrum
Action spectrum in photocatalytic ring hydrogenation of phenol was determined
based on the apparent quantum efficiency (AQE) recorded on a multi-wavelength irradiation
monochromator (MM-3, Bunkoukeiki Co., Ltd). Details of the methods of experimental
procedure and calculation were described in supplementary information.
3. Results and discussion
3.1. Effect of metal co-catalysts on hydrogenation of phenol
Fig. 1 shows the effects of co-catalysts (1 wt%) on the yields of cyclohexanone and
cyclohexanol formed in the ring hydrogenation of phenol over TiO2 having co-catalyst. No
ring hydrogenation of phenol occurred over bare TiO2 and the color of TiO2 changed from
white to blue just after photoirradiation (Fig. S1). The color change shows that Ti4+ in TiO2
was reduced to Ti3+ by photogenerated electrons. No products were formed when Au-, Ag-,
Cu-, Ru- and Ir-loaded TiO2 samples were used. These results indicate that bare TiO2 and
these metal-loaded TiO2 samples were inactive for ring hydrogenation of phenol. In contrast
to these samples, Pd-, Pt- and Rh-loaded TiO2 samples were active for hydrogenation of
phenol and yielded hydrogenated products, i.e., cyclohexanone and cyclohexanol. Rh-TiO2
showed much larger yields of cyclohexanone and cyclohexanol.
3.2. Ring hydrogenation of phenol in an aqueous suspension of Rh-TiO2
Since Rh exhibited an extraordinary co-catalyst effect on ring hydrogenation of

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phenol, Rh-TiO2 samples having various Rh contents were prepared and characterized. X-ray
diffraction patterns of Rh-TiO2 samples are shown in Fig. S2. Peaks assignable to
anatase-type TiO2 were observed, while peaks due to rutile phase were not detected. With an
increase in Rh loading, the peak due to Rh metal (41.1 degrees) gradually increased. Fig. S3
shows a TEM image (a), an HAADF-STEM-EDX image (b), EDX line scans (c) and EDX
mapping images (d: Rh-L, e: Ti-K and f: reconstructed overlay image) of an Rh-TiO2 sample.
These results indicate successful loading of Rh nanoparticles on the TiO2 surface.
These Rh-TiO2 samples having various Rh contents were used for ring
hydrogenation of phenol and the results are shown in Fig. 2. With an increase in the amount
of Rh up to 6 wt%, the yield of cyclohexanone decreased, whereas the yield of cyclohexanol
increased. Cyclohexanol is formed by hydrogenation of cyclohexanone as discussed later.
Therefore, these results indicate that an Rh co-catalyst accelerates both ring hydrogenation of
phenol to cyclohexanone and the subsequent hydrogenation of cyclohexanone to
cyclohexanol.
Fig. 3 shows the results of ring hydrogenation of phenol over 6 wt%Rh-TiO2.
Cyclohexanone was observed as the hydrogenated product just after photoirradiation. After
the yield of cyclohexanone reached the maximum at 30 min, the yield gradually decreased
and the yield of cyclohexanol increased at around 30 min. These behaviors of cyclohexanone
and cyclohexanol indicate that stepwise hydrogenations, i.e., phenol to cyclohexanone and
cyclohexanone to cyclohexanol, occur (discussed later). Phenol and cyclohexanone were
almost completely consumed after 45 min and 60 min, respectively.
After consumption of
phenol and cyclohexanone, cyclohexanol was obtained in a high yield (79%). As far as we
know, this is the first report on the production of cyclohexanol from phenol without the use of

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hydrogen gas and at room temperature.
Material balance (M.B.) was calculated using eqn (1):
M.B.= n(Cyclohexanol) + n(Cyclohexanone) + n(Phenol)
,
(1)
n (Phenol)
0
where n(phenol), n(cyclohexanone) and n(cyclohexanol) are the amounts of phenol,
cyclohexanone and cyclohexanol during the photocatalytic reaction, respectively, and
n0(phenol) is the amount of phenol before the photocatalytic reaction. As shown in Fig. 3, the
values of MB decreased in the early stage of the reaction until 30 min and were almost
constant at ca. 0.8 after 30 min. The former result suggests that another product was formed
in the early stage of the reaction. However, products other than cyclohexanone and
cyclohexanol were not detected in the reaction mixture, and a small amount of a
water-soluble by-product(s) might have been produced.
As also shown in Fig. 3, CO2 continuously evolved during photoirradaition,
indicating that oxalic acid continuously worked as a hole scavenger. On the other hand, the
behavior of H2 evolution was different from that of CO2 evolution; H2 evolution was
negligible for 90 min, during which time hydrogenation was observed, and was then
predominant after consumption of phenol and cyclohexanone. These results indicate that
hydrogenation of phenol and cyclohexanone occurred before H2 formation over the Rh
co-catalyst. In other words, photogenerated electrons are selectively used for hydrogenation
over the Rh co-catalyst as long as phenol and cyclohexanone are present in the reaction
system. The high selectivity toward hydrogenation of phenol and cyclohexanone suggests a
strong interaction between Rh species and the substrates (phenol and cyclohexanone), which
will be discussed in section 3.4.
We also noted an important feature of this reaction system; cyclohexanol was not

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consumed after complete consumption of phenol and cyclohexanone (Fig. 3). Since an excess
of oxalic acid was used, the oxalic acid remaining effectively consumed positive holes and
eliminated the possibility of re-oxidation of cyclohexanol. This feature is practically
important because there is no need for careful observation of the reaction to avoid
consumption of cyclohexanol formed.
3.3. Effects of reaction conditions on ring hydrogenation of phenol and expandability
The table shows the effects of the hole scavenger and solvent on the ring
hydrogenation of phenol over 6 wt%Rh-TiO2. First, the effect of the hole scavenger on ring
hydrogenation of phenol in an aqueous solution was examined. A high yield of cyclohexanol
(79%) was obtained after 60 min in water in the presence of oxalic acid (Entry 1) and the
hydrogenation was completed by 120-min photoirradiation (Entry 2). Durability tests showed
that 6 wt%Rh-TiO2 could be used at least three times (Entries 3 and 4). However, we noted
that H2 yields gradually decreased in the second and third uses, suggesting that Rh-TiO2 was
gradually deactivated. Strong adsorption of phenol is eliminated from the reason for the
deactivation because the photocatalyst was washed with water and water-soluble phenol can
be removed from the catalyst surface. One possibility for the deactivation is deposition of
oligomerization/polymerization products. No hydrogenated products were obtained in the
absence of Rh-TiO2 in the dark (Entry 6), indicating that no homogeneous thermal reaction of
phenol and oxalic acid occurred. Photochemical ring hydrogenation between phenol and
oxalic acid did not occur in the absence of Rh-TiO2 under light irradiation (Entry 5). We also
confirmed that H2 in the gas phase did not contribute to ring hydrogenation of phenol in the
presence of oxalic acid and Rh-TiO2 in the dark (Entry 21). The use of sodium oxalate instead

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of oxalic acid resulted in a large decrease in the reaction rate (Entry 15). This drastic change
suggests that pH of the reaction medium greatly affects the catalytic performance. In the case
of formic acid, a large amount of H2 evolved instead of the formation of hydrogenated
compounds (Entry 14). The use alcohols (methanol and 2-propanol) as hole scavengers
resulted in smaller rates of phenol conversion (Entries 12 and 13). When acetonitrile was
used as the solvent, no hydrogenation of phenol occurred even in the presence of oxalic acid
and a large amount of H2 evolved (Entry 20).
When alcohols were used as the solvents, the alcohols simultaneously acted as hole
scavengers for the photocatalytic reaction. In addition, effective hole-scavenging is expected
because a large amount of the alcohol is adsorbed on the TiO2 surface. However,
cyclohexanol yields in alcohols were smaller than that in water in the presence of oxalic acid
(Entries 16-19). These results mean that the reaction medium is also an important factor for
driving the ring hydrogenation of phenol and that an aqueous medium and oxalic acid as the
hole scavenger are a suitable combination for efficient hydrogenation of phenol. Based on
results of adsorption experiments, the importance of simultaneous adsorption of phenol and a
hole scavenger will be discussed in section 3.4.
The expandability of ring hydrogenation over Rh-TiO2 was also examined under the
best condition obtained, i.e., in water in the presence of oxalic acid. When 2-, 3- and 4-methyl
phenols (cresols) were used, the corresponding methyl cyclohexanols were obtained and the
reaction rates were dependent on the position of the methyl group (Entries 7-9).
3-(Trifluoromethyl)phenol and 4-(trifluoromethyl)phenol were hydrogenated to the
corresponding cyclohexanols in high yields (95% and > 99%) (Entries 10 and 11). The large
rates of product formation and large yields of phenols having an electron-withdrawing group

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suggest that this reaction proceeds through a negatively charged transition state and that the
electron-withdrawing group suppressed a side-reaction(s).
3.4. Adsorption experiments
Fig. 4 shows the results of some adsorption experiments to clarify the effects of hole
scavengers and solvents on the photocatalytic reaction. A large amount of oxalic acid
molecules was adsorbed on 6 wt%Rh-TiO2 in water in the absence of phenol (Entry C). A
certain amount of phenol was adsorbed in competitive adsorption with oxalic acid in water
(Entry A). The amount of phenol adsorbed on Rh-free TiO2 decreased to ca. two-thirds (Entry
B), indicating that ca. one third of phenol was adsorbed on Rh metals. A large amount of
oxalic acid adsorbed on the surface of Rh-TiO2 and a certain amount of phenol adsorbed on
Rh metals greatly contribute to effective hole-scavenging and ring hydrogenation of phenol
under light irradiation (Entry 1 in Table). When sodium oxalate was used instead of oxalic
acid, the value of pH of the aqueous suspension increased to 7.2 and the amounts of oxalate
and phenol adsorbed drastically decreased (Entry D). Entry D explains the small reaction rate
of ring hydrogenation of phenol in the presence of sodium oxalate (Entry 15 in Table). This
result also means that an acidic condition is indispensable for efficient ring hydrogenation of
phenol.
The adsorption behavior of phenol in non-aqueous conditions was also examined
(Entries E and F). When acetonitrile was used as the solvent in the presence of oxalic acid, a
large amount of oxalic acid was adsorbed; however, only a small amount of phenol was
adsorbed (Entry F). A negligible reduction of phenol and a large yield of H2 (Entry 20 in
Table) are attributed to the characteristic adsorption behaviors of oxalic acid and phenol in

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acetonitrile. In other words, in acetonitrile, decomposition of oxalic acid ((COOH)2 → H2 +
2CO2) only occurred on Rh metal. In methanol solvent in the absence of oxalic acid, only a
small amount of phenol was adsorbed on Rh-TiO2 probably because most of the surface was
covered with methanol (Entry E). The small rate of hydrogenation in methanol solvent (Entry
16 in Table) can be explained by the small amount of phenol adsorbed in methanol. We also
confirm that phenol adsorbed onto 6 wt%Rh-TiO2 in water (Entry A) was almost
quantitatively recovered by washing with methanol (Fig. S4). The table and Fig. 4 indicate
that most of the photocatalytic reactions were controlled by the amount of phenol adsorbed
and that the adsorption of phenol was affected by the solvent and pH of the suspension.
3.5. Mechanistic studies
Fig. 5 shows an action spectrum in ring hydrogenation of phenol over Rh-TiO2
together with absorption spectra of TiO2 and Rh-TiO2.
AQE in ring hydrogenation of
phenol showed the same tendency as that of photoabsorption of TiO2, indicating that ring
hydrogenation of phenol was induced by the photoabsorption of TiO2. Fig. 5 also shows that
the ring hydrogenation of phenol occurred with high efficiency of photon utilization (14%
AQE at 335 nm).
The time courses of phenol and products (Fig. 3) suggest that hydrogenation of
phenol to cyclohexanol proceeded via cyclohexanone. To confirm this, we examined the
reactivity of cyclohexanone and cyclohexanol in aqueous suspensions of 6 wt%Rh-TiO2 in
the presence of oxalic acid. Fig. 6 shows the results of the reaction of cyclohexanone. Just
after photoirradiation, the amount of cyclohexanone decreased, while cyclohexanol was
formed. Cyclohexanone was almost completely consumed and cyclohexanol was obtained in

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a high yield (98%) after 15 min. Fig. 7 shows the results of the reaction of cyclohexanol,
indicating that no dehydrogenation of cyclohexanol to cyclohenone occurred in the presence
of oxalic acid. These results (Figs. 3, 6 and 7) show that hydrogenation of phenol to
cyclohexanol proceeded via cyclohexanone under the present conditions. Formation of
cyclohexanone from phenol cannot be explained by a simple hydrogenation mechanism.
Based on the results obtained in this study, we propose the following processes of conversion
of phenol to cyclohexanone over an Rh co-catalyst (Scheme): 1) hydrogenation of phenol to
cyclohexadienol and then to cyclohexenol and 2) transformation of cyclohexenol to
cyclohexanone by keto-enol tautomerism. Cyclohexadienol and cyclohexenol were not
detected in this study, indicating that these compounds are unstable and their transformation
to cyclohexane occurs rapidly.
Fig. 8 shows the expected reaction process of ring hydrogenation of phenol to
cyclohexanol over Rh-TiO2: (1) photogenerated electrons (e-) and positive holes (h+) are
formed in the conduction and valence bands of TiO2 under UV light irradiation, (2) H+ is
reduced by e-, resulting in the formation of an active hydrogen species over Rh particles, and
oxalic acid is oxidized by h+, (3) two C=C double bonds of phenol are hydrogenated and
thus-formed cyclohexenol is rapidly converted to cyclohexanone, and (4) cyclohexanone is
hydrogenated to cyclohexanol.
4. Conclusions
We examined photoinduced ring hydrogenation of phenol over a metal-loaded titanium(IV)
oxide (TiO2) photocatalyst without the use of H2 gas and the effects of various parameters,
including metal co-catalysts loaded on TiO2, solvents and hole scavengers, on the ring

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hydrogenation. The combination of an Rh co-catalyst, water and oxalic acid is suitable for
ring hydrogenation of phenol. The yield of cyclohexanol increased with an increase in the Rh
content, and the apparent quantum efficiency reached 14% at 335 nm over 6 wt%Rh-TiO2.
Phenols having methyl and trifluoromethyl groups were converted to the corresponding
cyclohexanols over the Rh-TiO2 photocatalyst. Adsorption experiments showed that the most
of the photocatalytic reactions were controlled by the amount of phenol adsorbed and that the
adsorption of phenol was affected by the solvent and pH of the suspension.
Acknowledgements
This work was partly supported by JSPS KAKENHI Grant Numbers 17H03462 and
17H04967 and the Precursory Research for Embryonic Science and Technology (PRESTO),
supported by the Japan Science and Technology Agency (JST). This work was also supported
by MEXT-Supported Program for the Strategic Research Foundation at Private Universities
2014-2018, subsidy from MEXT and Kindai University. K. N. is grateful to the Japan Society
for the Promotion of Science (JSPS) for a Research Fellowship for young scientists
(15J11412). A.T. is grateful for financial support from the Faculty of Science and Engineering,
Kindai University.

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Figure captions
Fig. 1. Effects of metal co-catalysts on ring hydrogenation of phenol (50 µmol) to
cyclohexanone and cyclohexanol in an aqueous suspension of 1 wt%M-TiO2 for 120 min in
the presence of oxalic acid (450 µmol).
Fig. 2. Effect of the amount of Rh loading on ring hydrogenation of phenol. The reaction
conditions other than the Rh loading were the same as those described in the legend of Fig. 1.
Fig. 3. Time courses of the amounts of phenol remaining (■), cyclohexanone formed (●),
cyclohexanol formed (◆), H2 evolved (▼) and CO2 evolved (◣), and material balance (▲) in
an aqueous suspension of 6 wt%Rh-TiO2. The reaction conditions were the same as those
described in the legend of Fig. 1.
Fig. 4. Amounts of phenol and/or additive adsorbed onto 6 wt%Rh-TiO2 or TiO2 for 20 h in
the dark. The conditions other than the amount of the sample (200 mg) and temperature
(300-301 K) were the same as those described in the legend of Fig. 1.
Fig. 5. Absorption spectra of TiO2 and 6 wt%Rh-TiO2 (left axis) and an action spectrum in
ring hydrogenation of phenol (right axis). The reaction conditions were the same as those
described in the legend of Fig. 1.
Fig. 6. Time courses of the amounts of cyclohexanone remaining (●) and cyclohexanol
formed (◆) and material balance (▲) in an aqueous suspension of 6 wt%Rh-TiO2. The

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reaction conditions other than the starting substrate (cyclohexanone) were the same as those
described in the legend of Fig. 1.
Fig. 7. Time courses of the amounts of cyclohexanol remaining (◆) and cyclohexanone
detected (●) and material balance (▲) in an aqueous suspension of 6 wt%Rh-TiO2. The
reaction conditions other than the starting substrate (cyclohexanol) were the same as those
described in the legend of Fig. 1.
Fig. 8. The expected reaction process of ring hydrogenation of phenol to cyclohexanol over
Rh-TiO2 in the presence of oxalic acid.

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Scheme 1. Ring hydrogenation of phenol in an aqueous suspension of Rh-TiO2.

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Table 1
Results of hydrogenation of phenols to cyclohexanones and cyclohexanols over 6 wt%Rh-TiO2 under various conditions at 293 K.
Yield/µmol
Rh-
TiO2
UV
light
Time
/min
Gas
phase
H2
µmol
Entries
Substrates
Hole scavengers
Solvents
M.B.
1
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
2-Cresol
3-Cresol
4-Cresol
3-TFMPe
4-TFMPf
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
OAa
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Yes
Yes
Yes
Yes
No
On
On
On
On
On
No
On
On
On
On
On
On
On
On
On
On
On
On
On
On
No
60
120
120c
120d
60
60
90
90
60
60
60
60
60
60
60
60
60
60
60
60
60
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
0.6
38.5
38.6
42.8
41.4
n.d.
n.d.
15.6
39.7
36.2
47.0
50.4
0.6
0.80
0.79
0.80
0.79
-
25
2
OA
n.d.b
n.d.
1.0
176
133
110
0.7
n.d.
16
3
OA
4
OA
5
OA
n.d.
n.d.
16.1
n.d.
n.d.
n.d.
n.d.
5.3
6
OA
No
-
7
OA
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
0.61
0.83
0.79
0.95
1.00
0.86
0.80
0.94
0.86
0.64
0.77
0.85
0.88
-
8
OA
H2O
62
9
OA
H2O
H2O
34
10
11
12
13
14
15
16
17
18
19
20
21
OA
27
OA
H2O
27
Methanol
2-Propanol
Formic acid
SO g
H2O
42
H2O
22.7
3.9
2.0
6
H2O
0.7
209
0.6
257
273
73
H2O
6.5
n.d.
1.9
Methanol
Ethanol
2-Propanol
2-Pentanol
OA
Methanol
Ethanol
2-Propanol
2-Pentanol
ACN h
H2O
0.4
1.7
0.9
27.9
3.6
13.8
1.0
68
n.d.
2.1
n.d.
n.d.
83
i
Phenol
OA
Ar/H2
0.95
-
a
b
c
d
e
f
Oxalic acid. Not detected. Second use. The photocatalyst was washed with water and then dried in vacuo before the use. Third use. 3-(Trifluoromethyl)phenol.
4-(Trifluoromethyl)phenol. g Sodium oxalate. h Acetonitrile. i H2: 200 µmol.

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30
25
20
15
10
5
0
None
Au Ag Cu Ru Ir Pd Pt Rh
Metal co-catalyst
Fig. 1. Effects of metal co-catalysts on ring hydrogenation of phenol (50 µmol) to
cyclohexanone and cyclohexanol in an aqueous suspension of 1 wt%M-TiO2 for 120 min in
the presence of oxalic acid (450 µmol).

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60
50
40
30
20
10
0
1 2 3 4 5 6
Amount of Rh loading / wt%
Fig. 2. Effect of the amount of Rh loading on ring hydrogenation of phenol. The reaction
conditions other than the Rh loading were the same as those described in the legend of Fig. 1.

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800
600
400
200
CO2
H2
0
50
1
0.8
0.6
0.4
0.2
0
40
30
20
10
0
0
30
60
90
Irradiation time / min
Fig. 3. Time courses of the amounts of phenol remaining (■), cyclohexanone formed (●),
cyclohexanol formed (◆), H2 evolved (▼) and CO2 evolved (◣), and material balance (▲) in
an aqueous suspension of 6 wt%Rh-TiO2. The reaction conditions were the same as those
described in the legend of Fig. 1.

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12
10
8
200
160
120
80
40
0
6
4
2
0
Fig. 4. Amounts of phenol and/or additive adsorbed onto 6 wt%Rh-TiO2 or TiO2 for 20 h in
the dark. The conditions other than the amount of the sample (200 mg) and temperature
(300-301 K) were the same as those described in the legend of Fig. 1.

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100
80
60
40
20
0
Rh-TiO2
12
8
TiO2
4
0
300 350 400 450 500
Wavelength / nm
Fig. 5. Absorption spectra of TiO2 and 6 wt%Rh-TiO2 (left axis) and an action spectrum in
ring hydrogenation of phenol (right axis). The reaction conditions were the same as those
described in the legend of Fig. 1.

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70
60
50
40
30
20
10
0
1
0.8
0.6
0.4
0.2
0
0
10
20
30
Irradiation time / min
Fig. 6. Time courses of the amounts of cyclohexanone remaining (●) and cyclohexanol
formed (◆) and material balance (▲) in an aqueous suspension of 6 wt%Rh-TiO2. The
reaction conditions other than the starting substrate (cyclohexanone) were the same as those
described in the legend of Fig. 1.

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50
40
30
20
10
0
1
0.8
0.6
0.4
0.2
0
0
10
20
30
Irradiation time / min
Fig. 7. Time courses of the amounts of cyclohexanol remaining (◆) and cyclohexanone
detected (●) and material balance (▲) in an aqueous suspension of 6 wt%Rh-TiO2. The
reaction conditions other than the starting substrate (cyclohexanol) were the same as those
described in the legend of Fig. 1.

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Fig. 8. The expected reaction process of ring hydrogenation of phenol to cyclohexanol over
Rh-TiO2 in the presence of oxalic acid.