Photocatalytic hydrogenation of furan to tetrahydrofuran in alcoholic suspensions of metal-loaded titanium(IV) oxide without addition of hydrogen gas

Article abstract of DOI:10.1039/c7cp02891a

The use of metal co-catalysts broadens the application of photocatalytic reduction without the use of dihydrogen (H₂) gas. We examined photocatalytic hydrogenation of furan, a representative heterocyclic compound and a compound derived from biomass, in alcoholic suspensions of metal-loaded titanium(iv) oxide (TiO₂) under a H₂-free condition and we found that furan was almost quantitatively hydrogenated to tetrahydrofuran with a high apparent quantum efficiency of 37% at 360 nm when palladium was used as a co-catalyst. Effects of different metal co-catalysts, different amounts of the co-catalyst, the type of TiO₂, the type of alcohol, light wavelength and reusability for furan hydrogenation were investigated. Based on the results, the functions of TiO₂ and the co-catalyst and the reaction process are discussed.

Full text of DOI:10.1039/c7cp02891a

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Photocatalytic hydrogenation of furan to
tetrahydrofuran in alcoholic suspensions of
Cite this:DOI: 10.1039/c7cp02891a
metal-loaded titanium(IV) oxide without addition
of hydrogen gas†
b
Kousuke Nakanishi,a Atsuhiro Tanaka,
Keiji Hashimotob and
b
Hiroshi Kominami
*
The use of metal co-catalysts broadens the application of photocatalytic reduction without the use of
dihydrogen (H2) gas. We examined photocatalytic hydrogenation of furan, a representative heterocyclic
compound and a compound derived from biomass, in alcoholic suspensions of metal-loaded titanium(IV)
oxide (TiO2) under a H2-free condition and we found that furan was almost quantitatively hydrogenated
to tetrahydrofuran with a high apparent quantum efficiency of 37% at 360 nm when palladium was used
as a co-catalyst. Effects of different metal co-catalysts, different amounts of the co-catalyst, the type of
TiO2, the type of alcohol, light wavelength and reusability for furan hydrogenation were investigated.
Based on the results, the functions of TiO2 and the co-catalyst and the reaction process are discussed.
Received 3rd May 2017,
Accepted 2nd July 2017
DOI: 10.1039/c7cp02891a
rsc.li/pccp
satisfies almost all of the 12 proposed requirements for green
chemistry3 because of its characteristics shown below. First,
1. Introduction
Tetrahydrofurans (THFs) are important compounds as inter- TiO2 functions as a catalyst. The TiO2-photocatalyzed reaction
mediates of chemical products, and they are produced by occurs at room temperature. Solar energy can be used to drive
dehydration of diols and hydrogenation of corresponding the reaction. Since TiO2 is a typical solid catalyst, energy
furans.1,2 Hydrogenation of furans to THFs is more attractive consumption for the separation of TiO2 from the reaction
because furans can be produced from biomass. Hydrogenation mixture is much smaller than that for the separation of a
of furans is achieved by catalytic hydrogenation using a metal homogeneous catalyst. In addition, a TiO2 photocatalyst can
catalyst such as nickel or palladium (Pd) supported on a be used repeatedly free from a special re-activation process
suitable support.2 However, these catalyst systems require the because of its excellent chemical and physical stability. Therefore,
addition of an excess amount of dihydrogen (H2) gas as a hydrogen photocatalytic material transformation has been studied by many
source in a closed reactor. Therefore, a more environmentally researchers.4–6 Recently, photocatalytic reduction of organic com-
friendly catalytic reaction system for hydrogenation of furans pounds has attracted the attention of researchers,5,6 and much
utilizing a ‘‘greener’’ hydrogen source is desired.
interest has been shown in the reduction of nitrobenzenes to
Since titanium(IV) oxide (TiO2) is inexpensive and not toxic aminobenzenes.7–21 We recently found that benzonitrile and
for humans and the environment, it has been used for a long styrene were successfully hydrogenated in alcohol suspensions
time as an indispensable inorganic material such as a pigment of a palladium-loaded TiO2 (Pd-TiO2) photocatalyst, although the
and a UV absorber. Another important application of TiO2 is reduction potentials of benzonitrile and styrene are believed to be
the use as a photocatalyst. When TiO2 is irradiated by UV light, much higher than the potential of the conduction band of
charge separation occurs and thus-formed electrons in the TiO2.22,23 The reason why Pd-TiO2 exhibited good performance
conduction band and positive holes in the valence band cause in photocatalytic hydrogenation is the high ‘‘catalytic’’ activity for
reduction and oxidation, respectively. The photocatalytic reaction hydrogenation of unsaturated bonds such as CQC and CRN
bonds.26,27 Two results have shown that (1) hydrogenation occurs
a Interdisciplinary Graduate School of Science and Engineering, Kindai University,
without the use of H2 because alcohol works as an electron
and hydrogen source in the photocatalytic reaction, (2) the Pd
co-catalyst strongly contributes to the reduction, and (3) the
applicability of photocatalytic reduction is not limited by
the conduction band position of semiconductor photocatalysts.
We also reported other co-catalyst effects in chemoselective
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.
E-mail: hiro@apch.kindai.ac.jp
† Electronic supplementary information (ESI) available. Experimental procedure
and Fig. S1. See DOI: 10.1039/c7cp02891a
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of chlorobenzene. The amount of H2 as the reduction product
of protons (H+) was determined using a TCD-type gas chroma-
tograph (GC-8A, Shimadzu) equipped with an MS-5A column. A
multi-wavelength irradiation monochromator (MM-3, Bunkou-
keiki Co., Ltd) was used to obtain apparent quantum efficiency
(AQE), and light intensity was determined using a spectro-
radiometer (USR-45D, Ushio Inc.).
Scheme 1 Photocatalytic hydrogenation of furan to THF in a methanolic
suspension of metal (M)-loaded TiO2 under a H2-free condition.
reduction of (2,3-epoxypropyl)benzene to allylbenzene (silver co-
catalyst)24 and semi hydrogenation of alkynes to alkenes (copper
co-catalyst),25 indicating that photocatalytic H2-free reduction
can be broadened by the use of metal co-catalysts. In this study,
we examined photocatalytic hydrogenation of furan, a representative
heterocyclic compound, to THF in an alcoholic suspension
of metal-loaded TiO2 (Scheme 1) to expand the possibility of
photocatalytic hydrogenation. Here we report (1) the effects of
metal co-catalysts and (2) the effects of various parameters on
H2-free hydrogenation of furan, and (3) details of the reaction.
3. Results and discussion
3.1. Effects of different metal co-catalysts
In the hydrogenation of furan, H2 may be formed as the product of
reduction of H+ by photogenerated electrons (2H+ + 2eꢁ - H2).
Therefore, hydrogenation of furan competes with the reduction of
H+, and the selectivity of two products (THF and H2) is also an
important indicator for evaluation of photocatalysts as well as the
yields of THF and H2. Fig. 1 shows the effects of metal co-catalysts
(1.0 wt%) on the yields of THF and H2 produced in photocatalytic
hydrogenation of furan in methanolic suspensions of metal-loaded
TiO2 (MT-150A) for 10 min under Ar with irradiation of UV light
from an LED. When bare TiO2 was used as the photocatalyst, no
THF or H2 was formed. Just after photoirradiation, the color of TiO2
became blue, indicating that Ti4+ in TiO2 was reduced to Ti3+ by
photogenerated electrons. These results mean that without the aid
of a co-catalyst, photoirradiated TiO2 has no ability to reduce
(hydrogenate) a CQC double bond in a heterocyclic compound
as well as a CQC double bond in hydrocarbons.23 In the case of
Au-, Ag- and Cu-loaded TiO2 samples, no THF was formed,
indicating that these metals were inactive as co-catalysts for
furan hydrogenation. H2 was evolved over Au- and Ag-loaded
TiO2 samples. Formation of H2 will be discussed at the end of
this section. In contrast to these metal-loaded TiO2 samples, a
small amount of THF was formed when Pt-loaded TiO2 (Pt-TiO2)
was used, although a large amount of H2 was simultaneously
evolved. We noted that Pd-loaded TiO2 (Pd-TiO2) showed a
distinctive photocatalytic activity for the largest production of
THF and suppressed evolution of H2. As clearly shown, Pd-TiO2
possessed a favorable catalytic property in hydrogenation of furan
in contrast to Pt-TiO2. Bradley et al.28 reported that Pd can readily
adsorb a furan ring due to a strong interaction between the Pd
2. Experimental
2.1. Preparation of metal-loaded TiO2
Various metals as co-catalysts (Au, Ag, Cu, Pd and Pt) were loaded
on TiO2 (MT-150A, Tayca, Osaka, Japan) using the photodeposition
method. Some TiO2 samples were selected from Japan Reference
Catalysts (JRC-TIO series) registered at the Catalysis Society of
Japan and used to examine the effects of different kinds of TiO2.
Metal-loaded TiO2 powder and unloaded TiO2 powder were each
suspended in 10 cm3 of an aqueous methanol solution (10 vol%)
containing a metal source in a test tube. HAuCl4, AgNO3, CuCl2,
PdCl2 and H2PtCl6 were used as metal sources. Each test tube was
sealed with a rubber septum under argon (Ar) and then photo-
irradiated for 90 min at l 4 300 nm using a 400 W high-pressure
mercury arc (Eiko-sha, Osaka) with magnetic stirring in a water
bath continuously kept at 298 K. The resulting powder was washed
repeatedly with distilled water and dried for 1 h in vacuo.
2.2. Characterization
The morphology of the Pd-TiO2 particles was observed under a
JEOL JEM-2100F transmission electron microscope (TEM) operated
at 200 kV in the Joint Research Center of Kindai University.
2.3. Photocatalytic reaction
In a typical run, Pd-TiO2 (50 mg) was suspended in 5 cm3 of
methanol containing furan (40 ꢀ 4 mmol) in a test tube, which
was sealed with a rubber septum and then photoirradiated
under Ar at 298 K with the same high-pressure mercury lamp or
a UV-LED (UV-LED, PJ-1505-2CA, CCS Inc., Kyoto, maximum
energy at l = 365 nm). The amounts of furan and THF were
determined using an FID-type gas chromatograph (GC-2025,
Shimadzu) equipped with a DB-1 column. Chlorobenzene was
used as an internal standard sample. Chlorobenzene (7 mm3)
was added to the reaction solution (3 cm3). After the mixture
has been stirred for 7 min, furan and THF in the mixture
solution were analyzed. The amounts of furan and THF were
determined from the ratios of their peak areas to the peak area
Fig. 1 Effects of different metal co-catalysts (1.0 wt%) on the yields of THF
and H2 produced in photocatalytic hydrogenation of furan (40 ꢀ 4 mmol) in
methanolic suspensions of metal-loaded TiO2 (MT-150A) for 10 min under
irradiation of UV light from an LED.
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and p bonds in furan. The strong interaction between Pd and
furan is attributed to the high activity of Pd-TiO2.
As shown in Fig. 1, H2 was formed when Au, Ag, Pt and Pd
were used as co-catalysts. It is known that co-catalysts such as
Pt loaded on a photocatalyst act as sites for storage of photo-
generated electrons and as active sites for reaction, resulting in
reduced activation energy of the reaction and formation of
hydrogenated products.29,30 In previous papers on photocatalytic
NH3 decomposition in aqueous suspensions of TiO2 having
co-catalysts, we reported that the rate of H2 evolution was
drastically changed depending on the type of co-catalyst and
that the effect as a co-catalyst became smaller with an increase
in hydrogen over-voltage of the co-catalyst metal when used as
an electrode.31,32 A similar tendency was observed in the
evolution of H2 from methanol in the presence of furan in this
study as shown in Fig. 2. However, the use of a Pd co-catalyst
resulted in a very small yield of H2 that was much different from
the value predicted from the hydrogen overvoltage. The second
smallest value of hydrogen overvoltage means that the activation
energy for H2 evolution over Pd particles is rather small. The
much smaller H2 yield in furan hydrogenation indicates that the
Pd co-catalyst provides an excellent route with very small activa-
tion energy for hydrogenation of furan to THF.
Supported Pd nanoparticles are often used for catalytic
hydrogenation with H2 as a hydrogen source. In thermocatalytic
hydrogenation, H2 molecules are adsorbed dissociatively on Pd
particles, and thus-formed active hydrogen species are inserted
into the CQC double bond.33,34 Therefore, active hydrogen
species formed reductively from H+ in the photocatalytic process
and formed dissociatively from H2 gas would be essentially the
same. It can be concluded that hydrogenation of furan to
THF under the present conditions consists of two processes:
(1) photocatalytic production of active hydrogen species and
(2) thermocatalytic hydrogenation of furan over Pd particles.
As can be seen in Fig. 1 and 2, only the thermocatalytic
properties of metal particles decide whether hydrogenation of
furan to THF occurs or not. Strictly speaking, the observed
Fig. 3 Effects of amounts of Pd loading on TiO2 in photocatalytic hydro-
genation of furan (40 ꢀ 4 mmol) in methanolic suspensions for 5 min under
irradiation of UV light from a high-pressure mercury lamp.
hydrogenation should be called a combination reaction by
photocatalysis and thermocatalysis, although we call this reac-
tion simply photocatalytic hydrogenation hereafter.
Fig. 3 shows effects of the Pd loadings on the yield of THF
formed by photocatalytic hydrogenation of furan in methanolic
suspensions after 5 min photoirradiation from a high-pressure
mercury lamp. The yield of THF increased with an increase in
the amount of Pd up to 1.0 wt% probably because the number
of reduction sites for hydrogenation of furan increased with an
increase in the Pd content. However, the yield was saturated
over 1.0 wt%. There are some possible reasons for the saturation:
(1) dispersion of Pd on the TiO2 surface was saturated, (2) the
rate-determining step changed from the reduction process to the
oxidation process, and (3) the overall reaction was limited by
photon flux.
TEM photographs of bare TiO2 (MT-150A) and 1.0 wt% Pd-
TiO2 and the distribution of Pd particles in 1.0 wt% Pd-TiO2 are
shown in Fig. 4a–c, respectively. Fine Pd particles can be seen
in the TEM photograph (Fig. 4b), and the average diameter was
determined to be 3.4 nm, indicating that Pd nanoparticles were
deposited on the TiO2 surface using the photodeposition
method. The amount of THF (40 mmol) after photoirradiation
for 5 min was larger than the amount of Pd (4.7 mmol) loaded
on TiO2, indicating that Pd metal works as a catalyst for the
hydrogenation of furan.
3.2. Effects of reaction conditions on photocatalytic
hydrogenation of furan
The effects of various reaction conditions on photocatalytic
hydrogenation of furan to THF were investigated, and the
results are summarized in Table 1. First, the effects of different
kinds of commercial TiO2 were examined (entries 1–7). In addition
to MT-150A TiO2, six samples were selected from Japan Reference
Catalysts (JRC-TIO series) registered at the Catalysis Society of
Japan. Rutile TiO2 (R, entries 6 and 7) showed THF yields higher
than those by anatase TiO2 (A, entries 1–3) and those of TiO2
consisting of two phases (A/R, entries 4 and 5). In the case of
Fig. 2 Correlation between yield of H2 produced in photocatalytic hydro-
genation of furan and hydrogen overvoltage of metal electrodes. Reaction
conditions are shown in Fig. 1.
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the durability of the Pd-TiO2 photocatalyst in this reaction system,
Pd-TiO2 was used repeatedly. After reaction for 10 min (entry 9), Pd-
TiO2 particles were recovered by simple filtration from the reaction
mixture and were re-used. Entries 10 and 11 show that Pd-TiO2
photocatalysts were reusable at least twice without notable loss of
activity. Entries 12–14 show the results of three blank reactions:
(1) in the absence of Pd-TiO2 in the dark (non-catalytic thermal
reaction between furan and methanol, entry 12), (2) in the absence
of Pd-TiO2 with irradiation of light (photochemical reaction,
entry 13), and (3) in the presence of Pd-TiO2 in the dark (thermo-
catalytic reaction over Pd-TiO2, entry 14). These results indicate that
the Pd-TiO2 photocatalyst and photoirradiation were indispensable
for the hydrogenation of furan. In the dark at 298 K, THF was
formed in the presence of Pd-TiO2 and H2 (1 atm) (entry 15),
indicating that H2 was used for hydrogenation of furan over Pd-
TiO2. In other words, Pd particles loaded on TiO2 showed thermo-
catalytic activity for hydrogenation of furan as discussed earlier.
When the amount of H2 added in the reactor was reduced to
50 mmol, THF was not formed in the dark at 298 K (entry 16).
Results of the dark reactions under H2 (entries 15 and 16) mean
that a large excess of H2 is necessary to obtain a sufficient rate of
THF production at 298 K.
Fig. 4 TEM photographs of (a) TiO2 (MT-150A) and (b) 1.0 wt% Pd-TiO2,
and (c) distribution of Pd particles in 1.0wt% Pd-TiO2.
Dark reactions over metal-free TiO2 and TiO2 modified with other
photocatalytic hydrogenation of nitro compounds, rutile phase metal co-catalysts under H2 (1 atm) were negligible (entries 17–21).
TiO2 was active, though the reason is not clear.20,35 An increase To examine the effects of different kinds of alcohol, ethanol,
in Pd loading up to 1.0 wt% resulted in an increase in THF yield 2-propanol, 2-butanol, 2-pentanol and glycerol were chosen
(entry 8) as has already been shown in Fig. 3. Prolonging the (entries 22–26). Ethanol and glycerol have attracted much atten-
reaction time increased the yield as expected (entry 9). To examine tion as typical biomasses and by-products in biomass utilization
Table 1 Effects of various reaction conditions on photocatalytic production of THF from furan in alcoholic suspensions of Pd-TiO2 at 298 Ka
Phasec
SBETd/m2 gꢁ1
Pd/wt%
Alcohol
UV
Time/min
Gas phase
Yield/mmol
b
Entries
TiO2
1
2
3
4
5
6
7
8
JRC-TIO-1
JRC-TIO-8
JRC-TIO-13
JRC-TIO-4
JRC-TIO-11
JRC-TIO-3
MT-150A
MT-150A
MT-150A
MT-150A
MT-150A
—
A
A
A
72
313
59
54
97
48
93
93
93
93
93
—
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1.0
1.0
1.0
1.0
—
—
1.0
1.0
1.0
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
On
On
On
On
On
On
On
On
On
On
On
(Dark)
On
(Dark)
(Dark)
(Dark)
(Dark)
(Dark)
(Dark)
(Dark)
(Dark)
On
5
5
5
5
5
5
5
5
10
5
5
10
5
5
5
5
5
5
5
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
H2
H2g/Ar
H2
H2
H2
H2
H2
Ar
Ar
Ar
Ar
Ar
17
16
18
9
A, (R)
A, (R)
R
R
R
R
R
R
—
—
R
R
R
R
R
R
R
20
21
26
38
40
33
36
—
—
—
39
—
—
—
—
—
0.6
19
22
29
13h
35
9
10e
11f
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
—
—
MT-150A
MT-150A
MT-150A
MT-150A
MT-150A
MT-150A
MT-150A
MT-150A
MT-150A
MT-150A
MT-150A
MT-150A
MT-150A
93
93
93
93
93
93
93
93
93
93
93
93
93
—
1.0 (Au)
1.0 (Ag)
1.0 (Cu)
1.0 (Pt)
1.0
1.0
1.0
5
5
10
10
10
10
10
R
R
R
R
R
R
2-PrOH
2-BuOH
2-PnOH
Glycerol
On
On
On
On
1.0
1.0
a
Alcohol solution (5 cm3) of furan (40 ꢀ 4 mmol) and 50 mg of the photocatalyst under irradiation of UV light from a high-pressure mercury lamp.
b
c
JRC-TIO series registered at the Catalysis Society of Japan as Japan Reference Catalysts. MT-150A was supplied from Tayca. A: Anatase, R: Rutile,
d
e
f
g
h
minor form in bi-phase samples is shown in parentheses. Determined using the BET method. Second use. Third use. H2: 50 mmol. H2
formed: 6 mmol, 2-pentanone formed: 33 mmol.
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(saponification and transesterification of fats and oils). When these amount of furan before the photocatalytic reaction. In the initial
alcohols were used, THF was formed, indicating that various stage of the reaction, the value of MB was ca. 0.9. We examined
alcohols can be used as solvents and electron sources for furan adsorption on TiO2 and metal-loaded TiO2 in the dark
hydrogenation of furan. The results for ethanol and glycerol (Fig. S1, ESI†). The results indicated that furan was adsorbed on
indicate another important possibility of two alcohols: biomass both TiO2 and some metals (Pd, Pt and Au) and Pd-TiO2 showed
alcohols can be used as hydrogen sources without another the highest amount of adsorption. The value of MB in the early
H2-producing process for upgrading of another biomass stage can be attributed to the adsorption of furan on Pd-TiO2.
(hydrogenation of furan to THF in this study). In the reaction Formation of an intermediate(s) may also be one of the reasons for
in 2-pentanol, the oxidation product was analyzed as well as the value of MB. Since species other than furan and THF were not
THF and H2 (entry 25). 2-Pentanone was formed and the redox detected in GC, we cannot discuss the possibility of adsorption of
balance (RB) was calculated to be 0.97 from eqn (1):
an intermediate(s). As also shown in Fig. 5, H2 evolution was
predominant under excessive photoirradiation after consumption
of furan, indicating that active hydrogen species were continuously
formed over the Pd co-catalyst. We noted that the amount of THF
slightly decreased after complete consumption of furan. Since a
large excess of methanol used as a solvent effectively consumes
positive holes, the possibility of THF re-oxidation can be elimi-
nated. The results after consumption of furan suggest the occur-
rence of a reductive process of THF in which a part of the active
hydrogen species was probably used for catalytic hydrogenolysis of
THF, resulting in consumption of THF. Mizugaki et al. reported the
formation of butanol in the ring opening reaction of furfural that
has a furan ring.36 Since butanol was not detected in GC, butanol
was probably adsorbed on Pd-TiO2. The high yield of THF at 25 min
indicates that the activation energy of catalytic hydrogenolysis of
THF over Pd is larger than that of catalytic hydrogenation of furan.
Therefore, furan hydrogenation predominantly occurred at 298 K
when furan was present in the reaction system.
2 ꢂ nðTHFÞ þ nðH2Þ
RB ¼
(1)
nð2-pentanoneÞ
where n(THF), n(H2) and n(2-pentanone) are the amounts of
THF, H2 and 2-pentanone formed. The value indicates that
photogenerated electrons and positive holes were equally used
in the photocatalytic reaction.
3.3. Photocatalytic hydrogenation of furan
Fig. 5 shows time courses of furan remaining and THF and H2
formed in a methanolic suspension of 1.0 wt% Pd-TiO2 (MT-150A)
under deaerated conditions. The amount of furan decreased almost
linearly with photoirradiation, while THF as the hydrogenation
product of furan was formed with a decrease in the amount of
furan. After 25 min, furan was almost completely consumed and
THF was obtained in a high yield (97%). To evaluate the selectivity
of THF and intermediates in the furan hydrogenation, an indicator,
i.e., material balance (MB), was calculated by eqn (2):
3.4. Action spectrum and the reaction process
nðfuranÞ þ nðTHFÞ
MB ¼
(2)
An action spectrum is a strong tool for determining whether a
photoinduced process is the rate-determining step in the reac-
tion observed. To obtain an action spectrum in this reaction
system, hydrogenation of furan in methanolic suspensions of
1.0 wt% Pd-TiO2 (MT-150A) was carried out at 298 K under
irradiation of monochromated light from a Xe lamp. Apparent
quantum efficiency (AQE) at each centered wavelength of light
was calculated from the ratio of quadruple the amount of THF
formed and the amount of photons irradiated using eqn (3):
n0ðfuranÞ
where n(furan) and n(THF) are the amounts of furan and THF
during the photocatalytic reaction, respectively, and n0(furan) is the
4 ꢂ the amount of THF formed
AQE ¼
ꢂ 100
(3)
amount of incident photons
As shown in Fig. 6, AQE was in agreement with the absorption
spectrum of TiO2. Therefore, it can be concluded that hydro-
genation of furan in a methanolic suspension was induced by
photoabsorption of TiO2. As also shown in Fig. 6, AQE reached
37% at 360 nm, indicating that photocatalytic hydrogenation of
furan to THF occurred with a high efficiency of photon utiliza-
tion in addition to high selectivity and MB close to unity. This
result indicates that photoabsorption at 410 nm does not
contribute to the reaction, in other words, this photoabsorption
does not induce the band-gap excitation of TiO2. Electrons in
the trap site below the bottom of the conduction band of TiO2
have no potential to reduce protons to H2.
Fig. 5 Time courses of furan remaining, THF formed and material balance
of furan and THF in a methanolic suspension of 1.0 wt% Pd-TiO2 under
irradiation of UV light from an LED.
The expected reaction process of photocatalytic hydrogena-
tion of furan to THF over Pd-TiO2 is shown in Fig. 7: (1) by
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solvent, electron donor and hydrogen source. Among the
metal-loaded TiO2 samples examined in this study, Pd-loaded
TiO2 (Pd-TiO2) showed a distinctive photocatalytic activity for
the largest production of THF and suppressed evolution of H2,
with apparent quantum efficiency reaching 37% at 360 nm.
Hydrogenation of furan to THF under the present condition
consisted of two processes: (1) photocatalytic production of
active hydrogen species and (2) thermocatalytic hydrogenation
of furan over Pd particles. Biomass-related alcohols (ethanol,
butanol and glycerol) can also be used for hydrogenation of
furan. The results obtained in this study show that photocatalytic
hydrogenation is not limited to hydrocarbons and can be applied to
heterocyclic compounds.
Fig. 6 Absorption spectrum (right axis) and the action spectrum of TiO2
Conflict of interest
and 1.0 wt% Pd-TiO2 in the hydrogenation of furan (left axis).
There are no conflicts to declare.
Acknowledgements
This work was partly supported by JSPS KAKENHI Grant
Numbers 17H03462, and 17H04967. This work was also sup-
ported 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. A. T. is grateful for financial
support from the Faculty of Science and Engineering, Kindai
University.
Fig. 7 Expected reaction process of photocatalytic hydrogenation of
furan to THF over Pd-TiO2.
irradiation of UV light, photogenerated electrons (eꢁ) and
positive holes (h+) are formed in the conduction and valence
bands of TiO2, and methanol is oxidized by h+, (2) H+ is reduced
by eꢁ, resulting in the formation of active hydrogen species over
Pd particles and (3) two CQC double bonds of furan are
successively hydrogenated by the active hydrogen species over
Pd particles, resulting in the formation of THF. Higher adsorp-
tion ability of Pd is one of the most important reasons for the
large yield of THF in the photocatalytic hydrogenation of furan.
Since THF was formed in the presence of H2 in the dark at
298 K, the active hydrogen species would be essentially the
same as those formed in the photocatalytic process. Active
hydrogen species also induce other reactions, i.e., H2 evolution
by self-coupling and hydrogenolysis of THF; however, these
reactions are negligible as long as furan to be reacted with
active hydrogen species is present in the reaction system. The
negligible production of H2 in the photocatalytic hydrogenation
of furan means that the active species formed on Pd are very
active for hydrogenation of furan, and the equilibrium between
the active species formed on Pd and H2 in the gas phase greatly
shifts toward the left side (formation of the active species).
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