Photocatalytic hydrogenolysis of epoxides using alcohols as reducing agents on TiO2 loaded with Pt nanoparticles

Article abstract of DOI:10.1039/c4cc09307k

Photoexcitation (λ > 300 nm) of TiO₂ loaded with Pt particles promotes selective hydrogenolysis of epoxides using alcohols as reducing agents.

Full text of DOI:10.1039/c4cc09307k

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DOI: 10.1039/C4CC09307K
COMMUNICATION
Photocatalytic hydrogenolysis of epoxides with
alcohol as a reducing agent on TiO loaded with
2
Cite this: DOI: 10.1039/x0xx00000x
Pt nanoparticles†
a
a,b
a
Hiroaki Hirakawa, Yasuhiro Shiraishi,* Hirokatsu Sakamoto, Satoshi
c
d
a
Received 00th January 2012,
Accepted 00th January 2012
Ichikawa, Shunsuke Tanaka and Takayuki Hirai
DOI: 10.1039/x0xx00000x
www.rsc.org/
5
Photoexcitation (λ >300 nm) of TiO loaded with Pt particles to the H−M species (eq. 3). The photocatalytically generated
2
promotes selective hydrogenolysis of epoxides with alcohol as H−M species may promote hydrogenolysis of epoxides.
a reducing agent.
–
+
TiO + h
ν
→ e + h
(1)
(2)
(3)
2
Reductive ring opening (hydrogenolysis) of epoxides into the
corresponding alcohols is one of the most important organic
+
+
R R CHOH + 2h → R R CO + 2H
3
4
3 4
1
+
−
transformations. This facilitates selective hydration of alkenes
H + e + M → H–M
via the protection of C=C bond by epoxidation (Scheme 1). The
hydrogenolysis of epoxides is usually carried out with excess or
stoichiometric amount of metal hydrides (H−M; for example:
LiAlH and LiEt BH), which are explosive and toxic reducing
We found that the above hypothesis is realized by TiO loaded
2
with Pt particles (Pt/TiO ). The system produces several kinds
2
of ringꢀopened products with high selectivity (>72%). The
catalytic activity strongly depends on the size of Pt particles;
small Pt particles (ca. 3 nm) exhibit the best performance.
4
3
agents, with a concomitant formation of copious amount of
2
wastes. Catalytic hydrogenolysis is therefore ideal. Pd particles
supported on carbon, Al O , CaCO , AlO(OH), or polyurea are
2
3
3
3
a
b 200
wellꢀknown heterogeneous catalysts for hydrogenolysis. These
systems use metal hydride (H−M) species, in-situ generated on
the metal surface by the reaction with a reducing agent as active
species. They, however, need H or HCOONH as reducing
agents, which hold explosion or cost issue. Alternative systems
that promote efficient and selective hydrogenolysis with safe
and inexpensive reducing agent are therefore desired.
dPt = 2.9 ± 0.8 nm
2
4
100
0
0
2
4
6
8
10
Size / nm
2 2 2
Fig. 1 (a) TEM image of Pt /TiO prepared by H reduction at 673 K and
Scheme 1 Hydration of alkenes through epoxidation.
(b) size distribution of the Pt particles.
Here we report for the first time a catalytic system that
M /TiO [x (wt %) = M/TiO2 × 100] catalysts were prepared
x 2
promotes hydrogenolysis of epoxides with safe and inexpensive with JRCꢀTIOꢀ4 TiO (equivalent to P25, average size, 27 nm;
2
2
–1
alcohols as reducing agents. We employed semiconductor TiO2 BET surface area, 57 m g ; anatase/rutile ratio, 83/17 w/w)
loaded with metal particles (M/TiO ) under UV irradiation (
supplied from the Catalyst Society of Japan. Pt, Ag, or Pd
300 nm). Our hypothesis is in-situ generation of H−M species particles were loaded by impregnation of precursors followed
λ
2
>
6
with alcohol by photocatalysis: photoexcited TiO produces the by H reduction at designated temperature. ‡ Au particles were
electron (e ) and positive hole (h ) pairs (eq. 1). The h oxidize loaded by the deposition–precipitation method. § As shown in
alcohols and produce aldehyde (ketones) and H (eq. 2). The Fig. 1, a typical transmission electron microscopy (TEM) image
H are reduced by the e on the metal particles and transformed of Pt /TiO2 prepared by H2 reduction at 673 K shows Pt
2
2
–
+
+
7
+
4
+
–
2
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particles with an average size 2.9 nm. As shown in Fig. S1
The formation of H–Pt species by photocatalysis on Pt/TiO2
(
ESI†), Au /TiO , Ag /TiO , and Pd /TiO also contain metal is confirmed by diffuseꢀreflectance infrared FourVieierw tArrtaicnlesfOonrlimne
2
2
2
2
2
2
particles with similar sizes (3.7, 4.2, and 4.4 nm, respectively). (DRIFT) analysis. 2ꢀPrOH was adsorbed onto Pt /TiO in the
DOI: 10.1039/C4CC09307K
2 2
As shown in Fig. S2 (ESI†), diffuseꢀreflectance UVꢀvis spectra gas phase and photoirradiated with 360 nm monochromatic
of Au /TiO and Ag /TiO show characteristic band at ca. 550 light. As shown in Fig. S3 (ESI†), a distinctive absorption band,
2
2
2
2
1
1
–1
and 520 nm, respectively, assigned to the surface plasmon assigned to the H
resonance (SPR) absorption, whereas Pd /TiO and Pt /TiO
2
−
Pt species, appears at 1975 cm . This
8
clearly suggests that, as shown in Scheme 2a b, photocatalysis
→
2
2
2
show flat absorption at
the metal particles.
λ
>300 nm due to the light scattering by on Pt/TiO indeed produces H
−
Pt species.
2
4
Hydrogenolysis of transꢀstilbene oxide was carried out with
ꢀPrOH as a reducing agent. The reactions were performed by
photoirradiation (λ >300 nm) of a 2ꢀPrOH solution (5 mL)
a
b
trans−stilbene oxide
5
0
0
50
2
4
40
30
20
10
containing catalyst (10 mg) and epoxide (50 µmol) under N for
2
2
2
3
6
h. Table 1 summarizes the conversions of epoxide and yields
of stilbene ( ), 1,2ꢀdiphenylethanol ( ), and 1,2ꢀdiphenylethane
). Bare TiO (entry 1) gives a small amount of deoxygenation
30
trans−stilbene oxide
1
2
2
0
0
0
(
3
2
product (
1
, 2%), but does not afford the hydrogenolysis product
1
9
(
2). Au /TiO and Ag /TiO (entries 2 and 3) are also inactive.
2
2
2
2
3
Pd /TiO (entry 4) produces
2
with 54% yield, but also
produces a large amount of byproduct ( , 42%). In contrast,
Pt /TiO (entry 5) produces with quantitative yield (>99%).
Fig. 2 shows the timeꢀprofiles for the amounts of substrate and photoreaction of trans-stilbene oxide on (a) Pd
0
0
2
2
0
6
12
6
t / h
12
3
t / h
2
2
2
Fig. 2 Time-profiles for the amounts of substrate and products during
/TiO and (b) Pt /TiO
reduction at 673 K. The reaction conditions
2
2
2
2
catalysts prepared by H
2
products on Pd /TiO and Pt /TiO . On Pd /TiO (Fig. 2a), the
2
2
2
2
2
2
are identical to those in Table 1.
epoxide (black) is selectively transformed to
stage, but subsequently transformed to
Pt /TiO (Fig. 2b) selectively produces
2
(red) at the early
3
(blue). In contrast,
and does not promote
2
a
2
2
further reaction even after prolonged irradiation. In addition, as
shown in Table 1 (entry 5), the catalyst recovered after the
reaction exhibits almost the same
2 yields as the fresh one.
These data suggest that Pt/TiO specifically promotes efficient
2
and selective hydrogenolysis of epoxide and is reusable without
the loss of activity and selectivity.
c
b
Mechanism for hydrogenolysis on Pt/TiO is explained as
2
–
+
Scheme 2. Photoexcited TiO produces the e and h pairs. The
2
–
10
e overcome the Schottky barrier (φB) created at the interface
and are trapped by the Pt particles (a). The h oxidize alcohol
and produce H , while the e reduce H and create H−Pt species
+
+
–
+
(
b). Epoxide is adsorbed onto the Pt surface and undergoes a Scheme 2 Proposed mechanism for hydrogenolysis of epoxide on the
homolytic cleavage of the oxirane ring. Subsequent reaction of
2
photoexcited Pt/TiO with alcohol.
the intermediates with H−Pt species gives the product (c).
a
Table 1 Reactions of trans-stilbene oxide on various catalysts under photoirradiation
metal particle size
trans-stilbene oxide
yield / %
entry
catalyst
TiO
b
/
nm
conv. / %
1
2
0
0
3
0
0
0
42
0
1
2
3
4
5
2
2
<1
9
>99
>99
>99
>99
2 (E/Z = 1/3)
Au /TiO
Ag
Pd
Pt
st reuse
2
2
c
3.7 ± 0.5
4.2 ± 0.9
4.4 ± 0.8
2.9 ± 0.8
0
2
/TiO
2
8 (E/Z = 1/3)
0
c
2
/TiO
2
0
0
0
0
54
>99
>99
>99
c
2
/TiO
2
d
1
2
0
0
d
nd reuse
a
−2
b
c
Photoirradiation was carried out with a 2 kW Xe lamp (light intensity at 300−450 nm, 27.3 W m ). Determined by TEM observations. Prepared by
d
2
impregnation followed by H reduction at 673 K. Catalysts were reused after simple washing with 2ꢀPrOH followed by drying in vacuo.
2
| J. Name., 2012, 00, 1-3
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Au/TiO and Ag/TiO are inactive for hydrogenolysis of
epoxide (Table 1, entries 2 and 3). As reported, temperatureꢀ The Pt /TiO catalysts were prepared at different H reduction
The size of Pt particles strongly affects the cataVliyewticArtaiccletiOvniltinye.
2
2
1
2
2
2
DOI: 10.1039/C4CC09307K
2
programmed desorption (TPD) analysis revealed that Pt or Pd temperatures to create Pt particles with different sizes. As
surface promotes the oxirane ring cleavage of the adsorbed shown in Fig. 3a (circle), the size of Pt particles increases with
ethylene oxide via the electron transfer from metal, whereas Au a rise in temperature due to the migration and sintering of the
1
6
or Ag surface is inactive. This is consistent with no activity of particles; the treatment at 573, 673, 773, and 873 K creates Pt
Au/TiO and Ag/TiO . This suggests that, as shown in Scheme particles with 2.8, 2.9, 3.6, and 5.9 nm diameters, respectively.
2
2
2
b
→
c, the oxirane ring cleavage on the Pt surface is indeed These catalysts were employed for photoreaction (2 h) of trans
involved in the hydrogenolysis on Pt/TiO2. stilbene oxide. All of them produce with very high selectivity
As shown in Table 1 (entries 4 and 5), Pd/TiO promotes (>99%), but exhibit different activities. The bar data in Fig. 3a
ꢀ
2
2
hydrogenolysis as well as Pt/TiO because Pd particles are also show the
2
yield. The catalysts with ca. 3 nm Pt particles
active for oxirane ring cleavage. As shown in Fig. 2, Pt/TiO2 prepared at 573 and 673 K show high activity, but the catalysts
selectively produces , but Pd/TiO2 promotes subsequent with larger Pt particles show lower activity. Larger Pt particles
hydrogenolysis of –OH group of (formation of ). This is due have smaller surface area. The lower activity of these catalysts
to the high nucleophilicity of H–Pd species. It is well known may therefore be due to the inefficient oxirane ring cleavage of
2
1
2
2
2
3
1
7,18
that H atoms are adsorbed onto the Pd surface more strongly epoxide or formation of H–Pt species.
1
3
–
than onto the Pt surface. The e on the Pd particles are
therefore strongly donated to H , producing nucleophilic H−Pd
a
2
+
Table 2 Photoreaction of various epoxides on Pt
2
/TiO
b
species. This may promote hydrogenolysis of –OH group of
2
.
entry
1
substrate
t / h
product
yield / %
>99
Similar hydrogenolysis of –OH group by the H–Pd species is
6
1
4
observed for allyl alcohol (propylene formation) and benzyl
c
1
5
2
6
6
6
97
>99
99
alcohol (toluene formation), whereas the H–Pt species are
inactive. These imply that, on Pt/TiO , less nucleophilic H−Pt
3
2
species are inactive for hydrogenolysis of –OH group, resulting
c
4
in selective formation of ringꢀopened product (2).
O
5
6
6
76
80
2
selectivity / %
a
>
99
>99 >99
>99
5
4
3
2
1
0
0
0
0
0
0
10
8
d
6
d
7
6
6
84
84
81
86
6
4
d
8
2
d,e
9
10
10
0
5
73
673
773
873
H reduction temp. / K
2
b
d,e
1
0
2
selectivity / %
>99 >99
10
8
>
99 >99
>99
6
4
2
0
0
0
0
d,e
d,e
1
1
1
2
10
10
72
73
6
4
a
Reaction conditions: epoxide (50 µmol), Pt
2
/TiO
2
catalyst prepared by
(1 atm), light
Conversions of
epoxides in all runs are >99%. EtOH was used as a solvent in place of
2
H
2
reduction at 673 K (10 mg), 2ꢀPrOH (5 mL), N
2
b
irradiation (λ >300 nm), temperature (303 K).
0
c
0
.5 1.0
2.0
Pt loading, x / wt %
reduction temperature of Pt
b) amount of Pt loaded (x) for Pt /TiO catalysts on the
photoreaction (2 h) of trans-stilbene oxide. Reaction conditions are
identical to those in Table 1. Circles denote the average diameter of Pt
particles on the catalysts, and the numbers on the top of the figures wt %) were prepared by H reduction at 673 K. As shown in
3.0
4.0
d
e
2
ꢀPrOH. Epoxide (20 µmol). Catalyst (20 mg).
Fig. 3 Effect of (a) H
(
2
2
/TiO
2
catalysts and
2 yield during
x
2
The amount of Pt loaded also affects the catalytic activity.
The Pt /TiO catalysts with different Pt loadings (x = 0.5–4
x
2
2
denote the selectivity of
2
2. The H reduction temperature for the
catalysts (b) is 673 K. Typical TEM images of catalysts and size
distributions of their Pt particles are summarized in Fig. S1 (ESI†).
Fig. 3b (circle), these catalysts possess Pt particles with similar
diameters (2.8–3.1 nm). As shown by the bar data, the activity
increases linearly with the Pt loadings, where the selectivity of
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2
for all of the catalysts scarcely changes (>99%). The above
Gütschow, Helv. Chim. Acta., 2004, 87, 2597–2601; (
c
) T. Hamada
) M. R.
findings suggest that loading of large number of small Pt
particles (ca. 3 nm diameter) exhibit best catalytic performance
for hydrogenolysis of epoxide.
and Y. Kobayashi, Tetrahedron Lett., 2003, 44, 4347–4350; (
d
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The Pt/TiO catalyst is applicable for the hydrogenolysis of
several types of epoxides. Table 2 summarizes the results of
2
3
R. C. Larock, Comprehensive Organic Transformations, VCH, New
2
York, 1989, p. 505.
photoreactions on Pt /TiO2 in 2ꢀPrOH as a reducing agent.
(
a
) H. Sajiki, K. Hattori and K. Hirota, Chem. Commun., 1999, 1041–
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2
Reactions of stilbene oxides (entries 1 and 3), styrene oxides
5–8), and aliphatic epoxides (9–12) successfully produced the
b
9,
(
c
,
corresponding ringꢀopened alcohols with moderate to high
yields (72–99%). In particular, styrene oxides bearing reducible
halogen groups (entries 7 and 8) gave the products with high
yields (84%) while retaining their substituents. It must also be
noted that, as shown by entries 2 and 4, hydrogenolysis is also
successfully promoted when using EtOH as a reducing agent, a
very cheap alcohol which can be produced from biomass. This
suggests that economical hydrogenolysis process can be created
9
d
5
4
5
Y. Shiraishi, Y. Sugano, S. Tanaka and T. Hirai, Angew. Chem. Int.
Ed., 2010, 49, 1656–1660.
(
a
) Y. Shiraishi, Y. Takeda, Y. Sugano, S. Ichikawa, S. Tanaka and T.
Hirai, Chem. Commun., 2011, 47, 7863–7865; ( ) A. Maldotti, A.
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b
2
(
c
by the Pt/TiO system.
W. Wang and A. G. Stepanov, J. Phys. Chem. C, 2013, 117, 7690–
7702.
2
In summary, we found that UV irradiation of Pt/TiO with
2
alcohol promotes selective hydrogenolysis of epoxides. The
system offers several advantages over conventional processes:
6
7
8
Y. Shiraishi, H. Sakamoto, Y. Sugano, S. Ichikawa and T. Hirai, ACS
Nano, 2013, 7, 9287–9297.
(
i) safe and inexpensive alcohols can be used as a reducing
D. Tsukamoto, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka and T.
agent; and, (ii) the reaction is carried out under milder reaction
conditions (room temperature). The catalytic system presented
here based on photocatalytic in-situ generation of H–Pt species
may contribute to the catalyst design for photocatalysisꢀbased
green organic synthesis.
Hirai, J. Am. Chem. Soc., 2012, 134, 6309–6315.
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a
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, 1642−1649.
,
b
9
This work was supported by the Grantꢀin Aid for Scientific
4
Research (No. 26289296) from the Ministry of Education, 10 T. Uchihara, M. Matsumura, A. Yamamoto and H. Tsubomura, J.
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Electronic Supplementary Information (ESI) available: Experimental
details, supplementary data (Figs. S1–S4). See DOI: 10.1039/c000000x/
Pt /TiO (wt %) = 0.5, 1, 2, 3, and 4] were prepared as follows: TiO
1.0 g) and H PtCl ·6H
20 mL) and evaporated under stirring at 393 K for 12 h. The resultant
flow at the designated
temperature (573, 673, 773, or 873 K) with the heating rate and holding
3
b
†
16 Y. Shiraishi, Y. Kofuji, S. Kanazawa, H. Sakamoto, S. Tanaka, S.
‡
x
2
[x
2
Ichikawa and T. Hirai, Chem. Commun., 2014, 50, 15255–15258.
(
(
2
6
2
O (13, 27, 54, 82, or 111 mg) were added to water 17 The anataseꢀtoꢀrutile phase transition of P25 TiO
higher temperature is not the major factor for decreased activity. As
shown in Fig. S4 (ESI†), the anatase/rutile ratios of Pt /TiO prepared
by H reduction at 573, 673, and 773 K (82/18, 80/20, and 81/19 w/w,
respectively), as determined by XRD analysis (ref 18), are similar to
those of pure TiO (83/17), although the heating at 873 K increases
2
upon heating at
was dried under air flow and reduced under H
2
2
2
2
−
1
time being 2 K min and 2 h, respectively. Pd
prepared with Pd(NO (44 mg) or AgNO (32 mg) as a precursor.
Au /TiO was prepared as follows: HAuCl ·4H O (46 mg) was added to
water (50 mL). The pH of the solution was adjusted to 7 by an addition of
2 2 2 2
/TiO and Ag /TiO were
3
)
2
3
2
§
2
2
4
2
the ratio to 71/29. As shown in Fig. 3a, the catalyst prepared at 773 K
shows activity lower than that prepared at 573 and 673 K, even
though their anatase/rutile ratios are similar. This indicates that the
1
3
M NaOH. TiO
2
(1 g) was added to the solution and stirred vigorously at
53 K for 3 h. The solids were recovered by centrifugation, washed with
phase transition of P25 TiO
decreased activity of the catalysts prepared at higher temperatures.
) N. M. Williamson and A. D. Ward, Tetrahedron, 2005, 61, 155– 18 G. Ramis, G. Busca, C. Cristiani, L. Lietti, P. Forzatti and F. Bregani,
65; ( ) R. Löser, M. Chlupacova, A. Marecek, V. Opletalova and M. Langmuir, 1992, , 1744−1749.
2
support is not the major factor for the
water, and calcined at 673 K for 2 h under air flow.
1
(a
1
b
8
4
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