678
Table 1. Synthesis of PCA from L-Lys using various platinized
TiO2 photocatalysts
a
b
d
Conversion SPCA OPPCA
YH2
c
Photocatalyst
RPCA
/%
/%
/%
/¯mol
TiO2
mec-SiO2 + TiO2
dir-SiO2/TiO2
SiO2(0.5)/void/TiO2
SiO2(1.5)/void/TiO2
SiO2(3.0)/void/TiO2
100
100
14
98
96
51
52
26
43
50
46
57
59
®
70
70
70
27
27
0.2
22
25
23
75
63
2
72
50
57
e
f
96
aSelectivity of PCA production based on amount of consumed
b
c
L-Lys. Optical purity of L-PCA. Rate of PCA formation in
the unit of ¯mol h¹1. Yield of H2. Platinization via photo-
deposition was unsuccessful (see text). Not determined.
d
e
f
Figure 2. Time-course curves of H2 liberated from aqueous
methanol solutions by TiO2 (filled squares), SiO2(0.5)/void/
TiO2 (filled triangles), mec-SiO2 + TiO2 (open squares), and dir-
SiO2/TiO2 (open triangles) preirradiated in aqueous H2[PtCl6]
solutions.
irradiation was maintained at 25 « 0.5 °C by the use of a
thermostated water bath. After irradiation for 2 h, a portion
(0.2 cm3) of the gas phase of the sample was withdrawn with a
syringe and subjected to gas chromatographic analysis (GC,
Shimadzu GC-8A with an MS-5A column and a TCD detector)
for H2. The yield of enantiomers of PCA, as well as the amount
of unreacted L-Lys, was measured by HPLC (Shimadzu LC-6A
equipped with a Daicel Chiral-Pak MA(+) column and an
ultraviolet absorption detector).
Since platinum (Pt) deposits on the TiO2 surface are
required for the photocatalytic synthesis of L-PCA,13 all samples
were platinized (2 wt %) using two-step photodeposition. First,
a sample was suspended in water containing the required
amount of hydrogen hexachloroplatinate(IV) (H2[PtCl6]¢6H2O),
irradiated by
a 400-W mercury arc (Eiko-sha 400; ca.
¹2
25 mW cm at 300-400 nm) for 1.5 h, and then irradiated for
an additional 1.5 h in the presence of 50 vol % methanol.
Figure 2 shows the time-course curves of hydrogen (H2)
liberation from aqueous methanol solutions in the second step of
the platinization. Almost linear increase in the amount of H2 was
observed after some induction period for all the samples except
for dir-SiO2/TiO2, suggesting that reduction of platinum
complex to metallic state, to induce methanol dehydrogenation,
required 5-10 min irradiation. As shown in this figure, dir-SiO2/
TiO2 was almost inactive with negligible H2 liberation possibly
due to retardation of adsorption of substrates, methanol, and
H2[PtCl6], participating in the reaction onto the TiO2 surface by
the covering silica layer to result in practically no Pt deposition.
The activity of SiO2/void/TiO2 was almost the same as that of
bare TiO2 despite the presence of silica shell and even higher
than that of mec-SiO2 + TiO2. SEM observation of the sample
after the platinization shown in Figure 1d clearly indicates the
deposition of fine Pt particles onto TiO2 without any collapse of
the silica shells. A similar finding was observed in our previous
research, and this can be attributed to the presence of pores in
silica shell and void spaces between the shell and core TiO2
particles.10 These structures led to efficient mass transfers
through a silica shell to supply substrates that participate in this
reaction to the naked active surface of the TiO2 core.
For the photocatalytic reaction of redox-combined stereo-
selective synthesis of L-PCA from L-lysine (L-Lys), a Pt-loaded
photocatalyst (0.05 g as TiO2) was suspended in an aqueous
solution (5.0 cm3) containing L-Lys (100 ¯mol) and photoirra-
diated with a high-pressure mercury arc (Eiko-sha, 400 W) under
argon (Ar) under magnetic stirring (1000 rpm). The photo-
irradiation was performed through a cylindrical Pyrex glass filter
and a glass reaction tube (18 mm in diameter and 180 mm in
length) so that light of wavelength >290 nm reached the
suspension. The temperature of the suspension during photo-
Table 1 summarizes the results for the synthesis of L-PCA
from L-Lys by 2-h photoirradiation using various platinized
TiO2 photocatalysts. Photoirradiation of the TiO2 photocatalysts
suspended in an aqueous solution of L-Lys under Ar led to
the formation of PCA, as reported previously.14,15 Complete
consumption of L-Lys was achieved using TiO2 and also mec-
SiO2 + TiO2. These photocatalysts showed very similar results
in terms of selectivity (SPCA), optical purity (OPPCA), and the rate
of PCA formation (RPCA), suggesting that the mechanical mixing
of silica with TiO2 does not give any effect on this reaction
as only the TiO2 part was responsible for the photocatalytic
reaction. As expected, dir-SiO2/TiO2 showed poor photocata-
lytic activity to convert only 14% of L-Lys, thus proving that
direct coverage of the TiO2 surface with silica hinders the
activity of the TiO2 by prohibiting Pt deposition as well as L-Lys
adsorption onto the bare TiO2 surface. The SiO2(0.5)/void/TiO2
particles prepared with 0.5 h of silylation period showed the
performance almost the same as that of bare TiO2. Although the
selectivity was slightly lower than that of bare TiO2, SiO2/void/
TiO2 exhibited the highest OPPCA, 13% more than that of
platinized bare TiO2, among all the samples. In order to further
prove the effectiveness of the hollow core-shell structure, SiO2/
void/TiO2 with a thicker layer of silica shell was also prepared,
by extending the silylation period (1.5 and 3.0 h). The thickness
of the silica layer was increased to 14-32 at 1.5 h and 28-45
nm at 3.0 h from 9 to 10 nm for SiO2(0.5)/void/TiO2. While
SiO2(1.5)/void/TiO2 exhibited the best performance among the
tested samples, it seemed that the photocatalytic performance
(conversion, SPCA, OPPCA, and RPCA) was almost independent of
the silica shell thickness. This suggests that the silica shell
behaves as highly porous optically transparent penetration-free
layer which surrounds the TiO2 core and that this swollen
sponge-like silica layer controls the stereoselectivity of the
reaction.
Chem. Lett. 2012, 41, 677-679
© 2012 The Chemical Society of Japan