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catalyzed aldehyde–ethanol acetalization as DEE.[11a,12] In this
indirect process, besides the process complexity, the oxidation
step (currently using noxious oxidants such as manganese and
chromium compounds[13]) is highly unfriendly to the environ-
ment. Therefore, the development of a simple and green pro-
cess is urgently needed. Recently, Gusevskaya[4e] reported
a tandem aerobic oxidation-acetalization of ethanol using
Pd(OAc)2/Cu(OAc)2/p-TsOH as an efficient bifunctional catalyst.
Unlike the aerobic oxidation process, Milstein[4c] developed an
acceptorless dehydrogenation coupling (ADC) to convert alco-
hols into acetals, which exhibits a green, atom-economic fea-
ture because hydrogen atoms obtained from alcohols are con-
verted into useful H2. Unfortunately, Milstein’s process is un-
suitable for ethanol because of its low boiling point (liquid re-
action at a high temperature is required).[4c] Recently, we found
that under mild photocatalyisis conditions (typically at room
temperature) over Pt/TiO2(P25), primary C2-C6 alkyl alcohols
can readily undergo ADC reaction and generate acetals and H2
with high selectivity.[14] This process provides the possibility of
directly converting bioethanol into DEE in an atom-economical
green way.
Figure 1. TEM images of the photocatalysts: a) as-synthesized NaTiO3-NT,
b) HTiO3-NT obtained by ion exchange of NaTiO3-NT with HCl, c) TiO2-NT ob-
tained by calculating HTiO3-NT at 3008C, and d) TiO2-NR obtained by calcu-
lating HTiO3-NT at 4008C.
In this paper, we report that TiO2-nanotubes (NTs) and TiO2-
nanorods (NRs) loaded with platinum are highly photoactive
for the dehydrogenation CꢀO coupling of ethanol without
need for any oxidant. The reaction efficiently produces DEE
and H2 following a tandem dehydrogenation–acetalization
mechanism.
ethanol in argon atmosphere under UV irradiation at room
temperature. The experimental results are presented in Table 1.
Results showed that 1%Pt/NaTiO3-NT was inactive. Only
a small amount of DEE was detected after 9 h of irradiation,
with a very low conversion of ethanol of about 0.83%. The
simple ion exchange from Na+ to H+ dramatically activated
the photocatalyst, and about 27% of ethanol conversion was
achieved for 1%Pt/HTiO3-NT. Photoactivity was further im-
proved, and after 9 h of reaction, ethanol conversion reached
about 29% for both TiO2-NTs and TiO2-NRs. This value was
slightly higher than when using commercial 1%Pt/TiO2
(P25).[14] The reaction rates were high, i.e., 109.1 and
110.8 mmolgꢀ1 hꢀ1 for 1%Pt/TiO2-NT and 1%Pt/TiO2-NR, re-
spectively. Notably, the rate reached 157.7 mmolgꢀ1 hꢀ1 in rela-
tively low photocatalyst feeding, as described below. Most in-
terestingly, the present photocatalytic reaction was highly se-
lective, with>99% of reacted ethanol converted into DEE, or
nearly a stoichiometric reaction. Only trace amounts of carbon-
containing byproducts such as acetaldehyde, acetate acid, CO,
CO2, and CH4 were detected by GC for liquid and gas sample
(Figure S2). Notably, the data presented in Table 1 were ob-
tained after 9 h of reaction, exhibiting relatively low ethanol
conversion. Actually, ethanol conversion can reach 50% with
increased reaction duration (Figure 2), after which the reaction
was limited by thermodynamic equilibrium because the reac-
tion-derived water (as described below) can promote the re-
verse reaction.[17]
Results and Discussion
TiO2-NT and TiO2-NR were synthesized by a hydrothermal pro-
cess similar to a previously reported method.[15] Sodium tita-
nate NTs (NaTiO3-NTs) were initially prepared from a hydrother-
mal treatment of titania powder in an aqueous NaOH solution.
When excessive NaOH was used, titania powder was nearly
completely assembled as NaTiO3-NTs. These tubes were typical-
ly 800–1200 nm long and 8–10 nm in diameter (Figure 1a). To
transform NaTiO3-NT into TiO2-NT, NaTiO3-NT was ion-ex-
changed with hydrochloric acid and then calcined at desired
temperatures. Clearly, the ion exchange that resulted in titanic
acid NTs (HTiO3-NTs) produced a truncation of tubes of about
100 nm (Figure 1b). After calcination of HTiO3-NT at a relatively
low temperature (3008C), tubule structure showed no visible
change (Figure 1c), whereas the phase structure transformed
into anatase TiO2-NT (Figure S1). However, calcination at
a higher temperature (4008C) led to tubule collapse and reor-
ganization into rod-like morphology (Figure 1d; TiO2-NRs).
Compared with TiO2-NT obtained at 3008C, TiO2-NRs were
highly crystalline, as shown by a high-resolution TEM image
(inset in Figure 1d). A lattice fringe of 3.52 ꢁ can be clearly ob-
served, which corresponds to the (101) lattice planes of ana-
tase TiO2. Improvement in crystallization degree for TiO2-NRs
was also proven by X-ray diffraction analysis (Figure S1).
Photocatalysis behaviors of the synthesized NTs and NRs
were characterized for the dehydrogenation coupling reaction
of ethanol, with metal co-catalysts being loaded by in situ pho-
todeposition.[16] We performed photocatalytic reaction in neat
We also examined the effect of metal cocatalysts (Pt, Pd, Au,
and Rh) on photocatalyst performance based on TiO2-NRs.
Blank experiments with bare TiO2-NRs (without any metal load-
ing) showed that it is inactive for the present reaction, with no
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