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undesirable side reactions, thus moving towards the environ-
mental-friendly and atom-economic “green chemistry”. Further-
more, photo-driven reactions in many cases involve deep-
seated chemical transformations and result in high yields with
formance is of great importance for the further development
of photocatalytic 2,3-BD synthesis.
Herein we investigated the CÀC coupling of bioethanol over
Pt/TiO photocatalyst with anatase, rutile, brookite, and ana-
2
[
4]
high selectivity. More importantly, photo-driven reactions
often follow unique reaction channels and are available of
achieving complex, polycyclic or highly functionalized struc-
tase–rutile mixed crystalline structures, respectively. It was
found that the 2,3-BD selectivity increases significantly with
the decreasing of COH content in the reaction system. More-
over, the amount of COH shows a positive correlation with the
[5a]
tures from simple substrates and even products that cannot
[
5b]
be obtained by conventional ground-state thermal reactions,
opening new perspectives in the production of green hydro-
quantity of surface hydroxyl groups on TiO photocatalyst, indi-
2
cating that the oxidative CÀC coupling of ethanol may mainly
[
5c]
[5d]
gen energy and in the search of natural compounds.
,3-BD is a vicinal diol in which the two hydroxyl groups are
proceed by the COH formed in the reaction between photogen-
2
erated holes and surface hydroxyl groups. This was further cor-
bonded to adjacent C2 and C3 carbons, respectively. This
unique structure makes it highly difficult to be obtained by the
roborated over Pt/Degussa-P25-TiO photocatalysts through F
2
substitution of surface hydroxyl groups. Water was also found
[6]
conventional chemical synthesis. From the viewpoint of
photo-driven organic synthesis, however, 2,3-BD can be
achieved by a-CÀC coupling of two a-hydroxyethyl radicals
to be in favor of the CÀC coupling reaction.
Results and Discussion
(
CCH(OH)CH ) because the radical coupling pathway has shown
3
[
7]
great interest in the photo-driven process for the formation
Pure anatase (A–), rutile (R–), brookite (B–) TiO , and Degussa
2
[
8]
of the CÀC bond. It was reported in a few pioneering works
that the vicinal diols such as ethylene glycol and 2,3-BD were
selectively obtained in corresponding methanol and ethanol
aqueous solutions by photocatalytic a-CÀC coupling synchro-
mixed crystal (P25–) TiO were incipiently employed to explore
2
the discrepancies in product selectivity in the photocatalytic
oxidation of ethanol. In Figure 1, the selectivity of 2,3-BD and
overoxidation product (aldehyde, acetic acid, CO, CH , and
4
[7c,d,9]
nously with H2 evolution through water splitting.
H
is
CO ) versus the crystalline phase of TiO is shown. It is clear
2
2
2
a high-energy clean fuel, this method therefore provides
a green avenue for synthesizing 2,3-BD in one step especially if
the hydrous bioethanol is used as the feedstock. Nevertheless,
these reactions have not yet been well developed until recent-
ly because the only effective photocatalysts reported were per-
that the 2,3-BD selectivity increases in the order of Pt/R–TiO >
2
Pt/B–TiO >Pt/A–TiO >Pt/P25–TiO , and the overoxidation
2
2
2
product selectivity is on the contrary. This behavior indicates
that the formation of 2,3-BD through CÀC coupling of ethanol
is competitive to the overoxidation of ethanol into aldehyde,
acetic acid, CO, CH , CO , and so on. Interestingly, the ethanol
[7,9]
ishable metal sulfide semiconductors (typically ZnS)
instead
4
2
of stable metal oxide semiconductors (e.g., TiO , WO , ZnO,
conversion decreases from approximately 23% to approxi-
mately 5% with the increasing of 2,3-BD selectivity from ap-
proximately 2.6% to approximately 85% (Table S1 in the Sup-
porting Information), suggesting the difficulty in obtaining the
important purpose of “high selectivity at the high substrate
conversion” for catalysis. To find out the causes of the discrep-
ancies in selectivity and activity, some textural and structural
characterizations were conducted.
2
3
SnO , and so on). Thermodynamically, the higher conduction
2
band edges of metal sulfide are adverse to the injection of an
electron from the intermediate radicals by a current-doubling
[
10]
process, thus impeding the formation of overoxidation com-
pounds such as aldehydes, acids, and even CO , but facilitating
2
[11]
the CÀC coupling reactions. Over the metal oxide photocata-
lysts, the situation is just the opposite.
Recently, through modulating the crystalline structures and
pores of TiO , Lu et al. have found that over Pt/TiO photocata-
Scanning electron microscopy (SEM) images (Figure 2) exhib-
it that P25–, A–, and R–TiO are nanoparticles with increasing
2
2
2
lyst ethanol in aqueous solution can be transformed into 2,3-
BD with a high selectivity of approximately 97% under UV
light irradiation through CÀC coupling of CCH(OH)CH radi-
3
[12]
cals. Simultaneously, a moderate amount of H was evolved
2
because of the catalytic reduction of protons in the reaction
[12]
system on the Pt co-catalyst.
This is contradictory to the
above thermodynamic viewpoint and indicates that the reac-
tion pathways over metal oxide photocatalysts can be signifi-
cantly regulated by changing the adsorption–desorption kinet-
ics and the reaction kinetics. It also shows the possibility of
using stable metal oxide photocatalysts to realize the alcohol
CÀC coupling reactions. In view of the intriguing progress and
the wide potential applications of 2,3-BD, it is very urgent to
gain more insight into the ways in which how the photocata-
Figure 1. Selectivity of 2,3-BD and overoxidation products versus crystalline
lytic CÀC coupling reaction takes place over the Pt/TiO photo-
2
phase of TiO
aqueous ethanol solution (200 mL); TiO
300 W high-pressure Hg lamp (l=365 nm); 24 h; Ar purging.
2
over Pt/TiO
2
photocatalysts. Reaction conditions: 10 vol%
catalyst. Especially, the structure–selectivity relationship be-
2
decorated with 1 wt% Pt (0.5 g);
tween TiO2 based photocatalyst and CÀC coupling per-
ChemCatChem 2015, 7, 2384 – 2390
2385
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