Photoreductive transformation of fluorinated acetophenone derivatives on titanium dioxide: Defluorination vs. reduction of carbonyl group

Article abstract of DOI:10.1016/j.apcata.2015.10.033

Photoreductive transformation of mono- and di-fluoromethyl acetophenone (AP) derivatives on the P25 titanium dioxide (TiO₂) has been studied in deaerated ethanol solution under UV irradiation. 2-Monofluoromethyl AP (MFAP) was stable in the dark and existed as keto form in ethanol, whereas 64% of 2,2-difluoromethyl AP (DFAP) transformed into hemiketal form (photocatalytically inactive form) under the same condition. Under the UV irradiation with the TiO₂ particles, the reduction of MFAP afforded only the defluorinated ketone, while the reduction of DFAP provided not only defluorinated ketones but also a hydrogenated alcohol. The reduction of carbonyl group and defluorination of DFAP concurrently occurred on TiO₂, in which the formation of MFAP was observed as an intermediate of the sequential defluorinations. These two parallel reactions were initiated by electron transfer from the surface defect sites (Ti_sd) to DFAP adsorbed on the TiO₂ surface. A possible reaction mechanism for DFAP is proposed and discussed on the basis of thermodynamic data upon the C-F bond cleavage of anion radical species generated during the photocatalysis.

Full text of DOI:10.1016/j.apcata.2015.10.033

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Applied Catalysis A: General
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Photoreductive transformation of fluorinated acetophenone
derivatives on titanium dioxide: Defluorination vs. reduction of
carbonyl group
∗
∗
Shigeru Kohtani , Takuya Kurokawa, Eito Yoshioka, Hideto Miyabe
Department of Pharmacy, School of Pharmacy, Hyogo University of Health Sciences, 1-3-6, Minatojima, Chuo-ku, Kobe 650-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Photoreductive transformation of mono- and di-fluoromethyl acetophenone (AP) derivatives on the
P25 titanium dioxide (TiO2) has been studied in deaerated ethanol solution under UV irradiation. 2-
Monofluoromethyl AP (MFAP) was stable in the dark and existed as keto form in ethanol, whereas 64% of
Received 28 August 2015
Received in revised form 10 October 2015
Accepted 20 October 2015
Available online xxx
2
,2-difluoromethyl AP (DFAP) transformed into hemiketal form (photocatalytically inactive form) under
the same condition. Under the UV irradiation with the TiO2 particles, the reduction of MFAP afforded
only the defluorinated ketone, while the reduction of DFAP provided not only defluorinated ketones but
also a hydrogenated alcohol. The reduction of carbonyl group and defluorination of DFAP concurrently
occurred on TiO2, in which the formation of MFAP was observed as an intermediate of the sequential
defluorinations. These two parallel reactions were initiated by electron transfer from the surface defect
sites (Tisd) to DFAP adsorbed on the TiO2 surface. A possible reaction mechanism for DFAP is proposed
and discussed on the basis of thermodynamic data upon the C F bond cleavage of anion radical species
generated during the photocatalysis.
Keywords:
Titanium dioxide
Photocatalytic reduction
Hydrogenation
Defluorination
Acetophenone derivatives
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
resonance [10,11], photoacoustic [12,13], photoluminescence [14]
and diffuse reflectance IR [15] or UV–vis spectroscopy [16].
We have recently demonstrated that the P25 TiO2 exhibited
the excellent photocatalytic activity to hydrogenate several aro-
matic carbonyl compounds into corresponding alcohols under the
combination of UV light irradiation and deaerated conditions in
ethanol [17]. We have further examined the adsorptive and kinetic
behaviors on the reductive hydrogenation of AP derivatives, and
found that conduction band (CB) electrons or those trapped at the
Tisd on TiO2 actually take part in the photohydrogenation [18].
We have also found that most of the reaction rates depend on
the reduction potentials (Ered) of substrates and proposed that a
sequential two-step electron transfer leads to the hydrogenation
reaction [19]. However, contrary to our expectation, the hydro-
genation rate for 2,2,2-trifluoroacetophenone (TFAP) having an
electro-withdrawing trifluoromethyl group was much slower than
that for AP, though Ered for TFAP is sufficiently positive compared
to that for AP [18]. The photo-hydrogenation of TFAP followed the
first-order rate law because of the predominant formation of ketal
or hemiketal as inactive species in ethanol, i.e., only a few percent
of TFAP remained in the original keto form as active species [18].
Thus, the formation of ketal or hemiketal for TFAP in ethanol greatly
affects the kinetics on the photocatalysis.
Photocatalysis on semiconductors is defined as a light-driven
redox reaction at a solid/liquid or a solid/gas interface. In partic-
ular, photocatalysis is a unique catalytic methodology compared
to the conventional thermal or photochemical reactions because
a sequential multi-step electron transfer induced by accumulated
electrons and holes on surface of semiconductors can initiate some
characteristic reactions. This property makes it possible to be uti-
lized in water splitting [1,2], reduction of carbon dioxide [2,3],
synthetically useful organic reactions [3–5] and so on. It is well
known that titanium dioxide (TiO ) can accumulate electrons at
2
defective trap sites (Tisd) on the surface under UV photoexcita-
tion [6–8]. The accumulated electrons have been used for the
sequential multi-step electron transfer in the reduction of sub-
strates adsorbed on TiO , for example, hydrogenations of alkenes
2
and alkynes [3,9], aromatic nitro compounds [3–5], aldehydes and
ketones [3,4] etc. The electron accumulation on Tisd has been
physicochemically investigated by means of electron paramagnetic
∗ Corresponding authors. Fax: +81 78 304 2858.
E-mail addresses: kohtani@huhs.ac.jp (S. Kohtani), miyabe@huhs.ac.jp
(
H. Miyabe).
http://dx.doi.org/10.1016/j.apcata.2015.10.033
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>
(
99%), DFAP (SynQuest, >97%), TFAP (Sigma–Aldrich, 99%), AP-OH
TCI, >98%), TFAP-OH (Sigma–Aldrich, 98%).
DFAP-OH was synthesized by the TiO2 photocatalyzed hydro-
genation of DFAP in ethanol as follows: a suspended solution
containing DFAP (158 mg, 3 mmol) and P25 TiO2 (0.1 g) in ethanol
(
30 mL) was placed in a cylindrical glass cell (40 mm × 45 mm i.d.)
and sealed with a rubber septum. To the suspended solution was
passed a pure argon gas through the rubber septum for 30 min in
the dark, and then irradiated with UV light from a 300 W xenon
arc lamp (ILC Technology, CERMAX LX300) through a water fil-
◦
ter and cut-off filters (Toshiba UV35) at 32 C for 12 h. After the
irradiation (conversion 83%), the suspended solution was cen-
Fig. 1. AP derivatives and corresponding secondary alcohols.
trifuged to remove the TiO powders, and then evaporated ethanol
2
under reduced pressure to provide a pale yellow residue. Purifi-
cation was carried out by preparative thin-layer chromatography
Fluorine containing compounds are often used for pharmaceu-
tical and agrochemical reagents. Since the C F bond is one of the
strongest bonds, the C F bond activation/cleavage is a field of
current interest in organic chemistry [20], though less is known
about the catalytic methods. Photocatalytic reaction is one of the
promising ways to induce the C F bond activation/cleavage for
such fluorinated compounds under mild conditions. Kaprinidis
and Turro reported photosensitized defluorination of saturated
perfluorocarbons in the presence of various photosensitizing
amines [21]. Burdeniuc et al. also reported photoinduced catalytic
defluorination of perfluoroalkanes to give perfluoroalkenes using
decamethylferrocene [22] and mercury [23] as photosensitizers.
Moreover, photocatalytic defluorination of fluorinated substrates
has been examined using some semiconductor photocatalysts such
(
(
(
SiO , chloroform) to yield 10.4 mg of DFAP-OH as colorless oil
2
1
24% yield); H NMR (400 MHz, CDCl ): ı 2.44 (1H, br, OH), 4.83
3
1H, td, JHF = 10.2 Hz, JHH = 4.7 Hz, CH), 5.77 (1H, dt, JHF = 55.6 Hz,
13
JHH = 4.7 Hz, CF H), 7.26–7.42 (5H, m, C H5). C NMR (101 MHz,
2
6
CDCl ): ı 73.6 (t, J = 24 Hz, CH), 115.8 (t, J = 246 Hz, CF ), 127.1,
3
2
1
28.7, 129.0, 135.8 (t, J = 3.5 Hz, CAr-CH). 19F NMR (376 MHz, CDCl ):
3
ı = −127.7 (1F, ddd, JFF = 284 Hz, JFH = 56 Hz, JHH = 9 Hz), −128.3 (1F,
ddd, JFF = 284 Hz, JFH = 57 Hz, JHH = 11 Hz). MS (EI): m/z (relative
+
intensity) = 158 (M , 15%), 107 (100%), 79 (95%), 77 (63%), 51 (26%).
The data are consistent with the previous reports [29,30].
2.2. Prolonged UV irradiation experiment
Irradiation experiments were carried out for a mixture of the AP
as CdS [24], ZnS [24,25], metal-doped ZnS [26], and ␤-Ga O [27].
2
3
−1
derivatives (initial concentration range: 1–20 mmol L ) and TiO2
In general, these semiconductors used for the defluorination reac-
tions possess high CB energy. On the contrary, TiO2 does not have
such a high CB level. Therefore, TiO2 has not been regarded as an
active photocatalyst for the C F bond activation/cleavage.
In this paper, we report the photoreductive transformation of
mono- and di-fluoromethyl AP derivatives including C F bond
(
0.10 g) in deaerated ethanol solution (30 mL) under the irradiation
−2
◦
with UV light (>340 nm, light intensity: 790 mW cm ) at 32 C.
The details of irradiation experiment and GC analysis have been
described in our previous reports [17–19].
2.3. Pre-UV irradiation experiment
cleavage upon the UV irradiated P25 TiO . The structures of these
2
compounds are summarized in Fig. 1. We have already reported
that the photocatalytic reaction of AP and TFAP gives only the
hydrogenated alcohols such as 1-phenylethanol (AP-OH) and 1-
phenyl-2,2,2-trifluoroethanol (TFAP-OH) without defluorination,
respectively. Andrieux et al. reported that the electrochemical
reduction of 2-fluoromethylacetophenone (MFAP) has yielded AP
via the reductive defluorination [28]. Therefore, we expect that
the photocatalytic reduction of MFAP on TiO2 may afford the
defluorinated ketone. Moreover, it is interesting to note whether
the reaction of 2,2-difluoromethyacetophenone (DFAP) with two
fluorine atoms provides the hydrogenated alcohol of 1-Phenyl-
The details of this method have been described in our previ-
ous reports [18,19]. TiO2 (0.10 g) in ethanol (30 mL) was placed in
the cylindrical glass cell (40 mm × 45 mm i.d.) and sealed with the
rubber septum. Argon gas was passed into the suspended solution
through the rubber septum for 30 min. The degassed solution was
stirred in a water bath for 30 min to attain thermal equilibrium
◦
at 32 C in the dark. During the degassed solution was irradiated
−2
with UV light (>340 nm, light intensity: 1250 mW cm ) for 2 h, the
white color of TiO2 powder changed into blue–gray one. After con-
firming the sufficient color change, 300 ␮mol of the AP derivatives
was injected into this TiO2 suspended solution in the dark. Then,
electron transfer from the Tisd sites to adsorbed AP derivatives took
place and afforded reductive products. The amount of the products
was quantitatively analyzed by GC–MS [18,19].
2
,2-difluoroethanol (DFAP-OH) or the defluorinated ketone. In this
paper, we also propose a possible mechanism for the reduction
of DFAP and discuss the reaction route on the basis of thermody-
namic data upon the C F bond cleavage of anion radical species
generated during the photocatalysis. Additionally, keto/hemiketal
equilibrium was investigated by 1 F NMR spectroscopy to elucidate
the ratio of a keto form in ethanol, which is regarded as an active
form on the photocatalytic reduction.
3
. Results and discussion
9
3
.1. Hemiketal formation in ethanol
The photocatalytic hydrogenation of AP derivatives in ethanol
2
. Experimental
will be influenced by the ketal or hemiketal formation, because the
adsorptivity and reactivity on the TiO2 surface are greatly affected
by a concentration ratio of keto/hemiketal/ketal as reported for
TFAP [18]. Therefore, keto/hemiketal/ketal equilibrium for MFAP
and DFAP was firstly investigated by means of UV absorption
2.1. Materials
Polycrystalline TiO powder (Degussa P25, specific surface area:
2
2
−1
19
5
0 m g ) was purchased from Japan Aerosil and used as received.
and F NMR spectroscopy. Fig. 2 shows UV absorption spectra
HPLC grade ethanol used for a solvent was purchased from Nacalai
Tesque without further purification. The following organic reagents
were used as received: AP (Nacalai Tesque, >98.5%), MFAP (Alfa,
of MFAP in ethanol (Fig. 2(a)) and DFAP in the mixed solvent of
ethanol/CH CN (19/1) (Fig. 2(b)). The absorption spectrum of MFAP
3
with maximum wavelength at 243 and 280 nm did not change with
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Fig. 2. UV absorption spectra of (a) MFAP in ethanol and (b) DFAP in the mixed solvent of ethanol/CH3CN (19/1). The blue line for (b) indicates the absorption spectrum of
DFAP immediately after dilution of the CH3CN stock solution with ethanol. The inset is the decay signal of absorbance of DFAP at 250 nm. (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Time profiles of the decrease in DFAP keto form (black circles: ꢀ) and the
◦
formation of hemiketal (red squares: ᮀ) in C2D5OD at 32 C. The normalized con-
Fig. 3. 19F NMR spectra recorded at (a) 0.18 and (b) 4.67 h after preparation of DFAP
19
centrations were calculated by integration of area beneath the peaks on F NMR
◦
19
sample in C2D5OD at 32 C. Chemical shifts of the trifluoromethyl F peaks were
spectra recorded at several times after preparation of DFAP sample in C2D5OD. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
19
reported in ppm relative to the hexafluorobenzene F peak.
time, indicating that the keto form of MFAP is stable in ethanol in
the dark. On the other hand, the keto from of DFAP was not sta-
ble. The absorption spectrum of DFAP immediately after dilution
pattern was also reported in the production of hemiketal from 2-
acylpyridine derivatives [32]. In contrast, the two fluorine atoms
of ketal should be equivalent because of the free rotation of CHF2
group (Fig. S2 in ESI), which should cause a simpler F NMR spec-
trum. On the other hand, the small doublet peaks at 31.5 ppm
indicated by the arrow in Fig. 3 are ascribed to the hydrated form of
of the CH CN stock solution with ethanol is depicted by the blue
3
line in Fig. 2(b). This spectral shape with maximum wavelength at
19
2
50 nm was very similar to that of AP (ca. 10 nm red-shifted). How-
ever, the absorbance at 250 nm gradually decreased to the red line
with time as shown in the inset in Fig. 2(b). This absorbance change
suggests that DFAP reacts with ethanol to produce hemiketal and/or
ketal species. The decrease in absorbance continued during ca. 15 h,
which was much slower than that of TFAP (a few ten minutes) in
ethanol [18].
DFAP assigned by the D O addition experiment. Thus, we confirmed
2
that ketal was not formed from DFAP.
From the peak areas, time profiles of the decrease in keto form
and the formation of hemiketal can be illustrated in normalized
concentration as shown in Fig. 4. While the keto form decreased, the
hemiketal grew up to reach the equilibrium after ca. 15 h at which
the normalized concentration of keto/hemiketal was attained to be
0.35/0.64. Thus, ca. 64% of DFAP dissolved in ethanol transformed
into the hemiketal, whereas 35% of DFAP remained in the photoac-
tive keto form. Additionally, the normalized concentration of the
hydrated form was ca. 1% at equilibrium. The equilibrium constant
K([hemiketal]/[keto]) was 1.8 and the rates of forward and back
In order to gain further information about the transformation of
19
DFAP into hemiketal or ketal in ethanol, F NMR was measured in
19
deuterated ethanol (C D5OD). Fig. 3(a) and (b) are F NMR spec-
2
tra recorded at 0.18 and 4.67 h after preparation of DFAP sample in
C2D5OD. The strong doublet peak (JH–F = 56 Hz) at ca. 38.5 ppm is
ascribed to DFAP (keto form), whereas peaks at around 29–33 ppm
(
except for the small doublet peak at 31.5 ppm indicated by an
arrow) can be assigned to hemiketal having an asymmetric carbon
atom due to the appearance of eight peaks based on F–F and H–F
couplings. The doublet splitting pattern for each peak results from
H–F coupling (JH–F = 56 Hz). These doublets confirm the presence of
−1
reactions were 0.31 and 0.17 h , respectively.
3.2. Photocatalytic reduction under prolonged UV irradiation
␣
-hydrogen atom (CHF ) and not an enol form. The spectrum pat-
2
tern can be explained by the AB spin system with strong interaction
between the two fluorine atoms (coupling constant: JF–F = 280 Hz
and the difference of two chemical shifts: 2.48 ppm) [31]. The
hemiketal should form an intramolecular hydrogen bond between
fluorine atom and the OD group (Fig. S1 in ESI). Therefore, the
rotational motion of CHF2 group is restricted so that the two fluo-
rine atoms near an asymmetric center are not equivalent, leading
to the appearance of the AB type pattern. The similar spectrum
At first, we briefly summarize the characteristics of photocat-
alytic hydrogenation of AP and TFAP on the basis of the previous
reports [17–19]. The hydrogenation for both AP and TFAP occurs
quantitatively to give the alcohols AP-OH and TFAP-OH regardless
of the initial concentration of the substrates. The kinetic order of AP
varies from zero- to first-order depending on the concentration of
AP and follows the Langmuir-Hinschelwood expression, whereas
that of TFAP obeys the first order rate law because of the very low
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tions concurrently occurred on TiO , in which the formation of
2
MFAP was observed as an intermediate in the sequential defluo-
rinations. Because a sufficient concentration of photoactive keto
form of DFAP was attained in ethanol solution (ca. 35%) as shown
in Fig. 4, sequential multi-electron transfer events efficiently pro-
ceeded at the liquid/TiO2 interface in the photocatalytic reducition
of DFAP.
Interestingly, AP, produced by the defluorination of MFAP and
DFAP, did not react any more. No hydrogenation of AP giving AP-
OH occurred on TiO2 in these cases. This is possibly due to the
production of HF which would act as an inhibitor for the hydro-
genation of AP. In fact, we demonstrated that the photocatalytic
hydrogenation of AP (150 ␮mol) was completely inhibited by addi-
tion of 0.38 mol HF as well as 0.34 mol HCl into the TiO suspended
2
solution. Recently, Zhou et al. reported that the effects of adsorbed
F and Cl ions upon formaldehyde (HCHO) adsorption to anatase
TiO2 surface [33]. Their results indicated that the adsorbed F and
Cl ions could weaken the interaction between HCHO and a spe-
cific Ti site (five coordinated Ti site), leading to serious decrease in
the HCHO adsorption performance. In a similar manner, F and Cl
ions may adsorb to the specific Tisd sites, i.e. the five coordinate Ti
and oxygen vacancy sites [6–8], where are also regarded as binding
sites for carbonyl compounds [34–36]. Thus, F and Cl ions may be
a candidate for the catalyst poison in the reduction of AP.
◦
Fig. 5. Time profiles of (a) decay of MFAP and (b) formation of AP in ethanol at 32 C
under UV irradiation (>340 nm, 790 mW cm ).
−
2
The results of photocatalytic reduction of fluorinated AP deriva-
tives under prolonged UV irradiation after 8 h are summarized in
Table 1. The hydrogenation of AP (150 ␮mol) was completed within
4
h to yield the total amount of 147 ␮mol of AP-OH [18,19]. Con-
sidering that the single electron transfer takes place twice in the
photohydrogenation, the amount of reacted electrons (294 ␮mol) is
two times larger than the amount of AP-OH. On the other hand, the
reaction of MFAP (150 ␮mol) gave only the defluorinated ketone
AP (96 ␮mol), though the amount was much smaller than that of
AP-OH produced from the AP hydrogenation. The reason has been
explained in terms of the inhibition effect caused by the HF pro-
duction. The amount of reacted electrons (192 ␮mol) is also twice
larger than the defluorinated ketone AP because the single electron
transfer also takes place twice in the defluorination process. The
reduction of DFAP (150 ␮mol) provided the three reductive prod-
ucts DFAP-OH as well as MFAP and AP. The reacted electrons on the
defluorination of DFAP to AP (196 ␮mol) is four times larger than
the amount of AP production (49 ␮mol) because of the sequential
double defluorination processes. Taking into account of these pro-
cesses, the amount of reacted electrons for the defluorination of
DFAP was 216 ␮mol which corresponds to 77% to the total amount
of reacted electrons (274 ␮mol), while the residual electrons (23%)
was used for the reduction of carbonyl group to produce DFAP-OH.
Fig. 6. Decay of DFAP (open circles: ꢀ) and formation of DFAP-OH (red squares: ᭿),
◦
MFAP (green triangles: ꢀ), and AP (blue diamonds: ꢁ) in ethanol at 32 C under UV
−
2
irradiation (>340 nm, 790 mW cm ). (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this article.)
concentration of the photoactive keto form of TFAP (a few percent)
caused by the predominant ketal and hemiketal formation.
In marked contrast to AP and TFAP, the photoreduction of
carbonyl group of MFAP did not take place and the selective defluo-
rination newly proceeded. Photoreductive defluorination of MFAP
took place to form AP upon the UV irradiated TiO2 as shown in
Fig. 5. It shows the decrease in MFAP and the evolution of AP in the
deaerated conditions. For all initial concentrations, the amounts of
reacted MFAP were almost equal to those of the produced AP. Thus,
this reaction proceeded almost stoichiometrically. The time pro-
files for MFAP did not follow the first-order rate law as well as the
Langmuir-Hinshelwood kinetic expression. Because MFAP exists in
the original keto form, the initial reaction rates were considerably
fast regardless of the initial concentrations. However, the reaction
rates significantly slowed down within 2 h irradiation time.
Fig. 6 depicts time courses for degradation of DFAP and for pro-
ductions of DFAP-OH by the photoreduction of carbonyl group as
well as MFAP and AP by the photoreductive defluorination. The
photocatalytic reduction of DFAP proceeds in two different path-
ways: one is the hydrogenation to produce DFAP-OH and another is
defluorination of DFAP giving MFAP which is further defluorinated
to afford AP. Thus, DFAP provided not only a hydrogenated alco-
hol (DFAP-OH: 0.97 mmol/L) but also defluorinated ketones (MFAP:
3.3. Electron transfer efficiency on the pre-irradiated TiO2 surface
It is well known that there are various Tisd sites on TiO , e.g.,
2
the five coordinate Ti and the O vacancies and so on [6–8]. The
electronic energy of the Tisd states is almost located just below
the CB edge of TiO2 in the ranges of ca. 1 eV [8,37]. Therefore,
4+
electrons excited in the CB relax to the Ti
sites to yield the
sd
accumulated electrons (Tisd3+) which indicate the blue–gray color
with absorption band from visible to IR region [15,16]. Since the
electrons accumulated on the P25 TiO2 surface are quite stable
in the deaerated ethanol, the lifetime exceeds a few days in the
absence of electron acceptor [16,18]. Therefore, in this study, we
tested whether the accumulated electrons can transfer from the
Tisd to the fluorinated AP derivatives adsorbed on TiO . Further,
2
we evaluated the amount of transferred electrons in the injection
experiment using a pre-irradiated TiO2 suspension [18,19]. After
being confirmed the color change from white to blue–gray in the 2 h
pre-irradiation of TiO2 suspension, a sufficient amount of each AP
0.33 mmol/L and AP: 1.63 mmol/L) after 8 h irradiation. Both reac-
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Table 1
Reduction potentials of AP derivatives and the amounts of products and reacted electrons.
Substrate
E0 (V)a
Products
Prolonged irradiation for 8 hc
Injection after pre-irradiationd
product (␮mol)e
reacted electron (␮mol)
product (␮mol)f
reacted electron (␮mol)
AP
MFAP
DFAP
−2.14
AP-OH
AP
DFAP-OH
MFAP
AP
147
96
29
10
49
88
93
294
192
58
4.1
5.9
1.3
2.3
1.0
4.6
5.4
8.2
b
−1.54, −1.75
−1.46
11.8
2.6
(23%)
77%)
(21%)
(79%)
g
g
20
4.6
4.0
(
h
h
196
274
186
total
TFAP-OH
11.2
10.8
TFAP
−1.38
a
Reduction potential in CH3CN vs. SCE quoted from ref. [41] unless otherwise noted.
Quoted from ref. [28].
Carried out for a mixture of the substrate (150 ␮mol) and TiO2 (0.10 g) in deaerated ethanol (30 mL) under UV irradiation (>340 nm, 790 mW cm ).
b
c
−2
d
The molar amounts of products and reacted electrons were estimated at 20 h after injection of the substrate (300 ␮mol) into the 2 h pre-irradiated TiO2 suspension (0.1 g
◦
in 30 mL ethanol) at 32 C.
e
Determined by GC.
Determined by GC–MS.
f
g
−
+
The single electron transfer takes place twice in the single defluorination process (DFAP + 2e + 2H → MFAP + HF).
h
−
+
The single electron transfer takes place four times in the double defluorination process (DFAP + 4e + 4H → AP + 2HF).
Fig. 7. Time evolutions of the molar amount of DFAP-OH (red squares: ᮀ), MFAP
Fig. 8. Schematic illustration on the photocatalytic reduction of DFAP on the UV
irradiated TiO2 surface.
(
green triangles: ꢁ), AP (blue diamonds: ♦), and the total amount of the three prod-
ucts (black circles: ꢀ) after 300 ␮mol injection of DFAP into the 2 h pre-irradiated
◦
TiO2 suspension at 32 C. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
whole range of the surface Tisd because the reduction potential of
DFAP and MFAP (intermediate) is not so negative compared to that
of AP. Thus, the sequential defluorination and the reduction of car-
bonyl group can take place on the TiO2 surface by consuming the
electrons accumulated at the whole Tisd sites. It is worth noting
that the ratio of consumed electrons used for the reduction of car-
bonyl group (DFAP-OH: 23%) and defluorination (MFAP + AP: 77%)
is consistent with that of reacted electrons for the production of
DFAP-OH (21%) and the defluorination (MFAP + AP: 79%) during the
prolonged 8 h irradiation as listed in Table 1. This strongly suggests
that the photocatalytic reduction of DFAP proceeds on the Tisd of
derivative was injected into this suspension in the dark so that the
surface electron transfer from Tisd to the adsorbed AP derivatives
took place to afford the reduced products.
3+
When 300 ␮mol of DFAP was injected into the pre-irradiated
TiO2 suspension, the color change from blue–gray to white took
place rapidly and completed within 1 h (Fig. S3 in ESI). Therefore,
we concluded that all of the accumulated electrons were consumed
in the reduction of DFAP. Fig. 7 indicates plots of the molar amounts
of each product of MFAP, AP, DFAP-OH and the total amount of
products after the injection of DFAP. The amounts of each prod-
uct quickly grew up and reached at each constant value within 1 h,
which is in good agreement with the observation of color change
the TiO surface, which is similar to the case of other AP derivatives
2
[
18,19].
in the TiO suspension (Fig. S3 in ESI). The amounts of each product
2
and the total amount of electrons reacted in the reductive reac-
tions (11.2 ␮mol) are listed in Table 1. When an excess amount
of MFAP (300 ␮mol) was injected in the pre-irradiated TiO2 sus-
pension, the similar color change was also observed; the blue-gray
color of TiO2 was completely disappeared within 1 h. The total
amount of reacted electrons was 11.8 ␮mol in this case (Table 1).
Therefore, we conclude that the total amount of Tisd sites used for
the electron accumulation is in the range of 110–120 ␮mol/g for
the P25 TiO2 powder.
Fig. 8 illustrates the sequential reduction of DFAP on the TiO2
surface. Since the sufficient concentration of keto form of DFAP
exists in the ethanol solution, DFAP should be favorably adsorbed
on the Ti surface by an interaction between surface Ti sites and the
lone pair electrons on the carbonyl oxygen of DFAP [34–36]. The
adsorbed DFAP can be reduced by the electrons accumulated in the
3.4. A proposed reaction mechanism
We discuss the difference between the reactivities of the three
fluorinated AP derivatives in this section. The reduction of AP and
TFAP gave only the hydrogenated alcohols AP-OH and TFAP-OH,
respectively [17–19]. In contrast, the reduction of MFAP afforded
only the defluorinated ketone AP. At first, we would like to consider
the reduction mechanism of MFAP on the basis of the electroreduc-
tive dissociation of a series of ␣-substituted AP derivatives reported
by Andrieux et al. [28,38]. They dealt with the dissociation of leav-
−
−
−
−
ing groups (X ) such as halide ions (F , Cl , Br ), benzoate anion,
−
−
alkoxy anions, (CH O , C H5O ) etc. from a series of ␣-substituted
3
2
AP derivatives, in which two dissociative mechanisms were taken
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6
O
OH
OH
OH
e
H
H
CHF2
CHF2
CHF2
CHF2
RF
A
e
ketyl radical
DFAP-OH
O
stepwise
route
F
F
O
OH
O
CH2F
CHF2
CHF
CHF
DFAP
RF
MFAP
e
H
O
O
O
e
F
CHF
CHF
CHF
concerted
route
phenacyl radical
R
Scheme 1. A proposed reaction mechanism for the photocatalytic reductive transformation of DFAP on the UV irradiated TiO2.
Table 2
into account; one was a stepwise pathway, and another was a con-
certed one as below:
•
−
.
Thermodynamic parameters in the cleavage of the C F bond of RF
ERF/RF (V)a
−
DRF (eV)b
ꢀG (eV)c
◦
RX + e− → RX•−
RX → R• + X (stepwise)
•−
−
d
e
e
MFAP
DFAP
TFAP
−1.54 , −1.75
3.12–3.46
−1.5 to −0.9
−0.8 to −0.4
−0.3 to +0.1
d
f
−1.46
3.5–3.9
d
g
−1.38
3.9–4.3
RX + e− → R• + X (concerted).
−
a
Reduction potential in CH3CN vs. SCE.
b
c
Bond dissociation energy of C F bond of the fluorinated AP derivatives.
Standard free energy change in the cleavage reaction of RF
They reported that the electroreduction of MFAP to yield AP and
was classified into the stepwise pathway, whereas electroreduc-
•
−
:
−
•
F
◦
−
−
ꢁ
G = DRF + ERF/RF – EF• /F – TꢁS where TꢁS corresponds to the value of 0.18 eV at
◦
tion of 2-chloro- and 2-bromoacetophenones to give AP as well as
25 C [28].
−
−
d Quoted from [41].
Cl and Br was grouped into the concerted one. The main factor
to determine the dissociative mechanism was considered to be the
bond dissociation energy of C X bonds for the three halogen sub-
stituted AP derivatives (X = F, Cl, and Br). The electroreduction for
MFAP should lead to the stepwise pathway with generation of the
e
Quoted from [28].
f
Energy difference of {D (CH2F2) − D (CH3F) = 0.4 eV} was added to DRF of MFAP:
DRF (DFAP) = D (CH2F2) − D (CH3F) + DRF (MFAP).
g Energy difference of {D (CHF ) − D (CH F) = 0.8 eV} was added to D of MFAP:
3
3
RF
DRF (TFAP) = D (CHF3) − D (CH3F) + DRF (MFAP).
•
−
anion radical species (RX ) because of the strong C F bond, while
the electroreduction of the other two AP derivatives proceeded via
the concerted manner due to the weak C Cl and C Br bonds. We
also suppose that the photoreductive defluorination of MFAP on
The production of DFAP-OH cannot be explained by the concerted
route.
In order to gain further insight into the possible reaction path-
way, we focus on the standard free energy changes in the cleavage
TiO should take place through the stepwise mechanism. However,
2
the possibility of the concerted route cannot be entirely ruled out.
We found that the photocatalytic reduction of DFAP provided
not only the hydrogenated alcohol DFAP-OH but also defluorinated
ketones MFAP and AP. A proposed reaction mechanism can be
illustrated in Scheme 1. The sequential reductive reaction is firstly
initiated by the electron transfer from the Tisd to DFAP adsorbed
•
•
−
−
•
−
reaction of the anion radical RF (RF → R + F )[28,38],
◦
•−
•
−
– TꢁS
/F
ꢁG
= DRF + ERF/RF – EF
(1)
where DRF is the bond dissociation energy of C F bond of fluo-
•
−
−
rinated AP derivatives, ERF/RF and EF/F are standard reduction
potentials of the AP derivatives and fluorine atom (−2.63 V vs. SCE)
on the TiO surface [18,19]. It can be assumed that the one electron
2
reduction of DFAP leads to the two pathways to generate the anion
[40], respectively, and ꢁS is the entropy changes in the cleavage
of RF . The entropy term TꢁS corresponds to the formation of
•
−
−
•−
radical (RF ) as the stepwise route and to yield a phenacyl radical
•
(
R ) and F directly as the concerted route (Scheme 1). Although
two particles from one particle and therefore is almost constant
•
◦
the concerted route giving the phenacyl radical (R ) should not be
a major pathway due to the strong C F bond, we assume that the
phenacyl radical (R ) can be formed from the anion radical (RF
via an epoxide radical intermediate. Moreover, RF can give rise
to a ketyl radical and an anion (A ) species followed by DFAP-OH
and equal to 0.18 eV at 25 C [28]. The thermodynamic data of
•
−
◦
ERF/RF , DRF and ꢁG for MFAP have been reported in the elec-
•
•−
)
trochemical reduction process (Table 2) [28]. Furthermore, the
•
•
−
−
standard reduction potentials of ERF/RF undefined are obtained
−
from the literatures [28,41]. However, DRF for DFAP and TFAP
have not been reported. Therefore, we calculated these values
as below: (1) the bond dissociation energies of C F bonds for
CH F (D (CH F) = 4.76 eV), CH F (D (CH F ) = 5.18 eV), and CHF (D
through the protonation, the second electron transfer, and protona-
tion processes. These protonation steps would be associated with
the ethanol oxidation by valence band holes generated on the TiO2
surface. Recently, Panayotov et al. proposed the photo-oxidation
mechanism of methanol on rutile TiO2 nanoparticles by means
of FT-IR spectroscopy, in which the photogenerated holes play a
3
3
2
2
2
2
3
(CHF ) = 5.53 eV) are picked up from the literature [40], (2) energy
3
differences of D (CH F ) − D (CH F) and D (CHF ) − D (CH F) can
2
2
3
3
3
be roughly estimated to be 0.4 and 0.8 eV, respectively, and (3)
these energy differences are added to the value of DRF of MFAP
(DRF (MFAP)) by the following equations:
+
crucial role in the methanol oxidation and produce H and a new
−
[39]. In a similar manner, H+ and/or
surface hydroxyl group HO
br
−
HObr might be generated during the photo-oxidation of ethanol
on the P25 TiO2 surface and associated with the protonation steps.
DRF(DFAP) = D(CH F ) − D(CH F) + DRF(MFAP)
(2)
2
2
3
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7
DRF(TFAP) = D(CHF ) − D(CH F) + DRF(MFAP)
(3)
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3
3
◦
The estimated DRF and ꢁG values for DFAP and TFAP are also
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7
033.
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