Direct cyanomethylation of aliphatic and aromatic hydrocarbons with acetonitrile over a metal loaded titanium oxide photocatalyst

Article abstract of DOI:10.1039/c7cy00365j

A platinum-loaded TiO₂ (Pt/TiO₂) photocatalyst promoted cyanomethylation of aliphatic hydrocarbons, namely cyclohexane and cyclohexene, with acetonitrile, where the photogenerated hole oxidatively dissociates the C-H bond of both the acetonitrile and the aliphatic hydrocarbons to form each corresponding radical species before their radical cross-coupling. The Pt/TiO₂ photocatalyst was more active than the Pd/TiO₂ photocatalyst in these reactions. In contrast, the cyanomethylation of benzene was promoted by the Pd/TiO₂ photocatalyst or a physical mixture of the Pt/TiO₂ photocatalyst and a Pd catalyst supported by Al₂O₃, while it was hardly promoted by the Pt/TiO₂ photocatalyst alone. The temperature dependence of the reaction rate proved that the Pd nanoparticles on the TiO₂ photocatalyst thermally function as a metal catalyst. These results clearly suggest that the Pd metal catalyst is necessary for the cyanomethylation of benzene. However, in the cyanomethylation of aliphatic hydrocarbons, the catalytic effect of the metal particles was not observed, meaning that the radical coupling takes place without the metal catalysis. Thus, it is concluded that in the case of the benzene cyanomethylation the Pd nanoparticles play dual roles, as a catalyst to catalyse the substitution reaction of benzene with the cyanomethyl radical, and as an electron receiver to reduce the recombination of the photoexcited electrons and holes in the TiO₂ photocatalyst, although they could not contribute as a catalyst to the cyanomethylation of aliphatic hydrocarbons.

Full text of DOI:10.1039/c7cy00365j

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Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
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Direct cyanomethylation of aliphatic and aromatic hydrocarbons
with acetonitrile over metal loaded titanium oxide photocatalyst
Emiko Wada,a,b Tomoaki Takeuchi,a Yuki Fujimura,c Akanksha Tyagi,a Tatsuhisa Kato a,d and Hisao
Yoshida*a,e
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Platinum-loaded TiO2 (Pt/TiO2) photocatalyst promoted cyanomethylation of aliphatic hydrocarbons, namely cyclohexane
and cyclohexene, with acetonitrile, where the photogenerated hole oxidatively dissociates the C–H bond of both the
acetonitrile and the aliphatic hydrocarbons to form each radical species before the radical cross-coupling of them. The
Pt/TiO2 photocatalyst was more active than the Pd/TiO2 photocatalyst in these reactions. In contrast, the
cyanomethylation of benzene was promoted by the Pd/TiO2 photocatalyst or a physical mixture of the Pt/TiO2
photocatalyst and a Pd catalyst supported by Al2O3, while it was hardly promoted by the Pt/TiO2 photocatalyst alone. The
temperature dependence of the reaction rate proved that the Pd nanoparticles on the TiO2 photocatalyst function as a
metal catalyst in the dark process. These results clearly suggest that the Pd metal catalyst is necessary for the
cyanomethylation of benzene. However, in the cyanomethylation of aliphatic hydrocarbons, the catalytic effect by the
metal particles was not observed, meaning that the radical coupling takes place without the metal catalysis. Thus, it is
concluded that in the case of the benzene cyanomethylation the Pd nanoparticles play the dual roles, as the catalyst to
catalyse the substitution reaction of benzene with the cyanomethyl radical, and as the electron receiver to reduce the
recombination of the photoexcited electron and hole in the TiO2 photocatalyst, although they could not contribute as a
www.rsc.org/
catalyst
to
the
cyanomethylation
of
aliphatic
hydrocarbons.
al. reported that di-tert-butyl peroxide (DTBP) can activate
acetonitrile to form cyanomethyl radical for the cyanomethylation
of aliphatic amides.8c Chakraborty et al. developed the
cyanomethylation of aldehydes with Ni complex catalyst promoting
the C–H cleavage of acetonitrile at room temperature.4 In view of
the sustainable chemistry, catalytic process is more desirable.
However, in heterogeneous catalytic reaction systems, it has always
been pointed out that the products separation from the reaction
mixture is not easy. Thus, heterogeneous catalysts have been
desired for the direct cyanomethylation with acetonitrile, but it still
has not been achieved except for our previous study.10
In our laboratory, several photocatalytic reaction systems for
direct functionalization of alkene11 and aromatic compounds12,13
have been investigated by using metal loaded TiO2 photocatalysts
(M/TiO2, M = Pt or Pd). The Pt/TiO2 photocatalyst promotes
functionalization of benzene with water12 and ammonia13 to yield
phenol and aniline, respectively. On one hand, the Pd/TiO2
photocatalyst promotes the direct cyanomethylation of various
aromatic compounds with acetonitrile to yield banzyl cyanides.10
The Pd/TiO2 photocatalyst also forward reactions between benzene
and ethers to form phenylated ethers.14 In general, Pt nanoparticles
1. Introduction
Cyanomethylation is one of the useful synthesis methods to
provide intermediates of medicines, polymers, and so on. In order
to obtain cyanomethylated compounds with high yield,
functionalized nitriles such as trimethylsilyl acetonitrile,1
cyanoacetate salts2 and chloroacetonitrile3 are typical reagents, and
it is recognised that acetonitrile is not a useful nitrile source due to
the very week acidity. Thus the direct cyanomethylation with
acetonitrile has examined by using some kinds of base reagents
such as NaH,5 cesium pivalate6 and NaOH7 to induce C–H bond
dissociation of acetonitrile. However, available strong base
reactants are limited because they tend to promote side reactions.
On the other hand, radical initiators8 and metal complex
catalysts4,9 have been developed for the direct C–H bond activation
of acetonitrile as base-free cyanomethylation. For example, J. Li et
on
a semiconductor photocatalyst can much improve the
photocatalytic activity compared to other metals such as Pd. It is
considered that Pt nanoparticles function as a charge separator for
the photoexcited electron and hole because of the large work
function of Pt.15 However, it is noted that the direct
cyanomethylation of benzene hardly proceeds over the Pt/TiO2
photocatalyst, while it is well promoted over the Pd/TiO2
photocatalyst only under photoirradiation.10 This shows that the Pd
nanoparticles on the TiO2 photocatalyst itself played another role in
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addition to the well-known function as an electron receiver from sample. These results indicated that the contribution of the
the TiO2 photocatalyst, suggesting that the Pd nanoparticles would catalysis by the supported Pd and Pt metal particles was not so
contribute as a metal catalyst to this reaction in the presence of large or negligible in this reaction system. The presence of these
photocatalyst.10 In literature, some articles have described the additional powders reduced the product yields, which would
function of the metal particles as a catalyst coupled with the originate from the shading the photocatalyst, adsorbing some
photocatalysis.10,14,16
species, and so on.
In the present study, the photocatalytic cyanomethylation of
cyclohexane and cyclohexene were examined by using the M/TiO2
photocatalysts and their reaction mechanisms were discussed. In
addition, in order to clarify the function of the metal nanoparticles
in TiO2 photocatalyst, the physical mixture of the TiO2 photocatalyst
and a supported Pd or Pt metal catalyst was applied for the
cyanomethylations of cyclohexane, cyclohexene, and benzene.
Table 1 Results of the photocatalytic reaction tests for the
cyanomethylation of cyclohexane with acetonitrile over the
photocatalyst samples and some mixtures a
hν
+
CH3CN
+
H2
M/TiO2
Cyanomethyl
cyclohexane (CC)
Byproducts:
2. Results and discussion
2.1. Cyanomethylation of cyclohexane
Succinonitrile
Cyclohexane
methyl ketone
(CMK)
Bicyclohexyl
Cyclohexanone
(SN)
(BC)
(1)
hν
(1)
CH3CN
H2
+
+
M/TiO2
Entry Catalystb
Products /µmol
CC SN
10 16
Table 1 shows the result of the reaction between cyclohexane
and acetonitrile (eq. 1) over the photocatalyst samples. The Pt/TiO2
sample gave cyanomethyl cyclohexane (CC) as the desired product,
the amount of which was lower than that of succinonitrile (SN) as a
major by-product (Table 1, entry 1). A small amount of bicyclohexyl
(BC), cyclohexyl methyl ketone (CMK) and cyclohexanone were also
detected because of the coexistence of water in the reaction
mixture. Hydrogen was detected as a gaseous product although the
accuracy for quantitative determination might be low. The reaction
over the Pd/TiO2 sample gave the same products (Table1, entry 2),
but the yields of these products were much less than those over the
Pt/TiO2 sample, indicating that the Pt nanoparticles on the TiO2
photocatalyst can enhance the charge separation of excited
electrons and holes more efficiently and thus gave more products.14
In both cases, the selectivity for this cyanomethylation was not high
compared with the cyanomethylation of benzene in the previous
study.12 The pristine TiO2 photocatalyst exhibited much less activity
than metal loaded ones (Table 1, entry 3), showing that these metal
nanoparticles could act as a cocatalyst enhancing the electron–hole
separation.
1
BC
CMK
1.5
H2
1
2
3
4
Pt/TiO2
Pd/TiO2
TiO2
2.0
1.1
0.4
tr.
70
15
tr.
28
2.6
n.d.c
4.2
2.5 0.13 0.36
tr.d n.d.
0.27
4.0 0.58 0.87
Pt/TiO2 +
Pd(1.0)/Al2O3
Pt/TiO2 +
Pt(1.0)/Al2O3
Pt/TiO2 +
tr.
e
e
5
6
4.3
4.0
4.8 0.72 0.25
tr.
tr.
28
34
7.5 1.7
0.56
e
Al2O3
a
Reaction condition: The M(0.1wt%)/TiO2 photocatalyst 0.20 g,
cyclohexane 0.93 mmol (0.10 mL), acetonitrile 72 mmol (3.8 mL),
and water 5.6 mmol (0.10 mL). Reaction time was 1 h. Reaction
temperature was ca. 310 K. The wavelength of the irradiation light
was 365±20 nm and the light intensity was 27 mW cm−2 measured
b
at 360±15 nm. The loading amount of metal on the TiO2 sample
was 0.1 wt%, and the loading amount of metals on the Al2O3
c
d
e
support was 1.0 wt%. not detected. trace. A mixture of the
M(0.1wt%)/TiO2 photocatalyst of 0.20 g and the M(1.0 wt%)/Al2O3
sample or the bare Al2O3 sample of 0.20 g was used.
The effect of the addition of the M/Al2O3 sample to the Pt/TiO2
photocatalyst was also examined in this reaction system for the
discussion mentioned later to know the catalytic effect of the
loaded metals. According to the literature,17 in the presence of
platinum black, the catalytic activity of TiO2 photocatalyst is much
improved because physical contact of Pt particles with TiO2
photocatalyst enhances the charge transfer of photogenerated
electron from the TiO2 photocatalyst to decrease the recombination
of electron-hole pairs. To investigate the catalytic function of the
metal nanoparticles, the direct physical contract of metal
nanoparticles and TiO2 photocatalyst should be avoided. Thus, the
M/Al2O3 sample were employed here. Since in these supported
samples the metal nanoparticles were dispersed and stabilized on
the surface of the Al2O3 with large surface area and porous
structure, the nanoparticles could not contact with the TiO2 surface
so easily, allowing us to ignore the effect of the metal nanoparticles
as the electron receiver from the TiO2 photoctalyst. As listed in
Table 1 entries 4–6, the yield of CC decreased in the coexistence of
the Pd/Al2O3 sample, the Pt/Al2O3 sample, and the bare Al2O3
Since SN and BC were given by each photocatalyst, both
acetonitrile and cyclohexane molecule would be oxidized to by the
TiO2 photocatalysts to form each radical species, followed by the
radical homo-coupling reaction between them. As shown in our
previous study,10 acetonitrile can be oxidized to a cyanomethyl
radical by a Pd/TiO2 photocatalyst under the light. In the present
study, ESR measurement was carried out in order to confirm the
formation of radical species from cyclohexane in the presence of
the photocatalyst. Fig. 1 shows the ESR spectra of cyclohexane
solution containing PBN as a spin trap reagent in some conditions.
While no signals except for those of Mn2+ marker was observed
without the photocatalyst (Fig. 1a) or light (Fig. 1b), new triplet
signals were detected in the presence of the Pd/TiO2 photocatalyst
upon light irradiation (Fig. 1c). Although the hyperfine structure
was not clearly observed due to the week signals, these signals
would be a radical species generated by the photocatalyst. Judging
from the compounds in the cell under this condition, it can be
considered that the triplet signals could originate from the nitrogen
2 | J. Name., 2012, 00, 1-3
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of the PBN adduct shown in Fig. 1 with the g value =2.006 and AN
=
values of kH/kD(SN) and kH/kD(BC) were calculated in the similar
1.32 mT,18 indicating that the cyclohexene-PBN adduct would be ways. d Not detected.
detected. Signals assignable to the hydrogen radical adduct could
not be observed in this condition.
On the other hand, it should be considered that the kH/kD values
less than unity (inverse KIE) can provide more important
information of the reaction mechanism. When the reaction
between C6H12 and CD3CN was carried out, the kH/kD values for the
yields of CC and BC were 0.83 and 0.11, respectively over the
Pt/TiO2 sample (Table 2, entry 2) and 0.77 and 0.14, respectively
over the Pd/TiO2 sample (Table 2, entry 5), which can be recognised
as the inverse KIE. In this reaction, both cyclohexane and
acetonitrile would be oxidized by the photocatalytically-generated
hole to become corresponding radical species, i.e., the two
reactions for the radical formation competitively take place to
consume the limited number of the photogenerated holes. The use
of CD3CN instead of CH3CN would decrease the oxidation rate of
acetonitrile due to the high C–D bond dissociation energy, which
enhanced the oxidation of competitor instead, i.e., the oxidation
rate of C6H12 to form the cyclohexyl radical. Since the RDS in these
coupling reactions was the formation of cyclohexyl radical, both
formation rates of CC and BC increased in the presence of CD3CN as
a result. In the reaction between CH3CN and C6D12, since the C–D
bond dissociation of C6D12 was more difficult, the competitive
oxidation was enhanced, resulting that the concentration of
cyanomethyl radical increased. In consequence, the yield of SN
increased and the kH/kD values for SN production were lower than
unity such as 0.84 nad 0.94 (Table 2, entries 3 and 6). These results
clarified that the competitive hole oxidation of both cyclohexane
and acetonitrile took place to form their radical species.
Mn2+
Mn2+
(c)
PBN
(b)
(a)
321 323 325 327 329 331 333 335
Magnetic field / mT
Possible structure of
the spin adduct
Fig. 1 ESR spectra of the cyclohexane solution containing a spin
trap reagent (PBN) measured (a) after light irradiation for 120
min without the Pd/TiO2 sample, (b) in the presence of the
Pd/TiO2 sample in the dark, and (c) after light irradiation for 120
min in the presence of the Pd/TiO2 sample.
To discuss the reaction mechanism, some reaction tests were
carried out with deuterated reagents over the M/TiO2 samples.
When deuterated acetonitrile (CD3CN) was reacted with
cyclohexane (C6H12) over the Pt/TiO2 sample (Table 2, entry 2), the
yield of CC was almost similar to, or slightly larger than, the result
using normal reagents (C6H12 and CH3CN, Table 2, entry 1), i.e. the
kH/kD value for the CC formation was not larger than unity. On the
other hand, when deuterated cyclohexane (C6D12) was used for the
reaction, the yield of CC clearly decreased, i.e., the kH/kD value for
the CC formation was 1.9 (Table 2, entry 3). This kinetic isotopic
effect (KIE) clearly evidenced that the C–H bond activation of
cyclohexane would be the rate-determining step (RDS) to give CC in
this condition. The similar KIE was observed for the reaction over
the Pd/TiO2 sample (Table 2, entries 4–6), showing that the RDS
would be the same step as the case of the Pt/TiO2 sample. These
results were consistent with the order of the C–H bond dissociation
energy19a (Table S1): cyclohexane (416 KJ mol−1) > acetonitrile (406
KJ mol−1), where the RDS corresponded to the cleavage of the
stronger bond.
Considering the results of the ESR measurements and the
kinetic isotopic effect, the reaction mechanism for the
cyanomethylation of cyclohexane should be radical cross-coupling,
which would be a two photon process, as shown in Fig. 2. First,
photoexited electrons and holes generate on TiO2 photocatalyst
under photoirradiation. The holes on the TiO2 surface oxidize both
reactants, cyclohexane and acetonitrile, to form the corresponding
radical species and protons, which proceeds competitively. Then,
CC can be produced through the cross-coupling reaction between
the cyclohexyl radical and the cyanomethyl radical, while BC and SN
can be also formed via each homo-coupling reaction similarly.
Produced protons are reduced by the photoexcited electrons to
hydrogen radicals, followed by their radical coupling to form
molecular hydrogen. These reactions can be promoted by the Pt or
Pd loaded TiO2, although the former exhibits higher activity than
the latter. In the cyanomethylation of cyclohexane proceeding via
radical cross-coupling, the loaded metal would function as the
electron receiver to reduce the recombination of the photoexcited
electron and hole in the TiO2 photocatalyst.
Table 2 Results of the reaction tests with isotopic compounds for
the direct cyanomethylation of cyclohexane by the M(0.1
wt%)/TiO2 photocatalysts a
Pt or Pd
H2
Entry Catalyst Isotopic
Productsb /µmol
kH/kD
c
CH3CN
TiO2
H• •H
compound
CC
SN
16
3.0 19
BC
CC
SN
BC
Radical coupling
CH3CN
e−
Adsorption
1
none
Pt/TiO2 CD3CN
C6D12
10
12
5.2
2.0
-
-
-
h
Photoexcitation
CH2CN
Radical coupling
Reduction of H+
2
3
4
5
0.83
1.9
-
0.77
1.2
5.3
0.84
-
0.11
2.2
-
0.14
2.6
19
0.90
0.29
2.1
Oxidation of CH3CN
•CH2CN
none
Pd/TiO2 CD3CN
C6D12
2.0
2.6
1.7
2.5
n.d.d
2.6
-
H+
•
e−
H+
6
a
0.11
0.96
H•
e−
•CH2CN
The reaction condition was the same as that described in the
caption of in Table 1.
succinonitrile, BC= bicyclohexyl. kH/kD(CC) = (yield of CC with the
normal reagent) / (yield of CC with the deuterated reagent). The
b
Oxidation of cyclohexane Reduction of H+
Rateꢀdetermining step
CC= cyanomethyl cyclohexane, SN=
c
H•
H•
•CH2CN
•CH2CN
e−
h+
Adsorption
Photoexcitation
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determined by the difficulty of the radical formation through the C–
H bond dissociation. Thus, the RDS in the cyanomethylation of
cyclohexane was the formation of the cyclohexane radical, while
that in the cyanomethylation of cyclohexene was the formation of
cyanomethyl radical. The kH/kD value for the yield of bc was less
than unity in the reaction between cyclohexene and CD3CN,
indicating that the oxidative reactions of cyclohexene and
acetonitrile to form the corresponding radical species competitively
proceed in the similar way as discussed above.
Fig.
2
Proposed reaction mechanism of the direct
cyanomethylation between cyclohexane and acetonitrile over the
M/TiO2 photocatalyst.
It was also confirmed that the cyanomethylation of n-hexane
was also promoted by both the Pt/TiO2 photocatalyst and the
Pd/TiO2 photocatalyst (Table S2), where the former was more
active than the latter similar to the case of cyclohexane.
Table
3 Results of the photocatalytic reaction tests for the
cyanomethylation of cyclohexene with acetonitrile over the M/TiO2
photocatalyst samples or a mixture of the Pt/TiO2 and Pt/Al2O3
samples. a
2.2. Cyanomethylation of cyclohexene
hν
CH3CN
H2
(2)
hν
+
+
M/TiO2
+ CH3CN
+
H2
+
M/TiO2
Cyanomethyl
cyclohexane (CC)
Cyclohexenyl
acetonitrile (CA)
The direct cyanomethylation of cyclohexene with acetonitrile
(eq. 2) was examined to know the property of the photocatalytic
cyanomethylation system and to expand the application to
unsaturated hydrocarbon. Table 3 shows the results of the reaction
tests with the M/TiO2 samples. The main product was assigned to 2-
cyclohexenyl acetonitrile (CA) according to the following results; (i)
the retention time of the main product in the gas chromatograph
was close to but clearly different from that of a purchased reagent
of 1-cyclohexenyl acetonitrile, (ii) the mass fragments in the mass
spectrum of the main product were almost the same but not
identical to those of the purchased reagent of 1-cyclohexenyl
acetonitrile (Fig. S3), and (iii) it is fact that the C–H bond
dissociation energy at allylic position is the lowest in a cyclohexene
molecule19b (Table S1). Each photocatalyst produced CA, a desired
cyanomethylated cyclohexene, as the major product over. The
Pt/TiO2 sample produced higher amount of CA than the Pd/TiO2
sample (Table 3, entries 1 and 2). Since the pristine TiO2 sample
exhibited very small activity, the metal nanoparticles were
confirmed to function as the electron receiver enhancing the
electron–hole separation (Table 3, entry 3). The physical mixture of
the Pt/TiO2 sample and the Pd/Al2O3 sample did not exhibit higher
yield of CA than the Pt/TiO2 sample alone (Table 3, entry 4). The
byproducts were cyanomethyl cyclohexane (CC), some kinds of
homo-coupling products from cyclohexene, e.g., bicyclohexenyls
(bc) such as [1,1’-bi(cyclohexane)]-2,2’-diene, and the hydrated
compounds such as cyclohexylcyclohexene. The small amounts of
cyclohexenol and cyclohexenone were also detected because of the
presence of water. Interestingly, SN was not detected even though
large amount of acetonitrile exited in the reactor, which would
originate from the much lower C–H bond dissociation energy of the
allylic position of cyclohexenethe C–H bond of cyclohexene (350 kJ
Byproducts:
Cyclohexenylꢀ
cyclohexene
(1)
Cyclohexeꢀ
none (2)
Bicyclohexenyl
Succinonitrile
Cyclohexeꢀ
nol (3)
(bc)
(SN)
Entry Catalyst b
Products /µmol
CA
42
16
CC
5.4
7.8
SN
tr.c
Bc
24
8.4
2.4
1
H2
90
10
1.7
70
2
3
1
2
3
4
Pt/TiO2
Pd/TiO2
TiO2
n.d.
6.6
n.d.
n.d.
1.4
2.0
1.1
1.0
0.81
1.9
0.24
0.52
n.d. d
n.d. n.d.
4.1 n.d.
n.d.
30
Pt/TiO2
+
22
Pd(1.0)/Al2O
a
Reaction condition: Catalyst 0.20 g, cyclohexene 0.98 mmol (0.10
mL), acetonitrile 72 mmol (3.8 mL), and water 5.6 mmol (0.10 mL).
Other conditions were the same as those described in the caption
of Table 1. The loading amount of metal on the TiO2 sample was
0.1 wt%, and the loading amount of metals on the Al2O3 support
was 1.0 wt%.
b
c
d
e
trace.
Not detected.
A mixture of the
M(0.1wt%)/TiO2 photocatalyst of 0.20 g and the M(1.0 wt%)/Al2O3
sample or the bare Al2O3 sample of 0.20 g was used.
Table 4 Results of the reaction tests with isotopic compounds for
the cyanomethylation of cyclohexene over the Pd/TiO2 sample. a
Products b /µmol
kH/kD
CA
-
3.9
1.0
c
Isotopic
compound
Entry
CA
12
Bc
SN
bc
1
2
3
None
CD3CN
C6D10
9.1
3.1 18
12 2.0
n.d d
n.d.
n.d.
-
0.48
5.4
a Reaction condition: A Pyrex test tube (ca.70 mL) was used as a
reactor. Other conditions were the same as those described in
the caption of Table 1. b CA=2-cyclohexenyl acetonitrile, bc= the
sum of bicyclohexenyl including isomers, SN=succinonitrile. c
kH/kD = (yield with the normal compound) / (yield with the
deuterated reagent). d Not detected.
mol–1)17b than that of acetonitrile (406 kJ mol–1 17a (Table S1).
)
To discuss the reaction mechanism, the reaction tests with the
isotopic compounds was carried out over the Pd/TiO2 photocatalyst.
As shown in Table 4, the kH/kD value for the CA formation was 3.9
when deuterated acetonitrile (CD3CN) was used. This suggests that
the C–H bond activation of acetonitrile would be the rate-
determining step in this condition, which is clearly different from
the case of the cyanomethylation of cyclohexane mentioned above.
The RDS in these reactions seems related to the order of the C–H
bond dissociation energy of the reactants19b (Table S1), cyclohexane
(416 kJ mol−1) > acetonitrile (406 kJ mol−1) > cyclohexene (350 kJ
mol−1 at allylic position as the lowest one), i.e., the RDS would be
The radical formation from cyclohexene over the Pd/TiO2
sample was confirmed by ESR measurements. As shown in Fig. 4,
the sharp signals were detected in the presence of the Pd/TiO2
sample under photoirradiation, which means radical species can be
generated from cyclohexene by the photocatalyst. The signals
showed a double triplet with the g value of 2.006 and the hyperfine
coupling constant of AN=1.32 mT (triplet) and AH=0.21 mT (doublet),
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respectively. The possible spin adduct structure was suggested to
be the cyclohexene-PBN adduct shown in Fig. 3, indicating that the
cyclohexenyl radical was formed from cyclohexene. The signal
intensity of this spin adduct with cyclohexene was much larger than
that of the spin adduct with cyclohexane, consisting with the
radical formation rate expected from the C–H bond dissociation
energy19b of the cyclohexane (416 kJ mol−1) and cyclohexene (350 kJ
mol−1 at allylic position). Signals originating from the hydrogen
radical adduct could not be observed in this condition. Since the
radical formation of the cyanomethyl radical was confirmed with
the Pd/TiO2 photocatalyst in the previous study,10 it is proposed
that the reaction between cyclohexene and acetonitrile is a radical
cross-coupling between cyclohexenyl radical and cyanomethyl
radical.
Fig.
4
Proposed reaction mechanism of the direct
cyanomethylation between cyclohexene and acetonitrile over
the M/TiO2 photocatalyst.
2.3. Cyanomethylation of benzene
hν
CH3CN
H2
(3)
+
+
M/TiO2
In the previous study, we found that the cyanomethylation of
benzene (eq. 3) can be promoted by the hybrid Pd/TiO2 catalyst
consisting of Pd thermal catalyst and TiO2 photocatalyst.10 Here, it
was again confirmed that the reaction between benzene and
acetonitrile over the Pd/TiO2 sample gave benzyl cyanide (BnCN) as
the major product, and phenol (PhOH) and succinonitrile (SN) as the
minor products (Table 5, entry 2). The formation of SN suggested
that cyanomethyl radical would be actually generated through the
photocatalytic oxidation of acetonitrile before the homo-coupling.
In fact, ESR measurement elucidated the formation of the
cyanomethyl radical in the presence of the Pd/TiO2 sample under
photoirradiation.10 Since the biphenyl was not detected as a homo-
coupling product in this reaction condition, benzene would not be
oxidized to phenyl radical species. This is also supported by the fact
that the C–H bond activation energy19a of benzene (440 kJ mol−1 as
shown in Table S1) is much higher than that of acetonitrile (406 kJ
mol−1).
1.32 mT
0.21 mT
O
(c)
Mn2+
H
N
CH3
CH3
Mn2+
C
C
CH3
Ph
Possible structure of
the spin adduct
(b)
(a)
323
325
327
329
331
333
Magnetic field / mT
Fig. 3 ESR spectra of the cyclohexene solution containing PBN in
argon atmosphere, (a) measured after light irradiation for 20 min
without the Pd/TiO2 sample, (b) measured in the presence of the
Pd/TiO2 sample without light irradiation, (c) measured after light
irradiated for 10 min in the presence of the Pd/TiO2 sample.
Table 5 Results of the photocatalytic reaction tests for the
cyanomethylation of benzene with acetonitrile over the
photocatalyst samples and some mixtures. a
Considering from the KIE and the ESR signals, the reaction
between cyclohexene and acetonitrile can be explained by the
radical coupling mechanism involving two photoexcitation steps as
shown in Fig. 4. The photoformed hole oxidizes cyclohexene to
cyclohexenyl radical, in which the radical centre is at the allylic
position. The photoformed hole oxidizes also acetonitrile to
cyanomethyl radical, which is the rate determining step (RDS) in
this condition. Then, the cross-coupling proceeds between these
radicals, while homo-coupling reactions also undergo as the side
reactions. In the current reaction condition, since the cyclohexenyl
radical would be predominantly formed compared with the
cyanomethyl radical, CA and bc were produced as the major
products, but SN was rarely formed.
hν
+
CH3CN
+
H2
M/TiO
2
Benzyl cyanide
(BnCN)
Byproducts:
Succinonitrile
Phenol
(SN)
(PhOH)
Entry
Catalyst b
Pt/TiO2
Pd/TiO2
TiO2
Pt/TiO2 +
Pd(0.1)/Al2O3
Pt/TiO2 +
Pd(1.0)/Al2O3
Pt/TiO2 +
Pd(2.0)/Al2O3
Products /µmol
BnCN
0.08
4.0
PhOH
0.91
1.6
n.d.
0.35
SN
12
1.1
tr d
9.4
H2
1
2
3
4
20
10
1.1
13
H2
Pt or Pd
n.d.c
0.40
TiO2
H• •H
Radical coupling
e
e
e
e−
Adsorption
h+
Photoexcitation
5
6
4.4
4.6
0.89
1.8
2.8
3.7
CH2CN
Reduction of H+
Oxidation of
cyclohexene
0.81
1.0
Radical coupling
H+
e−
H+
H•
e
e−
•CH2CN
7
8
Pt/TiO2 + Al2O3
Pd(1.0)/Al2O3
0.08
n.d.
0.43
n.d.
5.8
n.d.
15
n.d.
Oxidation of CH3CN
Rateꢀdetermining step
Reduction of H+
a Reaction condition: The photocatalyst 0.20 g, benzene 1.1 mmol
(0.10 mL), acetonitrile 72 mmol (3.8 mL), and water 5.6 mmol
(0.10 mL). Other conditions were the same as those described in
H•
H•
CH3CN
e
h+
CH3CN
Adsorption
Photoexcitation
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the caption of Table 1. b The loading amount of Pt or Pd on the large band gap and thus the Pt/TiO2 photocatalyst was required to
TiO2 photocatalyst was 0.1 wt% and on the Al2O3 support was 0, generate the cyanomethyl radical species.
c
e
0.1, 1.0, or 2.0 wt%. Not detected. d trace. A mixture of the
Pt/TiO2 sample of 0.20 g and the Pd/Al2O3 sample of 0.20 g was
used.
The amount of the exposed metal atoms of the Pd
nanoparticles on the Al2O3 support were evaluated by the CO
adsorption experiment (Table S3). It was confirmed that the particle
size of the Pd nanoparticles in the Pd/Al2O3 samples were in the
range of 2.3–4.6 nm. It was found that the obtained amounts of
BnCN shown in Table 5 were almost consistent with the amount of
exposed metal atoms, where turnover frequency (TF) calculated
from the values were almost the same level in the range of 0.10–
0.15 h−1. It is suggested that the surface Pd atom is the active sites
for the catalytic step, the reaction between benzene and
cyanomethyl radical, during the photocatalytic cyanomethylation of
benzene.
On the other hand, it is notable that the reaction over the
Pt/TiO2 sample gave a very small amount of BnCN with a large
amount of SN (Table 5, entry 1), which means this photocatalyst
would be active at least for the generation of the cyanomethyl
radical. The pristine TiO2 sample scarcely produced the coupling
products (Table 5, entry 3), indicating that the metal nanoparticles
enhanced the photocatalytic reaction. As mentioned above, it is
generally accepted that metal nanoparticles can function as the
electron receiver to enhance the charge separation, and Pt
nanoparticles is one of the most typical electron receiver. In fact,
the Pt/TiO2 sample was more active than the Pd/TiO2 sample for
the cyanomethylation of aliphatic hydrocarbons. However, the
Pt/TiO2 sample promoted very slowly the cyanomethylation of
benzene with acetonitrile, which is also well consistent with the
previous study.10 Although the cyanomethyl radical was produced
by the both catalysts, BnCN as the cross-coupling product was
obtained only with the Pd/TiO2 sample, but hardly obtained with
the Pt/TiO2 sample. The Pd nanoparticles would play a special role
in the reaction. Since the particle size of Pt (2.1 nm) and Pd (2.2 nm)
on TiO2 photocatalysts were almost same (Table S3, entries 1 and 2),
the different performance of the photocatalyst should originate
from the properties of the metals. Thus, it was suggested that the
Pd nanoparticles would be necessary to catalyse at least one step in
the cyanomethylation of benzene, which would be the reaction
between benzene and cyanomethyl radical as a dark process. It was
also revealed that this hybrid catalyst consisting the Pd metal
catalyst and the TiO2 photocatalyst could be applied to other
aromatic compounds such as toluene and pyridine as reported in
the previous study,10 implying that the Pd/TiO2 hybrid catalyst
would promote the cyanomethylation of aromatic ring of these
compounds.
In order to clarify the catalytic property of the Pd nanoparticles
furthermore, temperature controlled reaction tests by using a
water bath were performed over the Pd/TiO2 sample. In these
reaction tests,
a reaction mixture of benzene (0.1 mL) and
acetonitrile (3.9 mL) was employed without the addition of the
small amount of water in order to achieve the similar selectivity to
that in the standard condition, since unfavourable by-products such
as phenol were found at higher temperature. It is expected that the
yield of BnCN will increase with the temperature if the Pd
nanoparticles supported on the TiO2 photocatalyst can function as a
metal catalyst. As a result, the BnCN yield increased with increasing
the reaction temperature, but the SN yield did not increase so much
as shown in Fig. 5. The apparent activation energy calculated from
the pseudo Arrhenius plot for the formation of BnCN was 43 kJ
mol−1, which seems high enough as the value for a thermal catalytic
process to produce BnCN. This step would be the rate determining
step among the thermal processes, which should be catalysed by
the Pd metal catalyst loaded on the TiO2 support. This step would
correspond to the addition-elimination step between benzene and
cyanomethyl radical proposed in the previous study.10
The KIE of this reaction over the Pd/TiO2 sample suggests that
the C–H bond dissociation of benzene to from benzene radical is
not involved for the production of BnCN.10 Therefore, it can be
explained that the Pd thermal catalyst promotes the reaction of
molecular benzene with the cyanomethyl radical to produce BnCN
and hydrogen radical, i.e., the formation of the transition state
having a sp3 carbon in the benzene ring and then the homolytic
dissociation of the C–H bond of the aromatic ring in the transition
state. If the electron rich Pd metal nanoparticles14 donates the
electron density to π* orbital of benzene, the molecular benzene
would be activated before the reaction with the cyanomethyl
radical. On one hand, the Pd nanoparticles are also expected to
accelerate the C–H bond homolytic dissociation of the aromatic ring
of the transition state.
In the present study, further experiments were carried out to
confirm the assumption that the reaction between benzene and
cyanomethyl radical requires the Pd nanoparticles as a catalyst. If
the Pd nanoparticles catalysed the reaction of benzene with
cyanomethyl radical, BnCN would be obtained even in the
coexistence of the Pd/Al2O3 sample as a supported metal catalyst
and the Pt/TiO2 sample as a photocatalyst for radical formation. The
results are listed in Table 5, entries 4–6. Whereas the amount of
BnCN was negligible small in the absence of the Pd/Al2O3 sample
(Table 5, entry 1), BnCN was obtained with the Pt/TiO2
photocatalyst and the Pd/Al2O3 catalyst (Table 5, entries 4–6). In
addition, the yield of BnCN was improved with the increase of the
loading amount of Pd on the Al2O3 support, but it was not efficiently
improved more than 1.0 wt% loading, although the formation of SN
was suppressed with higher loading amount of Pd on the Al2O3
support. It is noted that the mixture of the Pt/TiO2 sample and bare
Al2O3 support gave very small amount of the products (Table 5,
entry 7). Hence, these results obviously support that the Pd
nanoparticles catalyse the reaction between benzene and the
cyanomethyl radical, which would be not a photocatalytic process
but a catalytic process. The Pd/Al2O3 sample itself did not give any
product upon photoirradiation (Table 5, entry 8), confirming that
the Al2O3 support could not function as a photocatalyst due to the
In contrast, the apparent activation energy for the formation of
SN was 5.5 kJ mol−1 that would be acceptable as the activation
energy for the photocatalytic reaction,20 suggesting that SN was
formed by a photocatalysis through the homo-coupling of the
cyanomethyl radicals.
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2.4. The role of the Pd nanoparticles
Both of the Pt/TiO2 and Pd/TiO2 samples promoted the
cyanomethylation of cyclohexane and cyclohexene (Table 1, entries
1–2, and Table 3, entries 1–2) via radical cross-coupling reaction.
The addition of the Pd/Al2O3 samples to these reaction systems did
not improve the cyanomethylation reactions (Table 1, entries 4–6
and Table 3, entry 4), which means that the catalysis by the Pd
metal nanoparticles was not required in these cases. Hence, it can
be concluded that the cyanomethylation of cyclohexane and
cyclohexene are mainly promoted by the photocatalysis with the
M/TiO2 samples, but the metal catalysis would not contribute to the
reaction so much. In these reactions, the Pd nanoparticles would
contribute as an acceptor of photoexcited electron to prolong the
life time of the photoexcited charge carriers and enhance the
radical formation, like as the Pt cocatalyst mentioned above.
2.5
2.0
1.5
1.0
(b) SN
Ea: 5.5 kJ mol−1
(a) BnCN
Ea: 43 kJ mol−1
0.5
3.35
3.40
3.45
3.50
3.55
3.60
1/T (10−3 K−1)
However, the Pd metal catalyst was necessary for the
photocatalytic cyanomethylation of benzene with acetonitrile. This
would be related to the high C–H bond dissociation energy for
benzene molecule. This reaction was well promoted by the Pd/TiO2
hybrid catalyst (Table 5, entry 2) as well as the physical mixture of
the Pt/TiO2 photocatalyst and the Pd/Al2O3 catalyst (Table 5, entries
4–6). This fact evidenced that the supported Pd nanoparticles can
function as the catalyst for at least one step during the
cyanomethylation of benzene. The Pd nanoparticles would
accelerate the addition-elimination step, i.e., the substitution
reaction with cyanomethyl radical. On the other hand, it is also
considered that the Pd nanoparticles deposited on the TiO2
photocatalyst would also contribute as the electron acceptor for
the photoexcited electrons to improve the photocatalysis for the
formation of the cyanomethyl radical.
Fig. 5 Pseudo Arrhenius plots of (a) benzyl cyanide (BnCN) and
(b) succinonitrile (SN) yields with the Pd/TiO2 hybrid catalyst.
Reaction condition: The Pd/TiO2 sample 0.20 g, benzene 1.1
mmol (0.10 mL), and acetonitrile 73 mmol (3.9 mL). Reaction
time was 2 h. The irradiation condition was the same as that
described in the caption of Table 1.
Therefore, the reaction mechanism of the cyanomethylation of
benzene over the Pd/TiO2 hybrid catalyst should be concluded as
presented in Fig. 6. The hole generates on the TiO2 photocatalyst
under photoirradiation and oxidizes acetonitrile to form the
cyanomethyl radical and proton, which is the RDS in the whole
catalytic cycle of the reaction in the current condition.10 The
cyanomethyl radical can react with the adsorbed and activated
benzene on the Pd nanoparticles to form the complex having sp3
carbon in the transition state, as shown in Fig. 6. The C–H bond
dissociation at the sp3 carbon in the transition state is promoted on
the Pd nanoparticles, followed by the formation of BnCN with
In the past studies, the photocatalytic reactions for aromatic
functionalization such as amination of benzene11 and hydroxylation
of benzene12 were well promoted by Pt/TiO2 photocatalyst without
the Pd metal catalyst. In these reactions, ammonia and water
molecules are oxidized to the radical species by the photogenerated
holes and the substitution reaction of benzene with these radical
species would proceed to form the product and hydrogen radical
species without the Pd metal catalyst. This would be because these
radical species would have enough reactivity toward a molecular
benzene. On the other hand, the cyanomethyl radical might be
more stable and less reactive than these radical species such as the
surface hydroxyl radical and the amide radical, since the cyano
group in the cyanomethyl radical would withdraw the electron from
the radical centre to delocalise the radical property. Therefore, in
the photocatalytic cyanomethylation of benzene with acetonitrile,
the assist by the Pd catalyst would be necessary to promote the
substitution process of benzene by the cyanomethyl radical species.
Here, the Pd catalyst would activate benzene ring by adsorption
before the cyanomethyl radical attack and assist the hydrogen
abstraction from the intermediate. The Pd catalyst was not required
for the cyanomethylation proceeding via radical coupling, as such
the cyanomethylation of aliphatic hydrocarbons with acetonitrile,
because the reactants can be oxidized to the radical species having
enough reactivity for the radical coupling in nature. The assist by
the Pd metal catalyst would be effective only for the addition-
elimination process between aromatic ring and somewhat stable
radical species such as cyanomethyl radical.
releasing
a hydrogen radical. This hydrogen radical and the
photocatalytically formed radical can form a hydrogen molecule. It
is clear that the formation of BnCN is one photon process.
Pd
H2
CH3CN
TiO2
H•
•H
Radical coupling
e−
Adsorption
CH3CN
Photoexcitation
h+
Abstraction of
H radical
Oxidation of CH3CN
Rateꢀdetermining step
Transition state
•H
H+
e−
•CH2CN
Reduction of H+
Attack of cyanomethyl radical
Thermal catalysis
•H
•H
•CH2CN
•CH2CN
Pd
Adsorption
Fig.
6
Proposed photocatalysis and addition-elimination
mechanism for the cyanomethylation of benzene over the
Pd/TiO2 hybrid catalyst.
3. Conclusions
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The direct cyanomethylation of cyclohexane and were put into a beaker followed by stirring for 60 min at room
cyclohexene with acetonitrile by metal loaded photocatalysts temperature. The mixture was heated for evaporation to dryness
proceeds via radical coupling reaction, where the reaction around 373 K with stirring, and the obtained powder was calcined
takes place photocatalytically even without the aid of Pd metal at 723 K for 60 min. The obtained powder samples were referred to
catalysis. In these cases, the Pt and Pd nanoparticles on the as the M(x)/Al2O3 samples, where the metal loading amount (x) on
TiO2 photocatalyst would contribute as just an acceptor of the Al2O3 support was 0.1, 1.0 or 2.0 wt%.
photoexcited electrons to enhance the photocatalytic reaction
rate. These reactions are two photon processes.
4.2. Reaction tests
In contrast, the Pd/TiO2 hybrid catalyst or the physical
mixture of the Pt/TiO2 photocatalyst and the Pd/Al2O3 catalyst
are required to promote the reaction between benzene and
acetonitrile, where the Pd nanoparticle functions as a metal
catalyst for the reaction between benzene molecule and
cyanomethyl radical species through the addition-elimination
The photocatalytic reaction tests were typically carried out in
the following manner. The M/TiO2 sample (0.2 g) was put into the
quartz reactor (46 cm3) and pre-irradiated from the beneath (20
cm2) with the xenon lamp for 15 min in air to clean the catalyst
surface. After purging with argon, a mixture consisting of 0.1 mL of
benzene, cyclohexane or cyclohexene, 3.8 mL of acetonitrile and 0.1
process before completing the cyanomethylation. In the mL of distilled water, were introduced into the reactor. All reactants
cyanomethylation of benzene, the Pd nanoparticle on the TiO2
photocatalyst functions as not only an acceptor for the without further purification. The reaction test was carried out
for the reaction tests were of analytical grade reagents and used
photoexcited electron but also a metal catalyst. Thus, the Pd
nanoparticle is bifunctional cocatalyst. This is a one photon
process.
under the light of 365±20 nm in wavelength from the xenon lamp
for 60 min with cooling by an electric fan. The light intensity was
measured at 360±15 nm to be 27 mW cm−2. After the reaction, the
products in the gas phase were collected by a gas tight syringe and
then analysed by a GC-TCD (Shimadzu, GC-8A). The reaction mixture
was diluted with methanol and filtrated with a PTFE filter. The
filtrate was mixed with a methanol solution of n-decane as an
internal standard, and then analysed by a GC-MS (Shimadzu, GCMS-
QP5050A).
In this study, some rules in the photocatalytic organic
reactions were suggested, i.e., both of the reactivity of
reactant to become the corresponding radical species and the
reactivity of the radical species should be the important
factors for the determination of the reaction rate and the
product selectivity, and the metal nanoparticles can help these
reaction steps as an electron acceptor or a metal catalyst to
promote the photocatalytic coupling reactions. These simple
photocatalytic coupling reaction systems would widely provide
desirable applications in the field of chemical synthesis in view
of green and sustainable chemistry.
4.3. Electron spin resonance (ESR) measurement
In order to investigate the radical species formed during the
cyanomethylation of cyclohexane and cyclohexene, ESR spectra
were recorded by an X-band spectrometer (JEOL-RE2X) in the
presence of the Pd/TiO2 photocatalyst under photoirradiation.
Before the measurement, the Pd/TiO2 sample was pre-irradiated for
60 min in order to clean the sample surface. The trace amount of
the pre-irradiated Pd/TiO2 sample (0.02 g) was suspended in
cyclohexane or cyclohexene (1 mL; 9.3 mmol and 9.8 mmol,
respectively) with PBN (N-tert-Butyl-α-phenylnitrone, Tokyo
Chemical Industry, 98%, 0.67 mmol) as a spin trap reagent, then a
part of the solution was taken into a quartz sample cell. In order to
suppress the photodissociation of the cyclohexane and cyclohexene,
the light from a xenon lamp was limited to 400±20 nm in
wavelength with a band-pass filter. Especially, for measuring the
ESR spectra of the cyclohexene solution, oxygen was carefully
removed from the mixed solution of cyclohexene and PBN, and
then a part of the solution was transferred into a quartz sample cell.
After that, a trace amount of the Pd/TiO2 sample was added into
the solution, followed by argon purge. Other procedure and
condition were the same as mentioned above.
4. Experimental Section
4.1. Preparation of metal loaded TiO2 and Al2O3
Anatase TiO2 (JPC-TIO-8, 338 m2 g-1) supplied by Catalysis
Society of Japan was used as the photocatalyst. Metal nanoparticles
were loaded by a photodeposition method as below. TiO2 powder
was suspended in distilled water in a beaker and irradiated by the
light from a 300 W xenon lamp (PE300BUV) without any optical
filters (the intensity was 50 mW cm−2 measured at 360±15 nm) to
clean the catalyst surface for 60 min. After the pre-irradiation, a
stock solution of a metal precursor (PdCl2, Wako 99.0 %, dissolved
in an aqueous solution of HCl, or H2PtCl6, Wako 99.8 %, dissolved in
water) and 20 vol% of methanol as a hole scavenger were added
into the suspension, and then it was stirred for 15 min in the dark,
followed by photoirradiation from the xenon lamp without optical
filters for 60 min. The obtained sample was separated from the
suspension by suction filtration and washed with distilled water.
The prepared sample was dried at 323 K overnight in an electric
oven to obtain the metal loaded photocatalyst, i.e., the M/TiO2
samples. The metal loading amount of Pd or Pt was 0.1 wt%.
4.4. CO adsorption
The size of Pd and Pt nanoparticles loaded on the TiO2 and Al2O3
were measured by using a CO pulse method. The catalyst samples
used was 50 mg. As a pretreatment, metal loaded TiO2 samples
were heated at 473 K for 15 min in a flow of hydrogen (11%) and
argon mixture before measurements, while metal loaded Al2O3
samples were heated at 673 K in the same condition. The CO gas
was detected by GC-TCD (Shimadzu, GC-8A). Metal dispersion and
particle size were calculated from the adsorption amount of CO.
An impregnation method was employed for the preparation of
the metal loaded Al2O3 samples. The Al2O3 spherical granule
donated by Catalysis Society of Japan (JRC-ALO-07, 204 m2 g−1) was
milled with a mortar before use. The Al2O3 powder, distilled water,
and an aqueous solution of the metal precursor mentioned above
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Acknowledgements
This work was supported by JSPS KAKENHI Grant Numbers
25105723, and 16J03722. The authors thanks to Dr. H. Yuzawa
at Institute for Molecular Science for valuable comments.
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Catalysis Science & Technology
Page 10 of 10
View Article Online
DOI: 10.1039/C7CY00365J
Hisao Yoshida, Professor
Graduate School of Human and Environmental Studies, Kyoto University,
Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, JAPAN.
E-mail: yoshida.hisao.2a@kyoto-u.ac.jp
Graphical Abstract
The direct cyanomethylation of aliphatic hydrocarbons proceeds with Pt/TiO2 photocatalyst,
while that of benzene requires Pd/TiO2 hybrid catalyst. (18 words)
•CH2CN
●
•CH2CN
Radical
coupling
Catalysis
Pd
+h+
h+
e−
h
e−
e−
Pt or Pd
TiO2 Photocatalyst
Pd catalyst + TiO2 Photocatalyst