Photocatalytic secondary amine synthesis from azobenzenes and alcohols on TiO2 loaded with Pd nanoparticles

Article abstract of DOI:10.1039/c5nj00158g

Photoirradiation (λ > 300 nm) of TiO₂ loaded with Pd nanoparticles (ca. 2 wt%, 5 nm diameter) in water containing alcohols and azobenzene derivatives at room temperature successfully produces the corresponding secondary amines with high yields.

Full text of DOI:10.1039/c5nj00158g

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Journal Name
RSCPublishing
ARTICLE
Photocatalytic secondary amine synthesis from
azobenzenes and alcohols on TiO2 loaded with
Pd nanoparticles†
Cite this: DOI: 10.1039/x0xx00000x
Kaliyamoorthy Selvam,a Hirokatsu Sakamoto,a Yasuhiro Shiraishi*a,b and
Takayuki Hiraia
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Photoirradiation (λ >300 nm) of TiO2 loaded with Pd nanoparticles (ca. 2 wt %, 5 nm diameter) in water
www.rsc.org/
containing alcohols and azobenzene derivatives at room temperature successfully produces the
corresponding secondary amines with high yields. This is facilitated via consecutive photocatalytic and
catalytic reactions: (i) photocatalytic oxidation of alcohols (aldehydes formation) and reduction of
azobenzenes (anilines formation); (ii) catalytic condensation of the formed aldehydes with anilines on the
TiO2 surface (imines formation); and, (iii) photocatalytic hydrogenation of the formed imines (secondary
amines formation). This catalytic system successfully produces several kinds of secondary amines with
>80% yields.
Several heterogeneous catalytic systems have been proposed
with PdOx particles supported on Fe2O3,9 Pd particles supported
Introduction
Azobenzene and its derivatives are very important chemicals in
application to dyes, indicators, optical and electronic materials.1
They have tremendous carcinogenic risks.2 Decomposition
process of these compounds in industrial wastes is therefore
very important for the protection of environment. Several
catalytic or photocatalytic decomposition processes have been
proposed.3 Recently, as an alternative to the decomposition
processes, selective transformation of azobenzenes to valuable
chemicals and the reuse of the products for upstream processes
have attracted a great deal of attention from the viewpoint of
green and sustainable chemistry.4 Catalytic hydrogenation of
azobenzenes to anilines have mainly been studied.5 All of these
systems selectively produce anilines, but require expensive
reducing agents such as HCO2NH2 or relatively high reaction
temperatures (>363 K).
on boehmite nanofibers,10 Ag clusters supported on Al2O3,11
Ru(OH)x supported on Al2O3,12 Au particles supported on
TiO2,13 and Pd particles supported on MgO.14 All of these
systems selectively produce secondary amines, but require
relatively high reaction temperatures (>363 K).
Secondary amines are very important intermediates for the
synthesis of agricultural chemicals and pharmaceuticals.6 They
are usually synthesized by Nꢀalkylation of primary amines with
alkyl halides; however, this needs excess amount of inorganic
bases with a concomitant formation of copious amount of
wastes.7 An alternative clean way for the secondary amine
synthesis is the use of alcohols as alkylating agents with metal
catalysts.8 These systems promote consecutive catalytic steps in
one pot: (a) dehydrogenation of alcohol (aldehyde and H–metal
species formation); (b) condensation of the formed aldehyde
with amine (imine formation); and, (c) hydrogenation of the
imine by the H–metal species (secondary amine formation).
Scheme 1. Proposed mechanism for secondary amine production from alcohol
and azobenzene on a Pd/TiO2 catalyst under UV irradiation.
Herein, we report for the first time a catalytic system that
produces secondary amines from alcohols and azobenzenes at
room temperature. We use TiO2 loaded with Pd nanoparticles
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(Pd/TiO2) as a catalyst under UV irradiation (λ>300 nm). This ꢁmol), and benzyl alcohol (0.5 mL) under N2 atmosphere at
system promotes consecutive catalytic steps in one pot, as 303 K. Table 1 summarizes the conversions of azobenzene and
depicted in Scheme 1: (i) photocatalytic oxidation of alcohol the yields of products obtained by 4 h reaction. As shown by
(aldehyde formation) and reduction of azobenzene (aniline entry 1, the absence of catalyst scarcely promotes reaction.
formation); (ii) catalytic condensation of the formed aldehyde Addition of bare TiO2 (entry 2) produces very small amounts of
with aniline on the TiO2 surface (imine formation); and, (iii) hydrazobenzene (
2) and aniline (4). In contrast, as shown by
photocatalytic hydrogenation of the formed imine (secondary entries 3–6, Pdx/TiO2 catalysts produce the secondary amine
amine formation). The system successfully produces several
kinds of secondary amines with very high yields (>80%).
(
N
ꢀbenzylaniline,
1
) with high yields. Among them, Pd2/TiO2
with almost
(entry 5) shows the highest activity, producing
1
quantitative yield (98%). It is noted that the recovered Pd2/TiO2
catalyst, when reused for further photoreaction, shows almost
the same activity as the fresh one. This suggests that the
catalyst is reusable without the loss of activity and selectivity.
As shown by entries 7 and 8, other metal particles such as Au
Results and Discussion
Preparation and characterization of catalysts
The Pdx/TiO2 catalysts with different Pd loadings [
x (wt %) =
Pd/TiO2 × 100; = 0.5, 1, 2 and 4] were prepared by
x
and Pt, when loaded on TiO2, produce
1 with very low yields
impregnation of Pd(NO3)2 onto P25 TiO2 (anatase/rutile ratio =
~80/20; average particle size, 24 nm; BET surface area, 59 m2
g–1) followed by reduction with H2.15 As shown in Fig. S1
(ESI†), transmission electron microscopy (TEM) images of the
catalysts show reveal that all of the catalysts possess spherical
Pd particles. As summarized in Table 1, the size of Pd particles
(≤6%). These findings clearly suggest that the Pd/TiO2 catalyst
specifically promotes the formation of secondary amine from
alcohol and azobenzene.
Reaction mechanism
Secondary amine (1) is produced by tandem photocatalytic and
catalytic actions on Pd/TiO2 (Scheme 1): photoexcitation of
TiO2 produces the electron (e–) and positive hole (h+) pairs.
increases with the Pd loadings: average diameters for x = 0.5, 1,
2, and 4 catalysts were 2.6, 4.1, 5.0, and 6.5 nm, respectively.
As shown in Fig. S2 (ESI†), diffuseꢀreflectance UVꢀvis spectra
of the catalysts reveal that all of them show characteristic flat
TiO2 + h
ν
→ e– + h+
(1)
absorption at
particles.16
λ >300 nm due to the light scattering by the Pd
The h+ oxidize alcohol and produces aldehyde and H+.17
R1–CH2OH + 2h+ → R1–CHO + 2H+
(2)
Photocatalytic activity
The e– formed on the TiO2 conduction band are transferred to
the Pd particles. They reduce H+ and produce H atoms on the
particle surface (H–Pd species).18
Efficacy of Pd/TiO2 catalysts for secondary amine synthesis is
confirmed by the reaction of azobenzene and benzyl alcohol.
The reaction was performed by photoirradiation (
λ >300 nm) of
water (4.5 mL) containing catalyst (10 mg), azobenzene (12.5
Table 1. Results for photoreaction of azobenzene and benzyl alcohol on the respective catalystsa
Product yields / %
3 c
diameter of metal
particles / nm b
azobenzene
conv. / %
entry
catalyst
1 c
2 d
4 c
none
TiO2
0
0
0
0
7
0
0
0
8
1
2
3
4
5
15
Pd0.5/TiO2
Pd1/TiO2
Pd2/TiO2
reuse
2.6 ± 0.5
4.1 ± 0.9
5.0 ± 0.9
94
47
85
98
98
68
2
27
10
0
10
5
10
0
>99
>99
>99
>99
25
0
0
0
0
0
Pd4/TiO2
Au2/TiO2
Pt2/TiO2
6.5 ± 1.3
3.7 ± 0.5
3.1 ± 0.8
0
20
4
12
4
6
7
8
12
30
95
6
24
30
a Photoirradiation was carried out by a Xe lamp (500 W; light intensity at 300–400 nm, 27.2 W m–2). b Determined by TEM observations. TEM images of the
catalysts and size distribution of metal particles are summarized in Fig. S1 (ESI†). c = [product formed (ꢁmol)] / [initial amount of azobenzene (12.5 ꢁmol) ×
2] × 100. d = [product formed (ꢁmol)] / [initial amount of azobenzene (12.5 ꢁmol)] × 100.
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5NJ00158G
H+ + e− + Pd → H–Pd
Azobenzene is reduced to two equivalents of anilines (
(3)
Fig. S3 (ESI†) summarizes the timeꢀprofiles for the amounts of
substrate and products during photoreaction on the respective
catalysts. These data clearly reveal that the decreasing rates for
4
) by the
surface H–Pd species,19 via the formation of a hydrazobenzene
the amounts of substrate azobenzene and all intermediates (
2, 3,
(
2
) intermediate.
and ) on the Pd0.5, Pd1, and Pd4 catalysts are much lower than
4
those on Pd2/TiO2 (Fig. 1). This suggests that slower reductions
on these catalysts result in lower activity for secondary amine
formation. As shown by eqs. 4, 5, and 7, the reduction reactions
are promoted by the H–Pd species; therefore, the lower activity
of these catalysts is probably due to the smaller number of H–
Pd species formed. As shown by eqs. 2 and 3, the H–Pd species
are produced via the oxidation of alcohol, and the number of
H–Pd species depends on the amount of alcohol oxidized. To
roughly determine the number of H–Pd species produced, a
benzyl alcohol solution was photoirradiated with respective
R2–N=N–R2 + 2H–Pd → R2–NH–NH–R2 (2) + 2Pd
R2–NH–NH–R2 (2) + 2H–Pd → 2R2–NH2 (4) + 2Pd
(4)
(5)
Catalytic condensation of the formed aldehyde with aniline by
the Lewis acid sites on the TiO2 surface20 produces imine (
).
3

←
→
R1–CHO + R2–NH2
R1–CH=N–R2 (3) + H2O
(6)
The imine is hydrogenated by the H−Pd species to the product
secondary amine ( ).
1
catalysts for
4 h without azobenzene. The amount of
R1–CH=N–R2 (3) + 2H–Pd → R1–CH2–NH–R2 (1) + 2Pd (7)
benzaldehyde formed is summarized in Fig. 2. The Pd2/TiO2
catalyst produces the highest amount of aldehyde, and the
tendency for aldehyde formation is consistent with the activity
for secondary amine formation (Table 1, entries 3–6). These
data clearly suggests that Pd2/TiO2 produces the largest amount
of H–Pd species and, hence, exhibits the highest activity for
reduction reactions.
Fig. 1 shows the timeꢀprofiles for the amounts of substrate
and products during reaction of azobenzene and benzyl alcohol
on the Pd2/TiO2 catalyst. Decrease in the amount of azobenzene
(black) associates with a formation of secondary amine (
blue) and benzaldehyde (white). Photoreaction for 1.5 h
produces 25 mol , two equivalents of substrate azobenzene
(12.5 mol), indicating that two aniline units of azobenzene are
quantitatively converted to . During reaction, hydrazobenzene
, red), imine ( , green), and aniline (
intermediately. These data suggest that
1,
ꢀ
1
ꢀ
1
(
2
3
4
1
, orange) are produced
is indeed produced via
the above reaction sequence (eqs. 2–7). In addition, prolonged
irradiation (~3 h) does not affect the amount of
1
, suggesting
that further reaction of
1
does not occur.
1
25
20
15
10
5
50
Fig. 2 Amounts of benzaldehyde formed during photoirradiation of a benzyl
alcohol solution with respective catalysts in the absence of azobenzene.
3
The smaller number of H–Pd species on lower Pd loading
catalysts (Pd0.5, Pd1) is probably due to the insufficient charge
separation of e– and h+ pairs, as often observed for related
metal/semiconductor systems.21 In contrast, Pd4/TiO2 possesses
a larger number of Pd atoms, but produces smaller number of
H–Pd species. It is well known that a metal/semiconductor
4
2
0
0
0
3
1
2
t / h
Fig. 1 Time-dependent change in the amounts of substrate and products during
photoreaction of azobenzene and benzyl alcohol on Pd2/TiO2. The reaction
conditions are identical to those in Table 1.
interface creates a Schottky barrier (φB), and its height increases
with an increase in the amount of metal loaded.22 On Pd4/TiO2,
the φB height increased by higher Pd loading may suppress
smooth transfer of the conduction band e– to Pd, probably
resulting in insufficient charge separation. As a result of this,
Pd2/TiO2 with appropriate Pd loading facilitates efficient charge
separation and produces a larger number of H–Pd species, thus
promoting efficient secondary amine formation.
Effect of Pd loadings
As shown in Table 1 (entries 3–6), the Pd amount on TiO2
strongly affects the activity for secondary amine formation.
Pd2/TiO2 shows the highest activity, and the lower (Pd0.5, Pd1)
or higher Pd loading (Pd4) catalysts show decreased activity.
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of 1,1′ꢀazonaphthalene or azobenzenes bearing –CH3, –OCH3,
and –OH groups also gave the products with high yields
(≥80%). In addition, azobenzenes with reducible substituents
such as –CN and –COOH groups (entries 12 and 13) are also
successfully transformed to the secondary amines with high
yields (≥87%) while maintaining the substituents as they are. It
is noted that halogenꢀsubstituted azobenzene is unsuccessful for
secondary amine synthesis: reaction of Cl–substituted
azobenzene (entry 14) mainly gave a dechlorinated product
because the H−Pd species also behave as the active site for
dehalogenation.16 These results suggest that the Pd2/TiO2
catalyst is applicable for synthesis of several kinds of secondary
amines, although some cases are unsuccessful owing to the
subsequent reactions of substituents.
Table 2 Secondary amine synthesis from various alcohols and azobenzenes
(R–N=N–R) on Pd2/TiO2
a,b
t
/ h
yield
/ % c
run
alcohol
R–N=N–R
product
1
2
3
1
99
99
99
1.5
1.5
4
1.5
24
6
98
91
98
94
5 d
6 e
7 f
5
Conclusion
8g
16
2
80
90
We found that photoexcitation of Pd/TiO2 catalyst facilitates
oneꢀpot synthesis of secondary amines from alcohols and
azobenzenes via consecutive photocatalytic and catalytic
actions. As shown in Figure 1, the present system oxidizes large
amount of alcohols because large amount of H atoms of
alcohols are required for multiꢀstep hydrogenation of substrates
and intermediates (azobenzenes and imines). This negative
aspect must be addressed for practical application by
improvement of the process. Nevertheless, the present system
offers several advantages: (i) easily recoverable and recyclable
heterogeneous catalyst; (ii) mild reaction conditions (room
temperature and atmospheric pressure); and, (iii) waste
azobenzenes can be used as resources. The present system
based on the consecutive photocatalytic and catalytic actions,
therefore, have a potential to be a powerful method for oneꢀpot
green synthesis of secondary amines.
9
10
11g
12g
13
4
8
95
90
87
91
10
7
14g
5
38h
Experimental
Materials
a Reaction conditions: azobenzenes (12.5 ꢀmol), alcohols (0.5 mL, water (4.5
mL), catalyst (10 mg), Xe lamp (λ >300 nm), light intensity at 300–400 nm
(27.2 W m–2). b Conversions of azobenzenes in all runs are >99%. c = [product
formed (µmol)] / [initial amount of azobenzene derivatives (ꢁmol) × 2] × 100.
d Azobenzene (62.5 ꢀmol). e Catalyst (5 mg). f Light intensity at 300–400 nm
All of the reagents used were supplied from Wako, Tokyo
Kasei, and SigmaꢀAldrich, and used without further
purification. Water was purified by the Milli Q system. P25
TiO2 (JRCꢀTIOꢀ4) were kindly supplied from the Catalyst
Society of Japan.
g
h
(13.6 W m–2). Catalyst (15 mg). The yield of dehalogenated product (Nꢀ
benzylaniline) is 55%.
Pdx/TiO2 catalysts [x (wt %) = Pd/TiO2 × 100; x = 0.5, 1, 2,
Synthesis of various secondary amines
4] were prepared as follows: TiO2 (1.0 g) and Pd(NO3)2 (10.9,
21.9, 44.2, or 90.2 mg) were added to water (40 mL), and the
solvents were evaporated with vigorous stirring at 353 K for 12
h. The obtained powders were dried at 673 K under air flow
(0.5 L min–1) and then reduced at 673 K under H2 flow (0.2 L
min–1). The heating rate and the holding time at 673 K for these
treatments were 2 K min–1 and 2 h, respectively. Pt2/TiO2 was
prepared in a similar manner to Pdx/TiO2 with H2PtCl6 •6H2O
(54 mg) as a precursor.
The Pd2/TiO2 catalyst is applicable for the synthesis of various
types of secondary amines, as depicted in Table 2. Reactions of
aliphatic alcohols and azobenzene (entries 1–3) produce the
corresponding secondary amines with very high yields (99%),
as is the case for the benzyl alcohol/azobenzene system (entry
4). As shown by entry 5, a large scale reaction also gave the
product with high yield although relatively long reaction time is
required. As shown by entries 6 and 7, decrease in the catalyst
amount and the light intensity also prolongs the reaction time
for secondary amine formation. This indicates that the catalyst
amount and light intensity are very important factors for rapid
secondary amine synthesis. As shown by entries 8–11, reactions
Au2/TiO2 was prepared by
a deposition–precipitation
method as follows:23 HAuCl4 •4H2O (45.8 mg) was added to
water (50 mL). The pH of the solution was adjusted to 7 by an
addition of 1 M NaOH. TiO2 (1.0 g) was added to the solution
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DOI: 10.1039/C5NJ00158G
and stirred vigorously at 353 K for 3 h. The particles were
recovered by centrifugation and washed with water. They were
calcined at 673 K under an air flow (0.5 L min–1).
3
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Photoreaction
Each of the respective catalysts (10 mg) was added to a mixture
of water (4.5 mL) containing alcohol (0.5 mL) and azobenzene
4
5
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(Ed.), Pergamon Press, Oxford 1991, Vol. 8, p. 381.
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N2 gas. The tube was photoirradiated by a Xe lamp (500 W;
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λ >300 nm with magnetic stirring. The
temperature of the solution during photoirradiation was ca. 303
K, and the light intensity was 18.2 W m–2 (at 300–400 nm),
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This work was supported by the Grantꢀin Aid for Scientific
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by the Precursory Research for Embryonic Science and
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J. Name., 2012, 00, 1-3 | 5

New Journal of Chemistry
Page 6 of 7
ARTICLE
VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C5NJ00158G
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6 | J. Name., 2012, 00, 1-3
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Page 7 of 7
New Journal of Chemistry
View Article Online
DOI: 10.1039/C5NJ00158G
Manuscript title: Photocatalytic secondary amine synthesis from azobenzenes and
alcohols on TiO2 loaded with Pd nanoparticles
Authors: Kaliyamoorthy Selvam, Hirokatsu Sakamoto, Yasuhiro Shiraishi,* and Takayuki
Hirai
Graphical Abstract
Photoexcitation of TiO2 loaded with Pd particles produces secondary amines from azobenzenes
and alcohols with almost quantitative yields, via consecutive photocatalytic and catalytic
actions.