Activated Carbon Supported Ruthenium Nanoparticles Catalyzed Synthesis of Imines from Aerobic Oxidation of Alcohols with Amines

Article abstract of DOI:10.1007/s10562-016-1930-3

Abstract: Imines were synthesized from the cross-coupling of alcohols with amines catalyzed by activated carbon (AC) supported ruthenium nanoparticles under atmospheric molecular oxygen without aid of any additives. The readily prepared catalyst 5%Ru/AC showed good to excellent (yield > 90%) performances in the reaction of aromatic and heterocyclic alcohols with various amines, such as aromatic, aliphatic and heterocyclic amines. This protocol is simple, efficient, and environmentally friendly, and the catalyst can be easily recovered without major ruthenium loss. Graphical Abstract: Imines were synthesized from the cross-coupling of alcohols with amines catalyzed by activated carbon (AC) supported ruthenium nanoparticles under atmospheric molecular oxygen without aid of any additives. This protocol is simple, efficient, and environment friendly, and the readily prepared catalyst 5%Ru/AC showed good to excellent performances in the reaction of aromatic and heterocyclic alcohols with various amines, such as aromatic, and aliphatic amines. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1930-3

Catal Lett
DOI 10.1007/s10562-016-1930-3
Activated Carbon Supported Ruthenium Nanoparticles Catalyzed
Synthesis of Imines from Aerobic Oxidation of Alcohols
with Amines
Yuecheng Zhang1 · Fei Lu1 · Hong-Yu Zhang1 · Jiquan Zhao1
Received: 17 August 2016 / Accepted: 20 November 2016
© Springer Science+Business Media New York 2016
Abstract Imines were synthesized from the cross-cou-
pling of alcohols with amines catalyzed by activated car-
bon (AC) supported ruthenium nanoparticles under atmos-
pheric molecular oxygen without aid of any additives. The
readily prepared catalyst 5%Ru/AC showed good to excel-
lent (yield>90%) performances in the reaction of aromatic
and heterocyclic alcohols with various amines, such as aro-
matic, aliphatic and heterocyclic amines. This protocol is
simple, eicient, and environmentally friendly, and the cat-
alyst can be easily recovered without major ruthenium loss.
Keywords Aerobic oxidation · Ruthenium nanoparticles ·
Alcohols · Amines · Imines
Graphical Abstract Imines were synthesized from the
cross-coupling of alcohols with amines catalyzed by acti-
vated carbon (AC) supported ruthenium nanoparticles
under atmospheric molecular oxygen without aid of any
additives. This protocol is simple, eicient, and environ-
ment friendly, and the readily prepared catalyst 5%Ru/AC
showed good to excellent performances in the reaction of
aromatic and heterocyclic alcohols with various amines,
such as aromatic, and aliphatic amines.
1 Introduction
Imines are very important synthetic intermediates in vari-
ous reactions such as reduction, addition, cycloaddition,
and condensation, and also important compounds employed
in the synthesis of numerous biologically and pharmaceuti-
cally active compounds [1–4]. Due to the need for organic
and pharmaceutical synthesis, exploring new methods for
the synthesis of various imines is still an essential work in
chemical research.
Recently, a number of protocols for the synthesis of
imines have been developed such as oxidation of second-
ary amines [5–8], self-condensation of primary amines
with oxidant [9, 10], and oxidative cross-coupling of
alcohols with amines [11–13]. Among all of them, the
aerobic oxidative cross-coupling of alcohols with amines
to imines is the most attractive, because both alcohols and
amines are widely available, and the protocol is environ-
ment friendly due to water being the only by-product in
principle. Because of their easy separation and recovery,
various heterogeneous catalysts such as Pd [14, 15], Pt
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1930-3) contains supplementary
material, which is available to authorized users.
* Jiquan Zhao
zhaojq@hebut.edu.cn
1
School of Chemical Engineering and Technology,
Hebei University of Technology, Tianjin 300130,
People’s Republic of China
Vol.:(0123456789)
1 3

Y. Zhang et al.
[16], Au [11, 12, 17–20], Ag [21], and Ru [22, 23] based
catalysts were employed to achieve this transformation.
Good to excellent catalytic results have been achieved.
However, some procedures for the preparation of the
catalysts are complicated. And large amount of base
(10–130%) are needed to act as co-catalysts in practices
[15, 17, 20, 23]. Furthermore, high reaction temperature
and oxygen pressure were required to get high yields of
products in some cases.
2.2 Preparation of Ru/AC Catalysts
The x%Ru/AC (x% represents the weight percentage
of ruthenium in the catalyst) catalysts were prepared as
described in the literature [35]. For example, the 5%Ru/C
was prepared as follows: To 300 ml aqueous solution of
RuCl3 (3.3 mmol L−1) containing 15 ml of isopropanol
(for well dispersion of AC) was added aqueous solution of
NaOH (0.1 mol L−1) to adjust the pH of 7, then 1.90 g of
AC was added. The temperature of suspension was con-
trolled in a water bath at 25°C. To the suspension was
added 100 ml of NaBH4 aqueous solution (10 mmol) whose
temperature was also controlled to reduce the Ru precursor.
After stirring for 0.5 h, the suspension was iltered and the
residue was washed with 200 ml of water.
Some transition metal salts or complexes [24–28] were
also found to be active in the oxidative cross-coupling of
alcohols with amines to imines with molecular oxygen as
the terminal oxidant under mild conditions. However, addi-
tional catalytic or stoichiometric amounts of ligands, addi-
tives, and bases were generally required, which led to dif-
iculties in purifying the products.
The 3%Ru/AC, 7%Ru/AC, 10%Ru/AC catalysts were
prepared as the above procedures just by changing the acti-
vated carbon amounts to 3.33, 1.33 and 0.90 g, respectively.
As an important industrial carbonaceous material, acti-
vated carbon (AC) has many advantages, such as high
surface area, well-developed pore structure and low cost,
which make it an ideal catalyst support. On the other hand,
AC supported Ru(0) catalysts are readily prepared com-
pared to other heterogeneous catalysts, and very easily sep-
arated from the products in applications. Up till now, they
have been widely used in various reactions, such as hydro-
genation [29–31], oxidation [32], ammonia synthesis [33],
ammonia decomposition [34], but its application in the
aerobic oxidative cross-coupling of alcohols with amines
to imines has not been excavated to our best knowledge.
When a catalyst of ruthenium supported on AC (Ru/AC)
was tried in the aerobic oxidative cross-coupling of benzyl
alcohol with aniline to the corresponding imine, we found
that the reaction proceeded smoothly without addition of
any additives or promoters. Further investigation conirmed
this catalyst could tolerate various substrates and aford
diverse types of substituted imines in good to excellent
yields under mild conditions. Herein, we report the results.
2.3 Characterization of Catalysts
The metal content of the catalyst samples was determined
by a Perkin Elmer Opima 7300 V type ICP-AES instru-
ment. Before analysis, the sample was dried in vacuo at
60°C for 12 h and the accurately weighed sample (about
10 mg) was carefully added in a lask, and then 10 ml nitric
acid and 7.5 ml perchloric acid were added. The suspension
was heated at 120°C for 2 h to dissolve the sample, and the
obtained solution was subjected to analysis.
The X-ray difraction (XRD) patterns of the samples
were recorded with a Bruker AXS GMBH D8 FOCUS
X-ray difractometer using Cu Kα radiation (40 kV, 40 mA)
in the range 2θ=5°–90° with a scanning rate of 6° min−1
.
Transmission electron microscopy (TEM) was per-
formed using a JEM 2100F instrument which was equipped
with an energy dispersive X-ray (EDX) detector at an accel-
erating voltage of 200 kV. The specimens for TEM analy-
sis were prepared by ultrasonic dispersion in ethanol, and
then a drop of the resultant suspension was evaporated on a
lacey carbon/Cu grid.
2 Experimental
The nitrogen adsorption–desorption was carried out on a
Micromeritics ASAP 2020 M+C Surface Area and Poros-
ity Analyzer. Before analysis, the accurately weighed sam-
ple was carefully added in the tube equipped, and heated
at 150°C for 4 h in vacuo to remove the impurities of the
surface. Then the surface areas, total pore volumes and
pore sizes of the samples were measured at 77 K using the
instrument.
2.1 Materials and Apparatus
The AC was obtained from Arkema France, Co., Ltd.
RuCl3·3H2O was supplied by Xi’An Catalyst Chemical
Co., Ltd. The alcohols and amines were obtained from Alfa
Aesar China (Tianjin) Co., Ltd. All reagents were used as
received without further puriication.
1H MNR spectra of imines were recorded with TMS
as internal standard on a Bruker AC-P 400 spectrometer.
Oxidation reaction samples were analyzed on a Shandong
Lunan Ruihong Gas Chromatograph (SP-7800A) equipped
with a FID detector and a SE 54 column (30 m×0.5 μm).
2.4 Catalytic Oxidation
In a typical process, into a 10 ml two-necked round-bot-
tom lask equipped with a magnetic stirrer were added
1 3

Activated Carbon Supported Ruthenium Nanoparticles Catalyzed Synthesis of Imines from Aerobic…
5%Ru/AC (0.025 mmol, 5 mol%, 50 mg), benzyl alcohol
(0.5 mmol, 54 mg), aniline (0.55 mmol, 51 mg) and 5 ml
of toluene at room temperature successively. After which
the resulting reaction mixture was heated at 90°C for 15 h
under an oxygen atmosphere. The inal reaction conver-
sion and selectivity towards the corresponding imines
were determined by Gas Chromatograph. After comple-
tion of the reaction, the catalyst was separated by iltration
and washed with toluene. The iltrate was concentrated in
vacuo. Finally, the product was received by column chro-
matography of the reaction mixture on neutral alumina
using hexanes/EtOAc (20:1) as eluent and determined by
1H NMR analysis.
between benzyl alcohol and aniline in toluene. The reac-
tion proceeded smoothly and the major product identiied
was N-(phenyl)benzylamine, which is the target compound.
Then the efect of reaction parameters on the reaction was
investigated to enhance the selectivity towards the target
product. The results are summarized in Table 1. First, the
efect of the molar ratio of benzyl alcohol to aniline was
investigated with 5 mol% of catalyst loading. As shown in
Table 1, when the molar ratio of benzyl alcohol to aniline
was 1:1, the conversion of benzyl alcohol was 97% and the
selectivity of the target product was 94% (Table 1, entry 1).
GC-MS analysis revealed that the only by-product was ben-
zaldehyde. The selectivity increased gradually with chang-
ing the molar ratio of benzyl alcohol to aniline from 1:1 to
1:1.1, but the conversion of benzyl alcohol remained con-
stant (Table 1, entries 1–3). In the case of molar ratio of
1:1.1 both the conversion and the selectivity of the reaction
reached their maxima of 97% and 99% (Table 1, entry 3).
Further increasing the amount of aniline led to the decrease
of the conversion. For instance, the conversion was only
95% when the molar ratio of benzyl alcohol to aniline is
3 Results and Discussion
3.1 Catalytic Activity
In our initial studies, the heterogeneous 5%Ru/AC cata-
lyst was tested in the aerobic oxidative cross-coupling
Table 1 Optimization of the
reaction conditions
NH2
u
R
AC
/
H
O
N
+
+
2H2O
O2
B
A
a
Entry
Catalyst
(mol%)
nA/nB
1:1
Temp. (˚C)
Solvent
Time (h)
Conv.b (%)
Select.b (%)
1
5
5
5
5
–
5
5
5
5
5
5
4
3
2
5
90
90
90
90
90
80
100
110
90
90
90
90
90
90
90
Toluene
15
15
15
15
15
15
15
12
15
15
24
15
15
15
15
97
97
97
95
<1
87
97
98
92
69
37
85
63
27
63
94
2
1:1.05
1:1.1
1:1.2
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
Toluene
97
3
Toluene
>99
>99
–
4
Toluene
5
Toluene
6
Toluene
>99
>99
99
7
Toluene
8
Toluene
9
Benzotriluoride
1,4-Dioxane
Toluene
>99
93
10
11c
12
13
14
15d
–
Toluene
96
Toluene
95
Toluene
95
Toluene
96
Conditions benzyl alcohol (0.5 mmol), aniline (0.55 mmol), 5%Ru/AC (5 mol%), solvent (5 ml), oxygen
balloon
anA/nB is the molar ratio of benzyl alcohol to aniline
bConversation and selectivity were determined by GC based on area%
cUsing air as oxidant
dRecycle of 5%Ru/AC
1 3

Y. Zhang et al.
1:1.2 (Table 1, entry 4). Much excess of aniline will retard
the conversion of benzyl alcohol to bezaldehyde, though
it can drive the condensation of bezaldehyde with aniline
to N-(phenyl)benzylamine. Generally, increasing reaction
temperature can accelerate the reaction, and increase the
yield of N-(phenyl)benzylamine in the temperature range of
80–110°C (Table 1, entries 3, 6–8).
increase of ruthenium content. The 5%Ru/AC showed best
among all the evaluated catalysts. In the case of 5%Ru/AC
as catalyst with 5 mol% loading, the conversion of benzyl
alcohol reached up to 97% with a selectivity of the target
product of 99% (Table 1, entry 2). From the above results
we concluded that the optimal parameters for the aerobic
oxidative cross-coupling of benzyl alcohol with aniline to
N-(phenyl)benzylamine were: 5%Ru/AC, 5 mol%; molar
ratio of benzyl alcohol to aniline, 1:1.1; solvent, toluene;
reaction temperature, 90°C; oxygen balloon.
Control experiments were performed to evaluate the
necessity of the catalyst in the reaction of aniline with ben-
zyl alcohol. As shown in Table 1, the use of the catalyst is
essential. Less than 1% of benzyl alcohol was converted in
15 h when no catalyst is added (Table 1, entry 5). Increas-
ing catalyst loading led to fast reaction, both the conver-
sion of benzyl alcohol and the selectivity of target product
increased in the same reaction time (Table 1, entries 3,
12–13). When the 5%Ru/AC loading increased from 2 to
5 mol%, the conversion of benzyl alcohol increased from
27 to 97%, meanwhile, the selectivity of target product
increased from 95 to 97%.
To demonstrate the versatility of the 5%Ru/AC, the oxi-
dative cross-coupling of various alcohols with amines to
the corresponding imines was explored under the optimized
reaction conditions, and the results are shown in Table 3.
As shown in Table 3, various aromatic alcohols reacted
with diferent types of anilines to give the desired products
in excellent yields (GC), and the target products were sepa-
1
rated and characterized by H NMR. The electronic efect
on the aromatic ring is not signiicant. Benzyl alcohol
reacted with anilines with both electron-withdrawing and
electron-donating groups para or meta to amino group to
give the corresponding imines in good to excellent yields
under the optimized conditions (Table 3, entries 1–5, 7–8).
Similarly, aniline reacted with benzyl alcohols with both
electron-withdrawing and electron-donating groups para or
meta to hydroxymethyl group also aford the target prod-
ucts in very high yields (Table 3, entries 1, 15–19). The
reactants with an o-substituent showed poor reactivity com-
pared to those with an m- or p-substituent due to the steric
hindrance (Table 3, entries 4–6, 18–20). For instance, more
reaction time was required to get high yield of the imine in
the case of o-chloroaniline or o-luorobenzyl alcohol as a
reactant (Table 3, entry 6, 20). Furfuryl alcohol, one rep-
resentative of heterocyclic alcohols, reacted smoothly with
aniline to give the corresponding imine, too. The yield of
the imine reached 90% in 36 h (Table 3, entry 21).
The reaction was also run in benzotriluoride and 1,
4-dioxane, respectively. It was found that the reaction pro-
ceeded readily in benzotriluoride, but slightly slower than
in toluene (Table 1, entries 3, 9). However, both the con-
version and selectivity of the reaction were smaller in 1,
4-dioxane than in toluene (Table 1, entries 3, 10). In com-
parison, toluene is the best one among the three tested sol-
vents. Air as oxidant was also tested in the reaction, but the
reaction underwent poorly. No target product was detected
and the conversion of benzyl alcohol was only 37% after
24 h (Table 1, entry 11).
The efect on the reaction of the ruthenium content in
the catalyst was evaluated and the results are listed in
Table 2. In the cases of ruthenium content less than 5%,
the activities of the catalysts increased with the ruthenium
content. When the ruthenium content was higher than 5%,
the activities of the catalysts decreased gradually with the
Table 2 Evaluation of the
catalysts
NH2
H
O
u
R A
/ C
N
+
+
2H2O
O2
B
A
Entry
Catalyst
Time (h)
Conv.a (%)
Select.a (%)
1
2
3
4
3%Ru/AC
5%Ru/AC
7%Ru/AC
10%Ru/AC
15
15
24
24
95
97
84
70
98
99
98
98
Conditions benzyl alcohol (0.5 mmol), aniline (0.55 mmol), catalyst (5 mol%), toluene (5 ml), reaction
temperature (90°C), oxygen balloon
aConversation and selectivity was determined by GC based on area%
1 3

Activated Carbon Supported Ruthenium Nanoparticles Catalyzed Synthesis of Imines from Aerobic…
Table 3 Coupling of amines with alcohols
u
R A
/ C O
5
%
,
R2
2
+
R1
R2
H
O
R1
N
NH2
A
B
Time
(h)
Conv. a
(%)
Yield a, b
(%)
Entry
Alcohol
Amine
Imine
NH2
N
1
2
15
30
36
33
25
36
25
27
26
36
36
36
36
25
97
98
96(81)
96(85)
(74)
NH2
N
NH2
N
3
-
O
O
NH2
N
4
95
87(70)
92(80)
87(60)
94(71)
93(54)
95(73)
77(44)
94(77)
97(80)
87(53)
98(78)
Cl
l
l
C
N
C
l
NH2
C
5
99
NH2
N
6
99
Cl
Cl
NH2
N
7
99
OH
NH2
N
8
97
r
B
r
B
NH2
N
9
97(98)
94
NH2
N
10
11
12c
13
14c
NH2
N
N
95
NH2
98
N
NH2
96
N
99
NH2
OH
N
15
32
99
97(75)
The catalyst also showed good performance in the
oxidative cross-coupling of benzyl alcohol with vari-
ous aliphatic amines including both acyclic and cyclic
ones. (Table 3, entries 9–14). However, poor results
were received when the catalyst was applied to the cross-
coupling of aliphatic alcohols with aniline to imines
1 3

Y. Zhang et al.
Table 3 (continued)
Time
(h)
Conv. a
(%)
Yield a, b
Entry
Alcohol
Amine
Imine
(%)
N
O
O
16
17
18
19
36
36
33
32
99
96
99
99
93(71)
OH
N
NH2
Cl
92(79)
95(80)
97(82)
Cl
OH
N
F
F
F
F
OH
N
N
OH
20
21
36
36
36
36
36
18
96
98
25
41
37
98
94(70)
90(73)
24
F
F
O
N
N
N
O
OH
OH
22d
OH
23d, e
24d, e
25f
41(20)
37
OH
N
NH2
N
OH
95
Conditions alcohol (0.5 mmol), amine (0.55 mmol), 5%Ru/AC (5 mol%), toluene (5 ml), reaction temperature 90°C, oxygen balloon
aConversation and yield were determined by GC based on area%
bThe data in the parentheses is the isolated yields
cAmine 0.8 mmol
dReaction temperature 120°C
e5%Ru/AC 7.5 mol%
fbenzyl alcohol (10 mmol), amine (11 mmol), 5%Ru/AC (5 mol%), toluene (50 ml)
(Table 3, entries 22–24). Though increasing catalyst
loading and reaction temperature could improve the reac-
tion in some degree, the yield was still very low (Table 3,
entries 22, 24). The poor results can be ascribed to the
diiculty of oxidation of non-activated aliphatic alco-
hols to aldehydes. Therefore, it can be concluded that the
conversion of alcohols to aldehydes is the rate and yield
determining step in the cross-coupling of alcohols with
amines to imines. To demonstrate practicality of this
methodology, a multigram scale reaction was carried out
in the aerobic oxidative cross-coupling of benzyl alcohol
with aniline. Similar results were received when the load-
ing amount of benzyl alcohol was increased to 10 mmol
(Table 3, entries 1, 25).
1 3

Activated Carbon Supported Ruthenium Nanoparticles Catalyzed Synthesis of Imines from Aerobic…
Carbon 002
absorption
desorption
a:
b: 5% Ru/C
C
d
Carbon 100
c: 10% Ru/C
d: used 5% Ru/C
a
c
b
c
b
a
d
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
20
40
60
80
2θ
Fig. 1 Adsorption–desorption isotherms of catalyst samples. a AC, b
5%Ru/AC, c 10%Ru/AC, d 5%Ru/AC (used)
Fig. 2 XRD patterns of the catalyst samples. a AC, b 5%Ru/AC, c
10%Ru/AC, d 5%Ru/AC (used)
3.2 Characterization of Catalysts
The catalysts were characterized by XRD, TEM, and N2
adsorption–desorption analysis. The speciic surface areas
and porous structure of the AC, 5%Ru/AC, 10%Ru/AC, and
the used 5%Ru/AC were analyzed by N2 adsorption–des-
orption. The adsorption–desorption isotherms are shown
in Fig. 1. All of the samples exhibited a type IV Langmuir
isotherm according to the IUPAC classiication, which
indicated the existence of mesopores in the catalysts.
due to the deposition of some impurities in the pores of AC
during the catalytic run. However, these changes were not
too big, which can not be ascribed to the main reason lead-
ing to the deactivation of the recovered catalyst.
The XRD patterns of the AC, 5%Ru/AC, 10%Ru/AC
and the used 5%Ru/AC samples are shown in Fig. 2. Only
the difraction peaks of carbon (002) (2θ=26.5°), carbon
(100) (2θ=44.5°) were observed for these samples. The
difraction peaks for ruthenium species were not observed,
which indicated that Ru was inely dispersed on the AC
supports [36, 37].
The textural properties of the AC, 5%Ru/AC, 10%Ru/
AC, and the used 5%Ru/AC are summarized in Table 4.
The 5% Ru/AC and 10% Ru/AC showed a slightly lower
surface area compared to the activated carbon, and the
decrease value further decreased with the increasing of Ru
loading, indicating deposition of Ru particles in the pores
of AC. On the contrary, a small increase of pore diameter
was observed with the loading of Ru, possibly due to the Ru
crystallites plug the smaller pore of AC. Compared to the
fresh 5%Ru/AC, the used one exhibited decrease of surface
area and pore volume, and an increase of pore diameter,
The TEM images of the samples of 5%Ru/AC, 10%Ru/
AC and the used 5%Ru/AC are shown in Fig. 3 and the
ruthenium content of the catalyst samples determined by
ICP-AES and EDS are shown in Table S1, which directly
gave us the evidence of the dimensional and dispersion
information about the Ru particles in the catalysts. The
TEM images of the catalysts revealed that Ru particles
well dispersed on the surface of the pores. The Ru par-
ticle sizes in the 5%Ru/AC and 10%Ru/AC are about 1.1
and 1.9 nm, respectively, conirming that the Ru nano-
particles are of nanometer scale [38]. Besides, the Ru
particle sizes of the catalysts increased with the content
of Ru in the catalysts, from which we can conclude that
the particle sizes in the 3%Ru/AC are smaller than those
in the 5%Ru/AC. In the catalytic results we found that
5%Ru/AC performed best among the catalysts. Therefore,
we can say that the Ru nanoparticle sizes either larger or
smaller than 1.1 nm will lead to activity decrease of the
catalysts in the cross-coupling of alcohols with amines to
the corresponding imines. In other words, the catalysis
of the catalysts is very sensitive to the Ru nanoparticle
Table 4 Textural properties of the AC, the 5% Ru/AC, 10% Ru/AC,
5% Ru/AC (used)
Catalyst
SBETa (m2g−1
)
Vb (cm3g−1
)
dpc (nm)
AC
764
712
626
668
0.629
0.592
0.560
0.537
4.40
4.47
4.83
4.50
5%Ru/AC
5%Ru/AC (used)
10%Ru/AC
aBET surface area
bBJH cumulative desorption pore volume
cMean pore diameter=4 V/SBET
1 3

Y. Zhang et al.
Fig. 3 TEM images of the
catalyst samples a 5%Ru/AC, b
10%Ru/AC, c 5%Ru/AC (used)
and Ru particle size distribution
of them
a
5% Ru/C
1.1 0.2 nm
40
30
20
10
0
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Particle size (nm)
b
1.9 0.2nm
10% Ru/C
40
30
20
10
0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
Particles size (nm)
c
30
25
20
15
10
5
Used 5% Ru/AC
2.8+0.8nm
0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Particle size(nm)
sizes. The Ru nanoparticle sizes in the used 5%Ru/AC
is 2–5 nm, larger than those in the fresh one, indicating
the Ru nanoparticles aggregated during the catalytic run.
Besides, the leaking of Ru was smaller than 0.7% deter-
mined by ICP, but the activity of the recovered 5%Ru/
AC was only 63% of that of the fresh one. Therefore,
the aggregation of Ru nanoparticles during the catalytic
run is the main reason causing the deactivation of the
catalyst. The easy aggregation of Ru nanoparticles in the
5%Ru/AC catalyst is due to the weak ruthenium-carbon
interactions. Generally, strong metal-support interactions
can inhibit the aggregation or sintering of metal nanopar-
ticles [39].
4 Conclusions
In conclusion, Ru supported on AC as catalyst is very
eicient in the oxidative crossing-coupling of alco-
hols with anilines to the corresponding imines with O2
as oxidant without need of any additives. This protocol
can tolerate a wide range of substrates. Higher than 90%
yields (determined by GC) of imines were obtained in the
1 3

Activated Carbon Supported Ruthenium Nanoparticles Catalyzed Synthesis of Imines from Aerobic…
14. Kwon MS, Kim S, Park S, Bosco W, Chidrala RK, Park J (2009)
J Org Chem 74:2877–2879
cross-coupling of benzyl alcohols with various amines
including both acyclic and cyclic ones. The catalyst was
very readily prepared and recovered after reaction. No
obvious leaking of Ru took place during the catalytic run.
The characterization results revealed the elemental Ru in
5%Ru/AC is well dispersed, and presents as nanoparticles
with a diameter about 1.1 nm. Characterization results
also disclosed that the catalysis of the Ru/AC catalysts was
very sensitive to the Ru nanoparticle sizes in the catalysts.
Aggregation of Ru nanoparticles due to weak Ru–AC inter-
actions is the main reason leading to the deactivation of the
catalyst.
15. Cui W, Zhaorigetu B, Jia M, Ao W, Zhu H (2014) RSC Adv
4:2601–2604
16. He W, Wang LD, Sun CL, Wu KK, He SB, Chen JP, Wu P, Yu
ZK (2011) Chem Eur J 17:13308–13317
17. Soule′ JF, Miyamura H, Kobayashi S (2013) Chem Commun
49:355–357
18. Liu P, Li C, Hensen EJM (2012) Chem Eur J 18:12122–12129
19. Cui W, Xiao Q, Sarinac S, Ao W, Xie M, Zhu H, Bao Z (2014)
Catal Today 235:152–159
20. Zhang L, Wang W, Wang A, Cui Y, Yang X, Huang Y, Liu X, Liu
W, Son JY, Oji H, Zhang T (2013) Green Chem 15:2680–2684
21. Mielby J, Poreddy R, Engelbrekt C, Kegnæs S (2014) Chin J
Catal 35:670–676
22. Kim JW, He J, Yamaguchi K, Mizuno N (2009) Chem Lett
38:920–921
23. Cano R, Ramon DJ, Yus M (2011) J Org Chem 76:5547–5557
24. Zhang E, Tian H, Xu S, Yu X, Xu Q (2013) Org Lett
15:2704–2707
25. Kang Q, Zhang Y (2012) Green Chem 14:1016–1019.
26. Tian H, Yu X, Li Q, Wang J, Xu Q (2012) Adv Synth Catal
354:2671–2677
Acknowledgements The authors are grateful for inancial sup-
ports from the National Natural Science Foundation of China (No.
21276061), Natural Science Foundation of Hebei Province, China
(No. B2013202158), and Research Fund for the Doctoral Program of
Higher Education of China (No. 20121317110010).
27. Jiang L, Jin L, Tian H, Yuan X, Yu X, Xu Q (2011) Chem Com-
mun 47:10833–10835
References
28. Gnanaprakasam B, Zhang J, Milstein D (2010) Angew Chem Int
Ed 49:1468–1471
1. Murahashi SI (1995) Angew Chem Int Ed Engl 34:2443–2465
2. Kobayashi S, Mori Y, Fossey JS, Salter MM (2011) Chem Rev
111:2626–2704
3. Adams JP (2000) J Chem Soc Perkin Trans 1:125–139.
4. Gawronski J, Wascinska N, Gajewy J (2008) Chem Rev
108:5227–5252
5. Zhu B, Angelici RJ (2007) Chem Commun 2157–2159
6. Zhu B, Lazar M, Trewyn BG, Angelici RJ (2008) J Catal
260:1–6
7. Furukawa S, Suga A, Komatsu T (2014) Chem Commun
50:3277–3280
8. Furukawa S, Suga A, Komatsu T (2015) ACS Catal 5:1214–1222
9. Patil RD, Adimurthy S (2011) Adv Synth Catal 353:1695–1700
10. Chen B, Wang L, Gao S (2015) ACS Catal 5:5851–5876.
11. Sun H, Su FZ, Ni J, Cao Y, He HY, Fan KN (2009) Angew
Chem Int Ed 48:4390–4393
29. Gallezot P, Nicolaus N (1998) J Catal 180:51–55.
30. Heinen AW, Peters JA (2000) Carbohydr Res 328:449–457.
31. Hofer BW, Crezee E, Mooijman PRM, Langeveld van AD,
Kapteijn F, Moulijn JA (2003) Catal Today 79–80:35–41
32. Mori S, Takubo M, Makida K, Yanase T, Aoyagi S, Maegawa T,
Monguchi Y, Sajiki H (2009) Chem Commun 5159–5161
33. Rossetti I, Forni L (2005) Appl Catal A 282:315–320
34. Li L, Zhu ZH, Yan ZF, Lu GQ, Rintoul L (2007) Appl Catal A
320:166–172
35. Ohyama J, Sato T, Arai S, Satsuma A (2013) J Am Chem Soc
135:8016–8021
36. Huang Y, Ma Y, Cheng Y, Wang L, Li X (2015) Catal Commun
72:20–23
37. Zhu L, Zheng L, Du K, Fu H, Li Y, You G, Chen B (2013) RSC
Adv 3:713–719
38. Yang X, Wang X, Qiu J (2010) Appl Catal A 382:131–137
39. Zheng T, He J, Zhao Y, Xia W, He J (2014) J Rare Earths 32:
97–107
12. Kegnæs S, Mielby J, Mentzel UV, Christensen CH, Riisager A
(2010) Green Chem 12:1437–1441
13. Sindhuja E, Ramesh R (2014) Tetrahedron Lett 55:5504–5507
1 3