Highly efficient one-pot multi-directional selective hydrogenation and N-alkylation catalyzed by Ru/LDH under mild conditions

Article abstract of DOI:10.1016/j.apcata.2020.117536

Atomic economy, non-toxicity, harmlessness and multidirectional selectivity advocated by green chemistry have increasingly become a hot and difficult research topic. Herein, we present a highly efficient, one-pot tandem and easy-to-operate method through which we could directly produce a broad range of multi-directional selective hydrogenated amines or N-alkyl aliphatic amines using aromatic nitro compounds as raw materials. Ru/LDH with characteristics of layered mesoporous structure, well dispersed small Ru nanoparticles and LDH stabilization to the Ru NPs was employed as the catalyst. It is remarkable that multi-directional superb chemoselectivity to aromatic amines, alicyclic amines as well as N-alkyl aliphatic amines could be achieved with excellent catalytic activity and recyclability by tuning reaction conditions over 5wt%Ru/LDH-2. Additionally, this catalytic system also exhibited attractive activity and multi-directional chemoselectivity in the hydrogenation of quinoline and its derivatives with solvents of different polarity. Chemoselectivity to 5,6,7,8-tetrahydroquinoline derivatives could reach as high as 95.6 %.

Full text of DOI:10.1016/j.apcata.2020.117536

Journal Pre-proof
Highly efficient one-pot multi-directional selective hydrogenation and
N-alkylation catalyzed by Ru/LDH under mild conditions
Sishi Zhang (Investigation) (Data curation) (Writing - original draft),
Jie Xu (Methodology), Hongmei Cheng (Validation), Cuicui Zang
(Investigation), Bin Sun (Supervision), Heyan Jiang (Supervision)
(Conceptualization) (Writing - review and editing) (Funding
acquisition), Fengxia Bian (Supervision) (Funding acquisition)
PII:
S0926-860X(20)30129-0
DOI:
https://doi.org/10.1016/j.apcata.2020.117536
APCATA 117536
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
10 January 2020
14 March 2020
17 March 2020
Please cite this article as: Zhang S, Xu J, Cheng H, Zang C, Sun B, Jiang H, Bian F, Highly
efficient one-pot multi-directional selective hydrogenation and N-alkylation catalyzed by
Ru/LDH under mild conditions, Applied Catalysis A, General (2020),
doi: https://doi.org/10.1016/j.apcata.2020.117536

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Highly efficient one-pot multi-directional selective hydrogenation
and N-alkylation catalyzed by Ru/LDH under mild conditions
Sishi Zhanga, Jie Xua, Hongmei Chenga, Cuicui Zanga, Bin Suna, Heyan Jianga*,
Fengxia Biana*
Key Laboratory of Catalysis Science and Technology of Chongqing Education
Commission, Chongqing Key Laboratory of Catalysis and New Environmental
Materials, College of Environmental and Resources, Chongqing Technology and
Business University, Chongqing 400067, China
* Corresponding author. E-mail: orgjiang@163.com.
Graphical Abstract
Ru/LDH catalytic system exhibited excellent switchable chemoselectivity in the
one-pot hydrogenation/N-alkylation of a broad range of aromatic nitro compounds as
well as quinoline and its derivatives under rather mild conditions.
Highlight
 Layered mesoporous Ru/LDH was used in multi-directional hydrogenation &
N-alkylation.
 Hydrogenation and/or N-alkylation of nitrobenzenes by tuning reaction

conditions.
 Selectivity switches in quinolines hydrogenation by tuning reaction
conditions.
 Selectivity to 5,6,7,8-tetrahydroquinoline derivatives could reach as high as
95.6%.
Abstract: Atomic economy, non-toxicity, harmlessness and multidirectional
selectivity advocated by green chemistry have increasingly become a hot and difficult
research topic. Herein, we present a highly efficient, one-pot tandem and
easy-to-operate method through which we could directly produce a broad range of
multi-directional selective hydrogenated amines or N-alkyl aliphatic amines using
aromatic nitro compounds as raw materials. Ru/LDH with characteristics of layered
mesoporous structure, well dispersed small Ru nanoparticles and LDH stabilization to
the Ru NPs was employed as the catalyst. It is remarkable that multi-directional
superb chemoselectivity to aromatic amines, alicyclic amines as well as N-alkyl
aliphatic amines could be achieved with excellent catalytic activity and recyclability
by tuning reaction conditions over 5wt%Ru/LDH-2. Additionally, this catalytic
system also exhibited attractive activity and multi-directional chemoselectivity in the
hydrogenation of quinoline and its derivatives with solvents of different polarity.
Chemoselectivity to 5,6,7,8-tetrahydroquinoline derivatives could reach as high as
95.6%.
Keywords：One-pot tandem; Hydrogenation; N-alkylation; Ruthenium; Layered
double hydroxide
1. Introduction
In order to solve the environmental crisis brought about by traditional
chemical industry and cut down or phase out the adverse raw materials use and
by-products production, it is necessary to develop new one-pot tandem

processes with adjustable chemoselective characteristics to realize atomic
economy and improve the utilization of resources. As essential intermediates
and final products, aromatic amines, alicyclic amines as well as N-alkyl amines
are universally used to synthesize dyes, pharmaceuticals and other chemical
products [1-4]. The traditional synthesis of N-alkyl amines from aromatic nitro
compounds generally consists of two steps: the first step is hydrogenation of
aromatic nitro compounds to aromatic amines or alicyclic amines; the second
step is the N-alkyl reaction of amines with alkylating agents to form N-alkyl
amines [5,6]. However, the traditional alkylating agents in N-alkylation of
amines were alkyl halides, which are toxic and the by-products produced are
harmful to the environment [7-10]. Considering alcohols are cheaper, easier to
obtain and less harmful to the environment in chemical production, developing
and utilizing alcohols as alkylating agents to produce N-alkylated amines under
mild reaction conditions is an attractive research field in green chemistry
[11-14].
Many homogeneous transition metal catalysts, with the assistance of ligands
and/or bases, have been explored in selective hydrogenation and N-alkylation
reaction [15-18]. However, these catalytic systems were more or less difficult to
recycle. Heterogeneous one-pot multi-directional catalytic systems are
fascinating because of their potential in green synthetic applications. Seung
Geun Oh et al.[19] reported temperature controlled one-pot catalytic NO2
reduction, ring hydrogenation, and N-alkylation from aromatic nitro
compounds to generate alicyclic amines catalyzed by Ru/C-NaNO2 with harsh
o
conditions (170 C and 8.3 MPa H2 pressure). Therefore, it is obviously rather
challenging to develop novel heterogeneous catalysts to construct efficient
one-pot reactions for aromatic nitro compounds multi-directional selective
hydrogenation and N-alkylation with different alcohols to prepare more
value-added N-alkylated amines under mild conditions.
Layered double hydroxide (LDH) is a series of two-dimensional layered
compound, which is constituted by positively charged brucite-like layer and

interlayer anions and water molecules [20-24]. LDH as catalytic materials have
been studied widely because of its many excellent properties, such as adjustable
inherent composition, high thermal stability and good metal dispersion etc
[25-29]. Herein, Ru/LDH catalyst with characteristics of layered mesoporous
structure (SEM and BET), well dispersed small Ru NPs (2.5 nm, XRD and
TEM) and stabilization effect of LDH to Ru NPs (XPS) has been successfully
prepared. With this interesting new catalytic material, we discovered a highly
efficient and easy to operate one-pot tandem method by which we could
directly produce a broad range of multi-directional chemoselective
hydrogenated amines or N-alkyl aliphatic amines using aromatic nitro
compounds as the starting material under mild conditions. Multi-directional
excellent chemoselectivity to aromatic amines, alicyclic amines as well as
N-alkyl aliphatic amines could be achieved with attractive catalytic activity and
recyclability in different solvent, including water, ethanol and isopropanol, etc.,
over Ru/LDH catalytic system. It is worth mentioning that the substrate range
of the one-pot tandem hydrogenation N-alkylation catalytic system could be
extended to various aliphatic alcohols. Additionally, this catalytic system also
exhibited attractive activity and multi-directional chemoselectivity in the
hydrogenation of quinoline and its derivatives with solvents of different
polarity. Chemoselectivity to 5,6,7,8-tetrahydroquinoline derivatives could
reach as high as 95.6%.
2. Experimental section
2.1. Materials
All chemical reagents used were of analytical grade. RuCl3·3H2O and 5%Ru/C
were from Acros. NaBH4, urea, Mg(NO3)2·6H2O, Al(NO3)3·9H2O, 4Å MS, γ-Al2O3,
MgO and solvents were purchased from Chengdu Kelong Chemical Reagent Factory,
China. All substrates were purchased from Aladdin Chemical Co. Ltd. All organic
solvents were purified by standard methods. Deionized water was used in the

experiments. 4Å molecular sieve was calcined at 400 oC for 2 h before use. The purity
of hydrogen was over 99.99%.
2.2. Preparation of Mg-Al LDH
Mg-Al LDH was synthesized by hydrothermal method using Mg(NO3)2·6H2O and
Al(NO3)3·9H2O as raw materials and urea as precipitator. Mg(NO3)2·6H2O (23.07 g,
0.09 mol), Al(NO3)3·9H2O (16.89 g, 0.045 mol) and urea (37.83 g, 0.63 mol) were
dissolved in 150 mL of deionized water in a 500mL round-bottom flask, and heated to
105 oC for 10 h with stirring, then crystallized at 100 oC for 20 h. After crystallization,
the precipitate was filtered and washed with deionized water until the pH of the
o
filtrate was neutral, and then dried in a 60 C vacuum drying box for 12 h. A sample
of LDH-2 (Mg/Al molar ratio was 2) was obtained. LDH-4 and LDH-6 (Mg/Al molar
ratio of was 4 and 6) were prepared by increasing the amount of Mg(NO3)2·6H2O.
2.3. Preparation of Ru/LDH
The supported Ru/LDH catalysts were prepared through the following procedure.
Typically, 1 g of LDH-2, 26.12 mg of RuCl3·3H2O and 20 mL of deionized water
mixed evenly, heated to 45 oC and stirred for 12 h. 65 mg of NaBH4 was dissolved in
a small amount of water, dripped into the above solution and stirred for another 1 h.
The solid sample was filtered and washed with deionized water and acetone, dried at
60 oC for 12 h, and 1wt%Ru/LDH-2 was obtained. 3wt% Ru/LDH-2 and
5wt%Ru/LDH-2 were prepared by changing the amount of Ru content.
1wt%Ru/LDH-4, 1wt%Ru/LDH-6, 1wt%Ru/γ-Al2O3, and 1wt%Ru/MgO were
prepared with the same method.
2.4. Preparation of Ru/LDH-LTC and Ru/LDH-CTL
Ru/LDH-LTC was prepared by loading then calcination. Typically,
RuCl3/LDH-2 was calcined at 100 oC for 2 h, and then 400 oC for 3 h. The solid
was added into 20 mL deionized water, stirred for 12 h, dripped with a certain
amount of NaBH4 solution and stirred for another 1 h. After centrifugation, the
o
solid was washed with deionized water and acetone, and then dried at 60 C for
12 h.

Ru/LDH-CTL was prepared by calcination then loading. In general, the as
o
o
prepared LDH-2 was calcined at 100 C for 2 h, and then 400 C for 3 h. Ru
was loaded by using the same method described above. These two catalysts
were used to study the effect of LDH calcination and Ru loading order on the
catalytic performance.
2.5. Sample characterization
The crystallinity of the as-prepared catalyst samples was characterized by powder
X-ray diffraction (XRD) with a Rigaku D/MAX 2250 V diffractometer, using
monochromatic Cu Kα (λ= 0.154 nm) radiation with the voltage and electric current
set to 40 kV and 100 mA, respectively. The morphology and size of crystals were
imaged with a JEOL JSM-6510A scanning electron microscope (SEM). Transmission
electron microscope (TEM) images were obtained using an accelerating voltage of
200 kV on a JEOL-135 2010F transmission electron microscope. FT-IR spectra were
recorded on a Bruker Vector-22 spectro-photometer in the range 4000–400 cm−1.
Thermal Gravimetric (TG) analysis was carried out from 298 to 1073 K in nitrogen
atmosphere with a temperature ramp rate of 10 K min−1. X-ray photoelectron spectra
(XPS) were recorded on a Perkin-Elmer PHI ESCA system. X-ray source was
standard Mg anode (1253.6 eV) at 12 kV and 300 W. The Brunauer–Emmett–Teller
(BET) specific surface area was calculated using the BET equation in the relative
pressure between 0.05 and 0.25.
2.6. Catalytic one-pot multi-directional selective hydrogenation and N-alkylation
In a typical procedure, 1 mmol nitrobenzene, 40 mg 5wt%Ru/LDH, 0.5 g 4Å MS,
and 5 mL isopropanol were added into the 60 mL high pressure reactor equipped with
a pressure gauge, a magnetic stirrer, and an electric temperature controller. The
reactor was purged four times with hydrogen to remove the residual atmosphere,
followed by filling with 0.5 MPa H2, and then heated to a definite temperature for a
o
period of time. After the reactor was cooled to 25 C, the reaction mixture was
separated by centrifugation. After the experiment, the catalyst was filtered, washed
o
three times with isopropanol and dried overnight in a 60 C vacuum drying box.

Recycle experiments were carried out with the recovered catalyst. The resulting
solution was analyzed on Agilent 7820A gas chromatograph equipped with a flame
ionization detector. The identification of the obtained products was carried on an
Agilent 7890B-5975 MSD-GC Mass spectrometer equipped with HP-5 column along
with compared with known compounds.
2.7. Catalytic test in quinoline and its derivatives selective hydrogenation
The hydrogenation of quinoline was performed in a 60 mL high pressure reactor
equipped with a pressure gauge, a magnetic stirrer, and an electric temperature
controller. For the selective hydrogenation of quinoline, 40 mg 5wt%Ru/LDH and 1
mmol quinoline were dissolved in 5 mL solvent. Before the reaction, the high
pressure reactor was purged four times with hydrogen to remove residual atmosphere,
then pressurized to 3.0 MPa and set to stir. Upon heating the reactor up to the
designated temperature, the reaction time was noted. After the reactor was cooled to
o
25 C, the reaction mixture was separated by centrifugation. The resulting solution
was analyzed on Agilent 7820A gas chromatograph equipped with a flame ionization
detector. The identification of the obtained products was carried on an Agilent
7890B-5975 MSD-GC Mass spectrometer equipped with HP-5 column along with
compared with known compounds.
3. Results and discussion
3.1. Characterization of Ru catalysts
As revealed in Fig. 1, the XRD patterns of the LDH samples showed the typical
diffraction lines for LDH structure with interlayer carbonate. The characteristic peaks
of the (003), (006), (009), (015), (018), (110) and (113) lattice planes were at 2θ =
11.7°, 23.5°, 35.0°, 39.5°, 47.1°, 60.8° and 62.2°, respectively (JCPDS card 22-0700),
demonstrating that a kind of hydroxycarbonate of Mg and Al was successfully
synthesized [30]. In addition, the two diffraction peaks for (110) and (113) planes
demonstrated that these samples had typical highly crystalline LDH structure. When
the Mg/Al molar ratio was 2, the peak intensity of (003) plane was the highest and the

symmetry of the peak was the best, which indicated that LDH-2 had highest
crystallinity (Fig. 1a). Although LDH-4 and LDH-6 had typical LDH characteristic
peaks, the intensity of the (003) diffraction peak was lowered and the symmetry
decreased. In addition, the intensity of (006) diffraction peak was also significantly
weakened with a pronounced peak broadening, while a new peak appeared at 2θ =
20.0°. The diffraction peak for the (003) plane became broader with the increase of
Mg/Al molar ratio also supported the LDH crystallinity decrease (Fig. 1a-1c). The
diffraction peaks did not change significantly with the increase of Ru loading,
indicating that Ru NPs were uniformly dispersed on LDH with no significant effect
on the LDH crystal structure (Fig. 1a, 1d-1f) [31]. As shown in Fig. S1, the LDH
characteristic peaks became weak and wide after calcination, inferred the LDH
structure change and the crystallinity decrease. 1wt%Ru/LDH-LTC had additional
peaks at 2θ=28.3° and 54.5° in Fig.S1(c), which should be related to the (110) and
(211) planes of RuO2 (JCPDS card 06-0663). This might be attributed to the partial
decomposition of RuCl3 to produce RuO2 during calcination.
Fig. 2 showed SEM images of Mg-Al LDH. The layered structure LDH-2
sample in Fig. 2 (a) was formed with ultrathin platelets aggregation. The
Ru/LDH maintained the layered structure after metal loading (Fig. 2 (b)),
however, the platelets stacked in some degree. As shown in Fig. 2 (c, d),
Ru/LDH-CTL and Ru/LDH-LTC aggregated into flower-like structure. The
metal loading and calcination sequence did not significantly affect the
morphology of the catalyst.
The TEM characterization showed the morphology and particle size distribution of
Ru NPs in 5wt%Ru/LDH-2 (Fig. 3(a)). Ru NPs were highly dispersed on the surface
of LDH with the average sizes of 2.5 nm (Fig. 3(b)). The HRTEM image of
5wt%Ru/LDH in Fig. 3(c, d) showed that the measured lattice spacing of Ru was 0.21
nm, which was consistent with the reflection of the (101) lattice planes of metal Ru
[32].
The FT-IR spectrum of catalytic materials was used to identify the structural
characteristics such as interlayer anions, cations in brucite-like layers as well as Ru

NPs on LDH. The typical LDH strong absorption band at 3453 cm−1 was observed in
Fig. S2, which should be attributed to the stretching band of the metal hydroxyl
groups and water molecules from both the interlayers and brucite-like layers [33]. The
weak band centered at 1635 cm−1 should be ascribed to the bending mode of water
molecules [34]. The sharp peak at 1369 cm−1 could be related to the CO32- symmetric
stretching mode, and a weak one at 783 cm−1 could be attributed to out-of-plane
2-
deformation of interlayer CO3 . The remaining bands between 688 cm−1 and 450
cm−1 should be assigned to metal-O and metal-OH vibrations in the LDH lattice [35].
With the increase of Mg/Al molar ratio and the loading of Ru NPs, the characteristic
2-
peak of CO3 at 1369 cm−1 became weaker significantly, illustrated that the increase
2-
of Mg2+ and the loading of Ru NPs to the LDH surface or interlayer reduced the CO3
symmetric stretching [36].
The TG analysis of LDH-2 and 5wt%Ru/LDH-2 was listed in Fig. S3. Two obvious
weight loss stages were observed during the TG analysis. The first weight loss at
o
around 200 C was attributed to the loss of physically absorbed and interlayer water.
The mass loss caused by thermal decomposition at this stage was about 11.0% of the
o
o
total LDH. The second weight loss in the temperature range from 260 C to 530 C
could be attributed to the decomposition of CO32- into carbon dioxide and the removal
of water molecules from hydroxyl groups in brucite-like layers, resulted in the
collapse of the LDH structure and eventually formed MgAl mixed oxides [21]. The
weight loss in this progress was around 24.0%. The overall weight loss of LDH-2 was
o
about 35.0% in the temperature range from 25 C to 530 oC, which was equivalent to
2-
the total weight loss of water and CO3 . The weight loss of 5wt%Ru/LDH-2 was less
than that of LDH-2 because Ru NPs remained stable during this TG analysis process.
Fig. S4(a) showed the specific surface areas of LDH-2 and 5wt%Ru/LDH-2, which
were determined by the BET method. The N2-adsorption and desorption curves of
both LDH-2 and 5wt%Ru/LDH-2 conformed to type IV adsorption curve with H3
hysteresis loop, indicating the sample had mesoporous structure. The maximum
adsorption capacity (or plateau) did not appear on the adsorption curves at relative
higher pressures, and the adsorption capacity increased with the increase of relative

pressures, suggested that the mesoporous structure should be owing to the slit pore
structure formed by the agglomeration of LDH platelet. The BET surface areas of
LDH-2 and 5wt%Ru/LDH-2 were 78 and 75 m²/g, respectively. Fig. S4(b) exhibited
that the pore size distribution of 5wt%Ru/LDH-2 decreased slightly in comparison
with LDH-2, indicating Ru NPs were supported on the LDH-2. The average pore was
12.3 nm and 10.7 nm, respectively, with the BJH (Barrett-Joiner-Halenda) method.
The valence state of 5wt%Ru/LDH-2 catalyst was characterized by XPS. The
XPS full scan showed that the catalyst contains Ru, Mg, Al, C and O elements
(Fig. 4a). The Ru 3d XPS spectrum was displayed in Fig. 4b. The peak at
284.66 eV should be attributed to C 1s; the peaks at 281.49 eV and 285.43 eV
were assigned to Ru 3d5/2 and Ru 3d3/2; while the peaks at 282.47 eV and
286.29 eV should be reasonably ascribed to LDH metal hydroxyl groups
stabilization effect to Ru NPs. Therefore, Ru in the 5wt%Ru/LDH-2 mainly
existed in the metallic Ru(0) and Ruδ+ states. The Mg 1s peaked at 1304 eV was
assigned to Mg2+ in Fig. 4c. The Al 2p peaked at 74.4 eV was assigned to Al3+
in Fig. 4d [37].
3.2. Catalytic one-pot multi-directional selective hydrogenation and N-alkylation
Table 1 presented the catalytic data in one-pot multi-directional selective
hydrogenation and N-alkylation reaction of nitrobenzene (NB) with isopropanol
over various Ru catalysts. The multi-selective hydrogenation and N-alkylation
of NB with isopropanol involve the following directions: 1) reduction of nitro
group to amino group; 2) hydrogenation of aromatic ring to alicyclic; 3)
N-alkylation of amine with alcohol. In comparison with traditional MgO and
Al2O3 support, Ru NPs on LDH notably had the highest catalytic activity and
chemoselectivity to N-alkylation product IP-CHA (Table 1, entries 1-3). This
might be owing to the fact that LDH ultrathin platelets were beneficial to the
dispersion of Ru NPs, and the metal-O bond and metal-OH bonds in LDH were
beneficial to the Ru NPs stability and the catalytic performance improvement.
Considering the influence of calcination on the LDH surface alkalinity, and the

influence of metal loading and calcination sequence on the valence and
catalytic properties of metal NPs [38,39], we also investigated these factors.
However, the catalytic chemoselective hydrogenation and N-alkylation activity
and chemoselectivity were significantly reduced due to the agglomeration of
LDH, the decomposition of the brucite-like layer hydroxyl groups along with
the interlayer CO32- and the RuO2 formation after calcination (Table 1, entries 4
and 5). The Mg/Al molar ratio affects not only the crystal structure of LDH, but
also the surface alkalinity. Therefore, the influence of LDH Mg/Al molar ratio
on catalytic performance was examined. The catalytic N-alkylation selectivity
decreased with the Mg/Al molar ratio increase (Table 1, entries 1, 6 and 7). In
combination with XRD characterization in Fig. 1, the crystallinity of the LDH
carrier obviously affected the catalytic performance. The amount of metal
loading is the key factor to influence the metal dispersion, the number of
surface active sites as well as the catalytic performance of the catalyst.
Obviously, as the metal loading increase, the catalytic N-alkylation selectivity
increased (Table 1, entries 1, 8 and 9), confirming that the dispersibility of the
Ru NPs on the LDH had important effect on the catalytic performance. When
the metal loading increased to 5%, the chemoselectivity to N-alkylation of
amine could reach as high as 98.0% with just 0.5 MPa H2 (Table 1, entry 9). In
the absence of 4Å MS, Ru/LDH catalyst could also catalyze N-alkylation of
NB with just 44.0% IP-CHA chemoselectivity, and a large number of CHA,
DCHA and IP-AN were detected (Table 1, entry 10). This identified the
synergy effect between 4Å MS and Ru/LDH to promote the improvement of
catalytic IP-CHA selectivity. For comparison, we also tested the catalytic
performance of commercial available catalyst 5wt%Ru/C. Under the same
catalytic conditions, 5wt%Ru/C catalyst just achieved poor chemoselectivity in
the N-alkylation of NB with isopropanol (Table 1, entry 11). Heterogeneous
one-pot multi-directional catalytic systems are charming because they can
simplify experimental procedures, realize atomic economy and save time and
energy. After repeated attempts, we discovered a simple and convenient method

to control multi-directional selective hydrogenation and N-alkylation by tuning
reaction conditions. Except the 98.0% chemoselectivity to N-alkylation amine
in isopropanol (Table 1, entry 9); Hydrogenation of NB to AN could be
achieved quantitatively in water at 50 °C (Table 1, entry 12); CHA, which is
NB fully hydrogenated product, could also be achieved with chemoselectivity
up to 92.6% in ethanol at 100 °C (Table 1, entry 13). In short, above results
indicated that 5wt%Ru/LDH-2 perfectly achieved one-pot multi-directional
highly chemoselective hydrogenation and N-alkylation of NB under mild
conditions.
In order to clarify whether the hydrogenation and N-alkylation chemoselectivity
switch was caused by the kinetic control or by the action of the solvent to the catalyst,
we investigated the time-dependent catalytic performance in isopropanol, ethanol as
well as water over 5wt%Ru/LDH-2 catalyst. Fig. 5a showed the time-dependent
catalytic performance in selective hydrogenation and N-alkylation of NB in
isopropanol. The reduction of the nitro group reached 100% conversion with 80.7%
chemoselectivity to AN in 2 h. CHA, DCHA and IP-AN were also detected in the
product in 2 h. When the reaction time reached 4 h, the chemoselectivity to CHA
increased to 39.3% and the chemoselectivity to IP-CHA reached 53.0% accompany
with the selectivity to AN decreased to less than 1.0%. Further extending the reaction
time, continuous increase in the concentration of IP-CHA was achieved accompanied
with the decrease in chemselectivity to other products, and chemoselectivity to
IP-CHA could be increased to as high as 98.0% in 10 h. In ethanol (Fig. 5b), the nitro
group reached 94.0 % conversion with 68.0 % chemoselectivity to AN and 31.8 %
chemoselectivity to CHA in 2 h. The hydrogenation activity of NB in ethanol was
obviously higher than that in isopropanol; however, CHA did not continually convert
to N-alkylamine with the increase of time. Additionally, the hydrogenation behavior
in water was similar to the catalytic performance in ethanol (Fig. 6). Therefore, it
should be concluded that the selectivity switch of NB hydrogenation and N-alkylation
was controlled by both the kinetic control and the action of the solvent to the catalyst.
The recycling experiment of 5wt%Ru/LDH-2 on hydrogenation and N-alkylation of

NB in isopropanol was investigated in Fig. 7. It was apparent that 5wt%Ru/LDH-2
was very stable and could be reused at least 5 times with maintained high activity and
high IP-CHA chemoselectivity.
As presented in Table 2, one-pot multi-directional selective hydrogenation and
N-alkylation of different aromatic nitro compounds was tested. Substrates bearing
either electron-donating or electron-withdrawing substituents on the aryl ring of NB
could be hydrogenated and alkylated to corresponding aromatic amines, alicyclic
amines and the N-alkylated alicyclic amines with excellent switchable
chemoselectivity. All aromatic nitro compounds could almost completely be
o
hydrogenated to the corresponding aromatic amine at 50 C in water. The stepwise
o
hydrogenation to produce the fully hydrogenated amines could be achieved at 100 C
in ethanol. In addition, aromatic nitro compounds were completely hydrogenated and
o
almost completely N-alkylated at 130 C in isopropanol with just 0.5 MPa H2. In
comparison with NB multi-directional selective hydrogenation and N-alkylation, the
presence of methyl group at ortho position had no obvious influence on catalytic
activity and chemoselectivity (Table 2, entries 1-3 vs 4-6), and the N-alkylation
chemoselectivity could reach as high as 99.8%. When the methyl group was in the
meta or para position, the chemoselectivity to alicyclic amines was slightly improved
(Table 2, entries 8 and 11), while the chemoselectivity to N-alkylation alicyclic
amines decreased to 95.4% and 88.1%, respectively (Table 2, entries 9 and 12).
However, further enhancement of the electron donating effect in the ortho-substituted
NB resulted in a significant decrease in the catalytic chemoselectivity to both alicyclic
amine and N-alkylated alicyclic amine (Table 2, entries 14-15). When the substituents
were electron-withdrawing groups, the catalytic activity slightly decreased. When the
halogen substituted substrates were tested in the selective hydrogenation in ethanol or
N-alkylation in isopropanol, severe dehalogenation was observed and high reaction
temperature and alkaline of LDH might be responsible for the severe dehalogenation
(Table 2, entries 17-18, 20-21) [40-42]. When the substituent groups were aldehyde or
ketone, both nitro group and carbonyl group were hydrogenated in the selective
o
o
hydrogenation at 100 C or N-alkylation at 130 C (Table 2, entries 23-24, 26-27). In

short, 5wt%Ru/LDH-2 showed excellent catalytic performance in one-pot
multi-directional selective hydrogenation and N-alkylation of a broad range of
aromatic nitro compounds, and the extent of the chemoselectivity seemed to be subtly
affected by the substituent in the substrate.
We further studied the catalytic performance of one-pot chemiselective
hydrogenation and N-alkylation of different alkyl alcohols with NB in the
5wt%Ru/LDH-2 catalyst. Under optimized conditions, all the NB conversion
reached 100%, and aniline could be hardly detected in the product, which
indicated that 5wt%Ru/LDH-2 had high hydrogenation activity to both nitro
group and benzene ring. Compared with 98.0% N-alkylation selectivity in
o
isopropanol at 130 C and 0.5 MPa H2, 88.2% N-alkylation chemoselectivity
was achieved in 2-butanol (Table 3, entry 1 vs 2), indicating less sterically
hindered alcohols undergo faster N-alkylation. In order to achieve higher
N-alkylation activity and chemoselectivity, the reactions with primary alcohol
o
and tertiary alcohol were carried out at 150 C and 3 MPa H2. The N-alkylation
chemoselectivity of primary alcohol, including ethanol, n-propanol and
n-butanol, was 86.7%, 88.9% and 90.4%, respectively (Table 3, entries 3-5).
The stronger electron-donating ability of the alkyl group was the higher N-
alkylation activity of alcohol achieved. However, the N-alkylation
chemoselectivity in tert-butanol was only 21.0% (Table 3, entry 6). Although
tert-butyl group had strong electron-donating effect, steric hindrance mainly
affected the N-alkylation at this time. So, with the integrated electronic and
steric effect, the N-alkylation activity of NB and alkyl alcohols was secondary
alcohol > primary alcohol > tertiary alcohol. Usually, N-alkylation of amine with
alcohol involved alcohols dehydrogenation, imines formation, and imines
hydrogenation [11,12]. Considering tertiary butanol could also be used for the
N-alkylation over 5wt%Ru/LDH-2, we speculated that the mechanism in this system
included both alcohols dehydrogenation, imines formation along with imines
hydrogenation hydrogen process and direct dehydration process between amino
groups and alcohols [43]. Shortly, 5wt% Ru/LDH-2 exhibited attractive activity in

the N-alkylation of NB, and the application of this catalytic system could be
extended to various alkyl alcohols.
Quinoline and its derivatives have many biological activities and are widely
used in the design and synthesis of dyes, drugs, insecticides and antioxidants
[44-46]. However, using heterogeneous catalysts to regulate chemoselective
hydrogenation of quinoline and its derivatives under mild conditions is still a
daunting scientific challenge. Considering the excellent multi-directional
hydrogenation performance of 5wt%Ru/LDH-2, the chemoselective
hydrogenation of quinoline and its derivatives was also studied. Quinoline
could be easily hydrogenated to 1,2,3,4-tetrahydroquinoline at 50 °C with 94.7%
chemoselectivity in ethanol (Table 4, entry 1). Quinoline was completely
hydrogenated at 100 °C with 100% chemoselectivity in water (Table 4, entry 2).
Interestingly, quinoline could be hydrogenated to 5,6,7,8-tetrahydroquinoline
with up to 68.5% chemoselectivity in isopropanol in the presence of 4Å MS
(Table 4, entry 3). Catalytic hydrogenation chemoselectivity of
2-methylquinoline was lower than that of quinoline due to steric hindrance
effect (Table 4, entries 4-6). Unexpectedly, the main product of the
hydrogenation of 3-methylquinoline in ethanol was not the
1,2,3,4-tetrahydroquinoline
derivative
but
the
corresponding
5,6,7,8-tetrahydroquinoline (Table 4, entry 7). This phenomenon might be
explained that the steric hindrance formed by the Ru/LDH ultrathin platelets
aggregation and the methyl group in the 3-methylquinoline blocked the contact
of the pyridine ring with the Ru active center in ethanol. The multi-directional
hydrogenation chemoselectivity of 8-methylquinoline was similar to that of
quinoline (Table 4, entries 10-12). For comparison, 2,3-benzopyrrole and
1-benzothiophen were also examined (Table 4, entries 13-18). It's remarkable
that 2,3-benzopyrrole could be completely hydrogenated in water. However, for
1-benzothiophene, the conversion was rather low. The low activity may be
attributed to the affinity of sulfur atoms for its classical η1-S adsorption onto the
surface of Ru NPs [47-49].

The interesting chemoselectivity change in the quinoline hydrogenation with
different solvent was also preliminary discussed. Different solvent may affect
the adsorption properties of substrate on catalysts, resulting in difference in
activity and chemoselectivity in the liquid-phase reaction [50]. Table 5
examined the effect of different polarity (dielectric constant) solvents on the
chemoselective hydrogenation with quinoline under mild conditions [51-53].
5wt%Ru/LDH-2 catalyst showed the best hydrogenation activity in water,
mainly resulting in decahydroquinoline (Table 5, entry 1). Quinoline was
mainly hydrogenated to 1,2,3,4-tetrahydroquinoline in methanol and ethanol
(Table 5, entries 2-3). The chemoselectivity to 5,6,7,8-tetrahydroquinoline was
greatly increased in isopropanol and toluene (Table 5, entries 4-5), although the
conversion of quinoline was only 3.0% in toluene. In high polarity methanol
and ethanol solvent, π electrons of the benzene ring in quinoline could be
attracted by the solvent [45], thus, the coordination between the LDH surface
hydroxyl group and the benzene ring was inhibited and high chemoselectivity
toward 1,2,3,4-tetrahydroquinoline was obtained. With the further solvent
polarity increase, the catalytic hydrogenation activity evidently increased,
therefore, 1,2,3,4-tetrahydroquinoline could be further hydrogenated to generate
decahydroquinoline. With the lower polarity solvent isopropanol and toluene,
the interaction between solvent and benzene ring π electrons in quinoline was
weakened, resulting in the corresponding increase of chemoselectivity to
5,6,7,8-tetrahydroquinoline. Similar phenomenon in chemoselectivity change
was also observed in some previous quinoline hydrogenation literature [54].
The chemoselectivity to 5,6,7,8-tetrahydroquinoline in isopropanol further
increased with the introduction of 4Å MS (Table 5, entry 6). The N atom with
lone pair electrons was more likely to interact with the acid sites on the 4Å MS,
resulting in the reduction of quinoline N atom interaction with the LDH
hydroxyl groups in 5wt%Ru/LDH-2. Acid sites were beneficial to improve the
5,6,7,8-tetrahydroquinoline chemoselectivity, which was also reported in

previous literature [55]. In short, both solvent polarity and acid sites in 4Å MS
impacted the chemoselectivity change in quinoline hydrogenation.
4. Conclusions
In conclusion, Ru/LDH catalyst with characteristics of layered mesoporous
structure and well dispersed small Ru NPs has been successfully prepared. With the
guidance of atom economy, non-toxicity, harmlessness and energy saving in green
catalysis, we have discovered a highly efficient and easy to operate one-pot tandem
method by which we could directly produce a broad range of multi-directional
chemoselective hydrogenated amines or N-alkyl aliphatic amines using aromatic nitro
compounds as the starting material under mild conditions. By easily adjusting the
reaction conditions in the Ru/LDH catalytic system, multi-directional excellent
chemoselectivity to aromatic amines, alicyclic amines as well as N-alkyl aliphatic
amines could be achieved with excellent catalytic activity and recyclability. The
chemoselectivity switch of NB hydrogenation and N-alkylation was controlled by
both the kinetic control and the action of the solvent to the catalyst. Additionally, this
catalytic system also exhibited attractive activity and multi-directional
chemoselectivity in the hydrogenation of quinoline and its derivatives with solvents of
different polarity, and both solvent polarity and acid sites in 4Å MS impacted the
chemoselectivity change in quinoline hydrogenation.
CRediT author statement
Sishi Zhang：Investigation,Data curation, Writing- Original draft preparation, Jie Xu：Methodology,
Hongmei Cheng: Validation, Cuicui Zang: Investigation, Bin Su：Supervision, Heyan Jiang：
Supervision, Conceptualization, Writing-Reviewing and Editing, Funding acquisition, Fengxia Bian：
Supervision, Funding acquisition
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.

Acknowledgements
This work was financially supported by Natural Science Foundation Project of CQ
(No. cstc2018jcyjAX0735, cstc2019jcyj-msxmX0641), National Natural Science
Foundation of China (No. 21201184), Ministry of Education of Chongqing (No.
KJQN201900811), Chongqing Technology and Business University (950119090) and
Chongqing Key Laboratory of Catalysis and New Environmental Materials
(KFJJ2019082).
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Fig. 1. XRD pattern of (a) LDH-2, (b) LDH-4, (c) LDH-6, (d) 1wt%Ru/LDH-2, (e) 3wt%Ru/LDH-2 and (f)
5wt%Ru/LDH-2.
Fig. 2. SEM images of (a) LDH-2, (b) 5wt%Ru/LDH-2, (c) 1wt%Ru/LDH-CTL and (d) 1wt%Ru/LDH-LTC.

Fig. 3. (a) TEM image of 5wt%Ru/LDH-2. (b) Particle size distribution of Ru NPs. (c) HRTEM image of
5wt%Ru/LDH-2, (d) enlargement of (c).
Fig. 4. XPS spectra of 5wt%Ru/LDH-2: (a) survey spectra, (b) Ru 3d, (c) Mg 1s, (d) Al 2p.

Fig. 5. Time-dependent catalytic performance in selective hydrogenation and N-alkylation of NB over
5wt%Ru/LDH-2 catalyst (a) in isopropanol, (b) in ethanol. The reaction conditions were the same as in Table 1,
0.5 g 4Å MS, 130 oC, 0.5 MPa H2, reaction time 10 h.
Fig. 6. Time-dependent catalytic performance in selective hydrogenation of NB over 5wt%Ru/LDH-2 catalyst in
water. The reaction conditions were the same as in Table 1, 130 oC, 0.5 MPa H2, reaction time 10 h.

Fig. 7. Catalytic recycles in NB selective hydrogenation and N-alkylation in isopropanol over
5wt%Ru/LDH-2.The reaction conditions were the same as in Table 1, 0.5 g 4Å MS, 5 mL isopropanol, 130 oC, 0.5
MPa H2, reaction time 10 h.

Table 1 Catalytic one-pot multi-directional selective hydrogenation and N-alkylation over various
Ru catalysts. a
Entry
catalyst
Conv.
(%)
Selectivity(%) b
AN
CHA
IP-CHA
others f
1
1wt%Ru/LDH-2
1wt%Ru/MgO
1wt%Ru/Al2O3
100
100
100
0.3
9.1
89.0
79.6
79.2
23.3
0.0
1.6
2
2.8
7.6
10.0
11.0
36.3
17.9
3.3
3
1.8
8.0
4
1wt%Ru/LDH- CTL 100
1wt%Ru/LDH- LTC 100
12.8
65.5
8.7
26.8
16.5
3.0
5
6
1wt%Ru/LDH-4
1wt%Ru/LDH-6
3wt%Ru/LDH-2
5wt%Ru/LDH-2
5wt%Ru/LDH-2
5wt%Ru/C
100
100
100
100
100
100
100
100
85.0
48.2
92.2
98.0
44.0
75.2
0.0
7
17.4
1.1
31.4
0.1
3.0
8
6.6
9
0.8
0.1
1.1
10c
11
12d
4.0
22.7
0.7
29.3
23.6
0.0
0.5
5wt%Ru/LDH-2
5wt%Ru/LDH-2
100
7.4
0.0
13e
92.6
0.0
0.0
a
Reaction conditions: 1 mmol NB, The molar ratio of metal Ru/substrate was 1:50, 5 mL isopropanol,
0.5 g 4Å MS, 0.5 MPa H2, 130 oC, reaction time 10 h. b Conversion (Conv.) and selectivity (Sel.) were
determined by gas chromatography (GC) and the identification of the obtained products was carried on
an Agilent 7890B-5975 MSD-GC Mass spectrometer equipped with HP-5 column along with compared
with known compounds. c Without 4Å MS. d 5mL water, 50 oC, without 4Å MS. e 5 mL ethanol, 100 oC,
f
3 MPa H2, without 4Å MS. The main reaction products were aniline (AN), cyclohexylamine (CHA)
and N-isopropyl CHA (IP-CHA); dicyclohexylamine (DCHA) and N-isopropylaniline (IP-AN) were the
detected byproducts.

Table 2 Catalytic one-pot multi-directional selective hydrogenation and N-alkylation of
substituted nitrobenzenes catalyzed by 5wt%Ru/LDH-2.a
Sel.(%) b
Entry
A-J
Conv. (%)b
A-J a
100
7.4
A-J b
0.0
A-J c
0.0
others
0.0
0.0
1.1
0.0
0.0
0.0
0.0
0.0
3.2
0.0
0.0
0.0
0.0
0.0
8.0
0.0
0.0
4.5
0.0
2.9
0.0
1 c
2 d
3
4 c
5 d
100
100
100
100
98.6
100
93.0
100
100
94.0
98.5
100
100
100
100
99.8
100
100
75.9
100
100
A
A
A
B
B
B
C
C
C
D
D
D
E
E
E
F
92.6
0.1
0.0
0.8
98.0
0.0
99.8
9.0
0.2
91
0.0
6
0.1
0.1
99.8
0.0
7 c
8 d
100
1.9
0.0
98.1
0.0
0.0
9
1.4
95.4
0.0
10 c
11 d
12
13 c
14 d
15
16 c
17 d
18
19 c
20 d
21
100
2.1
0.0
97.9
1.0
0.0
10.9
99.9
11.8
12.1
99.5
5.1
88.1
0.0
0.1
88.2
0.0
0.0
79.9
0.0
0.5e
94.9e
0.0
0.0
95.5f
F
0.0
F
91.6
4.7
8.4
92.4e
0.0
G
G
G
0.0
50.3+49.7f
0.0
0.0

22 c
23 d
24
25 c
26 d
93.8
94.6
100
100
100
100
90.3
0.0
9.7g
0.0
0.0
3.8
0.9
2.3
3.3
2.1
H
H
H
I
96.2g
64.6g
0.0
24.5+72.2i
15.1i
0.0
28.6h
5.9
97.7
0.0
0.0
I
0.0
82.8j
27
I
0.0
a
Reaction conditions: 1 mmol substrate, 40 mg 5wt%Ru/LDH-2, 0.5 g 4Å MS, 5mL
b
o
isopropanol, 0.5 MPa H2, 130 C, reaction time 10 h. Conversion (Conv.) and selectivity
(Sel.) were determined by gas chromatography (GC) and
the identification of the obtained products was carried on an Agilent 7890B-5975 MSD-GC
Mass spectrometer equipped with HP-5 column along with compared with known
c
o
d
compounds. 5 mL of water, without 4Å MS, 50 C, reaction time of 6 h. 5 mL ethanol,
e
f
h
j
without 4Å MS, 100 oC, 3 MPa H2, reaction time 10 h.
cyclohexanamine.
g
N-isopropylcyclohexanamine.
4-aminocyclohexylmethanol.
1-(3-aminocyclohexyl)ethanol.
i
4-isopropylaminocyclohexylmethanol.
1-(3-(isopropylamino)cyclohexyl)ethanol.
Table 3 Catalytic one-pot multi-directional selective hydrogenation and N-alkylation of NB with
various alcohols catalyzed by 5wt%Ru/LDH-2.a
Entry Alcohol
Product
Conv.(%)b
Sel.(%) b
N-alkyl others
AN
CHA
amines
1 c
2 c
3
100
100
100
100
100
0.8
0.0
1.0
0.0
0.0
0.1
98.0
1.1
0.1
0.0
0.0
0.0
11.7
12.3
11.1
9.6
88.2
86.7
88.9
90.4
4
5
6
100
0.0
79.0
21.0
0.0
a Reaction conditions: 1 mmol NB, 40 mg 5wt%Ru/LDH-2, 5 mL alcohol, 0.5 g 4Å MS, 3 MPa H2, 150 oC,
b
reaction time 10 h. Conversion (Conv.) and selectivity (Sel.) were determined by gas chromatography
(GC) and the identification of the obtained products was carried on an Agilent 7890B-5975 MSD-GC Mass
spectrometer equipped with HP-5 column along with compared with known compounds. c 0.5 MPa H2, 130
oC.

Table 4 Catalytic multi-directional chemoselective hydrogenation of quinoline and its derivatives
catalyzed by 5wt%Ru/LDH-2.a
Sel.(%) b
Entry
K-Q
Conv.(%)b
K-Q a
94.7
25.0
0.0
K-Q b
5.3
K-Q c
0.0
1
98.6
94.8
100
K
K
K
L
2 c
3 d
4
5 c
6 d
7
8 c
9 d
10
11 c
12 d
13
14 c
15d
16
17 c
18 d
a
68.5
0.0
6.5
100
99.3
96.2
86.8
30.4
13.2
63.6
0.0
6.0
L
100
100
99.3
100
94.0
93.0
99.8
30.6
14.0
99.3
3.0
0.0
7.0
93.0
4.7
L
1.4
93.9
95.6
1.6
M
M
M
N
N
N
O
O
O
P
4.4
0.0
1.2
97.2
2.0
98.0
27.8
0.8
0.0
59.2
0.6
13.0
98.6
58.5
50.0
99.9
8.6
0.0
41.5
50.0
0.1
0.0
0.0
91.4
0.0
P
7.0
50.0
100
17.1
0.0
32.9
0.0
P
10.0
Reaction conditions: 1 mmol substrate, 40 mg 5wt%Ru/LDH-2, 5 mL
b
ethanol, 3 MPa H2, 50 oC, reaction time 8 h. Conversion (Conv.) and
selectivity (Sel.) were determined by gas chromatography (GC) and
the identification of the obtained products was carried on an Agilent
7890B-5975 MSD-GC Mass spectrometer equipped with HP-5 column along
c
with compared with known compounds. 5 mL isopropanol, 0.5g 4Å MS, 0.5
MPa H2,80 oC, reaction time 4 h. d 5 mL water, 3 MPa of H2, 100 oC, reaction
time 8 h.