Effects of Ni particle size on amination of monoethanolamine over Ni-Re/SiO2 catalysts

Article abstract of DOI:10.1016/S1872-2067(19)63302-4

Ni-Re/SiO₂ catalysts with controllable Ni particle sizes (4.5–18.0 nm) were synthesized to investigate the effects of the particle size on the amination of monoethanolamine (MEA). The catalysts were characterized by various techniques and evaluated for the amination reaction in a trickle bed reactor at 170°C, 8.0 MPa, and 0.5 h^?1 liquid hourly space velocity of MEA (LHSV_MEA) in NH₃/H₂ atmosphere. The Ni-Re/SiO₂ catalyst with the lowest Ni particle size (4.5 nm) exhibited the highest yield (66.4%) of the desired amines (ethylenediamine (EDA) and piperazine (PIP)). The results of the analysis show that the turnover frequency of MEA increased slightly (from 193 to 253 h^?1) as the Ni particle sizes of the Ni-Re/SiO₂ catalysts increased from 4.5 to 18.0 nm. Moreover, the product distribution could be adjusted by varying the Ni particle size. The ratio of primary to secondary amines increased from 1.0 to 2.0 upon increasing the Ni particle size from 4.5 to 18.0 nm. Further analyses reveal that the Ni particle size influenced the electronic properties of surface Ni, which in turn affected the adsorption of MEA and the reaction pathway of MEA amination. Compared to those of small Ni particles, large particles possessed a higher proportion of high-coordinated terrace Ni sites and a higher surface electron density, which favored the amination of MEA and NH₃ to form EDA.

Full text of DOI:10.1016/S1872-2067(19)63302-4

Chinese Journal of Catalysis 40 (2019) 567–579
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Effects of Ni particle size on amination of monoethanolamine over
Ni-Re/SiO2 catalysts
Lei Ma a,b,d, Li Yan a,#, An-Hui Lu b, Yunjie Ding a,c,
*
a Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
b State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, Liaoning, China
c State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
d University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Ni-Re/SiO2 catalysts with controllable Ni particle sizes (4.5–18.0 nm) were synthesized to investi-
gate the effects of the particle size on the amination of monoethanolamine (MEA). The catalysts
were characterized by various techniques and evaluated for the amination reaction in a trickle bed
reactor at 170 °C, 8.0 MPa, and 0.5 h–1 liquid hourly space velocity of MEA (LHSVMEA) in NH3/H2
atmosphere. The Ni-Re/SiO2 catalyst with the lowest Ni particle size (4.5 nm) exhibited the highest
yield (66.4%) of the desired amines (ethylenediamine (EDA) and piperazine (PIP)). The results of
the analysis show that the turnover frequency of MEA increased slightly (from 193 to 253 h–1) as the
Ni particle sizes of the Ni-Re/SiO2 catalysts increased from 4.5 to 18.0 nm. Moreover, the product
distribution could be adjusted by varying the Ni particle size. The ratio of primary to secondary
amines increased from 1.0 to 2.0 upon increasing the Ni particle size from 4.5 to 18.0 nm. Further
analyses reveal that the Ni particle size influenced the electronic properties of surface Ni, which in
turn affected the adsorption of MEA and the reaction pathway of MEA amination. Compared to
those of small Ni particles, large particles possessed a higher proportion of high-coordinated terrace
Ni sites and a higher surface electron density, which favored the amination of MEA and NH3 to form
EDA.
Received 24 October 2018
Accepted 8 January 2019
Published 5 April 2019
Keywords:
Particle size effects
Monoethanolamine
Amination reaction
Ni-Re/SiO2
Turn over frequency
© 2019, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
zine (PIP). The amination reaction of MEA is generally driven
by Ni-, Co-, or Cu-based metallic catalysts via a borrowing hy-
As important fine chemicals, ethylene amines are widely
used as solvents, stabilizers, and for the synthesis of pharma-
ceuticals, chelating agents, resins, surfactants, and fabric sof-
teners. In chemical industry production [1–4], the amination of
monoethanolamine (MEA) represents an economical and envi-
ronmentally benign method to prepare valuable ethylene
amines, mainly including ethylenediamine (EDA) and pipera-
drogen mechanism (Scheme 1) [5–8]: initially, MEA is dehy-
drogenated to form a 2-aminoacetaldehyde compound; next, an
imine is formed through condensation of the aldehyde with
ammonia or another MEA molecule; finally, the intermediate
imine is reduced to ethylene amines. During the catalytic ami-
nation process (Scheme 2), various competitive and consecu-
tive reactions can take place through the participation of prod-
* Corresponding author. Tel/Fax: +86-411-84379143; E-mail: dyj@dicp.ac.cn
# Corresponding author. Tel/Fax: +86-411-84379055; E-mail: yanli@dicp.ac.cn
This work was supported by the National Natural Science Foundation of China (21273227) and Strategic Priority Research Program of Chinese
Academy of Sciences (XDB17000000).
DOI: S1872-2067(19)63302-4 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 40, No. 4, April 2019

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Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
be influenced by various factors, including catalyst support,
particle size distribution, reaction substrate, and reaction con-
ditions. Therefore, previous studies of structure-activity rela-
tionships have not yet reached a common conclusion. Moreo-
ver, to the best of our knowledge, the effects of the metal parti-
cle size on the product distribution of the amination of alcohol
amines have never been investigated.
Scheme 1. Reaction mechanism for MEA amination.
In this work, SiO2 was selected as inert support in order to
avoid or reduce the complications associated with the interac-
tion between metal and support. Ni-Re/SiO2 catalysts (Ni 20
wt%, Re 1.5 wt%) with a wide range of Ni particle sizes
(4.5–18.0 nm) were prepared and characterized by N2 phy-
sisorption, H2 temperature programmed reduction (H2-TPR),
H2 temperature programmed desorption (H2-TPD), X-ray dif-
fraction (XRD), transmission electron microscopy (TEM), H2
chemisorption, as well as Fourier transform infrared (FT-IR)
spectroscopy measurements of adsorbed CO and MEA. We in-
vestigated the MEA amination over supported Ni-Re catalysts
in a trickle-bed reactor in order to understand the effects of the
Ni particle size on the catalyst activity and selectivity to prod-
uct amines.
uct amines in the condensation step. These reactions unavoid-
ably generate variety of by-products, including
a
N’-(2-aminoethyl)ethanolamine
N’-aminoethylpiperazine
(AEEA),
and
(AEP),
N’-(2-hydroxyethyl)piperazine (HEP). The formation of
by-products would result in economic losses and product sep-
aration issues. The development of efficient catalysts with a
high yield of desired amines (EDA and PIP) is thus an important
and challenging research target.
In the case of supported metal catalysts, the surface chemi-
cal properties and electronic structure of the metal particles
are influenced by their size [9–17]. Upon increasing the size of
the metal particles, the fraction of terrace sites increases line-
arly, while the fraction of step/corner sites decreases slowly.
These changes in the distribution of surface sites may alter the
coordination environment of surface metals, thus influencing
the adsorption and activation capabilities of the catalysts. The
effects of the metal particle size in catalysis have been investi-
gated for many structure-sensitive reactions. Typically, for the
Pd-catalyzed dehydrogenation of alcohols, Wang et al. [11]
concluded that a suitable ratio of surface atoms (terrace Pd) to
coordinatively unsaturated atoms (edge and corner Pd) favors
the adsorption and β-H activation steps. Jensen et al. [18] stud-
ied the effects of the Ni particle size of Ni/SiO2 catalyst in the
hydrodeoxygenation of phenol. They found that deoxygenation
and hydrogenation reactions are facilitated on step/corner and
terrace Ni sites, respectively. Zhu et al. [9] investigated the in-
fluence of the Ni particle size in m-cresol hydrodeoxygenation.
They reported that the selectivities to products are influenced
by the Ni particle size of Ni/SiO2 catalysts. For the amination
reaction, a few papers on particle size effects have been re-
ported. Shimizu et al. [19] reported the N’-alkylation of amines
with alcohols on Ni/Al2O3 catalysts in a slurry reactor at 144 °C.
They concluded that the turnover frequency (TOF) increases
with a decrease in Ni particle size, and low-coordinated Ni
combined with acid-base sites represents the active phase for
the dehydrogenation and hydrogen transfer steps. In catalytic
amination reactions, the effects of the metal particle size may
2. Experimental
2.1. Catalyst preparation
Ni-Re/SiO2 catalysts (Ni 20 wt%, Re 1.5 wt%) were pre-
pared by incipient-wetness impregnation. Silica (Qingdao Hai-
yang Chemical Co., Ltd., China) was used as support while
Ni(NO3)2·6H2O (99.9% purity) and NH4ReO4 (99.99% purity)
were employed as metal precursors. After impregnation, the
materials were dried at 120 °C for 4 h. The following
Ni-Re/SiO2 catalysts, with varying particle sizes, were prepared
under different calcination atmospheres and reduction tem-
peratures.
Catalyst 1: The materials were calcined in a tubular oven
under a flow of NO/Ar (1% v/v) at 450 °C, corresponding to a
space velocity of about 8.4 × 103 h–1. The calcined catalysts
were denoted as Ni-Re/SiO2-NO. Before reaction, the
Ni-Re/SiO2-NO catalysts were reduced in situ in a trickle-bed
reactor under a H2 flow of 100 ml·min–1 for 4 h at 440 °C. The
obtained catalysts were denoted as Ni-Re/SiO2-4.5.
Catalyst 2: The materials were calcined in a tubular oven
under a flow of Ar at 450 °C, corresponding to a space velocity
of about 3.6 × 103 h–1. The calcined catalysts were denoted as
I
II
III
Scheme 2. Main reaction pathways of MEA amination on Ni-Re/SiO2 catalysts. I, II, and III represent the amines produced under different reaction
depth.

Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
569
Ni-Re/SiO2-Ar. Before reaction, the obtained Ni-Re/SiO2-Ar
catalysts were reduced in situ in a trickle-bed reactor under a
H2 flow of 100 ml·min–1 for 4 h at 440 °C. The obtained catalysts
were denoted as Ni-Re/SiO2-10.5.
Catalyst 3: The materials were calcined in a tubular oven
under a still air atmosphere at 450 °C. The calcined catalysts
were denoted as Ni-Re/SiO2-Air. Before reaction, the obtained
Ni-Re/SiO2-Air catalysts were reduced in situ in a trickle-bed
reactor under a H2 flow of 100 ml·min–1 for 4 h at 440 °C. The
obtained catalysts were denoted as Ni-Re/SiO2-14.6.
Catalyst 4: The materials were calcined in a tubular oven
under a still air atmosphere at 450 °C. The calcined catalysts
were denoted as Ni-Re/SiO2-Air. Before reaction, the obtained
Ni-Re/SiO2-Air catalysts were reduced in situ in a trickle-bed
reactor under a H2 flow of 100 ml·min–1 for 4 h at 640 °C. The
obtained catalysts were denoted as Ni-Re/SiO2-18.0.
Monometallic Ni/SiO2 (Ni 20 wt%) and Re/SiO2 (Re 1.5
wt%) catalysts were also prepared for comparison. The prepa-
ration process was the same as that employed for the
Ni-Re/SiO2-14.6 catalyst.
TEM micrographs of the Ni-Re/SiO2 catalysts were acquired
in bright-field mode using a Tecnai G2 F30 S-Twin electron
microscope operated at 300 kV. The reduced catalysts were
passivated by exposing them to N2O/He (10% v/v) at 70 °C for
1 h. The samples were prepared by dispersion of the catalysts
powder on a carbon film-supported Cu grid. The particle size
distribution was determined by measuring the sizes of more
than 200 particles in different selected regions of the TEM im-
ages.
H2 chemisorption experiments were performed by a static
chemisorption method using a Quantachrome Autosorb in-
strument. The experiment was carried out using ~0.5 g of the
Ni-Re/SiO2 catalysts. The samples were reduced in situ in a
flow of H2. After reduction, the sample was evaluated for 1 h to
remove the residual H2 and cooled to room temperature. The
H2 adsorption isotherms were measured at room temperature.
The irreversible H2 uptake, determined from the total and re-
versible H2 uptakes, was used to calculate the amount of sur-
face metal sites [20]. The H/Ni ratio in the chemisorption ex-
periments was taken as 1. No detectable H2 uptake was ob-
served on the monometallic Re/SiO2 catalyst.
2.2. Catalyst characterization
CO- and MEA-adsorbed FT-IR measurements were recorded
on a Thermo Scientific Nicolet iS50 spectrometer, equipped
with a mercury-cadmium-tellurium (MCT) detector and oper-
ated at a resolution of 4 cm–1. For the CO-adsorbed FT-IR
measurements, 15–20 mg powder samples were compressed
into a self-supporting wafer and reduced in a cell with H2 for 1
h. Following reduction, the cell was evacuated to 10–2 Pa and
cooled to room temperature. Background spectra were col-
lected before CO adsorption. After cooling, CO gas was intro-
duced into the cell for 20 min at room temperature. Finally, the
cell was evacuated again and the spectra were recorded against
a background of the sample. For the MEA-adsorbed FT-IR
measurements, the cell was evacuated to approximately 40–50
Pa and cooled to 50 °C after reduction. The IR spectra of the
samples were recorded as backgrounds. MEA vapor was intro-
duced in the cell at 50 °C for about 10 min. After MEA adsorp-
tion, physically adsorbed MEA was desorbed in vacuum (40–50
Pa) at 50 °C for about 10 min. Next, the temperature of the cell
was increased at a heating rate of 10 °C·min–1 and the spectra
of the samples were recorded.
N2 physisorption isotherms were recorded on
a
Quantachrome Autosorb instrument. Before the measure-
ments, the samples were outgassed at 300 °C for 3 h under
vacuum. The total specific surface area and pore size distribu-
tion were calculated by Brunauer-Emmet-Teller (BET) and
Barrett-Joyner-Halenda (BJH) analyses. The total pore volume
was determined from the amount of N2 adsorbed at a P/P0 ratio
of 0.99.
The metal loadings of Ni or Re in the Ni-Re/SiO2 catalysts
were determined by inductively coupled plasma optical emis-
sion spectroscopy (ICP-OES). A Perkin Elmer Optima 7300 DV
spectrometer was used for elemental detection.
H2-TPR and H2-TPD measurements were carried out using
an Altamira Instruments AMI-300 analyzer equipped with a
thermal conductivity detector (TCD). For the H2-TPR meas-
urements, about 100 mg of calcined catalysts were placed in a
quartz reactor. To remove impurities, the catalysts were treat-
ed in a flow of Ar at 400 °C for 0.5 h, followed by cooling to 50
°C. Next, the temperature was increased to 900 °C at a heating
rate of 10 °C·min–1 in a 30 ml·min–1 flow of H2/Ar (10% v/v).
The amount of H2 consumption was measured by the TCD. For
the H2-TPD measurements, the catalysts were reduced for 4 h
in a H2 flow, followed by cooling to 30 °C in Ar flow. Afterward,
the catalysts were exposed to a flow of H2/Ar (10% v/v) for 1 h
at 30 °C. After being purged in Ar flow for 30 min at 40 °C, the
catalysts were heated linearly at 10 °C·min–1 to 800 °C in Ar
flow (30 ml·min–1). The amount of desorbed H2 was measured
with the TCD.
X-ray photoelectron spectroscopy (XPS) spectra of the cata-
lysts were recorded using a Thermo ESCALAB 250Xi spec-
trometer. A monochromatic Al Kα X-ray source of 15 kV was
employed. The ex situ reduced powder catalysts were pressed
into self-supported pellets. The chamber was evacuated to
about 7.1 × 10–5 Pa. The binding energy (BE) values were re-
ferred to the Si 2p line at 103.4 eV.
2.3. Activity measurements
The XRD patterns of the reduced Ni-Re/SiO2 catalysts were
The catalytic performance of the Ni-Re/SiO2 catalysts for
MEA amination was evaluated in a trickle-bed reactor. As de-
scribed previously [21,22], about 3.6 ml (1.9 g) of oxide state
catalysts were placed in the center of a stainless steel reaction
tube, and the residual space was filled with silica sand. Before
reaction, the catalyst was reduced in situ. Following reduction,
recorded using
a PANalytical X’Pert PRO diffractometer
equipped with a Cu Kα radiation source. After reduction, the
catalysts were passivated by exposing them to N2O/He (10%
v/v) at 70 °C for 1 h. The patterns were recorded with a step
size of 0.013° and a scan speed of 0.04° s–1.

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Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
the temperature was decreased to the reaction temperature.
The feed, consisting of MEA and liquid NH3 (molar ratio 1:10),
was injected into the reactor by a syringe pump. According to
the industrial production conditions, MEA amination was per-
formed at 170 °C under a total pressure of 8.0 MPa, a liquid
hourly space velocity (LHSVMEA)of 0.5 h–1, and a H2 gas hourly
space velocity (GHSV) of 60 h–1. The products were collected at
intervals of about 6–12 h and analyzed using a gas chromato-
graph (Agilent 7890) fitted with a flame ionization detector and
a DB-35 capillary column. The conversion and selectivity were
calculated using the following equations:
Table 1
Physical properties of SiO2 support and calcined Ni-Re/SiO2 catalysts.
Surface area a Pore volume b Pore diameter c
Catalyst
(m2·g–1
285
259
253
226
)
(ml·g–1
1.1
0.8
0.7
0.7
)
(nm)
12.5
12.3
12.5
12.6
SiO2
Ni-Re/SiO2-NO
Ni-Re/SiO2-Ar
Ni-Re/SiO2-Air
a
Calculated by the BET equation at a relative pressure (P/P0) of
0.05–0.30; b Total pore volume at P/P0 ∼0.990; c Calculated by the BJH
method using the desorption branch; d Determined by ICP-OES analysis.
nMEA,in - nMEA,out
ConversionMEA % =
 100


3. Results and discussion
nMEA,in
ni
3.1. Physicochemical properties of calcined Ni-Re/SiO2 catalysts
Selectivity % =
i 
 100

n

i
where nMEA,in and nMEA,out are the number of moles of MEA ini-
tially added and remaining in the products, respectively, while
ni represents the molar amount of compound i in the products.
In order to determine the TOF of MEA, the amount of used
catalysts was varied in order to keep the conversion at a low
level (below 20%). The effects of internal and external diffusion
were excluded in the experiments. The reaction temperature
and pressure remained the same, while the LHSVMEA was varied
from 3.0 to 17.9 h–1. The TOF values of MEA were calculated
using the following equations:
The porous nature of the SiO2 carrier and the calcined
Ni-Re/SiO2 catalysts was confirmed by the analysis of N2 ad-
sorption-desorption data. The N2 adsorption-desorption iso-
therms are shown in Fig. 1. All samples exhibited type IV iso-
therms with H1-shaped hysteresis loops, indicating the pres-
ence of uniform cylindrical mesoporous channels. The values of
the BET surface area, total pore volume, and pore diameter are
listed in Table 1. The SiO2 support exhibited a surface area of
285 m2·g–1, a pore volume of 1.1 cm3·g–1, and a pore diameter
around 12.5 nm. After being impregnated with metal species
and calcined under different atmospheres, the Ni-Re/SiO2 cat-
alysts showed surface areas in the range of 226–259 m2·g–1.
The pore volume and diameter were around 0.7–0.8 ml·g–1 and
12.3–12.6 nm, respectively. The decrease in surface area and
pore volume was due to the partial blockage of the mesopores
after Ni and Re were loaded on the support.
The interaction between metal species and SiO2 support was
investigated using H2-TPR measurements. As shown in Fig. 2,
the monometallic Re/SiO2 sample displayed a broad H2 con-
sumption peak in the range of 330–420 °C, corresponding to
the reduction process of ReOx with different oxidation states
[21,23]. The Ni/SiO2 sample showed two reduction peaks: (I) a
broad peak around 412 °C ascribed to the reduction of bulk NiO
particles weakly interacting with SiO2, and (II) a small peak at
nMEA,in - nMEA,out
r μmol g-c1at. h-1
=


mcat. t
nMEA,in - nMEA,out
TOF h-1
=


nNi t
where r is the intrinsic reaction rate of catalyst, mcat. denotes
the mass of catalysts, t is the reaction time, and nNi represents
the molar amount of surface Ni sites measured by H2 chemi-
sorption.
1200
SiO2
1100
1000
900
800
700
600
500
400
300
200
100
0
Ni-Re/SiO2-Air
Ni-Re/SiO2-Ar
Ni-Re/SiO2-NO
I
II
(5)
(4)
(3)
0.0
0.5
P/P0
1.0
(2)
(1)
100
200
300
400
500
600
700
800
Temperature (oC)
Fig. 1. N2 adsorption-desorption isotherms of SiO2 support and calcined
Ni-Re/SiO2-Air, Ni-Re/SiO2-Ar, and Ni-Re/SiO2-NO catalysts. The iso-
therms of SiO2, Ni-Re/SiO2-Air, and Ni-Re/SiO2-Ar were offset vertically
by 380, 268, and 120 cm3·g‒1 (STP), respectively.
Fig. 2. H2-TPR profiles of Re/SiO2 (1); Ni/SiO2 (2); Ni-Re/SiO2-Air (3);
Ni-Re/SiO2-Ar (4); and Ni-Re/SiO2-NO (5) catalysts.

Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
571
510 °C that was attributed to highly dispersed Ni2+ species that
were difficult to reduce [24–27]. After being modified with the
Re promoter, the Ni-Re/SiO2-Air catalyst showed a broad peak
at 420 °C. Among the Ni-Re/SiO2 catalysts pretreated in a flow
of different gases, the Ni-Re/SiO2-Ar sample showed a reduc-
tion peak at 455°C, while the Ni-Re/SiO2-NO catalyst displayed
a reduction peak at 420 °C and a shoulder at 530 °C. A shift of
the reduction peak to higher temperature was observed in the
Ni-Re/SiO2 catalysts pretreated using either Ar or NO. This
phenomenon may be attributed to the stronger interaction
between NiO and the SiO2 support, due to the high dispersion of
Ni species after calcination under dynamic Ar/NO atmosphere
[28–31]. The H2-TPR results indicate that the calcination at-
mosphere could influence the reducibility of NiO species and
the interaction between NiO and support in Ni-Re/SiO2 cata-
lysts.
(5)
(4)
(3)
(2)
(1)
Re
(100)
Re
Re
Re
(102)
Re
(110)
Re
(103)
(002) (101)
30
40
50
60
70
80
2 (o)
Fig. 3. XRD patterns of reduced catalysts. (1) Ni-Re/SiO2-4.5; (2)
Ni-Re/SiO2-10.5; (3) Ni-Re/SiO2-14.6; (4) Ni-Re/SiO2-18.0; (5) Ni/SiO2.
The Re diffraction lines are also shown as reference.
3.2. Particle size characterization for reduced catalysts
particle size distribution of the reduced Ni-Re/Al2O3 catalysts
was analyzed by TEM. The corresponding images are shown in
Fig. 4 and the average particle sizes are listed in Table 2. The
The crystal phase and dispersion of the Ni particles in the
reduced catalysts were further investigated by XRD and TEM
measurements. The XRD patterns are shown in Fig. 3. The pat-
tern of monometallic Ni/SiO2 is also shown for comparison. The
Ni/SiO2 catalyst exhibited three peaks at 44.5°, 51.9°, and 76.4°,
which are characteristics of the (111), (200), and (220) crystal
planes of metallic Ni (JCPDS 65-2865). In the Ni-Re/SiO2 cata-
lysts, the introduction of Re did not affect the position of the Ni
peaks. However, the Ni-Re/SiO2 catalysts that were thermally
treated with different calcination atmospheres or reduction
temperatures displayed different peak intensities, indicating
different average crystallite sizes of the Ni particles in the four
Ni-Re/SiO2 catalysts. Moreover, as shown in Fig. 3, no peaks
corresponding to Re species (JCPDS 65-7974) were detected in
any of the Ni-Re/SiO2 catalysts. This result could be ascribed to
the low loading and high dispersion of the Re species. The Ni
high-resolution TEM image of
a
metal particle in
Ni-Re/SiO2-10.5 is shown in the inset of Fig. 4(2). The metal
particle exhibited an interplanar spacing of 0.206 nm, which
was assigned to the lattice fringe of the Ni(111) plane. As
shown in Fig. 4, the size distributions obtained from Gaussian
fitting confirmed that the Ni particles were uniformly dispersed
on the SiO2 surface. The TEM results indicate that Ni-Re/SiO2
catalysts with various Ni particles sizes (4.5–18.0 nm) were
successfully synthesized. The number of surface Ni sites and
the metal dispersion values were calculated from the H2 chem-
isorption data and are summarized in Table 2. It should be
noted that H2 chemisorption was not detected on monometallic
Re/SiO2. Therefore, the H2 consumption on the Ni-Re/SiO2 cat-
alysts was used determine the number of surface Ni sites. For
(1)
(2)
(3)
(4)
Fig. 4. TEM images and particle size distributions of reduced catalysts. (1) Ni-Re/SiO2-4.5; (2) Ni-Re/SiO2-10.5; (3) Ni-Re/SiO2-14.6; and (4)
Ni-Re/SiO2-18.0.

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Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
Table 2
Particle size, surface Ni sites, and Ni dispersion values of reduced Ni-Re/SiO2 catalysts.
Loading a (wt%)
Particle size b (nm)
–1
Catalyst
Surface Ni sites c (μmol·gcat.
)
Ni dispersion c (%)
Ni
Re
1.4
1.4
1.4
1.5
Fresh
4.5
Spent
5.2
Ni-Re/SiO2-4.5
Ni-Re/SiO2-10.5
Ni-Re/SiO2-14.6
Ni-Re/SiO2-18.0
19.6
19.2
19.7
20.8
424.6
181.0
108.9
64.1
12.7
5.5
3.1
10.5
14.6
18.0
11.1
15.0
18.2
1.8
a Obtained from ICP-OES results; b Calculated from TEM images; c Calculated from H2 chemisorption results.
the Ni-Re/SiO2 catalysts, this number increased by about seven
times (from 64.1 to 424.6 μmol·gcat.–1) as the Ni particle size
decreased from 18.0 to 4.5 nm. This result could be ascribed to
the enhanced Ni dispersion in Ni-Re/SiO2 catalysts calcined in a
flow of either Ar or NO [28,29].
ports, the MEA amination reaction was always performed un-
der high pressure (> 12 MPa), because the formation of super-
critical NH3 at a high reaction pressure was deemed crucial to
improve the catalyst activity [35,36]. In conclusion, the
Ni-Re/SiO2-4.5 catalyst, with the lowest particle size (4.5 nm),
exhibited a higher yield of the desired EDA and PIP amines
under a lower reaction pressure (8.0 MPa) compared to those
of the catalyst systems previously used for MEA amination.
It should be noted that the activity of Ni-Re/SiO2 catalysts
depends on the number of surface Ni sites [19,37]. The de-
crease of the Ni particle size improves the dispersion and
number of surface Ni sites in Ni-Re/SiO2 catalysts. Therefore,
the NO-pretreated Ni-Re/SiO2-4.5 catalyst, with the largest
number of surface Ni sites (Table 2), exhibited the highest MEA
conversion under the same reaction conditions (Table 3). As
shown in Scheme 1, at the beginning there are two competitive
reaction pathways for MEA amination: (1) the reaction of MEA
with NH3 to form EDA, and (2) the self-amination reaction be-
tween two MEA molecules to form AEEA. Next, as the MEA
conversion increases, the EDA and AEEA amines take part in
the subsequent amination reactions to form other secondary or
tertiary amines, including PIP, DETA, HEP, and AEP. The I:II:III
molar ratio was used to measure the progress of the MEA ami-
nation reaction. As shown in Table 3, the I:II:III ratio decreased
with an increase in MEA conversion, indicating that higher
amounts of secondary (PIP and DETA) and tertiary (AEP and
HEP) amines were generated. Furthermore, we calculated the
ratio between primary (EDA) and secondary (AEEA, PIP, and
DETA) amines (Pri.:Sec.), which is representative of the product
distribution. As shown in Table 3, the Pri.:Sec. ratio increased
with an increase in Ni particle size. However, it is unclear
whether these differences were due to the reaction progress or
to the Ni particle size. To investigate the effects of the Ni parti-
cle size on the amination activity and product distribution, the
MEA conversion should be controlled at a low and similar level.
The effects of the Ni particle size are discussed in the following
section.
3.3. MEA amination results
The MEA amination reactions were carried out in a trick-
le-bed reactor at 170 °C and 8.0 MPa. The LHSVMEA was main-
tained at around 0.5 h–1, according to the industrial production
conditions [4,32,33]. The product amines collected at ~30 h
were used to assess the catalytic performance. The results of
the amination reaction over the Ni-Re/SiO2 catalysts are shown
in Table 3. Upon increasing the particle size from 4.5 to 18.0
nm, the MEA conversion decreased from 85.7% to 51.1% and
the selectivity to EDA showed a linear increase (from 51.5% to
65.6%). Moreover, decreasing of the particle size significantly
improved the selectivity to PIP, while also reducing the genera-
tion of the AEEA by-product. At the same time, the selectivities
to DETA, HEP, and AEP showed a slight increase as the particle
size of the Ni-Re/SiO2 catalysts decreased. The results show
that the Ni-Re/SiO2-4.5 catalyst, with the lowest metal particle
size, exhibited the highest yield (66.4%) of the desired EDA and
PIP amines. To the best of our knowledge, the value obtained in
this work is higher than those reported in previous studies or
patents. Zhang et al. [34] studied the amination of MEA on
Co/H-ZSM-5 catalysts and obtained a total (EDA and PIP) yield
of 42.5% at 230 °C and 6.0 MPa. Ding and coworkers [33] as-
sessed the catalytic performance of Ni-Re/Al2O3 catalysts for
MEA amination in a fixed bed reactor. The Ni-Re/Al2O3 catalyst
showed a yield of 52.3% at 160 °C and 8.0 MPa. Chang et al.
[32] described Ni-based catalysts for the MEA amination reac-
tion. The highest total yield of EDA and PIP was reported to be
41.0% at 180 °C and 16.5 MPa. Another example is Ni-Re/Al2O3
that showed an EDA and PIP total yield of 57.8% for the amina-
tion of MEA at 180 °C and 12.5 MPa [4]. According to the re-
Table 3
MEA amination results of Ni-Re/SiO2 catalysts with different particle sizes.
Selectivity (%)
II
Yield of desired
amines d (%)
Catalyst
Conversion (%)
I
III
Others a
I:II:III b
Pri.:Sec. c
EDA
AEEA
PIP
DETA
Ni-Re/SiO2-4.5
Ni-Re/SiO2-10.5
Ni-Re/SiO2-14.6
Ni-Re/SiO2-18.0
85.7
73.0
57.6
51.1
51.5
59.4
64.1
65.6
7.4
9.2
12.1
13.0
26.0
17.4
12.0
10.7
9.7
9.0
7.6
7.0
5.5
5.1
4.2
3.7
59/36/5
69/26/5
76/20/4
79/18/3
1.2
1.7
2.0
2.1
66.4
56.0
43.8
38.9
a HEP and AEP; b Ratio of different product amines: I (nEDA + nAEEA) / II (nPIP + nDETA) / III (nHEP + nAEP); c Ratio between primary (EDA) and secondary
(AEEA, PIP, and DETA) amines; d Yield of EDA and PIP.

Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
573
(1)
(3)
(2)
(4)
Fig. 5. TEM images and particle size distributions of spent catalysts. (1) Ni-Re/SiO2-4.5; (2) Ni-Re/SiO2-10.5; (3) Ni-Re/SiO2-14.6; (4) Ni-Re/SiO2-18.0.
The morphology and particle size of the spent Ni-Re/SiO2
catalysts were also characterized by TEM. The TEM images and
Ni particle size distributions are shown in Fig. 5 and Table 2.
The comparison of the fresh and spent Ni-Re/SiO2 catalysts
reveals that the Ni particle sizes showed very little changes
after amination.
amines. Furthermore, the EDA:AEEA ratio was used to study
the competitive reactions of carbonyl compounds
(2-aminoacetaldehyde) with NH3 and another MEA molecule
(Scheme 1). As shown in Table 4, the EDA:AEEA ratio increased
from 1.3 to 2.7 when the Ni particle size increased from 4.5 to
18.0 nm. As the selectivity to DETA of the four Ni-Re/SiO2 cata-
lysts was almost equal (4.7%–5.8%), the increase in EDA can-
not originate from the inhibition of the second amination reac-
tion of EDA. The higher EDA:AEEA ratio indicates that the con-
densation between 2-aminoacetaldehyde and NH3 was facili-
tated, or that the condensation between 2-aminoacetaldehyde
and MEA was inhibited during the first amination process
(Scheme 1). To further analyze the size effects on the reaction
pathway, the TOF values of EDA and AEEA formation on
Ni-Re/SiO2 catalysts are also listed in Table 4. Upon increasing
the size of the Ni particles, TOFEDA increased from 63 to 119 h–1,
while TOFAEEA remained almost constant (44–49 h–1). These
results indicate that the Ni particle size affected the competitive
reactions during the amination reaction. The amination be-
tween MEA and NH3 molecules to form EDA was more favora-
ble on larger Ni particles of the Ni-Re/SiO2 catalysts during the
first amination process.
3.4. Effects of Ni particle size on catalytic performance
The intrinsic reaction rates and TOFs measured for the
Ni-Re/SiO2 catalysts are shown in Table 4. The I:II:III ratios for
all catalysts were around 89/10/1, indicating that the amina-
tion reaction proceeded to a low and similar extent. The MEA
amination results are listed in Table S1. The intrinsic reaction
rate decreased by more than five times (from 83 to 15
mmol·gcat.–1·h–1) as the Ni particle size increased from 4.5 to
18.0 nm. The TOF of MEA conversion of the Ni-Re/SiO2 cata-
lysts showed a slight increase from 193 to 253 h–1 with the
increase of the Ni particle size, indicating that the larger size of
the Ni particles had a positive effect on the activity of the
Ni-Re/SiO2 catalysts. Shimizu et al. [19] reported that the TOF
of per surface Ni site increased with a decrease in Ni particle
size. Such difference might be due to the different substrates
and reaction pressure used. Turning to the product distribu-
tion, the Pri.:Sec. molar ratio increased linearly from 1.0 to 2.0
as the Ni particle size increased, demonstrating that the surface
Ni sites in larger particles favored the formation of primary
It is well known that the surface chemical properties and
electronic structure vary with the metal particle size [38,39].
To understand the relationship between metal particle size and
catalytic performance, the electronic state of the nanoparticles
of Ni-Re/SiO2 catalysts was studied using CO-adsorbed FT-IR
Table 4
Intrinsic reaction rates and TOF data of Ni-Re/SiO2 catalysts with different particle sizes.a
Intrinsic reaction rate
Catalyst
I:II:III
Pri.:Sec.
EDA:AEEA
TOFMEA (h–1
)
TOFEDA (h–1
)
TOFAEEA (h–1)
(mmol·gcat.–1·h–1
)
Ni-Re/SiO2-4.5
Ni-Re/SiO2-10.5
Ni-Re/SiO2-14.6
Ni-Re/SiO2-18.0
88/11/1
87/11/2
91/08/1
89/10/1
83
37
25
15
1.0
1.3
1.6
2.0
1.3
1.9
2.1
2.7
193
219
234
253
63
82
101
119
49
44
48
44
a These values were determined at similar MEA conversion rates. The details are shown in the Experimental section.

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Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
(1)
(2)
(1)
(2)
(3)
(4)
(3)
(4)
2250 2200 2150 2100 2050 2000
Wavenumber (cm-1)
Fig. 6. FT-IR spectra of CO adsorption on reduced Ni-Re/SiO2 catalysts.
(1) Ni-Re/SiO2-4.5; (2) Ni-Re/SiO2-10.5; (3) Ni-Re/SiO2-14.6; (4)
Ni-Re/SiO2-18.0.
100
200
300
400
Temperature (oC)
spectroscopy [40,41]. The FT-IR spectra are shown in Fig. 6. In
general, all samples showed two absorption peaks centered at
2170 and 2120 cm–1, due to the physisorbed CO species on the
Ni-Re/SiO2 catalysts [42,43]. The bands corresponding to line-
arly adsorbed CO on surface step/corner Ni sites with lower
coordination numbers were observed at 2067 and 2055 cm–1.
The band around 2041 cm–1 could be ascribed to linear CO ad-
sorbed on high-coordinated terrace Ni sites [44–46]. As indi-
cated by the CO-adsorbed FT-IR results, the relative intensity of
the band at 2041 cm–1 gradually increased with an increase in
metal particle size in the Ni-Re/SiO2 catalysts. The absorbance
ratios of the peaks at 2041 and 2055 cm‒1 are also listed in
Table S2. These ratios increased with an increase in Ni particle
size, indicating a greater proportion of surface Ni sites with
high coordination number. The CO-probed IR results indicate
that the electronic structure of the Ni-Re/SiO2 catalysts was
influenced by the Ni particle size, which was probably the main
reason for the difference in catalytic performance.
The H2-TPD method was used to further investigate the
surface properties of Ni in the Ni-Re/SiO2 catalysts. The H2-TPD
profiles and corresponding deconvoluted peaks are shown in
Fig. 7. No H2 desorption on reduced Re/SiO2 and unreduced
Ni-Re/SiO2 catalysts was detected by TCD. Therefore, the H2
amount detected in the Ni-Re/SiO2 catalysts was ascribed to
the hydrogen adsorbed on Ni sites. All samples showed a broad
peak in the range of 50–480 °C, which included several com-
ponents. Four peaks around 94, 170, 238, and 315 °C were ob-
tained after deconvolution. It has been reported that Ni atom at
different sites (terraces, steps, and corners) show different
hydrogen adsorption strength [9,47,48]. Hydrogen was weakly
adsorbed on high-coordinated terrace Ni. In contrast, corner Ni
sites with low coordination number showed the highest hy-
drogen adsorption strength. According to a previous study, the
peak at 94 °C was attributed to the desorption of weakly ad-
sorbed H2. The three other peaks at higher temperature could
Fig. 7. H2-TPD profiles of Ni-Re/SiO2 catalysts. (1) Ni-Re/SiO2-4.5; (2)
Ni-Re/SiO2-10.5; (3) Ni-Re/SiO2-14.6; (4) Ni-Re/SiO2-18.0.
be assigned to desorption of hydrogen from Ni at terrace, step,
and corner sites, respectively. The calculated proportions of
different Ni surface sites are shown in Fig. 8. The proportion of
terrace Ni sites increased linearly with the particle size, while
the proportion of step/corner Ni showed the opposite trend.
The H2-TPD results further confirm that the proportion of dif-
ferent surface Ni sites varied with the particle size of the
Ni-Re/SiO2 catalysts.
XPS was used to analyze the electronic structure of the
Ni-Re/SiO2 catalysts. The of Ni 2p XPS spectra the ex situ re-
duced Ni-Re/SiO2 catalysts and the deconvoluted peaks are
100
Terrace
Step
Corner
80
60
40
20
0
4
8
12
Particle size (nm)
16
20
Fig. 8. Proportion of different surface Ni sites obtained from the H2-TPD
results.

Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
575
decreased from 0.087 to 0.011 upon increasing the Ni particle
size, which was ascribed to the lower Ni dispersion. In conclu-
sion, the XPS results indicate that the electronic properties of
surface Ni sites were influenced by the particle size. The elec-
tron density of Ni increased with an increase in particle size.
Ni2+ 2p3/2
5000
Satellite
Ni2+ 2p1/2
Ni0
2p3/2
Satellite
Ni0 2p1/2
(1)
3.5. Discussion
Few previous reports investigated the effects of the metal
particle size on the amination of alcohol amines. In this study,
to focus on the effects of the Ni particle size, we employed inert
silica to eliminate support interferences. As shown in Fig. S1,
the activity of monometallic Ni/SiO2 decreased rapidly in the
initial reaction stage. According to previous studies, the deacti-
vation of Ni/SiO2 catalysts is due to the sintering of metallic Ni
particles under H2 and NH3 atmospheres at high pressure
[21,52,53]. Therefore, the Re promoter was introduced in the
Ni/SiO2 catalyst system in order to maintain the stability of the
Ni particle size. To assimilate the effect of Re, the metal loading
of Re in all Ni-Re/SiO2 catalysts was kept constant at 1.5 wt%
(Table 2). By adjusting the calcination atmosphere and reduc-
tion temperature, Ni-Re/SiO2 catalysts with varying Ni particle
sizes (4.5–18.0 nm, Table 2) were synthesized. The monome-
tallic Re/SiO2 catalyst showed no activity in MEA amination. In
addition, the XRD and high-resolution TEM results reveal that
the structure of Ni was not modified by Re. After amination, the
Ni particle sizes in spent Ni-Re/SiO2 catalysts (Table 2) showed
very little changes compared to the fresh catalysts.
2500
(2)
1000
(3)
1000
(4)
The catalytic performances of the Ni-Re/SiO2 catalysts (Ta-
ble 3) reveal that the MEA conversion and yield of desired
amines (EDA and PIP) increased with a decrease in Ni particle
size. The Ni-Re/SiO2-4.5 catalyst showed the highest yield
(66.4%). To the best of our knowledge, this is the highest MEA
amination yield reported to date. As the MEA amination reac-
tion is catalyzed by metallic Ni sites, the main reason for the
high MEA conversion is the increased surface Ni area of the
Ni-Re/SiO2-4.5 catalyst.
885 880 875 870 865 860 855 850
Binding energy (cm-1)
Fig. 9. Ni 2p XPS spectra of Ni-Re/SiO2-4.5 (1); Ni-Re/SiO2-10.5 (2);
Ni-Re/SiO2-14.6 (3); and Ni-Re/SiO2-18.0 (4).
Table 5
Binding energies and atomic ratios of Ni-Re/SiO2 catalysts.
The structure-activity relationship between Ni particle size
and MEA amination was then investigated. The experimental
results (Table 4) indicate that the TOFMEA and molar ratio of
primary to secondary amines increased with an increase in Ni
particle size of the Ni-Re/SiO2 catalysts. Based on the above
results, we conclude that increasing the size of Ni particles of
Ni-Re/SiO2 catalysts not only has a positive effect on their ac-
tivity for MEA amination, but also improves their selectivity to
primary amines. Further analysis reveals that the TOFEDA in-
creased from 63 h–1 for Ni-Re/SiO2-4.5 to 119 h–1 for
Ni-Re/SiO2-18.0. On the other hand, the Ni particle size had
only minor effects on the TOFAEEA value. These results indicate
that the Ni particle size affected the reaction pathway of the
amination reaction (Scheme 1). More specifically, increasing
the metal particle size could promote the amination reaction
between MEA and NH3 molecules over Ni-Re/SiO2 catalysts.
This phenomenon could be attributed to the changes in surface
electronic structure of the metal particles, as shown by
CO-adsorbed FT-IR, H2-TPD, and XPS measurements. The
change in electronic properties may have an impact on the ad-
Ni 2p3/2 (eV)
Ni:Si atomic Ni reduction
Catalyst
Ni2+
Ni0
ratio
degree (%)
Ni-Re/SiO2-4.5
Ni-Re/SiO2-10.5
Ni-Re/SiO2-14.6
Ni-Re/SiO2-18.0
855.5
855.4
855.3
855.2
853.7
853.0
852.7
852.5
0.087
0.049
0.013
0.011
27
32
36
39
shown in Fig. 9, while the curve-fitting data are listed in Table
5. The peaks around 860 and 879 eV were ascribed to the
shake-up satellite peaks of Ni2+ 2p3/2 and Ni2+ 2p1/2, respective-
ly [21,49]. It can be observed that the Ni0 2p3/2 peak shifts from
853.7 to 852.5 eV with an increase in size of Ni particles, prob-
ably due to the different atomic relaxation in large and small
nanoparticles [50,51]. The surface Ni sites showed a lower
electron density in small than large particles. Furthermore,
oxidation of the catalysts took place during their preparation,
and varying degrees of oxidation were observed in the four
catalysts. The decrease in particle size led to a significantly
higher proportion of Ni2+ (around 855 eV), indicating that small
Ni particles were more easily oxidized. The Ni:Si atomic ratio

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Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
sorption and/or dehydrogenation progress of MEA during
amination, leading to a different product distribution. Taking
Ni-Re/SiO2-4.5 and Ni-Re/SiO2-18.0 catalysts as an examples,
the adsorption strength of MEA would be lower on the
Ni-Re/SiO2-18.0 than Ni-Re/SiO2-4.5 catalyst, due to the dif-
ference in the electron density and the coordinatively unsatu-
rated environment of surface Ni atoms. After dehydrogenation
of MEA, the carbonyl compounds would be more easily de-
sorbed from the high-coordinated terrace Ni sites and trans-
ferred to the liquid phase to react with NH3 (Scheme 3), com-
pared to the same process on surface step/corner Ni sites. For
these reasons, the Ni-Re/SiO2-18.0 catalyst with a higher pro-
portion of terrace Ni sites was more active in the formation of
EDA.
To prove this hypothesis, we tested the adsorption strength
of MEA on Ni-Re/SiO2-4.5 and Ni-Re/SiO2-18.0 catalysts using
in situ FT-IR measurements. The adsorption of MEA on
Ni-Re/SiO2 took place at 50 °C and physically adsorbed MEA
was desorbed in vacuum at about 40–50 Pa. Next, the changes
in the MEA-adsorbed IR spectra were recorded during a tem-
perature programmed process in vacuum. The FT-IR spectra
are shown in Fig. 10. For comparison, we also recorded the IR
spectra of liquid and vapor MEA, shown in Fig. S2. Liquid MEA
showed three peaks at 1599, 1460, and 1358 cm–1, which were
assigned to the deformation vibration of NH, –CH2, and OH
groups, respectively [54]. An adsorption peak at 1625 cm–1 was
detected in the spectrum of MEA vapor. Combined with the
characteristic peaks of the CH stretching vibration at 2931 and
2864 cm–1 and the NH stretching vibration at 3334 cm–1, the
peak at 1625 cm–1 was assigned to the deformation vibration of
NH, which was blue-shifted compared to that in the spectrum
of liquid MEA. In the case of MEA vapor adsorbed on
Ni-Re/SiO2 catalysts (Fig. 10), NH deformation bands were
observed around 1630 cm–1. The peak intensity decreased
gradually with an increase in temperature, indicating that MEA
was desorbed from the surface of the Ni-Re/SiO2 catalysts. The
peak intensity of the Ni-Re/SiO2-4.5 catalyst (Fig. 10(a)) was
nearly level with the baseline when the temperature reached
approximately 100 °C, indicating that MEA was almost com-
pletely desorbed from surface of catalyst. However, in Fig.
(1)
(2)
Scheme 3. Schematic representation of the desorption and diffusion of MEA on the surface of (1) small and (2) large metal particles in Ni-Re/SiO2
catalysts during MEA amination.
(1)
(2)
0.01
0.01
50 AD
50 DE
55 DE
60 DE
65 DE
70 DE
75 DE
80 DE
85 DE
90 DE
95 DE
100 DE
50 AD
50 DE
55 DE
60 DE
65 DE
70 DE
75 DE
Baseline
1600
Baseline
1750
1700
1650
1550
1500
1450
1750
1700
1650
1600
1550
1500
1450
Wavenumber (cm-1)
Wavenumber (cm-1)
Fig. 10. FT-IR spectra of adsorbed MEA on Ni-Re/SiO2-4.5 (1) and Ni-Re/SiO2-18.0 catalysts (2).

Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
577
10(2), the peak disappeared at a lower temperature (70–75
°C), demonstrating that MEA desorbed more easily from the
Ni-Re/SiO2-18.0 catalyst. The MEA-adsorbed FT-IR results
prove that the adsorption strength of MEA was weaker on the
Ni-Re/SiO2-18.0 than Ni-Re/SiO2-4.5 catalyst.
In conclusion, increasing the Ni particle size in Ni-Re/SiO2
catalysts slightly improved the TOF of MEA amination, and
enhanced the selectivity to EDA. Further analyses indicate that
increasing the Ni particle size affected the reaction pathway by
promoting the amination between MEA and NH3 to form EDA.
In practical applications, the decrease in Ni particle size could
improve the utilization of Ni species by enhancing the metal
dispersion of the Ni-Re/SiO2 catalysts. Although the decrease in
Ni particle size partly reduced the TOFMEA, it could improve the
yield of desired amines. In our study, the Ni-Re/SiO2-4.5 cata-
lyst with Ni particle sizes of 4.5 nm exhibited the highest yield
of EDA and PIP amines.
The amination of MEA was a structure-sensitive reaction.
The variation of the Ni particle size affected the activity of the
Ni-Re/SiO2 catalysts and their product distribution during the
reaction. The experimental results indicate that the TOF values
of MEA amination were higher for larger Ni particles. For the
amination products, the TOFEDA and Pri.:Sec. values increased
with an increase in Ni particle size. Further analyses show that
the Ni particle size influenced the electron density and coordi-
nation environment of surface Ni sites in Ni-Re/SiO2 catalysts.
Increasing the Ni particle size enhanced the proportion of ter-
race Ni sites and increased the electron density of surface Ni
sites. Large Ni particles were more beneficial for the desorption
of intermediate products, thus facilitating the amination be-
tween MEA and NH3 to form EDA.
References
[1] G. F. Mackenzie, US Patent 2861995, 1958.
[2] R. Lichtenberger, F. Weiss, US Patent 3068290, 1962.
[3] P. H. Moss, N. B. Godfrey, US Patent 3037023, 1962.
[4] S. W. King, US Patent 5750790, 1998.
[5] A. Baiker, J. Kijenski, Catal. Rev. Sci. Eng., 1985, 27, 653–697.
[6] H. Kimura, Catal. Rev. Sci. Eng., 2011, 53, 1–90.
[7] K. Shimizu, Catal. Sci. Technol., 2015, 5, 1412–1427.
[8] Q. X. Li, G. Y. Zhang, S. Y. Peng, Chin. J. Catal., 2001, 22, 7–10.
[9] F. Yang, D. Liu, Y. Zhao, H. Wang, J. Han, Q. Ge, X. Zhu, ACS Catal.,
2018, 8, 1672–1682.
[10] L. Liu, A. Corma, Chem. Rev., 2018, 118, 4981–5079.
[11] Q. Zhang, W. Deng, Y. Wang, Chem. Commun., 2011, 47,
9275–9292.
[12] D. Baudouin, U. Rodemerck, F. Krumeich, A. D. Mallmann, K. C.
Szeto, H. Ménard, L. Veyre, J. P. Candy, P. B. Webb, C. Thieuleux, C.
Coperet, J. Catal., 2013, 297, 27–34.
4. Conclusions
In this study, we provided insights into the effects of the Ni
particle size of Ni-Re/SiO2 catalysts on MEA amination.
Ni-Re/SiO2 catalysts (Ni 20 wt%, Re 1.5 wt%) with variable Ni
particle sizes were prepared by adjusting the calcination at-
mosphere and reduction temperature. The Ni particle sizes of
the Ni-Re/SiO2 catalysts remained almost unchanged after the
amination reaction. The Ni-Re/SiO2-4.5 catalyst, with an aver-
age particle size of 4.5 nm, showed the highest yield (66.4%) of
the desired amines (EDA and PIP), which was higher than the
yields reported in previous studies and patents. The high MEA
conversion of the Ni-Re/SiO2-4.5 catalyst was due to its high Ni
dispersion and abundant surface Ni sites.
Graphical Abstract
Chin. J. Catal., 2019, 40: 567–579 doi: S1872-2067(19)63302-4
Effects of Ni particle size on amination of monoethanolamine over Ni-Re/SiO2 catalysts
Lei Ma, LiYan*, An-Hui Lu, Yunjie Ding *
Dalian Institute of Chemical Physics, Chinese Academy of Sciences; Dalian University of Technology;
University of Chinese Academy of Sciences
MEA
MEA
EDA
Ni-Re/SiO2
AEEA
EDA
AEEA
The reaction pathway of MEA amination by Ni-Re/SiO2 catalyst is affected by the Ni particle size. Larger Ni particles are more beneficial
for the amination of MEA and NH3 to form EDA.

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Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
[13] J. Z. Guo, Z. Y. Hou, J. Gao, X. M. Zheng, Chin. J. Catal., 2007, 28,
22–26.
[14] S. Y. Wang, B. Zeng, C. Li, Chin. J. Catal., 2018, 39, 1219–1227.
[15] Y. X. Chen, Z. W. Huang, G. Xiao, M. Zhen, J. M. Chen, X. F. Tang, Chin.
J. Catal., 2017, 38, 1588–1596.
2013152548 A1, 2013.
[34] Y. Zhang, G. Bai, X. Yan, Y. Li, T. Zeng, J. Wang, H. Wang, J. Xing, D.
Luan, X. Tang, L. Chen, Catal. Commun., 2007, 8, 1102–1106.
[35] A. Fischer, T. Mallat, A. Baiker, Angew. Chem. Int. Ed., 1999, 38,
351–354.
[16] Z. Chen, Y. Liang, D. S. Jia, Z. M. Cui, W. G. Song, Chin. J. Catal., 2017,
38, 651–657.
[17] R. J. Shi, F. Wang, X. L. Mu, N. Ta, Y. Li, X. M. Huang, W. J. Shen, Chin.
J. Catal., 2010, 31, 626–630.
[18] P. M. Mortensen, J. D. Grunwaldt, P. A. Jensen, A. D. Jensen, Catal.
Today, 2016, 259, 277–284.
[19] K. I. Shimizu, N. Imaiida, K. Kon, S. M. A. H. Siddiki, A. Satsuma, ACS
Catal., 2013, 3, 998–1005.
[20] R. C. Reuel, C. H. Bartholomew, J. Catal., 1984, 85, 63–77.
[21] L. Ma, L. Yan, A. H. Lu, Y. J. Ding, RSC Adv., 2018, 8, 8152–8163.
[22] L. Ma, K. C. Sun, M. Luo, L. Yan, Z. Jiang, A. H. Lu, Y. J. Ding, J. Phys.
Chem. C, 2018, 122, 23011–23025.
[23] S. R. Bare, S. D. Kelly, F. D. Vila, E. Boldingh, E. Karapetrova, J. Kas,
G. E. Mickelson, F. S. Modica, N. Yang, J. J. Rehr, J. Phys. Chem. C,
2011, 115, 5740–5755.
[24] C. Louis, Z. X. Cheng, M. Che, J. Phys. Chem., 1993, 97,
5703–5712.
[25] X. Kong, Y. Zhu, H. Zheng, X. Li, Y. Zhu, Y. W. Li, ACS Catal., 2015, 5,
5914–5920.
[26] B. Mile, D. Stirling, M. A. Zammitt, A. Lovell, M. Webb, J. Catal.,
1988, 114, 217–229.
[27] X. F. Wang, J. X. Chen, Chin. J. Catal., 2017, 38, 1818–1830.
[28] J. R. A. Sietsma, J. D. Meeldijk, M. Versluijs-Helder, A. Broersma, A. J.
Van Dillen, P. E. de Jongh, K. P. de Jong, Chem. Mater., 2008, 20,
2921–2931.
[29] J. R. A. Sietsma, H. Friedrich, A. Broersma, M. Versluijs-Helder, A. J.
Van Dillen, P. E. de Jongh, K. P. de Jong, J. Catal., 2008, 260,
227–235.
[36] G. Jenzer, T. Mallat, A. Baiker, Catal. Lett., 1999, 61, 111–114.
[37] J. H. Cho, J. H. Park, T. S. Chang, J. E. Kim, C. H. Shin, Catal. Lett.,
2013, 143, 1319–1327.
[38] J. M. Mayne, K. A. Dahlberg, T. A. Westrich, A. R. Tadd, J. W.
Schwank, Appl. Catal. A, 2011, 400, 203–214.
[39] Y. Yao, Z. Yan, L. Chen, Z. Zhou, L. Liu, D. W. Goodman, Catal. Lett.,
2012, 142, 1437–1444.
[40] C. Asokan, L. Derita, P. Christopher, Chin. J. Catal., 2017, 38,
1473–1480.
[41] H. J. Shen, X. Y. Wu, D. H. Jiang, X. N. Li, J. Ni, Chin. J. Catal., 2017, 38,
1597–1602.
[42] Y. K. Lee, S. T. Oyama, J. Catal., 2006, 239, 376–389.
[43] H. X. Cao, J. Zhang, C. L. Guo, J. G. Chen, X. K. Ren, Chin. J. Catal.,
2017, 38, 1127–1137.
[44] C. Hu, Y. Chen, P. Li, H. Min, Y. Chen, A. Tian, J. Mol. Catal. A, 1996,
110, 163–169.
[45] G. Poncelet, M. A. Centeno, R. Molina, Appl. Catal. A, 2005, 288,
232–242.
[46] M. B. Jensen, S. Morandi, F. Prinetto, A. O. Sjåstad, U. Olsbye, G.
Ghiotti, Catal. Today, 2012, 197, 38–49.
[47] A. S. Mårtensson, C. Nyberg, S. Andersson, Surf. Sci., 1988, 205,
12–24.
[48] B. Liu, M. T. Lusk, J. F. Ely, J. Phys. Chem. C, 2009, 113,
13715–13722.
[49] C. P. Li, A. Proctor, D. M. Hercules, Appl. Spectrosc., 1984, 38,
880–886.
[50] G. F. Liang, Y. Zhou, J. P. Zhao, A. Y. Khodakov, V. V. Ordomsky, ACS
Catal., 2018, 8, 11226–11234.
[30] J. R. A. Sietsma, J. D. Meeldijk, J. P. den Breejen, M. Versluijs-Helder,
A. J. Van Dillen, P. E. de Jongh, K. P. de Jong, Angew. Chem. Int. Ed.,
2007, 46, 4547–4549.
[31] S. L. Soled, E. Iglesia, R. A. Fiato, J. E. Baumgartner, H. Vroman, S.
Miseo, Top. Catal., 2003, 26, 101–109.
[51] Z. J. Zhao, F. Liu, L. M. Qiu, L. Z. Zhao, S. K. Yan, Acta Phys.-Chim. Sin.,
2008, 24, 1685–1688.
[52] J. Lif, M. Skoglundh, L. Löwendahl, Appl. Catal. A, 2002,
228,145–154.
[53] J. Lif, M. Skoglundh, L. Löwendahl, Appl. Catal. A, 2004, 274, 61–69.
[54] K. Segawa, S. Mizuno, Y. Maruyama, S. Nakata, Stud. Surf. Sci. Catal.,
1994, 84, 1943–1950.
[32] D. Chang, F. A. Sherrod, US Patent 5817593, 1998.
[33] L. Yan, Y. J. Ding, Y. Lv, X. B. Cheng, L. X. Ma, WO Patent
Ni-Re/SiO2催化剂中Ni颗粒尺寸对乙醇胺催化胺化反应的影响
马 雷a,b,d, 严 丽a,#, 陆安慧b, 丁云杰a,c,*
a中国科学院大连化学物理研究所, 洁净能源国家实验室(筹), 辽宁大连116023
b大连理工大学化工学院, 精细化工国家重点实验室, 辽宁大连116024
c中国科学院大连化学物理研究所, 催化基础国家重点实验室, 辽宁大连116023
d中国科学院大学, 北京100049
摘要: 以醇和氨/胺为原料采用催化胺化法合成有机胺长期以来广受关注, 该工艺反应副产物只有水, 符合现代绿色化工理
念. 作为工业应用的成功案例, 乙醇胺(MEA)催化胺化法目前已成为生产乙撑胺(包括乙二胺(EDA), 哌嗪(PIP)等)的主要工
艺. MEA催化胺化通常在过渡金属催化剂上进行, 如Ni基催化剂, 反应过程遵循“借氢机理”, 经历了“脱氢-缩合胺化-加氢”
三个过程.
目前, 关于MEA胺化催化剂的研究主要以专利为主, 据我们所知, 对于催化剂金属颗粒尺寸和胺化性能之间的构效关
系的研究极少, 特别是对于金属粒径对胺化产物分布的影响, 目前还未见报道. 因此, 本文的目的是合成具有不同Ni颗粒
尺寸的Ni-Re/SiO2催化剂, 通过滴流床反应器评价, 研究颗粒尺寸对MEA胺化活性和产物分布的影响. 采用控制催化剂焙
烧和还原条件的方法, 制备了四种不同Ni粒径的Ni-Re/SiO2催化剂; 随后采用N2物理吸附, H2程序升温还原, X射线衍射, H2

Lei Ma et al. / Chinese Journal of Catalysis 40 (2019) 567–579
579
化学吸附, 光电子能谱, 红外吸收光谱等对催化剂的孔结构、还原性、粒径分布、金属Ni活性比表面、分散度、表面Ni位
点类型和电子性质进行研究. MEA催化胺化反应在滴流床反应器上进行, 反应条件为170 ºC, 8.0 MPa, MEA液时空速0.5
h–1, NH3:MEA摩尔比10:1, H2含量2.5 mol%.
表征结果证明, 成功制备了具有不同Ni粒径, 且尺寸分布集中的Ni-Re/SiO2催化剂, Ni粒径分别为4.5, 10.5, 14.6, 18.0
nm. 评价结果表明, Ni颗粒4.5 nm的Ni-Re/SiO2催化剂具有最高的活性, MEA转化率高达85.7%, EDA和PIP收率为66.4%,
优于以往专利文献所报道的值. 结合表征结果分析, 这是由于Ni-Re/SiO2 (4.5 nm)上的Ni粒径小, 分散度高, 因此Ni活性比
表面积大, 为胺化反应提供了充足的催化位点. 进一步深入研究了Ni颗粒尺寸对MEA胺化反应的影响, 发现随着Ni粒径增
加, 底物MEA的转换频率(TOF)从193 h–1增加到253 h–1, 表明大颗粒有利于胺化反应. 此外, 随着粒径增加, 产物中伯胺和
仲胺的摩尔比从1.0增加到2.0, 产物EDA的TOFEDA值由63 h–1增加到119 h–1, 表明Ni粒径变化影响了MEA胺化反应途径和
产物分布, 增大粒径有利于EDA的生成, 从而提高了产物中的伯: 仲胺之比. 实验结果证明, Ni-Re/SiO2催化剂粒径影响了
颗粒表面Ni的配位环境, 从而改变了表面的电子结构. 增大Ni粒径可提高面位点Ni的比例和表面电子云密度, 导致中间产
物容易从Ni表面脱附, 从而有利于促进反应初始阶段MEA与NH3的胺化, 提高EDA选择性.
关键词: 颗粒尺寸效应; 乙醇胺; 胺化反应; 镍-铼/氧化硅; 转换频率
收稿日期: 2018-10-24. 接受日期: 2019-01-08. 出版日期: 2019-04-05.
*通讯联系人. 电话/传真: (0411)84379143; 电子信箱: dyj@dicp.ac.cn
#通讯联系人. 电话/传真: (0411)84379055; 电子信箱: yanli@dicp.ac.cn
基金来源: 国家自然科学基金(21273227); 中国科学院战略性先导科技专项(XDB17000000).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).
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