Effect of Re promoter on the structure and catalytic performance of Ni-Re/Al2O3 catalysts for the reductive amination of monoethanolamine

Article abstract of DOI:10.1039/c7ra12891f

In this paper, Ni/Al₂O₃ catalysts (15 wt% Ni) with different Re loadings were prepared to investigate the effect of Re on the structure and catalytic performance of Ni-Re/Al₂O₃ catalysts for the reductive amination of monoethanolamine. Reaction results reveal that the conversion and ethylenediamine selectivity increase significantly with increasing Re loading up to 2 wt%. Ni-Re/Al₂O₃ catalysts show excellent stability during the reductive amination reaction. The characterization of XRD, DR UV-Vis spectroscopy, H₂-TPR, and acidity-basicity measurements indicates that addition of Re improves the Ni dispersion, proportion of octahedral Ni²⁺ species, reducibility, and acid strength for Ni-Re/Al₂O₃ catalysts. The Ni15 and Ni15-Re2 catalysts were chosen for in-depth study. The results from SEM-BSE, TEM, and CO-TPD indicate that smaller Ni⁰ particle size and higher Ni⁰ surface area are obtained in the reduced Ni-Re/Al₂O₃ catalysts. Results from in situ XPS and STEM-EDX line scan suggest that Re species show a mixture of various valances and have a tendency to aggregate on the surface of Ni⁰ particles. During reaction, the Ni⁰ particles on the Al₂O₃ support are stabilized and the sintering process is effectively suppressed by the incorporation of Re. It could be concluded that sufficient Ni⁰ sites, the collaborative effect of Ni-Re, and brilliant stability contribute to the excellent catalytic performance of Ni-Re/Al₂O₃ catalysts for the reductive amination of monoethanolamine.

Full text of DOI:10.1039/c7ra12891f

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PAPER
Effect of Re promoter on the structure and catalytic
performance of Ni–Re/Al2O3 catalysts for the
reductive amination of monoethanolamine†
Cite this: RSC Adv., 2018, 8, 8152
b
a
bc
Lei Ma,abd Li Yan, An-Hui Lu
and Yunjie Ding
*
*
In this paper, Ni/Al2O3 catalysts (15 wt% Ni) with different Re loadings were prepared to investigate the effect
of Re on the structure and catalytic performance of Ni–Re/Al2O3 catalysts for the reductive amination of
monoethanolamine. Reaction results reveal that the conversion and ethylenediamine selectivity increase
significantly with increasing Re loading up to 2 wt%. Ni–Re/Al2O3 catalysts show excellent stability during
the reductive amination reaction. The characterization of XRD, DR UV-Vis spectroscopy, H2-TPR, and
acidity–basicity measurements indicates that addition of Re improves the Ni dispersion, proportion of
octahedral Ni2+ species, reducibility, and acid strength for Ni–Re/Al2O3 catalysts. The Ni15 and Ni15–Re2
catalysts were chosen for in-depth study. The results from SEM-BSE, TEM, and CO-TPD indicate that
smaller Ni0 particle size and higher Ni0 surface area are obtained in the reduced Ni–Re/Al2O3 catalysts.
Results from in situ XPS and STEM-EDX line scan suggest that Re species show a mixture of various
valances and have a tendency to aggregate on the surface of Ni0 particles. During reaction, the Ni0
particles on the Al2O3 support are stabilized and the sintering process is effectively suppressed by the
incorporation of Re. It could be concluded that sufficient Ni0 sites, the collaborative effect of Ni–Re, and
brilliant stability contribute to the excellent catalytic performance of Ni–Re/Al2O3 catalysts for the
reductive amination of monoethanolamine.
Received 29th November 2017
Accepted 4th February 2018
DOI: 10.1039/c7ra12891f
rsc.li/rsc-advances
In the reductive amination process, ethylene amines are
synthesized by contacting MEA and ammonia under hydrogen
Introduction
atmosphere in the presence of hydrogenation–dehydrogenation
metal catalysts. The reaction pathways for the transformation of
MEA to ethylene amines on a metal catalyst are postulated as
follows: rstly, dehydrogenation of MEA to 2-amino-
acetaldehyde; secondly, condensation with ammonia or an
amine to form imine; lastly, hydrogenation of imine to
amine.7,8,11,15 The hydrogenation and dehydrogenation
processes are catalyzed by the transition metal, while the
condensation process could be accelerated by acid or base
sites.16–19 The presence of small amount of hydrogen is found to
be necessary to maintain the activity of catalyst during the
reductive amination process.8,20,21 The dashed box in Fig. 1
indicates the main reaction path for the generation of EDA.
Moreover, the product amines could continue to take part in the
reductive amination process as reactants. The presence of
continuous reaction makes the pathways more complicated and
decreases the selectivity of the desired amine. As a result, the
production of various by-products increases the cost of sepa-
ration process. For this reason, it's important for the catalyst to
have both high activity and selectivity of the desired product,
such as EDA.
Ethylene amines is the collective name for ethylenediamine
(EDA), polyethylene polyamines, and the cyclic amines,
including piperazine (PIP), aminoethyl piperazine (AEP), and
hydroxyethyl piperazine (HEP) etc. As an important interme-
diate, ethylene amines have wide applications in the production
of pharmaceuticals, lube oil additives, chelating agents, fungi-
cides, oil eld chemicals, paper wet-strength resins, surfactants,
and fabric soeners. Reductive amination of monoethanol-
amine (MEA) to ethylene amines is a common synthesis route
which has been applied in industrial production process since
the 1960s.1–4 The reductive amination method has gained
increasing interest since it constitutes a green economy route
for the synthesis of ethylene amines.5–14
aState Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, No. 2 Linggong Road, Dalian 116024, P. R. China
bDalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, 116023, China. E-mail:
dyj@dicp.ac.cn; Fax: +86 411 84379143; Tel: +86 411 84379143
cState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, 457 Zhongshan Road, Dalian, 116023, China
dUniversity of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049,
China
Different catalytic metal systems such as Ni,17–19,21–24 Ni–
Re,25–29 Co,20,30,31 Cu,32–35 Fe,36–38 Rh,39,40 Ru,12,13,41,42 Pt,43–45 and
Pd46 etc. have been tested in the reductive amination of amino
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra12891f
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ratio of Ni : Al2O3 was 15 : 100) was synthesized by impregna-
tion of Al2O3 (Shandong Aluminum Corp., A-AS-04) with an
aqueous solution of Ni(NO3)2$6H2O (99.9% purity). Aer
impregnation, the catalyst was initially dried at room temper-
ature. The dried catalyst was then kept in an oven at 120 ꢀC for
4 h and in a muffle at 500 ꢀC for another 4 h. The calcined
catalyst was denoted as Ni15. The monometallic Re/Al2O3
catalyst with 2 wt% Re (the mass ratio of Re : Al2O3 was 2 : 100)
was synthesized by impregnation of Al2O3 with an aqueous
solution of NH4ReO4 (99.99% purity). The drying and roasting
processes were the same as before and the obtained catalyst was
denoted as Re2. Ni–Re/Al2O3 catalysts were prepared by co-
impregnation of the mixture of Ni(NO3)2$6H2O and NH4ReO4
aqueous solution on Al2O3. The Ni loading was xed as 15 wt%
and the Re loadings were 0.5, 1, 2, and 3 wt%, respectively. The
drying and roasting processes were the same as before. The
calcined Ni–Re/Al2O3 catalysts were denoted as Ni15–Rex, where
x represented the loading of Re. Before reaction, the calcined
catalysts were reduced in situ for 4 h at 390 or 500 ꢀC in H2
owing at 100 mL minꢁ1. The reduced catalysts henceforth was
Fig. 1 Reductive amination pathways of MEA.
and aliphatic alcohols. Ni-based catalytic system has proved to be
cost-effective and highly active for the amination reaction and
has been extensive applied in industry. In one of the represen-
tative research, Shimizu and his group have studied the inuence
of acid–base property and metal particle size of Ni/Al2O3 catalysts
on N-alkylation of amines with alcohols.18,19 They claried that
the coexistence of acidic and basic sites on Al2O3 is indispensable
for the reductive amination reaction. Moreover, surface metallic
Ni sites are proved to be active sites and the small Ni particles
with low-coordinated Ni0 atoms show higher turnover number
during the reaction. The effect of acidity or metallic surface area
on the catalytic performance of supported Ni catalyst in other
reductive amination reactions has also been reported by
researchers.17,21 Furthermore, for the reductive amination of
MEA, many patents have reported that the introduction of Re into
the Ni-based catalysts could not only improve the MEA conver-
sion but enhance the selectivity of EDA.25,26 Although the Ni–Re/
Al2O3 catalysts have been applied in industry for decades, studies
on the Ni–Re/Al2O3 catalytic system for reductive amination are
scarce and presented mainly in the patent literatures. The role of
Re promoter on the Ni–Re catalysts is still ambiguous. In order to
comprehend the catalytic processes and design novel catalyst
with better performance, it is of great signicance to study the
effect of Re in the Ni–Re catalysts on the behavior for the
reductive amination process.
designated Ni15–Rex–yR, where
y
was the reduction
temperature.
Catalyst characterization
N2 physisorption was measured at Quantachrome Autosorb-1
instrument. Before measurement, each catalyst was outgassed
at 300 ꢀC for 3 h under vacuum. The BET surface area was
calculated from the range P/P0 ¼ 0.05–0.30 in the adsorption
branch. The total pore volume was determined by the amount of
N2 adsorbed at a P/P0 of 0.99.
Total Ni or Re metal contents in calcined catalysts were
determined by inductively coupled plasma optical emission
spectroscopy (ICP-OES). Perkin Elmer Optima 7300 DV was
applied for elemental detection.
The X-ray diffraction (XRD) analysis of catalysts was per-
formed on a PANalytical X'Pert PRO diffractometer equipped
with Cu Ka1 radiation source. The data were recorded with
a scan speed of 0.04ꢀ sꢁ1 and a step size of 0.013ꢀ.
Diffuse reectance UV-vis (DR UV-vis) analysis of the
calcined catalysts was performed on a Varian Cary spectro-
photometer with BaSO4 used as a reference.
H2-temperature programmed reduction (H2-TPR) was per-
formed on an Altamira Instruments AMI-300 equipped with
a thermal conductivity detection (TCD) device. Typically,
100 mg of catalyst was placed in a quartz reactor. Firstly, the
We report herein that Al2O3 supported Ni–Re catalysts act as
heterogeneous catalysts for the reductive amination of MEA with
ammonia under hydrogen atmosphere in a trickle bed reactor. A
series of Ni–Re/Al2O3 catalysts with different loading of Re were
prepared by co-impregnating method. Combined with various
catalyst characterizations, the physicochemical properties and
catalytic performance of Ni–Re/Al2O3 were discussed in detail so
as to investigate the role of Re in the catalytic system.
ꢀ
sample was pre-treated at 400 C for 30 min in an Ar ow, fol-
lowed by cooling to 50 ꢀC. Next, the temperature was raised
from 50 ꢀC to 900 ꢀC with a heating rate of 10 ꢀC minꢁ1 in a 10%
H2/Ar ow (30 mL minꢁ1). The TCD was used to detect the
amount of H2 consumption.
Temperature programmed desorption (TPD) experiments of
NH3 were conducted using an Altamira Instruments AMI-300
equipped with a thermal conductivity detection (TCD) device.
Approximately 100 mg of catalyst was placed in a quartz reactor.
Experimental
Materials and catalysts
ꢀ
All the catalysts were prepared by incipient-wetness impregna-
Firstly, the catalyst was pre-reduced at 390 C for 1 h under
tion method. The Ni/Al2O3 catalyst with 15 wt% Ni (the mass 10% H2/Ar ow. Next, the catalyst was heated under stream of Ar
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at 600 ꢀC for 30 min then cooled to 50 ꢀC. Further, catalyst was between MEA : NH3 was 1 : 10. The liquid hourly space velocity
exposed to a ow (30 mL minꢁ1) of 5% NH3/Ar for 2 h. Aer (LHSV) of MEA in the reaction was about 0.5 hꢁ1. The H2 ow
being purged in Ar for 1 h, the catalyst was heated linearly at rate was controlled by mass owmeter, corresponding to
ꢁ1
10 C min to 600 C in an Ar ow (30 mL minꢁ1).
a space velocity of about 60 hꢁ1. Before reaction, the catalysts
ꢀ
ꢀ
CO chemisorption experiments were performed at 50 ꢀC were in situ reduced by pure H2 for 4 h at either 390 or 500 C.
using an Altamira Instruments AMI-300 equipped with thermal The reductive amination reactions were conducted at 170 ꢀC
conductivity detection (TCD) device. Typically, about 100 mg of and 8 MPa. The liquid phase products were collected at 5–11 h
catalyst was placed in a quartz reactor. Firstly, the catalyst was reaction intervals. Then, the samples were quantied by gas
heated at 400 C for 30 min in an Ar ow, followed by in situ chromatograph (Agilent 7890) tted with a ame ionization
reduction at 390 or 500 ꢀC for 4 h in a H2 ow. The H2 detector and
ꢀ
ꢀ
a DB-35 capillary column, and N,N-dime-
consumption was detected by TCD. The catalyst was then thylformamide as an internal standard.
purged with a He ow of 30 mL minꢁ1, at 400 or 510 ꢀC for 1 h,
respectively. The catalyst was cooled to 50 C and 10% CO/He were calculated by the equations as follows:
pulse chemisorption was initiated at 50 ꢀC. The catalyst was
The conversion of MEA and the selectivity of the products
ꢀ
nMEA;in ꢁ nMEA;out
Conversion of MEA ð%Þ ¼
ꢂ 100
then ushed with a He ow of 30 mL minꢁ1 for 30 min at 50 ꢀC.
Finally, CO-TPD pattern was recorded in the range of 50–800 ꢀC
at a heating rate of 10 ꢀC minꢁ1 in a He ow (30 mL minꢁ1). The
area under the TCD curve below the reduction temperature was
used for the calculation of chemisorbed CO volume.47,48 The Ni
metal surface area, dispersion, and Ni metal crystal sizes were
calculated using the equations described by Velu et al.49 As
a note, the reduction degrees were determined by the ratios of
the amounts of H2 consumption during H2-TPR to the theo-
retical values calculated by ICP-OES measurements. When
calculating the reduction degree of Ni in Ni15–Re2 catalyst, the
H2 consumption of Re in the Ni15–Re2 catalyst was supposed to
be equal to that in Re2 catalyst at the same reduction condition.
The in situ X-ray photoelectron spectroscopy (XPS) spectra
were recorded using Thermo ESCALAB 250Xi X-ray photoelec-
tron spectrometer. A monochromatic X-ray source of Al Ka of 15
kV was applied. Special regions were Al 2p, Ni 2p and Re 4f. The
charging effect was adjusted according to the standard Al 2p
line at 74.5 eV.50 The peak tting of Ni 2p3/2, Re 4f7/2, and Al 2p
primary peaks were selected for the quantication of surface
components. The samples were transferred to the XPS chamber
nMEA;in
ni
X
Selectivity of product i ð%Þ ¼
ꢂ 100
ni
where nMEA,in was the moles of MEA initially added, nMEA,out was
the moles of MEA that remained in the products, and ni was the
moles amount of compound i in the products.
Results and discussion
N2 adsorption–desorption
The BET equation was conducted to inspect the porous nature
of Al2O3 and Ni-based catalysts. Fig. 2 shows the N2 adsorption–
desorption isotherms of calcined Al2O3 support and catalysts.
All the isotherms were identied as type IV, which was charac-
teristic of mesoporous materials. The isotherms exhibited type
H4 hysteresis loops, indicating the existence of irregular
channel systems. The structural properties of support and
catalysts, such as surface area, pore volume, and average pore
diameter, are shown in Table 1. The BET specic surface area of
ꢀ
and in situ reduced at 500 C for 2 h under H2 ow. Aer each
reduction, the XPS chamber was evacuated to about 7.1 ꢂ 10ꢁ5
Pa and the XPS measurements were sequentially conducted.
The morphology of the catalysts was investigated by a JSM-
7800F scanning electron microscopy (SEM) instrument. The
segregation of Ni species was characterized by SEM in back-
scattered electron mode.
Transmission electron microscopy (TEM), scanning trans-
mission electron microscopy (STEM), and energy dispersive X-
ray (EDX) line scan were acquired with a Tecnai G2 F30 S-
Twin electron microscope, operating at 300 kV. The particle
size distribution of sample was measured from more than 20
TEM images which obtained at different selected regions.
Materials and catalysts
The details of the reaction method was described in the ESI.† In
short, the reductive amination of MEA was carried out in
a trickle bed reactor (9 mm inner diameter). About 3.4 mL
catalyst (about 2.4 g) was placed in the middle of the reactor and
the rest space was lled with about 22 mL silica sand. The raw
Fig. 2 N2 adsorption–desorption isotherms of Al2O3 support, Ni15,
material consisted of MEA and liquid ammonia. The mole ratio Ni15–Re0.5, Ni15–Re1, Ni15–Re2, and Ni15–Re3 catalysts.
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ꢀ
Al2O3 support was calculated from N2 isotherms at ꢁ196.6 C,
and was measured as 388.1 m2 gꢁ1. Aer modied with Ni or Re,
obvious decrease in surface area and pore volume was observed.
For Ni–Re doped catalysts, surface area values were in the range
of 231–268 m2 gꢁ1. The reduction of surface area and pore
volume aer impregnation may be caused by the partial
blockage of mesoporous structure by metal species.
XRD
X-ray diffraction experiments (XRD) analysis technique was
performed to identify the crystal structure of Al2O3 supported Ni
and Ni–Re catalysts. As Ni–Re catalysts exhibited similar spectra
(Fi. S1†) aer calcination, only Ni15 and Ni15–Re2 samples
(Fig. 3) were chosen to analyze the crystal structure. Meanwhile,
the spectrum of Al2O3 support was also shown for comparison.
For Al2O3 support, the XRD pattern showed the diffraction
peaks at 2q ¼ 37.6ꢀ, 39.5ꢀ, 45.9ꢀ, and 67.0ꢀ, corresponding to
(311), (222), (400), and (440) crystal planes of g-Al2O3 with low
crystallinity (JCPDS 29-0063), respectively. For Ni15 catalyst,
only weak diffraction line at 2q ¼ 42.3ꢀ originating from bulk
NiO (200) crystal plane (JCPDS 44-1159) was detected. In
contrast, the NiO diffraction peak at 2q value of 42.3ꢀ was not
observed on Ni15–Re2 sample, implying the good dispersion of
Ni species on the Al2O3 surface. Moreover, for Ni15 and Ni15–
Re2 catalysts, the broad peak at 66.5ꢀ, which was between the
values of 2q ¼ 67.0ꢀ for g-Al2O3 and 2q ¼ 65.5ꢀ for stoichio-
metric NiAl2O4 spinel (JCPDS 10-0339), can be indexed to non-
stoichiometric NiAl2O4 phase on the catalysts. It has been re-
ported that Ni species have a tendency to preserve monolayer
dispersion and form surface NiAl2O4-like species on the Al2O3
support until the content of Ni is higher than the threshold
Fig. 3 XRD patterns of Al2O3 support, calcined and 500 ꢀC reduced
Ni15, Ni15–Re2 catalysts.
was too weak to calculate the metal size, suggesting the high
dispersion of metallic Ni phase. Moreover, the diffraction peaks
of Re were absent in all XRD patterns of Ni–Re catalysts, because
of its lower loading content and weak crystallization, on the
other hand, also implying the good dispersion of Re species. On
the basis of the analysis results, it could be concluded that the
introduction of Re effectively improves the dispersion of Ni
species on the g-Al2O3 surface aer calcination and reduction
treatments.
DR UV-Vis spectroscopy
coverage.51–54
Diffuse reectance ultraviolet-visible (DR UV-Vis) spectroscopy
ꢀ
The XRD patterns of 390 C reduced Ni and Ni–Re catalysts was applied to research the symmetry and coordination of Ni
are shown in Fig. S1.† There were no signicant changes in the species in the catalysts. The reectance spectra are shown in
diffraction patterns as compared with the calcined samples. Fig. 4. It can be observed that the Al2O3 and Re2 samples
The metallic Ni diffraction peaks were not observed, ascribed to showed no absorption bands in the range of 350–800 nm, while
the low crystallinity, low reduction degree, or/and small particle ve bands can be observed from the spectrum of the Ni15
size of metallic Ni on the support. For 500 ꢀC reduced Ni15 and catalyst. The band appearing around 377 nm, ascribed to the d–
Ni15–Re2 catalysts (Fig. 3), diffraction peaks were observed at 2q d electron transitions in octahedral Ni2+ (Ni[O]), was a nger-
¼ 44.3ꢀ, 51.7ꢀ, and 76.1ꢀ, corresponding to (111), (200), and print for NiO lattice.51 The band at 410 nm was attributed to Ni
(220) crystal planes of metallic Ni (JCPDS 04-0850), respectively. [O] in surface NiAl2O4-like spinel.55 The adsorption bands in the
With the use of Scherer's formula, average metallic Ni size of region 550–650 nm as a doublet with maxima at 592 and 638 nm
6.2 nm was found for the Ni15–500R catalyst. However, for were assigned to the electron transitions of tetrahedral Ni2+ (Ni
Ni15–Re2–500R sample, the intensity of the metallic Ni peaks [T]) in bulk NiAl2O4 lattice. The adsorption band around 713 nm
Table 1 Structural properties of Al2O3 support and catalysts
Catalyst
Surface areaa (m2 gꢁ1
)
Pore volumeb (mL gꢁ1
)
Pore diameterc (nm)
Ni loadingd (wt%)
Re loadingd (wt%)
Al2O3
Ni15
Ni15–Re0.5
Ni15–Re1
Ni15–Re2
Ni15–Re3
388.1
229.7
268.1
256.4
238.3
231.6
0.690
0.482
0.536
0.572
0.481
0.408
5.6
6.5
5.6
5.7
6.5
5.5
—
—
—
0.3
0.7
1.4
2.2
12.5
12.3
12.2
11.7
11.5
a
Calculated by BET equation. b BJH desorption pore volume. c BJH desorption average pore volume. d Obtained from ICP-OES results.
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Fig. 5 Variation of the F(R) ratio (the ratio calculated from the
Kubelka–Munk functions intensities of the band at 377, 410, and 638
nm) with the Re mass content (ICP-OES results) for Ni and Ni–Re
catalysts.
Fig. 4 DR UV-Vis spectra of Al2O3 support, Re2, Ni15, Ni15–Re0.5,
Ni15–Re1, Ni15–Re2, and Ni15–Re3 catalysts.
support; the second range around 470–725 ꢀC could be ascribed
could be assigned to both d–d electron transition of Ni[O] in
NiO and Ni[T] in the NiAl2O4 lattice.55 When Re was incorpo-
rated to the Ni-based catalysts, an increase in the intensity of
adsorption bands around 377 and 410 nm was observed.
Meanwhile, the intensity of the bands at 592, 638, and 713 nm
to the reduction of surface NiAl2O4-like spinel, which was highly
ꢀ
dispersed on Al2O3 support; the third range above 725 C was
attributed to the reduction of bulk NiAl2O4 spinel, which had
a strong interaction with Al2O3 support and difficult to be
decreased with increase of Re content. The DR UV-Vis results reduced.51,57 The reduction degree of Ni15 was 92%.
indicated that more NiO and surface NiAl2O4-like spinel were
The reduction prole of Re2 sample displayed two reduction
ꢀ
ꢀ
generated and the proportion of bulk NiAl2O4 was reduced on peaks at 367 C and 482 C, similar to the results reported by
Bare et al.53 The two step reduction of Re2 may be involved in
the Ni–Re catalysts. Since the NiO diffraction peaks were not
the reduction of different Re valences. The reduction degree of
Re2 was calculated to be 85%, indicating rhenium oxide species
were incomplete reduced. The average valence of Re was
calculated to be about +1. The H2-TPR patterns of Ni–Re cata-
observed for the Ni–Re catalysts in the XRD patterns (Fig. 3), it
was reasonable to conclude that the NiO species were highly
dispersed on the Re promoted Ni–Re catalysts.
To further analyze the proportion of Ni[O] and Ni[T] in the
catalysts, the variation of the Kubelka–Munk function ratio of lysts were different from the Ni15 catalyst. The peaks below
F(R377)/F(R638) and F(R410)/F(R638) with the Re mass content for 470 ꢀC, which could be assigned to reduction of bulk-like NiO or
Ni and Ni–Re catalysts were plotted as a semi-quantitative
method. As results shown in Fig. 5, the ratio of F(R377)/F(R638
)
and F(R410)/F(R638) increased with Re content up to about
1.4 wt%, and started to level off with more Re loading. This
result indicates that the addition of Re promoter facilitates the
conversion of Ni[T] to Ni[O]. In addition, Ni[O] ions were re-
ported to be reduced more easily than Ni[T].56 The increase of Ni
[O] proportion may enhance the reducibility of Ni2+ in Ni–Re
catalysts. In conclusion, the addition of Re to Ni catalysts
generates more Ni[O] species and improves the proportion of
NiO and surface NiAl2O4-like spinel.
H2-TPR
The inuence of Re on the reducibility of Ni–Re/Al2O3 catalysts
was analyzed by H2-TPR measurements and the proles are
shown in Fig. 6. The corresponding H2 consumptions and
reduction degrees are listed in Table S1.† For the Ni15 catalyst,
three regions of reduction peaks can be observed. The rst
range at about 350–470 ꢀC can be assigned to the surface bulk-
Fig. 6 H2-TPR profiles of Re2, Ni15, Ni15–Re0.5, Ni15–Re1, Ni15–Re2,
like NiO species which was weakly interacted with Al2O3 and Ni15–Re3 catalysts.
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which is hardly reduced and inert for the amination reaction.21
The reduction degrees for Ni–Re/Al2O3 catalysts were around
95%. In conclusion, H2-TPR studies suggest that the introduc-
tion of Re signicantly improves the reducibility of Ni–Re
catalysts and inhibits the formation of bulk NiAl2O4 species.
Acid–base properties
As both the acidic and basic nature of the catalysts were re-
ported to be necessary for the reductive amination reaction,18,19
NH3-TPD, CO2-TPD, and PY-IR experiments were conducted to
investigate the acid–base character of catalysts. The NH3-TPD
proles are shown in Fig. 7. The spectrum of Al2O3 support
showed a doublet at 120 and 170 ꢀC. The peak at 120 ꢀC could be
attributed to desorption of residual physically absorbed NH3.
The second peak at 170 ꢀC was assigned to chemisorb NH3 from
weak acidic ꢀsites. For Al2O3 supported ꢀRe sample, desorption
peak at 170 C shied up to about 225 C, indicating the pres-
ence of Re enhanced the acid strength of Al2O3 support. The
NH3-TPD prole of Ni15 catalyst was similar to the Al2O3
support prole. Aer modied with Re, Ni–Re catalysts exhibi-
ted a broad peak in the range of 225–370 ꢀC, indicating the
introduction of Re improves the acid strength of Ni–Re cata-
Fig. 7 NH3-TPD profiles of Al2O3 support, Ni15, Re2, Ni15–Re0.5,
Ni15–Re1, Ni15–Re2, and Ni15–Re3 catalysts.
Ni–Re oxide species, shied to lower temperature and became lysts. Moreover, the intensity of desorption peak around 170 ꢀC
much stronger as the loading of Re increased from 0.5 to 3 wt%. increases with the increase of Re loading in Ni–Re catalysts.
Moreover, the redꢀuction peaks belonged to surface NiAl2O4-like
The pyridine adsorption infrared (IR) spectra (Fig. S2†)
species (470–725 C) were shied toward lower temperature as demonstrate that Al2O3 support mainly contains surface Lewis
the increase of Re loading, indicating the addition of Re acid sites. The impregnation of Ni or Re to the Al2O3 support
promoter decreased the reduction temperature of Ni–Re cata- doesn't affect the type of acidic centers. Furthermore, the basic
lysts. Furthermore, as the increase of Re loading, the peaks contents measured from the CO2-TPD results (Fig. S3 and Table
ꢀ
related to the reduction of bulk NiAl2O4 above 725 C became S2†) indicate that few basic sites exist on the Al2O3 support and
much weaker, indicating that the addition of Re can prevent the the impregnation of Ni or Re species has little impact on the
diffusion of Ni2+ into the Al2O3 lattice to form NiAl2O4 specie, base character of the catalysts.
Fig. 8 In situ XPS of (1) Ni 2p region for Ni15–500R and Ni15–Re2–500R catalysts, and (2) Re 4f region for Ni15–Re2–500R catalyst. The dash
lines in the spectra correspond to the Ni 2P or Re 4f7/2 peaks of different valance states.
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Table 2 Binding energies and atomic ratios for 500 ꢀC reduced Ni15 and Ni15–Re2 catalysts
a
Ni 2P3/2 (eV)
Ni2+
Ni 2P1/2a (eV)
Ni2+ Ni0
Re 4f7/2a (eV)
Atomic ratio
Ni/Al
Atomic ratio
Re/Ni
Catalyst
Ni0
Re0
Re3+
Re4+
Re7+
Ni15–500R
856.5 (70.9) 853.0 (29.1) 874.1 870.4 0.0725
—
—
—
—
—
Ni15–Re2–500R 856.5 (60.9) 853.3 (39.1) 874.2 870.6 0.0678
40.5 (4.6) 41.3 (25.1) 43.1 (21.0) 46.5 (49.3) 0.0836
a
BEs of Ni 2P or Re 4f7/2 species. The percentages of Ni or Re with different valance states are shown in parentheses.
the major Re species existed as a mixture of various oxidation
states aer reduction at 500 C. These multiple oxidation states
In situ XPS
ꢀ
In situ X-ray photoelectron spectroscopy (XPS) was used to
analyze the valance state of the elements and the surface
composition in the 500 ꢀC reduced Ni15 and Ni15–Re2 catalysts.
Fig. 8 presents the XPS spectra of Ni 2p and Re 4f regions from
the Ni15–500R and Ni15–Re2–500R samples. The curve tting
results of Ni 2p and Re 4f7/2 are listed in Table 2. The Ni15–500R
catalyst had two main peaks at 853.0 and 870.4 eV, corre-
sponding to the Ni0 2p3/2 and Ni0 2p1/2, respectively. Moreover,
the presence of peaks at 856.5 and 874.1 eV, respectively cor-
responding to the BEs of Ni2+ 2p3/2 and Ni2+ 2p1/2, indicating the
Ni species in the Ni15 catalyst were partially reduced. The peaks
at 862.7 eV and 881.2 eV were ascribed to the shake-up satellite
peaks of Ni2+ 2p3/2 and Ni2+ 2p1/2, respectively.50,51,59 It was
notable that the BE of Ni2+ 2p3/2 at 856.5 eV was higher than that
of published bulk NiO species (around 854.5 eV), indicating the
presence of NiAl2O4 species on the catalyst surface.50 For Ni15–
Re2–500R catalyst, the peak intensity of Ni0 2p3/2 and Ni0 2p1/2
was higher than the case in Ni15–500R catalyst. According to the
calculation results in Table 2, the reduction degree of Ni15–
500R catalyst was 29.1%, while that of Ni15–Re2–500R catalyst
increased to 39.1%, indicating the presence of Re enhanced the
reducibility of Ni species. Furthermore, from curve tting
results, the BE of Ni0 2p3/2 in Ni15–Re2–500R was 0.3 eV higher
than that in Ni15–500R catalyst. Chayakul et al. have reported
that d-band electron densities of Ni could be reduced by the
electron transformation from Ni to Re.58 The shi in BEs of Ni0
2p3/2 when comparing Ni15–500R and Ni15–Re2–500R provided
evidence of an interaction between Ni to Re species. In addition,
the surface Ni/Al ratios of catalysts were in the range of 0.0678 to
0.0725, which were lower than the theoretical value of 0.122,
because of the incorporation of Ni2+ into the Al2O3 lattice by the
strong interaction between the surface Ni and Al2O3 support. In
conclusion, the XPS results proved that the addition of Re
promotes the reduction of Ni2+ compounds and electron inter-
action existed between Ni and Re.
aer reduction might arise from the strong oxophilicity of Re
species. Bare et al. have studied the oxidized and reduced states
of Re over Re/Al2O3 using a combination of X-ray absorption ne
structure (XAFS) and theoretical approaches.53 They concluded
that the oxidized Re species still existed even aer reduction at
ꢀ
700 C under dry H2. Furthermore, for Ni15–Re2–500R catalyst,
the surface atomic ratio of Re/Ni was found to be 0.0836, which
was much higher than the theoretical value calculated by ICP-
OES (0.0377). This result indicates the enrichment of Re species
on the surface of Ni–Re/Al2O3 catalysts.
BSE-SEM and CO-adsorption
The dispersion of metal particles for the reduced Ni15 and
Ni15–Re2 catalysts at different temperature were further
examined by SEM in backscattered electron (BSE) image mode.
The images are shown in Fig. 9. It should be noted that BSE
image was used to detect differences in the atomic number
within the catalysts. The contrast between particles and support
was mainly linked to the atomic masses or atomic numbers of
the corresponding elements. For the Ni15–390R sample
(Fig. 9(1)), Ni containing particles with an average size of about
8 nm were observed on the surface of catalyst. When the
reduction temperature was increased to 500 ꢀC (Fig. 9(2)), more
Ni particles were detected on the sample and the particle sizes
slightly increased to about 10 nm, indicating the aggregation of
Ni under higher reduction temperature. For Re promoted Ni15–
Re2 catalyst (Fig. 9(3 and 4)), the particle sizes were mostly
The XPS spectrum of Re 4f for the Ni15–Re2–500R catalyst is
shown in Fig. 8(2). The separation of the spin–orbit Re 4f7/2–4f5/
doublet were xed for the curve tting of Re spectra. Four
2
valance states of Re were selected to get the best tting. The
deconvoluted peaks at 40.5, 41.3, 43.1, and 46.5 eV were corre-
sponded to the 4f7/2 BEs of Re0, Re3+, Re4+, and Re7+ compo-
nents, respectively.60–62 The concentration of Rex+ species was
calculated by the intensity of Re 4f7/2 and shown in Table 2. The
reduction degree and average valence of Re were calculated to
be about 30% and +5, respectively. The results indicated that
Fig. 9 BSE-SEM images of the reduced catalysts: (1) Ni15–390R, (2)
Ni15–500R, (3) Ni15–Re2–390R, and (4) Ni15–Re2–500R.
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Table 3 Ni particle sizes and dispersions measured by CO chemisorption and reduction degree
Reduction temperature Reduction degree
Ni0 dispersion Ni0 crystal
Catalyst
Ni15
(ꢀC)
(%)
CO uptake (mmol gcat.
)
Ni0 surface area (m2 gcat.
)
(%)
size (nm)
ꢁ1
ꢁ1
390
500
Ni-8.8
17.4
56.1
77.6
125.3
—
0.7
2.2
3.0
4.9
—
9.3
9.6
15.7
12.4
—
19.4
18.7
11.5
14.5
—
Ni-27.3
Ni-24.8
Ni-50.9
Re-47.7
Re-72.8
Ni15–Re2 390
500
390
500
Re2
—
—
—
—
around 4 and 6 nm aer reduced at 390 and 500 ꢀC, respectively. calculated from CO uptakes. This difference might arise from
The particle sizes of reduced Ni15–Re2 catalysts were much the decoration of Ni particles with NiAl2O4 species aer reduc-
smaller than those of reduced Ni15 catalysts, indicating the tion. The aluminate shells on the surface of metallic Ni particles
addition of Re promoter effectively improved the dispersion of might prevent the adsorption of CO gas, resulting in low CO
Ni aer reduction.
uptake. Salagre et al. have concluded that Ni crystallites surface
For supported Ni catalysts, metallic Ni sites had proved to be would be decorated with porous NiAl2O4 species during reduc-
active species for the dehydrogenation–hydrogenation steps. tion process.63 A similar model of NiAl2O4 structure aer
CO-TPD measurement was applied to study the inuence of Re reduction was proposed by Jerzy earlier.64 This speculation was
promoter on the metallic Ni site, metal dispersion, and average further supported by the XRD, H2-TPR, and XPS analyses which
Ni particle size of the Ni based catalyst. Ni15 and Ni15–Re2 showed that Ni species mainly existed as surface NiAl2O4 spinel
catalysts were chosen for comparison. The CO uptakes and in the catalyst. According to the CO-adsorption results, the
analogy computation results, corrected by the reduction addition of Re in the Ni based catalysts signicantly improves
degrees, are summarized in Table 3. The CO uptake results the Ni dispersion and generates more Ni0 sites aer reduction.
indicated that a trace amount CO was adsorbed on the surface
of Re2 catalyst. Moreover, since CO adsorption infrared (IR)
Reductive amination of MEA
spectra (Fig. S4†) proved that major CO species were linearly
Reductive amination of MEA was carried out in a trickle bed
adsorbed on the surface of Ni0 sites, the Ni : CO ratio in
reactor. The amine products collected aer reaction about 12 h
were analyzed and summarized in Table 4. Interestingly, it was
found that Ni15–Rex catalysts could efficiently catalyze the
chemisorption was taken as 1 to calculate the Ni0 surface area.47
As shown in Table 3, aer modied with Re promoter, the Ni0
surface areas of the 390 and 500 ꢀC reduced catalysts increased
reductive amination of MEA to EAD. The conversion and EDA
from 0.7 to 3.0 m2 gcat.ꢁ1 and 2.2 to 4.9 m2 gcat.ꢁ1, respectively.
selectivity increased to 36.6% and 63.0% (entry 5 in Table 4)
with Re content up 2 wt% (Ni–15–Re2–390R), and started to
level off with more Re loading (entry 6 in Table 4). In compar-
Furthermore, Re promoted Ni15–Re2 catalyst showed higher
dispersion and smaller particle size in comparison with the
monometallic Ni15 catalyst aer reduction.
ison, a very low conversion (7.6%) and EDA selectivity (40.0%)
It should be noted that the Ni0 particle sizes obtained from
were observed on Ni15–390R catalyst (entry 2 in Table 4), and no
detectable activity was observed on Re2–390R catalyst under the
previous XRD or BSE-SEM methods were smaller than those
Table 4 Catalytic results for the reductive amination of MEA
Selectivity (%)
Entry
Catalyst
Conversion (%)
EDAa
AEEAb
PIPc
DETAd
Otherse
1
Re2–390R
—
—
—
—
—
—
2
3
4
5
6
7
8
9
Ni15–390R
7.6
40.0
55.6
60.3
63.0
63.6
46.3
67.7
69.1
82.4
82.1
56.2
32.2
26.2
20.4
19.2
37.4
16.4
12.9
9.8
3.2
5.8
6.4
8.1
8.9
8.5
8.2
6.6
1.1
5.1
0.0
4.3
4.6
5.7
5.2
5.1
5.5
10.5
5.9
7.7
0.6
2.1
2.5
2.8
3.1
2.7
2.2
0.9
0.8
0.8
Ni15–Re0.5–390R
Ni15–Re1–390R
Ni15–Re2–390R
Ni15–Re3–390R
Ni15–500R
Ni15–Re2–500R
Ni–Re/Al2O3
Ni–Re/SiO2–Al2O3
Ni15–Re/Al2O3
22.3
27.6
36.6
37.6
14.5
43.2
45.4
53.0
50.4
f
g
10
11
h
4.4
a
b
c
d
e
Ethylenediamine. Aminoethylethanolamine. Piperazine. Diethylenetriamine. N-(2-Hydroxyethyl)piperazine and N-Aminoethylpiperazine.
f
g
h
Patent (US 5750790):26 170 ꢀC, 12.5 MPa, MEA : NH3 ¼ 1 : 10. Patent (US 6534441 B1):27 160 ꢀC, 20.0 MPa, MEA : NH3 ¼ 1 : 10. The pilot-
scale results of our Ni–Re/Al2O3 catalysts: 175 ꢀC, 17.5 MPa, MEA : NH3 ¼ 1 : 10.
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same reaction conditions (entry 1 in Table 4). The big difference activity. The above results indicate that the addition of Re not
between Ni15–Rex catalysts and Ni15 catalyst revealed that the only improves the yield of EDA but also inhibits the deactivation
presence of Re was critical for reductive amination. The addi- during the reductive amination process. In combination with
tion of Re not only signicantly improved the activity of Ni- the previously CO chemisorption results, the highly Ni0 surface
based catalysts but also enhanced the selectivity of desired area could correspond to the enhancement in the yield of EDA.
amines, such as EDA and PIP. In consideration of many parallel
Currently, the reports about MEA reductive amination were
reactions during reductive amination processes of MEA, the presented mainly in the patent literatures.26,27,65 The reaction
enhancement of the EDA yield was essential to avoid the diffi- results of Ni–Re catalysts reported by others were listed in Table
culty during products separating.
4 (entry 9 and 10). It should be noted that most of patent re-
The TOF of Ni–Re catalysts (Table S3†) with varied Re loading ported experiments and industrial processes were carried out
were estimated for reference according to the method reported under high reaction pressure (12–25 MPa). Fischer et al. have
by Shimizu.19 The TOF increases from 213 to 251 hꢁ1 with reported that ammonia would form a supercritical uid (critical
ꢀ
increasing the Re loading from 0.5 to 2 wt% and starts to level data of ammonia: Tc ¼ 132.4 C, Pc ¼ 11.48 MPa) in a x-bed
off with further increasing Re loading.
reactor under high pressure.9,10 The formation of supercritical
In order to compare the activity of Ni15 and Ni15–Re2 cata- ammonia is crucial to improve the selectivity of primary
lysts further, we increased the reduction temperature of Ni15 diamines. Unfortunately, our reaction pressure should not
ꢀ
and Ni15–Re2 catalysts from 390 to 500 C and the results are exceed 8 MPa due to the limitations of experimental conditions.
list in Table 4. The MEA conversion for the Ni15–500R catalyst In our subsequent pilot-scale study (entry 11), 50.4 percent
was still low and product distribution changed little (entry 7 in conversion could be achieved with a selectivity of 82.1 percent
ꢀ
Table 4). In contrast, the yield of EDA was further improved over for EDA at 175 C under 17.5 MPa.
the Ni15–Re2–500R catalyst (entry 8 in Table 4). Fig. 10 shows
the stability of Ni15–500R and Ni15–Re2–500R catalysts. The
results showed that the increase in reduction temperature could
increase the initial activity of catalysts. However, for Ni15–500R
catalyst, the MEA conversion decreased rapidly with reaction
time and nearly deactivated within 23 h. The EDA selectivity
declined and the by-product AEEA signicantly increased as the
activity dropped. For Ni15–Re2–500R catalysts, a slightly
decrease of conversion was also observed, but the catalysts
performance could reach steady state and maintain a higher
Characterization of the spent catalysts
XRD technique was used to investigate the crystalline structure
and morphology of the spent Ni15–500R and Ni15–Re2–500R
catalysts. XRD patterns are shown in Fig. 11. Both the spent
catalysts exhibited obvious diffraction lines at 2q ¼ 44.3ꢀ, 51.7ꢀ,
and 76.1ꢀ, characteristic of the metallic Ni phase. Average Ni0
particle sizes of 21.3 and 15.8 nm were found for the spent
Ni15–500R and Ni15–Re2–500R catalysts with the use of
Scherrer's equation, respectively. Combined with the previous
XRD patterns in Fig. 3, it was clear that aggregation and growth
of Ni0 particles occurred during the reductive amination
process.
The TEM micrographs and size distributions of Ni0 particles
of spent catalysts are shown in Fig. 12. It was found that Ni
particles for the spent Ni15–500R catalyst (Fig. 12(1 and 2)) were
heterogeneously dispersed and had a broad distribution of 40 to
550 nm. In addition, large Ni particles with sizes above 1 mm
Fig. 11 XRD patterns of spent Ni15–500R and Ni15–Re2–500R
Fig. 10 Stability of (1) Ni15–500R and (2) Ni15–Re2–500R catalysts.
8160 | RSC Adv., 2018, 8, 8152–8163
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enhances the NiO dispersion and facilitates the generation of
surface Ni[O] species in Ni–Re/Al2O3 catalysts.
Aer reduction, H2-TPR and in situ XPS results show that Re
promoter signicantly improves the reducibility of Ni. More-
over, as indicated by BSE-SEM and CO-adsorption results, Re
promoted Ni/Al2O3 catalysts have smaller Ni0 particle size and
more surface Ni0 sites compared with monometallic Ni/Al2O3
catalyst. The enhancement of the Ni reducibility could be
explained by two reasons. Firstly, as indicated by DR UV-Vis
results, the increase of surface NiO species and Ni[O] propor-
tion for Ni–Re catalysts makes the Ni2+ species more easily to be
reduced; secondly, the reduction of Ni2+ may be accelerated by
hydrogen spillover effects on Re atoms. It has been reported
that Re could promote the reduction of Co2+ or Ce4+ species by
hydrogen spillover.59,66 Moreover, both H2-TPR and in situ XPS
results indicate that Re species are incomplete reduced and
exhibit a mixture of various oxidation states aer reduction.
Furthermore, the acidity–basicity measurement results suggest
that the addition of Re improves the acid strength of catalysts.
The generation of new acid sites in bimetallic catalysts
comprised of noble metals and oxophilic metal promoters has
been reported in recent years.67,68 Some research reported that
the acid–base pair sites adjacent to the Ni0 sites assisted
important steps such as deprotonation of alcohol or hydrogen
transfer.18,19 The enhancement of acidic strength might accel-
erate the deprotonation of MEA or hydrogen transfer steps.
The characterization results of spent catalysts indicate that
Ni/Al2O3 catalyst suffers from serious Ni sintering during reac-
tion. The sintering of Ni particles on Al2O3 support in the
presence of NH3 and H2 has been reported by Lif et al.69 They
concluded that the presence of NH3 accelerates the rate of
sintering by weakening the interaction between Ni particle and
Al2O3 support. For Re modied Ni–Re/Al2O3 catalysts, the Ni
sintering process could be effectively suppressed. Moreover,
monometallic Re2 catalyst has proved to be inactive during
reaction. The reductive amination process is mainly catalyzed
by Ni0 sites. In combination with the reaction and character-
ization results of spent catalysts, the decline of activity could be
ascribed to the sintering of Ni particles during reaction. As
shown in Fig. 10(1) (Ni–500R), the MEA conversion drops by
a dramatic 86 percent aer reaction for about 30 h, compared to
the sampling result at 2 h. Meanwhile, serious Ni sintering is
observed from the TEM images of spent Ni–500R sample. For
Ni15–Re2–500R catalyst (Fig. 10(2)), the MEA conversion
decreases from 54% (2 h) to 38% (28 h) with the growth in Ni
particle size. Even more interestingly, for both catalysts, the
selectivity of products changes along with the catalyst activity.
In the initial period, the selectivities of EDA and PIP are higher
and selectivity of by-product AEEA is less than 10%. As the MEA
conversion decreases to about 36%, the PIP selectivity
Fig. 12 TEM images and particle size distributions of: (1) and (2) spent
Ni15–500R; (3) and (4) spent Ni15–Re2–500R. (5) STEM image of
spent Ni15–Re2–500R, and (6) EDX line scans across a large particle as
dedicated by the red line.
were oen observed from the TEM images at some selected
regions (Fig. S5†). The TEM results clearly indicated that serious
sintering of Ni0 particles happened during reaction for Ni15
catalyst. On the contrary, the Ni0 sintering process was effec-
tively suppressed in the presence of Re promoter. For spent
Ni15–Re2–500R catalyst (Fig. 12(3 and 4)), Ni0 particles more
uniformly dispersed and the sizes of Ni0 particles densely
distributed in a range of 10–60 nm. One should be noted that
the difference between the XRD and TEM results was due to the
low crystalline of Ni0 or the sizes of metal particles beyond the
applicable scope of Scherrer's equation. The elemental distri-
bution of Ni and Re for spent Ni15–Re2–500R catalyst was
examined by STEM-EDX line scan. As shown in Fig. 12(5 and 6),
similar trend was observed for the Ni and Re elements in the
spent catalysts. The EDX line scan results suggest that the
surface of Ni0 particles is rich in Re species.
Discussion
As indicated by characterization results, the addition of Re decreased and more AEEA generated. Along with further
promoter affects the structure and activity of Ni/Al2O3 catalysts decrease of activity, a sharp rise in AEEA selectivity is detected
during preparation process and MEA reductive amination. For and EDA selectivity decreases to about only 38%. This
calcined catalysts, XRD and DR UV-Vis results indicate that phenomenon could be explained by the continuous reaction
most of Ni species existed in the form of non-stoichiometric during MEA reductive amination. As shown in Fig. 1, PIP is
NiAl2O4 phase in Ni/Al2O3 catalyst. The presence of bulk NiO mainly generated from the amination of AEEA. For high active
phase is also detected by XRD. The introduction of Re effectively catalysts, there are sufficient Ni0 sites on the surface of catalysts
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and AEEA would proceed to generate PIP. With sintering of Ni
particles during reaction, the amount of surface Ni0 sites would
decline and AEEA selectivity starts to increase. Moreover,
competitive reaction between MEA of NH3 and MEA molecules
is in existence during reductive amination and the generation of
side product AEEA is inevitable. The reaction results indicate
that highly active Ni–Re/Al2O3 catalyst is benecial to enhance
the selectivities of EDA and PIP and reduce the generation of by-
product AEEA.
The interaction between Ni and Re has been conrmed in
recent years. Liu et al. studied the TiO2 supported Ni–Re cata-
lysts for selective hydrogenation of methyl esters.70 Their XPS
results suggested that surface enrichment of metallic Re species
in bimetallic Ni–Re catalysts caused the enhancement of
activity. Gebrestadik et al. investigated ReOx modied Ni cata-
lysts for the hydrogenolysis of glycidol.71 According to the XPS
results, they concluded that Re was decorated by Ni and the
collaborative effect between ReOx and Ni particles was bene-
cial for the production of 1,3-PD. Yang et al. also reported the
formation of surface Ni–Re alloy on the Ni–Re/SiO2 catalysts.67
Furthermore, they suggested that oxophilic Re played a crucial
role for the adsorption of O atom during reaction. In our
research, the XPS and STEM-EDX line scan results indicated the
electron transformation from Ni to Re and the enrichment of Re
species on the Ni0 particles surface. It was reasonable to spec-
ulate that surface of Ni0 particles was decorated with ReOx
species. The collaboration effect between Ni–ReOx may has an
inuence on the mechanism of MEA reductive amination. This
speculation will be studied in our further study.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21273227) and Strategic Priority
Research Program of Chinese Academy of Sciences, Grant No.
XDB17000000.
Notes and references
1 G. F. Mackenzie, US Pat., 2 861 995, The Dow Chemical
Company, 1958.
2 F. Weiss and R. Lichtenberger, US Pat., 3 068 290, Societe
d'Electro-Chimie d'Electro-Metallurgie et des Acieries
Electriques d'Ugine, 1962.
3 P. H. Moss and N. B. Godfrey, US Pat., 3 151 115, Jefferson
Chemical Company, 1964.
4 J. Johansson and J. Tornquist, US Pat., 3 766 184, Sweden No
Drawing Company, 1973.
ˇ
5 J. Pasek, P. Kondelik and P. Richter, Ind. Eng. Chem. Prod.
Res. Dev., 1972, 11, 333.
6 A. Baiker and W. Richarz, Ind. Eng. Chem. Prod. Res. Dev.,
1977, 16, 261.
7 C. M. Barnes and H. F. Rase, Ind. Eng. Chem. Prod. Res. Dev.,
1981, 20, 399.
8 A. Baiker and J. Kijenski, Catal. Rev., 1985, 27, 653.
9 A. Fischer, T. Mallat and A. Baiker, Angew. Chem., Int. Ed.,
1999, 38, 351.
Conclusions
10 A. Fischer, T. Mallat and A. Baiker, J. Catal., 1999, 182, 289.
¨
The Al2O3 supported Ni and Ni–Re catalysts were prepared by 11 A. Fischer, M. Maciejewski, T. Burgi, T. Mallat and A. Baiker,
impregnation method and evaluated for the reductive amina- J. Catal., 1999, 183, 373.
tion of MEA in a trickle bed reactor. The Ni–Re/Al2O3 catalysts 12 S. Imm, S. Bahn, L. Neubert, H. Neumann and P. M. Beller,
showed high activity, high EDA selectivity, and outstanding
Angew. Chem., Int. Ed., 2010, 49(8126), 8129.
¨
stability in comparison with the monometallic Ni/Al2O3 catalyst. 13 K. O. Marichev and J. M. Takacs, ACS Catal., 2016, 6, 2205.
The addition of Re promoter improved Ni dispersion and 14 G. Liang, A. Wang, L. Li, G. Xu, N. Yan and T. Zhang, Angew.
proportion of Ni[O] species on the surface of Ni–Re/Al2O3
Chem., Int. Ed., 2017, 56, 3050.
catalysts. The reducibility of Ni species was signicantly 15 H. Kimura, Catal. Rev., 2011, 53, 1.
improved by the Re promoter. Aer reduction, Ni–Re/Al2O3 16 Y. Zhang, G. Bai, X. Yan, Y. Li, T. Zeng, J. Wang, H. Wang,
catalysts exhibited smaller Ni particle size (4–6 nm) and more
J. Xing, D. Luan and X. Tang, Catal. Commun., 2007, 8, 1102.
Ni0 sites as compared to Ni/Al2O3 catalysts. Moreover, the 17 J. M. Jehng and C. M. Chen, Catal. Lett., 2001, 77, 147.
presence of Re improved the acid strength of catalysts. In situ 18 K. I. Shimizu, K. Kon, W. Onodera, H. Yamazaki and
XPS and STEM-EDX line scan results indicated that Re species
had a mixture of various oxidation states and tended to aggre- 19 K. I. Shimizu, N. Imaiida, K. Kon, S. M. A. H. Siddiki and
gate on the surface of Ni0 particles aer reduction. The adjacent
A. Satsuma, ACS Catal., 2013, 3, 998.
Ni0–ReOx sites might facilitate the dehydrogenation step of 20 J. H. Cho, J. H. Park, T. Chang, G. Seo and C. Shin, Appl.
MEA. The analysis results indicated that the deactivation of Ni/ Catal., A, 2012, 417, 313.
Al2O3 catalyst was mainly caused by the aggregation of Ni0 21 J. H. Cho, J. Park, T. Chang, J. Kim and C. Shin, Catal. Lett.,
particles during reaction. For Ni–Re/Al2O3 catalysts, the Ni0
2013, 143, 1319.
J. N. Kondo, ACS Catal., 2012, 3, 112.
particles were stabilized and sintering process was effectively 22 A. Baiker, D. Monti and S. F. Yuan, J. Catal., 1984, 88, 81.
suppressed by the presence of Re promoter. We concluded that 23 K. I. Shimizu, S. Kanno, K. Kon, S. Siddiki, H. Tanaka and
the abundant Ni0 sites, which combined with good stability of
Y. Sakata, Catal. Today, 2014, 232, 134.
Ni0 particles and synergic effect of Ni–Re, corresponded to the 24 J. H. Cho, S. H. An, T. S. Chang and C. H. Shin, Catal. Lett.,
excellent catalytic performance of Ni–Re/Al2O3 catalysts.
2016, 146, 811.
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RSC Advances
25 D. C. Best, US Pat., 4 123 462, Union Carbide Corporation, 49 S. Velu and S. K. Gangwal, Solid State Ionics, 2006, 177, 803.
1978.
50 C. P. Li, A. Proctor and D. M. Hercules, Appl. Spectrosc., 1984,
38, 880.
51 E. Heracleous, A. F. Lee, K. Wilson and A. A. Lemonidou, J.
Catal., 2005, 231, 159.
26 S. W. King, US Pat., 5 750 790, Union Carbide Corporation &
Plastics Technology Corporation, 1998.
27 W. J. Bartley, R. G. Cook, K. E. Curry and S. K. Mierau, US
Pat., 6 534 441, Union Carbide Corporation & Plastics 52 G. Li, L. Hu, J. M. Hill, G. Li, L. Hu and J. M. Hill, Appl. Catal.,
Technology Corporation, 2003. A, 2006, 301, 16.
28 Z. Pan, Y. Ding, D. Jiang, X. Li, G. Jiao and H. Luo, Appl. 53 S. R. Bare, S. D. Kelly, F. D. Vila, E. Boldingh, E. Karapetrova,
Catal., A, 2007, 330, 43.
29 L. Yan, Y. J. Ding, Y. Lv, X. B. Cheng and L. X. Ma, WO Patent
J. Kas, G. E. Mickelson, F. S. Modica, N. Yang and J. J. Rehr, J.
Phys. Chem. C, 2011, 115, 5740.
2013152548 A1, Dalian Institute of Chemical Physics, 54 B. C. Vicente, R. C. Nelson, A. W. Moses, S. Chattopadhyay
Chinese Academy of Sciences, 2013. and S. L. Scott, J. Phys. Chem. C, 2011, 115, 9012.
30 G. Jenzer, T. Mallat and A. Baiker, Catal. Lett., 1999, 61, 111. 55 A. Fouskas, M. Kollia, A. Kambolis, C. Papadopoulou and
31 L. Zhang, C. Yang, Q. Yao, Y. Hou, Q. Lu and J. Xu, Mater.
Manuf. Processes, 2014, 29, 738.
H. Matralis, Appl. Catal., A, 2014, 474, 125.
56 M. Wu and D. M. Hercules, J. Phys. Chem., 1979, 83, 2003.
32 J. Runeberg, A. Baiker and J. Kijenski, Appl. Catal., 1985, 17, 57 J. L. Ewbank, L. Kovarik, F. Z. Diallo and C. Sievers, Appl.
309. Catal., A, 2015, 494, 57.
33 W. Hammerschmidt, A. Baiker, A. Wokaun and W. Fluhr, 58 K. Chayakul, T. Srithanratana and S. Hengrasmee, Catal.
Appl. Catal., 1986, 20, 305. Today, 2011, 175, 420.
34 J. Kijenski, P. J. Niedzielski and A. Baiker, Appl. Catal., 1989, 59 G. Garbarino, C. Wang, I. Valsamakis, S. Chitsazan, P. Riani,
´
53, 107.
E. Finocchio, M. F. Stephanopoulos and G. Busca, Appl.
Catal., B, 2015, 174, 21.
35 P. R. Likhar, R. Arundhathi, M. L. Kantam and
P. S. Prathima, Eur. J. Org. Chem., 2009, 31, 5383.
36 T. Yan, B. L. Feringa and K. Barta, ACS Catal., 2015, 6, 381.
´
60 J. Okal, L. K. Pinski, L. Krajczyk and W. Tylus, J. Catal., 2003,
219, 362.
´
37 H. J. Pan, W. N. Teng and Y. Zhao, Chem. Commun., 2015, 51, 61 J. Okal, W. Tylus and L. K˛epinski, J. Catal., 2004, 225, 498.
11907.
62 G. Beamson, A. J. Papworth, C. Philipps, A. M. Smith and
R. Whyman, J. Catal., 2011, 278, 228.
63 P. Salagre, J. L. G. Fierro, F. Medina and J. E. Sueiras, J. Mol.
Catal. A: Chem., 1996, 106, 125.
38 B. Emayavaramban, M. Roy and B. Sundararaju, Chem.–Eur.
J., 2016, 22, 3952.
39 T. Takanashi, Y. Nakagawa and K. Tomishige, Chem. Lett.,
2014, 43, 822.
´
64 J. Zielinski, J. Catal., 1982, 76, 157.
40 A. Abdukader, H. M. Jin, Y. X. Cheng and C. J. Zhu, 65 D. Chang and F. A. Sherrod, US Pat., 5 817 593, The Dow
Tetrahedron Lett., 2014, 55, 4172.
Chemical Company, 1998.
41 X. Xie and V. H. Han, ACS Catal., 2015, 5, 4143.
42 J. G. Donaire, M. Ernst, O. Trapp and T. Schaub, Adv. Synth.
Catal., 2016, 358, 358.
43 K. K. Wu, W. He, C. Sun and Z. Yu, Tetrahedron, 2016, 72,
8516.
66 A. M. Hilmen, D. Schanke and A. Holmen, Catal. Lett., 1996,
38, 143.
67 F. F. Yang, D. Liu, H. Wang, X. Liu, J. Y. Han, Q. F. Ge and
X. L. Zhu, J. Catal., 2017, 349, 84.
68 D. Hibbitts, Q. Tan and M. Neurock, J. Catal., 2014, 315, 48.
¨
44 Y. Nakamura, K. Kon, A. S. Touchy, K. Shimizu and W. Ueda, 69 J. Lif, M. Skoglundh and L. Lowendahl, Appl. Catal., A, 2002,
ChemCatChem, 2015, 7, 921. 228, 145.
45 S. Siddiki, K. Kon and K. Shimizu, Chem.–Eur. J., 2013, 19, 70 K. Liu, J. Pritchard, L. Lu, R. van Putten,
14416.
M. W. G. M. Verhoeven, M. Schmitkamp, X. Huang,
L. Lefort, C. J. Kiely, E. J. M. Hensen and E. A. Pidko,
Chem. Commun., 2017, 53, 9761.
46 M. Ousmane, G. Perrussel, Z. Yan, J. M. Clacens, F. De
Campo and M. P. Titus, J. Catal., 2014, 309, 439.
47 A. Tanksale, J. N. Beltramini, J. A. Dumesic and G. Q. Lu, J. 71 F. B. Gebretsadik, J. Llorca, Y. Cesteros and P. Salagre,
Catal., 2008, 258, 366.
ChemCatChem, 2017, 9, 1.
48 S. Abate, K. Barbera, E. Giglio, F. Deorsola, S. Bensaid,
S. Perathoner, R. Pirone and G. Centi, Ind. Eng. Chem. Res.,
2016, 55, 8299.
This journal is © The Royal Society of Chemistry 2018
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