Role of acid sites and surface hydroxyl groups in isophthalonitrile hydrogenation catalyzed by supported Ni-Co catalysts

Article abstract of DOI:10.1039/c5ra01007a

meta-Xylylenediamine (m-XDA) is industrially produced by the hydrogenation of isophthalonitrile (IPN) using Raney Ni/Co and basic additives. Compared with Raney Ni/Co, the supported Ni/Co catalysts are safer and have better mechanical strength. This work aimed at studying the catalytic performance of the supported Ni-Co catalysts in hydrogenation of IPN to m-XDA. The active sites for the condensation side reactions were studied using Ni-Co catalysts supported on different oxides and with different loadings. It was found that the acid sites catalyzed the condensation reactions between intermediate imines and amines. Two types of acid sites existed on the supported Ni-Co catalysts, namely, the original acid sites of the support and new acid sites formed by Ni/Co aluminates. In addition to acid sites, the surface hydroxyl groups on the oxide supports also catalyzed the condensation reactions, but were not active for the hydrogenation reaction. By increasing the Ni-Co loading, the selectivity to m-XDA was significantly enhanced, which was attributed to the suppression of both acid sites and hydroxyl groups. Compared to the low-loading catalysts (5Ni-1.25Co/Al₂O₃ and 5Ni-1.25Co/SiO₂), the high-loading catalysts (20Ni-5Co/Al₂O₃ and 20Ni-5Co/SiO₂) increased the m-XDA selectivity from ～45.5 to 99.9%.

Full text of DOI:10.1039/c5ra01007a

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Role of acid sites and surface hydroxyl groups in
isophthalonitrile hydrogenation catalyzed by
supported Ni–Co catalysts†
Cite this: RSC Adv., 2015, 5, 26465
Chang Liu, Ruijun Hou and Tiefeng Wang*
meta-Xylylenediamine (m-XDA) is industrially produced by the hydrogenation of isophthalonitrile (IPN)
using Raney® Ni/Co and basic additives. Compared with Raney® Ni/Co, the supported Ni/Co catalysts
are safer and have better mechanical strength. This work aimed at studying the catalytic performance of
the supported Ni–Co catalysts in hydrogenation of IPN to m-XDA. The active sites for the condensation
side reactions were studied using Ni–Co catalysts supported on different oxides and with different
loadings. It was found that the acid sites catalyzed the condensation reactions between intermediate
imines and amines. Two types of acid sites existed on the supported Ni–Co catalysts, namely, the
original acid sites of the support and new acid sites formed by Ni/Co aluminates. In addition to acid sites,
the surface hydroxyl groups on the oxide supports also catalyzed the condensation reactions, but were
not active for the hydrogenation reaction. By increasing the Ni–Co loading, the selectivity to m-XDA was
significantly enhanced, which was attributed to the suppression of both acid sites and hydroxyl groups.
Received 18th January 2015
Accepted 3rd March 2015
DOI: 10.1039/c5ra01007a
2 3 2
Compared to the low-loading catalysts (5Ni–1.25Co/Al O and 5Ni–1.25Co/SiO ), the high-loading
www.rsc.org/advances
catalysts (20Ni–5Co/Al and 20Ni–5Co/SiO
2
O
3
2
) increased the m-XDA selectivity from ꢀ45.5 to 99.9%.
the Ni–Co bimetal can further enhance the catalytic perfor-
1
. Introduction
mance. A variety of oxides can be used as the support, among
2,15,20,21
meta-Xylylenediamine (m-XDA) is an important ne chemical which g-Al O , SiO and MgO are the most widely used.
2 3 2
intermediate that is widely used in the production of pharma- The product selectivity is also affected by the metal loading. A
ceuticals, disinfectants, rubber stabilizers, textile additives and high metal loading (>10 wt%) of the Ni-based supported catalyst
1,2
curing agents of epoxy. The industrial production of m-XDA is usually used to give a high selectivity (>90%) to primary
2
,9,13–15
uses the hydrogenation of isophthalonitrile (IPN) catalyzed by amines.
However, the underlying mechanism has not
1–3
Raney® Ni/Co catalysts with basic additives. Raney® Ni/Co been studied.
4–7
6,9,22,23
catalysts have a high selectivity to m-XDA, which can be
According to the reaction routes shown in Scheme 1,
addition to the hydrogenation reactions, condensation
in
1
further enhanced by adding other metal components.
Although the skeleton structure of Raney® catalysts favors a
high catalytic activity, it causes low mechanical strength and
bad regeneration capability, leading to a very high catalyst
8
consumption. In addition, a safety hazard also exists as the
Raney® catalysts are ammable when exposed to air. Recently,
the supported metal catalysts with metals from group VIII such
as Pd, Pt, Ru, Ni and Co as the active component have attracted
2,9–15
more attentions.
The supported catalysts have good
mechanical strength provided by the support, and the m-XDA
selectivity can be enhanced by optimizing both the metal
14,16–19
composition and the support.
Among Group VIII metals,
Ni and Co give high selectivity to m-XDA and a synergy effect of
Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of
Chemical Engineering, Tsinghua University, Beijing 100084, China. E-mail: wangtf@
tsinghua.edu.cn; Tel: +86-10-62794132
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra01007a
Scheme 1 Reaction network of IPN hydrogenation reactions.
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1
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reactions between the highly reactive intermediate imines and 20 wt% (x ¼ 2.5–20, y ¼ 0.625–5). Ni and Co were impregnated
amines are the main side reactions in IPN hydrogenation. The onto the support simultaneously. The impregnated samples
condensation reactions decrease the selectivity to m-XDA and were treated with an ultrasonic treatment for 1 h to facilitate
cause catalyst deactivation by forming higher amines and metal dispersion, and were aged overnight at room tempera-
1
7
ꢂ
blocking the catalytic sites. The bifunctional mechanism
ture. Then the catalyst samples were dried in air at 80 C for 6 h
ꢂ
reveals that in a nitrile hydrogenation system, the hydrogena- and calcined at 400 C for 4 h.
tion reactions are catalyzed by metal sites while condensation
Because the solubility of the nitrates and the water absorp-
reactions are catalyzed by acid sites. It was conrmed by the tion capacity of the supports were low, the catalysts with high
1
7
19
studies of Verhaak et al., Infantes-Molina et al., Cabello Ni–Co
loadings,
10Ni–2.5Co/Al
2
O
3
,
2 3
20Ni–5Co/Al O ,
24
20
et al. and Chen et al. that the acid–base properties of the 10Ni–2.5Co/SiO and 20Ni–5Co/SiO , could not be prepared in
2
2
support are crucial to the catalytic performance. The surface one impregnation. In this work, repeated impregnation (RI
acidity favors the conversion of nitrile but decreases the selec- method) of a calcined catalyst with a lower Ni–Co loading was
tivity to primary amines, and the surface basicity shows the used to prepare the catalysts with loading of x ¼ 10 and 20. For
opposite effects. Actually, oxide surfaces are usually covered by x ¼ 10, the required amount of Ni and Co nitrate solution was
hydroxyl groups, which are potential proton donors or accep- divided into two portions, and used to impregnate the support
2
5–27
tors,
and catalytic performance.
of the surface hydroxyl groups highly depends on the environ- (SI method) was also used to prepare 20Ni–5Co/Al O3 and
and play an important role in the adsorption property two times. For x ¼ 20, the catalyst preparation used four
28,29
However, the acid–basic property impregnations. For comparison, a single step impregnation
2
30
31
ment. Unless activated by agents, the surface hydroxyl 20Ni–5Co/SiO . In the SI method, the nitrate solution was
groups on SiO and g-Al O are very stable and do not show added to the support four times with drying at 80 C in air aer
Brønsted acidity and basicity by providing H or OH . In each impregnation, and then the sample was calcined. These
addition to the acid–basic property of the support, a metal- were denoted as 20Ni–5Co/Al (SI) and 20Ni–5Co/SiO (SI). The
support interaction also affects the catalytic performance. MgO supported catalysts were prepared similarly. The catalysts
2
ꢂ
2
2 3
+
ꢁ
2
O
3
2
18
Rode et al. studied a series of supported Ni catalysts and obtained were stored in vials and pre-reduced in H
2
ow
ow before
ꢁ1
ꢂ
reported that the Ni dispersion depends on the support. For the (70 mL min ) at 450 C for 5 h and passivated in N
gas-phase hydrogenation of benzonitrile and acetonitrile, the being used in the hydrogenation reactions.
total catalytic activity increases with decreasing Ni dispersion.
Gluhoi et al. revealed that new active sites formed at the metal-
support interface enhanced the activity of supported Ni cata-
2
32
2.2. Catalyst characterization
lysts in acetonitrile hydrogenation. The results in the literatures The BET surface area, pore volume and pore size distribution of
suggest that the support affects the catalytic performance by the unreduced catalysts and the supports were measured by N2
providing original acid sites, modulating the dispersion of adsorption with a Quantachrome Autosorb iQ and AsiQwin
active metal and forming new active sites. However, the instrument. The X-ray diffraction (XRD) patterns of the unre-
mechanism of the formation of new acid sites and their effects duced and reduced catalysts and supports were recorded on a
are still unclear. The effect of the nature of support on the Bruker D8 Advance powder X-ray diffratomer (40 kV, 40 mA)
catalyst composition and catalytic performance is also not well with a Cu K radiation source and a Ni lter in the 2q range of
a
ꢂ
known.
In this work, a series of g-Al
5–90 . Scanning electron microscopy (SEM, JEOL JEM-7401F)
2
O
3
and SiO
2
supported Ni–Co was used to observe the morphology of the unreduced cata-
catalysts with different metal loading were synthesized and lysts and supports.
evaluated for IPN hydrogenation. MgO supported catalysts were
The reducibility of the catalysts was tested by temperature
2
also tested for comparison. The preparation method was opti- programmed reduction (H -TPR) performed on a Quantachrome
mized to enhance the metal dispersion and catalytic perfor- ChemBET Pulsar TPR/TPD instrument. A 5% H /Ar mixture was
2
mance. The effects of preparation method, Ni–Co loading and used as the reducing gas. The consumption of H was measured
2
nature of support on the catalyst acidity and catalytic perfor- by a TCD. Ammonia temperature programmed desorption
mance, and the effect of acid sites and surface hydroxyls groups (NH -TPD) was also conducted on the Quantachrome ChemBET
3
ꢂ
on the side reactions were systematically studied.
Pulsar TPR/TPD instrument. The catalysts were reduced at 450 C
online before the measurement. A mixture of 5% NH /He was
3
used for the adsorption, and the signal of ammonia desorption
was recorded by a TCD. The acidic properties of some selected
catalysts were also characterized by FT-IR spectroscopic analysis.
2 3
O and The spectra were recorded with a NICOLET 6700 spectrometer
2
. Experimental
2
.1. Catalyst preparation
A series of Ni–Co supported catalysts, xNi–yCo/Al
xNi–yCo/SiO
prepared by incipient wetness impregnation using nitrates a gas dosing–evacuating system. The surface hydroxyl content of
Ni(NO ) $6H O, 98%, Alfa Aesar, Co(NO ) $6H O, 98.0%, Alfa the support was determined by thermogravimetric analysis
2
(x, y are the Ni and Co loading in wt%), were equipped with a heatable IR cell with KBr windows, connected to
(
3
2
2
3 2
2
Aesar) as the metallic precursors and g-Al O or SiO (Alfa Aesar, (TG-DTA) on a NETZSCH STA 409 PC in N . The support powders
2
3
2
2
ꢂ
ꢂ
ꢂ
ꢁ1
both ground to 40–80 mesh) as the support. The mass ratio of Ni were heated from 30 C to 120 C at 10 C min and held at
to Co was kept at 4 : 1, and the Ni loading was varied from 2.5 to 120 C for 10 min to remove the physically adsorbed water, and
ꢂ
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ꢂ
ꢁ1
ꢂ
ꢁ1
2
ꢁ1
then the sample was heated to 800 C min at 20 C min and in Table 1. The support g-Al
2 3
O had a surface area of 221 m g
3
ꢁ1
held for 10 min. The surface hydroxyl content of the supports was and a pore volume of 0.63 cm g , which decreased aer the
calculated according to weight loss in the second heating step.
33
introduction of Ni–Co. As the Ni–Co loading increased from
The coke properties of the spent catalysts were also determined x ¼ 0 to x ¼ 20, the surface area decreased from 221 to
ꢁ
1
2
ꢁ1
ꢁ1
by TG-DTA with an O
2
ow of 50 mL min
.
153 m g , and the pore volume decreased from 0.63 to
0
3
.41 cm g , while the average pore diameter remained almost
unchanged in the range of 7.88–7.92 nm. The BET results of
xNi–yCo/SiO and SiO are also included in Table 1. Overall, the
SiO supported catalysts had smaller specic surface areas,
similar pore volumes and larger pore sizes compared to their
counterparts on g-Al O . The loading of Ni and Co decreased
2.3. Catalytic reaction
2
2
The catalysts were evaluated for the hydrogenation of IPN in a
stainless steel autoclave (Weihai Chemical Machinery Co., Ltd.,
50 mL). To eliminate the inuence of external and internal
2
2
2
3
diffusion, the stirring speed of the magnetically driven impeller
the surface area and pore volume. The surface area and pore
volume of SiO
ꢁ
1
was set at 800 rpm, and the H
2
ow rate was set at 190 mL min
2
ꢁ1
3
ꢁ1
2
were 152 m g and 0.62 cm g , respectively,
ꢂ
by a mass ow meter. The reaction temperature was 80 C and
the pressure was 6.0 MPa. A mixture of 20 mL methanol (>99.5%,
Beijing Chemical Works) and 80 mL toluene (>99.5%, Beijing
Modern Oriental Fine Chemistry Co., Ltd.) was used as the
solvent, and 0.086 g NaOH (>96.0%, Beijing Chemical Works)
was used as a basic additive. For each experiment, the amount of
IPN (98%, J & K Chemical) was 2.9 g. The amount of catalyst was
adjusted to give 0.25 g Ni and 0.0625 g Co on the catalyst:
2
ꢁ1
and the values for 20Ni–5Co/SiO
2
were 100 m
.38 cm g , respectively. The pore size distribution was little
changed with the loading of Ni and Co. In agreement with the
g
and
3
ꢁ1
0
15,34
results in the literature,
the BET results suggested the
uniform dispersion of Ni–Co on the support, with part of the
pore structure blocked by Ni–Co particles at high metal
loadings.
3
.1.2. XRD. The XRD patterns of g-Al
duced xNi–yCo/Al catalysts are shown in Fig. 1(a). It was
conrmed by the XRD and H -TPR (discussed below in 3.1.4)
analysis that NiO, Co , NiCo and Ni/Co aluminates were
The XRD patterns of g-Al showed
little change aer the introduction of Ni–Co at loadings of x #
. The results were attributed to the low Ni–Co loading and
the overlapped peaks of Ni/Co oxides and g-Al O . At the
2 3
O and the unre-
2
5
6:25
2
O
3
Catalyst mass ¼
or
ðx; y in wt%Þ
(1)
x
y
2
3
5,36
O
4
2 4
O
3
the main phases.
2 3
O
Before each experiment, the catalyst was pre-reduced in H
ow of 70 mL min at 450 C for 5 h. Aer passivation in N
ow, the reduced catalyst was transferred to the reactant solution
in the autoclave. The system was purged with H ow for 30 min
under 300 rpm stirring to displace trapped air. Then the reactor
system was heated to 80 C under 0.3 MPa and pressurized to
2
.0 MPa within 5 min while the H ow rate and stirring speed
were set to the specied values. The time when the pressure
reached 6.0 MPa was considered as the zero time of the reaction.
The products were sampled online at time intervals of
2
ꢁ
1
ꢂ


2
5
2
3
2
Ni–Co loading of x > 5, the characteristic peaks of NiO, Co
and NiCo were detected. It should be pointed out that
some characteristic peaks of Co and NiCo overlap each
3 4
O
ꢂ
2 4
O
3
O
4
2 4
O
6
other, which made the two species hard to distinguish.
However, the two species had some particular characteristic
peaks besides the common peaks. In this study, the peak at
ꢂ ꢂ
1 was used to analyze Co O and the peak at 36 was used to
3 4
3
1
0 min and were analyzed by a gas chromatograph (GC 7900II,
analyze NiCo O . In addition, the two species were further
conrmed by the different reducibility measured by H -TPR
2
as discussed in 3.1.4. The crystal sizes were estimated by the
Scherrer equation and listed in Table 1. The crystal sizes of
2
4
Techcomp Instrument Company) equipped with a DB-1MS UI
capillary column (30 m ꢃ 0.25 mm ꢃ 0.25 mm, Agilent) and an
FID detector. The conversion of IPN and the selectivity to m-XDA
were calculated as:
NiO, Co
3
O
4
and NiCo
7.4 nm on 10Ni–2.5Co/Al
1.0 and 16.9 nm on 20Ni–5Co/Al O . Compared with
2
O
4
were, respectively, 10.8, 20.9 and
moles of converted IPN
IPN conversion ¼ moles of IPN feedstock ꢃ 100%
1
2
2
2 3
O
, and were, respectively, 11.4,
(2)
2
3
0Ni–5Co/Al O , more NiO and Co O and less NiCo O were
formed on 20Ni–5Co/Al O (SI), with much larger crystal sizes,
2
3
3
4
2
4
moles of carbon in a defined product
Product selectivity ¼
2
3
moles of carbon in converted IPN
which were 20.2, 32.2 and 29.6 nm for NiO, Co
NiCo , respectively. For further comparison, the 20Ni–5Co/
Al and 20Ni–5Co/Al (SI) catalysts were analyzed aer
3 4
O and
ꢃ 100%
(3)
2
O
4
2
O
3
2 3
O
The higher amines and other oligomers in the liquid reduction. The Ni crystal particle size was 8.6 nm on reduced
samples could not be detected by GC. To identify the heavier 20Ni–5Co/Al O , and it was 13.7 nm on 20Ni–5Co/Al O (SI).
2
3
2
3
species, some liquid samples were analyzed by mass spec- This trend was consistent with the results of the unreduced
trometry (MS, instrument model: Q Exactive).
catalysts.
The XRD patterns of SiO2 and xNi–yCo/SiO2 further
conrmed the effects of Ni–Co loading and the preparation
method on metal dispersion and formation of bimetallic oxides.
3
. Results and discussion
3.1. Catalyst characterization
These are shown in Table 1 and Fig. 1(b). Similar to the g-Al
.1.1. BET surface area. The physical properties of supported catalysts, NiO, Co , NiCo and the Ni/Co sili-
xNi–yCo/Al and g-Al (calcined before analysis) are listed cates were the main phases of Ni and Co on xNi–yCo/SiO . The
2 3
O
3
3
O
4
2 4
O
2
O
3
2
O
3
2
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Table 1 Physical properties of supported Ni–Co catalysts and supports
Crystal size (nm)
2
ꢁ1
3
ꢁ1
a
b
c
Ni–Co loading x (wt%)
S (m g
)
V
p
(cm g
)
D
p
(nm)
NiO
Co
3
O
4
2 4
Co NiO
d
0
221 (152)
218 (144)
209 (130)
190 (123)
153 (100)
159 (96)
0.63 (0.62)
0.60 (0.59)
0.57 (0.56)
0.52 (0.49)
0.41 (0.38)
0.39 (0.37)
7.92 (12.32)
7.92 (12.36)
7.88 (12.30)
7.88 (12.28)
7.90 (12.22)
7.88 (12.28)
—
—
—(19.9)
—(19.8)
20.9 (20.3)
21.0 (20.2)
32.2 (24.8)
—
—(20.9)
—(21.2)
17.4 (22.4)
16.9 (21.0)
29.6 (27.2)
d
2
5
1
2
2
.5
—(19.6)
—(20.6)
10.8 (22.4)
11.4 (21.6)
20.2 (34.9)
d
d
0
0
0(SI)
d
d
a
c
ꢂ
b
ꢂ
Crystal size of NiO was calculated from the characteristic peak at 43 . Crystal size of Co
3
O
4
was calculated from the characteristic peak at 31 .
ꢂ
d
Crystal size of NiCo
2
O
4
was calculated from the characteristic peak at 36 . The numbers outside the brackets are the results of xNi–yCo/Al
2
O
3
,
2
and those in the brackets are the results of xNi–yCo/SiO .
were formed on SiO than on g-Al O . These results indicated
2
2 3
that the RI method using g-Al O as support facilitated the
2
3
metal dispersion and formation of Ni–Co bimetallic oxides.
.1.3. SEM results. Fig. 2 shows the SEM images of g-Al
and the unreduced xNi–yCo/Al catalysts. The occules in the
image of g-Al were due to the partially amorphous
3
2 3
O
2 3
O
2 3
O
morphology of the support. Aer the loading of Ni–Co, no
signicant change in morphology was detected in the SEM
images, except for 20Ni–5Co/Al O (SI), on which large octahe-
2
3
dral crystal particles were observed. Consistent with the BET
and XRD results, the SEM images show that Ni and Co were
2 3
uniformly dispersed on the g-Al O support when the RI
method was used. In contrast, aggregation of Ni/Co oxides was
much more serious on 20Ni–5Co/Al O (SI) and this led to larger
2 3
crystal particles.
2
The SEM results of the xNi–yCo/SiO catalysts were similar to
that of xNi–yCo/Al O (Fig. S1†). It was conrmed by the SEM
2
3
images that the Ni–Co dispersion was uniform on the SiO2
support for the catalysts prepared by the RI method, while for
2
0Ni–5Co/SiO
.1.4. -TPR results. The H
catalysts are shown in Fig. 3. The results of Ni/Al
2
(SI), large crystal particles were observed.
3
O
H
2
2
-TPR proles of the xNi–yCo/
Al
2
3
2
O
3
and
Fig. 1 XRD results of: (a) xNi–yCo/Al
SiO and SiO
: (A) support, (B) x ¼ 2.5, (C) x ¼ 5, (D) x ¼ 10, (E) x ¼ 20, (F)
x ¼ 20 (SI).
2 3 2 3
O and g-Al O ; (b) xNi–yCo/
2
2
xNi–yCo/SiO catalysts at the loading of x ¼ 2.5–20 had similar
2
average crystal sizes of Ni/Co oxides, while 20Ni–5Co/SiO
2
(SI)
had more NiO and Co
dispersion.
O , less NiCo O , and lower metal
3 4 2 4
The crystal sizes of the xNi–yCo/SiO
erably larger than those of the xNi–yCo/Al
average, for the Ni–Co/Al O catalysts prepared by the RI
2
catalysts were consid-
2 3
O
catalysts. On
2
3
method, the crystal sizes of NiO, Co O and NiCo O were 11.1,
3
4
2 4
2
1.0 and 17.2 nm, respectively. For the SiO supported catalysts,
2
the corresponding average crystal sizes were 21.1, 20.1 and
2 4
1.4 nm. In addition, more NiO and Co and less NiCo O
Fig.
2
SEM images of (a) g-Al (b) 5Ni–1.25Co/Al
2
O
3
,
2 3
O ,
(c)
2
3
O
4
20Ni–5Co/Al
2 3
O
, and (d) 20Ni–5Co/Al O (SI).
2 3
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ꢂ
increase of Ni–Co loading, the single reduction peak at 370 C
splitted into two peaks and shied slightly to higher tempera-
ture. The splitted peaks were due to the different compounds of
Ni and Co, namely the monometallic oxides (NiO and Co O )
3
4
and the bimetallic oxide (NiCo
reducible. With the increase of Ni–Co loading, the ratio of NiO
and Co to NiCo increased and the reducibility decreased.
2 4
O ), with the latter being more
3
O
4
2 4
O
In addition, the reduction peak area increased with an increase
in the Ni–Co loading due to the increased amount of the Ni and
Co oxides. In the case of 20Ni–5Co/Al O (SI), the characteristic
2
3
peaks of NiO, Co O and Ni/Co aluminates were enhanced while
3
4
that of NiCo O was weakened. This further conrmed the
2
4
2 4
formation of less NiCo O and the lower metal dispersion on
2 3
20Ni–5Co/Al O (SI).
The dominant reduction peaks of xNi–yCo/SiO
2
were below
ꢂ
6
00 C, indicating the formation of less Ni and Co silicates on
ꢂ
SiO
2
. The peak at 300 C was attributed to the reduction of
ꢂ
NiCo O and that at 400 C was attributed to NiO and Co O .
2
4
3 4
For the xNi–yCo/SiO catalysts prepared by the RI method, the
2
peak locations and the relative peak areas were unchanged with
the Ni–Co loading. In accordance with the XRD results, the
H
2
-TPR results demonstrated that there were less NiCo
more NiO and Co on SiO than on g-Al . Overall, the
surface composition of xNi–yCo/SiO was less dependent on the
Ni–Co loading than that of xNi–yCo/Al
3.1.5. Catalyst acidity. The reduced xNi–yCo/Al O catalysts
2 4
O but
3
O
4
2
2 3
O
2
2 3
O .
Fig. 3
2 2 3 2 3 2 3
H -TPR profiles of (a) xNi–yCo/Al O , Ni/Al O and Co/Al O ;
2
3
(
b) xNi–yCo/SiO
2
: (1) x ¼ 2.5, (2) x ¼ 5, (3) x ¼ 10, (4) x ¼ 20, (5)
and g-Al O were tested by NH -TPD. The total acidity of each
2
3
3
x ¼ 20(SI).
catalyst and g-Al O was calculated from the desorbed amount
2
3
of NH
3
corresponding to the desorption peak in the range of
ꢂ
100–500 C (Fig. S2†). The results are summarized in Table 2.
Co/Al
2
O
3
(both at the loading of 5.0 wt%) are also included for
-TPR proles of Ni/Al and
The support g-Al
2
O
3
has some acidity generated from dehy-
comparison. As shown in the H
2
2
O
3
37,38
dration.
The loading of Ni–Co signicantly affected the
Co/Al
2
O
3
, the reduction peaks of Ni oxide were at 450 and
acidity. With an increase in the Ni–Co loading, the total acidity
ꢂ
ꢂ
7
00 C, and that of Co oxide was at 475 C. For all the xNi–yCo/
ꢁ1
increased from 0.180 mmol NH g for g-Al O to 0.224 mmol
3
2 3
Al O catalysts, the reduction of Ni/Co oxides had two peaks at
ꢁ1
2
3
NH g for 2.5Ni–0.625Co/Al O , and reached its maximum of
3
2 3
ꢂ
370 and 650 C, which were lower than the characteristic peaks
ꢁ1
0.474 mmol NH3
g
for 5Ni–1.25Co/Al O . With a further
2 3
of the monometallic Ni and Co catalysts. The H -TPR results
indicated the formation of Ni–Co bimetallic oxides, which had a
higher reducibility than the Ni and Co monometallic oxides.
The reduction peaks at 370 C was attributed to the reduc-
tion of NiO, Co
2
increase of the Ni–Co loading, the acidity decreased from
ꢁ1
0
NH
.474 mmol NH
3
g
2 3
for 5Ni–1.25Co/Al O to 0.160 mmol
ꢁ1
3
g
for 20Ni–5Co/Al
2
O
3
.
The poorly dispersed
ꢂ
ꢁ1
2
0Ni–5Co/Al
2
O
3
(SI) had an acidity of 0.264 mmol NH
3
g
,
ꢂ
3
O
4
and NiCo
2
O
4
, and that at 650 C was due to
which was larger than that of 20Ni–5Co/Al
that the aluminates, NiAl O and CoAl O , which were
2 3
O . It was reported
2
NiAl O4 and CoAl O , which were less reducible. With the
2
2 4
2
4
2 4
a
Table 2 Acidity and reaction results of xNi–yCo/Al
O
2 3
and xNi–yCo/SiO
2
Ni–Co loading,
x (wt%)
Acidity
(mmol NH
k
r
S
m-XDA
(%)
ꢁ1
ꢁ2
0.2 ꢁ0.2
ꢁ1
3
g
)
(10 mol
L
min
)
0
0.180 (0.014)
0.224 (0.087)
0.474 (0.061)
0.346 (0.029)
0.160 (0.023)
0.264 (0.045)
—
—
b
2
5
1
2
2
.5
0.6 (2.6)
2.1 (2.0)
2.4 (2.1)
2.8 (2.2)
2.2 (1.9)
22.0 (27.9)
45.5 (45.8)
96.5 (65.7)
99.9 (99.9)
90.3 (88.7)
b
b
0
0
b
b
0(SI)
a
ꢂ
Reaction conditions: 80 C, 6.0 MPa, catalyst in 200–400 mm containing 0.25 g Ni and 0.0625 g Co, 80 mL toluene and 20 mL methanol as solvent,
ꢁ1
b
2
Al
.9 g IPN feed, 0.086 g NaOH, 180 mL min hydrogen gas ow, 800 rpm stirring. The numbers outside the brackets are the results of xNi–yCo/
, and those in the brackets are the results of xNi–yCo/SiO
2
O
3
2
.
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Fig. 4 Catalytic performance of (a) and (b) xNi–yCo/Al
2
O
3
and (c) and (d) xNi–yCo/SiO
2
catalysts
x ¼ 2.5,
x ¼ 5,
x ¼ 10,
x ¼ 20,
x ¼ 20(SI), open and solid symbols represent conversion and selectivity, respectively.
conrmed by XRD and H -TPR analysis in this work, were Similar to the g-Al O supported catalysts, the preparation
2
2 3
3
7,39,40
mainly responsible for the enhanced acidity.
In NiAl
2
O
4
method also had effects on the catalyst acidity. The 20Ni–5Co/
(SI) catalyst had a stronger acidity than 20Ni–5Co/SiO
, and is present in the form of Al surface Nevertheless, all the Ni–Co/SiO catalysts had much lower
cations, which act as Lewis acid sites. Conrmed by the results acidity than their counterparts on g-Al , due to the much less
of FT-IR (Fig. S3†), all the acid sites on the reduced xNi–yCo/ formation of Ni/Co silicates, which were the source of the new
Al catalysts and g-Al were Lewis acid sites, consistent acid sites.
with the literature. However, the literatures
and CoAl
2
O
4
spinel, the positive charge of the surface is higher SiO
2
2
.
3
+
than on g-Al
2
O
3
2
2 3
O
2
O
3
2 3
O
38,40–42
had no
consensus about the effects of the metal on the acidity. Some
3
.2. Catalytic performance
researchers reported that the introduction of Ni enhanced the
40
acidity of alumina and silica-alumina support, while others
3.2.1. Effect of preparation method and Ni–Co loading.
3
8,41
reported the opposite effects.
In other studies, more The g-Al O supported catalysts were evaluated for IPN hydro-
2 3
42
complicated effects were reported. In this work, the enhanced genation under the same conditions to study the role of acid
acidity was attributed to the exposed Ni/Co aluminates. The sites by using catalyst with different preparation methods and
newly formed acid sites only showed acidity when they were Ni–Co loadings. For each experiment, the catalyst amount used
exposed without being covered by the loaded metals. Therefore, was adjusted to give the amounts of Ni and Co of 0.25 g and
the Ni–Co loading had two effects on the catalyst acidity, 0.0625 g, respectively. The conversion of IPN and selectivity to
namely, the formation and exposure of the Ni/Co aluminates. At m-XDA are plotted as a function of reaction time in Fig. 4. The
the Ni–Co loadings of x # 5, the effect of aluminates formation rate constant (k ) for IPN hydrogenation and selectivity to
r
was dominant, therefore the acidity increased with Ni–Co m-XDA over xNi–yCo/Al O based on the same metal amount are
2
3
loading. As the Ni–Co loading further increased to x > 5, the listed in Table 2. The reaction order (p) and rate constant (kr)
decrease in exposure of the aluminates became dominant were determined by tting the IPN concentration data in the
because some acid sites on the Ni/Co aluminates were covered rst 100 min as a function of the reaction time using eqn (4).
by Ni–Co, therefore the acidity decreased with increasing Ni–Co
dCIPN
p
loading.
The Ni–Co catalysts supported on SiO
support, were also analyzed by NH -TPD, as shown in Table 2.
The support SiO had very low acidity of 0.014 mmol NH
which was less than 10% of that of g-Al . At the Ni–Co loading
of x ¼ 2.5, the catalyst acidity increased to 0.087 mmol NH
Further increase of the Ni–Co loading reduced the catalyst
¼ ꢁk
r
C
IPN
(4)
dt
2
, a much less acidic
3
ꢁ
1
The reaction order of IPN was found to be in the range
.75–0.85. To further quantitatively compare the activity of
2
3
g ,
0
2 3
O
ꢁ
1
different catalysts, the reaction order was xed at 0.8. With
p ¼ 0.8, eqn (4) has a good agreement with the experimental data
3
g .
2
ꢁ
1
of all the catalyst, with R $ 0.99. For comparison, the calcula-
acidity to 0.023 mmol NH g at the Ni–Co loading of x ¼ 20.
3
tions with p ¼ 1.0 were also included in Table S1.† When the 1st
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the increase of Ni–Co loading decreased the acid amount, while
the catalytic activity and selectivity to m-XDA were enhanced.
For the xNi–yCo/Al O catalysts, there was a strong correlation
2
3
between the acid amount and the catalytic performance.
Verhaak et al. reported similar results that the acid sites on the
supported Ni catalyst were responsible for the condensation
17
reactions in acetonitrile hydrogenation. In the present work,
the enhanced activity and selectivity to m-XDA by increasing the
Ni–Co loading was due to the weakened adsorption of imines on
the catalytic sites caused by the suppressed acidity, which was
resulted from the decreased exposure of Ni/Co aluminates.
However, the catalytic performance of 2.5Ni–0.625Co/Al O was
2
3
an exception. Compared with 5Ni–1.25Co/Al
2 3
O , 2.5Ni–0.625Co/
Al had less acid amount, but had lower activity and selec-
2 3
O
tivity to m-XDA. Since there was more exposure of the support
surface at lower Ni–Co loading, the low activity and selectivity to
m-XDA over 2.5Ni–0.625Co/Al O should be attributed to the
2 3
effect of other sites on the support surface, which will be dis-
cussed in detail below.
3
.2.2. Effect of the nature of support. The SiO supported
2
catalysts were evaluated for IPN hydrogenation. The catalytic
results are listed in Table 2. Different from the g-Al supported
catalysts, the rate constant k based on the same metal amount
2 3
O
r
ꢁ
2
0.2 ꢁ0.2
ꢁ1
Fig. 5 Acidity, selectivity to m-XDA and rate constant of the supported remained in the range of 1.9–2.6 ꢃ 10 mol
L
min for
Ni–Co catalyst: (a) acidty and selectivity to m-XDA, (b) acidity and rate
constant k .
r
different Ni–Co loading and preparation method. With an
increase in Ni–Co loading, the selectivity to m-XDA increased from
27.9% over 2.5Ni–0.625Co/SiO2 to 99.9% over 20Ni–5Co/SiO2.
Similar to the g-Al O supported catalysts, 20Ni–5Co/SiO (SI) had
2
3
2
order was used, there was a large deviation between the calcu-
lated and measured IPN concentrations for 5Ni–1.25Co/Al and
, with R less than 0.95. The tting results with
were shown in Fig. S4.† It
should be pointed out that eqn (4) was only used for quantitative
comparison of the catalyst activity rather than providing reaction
kinetics. Using a xed reaction order of IPN for different catalysts,
the catalyst activity can be solely described by the rate constant.
a lower selectivity to m-XDA (88.7%) than 20Ni–5Co/SiO
The catalyst acid amount, rate constant k and selectivity to
m-XDA were plotted as a function of Ni–Co loading for the SiO
supported catalysts in Fig. 5. The Ni–Co/SiO catalysts had
much lower acidity than the Ni–Co/Al catalysts, and had its
2
.
2 3
O
r
2
2
0Ni–5Co/Al
2 3
O
2
p ¼ 0.8 and 1.0 for 20Ni–5Co/Al
2 3
O
2
2 3
O
highest acid amount at a lower loading of x ¼ 2.5. The catalytic
activity of Ni–Co/SiO was little affected by the Ni–Co loading,
2
while the selectivity to m-XDA increased with increasing Ni–Co
At the Ni–Co loading of x ¼ 2.5, the rate constant k
r
was
loading. According to the XRD and H
and Co and less NiCo and Ni–Co silicates were formed on
than on g-Al , and the metal composition of the Ni–Co/
catalysts was less dependent on the Ni–Co loading. As a
2
-TPR results, more NiO
ꢁ
2
0.2 ꢁ0.2
ꢁ1
ꢁ2
0
.6 ꢃ 10 mol
L
min , which increased to 2.1 ꢃ 10
3
O
4
2 4
O
0
.2 ꢁ0.2
ꢁ1
mol
L
min at Ni–Co loading of x ¼ 5. When the Ni–Co
SiO
SiO
2
2
2 3
O
loading further increased to x ¼ 10, 20 and 20(SI), the Ni–Co/
Al catalyst activity showed little change and the rate constant
2 3
O
result, xNi–yCo/SiO
acidity and consequently similar catalytic activity. The differ-
2
at all Ni–Co loadings (x ¼ 2.5–20) had low
ꢁ
2
0.2 ꢁ0.2
ꢁ1
k was within the range of 2.2–2.8 ꢃ 10 mol
L
min .
r
The selectivity to m-XDA was enhanced with increasing Ni–Co
loading. In particular, the selectivity to m-XDA over 20Ni–5Co/
ences in physicochemical properties of the xNi–yCo/SiO and
2
xNi–yCo/Al O catalysts, which had important effects on the
2
3
Al
.5Ni–0.625Co/Al
selective than 20Ni–5Co/Al
XDA. In this work, MS was used to identify the heavy species in
2
O
3
was 99.9%, which was 77.9% higher than that over
. The 20Ni–5Co/Al (SI) catalyst was less
, showing 90.3% selectivity to m-
catalytic performance, could be attributed to the weaker metal-
support interaction between Ni–Co and SiO
Compared with the xNi–yCo/Al catalysts, the acid amount
of xNi–yCo/SiO based on the same metal amount was much
2
2
O
3
2 3
O
2
.
2 3
O
2 3
O
2
43
the products. In our previous work, some typical MS spectra
showed that the enhanced formation of higher amines was
responsible for the low selectivity to m-XDA. Over 20Ni–5Co/
Al O , the condensation reactions were effectively suppressed
lower. However, the correlation between the m-XDA selectivity
and the Ni–Co loading was strong. To further investigate the
effect of Ni–Co loading, MgO supported Ni–Co catalysts were
also prepared and evaluated for IPN hydrogenation. The selec-
tivity to m-XDA was 37.2% over 5Ni–1.25Co/MgO, which was
2
3
and very little higher amines were detected.
To further analyze the effect of Ni–Co loading, the catalyst
acid amount, rate constant and selectivity to m-XDA based on
the same metal amount were plotted as a function of Ni–Co
loading in Fig. 5. The results showed that at the loading of x $ 5,
lower than that over its counterparts on g-Al
though MgO is a typical basic support and 5Ni–1.25Co/MgO had
no acidity (no signicant NH desorption peak was detected in
2 3 2
O and SiO , even
3
the NH
3
-TPD characterization). The above results, including the
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stronger interaction with Ni–Co, leading to the formation of a
larger amount of Ni/Co aluminates and higher catalyst acidity.
In addition, the metal dispersion was higher and the synergy
effects between Ni and Co by forming NiCo O were stronger on
2
4
g-Al
low Ni–Co loading.
.2.3. Effect of the surface hydroxyl groups. It was reported
2 3
O , which increased the selectivity to m-XDA, especially at
3
in previous studies that the acid sites are the main catalytic sites
for the condensation reactions in IPN hydrogenation. However,
the results in this work showed that the catalyst acidity was not
the only factor that affected m-XDA selectivity. As discussed in
3.2.1 and 3.2.2, low m-XDA selectivities were obtained over the
supported catalysts with low metal loadings and high amounts
of exposed support sites. In this work, the effects of these
support sites were further studied by introducing bare supports,
2 3 2
namely g-Al O and SiO , into the IPN hydrogenation system.
In the blank experiments of IPN hydrogenation using only
the support, both g-Al O and SiO had very low activity for the
2
3
2
conversion of IPN. In additional experiments, a mixture of
.25 g 20Ni–5Co/Al O (SI) and 8.75 g g-Al O was used. The
1
2
3
2 3
results are shown in Fig. 6. The catalytic results of the mixture of
.25 g 20Ni–5Co/SiO (SI) and 8.75 g SiO were also included.
Comparing the catalytic results, it was found that the rate
1
2
2
Fig.
Al
introduction of bare support.
6
Comparison of catalytic performance of (a) 20Ni–5Co/ constant k
and m-XDA selectivity of the mixture of 20Ni–5Co/
r
2
O
3
(SI) and (b) 20Ni–5Co/SiO
2
(SI) catalysts with or without the
ꢁ2
0.2 ꢁ0.2
ꢁ1
Al
2
O
3
(SI) and g-Al
2
O
3
(1.1 ꢃ 10 mol
L
min and 38.1%,
respectively) were lower than that of 20Ni–5Co/Al O (SI)
2
3
ꢁ
2
0.2 ꢁ0.2
ꢁ1
(
2.2 ꢃ 10 mol
L
min and 90.3%, respectively). Similar
results were obtained with the mixture of 20Ni–5Co/SiO (SI) and
2
catalytic result of 2.5Ni–0.625Co/Al O , indicated that the
2
3
2
SiO .
catalyst acid amount was not the only factor that affected the
m-XDA selectivity and some support sites other than acid sites
could catalyze the side reactions. It was clear that the amount of
exposed support sites decreased with the increase of Ni–Co
It has been well known that the surface hydroxyl groups,
which widely existed on oxide supports, played an important
role in adsorption and catalytic reaction.
25–29
To understand
the possible effect of the surface hydroxyl groups, additional
studies were carried out. According to the literature, the
loading, and for the results of xNi–yCo/SiO
2
, the correlation
44
between the amount of exposed support sites and m-XDA
selectivity was more consistent and crucial than that between
the acidity and m-XDA selectivity.
In conclusion, the selectivity to m-XDA was not only affected
by the catalyst acid amount, but also affected by other sites on
the support. The nature of support signicantly affected the
catalyst composition and physicochemical properties, and thus
concentration of hydroxyl groups on the SiO surface could
be modied by treatment at different temperatures
without signicant change of other surface properties. In this
study, the SiO
were each mixed with the reduced 20Ni–5Co/SiO
2
ꢂ
ꢂ
2
samples calcined at 400 C and 600 C for 4 h
(SI) and
2
evaluated in the IPN hydrogenation reaction. The results were
shown in Table 3. The BET surface area and the content of the
surface hydroxyl groups were also tested and included in
Table 3 and Fig. S5.† The calcination treatment had important
effect on the surface hydroxyl content and the catalytic
performance, while the effect on the BET surface area was
2
affected the catalytic performance. For xNi–yCo/SiO , the cata-
lyst acidity was in a very low range, and there was a more
consistent correlation between the amount of exposed support
sites and m-XDA selectivity. Compared with SiO , g-Al O has a
2
2 3
a
2
Table 3 Effect of calcination temperature on surface property, hydroxyl group content and catalytic performance of SiO
S
Acidity
(mmol NH
Surface hydroxyl content
(wt%)
S
m-XDA
(%)
2
ꢁ1
ꢁ1
Support
(m g
)
3
g
)
SiO
SiO
SiO
2
2
2
-untreated
152
149
150
0.014
0.005
0.004
0.59
0.33
0.12
28.1
42.3
65.4
b
-400b
-600
a
ꢂ
2 2
Reaction conditions: 80 C, 6.0 MPa, 1.25 g 20Ni–5Co/SiO (SI) + 8.75 g SiO , both in 200–400 mm, 80 mL toluene and 20 mL methanol as solvent,
b
ꢁ1
ꢂ
2
.9 g IPN feed, 0.086 g NaOH, 180 mL min hydrogen gas ow, 800 rpm stirring. SiO
2
-400 and SiO
2
-600 were the SiO
2
samples calcined at 400 C
ꢂ
and 600 C for 4 h, respectively.
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2 3 2 3
Fig. 7 TG-DTA results of the separated catalysts and supports after reaction: (a) 20Ni–5Co/Al O (SI) separated from the mixture, (b) g-Al O
separated from the mixture, (c) 20Ni–5Co/SiO (SI) separated from the mixture, (d) SiO separated from the mixture.
2 2
negligible and the acidity kept in a very low range. The system: the hydrogenation reaction occurs on the metal sites of
untreated SiO2 had a surface hydroxyl content of 0.59%, the catalyst, and the condensation reactions occur on the
ꢂ
which decreased to 0.33% on the SiO treated at 400 C, and support sites, including the acid sites and the surface hydroxyl
2
ꢂ
further decreased to 0.12% on the SiO treated at 600 C. With groups. Imines and amines are formed and desorbed from the
2
the decrease of the surface hydroxyl content, the m-XDA metal sites, diffused in the liquid phase and re-adsorbed on
selectivity increased from 28.1% to 65.4%. This suggested these support sites, and nally produce heavy species by the
that the surface hydroxyl groups were also the active sites for condensation reaction on the acid sites and hydroxyl groups on
condensation side reactions.
the support surface.
To further study the roles of the surface sites, the spent
catalyst and support were separated aer the reaction and
analyzed by TG-DTA. The results are shown in Fig. 7. The spent
4. Conclusions
2
0Ni–5Co/Al O (SI), 2.5Ni–0.625Co/Al O , 20Ni–5Co/SiO (SI)
2 3 2 3 2
2 3 2
The g-Al O and SiO supported Ni–Co bimetallic catalysts with
different metal loading were synthesized and evaluated for IPN
and 2.5Ni–0.625Co/SiO
2
catalysts were also analyzed for
comparison (Fig. S6†). As shown in the TG-DTA results, all the
spent catalysts and supports had weight loss peaks corre-
sponding to the oxidation of heavy species deposited on the
catalysts during the reaction. The weight loss peaks at the
different temperatures were due to different heavy compo-
nents formed on the catalyst. A smaller amount of heavy
species was formed on the more selective catalyst and the
weight loss peak appeared at lower temperature. For the spent
hydrogenation. The repeated impregnation method
(RI method) effectively facilitated metal dispersion as
compared with the single step impregnation method (SI
method). The physical morphology and crystal size of xNi–yCo/
Al O prepared by the RI method were almost independent of
2
3
the Ni–Co loading. The effect of catalyst acidity was conrmed
by the study of the effect of the preparation method and Ni–Co
loading of xNi–yCo/Al O . However, besides the acid sites on
2 3
mixture of 20Ni–5Co/Al
support were separately characterized by TG-DTA, showing
2 3
that the weight loss peaks of 20Ni–5Co/Al (SI) and g-Al O
2 3 2 3
O (SI) and g-Al O , the catalyst and
the catalyst, the surface hydroxyl groups on the oxide supports
also catalyzed the condensation reactions. The enhanced
selectivity to m-XDA by increasing the Ni–Co loading was
attributed to the combined effect of the suppressed acidity and
the decreased amount of exposed surface hydroxyl groups. The
optimal 20Ni–5Co/Al O and 20Ni–5Co/SiO catalysts prepared
2
O
3
ꢂ
ꢂ
occurred at 365 C and 485 C, respectively. Similar results
were obtained with the experiments using the spent mixture of
2
0Ni–5Co/SiO (SI) and SiO .
2 2
2
3
2
By combining the above results, the following reaction
mechanism was proposed for the IPN hydrogenation reaction
by the RI method both have 99.9% selectivity to m-XDA in the
presence of basic additives.
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2
1 R. K. Marella, K. S. Koppadi, Y. Jyothi, K. S. R. Rao and
D. R. Burri, New J. Chem., 2013, 37, 3229.
Acknowledgements
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506.
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discussion and English revision.
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