Nanotubular TiO2-supported amorphous Co-B catalysts and their catalytic performances for hydroformylation of cyclohexene

Article abstract of DOI:10.1016/j.catcom.2014.09.043

The nano-confinement effect of multiwall TNTs was utilized for preparing Co-B/TNTs.Co-B deposited on the surface of TNT and entered into the interspace between walls.Co-B/TNTs showed high catalytic activity and stability for hydroformylation.The leaching of catalytically active species from Co-B/TNTs was very small.

Full text of DOI:10.1016/j.catcom.2014.09.043

Catalysis Communications 59 (2015) 45–49
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Nanotubular TiO2-supported amorphous Co–B catalysts and their
catalytic performances for hydroformylation of cyclohexene
⁎
Xiaojing Hu, Yukun Shi, Yijie Zhang, Baolin Zhu, Shoumin Zhang, Weiping Huang
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China
The Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China
Tianjin Key Lab of Metal and Molecule-based Material Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 January 2014
Received in revised form 23 September 2014
Accepted 25 September 2014
Available online 2 October 2014
TiO2 nanotubes supported amorphous Co–B catalysts (Co–B/TNTs) were prepared via WI-CRP. The catalysts were
characterized with TEM, XPS, TPD, nitrogen adsorption and so on. The catalytic performances and stabilities of
Co–B/TNTs for hydroformylation of cyclohexene as well as the influence factors, such as reaction media and Co
loading, were investigated. The Co–B/TNTs not only exhibited better catalytic performances in ether than in
acetone and alcohol, but also showed higher activities and stabilities than TiO2 powder supported amorphous
Co–B under the same reaction conditions. The reuse of Co–B/TNTs was also tested, and the results showed that
the used catalysts maintained high catalytic activity.
Keywords:
Hydroformylation
TiO2 nanotubes
Amorphous Co–B
Cyclohexene
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
catalytic activities [12]. Multiwall structured TiO2 nanotubes (TNTs) pre-
pared via simple hydrothermal treatment have many distinct proper-
Nowadays, hydroformylation discovered by Roelen in 1938 is still
an attractive reaction. Several million tons of aldehydes are produced
annually by the reaction [1]. For decades, the Co-based [2,3], Ir-based
[4,5] and Rh-based [6] catalysts for hydroformylation are widely in-
vestigated. Indeed, the Co-based catalysts are much less expensive
than Rh-based and Ir-based catalysts, and are able to hydroformylate
olefins selectively to aldehydes effectively. Though the soluble cata-
lyst shows high catalytic activity and selectivity, it is very difficult to
separate the catalyst from the products, especially in the case of
high boiling aldehyde formation. Wiese and Obst have estimated the
annual financial loss in a 400 kt plant when just 1 ppm Rh/kg product
is lost at several million euros [7]. Therefore, exploring for a stable cat-
alytic system incorporating the advantages of both homogeneous and
heterogeneous catalysis should have been an interesting work. The
conversion of homogeneous catalyst into heterogeneous system or
sealing catalytically metal active species into supports may be more
favorable, which prevents catalytically active species from loss [8,9].
Recently, several Co metal catalysts supported on inorganic supports
have been investigated for the hydrogenation [10] and hydroformylation
[11]. Nanotubes have large SSA, which has been widely used as supports
for heterogeneous catalysts. Carbon nanotubes supported catalysts for
the hydroformylation of olefine have been reported, and show high
ties [13,14]. It is well known that this kind of TNT is constructed by
rolling up one (1 0 1) layer of the anatase structure along the [−101] di-
rection and the (1 0 1) facet-constructed space between walls is about
0.76 nm [15]. In our previous works we have utilized such structure for
preparing thermally stable TNTs and photocatalysts with good catalytic
performances [16–18]. In the present contribution we report amorphous
Co–B sealed in TNTs via WI-CRP and its catalytic activity for
hydroformylation.
2. Experimental
TiO2 nanotubes were synthesized by the hydrothermal treatment as
the previous report [19].
The Co–B/TNTs (X, T °C) were prepared via WI-CRP, where X is the
mass fraction of Co and T is calcination temperature. The synthesis of
Co–B/TNTs was performed as follows: 1.0 g TNTs were dispersed in
20 mL of aqueous Co(NO3)2 solutions (2 mol/L, 1 mol/L, 0.5 mol/L,
respectively) and vigorously agitated for 24 h. After low-energy soni-
cation for 2 h, the mixture was centrifuged and the obtained TNTs
were transferred into 100 mL flask. Then 20 mL of aqueous KBH4
solution (2 mol/L, 1 mol/L, 0.5 mol/L, respectively) was dropwise
added, while vigorously stirred, in ice-water bath and under argon at-
mosphere. After addition of aqueous KBH4 solution, the mixture was
continually stirred for another 1 h at 0 °C and room temperature, re-
spectively. The mixture was centrifuged and the black product was
washed to neutral with distilled water repeatedly, and then washed
⁎
Corresponding author. Tel.: +86 22 23502996; fax: +86 22 23502669.
E-mail address: hwp914@nankai.edu.cn (W. Huang).
http://dx.doi.org/10.1016/j.catcom.2014.09.043
1566-7367/© 2014 Elsevier B.V. All rights reserved.

46
X. Hu et al. / Catalysis Communications 59 (2015) 45–49
three times with ethanol. The product was filtered, dried at 40 °C for
12 h in vacuum, and marked as Co–B/TNTs (X). Part of the product
was calcinated in argon atmosphere at 300 °C for 2 h, which was
marked as Co–B/TNTs (X, 300 °C). For comparison, the Co–B/TiO2
was prepared under the same conditions except that commercial
TiO2 powder was substituted for TNTs, and a conventional amorphous
Co–B sample marked as Co–B was also prepared by the same reduc-
tion processes. The detailed process of characterization is showed in
the Supporting information S.1.
Hydroformylation with cobalt catalysts is usually carried out in the
temperature range of 140–180 °C. Here 150 °C was chosen as the reac-
tion temperature based on the DSC results and our previous work [2].
The hydroformylation of cyclohexene was carried out in a 250-mL stain-
less steel autoclave reactor with magnetic stirrer. In a typical experi-
ment, the required amount of catalyst, substrate and solvent was
charged into the reactor. The reactor was sealed, purged with nitrogen
three times, heated to the reaction temperature and pressurized with
gas (CO + H2) to the desired pressure while stirring. After the required
reaction time, the stirring was stopped. Then the reactor was cooled to
room temperature, and the pressure was carefully released. The reac-
tion mixture was withdrawn and analyzed using GC–MS or GC (Beifen
3420A chromatograph equipped with a 50 m × 0.53 mm SE-30 capillary
column and a FID, cf. S.7–10). After reaction, the reaction system was
centrifuged and the recovered solid catalyst was washed three times
with ethanol, and then dried at 40 °C for 12 h in vacuum for the next
reaction.
between walls of TNTs, which can be seen clearly by TEM. ICP-AES was
used to determine the contents of Co and B in as-prepared Co–B/TNTs
and Co–B. The atom ratio of Co to B is around 2 or 3 (Table 2), which
means that the Co and B may exist as Co2B or Co3B in samples though
there are other types of Co–B alloy [20].
Displayed in Fig. 1 are TEM images of conventional amorphous Co–B,
as-prepared and calcinated Co–B/TNTs (12.96). As shown in Fig. 1, the
conventional amorphous Co–B is an agglomeration of Co–B nanoparti-
cles (Fig. 1A) and the multiwall structure of TNT is clear (Fig. 1B–D).
For as-prepared Co–B/TNTs (12.96), amorphous Co–B not only deposit-
ed highly uniformly on the surface of TNTs, but also entered the inter-
space between layers (Fig. 1B–C). However, there were some black
spots in the as-prepared Co–B/TNTs (12.96), which should be amor-
phous Co–B. After calcination at 300 °C, the Co–B/TNTs (12.96, 300 °C)
(Fig. 1D) still kept its tubular structure, its multiwall structure became
clearer in comparison with the as-prepared ones. The multiwall struc-
ture of TNTs may play an important role in preventing amorphous Co–
B particles from aggregation or running off in the processes of
hydroformylation.
XPS was used to analyze Co-2p3/2 and B-1s in as-prepared Co–B/
TNTs (12.96). The binding energies of 778.8 eV and 780.8 eV arise
from Co-2p3/2 and 187.9 eV and 191.4 eV from B-1s (cf. Supporting in-
formation S.2). This means that the Co and B on the surface of as-
prepared Co–B/TNTs are present in both elemental and oxidized
states. The binding energy peaks at 778.8 eV and 187.9 eV are assigned
to the metallic Co species and elemental B, respectively. A small
amount of oxidized cobalt (780.8 eV) is attributed to CoO that has
been formed in the processes of preparation or measurement [21].
Fig. 2 is the magnetic hysteresis loop of as-prepared Co–B/TNTs
(12.96) and Co(NO3)2/TNTs at room temperature, which shows that
the as-prepared Co–B/TNTs (12.96) are of ferromagnetism. The satura-
tion magnetization (Ms), residual magnetism (Br) and coercivity (Hc)
3. Results and discussion
The SSA of as-prepared Co–B/TNTs (12.96) and TNTs is 180 m2/g
and 235 m2/g, respectively. The result can be ascribed to that amor-
phous Co–B deposits on the surface of TNTs or occupies the interspace
Fig. 1. TEM images of as-prepared Co–B (A), Co–B/TNTs (12.96) (B, C) and Co–B/TNTs (12.96, 300 °C) (D).

X. Hu et al. / Catalysis Communications 59 (2015) 45–49
47
(C). 2) The amorphous structure of Co–B supported by TNTs did not
change in the hydroformylation reaction at 150 °C for 3 h.
1.0
0.5
4
2
A
The DSC was used to characterize the thermally stability of as-
prepared Co–B/TNTs. The DSC thermogram of as-prepared Co–B/TNTs
(12.96) can be roughly divided into three exothermic sections in the
temperature range of 100–500 °C (cf. Supporting information S.3). A
sharp exothermic peak at 213 °C in the first section (100–224 °C)
might be relevance to the surface shrinkage of as-prepared Co–B/TNTs
(12.96). A similar result has been reported [23]. The second (224–
406 °C) and third (above 406 °C) sections should be related to the
phase transition of TNTs and agglomeration or crystallization of amor-
phous Co–B particles, which is confirmed by the XRD patterns of Co–
B/TNTs (12.96, 300 °C) (Fig. 3C).
In order to investigate the adsorption capacities of samples for CO
and H2, the CO- and H2-TPD were performed and the results are showed
in Fig. 4. As shown in the Fig. 4A-a, two distinguished CO desorption
peaks with maxima at 115 and 221 °C appear, which might be ascribed
to the desorptions from the outer and inner surfaces of TNTs, respective-
ly. Compared with TNTs, the as-prepared Co–B/TNTs (12.96) show
three CO desorption peaks (Fig. 4A-b), and the peak at 338 °C should
be assigned to the desorption from amorphous Co–B. In Fig. 4B, desorp-
tion peaks with maxima at 89 and 208 °C (Fig. 4B-c) should be the H2
desorptions from the outer and inner surfaces of TNTs, respectively.
For as-prepared Co–B/TNTs (12.96), four H2 desorption peaks that ap-
peared at around 89, 188, 329, and 369 °C (Fig. 4B-d) could be grouped
to desorptions from TNTs (peaks at 89, 188 °C) and amorphous Co–B
0.0
-0.5
-1.0
-200 -100
0
100 200
Magnetic Field(Oe)
0
0.050
0.025
0.000
-0.025
-0.050
B
-2
-4
-1000 -500
0
500 1000
Magnetic Field(Oe)
-800
-400
0
400 800
Magnetic Field(Oe)
Fig. 2. Magnetic hysteresis loop of as-prepared Co–B/TNTs (12.96) at room temperature.
Inset: (A)—magnified view of Co–B/TNTs at smaller field; (B)—magnetic hysteresis loop
of as-prepared Co(NO3)2/TNTs at room temperature.
of as-prepared Co–B/TNTs (12.96) are 4.785 emu/g, 0.3976 emu/g
and 34.04 Oe, respectively, which are all much larger than that of
Co(NO3)2/TNTs, implying formation of Co–B alloy, though the Ms of
as-prepared Co–B/TNTs (12.96) is much smaller than that of the
normal Co–B alloy (45.7 emu/g) [22]. It is well known that the Ms
increases with the increase of particle size, so do the Br and Hc. Due
to the nano-confinement effect of TNTs, the average size of amor-
phous Co–B particles that formed on the inner surfaces, especially
in the interlayer space of TNTs, is certainly smaller than that of regu-
lar Co–B, and it is reasonable that the Ms of Co–B/TNTs (12.96) is
smaller than that of the regular Co–B.
A
a: TNTs-CO
221
115
124
b: Co-B-TNTs-CO
209
Fig. 3 is the XRD patterns of as-prepared Co–B/TNTs (12.96), Co–B/
TNTs (12.96) used at 150 °C for 3 h and Co–B/TNTs (12.96, 300 °C).
The patterns of A, B and C all possess peaks of anatase TiO2 (JCPDS21-
1272), and the peaks at 25.24° and 48.1° are the diffractions of the
(101) and (200) crystal planes, respectively. As shown in Fig. 3C, there
is a very weak and broad diffraction peak at around 2θ = 44.7°, which
is a characteristic peak of crystal Co–B particles. Fig. 3 indicates: 1) The
Co–B in as-prepared Co–B/TNTs (12.96) (A) and Co–B/TNTs (12.96)
used at 150 °C for 3 h (B) were amorphous. After being calcined at
300 °C for 2 h, the TNTs supported amorphous Co–B turned into crystal
338
a
b
0
100
200
300
400
500
Temperature/°C
B
89
c: TNTs-H2
Co-B alloy
d: Co-B/TNTs-H2
208
A
369
329
188
B
C
d
c
0
10
20
30
40
(degree)
50
60
70
80
0
100
200
300
400
500
2θ
Temperature/°C
Fig. 3. XRD patterns of as-prepared Co–B/TNTs (12.96) (A), Co–B/TNTs (12.96) used at
150 °C for 3 h (B) and Co–B/TNTs (12.96, 300 °C) (C).
Fig. 4. (A): CO-TPD profiles of TNTs (a) and Co–B/TNTs (12.96) (b). (B): H2-TPD profiles of
TNTs (c) and Co–B/TNTs (12.96) (d).

48
X. Hu et al. / Catalysis Communications 59 (2015) 45–49
Table 1
molecule (III) was too large to form or nano-confinement effect of
Comparison of the catalytic activities of supported Co–B catalysts.a
Co–B/TNTs prevented it from forming. Entries 2, 6 and 7 display the
variation of catalytic activity of Co–B/TNTs with the Co loading
under the same reaction conditions; the total conversion of cyclo-
hexene and amount of alcohol increased with the increase of the
Co content. However, as we concentrate on the cyclohexene conver-
sion over per mole of Co, we can find that the catalytic activities of
Co–B/TNT catalysts increase firstly and then decrease with the in-
crease of Co loading. It is well known that there are some coordina-
tive unsaturated oxygen atoms on the inner and outer surface of
TNTs, which can be covered by Co2+ ions step by step (monolayer
adsorption) along with the increase of amount of Co2+ ions in the
processes of preparation. After monolayer adsorption, instead of
raising the dispersion of Co2+ ions, the excess Co2+ ions may block
tubes after reduction, which resulted in the decrease of catalytic ac-
tivity per mole of Co. As a matter of fact, the TiO2 nanotubes play an
important role in enhancing the catalytic activity per mole of Co,
which is confirmed by that the catalytic activity of Co–B/TNTs
(7.53) (Entry 6) is higher than that of pristine Co–B (Entry 8) though
the amounts of Co used in the reaction are almost identical.
Fig. 5 shows the variations of conversion and selectivity with reac-
tion time. The conversion of cyclohexene increased steadily up to 2 h,
and thereafter it remains constant. However, the selectivity to aldehyde
increased steadily up to 1 h and decreased steadily. If the catalytic hy-
drogenation of (I) was faster than the hydroformylation of cyclohexene,
the main product of the catalytic reaction should be alcohol (II), which
was, however, not back up by the results showed in Table 2 and Fig. 5.
The as-prepared Co–B/TNT catalyst has a good catalytic activity for
both hydroformylation and hydrogenation, and its catalytic activity for
hydroformylation is far higher than that for hydrogenation under the
optimized reaction conditions. After 2 h, nearly all of cyclohexene
turned into (I) and hydrogenation of (I) increased, which resulted in de-
crease in selectivity for (I).
In order to examine the difference in catalytic activity between
fresh and used catalyst, the recycle uses of as-prepared Co–B/TNTs
were carried out, and the results are showed in Fig. 6. The fresh Co–
B/TNTs (12.96) showed the best catalytic performance, over which
the conversion of cyclohexene was the highest (98.94%), but the se-
lectivity for aldehyde was the lowest (aldehyde: 67.64%, alcohols:
32.36%). As it was used secondly, its catalytic performance changed
slightly. The selectivity for aldehyde increased very much though
the conversion of cyclohexene decreased a little (93%). Furthermore,
the catalyst became stable onward, and the conversion as well as se-
lectivity did not change evidently. After reaction, the contents of Co
and B in the solution were detected by ICP, which are also showed
in Fig. 6. The data exhibit that the Co and B leaching from Co–B/TNTs
were very low and they decreased with recycle times. As discussed
above, a small amount of oxidized cobalt existed on the as-prepared
Entry Catalyst
Conversion of
cyclohexene (%)
Selectivity (%)
Aldehyde Alcohols
1
2
3
As-prepared Co–B/TNTs
(12.96)
Co–B/TNTs (12.96,
300 °C)
As-prepared Co–B/TiO2
(11.99)
83.17
12.93
68.69
81.81
100
93.02
18.19
0
6.98
a
Reaction condition: 5 mL cyclohexene, 65 mL THF, 1 g as-prepared catalyst, 150 °C,
t = 1 h, gas (CO:H2 = 2:1): 6.0 MPa at 100 °C.
(peaks at 329, 369 °C), respectively. The peaks at 329 and 369 °C sug-
gest that amorphous Co–B particles contained at least two kinds of Co-
based active sites for H2 [24].
Listed data in Table 1 are the catalytic activities of different forms of
titania supported Co–B and Co–B/TNTs (12.96, 300 °C). The as-prepared
Co–B/TNTs (12.96) are more active than both Co–B/TNTs (12.96,
300 °C) and as-prepared Co–B/TiO2 (11.99), and the calcination affects
the catalytic activity of Co–B/TNTs badly. As shown in DSC analysis,
the as-prepared Co–B/TNTs underwent shrinkage at 213 °C, this result-
ed in the fact that the SSA and the amount of catalytically active species
on the surface of Co–B/TNTs (12.96, 300 °C) were certainly smaller than
that of as-prepared Co–B/TNTs (12.96), which were unfavorable for cat-
alytic reaction. The SSA of TNTs is larger than that of the commercial
titania powder; the large SSA is favorable not only for dispersion of
amorphous Co–B, but also for the increase of absorption capacity of
catalyst for CO and H2. Thus, it is reasonable that the as-prepared Co–
B/TNTs show higher catalytic activity than the Co–B/TiO2.
There would be four products, aldehyde (I), alcohol (II), acetal
(III) and cyclohexane (IV), if all possible reactions took place equally
in the hydroformylation processes of cyclohexene (cf. Supporting in-
formation S.4). The catalytic performances of various catalysts for
hydroformylation of cyclohexene are listed in Table 2. The conver-
sion of cyclohexene over Co–B/TNTs is related to the CO/H2 ratio by
comparison between Entries 1 and 2. The conversion of cyclohexene
rises from 56.82% (Entry 1) to 99.15% (Entry 2) with the increase of
CO/H2 ratio from 1 to 2. The catalytic performances of as-prepared
Co–B/TNTs (12.96) in tetrahydrofuran, dimethoxyethane, ethanol
and acetone (Entry 2–5) imply that tetrahydrofuran is a more perfect
solvent for the hydroformylation of cyclohexene. It is noteworthy
that only in the case of ethanol used as solvent, the condensation
product formed (Entry 4, cf. Supporting information S.6–7), which
means that the condensation reaction between formed aldehyde
and alcohol did not take place in the reaction processes. The reason-
able interpretation for the results could be that the condensation
Table 2
Catalytic performances of various Co–B/TNTs under different conditions.a
Entry
Catalyst
Contents of Co (wt.%)
Atom ratio of Co to B
Solvent
Conver. of cyclohexene
Selectivity (%)
Aldehyde
Total (%)
Molesb
Alcohol
Acetal
1
2
3
4
5
6
7
8e
Co–B/TNTs
Co–B/TNTs
Co–B/TNTs
Co–B/TNTs
Co–B/TNTs
Co–B/TNTs
Co–B/TNTs
Co–B
12.96
12.96
12.96
12.96
12.96
7.53
3
3
3
3
3
2
3
2
THFc
THF
DME
Ethanol
Acetone
THF
56.82
99.15
92.28
77.19
11.79
91.88
49.49
88.86
12.67
30.07
24.08
17.19
4.00
43.53
30.04
35.50
99.97
65.28
84.13
6.62
93.86
78.17
94.88
80.20
0.03
34.72
15.87
0
6.14
21.83
5.12
19.8
0
0
0
93.38d
0
0
0
0
4.75
68.27
THF
THF
a
Reaction conditions: 5 mL cyclohexene, 65 mL solvent, 1 g as-prepared Co–B/TNTs, 150 °C, t = 3 h, gas (CO:H2 = 2:1): 6.0 MPa at 100 °C.
Moles of cyclohexene conversion and aldehyde hydrogenation over one mole of Co.
Gas (CO:H2 = 1:1): 6.0 MPa at 100 °C.
Acetal is the condensation product of aldehyde (II) and ethanol.
0.128 g as-prepared Co–B catalyst.
b
c
d
e

X. Hu et al. / Catalysis Communications 59 (2015) 45–49
49
120
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
0.25
0.20
0.15
0.10
0.05
0.00
conversion
aldehyde
alcohols
CoContent
B Content
conversion of cyclohexene
selectivity for aldehyde
1
2
3
4
5
0
1
2
3
4
recycle times
Time/h
Fig. 6. Influence of the cycling on the conversion, selectivity, and element leaching after
reaction. Reaction condition: 5 mL cyclohexene, 65 mL THF, 1 g as-prepared Co–B/TNTs
(12.96), 150 °C, t = 3 h, gas (CO:H2 = 2:1): 6.0 MPa at 100 °C.
Fig. 5. Variations of conversion/selectivity with reaction time. Reaction condition:
10 mL cyclohexene, 60 mL THF, 1 g as-prepared Co–B/TNTs (12.96), 150 °C, gas
(CO:H2 = 2:1): firstly, 6.0 MPa at 100 °C, and then charging gas (CO:H2 = 1) to
6.5 MPa every 1 h at 150 °C.
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Acknowledgments
This work is supported by the National Natural Science Foundation
of China (21373120, 21071086, 21271110 and 21301098), the 111
Project (B12015) and the Natural Science Foundation of Tianjin
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.09.043. These data include MOL files
and InChiKeys of the most important compounds described in this
article.