Cobalt immobilized on hydroxyapatite as a low-cost and highly effective heterogeneous catalyst for alkenes epoxidation under mild conditions

Article abstract of DOI:10.1039/C8RA07168C

Transition metal Co immobilized on hydroxyapatite with a loading of 0.05 wt% (denoted 0.05 wt% Co/HAP) could catalyze partial oxidation of cyclic alkenes, aromatic alkenes and aliphatic alkenes to yield epoxide products with excellent selectivity at 30 °C with O₂ and iso-butyraldehyde as co-oxidant. The TOF value was as high as 6261 h^?1 for epoxidation of cyclohexene. In addition, the prepared 0.05 wt% Co/HAP catalyst can be re-used at least 6 times without significant loss of catalytic activity and selectivity.

Full text of DOI:10.1039/C8RA07168C

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Cobalt immobilized on hydroxyapatite as a low-
cost and highly effective heterogeneous catalyst
for alkenes epoxidation under mild conditions†
Cite this: RSC Adv., 2018, 8, 37303
ab
Pagasukon Mekrattanachai,ab Changyan Cao,
Zhaohua Li,ab Huining Liab
*
ab
*
and Weiguo Song
Transition metal Co immobilized on hydroxyapatite with a loading of 0.05 wt% (denoted 0.05 wt% Co/HAP)
could catalyze partial oxidation of cyclic alkenes, aromatic alkenes and aliphatic alkenes to yield epoxide
products with excellent selectivity at 30 ꢀC with O2 and iso-butyraldehyde as co-oxidant. The TOF value
was as high as 6261 hÀ1 for epoxidation of cyclohexene. In addition, the prepared 0.05 wt% Co/HAP
catalyst can be re-used at least 6 times without significant loss of catalytic activity and selectivity.
Received 28th August 2018
Accepted 26th October 2018
DOI: 10.1039/c8ra07168c
rsc.li/rsc-advances
Epoxides are very important intermediates for the production of and high stability, but it is can be also modied through
ne chemicals and pharmaceuticals. Epoxidation of alkenes is substitutions of other anions, cations, or functional groups.11–13
one of the most effective routes to synthesize epoxides.1–3 In the This character makes HAP as an excellent support for metals
past decades, many different kinds of catalysts and reaction and makes the preparation for heterogeneous catalysts very
systems have been developed for alkenes epoxidation with simple. Based on the ion-exchange mechanism, various HAP-
molecular oxygen, hydrogen peroxide or organic oxidants. supported transition metals, such as Pd, Ru, Cu and Au, have
Homogeneous catalysts usually exhibited high activity and been prepared and used as heterogeneous catalysts for various
selectivity but they cannot be recycled, resulting in inconve- organic transformations, including oxidation and Suzuki–
nience for separation. In comparison, heterogeneous catalysts Miyaura cross-coupling.14–22 In our previous work, we also found
have the obvious advantage of recyclability. Apart from the well- an excellent catalytic system for the epoxidation of alkenes
known heterogeneous catalyst based on titanium silicalite (TS- based on 0.05 wt% of Ru species supported on HAP in the
1),4 catalysts based on noble metals (such as Au, Pd, Ru, etc.) presence of molecular oxygen and iso-butyraldehyde as co-
were usually used for highly selective epoxidation of alkenes.5–7 oxidant.23 However, Ru is noble metal which is expensive and
Scattered reports about using novel low-cost non-noble metal the reaction was operated at 60 ꢀC to complete the reaction.
catalysts, for example, tungsten oxide, gallium oxide and Therefore, we wondered other metals immobilized on HAP
graphitic oxide can also be found in the literature.8–10 However, might be more efficiency for alkenes epoxidation, especially for
their activity and selectivity remain relatively low. In order to Co because of the special oxidizing property reported in the
obtain high yields, these heterogeneous reaction systems were literatures.24–33
usually operated under harsh conditions, e.g. high temperature,
Herein, we indeed nd that Co immobilized on hydroxyap-
strong organic oxidizing agents. Regarding atom efficiency and atite even with very low loading of 0.05 wt% (denoted as
cost, it is highly desirable to develop a highly effective hetero- 0.05 wt% Co/HAP) is a highly effective heterogeneous catalyst
geneous catalyst for alkenes epoxidation under mild reaction for alkenes epoxidation under mild condition. It could catalyze
conditions.
partial oxidation of cyclic alkenes, aromatic alkenes and
Hydroxyapatite (HAP) which possess Ca2+ sites surrounded aliphatic alkenes to yield epoxide products with excellent
3À
by PO4
tetrahedral parallel to the hexagonal axis, have selectivity at 30 ꢀC with O2 and iso-butyraldehyde as co-oxidant.
attracted more attention as solid support for recyclable catalysts The TOF value could be reached as high as 6261 hÀ1 for epox-
due to their multiple functionalities. HAP is not only nontoxic idation of cyclohexene. Such value is among the highest active
heterogeneous catalyst for epoxidation of alkenes. In addition,
the prepared 0.05 wt% Co/HAP catalyst can be re-used at least 6
aBeijing National Laboratory for Molecular Sciences, Laboratory of Molecular
times without signicant loss of catalytic activity and selectivity.
Nanostructures and Nanotechnology, CAS Research/Education Center for Excellence
Various metals immobilized on HAP were prepared through
in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing
cation-exchange between metal ions in solution and calcium
ions in HAP,34,35 and then tested for epoxidation of cyclohexene
at 30 ꢀC with O2 and iso-butyraldehyde as co-oxidant in CH3CN
100190, China. E-mail: cycao@iccas.ac.cn; wsong@iccas.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available: Experimental details,
solvents effect and TEM image of catalyst. See DOI: 10.1039/c8ra07168c
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Table 2 Catalytic activities of Co immobilized on HAP with various Co
solvent. The metals loading in all of prepared catalysts were
determined by inductively coupled plasma atomic emission
spectroscopy (ICP-AES, Table S1†). No peaks of metals were
observed in the XRD patterns (Fig. S1†). As shown in Table 1,
0.05 wt% Co/HAP showed the highest conversion and selec-
tivity, even much higher than those noble metals of Ru and Pd.
The TOF value was calculated to be 6261 hÀ1 at 85% conversion,
which was nearly three times higher than that of 0.05 wt% Ru/
HAP. In addition, oxygen and iso-butyraldehyde co-oxidant were
both indispensable for this reaction system (Table 1, entries 7
and 8). The organic solvents with varying polarity also showed
obvious effect on the catalytic activity for cyclohexene epoxida-
tion. As shown in Table S2,† the moderate polarity solvent,
CH2Cl2, showed the highest activity in this system. The effect of
Co loading was also investigated and the results were presented
with TOF value (Table 2). When the higher Co loading (1, 3 and
5 wt% of Co) were immobilized on HAP, more cyclohexene-1-
one was observed and the % selectivity was slightly decreased.
This might be due to the more Co active sites in the reaction
system, the high possibility of over oxidized cyclohexene-1-one
product could take place. It can be seen that 0.05 wt% Co/
HAP showed the highest TOF value of 245 minÀ1 and 86%
selectivity for 1,2-epoxycyclohexane. This indicated that
0.05 wt% Co/HAP exhibited superior catalytic activity which
might be due to the low coordination of Co nanocluster on HAP.
The 0.05 wt% Co/HAP catalyst also exhibited excellent cata-
lytic property in epoxidation of other alkenes, including cyclic
alkenes, aromatic alkenes and aliphatic alkenes (Table 3). All
tested olens could be converted to epoxides with excellent
selectivities. Particularly, allylic alkenes and linear olens could
be converted to epoxide products with high yields under such
mild reaction condition, conrming the highly active of
0.05 wt% Co/HAP catalyst in this catalytic system.
loading in epoxidation of cyclohexenea
Entry
Catalyst
TOF (minÀ1
)
Conv.%
Sel.%
1
2
3
4
0.05 wt% Co/HAP
1 wt% Co/HAP
3 wt% Co/HAP
5 wt% Co/HAP
245
12
7
50
92
95
96
86
85
84
84
6
a
Reaction conditions: catalyst (20 mg), cyclohexene (1 mmol), CH3CN (5
mL) iso-butyraldehyde (5 mmol), 30 ꢀC, O2 balloon, 30 min reaction
time.
was 0.05 wt%, which was the same as the fresh catalyst.
Therefore, this conrmed the high stability of the Co/HAP
catalyst and the leaching of Co species was not observed.
Furthermore, the BET surface area aer the re-used catalyst was
only a little decrease compared with the fresh catalyst (Fig. S2†).
This might be due to the adsorption capacity of HAP support,
which could adsorb some organic molecules on its surface.
Based on the above results, the facile synthesis procedures,
extremely low percentage of Co, together with excellent catalytic
performances and stability make 0.05 wt% Co/HAP a low-cost
and highly effective heterogeneous catalyst for olens epoxi-
dation under mild reaction conditions.
Physical structure of 0.05 wt% Co/HAP was characterized by
various techniques in order to preliminary clarifying the excel-
lent catalytic property. In the XRD patterns, all the peaks of
0.05 wt% Co/HAP can be ascribed to the pure hexagonal struc-
ture HAP (JCPDS 74-0565).36,37 No peaks of Co species were
observed (Fig. 2a), suggesting the extremely low loading and
highly dispersion of Co species. In addition, no obvious dots or
nanoparticles were observed in the dark-eld TEM image
(Fig. S3†), while highly dispersed Co species can be clearly seen
Besides the high activity and selectivity, 0.05 wt% Co/HAP
showed the good recyclability. During recycles, the solid cata-
lyst was recovered from reaction mixture by centrifugation,
Table 3 Catalytic activity of 0.05 wt% Co/HAP in epoxidation of
various alkenes under optimal conditiona
ꢀ
washed with dichloromethane and then dried at 60 C under
Entry
1
Substrate
Time (h)
3.5
Conv.%
100
Sel.%
100
vacuum overnight. As shown in Fig. 1, the prepared 0.05 wt%
Co/HAP catalyst can be re-used at least 6 times without signif-
icant loss of catalytic activity and selectivity. Additionally, the Co
loading of the used 0.05 wt% Co/HAP aer the 6th run reaction
was determined by ICP-AES. The result showed that Co loading
2
3
5
6
100
77
100
100
Table 1 Catalytic activities of various metals immobilized on HAP in
epoxidation of cyclohexenea
4
5
24
20
87
88
93
96
Entry
Catalyst
TOF (hÀ1
)
Conv.%
Sel.%
1
2
3
4
5
6
7
8
0.05 wt% Co/HAP
0.05 wt% Ru/HAP
0.05 wt% Mn/HAP
0.05 wt% Cu/HAP
0.05 wt% Fe/HAP
0.05 wt% Pd/HAP
0.05 wt% Co/HAPb
0.05 wt% Co/HAPc
6261
2148
1614
1245
838
1786
3388
3094
85
51
47
47
54
47
46
42
93
88
90
88
83
64
78
11
6
7
15
30
66
70
100
100
8
6
92
100
9
14
17
73
70
100
95
10
a
Reaction conditions: catalyst (20 mg), cyclohexene (1 mmol), CH3CN (5
mL) iso-butyraldehyde (5 mmol), 30 ꢀC, O2 balloon, 2 h reaction time.
a
Reaction conditions: catalyst (20 mg), alkene (1 mmol), CH2Cl2 (5 mL),
iso-butyraldehyde (5 mmol), 30 ꢀC, O2 balloon.
b
Without oxygen balloon. c Without iso-butyraldehyde.
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bond of alkenes. Thus, the highly dispersed Co species on HAP
exhibited the superior catalytic activity.
Based on the above results and literatures about heteroge-
,
neous catalysts for alkenes epoxidation.19 39–41 We proposed
a plausible reaction mechanism for cyclohexene epoxidation
over our catalysis system (Scheme S1†). Firstly, acylperoxy
radicals were formed through homo and heterolytic cleavage of
oxygen molecules and iso-butyraldehyde; then, Co species react
with oxygen atom of acylperoxy radical, which was subsequently
coordinated with C]C bond of alkenes; nally, the oxidative
addition of acylperoxy radical to C]C bond take place, yielding
epoxide product companying the formation of butyric acid.
Conclusions
Co immobilized on hydroxyapatite with loading of 0.05 wt% was
found to be a highly effective heterogeneous catalyst for alkenes
epoxidation under mild conditions, e.g. reaction temperature at
30 ꢀC, O2 and iso-butyraldehyde as co-oxidant. Various alkenes,
including cyclic alkenes, aromatic alkenes and aliphatic alkenes
can be converted to epoxide products with excellent selectivity.
The TOF value for epoxidation of cyclohexene could be reached
as high as 6261 hÀ1. In addition, the prepared 0.05 wt% Co/HAP
catalyst also showed good recyclability, no signicant loss of
catalytic activity and selectivity were observed aer the 6th runs.
According to the excellent catalytic performances and high
stability of 0.05 wt% Co/HAP catalyst, this might be indicated
that Co species might exist as single atom or nanocluster with
low coordination environment. Additionally, the initial success
in achieving selective epoxidation over our prepared 0.05 wt%
Co/HAP catalyst might provide a new avenue for developing new
types of catalyst with small amount of metal loading.
Fig. 1 Reusability test of 0.05 wt% Co/HAP for epoxidation of cyclo-
hexene. Reaction conditions: catalyst (20 mg), cyclohexene (1 mmol),
iso-butyraldehyde (5 mmol), CH2Cl2 (5 mL), 30 ꢀC, 30 min. reaction
time, O2 balloon.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors thank the nancial supports from the National
Natural Science Foundation of China (NSFC 21333009,
21573244, 21573245) and the Youth Innovation Promotion
Association of CAS (2017049).
Notes and references
Fig. 2 (a) XRD patterns of synthesized HAP and 0.05 wt% Co/HAP, (b)
dark field TEM image of 0.05 wt% Co/HAP and EDS mapping images.
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