A multi-component polyoxometalate and its catalytic performance for CO2 cycloaddition reactions

Article abstract of DOI:10.1039/c5dt01046b

A multi-component polyoxometalate based on earth-abundant elements (NH₄)₁₀[Co₈(H₂O)₁₀V₁₀Mo₂₃O₁₀₄(OH)₆]·34.5H₂O (1) has been successfully obtained and characterized. Furthermore, compound 1 acted as a Lewis acid catalyst and promoted the conversion of carbon dioxide to a cyclic carbonate under mild reaction conditions.

Full text of DOI:10.1039/c5dt01046b

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DOI: 10.1039/C5DT01046B
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A Multi-component Polyoxometalate and its Catalytic
Performance for CO2 Cycloaddition Reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Shumin Chen, Ying Liu, Jipeng Guo, Pengzhen Li, Zhiyuan Huo, Pengtao Ma, Jingyang Niu* and
Jingping Wang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A multi-component polyoxometalate based on earth-abundant reported by Zhai,7b as catalyst whereas co-catalyst
elements (NH4)10[Co8(H2O)10V10Mo23O104(OH)6]·34.5H2O (1) has tetrabutylammonium bromide in polyethylene glycol (PEG) for
been successfully obtained and characterized. Further, compound coupling reaction of CO2 and propylene oxide or ethylene oxide at
1 could act as Lewis acid catalyst and promote the conversion of 120 oC. Then, Hu reported monosubstituted α-Keggin-type POMs
carbon dioxide to cyclic carbonate under mild reaction conditions.
[(n-C7H15)4N]x[α- GeW11M(H2O)O39] (M = CoII, MnII, NiII) for catalytic
conversion of CO2 to cyclic carbonate in 20108a and proposed the
mechanism of the cylcoaddition of CO2 to epoxides in 2012.8b
Recently, our group reported an example of transition metal
rhenium substituted POM [(CH3)4N]5H23[(PW11O39){Re(CO)3}3(µ3-
O)(µ2-OH)]4·24H2O,9 which catalyzed the conversion of CO2 to cyclic
carbonates under mild reaction conditions. Nevertheless, Re is
neither abundant nor inexpensive and thus very likely prohibitive
for use on a realistic scale. The gain of active, environmentally
benign, and recyclable catalysts are expected to be of great
industrial interest.
In recent years, carbon dioxide (CO2) concentration in the
atmosphere is deemed to be the main contributor to the
greenhouse effect and global warming. Therefore, the fixation and
transformation of CO2 has become a hot research topic.1 In
particular, the synthesis of cyclic carbonates by the coupling of
epoxide with CO2 has attracted an increasing attention due to the
potential applications in pharmaceutical and fine chemical
industries.2 Although CO2 is an attractive C1 building block,
efficiently chemical fixation of CO2 remains difficult due to its inert
nature. Over the past decades, various catalysts have been widely
used in CO2 cycloaddition reactions,3 but only under high
In continuation of our studies, we focused our attention on a new
strategy to address the problem by exploring efficient catalyst.
Multi-component systems, that are combined the intrinsic
properties of distinct elements into optimized devices for
synergistic effects and new applications, should be effective in
cycloaddition reactions. By mixing (NH4)6Mo7O24·4H2O, NH4VO3 and
CoCl2·2H2O in aqueous solution at pH value of 4.7, a novel POM
(NH4)10[Co8(H2O)10V10Mo23O104(OH)6]·34.5H2O was obtained
(Scheme 1), which displayed a two dimensional (2D) layer
framework built up of the [Co6(H2O)2V10Mo23O104(OH)6]14−
o
temperature (> 100 C) and pressure (> 2 MPa) as well as rigorous
separation and purification of the products. Therefore, it is still
needed to search for new catalysts, that are capable of catalysing
cycloaddition of CO2 with epoxides. In this field, polyoxometalates
(POMs) exhibit efficient catalyst activity, due to their unique
structural and compositional varieties.4 Generally, POMs were
utilized as Brønsted acid and oxidation catalysts.5 On the other
hand, the POMs decorated with transition metal moieties were
reported to act as Lewis acid catalysts. In 2004, a zinc-substituted
sandwich-type Na12[WZn3(H2O)2(ZnW9O34)2] was reported as
catalyst for CO2 cycloaddition reaction at moderate CO2 pressure
(0.4–1.0 MPa) in the temperature range 100–160 oC.6 Subsequently,
in 2005, Sakakura group reported that monosubstituted α-Keggin-
type POMs [(n-C7H15)4N]x[α-SiW11M(H2O)O39
] (M =
CoII, MnII)
o
catalyzed cyclic carbonate from CO2 at 150 C.7a In 2007, α2-Wells-
Dawson-type [(n-C4H9)4N]xP2W17M(Br)O61 (M = CoII, CuII, MnIII) were
Scheme 1 Synthetic procedure of 1. Color codes: the atoms of Co, V
and
O are brown, yellow and red, respectively. The MoO6
polyhedra is shown in LT cyan.
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two-up-two-down fashion and four VO4 tetrahedra filling the gaps
between the pairs of octahedra. The V(2) atom is found in the
center of the subunit (Fig. S3). It should be noted that V(6) occupies
special position with an occupancy factor of 0.5 for each. And the
{VMo7O28} (Fig. 2d) fragment can be considered as a derivative of
(B-β-VMo9O33) structure by removing one edge-shared {Mo3O13
groups. Each monomeric unit of 1 contains two truncated {Co3O13
}
}
cubane core (Fig. 2c), three Co (II) cations reside in the CoO6
octahedral geometry and three edge-sharing CoO6 octahedra
generate the triangular cobalt cluster. Furthermore, in the triplet
Co(1)–Co(2)–Co(3), Co(3) has a terminal water molecule, as showed
in Figure 2c. Such water molecules can be removed or replaced by
other ligands, the loss of the coordinated water molecules leads to
coordinatively unsaturated sites,12 where the Lewis acidic site may
be produced.
Fig. 1. Ball and stick representation of anion {Co8Mo23V10} viewed
along (left) the b-axis and (right) the a-axis.
polyanion (abbreviated as {Co6Mo23V10}) and {Co(H2O)4} bridges. It
is noteworthy that the labile aqua ligand of {Co(H2O)4} could be
removed during the activation stage prior to use in catalysis, which
may leave a free coordination position in the metal as Lewis acidic
site. Compound 1 with the 2D sheet, which has a high density of
well-oriented Lewis acid sites, is desirable for high catalytic activity
for the chemical fixation of CO2 into cyclic carbonates under mild
reaction conditions.
The polyanion is extended to 2D infinite network, in which each
{Co6Mo23V10} is linked to four others through {Co(H2O)4} bridges (Fig.
S1–S2). The six-coordinate Co(4) atom resides in octahedral
geometry, which is defined by six oxygen atoms from four
coordinated water and two terminal oxygen atoms of {V4.5Mo8O40}.
These coordinated water molecules of Co(4) could be expediently
removed, thus Co(4) could be used as a Lewis acid catalyst.
Successive 2D sheets are further stacked in the -A-B-A-B- type to
produce a 3D architecture framework with 1D channel along the b
axis, and that the active sites of the Lewis acid catalysts are well
aligned in the channels (Fig. S4). The accessible pores are about
4211.1 Å3 (23.9% of the unit cell volume) calculated from PLATON
Single-crystal X-ray analysis indicates that 1‡ crystallizes in the
orthorhombic space group pnnm, and consists of
a
[Co8(H2O)10V10Mo23O104(OH)6]10- (abbreviated as {Co8Mo23V10})
anion （Fig. 1）, 34.5 lattice water molecules and ten ammonium
cations. According to the bond valence sum (BVS) calculations,10 the
oxidation state of all Co, Mo and V atoms are +2, +6 and +5
respectively. (Table S2–S3). The lower BVS values of O7 (1.317) and
O14 (1.287) in the frameworks of the {VMo7O28} unit suggest
monoprotonation of these oxygen atoms. Similarly, the BVS values
also suggest monoprotonation of O56 (1.216) in {Co3O13} unit and
O58 (1.391) in {V4.5Mo8O40} unit (Table S4).
Table 1. Effect of reaction parameters on the coupling of CO2 and
epoxide catalyzed by catalyst 1 with ionic liquid 2a
O
O
Catalyst 1
O
CO2
+
O
R
ionic liquids 2
R
Cat.1
(mol%)
2
Pres.
(MPa)c
Temp.
(ºC)
Yield
(%)d
{Co6Mo23V10} is composed of two subunits {V4.5Mo8O40} and a
{VMo7O28} fragment linked by trinuclear CoII clusters, leading to a
banana-shaped structure with the idealized C2v symmetry (Fig. 2a).
In other words, {Co6Mo23V10} can also be considered as a double-
sandwich structure, where two {V4.5Mo8O40} and a {VMo7O28} are
Entry
Substrate
(mmol)
1b
2
--
1.5
1.5
1.5
1.5
1.0
1.5
1.5
1.5
1.5
70
60
65
70
70
70
70
70
70
36.5
36.8
63.7
76.1
68.3
79.0
71.3
63.3
0.00
8
8
8
8
8
8
8
8
--
0.28
0.28
0.28
0.28
0.31
0.28
0.28
0.28
3
4
separated by two distinct {Co3O13} triads. The subunit {V4.5Mo8O40
}
(Fig. 2b) is similar to the reported {V5Mo8O40}.11 The subunit
5
6
contains of eight edge-sharing {MoO6} octahedra forming a ring in a
7e
8f
9
10g
0.28
8
8
1.5
70
78.6
11h
0.28
1.5
70
86.2
aReaction conditions: epoxide (5 mmol), ionic liquid (8 mmol, 80
mg), reaction time 1 h. b1,2-epoxy-3-phenoxypropane (5 mmol, 680
uL), cInitial pressure at room temperature. dDetermined by GC using
Dimethyl phthalate as an internal standard, the selectivity were
over 99% in all cases. eThe 2nd run. fThe 3rd run. gglycidyl
methacrylate (5 mmol, 660 uL). hchloromethyloxirane (5 mmol, 390
uL).
Fig. 2 Polyhedral / ball-and-stick representation of a) {Co6Mo23V10};
b) {V4.5Mo8O40}; c) {Co3O13} and d) {VMo7O28}.
2 | J. Name., 2012, 00, 1-3
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analysis.13 The porous structure benefits guest molecules getting and catalyzed the cycloaddition of CO2 to epoxides under mild
into the channels interacted with catalyst site.
conditions, which paves an avenue for the design and synthesis of
Based on previous reports,3e 1 was used to catalyze the chemical novel environmentally benign catalytic system for chemical fixation
fixation of CO2 with glycidyl phenyl ether in the ionic liquid of of CO2.
pyrrolidinium bromide. The reactions were carried out under the
conditions described in Table 1. We explored the catalytic reaction
Notes and references
of the coupling of glycidyl phenyl ether (1a) with CO2 to produce 3-
phenoxy-1,2-propylene carbonate (2a). In the absence of 1 (Table 1,
entry 1), 1a was found to be able to convert into 2a with 36.5%
yield. With 0.28 mol% of 1 (entry 4), the yield was increased to
76.1% and over 99% selectivity. If addition of 1 was increased
incrementally from 0.28 to 0.31 mol%, the yield increased from 76.1
to 79.0 % (entries 6). The effect of the CO2 pressure was also
investigated (entries 5), the yield of 2a was decreased at a lower
pressure of 1.0 MPa. As shown in Table 1 (entries 2–4), the catalyst
system was quite sensitive to reaction temperature. When the
temperature was decreased from 70 °C to 60 °C, the yields
decreased from 76.1% to 36.8%. Consequently, the best yield of 2a
can be achieved under such reaction conditions as entry 4 in table
1. The possibility of recycling complex 1 in the synthetic process was
investigated. 1 was used to catalyze the cycloaddition of CO2 for 3
cycles, and showed a relative recyclability (entries 7 and 8). The IR
spectra of the recovered catalyst were similar with the fresh
catalyst (Fig. S7), indicating that the catalytic system was stable.
The catalytic activities of other epoxides like glycidyl methacrylate
and chloromethyloxirane were also tested under the same reaction
conditions. The results showed that 1 was active for these epoxides
as well (entries 10 and 11). The catalytic activity of 1 seemed to
depend on the substrates size. The yield of 1a, with dimensions of
10.0×4.4×3.2 Å3, was 76.1%. The yield of glycidyl methacrylate and
chloromethyloxirane, the smaller epoxides with dimensions of
8.8×4.4×2.4 Å3 and 3.5×3.2×2.2 Å3, were increased to 78.6% and
86.2% under similar conditions, respectively. Furthermore, highly
polar of glycidyl methacrylate and chloromethyloxirane also give
relative conversion.
‡
Crystal data for1: CSD 429071. Mr = 5935.64, orthorhombic,
space group Pnnm, a= 28.1420(16) Å, b= 24.2136(13) Å, c =
25.7921(14) Å, V= 17575.2(17) Å3, Z= 4, μ=2.922 mm−1, F(000) =
11412.0, GOOF = 1.117. Of 88020 total reflections collected,
15773 reflections are unique (R(int)= 0.0672). R1 = 0.0778, ωR2
= 0.2161 for 828 parameters and 15773 reflections [I>2σ(I)]
.
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They confirmed that the reaction occurred through attacks of the
catalyst to the epoxides and formed a CoIII radical intermediate.
Here, we supposed that the reaction mechanism was similar to that
reported by Manikandan et al. Firstly, the epoxide is activated by
coordination to the Co center (Lewis acid site), then, it is attacked
by Br- (Lewis base site) and the ring opens; Finally, the subsequent
interactions of the nucleophilic alkoxide intermediate with the
electrophilic CO2 form the cyclic carbonate. Control experiment
demonstrated that no detectable conversion was observed for the
model reaction in the absence of the pyrolidinium bromide
cocatalyst (Table 1, entry 9), which gave a preliminary evidence to
support this hypothesis.
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In this paper, a multi-component polyoxometalate based on earth-
abundant elements (Co, V, Mo) was synthesized at a cheaper cost
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