B. Ding et al.
Molecular Catalysis 500 (2021) 111342
requirements of green chemistry.
2. Experimental
Niobium-based compounds can be extensively studied as the cata-
lytic materials [17,18]. For examples, Nb-silicates offer more advan-
tages with respect to hydrolytic stability as compared with that of V and
Mo catalysts [19]. Especially, as potential oxygen donors, niobium
complexes have received increasing attention in oxidation reaction
recently [20]. The previous studies have confirmed that the peroxo
niobium complexes are able to release active oxygen either chemically
or on irradiation, thus participating in oxidation of various organic
substrates [21].
2.1. Materials
18-crown-6 (CE, C12H24O6, 98 wt.%) was provided by Bide Phar-
matech Ltd. Potassium hydroxide (KOH) and Hydrogen peroxide (H2O2,
30 wt.%) were supplied by Shanghai Titan Scientific Co., Ltd. Niobium
pentaoxide (Nb2O5), ammonium hydroxide (NH3⋅H2O, AR) and meth-
anol (CH3OH, AR) were purchased from Aladdin. Glycolic acid (GLY,
C2H4O3, 98 wt.%) was supplied by TCI (Shanghai) Development Co.,
Ltd.
Ionic liquid (IL) has been represented as a research hotspot recently
owing to their distinctive natures containing a wide liquid range, non-
volatility, ionicity and strong polarity [22–24]. These specific proper-
ties of ILs have opened up their applications in the industry as reaction
solvents, catalysts, electrolytes, sensors, enzyme stabilizers [25,26]. The
structures and properties of ILs can be easily designed and adjusted via
varying their cations and inorganic anions. Currently, a great deal of
functionalized ILs have been investigated, providing a promising
methodology to synthesize catalysts with high performance and excel-
lent recyclability [27]. As a subset of functionalized ILs, poly-
oxometallate anion-based ILs (POM-ILs) have been achieved and
employed in acid-catalysis and oxidative reaction with excellent cata-
lytic activity and reusability [28,29]. There is no doubt that the
appearance of the functionalized ILs will assure the ILs with different
particular capabilities, promoting more sustainable and reasonable
development of ILs [30,31]. In our previous research, a series of per-
oxoniobate (peroxotantalate)-based ILs have been utilized in epoxi-
dizing olefins and allylic alcohols [32,33]. These ILs catalysts showed
huge advantages in separation and recycling and demonstrated better
performance in catalytic epoxidation compared to tungsten-containing
catalysts. However, the preparation of these ILs normally needs
tedious procedures such as neutralization, ionic exchange and solvent
removal etc., which contains time-consuming multiple operations and
might bring impurities in ILs.
2.2. Catalyst characterization
The procedures for catalyst characterization can be found in Sup-
porting Information.
2.3. Catalyst preparation
2.3.1. Synthesis of (NH4)3[Nb(O2)4]
At first, Nb2O5⋅nH2O was prepared by sintering KOH and Nb2O5 in a
◦
molar ratio of 10 (n(KOH/Nb2O5) = 10) at 550 C for 6 h [41]. Subse-
quently, the mixture was dissolved in water and acidified by acetic acid
with pH reaching to 4. The white precipitate was obtained by filtration
and washed thoroughly, followed by drying at 60 ◦C for 1 h. Then
NH4-Nb ((NH4)3[Nb(O2)4]) was obtained from the reaction of the
Nb2O5⋅nH2O and H2O2, adjusting pH around 10 with NH3⋅H2O until the
solution became clear. Ethanol was poured into the clear solution to
precipitate the NH4-Nb ((NH4)3[Nb(O2)4]) as a white powder [42].
Anal. Calcd for NH4-Nb (274.97): Nb, 33.8. Found: Nb, 33.1. Number of
peroxide bonds: 3.6.
2.3.2. Preparation of (CE)6(NH4)4[Nb2O4(
μ
-O)(η
2-O2)2](CE-1)
The IL (CE-1) was obtained according to the following procedure.
Briefly, CE (0.79 g, 3 mmol) and NH4-Nb (0.27 g, 1 mmol) (CE/Nb =
3:1) were stirred in 10 mL of methanol at 60 ◦C for 24 h. The clear so-
lution was rotary evaporated to obtain the light yellow viscous liquid as
As a branch of noncovalent interplay and supramolecular chemistry,
host-guest interaction exists extensively between metal cations and
macrocyclic ethers [34,35]. In recent years, crown ethers have drawn
growing interest because they are capable of forming complexes selec-
tively with ionic species by host-guest interaction [36]. In the previous
studies, a crown ether complex cation ILs were employed for various
carbon-carbon bond-forming reactions, including Michael addition,
Heck reaction and Henry reaction [37]. It indicated that these ILs can be
recycled for many times without significant deactivation. Sequentially,
the numerous deep eutectic solvents have been developed based on
host-guest interactions [38,39]. However, up to now the specific ILs
formed by the host-guest interactions have seldom been reported for
catalysis, let alone oxidation reaction.
(CE)6(NH4)4[Nb2O4(
μ-O)(
η
2-O2)2], designated as CE-1 (Fig. S1). 1H
NMR (400 MHz, D2O): 3.33 (s, 0..12 H), 3.67 (s, 6..15 H), δ 4.79 (s, 1 H).
13C NMR (100 MHz, D2O): δ 70.21, 49.50 (Fig. S2). Anal. Calcd for
C
72H160N4Nb2O45 (1987.87): C, 43.46; H, 8.05; N, 2.81; Nb, 9.35.
Found: C, 42.70; H, 7.70; N, 2.50; Nb, 9.20. Number of peroxide bonds:
1.5 per Nb atom.
2.3.3. Preparation of (CE)6(NH4)4[Nb2O4(μ-O)(GLY)2] (CE-2)
The preparation of CE-2 was similar to that of CE-1. Firstly, CE (0.79
g, 3 mmol) was dissolved in 10 mL of methanol and NH4-Nb (0.27 g, 1
mmol) was then added to the above solution with the molar ratio of CE/
According to our previous work, organic acid-modified perox-
oniobate ILs had high reactivity toward the epoxidation reaction [33].
However, the synthesis of the specific ILs was rather tedious due to
multiple steps. As a continuous effort to optimize the catalytic applica-
tion of the ILs, in this work we have developed a novel type of ILs formed
by supramolecular interactions. The supramolecular ILs can be obtained
by a direct mixing of CE with the NH4-Nb ((NH4)3[Nb(O2)4]) with the
certain molar ratio in methanol. Afterwards, the methanol was evapo-
rated under the vacuum to generate the ILs without any by-products,
which was a much simpler approach in comparison to that of the pre-
vious work [33,40]. The supramolecular complexation of CE and
NH4-Nb has been proved by FT-IR spectra, differential scanning calo-
rimetry (DSC) and conductivity measurement etc. Notably, the easily
available ILs demonstrated the excellent catalytic performance for the
epoxidation under very mild conditions.
◦
Nb = 3:1. The resulting mixture was stirred at 60 C for 24 h until it
became a clear solution. Next, GLY (0.08 g, 1 mmol) was dissolved in 2
mL of methanol and added dropwise to the above solution. The mixture
was then agitated intensely at 40 ◦C for another 12 h, and the solvent
was removed with a rotary evaporator. The resultant IL was placed
under vacuum at 40 ◦C to obtain yellow viscous liquid ((CE)6(N-
H4)4[Nb2O4(
μ
-O)(GLY)2]), designated as CE-2 (Fig. S1). 1H NMR (400
MHz, D2O): 3.34 (s, 2 H), 3.68 (s, 44 H), δ 3.98 (s, 1 H), 4.79 (s, 8 H). 13
C
NMR (100 MHz, D2O): δ 179.84, 171.03, 72.37, 70.20, 61.71, 49.50
(Fig. S3). Anal. Calcd for C76H164N4Nb2O47 (2071.94): C, 44.01; H, 7.91;
N, 2.70; Nb, 8.98. Found: C, 42.27; H, 7.48; N, 2.66; Nb, 8.89. Number of
peroxide bonds: 0.5 per Nb atom.
For the sake of comparison, CE-2a and CE-2b were prepared in a
similar way but 2 mmol CE (CE/Nb = 2) or 4 mmol CE (CE/Nb = 4) was
charged to 10 mL of methanol solution of NH4-Nb, respectively, fol-
lowed by adding GLY (0.08 g, 1 mmol). The resultant solution was
agitated at 40 ◦C for 12 h, and then the solvent was removed to afford
CE-2a and CE-2b, respectively. CE-2a ((CE)4(NH4)4[Nb2O4(μ-O)
2