Langmuir
Article
Preparation of THAF. First, tetrahexylammonium bromide
(THABr) and the 717 strongly alkaline anion exchange resin were
exchanged in dry ethanol at room temperature several times until no
bromide ions in the filtrate were detectable by a silver nitrate test to
obtain tetrahexylammonium hydroxide (THAOH). Next, the
obtained THAOH solution was directly neutralized with an equal
molar amount of hydrofluoric acid for several hours and then dried
under vacuum for 1 h at 60 °C. The molten organic salt was obtained
composed of group V (V, Nb, Ta) and group VI (W, Mo)
metals.43 Due to their unique adjustable redox, acid−base
properties, and high stability,44 they play an important role in
the field of acid catalysis and catalytic oxidation. In particular,
Kholdeeva and co-workers have investigated the catalytic
performance of Nb-substituted or Ti-substituted Lindqvist
tungstates in the epoxidation with H2O2 and suggested that the
Nb(V) catalyst can significantly decrease the free-energy
barrier for the heterolytic oxygen transfer because of the
higher electrophilicity of the Nb sites.45 Nevertheless, group V
metals have still not gained much attention due to the low
activities of their precursors and the difficult synthesis. Among
them, due to the rich surface redox properties, surface charge,
and more accessible nanoscale,46−48 polyoxyniobate (PONb)
deserves to be paid more attention.
In view of the fact that few methods were used to synthesize
different metal oxo-clusters directly (e.g., [Nb6O19]8−), their
stability and the determination of structures are still facing
challenges. Organic acids or nitrogen-containing heterocycles
have been employed as ligands to coordinate with metal sites
to stabilize their oxo-clusters, especially for group IV metals
(Ti, Zr, and Hf).49−55 Among these oxo-clusters, the organic
ligands play vital roles; the coordination of ligands can stabilize
these metal oxo-clusters and prevent from further aggregation.
Both experimental results and density functional theory (DFT)
calculations have shown that functional ligands have a
significant influence on the physicochemical properties and
catalytic activities of synthesized oxo-clusters.56,57 However,
Nb oxo-clusters stabilized by organic ligands are still rarely
reported, let alone their catalytic properties.
In our previous work, we synthesized a niobium oxo-cluster
stabilized by the ionic liquid tetrabutylammonium lactate.58
Even if the Nb oxo-cluster was used for the oxidation of
various thioethers at the ppm level, it showed exceptionally
high catalytic activity. As a continuation of our efforts to design
an easily accessible, highly active, and stable Nb oxo-cluster for
oxidation catalysis, we report herein that the commercially
available molten organic salts like tetraethylammonium
fluoride (TEAF), tetrabutylammonium fluoride (TBAF),
tetrabutylammonium chloride (TBACl), tetrabutylammonium
acetate (TBAOAc), etc. can protect the Nb oxo-cluster from
aggregation. Among these organic salts, TBAF showed the best
stabilization effect on the Nb oxo-cluster and also had a
positive influence on the epoxidation of allylic alcohols and
olefins under mild conditions. Notably, fluoride ions played a
key role not only in stabilizing/modifying Nb sites by
coordination interaction but also in promoting the epoxidation
of allylic alcohols by forming hydrogen bonds between
fluorine-containing niobate and the OH group of allylic
alcohol.
1
as a colorless transparent liquid. H NMR (400 MHz, CDCl3, TMS)
δ 3.22 (m, 8H), 1.65 (m, 8H), 1.36 (m, 8H), 0.90 (t, 12H). 19F NMR
(600 MHz, CDCl3, CF3COOH) δ = −151.5.
Preparation of Nb Oxo-Clusters with Organic Salts. First, Nb
oxo-clusters were prepared with a molten organic salt as a stabilizing
reagent by adjusting the molar ratio of the molten salt to Nb.
Considering the synthesis of Nb-OC@TBAF-0.5 as an example, the
as-synthesized niobic acid (1.21 g, 2 mmol Nb) and an excess of H2O2
(4.0 mL, 30% aqueous solution) were mixed to get a clear peroxy-
niobic acid solution under the ice-bath condition. TBAF·3H2O (0.315
g, 1 mmol) was then added and stirred overnight. Afterward, the
solution was heated to 50 °C and water was evaporated for several
hours. Finally, a viscous yellow-green semisolid was obtained and
named Nb-OC@TBAF-0.5, where the molar ratio of TBAF/Nb was
equivalent to 0.5. 1H NMR (400 MHz, D2O, Me4Si) δ 3.16 (m, 8H),
1.54 (m, 8H), 1.31 (m, 8H), 0.91 (t, 12H). 13C NMR (100 MHZ,
D2O, Me4Si): δ 59.43, 24.84, 20.74, 14.07. Found: C, 30.86; H, 6.72;
N, 1.99; Nb, 28.3. Number of peroxy bonds: 0.11 per Nb atom.
Similarly, Nb-OC@TBAF-0.25 and Nb-OC@TBAF-1 oxo-clusters
were synthesized just by tuning the molar ratio of TBAF/Nb to 0.25
and 1, respectively.
For the sake of comparison, Nb-OC@TBACl-0.5, Nb-OC@
TBABr-0.5, Nb-OC@TBAOAc-0.5, and Nb-OC@TBAHSO4-0.5
were prepared by the same method, except that the organic salts
TBACl, TBABr, TBAOAc, and TBAHSO4 were adopted instead of
TBAF·3H2O, respectively. The as-synthesized catalysts were yellow
powders. Similarly, fluorides with different cations were also used to
prepare control catalysts. For example, NaF, TEAF, and THAF were
used as stabilizing agents to obtain the corresponding Nb-OC@NaF-
0.5, Nb-OC@TEAF-0.5, and Nb-OC@THAF-0.5.
Typical Procedure of Catalytic Epoxidation of Allylic
Alcohols. The procedure for the epoxidation of allylic alcohol is as
follows. The substrate allylic alcohol (1.0 mmol), 30% aqueous
hydrogen peroxide (1.0 mmol), and the oxo-cluster catalyst (0.02
mmol) were mixed in a 5 mL reaction flask with a magnetic stirrer.
The reaction mixture was stirred at 0 °C for a certain time. The
conversion of the substrate was analyzed by gas chromatography
(GC) using n-butyl alcohol as the internal standard. After the
reaction, the products and the residual substrate were extracted with
ethyl acetate three times as the organic salt-stabilized oxo-cluster was
completely immiscible in weak polar organic solvents. The upper
organic phase was separated and dried with sodium sulfate, followed
by GC analysis, while the lower aqueous phase was dried under
vacuum at 40 °C to obtain the recovered catalyst for the next run.
Reaction Kinetics. The reaction kinetics over the Nb-OC@
TBAF-0.5 catalyst was investigated using 3-methyl-2-butene-1-ol as a
model substrate, and the procedure is as follows. The internal
standard n-butanol was stirred vigorously under the desired
temperature (268−285 K) with a magnetic stirrer. The reaction was
detected by GC. For the determination of reaction orders, the
reaction mixture maintained a consistent volume by addition of H2O
in the kinetic studies considering that the reaction was carried out
under solvent-free conditions. The concentration of the substrate was
varied from 1.16 to 6.51 M, but the concentrations of H2O2 and the
catalyst were maintained at 3.88 and 0.085 M, respectively. As a result,
the logarithmic dependence of the epoxidation rate on the substrate
concentration was deduced. In another parallel experiment, the
epoxidation rate was also determined by changing the concentration
of H2O2 from 1.26 to 6.30 M, but the concentrations of the substrate
and the catalyst were maintained at 3.32 and 0.085 M, respectively.
Similarly, the reaction rate was achieved by changing the
concentration of the Nb-OC@TBAF-0.5 catalyst (calculated by
EXPERIMENTAL SECTION
■
The materials, characterizations, and preparation procedures of niobic
acid, (NH4)3[Nb(O2)4], and F-containing peroxo-niobium complexes
(C12H10N2)[NbF5(O2)] and (C9H8NO)2[NbF5(O2)]·3H2O are
provided in the Supporting Information.
Preparation of TBAHSO4. The molten organic salt TBAHSO4
was prepared by simple neutralization of TBAOH (25% aqueous
solution) with H2SO4 in water (1:1 molar ratio). The solution was
continuously stirred for a few hours and then dried under vacuum for
1 h at 60 °C. The molten organic salt was obtained as a transparent
and colorless solid at room temperature, which could be melted when
heated (mp 170−173 °C). 1H NMR (400 MHz, D2O, Me4Si) δ 3.09
(m, 8H), 1.54 (m, 8H), 1.24 (m, 8H), 0.83 (t, 12H).
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Langmuir 2021, 37, 8190−8203