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1,1’-(Butane-1,4-diyl)-bis(3-cyanopyridine)
di(bromide)
([D-3-
because the nonfunctionalized hybrid [D-Py]2.5PMoV2 also con-
sists of noncrystalline nanoparticles (Figure S3) with a consider-
ably high surface area of 37.1 m2 gÀ1, but presents a low
phenol yield. The above phenomena indicate that it is the
strong electronic effect between functional-group-tethered IL
dications and the PMoV2 anion that primarily affects the cata-
lytic activity, although the stacking pores among the nanopar-
ticles also play a passive role.
CNPy]Br2): The dicationic IL precursor [D-3-CNPy]Br2 (Scheme 1)
was prepared by dissolving 3-cyanopyridine (20 mmol) and 1,4-di-
bromobutane (10 mmol) in ethanol (20 mL) in a 25 mL Teflon-lined
autoclave. The mixture was treated at 908C for 48 h. The obtained
white solid, which was not dissolved in ethanol, was washed with
ethanol several times and dried (yield 43%). 1H NMR (300 MHz,
D2O, TMS): d=9.57 (s, 2H; 2(ÀCH)), 9.23 (d, 2H; 2(ÀCH)), 8.99 (d,
2H; 2(ÀCH)), 8.32 (m, 2H; 2(ÀCH)), 4.83 (s, 2H; 2(ÀCH2)), 2.24 ppm
(s, 2H; 2(ÀCH2)); 13C NMR (75.5 MHz, D2O, TMS): d=151.8, 151.0,
131.9, 116.9, 116.0, 64.4, 29.7 ppm. 1,1’-(Butane-1,4-diyl)-bis(4-cya-
nopyridine) di(bromide), denoted as [D-4-CNPy]Br2, was prepared
in the same way with 4-cyanopyridine and 1,4-dibromobutane
Conclusion
1
(yield 39%). H NMR (300 MHz, D2O, TMS): d=9.20 (d, 4H; 4(ÀCH)),
Nitrile-tethered hybrid catalyst [D-3-CNPy]2HPMoV2 (see
Scheme 1) heterogeneously catalyzes the aerobic oxidation of
benzene to phenol in the presence of ascorbic acid, and exhib-
its a high phenol yield plus superior reusability. The partially
negative nitrile N atoms exert an electronic effect on the
PMoV2 anion, thereby accounting for the high catalytic activity
of [D-3-CNPy]2HPMoV2. The present work provides not only
a superior catalyst for hydroxylation of benzene with O2, but
also some clues for designing more versatile heterogeneous
vanadium-containing polyoxometalates for selective aerobic
oxidation of organic substrates.
8.51 (d, 4H; 4(ÀCH)), 4.82 (s, 4H; 2(ÀCH2)), 2.22 ppm (s, 4H; 2(À
CH2)); 13C NMR (75.5 MHz, D2O, TMS): d=148.7, 134.2, 131.1, 117.0,
64.6, 29.9 ppm.
[D-3-CNPy]2HPMoV2: The obtained IL precursor [D-3-CNPy]Br2
(5.0 mmol) was added to an aqueous solution of H5PMo10V2O40
(2.0 mmol), and then the mixture was stirred at room temperature
for 24 h. The formed yellow precipitate was isolated by filtration
and washed with water three times, followed by drying in
a vacuum. Elemental analysis calcd (wt%): C 16.98, N 4.95, H 1.46;
found: C 17.36, N 5.20, H 1.77. The thermogravimetric profile indi-
cated that [D-3-CNPy]2HPMoV2 was stable up to 2408C (Figure S1).
[D-4-CNPy]2HPMoV2 was prepared in the same way with [D-4-
CNPy]Br2 and H5PMo10V2O40. Elemental analysis calcd (wt%): C
16.98, N 4.95, H 1.46; found: C 17.23, N 5.13, H 1.58.
Experimental Section
1,1’-(Butane-1,4-diyl)-bis-pyridine di(bromide) denoted as [D-Py]Br2
was synthesized according to the previous literature.[33] [D-
Py]2.5PMoV2 was synthesized by a procedure similar to that for [D-
3-CNPy]2HPMoV2. Elemental analysis calcd (wt%): C 18.50, N 3.08, H
1.98; found: C 18.00, N 2.86, H 2.06.
Materials and methods
1
All chemicals were of analytical grade and used as received. H and
13C NMR spectra were measured with a Bruker DPX 500 spectrome-
ter at ambient temperature in D2O using TMS as internal reference.
Elemental analyses (C, H, and N) were performed on a CHN ele-
mental analyzer (Vario EL cube). Nitrogen adsorption–desorption
was performed at the temperature of liquid nitrogen using a BEL-
SORP-MINI analyzer. The samples were degassed at 1508C to
a vacuum of 10À3 Torr before analysis. Fourier transform infrared
(FTIR) spectra were recorded on a Nicolet iS10 FTIR instrument (KBr
disks) in the 4000–400 cmÀ1 region. Electron spin resonance (ESR)
spectra were recorded on a Bruker EMX-10/12 spectrometer at the
X-band. X-ray diffraction (XRD) measurements were made with
a SmartLab diffractometer (Rigaku Corporation) equipped with
a 9 kW rotating-anode Cu source at 40 kV and 200 mA, from 5 to
508 with a scan rate of 0.28 sÀ1. Field-emission scanning electron
microscopy (FESEM, Hitachi S-4800, accelerated voltage: 5 kV) with
energy-dispersive X-ray spectrometry (EDS, accelerated voltage:
20 kV) was used to examine the morphology and Mo/V molar ratio
of the catalyst. TEM images were obtained by using a JEOL JEM-
2010 (200 kV) TEM instrument.
1,1’-Methylenebis(4-dimethylaminopyridinium) dichloride and 1,1’-
methylenebis(3-aminopyridinium) dichloride were synthesized ac-
cording to the previous literature,[22] and were denoted as [D-4-
N(Me)2Py]Cl2
and
[D-3-NH2Py]Cl2,
respectively.
[D-4-
N(Me)2Py]2.5PMoV2 was synthesized by a procedure similar to that
for [D-3-CNPy]2HPMoV2. Elemental analysis calcd (wt%): C 18.92, N
5.89, H 2.31; found: C 19.06, N 5.95, H 2.37. [D-3-NH2Py]2HPMoV2
was synthesized in the same way. Elemental analysis calcd (wt%):
C 12.35, N 5.24, H 1.36; found: C 12.57, N 5.41, H 1.44.
Catalytic test
The hydroxylation of benzene was performed in a temperature-
controllable pressured titanium reactor (100 mL) equipped with
a
mechanical stirrer. In a typical experiment, catalyst (0.1 g,
0.2 mol%), ascorbic acid (0.60 g), and benzene (2 mL) were added
successively to an aqueous solution of acetic acid (80 vol%,
25 mL). After the system was charged with O2 (2.0 MPa) at room
temperature, the hydroxylation reaction was conducted at 1008C
for 10 h with vigorous stirring. After the reaction, 1,4-dioxane was
added to the product mixture as an internal standard for product
analysis. The mixture was analyzed by a gas chromatography (GC)
with a flame ionization detector and a capillary column (SE-54; SE-
54; 30 mꢁ0.32 mmꢁ0.25 mm). Under the reaction conditions,
phenol was the only product detected by GC, and the commonly
seen by-products (catechol, hydroquinone, and benzoquinone)
were not found. After the first run of the test, the reaction mixture
was centrifuged and the recovered solid catalyst was washed with
acetic acid, dried in a vacuum, and then reused in the next run.
Catalyst preparation
H5PMo10V2O40 (PMoV2) was prepared according to previous litera-
ture reports.[17,32] MoO3 (16.59 g) and V2O5 (2.1 g) were added to
deionized water (250 mL). The mixture was heated to the reflux
temperature under vigorous stirring with a water-cooled condens-
er, then at 1208C an 85 wt% aqueous solution of H3PO4 (1.33 g)
was added dropwise to the reaction mixture. When a clear orange-
red solution appeared, it was cooled to room temperature. The
orange-red powder PMoV2 was obtained by evaporation of the so-
lution to dryness, followed by recrystallizing for purification.
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ChemPlusChem 2014, 79, 1590 – 1596 1595