S. Fan et al. / Journal of Molecular Catalysis A: Chemical 404 (2015) 186–192
187
ratio characteristic, are available for catalysis [44]. Therefore,
poly(vinylpyridine) spheres with monodispersed nanomorphology
have been introduced to improve the catalytic activity in the reac-
tion system.
collected with centrifugation and washed with absolute ethanol
several times in order to remove HCl and free iron ions. Finally, the
yellow powder catalyst was dried under vacuum at 40 C for 12 h.
◦
Here, we report
a
novel poly(4-vinylpyridine) supported
2.4. General procedure for the oxidation of benzylic methylenes
Fe(III)/TBHP (tert-butylhydroperoxide) catalytic system for ben-
zylic oxidation. A series of monodispersed poly(4-vinylpyridine)
spheres as solid supports to load heterogeneous Fe(III) catalysis
was evaluated. The synthetic monodispersed poly(4-vinylpyridine)
supported Fe(III) catalyst possessed high dispersion in the reac-
tion system taking full advantage of its uniform sizes and
nanomorphologies. The novel poly(4-vinylpyridine) supported
Fe(III) catalyst showed excellent catalytic performance toward
benzylic oxidation. Furthermore, the pyridine moiety of the syn-
thesized catalyst was able to function as an organic base, which
significantly reduced the amount of pyridine in the system. There-
fore the design of monodispersed poly(4-vinylpyridine) supported
Fe(III) catalyst provides a novel approach to advanced catalysts for
organic systhesis.
ꢀ
In
a
typical
procedure,
a
mixture
of
4,4 -
and
difluorodiphenylmethane
(178.4 L,
1.0 mmol)
P4VPDVB2.5–40%-Fe(III) catalyst (2.0 mol%), pyridine (8.0 L,
0.1 mmol), TBHP (5.0–6.0 M in decane, 545.0 L, 3.0 mmol) and
1.0 mL acetonitrile was dissolved in a 25 mL single-necked flask
fitted with a reflux condenser. The mixture was heated at 80 C for
24 h under air atmosphere in an oil bath. Then the mixture was
◦
◦
cooled to 25 C and centrifuged to get a catalyst and supernatant
solution. Then the solution was analyzed by Agilent 7890/5975C-
GC/MSD using nitrobenzene as an internal standard. A calibration
curve for each reactant and product has been built, which is
1
included in the ESI†. The in situ H NMR monitoring of the reaction
conversion and yield was performed in the DMSO-d6 solvent, the
crude sample of entry 11 in Table 3 was filtered for direct 1H NMR
analysis (see ESI†). Furthermore, the products of entries 1 and 7
in Table 2 were isolated through column chromatography and the
isolated yields were provided.
2
. Experimental
2.1. Materials
All substrates and solvents were purchased from
2.5. Reusability of the catalyst
Sigma–Aldrich, Alfa Aesar or Aladdin. 4-Vinylpyridine monomer
96%, 4VP; Aldrich) were stabilized with 100 ppm hydro-
quinone and used under further purification. Divinylbenzene
80 mol% DVB; Aldrich), Aliquat 336 (Mn = 404.17; Aldrich),
(
On completion of each reaction cycle, the catalyst was cen-
trifuged from solution mixture and then washed with ethanol
(5.0–10.0 mL) several times. After being dried in an oven at 40 C
◦
(
poly(ethyleneglycol)methacrylate
solution (Mn = 2080; 50 wt% in H O) and 2,2 -Azobis(2-
(PEGMA)
macromonomer
under vacuum for 6 h, the catalyst was reused.
ꢀ
2
methylpropionamidine)dihydrochioride (97%, AIBA; Aldrich)
2.6. Characterization
were used as received. FeCl ·6H O were purchased from Alfa
3
2
Aesar and TBHP (tert-butylhydroperoxide) (5.0–6.0 M; Aldrich)
Field-emission scanning electron microscopy (FESEM) pho-
was preserved in decane.
tographs were observed by a SUPRA 55 (Zeiss, Germany)
instrument operated at acceleration voltage 10 kV. The iron con-
tent of the P4VPDVB10%-Fe(III) complex was measured by atomic
absorption spectroscopy (Varian 240FS). An FEI Tecnai F20 electron
microscope with EDX operating at an acceleration voltage of 200 kV
and a ZEISS SUPRA55 instrument operating at an acceleration volt-
age of 10 kV were applied to characterize the morphology of the
product. The spectra of Fourier transform infrared spectroscopy
(FTIR) were obtained on a NICOLET 6700 infrared spectrophotome-
ter using potassium bromide (KBr) disk samples. Thermal gravity
analysis (TGA) curves were investigated using a Netzsch STA449F3
2
.2. Preparation of P4VPDVB5–40% spheres
The monodispersed poly(4-vinylpyridine) (P4VP) spheres were
achieved according to the literature synthesis of monodispersed
poly(2-vinylpyridine) (P2VP) spheres [45]. In a typical emulsion
polymerization procedure, the Aliquat 336 surfactant (0.5 g) and
the PEGMA stabilizer (1.0 g) were dissolved in deionized water
(
80.0 g). Then the transparent solution turned into milk white
emulsion under supersonic conditions. The emulsion was stirred
at 300 rpm in a three-necked round-bottomed flask. The flask fit-
ted with a reflux condenser was sealed and the solution was
degassed with nitrogen cycles at ambient temperature. A mix-
ture of 4VP (5.0 g) and 2.5–40 wt% DVB (based on 4VP monomer)
cross-linker was then added. Meanwhile the solution was stirred
◦
instrument with the heating rate of 10 C/min. The X-ray photoelec-
tron spectroscopy (XPS) spectra were measured on a Kratos AXIS
Ultra DLD photoelectron spectroscope. The binding energies were
adjusted by placing the C1s binding energy at 284.8 eV from adven-
titious carbon. The gas chromatographic (GC) data was obtained by
Agilent 7890/5975C-GC/MSD, fitted with a capillary column (col-
◦
continuously and heated at 60 C. After 20 min, the initiator solu-
◦
tion (0.05 g in 9 mL distilled water) was added dropwise into the
above-mentioned solution. The solution was then stirred for 24 h.
The P4VPDVB2.5–40% spheres were synthesized and purified with
centrifugation at 9000 rpm for 30 min, then washed with ethanol
and distilled water several times so that the residual monomer was
removed. Furthermore, the P4VPDVB2.5–40% microgels were dried
under vacuum for 12 h.
umn: HP-5MS;injection temperature: 50 C).
3. Results and discussion
3.1. Synthesis of the monodispersed spheres supported Fe(III)
In an initial study, P4VPDVB2.5–40% complexes with different
weight percentages of the DVB cross-linker were synthesized and
used as the polymer supports for FeCl ·6H O. The porous structures
2.3. P4VPDVB5–40%-Fe(III) catalyst preparation
3
2
were provided by DVB in a poly(4-vinylpyridine) network [46].
The iron content of P4VPDVB2.5–40%-Fe(III) was determined by AAS
(atomic absorption spectroscopy) analysis. The P4VPDVB spheres
with 2.5 wt%, 5 wt%, 10 wt%, 20 wt% and 40 wt% DVB offered the key
advantage of high Fe(III) loading on P4VPDVB2.5–40%-Fe(III) catalyst
which was 2.01, 1.91, 1.84, 1.80 and 1.55 mmol/g, respectively. The
Powdered P4VPDVB2.5–40% spheres 5.0 g were dispersed insat-
urated solution of FeCl ·6H O in absolute ethanol with 1.0 mL of
3
2
1
N HCl aqueous solution (pH 2) under supersonic conditions for
high dispersion. The above solution was stirred at 300 rpm for
1
◦
2 h and heated at 70 C in an oil bath. The resulting product was