3
06
N.-T. Le et al. / Applied Catalysis A: General 464–465 (2013) 305–312
unsupported V O5 catalyst also exhibited low HMF conversion [24]
2
while V O5/TiO catalysts (V∼1.32–7.33%, TiO -64% anatase + 36%
2
2
2
rutile) showed 81% HMF conversion with a good selectivity to
DFF. Navarro et al. [25] reported the preparation of vanadyl-
immobilized onto polymers [poly(4-vinylpyridine) cross-linked
with 33% divinylbenzene] using vanadylacetylacetonate (VOacac)
as a vanadyl ion source and examined the catalytic efficiency. Even
though vanadium species coordinated to polymers exhibited better
catalytic performance than homogeneous VOacac, high vanadium
leaching was reported. This leaching was related to the loss of
catalytic activity during recycling tests. The literature reports also
revealed the similar problems when vanadyl ions are immobilized
on Nafion support [28] and insoluble polymer support [29].
Recently, Pang et al. [30] reported that VOSO4 in combination
with Cu(NO3)2 exhibited interesting catalytic properties in the
liquid-phase homogeneous aerobic oxidation of HMF into DFF.
When Cu(NO3)2 was used alone, the conversion of HMF was
found to be trace. When VOSO4 was applied alone, it showed poor
catalytic activity (10% HMF conversion and 31% selectivity to DFF).
When both are combined (1:1 mole ratio), DFF formed at very
high selectivity (99%) with full conversion. When other vanadium
compounds [NaVO , VOPO , VO(OEt) , VO(acac) , and VO(pic) ,
3
4
3
2
2
[
bis(pyridine-2-carboxylato)oxo-vanadium(IV)] were used to
Scheme 1. Preparation of VO2+ and Cu2+ ions-immobilized carbon catalysts.
replace VOSO , the HMF conversion was less than 33%. Similarly,
4
with other metal nitrates [Ni(NO ) , Co(NO ) , Ce(NO ) , NaNO ,
3
2
3 2
3 3
3
material are denoted by C and CS respectively. Subsequently, the
CS was exchanged with excess amount of metal ions. In a typi-
or Fe(NO ) ] the conversion obtained in the range 21–78%. Facile
3
3
oxidation of V(IV) into V(V) by Cu component was proposed by the
authors.
cal preparation, 1.0 g of Cu(NO ) ·3H O and 1.0 g of VO(SO )·xH O
3 2
2
4
2
were dissolved in 50 mL water containing 3.0 g of sulfonated carbon
at room temperature and stirred for 3 h. In the catalyst prepara-
tion and reactions, all chemicals are EP reagent grade except for
sulfuric acid (98% commercial grade). To vary the metal ion ratio,
the relative quantities of metal precursors were changed. Finally
The
intriguing
performance
of
the
homogeneous
[
VOSO –Cu(NO ) ] catalytic system [30] prompted us to devise
4
3 2
2+
2+
new solid catalysts containing immobilized VO and Cu ions.
Taking account of catalytically active cation immobilization,
carbon materials which present cation exchange capability, struc-
tural rigidity and thermal stability can be considered as suitable
supporting materials [31]. Particularly, the facile sulfonation of
specific carbon materials derived from the controlled carboniza-
tion of glucose or sugar has been well demonstrated [32–34].
In these materials, it has been proposed that, stable sulfonated
groups could be formed with poly aromatic carbons. A similar
controlled carbonization of ion exchange resin of polystyrene
chain was performed to obtain poly aromatic type carbon species
with graphite like stacking structure [35–37]. In the present study,
sulfonated carbon material based on the commercial ion exchange
◦
the ion-exchanged catalysts were dried at 80 C for 12 h and used
for catalytic reactions. Various stages of catalyst preparation are
depicted in Scheme 1.
2.2. Catalytic oxidation of 5-hydroxymethyl-2-furfural
The general scheme of aerobic oxidation of HMF into DFF is
depicted in Scheme 2. In a typical reaction, 300 mg catalyst, 300 mg
HMF and 30 mL acetonitrile were charged in the high pressure
◦
stainless steel reactor and air (40 bar) was introduced at 140 C.
2+
2+
resin is prepared to immobilize VO and Cu ions. Their catalytic
efficiency toward the aerobic oxidation of HMF is examined. The
metal ion compositions are varied to find out the optimum ratio.
The changes in the morphology of the catalysts are studied with
characterization methods such as XRD, TEM, Raman spectroscopy
and elemental analysis. The catalytic properties are discussed with
the characterization results.
After 4 h of reaction, the reactor was cooled, the pressure was
released and the catalyst was separated by centrifugation. The sep-
arated catalyst was washed with acetonitrile and re-used without
any further treatment. The product analysis was performed using
HPLC system equipped with BIO-RAD Aminex HPX-87H ion exclu-
sion column (300 mm × 7.8 mm), the eluent 0.1% H3PO4, flow rate
◦
0.5 mL/min, column temperature 60 C, UV–Vis (254 nm) detec-
tor. To prepare samples for HPLC analyses, the liquid product was
diluted 10 times with eluent. All major products were identified by
comparing their retention time with those of authentic compounds
2
. Experimental
(15.1 min, 20.8 min, 21.6 min, 32.7 min and 39.8 min for FDCA, FFCA,
2.1. Catalyst preparation
HMFCA, HMF and DFF, respectively).
In the first step, the controlled carbonization of commercial
2.3. Catalyst characterization
ion exchange resin (IER, Amberlyst-15) was performed to obtain
the carbon support following the known method [35–37]. This
involves thermal treatment of the IER sample at 500 C under
Chemical analyses of catalysts were performed by the induc-
tively coupled plasma–atom emission spectroscopy (ICP-AES) with
Thermo Scientific ICAP 6500 instrument. Sulfur content was deter-
mined by the elemental analysis. The surface area measurements
were performed using a Micromeritics ASAP 2040 system. The
X-ray powder diffraction (XRD) patterns were acquired with a
Rigaku D/MAX-2200V diffractometer (Cu K␣ line). The transmis-
sion electron microscopy (TEM) investigations were performed on
◦
N2 gas flow for 7 h. The obtained carbon material was immersed
in concentrated sulfuric acid at room temperature for 3 h and
◦
refluxed at 170 C for 5 h under flowing N . After treating with
2
sulfuric acid, the sample was washed with distilled water until
the pH of the washings became neutral and dried in air circu-
◦
lating oven at 80 C. The carbonized resin and sulfonated carbon