1
24
A. Mobaraki et al. / Applied Catalysis A: General 472 (2014) 123–133
2.2. Preparation of materials
Preparation of Fe O @SiO . The synthesis of Fe O @SiO was
3
4
2
3
4
2
achieved using the procedure described by Luo and co-workers
28]. This procedure involved a synthetic strategy based on the
hydrolysis and condensation of tetraethoxysilane (TEOS) on the
[
SO H
SO
Si
H
3
3
surface of Fe O4 magnetic nanoparticles. In a typical prepara-
3
tion procedure, ferric chloride hexahydrate FeCl ·6H O (11.0 g,
3
2
OH OH
Si Si
OH OH
Si Si
OH OH
Si Si
4
2
0.7 mmol) and ferrous chloride tetrahydrate FeCl ·4H O (4.0 g,
2
2
0.1 mmol) were dissolved in deionized water (250 mL) under
◦
nitrogen atmosphere with mechanical stirrer at 85 C. The pH value
of the solution was adjusted to 9–11 using aqueous NH3 (25%). After
continuous stirring for 4 h, the magnetite precipitates were washed
with distilled water until the pH value descended to 7.0. The black
precipitate (Fe O ) was collected with a permanent magnet at the
Si
O
O
O
O
O
O
O
Silicious based solid acids
:
:
H O
3
4
2
bottom of the reaction flask. The silica coated core–shell magnetic
nanoparticles (Fe O @SiO MNPs) were prepared by an ultrasonic
Reaction partners
3
4
2
pre-mixing of a dispersion of the above black precipitate (2.0 g)
with ethanol (400 mL) for approximately 30 min at room temper-
ature. Then, aqueous NH3 (25%, 12 mL) and TEOS (4.0 mL) were
slowly added successively. The resulting solution was mechani-
cally stirred continuously for 24 h, after which the black precipitate
product (Fe O @SiO ) was collected by magnetic separation and
Scheme 1. Schematic representation of interaction between water and silanol sur-
face and sulfonic acid active sites of solid acids in water-generating reactions.
as we know, there is not any report about surface hydrophobicity
on sulfonic acid-functionalized Fe O @SiO core–shell magnetic
nanoparticles; hence, this preliminary work represents the first
and a unique example including a hydrophobic Fe O @SiO @SO H
type magnetic material for catalysis in water-generating reac-
tions; thus, offered
explored.
3
4
2
3
4
2
washed with ethanol (3× 15 mL) and dried under vacuum overnight
at room temperature (Scheme 2).
3
4
2
3
Preparation of Fe O @SiO @Et-PhSO H (1). The surface
3
4
2
3
functionalization of the silica coated magnetic nanoparticles
with sulfonyl groups was carried out by adding 2-(4-
a new approach to be researched and
chlorosulfonylphenyl)ethyltrimethoxysilane
(CSPETS,
0.4 g,
As part of our efforts in exploring novel heterogeneous
catalysts for organic reactions [23–27], we have designed, pre-
pared and characterized two novel water-tolerant and sulfonic
1.23 mmol) to dry toluene (35 mL) containing silica-coated mag-
netic nanoparticles (1.0 g). The resulting mixture was stirred for
24 h and then washed with toluene (2× 15 mL) and distilled water.
acid organic–inorganic hybrid catalysts based on Fe O @SiO
Finally, the solid was suspended in H SO (1 M) solution for 2 h,
2 4
3
4
2
core–shell magnetic nanoparticles: Fe O @SiO @Et-PhSO H (1,
washed several times with water and dried at room temperature
under vacuum overnight to give the corresponding catalyst 1
(Scheme 2).
3
4
2
3
Scheme 2) and Fe O @SiO @Me&Et-PhSO H (2, Scheme 2); also,
3
4
2
3
the acidity, hydrophobicity, water-toleration and utility of these
catalysts were investigated. In this context, we wish to show that
in water-generating reactions involving both hydrophobic and
hydrophilic reaction partners, catalyst 2 with hydrophobic char-
acter on the surface is much more active and much more robust
than catalyst 1.
Preparation of Fe O @SiO @Me&Et-PhSO H (2). This procedure
3
4
2
3
involved a synthetic strategy based on the co-condensation of
CSPETS and trimethoxymethylsilane (TMMS) on the silica coated
magnetic nanoparticles. In a typical preparation procedure, CSPETS
(0.2 g, 0.62 mmol) and TMMS (0.2 g, 1.47 mmol) were added to dry
toluene (35 mL) containing silica-coated magnetic nanoparticles
(
1.0 g). The resulting mixture was stirred for 24 h and then washed
with toluene (2× 15 mL) and distilled water. Finally, the solid was
suspended in H SO (1 M) solution for 2 h, washed several times
2
. Experimental
2
4
2.1. Chemicals and characterizations
with water and dried at room temperature under vacuum overnight
to give the corresponding catalyst 2 (Scheme 2).
All chemicals were purchased from Merck and Aldrich Chemi-
cal Companies. Melting points were determined on a Büchi melting
2
.3. Acidity of the catalysts (1 and 2)
1
point B-540 apparatus. NMR spectra were recorded at 400 ( H) and
3
1
00.6 (1 C) MHz, respectively, on a commercial Bruker DMX-400
The concentration of sulfonic acid groups was quantitatively
instrument using DMSO-d6 as solvent. IR spectra were recorded
on an ABB Bomem Model FTLA 2000 spectrophotometer using KBr
discs. The magnetic measurement of samples was carried out in a
vibrating sample magnetometer (VSM) (4 in., Daghigh Meghnatis
Kashan Co., Kashan, Iran) at room temperature. X-ray diffraction
estimated by ion-exchange pH analysis [8,28,29]. The catalyst
50 mg) was added to an aqueous solution of NaCl (1 M, 25 mL), and
the resulting mixture was stirred for 3 days, after which titration
by NaOH (0.05 M) was carried out on the above obtained solu-
(
tions. The acid amount of 1 and 2 was determined to be 2.22 and
(
XRD) patterns were recorded by an Xpert MPD, X-ray diffrac-
−1
0
.70 mmol g , respectively.
tometer using Cu K␣ radiation. Thermogravimetric and differential
thermal analysis (TG-DTA) was carried out using a thermal gravi-
metric analysis instrument (NETZSCH TG 209F1 Iris) with a heating
2
3
.4. General procedure for the one-pot preparation of
,4-dihydropyrimidin-2-ones/thiones
◦
−1
rate of 10 C min . SEM was carried out on a VEGA\\TESCAN-LMU
instrument. Transmission electron microscope, TEM (Philips CM-
A mixture of aldehyde (2 mmol), methyl acetoacetate (2 mmol),
1
0) was also used to obtain TEM images. Elemental analyses for C,
urea/thiourea (2.4 mmol) and catalyst 2 (7.1 mg, 0.5 mol %) (in the
case of thiourea, 1 mol % of the catalyst was used) was stirred
H and S were performed using a Heraeus CHN-O Rapid analyzer.
The N2 and H O-sorption was carried out in a Belsorp-mini-BEL
◦
2
at 100 C for an appropriate time under solvent-free condition
Table 3). The progress of the reaction was monitored by thin layer
Japan, Inc. at 298 K (see ESI).
(