82
J. Adamiak / Journal of Molecular Catalysis A: Chemical 407 (2015) 81–86
also nitrated by charged-transfer mechanism in the presence of
tetranitromethane in acetonitrile [7].
temperature in CDCl3. Textural properties of the catalysts sur-
tion/desorption experiments were made for dinitrogen within the
p/p0 range of 0.02÷1.0 at −196 ◦C. The acidity of the catalyst was
determined by measurements of initial electrode potential and
potentiometric titration method [14,15]. Certain 0.2 g of catalyst
was suspended in 25 mL of acetonitrile and the system was magnet-
ically stirred for 1 h. The suspension was titrated with a solution of
0.01 N n-butylamine in acetonitrile. Then the variation in the elec-
trode potential was measured with pH meter, using a standard glass
electrode. This method enables determination of the total number
of acid sites and their distribution.
Apart from different nitrating mixtures, application of solid acid
meet the recent ecological and safety requirements, and they can
example, in the presence of acidic montmorillonite clay impreg-
nated with anhydrous cupric nitrate, nitroanisoles with 98% yield
obtained in the reaction with P2O5/SiO2 and fuming nitric acid
[9]. Mesoporous silica impregnated with cesium exchanged Keggin
type heteropolyacids (HPAs) were used as dispersed catalysts in the
mixture of nitric acid (70%) and anisole [10]. The highest yield of
nitroanisoles (ca. 60%) was obtained with cubic silica impregnated
with cesium exchanged polyphosphotungstic acid after 24 h.
So far, the catalysts combining catalytic properties of P2O5/SiO2
(good selectivity [9]) with supported HPAs (high conversion
and availability [10]) through mixing of phosphoric acid with
ammonium molybdate, followed by impregnation of the obtained
solution on the silica gel, and followed by heating, were successfully
prepared. As a result, mixed catalysts composed of PO43− ions built
Preparation of P/Mo/SiO2. The catalysts were prepared according
to the scheme:
SiO
2
12(NH4)6Mo7O24 · 4 H2O+7 H3PO4 → 7 H3PMo12O40+72 NH3
T
+84 H2O
(1)
The solution of ammonium molybdate tetrahydrate and phosphoric
acid in 3% H2O2 was applied by wet impregnation method on SiO2
(grains 0.7–1.2 mm). Then, the catalysts were dried at 150 ◦C. Next,
it was heated at 300 ◦C for ca. 16 h. To obtain 5 g of the catalysts,
the amounts of substrates are as follows:
in the structure of surface Mo
O
3∞
lattice (heteropolyanion-like
∞
species) were obtained. Using various amounts of the molybde-
num species, it has been possible to simultaneously manipulate
the catalyst acidity and activity.
P/15Mo/Si
– 0.92 g (0.7 mmol) of ammonium molybdate
tetrahydrate, 7 mL of 3% H2O2, 4.25 g of SiO2, 0.043 g (0,4 mmol)
of H3PO4,
To the best of my knowledge, there is still lack of reports on
application of solid acids catalysts in anisole nitration, despite the
fact that the products of anisole nitration are widely used in phar-
maceutical, perfumery and dye industry. To fill this gap, in this
work the application of solid catalysts for anisole mononitration
under mild conditions without extra additives (e.g. nitrites) is fully
described. Importantly, the obtained nitroanisoles are not contami-
nated with nitrophenols. Moreover, possible reaction mechanisms
depending on various concentrations of nitric acid used are dis-
cussed. Conclusions from this work will be useful for both designing
novel catalysts and choosing proper conditions for nitration of
other activated aromatics.
P/45Mo/Si
– 2.76 g (2.2 mmol) of ammonium molybdate
tetrahydrate, 4.5 mL of 3% H2O2, 2.75 g of SiO2, 0.13 g (1.3 mmol)
of H3PO4.
Nitration process. 1,2-dichloroethane as a solvent (10 mL), the
catalyst (1 g) and 0.3 mL (0.0069 mol) of fuming HNO3 or 0.5 mL
(0.0069 mol) of 65% HNO3, were introduced into a three-necked
flask equipped with a mechanic stirrer and a dropping funnel.
Anisole (0.5 mL, 0.0046 mol) and the solvent (5 mL) were placed in
the dropping funnel. Upon dropwise addition of the funnel contents
(for ca. 10 min), the reaction was continued at room temperature for
30 min. On completion of the reaction, the catalyst was filtered and
washed off with the solvent. The filtrate was shaken with aqueous
sodium bicarbonate (NaHCO3), then with water, and finally dried
over magnesium sulfate (MgSO4). The selectivity to a particular
product was expressed as the amount of this product divided by
a total amount of all products, multiplied by 100.
2. Experimental
Phosphomolybdic acid hydrate was purchased from
Sigma–Aldrich. Materials purchased from AvantorTM Performance
Materials: anisole (pure), ammonium molybdate tetrahydrate
[(NH4)6Mo7O24·4H2O] (pure), 1,2-dichloroethane (pure), phos-
phoric acid (85%, pure), magnesium sulfate anhydrous (pure),
sodium hydrogen carbonate (pure). Silica gel (SiO2) was purchased
from Ma˛twy, Poland. Fuming nitric acid was obtained in distillation
from mixture of 65% nitric acid and 98% sulfuric acid.
Analysis of the reaction products was made using gas chro-
matography with a GC 17A, Shimadzu Corp. and a Rxi-5Sil MS
(30m × 0.32 mm × 1.0 um) column. The sample of post-reaction
mixture was dissolved in 1,2-dichloroethane. The quantitative
composition was determined by internal standard method using
peak areas and chlorobenzene as the internal standard (IS). The
composition of the product was confirmed by GC/MS.
3.1. Catalysts characterization
To determine the type of the surface domains of the catalysts,
Raman (Fig. 1.I), XRD (Fig. 1.II) and 31P MAS NMR (Fig. 1.III) analysis
were made.
As a reference for spectral analysis, the solid composed of silica
supported phosphomolybdic acid (HPM/Si) was prepared under the
It is postulated that reaction between HPM units and a hydrox-
ylated silica surface occurs and ion pairs with one (( SiOH2)+
+
Raman spectra were recorded on a Nicolet Almega Dispersive
Raman Spectrometer. Infrared spectra (IR) were recorded using
(H2PMo)–) or more silanol (( Si)m (H3-mPMo)m–) via simple pro-
ton transfer are formed [16]. Due to these reactions, Keggin units
undergo slight deformations and the bands for HPM/Si are shifted
in comparison to the bands of bulk HPM (996, 980, 900 and
600 cm−1[17]). Since the spectra for P/Mo/Si are similar to spectra
for HPM/Si, HPM formation in P/Mo/Si can be confirmed. More-
over, the bands at 820 and 994 cm−1 indicate the presence of
orthorhombic molybdenum oxide (␣-MoO3) and it can suggest that
Nicolet 6700 interferometer (4000–400 cm−1, resolution 4 cm−1
)
with ATR. X-ray powder diffraction patterns were recorded on a
Bruker D8 Discover diffractometer. The solid-state 31P MAS NMR
was performed on Bruker Avance II 500 MHz spectrometer. The
spinning frequency was 5 kHz. All measurements were carried
out at room temperature using P(Ph)3 as standard reference to
obtain the chemical shift of the solid materials. 1H NMR spec-
tra were recorded on Varian Merkury 400 spectrometer at room
PO4 ions are built in the Mo
terns (Fig. 1.II a and c) show the crystalline nature of the catalysts.
O
lattice irregularly. The XRD pat-
∞
3∞