R.L. Martins et al. / Journal of Molecular Catalysis A: Chemical 330 (2010) 88–93
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7. But CHCl3 is far from being considered a perfect probe molecule
because of its dissociation leading to formates and surface modifi-
cations with chlorine ions, as observed by Gordymova and Davydov
[7].
La2O3 (JMC303 Specpure) and Al2O3 (Harshaw A13996) were
used without any treatment. ZrO2 was obtained through the cal-
cination of zirconium hydroxide (MEL Chemicals) at 500 ◦C. TiO2
was prepared by the hydrolysis of titanium isopropoxide (Aldrich)
under inert environment followed by drying and calcination at
550 ◦C.
Because CO2 is acidic, it adsorbs specifically on basic sites of
metal oxides, leading to the formation of several species. On −basic
hydroxyl groups, for instance, H-carbonate species, HO–CO2 are
formed. On basic oxygen ions, different kinds of carbonate species
can be formed depending or not on the participation of neighboring
metal ions to the adsorption site. So, unidentate, bidentate, bridged
and others ill-defined species in which three oxygen atoms inter-
act with metal ions such as polydentate species can be formed.
The localization of these species either on the surface, unidentate,
bidentate or bridged species, or in the bulk, polydentate species,
spectroscopic methods, like temperature programmed desorption
or microcalorimetry as CO2 interaction can result not only from
adsorption on basic sites but also from its reaction with the bulk.
Bernal et al. [8,9], for instance, have shown that atmospheric CO2
not only convert lanthanum oxide into surface carbonate but also
into bulk carbonates. Also, CO2 cannot be considered an inert gas,
CeO2. In spite of these disadvantages (diversity of carbonate species
formed, possibility of reoxidation, and formation of bulk carbon-
ates) CO2 has been extensively used as probe for the determination
of the relative basicity of different metal oxides.
Lima et al. [10] have reported a study on characterization of
basic properties based on the 13C CP/MAS NMR chemical shifts
observed for nitromethane chemisorbed on basic oxides. They used
the compounds formed by the reaction of nitromethane with a
methanolic solution of NaOH to characterize the species formed
when this probe molecule chemisorbs on basic oxides. They clas-
sified the basic strength of the solid sites as weak, intermediate
and strong basic sites, when spectra similar to that of physisorbed
nitromethane, sodium salt of aci-anion nitromethane and sodium
salt of methazonic acid were respectively obtained.
2.2. Infrared measurements
Infrared experiments were conducted using a Fourier Trans-
form spectrometer, Perkin Elmer 2000, self supported wafers
(9.8 mg cm−2 “thickness”), and a Pyrex cell with CaF2 windows.
During the sample pretreatment and gas adsorption the cell was
attached to a vacuum glass system. The spectral domain was
between 4000 and 1000 cm−1 with a 4 cm−1 resolution. The zeolite
and oxide samples were pretreated at 400 ◦C for 4 h and 500 ◦C for
1 h, respectively, in vacuum up to 10−5 Torr. After cooling down to
room temperature the infrared spectrum was recorded and used as
background for the nitromethane adsorption experiments. Adsorp-
tion studies were conducted by exposing the pretreated wafers
to 10 Torr of nitromethane at 100 ◦C during 30 min, followed by
evacuation up to 10−5 Torr, at the same temperature. The diffuse
reflectance Fourier transform infrared spectroscopy (DRIFT) spec-
tra of the sodium salt of methazonic acid and of the sodium salt of
aci-anion nitromethane were obtained for comparative purpose.
The synthesis of the sodium salt of methazonic acid was per-
formed by using 0.1 mol of nitromethane added to 0.2 mol of NaOH
in 50 cm3 of CH3OH at 50 ◦C for 2 h. The sodium salt of aci-anion
nitromethane was prepared by adding a solution of NaOH (0.1 mol)
in 50 cm3 of methanol to 0.1 mol of nitromethane at 0 ◦C. In both
synthesis methanol was then removed by distillation under vac-
uum and the solid dried at 80 ◦C.
In the present work, infrared spectroscopy was used as an ana-
lytical tool to determine the basic properties of cation-exchanged
X zeolites, alkali-promoted MgO, and different metal oxides (MgO,
La2O3, Al2O3, ZrO2, TiO2) using nitromethane as a probe molecule.
This study is based on the interaction between acid hydrogen
atoms of nitromethane and oxygen atoms of basic solids gener-
ally used as catalysts. According to Lima et al. [10], nitromethane,
as shown in Fig. 1, presents two resonances forms one of them
is called aci-anion (1). These forms could react with a NaOH
methanolic solution producing either the sodium salt of aci-anion
nitromethane (2) or the sodium salt of methazonic acid (3). The
authors classified the sites of the solids in relation to basicity as
weak, intermediate and strong, depending on the product formed
after the exposure of the solid to nitromethane. Weak basic sites
produce aci-anion nitromethane while the sodium salt of aci-anion
nitromethane and the sodium salt of methazonic acid were formed
tively. Products (2) and (3) were synthesized and their IR spectra
were measured in order to interpret the spectra obtained after
nitromethane adsorption on zeolites and metal oxides.
Fig. 2 shows the spectrum of pure nitromethane. The nitro
group of nitromethane presents two resonances forms that vibrate
asymmetrically causing a strong absorption at 1560 cm−1 and sym-
metrically causing a weaker absorption at 1378 cm−1 [11,12].
The CH3 symmetric and asymmetric bending usually appear at
1375 and 1465 cm−1, however due to the electronegativity of the
neighbor nitro group they appear at 1404 and 1427 cm−1, respec-
tively. This electronegative group also influences the CH3 rock
vibration of nitromethane, absorbing at 1101 cm−1, this frequency
is quite shifted compared to those of aliphatic chains.
2. Experimental
2.1. Material preparation
A commercial MgO (Merck) was hydrolyzed in order to prepare
Mg(OH)2 according to the following procedure. A mixture of com-
mercial MgO and water was prepared using a solid ratio of 10 wt%,
and was kept under continuous stirring by 24 h at room tempera-
ture. The suspension was then evaporated and dried at 100 ◦C for
16 h. A MgO sample was obtained by calcination of the prepared
Mg(OH)2 at 500 ◦C under vacuum using a heat ratio of 3 ◦C min−1
.
Alkali-promoted MgO samples were prepared by wet impregna-
tion of the synthesized Mg(OH)2 with the corresponding aqueous
solutions of alkali nitrate (Li, Na, K) and acetate (Cs). The nominal
load of alkali cation was based on a cation/Mg molar ratio of 0.05.
The doped-oxides were calcined in situ at 500 ◦C in the presence of
synthetic air.
The zeolite samples used in this work were prepared from a
sodium faujasite X (silica alumina molar ratio = 2.3) using the fol-
lowing ion exchange procedure. The parent zeolite was exchanged
twice with an alkali chloride (K, Cs) or nitrate (Li) solution at 80 ◦C
for 1 h, using a molar ratio of alkali ion in solution to total cations in
the zeolite equal to 0.76 in each step. After each exchange step, the
materials were filtered, washed with hot water and dried at 120 ◦C.
Fig. 3 shows the infrared spectra of nitromethane chemisorbed
on alkali-exchanged X zeolites. The asymmetric stretching vibra-
tion of the nitro group (1560 cm−1) is the most intense band and
its intensity varies with the nature of the compensating cation.