144
V. Palermo et al. / Journal of Molecular Catalysis A: Chemical 373 (2013) 142–150
PMoLa: 0.080 mmol, and PMoY: 0.082 mmol. The first step in the
impregnation technique was to contact the HPA solution with the
support, S and/or NH2–S (500 mg), with H2O (0.50 mL). The impreg-
nated solids were left without stirring for 24 h, to help the diffusion
of HPA into the supports. Then they were shaken for 24 h. Finally,
the impregnating solutions were separated from the solids and
dried at 20 ◦C for 12 h. The nomenclature of the new catalysts is:
(12 mg), diphenyl sulfide (Aldrich, 1 mmol, 186 mg), and ethanol
(Soria, 4 mL) were stirred at 25 ◦C. Then aqueous H2O2, 35% (w/v)
(1.5 mmol, 0.15 mL), was added. The progress of the reaction was
monitored by TLC. After the end point of reaction, it was treated
with dichloromethane and water (2 × 3 mL). The organic phase was
dried on anhydrous Na2SO4 and concentrated to obtain diphenyl
sulfoxide.
(b) Diphenyl sulfone was obtained using bulk HPA (12 mg),
diphenyl sulfide (Aldrich, 1 mmol, 186 mg), 35% (w/v) H2O2 (7.5
mmol, 0.75 mL), and ethanol (Soria, 4 mL) at 50 ◦C.
- when S was used as support, they were named PMoB–S, PMoBi–S,
PMoLa–S, and PMoY–S;
- when NH2–S was used as support, they were named
PMoB–NH2–S, PMoBi–NH2–S, PMoLa–NH2–S, and PMoY–NH2–S.
2.3.2. Heterogeneous reactions
(a) Supported catalyst (50 mg), diphenyl sulfide (Aldrich, 1
mmol, 186 mg), and ethanol (Soria, 4 mL) were stirred at 25 ◦C.
Then 35% (w/v) H2O2 (1.5 mmol, 0.15 mL) was added. The progress
of the reaction was monitored by TLC. After the reaction end
point, the catalyst was recovered by centrifugation and washed
with dichloromethane. The reaction mixture was treated with
dichloromethane and water (2 × 3 mL). The organic phase was dried
on anhydrous Na2SO4 and concentrated to obtain diphenyl sul-
foxide.(b) Equally, diphenyl sulfone was obtained using 0.75 mL of
H2O2 35% (w/v) at 50 ◦C.
On the other hand, and with the only objective of comparing
the catalytic behavior, PMoY was impregnated on NH2–S using
H2O2 (35%, w/v) instead of water. The catalyst obtained was labeled
PMoY–NH2–S(P). This test was made to confirm that water or
hydrogen peroxide does not produce any changes in the catalytic
behavior of the final material.
The modifications of the amount of HPA in each case were eval-
uated by quantitative element analysis and acidic properties.
2.2. Catalyst characterization
The performance of the catalysts was evaluated quantitatively
by the conversion of diphenyl sulfide (Conv.%), and the selectivity
of diphenyl sulfoxide and diphenyl sulfone (Sel.%).
2.2.1. Quantitative element analysis
The composition of experimental bulk HPA was determined by
means of the inductively coupled plasma atomic emission spec-
troscopy (ICP-AES) technique using a Shimadsu 1000 III.
3. Results and discussion
2.2.2. UV–visible spectroscopy
3.1. Characterization of catalysts
Subsequent to the contact with the support, the impregnated
solution was analyzed by ultraviolet spectroscopy (UV), using a
Perkin Elmer Lambda 35 UV–vis double beam spectrophotometer,
in the range 200–1100 nm.
high acidity. Therefore, they can be used to catalyze attractive reac-
tions. For example, they can be used to replace classical acids, such
as sulfuric acid, with some advantages, such as a lower corrosion
and a lower production of wastes, thus leading to eco-efficient pro-
cesses [22]. In the Keggin structure there are four oxygen types: Oa
surrounds the central tetrahedral P, and links P and Mo together; Ob
connects MoO6 octahedra by the corners; Oc shares the octahedra
edges; and terminal oxygen Od is bonded to only one Mo atom. Each
2.2.3. Textural properties
The specific surface area (SBET), pore volume, and the mean pore
diameter of the supported catalysts were determined by nitrogen
adsorption/desorption technique using Micromeritics ASAP 2020
equipment at liquid-nitrogen temperature. The sample was previ-
ously degassed at 100 ◦C for 1 h.
3−
can be inscribed into a sphere of center
Keggin anion [XM12O40
]
X and radius close to 0.52 nm [23] (radius = mean of the 12X–Od
distances, and X = P in our case). M atoms are located at the corners
of an octahedron cube, which are just moved from centers of the
2.2.4. Fourier transform infrared spectroscopy
Thermo Nicolet IR.200 equipment was employed, using pellets
with BrK, and a measuring range of 400–4000 cm−1
.
side of the associated cube.
3−
Although the [PMo12O40
]
anion is compact, it can accommo-
2.2.5. Potentiometric titration
date heteroatoms that differ greatly in size of M (Mo in our case)
˚
Catalyst acidity was determined by means of potentiometric
titration of a suspension of the solid in acetonitrile, using a solu-
tion of n-butylamine in acetonitrile (0.025 N) in a Metrohm 794
Basic Titrino apparatus with a double junction electrode.
shown by the range of covalent radius for atoms such as 0.82 A (B)
˚
to 1.70 A (Bi). Then, PMoB, PMoBi, PMoLa, and PMoY can present
interesting characteristics as catalysts since they can have Lewis
and/or Brønsted acidity. To the best of our knowledge, their real
structure has been characterized by different techniques. Table 1
shows the atomic properties of Mo, B, Bi, La, and Y as elements,
which were used to replace one Mo atom in the Keggin primary or
secondary structures [24].
2.3. Catalytic activity test
2.3.1. General
All reagents were purchased from Merck and Aldrich and used
without further purification. All yields refer to isolated products
after purification. Products were characterized by spectroscopy
data (1H NMR). The NMR spectra were recorded on Bruker 200 MHz
equipment. The NMR spectra were measured in CDCl3 relative to
TMS (0.00 ppm).
The organic phase was dried on anhydrous Na2SO4 and fil-
tered for its analysis by gas chromatography using Varian Scan
3400 cx equipment. The product distribution was quantified by
a Shimadzu C-R34. Reactions were monitored by thin layer chro-
matography (TLC) analyses. Homogeneous reactions: (a) Bulk HPA
Taking into account the atomic mass, it could be supposed that Y
incorporation produces less structural modifications in the Keggin
primary structure (for example, bond between O or Mo addenda),
given that both Mo and Y have similar atomic mass (Table 1, entries
1 and 3). The extreme behavior is presented by B and Bi (Table 1,
entries 2 and 5); they show values of 10.81, almost nine times
smaller than Mo, and Bi, with 208.98, is twice higher than Mo. With
respect to the ionic radius (Table 1) it might be said that 3 B atoms
are necessary to replace one Mo, if not just one La can take the
place of one Mo. This supposes a strong effect on the bond that is
opposite to the electronegativity of doping metals. In this regard,