N. Candu et al. / Journal of Catalysis 287 (2012) 76–85
77
this sol–gelsynthesis route has permittedthe synthesis of Al- and Ti-
containing mesoporous silica materials that have been found to be
catalytically active for acid and redox processes, respectively. Re-
cently, we reported that Atrane route to synthesis of Sn-MCM-41
[10] and Al-UVM-7 (UVM = University of Valencia Materials) [11]
is an effective solution for the preparation of well dispersed acid
Sn and Al triflate species into the framework of the mesoporous sil-
ica. The obtained materials were active and highly selective into a
number of important fine chemicals synthesis as the acylation of
to age at room temperature for 4 h. The final mesostructured pow-
der was filtered off, washed with water and ethanol, and air-dried.
In order to open the pore system, the as-synthesized solid was
heated at 550 °C (1 °C/min) under static air atmosphere for 7 h.
In all cases, the molar ratio of the reagents in the mother liquor
was adjusted to (2 ꢂ d) Si/d Sc/7 TEA/0.52 CTABr/180 H2O (where
n = d/(2 ꢂ d) = 0.007, 0.010, 0.016, 0.025, and 0.1).
The second step corresponds to the formation of Sc triflate com-
plexes at the Sc-UVM-7 surface. In a typical synthesis leading to
ScOTf-UVM(50) sample, 1 g of Sc-UVM(50) was suspended in a
mixture of triflic acid (5 g) and methanol (35 ml). This mixture
was stirred under reflux for 4 h at ca. 70 °C. The resulting porous
samples were collected by filtration, washed with methanol to
eliminate any excess triflic acid, and air-dried.
aromatic sulfonamides, the synthesis of (dl)-[a]-tocopherol, or the
synthesis of non-ionic surfactants structures [10,12]. In spite of
the well-known similarity between Sc(III) and Al(III) (given by its
small ionic radius (0.73 Å) and identical charge (III)), reports dealing
with the replacement of Si for Sc are extremely limited. In fact, as far
as we known, only one recently report describing the incorporation
of Sc sites in ZSM-5 zeolite network was published [13]. In a similar
way, no examples of Sc inclusion in the framework of mesoporous
silica materials exist and only some examples of incorporation
through impregnation with complex solutions or grafting being de-
scribed [14]. On the other hand, according to the studies of Kobay-
ashi et al. [15], scandium triflate is a water-compatible strong
Lewis acid with a large applicability in organic synthesis.
2.2. Catalyst characterization
All solids were analyzed for Si, Sc, and S by electron probe
microanalysis (EPMA) using a Philips SEM-515 instrument. Si/Sc
and S/Sc molar ratio values averaged from EPMA data are summa-
rized in Table 1. X-ray powder diffraction (XRD) data were re-
corded on a Seifert 3000TT h–h diffractometer using Cu K
a
Taking into account this information and with the aim to create
robust catalysts with strong Lewis acidity for the 4,40-MDA synthe-
sis from aniline and 4-aminobenzylalcohol, we prepared a series of
ScOTf-UVM-7 mesoporous bimodal catalysts. The Atrane method
allows the Sc incorporation inside the UVM-7 silica walls in a first
step, then, through heating the Sc-containing mesoporous silicas in
solutions of triflic acid, the formation of Sc triflate complexes at the
materials surface is performed.
radiation. Low-angle patterns were collected in steps of 0.02°
(2h) over the angular range 1–10° (2h) for 25 s per step. In order
to detect the presence of some crystalline bulk phase, additional
patterns were collected with a bigger scanning step [0.05°(2h))
over a wide angular range (10–70°(2h)] and a lower acquisition
time (10 s. per step). The Sc2O3 particle size was estimated using
the Scherrer equation assuming spherical particles. For this pur-
pose, XRD patterns with
a good statistic (scanning step of
0.02°(2h) and 20 s. per step) were recorded over specific angular
ranges around the (222), (440), and (622) reflections of the
Sc2O3 cubic phase (JCPDS 43-1028). Transmission electron micros-
copy (TEM) images were acquired with a JEOL JEM-1010 instru-
ment operating at 100 kV. Surface area and pore size values were
calculated from nitrogen adsorption–desorption isotherms
(ꢂ196 °C) recorded on a Micromeritics ASAP-2010 instrument. Cal-
cined samples were degassed for 15 h at 130 °C and 10ꢂ6 Torr be-
fore analysis. Surface areas were estimated according to the BET
model, and pore size dimensions were calculated using the BJH
method. FTIR spectra were collected on a Nicolet 4700 spectrome-
ter (200 scans with a resolution of 4 cmꢂ1) using self disks of 1%
sample in KBr. NH3-DRIFT spectra were collected with the same
type of spectrometer using the following program: treatment of
the catalysts under a He flow (30 mL minꢂ1) for 1 h at room tem-
perature and then for another hour at 150 °C after heating the cat-
alysts with a slope rate of 5 °C minꢂ1. After cooling at room
temperature, a flow of NH3 (10% in He) of 30 mL minꢂ1 was flushed
for 30 min. Then, a He flow (30 mL minꢂ1) was flushed for 30 min
and the first spectrum was recorded. Other spectra were collected
after heating the samples in He till 150 °C with a slope rate of
5 °C minꢂ1. Spectra were collected at 50, 100, and 150 °C, respec-
tively. 45Sc NMR static spectra were recorded on a Varian Unity
300 spectrometer at 97.15 MHz. The chemical shifts were refer-
enced to ScCl3 aqueous solution at 0 ppm.
2. Experimental
2.1. Catalyst preparation
All reagents were used as received from Sigma–Aldrich (tetra-
ethyl orthosilicate (98%) [TEOS], scandium oxide (99.9%) [Sc2O3],
triethanolamine (99%) [N(CH2–CH2–OH)3, hereinafter TEA], cetyl-
trimethylammonium bromide (99%) [CTABr], triflic acid (98%),
and methanol (99.8%)).
ScOTf-UVM-7 porous bimodal catalysts were prepared in a two-
step strategy (i.e., Atrane method) based on the use of complexes
that include TEA-related ligand species (named Atrane ligands)
as hydrolytic inorganic precursors. The formation of the hierarchic
bimodal porosity occurs during the first step. The pH provided by
the triethanolamine–water medium (pH ꢀ 9) favors the nucleation
and growth of mesoporous nanoparticles and the subsequent
aggregation leads to a controlled textural-like meso/macroporosity
[16–18]. The resulted solids have been denoted as Sc-UVM(x)
(x = Sc/Si molar ratio ꢁ 103). The second step corresponds to the
heating the Sc-containing mesoporous silicas in solutions of triflic
acid with the formation of Sc triflate complexes at the materials
surface. The obtained catalysts were designated ScOTf-UVM(x)
(x = the final Sc/Si atomic ratio after reaction with triflic
acid ꢁ 103).
Details of a typical synthesis leading to Sc-UVM(50) sample can
be described as follows: a mixture (suspension) of Sc2O3 (0.11 g,
8.23 ꢁ 10ꢂ4 mol) and TEOS (11 mL, 0.05 mol) was slowly added
to liquid TEA (23 mL, 0.173 mol). This mixture was heated at
150 °C for 10 min (with stirring) leading to a solution containing
Sc and Si-atrane complexes. After cooling of the previous solution
to 90 °C, CTABr (4.689 g, 0.013 mol) was added under continuous
stirring to favor the surfactant dissolution. This solution was for-
ward cooled to 60 °C and mixed with water (80 mL, 4.44 mol).
After a few seconds, a white powder appeared and was allowed
2.3. Catalytic tests
Activity tests in batch mode were carried out as described in the
following procedure: aniline (1–2 mmol, 93–186 mg) was mixed
with 4-aminobenzylalcohol (1 mmol, 123 mg) in 3 ml of solvent
(acetonitrile, THF, hexane, or ethanol) into a vial glass of 20 mL
capacity. To this mixture, the catalyst (10–30 mg) was added and
heated up to 80 °C, under stirring (1.400 rpm), for 2–24 h. For com-
parison, tests with homogeneous Sc triflate (99%, Aldrich) and SAC-
13 (13 wt.% Nafion/silica: 0.15 meq H+/g; Ssp (m2/g) = 400; Dp