C. Aprile et al. / Journal of Catalysis 264 (2009) 44–53
45
(under dry N2 flow) followed by a step at 540 °C during 6 h (under
dry air flow). Then the solid is cooled at room temperature. The fi-
nal catalyst contains 2.1 wt% (expressed as TiO2) based on chemi-
cal analysis. This solid has a specific surface of 1090 m2 ꢁ gꢂ1 with
an average pore size distribution of 35 to 38 Å, and has a band in
the UV–Vis spectrum centered at 220 nm.
OOH
(a)
(b)
OH
O
O
+
+
+
OH
+
O2 +
The preparation of a silylated Ti-MCM-41 catalyst was per-
formed as follows. Typically, 2.0 g of Ti-MCM-41 was dehydrated
at 100 °C and 10ꢂ3 Torr for 2 h. The sample was cooled, and at
room temperature a solution of 1.88 g of hexamethyldisilizane
[(CH3)3Si–NH–Si(CH3)3] in 30 g of toluene was added. The resulting
mixture was refluxed at 120 °C for 90 min and washed with tolu-
ene. The end product was dried at 60 °C. The Ti in solid is
2.0 wt% (expressed as TiO2) based on chemical analysis. This solid
has a specific surface of 965 m2 ꢁ gꢂ1, and has a band in the UV–Vis
spectrum centered at 220 nm. Additionally, the spectrum of 29Si-
MAS-RMN presents a resonance band at ꢂ10 ppm assigned to
the presence of Si–C bonds.
For comparison purposes samples of Au/MCM-41 and Au/Ti-
MCM-41 have also been prepared. The preparation of a purely sili-
ceous MCM-41 mesoporous material (Si/Al molar ratio = 1) was
carried out as follows. Typically, 5.0 g of C16TABr was diluted in
33.5 g of water at 40 °C. In parallel, 8.65 g of 25 wt% TMAOH solu-
tion was dispersed on 0.96 g of aerosil. Then, the last suspension
was added to the initial C16TABr solution at room temperature.
Thus, 4.52 g of the aerosil contained in the resultant solution was
hydrolyzed under stirring (350 rpm) at room temperature. The
formed gel was maintained under stirring for 1 h until complete
homogenization. Then, the gel (pH 13.8) was disposed into an
internally Teflon-covered autoclave and heated under autogeneous
pressure at 135 °C for 24 h. The obtained solid was filtered off,
washed with abundant water, and oven dried at 60 °C for 12 h.
Finally, the elimination of organic occluded into the solid was
performed by thermal treatment (calcination) into a tubular quartz
reactor where the temperature was increased from room temper-
ature to 540 °C (under dry N2 flow) followed by a step at 540 °C
for 6 h (under dry air flow). The Si-MCM-41 sample obtained pre-
sents a specific surface area of 1000 m2 ꢁ gꢂ1, with an average pore
size distribution of 35 to 38 Å.
Deposition of gold (Au) particles onto the surface of both Si-
MCM-41 and Ti-MCM-41 mesoporous materials was performed
by adaptation of the deposition–precipitation method. Typically,
1.2 g of HAuCl4 ꢁ 3H2O was diluted in 100 ml of water (Milli-Q
Quality) and the pH of the obtained solution was modified with
drop to drop addition of 0.2 M of NaOH solution until pH 7 to 8
was reached. Then, the mentioned Au-containing solution was
added to a suspension of 6.0 g of MCM-41-type material in
200 ml of water (Milli-Q Quality) under continuous and vigorous
stirring at room temperature. The mixture was maintained under
stirring at room temperature for 15 to 16 h. After that, the solid
was recovered by filtration, exhaustively washed with distilled
water and dried in oven at 100 °C for approximately 12 h. The Au
loading on the MCM-41-type supports was 4.5 and 2.3 wt% for
Au/MCM-41 and Au/Ti-MCM41 catalytic samples, respectively,
based on XRF measurements.
Scheme 1. Existing commercial route (a) and alternative cascade-type reaction (b)
for propene epoxidation with TBHP.
to PO at 200 °C [18]. Alkene conversions of 3.2% and PO selectivity
of 93.5% have been reported by Haruta research group using
Au/Ti-MCM-41 materials as catalyst [19,20], while lower epoxide
yields have been attained when working with Au/SBA-15 as cata-
lysts [21]. In all the above cases, besides the low PO yields and
the strong deactivation of the catalyst, one of the major drawbacks
is to work with H2 and O2 mixtures close to the explosion threshold
in order to achieve greater PO yields.
The main afore-mentioned disadvantages in propene epoxida-
tion can be overcome by ‘‘in situ” generation of the hydroperoxide
from a sacrificial hydrocarbon molecule under mild reaction
conditions.
It will be shown here that by combining Au/CeO2 and a silylated
Ti-MCM-41, while using AIBN as initiator, it is possible to carry out
in one pot the formation of the organic hydroperoxide of an iso-al-
kane (3-methyl-pentane) with O2 and to transfer the oxygen from
the hydroperoxide to the alkene (1-octene) to produce the corre-
sponding epoxide with good selectivities. It will be also shown that
the procedure can be extended to other hydrocarbons such as eth-
ylbenzene and iso-propylbenzene (cumene).
2. Experimental
2.1. Catalysts preparation
Synthesis of meso-structured CeO2 was performed following
the procedure reported in Ref. [11]. Deposition of the gold particles
on CeO2 support was carried out by a deposition–precipitation
method with HAuCl4 ꢁ 3H2O as the source of Au, following the
experimental procedure detailed in [22]. Thus, a solution of 0.2 M
of NaOH was added (drop by drop) to 0.6 g of HAuCl4 ꢁ 3H2O di-
luted in 70 ml of water (Milli-Q quality) until a pH 10 was reached.
This solution of Au salt in water (pH 10) was added to a container
holding 5.7 g of CeO2 in 200 ml of water (Milli-Q quality), under
continuous, vigorous agitation. The mixture so obtained was con-
tinuously stirred at room temperature for 15 to 16 h. Once the solid
had been recovered by filtration, it was washed thoroughly with
water and oven dried at 100 °C for approximately 12 h. The Au/
CeO2 material thus synthesized contained approximately 2.5 wt%
of Au based on XRF and chemical analysis of the solid.
The Ti-MCM-41 (2.1 wt% of TiO2) mesoporous material was pre-
pared as reported in Refs. [23,24] starting from 3.11 g of cetyl-
trimethylammonium bromide (C16TAB) dissolved in 20.88 g of
water. Then, 5.39 g of tetramethylammonium hydroxide (TMAOH)
and 0.21 g of titanium tetraethoxide (TEOT) were added to the
above-mentioned solution, and the system was stirred until the
titanium compound was fully dissolved. Silica (3.43 g) was then
added, giving rise to a gel that was stirred at room temperature
for 1 h at 250 rpm. The resulting mixture was placed into auto-
claves and heated at 100 °C under autogenous pressure for 48 h.
Following this time, a solid was recovered by filtration, washed
thoroughly with distilled water, and dried at 60 °C during 12 h.
The solid material was placed in a tubular quartz reactor where
the temperature is increased from room temperature to 540 °C
2.2. Catalyst characterization
Phase purity of the catalysts was determined by X-ray diffrac-
tion (XRD) in a Philips X’Pert MPD diffractometer equipped with
a PW3050 goniometer (CuKa radiation, graphite monochromator),
provided with a variable divergence slit and working in the fixed
irradiated area mode. 29Si MAS NMR spectra of MCM-41 materials
were recorded at a spinning rate of 5 kHz on a Varian VXR 400S WB
spectrometer. Diffuse reflectance UV–Vis (DRUV) spectra of