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though we cannot exclude the sponge effect, which could be
responsible for the improvement in activity, another factor is
more apparent.
Experimental Section
Materials
The most important property of the materials for the cataly-
sis investigated is acidity. Generally speaking, Friedel–Crafts re-
actions cannot be catalyzed by weak Lewis acid sites of pure
silica materials,[33] such as MCM-41,[38] SBA-15,[39] and TUD-1.[26]
However, the Friedel–Crafts benzylation of benzene can be cat-
alyzed by silica-based catalysts, either pure Lewis acids such as
AlCl3/MCM-41 or pure Brønsted acids such as Nafion-embed-
ded silica.[35] Furthermore, it has been reported that there is
a synergy between Lewis and Brønsted acid sites with an ap-
propriate ratio,[40] that is, from 0.7 to 1 Brønsted/Lewis ratio.
In the HY/TUD-1 composite, the NH3-TPD and pyridine ad-
sorption in combination with FTIR studies indicate that the
mesoporous matrix reduces the total number of Brønsted acid
sites of zeolite Y, probably by forming new bonds between the
zeolite and the silica matrix of TUD-1. Thus, the Brønsted/Lewis
ratio changes from 1.7 in the parent HY to 0.67 in YTUD-40,
which is close to the optimized ratio reported previously. Thus,
the overall reaction rate of the HY/TUD-1 composite per zeolite
amount is higher than that of the parent HY (Figure 8).
The following chemicals were obtained and used without further
treatment: tetraethyl orthosilicate (TEOS, >98%, Acros Organics),
triethanolamine (TEA, 97, Acros Organics), tetraethylammonium hy-
droxide (TEAOH, 35%, Aldrich), benzene (anhydrous, 99.8%,
Sigma–Aldrich), and benzyl alcohol (anhydrous, 99.8%, Sigma–Al-
drich). Zeolite Y (CBV-600) with a Si/Al ratio of 5.2 and crystal size
around 250 nm was obtained from Zeolyst.
Synthesis
Four samples of the HY zeolite/TUD-1 composite were prepared by
adding commercial zeolite Y to the synthesis mixture of TUD-1.
The samples were labeled YTUD-x, with x representing a weight
percentage of 10, 20, 40, or 60% of HY zeolite. In a typical synthe-
sis method, deionized water was added to TEA and the mixture
was shaken by hand for a few minutes until a pale yellow mixture
was obtained. Then, TEOS was added dropwise with stirring. A sus-
pension of zeolite Y in NH3 (to avoid aggregation of the zeolite
crystals) was added to the previous mixture under vigorous stir-
ring. Finally, tetraethylammonium hydroxide was added dropwise.
The obtained mixture was stirred until gelation. The final synthesis
mixture has a molar ratio composition of SiO2/TEA/TEAOH/H2O=
1:1:0.5:11. The obtained gel was aged at RT for 24 h and then
dried at 371 K for another 24 h. The obtained solid was gently
ground, hydrothermally treated in a 50 mL Teflon-lined stainless
steel autoclave at 451 K under autogenous pressure for 4 h, and fi-
nally calcined at 873 K for 10 h (heating rate: 1 KminÀ1) in static air.
Furthermore, the changes in acidity are in agreement with
the reduction in the rate of deactivation of the composite
compared to the parent HY zeolite.[41] Zeolite Y deactivates
sharply in liquid phase hydrocarbon reactions even at mild
temperatures, which is due to coke formation.[42–44] We thus
suggest that the main benefit of the thin, highly porous TUD-
1 layers (ensuring accessibility to the HY surface) in the perfor-
mance of HY zeolite in benzylation reactions is the inhibition
of a considerable number of strongly Brønsted acid sites,
which prevents coke formation and zeolite deactivation. This
suggestion is in agreement with the work of Dong et al.[45] and
Weckhuysen et al.,[46] in which Brønsted acid sites are reported
to be responsible for coke formation on the surface of zeolites.
This effect has also been observed in studies combining
beta zeolite and TUD-1[22,25] as well as ITQ-2 and TUD-1.[24] De-
tailed IR studies of the beta zeolite/TUD-1 composites have
shown that hydrogen bonded silanols are more dominant in
the composite materials compared to TUD-1. Furthermore, the
composite with approximately 40% beta zeolite contained the
highest number of acid sites with medium acidity, which coin-
cided with the highest activity in the cracking of n-hexane.[24]
Characterization
Powder XRD patterns were measured with a Philips PW-1840 X-ray
diffractometer equipped with a graphite monochromator using
CuKa radiation (l=0.1541 nm). The samples were scanned at 2q=
0.1–808 with steps of 0.028. Instrumental neutron activation analy-
sis was used for the determination of the chemical composition at
the THER nuclear reactor at the Delft University of Technology,
with a thermal power of 2 MW and a maximum neutron flux of 2ꢁ
1017 mÀ2 sÀ1. The N2 adsorption–desorption isotherms were record-
ed on a Quantachrome’s AUTOSORB-6B at 77 K. Samples were
evacuated previously at 623 K for 16 h. The pore size distribution
was calculated from the adsorption branch by using the BJH
model. The BET method was used to calculate the surface area of
the samples, whereas the mesopore volume and external surface
area were calculated by using the t-plot method. 27Al MAS NMR ex-
periments were performed at a magnetic field of 9.4 T on a Varian
VXR 400 S spectrometer operating at 104.2 MHz with a pulse
width of 3.2 ms; 4 mm zirconia rotors were used, with the spinning
speed set to 8 kHz. The chemical shifts were determined by using
TMS as an external standard and set to 0 ppm. A total of
1000 scans were collected by using a sweep width of 20000 Hz
and an acquisition delay of 20 s. The IR spectra were recorded on
a Bio-Rad 176C spectrophotometer. Thin wafers of the samples
(weighing ꢀ15 mg) were prepared by using a SPECTA press and
applying a pressure of 3 tonscmÀ2. Samples were mounted on
a Cu sample holder equipped with a resistive heating element and
a type K thermocouple. All samples were activated at 800 K for 4 h
in air before spectral collection by averaging 200 scans at 8 cmÀ1
resolution. NH3-TPD was performed on a Micromeritics TPR/TPD
2900 apparatus equipped with a thermal conductivity detector.
Conclusions
HY/TUD-1 composites were prepared by adding commercial
HY zeolite to the synthesis mixture of the TUD-1 mesoporous
material. The characterization data indicate the formation of
a thin layer of the 3 D TUD-1 matrix surrounding the zeolite
crystals. The catalytic activity of HY/TUD-1 was higher than
that of commercial HY zeolite in the Friedel–Crafts benzylation
of benzene at 353 K. Furthermore, HY/TUD-1 showed improved
stability compared to the parent HY zeolite. The improved per-
formance of the composite is likely related to beneficial
changes in the acidity of the HY crystals through encapsulation
by TUD-1.
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ChemCatChem 2013, 5, 3156 – 3163 3161