F. Shang et al. / Catalysis Communications 12 (2011) 739–743
741
Fig. 4. IR spectra of pyridine adsorption on Al-MCM-41 (a), Al-MCM-41-NH
Al-MCM-41-NH -10% (c) at 373 K.
2
-3% (b) and
2
are lacking in this sample. Therefore, it is supposed that appropriate
amount of amine (basic sites) could friendly coexist with the acid on
the surface of the solid without destroying the original acid. However,
densely populated amines will destroy the acid sites because of the
neutralization.
Fig. 3. 29Si MAS NMR spectra of Al-MCM-41-NH
-3% (a), and Al-MCM-41-NH -5% (b).
2 2
NH
and strength of acidic sites. The NH
shown here) shows only one peak at 190–200 °C, which may be
corresponding to the desorption of NH from the weak acid sites of the
3
-TPD measurement was employed to estimate of the number
3
-TPD profile of Al-MCM-41 (not
ordered mesoporous materials [22]. In addition, as shown in Table 1,
with the introduction of organic functional groups, the BET specific
surface area, pore size and pore volume gradually decreases. The results
imply that organic moieties are successfully incorporated by grafting in
toluene.
3
materials. The lack of strong acid on Al-MCM-41may be due to the
amorphous nature of the pore wall of the mesoporous materials. The
number of acidic sites in Al-MCM-41 is 0.60 mmol/g, which is similar
to the results materials reported [26].
3
.2. Confirmation of the base and acid sites
3
.3. Catalytic properties
Solid-state 29Si MAS NMR spectra of samples in Fig. 3 show two
3
distinct peaks at about −100 ppm due to Q (Si(OSi)
3
OH) and −110 ppm
The acid-basic properties of the bifunctionalized catalysts were
4
due to Q (Si(OSi)
display a resonance peak at −65 ppm, assigned to T (RSi(OSi)
peak is due to the Si atoms in the organosilane APTES. It is also seen that
sample Al-MCM-41-NH -5% possesses a higher intensity of T sites than
sample Al-MCM-41-NH -3%, demonstrating a higher content of amine
4
) silicon sites. In addition to these peaks, samples
first evaluated in two tandem reactions: the catalytic conversion of
benzaldehyde dimethyl acetal (1) into benzylidene ethyl cyanoacetate
(3) and (2-nitrovinyl) benzene (4), as shown in Table 2 and Table 3.
These reaction sequences involve two separate steps: an acid-catalyzed
deprotection to give the intermediate benzaldehyde (2) and the
subsequent base-catalyzed Knoevenagel reaction or nitroaldol
(Henry) reaction to yield benzylidene ethyl cyanoacetate or 2-nitrovinyl
benzene. Deacetalization is used to recover protected aldehydes in
organic chemistry.
3
3
). This
2
2
groups in the former than that in the latter [23]. Quantification of amine
loading in some materials was performed using elemental analysis (CHN).
The results indicate that Al-MCM-41, Al-MCM-41-3% and Al-MCM-41-5%
2
contains 0, 0.43 and 0.62 mmol/g of NH , respectively (Table 1).
Pyridine is employed as a test molecule to confirm the acidic sites
on the selected samples [24,25]. IR spectra of pyridine adsorption on
One-pot synthesis of benzylidene ethyl cyanoacetate (3) from
benzaldehyde dimethylacetal (1) was demonstrated over various
catalysts (Table 2). From the table, benzaldehyde is the main product
(entry 1) over Al-MCM-41 and only a negligible formation (b1%) of 3
Al-MCM-41, Al-MCM-41-NH
2 2
-3% and Al-MCM-41-NH -10% at 373 K
were shown in Fig. 4. The peaks associated with pyridine coordinated
−1
to Lewis acid sites (~1455, 1623 and 1490 cm ) and to Brönsted acid
2
was formed over MCM-41-NH -3% (entry 2). Thus the monofunctio-
−1
sites (~1546, 1639 and 1490 cm ) can be clearly seen in Al-MCM-41
and Al-MCM-41-NH -3%, indicating that aluminium incorporation in
the framework of MCM-41 generates both Bronsted and Lewis sites.
However, for Al-MCM-41-NH -10%, the peaks associated with acid
nalized catalyst on its own cannot perform the one-pot cascade
reaction. Addition of equivalent amounts of structurally similar free
acid (p-toluenesulfonic acid) or base (tert-butylamine) to bifunctio-
nalized catalysts cannot catalyze the tandem reaction sufficiently, as
these homogeneous species diffuse into the pore channels and then
destroy the catalytic sites by formation of ion pairs (entry 7 and entry
2
2
sites intensively weaken or even disappear implying that the acid sites
Table 1
2
8). Al-MCM-41-NH -1% gave almost 100% conversion of benzaldehyde
dimethylacetal within 40 minutes, but the yield to the final product 3
The structure parameters and elemental analysis results of samples.
Pore sizea
(nm)
N contentb
(mmol/g)
(42.8%) is low, indicating the lack of base sites on this catalyst (entry 3).
Remarkably, Al-MCM-41-NH -3% showed the highest activity in
2
Samples
BET surface area
Pore vol.
2
−1
(cm3
−1
(
m
g
)
g
)
tandem deacetalization–Knoevenagel condensation. It gave almost
100% conversion of benzaldehyde dimethylacetal and afforded 3 in
yield of 97.3% (entry 4). The conversion of benzaldehyde dimethylacetal
decreased with raising concentration of organosilanes in the catalysts
(entries 5 and 6). The probability of neutralization from acid and base
Al-MCM-41
Al-MCM-41-NH
Al-MCM-41-NH
1250
934
746
1.23
0.88
0.61
2.91
2.70
2.45
0
0.43
0.62
2
-3%
-5%
2
a
Calculated by using the BJH model on the adsorption branch of the isotherms.
Based on the N elemental analyses.
b