MCM-41–Quaternary organic tetraalkylammonium hydroxide composites as
strong and stable Brønsted base catalysts
Isabel Rodriguez, Sara Iborra, Avelino Corma,* Fernando Rey and José L. Jordá
Instituto de Tecnología Química (UPV-CSIC), Avda. los Naranjos, s/n, 46022, Valencia, Spain.
E-mail: itq@upvnet.upv.es
Received (in Cambridge, UK) 13th January 1999, Accepted 23rd February 1999
Table 1 Knoevenagel condensation between benzaldehyde 1 (10 mmol)
and ethyl cyanoacetate 2a (8 mmol) in the presence of MCM-41OH (80 mg)
at 60 °C under N2
Quaternary organic tetraalkylammonium hydroxide grafted
on MCM-41 is a strong Brønsted base catalyst, and
represents a useful alternative to soluble bases in reactions
such as Knoevenagel condensations, Michael additions and
aldol condensations because of their high catalytic activity
under mild conditions, and good stability.
Anchored
NMe4OHa
Intial rateb/
1024
TONc/
1023
Yield
of 3a
Catalyst
MCM-41OH1d
MCM-41OH2d
MCM-41OH1f
MCM-41OH2f
NMe4OHf
1.22
0.88
1.22
0.88
0.064h
17.8
11.0
217
163
208i
1.45
1.25
17.8
18.6
32.5
88e
77e
The development of environmentally friendly solid catalysts for
the production of fine chemicals is becoming an area of growing
interest, and recyclable solid base catalysts are needed. Alkali
ion-exchanged zeolites,1,2 sepiolites,3 alkaline oxides supported
on microporous 4,5 and mesoporous aluminosilicates,6 alkaline
earth solids such as magnesium oxide,7 aluminium–magnesium
mixed oxides derived from hydrotalcites8 and organic resins9
can cover a wide range of basic strengths. The recently
discovered family of mesoporous MCM-41 materials, with the
possibility of being prepared with a wide range of pore
dimensions, provides a unique inorganic support for introduc-
ing basic sites in a post-synthesis step and expanding the
possibilities of the above described solid catalysts. Indeed, the
reported anchored amines on MCM-41,10 e.g. the immobilized
1,5,7-triazabicyclo[4.4.0]dec-5-ene on MCM-41,11 have been
shown to be efficient base catalysts. However, excluding
anionic resins, the basicity of the solid inorganic catalysts
developed up to now is associated with Lewis sites, and there is
a need for stable catalysts containing Brønsted basic sites.
Here we report the preparation of a strong Brønsted basic
catalyst obtained by preparing an inorganic–organic composite
formed by an organic ammonium quaternary salt anchored on
the surface of pure silica MCM-41. The truly catalytic
behaviour of this material as well as the recycling character-
istics is shown for Knoevenagel condensations, Michael
additions and aldol condensations.
95g
90g
100g
a
b
In millimoles of NMe4OH per gram MCM-41. In moles of product
per minute per gram of catalyst. c Turn over number in moles of product per
minute per millimole of NMe4OH. d CHCl3 as solvent (5 ml). e 2 h reaction
time. f Without solvent. g 0.5 h reaction time. h NMe4OH added (in mmol).
i In mol min21
.
all samples was close to 1. 29Si MAS NMR spectroscopy and
elemental analysis of the catalysts in the chloride and hydroxide
forms show that no leaching of the anchored tetraalkyl-
ammonium occurs during the anionic exchange of chloride by
hydroxide anions.
MCM-41OH1 was tested as a basic catalyst for the
Knoevenagel condensation of benzaldehyde 1 with ethyl
cyanoacetate 2a. The reaction progress was monitored by
withdrawing aliquots which were analyzed by GC, and the
products were identified by GC–MS. The reaction was carried
out in CHCl3 at 60 °C under nitrogen atmosphere using the
conditions shown in Table 1. After 2 h the conversion of ethyl
cyanoacetate was 88% with 100% selectivity for the condensa-
tion product 3a (Scheme 1).
When the condensation was carried out under the same
reaction conditions described above, but in the presence of an
MCM-41 sample containing a different amount of anchored
tetraalkylammonium quaternary salt (MCM-41OH2), it was
possible to see that the initial rate was proportional to the
amount of anchored tetraalkylammonium hydroxide (Table 1),
indicating that a highly homogeneous strength distribution of
basic sites is obtained on these catalysts and there is total
accessibility of reactants to the active centres.
The condensation was also carried out without solvent under
the reaction conditions described above, and the yield, initial
rate and TON obtained are given in Table 1. When the
Knoevenagel condensation was carried out using other active
methylene compounds, i.e. phenylsulfonylacetonitrile 2b, in the
presence of MCM-41OH1 at 70 °C, the reaction was completed
within 1 h with 100% selectivity for phenylsulfonylcinnamoni-
trile 3b.
Pure silica MCM-41 with 3.7 nm pore diameter was
synthesised following a well-known method from the lit-
erature.12
The functional tetraalkylammonium groups were anchored
on the Si-MCM-41 surface by reacting 3-trimethoxysilyl-
propyl(trimethyl)ammonium chloride (SiNR4Cl) with hydroxy
groups located at the surface. This was achieved by contacting
the dehydrated MCM-41 material with a solution of the
appropriate amount of SiNR4Cl (provided by ABCR GMBH,
50 wt% in MeOH) in CHCl3. This slurry was stirred for 1 h to
allow the SiNR4Cl to diffuse inside of the pores. Then, Et3N
(molar ratio Et3N/SiNR4Cl = 2) in CHCl3 was added to the
above slurry. The final liquid/solid mass ratio was 20. The
reaction was performed for 12 h and functionalized MCM-41
was obtained by filtration, exhaustive washing with CHCl3 and
CH2Cl2 and drying at 60 °C overnight. The MCM-41 structure
was preserved.
The exchange of chloride by hydroxide anions was carried
out by contacting the functionalized MCM-41 samples with a
0.21 m methanolic solution of NMe4OH at room temperature
for 10 min using a liquid/solid mass ratio of 50. The solid was
recovered by filtration and extensively rinsed with MeOH,
followed by vacuum drying, and the MCM-41 structure was
preserved. The degree of exchange was calculated by titration of
the NMe4OH solution after exchange. The OH/N ratio found for
O
R1
Ph
R1
Ph
C
H2C
H
R2
H
R2
1
2a,b
3a,b
a R1 = CN, R2 = CO2Et
b R1 = CN, R2 = SO2Ph
Scheme 1
Chem. Commun., 1999, 593–594
593
Rodriguez, Isabel
