to the IUPAC nomenclature, which are typical for mesoporous
materials. The BET surface area was found to decrease con-
tinuously with the loading of the organics. Pore size distribution
measurements showed a slight decrease and broadening of the
average pore diameter with the anchoring of the organics. Table 1
provides a summary of surface area and pore volume for some of
the perfluoroalkylsulfonic acid containing MCM-41 and SBA-15.
FT-IR spectra (Nicolet 710 FT-IR spectrophotometer) of self-
supporting wafers of the solids were recorded at room temperature
in sealed cells after degassing under vacuum at 200 or 300 °C for
1 h. Fig. 2 shows a typical example in which the presence of a band
at 1420 cm21 characteristic of n(SNO) stretching vibrations of
undissociated 2SO3H groups previously reported in Nafion® films
and triflic acid can be observed.11 The intensity of the band was
significantly maintained with increasing degassing temperatures
even up to 300 °C indicating strong binding of the perfluoroalkyl-
sulfonic acid groups onto the surface of the mesoporous silicas and
its thermal stability. Other bands observed at 1650 and 1760 cm21
were assigned according to the IR studies on Nafion® to the
vibrations of absorbed and protonated water molecules, re-
spectively.11
In a preliminary screening, the catalytic performance of the
hybrid MCM-41 materials was determined for the esterification of
alcohols with long chain aliphatic acid.‡ Blank reactions performed
under identical conditions without any catalyst and also with pure
siliceous MCM-41 showed negligible conversion of the acid. All
the catalysts showed very high activity for the esterification of
octanoic acid with ethanol (Table 1). Conversion of octanoic acid
reached almost 95% in 6 h. Comparison under identical reaction
conditions showed that the hybrid organic–inorganic mesoporous
silica catalysts possess more than double activity than the
commercial Nafion® silica composite (Nafion® SAC-13, Aldrich)
and Nafion silica co-gels in spite of the much higher density of
sulfonic groups of the latter catalysts. Per acid site the turnover
Fig. 3 Esterification of octanoic acid with ethanol at 60 °C in the presence
of MCM-41 containing perfluorosulfonic acid groups for 5 consecutive
runs.
frequency of sulfonic acid groups anchored on MCM-41 (9.7 3
1023 s21) is one order of magnitude higher than that of commercial
Nafion® silica (1.1 3 1023 s21) composites.
To check the reusability of the catalysts fresh reactants (0.15 g
acid and 1.0 g ethanol) were added after 24 h reaction and the
progress of the reaction was monitored by taking small aliquots of
samples at different time intervals. Fig. 3 shows the results for 5
successive runs. It can be seen that the catalyst converts almost 90%
of the acid in less than 6 h for 5 consecutive runs. Alternatively, if
the solid catalyst was filtered after each run, washed with CH2Cl2,
dried and reused, an approximately 25–35% decrease in activity
was observed between the 1st and 2nd use but then it remains fairly
constant for consecutive runs.
Thus, we have developed a single-step method of preparing
hybrid organic–inorganic mesoporous silica catalysts function-
alized with terminal perfluorosulfonic acid groups for which no
oxidation step is required. The catalysts analogous to Nafion® with
completely fluorinated tethers showed very high activity and can be
reused repeatedly without any observed loss of activity.
Table 1 Physical properties and catalytic activity of the catalysts in the
esterification of octanoic acid with ethanol
Amount
of 1 per g
of solid
C8 acid
conversiona
(%)
S content/
mmol g21
SBET /
m2 g21
Notes and references
Sample
† Calcined MCM-41 or SBA-15 (2.0 g) was evacuated at 120 °C for 8 hours,
cooled to room temperature and to this added a solution of 1 (1.0 g) in 50
ml dry toluene. The mixture was refluxed for 4 hours, the solid was filtered
and washed thoroughly with toluene and finally dried at 100 °C
overnight.
‡ 5 g of ethanol and 150 mg of octanoic acid were heated in a three-necked
flask at 60 °C and then 100 mg of the catalyst was added. Samples were
collected at different time intervals and analysed by gas chromatography
(HP5890, HP-5 column).
MCM-41
—
—
0.16
0.45
0.25
0.51
29%b
10–20%b
1030
912
680
820
780
77
< 1.0
34.7
88.0
63.8
87.1
57.7
37.3
MFS-10 (MCM)
MFS-1 (MCM)
SFS-2 (SBA)
SFS-1 (SBA)
Nafion-SiO2 gel
SAC-13
0.5
1.5
1.0
1.5
—
—
200c
a At 3 h. b Amount of Nafion® loadings. c From manufacturer’s data.
Reaction temp 60 °C. Ethanol/acid (mol. ratio) 105. Catalyst wt 100 mg.
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Fig. 2 FT-IR spectra of perfluorosulfonic MCM-41 sample at different
treatment temperatures.
C h e m . C o m m u n . , 2 0 0 4 , 9 5 6 – 9 5 7
957