Sulfonic acid functionalized mesoporous MCM-41 silica as a convenient catalyst for Bisphenol-A synthesis

Article abstract of DOI:10.1039/b107155f

Sulfonic acid groups anchored to the surface of mesoporous MCM-41 silica have been identified with S K-edge XANES spectra and the material is an efficient catalyst for the liquid phase condensation of phenol with acetone to form Bisphenol-A with high selectivity.

Full text of DOI:10.1039/b107155f

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Sulfonic acid functionalized mesoporous MCM-41 silica as a
convenient catalyst for Bisphenol-A synthesis
Debasish Das,a Jyh-Fu Leeb and Soofin Cheng*a
a Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.
E-mail: chem1031@ccms.ntu.edu.tw
b Research Division, Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
Received (in Cambridge, UK) 7th August 2001, Accepted 18th September 2001
First published as an Advance Article on the web 5th October 2001
Sulfonic acid groups anchored to the surface of mesoporous
MCM-41 silica have been identified with S K-edge XANES
spectra and the material is an efficient catalyst for the liquid
phase condensation of phenol with acetone to form Bis-
phenol-A with high selectivity.
The –SH groups were converted into –SO3H groups by mild
oxidation with H2O2 (stirring for 24 h at 60 °C with excess
oxidant). The solid was filtered off, washed with water and
ethanol, followed by acidification with 0.1 M H2SO4 followed
by thorough washing with water to remove all traces of liquid
acid. The solid material was finally dried at 60 °C overnight.
Sulfur loading of the solid material was determined by
elemental analysis (Heraeus) and was typically in the range
0.9–1.8 meq. g21 of solid. Thermogravimetric analysis (Du
Pont 950) shows the material to be more hydrophobic than
MCM-41. While calcined pure silica MCM-41 shows ca. 5%
weight loss in the range 30–100 due to water removal, sulfonic
acid functionalized MCM-41 (denoted MCM-SO3H) showed
only < 1% weight loss in this region. Loss of sulfonic acid
groups starts at ca. 200 °C and is completed by 325 °C. The
specific surface area and pore volume of the pure silica MCM-
We report here that sulfonic acid functionalized mesoporous
MCM-41 silica can be an efficient catalyst for the condensation
of phenol and acetone at relatively low temperature to
synthesize Bisphenol-A with a very high selectivity. Bisphenol-
A is an important raw material for polymer and resin
production, and it is produced industrially using ion-exchange
resins such as Amberlyst.1 However, thermal stability and
fouling of the resins are major problems for these catalysts and
the search for thermally stable and regenerable solid acid
catalysts are continuing.
Zeolites have been increasingly used in the synthesis of fine
chemicals due to their high surface area and confined domains.2
However, the suitability of zeolites for the conversion of larger
substrates has been severely limited by the available dimensions
of their pores, which are in the micropore region with pore
mouth diameter usually less than 1 nm.3 For reactions to occur
within the pores, the reaction substrates and products must be
allowed to diffuse smoothly through the zeolite pores. The
discovery of surfactant–templated mesoporous silica materials
such as MCM-41 (hexagonal P6m symmetry) and MCM-48
(cubic Ia3d symmetry) with controlled pore diameters (2–10
nm) and topologies has extended the suitability of these
materials for the conversion of larger substrates.4 They are very
promising for the application as catalysts and/or catalyst
supports and are likely to offer potential improvements in
reaction selectivity in the conversion of larger substrate
molecules in their well-defined channels with narrow pore size
distributions. However, in spite of these favorable pore
dimensions, the acidity of Al-substituted mesoporous materials
such as Al-MCM-41, is much weaker than that of microporous
zeolites.5 To overcome this drawback, the synthesis of in-
organic–organic hybrid mesoporous materials with alkyl-
sulfonic acid groups has been reported recently.6–9 These hybrid
materials can be synthesized either by direct one-step synthesis
or via secondary silylation. Thus, MCM-SO3H prepared by
direct synthesis and secondary silyation is reported to be highly
efficient in the synthesis of bisfurylalkanes6 and also in the
esterification of glycerol with fatty acids.7,9 However, as the
organic functional groups may be damaged or destroyed during
the template removal process, the secondary silylation tech-
nique seems to offer a better alternative for preparing mesopor-
ous acid catalysts.
41 material was reduced from 1035 m2 g21 and 0.85 cm3 g21
,
respectively, to 690 m2 g21 and 0.42 cm3 g21 after anchoring
the sulfonic acid groups. Although the pore size distribution
curve (Fig. 1) becomes somewhat broad after sulfonic acid
group introduction and the peak maximum shifts from 27 Å
(pure MCM-41) to 22 Å, the pore size distribution still remains
very narrow with major fractions of pores lying within the 7 Å
region. These results indicate that anchoring sulfonic groups to
the channels of MCM-41 reduced the pore diameter and volume
of MCM-41 but did not damage the mesoporous structure. This
was also affirmed by the retention of sharp diffraction peaks in
the corresponding XRD pattern.
To obtain good catalytic activity it is imperative that the thiol
groups can be effectively oxidized to active sulfonic acid
groups. To monitor this process, the oxidation states of sulfur
were examined by studying the S K-edge XANES spectra of the
sample before and after oxidation. The XANES experiments
were performed at Beam line 15B at the Synchrotron Radiation
Research Center facility at Hsinchu, Taiwan. Standard operat-
ing conditions were 1.5 GeV and 200 mA beam current and the
Pure silica MCM-41 was synthesized according to the
method described earlier.10 Surface functionalization with
sulfonic acid groups was carried out according to the method
described in the literature.7,8 Typically, 3.0 g of the freshly
calcined MCM-41 sample was evacuated at 150 °C and then an
excess of 3-mercaptopropyltrimethoxysilane (MPTS) in dry
toluene was introduced. The mixture was refluxed for 6 h and
the solid was filtered off, washed with dry toluene and air-dried.
Fig. 1 Pore size distribution of (a) calcined pure silica MCM-41 and (b)
MCM-41 functionalized with sulfonic acid groups.
2178
Chem. Commun., 2001, 2178–2179
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b107155f

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Table 1 Synthesis of Bisphenol-A with sulfonic acid functionalized MCM-
photon energies were calibrated using the L-edge of pure Mo
foil. Fig. 2 shows the S K-edge XANES spectra of the thiol
functionalized samples before and after H2O2 oxidation. It has
been reported that both the energy position (Ek) and intensity of
the S s?p transition peak (white-line) are sensitively related to
the oxidation state of the S atom.11 Studies with model
compounds showed an increase of Ek and amplitude of the
white-line with increasing formal oxidation states of S. Thus,
for unoxidized sample containing thiol groups the white-line
appears at 2472 eV corresponding to sulfur in the reduced state.
After H2O2 oxidation the white-line shifts to higher energy
(2481 eV) which corresponds to sulfonic acid with S in the +5
state.11,12 It can be seen from Fig. 2 that most of the anchored
thiol groups have been oxidized to sulfonic acid groups under
the above oxidation conditions.
41 silicasa
Phenol
conversion (%) (%)
Selectivityb
Entry
Catalyst
1
2
3
4
5
6
—
0
5
7
< 5
30
17
—
55
—
10
92
90
H-betac
HYd
HZSM-5e
MCM-SO3H
MCM-SO3Hf
a Reagents and conditions: Phenol 4.7 g, acetone 0.58 g, (molar ratio =
5+1), catalyst 50 mg, 70 °C, 24 h. b Selectivity to p,pA-Bisphenol-A. c Si/Al
=
50. d Si/Al
XANES).
= =
10.6. e Si/Al 80. f Incomplete oxidation (from
the selectivity to Bisphenol-A was much lower than that
obtained with MCM-SO3H. Furthermore, leaching of sulfur
during the catalytic reaction was found to be negligible with
MCM-SO3H. Elemental analysis of the catalyst before reaction
indicated ca. 3.4% sulfur and 5.6% carbon content. After
reaction at 70 °C, these values were 3.2% S and 9.5% C. The
slight decrease in sulfur content is mainly due to the increase in
weight from adsorption of organic species. On the other hand, it
is noted that optimization of the oxidation process in the
preparation of MCM-SO3H is essential to achieve high catalytic
activity. For samples with similar sulfur loadings but with
incompletely oxidized thiol groups (dotted line in Fig 2) the
activity was found to be much lower (see Table 1). XANES
spectra of the samples with incompletely oxidized sulfur species
showed a number of peaks around 2472 eV, suggesting the
presence of lower valent sulfur species such as sulfides and
disulfides.11,12 It has also been observed that at higher sulfur
loadings ( > 1.5 meq. g21 solid) part of the sulfur remains in the
reduced form even after prolonged oxidation, probably due to
formation of sulfides and disulfides.
Fig. 2 S K-edge XANES spectra of thiol functionalized MCM-41 materials
(a) before oxidation, (b) after oxidation and (c) sample with incomplete
oxidation of thiol groups.
In conclusion, sulfonic acid anchored MCM-41 has been
found to be very effective in the synthesis of p,pA-Bisphenol-A
with very high selectivity at relatively low reaction tem-
peratures. S K-edge XANES studies can be used as a convenient
tool for easy monitoring of the oxidation state of sulfur in the
samples.
Condensation of phenol and acetone to Bisphenol-A (Scheme
1) was carried out at temperatures of 70–125 °C with the
sulfonic acid functionalized MCM-41 silicas. Several other
acidic zeolites such as H-ZSM-5, H-Y and H-beta have also
been used for comparison. The products were analyzed and
identified using a Chrompak 9000 gas chromatograph and HP
6890 GC-MS, and the results are given in Table 1. No
conversion of phenol was observed in the absence of catalyst.
While acidic zeolites like H-ZSM-5, H-Y and H-beta showed
negligible activity at 70 °C, MCM-SO3H showed 30% phenol
conversion (cf. 40% maximum theoretical conversion at a
phenol+acetone molar ratio of 5+1) with > 90% selectivity
towards p,pA-Bisphenol-A; the other product was o,pA-Bis-
phenol-A. The conversion was found to increase with the
reaction temperature, but the selectivity remained almost
unchanged up to 120 °C. Chroman and trisphenols were not
detected at temperature below 125 °C, but small amounts
( < 1%) of these byproducts were detected at or above 125 °C.
Recently, the synthesis of Bisphenol-A over heteropolyacid
encapsulated MCM-41 has been reported.13 However, 12-tung-
stophosphoric acid encapsulated MCM-41 was found to be
catalytically active only at temperatures above 120 °C. Due to
this high reaction temperature, several by-products such as
alkylated phenols and chroman derivatives were formed, and
Financial support was provided by the Chinese Petroleum
Corporation and the Ministry of Education, Taiwan.
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Scheme 1
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