Highly ordered acid functionalized SBA-15: A novel organocatalyst for the preparation of xanthenes

Article abstract of DOI:10.1039/c1cc11007a

Post-synthesis modification of SBA-15 has been carried out to design highly ordered acid functionalized hybrid mesoporous organosilica, AFS-1. This material has been used as an efficient heterogeneous organocatalyst for the syntheses of xanthenes under mi

Full text of DOI:10.1039/c1cc11007a

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Highly ordered acid functionalized SBA-15: a novel organocatalyst for
the preparation of xanthenesw
Mahasweta Nandi,ab John Mondal,a Krishanu Sarkar,a Yusuke Yamauchic and
Asim Bhaumik*a
Received 21st February 2011, Accepted 8th April 2011
DOI: 10.1039/c1cc11007a
Post-synthesis modification of SBA-15 has been carried out to
design highly ordered acid functionalized hybrid mesoporous
organosilica, AFS-1. This material has been used as an efficient
heterogeneous organocatalyst for the syntheses of xanthenes under
mild conditions in the absence of any other metal co-catalyst.
Xanthenes are a class of compounds which are hugely recognized
in the field of medicine and organic chemistry due to their
antiviral,1 antibacterial2 and anti-inflammatory3 activities.
They are the source of a class of brilliant fluorescent dyes4
and are used extensively in laser technology5 and pH sensitive
fluorescent materials.6 Several methods for the syntheses of
Scheme 1 Schematic diagram for acid functionalization of SBA-15.
substituted xanthene via reaction of 2-naphthol with aryl
aldehydes under strongly acidic conditions are reported in
the literature.7 Most of these syntheses either involve the use of
metal containing catalysts8 or suffer from serious drawbacks
via consecutive surface functionalization with 3-aminopropyl
triethoxysilane (3-APTES) followed by the condensation of the
surface –NH2 groups with the aldehyde group of 4-formyl-benzoic
acid (Scheme 1). Previously –CO2H and –SO3H functionalized
mesoporous SBA-15 has been reported by co-condensation13
and post-synthesis grafting14 techniques, respectively. However,
incorporation of the –CO2H group into a mesoporous silica frame-
work by condensation between an aldehyde and a primary amine
has not been studied so far. Increasing importance of metal-free
catalysis, on the other hand, motivated us to use this organically
modified acid functionalized mesoporous silica (AFS-1) as a
catalyst for the synthesis of benzoxanthene from a mixture of
aromatic aldehyde and 2-naphthol. To the best of our knowledge,
organocatalysis promoted by a –CO2H functionalized mesoporous
silica for the synthesis of xanthene has not been explored so far.
Mesoporous silica, SBA-15, has been synthesized following
the reported procedure.15 It is modified with 3-APTES by
stirring 0.1 g of SBA-15 with 0.18 g (0.813 mmol) of the former
in chloroform at 298 K for 12 h under a N2 atmosphere.
The resulting white solid is filtered, washed repeatedly with
chloroform and dichloromethane (DCM), and finally dried in
air. This 3-APTES functionalized SBA-15 is refluxed with 0.15 g
(1 mmol) of 4-formylbenzoic acid dissolved in methanol (20 ml)
for 6 h at 333 K and the final product is collected by filtration,
repeatedly washed with hot methanol until all the unreacted
carboxylic acid is removed and finally dried in a vacuum
desiccator. In a typical catalytic cycle, a mixture of 2 mmol
2-naphthol and 1 mmol aromatic aldehyde is stirred with 20 mg
of AFS-1 in the presence of 5 ml dry DCM at 298 K for the
like long reaction time, low yield, vigorous reaction conditions
and so on.9 Owing to the toxic effects and deactivation
associated with metal-leaching during the reaction, increasing
interests are being paid for the development of organocatalysts
devoid of metal ions.10 Transformations involving an organo-
catalyst can be considered as a green process but they are
mostly homogeneous in nature.11 Thus catalyst recovery and
its reuse is an important drawback associated with an organo-
catalytic reaction. To avoid these problems heterogeneous organo-
catalysts can be very promising, particularly those based on
mesoporous silica due to their exceptional surface area, tuneable
nano-scale pores, and exciting host–guest chemistry.12 Thus
the syntheses of organically modified mesoporous silica with
new framework structures are highly desirable. In this commu-
nication, we report a new surface –CO2H group functionalized
mesoporous silica, with a 2D-hexagonal mesoporous structure
a Department of Materials Science, Indian Association for the
Cultivation of Science, Jadavpur, Kolkata-700 032, India.
E-mail: msab@iacs.res.in; Fax: +91-33-2473-2805;
Tel: +91-33-2473-4971
b Integrated Science Education and Research Centre, Siksha Bhavana,
Visva-Bharati, Santiniketan-731235, India
c World Premier International (WPI) Research Center for Materials
Nanoarchitectonics (MANA), National Institute for Materials
Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
w Electronic supplementary information (ESI) available: TEM,
TG-DTA, solid state 13C MAS NMR and recycling tests of AFS-1 and
1H and 13C NMR of the reaction products. See DOI: 10.1039/c1cc11007a
c
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Fig. 1 Low angle XRD pattern of SBA-15 (a) and AFS-1 (b).
desired time. After completion of the reaction, the catalyst is
filtered from the reaction mixture, washed thoroughly with
DCM and the filtrate is evaporated to dryness under reduced
pressure. The crude solid product obtained is crystallized from
ethanol. The recovered catalyst is washed with ether and dried
at 353 K for 2 h prior to use for the next cycle.
Fig. 2 Nitrogen adsorption–desorption isotherm of SBA-15 (a) and
AFS-1 (b). Inset: pore size distribution using the BJH method.
Barett–Joyner–Halenda (BJH) method is shown in the inset of
Fig. 2. Estimated pore width for SBA-15 is 10.5 nm whereas for
AFS-1 the pore size is reduced to 9.16 nm. These data are in good
agreement with the pore widths depicted by the TEM image
analysis and XRD studies.
Quantitative estimation shows that the loading of 3-APTES
in 3-APTES-SBA-15 and 4-formylbenzoic acid in AFS-1 are
0.0075 mol gÀ1 and 0.0045 mol gÀ1 respectively. These values can
be correlated to ca. 82% conversion of the amino groups to
corresponding Schiff-base in AFS-1. The small-angle powder
X-ray diffraction (PXRD) patterns of mesoporous SBA-15
and AFS-1 are shown in Fig. 1. For SBA-15 (Fig. 1a) four
well-resolved diffraction peaks in the 2y region of 0.6–2.07 can
be observed, which can be indexed to the 100, 110, 200 and
210 reflections16 corresponding to a two-dimensional hexagonal
mesostructure. When the silica framework is functionalized with
the acid groups a decrease in the intensity of the peaks is
observed, however the 2D-hexagonal ordering is retained. This
decrease in the peak intensities is attributed to the lowering of
local order, viz. variations in the wall thickness or reduction of
scattering contrast between the channel wall of the silicate
framework.17 The TEM images (ESIw, Fig. S1) of mesoporous
SBA-15 and acid functionalized material AFS-1 show uniform
and long range ordering of large mesopores throughout the
respective specimens. The corresponding FFT diffractogram
(inset of Fig. S1a, ESIw) further suggests the presence of hexa-
gonally arranged pores.18
13C and 29Si MAS NMR experiments often provide useful
information regarding the chemical environment and the pre-
sence of an organic functional group in the hybrid frameworks.
The 13C CP MAS NMR spectrum (ESIw, Fig. S2) for AFS-1
exhibits three strong signals at 9.1, 21.8 and 43.5 ppm. Sharp
peaks could be assigned to the aliphatic propyl group attached
with Si, C1, C2 and C3 respectively (Scheme 1). Four other
broad signals with maximums at 62.8, 130.6, 140.1 and 164.2
could be attributed to imine-C, benzene rings and –CO2H
groups, respectively. This result indicates that the functional
group bearing the acid moiety is covalently grafted inside the
pore walls of AFS-1. On the other hand the 29Si MAS NMR
spectra of AFS-1 (Fig. 3) show two broad peaks with chemical
Nitrogen-sorption studies carried out on the calcined
mesoporous SBA-15 and AFS-1 at 77 K are shown in Fig. 2.
Both the samples show type-IV adsorption–desorption isotherms
with a very large H2 type hysteresis loop in the 0.5 to 0.8 P/P0
range. The sharpness of the adsorption step in these isotherms
suggest the relative order in mesopore size and the characteristic
hysteresis loop account for the large-tubular pores of SBA-15.18
The surface area of SBA-15 (Fig. 2a) is 610 m2 gÀ1 and that for
AFS-1 is reduced to 250 m2 gÀ1 (Fig. 2b). The values of P/P0
near the inflection point are closely related to the pore-widths
which lie in the mesopore range and the sharpness of these steps
indicates the uniform size distribution of the mesopores.18 Pore
size distribution of both the samples estimated employing the
Fig. 3 29Si MAS NMR spectra of AFS-1.
c
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In conclusion, we have shown that new functionalized meso-
porous material AFS-1 can be used as an extremely stable and
reusable catalyst for the preparation of xanthenes from a mixture
of b-naphthol and aromatic aldehyde. This reaction proceeds
smoothly at room temperature in the absence of any other metal-
containing species and the catalytic efficiency is restored in
repeated reaction cycles. This is the first example of organo-
catalysis promoted by a –COOH functionalized mesoporous
silica for the synthesis of xanthenes and it opens up a new
direction for the development of catalysts devoid of toxic metals
and their further applications in various organic transformations.
JM wishes to thank CSIR, New Delhi, for Senior Research
Fellowship.
Scheme 2 General protocol for xanthene synthesis over AFS-1.
shifts at –61.5 and –110.0 ppm, which could be attributed to the
T3 ((OH)(OSi)3Si–R) and Q4 (Si(OSi)4) species, respectively.19
Presence of high concentration of Q4 and T3 species in the
NMR spectrum of AFS-1 suggested a considerably defect-less
cross-linked network in the material. Thermal analysis of SBA-
15, 3-APTES-functionalized SBA-15 and AFS-1 (ESIw, Fig. S3
and S4) has been carried out to compare the stability of the
catalyst, which is an important issue associated with a hetero-
geneous catalytic system. For the synthesis of xanthene, we have
carried out the reactions at room temperature, but AFS-1 is
found to have sufficient thermal stability to carry out other
organocatalytic reactions, which can occur at higher temperature.
The total weight loss for the material upto 773 K is found to be
16% which takes place in two steps. The first step is associated
with the removal of physisorbed water, whereas the second
step involving combustion of the organic functionality present
in the organosilane framework starts above 473 K.
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AFS-1 is investigated for its catalytic activity for the prepara-
tion of xanthene in the absence of any other metal co-catalyst. It
is found that condensation between aromatic aldehydes (1a–e)
and 2-naphthol (2) at room temperature (Scheme 2, Table 1)
produces 14-aryl-14H-dibenzo [a,j] xanthenes in good yields
over our AFS-1 material. The products are characterized by 1H
and 13C NMR spectroscopy and the data are given in ESI.w
The heterogeneous nature of the catalyst has been confirmed
by performing in situ filtration experiment for the reaction
between 4-nitrobenzaldehyde and 2-naphthol, as a representative
case. The catalyst has been separated from the reaction
mixture by filtration after 1 h (30% conversion). The reaction
was continued with the filtrate for another 4 h but no further
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repeating cycles (ESIw, Fig. S5). No appreciable loss of
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turnover numbers (TON, Table 1). Thus we see that AFS-1
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This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6677–6679 6679