Mesoporous silicophosphates

Article abstract of DOI:10.1016/S0025-5408(98)00147-0

Mesoporous silicophosphates containing up to 22% phosphorus were prepared using cationic surfactants. A mesoporous silicophosphate of the approximate composition Si₁₉P₄O₄₈ had a surface area of 770 m²g^-1, after removal of the template. A thermal transformation from a lamellar to a hexagonal structure was observed in this solid. The mesoporous silicophosphate phases were characterized by infrared and NMR spectroscopy.

Full text of DOI:10.1016/S0025-5408(98)00147-0

Materials Research Bulletin, Vol. 33, No. 10, pp. 1549–1554, 1998
Copyright © 1998 Elsevier Science Ltd
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PII S0025-5408(98)00147-0
MESOPOROUS SILICOPHOSPHATES
Neeraj, M. Eswaramoorthy, and C.N.R. Rao*
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced
Scientific Research, Jakkur P.O., Bangalore 560 064, India
(Communicated by M. Greenblatt)
(Received February 4, 1998; Accepted February 4, 1998)
ABSTRACT
Mesoporous silicophosphates containing up to 22% phosphorus were prepared
using cationic surfactants. A mesoporous silicophosphate of the approximate
2
Ϫ1
composition Si P O had a surface area of 770 m g , after removal of the
1
9 4 48
template. A thermal transformation from a lamellar to a hexagonal structure
was observed in this solid. The mesoporous silicophosphate phases were
characterized by infrared and NMR spectroscopy. © 1998 Elsevier Science Ltd
KEYWORDS: A. ceramics, A. microporous materials, D. catalytic properties,
D. surface properties
INTRODUCTION
Since the first reported synthesis of mesoporous silica [1], several mesoporous oxides have
been prepared using ionic as well as neutral templates [2]. Some examples of these are ZrO2
[
3], TiO [4], SnO [4], Al O [5], and Nb O [6]. Mesoporous aluminophosphate [7] and
2 2 2 3 2 5
aluminoborate [8], which are analogous to silica, have also been prepared and characterized.
We were interested in investigating whether mesoporous silicophosphates could be prepared
by using suitable templates. Crystalline and glassy silicophosphates are known, and in some
of them, silicon occurs with octahedral coordination [9–11]. Silicophosphates find applica-
tions as hosts for fast ionic conductors and optical fibers [11] and as catalysts [12]. In this
communication, we report the successful synthesis of mesoporous silicophosphates with
various phosphorus content, by using cationic surfactants.
*
To whom correspondence should be addressed.
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TABLE 1
Mesoporous Silicophosphates
Preparative condition
b
Acid
(mmol)
SA
(mmol)
Surface area
a
2 Ϫ1
Sample
pH
Composition
(m g
)
1
2
3
4
H PO (18)
CTAB(8)
CPB(7)
CPB(7)
CTA(8)
11
SiO ⅐0.1SA
1100
965
3
2
2
H PO (22)
3.5
3.5
3.5
23SiO ⅐P O ⅐3.8SA⅐3H O
3
4
2
2
5
2
c
H PO (18)
9.4SiO ⅐P O ⅐3.0SA⅐1H O
770
3
2
2
2
5
2
d
H PO (22)
3SiO ⅐P O ⅐0.9SA⅐0.7H O
130
3
2
2
2
5
2
a
Of the as-synthesized material.
Of the samples calcined at 673 K. In sample 3, over 80% of the template was removed on
b
calcination.
c
H PO with CTAB under similar conditions gave a product with 7% P.
Not mesoporous.
3
2
d
EXPERIMENTAL
In the preparation of mesoporous silicophosphates, we used tetraethylorthosilicate (TEOS)
and phosphoric or phosphinic acid as the starting materials and cetyltrimethlammonium
bromide (CTAB) and cetylpyridinium bromide (CPB) as the surfactants (SA). The prepara-
tion was carried out in acidic media, unlike that of silica, which is carried out in a basic
medium. In a typical synthesis, 7–8 mmol SA were added to water. This solution was stirred
for 20 min, then 2.8 mL HCl (5.6 M) was added. TEOS (16 mmol) was added to the SA
solution, with stirring for 15 min, followed by the addition of H PO /H PO (ϳ20 mmol).
3
4
3
2
The pH of the resulting mixture was adjusted to 3.5 by adding 5 M NaOH solution, and
stirring continued until a gel was obtained. The gel was aged at ambient temperature for 20 h.
The final product was filtered and dried at 373 K for 4 h.
Thermogravimetric analysis (TGA) of the dried product was carried out using a Mettler-
Toledo TG850 instrument. Energy dispersive X-ray (EDX) analysis was performed with a
Leica scanning electron microscope. X-ray diffraction (XRD) patterns were recorded with a
Rich Seifert instrument. Transmission electron microscopy (TEM) images were obtained
with a Jeol JEM-3010 instrument.
RESULTS AND DISCUSSION
The various compositions prepared by us, together with some preparative details are listed in
Table 1. From this table, we see that phosphorus was incorporated into all the preparations
carried out at a pH of 3.5. Higher pH led to the formation of pure mesoporous silica,
independent of the presence of an initial excess of H PO /H PO . The extent of the
3
4
3
2
incorporation of phosphorus depended on the phosphorus source and the SA; H PO –CTAB
3
4
and H PO –CPB combinations seemed to favor the incorporation. We also found that the
3
2
bromide of the SA was exchanged by chloride from HCl during the preparation. Without the
addition of HCl, however, we failed to obtain the silicophosphates.
In Figure 1, we show the XRD patterns of the as-synthesized mesoporous silicophosphates
samples 3 and 4 along with the XRD pattern of the mesoporous hexagonal phase of silica

Vol. 33, No. 10
MESOPOROUS SILICOPHOSPHATES
1551
FIG. 1
XRD patterns of the as-synthesized silicophosphates samples (1, 3, and 4) and the calcined
samples of 1 and 3. For a description of the compositions, see Table 1.
(sample 1) with a d100 of 3.87 nm. The diffraction patterns of samples 3 and 4 are quite
similar to that of silica, giving the (100) reflection at d values between 3.8 and 3.9 nm. The
XRD pattern of sample 3 has some features of the lamellar phase, and TEM images show the
presence of the hexagonal phase admixed with a small proportion of the lamellar phase.
Calcination of the samples at 673 K for 4 h removed most of the surfactant, as determined
by TGA and elemental analysis. Infrared spectra showed the near-absence of the bands of the
template. The mesoporous structure was fully retained in samples 1, 2, and 3 after calcination
at 673 K, as seen from the XRD patterns (see Fig. 1). The mesoporous structure of sample
4
(with ϳ40% P), however, was destroyed on calcination at 673 K. The phosphorus content
of the silicophosphates after calcination was close to the values given in Table 1. The
composition of sample 3 after calcination was Si P O . It appears that silicon can be
1
9 4 48
substituted up to a maximum of 25% by phosphorus, in the hexagonal mesophorous phase.

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NEERAJ et al.
Vol. 33, No. 10
FIG. 2
TEM image of silicophosphate sample 3 (a) during calcination and (b) after calcination at 673 K.
The BET surface areas of samples 1, 2, and 3 calcined at 673 K were 1150, 965, and 770
2
Ϫ1
m g , respectively.
It was mentioned earlier that sample 3 contained a mixture of lamellar and hexagonal
forms. On heating at 673 K, the lamellar phase transformed into the hexagonal phase. In
Figure 2, we show the TEM images of sample 3 (with 22% P) to demonstrate the presence
of both the lamellar and hexagonal phases during transformation and the crystalline nature of
the fully transformed hexagonal phase. Such a lamellar-to-hexagonal transformation in the
solid state is interesting. It appears that the transformation is favored by the partial elimi-
nation of the template on heating the lamellar phase, which enables the structure to curl up
to produce the cylindrical structure.
The TGA curves, chemical analysis, and compositional dependence of the thermal stabil-
ities of the mesoporous structures indicated that phosphorus was incorporated into the silica
framework. Further evidence of the incorporation of phosphorus was obtained by NMR and
3
1
29
infrared spectroscopy. We examined the P and Si MAS NMR spectra of the mesoporous
silicophosphates. Both spectra changed during calcination in the case of sample 3, as shown
3
1
in Figure 3. The P signals of the calcined sample 3 (relative to H PO ) are at –13.0, –23.1,
3
4
and –34.4 ppm due to OP(OSi or P) (OH) , OP(OSi or P) (OH), and OP(OP)3Ϫx(OSi)x,
2
2

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MESOPOROUS SILICOPHOSPHATES
1553
FIG. 3
Si and P MAS NMR spectra of sample 3 (a) before and (b) after calcination.
2
9
31
2
9
respectively [11]. The Si signals of this silicophosphate are at –91 (Q , Q ), –104 (Q ) and
1
2
3
–
111 ppm (Q ) [10,13,14], the slight shift to negative values arising from the incorporation
4
of phosphorus. It appears that silicon was present only as a 4-coordinated species in these
silicophosphates. The infrared spectrum of calcined sample 3 showed a strong band at 1080
Ϫ1
cm due to a coupled Si–O and P–O stretching vibration. Bands due to the bending
Ϫ1
vibration of Si–O–P/Si–O–Si/P–O–P units are found at 815 and 455 cm [15]. Thus, the
present study demonstrated that the mesoporous phases of silicophosphates in which phos-
phorus is an integral part of the silica framework can be prepared. The use of these materials
as hosts in catalysis is being explored.
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