ORGANOINORGANIC COMPOSITES BASED ON TETRAETHOXYSILANE
337
Table 3. SSC, TSC, and D values for organoinorganic composites with respect to noble metal ions
SSC, mg/g (TSC, mg/g)
Pt(IV)
D, cm3/g
Sample
Pd(II)
Au(III)
Pd(II)
Pt(IV)
Au(III)
1
2
3
4
142 (690)
139 (495)
182 (495)
370 (640)
870 (1270)
582 (910)
760 (910)
704 (1180)
110 (1280)
115 (920)
140 (920)
302 (1190)
500
1030
860
6900
6150
200
500
20500
89000
1030
1430
1400
The composition of the precipitates formed in the ing vibrations at 1500–1480 cm–1 and pyridine vibra-
alkaline hydrolysis of TEOS is virtually independent of tions at 1600, 1580, 1490, and 1020 cm–1.
the composition of the initial mixture. In TEOS–PVPyr
(PVIm) systems (Table 2, samples 2 and 3), we have
approximately one nitrogen-containing monomer unit
per two silicon atoms, and, in the TEOS–PVP system
(Table 2, sample 4), this ratio is 1 : 1.
According to [5], the composition of composites
obtained in the hydrolysis of TEOS in the presence of
PVP and poly-1-vinyl-2-pyrrolidone was close to
equimolar, whereas PVIm gave products with two imi-
dazole monomer units per one silicon atom. This differ-
ence was justly explained in [5] by changes in the basic-
ity of the nitrogen-containing component. The basicity
constants K+BH of VPyr, VIm, and VP are 1.7, 7.52,
and 5.62, respectively.
We did not observe any dependence of the composi-
tion of reaction products on K+BH . The composition of
the product from PVPyr becomes close to equimolar as
the conditions of the hydrolysis of TEOS change. For
instance, the hydrolysis of TEOS in an aqueous–ethan-
olic solution in the presence of PVPyr, whose basicity
is low, results in the formation of a precipitate in two
days, and the composition of the product formed is
close to equimolar (Table 2, experiment 1). This means
that the composition of the products largely depends on
the rate of TEOS hydrolysis. Indeed, the rate of alkaline
TEOS hydrolysis is much higher than the rate of
hydrolysis in water or under acid catalysis conditions.
The formation of polysiloxane structures and their
interaction with polybases therefore occur at a higher
rate than similar reactions in the absence of alkali.
The elemental analysis data (Table 2) show that the
hydrolysis of TEOS in the presence of nitrogen bases
yields insoluble products of the compositions (1) SiO2 ·
PVPyr, (2) (SiO2)2 · PVPyr, (3) (SiO2)2 · PVIm, and
(4) SiO2 · PVP.
According to [5], the formation of organoinorganic
composites in aqueous media is caused by the partici-
pation of free silanol Si–OH groups in polymer–poly-
mer interactions between silicon dioxide being formed
and polymeric N-bases with the formation of H-bonds.
The IR spectra of all composites contain an intense
absorption band at 1200–1250 cm–1 characteristic of
the Si–O bond. The IR spectrum of the PVP-based
composite contains a shoulder at 1630 cm–1, and, in the
spectra of PVIm and PVPyr, azole ring absorption
bands shift from 1480 to 1520 cm–1. These changes in
the IR spectra of insoluble hydrolysis products are
unambiguous evidence that the pyridine nitrogen atom
participates in the formation of bonds with the inor-
ganic component of hybrid polymers.
According to the scanning electron microscopy
data, samples 1–4 possess a homogeneous surface
structure. Composite particles consist of globules; the
predominant diameter of regularly shaped globules is
300 nm.
The sorption activity of the hybrid composites with
respect to Pd(II), Pt(IV), and Au(III) ions was studied
in hydrochloric acid solutions, where these ions are
present as acido complexes [PdCl4]2–, [PtCl6]2–, and
[AuCl4]– [12, 13]. An increase in the concentration of
HCl from 0.1 to 5.0 mol/l decreases the degree of metal
extraction insignificantly, which may be related to an
increase in the competing coordination of chlorine
anions to active polymer nitrogen atoms. The static
sorption capacities (SSC) and interphase distribution
coefficients (D) were calculated from the sorption iso-
therms obtained in 1 M HCl solutions. They show that
the hybrid composites exhibit the highest sorption
activity with respect to platinum(IV) ions (Table 3).
This is in agreement with the well-known ability of
platinum(IV) to form stabler complexes with N-ligands
compared with Pd(II) andAu(III) ions [12, 13]. Most of
the known complex-forming N-functional sorbents [14,
15], including silicon-containing sorbents [16], exhibit
similar behavior.
In order to estimate the degree of filling of the func-
tional groups of the composites with noble metal ions
under saturation conditions, we calculated the theoreti-
cal total sorption capacities (TSC) of the synthesized
sorbents with respect to the ions of the elements to be
extracted (Table 3). The calculations were based on the
assumption of the formation of 1 : 1 complexes (com-
The IR spectra of the initial polymers (PVPyr, posite structural unit : metal) in the matrices of the sor-
PVIm, and PVP) and composites synthesized in this bents. The molecular weight of the structural unit was
work contain bands of C=C and C=N azole ring stretch- calculated from the elemental analysis data (Table 2). A
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 3 2007