Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions
The method of synthesis of silica gels containing small additives of Al and Zr ions was developed. The porous structure of the synthesized Al,Zr-silica gels has been characterized by the adsorption method. It was found that simultaneous modification of silica gels by aluminium and zirconium ions lea...
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Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine
2002
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Surface| _version_ | 1869291168298172416 |
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| author | Bondar, L. A. |
| author_facet | Bondar, L. A. |
| author_institution_txt_mv | [
{
"author": "L. A. Bondar",
"institution": "Інститут хімії поверхні НАН України"
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| author_sort | Bondar, L. A. |
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| description | The method of synthesis of silica gels containing small additives of Al and Zr ions was developed. The porous structure of the synthesized Al,Zr-silica gels has been characterized by the adsorption method. It was found that simultaneous modification of silica gels by aluminium and zirconium ions lead to the formation of micro- and ultramicroporous samples in neutral media of precipitation. The parameters of porous structure of Al,Zr-silica gels are considerably dependent on the modified ions contents. A supposition was made that the formation of porous structure was caused by mutual stabilizing effect of Al and Zr additives on silica gel globules growth at sol-gel stages of synthesis. The nature of surface active sites of the modified silica gels was studied by the diffuse-reflectance IR-spectroscopy with the use of adsorption of deuteroacetonitrile. |
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26
SYNTHESIS AND PHYSICOCHEMICAL PROPERTIES
OF SILICA GELS WITH SMALL ADDITIVES
OF ALUMINIUM AND ZIRCONIUM IONS
L.А. Bondar
Institute of Surface Chemistry, National Academy of Sciences
Gen. Naumov Str. 17, 03680 Kyiv-164, UKRAINE
Abstract
The method of synthesis of silica gels containing small additives of Al and Zr ions was
developed. The porous structure of the synthesized Al,Zr-silica gels has been characterized by
the adsorption method. It was found that simultaneous modification of silica gels by aluminium
and zirconium ions lead to the formation of micro- and ultramicroporous samples in neutral
media of precipitation. The parameters of porous structure of Al,Zr-silica gels are considerably
dependent on the modified ions contents. A supposition was made that the formation of porous
structure was caused by mutual stabilizing effect of Al and Zr additives on silica gel globules
growth at sol-gel stages of synthesis. The nature of surface active sites of the modified silica
gels was studied by the diffuse-reflectance IR-spectroscopy with the use of adsorption of
deuteroacetonitrile.
Introduction
At present development of the methods for synthesis of new type of adsorbents,
catalysts and supports with properties of strong acids and special characteristics of their porous
structure is urgent problem. It is known that for many catalytic processes the systems with
microporous and ultramicroporous structure are more efficient because of the effect of shape
selectivity [1]. In this connection the investigations directed toward the preparation of new
micro-, meso- and ultramicroporous materials, exhibiting molecular-sieve selectivity and strong
acid surface sites, on the basis of relatively cheap silica gels are of considerable interest. At
present, there are only separate data concerning synthesis and study of ultramicroporous silica
gels in the scientific literature [2, 3]. The synthesis of such samples may be carried out under
conditions of minimum rate of polycondensation (acidic media of gel precipitation and
washing), considerable capillary forces (low drying temperature), a stabilizing effect of
zirconium ions over a globule sizes. It was earlier shown [4-6] that addition of the small
amounts of Al, Ti or Zr ions (up to 5-6 wt.% of oxide) to silica gels at the sol-formation stage
would result in increasing of their specific surface area, reducing in pore volume and diameter,
and the formation of supermicroporous silica gels in neutral medium of precipitation. This
phenomenon can be explained by a stabilizing effect of the above-mentioned ions on the silica
gel globules growth at sol-gel stages of the synthesis. It was established that dehydroxylation
of the silica gels modified by the Al and Zr ions lead to formation of Lewis acidic sites on their
surface [7]. Small additives of these ions were found to improve considerably the silica gel
27
stability under hydrothermal conditions [8-10]. Thus, the silica gels modification with small
additives of one of the mentioned ions can considerably vary both porous structure and nature
of the surface active sites. The investigation of the mutual influence of ions of two or several
metals on the texture and surface structure of silica gel is of great interest and will result in the
development of new materials with unusual combination of molecular-sieve and acid-base
properties.
The aim of this work is to elaborate the methods of synthesis of new modified silica
gels containing Al and Zr ions simultaneously and investigation of their porous structure and
nature of surface active sites.
Experimental
Al, Zr-silica gels were prepared by the method similar to that described in the
works [4, 5]. Sodium silicate, sulfuric acid, aluminium sulfate, and zirconium oxochloride were
used as starting reagents. Sols were carried out by interaction of the numbered solutions at
pH=7. After setting and syneresis the gels were cut, washed out by distilled water up to
absence in washing water of SO4
2- ions (test on BaCl2). The washed gels were dried at 373K or
ambient temperature. Samples of one series had the approximately identical content of Al ions
and differed in the content of Zr ions. The specific surface areas of the obtained samples were
determined from the adsorption isotherms of methanol vapours by the BET method. Volume
of sorption pores was designed on amount of adsorption at P/Ps =0.92. The investigations of
surface active sites of the synthesized silica gels were carried out by diffuse reflectance infrared
spectroscopy [7]. Deuterated acetonitrile CD3CN was used as a test for Lewis acidic sites.
Results and discussion
Chemical composition and the parameters of the porous structure of obtained samples
are given in Table 1. Data presented in this Table show that all samples of series I contain
approximately 2.3 wt. % Al2O3 . Increasing the zirconium ion content in these samples to
6.2 wt. % ZrO2 leads to increasing in specific surface area (S). Further increasing ZrO2 content
up to 7.76 wt. % will result in reducing S. In both cases the pore volume has been gradually
decreased. The growth of the zirconium ion content in silica gel samples is accompanied by the
following changes in isotherms of adsorption of methanol vapour: at low relative pressures it
results in an increase in adsorption values; at high relative pressures the adsorption values
decrease; and the hysteresis loop characteristic of the capillary condensation disappears
(Fig. 1). Similar regularities were previously observed at Al or Zr ions inclusion into silica gels.
The increase of zirconium ions content in Al,Zr-silica gels containing near 3.5 and
4.7 wt. % Al2O3 (II and III series of samples, respectively) makes less effect on the parameters
of porous structure - specific surface area remains practically constant or slightly reduced and
the pore volume tends to diminishing (Table 1). Increase in zirconium ion concentration
Al,Zr-silica gels, contained approximately 5.7 wt.% of Al2O3 (IV series of samples), resulted in
reducing of both specific surface area and pore volume (Table 1). The adsorption isotherms of
methanol vapour measured for samples of series II-IV are of the 1st type, which is typical for
microporous adsorbents [11].
28
It is known that silica gel represents a porous solid consisting from globules of
SiO2 [12, 13]. The formation and growth of silica gel globules proceed through the reaction of
polycondensation of silicic acid, the aggregation of globules formed, and the
dissolution-precipitation of silica. The globules finite size and density of their packing depend
on the synthesis conditions and define the specific surface area and pore volume of the samples
obtained.
The small amounts of Al or Zr ions added into silica gels at the sol-formation stage
have a stabilizing effect on the SiO2 globules sizes, i.e. the presence of these ions during
synthesis delays growth of SiO2 globules [4. 5]. According to [4, 5], the stabilizing effect of Al
or Zr ions on the size of SiO2 globules is due to the following factors: a decreased solubility of
SiO2 in the presence of Al or Zr ions, the isomorphous replacement of silicon ions in the SiO2
matrix by aluminium ions, and an increase in the hydrophilicity of SiO2 surface. For silica gels
of a series I stabilizing influence of Al ions on the globules probably is not enough for
obtaining of samples with the smallest globule sizes. The introduction of zirconium ions into
samples of this series renders additional stabilizing influence on the globules SiO2 that is
accompanied by diminution of their sizes and is displayed in increasing of specific surface area.
Table 1. Chemical composition and parameters of porous structure of Al, Zr-silica gels*
Series and Content, wt. % S, Vs,
sample number Al2O3 ZrO2 m2/g cm3/g
I-1
I-2
I-3
I-4
I-5
I-6
II-1
II-2
II-3
II-4
III-1
III-2
III-3
IV-1
IV-2
IV-3
IV-4
IV-5
2.24
2.24
2.24
2.49
2.50
2.64
3.21
3.61
3.53
3.53
4.71
4.71
4.72
5.97
5.76
5.90
5.63
5.48
-
1.13
2.20
3.85
6.20
7.76
-
1.04
2.05
4.10
-
2.00
4.00
-
1.13
4.24
5.64
8.42
510
540
645
660
690
630
730
710
710
710
790
730
620
650
640
540
490
520
0.49
0.47
0.41
0.35
0.34
0.30
0.37
0.33
0.33
0.29
0.33
0.28
0.28
0.28
0.27
0.22
0.22
0.24
*Samples of series I were prepared by drying of the washed gels at 373К
and samples of series II-IV – by drying at 293К.
SiO2 globules, which are smaller on their sizes, are packed more densely at drying that
results in diminution of volume of sorption pores (Table 1). The diminution of specific surface
area of the sample I-6 can be caused by formation of a part of very fine pores inaccessible for
29
adsorption of methanol molecules, or by formation of small quantity of a separate phase of
zirconium dioxide [7].
For silica gels of series II and III, distinguished by the major content of Al ions, the
stabilizing influence of the latter on the globules SiO2 is also more that results in preparation of
materials with higher specific surface area (samples II-1 and III-1, Table 1). Probably, that is
why the influence of Zr ions on the globules of SiO2 and parameters of the porous structure of
samples of these series is lesser displayed. The silica gels of a series IV contain the greatest
quantity of Al ions (Table 1). It results in formation during synthesis fineporous structures,
which are accessible only partially for adsorption of methanol vapours. The introduction of Zr
ions in these adsorbents results in diminution of specific surface area and volume of pores. The
reason of this can be an arrangement of Zr ions on a surface of SiO2 globules. It is known that
the zirconium ions injected into silica gel at a stage of precipitation are displaced mainly on a
surface of SiO2 globules [7]. Probably, in Al,Zr-silica gels the Zr ions are also displaced on a
surface of globules, blocking thus inlets into the narrowest pores and creating steric hindrances
for adsorption of molecules. Thus, in some samples the ultramicropores, which are inaccessible
for adsorption of large molecules, can be formed. In order to check such assumption, the
adsorption isotherms of carbon tetrachloride vapours were measured on the separate samples.
It follows from the data of Table 2 that the porous structure of all samples studied differs in
their access for methanol molecules (molecules diameter of 0.34 nm) and to carbon
tetrachloride (0.69 nm), i.e. the molecular-sieve selectivity in relation to these adsorbates take
place for these adsorbents. However, the molecular-sieve selectivity is the most evidently
displayed in the case of samples comprising the greatest amount of Al ions (Fig. 2). For these
samples an adsorption of CCl4 vapours was not practically observed, i.e. their pores diameter
was less than 0.69 nm.
Thus, the simultaneous introduction of the small additives of Al and Zr ions into silica
gels leads to the formation of micro- and ultramicroporous samples in a neutral precipitation
medium, which is peculiar for the formation of mesoporous samples.
It is known that even the smallest inclusions of metal ions can considerably improve adsorptive
properties of silica gels and in some cases impart acidic character and catalytic activity to their
surfaces. At the same time the attempts to separate acidic OH groups in the samples containing
even great amounts of metals were failed [14, 15].
It may be explained by low concentration of acidic OH-groups and by difficulty of their
separating against a background of the strong absorption of the terminal ºSiOH groups. In this
work we have attempted to study the Lewis acidic sites of the prepared adsorbents using
diffuse-reflectance IR-spectroscopy [16]. Molecules of deuterated acetonitrile were used to
test acidic sites. The first series of samples having approximately the same content aluminium
ions and differing by zirconium ions content and reference specimen with only aluminium were
chosen for the study. Two absorption bands with maxima 2324 and 2270 cm-1 were observed
in IR-spectrum of the reference sample (Fig. 3). Evacuation at ambient temperature resulted in
a reducing of 2270 cm-1 band intensity and at 373К – to its disappearance in this sample. At the
same time the band 2324 cm-1 becomes more intensive and shifts towards high frequencies
range.
30
Fig. 1. The adsorption isotherms of methanol vapours on Al,Zr-silica gels of series I containing
0 (1), 1.13 (2), 3.85 (3), and 7.76 (4) wt. % ZrO2.
Table 2. Values of methanol and carbon tetrachloride adsorption by Al,Zr-silica gels*
Series and Content, wt. % CH3OH, CCl4,
sample number Al2O3 ZrO2 cm3/g cm3/g
II-1
II-2
II-3
II-4
IV-1
IV-2
IV-3
IV-4
IV-5
3.21
3.61
3.53
3.53
5.97
5.76
5.90
5.63
5.48
-
1.04
2.05
4.10
-
1.13
4.24
5.64
8.42
0.33
0.28
0.28
0.26
0.25
0.24
0.21
0.18
0.19
0.24
0.20
0.20
0.11
0.02
0.02
0.01
0.01
0.04
*Calculated from the isotherms of adsorption at P/Ps=0.5.
a, mmol/g
P/Ps
1
2
0 0.2 0.4 0.6 0.8
12
10
8
6
4
2
3
4
31
Fig. 2. The adsorption isotherms of methanol (1) and carbon tetrachloride (2) vapours on
Al,Zr-silica gels of the series IV, containing 0 (а) and 4.24 (b) wt. % ZrO2.
As it was reported in the previous work [7], there are two types of active sites on the
dehydroxylated at 623К Al-silica gel surface - terminal ºSiOH groups and three-coordinated
silicon ions. As it was shown in that work, an introduction of small additives of Al ions into a
skeleton of silica gel proceed through Si4+ ions isomorphous replacement for Al3+ and
Si(OH)Al acid bridging groups are present on the surface.
In line with the assumption [7], dehydroxylation of silica gels, comprising aluminium
ions additives, is performed owing to the removal of a proton of bridging Si(OH)Al group and
hydroxyl of the neighbouring terminal ºSiOH group:
OH
Si Al
O
Si
H
+ Si Al
O
Si+
(1)
- H2O
b
a 0.4
0.2
0.4
0.2
0 0.2 0.4 0.6 0.8
2
1
1
a, cm3/g
2
P/Ps
1
32
Fig. 3. IR-spectra of deuterated acetonitrile on Al,Zr-silica gels of series I containing 0 (a) and
1.13 (b) wt. % ZrO2 after adsorption (1) and subsequent evacuating at 293К (2) and
393К (3). Samples were previously outgased at 623К.
One can suppose that the band 2270 cm-1 in IR-spectra (Fig. 3) appertains to molecules
of CD3CN, physically adsorbed onto terminal ºSiOH surface group. Evacuation of this sample
at the ambient temperature and at 373К leads to removal of CD3CN molecules from these
groups. Other absorption band with the maximum 2328 cm-1 is evidently related with CD3CN
molecules, retained on the surface Lewis acidic sites - three-coordinated silicon ions.
Inclusion of small amounts of zirconium ions (1.13-3.85 wt. % of ZrO2), equally with
aluminium ions, results in changes in IR-spectra. In IR-spectra of the sample with adsorbed
deuterated acetonitrile, recorded after degassing at 293 and 393К, along with 2328 cm-1
absorption band a new band at 2305 сm-1 appears. The first band is related to CD3CN
molecules, interacting with three-coordinated silicon ions, whose formation is evidently
proceeding by the same mechanism (1) as for the silica gels with the additives of aluminium
ions. Based on these facts, one can assume the similar (like in Al-silica gels) route of
isomorphic replacement of Al3+ for Si4+ in Al,Zr-silica gels and the bridging acidic Si(OH)Al
groups are present on the surface of non-dehydroxylated samples. The second band
(2305 cm-1) is probably attributed to new Lewis acidic sites - coordinatively-unsaturated
zirconium ions [17]. These sites in Al,Zr-silica gels were evidently formed while removing
molecules of water from the interior coordination sphere of octahedrally-coordinated
zirconium ions:
33
[ZrO4(H2O)2]4- ® [ZrO4]4- + 2H2O (2)
The mechanism of the Lewis acidic sites formation was earlier proposed for silica gels with the
additives of zirconium ions [7].
Thus, dehydroxylation of Al,Zr-silica gels containing 1.13-3.85 wt. % of ZrO2 is
carried out by two mechanisms (1) and (2) simultaneously and lead to the formation on a
surface of two types of the Lewis acid sites.
Conclusions
The obtained in this work experimental data demonstrate that simultaneous
modification of silica gel by Al and Zr ions at sol-formation stage of synthesis leads to the
preparation of micro- and ultramicroporous adsorbents in a neutral precipitation medium. It is
evidently can be attributed to the mutual stabilizing effect of Al and Zr ions on SiO2 globules
growth. Upon Al,Zr-silica gels synthesis the min-sized globules were formed which were
closely packed at drying. Dehydroxylation of the prepared Al,Zr-silica gels leads to the
formation on the surface of the Lewis acidic sites of two types - coordination-unsaturated ions
of Si and Zr. On the analogy with previously derived Al- and Zr-containing samples, one can
suppose that in the synthesized Al,Zr-silica gels Al ions are included in SiO2 structure via
isomorphic substitution of Al3+ for Si4+ but Zr ions are arranged on globule surface.
Acknowledgement
The author thanks Prof. L.M. Kustov and his associates for the interest to this work
and help in recording of IR-spectra.
References
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34
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L.А. Bondar
L.А. Bondar
L.А. Bondar
L.А. Bondar
Conclusions
|
| id | oai:ojs.pkp.sfu.ca:article-80 |
| institution | Surface |
| keywords_txt_mv | keywords |
| language | English |
| last_indexed | 2025-07-22T19:30:06Z |
| publishDate | 2002 |
| publisher | Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine |
| record_format | ojs |
| resource_txt_mv | surfacezbircomua/0c/c2a799b288982da6c47df2f47fafa90c.pdf |
| spelling | oai:ojs.pkp.sfu.ca:article-802018-11-27T09:42:19Z Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions Bondar, L. A. The method of synthesis of silica gels containing small additives of Al and Zr ions was developed. The porous structure of the synthesized Al,Zr-silica gels has been characterized by the adsorption method. It was found that simultaneous modification of silica gels by aluminium and zirconium ions lead to the formation of micro- and ultramicroporous samples in neutral media of precipitation. The parameters of porous structure of Al,Zr-silica gels are considerably dependent on the modified ions contents. A supposition was made that the formation of porous structure was caused by mutual stabilizing effect of Al and Zr additives on silica gel globules growth at sol-gel stages of synthesis. The nature of surface active sites of the modified silica gels was studied by the diffuse-reflectance IR-spectroscopy with the use of adsorption of deuteroacetonitrile. The method of synthesis of silica gels containing small additives of Al and Zr ions was developed. The porous structure of the synthesized Al,Zr-silica gels has been characterized by the adsorption method. It was found that simultaneous modification of silica gels by aluminium and zirconium ions lead to the formation of micro- and ultramicroporous samples in neutral media of precipitation. The parameters of porous structure of Al,Zr-silica gels are considerably dependent on the modified ions contents. A supposition was made that the formation of porous structure was caused by mutual stabilizing effect of Al and Zr additives on silica gel globules growth at sol-gel stages of synthesis. The nature of surface active sites of the modified silica gels was studied by the diffuse-reflectance IR-spectroscopy with the use of adsorption of deuteroacetonitrile. The method of synthesis of silica gels containing small additives of Al and Zr ions was developed. The porous structure of the synthesized Al,Zr-silica gels has been characterized by the adsorption method. It was found that simultaneous modification of silica gels by aluminium and zirconium ions lead to the formation of micro- and ultramicroporous samples in neutral media of precipitation. The parameters of porous structure of Al,Zr-silica gels are considerably dependent on the modified ions contents. A supposition was made that the formation of porous structure was caused by mutual stabilizing effect of Al and Zr additives on silica gel globules growth at sol-gel stages of synthesis. The nature of surface active sites of the modified silica gels was studied by the diffuse-reflectance IR-spectroscopy with the use of adsorption of deuteroacetonitrile. Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine 2002-06-12 Article Article application/pdf https://surfacezbir.com.ua/index.php/surface/article/view/80 Surface; No. 7-8 (2002): Chemistry, Physics and Technology of Surface; 26-34 Поверхность; № 7-8 (2002): Химия, физика и технология поверхности; 26-34 Поверхня; № 7-8 (2002): Хімія, фізика та технологія поверхні; 26-34 3154-8091 3154-8083 en https://surfacezbir.com.ua/index.php/surface/article/view/80/78 Авторське право (c) 2002 L.А. Bondar |
| spellingShingle | Bondar, L. A. Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions |
| title | Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions |
| title_alt | Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions |
| title_full | Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions |
| title_fullStr | Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions |
| title_full_unstemmed | Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions |
| title_short | Synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions |
| title_sort | synthesis and physicochemical properties of silica gels with small additives of aluminium and zirconium ions |
| url | https://surfacezbir.com.ua/index.php/surface/article/view/80 |
| work_keys_str_mv | AT bondarla synthesisandphysicochemicalpropertiesofsilicagelswithsmalladditivesofaluminiumandzirconiumions |