Alumina and silica preparation: Via a sol-gel route with carbon dioxide as a pH regulator-a new method for green catalysis

Article abstract of DOI:10.1039/c8nj00865e

This paper presents a new preparation method of SiO₂ and Al₂O₃via a sol-gel route with CO₂ as a pH regulator. The influence of carbon dioxide as a pH moderator on the textural parameters is demonstrated. The properties of the oxide xerogels were investigated using SEM, TEM, FT-IR, thermal analysis techniques, and low temperature nitrogen adsorption measurements to describe their structural evolution. The results illustrate differences in the final textural properties of the systems obtained depending on the pH regulator used in the synthesis. The method proposed is doubly beneficial as it eliminates the use of (organic or inorganic) acids and represents an interesting and new application of carbon dioxide.

Full text of DOI:10.1039/c8nj00865e

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Alumina and silica preparation via sol-gel route with carbon
dioxide as pH regulator - a new way for green catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
b
c
Robert Przekop,* Bogna Sztorch , Mariusz Pietrowki , Michał Zieliński and Agnieszka Martyła
DOI: 10.1039/x0xx00000x
2 2 3 2
This paper presents a new preparation method of SiO and Al O via sol-gel route with CO as a pH regulator. The influence
of carbon dioxide as a pH moderator on textural parameters was demonstrated. The oxide xerogel properties were
investigated using SEM, TEM, FT-IR, thermal analysis techniques, and low temperature nitrogen adsorption to describe its
structural evolution. The results illustrate differences in the final textural properties of the systems obtained depending on
the pH regulator used in the synthesis. The method proposed is doubly beneficial as it eliminates the use of (organic or
inorganic) acids and is an interesting and new application of carbon dioxide.
www.rsc.org/
product.
Introduction
We propose a significant improvement in the method of
synthesis of oxide gels SiO
commonly used acids and their replacement with carbon
dioxide for pH regulation. The use of CO brings a number of
2 2 3
and Al O based on elimination of
In the area of sol-gel synthesis there is a continuous demand
for new, simple and effective techniques of preparation.
Presently the most often applied method of pH regulation in
sol-gel syntheses involves the use of strong mineral acids such
2
advantages, including the elimination of problems related to
the use of organic or mineral acids. Moreover the method
proposed is consistent with the principles of green chemistry
1
,2
3,4
5
as HCl , HNO3 and less frequently H SO . The use of these
2
4
acids carries some serious difficulties related to the removal of
anionic residues that can undergo strong adsorption or
permanent attachment to the structure of synthesized
material. In extreme situations it can lead to obtaining
undesired products, insoluble compounds like calcium of
magnesium sulfates. Moreover, in order to remove the anionic
residues by desorption or decomposition and thermal
desorption, it is necessary to use elevated temperature (even
2
and rational synthesis as employs gas CO which is inexpensive
and widely available. Other important advantages of the
method proposed is the lack of post-synthesis waste and easy
drying of the final product. Of course a new way of using CO
in material synthesis is also worth mentioning.
2
Experimental
6
up to 650 °C). Other methods for removal of anionic residues
Materials
are based on the use of organic acids, either monocarboxylic
7
,8
9
like acetic acid or formic acid or dicarboxylic acids like citric
The precursors used for the syntheses were the alcoholates:
tetraethoxysilane (97%, Sigma Aldrich) and aluminum
isopropoxide (98%, Acros Organics). Gas carbon dioxide
1
0
11
12
acid , tartaric acid or other ones. The use of organic acids
entails problems related to the interaction of the acid with
1
3
precursors, e.g. formation of salts like acetates or strong
interactions with the basic components of the matrix that
would hinder desorption of the acid from the final product. To
solve these problems also elevated temperature (400-600 °C)
and oxidizing atmosphere are needed to obtain pure final
(
99.99%, Linde) and acetic acid (99.8%, Sigma Aldrich) were
used as pH regulators. The reaction medium was doubly
-
distilled water of electric conductivity <1.6 µScm .
1
Preparation
The syntheses were carried out in a glass reactor equipped
with a magnetic stirrer, the reaction mixture was heated by a
heating mantle with a temperature controller ensuring
temperature stabilization to
±2 °C. The setup for the syntheses
is shown schematically in Figure 1.
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2010 V2.00 D made by Micromeritics, using 200-300 mg of Capillary tension caused by water and solvents vapor, could be
materials with the grain size fraction between 0.1-0.2 mm. responsible for the production of considerable stress inside
1
Prior to nitrogen adsorption all samples were outgassed at the sol–gel matrix. Also, the presence of incompletely
4
o
2
00-300 C at 0.4 Pa till constant weight. Both adsorptive and hydrolyzed precursor of gel cannot be excluded. In the
desorptive branches of the isotherm were taken in the range synthesis of SiO gels without a pH moderator (Figure 2 D) the
of p/p 0-1. sample was transparent but with distinct milky inclusions that
could come from incompletely hydrolyzed precursor. The SiO
sample obtained with the use of gas CO (Figure 2 F) showed
2
0
2
FTIR Spectroscopy
2
FT-IR measurements were carried out using a Thermo lower tendency to breaking were transparent but with the
Scientific Nicolet iS50 FT-IR spectrometer. Four mg samples iridescent effect. This effect can be related to the capturing of
were mixed with 200 mg KBr and pressed at 10 tons press into some amounts of CO
the form of discs. FTIR analysis was made for gel samples dried
at room temperature.
2
in the gel structure (cf. FTIR spectra).
Results and discussion
Figure
2 presents photos of xerogels dried at room
temperature. The differences in transparency of the materials
obtained depending on the method of synthesis are well
2
visible. The samples of Al(O)OH and SiO obtained with the use
of acetic acid showed the lowest tendency to breaking upon
drying and formed transparent monoliths (Figure 2 B, E). In the
synthesis of aluminum gel, the lack of pH moderator (Figure
2
2A) or the use of CO (Figure 2C) resulted in the loss of
transparency and a tendency towards formation of small
fragments. These features can suggest weakening of the gel
cross-linking and thus a decrease in its mechanical strength.
Another more probable explanation of this effect is an
increase in the rate of the solvent evaporation.
Fig. 3. DTG and TG curves for silica (A) and alumina samples
B).
(
The gel samples dried at RT were subjected to
thermogravimetric analysis (Figure 3). For the samples of
aluminum gel hydrolyzed and condensed with the use of acetic
acid (Figure 3 A) three mass loss signals are obtained, the first
o
related to the removal of water (102 C), the second related to
the removal of residues of isopropyl alcohol or incompletely
o
hydrolyzed precursor or weakly bound acid (217 C) and the
most intensive signal related to the removal of strongly bound
o
acetic acid (345 C). The strong binding of the acid is a
consequence of interaction with amphoteric and basic
hydroxyl groups present in the bulk and on the surface of
Fig. 2. Photos of xerogels dried at RT.
(A) Al; (B) Al-Ac; (C) Al-CO ; (D) Si; (E) Si-Ac; (F) Si-CO .
2
2
7
aluminum gel. Another reason for so much content of organic
1
5-17
compounds is a more compact microporous gel structure.
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8
-1
This effect has also been observed by the authors of . The the presence of a strong band at 2400-2450 cm assigned to
total mass loss of sample Al-Ac is 60.3 wt.%, while that of the the carbon dioxide bound in the silica gel, in the spectrum of
sample obtained with the use of CO
2
as a pH regulator (Al-CO
2
)
2
sample Si-CO . Because of this observation it can be inferred
is just 23.2 wt.% and is close to that recorded for the gel that in contrast to the aluminum samples, the mass loss
synthesized without acidity regulators (20.5 wt.%). For samples observed for silica samples in DTG in the range 200-450 °C can
2
Al and Al-CO we observe only the mass loss related to the be attributed to the removal of residues of isopropyl alcohol
o
removal of water (~100 C) and a less intensive signal and carbon dioxide. As to the threefold difference in the mass
corresponding to the removal of strongly bound water loss between the systems compared, we can give two main
(
dehydroxylation) and the residues left after hydrolysis of reasons. The interaction of acetic acid with SiO
2
and in
precursor (~380-400 C), Figure 3 A. No analogous results were particular with AlOOH was much stronger. Apart from physical
observed for silica gel (Figure 3 B). The use of CO in the interactions, also chemical ones take place, in particular with
o
2
synthesis of silica gel leads to a significant decrease in the total amphoteric oxide (AlOOH). Another reason is that in the
mass loss, from 14.8 wt.% for SiO obtained without a pH presence of acetic acid some amount of alcohol undergoes
2
regulator (Si) to 5.1 wt.% when CO was a pH regulator (Si- esterification and the ester molecules of greater size can be
2
CO ). As follows from the above analysis, the samples obtained captured in the structure of porous matrix.
2
with the use of acetic acid contain large amount of organic
impurities that can be removed only by high-temperature
thermal treatment. The proposed method of using CO as a pH
2
regulator is devoid of this drawback. The gels obtained with
CO are pure and without ballast introduced by organic acids.
2
o
Fig. 4. FT-IR spectra of xerogels samples dried at 90 C. (A) Al;
2 2
(B) Al-CO ; (C) Si; (D) Si-CO .
In order to detect structural differences between Al(O)OH and
SiO gels obtained with CO as a pH regulator and without any
Fig. 5. Transmission electron microscope (A-D) and scanning
2
2
electron microscope (E,F) images of xerogels samples dried at
o
0 C. (A) Si; (B) Al; (C) Si-CO
pH regulator, their FT-IR spectra were recorded. Comparison
of the FT-IR spectra of the dried samples reveals significant
9
2
; (D) Al-CO
2
; (E) Si-CO
2
; (F) Al-CO
2
.
differences in the interaction of CO
For aluminum oxide (Figure 4 A, B) no essential differences
were noted between the spectra of Al and Al-CO . For Al-CO
the intensity of bands assigned to the stretching vibrations of
2
with these two matrices.
Figure 5 presents the SEM and TEM images of the gel samples
obtained without a pH regulator and with CO . For the silica
without acid (Figure 5 A) the structure is heterogenic with a
mixed pore size distribution. The sample Si-CO shows a
morphology similar to that of typical microporous silica gels
Figure 5 C). In the micrometric scale, the xerogel particles
morphology is similar to that of standard samples of Al(O)OH
Figure 5 E) and silica gel (Figure 5 F).
2
2
2
2
-1
2
CH in the range 3300-3100 cm is slightly lower. These bands
correspond to the residues of isopropyl alcohol captured in the
gel structure or incompletely hydrolyzed precursor. More
significant differences were observed between the spectra of
(
(
2
samples Si and Si-CO (Figure 4 C, D). The main difference is
4
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o
were calcined at 500 C in order Sample Si-500, obtained with no pH regulator, is a typical
Samples of Al(O)OH and SiO
2
to obtain the corresponding oxides. The results of low- mesoporous material whose isotherm shows features typical
1
9
temperature nitrogen adsorption/desorption studies show of Type IV(a) isotherm (Figure 6 A). The presence of a
that for SiO the gas CO used as a pH regulator acts also as the hysteresis loop of Type H2(b) evidences the pore blocking,
agent promoting development of textural features, the surface which is observed for bottle pores of rather narrow necks. The
2
2
2
-1
2 -1
area increases from 575 m g for sample Si to 613 m g for pore size distribution covers the range from 3 to 14 nm, with
sample Si-CO (Table 2). The use of CO instead of acetic acid no distinct maximum (Figure 6 B).
leads to an increase in surface area by nearly 80%. The textural The isotherm recorded for sample Si-Ac is type I(b) according
2
2
1
9
changes are particularly pronounced in the character of the to the recent IUPAC classification and is typical of materials
low-temperature nitrogen adsorption/desorption isotherm, having pore size distribution over a broader range including
Figure 6, 7.
wider micropores and possibly narrow mesopores (<~2.5 nm).
Poorly marked but present hysteresis loop covers practically
the whole range of pressure p/p , which confirms the
Table 2. The surface area and pore diameters in Al O and SiO
2
o
after calcination at 500 C.
3
2
0
occurrence of rather wide micropores and the capillary
condensation process taking place in very narrow mesopores.
The porous structures of samples Si and Si-Ac are much
different, sample Si, with no pH regulator, is a typical
mesoporous material of a wide pore size distribution, sample
Si-Ac, with acetic acid as a pH regulator, is a typical
Sample
BET surface
area
Average pore
diamiter*
[nm]
Cumulative pore
volume*
2
-1
3 -1
[
m g ]
575
397
613
277
337
243
[cm g ]
0.758
0.035
0.303
0.509
0.465
0.425
Si
10.1
Si-Ac
2.9
microporous material. The use of CO as a pH regulator brings
2
Si-CO
Al
2
3.7
significant changes in the surface area and in the porous
structure of the material, which induce changes in the
isotherm shape to type IV(a) and in the hysteresis loop shape –
type H2(a). Such an isotherm is typical of mesoporous
materials having the system of pores for which network effects
are important. The very steep desorption branch, which is a
characteristic feature of H2(a) loops, can be attributed either
to pore-blocking/percolation in a narrow range of pore necks
or to cavitation-induced evaporation. Sample Si-CO2 is
characterized by very narrow pore size distribution, the
majority of them have diameter between 3 and 4 nm, the
5.8
Al-Ac
4.3
Al-CO2
5.41
*
calculated by BJH from the desorption branch of the isotherm
mean pore diameter is 3.7 nm. Thus, Si-CO sample, similarly
2
as Si, is mesoporous material, however, in contrast to Si, of
very narrow pore size distribution.
For SiO samples the presence and type of pH regulator used in
2
the synthesis has a great impact on the surface area and
structure of pores, while for Al O this impact is much weaker.
2
3
Introduction of acetic acid as a pH regulator causes an increase
-1
2
in the surface area of Al O3 from 277 to 337 m g
2
accompanied by a reduction in the mean pore size from 5.8 to
.3 nm (Table 2). The use of CO as a pH regulator in the
4
2
synthesis of Al-CO sample leads to a reduction of surface area
2
2
to 243 m g when compared to that of sample Al (Al O with
-1
2
3
2
-1
no pH regulator) of 277 m g . This small reduction in surface
area is also manifested in the course of nitrogen
adsorption/desorption isotherm and in the pore size
distribution curves (Figure 7 A, B). All isotherms are of type
IV(a) while the hysteresis loops are of type H2(b) typical of
mesoporous materials.
Fig. 6. Low temperature nitrogen adsorption isotherms and
o
pore size distribution of SiO
2
samples calcined at 500 C. Curve
1
– Si; curve 2 – Si-Ac; curve 3 – Si-CO .
2
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suggest that SiO gel is able to capture gas CO in its structure.
This effect can be useful in a number of applications, e.g. to
increase the heat absorption by silica glass or to increase
absorption of infrared radiation. The most important feature
and advantage of the proposed method is stimulation of a new
branch of sol-gel synthesis in which pH could be regulated or
one-pot carbonate systems could be obtained with using gas
2
CO . It was not our aim to obtain a specific porous structure
but we wanted to eliminate the organic solvents and acids
from the method of synthesis.
Conflicts of interest
The authors declare no conflict of interest.
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1
2
7
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Fig. 7. Low temperature nitrogen adsorption isotherms and
distribution of pores recorded for Al
2
O
3
samples calcined at
o
00 C. Curve 1 – Al; curve 2 – Al-Ac; curve 3 – Al-CO
9
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2
as a pH
1
1
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regulator in the synthesis of SiO leads to material of large
2
,
surface area and narrow pore distribution in the range of
mesopores, which cannot be achieved when using acetic acid
1
as a pH regulator or no pH regulator. The effect of CO
2
on
Al
The silica gel obtained in the presence of CO
Al obtained without a pH regulator than that obtained with
2
O
3
samples is less pronounced than for SiO
2
, but important. 14 X.R. Deng, Q.H. Zhang, X.Y. Lei, W. Yang, H.H. Hui, J. Wang
and J. Shen, J. Sol-Gel Sci. Technol., 2015, 73, 242-249.
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2
resembles more
1
1
1
1
2 3
O
the use of acetic acid.
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gels of 19 M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F.
8, 1-10.
It has been shown that the use of gas CO
2
as a pH regulator
1
permits effective synthesis of Al
2
O
3
and SiO
2
comparable or better textural parameter than those of the
samples obtained with acetic acid as a pH regulator. The use of
Rodriguez-Reinoso, J. Rouquerol and K.S.W. Sing, Pure. Appl.
Chem., 2015, 87, 1051-1069.
2
gas CO as a pH regulator permits synthesizing gels devoid of
the impurities that remain in the samples obtained with the
use of acetic acid and whose removal needs thermal treatment
at high temperatures. As follows from low-temperature
nitrogen adsorption studies, the proposed method of synthesis
significantly changes the textural parameters of SiO
2
which
from microporous becomes mesoporous. The FT-IR results
6
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