Synthesis, Structural Characteristics, and Adsorption Properties of Macroporous Alumina

Article abstract of DOI:10.1134/S0036024419080144

Abstract: Macroporous alumina is synthesized via precipitation with sulfuric acid from a sodium aluminate solution. It is established that introducing a coagulating agent (sodium chloride) preventing macrogel formation makes it possible to obtain highly porous samples of alumina with specific surface areas of ~100 m²/g, a large pore volume of 1.6–1.7 cm³/g, and an average pore diameter of 50–75 nm. According to the adsorption isotherms and the pH dependence of hemoglobin adsorption that such samples have a high capacity for protein macromolecules. The maximum adsorption of hemoglobin is ~500 mg/g, and is observed at pH 6–7. Adsorption falls substantially when the pH is reduced to 4 or increased to 9, and it is close to zero at?pH?11.

Full text of DOI:10.1134/S0036024419080144

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2019, Vol. 93, No. 8, pp. 1589–1591. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Zhurnal Fizicheskoi Khimii, 2019, Vol. 93, No. 8, pp. 1249–1251.
PHYSICAL CHEMISTRY
OF SURFACE PHENOMENA
Synthesis, Structural Characteristics, and Adsorption Properties
of Macroporous Alumina
T. D. Khokhlovaa,*
a Department of Chemistry, Moscow State University, Moscow, 119991 Russia
*e-mail: khokhlova939@gmail.com
Received November 13, 2018; revised November 13, 2018; accepted December 12, 2018
Abstract—Macroporous alumina is synthesized via precipitation with sulfuric acid from a sodium aluminate
solution. It is established that introducing a coagulating agent (sodium chloride) preventing macrogel forma-
tion makes it possible to obtain highly porous samples of alumina with specific surface areas of ~100 m2/g, a
large pore volume of 1.6–1.7 cm3/g, and an average pore diameter of 50–75 nm. According to the adsorption
isotherms and the pH dependence of hemoglobin adsorption that such samples have a high capacity for pro-
tein macromolecules. The maximum adsorption of hemoglobin is ~500 mg/g, and is observed at pH 6–7.
Adsorption falls substantially when the pH is reduced to 4 or increased to 9, and it is close to zero at pH 11.
Keywords: macroporous alumina, synthesis, structure, hemoglobin adsorption
DOI: 10.1134/S0036024419080144
INTRODUCTION
In this work, macroporous alumina was synthe-
sized via precipitation with acid from a sodium alumi-
nate solution (in analogy with the synthesis of silica
from sodium silicate [11]). The effect introducing
sodium chloride into the reaction mixture had on the
structural characteristics of alumina was considered.
The adsorption properties of alumina are given for an
example of the adsorption isotherms and pH depen-
dence of protein (hemoglobin) adsorption.
Oxide macroporous adsorbents with a high specific
surface areas and large pore volumes are used for con-
centrating biopolymers and their immobilization,
chromatography, purification, and removal from solu-
tions [1–4]. The synthesis of alumina via the sol-gel
approach allows us to obtain adsorbents with relatively
small average pore diameters (D) of 2 to 20 nm [5–7].
Such diameters of pores are not suitable for the protein
adsorption. Macroporous alumina with large pore vol-
umes (V) of up to 4 cm3/g and specific surface areas
(S) of 200–500 m2/g can be obtained via the precipi-
tation of aluminum hydroxide by halohydrins from
sodium aluminate solutions [8]. The macroporous
alumina obtained via the hydrothermal treatment of
γ-Al2O3 was used in [9] to immobilize enzymes.
EXPERIMENTAL
Synthesis of Alumina
Aluminum chloride (AlCl3 · 6H2O), sodium
hydroxide, sulfuric acid, and sodium chloride were
used to obtain alumina. Sodium chloride (5 or 12%)
and a solution of sodium hydroxide in amounts equiv-
alent to sodium aluminate were added to an aqueous
solution of aluminum chloride. The precipitation of
aluminum hydroxide from the prepared solution of
sodium aluminate was conducted at 23 or 60°C by
addtion of sulfuric acid solution until pH 7. For com-
parison, the same synthesis was conducted at a tem-
perature of 23°C without addition of sodium chloride.
The precipitated aluminum hydroxide was aged for
one day and then rinsed until no more sulfate or chlo-
ride ions were detected in filtrates. Finally, the sam-
In [10], Iler showed that introducing an electrolyte
when synthesizing silica from sodium silicate allowed
silica powders to be obtained with pore volumes much
greater than those of silica gel. A highly porous adsor-
bent with a pore volume (V) of around 2–4 cm3/g was
obtained via the precipitation of silica from sodium
silicate by sulfuric acid in the presence of sodium chlo-
ride [11]. The adsorption of proteins (hemoglobin and
gamma globulin) on such high-porous silica (S =
580 m2/g, V = 2.1 cm3/g, and D = 14 nm) was consid-
erably greater than on industrial silica gel (S =
600 m2/g, V = 0.8 cm3/g, and D = 5.5 nm) [12]. Mag-
netic composites of Fe2O3/SiO2 containing highly
porous silica precipitated with sodium chloride had a
strong capacity for cytochrome c and hemoglobin [13]. ples were dried at 160°C for 6 h.
1589

1590
KHOKHLOVA
pore volume (V, cm3/g), determined from the conden-
sation of water; specific surfaces (S, m2/g) determined
from the desorption of heat; and average pore diame-
ters (D, nm), calculated as D = 4V/S. Sample 1, syn-
thesized without adding sodium chloride to the reac-
tion mixture, had a relatively small volume of pores:
0.42 cm3/g. Upon precipitation in the presence of
sodium chloride, the pore size of aluminum oxide
grew by ~300%. The specific surface areas of the
samples differed slightly, so the average pore diame-
ter grew. We can see from the three samples obtained
with addition of sodium chloride that changing its
content from 5 to 12% and the temperature of the
reaction mixture from 23 to 60°C had a negligible
effect on the structural characteristics of alumina.
We assume that introducing the electrolyte (sodium
chloride) led to the coagulation of aluminum hydrox-
ide as microgel particles without the formation of a
macrogel. During drying, the particles of microgel
were subjected to less contraction than the macrogel.
As a result, aluminum oxide with larger volume and
pore diameters formed.
Table 1. Conditions of synthesis and structural characteris-
tics of Al2O3 samples: S, V, D, and Am at pH 7.0
S,
V,
Am,
mg/g
Synthesis
conditions
D,
nm
Sample
1
m2/g cm3/g
Without NaCl
124
0.42
13
35
23°C
2
3
4
5% NaCl 23°C
5% NaCl 60°C
12% NaCl 23°C
87
115
130
1.64
1.71
1.58
75
59
48
—
380
500
Determining Specific Surface Area and Pore Volume
The specific surface area of the synthesized sam-
ples was measured via the thermal desorption of nitro-
gen [14]. The volume of pores was determined from
the kinetics of saturated water vapor condensation in a
desiccator.
Hemoglobin Adsorption
Bovine hemoglobin (Sigma) was used in this work.
The isotherms of hemoglobin adsorption were mea-
sured in a phosphate buffer solution (0.01 M) contain-
ing NaCl (0.15 M), and the pH of the solution was 4–
11. Samples weighing 10 mg were kept in 5 mL of
hemoglobin solution at a temperature of 20°C for 2–
4 days with periodic stirring until equilibrium was
reached. The concentration of protein in the solution
was determined using a KFK-3 photometer at a wave-
length of 370 nm.
Figure 1 presents the isotherms of hemoglobin
adsorption (pH 7) obtained for samples 1 and 3 syn-
thesized at 60°C without and with 5% of NaCl,
respectively. Table 1 gives the values of maximum
adsorption (Am) of hemoglobin for three samples: Am is
380 and 500 mg/g for samples 3 and 4, respectively,
synthesized with sodium chloride. This is one order of
magnitude higher than for sample 1 obtained without
adding NaCl (Am = 35 mg/g). Raising the average
diameter of alumina pores from 13 to ~50–60 nm by
adding NaCl when the specific surfaces differ slightly
thus increases the availability of the surface for the
adsorption of hemoglobin globules with hydrody-
namic diameter Dh of 6.2 nm [15].
RESULTS AND DISCUSSION
Table 1 presents the structural characteristics of
alumina samples obtained under different conditions:
200
250
2
2
3
160
200
1
120
80
150
100
4
40
1
50
5
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
C, mg/mL
0
0.1
0.2
0.3
0.4
0.5
0.6
C, mg/mL
Fig. 2. Isotherms of hemoglobin adsorption on sample 4 of
alumina at different pH values: (1) 4, (2) 6, (3) 7, (4) 9,
(5) 11. Hemoglobin was dissolved in a phosphate buffer
solution (0.01 M) containing NaCl (0.15 M).
Fig. 1. Isotherms of hemoglobin adsorption on samples 1
and 3 of alumina (curves 1 and 2 respectively). Hemoglo-
bin was dissolved in a phosphate buffer solution (0.01 M)
containing NaCl (0.15 M); pH 7.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 93
No. 8
2019

SYNTHESIS, STRUCTURAL CHARACTERISTICS, AND ADSORPTION PROPERTIES
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Translated by N. Saetova
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 93 No. 8 2019