Chemical modification of alumina and silica with alkylphosphonic acids and their esters

Article abstract of DOI:10.1023/A:1013067409225

Alumina and silica were modified with alkylphosphonic acids and diethyl butylphosphonate. The boundaries of hydrolytic stability of the obtained surface-modified materials were determined.

Full text of DOI:10.1023/A:1013067409225

«
P. G. Mingalyov, M. V. Buchnev, and G. V. Lisichkin
Russian Chemical Bulletin, International Edition, Vol. 50, No. 9, pp. 1693—1695, September, 2001
1693
Brief Communications
Chemical modification of alumina and silica with alkylphosphonic
acids and their esters
Department of Chemistry, M. V. Lomonosov Moscow State University,
Leninskie Gory, 119899 Moscow, Russian Federation.
Fax: +7 (095) 939 0126. E-mail: glo@petrol.chem.msu.ru
Alumina and silica were modified with alkylphosphonic acids and diethyl butylphosphonate.
The boundaries of hydrolytic stability of the obtained surface-modified materials were
determined.
Key words: alumina, silica, alkylphosphonic acids, modification, hydrolysis.
Chemical modification of the surface of oxide mate-
rials is a widespread technique for synthesis of heteroge-
neous metallocomplex catalysts and efficient and selec-
tive sorbents.1 Porous silica is the most widely used for
modification, and alumina is used more rarely. Organo-
silicon compounds are used most often as modifiers.
However, they do not provide, in many cases, a high
surface density of grafted functional groups. Therefore,
it is of interest to test other classes of organoelement
compounds as modifiers. The main criteria of selection
of new types of modifiers are selectivity of the reaction
between active groups of the support surface and modi-
fier and the hydrolytic, thermal stability of the bond
between them, and availability of the modifier.
with phosphorus trichloride followed by the hydrolysis of the
products.6 Diethyl ester of n-butylphosphonic acid was synthe-
sized by the alkylation of sodium diethyl phosphite with n-butyl
bromide. The ester obtained was hydrolyzed to n-butyl-
phosphonic acid by boiling with 10% HCl until a homogeneous
,2
liquid was obtained.
Alumina (specific surface area 210 m2 g–1, average pore
diameter 10—12 nm) was modified from a boiling toluene
solution of the corresponding phosphonic acid with azeotropic
distillation off of water. The amount of the modifier was large
enough to ensure a grafting density of 10 groups per nm2 (for
all acids used and diethyl ester of n-butylphosphonic acid) and
4
groups per nm 2 (for all acids used). The reaction was
performed for 2 days. The samples obtained were successively
washed with toluene, 1,4-dioxane, a 1,4-dioxane—water (1 : 1)
mixture, and distilled water and then once more with the
aqueous-dioxane mixture, dioxane, and hexane. Then the
samples were dried on a glass filter at ∼20 °C. Silica was
It has recently3—5 been shown that alkylphosphonic
acids can be used as modifiers of the alumina surface.
In this work, we studied the hydrolytic stability of
alkylphosphonic layers of the surface of modified alu-
mina and silica.
2
–1
modified (Silochrom Ñ-120, specific surface area 110 m
g
,
average pore diameter 40 nm) using a similar procedure, and
the amount of the modifier was large enough to provide a
grafting density of 4 groups per nm2.
The hydrolytic stability of the samples obtained were stud-
Experimental
ied using solutions of H SO or NaOH in an aqueous-dioxane
2
4
(
2 : 1) mixture with different pH values (2, 3, 4, 5.5 (distilled
tert-Butylphosphonic and isopropylphosphonic acids were
water), and 12). A weighted sample (0.05 g) was placed into a
prepared by the reaction of the corresponding alkyl chlorides
solution (10 mL) and left for 48 h at ∼20 °C with periodical
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1613—1615, September, 2001.
066-5285/01/5009-1693 $25.00 ©ꢀ2001 Plenum Publishing Corporation
1

1
694
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001
Mingalyov et al.
Table 1. Data on hydrolytic stability of alumina modified with
Table 2. Pore volume of samples of modified alumina
isopropylphosphonic acid
Modifier
A
B
V
Modifier
pH
C (%)
ρ
/nm–2
/number
of groups
per 1 nm
/mL g–1
/
number of groups
per 1 nm2
2
t
t
Bu PO(OH)
Without hydrolysis
3.3
1.6
2.2
2.4
2.8
1.4
3.3
2.4
2.5
2.6
3.0
2.2
4.9
1.7
2.9
4.4
4.5
3.1
2.1
1.0
1.4
1.5
1.8
0.8
2.9
2.0
2.1
2.2
2.6
1.8
3.3
1.1
1.8
2.9
3.0
2.0
Bu PO H
4
10
4
10
4
2.1
2.7
2.9
3.8
3.3
9.1
0.9
0.45
2
2
2
2
2
2
3
2
2
t
t
Bu PO(OH)
2
3
4
5.5
Bu PO H
3
t
i
Bu PO(OH)
Pr PO H
0.48
0.44
0.35
0.25
0.56
0.58
3
2
t
i
Bu PO(OH)
Pr PO H
3
2
t
n
Bu PO(OH)
Bu PO H
3
2
2
t
n
Bu PO(OH)
12
Bu PO H
10
10
3
i
n
Pr PO(OH)
Pr PO(OH)
Pr PO(OH)
Pr PO(OH)
Without hydrolysis
Bu PO(OEt)
Initial
2
2
i
2
3
4
5.5
12
2
i
2
2
2
2
i
Notes. A is the amount of the modifier large enough to ensure a
high grafting density; B is the obtained grafting density.
i
Pr PO(OH)
i
Pr PO(OH)
n
Bu PO(OH) Without hydrolysis
2
n
into a fine dust (the size of most particles did not
exceed 2 µ). Then, in the case of a great excess of the
modifier, the obtained grafting density for isopropyl-
phosphonic and, especially, n-butylphosphonic acids is
very high and reaches 9 groups per nm2 (Table 2). At
the same time, the acids used (tert-butylphosphonic and
n-butylphosphonic) are not prone to polymerization
under the modification conditions.
These facts force us to assume that phosphonic acids
react with the alumina surface to form aluminum
phosphonates that are insoluble in standard solutions.
However, in order to form the phosphonate polylayer,
the diffusion of acid through the already formed salt
layer is needed. Evidently, the more bulky radical has
the acid, the more hindered is the diffusion. Therefore,
the maximum grafting density decreases on going from
n-butylphosphonic acid to isopropylphosphonic acid and
further to tert-butylphosphonic acid. It can be assumed
that, in the latter case, the highest grafting density
virtually corresponds to the monolayer coating.
Bu PO(OH)
2
3
4
5.5
12
2
2
2
2
2
n
Bu PO(OH)
n
Bu PO(OH)
n
Bu PO(OH)
n
Bu PO(OH)
stirring. Then the sample was washed with distilled water to the
neutral reaction in washing waters and dried at ∼20 °C on a
glass filter.
The total volume of the pores was determined by measuring
the increase in the weight of the sample after the pores were
filled with benzene. The oxide samples were stored in a desic-
cator filled with benzene vapor until a constant weight was
achieved. The results are presented in Table 1.
2
The grafting density ρ (nm ) was calculated using the
2
following formula :
5
ρ = (6•10 p )/[(1200n – M´p )S ],
Ñ
Ñ
Ñ
where pC is the mass percentage of carbon in the studied
sample, nC is the number of C atoms in the grafted part of the
modifier molecule, S is the specific surface area of the support
The destruction of the alumina surface should be
accompanied by a strong decrease in the pore volume of
the support. The data on the pore volume of the modi-
fied samples presented in table 2 confirm our assump-
tion about the formation of the polylayer of Al
phosphonates.
2
–1
(
m
g
), and M´ is the reduced molecular weight of the
2
modifier calculated by the formula
M´ = M – nMx + 17n – 18F,
where M is the molecular weight of the modifier, n is the
number of groups leaving the modifier molecule, Mx is the
molecular weight of the leaving group, and F is the average
number of bonds formed by the modifier molecule with the
The modification of alumina with alkyl esters of
phosphonic acids must not result in similar destruction.
We modified alumina with an excess of diethyl n-butyl-
phosphonate. The modification was successful (the ob-
tained grafting density was 0.9 nm–2), however, the
granulometric composition of the support and its pore
volume (see Table 1) remained almost unchanged.
Study of the hydrolytic stability of modified alumina
samples. The hydrolytic stability of the materials ob-
tained is a very important characteristic. Therefore, we
studied it for the alumina samples treated with the
phosphonic acids at different pH values. The results
obtained for the modified alumina are presented in
Tables 2 and 3. These data show that the samples
obtained are stable in a neutral medium, however, their
2
surface (in our case, we accepted F = 1.5 ).
Results and Discussion
Modification of alumina with phosphonic acids. The
modification reaction occurred successfully (see Table 1).
All obtained alumina samples (modified by both
phosphonic acids and diethyl butylphosphonate) are not
wetted with water, which indicates hydrophobicity of
their surface preventing water incorporation into sup-
port pores. Relatively large particles of alumina
(
0.2—0.3 mm) taken for modification were transformed

Chemical modification of alumina and silica
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001 1695
Table 3. Data on hydrolytic stability of silica modified with
tert-butylphosphonic acid
Thus, in this work, we showed a possibility of the
chemical modification of alumina and silica with
alkylphosphonic acids and their esters.
pH
Ñ (%)
ρ
2
/
number of groups per 1 nm
References
Without hydrolysis
2
4
1.0
0.5
0.5
0.8
0.5
1.5
0.7
0.7
1.3
0.8
1
2
. R. K. Iler, The Chemistry of Silica: Solubility, Polymerization,
Colloid and Surface Properties and Biochemistry of Silica,
J. Wiley, New York—Chichester—Brisbane—Toronto, 1979].
. Modifitsirovannye kremnezemy v sorbtsii, katalize i khromato-
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5
1
.5
2
stability is lower in acidic and alkaline media. It can be
assumed that the support is dissolved in these media.
However, in media with the neutral reaction phosphonic
acids can successfully be used as modifiers for alumina.
Modification of silica with tert-butylphosphonic acid.
We attempted to modify silica with tert-butylphosphonic
acid (see Table 3). It is seen that the modification was
(
in Russian).
3
. P. Adler and J. Startmann, FATPEC Congr., 1998, 24th,
vol. B, 1233.
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successful, the grafting density is reasonably high
2
(
1.5 groups per nm ), and the grafted layer is stable in a
neutral medium. The granulometric composition of silica
remained unchanged after modification.
Received November 14, 2000;
in revised form March 13, 2001