Nature of Catalytic Activity of Defective Calcium Hydroxyapatites

Article abstract of DOI:10.1023/A:1024690226430

The nature of the catalytic activity of defective calcium hydroxyapatites was studied. The activity of synthesized defective hydroxyapatites in vapor-phase dehydration of trimethylcarbinol, methylphenylcarbinol, α-hydroxyisobutyric acid, and in decomposition of 4,4-dimethyl-1, 3-dioxane into isoprene was analyzed.

Full text of DOI:10.1023/A:1024690226430

Russian Journal of Applied Chemistry, Vol. 76, No. 2, 2003, pp. 226 228. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 2, 2003,
pp. 234 236.
Original Russian Text Copyright
2003 by Dykman.
CATALYSIS
Nature of Catalytic Activity of Defective
Calcium Hydroxyapatites
A. S. Dykman
Vserossiiskii Nauchno-issledovatel’skii Institut Neftekhimicheskikh Protsessov Joint-Stock Company,
St. Petersburg, Russia
Received August 1, 2002
Abstract The nature of the catalytic activity of defective calcium hydroxyapatites was studied. The activ-
ity of synthesized defective hydroxyapatites in vapor-phase dehydration of trimethylcarbinol, methylphenyl-
carbinol, -hydroxyisobutyric acid, and in decomposition of 4,4-dimethyl-1,3-dioxane into isoprene was
analyzed.
Calcium phosphates, which have been described as
catalysts and supports for catalysts for hydration and
dehydration, vapor-phase hydrolysis, isomerization,
and alkylation [1, 2], were applied to industrial syn-
thesis of isoprene (in the second stage of decom-
position of 4,4-dimethyl-1,3-dioxane, DMD) [2 4].
We used in this process basic calcium phosphates
whose X-ray pattern is typical of stoichiometric cal-
cium hydroxyapatite Ca (PO ) (OH) (molar ratio
2-methyl butanol [10], and esters [11], and in other
thermocatalytic reactions [12] have been published.
Bett et al. [7], having generalized data on the com-
position and catalytic properties of DCHA, concluded
that their acidity and catalytic activity result from
the presence of protons in their structure. Protons in
DCHA are in the form of the hydrophosphate ions,
HPO2 [2, 3, 5, 6]. As a proof of the hypothesis, data
4
on dehydration of butanol-2 in the presence of a num-
ber of DCHA with varied degree of defectiveness
have been reported [10]. According to these data,
the activity of the catalysts grows with decreasing
CaO/P O ratio in a sample, i.e., with increasing
1
0
4 6
2
CaO/P O = 3.34), but differ from it in their composi-
2
5
tion: the CaO/P O ratio in these compounds varies
2
5
from 2.66 to 3.34. It has been shown in numerous
studies [2 7] that such calcium phosphates belong
to the series of defective calcium hydroxyapatites
2
5
amount of hydrophosphate ions in the structure of
hydroxyapatites [7, 13].
(
DCHA) with composition Ca10 xH (PO ) (OH)
or
x
4 6
2 x
Ca10 x (HPO ) (PO ) (OH)2 x (where 0
x
2).
4 x
4 6 x
However, it has been shown by means of IR spec-
troscopy [4] that, within the operation temperature
range of DCHA-based catalysts (250 500 C), hydro-
phosphate ions react with one another rather rapidly
(within several hours) to be converted into condensed
These compounds, which are derivatives of the
stoichiometric Ca (PO ) (OH) hydroxyapatite, have
1
0
4 6
2
been given their name because of the presence of a
calcium defect (calcium vacancy) in the structure. To
maintain the charge balance, one proton is incor-
porated into the lattice instead of each absent calcium
ion and one hydroxyde ion is removed [3, 6, 7].
(
n + 2)
phosphate ions P O
, with the catalyst retaining
n
3n + 1
its initial activity. Consequently, the acidity and cat-
alytic activity of DCHA cannot be directly determined
by the presence of HPO2 groups [4].
4
Commercial DCHA-based catalysts developed with
the author’s participation (conventionally designated
as KF-70, KBF, and KBF-76) were successively in-
troduced into industrial practice and have been used
at Russian synthetic-caoutchouc plants in the second
stage of isoprene synthesis from isobutylene and
formaldehyde.
It seems more correct to suggest that the acidity
and catalytic activity of DCHA are determined by
the presence of a film of phosphoric acid formed on
the catalyst surface in the reaction of condensed phos-
phates with water at elevated temperature. The hy-
drolytic decomposition of condensed phosphates can
be schematically represented by the reaction
The KF-70, KBF, and KBF-76 catalysts have
aroused considerable interest of researchers. Results
obtained in studying their catalytic activity in dehy-
dration of aliphatic alcohols [8], isoamyl alcohol [9],
n + 2)
(
n + 1)P O (
+ H O
3n + 1(solid) 2 (vapor)
n
(
n + 1)
(n + 2)Pn 1
O
+ 2H PO4 (vapor).
3n 2(solid) 3
1
070-4272/03/7602-0226 $25.00 2003 MAIK Nauka/Interperiodica

NATURE OF CATALYTIC ACTIVITY OF DEFECTIVE CALCIUM HYDROXYAPATITES
227
It seems that the greater the amount of phosphoric
acid in the film, the greater the amount of acid re-
moved on treating the catalyst with water vapor.
Table 1. Physicochemical properties of synthesized DCHA
samples with varied degree of defectiveness
Sample
no.
CaO/P O ,
mol/mol
Bulk density,
2
5
2
S , m g
sp
1
The aim of this study was to reveal the relationship
between the composition of defective calcium hy-
droxyapatites, the amount of phosphoric acid they
evolve, and their catalytic activity in dehydration of
alcohols and acids and in decomposition of DMD.
3
g cm
1
2
3
4
2.72
3.04
3.26
3.34
86
85
71
79
0.62
0.64
0.68
0.69
EXPERIMENTAL
As objects of study served DCHA samples with
varied defectiveness. The physicochemical parameters
of these compounds are listed in Table 1. According
to X-ray diffraction analysis, the samples have phase
composition of hydroxyapatite.
Table 2. Conversion of trimethylcarbinol, methylphenyl-
carbinol, -hydroxy isobutyric acid, and 4,4-dimethyl-1,3-
dioxane at various CaO/P O5 ratios in DCHA (weight
2
ratios: H O : trimethylcarbinol = 1 : 1, H O : methylphenyl-
2
2
carbinol = 1 : 1, H O : -hydroxyisobutyric acid = 4 : 1,
2
H O : DMD = 2 : 1)
2
To determine the amount of phosphoric acid re-
leased by a catalyst, we put the samples in a quartz
reactor, which was placed, in turn, in an electric fur-
nace. Distilled water was delivered to the reactor
through an evaporator, using a controlled-volume
pump. The water vapor released from the reactor was
condensed in a condenser. We determined the content
of phosphoric acid in the condensate by the procedure
described in [14]. The method used allows phos-
phorus(V) to be determined only as orthophosphate
ions, and, therefore, to convert condensed phosphates
into orthophosphates, we boiled the samples before
measurements.
Conversion (%) at indicated
volumetric rate of raw
CaO/P O ,
mol/mol
2
5
1
T,
C
material supply, h
1
2
4
6
8
Trimethylcarbinol
3
00
2.72
99.7 97.0 91.2 63.0
93.0 74.0 68.5 44.6
3
3
3
.04
.26
.34
30.9 19.8 16.4
7.6 9.8 4.4
9.7
1.1
Methylphenylcarbinol
We studied the activity of DCHA as catalysts in
dehydration of trimethylcarbinol, methylphenylcar-
binol, and -hydroxyisobutyric acid and in decom-
position of DMD. We tested the catalysts on the in-
stallation described above. The raw material was de-
livered into the reactor in a mixture with water, the
duration of an experiment was 2 h. The catalyzates
were analyzed by gas-liquid chromatography.
2
50
2.72
99.5
60.3
21.6
6.3
3
3
3
.04
.26
.34
-Hydroxyisobutyric acid
2
80
20
2.72
97.7
54.9
15.3
3
3
.04
.26
The dependence of the amount of H PO released
3
4
2
by 1 m of the DCHA surface on the CaO/P O ratio
4,4-Dimethyl-1,3-dioxane
2
5
is given in the figure. As the CaO/P O ratio de-
2
5
3
2.72
98.1 78.9 52.2
53.3 32.1 15.2
30.9 23.2
28.3 22.5
32.0
10.4
3.9
creases, i.e., as the content of condensed phosphates
increases, the amount of released phosphoric acid
grows. The greatest amount of H PO was observed
3
3
3
.04
.26
.34
8.7
8.5
3
4
3.3
for the sample with the maximum defectiveness (sam-
ple no. 1). Stoichiometric hydroxyapatite (sample
no. 4) hardly releases any phosphoric acid.
It was found that defective calcium hydroxyapatite
Data on the catalytic activity of DCHA samples in
dehydration of trimethylcarbinol, methylphenylcar-
binol, and -hydroxyisobutyric acid (conversion of
with the highest content of condensed phosphates (sam-
ple no. 1) is the most active in the reactions under
study and a sample of stoichiometric calcium hydroxy-
apatite has the lowest activity. Thus, the activity of
DCHA in dehydration of alcohols and acids and in
-hydroxyisobutyric acid into methacrylic acid), and
in decomposition of DMD are listed in Table 2.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 2 2003

228
DYKMAN
(4) The catalytic activity of calcium hydroxyap-
atites in dehydration of trimethylcarbinol, methyl-
phenylcarbinol, and -hydroxyisobutyric acid and
decomposition of 4,4-dimethyl-1,3-dioxane grows
with increasing amount of phosphoric acid released
by the catalysts.
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1
2
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A vs. CaO/P O molar ratio. Temperature 300 C. Water
2
5
supply (vol/vol of catalyst per 1 h): (1) 16, (2) 8, (3) 4,
and (4) 2.
3
4
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decomposition of DMD grows with increasing amount
of released phosphoric acid, which seems to be pres-
ent in the form of a film on the catalyst surface and is
removed when the catalyst is treated with water vapor.
To elucidate the part played by DCHA in the cat-
alytic reactions under study, we carried out exper-
iments on dehydration of trimethylcarbinol and de-
composition of DMD on quartz, to which phosphoric
acid was delivered during the experiment. In this case,
we hardly observed any dehydration of trimethyl-
carbinol or decomposition of DMD. This suggests that
phosphoric acid released by DCHA is present on its
surface as a relatively firmly adhering film, which
determines the catalytic activity of calcium phosphates
belonging to the hydroxyapatite structure.
5
6
7
. Bett, J.A.S., Christner, L.G., and Hall, W.K., J. Am.
Chem. Soc., 1967, vol. 89, pp. 5535 5541.
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tekhnicheskikh izdelii, NTRS, Moscow, 1989,
no. 2, pp. 3 5.
CONCLUSIONS
(1) A set of calcium phosphate catalysts with var-
1
0. Mazaeva, V.A., Slivinskii, E.V., Bol’shakov, D.A.,
and Loktev, S.M., Neftekhimiya, 1990, vol. 30, no. 3,
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ious molar ratios CaO/P O has been synthesized.
2
5
These catalysts are defective calcium hydroxyapa-
tites with composition Ca10 (HPO ) (PO ) x(OH)
x
4 x
4 6
2
x
11. Smagin, V.M., Popov, S.V., Kamneva, S.A., and
Strel’chik, B.S., Kinetika-4: Vsesoyuznaya konferen-
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Kinetics of Heterogeneous Catalytic Reactions), Yaro-
slavl, 1988, Moscow, 1988, pp. 169 170.
(0
x
2).
2) Hydrophosphate ions HPO2 cannot be directly
(
4
responsible for the catalytic activity of defective cal-
cium hydroxyapatites, since they are converted to
(
n + 2)
condensed phosphate ions P O
at the working
12. Golovko, L.V. and Kashina, S.V., Proizv. Ispol’z.
Elastomerov, 1990, no. 4, pp. 9 11.
n
3n + 1
temperatures of the catalyst (250 500 C).
1
3. Bett, J.A.S. and Hall, W.K., J. Catal., 1968, vol. 10,
(
3) The catalytic activity of defective calcium hy-
pp. 105 112.
droxyapatites is determined by the release of phos-
phoric acid formed in the reaction of condensed phos-
phates with water. The acid is presumably present on
the catalyst surface in the form of a film.
1
4. Shafran, I.G., Pavlova, M.V., and Titova, S.A., Khi-
micheskie reaktivy i preparaty: Trudy IREA (Chemical
Reagents and Preparations: Proc. IREA), Issue 28,
Moscow, 1966.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 2 2003