Synthesis of Monetite from Calcium Hydroxyapatite and Monocalcium Phosphate Monohydrate under Mechanical Activation Conditions

Article abstract of DOI:10.1134/S0036023619090171

Abstract: A powder of monetite СаНРО₄ with a particle size of 100–300 nm was synthesized from monocalcium phosphate monohydrate Ca(H₂PO₄)₂ ? H₂O and calcium hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ in an acetone medium upon mechanical activation in a planetary mill. According to X-ray powder diffraction data, after heat treatment in the range 900–1100°С, the phase composition of the samples was represented by calcium β-pyrophosphate β-Ca₂P₂O₇. The synthesized powder can be used for producing resorbable calcium phosphate ceramic materials.

Full text of DOI:10.1134/S0036023619090171

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2019, Vol. 64, No. 9, pp. 1088–1094. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Zhurnal Neorganicheskoi Khimii, 2019, Vol. 64, No. 9, pp. 916–922.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Synthesis of Monetite from Calcium Hydroxyapatite
and Monocalcium Phosphate Monohydrate
under Mechanical Activation Conditions
T. V. Safronovaa, *, I. S. Sadilova, K. V. Chaikuna,
T. B. Shatalovaa, and Ya. Yu. Filippova
aMoscow State University, Moscow, 119991 Russia
*e-mail: t3470641@yandex.ru
Received January 11, 2019; revised February 17, 2019; accepted March 15, 2019
Abstract—A powder of monetite СаНРО4 with a particle size of 100–300 nm was synthesized from monocal-
cium phosphate monohydrate Ca(H2PO4)2 ⋅ H2O and calcium hydroxyapatite Ca10(PO4)6(OH)2 in an ace-
tone medium upon mechanical activation in a planetary mill. According to X-ray powder diffraction data,
after heat treatment in the range 900–1100°С, the phase composition of the samples was represented by cal-
cium β-pyrophosphate β-Ca2P2O7. The synthesized powder can be used for producing resorbable calcium
phosphate ceramic materials.
Keywords: powder, phase transformations, calcium pyrophosphate, ceramics
DOI: 10.1134/S0036023619090171
Calcium phosphates are used as nutritional supple- it is a nonresorbable calcium phosphate, known as the
ments [1] and catalysts for some chemical reactions [2, inorganic component of bone tissue [18, 19]. For the
3], in agriculture as fertilizers [4], as well as for creating treatment of bone defects, materials based on tricalcium
materials with unique properties. Materials with lumi- phosphate Са3(РО4)2 with the molar ratio Ca/P = 1.5
nescent properties based on calcium pyrophosphate
Са2Р2О7 are known [5, 6]. Research is underway on
are widely used [20]. Researchers also pay attention to
calcium phosphates with Са/Р = 1 because of their
the development and application of biocompatible inherent biocompatibility and higher resorbability.
materials based on various calcium phosphates, includ- The ratio Са/Р = 1 is inherent to the following calcium
ing calcium hydroxyapatite Ca10(PO4)6(OH)2, trical- phosphates: Са2Р2О7, Са2Р2О7 ⋅ хН2О, СаНРО4, and
cium phosphate Са3(РО4)2, calcium pyrophosphate СаНРО4 ⋅ 2Н2О. High-temperature calcium pyro-
Са2Р2О7, octacalcium phosphate Са8(НРО4)2(РО4)4 ⋅ phosphate in the form of γ- and β-Са2Р2О7 can be
5Н2О, brushite СаНРО4 ⋅ 2Н2О, monetite СаНРО4, produced through thermal conversion of hydrated
and/or hydrogen calcium phosphates, as well as
through heterogeneous reactions in powder mixtures,
including compounds with the Са/Р ratio larger and
smaller than 1, for example, by reactions (1) [17], (2)
[21], or (3) [22]:
and hydrated calcium pyrophosphate Са2Р2О7 ⋅ хН2О,
for the treatment of bone defects [7–9].
High-quality calcium phosphate powders are syn-
thesized through chemical deposition, thermal con-
version, solid-state procedure [10]. In synthesis, addi-
tional treatments, such as ultrasonic and hydrother-
mal treatment [11] and mechanical activation [12], are
used. Synthesis of calcium phosphates is carried out in
various media, including those mimicking the ionic
composition of blood plasma [13]. To produce biocom-
patible composites, calcium phosphate powders syn-
thesized from mixed-anion solutions are used [14–17].
(1)
(2)
2СаНРО4· 2Н2О = Са2Р2О7 + 5Н2О,
Са2Р2О7 ⋅ xН2О = Са2Р2О7 + xН2О,
2СаСО3 + 2(NH4)2HPO4
= Са2Р2О7 + 4NH3 + 2Н2О+ 2CO2.
(3)
Synthesis of hydrated hydrogen calcium phos-
phates underlies the production of brushite/monetite
cement [23]. The synthesis of brushite or monetite
Regenerative treatment methods require resorb-
able materials. The solubility of biocompatible cal-
cium phosphates increases with a decrease in the Ca/P
requires the simultaneous presence of Ca2+ (from a
–
molar
ratio.
For
calcium
hydroxyapatite
basic compound) and
(from an acidic com-
H2PO4
Ca10(PO4)6(OH)2, the molar ratio is Ca/P = 1.67, and
pound) in the reaction zone:
1088

SYNTHESIS OF MONETITE
1089
–
+
Ca2+ + H2PO4 + 2H2O → CaHPO4 ⋅ 2H2O + H .
mill. The duration of mechanical activation at a revo-
lution rate of 600 rpm was 20 min. After the treatment
in the planetary mill, the powder was dried in air at
room temperature for 2 h. After drying, the powders were
passed through a sieve with a mesh size of 200 μm. From
the obtained powders, compact powder pellets 12 mm
in diameter and 2–3 mm in height were made at a
pressing pressure of 100 MPa without the use of a tem-
porary technological binder using a Carver Laboratory
Press model c hand press (United States). The result-
ing pellets were sintered in a furnace at different tem-
peratures in the range 900–1100°C (heating rate
5 K/min, holding at a given temperature for 2 h, fur-
nace cooling).
(4)
Alkaline calcium sources—tricalcium phosphate
Са3(РО4)2, calcium hydroxyapatite Ca10(PO4)6(OH)2,
tetracalcium phosphate Са4(РО4)2О, calcium oxide
СаО, and calcium hydroxide Са(ОН)2—as well as
acidic components—phosphoric acid Н3РО4, mono-
calcium phosphate monohydrate Ca(H2PO4)2 ⋅ H2O,
sulfuric acid H2SO4, pyrophosphoric acid H4P2O7,
and citric acid C3H5O(COOH)3—are presented in
review [23]. The choice of components for synthesis
calcium phosphate cement, which is intended to be
used in situ in non-invasive methods for compensa-
tion of bone defects, takes into account the nature of
the resulting by-product of the reaction. The use of
most of these components taken in any combination
leads to the formation of water as a reaction by-prod-
uct, which is nontoxic for body tissues.
Linear shrinkage and geometric density of ceramic
samples were determined by measuring their weight
and dimensions (with an accuracy of 0.05 mm)
before and after sintering.
X-ray powder diffraction (XRD) analysis of the
initial powders, as-synthesized powder mixtures after
mechanical activation, and the samples after sintering
was carried out on a Rigaku D/Max-2500 diffractom-
eter with a rotating anode (CuKα radiation). For qual-
itative phase analysis, the ICDD PDF2 database [24]
was used.
Thus, brushite CaHPO4 ⋅ 2H2O or monetite СаНРО4
can be synthesized by reacting solutions of corre-
sponding salts or in pastes in the course of formation
of cement stone. The synthesis of hydrogen calcium
orthophosphate with the molar ratio Са/Р = 1 (mon-
etite СаНРО4) under mechanical activation condi-
tions has not been described to date, although this
method affords active powders as a feedstock for pro-
duction of ceramics or composites with a polymeric
matrix.
The present work focuses on the synthesis of cal-
cium phosphate with the molar ratio Са/Р = 1 (mon-
etite СаНРО4) from poorly soluble compounds—an
acidic calcium salt (monocalcium phosphate mono-
hydrate Ca(H2PO4)2 ⋅ H2O) and a basic calcium salt
(calcium hydroxyapatite Ca10(PO4)6(OH)2)—under
mechanical activation conditions. The use of this pair
of precursors enables the production of the powder
free of toxic by-products.
Simultaneous thermal analysis (TA) was carried
out on a NETZSCH STA 409 PC Luxx thermal ana-
lyzer at a heating rate of 10 K/min. The sample weight
was 10 mg. The composition of the gas phase that
forms upon the decomposition of the samples was
studied using a NETZSCH QMS 403C Aëolos quad-
rupole mass spectrometer coupled with a NETZSCH
STA 409 PC Luxx thermal analyzer. Mass spectra
(MS) were recorded for mass numbers 18 and 17
(Н О+ and ОН+), 44 ( +).
CO2
2
The microstructure of the samples was studied by
scanning electron microscopy on a Carl Zeiss LEO
SUPRA 50VP electron microscope (field emission
source); secondary electron images were acquired at
an accelerating voltage of 3–20 kV (SE2 detector).
A layer of chromium was deposited onto the surface of
the samples (up to 10 nm).
EXPERIMENTAL
The amounts of initial salts for the synthesis were
determined by reaction (5) leading to calcium phos-
phate with Са/Р = 1:
4Ca(H2PO4)2 ⋅ H2O
+ Ca10(PO4)6(OH)2 = 14СаНРО4+ 6Н2О.
RESULTS AND DISCUSSION
(5)
Figure 1 shows the X-ray diffraction patterns of the
initial reagents and synthesized monetite СаНРО4,
which indicate that after mechanical activation of the
powder mixture in acetone for 20 min, all initial
reagent were exhausted to form monetite СаНРО4
(card PDF 9-80). Indeed, reaction (5) shows that the
water amount in the reaction of monocalcium phos-
phate monohydrate Ca(H2PO4)2 ⋅ H2O (card PDF 9-
347) and hydroxyapatite Ca10(PO4)6(OH)2 (card PDF
74-566) is sufficient for the formation of monetite СаН-
РО4. Although both initial compounds have low solubil-
ity (~17 g/L for Ca(H2PO4)2 ⋅ H2O and ~0.0003 g/L for
The calculated amounts of hydroxyapatite
Ca10(PO4)6(OH)2 (CAS no. 1306-06-5, puriss. p.a.
≥90%, Riedel-deHaen, Sigma-Aldrich Laborchemi-
kalien, 04238, lot 70080, Germany) and monocal-
cium phosphate monohydrate Ca(H2PO4)2 ⋅ H2O
(CAS no. 10031-30-8 puriss. p.a. ≥85%, Sigma-
Aldrich) were placed in zirconia containers. To a pow-
der mixture, zirconia milling media were added to the
weight ratio 1 : 5. Acetone (State Standard GOST
2603-79) was added into the containers with the pow-
ders, which were then sealed and fixed in a planetary
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 64 No. 9 2019

1090
SAFRONOVA et al.
Ca(H2PO4)2 · H2O
(а) Ca(H2PO4)2 · H2O
(b) Ca10(PO4)6(OH)2
(c) CaHPO4
9000
6000
3000
0
2000
Ca10(PO4)6(OH)2
1500
1000
500
100 μm
0
4000
3000
2000
1000
0
CaHPO4
0
10
20
30
40
50
60
70
2θ, deg
2 μm
Fig. 1. X-ray powder diffraction patterns of initial com-
pounds—Ca(H PO ) ⋅ H O (card PDF 9-347), and
2
4 2
2
Ca (PO ) (OH) (card PDF 74-566)—and the synthe-
10
4 6
2
sized calcium phosphate СаНРО (card PDF 9-80).
4
Ca10(PO4)6(OH)2) [25]), and the amount of water in
the reaction zone is small (even with taking into
account some amount of water contained in acetone),
mechanical activation provides the occurrence of
reaction (5). It should be noted that the solubility of
monetite СаНРО4 is intermediate between the solubili-
ties of hydroxyapatite Ca10(PO4)6(OH)2 and monocal-
cium phosphate monohydrate Ca(H2PO4)2 . H2O, being
~0.048 g/L. However, the higher solubility of monocal-
cium phosphate monohydrate Ca(H2PO4)2 ⋅ H2O as
compared with the solubility of calcium hydroxyapa-
tite and monetite seems to provide, in aqueous seg-
ments (drops) under mechanical activation condi-
tions, the pH level favorable for the formation and per-
sistence of monetite СаНРО4 rather than
hydroxyapatite.
2 μm
Fig. 2. SEM images of initial compounds (a) Ca(H PO ) ⋅
2
4 2
H O and (b) Ca (PO ) (OH) and (c) synthesized cal-
2
10
4 6
4
2
cium phosphate СаНРО .
Ca10(PO4)6(OH)2 particles is 100–200 nm (Fig. 2b).
The size of the synthesized monetite СаНРО4 parti-
cles can be estimated at 100–300 nm (Fig. 2c). Two
Figure 2 shows SEM images of the initial powders
and synthesized monetite СаНРО4. The size of mono- types of particles—of columnar morphology and with
the shape close to isometric—are observed in the fig-
ure. It is known that monetite CaHPO4 obtained from
calcium phosphate monohydrate Ca(H2PO4)2 ⋅ H2O
plates is rather large, being 50–300 μm at a thickness
of 15–20 μm (Fig. 2a). The size of hydroxyapatite solutions is characterized by a lamellar particle mor-
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SYNTHESIS OF MONETITE
1500
1091
m/m0, %
(а)
1100°C
100
1000
500
0
CaHPO4
95
90
85
80
Ca10(PO4)6(OH)2
300
200
100
0
1000°C
Ca(H2PO4)2 · H2O
75
0
200
400
600
800
1000
T, °C
.
Fig. 3. TA of initial compounds Ca(H PO ) H O and
2
4 2
2
900°C
Ca (PO ) (OH) and synthesized calcium phosphate
300
200
100
0
10
4 6
2
СаНРО .
4
I, A
6 × 10–9
5 × 10–9
4 × 10–9
3 × 10–9
2 × 10–9
0
10
20
30
40
50
60
70
Ca(H2PO4)2 · H2O
Ca10(PO4)6(OH)2
CaHPO4
2θ, deg
(b)
Card PDF 81-2257
0
200
400
600
800
1000
T, °C
.
Fig. 4. MS of initial compounds Ca(H PO ) H O and
2
4 2
2
Ca (PO ) (OH) and synthesized calcium phosphate
10
4 6
2
СаНРО for m/z = 18.
4
phology [26]. In this case, for monetite СаНРО4 syn-
thesized under mechanical activation conditions from
poorly soluble calcium phosphates in the presence of
acetone, the lamellar morphology of particles is not
observed. This result can be caused by the fact that chem-
ical reaction (5) of synthesis of monetite СаНРО4 occurs
on the surface of hydroxyapatite Ca10(PO4)6(OH)2.
We believe that, under the described conditions, het-
erogeneous synthesis occurs at the solid/liquid inter-
face, where solid is calcium hydroxyapatite and liquid
is an aqueous solution of monocalcium phosphate
monohydrate Ca(H2PO4)2 ⋅ H2O. Acetone functions
as a disaggregation medium ensuring the mobility of
milling bodies, powder particles, and aqueous solution
segments (drops). A certain increase in size of mone-
0
10
20
30
40
2θ, deg
50
60
70
Fig. 5. X-ray powder diffraction patterns of ceramics from
.
the СаНРО powder synthesized from Ca(H PO ) H O
4
2
4 2
2
and Ca (PO ) (OH) after heat treatment at (a) 900,
10
4 6
2
1000, and 1100°С and (b) and line diagram for card PDF
81-2257 (β-Ca P O ).
2
2 7
tite СаНРО4 particles as compared with the size of cal-
cium hydroxyapatite Ca10(PO4)6(OH)2 particles can
be caused by the fact that one mole of hydroxyapatite
Ca10(PO4)6(OH)2 (М = 1004 g/mol) with a density of
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 64 No. 9 2019

1092
SAFRONOVA et al.
3.16 g/cm3 gives rise to 14 moles of monetite (М =
136 g/mol) with a density of 2.93 g/cm3. Thus, the vol-
ume of a separate particle can increase approximately
twofold.
(а)
D/D0, %
100
The data of thermal analysis of the initial compounds
and synthesized monetite СаНРО4 are shown in Figs. 3
and 4. The TA and MS data for the initial compounds—
Ca(H2PO4)2 ⋅ H2O and Ca10(PO4)6(OH)2—are given
for comparison. Comparison of TA and MS data con-
firm the absence of the initial compounds in the syn-
thesized powder, which is consistent with the XRD
data (Fig. 1). For monetite СаНРО4, reaction (6) of its
transformation to pyrophosphate Ca2P2O7 occurs at
400°С.
80
60
40
20
0
(6)
2CaHPO4 = Ca2P O7 + H2O.
2
Before sintering 900
1000
1100
The total weight loss of monetite СаНРО4 powder
on heating was ∼7% (Fig. 3). According to MS data,
the weight loss of monetite СаНРО4 is due to water
release in the range 300–475°С (Fig. 4).
T, °C
(b)
ρ, g/cm3
According to XRD data (Fig. 5), the phase composi-
tion of ceramics after test heat treatments in the tempera-
ture range 900–1100°С is represented by β-Са2Р2О7.
2.0
1.5
1.0
0.5
0
Figure 6 presents (a) the linear shrinkage and (b)
density of ceramic samples before and after sintering.
The density of the sample compacted at 100 MPa from
the synthesized powder was 45% of the theoretical
density of β calcium pyrophosphate (3.09 g/cm3). The
linear shrinkage of the samples after heat treatment at
900°С was 17% and after heat treatment at 1100°С,
25%. The density of the samples after heat treatment
at 1000–1100°С was 74–76% as compared with the
theoretical density of β calcium pyrophosphate
(3.09 g/cm3).
Figure 7 shows the microstructure of ceramics
based on β-Са2Р2О7 after heat treatment at different
temperatures in the range 900–1100°С. The grain size
after heat treatment at 900–1000°С is 2–4 μm. After
annealing at 900°С the structure of the ceramics looks
more loose and porous. The ceramic grain size after
sintering at 1100°С is 5–10 μm.
Before sintering 900
1000
1100
T, °C
Fig. 6. (a) Linear sizes and (b) density of ceramics from the
.
СаНРО powder synthesized from Ca(H PO )
H O
2
4
2
4 2
and Ca (PO ) (OH) after heat treatment at 900, 1000,
10
4 6
2
and 1100°С.
CONCLUSIONS
The powder of monetite СаНРО4 with a particle
size of 100–300 nm synthesized from monocalcium
phosphate monohydrate Ca(H2PO4)2 · H2O and cal-
cium hydroxyapatite Ca10(PO4)6(OH)2 under mechan-
ical activation conditions can be used for producing
ceramic materials with the phase composition con-
taining the biocompatible bioresorsable phase, cal-
cium pyrophosphate β-Са2Р2О7.
Ceramic calcium phosphate materials obtained
from the synthesized powder, containing a bioresorb-
able and biocompatible phase of calcium pyrophos-
phate β-Ca2P2O7, can be recommended for creating
bone implants.
ACKNOWLEDGMENTS
In addition, the synthesized powder can be used as
a filler to create materials with a polymer matrix, as
The work was accomplished using equipment purchased
well as for doping with appropriate ions and as a matrix at the expense of the Moscow State University Develop-
when creating luminescent materials.
ment Program.
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SYNTHESIS OF MONETITE
AUTHOR CONTRIBUTIONS
1093
(а) 900°С
T.V. Safronova formulated a goal, designed the experi-
ment, and wrote the paper; I.S. Sadilov and K.V. Chaikun
synthesized samples and interpreted the XRD data;
T.B. Shatalova performed thermal analysis (TA, MS) and
interpreted the results; Ya.Yu. Filippov conducted electron
microscopic studies of the synthesized powders and sam-
ples of ceramic materials. All authors participated in the
discussion of the results.
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The authors declare no conflict of interests.
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Translated by G. Kirakosyan
https://doi.org/10.1007/s11148-016-9877-x
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