- Email: [email protected]
Preparation of alkaline earth phosphates with sol containing sodium alginate and sodium diphosphate
Journal of Colloid and Interface Science 295 (2006) 141–147 www.elsevier.com/locate/jcis
Preparation of alkaline earth phosphates with sol containing sodium alginate and sodium diphosphate Shigeru Sugiyama a,∗ , Minako Fujii a , Kazuya Fukuta a , Kazunori Seyama a , Ken-Ichiro Sotowa a , Naoya Shigemoto b a Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770-8506, Japan b Shikoku Research Institute Inc., Yashima-nishi, Takamatsu 761-0192, Japan
Received 23 June 2005; accepted 4 August 2005 Available online 12 September 2005
Abstract Magnesium hydrogen phosphate, calcium hydroxyapatite, and strontium hydroxyapatite were successfully prepared from sol consisting of sodium alginate and Na4 P2 O7 with Mg2+ , Ca2+ , and Sr2+ in the corresponding nitrates, respectively. It is revealed that the order of the addition of those substrates and the role of sodium alginate are important factors for the preparation of desired phosphate compounds. According to the previous paper on the preparation of calcium hydroxyapatite, sodium alginate was mixed with aqueous Na4 P2 O7 , followed by the addition of the aqueous divalent cations, resulting in the poor formation of the target phosphates. However, as a revised sol–gel technique, sodium alginate was added to the mixture of Na4 P2 O7 and aqueous Mg2+ and Sr2+ , resulting in a rather favorable formation of MgHPO4 and strontium hydroxyapatite, respectively, while the sol thus obtained was stable within a few days. However for aqueous Ca2+ , calcium hydroxyapatite could not be obtained through the revised sol–gel technique. In the preparation of magnesium hydrogen phosphate, sodium alginate contributes mainly to the sol formation of the precursor. The ion exchange between Na+ in sodium alginate and aqueous Ca2+ was important for the preparation of calcium hydroxyapatite. In contrast, the reaction of sodium alginate with the mixture of Na4 P2 O7 and aqueous Sr2+ afforded strontium hydroxyapatite at the specific ratio of those three substrates. The structure of calcium and strontium phosphates prepared from the revised sol–gel process evidently depended on the amount of sodium alginate introduced into the mixture of Na4 P2 O7 and the corresponding divalent cations. 2005 Elsevier Inc. All rights reserved. Keywords: Sol–gel; Sodium alginate; Alkaline earth phosphates; Apatites
1. Introduction It has been generally accepted that alkaline earth phosphates show various unique properties as catalysts and immobilization reagents of various harmful compounds such as aqueous heavy metals and ammonium. Calcium and strontium hydroxyapatites (abbreviated as CaHAp and SrHAp, respectively), which are in a stoichiometric form, Ca10 (PO4 )6 (OH)2 and Sr10 (PO4 )6 (OH)2 , respectively, have received attractive attention as catalysts for the dehydration of alcohols [1–3] and the oxidative dehydrogenation and the partial oxidation of methane [4,5], ethane [6,7], and propane [8,9]. Furthermore, these hy* Corresponding author. Fax: +81 655 7025.
E-mail address: [email protected] (S. Sugiyama). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.08.014
droxyapatites favorably immobilize various divalent heavy metals dissolved in aqueous solution [10–12]. In some cases, these hydroxyapatites immobilized with heavy metals reveal excellent activities for the oxidative dehydrogenation and the partial oxidation of alkanes [13–15]. Magnesium cations cannot form the hydroxyapatite structure [16] while magnesium hydrogen phosphate (MgHPO4 ) can be used in the continuous removal/recovery process for aqueous ammonium [17]. For example, MgHPO4 can favorably remove aqueous ammonium from wastewater to afford magnesium ammonium phosphate (MgNH4 PO4 :MAP). Although the resulting MAP has been used as a slow-acting fertilizer, it has been recently shown that MAP can easily be converted to MgHPO4 through elimination of NH3 [17]. Therefore magnesium hydrogen phosphate can be used repeatedly for a continuous removal/recovery process for aqueous ammonium. Magnesium hydrogen phosphate is the
142
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
usual commercial product prepared from double decomposition of disodium phosphate with magnesium salts in aqueous solution [18]. Hydroxyapatites are generally prepared from the precipitation produced by the reaction of ammonium hydrogen phosphate with the corresponding divalent cations in aqueous solution [12,19,20]. The preparation of CaHAp from a partial substitution with Ca2+ of Mg2+ in magnesium–calcium hydroxyapatite has also been attempted [21]. Furthermore CaHAp can be prepared from hydrothermal conditions [22]. However, those preparation procedures afford bulk materials but not thinlayer phosphates. Since those phosphates are advanced materials such as sensor [23], conductor [24], and adsorbent [25] together with catalysts and immobilization reagents as noted above, the development of the preparation procedure of thinlayer phosphates is important. As one of the preparation procedures of various layer materials, the dip-coating technique based on the sol–gel method has been suggested [26] and the preparation of CaHAp by the sol–gel method has also been reported [27–31]. However, to the best of our knowledge, various researchers have focused on the sol–gel preparation of CaHAp itself while the preparation of MgHPO4 and SrHAp by the sol– gel technique developed from that of CaHAp has not been reported. In the present study, the sol–gel technique reported for the preparation of CaHAp was developed for the preparation of MgHPO4 and SrHAp. The revised preparation method of those phosphates by the sol–gel technique was also examined.
Fig. 1. Flowchart for the preparation of alkaline earth phosphates through a sol–gel route.
2. Experimental All the chemicals were purchased from Wako Pure Chemicals, Osaka, Japan, and used as supplied. Standard solutions for ICP measurement were obtained from Kanto Kagaku, Tokyo, Japan. In the present study, the preparation procedure of CaHAp by a sol–gel route [27,28] was developed to that of MgHPO4 and SrHAp. Typical preparation procedures of CaHAp are as follows. The starting sol was prepared by dissolving Na4 P2 O7 ·10H2 O (4.17 g, 9.35 mmol) into 5% sodium alginate (Na-Alg = 12.5 g (1.21 mmol) + H2 O = 237.5 g). Since the resulting sol was too viscous to stir homogeneously, 8.42 g of the sol, in which 0.04 mmol Na-Alg and 0.31 mmol Na4 P2 O7 ·10H2 O were contained, was dissolved into 20 ml H2 O, followed by purging with N2 in the solution for 1 h to obtain the homogeneous and degassed sol. Into the sol, calcium nitrate solution (Ca(NO3 )2 ·4H2 O = 1.89 g (8.0 mmol) + H2 O = 17 ml) was added at pH 10 adjusted with aq NH3 solution [29]. The sol thus obtained was evaporated at 353 K and dried at 373 K for 3 h under vacuum to the dried gel. The dried gel was calcined at 1173 K for 1 h. In order to remove the oxide in the calcined solid, the solid was washed with 1% NH4 Cl solution, followed by washing with water, and dried at 373 K overnight. It should be noted that calcium chlorapatite (Ca10 (PO4 )6 Cl2 ) but not CaHAp was prepared when a mixture of CaCl2 and Ca(CH3 COO)2 , which has been used in the previous papers [27,28], was employed for Ca(NO3 )2 [29]. A flowchart depicting the various steps involved in the present procedure is illustrated in Fig. 1. In order to improve the sol–gel procedure, another procedure described in Fig. 2
Fig. 2. Flowchart for the preparation of alkaline earth phosphates through a revised sol–gel route.
was also examined (see below). In the preparation of MgHPO4 and SrHAp, Mg(NO3 )2 ·6H2 O and Sr(NO3 )2 were employed for Ca(NO3 )2 ·4H2 O, respectively. The solution after the filtration was analyzed by ICP (Seiko SPS 1500). Powder X-ray diffraction (XRD) of those solids thus obtained was recorded with Rigaku RINT2500X using monochromatized CuKα radiation at 40 kV and 100 mA. In order to analyze amorphous compounds, solid-state 31 P magic-angle spinning nuclear magnetic resonance (31 P MAS NMR) was also employed (Bruker AVANCE DSX300) with an external reference of (NH4 )2 HPO4 at 1.33 ppm and a spinning rate of 7 kHz. The particle size of phosphates prepared was measured in ethanol solution by a laser diffraction/dispersion method (Microtrac FRA, Nikkiso, Tokyo). 3. Results and discussion 3.1. Preparation of magnesium hydrogen phosphate In order to prepare MgHPO4 through the sol–gel method shown in Fig. 1, 42.5 mmol Mg(NO3 )2 ·6H2 O (10.9 g) was
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
Fig. 3. XRD of the solids obtained from the sol–gel procedures for MgHPO4 at various pH.
employed for 8.0 mmol Ca(NO3 )2 ·4H2 O. It should be noted that an excess amount of Mg2+ compared to that of Ca2+ was employed in the present study since the employment of 8 mmol Mg(NO3 )2 ·6H2 O for the corresponding calcium nitrate in the preliminary experiment resulted in insufficient recovery of the inorganic compound. Fig. 3 shows XRD patterns of the solids prepared at pH 7, 8, and 9. The more basic conditions at pH 10 resulted in the formation of Mg(OH)2 precipitated from Mg(NO3 )2 solution. Under the conditions at pH 7 and 8 (Figs. 3A and 3B), the formation of MgO (JCPDS 45-0946) was observed while no peaks from the corresponding phosphates were detected. It should be noted that evident XRD signals due to MgNH4 PO4 ·6H2 O (JCPDS 15-0762) together with those from MgO were detected in the solid prepared at pH 9 but after treatment with NH4 Cl solution for 5 min (Fig. 3C). The detection of MgNH4 PO4 ·6H2 O after treatment with NH4 Cl is equivalent to that of MgHPO4 as shown in [17] MgHPO4 + NH3 + 6H2 O → MgNH4 PO4 ·6H2 O.
(1)
For example, equimolar reaction of MgHPO4 ·3H2 O with NH4 Cl (both 8.15 mmol) in 100 ml H2 O at 298 K affords MgNH4 PO4 ·6H2 O selectively with 77% conversion. Therefore the application of the preparation procedure of CaHAp to MgHPO4 is possible. However, the signals due to MgNH4 PO4 ·6H2 O are very weak, indicating that MgHPO4 is not sufficiently formed from the present procedure. As described in Fig. 1, aqueous Mg2+ species are added into the sol containing phosphate species. The diffusion of Mg2+ into the sol is too difficult to favorably contact with phosphate species, resulting in the insufficient formation of MgHPO4 . Therefore another preparation procedure shown in Fig. 2, in which Mg(NO3 )2 reacts first with Na4 P2 O7 , followed by the addition of Na-Alg to form the sol, was examined. At first, the reaction of Mg(NO3 )2 with Na4 P2 O7 was examined. To
143
Fig. 4. XRD of the solids obtained from the reaction of Mg(NO3 )2 with Na4 P2 O7 . (A) After drying. (B) Calcined at 773 K for 1 h. (C) Calcined at 1173 K for 1 h.
the aqueous solution (50 ml) of Mg(NO3 )2 ·6H2 O (10.9 g, 42.5 mmol), the aqueous solution (50 ml) of Na4 P2 O7 ·10H2 O (4.73 g, 10.6 mmol) was added and stirred for 1 h, followed by filtration. Under the present conditions, the atomic ratio of Mg/Na was adjusted to be unity. From ICP analysis of the filtrate, 82.6% of Mg2+ and 94.6% of PO3− 4 were removed from the solution and the extension of the stirring times to 3 h had no evident influence on the removal rate. The solid thus obtained was dried and calcined at 773 K for 1 h, followed by recalcination at 1173 K for 1 h and XRD of those samples are described in Fig. 4. Before calcination, peaks due to MgHPO4 ·1.2H2 O (JCPDS 49-0752) were detected (Fig. 4A). The calcination at 773 and 1173 K resulted in the formation of amorphous compound and Mg2 P2 O7 (JCPDS 32-0626), respectively (Figs. 4B and 4C). Essentially identical XRD patterns were obtained from the results prepared with the atomic ratio Mg/Na = 4/1, 2/1, and 1/4. Therefore it is evident that the target phosphate can be obtained through the reaction of Mg(NO3 )2 with Na4 P2 O7 while the calcination at 1173 K results in the formation of Mg2 P2 O7 through the dehydration of MgHPO4 . In order to obtain the sol, Na-Alg was added to aqueous solution (100 ml) containing Mg(NO3 )2 ·6H2 O (5.13 g, 20.0 mmol) and Na4 P2 O7 (2.23 g, 5.00 mmol) as described in Fig. 2 but with the calcination temperature at 773 K. The atomic ratio of Mg2+ /Na-Alg was adjusted to be 1.0, 0.5, and 0.33. XRD patterns of the solids calcined at 773 K for 1 h followed by washing with 1% NH4 Cl are shown in Fig. 5. Compared with the results shown in Fig. 3C, peaks with great intensity due to MgNH4 PO4 ·6H2 O, indicating the formation of MgHPO4 , were detected from Mg2+ /Na-Alg = 1.0, together with MgO, which can be removed with additional NH4 Cl treatment (Fig. 5A). The particle size of this solid was 24.5 µm. It should be noted that the employment of the small amount of Na-Alg resulted in the formation of Mg2 P2 O7 (Figs. 5B and 5C). Since the sol obtained through the revised
144
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
Fig. 5. XRD of the solids obtained from the sol prepared from the revised sol–gel route for MgHPO4 . (A) Mg:Na:Na-Alg = 1:1:1. (B) Mg:Na:Na-Alg = 1:1:0.5. (C) Mg:Na:Na-Alg = 1:1:0.33.
sol–gel route remained stable at least within 1 week, the sol can be used for the dip-coating materials for thin-layer MgHPO4 . It is worthwhile to note that MgHPO4 was certainly detected by XRD on a glass plate covered with the present sol but followed by the calcination at 773 K for 3 h (not shown). It should be noted that evident XRD signals were obtained after the calcination at 773 K of the solid obtained from the Mg(NO3 )2 – Na4 P2 O7 –Na-Alg system (Fig. 5) while the amorphous phase was detected after calcination at 773 K of the solid obtained from the Mg(NO3 )2 –Na4 P2 O7 system (Fig. 4B). The present results reveal that the addition of Na-Alg directly contributes to the enhancement of the crystallinity of MgHPO4 . 3.2. Preparation of calcium hydroxyapatite Since MgHPO4 could be favorably prepared with the revised sol–gel route (Fig. 2), the preparation of CaHAp according to Fig. 2 was examined. At first, the reaction of Ca(NO3 )2 with Na4 P2 O7 was observed. To the aqueous solution (34 ml) of Ca(NO3 )2 ·4H2 O (4.02 g, 17.0 mmol), the aqueous solution (34 ml) of Na4 P2 O7 ·10H2 O (3.80 g (8.51 mmol), 2.27 g (5.09 mmol), 1.90 g (4.26 mmol), or 1.27 g (2.85 mmol)) was added at pH 10 and stirred for 1 h, followed by filtration. Under the present conditions, the atomic ratio of Ca/P was adjusted to be 1/1, 1.67/1 (corresponding to the atomic ratio for the stoichiometric CaHAp), 2/1, or 3/1, respectively. XRD patterns of the solids thus obtained showed essentially identical amorphous phase regardless to the atomic ratio after drying at 353 K overnight (Fig. 6A for Ca/P = 2/1). It should be noted that, after the calcination at 1073 K for 1 h, Ca2 P2 O7 (JCPDS 09-0346) but not CaHAp was detected with XRD regardless of the atomic ratio of Ca/P (not shown), al-
Fig. 6. XRD of the solids before and after the addition of Na-Alg to aqueous solution of Ca(NO3 )2 and Na4 P2 O7 . (A) Before the addition of Na-Alg to the mixture (Ca/P = 2/1) but dried at 373 K overnight. (B–F) After addition of Na-Alg to the mixture (Ca/P = 2/1) but dried at 1173 K for 1 h.
though CaHAp was certainly detected in the solid prepared from the sol–gel route shown in Fig. 1 [27–29]. In order to obtain the sol, Na-Alg was added to aqueous solution (64 ml) containing Ca(NO3 )2 ·4H2 O (4.02 g, 17.0 mmol) and Na4 P2 O7 (1.90 g, 4.26 mmol) by the atomic ratio of Ca/P/Na-Alg = 2/1/2.1–2.7 as described in Fig. 2, followed by the calcination temperature at 1173 K for 1 h. However, CaHAp was not detected with XRD while the formation of various calcium compounds such as Ca2 P2 O7 , Ca3 (PO4 )2 (JCPDS 09-0169), CaO (JCPDS 48-1467), and NaCaPO4 (JCPDS 29-1193) were observed (Figs. 6B–6F). Therefore, CaHAp cannot be prepared from the revised sol–gel route (Fig. 2). The results described above show that the reaction of Ca(NO3 )2 and Na4 P2 O7 affords CaHPO4 , which is converted to Ca2 P2 O4 through dehydration after the calcination, or Ca2 P2 O7 directly through ion exchange of Na+ on the pyro-phosphate with Ca2+ . However, the solid obtained after the reaction of Ca(NO3 )2 and Na4 P2 O7 afforded an amorphous compound even after calcination at 573 K. Therefore, 31 P MAS NMR was employed for the identification of the solids obtained from the reaction of Ca(NO3 )2 and Na4 P2 O7 with an atomic ratio of Ca/P = 1.67 but after drying at 375 K (Fig. 7A) and the calcination at 423 and 573 K (Figs. 7B and 7C, respectively. With increasing calcination temperature, the shapes of NMR were evidently changed, indicating that various phosphate species were formed during the calcination. From our analyses with 31 P MAS NMR, a single signal due to CaHPO4 ·2H2 O was observed at 1.15 ppm, respec-
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
145
tively, while a main signal with a small peak due to Ca3 (PO4 )2 , Ca8 H2 (PO4 )6 ·5H2 O, Ca2 (H2 PO4 )2 , and Ca2 P2 O7 was detected at 0.11 with 4.78, 3.20 with −0.28, −4.46 with −1.17, and −10.54 ppm with −8.57 ppm, respectively. Therefore, as shown in Fig. 7A, the direct formation of CaHPO4 from the reaction of Ca(NO3 )2 and Na4 P2 O7 can be rejected. It should be noted that the intensity of the signal detected at 10–0 ppm was enhanced with increasing calcination temperature while the candidate for the phosphate species, main signal of which can be detected at those ranges, may be Ca8 H2 (PO4 )6 ·5H2 O. It has been reported that Ca8 H2 (PO4 )6 ·5H2 O can be converted to various phosphates by calcination as follows [32]: Ca8 H2 (PO4 )6 ·5H2 O → Ca5 (PO4 )3 OH + 1.5Ca2 P2 O7 + 5.5H2 O (423–673 K),
(2)
2Ca5 (PO4 )3 OH + Ca2 P2 O7 → 4Ca3 (PO4 )2 + H2 O (923–1173 K).
(3)
Therefore,
Fig. 7. 31 P MAS NMR of the solids obtained from the reaction of Ca(NO3 )2 and Na4 P2 O7 , but after the drying at 375 K (A) and the calcination at 423 and 573 K (B and C, respectively).
Ca8 H2 (PO4 )6 ·5H2 O → 2Ca3 (PO4 )2 + Ca2 P2 O7 + 6H2 O (423–1173 K). (4) Based on Eq. (4), the formation of Ca2 P2 O7 and Ca3 (PO4 )2 shown in Fig. 6 can be explained. Since the preparation of CaHAp through the sol–gel route (Fig. 1) was possible [27–29], the reaction shown in Fig. 1 may proceed through the cation exchange of Na+ in Na-Alg with Ca2+ from Ca(NO3 )2 to form calcium alginate (Ca-Alg) followed by the reaction of Ca-Alg with Na4 P2 O7 to form CaHAp. Since the cation exchange of Na-Alg with Ca2+ to form Ca-Alg has already been reported [33], it should be confirmed that the reaction of Ca-Alg with Na4 P2 O7 affords CaHAp. Into the solution (34 ml) of Ca-Alg (1.90 g), an aqueous solution (34 ml) of Na4 P2 O7 (1.90, 0.95, or 0.63 g) was added (the atomic ratio of Ca/P = 1/1, 2/1, or 3/1, respectively) and the sol thus obtained was stirred for 1 h at 298 K. The solids obtained from the separation of the sol with a centrifuge was washed and dried at 353 K overnight. XRD patterns of the solids prepared with Ca/P = 1/1, 2/1, and 3/1 are described in Figs. 8A–8C, respectively. The solid prepared with Ca/P = 1/1 was identified as CaNa2 H2 (PO4 )2 (JCPDS 380410) (Fig. 8A) while the amorphous phase was detected from the solids prepared with Ca/P = 2/1 and 3/1 (Figs. 8B and 8C, respectively). When the solid prepared with Ca/P = 3/1 was calcined at 773 K for 1 h, the carbonated CaHAp was detected as shown in Fig. 8D (JCPDS 19-0272). The increase of the calcination temperature from 873 K for 3 h to 1173 K for 1 h resulted in the formation of crystalline CaHAp (JCPDS 09-0432) (Figs. 8E and 8F, respectively) while the particle size of the solid shown in Fig. 8F was 33.1 µm. Therefore the cation exchange of Na+ in Na-Alg with Ca2+ from Ca(NO3 )2 to form Ca-Alg should be a key step for the preparation of CaHAp through the sol–gel route (Fig. 1). 3.3. Preparation of strontium hydroxyapatite In order to prepare strontium hydroxyapatite (SrHAp) through the sol–gel method shown in Fig. 1, the starting sol was prepared by dissolving Na4 P2 O7 ·10H2 O (4.17 g, 9.35 mmol)
Fig. 8. XRD of the solids obtained from the reaction of Ca-Alg with Na4 P2 O7 . (A–C) After drying but the solid prepared with Ca/P = 1/1, 2/1, and 3/1, respectively. (D–F) After calcination at 773 K for 1 h, 873 K for 3 h, and 1173 K for 1 h, respectively, of the solid prepared with Ca/P = 3/1.
into 5% sodium alginate (Na-Alg = 12.5 g (1.21 mmol) + H2 O = 237.5 g). Since the resulting sol was too viscous to stir homogeneously, 8.42 g of the sol, in which 0.04 mmol Na-Alg and 0.31 mmol Na4 P2 O7 ·10H2 O were contained, was dissolved into 20 ml H2 O, followed by purging with N2 in the solution for 1 h to obtain the homogeneous and degassed sol. Into the sol, strontium nitrate solution (Sr(NO3 )2 = 1.80 g (8.5 mmol) + H2 O = 17 ml) was added at pH 10 adjusted with aq NH3 solution [29]. The sol thus obtained was evaporated at 353 K and dried at 373 K for 3 h under vacuum to the dried gel. The
146
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
at least within 1 week, indicating that the sol can be used for the dip-coating materials for thin-layer SrHAp. 4. Conclusion In the present paper, it is revealed that magnesium hydrogen phosphate and strontium hydroxyapatite can be prepared from the revised sol–gel preparation procedure while calcium hydroxyapatite cannot be prepared from the revised procedure. Since the revised sol–gel preparation procedure of magnesium and strontium phosphates affords stable sol, the present preparation can be adapted to the efficient dip-coating technique. As described briefly in the present paper, the present preparation procedure has certain advantages for the formation of a thin layer of these phosphate species on a glass plate. Acknowledgments This work has been partly funded by Mukai Science and Technology Foundation to S.S., to which our thanks are due. References
Fig. 9. XRD of the solids before and after the addition of Na-Alg to aqueous solution of Sr(NO3 )2 and Na4 P2 O7 . A Before the addition of Na-Alg to the mixture (Sr/P = 2/1) but dried at 373 K overnight. (B–F) After the addition of Na-Alg to the mixture (Sr/P = 2/1) but dried at 1173 K for 1 h.
dried gel was calcined at 1173 K for 1 h. In order to remove the oxide in the calcined solid, the solid was washed with 1% NH4 Cl solution, followed by washing with water and dried at 373 K overnight. XRD patterns of the solid thus obtained were not a single phase of SrHAp but a mixture of SrHPO4 (JCPDS 70-1215) and SrHAp (JCPDS 33-1348) (not shown). In order to examine the possible preparation of SrHAp with the revised sol–gel route, the preparation of SrHAp according to Fig. 2 was investigated. To the aqueous solution (34 ml) of Sr(NO3 )2 (3.60 g, 17.0 mmol), the aqueous solution (34 ml) of Na4 P2 O7 ·10H2 O 1.90 g (4.26 mmol) was added at pH 10 and stirred for 1 h. It should be noted that the atomic ratio of Sr/P was adjusted to be 2/1. Then, in order to obtain the sol, Na-Alg was added to aqueous solution (64 ml) containing Sr(NO3 )2 and Na4 P2 O7 by the atomic ratio of Sr/P/Na-Alg = 2/1/2.1–2.7 as described in Fig. 9 but with a calcination temperature at 1173 K for 1 h. It was evident that the addition of a small amount of Na-Alg by Sr/P/Na-Alg = 2/1/2.3 resulted in the formation of Sr2 P2 O7 (JCPDS 24-1011) and/or Sr3 (PO4 )2 (JCPDS 24-1008) as shown in Figs. 9B–9D. However, the addition of Na-Alg by Sr/P/Na-Alg = 2/1/2.4 and 2/1/2.5 resulted in the formation of SrHAp only (Figs. 9E and 9F). The particle size of the solid described in Fig. 9F was 71.6 µm and evidently greater than those of corresponding magnesium and calcium phosphates described above. Finally it should be noted that the sol obtained through the revised sol–gel route remained stable
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22] [23]
J.A.S. Bett, L.G. Christner, W.K. Hall, J. Catal. 13 (1969) 332. Y. Imizu, M. Kadoya, H. Abe, Chem. Lett. (1982) 1649. H. Monma, J. Catal. 75 (1982) 200. S. Sugiyama, T. Minami, H. Hayashi, M. Tanaka, N. Shigemoto, J.B. Moffat, J. Chem. Soc. Faraday Trans. 92 (1996) 293. S. Sugiyama, T. Minami, T. Higaki, H. Hayashi, J.B. Moffat, Ind. Eng. Chem. Res. 36 (1997) 328. S. Sugiyama, Y. Iguchi, H. Nishioka, T. Miyamoto, H. Hayashi, J.B. Moffat, J. Mater. Chem. 7 (1997) 2483. S. Sugiyama, K. Abe, T. Miyamoto, H. Hayashi, J.B. Moffat, J. Mol. Catal. A Chem. 130 (1998) 297. S. Sugiyama, E. Nitta, H. Hayashi, J.B. Moffat, Appl. Catal. A 198 (2000) 171. S. Sugiyama, T. Shono, E. Nitta, H. Hayashi, Appl. Catal. A 211 (2001) 123. T. Suzuki, T. Hatsushika, Y. Hayakawa, J. Chem. Soc., Faraday Trans. 1 77 (1981) 1959. P. Zhang, J.A. Ryan, L.T. Bryndzia, Environ. Sci. Technol. 31 (1997) 2673. S. Sugiyama, T. Moriga, M. Goda, H. Hayashi, J.B. Moffat, J. Chem. Soc. Faraday Trans. 92 (1996) 4305. Y. Matsumura, J.B. Moffat, Catal. Lett. 17 (1993) 197. S. Sugiyama, H. Mitsuoka, T. Shono, T. Moriga, H. Hayashi, J. Chem. Eng. Jpn. 36 (2003) 210. S. Sugiyama, T. Shono, D. Makino, T. Moriga, H. Hayashi, J. Catal. 214 (2003) 8. T. Suzuki, T. Hatsushika, M. Miyake, J. Chem. Soc. Faraday Trans. 1 78 (1982) 3605. S. Sugiyama, M. Yokoyama, H. Ishizuka, K.-I. Sotowa, T. Tomida, N. Shigemoto, J. Colloid Interface Sci. (2005), in press. T. Staffel, in: B. Elvers, S. Hawkins, G. Schulz (Eds.), Ullmann’s Encyclopedia of Industrial Chemistry, vol. 19, VCH Verlagsgesellschaft, Germany, 1991, pp. 497–498. E. Hayek, H. Newesely, Inorg. Synth. 7 (1963) 63. A. Bigi, E. Foresti, F. Marchetti, A. Ripamonti, N. Roveri, J. Chem. Soc. Dalton Trans. (1984) 1091. A. Yasukawa, T. Yokoyama, K. Kandori, T. Ishikawa, Colloids Surf. A 238 (2004) 133. J.C. Rendon-Angeles, K. Yanagisawa, N. Ishizawa, S. Oishi, J. Solid State Chem. 151 (2000) 65. N. Masayuki, T. Nishino, T. Saeki, Sensors Actuat. 15 (1988) 145.
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
[24] L. El-Hammari, A. Laghzizil, P. Barboux, A. Saoiabi, K. Lahlil, J. Solid State Chem. 177 (2004) 134. [25] A. Nzihou, B. Adhikari, R. Pfeffer, Ind. Eng. Chem. Res. 44 (2005) 1787. [26] S. Sakka, in: M. Tomozawa, R. Doremus (Eds.), Treatise on Materials Science and Technology, Academic Press, New York, 1982, pp. 129–167. [27] S. Suzuki, H. Nakane, M. Ohshima, M. Takahashi, Y. Hikichi, J. Soc. Mater. Sci. Jpn. 41 (1992) 939. [28] H. Iwasaki, Y. Kaneko, Zairyo 37 (1988) 60.
147
[29] S. Sugiyama, M. Fujii, H. Hayashi, K. Kusakabe, Inorg. Chem. Commun. 7 (2004) 569. [30] A. Jillavenkatesa, R.A. Condrate Sr, J. Mater. Sci. 33 (1998) 4111. [31] A. Bigi, E. Boanini, K. Rubini, J. Solid State Chem. 177 (2004) 3092. [32] J.C. Elliott, Structure and Chemistry of the Apatites and Other Calcium Orthophosphates, in: Studies in Inorganic Chemistry, vol. 18, Elsevier, Amsterdam, 1994, pp. 18–20. [33] O. Smidsrod, Faraday Discuss. Chem. Soc. 57 (1974) 263.
Preparation of alkaline earth phosphates with sol containing sodium alginate and sodium diphosphate Shigeru Sugiyama a,∗ , Minako Fujii a , Kazuya Fukuta a , Kazunori Seyama a , Ken-Ichiro Sotowa a , Naoya Shigemoto b a Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770-8506, Japan b Shikoku Research Institute Inc., Yashima-nishi, Takamatsu 761-0192, Japan
Received 23 June 2005; accepted 4 August 2005 Available online 12 September 2005
Abstract Magnesium hydrogen phosphate, calcium hydroxyapatite, and strontium hydroxyapatite were successfully prepared from sol consisting of sodium alginate and Na4 P2 O7 with Mg2+ , Ca2+ , and Sr2+ in the corresponding nitrates, respectively. It is revealed that the order of the addition of those substrates and the role of sodium alginate are important factors for the preparation of desired phosphate compounds. According to the previous paper on the preparation of calcium hydroxyapatite, sodium alginate was mixed with aqueous Na4 P2 O7 , followed by the addition of the aqueous divalent cations, resulting in the poor formation of the target phosphates. However, as a revised sol–gel technique, sodium alginate was added to the mixture of Na4 P2 O7 and aqueous Mg2+ and Sr2+ , resulting in a rather favorable formation of MgHPO4 and strontium hydroxyapatite, respectively, while the sol thus obtained was stable within a few days. However for aqueous Ca2+ , calcium hydroxyapatite could not be obtained through the revised sol–gel technique. In the preparation of magnesium hydrogen phosphate, sodium alginate contributes mainly to the sol formation of the precursor. The ion exchange between Na+ in sodium alginate and aqueous Ca2+ was important for the preparation of calcium hydroxyapatite. In contrast, the reaction of sodium alginate with the mixture of Na4 P2 O7 and aqueous Sr2+ afforded strontium hydroxyapatite at the specific ratio of those three substrates. The structure of calcium and strontium phosphates prepared from the revised sol–gel process evidently depended on the amount of sodium alginate introduced into the mixture of Na4 P2 O7 and the corresponding divalent cations. 2005 Elsevier Inc. All rights reserved. Keywords: Sol–gel; Sodium alginate; Alkaline earth phosphates; Apatites
1. Introduction It has been generally accepted that alkaline earth phosphates show various unique properties as catalysts and immobilization reagents of various harmful compounds such as aqueous heavy metals and ammonium. Calcium and strontium hydroxyapatites (abbreviated as CaHAp and SrHAp, respectively), which are in a stoichiometric form, Ca10 (PO4 )6 (OH)2 and Sr10 (PO4 )6 (OH)2 , respectively, have received attractive attention as catalysts for the dehydration of alcohols [1–3] and the oxidative dehydrogenation and the partial oxidation of methane [4,5], ethane [6,7], and propane [8,9]. Furthermore, these hy* Corresponding author. Fax: +81 655 7025.
E-mail address: [email protected] (S. Sugiyama). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.08.014
droxyapatites favorably immobilize various divalent heavy metals dissolved in aqueous solution [10–12]. In some cases, these hydroxyapatites immobilized with heavy metals reveal excellent activities for the oxidative dehydrogenation and the partial oxidation of alkanes [13–15]. Magnesium cations cannot form the hydroxyapatite structure [16] while magnesium hydrogen phosphate (MgHPO4 ) can be used in the continuous removal/recovery process for aqueous ammonium [17]. For example, MgHPO4 can favorably remove aqueous ammonium from wastewater to afford magnesium ammonium phosphate (MgNH4 PO4 :MAP). Although the resulting MAP has been used as a slow-acting fertilizer, it has been recently shown that MAP can easily be converted to MgHPO4 through elimination of NH3 [17]. Therefore magnesium hydrogen phosphate can be used repeatedly for a continuous removal/recovery process for aqueous ammonium. Magnesium hydrogen phosphate is the
142
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
usual commercial product prepared from double decomposition of disodium phosphate with magnesium salts in aqueous solution [18]. Hydroxyapatites are generally prepared from the precipitation produced by the reaction of ammonium hydrogen phosphate with the corresponding divalent cations in aqueous solution [12,19,20]. The preparation of CaHAp from a partial substitution with Ca2+ of Mg2+ in magnesium–calcium hydroxyapatite has also been attempted [21]. Furthermore CaHAp can be prepared from hydrothermal conditions [22]. However, those preparation procedures afford bulk materials but not thinlayer phosphates. Since those phosphates are advanced materials such as sensor [23], conductor [24], and adsorbent [25] together with catalysts and immobilization reagents as noted above, the development of the preparation procedure of thinlayer phosphates is important. As one of the preparation procedures of various layer materials, the dip-coating technique based on the sol–gel method has been suggested [26] and the preparation of CaHAp by the sol–gel method has also been reported [27–31]. However, to the best of our knowledge, various researchers have focused on the sol–gel preparation of CaHAp itself while the preparation of MgHPO4 and SrHAp by the sol– gel technique developed from that of CaHAp has not been reported. In the present study, the sol–gel technique reported for the preparation of CaHAp was developed for the preparation of MgHPO4 and SrHAp. The revised preparation method of those phosphates by the sol–gel technique was also examined.
Fig. 1. Flowchart for the preparation of alkaline earth phosphates through a sol–gel route.
2. Experimental All the chemicals were purchased from Wako Pure Chemicals, Osaka, Japan, and used as supplied. Standard solutions for ICP measurement were obtained from Kanto Kagaku, Tokyo, Japan. In the present study, the preparation procedure of CaHAp by a sol–gel route [27,28] was developed to that of MgHPO4 and SrHAp. Typical preparation procedures of CaHAp are as follows. The starting sol was prepared by dissolving Na4 P2 O7 ·10H2 O (4.17 g, 9.35 mmol) into 5% sodium alginate (Na-Alg = 12.5 g (1.21 mmol) + H2 O = 237.5 g). Since the resulting sol was too viscous to stir homogeneously, 8.42 g of the sol, in which 0.04 mmol Na-Alg and 0.31 mmol Na4 P2 O7 ·10H2 O were contained, was dissolved into 20 ml H2 O, followed by purging with N2 in the solution for 1 h to obtain the homogeneous and degassed sol. Into the sol, calcium nitrate solution (Ca(NO3 )2 ·4H2 O = 1.89 g (8.0 mmol) + H2 O = 17 ml) was added at pH 10 adjusted with aq NH3 solution [29]. The sol thus obtained was evaporated at 353 K and dried at 373 K for 3 h under vacuum to the dried gel. The dried gel was calcined at 1173 K for 1 h. In order to remove the oxide in the calcined solid, the solid was washed with 1% NH4 Cl solution, followed by washing with water, and dried at 373 K overnight. It should be noted that calcium chlorapatite (Ca10 (PO4 )6 Cl2 ) but not CaHAp was prepared when a mixture of CaCl2 and Ca(CH3 COO)2 , which has been used in the previous papers [27,28], was employed for Ca(NO3 )2 [29]. A flowchart depicting the various steps involved in the present procedure is illustrated in Fig. 1. In order to improve the sol–gel procedure, another procedure described in Fig. 2
Fig. 2. Flowchart for the preparation of alkaline earth phosphates through a revised sol–gel route.
was also examined (see below). In the preparation of MgHPO4 and SrHAp, Mg(NO3 )2 ·6H2 O and Sr(NO3 )2 were employed for Ca(NO3 )2 ·4H2 O, respectively. The solution after the filtration was analyzed by ICP (Seiko SPS 1500). Powder X-ray diffraction (XRD) of those solids thus obtained was recorded with Rigaku RINT2500X using monochromatized CuKα radiation at 40 kV and 100 mA. In order to analyze amorphous compounds, solid-state 31 P magic-angle spinning nuclear magnetic resonance (31 P MAS NMR) was also employed (Bruker AVANCE DSX300) with an external reference of (NH4 )2 HPO4 at 1.33 ppm and a spinning rate of 7 kHz. The particle size of phosphates prepared was measured in ethanol solution by a laser diffraction/dispersion method (Microtrac FRA, Nikkiso, Tokyo). 3. Results and discussion 3.1. Preparation of magnesium hydrogen phosphate In order to prepare MgHPO4 through the sol–gel method shown in Fig. 1, 42.5 mmol Mg(NO3 )2 ·6H2 O (10.9 g) was
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
Fig. 3. XRD of the solids obtained from the sol–gel procedures for MgHPO4 at various pH.
employed for 8.0 mmol Ca(NO3 )2 ·4H2 O. It should be noted that an excess amount of Mg2+ compared to that of Ca2+ was employed in the present study since the employment of 8 mmol Mg(NO3 )2 ·6H2 O for the corresponding calcium nitrate in the preliminary experiment resulted in insufficient recovery of the inorganic compound. Fig. 3 shows XRD patterns of the solids prepared at pH 7, 8, and 9. The more basic conditions at pH 10 resulted in the formation of Mg(OH)2 precipitated from Mg(NO3 )2 solution. Under the conditions at pH 7 and 8 (Figs. 3A and 3B), the formation of MgO (JCPDS 45-0946) was observed while no peaks from the corresponding phosphates were detected. It should be noted that evident XRD signals due to MgNH4 PO4 ·6H2 O (JCPDS 15-0762) together with those from MgO were detected in the solid prepared at pH 9 but after treatment with NH4 Cl solution for 5 min (Fig. 3C). The detection of MgNH4 PO4 ·6H2 O after treatment with NH4 Cl is equivalent to that of MgHPO4 as shown in [17] MgHPO4 + NH3 + 6H2 O → MgNH4 PO4 ·6H2 O.
(1)
For example, equimolar reaction of MgHPO4 ·3H2 O with NH4 Cl (both 8.15 mmol) in 100 ml H2 O at 298 K affords MgNH4 PO4 ·6H2 O selectively with 77% conversion. Therefore the application of the preparation procedure of CaHAp to MgHPO4 is possible. However, the signals due to MgNH4 PO4 ·6H2 O are very weak, indicating that MgHPO4 is not sufficiently formed from the present procedure. As described in Fig. 1, aqueous Mg2+ species are added into the sol containing phosphate species. The diffusion of Mg2+ into the sol is too difficult to favorably contact with phosphate species, resulting in the insufficient formation of MgHPO4 . Therefore another preparation procedure shown in Fig. 2, in which Mg(NO3 )2 reacts first with Na4 P2 O7 , followed by the addition of Na-Alg to form the sol, was examined. At first, the reaction of Mg(NO3 )2 with Na4 P2 O7 was examined. To
143
Fig. 4. XRD of the solids obtained from the reaction of Mg(NO3 )2 with Na4 P2 O7 . (A) After drying. (B) Calcined at 773 K for 1 h. (C) Calcined at 1173 K for 1 h.
the aqueous solution (50 ml) of Mg(NO3 )2 ·6H2 O (10.9 g, 42.5 mmol), the aqueous solution (50 ml) of Na4 P2 O7 ·10H2 O (4.73 g, 10.6 mmol) was added and stirred for 1 h, followed by filtration. Under the present conditions, the atomic ratio of Mg/Na was adjusted to be unity. From ICP analysis of the filtrate, 82.6% of Mg2+ and 94.6% of PO3− 4 were removed from the solution and the extension of the stirring times to 3 h had no evident influence on the removal rate. The solid thus obtained was dried and calcined at 773 K for 1 h, followed by recalcination at 1173 K for 1 h and XRD of those samples are described in Fig. 4. Before calcination, peaks due to MgHPO4 ·1.2H2 O (JCPDS 49-0752) were detected (Fig. 4A). The calcination at 773 and 1173 K resulted in the formation of amorphous compound and Mg2 P2 O7 (JCPDS 32-0626), respectively (Figs. 4B and 4C). Essentially identical XRD patterns were obtained from the results prepared with the atomic ratio Mg/Na = 4/1, 2/1, and 1/4. Therefore it is evident that the target phosphate can be obtained through the reaction of Mg(NO3 )2 with Na4 P2 O7 while the calcination at 1173 K results in the formation of Mg2 P2 O7 through the dehydration of MgHPO4 . In order to obtain the sol, Na-Alg was added to aqueous solution (100 ml) containing Mg(NO3 )2 ·6H2 O (5.13 g, 20.0 mmol) and Na4 P2 O7 (2.23 g, 5.00 mmol) as described in Fig. 2 but with the calcination temperature at 773 K. The atomic ratio of Mg2+ /Na-Alg was adjusted to be 1.0, 0.5, and 0.33. XRD patterns of the solids calcined at 773 K for 1 h followed by washing with 1% NH4 Cl are shown in Fig. 5. Compared with the results shown in Fig. 3C, peaks with great intensity due to MgNH4 PO4 ·6H2 O, indicating the formation of MgHPO4 , were detected from Mg2+ /Na-Alg = 1.0, together with MgO, which can be removed with additional NH4 Cl treatment (Fig. 5A). The particle size of this solid was 24.5 µm. It should be noted that the employment of the small amount of Na-Alg resulted in the formation of Mg2 P2 O7 (Figs. 5B and 5C). Since the sol obtained through the revised
144
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
Fig. 5. XRD of the solids obtained from the sol prepared from the revised sol–gel route for MgHPO4 . (A) Mg:Na:Na-Alg = 1:1:1. (B) Mg:Na:Na-Alg = 1:1:0.5. (C) Mg:Na:Na-Alg = 1:1:0.33.
sol–gel route remained stable at least within 1 week, the sol can be used for the dip-coating materials for thin-layer MgHPO4 . It is worthwhile to note that MgHPO4 was certainly detected by XRD on a glass plate covered with the present sol but followed by the calcination at 773 K for 3 h (not shown). It should be noted that evident XRD signals were obtained after the calcination at 773 K of the solid obtained from the Mg(NO3 )2 – Na4 P2 O7 –Na-Alg system (Fig. 5) while the amorphous phase was detected after calcination at 773 K of the solid obtained from the Mg(NO3 )2 –Na4 P2 O7 system (Fig. 4B). The present results reveal that the addition of Na-Alg directly contributes to the enhancement of the crystallinity of MgHPO4 . 3.2. Preparation of calcium hydroxyapatite Since MgHPO4 could be favorably prepared with the revised sol–gel route (Fig. 2), the preparation of CaHAp according to Fig. 2 was examined. At first, the reaction of Ca(NO3 )2 with Na4 P2 O7 was observed. To the aqueous solution (34 ml) of Ca(NO3 )2 ·4H2 O (4.02 g, 17.0 mmol), the aqueous solution (34 ml) of Na4 P2 O7 ·10H2 O (3.80 g (8.51 mmol), 2.27 g (5.09 mmol), 1.90 g (4.26 mmol), or 1.27 g (2.85 mmol)) was added at pH 10 and stirred for 1 h, followed by filtration. Under the present conditions, the atomic ratio of Ca/P was adjusted to be 1/1, 1.67/1 (corresponding to the atomic ratio for the stoichiometric CaHAp), 2/1, or 3/1, respectively. XRD patterns of the solids thus obtained showed essentially identical amorphous phase regardless to the atomic ratio after drying at 353 K overnight (Fig. 6A for Ca/P = 2/1). It should be noted that, after the calcination at 1073 K for 1 h, Ca2 P2 O7 (JCPDS 09-0346) but not CaHAp was detected with XRD regardless of the atomic ratio of Ca/P (not shown), al-
Fig. 6. XRD of the solids before and after the addition of Na-Alg to aqueous solution of Ca(NO3 )2 and Na4 P2 O7 . (A) Before the addition of Na-Alg to the mixture (Ca/P = 2/1) but dried at 373 K overnight. (B–F) After addition of Na-Alg to the mixture (Ca/P = 2/1) but dried at 1173 K for 1 h.
though CaHAp was certainly detected in the solid prepared from the sol–gel route shown in Fig. 1 [27–29]. In order to obtain the sol, Na-Alg was added to aqueous solution (64 ml) containing Ca(NO3 )2 ·4H2 O (4.02 g, 17.0 mmol) and Na4 P2 O7 (1.90 g, 4.26 mmol) by the atomic ratio of Ca/P/Na-Alg = 2/1/2.1–2.7 as described in Fig. 2, followed by the calcination temperature at 1173 K for 1 h. However, CaHAp was not detected with XRD while the formation of various calcium compounds such as Ca2 P2 O7 , Ca3 (PO4 )2 (JCPDS 09-0169), CaO (JCPDS 48-1467), and NaCaPO4 (JCPDS 29-1193) were observed (Figs. 6B–6F). Therefore, CaHAp cannot be prepared from the revised sol–gel route (Fig. 2). The results described above show that the reaction of Ca(NO3 )2 and Na4 P2 O7 affords CaHPO4 , which is converted to Ca2 P2 O4 through dehydration after the calcination, or Ca2 P2 O7 directly through ion exchange of Na+ on the pyro-phosphate with Ca2+ . However, the solid obtained after the reaction of Ca(NO3 )2 and Na4 P2 O7 afforded an amorphous compound even after calcination at 573 K. Therefore, 31 P MAS NMR was employed for the identification of the solids obtained from the reaction of Ca(NO3 )2 and Na4 P2 O7 with an atomic ratio of Ca/P = 1.67 but after drying at 375 K (Fig. 7A) and the calcination at 423 and 573 K (Figs. 7B and 7C, respectively. With increasing calcination temperature, the shapes of NMR were evidently changed, indicating that various phosphate species were formed during the calcination. From our analyses with 31 P MAS NMR, a single signal due to CaHPO4 ·2H2 O was observed at 1.15 ppm, respec-
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
145
tively, while a main signal with a small peak due to Ca3 (PO4 )2 , Ca8 H2 (PO4 )6 ·5H2 O, Ca2 (H2 PO4 )2 , and Ca2 P2 O7 was detected at 0.11 with 4.78, 3.20 with −0.28, −4.46 with −1.17, and −10.54 ppm with −8.57 ppm, respectively. Therefore, as shown in Fig. 7A, the direct formation of CaHPO4 from the reaction of Ca(NO3 )2 and Na4 P2 O7 can be rejected. It should be noted that the intensity of the signal detected at 10–0 ppm was enhanced with increasing calcination temperature while the candidate for the phosphate species, main signal of which can be detected at those ranges, may be Ca8 H2 (PO4 )6 ·5H2 O. It has been reported that Ca8 H2 (PO4 )6 ·5H2 O can be converted to various phosphates by calcination as follows [32]: Ca8 H2 (PO4 )6 ·5H2 O → Ca5 (PO4 )3 OH + 1.5Ca2 P2 O7 + 5.5H2 O (423–673 K),
(2)
2Ca5 (PO4 )3 OH + Ca2 P2 O7 → 4Ca3 (PO4 )2 + H2 O (923–1173 K).
(3)
Therefore,
Fig. 7. 31 P MAS NMR of the solids obtained from the reaction of Ca(NO3 )2 and Na4 P2 O7 , but after the drying at 375 K (A) and the calcination at 423 and 573 K (B and C, respectively).
Ca8 H2 (PO4 )6 ·5H2 O → 2Ca3 (PO4 )2 + Ca2 P2 O7 + 6H2 O (423–1173 K). (4) Based on Eq. (4), the formation of Ca2 P2 O7 and Ca3 (PO4 )2 shown in Fig. 6 can be explained. Since the preparation of CaHAp through the sol–gel route (Fig. 1) was possible [27–29], the reaction shown in Fig. 1 may proceed through the cation exchange of Na+ in Na-Alg with Ca2+ from Ca(NO3 )2 to form calcium alginate (Ca-Alg) followed by the reaction of Ca-Alg with Na4 P2 O7 to form CaHAp. Since the cation exchange of Na-Alg with Ca2+ to form Ca-Alg has already been reported [33], it should be confirmed that the reaction of Ca-Alg with Na4 P2 O7 affords CaHAp. Into the solution (34 ml) of Ca-Alg (1.90 g), an aqueous solution (34 ml) of Na4 P2 O7 (1.90, 0.95, or 0.63 g) was added (the atomic ratio of Ca/P = 1/1, 2/1, or 3/1, respectively) and the sol thus obtained was stirred for 1 h at 298 K. The solids obtained from the separation of the sol with a centrifuge was washed and dried at 353 K overnight. XRD patterns of the solids prepared with Ca/P = 1/1, 2/1, and 3/1 are described in Figs. 8A–8C, respectively. The solid prepared with Ca/P = 1/1 was identified as CaNa2 H2 (PO4 )2 (JCPDS 380410) (Fig. 8A) while the amorphous phase was detected from the solids prepared with Ca/P = 2/1 and 3/1 (Figs. 8B and 8C, respectively). When the solid prepared with Ca/P = 3/1 was calcined at 773 K for 1 h, the carbonated CaHAp was detected as shown in Fig. 8D (JCPDS 19-0272). The increase of the calcination temperature from 873 K for 3 h to 1173 K for 1 h resulted in the formation of crystalline CaHAp (JCPDS 09-0432) (Figs. 8E and 8F, respectively) while the particle size of the solid shown in Fig. 8F was 33.1 µm. Therefore the cation exchange of Na+ in Na-Alg with Ca2+ from Ca(NO3 )2 to form Ca-Alg should be a key step for the preparation of CaHAp through the sol–gel route (Fig. 1). 3.3. Preparation of strontium hydroxyapatite In order to prepare strontium hydroxyapatite (SrHAp) through the sol–gel method shown in Fig. 1, the starting sol was prepared by dissolving Na4 P2 O7 ·10H2 O (4.17 g, 9.35 mmol)
Fig. 8. XRD of the solids obtained from the reaction of Ca-Alg with Na4 P2 O7 . (A–C) After drying but the solid prepared with Ca/P = 1/1, 2/1, and 3/1, respectively. (D–F) After calcination at 773 K for 1 h, 873 K for 3 h, and 1173 K for 1 h, respectively, of the solid prepared with Ca/P = 3/1.
into 5% sodium alginate (Na-Alg = 12.5 g (1.21 mmol) + H2 O = 237.5 g). Since the resulting sol was too viscous to stir homogeneously, 8.42 g of the sol, in which 0.04 mmol Na-Alg and 0.31 mmol Na4 P2 O7 ·10H2 O were contained, was dissolved into 20 ml H2 O, followed by purging with N2 in the solution for 1 h to obtain the homogeneous and degassed sol. Into the sol, strontium nitrate solution (Sr(NO3 )2 = 1.80 g (8.5 mmol) + H2 O = 17 ml) was added at pH 10 adjusted with aq NH3 solution [29]. The sol thus obtained was evaporated at 353 K and dried at 373 K for 3 h under vacuum to the dried gel. The
146
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
at least within 1 week, indicating that the sol can be used for the dip-coating materials for thin-layer SrHAp. 4. Conclusion In the present paper, it is revealed that magnesium hydrogen phosphate and strontium hydroxyapatite can be prepared from the revised sol–gel preparation procedure while calcium hydroxyapatite cannot be prepared from the revised procedure. Since the revised sol–gel preparation procedure of magnesium and strontium phosphates affords stable sol, the present preparation can be adapted to the efficient dip-coating technique. As described briefly in the present paper, the present preparation procedure has certain advantages for the formation of a thin layer of these phosphate species on a glass plate. Acknowledgments This work has been partly funded by Mukai Science and Technology Foundation to S.S., to which our thanks are due. References
Fig. 9. XRD of the solids before and after the addition of Na-Alg to aqueous solution of Sr(NO3 )2 and Na4 P2 O7 . A Before the addition of Na-Alg to the mixture (Sr/P = 2/1) but dried at 373 K overnight. (B–F) After the addition of Na-Alg to the mixture (Sr/P = 2/1) but dried at 1173 K for 1 h.
dried gel was calcined at 1173 K for 1 h. In order to remove the oxide in the calcined solid, the solid was washed with 1% NH4 Cl solution, followed by washing with water and dried at 373 K overnight. XRD patterns of the solid thus obtained were not a single phase of SrHAp but a mixture of SrHPO4 (JCPDS 70-1215) and SrHAp (JCPDS 33-1348) (not shown). In order to examine the possible preparation of SrHAp with the revised sol–gel route, the preparation of SrHAp according to Fig. 2 was investigated. To the aqueous solution (34 ml) of Sr(NO3 )2 (3.60 g, 17.0 mmol), the aqueous solution (34 ml) of Na4 P2 O7 ·10H2 O 1.90 g (4.26 mmol) was added at pH 10 and stirred for 1 h. It should be noted that the atomic ratio of Sr/P was adjusted to be 2/1. Then, in order to obtain the sol, Na-Alg was added to aqueous solution (64 ml) containing Sr(NO3 )2 and Na4 P2 O7 by the atomic ratio of Sr/P/Na-Alg = 2/1/2.1–2.7 as described in Fig. 9 but with a calcination temperature at 1173 K for 1 h. It was evident that the addition of a small amount of Na-Alg by Sr/P/Na-Alg = 2/1/2.3 resulted in the formation of Sr2 P2 O7 (JCPDS 24-1011) and/or Sr3 (PO4 )2 (JCPDS 24-1008) as shown in Figs. 9B–9D. However, the addition of Na-Alg by Sr/P/Na-Alg = 2/1/2.4 and 2/1/2.5 resulted in the formation of SrHAp only (Figs. 9E and 9F). The particle size of the solid described in Fig. 9F was 71.6 µm and evidently greater than those of corresponding magnesium and calcium phosphates described above. Finally it should be noted that the sol obtained through the revised sol–gel route remained stable
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22] [23]
J.A.S. Bett, L.G. Christner, W.K. Hall, J. Catal. 13 (1969) 332. Y. Imizu, M. Kadoya, H. Abe, Chem. Lett. (1982) 1649. H. Monma, J. Catal. 75 (1982) 200. S. Sugiyama, T. Minami, H. Hayashi, M. Tanaka, N. Shigemoto, J.B. Moffat, J. Chem. Soc. Faraday Trans. 92 (1996) 293. S. Sugiyama, T. Minami, T. Higaki, H. Hayashi, J.B. Moffat, Ind. Eng. Chem. Res. 36 (1997) 328. S. Sugiyama, Y. Iguchi, H. Nishioka, T. Miyamoto, H. Hayashi, J.B. Moffat, J. Mater. Chem. 7 (1997) 2483. S. Sugiyama, K. Abe, T. Miyamoto, H. Hayashi, J.B. Moffat, J. Mol. Catal. A Chem. 130 (1998) 297. S. Sugiyama, E. Nitta, H. Hayashi, J.B. Moffat, Appl. Catal. A 198 (2000) 171. S. Sugiyama, T. Shono, E. Nitta, H. Hayashi, Appl. Catal. A 211 (2001) 123. T. Suzuki, T. Hatsushika, Y. Hayakawa, J. Chem. Soc., Faraday Trans. 1 77 (1981) 1959. P. Zhang, J.A. Ryan, L.T. Bryndzia, Environ. Sci. Technol. 31 (1997) 2673. S. Sugiyama, T. Moriga, M. Goda, H. Hayashi, J.B. Moffat, J. Chem. Soc. Faraday Trans. 92 (1996) 4305. Y. Matsumura, J.B. Moffat, Catal. Lett. 17 (1993) 197. S. Sugiyama, H. Mitsuoka, T. Shono, T. Moriga, H. Hayashi, J. Chem. Eng. Jpn. 36 (2003) 210. S. Sugiyama, T. Shono, D. Makino, T. Moriga, H. Hayashi, J. Catal. 214 (2003) 8. T. Suzuki, T. Hatsushika, M. Miyake, J. Chem. Soc. Faraday Trans. 1 78 (1982) 3605. S. Sugiyama, M. Yokoyama, H. Ishizuka, K.-I. Sotowa, T. Tomida, N. Shigemoto, J. Colloid Interface Sci. (2005), in press. T. Staffel, in: B. Elvers, S. Hawkins, G. Schulz (Eds.), Ullmann’s Encyclopedia of Industrial Chemistry, vol. 19, VCH Verlagsgesellschaft, Germany, 1991, pp. 497–498. E. Hayek, H. Newesely, Inorg. Synth. 7 (1963) 63. A. Bigi, E. Foresti, F. Marchetti, A. Ripamonti, N. Roveri, J. Chem. Soc. Dalton Trans. (1984) 1091. A. Yasukawa, T. Yokoyama, K. Kandori, T. Ishikawa, Colloids Surf. A 238 (2004) 133. J.C. Rendon-Angeles, K. Yanagisawa, N. Ishizawa, S. Oishi, J. Solid State Chem. 151 (2000) 65. N. Masayuki, T. Nishino, T. Saeki, Sensors Actuat. 15 (1988) 145.
S. Sugiyama et al. / Journal of Colloid and Interface Science 295 (2006) 141–147
[24] L. El-Hammari, A. Laghzizil, P. Barboux, A. Saoiabi, K. Lahlil, J. Solid State Chem. 177 (2004) 134. [25] A. Nzihou, B. Adhikari, R. Pfeffer, Ind. Eng. Chem. Res. 44 (2005) 1787. [26] S. Sakka, in: M. Tomozawa, R. Doremus (Eds.), Treatise on Materials Science and Technology, Academic Press, New York, 1982, pp. 129–167. [27] S. Suzuki, H. Nakane, M. Ohshima, M. Takahashi, Y. Hikichi, J. Soc. Mater. Sci. Jpn. 41 (1992) 939. [28] H. Iwasaki, Y. Kaneko, Zairyo 37 (1988) 60.
147
[29] S. Sugiyama, M. Fujii, H. Hayashi, K. Kusakabe, Inorg. Chem. Commun. 7 (2004) 569. [30] A. Jillavenkatesa, R.A. Condrate Sr, J. Mater. Sci. 33 (1998) 4111. [31] A. Bigi, E. Boanini, K. Rubini, J. Solid State Chem. 177 (2004) 3092. [32] J.C. Elliott, Structure and Chemistry of the Apatites and Other Calcium Orthophosphates, in: Studies in Inorganic Chemistry, vol. 18, Elsevier, Amsterdam, 1994, pp. 18–20. [33] O. Smidsrod, Faraday Discuss. Chem. Soc. 57 (1974) 263.