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Topotactic exchange and intercalation of calcium phosphate
Solid State Sciences 6 (2004) 1245–1250 www.elsevier.com/locate/ssscie
Topotactic exchange and intercalation of calcium phosphate Cicero B.A. Lima a , Claudio Airoldi b,∗ a Departamento de Química, Universidade Estadual do Rio Grande do Norte, Caixa Postal 70, 59633-010 Mossoró, Rio Grande do Norte, Brazil b Instituto de Química, Universidade Estadual de Campinas, Caixa Postal 6154, 13084-971 Campinas, São Paulo, Brazil
Received 12 February 2004; received in revised form 16 June 2004; accepted 23 June 2004 Available online 11 September 2004
Abstract The precursor (NH4 )2 Ca(H2 PO4 )2 ·H2 O (CaAP) compound was obtained by combining a calcium chloride solution with dibasic ammonium phosphate. After submitting it to a thermal treatment, crystalline calcium phosphate, Ca(H2 PO4 )2 ·H2 O (CaP) was isolated. X-ray diffraction patterns for this compound indicated good crystallinity, with a peak at 2θ = 12.8◦ , to give an interlamellar distance of 697 pm, which changed to 1550 pm, when the reaction employed phenylphosphonic acid, and to 1514 pm when intercalated with methylamine. Phosphorus and calcium analysis from colorimetric and gravimetric methods gave for CaP 24.2 and 15.8%, respectively, to yield a P:Ca molar ratio equal to two. The phosphorus nuclear magnetic resonance presented a peak centered at −1.23 ppm, in agreement with the existence of phosphate groups in protonated form. CaAP showed a mass loss of 21.2% in the 466 to 541 K interval due to ammonia and water elimination to yield Ca(PO3 )3 , and CaP can be dehydrated at 440 K for 6 h. A topotactical exchange occurred when CaP is intercalated with methylamine or reacted with phenylphosphonic acid to yield the phosphonate compound and the infrared spectrum of the resulting compound clearly showed the presence of PO4 and PO3 groups. The topotactic exchange was also demonstrated by X-ray diffractometry in following the stages of decomposition from 527 to 973 K. 2004 Elsevier SAS. All rights reserved. Keywords: Calcium phosphate; Topotactic exchange; Ethylamine; Phenylphosphonic acid
1. Introduction Crystalline layered organic α-zirconium phosphate derivatives, represented most commonly by the phosphonate compounds, with the general formula Zr(RPO3 )2 ·H2 O, with R being an organic moiety, can be synthesized by using the appropriate phosphonic acid as reagent. Taking into account that such classes of compounds started with zirconium, then, several features associated with the chemistry of αzirconium phosphonates have been well developed. Previous host applications in intercalation processes were carried out through neutralization of Brønsted and Lewis centers available on the inorganic layers. These investigations were focused on ionic conductivity, ion exchange properties, photochemical reactions and catalysis. Those are some examples * Corresponding author.
E-mail address: [email protected] (C. Airoldi). 1293-2558/$ – see front matter 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.06.009
for applications of such layered compounds. Organic derivative preparations from γ -zirconium phosphates are also reported, with direct synthetic methods based on topotactic reactions between pre-formed γ -zirconium phosphate and phosphonic acids or by using propylene oxide. Meanwhile, the same synthetic procedure directed to α-zirconium phosphonate derivatives is based on the pre-formed α-zirconium phosphate precursor [1–4]. Changing from the well-known zirconium compounds to calcium, no report describes any derivative formed when employing synthetic topotactic reactions between calcium phosphate and phosphonic acid solutions or by using the alkylmonoamine solutions. However, layered metal phosphonates can also be formed by reacting metal hydroxides with molten phosphonic acid. Application of this synthetic method avoids the employment of extreme experimental condition [5–10]. A synthetic topotactic procedure may lead to a variety of possible products, depending on the extent to which the
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phosphate can be replaced by phenylphosphonate groups. During the synthesis a possible type of product can be a single derivative phase, containing both phenylphosphonate and phosphate groups dispersed between the layers. As expected, when a phosphate compound reacts with phosphonic acid, the O3 PO4 group can be substituted in the lamellar structure by the equivalent O3 PR group [1,3,11,12]. Under such conditions, the formation of a single phase, for example, from this mixture of reactants to give a phosphate– metal–phosphonate arrangement, requires coprecipitation of the cation with phosphate and phosphonate. Thus, a simple physical mixture can be obtained from an α-phosphate of the cation and phenylphosphonate [11,13,14]. Hypothetically, the mixture of compounds can be prepared with various interlamellar arrangements containing functional groups, usually, an organic R group and inorganic H or OH groups. The interlamellar distance for this simple mixture of phosphate–phosphonate solid phase is located in between pure phosphate and phenylphosphonate compounds. As an illustration, the interlamellar distance for titanium phenylphosphonate is 1520 pm, while for the phosphate of the same metal it is equal to 760 pm. The preparation of organic γ -zirconium phosphonate derivatives using topotactic reactions with phosphonic acids in 1:1 water– acetone solutions was reported several years after elucidation of the lamellar structure of γ -tetravalent metal phosphonates [11,15]. A general procedure to synthesize metal organophosphonate compounds consists in reacting, at low temperature, the metallic salt, normally in the form of chloride or sulphate, with the desired acid. For example, the precursor calcium phenylphosphonate, Ca(HO3 PC6 H5 )2 ·2H2 O, can be prepared by mixing a phosphonic acid solution with dihydrated calcium chloride, at a pH between 5 and 6, with a sodium hydroxide solution [5,7]. The present investigation deals with the topotactic synthesis of calcium phenylphosphonate derivatives by reacting calcium phosphate and phenylphosphonic acid, and its intercalated methylamine compound.
2. Experimental 2.1. Reagents Reagent grade materials were used throughout for the syntheses, including calcium chloride, ammonium phosphate, sodium hydroxide, phenylphosphonic acid and methylamine. 2.2. Synthesis Hydrate calcium phosphate, Ca(H2 PO4 )2 ·H2 O (CaP), was synthesized by slowly adding a dilute solution of calcium chloride dihydrate to a 1.50 mol dm−3 dibasic ammonium phosphate solution. The stirred suspension was main-
tained at 360 K for 1 h, after which time the solid settled out, was filtered and dried at 320 K. The intermediate solid, (NH4 )2 Ca(HPO4 )2 ·H2 O (CaAP), was heated at 440 K in a muffle to ammonia elimination. The preparative processes for these compounds are summarized in the following reactions [16,17]: CaCl2 ·2H2 O + 2(NH4 )2 HPO4 → (NH4 )2 Ca(HPO4 )2 ·H2 O + 2NH4 Cl + H2 O, (NH4 )2 Ca(HPO4 )2 ·H2 O → Ca(H2 PO4 )2 ·H2 O + 2NH3 . 2.3. Topotactic exchange This exchange process consisted in reacting phenylphosphonic acid with calcium phosphate in aqueous solution at 350 K under reflux for 24 h. The suspension formed was centrifuged and the solid was dried at 320 K for 24 h, as represented by [2,18,19] Ca(H2 PO4 )2 ·H2 O + C6 H5 PO3 H2 → Ca(H2 PO4 )(C6 H5 PO3 H)·H2O + H3 PO4 . 2.4. Intercalation From the intercalation of the desired amine, methylamine, into the host calcium phosphate, the corresponding isotherms as a function of time or concentration were defined by using the batch method, whose equilibrium was performed at the solid–liquid interface. To obtain these isotherms, different methylamine solutions, varying from 4.0 × 10−3 to 0.50 mol dm−3 , were orbitally stirred with calcium phosphate in polyethylene flasks for 24 h. The suspension was centrifuged and the solid was dried at 320 K for 12 h. The intercalation process can be generally represented by [17,20,21] Ca(H2 PO4 )2 ·H2 O + xH2 NCH3 → Ca(H2 PO4 )2 · x(H2 NCH3 )2 · (1 −x)H2O + xH2 O. 2.5. Analytical determinations Calcium content in the host matrix was analyzed from a 0.10 g sample dissolved in concentrated nitric acid. By adding ammonium nitrate to this solution, followed by ammonium molybdate, the phosphorus component was precipitated as ammonium phosphomolybdate. After filtration of the solid, samples of the supernatant were titrated with a EDTA standard solution, using Calcon as indicator. The amount of phosphorus was also colorimetrically analyzed in the nitric acid solution by using ammonium molybdate and ascorbic acid. After the solution had stood for 24 h, the absorbance at 860 nm was measured and the element was determined from a standard analytical curve [3,22]. 2.6. Physical measurements Infrared spectra with 4 cm−1 resolution were obtained using a Perkin-Elmer model 1600 FTIR spectrophotometer,
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using samples pressed into KBr pellets. Thermogravimetric curves were recorded on a DuPont model 1090 B apparatus coupled to a thermobalance, model 951, on heating from room temperature to 1273 K, at a heating rate of 0.16 K s−1 in an argon flow of 1.67 cm3 s−1 , for samples varying in mass from 15.0 to 30.0 mg. X-ray diffraction patterns were performed with nickel-filtered Cu-Kα radiation on a Shimadzu model XD3-A diffractometer (30/20 kV/mA). The nuclear magnetic resonance spectra of the solid materials were obtained on an AC 300/P Bruker spectrometer at room temperature at 121.0 and 75.5 MHz for phosphorus and carbon. A pulse repetition time of 3 s and a contact time of 3 ms were used. Fig. 1. 31 P NMR spectrum of calcium phosphate in the solid phase.
3. Results and discussion Calcium and phosphorus elemental analysis determinations for the synthesized compound gave 15.9 and 24.6%, respectively, values which are very close to the expected amounts, 15.8 and 24.2%, for the proposed formula Ca(H2 PO4 )2 ·H2 O. From these percentage values the corresponding number of moles of each element was calculated to give phosphorus to calcium molar ratio equal to two. The nuclear magnetic resonance of phosphorus-31 for calcium phosphate in the solid state presented a peak centered at −1.23 ppm, as shown in Fig. 1, indicating that the phosphate groups are in the protonated form. The main peak is followed by symmetrical and less intense signals, characterizing the satellite peaks. These are due to anisotropy, recognized by the symmetric positions and also because the distance up to the central peak varies with the speed of rotation of the sample tube [10,23,24]. The X-ray diffraction powder patterns for the synthetic compound, after reacting with phenylphosphonic acid, and the corresponding intercalated form with methylamine are shown in Fig. 2. Sharp peaks indicate crystallinity of the resulting solids, and the peak at 2θ = 12.8◦ for the 002 reflection plane corresponds to an interlayer distance of 697 pm for the original lamellar compound, as shown in Fig. 2a. For comparison, the phenylphosphonic derivative showed the appearance of a peak at 2θ = 5.8◦ that gives an interlamellar distance of 1514 pm. This increase in interlayer distance is in agreement with the existence of phosphonate groups, after reaction with the precursor phosphate in aqueous solution, as shown in Fig. 2b. However, other peaks similar to those found for phosphonate are also present in the phenylphosphonate diffraction patterns. Based on these data it is reasonable to suggest a mixture of phenylphosphonate and phosphate in the newly synthesized compound that implies the presence of free OH groups bonded in the inorganic backbone. In a subsequent step, the methylamine was intercalated and the interlamellar distance increases to 2θ = 5.9◦ to give 1545 pm in relation to the original host, as is shown in Fig. 2c. Thus, when calcium phosphate and phosphonate are present, the amine solution established equilibrium at
Fig. 2. X-ray diffraction patterns for calcium phosphate (a), after reacting with phenylphosphonic acid in aqueous solution (b) and when intercalated with methylamine (c).
solid/liquid phase. In this case, the amine is preferentially intercalated into favorable host positions, due to acid–base interactions, then it is expected its inclusion into calcium phosphate rather than the calcium phenylphosphonate derivative [3,8,11,15]. The infrared spectrum for calcium ammonium phosphate, (NH4 )2 Ca(H2 PO4 )2 ·H2 O, is shown in Fig. 3a. A low broad peak at 3500 cm−1 is attributed to OH stretching group vibration that is a good source to give information about the presence of coordinated water molecules in the inorganic structure. Two other very weak bands, related to N–H bonds refer to the ammonium cation in the compound and appear at 3000 and 2870 cm−1 . When the compound CaAP is heated, ammonia is eliminated to yield the phosphate
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Fig. 4. Infrared spectra of hydrated calcium phenylphosphonate (a) and the methylamine intercalated compound (b).
Fig. 3. Infrared spectra of calcium ammonium phosphate (a), calcium phosphate (b), calcium phosphate intercalated with methylamine (c) and calcium phosphate after reacting with phenylphosphonic acid (d).
CaP, which infrared spectrum does not indicate the presence of any band related to N–H bond, as shown in Fig. 3b. Identically, the same set of bands was observed even when methylamine was previously intercalated before reacting with phenylphosphonic acid into this latter compound. The characteristic main bands for phosphate groups are located at 1330 and 1010 cm−1 . The infrared spectrum for the product of methylamine–calcium phosphate reaction is shown in Fig. 3c. The bands correspond to symmetric and asymmetric N–H deformation modes are observed in the 3380– 3300 cm−1 interval. Very weak bands in the 3000–2900 cm−1 range are related to the symmetric and asymmetric C– H ring while the sharp band at 1437 cm−1 corresponds to the stretching of the C–C double bond [5,16,17,25]. Hydrated calcium phenylphosphonate, Ca(HO3 PC6 H5 )2 · 2H2 O, was previously obtained through the reaction of phenylphosphonic acid with calcium chloride. This crystalline lamellar compound was intercalated with a series of alkylmonoamines and in this intercalation process the corresponding amine can replace the water molecules bonded to the inorganic layer [7,26]. The infrared spectrum of the hydrated calcium phenylphosphonate and when intercalated with methylamine are shown in Fig. 4a and 4b, respectively. As expected, both spectra presented a series of common bands. Thus, the band at 1438 cm−1 and the medium bands in the 720–644 cm−1 interval are characteristic of the phenyl ring. The bands that appeared in the 1340– 1017 cm−1 range are assigned to the PO3 group. The other bands in the 3000–2900 cm−1 range are related to symmet-
ric and asymmetric carbon–hydrogen stretching vibrations. Moreover, the appearance of absorption bands in the 3400– 3300 cm−1 interval might correspond to the symmetric and asymmetric nitrogen–hydrogen stretching bands, and an absence of oxygen–hydrogen bonds on the phosphonate groups [27–29]. By comparing Figs. 4b and 3c, identical sequences of bands are observed, indicating the success of preparation through distinct routes. The infrared spectrum of calcium phosphate, after reacting with phenylphosphonic acid is shown in Fig. 3d. The bands that appeared at 1105, 1080 and 1017 cm−1 are attributed to PO3 group and the deformation band of the P–O bond appeared at 1144 cm−1 , and for the ammonium compound very weak bands at 2950–2850 cm−1 appeared, which may be attributed to N–H stretching frequencies. Thus, the reaction of formation of the organic derivative between calcium phosphate and phenylphosphonic acid can be considered as an exchange process by involving the corresponding monovalent anions [12]. The substitution reaction can be directly obtained on a lamellar exfoliated surface, followed when species interdiffused into the interlamellar region. In this case, as a result of the reaction with phenylphosphonic acid, an increase in organic groups are bonded in the exfoliated inorganic lamella, to form a calcium phosphate–phosphonate mixture, as represented by Ca(H2 PO4 )(C6 H5 PO3 H)·H2 O. The proposed mechanism of the topotoctic reaction is shown in Fig. 5. As represented in Fig. 5a, initially one of the oxygen atoms of the anion O2 P(OH)2 − is freed from calcium atom coordination, followed by a hydrolytic substitution by the oxygen atoms of the new anion O2 P(OH)2 C6 H5 − over the available site of coordination. Part b illustrated the participation of the amine, when an oxygen atom of the precursor anion O2 P(OH)2 − is free from the calcium coordination. The topotactical mechanism of reaction with methylamine is similar, however, after hydrolytic substitution, the amine removes a water molecule and, consequently, such water molecule leaves the phosphate group, enabling the phosphonate group to bond. In such a process, beyond the phosphonate species O2 P(OH)2 C6 H5 − , also is simulta-
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Fig. 5. Mechanism proposed for the topotaxic reaction between calcium phosphate and phenylphosphonic acid. Fig. 7. X-ray diffraction patterns of hydrated calcium phenylphosphonate after heating at different temperatures: 523 (a), 573 (b), 673 (c), 773 (d) and 973 K (e). The symbol ◦ in the diffractogram indicates the presence of Ca2 P2 O7 .
(a)
temperature, to form as a final residue calcium pyrophosphate, Ca2 P2 O7 . The observed stage of decomposition from 460 to 500 K can be assigned to ammonia elimination, to give Ca(H2 PO4 )2 ·H2 O. The next step does not distinguish the dehydration and the OH condensation to form water, in the 500– 700 K range, to give Ca(PO3 )2 compound. The last stage not shown in the thermogravimetric curve should be related to the loss of phosphorus oxide to form a final residue of pyrophosphate, over 900 K. Based on this percentage of 21.6% the newly synthesized compound has the formula established as (NH4 )2 Ca(H2 PO4 )2 ·H2 O [5,7,16]. The proposed mechanism of decomposition can be formulated as follows: (NH4 )2 Ca(HPO4 )2 ·H2 O −2NH3 Ca(H2 PO4 )2 ·H2 O 460–500 K −3H2 O Ca(PO3 )2 500–700 K 1 1 Ca2 P2 O7 + P2 O5 . 2 2 >900 K
(b) Fig. 6. Thermogravimetric curves of calcium ammonium phosphate (a) and calcium phosphate (b).
neously presented the phosphate anions O2 P(OH)2 − bonded to the crystalline structure [15]. The thermogravimetric curve of calcium ammonium phosphate is shown in Fig. 6a, presenting a mass loss of 21.6% in the 460–700 K range, which corresponds to a displacement of ammonia and water from the crystalline compound to form Ca(PO3 )2 . The next expected step should be related to the loss of phosphorus oxide, P2 O5 , at higher
The crystalline lamellar Ca(H2 PO4 )2 ·H2 O compound lost one mole of water of hydration and two other due to condensation of OH groups bonded to phosphate with a mass loss of 17.8%, in the same region, between 600–800 K, to give Ca(PO3 )2 , as shown in Fig. 6b. It is also expected to end during the heating the pyrophosphate, as proposed before. This proposed mechanism of condensation was previously observed for Ca(HO3 PC6 H5 )2 ·2H2 O [7], which compound should be also formed during the topotactic synthesis. To elucidate the changes during this process to end up in pyrophosphate, both the phosphate and phosphonate compounds were submitted to different temperatures and the X-ray powder patterns recorded. The same set of patterns was observed and for phosphonate is shown in Fig. 7. Thus,
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when this compound was heated at 523 K the water molecules were removed to form the anhydrous crystalline compound, as shown in Fig. 7a. The lamellar structure collapses in the 573–673 K range due to phenyl groups removal and, consequently, the lost of crystallinity, as shown in Fig. 7b and 7c. Finally, the crystalline arrangement starts to appear at 773 K, as illustrated by the X-ray patterns in Fig. 7d, a process which is more evident with an intensification of crystallinity at 973 K. At this temperature calcium pyrophosphate is formed and the characteristic peaks of the compound are indicated in Fig. 7e.
4. Conclusion Topotactic exchange was obtained when calcium phosphate reacted with phenylphosphonic acid to give a compound similar to hydrated calcium phenylphosphonate. This fact was observed also when calcium phosphate reacted with the methylamine intercalated compound. In this process the intercalation caused an increase in the interlamellar distance in relation to the original Ca(H2 PO4 )2 ·H2 O compound. The infrared spectrum for calcium phosphate, after intercalation with methylamine, showed clearly bands which are attributed to PO3 groups. However, the new compound presented bands related to PO4 and PO3 groups, when the compound Ca(H2 PO4 )2 ·H2 O reacted with phenylphosphonic acid. Beyond the known route involving the reaction between calcium phenylphosphonate with methylamine, the present investigation illustrates a new way to form phosphonate derivatives starting with pre-formed calcium phosphate.
Acknowledgement The authors are indebted to CNPq for fellowships and to FAPESP for financial support.
References [1] A. Clearfield, Progr. Inorg. Chem. 47 (1998) 47. [2] M. Ogawa, K. Kuroda, Chem. Rev. 95 (1995) 399. [3] B.G.B. Hix, S.J. Kitchin, K.D.M. Harris, J. Chem. Soc., Dalton Trans. (1998) 2315. [4] U. Costantino, M. Casciola, L. Massinelli, M. Nocchetti, R. Vivani, Solid State Ionics 97 (1997) 203. [5] Y. Murakami, H. Imai, J. Ceram. Soc. Jpn. 100 (1992) 439. [6] A.H. Mahmoudkhani, V. Langer, Solid State Sci. 3 (2001) 519. [7] C.B.A. Lima, C. Airoldi, Solid State Sci. 4 (2002) 1321. [8] G. Alberti, U. Costantino, J. Mol. Catal. 27 (1984) 235. [9] B. Shpeizer, D.M. Poojary, K. Ahn, C.F. Runyan, A. Clearfield, Science 266 (1994) 1357. [10] J.L. Miquel, L. Facchini, A.P. Legrard, C. Rey, J. Lemaitre, Colloids Surf. 45 (1990) 427. [11] E. Jaimez, A. Bortun, G.B. Hix, J.R. Garcia, J. Rodriguez, R.C.T. Slade, J. Chem. Soc., Dalton Trans. (1996) 2285. [12] F. Fredoveil, D. Massiot, P. Janvier, F. Ginge, M. Bujoli-Doenff, M. Evain, A. Clearfield, B. Bujoli, Inorg. Chem. 38 (1999) 1831. [13] A. Bortun, E. Jaimez, R. Leavona, J.R. García, J. Rodriguez, Mater. Res. Bull. 30 (1995) 413. [14] A.N. Christensen, E.K. Andersen, I.G.K. Andersen, G. Alberti, M. Nielsen, M.B. Lehmann, Acta Chem. Scand. 44 (1990) 865. [15] G. Alberti, E. Giontella, S.M. Mascarós, Inorg. Chem. 36 (1997) 2844. [16] A. Goni, J. Rius, M. Insausti, L.M. Lezama, J.L. Pizarro, M.I. Arriortua, T. Rojo, Chem. Mater. 8 (1996) 1052. [17] J. Fraissard, J. Etienne, J. Bull. Soc. Fr., Minéral. Crystallogr. 90 (1967) 162. [18] G. Alberti, T. Bein (Eds.), Comprehensive Supramolecular Chemistry, vol. 7, Pergamon, New York, 1996, . [19] U. Costantino, in: A. Clearfield (Ed.), Inorganic Ion Exchange Materials, CRC Press, Boca Raton, FL, 1982. [20] L.M. Nunes, C. Airoldi, Thermochim. Acta 328 (1999) 297. [21] K.G. Bhattachatyya, Langmuir 8 (1992) 2284. [22] A.I. Vogel, A Text-Book of Quantitative Inorganic Analysis: Theory and Practice, fourth ed., Longmans, London, 1978, p. 320, 756. [23] W.P. Rothwell, J.S. Waugh, J.P. Yesirowski, J. Amer. Chem. Soc. 10 (1980) 2637. [24] S. Carlino, M.J. Hudson, W.J. Locke, J. Mater. Chem. 7 (1997) 813. [25] C. Airoldi, S. Roca, J. Solution Chem. 22 (1993) 707. [26] C.B.A. Lima, C. Airoldi, Thermochim. Acta 400 (2003) 51. [27] G. Cao, T.E. Mallouk, Inorg. Chem. 30 (1991) 1434. [28] Y. Zhang, K.J. Scott, A. Clearfield, Chem. Mater. 5 (1993) 495. [29] D.J. Machlan, K.R. Morgan, J. Phys. Chem. Mater. 94 (1990) 7656.
Topotactic exchange and intercalation of calcium phosphate Cicero B.A. Lima a , Claudio Airoldi b,∗ a Departamento de Química, Universidade Estadual do Rio Grande do Norte, Caixa Postal 70, 59633-010 Mossoró, Rio Grande do Norte, Brazil b Instituto de Química, Universidade Estadual de Campinas, Caixa Postal 6154, 13084-971 Campinas, São Paulo, Brazil
Received 12 February 2004; received in revised form 16 June 2004; accepted 23 June 2004 Available online 11 September 2004
Abstract The precursor (NH4 )2 Ca(H2 PO4 )2 ·H2 O (CaAP) compound was obtained by combining a calcium chloride solution with dibasic ammonium phosphate. After submitting it to a thermal treatment, crystalline calcium phosphate, Ca(H2 PO4 )2 ·H2 O (CaP) was isolated. X-ray diffraction patterns for this compound indicated good crystallinity, with a peak at 2θ = 12.8◦ , to give an interlamellar distance of 697 pm, which changed to 1550 pm, when the reaction employed phenylphosphonic acid, and to 1514 pm when intercalated with methylamine. Phosphorus and calcium analysis from colorimetric and gravimetric methods gave for CaP 24.2 and 15.8%, respectively, to yield a P:Ca molar ratio equal to two. The phosphorus nuclear magnetic resonance presented a peak centered at −1.23 ppm, in agreement with the existence of phosphate groups in protonated form. CaAP showed a mass loss of 21.2% in the 466 to 541 K interval due to ammonia and water elimination to yield Ca(PO3 )3 , and CaP can be dehydrated at 440 K for 6 h. A topotactical exchange occurred when CaP is intercalated with methylamine or reacted with phenylphosphonic acid to yield the phosphonate compound and the infrared spectrum of the resulting compound clearly showed the presence of PO4 and PO3 groups. The topotactic exchange was also demonstrated by X-ray diffractometry in following the stages of decomposition from 527 to 973 K. 2004 Elsevier SAS. All rights reserved. Keywords: Calcium phosphate; Topotactic exchange; Ethylamine; Phenylphosphonic acid
1. Introduction Crystalline layered organic α-zirconium phosphate derivatives, represented most commonly by the phosphonate compounds, with the general formula Zr(RPO3 )2 ·H2 O, with R being an organic moiety, can be synthesized by using the appropriate phosphonic acid as reagent. Taking into account that such classes of compounds started with zirconium, then, several features associated with the chemistry of αzirconium phosphonates have been well developed. Previous host applications in intercalation processes were carried out through neutralization of Brønsted and Lewis centers available on the inorganic layers. These investigations were focused on ionic conductivity, ion exchange properties, photochemical reactions and catalysis. Those are some examples * Corresponding author.
E-mail address: [email protected] (C. Airoldi). 1293-2558/$ – see front matter 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.06.009
for applications of such layered compounds. Organic derivative preparations from γ -zirconium phosphates are also reported, with direct synthetic methods based on topotactic reactions between pre-formed γ -zirconium phosphate and phosphonic acids or by using propylene oxide. Meanwhile, the same synthetic procedure directed to α-zirconium phosphonate derivatives is based on the pre-formed α-zirconium phosphate precursor [1–4]. Changing from the well-known zirconium compounds to calcium, no report describes any derivative formed when employing synthetic topotactic reactions between calcium phosphate and phosphonic acid solutions or by using the alkylmonoamine solutions. However, layered metal phosphonates can also be formed by reacting metal hydroxides with molten phosphonic acid. Application of this synthetic method avoids the employment of extreme experimental condition [5–10]. A synthetic topotactic procedure may lead to a variety of possible products, depending on the extent to which the
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phosphate can be replaced by phenylphosphonate groups. During the synthesis a possible type of product can be a single derivative phase, containing both phenylphosphonate and phosphate groups dispersed between the layers. As expected, when a phosphate compound reacts with phosphonic acid, the O3 PO4 group can be substituted in the lamellar structure by the equivalent O3 PR group [1,3,11,12]. Under such conditions, the formation of a single phase, for example, from this mixture of reactants to give a phosphate– metal–phosphonate arrangement, requires coprecipitation of the cation with phosphate and phosphonate. Thus, a simple physical mixture can be obtained from an α-phosphate of the cation and phenylphosphonate [11,13,14]. Hypothetically, the mixture of compounds can be prepared with various interlamellar arrangements containing functional groups, usually, an organic R group and inorganic H or OH groups. The interlamellar distance for this simple mixture of phosphate–phosphonate solid phase is located in between pure phosphate and phenylphosphonate compounds. As an illustration, the interlamellar distance for titanium phenylphosphonate is 1520 pm, while for the phosphate of the same metal it is equal to 760 pm. The preparation of organic γ -zirconium phosphonate derivatives using topotactic reactions with phosphonic acids in 1:1 water– acetone solutions was reported several years after elucidation of the lamellar structure of γ -tetravalent metal phosphonates [11,15]. A general procedure to synthesize metal organophosphonate compounds consists in reacting, at low temperature, the metallic salt, normally in the form of chloride or sulphate, with the desired acid. For example, the precursor calcium phenylphosphonate, Ca(HO3 PC6 H5 )2 ·2H2 O, can be prepared by mixing a phosphonic acid solution with dihydrated calcium chloride, at a pH between 5 and 6, with a sodium hydroxide solution [5,7]. The present investigation deals with the topotactic synthesis of calcium phenylphosphonate derivatives by reacting calcium phosphate and phenylphosphonic acid, and its intercalated methylamine compound.
2. Experimental 2.1. Reagents Reagent grade materials were used throughout for the syntheses, including calcium chloride, ammonium phosphate, sodium hydroxide, phenylphosphonic acid and methylamine. 2.2. Synthesis Hydrate calcium phosphate, Ca(H2 PO4 )2 ·H2 O (CaP), was synthesized by slowly adding a dilute solution of calcium chloride dihydrate to a 1.50 mol dm−3 dibasic ammonium phosphate solution. The stirred suspension was main-
tained at 360 K for 1 h, after which time the solid settled out, was filtered and dried at 320 K. The intermediate solid, (NH4 )2 Ca(HPO4 )2 ·H2 O (CaAP), was heated at 440 K in a muffle to ammonia elimination. The preparative processes for these compounds are summarized in the following reactions [16,17]: CaCl2 ·2H2 O + 2(NH4 )2 HPO4 → (NH4 )2 Ca(HPO4 )2 ·H2 O + 2NH4 Cl + H2 O, (NH4 )2 Ca(HPO4 )2 ·H2 O → Ca(H2 PO4 )2 ·H2 O + 2NH3 . 2.3. Topotactic exchange This exchange process consisted in reacting phenylphosphonic acid with calcium phosphate in aqueous solution at 350 K under reflux for 24 h. The suspension formed was centrifuged and the solid was dried at 320 K for 24 h, as represented by [2,18,19] Ca(H2 PO4 )2 ·H2 O + C6 H5 PO3 H2 → Ca(H2 PO4 )(C6 H5 PO3 H)·H2O + H3 PO4 . 2.4. Intercalation From the intercalation of the desired amine, methylamine, into the host calcium phosphate, the corresponding isotherms as a function of time or concentration were defined by using the batch method, whose equilibrium was performed at the solid–liquid interface. To obtain these isotherms, different methylamine solutions, varying from 4.0 × 10−3 to 0.50 mol dm−3 , were orbitally stirred with calcium phosphate in polyethylene flasks for 24 h. The suspension was centrifuged and the solid was dried at 320 K for 12 h. The intercalation process can be generally represented by [17,20,21] Ca(H2 PO4 )2 ·H2 O + xH2 NCH3 → Ca(H2 PO4 )2 · x(H2 NCH3 )2 · (1 −x)H2O + xH2 O. 2.5. Analytical determinations Calcium content in the host matrix was analyzed from a 0.10 g sample dissolved in concentrated nitric acid. By adding ammonium nitrate to this solution, followed by ammonium molybdate, the phosphorus component was precipitated as ammonium phosphomolybdate. After filtration of the solid, samples of the supernatant were titrated with a EDTA standard solution, using Calcon as indicator. The amount of phosphorus was also colorimetrically analyzed in the nitric acid solution by using ammonium molybdate and ascorbic acid. After the solution had stood for 24 h, the absorbance at 860 nm was measured and the element was determined from a standard analytical curve [3,22]. 2.6. Physical measurements Infrared spectra with 4 cm−1 resolution were obtained using a Perkin-Elmer model 1600 FTIR spectrophotometer,
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using samples pressed into KBr pellets. Thermogravimetric curves were recorded on a DuPont model 1090 B apparatus coupled to a thermobalance, model 951, on heating from room temperature to 1273 K, at a heating rate of 0.16 K s−1 in an argon flow of 1.67 cm3 s−1 , for samples varying in mass from 15.0 to 30.0 mg. X-ray diffraction patterns were performed with nickel-filtered Cu-Kα radiation on a Shimadzu model XD3-A diffractometer (30/20 kV/mA). The nuclear magnetic resonance spectra of the solid materials were obtained on an AC 300/P Bruker spectrometer at room temperature at 121.0 and 75.5 MHz for phosphorus and carbon. A pulse repetition time of 3 s and a contact time of 3 ms were used. Fig. 1. 31 P NMR spectrum of calcium phosphate in the solid phase.
3. Results and discussion Calcium and phosphorus elemental analysis determinations for the synthesized compound gave 15.9 and 24.6%, respectively, values which are very close to the expected amounts, 15.8 and 24.2%, for the proposed formula Ca(H2 PO4 )2 ·H2 O. From these percentage values the corresponding number of moles of each element was calculated to give phosphorus to calcium molar ratio equal to two. The nuclear magnetic resonance of phosphorus-31 for calcium phosphate in the solid state presented a peak centered at −1.23 ppm, as shown in Fig. 1, indicating that the phosphate groups are in the protonated form. The main peak is followed by symmetrical and less intense signals, characterizing the satellite peaks. These are due to anisotropy, recognized by the symmetric positions and also because the distance up to the central peak varies with the speed of rotation of the sample tube [10,23,24]. The X-ray diffraction powder patterns for the synthetic compound, after reacting with phenylphosphonic acid, and the corresponding intercalated form with methylamine are shown in Fig. 2. Sharp peaks indicate crystallinity of the resulting solids, and the peak at 2θ = 12.8◦ for the 002 reflection plane corresponds to an interlayer distance of 697 pm for the original lamellar compound, as shown in Fig. 2a. For comparison, the phenylphosphonic derivative showed the appearance of a peak at 2θ = 5.8◦ that gives an interlamellar distance of 1514 pm. This increase in interlayer distance is in agreement with the existence of phosphonate groups, after reaction with the precursor phosphate in aqueous solution, as shown in Fig. 2b. However, other peaks similar to those found for phosphonate are also present in the phenylphosphonate diffraction patterns. Based on these data it is reasonable to suggest a mixture of phenylphosphonate and phosphate in the newly synthesized compound that implies the presence of free OH groups bonded in the inorganic backbone. In a subsequent step, the methylamine was intercalated and the interlamellar distance increases to 2θ = 5.9◦ to give 1545 pm in relation to the original host, as is shown in Fig. 2c. Thus, when calcium phosphate and phosphonate are present, the amine solution established equilibrium at
Fig. 2. X-ray diffraction patterns for calcium phosphate (a), after reacting with phenylphosphonic acid in aqueous solution (b) and when intercalated with methylamine (c).
solid/liquid phase. In this case, the amine is preferentially intercalated into favorable host positions, due to acid–base interactions, then it is expected its inclusion into calcium phosphate rather than the calcium phenylphosphonate derivative [3,8,11,15]. The infrared spectrum for calcium ammonium phosphate, (NH4 )2 Ca(H2 PO4 )2 ·H2 O, is shown in Fig. 3a. A low broad peak at 3500 cm−1 is attributed to OH stretching group vibration that is a good source to give information about the presence of coordinated water molecules in the inorganic structure. Two other very weak bands, related to N–H bonds refer to the ammonium cation in the compound and appear at 3000 and 2870 cm−1 . When the compound CaAP is heated, ammonia is eliminated to yield the phosphate
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Fig. 4. Infrared spectra of hydrated calcium phenylphosphonate (a) and the methylamine intercalated compound (b).
Fig. 3. Infrared spectra of calcium ammonium phosphate (a), calcium phosphate (b), calcium phosphate intercalated with methylamine (c) and calcium phosphate after reacting with phenylphosphonic acid (d).
CaP, which infrared spectrum does not indicate the presence of any band related to N–H bond, as shown in Fig. 3b. Identically, the same set of bands was observed even when methylamine was previously intercalated before reacting with phenylphosphonic acid into this latter compound. The characteristic main bands for phosphate groups are located at 1330 and 1010 cm−1 . The infrared spectrum for the product of methylamine–calcium phosphate reaction is shown in Fig. 3c. The bands correspond to symmetric and asymmetric N–H deformation modes are observed in the 3380– 3300 cm−1 interval. Very weak bands in the 3000–2900 cm−1 range are related to the symmetric and asymmetric C– H ring while the sharp band at 1437 cm−1 corresponds to the stretching of the C–C double bond [5,16,17,25]. Hydrated calcium phenylphosphonate, Ca(HO3 PC6 H5 )2 · 2H2 O, was previously obtained through the reaction of phenylphosphonic acid with calcium chloride. This crystalline lamellar compound was intercalated with a series of alkylmonoamines and in this intercalation process the corresponding amine can replace the water molecules bonded to the inorganic layer [7,26]. The infrared spectrum of the hydrated calcium phenylphosphonate and when intercalated with methylamine are shown in Fig. 4a and 4b, respectively. As expected, both spectra presented a series of common bands. Thus, the band at 1438 cm−1 and the medium bands in the 720–644 cm−1 interval are characteristic of the phenyl ring. The bands that appeared in the 1340– 1017 cm−1 range are assigned to the PO3 group. The other bands in the 3000–2900 cm−1 range are related to symmet-
ric and asymmetric carbon–hydrogen stretching vibrations. Moreover, the appearance of absorption bands in the 3400– 3300 cm−1 interval might correspond to the symmetric and asymmetric nitrogen–hydrogen stretching bands, and an absence of oxygen–hydrogen bonds on the phosphonate groups [27–29]. By comparing Figs. 4b and 3c, identical sequences of bands are observed, indicating the success of preparation through distinct routes. The infrared spectrum of calcium phosphate, after reacting with phenylphosphonic acid is shown in Fig. 3d. The bands that appeared at 1105, 1080 and 1017 cm−1 are attributed to PO3 group and the deformation band of the P–O bond appeared at 1144 cm−1 , and for the ammonium compound very weak bands at 2950–2850 cm−1 appeared, which may be attributed to N–H stretching frequencies. Thus, the reaction of formation of the organic derivative between calcium phosphate and phenylphosphonic acid can be considered as an exchange process by involving the corresponding monovalent anions [12]. The substitution reaction can be directly obtained on a lamellar exfoliated surface, followed when species interdiffused into the interlamellar region. In this case, as a result of the reaction with phenylphosphonic acid, an increase in organic groups are bonded in the exfoliated inorganic lamella, to form a calcium phosphate–phosphonate mixture, as represented by Ca(H2 PO4 )(C6 H5 PO3 H)·H2 O. The proposed mechanism of the topotoctic reaction is shown in Fig. 5. As represented in Fig. 5a, initially one of the oxygen atoms of the anion O2 P(OH)2 − is freed from calcium atom coordination, followed by a hydrolytic substitution by the oxygen atoms of the new anion O2 P(OH)2 C6 H5 − over the available site of coordination. Part b illustrated the participation of the amine, when an oxygen atom of the precursor anion O2 P(OH)2 − is free from the calcium coordination. The topotactical mechanism of reaction with methylamine is similar, however, after hydrolytic substitution, the amine removes a water molecule and, consequently, such water molecule leaves the phosphate group, enabling the phosphonate group to bond. In such a process, beyond the phosphonate species O2 P(OH)2 C6 H5 − , also is simulta-
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Fig. 5. Mechanism proposed for the topotaxic reaction between calcium phosphate and phenylphosphonic acid. Fig. 7. X-ray diffraction patterns of hydrated calcium phenylphosphonate after heating at different temperatures: 523 (a), 573 (b), 673 (c), 773 (d) and 973 K (e). The symbol ◦ in the diffractogram indicates the presence of Ca2 P2 O7 .
(a)
temperature, to form as a final residue calcium pyrophosphate, Ca2 P2 O7 . The observed stage of decomposition from 460 to 500 K can be assigned to ammonia elimination, to give Ca(H2 PO4 )2 ·H2 O. The next step does not distinguish the dehydration and the OH condensation to form water, in the 500– 700 K range, to give Ca(PO3 )2 compound. The last stage not shown in the thermogravimetric curve should be related to the loss of phosphorus oxide to form a final residue of pyrophosphate, over 900 K. Based on this percentage of 21.6% the newly synthesized compound has the formula established as (NH4 )2 Ca(H2 PO4 )2 ·H2 O [5,7,16]. The proposed mechanism of decomposition can be formulated as follows: (NH4 )2 Ca(HPO4 )2 ·H2 O −2NH3 Ca(H2 PO4 )2 ·H2 O 460–500 K −3H2 O Ca(PO3 )2 500–700 K 1 1 Ca2 P2 O7 + P2 O5 . 2 2 >900 K
(b) Fig. 6. Thermogravimetric curves of calcium ammonium phosphate (a) and calcium phosphate (b).
neously presented the phosphate anions O2 P(OH)2 − bonded to the crystalline structure [15]. The thermogravimetric curve of calcium ammonium phosphate is shown in Fig. 6a, presenting a mass loss of 21.6% in the 460–700 K range, which corresponds to a displacement of ammonia and water from the crystalline compound to form Ca(PO3 )2 . The next expected step should be related to the loss of phosphorus oxide, P2 O5 , at higher
The crystalline lamellar Ca(H2 PO4 )2 ·H2 O compound lost one mole of water of hydration and two other due to condensation of OH groups bonded to phosphate with a mass loss of 17.8%, in the same region, between 600–800 K, to give Ca(PO3 )2 , as shown in Fig. 6b. It is also expected to end during the heating the pyrophosphate, as proposed before. This proposed mechanism of condensation was previously observed for Ca(HO3 PC6 H5 )2 ·2H2 O [7], which compound should be also formed during the topotactic synthesis. To elucidate the changes during this process to end up in pyrophosphate, both the phosphate and phosphonate compounds were submitted to different temperatures and the X-ray powder patterns recorded. The same set of patterns was observed and for phosphonate is shown in Fig. 7. Thus,
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when this compound was heated at 523 K the water molecules were removed to form the anhydrous crystalline compound, as shown in Fig. 7a. The lamellar structure collapses in the 573–673 K range due to phenyl groups removal and, consequently, the lost of crystallinity, as shown in Fig. 7b and 7c. Finally, the crystalline arrangement starts to appear at 773 K, as illustrated by the X-ray patterns in Fig. 7d, a process which is more evident with an intensification of crystallinity at 973 K. At this temperature calcium pyrophosphate is formed and the characteristic peaks of the compound are indicated in Fig. 7e.
4. Conclusion Topotactic exchange was obtained when calcium phosphate reacted with phenylphosphonic acid to give a compound similar to hydrated calcium phenylphosphonate. This fact was observed also when calcium phosphate reacted with the methylamine intercalated compound. In this process the intercalation caused an increase in the interlamellar distance in relation to the original Ca(H2 PO4 )2 ·H2 O compound. The infrared spectrum for calcium phosphate, after intercalation with methylamine, showed clearly bands which are attributed to PO3 groups. However, the new compound presented bands related to PO4 and PO3 groups, when the compound Ca(H2 PO4 )2 ·H2 O reacted with phenylphosphonic acid. Beyond the known route involving the reaction between calcium phenylphosphonate with methylamine, the present investigation illustrates a new way to form phosphonate derivatives starting with pre-formed calcium phosphate.
Acknowledgement The authors are indebted to CNPq for fellowships and to FAPESP for financial support.
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