5535
Studies of the Hydrogen Held by Solids. XII. Hydroxyapatite Catalysts J. A. S. Bett, L. G. Christner, and W. Keith Hall Contribution from the Mellon Institute, Pittsburgh, Pennsylvania. Received April 26, 1967 Abstract: A series of hydroxyapatite catalysts of varying Ca/P ratios were prepared and characterized by a number of physical methods. All of these catalysts were nonporous crystalline solids whose BET Nz surface areas were in fair agreement with areas calculated from their electron micrographs. It was shown that most of this area stemmed from edges of the ab planes stacked along the c axis. Direct assay of catalyst hydrogen, when compared with the known variations in Ca/P ratio, made it possible to discriminate among the several possible ways by which the stoichiometry can be maintained. It was concluded that for each Ca2+removed, two protons were introduced into the solid; these probably resided (temporarily) on phosphate groups. On mild heating to remove HzO, about half of the phosphate hydrogen was removed by reaction with structural OH groups; on heating to 500°, most of the remainder was lost through formation of intracrystalline pyrophosphate; and on heating above 800", this reacted with the residual structural OH groups as the structure rearranged to /?-tricalciumphosphate. studies of the reaction of hydroxyapatites with triphenylcarbinol, tritolylcarbinol, and trianisylcarbinol showed that surface acidity was developed as the preparations were made calcium deficient, and that the acidity was much higher with catalysts after mild heating than with those heated to 500". In no case, however, was the surface carbonium ion concentration as high as the residual phosphate hydrogen calculated to be in unit cells adjacent to the surface. Rates of alcohol dehydration correlated with increasing acidity and with increasing calcium deficiency.
B
ecause it is the mineral phase in bone, the structure of the bands used to identify phosphate hydrogen (POH). This identification, together with the quanand properties of the hydroxyapatites have been titative determination of P2074-, led Winand, et the subject of many investigations. The unit cell of the parent compound, stoichiometric hydroxyapatite, to suggest that the charge balance was maintained in nonstoichiometric preparations by the net addition of 3[Ca3(P04)2].Ca(OH)2, has been established by both one proton and removal of one structural OH per and neutron diffraction,2a but the calciummissing Ca2+. However, Posner, et a1.,6 pointed deficient hydroxyapatites are of greater biological interest because the Ca/P ratio in bone is nearer to 1.5 out that the available data do not justify this hypothesis than to 1.67. (Some carbonate is always present.) and opined that missing hydroxyl ions would allow collapse of the hydroxyapatite structure. It is shown Because of this flexibility of stoichiometry, bone hydroxyapatite acts as a buffer to the composition of herein that Winand's hypothesis nevertheless approximates the real situation more closely than any other blood. Controversy has centered about the h y p o t h e ~ i s ~ , ~simple model. that caicium phosphates form an isostructural series Recently, Brown and co-workers? have suggested a from stoichiometric hydroxyapatite (Ca/P = 1.67) further possibility, viz., that calcium-deficient hydroxyto tricalcium phosphate (CajP = 1.5). An alternative apatites are epitaxial intergrowths of octacalcium (adsorption) theory, which proposed that calciumphosphate, Ca/P = 1.33, and stoichiometric hydroxydeficient compounds were formed by the adsorption of apatite, Ca/P = 1.67, to give the observed low Ca/P biphosphate ions on the surface of hydroxyapatite, ratio. Berrys has shown that both Winand's hypothehad been a d ~ a n c e d but , ~ this was rejected by Posner4 sis and the lamellar structure of Brown will explain the on the basis that the available surface area is insufformation of P2074-and the weight loss at 800" associficient in some calcium-deficient preparations to explain ated with loss of HzO, but that neither infrared data the over-all composition. Posner preferred a statistical nor thermal analyses give any indication of octacalcium model in which Ca2+ ions were removed from column phosphate in calcium-deficient hydroxyapatites. positions of hydroxyapatite, the deficiency being Our principal interest in the hydroxyapatite system balanced by the addition of two protons per calcium has been in its potential as a model heterogeneous removed. Infrared identification of both structural catalyst in which the stoichiometry could be changed hydroxyls and acid phosphate groups in calciumwithout altering the crystal structure. Bowman and deficient hydroxyapatites has been claimed by two Piaseckyg have shown that basic calcium phosphates group^,^,^ but the workers did not agree on the location effect the dehydrogenation and dehydration of alcohols, and that the selectivity for these reactions is controlled (1) W. F. Neurnan and M. W. Neurnan, Chem. Reo., 53, 1045 by the composition. However, their work did not (1953). provide a basis for understanding the origin of the (2) (a) M. I. Kay, R. A. Young, and A. S . Posner, Nature, 204,1050 selectivity. Data on the influence of the stoichiometry (1964); (b)A. S . Posner, A. Perloff, and A. F. Dorio, Acta Cryst., 11, 308 (1958). of hydroxyapatite on dehydration are given herein, (3) (a) F. K. Cameron and J. M. Bell, J . A m . Chem. SOC.,27,1512 (1905); (b) S . B. Hendricks and W. L. Hill, Proc. Natl. Acad. Sci. U.S., 36, 731 (1950). (4) A. S . Posner and A. Perloff, J . Res. Natl. Bur. Std., 58,279 (1957); A. S . Posner and S . R. Stephenson, J . Dental Res., 31, 371 (1952). (5) (a) L. Winand and G . Duyckaerts, Bull. SOC.Chim. Belges, 71, 142 (1962); (b) see also L. Winand, Anal. Chim., 6 , 62 (1961).
(6) A. S . Posner, J. M. Stutman, and E . R. Lippincott, Nature, 188, 486 (1960); 193, 368 (1962). (7) W. Brown, J. P. Smith, J. R. Lehr, and A. W. Frazier, ibid., 196, 1048 (1962). (8) E. E. Berry, J . Inorg. Nucl. Chem., 29, 317 (1967). (9) R . S . Bowman and L. J. Piasecky, U. S . Patent 3,149,082 (1964).
Bett, Christner, HaN / Hydroxyapatite Catalysts
5536 CO/P
\
Table 1. Chemical Analyses of Representative Hydroxyapatite Preparationsn
163
Initial solution 1.67 1.61 1.58
I60
1.58
600
700
800 Tempernlure, 'C
900
Figure 1. Thermogravimetric analysis of calcium-deficient hydroxyapatites.
and the effects of composition on the selectivity will be the subject of a later paper,
(10) A. Gee and V. R. Dietz, J . Am. Chem. SOC.,77, 2961 (1955).
7
1.68 =k 0.008 1.62 f 0.008 1.60 =k 0.008
a The authors wish to express their gratitude to Dr. A. S. Posner in whose laboratory these analyses were performed.
identified, however, by their behavior on heating above 780". While stoichiometric hydroxyapatite maintained its structure virtually intact to 1000", increasing calcium deficiency was indicated by the appearance of increasing amounts of /3-tricalcium phosphate. The intensity of the strongest lines of each phase are given in arbitrary units in Table I1 (as peak height above background). The ratio of these peaks reflects qualitatively the increase in the p-tricalcium phosphate phase. Table 11. Formation of ~-TricalciumPhosphate in Hydroxyapatites Heated to 1OOO"
Ca/P
Intensity, 31 1 reflection,a hydroxyapatite
Intensity, 2.0.10, 217b reflections, p-tricalcium phosphate
1.67 1.60 1.58
84 43 52
2 21 39
Experimental Section Preparation and Characterization of Catalysts. Preparation, The hydroxyapatites were made by titrating concentrated H3PO4 into saturated Ca(OH)2. From the volume and concentration of the latter, the volume of H3POa required to give a desired Ca/P ratio was slowly stirred into the Ca(0Hh at room temperature. Pyrex vessels were used and the reagents were mixed under NP. A fine gel-like precipitate appeared immediately, and this suspension was reduced to dryness during 15 hr over a hot plate. The pH of the solution fell from 12.6 to 11.6 during titration for hydroxyapatite and to somewhat lower values for calcium-deficient preparations. The products, white crystalline solids, were sieved for catalytic studies to 30-60 mesh. In a typical preparation (stoichiometric hydroxyapatite), 59.8 ml of 0.769 MH8POawas added dropwise in 1 hr to 3.67 1. of saturated (0.0211 M ) Ca(OH)2. The large volume of the Ca(OH)2 solution prevented the temperature from rising perceptibly from room temperature (22") during titration. In order to reduce minor impurities, CaO was prepared by calcining Fisher certified CaC03in a quartz vessel at 900". The resulting oxide was dissolved in deionized HzO and stored in a plastic container. Fisher certified orthophosphoric acid was purified by passage through an ion-exchange column of Amberlite IR-120 in the acid form. Spark spectra showed the total cationic impurities (except Si) in the final catalysts to be less than 500 ppm, principally Na and AI; Si occurred in some samples up to 0.1 %. These impurities undoubtedly stemmed from the Pyrex containers used in the preparations. Characterization. Because of the rather small changes in stoichiometry and properties of the calcium-deficient hydroxyapatites, a variety of chemical and physical measurements were made. The results of these diverse measurements, which are given below, are consistent with a description of our preparations as a series of hydroxyapatites of constant structure and similar morphology, but with stoichiometry varying from Ca/P = 1.67 to 1.57. Analytical Methods. Calcium was determined by standard EDTA titration techniques and phosphate by the differential spectrophotometric technique of Gee and Dietz.lo The calcium to phosphorus ratios were found to correspond t o the ratios expected from the starting materials to within =kO.Ol. Analyses of aliquots of the same samples, in our laboratory and in Posner's, agreed within the same limits. Some representative data are recorded in Table I. X-Ray Diffraction. In accord with the frequently recorded observation' , Z b that both hydroxyapatite and calcium-deficient hydroxyapatites yield closely similar X-ray diffraction patterns, all preparations between Ca/P = 1.67 and 1.57 (our most calciumdeficient preparation) gave fairly well-resolved patterns similar to hydroxyapatite, both when dried over a hot plate at 175" and when heated to 600". Preparations with different Ca/P ratios could be
Ca/P atom ratioChemical analysis
a
Not completely resolved from 112.
190 X I~TCPIIEA -2 49 75
Coincident.
Thermogravimetric Analysis. The curves of Figure 1 are consistent with those of other workers,5b911as well as with the X-ray data. Brasseur'l reported an inflection in the weight loss curve near 800°, corresponding to the loss of about half a molecule of water per calcium deficiency in a sample with Ca/P = 1.5, while the inflection was absent in stoichiometric hydroxyapatite. Winand5b identified this loss with that accompanying the removal of structural OH in the transition to p-tricalcium phosphate. The data in Figure 1 showed an increasing weight loss with calcium deficiency corresponding to approximately 0.5 mole of water per calcium deficiency (Table 111). Table 111. Thermogravimetric Analyses of Hydroxyapatites"
Ca/P
From each mole of 3[Ca3(PO4)~1Ca(OH)z Loss at 780°, Ca2+ deficiency, moles of moles Hz0
Ratio, H20/ACaZ+ ~
1.67 1.63 1.61 1.60 1.58 1.57
0 0.23 0.34 0.40 0.52 0.57
0 0.14 0.22 0.24 0.29 0.29
... 0.61 0.64 0.60 0.56 0.51
a Weight loss at 780". Water is lost mainly by the reaction P207*- 20H- +. H 2 0 2POa3- as hydroxyapatite is converted to p-tricalcium phosphate; see ref 5 .
+
+
Infrared Spectroscopy. An attempt was made to identify two types of O H bands: the structural O H groups and the acidic POH presumed to be present in the nonstoichiometric preparations. Spectra from preparations pressed into KBr disks agreed well with those of Winand and Duyckaerts,S taken in the same way. The strong band due to the structural OH in the 3550-cm-l region was readily identified and confirmed by deuteration. The weak bands (11) H.Brasseur, Bull. SOC.Chim. Belges, 62, 383 (1953).
Journal of the American Chemical Society 1 89:22 / October 25, 1967
Table 1V. Behavior of the Band at 875 Cm-I (POH) Deformation as a Function of Calcium Deficiency Relative Ca/P
POH intensity
1.67
0.15
1.60
0.42
1 58
57
n
Band at 875 cm-' normalized t o the P O F harmonic at 2000 cm-1 to correct for sample concentration in KBr disk. at 875 and 2400 cm-', which they assigned t o POH, were evident but poorly resolved. The area of the former, normalized t o the area of the Pola-harmonic a t 2000 cm-1, paralleled the degree of nonstoichiometry (Table IV). However, it was impossible t o confirm that these were OH vibrations by deuteration, even though attempts were made in Nujol and halocarbon mulls, as well as with pressed disks of pure catalyst under high-vacuum conditions. Possibly this was because exchange of these intracrystalline hydrogens did not occur with either D90or D. at temperatures where condensation of orthophosphate t o pyrophosphate groups was not rapid. Nevertheless, we must agree with Posner, ef a/.,@that these band assignments d o not establish the presenceof POH. However, our work does not support the conclusions drawn by Posner' from his published data, that the POH bands occur at 3330-3400 cm-'. We see these bands but find that they disappear in uocuo at temperatures ( 300") where other measurements indicate POH should be stable ( d e Infra). Electron Microscopy and Diffraction from Selected Single Crystals. The morphology of our samples was established by transmission electron microscopy and electron diffraction t o be predominantly well-crystallized prisms of average dimensions, 0.02 X 0.02 X 0.07 p. The size distributions were in all cases fairly narrow. The crystals were assumed t o be hexagonal prisms, and electron diffraction showed that most of the exposed area was from the edges of the ab planes stacked along the c direction (Figure 2b). This crystal habit corresponds closely t o that described by Frank, e: a/.," for apatite crystals in human enamel, rather than the hexagonal platelets described by Neuman and Neuman' for bone apatite. Values from conventional BET surface area measurements are listed in Table V,where they are compared with values calculated from the electron micrographs. The areas were calculated by summing the surface area per particle and the weight per particle over the distribution shown on the electron micrographs, assuming a hexagonal prism geometry. The agreement with BET data could only be achieved with a solid made up of microscopic single crystals. Thus, all the pores are large and intercrystalline; indeed, the shape of the BET isotherms indicated that most of the pores had radii greater than 100 A. The higher area of the stoichiometric sample reflected its slightly smaller crystallite size.
