Influence of Magnesium Substitution on the Basic Properties of

ARTICLE pubs.acs.org/JPCC

Influence of Magnesium Substitution on the Basic Properties of Hydroxyapatites Sarah Diallo-Garcia,†,‡ Danielle Laurencin,§ Jean-Marc Krafft,†,‡ Sandra Casale,†,‡ Mark E. Smith,^ Helene Lauron-Pernot,†,‡ and Guylene Costentin*,†,‡ †

UPMC, University Paris 06, UMR 7197, Laboratoire Reactivite de Surface, F-75005 Paris, France CNRS, UMR 7197, Laboratoire Reactivite de Surface, F-75005 Paris, France § Institut Charles Gerhardt de Montpellier, UMR 5253, CNRS-UM2-ENSCM-UM1, Universite de Montpellier 2, CC 1701, Place Eugene Bataillon, 34095 Montpellier cedex 5, France ^ Department of Physics, University of Warwick, CV4 7AL Coventry, United Kingdom ‡

bS Supporting Information ABSTRACT: Magnesium-substituted hydroxyapatites (HAP) have been prepared to evaluate the influence of cationic substitution on the surface basic properties of HAP. Despite successful introduction of some of the magnesium cations into the HAP structure (as evidenced by XRD, infrared, and Raman), no influence of low magnesium content in MgxHAP samples (x e 1) on the basic conversion of 2-methylbut-3-yn-1-ol was found, which can be explained by the surface content of magnesium being low, as evidenced by CO adsorption. At higher magnesium content, a higher amount of magnesium could be detected on the surface, but this resulted in a structural disorder leading to either nonstoichiometry or eventually the formation of the phase whitlockite. In this case, the associated relative decrease of the amount of basic sites, as well as the possible influence of the enhanced surface concentration of acidic POH groups, are responsible for the lower intrinsic basicity of the related samples. In contrast, preliminary results indicate that an enhancement of the basic reactivity is observed on substituting calcium for strontium.

1. INTRODUCTION Calcium hydroxyapatite (Ca10(PO4)6(OH)2, CaHAP), the main mineral component of bone tissue and teeth, is a calcium phosphate that is largely studied in the biological field due to its numerous applications as bone regeneration materials, food supplements, or drug vectors. Recently, the interest in this material was extended to the field of acidbase18 or bifunctional911 heterogeneous catalysis. Such a large field of applications is related to the biocompatibility (thus eco-compatibility) as well as to the versatility of this material. Indeed, its properties can be tuned by modifying the Ca10(PO4)6(OH)2 lattice composition through the numerous possible ionic substitutions,12 on either the Ca cationic position (Ba, Zn, Cd, Mg, Sr, etc.) or the anionic sites, PO43 (VO43, HPO42, CO32) or OH (CO32, F). These substitutions influence the thermal stability, textural properties, and surface reactivity. For instance, magnesium-substituted hydroxyapatite is known to have an improved bioactivity compared to CaHAP,13,14 probably associated with modifications of its physicochemical properties.13,15,16 However, there have been very few attempts to rationalize the influence of such cationic substitution on the surface properties of hydroxyapatites, despite the fact that the analysis of the modifications induced by such substitutions on the structural and surface properties of the system could provide deeper insight on r 2011 American Chemical Society

how the surface of HAP reacts at molecular level when it is used as catalyst. Although surface acidic sites, Ca2+ and POH, have been characterized from adsorption of probe molecules,8,1719 the nature of those really involved as acidic active sites is still under debate.1,17,20 Moreover, there is a growing interest in hydroxyapatites as basic heterogeneous catalysts,3,4 mainly due to their unique performance to convert selectively ethanol into n-butanol.68,21,22 Nevertheless, the characterization of surface basic sites, possibly OH23,24 or PO432527 remains very scarce.20 In fact, up until now, the results are mainly discussed in relation with Ca/P ratio with no attempts to identify the active acidbase pair involved in the basic reactivity.23,28 The lack of a universal probe molecule to characterize basic sites,29 means that model catalytic reactions such as 2-methylbut3-yn-1-ol (MBOH) conversion30 (which is considered as the gold standard to evaluate the surface basicity of oxides)31 appear to be an interesting alternative to characterize the basic properties of HAP. Moreover, the study of the influence of cationic substitutions on the basic properties of these systems could be a good way not only to tune the basic reactivity but also to evaluate Received: September 27, 2011 Revised: November 3, 2011 Published: November 03, 2011 24317

dx.doi.org/10.1021/jp209316k | J. Phys. Chem. C 2011, 115, 24317–24327

The Journal of Physical Chemistry C

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Table 1. Sample Labelling of Non-Substituted CaHAP, MgxHAP (Ca10xMgx(PO4)6(OH)2) Samples and OCP Sample, Sample Compositions, Expressed as Ca/P, Mg/P, and (Ca + Mg)/P Ratio Calculated from ICP Analysis (Nominal Expected Composition Are Reported in Brackets), Specific Surface Areas, and MBOH Conversion sample name

x

Mg/P

Ca/P

(Mg + Ca)/P

specific surface

MBOH

area (m2g1)

conversion (%)

CaHAP-1

0

1.68 (1.67)

1.68 (1.67)

89

68

CaHAP-2

0

1.67 (1.67)

1.67 (1.67)

52

44

1.62 (1.62)

1.66 (1.67)

71

59.5 23 30 30

Mg0.25HAP

0.25

Mg0.5HAP-1 Mg0.5HAP-2

0.5 0.5

0.077 (0.082)

1.58 (1.58)

1.66 (1.67)

26 31

Mg0.75HAP-1

0.75

0.104(0.125)

1.58 (1.54)

1.68 (1.67)

31.5

Mg0.75HAP-2

0.75

0.107(0.125)

1.58 (1.54)

1.69 (1.67)

25

18

Mg0.75HAP-3

0.75

42.5

35 14

0.042 (0.042)

Mg1HAP-1

1

0.135 (0.167)

1.55 (1.5)

1.68 (1.67)

23

Mg1HAP-2

1

0.136(0.167)

1.52(1.5)

1.66 (1.67)

33

33

Mg1.25HAP

1.25

0.142 (0.208)

1.45 (1.46)

1.60 (1.67)

31

1.5

Mg1.5HAP-1 Mg1.5HAP-2

1.5 1.5

0.206 (0.25) 0.185 (0.25)

1.42 (1.42) 1.44 (1.42)

1.63 (1.67) 1.62 (1.67)

229 64

68 33

Mg1.54HAP

1.54

0.197 (0.257)

1.38(1.41)

1.59 (1.67)

28

6.5

2

0.249 (0.333)

1.36 (1.33)

1.61 (1.67)

203

30

Mg2HAP dried OCP

0

1.38 (1.33)

1.38 (1.33)

17

3.5

heated OCP

0

1.38 (1.33)

1.38 (1.33)

15

3.6

if the surface OH groups could act as basic sites. Indeed, because magnesium was recently shown to selectively replace calcium on the crystallographic Ca(II) site of hydroxyapatite,32 which is the only cationic site to be directly bonded to the potentially basic OH groups, its incorporation in the hydroxyapatite lattice may influence the basic strength of the neighboring OH group. Moreover, it is worth noting that Mg substitution in CaHAP led to distortions of the local structure around the cation in the site(II) environment,32 and, thus, an impact the reactivity of the neighboring OH group can be expected. Hence, in the present work, magnesium hydroxyapatites were synthesized. The incorporation of this cation in the hydroxyapatite lattice was verified by chemical analysis, and thorough structural characterization, and the accessibility of cationic sites on the surface was probed by CO adsorption followed by FTIR. Finally, the influence of magnesium substitution on the basic properties of the system was evaluated thanks to the MBOH reaction test.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Pure calcium hydroxyapatites sample (CaHAP) and magnesium- (MgxHAP) substituted HAP samples with formulation Ca10xMIIx(PO4)6(OH)2 (with x ranging from 0.25 to 2) were prepared following the precipitation method already described in literature.3335 A first aqueous solution was obtained from mixing Mg(NO3)2 3 6H2O to Ca(NO3)2 3 4H2O ([Mg] + [Ca] = 0.216 mol 3 L1) and its pH value adjusted to 1036 by addition of 1 mol 3 L1 NH4OH. It was brought to 353 K under nitrogen atmosphere in a Teflon flask to avoid any silicon contamination from the glassware in the final product.37 A second aqueous solution of (NH4)H2PO4 ([P] = 0.130 mol 3 L1), adjusted to pH 10 by addition of concentrated NH4OH, was then added dropwise to the first one (∼2.2 mL 3 min1). A white precipitate was formed. The heating under stirring is then maintained for 4 h under reflux with periodic addition of NH4OH to

keep the pH above 9.0: between 2.5 and 4.0 mL of 30% NH4OH was added altogether, for a final pH value between 9.5 and 10. To avoid deep carbonation of the materials, the addition and maturation steps were performed under a continuous nitrogen flow. The reaction medium was then left to cool down to room temperature, and centrifuged to separate the white precipitate. The solid was then washed several times with distilled water, and dried at 373 K under vacuum overnight. Finally, the dried samples were heated under argon flow (150 mL 3 min1) up to 623 K (5 C 3 min1) and maintained at this temperature for 90 min. To obtain a large variety of samples, many compositions were screened, some of them being reproduced several times. It was concluded reasonably good reproducibility was achieved providing that the addition rate of the (NH4)H2PO4 solution was carefully controlled, which is consistent with the influence of this parameter in the nucleation growth process already reported by Kandori et al.38 Indeed, two samples prepared without any fine control of this parameter, Mg1.5HAP-1 and Mg1.54HAP, exhibit quite different crystallinity and specific areas (Table 1). Despite their peculiar properties, they were retained in this study to enrich discussion about the key parameters governing the basicity. An octacalcium phosphate sample (OCP), Ca8(HPO4)2(PO4)4 3 5H2O,39 was also prepared by varying the proportions between precursors (Ca/P = 1.33). An aqueous solution of Ca(CH3COO)2 ([Ca] = 0.133 mol 3 L1 was instantaneously added to an aqueous solution of NH4H2PO4 ([P] = 0.1 mol 3 L1) under stirring. The reaction medium (pH 5) was then brought to 333 K under an N2 atmosphere. The total heating and stirring time was 2 h. The sample was centrifuged then washed several times with distilled water and dried at 373 K overnight. Part of this dried OCP sample was then heated following the procedure described above. Both samples will be hereafter referred as to as dried and heated OCP samples. 2.2. Characterization. X-ray powder diffractograms were recorded with a Siemens diffractometer equipped with a Copper 24318

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The Journal of Physical Chemistry C anode generator (λ = 1.5418 Å). Diffractograms were recorded by 0.02 steps in the 2θ range of 1085 (or 385 for the dried OCP and OCP samples). Crystalline phases were identified from comparison with ICDD data files. Elemental analyses were performed by ICP-AES by the “Service Central d’Analyse” of the CNRS (Vernaison, France). Specific surface areas were measured by adsorption of N2 on a Micromeritics (ASAP 2010) apparatus. Samples were outgassed at 573 K overnight before adsorption, and specific surface areas were calculated from the BET method. DTA-TGA analyses were performed on a SEIKO SSC 5200H apparatus heating 20 mg sample under air flow (100 mL 3 min1) up to 1273 K (278 K 3 min1). TEM characterization was performed with a JEOL JEM 1100 using a 100 keV electron beam. The samples were suspended in ethanol, dispersed with microwaves, and then rapidly supported on a copper grid. Raman spectra of the samples were collected from a KAISER Optical system equipped with a charge coupled detector (CCD) and a LASER with λ = 785 nm (P = 1012 mW, resolution = 4 cm 1 accumulation time = 30 s, 30 scans per spectrum). A microscope with a X50 lens was used. Infrared spectra were obtained from self-supported pellets (2025 mg) placed in a quartz cell equipped with CaF2 windows and connected to a vacuum line allowing thermal treatments and adsorptiondesorption experiments to be carried out in situ. Wafers were first pretreated: after a first ramp to 623 K (5 K 3 min1) under argon flow (20 mL 3 min1), they were kept at this temperature and atmosphere for 90 min, then cooled down to 573 K, where the vacuum was performed until a residual pressure ∼106 Torr was reached, and further transferred to the beam zone that was maintained at about 100 K with liquid nitrogen. Spectra were recorded at 100 K before and after introduction of increasing doses of CO gas (up to a final equilibrium pressure of 0.8 Torr) using a Bruker FTIR Vector 22 spectrometer, equipped with an MCT detector (resolution 2 cm1, 128 scans per spectrum). The spectra of adsorbed molecules are reported in absorbance, after subtraction of the background of the sample before the adsorption. 2.3. MBOH Catalytic Test. MBOH conversion experiments were performed in an automated differential flowing microreactor. The catalyst (25 mg) was put on a porous glass, in the center of a U shape quartz tube of 10 mm internal diameter. The temperature of the catalyst bed was controlled by a thermocouple located close to the wall of the quartz tube. Except in the case of the dried OCP sample, samples were first pretreated under a nitrogen flow (20 mL 3 min1) up to 623 K (5 K 3 min1), maintained at this temperature for 90 min, then the temperature was decreased under nitrogen flow to 413 K, the reaction temperature. The reactant feed that was composed of MBOH diluted by bubbling nitrogen (50 mL 3 min1) in liquid MBOH (Fluka, 99.9%) at 293 K (MBOH partial pressure of 1.73 kPa) was then allowed to pass through the catalyst. It was checked that no diffusional limitation occurs up to about 60% conversion. Reaction products were analyzed every 2 min using a Perichrom gas chromatograph equipped with a FID detector and a 15% TCEPE/Chromosorb P column (2 m length). Acetone and acetylene were the only products detected. The partial pressure of each product Pi was calculated from chromatographic area measurements, using the appropriate response coefficient and the value of the initial partial pressure of MBOH in the feed PMBOH. Conversion τ (in %) is

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Figure 1. XRD patterns of CaHAP-1, Mg0.75HAP-3, Mg1.54HAP, heated OCP, dried OCP, Mg1.54HAP further calcined at 1173 K. Right insert: zoom on the 2θ = 2229 range to evidence the shift of the (002) diffraction lines upon Mg incorporation compared to CaHAP-1 sample. Left insert: low angles (2θ = 310) diffraction patterns of dried and heated OCP samples.

given by:

τ% ¼

1

∑

i6 ¼ MBOH

αPi

PMBOH

with α ¼

1 for acetone and acetylene 2

Selectivities in acetone and acetylene are given by: Si% = /2Pi/∑i6¼MBOHαPi.

3. RESULTS 3.1. Structural Characterization. Chemical Composition. From the elemental analysis, the Ca, Mg, and P contents were determined. Note that the C and N contents were found to be below 0.3 and 0.1 wt % respectively meaning that the content in nitrate or carbonate impurities is very low. As reported in Table 1, for x e 1 samples, the (Ca + Mg)/P ratios are in very good agreement with the 1.67 expected value. However, even though most of magnesium has been incorporated inside the compound, for almost all samples, the Mg/P values are lower than expected. Such magnesium deficiency was already reported for a similar preparation procedure.15 Despite this lack of magnesium, up until x e 1 this phenomenon is compensated by higher calcium content, resulting in stoichiometric hydroxyapatites. However, for higher magnesium content samples (x > 1) Mg/ P ratios are too low to enable compensation resulting in a global cationic deficiency with (Ca + Mg)/P lower than 1.63. Such lack of magnesium could be related either to the formation of other related phases such OCP or β-TCP (Ca3(HPO4)x(PO4)2x/3)40 or to that of non stoichiometric hydroxypatites.40,41 In the latter case, the cationic deficiency material would be associated with 24319



Figure 2. (a) Raman spectra of CaHAP-2, Mg0.5HAP-2, Mg1 HAP-1, Mg1.5HAP-2, Mg2HAP) Mg1.54HAP, Mg1.54HAP further calcined at 1173 K. (b) Correlation between the fwmh (full width at middle height) of the Raman band at 960 cm1 and the magnesium substitution level (x value). The error in the fwmh measurement is estimated to 1.5 cm1.

incorporation of anions of lower charge than that of PO43, such as HPO42, CO32, or SiO32 to achieve the charge balance.12 Note that, in the present study, the introduction of HPO42 anions is the most likely according to chemical analyses because the two latter anions are mostly avoided thanks to the efficient use of the Teflon flask and of the inert atmosphere during synthesis. Finally, the OCP samples exhibit a Ca/P ratio (1.38) slightly higher than that expected from the nominal one for OCP structure (Ca/P = 1.33). Such a difference may be due to partial transformation into the HAP phase because OCP is considered as a possible precursor in the HAP formation.39 Structure. XRD. Figure 1 reports the XRD patterns of a few representative samples. From these data, MgxHAP samples exhibit the hydroxyapatite HAP structure (ICDD pattern 01 0749780(A)). The slight shifts of the diffraction lines (see in the insert on the right of diffractogramms of CaHAP-1 and Mg0.75HAP-3) observed for Mg substituted samples are due to the slight contraction of the cell parameters of HAP, in agreement with the smaller ionic radius of the magnesium cation compared to calcium. XRD thus proves that magnesium cations have indeed been incorporated in the HAP structure. As expected from its preparation conditions, the dried OCP sample prepared at pH ∼5 exhibits the OCP structure: even though its diffraction pattern only slightly differs from those of HAP samples, on the one hand the appearance of a new line at 2θ = 4.8 (insert on the left), and on the other hand the absence

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of the line at 2θ = 10.8, which are characteristics of OCP39 (ICDD pattern 000440778(I)) and HAP structures respectively indicate that crystalline OCP has been obtained. In contrast, the corresponding heated OCP sample has lost the crystalline structure of OCP and its powder pattern has become more similar to that of HAP, in agreement with the low thermal stability of OCP structure and its well-known tendency to transform into HAP upon heat-treatment.39 From this result and taking into account the Ca/P global ratio of 1.38 which is even lower than the limit value (Ca/P = 1.5) expected for a nonstoichiometric HAP sample,40 the heated OCP sample is probably a mixture of amorphous OCP and of crystalline nonstoichiometric HAP. Finally, it should be noted that although the XRD pattern of Mg1.54HAP sample exhibits only classical diffraction lines associated to a HAP structure, new diffraction lines associated with the crystallization of a Mg-whitlockite phase Ca3yMgy(HPO4)z(PO4)22z/3 (Ca/P = 1.5)4245 (ICDD pattern 01.070 2065(I)) are detected upon heating the sample up to 1173 K. At this point, it is still questionable whether the related Mg1.54 HAP sample (Ca/P = 1.59) is associated with nonstoichiometric HAP, which partially decomposes into a whitlockite phase at high temperature or whether it is made of mixture of a pre-existing amorphous whitlockite phase and of crystalline nonstoichiometric MgxHAP. RAMAN. Raman spectroscopy being more sensitive than XRD, it could allow to investigate the efficiency of magnesium introduction in the structure as well as the possible presence of a whitlockite phase. In fact, from the spectra reported in part a of Figure 2, all samples, including the Mg1.54HAP sample, exhibit the classical band at 960 cm1 assigned to the ν1 vibration mode of PO43 groups of hydroxyapatite (as well as the expected ν2, ν3, ν4 vibration modes).46 In addition, as already observed by XRD, the presence of crystalline whitlockite in the Mg1.54HAP sample heated at 1173 K is confirmed by the additional presence of the two characteristic Raman peaks at 959 and 975 cm1.47 Furthermore, except in the case of Mg1.54HAP, a broadening of the band at 960 cm1 progressively occurs with increasing magnesium content, which can be related to the appearance and increase of a new vibration band centered at 950 cm1 (decompositions not shown). This feature could be related to the Mgsubstitution in the HAP lattice, or perhaps to the presence of another substituted calcium phosphate phase such as β-TCP (Ca3zMgz(HPO4)x(PO4)2x/3), according to the literature.48 However, the ν4ν2 value measured (130150 cm1) from the spectra is more consistent with the value expected for hydroxyapatite (120 cm1) rather than that expected for β-TCP (55 cm1).48 This suggests that the broadening is most probably due to a crystalline disorder enhanced by cationic substitution.49 Indeed, in a similar way, the substitution of PbII+ in HAP (which occurs on cationic site II, just as in the case of magnesium32) was also shown to induce an additional contribution at 928 cm1.50 This direct influence of magnesium substitution on the vibrational spectra is actually confirmed by the linear relationships between the full width at midheight of the 960 cm1 band and the magnesium content (part b of Figure 2). Note that, once again, the Mg1.54HAP sample does not follow the trends observed for most of the other samples, suggesting that this sample is already a mixture of HAP (with substitution level close to x = 0.5, as deduced from part b of Figure 2) and of an amorphous whitlockite phase, which contributes much less to the intensity of 24320



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Figure 3. DTA curves of dried MgxHAP samples (x = 0.5, 0.75, 1.54) and TG curve for x = 0.5, recorded under nitrogen flow in the 3001300 K range.

Figure 4. TEM pictures of a) CaHAP-1, b) Mg0.25HAP, c) Mg1.54HAP, d) Mg2HAP.

the phosphate vibration bands in Raman, and can thus not be unambiguously identified on the Raman spectrum. DTA-TGA. The transformation of HAP samples upon heating can be followed by thermogravimetric studies. As observed from the comparison of the DTA and TG curves (Figure 3) of the dried MgxHAP samples, the main weight loss takes place below 623 K. It corresponds to water desorption, and is

surprisingly associated with a first exothermic phenomenon already observed by Yasukawa et al.,42 then followed by the expected endothermic behavior related to water release. The exothermicity of the first weight loss may be due to the fact that the heating of the MgxHAP phases in the presence of physisorbed water could first induce a structuring effect of the hydroxyapatite lattice. 24321


The Journal of Physical Chemistry C On the basis of the TGA, the final thermal treatment of the prepared catalysts was set to 623 K, as this temperature creating well-defined systems but avoiding severe sintering or progressive transition of the phase,42 which would occur at higher temperatures. Indeed, additional minor weight losses can be observed in the 9701100 K temperature range. From the literature, these could be related either to the condensation of the HPO42‑ associated to cationic deficiency,51 (2 HPO42 a P2O74 + H2O) or to the decomposition of the partial HAP into a Mgcontaining β-tricalcium phosphate (βTCP) (Mg,Ca)3(PO4)2.42 Considering that i) no clear correlation between this weight loss and the Ca/P ratio reported in Table 1 could be evidenced, and that ii) the temperature of these additional weight losses decreases as the Mg content increases, as previously observed in the literature,17,42 they most likely correspond to the decomposition of MgxHAP into β-Ca3‑xMgx(PO4)217 or even, in the case of Mg1.54HAP, to the crystallization of Mgwhitlockite Ca3yMgy(HPO4)z(PO4)22z/3 phase,4245 whose structure is very close to β-Ca3xMgx(PO4)2, as also suggested from XRD and Raman characterization.52 3.2. Surface Properties. Textural and Morphological Properties. N2 Physisorption. In preparation methods of MgxHAP phases, which start from calcium and magnesium hydroxide precursors, the incorporation of magnesium was found to induce a decrease in crystallinity, and thus either an increase of the specific surface areas (>90 m2g1),16,49,53 or almost no change in their value.54 In contrast, here, the incorporation of a limited magnesium content globally results in a decrease of specific surface areas, from 52 to 89 m2g1 for CaHAP to 2371 m2g1 for MgxHAP samples prepared in the same synthesis conditions. This result is in line with the effect reported for similar precursors by Chaudry et al.43 For some of the samples with a higher magnesium content, an increase of surface area is observed (Mg1.5 HAP and Mg2HAP); such an observation had also already been reported,43 and it was proposed that this is because a high Mg content could severely retard crystallization and growth of particles in solution. Electron Microscopy. Consistently, TEM pictures (Figure 4) also show that the morphology and size of the crystallites are impacted by the incorporation of magnesium. In the absence of magnesium, classical rod-shaped crystallites are observed with lengths between 50 and 300 nm (part a of Figure 4). These elongated rods are stacked on each other, mainly along their length, to form agglomerates.55 Upon magnesium incorporation, a slight increase of particle size is observed (∼400 nm for Mg0.25HAP) (part b of Figure 4), which is consistent with the decrease of specific area observed above for low magnesium content. It is noteworthy that the morphology is even more impacted for high magnesium content samples: for instance, in both Mg1.54HAP and Mg2HAP samples (part c of Figure 4 and part d of 3), rods appear much frayed and are stacked together to form filigree agglomerates. If such organization may explain the very high surface area of the Mg2HAP sample, the presence of spherical particles (diameter ∼50 nm) that are also observed in the case of Mg1.54HAP sample finally results in unmodified average surface area compared to low content MgxHAP. The presence of such spherical particles was already reported for Mgwhitlockite.43 Such evidence of whitlockite-type particles reinforces the conclusion that, as shown by XRD and Raman results, the Mg1.54HAP sample is a mixture of both crystalline HAP and amorphous whitlockite phases.

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Figure 5. (a) FTIR spectra of self-supported wafers in the 40001250 cm1 range of CaHAP-2, Mg0.5HAP-2, Mg1HAP-1, Mg1.5HAP-2, Mg2HAP. (b) Shift of the maximum of the IR band related to structural OH upon magnesium incorporation in CaHAP-2, Mg1HAP-1, Mg2HAP.

Acido Basic Properties. Infrared. Infrared can provide information on both structural and surface levels. In particular, the accessibility of magnesium cations at the surface, as well as acidic surface properties, can be investigated by following the adsorption of CO probe molecules at low temperature. First, the as-prepared samples were characterized by IR spectroscopy. IR bands characteristic of HAP structures are observed for all the samples, as shown in part a of Figure 5. The magnesium introduction results in some minor modifications with respect to the pure calcium HAP phase. Indeed, according to part b of Figure 5, the maximum of the νOH band at 3572 cm1 assigned to structural OH groups in the bulk of CaHAP is progressively shifted to lower wavenumbers with increasing magnesium content (to reach 3565 cm1 for Mg2HAP). The shift of the structural OH band can be related to its proximity to Mg2+ (which preferentially enters the Ca(II) site close to the OH).32 In addition, for higher magnesium content, it can be hypothesized that the structural disordering occurring when going from stoichiometric to under stoichiometric hydroxapatites may also participate to 24322



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Figure 6. (a) FTIR spectra in the 22302130 cm1 range upon adsorption at 77 K of 27.108 mol of CO on CaHAP-2, Mg0.5HAP-2, Mg1HAP1, Mg1.5HAP-2, Mg2HAP. (b) Decomposition of the band related to CO adsorbed on the surface for CaHAP-2 into three components at 2173, 2182, and 2185 cm1 assigned to CO in interaction with surface POH, Ca2+(II) and Ca2+(I), respectively. (c) Decomposition of the band related to CO adsorbed on the surface for Mg1HAP-1 into four components at 2173, 2182, 2185, and 2198 cm1 assigned to CO in interaction with surface POH, Ca2+(II), Ca2+(I), and Mg2+, respectively.

the broadening of the OH band because it is known to affect the ordering of structural OH groups.5658 Although accurate comparison between the intensities of the OH bands of different samples needs to be done carefully (here the intensities in the spectra have all been normalized from the wafer weight used for IR analysis), it seems clear that the relative intensity of the OH band strongly decreases as the Mg content increases (CaHAP2 and Mg1HAP-1 spectra compared to Mg2HAP spectrum). In contrast, the weak band around 3680 cm1 (assigned to the OH stretching vibration mode of POH groups20,5961) is enhanced for the highly nonstoichiometric sample (Ca+Mg/ P = 1.61) (Mg2HAP spectrum in parts a and b of Figure 5). Thus, a change in the relative intensities of both bands occurs

when switching from stoichiometric to nonstoichiometric samples. This feature is consistent with the expected decrease in the number of structural OH groups and appearance of bulk POH occurring in cationic deficient hydroxyapatites.1,62 Note that the presence of a weak 3680 cm1 band on all the samples, including the stoichiometric ones, indicates protonation of surface PO43 ions.20,5961 Combination and overtone bands of PO43 in the 1900 2200 cm1 area1,63 are also less defined with increasing magnesium content, which could be indicative of a loss of symmetry of the PO43 tetrahedra (part a of Figure 5) because of structural disorder in the HAP lattice, as already mentioned with the band relative to structural OH groups. 24323


The Journal of Physical Chemistry C Changes in these IR spectra in the presence of CO were then analyzed. Introduction of incremental doses of CO (up to 27.108 mol) results in the appearance of a vibration band at 2181 cm1, whose maximum is shifted to higher wavenumbers with increasing magnesium content (2186 cm1) (part a of Figure 6). As shown in part b of Figure 6 for the CaHAP sample, this contribution can in fact be decomposed into several components. Indeed, in addition to the component centered at 2173 cm1, which can be assigned to the vibration of CO molecules interacting with surface acidic POH groups,54 the two major bands at ca. 2185 and 2182 cm1 can be assigned to the CO adsorbed on inequivalent calcium sites at the surface.54 Indeed, the related wavenumbers are very consistent with the data reported for CO adsorbed on Ca exchanged zeolites.64,65 In the case of Mg1HAP, a similar decomposition was carried out (part c of Figure 6), which shows that the shift of the maximum of the band with respect to nonsubstituted HAP actually results from the decrease of the relative intensity of the component at 2182 cm1. Given that magnesium cations have been shown to preferentially substitute on the Ca(II) sites,32 this feature may indicate that it is the interaction of CO with the Ca(II) site which leads to the band at 2182 cm1. As a result, the other band (at 2185 cm1) could correspond to the Ca(I) site. Consistently, a fourth component is present at high wavenumber (2198 cm1) on the spectra of MgxHAP phases, which progressively increases with the Mg content, being clearly detected as a shoulder from x = 0.5 onward. This contribution undoubtedly originates from surface Mg2+ sites.54 Such a high-frequency value is indeed very similar to that reported for CO adsorbed on the Mg2+3C cations with the lowest coordination in MgO,66 and the blue shift with respect to the Ca2+ contributions is also consistent with higher charge/radius ratio of Mg2+ cations compared to Ca2+. However, despite this evidence of the presence of some magnesium cations at the surface of HAP, their number remains very small, especially for an x value lower than x = 0.5. It should be noted that, as already reported,54 a further increase in the doses of CO (Figure S1 of the Supporting Information) results in a red shift of the CO vibration bands, which is indicative of superimposition of additional components. These may be related to the formation of Mg2+(CO)2,67 Ca2+(CO)2, and Ca2+(CO)365 entities at the surface but also because of the increasing perturbation of the vibration band at 3680 cm1 band, to the increase in the interaction of CO with surface acidic P OH groups.54 From these results, it appears that four types of acidic site are thus detected at the surface: in addition to the two calcium sites, and to the magnesium surface cations (for x > 0.5), which are all Lewis acidic sites, OH terminated surface phosphate groups also provide BrØnsted acidic properties.6,20 Note that such direct evidence of surface accessibility of Lewis acidic sites by CO adsorption could not be achieved using pyridine17 probably due to its larger probe molecule size. Catalytic Properties. In all of the catalytic tests carried out on these samples, acetone and acetylene were found to be the only products resulting from the MBOH decomposition, which is indicative of basic properties of the HAP surfaces. Such basic behavior is consistent with the good performances reported for this kind of system in the conversion of ethanol in n-butanol.8 In agreement with the basic MBOH decomposition pathway, these products are formed in equimolar ratio. The MBOH conversion (reported in Table 1) is a reaction that depends on both the density of basic sites and on their strength, so it can thus be used to compare the basic properties of the different surfaces.

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Figure 7. Conversion of MBOH per square meter (%) of surface introduced into the reactor versus magnesium or strontium content (x) introduced in the preparation. (0 CaHAP, MgxHAP, ) Sr1.54HAP, O dried and heated OCP).

Figure 8. Conversion of MBOH (%) versus specific areas for 0 CaHAP, ΔMgxHAP, ) Sr1.54HAP, O dried and heated OCP. Samples exhibiting a negative (second group) or positive (Sr1.54HAP) deviation to the linear relationships (first group) are colored in black and gray, respectively.

The conversion level per square meter for the catalysts exhibiting a conversion lower than 60% (thus not limited by diffusion) is presented on Figure 7. Compared to CaHAP-2 sample, MgxHAP samples with x e 1 are not at all or only slightly impacted by the presence of Mg, whereas those with x > 1 have a significantly lower basic reactivity. It should also be noted that both dried (tested without any activation at 623 K prior to activity measurement at 413 K) and heated OCP samples appear much less active than CaHAP samples with a basic reactivity quite close to that of corresponding to MgxHAP phases with x >1. To point out the influence of specific surface area, Figure 8 presents the MBOH conversion as a function of the specific surface area. Once again, two types of behavior related to MgxHAP samples can be described. The first one gathers MgxHAP samples (mainly with x e1), for which a linear dependence of conversion level upon specific surface area is observed. Note that CaHAP samples can also be associated with this group of samples. The second group includes MgxHAP (x > 1) but also the OCP samples: they exhibit a lowered conversion than that expected from the linear relationship. 24324



4. DISCUSSION In this article, the impact of the substitution of magnesium on HAP structures is evaluated to determine how this system works at a molecular level. From the structural characterization reported (chemical analyses, XRD, Raman, IR), it appears that magnesium has been successfully incorporated into the HAP structure. On the one hand, low magnesium contents do not lead to important modifications of the HAP structure, with preservation of the Ca+Mg/P stoichiometry. On the other hand, for higher magnesium contents, the induced structural disorder results in nonstoichiometric HAP samples, or even in the formation of a whitlockite phase,45 as shown in the case of Mg1.54HAP. CO adsorption and catalytic properties are very sensitive tools to analyze the influence of substitution level on surface properties of HAP. On the one hand, the presence of magnesium on the surface could be probed by adsorption of CO at low temperature followed by FTIR. It is in fact shown that for low magnesium content, the amount of surface magnesium cations accessible to CO molecule remains quite low (which is not fully surprising given that some of the magnesium is actually inside the bulk HAP lattice). On the other hand, from the MBOH conversion test, which led to the formation of acetone and acetylene products, it appears that CaHAP and MgxHAP samples exhibit a basic surface reactivity. It should be noted that there are actually very few studies aimed at describing how hydroxyapatites work as basic heterogeneous catalysts.8 From the present work, two groups of samples can be discriminated depending on their catalytic behavior. In the first group, all samples have the HAP structure, with M/ P = 1.67. Moreover, from TEM characterization, they all show very similar morphologies with elongated rods. Thus, it can be concluded that they all exhibit the same nature and density of active sites.36 Consequently, despite the introduction of magnesium cations in the HAP structure (as shown by XRD and IR), even for samples of low magnesium content, no influence of magnesium substitution on the surface catalytic properties is detected. This result may in fact be related to the very low number and exposure of these magnesium cations at the surface, as evidenced by CO adsorption experiments. The second group with lowered basic reactivity compared to the first group corresponds to samples of higher magnesium content (x > 1) and to the dried and heated OCP compounds. Taking into account the decreasing basic character of MgOH compared to CaOH,68 the decrease in basic reactivity could at first appear to prove the involvement of OH groups as basic sites. However, because the observed decrease (Figure 7) is not really gradual as the Mg content increases, another explanation for this drastic loss of basicity should thus also be considered. From the characterization mentioned above, in this second group, all samples exhibit a Ca/P value < 1.67. Calcium deficienthydroxyapatites can be associated to both the incorporation of HPO42 anions in substitution to PO43, and to the creation of OH vacancies,1 (which eventually result in the incorporation of water in the structure)41 leading to the formula Ca10x 0 x(HPO4)x(PO4)6x(OH)2x 0 x,nH2O (0 e x e 1). It should be noted that the impact of nonstoichiometry was observed on the relative intensities of the IR bands and is consistent with this formula. Thus, under-stoichiometric hydroxyapatites exhibit lower concentration of both PO43 and OH groups compared to stoichiometric hydroxyapatites. Considering that surface basic sites are expected to be either OH23,24or PO432527 groups, and assuming that

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the evolution of the composition of the surface follows the same trends as the bulk, the lower basic reactivity of under stoichiometric HAP samples compared to that of stoichiometric ones could be simply related to the decrease of the relative amount of basic sites present. Another possible reason for a low Ca/P ratio is the presence in the samples of a mixture of HAP phase and other poorly crystallized calcium phosphates phases, such as OCP (Ca8(HPO4)2(PO4)4 3 5H2O), which is considered as a HAP precursor, β-Ca3xMgx(PO4)2, or even whitlockite (Ca3yMgy(HPO4)z(PO4)22z/3).17,4245 In the present study, among the of Mgx HAP samples heated to 623 K, the presence of whitlockite could only be clearly evidenced in the Mg1.54HAP sample. It should be noted that in all these related calcium phosphates phases, no structural OH group likely to generate surface basicity is present, which could explain the lower intrinsic basicity of the related materials. Moreover, as already mentioned in the case of real under-stoichiometric hydroxypatites Ca10x 0 x(HPO4)x(PO4)6x(OH)2‑x 0 x,nH2O, the presence of HPO42 groups in the OCP and whitlockite phases could also provide BrØnsted acidity,1 which would then modulate the acidic-basic properties of the surface at the expense of basic reactivity. Indeed, even though the presence of such surface HPO42 groups is not exclusively related to bulk properties (nonstoichiometry or formation of other calcium phases),43,52,62 because the presence of an IR band around 3680 cm1 can also be observed even for stoichiometric compounds (parts a and b of Figure 5) due to protonation of surface PO43 ions,20,5961 it was underlined that the relative intensity of the 3680 cm1 band is much more important for the highly non stoichiometric Mg2HAP compound. Thus, for high magnesium content samples, the decrease of basic reactivity is rather ascribed to a decrease of the amount of basic sites, which is related to structural modifications rather than as direct evidence of lower intrinsic basicity of the OH groups close to Mg2+ compared to those close to Ca2+. From this point of view, the study of other cationic substitution, such as strontium substituted HAP that are known to exhibit catalytic properties9,20,22 could help to get more insights into the intrinsic catalytic behavior of the lone hydroxyapatite phase. Indeed, strontium is easily incorporated into the Ca HAP lattice69 (as the ionic radius of Sr2+ is more similar to that of Ca2+), and for a substitution level higher than 6 at %, Sr preferentially enters the Ca(II) site.70 From a very preliminary study on a Sr1.54HAP sample (Figure S2 in the Supporting Information for its preparation and characterization), the HAP stoichiometric structure is maintained upon incorporation of higher levels of strontium compared to magnesium. Thus, the increased reactivity of Sr1.54HAP in MBOH conversion compared to that of CaHAP (Figures 7 and 8) should directly be linked to the substitution effect, and thus possibly, to the enhanced reactivity of OH close to strontium compared to those close to calcium.

5. CONCLUSIONS It was shown that HAP substituted by magnesium can lead to two different types of samples and thus catalytic behavior depending on the magnesium content. For low Mg contents (x e 1), despite the successful introduction of the Mg in the HAP structure, no major change in the structural properties or (Mg + Ca)/P ratio could be observed. Furthermore, IR studies showed that in those samples, very little Mg was actually accessible on the surface. This could explain why 24325


The Journal of Physical Chemistry C these samples exhibit no or very little difference in basic reactivity with respect to the classic non substituted CaHAp. By contrast, a higher magnesium content results in severe changes in both structure and stoichiometry of the materials, even leading to the creation upon thermal treatment of other OH deprived phases to stabilize the system. It is well-known that under-stoichiometry leads to a decrease of both OH and PO43 groups, which are the two potential surface basic sites. Whether linked to under-stoichiometry or to the existence of another phase, this drop of the number of possible basic sites is reflected by the drastic decrease of basic reactivity. The preliminary study of strontium-substituted hydroxyapatites for which stoichiometry is easily maintained due to better stability of CaHAP toward Sr substitution, indicates that this system could be very promising not only to enhance the catalytic performance of HAP systems for applications in basic heterogeneous catalysis, but also to definitively establish the implication of OH groups as active sites. Work is currently in progress to gain deeper insight into the identification of active sites and to evaluate the impact of cationic substitution in HAP compounds in other catalytic reactions of application interest such as ethanol conversion.

’ ASSOCIATED CONTENT

bS

Supporting Information. FTIR spectra in the 2230 2100 cm1 range recorded after adsorption of CO at 77 K (Peq = 0.8 Torr) on CaHAP-2, Mg0.5HAP-2, Mg1HAP-1, Mg1.5 HAP-2, Mg2HAP; preparation procedure, XRD diffractogram, and chemical composition of a Sr1.54HAP sample. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +33 1 44 27 60 05, fax: +33 1 44 27 60 33, e-mail: [email protected].

’ ACKNOWLEDGMENT S. Aitbachir is thanked for his help in the catalytic tests. D.L. acknowledges the 6th European Community Framework Program for a Marie Curie IEF. ’ REFERENCES (1) Joris, S. J.; Amberg, C. H. J. Phys. Chem. 1971, 75, 3167–3171. (2) Monma, H. J. Catal. 1982, 75, 200203. (3) Sebti, S.; Solhy, A.; Tahir, R.; Smahi, A. Appl. Catal., A 2002, 235, 273–281. (4) Zahouily, M.; Abrouki, Y.; Bahlaouan, B.; Rayadh, A.; Sebti, S. Catal. Comm. 2003, 4, 521–524. (5) Tahir, R.; Banert, K.; Solhy, A.; Sebti, S. J. Mol. Catal. A, Chem 2006, 246, 39–42. (6) Tsuchida, T.; Sakuma, S.; Takeguchi, T.; Ueda, W. Ind. Eng. Chem. Res. 2006, 45, 8634–8642. (7) Tsuchida, T.; Yoshioka, T.; Sakuma, S.; Takeguchi, T.; Ueda, W. Ind. Eng. Chem. Res. 2008, 47, 1443–1452. (8) Tsuchida, T.; Kubo, J.; Yoshioka, T.; Sakuma, S.; Takeguchi, T.; Ueda, W. J. Catal. 2008, 259, 183–189. (9) Sugiyama, S.; Iguchi, Y.; Minami, T.; Hayashi, H.; Moffat, J. B. Catal. Lett. 1997, 46, 279–285. (10) Sugiyama, S.; Nitta, E.; Hayashi, H.; Moffat, J. B. Catal. Lett. 1999, 59, 67–72.

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(11) Sugiyama, S.; Osaka, T.; Hirata, Y.; Sotowa, K. I. Appl. Catal., A 2006, 312, 52–58. (12) Cazalbou, S.; Combes, C.; Eichert, D.; Rey, C. J. Mater. Chem. 2004, 14, 2148–2153. (13) Landi, E.; Logroscino, G.; Proietti, L.; Tampieri, A.; Sandri, M.; Sprio, S. J. Mater. Sci: Mater. Med. 2008, 19, 239–247. (14) Gomes, S.; Renaudin, G.; Jallot, E.; Nedelec, J. M. Appl. Mater. Interf. 2009, 1, 505–513. (15) Bigi, A.; Falini, G.; Foresti, E.; Ripamonti, A.; Gazzano, M.; Roveri, N. J. Inorg. Biochem. 1993, 49, 69–78. (16) Suchanek, W. L.; Byrappa, K.; Shuk, P.; Riman, R. E.; Janas, V. F.; TenHuisen, K. S. Biomaterials 2004, 25, 4647–4657. (17) Tanaka, H.; Watanabe, T.; Chikazawa, M. J. Chem. Soc. Faraday Trans. 1997, 93, 4377–4381. (18) Zaki, M. I.; Knozinger, H.; Tesche, B. Langmuir 2006, 22, 749–755. (19) Pekounov, Y.; Chakarova, K.; Hadjiivanov, K. Mater. Sci. Eng., C 2009, 29, 1178–1181. (20) Ishikawa, T.; Saito, H.; Yasukawa, A.; Kandori, K. J. Chem. Soc., Faraday Trans. 1993, 89, 3821–3825. (21) Tsuchida, T.; Yoshioka, T.; Sakuma, S.; Takeguchi, T.; Ueda, W. Ind. Eng. Chem. Res. 2008, 47, 1443–1452. (22) Ogo, S.; Onda, A.; K., Y. Appl. Catal., A 2011, 402, 188–195. (23) Bailly, M. L.; Chizallet, C.; Costentin, G.; Krafft, J. M.; LauronPernot, H.; Che, M. J. Catal. 2005, 235, 413–422. (24) Chizallet, C.; Petitjean, H.; Costentin, G.; Lauron-Pernot, H.; Maquet, J.; Bonhomme, C.; Che, M. J. Catal. 2009, 268, 175–179. (25) Aramendia, M. A.; Borau, V.; Jimenez, C.; Marinas, J. M.; Romero, F. J. React. Kinet. Catal. Lett. 2000, 69, 311–316. (26) Sebti, S.; Smahi, A.; Solhy, A. Tetrahedron Lett. 2002, 43, 1813– 1815. (27) Zahouily, M.; Mounir, B.; Charki, H.; Mezdar, A.; Bahlaouan, B.; Ouammou, M. ARKIVOC 2006, xiii, 178–186. (28) Chizallet, C.; Bailly, M. L.; Costentin, G.; Lauron-Pernot, H.; Krafft, J. M.; Bazin, P.; Saussey, J.; Che, M. Catal. Today 2006, 116, 196–205. (29) Lavalley, J. C. Catal. Today 1996, 27, 377–401. (30) Lauron-Pernot, H.; Luck, F.; Popa, J. M. Appl. Catal., A 1991, 78, 213–225. (31) Lauron-Pernot, H. Catal. Rev. 2006, 48, 315–361. (32) Laurencin, D.; Almora-Barrios, N.; de Leeuw, N. H.; Gervais, C.; Bonhomme, C.; Mauri, F.; Chrzanowski, W.; Knowles, J. C.; Newport, R. J.; Wong, A.; Gan, Z.; Smith, M. E. Biomaterials 2011, 32, 1826–1837. (33) Bertoni, E.; Bigi, A.; Cojazzi, G.; Gandol, M.; Panzavolta, S.; Roveri, N. J. Inorg. Biochem. 1998, 72, 29–35. (34) Ben Abdelkader, S.; Khattech, I.; Rey, C.; Jemal, M. Thermochim. Acta 2001, 376, 25–36. (35) Sugiyama, S.; Shono, T.; Makino, D.; Moriga, T.; Hayashi, H. J. Catal. 2003, 214, 8–14. (36) Tsuchida, T.; Kubo, J.; Yoshioka, T.; Sakuma, S.; Takeguchi, T.; Ueda, W. J. Jap. Petrol. Inst. 2009, 52, 51–59. (37) Markovic, M.; Fowler, B. O.; Tung, M. S. J. Res. Natl. Inst. Technol. 2004, 109, 553–567. (38) Kandori, K.; Fudo, A.; T., I. Colloids Surf., B 2002, 24, 145–153. (39) Tseng, Y. H.; Zhang, J.; Lin, K. S. K.; Mou, C. Y.; Chan, J. C. C. Solid State Nuclear Resonance 2004, 26, 99–104. (40) Elliot, J. C. Structure and Chemistry of the Apatites and Other Calcium Orthophosphates; Elsevier: Amsterdam, Neth.; 1994; Vol. 18. (41) Wilson, R. M.; Elliot, J. C.; Dowker, S. E. P. JSSC 2003, 174, 132–140. (42) Yasukawa, A.; Ouchi, S.; Kandori, K.; Ishikawa, T. J. Mater. Chem. 1996, 6, 1401–1405. (43) Chaudhry, A. A.; Goodall, J.; Vickers, M.; Cockcroft, J. K.; Rehman, I.; Knowles, J. C.; Darr, J. A. J. Mater. Chem. 2008, 18, 5900– 5908. (44) Correia, R. N.; Magalhaes, M. C. F.; Marques, P. A. A. P.; Senos, A. M. R. J. Mater. Sci: Materials in Medicine 1996, 7, 501–505. 24326



ARTICLE

(45) Fadeev, I. V.; Shvorneva, L. I.; Barinov, S. M.; Orlovskii, V. P. Inorg. Mater. 2003, 39, 947–950. (46) Silva, C. C.; Pinheiro, A. G.; Miranda, M. A. R.; G oes, J. C.; Sombra, A. S. B. Solid State Sci. 2003, 5, 553–558. (47) Ionov, D. A.; Hofmann, A. W.; Merlet, C.; Gurenko, A. A.; Hellebrand, E.; Montagnac, G.; Gillet, P.; Prikhodko, V. S. Earth and Planetary Science Letters 2006, 244, 201–217. (48) Cusco, R.; Guitian, F.; de Aza, S.; Artus, L. J. Europ. Ceramic Soc. 1998, 18, 1301–1305. (49) Sprio, S.; Pezzotti, G.; Celotti, G.; Landi, E.; Tampieri, A. J. Mater. Res. 2005, 20, 1009–1016. (50) Hadrich, A.; Lautie, A.; Mhiri, T. Spectrochim. Acta 2001, A 57, 1673–1681. (51) Monma, H.; IUeno, S.; Kanazawa, T. J. Chem. Tech. Biotechnol. 1981, 31, 15–24. (52) Gopal, R.; Calvo, C.; Ito, J.; Sabine, W. K. Can. J. Chem. 1974, 52, 1155–1164. (53) Yasukawa, A.; Ouchi, S.; Kandori, K.; Ishikawa, T. J. Mater. Chem. 1996, 6, 1401–1405. (54) Bertinetti, L.; Tampieri, A.; Landi, E.; Martra, G.; Coluccia, S. J. Eur. Ceramic Soc. 2006, 26, 987–991. (55) Bertinetti, L.; Tampieri, A.; Landi, E.; Ducati, C.; Midgley, P. A.; Coluccia, S.; Martra, G. J. Phys. Chem. C 2007, 111, 4027–4035. (56) Matthew, M.; Tagaki, S. Res. Natl. Inst. Stand. Technol. 2001, 106, 1035–1044. (57) Wang, L.; Nancollas, G. H. Chem. Rev. 2008, 108, 4628–4669. (58) Pan, H. B.; Li, Z. Y.; Wang, T.; Lam, W. M.; Wong, C. T.; Darvell, B. W.; Luk, K. D. K.; Hu, Y.; Lu, W. W. Cryst. Growth Des. 2009, 9, 3342–3345. (59) Ishikawa, T.; Wakamura, M.; Kondo, S. Langmuir 1988, 5, 140–144. (60) Ishikawa, T.; Wakamura, M.; Kondo, S. Langmuir 1989, 5, 140–144. (61) Tanaka, H.; Yasukawa, A.; Kandori, K.; Ishikawa, T. Colloids Surf., A 2002, 204, 251–259. (62) Cheng, Z. H.; Yasukawa, A.; Kandori, K.; Ishikawa, T. Langmuir 1998, 14, 6681–6686. (63) Tanaka, H.; Futaoka, M.; Hino, R. J. Colloid Interface Sci. 2004, 269, 258–363. (64) Li, P.; Xiang, Y.; Grassian, V. H.; Larsen, S. C. J. Phys. Chem. B 1999, 103, 5058–5062. (65) Hadjiivanov, K.; Ivanova, E.; Knozinger, H. Microporous Mesoporous Mater. 2003, 58, 225–236. (66) Coluccia, S.; Baricco, M.; Marchese, L.; Martra, G.; Zecchina, A. Spectrochim. Acta 1993, 49A, 1289–1298. (67) Spoto, G.; Gribov, E. N.; Ricchiardi, G.; Damin, A.; Scarano, D.; Bordiga, S.; Lamberti, C.; Zecchina, A. Prog. Surf. Sci. 2004, 76, 71–146. (68) Petitjean, H.; Krafft, J. M.; Che, M.; Lauron Pernot, H.; Costentin, G. Phys. Chem. Chem. Phys. 2010, 12, 14740–14748. (69) Bigi, A.; Boanini, E.; Capuccini, C.; Gazzano, M. Inorg. Chim. Acta 2007, 360, 1009–1016. (70) Terra, J.; Rodrigues Dourado, E.; Eon, J. G.; Ellis, D. E.; Gonzalez, G.; Malta Rossi, A. Phys. Chem. Chem. Phys. 2009, 11, 568–577. (71) Yasukawa, A.; Ueda, E.; Kandori, K.; Ishikawa, T. J. Colloid Interface Sci. 2005, 288, 468–474.

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Influence of Magnesium Substitution on the Basic Properties of

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