Investigations of a Sodium− Polyacrylate-Containing System Yielding

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Investigations of a Sodium-Polyacrylate-Containing System Yielding Nanosized Boehmite Particles Yannick Mathieu, Se´verinne Rigolet, Valentin Valtchev, and Be´ne´dicte Lebeau* Laboratoire de Mate´riaux a` Porosite´ Controˆle´e, UMR-7016 CNRS, ENSCMu, UniVersite´ de Haute Alsace, 3, rue Alfred Werner, 68093 Mulhouse Cedex, France ReceiVed: July 21, 2008; ReVised Manuscript ReceiVed: September 29, 2008

The formation of nanosized boehmite particles under hydrothermal conditions from a sodium polyacrylate (NaPa) containing system was studied. Precursor mixtures prior to hydrothermal treatment were in particular investigated by DLS and multinuclear (27Al and 23Na) liquid and solid state NMR spectroscopy to get a better understanding of the formation of boehmite particles and the role of the NaPa polymer. The final product was also studied by solid state NMR. Experimental data showed that amorphous particles were first formed for a pH ranging from 9.5 to 10.2 via polycondensation reactions between octahedral species followed by a crystallization step leading to the formation of boehmite particles (pH g 10.5). It was found that the agglomeration state of the boehmite primary particles formed before hydrothermal treatment is strongly dependent to the pH. After the hydrothermal treatment, the agglomeration state of boehmite particles was limited due to the formation of a complex between the boehmite surface and carboxylate groups of sodium polyacrylate thus preventing particles from coalescence with each other. Introduction The synthesis of oxide nanoparticles with controlled morphology is of great interest due to their technical importance in different fields such as catalysis, coating, optic, electronic, or medical applications. Among aluminum oxides, boehmite (γAlOOH) was often used as the starting material for the preparation of γ-Al2O3 (transitional alumina) which is widely used in catalysis,1 coatings,2 or membranes for the liquid or gas filtration process.3 Controlling the size and shape of boehmite particles is hence a key parameter to improve the properties and potentialities of the final alumina material. One of the strategies frequently reported to provide nanosized boehmite particles is the hydrothermal synthesis of an aluminum aqueous solution previously neutralized with alkali or acid.4-8 However, despite the relative simplicity of this way, a major drawback of the hydrothermal treatment is the strong agglomeration of the nanoparticles. Numerous organic compounds including surfactants and polymers are known to be efficient to control the size or morphology and, in particular, to prevent nanoparticles from coalescing with each others.9-13 Our previous work has demonstrated the effect of a biocompatible polymer, sodium polyacrylate (NaPa), on the boehmite nanoparticles obtained after the neutralization of an aqueous solution of AlCl3 followed by hydrothermal treatment at 160 °C during 24 h.14 This polymer was employed as a size/morphology controlling agent to provide nonagglomerated rounded boehmite nanoparticles ranging in size between 15 and 40 nm with respect to the conventional synthesis leading to strongly agglomerated boehmite nanoparticles in the form of tiny plates (60-80 nm). The size and the agglomeration state of boehmite nanoparticles obtained in the NaPa-containing system were strongly dependent on the pH of the precursor mixture. The prolongation of the hydrothermal synthesis time from 24 to 168 h resulted also in substantial changes of crystal morphology. Fibers up to 2000 nm longer and 5-8 nm in diameter were obtained after a hydrothermal treatment at 160 °C during 168 h.

Numerous studies have already showed that NaPa is an efficient crystal growth inhibitor that can be used to control the size of organic and inorganic crystals.15-18 For instance, NaPa is known to inhibit the growth and the crystallinity of octacalcium crystals obtained during the hydrolysis process of R-tricalcium phosphate probably though an interaction between carboxylate groups of NaPa molecules with calcium ions.15,16 The presence of NaPa during the formation of L-asparagine provided needle-like crystals instead of the typical prismatic ones.17 Similarly, Ahmadi et al.18 suggest that the polymer NaPa acts as a capping material stopping the growth of the particles at a small size distribution and also preventing individual colloidal particles from coalescing with each other. Although the polymer NaPa was found to control the growth of boehmite nanoparticles, the mechanism of interactions between NaPa molecules and aluminic species has not yet been clearly elucidated.14 Therefore, the understanding of these interactions would be an important step toward the preparation of nanosized alumina oxide materials with tuned properties. It is worth underlining the extreme complexity of aqueous chemistry of aluminum due to the wide range of aluminum species which could be formed as a function of several experimental parameters such as pH, temperature, ionic strength, or type of anion. This probably explains why few studies devoted to aluminic species involved in the mechanism of formation of aluminum hydroxides and oxyhydroxides have been reported. Hsu and Bates19 first suggested that the formation of these materials is probably due to progressive condensation of hexamer [Al(H2O)6]3+ species. Further, Bottero et al.20 have studied by liquid 27Al NMR the mechanism of formation of aluminum trihydroxides such as gibbsite or bayerite from an aluminum chloride solution progressively neutralized by sodium hydroxide and underlined that these materials are not formed by condensation of hexamers but by solid state structural rearrangements between Al13 species. This was strengthened by the results obtained by Fu et al.21 who also identified three new Al clusters

10.1021/jp806404e CCC: $40.75  2008 American Chemical Society Published on Web 10/31/2008

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formed because of the thermal degradation of Al13 Keggin species. As suggested by the authors, these clusters may be involved in the mechanism pathway. Vogels et al.22 studied the aging of the Keggin polycation in acidified aluminum trisecbutoxyde with 27Al NMR experiments and postulated also the existence of large polymers due to rearrangement of Keggin species. These large clusters consist of hexameric rings and were not observable by NMR. Morgado et al.23 who studied aluminum gel precursors obtained from base hydrolysis of an aluminum nitrate solution used to obtain fiber-like microcrystalline boehmites showed the presence of tetrahedrally coordinated aluminum from Al13 Keggin species and suggested that the resulting gel precursor is built by the consolidation or agglomeration of Al13 species. Thus, to complete our previous work, an investigation of the precursor mixtures prior to hydrothermal treatment was needed to get more information about the formation of boehmite particles and the role of the NaPa polymer. The precursor NaPa-AlCl3 solutions used to obtain nanosized boehmite particles were first studied by pH-metric titrimetry and DLS to follow the formation of nanoparticles as a function of pH during the neutralization process by sodium hydroxide. A main part of this work was devoted to the study of precursor NaPa-AlCl3 mixtures as a function of pH by 27Al liquid state NMR and by 27Al solid state NMR performed on the freezedried precursor. The evolution of the aluminic species present in the solution and in the solid phase was also followed by 27Al solid-liquid state NMR. The final product obtained after a 24 h hydrothermal treatment at 160 °C was investigated by solid state NMR (27Al, 23Na). All of these studies allowed us to propose a mechanism pathway for the boehmite formation in the NaPacontaining system. Experimental Section Synthesis. Sodium polyacrylate (NaPa) 2100 (Fluka) and aluminum chloride hexahydrate (Avocado) were used without further purification. A typical synthesis procedure includes dissolution of 9 g of NaPa 2100 in 75 mL of an aqueous solution of aluminum chloride 0.1 M. The resulting mixture was mechanically stirred at room temperature for 24 h. Sodium hydroxide (solution 5 M) was added dropwise to the AlCl3 + NaPa mixture to increase the pH up to the desired value, and the precursor mixture was stirred for 10 more minutes. The precursor mixtures prepared as a function of pH are listed in the Table 1. Two reference aqueous solutions of 0.057 M sodium polyacrylate 2100 and 0.1 M AlCl3 were also prepared. To get the final boehmite material, the precursor mixture obtained at pH ) 10.5 was transferred in a PTFE-lined stainless TABLE 1: Particle Size, Polydispersity Index, and Turbidity of the AlCl3 + PaNa Mixture as a Function of pH pH 5.6-5.8a 7 8 9 9.5 10 10.2 10.2 (20 min)b 10.5

VNaOH 5 M particle size polydispersity (mL) (nm) index turbidity 0 3.2 4.8 6 6.2 6.6 7 7 7.3

1–1.5 1–1.5 1–1.5 2–3 25–40 20–40 40–65 50–100 30–90

0.41 0.53 0.50 0.58 0.55 0.33 0.29 0.42 0.44

low low low low middle middle middle high high

a The starting pH of the precursor mixture without NaOH addition will be denoted pH0. b Particle size and polydispersity index after a 20 min step at pH ) 10.2.

Figure 1. pH-metric curves of (a) 0.057 M sodium polyacrylate 2100 aqueous solution, (b) 0.1 M AlCl3 aqueous solution, and (c) the AlCl3 + NaPa mixture aged 24 h with 5 M NaOH.

steel autoclave and heated under stirred conditions (15 rpm) at 160 °C for various times (3, 17, and 168 h). The colloidal suspension was washed by dialysis with a Sartorius Slice 200 Benchtop instrument equipped with a 100 KD polyethersulfone membrane. The amount of distilled water per washing was 5 L. The solid was recovered by centrifugation (1 h, 25 000 rpm) and dried overnight at 80 °C. Characterization. A Hanna HI991001 pH meter with a combined glass electrode operating in the pH range 1-14 was employed for the pH measurements. Powder diffraction patterns were recorded on a STOE STADI-P diffractometer equipped with a linear positionsensitive detector (6° 2θ) in Debye-Scherrer geometry and employing Ge monochromated Cu KR1 radiation (λ ) 1.5406 Å). XRD experiments were performed on the solid obtained after a 1 h centrifugation at 25 000 rpm of the precursor mixtures and drying overnight at 80 °C. Particle size analysis was performed by dynamic light scattering (DLS) with a Malvern ZetaSizer Nano ZS instrument equipped with a Peltier heating/cooling element (4-88 °C). An aliquot of the colloidal suspension was collected for the analysis, and the measurement was repeated 5 times with an interval of 180 s between each measurement. For particle size analysis as a function of temperature, the temperature ramp was 10 degrees per min, and before each measurement, a step of 5 min was observed. Particle size distributions were calculated using a nonnegative least square (NNLS) algorithm. 27Al (I ) 5/2) and 23Na (I ) 3/2) NMR spectroscopies were used to characterize the short-range atomic arrangement in the precursor solution. The spectra were recorded at room temperature on a Bruker Avance II 400 spectrometer operating at B0 ) 9.4 T (Larmor frequency ν0 ) 104.26 and 105.8 MHz, respectively) with a 10 mm Bruker BBO probehead. To avoid modifying the precursor mixtures, the samples were held in an 8 mm tube which was inserted into a 10 mm tube containing D2O as field frequency lock. A 7.7 and 4.5 µs pulse width were used corresponding, respectively, to a flip angle of π/12 for 27Al and π/8 for 23Na to ensure selective excitation of the central transition. 27Al spin-lattice relaxation times (T1) of 0.1 and 125 s for AlCl3 0.1 M reference aqueous solution and the NaPa + AlCl3 mixture, respectively, were measured with the inversion-recovery pulse sequence, indicating that 0.5 and 15 s recycle delays were more than enough to avoid saturation. A 1 s recycle delay was used for the 23Na NMR analyses. Typically, 320 scans were recorded for the 27Al experiments and 128 scans for the 23Na. Spectra were corrected for background signals arising from the aluminum and the natrium in the probe, using a blank consisting of dilute NaPa in H2O

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Figure 2. Dynamic light scattering results for the mixture NaPa + AlCl3 (NaPa/Al ) 0.57) as a function of pH.

Figure 3. XRD patterns of the solid obtained from precursor mixtures as a function of pH: (a) 9.5 e pH e 10.2 and (b) 10.5 e pH e 11.

TABLE 2: Particle Size and Polydispersity Index of the AlCl3 + PaNa Mixture Obtained at pH ) 10.5 as a Function of Temperature T (°C)

particle size (nm)

polydispersity index

25 35 45 55 65 75 160a

30-90

0.44 0.42 0.41 0.31 0.26 0.33 0.29

30-80 25-70 20-60 25-40

a

Particle size and polydispersity index after a 3 h hydrothermal treatment at 160 °C.

and H2O, respectively, whose spectrum was recorded under the same conditions as those used for sample. Chemical shifts reported thereafter are relative to Al(H2O)63+ and Na(NO3)3. 27Al (23Na) magic angle spinning (MAS) NMR experiments were performed at room temperature on the same spectrometer at ν0 ) 104.3 (105.8) MHz. Single-pulse experiments were recorded with a double channel 4 mm Bruker MAS probe, a spinning frequency of 12 kHz, a 0.5 (1.0) s recycling delay, and a 0.7 (1.0) µs pulse width corresponding to a flip of π/12 (π/8). Typically, 6000 (150) scans were recorded, and the chemical shifts reported are relative to Al(H2O)63+ (a NaCl saturated solution). The experiments were performed on the solid obtained after a 24 h freeze-drying of the precursor mixtures which were previously frozen in N2 liquid within 10 min. 27Al solid-liquid MAS NMR experiments were performed with a double-channel 7 mm Bruker probe. The precursor mixtures are introduced into insert in Kel-f which are then sealed to avoid any possible flight (leak) during the rotation. This insert was placed in a 7 mm rotor zirconium and put in rotation at a spinning frequency of 3 kHz. The spectra were recorded with

Figure 4. 27Al solution NMR spectra of the 0.1 M reference aqueous solution of AlCl3 (a) and the AlCl3 + NaPa mixture (b) aged for 24 h.

a 1.0 recycling delay and 2 µs pulse width corresponding to a flip of π/12. Spectra were corrected using a blank consisting of dilute NaPa in H2O into an insert placed in a rotor, whose spectrum was recorded under the same conditions as those used for sample. Results 1. Precursor Mixtures and Reference Solutions. The pHmetric titration curves of the reference aqueous solutions (a) 0.057 M sodium polyacrylate 2100, (b) 0.1 M AlCl3. and (c) the precursor AlCl3 + NaPa mixture aged 24 h (NaPa/Al ) 0.57) with 5 M NaOH are plotted in Figure 1. The pKa of the sodium polyacrylate 2100 (Figure 1a) was found to be around 4.5 which is in agreement with values reported in the literature.24,25 For a pH ranging from 4.5 to about 12, the polymer is fully ionized thus giving way to possible interactions between negatively charged NaPa molecules and positively charged aluminic species. For the 0.1 M AlCl3 aqueous solution, the pH-metric titration curve exhibits four distinct parts (Figure 1b). At the start of the dosing (VNaOH < 0.5 mL), the little inflection is related to the hydrolysis of the monomeric species [Al(H2O)6]3+. In the second part (0.5 e VNaOH e 5 mL), the pH was nearly the same around 4, and the formation of octahedral aluminic species, i.e,

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TABLE 3: Assignment of the 27Al NMR Spectra Peaks of the Aluminic Species species

nature

chemical shift (ppm)

assignment

ref

3+

[Al(H2O)6] [Al(OH)(H2O)5]2+ [Al(OH)2(H2O)4]+ [Al2(OH)2(H2O)8]4+ [Al3O2(OH)4(H2O)8]+ [Al13O4(OH)24(OH2)12]7+ [Al13O4(OH)24(OH2)12]7+

monomer

0

octahedral Al

21, 30-32

dimer trimer Keggin ion Keggin ion

≈ 4-4.2 ≈4 ≈ 10 ≈ 60-65

octahedral Al octahedral Al octahedral Al tetrahedral Al

30 33 20, 32, 34 20, 32, 34

monomeric, dimeric, trimeric, and Keggin species occurred. Then, the major inflection (5 e VNaOH e 9.5 mL) corresponds to the polycondensation reactions between the aluminic species giving rise to the formation of Al hydroxide (bayerite, gibbsite) or oxyhydroxide (pseudoboehmite) depending on the pH. After the precipitation step (VNaOH > 9.5 mL), a slight decrease of the turbidity was observed, which is related to the formation of [Al(OH)4]- monomers due to gradual dissolution of the asformed alumina.26 The behavior for the pH-metric titration curve of the AlCl3 + NaPa mixture is slightly different. After a 24 h aging of the AlCl3 + NaPa mixture, the pH0 was around 5.6-5.8 thus giving favorable conditions to the formation of monomers, dimers, trimers, and, in particular, Keggin species. The formation of a complex between carboxylate groups of the fully ionized NaPa and aluminic species probably via a bidendate coordination bond as it was already mentioned by Bouyer et al.27,28 with lanthanum hydroxide nanoparticles occurred via the following reaction:

The complexation behavior of carboxylate groups was mentioned in the literature for numerous metallic cations including Ca2+, Cu2+, Zn2+, Ni2+, or Al3+.29 Two minor inflexions were observed for the pH-metric titration curve of the AlCl3 + NaPa mixture and were attributed to the formation of octahedral aluminic species (0.5 e VNaOH e 5.3 mL) and to the polycondensation reactions (5.3 e VNaOH e 11), respectively. In a pH range from 9 to 10.5, a stable colloidal suspension was obtained, whereas for pH g 11, a strong increase of the turbidity was detected resulting in the collapse of the colloidal suspension and flocculation of the nanoparticles within 30 min. A DLS analysis was performed for the initial AlCl3 + NaPa mixture as a function of initial pH before hydrothermal treatment to follow the formation of nanoparticles (Figure 2). The particle size and polydispersity index as a function of pH for the AlCl3 + NaPa mixture were summarized in Table 1. In a pH range from 5.8 to 9, just a peak centered at about 1.5 nm was observed. According to the DLS analysis performed on sodium polyacrylate 2100 in water as a function of pH (2 e pH e 12), this particle class is related with free NaPa molecules. Although just a single distribution was observed in this pH range, the polydispersity index of the initial mixture continuously increased which is due to a high number of oligomeric species in the solution during the neutralization process. As can be seen, higher pH resulted in an increase of the particle size which is related to the nanoparticles formation. This observation was confirmed and strengthened by the increase of the turbidity of the initial mixture for a pH ranging from 9 to 10.5. Waiting 20 min at a pH of 10.2 results in a strong increase of the turbidity confirmed by an increase of the polydispersity index from 0.29 to 0.42

underlining agglomeration phenomena of primary particles. For pH upper to 10.5, the turbidity was too strong for a DLS analysis. It should be stressed that all syntheses performed with an initial pH between 9 and 10.5 yielded stable colloidal suspensions after hydrothermal treatment in contrast to the strongly agglomerated particles obtained for pH higher than 10.5. We also tried to perform the same experiments on the 0.1 M AlCl3 reference aqueous solution. However, in contrast to the NaPa-containing system, only a few drops of 5 M NaOH was enough to give a thick and milky solution resulting in a very fast flocculation of the nanoparticles within 5 min thus giving samples not suitable for a DLS analysis. XRD experiments performed on the solid obtained from precursor mixtures as a function of pH clearly underlined that amorphous nanoparticles were first formed in the pH range 9.5-10.2 (Figure 3a). The formation of pseudo-boehmite nanoparticles occurred for a pH ranging from 10.2 to 10.5 (Figure 3b). The effect of temperature from 25 to 75 °C on the AlCl3 + PaNa precursor mixture obtained at pH ) 10.5 was also investigated by DLS (Table 2). As can be seen, for a temperature ranging from 25 to 45 °C, no significant change for the particle size was observed (30-90 nm). By increasing the temperature from 45 to 75 °C, the width of the distribution became narrower

Figure 5. 27Al solution NMR spectra of the AlCl3 + NaPa mixture as a function of pH.

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Figure 7. 27Al MAS NMR spectra of the freeze-dried AlCl3 + NaPa mixture as a function of pH.

TABLE 5: 27Al Chemical Shifts of the Freeze-Dried Precursor Mixtures as a Function of the pH

Figure 6. 27Al solid-liquid MAS NMR spectra of the AlCl3 + NaPa mixture as a function of pH.

TABLE 4: 27Al Chemical Shifts of the Precursor Mixtures as a Function of the pH ref

pH of the AlCl3 + PaNa mixture

δ (ppm)

(a) (b) (c) (d) (e) (f)

5.7 7 8 9 10 10.5

-1.8 0.6 0 2.5 3.4 5

from 30-90 nm at 45 °C to 20-60 nm at 75 °C suggesting a decrease of the particle size or/and the agglomeration state. This was strengthened by the decrease of the polydispersity index from 0.44 at 25 °C to 0.33 at 75 °C. 27Al liquid NMR spectra of the aluminum chloride 0.1 M reference aqueous solution and the NaPa + AlCl3 mixture after aging for 24 h are shown in Figure 4. As can be expected for the AlCl3 0.1 M reference aqueous solution, a relatively sharp signal corresponding to monomers with Al in octahedral coordination was detected. After a 24 h aging of the NaPa + AlCl3 mixture, two broad signals at about 0 and 62-65 ppm were detected. It is worth mentioning that usually 27Al NMR signals observed for monomers, dimers, trimers, and Keggin species exhibit relatively sharp signals. After a 24 h aging of the NaPa + AlCl3 mixture, the pH was around 5.8 thus offering favorable conditions for several aluminic species to occur, especially Al13 polycations. The chemical shift of the aluminic species identified by 27Al liquid NMR is listed in Table 3. Chemical shifts between 0 and 4 ppm were only attributed to octahedral Al in monomers, dimers, and trimers, respectively. This suggests that the broad signal around 0 ppm is due to a chemical equilibrium between these octahedral species. Furthermore, at ambient temperature, it was actually not possible to detect the signal at 10 ppm attributed to octahedral Al in the Keggin structure. Indeed, due to a strong electric field, these octahedrons are strongly distorted thus giving a nonsymmetrical environment resulting, along with quadrupolar relaxation, in a significant broadness of the signal which cannot be detected at ambient temperature.30 The broad chemical signal around 62-65

ref

pH of the AlCl3 + PaNa mixture

δ (ppm)

(a) (b) (c) (d) (e) (f)

5.7 7 8 9 10 10.5

-3.7 -2.1 2.3 4.4 6.2 8

TABLE 6: Half-Width of the Resonance Line of the 23Na NMR Spectra of the Polymer NaPa and the NaPa-Containing Boehmite Samples Obtained after 3, 24, and 168 h Hydrothermal Treatment at 160 °C sample

time (h)

∆1/2 (Hz)

polymer NaPa NaPa-containingboehmite

/ 3 24 168

1085 840 553 404

ppm is only attributed to Keggin Al13 species. The interaction between the carboxylate groups of NaPa molecules and the aluminic species might also change slightly the chemical shift of the above-discussed species. Further experiments were also performed by 23Na liquid NMR on the reference NaPa aqueous solution (0.057 M) and on the NaPa + AlCl3 mixture after aging for 24 h (see Supporting Information). In contrast to our reference (NaPa in water, δ ) 0 ppm), a slight shift of the peak was observed in the NaPa + AlCl3 mixture after aging for 24 h (δ ) 0.2 ppm) which is due to a decrease of the pH from 7.5 (NaPa in water) to 5.6-5.8 (NaPa + AlCl3 mixture after aging for 24 h). Another significant step was to perform an 27Al liquid NMR investigation of the AlCl3 + NaPa precursor mixture as a function of pH, in particular, to study the aluminic species involved in the formation of nanoparticles (Figure 5). As was shown previously (Figure 4b), for pH0 ) 5.7, two broad signals were detected around 0 ppm and 62-65 ppm attributed to octahedral species. As the pH increased from 5.7 to 9.5, the amount of the Al13 species gradually decreased and disappeared at pH ) 9.5 while a new peak occurred at 80 ppm which was assigned to the Al(OH)4- monomer. The amount of the Al(OH)4- species strongly increased from pH ) 9.5 to 11, while the last aluminic species corresponding to Al in octahedral coordination (monomers, dimers, and trimers) disappeared for pH up to 10. It is well-known that the formation of the Al(OH)4-

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Figure 8. XRD patterns of boehmite samples synthesized with a NaPa/Al ratio of 0.57 (a) and without NaPa (b). Inset: the colloidal suspension obtained with NaPa (a) and with respect to reference sample without NaPa (b).

Figure 9. 27Al MAS NMR spectrum of the NaPa-containing boehmite sample obtained after a 24 h hydrothermal treatment at 160 °C.

Figure 10. 23Na MAS NMR spectra of the polymer NaPa (a) and the NaPa-containing boehmite samples obtained after a 3, 24, and 168 h hydrothermal treatment at 160 °C (b).

monomer occurs in basic media because of the gradual dissolution of Al hydroxide or oxyhydroxide as the pH increases.26 All of these precursor mixtures were further analyzed by 27Al solid-liquid NMR to support previous results obtained by the 27Al solution NMR investigation and also to follow the formation of the octahedral sites of the solid phase (Figure 6).

The same broad signal around 0 ppm attributed to octahedral species was again observed at pH0 ) 5.7. As the pH increased from 5.7 to 10.5, a slight shift of this signal occurred from -1.8 ppm to 5 ppm related to the gradual formation of the octahedral sites (Table 4). It is worth mentioning that the 27Al solid-liquid NMR technique is more suitable to reveal the solid species. Indeed, a strong enhancement of the signal attributed to the octahedral sites was observed thus leading to some difficulties in detecting the Keggin species in the precursor mixtures between pH ) 5.7 and 8. However, increasing the pH from 5.7 to 8 results in a slight dissymmetry of the peak which may be related to the disappearing of the Keggin species. No changes were observed for the Al(OH)4- species which were again detected at δ ) 80 ppm for pH ) 9.5. The gradual formation of the octahedral sites was also studied by MAS 27Al solid NMR performed on the freeze-dried precursor mixtures as a function of the pH (Figure 7). As the pH increased from 5.8 to 10.5, a slight shift of the broad signal from -3.7 to 8 ppm was observed (Table 5). This trend toward higher chemical shift values as the pH increases is again related to the gradual formation of the octahedral sites thus supporting previous results obtained in the 27Al solid-liquid investigation. As was mentioned numerous times in the literature, a chemical shift value of 7.5/8 ppm is characteristic of the octahedral sites of a boehmite-type material.35 2. Resulting Mixtures Obtained after Hydrothermal Treatment. Synthesis performed with a molar ratio NaPa/Al ) 0.57 yielded colloidal suspension of rounded boehmite nanoparticles after a 24 h hydrothermal treatment at 160 °C. As shown in Figure 8, both NaPa-free and NaPa-containing samples exhibited the characteristic of boehmite X-ray diffraction patterns.36,37 Significant changes in boehmite characteristics with respect to the sample synthesized without NaPa were observed. The NaPacontaining system yielded small boehmite fibers having a length between 20 and 50 nm. In contrast, the NaPa-free system led to the formation of strongly agglomerated platelike crystals having a size between 60 and 80 nm and specific surface area of 105 m2 · g-1. The previous results demonstrated also via IR spectroscopy and zeta potential measurements that sodium polyacrylate is anchored to the boehmite crystallite giving a negative surface charge thus avoiding abundant aggregation phenomena.14 3. Final Product. A characteristic 27Al MAS NMR spectrum of a boehmite-type material was obtained after a 24 h hydrothermal treatment of the AlCl3 + PaNa precursor solution at pH ) 10.5 showing only octahedral sites at δ ) 7.7 ppm (Figure 9).

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Figure 11. TEM micrographs of the boehmite sample synthesized at 160 °C for (a) 3 h and (b) 168 h.

TABLE 7: Scattering Intensity of the First Distribution Observed by Dynamic Light Scattering for the Mixture NaPa + AlCl3 (NaPa/Al ) 0.57) as a Function of pH pH

scattering intensity (%)

5.8 7 8 9 9.5 10 10.2

12 7 5 5 2 0 0

23Na

MAS NMR was also performed on the polymer NaPa and the NaPa-containing boehmite samples obtained after a 3, 24, and 168 h hydrothermal treatment at 160 °C (Figure 10). For the polymer NaPa, just one peak was observed at -10.3 ppm (Figure 10a). After a 3 h hydrothermal treatment at 160 °C, the same peak was again observed but for a higher chemical shift (δ ) -9.1 ppm) thus confirming the presence of adsorbed NaPa molecules on the boehmite surface. Increasing the synthesis time from 3 to 168 h resulted in a shift of the peak toward higher chemical shift values from -9.1 to -1.7 ppm. Furthermore, a closer look at these spectra reveals also a decrease of the half-width of the peak by increasing the synthesis time (Table 6). MQMAS NMR experiment was also performed on the NaPa-containing boehmite sample after a 24 h hydrothermal treatment at 160 °C showing only one sodium site (see Supporting Information). It is worth mentioning that longer synthesis time results in an increase of the boehmite fiber length from 20-40 nm (Figure 11a) at 3 h to 2-3 µm at 168 h (Figure 11b) which is correlated to a significant improvement of the crystallinity as observed by XRD. A careful observation of TEM micrographs revealed a ribbon-like morphology for the long fibers. The change in morphology and size from 20-40 nm fiber to 2-3 µm ribbon with hydrothermal treatment duration is probably accompanied by a reorganization of the polymer chains, which induces a change of the Na environment. After a 168 h hydrothermal treatment, the shift of the 23Na chemical shift value associated with the decrease of the resonance half-width may be related to a more homogeneous environment of the sodium atoms when NaPa chains are in interaction with boehmite ribbons. Discussion As was mentioned in DLS and XRD analysis performed on the AlCl3 + NaPa mixtures as a function of pH (Figures 2 and 3a), the formation of amorphous nanoparticles occurs in the pH range 9-10. For pH ) 10, a stable colloidal suspension of particles having a size ranging from 20 to 40 nm was obtained

in contrast to the completely agglomerated particles obtained in the NaPa-free system. Nevertheless, as the pH was gradually increased, the stability of the colloidal suspension was not maintained. Indeed, both the particle size observed by DLS and the turbidity of the solution are increased for pH up to 10 which is related to an agglomeration of primary particles. This suggests that the size of boehmite particles and the agglomeration state obtained after hydrothermal treatment of several hours are partially governed by the pH of the initial AlCl3 + NaPa mixture. The formation of pseudo-boehmite nanoparticles was detected at pH ) 10.5 (Figure 3b). Results obtained by 27Al solution NMR investigation of the precursor mixture as a function of pH underlined also that the formation of nanoparticles seems to occur after pH ) 9. Indeed, due to the dissolution of the amorphous phase, a small amount of the monomer Al(OH)4- was detected at pH ) 9.5. Consequently, as was already shown by DLS analysis, the formation of primary particles seems to occur between pH ) 9 and 10. The scattering intensity of the first distribution (between 1 and 3 nm) of the mixture NaPa + AlCl3 (NaPa/Al ) 0.57) as a function of pH was summarized in Table 7. As can be seen, the scattering intensity gradually decreases as a function of pH. One finds that this distribution was probably only due to free NaPa molecules, but it could also be related to Keggin species. Indeed, the size of the Keggin-type species is known to be around 1 nm.34 It is relatively difficult to detect these species by DLS due to the presence of free NaPa molecules between 1 and 3 nm, but measurements performed by NMR experiments clearly underlined the presence of Al13 species between pH ) 5.8 and 8. It is also necessary to stress that the Keggin species may not be involved in the mechanism pathway because the corresponding peak located at 62-65 ppm completely disappeared for pH > 8. Keggin species are known to occur typically for a hydrolysis ratio between 0.5 and 2.5.20,32,34 For such a ratio (0.5 < h < 2.5), no increase of the particle size was detected by DLS in the NaPacontaining system. According to our results and on the basis of previous works,14 amorphous particles were the first formed due to polycondensation reactions between octahedral species (monomers, dimers, or trimers species). Then, as the degree of sursaturation was sufficient, a nucleation and crystallization step occurred leading to the formation of partially crystallized pseudo-boehmite nanoparticles. After hydrothermal treatment, the final pH of the NaPacontaining system decreases and ranges between 8 and 8.5. This

Sodium-Polyacrylate-Containing System

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18391

Figure 12. Proposed mechanism for the boehmite formation in the NaPa-containing system.

is due to the thermohydrolysis of the aluminic species via the following reaction38 [Al(OH2)6]3+ + hH2O f [Al(OH)h(OH2)6-h](3-h)+ + xhH+solv

It is worth mentioning that for elements having a charge z up to 4, the hydrolysis reaction was spontaneous (∆G° < 0). Nevertheless, in the case of aluminum (z ) 3), it is necessary to increase the temperature of the mixture to start the hydrolysis () forced hydrolysis or thermohydrolysis).39 During the hydrothermal treatment, the hydrolysis reaction is closed to the thermodynamic equilibrium thus leading to a slight decrease of the pH. The following mechanism for the boehmite formation in the NaPa-containing system was proposed (Figure 12). During the aging process of the AlCl3 + NaPa mixture, since the pH > pKa, interaction between carboxylate groups of the fully ionized polymer and octahedral species occurred leading to the formation of a complex. As the pH increased, amorphous nanoparticles were the first formed for a pH ranging from 9.5 to 10.2. Once a sufficient sursaturation degree was reached, the nucleation and crystallization step started leading to the formation of pseudoboehmite nanoparticles. The decrease of the agglomeration state observed during the increase of the temperature by DLS (Table 2) may be due to the complexation of the boehmite surface by the NaPa polymer thus giving negatively charged nanoparticles. Hence, the polymer is believed to have mainly two functions: first, it stops the growth of the particles at a small particle size distribution and then it prevents primary particles from coalescence with each other by capping these nanoparticles with negatively charged NaPa molecules thus giving a stable colloidal suspension after the hydrothermal treatment. It is also necessary to stress that, for a pH up to 11, a flocculation of the nanoparticles occurred after the hydrothermal treatment suggesting that the stability of the system is strongly related to the pH. Conclusion The present study reports on the investigation of precursor mixtures used to obtain nanosized boehmite particles prepared

under hydrothermal conditions. Dynamic light scattering experiments prior to hydrothermal treatment underlined that the nanoparticle formation occurred for a pH ranging from 8.5/9 to 11. As was shown by liquid 27Al NMR experiments as a function of pH, octahedral species, i.e., monomers, dimers, and trimers, are the only aluminic species involved in the mechanism pathway of the boehmite formation. The presence of the polymer yields to a stable colloidal suspension of boehmite particles after the hydrothermal treatment at 160 °C during 24 h of the precursor mixture obtained at pH ) 10.5. During the synthesis, the polymer agent NaPa is believed to have two effects. First, the pH of the precursor mixture was partially controlled by the presence of the polymer NaPa. Then, it leads to the formation of a complex between the boehmite surface and carboxylate groups of sodium polyacrylate thus preventing particles from coalescence with each other. The results obtained in this work may be helpful for a better understanding of the boehmite mechanism pathway and also for the preparation of boehmite nanoparticles with tuned properties. Supporting Information Available: Some NMR spectra. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Kaya, C.; He, J. Y.; Gu, X.; Butler, E. G. Microporous Mesoporous Mater. 2002, 54, 37. (2) Hwang, K. -T.; Lee, H. -S.; Lee, S. -H.; Chung, K. -C.; Park, S. -S.; Lee, J. -H. J. Eur. Ceram. Soc. 2001, 21, 375. (3) Khalil, K. M. S. J. Catal. 1998, 178, 198. (4) Bugosh, J. J. Phys. Chem. 1961, 65, 1789. (5) Sterte, J. P.; Otterstedt, J. E. Mater. Res. Bull. 1986, 21, 1159. (6) Brusasco, R.; Gnassi, J.; Tatian, C.; Baglio, J.; Dwight, K.; Wold, A. Mater. Res. Bull. 1984, 19, 1489. (7) Music´, S.; Dragcˇevic´, Ð.; Popovic´, S.; Vdovic´, N. Mater. Sci. Eng. 1998, B52, 145. (8) Music´, S.; Dragcˇevic´, Ð.; Popovic´, S.; Vdovic´, N. Mater. Chem. Phys. 1999, 59, 12. (9) Kuang, D. B.; Fang, Y. P.; Liu, H. Q.; Frommen, C.; Fenske, D. J. Mater. Chem. 2003, 13, 660.

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