Dissolution of Silica and Formation of a Dispersed Phase Induced at

Induced at Low pH by the Association of Soluble. Aluminum Ionic ... Shams University, Cairo, Egypt ... resulting from the association of these soluble...
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Langmuir 1998, 14, 1072-1080

Dissolution of Silica and Formation of a Dispersed Phase Induced at Low pH by the Association of Soluble Aluminum Ionic Species with Solid Silica Gel M. Bouallou,† L. Vielvoye,‡ G. M. S. El Shafei,§ and W. E. E. Stone*,‡ Unite´ de Chimie des Interfaces and Unite´ de Physico-Chimie Mine´ rale (MRAC-Tervuren), Universite´ Catholique de Louvain (UCL), Place Croix du Sud 2/18, B-1348 Louvain-la Neuve, Belgium, and Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt Received July 7, 1997. In Final Form: December 1, 1997 It is shown that the overall behavior of silica in the presence of hydrolyzed Al solutions (pH range 3-4) is drastically modified: the total amount of dissolved silica and the initial rate at which silica passes into solution are increased compared to what is observed for a porous silica in water. By using a simple spectrophotometric approach, an estimate of the amount of monomeric and polymeric silica aqueous species can be obtained as a function of time. For all hydrolyzed solutions, the presence of a dispersed phase is also observed. Furthermore, when it is treated with these Al solutions the initial silica gel has its texture, surface charge, and global reactivity modified. This complex chain of events which occurs at acidic pH is in fact triggered by the presence in solution of highly charged Al13 polymeric cations, which after adsorption onto the silica sample depolymerize in a process which involves the surface hydroxyls. This leads to the inclusion of tetrahedral Al in the silica famework and the presence of surface-coordinated octahedral Al. This globaly leads through a weakening of the Si-O bonds to an increased release of silica into solution. The Al cations left in solution interact with the aqueous Si species, leading with time to the formation of a metastable silica-aluminum dispersed phase which differs strongly from the treated silica beads. These interactions between Al hydrolyzed solutions and solid silica are therefore quite effective in spatially redistributing the initial silica beads. Many of these conclusions have been made possible by the use of solid-state NMR.

Introduction Many geochemical and industrial processes involve the transfer of chemical elements and matter between solid and fluid phases. At low temperature (25 °C) most processes such as adsorption, dissolution, precipitation, and so forth are surface-controlled processes. Much work has therefore been devoted (both experimentally and theoretically) to the study of the surface structure and chemistry of minerals.1 In the dissolution of solids, surface species and ionized groups can drastically modify the surface reactivity, surface hydrolysis, and bridging bond strengths. Their role in this field has amply been emphasized.2,3 The solubility of quartz and amorphous silica has also been abundantly studied over the years,2 but still many problems remain, such as, for example the role played by certain individual aqueous components in solution. The interaction of these components with silica surfaces can indeed profoundly affect the bulk reactivity of the solid. In this paper it is shown how at acid pH the addition of hydrolyzed Al solutions to a porous silica gel solid modifies the amount of silica which is dissolved and gives rise to a dispersed solid phase. The implications † Unite ´ de Chimie des Interfaces, UCL. Present address: Dpt. de Chimie, Faculte´ des Sciences, Universite´ Mohamet I, Oujda, Marocco. ‡ Unite ´ de Physico-Chimie Mine´rale (MRAC-Tervuren), UCL. § Ain Shams University.

(1) Mineral Water Interface Geochemistry; Hochella, M. F., White A. F., Eds.; Reviews in Mineralogy; Mineralogical Society of America: Washington, DC, 1990; Vol. 23. (2) Dove, P. M.; Rimstidt, J. D. Silica; Heaney, P. J., Prewitt, C. T., Gibbs, G. V., Eds.; Reviews in Mineralogy; Mineralogical Society of America; Washington, DC, 1994; Vol. 29, p 259. (3) House, W. A.; Orr, D. R. J. Chem. Soc., Faraday Trans. 1992, 88, 233.

resulting from the association of these soluble aluminum ionic species with silica gel were presented in a previous paper,4 where it was shown how the surface properties of silica in terms of surface charge density and reactivity can under these conditions be profoundly modified. Experimental Section 1. Reagents. The silica sample (silica beads, diameter around 1 mm) used here consisted of a mesoporous silica gel 254 purchased from W.R. Grace-USA (99.75% SiO2; BET specific surface area, 542 m2/g; total pore volume, 0.83 mL/g). The work was conducted on Al solutions having a hydrolysis level, R ) OH/Al, between 0 and 2 (pH ) 3.1-4),5 that is, having variable amounts of monomeric and Al13 ions in solution (ref 4 and references therein). The initial system (R ) 0) is formed by contacting at room temperature a constant amount of silica, 16 g/L, with an unhydrolyzed Al(NO3)3‚9H2O solution (Merck p.a.) such that the final aluminum concentration was 0.1 M. To obtain the various hydrolysis levels, this initial system is (under vigorous stirring) titrated by slowing adding (1 mL/min) NaOH up to the desired OH/Al ratio (during this preparation the pH corresponds within experimental errors to the one observed for the hydrolyzed Al solution alone). After complete addition of NaOH, the solutions plus silica are placed in polyethylene bottles and submitted to continuous agitation, at room temperature, for various lengths of time (“contact time” ranging from 1 day to 20 months). For analysis, the silica beads and liquid phase are separated by decantation. In certain cases (see below), a dispersed phase is observed visually to be present in the liquid phase. This dispersed phase is separated from the liquid phase (after the corresponding contact time) by using a 1.2-µm Millipore filter (placed in a 2-cmdiameter holder connected to a water-aspirator vacuum pump). (4) Stone, W. E. E.; El Shafei, G. M. S.; Sanz, J.; Selim, S. A. J. Phys. Chem. 1993, 97, 10127. (5) Changui, C.; Stone, W. E. E.; Vielvoye, L.; Dereppe, J. M. J. Chem. Soc., Dalton Trans. 1990, 1723.

S0743-7463(97)00750-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/13/1998

Dissolution of Silica and Formation of a Dispersed Phase These two recovered solid phases are thoroughly washed with MilliQ water and left to dry at room temperature between filter papers. The time separating the recovery of the solids and their experimental characterization will be termed “aging” (i.e. left as such in the powdered form at room temperature for periods ranging between 1 day and 18 months). In the following, Altreated silica samples will be called solid A, and the dispersed phase will be called solid B. 2. Analyses. A series of analyses were performed on both the liquid and solid phases. For the liquid phase, a colorimetric technique was used to characterize the silica having passed into solution. The total Al and Si contents of the two solid phases were determined after dissolution by atomic absorption/emission spectroscopy. The solids were also characterized by N2 isotherms and solid-state NMR. Some electron micrographs were obtained for solid B. Silica in Solution. The method used is the well-known reaction6 occurring between silica in solution and molybdic acid; the procedure used was based on the one proposed by Voinovitch et al.7 Two routes can be followed: (a) formation of a yellow nonreduced β silicomolybdic complex [absorbance measurements (at 400 nm using a Bausch & Lomb Spectronic-700 apparatus) can be carried out fairly rapidly after addition of the molybdate reagent, allowing the kinetics of the reaction to be followed] and (b) formation of a blue reduced β silicomolybdic complex [the method consists in first waiting for the full development (10 min) of the yellow color and then, after reduction, waiting for the full development (30 min) of the blue color (measured at 830 nm)]. The results below will often be given as xr, the fraction of silica having reacted (at a certain time) after addition of molybdic acid. This fraction is given by the ratio of the absorbance measured at that particular time to the one measured at the absorbance plateau. Chemical Analysis of the Solids. The Al and Si contents of solids A and B were measured adopting the chemical analysis method of Bernas,8 that is, dissolution by HF followed by a determination of elements by AAS (Varian AA-300) and AES (SMI Spectra Span V). Texture Properties. N2 adsorption/desorption isotherms were obtained volumetrically at 77 K with a computer-interfaced Sorptomatic ASAP 2000 using high-purity (99%) nitrogen gas. Electron Microscopy. Solid B samples were prepared (by simple air drying) on grids covered by a carbon layer and examined using a JEM-100-C TEMSCAN electron microscope. NMR Spectroscopy. Solid-state NMR 27Al and 29Si spectra were obtained, at room temperature, at respectively 78.206 and 59.63 MHz on a Bruker MSL-300 spectrometer. Spectra were examined under MAS (4 and 13 kHz) and sometimes crosspolarization (CP-MAS) conditions. For 27Al, a π/12 excitation pulse was used with a recycle time of 1 s; for 29Si the excitation pulse and recycle time were respectively 4 µs and 5 s. 27Al chemical shifts were referred to liquid [Al(H2O)6]3+; those of 29Si were measured with respect to TMS.

Results and Discussion 1. Dissolution of Silica. As a matter of illustration, results obtained9 for the amount of silica in solution versus contact time as measured with the reduced silicomolybdic complex method are given in Figure 1. The various curves correspond to filtered (1.2 µm Millipore) solutions of different R values. Also shown are the results obtained when the silica gel sample is immersed in deionized water at pH ) 4. The total amount of dissolved silica reported in Figure 1 is obtained by the reduced method, which allows sufficient reaction time (40 min; see above) so that hopefully, for a given contact time, most of the silicacontaining aqueous species present have been detected. In any case, these results clearly show that, in the presence (6) Alexander, G. B. J. Am. Chem. Soc. 1954, 76, 2094. (7) Voinovich, I. A.; Debras-Guedon, J.; Lourier, J. In L’Analyse des Silicates; Herman, M., Ed.; Presses Universitaires: Paris, 1962. (8) Bernas, B. Anal. Chem. 1968, 40, 1682. (9) El Shafei, G. M. S. Ph.D. Thesis, Universite´ Catholique de Louvain, Louvain-la Neuve, Belgium, 1991.

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Figure 1. (A) Amount of silica found in solution (as determined by the reduced silicomolybdic complex method) as a function of the contact time between silica gel 254 and water (pH ) 4) and Al hydrolyzed solutions of different R values. (B) Comparison of results (obtained as in Figure 1a) between silica gel 254 and water (pH ) 4) and a nonhydrolyzed Al solution (R ) 0).

of R ) 1.0, 1.5, and 2.0 hydrolyzed Al solutions, the observed total amount of dissolved silica is much larger compared to what is found for the reference pH ) 4 water system. It is also observed that, in the presence of Al, the initial rate (ppm of SiO2/day) at which silica passes into solution is increased (by a factor of 4). In order to gain more insight into the various types of aqueous silica species present in solution, the nonreduced silicomolybdate method was then used, as the final absorbance plateau can be measured much more rapidly (,40 min) than in the previous case; it therefore allows (to a certain extent) us to discriminate between species reacting with molybdate at different rates. These results are presented below. In order to optimize the experimental procedure for the nonreduced silicomolybdate method and obtain reference curves relative only to silica monomers (monosilicic acid), a series of standard solutions (2-10 ppm of Si) were examined. The results given in Figure 2 are obtained with the molybdate concentration well in excess (4 g/L).

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Figure 2. Variation of absorbance as a function of reaction time for various monosilicic standard solutions (nonreduced silicomolybdic complex method).

Figure 3. Fraction xr of silica having reacted with the molybdate reagent (nonreduced silicomolybdic complex method) for a R ) 2 Al solution for various contact times (as indicated in the figure; h ) hours; d ) days).

It is observed that, in this Si concentration range, the absorbance plateau is reached fairly quickly (well within 3 min). Relative to monosilicic acid, the reaction is of first order with the rate constant 2.2 ( 0.2 min-1, in accordance with values given in the literature.10 Nonhydrolyzed Al Solutions. By applying this technique to solutions taken from the silica gel sample placed either in water (pH ) 4) or in a nonhydrolyzed Al solution (R ) 0), it is observed that, whatever the time of contact between

Bouallou et al.

Figure 4. Variation of absorbance (nonreduced silicomolybdic complex method) as a function of reaction time for filtered (open symbols) and unfiltered samples (closed symbols) taken from a R ) 2, 5-month contact time solution: (a) treated with KOH; (b) not treated with KOH. Table 1. Variation with Contact Time of the Amount of Monomeric and Polymeric Silica Species Found in the System Silica Gel-Al solution R ) 2.0, As Deduced from an Analysis of the Absorbance Measurements of the Nonreduced Silicomolybdic Complex (See Text) contact time

ppm SiO2 total mono. + poly.

monomer % SiO2

polymer % SiO2

monomer/ polymer

12 h 7 days 2 months 5 months 7 months 14 months 20 months

29 177 536 1120 1205 1190 1059

100 70 30 20 18 18 15

0.0 30 70 80 82 82 85

2.33 0.43 0.25 0.22 0.22 0.18

solid and solution, the shape of the absorbance curves is identical to that observed for the standard solutions containing only silica monomers (see Figure 2). These solutions therefore contain negligible amounts of highly polymerized soluble Si species. Hydrolyzed Al Solutions. (a) The same technique was applied to the hydrolyzed Al solutions, and the results obtained for the filtered R ) 2 solution are shown in Figure 3. Different contact times between solid A and its solution have been examined. It is observed that, for contact times up to 12 h, the absorbance curves are identical to what is observed for a solution containing only monomers (see Figure 2). After 7 days, the reaction time increases gradually with contact time and the reaction is no longer of first order. As suggested by others,10 this behavior corresponds to the appearance with time of soluble polymeric silica species. In the presence of Al hydrolyzed solutions the silica solid therefore dissolves by releasing monomeric species (plus some small oligomers which react quickly with the molybdate reagent), the proportion of which decreases with contact time at the expense of more condensed forms. Without an exact knowledge of the complex kinetic behavior involving the reaction of polymeric species of various sizes with molybdate, it is difficult (10) Iler, R. K. In The Chemistry of Silica; Wiley: New-York, 1979.

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Figure 5. Electron micrographs of solid B (aged 2 months): (a) R ) 0.5 (48 h); (b) R ) 2.0 (48 h) (in both cases the lengths of the flocs shown are around 1 µm).

to completely analyze these curves. As a first approximation, the system can be subdivided into “monomers” (plus some small polymers), which react with molybdate within 3 min, plus “soluble polymeric species”, which give rise to the near-linear response observed for longer times. Extrapolation to time zero of this long-time component provides an estimate for the remaining monomeric content. It can indeed be shown that a monomer-only curve (i.e. similar to those obtained from the silica standard solutions) can be obtained by subtracting from the experimental curve a line passing through the origin of axes and parallel to this linear polymeric component. With this approach (which is however of low precision), variation with contact time of percentage values of “monomeric” and “soluble polymeric” species found in solution can be deduced. These values are given in Table 1 together with the total amount of silica found in solution. After 7 months

this R ) 2 sample reaches a “solubility” plateau of about 1200 ppm (which is an order of magnitude higher than that of pure silica). However, as shown in Figure 3, the system is still evolving, as curves corresponding to 7 months and more have continuously declining long-term slopes. This indicates that the degree of cross-linking of the “polymeric” species is continuously increasing. These various effects illustrated in Figure 3 do not result from an aging process of the initial silica monomers present in the system. It was indeed checked that a short-contacttime (48 h) solution retains its original monomeric-type curve after a storage period of 7 months. The induction period corresponding to the disappearance of monomers on storage is indeed very long at low pH and silica concentration (50 ppm).10 (b) As the dissolution of the silica gel proceeds in the presence of hydrolyzed Al solutions, a dispersed solid phase

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Figure 6. Total Al content for solid A and corresponding solid B (48 h) for different hydrolysis ratios R (as indicated on the figure). Table 2. Al Chemical Analyses of Solid A and Corresponding Solid B (Contact Time, 48 h; Aged 10 days) solid A

solid B

sample (48 h)

[Al] (mM/g)

[Al] (µM/m2)

Si/Al

[Al] (mM/g)

[Al] (µM/m2)

Si/Al

H2O R ) 0.0 R ) 0.05 R ) 1.0 R ) 1.5 R ) 2.0

0.015 0.08 0.87 1.14 1.31 1.47

0.03 0.15 1.77 2.38 3.06 3.29

1168 n.d. 15.8 11.9 10 10

0.95 1.48 1.69 2.09

2.13 3.67 4.02 5.55

14.6 8.7 7.5 5.8

(solid B) is visually observed to appear within the liquid phase. This solid B could of course be characterized in terms of its nature, particle size distribution, local spatial organization, and so forth by the use of various elaborate techniques (such as NMR, small-angle scattering experiments, and so forth). The simple spectrophotometric approach used here can however be used to obtain some preliminary information, as shown below. Figure 4 shows results obtained for a R ) 2 solution maintained in contact for 5 months with silica gel. Curves b1 and b2 correspond respectively to filtered (1.2 µm Millipore) and unfiltered solutions. As both curves are identical and reach a similar plateau value, it seems that, under these conditions, the molybdic reagent does not react with colloidal particles larger than 1.2 µm. When KOH (which completly depolymerizes silica into monomeric species) is added to the filtered solution, curve a1 is obtained. This curve is the one expected for a monomer-only solution. Again the plateau value is unchanged, indicating that for this 1.2 µm filtered solution all silica-containing species are able to react with the molybdate. However, when KOH is added to the unfiltered solution (curve a2), the plateau value is at a higher absorbance value. The difference between the plateau values of a2 and b2 corresponds to colloidal particles larger than 1.2 µm present in solution. The behavior depicted in Figure 4 is also observed for a shorter contact time of 2 months and for both R ) 2 and R ) 1.5

Figure 7. 27Al NMR MAS spectra of solid B (48 h, aged 1 month) as a function of R.

solutions. However for solutions obtained with R ) 1 and R ) 0.5, the b2 nonfiltered curves reach plateau values which are above those of the b1 curves. This indicates that, for a given contact time, the particles formed in the presence of less hydrolyzed Al solutions are smaller or less well-organized and therefore more susceptible to being involved in the formation of silicomolybdic complexes. For R ) 0 and H2O (pH ) 4) solutions (which do not show the presence of a dispersed phase), all curves are similar irrespective of treatment (filtered and nonfiltered and with and without KOH). Finally, it is should be noted that, under our conditions, the total amount of soluble silica (monomers plus polymers), as deduced by the reduced silicomolybdic complex method, is systematically higher than that given by the nonreduced complex method. This could be due to stray additional reduction reactions involving the molybdate reagent itself. (c) As observed here, it is well-established that the final solubility of amorphous silica gel in pure water is around 100 ppm.10 In this case the evolution with time of the amount of silica found in solution is analyzed2 in terms of the global reaction SiO2(solid) + 2H2O f H4SiO4 (solution), following the principle of detailed balancing. This principle (which links kinetics to thermodynamics) states that, at equilibrium, the rates of forward (i.e. dissolution) and reverse (i.e. precipitation) processes are

Dissolution of Silica and Formation of a Dispersed Phase

Figure 8. 27Al NMR MAS spectra of solid A and solid B: (a) comparison between solid A (s) and solid B (- - -) (R ) 2, 48 h, aged 1 month); (b) comparison between solid B (R ) 2, aged 1 month) at the contact times 48 h (- - -) and 11 months (s).

equal for each elementary reaction involved in the global process and therefore implies microscopic reversibility.11 It can therefore only be applied to certain well-understood, simple, model cases. For the systems studied here, it is observed that in the presence of Al the initial rates of dissolution are increased. This implies that the initial silica solid has been somehow modified by the presence of Al. Furthermore, when R ) 1.5 and 2.0, the increased equilibrium “solubility plateau” values are only, in these batch experiments, reached after several months. The literature data concerning the effects of multivalent cations on the dissolution rates and solubility of amorphous silica, at room temperature and low pH, are scarce. At near-neutral or basic pH, Al is repeatedly reported2 as leading to a drastic reduction in dissolution rates. Recently it has however been shown that, in the case of the dissolution of kaolinite12 and motmorillonite13 at low pH, the adsorption of Al does not interfere with the global dissolution reaction of that particular mineral. At low pH (3-6), dissolved constituents such as Fe3+ ions can interact with aqueous silica.14 It is then proposed that (11) Lasaga, A. C. Kinetics of Geochemical Processes; Lasaga, A. C., Kirkpatricks, R. J., Eds.; Review in Mineralogy; Mineralogical Society of America: Washington, DC, 1981; Vol. 8, p 1. (12) Wieland, E.; Stumm, W. Geochim. Cosmochim. Acta 1992, 56, 3339. (13) Furrer, G.; Zysset, M.; Charlet, L.; Schindler, P. W. Metal Compouds in Environment and Life 4; Merian, E., Haerdi, W., Eds.; Science and Technology Letters; Science Reviews Inc.: Wilmington, DE, 1991; p 89. (14) Olsen, L. L.; Melia, C. R. J. Inorg. Nucl. Chem. 1973, 35, 1977.

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these solutes, which react with H4SiO4 to form soluble complexes, enhance the forward-dissolution process of silica. In this study, the enhanced reaction rate and increased total amount of dissolved silica observed for silica gel when in the presence of solutions rich in highly charged Al polymers (R ) 1 to 2) (see Figure 1a) probably result from a complex process involving several concomitant effects. In this case, these can be assigned to insertion of Al in the silica framework and increase of the initial surface electric charge;4 aluminum adsorption at the surface and possible reconstitution of a secondary mixed Al phase;4 solution complex-forming reactions between silica species having been discharged from the solid and aluminum species in solution; and polymerization of these heterogeneous complexes, leading to the formation of solid B. Solid-state NMR indeed clearly shows4 the insertion of Al in the tetrahedral sites of the starting silica material. This leads to an increase in surface charge and formation of SiO-Al bonds; both effects lead by polarization to a weakening of the Si-O bonds.3,15-17 The presence at the surface of octahedrally adsorbed Al and/or a secondary mixed Al phase is shown by solid-state NMR4 and also by electrophoretic measurements conducted on solid A which indeed show a surface charge reversal effect. The textural properties of solid A also show (see below) a decrease in surface area indicative of an adorption effect. We have no spectroscopic proof concerning the formation of aqueous silica-aluminum species. For our samples, the concentration of silica species was too low to be detected by 29Si NMR with the equipment at our disposal. Such silicaaluminum species have however been detected by others.18 Their thermodynamic stability in an aqueous solution (low pH, low concentrations in Si and Al, with only monomeric Al present) has also recently been reexamined.19 The above effects are especially effective when the degree of hydrolysis of the Al solution is sufficient, that is, when highly charged Al cationic polymers are present. When these are not present, such as for R ) 0, the solubility plateau value is similar to the one found for silica gel in water but is however reached at a faster rate (see Figure 1b). This increase in the case R ) 0 could be due to the observed4 small insertion of tetrahedral Al and therefore the increase in surface electric charge density, which has been shown to be linearly related to the rate of dissolution of silica.20 In our opinion, there is to date no straightforward answer to the general question of whether Al enhances or decreases the dissolution of amorphous silica. Each case should be treated according to its own merits. In this problem of the dissolution of silica, the parameters to be considered are numerous: pH, texture, and past history of the solid,21 nature of the cation, presence in solution of highly charged polymeric species, and so forth, which are all important in determining the microscopic mechanisms leading to the passage of silica in solution. As recently stated,22 direct spectroscopic or microscopic (15) Casey, W. H.; Westrich, H. R.; Massis, T.; Banfield, J. F.; Arnold, G. W. Chem. Geol. 1989, 78, 205. (16) Brady, P. V.; Walther, J. V. Geochim. Cosmochim. Acta 1989, 53, 2823. (17) Guy, C.; Schott, J. Chem. Geol. 1980, 78, 181. (18) Swaddle, T. W.; Salerno, J.; Tregloan, P. A. Chem. Soc. Rev. 1994, 23, 319. (19) Pokrovski, G. S.; Schott, J.; Harrichoury, J. C.; Sergeyev, A. S. Geochim. Cosmochim. Acta 1996, 60, 2495. (20) Fleming, B. A. J. Colloid Interface Sci. 1986, 110, 40. (21) Van Capellen, P. Chem. Geol. 1996, 132, 125. (22) Dove, P. M. In Chemical Weathering Rates of Silica Minerals; White, A. F., Brantley, S. L., Eds.; Reviews in Mineralogy; Mineralogical Society of America: Washington, DC, 1996; Vol. 31, p 235.

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Table 3. Textural Parameters Deduced from N2 Adsorption-Desorption Isotherms for Solid A and Corresponding Solid B (ABET ) BET Surface Area; At ) Surface Area from t-plots; Vp ) Total Pore Volume; Dm ) Mean Pore Diameter; CBET ) BET Energy Constant; Dmean, Acum, and Vcum Are Respectively the Mean Pore Diameter, the Cumulative Surface Area, and the Pore Volume, As Calculated by the BJH Method24) BJH method (ads) sample

ABET (m2/g)

At (m2/g)

VP (mL/g)

SiO2-254 H2O, pH4 R ) 0.0 (2 days) R ) 0.5 (2 days) R ) 1.0 (2 days) R ) 1.5 (2 days) R ) 2.0 2 days 2 months 15 months

542 548 559 490 478 429

547 541 547 487 486 421

0.83 0.80 0.81 0.76 0.77 0.67

450 418 496

435 403 469

R ) 0.5 (2 days) R ) 1.0 (2 days) R ) 1.5 (2 days) R ) 2.0 2 days 2 months 11 months

444 404 420 377 409 609

Dm ) 4Vp/ABET (Å)

CBET*

Dmean (Å)

Acum (m2/g)

Vcum (mL/g)

Solid A 61.9 62.1 62.4 62.2 64.5 62.8

79 85 87 86 80 87

50.0 48.6 50.3 50.7 49.5 50.3

629 632 637 572 584 493

0.86 0.85 0.88 0.80 0.80 0.69

0.72 0.66 0.76

64.2 64.6 61.2

94 93 101

50.3 50.3 49.9

534 494 566

0.75 0.71 0.78

469 430 429

0.74 0.68 0.69

Solid B 66.5 67.1 65.7

68 67 78

50.6 50.8 51.7

544 495 505

0.77 0.71 0.72

397 385

0.61 0.70 0.85

64.8 68.2 55.7

70 101 89

49.5 60.3 50.2

462 474 703

0.64 0.71 0.88

measurements associated with usual indirect approaches should be carried out to describe and understand the many complex processes involved between silica and cations in solution. 2. Dispersed Phase. As indicated above, a dispersed phase (solid B) is formed when silica gel 254 is placed, under agitation, in hydrolyzed Al solutions. It is important to note that solid B only appears when the Al solutions are hydrolyzed by addition of NaOH. When the silica sample is simply left in water at pH ) 4 or in a R ) 0 Al solution, no solid B is observed whatever the contact time. This observation is to be related to the fact that in these particular solutions no highly charged Al polymeric species (such as Al13) are present. These have been shown4 to be decisive in the transformation of the properties of the original silica solid. The results and discussions given below relate to a certain limited number of characterizations of solid B. By comparing the results obtained with those of solid A, some elements concerning the origin of this dispersed phase can be put forward. Indeed solid B could be thought of as either resulting from a simple destruction of solid A or finding its origin in an association of certain species present in solution. X-ray Diffraction and Electron Microscopy. Both solid A and solid B are amorphous, as confirmed by their X-ray diffraction patterns. Certain particles of solid B were examined by electron microscopy. Figure 5 shows some typical morphological features obtained for samples aged 2 months recovered from systems having (a) R ) 0.5 (48 h) and (b) R ) 2.0 (48 h). It is seen that in addition to the expected silica-type flocs observed in the case of R ) 2, rod- and platelet-form particles are observed for the less hydrolyzed solution. Such particular features have been reported in a different but related field, the sol-gel preparation of silica-alumina catalysts.23 Chemical Analyses. Results for the Al content of the solids (48 h) are given in Table 2. The values for the H2O sample are those which correspond to the amount of tetrahedral Al found as an impurity in the starting silica material. One of the main features distinguishing solid A from solid B is the amount of Al, which is always larger for solid B (see Figure 6). In both cases, the Al content increases with R. (23) Saˆrbu, C.; Delmon, B. Private communication.

Texture. Both solids show type IV N2 adsorptiondesorption isotherms, exhibiting, over the 0.51-0.85 P/P0 range, sloping hysteresis loops of type H2. The parameters deduced from an analysis of these isotherms for the two solids are given in Table 3. Compared to what is found for solid A, the BET surface areas, C constants, and total pore volume of solid B (contact time, 48 h) are slightly smaller (10-20%). It should also be noted that compared to the initial silica gel 254, all Al-treated samples (48 h) have smaller ABET and VP values. With contact time, these values however increase back to values close to those of silica 254, especially in the case of solid B. This evolution in the textural properties of solid B also shows up in the NMR results discussed below. Using the BJH approach24 for cylindrical-type pores, pore size distribution curves can be established which show no appreaciable differences between the two solids; that is, both have a similar large range of pore sizes between 2 and 10 nm. It is however observed that the number of pores below 5 nm decreases for both solids (48 h) as the degree of hydrolysis of the Al solution increases. Solid-State NMR. Solid B was examined by 27Al and 29 Si NMR MAS. Certain samples were examined under CP-MAS conditions. 27Al NMR. The effect of the degree of hydrolysis of the Al solutions is depicted in Figure 7 for washed samples, with the contact time 48 h, aged for 1 month. The spectra consist of a relatively fine tetrahedral line located at around 55 ppm together with a nonsymmetrical octahedral line around 1 ppm. It is observed that the total line intensity increases with R, in accordance with chemical analyses results. This increase in total line intensity is due essentially to an increase of the octahedral line together with contributions appearing around 35 ppm and to the left ((100 ppm) of the tetrahedral line. When the spectra are recorded under static conditions (i.e. no MAS), very broad lines are observed (extending between (300 ppm), from which emerge quite distinctly a narrow line at 55 ppm, indicating that the corresponding Al nuclei are located in fairly isolated symmetrical sites. When the MAS spectrum of solid B, 48 h, is compared (see Figure 8a) to that of the corresponding solid A, from which the (24) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373.

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Figure 10. Comparison between MAS and CP-MAS 27Al NMR spectra of solid B, R ) 2, 48 h, aged 18 months. Figure 9. 27Al NMR MAS spectra of solid A and solid B: (a) comparison between solid B (R ) 2, 48 h) aged 1 (- - -) and 18 months (s); (b) comparison between solid A (s) and corresponding solid B (- - -) (R ) 2, 60 days, aged 6 months).

dispersed phase is issued, certain differences appear: (1) the total line intensity is larger than that of solid A (in accordance with chemical analyses) and (2) the percentage of tetrahedral Al is higher. These differences (observed for a short 48-h contact time) therefore seem to suggest that on a local scale the distribution of Al is not identical for the two solids. However, with increasing contact time the spectra of solid B alter: (1) the line widths decrease notably and (2) the relative amount of AlT/Al0 decreases (see Figure 8b). This clearly indicates that, while remaining different from solid A, solid B undergoes with contact time a redistribution of sites and/or structure. Changes in the Al spectra are once again observed when solid B is left to age in the powdered form at room temperature. Figure 9a shows the spectral change for solid B (R ) 2, 48 h) aged respectively 1 and 18 months. In fact, after an age period of about 6 months, the differences observed between the Al spectra of solid A and the corresponding solid B have been completely wiped out (see Figure 9b). These results all point very qualitatively to the evolutionary processes which are taking place in the local and structural organization of solid B. However it is evident that when solid B first appears in the system, the local organization, with respect to Al anyhow, is quite different from that of the solid from which it is issued. A cross-polarization experiment H f Al for solid B (R ) 2, 48 h) clearly confirms as suggested above that the octahedral Al are in strong interaction with surrounding protons (see Figure 10). The tetrahedral Al however are not completely isolated from protons, as a small signal around 55 ppm is still visible. This is probably due to the open structure of the solid with the result that a certain proportion of the Al ions inserted in tetrahedral silica sites remain in close proximity to surface protons. The inclusion of metal ions into a silica matrix is well-

documented. Iron in silica for instance has been studied25 by EPR, and different configurations for the iron sites have been identified. They go from purely substitutional tetrahedral sites to various interstitial positions corresponding to highly distorted tetrahedral and octahedral sites. Al can occupy sites going from defects in the lattice,26 surface-coordinated positions,4 or occluded 6- to 4-coordinated sites within the pores of high surface materials.27 It is therefore not surprising that, for amorphous solids such as those formed here, a large distribution of sites is possible. This is reflected by the NMR spectra which present broad and asymmetric lines, which makes the interpretation in terms of exact local configurations particularly difficult. Distorted crystallographic sites lead for Al to large quadrupolar interactions, the effects of which can only be decreased in MAS by working at very high magnetic fields. To this effect certain samples were examined at a somewhat higher magnetic field, 11.744 T compared to 7.043 T. When these spectra are compared to the spectra shown above, only small changes are observed: the asymmetry of the octahedral line is slightly attenuated, and the presence of a small 35 ppm “line” is less obvious, which seems to indicate that this “line” is in fact an anisotropic effet of the tetrahedral Al. In any case, this confirms the presence in our samples of a distribution of sites which are highly disorded especially for octahedral Al. 29 Si NMR. Modifications in the stuctural conformation of the silica matrix can in principle be detected by 29Si NMR. Unfortunately (due to technical problems), only aged samples (>10 months) of the dispersed phase were examined. The heterogeneity in site distribution discussed above is also reflected in these 29Si NMR spectra. Compared to what is observed4 for the starting silica sample 254, which gives the usual Si NMR spectrum obtained for silica gel (i.e. three lines corresponding to (25) Rossman, G. R. Silica; Heaney, P. J., Prewitt, C. T., Gibbs, G. V., Eds.; Reviews in Mineralogy; Mineralogical Society of America: Washington, DC, 1994, Vol. 29, p 433. (26) Humbert, B. Ph.D. Thesis, Universite´ de Nancy, Nancy, France, 1991. (27) Klinowski, J. Annu. Rev. Mater. Sci. 1988, 18, 189.

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ms, indicates that the “nonproton” Q4 sites are quickly recovered in the spectra, as anticipated for open structures.

Figure 11. 29Si NMR spectra of solid B, R ) 2, 60 days, aged 15 months: (MAS) bottom spectrum; (CP-MAS spectra for different contact times) top spectra.

Si(Si)3 and surface Si(Si)2OH and Si(Si)(OH)2 sites), the spectra obtained for solid B are more difficult to decipher. This is due to the presence in a very narrow frequency range of many different lines issued from different sites and/or combinations of the three elements (Si, Al, H) present in the sample. Furthermore, as expected for these particular samples, no difference between the 29Si spectra of solid A and those of aged solid B is observed. All lines remain in the strongly cross-linked region, that is, -95 to -115 ppm. The cross-polarization (CP) experiments H f Si which were performed only help in localizing certain particular regions which are close to OH groups. Exploration (see Figure 11) of the short CP contact region, 1-5

Conclusions It has been shown that, compared to what is observed in pure water at pH ) 4, the behavior of silica in the presence of hydrolyzed Al solutions is drastically modified: from R ) 1 to 2, the total amount of dissolved silica is much larger, and from R ) 0 to 2, the initial rate at which silica passes into solution is increased. By using a simple spectrophotometric approach, an estimate of the amount of monomeric and polymeric silica aqueous species can be obtained as a function of time. For the lesshydrolyzed solutions (R ) 0.5 and 1.0) the silica polymeric species formed for a given contact time have a much smaller degree of polymerization than those found in R ) 1.5 and 2.0 solutions. For all hydrolyzed solutions, the presence of a dispersed phase is observed. Furthermore, when treated with these Al solutions, the initial silica gel has its texture, surface charge, and global reactivity modified. This complex chain of events which occurs at acidic pH is in fact triggered by the presence in solution of highly charged Al13 polymeric cations, which after adsorption onto the silica sample depolymerize in a process which involves the surface hydroxyls.4 This leads to the inclusion of tetrahedral Al in the silica famework, the presence of surface-coordinated octahedral Al, and possibly an amorphous silicaaluminum phase. This globaly leads through a weakening of the Si-O bonds to an increased release of silica into solution. The Al cations in solution (in the presence of silica gel, at R ) 2, after 60 days, about 2.5% Al13 are still present in solution, whereas, at R ) 0.5 after 24 h, all the Al13 is consumed, leaving only monomers)5 probably interact strongly with some of the aqueous Si species.28 With contact time this leads to the formation of a metastable silica-aluminum dispersed phase, which, as demonstated, differs from the treated silica beads by a different chemical Si/Al ratio, a different distribution of sites for the Al ions, and different textural properties. These interactions between Al hydrolyzed solutions and solid silica are therefore quite effective in spatially redistributing the initial silica beads. Acknowledgment. We gratefully acknowledge support from the Services de Programmation de la Politique Scientifique-Belgium. One of us (M.B.) would like to thank le Ministe`re de l’Education Nationale du Maroc for financial support. This work forms part of a joint effort supported by a CE COST D5/003/94 Action. All authors would like to thank Prof. P. Rouxhet (UCL Belgium), Prof. S. A. Selim (Ain Shams University Egypt), and Prof. J. Y. Bottero (Cerege Aix-Marseilles III France) for continuous interest and support. Thanks also to Dr. J. Sanz (CSIC Madrid Spain) for helping us with various aspects of this work. LA970750V (28) Exley, C.; Birchall, J. D. Geochim. Cosmochim. Acta 1995, 59, 1017.