Characterization, Surface Area, and Porosity Analyses of the Solids

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Characterization, Surface Area, and Porosity Analyses of the Solids Obtained by Acid Leaching of a Saponite Miguel Angel Vicente,*,†,‡ Mercedes Sua´rez,§ Juan de Dios Lo´pez-Gonza´lez,‡ and Miguel Angel Banãres-Munõz† Departamento de Quı´mica Inorga´ nica, Facultad de Ciencias Quı´micas, Universidad de Salamanca, Plaza de la Merced, s/n, 37008-Salamanca, Spain, Departamento de Quı´mica Inorga´ nica, Facultad de Ciencias, Universidad Nacional de Educacio´ n a Distancia, Senda del Rey, s/n, 28040-Madrid, Spain, and Area de Mineralogıá y Cristalografıá, Departamento de Geologıá, Facultad de Ciencias, Universidad de Salamanca, Plaza de la Merced, s/n, 37008-Salamanca, Spain Received June 23, 1995. In Final Form: October 5, 1995X New deposits of saponite have been described recently in the Madrid Basin (Madrid and Toledo provinces, Central Spain), a zone very rich in Mg silicates, especially in sepiolite. This is the first time that the high surface area saponitic material from Yunclillos deposit (Toledo province) is described. The acid treatment of this material with diluted solutions of HCl (0.62 and 1.25% by weight) at 25 °C for 2, 6, 24, and 48 h has been carried out. Although the conditions employed in the leaching are very soft, most of the octahedral sheet of the clay is dissolved, as indicated by the high removal of Mg cations and corroborated by the changes observed in the FT-IR spectra and by the simplification in the profile of the TGA-DTA curves of the solids obtained after the treatments. The destruction of the saponite structure by the treatments results in the generation of free silica. The surface area of natural saponite (161 m2 g-1) is doubled in the leached solids, even under the mild conditions considered, reaching a maximum value of 392 m2 g-1. The texture of the free silica generated after the saponite ordered structure affects the properties of the activated solids, as shown by the numerical analyses of the nitrogen adsorption-desorption isotherms of these solids, mainly made by using the “t” and “f” plots.

Introduction The surface properties of clay minerals are very important because of their great influence in the adsorbing and catalytic behavior of these minerals. Although a layered clay mineral, such as montmorillonite or saponite, can reach a maximum theoretical surface area of about 800 m2 g-1, the external surface area observed in natural minerals is much lower, due to the aggregation of clay particles, the presence of mineral impurities, and the existence of tetrahedral and octahedral substitutions and octahedral vacancies that must be compensated by exchange cations. The acid treatments increase the surface area of clay minerals by disaggregation of clay particles, elimination of several mineral impurities, removal of metal-exchange cations, and proton exchange, reasons for which they are usually known as “activation treatments”. Thus, the acid treatments, together with the thermal treatments, have been much used not only for increasing the surface area of clay minerals but also for obtaining porous solids with high amounts of acid centers, factors that affect the catalytic applications of the solids obtained. The acid treatments of clay minerals, especially sepiolite and montmorillonite, have been widely studied for years, and solids with interesting properties are so obtained. The solids obtained after acid attacks are mainly composed * All correspondence should be sent to Dr. Miguel Angel Vicente-Rodrı´guez, Departamento de Quı´mica Inorgańica, Facultad de Quı´mica, Universidad de Salamanca, Plaza de la Merced, s/n, 37008-Salamanca, Spain. FAX: 34-23-29 44 89. E-mail: [email protected]. † Departamento de Quı´mica Inorga ´ nica, Facultad de Quı´micas, Universidad de Salamanca. ‡ Departamento de Quı´mica Inorga ´ nica, Facultad de Ciencias, Universidad Nacional de Educacioń a Distancia. § Area de Mineralogıá y Cristalografıá, Departamento de Geologıá, Facultad de Ciencias, Universidad de Salamanca. X Abstract published in Advance ACS Abstracts, December 15, 1995.

0743-7463/96/2412-0566$12.00/0

of impure silica gels. These silica gels obtained from the tetrahedral sheet of the silicate have textural properties that permit these solids to reach high values of surface area.1-8 These high surface area silica gels are competitive in different industrial uses with silica gels obtained by other methods, such as precipitation of sodium silicates. Saponite is a trioctahedral smectite, originated as sedimentary rock and as a product of the hydrothermal alteration and weathering of basalts and ultramafic rocks. In a very pure Ca saponite from Koza´kov (Czechoslovakia), Suquet et al.9 found a monoclinic unit cell 5 × 9 Å, β ) 97-100°, basal spacing 15 Å. In some aspects, saponite is very similar to vermiculite. Thus, saponite has a negatively charged tetrahedral sheet, partially balanced by the positive charge in the octahedral sheet.10 New important deposits of saponite have been recently described in the literature.11-15 The adsorbent and catalytic properties of saponite have been studied in recent years, and it has been found that (1) Mendioroz, S.; Pajares, J.; Benito, I.; Pesquera, C.; Gonza´lez, F.; Blanco, C. Langmuir 1987, 3, 676. (2) Cetisli, H.; Gedikbey, T. Clay Miner. 1990, 25, 207. (3) Komadel, P.; Schmidt, D.; Madejova´, J.; Cicel, B. Appl. Clay Sci. 1990, 5, 113. (4) Suquet, H.; Chevalier, S.; Marcilly, C.; Barthomeuf, D. Clay Miner. 1991, 26, 49. (5) Pesquera, C.; Gonza´lez, F.; Benito, I.; Blanco, C.; Mendioroz, S.; Pajares, J. A. J. Mater. Chem. 1992, 2, 907. (6) Vicente Rodrı´guez, M. A.; Lo´pez Gonza´lez, J. de D.; Banãres Munõz, M. A. Clay Miner. 1994, 29, 361. (7) Vicente Rodrı´guez, M. A.; Sua´rez Barrios, M.; Lo´pez Gonza´lez, J. de D.; Banãres Munõz, M. A. Clays Clay Miner. 1994, 42, 724. (8) Vicente Rodrı´guez, M. A.; Lo´pez Gonza´lez, J. de D.; Banãres Munõz, M. A. Microporous Mater. 1995, 4, 251. (9) Suquet, H.; de la Calle, C.; Pezerat, H. Clays Clay Miner. 1975, 23, 1. (10) de la Calle, C.; Suquet, H. Rev. Miner. 1988, 19, 455. (11) Post, J. L. Clays Clay Miner. 1984, 32, 147. (12) Galań, E.; Alvarez, A.; Esteban, M. A. Appl. Clay Sci. 1986, 1, 295. (13) Kodama, H.; de Kimpe, C. R.; Dejou, J. Clays Clay Miner. 1988, 36, 102. (14) April, R. H.; Keller, D. M. Clays Clay Miner. 1992, 40, 22. (15) Beaufort, D.; Meunier, A. Clay Miner. 1994, 29, 47.

© 1996 American Chemical Society

Solids Obtained by Acid Leaching of a Saponite

this silicate has an excellent ability to form pillared clays and good catalytic properties.16-23 But the behavior of this mineral under acid treatment has not been studied. Only recently, we have reported the acid activation of the high iron content saponite from Griffith Park (California), describing the physicochemical characterization of the solids obtained and the development of the specific surface area.7,8 In the present paper, the acid leaching of a high surface area natural saponite, obtained from a new deposit of this mineral, is reported. The physicochemical and textural transformations occurring when different concentration HCl solutions and different times of treatment are used in its activation treatments are described. Elemental analyses, X-ray diffraction, FT-IR spectroscopy, thermal analyses, and a complete study of the nitrogen adsorptiondesorption isotherms of all the solids are used for elucidating the results obtained. Experimental Section Materials. The clay mineral used in this work is a natural saponite from the Yunclillos deposit (Toledo province, Spain), supplied by TOLSA, SA. This deposit is situated in the Madrid Basin, in the center of Spain. This basin is very rich in Mg silicate deposits, Vallecas sepiolitesthe biggest sepiolite deposit in the worldsbeing the most important of them. The presence of smectite materials in this basin was first reported about 20 years ago, but it was only in 1986 when the first bentonitic deposit in this basin was carefully studied, when Galań et al. described the saponite deposit in Cabanãs de la Sagra, together with the technical properties and suitable uses of this material. (See ref 12 and references therein.) The Yunclillos deposit, situated near the deposit described by Galań et al., was studied later. In the present work, the natural powder material from this deposit is used for the acid leaching treatments. This material is composed of more than 90% saponite; sepiolite, quartz, and feldspars, in low quantities, are observed as impurities. Its e2 µm fraction, obtained by decantation of this material, was used for obtaining the structural formula of saponite. Equipment and Methods. Elemental analyses of the solids were carried out by plasma emission spectroscopy, using a PerkinElmer emission spectrometer, model Plasma II. In preparation for analysis, the solids were digested under pressure, with a mixture of nitric and hydrofluoric acids, in a PTFE autoclave. The cation-exchange capacity (CEC) was determined by using a Bouat apparatus, after NH4+-exchanged samples were prepared by treatment with ammonium acetate. Five grams of the clay was treated with 150 mL of 0.62 and 1.25 wt % solutions of HCl (0.17 and 0.34 M, respectively), at 25 °C for 2, 6, 24, and 48 h, with mechanical stirring. The solids obtained after the treatments were washed, dried at 50 °C, and kept over H2SO4 solutions. The filtrates obtained after washing until no chloride anions could be detected were analyzed by plasma emission spectroscopy. X-ray powder diffraction patterns were obtained from a Siemens D-500 diffractometer, employing the Cu KR line (λ ) 1.540 Å) and operating at 40 kV and 30 mA (1200 W). The equipment has a graphite monochromator and a DACO-MP data station. FT-IR spectra were recorded in the region 4000-350 cm-1 in a Perkin-Elmer 1730 infrared Fourier transform spectrometer, equipped with a 3700 data station, using the KBr pellet technique. (16) Urabe, K.; Sakurai, H.; Izumi, Y. J. Chem. Soc., Chem. Commun. 1986, 1074; 1988, 1520. (17) Watanabe, T.; Sato, T. Clay Sci. 1988, 7, 129. (18) Schoonheydt, R. A.; Leeman, H. Clay Miner. 1992, 27, 249. (19) Usami, H.; Tagaki, K.; Sawaki, Y. Chem. Lett. 1992, 1405. (20) Chevalier, S.; Franck, R.; Lambert, J.-F.; Barthomeuf, D.; Suquet, H. Appl. Catal., A 1994, 110, 153. (21) Chevalier, S.; Franck, R.; Lambert, J.-F.; Barthomeuf, D.; Suquet, H. J. Chem. Soc., Faraday Trans. 1994, 90, 667. (22) Lambert, J.-F.; Chevalier, S.; Franck, R.; Barthomeuf, D.; Suquet, H. J. Chem. Soc. Faraday Trans. 1994, 90, 675. (23) Suquet, H.; Franck, R.; Lambert, J.-F.; Elssas, F.; Marcilly, C.; Chevalier, S. Appl. Clay Sci. 1994, 8, 349.

Langmuir, Vol. 12, No. 2, 1996 567 Table 1. Cations Dissolved by Acid Treatments, Expressed as Percentage, in Oxide Form, of the Original Content in the Samples time (h)

SiO2

MgO

Al2O3

Fe2O3

CaO

Na2O

K2O

2 6 24 48

0.87 1.25 1.40 1.19

31.67 40.51 51.18 52.25

0.62 wt % HCl 8.16 3.93 12.06 2.16 16.82 2.50 16.33 3.17

75.31 78.44 78.56 79.00

39.27 40.49 38.78 40.49

4.78 4.43 4.08 3.82

2 6 24 48

1.02 1.21 0.95 0.87

41.37 56.29 61.75 75.22

1.25 wt % HCl 13.01 8.36 22.30 17.58 35.23 28.65 36.97 27.28

77.83 81.58 85.81 86.67

45.61 43.90 43.90 46.59

5.26 5.04 5.44 5.04

Thermal analyses were performed on Perkin-Elmer analyzers, TGS-2 and 1700 for gravimetric and differential thermal analyses, respectively; both were connected to a 3600 data station. All measurements were carried out at a heating rate of 10 °C min-1 under a flow of air of 35 mL min-1. Al2O3 was used as reference for the DTA measurements. Specific surface area and textural analyses of the original and activated samples were determined from the corresponding nitrogen adsorption-desorption isotherms at 77 K, obtained from a Micromeritics ASAP 2000 analyzer, after the samples were outgassed at 110 °C for 8 h. The relative error in these measurements is about 5%.

Results and Discussion Chemical Analyses. The weight percent of natural saponite was as follows: SiO2, 52.90; Al2O3, 4.35; Fe2O3, 1.18; MgO, 18.24; TiO2, 0.23; MnO, 0.03; CaO, 1.01; Na2O, 0.11; K2O, 0.55; loss by ignition, 20.8 wt %. In the e2 µm fraction of saponite, the obtained weight percent was as follows: SiO2, 49.45; Al2O3, 4.72; Fe2O3, 1.29; MgO, 24.34; TiO2, 0.20; MnO, 0.03; CaO, 0.78; Na2O, 0.07; K2O, 0.44; loss by ignition, 18.31 wt %. The structural formula of saponite has been calculated from the chemical composition of the e2 µm fraction, considering that the amount of impurities in this sample is very low; only a very small amount of sepiolite and traces of quartz are detected in this fraction. The formula thus obtained is (Si3.71Al0.29)O10(Mg2.58Fe3+0.07Al0.13Mn0.002Ti0.009)(OH)2[Mg0.12Ca0.062Na0.010K0.042]. The tetrahedral sheet has a charge of -0.29, and the octahedral sheet has a charge of -0.20, which gives an interlayer charge of -0.49, high for an interlayer charge, but lower than the value of -0.60 usually assumed as a maximum for the trioctahedral smectites.24 This sample resembles that of Milford (UTAH) by virtue of the distribution of charge between the tetrahedral and octahedral sheets and the high magnesium content.24 The charge balance is good, because the exchange cations give a positive charge, +0.416. The amount of Mg(II) as exchangeable cation has been calculated considering the cation-exchange capacity (CEC) of the sample, which was found to be 115 mequiv/100 g. The obtained formula indicates that the chemical composition of this sample is similar to that of other saponites described in the literature.11,15,24 The amounts of cations of the solid that are removed in each acid treatment are given in Table 1. As observed, Ca(II) is immediately dissolved in a high percentage of about 80%, while Na(I) is dissolved only in about 50%, and most of the K(I) remains in the solid. The amounts of these cations that are removed correspond to their presence in the sample as exchangeable cations, while (24) Newman, A. C. D.; Brown, G. The chemical Constitution of clays. In Chemistry of Clays and Clay Minerals; Newman, A. C. D., Ed.; Monograph No. 6; Mineralogical Society: New York, 1987; pp 1-128.

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Figure 1. Plot of the amount of cation dissolved (XMe), for Mg(II), Al(III), and Fe(III), as a function of xt.

the amounts that remain in the activated solidssimportant quantities of K(I) and Na(I) and only a small amount of Ca(II)smust be due to their presence as insoluble impurities, mainly small quantities of feldspars, which contain these cations. The octahedral cations are progressively removed, depending on the acid concentration and on the time of treatment. The main octahedral cation, Mg(II), is dissolved in a high percentage, about 75%, under the more intense conditions considered. Fe(III) and Al(III), this last situated in both octahedral and tetrahedral positions, are dissolved in smaller amounts, 30 and 35%, respectively. It must be considered that the data on dissolution of these cations have a small relative error, because of their presence as constituents of feldspars (Al) and sepiolite (Fe and Al). The sepiolite present in the sample is not attacked very much by the acid treatments, as evidenced by X-ray diffraction and according to Vicente Rodrı´guez et al.6 Silica is removed in only about 1% in each treatment, due to its insolubility in acid solutions. The removal of Mg(II), Al(III), and Fe(III) cations is linear with respect to the time of treatment. Figure 1 shows that these two magnitudes can be related by an equation of the type XMe ) kxt, XMe being the amount of a cation Me removed in a time t, a trend observed in kinetic studies of acid treatment of other silicates.2,25 More experiments must be carried out for obtaining valid kinetic results. The removal of the Mg(II) octahedral cations in Yunclillos saponite is comparable to that observed for this cation during acid treatment of griffithite (ferrous saponite), in which 82.1% of Mg(II) is removed during treatment with 1.25% HCl for 48 h.7 In the same paper, we reported that Fe(III) is more resistant than Mg(II) to removal by acid leaching when griffithite is treated. There, 46.8% of iron was removed during treatment with 1.25% HCl for 48 h (82.1% of Mg removed under the same conditions, as indicated before). It seems that, in Yunclillos saponite, Fe(III) is even more resistant to acid attack than in griffithite, but the small amount of this cation in the sample now studied and its possible presence in the small amount of sepiolite that impurifies the saponite must be taken into account, because the data obtained about this cation could have a big relative error. By contrast, much more drastic conditions are usually used for acid activation of montmorillonite. A dissolution (25) Corma, A.; Mifsud, A.; Pe´rez, J. Clay Miner. 1986, 21, 69.

Figure 2. (A) XRD of natural material with the assignment of the main peaks of saponite and identification of impurities (Sep ) sepiolite, Q ) quartz, F ) feldspars). (B) XRD of natural saponite (a) and samples treated with 0.62% HCl for 6 h (b) and with 1.25% HCl for 6 h (c) and 48 h (d).

of only 19% of Al(III) and 1% of Mg(II) was found when this silicate was treated with 4 mol L-1 HCl at 80 °C for 1 h.1 We have reported that only 2.5% of Mg cations are removed when Gador montmorillonite is treated under the conditions now used for the treatment of saponite.7 These results confirm once more the influence of the chemical composition of these silicates in their attackability by acid solutions. X-ray Diffraction. The X-ray diffractogram of the natural material agrees with that usually found in the literature for saponite (JCPDS files 11-56 and 13-86). The assignment of the reflection peaks corresponding to this silicate has been made following that given by other authors for samples previously described.9,11,26 Quartz, feldspars, and sepiolite, this last detected as a shoulder at 12.1 Å in the 001 reflection peak of saponite, are observed as impurities. The powder X-ray diffraction patterns of natural and activated samples are given in Figure 2B. The loss of crystallinity in the samples when the acid treatment progresses can be seen in the intensity and width of the 001 reflection peak, which appears at about 15.0 Å and becomes broader and less intense as the time of treatment progresses. The structure of saponite is maintained in all the series when treatment is with 0.62% HCl and appears destroyed after 24 h of treatment when the 1.25% HCl series is considered. The appearance and increase in intensity of a broad band in the range 2Θ ) 20-30° indicates the formation of free silica. This silica is generated by destruction of the tetrahedral sheet of silicate. In the series treated with 1.25% HCl, the loss of crystallinity and the appearance of amorphous silica were similar to those in the above-mentioned series. With increasing acid concentration, the time required to produce (26) Brindley, G. W. Order-Disorder in Clay Mineral Structures. In Crystal Structures of Clay Minerals and their X-Ray Identification; Brindley, G. W., Brown, G., Eds.; Monograph No. 5; Mineralogical Society: London, 1980; pp 126-195.


Figure 3. DTA and TGA curves of natural saponite (a) and samples treated with 0.62% (b) and 1.25% (c) HCl for 48 h.

the observed effects decreases. The decrease in intensity in the 001 reflection peak of saponite makes the quartz peak at 3.33 Å the most intense in the pattern of activated solids. The small amount of sepiolite observed in the parent mineral is not removed by acid treatment under the conditions considered, as can be clearly seen by the peak at 12.2 Å in Figure 2B. Also the small amount of feldspar present in the natural sample remains in the activated solids, as can be observed by the peak at 3.22 Å in Figure 2B. Thermal Analyses. The thermal analyses confirm the transformation of saponite into hydrated silica by acid leaching. Curves obtained for both natural and activated solids are given in Figure 3. The behavior of the natural saponite agrees with that reported by Mackenzie27 for this silicate: an initial weight loss between 30 and 200 °C, due to the interlayer water departure (14% weight loss in TG curve), with a corresponding endothermic effect in the DTA curve centered at 145 °C; a modest weight loss (5%) between 200 and 800 °C; and a last effect due to dehydroxylation (825 °C) and crystallization (845 °C), with a weight loss of 3.5%. The residue obtained after calcination has been identified by XRD as enstatite. Mackenzie indicates that MgSiO3, enstatite, is formed by calcination of saponite, just as it occurs in other magnesic silicates such as sepiolite. The form of the curves changes when saponite is transformed in hydrated silica; the removal of octahedral cations makes the effect due to the interlayer water disappeared. Thus, the thermal curves of activated samples are very simple and only a broad peak centered (27) Mackenzie, R. C. Simple phyllosilicates based on Gibbsite- and Brucite-like sheets. In Differential Thermal Analysis; Mackenzie, R. C., Ed.; Academic Press: London, 1970; Vol. I, pp 497-537.

Langmuir, Vol. 12, No. 2, 1996 569

at 125-130 °C corresponding to the dehydration of silica and the dehydroxylation and later crystallization of the original remaining structure at 845 °C can be observed. The temperature of the final phase change, due to the loss of hydroxyl groups, is characteristic of each particular sample, depending to a large degree on the chemical composition of the octahedral sheet of silicate. The usual trend is an increase in the dehydroxylation temperature from vacancies to the presence of Fe, Al, or Mg in the octahedral sheet of saponite. Thus, dehydroxylation temperatures of 857 °C (Cornwall, England), 858 °C (Allt Ribein, Scotland), and 882 °C (Krugersdorp, South Africa) have been referred to for magnesic saponites.27 The dehydroxylation temperature of 825 °C found for Yunclillos saponite also follows this trend. As an exception to this trend, the dehydroxylation of Grosslattengrun (Germany) magnesic saponite has been observed at 818 °C.27 For the ferromagnesic Griffith Park (United States) saponite, a dehydroxylation temperature of 830 °C has been reported recently.7 Also the existence of a high number of vacancies in the octahedral sheet makes the dehydroxylation temperature decrease. In Yunclillos saponite, the number of vacancies is 0.21, as can be observed in the structural formula. Usually, the destruction of the silicate structure and its transformation into free silica make the dehydroxylation temperature decrease; the more intense is the acid attack, the lower is the dehydroxylation temperature. But, in Yunclillos saponite, the dehydroxylation temperature is almost constant, being 825 °C both for natural saponite and for very attacked samples, in which the amount of free silica is greater than that of saponite. The delamination and disaggregation of particles when the treatment progresses, as well as the formation of hydrated silica in the more intensely treated samples, cause an increase in the content of adsorbed water in activated samples in comparison with the natural silicate. This effect compensates the loss of structural water when octahedral cations, especially Mg(II), are removed by the acid leaching. The total weight loss in all acid-activated samples reaches values between 18 and 21% of their weight. IR Spectroscopy. FT-IR spectroscopy also confirms the destruction of the saponite structure and the generation of free silica during acid leaching. The spectra of natural saponite and two treated samples are shown in Figure 4. The bands characteristic of the tetrahedral sheet of silicate (1200-1000, 658, 458 cm-1) disappear with acid leaching. At the same time, bands corresponding to free silica appear (1200-1000, 795, 465 cm-1). The band at 1200-1000 cm-1, assigned to Si-O-Si bonds, appears both in natural and in treated saponite, but its form is different in both cases, from that characteristic of a silicate tetrahedral sheet to that characteristic of free silica. The same trend is observed in the band at 448 cm-1, with a shoulder at 527 cm-1, due to Si-O-Si and Si-O-Mg bonds, which in activated samples corresponds to free silica. The changes in the silicate can also be observed in the bands corresponding to its octahedral sheet. Thus, the band at 3430 cm-1, corresponding to water bonded to the octahedral cations, especially Mg(II), decreases in intensity when these cations are removed by the acid leaching, while the band at 3678 cm-1, assigned to the hydroxyl groups, almost disappears. Consequently, only a wide band centered at about 3420 cm-1, corresponding to water adsorbed by free silica, is observed in this region in the activated solids. The formation of silica is faster when the concentration of the acid used in the treatments increases, in agreement with the results given by other

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Vicente et al.

Figure 5. Effect of time of treatment, for each acid concentration, on the BET surface area of saponite.

Figure 4. FT-IR spectra of natural saponite (a) and samples treated with 0.62% (b) and 1.25% (c) HCl for 48 h. Table 2. Surface and Textural Data on the Different Solidsa sample natural saponite treated saponite 0.62% HCl, 2 h 0.62% HCl, 6 h 0.62% HCl, 24 h 0.62% HCl, 48 h 1.25% HCl, 2 h 1.25% HCl, 6 h 1.25% HCl, 24 h 1.25% HCl, 48 h

SBET Vads Scum Vtot Vcum dp (m2 g-1) (mL g-1) (m2 g-1) (mL g-1) (mL g-1) (nm) 161

105

134

0.166

0.152

0.42

275 288 309 318 312 324 374 392

149 152 160 155 178 169 163 165

244 244 255 243 324 288 300 249

0.231 0.238 0.251 0.241 0.277 0.264 0.255 0.257

0.215 0.217 0.225 0.205 0.283 0.247 0.219 0.188

0.35 0.33 0.33 0.31 0.36 0.37 0.28 0.26

a S BET ) specific surface area; Vads ) volume adsorbed at P/P0 ) 0.98; Scum ) cumulative surface area; Vtot ) total pore volume; Vcum ) cumulative pore volume; dp ) average pore diameter.

techniques. The band at 1635 cm-1, due to the bending vibration mode of water, was maintained but decreased in intensity after the treatment. Bands at 693 and 656 cm-1 were assigned to Al-OH and Mg-OH bonds, these bands clearly decreasing in intensity when the treatment progresses and Mg(II) cations are removed. Surface Area Measurements. The specific surface areas (S) of the solids have been calculated after the corresponding nitrogen adsorption isotherms by using the BET method. Table 2 and Figure 5 show the evolution of this magnitude with acid treatment. Natural saponite has a surface area of 161 m2 g-1, much higher than that usually found in natural saponites. The value of S increases with the intensity of the acid treatment: it increases continuously with the time of treatment in each of the two series considered, and higher values are obtained in the series treated with 1.25 wt % HCl than in the 0.62%

HCl series. The maximum value, 392 m2 g-1, is obtained when treatment is with 1.25% HCl for 48 h, the most intense of the treatments, which suggests that more intense treatments would permit one to obtain solids with higher values of S. The volume of nitrogen adsorbed at relative pressure P/P0 ) 0.98 also increases from natural to activated samples, from 105 mL g-1 (STP conditions) in natural saponite to a maximum of about 170 mL g-1 in the activated samples. The increase in the value of surface area is especially significant at the beginning of treatment, after 2 or 6 h; while with longer treatments, 24 or 48 h, the surface area continues increasing, but in smaller amounts (Figure 5). This figure suggests that the surface area should be higher in samples that are more activated, i.e., obtained by using more concentrated HCl solutions and/or extending the time of treatment. As observed, all the leached samples have surface areas higher than 275 m2 g-1, even in the case of treatement with a solution as diluted as 0.62% (0.17 mol L-1) for only 2 h. These values of surface area are comparable with those recently reported for griffithite.7 The increase of surface area observed in this ferromagnesic saponite is from 35 m2 g-1 in the natural silicate to 367 m2 g-1 in the sample treated with 1.25 wt % HCl for 48 h. The surface area of Yunclillos saponite is increased 2.5 times by the acid treatments while the area of griffithite becomes multiplied 10-fold. The development of this magnitude is different because of the very different areas of the natural samples; the surface area of activated solids is similar in both cases. On an Al smectite, an increase of only 10 m2 g-1 (10%) was obtained when Gador montmorillonite was treated under the same conditions, an increase attributable to impurities and exchange cation removal, because montmorillonite is almost inert under these conditions. By plotting the surface area versus the amount of Mg(II) removed, an almost straight line is obtained, which indicates that the increase in surface area directly depends on the intensity of the acid treatment and on the removal of the octahedral sheet of silicate. Similar results are obtained when different magnesic silicates are treated, both with fibrous and with layered structure: sepiolite, saponite, vermiculite and even a ferromagnesic saponite, such as griffithite. By contrast, aluminic silicates, such as montmorillonite and palygorskite, are almost inert under the same conditions, which indicates that much more intense acid treatments are required when aluminic silicates are activated for obtaining a comparable development in their surface properties. Textural Changes. The porosity of the solids was studied by numerical analyses of their nitrogen adsorp-


Langmuir, Vol. 12, No. 2, 1996 571

Figure 6. Nitrogen adsorption-desorption isotherms of natural saponite (a) and samples treated with 0.62% (b) and 1.25% (c) HCl for 48 h. Figure 8. “f” plot of samples treated with 0.62% (a) and 1.25% (b) HCl for 48 h compared to natural saponite as reference.

Figure 7. “t” plot of natural saponite (a) and samples treated with 0.62% (b) and 1.25% (c) HCl for 48 h.

tion-desorption isotherms. A computer program28 was used for analyzing the isotherms and for obtaining the “t” 29 and the “f” 30 plots. Some textural data of the different solids are given in Table 2. The nitrogen adsorption-desorption isotherms of all samples are similar to type II of the BDDT classification,31 but a clear change both in the form of the isotherm and in the form of the hysteresis loop can be seen from natural saponite to activated samples (Figure 6). The form of the isotherms changes both in the zone of lower relative pressures, that is, in the region corresponding to microporosity, and in the high relative pressures with the indicated change in the form of the hysteresis loop, all these changes suggesting modifications in the texture of the solids. These modifications can be observed in the corresponding t plots (Figure 7). Extrapolation of the t plot of natural saponite does not pass through 0, indicating a microporosity of about 10 mL g-1 (SPT conditions) for this sample. This value increases continuously in each series, reaching values of 15 and 30 mL g-1, approximately, when considering the samples treated with 0.62 and 1.25% HCl for 48 h, respectively. In Figure 8, natural saponite and samples treated with 0.62 and 1.25% HCl for 48 h are compared by using the f plot. In this plot, the amount of nitrogen adsorbed by the activated samples is compared with that adsorbed by (28) Rives, V. Adsorp. Sci. Technol. 1991, 8, 95. (29) Lippens B. C.; de Boer, J. H. J. Catal. 1965, 4, 319. (30) Gregg, S. J. J. Chem. Soc., Chem. Commun. 1975, 699. (31) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London & New York, 1982.

natural saponite, used as reference, as a function of P/P0. The adsorption process is higher in the activated solids than in the natural saponite. The main differences in the amount of nitrogen adsorbed are observed in the region corresponding to low relative pressures, where the activated solids adsorb about 2.5 times the amount of nitrogen adsorbed by the natural sample. These differences decrease in the region of high relative pressures, in which activated samples adsorb about 1.5 times more nitrogen than the natural sample. As the region of low relative pressures corresponds to microporosity, it can be concluded that the creation of microporosity takes place during acid treatment. This result agrees with the tendency observed in the shape of the nitrogen adsorption-desorption isotherms and is confirmed by the pore size distribution in the different solids. The textural evolution of montmorillonite under acid treatment has been widely studied. As it is well-known, montmorillonite is a layered silicate, by reason of which its structure is similar to that of saponite and the results obtained when treating both silicates would be compared. But there is a very important difference between these two silicates: the aluminic composition of montmorillonite gives it a high resistance to acid leaching, while saponite, due to its magnesic compositionsferromagnesic in the case of griffithitesis more easily altered by acid solutions. Thus, no changes in the structure of Gador montmorillonite have been observed when it has been treated under the conditions used in the present work.7 Recently, an increase in the mesoporosity has been described when montmorillonite has been treated with HCl under aggressive conditions (1-5 mol L-1 HCl, reflux heating for 1 h).1,5 If more concentrated solutions are used (6-8 mol L-1 HCl, reflux heating for 1 h), a passivation effect produced by the silica created during acid treatment has been reported, together with a change from type H3 to H2 in the hysteresis loops in the corresponding nitrogen isotherms. About saponite acid activation, the creation of microporosity during griffithite treatment has been reported recently.8 In this ferromagnesic saponite, the creation of microporosity is responsible, together with the disaggregation and delamination of the particles of the solid, for a very large increase in the surface area, from 35 m2 g-1 in natural saponite to 350 m2 g-1 in activated samples. The results now obtained when Yunclillos saponite is treated with HCl solutions are very different from those reported for montmorillonite and similar to those reported

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for griffithite. Especially the increase in microporosity observed during the treatment is similar to the results found for ferromagnesic saponite. These differences are probably due to the texture of the free silica obtained after the acid treatments (mild conditions for saponite and severe attack for montmorillonite). Final Remarks The acid leaching under very soft conditions results in very important transformations in the structure of Yunclillos saponite. Its octahedral cations are almost completely removed, and the attack on its tetrahedral sheet yields noncrystalline hydrated free silica, which affects the texture of the solids obtained after the acid treatments. This free silica is mainly responsible for the development in the surface properties of these solids, such as specific surface area and porosity. Thus, the surface area in the resulting solids is considerably high, more than 300 m2 g-1 (161 m2 g-1 in the natural sample). An important increase in microporosity is observed when the acidactivated samples are compared with the natural saponite. The acid leaching permits one to obtain easily high surface area acid-activated saponite, by reason of which the

Vicente et al.

adsorbent and catalytic properties of these solids are being investigated. In this sense, their potential applicability as cracking catalysts is now being studied. The data given in this paper, together with numerous literature data, indicate that the chemical composition of a smectite has large influence on its attackability by acid solutions, the octahedral sheet of magnesic and ferromagnesic smectites (saponite, iron saponite) being much more easily dissolved by acid leaching than that of aluminic silicates (montmorillonite). Also the increase in surface area when silicates are treated by acid leaching is very closely tied to their chemical composition, being related with the removal of the octahedral cations and the consequent destruction of their ordered structures, processes that result in the generation of free silica. Acknowledgment. The authors thank Drs. Marıá Luisa Rojas, Rosa Martıń Aranda, and Antonio Lo´pezPeinado (UNED University, Madrid) for the surface area measurements. The financial support of Junta de Castilla y Leoń (J0W0 Project) is also acknowledged. LA950501B

Characterization, Surface Area, and Porosity Analyses of the Solids

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