with a1-oligomers - The Clay Minerals Society

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Abstract--Griffithite, a high Fe content saponite (Griffith Park, California) was pillared with A1 polymeric solutions, using different A1/clay ratios. The cationĀ ...
Clays and Clay Minerals, Vol. 45, No. 5, 761-768, 1997.

C H A R A C T E R I Z A T I O N OF THE SOLIDS O B T A I N E D B Y PILLARING OF GRIFFITHITE (HIGH IRON C O N T E N T SAPONITE) WITH A1-OLIGOMERS MIGUEL ANGEL VICENTE,1 MERCEDES SUAREZ,2 MIGUEL ANGEL BAlqARES--MuIqozt AND JOSE MARTIN--PozAs 2 1Departamento de Qufmica Inorgfinica, Facultad de Qufmica, Universidad de Salamanca, Plaza de la Merced S/N, 37008-Salamanca, Spain 2 Area de Mineralogfa y Cristalograffa, Departamento de Geologfa, Facultad de Ciencias, Universidad de Salamanca, Plaza de la Merced S/N, 37008-Salamanca, Spain Abstract--Griffithite, a high Fe content saponite (Griffith Park, California) was pillared with A1 polymeric solutions, using different A1/clay ratios. The cation exchange began when Al-polycation solutions were added, being completed during the dialysis of the samples. Pillared solids were obtained by calcination of intercalated precursors at 500 ~ The content of A120 3 increased from 7.35% in the natural griffithite to about 14% in the pillared samples, equivalent to the fixation of about 1.4 mmol A1 per g of clay. The surface areas of the pillared griffithite were between 230-300 m2 g-1. The intercalation and pillaring of griffithite were easier than that of a less-crystalline nonferrous saponite. Key Words----All3-Keggin Polycation, Griffithite, Iron-Saponite, Pillaring, Porosity, Saponite. INTRODUCTION Intercalation of layered clays by bulk inorganic polycations and calcination of the intercalated precursors yield porous solids with regular porosity and a high number of acid sites. Polycations of different elements, such as A1, Zr, Ga, Cr or Ti, have been used. Montmorillonite is the most common layered silicate and it has been the most used in pillaring studies. The pillared solids improve the thermal stability of natural clays and have good catalytic properties (Figueras 1988; Occelli 1988). Saponite, a magnesic smectite, is a product of the hydrothermal alteration and weathering of basalts and ultramafic rocks. This mineral is much less common than the aluminous smectite montmorillonite. Saponite is mainly tetrahedraUy charged. Its octahedral sites are occupied by Mg(II) cations, but in its ferrous variety Fe(II,III) they substitute Mg(II) octahedral cations; this substitution is important when the ratio Mg/Fe is lower than 5:1 (De la Calle and Suquet 1988). Although montmorillonite is the preferred clay mineral used in pillaring studies (Lahav et al. 1978; Figueras et al. 1990; Fetter et al. 1994; Ge et al. 1994; Kloprogge et al. 1994; Lahodny-Sarc and Khalaf 1994; Mokaya and Jones 1994; Storaro et al. 1995), saponite has also been intensely studied in the recent years. Both natural saponites (Usami et al. 1992; Chevalier et al. 1992; Chevalier, Franck, Suquet et al. 1994; Chevalier, Franck, Lambert et al. 1994; Schoondheydt et al. 1992, 1993, 1994; Li et al. 1993; Malla and Komarneni 1993; Lambert et al. 1994; Suquet et al. 1994; Bergaoui, Lambert, Suquet and Che 1995) and synthetic saponites (Bergaoui, Lambert, Franck et Copyright 9 1997, The Clay MineralsSociety

al. 1995; Bergaoui, Lambert, Vicente-Rodrfguez et al. 1995) have been used in these studies, and new data about the mechanism of pillaring process have been obtained. Applicability of pillared saponite as catalysts (Usami et al. 1992; Chevalier, Franck, Lambert et al. 1994; Suquet et al. 1994) and in metal retention (Bergaoui, Lambert, Suquet and Che 1995) has been investigated. Griffithite (Griffith Park, California) is a high Fe content saponite with a ratio Mg:Fe of 1.85. The acid activation of this clay has been recently reported (Vicente Rodrfguez et al. 1994, 1995). In the present study, it was intercalated with AI oligomers. The pillared solids Were obtained by calcination of intercalated precursors at 500 ~ At the same time, a nonferrous saponite from Yunclillos (Toledo, Spain) was also pillared. The pillaring process and the textural properties of the solids obtained by pillaring of both saponites were compared. EXPERIMENTAL Pillaring of Saponite

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Griffithite was obtained by aqueous decantation of the < 2 wm fraction of the basaltic rock from Griffith Park deposit (California, supplied by Minerals Unlimited, Ridgecrest, California). The structural formula of the < 2 p.m sample was found to be: [5i6.92 Al1.08] 020 (OH)4 [Mg2.92 Fel.58 A10.28Tio.o4Mno.06] [Ca0.62Na0.20K0.04] and its cation exchange capacity (CEC) was 0.86 meq/g (Vicente Rodrlguez et al. 1994, 1995). Griffithite was intercalated with A1 oligomeric solutions, obtained by hydrolysis of A1C13.6H20 with

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cination of the intercalated solid in the same conditions as described for griffithite. Techniques

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Figure 1. XRD patterns of samples before (b) and after (a) dialysis in griffithite 7.5 series.

NaOH. The ratio OH-lAP + was 2.2 and the volume of the pillaring solutions was 450 mL. Under these conditions, most of A1 polymerizes to [ml1304(OH)24 (H20)12] 7+, a polycation designated as Keggin ion or A113, but other A1 species also exist in solution (Fu et al. 1991). These solutions were maintained at room temperature for 24 h and added to previously prepared suspensions of 6 g of the clay in 550 mL of water, followed by stirring for 24 h. Ratios of 2.5, 5.0 and 7.5 mmol A1 per g of clay were used. The samples were then centrifuged and washed by dialysis until chloride-free. Then they were centrifuged and dried at 60 ~ giving the intercalated solids (designated as IS2.5GR, IS5.0GR and IS7.5GR; the numbers refer to the A1/clay ratios). Calcination at 500 ~ for 4 h, with a heating rate of 1 ~ min -~ from room temperature up to 500 ~ gave the pillared compounds (designated as PS2.5GR, PS5.0GR and PS7,5GR). The < 2 p~m fraction of the nonferrous saponite from Yunclillos deposit (Toledo, Spain, supplied by TOLSA, Madrid) was also obtained by aqueous decantation. The structural formula of this sample is [Si7.42Alo.58102o(OH)4 [Mgs.16Fe0.14A10.26Ti0.018Mn0.004][Mg024Ca0.124Na0.020Ko.084], and its CEC is 1.15 meq/g (Vicente et al. 1996). This sample was intercalated with A1 oligomer solutions following the described procedures and using a 2.5 mmol A1 per g of clay ratio (IS2.5YU sample). Pillared Yuncfillos saponite (PS2.5YU sample) was obtained by cal-

Elemental analyses were carried out by plasma emission spectroscopy, using a Perkin-Elmer emission spectrometer, model Plasma II. Previously, the solids were digested under pressure, in a nitric-hydrofluoric acid mixture, in a polytetrafluoroethylene (PTFE) autoclave. X-ray diffractograms were obtained on a Siemens D-500 diffractometer at 40 kV and 30 m A (1200 W) with filtered CuKa line. The equipment is connected to a DACO-MP microprocessor and uses Diffract-AT software. For obtaining X-ray diffraction (XRD) patterns before and after dialysis, a few drops were taken from the suspensions and oriented films were prepared. Intercalated and pillared solids were studied by the powder method. Fourier transform infrared (FTIR) spectra were obtained in the region 4000-350 cm i on a Perkin-Elmer 1730 FTIR spectrometer, equipped with a 3700 data station, by the KBr pellet technique. Specific surface areas were determined from the corresponding nitrogen isotherms at 77 K, obtained from a Micromeritics ASAP 2010 analyzer, after outgassing the samples at 110 ~ for 8 h to a residual pressure of 10 5 m m Hg. The BrunauerEmmett-Teller (BET) method was used for the calculations. RESULTS AND DISCUSSION The intercalation and pillaring processes were studied by XRD at 4 different stages: 1) after addition of Al13 solution and stirring for 24 h (studied as oriented film); 2) after dialysis (also studied as oriented film); 3) the intercalated solids; and 4) the pillared solids. The diffractograms show that the ion exchange process begins, in all cases, during the addition of Al13 solutions, and it is completed after the dialysis process (Figure 1). For griffithite, reflections at 18.66, 19.44 and 18.75 ,~ appear after addition of the Al~3 solution (Table 1). These peaks have shoulders at lower spacings, thus proving that not all the sheets have been intercalated. In Yunclillos saponite, there is a broad peak centered at 16.95 ]k, indicating that the ion exchange process is less complete than in griffithite. After dialysis, the basal spacings remain constant in griffithite (18.71, 19.07 and 18.91 ,~), while in Yunclillos saponite, the spacing increases until 18.33 A. The peaks are now narrower and more symmetric than before dialysis, indicating that the washing of the samples completes the intercalation process, as has been observed by other authors (Lahav et al. 1978; Figueras et al. 1990; Fetter et al. 1994). The intercalated and pillared compounds show similar XRD patterns in all the series considered (Figure 2). In griffithite, the basal spacings of intercalated solids is about 18.8 ,~, which corresponds to the inter-

Vol. 45, No. 5, 1997

Pillaring of griffithite with Al-oligomers

Table 1. Basal spacings of 001 reflection of oriented films and powder samples and FWHM index of 001 reflection peak for powder samples. Sample

Basal spacing (~)

BD2.5GR AD2.5GR IS2.5GR PS2.5GR BD5.0GR AD5.0GR IS5.0GR PS5.0GR BD7.5GR AD7.5GR IS7.5GR PS7.5GR BD2.5YU AD2.5YU IS2.5YU PS2.5YU

18.66 18.71 18.70 17.42 19.44 19.07 18.87 17.58 18.75 18.91 18.89 17.73 16.95 18.33 18.07 17.43

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FWHM (~

0.576 0.673 0.531 0.617 0.523 0.646 1.345 1.525

Key: BD = oriented film before dialysis; AD = oriented film after dialysis; IS = intercalated solids; PS = pillared solids; GR = griffithite; YU = Yunclillos saponite. I

c a l a t i o n o f All3 p o l y c a t i o n s w i t h t h e i r m a j o r axis perp e n d i c u l a r to the layers o f the clay. T h e v a l u e decreases in p i l l a r e d solids to a b o u t 17.6 ,~ after c a l c i n a t i o n at 5 0 0 ~ In Yunclillos saponite, the i n t e r c a l a t e d sample h a s a b a s a l s p a c i n g o f 18.1 ,~, d e c r e a s i n g to 17.4 ,~ in the pillared sample. T h e full w i d t h s at h a l f m a x i m u m ( F W H M index), g i v e n in Table 1, i n d i c a t e that w e l l - o r d e r e d i n t e r c a l a t i o n c o m p o u n d s are obtained, the solids o b t a i n e d b y t r e a t m e n t o f griffithite b e i n g m o r e c r y s t a l l i n e t h a n t h o s e o b t a i n e d f r o m Yunclillos saponite. F o r the pillared c o m p o u n d s , the i n t e n s i t y o f the d ( 0 0 1 ) p e a k is similar in the 3 series. I n Table 2 the 0 0 1 , 0 0 2 , 0 0 4 a n d 0 0 6 s p a c i n g s o f i n t e r c a l a t e d a n d p i l l a r e d s a m p l e s are given. T h e p r o d u c t ( B r a g g I i n d e x X B a s a l spacings) i n d i c a t e s that s a m p l e s h a v e g o o d crystallinity, the greater d e v i a t i o n s c o r r e s p o n d i n g to 0 0 6 reflection. T h e c r y s t a l l i n i t y is s i m i l a r in the 3 series c o n s i d e r e d a n d b e t t e r for i n t e r c a l a t e d t h a n for pillared samples. T h e F T I R spectra o f natural a n d i n t e r c a l a t e d saponite are d i s p l a y e d in F i g u r e 3. O n l y s m a l l d i f f e r e n c e s are o b s e r v e d b e t w e e n the r a w a n d the i n t e r c a l a t e d saponite, d i f f e r e n c e s that c o n f i r m the i n t e r c a l a t i o n process. A t 3 6 1 6 c m ~, a b a n d is clearly o b s e r v e d in int e r c a l a t e d solids w h i l e it a p p e a r e d o n l y as a s h o u l d e r in griffithite, this b a n d b e i n g a s s i g n e d to A1-O-H stretch. T h e b a n d at 519 c m -1, c o r r e s p o n d i n g to the Si-O-A1 m o d e , i n c r e a s e s w i t h i n t e r c a l a t i o n , b e i n g esp e c i a l l y significant in the s a m p l e i n t e r c a l a t e d w i t h 2.5 m m o l A1 p e r g o f clay, w h i l e in natural griffithite it appears o n l y as a shoulder. A t 9 2 0 c m -1, a s h o u l d e r d u e to ( M g , A 1 ) - O H b o n d s c a n b e o b s e r v e d in the int e r c a l a t e d solids ( K l o p r o g g e et al. 1994). T h e s e m o d e s are m o r e i n t e n s e in the i n t e r c a l a t e d t h a n in the natural

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:

30

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4U

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Figure 2. XRD patterns of samples intercalated (a) and pillared (b) in griffithite 2.5 series.

griffithite; the d i f f e r e n c e s b e t w e e n the n a t u r a l a n d the i n t e r c a l a t e d s a m p l e s c a n b e clearly o b s e r v e d b e c a u s e o f the low c o n t e n t o f AI in the natural sample. A t the s a m e time, the b a n d c o r r e s p o n d i n g to F e - O H b o n d s , w h i c h a p p e a r s at 5 8 4 c m ~ in raw griffithite, is n o t o b s e r v e d i n the i n t e r c a l a t e d solids. T h e A1 c o n t e n t in n a t u r a l a n d i n t e r c a l a t e d solids is g i v e n in Table 3 a n d i n c r e a s e s in i n t e r c a l a t e d griffithite b y a b o u t 7 % w i t h r e s p e c t to the r a w material. T h e p e r c e n t i n c r e a s e s c o r r e s p o n d to the fixation o f a b o u t 1.4 m m o l A1 p e r g o f clay. T h i s a m o u n t is v e r y s i m i l a r in all s a m p l e s a n d i n d e p e n d e n t o f the c o n c e n t r a t i o n o f A1 in the i n t e r c a l a t i n g solutions. In Yunclillos saponite, a 6 . 9 3 % i n c r e a s e in A1203 c o n t e n t is o b s e r v e d in

Table 2. The 001,002, 004 and 006 spacings of intercalated and pillared samples. Sample

d(.0J)l) (A)

d(002) (]~)

d(0()4) (A)

d(O06) (A)

IS2.5GR PS2.5GR IS5.0GR PS5.0GR IS7.5GR PS7.5GR IS2.5YU PS2.5YU

18.70 17.42 18.87 17.58 18.89 17.73 18.07 17.43

9.47 8.97 9.47 8.95 9.49 8.97 t t

4.57 4.56 4.59 4.55 4.59 4.58 4.53 4.52

3.17 3.20 3.17 3.19 3.19 3.20 3.20 3.16

t Masked by a mica peak at 9.9 ]~.

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