The formation of Mg-saponites is explained by a model in which an increased ... CEC meq/100 g d,,o, (~). LTSAP. Fluid Wash. M. Mg 2'. Total. 20"C b (fl0. NA.
Clays and Clay Minerals, Vol. 42, No. I, i 8 - 2 2 , I994.
C H A R A C T E R I Z A T I O N OF Mg-SAPONITES SYNTHESIZED FROM GELS C O N T A I N I N G A M O U N T S OF Na t, K § Rb § Ca 2§ Ba 2+, OR Ce 4+ EQUIVALENT TO THE CEC OF THE SAPONITE* J. THEO KLOPROGGE,l** JOHAN BREUKELAAR, 2 JOHN W. GEUS, 3 AND J. BEN H. JANSEN I*** Department of Geochemistry, Institute of Earth Sciences, University of Utrecht Budapestlaan 4, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands 2 Koninklijke/Shell-Laboratorium Amsterdam (Shell Research B.V.) P.O. Box 3003, 1003 AA Amsterdam, The Netherlands 3 Department of Inorganic Chemistry, University of Utrecht P.O. Box 80.083, 3508 TB Utrecht, The Netherlands Abstract--Saponites were hydrothermally grown in the presence of amounts of NH4 + , Na § , K § Rb +, Ca 2+, Ba 2+, and Ce 4+ equivalent with the CEC of the saponite (155 meq/100 g), with or without F , at a temperature of 200~ for 72 hr. X R D and CEC data revealed the formation of a two-water-layer saponite with mainly Mg 2§ as interlayer cation. Dehydration occurred between 25* and 450.C and dehydroxylation occurred in two steps between 450 ~ and 790"C and between 790* and 890.C. The relatively small length of the b-axis between 9.151 and 9.180/~ is explained by considerable octahedral AI substitution (between 0.28 and 0.70 per three sites) and minor tetrahedral AI substitution (between 0.28 and 0.58 per four sites). Under the synthesis conditions applied in this study, less than 13% of the interlayer sites are occupied by Na*, K +, and Rb+; between 13.3% and 21% by Ca ~+ and BaZ+; while N H 4 + gives the highest value at 34%. The remaining sites are mainly filled by Mg ~+. Ce 4+ is not found in the saponite structure due to the formation of cerianite, CeO2. The presence of F - had little influence on the saponite composition. The formation of Mg-saponites is explained by a model in which an increased bayerite formation resulting in a higher octahedral ml 3+ substitution and more Mg 2+ in solution. Mg 2+ is preferentially incorporated compared with the other interlayer cations due to its smallest ionic radius in combination with its 2+ charge. Key Words--Cation exchange capacity, Saponite, Synthesis, Thermal analysis, X-ray powder diffraction.
INTRODUCTION T h e r e s e a r c h o f s y n t h e t i c s a p o n i t e s h a s m a i n l y foc u s e d o n N a - s a p o n i t e s ( K o i z u m i a n d Roy, 1959; Suq u e t et al., 1977; Iwasaki et al., 1989) a n d o n t h e i r s a t u r a t i o n w i t h v a r i o u s o t h e r c a t i o n s in t h e i n t e r l a y e r r e g i o n ( S u q u e t e t a l . , 1977; 1981a, 1981b, 1982, 1987). Recently, K l o p r o g g e (1992) r e p o r t e d o n the direct synt h e s i s o f a m m o n i u m - s a p o n i t e , w h i c h is i n t e r e s t i n g as a p o t e n t i a l acid catalyst after c o n v e r s i o n o f t h e NH4 + to H +. T h i s s y n t h e s i s d i m i n i s h e s t h e catalyst p r e p a r a t i o n r o u t e b y o n e step, t h e a m m o n i u m c a t i o n exchange. A m a j o r d r a w b a c k o f this s y n t h e s i s was t h a t t h e i n t e r c a l a t i o n o f AI 3+ in t h e i n t e r l a y e r c o u l d n o t be controlled, resulting in non-swelling s a p o n i t e s w i t h only low a m o u n t s o f NH4 + and, thus, low catalytic activity. S a p o n i t e is a t r i o c t a h e d r a l 2:1 s m e c t i t e w i t h a n ideal c o m p o s i t i o n g i v e n b y M~Mg3AI~Si4_~O~0(OH,F)2,
* This paper is a joint contribution of the Debye Institute, University of Utrecht, The Netherlands and Shell Research B.V. ** Present address: TPD-TNO, Department of Inorganic Materials Chemistry, P.O. Box 595, 5600 AN Eindhoven, The Netherlands. *** Present address: Bowagemi b.v., Prinses Beatrixlaan 20, 3972 AN Driebergen, The Netherlands. Copyright 9 1994, The Clay Minerals Society 18
where M represents one equivalent of the interlayer cation, e.g., N a +, K +, R b + , NH4 +, o r 1/2 C a 2+ , 1/2 Ba 2+ , l/z M g 2+, or e v e n 1/3 A13+; a n d w h e r e x c a n range f r o m a p p r o x i m a t e l y 0.3 to 0.6. A single 2:1 layer is n o r m a l l y o r g a n i z e d w i t h a c e n t r a l sheet o f o c t a h e d r a l l y coordin a t e d M g z+ w i t h sheets o f t e t r a h e d r a l l y c o o r d i n a t e d Si 4+ o n b o t h sides. T h e partial s u b s t i t u t i o n o f SP + b y A13+ causes the t e t r a h e d r a l sheet to h a v e a n o v e r a l l n e g a t i v e charge, w h i c h is c o m p e n s a t e d by i n t e r l a y e r cations. Actually, s u b s t i t u t i o n o f A13+ also o c c u r s at o c t a h e d r a l a n d i n t e r l a y e r sites. S u b s t i t u t i o n o f M g 2+ at t h e i n t e r l a y e r sites m a y a d d i t i o n a l l y p r o c e e d d u r i n g t h e h y d r o t h e r m a l synthesis. S a p o n i t e s w i t h N a +, C a 2+ , M g 2+, o r Ba 2+ as i n t e r l a y e r c a t i o n s c o n t a i n at a relative h u m i d i t y o f a p p r o x i m a t e l y 6 0 % two w a t e r layers resulting in a basal spacing o f a p p r o x i m a t e l y 1 5 - 1 6 A, w h e r e a s s a p o n i t e s w i t h i n t e r l a y e r c a t i o n s such as K + o r NH4 + h a v e o n l y o n e w a t e r layer, r e s u l t i n g in a basal s p a c i n g o f a p p r o x i m a t e l y 12.5 ~ ( S u q u e t et al., 1975). T h e o b j e c t i v e o f this i n v e s t i g a t i o n was to identify t h e influence o f v a r i o u s i n t e r l a y e r c a t i o n s a n d F o n the s y n t h e s i s in o r d e r to p r o h i b i t t h e i n c o r p o r a t i o n o f A13+ in t h e interlayer, as d e s c r i b e d for N H 4 - s a p o n i t e (Kloprogge, 1992; Kloprogge et aL, 1993). T h e r e f o r e , t h i s investigation reports the synthesis of saponites from s t o i c h i o m e t r i c gels c o n t a i n i n g N a +, K +, R b +, C a z+,
Vol. 42, No. 1, 1994
Mg-Saponites synthesized from gels
Table 1. Experimental runs at 200~ for 72 hr. Run LTSAP
pH Fluid
pH Wash
NA K CA BA CE NHF NAF KF RBF CAF BAF
4.46 4.41 -4.47 4.33 4.43 4.41 4.42 4.62 4.32 4.43
4.53 4.48 4.48 4.58 4.54 4.54 4.52 4.50 4.70 4.52 4.56
M
CEC meq/100 g Mg2' Total
8.4 1 3 4 . 0 10.5 140.0 13.3 101.4 21.2 77.4 0.0 74.3 40.0 19.8 4.8 85.0 13.5 74.4 16.0 76.1 13.4 75.5 21.2 88.0
142.4 150.5 114.7 98.6 74.3 58.8 89.8 87.9 92.1 88.9 109.2
d,,o, (~) 20"C
14.1 15.3 14.6 14.7 14.7 14.7 --15.1 14.0 --
Approximately 125 g of the above gel was hydrothermally treated for 72 hr at 200~ under autogenous water pressure. Kloprogge et al. (1993) have shown that under these conditions a crystalline yield near 100% is reached. After cooling, the solids were separated from the coexisting hydrothermal fluid, washed twice with distilled water to r e m o v e possible free salts, centrifuged, and dried overnight at 120~ Characterization o f the solid product was based on the fraction smaller than 64 tzm (Kloprogge, 1992; Kloprogge et aL, 1993). The coexisting hydrothermal fluid was analyzed by ICP-AES. The pH o f the coexisting hydrothermal fluid at room temperature, as well as the pH of the water after washing the solid for the first time, were measured with a Consort P514 pH meter. X R D patterns were recorded with a Philips PW 1050/ 25 diffractometer, using C u K a radiation. Heating stage X R D was carried out at 350~ using a H T Guinier C u K a I (Enraf Nonius FR553) focusing powder camera. T G A was made with a du Pont 1090 Thermal Analyzer using a heating rate of 10~ under a N 2 flow of 50 ml/min. Elemental analyses o f the solid products were obtained by XRF. The cation exchange capacity (CEC) was determined by exchanging the product with a solution o f 1 N a m m o n i u m chloride brought to pH 7 by addition of a m m o n i u m hydroxide. The exchanged solution was analyzed with ICP-AES for the interlayer cations, including i g 2+ and AP +.
b (fl0
9.163 9.180 9.169 9.173 9.172 9.164 9.157 9.152 9.173 9.151 9.160
Ba 2+, or Ce 4+ as interlayer cations that may be replaced during the synthesis by Mg 2§ or even AP+; the influence o f F - on the synthesis of saponite; and the influence of partial replacement o f hydroxyl groups by F on the saponite characteristics. Incorporation o f Mg 2§ and A13+ on the interlayer must be reflected by changes in the 2:1 layer octahedral and tetrahedral substitution due to the stoichiometric starting composition. The synthetic products were characterized by X-ray powder diffraction (XRD), X-ray fluorescence (XRF), thermogravimetric analysis (TGA), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The data will be compared with those reported by Suquet et al. (1975, 1981a, 1981b), who exchanged Nasaponite to saturation with the above cations. EXPERIMENTAL METHODS A homogeneous powder mixture of amorphous silica (SiO2), aluminum triisopropylate (AI[OCH(CH3)2]3), and magnesium acetate-tetrahydrate ([CH3COO]2Mg. 4H20) (Kloprogge, 1992; Kloprogge et al., 1993) was mixed with a solution containing the desired cation in the form o f a hydroxide or fluoride salt. The resulting stoichiometric gel had the theoretical saponite composition o f Mo.6Mg3mlo.6Si3.4Oto(On)2 , where M represents one equivalent of the interlayer cation, i.e., Na § K +, Rb +, Ca 2+, Ba 2+, or Ce 4+ .
19
RESULTS The results of the different runs are summarized in Tables 1, 2, and 3. The runs are denoted, for example, by L T S A P N A , when the run was performed with Na § as the ion considered to be taken up at the interlayer positions. When F - ions were also present, the run is indicated by LTS A P N A F . X R D o f the synthesis products o f all experiments revealed mainly (hkl) saponite reflections. The (001) reflections are very weak (LTSAPNA, -K, -CA, -BA, -CE, -NHF, -RBF, -CAF), or absent (LTSAPNAF, -KF, -BAF). Experiment LTSAPCE led to an X-ray pattern
Table 2. X-ray fluorescence analyses of the saponite bulk samples. LTSAP
SiO_, wt.%
AI_,O~ wt.%
MgO wt.%
Na20 wt.%
NA K CA BA CE NHF NAF KF RBF CAF BAF
51.77 52.63 54.13 54.34 55.84 52.84 51.99 53.06 52.41 55.19 51.34
9.16 9.47 9.01 9.35 9.64 11.98 12.05 12.45 9.75 9.07 13.04
24.71 24.71 23.55 21.23 19.40 21.72 22.88 22.22 22.22 23.05 19.40
bd
Run
bd = below detection limit.
K,O wt.%
Rb~O wt.%
CaO wt.%
BaO wt.%
CeO, ",r %
F wt.%
0.57 0.31 2.04 2.10 bd 0.78 2.94 0.33 2.31
0.06 0.06 0.11 0.20 0.02 0.03
20
Kloprogge, Breukelaar, Geus, and Jansen
Clays and Clay Minerals
Table 3. Structural formulae of the Mg-saponites, based on XRF and CEC data. Run LTSAP
Formula
NA K
Nao.oaMgo.27(Mg2.samlo.2s[[]o.,4)(Alo.s7Si3.a3)O~o(OH)2 Ko.04Mgo.27(Mgz.57Alo.29130.t,)(Alo.58Si3.42)O,o(OH)2
CA
Cao.03Mgo. L9(Mg2., ~A10.391[]0 20)(A10.4,Si3.59)O,o(OH)2
BA CE NHF NAF
Bao.o,Mgo.~5(Mg2.zgAlo.47Ulo2,)(Alo.3sSi362)O,o(OH)2 Ceo.ooMgo~4(MgzIIAlo6oE]o~,9)(Alo.zsSi3.72)Oio(OH)2 (NH4)o.~Mgo.o4Alo.oT(Mgz.zTAlo.49[]o.24)(Ato.4sSi3.ss)Olo(OH)l.ogFo.ol Nao.o2Mgo.16Alo.o4(Mgz.z4Alo.5oDo.26)(mlo.46Si3.54)Oio(On)l.99Fo.ol
KF
Ko.05Mgo.~4Alo.05(Mg2.23A10 5~l"qo.26)(Alo.49Si3.5,)O~o(OH),.98Fo.o2
RBF CAF BAF
Rbo.06Mgo 14(Mgz~6Alo.49[S]o.25)(Alo.34Si3.66)Oto(On)t.96Fo.04 Cao.03Mgo.~4(Mg2.34Alo ,4[]o.22)(Alo.a4Si3.66)O to(OH) L99Fo.o, Bao.o,Mgo ~7(Mg~.95Alo.70t-qo3~)(Alo42Si~.~s)O~o(OH)L99Fo.o~
showing additionally 2 wt. % (calculated from the a m o u n t of Ce in gel) cerianite, CeOa (Figure 1). The presence of corundum (-
