Clays and Clay A4inerals. Vol. 32, No. 2, 99-107, 1984.

CROSS-LINKED SMECTITES. III. SYNTHESIS A N D PROPERTIES OF H Y D R O X Y - A L U M I N U M HECTORITES

A N D FLUORHECTORITES

J. SHABTAI, MARIA ROSELL, 1 AND M. TOKARZ 2

Department of Fuels Engineering, University of Utah Salt Lake City, Utah 84112

Abstract--Interaction of La 3§ and Ce3+-exchanged hectorites with oligomeric hydroxy-A1 cations results in the formation of cross-linked hectorites possessing moderately high surface areas (-220-280 mV~) and high thermal stability. The basal spacings of these products were generally in the range 17.0-18.0 A, the exact d(001) value depending on the age of the hydroxy-A1 oligomeric solution and on the pH of the starting hectorite dispersion. Interaction of Li- or Ce-fluorhectorite with hydroxy-A1 oligocations produced the corresponding cross-linked fluorhectorites, which showed markedly higher basal spacings (18.2-20.0 /~ for air-dried samples), surface areas (-300-380 m2/g), and thermal stability, as compared with those of the cross-linked hectorites. The cross-linking agent applied in the synthesis of the bydroxy-Al hectorite and fluorhectorite products consisted of a solution of hydroxy-A1 oligomers aged for periods of 7 to 27 days. A constant ratio of 2.0 mmole A1/g of smectite was used in all preparations. The high basal spacings and porosity of the newly synthesized products are consistent with a structure similar to that previously proposed for cross-linked hydroxy-A1 montmoriUonite. Key Words--Cross-linking, Fluorhectorite, Hectorite, Hydroxyaluminum, Molecular sieve, Surface area, Synthesis, Thermal stability.

INTRODUCTION

The process of smectite cross-linking has recently attracted considerable attention (Pinnavaia, 1983). It provides a versatile method for preparing a new class of molecular sieves which are structurally different from natural and synthetic zeolites and which offer new pos- sibilities for application as catalysts and adsorbents (Shabtai, 1979, 1980; Shabtai and Lahav, 1980; Vaughan and Lussier, 1980; Vaughan et al., 1979, 1981a, 1981b; Lussier et al., 1980).

Cross-linked smectites (CLS), referred to by some authors also as pillared interlayered clays (PILC), are characterized by fixed porosity and are fundamentally different from smectite systems obtained by conven- tional intercalation of neutral molecules in the inter- lamellar space. The cross-linking process involves in- teraction of the polyanionic smectite layers with di- or polycationic species derived from suitable organic or inorganic compounds. With inorganic cross-linking agents, thermally stable CLS products can be derived (Brindley and Sempels, 1977; Lahav et al., 1978; Ya- manaka and Brindley, 1979; Vaughan and Lussier, 1980).

In 1971 Barter and Jones prepared porous forms of montmorillonite, hectorite, and fluorhectorite by ion exchange with tetramethylamrnonium and ethylene- d iammonium cations. The products, however, had very

On leave of absence from EKA AB, Surte, Sweden, 1982. z On leave of absence from the University of Mining and

Metallurgy, Krakow, Poland, 1981 - 1983.

small interlamellar spacings (3-4 ~k) and showed low resistance to swelling. Subsequently, cross-linked or- gano-smectite (CLOS) systems possessing larger inter- layer spacings (Ad(001) > 5 /~)3 and higher resistance to swelling were prepared by using di- or polycations derived from rigid or cage-like amines as cross-linking agents. The first bona fide CLOS system of this type (Ad(001) = 5.1 ~; lateral pore size, ~6.0 ~,) was pre- pared by ion exchange of Na-montmoril lonite with the d i a m m o n i u m ion of 1,4-diazabicyclo[2,2,2]octane (Shabtai et al., 1976; Mortland and Berkheiser, 1976). This system showed typical molecular sieve properties and high catalytic activity for esterification of carbox- ylic acids with alkanols. Products with larger Ad(001) values were prepared by using bulkier cross-linking di- or tetracationic species derived from 1,4-diaminoad- amantane , te trakis (p-aminophenyl )methane , and 2,2',6,6'-substituted benzidines. A general feature of such CLOS systems is that simultaneous vertical at- tachment of cross-linking polycationic species to an- ionic sites of opposite aluminosilicate surfaces be- comes requisite due to steric and electronic effects which exclude alternative orientation modes (Shabtai, 1979).

Derivatives showing some similarities to CLS sys- tems were also prepared by interaction of Na-hectorite with Fe 2+- and Cu 2§ 1,10-phenanthroline chelates, or

3 The term Ad(001), recently introduced by Pinnavaia et al. (1979), denotes the interlayer spacing in smectites; Ad(001) = d(001) - t, where t is the thickness of the smectite unit layer.

Copyright �9 1984, The Clay Minerals Society 99

100 Shabtai, Rosell, and Tokarz Clays and Clay Minerals

with Fe 2+-, Cu 2+-, and Rh2+-tris bipyridyl complexes (Berkheiser and Mortland, 1977; Traynor et al., 1978). The preparat ion of such derivatives involved ion-ex- change with the complex ion, accompanied and/or fol- lowed by intersalation of excess complex salt beyond the exchange capacity of the smectite. In another study (Loeppert et aL, 1979), intersalated phases were pre- pared by interaction of Na-montmori l loni te or n-bu- ty lammonium vermiculite with sulfate salts of Fe 2+-, Co 2§ or Ni2+-tris bipyridyl or tris 1,10-phenanthroline complexes. Products containing 1.0 cation-exchange capacity (CEC) equivalent of the complex ion collapsed upon firing at 550~ whereas products containing an excess of intercalated complex salt remained expanded to 18 ~ (for montmori l loni te systems) and 29 /~ (for vermiculite derivatives) after the same heat treatment, due to the presence of residual, inorganic decomposi- tion products in the interlamellar space. There was no indication as to the possible catalytic activity of such systems, prior to or after firing.

In an interesting recent development, cationic hy- drogenation catalyst precursors, e.g., Rh(PPh3)+n, where n = 2 or 3, have been intercalated in hectorite (Pin- navaia et al., 1979; Quayle and Pinnavaia, 1979; Ray- thatha and Pinnavaia, 1981, 1983; Pinnavaia, 1983). Such systems showed good catalytic activity for hy- drogenation of alkenes, alkadienes, and alkynes, and considerable difference in reaction specificity when compared to the same rhodium complex in solution. Hydroformylat ion catalysts, based on intercalated RhH(CO)x(P-P+)2 complexes (where P-P+ denotes a phosphonium-phosphine ligand), were also prepared (Pinnavaia, 1982; Farzaneh and Pinnavaia, 1983). The application o f the above intercalated systems as cata- lysts necessitates the use of a solvent, e.g., methanol, which for Rh(PPh3)+n-hectorite causes l imited swelling (Ad(001) = 7.7 /k), allowing the reactant to penetrate into the interlamellar space. Although such metal com- plex-smectite catalysts cannot be considered as CLS systems per se, their potential importance for func- t ionalization of inorganic cross-linked frameworks (see below) is obvious.

A turning point in the synthesis of bona fide CLS molecular sieves was achieved by cross-linking of smectites with fully inorganic, positively charged species, e.g., hydroxy-A1 oligocations (Brindley and Sempels, 1977; Lahav et al., 1978; Vaughan et al., 1979, 1981 a, 198 l b; Shabtai and Lahav, 1980; Vaughan and Lussier, 1980; Brindley and Kao, 1980) or tetra- meric hydroxy-Zr cations (Yamanaka and Brindley, 1979). The cross-linked products showed high thermal stability, considerable surface areas, and intrinsic cat- alytic activity. When properly functionalized, such CLS molecular sieve catalysts showed increased activity and selectivity for certain types of reactions. For example, acidic (H +, La 3+, and Ce 3+) forms of cross-linked mont-

mori l lonite showed high catalytic cracking activity (Shabtai, 1980; Shabtai et al., 1981).

In continuation of previous work on the preparation of CLS systems, a study of the cross-linking of hec- torites and synthetic fluorhectorites with hydroxy-Al oligocations was undertaken. The present paper reports on the synthesis of cross-linked hectorites and fluor- hectorites and their characterization by X-ray powder diffraction, surface area and thermal stability deter- minations, transmission electron microscopy, and el- emental analysis.

EXPERIMENTAL

Start ing materials

Hectorites. A natural Na-hectorite sample from Hec- tor, California, was obtained from The Clay Minerals Society. Some quartz and calcite present as impurit ies was removed by fractionation, using conventional sedimentat ion techniques. The < 1-#m fraction used in the study contained only traces of the above im- purities as determined by X-ray powder diffraction analysis. Homoionic Ce 3+- and La3*-exchanged hec- torites were prepared by subjecting the purified Na- hectorite to repetitive (4 times) ion exchange at 90~ with 0.4 N aqueous CeC13 and LaC13 solutions, re- spectively. The Ce 3+- and La3+-exchanged hectorites were washed with deionized water until chloride-free and air dried. Basal spacings of the air-dried Na-, Ce-, and La-hectorite samples were 12.7, 14.7, and 14.6 /k, respectively; the surface areas of all samples were less than t00 mVg. Elemental analysis of the Ce- and La-hectorites showed a Ce content of 10.6% and a La content of 9.40%, respectively (see also Table 3).

Fluorhectorites. Li-fluorhectorite samples were pre- pared according to a method described by Barrer and Jones (1970). The solid state synthesis was performed using reagent grade LiF, MgO, MgF2, and SiO2 gel as reactants. Prior to the reaction, SiO2 and MgO were ignited at 800~ for 3 hr, and LiF and MgF2 were dried at 150~ for 3 hr. The four reactants were then inti- mately mixed by grinding in an appropriate molar ratio (LiF:MgO:MgF2:SiO2 = 1:2:1:4). This reactant mix- ture was heated in a closed plat inum crucible at 800~ for 24 hr. The product was cooled to 500~ at a con- trolled rate of 30~ and then transferred to a des- iccator and allowed to cool to room temperature. The Li-fluorhectorite product was washed with hot deion- ized water and thoroughly purified by sedimentation. X-ray powder diffraction analysis of the < 1-urn frac- tion of the product showed the presence of only minor amounts (<5%) of impurities, mainly quartz, amphi- boles, and, in some samples, LiaSiF 6. Some samples of Li-fluorhectorite were prepared by an alternative pro- cedure in which mixtures of the above reactants were heated at 850~ for 2 hr to form melts (Barrer and

Vol. 32, No. 2, 1984 Properties of hydroxy- and fluorhectorites 101

5

\ ' 4 , \ \ \ \ \ \ \ \ \ \ " \ " ; , \ \ \ \ \ \ \ \ \ \ \ \

Figure 1. Apparatus for preparation of cross-linked smectites. 1 = 20-liter container with smectite dispersion; 2 = 500-ml container with cross-linking agent; 3 = tubing pump (flow rate range, 5-1500 ml/min); 4 = tubing pump (flow rate range, 1- 1000 ml/hr); 5 = mixing chamber (250 ml); 6 = magnetic stirrer; 7 = 20-liter receiver for CLS product.

Jones, 1970). Otherwise, the same procedure as de- scribed above was used. The final purity of the < 1-t~m Li-fluorhectorite product obtained by the two proce- dures was essentially the same (-95%). Changes in the composition of the starting reactant mixture, e.g., in- creases in the molar amounts of LiF and MgF2, gave products which, after washing out of the excess soluble fluorides, were similar to those obtained by using the above indicated molar ratios. Purified Li-fluorhectorite fractions showed d(001) values in the range of 12.4- 12.8 • and very low N 2 (BET) surface areas (< 10 m2/ g).

Hydroxy-Al oligomeric solution

The method of preparation of the cross-linking agent, consisting of a solution of hydroxy-Al oligomers, was the same as previously described (Lahav et aL, 1978; Shabtai and Lahav, 1980). It involved gradual addition of aqueous 0.2 M NaOH to a 0.2 M A1C13 solution until a final pH of 4.35 was attained, corresponding to an OH/A1 ratio of 1.85. This solution was aged at room temperature for 1-4 weeks prior to its use.

Preparation of cross-linked smectites

The eross-linking experiments were carried out in a recently designed apparatus (to be described elsewhere) equipped with reactant containers, two tubing pumps, a mixing chamber, and a receiver for the CLS product (Figure 1). A dilute aqueous dispersion of the starting smectite was prepared by prolonged mixing (15 hr) with a magnetic stirrer (in some cases dispersion for a period of 5-15 rain in an ultrasonic bath was used). The pH of separate batches of the smectite dispersion was adjusted prior to the cross-linking step to different values in the range 4.5-7.8 using aqueous 0.1 M po- tassium hydrogen phthalate, in conjunction with either aqueous 0.1 N NaOH or 0.05 N HCI. The two reac-

tants, i.e., the buffered smectite dispersion (A) and the aged cross-linking solution (B), were pumped at ap- propriate relative flow rates to the mixing chamber where fast cross-linking and flocculation took place. A ratio of 2 mmole A1 per gram of smectite was applied in all of the preparations. The cross-linked smectite formed in the mixing chamber was continuously re- moved from the reaction zone and collected in the product receiver. After completion of the reaction (usu- ally 3-5 rain), the product mixture was allowed to stand for 15 hr, the supernatant was syphoned off, and the solid precipitate was washed with deionized water until chloride-free and freeze-dried.

Analysis of CLS products

X-ray powder diffraction (XRD) analyses of cross- linked smectites were performed with oriented samples prepared by spreading about 0.5 ml of a water suspen- sion of the smectite on a glass slide and drying the slide at room temperature. Homoionic smectites, used as starting materials for preparing the CLS systems, were analyzed for purity using powdered samples. The XRD patterns were obtained with a Norelco diffractometer using CuKa radiation and a graphite monochromator.

Surface areas of CLS samples were determined using an Orr Surface Area Pore Volume Analyzer, Model 2100 (Micrometrics Instrument Company). The sam- ples were first degassed at 230~176 (0.001 tort) and their surface areas determined at liquid nitrogen tem- perature, using N2 as the sorbate. Inasmuch as multi- layer adsorption of N2 (kinetic diameter = 3.64/k) in CLS systems is probably prevented due to steric re- strictions, the Langmuir equation, which is more ap- propriate for monolayer adsorption, was used in the surface area calculations.

The thermal stability of the CLS products was ex- amined by subjecting them to heat treatment at 250 ~

102 Shahtai, Rosell, and Tokarz Clays and Clay Minerals

and 400~ and then determining their basal spacings and surface areas. The samples were pretreated by heat- ing in a Pyrex glass-tube reactor under a flow of dry N2 for 3 hr.

Some CLS samples were also examined by trans- mission electron microscopy (TEM), using the thin sec- tion technique. The powder samples were first embed- ded in Spurr resin (Spurr, 1969) and then sectioned with a diamond knife on a Porter-Blum MT-2 micro- tome. Micrographs were taken with a Philips, Model EM400HTG electron microscope.

Elemental analysis of the starting smectites and of the CLS products were performed by inductively cou- pled plasma spectrometry (ICPS). Samples were dis- solved in a mixture of HF, HC1, HNO3, and HC104 and analyzed with a Bausch & Lomb ICPS apparatus using as an excitation source an argon plasma (flame temperature, ~ 10,000~ Duplicate analyses of sev- eral samples showed very good reproducibility. The fluoride content was separately determined with a spe- cific ion electrode (Orion & Orion Meter, Model 407A), using the method of standard additions. SiO2 was sep- arately determined by the molybdenum blue colori- metric method (Kolthoff and Elving, 1962).

RESULTS AND DISCUSSION

La s+- and Ce3§ cross-linked with hydroxy-Al oligomers (La-AI- CLH and Ce-AI- CLH)

The basal spacings and surface areas of La 3§ and Ce3+-exchanged hectorites, cross-linked with hydroxy- A1 oligomers, were investigated as a function of the pH (4.5 to 7.8) of the starting hectorite dispersion. The d(001) and surface areas of the cross-linked products (La-A1-CLH and Ce-A1-CLH) obtained from hectorite dispersions having different pH are summarized in Ta- ble 1 and in Figures 2 and 3. As seen, the pH of the starting RE3§ hectorite dispersion had a marked effect on the d(001) value as well as on the shape and intensity of the XRD peaks. Likewise, the pH influenced the surface areas of the RE-A1-CLH products. For La-AI-CLH, d(001 ) values of 17.0 to 17.4 /~ were found for the pH range of 4.8-5.5 /~, using a 22-day aged hydroxy-A1 oligomeric solution in the cross-linking step. The highest value (17.4/k) was ob- served using a starting dispersion with pH = 5.0, whereas an increase in the pH to 7.8 caused sharp decrease in d(001). Furthermore, the XRD peaks of products formed at pH 4.8-5.5 were much sharper and more intense than those of products formed at pH 7.8 (Figure 2). The surface areas showed a similar trend, with values of 220-276 mE/g for products formed at pH 4.8-5.5 and a considerably lower value (168 mV g) for those formed at pH 7.8.

For Ce-A1-CLH (Figure 3), d(001) gradually in- creased (from 16.0 to 17.5 ~) with increase in the pH of the starting hectorite dispersion, and then remained

Table l. Basal spacings and surface areas of La-AI-CLH and Ce-At-CLH as a function of the pH of the hectorite disper- sion.'

p H o f the hectori te Surface area 4

Sample d ispers ion 2 d(00 l), (•)3 (rnVg)

La-AI-CLH/32 4.8 17.0 276 La-AI-CLH/33 5.0 17.4 260 La-A1-CLH/30 5.5 17.0 220 La-A1-CLH/36 7.8 14.1 168 ...............................................................................................................................................

Ce-A1-CLH/20 4.5 16.0 122 Ce-A1-CLH/19 5.4 16.9 253 Ce-AI-CLH/58 6.0 17.3 236 Ce-A1-CLH/23 6.5 17.5 227 Ce-A1-CLH/22 7.5 17.2 149

l The starting La 3§ and CeS+-hectorite dispersions were pre- pared by mixing with an ultrasonic bath at 25~ for 5 min (see Experimental).

2 The pH values of the srnectite dispersion were adjusted prior to the cross-linking step (see Experimental); the pH of the 22-day aged oligomeric hydroxy-A1 solution was 4.4 in all experiments.

3 Measured with air-dried samples. 4 Measured after preheating the sample at 250~ (0.001 torT).

only slightly lower (17.2 ~) for pH = 7.5. The surface areas were in the range of 227-253 mZ/g for products formed at pH 5.4-6.5, but were considerably lower for those formed at higher or lower pH. Ultrasonic dis-

17.4 ~,~.~/ 1 17.0A 1

pH

7.8

5.5

5.0

4.8

I I I I 12 10 8 6 4

28

Figure 2. X-ray powder diffraction patterns of La-A1-CLH as a function of the pH of the starting hectorite dispersion (CuKa radiation).

Vol. 32, No. 2, 1984 Properties of hydroxy- and fluorhectorites 103

17.2A

pH

7.5

6.5

6.0

5.4

4.5

12 10 8 6 4

20

Figure 3. X-ray powder diffraction patterns of Ce-A1-CLH samples as a function of the pH of the starting hectorite dis- persion (CuKa radiation).

persion of the starting hectorite for a short period of time (5-15 min) resulted in CLH products with some- what higher d(001) and surface areas than those ob- tained by prolonged dispersion with a magnetic stirrer.

The effect of the age of the hydroxy-A1 oligomeric solution upon the basal spacings and surface areas of the CLH products was also examined by cross-linking of La-hectorite dispersions with hydroxy-A1 solutions aged for different lengths of time, i.e., 7, 15, 22, and 27 days, under otherwise identical conditions. As seen in Table 2, aging for a period of 15 days produced La- A1-CLt-I having both a relatively large d(001) value (17.8 A) and a high surface area (260 m2/g). Longer aging periods, e.g., 22-27 days, resulted in a decrease in the surface area.

It should be noted that the data in Table 1 relate to La-A1-CLH and Ce-AI-CLH samples prepared by cross- linking with a hydroxy-A1 oligomeric solution aged for 22 days. Somewhat higher surface areas and basal spac- ings, i.e., 17.8-18.0/~, were found for products formed at pH 5-6.5 of the starting hectorite when the cross- linking agent was aged for 14-15 days (see, e.g., La- A1-CLH/40, Table 2). Heat treatment of La-A1-CLH samples up to 400~ under a flow of dry nitrogen, or under vacuum, caused only minor changes in basal spacings and surface areas.

Table 2. Basal spacings and surface areas of La-A1-CLH as a function of the age of the hydroxy-Al oligomeric solution.

Age of solution d(Q01) Surface area 2

Sample ~ (days) (A) (m2/g)

La-A1-CLH/42 7 16.8 251 La-A1-CLH/40 15 17.8 260 La-A1-CLH/30 22 17.0 220 La-A1-CLH/41 27 18.0 218

x The pH of the starting La-hectorite dispersion was 5.5 in all the preparations.

2 Measured after preheating the sample at 250~ (0.001 ton').

Table 3 summarizes the elemental composition of the starting La- and Ce-hectorites and of some typical La-A1-CLH and Ce-A1-CLH products derived by cross- linking with hydroxy-A1 oligomers. The La- and Ce- hectorites contained a high concentration of RE 3+ ion, i.e., 9.40% La and 10.64% Ce, respectively. Cross-link- ing resulted in a major decrease in La and Ce and a sharp increase in A1. Mg decreased slightly, indicating that it was not significantly extracted from the hectorite layers under the moderately acidic conditions (pH of aqueous 0.4 N t aC13 = 4 .5 ; pH of aqueous 0.4 N CeC13 = 3.8) of the ion exchange. The presence of sig- nificant residual La 3+ and Ce 3+ in the A1-CLH products indicates that these ions were not completely displaced during the cross-linking process when using 2.0 mmole A1/g RE3+-exchanged hectorite. As anticipated, in some experiments with lower A1/hectorite ratios the concen- tration of residual La 3+ or Ce 3§ ion in the cross-linked products was higher than that found for the RE-A1- CLH samples in Table 3.

Table 3. Elemental composition of RE3+-hectorites and of some derived RE-A1-CLH products.

Component La-AI- Ce-AI- Ce-AI- (wt. %) La-hectorite CLH/332 Ce-hectorite CLH/193 CLH/234

A1203 0.62 9.30 0.41 9.15 8.61 F%O3 0.24 0.26 0.22 0.24 0.21 MgO 27.92 26.13 26.92 24.20 24.27 CaO 0.68 0.10 0.08 0.04 0.03 Na20 0.18 0.14 0.19 0.20 0.15 1(20 0.10 0.10 0.11 0.12 0.09 Li20 0.54 0.49 0.49 0.39 0.47 La 9.40 2.50 -- -- -- Ce -- -- 10.64 4.65 5.10

A ratio of 2.0 mmole A1/g of RE3*-hectorite was used in the preparation of all RE-A1-CLH products.

2 Cross-linking was performed using a La3+-hectorite dis- persion having pH 5.0.

3 Cross-linking was performed using a Ce3+-hectorite dis- persion having pH 5.4.

4 As in footnote 3, but pH 6.5.

104

]a.aA

Shabtai, RoseU, and Tokarz Clays and Clay Minerals

Temp,~ ~

4 0 0 "

250 ~

25 ~

12 10 8 6 4

2 e

Figure 4. X-ray powder diffraction patterns of a typical Li- A1-CLFH sample as a function of heat-treatment temperature (CuKa radiation).

18.3 ~ , ~

19.1~,

Temp., ~

400 ~

250 ~

25 o

1 12 10 8 6 4

20 Figure 5. X-ray powder diffraction patterns of a typical Ce- A1-CLFH sample as a function of heat-treatment temperature (CuKt~ radiation).

Li +- and Ce3+-fluorhectorites cross-linked with hydroxy-Al oligomers (Li-AI-CLFH and Ce-Al-CLFH)

A number of synthetic Li-fluorhectorite samples (see Experimental) and a Ce3+-exchanged fluorhectorite sample were cross-linked with the previously described hydroxy-A1 oligomeric solution, using a 2.0 mmole A1/ g fluorhectorite ratio. The basal spacings and surface areas of some typical Li-A1-CLFH and Ce-A1-CLFH products are summarized in Table 4. The d(001) values for air-dried (at 25~ samples of these products are in the range of 18.2 to 20.0/k, higher than those found for the cross-linked hectorites (see Tables 1 and 2). Heat treatment at 250 ~ and 400~ generally resulted in relatively small decreases in d(001) values, e.g., 0 .5- �9 5 ]k. Typical X R D patterns and their change with pretreatment temperatures for Li-A1-CLFH and Ce- AI-CLFH products are given in Figures 4 and 5, re- spectively. As seen, the X R D peaks of the d(001) re- flection are very sharp and intense, possibly indicating that synthetic fluorhectorites underwent more effective cross-linking than natural hectorites. The surface areas of the Li-A1-CLFH and Ce-A1-CLF samples (Table 4) after pretreatment at 250~ were ~ 300-380 m2/g, con- siderably higher than those of cross-linked natural hec- torites (Tables 1 and 2).

Table 5 summarizes the elemental composi t ions of

the synthetic Li-fluorhectorite and o f the CeS+-ex - changed fluorhectorite. Also included are data on the composition of two cross-linked products, Li-AI-CLFH and Ce-A1-CLFH, derived from these starting smec- tiles. The composi t ion of Li-fluorhectorite corresponds closely to that anticipated for Li(MgsLi)SigO20F4 which in turn is very close to the formula previously proposed for this synthetic smectite (Barter and Jones, 1970). As expected, the cross-linked derivative (Li-A1-CLFH) contains a large amount of alumina and the li thium is considerably reduced due to displacement during the cross-linking process. The change in composi t ion re- sulting from cross-linking of Ce-fluorhectorite follows the same pattern, i.e., the cross-linked product (Ce-A1- CLFH) contains a significant amount o f incorporated alumina, and the exchangeable ion (Ce 3+) is markedly reduced. The presence o f some residual l i thium in Ce- A1-CLFH indicates that some of this element is present in the layer structure of the fluorhectorite.

Some of the A1-CLH and AI-CLFH products were examined by transmission electron microscopy (TEM). A typical micrograph o f a Li-A1-CLFH sample (Li-A1- CLFH/39, Table 4) is shown in Figure 6. In agreement with the XRD results (Table 4), basal spacings of ap- proximately 19-20 ~ were discerned. No satisfactory micrographs were obtained for the Li-fluorhectorite, prior to cross-linking.

Vol. 32, No. 2, 1984 Properties of hydroxy- and fluorhectorites 105

Table 4. Basal spacings and surface areas of typical Li-A1- CLFH and Ce-AI-CLFH products as a function of pretreat- ment temperature.

Pretreatment temp. (~ Cross-linked fluorhectorite 4

Surface area 3 d(001) (/~) (mVg)

252 250 400 250 400

Li-A1-CLFH/38 20.0 18.4 18.5 373 278 Li-At-CLFH/39 18.8 18.3 18.3 343 323 Li-AI-CLFH/48 18.9 18.5 18.4 297 206 Li-A1-CLFH/51 19.6 17.5 17.0 338 280 Li-A1-CLFH/52 18.2 18.5 17.2 296 279 Ce-A1-CLFH/455 19.1 18.3 18.1 338 283

1 d(001) and surface areas were determined after heat pre- treatment of the A1-CLFH products at 250 ~ or 400~ for 3 hr under a stream of dry nitrogen.

2 Air-dried samples (at 25~ 3 Surface areas could be slightly higher if the small amounts

(<5%) of nonporous impurities present in the samples are taken into account.

4 Derived by cross-linking of different Li-fluorhectorite dis- persions (generally pH = 5.0-6.0) with hydroxy-A1 oligomers (2.0 mmole Al/g smectite). Numbers following the slash = batch number.

5 Prepared by cross-linking of a Ce3+-fluorhectorite disper- sion (pH = 5.0) with hydroxy-A1 oligomers (2.0 mmole A1/g smectite).

Structural features o f cross-linked hectorites and fluorhectorites

A structural mode l for cross-l inked hydroxy-A1 mon tmor i l l on i t e (AI-CLM) which is also applicable to the above A1-CLH and A1-CLFH was proposed by Lahav et al. (1978) and in more detail by Shabtai (1979). It consists o f mon tmor i l l on i t e uni t layers cross-l inked by means ofpolycat ionic, hydroxy-A1 ol igomeric species o f essentially un i fo rm size. To account for the Ad(001)

Table 5. Elemental composition of Li- and Ce3+-fluorhec - torites and of derived cross-linked products.l'Z

Smectite sample

Component, Li-fluor- Ce-fluor- (wt. %) hectorite Li-AI-CLFH hectorite Ce-AI-CLFH

SiO 2 59.90 57.60 57.40 56.10 AI203 0.07 8.37 0.07 7.29 Fe203 0.03 0.02 0.06 0.04 MgO 27.17 24.23 26.15 23.75 CaO 0.05 0.03 0.04 0.02 Na20 0.03 0.02 0.07 0.06 K20 0.01 0.01 0.02 0.02 Li20 4.09 1.29 1.49 1.21 Ce -- -- 6.05 2.94 F 9.50 8.40 9.20 8.10

Total 3 100.85 99.97 100.55 99.53

1 The hydroxy-Al oligomeric solution used in the prepa- ration of the cross-linked products had a pH = 4.4.

2 The pH of the starting Li-fluorhectorite dispersion was 5.4; that of the starting Ce-ltuorhectorite dispersion was 5.0.

a Excluding water.

Figure 6. Transmission electron micrograph of a typical cross- linked Li-fluorhectorite sample (Li-A1-CLFH/39).

va lue o f 9 .0-9.5 /k for A1-CLM, it was proposed that the c r o s s - l i n k i n g spec ies c o n s i s t s o f two s t a cked [A16(OH),2(H20)~2] 6+ ions wi th a combined height o f ~ 9.4/~. Recently, P innava ia (1983) suggested that the c r o s s - l i n k i n g spec ies is an A1,304(OH)2s 3+ or Al~304(OH)247+ cation, on the basis o f A127 neut ron magnet ic resonance and po ten t iomet r i c t i t rat ion data, which indicated the presence o f such species in hy- droxy-A1 o l igomef ic solutions hav ing OH-/A13+ ratios s imilar to those appl ied in the prepara t ion o f cross- l inked smectites (Johansson, 1960; Bottero et al., 1980). I m m o b i l i z e d All3 ol igocations should have a ver t ical d imens ion (~9 .5 /~) s imilar to that o f the previous ly proposed two-stacked oligocations.

As did Shahtai (1980) and Shahtai and Lahav (1980), we found that Na- or Li-smect i te forms can he cross- l inked to a s to ichiometf ic extent by using an equiva len t a m o u n t or excess o f hydroxy-A1 oligocations, whereas other forms, in part icular Ce3+-exchanged smectites, undergo only part ial cross-linking, i.e., incomple te re- p lacement o f the exchangeable cations. Assuming the max ima l extent o f cross-l inking found for La3+-mont - mori l loni te (~95%) and assuming an equiva len t area o f 70 /~2/charge for mon tmor i l l on i t e and a d iamete r o f 11.7 /k (Brindley and Sempels, 1977) for the hy- droxy-A1 ol igocat ion (stacked A16 ions or AI~3 oligo- cation), the average lateral (interpillar) distance in the La-A1-CLM should be about 12/k. Recent adsorpt ion data (Tsai, 1983) using polycyclic compounds o f var- ious critical diameters , i.e., coronene (11.1 ~), octae- thylporphyrin (15.3/~), and tetraphenylporphyrin (19.0 /~), are in good agreement wi th this calculation, show- ing that for max ima l ly cross- l inked La-A1-CLM the

106 Shabtai, Rosell, and Tokarz Clays and Clay Minerals

P r e d o m i n a n t lateral pore sizes are in the range 11-15 ~ .

Fo r La-A1-CLH, based on an equ iva len t area of 98 ~Z/charge for hector i te (Lagaly and Weiss, 1969) and a 75% extent o f cross-l inking with hydroxy-A1 oligo- cations, the average lateral pore size should be ~ 19.5 ~ ; whereas for Li-A1-CLFH, on the basis o f an equiv- alent area o f ~ 7 0 AVcharge and an extent o f cross- l inking o f 95%, the calculated average lateral distance is ~ 16.4 ~ . These lateral pore sizes are considerably larger than typical critical pore sizes found in Y- and X- type zeolites. Accordingly, recent catalytic cracking studies wi th A1-CLH, A1-CLFH, and A1-CLM (Mc- Cauley, 1983) showed that these catalysts are 1 to 2 orders o f magni tude more act ive than CeY-type zeolite for the cracking o f large polycyclic naph thenoa roma t - ics, e.g., dodecahydro t r iphenylene (kinetic diameter , 11.2 A).

A C K N O W L E D G M E N T S

We thank E K A AB, Surte, Sweden, for financial sup- por t o f this project and Dr. W. M. Hess, Elect ron Optics Labora tory , Br igham Young Univers i ty , for the T E M examina t i on o f the smect i te samples.

R E F E R E N C E S

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Vol. 32, No. 2, 1984 Properties ofhydroxy- and fluorhectorites 107

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(Received 18 June 1983; accepted 10 October 1983)

Pe31OMe--Pe3yJ/bTaTOM B3aMHo~e~ICTBH~I FeKTOpHTOB, OtMeHeHHblX La 3§ n Ce 3+ c OJIHFoMepHqeCKHMH KaTnOHaMn rn~porcn-A1, nB3~aoc~ qbopMapoBanne nonepeqHo-cBzaann~ix reKTOpnTOB C yMepenno 6ozb- mnMa nnoma~aMH noBepxnocTn (~220-280 MVr) ~ 6onbtuofi TepMnqecKo~ CTa6nn~nOCTb~3. 1-IpoMeXy- TOqa~Ie paccToaHr~Z aTaX npo~yKTOB Haxo~nnnc~ B o6meM Mex~y 17,0 H 18,0 ~; TOqnoe 3naqeHHe d(001) 3aB}ICnJio OT Bo3pacTa o3nroMepHqecKoro pacTBOpa FH~poKcH-A1 rI OT pH naqaJqbHOITt ~ncnepcnn FeKTOpHTa. B3aHMo)Ie~CTBrie Li- nan Ce-d~J~yoprerTOpHTa C OJTtlFOKaTHOHaMH rn~poKcH-A1 ~aBa31o COOTBeTCTByIomHe nonepeqHo-ca~3aHHbie qbJIyopreKTOpaTbI, KOTOpble riMeJIH 3HaqnTeJIbHO 6OJIbtUHe npoMe~ZyTOqHbIe pacc- TOaHH~ (18,2--20,0 /~ ~na o6paattOB, ocymeHHbXX a BOa)Iyxe), nJIoma~a noBepXHOCTH (300--380 M2/r), a TepMtlqecrrle CTatHJIbHOCTH I10 cpaBHenHIO CO COOTBeTCTByIOLI~HMH 3HaqHHflMH ~.rI~l IIOIIepeqHo-CB~I3aHHblX reKTOpnTOB. HonepeqHo-caaa~iBa~o~ee aemecTao, acno~I~3oaaHnoe )~Ji~ CnHTeaa rnApoKcrI-A1 reKToprlTa rI qbJIyopreKToprlTa, COCTOa~IO rI3 pacTaopa OJI~IrOMepoa rr~porcn-A1 rlocJIe cTapeHna qepe3 7 ao 27 ~Hefi. HOCTOZHHOe OTHomenae 2,0 MMO~Ib A1 Ha I'paMM CMeKTHTa HcnoYlb3OBaJIocb BO Bcex IIO~rOToarax. 13oJibmHe 3HaqeHn~ npoMe~KyTOqm, IX paccToflHrfft rI IIOpOaaTOCTH noao-crlsTeanpoaaI~m,ix IIpo~yKTOB Haxo~ITCff B cor~lacrm co cTpyrTypOft noxo~Ke~I Ha paHee npe~no~IoxeHHyIO cTpyKTypy AJIa nonepeqHo-ca~3aHnoro rH~oKcla-A1 MOHTMOp!,I.rlJIOIt~ITa. [E.G.]

Resiimee--Die Wechselwirkung yon La 3+- und Ce3+-ausgetauschten Hektoriten mit oligomeren Hydroxy- A1-Kationen f'tihrt zur Bildung von kreuzweise verkntipften Hektoriten, die eine mal3ig groBe Oberfltiche (etwa 220-280 m2/g) und eine hohe thermische Stabilittit besitzen. Der Basisabstand dieser Produkte reichte im allgemeinen yon 17,0-18,0 ~. Der exakte d(001)-Wert hing vom Alter der oligomeren Hydroxy- A1-LSsung und vom pH der Hektorit-Ausgangssubspension ab. Die Wechselwirkung yon Li- oder Ce- Fluorhektorit mit Hydroxy-A1-Oligokationen ftihrte zu entsprechenden kreuzweise verkniipften Fluor- hektoriten, die einen beachtlich grSl3eren Basisabstand (18,2-20,0 A i'dr Luft-getrocknete Proben), eine grtl3ere Oberfltiche (etwa 300-380 m2/g) und eine hShere thermische Stabilittit im Vergleich zu den kreuzweise verkntipften Hektoriten zeigte. Das Agens, das zur kreuzweisen Verkniipfung bei der Synthese yon Hydroxy-A1-Hektorit und Fluor-Hektorit-Produkten verwendet wurde, bestand aus einer LSsung yon Hydroxy-A1-Oligomeren, die tiber einen Zeitraum von 7 his 27 Tagen gealtert wurden. Ein konstantes Smektit-Verhtiltnis yon 2,0 mMol Al/g wurde in allen Versuchen verwendet. Die grol3en Basisabstfinde und die Porosittit der neu synthetisierten Produkte stimmen mit einer Struktur tiberein, die vor kurzem ftir kreuzweise verkntipften Hydroxy-AI-Montmorillonit vorgeschlagen wurde. [U.W.]

Rtsumt--L'interaction d'hectorites 6changtes avec La 3+ et Ce 3+ avec des cations oligomtriques hydroxy- A1 rtsulte en la formation d'hectorites ~ liens croists posstdant des aires de surface modtr tment 6levtes (~220-280 m2/g) et une haute stabilit6 thermique. Les espacements de base de ces produits 6talent gtntralement compris entre 17,0-18,0 ,~, la valeur exacte de d(001) dtpendant de l'fige de la solution oligomtrique hydroxy-Al et du pH de la dispersion d'hectorite de dtpart. L'interaction de fluorhectorite- Li ou -Ce avec des oligocations hydroxy-A1 a produit des fluorhectorites h liens croists correspondantes qui montraient des espacements de base (18,2-20,0/k pour des 6chantillons sechts gt Fair), des aires de surface (~300-380 m2/g) et une stabilit6 thermique nettement plus 61evts, en comparaison avec ceux d'hectorites ~ liens croists. L'agent produisant des liens croists appliqu6 dans la synth~se des produits hectorite hydroxy-A1 et fluorhectorite consistait en une solution d'oligom~res hydroxy-A1 vieillie pour des ptriodes de 7 ~t 27 jours. Une proportion constante de 2,0 mmole A1/g de smectite 6tait employte dans toutes les prtparations. Les espacements de base 61evts et la porosit6 des produits nouvellement synthetists sont fidtles gt une structure semblable h celle proposte pr tc tdemment pour une montmorillonite hydroxy-A1 ~ liens croists. [D.J.]