CROSS-LINKED SMECTITES. V. SYNTHESIS AND … 35/35-6-429.pdf · HYDROXY-SILICOALUMINUM MONTMORILLONITES AND FLUORHECTORITES ... Abstract--Solutions containing hydroxy-SiA1 ... based

Clays and Clay Minerals, Vol. 35, No. 6. 429439 , 1987,

CROSS-LINKED SMECTITES. V. SYNTHESIS AND PROPERTIES OF HYDROXY-SILICOALUMINUM MONTMORILLONITES

AND FLUORHECTORITES

JOHAN STERTE 1 AND JOSEPH SHABTAI 2

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

Abstract--Solutions containing hydroxy-SiA1 (HSA) oligocations were prepared by two procedures: (1) treatment of a mixture of orthosilicic acid and A1C13 with aqueous NaOH, followed by aging of the product; and (2) preliminary preparation and aging of hydroxy-Al~3 oligocations followed by reaction of the latter with orthosilicic acid. Ion exchange of Na,Ca-montmorillonite with HSA oligocations yielded pillared, crossqinked montmorillonites (designated as HSA-CLM) showing a maximum d(001) value of 19.5 ,~ for air-dried samples, and maximum surface areas of -500 m2/g after outgassing at 250~ -3 tort. Corresponding ion exchange of Li-fluorhectorite yielded HSA fluorhectorites (HSA-CLFH) showing a maximum d(001) value of 19.0/~ and a surface area of 355 mVg. Calculated structural formulae for the HSA-CLM and HSA-CLFH products, based on elemental analysis, showed a gradual increase in the Si/A1 ratio in the intercalated HSA oligocations with increasing Si/A1 ratio in the pillaring solution. Optimum d(001) values and surface areas of HSA-CLM and HSA-CLFH products were obtained using method 2 and applying a ratio of 1.6-2.5 mmole (Si)A1/g smectite.

The thermal stabilities of HSA-CLM and HSA-CLFH products were determined by heat treatment between 250 ~ and 700~ and subsequent measurement of the d(001) values and surface areas. HSA-CLFH products showed the unusual behavior of increase of d(001) with increase in temperature from 400 ~ to 500~ and essential constancy of d(001) from 500 ~ to 600~ The HSA-CLM products showed a gradual decrease in surface area, whereas the HSA-CLFH products prepared with a Si/A1 ratio of 1.04-2.18 in the pillaring solution showed constant surface areas with increasing temperature from 250 ~ to 600~ HSA-CLM and HSA-CLFH show sharply higher acidities compared with those of reference A1-CLM and A1-CLFH samples obtained by pillaring with hydroxy-Al~3 oligocations. This increased acidity is probably due to the presence of acidic, surface silanol groups in the HSA oligocations.

Key Words--Cross-linked smectite, Fluorhectorite, Hydroxy-SiA1 oligocations, Pillared montmorillonite, Thermal stability.

I N T R O D U C T I O N

Earlier work on the synthesis, structure, and properties o f cross-l inked smecti tes (CLS), described also as pillared clays, has been reviewed by Pinnavaia (1983), Shabtai et al. (1984b), and Tokarz and Shabtai (1985). Major addi t ional progress in this field has recently been repor ted by several authors; in particular, the range o f pillaring agents and preparat ive condi t ions has been considerably enlarged, and efforts to elucidate the structure o f CLS materials and improve their properties have been intensified. P innavaia et al. (1985b) repor ted the prepara t ion o f large-size hydroxy-Cr oligocat ions by hydrolysis at 95~ o f a Cr(NO3)3 solut ion with aqueous Na2CO3, using a base : Cr mola r ratio o f 1.5-2.5 and a hydrolysis t ime of 6-36 hr. Cross-l inking o f N a - m o n t m o r i l l o n i t e with such oligocations yielded a hydroxy-Cr mon tmor i l lon i t e product , [d(001) = 26.8 /~ (25~ 21.0 A (500~ surface area = 433 m2/g], which showed significant catalytic dehydrogenat ion ac-

' Research associate on leave from Chalmers University of Technology, Gothenburg, Sweden.

2 To whom correspondence should be addressed.

t ivity. Sterte (1986) prepared a TiO2-pil lared mont - mor i l loni te which had an unusually high basal spacing o f ~ 28 ~, and which was thermal ly and hydro thermal ly stable to 700~ The TiO2-CLS mater ia l was prepared in two steps, i.e., pillaring with ol igomeric Ti-conta ining cations obta ined by partial hydrolysis o f TiC14 in aqueous HC1, fol lowed by further in si tu hydrolysis o f the oligocations and heat ing to 700~ to produce stable TiO2 cross-links between the smect i te layers.

Recently, Shabtai and Fijal (1986) repor ted the synthesis o f a novel class o fhydroprocess ing catalysts having a CLS structure. Three different types o f catalysts were prepared: (1) pil lared smecti tes containing catalytically active hydroxy-M or sulfhydryl-M (where M -- Mo, Cr, Ni, Co, or other t ransi t ion metal) o l igomers or ol igocations intercalated in the in ter lamel lar space between the cross-links; (2) catalysts prepared by pillaring o f smecti tes with catalytically active, m ixed oligocations conta ining A1 in combina t ion with one or more transi t ion metals (e.g., hydroxy-CrA1, hydroxy- NiA1); and (3) catalysts consist ing o f a smect i te cross- l inked with AI203 (derived by heat t rea tment o f intercalated hydroxy-A1 oligocations) and containing M o oxide supported on the stabil ized A1203 cross-links. In

Copyright '~" 1987, The Clay Minerals Society 429

430 Sterte and Shabtai Clays and Clay Minerals

addit ion to catalytically active oligomeric components, and in some cases also to simpler, exchangeable tran- sition metal cations, the above catalysts contained considerable protonic acidity and showed high activity for hydrocracking of long-chain paraffins and bicyclic naphthenes. Yamanaka et al. (1984) prepared a Fe203- pillared montmori l loni te by ion-exchange of the smec- tire with a solution containing trinuclear acetatohy- droxo-Fe(III), i.e., [F%(OCOCH3)7OH] +, cations. The product, after calcining at 500~ showed a d(001) value of 16.7 ,~ and a surface area of 280 m2/g.

Another direction of recent research has been the preparation of SiO2-intercalated smectites. Endo et al. (1980) and Pinnavaia el al. (1983) reported the introduction of silica in montmori l loni te and other clays by (1) ion exchange of the swollen smec t i t e wi th t r i s (ace t y lace tona to ) s i l i con cat ions (Si(acac)3+), followed by hydrolysis of the latter, or (2) in situ reaction of acetylacetone-solvated smectites with SIC14. Calci- nation of the resulting Si(acac)3--exchanged smectites resulted in carbon-free, SiO2-intercalated smectites showing relatively low surface areas (40-190 m2/g) and a maximum d(001) value of 12.6 ]k, indicating the presence ofmonolayer siloxane chains between the layers. In an important subsequent development, Lewis et al. (1985) pillared the smectite with polyhedral oli- gosilsesquioxane compounds, which upon calcination gave rise to two-layer silica structures between the layers. The pillaring agent consisted of one or more compounds having the general formula (ZSiO~.5)o(OSiZ2)m, in which Z is an organic moiety serving as an ion- exchange or coordinating group. Typically, 4-(2-tri- chloros i ly le thyl)pyr id ine or 2-(2-s i lylethyl)pyridine were subjected to hydrolysis, and the resulting oligo- silsesquioxane pillaring agent was intercalated in the smectite at a pH < 6. Calcination at 350~176 yielded pillared products with interlayer spacing, Ad(001), 3 of 6.6--10.2 A and surface areas of 140-400 m2/g. A Pt- loaded SiO2-pillared montmori l loni te showed moderate catalytic activity for hydroisomerization. In a different approach to silicon incorporation in the pillaring component of CLS systems, Atkins and Ashton (1985) silanized the A1203 pillars of calcined hydroxy-A1 smectites by reacting the latter with tetraethoxysilane. On the other hand, Occelli (1986) reported the use of submicrometer-size A1203-coated silica to produce A1203/SiOz-pillared montmoril lonite.

In view of the above studies it was of interest to synthesize well-defined hydroxy-SiA1 (HSA) oligocations for use as pillaring agents. Calcination of CLS materials intercalated with such mixed oligocations should result in the formation of products having stable SIO2-A1203 pillars. Of particular interest as a model were H S A o l igoca t ions based on a core o f

3 See Pinnavaia et al. (1979).

Al1304(OH)247+ oligocations, which have been structurally well characterized. Such hydroxy-All3 oligocations (Johansson, 1960), designated as HA, form when a strong base, e.g., NaOH, is added to an aqueous solution of an a luminum salt, e.g., a luminum chloride. The maximal concentration of HA oligocations is obtained at pH ~4.3 (Gunnarsson and Nilsson, 1983). The percentage of A1 present in this form is also de- pendent on the total AI concentration, the equilibration t ime and temperature, and the rate of neutralization (Turner and Ross, 1969; Hodges and Zetazny, 1983). These authors found that the relative amount of A1 present as HA oligocations is lower in dilute solutions and that the total amount present in this form decreases with time. These same authors demonstrated that the concentration and stability of HA oligocations in- creases if the rate of neutralization is low.

Wada and Wada (1980) showed that Si can be in- corporated in the structure of oligomeric hydroxy-Al ions by reacting a mixture of orthosilicic acid, aluminum chloride, and sodium hydroxide. Several possible mechanisms for this reaction were discussed by Luciuk and Huang ( 1974); for formation of a HSA oligocation, structurally based on a hydroxy-Al~3 oligocationic in- termediate, the predominant pathway indicated by Wada and Wada (1980) was:

[Al1304(OI--I)23(H20)1217+-OH + St(OH)4 [Al~304(OH)23(H~O)I217--O-Si(OH)3 + H20. (1)

They suggested, however, some contribution of a second reaction (2) as indicated by a slight increase in the acidity of the product mixture during the equilibration period:

[AI,304(OH)z4(H20),~]7--OH2 + Si(OH)4 [Al1304(OH)24(H20),d6+-O-Si(OH)3 + H30 +. (2)

Inasmuch as the main reaction (1) proceeded with retention of the oligocationic charge, even if the silanization process affected several OH groups, the HSA oligocation produced could be considered as a suitable species for smectite cross-linking. Thus, intercalation of mixed HSA oligocations in the interlamellar space of the smectite, followed by calcination to yield sta- bilized SiO2-A1203 pillars, could result in a CLS product of augmented thermal stability and acidity. On the other hand, Holy (1983) showed that surface silanol groups react smoothly with various catalytically active compounds, thereby allowing for introduction of new active functional groups in pillared smectite catalysts, including such needed for shape-selective organic and biochemical reactions at low (<300~ temperature (Shabtai, 1979).

The present paper reports the synthesis and characterization of CLS materials prepared by controlled cross-linking of montmori l lonite and synthetic fluorhectorite with hydroxy-SiAl oligocations prepared by two different methods. The effects of the Si/AI ratio in

Vol. 35, No. 6, 1987 Hydroxy-silicoaluminum montmorillonite and fluorhectorite 431

the pillaring solut ion and of the pillaring solut ion/ smect i te ratio upon the proper t ies o f der ived CLS products are also repor ted.

E X P E R I M E N T A L

Starting materials A sample of Na,Ca-montmorillonite from Belle Fourche,

South Dakota (commercial designation, Accofloc 350) was obtained from American Colloid Company. Elemental analysis of the < 2-urn fraction gave the following structural formula for this sample:

Na024Ko osCa0 ~ 6(Mgo 51Fe0 36 A I 3.16)(Alo. I 8Si7 82)020(0H)4 ,

which corresponded to a formula weight of 744 and a cation- exchange capacity (CEC) of 79 meq/100 g.

Li-fluorhectorite was prepared according to the method described by Barrer and Jones (1970) and Shabtai et al. (1984b). Although no impurities were found by X-ray powder diffrac- tion (XRD) analysis of the <2-um fraction of the Li-fluorhectorite product, the elemental analysis indicated the possible presence of a minor amount (<2%) of impurities. The unit-cell formula calculated from the elemental analysis was Lioss(Mg5 tsLio85)SisO30F~, close to that reported by BaiTer and Jones (1970) and corresponding to a formula weight of 758 and a CEC of ~ 110 meq/100 g.

Hyclroxy-SiAl oligomeric solution The preparation of the oligomeric HSA solution, used as

pillaring reactant, was carried out by two methods. Method A was essentially the same as that described by Wada and Wada (1980). Monomeric orthosilicic acid solutions were prepared by diluting a calculated amount of tetraethyl orthosilicate, dissolved in 50 ml of ethanol, with 20 liters ofdeionized water. Solutions having Si(OH)~ concentrations of 2.55 x 10 4, 4.99 x 10 ", and 10.46 x 10 -4 M were prepared. An equal amount of aqueous 0.2 M A1CI3 was then gradually added to each of the Si(OHh solutions, resulting in product solutions having various Si/AI ratios. A reference A1C13 solution containing no Si(OH)4 was also prepared. The AI concentration in all product solutions was constant at 4.8 x l0 -~ M. An aqueous NaOH solution (0.2 M) was then added to these solutions with constant stirring at a rate of about 1 cm3/ rain until a final OH/AI ratio of 2.0 was reached. The resulting solutions were then aged in polyethylene bottles for different periods of time from 2 hr to 30 days. The pH of the solutions was measured both immediately after preparation and after aging. The initial pH values were between 4.7 and 5.0, and were slightly higher in solutions with low Si/A1 ratios. After 10 days of aging, the pH values generally decreased to 4.3- 4.5. Relevant Si/AI ratios calculated by Wada and Wada (1980) for HSA oligocations present in solutions prepared by this method are given in Table 1.

Inasmuch as method A involved handling very large volumes of solutions, the following alternative method (B) was developed and used: A solution containing hydroxy-A1 oligocations, having an OH/A1 ratio of 1.85 was prepared by the method of Lahav et al. (1978) and Shabtai and Lahav (1980) and aged for 18 days. After aging, four separate 50-ml samples were withdrawn and diluted with 0 (no dilution), 100, 200, or 400 ml of deionized water. To these solutions, calculated amounts of tetraethyl orthosilicate, i.e., 0.0, 0.37, 0.74, and 1.49 g, respectively, each dissolved in 50 ml of ethanol, were added with constant stirring over a period of 5 hr, to produce HSA-contalning solutions having Si/AI weight ratios of 0.0, 0.5, 1.0, and 2.0. These solutions were then left to stand for one day before use. Although all solutions, except the reference solution, were saturated with respect to Si(OH)4, no precipitation was observed during the preparation period

Table 1. Estimated Si/AI ratios in hydroxy-SiA1 (HSA) oligocations as a function of the Si/A1 ratio in the parent solution and of aging time.

Calculated Si/AI ratio in HSA oligocations

Si/AI ra~io in Aging time: parent solution 2 hr 10 days 30 days

0.53 0.22 0.34 0.47 1.04 0.33 0.41 0.47 2.18 0.59 0.57 0.62

Si/Al ratio calculated by dividing the amount of Si retained on a cation exchanger by the amount of oligomeric AI in solution (Wada and Wada, 1980).

or as long as 3 days afterwards. Solutions left for longer periods, e.g., 3-5 days, however, became opalescent; some pre- cipitate formed in HSA solutions having higher Si/A1 ratios (e.g., 1.0 and 2.0). For this reason HSA solutions prepared by method B were used as cross-linking reactants within 1- 2 days of their preparation.

Cross-linking procedure

Cross-linking was performed using the apparatus and procedure described by Shabtai et al. (1984b). The clay sample (1.37 g) was dispersed in 560 ml of deionized water by magnetic stirring. From 5.7 to 22.9 liters of solutions containing HSA oligocations prepared by method A were used as pillaring reactants to obtain ratios of 2.0 to 9.0 mmole Al/g of smectite. With solutions prepared by method B, volumes of only 50-450 ml were used to cross-link 1.4-g smectite samples, corresponding to ratios of 1.3-2.5 mmole A1/g of smec- rite. After completion of the reaction (usually 5-15 rain), the product was allowed to stand in contact with the solution for 15 hr. The precipitated product was then separated from the solution by centrifugation, washed with deionized water until chloride-free, and air- or freeze-dried.

Analysis of products

Oriented samples of products were analyzed by XRD. Sur- face areas were measured by nitrogen adsorption at liquid nitrogen temperature. The thermal stability of cross-linked products was examined by determining the basal spacings and surface areas of samples heat-treated at 250~176 for 3 hr, under dry nitrogen. Elemental analyses were obtained by in- ductively coupled plasma spectrometry (ICPS). Fluorine con- tents were determined with a specific ion electrode, and SiO2 was separately determined by the molybdenum blue colori- metric method. The analytical methods used have been described in detail elsewhere (Shabtai et aL, 1984b).

Ammonia adsorption was measured using a model 2000 microbalance (Cahn Instruments, Inc.). About 540 mg of the pillared smectite was first outgassed at 400~ overnight under a flow of nitrogen. The sample was then cooled to 100~ the flow gas was changed to 5.4% o f N H 3 in N2, and the amount of NH3 adsorbed was measured after 2-hr equilibration at this temperature. Using the same gas mixture, NH3 adsorption was then measured after 20-min equilibration, at 25~ temperature increments to 250~

R E S U L T S A N D D I S C U S S I O N

Hydroxy-SiAl cross-linked montmorillonites dHSA-CLM)

Dependence o f properties upon Si /Al ratio in the pillaring reactant. H S A - C L M produc t s were p repared according to m e t h o d A (vide supra) by cross- l inking por-


Table 2. Change in basal spacing and surface area of hydroxy-SiAl cross-linked montmorillonite products as a function of the Si/A1 ratio in the cross-linking oligomeric solution.

Si/AI ratio in Method o f h y d r o x y - S i A l so lu - Surface area-" d(0.01) ~

preparat ion I t ion (m2/g) (A)

A 0.0 458 19.2 A 0.53 369 19.3 A 1.04 324 19.5 A 2.18 278 19.0 B 0.0 499 17.6 B 0.5 498 17.7 B 1.0 460 17.2 B 2.0 343 17.0

See Experimental section. 2 Measured after outgassing the samples under vacuum

overnight at 250~ 3 Measured with oriented samples, air-dried at 25~

tions ofa Na,Ca-montmoril lonite dispersion with HSA oligomeric solutions having Si/AI ratios of 0.53, 1.04, and 2.18. A reference sample cross-linked with hydroxy-Al oligocations only (Si/A1 ratio = 0) was also prepared under otherwise the same conditions. The OH/A1 ratio of each HSA solution was adjusted to 2.0, and the aging period was kept at 10 days. The total amount of HSA solution applied in each preparation was such as to contain 7.0 mmole A1/g of montmoril-

lonite. A second series of HSA-CLM products was prepared

according to method B (vide supra) by cross-linking other portions of the above montmoril lonite dispersion with HSA oligomeric solutions having Si/A1 ratios of 0.5, 1.0, and 2.0. In these preparations, the OH/A1 ratio of the HSA solutions was adjusted to 1.85, and the amount of HSA solution used in each case was such as to contain 2.5 mmole A1/g of montmorillonite. A reference sample cross-linked with hydroxy-A1 oligocations only (Si/A1 ratio = 0) was prepared also by this

method.

The d(001) and surface area values of the above eight products are summarized in Table 2. The d(001) values of air-dried samples prepared by method A were in the narrow range 19.0-19.5 A and were independent of the Si/A1 ratio in the pillaring solution. On the other hand, the surface areas of the samples after heating under vacuum at 250~ decreased considerably with increase in the Si/AI ratio. Samples prepared by method B showed d(001) values of 17.0-17.7 ~, which were somewhat lower than those of the materials produced by method A; however, the samples had markedly higher surface areas (343-498 mVg) compared with those prepared by method A (278-369 m2/g). The basal spacings of all products were similar to those previously reported for pillared hydroxy-A1 montmoril lon- ites (Tokarz and Shabtai, 1985) and indicate that the dimension of the HSA cross-linking species (before calcination, vide infra) in a direction vertical to the smectite layers is similar to that of hydroxy-Al~3 oligocations. The moderate decrease in surface area of HSA-CLM products with an increase in Si/A1 ratio was probably due to a gradual decrease in the lateral (in- terpillar) distance and an attendant decrease in interlamellar pore volume and adsorptive surface, resulting from a gradual increase in the extent of space-consum- ing substitution o f - O H groups by bulkier -OSi(OH)3 groups in the pillaring species.

The elemental compositions of the starting Na,Ca- montmoril lonite and of the pillared products given in Table 2 are summarized in Table 3. The SiO2 content of products derived by method A increased only slightly relative to that of the reference hydroxy-A1 mont- morilloni~e (AI-CLM) with an increase in Si/A1 ratio in the pillaring solution. For products prepared by method B, the increase in SiO2 content is somewhat more pronounced. To evaluate more accurately the extent of incorporation of Si into the mixed HSA pillars, the data in Table 3 were analyzed further. Using the structural composition of the starting Na,Ca-mont-

Table 3. Elemental composition (wt. %) of hydroxy-SiA1 cross-linked montmorillonite (HSA-CLM) products2

H S A - C L M . me t hod A 2 H S A - C L M , me thod B 2

Na ,Ca-M 3 AI -CLM' (0.53) (1.04) (2.18) A1-CLM' (0.5) (1.0) (2.0)

Oxide s SiO2 65.56 59.73 60.17 60.77 60.94 61.73 62.05 62.44 63.15 AI203 23.78 33.46 33.64 33.03 33.52 31.66 31.62 31.50 30.86 Fe20 ~ 4.06 4.07 3.96 3.72 3.57 4.04 3.91 3.75 3.70 MgO 2.86 2.48 2.06 2.25 1.94 2.22 2.13 2.06 2.03 CaO 1.29 0.03 0.03 0.02 0.02 0.05 0.04 0.04 0.04 Na20 2.08 0.11 0.07 0.08 0.07 0.14 0.12 0.10 0.11 K20 0.37 0.11 0.07 0.11 0.06 0.15 0.13 0.10 0.10

' Samples were dried at 110~ overnight prior to analysis. Numbers in parentheses indicate Si/A1 ratio in HSA solution used for cross-linking.

3 Starting Na,Ca-montmorillonile sample (see Experimental). 4 Reference samples of pillared hydroxy-Al montmorillonites prepared under the conditions of methods A or B (no Si

present in pillaring solution). 5 For easier comparison, wt. % of components are normalized to a total of 100%, excluding chemically bonded water.

Vol. 35, No. 6, 1 9 8 7 Hydroxy-silicoaluminum montmorillonite and fluorhectorite 433

Table 4. Calculated structural formulae for cross-linked hydroxy-SiAl montmorillonite (HSA-CLM) products.

Si/AI ratio Si/AI ratio in pillaring in pillaring

solution (method) ~ Calculated structural formula-' oligocations s Charge/Al a tom 4

(Na,Ca-M) ~ 0 . 0 ( A ) 6

1.04 (A) 2.18 (A) 0.0 (B) ~ 0.5 (B) 1 . 0 ( B )

2.0 (B)

Na0.12Ko.o t Cao.os(Mgo.25Fe0 lsAll.ss)(Si3.glAlo.og)O 1o(OH)2 -- __ Ale.9~Nao o lKo.o~(Mgo.z4F%.2oAll.~a)(Si3.gjAlo.og)Olo(OH)2 -- 0.27 Sio 39All osNao.o,Ko.ol(Mgo.24Feo.zoA1, s8)(Si3.9 jAlo.og)O t o(OH)2 0.36 0.26 Sio 79A1~ ssNao o~K~.m(Mgo :2Feo.2~Ali.ss)(Sis.gjAlo o9)O re(OH)2 0.58 0.20 AI(> .,2Nao.o2K<,o~ (Mg~ 2, F% 2oA1~.58)(Si3 9~ Alo.og)O~o(OH)2 -- 0.42 Sio46ALo96Nao.o2Ko.ol(Mgo22Feo.21All.ss)(Si3.91Aloog)Olo(OH)~ 0.46 0.29 Sio 66A1LosNao.o2Ko Ol (Mgo.,3Feo.21All. 5 s)(Si39 i Alo.o9)Oto(OH)2 0.6 3 0.24 Sio. 78A1~ .o3Nao o2Ko.o,(Mgo.23 Feo 2~ AlL. ss)(Si3.9~Alo o9)0 ~ o(OH)2 0.7 6 0.24

l Numbers indicate Si/A1 ratio in hydroxy-SiA1 solution used for crossqinking by method A or B. 2 For method of calculation, see text. 3 Si/A1 ratio in the pillaring oligocations, as calculated from structural formulae. 4 Positive charge per A1 atom in the pillaring oligocations as derived from structural formulae.

Na, Ca-montmorillonite used as starting material. 6 Reference solutions containing hydroxy-A1 oligocations only (no Si added).

morillonite and assuming constant tetrahedral and octahedral occupancy of Si and A1, respectively, the structural fo rmulae o f the two reference hydroxy-A1 montmoril lonite products (Table 3, footnote 4; and Table 4) can be calculated. By further assuming, on a charge basis, a constant occupancy of Mg and Fe in the octahedral layer, the approximate structural formulae of the HSA-pillared montmorillonites were calculated, as summarized in Table 4. The calculated Si/ A1 ratio in the interlayer HSA oligocations increased with increasing Si/A1 ratio in the pillaring HSA solutions used in the cross-linking procedure A or B. That this relationship is not clearly discerned in Table 3, in which the elemental composition of the total pillared products is given on a weight basis, is probably due to the decrease in the charge per AI atom in the HSA oligocations with increase in the Si/AI ratio (Table 4). Such a charge reduction may have been due to a small contribution of reaction (2), leading to formation of HSA oligocations, e.g., [ A l ~ 3 0 4 ( O H ) 2 4 ( H z O ) , I ] 6 + - O -

Si(OH)3 (see Introduction). The decrease in charge per A1 atom should have resulted in the interlamellar incorporation of a somewhat larger amount of A1 (in the form of HSA oligocations) than that anticipated from incorporation of HSA oligocations formed by reaction (1) only, thus affecting to some extent the total Si/AI ratio in the pillared product. Although the calculated structural formulae of HSA-CLM products (Table 4) are based on several assumptions and are, consequent- ly, only approximate, they provide more explicit in- formation on the intercalation of HSA cations in the pillared products than is indicated by elemental analysis of the latter on a weight basis (Table 3). It is also noteworthy that the Si/A1 ratios for pillared samples prepared by method A (Table 4) are fairly close to those estimated previously by Wada and Wada (1980) for HSA oligocations in the corresponding pillaring solutions (Table 1). Further, for approximately equal Si/ A1 ratios in the pillaring solutions, method B yielded

products having higher Si/A1 ratios in the pillaring HSA oligocations, as compared with those obtained by method A.

Dependence of properties upon pillaring reactant/smec- rite ratio and HSA aging time. HSA-CLM products were also prepared by cross-linking equal portions of a Na,Ca-montmoril lonite dispersion with different amounts of the pillaring HSA solution, prepared by method A. The amounts of the HSA solution were changed within the range corresponding to 2-8 mmole A1/g of smectite. In all preparations the Si/A1 and OH/ A1 ratios in the HSA solution were kept constant at 0.53 and 2.0, respectively, and the solution was aged for 10 days at 25~

Other HSA-CLM products were synthesized by cross- linking equal portions of the smectite dispersion with different amounts ofa HSA solution prepared by method B. The amounts of the pillaring HSA solution were changed to correspond to HSA/smecti te ratios in the pillaring step of 1.3, 1.6, 2.0, and 2.5 mmole A1/g of smectite. In these preparations the Si/AI and OH/A1 ratios were kept constant at 0.50 and 1.85, respectively, and the aging time was 10 days at 25~

The d(001) values and surface areas of the HSA- CLM products are summarized in Table 5. The d(001) values of products prepared by method A increased gradually with an increase of the HSA/smecti te ratio from 2.0 to 8.0 mmole Al/g smectite. The surface areas of the same products increased in a more pronounced fashion, i.e., from 85 mVg for a ratio of 2.0 to 431 m2/g for a ratio of 8.0. For HSA-CLM products prepared by method B optimal properties, e.g., d(001) = 18.1 /~ and surface area = 496 mZ/g, were obtained by using a HSA/smectite ratio of about 2 mmole A1/g of smectite, which is much lower than the ratios of 7-8 mmole A1/g smectite needed for optimizing the properties of products prepared by method A. These results indicate that method B, which involves initial prepa-


Table 5. Change in basal spacing and surface area of hydroxy-SiA1 cross-Iinked mommorillonite products as a function of the hydroxy-SiA1 (HSA) pillaring solution/smectite ratio.

Method of H SA/smectite preparation of ratio (mmole d(091): Surface area 3 HSA solulion' AI/g smectite) (A) (me/g)

A 2.0 15.4 85 A 3.0 17.1 173 A 4.0 18.8 228 A 5.0 19.2 255 A 6.0 19.3 318 A 7.0 19.5 417 A 8.0 19.6 431

B 1.3 16.0 341 B 1.6 18.2 434 B 2.0 18.1 496 B 2.5 17.1 498

Two different methods (A and B) were used in the preparation of the oligomeric HSA solutions (see Experimental).

2 Basal spacing delermined on oriented samples dried at 25~

3 Surface area measured after outgassing overnight at 250~

ration and aging of hydroxy-A1 oligocations followed by silanization (silicalization) of the latter, is preferable compared with method A in which the oligomerization and silicalization reactions are performed simulta- neously, using very large volumes of aqueous solutions. The HSA/smectite ratio corresponding to about 2 mmole A1/g of smectite is close to the ratio needed for stoichiometric ion exchange of the Na,Ca-montmorillonite with hydroxy-SiA1 oligocations. Tokarz and Shabtai (1985) found a similar ratio of 2.0-2.25 mmole A1/g of smectite to be optimal also for preparation of hydroxy-A1 montmorillonites.

The effect of aging the HSA solution, prepared by method A, upon product properties was examined by comparing the basal spacing and surface area of a series of HSA-CLM samples obtained by pillaring of Na,Ca- montmoril lonite with HSA solutions aged from 2 hr to 30 days (Table 6). In all preparations the amount of l iSA solution used as pillaring reactant corresponded to the use of 7.0 mmole A1/g smectite. Both the d(001) and surface area values increased with increasing aging time to about 10 days. The properties did not change significantly at longer aging times.

In view of the advantages of method B for preparation of the HSA pillaring solution (vide supra), this method was further optimized for time-saving preparation of HSA-CLM products. The initial step of pre- paring a solution containing hydroxy-A1 oligocations was performed by aging at 95~176 for 6-8 hr (To- karz and Shabtai, 1985). The fast-aged hydroxy-A1 solution was then reacted with orthosilicic acid according to method B (see Experimental) to yield a HSA oligomeric solution (Si/A1 ratio = 1.0), which was used as a pillaring agent without further aging. HSA-CLM products prepared by this procedure, using 1.6 to 2.0

Table 6. Change in basal spacing and surface area of hydroxy-SiAl (HSA) cross-linked montmorillonite products as a function of the hydroxy-SiA1 aging time.

Aging lime oFHSA d(Q01) 3 Surface area 4 solution ~ 2 (A) (m2/g)

2 hr 16.4 270 5 days 16.4 319

10 days 19.3 369 20 days 18.2 352 30 days 18.6 388

Solutions aged in polyethylene bottles at 25~ 2 Prepared by method A; Si/A1 ratio = 0.53; OH/A1 ratio =

2.0. 3 Measured on oriented samples, air-dried at 25~ 4 Determined after outgassing the HSA-CLM sample over-

night at 250~

/

mmole A1/g of smectite in the pillaring step, showed basal spacings of 18.2-18.7 A and a very high surface area (480-505 m2/g), after outgassing at 250~

Hydroxy-SiAl cross-linked fluorhectorites (HSA-CLFH)

Shabtai et al. (1984b) showed pillared hydroxy-A1 fluorhectorites to have high thermal stability. There- fore, cross-linked hydroxy-SiA1 fluorhectorites, designated as HSA-CLFH, were prepared (method A) by reacting separate portions of a Li-fluorhectorite dispersion with HSA solutions having Si/A1 weight ratios of 0.53, 1.04. and 2.18. In all preparations the OH/AI ratio in the HSA solutions was 2.0 and the aging period was 10 days. The amount of l iSA solution used in each preparation was such as to contain 7.0 mmole Al/g fluorhectorite. The resulting HSA-CLFH products showed d(001) values of 17.3-19.0 A (for air-dried samples) and surface areas of 190-355 m2/g after outgassing at 250~ under vacuum.

Thermal stability and acidity o f HSA - C L M and HSA- CLFH products

The thermal stabilities of HSA-CLM and HSA-CLFH products, obtained by cross-linking corresponding smectites with HSA solutions that had been prepared by method A, were determined by measuring the basal spacings and surface areas of the products after heat treatment between 250 ~ and 600~ (Table 7). The d(001) values for HSA-CLM products after heat treatment in this temperature range were between 17.1 and 18.3 A, similar to those of the reference AI-CLM sample (with no Si added). The d(001) values for the HSA-CLFH products, however, were clearly higher than those of the reference A1-CLFH sample (with no Si added) heated to the same temperatures. Moreover, the HSA-CLFH products showed an unexpected increase in d(001) with increase in heat-treatment temperature from 400 ~ to 500~ and only a negligible decrease between 500 ~ to 600~ This behavior is particularly noticeable for HSA- CLFH having high Si/A1 ratios (1.04-2.18) in the pil-

Vol. 35, No. 6, 1987 Hydroxy-silicoaluminum montmorillonite and fluorhectorite 435

Table 7. Basal spacings ofhydroxy-SiAl cross-linked montmorillonite (HSA-CLM) and hydroxy-SiA1 cross-linked fluorhectorite (HSA-CLFH) products as a function of heat treatment temperature and Si/AI ratio in the pillaring hydroxy-SiA1 solution.

H S A - C L M 3 H S A - C L F H 2 T e m p . AI- AI-

(~ C L M ~ (0 .53) (1 .04) (2 .18) C E F H ~ (0 .53) (1 .04) (2 .18)

25 19.2 19.3 19.5 19,0 17.7 19.0 17,7 17.3 250 18.0 18.0 18.3 17.7 17 .1 18.2 17.8 17.2 400 17.3 17.7 17.3 17.0 17.0 18.5 17.9 17.3 500 17.2 17.2 17.4 17.5 17 .1 18.8 18,8 19.6 600 17.0 17.1 17 .1 17.3 17 .1 18.3 18.3 18.9

Products obtained by cross-linking the smectites with hydroxy-SiAl (HSA) solutions prepared by method A. All HSA solutions contained 7.0 mmole A1/g smectite.

:z Numbers in parentheses indicate Si/A1 ratio in the HSA solution used as cross-linking reactant.

3 Reference sample of pillared hydroxy-A1 montmorillonite (no Si added) prepared under identical conditions (footnote 1),

4 Reference sample of pillared hydroxy-Al fluorhectorite (no Si added) prepared under identical conditions (footnote 1).

5 All samples were heated in a tube reactor for 3 hr at indicated temperature under a flow of dry nitrogen, prior to the d(001) measurement.

laring solutions. Heat ing the H S A - C L M products to 700~ resulted in decreased d(001) values o f 16.0-16.5 A, whereas H S A - C L F H products heated at that temperature showed no clear d(001) reflections.

Two samples each of H S A - C L M and H S A - C L F H were also synthesized by cross-linking Na ,Ca-mon t - mor i l loni te and Li-fluorhectori te, respectively, with H S A solutions (Si/AI ratios o f 1.0 or 2.0) prepared according to me thod B (see Exper imenta l and Table 2). Hea t t rea tment o f these pil lared products at 600 ~ 650~ resulted in large, contract ion-res is tant basal spacings of 18.0-18.7 A for the H S A - C L M and 18.9- 19.7 A for the H S A - C L F H products.

The changes in surface areas o f the H S A - C L M and H S A - C L F H products (der ived by me thod A, Table 7) as a funct ion o f heating tempera ture and Si/A1 ratio in the pillaring solution, are summar ized in Figures 1 and 2. The surface area of H S A - C L M products gradually decreased with increase in heat ing tempera ture from 250 ~ to 700~ The decrease was relat ively fast for a Si/A1 ratio o f 0.53 but slower for a Si/A1 ratio o f 1.04 or 2.18 in the pillaring HSA solution. The behav io r o f H S A - C L F H products was quite different in that the surface area of such products (in part icular those prepared with a Si/AI ratio = 1.04 or 2.18 in the pillaring H S A solution) r emained essentially constant at heating tempera tures o f 250~176 All samples showed a decrease in surface area at 700~

The thermal stability o f CLS products probably de- pends on the structure and propert ies o f both the smecti te c o m p o n e n t and the pillaring agent. Dehydra t ion and differential thermal analysis (DTA) studies o f nor-

I I I I I

400 - Si/,41 Ratio

0 . 5 3

E 1 .04

300

2 .18

200

tO0 L J i I I I 200 300 400 500 600 700

H E A T - T R E A T M E N T T E M P E R A T U R E (~ Figure 1. Change in surface area of hydroxy-SiAl montmorillonites as a function of Si/AI ratio in the hydroxy-SiA1 pillaring solution and heat-treatment temperature.

mal montmor i l lon i t e s (i.e., mon tmor i l lon i t e containing small amount s o f Fe and Mg in the octahedral layer), including a montmor i l lon i te from Belle Fourche, South Dakota , indicate rapid dehydroxylat ion, i.e., loss o f structural O H groups, beginning at about 500~ and ending at 700-800~ (Grim, 1968). The dehydroxylat ion o f such dioctahedral smecti tes below 800~ however , was accompanied by only mino r changes in the f ramework of the layer, inasmuch as the structure was most ly destroyed above 900~ For hectofi te , a t r ioctahedral smect i te having some OH-groups replaced by fluoride, dehydroxyla t ion began only at - 7 0 0 ~ and was not comple te even at 930~ In synthetic f luorhectori te all structural O H is replaced by fluoride; hence, dehydroxyla t ion should not occur. Further, G r i m (1968) indicated that a l though diocta-

J I I ~ I I |

500 t 2 E

4 0 0 S i / ~ l Ra t i o o.53 ~

300

1.04 u. �9 �9

200 2.18

100

0 [ I i i I 1 200 300 400 500 600 700

HEAT-TREATMENT TEMPERA TURE ( ~

Figure 2. Change in surface area of hydroxy-SiAl fluorhectorites as a function of Si/A1 ratio in the hydroxy-SiA1 pillaring solution and heat-treatment temperature.


4.0

s

-~ 3.0 o

O 2.o

O C~

m 1.0

i SI/AI r a t i o

2.18

1.04 0.53

0.00 (AI-CLM

0 I I I I O0 150 200 250

TEMPERA TURE (~

Figure 3. Equilibrium adsorption of ammonia on hydroxy- SiAl montmorillonite products as a function of temperature and Si/AI ratio in the hydroxy-SiAl pillaring solution.

4.0 t I I I

-~ 3.0

~ E

Z S I / A I 0 r a t i o 2.01 ~. 2.18

1.04 i 0 .53

1,0

- ~ A -

Z

l -CLFH) 0.00

0 [ I I l 100 150 200 250

TEMPERATURE (~

Figure 4. Equilibrium adsorption of ammonia on hydroxy- SiAl fluorhectorite products as a function of temperature and Si/A1 ratio in the hydroxy-SiAl pillaring solution.

hedral smectites, e.g., montmorillonite, form a stable anhydride persisting at temperatures as high as ~ 900~ trioctahedral smectites, e.g., hectorite, do not form such an anhydride�9 Apparently, no detailed studies of high- temperature (>700~ structure changes in hectorite or fluorhectorite have been reported. For heat treatment at -< 600~ however, the observed higher thermal stability of HSA-CLFH than that of corresponding HSA-CLM products, as evidenced by the more constant d(001) values and surface areas of the former at _<600~ (Table 7; Figures 1 and 2) could have been due largely to the higher resistance to dehydroxylation of the fluorhectorite component as compared with that of the montmoril lonite component in the corresponding pillared systems. This result is consistent with the previously observed higher thermal stability of hydroxy-A1 pillared fluorhectorites compared with that of corresponding pillared montmorillonites (Shabtai et al., 1984b). Further, the markedly higher values of the basal spacings for HSA-CLFH products at 500~176 as compared with those of the reference hydroxy-A1 fluorhectorite (Table 7) suggest that the incorporation of Si into the oligocationic pillars increased the thermal stability of the pillared products. This interpretation is supported by the higher d(001) values and better constancy of the surface areas of HSA-CLFH products prepared with pillaring solutions having Si/AI ratios of 1.04-2.18, compared with those having a lower Si/ A1 ratio of 0.53 (Table 7; Figure 2).

The total acidities of the HSA-CLM and HSA-CLFH products listed in Table 7 were determined as a function of temperature and Si/AI ratio in the pillaring solution, using NH 3 adsorption (see Experimental). Re-

sults obtained for the HSA-CLM products and the reference A1-CLM sample are given in Figure 3; corresponding data obtained for the HSA-CLFH products and the reference A1-CLFH sample are summarized in Figure 4. As seen in Figure 3, the total acidity of the HSA-CLM samples, as reflected by the NH3 adsorption level, was considerably higher than that of the reference A1-CLM sample. Among HSA-CLM products the acidity increased with increase in the Si/AI ratio in the HSA pillaring reactant, i.e., in the pillaring oligocations. Further, the data in Figure 3 indicate that the ratio of the total NH 3 adsorption, i.e., the total acidity of each HSA-CLM sample vs. that of the A1-CLM sample (Si/A1 = 0) increased with increase in temperature, indicating that HSA-CLM products have a higher concentration of strong acid sites as compared with the reference A1-CLM sample which was obtained by pillaring with Si-free, hydroxy-A1 oligocations. Similar trends are seen in Figure 4, except that the differences in total acidities for HSA-CLFH vs. the reference A1- CLFH product are even larger compared with those for the corresponding pillared montmorillonites (Fig- ure 3). The increase in the acidity of HSA-pillared products with increase in the Si/A1 ratio of the pillaring solution indicates the presence of acidic silanol groups in the hydroxy-SiA1 cross-links. By analogy with the surface chemistry ofH*-forms ofzeolites (Ward, 1976), silanol groups possessing protonic acidity were probably present at the surface of the hydroxy-SiAl pillars (see below). Dehydroxylation of the pillars at high temperature should have converted a major part of such groups into Lewis acid groups, in accordance with previously proposed schemes (Ward, 1976), as follows:

Vol. 35, No. 6, 1 9 8 7 Hydroxy-silicoaluminum montmoriUonite and fluorhectorite 437

14 H

o x /o /o N io /o -.zo o\ io\~io ~ /o Si AI $[ A] ~ Si AI Si AI

o / \oo / \oo / \oo / \o HZ " ~ o / \oo / \oo / \oo / \o

Both Bronsted and Lewis acidity in pillared clays were previously reported by Shabtai et al. (1984a) and Occelli (1986).

Structural features o f l iSA-pi l lared smectites

Earlier schematic models of cross-linked smectites (Shabtai, 1979; Vaughan and Lussier, 1980; Pinnavaia, 1983; Shabtai et al., 1984a) did not provide exact in- formation on the structure of the pillaring species, nor did they indicate clearly the mode of chemical interaction between the smectite layers and the pillaring component or the dependence of this interaction upon heat treatment. Recent solid-state 29Si and ~7A1 magic angle spinning-nuclear magnetic resonance (MAS- NMR) studies, however, have provided data relevant to the structure of CLS products. Plee et al. (1985), for example, interpreted 29Si and 27A1 MAS-NMR spectra of hydroxy-A1 hectorite, Laponite (a synthetic hectorite), and beidellite before and after mild calcination (300~176 as follows: (1) Pillaring of all clays involved intercalation of the [AI~304(OH)24(H20),2] 7+ species, previously identified by Bottero et al. (1980) as the main oligocation present in base-hydrolyzed A1C13 solutions having an OH/AI ratio between 1.5 and 2.5. In the non-calcined pillared products only electrostatic bonding exists between the negatively charged layers and the pillaring oligocations. (2) Mild calcination (300~176 of pillared smectites containing no tetrahedral substitution, e.g., hectorite and Laponite, caused no structural changes in the clay tetrahedral layer or in the hydroxy-A1 oligocation. (3) Mild calcination of pillared beidellite, a clay characterized by a tetrahedral seat of charge, produced stable covalent bonds between the clay layers and the Al13 pillars and major structural changes both in the clay layers and in the hydroxy-Al~3 oligocations. According to Plee et al. ( 1985), heat-induced formation o f new A1-O-AI bonds was achieved between some inverted AI tetrahedra in the clay tetrahedral layer (see Edelman and Favejee, 1940) and aluminol groups in the hydroxy-Al oligocation. The subsequent 27A1 and 298i MAS-NMR study by Pinnavaia et al. (1985a) showed that the reactivity of the smectite tetrahedral layer, as reflected in its ten- dency to form stable covalent bonds with the hydroxy- AI oligocations, did not depend on the origin and lo- cation of the negative charges alone, but also on the smectite composition. Thus, pillaring of octahedrally charged synthetic fluorhectorite, followed by mild calcination (350~ resulted in the formation of stable A1-O-Si linkages between AI sites in the hydroxy-A113 oligocations and SiO4 tetrahedra in the smectite tetrahedral layer. No such bonding was found for pillared montmori l loni te or hectorite, and Pinnavaia et al.

(A)

� 9174 AI Si

@�9 H~O O

(B) OH

) Figure 5. Molecular models of(A) an [AI~ 304(OH)24(H20)~z] 7+ oligocation, and (B) a hydroxy-SiA1 oligocation derived from (A) by partial substitution of--OH with -Si(OH)3 groups.

(1985a) ascribed the higher reactivity of the fluorhectorite to the presence of F atoms, which labilize the Si-O bonds in the tetrahedral layer and promote cou- pling with the intercalated oligocations.

In the present study, the observed extraordinarily high thermal stability of HSA-pil lared fluorhectorites (as high as 600~ see Table 7 and Figure 4) indicates the formation of strong covalent bonds between the HSA oligocations and the tetrahedral layer of fluorhectorite, in agreement with the findings of Pinnavaia


, ) / 61. "AJ2

/ S i - /S i - -

/ r k

Oxo-AI core oF HSA oligomer

Cross-linking Si sites

(ex-Si(OH) 3 groups)

Inverted SiO 4 tetrahedra

Smectite layer

Figure 6. Proposed schematic model for cross-linking interaction of a hydroxy-SiA1 oligocation with the tetrahedral layer of fluorhectorite, ex-Si(OH) 3 groups - groups that have changed during calcination (see text).

et al. (1985a) for the Al~O~-pillared form of this smec- rite. The high efficiency of preparative method B, which involved preliminary preparation and aging of hydroxy-Al~3 oligocations followed by silanization (silicalization) of the latter, indicates that the HSA oligocations probably consisted of an AI~3 oligocationic core and peripheral -Si(OH) 3 groups, as proposed in the structural model shown in Figure 5B. The arbitrarily selected number of-Si(OH)3 groups per HSA oligocation in Figure 5B is six, but it could be lower or higher (theoretically as high as 24; see reaction (1)) depending on the Si/A1 ratio in the pillaring solution and other experimental factors, e.g., temperature. Based on the model for pillar-smectite bonding in A1-CLFH (hydroxy-A1 fluorhectorite), proposed by Pinnavaia et al. (1985a), a similar interaction between the fluorhectorite tetrahedral layer and HSA oligocations leading to formation of new Si-O-Si bonds could have oper- ated in the formation of the HSA-CLFH products of the present study, as shown schematically in Figure 6. This proposed model involves bonding between inverted SiO4 tetrahedra and -Si(OH)3 groups in the HSA oligocation. Calcination and attendant cross-linking was probably accompanied by considerable dehydroxylation of silanol and aluminol groups in the HSA oligocation. Further, for HSA oligocations having a low Si/AI ratio, part of the bonds between the tetrahedral layer and the oligocations may have involved aluminol groups in the latter. A structural model for HSA-CLFH products is proposed in Figure 7. The model accounts not only for a thermally stable system formed by Si-

si s[ , / 6 0

0

0 0 H §

Si /S! Si Si

H e O' 0 H + H* O / O'

~O |

P P H* 9 9 H+ ~i gi- ~i si

Figure 7. Proposed schematic structural model for fluorhectorite cross-linked with hydroxy-SiA1 oligocations (HSA- CLFH). Dashed and solid lines indicate schematically octahedral and tetrahedral sheets in smectite unit layer.

O-Si cross-linking, but also explains the high acidity of HSA-pillared smectites (Figures 3 and 4) by partial ionization of A1-O-Si(OH)3 groups to produce protonic acidity. Whereas the generation of Br6nsted acidity in calcined hydroxy-Al~3 pillared smectites can be ascribed to hydrolysis, dissociation of coordinated water molecules, or dehydroxylation-rearrangements within the hydroxy-Al ol igocat ions at high tempera ture (Vaughan and Lussier, 1980; Pinnavaia et al., 1984; Occelli, 1986), the markedly higher acidity of HSA- pillared montmorillonites and fluorhectorites observed in this study as compared with that ofhydroxy-A1 pillared forms of the same smectites (Figures 3 and 4, respectively), was apparently due primarily to the acidic silanol groups in the HSA oligocations.

The use of very low ca lc ina t ion tempera tures (~350~ in the MAS-NMR studies reported above may have been dictated by the low thermal stability of the pillared clay samples, some of which have been prepared by an apparently inefficient method, e.g., di- alysis of a concentrated clay-hydroxyaluminum slurry (Plee et al., 1985), rather than by pillaring with appro- priately aged hydroxy-Al oligomeric solutions. To de- termine whether high-temperature treatment could result in cross-linking of smectites without tetrahedral substitution, e.g., montmorillonite, pillared samples that can withstand temperatures >_ 500~ must be prepared for NMR studies. Methods for preparation of such thermally stable pillared smectites having d(001) values between 17 and 18.5 A and surface areas between 300 and 400 m2/g after calcination at 5000C were previously described by Tokarz and Shabtai (1985). NMR studies of thermally stable hydroxy-SiA1 smectites are presently in progress in this department.

ACKNOWLEDGMENTS

We thank the Chalmers University of Technology, Gothenburg, Sweden (through Prof. J. E. Otterstedt) for partial financial support, and Prof. F. E. Massoth, University of Utah, for helpful suggestions on the cat- alyst acidity measurements.

Vol. 35, No, 6, 1987 Hydroxy-silicoaluminum montmorillonite and fluorhectorite 439

R E F E R E N C E S

Atkins, M. P. and Ashton, A. G. (1985) Novel pillared interlayered clays, processes for their production and uses thereof: European Patent 4ppl. 85300040.4, Aug. 7, 1985, 20 pp.

Barter, R. M. and Jones, D. L. (1970) Chemistry of soil minerals. Part VIII. Synthesis and properties of fluorhectorites: J. Chem. Soc. (A), 1531-1537.

Bottero, J. Y., Cases, J. M., Fiessinger, F., and Poirier, J. E. (1980) Studies of hydrolyzed aluminum chloride solutions. 1. Nature of aluminum species and composition of aqueous solutions: Z Phys. Chem. 84, 2933-2939.

Edelman, C. H. and Favejee, J. C. L. (1940) On the crystal structure of monlmorillonite and halloysite: Acta Crystal- logr. 102, 417-431.

Endo, T., Mortland, M. M., and Pinnavaia, T. J. (1980) Intercalation of silica in smectite: Clays & Clay Minerals 28, 105-110.

Grim, R. E. (1968) (Tay Mineralogy: McGraw-Hill, New York, 278-352.

Gunnarsson, L. and Nilsson, R. (1983) Polyaluminumsul- fat-eft redskap for pappersmakaren: Cellulosa No. 2, 22- 25.

Hodges, S. C. and Zelazny, L. W. (1983) Interactions of dilute, hydrolyzed aluminum solutions with clays, peat and resin: Soil Sci. Soc. Amer. J. 47, 206-212.

Holy, N. L. (1983) Polymer-bound phosphine catalysts: in Homogeneous Catalysis with Phosphine Complexes, L. H. Pignolet, ed., Plenum Press, New York, 447-448.

Johansson, G. (1960) On the crystal structures of some basic aluminum salts: ,4eta Chem. Scan. 14, 771-773.

Lahav, N., Shani, U., and Shab~ai, J. (1978) Cross-linked smectites. I. Synthesis and properties ofhydroxy-aluminum montmorillonite: Clays & Clay Minerals 26, 107-115.

Lewis, R. M., Ott, K. C., and Van Santen, R. A. (1985) Silica-clay complexes: U.S. Patent 4,510,257, Apr. 9, 1985, 10 pp.

Luciuk, G. M. and Huang, P.M. (1974) Effect ofmonosilicic acid on hydrolytic reactions of aluminum: Soil Sci. Soc. Amer. Proc. 38, 235-244.

Occelli, M. L. (1986) New routes to the preparation of pillared montmorillonite catalysts: J. Mol. Catal. 35, 377- 389.

Pinnavaia, T. J. (1983) Intercalated clay catalysts: Science 220, 365-371.

Pinnavaia, T. J., Landau, S. D., Tsou, M. S., and Johnson, I. D. (1985a) Layer cross-linking in pillared clays: Z Amer. Chem. Soc. 107, 7222-7224.

Pinnavaia, T. J., Mortland, M. M., and Endo, T. (1983) Silica-clay complexes: U.S. Patent 4,367,163, Jan 4, 1983, 9 pp.

Pinnavaia, T. J., Raythatha, R., Guo-Shuh Lee, R., Halloran, L. J., and Hoffman, J. F. (1979) Intercalation of catalytically active metal complexes in mica-type structures. Rhodium hydrogenation catalysts: Z Amer. Chem. Soc. 101, 6891-6897.

Pinnavaia, T. J., Tsou, M. S., and Landau, S. D. (1985b) New chromia-pillared clay catalysts: J. Amer. Chem. Soc. 107, 4783-4785.

Pinnavaia, T. J., Tsou, M. S., Landau, S. D., and Raythatha, R .H. (1984) On the pillaring and delamination ofsmec- tite clay catalysts by polyoxocations of aluminum: J. Mol. Catal. 27, 195-212.

Plee, D,, Borg, F., Gatineau, L., and Fripiat, J. J. (1985) High-resolution solid-state 27A1 and zgsi nuclear magnetic resonance study of pillared clays: J. Amer. Chem. Soc. 107, 2362-2369.

Shabtai, J. (1979) Zeolites and cross-linked silicates as me- dia for selective catalysis: Chim. Ind. 61, 734-741.

Shabtai, J. and Fijal, J. (1986) A novel class of hydropro- cessing catalysts and preparation methods: U.S. Patent 4,579,832, Apr. 1, 1986, 12 pp.

Shabtai, J. and Lahav, N. (1980) Cross-linked montmorillonite molecular sieves: U.S. Patent 4,216,188, 6 pp.

Shabtai, J., Massoth, F. E., Tokarz, M., Tsai, G. M., and McCauley, J. (1984a) Characterization and molecular shape selectivity of cross-linked montmorillonite (CLM) catalysts: in Proc. 8th Internat. Congress Catal., Berlin, 1984, Vol. IV, Verlag-Chemie, Weinheim, 735-745.

Shabtai, J., Rosell, M., and Tokarz, M. (1984h) Cross-linked smectites. III. Synthesis and properties of hydroxy-aluminum hectorites and fluorhectorites: Clays & Clay Minerals 32, 99-107.

Sterte, J. (1986) Synthesis and properties of titanium oxide cross-linked montmorillonite: Clays & Clay Minerals 34, 658-664.

Tokarz, M. and Shabtai, J. (1985) Cross-linked smectites. IV. Preparation and properties of hydroxyaluminum-pillared Ce- and La-montmorillonites and fluorinated NH4 +- montmorillonites: Clays & Clay Minerals" 33, 89-98.

Turner, R. C. and Ross, G.J . (1969) Conditions in solution during the formation of gibbsite in dilute aluminum salt solutions. III. Hydroxyaluminum products of reactions during the neutralization of aluminum chloride solutions with sodium hydroxide: Can. J. Soil Sci. 49, 389-396.

Vaughan, D. E. W. and Lussier, R.J . (1980) Preparation of molecular sieves based on pillared interlayered clays (PILC): in Proc. 5th Internat. Conf. Zeolites, Naples, 1980, L. V. C. Rees, ed., Heyden, London, 94-101.

Wada, S. I. and Wada, K. (1980) Formation, composition and structure of hydroxy-aluminosilicate ions: J. Soil Sci- ence 31, 457-467.

Ward, J. H. (1976) Infrared studies of zeolite surfaces and surface relations: in Zeolite Chemistry and Catalysis, J. A. Rabo, ed., Amer. Chem. Soc., Washington, D.C., 118-284.

Yamanaka, S., Doi, T., Sako, S., and Hattori, M. (1984) High surface area solids obtained by intercalation of iron oxide pillars in montmorillonite: Mat. Res. Bull. 19, 161- 168.

(Received 29 January 1987, accepted 16 May 1987," Ms. 1634)

Documents

CROSS-LINKED SMECTITES. V. SYNTHESIS AND … 35/35-6-429.pdf · HYDROXY-SILICOALUMINUM MONTMORILLONITES AND FLUORHECTORITES ... Abstract--Solutions containing hydroxy-SiA1 ... based