Zirconium Pillared Montmorillonite: Influence of Reduced Charge of the Clay

97

Zirconium Pillared Montmorillonite : Influence of Reduced Charge of the Clay

E.M. Farfan-Toms and P. Grange Unit6 de Catalyse et Chimie de MatCriaux DivisCs, UniversitC Catholique de Louvain, Place Croix du Sud 2, boite 17, 1348 Louvain-la-Neuve (Belgium) ABSTRAa Lithium introduced in the structure of the clay allows to control the density of the pillars and the strength of interaction between the pillar and the clay layer. At low calcination temperature, the interlayer distances and the surface area increased. The thermal stability of the clay, calcined at temperature higher than 4Oo0C, drastically decreases.

INTRODUCTION The intercalation of large size inorganic complexes between the layers of montmorillonite allows to prepare thermally stable microporous solids. Brindley and Sempels (l), Vaughan et al. (2) and Shabtai (3) have shown that the experimental conditions of A1 intercalation influences the physicochemicalproperties of the clay. The nature, amount and spacial distribution of the pillars change the thermal stability, texture and acidity of the pillared clays. For example, Rausch and Bale (4) have reported that the OWAl ratio modifies the structure of the A1 complex and that monomeric [Al(OH)x(H20)6-x]3-x' or polymeric [Al1304(OH)a(H20)12]species can be obtained. Clearfield ( 5 ) demonstrated that the polymerisation state of Zr species depends on the temperature, concentration and pH of the solutions. In any case, the height of pillars is largely controlled by the polymerisation state of the intercalated complexes. However, in order to maintain the accessibility of the inner surface, the density or spacial distribution of the pillars has to be controlled. This parameter has been studied by Plee et a1 (5), and Shabtai et a1 (7) for A1 pillared clays and Farfan-Toms et al(8) for zirconium. The distribution of the pillars may be controlled by the polymerization degree of the complex and by a precise adjustment of the charge density of the clay through a partial blocking of the exchangeable sites. The introduction of small cations (Li+, Mg2+ or Al3+) able to migrate at low temperatures to vacant sites in the octahedral layer and to partially neutralize the net octahedral charge due to isomorphic substitution represents the simplest way to prepare clays with reduced charge. This phenomenon is known as the Hoffman-Klemeneffet (9). Later Calvet et a1 (10) and Clementz et al(l1,12) demontrated that the charge decrease directly depends on the amount of Li ions introduced into the exchangeable sites. It is expected that the application of the Hoffman-Klemen effect will allow to modify the density of the pillars in Zr pillared montmorillonite. This paper reports the influence of Li concentration on the physico-chemicalproperties of Zr pillared montmorillonite.

98 E. M. Farfan-Torres and P. Grange

EXPERIMENTAL METHODS Preparation of the S ~les D The Li ions were introduced in two different ways: either before or after Zr intercalation. The montmorillonite (Weston L-Eccagun) was first exchanged with NaCl (1N) and washed. Two montmorillonites with reduced charge were prepared following the Brindley and Ertem method (13). Part of the Na+ montmorillonitewas first saturated with LiCl (1N) and washed. The Li+ clay thus obtained and Na+ clay suspension were stirred for 24 hours at 25OC and dried on glass plate. The films were then heated at 22OOC for 24 h in order to allow Li diffusion in the clay structure. Two different Li concentrations (F4.4 and F=0.6) were used. The Na+Li+ modified montmorillonite were dispersed in water acetone solution (l/l). The ZrOC12,8H20 solution was added to the Na+Li+ montmorillonite (0.02g.l-1;Zr/Clay=S.CEC). The suspension was stirred with NaOH solution (0.1 N) up to a OHEr ratio of 0.5. The final pH of the suspension was 1.85. After two hours of reaction at 4OoC the Zr pillared clay was washed up to constant conductivity of the solution, freeze-dried and calcined at different temperaturesup to 700°C (Em-02 and EIII-03). For comparison, Zr montmorillonite have been prepared with non modified Na+ montmorillonite (EIII-01). In addition, after calcination at 4OO0C, Li was also introduced in this sample (EIII-04). PHYSICO-CHEMICAL CHARACTERIZATION The Si-Al-Zr content of the clays, before and after pillaring, was determined by X-ray fluorescence (XRF-Philips PW 1450).The other elements were analyzed by Atomic Absorption (AAVarian techtron AA-5) after sulfofluorhydric leaching. The dichroic properties of the Li modified montmorillonite were followed by orientation of thin films (0-45') in the IR beam (FTIR Brucker IFS 88). The cationic exchange capacity (CEC) of the samples calcined at 400OC was evaluated. The basal spacing (d 001) (DRX-Kristalloflex-805 Siemens) and the surface area (MicromeriticsASAP 2400) was obtained on the solids calcined at different temperatures. X-Ray diffraction patterns have also been obtained after ethylenglycol saturation of selected samples. High resolution transmission electron microscopy (HREM) was performed (Jeol 100 CX Temscan) on ultrathin preparations (LKB Ultratome type 8802A). TPD (NH3) and infrared spectroscopy (pyridine)allowed to evaluate the acid properties of the solid calcined at 400 and 600OC. EXPERIMENTAL RESULTS The chemical composition and the cationic exchange capacity of the solids are reported in table 1. The relatively high Na content of the Zr modified pillared montmorilloniteshas to be noted (EIII02, EIII-03). Figures 1 and 2 report the IR spectra of the Na+ montmorillonite (A) and the Li+ modified montmorillonite [Na+Li+0.4 (B); Na+Li+0.6 (C)] before calcination (l), after calcination at 22OOC (2) and orientation of the sample in the IR beam (3). These figures illustrate the OH stretchnig vibration (fig. 1) and the bending (fig. 2) vibrations of the non pillared samples.

Zirconium Pillared Montmorillonite 99

Table 1. Chemical composition and CEC of the samples. Na+ mont Na++-Li+mont. Na+-Li+mont EIII-01 Em-02 ZIII-03 EIII-04 F = 0.4 F = (0.6)

E:3.66 :

63.98 22.35 3.17 0.86 0.09 0.11 1.63 0.55

64.41 22.72 3.08 0.86 0.12 0.09 1.20 0.70

10.31

7.26

6.78

0.83 0.1 1 0.05 2.74

86

50

37

49.20 51.73 52.69

20.35 11.06

48.26 18.40 18.18 16.37 2.39 2.63 2.53 0.72 0.66 0.75 0.05 0.06 0.05 0.03 0.03 0.03 0.12 0.20 0.18 0.38 0.47 0.39 17.35 17.35 20.30 8.84 7.73 11.14

48*

40*

16.40 2.24 0.59 0.08 0.00 0.05

37*

47*

Calcined at 4OOOC

g

I

a c

i /mi't

Fig. 2 Fig. I A : Na+ montmorillonite; B :Na+Li+(0.4) montrnorillonite;C : Na+Li+(0.6) montmorillonite. 1 :Fresh sample; 2 : montmorillonite treated at 220°C; 3 : wafer rotated at 45°C. The DRX spectra of the solids calcined up to 500°C are illustrated on fig. 3. The position of the d 001 diffraction line versus thecalcination temperature for the Na+ montmorillonite (Em-01) and the Zr pillared modified clays (Em-02, EIII-03) is reported on fig. 4. The same evolution of the basal spacing (d 001) for the pillared montmorillonite in which the r is illustrated in fig. 5. It has to be mentioed that, after saturation of Li has been introduced after the Z the solids by ethylene glycol, the interlayer distance of the samples calcined at 400°C is always slightly higher than before saturation. It has to be noted that the introduction of Li into the structure of the clay before pillaring and a calcination temperature lower than 300°C increase the surface area of the solids. A calcination temperature higher than 500°C gives amorphous solids. The Li clay structure collapses. In addition, these solids treated at 700°C present the same surface area as the Na montmorillonite.

100 E. M. Farfan-Torres and P. Grange

&

Q

20

18

10

2

10

2 10

L l

2 10

16 I

200

0

2

LOO

600

I BM)

T IW

28

Fig. 4 : (d.OO1) Fig. 3 : DRX The specific surface area of the solids calcined at different temperatures, up to 7OO0C, is

16

0

The amount of NH3, desorbed up to 40O0C, for the solids calcined at 400 and 600OC is reported in table 2. For the samples EIII-02 and EIII-03, a large amount of adsorbed N H 3 still remains strongly Table 2 :TPD :NH3 desorbed up to 400OC. samples

EIII-01 EIII-02 ~m-03 Em-04 Na+Mont. (Fa) Na+Li+Mont (F4.4) Na+Mont. (F=0.6)

NH3 (pmo1.g-1 at 400OC) 4oooC 6OOOC 329.0 272.0 249.0 165.0 390.0 200.0 49.0 29.0 75.0 31.5 54.0 25.5

E:!

Zirconium Pillared Montmorillonite

101

adsorbed at 400°C. In addition, it is observed that the Li modification does not drastically change the total acidity of the pillared clays. The FITR analysis of adsorbed pyridine evidences the same evoltuion for all the samples. The normalized intensities of the Br6nsted sites (1540 cm-I), Lewis sites (1448 cm-l) as well as the Lewis-Bronsted ratio with the outgazing temperature is reported in kble 3. Table 3 :FTIR (pyridine) : normalized intensities (x 10-3) B. (154Ocm-1)

Samples

150

300

400

EIII-01 EIII-02 EIII-03 EIII-04

5.9 7.7 9.6 7.9

4. 6.: 6.6 6.9

.7 3.1 4.0 4.0

UB

L(148~m-~) 150 300 400

100

300

400

14.0 9.4 16.9 11.1 13.3 8.8 16.6 11.2

3.44 2.75 2.05 2.80

3. 2:. 2.02 2.32

% 2.17

20.3 21.2 19.6 22.3

2.80

DISCUSSION Structure and Droperties of the modified montmorillonitg The IR spectra of the reduced charge montmorillonite (fig.1) indicates that, after heating at 22OoC, the OH stretching vibration (3630 cm-1) is shifted to 3636 cm-l for the Na+Li+ montmorilloniteF S . 4 and 3639 cm-1 for the F S . 6 samples. This shift is more pronounced when the film is oriented at 45" in the IR beam. This suggests the dichroic character of this band. In addition, a shoulder at 3670 and 3700 cm-l appears. Prost and Calvet (10,14) attributed this dichroic band to OH groups perpendicular to the plan. The orientation change of the OH groups has been correlated to the interlayer cation migration in the octahedral cavities of the clay structure. The Li migration into a vacant site close to isomorphic substitution has to be linked to the inversion of the OH groups (10). The 3670 cm-l band is due to Al-Li-Mg configuration and the 3640 cm-l one associated with Al-Li-Al. Vedder (15) attributes the shoulder at 3700 cm-* to the Mg-Li-Mg structure. It has also been observed that Li introduction in octahedral sites induces a shift in the bending zone (10). The linear variation of the CEC with the Li concentration also supports the incorporation and migration of the Li cations in the octahedral vacant sites. Influence of Li on the structure and thermal stabilitv of Zr montmorilloni& A small increase of the (d 001) basal spacing is observed for the Li containing Zr pillared clays. However, the thermal stability of these solids drastically decrease. At high temperature, the collapse of the strucutre is also supported by the decrease of the surface area which is, at 700OC, almost identical to those measured for the montmorillonite. Different hypothesis may be proposed to explain the increase of the interlayer distance at low temperature: (i) a better polymerization of the intercalated complex; (ii) a modification of the dismbution of the pillars; (iii) a lower interaction between the pillar and the silica layer. The first hypothesis may easily be eliminated since the small variation of the height of the pillars (less than 1 A) cannot be explained by structural changes of the

102 E. M. Farfan-Torres and P. Grange

polymeric species introduced between the layers. In addition, the experimental conditions of the synthesis (pH, temperature, time) are always identical and the hydrolysis-polymerizationprocess of the zirconium salt should be identical. On the contrary, the important decrease of the charge of the clay may change the interaction strength between the polymer and the clay layer. This last assumption is strongly supported by the variation of interlayer distance of the Zr pillared after ethylene glycol saturation. Two hypothesis may explain the poor thermal stability evidenced by the collapse of the structure at 500°C : (i) the decrease of the CEC could induce the intercalation of smaller amounts of pillars regularly distributed; (ii) the distribution of the pillars could be less homogeneous. Based on the chemical analysis, it has been shown that the content of the different solids change by 3%. Assuming that the structure of the zirconium complex is represented by [(Zr40H)i4(H20)10]2+,a minimum of 4 moles of z r O 2 will be produced upon calcination. Four moles give 2000 meq charge, and 1000 meq for dimer species. The values of the observed basal distances seem to indicate that a dimer complex is intercalated. In addition, results of table 1 show that 45%, 60% and 50% of the sites are neutralized for the samples EIII-01, EIII-02 and EIII-03 respectively. The large number of exchanged sites could be correlated with the large distance between the sites, the loss of thermal stability being due to the low number of pillars and not to the poor spatial distribution of the complexes. At low calcination temperature, the decrease of the charge density which induces a larger distance between the layers and an enhancement of the distances between the pillars brings a better accessibility to the inner surfaces and this explains the high surface area of these pillared montmorillonites below 300OC. However, the surface area is drastically influenced at higher thermal treatment. The introduction of Li after pillaring (EIII-04) allows to explain part of this behaviour. For this solid, the basal spacing is not changed at low temperature, but the calcination decreases the thermal stability and the porosity. This is exclusively due to the lithium as the zirconium oxide pillars were present inside the layer before the modification of the clay. Such behaviour was observed for montmorillonite saturated with ammonium (16). For smectites, in which the charge is produced by isomorphic substitution in the tetrahedral layer, N a + are adsorbed on Si-0-A1 groups (beidellite for example). For montmorillonite, in which the charge is mainly due to octahedral substitution, migrates to octahedral layers (in the same way as Li) and induces dehydroxylation at lower temperature. In addition, Li acts as flux and improves the sintering of the clays. ACidiQ The Li diffusion in the clay structure slightly enhances the acidity of the Zr pillared montomorillonite as shown by the variation of the amount of desorbed NH3 We also observed a parallel decrease of the Lewis and increase of the Bransted sites. The total acidity of the EIII-02 and EIII-03 samples is reduced as compared with the pure Zrmontmorillonite. However, the acid strength is enhanced. The lowest charge on the surface layer could explain this behaviour.

Zirconium Pillared Montmorillonite 103

CONCLUSIONS The diffusion of Li+ in the octahedral cavities of the Na+montmorilloniteallows to control the density of the pillars of the Zr pillared montmorillonite. The solids, stable up to 30O0C, have larger surface area basal distancy than the pure Zr montmorillonite. The distance between the pillars increases while the interaction strength between the pillars and the clay layer decreases. However, the thermal stability of the Li-Zr pillared clays is drastically influenced after calcination at temperatures higher than 400OC. This is mainly due to Li acting as flux. AKNOWLEDGMENTS The financial support of the SPPS (Service de la Programmation de la Politique Scientifique), Belgium, is gratefully acknowledged. E.M. Farfan-Torres thanks the CGRI (Commissariat GCnCral de la CommunautC FranGaise de Belgique) for her grant. REFERENCES 1 G.W. Brindley and R.E. Sempels, Clay Miner., 12 (1977), 229-236. 2 D.E.W. Vaughan, R.Y. Lussier and J.S. Magee, U.S. Patent 4;176,090 (1979), 7 pp. 3 J. Shabtai, Chim. Ind., 61 (1979), 734-741. 4 W. Rausch and H.D. Bale, J. Chem. Phys., 40 (1964), 3891. 5 A. Clearfield, Inorg. Chem., 3 (1964), 146-148. 6 D. Plee, F. Borg, L. Gatineau and J.J. Fripiat, J. Am. Chem. Soc., 107 (1985), 2362-2369. 7 J. Shabtai, M. Rose11 and M. Tokarz, Clays Clay Miner., 32 (1984), 99-107. 8 E.M. Farfan-Torres and P. Grange, Preparation of Catalysts V; Elsevier, in press. 9 V. Hoffmann and R. Klemen, Z. Anorg. Allg. Agron., 13 (1950), 269-327. 10 R. Calvet and R. Prost, Clays Clay Miner., 19 (1971), 175-186. 1 1 D.M. Clementz, M.M. Mortland and T.J. Pinnavaia, Clays Clay Miner., 22 (1974), 49-57. 12 D.M. Clementz and M.M. Mortland, Clays Clay Miner. 22 (1974), 223-229. 13 G.W. Brindley and G. Ertem, Clays Clay Miner., 19 (1971), 399-404. 14 R. Prost and R. Calvet, C.R. Hebd. SCanc. Acad. Sci. Pans, 269 (1969), 539-541. 15 W. Vedder, Amer. Mineralogist, 49 (1964), 736-768. 16 B. Chourabi and J.J. Fripiat, Clays Clay Miner., 29 (1981), 260-268.