Preparation and characterization of zirconium pillared laponite and hectorite

MICROPOROUS MATERIALS ELSEVIER

Microporous Materials 6 (1996) 27-36

Preparation and characterization of zirconium pillared laponite and hectorite P. Cool *, E.F. Vansant Laboratory of Inorganic Chemistry, Department of Chemistry, University of Antwerp ( U.I. A. ), Universiteitsplein 1, B-2610 Wilrijk, Belgium Received 19 June 1995; accepted 13 September 1995

Abstract

The Zr-pillaring of natural hectorite and synthetic laponite clay was investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) to characterize the different substrates. The difference in particle size of both clays is responsible for their differences in crystallinity, surface area (SA) and micropore volume (#PV) and consequently for their pillared forms too. Pre-adsorption of ethylenediamine in the interlayer space of laponite is performed in order to reduce the important contribution of edge-to-face and edge-to-edge stacking of the clay layers and creating a more homogeneous substrate for pillaring. As is proven by XRD, TGA and FTIR, ethylenediamine is exchanged completely for the Zr-pillaring precursors during the pillaring reaction, Surface areas and micropore volumes of, respectively, 482 mZ/g and 0.34 cm3/g for Zr-pillared laponite and 171 m2/g and 0.064 cm3/g for Zr-pillared hectorite after calcination are obtained. Through pillaring, especially small pores (<0.71 nm), additional secondary micropores are formed with laponite while for hectorite, pores over a broad size range are observed. The prepared pillared interlayered clays (PILCsl were tested for their gas adsorption behaviour. Gas adsorption measurements on Zr-laponite reveal high adsorption capacities for N, and 02 with a low N2/O 2 selectivity at 0c~C.

Keyword~. Laponite; Hectorite; Pillared clay; Amine: Gas adsorption

1. Introduction

The preparation of pillared interlayered clays (PILCs) has been investigated intensively because of their interesting properties in the field of catalysis [1] and adsorption [2]. The intercalation of several different types of pillars between the clay layers makes these substrates suitable for specific applications [3,4]. The first extensive studies on PILCs appeared * Corresponding author. 0927-6513/96/$15.00© 1996 ElsevierScienceB.V. All rights reserved SSD1 0927-6513(95)00080-1

around 1980, during the oil crisis, because of their appropriate pores for cracking processes. The preparation of alumina- and zirconia-pillared montmorillonite [5-13] was studied in detail. Pillared clays are formed by an exchange reaction of a pillaring solution, which contains large oligomeric hydroxy metal cations, with a smectite clay. The charge-compensating cations between the clay sheets exchange for those oligomers. Upon heating, dehydration and dehydroxylation of the oligomers occur. Metal oxide clusters, the so-called 'pillars', are formed between the clay layers. A

28

P. Cool, E.F Vansant/Microporous Materials 6 (1996)27-36

2-dimensional stable microporous material with a high surface area is then formed. In this study, the clay minerals laponite and hectorite are used. Both clays have a similar structure but differ in their layer diameter. For laponite the layer diameter is ca. 0.03 pm, while for hectorite this is of the order of 2 pm as for most natural clays [14]. Because of these small laponite layers, a random stacking of particles (tactoids) in a predominantly edge-to-face and edge-to-edge way can be observed, yielding a high surface area (SA=350 m2/g) and micropore volume (#PV= 0.243 cm3/g). However, hectorite has a higher crystallinity mainly due to a face-to-face orientation of the large sized plates and results in a low surface area and micropore volume• In pure laponite, the supermicropores (between 1.7 and 2 nm) dominate due to the irregular stacking. By a preadsorption of ethylenediamine in the interlayer space, the face-to-face orientation of the clay plates will increase, creating a more homogeneous substrate for pillaring. By a Zr-pillaring reaction, the amines completely exchange for the Zr-pillaring precursors resulting in a significant increase in the surface area and the micropore volume. The existence of a combined micro- and supermicroporosity in the Zr-laponite, makes it a good substrate for gas adsorption applications.

2.2. Characterization techniques 2.2.1. Nitrogen adsorption isotherms In order to determine the specific surface area and the micropore volume of the materials, nitrogen adsorption-desorption isotherms at -196°C were recorded on a Quantachrome Autosorb-1MP automated gas adsorption system. Prior to analysis the samples were degassed at 200°C during 16 h in a vacuum furnace. Surface areas were calculated using the BET-equation, since all isotherms could be classified as type II [15]. The micropore volumes of the samples were calculated using the t-method of De Boer [15]. The pore size distributions (PSD) were obtained according to the method of Zhu et al. [16, 17].

2.2.2. X-ray diffraction A Philips X-ray PW 1840 powder diffractometer (Ni-filtered CuK, rad., 45kV, 30mA, 2= 1.5405 A) was used to determine the basal spacings of the samples. Samples were prepared by airdrying a clay slurry on a glass plate.

2.2.3. Thermal analysis Thermogravimetric analysis of the treated clays was performed on a Mettler TG 50/TA 3000 thermobalance, controlled by a TC10A microprocessor. Samples were heated at a rate of 10°C/min under a N2-flow (150 ml/min).

2.2.4. Fourier transform infrared (FTIR)

2. Experimental 2.1. Materials The laponite RD was supplied by Laporte Inorganics. The idealised unit cell formula is the following: (Naff.5 •nH20 ) (Mg2.~ Li~.5) (Si 4+)s(OH)4020. The hectorite clay (SHCa-1) was obtained from the Source Clay Repository of the Clay Minerals Society, with the following idealised unit cell: (Na~.6• n H 2 0 ) ( A l o . 03+ 4 M g s . 32+ L l o . 7"+) ( S 1 "4+ ) 8 F 2 (0H)202o.

The chemicals ethylenediamine (>99%) and ZrOC12"8H/O techn, were obtained from Fluka Chemica.

The FTIR study was carried out on a Nicolet 20 SXB FTIR-spectrometer. 200 scans were recorded in the mid-IR range (400-4000 cm -1) with a resolution of 4 cm-1. A transmission FTIR cell was built, allowing heating and evacuation of a sample over a desired temperature range. An amount of 0.01 g of the PILC sample was spread out on a prepared disc of 0.4 g KBr and subsequently pressed, so that a double layered KBr/PILC pellet was formed.

2.2.5. Gas adsorption Adsorption experiments were carried out in a glass dynamic-volumetric adsorption gas apparatus [18]. Prior to the adsorption experiments the sample was degassed at 150°C during 16 h under

P. Cool, E.F Vansant/Microporous Materials 6 (1996) 27 36

vacuum. Subsequently, the samples were equilibrated to the desired adsorption temperature during 1 h.

2.2.6. Element analysis A JEOL JCXA 733 electron microprobe analyser (EPMA) was used to determine the Zr-content in the PILCs. 2.3. Synthesis of the pillared clays 2.3.1. Amine intercalation An amount of 3 g of clay was stirred overnight ( 16 h) in a 500 ml 0.3 M ethylenediamine solution. The starting pH of this suspension was 10.8. At this pH about 49.6% is present in the neutral ethylenediamine form and the same amount of ethylenediamine is single protonated. After centrifugation, the amine intercalated clay was air-dried. A few milligrams of this product, used for characterization purposes, were washed with distilled water until a pH of 7. The sample was named as: en-L and en-H for amine intercalated laponite and hectorite, respectively. 2.3.2. Zr-pillaring reaction ZrOCI2.8H20 (0.1 M) was refluxed for 24 h. After reflux, the pH of the solution was 0.95.2.5 g en-L or en-H were added to a 250ml pillaring solution (10mmol Zr/g clay) and stirred for another 24 h. After reaction, the pillaring solution was slightly yellow, while the pH increased. The treated clay sample was separated by centrifugation, washed till C1--free and air dried. Uncalcined samples and samples calcined for 2 h at 500"C with a heating rate of 10°C/min were investigated.

3. Results and discussion

3.1. Amine intercalation The adsorption of ethylenediamine between the clay sheets can be seen in the XRD. Fig. 1a and b represent the X-ray diffractograms of the clays before and after treatment with amine. Because of the random orientation of laponite plates, pores

29

over a very broad range exist. This explains the appearance of a broad XRD-peak of low intensity in Fig. 1a. A pre-adsorption of ethylenediamine in the interlayer space of laponite favors a face-toface orientation of the clay plates, reflecting an enhanced intensity in the X-ray diffractogram of en-L. An XRD peak appears with its maximum at 7 20 corresponding to a do01 distance of 1.32 nm for en-L. Hectorite, with its larger layer size, has a much higher crystallinity because of predominantly faceto-face orientation of the plates. This results in a sharp XRD peak in Fig. lb. After amine intercalation of hectorite, a peak maximum at a dool distance comparable to that of en-L is obtained. This dool value is in agreement with literature results for amine intercalated clays. If the organic amine forms a monolayer in the interlayer space of the clay, a dool distance between 1.25 and 1.35 nm is reported [19]. Ethylenediamine adsorption on montmorillonite indeed forms a monolayer with the organic chain parallel to the clay layers [20,21]. Evidence for the amine uptake can also be found in the TGA curves of en-L and en-H samples. Normalised DTG curves for L and en-L are shown in Fig. 2. Peaks below 100c'C correspond to the dehydration of the clays. As shown by the increase of the band at 100c'C, amine intercalation in laponite induces a certain hydrophillicity. A good thermal stability is found for both en-L and en-H, the dehydroxylation peak appears above 700C. In comparison with the original clays, additional peaks of the ethylenediamine are found. Two steps can be distinguished. The first appears around 2 0 0 C and is attributed to the breakdown and removal of the organic amine. The higher temperature peak, for en-H around 4 5 0 C and for en-L around 600C. represents the removal of occluded material from the collapsed pillared structures. This occluded material consists of carbon that is retained in the pores of the clay till higher temperatures. In the temperature range between 200"C and 4 5 0 C (600°C) the samples en-H (en-L) become black, which indicates carbon deposition. Further evidence for the amine uptake can be found in the IR spectra. Fig. 3 represents the spectra of L and en-L. Typical amine vibrations

30

P. Cool, E.F Vansant/Microporous Materials 6 (1996)27-36

(b) ~

,

,

2

,

,

,

,

G

4

,

J

8

,

,

10

,

,

12

' ~

' ;

d~r.2 lhda

'

~

' ~

=-H

' ,~ ',~

deg'~~o

Fig. 1. (a) X-ray diffractograms of laponite (L) and amine-intercalated laponite (en-L). (b) X-ray diffractograms of hectorite (H) and amine-intercalated hectorite (en-H).

Zr -L

en~ en-L

\ E

t

i

I~o 2oo 3~o 4c;o 560 5~o 7oo 800 temp. (degr.C)

4000

3 5 0 0 3 0 0 0 2500 2000

1500

i000

500

waven~er

Fig. 2. Normalised DTG-curves of laponite (L) and amineintercalated laponite (en-L).

Fig. 3. IR-spectra of laponite (L), amine-intercalated laponite (en-L) and Zr-laponite (Zr-L).

in en-L can be found: vas C H (2935 cm-1), vs C H (2852 cm-1), 6 N H scissoring (at 1620 cm -1 as a shoulder of the intense H20-bending vibration at slightly higher wavelength), 6 C H scissoring (1465 cm -1) and 6 C H wagging (1355 cm 1). The peak at 3685 cm -1, due to free hydroxyl groups in laponite, decreases in intensity after the ethylenediamine intercalation. This means that part of these

O H groups have dissociated to protonate the - N H 2 function to form - N H ; species in the interlayer space. Similar characteristics were found in the I R spectrum of en-H. These amine-modified clays were used as substrates in the pillaring process. Compared to pure laponite, amine intercalated laponite forms a more homogeneous substrate for pillaring because of

31

P. Cool, E.F Vansant/Microporous Materials 6 (1996) 27 36

the increased crystallinity by amine-monolayer formation in the interlayer space. This will have its repercussion on the final surface area and micropore volume of the synthesized Zr-PILC. When compared to results obtained by pillaring the pure laponite, we can conclude that an ethylenediamine pre-adsorption results in an increase in surface area and micropore volume of respectively 86 mZ/g and 0.059 cm3/g for the Zr-PILC.

£ Zr-L

/

3.2. Z r - P I L C

XRD, TGA and IR results show the complete exchange of the pre-adsorbed ethylenediamine on laponite and hectorite for the Zr-pillaring species. An indication for this exchange is also the fact that the pillaring solution is slightly yellow after the reaction. In Fig. 3 the IR spectra of laponite, en-L and Zr-L are compared. The peaks due to the amine disappear after the Zr-pillaring. The peak at 3685 cm 1 due to free hydroxyl groups in laponite, also disappears, reflecting that the protons of these hydroxyl groups have been exchanged for Zr-species. No traces of amine were detected in the thermogravimetric analysis. An extremely high thermal stability ( T > 8 0 0 ° C ) of the Zr-PILCs was observed. After the pillaring process the X-ray diffractograms reflect broad peaks for both clays (Fig. 4). The doo~ spacing varies between 1.76-2.21 nm for Zr-L and 1.26-2.21 nm for Zr-H. After a subtraction of the clay layer thickness (0.96nm), the interlayer spacing ranges between 0.8 and 1.25 nm for Zr-L and 0.3 and 1.25nm for Zr-H. The predominant specie in zirconium oxychloride solutions is the zirconium tetrameric cation with the general formula [ Z r 4 ( O H ) 8 ( H z O ) 1 6 C l z ] (8- z)+ [22]. The dimensions are 0.89 nm in width and length and 0.58 nm in thickness. Under reflux conditions a parallel intercalation between the clay sheets occurs after a 3-dimensional polymerization of the tetramers. A 2-dimensional layer composed of a maximum of four tetramers linked by olation is formed, followed by an oxolation reaction between three 2-dimensional layers. In this way a pillaring specie consisting of twelve tetramers can

I]

2

4

6

8 10 degr.2 {het~

12

14

16

Fig. 4. X-ray diffractograms of Zr-laponite (Zr-L) and Zr hectorite ( Zr-H ).

be formed with the upper limit of size before precipitation. The thickness of the polymer can be calculated using 0.25 nm as the diameter of the oxygen anion: 3.0.58 n m - 2 . 0 . 2 5 nm or 1.24 nm. This value is in good agreement with the experimentally observed maximum interlayer spacing of 1.25 nm for Zr-L and Zr-H. Elemental analysis shows very high Zr contents, viz. 36.9% for Zr-L and 34.9% for Zr-H. This indicates that the average charge on the Zr-complex ions must be very low. This charge can be calculated since the pH of the pillar solution is known (pH=0.95). A liberation of 4.4 H + per tetramer is required. Therefore, the tetrameric ion will possess a charge of 3.6 ~ and can be represented by the formula [Zr4(OH)8(H20)16C14.4] 3"6-. The slightly higher Zr content for laponite is possibly due to the smaller dimensions of the clay plates. The plates possess more broken bonds at the edges yielding a higher CEC and enabling laponite to exchange more Zr species from the solution. In addition, ion-exchange is favored in the case of laponite because of the diffusion-controlled character of the process. Also for montmorillonite with different particle sizes, Figueras et al. [11] noticed the influence of diffusion on the zirconium ionexchange. In Table 1 the surface areas (SA) and micropore

P. Cool E.F. Vansant/Microporous Materials 6 (1996) 27-36

32

Table 1 Surface area and porosity data of laponite, hectorite and their Zr-pillared forms

SA(BET) (mZ/g) /~PV (cm3/g)

L

Zr-L not calcined

Zr-L calcined

H

Zr-H not calcined

Zr-H calcined

342 0.244

692 0.395

482 0.340

57 0.016

303 0.159

171 0.064

volumes (/~PV) of the clays and their pillared forms are given. Laponite already possesses a high SA and/~PV. Via Zr-pillaring the SA and/~PV of laponite and hectorite have been greatly increased. For Zr-laponite very high values can be reached, comparable to zeolites (zeolite X and Y: #PV = 0.38 cm3/g). These high ~tPV values for L and Zr-L are a direct result of the shapes of the isotherms. Fig. 5 represents the N 2 adsorption-desorption isotherms at -196°C of the pillared samples (not calcined). The adsorption isotherm of Zr-hectorite can be classified as type II, the same classification as for the unpillared hectorite. However, the isotherm shapes for laponite and Zr-laponite are distorted type I curves. This distortion is noticed in the relative pressure range 0.1 to 0.5, which is typical for supermicropores and small mesopores due to a non-ideal stacking of aggregates. For relative pressure values higher than 0.5 the typical

type I shape is followed. Due to these large micropores, the Langmuir equation (based on monolayer adsorption) fails and the BET type multilayer adsorption becomes a more accurate approach. Because of this distortion in the type I isotherm, the t-plot will only have a linear region at higher t-thickness, corresponding to pressures starting from 0.8. This means that the experimental #PV includes not only the volume of the real micropores (IUPAC <2 nm), but also the volume of the small mesopores in the material. The micropore size distributions for L and Zr-L also prove the importance of these large micropores. The pore size distributions shown in Fig. 6 and Fig. 7 are calculated by the method of Zhu et al. [16,17]. In this method the N2-adsorption isotherm is replotted on a log P/Po scale so that it represents the adsorption potential. This curve exhibits steps which are thermodynamically related to the filling of pores of a certain size. After taking

300-

250-

~200v 150100 o >

50 0

o

o;i

o12

0;3 o~, 0;5 0;6 05 relotive pressure (P/Po)

0:8

o.'9

Fig. 5. N z adsorption-desorption isotherms at -196°C on Zr-laponite ([], Zr-L) and Zr-hectorite ([, Zr-H).

P. Cool E.F. Vansant/Microporous Materials 6 (1996) 27.-36

33

0.2

0.180.16 .~0.14 0

~0.12 ®

E 0.1 -6 > 0.08 0.06

0.04

0.02 <0.71

[ ~v'~-~ L

'0.71-1.06 '1.06 -1.42'1.42 -1.77 '1.77 -2.12 Pore-size range (nm) ~ ' ~ Zr-I_ n.c. ~

7r-I_ c.

]

Fig. 6. Pore-size distributions of laponite (L), Zr-laponite not calcined (Zr-L n.c.) and Zr-laponite calcined (Zr-L c.).

0.05

0.04 ..

//.

u

~0.03

f//

23 0

> 0.02

0 (3_

0.01

<0.71 H

'0.71 -1.06'1.06 -1.42'1.42-1 77'1.77-2.12 Pore-size range (nm) ~

Zr-H n.c. ~

Zr-H c.

Fig. 7. Pore-size distributions of hectorite (H), Zr-hectorite not calcined (Zr-H n.c.) and Zr-hectorite (Zr-H c.).

the derivative d V/d log(p/po), the location of these steps becomes clearer. The micropore range can be subdivided in five pore groups adsorbing 1 to 5 layers. By locating the main maxima in the derivative curve of a P I L C with known but

different slit widths, the pore-size range can be correlated with a certain P/Po range. For both clay substrates, pores between 0.71 and 1.06 nm have a large contribution to the micropore volume. Probably pores of this size are

34

P. Cool, E.F. Vansant/Microporous Materials 6 (1996) 27 36

formed because of gaps in the face-to-face stacking of collapsed or non-intercalated clay layers. The thickness of one clay layer is normally 0.96 nm, however higher values can be obtained when cations are trapped between the layers. The pores in the range of 0.71 to 1.06 nm are common for all types of clays and pillared clays [23,24]. In the case of laponite, pores in the secondary micropore range have a large contribution. For hectorite, porosity is not so important over the whole poresize range. Through pillaring mainly small pores <0.71 nm and pores in the range 1.42-2.12 nm are formed for Zr-laponite as well as for Zr-hectorite. As shown in Table 1, a calcination of the Zr-pillared clays at a heating rate of 10°C/min followed by an isothermal treatment of 2 h at 500°C, has a negative influence on the surface area and the micropore volume. For Zr-L mainly the surface area is decreased by a calcination step (about 30%), while for Zr-H the micropore volume decreases by about 60%. By the calcination the pore volume decreases over the whole pore-size range, mainly for the pillared Zr-hectorite compared to Zr-laponite (Fig. 6 and Fig. 7). Each pore-size range in Zr-hectorite (calcined) has about the same contribution to the total micropore volume. This is consistent with X R D results, where the pore dimensions cover a very broad range. In Zr-laponite (calcined) a combination of very small pores and larger secondary micropores are present. This makes the PILC sample a good substrate for gas adsorption because the larger pores provide a good transport of gas molecules to the smaller pores. Gas-adsorption isotherms were taken on Zr-pillared as well as on unpillared clays for nitrogen and oxygen at - 1 9 6 ° C and cyclohexane, carbon monoxide and carbon dioxide at 0°C. The results shown in Fig. 8 and Fig. 9 reveal that the Zr-pillared clays have a significantly higher adsorption capacity compared to the unpillared analogues. At - 196°C, the same shape of isotherm can be observed before and after pillaring. However, for the oxygen adsorption on Zr-laponite, in comparison with the unpillared laponite, an upward deviation in the isotherm can be seen between 0.05 and

(~llg(Iry)

(mmoi/gclry)

8O

(a)

25-

1 z,-,

20-

15-

10-

~

5-

Z.r -L.

~

+ L

i O! 0

0.1

0.2

0.$

0.4

0.6

P (arm) + N2

~

02

n~sd|/gdry) (mrnallgdr y) 10"

(b)

z,-%~

Zr-L

1.6 1.4 1.2 1 0.8 0.8 0.4 0.2 0

0.1

0.2

0.3

0.4

0.5

P(atrn) (3 Cyolohexane

+ C02

'1' CO

Y~o~lohexlme I Y~.,O2,GO

Fig. 8. (a) N 2 and 0 2 adsorption isothermsat 196°Con laponite (L) and Zr-laponite (Zr-L). (b) Cyclohexane, CO2 and CO adsorption isotherms at 0°C on laponite (L) and Zr-laponite (Zr-L). 0.15 atm. This additional adsorption capacity is due to the high pore-volume of the large micropores (capillar condensation). For the cyclohexane adsorption, at low pressures an important increase in capacity for the Zr-pillared samples can be observed. This can be due to the higher amount of small pores < 0.71 nm. An additional adsorption force will be present when the size of the adsorbed cyclohexane molecules (0.6 nm) fits in these pores (<0.71 nm). Further experiments were carried out for nitrogen and oxygen on Zr-laponite at 0°C. Capacities of 0.286 mmol/g N 2 and 0.219 mmol/g 02 at an equilibrium pressure of 0.45 atm were obtained. In general the Zr-laponite is a good substrate with respect to the high adsorption capacities but no significant difference in the adsorption selectivity for N2 and 02 was detected.

P. Cool, E.F Vansant/Microporous Materials 6 (1996) 27-36 n (ad$/gdry) (mmol/gdry) 20 Zf-H

16

(a) H

10

G~__~

~

~.,

~

,~

H

O 0.1

0.2

0.$

0.5

0.4

P (atm) ~' N2

02

+

n (RdIIgdry) (mmollgdry) 6

1

z~-H

(b)

z,-H

o.,

0.4

o. ~ ~ - ~ . ~ 0

-

OA

,~

,-----~

0.2

0.3

0.4

/o 0.5

P (atrn) o

Cyolohexane

+

C02

;'

35

exchanged completely for the Zr-pillaring species as was proven by X R D , T G A and F T I R . P I L C s with a high thermal stability were obtained. With Zr-pillaring, the BET surface areas and micropore volumes o f both laponite and hectorite have been increased significantly. For Zr-laponite even very high #PV values, c o m p a r a b l e to zeolites, were reached. N2 adsorption isotherms at - 1 9 6 ~ C together with pore-size distributions o f laponite and Zr-laponite indicate the existence o f supermicropores formed by a non-ideal stacking o f clay plates. Zr-hectorite possesses pores over a very b r o a d pore-size range. Zr-pillaring o f laponite, especially forms smaller micropores (<0.71 nm) and larger secondary micropores. The c o m b i n a t i o n o f these pores, together with its high surface area and micropore volume, makes the Zr-laponite a promising substrate for gas adsorption. High capacity properties for N 2 and 02 with lo~' N 2 / 0 2 selectivity at 0°C were obtained.

CO

YT ; GyOlohexenl / Y2 ; C02, GO

Fig. 9. (a) N 2 and 02 adsorption isotherms at - 196°C on hectorite (H) and Zr-hectorite (Zr-H). (b) Cyclohexane, CO2 and CO adsorption isotherms at 0°C on hectorite (H) and Zr-hectorite (Zr-H). Future w o r k will focus on further modification o f the Z r - P I L C substrates to introduce specific adsorption sites in the interlayer space in order to improve the selectivity towards specific gas molecules.

Acknowledgment P. Cool acknowledges the N F W O / F N R S financial support as a research assistent.

for

References [1] [2] [3] [4]

4. Conclusion

[5]

N a t u r a l hectorite and synthetic laponite clays were used as substrates for Zr-pillaring. Both clays are similar in structure but differ in particle size. The laponite plates are very small and are totally disordered while hectorite has a higher crystallinity. Pre-adsorption o f ethylenediamine is performed to increase the face-to-face organisation o f the plates creating a m o r e h o m o g e n e o u s substrate for pillaring. The ethylenediamine can be

[6] [7] [8] [9] [10]

T.J. Pinnavaia, Science, 220 (1983) 365. J. Shabtai, U.S. Patent, 4238 364 (1980). R. Burch, Catal. Today, 2 (1988) 297. M.P. Atkins, in I.V. Mitchell (Editor), Pillared Layered Structures: Current Trends and Applications, Elsevier, Essex, 1990, pp. 159. N. Lahav, I. Shani and I. Shabtai, Clays Clay Miner., 26 (1978) 107. T.J. Pinnavaia, M.S. Zhou, S.D. Landau and R.H. Raythatha, J. Mol. Catal., 27 (1984) 195. D. Tichit, F. Fajula, F. Figueras, B. Ducourant, G. Mascherpa, C. Gueguen and J. Bousquet, Clays (:lay Miner., 36 (1988) 369. R.A. Schoonheydt, J. Van Den Eynde, H. Yubbax, H. Leeman, M. Stuyckens, I. Lenotte and W.E.E. Stone, Clays Clay Miner., 41 (1993) 598. S. Yamanaka and G.W. Brindley, Clays Clay Miner., 27 (1979) 119. R. Burch and C.I. Warburton, J. Catal., 97 (1986) 503.

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P. Cool, E.F. Vansant/Microporous Materials 6 (1996) 27-36

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