Microporous Materials, 1 (1993) 237-245 Elsevier Science Publishers B.V., Amsterdam

237

Single step dealumination of zeolite beta precursors for the preparation of hydrophobic adsorbents

Elodie Bourgeat Lami”, Francois Fajula”**, Didier Anglerotb and Thierry Des Couriere9

“Laboratoire de Chimie Organique Physique et CinLtique Chimique Appliqukes. URA 418 CNRS, ENSCM, 8. rue de 1 ‘Ecole Normale, 34053 Montpellier, France

“Groupement de Recherches de Lacq. Groupe Elf Aquitaine, B.P. 34, Lacq 64170 Artix, France

‘Centre de Recherches ElfAntar France, B. P. 22,69360 Solaize, France

(Received 19 January 1993; accepted 17 February 1993)

Zeolite beta precursors containing the occluded organic template have been dealuminated by reaction with nitric acid solution at reflux temperature. Si/Al ratios greater than 1000 can be obtained in a single step, without significant loss of crystallinity, porous volume and thermal stability. The dealuminated zeolite contains mesopores and three types of silanol defect groups. The latter are readily annealed by calcination. Dealuminated zeolite beta exhibits a strong hydrophobic/orga- nophilic character.

Keywords: zeolite beta; dealumination; silanol groups; adsorption; hydrophobicity

Introduction

The hydrophobic/organophilic character of high-silica zeolites has been long recognized [l-3]. Their sieving properties, operating at the molecular level, and their excellent chemical, thermal and hydrothermal stability suggest that these materials may have technological potential as adsorbents in separation and purification processes in aqueous or wet media [4-73. The degree of hydrophobicity of zeolites is directly dependent on their aluminium content. For a limited number of the structures known to date, the aluminium content may be adjusted during the hydrothermal synthesis. This is the case, for instance, for zeolites ZSM-5 and ZSM-11 (MFI and MEL structural types) which can be readily synthesized as purely silicic forms. However, except for the previous examples, most of the siliceous zeolite types that can be produced in that way feature low intracrystalline void vol-

* Corresponding author.

umes and unidirectional porosities which consti- tute main drawbacks for practical uses. Large pore high-silica zeolites with three-dimensionally con- nected channels and high potential sorption capac- ity, such as faujasite (FAU), zeolite beta (BEA) or ZSM-20, are usually prepared through post- synthesis dealumination procedures that involve reaction with mineral acids or chelating agents, isomorphous substitution of framework aluminium by silicon atoms or hydrothermal treatments com- bined with acid leaching. With all of the methods mentioned above, the experimental conditions appear criticaland must be systematically adjusted to the composition, structure and/or texture of the parent material in order to preserve the integrity of the crystallinity. In addition, complete removal of the framework aluminium requires repeated treatments with intermediate ion-exchange and/or calcination steps. We report here on a very simple method used for the dealumination of zeolite beta which consists of a single treatment of the as-

0927-6513/93/$6.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved.

238 E. Bourgeat Lami et al. 1 Microporous Mater. 1 (1993) 237-245

synthesized precursor by concentrated nitric acid solution. The materials thus obtained are highly crystalline, thermally stable and their composition may be tuned by varying the acid concentration.

Experimental

Materials and procedures

The starting zeolite was synthesized in the pres- ence of sodium and tetraethylammonium (TEA) cations according to the procedures described in the patent of Wadlinger et al. [S]. It exhibited the following characteristics: Si/Al = 16, Na/Al = 0.03, TEA/Al = 1.65, spherulitic crystals with an average diameter of 0.6 pm. The zeolite was dealuminated by dispersing 500 mg samples in 50 ml of a solution of nitric acid, the concentration of which was varied between 0.1 and 13 N [9]. The suspension was then heated to 80°C with stirring and main- tained at this temperature for 4 h. After cooling to room temperature the crystals were recovered by filtration, washed with deionized water, oven-dried at 70°C overnight and then stored under ambient conditions. The composition of the dealuminated solids was determined by elemental analysis and thermal analysis using a combined TG-DTG-DTA instrument [lo].

Characterization studies

X-ray powder diffraction patterns were recorded using CuKa! monochromated radiation (CGR Theta 60 instrument). The crystallinity of the samples was evaluated by comparison of the area of the most intense diffraction peak at 22.4” 2 theta to that of the parent zeolite taken as 100% crystal- line. Unit cell parameters (_+ 0.01 A) have been calculated by a double refinement method [l l] using the peak indexation in the tetragonal system proposed by Higgins et al. [12].

Infrared spectra were recorded with a FT-IR Nicolet 320 spectrometer (resolution 2 cm-‘). The framework region (400-1200 cm-‘), was investi- gated using KBr pellets containing 1% of zedlite. For the hydroxyl region (2500-4000 cm-‘), self- supporting wafers (10 mg/cm’) were prepared and activated, up to 500°C in vacuum, in the IR cell.

MAS NMR spectra of 2gSi (at 58.6 MHz) and *‘Al (at 78.2 MHz) were taken with a Bruker AM 300 spectrometer [13,14].

Adsorption isotherms for nitrogen at 77 K were obtained with a laboratory-made static volumetric instrument. The porous volumes and contributions from the external and mesoporous surfaces were ,derived from the as plots using the method recom- mended by Sing [ 151.

Equilibrium sorption capacities at room temper- ature for water (at P/P, = 0.17), n-hexane (at P/P., = 0.2) and n-butanol (at P/P, = 0.1) have been deter- mined by gravimetry using a Setaram B 85 balance according to the procedure described in detail elsewhere [ 161.

Ultrathin (300-500 A) sections of zeolite grains embedded in resin were observed under a Jeol 100 C transmission electron microscope (TEM).

Results and discussion

Composition of the acid-treated zeolites

Related works published recently demonstrate that zeolite beta can be dealuminated by treatment with ammonium hexafluorosilicate or silicon tetra- chloride [ 171, by exposure to a solution of a mineral acid [18,19] or by steaming followed by acid leaching [19]. In all of these studies the starting zeolite, with an initial Si/Al ratio below 15, had been subjected to an oxidative calcination, to remove the template, followed by ammonium ion-exchange, to remove the residual sodium ions, prior to the dealumination treatment and the final Si/Al ratios attained were in the range 20-l 10.

In the present study, the dealumination pro- cedure has been applied to the solid in the as- synthesized form, i.e., containing both TEA and Na. Separate portions of zeolite have been con- tacted for 4 h, at 80°C with HN03 solutions of increasing concentration. The influence of the acid concentration on the composition of the treated materials is presented in Figs. 1 to 3. It is seen that the aluminium content of the final zeolite decreases monotonically as the acid concentration increases and falls below the analytical limit after treatment with concentrations greater than 11.5 molar. It is

E. Bourgeat Lami et al. / Microporous Mater. I (1993) 237-245

.loo f

+ *

o-0 -=--n q

b

a-o--- 80 e X

I \

w - 80

0.05 a

n a \

+ .

: - \

.

z a03 - 8

II . E

L ’ ’ ’ b ’ ’ ’ ’ ’ ’ .\

I+-. L 0 2 4 (1 8 10 12

[H30*] (moWI)

Fig. 1. Variation of the composition and XRD crystallinity of zeolite beta as a function of the concentration of acid.

101 I

[H30*] (mol./l)

Fig. 2. Organic and water content of zeolite beta as a function of the concentration of acid.

noteworthy that dealumination resulted only in a 20% decrease of the intensity of the XRD peaks (Fig. 1). From the TGA weight losses, the amounts of occluded TEAOH, TEA+ and water retained by the solids at each stage have been calculated. The plots in Fig. 2 reveal that the amount of occluded TEAOH decreased from 8.5 wt% to 0.5 wt% after treatment with a 0.1 N acid solution while that of water increased from 2.9 wt% to 5.5 wt% indicating that part of the intracrystalline space liberated by the occluded organic had been

239

3- /

- /

‘- /. .

. I ,I I I II

1 3 5 7

AI/ 100 Td

Fig. 3. Relationship between the amount of aluminium and the amount of TEA cations (amounts expressed per 100 tetrahedra) in acid-treated zeolites.

filled by water. At this stage dealumination already occurred as the Si/Al ratio increased to 19.5. Such a ready dealumination of zeolite beta after reaction with an acid solution of relatively low concen- tration has been pointed out by Briscoe et al. [18], Using H-beta with Si/Al of 14 as starting material, they obtained a zeolite with a Si/Al ratio of -30 after reaction in 0.1 N hydrochloric acid at room temperature. Independent experiments using hydrochloric acid showed that nitric acid was more efficient in removing aluminium from the frame- work of beta. The result of Briscoe et al. would then suggest that, at low acid concentrations, the protonic form of zeolite beta is more prone to dealumination than the TEA-Na form. Upon increasing the dealumination level, the amounts of TEA+ and water decreased continuously. The ratio TEA+/Al was always equal to 1 (Fig. 3) demon- strating a simultaneous removal of the framework aluminium atom and of the organic counteranion associated with it, without the intermediate forma- tion of a mixed H-TEA form. This phenomenon can be tentatively explained in the light of previous NMR work reported by us [14] which showed that, in the presence of protons, the aluminium tetrahedral sites in zeolite beta were highly dis- torted due to the strong electron affinity of the protons. Under the conditions of the acid attack we used here, the AlO;H+ protonic sites, if formed, would be unstable, and the T-O bonds readily

240

hydrolyzed. At variance with that observed with water and TEA+, the TEAOH content of the samples increased to 1.5 wt% and remained con- stant as the dealumination level increased. The peculiar behaviour of the solid treated with 0.1 N acid, namely an optimum water content and a minimum TEAOH content, was reproducible and can be attributed to a change in the sorption character of the zeolite from hydrophilic to hydrophobic.

Structural and textural properties

As indicated previously, the intensities of the XRD peaks and of the infrared bands in the region of lattice vibrations did not change significantly upon dealumination. Actually, all the signals became narrower, suggesting a higher degree of crystallinity. Significant changes in the peak posi- tions were nevertheless noticed, in agreement with the change in chemical composition of the zeolite.

Figure 4 reports the evolutions, as a function of the aluminium fraction, of two infrared frequencies characteristic of the stretching (signal at 782-798 cn-‘) and ring breathing (signal at 568-576 cm-‘) vibrations of the lattice. They both increased monotonically as the lattice became richer in silica. The frequency increase versus alu- minium content was nevertheless non-linear, as is generally the case for zeolites containing unequiva- lent crystallographic sites, such as mordenite [20],

c 796-m - 576

A _ A

794- A+ - 574

_ . - ‘ii 3

7 790- Cm -572 ’

I ,’

.

766- A

1 OA

- 570

. A

762 - .

- 566

I I I I I I

0.01 0.03 0.06

Ill = Al / Al + SI

Fig. 4. Variation of the infrared frequency (+2 cm-‘) of the symmetric stretching (squares) and ring breathing vibration (triangles) as a function of the ahnninium content in zeolite beta.

E. Bourgeat Lumi et al. / Microporous Mater. I (1993) 237-245

offretite [21,22] and mazzite [22]. As previously reported for the dealumination of the two latter zeolites [22], the bending vibration frequencies in zeolite beta were found to be nearly constant (462 + 2 and 432 + 2 cm- ‘) in the whole range of compositions investigated here.

The dependency of the unit cell dimensions on aluminium content, presented in Fig. 5, reveals that the lattice contraction that occurred upon dealumi- nation was anisotropic. It affected only the c parameter, which shrank by 1% during the process, while the a parameter remained practically con- stant. Unit cell parameters are known to be depen- dent on the nature of the counterions and the presence of aluminium in extra-framework posi- tions. Both influences can be reasonably neglected here since the framework negative charges are only balanced by TEA+ cations (see Fig. 3) and “Al MAS NMR measurements revealed the sole pres- ence of tetrahedral framework Al in the samples. Previous related works in our group [23,24] have shown that the a and c parameters decrease when the TEA cations are substituted for protons or alkali cations whereas a increases and c remains constant during steam dealumination. The data of Fig. 5 could then be tentatively explained by a compensation effect between the decrease of the absolute amount of TEA present in the samples (that would result in a decrease of both a and c) and the creation of Al-depleted sites (that would

26.67 -o 0

•1 0 II

q - 00 _

26.57 - .’

*(:

u 26.47 -

/ .

26.37 -

12A6 .

I242 A

12.39t

Fig. 5. Evolution of the unit cell dimensions (+O.Ol A) as a function of the aluminium content.

E. Bourgeat Lami et al. 1 Microporous Mater. I (1993) 237-245

increase a without modifying c as observed in steam-dealuminated beta).

The texture of a zeolite sample, 80% XRD crystalline, dealuminated by reaction with 13 N

acid has been characterized by nitrogen adsorption and TEM. While the isotherm obtained on the starting zeolite (calcined at 550°C in order to remove the TEA) was of type I, the isotherm of the dealuminated material was characterized by a hysteresis loop with a lower closing point at about P/P,=O.42 indicating the presence of mesopores not directly connected to the exterior of the crys- tals, as in the case of steam-dealuminated zeolites [25,26]. The acid treatment induced a net decrease of the microporous volume and BET surface area (Table 1) while it increased the contribution of the external and mesopore surface, derived from the as plots. The volume of mesopores amounted to 0.16 ml/g. The micrographs of the ultrathin micro- tome sections showed that these mesopores were in the form of cylinders loo-150 A long and 30-50 A diameter. At variance with what was reported by Ajot et al. [19] for steam-dealuminated beta zeolites synthesized in fluoride medium, no particu- lar orientation of the mesopores could be evidenced.

Infrared characterization of the hydroxyl groups

The IR spectrum of the hydroxyl region of a protonic form of zeolite beta (Si/Al = 12.8) has been discussed in detail recently [14]. The spectrum exhibits five main features related to the structural Al-OH-Si groups (at 3615 + 5 cm- ‘), hydrolyzed aluminium species (at 3665 cm- ‘), terminal silanols (at 3740+ 10 cm-l), alumina-like phases (at 3780 cm-l) and hydrogen-bonded silanols (broad

TABLE 1

241

signal in the range 3750-3300 cm-’ appearing as a halo which superimposes the baseline). The very same signals were detected on the calcined parent zeolite used in the present study (Fig. 6a, the broad band characteristic of H-bonded silanols is not visible here, due to the restricted range of wave- numbers of the figure). The bands at 3665 and 3780 cm- ’ became hardly visible after an acid treatment at a concentration greater than 4 N, namely for Si/Al ratios greater than 30. The removal of aluminium caused a progressive decrease of the intensity of the signal of the struc- tural Al-OH-Si groups, a narrowing of its width and a shift towards lower wavenumbers (Fig. 7). Such a behaviour would be consistent with the sequential removal of aluminium atoms from different crystallographic sites, i.e., from sites with different TOT angles [27], in agreement with what has been proposed by Perez-Pariente et al. [28] who studied the dealumination of zeolite beta by using 27A1 and “Si NMR spectrosdopy. Careful examination of the signals due to the silanol groups revealed that the peak at 3740 f 10 cm- ’ consisted, in fact, of three different signals, centered’ at (i) 3747f2cm-‘, (ii) 3745f2cm-’ gnd (iii) 3736f2 cm-‘. The three signals, which are discernable on the spectrum of the calcined parent zeolite reported in Fig. 6a, have also been identified in steam-dealuminated faujasite by Janin et al. [29]. The latter authors have attributed signal (i) to silanol groups attached to amorphous silica- alumina debris, signal (ii) to silanol groups attached to extra-framework silicon-rich debris and signal (iii) to terminal framework silanols. Although no evidence for the presence of extra-framework debris was obtained in our samples after acid treatment, the generation of such species during

Nitrogen porous volumes and surface areas of the parent and dealuminated zeolites beta

Sample @i/Al) Total Micropore BET surface as surface volume volume area a&

(ml/g) (ml/g) (m’/g) (m’/g)

Beta (16) 0.29 0.25 600 75 Beta (> 1000) 0.36 0.21 450 150 Beta C’ (> 1000) 0.36 0.18 430 150

‘Sample calcined at WC for 10 h. bContribution form the external and mesopore surfaces.

242 E. Bourgeat Lami et al. / Microporous Mater. I (1993) 237-245

\ a

3745

x2

f

x2

3750 3650 I cm-t)

3750 3650

I cm-1 1 Fig. 6. Infrared spectra of the hydroxyl region of zeolite beta. (a) Parent material calcined at XtO’T, (b) sample treated with 4.2 N HNOs (Si/Al=33), (c) sample dealuminated with 13 N HNOs (Si/Al> lOOtI), (d) sample c calcined for 6 h at 5OOT, (e) sample c calcined for 2 h at 600°C and (f) sample c calcined for 10 h at 900°C.

a01 0.03 0.06

Fig. 7. Infrared frequency of the stretching vibration of the structural hydroxyl groups as a function of the aluminium content.

the activation of the IR wafer cannot be ruled out. It is indeed known that calcination at 500°C of protonic forms of zeolite beta with intermediate Al contents leads to some dealumination of the frame- work [ 14,23,28].

An acid treatment with 4.2 N HNO, (leading to a Si/Al ratio of 33), resulted in a net decrease of the contribution of the peak at 3747 cm-’ (Fig. 6b), confirming its assignation to silanol groups in the vincinity of aluminium atoms, and an increase of the broad band between 3750 and 3300 cm- ‘, indicating the creation of silanol defect groups interacting mutually through hydrogen bonds. After intensive dealumination in 13 N acid (Si/Al>lOOO), the component at 3747 cm-’ disap- peared and the band at 3738 cm- ’ became pre-

E. Bourgeat Lami et al. / Microporous Mater. I (1993) 237-245 243

dominant (Fig. 6~). Calcination of the sample under vacuum in the infrared cell at 500°C for 6 h greatly reduced the intensity of the signal in the region 3300-3750 cm- ’ (Fig. 6d) providing evidence for the facile annealing of the hydrogen-bonded silanol groups [30,3 l] created upon aluminium extraction. The intensity of the peak at 3738 cm- ’ also decreased, but complete elimination of the two latter signals required a calcination at 600°C for 2 h under flowing dry air (Fig. 6e). The only type of silanol group detected on the surface of this material appeared at a frequency of 3745 cm-’ and would then correspond, according to Janin et al. [29], to silanols attached to non-zeolitic silica zones. Finally, after calcination of this sample for 10 h at 900°C under flowing dry air, the intensity of absorption of the OH-stretching region became very weak (Fig. 6f), with only the peak at 3745 cm-’ being detected. It is noteworthy that the material calcined at 900°C was still highly crystalline and suffered little loss in sorption capac- ity and surface area (last entry of Table 1).

2gSi MAS NMR spectroscopy

The 2gSi MAS NMR spectrum of the parent TEA-beta is given in Fig. 8a. Four different compo- nents centered at - 94, - 104, - 110 and - 114 ppm can be distinguished. This spectrum closely resem- bled the one reported by Perez-Pariente et al. [28] who attributed the lines at - 110 and - 114 ppm to silicon atoms in two different crystallographic sites, surrounded by four silicons, the line at -104 ppm to Si surrounded by 3 Si and 1 Al and/or 3 Si and 1 OH and the line at -94 to Si surrounded by 2 Si and 2 Al and/or 2 Si and 2 OH. The ‘H/2gSi cross-polarization technique (Fig. 8b) led to a narrowing of the - 110 ppm line and a better resolution of the -104 ppm signal. After intensive dealumination (Si/Al > lOOO), the main difference in the 2gSi NMR spectrum was a decrease of the intensity and a 2 ppm shift to higher field of the signals above -110 ppm (Fig. 8~). Under cross-polarization conditions, the intensity of the lines at -92 and -102 ppm were enhanced. This agrees well with their assignation to silicon atoms linked to hydroxyl groups. In addition, the NMR signal exhibited a very broad

-102

k _A,& b

6 I ppm 1

Fig. 8. 2gSi MAS NMR spectra of (a) parent zeolite beta, (b) parent zeolite beta under 1H/2gSi cross-polarization condi- tions, (c) dealuminated zeolite (Si/Al> 1000) and (d) dealumi- nated zeolite under ‘H/“Si cross-polarization conditions.

background in the range - 80 to - 120 ppm sug- gesting the presence of an amorphous material.

The NMR data confirm then the infrared study presented above. As expected, after acid treatment, the zeolite lattice contains a high density of defect sites and siliceous non-zeolitic zones. The material remains, however, highly crystalline and thermally stable, allowing the structural rearrangement and the annealing of hydrogen-bonded and isolated silanols by calcination.

Sorption properties of dealuminated beta

Equilibrium sorption capacities for water, n- hexane and n-butanol as a function of the composi- tion of the dealuminated solids are presented in Table 2, Data have been collected on samples calcined at 550°C after the dealumination treat- ment in order to decompose the residual organic. On all materials, the amounts of water sorbed were much smaller than those of n-hexane, demonstrat- ing the hydrophobic/organophilic character of zeo- lite beta. Upon extensive dealumination, the equilibrium loading of n-hexane decreased, as did, the microporous volume measured by nitrogen

244 E. Bourgeat Lumi et al. / Microporous Mater. I (1993) 237-245

TABLE 2

Equilibrium sorption capacities as a function of the composition of the acid-dealuminated zeolites beta

Si/Al CB,O+l’ Amounts sorbed (wt%)

water n-hexane n-butanol

23 0.6 5.0 17 37 4.5 4.1 18 82 5.7 2.6 19

135 7.2 1.5 16 425 9 I.0 14 11

>I000 13 I.0 12 12 >lOOOb - 0.3 12

YZoncentration (mol/l) of acid for the dealumination treatment. “Sample calcined at 900°C for 10 h.

extensive crystallographic faulting generating interplanar defects terminated by hydroxyl groups. The additional defects created upon dealumination would be then readily accomodated by the struc- ture, without loss of crystal stability. Based on the equilibrium sorption capacities determined with pure sorbates in the gas phase, dealuminated zeolite beta shows a pronounced hydrophobic/organophi- lit character. Studies ‘dealing with the use of this material for the separation of organic substances from water will be presented in the near future.

Acknowledgements

adsorption (Table l), while the ability of the zeolite to retain water was drastically reduced. The dehy- droxylation of the surface by calcination at 900°C led to a further decrease of the water sorption capacity, which was in line with the removal of adsorption sites associated with the relatively polar silanol groups. The sorption capacities for n-buta- no1 confirmed the hydrophobic character of dea- luminated beta.

The authors thank M.A. Nicolle for the synthesis of the parent zeolite, R. Dutartre for the TEM micrographs and F. Di Renzo for suggestions and helpful discussions.

References

1 2

N.Y. Chen, J. Phys. Gem., 80 (1976) 60. E.M. Flanigen, J.M. Bennet, R.W. Grose, J.P. Comen, R.L. Patton, R.M.. Kirschner and J.V. Smith, Nature, 271 (1978) 512.

Conclusions

Zeolite beta precursors containing the organic template can be readily dealuminated by reaction with concentrated nitric acid, without significant loss of crystallinity and sorption capacity. Using the experimental conditions presented in this work, the final composition of the zeolite can be tuned by varying a single parameter: The concentration of the acid. Acid-dealuminated zeolite beta contains mesopores, the volume of which amounts to approximately 50% of the total porous volume, and a large density of silanol defects. It is proposed that the latter are of, at least, three types - terminal framework silanols, isolated silanols attached to non-zeolitic zones and hydrogen-bonded silanols - and exhibit a different reactivity towards annea- ling. It is noteworthy that these defects do not affect the thermal stability of the zeolite. Such a situation could have to do with the unique struc- ture of zeolite beta which is characterized by

3

4

5 6 7

8

9

10

11 12

13

14

15

16

D.H. Olson, W.O. Haag and R.M. Lago, J. Catal., 61 (1980) 390. B. Gunxel, J. Weitkamp, S. Ernst, M. Neuber and W.D. Deckwer, Chem. Ing. Tech., 61 (1989) 66. R.M. Dessau, ACS Symp. Ser., 135 (1980) 124. D.R. Garg and J.P. Ausikaikis, Can. Pat. 1195258 (1985). J. Weitkamp, S. Ernst, B. Gunzel and W.D. Decker, Zeolites, 11 (1991) 314. R.L. Wadlinger, G.T. Kerr and E.J. Rosinski, US. Pat. 3 308 069 (1967). E. Bourgeat-Lami, F. Fajula, T. Des Courieres and D. Anglerot, Fr. Pat. Appl. 90 14 749 (1990). E. Bourgeat-Lami, F. Di Renxo, F. Fajula, P.H. Mutin and T. Des Courieres, J. Phys. Chem., 96 (1992) 3807. 0. Lindqvist and F. Wengelin, Ark. Kemi, 238 (1967) 179. J.B. Higgins, R.B. Lapierre, J.L. Schlenker, A.C. Rohrman, J.D. Wood, G.T. Kerr and W.J. Rohrbaugh, Zeolites, 8 (1988) 446. P. Massiani, B. Chauvin, F. Fajula, F. Figueras and C. Gueguen, Appl. Catal., 42 (1988) 105. E. Bourgeat-Lami, P. Massiani, F. Di Renzo, P. Espiau, F. Fajula and T. Des Courieres, Appl. Catal., 72 (1991) 139. K.S.W. Sing, in D.H. Everett and R.H. Ottewill (Eds.), Surface Area Determination, Butterworth, London, 1970, p. 25. B. Chauvin, F. Fajula, F. Figueras, C. Gueguen and J. Bousquet, .I. Catal., 111 (1988) 94.

E. Bourgeat L.ami et al. / Microporous Mater. 1 (1993) 237-245 245

17 J. Weitkamp, M. Sakuth, C. Chen and S. Ernst, J. Chem. Sot., Chem. Commun., (1989) 1908.

18 N.A. Briscoe, J.L. Casci, J.A. Daniels, D.W. Johnson, M.D. Shannon and A. Stewart, Stud. Surf. Sci. Catal., 49A (1989) 1.51.

19 H. Ajot, J.F. Joly, J. Lynch, F. Raatz and Ph. Caullet, Stud. Surf. Sci. Catal., 62 (1991) 583.

20 R.W. Olson and L.D. Rollmann, Inorg. Chem., 16 (1977) 65 1. 21 C. Femandez, J.C. Vedrine, J. Grosmangin and G. Szabo,

Zeolites, 6 (1986) 484. 22 E. Ponthieu, P. Grange, J.F. Joly and F. Raatz, Zeolites, 12

(1992) 395. 23 E. Bourgeat-Lami, P. Massiani, F. Di Renzo, F. Fajula and

T. Des Courieres, Catal. Z.&t., 5 (1990) 265. 24 M. Derewinski, F. Di Renzo and F. Fajula, to be published.

25 J. Lynch, F. Raatz and P. Dufresne, Zeolites, 7 (1987) 333.

26 B. Chauvin, P. Massiani, R. Dutartre, F. Figueras, F. Fajula and T. Des Courieres, Zeolites, 10 (1990) 174.

27 A.G. Pelmenshchikov, E.A. Paukshtis, V.G. Stepanov, V.I. Pavlov, E.N. Yurchenko, K.G. Ione, G.M. Zhidomirov and S. Beran, J. Phys. Chem., 93 (1989) 6725.

28 J. Perez-Pariente, J. Sanz, V. Fornes and A. Corma, J. Catal., 124 (1990) 217.

29 A. Janin, M. Maache, J.C. Lavalley, J.F. Joly, F. Raatz and N. Szydlowsky, Zeolites, 11 (1991) 391.

30 R.M. Dessau, K.D. Schmitt, G.T. Kerr, G.L. Woolery and L.B. Alemany, J. Catal., 104 (1987) 484.

31 V. Bolis, L. Marchese, S. Coluccia and B. Fubini, Ads. Sci. Techn., 5 (1988) 239.