Factors affecting the steam dealumination of zeolite omega

Factors affecting the steam dealumination of zeolite omega B. Chauvin, P. Massiani, R. Dutartre, F. Figueras and F. Fajula Laboratoire de Chimie Or~anique Physique et Cin~tique Chimique Appliqudes U.A. 418 du C.N.R.S.-E.N.S.C.M. iv1ontpellier, France and T. Des Courieres Centre de Recherche ELF France, Saint Symphorien D'Ozon, France. Three samples of zeolite omega with varying residual contents of sodium have been dealuminated by combined steam and acid treatments. The hydrolysis of the AI-0 bonds is a fast process that occurs mainly during the initial period of heating above 500° under steam. Sodium ions inhibit the reaction. The steamed solids contain mesopores --- 100 A in diameter, but their porosity is not available to sorbents because of the presence of debris in which aluminum is in tetrahedral and octahedral configurations. Acid leaching removes all the nonframework tetrahedral aluminium and part of the octahedral one. This removal permits access to the microporous and mesoporous voids. Increasing the severity of the steam treatment increases the quantity of the disloged material that cannot be removed by the acid, but has little effect on the texture of the final solids. The TEM observations and the volumetric measurements suggest that the mesopores are not directly linked to the surface of the grains and are connected to each other by narrow necks. Keywords: Zeolite omega; steam dealumination; 29Si and 27AI n.m.r.; porosity; transmission electron microscopy

INTRODUCTION

EXPERIMENTAL

Steam dealumination of zeolites by combined hydrothermal and acid treatments is known to produce favorable changes in their catalytic performances. The decrease of the number of catalytic sites is largely compensated for by the improvement of the thermal and hydrothermal stabilities, the enhancement of the acid strength, and the increase in sorption capacity and diffusion rates. The latter effects, which result from the creation of a secondary pore system, are particularly pronounced in zeolites featuring unidimensional porosities. Zeolite omega is one of those systems, and we have shown recently 1 that the main disadvantage exhibited by omega-based catalysts in the cracking and hydroconversion of hydrocarbons, namely, a severe activity decay due to coking, could be circumvented by steam dealumination. The aim of the work presented here was to investigate thoroughly the influence of the experimental parameters of the steam-dealumination procedure on the chemical and structural characteristics of the solids.

Materials

Address reprint requests to Dr. F. Fajula at the Laboratoire de Chimie Organique Physique et Cin~tique Chimique Appliqu~es U.A. 418 du C.N.R.S.-E.N.S.C.M.; 8 rue de I'Ecole Normale, 34053 Montpellier, C~dex 1, France. Received 9 May 1989; accepted 10 July 1989 (~) 1990 Butterworth Publishers 174

ZEOLITES, 1990, Vol 10, March

The parent zeolite was synthetized in the Na-TMA (TMA = tetramethylammonium) system as described previously. 2 It appeared as hexagonal single crystals of 3 l~m in length and 1 vm in width. Figure 1 shows the topology of zeolite omega. T h e framework consists of a stacking of 14-hedral gmelinite cages skzring 6-membered rings. T h e microporous volume available to sorbents corresponds to a unidirectional near cyclindrical channel running parallel to the c-axis and accessible through a 12-membered oxygen ring. a,4 The parent material was submitted to three different procedures of calcination (in air) and ion exchange (with ammonium salt) in order to prepare solids with varying amounts of residual sodium. T h e details of the procedures and the sodium content of the corresponding zeolites are given in Table I.

Dealumination procedures The steaming step was performed on ca. 1 g of zeolite placed in a quartz reactor and using a bed geometry of 3 cm in diameter and 0.3 cm in height. The reactor was swept with dry air (80 ml/min) and its temperature increased (l°C/min) up to 500°C. After 4 h, the air flow was stopped and replaced by a flow of steam (1 atm., volume of steam/volume of zeolite x

Steam dealumination of zeolite omega: B. Chauvin et al.

/ SSR

\

So

b

Figure I Structure of zeolite omega: (a) assembly of gmelinite cages in the c direction; (b) projection viewed along (001). Sites A located in the four-membered rings of the gmelinite units (S4R); sites B located in the six-membered rings (S6R)

time = 1Oh-1). The temperature was raised (2°C/min) and maintained at the desired value for times ranging from 0 up to 20 h. Table 2 shows the various combinations of temperatures and times investigated. The vapor supply was then stopped, and the steamed solid was allowed to cool down (20°C/min) under flowing air. The acid leaching (AL) treatment consisted in refluxing the steamed zeolites at 80°C in 0.05 M nitric acid solution for 18 h using a solid-to-liquid weight ratio of 1:50. The solids were recovered by filtration, washed, and oven-dried overnight at 200°C. In the following, sample indentification will be made by indicating (as a prefix) the residual amount of sodium (Table I) of the starting material, the number of the steaming procedure (Table 2), and the number of acid treatments performed on it. For example, (1.3)g) 6ALAL will refer to a zeolite containing a residual amount of sodium of 1.3 wt%, steamed at 700°C for 20 h (procedure 6), and acid leached twice in nitric acid. Table 1

Characterization studies X-ray diffraction patterns were recorded on a CGR Theta 60 instrument using CuKot monochromated radiation. Pb(NO3)2 was used as internal standard, and the unit cell parameters were calculated by a double refinement method. 5 Infrared spectra of wafers made by pressing 0.5 mg of zeolite in 300 mg of KBr were taken on a Beckman Microlab 600 computing double-beam spectrophotometer. The framework (1200-600 cm -1) region was scanned in order to determine the position (+_ 3 cm -1) of the characteristic stretching of the six-membered rings of the lattice (V6R). Uhrathin (300-500/~) sections of the grains embedded in resin were observed under a Jeol 100 C transmission electron microscope (TEM). Nitrogen and n-hexane sorption capacities were measured by volumetry and gravimetry, respectively, as described elsewhere. 6 The values determined at relative pressures P/Po = 0.2 and P/Po = 0.9 were taken as the microporous and total volumes, respectively. 298i magic angle spinning n.m.r (MAS) spectra were recorded at 39.73 MHz at room temperature on a Bruker CXP 200 spectrometer operating in the Fourier transform mode. The samples were spun at 3 kHz in a conical rotor made of delrin. A one-cycletype measurement was used. Time intervals between ~/2 pulses were 3 s and 1000-3000 free-induction decays were accumulated per sample. Chemical shifts (in ppm) were measured from tetramethylsilane as an external standard. Framework Si/A1 ratios, (Si/A1)v, were calcul;/ted f r o m the 29Si n.m.r, spectra using classical procedures, v The precision of the determination was better than 10% for (Si/Al)v values lower than 10. For higher values of this ratio, the determination was not as accurate because of the low amount of framework aluminum and the possible presence of silanol groups. Global Si/AI ratios, (Si/A1)G, and unit cell contents were calculated from the elemental analyses performed by atomic absorption (S.C.A.-C.N.R.S. in Solaize). 27A1 MAS n.m.r, spectra were obtained at 104.27 MHz on a Bruker MSL 400 spectrometer equipped with a double-bearing MAS probe free of aluminium. The rotor, made of zirconia, was spun at 4.2 kHz. Short radio frequency pulses of 2 ~ were used. A number of 3000 free-induction decays were accumulated per sample at a repetition time of 2 s. AI (OH2) ~+

Characteristics of the parent (as-made) and exchanged (starting) materials

Treatment a

wt% Na

Unit cell content

•6R b

(cm-1) None C EC CEC

5.4 5.3 1.3 0.3

6.5 Na, 6.4 Na, 1.6 Na, 0.4 Na,

1.5 TMA AIsSi2a072 15 H20 1.6 H AI8Si2807217 H20 6.4 H AleSi2s07216 H20 7.6 H AI8Si2807217 H20

616 618 619 620

Unit cell parameters c (A) ao

18.26 18.25 18.25 18.23

Co

7.64 7.64 7.63 7.62

a E: Exchange in 1M ammonium nitrate at 80°C for 4 h; C: calcination in flowing dry air (100 ml/min) at 500°C for 10 h b Infrared frequency of the stretching of the six-rings (+_ 3 cm -1) c _+ 0.02 A

ZEOLITES, 1990, Vol 10, March

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Steam dealumination of zeolite omega: B. Chauvin et al.

Table 2 Combinations of temperatures and times investigated during the steam dealumination of zeolite omega (see text) Steam treatment (no.)

Temperature (°C)

1 2

Time (h)

600

2

3 4 5 6

600 700 700 700 700

20 0 2 5 20

7 8

820 910

2 .2

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was used as external reference to measure the chemical shifts.

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RESULTS I 80

Parent and exchanged materials The unit cell content of the parent and exchanged starting materials are given in Table 1. The various calcination and exchange treatments resulted in a total elimination of the organic cations and did not modify the silicon and aluminum contents. The extent of sodium removal was greater when the exchange procedure was performed on the solid free of T M A ions (i.e. previously calcined). All four solids exhibited X-ray diffraction patterns and MAS n.m.r. Spectra characteristics of pure, highly crystalline zeolite omega. (Figures 2 and 3). The 27A1MAS n.m.r, spectrum of the parent zeolite (Figure 3a) exhibited two peaks in the 55-65 ppm region due to aluminum atoms located in the two n o n e q u i v a l e n t c r y s t a l l o g r a p h i c sites o f t h e framework s corresponding to the four-membered rings (S4R, site A) and six-membered rings (S6R, site B) of the gmelinite columns (Figure I). The A1A/AIB ratio was found equal to 1.25. Since there are two times more A sites than B sites in the lattice, this result indicates a preferential incorporation of aluminum in the six-membered rings of the structure as discussed elsewhere. 9

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Figure 3 MAS n.m.r, spectra of the parent zeolite: (a) 27AI n.m.r, at 104.27 MHz; (b) 29Si n.m.r, at 39.73 MHz; (c) chemical shifts of the various All and Si (nAI)i configurations

The29Si MAS n.m.r, spectrum (Figure 3b) consisted of the sum of eight peaks, centered at chemical shifts specified in Figure 3c. T h e peaks corresponded to various Si(nA1)i configurations where i is the type of crystallographic site (A or B) and n the number of aluminum terahedra in the first coordination shell (n -- 0-3; signals due to n = 4 were not detected.) The decomposition of the 29Si n.m.r, signal and the calculation o f the framework Si/AI ratio were achieved as described previously. 1° A value of this ratio equal to 3.5 was found for the as-made material, identical to that obtained by chemical analysis. No loss of crystallinity or extraction of aluminum from the framework was observed on the materials calcined and exchanged. The decomposition of the T M A and the removal of sodium ions resulted only in a slight decrease of the unit cell parameters and increase of the infrared frequency of the six-ring stretching (Table 1).

Effect of steaming

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176

ZEOLITES, 1990, Vol 10, March

Up to a steaming temperature of 820°C (procedures 1-7, Table 2), no loss of crystallinity was evidenced by X-ray diffraction. All materials became amorphous after stehming at 910°C for 2 h (procedure 8). The evolutions of the structural parameters and framework Si/A1 ratios are given in Figures 4 and 5. Typical MAS n.m.r, spectra are presented in Figures 6 and 7. Increasing the framework dealumination level (Figure 4a) resulted in a decrease of the unit cell parameters (Figure 4b) (the evolution of Co, not shown, was in all points parallel to that of a0) and in an increase of the infrared frequencies of the structure-

Steam dealumination of zeofite omega: B. Chauvin et al.

sensitive framework vibrations (Figure 4c), as is generally observed with most zeolites. In the 29Si MAS n.m.r, spectra, steam dealumina8 40 A

U.

20

b

tion led to a partial - or eventually complete decrease of the intensity o f the signals due to the Si(3AI), Si(2AI), and Si(1AI) configurations (Figure 6). The remaining peaks were shifted to higher fields as usually observed. 7 Simultaneously broad signals around 30 ppm, and 0 ppm appeared in the 27A1 MAS n.m.r, spectra due to extraframework tetrahedral and octahedral aluminic species, ]1 respectively (Figure 7). The TEM micrograph of a thin section of sample (0.3) fl 6 is presented in Figure 8a. T h e hydrothermal treatment caused the formation of holes, slightly

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ZEOLITES, 1990, Vol 10, March

177

Steam dealumination of zeolite omega: B. Chauvin et al.

a

b x4

X4

Figure 8 TEM micrographs of thin sections of samples: (a) (0.3)Q 6; (b) (0.3)Q 6 ALAL (two magnifications presented)

elo.ngated in the c direction, with an average size of 100 A. The holes were homogeneously distributed in the crystal. Large areas with well-resolved (100) planes, free of dislocations or stacking faults, are clearly visible in the micrograph. The influence of the severity of the steam treatment on the extent of dealumination is summarized in Figure 4 for sample (0.3)ffL When the solids were maintained for 2 h under steam, the framework Si/AI ratio increased monotonically from 18 to 35, while increasing the temperature from 600 to 820°C. On the other hand, at 700°C, the dealumination level increased slightly with the steaming time up to 5 h of treatment [the (Si/Al)v increasing from 25 to 35] and then remained constant for longer steaming periods up to 20 h. It is also clear, from Figure 7, that the contribution of the 30 ppm signal of the 27A1 MAS n.m.r, increased as the severity of the treatment increased. The effect of the residual sodium content is apparent from Figures 5-7. All other conditions being

178

ZEOLITES, 1990, Vol 10, March

kept constant, the extent of dealumination decreased as the amount of sodium of the sample increased. The sodium ions led apparently to an inhibition of the dealumination process as evidenced by the data obtained on the (1.3)Q sample. Thus, equivalent framework Si/A1 ratios around 15 + 2 (Figure 5a) and structural characteristics (Figure 5b and c) were reached under mild (600°C, 2 h) or harder (700 °, 20 h) steaming conditions.

Effect of acid leaching The steamed materials exhibited smaller sorption capacities than did the starting zeolites, indicating a severe blocking of the porosity by the dislodged species (Table 3). A first acid treatment resulted in all cases in a drastic increase of the microporous (measured at P/Po = 0.2) and total (measured at P/Po = 0.9) sorption capacities (Table 3). T h e microporous volume available to nitrogen was significantly higher than that of the parent zeolite. Half of it was

Steam dealumination of zeolite omega: B. Chauvin et al. Table 3

Si/AI ratios (global and framework) and sorption capacities of the omega samples: effect of steaming and of acid leaching Sorption capacity (ml/g)

Sample

(0.3)Q (1.3)Q (1.3)9 (1.3)~ (1.3)Q (1.3)~ (1.3)~ (0.3)Q (0.3)Q (0.3;9 (0.3)~ (0.3)Q (0.3)~ (0.3)Q

Si/AI

Nitrogen

Framework

Global

Micropores

Total

Micropores

Total

3.5 13 13 13 17 17 17 30 30 35 35 -

3.5 3.5 7.0 12.0 3.5 7.1 11.5 3.5 7.0 9.5 3.5 7.1 7.6 7.6

0.08 0.02 0.20 0.22 0.02 0.18 0.19 . 0.18 0.01 0.19 0.14

0.09 0.04 0.30 0.40 0.03 0.32 0.40 . 0.37 0.03 0.34 0.44

0.08 0.01 0.10 0.12 0.10 0.10

0.08 0.01 0.21 0.29 0.20 0.30

0.09 0.13 0.09 0.15 0.08

0.18 0.30 0.09 0.30 0.36

2 2AL 2ALAL 6 6AL 6ALAL 4 4AL 4ALAL 5 5AL 5ALAL 5ALALAL

accessible to n-hexane. T h e sorption-desorption isotherms for nitrogen at 77 K, which were totally reversible on the starting and steamed materials, became type IV of the BET classification after the acid treatment (Figure 9). The desorption branch, signaling the presence of mesopores (with no welldefined structure), joined the adsorption branch after a steep fall at a relative pressure near 0.4. Taking the difference between the amount sorbed at P/Po -- 0.2 and 0.9 as an estimation of the free volume of the mesopores, it was calculated that the latter corresponded to 0.1 ml/g and was accessible to both nitrogen and hydrocarbons. A second main effect of the first acid leaching was

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P z

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0.25

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n-hexane

Adsorption-desorption isotherms for nitrogen at 77 K:

(a) (1.3)Q 2; (b) (1.3)~ 2AL; (c) (1.3)~ 2 ALAL

.

.

to increase the global Si/A1 ratios to a value around 7 in all samples (Table 3). This corresponded to the elimination of 3.5 aluminum atoms per unit cell of solid. This value, which was independent of the framework Si/A1 ratio, suggests that an equilibrium had been reached under our conditions. As shown in Figure 7, acid leaching resulted in the removal of most of the tetrahedral extraframework aluminum. The quantity of octahedral aluminic species was much less affected by the treatment. In the 29Si MAS n.m.r, spectra (Figure 6), the signals were better resolved than those observed after steaming, but no further dealumination of the framework could be evidenced. This was confirmed by X-ray diffraction and infrared spectroscopy. When the acid attack was repeated, additional aluminic species were removed from the solids as indicated by the increase of the global Si/A1 ratio (Table 3). Interestingly the global Si/A1 ratio achieved decreased when the framework Si/A1 ratio increased. This subsequent aluminum extraction led to a significant increase of the nitrogen and hydrocarbon sorption capacities of the mesopores up to 0.2 ml/g, whereas the microporous volumes were hardly changed (Figure 9 and Table 3). In the TEM micrographs of the specimens submitted to two acid attacks, higher densities of holes were detected but their size was not modified (Figure 8b). The (100) plane images remained highly ordered, ruling out framework structural defects. The comparison of the TEM micrographs of various samples revealed, moreover, that, provided the framework Si/AI ratio be higher than ca. 15, the texture (size and density of the holes) was identical on all steamed and acidleached omega zeolites. This observation correlates well with the fact that, after two acid leachings, all the materials we studied gave nearly identical nitrogen microporous and total sorption capacities (Table 3). A third acid leaching led to a decrease of the microporous volume (Table 3), a decrease of the intensity of the X-ray diffraction peaks, and the

ZEOLITES, 1990, Vol 10, March

179

Steam dealuminafion of zeolite omega: B. Chauvin et al.

a~)pearance of diffuse backgrounds in the XRD and i MAS n.m.r, spectra as exemplified in Figure I0. The variot~s remaining peaks were, nevertheless, very narrow. The final solids should then be regarded as mixtures of amorphous material and highly crystalline zeolitic zones. It was hard to determine if the latter had been further dealuminated by the acid due to the poor quality of the spectra. Chemical analysis revealed no change of the global Si/AI ratio (Table 3).

DISCUSSION The data presented here allow for a qualitative and quantitative description of the various events that occur during the dealumination of zeolite omega by successive steam and acid treatments. In the presence of water vapor, at high temperature, framework Al-O bonds are hydrolysed. The high density of aluminium atoms in the original zeolite means that not only individual atoms but also large portions of crystalline material containing both silicon and aluminum atoms are disconnected from the tridimensional lattice. From the average size of the cavities formed (5.105 ~a), it may be estimated that these detatched zones contain up to 200 unit cells of zeolite omega. The latter rearrange into an amorphous silicoaluminate phase, denser than the zeolite framework, which remains trapped in the holes thus created. The zeolite zones are highly crystalline, fault-free, and exhibit contracted unit cells. These facts are consistent with the formation of new Si-0-Si bonds in the lattice, with silicon atoms taking positions in the vacated tetrahedral sites. ~; The 27AI n.m.r, spectra of the steamed zeolites omega resemble those reported for hydrothermally treated amorphous silica-alumina and Y (Refs. 12-14)

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.

4

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.

.

.

12

c.K

i

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28

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-80

,

,

,

-100

,

,

-120

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Figure 10 X-ray powder diffraction patterns and 29Si AlAS n.m.r, spectra at 39.73 MHz of samples: (a) (0.3)Q 5; (b)

(0.3)Q 5AL; (c) (0.3)Q 5ALAL; (d) (0.3Q 5ALALAL

180

ZEOLITES, 1990, Vol 10, March

except for the presence of a doublet at 55-65 ppm in omega (see above). For the Y zeolite, Corma et al. is concluded that at least four types of aluminum atoms were present: framework and nonframework tetrahedrally coordinated atoms (giving n.m.r, signals at 60 and 30 ppm, respectively), oct~hedral extraframework aluminum (signal at 0 ppm), and tetrahedrally coordinated aluminum in an amorphous silicaalumina phase, the n.m.r, signal of it being superposed to that of the aluminum in the zeolite framework (signal at -= 54 ppm (Ref. (13)). Our data did not show evidence of the latter signal. T h e extent of framework dealumination depends on the severity of the steam treatment. In the particular case of omega, the dealumination seems to be a fast process, occurring mostly in the period during which the temperature is raised from 500°C to the final temperature under 1 atmosphere of steam. For instance, at 700°C, six aluminum atoms per unit cell are extracted during this initial period wheras only one additional atom is extracted during the subsequent 20 h. The presence of residual sodium ions strongly inhibits the dealumination process, in agreement with the higher resistance to hydrolysis of the AI-0-Na bonds compared with the A I - 0 - H bonds already observed for Y (Ref. 11) and ZSM-5 (Ref. 15) zeolites. The presence of dislodged material in zeolite omega severely restricts the access to the intracrystalline micro and mesoporous voids, and acid treatment is therefore required to liberate the porosity. Even for highly dealuminated frameworks, it is our experience 1° that mild conditions (low acid concentrations and slow addition rates) must be used in order to avoid complete amorphization of the structure. The reasons for the low acid resistance of dealuminated zeolites omega is not fully understood. Quantitative analysis of the effect of two successive acid leachings (performed under the conditions described in the Experimental section) is presented in Figure 11, where the number of framework (AIr), removable (A1R), and nonremovable (A1NR) aluminum atoms per unit cell have been plotted as a function of the framework Si/A1 ratio. It is seen that the quantity of aluminum that cannot be extracted out of the solid by the acid increases with the (Si/AI)F ratio. In other words, when the severity of the steam treatment increases, more and more refractory species formed in the solid. On the other hand, the number of acid-removable aluminum atoms is less affected. This number always exceeds that of framework AI atoms] Aluminum-extractable species cannot, thus, all correspond to cationic species balancing the framework negative charges. They must consist mostly in dissolved parts of amorphous silica-alumina debris. T h e drastic effect of the acid leaching on the sorption properties indicates that the extraframework aluminum blocks both the zeolite channels and the mesopores. The effect on the structural microporosity is easily understood in view of the topology of the omega framework. T h e micropores

Steam dealumination of zeolite omega: B. Chauvin et al.

dealumination in systems with initial Si/A1 ratios smaller than 4 is independent of the topology of the parent zeolite.

s

CONCLUSIONS

,,i

2

I

I 15

I

I 25

I

I 35

(Si/A')F Figure 11 Unit cell contents in framework (AIF), extraframework removable (AIR) and extraframework nonremovable (AINR) aluminium of steamed and acid-leached (t~wice) samples with varying (Si/AI)F

consist of tubular channels that can be blocked by very small amounts of extracted material. Even the presence of cations has been shown to limit the access to the microporous volume of omega. 8'16 The high efficiency of the acid treatment in liberating the microporosity suggests that the entities that block the channels are not tightly retained and they can be regarded, intuitively, as small, exchangeable ionic species. 10 The large increase of the accessibility of the mesopores upon acid leaching shows that the latter are connected to the exterior of the crystals by restrictions. The TEM observations confirm that very few of the holes are linked directly to the surface of the crystals. In that respect, the secondary mesporous system created by dealumination of omega seems very similar to that presented by Lynch ei-al, iv for steamed Y. T h e above authors estimated the size of the restrictions between mesopores as smaller than 50 /~. Their assumption was based on the interpretation of the steep fall of the desorption branch of the nitrogen isotherm near P/Po = 0.4. This effect is believed to be due to the tensile-strength effect, is which leads to the catastrophic desorption, at P/Po = 0.42 for nitrogen, of mesopores connected to the exterior by necks smaller than 50/~ in diameter. It is interesting to note that zeolites with very different topologies such as Y (tridimensional porosity), offretite (bidimensional porosity), and omega (unidimensional porosity) behave so similarly upon dealumination, as far as their final texture is concerned (Refs. 17 and 19, and this work): They all feature the steep desorption at P/Po "~ 0.4 in the nitrogen isotherm and mesopores with sizes centred around 100-150 /~ in the TEM micrographs. If the above assumptions are correct, this would mean that the geometry of the secondary pore system generated by

Experimental factors that determine the composition and the texture of steam-dealuminated zeolite omega have been studied. Most of the framework dealumination takes place while the temperature is increased above 500°C under steam up. to the final temperature. At this stage, holes i00 ~ in diameter are created, which retain the material disconnected in the form of an amorphous silica-alumina phase. Increasing the severity of the treatment does not provoke additional creation of mesopores and results only in a slight additional dealumination. Sodium ions severely inhibit the hydrolysis of the AI-0 bonds. Part of the extracted material is present as exchangeable and/or acid-soluble species. The subsequent acid treatments liberate the micro and mesoporous volumes. Under the conditions of our study, the structural microporosity available to hydrocarbons was equal to 0.12 _ 0.02 ml/g and the volume in the mesopores corresponded to 0.18 _ 0.02 ml/g. These values were almost independent of the dealuruination conditions. The severity of the treatment determines the amount of aluminum-containing debris that are refractory to the acid attack. As a rule, higher steaming temperatures lead to higher framework Si/A1 ratios but to smaller global ratios after the acid-leaching procedure. Finally, the secondary porous network created by steaming is described as nearly spherical holes (= 100 /~) connected by narrow necks. The remaining crystalline fraction of dealuminated zeolite is free of dislocations or structural defects.

REFERENCES 1 Chauvin, B., Rajoafanova, V., Coq, B., Figueras, F., Fajula, F. and Des Couri~res, T. to be published in Proceedings of the 8th International Zeolite Conference, Amsterdam, July 1989 2 Fajula, F., Figueras, F., Moudafi, L., Vera Pacheco, M., Nicolas, S., Dufresne, P. and Gueguen, C. French Pat. Appl. 85 0772 (1985) 3 Galli, E. Cryst. Struct. Commun. 1974, 3, 339 4 Rinaldi, R., Pluth, J. J. and Smith, J. V. Acta Crystallogr. 1975, B31, 1603 5 Lindqvist, O. and Wengelin, F. Arkiv. Kemi. 1967, 28, 179 6 Chauvin, B., Fajula, F., Figueras, F., Gueguen, C. and Bousquet, J. J. Catal. 1988, 111, 94 7 Thomas, J. M. and Klinowski, J. Adv. Catal. 1985, 33, 199 8 Klinowski, J. and Anderson, M. W. J. Chem. Soc., Faraday Trans. / 1986, 82, 569 9 Massiani, P., Fajula, F., Figueras; F. and Sanz, J. Zeolites 1988, 8, 332 10 Massiani, P., Chauvin, B., Fajula, F., Figueras, F. and Gueguen, C. Appl. Catal. 1988, 42, 105 11 McDaniel, C. V. and Maher, P. K. in Zeolite Chemistry and Catalysis, ASC Monograph 171 (ed. Rabo, J. A.) Am. Chem. Soc., Washington, DC, 1976, Chap. 4 12 Gilson, J. Po, Edwards, G. C., Peters, A. W., Rajagopalan, K., Wormsbecher, R. F., Roberie, T. G. and Shatlock, M. P. J. Chem. Soc., Chem. Commun. 1987, 91

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Steam dealumination of zeolite omega: B. Chauvin et al. 13 Corma, A., Fornes, Vo, Martinez, A. and Sanz, J. ACS Sym. Ser. 375, Am. Chem. Soc., Washington, DC, 1988, p. 17 14 SamosoR, A., "Lippmaa, E., Englehardt, G., Lohse, U. and Jerschkewitz, M. G. Chem. Phys. Lett. 1987, 134, 589 15 Sano, T., Suzuki, K., Okado, H., Fujisawa, K., Kawamura, K., Ikai, S., Hagiwara, H. and Takaya, H. Stud. Surf. Sci. Catal. 1987, 34, 613

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16 Cole, J. F. and Kouwenhoven, H. W. Adv. Chem. Ser. 1973, 121,583 17 Lynch, J., Raatz, F. and Dufresne, P. Zeolites 1987, 7, 333 18 Gregg, S. and Sing, K. in Adsorption, Surface Area and Porosity, Academic Press, London, 1982 19 Hernandez, F., Ibarra, R., Fajula, F. and Figu.eras, F. Acta Phys. Chem. 1985, 31, 81