physicochemical and catalytic properties of a modified ... - CiteSeerX

Clay Minerals (1994) 29, 123-131

PHYSICOCHEMICAL AND CATALYTIC PROPERTIES A MODIFIED NATURAL CLINOPTILOLITE A. ARCOYA,

J. A. GONZALEZ*,

N. TRAVIESO*

OF

AND X . L . S E O A N E

Instituto de Catdlisis y Petroleoqu[mica, CSIC, Campus Universidad Aut6noma, Cantoblanco, 28049 Madrid, Spain, and *Centro de Investigaciones Qufmica, MINBAS, Washington I69, Cerro, La Habana, Cuba (Received 10 February 1992; revised 1 February 1993) A B S T R A C T : Samples of natural clinoptilolite modified by treatment with NH4C1 or HCI solutions, followed by thermal treatments, have been characterized, and their catalytic activity evaluated, in o-xylene isomerization and ethanol dehydration reactions. The substitution of the compensating cations by NH4~ does not produce structural changes in the original material, but it opens the channels and increases its acidity and thermal stability. The treatment with HCI increases both the acidity and the effective diameter of the channels and pores but it produces an important loss of zeolite phase. Calcination of the acidic forms above 973 K leads to the breakdown of the zeolite structure. Catalytic activity of the samples is related to the surface acidity. For the original and NH4Cl-treated samples, however, the conversion of o-xylene is limited by the access of the reactant inside the channels of the zeolite. A comparative study with modified sepiolite in dehydration of ethanol has also been performed. The availability and relatively low cost of natural zeolites has stimulated much research on their physicochemical properties (Breck, 1980). Clinoptilolite, of the heulandite family, is one of the m o r e siliceous natural zeolites. Its basic structure, studied among others by K o y a m a & Takeuchi (1977) and Galli et al. (1983), consists of a twodimensional system of three types of channels, with sizes of 4.4 • 7.2, 4.0 • 5.5 and 4.1 • 4 / ~ respectively. Although clinoptilolite is available in deposits in relatively high purity, extensive application has not yet b e e n achieved. A t present, it is mainly used as an adsorbent, its application in catalysis being rather limited (Miklosy et al., 1983; Sakoh et al., 1985; Gonzgtlez et al., 1988). Zeolites can be used as catalysts mainly for reactions which require the participation of an acidic function. Their activity and selectivity are frequently related to lattice topology, dimensions of the cavities and channels and surface acidity. In natural zeolites, however, these parameters can be affected by the nature of the compensating cations in the framework ( R a b o et al., 1961; Barthomeuf, 1984), so that the catalytic properties of a given zeolite depend, to a certain extent, on its particular origin. Acid forms of zeolites are conventionally obtained either by ion exchange with NH~- or by leaching with mineral acids, followed by calcination. These treatments modify

the acidity and chemical composition of the starting material and, eventually, also its texture and structure, and therefore its catalytic properties (Tsitsishvili, 1979). Some physicochemical properties of clinoptilolite resemble those of sepiolite which is a natural fibrous magnesium silicate clay mineral with channels of approximate dimensions 10.8 • 4.0 in a cross-section perpendicular to the length of the fibre (Corma & Perez-Pariente, 1987). A n important difference exists between both materials, namely, the ion exchange capacity of clinoptilolite (2.7 mEq/g) which is far higher than that of sepiolite (8 mEq/100 g). For this reason, clinoptilolite and sepiolite can be expected to have different catalytic behaviours. The purpose of this work was to investigate the physicochemical properties of a natural clinoptilolite from Castilla (Cuba), which has been modified by ion exchange or acidic treatments. The catalytic behaviour of the prepared samples was evaluated in the o-xylene isomerization and ethanol dehydration. These reactions take place on acid sites (Ward & Hansford, 1969; Jacobs, 1982) but they are also sensitive to the texture and structure of the catalyst (Jacobs & Martens, 1986). O n the other hand, as sepiolite is an effective catalyst for dehydration of ethanol (Dandy & Nadiye-Tarbiruka, 1982), comparative experiments were performed with samples of the

9 1994 The Mineralogical Society

124

A. Arcoya et al.

material modified by ion exchange treatment.

or acid

solution (5 ml solution/g sepiolite) at reflux temperature for 2 h, and removal of CI- by washing with water. Catalysts C-1 and C-2 were dried at 393 K and calcined at 673 K for 3 h.

EXPERIMENTAL

Sample preparation The natural clinoptilolite used has a degree of purity higher than 85% ( R o q u e - M a l h e r b e et al., 1990) mordenite, calcite, and or-quartz being the major impurities. The chemical composition of the mineral (wt%) is: SiO2 68.05; A1203 12.3; TiO2 0.40; Fe203 1.34; C a O 3.70; M g O 1.03; N a 2 0 0.43; K 2 0 1.70 and H 2 0 11.06. The starting material was ground to < 0 . 3 m m and then washed with distilled water (sample CL). The rest of the samples were prepared according to the following procedures: Sample S-1. Ion exchange with 1 i NH4C1 solution (5 ml solution/g zeolite) at reflux temperature for 5 h. In order to attain a high degree of exchange, the solution was replaced five times. A f t e r filtering, the solid was washed until CI- ions were totally r e m o v e d , dried at 393 K and calcined at 673 K for 3 h in a stream of air. Sample S-2. Five successive leachings with 5 i HCI solution (5 ml solution/g zeolite) at reflux temperature for 5 h each time. The solid particles were then washed until free of acid and dried at 393 K. Sample S-3. In order to extract A P + and produce appreciable modifications in the material, one fraction of CL-2 was calcined at 973 K in an air stream for 4 h. Previously, it was verified that calcination at 873 K does not produce structural changes, while at 1073 K, complete collapse of the zeolite occurs. The calcined sample was treated with 0.25 i HCI solution to r e m o v e AI 3+ ions segregated and then washed and dried at 393 K. The samples of modified sepiolite were prepared from a high grade ( > 9 5 % ) mineral from Vallecas (Spain). A f t e r washing, the material was calcined at 823 K for 4 h to stabilize its structure (sample SE). F r o m SE two catalysts were prepared as follows: Catalyst C-1. Three successive treatments with 1 N NH4CI aqueous solution (5 ml solution/g sepiolite) at reflux temperature for 2 h each. The solid was then washed until C1- ions were totally eliminated. Catalyst C-2. Leaching with 3 N HCI aqueous

Characterization methods The n u m b e r of ions extracted from the zeolites was determined by atomic absorption. The SiO2/ A I 2 0 3 molar ratio in the samples was measured by chemical analysis and energy dispersive X-ray microanalysis ( E D X - S E M ) . X-ray diffraction ( X R D ) patterns were p e r f o r m e d in a powder diffractometer, using Cu-Kct radiation. Infrared (IR) spectra were recorded using the KBr technique. Surface areas were obtained by the B E T m e t h o d from N2 adsorption isotherms at 77 K. The samples were previously outgassed at 623 K and 10 -5 Torr. Surface acidity was measured by temperature p r o g r a m m e d desorption ( T P D ) of NH3 in a katharometric device. Since the NH3 molecule can penetrate into the channels and pores of clinoptilolite (kinetic diameter = 2.6 /~), desorbed a m m o n i a accounts for the n u m b e r and strength of the acid sites on both internal and external surface of the zeolite crystals. The samples were pretreated in a stream of H e at 623 K for 2 h, cooled to r o o m temperature under H e and then saturated under flow of a m m o n i a gas. A f t e r removal of the physisorbed NH3 at 373 K in H e stream, T P D profiles were recorded heating the samples at 10 K/min from 373 to 973 K under a flow of H e (60 ml/min). Simultaneously, desorbed NH3 was absorbed on 0.1 u HCI and then titrated with 0.1 M N a O H .

Thermal stability To compare the thermal stability of the samples, fractions of CL, S-1 and S-2 were calcined at 873, 973 and 1073 K for 4 h in an air stream, heating at 5 K/min. The relative intensity of the X R D peak at 20 = 30.05 ~ was taken as a measure of the crystallinity of the zeolite.

Catalytic activity Catalytic activity measurements were carried out in a conventional fixed-bed flow reactor (1/4" O D ) containing 0.3 g of catalyst, operating at atmospheric pressure. The feed was N 2 (7 ml/min) saturated with the reactant at 298 K. O-xylene isomerization was p e r f o r m e d between

Properties of modified clinoptilolite 575 and 673 K, and ethanol dehydration at 573 K. Prior to the reaction, catalyst samples were heated at the reaction temperature for 1 h under a flow of nitrogen. The products were analysed by gas chromatography, a packed column with 10 wt% polyethyleneglicoladipate on Chromosorb W being used for isomerization products. Dehydration products, on the other hand, were separated using a Porapak Q column.

RESULTS

AND DISCUSSION

125

Takruchi, 1977; Ward, 1984). For Mg, it is possible that some Mg 2+ occupies non-exchangeable positions in the structure, such as the crystallographic planes (200) (Galabova & Haralampiev, 1979). The acid treatment (sample S-2) removes the exchangeable cations, all the Fe 3+ and some of the A13+ . However, only part of the Ca 2+ present (43 mole%) is extracted as reported by Gonzhlez et al. (1982) for this type of material. For this sample the SIO2/AI203 molar ratio increases to 35.6 (Table 1), while for S-3, further dealumination takes place due to the calcination step.

Chemical composition o f the samples Plots of Figs. l a and l b give the degree of removal of individual ions in the samples S-1 and S-2 respectively. In S-l, more than 95 mole% of Ca 2+, Na + and K + ions were exchanged, while the degree of exchange for Mg 2+ was >80 mole%. The high degree of exchange obtained is a consequence of the property of the NI-I~ ions to bond to the oxygens of the zeolite by means of ionic and hydrogen bonds (Ceranic et al., 1985). The fact that only 1.6 mole% of Fe 3+ ions present in the original material was extracted is probably due to the fact that Fe3§ is replacing tetrahedral A P + in the framework (Roque-Malherbe et al., 1990). The different removal rate of the cations (Fig. la) can probably be related to their nature, location in the channels and coordination in the zeolite framework (Breck, 1974; Koyama &

X-ray diffraction and IR spectra The X R D pattern in Fig. 2a shows the high crystallinity of the starting material. The more characteristic lines of clinoptilolite (20 = 9.92 ~ 22.43 ~ and 30.50 ~) are observed. Other less intense lines can be identified as the main reflections of mordenite (20 = 25.8 ~ and 0~TA.LE 1. Characterization of the clinoptilolite samples. SIO2/A1203 SA Vp Acidity (mole/mole) (m2/g) (cc/g) (mEqNH3/g)

Sample CL S-1 S-2 S-3

9.5

10.2 35.6 48.3

51 256 200 120

0.082

1.10

0.162

2,10

0.183 0.192

1.00 0.25

loo

.- so 0

,o

a

~ 20 m

0

I

~

b

m

2 3 4 5 Number of t r e a t m e n t s

1

2 3 4 5 Number of t r e a t m e n t s

Fm 1. Removal of K+(9 Na+(A), Ca2+ (Q), Mg2+ (A) and Fe3+ (V), as a function of the number of treatments with (a) NHaCI and (b) HCI solutions.

126

A. Arcoya et al.

quartz (20 = 20.8 ~ which are p r e s e n t as impurities. T h e reflection at 20 = 22.43 ~ is intensified because it is c o m m o n to the (132) a n d (004) planes of clinoptilolite. T h e NH4C1 t r e a t m e n t followed by t h e r m a l d e c o m p o s i t i o n of the NH4 + form at 673 K (sample S-I) does n o t p r o d u c e any structural changes d e t e c t a b l e by X R D with r e g a r d to the C L sample (Fig. 2b), while the acid t r e a t m e n t p r o d u c e s i m p o r t a n t modifications as s h o w n in Fig. 2c. In this case, the d e c r e a s e of the intensity of the lines a n d the a p p e a r a n c e of a b r o a d low baseline are indicative of t h e t r a n s f o r m a t i o n of

F-

I 29

I 25

I 21

zeolite into a n a m o r p h o u s phase. F r o m the 044 reflection at 30.05 ~ 20 a n d using the S h e r r e r e q u a t i o n the average crystal size for S-2 was calculated to b e 114 ~,, smaller t h a n t h a t f o u n d for C L a n d S-1 (177/~,). A f t e r calcination of the original sample (CL) at 873 K, clinoptilolite lines were still p r e s e n t b u t t h e i r intensities were very weak (Fig. 2d). Since the t h e r m a l stability of the clinoptilolite is closely related to the n a t u r e of the m a j o r cation p r e s e n t in the f r a m e w o r k , the b r e a k d o w n of the structure of sample C L at a lower t e m p e r a t u r e t h a n t h a t r e p o r t e d by B r e c k (1974) for o t h e r clinoptilolites,

I 17

I 13

I I 9

28 FIG. 2. X-ray diffraction patterns of the clinoptilolite samples: (a) CL; (b) S-l; (c) S-2; (d) CL calcined at 873 K; (e) S-1 calcined at 973 K; (f) S-2 calcined at 973 K (S-3).

Properties of modified clinoptilolite

127

accompanied by dealumination (Spojakina et al., 1985).

Nitrogen adsorption

u c 0

l 1300

1000 700 400 Wovenumber (cm -1)

FIG. 3. Infrared spectra of the clinoptilolite samples. can be attributed, according to K o y a m a & Takeuchi (1977), to the lower value of K+/Ca 2+ ratio in zeolite CL. The diffraction patterns of S I and S-2 calcined at 873 K are shown in Figs. 2b and 2c, respectively. In these two samples, the substitution of exchangeable cations of the framework by H + probably results in a less distorted and therefore more stable structure than that of the natural material. A f t e r calcination at 973 K, however, these samples show a decrease in the intensity of the characteristic peaks of the zeolite (Figs. 2e and 2 0 and at 1073 K, the collapse of the structure is complete. This behaviour of the acidic forms is in good agreement with that observed by Barrer & Marki (1964) for the mineral from M o u n t H e c t o r (California). Dealumination and destruction of the structure of samples S-2 and S-3 are also suggested by the I R spectra of Fig. 3. The intensity of the clinoptilolite characteristic band at 615 cm -1 decreases with the severity of the treatment, while the band at 450 cm -x, characteristic of amorphous silica, increases. O n the other hand, the shift of the zeolite band from 1070 to 1110 cm -1 denotes a partial structural breakdown,

Data in Table 1 show that the adsorption capacity of the treated samples is higher than that of the original clinoptilolite. The relatively low values of specific surface area (SA) and pore volume (Vp) found for C L (51 m2/g and 0.082 cm3/g, respectively) are due to the fact that N 2 cannot enter into the pores of the natural zeolite and it is mainly adsorbed on the external surface of the solid (Tsitsishvili, 1973). O n the other hand, the increase in SA and Vp values for S-1 is consistent with the opening of the windows produced by the substitution of metal cations by H + . For sample S-2, in which the countercations were also r e m o v e d , an increase in SA and Vp with respect to C L is also observed. Nevertheless, the lower SA and the higher Vp values in comparison with those of the highly crystalline protonated clinoptilolite (S-I) can possibly be related to both the dealumination and the breakdown of part of the structure of the zeolite, as shown by X R D (Fig. 2c). This is consistent with the fact that when sample S-2 is calcined to give S-3, a loss of both surface area and zeolite phase occurs.

Surface acidity The T P D profiles in Fig. 4 show the distribution of acid sites in the samples. The untreated material exhibits a single desorption peak at 493 K (LTP) associated with the weak acid sites and a shoulder at 653 K, while the treated samples show another peak at a higher temperature (HTP) ascribed to strong acid sites. On the other hand, the comparison of the amounts of ammonia desorbed from the samples (Table 1) indicates that the treatments used do not only change the acidity spectra, but also the total number of acid sites. The treatment with NH~- and the subsequent deammoniation at 673 K increases both the number (2.1 m E q NH3/g) and strength of the stronger acid sites of the original material (T~,ax H T P = 873 K in comparison with 653 K for C L in Fig. 4). O n the other hand, the acid treatment (sample S-2) only increases the acid strength (T~ax H T P = 880 K) as a consequence of the partial dealumination of the zeolite. For the

128

A. Arcoya et al. calcined sample (S-3), there is a substantial decrease in total acidity (0.25 mEq/g), although it exhibits a small n u m b e r of strongest sites (Tmax H T P = 930 K). This is due to the loss of zeolite phase and the more extensive dealumination of the remaining zeolite crystal (Barthomeuf, 1984).

A I l

II Catalytic activity

Ii -

~

"~

E E

i/~I

I

il..".\; c,/

~,"

/

/x

\

:

!I

/i -2

o

...\ .,.--..

373

"\.. \

673 973 Ternperoture (K)

FI~. 4. Temperature programmed desorption profiles of ammonia of the clinoptilolite samples.

100 a

a

,,~

Table 2 s u m m a r i z e s molar conversion of o-oxylene (X) and yields (Yi) to m + p-xylenes, trimethylbenzenes (TMB) and toluene (Tol) after 10 min reaction time. The treated samples give higher conversion than natural clinoptilolite and, in particular, S-2 is by far the more active catalyst. However, for these samples the catalytic activity decreases with the time on stream, as illustrated in Fig. 5. Carbonigenic activity of the zeolite is a property that is closely related to the accessible n u m b e r and strength of acid sites on the surface of the zeolite (Jacobs, 1977). O n the other hand, it is known that o-xylene in the presence of acid catalysts may undergo two major different reactions (1) isomerization via intramolecular mechanism and (2) disproportionation through in intramolecular pathway. The relatively very low overall activity of the a a

60-

A

a-

i

a ~-

.a

O~o

b

2o-

L

o

o - -

n-----n

- -

I

1

a / a -

_a ~'-

w

!

X

c -a/A/

-~.o

i ~"~

0

r

~vm, I I 80 120 hO 80 t i m e on s t r e a m (min)

I 120

FIG. 5. Conversion (X%) of o-xylene (O) and selectivities (S i % ) toward isomerization (A) and disproportionation (V) products vs. time on stream for the samples: (a) CL; (b) S-I; (c) S-2; (d) S-3.

129

Properties of modified clinoptilolite TABLE2. Isomerizationof o-xyleneon clinoptilolite.

TABLE3. Dehydration of ethanol on sepiolite.

Catalyst

CL

S-1

S-2

S-3

Catalyst

SE

C-1

C-2

T (K) Conversion(mole%) Yields (mole %) of: m + p-xyl. Tol TMB

673 4.5

673 44.0

581 63.0

673 26.0

4.9

7.8

1.2

4.5 0.0 0.0

33.4 8.2 2.4

42.3 9.2 11.5

12.2 6.7 6.8

Conversion(mole%) Yields (mole %) of: Ethylene Ether

2.6 2.3

4.8 3.0

0.8 0.4

natural zeolite (CL) can be explained by the inaccessibility of the reactant (kinetic diameter = 7 ,~) inside the channels and pores rather than by the lack of acidity, Despite the acidity values of the treated samples (Table 2), the fact that S-1 is far less active than S-2 indicates that the reactant molecules have still limited access to the zeolite even after removal of exchangeable cations from the framework. On the other hand, because of the lack of space in clinoptilolite to accommodate the transition state in the transalkylation reaction, the small amounts of trimethylbenzenes present in the products from S-1 are probably produced on the external surface sites of the crystals. Moreover, since transalkylation produces toluene and trimethylbenzenes in the proportion 1:1, the value of the ratio YToJ/YTMB (Table 2) indicates that, in this case, toluene is mainly formed by cracking of o-xylene. For the samples S-2 and S-3, the collapse of part of the zeolite framework produces an increase in the intercrystailine space (secondary porosity) which enables the o-xylene to reach most of the acid sites, thus producing not only isomerization products but also significant amounts of trimethyibenzenes and toluene via the bulky diarylmethane intermediate (Guisnet & Gnep, 1984). Plots in Fig. 5 show that deactivation of the catalyst samples affects the various reaction products in a different way. For S-l, where disproportionation is very small, selectivities toward m + p-xylenes (Siso) and toluene + trimethylbenzenes (Sois) remain constant. For S-2 and S-3, however, the decay of transalkylation is faster than the isomerization and therefore the ratio Slso./SDis increases with the time on stream. In order to examine the effect of total acidity on the catalytic activity without steric hindrance of the reactant, the samples S-1 and S-2 were tested in the dehydration of ethanol. This molecule has

greater access than o-xylene to the channels of the zeolite. Both samples exhibited dehydration activity yielding ethylene, ethylether and water as the reaction products. Figure 6 shows the overall dehydration activity and the yields to ethylene (Yo) and ethylether (YE) vs. time on stream. As expected (Jacobs, 1982) from the acidity values in Table 1, catalyst S-1 is the more active and the less stable. Because transformation of ethanol into ether on small pore zeolites is restricted (Venuto & Landys, 1968), its formation in this case is surprising. On the basis of the general reaction pathway proposed by Kn6zinger & KOhne (1966) and taking into account the void dimension of the channels of the zeolite, ether is probably formed on the external surface of the crystals (secondary porosity). This statement was verified by Rodriguez (1987) by poisoning the external surface of the catalyst with pyridine prior to the reaction. Under these conditions, ether was not produced. A similar behaviour was observed for the samples of modified sepiolite, although the overall dehydration activity was in all cases lower than that exhibited by clinoptilolite. The different values of conversion found (Table 3) indicate that dehydration of ethanol is a reaction which is sensitive to slight changes in the surface acidity, produced by the treatments given to the sepiolite. The overall activity of the sample S-2, lower than that of SE, can be related to the loss of active sites by the partial transformation of sepiolite into microporous silica during the HCI treatment (Fernandez Alvarez, 1972). CONCLUSIONS Natural clinoptilolite from Castilla (Cuba) has low thermal stability and is only slightly active in o-xylene isomerization, because its counter-

A. Arcoya et al.

130

100

a 9-.-

•

60

b I

y ~ ~~

0 ~ 0 ~ 0 V------V-------V

0 V

2O

0

1

I

I

I

I

I

60

120

180

60

120

180

time

on stream (rnin)

FIG. 6. Conversion (X%) of ethanol (O) and yields (Yi%) of ethylene (/~) and diethylether (V) vs. time on stream for the samples: (a) S-I; (b) S-2.

cations do n o t p e r m i t access of the r e a c t a n t to the channels. T h e r e m o v a l of cations by NH~- e x c h a n g e , followed by d e a m m o n i a t i o n at 673 K, does n o t p r o d u c e structural changes b u t increases the effective d i a m e t e r of the channels. T h e acid t r e a t m e n t increases the o p e n i n g of the c h a n n e l s a n d pores, b u t also p r o d u c e s a n i m p o r t a n t loss of zeolite phase. Acid catalysts o b t a i n e d from these t r e a t m e n t s , which are t h e r m a l l y stable at least up to 873 K, can b e used for isomerization of xylene a n d d e h y d r a t i o n of ethanol. H e a t i n g a b o v e 973 K leads to the b r e a k d o w n of t h e zeolite samples. Modified clinoptilolites are m o r e effective d e h y d r a t i o n catalysts t h a n sepiolite. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the Comisi6n Mixta Hispano-Cubana. A.A. and X.L.S. thank the CICYT (Project Mat90-0808) for financial aid. REFERENCES BARBER R.M. & MARKI M.B. (1964) Molecular sieve sorbents from clinoptilolite. Can. J. Chem. 12, 1481-1487. BARTHOMEUFD. (1984) Acidic catalysis with zeolites. Pp. 317-346 in: Zeolites: Science and Technology. Martinus Nijhoff Publ., The Hague. BRECK D.W. (1974) Zeolites Molecular Sieves. WileyInterscience, New York. BRECKR.C. (1980) Potential Uses of Natural and Synthetic Zeolites in Industry. Pp. 391--422 in: The Properties"and Applications of Zeolites. The Chemical Society, London. CERANICT., VU(~INICD., DRZAJ B. & HO(SEVARS. (1985) Structural and thermal properties of exchanged forms of

clinoptilolite from Zlatokop (Vranje), Yugoslavia. Pp. 359-365 in: Zeolites, Synthesis, Structure, Technology and Application. Elsevier, Amsterdam. CORMAA. & PEREZ-PARIENTEJ. (1987) Catalytic activity of modified silicates: I. Dehydration of ethanol catalysed by acidic sepiolite. Clay Miner. 22, 423-433. DANDY A.J. & NADIYE-TARBIRUKAM.S. (1982) Surface properties of sepiolite from Amboseli, Tanzania, and its catalytic activity for ethanol decomposition. Clays Clays Miner. 30, 347-352. FERNANDEZ ALVAREZT. (1972) Activaci6n de la sepiolita con acido clorhidrico. Bol. Soc. Esp. Ceram. Vidr. 11, 365-374. GALABOVA I.M. & HARALAMP1EVG.A. (1979) Oxygen enrichment of air on alkaline forms of clinoptilolite. Pp. 121-132 in: The Properties and Application of Zeolites. Chemical Society, London. GALLI E., GOTTARDI G., MAYER H., PREISINGER A. & PASSAGLIA E. (1983) The structure of potassiumexchanged heulandite at 293,373 and 593 K. Actu Cryst. B39, 189-197. GONZALEZ J.A., ROMANOSKYB.V. & TOPCHIEVA. (1982) Catalytic properties of natural zeolites. I. Conversion of aromatic hydrocarbons. Kinet. Katal. 23, 149%1503. GONZALEZJ.A., TRAV1ESON., BALMAYORM., ARCOYAA. & SEOANEX.L. (1988) Activated Catalftica de una clinoptilolita de Tasajeras (Cuba). Actas del XI Simposio Iberoamerieano de Catdlisis, Guanajuato (Mexico), 605~611. GUISNET N. & GNEP N.S. (1984) Zeolites as catalysts in xylene isomerization processes. Pp. 571-582 in: Zeolites Science and Technology. Martinus Nijhoff Publ., The Hague. JACORS P.A. (1977) Carboniogenic Activity of Zeolites. Elsevier, Amsterdam. JACOBS P.A. (1982) Acid zeolites: an attempt to develop unifying concepts. Catal. Rev. Sci. Eng. 24, 415-444. JACOBS P.A. & MARTENSJ.A. (1986) Exploration of the void size and structure of zeolites and molecular sieves

Properties of modified clinoptilolite using chemical reactions. Pp. 23-32 in: New Developments in Zeolite Science and Technology. Elsevier, Amsterdam. KN6ZtNGER H. & KONEN R. (1966) The dehydration of alcohols over alumina. J. Catal. 5,264-270. KOYAMA K. & TAKEUCHI Y. (1977) Clinoptilolite: the distribution of potassium atoms and its role in thermal stability. Z. Krist 145,216-239. MIKLOSYE., PAPPJ. & KALLOD. (1983) Xylene iosomerization on H-mordenites and H-clinoptilolites, Zeolites 3, 139-147. RABO J. A., PICKERT P. T., STAMIRESD. N. & BOYLE J. (1961) Molecular sieve catalysts in hydrocarbon reactions. Proc. 2nd Int. Cong. Cat. 2, 2055-2073. RODRIGUEZ G. (1987) Propiedades fisicoquimicas y aplicaciones industriales de la clinoptilolita natural. PhD thesis, Univ. La Habana, Cuba. ROQUE-MALHERBE R., DIAZ-AGUILA C., REGERA-Ru/z E., FUNDORA-LLITERAS J., LOPEz-COLADO L., & HERNANOEZ-VELEZ M. (1990) The state of iron in natural zeolites: A M6ssbauer study. Zeolites 10, 685-689,

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SAKOH H., NITI'AM. & AOMURAK. (1985) Catalytic activity and selectivity of modified clinoptilolites for conversion of methanol to light olefins. Appl. Catal. 16, 249-253. SPOJAKINA A., TSOLOVSKI I., KOSTOVA N., PoPov T., KRUSTEV S. & SnOPOV (1985) Study of the properties of clinoptilolite modified by transition metal ions. Pp. 367373 in: Zeolites, Synthesis, Structure, Technology and Applications. Elsevier. Amsterdam. TSITSlSHVILI G.V. (1973) Physicochemical properties of high silica L and clinoptilolite zeolites. Pp. 291-298 in: Molecular Sieves. ACS, Washington DC. TSITSISHVlLI G.V. (1979) Natural Zeolites, pp. 37-49. Metsnierebra, Tbilisi. VENUTO P.B. & LANDYSP.S. (1968). Organic catalysis over crystalline aluminosilicates. Adv. Catal. 18, 259-371. WARD J.W. (1984) Molecular sieve catalysts. Pp. 272-388 in: Applied Industrial Catalysis 3, Academic Press, London. WARD J.W. & HANSFORD R.C. (1969). The nature of active sites on zeolites. IX. Sodium hydrogen zeolite. J. Catal. 13, 364--372.