M O L E C U L A R - S I E V E P R O P E R T I E S OF

S Y N T H E T I C C H A B A Z I T E S

V. E . S k a z y v a e v , S. S. K h v o s h c h e v , a n d S. P . Z h d a n o v

UDC 541.183:661.183.6

Synthetic potassium chabazites were first prepared and studied in [1, 2]. The crystallization con- ditions for synthetic sodium, potassium, and potass ium-sodium ehabazites were studied in detail later [3-5]. X-ray data [6] showed that in their structure synthetic sodium, potassium, and potass ium-sodium ehabazites are similar to each other and to natural chabazite. A more detailed comparison of the x- ray character is t ics of natural and synthetic chabazites shows that those zeolites are not completely identical and, it would seem, should have different adsorption propert ies . In the present paper the results are presented of an investigation into the molecular-sieve propert ies of various synthetic chabazites and ce r - tain natural chabazites from different deposits.

E X P E R I M E N T A L M E T H O D

The specimens of synthetic zeolites were prepared from sodium (type E zeolites), potassium (K-G zeolites) and potass ium-sodium (type K zeolites) s i l ica-a lumina gels under the conditions given in [3-5].

Since natural chabazites are almost pure calcium aluminosilicates, to compare their propert ies with the synthetic aluminosilicates the lat ter were also prepared in their Ca forms by ion exchange. Sodium-calcium chabazites were also studied. The compositions of the specimens studied are given in Table 1. Certain discrepancies in the aluminum contents and the equivalent cation contents in the zeolite lattice are probably due to analytical e r r o r s .

H20, N2, CH3OH, CzH~OH, C3HrOH, and C4H9OH were chosen as adsorbates. The adsorption of HzO and the alcohols was measured at 20 ~ and that of N 2 at liquid-nitrogen temperature. Before the mea- surements all the zeolite specimens were evacuated at 400 ~ to 5.10 -5 mm Hg for 20 h. All the measure- ments were made on a vacuum balance unit.

D r s c u s s I O N OF R E S U L T S

Figure 1 shows the adsorption isotherms for HzO, N 2, and certain alcohols on the Ca form of type E zeolite and on natural chabazite. There is a substantial difference in the molecular-sieve propert ies of the Ca form of zeolite E and natural chabazite (see Fig. 1 and Table 2). If the volume of the intracrystal- line voids of type E zeolite (zeolite CaE2) is available to the molecules of H20, N 2, CH~OH, C2H5OH, and C3HrOH then only H20, N2, and CH3OH molecules penetrate readily into the voids of natural chabazite, while the adsorption of CzH5OH and C3H~OH takes place extremely slowly. In the case of propanol equilib- rium values for adsorption cannot generally be obtained, even after maintaining the specimen in contact with the adsorbate for 120 h. Molecules of CtH9OH diffuse slowly in the voids of zeolite CaE, whereas butanol is scarcely adsorbed at all on natural chabazite (Table 2).

Thus, judging from the molecular-sieve properties, the effective diameters of the openings in zeolite CaE are markedly higher than for natural chabazites. In addition, the specimens of natural chaba- zites f rom different deposits (Czechoslovakia and the Sarbai deposit) scarcely differ at all in molecular- sieve propert ies (Figs.1 and 2, and Table 2). Consequently, CuE zeolites are not complete structural

I. V. Grebenshchikov Institute of Silicate Chemistry, Academy of Sciences of the USSR, Leningrad. Translated from Izvestiya Akademit Nauk, Seriya Khimicheskaya, No. 1, pp.19-24, January, 1976. Ori- ginal article submitted February 18, 1975.

�9 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

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TABLE 1. Chemical Composition of Specimens Studied

Specimen Chemical composition

Natural chabazite N(Czeqhqslqvakia)

aturai cnaoazite

CaK CaE~ CaE1 Na, CaK -- G Na,Ca- 9habaz[te

(Czechoslovakia)

0,87 CaO- 0,05 Na~0.0,09 K ~0. Al~0a. 4,53 SiO~

0,90 Ca0.0,06 Na~O.0,03 Ke0.AleOa.4,tt Si0~

0,94 CaO .0,04 Na~O .0,081420. AleOs.4,35 Si02 0,93 Ca0.0,08 Ke0.Al~O~.4,76 SiO~ t ,0i CaO.0,04 Na=O-Ah0a.4,5 SiO~ t ,05 CaO.0,O6 Na~O.AI~O~.4,04 Si0~ 0, t8 Ca0.0,79 Na~O.0,09 KeO.AhOa.4,42 SiO~ 0,31 CAO.0,73 Na~O.Al~Oa.4,53 SiO~

~jC

Oj3

O~Z

071,

na/g I

o7a

~.o--o ~ q O~Z

g~5 "7~0

E

o~ 3 ~ 6=;~~ ~ z ox 3

o~z[ o~2

071 I I O~l, 0 Off

.'7

ma/g o,

O~Z

0~1

015 1,0 0 P/&

]SE 0,3

q

op2

I I . .A

f~ Q 0~5 l f l P/Ps

Fig. 1

% -

:mS/g

I !

5 . r - ~ - - ~ - ~ - o - - ~ j

075 f~O Tr

!

0 ~ 1 ~ 6 =

0 0~5 I

fTg

a, cma/g

073

1

g~ 2

5 /7 l _ ,, ,

gig l~O P/Ps

Off

07Z

O 0,5 ljO P/Ps

Fig. 2 Fig. 1. Adsorption i so the rms for H20 (1), CH3OH (2), C2H.~OH (3), CaHTOH (4) at 20 ~ and for N 2 (6) at -- 196 ~ on zeol t tes . I) CaE1; II) natural chabazite (Sarbai, USSR); HI) CaE2; IV) CaK.

Fig. 2. Adsorption i so therms for H20 (1), CH3OH (2), C2HsOH (3) CtHgOH (5) at 20 ~ and for N 2 (6) at -- 196 ~ on zeol t tes . I) Natural chabazite (Czechoslovakia); II) Na, Ca-chabazite (Czechoslovakia; III) C a K - G ; IV)Na, CaK-G.

analogs of natura l chabazt tes . This is also conf i rmed by the differences in the thermal stability of natural chabazites and type E zeoli tes. Whereas the natural chabazites retain their adsorption capacity for H20 completely in both the Ca and the N a - C a fo rms after calcining at 400 ~ (see Fig. 2), according to [7] only specimens having a Ca content > 70-75% of the N a - C a E zeo!ites are thermal ly stable.

Of the synthetic zeoli tes in the chabazite group, however, there would appear to be specimens which are complete analogs of natural ehabazites. These include, in par t icular , type K-G chabazttes p repared f rom potass ium s i l i c a - a l u m i n a gels [4]. Figure 2 shows the adsorption isotherms for H20, N 2, and cer tain alcohols on zeol[te K-G in the Ca form. On comparing F igs .1 and 2 and Table 2 it is

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TABLE 2. Values for the Adsorption of Alcohols on Zeolites (cm3/g) for P/Ps = 0.5 after Exposure in Adsorbate Vapor for 5, 50, and 100h

Specimen

Natural. ch~baz.iCe. (Czecnosmval(m)

Natural chabazite (Sarbai, USSR)

CaK -- G CaK CaEe CaE~ Na,CaK-- G Na,Ca-.chabazite

(Czechoslovakia)

; r Z ~ ' ' i i Z/~I, , , Z0 ' J ' i /61 i i , / ~ i _~ i 8' t , , ~/I ~ ,

Fig. 3. Ionization x - r a y diagrams of chabazi te-group zeoli tes. 1) Natural chabazite (Czechoslovakia); 2) Ca_K-G; 3) CaE2; 4) CaK.

C~HsOE

5 50

0,06 0,20

0,i2 0~24

0,12 0,21 0,29 0,29 0,30 0,30 0,26 0,26 0,23 0,23 0,26 0,26

0,28

0,28

0,23 0,29 0,30 0,26 0,23 0,26

0,02

0,02

0,04 0,23 0,25 0,t5 0,06 0,14

C , H , O H

0,07

0,10

0,I0 0,23 0,25 0,15 0,13 0,2t

C , H g O H

i00

0,10

0,14

0,12 0,23 0,~5 0,i5 0,t4 0,25

5 50

0,01 0,02

0,01 0,02

0,0t 0,02 0,05 0,08 0,05 0,t6 0,02 0,04 0,0t 0,02 0,0t 0,02

100

0,02

0,02

0,02 0,t0 0,20 0,07 0,02 0,02

seen that there are no marked differences in the molecu la r - s ieve proper t ies of CaK-G zeolite and natural chabazi tes . Fur the rmore , zeoli tes K-G, like natural chabazites, are thermal ly stable when calcined in vacuum at 400 ~ , not only in the Ca form but also in the C a - Na fo rm.

Fur the rmore , K - N a zeolites of the chabazite group which crys ta l l ize f rom mixed K - N a s i l ica--alumina gels (type K zeolites) [5] approach type E zeoli tes in their adsorption cha rac te r i s t i c s . As may be seen f rom Fig .1 and Table 2, the Ca forms of type E and K zeolites scarce ly differ at all.

The differences observed in the molecu la r - s ieve proper t ies of the same ionic fo rm of difference specimens of chabazite-type zeolites may be caused by a cer tain difference in the aluminum-- silicon--oxygen f ramework itself, combined, for example, with different deformations of the individual elements in the lattice (in par t icular , the windows joining the intracrystal l ine voids), and also with differences in the positions of the ion-exchange cations and in their number, as determined by the Si/A1 ratio in the zeo- lite latt ice.

Synthetic Na chabazites (type E zeolites) have a variable Si/A1 ratio, which var ies over the range 2-2.5 according to the

conditions of preparat ion. A comparison of the data presented in Fig. 1 and Table 2 shows that a decrease in the Si/A1 ratio in the lattice of type E zeolites leads to a cer tain decrease in the volume of the intra- crystal l ine space available to C3H~OH molecules and to lower adsorption kinetics of the l a rge r C4H9OH molecules . With the increase in the number of Ca 2+ cations caused by the reduction in Si/A1 ratio, the possibili ty of the windows�9 being blocked by Ca 2+ ions evidently increases . It should be noted that although the molecular -s ieve proper t ies of type E zeoli tes depend on the Si/A1 rat io in the crys ta ls , the difference between those zeolites and zeolite K-G and natural chabazite is retained for both specimens. The thermal stability of the N a - C a fo rms of both specimens of type E zeoli tes also remains lower compared with the corresponding fo rms of zeolite K-G and natural chabazite.

On comparing the resul ts obtained we may conclude that the molecu la r - s ieve proper t ies of E, K, K-G zeoli tes and natural chabazites would not seem to be associated with differences in the Si/A1 rat io and, consequently, with differences in the number of cations in their unit cel ls .

The chabazi te-group zeoli tes studied in the present investigation can be divided into two subgroups with respec t to their molecu la r - s ieve proper t ies and thermal stability. One subgroup should include the synthetic K chabazites (K-G type zeolites) and the natural chabazites, and the other should include the synthetic Na and K - N a chabazites (type E and K zeolites). The differences between these subgroups are also revealed quite definite in the x - r a y charac te r i s t i c s of the zeol i tes . The reduction in the intensities and the marked broadening of the principal diffraction maxima for type E and K zeolites, compared with K-G zeolite and natural chabazite (Fig.3), would seem to indicate that the a l u m i n u m - s i l i c o n - o x y g e n f ramework of type E and K zeoli tes is deformed more appreciably.

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The different degree of deformation of the a luminum--si l icon--oxygen f ramework of different spe- c imens of chabaztte should probably also lead to different deformation of the e igh t -membered r ings of oxygen atoms through which the adsorbate molecules penetra te into the in t racrys ta l l tne voids. It is diffi- cult to imagine that all the e igh t -membered rings in each given chabaztte specimen are deformed identi- cally; there is evidently a cer ta in distribution of these r ings in the c rys ta l s with respect to the nature and degree of deformation. In the more deformed s t ruc tu res the probabil i ty is increased for the appear- ance of e igh t -membered r ings having effective d iameters both large and small compared with a cer tain average value charac te r i s t i c of more ordered s t ruc tu res . Since the rat io of the dimensions of an eight- membered :ling and an adsorbate molecule de termines the possibi l i ty of diffusion in the voids of the zeolite, the more deformed s t ruc tu res should be also more open, which is conf i rmed on compar ing the adsorption and x - r a y data for the chabazite specimens .

It should be taken into account that the configuration of the windows may va ry substantially during the adsorption p roce s s and these changes are different for different adsorbates [8, 9]. The position is still more compi[icated where ton-exchange cations are distr ibuted in the windows. In this case, when mole - cules which react s trongly with the cations are adsorbed, the displacement of the cations and a change in their position may take place. All this leads to the effective d iameter of a window not remaining a con- stant value but depending on the nature of the adsorbate molecule. Probably with just these c i r cums tances is also associa ted the pecul iar r eve r sa l of the molecu la r - s i eve effect [10] on p rogress ing f rom the Ca fo rm of nat~ral chabazite and zeolite K-G to N a - C a zeolite (see Fig. 2, T~ole 2).

On replacing ~ 70% Ca 2+ by Na for both zeolt tes the sorption space available to N 2 molecules, is appreciably reduced (see Fig.2) , which agrees with data for natural chabazites [2]. In addition, the con- dtttons for ~he diffusion of ethanol and propanol molecules in the voids of such ehabazites is facil i tated substantially, which is seen on compar ing the kinetic data in Table 2. For ethanol, equil ibrium is es tab- l ished fa i r ly rapidly and therefore the equil ibrium iso therms may be obtained completely.

Thus, if we judge f rom the molecu la r - s ieve effect for N 2 then we may conclude that the effective window diameters are decreased markedly when Ca z+ is exchanged for Na +. However, if we commence f rom the change in adsorption kinet ics of the alcohols then the conclusion should be direct ly opposite, m the effective window diameters with respec t to the molecules of the alcohols should be increased when Ca 2+ ts replaced by Na +.

The explanation of the resul ts obtained may be associa ted with differences in the possibi l i t ies of cation migrat ion in both cases during the adsorption p rocess itself. In the general case the interaction energ ies of N 2 molecules with cations of the zeoli tes are small and would seem to be insufficient to al ter the posi t ior of the cations markedly during adsorption. Therefore , as was to be expected the replacement of Ca 2+ by 2Na + leads to more ideal blocking of the openings and to a decrease in the number of voids available to the nitrogen molecules .

The appreciably higher adsorption energies of the alcohols probably make it possible for the cations of the molecules being adsorbed to be displaced f r o m their posit ions in the e igh t -membered oxygen r ings during the adsorption p r o c e s s . Such displacements are achieved more readi ly in the case of the univalent Na + cations (less stably bound with the lattice), which also ensures more favorable conditions for the alcohol molecules to diffuse when they are adsorbed on N a - C a chabazi tes .

S U M M A R Y

I . The molecu la r - s i eve proper t i es of a number of synthetic and natural chabazi tes have been investigate :l.

2. Charac ter i s t ic differences have been revea led in the molecu la r - s ieve proper t ies , thermal stability, ~,nd x - r a y cha rac te r i s t i c s between the two groups of the zeolt tes studied.

3. A reve r sa l of the molecu la r - s i eve effect on pass ing f rom the calc ium to the s o d i u m - c a l c i u m forms of certain synthetic and natural chabazites has been observed.

1. 2. 3.

L I T E R A T U R E C I T E D

R. M. B a r r e r and I. W. Baynham, J . Chem. Soc. , 2892 (1956). R. M. B u r r e r and I. W. Baynham, J . Chem. Soc., 2882 (1956). S. :P. Zhdanov and N. N. Buntar ~, Dokl. Akad. Naak SSSR, 147~ 1118 (1962).

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4Q

5. 6.

7.

81 9.

10.

S. P. Zhdanov and M. E. Ovsepyan, Dokl. Akad. Nauk SSSR, 157, 913 (1964). S. P. Zhdanov and B. G. Novikov, Izv. Akad. Nauk SSSR, Set. Khim. Nauk, 1, 44 (1966). S. P. Zhdanov, N. N. Buntar'-Samuelevich, and M. E. Ovsepyan, Dokl. Akad-. Nank SSSR, 161, 384 (1964). S. P. Zhdanov, V. E. Skazyvaev, S. S. Khvoshchev, and N. N. Samuelevich, Zh. PriM. Khim., 1, 39 (1975). I'. V. Smith, F. Rinaldi, and L. S. Dent Glasser, Acta Cryst . , 16, 45 (1963). I. H. Fang and I. V. Smith, J. Chem. Soc., 3749 (1964). S. S. Khvoshchev, V. E. Skazyvaev, and S. P. Zhdanov, Adsorption and Porosity [in Russian], Nauka (1975).

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