13 Synthesis of Thermodynamically Stable Zeolites in the N a O - A l O - S i O - H O System Downloaded by UNIV OF ARIZONA on November 13, 2012 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch013
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E. E. SENDEROV and Ν. I. KHITAROV V. I. Vernadsky Institute of Geochemistry and Analytical Chemistry, USSR Academy of Sciences, Moscow, V-334, Vorobiovskoe shosse, 47-A, USSR Zeolites natrolite and analcime (composition of the latter is close to its ideal formula, NaAlSi O · H O) have been ob tained in the N a O - A l O - S i O - H O system under condi tions hampering the formation and conservation of metastable crystals. Among sodium zeolites, only these 2 phases appear truly stable; formation of the others is a result of metastable growth from highly reactive starting materials. 2
2
2
3
2
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2
Τ η the Na20-Al203-Si02-H 0 system, a great number of artificial zeolites may be obtained. Their synthesis is mainly carried out below 200°-300°C using amorphous starting materials, gel-like mixtures and glasses. High supersaturation of a solution arising in a reactor with such a charge and slow rates of crystal formation under low-temperature zeolite synthesis conditions make development and conservation of meta stable states a rule rather than an exception (2, 17). When starting with highly reactive gel-like mixtures, the probability of metastable crystallization, as Fyfe (6) pointed out, is particularly high. In that case, a system should have an initial maximum free energy excess with respect to the final stable state. This increases the number of possible intermediate metastable states (and phases corresponding to them) through which the system passes to the end state according to Ostwald's law. Following Goldsmith's idea (7), during this transition the "simplicity" of crystal structure decreases in successively formed solid phases. Nucleation is the slowest for the most complex and ordered lattices, which form after the others. Prolongation of a run makes preservation of the intermediate phases possessing only relative stability more difficult. Mixing a charge in a 2
A
149 In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
150
MOLECULAR SIEVE ZEOLITES
1
reactor should cause a similar effect, as well as the use of catalysts which accelerate stable phase nucleation. The appearance of crystals with a slow nucleation rate should be favored by introducing its seeds, and that of intermediate phases should be averted by using less reactive crystalline starting materials instead of amorphous ones. In the latter case, however, the time necessary for new phase formation becomes considerably longer. Catalyzing influence on reactions in silicate systems may be excited by an increase of crystal-forming solution alkalinity. This is evidenced by zeolite synthesis experience (17) bell and Fyfe (3)
and Kerr (8).
and by direct experiments of Camp-
However, an increase of p H may affect
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not only growth rate, but displacement of desilication reactions towards products poorer in silica (15, 16).
These 2 effects must be distinguished
when analyzing the influence of alkalinity growth. In spite of the synthesis in the N a 2 0 - A l 2 0 3 - S i 0 2 - H 0 system of a 2
great number of zeolites of different structural groups, reliable communications on artificial natrolite, N a A l S i O i o · 2 H 0 , were lacking un2
til recently (18).
2
2
3
This might seem strange because of extensive investiga-
tion of the system and wide abundance of the zeolite in nature. Difficulties with natrolite synthesis were supposed to be conditioned by the complexity of its structure (12),
particularly by ordered Si and A l distribution
in it. The ordering also explains the constancy of natrolite composition, unlike the majority of artificial sodium zeolites.
Natrolite appears to
differ by the slowness of its nucleation from disordered, possibly less stable phases.
The factors preventing metastable
growth should
be
taken into account in the formation of natrolite. Gels of ( 1 — 2 ) N a 0 · A 1 0 2
2
3
· 3Si0
2
+
aq composition seeded with
natrolite were prepared to synthesize it (18).
Natrolite had been crys-
tallized in the range of about 1 0 0 ° - 2 0 0 ° C under saturated vapor of crystal-forming solution pressure
(Table
I).
In this range, the
zeolite
( commonly with analcime ) appeared instead of chabazite and garronite, which crystallize from the same mixtures in absence of the seeds
(17).
N o difference was noticed between the synthetic and natural natrolite. Chabazite and garronite also were formed in runs with the seeds but increased alkalinity resulting from concentration of relative N a 0 con2
tents in the initial mixture caused natrolite substitution for them. desilication reactions did not cause this substitution because
The
chabazite
and garronite may be as poor in silica as natrolite. Another way to obtain natrolite is the use of natural sodium aluminosilicates—nepheline,
( N a , K ) A l S i 0 , and albite, N a A l S i 0 — s e p a r a t e l y 4
3
8
and in mixtures, in a starting charge which was treated with neutral ( H 0 and 0.2IV N a C l ) and alkaline ( 0 . 2 N N a O H ) solutions. The charge 2
was placed in a rocking autoclave in which a stainless steel ball mixed the contents. Pressure was approximately 300 arm; duration ranged from
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
13.
SENDEROV
A N D
KHTTAROv
Table I. Na^O/AWs in a Gel
Thermodynamically
Natrolite Synthesis from Seeded Gels Duration, Days
Cone., Wt. %
Products'
1
120°C 2 1.5 2 1.5 2
5 20 20 39 39
1.5 2 1.2 1.5 1
5 5 20 20 40
103 103 103 103 103
nt am am an nt
+ + + + +
an nt nt nt an
19 19 19 19 19
an an an an nt
+ + + + +
nt nt nt nt an
Downloaded by UNIV OF ARIZONA on November 13, 2012 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch013
180°C
an=analcime, nt =natrolite.
» am=amorphous matter, t;c
a
4
Si0 /Al 0 252Analcime Figure
1.
ft
6 2
151
Stable Zeolites
2
s
4
6
Si0 /Al 0
3
2
Natrolite
2
3
11111 I C a n c r t m t e
Recrystallization of mineral mixtures treated with neutral (a) and alkaline (b) solutions
s e v e r a l w e e k s t o a b o u t 100 days. R e l i a b l y i d e n t i f i e d n e w phases w e r e o b t a i n e d o n l y f o r t h e l i m i t e d fields s h o w n i n F i g u r e 1. A l k a l i n e m e d i a f a v o r e d n a t r o l i t e c r y s t a l l i z a t i o n , w h i c h arose i n a p p r o x i m a t e l y t h e same
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
152
MOLECULAR SIEVE ZEOLITES
1
t e m p e r a t u r e r a n g e as f r o m the seeded gels. N a t r o l i t e as s t a r t i n g m a t e r i a l d e c o m p o s e s at n e a r l y 300 ° C , g i v i n g rise to h y d r o x y c a n c r i n i t e . T h i s p e r m i t s d e m a r c a t i o n o f the b o u n d a r y o f natrolite's s t a b i l i t y field at r o u g h l y 250°C. T h u s , n a t r o l i t e replaces g a r r o n i t e a n d c h a b a z i t e w i t h i n c r e a s i n g a l k a l i n i t y w h e n c r y s t a l l i z i n g f r o m seeded gels, a n d the latter 2 zeolites are n o t f o r m e d f r o m m i n e r a l s . A l l this suggests that n a t r o l i t e is m o r e t h e r m o d y n a m i c a l l y stable. T h e c o m p o s i t i o n of a n a l c i m e f o r m e d at r e c r y s t a l l i z a t i o n of the m i n e r a l m i x t u r e s c a n not v a r y as w i d e l y as i n synthesis f r o m Downloaded by UNIV OF ARIZONA on November 13, 2012 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch013
gels a n d glasses.
T h i s suggests
t h a t a n a l c i m e s of some
m i g h t g r o w as metastable phases.
amorphous compositions
T o d e t e r m i n e this p o s s i b i l i t y , s p e c i a l
e x p e r i m e n t s w e r e c o n d u c t e d i n w h i c h c a l c i n e d gels, N a 0 · A 1 0 2
2
3
· (2.5-
1 3 ) S i 0 , w e r e treated w i t h w a t e r a n d N a O H solutions of different c o n 2
centrations (10).
M i x i n g of a charge m a y b e u s e d d u r i n g t h e e x p e r i m e n t s ,
as i n the case o f n a t r o l i t e . H o w e v e r , this f a c t o r c o m b i n e d w i t h c h a n g i n g t h e c r y s t a l l i z a t i o n t i m e over a r a n g e of s e v e r a l weeks d i d n o t p r o v e as great a n influence as v a r i a t i o n of the s o l u t i o n a l k a l i n i t y .
2n Ο cti c ο
S
0,2 π
χ—ο
0,02 Π
/ /
I
cf|-*^c£^-
Μ \
i 6
8
Si0 /Al 0 2
2
10
3
Figure 2. Si0 /Al O ratio in initial gels (X) and analcimes crystallized at 250°C (circtes). Open circles = Si0 /Al O ratio in analcime in experi ments with mixing, closed ones = without mixing. Each horizontal line connects the point of an initial gel with the point of analcimes grown from this gel. Points of crystal compositions formed from the same gels in different solutions are connected by dashes. A dot-and-dash line shows the SiO^/AlgO^ ratio in the ideal formula of analcime. 2
t
s
2
g
e
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
13.
SENDEROV
KHiTAROv
A N D
S o m e results of
Thermodynamically
the d e t e r m i n a t i o n of
s h o w n i n F i g u r e 2. T h e i n i t i a l S i 0 / A l 0 2
2
3
153
Stable Zeolites
a n a l c i m e compositions
are
r a t i o i n gels a n d i n a n a l c i m e s
g r o w n f r o m t h e m at 2 5 0 ° C is i n d i c a t e d o n the abscissa. A n a l c i m e c o m position was measured w i t h an accuracy i n S i 0 / A l 0 2
u s i n g the S a h a (14) x-ray d a t a .
2
and Coombs and Whetten (5)
v a l u e of
3
±0.2
method based
on
C o m p o s i t i o n of a s o l u t i o n a c t e d o n gels is s h o w n b y the
ordinate. Analcimes crystallized i n more concentrated closer to the i d e a l f o r m u l a , w h e r e the S i 0 / A l 0 2
2
NaOH 3
solutions are
r a t i o is 4.
If i n the
experiments N a O H i n f l u e n c e d d i s p l a c e m e n t of e q u i l i b r i u m , s i l i c a - d e f i cient species, w h e r e S i 0 / A l 0 Downloaded by UNIV OF ARIZONA on November 13, 2012 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch013
2
2
3
^ 3, w o u l d arise. L i m i t s for S i 0 / A l 0 2
i n a n a l c i m e s a c c o r d i n g to o u r n e w d a t a are 2.8 a n d 8.2 (10).
2
3
W i t h an
a l k a l i n i t y increase, the d i s p l a c e m e n t is l i m i t e d , a n d not o n l y h i g h - S i 0
2
species b u t l o w - S i 0 ones d i s a p p e a r . I n s t e a d of the latter, f e l d s p a t h o i d s 2
w e r e c r y s t a l l i z e d . T h e a l k a l i n i t y c h a n g e i n f l u e n c e d the k i n e t i c s of the process a n d c a u s e d the d i s a p p e a r a n c e of less stable a n a l c i m e s of f r i n g e compositions. T h e i d e a l s t o i c h i o m e t r i c c o m p o s i t i o n is a l i m i t to w h i c h a n a l c i m e s t e n d f r o m b o t h sides i n experiments u p to 4 0 0 ° C .
A t that a n d h i g h e r
temperatures, p r o l o n g a t i o n of runs gives rise to a n a l c i m e s w i t h
lower
contents of s i l i c a , even w h e n w a t e r reacts w i t h gels e n r i c h e d i n S i 0 . 2
T h i s is i l l u s t r a t e d b y d a t a o n c r y s t a l l i z a t i o n of N a 0 · A 1 0 2
2
3
· 4.6 S i 0
2
+
H 0 ( T a b l e I I ). A b s e n c e of s o d i u m h y d r o x i d e i n the s o l u t i o n w i t h w h i c h 2
the gels are t r e a t e d a l l o w s neglect of the role of the d e s i l i c a t i o n reactions. T h e most p r o b a b l e reason for the constancy of c o m p o s i t i o n is the o r d e r i n g d i s t r i b u t i o n of S i a n d A l i n the lattice of s u c h a n a l c i m e
(11).
H e r e the a n a l o g y w i t h n a t r o l i t e a n d other groups of f r a m e w o r k silicates, feldspars, for w h i c h l o w - t e m p e r a t u r e o r d e r e d varieties are k n o w n , is reasonable. A n a l c i m e of the N a A l S i 0 2
6
· H 0 c o m p o s i t i o n appears to b e 2
the l o w - t e m p e r a t u r e v a r i e t y , stable u p to 4 0 0 ° C . D e v i a t i o n f r o m a defin i t e c o m p o s i t i o n i n d i c a t e s the b e g i n n i n g of d i s o r d e r . Table II.
S i 0 / A l 0 Ratio in Analcime Formed from N a 0 · A 1 0 • 4.6 S i 0 + H 0 2
2
a
T h e order must
2
3
2
3
2
Temp., °C
Duration, Days
250 300 350 400 400 450 450
14 10 15* 2 14° 3 14* a
2
SiOi/AWi 4.7 4.4 4.5 4.6 3.7 4.4 3.7
Experiments with mixing.
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
154
M O L E C U L A R
SIEVE
ZEOLITES
remain unchanged at all temperatures lower than 400 °C. It further means that, if the definite composition reflects the ordering, it must remain stable down to the lowest temperature of analcime formation which is likely to reach ambient temperature. Thus, the factors making metastable growth difficult force the phases easily formed from gels—garronite, chabazite, analcime solid solutions— to disappear. Mordenite crystallization also is connected with metastable growth and equilibria, not with quartz but with less stable forms of silica (4). All this suggests that among the zeolites in the N a 0 - A l 0 S i 0 - H 0 system the only thermodynamically stable phases are natrolite and the N a A l S i 0 · H 0 analcime (up to 400°C). This is indirectly supported by the very wide abundance of the 2 zeolites in nature. For the other sodium zeolites, grown metastably, it is possible to define sequences of increasing relative stability. Replacement of one phase by another may be influenced by the increase of crystallization time that was noticed by many investigators (I, 9, 13, 19) and by the increase of solution alkalinity and temperature when the latter 2 param eters accelerate rates of reactions. Here the sequences are: garronite (NaP) - » analcime (solid solution), tetragonal P2 (more ordered) being more stable than cubic PI; in the poorest in silica field, NaX -> NaA -> NaP. 2
2
2
3
2
2
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Literature Cited (1) Barrer, R. M., Baynham, J. M., Bultitude, F. W., Meier, W. M., J. Chem. Soc. 1959, 195. (2) Breck, D. W., Flanigen, E. M., Proc. Conf. Mol. Sieves, London, 1967. (3) Campbell, A. S., Fyfe, W. S., Am. Mineralogist 1960, 45, 463. (4) Coombs, D. S., Ellis, A. F., Fyfe, W. S., Taylor, A. M., Geochim. Cosmochim. Acta 1959, 17, 53. (5) Coombs, D. S., Whetten, J. T., Bull. Geol. Soc. Am. 1967, 78, 153. (6) Fyfe, W. S.,J.Geol. 1960, 68, 553. (7) Goldsmith, J. R.,J.Geol. 1953, 61, 439. (8) Kerr, G. T.,J.Phys. Chem. 1966, 70, 1047. (9) Ibid., 1968, 72, 1385. (10) Khundadze, A. G., Senderov, E. E., Khitarov, Ν. I., Geokhim. 1970, in press. (11) Knowles, C. R., Rinaldi, F. F., Smith, J. V., Ind. Mineralogist 1965, 6, 127. (12) Meier, W. M., Z. Krist. 1960, 113, 430. (13) Regis, A. J., Sand, L. B., Calmon, C., Gilwood, M. E.,J.Phys. Chem. 1960, 64, 1567. ( 14) Saha, P., Am. Mineralogist 1959, 44, 300. (15) Senderov, Ε. E., "Zeolites, Their Synthesis, Properties, and Applications," p. 165, Nauka, 1965. (16) Senderov, Ε. E., Geokhim. 1966, Ν 5, 600. (17) Ibid., 1968, Ν 1, 3. (18) Senderov, Ε. E., Khitarov, Ν. I., Geokhim. 1966, Ν 12, 1398. (19) Taylor, A. M., Roy, R., Am. Mineralogist 1964, 49, 656. RECEIVED January 21,
1970.
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
