Zeolite faujasite
Transcript of Zeolite faujasite
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Clays and Clay Minerals
197 3 Vol. 21 pp. 379-389. Pergamon Press. Printed in Grea t Britain
P R O P E R T I E S O F T H E Z E O L I T E F A U J A S I T E
S U B S T I T U T I O N A L S E R I E S :
A R E V I E W W I T H N E W D A T A
DENNIS N. STAMIRES
Internat ional A pplied Science, Inc. , 4630 Campus Dr ive, New por t Beach, C alifornia 92660, U.S.A.
Received 26 S e p te mb e r 1972)
A bs t r ac t - - N ew inf or mat ion i s p r esen ted r e la t ing to the hydr o the r mal s tab i l ity , l a t t i ce pa r amete rs , and
adsor p t ive , e lec tr ica l, and ca ta ly t ic p r oper t ie s o f syn the t ic and na tur a l f au jas i te . P r esen t concepts con-
ce r n ing the na tur e and r e la tionsh ip of synthe t ic and na tur a l f au jas ite a r e r es ta ted to be cons i s ten t w i th
the exper imen ta l ev idence and the d eve loped phys ica l mode l. T he major s t r uc tur a l and phys icochemi-
cal prope r t ies ref lect the c lose s imilar i t ies and smoo th gradations expec ted of su bst i tut ional mem bers
of a con tinuous ser ies . The exis t ing divis ion of the range 2-6) of SiO2/A1203 mole rat ios S/A) at 3 into
two comp osit ional subranges is shown to be unjust if iable and rathe r mis leading. Ind ividual com posi-
t ions f rom these tw o sub ranges do n ot represe nt dis t inct ly dif ferent zeoli te species ; ins tead, i t is
demon s t r a ted tha t they a r e mem ber s of a cont inuous se r ies w i th smooth ly changing p r oper t ie s over
the stud ied rang e of SiO2/AI~O3.Some of the pr oper t ie s o f the na tur a l minera l f au jas i te a re f ound to be
very s imilar to th ose of the synthet ic analogs with the same SiO2/A1203 mole rat io.
Final ly , a plea is made fo r a uniform nom enclature which wil l serve to identify specif ic s ingle)
com posit ions ins tead of ranges of com posit ions) ; ref lect the con tinuity, c lose s imilar it ies an d inter -
relat ionships in their main s tructural and physicoc hem ical prope r t ies ; and also identify the cha racter is-
t ics of the faujas i te group.
INTRODUCTION
H IS TO R IC A LL Y , e x p e r i m e n t a l w o r k o n c r y s t a l l i n e
a l u m i n o s i l ic a t e s o f t h e f a u j a s i t e t y p e s t a r t e d w h e n
D a m o u r 1 84 8) p u b l i s h e d a c h e m i c a l a n a l y s i s o f
f a u j a s i t e . S y n t h e t i c z e o l i t e s d e s i g n a t e d a s X a n d Y
w e r e p r o d u c e d b y M i l t o n 1 95 9) a n d B r e c k 1 96 4).
T h e X - r a y w o r k o f B a u r 1 96 4), B e r g e r h o f f
et al.
1 95 6) a n d B r o u s s a r d a n d S h o e m a k e r 1 96 0) a n d t h e
a d s o r p t i o n s t u d i e s o f B a r r e r e t a l . 1957) es tab-
l i s h e d t h e s t r u c t u r a l i d e n t i t i e s o f t h e s e s y n t h e t i c
z e o l i t e s a n d t h e n a t u r a l m i n e r a l . H o w e v e r , t h e l o c a -
t i o n o f c e r t a i n c a t i o n s i n t h e a l u m i n o s i l i c a t e
f r a m e w o r k a n d t h e r e l a t i v e d e g r e e o f S i , A 1) o r d e r -
i n g in t h e l a t t i c e s ti l l r e m a i n u n c e r t a i n a n d m a y b e
d i f f e r e n t i n t h e s y n t h e t i c a n d n a t u r a l s p e c i e s .
I t i s p o s s ib l e t o s y n t h e s i z e h y d r o t h e r m a l l y a c o n -
t i n u o u s s e r i e s o f f a u j a s i t e z e o l i t e s w i t h u n i f o r m
c o m p o s i t i o n a l w a r i at i o n . B r e c k 1 96 4) h a s d i v i d e d
t h i s s e r i e s in t o t w o c o m p o s i t i o n a l r a n g e s , o n e a
h i g h a l u m i n a s p e c i e s d e s i g n a t e d a s t y p e X , t h e
o t h e r a h i g h s i l i c a s p e c i e s d e s i g n a t e d a s t y p e Y .
O v e r t h e e n t i r e s i l i c a - a l u m i n a r a n g e f r o m 2 --6 , t h e
u n i t c e l l d i m e n s i o n s f o r z e o l i t e s in b o t h r a n g e s w e r e
f o u n d B r e c k a r id F l a n i g e n , 1 96 8) t o v a r y l i n e a r l y
w i t h t h e d e g r e e o f r e p l a c e m e n t o f S i b y A 1 i n th e
c r y s t a l f r a m e w o r k . A S IO 2 /A 1 20 3 m o l e r a t i o h e r e -
a f t e r r e f e r r e d t o a s S / A ) o f t h r e e w a s c h o s e n
B r e c k , 1 9 6 4 ) a s t h e b o u n d a r y b e t w e e n t y p e X a n d
t y p e Y . I t h a s a l s o b e e n s t a t e d B r e c k , 1 96 4) t h a t
Z e o l i t e X i s d i s t i n c t l y d i f f e r e n t f r o m Z e o l i t e Y a n d
t h a t b o t h a r e d i f f e r e n t f r o m n a t u r a l f a u j a s i t e . T h u s ,
a c c o r d i n g t o t h i s s c h e m e , t h e r e o c c u r s w i t h i n t h e
s u b s t i t u t i o n a l s e r i e s t h r e e d i s t i n c t l y d i ff e r e n t z e o -
l i te s p e c i e s ; n a m e l y , X , Y a n d n a t u r a l f a u j a s i t e . T o
j u s t i f y t h is d i s t i n c t i o n B r e c k 1 96 4) s t a t e d t h a t X -
r a y d a t a s t r u c t u r a l p r o p e r t i e s ) h a d t o b e c o n s i -
d e r e d t o g e t h e r w i t h o t h e r p r o p e r t i e s , s u c h a s c h e m -
i c a l c o m p o s i t i o n , h y d r o t h e r m a l s t a b i l i t y , a d s o r p -
t i o n , a n d i o n e x c h a n g e .
T h i s p a p e r r e v i e w s t h e e a r l ie r w o r k a n d p r e s e n t s
n e w i n f o r m a t i o n w h i c h e s t a b l i s h e s t h e c l o s e s i m i -
l a r i t y i n t h e h y d r o t h e r m a l s t a b i l i t y , a d s o r p t i o n a n d
o t h e r p r o p e r t i e s o f b o t h s y n t h e t i c a n d n a t u r a l f a u -
j a s i t e . It i s a l s o s h o w n t h a t t h e r e i s n o j u s t if i c a t i o n
w i t h i n t h e s e p r o p e r t i e s o f t h e s u b s t i t u t i o n a l s e r i e s
t o w a r r a n t a d i v i s i o n a t a S / A o f t h r e e , o r a t a n y
o t h e r p o i n t .
T h e m a i n c r i te r i a u s e d b y B r e c k 1 96 4) f o r t h e
379
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380 DENNIS N STAMmES
distinction between X, Y and natural faujasites
were hydrothermal stability, ionic conductivity,
and sorption properties. In this paper, these same
properties and additional structural data support
the lack of any such distinction and demonstrate
the similarities in the properties o f the substitu-
tional members of the faujasite series. Further,
these new data should help resolve the conflicts in
the conclusions of Breck 1964); Dempsey, KiJhl
and Olson 1969); Wright, Rupert and Granquist
1968, 1969); recently summarized by Smith 1971).
MATERIALS AND METHODS
The zeolites were synthesized from pure chemi-
cals by the procedures of Barrer 1959); Milton
1959) and Breck 1964). Samples were analyzed by
X-ray diffraction and fluorescence, chemical
analysis, and by the electron microprobe. Repeat
preparations were made at each desired composi-
tion, and those samples with the highest crystallin-
ity and most uniform crystals were selected for the
detailed study. Special care was taken to eliminate
occlusion of free sodium silicate in the zeolite
crystals. The Na20/A1203 mole ratio was kept close
to unity by ion exchange with NaCI. The selected
zeolites contained less than 20 ppm Fe and less than
80 ppm Ca and Mg. The S A was determined both
by wet chemical analysis and by X-ray fluores-
cence.
The samples usually were activated by room
temperature desorption under vacuum for several
hours followed by a slow temperature increase
about 2~ to the 400--450~ range where the
samples were kept at approximately 10 7 orr for
periods of 8-16 hr until the dehydration was com-
plete. A McBain balance and a mass spectrometer
were used routinely to establish complete dehydra-
tion of the samples and to avoid unnecessarily pro-
longed heating. The samples with higher AI con-
tents required longer heating under vacuum to re-
lease the last traces of water.
The X-ray measurements were done with CuKa
radiation at scanning rates of 2~ and 0.25 ~ 20/min.
An average value of the unit cell parameter a was
obtained from several diffraction peaks for each
sample. The 9 values were corrected by the known
reflections of either NaC1 or metal oxides, which
were mixed together with a portion of zeolite
sample.
Ion exchange was usually done by percolation;
i.e. a concentrated solution of the salt of the ex-
changing ion was allowed to flow through the zeo-
lite sample.
EXPERIMENTAL RESULTS AND DISCUSSION
Hydrothermal stabil i ty
The hydrothermal stability of each sample, over
a wide range of S/A, was determined and compared
with results available in the literature. In order to
make the comparison valid, the same experimental
conditions Breck, 1964) were used: steaming the
samples at 410~ and atmospheric pressure for 3 hr.
The steamed samples were activated by heating at
460~ and 10 7 torr by the monitoring procedure al-
ready described. Oxygen sorption capacity at
-183~ and 100 torr was measured on each sample
before and after steaming.
Four samples of the same S/A were studied to
establish the reproducibility of the method. The
first few experiments indicated that a steaming
period of 3 hr was not sufficient and that up to 5 hr
was required to obtain equilibrium. Oxygen sorp-
tion capacities and X-ray intensities of the steamed
samples were used to establish this equilibrium.
Non-reproducible results were obtained with com-
mercial synthet ic faujasite with Fe- content in the
range of 0.05-0.1 per cent and with samples con-
taining appreciable occluded sodium silicate; i.e.
non-reproducibility correlated with impurity con-
tent. Preliminary results indicate that both iron
which on heating forms the ferromagnetic species
Fe304, Singer and Stamires, 1965) and occluded
sodium silicate enhance the hydrothermal decom-
position of the crystalline zeolite. Thus, the pres-
ence of these impurities causes a considerable vari-
ation in the stability data.
A sample of natural faujasite Kaiserstuhl oc-
currence) was included for comparison with the
series of synthetic samples. The sample was con-
verted to the sodium form remaining Ca and Mg
was less than 0.03 weight per cent) by repeated ion
exchange, and ground to a fine powder before use.
The results are presented in Fig. 1, where the per-
cent crystallinity retained after steaming is plotted
as a function of S/A. For comparison, the data of
Breck 0964) are also shown. All data were corre-
lated using third degree polynomials to determine
the r.m.s, deviation, and the most probable values.
These results also appear in Fig. 1.
Although it is questionable if statistical treatment
of the Breck 1964) data is justifiable because of the
large spread, it is interesting to note that the smooth
curve obtained from the correlated data closely re-
sembles that of the new equilibrium results. Thus, it
can be concluded that a smooth continuous curve is
indeed the best representation for the data. The plot
of the new equilibrium results, without the large
spread of data, lies generally below the Breck
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Zeolite, faujasite, substitutional series: a review with new data 381
curve, as would be expected from the increased
severity of the steaming process. The plots con-
verge at ratios near 5. The position of the natural
faujasite in the plot is in accord with its composi-
tion. Thus, the stability data demonstrate the exis-
tence of a continuous substitutional series with
natural faujasite as a member.
The crystallinity remaining in these samples after
steaming was also measured by X-ray diffraction
techniques and was found to vary with the S/A in
much the same way as the retained crystallinity
shown in Fig. 1.
Mechanism of structural degradation by steaming
The drastic changes induced by steaming can be
viewed as similar to the changes occurring during
the heating of NH4*-exchanged faujasite. The hy-
drolysis of the A104/2)-Na+ site results in the for-
mation of a hydroxyl group associated with the
AIO412)- etrahedron. The distribution of the charge
density in the hydroxyl group varies according to
its location and crystal environment. In the high
alumina members of the series where there are a
large number of Na cations on type iii) sites
Breck and Flanigen, 1968), the hydrolysis results in
the production of a substantial number of protons
in the supercage. These protons are probably as-
m
z
o
DO I 1
/
o EXPERIMENTAL ALUESOF R.F. J
9 CALCULATEO ALUESRASEDON RMS
DEVIATIONSOF D.E. DATA
~, PRESENT XPER{MENTAL ATA
~ATURALFAUJASITE
1 O 1.5
E.15 310 t
.15 4.10 4.15 5.0 5.15
Fig. 1. Variation of hydrothermal stability with varying
SiOdA120~ mole ratio. Experimental values of Breck
1964), these data after statistical analysis and correlation,
and new equilibrium results.
sociated with the oxygen 4-membered rings which
constitute very weak cation sites. This association
in turn makes these protons very reactive, to the
extent that they can hydrolyze part of the
alumino-silicate framework. At the same time,
some of the Na probably migrates into the sodalite
cages, or disperses in the remaining portion of the
crystals.
The mechanism by which these chemical changes
proceed has been ascertained earlier Szymanski,
Stamires and Lynch, 1960; Rabo et al. 1960) and
further substantiated by Uytterhoeven, Christner
and Hall 1965).
In addition, Stamires 1965) has studied the e.s.r.
spectra of stabilized atomic hydrogen produced by
-/-irradiation of synthet ic zeolites and the interac-
tion of such hydrogen with the crystal lattice at
liquid helium or nitrogen temperatures. In the pres-
ent work, the e.s.r, spectra of samples, exposed to
steam only for short periods followed by vacuum
activation at 460~ and -/-irradiation, showed a
large amount of hydrogen atoms present. This hy-
drogen is introduced by the replacement in the
structure of some Na + by protons resulting from
the steam hydrolysis. Then, the protons react with
the lattice oxygens to form hydroxyl groups which
further by radiolysis produce hydrogen atoms and
lattice-oxygen radicals. In high-A1 samples, the
hyperfine splitting constant of the spectrum of ad-
sorbed atomic hydrogen is smaller approximately
498 G) than the value of approximately 508 G for
the free hydrogen atom. No changes were observed
in the G-values; however, the line width increased
with decreasing silica content. The brodening is at-
tributed to unresolved hyperfine lines resulting
from a distribution of various trapping sites Sta-
mires and Turkevich, 1964; Stamires, 1968) within
the unit cell. As the overall effect observed in the
e.s.r, spectra includes both short and long term in-
teractions, Stamires and Turkevich, 1963) it ap-
pears that the distribution of the strengths of the
individual hydroxyl groups probably varies from
one cell to the other. However, the largest differ-
ences are observed among the extreme members in
the silica-alumina range investigated.
The decrease in the hyperfine splitting constant is
attributed to the strong electric field in the super-
cage produced primarily by the high density of sur-
rounding protons and sodium ions. The close prox-
imity of the A104jE)- sites with which the hyd-
rogens are associated and the high reactivity of
those hydrogens are responsible for the structural
changes in the crystal. The strong electric field in
the supercage tends to expand the electronic wave
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382
DENNIS N ST MIRES
f u n c t i o n o f t h e u n p a i r e d e l e c t r o n i n t h e h y d r o g e n
a t o m s o a s t o d e c r e a s e t h e c h a r g e d e n s i t y a t t h e
n u c l e u s , a n d h e n c e t o d e c r e a s e t h e h y p e r t i n e c o n -
s t a n t . H o w e v e r , w h e n h y d r o g e n a t o m s a r e p r o -
d u c e d o n s t e a m e d s a m p l e s w i t h v e r y l o w a l u m i n a
c o n t e n t s , t h e c o n c e n t r a t i o n o f a t o m i c h y d r o g e n i s
c o n s i d e r a b l y s m a l le r a n d t h e h y p e r f i n e c o n s t a n t i s
c l o s e r t o t h a t o f f r e e a t o m i c h y d r o g e n ( a p p r o x -
i m a t e l y 5 0 4 G ) . F r o m e l e c t r o s t a t i c c o n s i d e r a t i o n s ,
i t a p p e a r s t h a t t h e c e n t e r o f t h e s o d a l i te u n i t w o u l d
b e a s u i t a b le t r a p p i n g s i t e f o r s u c h t y p e s o f h y d -
r o g e n .
I n s o m e f a u j a s it e s a m p l e s w i t h i n te r m e d i a t e S / A ,
t h e e . s . r , s p e c t r a s h o w e d a d o u b l e t w i t h h y p e r f i n e
l i n e s c o r r e s p o n d i n g t o t h e t w o e x t r e m e c a s e s d i s -
c u s s e d a b o v e ; i . e . h y d r o g e n a t o m s t r a p p e d i n t h e
s u p e r c a g e a n d i n t h e s o d a l i t e u n i t. A l t h o u g h t h e a p -
p e a r a n c e o f t h i s d o u b l e t a l l o w s a g r o s s d i s t i n c t i o n
t o b e m a d e b e t w e e n h y d r o g e n s l o c a t ed i n t h es e t w o
d i f f e r e n t s i t e s ( t h e s e t w o s i t e s c o u l d b e i d e n t i f i e d
w i t h t h o s e f o u n d i n t h e i . r . w o r k c o r r e s p o n d i n g t o
t h e 3 5 5 0 a n d 3 6 5 0 c m -1 a b s o r p t i o n f r e q u e n c i e s ) , t h e
u n r e s o l v e d h y p e r f i n e s t r u c tu r e o f t h e i n d i v id u a l
l i ne s o f t he do ub le t i nd i ca t e s t ha t t he re i s s ti ll
a n o t h e r d i s t r i b u t i o n o f s i t e s w i t h i n t h e i n d i v i d u a l
s o d a l i t e a n d s u p e r c a g e u n i t s . T h i s c o n c l u s i o n i s i n
c o n t r a s t t o t h e i n f o r m a t i o n o b t a i n e d f r o m i .r . ( U y t -
t e r h o e v e n , C h r i s t n e r a n d H a l l , 1 9 65 ) a n d X - r a y d i ff -
r a c t i o n ( O l s o n a n d D e m p s e y , 1 96 9) , w h e r e o n l y t w o
t y p e s o f h y d r o g e n h a v e b e e n i d e n t i f i e d ( t h o s e a s -
s o c i a t e d w i t h t h e 0 3 a n d O l l a t t i c e o x y g e n s ) .
T h e r e s o l u t i o n o f i .r . is l im i t e d w h e n a p p l i e d t o
p o l y c r y s t a l l i n e s o l i d s , p a r t i c u l a r l y i n c a s e s w h e r e
t h e p e r tu r b a t i o n o n s o m e o f t h e h y d r o g e n s , p r o -
d u c e d b y t h e e l e c t r i c fi el d o f t h e e n v i r o n m e n t , i s
r e l a t i v e l y s m a l l . I n t h a t c a s e s m a l l s h i f t s , d u e t o
s e c o n d o r d e r e f f e ct s i n th e h y d r o x y l b a n d s , w i ll n o t
b e s e e n i n th e i .r . s p e c t r a . F u r t h e r , X - r a y a n a l y s i s i n
i t s e lf i s n o t s e n s i t i v e e n o u g h t o l o c a t e a n d d i f f e r e n -
t ia t e t h e s t r u c t u r a l p o s i t i o n s o f t h e h y d r o g e n a t o m s .
E a r l i e r c o n c l u s i o n s r e g a r d i n g t h e p o s i t i o n s o f t h e
h y d r o g e n s w e r e b a s e d o n i n d i r e c t i n f o r m a t i o n i n
X - r a y s t u d i e s .
T h e e x c h a n g e o f s o d i u m f o r h y d r o g e n i o n s i n -
d u c e d b y s t e a m i n g a p p e a r s t o p r o d u c e t h e s a m e
t y p e o f p r o d u c t a s t h e t h e r m a l a c t i v a t i o n o f t h e c o r -
r e s p o n d i n g N H 4 f o r m . B a s e d o n s i m i la r it i es i n t h e
e . s . r , s p e c t r a , a s i m i l a r m e c h a n i s m f o r t h e f o r m a -
t i o n o f h y d r o g e n a n d i t s s t a b i l i z a t i o n i n t h e z e o l i t e
s t r u c t u re a p p e a r s t o o p e r a t e a l s o i n th e c a s e o f t h e
N H 4 f a u j a s i t e s d u r i n g t h e i r t h e r m a l a c t i v a t i o n .
H o w e v e r , b o t h t h e w i d t h o f th e e . s .r , l i ne s a n d
h y p e r f in e s p l it ti n g c o n s t a n t s w e r e f o u n d t o b e
s t r o n g l y d e p e n d e n t o n t h e a c t i v a t i o n t e m p e r a t u r e
a n d p u r i t y o f t h e z e o l i t e c r y s t a l s . A d e t a i l e d d i s c u s -
s i o n o f t h e e . s . r , s p e c t r a o f a t o m i c h y d r o g e n i n
s y n t h e t i c f a u j a s i t e s w i l l b e p r e s e n t e d e l s e w h e r e
( S t am i r e s , i n p r e ss ) . H o w e v e r , i t i s w o r t h m e n t i o n -
i n g h e r e t h a t h y d r o g e n i n t h e h i g h s i l ic a s a m p l e s i s
s t ab i l i z ed i n t he sm a l l e r c av i t i e s ( soda l i t e un i t s ) .
D u e t o t h e r e l a t i v e l y w e a k e l e c t r i c f i e l d a n d l a r g e
s e p a r a t i o n b e t w e e n t h e A 1 0 4 /., )- u n i t s , t h e h y d r o g e n
i n th i s e n v i r o n m e n t i s c o n s i d e r a b l y l e s s c h e m i c a l ly
r e a c t i v e . I n c o n t r a s t , h y d r o g e n t r a p p e d i n t h e
s u p e r c a g e s o f th e h i g h - a lu m i n a m e m b e r s o f t h e
s e r i e s i s m o r e r e a c t i v e a n d i s r e s p o n s i b l e f o r t h e
d e s t r u c t i o n o f t h e c r y s t a l . I n s a m p l e s w i t h i n t e r -
m e d i a t e o r l o w A 1 c o n t e n t , s o m e o f t h e N a a n d
p r o b a b l y t h e p r o t o n s c a n b e s t a b i l i z e d i n t h e s m a l l
c a v i t i e s ; t h u s , s o m e o f t h e c r y s t a l l i n i t y is r e t a in e d .
I n o t h e r c a s e s , w h e n t h e h y d r o l y z e d z e o l i te i s in
p h y s i c a l c o n t a c t w i t h a n o t h e r a l u m i n o - s i l i c a t e
p h a s e , s u c h a s k a o l i n i t e o r h a l l o y s i t e , b o t h t h e e x -
c e s s s o d i u m a n d h y d r o g e n p r o b a b l y d i f f us e a n d b e -
c o m e s t a b i l i z e d i n t h e n o n - z e o l i t e p h a s e . T h e s e
e f f e c t s , w h i c h p r o b a b l y i n v o l v e i o n e x c h a n g e , d i f fu -
s i o n a n d h y d r o l y s i s i n t h e s o l i d s t a t e , a r e e n h a n c e d
b y t h e h y d r o t h e r m a l c o n d i t i o n s . T h i s c h a n g e i n
r e a c t i v i t y i s i l l u s t r a t e d b y t h e v a r i a t i o n w i t h A I -
c o n t e n t o f th e s t e a m s t a b i li t y c u r v e . T h e s u b s t i t u -
t i o n o f S i f o r A 1 i n t h e s t r u c t u r e r e s u l t s i n a g r a d u a l
i n c r e a s e o f s t e a m s t a b i li t y o f t h e z e o l i t e s a m p l e s , a s
s h o w n i n F i g . 1 . S i m i l a r b e h a v i o r i s t o b e e x p e c t e d
i n t h e v a r i a t i o n w i t h A l - c o n t e n t o f t h e t h e r m a l s t a -
b i l i t y o f t h e N H 4 * - e x c h a n g e d f a u j a s i t e s e r i e s .
onic conduction
S t a m i r e s ( 1 9 6 0 , 1 9 6 2 ) ; F r e e m a n a n d S t a m i r e s
(1 96 1 ) ; S c h o o n h e y d t a n d U y t t e r h o e v e n (1 96 9) h a v e
s h o w n t h a t t h e c u r r e n t - c a r r y i n g s p e c i e s i n s y n t h e t ic
z e o l i t e s a r e c a t i o n s m i g r a t i n g i n t h e s t r u c t u r e b y a
j u m p m e c h a n i s m . F u r t h e r , t h e a c t i v at i o n e n e r g y
f o r c o n d u c t i o n i s d u e t o th e e l e c t r o s t a t i c i n t e r a c t i o n
of t he ca t i on wi th i t s s i t e i n t he a lum ino- - s i l i c a t e
f r a m e w o r k . T h i s s i t u a t i o n i s p a r t i c u l a r l y a p p l i c a b l e
i n c a s e s w h e r e a r e l a t i v e l y s m a l l i o n ( f o r e x a m p l e ,
N a ) , i s l o c a t e d i n a n o p e n f r a m e w o r k w h i c h h a s a
v e r y m i n o r s t e r i c e f f e c t o n t h e m i g r a t i o n o f t h e c a -
t i o n in t h a t s t r u c t u r e . S i n c e t h e n e g a t i v e c h a r g e o f a
c a t i o n s i t e m u s t b e d i s t r i b u t e d o v e r t h e o x y g e n s
b o u n d t o a g i v e n a l u m i n u m , t h e g e o m e t r i c a l c o n -
f i g u r a t i o n o f t h e c a t i o n / c a t i o n - s i t e p a i r d e t e r m i n e s
t h e m a g n i t u d e o f t h e c o u l o m b i c i n t e r a c t i o n . I n t h i s
c a s e , t h e c o n f i g u r a t i o n s h o u l d b e a n i s o t r o p i c w i t h
r e s p e c t t o t h e u n i t c e l l d i a g o n a l .
F r o m X - r a y i n v e s ti g a t io n s ( B r o u s s a r d a n d
S h o e m a k e r , 1 96 0 ; B r e c k , 1 96 4 ) i t h a s b e e n e s t a b -
l i sh e d t h a t t h e s o d i u m i o n s i n f a u j a s i t e h a v e a t l e a s t
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7/23/2019 Zeolite faujasite
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383
t w o d i f f e r e n t c r y s t a l l o g r a p h i c p o s i t i o n s . F o r e x a m -
p l e , in h i g h - A l - f a u j a s i t e s , 1 6 N a i o n s a r e l o c a t e d
w i t h i n t h e h e x a g o n a l p r i s m s [ t y p e ( i ) ] o f t h e
d o u b l e -5 m e m b e r o x y g e n r in g s , a n d 3 2 N a i o n s a r e
a s s o c i a t e d w i t h t h e s i n g le - 6 m e m b e r o x y g e n r i n g s
[ t y p e ( ii )] . T h e r e s t o f t h e N a i o n s h a v e b e e n a s -
s u m e d t o r e s i d e o n t h e s u r f a c e o f t h e l a r g e c a v i t i e s ,
p r o b a b l y a s s o c i a t e d w it h t h e fo u r m e m b e r o x y g e n
r i n g s [ t y p e ( i i i ) ] . S i m p l e e l e c t r o s t a t i c c a l c u l a t i o n s
r e v e a l * t h a t i o n - f r a m e w o r k c o u l o m b i c i n t e r a c t i o n s
f o r s o d i u m i o n s l o c a t e d i n th e s e t h r e e d i s c r e t e c r y s -
t a l l o g r a p h i c l o c a t i o n s ( s p e c i f i c a l ly , t h e i n t e r a c t i o n
o f an N a i o n a s s o c i a t e d w i t h a d o u b l e 6 - m e m b e r o x -
y g e n r i n g , a s i n g l e 5 - m e m b e r r i n g a n d a 4 - m e m b e r
o x y g e n r i n g ) s h o u l d b e c o n s i d e r a b l y d i ff e r e n t
( D e m p s e y , 1 9 6 8 ) .
B y s y n t h e s i s o f a s e r i e s o f z e o l i t e s , t h e c a t i o n
d e n s i t y c a n b e u n i f o r m l y d e c r e a s e d b y d e c r e a s e i n
t h e A 1 s u b s t i t u t i o n . I n t h i s w a y , t h e c a t i o n p o p u l a -
t i o n o f o n e o r m o r e o f t h e t h r e e c a t i o n s i t e s m e n -
t i o n e d a b o v e c a n b e p r e f e r e n t i a l l y d e p l e t e d . A c o r -
r e s p o n d i n g v a r i a t i o n s h o u l d o c c u r i n t h e a c t i v a t i o n
e n e r g y r e q u i r e d f o r t h e o v e r a l l c o n d u c t i o n p r o c e s s .
B r i e fl y , N a i o n s l o c a t e d o n t h e s u r f a c e o f t h e l a r g e
c a v i t i e s ( a s s o c i a t e d w i t h t h e 4 - m e m b e r o x y g e n
r i n g s ) a r e t h e l e a s t s t r o n g l y b o u n d a n d w i l l t h e r e -
f o r e r e q u i r e t h e l e a s t a m o u n t o f e n e r g y t o m i g r a t e .
I n z e o l i t e s w i t h h i g h c a t i o n d e n s i t i e s , t h e r a t e d e t e r -
m i n i n g p r o c e s s f o r t h e m i g r a t i o n o f th e c u r r e n t c a r -
r i e r s i n a n e x t e r n a l e l e c t r i c f ie l d i s d u e t o s u c h
w e a k l y b o u n d c a t i o n s . A s t h e c a t i o n d e n s i t y i s
d e c r e a s e d t o w a r d s t h e h i g h s i li c a e n d o f th e s e r i e s ,
t h e l o w e n e r g y c a t i o n s i t e s a r e d e p l e t e d , a n d t h e
m o r e s t r o n g l y b o u n d c a t i o n s s t a r t t o c o n t r i b u t e a n d
h e n c e d e t e r m i n e t h e r a te o f t h e c o n d u c t i o n p r o c e s s .
C o n s e q u e n t l y , t h e c h a n g e i n t h e a c t i v a t i o n e n e r g y
f o r c o n d u c t i o n i s a d i r e c t m e a s u r e o f t h e d i f f e r -
e n c e s i n t h e e n e r g y a s s o c i a t e d w i t h t h e d i f f e r e n t
k i n d s o f c a t i o n / c a t i o n - s i t e p a i rs .
*A s implified model of the locat ion of ca t ions in the unit
cell is considered ; namely, the associat ion of Na ions with
double 5-member oxygen r ings, s ingle 5-member o xygen
r ings and 4-member oxygen r ings . However , project ion
through the sodali te unit and the supercage results in
actua l sites of type. ( i) and (ii) , locate d within th e sod alite
unit. Typ e ( i) located in th e hex agonal pr ism, typ e ( i i) lo-
cated in the supercage an d ty pe ( i i i) a lso in the supercage.
In par t icular , we are concerne d here with th e dif ference in
the bon ding energy of the Na ions in the type ( i i i) locat ion
which is the leas t s table posi t ion (due mainly to the v ery
small coulombic interact ion) as compared to the bonding
energy at th e typ e ( i) and ( ii ) s i tes which are consid erably
more s table , with higher energy of interact ion.
tThis pr obably i s the aver age of the ind iv idua l cont r ibu-
t ions , such as coulom bic, repuls ive, polar izat ion and small
amounts due to steric effects.
I n z e o l i t e s w i t h lo w c a t i o n d e n s i t i e s , t h e N a i o n s
a r e p r i m a r i l y a s s o c i a t e d w i t h t h e s i n g l e a n d d o u b l e
6 - m e m b e r o x y g e n r i n g s ; t h e r e f o r e , t h e a c t i v a t i o n
e n e r g y i s c o n s i d e r a b l y h i g h e r . T h i s s i t u a t i o n w a s
s h o w n i n t h e r e s u l t s p u b l i s h e d e a r l i e r ( s e e F i g . 3, o f
F r e e m a n a n d S t a m i r e s , 1 96 1) w h i c h i l l u s t ra t e t h e
u n i f o r m c h a n g e i n e n e r g y t a s s o c i a t e d w i t h
c a t i o n / c a t i o n - s i t e p a i r s a m o n g t h e d i f f e re n t c r y s t a l -
l o g r a p h i c s i t e s . I t w a s o n t h i s b a s i s t h a t a p h y s i c a l
m o d e l w a s d e v e l o p e d a n d o n p u r e l y e n e r g e t ic c o n -
s i d e r a t i o n s i t w a s p o s s i b l e t o d i f f e r e n t i a t e t h e
c a t i o n s i n t h e s y n t h e t i c f a u j a s i t e i n t o t h r e e t y p e s .
H o w e v e r , B r e c k ( 1 9 6 4 ) a n d B r e c k a n d F l a n i g e n
(1 96 8) h a v e c o n c l u d e d t h a t t h e s e d a t a p r o v i d e e v i -
d e n c e f o r d i v i d i n g t h e s u b s t i t u t i o n a l s e r i e s a t a n
S / A o f t h r e e i n t o t w o s u b - g r o u p s , d e s i g n a t e d a s X
a n d Y . I n f a c t , t h e r e i s a s m o o t h v a r i a t i o n in t h e
a c t i v a t i o n e n e r g y w i t h S / A , w i t h n o d e t e c t a b l e d i s -
c o n t i n u i t y . T h e i n f le c t i o n p o i n t ( t h e p o i n t w h e r e t h e
a c t i v a t i o n e n e r g y c h a n g e s m o s t d r a s t i c a l l y w i t h c a -
t i o n d e n s i t y ) i n t h e c o n d u c t i v i t y - A 1 c o n t e n t p l o t i s
c l o s e t o a n S / A o f 4 , n o t 3 . T h i s p o i n t i s c o n v i n c -
i n g l y i l l u s t r a t e d i f t h e f i r s t d e r i v a t i v e o f t h e a c t i v a -
t i o n e n e r g y i s p l o t t e d a g a i n s t t h e S / A .
I n F i g . 2 th e a c t i v a t i o n e n e r g y ( A H ) a n d i t s fi rs t
d e r i v a t i v e a r e p l o t t e d a g a i n s t S / A . T h e a r b i t r a r y d i -
v i s i o n a t a r a t i o o f 3 i s a l s o i n d i c a t e d . T h e s e d a t a
c l e a r l y s h o w t h a t s u c h a d i v i s i o n i s n o t j u s t i f i e d .
3 0
d [ ~ H l
2 .O
0 . 2
0
No
Fig. 2. Faujas i te conductivi ty as a function of
s i l ica-alum ina rat io, Freem an an d Stamires (1961). The
broken l ine represents the plot of the f ir s t der ivat ive of
(AH) w ith respec t to SiOJA1203 mole rat io.
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7/23/2019 Zeolite faujasite
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384
DENNIS N STAMIRES
F u r t h e r m o r e , t h e d is t in c t io n o f X f r o m Y a s t w o
d i f f e r e n t z e o l i t e s p e c i e s w h e r e X a n d Y r e f e r t o
c o m p o s i t i o n a l r a n g e s ) , c e r ta i n l y i s n o t s u g g e s t e d b y
t h e r e s u lt s . I f o n e h a d t o u s e t h e c o n d u c t i v i t y d a t a
t o d i v i d e a r b i tr a r i l y t h e s e r i e s i n t o t w o g r o u p s , t h e n
a n S / A o f a p p r o x i m a t e l y 4 w o u l d b e a l o g i c a l
c h o i c e , s i n c e i n t h i s r e g i o n t h e c o n d u c t i o n p r o c e s s
c h a n g e s m o s t d r a s t i c a l l y w i t h c a t i o n d e n s i t y .
A s i n t h e c a s e o f s t e a m s t a b i l i t y , t h e e l e c t r o n i c
p r o p e r t i e s o f s y n t h e t i c f a u j a s i t e s a l s o v a r y
u n i f o r m l y a n d c o n t i n u o u s l y w i t h c o m p o s i t i o n .
Adsorp t ion proper t i es
A m o s t i m p o r t a n t p r o p e r t y , w h i c h h a s b e e n u s e d
e x t e n s i v e l y t o c h a r a c t e r i z e t h e v a r i o u s z e o l i t e
s p e c i e s , is th e a d s o r p t i o n o f g a s e s a n d v a p o r s .
T r i e t h y l a m i n e a d s o r p t i o n h a s b e e n u s e d b y B r e c k
t o d i s t i n g u i s h c a l c i u m - e x c h a n g e d Z e o l i t e X f r o m
s i m i la r l y t r e a t e d Z e o l i te Y . F u r t h e r , W r i g h t , R u p e r t
a n d G r a n q u i s t 1 9 69 ), h e r e a f t e r r e f e r r e d t o a s
W R G , h a v e r e p o r t e d s i m il ar e x p e r i m e n t s u s i n g t h e
N a f o r m s o f t w o s y n t h e t i c f a u j a s i t e s w i t h S / A o f
2 .4 and 4 .8 .
I n th e p r e s e n t w o r k , t h e m e a s u r e m e n t s h a v e
b e e n e x t e n d e d t o c o v e r t h e w h o l e r an g e o f S / A a n d
t o i n c l u d e a s a m p l e o f n a t u r a l f a u j a s i t e i n i t s
s o d i u m f o r m . E q u i l ib r i u m d a t a w e r e o b t a i n e d f r o m
e x p e r i m e n t s w h e r e t h e a d s o r p t i o n o f t r i e th y l a m i n e
a t r o o m t e m p e r a t u r e w a s m e a s u r e d a s a f u n c t i o n o f
v a p o r p r e s s u r e . E q u i l i b r iu m w a s r e a c h e d , a t t h e
l o w e r v a p o r p r e s s u r e s , a f t e r 8 - 1 0 h r . I n o r d e r t o
c o m p a r e
W R G
a n d p r e s e n t r e s u l t s , t h e s a m e e x -
p e r i m e n t a l c o n d i t i o n s w e r e u s e d . T h e d a t a a r e
c r o s s - p l o t t e d i n F i g . 3 . A l l t h e s a m p l e s s t u d i e d a d -
s o r b e d a b o u t t h e s a m e a m o u n t o f tr i e th y l a m i n e ,
w i t h i n e x p e r i m e n t a l e r r o r . S i n c e t h e s i z e o f t h e u n i t
c e l l d e c r e a s e s w i t h S / A , a d s o r p t i o n w o u l d b e e x -
p e c t e d t o d e c r e a s e w i th i n c r e as i n g S / A . P r o b a b l y
t h e v a r i a t i o n i n t h e n u m b e r a n d d i s t r i b u t i o n o f t h e
c a t i o n s i n t h e u n i t ce l l o f f s e t s t h i s t re n d . T h e i m p o r -
t a n t p o i n t t o n o t e h e r e i s t h a t a l l t h e s a m p l e s , i n c l u d -
i n g n a t u r a l f a u j a s i t e , s h o w w i t h i n e x p e r i m e n t a l
e r r o r) a b o u t t h e s a m e a d s o r p t i v e c a p a c i t y . T h e tw o
m e a s u r e m e n t s r e p o r t e d e a r l ie r b y W R G a r e i n
a g r e e m e n t w i t h t h e p r e s e n t r e s u l t s .
A c c o r d i n g l y , a d i v i s i o n b a s e d o n t h e a d s o r p t i o n
o f t r ie t h y l a m i n e o f t h e s u b s t i t u t i o n a l s e r i e s a t a n y
S / A i n t h e r a n g e 2 - 6 i s n o t j u s t i f i a b l e . F u r t h e r ,
n a t u r a l f a u j a s i t e c a n n o t b e d i s t i n g u i s h e d a s a d i f fe r -
e n t t y p e o f z e o l i t e o n t h e b a s i s o f s u c h d a t a . S i m i -
l a r ly , t h e a d s o r p t i v e c a p a c i t i e s o f th e s u b s t i t u t io n a l
s e r i e s a n d n a t u r a l f a u j a s i t e f o r o x y g e n a n d w a t e r a t
- 1 8 3 ~ a n d a t r o o m t e m p e r a t u r e re s p e c t i v e l y a t
t h e s a m e v a p o r p r e s s u r e s ) , w i t h in t h e e x p e r i m e n t a l
R E S E N T D T
D N A T U R A L F A U J A S I T E
s ~ o
~ - ~ . . . . . . . . o )
Fig. 3. Equilibrium capa city for triethylamine adsorption
at 3 tort and ro om temperature as a function o f the
SiO2/A1203mo le ratio.
e r r o r , w e r e f o u n d t o b e a b o u t t h e s a m e . T h u s , t h e
p o r e v o l u m e , a s i n d i c a t e d b y t h e s e a d s o r p t i v e
c a p a c i t i e s , i s a b o u t t h e s a m e i n t h e s y n t h e t i c a n d
n a t u r a l m e m b e r s o f t h e f a u j a s i t e g r o u p .
I t i s v e r y i m p o r t a n t i n c o m p a r i n g t h e v a r i o u s
p h y s i c o c h e m i c a l p r o p e r t i e s o f t h e s u b s t i t u t i o n a l
s e ri e s o f f a u ja s i te s t o u s e t h e s a m e h o m o c a t i o n i c
f o r m s i n t h e e x p e r im e n t s . T h e t y p e o f c a t i o n s i ze s
a n d c h a r g e ) a n d t h e p e r c e n t e x c h a n g e a f f e c t d r a s t i -
c a l l y t h e a d s o r p t i o n c h a r a c t e r i s t i c s o f t h e z e o l i t e
c r y s t a l s . T h e d a t a p r e s e n t e d b y B r e c k 1 9 64 ) o n th e
t r i e t h y l a m i n e a d s o r p t i o n b y s y n t h e t i c f a u j a s i t e s
w e r e o n c a l c iu m f o r m s a p p r o x i m a t e l y 8 5 p e r c e n t
e x c h a n g e d ) .
Var ia t ion o f un i t ce l l d imens ion wi th compos i t ion
T h e r e p l a c e m e n t o f a l u m i n u m f o r s i l i c o n i n t h e
s t r u c tu r e h a s b e e n d i s c u s s e d b y B r e c k 1 96 4). F r o m
s i m p le g e o m e t r i c a l c o n s i d e r a t io n s o f t h e S i - O a n d
A I - O b o n d d i s t a n c e s i. e. 1 - 6 05 a n d 1 .7 5 7 ~ , r e s p e c -
t i v e ly ) , it is e x p e c t e d t h a t t h e c u b i c l a t t i c e p a r a m e -
t e r a s h o u l d d e c r e a s e s m o o t h l y w i th i n c re a s i n g
S / A . T h i s g e n e r a l c o n c e p t w a s i l l u st r at e d b y B r e c k
a n d F l a n i g e n 1 9 68 ), h e r e a f t e r r e f e r r e d t o a s B F
w h o e x a m i n e d 3 7 s a m p l e s o f s y n t h e t i c f a u j a s i te s o f
v a r y i n g S / A . T h e y h a v e s h o w n b y a l e a s t s q u a r e f it
t h a t a v a r i e s l i n e a r ly w i t h t h e n u m b e r o f A 1 a t o m s
in t he un i t c e l l . B F r e p o r t e d a n e x p e r i m e n t a l e r r o r
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o f - + 0 .0 0 5 . ~ i n a a n d - + 0 .5 w t i n t h e s i l i c a a n d
a l u m i n a c o n t e n t s .
M o r e r e c e n t l y , W R G r e p o r t e d a v a l u e s f o r
s y n t h e t i c fa u j a s i t e s s y n t h e s i z e d f r o m h y d r o u s
s o d i u m s y s t e m s s i m i la r t o t h o s e u s e d b y B F . T h e
e s t i m a t e d p r o b a b l e e r r o r in t h e d a t a o f W R G w a s
+ 0 . 0 1 i n a a n d - +- 0.0 0 5 ~ i n t h e m o l e f r a c t i o n o f
A I O 2- . I n t h e p a p e r o f F r e e m a n a n d S t a m i r e s
( 19 6 1 ), s e v e r a l a v a l u e s w e r e a l s o g i v e n f o r s y n t h e -
t i c f a u j a s i t e s . T h e s e d a t a w e r e o b t a i n e d i n t h e v e r y
e a r l y s t a g e s o f f a u j a s i t e s y n t h e s i s a n d t h e t e c h n i -
q u e s f o r a a n d S / A d e t e r m i n a t i o n w e r e n o t y e t w e l l
r e f i n e d ; t h e r e f o r e , t h e e x p e r i m e n t a l e r r o r w a s
l a r g e r , c l o s e t o - 4 :0 . 2 i n t h e S / A v a l u e s .
I n t h i s w o r k , s a m p l e s w e r e s y n t h e s i z e d a n d
s t u d i e d i n a m a n n e r s i m i l a r t o t h e w o r k o f
B F
a n d
W R G . T h e e s t i m a t e d p r o b a b l e e r r o r i n t h e p r e s e n t
d a t a i s - 0 . 1 i n t h e S / A a n d - + 0 . 0 0 5 ~ i n t h e a
v a l u e s .
T h e r e s u l t s a r e c o m p a r e d i n F i g . 4 . W i t h i n t h e
e x p e r i m e n t a l v a r i a t io n , t h e r e i s a g r e e m e n t b e t w e e n
t h e r e s u l ts o f t h e s e f o u r i n d e p e n d e n t g r o u p s . T h e s e
s t u d i e s r e p r e s e n t w o r k d o n e o v e r a t l e a s t t h e l a s t
1 0 y r , w i t h v a r i o u s a n a l y t i c a l t e c h n i q u e s a n d l e v e l s
o f s o p h i s t i c a t i o n .
B F
( 1 9 6 8 ) r e p o r t e d p r e c i s i o n
m e t h o d s f o r d e t e r m i n i n g a w h i c h a r e u s e d
r o u t i n e l y t o i d e n t i f y f a u j a s i t e s a m p l e s .
B F a l s o p r e s e n t e d d a t a o n a v a l u e s o f d e h y d -
r a t e d c a l c i u m - e x c h a n g e d s y n t h e t i c f a u ja s i t e s . I n
c o n t r a s t t o t h e h y d r a t e d s o d i u m f o r m s , a i n t h i s
c a s e d i d n o t v a r y l i n e a r l y w i t h S / A , a n d t h e d a t a
ser ies : a review with new da ta
385
d i s p l a y e d t w o d i s c o n t in u i t e s , a t a p p r o x i m a t e l y 6 2
a n d 7 7 A 1 p e r u n i t c e l l. T h e s e d i s c o n t i n u i t i e s w e r e
i n t e r p r e t e d b y B F a s d u e t o f r a m e w o r k d i s t o r t i o n
a s s o c i a t e d w i t h m o v e m e n t o f C a i on s . D e h y d r a t i o n
c a u s e s t h e c a l c i u m i o n s t o m o v e t o s t r u c t u r a l p o s i -
t i o n s w h i c h a r e m o r e f a v o r a b l e e n e r g e t i c a l l y . T h e
m e c h a n i s m r e s p o n s i b l e f o r th e s e d i s c o n t i n u i t i e s
m a y b e m u c h m o r e c o m p l i c a t e d t h a n d e s c r i b e d ,
s i n c e t h e s a m p l e s c o n t a i n e d a p p r e c i a b l e a m o u n t s
o f r e s i d u a l N a , a n d s i n c e h e a t i n g i n a i r a t 40 0 ~ i s
n o t t h e b e s t w a y t o a c t i v a t e c a l c i u m - e x c h a n g e d
z e o l i t e s . A l s o , if a v a l u e s f o r N a a n d C a f o r m s o f
t h e s a m e s a m p l e s a r e c o m p a r e d , a a l s o c h a n g e s a s
N a i s e x c h a n g e d f o r C a i n s a m p l e s c o n t a i n i n g t h e
s a m e a m o u n t o f A 1. T h i s d e p e n d e n c y i n t r o d u c e s
a n o t h e r v a r i a b l e , d e g r e e o f i o n e x c h a n g e , i n t o t h e
d e t e r m i n a t i o n o f a . H o w e v e r , t h e i m p o r t a n t f a c t o r
t o b e n o t e d h e r e i s t h a t t h e t e c h n i q u e s u s e d b y
B F
w e r e s e n s i t i v e a n d q u i t e r e l i a b l e i n o r d e r t o b e a b l e
t o e s t a b l i s h t h e s e d i f f e r e n c e s i n th e a v a l u e s .
D e s p i t e t h e a g r e e m e n t s h o w n i n F i g . 4 , a c o m p l i -
c a t i o n i s i n t r o d u c e d b y t h e r e s u l t s o f D e m p s e y ,
K f i h l a n d O l s o n ( 1 96 9 ), h e r e a f t e r r e f e r r e d t o a s
D K O w h o h a v e a l s o s t u d i e d t h e v a r i a t i o n o f a w i t h
A I c o n t e n t f o r h y d r a t e d N a - f a u j a s i t e . I n c o n t r a s t t o
a l l p r e v i o u s l y p u b l i s h e d d a t a , D K O f o u n d t h r e e
d i s c o n t i n u i t i e s i n a , n e a r 5 2 , 64 , a n d 8 0 A 1 a t o m s p e r
u n i t c e l l . D K O s u g g e s t e d t h a t a t l e a s t t h r e e d i f f e r-
e n t f a u j a s i t e p h a s e s e x i s t i n t h e S / A r a n g e s t u d i e d .
T h e d i s c o n t i n u i ty o b s e r v e d a t t h e h i g h e s t S i co n -
t e n t w a s a s s u m e d t o b e d u e t o t h e p r e s e n c e o f
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in the hydra ted sodium (homocationic) form . Data o f Breck and Flanigen (1968) ; Wright , Ruper t and
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a m o r p h o u s s il ic a o c c l u d e d i n t h e s a m p l e s . DKO d o
n o t d e s c r i b e t h e t e c h n i q u e s u s e d i n s y n t h e s i z i n g
t h e ir s a m p l e s . H o w e v e r , i f th e y u s e d t h e t e a c h i n g s
o f B a r r e r ( 1 9 59 ) a n d B r e c k ( 19 6 4) f o r t h e s y n t h e s i s
o f f a u ja s i te s , t h e n t h e i r d a ta w o u l d b e e x p e c t e d t o
be s im i l a r t o t he r e su l t s , i n F ig . 4 , o f BF WRG a n d
t h e p r e s e n t p a p e r . F u r t h e r , i t i s h i g h l y i m p r o b a b l e
t h a t t h e i r o b s e r v e d d i s c o n t i n u i t i e s r e p r e s e n t e x p e r i -
m e n t a l a r t i f a c t s . T h e s e i r r e g u l a r i t i e s p r o b a b l y r e -
p r e s e n t r e a l e ff e c t s c h a r a c t e r i s t i c o f t h e n a t u r e o f
t h e DKO s a m p l e s . I n t h e a b s e n c e o f a d d i t io n a l i n -
f o r m a t i o n , i t m u s t b e c o n c l u d e d t h a t t h i s d i s c r e -
p a n c y i s d u e t o d i f f e r e n c e s i n t h e
DKO
s a m p l e s
c a u s e d b y v a r i a t i o n s i n s y n t h e s i s a n d p r e p a r a t i o n
t e c h n i q u e s .
DKO e x p l a i n t h e s e d i s c o n t i n u i t e s o n t h e b a s i s o f
v a r i a t i o n s i n ( St, A 1 ) o r d e r i n g . S i n c e s u c h o r d e r i n g
w i l l t e n d t o d e c r e a s e a , t h e f a c t t h a t a l l t h e
DKO
d a t a l i e b e l o w t h e s t r a i g h t l i n e o f F i g . 4 s u g g e s t s
t h a t th e s a m p l e s u s e d b y BF WRG a n d t h e p r e s e n t
a u t h o r c o r r e s p o n d t o h i g h e r ( S i, A I ) o r d e r i n g .
F u r t h e r , t h e a d a t a i n d i c a t e t h a t n a t u r a l f a u j a s i t e
h a s t h e h i g h e s t o r d e r . T h r e e p r e v i o u s l y r e p o r t e d
v a l u e s o f a f o r t h e n a t u r a l m i n e r a l a n d t h e p r e s e n t
m e a s u r e m e n t a r e s h o w n i n F i g . 4 . T h e p r e s e n t r e -
s u l t w a s o b t a i n e d o n h y d r a t e d h o m o c a t i o n i c
N a - f a u j a s i t e c r y s t a l s w h i c h w e r e u s e d i n th e o t h e r
e x p e r i m e n t s d e s c r i b e d i n t h is p a p e r . S / A w a s f o u n d
t o b e 4 . 5 5 - + 0 . 0 1 a n d t h e c u b i c l a t t i c e p a r a m e t e r a ,
2 4- 66 _+ 0 . 0 0 5 . ~; t h e s e v a l u e s w e r e e s t a b l i s h e d a f t e r
s e v e r a l d e t e r m i n a t i o n s . T h e s e v a l u e s a r e v e r y c l o s e
t o t h o s e o f
BF
a n d B e n n e t t a n d S m i t h ( 1 9 7 2 ) . A l l
f o u r v a l u e s a r e p r e s e n t e d i n F i g . 4 , e n c l o s e d i n a
c i rc l e f o r e m p h a s i s . T h e BF v a l u e o f a l i e s c l o s e t o
t h e s t r a i g h t li n e o b t a i n e d f r o m t h e a v a l u e s f o r t h e
s y n t h e t i c s a m p l e s . I n a l l c a s e s , t h e n a t u r a l c r y s t a l s
a p p e a r t o b e m o r e ( S t, A 1 ) o r d e r e d t h a n t h e s y n t h e -
t i c m a t e r i a l , p r o b a b l y d u e t o m o r e i d e a l n a t u r a l
g r o w t h c o n d i t io n s . A l t h o u g h r e a s o n a b l e , t h e r e i s n o
d i r e c t e x p e r i m e n t a l e v i d e n c e t o s u b s t a n t i a t e t h i s
p o s t u l a t e d o r d e r i n g .
T h e a v a l u e s s h o w n i n F i g . 4 r e p r e s e n t a b o u t 6 0
s a m p l e s s y n t h e s i z e d a n d a n a l y z e d i n d e p e n d e n t l y
b y f o u r o r f iv e g r o u p s o f w o r k e r s a t d i f f er e n t t im e s .
I t c a n b e c o n c l u d e d t h a t w h e n f a u j a s i t e s a r e s y n -
t h e s i z e d b y s t a n d a r d p r o c e d u r e s ( B a r r e r , t 9 5 9 ,
B r e c k , 1 9 64 ), t h e n t h e a v a l u e s v a r y l in e a r l y a n d
c o r r e l a t e q u i t e w e l l w i t h t h e a l u m i n u m c o n t e n t o f
t h e c r y s ta l s . T w o o u t o f th e f o u r r e p o r t e d v a l u e s o f
a f o r n a t u r a l f a u j a s i t e a r e , w i t h i n t h e e x p e r i m e n t a l
e r r o r , c l o s e t o t h e s t r a i g h t l i n e o b t a i n e d f o r t h e
s y n t h e t i c s a m p l e s . N e v e r t h e l e s s , t o a f i r s t o r d e r
a p p r o x i m a t i o n i n s o f a r a s t h e v a r i a t i o n o f a w i t h A 1
c o n t e n t i s c o n c e r n e d , i t c a n b e c o n c l u d e d t h a t t h e
ST MIRES
d i f fe r e n c e b e t w e e n n a t u r a l a n d s y n t h e t i c s a m p l e s i s
qu i t e sm a l l .
T h e l o c a t i o n o f t h e c h a r g e - b a l a n c i n g c a t i o n s i n
t h e s t r u c t u r e , t h e ( S t , A 1 ) o r d e r i n g , a n d t h e d i s t r i b u -
t io n o f t h e a d s o r b e d m o l e c u l e s m a y b e d i f f e re n t i n
t h e n a t u r a l a n d s y n t h e t i c s a m p l e s .
F r o m a s t a t i s t i c a l p o i n t o f v i e w , t h e d a t a p r e -
sen t ed i n F ig . 4 (pa r t i cu l a r l y t he
BF
a n d p r e s e n t r e -
s u l t s ) a p p e a r t o b e i n a c c o r d . D e s p i t e t h i s a c c o r d ,
t h e c o n f l i c t in g DKO r e s u l t s s h o w t h a t t h e v a r i a t i o n
o f a w i t h c o m p o s i t i o n i s n o t a c l o s e d i ss u e . F u r t h e r
e x p l o r a t i o n i s n e c e s s a r y t o y i e l d a d e f i n i t i v e
a n s w e r .
Catalytic properties
C a t a l y t ic b e h a v i o r , a l t h o u g h c o m p l i c a t e d t o a s -
c e r t a i n , is a n o t h e r c h a r a c t e r i s t i c p r o p e r t y o f t h e
s u b s t i t u t i o n a l s e r i e s . T h e m a i n f a c t o r s w h i c h d e t e r -
m i n e t h e p e r f o r m a n c e o f a z e o l i t e c a t a l y s t a r e :
s i l i c a - a l u m i n a r a t i o ; p e r c e n t e x c h a n g e a n d n a t u r e
o f th e e x c h a n g e c a t i o n ; m e t h o d o f p r e p a r a t i o n ; a n d
t y p e o f m a t r i x ( t h e z e o l i t e i s u s u a l l y c o n t a i n e d i n
s u p p o r t i n g m a t r i x ) . G o o d c a t a l y s t s h a v e b e e n p r e -
p a r e d f r o m f a u j a s i t e s w i t h b o t h h i g h - a n d l o w - A 1
c o n t e n t s .
K i m b e r l i n a n d G l a d r o w ( 1 9 6 1 ) d e m o n s t r a t e d e x -
c e l l e n t c a t a l y t i c p r o p e r t i e s o f z e o l i t i c c a t a l y s t s
u s i n g s y n t h e t i c f a u j a s i t e w i t h S / A o f 2 . 7 . S u b s e -
q u e n t l y , R a b o et al. (1966) pub l i shed r e su l t s on
c a t a l y t i c p r o p e r t i e s o f s y n t h e t i c f a u j a s i t e s w i t h S / A
i n t h e r a n g e 3 - 6 , a n d c l a i m e d t h a t t h e c a t a l y t i c
p r o p e r t i e s o f th e s e z e o l i te s w e r e u n i q u e a n d d i f f e r-
e n t f r o m t h o s e s t u d ie d b y K i m b e r l i n a n d G l a d r o w
( 19 61 ). T h i s a s s e r t i o n w a s i n l in e w i t h B r e c k s c o n -
c e p t t h a t Y z e o l i t e i s d i f f e r e n t f r o m 3 ( . B o t h o f
t h e s e g r o u p s o f w o r k e r s f a i le d t o i n d i ca t e t h e p e r -
c e n t e r r o r i n v o l v e d i n t h e S / A u s e d i n t h e i r c l a im s .
A s i s s h o w n i n F i g . 5 , t h e c a t a l y t i c a c t i v i t y , f o r
c r a c k i n g c u m e n e a t 4 0 0~ o f a s er ie s o f M g e x -
c h a n g e d f a u j a s i t e c a t a l y s t s v a r i e s s m o o t h l y w i t h
S / A . T h e t e c h n i q u e s u s e d f o r t h e s e c a t a l y t i c m e a s -
u r e m e n t s w e r e s i m i l a r t o t h o s e o f e a r li e r s t u d i e s
( T u r k e v i c h , N o z a k i a n d S t a m i r e s , 1 9 6 4) w h e r e
o t h e r t y p e s o f z e o li t e c a t a l y s t s w e r e i n v e s t i g a t e d . A
d i s c u s s i o n o f t h e m e c h a n i s m i n v o l v in g t h e e f fe c t o f
S / A o n t h e c a t a l y t ic a c t i v it y a n d o t h e r r e l a te d p h y s -
i c a l a n d s t r u c t u r a l p r o p e r t i e s i s b e y o n d t h e s c o p e o f
t h i s p a p e r a n d w i l l b e p r e s e n t e d e l s e w h e r e ( S t a -
m i r e s , t o b e p u b l i s h e d ) . T h e i s s u e c o n s i d e r e d h e r e
i s t h e f a c t t h a t c a t a l y t i c a c t i v i t y f o r c u m e n e c r a c k -
i n g ( w i t h i n t h e e x p e r i m e n t a l e r r o r ) v a r i e s a l m o s t
l i n e a r ly w i t h S / A i n t h e r a n g e o f 2 - - 4 . S i m i l a r ly , a
l i n e a r v a r i a t i o n i s o b s e r v e d i n t h e r a n g e o f 4 - 6 . A l -
t e r n a t i v e l y , o n e c o u l d r e p r e s e n t t h e w h o l e r a n g e
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C A T A L Y S TS B Y M A G N E S IU M X C H A N G E D
F A U J A S I T E A T A L Y S T S
( 8 4 19 5 ' ~ ' M g - - F A U JA S IT E
8 5 } C A T A L Y S T S
Fig. 5. V ariation of the catalytic activity fo r cumene cracking at 400~ with si l ica-alumina ratio. The
samples were activated at 450~ prior to catalytic measurements.
s t u d i e d ( 2 - 6 ) b y t w o s t r a i g h t l in e s c o r r e s p o n d i n g t o
t h e t w o r a n g e s ( 2 - 4 ) a n d 4 - - 6 ) a n d i n t e r s e c t in g a p -
p r o x i m a t e l y a t S / A o f 4 .
T h e c o n t i n u o u s v a r i a t i o n o f c a t a l y t i c a c t i v i t y
w i t h n o d i s c o n t i n u i t y a t S / A o f 3 i s i n a c c o r d w i t h
t h e o t h e r p r o p e r t i e s d i s c u s s e d e a r l ie r . C a t a l y t i c a c -
t iv i ty f o r o t h e r t y p e s o f h y d r o c a r b o n r e a c t io n s m a y
s h o w a d i f f e r e n t t y p e o f d e p e n d e n c e o n t h e S / A ;
h o w e v e r , a s h a r p d i s c o n t i n u i t y a t 3 i n t h e S / A ( 2 - 6 )
range i s un l i ke ly .
O N L U S I O N S
C r i t ic a l r e v i e w o f t h e l it e r a tu r e a n d n e w s t u d i e s ,
d e s c r i b e d i n t h i s p a p e r , e s t a b l i s h t h e s m o o t h , c o n -
t i n u o u s v a r i a t i o n w i t h c o m p o s i t i o n ( A l - c o n t e n t ) o f
t h e f o l l o w i n g p r o p e r t i e s o f a s u b s t it u t i o n a l s e r i e s o f
s y n t h e t i c f a u j a s i t e s :
( 1) R e t e n t i o n o f c r y s t a l l in i t y u p o n s t e a m -
d e g r a d a t i o n a t 4 1 0 ~
(2 ) A c t i v a t i o n e n e r g y o f t h e c o n d u c t i o n p r o c e s s ;
( 3) E q u i l i b r i u m a d s o r p t i o n o f t r ie t h y l a m i n e ;
(4 ) Un i t c e l l d im ens ion , a ; and
( 5) C a t a l y t i c p r o p e r t i e s , p a r t i c u l a r ly t h e c r a c k i n g
o f c u m e n e .
O n t h e b a s i s o f s t e a m - s t a b i l i t y , t r i e t h y l a m i n e a d -
s o r p t i o n , a n d a v s c o m p o s i t i o n , t h e n a t u r a l c o u n -
t e r p a r t ( K a i s e r s t u h l o c c u r r e n c e ) i s s h o w n t o b e a
m e m b e r o f t h e s e r ie s , a l th o u g h p e r h a p s m o r e
h i g h l y - o r d e r e d .
T h e c o n f l i c t b e t w e e n t h e d i s c o n t i n u i t i e s s h o w n
b y
D K O
a n d t h e l in e o f F i g . 4 (b a s e d o n
B F , W R G ,
F r e e m a n a n d S t a m i r e s , a n d t h e p r e s e n t w r i t e r ) i s
u n r e s o l v e d . A d d i t i o n a l w o r k a t a sti ll h i g h e r l e v e l o f
soph i s t i c a t i on wi l l be r equ i red fo r a de f in i t i ve
a n s w e r .
F u r t h e r , f a u j a s i t e s a m p l e s s y n t h e s i z e d w i t h s i m i -
l a r h y d r o t h e r m a l t e c h n i q u e s s h o w c o ; n si s te n t a v a l-
u e s a t a g i v e n c o m p o s i t i o n , a n d a v a r i e s l i n e a r l y
w i t h t h e a l u m i n u m c o n t e n t o f t h e c r y s t a l s .
I n t h e l i g h t o f t h e s e f a c t s , t h i s w r i t e r p r o p o s e s
tha t t he ex i s t i ng a r t i f i c i a l d iv i s ion o f t he f au j a s i t e
s e r ie s a t a S / A o f 3 b e e l i m i n a t e d , a n d a n e w
n o m e n c l a t u r e a d o p t e d . T h i s n e w n o m e n c l a t u r e
s h o u l d b e c a p a b l e o f r e p r e s e n t i n g p r e c i s e l y o n e
p a r t i c u l a r c o m p o s i t i o n , r a t h e r t h a n t h e w i d e r a n g e
o f c o m p o s i t io n s e n c o m p a s s e d b y t h e t r ad e n a m e s
p r e s e n t l y i n u s e . F u r t h e r , t h i s n e w n o m e n c l a t u r e
, shou ld c l ea r ly r e f l e c t t he ac tua l s im i l a r i t i e s and t he
s m o o t h g r a d u a l v a r i a t i o n i n t h e m o s t i m p o r t a n t
p h y s i c o c h e m i c a l p r o p e r ti e s o f t h e c o n t i n u o u s
se r i e s and t he c lose s im i l a r i t i e s wi th t he i r na tu ra l
c o u n t e r p a r t f a u j a s i t e .
S u c h a u n i f o r m s y s t e m f o r n a m i n g t h e v a r i o u s
z e o l i t e m e m b e r s i n t h e c o n t i n u o u s s u b s t i t u t i o n a l
f a u j a s i t e s e r i e s c o u l d b e o n e s i m i l a r t o th a t a l r e a d y
s u g g e s t e d b y W R G .
R E F E R E N E S
Barter, R. M., Baynham, J. W., Bulti tude, F. W. and
Meier, W. M , (1959) Hydrothermal chemistry o f si l i-
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R 6 s u m 6 - O n p r6 s e n te d a n s c e t r a v a i l d e n o u v e l l e s in fo rm a t io n s s e r a p p o r t a n t ~ l a s t a b il i t 6 h y d ro -
the rm ale , le s pa ram~ tres de r6sea u , le s p ropri6 t6s d 'ads orp t ion e t le s p ropri6 t6s 61ec tr iques e t ca ta ly t t -
q u e s d ' & h a n t i l l o n s d e f a u ja s i t e s y n th 6 t iq u e e t n a tu re ll e . L e s c o n c e p t s a c tu e l s c o n c e r n a n t l a n a tu re
e t l e s r e l a t io n s e x i s t a n t e n t r e l a f a u ja s i t e s y n th 6 t iq u e e t l a f a u ja s i t e n a tu re l l e s o n t r e p r i s p o u r 6 t r e e n
a c c o rd a v e c l e s p re u v e s e x p 6 r im e n ta l e s e t l e m o d u le p h y s iq u e d 6 v e lo p p 6 . L e s p ro p r i6 t6 s s t ru c tu ra l e s
e t p h y s ic o -c h im iq u e s p r in c ip a le s r e f l~ te n t d e s s im i l i t u d e s & ro i t e s e t d e s g ra d a t io n s r6 g u l ib re s q u e l ' o n
p e u t a t t e n d re e n t r e d e s t e rm e s d e s u b s t i t u tio n q u i fo rm e n t u n e s 6 ri e c o n t in u e . O n d iv i s e a c tu e l l e -
m e n t l e d o m a in e d e s r a p p o r t s m o la i r e s S IO 2/A 12 0 3 (S /A ) , q u i s ' 6 t e n d d e 2 ~ t 6 , h l a v a l e u r 3 ; c e t t e
d iv i s io n e n d e u x s o u s d o m a in e s d e c o m p o s i t io n a p pa ra i't n o n ju st i f i6 e e t r i s q u e p lu t6 t d ' i n d u i re e n
e r re u r . L e s c o m p o s i t io n s in d iv id u e l l e s p r i s e s d u n s c e s d e u x s o u s d o m a in e s n e r e p r6 s e n te n t p u s d e s
ty p e s d e z 6 o l i t e s d i s t in c t e m e n t d i f f6 re n t s ; e n r e a l i t 6 , o n d 6 m o n t re q u ' i l s ' a g i t d e t e rm e s d ' u n e s & ie
c o n t in u e , a v e c d e s p ro p r i6 t6 s c h a n g e a n t g ra d u e l l e m e n t s u r t o u te l ' 6 t e n d u e S iO J A l2 0 3 & u d i6 e . C e r -
t a in e s d e s p ro p r i& & d e l a f a u ja s i t e n a tu re l l e o n t 6 t6 t ro u v 6 e s t r~ s s e m b la b le s h c e l l e s d ' a n a lo g u e s
s y n th 6 t iq u e s a y a n t l e m ~ m e ra p p o r t m o la i r e S iO 2 /A 1 2 0. v
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7/23/2019 Zeolite faujasite
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Z e o l i t e , f a u ja s i t e , s u b s t i t u tio n a l s e r i e s : a r e v ie w w i th n e w d a ta
F in a le m e n t , o n e x p r im e l e v o e u q u e s o i t c r~ 6e u n e n o m e n c la tu re u n i fo rm e q u i s e rv i r a h id e n t if i e r
d e s c o m p o s i t io n s s p 6 c i f iq u e s (u n iq u e s ) a u l i e u d e d o m a in e s d e c o m p o s i t io n s ~ r e n d re c o m p te d e l a
con t inu i tY, des s imil i tudes ~ tro i te s e t des in te rre la t ions da r ts le s p ropri~ t~s s t ruc tu ra les e t phys ico-
c h im iq u e s p r in c ip a le s A id e n t if i e r ~ g a le m e n t l e s c a ra c t~ r i s tiq u e s d u g ro u p e d e l a f a u ja s i t e .
389
K u r z r e fe ra t -N e u e E rk e n n tn i s s e b e z i ig l i c h d e r h y d ro th e rm a le n S ta bi li t~ it , d e r G i t t e rp a ra m e te r , s o w ie
d e r a d s rp t iv e n , e l e k t r i s e h e n u n d k a ta ly t i s c h e n E ig e n s c h a f t e n y o n s y n th e t i s c h e m u n d n a t i i r l i c h e m
F a u ja s i t w e rd e n m i tg e te i lt . G e g e n w i i r t i g e V o rs t e l lu n g e n , d i e d i e N a tu r u n d d a s V e rh i i l t n is y o n s y n -
th e t i s c h e m u n d n a t i i f l i c h e m F a u ja s i t b e t r e f f e n , w e rd e n a l s im E in k la n g m i t d e n e x p e r im e n te l l e n E rg e b -
n i s s e n u n d d e m e n tw o f fe n e n p h y s ik a l i s c h e n Mo d e l l b e fu n d e n . D ie w ic h t ig s t e n s t ru k tu re l l e n u n d
p h y s ik o c h e m is c h e n E ig e n s c h a f t e n s p ie g e ln d i e g rofS e A h n l i c h k e i t u n d d e n f l i e g e n d e n l ]b e rg a n g w id e r ,
d i e f i i r d ie S u b s t i tu t io n s g l i e d e r e in e r k o n t in u ie r l ic h e n R e ih e z u e rw a r t e n s ind . D ie b e s t e h e n d e A u f -
t e i lu n g d e s B e re i c h e s (2 b i s 6 ) d e r S iO d A l2 0 3 -Mo le k u la rv e rh ~ l tn i s s e (S /A ) b e i 3 in z w e i U n te rb e r
e i c h e d e r Z u s a m m e n s e tz u n g w i rd a l s u n g e re c h t fe r t ig t u n d e h e r i r r e f f ih re n d h in g e st e l i t. D ie in d iv id u e l-
l e n Z u s a m m e n s e tz u n g e n in n e rh a lb d i e s e r b e id e n U n te rb e re i c h e s t e l l e n k e in e e in d e u t ig v e r s c h ie d e n e n
Z e o l i th -A r te n d a r . E s w i rd v i e lm e h r g e z e ig t d a b s i e i n n h e rh a lb d e s u n te r s u c h te n S iO d A l2 0 ~ -B e re i c h e s
Glie der e ine r kon t inu ie r l ichen R e ihe m it fl ie l?,end wec hse lnd en E igensc haf ten shad .
E in ig e d e r E ig e n s c h a f t e n d e s n a t i i f l i c h e n Min e ra l s F a u ja s i t e rw ie s e n s i c h a l s d e n je n ig e n d e r
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