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Chapter 4 The Preparation of Molecular Sieves
77
Chapter 4
THE PREPARATION OF MOLECULAR SIEVES
. Jansen
A. S y n t h e s i s o f z e o l i t e s
J. C
B. S y n t h e s i s o f A1P04-based m o l e c u l a r s i e v e s
S.T. W i l s o n
A. S y n t h e s i s o f z e o l i t e s
J.C. Jansen U n i v e r s i t y o f Technology D e l f t , L a b o r a t o r y o f Organic Chemistry, J u l i a n a l a a n 136, 2628 BL D e l f t , The N e t h e r l a n d s
I . INTRODUCTION a . General
Nature
provided
mankind w i t h z e o l i t e s ( r e f . 1 ) . Massive z e o l i t e d e p o s i t s have
been d i s c o v e r e d a t many p l a c e s i n t h e w o r l d ( r e f . 2). The o c c u r r e n c e o f n a t u r a l zeolites
can
systems ( r e f s Natural
.
be
assigned
3,4) .
zeolites
t o c e r t a i n g e o l o g i c a l environments o r h y d r o l o g i c a l
generally
form
by
reaction
of
mineralizing
s o l u t i o n s w i t h s o l i d a l u m i n o s i l i c a t e s . The main s y n t h e s i s parameters the
c o m p o s i t i o n o f t h e h o s t r o c k and i n t e r s t i t i a l s o l u t i o n s ; pH
-
aqueous are:
(i)
10, ( i i ) t h e
t i m e ; thousands o f y e a r s and ( i i i ) t h e temperature; o f t e n < 100 'C. The
first
systematic
s t u d i e s on z e o l i t e s y n t h e s i s c o u l d t h u s be g u i d e d by
t h e g e o l o g i c a l and m i n e r a l o g i c a l f i n d i n g s o f t h e n a t u r a l s p e c i e s ( r e f . 5 ) . From 1946
on
many a d d i t i o n a l z e o l i t e t y p e s w i t h o u t a n a t u r a l c o u n t e r p a r t have been
s y n t h e s i z e d ( r e f . 6 ) . The e v o l u t i o n i n t h e
preparation
of
one
of
the
most
s t u d i e d z e o l i t e s i s i l l u s t r a t e d i n F i g u r e 1 by t h e number o f papers and p a t e n t s on t h e m a t e r i a l denoted as ZSM-5 ( r e f . 7 and S e c t i o n
XI1 o f t h i s c h a p t e r ) .
78
NUMBER OF REPORTS
25 20-
15-
L
10-
-
.
5-
o = m n , r - , I
1972
1975
,
,
,
,
,
1985
1980
1990
Fig. 1. The annual number of papers ( 0 )and patents (B) on the preparation of zeolite ZSM-5 since the first publication in 1972.
Throughout the last four decades molecular sieves were mainly prepared by precipitationlcrystallization of an aqueous mixture of reagents at 6 < pH < 14 and temperatures between 100-200 OC. As shown in Scheme 1 a relatively large effort is needed on the optimal preparation procedure of the reactant mixture, EFFORT
1
3
Scheme 1. The effort for the preparatory isolation (3) versus time.
(1).
TIME
the reaction (2) and the
whereafter the hydrothermal reaction process ( 2 ) runs autoclaved for a few days or weeks without manual intervention. Isolation ( 3 ) o f the crystalline material i s a simple final step in the synthesis procedure.
79
of
The z e o l i t e s y n t h e s i s f i e l d i s n o t o n l y extended and r e f i n e d by u s e f u l d a t a modern z e o l i t e c h a r a c t e r i z a t i o n and a p p l i c a t i o n t e c h n i q u e s b u t a l s o by
i n t e r f a c i n g areas o f p h y s i c a l , chemical and mathematical science, see Scheme 2 .
NMR Modern chemicals and p h y s i c a l methods
Nucleation/
Computational
c r y s t a l 1 iz a t i o n
model ing
Scheme 2. Areas o f chemical, p h y s i c a l and mathematical
science
interfacing
the z e o l i t e preparation f i e l d .
Studies i n the
sol-gel
chemistry
and
NMR
analysis
area
have
contributed
s u b s t a n t i a l l y t o t h e knowledge o f t h e hydrothermal r e a c t i o n process. Papers and r e v i e w s r e g a r d i n g s u b j e c t s w i t h i n t h e d i f f e r e n t areas
which
are
mentioned i n Scheme 2 and which a r e o f i n t e r e s t f o r z e o l i t e s y n t h e s i s a r e g i v e n i n Table 1. Besides
the
annually
efforts o f "zeolite synthesis
of
porous
new z e o l i t e
scientists"
in
the
preparations t h e extensive exploratory last
decade
has
resulted
in
the
m a t e r i a l s l i k e t h e A1P04-group ( p a r t B o f t h i s c h a p t e r ) ,
the metal-sulfides ( r e f .
30)
and
the
clathrasils
(ref.
31).
Accordingly,
z e o l i t e s y n t h e s i s appears t o remain a p r o m i s i n g area f o r f u t u r e r e s e a r c h . The c r y s t a l l i n i t y o f d i f f e r e n t s y n t h e s i s p r o d u c t s Plate
1.
The
is
well
illustrated
in
morphology and forms o f t h e c r y s t a l s g i v e a f i r s t i n d i c a t i o n o f
t h e t y p e o f z e o l i t e p r e s e n t and t h e p u r i t y o f t h e p r o d u c t .
80
Table 1. Examples of subjects from areas o f physical, chemical and mathematical science which delivered contributions t o the knowledge o f t h e zeolite synthesis process together with references. Area
Subject
Reference
Sol -gel chemistry
Hydrolysis and condensation o f silicates The sol-gel process
8,9 10
NMR
Structure of (a1umino)silicate-clusters in solution
11,12
Computational model i ng
Lattice energy calculation Local interactions in lattice
13
Modern chemical and physical methods
A1 koxides as reagents Fluorides as reagents and mineralizing agents Gravity - reduced - elevated CVD (chemical vapour deposition) Microwave
15
Nucleationlcrystallization theorypractise Zeal i t e Characterization Appl i cat i on
14
16 17
18 19 20
Mathematical analyses o f zeolite Crystallization. A review Are the general laws o f crystal growth applicable to zeolite synthesis
21
ZSM-5/- 1 1 intergrowth Catalysis - The catalytic site activity - The catalytic properties and the crystal size
23
22
24,25,26 27
81
b . T h i s chapter I n t h i s p a r t of t h e c h a p t e r t h e p r e p a r a t i o n o f two subgroups o f t h e micro-porous t e c t o s i l i c a t e s (see Chapter 3 ) i.e. t h e a l u m i n o s i l i c a t e s and s i l i c a t e s , b o t h i n c l u d i n g t h e c l a t h r a s i l s , w i l l be present ed. The d i v i s i o n between a l u m i n o s i l i c a t e s
and
silicates
i s o f t e n discussed on A l - p o o r r a t h e r
th an A l - f r e e l e v e l ( r e f . 3 0 ) . The
a l u m i n o s i l i c a t e s , s t a r t i n g f r o m Si/A1 r a t i o 1 up t o e.g. Si/A1 r a t i o o f
10000, do r e v e a l t h e presence o f A1 i n s y n t h e s i s , i n c h a r a c t e r i z a t i o n
as
well
as i n a p p l i c a t i o n , see F i g . 2 ( r e f . 31). The A l - p o o r zeo i t e s show no, a t l e a s t no d e t e c t i b l e , Al-dependent behaviour and a r e t h e r e f o r e , t o g e t h e r w i t h t h e A l - f r e e m a t e r i a l s , denoted as s i l i c a t e s . The presence o f aluminium, t h e g u e s t - h o s t i n t e r a c t i o n and t h e n u c l e a t i o n and crystallization
11
contribute
to
the
synthesis
event s
which
are
c h r o n o l o g i c a l l y d e s c r i b e d i n S e c t i o n s I 1 t o V I I o f t h i s chapt er. S e c t i o n V I I I i s focussed on t h e r e a c t i o n parameters. I n S ec t io ns I X and X t h e s i l i c a t e s and c l a t h r a s i l s a r e present ed. Examples
of
research
syntheses
performed w i t h c e r t a i n procedures and/or
m i x t u r e c omp os it i o n s a r e l i s t e d i n S e c t i o n X I . S ec t io ns X I 1 and X I 1 1 c o n t a i n l i t e r a t u r e sources on z e o l i t e p r e p a r a t i o n s and t h e re f e re nc es , r e s p e c t i v e l y .
10
F i g . 2. The
100
1000
10000
r e l a t i v e c a t a l y t i c a c t i v i t y o f H-ZSM-5 versus A1 c o n t e n t on ppm
s c a l e ( r e f . 31).
82
Plate 1. The crystalline nature o f zeolites. a) Single crystals o f zeolite A and b) and c) o f analcime and o f natrolite, respectively. d) A batch o f zeolite L, e) typical needle aggregates o f zeolite mordenite and f ) o f Nu-10.
83
I I . PREPARATORY a. R e a c t a n t s The
chemical
sources
which a r e i n p r i n c i p l e needed f o r z e o l i t e syntheses a r e
g i v e n i n Table 2. T a b l e 2. Chemical sources and t h e i r f u n c t i o n i n z e o l i t e s y n t h e s i s . Sources
Function(s)
Si02
P r i m a r y b u i l d i n g u n i t ( s ) o f t h e framework
A102
O r i g i n o f framework charge
OH-
Mineralizer, guest molecule
A1 k a l i c a t i o n , t e m p l a t e
C o u n t e r i o n o f framework charge, g u e s t m o l e c u l e
Water
Solvent, guest molecule
Within
each
t y p e o f source a v a r i e t y o f chemicals ( r e f . 32), has been used as
t h e d i f f e r e n c e s i n p h y s i c a l n a t u r e and chemical i m p u r i t i e s the
zeolite
synthesis
kinetics
(ref.
34)
and
strongly
influence
sometimes t h e p r o p e r t i e s as
c a t a l y s t s ( r e f s . 24-27). Data
on
t h e s p e c i f i c a t i o n s o f r e g u l a r l y used chemical sources a r e g i v e n i n
t h e f o l l o w i n g survey. - SiO -sources
2
Recent s y n t h e s i s
papers
of
the
Proceedings
of
the
International
Zeolite
Conferences ( r e f s . 34-37) and o f o t h e r z e o l i t e conferences ( r e f s . 38-40) r e v e a l t h a t f o r l a b o r a t o r y s c a l e p a r t i c u l a r S i - s o u r c e s a r e o f t e n used, see T a b l e 3a. Depending
upon
the p a r t i c u l a r synthesis a c e r t a i n Si-source might favour a
s p e c i f i c c r y s t a l l i z a t i o n . For i n s t a n c e , t h e A e r o s i l 200 p r o d u c t can be dissolved
compared
to
the
Optipur
and
d i f f e r e n c e i n p a r t i c l e s i z e , see F i g u r e 3. influence
the
rate
of
nucleation
Gold As
the
impurities
are
more
than
rate
of
dissolution
can
and c r y s t a l l i z a t i o n ( r e f . 41) t h e p r o d u c t
f o r m a t i o n can be a f f e c t e d . A t t h e same t i m e t h e A1 replacing
readily
l a b e l m a t e r i a l because o f t h e
10000
times
and
other
potentially
Si
h i g h e r i n t h e A e r o s i l 200
p r o d u c t compared t o t h e O p t i p u r and Gold l a b e l m a t e r i a l s . The
influence
of
i m p u r i t i e s can change t h e c r y s t a l f o r m ( r e f . 42) and t h e
chemical p r o p e r t i e s ( r e f s . 2 4 - 2 6 ) .
84
Ta ble 3. S p e c i f i c a t i o n s and t h e r e f e r e n c e s o f r e c e n t , r e g u l a r used sources, and high-p u r e ,
*
S i - , and
*
Al-sources.
Specifications S i -s o urc e ( a )
Phys.
Reference
Chem.
A1 -s o urc e ( b )
manufacturer
i m p u r i t i e s (ppm)
Si 7 icon compounds S i (OCH3)4
Tetramethylorthosilicate
liquid
Na,Ca < . 5
liquid
A1,Pt < .2
Merck
(TMOS) S i (OC2H5 14 Tetraethylorthosilicate (TEOS)
Na2Si03.9H20
A1 < 200
"
( o r Na2H2Si04.8H20)
Fe < 120
Q u a r t z Co.
Na20 11%, S i 0 2 29% Water g l a s s
T i < 60
Colloidal silica L u ~ o x - A S- 40 SiO
NH4
I
heavy m e t a l s < 50
A1 < 500
sol
DuPont de Nemours
2r
40 w t % (counterion)
Fe
LU ~ O X -H S -4 0
< 50
Ti B < 10
Si0 2 40 w t % Na'
N" Phi 1 id e l p h i a
(counterion)
Fumed silica A e ro s i 1-200
Dp
-
A1 < 10 12 nm
CAB-0-SIL M-5
* Silica Optipur Gold l a b e l
- 200 pm Dp - 800 pm
Dp
Fe < .6
Degussa
T i < 10
BDH
A1 < .001
Merck
Fe < 0.01
Aldrich
85 Table 3, c ont in u e d
-
Riedel de Hahn
NaA102 Fe < 4
Na20 54%
C a r l o Erba BDH L t d .
Sodium alumin at e
- AlOOH Pseudo-boehmite A1203 70% H20 30%
Dp
- mm Fe < 4 T i < 40
Vista
Fe < 3
Merck
Fe < 0.01
Baker
Catapal - B
-
A1(OH)3
G i bbsi t e
- A1 (N03)3.9H20
Dp
- A1203
*
-
nm
Aluminiumoxide ( U l t r e x )
T h ere f o re , a c a r e f u l c h o i c e o f t h e r e a c t a n t s i s
needed.
The
h i g h grade
Si-
a l k o x i d e s o f which even double a l k o x i d e s l i k e -Si-0-A1- a r e a v a i l a b l e ( r e f . 43) do n o t
have
the
above
d i s c u s s e d disadvantages,
except
for
the
rate
of
h y d r o l y s i s o f t h e a l k o x i d e groups.
F i g . 3.\ SEM photographs of (a) O p t i p u r , (b) Gold l a b e l , and (c) A e r o s i l 200.
86
- A102 Often
source used
Al-sources,
collected
from
t h e same r e f e r e n c e s as g i v e n f o r t h e
S i - s o u r c e s , a r e l i s t e d i n T a b l e 3b t o g e t h e r w i t h t h e
main
chemical
impurity.
Though t h e v e r y p u r e A1203 p r o d u c t c o n s i s t s o f s m a l l p a r t i c l e s i t i s n o t e a s i l y d i s s o l ved. - Alkali c a t i o n / t e m p l a t e The i n o r g a n i c c a t i o n s i n t h e z e o l i t e s y n t h e s i s a r e m a i n l y a l k a l i n e o r ions.
ammonium
The o r g a n i c c a t i o n s / t e m p l a t e s used may be d e v i d e d i n charged and n e u t r a l
molecules c o n t a i n i n g f u n c t i o n a l atoms o r groups. The l a r g e
number
of
organic
molecules
listed
in
several
p u b l i c a t i o n s ( r e f s . 44, 45) t o g e t h e r w i t h t h e s p e c i f i c z e o l i t e p r o d u c t
formed.
To
used
illustrate
in in
zeolite
synthesis
is
extensively
g e n e r a l t h e v a r i a t i o n i n o r g a n i c t e m p l a t e m o l e c u l e s some o f
t h e more common t e m p l a t e s a r e l i s t e d i n Table 4. -
on-
Most z e o l i t e syntheses a r e performed under b a s i c mineralizing
agent.
A
second
agent
is
F-
conditions (refs.
using
OH-
as
a
16 and 46) o f which t h e
d i f f e r e n t n a t u r e compared t o OH' w i l l be d i s c u s s e d i n t h e s e c t i o n
on
reaction
parameters. Both anions a r e t h e c o u n t e r i o n o f t h e i n o r g a n i c o r o r g a n i c c a t i o n s used the
for
syntheses. Depending upon t h e q u a l i t y o f t h e m i n e r a l i z i n g agent i m p u r i t i e s
such as A13'
and Fe3' a r e p r e s e n t a t ppm un t s c a l e .
- The overall
reactant mixture
I n q e n e r a l t h e chemical b e h a v i o u r o f imDuri i e s 1 ike Fe3' importance
compared
to
A13'
in
high
Si/A1
zeolites
c a t a l y s i s when based on Bronsted a c t i v i t y . However,
in
and T i 4 + a r e o f m i n o r i n t h e heterogeneous the
case
of
an
all
s i l i c a z e o l i t e , o r m o d i f i e d z e o l i t e s l i k e B-ZSM-5, Fe-ZSM-5 and Ga-ZSM-5 t r a c e s o f A13+ f r o m r e a c t a n t s as g i v e n i n T a b l e 3 may p l a y an unexpected dominant r o l e
in
t h e B r o n s t e d a c t i v i t y ( r e f s . 24-26). E x t e n s i v e i n f o r m a t i o n on t h i s p o i n t i s
g i v e n i n Chapter 5 on t h e m o d i f i c a t i o n o f influence
of
zeolites.
i m p u r i t i e s f r o m r e a c t a n t s i s K'.
r e t a r d e d by f a c t o r s when
'K
Another
example
of
the
The c r y s t a l l i z a t i o n t i m e can be
i s p r e s e n t i n t h e syntheses o f e . g . z e o l i t e Na-A o r
Na-ZSM-5 ( r e f s . 4 7 , 4 8 ) . I m p u r i t i e s l i k e t r i v a l e n t metal i o n s sometimes change p h y s i c a l c o n d i t i o n s i n t h e r e a c t i o n m i x t u r e i n d i c a t e d by t h e c r y s t a l f o r m o r morphology ( r e f . 42).
87
Table 4. Type of o r g a n i c templates, Organic
f u n c t i o n a l atoms/groups and r e f e r e n c e s .
-__
F u n c t i o n a l atom/group
templ a t e
Ref.
Organic
F u n c t i o n a l atom/group
Ref
templ a t e
-
~
amine
49
n
-NC" n=4,5
N
50 51
3
52
d i -amine
N
~
N 53
54
ammon ium ' P C
' w n n=4,5
-is -+n Cn/NLCn n=4,5
G5
I
I
C-OH
e r y t h r i to1
:N- ( Cn-OH) n=2,3
amine t alcohol
66
)(
~=1-3 ammonium
I
t
a1 cohol
67
-~+-c,-oH n=2
55 acetal
5G amine
-NGO
t ether
69
57 58
di-ammonium
C-OH HO-C-C-C-OH
penta-
N - o x i d e t ammonium
59 phosphonium tri-ammonium
+ A + _ 70
-0-NGN
71
I
-P+I
60
NA +G N
amine t ammonium a1 cohol
Cn-OH
HO-Cn-OH
63 di-phosphonium
n=2-6 HY
tri -01
72
62
n=1-6 d i -01
61
YH
c-c-c I OH
64
-!-C6-y I+
-
I+
73
88
b . The r e a c t i o n v e s s e l and h y d r o t h e r m a l c o n d i t i o n s
Depending OC,
upon
the
r e a c t i o n t e m p e r a t u r e chosen, m a i n l y between 60 5.
r e a c t i o n v es s e l s can v a r y as shown i n T able
aut o c lav es ,
Various
and 300
OC
Teflon
inserted
see F i g . 4a, t o g e t h e r w i t h r e l a t i v e l y l o w p r i c e d ( s t a i n l e s s s t e e l )
and h i g h p r i c e d ( r e e n f o r c e d p o l y e t h e r e t h e r k e t o n ) a u t o c l a v e s a r e shown
in
Fig.
4b and 4c. To f o l l o w t h e c o u r s e o f t h e events t a k i n g p l a c e i n t h e s y n t h e s i s m i x t u r e ,
a
l o o k t h r o u g h aut o c l a v e , see F i g . 4d, can be used. I t was concluded, u s i n g such ex periment a l c o n d i t i o n s , t h a t n u c l e a t i o n and c r y s t a l l i z a t i o n o f z e o l i t e ZSM-5 o c c urred on and in, r e s p e c t i v e l y , g e l spheres o f about 2 mm ( r e f . 42). Another p o s s i b i l i t y t o m o n i t o r s y n t h e s i s event s i n s i t u i s of
IR
the
application
i n t e r n a l r e f l e c t a n c e u s i n g a c r y s t a l embedded i n an aut oclave, as shown
i n F i g . 5 ( r e f s . 74, 7 5 ) .
T a ble 5. Regular
used
lab-scale
r e a c t i o n vessels, t h e t y p i c a l i m p u r i t i e s and
t e mp er a t u r e range. Re ac t io n v es s el
Volume
Impurity
Temperature
P1 a s t i c b o t t l e
( 1 1
Zn2+
Stainless s t e e l autoclave S t a i n 1 ess s t e e l t
< 5 1
Fe3',Cr3'
< 2 1
n u c l e i o f preceding
< 100 o c >> 200 o c < 200 O C
< 5 ml
Si
teflon lining
synthesis
Quartz a u t o c l a v e
< 200
oc
The a ut o c lav es must be f i l l e d between 30 and 70 v o l % i n t h e case o f an aqueous r e a c t i o n m i x t u r e between 100 and 200 O C t o m a i n t a i n a l i q u i d phase ( r e f . 7 6 ) . C leaning o f t h e r e a c t i o n v e s s e l s can be considered i n some cases, teflon-lined
au t o c l a v e s .
As
memory
synthesis i n c a v i t i e s o f the t e f l o n ex periment s
it
is
important
effects wall
can
caused be
by
e.g.
the n u c l e i o f preceding
encountered
in
subsequent
t o c l e a n t h e vessel w i t h e i t h e r HF and w a t e r a t
room t emp era t u re o r NaOH and w a t e r a t t h e r e a c t i o n t emperat ure.
89
1
MACROSCOPE
Fig. 4. Different autoclaves for laboratory use. a) Teflon lined autoclaves u p t o 1000 ml, b ) s t a i n l e s s steel autoclave of 25 ml, c) "Arlon" (polyetheretherketon) reenforced with carbon fiber or glass f i b e r ) autoclaves and d ) s t a i n l e s s steel look through autoclave with quartz windows and Teflon inserts in exploded view together with a schematic drawing of the experimental s e t up.
' R beam
Fig. 5. Parr mini autoclave. I R internal reflection via a crystal embedded in the autoclave makes monitoring of z e o l i t e synthesis events possible ( r e f . 74).
90
111. ZEOLITE PRODUCT VERSUS THE SYNTHESIS MIXTURE a. Two s y n t h e s i s examples The
f i n e t u n i n g and d i f f e r e n c e s i n t h e p r e p a r a t i o n o f each z e o l i t e t y p e i s t o o
complex t o be d is c u s s e d i n t h i s i n t r o d u c t i o n on t h e s y n t h e s i s o f specific
parts
of
the
chapter,
however,
is
chosen
for
a
zeolites. In more d e t a i l e d
p r e s e n t a t i o n o f t h e s y n t h e s i s o f two s u b s t a n t i a l d i f f e r e n t z e o l i t e t ypes, zeolite
Na-A
and
i.e.
z e o l i t e TPA-ZSM-5. The two z e o l i t e s p r e s e n t r o u g h l y a l l t h e
groups i n which z e o l i t e t y p e s a r e d i v i d e d ( r e f . 77). The s y n t h e s i s m i x t u r e s and chemical and p h y s i c a l p r o p e r t i e s o f b o t h z e o l i t e s a r e g i v e n i n T able 6. T able 6. The s y n t h e s i s m i x t u r e s , p h y s i c a l and chemical p r o p e r t i e s
of
zeolites
Na-A and TPA-ZSM-5 ( r e f s . 78-80). Na-A
TPA-ZSM-5 - An example o f s y n t h e s i s m i x t u r e s -
(molar oxyde r a t i o ) 1
Si02
.5
2'3 Na20 TPA20
1 17
H2°
1
< .14 .16 .3 49
> 150
- P h y s i c a l and chemical p r o p e r t i e s -
3D, h o l e s connected v i a windows 1.28 .37
Na',
H20
p o r e arrangements
3 d e n s i t y (g/cm ) p o r e volume (cm3/ g)
ZD, i n t e r s e c t i n g
channels 1.77 .18
1 a t t ic e s t a b i 1iz a t ion
TPA'
1
Si/A1
>
1 ow
Bronsted a c t i v i t y
high
hy dro phy l i c
affinity
hydrophobic
12
91
b. Z e o l i t e product versus synthesis mixture
The most simple zeolite product composition can be given by the overall Si/A1 ratio and the cation type/content. More often the unit cell composition of the zeolite crystal is expressed, e.g. Na-A:
Na12[A112Si12048].27
H20
*
At higher loadings than 4 Al/uc, is replaced by the smaller cation Na' (ref. 81).
TPA' TPA-ZSM-5: 4 TPA[AlnSig6-n0192]H20 * n 5 4
The zeolite reaction mixture i s often formulated in the molar oxyde ratio of the reactants, e.g. SiO2:Al2O3:Na20:(TPA20):H2O. The ratios o f H20/Si02, OH-/Si02, Si02/A1203 and (Na20/TPA20) then give an impression of the concentration, solubility and the expected zeolite types, respectively (ref. 82). Correlation between the synthesis mixture and the product can be obtained from ternary composition diagrams (see Fig. 6a,b) (refs. 83-86), or from graphs of crystallization fields of zeolite types as a function of reactant ratios, see Fig. 6c and Section XI.b.3 SIO) A
a
t ilOl
Fig. 6. Zeolite product versus the synthesis mixture. a) and b ) , ternary compositon diagrams with an inorganic and organic cation/template, respectively. c) Crystallization fields, indicating (0) ZSM-5, (m) ZSM-35, and (A) ZSM-39 (ref. 87).
92
The p r o d u c t f i e l d s a t c e r t a i n P,T, experiment a l
data
depicted
in
Fig.
are
6,
obtained
which a r e n o t always expected f rom a thermodynamic p o i n t o f
view. As t h e i n e v i t a b l y heterogeneous s y n t h e s i s m i x t u r e c o n t a i n s with
different
f rom
micro-domains
r e a c t a n t r a t i o s , k i n e t i c parameters m i g h t induce o t h e r p r o d u c t
phases t h a n t h o s e d e r i v e d f r o m t h e t e r n a r y s y n t h e s i s c o m p o s i t i o n diagram. Because p a r t i c u l a r l y t h e n u c l e a t i o n i s k i n e t i c a l l y det ermined i t i s t hus o f i n t e r e s t t o understand t h e d i f f e r e n t f a c t o r s , e.g. t y p e o f S i - s o u r c e , c a t i o n , A1-source, a d d i t i v e s and p h y s i c a l parameters, i n f l u e n c i n g t h e k i n e t i c st age o f n u c l e a t i o n . The i n f l u e n c e o f t h e s e f a c t o r s can be recognized i n t h e events o f t h e z e o l i t e hereafter i n detail.
T a ble 7 . The subsequent preparation.
subsequent
p r e p a r a t i o n which a r e g i v e n i n T able 7 and discussed
events
present
i n the
Temperature
Subsequent e vent s
Low (< 60 OC)
Reactant s o l u t i o n s
course
of
the
zeolite
Reactant m i x t u r e - g e l f o r m a t i o n Low h i g h (< 60 O C < 200 O C ) +
+
Gel rearrangement Dissolution o f gel Dissociation o f s i l i c a t e
High (< 200
OC)
P r e - n u c l e a t i o n phase Nucleation C r y s t a l l i z a t on
Low (< 60 OC)
I s 0 1a t i o n
I V . THE LOW TEMPERATURE REACTION MIXTURE a. Introduction
The r e a c t i o n m i x t u r e e v e n t s o c c u r r i n g a t l o w t emperat ure disc u s s ed f o r two reasons.
(<
60
'C)
will
be
93
i) Reaction mixtures are prepared at low temperature. Drastic chemical and
physical changes take place then. i i ) Substantial knowledge about the zeolite reaction mixture at low temperature has been obtained using characterization methods such as the molybdate method (ref. 88), the paper chromatography method (ref. 89), the tri-methylsilylation method (ref. go), IR- and laser-Raman spectroscopy (ref. 91), single crystal structure analysis (refs. 92, 93) and the NMR technique (ref. 94). Mostly, starting reaction mixtures typically consist of a gel phase and a liquid phase which means that nucleation is initiated at high temperature by the presence o f a residual gel phase, though there are a few exceptions (refs. 91, 95). The (a1umino)silicate gel phase consists of either a homogeneous dispersed phase of branched chains of sol particles, see Fig. 7I, or a more separated solid phase of an ordered aggregate of sol particles (like opals), see Fig. 711(refs. 96, 97).
I
I1
Schematic representation
Micrograph picture
Fig. 7. Alkaline gel forms. Schematic representation and micrograph picture of I ) a dispersed low density gel (ref. 96b) of branched chains of sol particles and 11) a separated high density gel form resulting in spheres consisting of an ordered aggregate o f sol particles (like opals)
.
94
Si(IV)
Si(lV) O h
80 60 40
20 0
a)
6
a
10
12
pH
O.lm Si(IV)
Fig. 8. S i l i c a t e d i s t r i b u t i o n v e r s u s pH a t c o n c e n t r a t i o n ( r e f . 98).
a)
high
and
b)
low s i l i c a t e
The pH o f t h e l i q u i d phase, i n t h e case o f OH- as t h e m i n e r a l i z i n g agent, l i e s g e n e r a l l y between 8-12. As d e p i c t e d i n F i g . 8 t h e most abundant f orm(s) o f S i sp ec ies a t r e l a t i v e l y h i g h pH a r e t h e monomeric i o n s , whereas a t l o w e r pH v a l u e monomeric n e u t r a l S i - s p e c i e s can be formed, when t h e S i - c o n c e n t r a t i o n i s low. A t h i g h c o n c e n t r a t i o n , however, c y c l i c
tetramers
a r e most
abundant
species
( r e f . 98).
b . Hydrolysis and condensation o f silicate Monomers
and
ol i g o m e r s
i n s o l u t i o n a r e i n e q u i l i b r i u m w i t h t h e g e l phase. A t
t h i s ambient s t ag e o f t h e r e a c t i o n m i x t u r e released e.g.
from
monomeric
silica
species
can
be
t h e g e l v i a h y d r o l y s i s r e a c t i o n s and a r e p r e s e n t i n s o l u t i o n as
Si(OH)30- and Si(OH),022-.
The d i s s o l u t i o n o f t h e g e l i s promoted by
the
OH'-coordination o f s i l i c o n above f o u r , t h u s weakening t h e o t h e r s i l o x a n e bonds t o t h e g e l network. T h i s n u c l e o p h i l i c mechanism i s p r e s e n t e d t o o c c u r v i a a SN2-Si t r a n s i t i o n s t a t e as shown i n Scheme 3a ( r e f . 9 ) .
95
Scheme 3. a) H y d r o l y s i s and b) condensation mechanism o f s i l i c a t e species a t room temperature.
gel phase
1 Relative
<
branching
> of SiOH
Scheme 3c. Growth s i t e i n t h e g e l phase f o r monomers from s o l u t i o n .
The mechanism o f t h e condensation r e a c t i o n s
in
aqueous
systems
at
high
pH
i n v o l v e s t h e a t t a c k o f a n u c l e o p h i l i c d e p r o t o nat ed s i l a n o l group on a monomeric n e u t r a l species as r e p r e s e n t e d i n Scheme 3b ( r e f . 9 ) . The a c i d i t y o f t h e s i l a n o l group depends on t h e number and t y p e of s u b s t i t u e n t s on t h e s i l i c o n - a t o m . The more s i l i c o n s u b s t i t u e n t s a r e present , the
more
a c i d i c t h e OH-groups o f t h e c e n t r a l s i l i c o n atom. As shown i n Scheme
3c, a t h i g h pH t h e most f a v o u r a b l e p o l y m e r i z a t i o n i s t h e r e a c t i o n between l a r g e most h i g h l y branched species and t h e monomer s i l i c a species. A t more
specific
neutral bonding
pH,
hydrolysis
configurations,
and see
condensation
of
pe nt a c oord inat e s t a t e o f S i , i l l u s t r a t e d i n Scheme 3, i s 99 ).
clusters,
containing
F i g . 9, i n d i c a t e t h a t i n v e r s i o n i n t h e not
essential
(ref.
96
+ I
I
+
F ig . 9. Condensation o f octamers,
The
pentacoordinate
silicon
H20
with r e t e n t i o n o f t h e configuration.
intermediate
state
is,
however,
conf irmed
c r y s t a l l o g r a p h i c a l l y ( r e f . 100). Condensation
of
m i x t u r e i s above t h e
the
monomers
isoelectric
lead, point
as of
the silica
pH o f t h e z e o l i t e s y n t h e s i s
(ref.
t o ramified c l u s t e r s can be r e o r g a n i z e d i n t o fewer l a r g e r p a r t i c l e s w i t h a
c l u s t e r s . Such co rres ponding r e d u c t i o n i n s u r f a c e energy, according t o
the
101)
Ostwald
ripening
p r i n c i p l e . The s t r u c t u r a l e v o l u t i o n o f a g r owing c l u s t e r i s s c h e m a t i c a l l y g i v e n i n F i g . 10.
F i g . 10. S t r u c t u r a l e v o l u t i o n o f s i l i c a t e c l u s t e r s .
97
c. Evidence f o r silicate clusters In the course of the gel dissolution the monomers form dimers, according to 29Si-NMR studies (ref. 94), via condensation reactions whereafter trimers and tetramers, cyclic trimers and tetramers and higher order rings are observed as condensation products, see Fig. 11.
Fig. 11. Numerous oligomers characterized in solution at low temperature by *'Si -NMR (ref. 101).
Evidence for the existence of e.g. double four rings resulted from the s ngl e crystal structure analysis of so-called pseudo-A, a material, not a zeo i t e , crystallized at ambient temperature from a mixture of SiOp, TBAOH and H20 see Fig. 12 (ref. 93).
Fig. 12. Model of a part of the framework of pseudo-A; the double four ring units are indicated by asterisks (*).
The silicate species identified in the liquid phase by a.0. NMR, SAXS (ref. 102) and IR, (ref. 103) are products in a simple reaction mixture of Si02, NaOH and H20 at room temperature.
98
The i n t e r a c t i o n o f a l k a l i - i o n s i n such suggested
(ref.
systems
is
not
clear.
is
It
104) t h a t t h e o r d e r e d h y d r a t i o n sphere o f a.0. Na'
often
stabilizes
s i l i c a t e species. Recent NMR r e s u l t s i n d i c a t e t h a t i n t e r a c t i o n between
cations
and s i l i c a t e s p e c i e s ( r e f . 105) do occur. An o r g a n i c c a t i o n / t e m p l a t e added as i n g r e d i e n t ( s ) mixture
to
the
simple
reaction
shows i n t y p i c a l experiments a c c o r d i n g t o NMR measurements i n t e r a c t i o n
w i t h t h e g e l and s i l i c a t e species, r e s p e c t i v e l y ( r e f s . 106-108). However, t h e h i g h l y c o m p l i c a t e d s e t o f i n t e r a c t i o n s and f a s t changing e q u i l i b r i a , due t o t h e i n c r e a s e d number o f t y p e o f s p e c i e s a f t e r a d d i t i o n o f t e m p l a t e and/or A13'
has
n o t been u n r a v e l l e d y e t .
V. THE TEMPERATURE RAISE OF THE REACTION MIXTURE Temperature
raise,
< 60
from
OC
up t o < 200 O C , can be p e r f o r m e d i n s e v e r a l
ways as shown i n F i g . 13 f o r one t y p e o f a u t o c l a v e and
reaction mixture.
The
d i f f e r e n t h e a t i n g r a t e s a r e achieved i n s t a t i c systems. 200 'C
100
-
501// 0
0
5 ml
t (min)
2
4
6
8
10
12
F i g . 13. D i f f e r e n t h e a t i n g r a t e s f o r one t y p e o f a u t o c l a v e a c h i e v e d
by
(a)
microwave, (b) h o t sand b a t h and (c) h o t a i r oven. The s i z e o f t h e a u t o c l a v e , t h e v i s c o s i t y o f t h e r e a c t i o n m i x t u r e and t h e way o f a g i t a t i n g e.g.
static,
tumbling
o r t u r b o s t i r r i n g a r e f a c t o r s modulating t h e
temperature r a i s e o f t h e r e a c t i o n mixture. During
the
temperature
raise
of
the
reaction
mixture
f r o m ambient t o
r e a c t i o n c o n d i t i o n s p r i m a r y events a r e : - A c c e l e r a t e d d i s s o l u t i o n o f t h e g e l i n t o monomeric s i l i c a t e species. - D i s s o c i a t i o n o f s i l i c a t e o l i g o m e r s i n s o l u t i o n and i n c r e a s e
measured
by
NMR up t o
-
of
monomers
as
100 O C ( r e f s . 109-112). As shown i n F i g . 14 a model
s t u d y w i t h NMR on t r i m e t h y l s i l y l a t e d s i l i c a t e c o n f i r m s ( r e f . 109) a s h i f t o f t h e s i l i c a t e anion e q u i l i b r i u m f r o m r e l a t i v e h i g h - m o l e c u l a r , m a i n l y d o u b l e f o u r r i n g s , t o l o w - m o l e c u l a r w e i g h t , monomers and dimers.
99 ( % I mol
.
.
y c u b i c oc!amer \
€0
\
monomer,
R
* \ :
‘\
- --.
dim-er0 0
20
60
40
00
100
‘C
changes i n c o m p o s i t i o n
F i g . 14. Main
(X mol) o f t r i m e t h y l s i l y l a t e d s i l i c a t e
s o l u t i o n versus temperature.
- Higher
concentration
and
mobility
of
monomeric
s i l i c a t e - and e v e n t u a l l y
a l u m i n a t e species. - Association o f primary b u i l d i n g u n i t s . - P o s s i b l e n u c l e a t i o n and c r y s t a l l i z a t i o n o f unwanted ( m e t a s t a b l e ) phases. Some secondary events a r e : - The s t a r t o f t h e d e g r a d a t i o n
o f q u a t e r n a r y ammonium i o n s , which s u b s t a n t i a l i n a ZSM-5 s y n t h e s i s ( r e f . 4 2 ) as d e p i c t e d i n F i g . 15.
- S t a r t o f t h e d r o p i n pH caused by t h e Hoffman d e g r a d a t i o n .
100.; \
%
:TPA
\
t
i
\
50
‘, .‘A
.. .
- - t r - - - - > _ - -
- - - -._ -
-_
0. . -‘(hr)
F i g . 15. D e g r a d a t i o n o f tetrapropylammonium v e r s u s t i m e .
can
be
100
V I . THE HIGH TEMPERATURE REACTION PROCESS
a. Introduction reaction
The main e v e n t o c c u r r i n g i n t h e s y n t h e s i s m i x t u r e a t t h e is
the
formation
of
zeolites
from
amorphous
material.
temperature
Chemical r e a c t i o n
processes a c c e l e r a t e d by t h e h i g h t e m p e r a t u r e l e a d t o :
i) f u r t h e r r e o r g a n i z a t i o n o f t h e l o w t e m p e r a t u r e s y n t h e s i s m i x t u r e ; ii)w h e r e a f t e r p r i m a r y (homogeneous
or
c r y s t a l s ( r e f . 113)) n u c l e a t i o n ; iii) and f i n a l l y , p r e c i p i t a t i o n (based
heterogeneous) on
and
reactions)
secondary as
a
(seed
form
of
crystallization.
b. Nucleation At
t h e high temperature o f t h e r e a c t i o n mixture the z e o l i t e c r y s t a l l i z a t i o n i s
expected a f t e r an i n d u c t i o n p e r i o d i n which t h e n u c l e a t i o n o c c u r s . induction period the
gel
During
the
and s p e c i e s i n s o l u t i o n ( a f o r e m e n t i o n e d i n t h e l o w
t e m p e r a t u r e s e c t i o n ) r e a r r a n g e from a c o n t i n u o u s changing phase o f monomers and clusters,
e.g.
polysilicates
and
aluminosilicates.
These c l u s t e r s f o r m and
d i s a p p e a r t h r o u g h i n h o m o g e n e i t i e s i n t h e s y n t h e s i s m i x t u r e v i a c o n d e n s a t i o n and hydrolysis
processes.
The
c o n t i n u o u s d i s s o l u t i o n o f t h e g e l phase i n c r e a s e s ,
however, t h e amount o f c l u s t e r s and t h e p o s s i b i l i t y o f f u r t h e r the
clusters
and c a t i o n s .
In
the
s t a b l e . N u c l e i o f c e r t a i n dimensions, e.g. and
-
-
association
of
o f t h i s p r o c e s s p a r t i c l e s become
course
10 A f o r z e o l i t e Na-A
(ref.
114)
20 A f o r z e o l i t e ZSM-5 ( r e f . 115), a r e formed and c r y s t a l l i z a t i o n s t a r t s .
c. Crystallization The
l i n e s a l o n g which ideas on z e o l i t e c r y s t a l f o r m a t i o n a r e developed, e i t h e r
based on b u l k and macroscopic o b s e r v a t i o n s o r on are
described occur
mechanistic
scale
i n t h i s paragraph. Four cases o f n u c l e a t i o n and c r y s t a l l i z a t i o n
a r e s c h e m a t i c a l l y p r e s e n t e d i n Table 8. might
molecular
in
clear
synthesis
r e a c t i o n m i x t u r e s where ( b )
(a)
Zeolite
crystallizations,
which
s o l u t i o n s , o r , more o f t e n , i n heterogeneous
highly
dispersed
or
(c)
dense
gel
forms
are
p r e s e n t , see a l s o F i g . 7. I n some occasions ( d ) m e t a s t a b l e s o l i d phases undergo transformation during synthesis.
Homogeneous
n u c l e a t i o n whereafter
crystal -
l i z a t i o n has been observed i n ( a ) c l e a r s o l u t i o n e x p e r i m e n t s ( r e f s . 91, 9 2 ) .
101
Table 8. Four cases of crystal growth environment representation of nucleation and crystal 1 ization. Crystal growth environment
(a)
Clear solution
(c)
Separated high density gel
(d)
Solid phase
and
schematic
Nucleation Crystallization (+) ( a )
k
H
Fig. 16. a) Powder and b) a twinned elongated prismatic crystal o f ZSM-5 from a dispersed gel phase and c) a cubic form of ZSM-5 from a dense gel phase.
102
Nucleation
(heterogeneous) o c c u r s a t t h e l i q u i d - g e l i n t e r f a c e i n t h e d i s p e r s e d
g e l - s o l u t i o n m i x t u r e s (b) ( r e f . 108). The forms o f t h e c r y s t a l l i z a t i o n p r o d u c t s in
the
case
of
a
dispersed
gel
phase
a r e shown f o r ZSM-5 i n F i g . 16a,b.
S i m i l a r l y t o t h e c l e a r synthesis solutions, the driving force for c r y s t a l l i z a t i o n i s equal i n a l l d i r e c t i o n s as shown i n T able 8a,b. I n t h e case o f a dense g e l phase p r e s e n t i n t h e s y n t h e s i s m i x t u r e , see T able 8c, c r y s t a l lization
proceeds
g e l ( r e f . 42) as shown s c h e m a t i c a l l y i n F i g . 17.
into t h e
D e v i a t i n g c r y s t a l forms compared t o c r y s t a l forms f r o m d i s p e r s e d a r e t h e n observed, as shown i n F i g . 16c.
gel
systems
G enera lly , t h e t y p i c a l f o r m and morphology o f a z e o l i t e c r y s t a l r e v e a l s n o t i n f o r m a t i o n on t h e t y p e o f t h e z e o l it e formed b u t a1 so on t h e c r y s t a l
only
growth h i s t o r y , as shown above. a/c
ratios
v i e w s on gelsphere surface
Pyramidal crystals
perpendicular
basal piano 2nd afc piano
@
3
1
.4
.8
.6
.7
.7
along
ib gel s p h e r e
0
a/c r a t i o s o f d e v e l o p i n g c r y s t a l s and schematic drawing o f g ro w t h process i n t h e g e l spheres.
F ig . 17. Average
As
a
l i q u i d phase
is
c o n t i n u o u s l y p r e s e n t between t h e d i s s o l v i n g dense g e l
phase and t h e gro w i n g c r y s t a l , t h e c r y s t a l l i z a t i o n i s , however,
still
solvent
mediated. When a me t a s t a b l e s o l i d phase, e . g . a z e o l i t e , i s p r e s e n t i n mixture,
a
transformation
the
synthesis
i n t o a more s t a b l e phase i s p o s s i b l e , a c c o r d i n g t o
t h e Ostwald r u l e o f s u c c e s s i v e t r a n s f o r m a t i o n s ( r e f . 116).
103 The
nucleation
and c r y s t a l l i z a t i o n o f t h e new phase, i l l u s t r a t e d i n T a b l e 8d,
occurs i n t h e s u p e r s a t u r a t e d s o l u t i o n
generated
by
the
dissolution
of
the
f o r m e r phase ( r e f . 117).
I n t h e l a s t t h r e e cases o f Table 8 dynamic steps
of
equilibria
between
successive
d i s s o l u t i o n , i o n t r a n s p o r t a t i o n and p r e c i p i t a t i o n , can be r e c o g n i z e d
( r e f . 118). E s p e c i a l l y , t h e precipitation/crystallization s t e p , i.e.
the
type
o f c r y s t a l b u i l d i n g u n i t s and t h e way o f c r y s t a l growth on m o l e c u l a r l e v e l , has been s u b j e c t t o many s t u d i e s .
d. Crystal building units A t l e a s t t h r e e t y p e s o f c r y s t a l b u i l d i n g u n i t s have been
suggested
which
are
d e s c r i b e d be1 ow. d.1. The Drimarv b u i l d i n q u n i t t h a t p r i m a r y b u i l d i n g u n i t s , i .e. t e t r a h e d r a l monomeric s p e c i e s , can
Arguments
be i n v o l v e d i n t h e c r y s t a l l i z a t i o n are: i ) The g e n e r a l view from c r y s t a l growth t h e o r i e s t h a t c r y s t a l s a r e formed v i a p r i m a r y b u i l d i n g u n i t s ( r e f . 119);
ii) The general view i n s o l / g e l c h e m i s t r y ( r e f s . 8, 10) t h a t t h e most f a v o u r e d condensation r e a c t i o n occurs between a monomeric and p o l y m e r i c s p e c i e s . I n terms
o f t h e z e o l i t e c r y s t a l l i z a t i o n : between a p r i m a r y b u i l d i n g u n i t and
a c r y s t a l s u r f a c e ; see S e c t i o n IVb; iii) At
raising
increases
temperatures (ref.
measurements
109)
at
( t i l l 200
(up the OC)
t i l l 100 OC) t h e c o n c e n t r a t i o n o f monomers expense are
not
of
clusters.
actually
Though
performed,
e x p e r i m e n t a l r e s u l t s might i n d i c a t e t h a t a t r e a c t i o n
in
situ
the
above
temperatures
mainly
monomers a r e p r e s e n t ; i v ) S t u d i e s on t h e c r y s t a l l i z a t i o n o f z e o l i t e have shown t h a t t h e g r o w t h o f z e o l i t e occurs by a s u r f a c e r e a c t i o n o f monomeric a n i o n s ( r e f . 120). d.2. As
a
A t y p i c a l c l u s t e r as b u i l d i n q u n i t shown
in
Chapter
r e l a t i v e l y low several
decades
(5
3
of
this
16-Si-tetrahedra) ago
book
secondary b u i l d i n g u n i t s (SBU’s) a r e
polymer
units.
SBU’s
and f u r t h e r p h y s i c a l f e a t u r e s o f t h e z e o l i t e s . A t t h e same t i m e non-chiral
independent
were
introduced
( r e f . 121) and used s i n c e t o p r e s e n t s t r u c t u r a l ( r e f . 6 )
SBU’s a c t i n g as
u n i t s can generate a c e r t a i n z e o l i t e s t r u c t u r e . It i s ,
however, though t h e SBU’s show sometimes a s u p e r f i c i a l resemblance t o
silicate
104
anions,
not
t h a t SBU's a r e t h e b u i l d i n g b l o c k s o f t h e growing c r y s t a l
likely
( r e f . 1 2 2 ) . On t h e o t h e r hand, t h e b u i l d i n g o f t h e porous and d i f f e r e n t z e o l i t e frameworks w i t h monomers c o n d e n s a t i n g i n t h e r i g h t t o p o l o g y seems l e s s f a v o u r a b l e compared t o a t y p i c a l c l u s t e r b u i l d i n g u n i t ( r e f .
123).
From t h i s
p o i n t o f view s u g g e s t i o n s a r e r a i s e d about a t y p i c a l o r common c l u s t e r b u i l d i n g u n i t f o r a l l z e o l i t e structures. d.3. The c a t i o n t e m l a t i n q theor! Organic
as
well
water-ordering,
as
inorganic
properties.
cations
Typical
show
structure
directing,
c r y s t a l s t r u c t u r e a n a l y s i s o f o r g a n i c w a t e r c l a t h r a t e d c a t i o n s ( r e f . 124). wat e r
i.e.
examples a r e g i v e n i n a r e v i e w o f s i n g l e The
mo lec ules c o m p r i s i n g a t e t r a h e d r a l n e t work i n t h e f i r s t l a y e r around t h e
c a t i o n m i g h t be p a r t l y r e p l a c e d by s i l i c a t e and aluminat e
anionic
tetrahedra.
The c l a t h r a t e d c a t i o n s m i g h t s e r v e t h i s way as c r y s t a l b u i l d i n g u n i t s . An example o f such a t e m p l a t i n g / c l a t h r a t i n g r o l e i s t h e f o r m a t i o n s o d a l i t e w i t h tetramethylammonium (TMA')
of
c a t i o n s ( r e f . 125).
The h i g h t e mp era t u r e events, d i s c u s s e d above, a r e summarized i n Scheme 4.
met as t ab 7 e phase
I
ion t r a n s p o r t a t i o n
I
II
s t a b l e phase
I
I
-
gel or
c
small c l u s t e r s
I
I
I
hydrolysis
I
association
condensat ion
Scheme 4. Re p r e s e n t a t i o n o f crystallization.
I
1
precipitation
successive
steps
in the evolution o f zeolite
105
e. Nucleation-crystal 7 iration kinetics
Nucleation and crystal1 ization events are generally illustrated on characteristic S-shaped crystallization curves (ref. 126). The yield (wt % of crystalline material), often determined by indirect methods, plotted against time gives an impression of the nucleation and crystallization time and certain reaction temperatures. More accurate information on the crystallization kinetics can be provided when, based on crystal size and size distribution, the linear crystal growth rate and the rate of nucleation can be determined. Of the studies (ref. 127) on zeolite crystallization, one contribution (ref. 128) reporting on a method to collect kinetic data is briefly described here.
20
30
40
x 100
5
0
10
a
**
,''
b
,~*opr'-l'OOX conversion of
]50the
.lo2
dt
zo!
A
t7\ p
p/"
10
0 6-A 0 40
> O ,\{
80
120
,
160
Time j h I
I
I
I
200
mars of crystals
C
240
Fig. 19. a) Histogram o f the crystal size distribution in the final product, b) diameter of the largest crystals o f different unfinished crystallization runs versus time, resulting in the crystal growth rate graph and c) (i) the nucleation rate (number o f crystals of each unfinished crystallization run versus time) together with (ii) the yield curve.
106
A number of i d e n t i c a l synthesis experiments, b u t d i f f e r i n g i n t o t a l synthesis time, were performed. The average diameter o f t h e l a r g e s t c r y s t a l s which could be c o l l e c t e d from t h e various products was measured. I n t h e case o f z e o l i t e Na-X i t was found t h a t i n a p l o t o f c r y s t a l s i z e versus time t h e l i n e a r c r y s t a l growth r a t e (.5 A L / A t ) was constant, i r r e s p e c t i v e o f t h e c r y s t a l
size,
even u n t i l near exhaustion o f t h e c r y s t a l b u i l d i n g u n i t s , see Fig. 19b. The n u c l e a t i o n time can be determined now product
for
any
crystal
in
the
final
o f t h i s Na-X c r y s t a l l i z a t i o n , knowing the growth r a t e . For instance, a
c r y s t a l o f 16.5 pn nucleated a t t
- 90
h.
Together
with
the
particle
size
d i s t r i b u t i o n curve, Fig. 19a, t h e r a t e o f n u c l e a t i o n was found, see F i g . 19c. The n u c l e a t i o n r a t e curve and the
c a l c u l a t e d from both the growth r a t e and p a r t i c l e s i z e d i s t r i b u t i o n curve, i n d i c a t e t h a t as soon as t h e c r y s t a l l i z a t i o n s t a r t s the chemical n u t r i e n t s are consumed f o r c r y s t a l growth. The formation o f conclusion, i t can crystal1ine
yield
curve
f r e s h n u c l e i i s from be said t h a t z e o l i t e
product
can d e l i v e r
then on l a r g e l y suppressed. I n synthesis, r e s u l t i n g i n a good
accurate
i n f o r m a t i o n on
n u c l e a t i o n and
c r y s t a l 1iz a t i o n k i n e t i c s . f . Energy o f a c t i v a t i o n
Though z e o l i t i c m a t e r i a l can be prepared a t low temperature (20-60 'C) most n u c l e a t i o n and c r y s t a l l i z a t i o n processes are performed a t temperatures between 60 and 250
OC.
The choice o f t h e r e a c t i o n temperature i s governed by t h e energy
o f a c t i v a t i o n r e q u i r e d f o r the z e o l i t e c r y s t a l l i z a t i o n .
Table 9 shows the energy o f a c t i v a t i o n (E,)
as
a
function
of
the
Si/Al
ratio. Table 9. Ea's o f d i f f e r e n t z e o l i t e framework types and S i / A l r a t i o s . Guest molecule Na';
H20
TPA+
TPA+ Na';
H20
Ea (kcal/mol)
Framework
Si/A1
Y
1.5
11.8
MF I MF I MF I
1.8
12.3
2.2
14.1
2.5
15.6
30 W
80
7 11 18
Ref. 129
a
b C
107
I t appears t h a t t h e Ea's a r e n o t r e l a t e d t o d i f f u s i o n o f c r y s t a l b u i l d i n g u n i t s
in
(Ea ( d i f f . ) < 5 k c a l m o l - l ) b u t t o condensation r e a c t i o n s between
solution
t h e c r y s t a l s u r f a c e and c r y s t a l b u i l d i n g u n i t . As shown i n T a b l e 9 Na-X
changes
as
a
Ea
the
of
f u n c t i o n o f t h e S i / A l r a t i o which i n d i c a t e s t h a t t h e more
s i l i c i o u s the zeolite, the
larger
the
Generally,
Ea.
this
trend
is
also
observed between d i f f e r e n t z e o l i t e s , a l t h o u g h t h e c o n t r i b u t i o n t o Ea o f c a t i o n s and t e m p l a t e s , as shown f o r ZSM-5, can be s u b s t a n t i a l .
VII. ISOLATION OF THE ZEOLITE PRODUCT Products o f z e o l i t e p r e p a r a t i o n s can be composed o f e i t h e r phase, quartz,
a
one
pure
zeolitic
m i x t u r e o f z e o l i t i c phases o r a m i x t u r e o f a z e o l i t i c phase and e.g. cristobalite
or
gel
phase.
Mostly
the
product
is
isolated
by
d e c a n t a t i o n / c e n t r i f u g a t ion o r f i1t r a t i o n .
I f the product consists o f c r y s t a l s w i t h a uniform recognized
as
characteristic
for
the
expected
crystal
form
which
is
p r o d u c t , t h e z e o l i t e can be
separated by d e c a n t i n g t h e mother l i q u o r f o l l o w e d by washing w i t h w a t e r .
If there precipitated dissolution
is, as
however, e.g. some g e l phase p r e s e n t , t h i s may be e i t h e r c o a
separate
phase
e l e v a t e d temperature i s s t r o n g l y zeolite.
or
adsorbed
on
the
crystals.
Careful
o f t h e g e l phase w i t h e.g. a d i l u t e b a s i c OH' s o l u t i o n a t s l i g h t l y
Especially
in
the
advisable
case
of
prior
adsorbed
to gel
the on
isolation
of
the
t h e c r y s t a l surface
elemental a n a l y s i s (AAS, I C P o r EMPA) i s r e q u i r e d t o c o n t r o l t h e Si/A1 r a t i o o f the
c r y s t a l s b e f o r e and a f t e r t h e washing procedure ( r e f . 130). The f i n a l s t e p
i n t h e z e o l i t e p r e p a r a t i o n i s t h e d r y i n g o r c a l c i n a t i o n procedure
after
which
t h e z e o l i t e v o i d volume i s f r e e f o r d i f f e r e n t m o d i f i c a t i o n and/or a p p l i c a t i o n .
VIII. REACTION PARAMETERS a. Introduction
The t y p e o f r e a c t a n t s , t h e way t h e r e a c t a n t m i x t u r e i s made, temperature formation.
typically
affect
the
crystallization
the
kinetics
pH and
and
the
product
108
Furthermore
the pre-treatment o f the reaction mixture, the addition o f c r y s t a l
growth i n h i b i t o r s , t h e r e a c t i o n m i x t u r e t e m p e r a t u r e t r a j e c t o r y and t h e
use
of
seeds have an i n f l u e n c e on t h e z e o l i t e p r e p a r a t i o n . Some aspects o f t h e t y p e o f t h e above mentioned f a c t o r s a r e d i s c u s s e d i n t h e f o l l o w i n g paragraphs. I l l u s t r a t i o n s a r e m a i n l y g i v e n on t h e z e o l i t e A and ZSM-5 formation.
b. The 5i-source As mentioned i n
Section
I1 of
this
chapter
the
different
types
of
the
S i - s o u r c e s c o n t a i n i m p u r i t i e s which may a f f e c t z e o l i t e c r y s t a l l i z a t i o n . Another parameter, t h e s p e c i f i c s u r f a c e a r e a o f t h e s e sources, can r e s u l t i n d i f f e r e n t nucleation
c r y s t a l l i z a t i o n t i m e s as shown f o r z e o l i t e A i n F i g . 20a ( r e f .
and
4 7 ) . The s h o r t e r i n d u c t i o n and c r y s t a l l i z a t i o n t i m e s l e a d t o more
and
smaller
c r y s t a l s , see F i g . 20b.
.-
-
/,..
Silica source
I
I1 111
a
I
1
2
3
4
5
6
7
Crystals r e 1 . number size 48 .7 30 2.6 15 4.8
b
8
tlme(h)
Fig. 20. a) The y i e l d o f z e o l i t e A versus t i m e o f d i f f e r e n t s i l i c a
sources.
b) The s p e c i f i c s u r f a c e a r e a s o f t h e s i l i c a sources ( I > I 1 > 111) r e s u l t i n d i f f e r e n t amounts and s i z e s o f c r y s t a l s .
c. The type o f template Many t y p e s o f t e m p l a t e chapter).
The
are
surprising
regularly
t y p e o f z e o l i t e framework formed i s template
can
used
(see
e.g.
Section
I1 o f t h i s
performance o f c e r t a i n t e m p l a t e s on s t a b i l i z i n g t h e illustrated
in
Table
10.
One
type
of
be used t o c r y s t a l l i z e v a r i o u s z e o l i t e s whereas t h e same t y p e o f
z e o l i t e may be c r y s t a l l i z e d w h i l e u s i n g d i f f e r e n t t e m p l a t e s .
109
T a b l e 10. S i n g l e
and m i x t u r e
of
i n t h e preparation o f
templates/cations
d i f f e r e n t z e o l i t e types. Single
Zeolite
Ref.
Mixture o f
temp1 a t e
/
TMA'
Zeol it e
Ref.
x,
Y
135
L (+ K')
136 136
template/cation
Sodalite
--A,
131 TMA',
Na'
Gismondine 132
Sodalite,
P, S and R ZSM-6 and
137
ZSM-47
TPA+ Na'
\
/
TEA
133 ZSM-5
-
EDA Ethanolamine
134
138
ZSM-5
139
j ,
Propanol ami n
- Omega
Na'
A1 coho1 Glycerol Morphol i n e Hexanediol TPA
-
The r o l e o f t h e s i n g l e t e m p l a t e / c a t i o n i n
stabilizing
subunits
of
different
z e o l i t e types i s n o t unravelled y e t .
A common f a c t o r , however, appears t o be t h e s i z e diameter
in
the
structures
of
r e s p e c t i v e l y , and t h e d i a m e t e r o f
-
of
a
certain
free
void
6.8 A and 7.0 A, 6.7 A o f t h e t e m p l a t e TMA', see F i g . 21.
sodalite
and
gismondine,
110
Fig. 21. Models
of
a)
the
s o d a l i t e and b) t h e gismondine v o i d and t h e v o i d
f i l ler/template/cation TMA+.
Although
TPA'
and Na' are r a t h e r d i f f e r e n t templates/cations a common f a c t o r
might be the s t a b i l i z a t i o n o f voids ( e i t h e r i n t e r s e c t i o n o f channels o r channel windows), see Fig. 22.
Fig. 22. View along s t r a i g h t channels o f w i r e model o f ZSM-5 with e i t h e r TPA' (*) o r hydrated Na' (0) on i n t e r s e c t i o n s o f channels and channel windows, r e s p e c t i v e l y .
Charged temp1 ates
compensate
negative
framework charges, due t o isomorphous
by A13'. A range o f S i / A l r a t i o s i s possible, see Scheme If, however, t h e number o f charged templates r e q u i r e d f o r charge compensation cannot be accommodated f o r dimensional reasons t h e zeol i t e substitution o f Si4'
5.
111
combines charged t e m p l a t e s w i t h e.g. Na'. for
one
Sodalite
zeolite
type
are
T h i s way, s t i l l v a r i o u s Si/A1
ratios
as shown i n Scheme 5 f o r z e o l i t e ZSM-5.
possible
can be prepared w i t h two d i f f e r e n t Si/A1 r a t i o s .
ZSM-5
Si/A1
Soda1 it e
TPA+
23 - <10000
TMA+
TPAt/Nat
23 - 11
Na'
Si/A1
Na'
11
Scheme 5. D i f f e r e n t Si/A1 r a t i o s f o r ZSM-5 and s o d a l i t e .
d. The reactant mixture The way
reactant
mixtures
are
made,
e.g.
r e a c t a n t s , t h e s t i r r i n g and g e l aging can r e s u l t
the
addition
sequence
i n method-dependent
o f the factors
i n f l u e n c i n g n u c l e a t i o n . As shown i n F i g . 23 c r y s t a l s o f z e o l i t e A s t a r t e d growing i n a z e o l i t e X s y n t h e s i s m i x t u r e w h e r e a f t e r z e o l i t e X c r y s t a l s s t a r t e d growing on and o v e r t h e z e o l i t e A c r y s t a l ( r e f s . 140, 141).
F i g . 23. Overgrowth o f z e o l i t e X o n t o z e o l i t e A.
112
Though t h e thermodynamic v a r i a b l e s were c o r r e c t l y chosen t o prepare z e o l i t e X, s y n t h e s i s m i x t u r e s o f z e o l i t e A and X, g i v e n below, do have comparable elements and a p p a r e n t l y l o c a l k i n e t i c f a c t o r s i n i t i a t e d t h e s y n t h e s i s o f z e o l i t e A. Na2Si03.9H20
NaA102 T r i e t h a n o l a m i n e
H20
z e o l i t e A:
.4
.1
.7
28
z e o l i t e X:
.4
.05
.7
28'
Ref. (142)
\
(molair)
e. The pH e. I.
Introduction
The pH and t h e s o l u b i l i t y o f r e a c t a n t s i n t h e s y n t h e s i s m i x t u r e a r e governed by t h e presence o f OH- o r F - .
111 F- compared t o OH- i s t h e h i g h e r s o l u b i l i t y o f e.9. Fe and T i " and t h e c o n d e n s a t i o n c a p a b i l i t y f o r e.g. Ge". A too high c o n c e n t r a t i o n o f F - , however, p r e v e n t s t h e polycondensat ion mechanisms. A compromise between s o l u b i l i t y o f c e r t a i n elements and i n h i b i t i o n o f z e o l i t e framework formation leads to Fs y n t h e s i s m i x t u r e s which a r e l e s s su pers a t u ra t e d t h a n OH- media. Hence, o n l y a few z e o l i t e t y p e s a r e obt ained, u n t i l now ( r e f . 16). An
advantage
of
e.2. OH-
Raising
pH
of
synthesis
mixtures
using
OH-,
mainly
c r y s t a l l i z a t i o n o f a c e r t a i n z e o l i t e i n a p o s i t i v e way
within
influences the
the
synthesis
A and z e o l i t e ZSM-5, r e s p e c t i v e l y , i n c r e a s i n g t h e pH shows an i n c r e a s e i n t h e c r y s t a l l i z a t i o n r a t e . The OH- i s a s t r o n g m i n e r a l i z i n g agent f o r b r i n g i n g r e a c t a n t s i n t o s o l u t i o n . field.
As
depicted
in
Fig.
24a
and
b
for
zeolite
The h i g h e r t h e pH and t h u s t h e c o n c e n t r a t i o n o f d i s s o l v e d
reactants
t h e r a t e o f c r y s t a l g r o w t h o f z e o l i t e s i s enhanced ( r e f s . 47, 143).
the
more
113
Zeolite A &]
b
a
F i g . 24. The
influence
of
a l k a l i n i t y on
a)
zeolite
A
and
b)
ZSM-5
c r y s t a l 1 iz a t i on.
e.3. F A f t e r t h e f i r s t p u b l i c a t i o n s on s y n t h e s i s w i t h F- ( r e f . 4 6 ) have
been
undertaken
to
investigate
the
extensive
studies
e f f e c t o f F- and p o s s i b i l i t i e s i n
z e o l i t e s y n t h e s i s ( r e f s . 16, 4 6 ) . Replacing OH- by F- w i t h e.g. NH4HF, NH4F and
BF3 t h e pH values o f t h e s y n t h e s i s m i x t u r e l i e s g e n e r a l l y between 3 and 10. A t y p i c a l synthesis formulation i s given i n Section
zeolites
obtained
XI1
of
this
chapter.
The
so f a r by t h i s r o u t e a r e s i l i c a - r i c h m a t e r i a l s o f w h i c h t h e
s t r u c t u r e t y p e s are:
ZSM-5 F e r r ie r it e Theta-1 and ZSM-23
f . The temperature I t has been shown f o r many z e o l i t e s t h a t r a i s i n g s y n t h e s i s t e m p e r a t u r e s
a certain zeolite
synthesis
field
within
increases the c r y s t a l growth r a t e ( r e f s .
47, 1 4 4 , 1 4 5 ) . As shown i n F i g . 25a and b f o r z e o l i t e A and z e o l i t e ZSM-5, i n c r e a s i n g temperature i n f l u e n c e s t h e c r y s t a l growth r a t e whereas i n t h e case o f z e o l i t e A t h e c r y s t a l s i z e does n o t change s u b s t a n t i a l v a r i a t i o n s i n t h e ZSM-5 p r o d u c t .
s i g n i f i c a n t l y compared
to
114
.
Crystol size
(pm)
200'C 165
'C
170 ' C
150 'C
10
a
i Time [hours]
bo
10
20
30
t(h)
Fig. 25. Influence of temperature on the crystallization of a) zeolite A and b) zeolite ZSM-5.
I X . ALL S I L I C A MOLECULAR SIEVES
a . Introduction
Two preparation routes can be followed to obtain all silica molecular sieves: i) A direct synthesis to crystallize molecular sieves with a Si02 composition and well known zeolite topologies. i i ) A secondary synthesis. After the direct synthesis of a zeolite a dealumination procedure, e.g. steaming (ref. 146), ammonium silicon hexafluoride (ref. 147) or silicon tetrachloride (ref. 148) can lead to an all silica molecular sieve. b. Synthesis
Though the neutral all silica molecular sieves do formally not need to be stabilized with cations the silica structures usually contains the cations used in the synthesis. For example, tetrapropylammonium for silicalite-1, tetrabutylammonium for silicalite-2, and tetraethylammonium for silica-ZSM-12. Recently, amines, di-amines (ref. 149) and poly-amines (ref. 150) have been used as templates. Table 11 contains a list of all silica molecular sieves with two examples of synthesis recipes and references.
115
Table 11. All silica zeolites, recipes and references. Product
Ref
Sil ica-ZSM-48
150
Recipe: 14.6 g of triethylenetetramine is dissolved in 18 ml H20 whereafter the solution is stirred into dry 1.2 g Si02. The smooth dispersion is then autoclaved between 120-180 OC for 28-105 days, respectively. Sil ica-ferrierite Recipe: 0.75 g Si(OC3)4 is added to a solution of 1.2 g ethyldiamine (EDA) in 10 ml H20. After adding 2 ml 1 M aqueous boric acid the solution is sealed in a silica tube and heated at - 170 O C for 56 days. Sil ical ite-1 (Sil ica-ZSM-5) Sil ical ite-2 (Silica-ZSM-11) Sil ica-ZSM-22
151
152 153 149
c . Remark
The main property of the silica molecular sieves is the strong hydrophobic character of the pores. The preferential uptake o f e.g. traces o f organic compounds (ref. 152) from water, which is not accommodated (ref. 154), in silicalite-1 is a good example.
X. CLATHRASILS or actually silicates and zeolite molecular sieves? a. Introduction
The name "clathrasil" has been introduced for a subclass of porous tectosilicates different from zeolites. The windows of the framework, connecting the cages, are too small to let guest species, stabilized during the synthesis, pass.
116
This characteristic of a clathrate together with the all silica composition is considered as specific for the members of the clathrasils (ref. 29). There are, however, exceptions. The recently synthesized decadodecasil-3R (DD-3R) (ref. 155) contains windows of eight-rings of oxygen, indicating that diffusion of small molecules through the porous structure is possible after calcination. This structure can therefore be considered to form an interface between the clathrasils and the silica molecular sieves. A modified type of DD-3R denoted as Sigma-1 (ref. 156) can, however, be seen as a link between clathrasils and zeolites, because some Si-framework sites are isomorphously substituted by Al. Finally, there is a novel tectosilicate, Sigma-2 (ref. 157). The recently solved structure of which two different polyhedra, see Fig. 26, have not been found before, reveals eight-rings of oxygen and cages with a free diameter of 75 nm. Sigma-2 has been prepared in the silicalite as well as in the zeolite form and can thus be considered as an intermediate between clathrasils, zeolites and silicates.
Fig. 26. The nonahedral and eikosahedral cages o f Sigma-2.
b . Experimental
The clathrasils can be synthesized generally from .5 M silica, prepared by hydrolyzing an alkoxysilane, e.g. Si(OCH3)4, in solutions containing an amine as guest template molecule. The syntheses are mainly carried out between 160-240 OC. The clathrasils, together with detailed synthesis data and products expected, are given in Table 12.
117
w i t h r e f e r e n c e s t o s y n t h e s i s p r e s c r i p t i o n s and two examples o f s y n t h e s i s r e c i p e s .
T a ble 12. C l a t h r a s i l s
Product
Ref.
Me1anophlogi t e
158
Dodecasil 3C
159
Dodecasil 1H
160
S i 1ica-soda1 it e
161
Sigma- 1
156
DD-3R ( s i l i c a )
155 Recipe: 0.75 g Si(OCH3)4 i s added t o a s o l u t i o n o f 1.2 g e thylenediamine (EDA) i n 10 m l H20. A f t e r adding 350 mg l-aminoadamantane t h e s o l u t i o n i s sealed i n a s i l i c a tube and heated a t 170
-
OC
for
70 days.
Sigma-2 ( s i l i c a and a l u m i n o s i l i c a t e )
157
S y n t h e s i s example: The molar oxyde r a t i o o f t h e s y n t h e s i s system i s : Na20 AN
2'3 Si02
3 20
(1-adamantanamine)
(0.6) 60
(Al-wire) (colloidal silica)
2400 H2° The system was c r y s t a l l i z e d a t 180 O C and c o n t i n u o u s l y s t i r r e d f o r a few days.
c. Remark
The t e m p l a t i n g r o l e o f some o f t h e guest molecules i s i l l u s t r a t e d i n F i g . 26. Polyhedra o f d i f f e r e n t c l a t h r a s i l s a r e f i l l e d w i t h a guest molecule. As o n l y 4-, 5 - o r 6 - r i n g f a c e s a r e p r e s e n t i n most o f t h e polyhedra i t l o o k s crystal
building
i s t o o l a r g e t o pass t h r o u g h one o f t h e r i n g s . The therefore
like
the
u n i t s have formed around t h e guest molecule as t h i s molecule clathrate
formation
might
be o b t a i n e d and based on s i n g l e b u i l d i n g u n i t s i n s o l u t i o n and/or a t
t h e growing c r y s t a l s u r f a c e ( r e f . 162).
118
Fig. 27. Orientation o f various guest molecules in clathrasils (ref. 163). a) H3CNH2 in [ 5 126 2 ] and b) adamantylamine in [51268] of melanophlogite and dodecasil l H , respectively.
OF SYSTEMATIC RESEARCH in the field of preparation to reach various objectives
X I . EXAMPLES
molecular
sieves
a . Introduction
A main thrust of research is: - to synthesize new molecular sieves - further optimization of recipes - to gain knowledge on the essential functions of reactants, e.g. structure directing role of cation/templ ate - to prepare relative large single crystals for fundamental studies The list can be longer, however, the examples given below illustrate generally the purpose and variety in the research o f molecular sieves preparation. b. Research examples
Objective 1)
Preparation o f zeolites
2) Preparation o f zeolites 3)
Investigation of crystal1 ization fields with pyrrolidine as template
Parameter ( s 1 Non-aqueous solvents F - as mineralizing agent
Na20-A1 203- Si O2 -H20 system was varied
119
b. Research examples (continued)
Objecti ve
Parameter(s)
4a) Investigation o f template-zeolite interaction 4b) Directing role o f template in the crystal1 ization
Systematic variation of template Use o f bis-quaternary ammonium compounds
5)
Large single crystals
Know1 edge on nucl eation/ crysta1 1 i zati on
6)
Morphology and form of zeol i te products
Change o f [Si02], temp1 ate, cation or additives
b.1. The use o f non-aqueous solvents
In contrast to the rich crop of zeolite types synthesized in aqueous systems the results in non-aqueous solvents are poor (refs. 164, 165). Solvents used, o f which the choice was a.0. based on boiling point (100-200 OC) and relative permittivity (10-45) (water: 78), are given in Table 13.
Table 13. Zeolite products formed. Solvents ~
Glycol Glycerol DMSO Sul fol ane C6C7 alcohol Ethanol
K+
Na' ~
~
HS HS HS
HS HS
HS
HS: Hydroxysodali te.
Lit
Cat+
120
Generally mixtures within the following molar oxyde ratio were used: Me0
1-20 1 1-100
2'3
Si02 Solvent 5-350 0.1-10 and MeO/Si02 As shown in the Table zeolite products could only be obtained in the case of .'aN The use o f other inorganic and organic cations was not successful. As the boiling point is a less critical factor than high relative permittivity (reduces the Coulomb force between ions and polar compounds thus enhancing dissolution) other non-aqueous solvents for zeolite crystallization which might be subject to zeolite synthesis tests are given below.
Non-aqueous solvent
Er
~~
formic acid formamide hydrogen peroxide hydrocyanic acid
b.2. The use o f
57 84 93 95
F- ( r e f . 166)
The compositional ratios of the reaction mixtures used were: A1 or B/Si F/Si Templ./Si H20/Si in molar ratio 0-0.5 0.05-6 0.05-6 4-500 with the pH of the mixtures between 1.5-10. The reaction mixtures were heated at 60-250 O C and autoclaved for a few hours t o a few months. After isolation the products were washed with water and dried. A typical example o f a " F - " synthesis of ZSM-5 is given below: - Reaction mixture composition: 36 g Ammonium aluminiumsilicate (Si/Al 7; NH4/A1 1) pH = 7 18.5 g NH4F 33.2 g TPABr t = 172 " C 180 g H20 time = 11 days
-
-
121
- Product Unit cell composition: 1.8 NH4+ t 4.1 TPA+ [A12~9Si93~101921 Crystal size 30 x 12 pm Advantages and differences using F - instead o f OH- as concluded so far: - Low pH compared to OH- Incorporation in the framework of elements sparingly soluble in alkaline medium, e.g. Fe111 - Synthesis without alkaline cations - New possibility to directly incorporate cations as NH4+ and divalent cations such as Co2' as well - Good stability of usual templates such as TAA' in this medium - Highly crystalline materials b.3. Pyrrolidine as template (ref. 87)
The crystallization o f zeolites in the system Na2O-Al2O3-SiO2-H20 t pyrrolidine as a template was studied. The reaction mixture compositions used are given in Table 14 in molar oxyde ratio.
Table 14. Reaction mixture ranges in molar oxyde ratios. Na20 A1 i03 Si02
0.05-0.5 0.002-0.05
H2S04 H2°
0-0.4 20-80
1
H t
-
0.7 pyrrolidine
Two procedures were used: I. To a stirred aluminium sulphate solution, calculated amounts of sodium silicate, sulphuric acid and pyrrolidine were added dropwise. 11. Calculated amounts of aluminium nitrate, colloidal silica and pyrrolidine were added to a stirred sodium hydroxide solution.
122
Si02/A1 203
A
4
t
A
A
A
H20/Si O2
F i g . 28. C r y s t a l l i z a t i o n
fields
of
product
Si02/A1203 v e r s u s H20/Si02 o f
ZSM-39 (A), ZSM-48 (+) and KZ-1 ( A ) .
The f o l l o w - u p o f b o t h procedures was t o a u t o c l a v e t h e r e a c t i o n m i x t u r e f o r 7 - 4 0 h a t 4 2 3 - 4 3 5 K w i t h s t i r r i n g . A f t e r i s o l a t i o n t h e p r o d u c t was washed w i t h w a t e r and d r i e d .
A
whereas
The main c o n c l u s i o n o f t h e s t u d y i s t h a t p u r e ZSM-5, ZSM-35, ZSM-39,
ZSM-48
The r e s u l t s o f t h e experiments a r e g i v e n i n F i g . 6 f o r p r o c e d u r e t h e r e s u l t s o f t h e experiments w i t h p r o c e d u r e B a r e g i v e n i n F i g . 28. and
KZ-l
can be c r y s t a l l i z e d w i t h p y r r o l i d i n e i n t h e a f o r e m e n t i o n e d s y n t h e s i s
system. No common f a c t o r , based on t h e use o f p y r r o l i d i n e , c o u l d be
recognized
i n the various z e o l i t e products. b.4.a. The
use
of
bis-quaternary
ammonium compounds
in
molecular
The o b j e c t i v e i n t h i s study was t h e s y s t e m a t i c v a r i a t i o n
of
template
sieves
s y n t h e s i s ( r e f . 58)
synthesis.
An
example
of
the
synthesis
is
given
in
the
below t o g e t h e r w i t h t h e
p r o d u c t s formed, see Table 15. The g e n e r a l f o r m u l a o f t h i s b i s - q u a t e r n a r y t e m p l a t e ( T ) i s :
123
T a b l e 15. S y n t h e s i s
mixtures
and
product
formation
with
bis-quat
as
template. Synthesis conditions
Product f o r m a t i o n
m o l a r oxyde r a t i o Si02
X
60
1
Zeol it e phases
2'3 Na20
10
3
ZSM-39
TBr2
10
4
ZSM-12
576
EU- 1
7,8
ZSM-23
H2°
3000
S i l i c a phases 3
EU-4
499
EU-2
( w i t h o u t A1 203)
The
reaction
conditions
were
180 O C , t h r e e days and c r a s h - c o o l i n g a f t e r t h e
s y n t e h s i s was t e r m i n a t e d . b . 4 . b . Another example (ref. 167) Systematic v a r i a t i o n o f t h e c h a i n l e n g t h o f t h e t e m p l a t e (T) g i v e n below i n t h e general formula
r e s u l t e d i n t h e p r o d u c t s , g i v e n i n Table 16. T a b l e 16. Z e o l i t e f o r m a t i o n s , o b t a i n e d w i t h b i s - q u a t . X
2-5
Zeol it e phase Ferrierit e ZSM-5
5-6
ZSM-5
7-10
ZSM- 11
124 The f u l l synthesis d e s c r i p t i o n i s given i n r e f . 167. b . 5 . The synthesis of relatively large single crystals of molecular sieves b . 5 . 1 . Znt roduct ion P e r t a i n i n g t o e.g. the v i s c o s i t y o f
the
synthesis
mixture
several
systems,
c l e a r s o l u t i o n , d i l u t e d gel and dense gel phase have been i n v e s t i g a t e d . b . 5 . 2 . Crystallization of ZSM-22 from a clear solution [ref. 149) I n a t y p i c a l experiment tetramethoxysilane was hydrolyzed i n 3 M diethylamine
(DEA) according t o the f o l l o w i n g reactions:
Si(OCH3)4
+
2 H20
-------- > Si02 + 4 CH30H -------DEA > 2 Si02(C2H5)2NH 180 OC 100 days
Single
crystals
of
silica-ZSM-22 o f 45 x 100 x 225 pm were i s o l a t e d and used
f o r s t r u c t u r e determination.
b . 5 . 3 . Synthesis of elongated prismatic ZSM-5 crystals The o b j e c t i v e o f t h i s study was t o Systems
using
Na'-TPA+,
Li'-TPA+
obtain
large
and NH4'-TPA'
single
crystals
of
ZSM-5.
were i n v e s t i g a t e d applying a
r e a c t i o n mixture given i n molar oxyde r a t i o f o r e.g. NH4'-TPA':
TPA20 (NH4)20 23' Si02 H2°
4 123
T = 453 K
1 59 2280
t = 7 days
Products A1 kal i n e - f r e e ,
homogeneous elongated p r i s m a t i c s i n g l e c r y s t a l s o f ZSM-5 o f 350
pm i n l e n g t h a t maximum ( r e f . 168).
125
b.5.4. Synthesis o f cubic shaped s i n g l e c r y s t a l s o f ZSM-5 ( r e f . 169) The s y n t h e s i s o f t h i s t y p e
of
crystals
developed
recently
(ref.
169)
was
s u b j e c t o f a s t u d y on t h e c r y s t a l growth h i s t o r y ( r e f . 42) o f t h i s t y p e o f c r y s t a l s . The o b j e c t i v e was: t o p i n p o i n t t h e d r i v i n g f o r c e s which change t h e ZSM-5 c r y s t a l f o r m from e l o n g a t e d p r i s m a t i c i n t o c u b i c . The c r y s t a l growth h i s t o r y s t udy r e v e a l e d t h a t t h e c u b i c c r y s t a l growth o c c u r r e d i n a dense g e l phase, P e r f e c t s i n g l e c r y s t a l s up t o 500 pm o f z e o l i t e ( E M - 5 ) and a l l s i l i c a (silicalite-1)
m o l e c u l a r s i e v e type, see F i g . 16c, c o u l d be o b t a i n e d u s i n g t h e
f o l l o w i n g mo lar oxyde r a t i o : ZSM-5
S i l i c a l it e - 1
S i O2
12
12
2'3 Na20
1 44
44
TPA20
44
44
2000
2000
H2°
A f t e r 5 days produc t .
at
180
OC
crystals
could
be i s o l a t e d and s e l e c t e d f rom t h e
b.5.5. Synthesis o f s i n g l e c r y s t a l s o f z e o l i t e A and X ( r e f . 142) S i n g l e c r y s t a l s o f z e o l i t e A and X up t o 100-500 pm i n s i z e c o u l d
be
obtained
u s i n g t h e f o l l o w i n g procedures. Procedure f o r z e o l i t e A: S o l u t i o n I: 100 g Na2Si03.9H20 i n 350 m l H20 t 50 m l TEA S o l u t i o n 11: 80 g NaA102 i n 350 m l H20 t 50 m l TEA Both s o l u t i o n s a r e f i l t e r e d w i t h m i l i p o r e f i l t e r s , w h e r e a f t e r s o l u t i o n I 1 i s added t o s o l u t i o n I w i t h s t i r r i n g . The c r y s t a l l i z a t i o n i s perf ormed a t 75-85 OC f o r 2-3 weeks, w i t h o u t s t i r r i n g . Procedure f o r z e o l i t e X: Identical
to
the
procedure
for
z e o l i t e A,
o n l y 40 g o f NaA102 i s used i n
s o l u t i o n I 1 now. The c r y s t a l l i z a t i o n t i m e i s 3 - 5 weeks.
126 Remark C a r e f u l f i l t e r i n g o f t h e s t a r t i n g s o l u t i o n s s u b s t a n t i a l l y reduces t h e amount o f heterogeneous n u c l e i such as d u s t and f o r e i g n p a r t i c l e s i n t h e c h e m i c a l s . The l o w e r t h e number o f n u c l e i , t h e l a r g e r t h e c r y s t a l s .
starting
b . 6 . Morphology and f o r m o f m o r d e n i t e and ZSM-5 The
morphology
and/or
form
of
zeolite
crystals
appear
generally
to
be
i n f l u e n c e d by: - [Si02] - Guest m o l e c u l e t y p e - C a t i o n ( r e f . 171) - Crystal growth i n h i b i t o r s
A
frequently
observed c r y s t a l f o r m o f m o r d e n i t e i s t h e n e e d l e f o r m ( w i t h p o r e
channel system p a r a l l e l t o n e e d l e d i r e c t i o n ) , see F i g . 29a.
F i g . 29. D i f f e r e n t
forms o f m o r d e n i t e . The n e e d l e f o r m a), t h e
forms b) and c ) and
the
disk
form
d).
The
pore
ntermed a t e d rection i s
i n d i c a t e d by a b a r ( r e f . 85).
As shown i n F i g . 29b, c and d, c o m p l e t e l y d i f f e r e n t c r y s t a l forms o f can
be
prepared.
According
to
mordenite
t h e s y n t h e s i s system used ( r e f . 85) t h e main
i n f l u e n c e i n t h e shape o f t h e c r y s t a l s seems t o be t h e [ S i 0 2 ] . The
higher
the
[Si02], i . e . t h e more t h e c r y s t a l l i z a t i o n o c c u r s i n a dense g e l , t h e more t h e e l o n g a t e d f o r m i s reduced and changed i n t o a d i s k form.
127
The
increase
i n p o r e e n t r i e s and decrease i n p o r e l e n g t h g o i n g f r o m n e e d l e t o
d i s k f o r m i s e v i d e n t and may be o f i n t e r e s t i n c a t a l y s i s ( r e f . 170). The
elongated
prismatic
f o r m i s t h e most f r e q u e n t l y found c r y s t a l f o r m o f
Z S M - 5 . Changing t h e [Si02] can change t h e c r y s t a l f o r m as shown i n F i g . 30a and
b f o r r e l a t i v e l y l o w and h i g h [Si02]c o n c e n t r a t i o n s , r e s p e c t i v e l y .
F i g . 30. The e l o n g a t e d p r i s m a t i c c r y s t a l f o r m (a) and t h e c u b i c c r y s t a l f o r m (b) o f z e o l i t e ZSM-5.
Changing t h e t e m p l a t e t y p e , i . e . r e p l a c i n g TPA' ammonium
ion,
f o r the divalent
bi-quaternary
hexapropyl-1,6-hexanediammonium, r e s u l t e d i n d i f f e r e n t c r y s t a l
forms f o r l o w as h i g h [Si02] as w e l l , see F i g . 31a and b.
F i g . 31. The
modification o f
the
c r y s t a l f o r m a t low (a) and a t h i g h (b)
[ S i 0 2 ] o f z e o l i t e ZSM-5 prepared w i t h b i q u a t as t e m p l a t e .
I n t h e case o f an a d d i t i v e ( i n h i b i t o r ) l i k e b o r i c a c i d an enrichment o f c r y s t a l faces i n t h e c - d i r e c t i o n was observed as shown i n F i g . 32.
128
F ig . 32. A d d i t i o n a l
crystal
face
compared
(001)
to
regular
elongated
p r i s m a t i c f o r m o f z e o l i t e ZSM-5.
XII. LITERATURE SOURCES PERTAINING ZEOLITE PREPARATION ASPECTS Though most o f t h e l i t e r a t u r e
sources
are
given
in
XIII, a more
Section
extended l i s t o f sources i s g i v e n below f o r reasons o f c l a r i t y and ease.
A
Chemical Abstracts literature
search
in
the
Chemical
A b s t r a c t s (CA) can be s u c c e s s f u l when
C o n t r o l l e d Vocabulary Index Terms (CVIT's) a r e used. As CVIT's a f t e r 1976 a r e n o t o n l y assigned t o words i n t h e t i t l e and t h e a b s t r a c t , b u t a l s o t hroughout the t e x t
of
thoroughly.
the The
paper choice
(open of
literature
CVIT's
must
or be
patent) correct.
the
search w i l l
be
I n t h e case t h e word
" s y n t h e s i s " i s used i n s t e a d o f " p r e p a r a t i o n " t h e main p a r t o f t h e search " h i t s "
w i l l p e r t a i n t o r e a c t i o n s w i t h t h e a i d o f z e o l i t e s whereas t h e p r e p a r a t i o n o f z e o l i t e s i s then d i f f i c u l t t o e x t r a c t . - Proceedings o f I n t e r n a t i o n a l Zeo7 i t e Conferences (IZC)
1. " Molec ular Sieves", SOC. Chem. I n d . , London, 1968; Proceedings I Z C , London, U.K.,
of
the
1st
1967.
2. " Mo lec ular Sieves I and I I " , Adv. Chem. Ser.,
101 and 102, ACS,
Washington,
D.C., 1971; Proceedings o f t h e 2nd I Z C , Worcester, Mass., U . S . A . , 1970. 3. " Mo lec ular Sieves", Adv. Chem. Ser., 121, ACS, Washington, D.C., 1973; W.M. Meier
and
J.B.
S w i t z e r l a n d , 1973.
Uytterhoeven,
Eds.,
Proceedings
o f t h e 3 r d IZC, Zurich,
129
4. " M o l e c u l a r S i e v e s - I I " , ACS Symp. Ser., 40, ACS, Washington, D.C.,
1977; J.R.
K a t z e r , Ed., Proceedings o f t h r e 4 t h I Z C , Chicago, Ill.,U.S.A., 5. "Proceedings
of
the
5th
International
London, P h i l a d e l p h i a , Rheine, 1980; L.V.C.
1977.
Conference
on z e o l i t e s " , Heyden,
Rees, Ed.,
Proceedings o f t h e 5 t h
I Z C , Naples, I t a l y , 1980. 6. "Proceedings o f t h e 6 t h I n t e r n a t i o n a l Conference on Z e o l i t e s " , B u t t e r w o r t h s , Guildford,
1984;
D.
Reno, Nev., U.S.A.,
Olson and A. B i s i o , Eds., Proceedings o f t h e 6 t h I Z C ,
1983.
7 . New Developments
in
Zeolites
Science
and
Technology", Kodansha, Tokyo,
E l s e v i e r , Amsterdam, Oxford, New York, Tokyo, 1986, Stud. S u r f . S c i . C a t a l . , 28;
Y. Murakami, A. I i j i m a and J.W. Ward, Eds., Proceedings o f t h e 7 t h I Z C ,
Tokyo, Japan, 1986. 8. " Z e o l i t e s :
Facts,
F i g u r e s , F u t u r e " , E l s e v i e r , Amsterdam, Oxford, New York,
Tokyo, 1989, Stud. S u r f . S c i . C a t a l . , 49; P.A. Jacobs and R . A .
van
Santen,
Eds., Proceedings o f t h e 8 t h I Z C , Amsterdam, N e t h e r l a n d s , 1989. - S y n t h e s i s p a r t i n r e c e n t i n t e r n a t i o n a l conferences
"Zeolites,
Synthesis,
Structure,
Technology
Amsterdam, Oxford, New York, Tokyo, 1985,
Stud.
and
Application",
Surf.
Sci.
Catal.,
Elsevier, 24;
B.
D r z a j , S. Hocevar and S. P e j o v n i k , Eds. " I n n o v a t i o n i n Z e o l i t e M a t e r i a l s Science",
Elsevier,
Amsterdam,
Oxford,
New
York, Tokyo, 1988, Stud. S u r f . S c i . Catal.,
37; P.J. Grobet, W.J. M o r t i e r , E.F.
Vansant and G. S c h u l z - E k l o f f , Eds. "Zeolite
Synthesis",
ACS
Symp.
Ser., 398, ACS, Washington, D.C.,
1989; M.L.
O c c e l l i and H.E. Robson, Eds.
-
Journal
Z e o l i t e s , L.V.C.
Rees and
R.
von
Ballmoos,
Eds.,
Publishers,
Butterworth,
Heinemann, Stoneham, MA, U.S.A.
"Zeolite
Molecular
Sieves",
S t r u c t u r e , Chemistry and Use, John W i l e y & Sons,
New York, London, Sydney, Toronto, 1974; D.W. "Hydrothermal R.M.
Chemistry
B a r r e r FRS.
Breck.
o f Z e o l i t e s " , Academic Press, London, New York, 1982;
130
"Synthesis Oxford,
of
High-Silica Alumiosilicate
New York,
Tokyo,
Zeolites",
Elsevier,
Amsterdam,
1987, Stud. Surf. Sci. Catal., 33; P.A. Jacobs and
J.A. Martens, Eds.
"Molecular
Sieves,
P r i n c i p l e s o f Synthesis and I d e n t i f i c a t i o n " , Van Norstrand
Reinhold, New York, 1989; R. Szostak. "An I n t r o d u c t i o n t o Z e o l i t e Molecular Sieves", John Wiley and Sons, Chichester, 1988, A. Dyer. ACKNOWLEDGMENT. I l i k e t o thank D r . H. Kouwenhoven f o r reading the manuscript.
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87 88
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134
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135
132 133
134
135 136 137 138 139 140 141 142 143 144 145 146 147 148
149 150 151 152 153 154 155 156
157 158 159 160 161 162 163
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136
164 W.A. van Erp, H.W. Kouwenhoven and J.M. Nanne, Z e o l i t e s , 7, 1987, 286-288. 165 Xu Wenyang, L i Jianquan, L i Wengyuan, Zhang Huiming and L i a n g Bingchang, Z e o l i t e s , 9, 1989, 468-473. 166 J.L. Guth, H. K e s s l e r and R . Wey, New Route t o P e n t a s i l - t y p e Z e o l i t e s u s i n g a non A l k a l i n e Medium i n t h e Presence o f F l u o r i d e I o n s , i n : Y. Murakami, A. I i j i m a and J.W. Ward (Eds.), New Developments i n Z e o l i t e Science and Technology, Proc. 7 t h I n t . Conf. on Z e o l i t e s , Tokyo, Japan, Aug. 17-22, 1986, Kodansha, Tokyo and E l s e v i e r , Amsterdam, 1986, pp. 121-128. 167 E.W. V a l y o c s i k and L.D. Rollmann, Z e o l i t e s , 5, 1985, 123-125. 168 U. M u l l e r and K.K. Unger, Z e o l i t e s , 8, 1988, 154-156. 169 H. Lermer, M. Draeger, J. S t e f f e n and K.K. Unger, Z e o l i t e s , 5 , 1985, 131- 134. 170 C.W.R. Engelen, u n p u b l i s h e d r e s u l t s . 171 D.E.W. Vaughan, Secondary C a t i o n E f f e c t s on Sodium and Potassium Z e o l i t e = 9, i n : M.M.J. Tracy, J.M. Thomas and J.M. White Synthesis a t S i / A l (Eds.), Mat. Res. So$. Symp. Proc., M i c r o s t r u c t u r e and P r o p e r t i e s o f Pittsburgh, C a t a l y s t s , V o l . 111, Nov. 30-Dec. 3, 1987, Boston, M.R.S., U.S.A., 1988, pp. 89-100.
Chapter 4
THE PREPARATION OF MOLECULAR SIEVES
. Jansen
A. S y n t h e s i s o f z e o l i t e s
J. C
B. S y n t h e s i s o f A1P04-based m o l e c u l a r s i e v e s
S.T. W i l s o n
A. S y n t h e s i s o f z e o l i t e s
J.C. Jansen U n i v e r s i t y o f Technology D e l f t , L a b o r a t o r y o f Organic Chemistry, J u l i a n a l a a n 136, 2628 BL D e l f t , The N e t h e r l a n d s
I . INTRODUCTION a . General
Nature
provided
mankind w i t h z e o l i t e s ( r e f . 1 ) . Massive z e o l i t e d e p o s i t s have
been d i s c o v e r e d a t many p l a c e s i n t h e w o r l d ( r e f . 2). The o c c u r r e n c e o f n a t u r a l zeolites
can
systems ( r e f s Natural
.
be
assigned
3,4) .
zeolites
t o c e r t a i n g e o l o g i c a l environments o r h y d r o l o g i c a l
generally
form
by
reaction
of
mineralizing
s o l u t i o n s w i t h s o l i d a l u m i n o s i l i c a t e s . The main s y n t h e s i s parameters the
c o m p o s i t i o n o f t h e h o s t r o c k and i n t e r s t i t i a l s o l u t i o n s ; pH
-
aqueous are:
(i)
10, ( i i ) t h e
t i m e ; thousands o f y e a r s and ( i i i ) t h e temperature; o f t e n < 100 'C. The
first
systematic
s t u d i e s on z e o l i t e s y n t h e s i s c o u l d t h u s be g u i d e d by
t h e g e o l o g i c a l and m i n e r a l o g i c a l f i n d i n g s o f t h e n a t u r a l s p e c i e s ( r e f . 5 ) . From 1946
on
many a d d i t i o n a l z e o l i t e t y p e s w i t h o u t a n a t u r a l c o u n t e r p a r t have been
s y n t h e s i z e d ( r e f . 6 ) . The e v o l u t i o n i n t h e
preparation
of
one
of
the
most
s t u d i e d z e o l i t e s i s i l l u s t r a t e d i n F i g u r e 1 by t h e number o f papers and p a t e n t s on t h e m a t e r i a l denoted as ZSM-5 ( r e f . 7 and S e c t i o n
XI1 o f t h i s c h a p t e r ) .
78
NUMBER OF REPORTS
25 20-
15-
L
10-
-
.
5-
o = m n , r - , I
1972
1975
,
,
,
,
,
1985
1980
1990
Fig. 1. The annual number of papers ( 0 )and patents (B) on the preparation of zeolite ZSM-5 since the first publication in 1972.
Throughout the last four decades molecular sieves were mainly prepared by precipitationlcrystallization of an aqueous mixture of reagents at 6 < pH < 14 and temperatures between 100-200 OC. As shown in Scheme 1 a relatively large effort is needed on the optimal preparation procedure of the reactant mixture, EFFORT
1
3
Scheme 1. The effort for the preparatory isolation (3) versus time.
(1).
TIME
the reaction (2) and the
whereafter the hydrothermal reaction process ( 2 ) runs autoclaved for a few days or weeks without manual intervention. Isolation ( 3 ) o f the crystalline material i s a simple final step in the synthesis procedure.
79
of
The z e o l i t e s y n t h e s i s f i e l d i s n o t o n l y extended and r e f i n e d by u s e f u l d a t a modern z e o l i t e c h a r a c t e r i z a t i o n and a p p l i c a t i o n t e c h n i q u e s b u t a l s o by
i n t e r f a c i n g areas o f p h y s i c a l , chemical and mathematical science, see Scheme 2 .
NMR Modern chemicals and p h y s i c a l methods
Nucleation/
Computational
c r y s t a l 1 iz a t i o n
model ing
Scheme 2. Areas o f chemical, p h y s i c a l and mathematical
science
interfacing
the z e o l i t e preparation f i e l d .
Studies i n the
sol-gel
chemistry
and
NMR
analysis
area
have
contributed
s u b s t a n t i a l l y t o t h e knowledge o f t h e hydrothermal r e a c t i o n process. Papers and r e v i e w s r e g a r d i n g s u b j e c t s w i t h i n t h e d i f f e r e n t areas
which
are
mentioned i n Scheme 2 and which a r e o f i n t e r e s t f o r z e o l i t e s y n t h e s i s a r e g i v e n i n Table 1. Besides
the
annually
efforts o f "zeolite synthesis
of
porous
new z e o l i t e
scientists"
in
the
preparations t h e extensive exploratory last
decade
has
resulted
in
the
m a t e r i a l s l i k e t h e A1P04-group ( p a r t B o f t h i s c h a p t e r ) ,
the metal-sulfides ( r e f .
30)
and
the
clathrasils
(ref.
31).
Accordingly,
z e o l i t e s y n t h e s i s appears t o remain a p r o m i s i n g area f o r f u t u r e r e s e a r c h . The c r y s t a l l i n i t y o f d i f f e r e n t s y n t h e s i s p r o d u c t s Plate
1.
The
is
well
illustrated
in
morphology and forms o f t h e c r y s t a l s g i v e a f i r s t i n d i c a t i o n o f
t h e t y p e o f z e o l i t e p r e s e n t and t h e p u r i t y o f t h e p r o d u c t .
80
Table 1. Examples of subjects from areas o f physical, chemical and mathematical science which delivered contributions t o the knowledge o f t h e zeolite synthesis process together with references. Area
Subject
Reference
Sol -gel chemistry
Hydrolysis and condensation o f silicates The sol-gel process
8,9 10
NMR
Structure of (a1umino)silicate-clusters in solution
11,12
Computational model i ng
Lattice energy calculation Local interactions in lattice
13
Modern chemical and physical methods
A1 koxides as reagents Fluorides as reagents and mineralizing agents Gravity - reduced - elevated CVD (chemical vapour deposition) Microwave
15
Nucleationlcrystallization theorypractise Zeal i t e Characterization Appl i cat i on
14
16 17
18 19 20
Mathematical analyses o f zeolite Crystallization. A review Are the general laws o f crystal growth applicable to zeolite synthesis
21
ZSM-5/- 1 1 intergrowth Catalysis - The catalytic site activity - The catalytic properties and the crystal size
23
22
24,25,26 27
81
b . T h i s chapter I n t h i s p a r t of t h e c h a p t e r t h e p r e p a r a t i o n o f two subgroups o f t h e micro-porous t e c t o s i l i c a t e s (see Chapter 3 ) i.e. t h e a l u m i n o s i l i c a t e s and s i l i c a t e s , b o t h i n c l u d i n g t h e c l a t h r a s i l s , w i l l be present ed. The d i v i s i o n between a l u m i n o s i l i c a t e s
and
silicates
i s o f t e n discussed on A l - p o o r r a t h e r
th an A l - f r e e l e v e l ( r e f . 3 0 ) . The
a l u m i n o s i l i c a t e s , s t a r t i n g f r o m Si/A1 r a t i o 1 up t o e.g. Si/A1 r a t i o o f
10000, do r e v e a l t h e presence o f A1 i n s y n t h e s i s , i n c h a r a c t e r i z a t i o n
as
well
as i n a p p l i c a t i o n , see F i g . 2 ( r e f . 31). The A l - p o o r zeo i t e s show no, a t l e a s t no d e t e c t i b l e , Al-dependent behaviour and a r e t h e r e f o r e , t o g e t h e r w i t h t h e A l - f r e e m a t e r i a l s , denoted as s i l i c a t e s . The presence o f aluminium, t h e g u e s t - h o s t i n t e r a c t i o n and t h e n u c l e a t i o n and crystallization
11
contribute
to
the
synthesis
event s
which
are
c h r o n o l o g i c a l l y d e s c r i b e d i n S e c t i o n s I 1 t o V I I o f t h i s chapt er. S e c t i o n V I I I i s focussed on t h e r e a c t i o n parameters. I n S ec t io ns I X and X t h e s i l i c a t e s and c l a t h r a s i l s a r e present ed. Examples
of
research
syntheses
performed w i t h c e r t a i n procedures and/or
m i x t u r e c omp os it i o n s a r e l i s t e d i n S e c t i o n X I . S ec t io ns X I 1 and X I 1 1 c o n t a i n l i t e r a t u r e sources on z e o l i t e p r e p a r a t i o n s and t h e re f e re nc es , r e s p e c t i v e l y .
10
F i g . 2. The
100
1000
10000
r e l a t i v e c a t a l y t i c a c t i v i t y o f H-ZSM-5 versus A1 c o n t e n t on ppm
s c a l e ( r e f . 31).
82
Plate 1. The crystalline nature o f zeolites. a) Single crystals o f zeolite A and b) and c) o f analcime and o f natrolite, respectively. d) A batch o f zeolite L, e) typical needle aggregates o f zeolite mordenite and f ) o f Nu-10.
83
I I . PREPARATORY a. R e a c t a n t s The
chemical
sources
which a r e i n p r i n c i p l e needed f o r z e o l i t e syntheses a r e
g i v e n i n Table 2. T a b l e 2. Chemical sources and t h e i r f u n c t i o n i n z e o l i t e s y n t h e s i s . Sources
Function(s)
Si02
P r i m a r y b u i l d i n g u n i t ( s ) o f t h e framework
A102
O r i g i n o f framework charge
OH-
Mineralizer, guest molecule
A1 k a l i c a t i o n , t e m p l a t e
C o u n t e r i o n o f framework charge, g u e s t m o l e c u l e
Water
Solvent, guest molecule
Within
each
t y p e o f source a v a r i e t y o f chemicals ( r e f . 32), has been used as
t h e d i f f e r e n c e s i n p h y s i c a l n a t u r e and chemical i m p u r i t i e s the
zeolite
synthesis
kinetics
(ref.
34)
and
strongly
influence
sometimes t h e p r o p e r t i e s as
c a t a l y s t s ( r e f s . 24-27). Data
on
t h e s p e c i f i c a t i o n s o f r e g u l a r l y used chemical sources a r e g i v e n i n
t h e f o l l o w i n g survey. - SiO -sources
2
Recent s y n t h e s i s
papers
of
the
Proceedings
of
the
International
Zeolite
Conferences ( r e f s . 34-37) and o f o t h e r z e o l i t e conferences ( r e f s . 38-40) r e v e a l t h a t f o r l a b o r a t o r y s c a l e p a r t i c u l a r S i - s o u r c e s a r e o f t e n used, see T a b l e 3a. Depending
upon
the p a r t i c u l a r synthesis a c e r t a i n Si-source might favour a
s p e c i f i c c r y s t a l l i z a t i o n . For i n s t a n c e , t h e A e r o s i l 200 p r o d u c t can be dissolved
compared
to
the
Optipur
and
d i f f e r e n c e i n p a r t i c l e s i z e , see F i g u r e 3. influence
the
rate
of
nucleation
Gold As
the
impurities
are
more
than
rate
of
dissolution
can
and c r y s t a l l i z a t i o n ( r e f . 41) t h e p r o d u c t
f o r m a t i o n can be a f f e c t e d . A t t h e same t i m e t h e A1 replacing
readily
l a b e l m a t e r i a l because o f t h e
10000
times
and
other
potentially
Si
h i g h e r i n t h e A e r o s i l 200
p r o d u c t compared t o t h e O p t i p u r and Gold l a b e l m a t e r i a l s . The
influence
of
i m p u r i t i e s can change t h e c r y s t a l f o r m ( r e f . 42) and t h e
chemical p r o p e r t i e s ( r e f s . 2 4 - 2 6 ) .
84
Ta ble 3. S p e c i f i c a t i o n s and t h e r e f e r e n c e s o f r e c e n t , r e g u l a r used sources, and high-p u r e ,
*
S i - , and
*
Al-sources.
Specifications S i -s o urc e ( a )
Phys.
Reference
Chem.
A1 -s o urc e ( b )
manufacturer
i m p u r i t i e s (ppm)
Si 7 icon compounds S i (OCH3)4
Tetramethylorthosilicate
liquid
Na,Ca < . 5
liquid
A1,Pt < .2
Merck
(TMOS) S i (OC2H5 14 Tetraethylorthosilicate (TEOS)
Na2Si03.9H20
A1 < 200
"
( o r Na2H2Si04.8H20)
Fe < 120
Q u a r t z Co.
Na20 11%, S i 0 2 29% Water g l a s s
T i < 60
Colloidal silica L u ~ o x - A S- 40 SiO
NH4
I
heavy m e t a l s < 50
A1 < 500
sol
DuPont de Nemours
2r
40 w t % (counterion)
Fe
LU ~ O X -H S -4 0
< 50
Ti B < 10
Si0 2 40 w t % Na'
N" Phi 1 id e l p h i a
(counterion)
Fumed silica A e ro s i 1-200
Dp
-
A1 < 10 12 nm
CAB-0-SIL M-5
* Silica Optipur Gold l a b e l
- 200 pm Dp - 800 pm
Dp
Fe < .6
Degussa
T i < 10
BDH
A1 < .001
Merck
Fe < 0.01
Aldrich
85 Table 3, c ont in u e d
-
Riedel de Hahn
NaA102 Fe < 4
Na20 54%
C a r l o Erba BDH L t d .
Sodium alumin at e
- AlOOH Pseudo-boehmite A1203 70% H20 30%
Dp
- mm Fe < 4 T i < 40
Vista
Fe < 3
Merck
Fe < 0.01
Baker
Catapal - B
-
A1(OH)3
G i bbsi t e
- A1 (N03)3.9H20
Dp
- A1203
*
-
nm
Aluminiumoxide ( U l t r e x )
T h ere f o re , a c a r e f u l c h o i c e o f t h e r e a c t a n t s i s
needed.
The
h i g h grade
Si-
a l k o x i d e s o f which even double a l k o x i d e s l i k e -Si-0-A1- a r e a v a i l a b l e ( r e f . 43) do n o t
have
the
above
d i s c u s s e d disadvantages,
except
for
the
rate
of
h y d r o l y s i s o f t h e a l k o x i d e groups.
F i g . 3.\ SEM photographs of (a) O p t i p u r , (b) Gold l a b e l , and (c) A e r o s i l 200.
86
- A102 Often
source used
Al-sources,
collected
from
t h e same r e f e r e n c e s as g i v e n f o r t h e
S i - s o u r c e s , a r e l i s t e d i n T a b l e 3b t o g e t h e r w i t h t h e
main
chemical
impurity.
Though t h e v e r y p u r e A1203 p r o d u c t c o n s i s t s o f s m a l l p a r t i c l e s i t i s n o t e a s i l y d i s s o l ved. - Alkali c a t i o n / t e m p l a t e The i n o r g a n i c c a t i o n s i n t h e z e o l i t e s y n t h e s i s a r e m a i n l y a l k a l i n e o r ions.
ammonium
The o r g a n i c c a t i o n s / t e m p l a t e s used may be d e v i d e d i n charged and n e u t r a l
molecules c o n t a i n i n g f u n c t i o n a l atoms o r groups. The l a r g e
number
of
organic
molecules
listed
in
several
p u b l i c a t i o n s ( r e f s . 44, 45) t o g e t h e r w i t h t h e s p e c i f i c z e o l i t e p r o d u c t
formed.
To
used
illustrate
in in
zeolite
synthesis
is
extensively
g e n e r a l t h e v a r i a t i o n i n o r g a n i c t e m p l a t e m o l e c u l e s some o f
t h e more common t e m p l a t e s a r e l i s t e d i n Table 4. -
on-
Most z e o l i t e syntheses a r e performed under b a s i c mineralizing
agent.
A
second
agent
is
F-
conditions (refs.
using
OH-
as
a
16 and 46) o f which t h e
d i f f e r e n t n a t u r e compared t o OH' w i l l be d i s c u s s e d i n t h e s e c t i o n
on
reaction
parameters. Both anions a r e t h e c o u n t e r i o n o f t h e i n o r g a n i c o r o r g a n i c c a t i o n s used the
for
syntheses. Depending upon t h e q u a l i t y o f t h e m i n e r a l i z i n g agent i m p u r i t i e s
such as A13'
and Fe3' a r e p r e s e n t a t ppm un t s c a l e .
- The overall
reactant mixture
I n q e n e r a l t h e chemical b e h a v i o u r o f imDuri i e s 1 ike Fe3' importance
compared
to
A13'
in
high
Si/A1
zeolites
c a t a l y s i s when based on Bronsted a c t i v i t y . However,
in
and T i 4 + a r e o f m i n o r i n t h e heterogeneous the
case
of
an
all
s i l i c a z e o l i t e , o r m o d i f i e d z e o l i t e s l i k e B-ZSM-5, Fe-ZSM-5 and Ga-ZSM-5 t r a c e s o f A13+ f r o m r e a c t a n t s as g i v e n i n T a b l e 3 may p l a y an unexpected dominant r o l e
in
t h e B r o n s t e d a c t i v i t y ( r e f s . 24-26). E x t e n s i v e i n f o r m a t i o n on t h i s p o i n t i s
g i v e n i n Chapter 5 on t h e m o d i f i c a t i o n o f influence
of
zeolites.
i m p u r i t i e s f r o m r e a c t a n t s i s K'.
r e t a r d e d by f a c t o r s when
'K
Another
example
of
the
The c r y s t a l l i z a t i o n t i m e can be
i s p r e s e n t i n t h e syntheses o f e . g . z e o l i t e Na-A o r
Na-ZSM-5 ( r e f s . 4 7 , 4 8 ) . I m p u r i t i e s l i k e t r i v a l e n t metal i o n s sometimes change p h y s i c a l c o n d i t i o n s i n t h e r e a c t i o n m i x t u r e i n d i c a t e d by t h e c r y s t a l f o r m o r morphology ( r e f . 42).
87
Table 4. Type of o r g a n i c templates, Organic
f u n c t i o n a l atoms/groups and r e f e r e n c e s .
-__
F u n c t i o n a l atom/group
templ a t e
Ref.
Organic
F u n c t i o n a l atom/group
Ref
templ a t e
-
~
amine
49
n
-NC" n=4,5
N
50 51
3
52
d i -amine
N
~
N 53
54
ammon ium ' P C
' w n n=4,5
-is -+n Cn/NLCn n=4,5
G5
I
I
C-OH
e r y t h r i to1
:N- ( Cn-OH) n=2,3
amine t alcohol
66
)(
~=1-3 ammonium
I
t
a1 cohol
67
-~+-c,-oH n=2
55 acetal
5G amine
-NGO
t ether
69
57 58
di-ammonium
C-OH HO-C-C-C-OH
penta-
N - o x i d e t ammonium
59 phosphonium tri-ammonium
+ A + _ 70
-0-NGN
71
I
-P+I
60
NA +G N
amine t ammonium a1 cohol
Cn-OH
HO-Cn-OH
63 di-phosphonium
n=2-6 HY
tri -01
72
62
n=1-6 d i -01
61
YH
c-c-c I OH
64
-!-C6-y I+
-
I+
73
88
b . The r e a c t i o n v e s s e l and h y d r o t h e r m a l c o n d i t i o n s
Depending OC,
upon
the
r e a c t i o n t e m p e r a t u r e chosen, m a i n l y between 60 5.
r e a c t i o n v es s e l s can v a r y as shown i n T able
aut o c lav es ,
Various
and 300
OC
Teflon
inserted
see F i g . 4a, t o g e t h e r w i t h r e l a t i v e l y l o w p r i c e d ( s t a i n l e s s s t e e l )
and h i g h p r i c e d ( r e e n f o r c e d p o l y e t h e r e t h e r k e t o n ) a u t o c l a v e s a r e shown
in
Fig.
4b and 4c. To f o l l o w t h e c o u r s e o f t h e events t a k i n g p l a c e i n t h e s y n t h e s i s m i x t u r e ,
a
l o o k t h r o u g h aut o c l a v e , see F i g . 4d, can be used. I t was concluded, u s i n g such ex periment a l c o n d i t i o n s , t h a t n u c l e a t i o n and c r y s t a l l i z a t i o n o f z e o l i t e ZSM-5 o c c urred on and in, r e s p e c t i v e l y , g e l spheres o f about 2 mm ( r e f . 42). Another p o s s i b i l i t y t o m o n i t o r s y n t h e s i s event s i n s i t u i s of
IR
the
application
i n t e r n a l r e f l e c t a n c e u s i n g a c r y s t a l embedded i n an aut oclave, as shown
i n F i g . 5 ( r e f s . 74, 7 5 ) .
T a ble 5. Regular
used
lab-scale
r e a c t i o n vessels, t h e t y p i c a l i m p u r i t i e s and
t e mp er a t u r e range. Re ac t io n v es s el
Volume
Impurity
Temperature
P1 a s t i c b o t t l e
( 1 1
Zn2+
Stainless s t e e l autoclave S t a i n 1 ess s t e e l t
< 5 1
Fe3',Cr3'
< 2 1
n u c l e i o f preceding
< 100 o c >> 200 o c < 200 O C
< 5 ml
Si
teflon lining
synthesis
Quartz a u t o c l a v e
< 200
oc
The a ut o c lav es must be f i l l e d between 30 and 70 v o l % i n t h e case o f an aqueous r e a c t i o n m i x t u r e between 100 and 200 O C t o m a i n t a i n a l i q u i d phase ( r e f . 7 6 ) . C leaning o f t h e r e a c t i o n v e s s e l s can be considered i n some cases, teflon-lined
au t o c l a v e s .
As
memory
synthesis i n c a v i t i e s o f the t e f l o n ex periment s
it
is
important
effects wall
can
caused be
by
e.g.
the n u c l e i o f preceding
encountered
in
subsequent
t o c l e a n t h e vessel w i t h e i t h e r HF and w a t e r a t
room t emp era t u re o r NaOH and w a t e r a t t h e r e a c t i o n t emperat ure.
89
1
MACROSCOPE
Fig. 4. Different autoclaves for laboratory use. a) Teflon lined autoclaves u p t o 1000 ml, b ) s t a i n l e s s steel autoclave of 25 ml, c) "Arlon" (polyetheretherketon) reenforced with carbon fiber or glass f i b e r ) autoclaves and d ) s t a i n l e s s steel look through autoclave with quartz windows and Teflon inserts in exploded view together with a schematic drawing of the experimental s e t up.
' R beam
Fig. 5. Parr mini autoclave. I R internal reflection via a crystal embedded in the autoclave makes monitoring of z e o l i t e synthesis events possible ( r e f . 74).
90
111. ZEOLITE PRODUCT VERSUS THE SYNTHESIS MIXTURE a. Two s y n t h e s i s examples The
f i n e t u n i n g and d i f f e r e n c e s i n t h e p r e p a r a t i o n o f each z e o l i t e t y p e i s t o o
complex t o be d is c u s s e d i n t h i s i n t r o d u c t i o n on t h e s y n t h e s i s o f specific
parts
of
the
chapter,
however,
is
chosen
for
a
zeolites. In more d e t a i l e d
p r e s e n t a t i o n o f t h e s y n t h e s i s o f two s u b s t a n t i a l d i f f e r e n t z e o l i t e t ypes, zeolite
Na-A
and
i.e.
z e o l i t e TPA-ZSM-5. The two z e o l i t e s p r e s e n t r o u g h l y a l l t h e
groups i n which z e o l i t e t y p e s a r e d i v i d e d ( r e f . 77). The s y n t h e s i s m i x t u r e s and chemical and p h y s i c a l p r o p e r t i e s o f b o t h z e o l i t e s a r e g i v e n i n T able 6. T able 6. The s y n t h e s i s m i x t u r e s , p h y s i c a l and chemical p r o p e r t i e s
of
zeolites
Na-A and TPA-ZSM-5 ( r e f s . 78-80). Na-A
TPA-ZSM-5 - An example o f s y n t h e s i s m i x t u r e s -
(molar oxyde r a t i o ) 1
Si02
.5
2'3 Na20 TPA20
1 17
H2°
1
< .14 .16 .3 49
> 150
- P h y s i c a l and chemical p r o p e r t i e s -
3D, h o l e s connected v i a windows 1.28 .37
Na',
H20
p o r e arrangements
3 d e n s i t y (g/cm ) p o r e volume (cm3/ g)
ZD, i n t e r s e c t i n g
channels 1.77 .18
1 a t t ic e s t a b i 1iz a t ion
TPA'
1
Si/A1
>
1 ow
Bronsted a c t i v i t y
high
hy dro phy l i c
affinity
hydrophobic
12
91
b. Z e o l i t e product versus synthesis mixture
The most simple zeolite product composition can be given by the overall Si/A1 ratio and the cation type/content. More often the unit cell composition of the zeolite crystal is expressed, e.g. Na-A:
Na12[A112Si12048].27
H20
*
At higher loadings than 4 Al/uc, is replaced by the smaller cation Na' (ref. 81).
TPA' TPA-ZSM-5: 4 TPA[AlnSig6-n0192]H20 * n 5 4
The zeolite reaction mixture i s often formulated in the molar oxyde ratio of the reactants, e.g. SiO2:Al2O3:Na20:(TPA20):H2O. The ratios o f H20/Si02, OH-/Si02, Si02/A1203 and (Na20/TPA20) then give an impression of the concentration, solubility and the expected zeolite types, respectively (ref. 82). Correlation between the synthesis mixture and the product can be obtained from ternary composition diagrams (see Fig. 6a,b) (refs. 83-86), or from graphs of crystallization fields of zeolite types as a function of reactant ratios, see Fig. 6c and Section XI.b.3 SIO) A
a
t ilOl
Fig. 6. Zeolite product versus the synthesis mixture. a) and b ) , ternary compositon diagrams with an inorganic and organic cation/template, respectively. c) Crystallization fields, indicating (0) ZSM-5, (m) ZSM-35, and (A) ZSM-39 (ref. 87).
92
The p r o d u c t f i e l d s a t c e r t a i n P,T, experiment a l
data
depicted
in
Fig.
are
6,
obtained
which a r e n o t always expected f rom a thermodynamic p o i n t o f
view. As t h e i n e v i t a b l y heterogeneous s y n t h e s i s m i x t u r e c o n t a i n s with
different
f rom
micro-domains
r e a c t a n t r a t i o s , k i n e t i c parameters m i g h t induce o t h e r p r o d u c t
phases t h a n t h o s e d e r i v e d f r o m t h e t e r n a r y s y n t h e s i s c o m p o s i t i o n diagram. Because p a r t i c u l a r l y t h e n u c l e a t i o n i s k i n e t i c a l l y det ermined i t i s t hus o f i n t e r e s t t o understand t h e d i f f e r e n t f a c t o r s , e.g. t y p e o f S i - s o u r c e , c a t i o n , A1-source, a d d i t i v e s and p h y s i c a l parameters, i n f l u e n c i n g t h e k i n e t i c st age o f n u c l e a t i o n . The i n f l u e n c e o f t h e s e f a c t o r s can be recognized i n t h e events o f t h e z e o l i t e hereafter i n detail.
T a ble 7 . The subsequent preparation.
subsequent
p r e p a r a t i o n which a r e g i v e n i n T able 7 and discussed
events
present
i n the
Temperature
Subsequent e vent s
Low (< 60 OC)
Reactant s o l u t i o n s
course
of
the
zeolite
Reactant m i x t u r e - g e l f o r m a t i o n Low h i g h (< 60 O C < 200 O C ) +
+
Gel rearrangement Dissolution o f gel Dissociation o f s i l i c a t e
High (< 200
OC)
P r e - n u c l e a t i o n phase Nucleation C r y s t a l l i z a t on
Low (< 60 OC)
I s 0 1a t i o n
I V . THE LOW TEMPERATURE REACTION MIXTURE a. Introduction
The r e a c t i o n m i x t u r e e v e n t s o c c u r r i n g a t l o w t emperat ure disc u s s ed f o r two reasons.
(<
60
'C)
will
be
93
i) Reaction mixtures are prepared at low temperature. Drastic chemical and
physical changes take place then. i i ) Substantial knowledge about the zeolite reaction mixture at low temperature has been obtained using characterization methods such as the molybdate method (ref. 88), the paper chromatography method (ref. 89), the tri-methylsilylation method (ref. go), IR- and laser-Raman spectroscopy (ref. 91), single crystal structure analysis (refs. 92, 93) and the NMR technique (ref. 94). Mostly, starting reaction mixtures typically consist of a gel phase and a liquid phase which means that nucleation is initiated at high temperature by the presence o f a residual gel phase, though there are a few exceptions (refs. 91, 95). The (a1umino)silicate gel phase consists of either a homogeneous dispersed phase of branched chains of sol particles, see Fig. 7I, or a more separated solid phase of an ordered aggregate of sol particles (like opals), see Fig. 711(refs. 96, 97).
I
I1
Schematic representation
Micrograph picture
Fig. 7. Alkaline gel forms. Schematic representation and micrograph picture of I ) a dispersed low density gel (ref. 96b) of branched chains of sol particles and 11) a separated high density gel form resulting in spheres consisting of an ordered aggregate o f sol particles (like opals)
.
94
Si(IV)
Si(lV) O h
80 60 40
20 0
a)
6
a
10
12
pH
O.lm Si(IV)
Fig. 8. S i l i c a t e d i s t r i b u t i o n v e r s u s pH a t c o n c e n t r a t i o n ( r e f . 98).
a)
high
and
b)
low s i l i c a t e
The pH o f t h e l i q u i d phase, i n t h e case o f OH- as t h e m i n e r a l i z i n g agent, l i e s g e n e r a l l y between 8-12. As d e p i c t e d i n F i g . 8 t h e most abundant f orm(s) o f S i sp ec ies a t r e l a t i v e l y h i g h pH a r e t h e monomeric i o n s , whereas a t l o w e r pH v a l u e monomeric n e u t r a l S i - s p e c i e s can be formed, when t h e S i - c o n c e n t r a t i o n i s low. A t h i g h c o n c e n t r a t i o n , however, c y c l i c
tetramers
a r e most
abundant
species
( r e f . 98).
b . Hydrolysis and condensation o f silicate Monomers
and
ol i g o m e r s
i n s o l u t i o n a r e i n e q u i l i b r i u m w i t h t h e g e l phase. A t
t h i s ambient s t ag e o f t h e r e a c t i o n m i x t u r e released e.g.
from
monomeric
silica
species
can
be
t h e g e l v i a h y d r o l y s i s r e a c t i o n s and a r e p r e s e n t i n s o l u t i o n as
Si(OH)30- and Si(OH),022-.
The d i s s o l u t i o n o f t h e g e l i s promoted by
the
OH'-coordination o f s i l i c o n above f o u r , t h u s weakening t h e o t h e r s i l o x a n e bonds t o t h e g e l network. T h i s n u c l e o p h i l i c mechanism i s p r e s e n t e d t o o c c u r v i a a SN2-Si t r a n s i t i o n s t a t e as shown i n Scheme 3a ( r e f . 9 ) .
95
Scheme 3. a) H y d r o l y s i s and b) condensation mechanism o f s i l i c a t e species a t room temperature.
gel phase
1 Relative
<
branching
> of SiOH
Scheme 3c. Growth s i t e i n t h e g e l phase f o r monomers from s o l u t i o n .
The mechanism o f t h e condensation r e a c t i o n s
in
aqueous
systems
at
high
pH
i n v o l v e s t h e a t t a c k o f a n u c l e o p h i l i c d e p r o t o nat ed s i l a n o l group on a monomeric n e u t r a l species as r e p r e s e n t e d i n Scheme 3b ( r e f . 9 ) . The a c i d i t y o f t h e s i l a n o l group depends on t h e number and t y p e of s u b s t i t u e n t s on t h e s i l i c o n - a t o m . The more s i l i c o n s u b s t i t u e n t s a r e present , the
more
a c i d i c t h e OH-groups o f t h e c e n t r a l s i l i c o n atom. As shown i n Scheme
3c, a t h i g h pH t h e most f a v o u r a b l e p o l y m e r i z a t i o n i s t h e r e a c t i o n between l a r g e most h i g h l y branched species and t h e monomer s i l i c a species. A t more
specific
neutral bonding
pH,
hydrolysis
configurations,
and see
condensation
of
pe nt a c oord inat e s t a t e o f S i , i l l u s t r a t e d i n Scheme 3, i s 99 ).
clusters,
containing
F i g . 9, i n d i c a t e t h a t i n v e r s i o n i n t h e not
essential
(ref.
96
+ I
I
+
F ig . 9. Condensation o f octamers,
The
pentacoordinate
silicon
H20
with r e t e n t i o n o f t h e configuration.
intermediate
state
is,
however,
conf irmed
c r y s t a l l o g r a p h i c a l l y ( r e f . 100). Condensation
of
m i x t u r e i s above t h e
the
monomers
isoelectric
lead, point
as of
the silica
pH o f t h e z e o l i t e s y n t h e s i s
(ref.
t o ramified c l u s t e r s can be r e o r g a n i z e d i n t o fewer l a r g e r p a r t i c l e s w i t h a
c l u s t e r s . Such co rres ponding r e d u c t i o n i n s u r f a c e energy, according t o
the
101)
Ostwald
ripening
p r i n c i p l e . The s t r u c t u r a l e v o l u t i o n o f a g r owing c l u s t e r i s s c h e m a t i c a l l y g i v e n i n F i g . 10.
F i g . 10. S t r u c t u r a l e v o l u t i o n o f s i l i c a t e c l u s t e r s .
97
c. Evidence f o r silicate clusters In the course of the gel dissolution the monomers form dimers, according to 29Si-NMR studies (ref. 94), via condensation reactions whereafter trimers and tetramers, cyclic trimers and tetramers and higher order rings are observed as condensation products, see Fig. 11.
Fig. 11. Numerous oligomers characterized in solution at low temperature by *'Si -NMR (ref. 101).
Evidence for the existence of e.g. double four rings resulted from the s ngl e crystal structure analysis of so-called pseudo-A, a material, not a zeo i t e , crystallized at ambient temperature from a mixture of SiOp, TBAOH and H20 see Fig. 12 (ref. 93).
Fig. 12. Model of a part of the framework of pseudo-A; the double four ring units are indicated by asterisks (*).
The silicate species identified in the liquid phase by a.0. NMR, SAXS (ref. 102) and IR, (ref. 103) are products in a simple reaction mixture of Si02, NaOH and H20 at room temperature.
98
The i n t e r a c t i o n o f a l k a l i - i o n s i n such suggested
(ref.
systems
is
not
clear.
is
It
104) t h a t t h e o r d e r e d h y d r a t i o n sphere o f a.0. Na'
often
stabilizes
s i l i c a t e species. Recent NMR r e s u l t s i n d i c a t e t h a t i n t e r a c t i o n between
cations
and s i l i c a t e s p e c i e s ( r e f . 105) do occur. An o r g a n i c c a t i o n / t e m p l a t e added as i n g r e d i e n t ( s ) mixture
to
the
simple
reaction
shows i n t y p i c a l experiments a c c o r d i n g t o NMR measurements i n t e r a c t i o n
w i t h t h e g e l and s i l i c a t e species, r e s p e c t i v e l y ( r e f s . 106-108). However, t h e h i g h l y c o m p l i c a t e d s e t o f i n t e r a c t i o n s and f a s t changing e q u i l i b r i a , due t o t h e i n c r e a s e d number o f t y p e o f s p e c i e s a f t e r a d d i t i o n o f t e m p l a t e and/or A13'
has
n o t been u n r a v e l l e d y e t .
V. THE TEMPERATURE RAISE OF THE REACTION MIXTURE Temperature
raise,
< 60
from
OC
up t o < 200 O C , can be p e r f o r m e d i n s e v e r a l
ways as shown i n F i g . 13 f o r one t y p e o f a u t o c l a v e and
reaction mixture.
The
d i f f e r e n t h e a t i n g r a t e s a r e achieved i n s t a t i c systems. 200 'C
100
-
501// 0
0
5 ml
t (min)
2
4
6
8
10
12
F i g . 13. D i f f e r e n t h e a t i n g r a t e s f o r one t y p e o f a u t o c l a v e a c h i e v e d
by
(a)
microwave, (b) h o t sand b a t h and (c) h o t a i r oven. The s i z e o f t h e a u t o c l a v e , t h e v i s c o s i t y o f t h e r e a c t i o n m i x t u r e and t h e way o f a g i t a t i n g e.g.
static,
tumbling
o r t u r b o s t i r r i n g a r e f a c t o r s modulating t h e
temperature r a i s e o f t h e r e a c t i o n mixture. During
the
temperature
raise
of
the
reaction
mixture
f r o m ambient t o
r e a c t i o n c o n d i t i o n s p r i m a r y events a r e : - A c c e l e r a t e d d i s s o l u t i o n o f t h e g e l i n t o monomeric s i l i c a t e species. - D i s s o c i a t i o n o f s i l i c a t e o l i g o m e r s i n s o l u t i o n and i n c r e a s e
measured
by
NMR up t o
-
of
monomers
as
100 O C ( r e f s . 109-112). As shown i n F i g . 14 a model
s t u d y w i t h NMR on t r i m e t h y l s i l y l a t e d s i l i c a t e c o n f i r m s ( r e f . 109) a s h i f t o f t h e s i l i c a t e anion e q u i l i b r i u m f r o m r e l a t i v e h i g h - m o l e c u l a r , m a i n l y d o u b l e f o u r r i n g s , t o l o w - m o l e c u l a r w e i g h t , monomers and dimers.
99 ( % I mol
.
.
y c u b i c oc!amer \
€0
\
monomer,
R
* \ :
‘\
- --.
dim-er0 0
20
60
40
00
100
‘C
changes i n c o m p o s i t i o n
F i g . 14. Main
(X mol) o f t r i m e t h y l s i l y l a t e d s i l i c a t e
s o l u t i o n versus temperature.
- Higher
concentration
and
mobility
of
monomeric
s i l i c a t e - and e v e n t u a l l y
a l u m i n a t e species. - Association o f primary b u i l d i n g u n i t s . - P o s s i b l e n u c l e a t i o n and c r y s t a l l i z a t i o n o f unwanted ( m e t a s t a b l e ) phases. Some secondary events a r e : - The s t a r t o f t h e d e g r a d a t i o n
o f q u a t e r n a r y ammonium i o n s , which s u b s t a n t i a l i n a ZSM-5 s y n t h e s i s ( r e f . 4 2 ) as d e p i c t e d i n F i g . 15.
- S t a r t o f t h e d r o p i n pH caused by t h e Hoffman d e g r a d a t i o n .
100.; \
%
:TPA
\
t
i
\
50
‘, .‘A
.. .
- - t r - - - - > _ - -
- - - -._ -
-_
0. . -‘(hr)
F i g . 15. D e g r a d a t i o n o f tetrapropylammonium v e r s u s t i m e .
can
be
100
V I . THE HIGH TEMPERATURE REACTION PROCESS
a. Introduction reaction
The main e v e n t o c c u r r i n g i n t h e s y n t h e s i s m i x t u r e a t t h e is
the
formation
of
zeolites
from
amorphous
material.
temperature
Chemical r e a c t i o n
processes a c c e l e r a t e d by t h e h i g h t e m p e r a t u r e l e a d t o :
i) f u r t h e r r e o r g a n i z a t i o n o f t h e l o w t e m p e r a t u r e s y n t h e s i s m i x t u r e ; ii)w h e r e a f t e r p r i m a r y (homogeneous
or
c r y s t a l s ( r e f . 113)) n u c l e a t i o n ; iii) and f i n a l l y , p r e c i p i t a t i o n (based
heterogeneous) on
and
reactions)
secondary as
a
(seed
form
of
crystallization.
b. Nucleation At
t h e high temperature o f t h e r e a c t i o n mixture the z e o l i t e c r y s t a l l i z a t i o n i s
expected a f t e r an i n d u c t i o n p e r i o d i n which t h e n u c l e a t i o n o c c u r s . induction period the
gel
During
the
and s p e c i e s i n s o l u t i o n ( a f o r e m e n t i o n e d i n t h e l o w
t e m p e r a t u r e s e c t i o n ) r e a r r a n g e from a c o n t i n u o u s changing phase o f monomers and clusters,
e.g.
polysilicates
and
aluminosilicates.
These c l u s t e r s f o r m and
d i s a p p e a r t h r o u g h i n h o m o g e n e i t i e s i n t h e s y n t h e s i s m i x t u r e v i a c o n d e n s a t i o n and hydrolysis
processes.
The
c o n t i n u o u s d i s s o l u t i o n o f t h e g e l phase i n c r e a s e s ,
however, t h e amount o f c l u s t e r s and t h e p o s s i b i l i t y o f f u r t h e r the
clusters
and c a t i o n s .
In
the
s t a b l e . N u c l e i o f c e r t a i n dimensions, e.g. and
-
-
association
of
o f t h i s p r o c e s s p a r t i c l e s become
course
10 A f o r z e o l i t e Na-A
(ref.
114)
20 A f o r z e o l i t e ZSM-5 ( r e f . 115), a r e formed and c r y s t a l l i z a t i o n s t a r t s .
c. Crystallization The
l i n e s a l o n g which ideas on z e o l i t e c r y s t a l f o r m a t i o n a r e developed, e i t h e r
based on b u l k and macroscopic o b s e r v a t i o n s o r on are
described occur
mechanistic
scale
i n t h i s paragraph. Four cases o f n u c l e a t i o n and c r y s t a l l i z a t i o n
a r e s c h e m a t i c a l l y p r e s e n t e d i n Table 8. might
molecular
in
clear
synthesis
r e a c t i o n m i x t u r e s where ( b )
(a)
Zeolite
crystallizations,
which
s o l u t i o n s , o r , more o f t e n , i n heterogeneous
highly
dispersed
or
(c)
dense
gel
forms
are
p r e s e n t , see a l s o F i g . 7. I n some occasions ( d ) m e t a s t a b l e s o l i d phases undergo transformation during synthesis.
Homogeneous
n u c l e a t i o n whereafter
crystal -
l i z a t i o n has been observed i n ( a ) c l e a r s o l u t i o n e x p e r i m e n t s ( r e f s . 91, 9 2 ) .
101
Table 8. Four cases of crystal growth environment representation of nucleation and crystal 1 ization. Crystal growth environment
(a)
Clear solution
(c)
Separated high density gel
(d)
Solid phase
and
schematic
Nucleation Crystallization (+) ( a )
k
H
Fig. 16. a) Powder and b) a twinned elongated prismatic crystal o f ZSM-5 from a dispersed gel phase and c) a cubic form of ZSM-5 from a dense gel phase.
102
Nucleation
(heterogeneous) o c c u r s a t t h e l i q u i d - g e l i n t e r f a c e i n t h e d i s p e r s e d
g e l - s o l u t i o n m i x t u r e s (b) ( r e f . 108). The forms o f t h e c r y s t a l l i z a t i o n p r o d u c t s in
the
case
of
a
dispersed
gel
phase
a r e shown f o r ZSM-5 i n F i g . 16a,b.
S i m i l a r l y t o t h e c l e a r synthesis solutions, the driving force for c r y s t a l l i z a t i o n i s equal i n a l l d i r e c t i o n s as shown i n T able 8a,b. I n t h e case o f a dense g e l phase p r e s e n t i n t h e s y n t h e s i s m i x t u r e , see T able 8c, c r y s t a l lization
proceeds
g e l ( r e f . 42) as shown s c h e m a t i c a l l y i n F i g . 17.
into t h e
D e v i a t i n g c r y s t a l forms compared t o c r y s t a l forms f r o m d i s p e r s e d a r e t h e n observed, as shown i n F i g . 16c.
gel
systems
G enera lly , t h e t y p i c a l f o r m and morphology o f a z e o l i t e c r y s t a l r e v e a l s n o t i n f o r m a t i o n on t h e t y p e o f t h e z e o l it e formed b u t a1 so on t h e c r y s t a l
only
growth h i s t o r y , as shown above. a/c
ratios
v i e w s on gelsphere surface
Pyramidal crystals
perpendicular
basal piano 2nd afc piano
@
3
1
.4
.8
.6
.7
.7
along
ib gel s p h e r e
0
a/c r a t i o s o f d e v e l o p i n g c r y s t a l s and schematic drawing o f g ro w t h process i n t h e g e l spheres.
F ig . 17. Average
As
a
l i q u i d phase
is
c o n t i n u o u s l y p r e s e n t between t h e d i s s o l v i n g dense g e l
phase and t h e gro w i n g c r y s t a l , t h e c r y s t a l l i z a t i o n i s , however,
still
solvent
mediated. When a me t a s t a b l e s o l i d phase, e . g . a z e o l i t e , i s p r e s e n t i n mixture,
a
transformation
the
synthesis
i n t o a more s t a b l e phase i s p o s s i b l e , a c c o r d i n g t o
t h e Ostwald r u l e o f s u c c e s s i v e t r a n s f o r m a t i o n s ( r e f . 116).
103 The
nucleation
and c r y s t a l l i z a t i o n o f t h e new phase, i l l u s t r a t e d i n T a b l e 8d,
occurs i n t h e s u p e r s a t u r a t e d s o l u t i o n
generated
by
the
dissolution
of
the
f o r m e r phase ( r e f . 117).
I n t h e l a s t t h r e e cases o f Table 8 dynamic steps
of
equilibria
between
successive
d i s s o l u t i o n , i o n t r a n s p o r t a t i o n and p r e c i p i t a t i o n , can be r e c o g n i z e d
( r e f . 118). E s p e c i a l l y , t h e precipitation/crystallization s t e p , i.e.
the
type
o f c r y s t a l b u i l d i n g u n i t s and t h e way o f c r y s t a l growth on m o l e c u l a r l e v e l , has been s u b j e c t t o many s t u d i e s .
d. Crystal building units A t l e a s t t h r e e t y p e s o f c r y s t a l b u i l d i n g u n i t s have been
suggested
which
are
d e s c r i b e d be1 ow. d.1. The Drimarv b u i l d i n q u n i t t h a t p r i m a r y b u i l d i n g u n i t s , i .e. t e t r a h e d r a l monomeric s p e c i e s , can
Arguments
be i n v o l v e d i n t h e c r y s t a l l i z a t i o n are: i ) The g e n e r a l view from c r y s t a l growth t h e o r i e s t h a t c r y s t a l s a r e formed v i a p r i m a r y b u i l d i n g u n i t s ( r e f . 119);
ii) The general view i n s o l / g e l c h e m i s t r y ( r e f s . 8, 10) t h a t t h e most f a v o u r e d condensation r e a c t i o n occurs between a monomeric and p o l y m e r i c s p e c i e s . I n terms
o f t h e z e o l i t e c r y s t a l l i z a t i o n : between a p r i m a r y b u i l d i n g u n i t and
a c r y s t a l s u r f a c e ; see S e c t i o n IVb; iii) At
raising
increases
temperatures (ref.
measurements
109)
at
( t i l l 200
(up the OC)
t i l l 100 OC) t h e c o n c e n t r a t i o n o f monomers expense are
not
of
clusters.
actually
Though
performed,
e x p e r i m e n t a l r e s u l t s might i n d i c a t e t h a t a t r e a c t i o n
in
situ
the
above
temperatures
mainly
monomers a r e p r e s e n t ; i v ) S t u d i e s on t h e c r y s t a l l i z a t i o n o f z e o l i t e have shown t h a t t h e g r o w t h o f z e o l i t e occurs by a s u r f a c e r e a c t i o n o f monomeric a n i o n s ( r e f . 120). d.2. As
a
A t y p i c a l c l u s t e r as b u i l d i n q u n i t shown
in
Chapter
r e l a t i v e l y low several
decades
(5
3
of
this
16-Si-tetrahedra) ago
book
secondary b u i l d i n g u n i t s (SBU’s) a r e
polymer
units.
SBU’s
and f u r t h e r p h y s i c a l f e a t u r e s o f t h e z e o l i t e s . A t t h e same t i m e non-chiral
independent
were
introduced
( r e f . 121) and used s i n c e t o p r e s e n t s t r u c t u r a l ( r e f . 6 )
SBU’s a c t i n g as
u n i t s can generate a c e r t a i n z e o l i t e s t r u c t u r e . It i s ,
however, though t h e SBU’s show sometimes a s u p e r f i c i a l resemblance t o
silicate
104
anions,
not
t h a t SBU's a r e t h e b u i l d i n g b l o c k s o f t h e growing c r y s t a l
likely
( r e f . 1 2 2 ) . On t h e o t h e r hand, t h e b u i l d i n g o f t h e porous and d i f f e r e n t z e o l i t e frameworks w i t h monomers c o n d e n s a t i n g i n t h e r i g h t t o p o l o g y seems l e s s f a v o u r a b l e compared t o a t y p i c a l c l u s t e r b u i l d i n g u n i t ( r e f .
123).
From t h i s
p o i n t o f view s u g g e s t i o n s a r e r a i s e d about a t y p i c a l o r common c l u s t e r b u i l d i n g u n i t f o r a l l z e o l i t e structures. d.3. The c a t i o n t e m l a t i n q theor! Organic
as
well
water-ordering,
as
inorganic
properties.
cations
Typical
show
structure
directing,
c r y s t a l s t r u c t u r e a n a l y s i s o f o r g a n i c w a t e r c l a t h r a t e d c a t i o n s ( r e f . 124). wat e r
i.e.
examples a r e g i v e n i n a r e v i e w o f s i n g l e The
mo lec ules c o m p r i s i n g a t e t r a h e d r a l n e t work i n t h e f i r s t l a y e r around t h e
c a t i o n m i g h t be p a r t l y r e p l a c e d by s i l i c a t e and aluminat e
anionic
tetrahedra.
The c l a t h r a t e d c a t i o n s m i g h t s e r v e t h i s way as c r y s t a l b u i l d i n g u n i t s . An example o f such a t e m p l a t i n g / c l a t h r a t i n g r o l e i s t h e f o r m a t i o n s o d a l i t e w i t h tetramethylammonium (TMA')
of
c a t i o n s ( r e f . 125).
The h i g h t e mp era t u r e events, d i s c u s s e d above, a r e summarized i n Scheme 4.
met as t ab 7 e phase
I
ion t r a n s p o r t a t i o n
I
II
s t a b l e phase
I
I
-
gel or
c
small c l u s t e r s
I
I
I
hydrolysis
I
association
condensat ion
Scheme 4. Re p r e s e n t a t i o n o f crystallization.
I
1
precipitation
successive
steps
in the evolution o f zeolite
105
e. Nucleation-crystal 7 iration kinetics
Nucleation and crystal1 ization events are generally illustrated on characteristic S-shaped crystallization curves (ref. 126). The yield (wt % of crystalline material), often determined by indirect methods, plotted against time gives an impression of the nucleation and crystallization time and certain reaction temperatures. More accurate information on the crystallization kinetics can be provided when, based on crystal size and size distribution, the linear crystal growth rate and the rate of nucleation can be determined. Of the studies (ref. 127) on zeolite crystallization, one contribution (ref. 128) reporting on a method to collect kinetic data is briefly described here.
20
30
40
x 100
5
0
10
a
**
,''
b
,~*opr'-l'OOX conversion of
]50the
.lo2
dt
zo!
A
t7\ p
p/"
10
0 6-A 0 40
> O ,\{
80
120
,
160
Time j h I
I
I
I
200
mars of crystals
C
240
Fig. 19. a) Histogram o f the crystal size distribution in the final product, b) diameter of the largest crystals o f different unfinished crystallization runs versus time, resulting in the crystal growth rate graph and c) (i) the nucleation rate (number o f crystals of each unfinished crystallization run versus time) together with (ii) the yield curve.
106
A number of i d e n t i c a l synthesis experiments, b u t d i f f e r i n g i n t o t a l synthesis time, were performed. The average diameter o f t h e l a r g e s t c r y s t a l s which could be c o l l e c t e d from t h e various products was measured. I n t h e case o f z e o l i t e Na-X i t was found t h a t i n a p l o t o f c r y s t a l s i z e versus time t h e l i n e a r c r y s t a l growth r a t e (.5 A L / A t ) was constant, i r r e s p e c t i v e o f t h e c r y s t a l
size,
even u n t i l near exhaustion o f t h e c r y s t a l b u i l d i n g u n i t s , see Fig. 19b. The n u c l e a t i o n time can be determined now product
for
any
crystal
in
the
final
o f t h i s Na-X c r y s t a l l i z a t i o n , knowing the growth r a t e . For instance, a
c r y s t a l o f 16.5 pn nucleated a t t
- 90
h.
Together
with
the
particle
size
d i s t r i b u t i o n curve, Fig. 19a, t h e r a t e o f n u c l e a t i o n was found, see F i g . 19c. The n u c l e a t i o n r a t e curve and the
c a l c u l a t e d from both the growth r a t e and p a r t i c l e s i z e d i s t r i b u t i o n curve, i n d i c a t e t h a t as soon as t h e c r y s t a l l i z a t i o n s t a r t s the chemical n u t r i e n t s are consumed f o r c r y s t a l growth. The formation o f conclusion, i t can crystal1ine
yield
curve
f r e s h n u c l e i i s from be said t h a t z e o l i t e
product
can d e l i v e r
then on l a r g e l y suppressed. I n synthesis, r e s u l t i n g i n a good
accurate
i n f o r m a t i o n on
n u c l e a t i o n and
c r y s t a l 1iz a t i o n k i n e t i c s . f . Energy o f a c t i v a t i o n
Though z e o l i t i c m a t e r i a l can be prepared a t low temperature (20-60 'C) most n u c l e a t i o n and c r y s t a l l i z a t i o n processes are performed a t temperatures between 60 and 250
OC.
The choice o f t h e r e a c t i o n temperature i s governed by t h e energy
o f a c t i v a t i o n r e q u i r e d f o r the z e o l i t e c r y s t a l l i z a t i o n .
Table 9 shows the energy o f a c t i v a t i o n (E,)
as
a
function
of
the
Si/Al
ratio. Table 9. Ea's o f d i f f e r e n t z e o l i t e framework types and S i / A l r a t i o s . Guest molecule Na';
H20
TPA+
TPA+ Na';
H20
Ea (kcal/mol)
Framework
Si/A1
Y
1.5
11.8
MF I MF I MF I
1.8
12.3
2.2
14.1
2.5
15.6
30 W
80
7 11 18
Ref. 129
a
b C
107
I t appears t h a t t h e Ea's a r e n o t r e l a t e d t o d i f f u s i o n o f c r y s t a l b u i l d i n g u n i t s
in
(Ea ( d i f f . ) < 5 k c a l m o l - l ) b u t t o condensation r e a c t i o n s between
solution
t h e c r y s t a l s u r f a c e and c r y s t a l b u i l d i n g u n i t . As shown i n T a b l e 9 Na-X
changes
as
a
Ea
the
of
f u n c t i o n o f t h e S i / A l r a t i o which i n d i c a t e s t h a t t h e more
s i l i c i o u s the zeolite, the
larger
the
Generally,
Ea.
this
trend
is
also
observed between d i f f e r e n t z e o l i t e s , a l t h o u g h t h e c o n t r i b u t i o n t o Ea o f c a t i o n s and t e m p l a t e s , as shown f o r ZSM-5, can be s u b s t a n t i a l .
VII. ISOLATION OF THE ZEOLITE PRODUCT Products o f z e o l i t e p r e p a r a t i o n s can be composed o f e i t h e r phase, quartz,
a
one
pure
zeolitic
m i x t u r e o f z e o l i t i c phases o r a m i x t u r e o f a z e o l i t i c phase and e.g. cristobalite
or
gel
phase.
Mostly
the
product
is
isolated
by
d e c a n t a t i o n / c e n t r i f u g a t ion o r f i1t r a t i o n .
I f the product consists o f c r y s t a l s w i t h a uniform recognized
as
characteristic
for
the
expected
crystal
form
which
is
p r o d u c t , t h e z e o l i t e can be
separated by d e c a n t i n g t h e mother l i q u o r f o l l o w e d by washing w i t h w a t e r .
If there precipitated dissolution
is, as
however, e.g. some g e l phase p r e s e n t , t h i s may be e i t h e r c o a
separate
phase
e l e v a t e d temperature i s s t r o n g l y zeolite.
or
adsorbed
on
the
crystals.
Careful
o f t h e g e l phase w i t h e.g. a d i l u t e b a s i c OH' s o l u t i o n a t s l i g h t l y
Especially
in
the
advisable
case
of
prior
adsorbed
to gel
the on
isolation
of
the
t h e c r y s t a l surface
elemental a n a l y s i s (AAS, I C P o r EMPA) i s r e q u i r e d t o c o n t r o l t h e Si/A1 r a t i o o f the
c r y s t a l s b e f o r e and a f t e r t h e washing procedure ( r e f . 130). The f i n a l s t e p
i n t h e z e o l i t e p r e p a r a t i o n i s t h e d r y i n g o r c a l c i n a t i o n procedure
after
which
t h e z e o l i t e v o i d volume i s f r e e f o r d i f f e r e n t m o d i f i c a t i o n and/or a p p l i c a t i o n .
VIII. REACTION PARAMETERS a. Introduction
The t y p e o f r e a c t a n t s , t h e way t h e r e a c t a n t m i x t u r e i s made, temperature formation.
typically
affect
the
crystallization
the
kinetics
pH and
and
the
product
108
Furthermore
the pre-treatment o f the reaction mixture, the addition o f c r y s t a l
growth i n h i b i t o r s , t h e r e a c t i o n m i x t u r e t e m p e r a t u r e t r a j e c t o r y and t h e
use
of
seeds have an i n f l u e n c e on t h e z e o l i t e p r e p a r a t i o n . Some aspects o f t h e t y p e o f t h e above mentioned f a c t o r s a r e d i s c u s s e d i n t h e f o l l o w i n g paragraphs. I l l u s t r a t i o n s a r e m a i n l y g i v e n on t h e z e o l i t e A and ZSM-5 formation.
b. The 5i-source As mentioned i n
Section
I1 of
this
chapter
the
different
types
of
the
S i - s o u r c e s c o n t a i n i m p u r i t i e s which may a f f e c t z e o l i t e c r y s t a l l i z a t i o n . Another parameter, t h e s p e c i f i c s u r f a c e a r e a o f t h e s e sources, can r e s u l t i n d i f f e r e n t nucleation
c r y s t a l l i z a t i o n t i m e s as shown f o r z e o l i t e A i n F i g . 20a ( r e f .
and
4 7 ) . The s h o r t e r i n d u c t i o n and c r y s t a l l i z a t i o n t i m e s l e a d t o more
and
smaller
c r y s t a l s , see F i g . 20b.
.-
-
/,..
Silica source
I
I1 111
a
I
1
2
3
4
5
6
7
Crystals r e 1 . number size 48 .7 30 2.6 15 4.8
b
8
tlme(h)
Fig. 20. a) The y i e l d o f z e o l i t e A versus t i m e o f d i f f e r e n t s i l i c a
sources.
b) The s p e c i f i c s u r f a c e a r e a s o f t h e s i l i c a sources ( I > I 1 > 111) r e s u l t i n d i f f e r e n t amounts and s i z e s o f c r y s t a l s .
c. The type o f template Many t y p e s o f t e m p l a t e chapter).
The
are
surprising
regularly
t y p e o f z e o l i t e framework formed i s template
can
used
(see
e.g.
Section
I1 o f t h i s
performance o f c e r t a i n t e m p l a t e s on s t a b i l i z i n g t h e illustrated
in
Table
10.
One
type
of
be used t o c r y s t a l l i z e v a r i o u s z e o l i t e s whereas t h e same t y p e o f
z e o l i t e may be c r y s t a l l i z e d w h i l e u s i n g d i f f e r e n t t e m p l a t e s .
109
T a b l e 10. S i n g l e
and m i x t u r e
of
i n t h e preparation o f
templates/cations
d i f f e r e n t z e o l i t e types. Single
Zeolite
Ref.
Mixture o f
temp1 a t e
/
TMA'
Zeol it e
Ref.
x,
Y
135
L (+ K')
136 136
template/cation
Sodalite
--A,
131 TMA',
Na'
Gismondine 132
Sodalite,
P, S and R ZSM-6 and
137
ZSM-47
TPA+ Na'
\
/
TEA
133 ZSM-5
-
EDA Ethanolamine
134
138
ZSM-5
139
j ,
Propanol ami n
- Omega
Na'
A1 coho1 Glycerol Morphol i n e Hexanediol TPA
-
The r o l e o f t h e s i n g l e t e m p l a t e / c a t i o n i n
stabilizing
subunits
of
different
z e o l i t e types i s n o t unravelled y e t .
A common f a c t o r , however, appears t o be t h e s i z e diameter
in
the
structures
of
r e s p e c t i v e l y , and t h e d i a m e t e r o f
-
of
a
certain
free
void
6.8 A and 7.0 A, 6.7 A o f t h e t e m p l a t e TMA', see F i g . 21.
sodalite
and
gismondine,
110
Fig. 21. Models
of
a)
the
s o d a l i t e and b) t h e gismondine v o i d and t h e v o i d
f i l ler/template/cation TMA+.
Although
TPA'
and Na' are r a t h e r d i f f e r e n t templates/cations a common f a c t o r
might be the s t a b i l i z a t i o n o f voids ( e i t h e r i n t e r s e c t i o n o f channels o r channel windows), see Fig. 22.
Fig. 22. View along s t r a i g h t channels o f w i r e model o f ZSM-5 with e i t h e r TPA' (*) o r hydrated Na' (0) on i n t e r s e c t i o n s o f channels and channel windows, r e s p e c t i v e l y .
Charged temp1 ates
compensate
negative
framework charges, due t o isomorphous
by A13'. A range o f S i / A l r a t i o s i s possible, see Scheme If, however, t h e number o f charged templates r e q u i r e d f o r charge compensation cannot be accommodated f o r dimensional reasons t h e zeol i t e substitution o f Si4'
5.
111
combines charged t e m p l a t e s w i t h e.g. Na'. for
one
Sodalite
zeolite
type
are
T h i s way, s t i l l v a r i o u s Si/A1
ratios
as shown i n Scheme 5 f o r z e o l i t e ZSM-5.
possible
can be prepared w i t h two d i f f e r e n t Si/A1 r a t i o s .
ZSM-5
Si/A1
Soda1 it e
TPA+
23 - <10000
TMA+
TPAt/Nat
23 - 11
Na'
Si/A1
Na'
11
Scheme 5. D i f f e r e n t Si/A1 r a t i o s f o r ZSM-5 and s o d a l i t e .
d. The reactant mixture The way
reactant
mixtures
are
made,
e.g.
r e a c t a n t s , t h e s t i r r i n g and g e l aging can r e s u l t
the
addition
sequence
i n method-dependent
o f the factors
i n f l u e n c i n g n u c l e a t i o n . As shown i n F i g . 23 c r y s t a l s o f z e o l i t e A s t a r t e d growing i n a z e o l i t e X s y n t h e s i s m i x t u r e w h e r e a f t e r z e o l i t e X c r y s t a l s s t a r t e d growing on and o v e r t h e z e o l i t e A c r y s t a l ( r e f s . 140, 141).
F i g . 23. Overgrowth o f z e o l i t e X o n t o z e o l i t e A.
112
Though t h e thermodynamic v a r i a b l e s were c o r r e c t l y chosen t o prepare z e o l i t e X, s y n t h e s i s m i x t u r e s o f z e o l i t e A and X, g i v e n below, do have comparable elements and a p p a r e n t l y l o c a l k i n e t i c f a c t o r s i n i t i a t e d t h e s y n t h e s i s o f z e o l i t e A. Na2Si03.9H20
NaA102 T r i e t h a n o l a m i n e
H20
z e o l i t e A:
.4
.1
.7
28
z e o l i t e X:
.4
.05
.7
28'
Ref. (142)
\
(molair)
e. The pH e. I.
Introduction
The pH and t h e s o l u b i l i t y o f r e a c t a n t s i n t h e s y n t h e s i s m i x t u r e a r e governed by t h e presence o f OH- o r F - .
111 F- compared t o OH- i s t h e h i g h e r s o l u b i l i t y o f e.9. Fe and T i " and t h e c o n d e n s a t i o n c a p a b i l i t y f o r e.g. Ge". A too high c o n c e n t r a t i o n o f F - , however, p r e v e n t s t h e polycondensat ion mechanisms. A compromise between s o l u b i l i t y o f c e r t a i n elements and i n h i b i t i o n o f z e o l i t e framework formation leads to Fs y n t h e s i s m i x t u r e s which a r e l e s s su pers a t u ra t e d t h a n OH- media. Hence, o n l y a few z e o l i t e t y p e s a r e obt ained, u n t i l now ( r e f . 16). An
advantage
of
e.2. OH-
Raising
pH
of
synthesis
mixtures
using
OH-,
mainly
c r y s t a l l i z a t i o n o f a c e r t a i n z e o l i t e i n a p o s i t i v e way
within
influences the
the
synthesis
A and z e o l i t e ZSM-5, r e s p e c t i v e l y , i n c r e a s i n g t h e pH shows an i n c r e a s e i n t h e c r y s t a l l i z a t i o n r a t e . The OH- i s a s t r o n g m i n e r a l i z i n g agent f o r b r i n g i n g r e a c t a n t s i n t o s o l u t i o n . field.
As
depicted
in
Fig.
24a
and
b
for
zeolite
The h i g h e r t h e pH and t h u s t h e c o n c e n t r a t i o n o f d i s s o l v e d
reactants
t h e r a t e o f c r y s t a l g r o w t h o f z e o l i t e s i s enhanced ( r e f s . 47, 143).
the
more
113
Zeolite A &]
b
a
F i g . 24. The
influence
of
a l k a l i n i t y on
a)
zeolite
A
and
b)
ZSM-5
c r y s t a l 1 iz a t i on.
e.3. F A f t e r t h e f i r s t p u b l i c a t i o n s on s y n t h e s i s w i t h F- ( r e f . 4 6 ) have
been
undertaken
to
investigate
the
extensive
studies
e f f e c t o f F- and p o s s i b i l i t i e s i n
z e o l i t e s y n t h e s i s ( r e f s . 16, 4 6 ) . Replacing OH- by F- w i t h e.g. NH4HF, NH4F and
BF3 t h e pH values o f t h e s y n t h e s i s m i x t u r e l i e s g e n e r a l l y between 3 and 10. A t y p i c a l synthesis formulation i s given i n Section
zeolites
obtained
XI1
of
this
chapter.
The
so f a r by t h i s r o u t e a r e s i l i c a - r i c h m a t e r i a l s o f w h i c h t h e
s t r u c t u r e t y p e s are:
ZSM-5 F e r r ie r it e Theta-1 and ZSM-23
f . The temperature I t has been shown f o r many z e o l i t e s t h a t r a i s i n g s y n t h e s i s t e m p e r a t u r e s
a certain zeolite
synthesis
field
within
increases the c r y s t a l growth r a t e ( r e f s .
47, 1 4 4 , 1 4 5 ) . As shown i n F i g . 25a and b f o r z e o l i t e A and z e o l i t e ZSM-5, i n c r e a s i n g temperature i n f l u e n c e s t h e c r y s t a l growth r a t e whereas i n t h e case o f z e o l i t e A t h e c r y s t a l s i z e does n o t change s u b s t a n t i a l v a r i a t i o n s i n t h e ZSM-5 p r o d u c t .
s i g n i f i c a n t l y compared
to
114
.
Crystol size
(pm)
200'C 165
'C
170 ' C
150 'C
10
a
i Time [hours]
bo
10
20
30
t(h)
Fig. 25. Influence of temperature on the crystallization of a) zeolite A and b) zeolite ZSM-5.
I X . ALL S I L I C A MOLECULAR SIEVES
a . Introduction
Two preparation routes can be followed to obtain all silica molecular sieves: i) A direct synthesis to crystallize molecular sieves with a Si02 composition and well known zeolite topologies. i i ) A secondary synthesis. After the direct synthesis of a zeolite a dealumination procedure, e.g. steaming (ref. 146), ammonium silicon hexafluoride (ref. 147) or silicon tetrachloride (ref. 148) can lead to an all silica molecular sieve. b. Synthesis
Though the neutral all silica molecular sieves do formally not need to be stabilized with cations the silica structures usually contains the cations used in the synthesis. For example, tetrapropylammonium for silicalite-1, tetrabutylammonium for silicalite-2, and tetraethylammonium for silica-ZSM-12. Recently, amines, di-amines (ref. 149) and poly-amines (ref. 150) have been used as templates. Table 11 contains a list of all silica molecular sieves with two examples of synthesis recipes and references.
115
Table 11. All silica zeolites, recipes and references. Product
Ref
Sil ica-ZSM-48
150
Recipe: 14.6 g of triethylenetetramine is dissolved in 18 ml H20 whereafter the solution is stirred into dry 1.2 g Si02. The smooth dispersion is then autoclaved between 120-180 OC for 28-105 days, respectively. Sil ica-ferrierite Recipe: 0.75 g Si(OC3)4 is added to a solution of 1.2 g ethyldiamine (EDA) in 10 ml H20. After adding 2 ml 1 M aqueous boric acid the solution is sealed in a silica tube and heated at - 170 O C for 56 days. Sil ical ite-1 (Sil ica-ZSM-5) Sil ical ite-2 (Silica-ZSM-11) Sil ica-ZSM-22
151
152 153 149
c . Remark
The main property of the silica molecular sieves is the strong hydrophobic character of the pores. The preferential uptake o f e.g. traces o f organic compounds (ref. 152) from water, which is not accommodated (ref. 154), in silicalite-1 is a good example.
X. CLATHRASILS or actually silicates and zeolite molecular sieves? a. Introduction
The name "clathrasil" has been introduced for a subclass of porous tectosilicates different from zeolites. The windows of the framework, connecting the cages, are too small to let guest species, stabilized during the synthesis, pass.
116
This characteristic of a clathrate together with the all silica composition is considered as specific for the members of the clathrasils (ref. 29). There are, however, exceptions. The recently synthesized decadodecasil-3R (DD-3R) (ref. 155) contains windows of eight-rings of oxygen, indicating that diffusion of small molecules through the porous structure is possible after calcination. This structure can therefore be considered to form an interface between the clathrasils and the silica molecular sieves. A modified type of DD-3R denoted as Sigma-1 (ref. 156) can, however, be seen as a link between clathrasils and zeolites, because some Si-framework sites are isomorphously substituted by Al. Finally, there is a novel tectosilicate, Sigma-2 (ref. 157). The recently solved structure of which two different polyhedra, see Fig. 26, have not been found before, reveals eight-rings of oxygen and cages with a free diameter of 75 nm. Sigma-2 has been prepared in the silicalite as well as in the zeolite form and can thus be considered as an intermediate between clathrasils, zeolites and silicates.
Fig. 26. The nonahedral and eikosahedral cages o f Sigma-2.
b . Experimental
The clathrasils can be synthesized generally from .5 M silica, prepared by hydrolyzing an alkoxysilane, e.g. Si(OCH3)4, in solutions containing an amine as guest template molecule. The syntheses are mainly carried out between 160-240 OC. The clathrasils, together with detailed synthesis data and products expected, are given in Table 12.
117
w i t h r e f e r e n c e s t o s y n t h e s i s p r e s c r i p t i o n s and two examples o f s y n t h e s i s r e c i p e s .
T a ble 12. C l a t h r a s i l s
Product
Ref.
Me1anophlogi t e
158
Dodecasil 3C
159
Dodecasil 1H
160
S i 1ica-soda1 it e
161
Sigma- 1
156
DD-3R ( s i l i c a )
155 Recipe: 0.75 g Si(OCH3)4 i s added t o a s o l u t i o n o f 1.2 g e thylenediamine (EDA) i n 10 m l H20. A f t e r adding 350 mg l-aminoadamantane t h e s o l u t i o n i s sealed i n a s i l i c a tube and heated a t 170
-
OC
for
70 days.
Sigma-2 ( s i l i c a and a l u m i n o s i l i c a t e )
157
S y n t h e s i s example: The molar oxyde r a t i o o f t h e s y n t h e s i s system i s : Na20 AN
2'3 Si02
3 20
(1-adamantanamine)
(0.6) 60
(Al-wire) (colloidal silica)
2400 H2° The system was c r y s t a l l i z e d a t 180 O C and c o n t i n u o u s l y s t i r r e d f o r a few days.
c. Remark
The t e m p l a t i n g r o l e o f some o f t h e guest molecules i s i l l u s t r a t e d i n F i g . 26. Polyhedra o f d i f f e r e n t c l a t h r a s i l s a r e f i l l e d w i t h a guest molecule. As o n l y 4-, 5 - o r 6 - r i n g f a c e s a r e p r e s e n t i n most o f t h e polyhedra i t l o o k s crystal
building
i s t o o l a r g e t o pass t h r o u g h one o f t h e r i n g s . The therefore
like
the
u n i t s have formed around t h e guest molecule as t h i s molecule clathrate
formation
might
be o b t a i n e d and based on s i n g l e b u i l d i n g u n i t s i n s o l u t i o n and/or a t
t h e growing c r y s t a l s u r f a c e ( r e f . 162).
118
Fig. 27. Orientation o f various guest molecules in clathrasils (ref. 163). a) H3CNH2 in [ 5 126 2 ] and b) adamantylamine in [51268] of melanophlogite and dodecasil l H , respectively.
OF SYSTEMATIC RESEARCH in the field of preparation to reach various objectives
X I . EXAMPLES
molecular
sieves
a . Introduction
A main thrust of research is: - to synthesize new molecular sieves - further optimization of recipes - to gain knowledge on the essential functions of reactants, e.g. structure directing role of cation/templ ate - to prepare relative large single crystals for fundamental studies The list can be longer, however, the examples given below illustrate generally the purpose and variety in the research o f molecular sieves preparation. b. Research examples
Objective 1)
Preparation o f zeolites
2) Preparation o f zeolites 3)
Investigation of crystal1 ization fields with pyrrolidine as template
Parameter ( s 1 Non-aqueous solvents F - as mineralizing agent
Na20-A1 203- Si O2 -H20 system was varied
119
b. Research examples (continued)
Objecti ve
Parameter(s)
4a) Investigation o f template-zeolite interaction 4b) Directing role o f template in the crystal1 ization
Systematic variation of template Use o f bis-quaternary ammonium compounds
5)
Large single crystals
Know1 edge on nucl eation/ crysta1 1 i zati on
6)
Morphology and form of zeol i te products
Change o f [Si02], temp1 ate, cation or additives
b.1. The use o f non-aqueous solvents
In contrast to the rich crop of zeolite types synthesized in aqueous systems the results in non-aqueous solvents are poor (refs. 164, 165). Solvents used, o f which the choice was a.0. based on boiling point (100-200 OC) and relative permittivity (10-45) (water: 78), are given in Table 13.
Table 13. Zeolite products formed. Solvents ~
Glycol Glycerol DMSO Sul fol ane C6C7 alcohol Ethanol
K+
Na' ~
~
HS HS HS
HS HS
HS
HS: Hydroxysodali te.
Lit
Cat+
120
Generally mixtures within the following molar oxyde ratio were used: Me0
1-20 1 1-100
2'3
Si02 Solvent 5-350 0.1-10 and MeO/Si02 As shown in the Table zeolite products could only be obtained in the case of .'aN The use o f other inorganic and organic cations was not successful. As the boiling point is a less critical factor than high relative permittivity (reduces the Coulomb force between ions and polar compounds thus enhancing dissolution) other non-aqueous solvents for zeolite crystallization which might be subject to zeolite synthesis tests are given below.
Non-aqueous solvent
Er
~~
formic acid formamide hydrogen peroxide hydrocyanic acid
b.2. The use o f
57 84 93 95
F- ( r e f . 166)
The compositional ratios of the reaction mixtures used were: A1 or B/Si F/Si Templ./Si H20/Si in molar ratio 0-0.5 0.05-6 0.05-6 4-500 with the pH of the mixtures between 1.5-10. The reaction mixtures were heated at 60-250 O C and autoclaved for a few hours t o a few months. After isolation the products were washed with water and dried. A typical example o f a " F - " synthesis of ZSM-5 is given below: - Reaction mixture composition: 36 g Ammonium aluminiumsilicate (Si/Al 7; NH4/A1 1) pH = 7 18.5 g NH4F 33.2 g TPABr t = 172 " C 180 g H20 time = 11 days
-
-
121
- Product Unit cell composition: 1.8 NH4+ t 4.1 TPA+ [A12~9Si93~101921 Crystal size 30 x 12 pm Advantages and differences using F - instead o f OH- as concluded so far: - Low pH compared to OH- Incorporation in the framework of elements sparingly soluble in alkaline medium, e.g. Fe111 - Synthesis without alkaline cations - New possibility to directly incorporate cations as NH4+ and divalent cations such as Co2' as well - Good stability of usual templates such as TAA' in this medium - Highly crystalline materials b.3. Pyrrolidine as template (ref. 87)
The crystallization o f zeolites in the system Na2O-Al2O3-SiO2-H20 t pyrrolidine as a template was studied. The reaction mixture compositions used are given in Table 14 in molar oxyde ratio.
Table 14. Reaction mixture ranges in molar oxyde ratios. Na20 A1 i03 Si02
0.05-0.5 0.002-0.05
H2S04 H2°
0-0.4 20-80
1
H t
-
0.7 pyrrolidine
Two procedures were used: I. To a stirred aluminium sulphate solution, calculated amounts of sodium silicate, sulphuric acid and pyrrolidine were added dropwise. 11. Calculated amounts of aluminium nitrate, colloidal silica and pyrrolidine were added to a stirred sodium hydroxide solution.
122
Si02/A1 203
A
4
t
A
A
A
H20/Si O2
F i g . 28. C r y s t a l l i z a t i o n
fields
of
product
Si02/A1203 v e r s u s H20/Si02 o f
ZSM-39 (A), ZSM-48 (+) and KZ-1 ( A ) .
The f o l l o w - u p o f b o t h procedures was t o a u t o c l a v e t h e r e a c t i o n m i x t u r e f o r 7 - 4 0 h a t 4 2 3 - 4 3 5 K w i t h s t i r r i n g . A f t e r i s o l a t i o n t h e p r o d u c t was washed w i t h w a t e r and d r i e d .
A
whereas
The main c o n c l u s i o n o f t h e s t u d y i s t h a t p u r e ZSM-5, ZSM-35, ZSM-39,
ZSM-48
The r e s u l t s o f t h e experiments a r e g i v e n i n F i g . 6 f o r p r o c e d u r e t h e r e s u l t s o f t h e experiments w i t h p r o c e d u r e B a r e g i v e n i n F i g . 28. and
KZ-l
can be c r y s t a l l i z e d w i t h p y r r o l i d i n e i n t h e a f o r e m e n t i o n e d s y n t h e s i s
system. No common f a c t o r , based on t h e use o f p y r r o l i d i n e , c o u l d be
recognized
i n the various z e o l i t e products. b.4.a. The
use
of
bis-quaternary
ammonium compounds
in
molecular
The o b j e c t i v e i n t h i s study was t h e s y s t e m a t i c v a r i a t i o n
of
template
sieves
s y n t h e s i s ( r e f . 58)
synthesis.
An
example
of
the
synthesis
is
given
in
the
below t o g e t h e r w i t h t h e
p r o d u c t s formed, see Table 15. The g e n e r a l f o r m u l a o f t h i s b i s - q u a t e r n a r y t e m p l a t e ( T ) i s :
123
T a b l e 15. S y n t h e s i s
mixtures
and
product
formation
with
bis-quat
as
template. Synthesis conditions
Product f o r m a t i o n
m o l a r oxyde r a t i o Si02
X
60
1
Zeol it e phases
2'3 Na20
10
3
ZSM-39
TBr2
10
4
ZSM-12
576
EU- 1
7,8
ZSM-23
H2°
3000
S i l i c a phases 3
EU-4
499
EU-2
( w i t h o u t A1 203)
The
reaction
conditions
were
180 O C , t h r e e days and c r a s h - c o o l i n g a f t e r t h e
s y n t e h s i s was t e r m i n a t e d . b . 4 . b . Another example (ref. 167) Systematic v a r i a t i o n o f t h e c h a i n l e n g t h o f t h e t e m p l a t e (T) g i v e n below i n t h e general formula
r e s u l t e d i n t h e p r o d u c t s , g i v e n i n Table 16. T a b l e 16. Z e o l i t e f o r m a t i o n s , o b t a i n e d w i t h b i s - q u a t . X
2-5
Zeol it e phase Ferrierit e ZSM-5
5-6
ZSM-5
7-10
ZSM- 11
124 The f u l l synthesis d e s c r i p t i o n i s given i n r e f . 167. b . 5 . The synthesis of relatively large single crystals of molecular sieves b . 5 . 1 . Znt roduct ion P e r t a i n i n g t o e.g. the v i s c o s i t y o f
the
synthesis
mixture
several
systems,
c l e a r s o l u t i o n , d i l u t e d gel and dense gel phase have been i n v e s t i g a t e d . b . 5 . 2 . Crystallization of ZSM-22 from a clear solution [ref. 149) I n a t y p i c a l experiment tetramethoxysilane was hydrolyzed i n 3 M diethylamine
(DEA) according t o the f o l l o w i n g reactions:
Si(OCH3)4
+
2 H20
-------- > Si02 + 4 CH30H -------DEA > 2 Si02(C2H5)2NH 180 OC 100 days
Single
crystals
of
silica-ZSM-22 o f 45 x 100 x 225 pm were i s o l a t e d and used
f o r s t r u c t u r e determination.
b . 5 . 3 . Synthesis of elongated prismatic ZSM-5 crystals The o b j e c t i v e o f t h i s study was t o Systems
using
Na'-TPA+,
Li'-TPA+
obtain
large
and NH4'-TPA'
single
crystals
of
ZSM-5.
were i n v e s t i g a t e d applying a
r e a c t i o n mixture given i n molar oxyde r a t i o f o r e.g. NH4'-TPA':
TPA20 (NH4)20 23' Si02 H2°
4 123
T = 453 K
1 59 2280
t = 7 days
Products A1 kal i n e - f r e e ,
homogeneous elongated p r i s m a t i c s i n g l e c r y s t a l s o f ZSM-5 o f 350
pm i n l e n g t h a t maximum ( r e f . 168).
125
b.5.4. Synthesis o f cubic shaped s i n g l e c r y s t a l s o f ZSM-5 ( r e f . 169) The s y n t h e s i s o f t h i s t y p e
of
crystals
developed
recently
(ref.
169)
was
s u b j e c t o f a s t u d y on t h e c r y s t a l growth h i s t o r y ( r e f . 42) o f t h i s t y p e o f c r y s t a l s . The o b j e c t i v e was: t o p i n p o i n t t h e d r i v i n g f o r c e s which change t h e ZSM-5 c r y s t a l f o r m from e l o n g a t e d p r i s m a t i c i n t o c u b i c . The c r y s t a l growth h i s t o r y s t udy r e v e a l e d t h a t t h e c u b i c c r y s t a l growth o c c u r r e d i n a dense g e l phase, P e r f e c t s i n g l e c r y s t a l s up t o 500 pm o f z e o l i t e ( E M - 5 ) and a l l s i l i c a (silicalite-1)
m o l e c u l a r s i e v e type, see F i g . 16c, c o u l d be o b t a i n e d u s i n g t h e
f o l l o w i n g mo lar oxyde r a t i o : ZSM-5
S i l i c a l it e - 1
S i O2
12
12
2'3 Na20
1 44
44
TPA20
44
44
2000
2000
H2°
A f t e r 5 days produc t .
at
180
OC
crystals
could
be i s o l a t e d and s e l e c t e d f rom t h e
b.5.5. Synthesis o f s i n g l e c r y s t a l s o f z e o l i t e A and X ( r e f . 142) S i n g l e c r y s t a l s o f z e o l i t e A and X up t o 100-500 pm i n s i z e c o u l d
be
obtained
u s i n g t h e f o l l o w i n g procedures. Procedure f o r z e o l i t e A: S o l u t i o n I: 100 g Na2Si03.9H20 i n 350 m l H20 t 50 m l TEA S o l u t i o n 11: 80 g NaA102 i n 350 m l H20 t 50 m l TEA Both s o l u t i o n s a r e f i l t e r e d w i t h m i l i p o r e f i l t e r s , w h e r e a f t e r s o l u t i o n I 1 i s added t o s o l u t i o n I w i t h s t i r r i n g . The c r y s t a l l i z a t i o n i s perf ormed a t 75-85 OC f o r 2-3 weeks, w i t h o u t s t i r r i n g . Procedure f o r z e o l i t e X: Identical
to
the
procedure
for
z e o l i t e A,
o n l y 40 g o f NaA102 i s used i n
s o l u t i o n I 1 now. The c r y s t a l l i z a t i o n t i m e i s 3 - 5 weeks.
126 Remark C a r e f u l f i l t e r i n g o f t h e s t a r t i n g s o l u t i o n s s u b s t a n t i a l l y reduces t h e amount o f heterogeneous n u c l e i such as d u s t and f o r e i g n p a r t i c l e s i n t h e c h e m i c a l s . The l o w e r t h e number o f n u c l e i , t h e l a r g e r t h e c r y s t a l s .
starting
b . 6 . Morphology and f o r m o f m o r d e n i t e and ZSM-5 The
morphology
and/or
form
of
zeolite
crystals
appear
generally
to
be
i n f l u e n c e d by: - [Si02] - Guest m o l e c u l e t y p e - C a t i o n ( r e f . 171) - Crystal growth i n h i b i t o r s
A
frequently
observed c r y s t a l f o r m o f m o r d e n i t e i s t h e n e e d l e f o r m ( w i t h p o r e
channel system p a r a l l e l t o n e e d l e d i r e c t i o n ) , see F i g . 29a.
F i g . 29. D i f f e r e n t
forms o f m o r d e n i t e . The n e e d l e f o r m a), t h e
forms b) and c ) and
the
disk
form
d).
The
pore
ntermed a t e d rection i s
i n d i c a t e d by a b a r ( r e f . 85).
As shown i n F i g . 29b, c and d, c o m p l e t e l y d i f f e r e n t c r y s t a l forms o f can
be
prepared.
According
to
mordenite
t h e s y n t h e s i s system used ( r e f . 85) t h e main
i n f l u e n c e i n t h e shape o f t h e c r y s t a l s seems t o be t h e [ S i 0 2 ] . The
higher
the
[Si02], i . e . t h e more t h e c r y s t a l l i z a t i o n o c c u r s i n a dense g e l , t h e more t h e e l o n g a t e d f o r m i s reduced and changed i n t o a d i s k form.
127
The
increase
i n p o r e e n t r i e s and decrease i n p o r e l e n g t h g o i n g f r o m n e e d l e t o
d i s k f o r m i s e v i d e n t and may be o f i n t e r e s t i n c a t a l y s i s ( r e f . 170). The
elongated
prismatic
f o r m i s t h e most f r e q u e n t l y found c r y s t a l f o r m o f
Z S M - 5 . Changing t h e [Si02] can change t h e c r y s t a l f o r m as shown i n F i g . 30a and
b f o r r e l a t i v e l y l o w and h i g h [Si02]c o n c e n t r a t i o n s , r e s p e c t i v e l y .
F i g . 30. The e l o n g a t e d p r i s m a t i c c r y s t a l f o r m (a) and t h e c u b i c c r y s t a l f o r m (b) o f z e o l i t e ZSM-5.
Changing t h e t e m p l a t e t y p e , i . e . r e p l a c i n g TPA' ammonium
ion,
f o r the divalent
bi-quaternary
hexapropyl-1,6-hexanediammonium, r e s u l t e d i n d i f f e r e n t c r y s t a l
forms f o r l o w as h i g h [Si02] as w e l l , see F i g . 31a and b.
F i g . 31. The
modification o f
the
c r y s t a l f o r m a t low (a) and a t h i g h (b)
[ S i 0 2 ] o f z e o l i t e ZSM-5 prepared w i t h b i q u a t as t e m p l a t e .
I n t h e case o f an a d d i t i v e ( i n h i b i t o r ) l i k e b o r i c a c i d an enrichment o f c r y s t a l faces i n t h e c - d i r e c t i o n was observed as shown i n F i g . 32.
128
F ig . 32. A d d i t i o n a l
crystal
face
compared
(001)
to
regular
elongated
p r i s m a t i c f o r m o f z e o l i t e ZSM-5.
XII. LITERATURE SOURCES PERTAINING ZEOLITE PREPARATION ASPECTS Though most o f t h e l i t e r a t u r e
sources
are
given
in
XIII, a more
Section
extended l i s t o f sources i s g i v e n below f o r reasons o f c l a r i t y and ease.
A
Chemical Abstracts literature
search
in
the
Chemical
A b s t r a c t s (CA) can be s u c c e s s f u l when
C o n t r o l l e d Vocabulary Index Terms (CVIT's) a r e used. As CVIT's a f t e r 1976 a r e n o t o n l y assigned t o words i n t h e t i t l e and t h e a b s t r a c t , b u t a l s o t hroughout the t e x t
of
thoroughly.
the The
paper choice
(open of
literature
CVIT's
must
or be
patent) correct.
the
search w i l l
be
I n t h e case t h e word
" s y n t h e s i s " i s used i n s t e a d o f " p r e p a r a t i o n " t h e main p a r t o f t h e search " h i t s "
w i l l p e r t a i n t o r e a c t i o n s w i t h t h e a i d o f z e o l i t e s whereas t h e p r e p a r a t i o n o f z e o l i t e s i s then d i f f i c u l t t o e x t r a c t . - Proceedings o f I n t e r n a t i o n a l Zeo7 i t e Conferences (IZC)
1. " Molec ular Sieves", SOC. Chem. I n d . , London, 1968; Proceedings I Z C , London, U.K.,
of
the
1st
1967.
2. " Mo lec ular Sieves I and I I " , Adv. Chem. Ser.,
101 and 102, ACS,
Washington,
D.C., 1971; Proceedings o f t h e 2nd I Z C , Worcester, Mass., U . S . A . , 1970. 3. " Mo lec ular Sieves", Adv. Chem. Ser., 121, ACS, Washington, D.C., 1973; W.M. Meier
and
J.B.
S w i t z e r l a n d , 1973.
Uytterhoeven,
Eds.,
Proceedings
o f t h e 3 r d IZC, Zurich,
129
4. " M o l e c u l a r S i e v e s - I I " , ACS Symp. Ser., 40, ACS, Washington, D.C.,
1977; J.R.
K a t z e r , Ed., Proceedings o f t h r e 4 t h I Z C , Chicago, Ill.,U.S.A., 5. "Proceedings
of
the
5th
International
London, P h i l a d e l p h i a , Rheine, 1980; L.V.C.
1977.
Conference
on z e o l i t e s " , Heyden,
Rees, Ed.,
Proceedings o f t h e 5 t h
I Z C , Naples, I t a l y , 1980. 6. "Proceedings o f t h e 6 t h I n t e r n a t i o n a l Conference on Z e o l i t e s " , B u t t e r w o r t h s , Guildford,
1984;
D.
Reno, Nev., U.S.A.,
Olson and A. B i s i o , Eds., Proceedings o f t h e 6 t h I Z C ,
1983.
7 . New Developments
in
Zeolites
Science
and
Technology", Kodansha, Tokyo,
E l s e v i e r , Amsterdam, Oxford, New York, Tokyo, 1986, Stud. S u r f . S c i . C a t a l . , 28;
Y. Murakami, A. I i j i m a and J.W. Ward, Eds., Proceedings o f t h e 7 t h I Z C ,
Tokyo, Japan, 1986. 8. " Z e o l i t e s :
Facts,
F i g u r e s , F u t u r e " , E l s e v i e r , Amsterdam, Oxford, New York,
Tokyo, 1989, Stud. S u r f . S c i . C a t a l . , 49; P.A. Jacobs and R . A .
van
Santen,
Eds., Proceedings o f t h e 8 t h I Z C , Amsterdam, N e t h e r l a n d s , 1989. - S y n t h e s i s p a r t i n r e c e n t i n t e r n a t i o n a l conferences
"Zeolites,
Synthesis,
Structure,
Technology
Amsterdam, Oxford, New York, Tokyo, 1985,
Stud.
and
Application",
Surf.
Sci.
Catal.,
Elsevier, 24;
B.
D r z a j , S. Hocevar and S. P e j o v n i k , Eds. " I n n o v a t i o n i n Z e o l i t e M a t e r i a l s Science",
Elsevier,
Amsterdam,
Oxford,
New
York, Tokyo, 1988, Stud. S u r f . S c i . Catal.,
37; P.J. Grobet, W.J. M o r t i e r , E.F.
Vansant and G. S c h u l z - E k l o f f , Eds. "Zeolite
Synthesis",
ACS
Symp.
Ser., 398, ACS, Washington, D.C.,
1989; M.L.
O c c e l l i and H.E. Robson, Eds.
-
Journal
Z e o l i t e s , L.V.C.
Rees and
R.
von
Ballmoos,
Eds.,
Publishers,
Butterworth,
Heinemann, Stoneham, MA, U.S.A.
"Zeolite
Molecular
Sieves",
S t r u c t u r e , Chemistry and Use, John W i l e y & Sons,
New York, London, Sydney, Toronto, 1974; D.W. "Hydrothermal R.M.
Chemistry
B a r r e r FRS.
Breck.
o f Z e o l i t e s " , Academic Press, London, New York, 1982;
130
"Synthesis Oxford,
of
High-Silica Alumiosilicate
New York,
Tokyo,
Zeolites",
Elsevier,
Amsterdam,
1987, Stud. Surf. Sci. Catal., 33; P.A. Jacobs and
J.A. Martens, Eds.
"Molecular
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