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Hydrothermal synthesis and crystallization of zeolites
Prog. Crystal Growth and Charact. 1990, Vol. 21. pp. 29-70 Printed in Great Britain. All rights resewed
0
01463535/90 $0.00 + +I 1991 Pergamon Press plc
HYDROTHERMAL SYNTHESIS AND CRYSTALLIZATION OF ZEOLITES Xu Qinhua Department
1. The natural
zeolites
sedimentary
deposits.
atory technique
have
formed
Aizhen
INTRODUCTION
under aqueous,
These conditions
of synthesis.
drew the attention
and Yan
of Chemistry, Nanjing University, Nanjing 210008, China
alkaline
conditions
are typical of hydrothermal
The role of water es a mineralising
of mineralogists
and chemists
in matrices systems
catalyst,
to hydrothermal
of basalt rock and as
and have guided the labor-
aided by alkaline
conditions,
end synthesis.
Systematic
reactions
in the early 1940’s, the first made being enalcime, mordenite zeolites (1) also with the framework of that subsequently termed ZK-5 (23).
studies began in Barrer’s laboratory
with the edingtonite ZK-5
framework
was thus the first zeolite without
a natural counterpart
to be synthesized.
in the laboratory synthesis has come by duplicating the conditions which produced natural zeolites. However, the one condition that we can never duplicate is crystallization time of lOU0 years or more. Thus laboratory systems operate at high pH(> 12, usually > 14) and higher temperature, and produce smaller, less-perfect crystal. So the majority of the synthetic zeolites are formed under non-equilibrium conditions and are metastable phases. Table l-3 show the natural zeolites whose structures were successfully duplicated from silica / alumina gels end alkali metal, elke. line earth metal or alkylammonium cations respectively (4). We can expect more examples in the future of discovery of natural counterparts of existing synthetic zeolites. Table 4 lists several synthetic zeolites with structures reasonable well defined and which have no counterpart in natural zeolites. Now the hydrothermal synthesis of approximately 150 zeolites from eluminosilicate systems has been reported in the published literature. New synthetic zeolites phases will continue to appear, but only about 60 differ. ent structures are known (5). Much of success
In the recent years many efforts have been made to improve the understanding synthesis
and
aluminasilicate
to get a deeper molecular
sieves.
insight Many
into
the
excellent
chemical
reactions
review papers
which
of the process lead
of zeolite
to the formation
of
on zeolite synthesis have been published
in the literature (4-12). In this paper the discussion is restricted to three-dimensional crystalline networks. Because faujasite, A and ZSM-5 zeolites are the most important in industrial application, most of the metioned problems will be discussed for the crystallization of these zeolites. We intend to review the main factors of influence on hydrothermal synthesis and crystallization involving temperature, alkalinity, chemical environments, end action of template agents. The metestability of zeolite crystals and recrystallization between xeolites with different structures, the mechanism of zeolite crystallization by nucleation from gel or solution, the kinetics and growth of zcolite crystallization in non-spontaneous or spontaneous system will be presented in this review.
PICG-B*
29
Xu Qinhua
30
Table
1
Synthetic
Zeolite
Table
counterparts Alkali
of natural
zeolites-alkaline
Investigator
metal Date
Mordenite
Na
RM
Analcime
Na
R.M .Barrer
1949
Phillipsite
Na
R.M .Barrer
1951
Cancrinite
Na
R.M.Barrer
1952
Natrolite
Na
R.M .Barrer
1952
Faujasite
Na
R.M .Milton
1956
Gmelinite
Na
R.M .barrer
1959
Chabazite
Na
R.M .Milton
1960
Erionite
K.Na
D.W .Breck
1960
Clinoptilolite
Li
L.L Ames,jr
1963
Ferricrite
Na
E.E.Senderov
1963
Gismondine
Na
A.M.TaylorkR.Rov
1964
Bikitalite
Li
D.J.Drysdale
1971
Edingtonite
K
R.M.Barrer
1974
2
Synthetic
counterparts
Zeolite
Table
and Yan Aizhen
Alkali
.Barrer
1948
of natural
zeolites-
investigator
alkaline Date
Heuiandite
Ca
R.Roy
1960
Scolecite
Ca
R.Roy
1960
Wairakite
Ca
R.Roy
1960
Epistilbite
Ca
R.Roy
1960
Garronite
Ca
R.M .Barrer
1961
Thomsonite
Ca
R.M .Barrer
1961
Yugawaralite
Sr
R.M .Barrer
1964
Clinoptilolite
Sr
R.M.Barrer
1964
Harmotone
Ba
R.M .Barrer
1964
3
Sythetic
Zeolite
Offretite Levynite Mazzite
a
counterparts
of natural
Alkali
TMA,‘Na,K [CH,N 0 Nl+,K TMA,‘Na
TMA,N(CH,),OH
zeolites-alkyl Investigator
earths
ammonium Date
H.E Robson
1966
G.T.Kerr
1969
R.M.Barrer
1970
Hydrothermal
Table 4
synthesis and crystallization of zeolites
Natural zeolitcs with no synthetic counterpart Year discovered
Zeolite
1756 1785 1813 1822 1825 1893 1896 1905 1909 1942 1960 1977
Stilbite Laumontite Mesolite Brcwsteritc Herschelitc Kehoeite Gonnardite Dachiardite Stellerite Viseite Paulingite Merlinoite
2. FACTORS AFFECTING THE HYDROTHERMAL SYNTHESIS AND CRYSTALLIZATION The xeoliter generally can be prepared by mixing sodium metasilicate and sodium aluminate solutions. The aluminorilicate gels are usually formed on mixing soluble silicates and soluble aluminates. In eolite synthesis one may consider the stages given below: Stages in xeolite synthesis Reactant
I Reactant mixtures
1 Nucleation Crystal
growth
The primary building block of the zeolitc structure is tetrahedron of four oxygen atoms surrounding a central silicon atom - (SiOJ*-, (fig. la (5)). These are connected through their corners of shared oxygen atoms to form a wide range of small secondary building unites (fig. lb). They are interconnected to form a wide range of polyhedra (fig. lc), which in turn connect to form the infinitely extended frameworks of the various specific xeolite crystal structures (fig. Id). In these figs. la - Id showing structure of xeolites, the corners of the polyhedra represent Si or Al atoms, and the connecting lines represent the shared oxygen atoms. Individual structures may comprise only one basic unit or many of them. A record is held by the minerial paulingite which contains five such polyhedra. Different combination of the same secondary build unit may give numerous distinctive, fig. 2 shows an example of three zeoliter that have the same structural polyhedron ( cube-octahedron), but probably form from smaller ring unites.
31
32
Xu Qinhua
(a)
PRIMARY
I
(b
UNITS
TERTIARY OR
ZEoLITE
UNITS
BUILDING
FOLY
cd)
UNITS
SECONDARY
tc)
and Yan Aizhen
HEDRA
STRUCTURES
PAULINGITE
MELANOPLOGITE
Fig.1.
Monomers
a); polymerize
in turn combine These (d) attach
Thermodynamic composition tained
in
variables
to growing
mixtures.
hydrothermal
thermodynamically
reactions
determined
reactant
prior
to crystallization,
fluences
are described
and
because,
variables The
b);
or first form clusters crystal
entities.
are temperature, appears kinetical
and physical
nature
(5)
alkalinity
do not neccessarily
nucleation
controlled.
their chemical
surfaces
to larger
of zeolites
These
rings and chains
units c); or sheet structures.
crystal
agglomerate
in the synthesis
of the reactant
to low-molecular-weight
to form polyhedral
which subsequently
RHO
to
(pH)
determine be
variables
kinetically include
(14, 15).
and
chemical
the products
ob-
rather
the
Some aspects
than
treatment
of
of these in-
below.
2.1. Temperature Zeohte
formation
idence
an upperlimit
mordenite
of 623 K has
to 703 K,clinoptilolite
The temperature zeolite,
has been observed
range
over a considerable been
for crystallization
has
L, omega
and mordenite,
of temperatures.
Analcime has been to 648 K (12).
to 643 K and ferricrite tended
from 298 K to 398 K for the aluminum-rich
Si / Al zeolites,
range
suggested.
to increase
zeolites,
with
Based synthesized
incresing
Si/
on geological
ev-
at least
639 K,
Al ratio
in the
from 373 K to 423 K for the intermediate
and from near 398 K to 473 K for the high silica zeolites
as
Hydrothermal
exemplified
This is consistent
by ZSM-5.
temperature
The higher
(16).
with the suggested
the synthesis
The most
end syntheris
to be the water
porous
zcolitee,
content
such
end
A, X, Y,
end ZK-5, with pore volumes in the range. 0.25 to 0.36 cm’/ g of crystal do not form Silica-rich phases such as mordenite, L, omega, ZSM-5, -11 end much above 373K. and -2, all with rather low intracrytalline porosities (0.15 - 0.20 cm’/ g), arc usually
et temperatures
rate of nucleation shown
in the range
and crystal
crystallize.
of pore volume
tends
Chebezite,
beat made
which
relationship
the smaller
porosity
RHO
zeolites
temperature
intrecrystelline et temperatures -39, and KZ-1
of any
33
synthesis and crystallization of zeolites
373 K to 473 K (16). The rate of nucleation
growth.
The temperature
can obviously
and the linear
rate of crystal
effect growth
are
as follows: dN/
dt = A(exp(Et)-1)
(1)
r = kf The coefficients
of crystal
with retsing
temperature.
growth
the temperature
The curves xed crystals,
showing
5) and k (in table 6) all increase
(coefficient
Is elevated,
that
with other
against
for mordenitc
the temperature zeolites,
k)and
the induction
yield of crystals
as illustrated
It is also observed that found
Cz)
k, A end E gn table
linear rates
When
the
rates of nucleation
with temperature,
(coefticiernts
time of s-shaped
crystallization
time are characteristically
sigmoid
strongly
affects
the induction
such as NaY by Xu Qinhue
time.
(20) or ZSM-5
This behaviour by Cheo
FAUIASITE
An example
of three zcolites
(cube-octahedron),
curves in shape,
that
increase
is shortened. in absence
of
is typical
of
in fig. 3 (19).
(TYPE
Fig.2.
indicating
A end E) both
X. Y)
that have the same structural
but probably
form from smaller
polyhedron
ring units.
(5)
(21).
34
Xu Qinhua and Yan Aizhen
t
TIME(h)
I
TIMECDAYS)
Fig.3
The characteristic N,
sorption
s-shaped
Table
curves
H,O / Na,O
illustrated
for mordenite
the yield of mordenite.
5. k,A and E for zeolite NaA of alkalinity
T(k)
crystallization
was used to estimate
as function
and temperature(l7) Kbmh-‘) Ax lo6 ~~____ 0.050 20
E ~~__.._ 0.102
343
20
343
30
0.027
13.2
0.033
343
40
0.017
9
0.0115
333
20
0.03
1
0.062
343
20
0.05
20
0.102
353
20
0.06
200
0.132
Table 6. Linear
rates K= 0.5 Al / At for growth 343 _---___ 0.0175
of NaX
353
363
373
0.0375
0.0625
0.1071
(18)
Hydrothermal
synthesis and crystallization of zeolites
35
of analysing the nucleation and crystal growth parts of such s-shaped cwvcs was developed by Zhdanov and Samulevich in the case of zeolitc NaX (18). Since the nucleation process is rate-determining during the induction period, the apparent activation energy for nucleation, En, can be c&ukted by the following equation (22)z
A method
dln (I/ -I-
8)
En
d(1 / Z-1
(3)
R
Where 0 is the induction time i.e., the point on the crystallization curve where the conversion into the crystalline phase just begins. The apparent activation energy for nucleation of phlllipsite was given as 13.5 and 14.3kcal/ mol with and without stirring respectively (23), while for the Al-free end member of the ZSM-5 series (silicalite 1) this energy was given as 9.1 kcal/ mol (21). The En value for NaY was obtained as 17 kcal/ mol(20).
Fig.4. Crystallization tields of some aqueous calcium aluminosilicate compositions On left (a, b and c) CaO.AlrO,.nSiO,+aq; on right 3Ca0.Als0,.nSi0,+aq. A = boehmite, Al,O,.H,O F = anorthite J - Ca-epistlbite B-corundum, AlsO, H - a-cristobalite K - hydrogrossular D - Ca-analcime, P - hexagonal dimorph N = Ca-aluminate, tetragonal of anorthitc 3CaO.A&0,.6HsO E = Ca-analcime, cubic I = Ca-thomsonite 0 = unidentified Temperatures are in C . Q = Ca-mordenite 2.2. Chemical composition The zeolite framework
which crystallizes from a given batch is determined
mainly by its composition
Xu Qinhua
36
and its temperature, summerized
shown
given in fig.5.
An interesting
much stronger
influence
The primary
Experiments
in fig.4.
in the well-known
and Yan Aizhen
crystallization fact which
covering
diagrams
of the composition
Table
7
Primary
Mole ratio
Primary
mixture
are usually
of such a diagram
is that the content
are shown
of the composition
of alkali
is
has a
ratio.
in table 7.
of reaction
mixture
(30)
influence
SiO, / AlsO,
framework
H,O / SiO,
rate, crystallization
OH-/
silicate
compositon mechanism.
molecular
weight,
Na* / SiO,
structure,
R,N+/
framework
aluminum
is present
as a major
SiO,
of compositions
An example
zeolite than the SiO, / AlxO,
of reaction
influence
SiO,
range
can be seen from this diagram
on the kind of the crystallizing
influences
a broad
(12, 14, 29).
cation
OH-
concentration
distribution. content.
2.2.1 Water In hydrothermal recognized OH-
chemistry
for
a long
ions in particular.
synthesis
zeolite
vent powers crystal
water
Water
as a guest mixing
siliceous
content
increases.
zeolite
is such
more.
For
synthesis
zeolites
of ZSM-5
water
which
is the basis which
transport
concerntration
can crystallize
the cavities
given
and
It has been
in this
chemistry;
the host lattice. and
facilitate
forming
role
by
in zeolite
The zeolite
systems
will not form
water
the good nucleation
soland
a type of solid solu-
in absence
of a “guest”
(27).
more organophilic
water
assisted
In hydrothermal
aluminosilicates
progressively
that zeolite
stabilizes
of materials
as well as water
magmas.
(12, 26),
of hydrothermal
zeolite.
by filling
porous
of crystallizing
mineralizer
anhydrous
crystals
that
become
This means
example,the
species
and
which may be a salt molecule
The highly
property
the unchanged
stablizes effect
component
is an excellent
mineralizing
promote
The stabilizing
molecule,
water
is essential leaving
of water
growth.
that
This
may then be removed
tion.
water
time
stabilizes
and more hydrophobic
them less and intercrystalline
or the degree
of dilution
is of minor
out of gels with an extremely
wide range
as the silica species
organic importance
for the
of H,O / SiO,
ratio
@ram 7 to 22) (28). 2.2.2 Alkalinity
(pH)
In the inorganic mineralizing OH-
system, role
concerntration
period
before
Table
5 also shows
of
the alkalinity free
will generally
viable nuclei that
OH-
as (OHj2
is also
amphoteric crystal
growth
been
bring
increasing
discussed
about
in determining in
detail
an accelerated
alkalinity
by
crystal
the crystallization Barrer
growth
(24).
rate.
The
An
increasing
and a shortened
induction
The
OH-
into solution.
pH can be attributed
temperature
influences
the rate of nucleation
Fig.4 indicates that the induction time decreases with pH. The induction time was reported to vary
rate of crystallization
mineralizer.
and hydroxides
with raising
at constant
at constant alkalinity. slope tend to increase
and the maximum
a powerful oxides
is the key parameter
has
are formed.
like increasing temperature strongly and the maximum inversely
OH-
ion
as (OH)‘.75(23). is a good
The decrease at least in part
complexing
in nucleation to the much
agent
which
can
time and enhanced greater
concerntration
bring rate of of
Hydrothermal
synthesis
and crystallization
37
of zeolites
In alkaline media the sohbility of riliea increases nearly exponentially with dlssaolved species. eoncertration of alkali, end according to the ratios of M,O / SIO, in the resultant mixture e range of dlicatc enionr may appenr of various degree of oligomtrisetion. In the case of alumina there is et high pH minimal oligomeriretion, the dominant anion always being Al(OH);(12). The high alkalinity tenses e high supersaturation of silicate end aluminate end the formation of a large number of nuclei. The growth of these nuclei proceeds until the eluminium in the gel is exhausted, so alkaline media thus enable ready mixing of reactants end facilitate nucleation and crystal growth. High pH (> 14) is necessary to initiate crystallization, but during crystal growth high pH reduces silica content of the product. Teble 8 shows some examples for the crystallization of Y zeolitea with high Si / Al ratio in dependence on different exeees elkelinities (Ne-Al) given by (NeOH)-(NeAlOr) in the batch.
/‘\ /’ ( **’ \* ’ I,‘/’
I
Fig.5.
Table 8
CHABASITE p1 + CIMELIbIIWI
Cryetellizetion diagram for the system Ne,O.AJO,SiO,H,O fore temperature of 363k end (Al,O,+SiOr)/ H,O = 0.0005 Dependence of the Si / Al ratio of zeolite Y grown from batches
with different excess alkalinity @In-Al) using seeds of zeolite X with r0 - 0.9pm Composition SiO, Al,O, 20 20 20 20 30 30 30
2.2.3
Composition
of the batch
(Na-AI) SiO 1
0.76 0.79 0.72 0.71 0.70 0.695 0.69
H,o
%
Y
% NePl
A1,O3 780 780 780 780 780 780 780
(25)
of the product Crystallization Si
emorph
xi
time
(h) 100 96 100 95 100 100 98
4 5 2
2.78 2.92 2.97 3.00 3.10 3.14 3.24
96 144 187 312 690 624 695
Si / Al ratio
Compoeition is a variable for any given framework structure. The synthesis conditions to give high silica zeolites, are now known clearly as a rule of thumb, any zeolite becomes more useful es e catalyst or an adsorbent where acid resistance and thermal stability are critical. Framework composition can often
Xu Qinhua and Yan Aizhen
38
be varied
by simply
thetic faujasites It is observed Si / Al near structures
changing
that
the crossover
to
those
containing
This suggests
Si / Al of 5, 5-ring In contrast,
several
faster
is opposite etc, in which
when,
an
have
all other
to the normal
factors
high Si / Al ratios
selectivity
fraction
of
from
5-rings
of Al decreases
and the syn-
hydrophilic
to hydropholic
features (for
example,
below one per 6-ring
Y -
omega
corresponding
All data remaining
the influence
of the aluminum
tend to show
that
equal,
behaviour
give a system
the aluminium
of zeolites
with higher
content
with lower
viscosity
content
of pH upon
for mordenite Table Framework
Lindc
A A Sodalitc Sodalitc
curves
synthesis
9. Variable
Name
A
of the solution
of yield against
composition
and a lower
time
in zeolites Surface
property
References
2
hydrophilic
14
N-A
2.5 to 6
hydrophilic
31
ZK-4
2.5 to 4
hydrophilic
32
Solalite
2
hydrophilic
14
10
hydrophilic
33 14
TMA
sodalite
Faujasitc
Linde
X
2 to 3
hydrophilic
Faujasite
Linde
Y
3 to 6
hydrophilic
14
hydrophobic
34
ZSM-5
ZSM-5
5 to infinity
of
of ZSM-5 This
such as A, X, Y
at 573K(193.
SiO, / AlrO, A
to an
of the gel is lower.
Si / Al ratio,
TIME(h)
Influence
-M
on the rate
the rate of crystallization
rate.
Fig.6.
at an
from 4, 6, and I-ring
(35).
investigated
hydrothermal
A, ZSM-5
mixture.
are listed in table 9.
in the xcollte structural
increasing
(9, 36, 37, 38).
of the reaction
surface
to the change
is favoured
workers
ratio
SiO, / Also,
that as the fraction
formation
crystallization
becomes
AlsO,
in the zeolitc
7.5 to 10, corresponds
-ZSM-5).
ZSM-5
the SiO,/
(X or Y type) with diferrent
reaction
Hydrothermal synthesis and crystallization of zeolites
2.3 Structure-directing
39
role of inorganic and organic cations - templatiag action
Cations present in a reaction mixture are often the dominant factor determining which zaolite structure is obtained (14). Flanigen (39) pointed out that the two basic roles of a cation are: 1). a limited structure dlrcctlag role and 2). a general role of providing a source of hydroxyl ion and stabilizlag the formation of sol-like alumiaosilicate species. The cation is said to have a structure directing function which influeaces the structure of the zcolite which forms. Such a structure directing effect has been termed ‘ternplating’. More recently, the concept of cation templatiag in zeohte synthesis has been developed and summerized by Rollmaa (30). The unique structural characteristics of zeolite frameworks coamining polyhedral cage have led to postulate that the cations may thus serve as ceatres or templates around which the silicate, aluminate or alumiaosilicate species aggregate, forming, with the cations, precusors to specific germ nuclei. The cation stabiLizes the formation of structural subunits which are the precursors or nucleating species in crystallization. For example, sodium and hydrated sodium ions were suggested to be responsible for the formation of D4R, D6R, gmelinite and sodalite cages. On the other hand, potassium, barium and rubidium were believed responsible for directing caacriaite cages (12, 24, 39). The templatiag theory is based on a stereospecificity which can not be separated from chemistry of the cation. The relative sizes of the polyhedral cages and the related specific cations are shown in table 10. A good tit for the anhydrous diameter is observed for the TMA ion in the gmeliaite and sodalite cages, the K, Ba, and Rb ions in the caacriaite cage and for the diameter of the hydrated Na ion in the gmeliaite and sodalite cages. Table 10 also shows that the Ba and Rb ions can substitute for K, and TMA can substitute for the hydrated Na ion in their structure-forming roles. This analysis knds to support the cation-templatiag concept for most of the polyhedral cages considered. Table 10 Cation Specific Building Units in Zeolite Structures
(39)
Specific Cation Building Unit
Diamcter,A
D-4 Q Soda&
Free Dimension ,A 2.3 11.4 6.6
Na Na Na or TMA
Anhydrous (Crystal) 2.0 2.0 2.0 (Na),6.9
Gmeliaite
6.0 x 7.4
Naor TMA
CrMA) 2.0 (Na),6.9
Caacriaite
3.5-5.0
K,Ba,or Rb
D-6
3.6
Na,K,Sr, or Ba
CrMA) 2.8 W2.7 0W3.0 (Rb) 2.0-2.8
Hydrated 7.2 7.2 7.2(Na), 7.3 CTMA) 7.2 Wa),7.3 CrMA) 6.6Q),8.1 Oa),6.6 IRb) 7.2-8.2
An excellent review about the role of organic substances in the synthesis of zeohtes has been published by Lok et al(40). In the early 1960’s, the TMA cation was introduced as the first organic cation to be used in zcolitc syathesis(3la,32). The synthesis of Omega zeolite is strongly favoured by the presence of TMA* ions, as illustrated by Xu Qiahua(41). Table 11 shows that the zeolitc G can be obtained only from gels containing both Na’aad Me,N+ (TMA+) as templates, cations Na* and Me+N+ play an important part in the synthesis of omega zeolite. Either Me,N+or Nat alone will not form this zeolite. The result showed that aaalcium would be formed when the ratio of (Me+N),O to Na,O + (Me,N),O was less than 0.06, and that sodalite hydrate would be produced when the ratio was more than 0.13,
Xu Qinhua and Yan Aizhen
40
zeolite omega affinity
was formed
of 14-hedral
which can occupy the ratio an
smaller
of Na*
example
of
templating
action.
This can be explained
necessarily cation.
Kacirek
and Lechert
pentasils
as follows:
space
available
The use of TMA+
ZSM-5 Table
in forming
(43).
/ ~a,O+(Me,N),O]
* A&O,
(M e,N),O
/
l
0.273
0.064
n
0.363
0.085
n
3.80
0.545
0.130
n
3.43 1.96
0.822
HSC’
2.29
0.193 0.540
0
4.25
1
into
using
the nucleation topology
the TPA+ is needed
process around
the channel
NMR
measurements
intersections that
see table
and
formation,
(41)
HS
has had two major (SiO,/
Also,>
impacts.
First is
120) region.
Second,
13. by Argauer
and
in the nucleation
the organic thus
88HsO
hydrate.
to alumina
was reported
whereby
In the case of ZSM-5
which
silica
preferably
itself
facilitate
such as the
HS+@)
ions in zeolite synthesis
the high
type have been reported,
the TPA+ion
[(Na,O+(Me,N),O] 0
of a; c) HS-sodalite
ammonium
bases
zeolites,
Zolite Type -a”
3.89
science
neu.
in some detail by
lOSi0,.
4.00
quaternary
charge
on the type of zeolite
12.12 Na,O+(Me,N),O]
amount
of these
and charge
organic
a+ (@’ a+n
of ZSM-5
type.
that
silica-rich
0.020 0.043
geometric
structure
12 shows
0.083 0.183
structure
During
Table
with extremely
the zeolite
and Fu-1.
@fe,N)O
b) @)-small
that
(33).
can be
framework
zeolite A was later examined
which
in the
Al ratio number
as template
4.17 4.07
of zeolite
The synthesis
acts both
(moles x IO”__ 0
The use of the other
ticular
of Me,N
the anionic
common
grown
are incorporated
for only a limited
(moles x 10-r) 4.25
a) a-analcium;
been shown
and Meier
Si/
of
of new
of more
of A zeolites
A with higher
therefore,
So TMA*
silica-rich
espicially
of the ratio
Na,O
many other
and
and also zeolite Nu-1
11. Influence
zeolite
there is room
Si, less Al).
(413k, 92 h). gel composition:
the extension
because
in the framework
novel structures,
and ZSM-11,
ions,
This is
especially
of the formation
and the synthesis
when large ions such as TMA+
usual,
(42) and Baerlochev
of several
formed
than
TMA
Therefore,
of zeolite n.
substance,
of the composition
the
Na+ ions,
unit structures.
organic
group,
because,
the smaller
in the synthesis
for the initiation
the Mobil
changes
Adding
have to be low (more
tralizing
the formation
series from
while
of smaller of
template
This is perhaps
is great,
range
addition
the
Dramatic
will be more silica-rich
0.06 to 0.13.
a definite
an important
been observed.
ions in the pore
would
Recently
compositions.
from
in n zeolite
for the formation
fall within
as the ZK and ZSM
of TMA’have
A produced large
is also needed
ions must
with unusual
obtained.
cavities,
is ranging
for the Me,N*
ions has become
zeolite structures presence
cages
to Me,N+
tetraalkylammonium zeolites
only when the ratio
gmelinite
provides the TPA*
cation
Landoit
(34).
Further
step of the crystallization
organizes
the initial
oxide
building
ion is obviously
tetrahedra block
it has process
into a par-
for a particular
used as a template
around
are formed.
the TPA+
It has been proved by Boxhoorn et al (44) by “C MAS ion is localized in the intersections of the channels of the ZSM-5
structure. Now it is possible started suggested
a systematic that
to synthesize study
the hydrated
ZSM-5
of ZSM-5 Na*ions
in the absence synthesis
of any organic
compounds
are able to function
as a template
(45).
Nastro et al (46) system. They for the formation of following
in the RJa, K),O.(Al,O,)x.(SiO,)y.(H,O)z
Hydrothermal
Table
12
Zeolite
TMA*
Some oolitc
synthera
type
Sodslite
Nm+tTMA+
41
synthesis and crystallization of zeolites
in prcsen~c
of orS~nic
barer (12)
C~tionl
Zeolitc
type
Li++Cs’tTMA*
Cmncrinite
Ciimmndinc
Li-ABW
SodmUte
Zcolite
Cnxrinite
Edinrtonite
Gimondine
AUlkiiC
Zeolitr
A
bsobpcs
N-A,
Offretite
cs,ZK-4) Pnisite Zeolite
TMA-E
Mnrritc
(reolites
Zeolite
EAB)
AI-V
IX-P)
(0)
Zditc
ZK-5 QtPL)
Zcolitc
ZSM-2
Gbmondine
D,ZSM-4)
(Nm,TM
A
Sod&e N~+tK++bcnrgltrimethyl~mmoninm
Erionite
GmtYpcr
Lorod
N mud Z-21)
CATMA*
ZoDlite Nu-1
Na++
Zmlite
Na++OiuO
Fu-1
Sodalitc
D&a
Lord
N~*tncopen~ltrimethyhmmonillm
Losod
Na+tCR,N+
Zeolitc
Giamondine Gmclinite
Bl%TMA+
Erionite Zeolitc L Erionitc
S++Nm++TNA*
N+CH,
Nl*tprimnrp.
n-~lkylmniner
(CI to C,J
Zeolitc
ZSM-5
Zcolite
ZSM-5
Oilretitc Na*tbopropylmninc, Chmbarite dibnlylmnine Zeolitc
ZSM-5
Zeolitc
ZSM-5
Zeolite
ZSM-5
zeo1ita
ZSM-8
Zeolitc
ZSY-11
N~*t+bcnryltriphenyl~mmonium
Zsolits
ZSM-11
N~+ttstrbotylphorphoninm
Zeolitr
ZSM-11
NH,ttelr~bntylwnmonium
Zeolite
ZSM-11
K++CH,&
Zdits
ZSM-10
Zeolito
ZSM-12
kn,
N~*tK*ttctnethyl~mmonium (with
(CI(OH),PC(OE),,AI(OH,)
(MPL)
(MEL)
ZK-5
KFL)
Lcsynite
@ dipropylmnine
or
(MFL)
Xu Qinhua and Yan Aizhen
42
Table 13.
Organic. zeolite strwtwe reltionehipaQ0)
Otgank TEA
structurs P ZSM-8
Metbyltrietbylammoulum TP.4 n-Propylamine TBA Choline
TMA+TBA
ZSM-12 ZSM-20 Mordenite ZSM-25 ZSM-12
structure type ? ZSM-5 ? FWljfl8ite Mordenite 7 I
ZSM -5 ZSM -5 ZSM-I1
ZSM-5 ZSM-5 ZSM-II
ZSM-38 ZSM-34 ZSM-43 CZH-b ZSM-39
Fe&rite Erionite-oflretitc
z&39
ZSM-39 ZSM -48
ZSM-12?
ZSM-35 ZSM-21 ZSM-23
Ferrletite Ferrierite ?
1,2-Diaminoethane
ZSM-5 ZSM-21 ZSM-35
ZSM-5 Ferrierite Ferrierite
1,3-Diaminopropane
ZSM-35 ZSM-5
Fertierite,ZSM-5
1,4-Diaminobutane
ZSM-35 ZSM-5
Ferrierite,ZSM-5
l,!i-Diaminopentane 1,6-Diaminoherane 1,7-Diamlnoheptsne
ZSM-5 ZSM-5 ZSM-11
ZSM-5 ZSM-5 ZSM-11
1,8-Disminooctane
ZSM-11 ZSM-48
ZSM-11
1,9-Diaminononane l,lO-Diaminodeonne
ZSM-11 ZSM-11
ZSM-II ZSM-11
TMA+n-propylamlne
Pyrrolidine
DDO
ZSM-10 ZK-5
ZSM-39
? ?
MD0
ZK-20
Levy&e
MQ
LZ-132 Nu-3
Levynite Levynite
TQA
ZSM-18 LOSOD
1 LOSOD
ZSM-30
?
Mordenite
M ordenlte
BP Dihexamethyleoet&mine Neopentyinmine
Hydrothermal
synthesis and crystallization of zeolites
43
It secondary building units (SBU), which arc common to ZSM-5 and also to the mordenite rtructure. was propoacd that in a suitable chemical cnvironmment Nat ions allow the very slow araembly of these units to give the precursor structures of ZSM-5 nuclei. (fig.7) Using “Si and -Al MAS NMR, IR, thermal and texture analysis Scholle and coworkers (47) have confirmed the presence of increasing amounts of TPA-ZSM-5 entities with dimensions lear than or comparable to an unit cell at early stager of the crystallization. The concentration of these. nuclei can be followed using the intensity of the lattice vibration at 550 cm-l, rpeciflc for ZSM-5 zeolite (48, 49). The change in concentration of nuclei and crystals formed during the rynthesir i8 showed in tlg. 8. It ir clear that just for any other zeolite cryrtallization, ZSM-5 also crystallizes according to the successive nucleation and growth processes.
Fig.7. The SUB of ZSM-5 zeolite.
SYNTHESIS TIME (MYS) Fig.8.
(c) nucleation
and (a) growth curves or ZSM-5
synthesis
Curve (a) represents X-ray crystnlinity and curve (b) the Ilk cryrtallanity; the nucleation curve (c) Is the difference
between
(b) and (a) (28).
44
2.4.
Xu Qinhua
and Yan Aizhen
The nature of reactants (50)
The variety
of species
When hydrated silica
zeolite
silicas
were
sodium NaX
was
used.
gismondine
used
14
Some
Charge
(51, 52), especially
formed
at 371K
“inactive”
much
sources
The “activity”
of very small amount Table
of the alumina,
silicates
The
type NaP.
as sources
silica and base
sodium
more
under
the
of the soluble
metasilicate
readily
than
appeared
14 (12).
were the source
“inactive’
of synthesis
silicates
in table
solid
and
resulted
to depend
of
colloidal
primarily
in
on the presence
of Al in them. sources
of
cations,
aluminium
and
silicon
in
zeolite
(12)
crystallization
compensating
cations
Silicon
Aluminium
-
--
--___--
Alkali
metal
Alkaline
hydroxides
earth
and
Metal
hydroxids
oxides
and
hydroxides Salts (Fluorides, halides, carbonates, phosphates, Organic
especially
Silicates A10
l
Silica
sob
Glasses
Silica
gels
Sediments Minerols,eopeoially
Silica and other synthetic gloeseo
clay
minerals,
felspathoids,
hydroxide,
and
other
glass
Silicon felspars
Tuffs
zeolites
esters and
Minerals
quaternary
volcanic including
minerals, felspars
and
Mixtures
aluminates
of two
of the
silicate
Water
bases Silicates
and
hydrates
OH
Al salts
and
ammonium
AlrO,,
Al alkoxides
sulphates,etc.)
bases
--
aluminates
Al(OH),
oxides
Other
pentahydrate,
when
conditions
sodium
has been listed
glasses clay
felspathoids, and
other
zeolites
or more
Basalts
above.
and
mineral
mixtures Sediments Combinations more
In alkaline NMR
sodium
indicates
while above
aluminate
that
pH 10.5, Al(OH);
polymeric
anions
the extent
of which
NMR plained
solutions
(AlfH,O),)‘+,
study
at pH values
is AHOH);,
and Al(OH),.nH,O In alkaline silicate
The viscosity
glass indicates
can be expected
by the presence
anion
(Al(OH),(II,O),)+ is exclusively present.
are common.
(55) of a solution
the important
to vary
of Hz0
of anionic
of water with
+ D,O
concentrations of KOH
(8M),
a high degree
of the Na silicate SiO,
“Al
A
was ex-
species as follows:
1.45M 0.22M 0.16M cyclic trimer
0.08M
linear
trimer
0.07M
linear
tetramer
0.03M
show
7,
free alkali. (0.02M)
dimer
clearly
below
of polymerization, and
(3M) and Cr”
cyclic trimer
now exsit that
or
are important species (54); solutions, on the other hand,
monomer
substituted
Data
of two
of the above
(table
15) that
the crystallization
of ZSM-5
from
the usual
gel is de-
Hydrothermal
45
synthesis and crystallization of zeolites
pendent on, among other factors, the nature of the silica source. Silica sources initially containing high amounts of the silicate monomer crystallize faster than gels in which silica is present in a higher polymeric form. It can therefore, be concluded that the rate of dissolution or of depolymerixation of As silica, which is known to be slow, intervenes in the rate-determining step for nucleation of ZSM-5. the crystal growth is influenced in the same way (table 15) this indicates that it’s rate is also determined by the concentration of monomeric silicate. Table 15. Influence of the nature of the silica source on the crystallization rate of ZSM-5 (28) Silica source
Nature
Induction period @)
“ m ’ 50% to reacn crystallinity 01)
Ref. 15
Wster glass
Monomeric
25
40
GCl
Polymeric
60
140
15
QUO
Precipitmtcd
8
12.5
62
Csbosil
Fumed
10.0
62
Ludox
ColIcAds ml
4.1
5.5
62
siucste
Monomeric
3.s
4.0
62
silica
In the synthesis of ZSM-5 zcolite, as a general rule, the TPA silicate contains a much smaller number of discrete species compared with corresponding sodium silicate system. In a real synthesis mixture form ZSM-5 with the molar composation (~PA),O),.~a,O),.(A~O~.(SiO,),,,(H,O),,,,. “Si NMR shows that many silicate species,from monomers to highly branched polysilicates, are present (56). The trans. formation of a Qissilieate species into a real precusor building of ZSM-5 reolite is shown in fig. 9 (57).
-’
\0----/
Fig. 9.
2.5
Transformation of a Qi, silicate species into a ZSM-5 building (62) Block Q2 end silicon atom; Q:Q” linear trimer silicate Qisa double five-ring silicate
Metastability
and recrystallization
The zeolites are prepared at irreversible, non-equilibrium and super saturated conditions. So the majority of these zcolites are not the true equilibrium phases. In the system Na,O, Also,, SiO,, H,O only two zeolites phases are probably equilibrium phases, these are analcime and mordenite. The exper. tmental results indicate that the less stable the phase the greater the chance in which initially it will nucleate and grow fast, but it’s chance of subsequent survival is less. Two xeolites, one which is minerial
Xu Qinhua and Yan Aizhen
46
that has formed and
and exsited
is synthesized
changes
in chemical
the ordering stable
composition.
conditions,
be transformed
that from
an aluminosilicate
depending
on crystallization
cage structures to denser
structures
growth
conditions Y and
transformation
is the predominant
diffractograms
of solid
hydroxide heating
in the liquid of zeolite
ture of molite formation found.
A and zeolite zeolite P,
This means
of intermediate
at three
appears
(14).
reaction
of zeolite work
(tr=
that the transfomation
(13) described 213 h and
However,
structural
the diffraction
of zeolite
A into zeolite
t,=
309 h).
the course
Similarly, of a mix-
of the transXI in fig. 10).
A and zeolite
place
x-ray
mr of sodium
(diffractogram
of zeolite
P takes
mol/
and
solid-solid
in the formation
during
form
peaks
nucleation
as fig. 10 that
2050
at 355 K for 213 h results
Large
into zeolite P or that
or even, that
containing
0, t,=
of zeolites, (58).
and tend to transform
indicated
A particles,
mixture
as the predominant only
results
given
This means
types
that zeolite A can be transformed
times
to another under
(14).
two or more phase
to
or equivalents.
may,
types of zeolites
in solution
VIII in fig. 10).
presented,
which
and
as related
time of crystallization)
experimental Subotic’s
of NaOH
Pt( diffractogam
diffractograms
unstable
off the reaction
A in 2.05 M soliution
process,
In all the x-ray
drawn
relatives
structure
one can obtain
at the surface
process
samples phase,
start
of disorder
and it is ease of conversion
alkalinity,
It is known
structurally
due to ordering
no mineral
more stable
S are relatively Some
have
metastable
composition,
(59).
differences
a high degree
zeolites
represent
(temperature,
media
P particles
zeolite
thermodynamically
as zeolite P (14, 59).
of zeolite
shows
of the synthetic
gel of desired
obvoiusly
chabazite
zeolites
into other
in alkaline
time, and the other which is related
exhibit
of the synthesized
synthetic
such as zeolites
into hydroxysodalite crystal
Many
Some
can
Synthesized
due to the metastability
species.
of geology
in the laboratory,
in the mineral.
This is probably more
over long periods
rapidly
without
P, can be
the formation
solids.
I% 17
18
15
14
13
12
11
10
9
8
7
6
5
t
Fig. 10.
Ifzeolite
A remains
in 3 days. zeolite
X-ray diffrecrograms solution at 355K in prolonged
Similarly,
Y to omega
zeolite
of solid samples
contact
X appears
has been observed
from 17 h to 63 h (60).
obtained
with the mother to be metastable in certain
by heating
liquor,
recrystallization
with respect
gel composition
zeolite A in 2.05M
to zeolite
when
to zeolite P (14).
the digestion
NaOH
P may occur
Conversion
of
time is extended
Hydrothermal
47
synthesis and crystallization of zeolites
The metartablility of crystalline zeolitcs was studied also by Xu Qinhur (61), and pointed out that the change of metastable zeolite to a more stable one, depends upon the intrinsic properties of the zcolite and also the chemical environment in which it exists. For example, added 50 wt% of crystalline Y zaolite, based on the amount of zeolite produced, to the aluminosilicate gel with the composition of (1.70 88H,O, which is the composition of the reaction mixture Na,O + 0.35 (Me,N),O). AGO,. lOSi0, The x-ray powder diffraction showed the characteristic pattern of the Y of omega zcolite formation. type after the gel had crystallized at 413K for 10 h, but the pattern is entirely of the omega type after the gel had crystallized for 12 h. After 24 h the process was complete (fig. 11). This means that the trans. formation of zcolite Y into omega takes place without the formation of intermediate
solids.
J
Fig. 11. X-ray diffraction pattern for Y turning to Omega zcolitc.(41) a. crystallization 10 h; b. crystallization 12 h; c. crystallization 24 h. The process and mechanism of the transformation of NaA to NaY zeolite in the chemical environment of NaY formation have been studied in ref. 20, 25 and 61. By adding NaA zeolite to the crystallization environment of NaY zeolite formation, the process of transformation or recrystallization may be represented as: Amorphous aluminosilicate gel NaY ----> ------> NaY NaA
NaA
In fact, the recrystallization of NaA zeolite does not occur until the gel crystallization is nearly complete. Scanning electron microscopic observations and the recrystallization curves are consistent with the mechanism that includes a degradation and dissolution of surface NaA crystal. The nucleation and crystal growth of NaY take place directly at the surface of NaA crystals. SEM also show that the formation of the smaller spherical NaY crystals (-0.5 pm) occurred at the surface of the larger cubic NaA crystals (.0.5 am). The nutrient for NaY crystallization is being supplied by the dissolution of NaA crystal surface. During recrystallization, the reaction process from the surface to the interior of the NaA crysals, finally the NaA crystals are converted completely to smaller NaY crystals.
Xu Qinhua and Yan Aizhen
3. MECHANISM
During
past
the
crystallization.
63-73).
two
Recently
also highly
much
siliceous
attention.
and Lechert
in the growth mechanism.
have proposed more complex
Derouane
formation
of 4 and
(monomeric
transformation
on the silica source
et al. (10) while
Roozeboom
studying
6 ring
and polymeric)
ions from
that
other
both
investigators
from
the amorphous
the
phase
ion trans-
of a solution
(22, 38, 52, 64-69) silicate
and possibly
by the dissolution
the liquid
(69)
been dis-
for crystallization
in support
phase
are important
Y synthesis indicated
systems have
et al (15, 18), Sand et
of aluminate, supplied
mechanism
system,
evidences
Zhdanov
and the gel formulation
A, X and
vs 5 ring
of zeolites
evidences
of the polymerization suggested
of zeolite
(15, 22, 38, 39, 43, 52,
the role of liquid
presented
the ions being continuously
phase
zeolite depending
and
mechanism
(38, 43) and mordenite
type zeolites.
et al. (63) and later many
phase,
the
for the synthesis
demonstrated
nuclei
et al. (43) have
and solid hydrogel
of ZSM-5
by contrast,
zeolite
Barrer
on
was studied
(70, 71) have been presented
to be the result
ions in the liquid
id gel material. mechanism
Initially
the nucleation
appeared
mechanisms
A and faujasite
(67) have,
of different
have
A and faujasite
such as the ZSM-5
different
in studying
CRYSTALLIZATION
papers
(74) and McNicol
transformation
al (22, 26), Kacirek transport
of
of zeolite
zeolites,
Two
Breck and Flanigen
by solid phase formation
a number
In most cases the synthesis
have received cussed.
decades
OF ZEOLITE
of the sol.
ion transformation
in studying
the synthesis
used.
gels, proposed
dissolution
aluminosilicate
a mechanism
of some
gels.
silicon
for the
containing
The hydroxylated
ions con-
dense with Al(OH); to form a large variety of complex aggregates. The mechanism of zeolite synthesis from alkaline aluminosilicate gels has continuously puzzled zolite chemists. The reason is the complicated reaction “skeleton”
medium
from which
with a liquid
processes
which
phase
occur
during
the zeolite crystallizes
containing the induction
during
induction
remains
an enigma
studies
on the mechanism
of zeolite
the autocatalysis role of solid
system
period,
is still being
insufflcienently
discussed
studied
the determination quantitative
phases
posed
a solid
experiments, Guth
data
no spectral
transformation
flicting. @RS),
and aluminate Roozeboom X-ray
synthesis. zeolite
crystallization,
structure in favour
(XRD) showed was
and
However,
Angel1
that the AI(
detected
of a solution
and
in the liquid
the solid
chemical some
transport
of solute
and liquid
crystallization
in NaOH phases
in the liquids species
mechanism.
(band
by Laser at 621 cm-‘)
appeared
species.
in the
But
and thus pro. similar
mechanism.
complexes
in solution
with the spectra
investigations
(Al, Si, Na) for studying
in
spectroscopic
transport
solution
in
species, and
(68), performing
of aluminosilicate
remains
of the difficulties
aluminosilicata
and Flank
of the
aluminosillcate
Some Raman
for a solution
So far, Raman
analysis solute
during
the nature
of the system
are some
of gels.
phase
ions separatly
phase
In
such as
The question
ions and silicoaluminate
the exsitence
in solution.
et al. (10) examined
diffraction
Their results
together
of the system
to kinetics,
studied.
These
structure
for the occurrence
and aluminate
are referred
of the liquid
the
of zeolite synthesis.
in the heterogenous
in the liquid
changes
evidence
ions exsiting
crystals
aluminate
and chemical
mechanism.
of silicate
quastions
gel
for tracing
The behavior
the mechanism
crystallization.
suggest
aluminosilicate
effect, on the rate of crystallization,
The composition
of spectral
indirect
the spectra
crystallization.
of zeolite
changes
sodium
to find a technique
have been insufficiently
and state of silicate,
spectroscopic
an evidence
before
alkalinity
which
with zeolite nuclei
et al. (80) reported
silicate
any
phase gave
by comparing
(75 - 79).
of zeolite
(70, 71) observed
an amorphous
It is difficult
the main
in the formation
of the structure
(80, 81) and 29Si NMR McNicol
process, effects,
in connection
determination
period
crystallization,
and seeding
and liquid
---
ions.
and is the key to understand
of the crsystallization
of the induction
similar
of
have been con-
Raman
Spectroscopy
A, X and disappeared course
Y zeolites before
of X and
Y
Hydrothermal
3.1.
Crystallization
49
synthesis and crystallization of zeolites
by solid phase transformation
mechanism
On the basis of electron microscopic studies and chemical analysis of aluminosilicate gels, the crystallization occurs from the solid gels phase (74, 75). The induction period was postulated to be a time during which the nuclei formed in the solid phase growing into a definite size. The elemental composition of the crystalline polite was almost identical to that of the initial solid phase extracted from the gel (15). During crystallizatioin neither the hydrogel phase is dissolved nor the formation and growth of crystal nuclei is occured in the liquid phase. The luminescence and Raman spectroscopy were used to study structural and chemical aspects of gel growth of A and faujasite type crystals (71). Their results are consistant with a solid-phase transformation of the solid amorphous network into zeolite crystals as shown in fig. (12 - 14). EOD)
4T,(4~
so0
1
,
400
500
I
do0
700
go0
WAVELENGTH Fig.12.
Phosphorescence
I Fig.
13. Eu’+ phosphorescence
(a) and excitation
(b) spectra of Fe’* in zeolites
-
&’ IN DEL
-
Et’ IN CRYSTALS
I
I
a00
700
WAVELENOTHCum) in gel and Linde A crystals, measured
(71)
L
ma at room
temperature
(71)
50
Xu Qinhua and Yan Aizhen
LINDE A FRAMEWORK
YlBRATIoN
7& I
I
I
750
500
250
WAVE
Fig.
14.
Raman
NUMBER
(cm-9
spectra of crystallizing gels: (CHa,N* / Linde INITIAL
ALUMINGSILICATE
A system
GEL
L-TETRADRON I,
BUILDING
BLOCK
REARRANGEMENT HYDRATED
FURTHER
ARROUND
SPBCIES
CONDENSATION
REACTION
ZBOLITE CRYSTALS
GR0WT.H OP CRYSTALS I
Pig.
15.
Schematic
rcpzescntstion
phase transformation
of the crystallization
mechanism
(82)
by a solid
(71)
51
Hydrothermal synthesis and crystallization of zeolites
Trace amount of Fe’+ ions aubstitutcd for Al% the tetrahedral eluminosilicatc gel framework exhibit characteristic phoaphorcscence spectra which have been used to follow the build up of the zeolite fremework (fig. 12). Phorphorescenee spectra of exchanged Eat’+ cations fig. 13) end Reman spectra of (CH,),N* cations present in the solid phase of gel (fig. 14) indicate that no zcolite cages exsit in this phase during the induction period. Reman spectra of the liquid pherc of the gel syrtem show only the The crystallization by e solid phase transformation presence of SiO,.(OH); end Al(OH); anions. mecheniem may be deecribed cchemetieally in fig. 15 (81). When the reactants are mixed, silicate end aluminate ions begin to link end polymerize and initial aluminosilicete gel is formed. Simultaneously, the liquid phase of gel is also formed, but the component concentrations in the liquid phase of gel would not change end remain constant, and liquid phase ion transportation would not occur on the nucleation and growth of crystalli zetion processes. The initial aluminosilicetc gel is dissolved and rearranged due to the action of OH- ions to form the basic building These primary building blocks then rearrange around the blocks of the growing zcolite crystals. More polyhedra erc futher formed and polymerize hydrated species (cations) to form e polyhedron. and combine each other to form e zeolitc crystal. Therefore, the gel formation is the major nucleation controlling process which affects the crystal size end morphology of zeolite phases. The zeolite crystals are generally uniform end smell in size (several The high microns),end are often euhcdrel. A larger number of nuclei arc formed during gel formation. degree of supersaturation with respect to the ionic species present leads to rapid heterogenous nucleation. After the initial rapid growth, continued crystal growth is extremely difticult to maintain. Seeding experiments end experiments involving repleniehment of the nutrient gel have not produced any significant increase in crystal aim. 3.2.
Crystallization
by a solution transport
mechanism
Kerr (64, 65) reported on crystallization of zeolites A end growth from solution. A direct solid-solid transformation uct does not occur. Amorphous solid diBBOlvCB rapidly in species. The concentration of this epecies remains constant
X in specificaystems in which he postulated of amorphous substrate to crystalline prodthe alkaline solution to form II soluble active during most of the growth period.
Angel1 et al. (68) have determined the mechanistic pathway in 4A synthesis, by using several different characterization techniques es a function of time. The evidence aLso supports e solution trenaport mechanism. The date obtained from the Reman spectra are summarized in table 16 and fig.16. These results, perticulnrly during the crystallization BtagC can be contreated with those reported by McNicol in previous section, who obacrved no changes with time other then the appearance of zeolite A. The spectra (ng. 16) show that conversion to framework silica occurs from precursor material. This may consist of local. ized concentrations of silica gel formed in the initial mixing of reactants, or by precipitation from e supersaturated solution, or un aluminosilicate gel with a variable degree of crosr-linking. In the early stages, i.e. during induction, the alumina concentration decreased after crystallization of zeolite A has set in. From the rerults of Reman spectroscopy end chemical Roozeboom (10) et al., aluminate disappeared from solution end ia incorporated in the solid to the silicate concentration of the liquid phase, this concentration decreaaed less drastically able low levels. Whereas, in case of both zeolite X end zeolite Y in the first 2-3 hours, i.e.
3 h when analysis by phase. As to comperbefore any
Xu Qinhua and Yan Aizhen
52
LIQUID A -INITIAL
Fig. 16.
Raman
PHASE
cm
spectra
A - INITIAL PHASE B-AFTER SHBS. DIGESTION C-AFIER 4HIlS.
i&7-- 200 of solid and liquid
I
I
,
I
1000
phases
1
,
cm+
500
I
200
in zeolite A synthesis
(68)
6 6
200
400
600
mo
FREQWENCY SHIFT (cm -9 Fig.
17.
Raman
spectra
of solid phaac in zcolite X synthesis
(IO)
Hydrothermal
synthesis and crystallization of zeolites
53
crystalliter were detected, aluminate was disappearing from rolution and incorporated in the amorphous aluminosilicate phase. In the same period an increase in silicate concentration in the liquid and a decrease in silicon concentration of the solid phase have been found. From these data it is clear that in the grst 2-3 h of the crystallization or nucleation aomc silicon containing ions (monomeric or polymeric) were dissolved from the amorphous (alumina) silicate gel. Tlttse hydroxylated iona apparently condensed with the Al(OH); ions present to form a large variety of aggregates, which may be the nuclei for crystal growth. Thus the whole mechanism of zeollte A, X and Y formation is a solution transport meohnnism. The fig. 17 and 18 give the resulting Raman spectra for the solid and liquid phases in zeolite X synthesis after 0,2,4,6, and 8 h of crystallization. Table 16 Laser Raman Spectroscopy
data as a function of
time during zcolite A synthesis aging time,h.
tryst. time,h.
band position,cm-’
(68)
band intensity and shape
-
species
liquid phase 0
0
1 2 3 4 0.5
0 0 0 0 1
0.5
2
0.5 0.5
3 4
620 400-500,700-800 unchanged unchanged unchanged unchanged 620 450,880 620 400-500,700-800 unchanged 620 400-500,700-800
rtrong,sharp very weak unchanged unchanged unchanged unchanged medium, sharp weak weak weak unchanged very weak weak, broad
-
aluminate siltcad) unchanged unchanged unchanged unchanged alum inate ailicafl) aliminate ailioad) unchanged aluminate silica(?)
solid phase 0
PICG-C
0
1 2 3 4 0.5
0 0 0 0 1
0.5 0.5 0.5
2 3 4
405 450 800 unchanged unchanged unchanged unchanged 450 800 unchanged 490 450,800 490 340,700,1040
weak
strong, broad weak, broad unchanged unchanged unchanged unchanged strong, broad weak, broad unchanged new shoulder weak, broad strong, sharp weak
silica silica silica unchanged unchanged unchanged unchanged silica silica unchanged zcolite A silica zeolitc A zelitea A
Xu Qinhua
54
and Yan Aizhen
hr
0
4 6
1
’
400
500
700
600
600
900
1
FRFLQUENCY SHIFT (Cm-‘)
Fig. In fig. 18, silicate silicate anions,
It means disappears
silicate
from
cm-‘(monosilicate)
appeared
A large
variety
nucleation.
(D-4-rings
When
silicate
while the free silica species liquid
as silica source,
species
and D-6-rings).
phase
which
in the solution
has
too
Schematic
are mixed phases
this, both
the aluminosilicate equilibrium
aluminosilicate)
actions
of polymeric
by Angel1 and Flank of their
with bands
initial
silicate
species
into
(68) that aluminate
zeolite
(for
around
450 and
A synthesis
gel.
does not give rise to any signal
800 This
in the gel when
low
prevents
the observation
concentration
representation
due
of specific Raman
to it is growth
of the crystallization
to large
peaks species
by a solution
transport
gels are formed
rapidly. Owing to
is given in fig. 19 (15).
and
aluminate
phase,
ions present
Each
the reactants
heating
are due to the depolymerization
in the separated
of complex
The solid and liquid the
(10)
silica gel.
during
mechanism
phase in zeolite X synthesis
This is also observed
ions.
the liquid
may be due to the sodium used colloidal
of liquid
species with the bands of 936, 777 and 448 cm-‘, and dimeric of 600 cm-‘, exsited in the liquid phase of X and Y zeolite after some 2-3 h
that these bands
and dimeric
621 cm-‘)
spectra
ions of monomeric
with the band
of synthesis. monomeric
Raman
18.
the gels,
depend their
the synthesis,
and silicate
product
the initial
ions are always
concentrationa
of
the
ions
of amorphous which
ions in the liquid
the ions increases,
giving
by the solution
equilibrium.
present in the liquid phase
on the composition increases,
aluminosilicate
gels are connected
solubility
and aluminosilicate between
during
of aluminosillca
(the
solubility
aluminosilicate
of aluminosilica
product
of
gels
amorphous
and temperature.
While
leads to increased concentration of the silicate, As a result, the probability of condensation rephase.
rise to the formation
of primary
aluminosilicate
blocks
(4-
Hydrothermal
synthesis and crystallization of zeolites
55
and &member rings) and crystal nuclei. The formation and growth of crystal nuclei lead to the exhausting simplest silicate, aluminate, and aluminoeilicate ions in the liquid phase, and the equilibrium state is reached by dissolving of the solid phase. The average rate of crystal growth at a given temperature depends on the concentrations and aluminate ions in the liquid phase of gel, as shown in table 17 (15).
of the silicate
HYDRATED SPECIES~ N;, OH-, AI(O (Ho&. Si CO);, (Al, 0, Si, OH) -ANIONS O.l<[Si].
IA~lXlO’<
6
(APPROXIMATELY) HEATING
osi
.A1
633HYDRATED~A~ONS 15Si:Al
hSSOLUTION OP AMORPHOUS
ONDENSATION REACITONS
RATE PHASE TEME
GROWTH IN 91735 AND
DEClkEASE OF CONCENTRATIONS
Fig. 19.
Schematic representation of the crystallization a solution transport mechanism (15).
by
Table 17 Component concentration in the liquid phase of gels(mols / 1) and rate of growth of zeolite A crystal at 363 K samples
NaOH
Al,%
SiO,
(Al)(Si)x 10’
266-l 266-2 266-3
0.560 0.460 0.428
0.0266 0.0258 0.0208
0.0103 0.0085 0.0092
5.46 4.00 3.80
3.05 2.25 1.80
266-4
0.302
0.0104
0.0108
2.25
1.25
V,,,
(15)
cc/ day
.
56
Xu Qinhua
As the nuclei and crystals of crystals
depends
grow at the expense
on that of liquid Table
phase,
18
and Yan Aizhen
of component
as confirmed
Dependence
in liquid
of gel liquid
phase
of gels, the composition 18.
(15)
Si / Al in zeolite SiO 1
X crystais
Na 1 G ,iC SiO 1
AlsO s
(mole / kg)
faujasite
phase
phase
Samples
The data
phase
of Si / Al ratio of zeolite X
on the composition
Concentrations
of the liquid
by the data given in table
Na,O
A’&‘,
SiO,
196-1
0.702
0.0124
0.397
32.0
1.75
1.63
196-2
0.825
0.0114
0.304
26.6
2.68
1.38
196-3
0.926
0.0105
0.220
21.0
4.17
1.33
196-4
0.955
0.0116
0.174
15.0
5.42
1.15
196-5
1.200
0.0202
0.117
5.8
10.05
1.07
the growth
of the Si/
given
in table
is directly
of the gels.
tion of silicates gave the evidence
18 indicates
associated Changes
and
that
with increases in component
aluminates
and
for the crystallization
in the SiO,
concentrations
from
in liquid
silica sol during
of zeolites
phase
crystallization.
by a solution
AQUEOUS BASIC SOLUTION
SILICA SGL
Al ratio
concentration
transport
in the crystal
and
Si / Al ratio
of gel were obtained These
experimental
mechanism.
BASIC ALUMINA SaUlION
1 SOL OF REACTANTS i
PRECIPITATION
1
1 AMORPHOUS GEL SOLID
1 1
L
NUCLEATION (1)
tn, NUCLEI IN GEL
NUCLEI IN SOLUTION
I
I
TRANSPORT GF MONOGLIGGMERS IN SOLUTION
GEL
REARRANGEMENT
GROWTH
4
CRYSTALS
Fig.20.
Schematic
representation
(I ) nucleation
of zeloite synthesis
from solution
of synthetic in the liquid
according
or (II ) in the gel (28)
to
from soluresults
Hydrothermal
Table 19
57
synthesis and crystallization of zeolites
Comparison
of the parameters and operating
conditions for syntheses of type A and B for ZSM-5 zeolites (43)
starting material4 Aluminium Silicon
Al metal Active SiO, (solid, polymeric)
TPA source
3.3.
Ingredient ratios(molar) Si/ Al Al/ Na (Si+Al) / TPA H,O / (Si+Al)
A-l 15.6 0.482 1.79 15.4
Synthesis conditions Pyrex tubes Temperature / k Time required for 100% crystallinity
hydrothermal 393 300-330 h
Crystallization
TPAOH
A&&SO,), 18HsG Na silicate (liquid, mono-and polysilicate anions) TPABr
A-2 13.9 0.827 1.80 15.2
B-l 45.1 0.016 9.19 21.6
l
B-2 45.2 0.016 9.80 27.6
hydrothermal 393 45-50 h
by two different mechanisms in the same system
The zcolitc crystallization may occur following two different mechanisms, in which nucleation occurs in the hydrogel phase or in the liquid phase. The successive events in both mechanisms are shown schematically in fig. 20 (28). Derouane etal. (43) were able to synthcsixc ZSM-5 by either the solution method (method A) or the gel nucleation method (method B), depending mainly on the relative conccntrations of the reactants and the nature of the silica source, to studies on the mechanism of xeolite crytallization. The comparison of the parameters and operating conditions for synthesis of type A and type B are listed in table 19. The results for type A and B measured by XRD, TGA, SEM and chemical analysis are also listed in table 20 and 21 respectively. Summarizing, synthesis of type A and B for ZSM-5 xeolite are two distinct ways from a solution or gel nucleation mechanism, respectively. It seems that a high basicity and / or the presence of foreign ions in the nutrient and also the polymeric nature of the silica source and the Si / Al ratio of the gel direct the reaction to solution nucleation, and that TPA content and degree of dilution are less important as determining parameters for the nucleation mechanism. Therefore, two different mechanisms occur in the synthesis of ZSM-5 zeolites, depending on the source of silica and the Si / Al, Al/ Na and (Si+Al)/ TPA ratios in the reaction mixture. The first is a liquid phase ion transportation process (solution transport mechanism) in which few nuclei are formed and large crystallite4 are obtained. The second is a solid hydrogel phase transformation process (solid phase transformation mechanism) in which numerous nuclei are formed, leading to polycrystalline aggregates.
Xu Qinhua
58
Table
20 Experimental
intermediate
function
(43) Conclusions
crystallinity
after
time. is achieved
Si / Al ratio
increases
The
Liquid
progressively
in the solid
crystallization analysis
as a
320 h at 393K
The
analysis
is studied of synthesis
100%
Thermal
on type A Synthesis of conclusions
Observation Crystallinity
diffraction
Chemical
observations
phases-summary
Technique X-ray
and Yan Aizhen
phase
as
proceeds.
amount
decomposed
TPA
intermediate
is progressively
incorporated
to the crystallinity
of the solid
mass
probably
occurs
of TPA
is proportional
phase
transport
phase
in the zeolite
framework; one
there
TPA
is about
entity
per
channel
intersection Scanning
Gel
electron
and
zeolite
taneously
microscopy
disapears
Table
21 Experimental
Crystallinity
phase
about The
of nuclei; mass
transport
occurs
on type B synthesis of conclusions
(43) Conclusions
is followed of synthesis
crystallinity
analysis
number
liquid probably
grow
- summary
Small
Obserbation
function
Chemical
simul
gel
observations
phases
Technique diffraction
are
The
as the crystallites
intermediate
X-ray
phases
observed.
as a
time.
is achieved
100%
after
48 h at 393 k Si/
Al ratio
the solid until
phase
near
stays
constant
during
in
the synthesis
100 % crystallinity
Crystallization
probably
occurs
by solid
of the
hydrogel
Large
amounts
transformation
is reached Thermal
analysis
The amount increases Large
amounts
present
Scanning microscopy
eletron
of TPA
with at low
An amorphous throughout Crystalhtes in near
in the solid
the crystallinity of TPA X-ray
phase
are
already
are
formed;
are
about
of small
4 TPA
entities
per unit cell in the crystalline material
is observed
Crystallites
apparent crystallized
the only material
nuclei
there
crystallinity
the synthesis. 100%
are
grow
gel by solid
of the latter
100%
within transformation
59
Hydrothermal synthesis and crystallization of zeolites
4.
KINETICS
AND GROWTH
OF ZEOLITE CRYSTALLIZATION
Kinetic studies of zeolite growth have been carried out mostly for A and X type zeolltea. The growing of A and X-type zeolltes has to be regarded as an autocatalytio process and the formation of zeolites is accelerated by nuclei already present (64, 65, 74, 83, 84). From the experiments using nuclei the growth can be regarded as a reaction of first order, whereas the nucleation is a reaction of high order. A typical kinetic experiment for the formation of zeolite A gave the rate of formation of crystalline product to be approximately close to the first-order kinetics with respect to the quantity of crystalline zeolite present (64): dz/dt
= kz
(4)
constant. where z = per cent of zeolite. in solid phase at time t and k is a proportionality The rate of any crystallization process is determined by the rate of nuclei formation and crystal growth. Equation 4 indicates the increase in either the linear rate of crystal growth or the rate of nuclei formation during the period of crystallization. The autocatalytic nature of the crystallization process cannot be attributed to a change in the crystal growth in the course of crystallization, as shown in fig. 21 (15). The rate of crystal growth is greatest at the beginning of crystallization, and gradually decreases as the process advances. The time passed from the beginning of crystallization to the formation of nuclei of crystals of each size can be calculated from the data represented by the curves in fig. 22 (15). The formation of nuclei takes place during the entire process of crystallization, but the rate of nucleation increases only during it’s initial stage. These curves can be described by a common equation in the form z * kt’. The values of the constants k and n are easily calculated from the plot of log z vs. log t as it is observed from fig. 23, the calculated points agree quite well with the experimental curves (15). The calculations confirm that the autocatalytic growth of the mass of crystals can take place during the induction period. Such an increase in the nucleation rate can be explained by postulating that not only the alumonosilieate blocks formed in the liquid phase but the similar blocks with ordered structure occurring in the gel skeleton can be the nuclei of crystals.
CRYSTAL LENGTH (r) Fig. 21.
Crystal size distribution of zeolite A in the product of crystallization (363k) (15)
Xu Qinhua and Yan Aizhen
60 loo-
aoe II
-
400- 1.0
40
0
120
go
200
160
ITMe (h) Fig.
22. Growth
of the number
of nuclei
in the rate of their formation during
crystallization
(1) and change
(2)
of xeolite A (15)
TIME (h) Fig.
Crystallization
23
of zeolites
l.Zeolite
A, 373 k
Z.Zeolite
X, 373 k@reck
3.Mordenite,
Kinetic
models
In the induction nucleation.
of crystallization
period
A kinetic
zeolite
on the crystal
where
k is related
to the composition
of the zeolite NH,-ZSM-5,
to a certain
extent.
law of the order A systematic
It was shown
2 / 3 under
study
that
the growth
of crystallization
complete
of omega
region
T-type
form Z = kt’
there
were
no spontaneous
zeolite was suggested
as
kx”’
of nucleation of the crystals growth
zeolite
calculated
system
gel, at a particular
the separation
approximately
phase
of rod-like =
of original
OPoints nucleation
or amorphous growth dx/dt
crystals
and Quoben)
curves
in non-spontaneous
of T-type
model
of time (15)
and Flanigen)
573 k(Dominc
--Experimental 4.1.
as a function
temperature
and growth
(85).
By adding
of NH,-ZSM-5
of xeolite NH,-ZSM-5
seed
was made obeys a kinetic
conditions.
in (TMA),O-Na,O-AJO,-SiO,-H,O
system
at
Hydrothermal synthesis and crystallization of zeolites
413 K was reported and the kinetics crystallization was presented.
of crystal
growth
of omega
61 zeolitc in non-spontaneous
The kinetics of the process of growth of the faujasite from amorphous aluminosilicate gel has been investigated by Lcchert et al.(67). In a wide range of crystallization, the growth obeys a kinetic law of the order 2 / 3, describing in detail as follows. Starting with the conception that the increase of the rate of crystal growth is proportional to the free surface O(x) of particles being available one obtains the equation of the kinetics of the process of zeolite growth. dx / dt = 3kx”‘xi”r,’
6)
where x = the mole fraction of the growing zeolite crystals. xs = the mole fraction of seed crystals added. rs = the average radius of seed crystals (nuclei). By rearangement and integration of x gives X xI x = xa + 3k-r + 2ka+’ ‘0
ro
+ k+’ ro
For the reaction constant or rate constant k follows that (7) k has the dimension (lenth / time), therefore, it describes the linear growth of a particle in a time unit. It can be shown that the growing of faujasite crystals from nuclei (using seed crystals) is possible in concentration ranges where nucleation of species is negligible. In a wide range of crystallization, the growth obeys a kinetic law of the order 2 / 3. From the values given by Lechert (86) it can be concluded as follows: 1. The crystallization time, t, increases with increasing Si / Al ratio in the final products, which resultx in decreasing value of k. 2. The growth of the crystals only occurs at the interface of the crystals and the solution, and a direct transition of the gel into the crystallization state must be excluded. 3. The apparent activation energy for crystal growth increases with incresing Si/ Al ratio in the final products. 4. The rate of the crystal growth is proportional to the actual surface O(x) of the crystals already present in the reaction mixture. 5. In case of the diffusion as the rate-determining step, the dependence of the crystallization rate on the surface of the crystals in the reaction mixture should not be realized. The rate-determining step seems to be given by the connection of silicate species. 4.2.
Kinetic models of crystallization
in spontaneous
nucleation system
Spontaneous nucleation system is the system without adding any seed crytals during synthesis of zcolitcs or gels. The curves showing yield of crystals against time are characteristically sigmoid in shape. This behavior has been observed for zeolitc A (83), X and Y (52, 67, 86),sodalite hydrate (27), @Ja-K)-phillipsites (23), zeolite ZK-5 (3), omega (87), Na-ferricrite (88) and mordenite (19). As a law of crystal growth for such system Meise and Schwochow (17) have formulated the expression r -j7t (8)
PICG-C’
Xu Qinhua
62
and assumed action
that
time t.
suggests
the crystals
A rough
are spherical
estimate
an e function
and Yan Aizhen
configurations
of the percent
increse
as a law of nucleation,
where
also been represented
2
r increases
of particles
in proportion
to re-
N in the reaction
time t
A and
A(eB’
3%.are cocfficicnts.
by the model-based
reaction
1)
-
The CUTYCSof crystal
growth
against
time have
(89):
= l-exp(-_,c”)
z1
The
radius
of the form dN -_ dt
Z
whose
in the number
ratio
Z,/
reaction
time
formed
is of the
Z,
mass
t the normalized
of crystal
Z,
contribution
at time (less than t) in the interval
d(Z,/
Z,)
at time d(Z,/
dr
t to the
mass
Zr) to the final
Z, in the final
yield
Z, arising
product.
only
At
from
nuclei
is
= 4 / 3n~3(t-r)3A(e”‘-l)dt
Beaction
was assumed
to be complete
all values
of r between
0 and t gave
(10) after a time t, when Z,/
1. Integration
Z,=
of equation
(10) over
(11) Experimental 5.
values
of constants
The experimental
arc illustrated
A, B, and E vs. alkalinity
and calculated
in fig. 24 (17).
The crystal
size growth
histogram
of crystal
size distribution
scope measurements
growth
reacted
for different
growth
rate for crystals
towards
zero.
intervals
Curve
of fig. 26.
This rate passes
which stant
derived
from
pendent
begins
curve
when
of crystal
A crystallization
of time
from
the beginning the average
the linear
of
results
Curve samples
The period of crystals.
to decrease.
It implies
that
growth growth
linear
the time for rate curve 2
curve
of delayed
linear
by microof nutrient
asymptotically
From
crystal
The stage
1 of fig. 25
of constant
declined
the crystallization
process.
the
was obtained
in identical
1, the rate of linear
ac-
and
3 in fig. 26, remains
con-
crystal
mass
rates
are inde.
size. kinetic
model
hydrothermal
Na,O-HMDA-M,O,-SiO,-H,) M-Si-ZSM-5
comparing
growth
mixture
curve of fig. 25 gives the nucleation
When
for the spontaneous
nucleation
- B, Al, Ga, Ti, V, Cr and Fe) has been suggested tigation
of reaction
115 h and thereafter
of the crystallization
rate has begun
observed
size distribution
and crystal
in fig. 25 (18).
size distribution
1 and 2 of fig. 25 with curve stage
nucleation
on the curve
of heating.
for about
a maximum.
point
crystals
the size distribution
through
are shown Each
largest
size extended
of each mode
all over the autocatalytic
growth
of the lo-20
both
and particle
crystallization
in the final product
2 in fig. 25 describes
of crystals
during
crystals.
are given by Meise in table
of the reaction
as time rised,
zeolite
rates of largest
of different
nucleation
that
of NaX
of diameters
and temperature
for the course
It is concluded
celerate.
and 26 gives the linear
curves
type zeolites,
(figs. 27 and 28).
synthesis at obtained
4233.
of
by Feng this
The
by calculation
system Shouhua
type
of
crystallization from
the model,
of M-Si-ZSM
-5 type zeolites
@l
et al (90) on the basis of the inveszeolites
in
kinetic
curves
agree
the for
system the
of
various
well with the experimental
Hydrothermal
synthesis
and crystallization
of zeolites
-r.T EXPERIMENTAL _--
I
TBfE
CALCULATED
63
CURVES CURVES
(min)
_‘== EXPERIMENTAL . . ..* CURVES
0
1
4
2 CRYSTAL
Fig. 24.
DlAMETER
5
6
7
tr)
Influence of alkianity on zeolite A crystallization (top) and on crystal sizedistribution (bottom), showing agreement between experimenial and calculated values (17)
64
Xu Qinhua and Yan Aizhen
TIME(h)
Fig.
25.
Crystal
size growth of Na-X
of the gel(l)
and the histogram
in the final product
zeolite during of crystal
crystallization
size distribution
(2).
80 120 loo !m
240
TIME th)
Fig.
26.
Nucleation Curved)
kinetic
for Na-X
curve(2)
and the crystal
zeolite calculated
mass
from data
growth
of Fig.25.(18)
Hydrothermal
synthesis and crystallization of zeolites
*
ee d
0
Fig. 27.
50 40 30 al 10 CRYSR+LLIZATIoW TIME (h)
Crystallization kinetic curves for B-,AlSilica sourcfzwaterglass- II B-Si-ZSM-5 0 Al-Si-ZSM-5 Q Ga-Si-ZSM-5 e
60
and Ga-Si-ZSM-5.
203040050 CRYSTALLIZATH)NTIME (h) Pig.
26.
Crystallization
kinetic
CUTYCI for
Silica souruzwaterglass-D[ Ti(m)-Si-ZSM-5, 0 V(m)-Si-ZSM-5, 0 Cr(m)-Si)ZSM-5. 0 Fe(m)-Si-ZSM-5, e
Ti(m)-;
V(m)-,Cr(m)-
and Fe(m)-Si-ZSM-5
65
Xu Qinhua and Yan Aizhen
66
The rate of nucleation
can be expressed
by the equation (12)
dz’ / dt = k,t=g,(t) where Z’is
the number
the crystallization The crystal
of nuclei;
k,is the rate constant
of nucleation;
01is a exponental
constant;
and t is
time.
mass growth
rate in the following
form is obtained (13)
assuming
the radius
of a crystal
at time t, which
and k, as a constant
related
The molar
of the crystalline
phous
fractions
solid
gel phase
g, respectively, mole number
to the crystal
per
so that
unit
of zeolite crystals,
zeolite,
volume
g,+g,+g,=
nucleated
at time z(t 2 z), as r( r, t), it’s mass as g( z, t)
shape and density
of zeolite crystal.
of the corresponding
in the
If M is the molecular
1.
the crystal
liquid
crystallization
growth
system
weight
component are
of the zcolite,
rate can be expressed
and of the amor-
expressed and
as
g,,
g, and
c n, is the total
by the equation: (14)
dgt (1)
or -
= kg,O)Y,
df
(1)
(15)
where k is the crystal
growth
As described
the course
above,
crystallization nucleation main
and crystal
growth
of zeolites
is observed growth,
the
thoretical
aspects
study
one may expect cal ingenuity
the future include
reaction
The use of nonaqueous
It was also reported
in surface
been found
zeolite
VPI-5,
not exceed
stages
in the course
linearly
increases
become
with
and
various
been suggested.
The
of zeolite synthesis.
the crystallization
proportionally
phase
The
t in the
time
to the surface
area
of
mechanism.
relatively
Because
Different
have
sophisticated,
of the almost
more novel structures,
provided
the
unlimited
quantitative
and
numbers
of zeolite,
effort
and chemi-
sufficient
phases
in the AlPO,
system
12-membered
zeolites, by other
in the synthesis
in which elements,
5% replacement
will continue
systems
recently
with 18-membered
studies
in zeolite synthesis
and the exploration
has been explored Al or Si atom
in aluminosilicate
have been synthesized
of either
Si or Al.
of widely
open
net-
(91).
(14, 90).
This might
zeolite
frame-
Framework
sub-
still be an interesting
and catalysis.
zeolite
reported
path media
replaced
chemistry
some other
has been
of many
that heteroatom
or totally
will probably
New synthetic years
has
more attention.
synthesis
systems
on the basis of liquid
synthesis
much
process.
nucleation
increases growth
for calculation.
is deployed.
challenges
subject
of crystal
almost
of zcolite
radius
introduced
is a complicated
are the two main
crystal
i.e. the rate
still require
work is partially
than
that
in the system,
Although,
stitution
synthesis
for the non-spontaneous
presented
works.
of zeolite
models
crystallization
Most
and y,(t) is the function
kinetic
conclusion
zeolites
rate constant
by Wilson
by Davies ring pores,
ring pores.
to appear;
have been described. and Flanigen
et al (94). which
Their success
They
Beside the aluminosilicate zeolite in the recent The most promissing new class of zeolites has (92, 93). have
is the first molecular will certainly
fostor
A new large-pore
synthesized
an AlPO,
zeolite
sieves with pores consisting synthesis
endeavours
material
material,
named of greater
worldwide.
Hydrothermal
67
synthesis and crystallization of zeolites
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69
70
Xu Qinhua, Professor of Chemistry Department in Nanjing University (Chemistry Department, Nanjing University, Nanjing 210008, P.R. China), graduated from Jinling College in 1951. Her teaching and research are mainly in the field of Physical Chemistry She has given courses on “Colloidol Chemistry”, “Adsorption”, “Catalysis”, and “Zeolite Chemistry” in Nanjing University; and published about 70 papers on the synthesis, adsorption and catalysis of zeolites. She obtained the National Prize for Zeolite Research in 1965 and 1978. She attended the 5th, 7th and 8th International Conference on Zeolites.
Yan Aizhen is an associate Professor of Chemistry Department in Nanjing University (Chemistry Department, Nanjing University, Nanjing 210008, P.R. China), where she graduated. Her interests are Physical Chemistry, Colloidal Chemistry and Radiochemistry. She has given courses on “Inorganic Chemistry”, “Radiochemistry”, “Catalysts and Catalysis” and “Adsorbents and its Applications”. Currently her research aspects include ion-exchange, adsorption science of micropore solids, especially zeolite and aluminophosphate molecular sieves, and its application to sorption refrigeration. She has published numerous articles and presented many papers at the National Conference and International Symposium on Zeolites. She is a member ofthe Chemistry and Chemical Engineering Society, and Petroleum Engineering Society of China.
0
01463535/90 $0.00 + +I 1991 Pergamon Press plc
HYDROTHERMAL SYNTHESIS AND CRYSTALLIZATION OF ZEOLITES Xu Qinhua Department
1. The natural
zeolites
sedimentary
deposits.
atory technique
have
formed
Aizhen
INTRODUCTION
under aqueous,
These conditions
of synthesis.
drew the attention
and Yan
of Chemistry, Nanjing University, Nanjing 210008, China
alkaline
conditions
are typical of hydrothermal
The role of water es a mineralising
of mineralogists
and chemists
in matrices systems
catalyst,
to hydrothermal
of basalt rock and as
and have guided the labor-
aided by alkaline
conditions,
end synthesis.
Systematic
reactions
in the early 1940’s, the first made being enalcime, mordenite zeolites (1) also with the framework of that subsequently termed ZK-5 (23).
studies began in Barrer’s laboratory
with the edingtonite ZK-5
framework
was thus the first zeolite without
a natural counterpart
to be synthesized.
in the laboratory synthesis has come by duplicating the conditions which produced natural zeolites. However, the one condition that we can never duplicate is crystallization time of lOU0 years or more. Thus laboratory systems operate at high pH(> 12, usually > 14) and higher temperature, and produce smaller, less-perfect crystal. So the majority of the synthetic zeolites are formed under non-equilibrium conditions and are metastable phases. Table l-3 show the natural zeolites whose structures were successfully duplicated from silica / alumina gels end alkali metal, elke. line earth metal or alkylammonium cations respectively (4). We can expect more examples in the future of discovery of natural counterparts of existing synthetic zeolites. Table 4 lists several synthetic zeolites with structures reasonable well defined and which have no counterpart in natural zeolites. Now the hydrothermal synthesis of approximately 150 zeolites from eluminosilicate systems has been reported in the published literature. New synthetic zeolites phases will continue to appear, but only about 60 differ. ent structures are known (5). Much of success
In the recent years many efforts have been made to improve the understanding synthesis
and
aluminasilicate
to get a deeper molecular
sieves.
insight Many
into
the
excellent
chemical
reactions
review papers
which
of the process lead
of zeolite
to the formation
of
on zeolite synthesis have been published
in the literature (4-12). In this paper the discussion is restricted to three-dimensional crystalline networks. Because faujasite, A and ZSM-5 zeolites are the most important in industrial application, most of the metioned problems will be discussed for the crystallization of these zeolites. We intend to review the main factors of influence on hydrothermal synthesis and crystallization involving temperature, alkalinity, chemical environments, end action of template agents. The metestability of zeolite crystals and recrystallization between xeolites with different structures, the mechanism of zeolite crystallization by nucleation from gel or solution, the kinetics and growth of zcolite crystallization in non-spontaneous or spontaneous system will be presented in this review.
PICG-B*
29
Xu Qinhua
30
Table
1
Synthetic
Zeolite
Table
counterparts Alkali
of natural
zeolites-alkaline
Investigator
metal Date
Mordenite
Na
RM
Analcime
Na
R.M .Barrer
1949
Phillipsite
Na
R.M .Barrer
1951
Cancrinite
Na
R.M.Barrer
1952
Natrolite
Na
R.M .Barrer
1952
Faujasite
Na
R.M .Milton
1956
Gmelinite
Na
R.M .barrer
1959
Chabazite
Na
R.M .Milton
1960
Erionite
K.Na
D.W .Breck
1960
Clinoptilolite
Li
L.L Ames,jr
1963
Ferricrite
Na
E.E.Senderov
1963
Gismondine
Na
A.M.TaylorkR.Rov
1964
Bikitalite
Li
D.J.Drysdale
1971
Edingtonite
K
R.M.Barrer
1974
2
Synthetic
counterparts
Zeolite
Table
and Yan Aizhen
Alkali
.Barrer
1948
of natural
zeolites-
investigator
alkaline Date
Heuiandite
Ca
R.Roy
1960
Scolecite
Ca
R.Roy
1960
Wairakite
Ca
R.Roy
1960
Epistilbite
Ca
R.Roy
1960
Garronite
Ca
R.M .Barrer
1961
Thomsonite
Ca
R.M .Barrer
1961
Yugawaralite
Sr
R.M .Barrer
1964
Clinoptilolite
Sr
R.M.Barrer
1964
Harmotone
Ba
R.M .Barrer
1964
3
Sythetic
Zeolite
Offretite Levynite Mazzite
a
counterparts
of natural
Alkali
TMA,‘Na,K [CH,N 0 Nl+,K TMA,‘Na
TMA,N(CH,),OH
zeolites-alkyl Investigator
earths
ammonium Date
H.E Robson
1966
G.T.Kerr
1969
R.M.Barrer
1970
Hydrothermal
Table 4
synthesis and crystallization of zeolites
Natural zeolitcs with no synthetic counterpart Year discovered
Zeolite
1756 1785 1813 1822 1825 1893 1896 1905 1909 1942 1960 1977
Stilbite Laumontite Mesolite Brcwsteritc Herschelitc Kehoeite Gonnardite Dachiardite Stellerite Viseite Paulingite Merlinoite
2. FACTORS AFFECTING THE HYDROTHERMAL SYNTHESIS AND CRYSTALLIZATION The xeoliter generally can be prepared by mixing sodium metasilicate and sodium aluminate solutions. The aluminorilicate gels are usually formed on mixing soluble silicates and soluble aluminates. In eolite synthesis one may consider the stages given below: Stages in xeolite synthesis Reactant
I Reactant mixtures
1 Nucleation Crystal
growth
The primary building block of the zeolitc structure is tetrahedron of four oxygen atoms surrounding a central silicon atom - (SiOJ*-, (fig. la (5)). These are connected through their corners of shared oxygen atoms to form a wide range of small secondary building unites (fig. lb). They are interconnected to form a wide range of polyhedra (fig. lc), which in turn connect to form the infinitely extended frameworks of the various specific xeolite crystal structures (fig. Id). In these figs. la - Id showing structure of xeolites, the corners of the polyhedra represent Si or Al atoms, and the connecting lines represent the shared oxygen atoms. Individual structures may comprise only one basic unit or many of them. A record is held by the minerial paulingite which contains five such polyhedra. Different combination of the same secondary build unit may give numerous distinctive, fig. 2 shows an example of three zeoliter that have the same structural polyhedron ( cube-octahedron), but probably form from smaller ring unites.
31
32
Xu Qinhua
(a)
PRIMARY
I
(b
UNITS
TERTIARY OR
ZEoLITE
UNITS
BUILDING
FOLY
cd)
UNITS
SECONDARY
tc)
and Yan Aizhen
HEDRA
STRUCTURES
PAULINGITE
MELANOPLOGITE
Fig.1.
Monomers
a); polymerize
in turn combine These (d) attach
Thermodynamic composition tained
in
variables
to growing
mixtures.
hydrothermal
thermodynamically
reactions
determined
reactant
prior
to crystallization,
fluences
are described
and
because,
variables The
b);
or first form clusters crystal
entities.
are temperature, appears kinetical
and physical
nature
(5)
alkalinity
do not neccessarily
nucleation
controlled.
their chemical
surfaces
to larger
of zeolites
These
rings and chains
units c); or sheet structures.
crystal
agglomerate
in the synthesis
of the reactant
to low-molecular-weight
to form polyhedral
which subsequently
RHO
to
(pH)
determine be
variables
kinetically include
(14, 15).
and
chemical
the products
ob-
rather
the
Some aspects
than
treatment
of
of these in-
below.
2.1. Temperature Zeohte
formation
idence
an upperlimit
mordenite
of 623 K has
to 703 K,clinoptilolite
The temperature zeolite,
has been observed
range
over a considerable been
for crystallization
has
L, omega
and mordenite,
of temperatures.
Analcime has been to 648 K (12).
to 643 K and ferricrite tended
from 298 K to 398 K for the aluminum-rich
Si / Al zeolites,
range
suggested.
to increase
zeolites,
with
Based synthesized
incresing
Si/
on geological
ev-
at least
639 K,
Al ratio
in the
from 373 K to 423 K for the intermediate
and from near 398 K to 473 K for the high silica zeolites
as
Hydrothermal
exemplified
This is consistent
by ZSM-5.
temperature
The higher
(16).
with the suggested
the synthesis
The most
end syntheris
to be the water
porous
zcolitee,
content
such
end
A, X, Y,
end ZK-5, with pore volumes in the range. 0.25 to 0.36 cm’/ g of crystal do not form Silica-rich phases such as mordenite, L, omega, ZSM-5, -11 end much above 373K. and -2, all with rather low intracrytalline porosities (0.15 - 0.20 cm’/ g), arc usually
et temperatures
rate of nucleation shown
in the range
and crystal
crystallize.
of pore volume
tends
Chebezite,
beat made
which
relationship
the smaller
porosity
RHO
zeolites
temperature
intrecrystelline et temperatures -39, and KZ-1
of any
33
synthesis and crystallization of zeolites
373 K to 473 K (16). The rate of nucleation
growth.
The temperature
can obviously
and the linear
rate of crystal
effect growth
are
as follows: dN/
dt = A(exp(Et)-1)
(1)
r = kf The coefficients
of crystal
with retsing
temperature.
growth
the temperature
The curves xed crystals,
showing
5) and k (in table 6) all increase
(coefficient
Is elevated,
that
with other
against
for mordenitc
the temperature zeolites,
k)and
the induction
yield of crystals
as illustrated
It is also observed that found
Cz)
k, A end E gn table
linear rates
When
the
rates of nucleation
with temperature,
(coefticiernts
time of s-shaped
crystallization
time are characteristically
sigmoid
strongly
affects
the induction
such as NaY by Xu Qinhue
time.
(20) or ZSM-5
This behaviour by Cheo
FAUIASITE
An example
of three zcolites
(cube-octahedron),
curves in shape,
that
increase
is shortened. in absence
of
is typical
of
in fig. 3 (19).
(TYPE
Fig.2.
indicating
A end E) both
X. Y)
that have the same structural
but probably
form from smaller
polyhedron
ring units.
(5)
(21).
34
Xu Qinhua and Yan Aizhen
t
TIME(h)
I
TIMECDAYS)
Fig.3
The characteristic N,
sorption
s-shaped
Table
curves
H,O / Na,O
illustrated
for mordenite
the yield of mordenite.
5. k,A and E for zeolite NaA of alkalinity
T(k)
crystallization
was used to estimate
as function
and temperature(l7) Kbmh-‘) Ax lo6 ~~____ 0.050 20
E ~~__.._ 0.102
343
20
343
30
0.027
13.2
0.033
343
40
0.017
9
0.0115
333
20
0.03
1
0.062
343
20
0.05
20
0.102
353
20
0.06
200
0.132
Table 6. Linear
rates K= 0.5 Al / At for growth 343 _---___ 0.0175
of NaX
353
363
373
0.0375
0.0625
0.1071
(18)
Hydrothermal
synthesis and crystallization of zeolites
35
of analysing the nucleation and crystal growth parts of such s-shaped cwvcs was developed by Zhdanov and Samulevich in the case of zeolitc NaX (18). Since the nucleation process is rate-determining during the induction period, the apparent activation energy for nucleation, En, can be c&ukted by the following equation (22)z
A method
dln (I/ -I-
8)
En
d(1 / Z-1
(3)
R
Where 0 is the induction time i.e., the point on the crystallization curve where the conversion into the crystalline phase just begins. The apparent activation energy for nucleation of phlllipsite was given as 13.5 and 14.3kcal/ mol with and without stirring respectively (23), while for the Al-free end member of the ZSM-5 series (silicalite 1) this energy was given as 9.1 kcal/ mol (21). The En value for NaY was obtained as 17 kcal/ mol(20).
Fig.4. Crystallization tields of some aqueous calcium aluminosilicate compositions On left (a, b and c) CaO.AlrO,.nSiO,+aq; on right 3Ca0.Als0,.nSi0,+aq. A = boehmite, Al,O,.H,O F = anorthite J - Ca-epistlbite B-corundum, AlsO, H - a-cristobalite K - hydrogrossular D - Ca-analcime, P - hexagonal dimorph N = Ca-aluminate, tetragonal of anorthitc 3CaO.A&0,.6HsO E = Ca-analcime, cubic I = Ca-thomsonite 0 = unidentified Temperatures are in C . Q = Ca-mordenite 2.2. Chemical composition The zeolite framework
which crystallizes from a given batch is determined
mainly by its composition
Xu Qinhua
36
and its temperature, summerized
shown
given in fig.5.
An interesting
much stronger
influence
The primary
Experiments
in fig.4.
in the well-known
and Yan Aizhen
crystallization fact which
covering
diagrams
of the composition
Table
7
Primary
Mole ratio
Primary
mixture
are usually
of such a diagram
is that the content
are shown
of the composition
of alkali
is
has a
ratio.
in table 7.
of reaction
mixture
(30)
influence
SiO, / AlsO,
framework
H,O / SiO,
rate, crystallization
OH-/
silicate
compositon mechanism.
molecular
weight,
Na* / SiO,
structure,
R,N+/
framework
aluminum
is present
as a major
SiO,
of compositions
An example
zeolite than the SiO, / AlxO,
of reaction
influence
SiO,
range
can be seen from this diagram
on the kind of the crystallizing
influences
a broad
(12, 14, 29).
cation
OH-
concentration
distribution. content.
2.2.1 Water In hydrothermal recognized OH-
chemistry
for
a long
ions in particular.
synthesis
zeolite
vent powers crystal
water
Water
as a guest mixing
siliceous
content
increases.
zeolite
is such
more.
For
synthesis
zeolites
of ZSM-5
water
which
is the basis which
transport
concerntration
can crystallize
the cavities
given
and
It has been
in this
chemistry;
the host lattice. and
facilitate
forming
role
by
in zeolite
The zeolite
systems
will not form
water
the good nucleation
soland
a type of solid solu-
in absence
of a “guest”
(27).
more organophilic
water
assisted
In hydrothermal
aluminosilicates
progressively
that zeolite
stabilizes
of materials
as well as water
magmas.
(12, 26),
of hydrothermal
zeolite.
by filling
porous
of crystallizing
mineralizer
anhydrous
crystals
that
become
This means
example,the
species
and
which may be a salt molecule
The highly
property
the unchanged
stablizes effect
component
is an excellent
mineralizing
promote
The stabilizing
molecule,
water
is essential leaving
of water
growth.
that
This
may then be removed
tion.
water
time
stabilizes
and more hydrophobic
them less and intercrystalline
or the degree
of dilution
is of minor
out of gels with an extremely
wide range
as the silica species
organic importance
for the
of H,O / SiO,
ratio
@ram 7 to 22) (28). 2.2.2 Alkalinity
(pH)
In the inorganic mineralizing OH-
system, role
concerntration
period
before
Table
5 also shows
of
the alkalinity free
will generally
viable nuclei that
OH-
as (OHj2
is also
amphoteric crystal
growth
been
bring
increasing
discussed
about
in determining in
detail
an accelerated
alkalinity
by
crystal
the crystallization Barrer
growth
(24).
rate.
The
An
increasing
and a shortened
induction
The
OH-
into solution.
pH can be attributed
temperature
influences
the rate of nucleation
Fig.4 indicates that the induction time decreases with pH. The induction time was reported to vary
rate of crystallization
mineralizer.
and hydroxides
with raising
at constant
at constant alkalinity. slope tend to increase
and the maximum
a powerful oxides
is the key parameter
has
are formed.
like increasing temperature strongly and the maximum inversely
OH-
ion
as (OH)‘.75(23). is a good
The decrease at least in part
complexing
in nucleation to the much
agent
which
can
time and enhanced greater
concerntration
bring rate of of
Hydrothermal
synthesis
and crystallization
37
of zeolites
In alkaline media the sohbility of riliea increases nearly exponentially with dlssaolved species. eoncertration of alkali, end according to the ratios of M,O / SIO, in the resultant mixture e range of dlicatc enionr may appenr of various degree of oligomtrisetion. In the case of alumina there is et high pH minimal oligomeriretion, the dominant anion always being Al(OH);(12). The high alkalinity tenses e high supersaturation of silicate end aluminate end the formation of a large number of nuclei. The growth of these nuclei proceeds until the eluminium in the gel is exhausted, so alkaline media thus enable ready mixing of reactants end facilitate nucleation and crystal growth. High pH (> 14) is necessary to initiate crystallization, but during crystal growth high pH reduces silica content of the product. Teble 8 shows some examples for the crystallization of Y zeolitea with high Si / Al ratio in dependence on different exeees elkelinities (Ne-Al) given by (NeOH)-(NeAlOr) in the batch.
/‘\ /’ ( **’ \* ’ I,‘/’
I
Fig.5.
Table 8
CHABASITE p1 + CIMELIbIIWI
Cryetellizetion diagram for the system Ne,O.AJO,SiO,H,O fore temperature of 363k end (Al,O,+SiOr)/ H,O = 0.0005 Dependence of the Si / Al ratio of zeolite Y grown from batches
with different excess alkalinity @In-Al) using seeds of zeolite X with r0 - 0.9pm Composition SiO, Al,O, 20 20 20 20 30 30 30
2.2.3
Composition
of the batch
(Na-AI) SiO 1
0.76 0.79 0.72 0.71 0.70 0.695 0.69
H,o
%
Y
% NePl
A1,O3 780 780 780 780 780 780 780
(25)
of the product Crystallization Si
emorph
xi
time
(h) 100 96 100 95 100 100 98
4 5 2
2.78 2.92 2.97 3.00 3.10 3.14 3.24
96 144 187 312 690 624 695
Si / Al ratio
Compoeition is a variable for any given framework structure. The synthesis conditions to give high silica zeolites, are now known clearly as a rule of thumb, any zeolite becomes more useful es e catalyst or an adsorbent where acid resistance and thermal stability are critical. Framework composition can often
Xu Qinhua and Yan Aizhen
38
be varied
by simply
thetic faujasites It is observed Si / Al near structures
changing
that
the crossover
to
those
containing
This suggests
Si / Al of 5, 5-ring In contrast,
several
faster
is opposite etc, in which
when,
an
have
all other
to the normal
factors
high Si / Al ratios
selectivity
fraction
of
from
5-rings
of Al decreases
and the syn-
hydrophilic
to hydropholic
features (for
example,
below one per 6-ring
Y -
omega
corresponding
All data remaining
the influence
of the aluminum
tend to show
that
equal,
behaviour
give a system
the aluminium
of zeolites
with higher
content
with lower
viscosity
content
of pH upon
for mordenite Table Framework
Lindc
A A Sodalitc Sodalitc
curves
synthesis
9. Variable
Name
A
of the solution
of yield against
composition
and a lower
time
in zeolites Surface
property
References
2
hydrophilic
14
N-A
2.5 to 6
hydrophilic
31
ZK-4
2.5 to 4
hydrophilic
32
Solalite
2
hydrophilic
14
10
hydrophilic
33 14
TMA
sodalite
Faujasitc
Linde
X
2 to 3
hydrophilic
Faujasite
Linde
Y
3 to 6
hydrophilic
14
hydrophobic
34
ZSM-5
ZSM-5
5 to infinity
of
of ZSM-5 This
such as A, X, Y
at 573K(193.
SiO, / AlrO, A
to an
of the gel is lower.
Si / Al ratio,
TIME(h)
Influence
-M
on the rate
the rate of crystallization
rate.
Fig.6.
at an
from 4, 6, and I-ring
(35).
investigated
hydrothermal
A, ZSM-5
mixture.
are listed in table 9.
in the xcollte structural
increasing
(9, 36, 37, 38).
of the reaction
surface
to the change
is favoured
workers
ratio
SiO, / Also,
that as the fraction
formation
crystallization
becomes
AlsO,
in the zeolitc
7.5 to 10, corresponds
-ZSM-5).
ZSM-5
the SiO,/
(X or Y type) with diferrent
reaction
Hydrothermal synthesis and crystallization of zeolites
2.3 Structure-directing
39
role of inorganic and organic cations - templatiag action
Cations present in a reaction mixture are often the dominant factor determining which zaolite structure is obtained (14). Flanigen (39) pointed out that the two basic roles of a cation are: 1). a limited structure dlrcctlag role and 2). a general role of providing a source of hydroxyl ion and stabilizlag the formation of sol-like alumiaosilicate species. The cation is said to have a structure directing function which influeaces the structure of the zcolite which forms. Such a structure directing effect has been termed ‘ternplating’. More recently, the concept of cation templatiag in zeohte synthesis has been developed and summerized by Rollmaa (30). The unique structural characteristics of zeolite frameworks coamining polyhedral cage have led to postulate that the cations may thus serve as ceatres or templates around which the silicate, aluminate or alumiaosilicate species aggregate, forming, with the cations, precusors to specific germ nuclei. The cation stabiLizes the formation of structural subunits which are the precursors or nucleating species in crystallization. For example, sodium and hydrated sodium ions were suggested to be responsible for the formation of D4R, D6R, gmelinite and sodalite cages. On the other hand, potassium, barium and rubidium were believed responsible for directing caacriaite cages (12, 24, 39). The templatiag theory is based on a stereospecificity which can not be separated from chemistry of the cation. The relative sizes of the polyhedral cages and the related specific cations are shown in table 10. A good tit for the anhydrous diameter is observed for the TMA ion in the gmeliaite and sodalite cages, the K, Ba, and Rb ions in the caacriaite cage and for the diameter of the hydrated Na ion in the gmeliaite and sodalite cages. Table 10 also shows that the Ba and Rb ions can substitute for K, and TMA can substitute for the hydrated Na ion in their structure-forming roles. This analysis knds to support the cation-templatiag concept for most of the polyhedral cages considered. Table 10 Cation Specific Building Units in Zeolite Structures
(39)
Specific Cation Building Unit
Diamcter,A
D-4 Q Soda&
Free Dimension ,A 2.3 11.4 6.6
Na Na Na or TMA
Anhydrous (Crystal) 2.0 2.0 2.0 (Na),6.9
Gmeliaite
6.0 x 7.4
Naor TMA
CrMA) 2.0 (Na),6.9
Caacriaite
3.5-5.0
K,Ba,or Rb
D-6
3.6
Na,K,Sr, or Ba
CrMA) 2.8 W2.7 0W3.0 (Rb) 2.0-2.8
Hydrated 7.2 7.2 7.2(Na), 7.3 CTMA) 7.2 Wa),7.3 CrMA) 6.6Q),8.1 Oa),6.6 IRb) 7.2-8.2
An excellent review about the role of organic substances in the synthesis of zeohtes has been published by Lok et al(40). In the early 1960’s, the TMA cation was introduced as the first organic cation to be used in zcolitc syathesis(3la,32). The synthesis of Omega zeolite is strongly favoured by the presence of TMA* ions, as illustrated by Xu Qiahua(41). Table 11 shows that the zeolitc G can be obtained only from gels containing both Na’aad Me,N+ (TMA+) as templates, cations Na* and Me+N+ play an important part in the synthesis of omega zeolite. Either Me,N+or Nat alone will not form this zeolite. The result showed that aaalcium would be formed when the ratio of (Me+N),O to Na,O + (Me,N),O was less than 0.06, and that sodalite hydrate would be produced when the ratio was more than 0.13,
Xu Qinhua and Yan Aizhen
40
zeolite omega affinity
was formed
of 14-hedral
which can occupy the ratio an
smaller
of Na*
example
of
templating
action.
This can be explained
necessarily cation.
Kacirek
and Lechert
pentasils
as follows:
space
available
The use of TMA+
ZSM-5 Table
in forming
(43).
/ ~a,O+(Me,N),O]
* A&O,
(M e,N),O
/
l
0.273
0.064
n
0.363
0.085
n
3.80
0.545
0.130
n
3.43 1.96
0.822
HSC’
2.29
0.193 0.540
0
4.25
1
into
using
the nucleation topology
the TPA+ is needed
process around
the channel
NMR
measurements
intersections that
see table
and
formation,
(41)
HS
has had two major (SiO,/
Also,>
impacts.
First is
120) region.
Second,
13. by Argauer
and
in the nucleation
the organic thus
88HsO
hydrate.
to alumina
was reported
whereby
In the case of ZSM-5
which
silica
preferably
itself
facilitate
such as the
HS+@)
ions in zeolite synthesis
the high
type have been reported,
the TPA+ion
[(Na,O+(Me,N),O] 0
of a; c) HS-sodalite
ammonium
bases
zeolites,
Zolite Type -a”
3.89
science
neu.
in some detail by
lOSi0,.
4.00
quaternary
charge
on the type of zeolite
12.12 Na,O+(Me,N),O]
amount
of these
and charge
organic
a+ (@’ a+n
of ZSM-5
type.
that
silica-rich
0.020 0.043
geometric
structure
12 shows
0.083 0.183
structure
During
Table
with extremely
the zeolite
and Fu-1.
@fe,N)O
b) @)-small
that
(33).
can be
framework
zeolite A was later examined
which
in the
Al ratio number
as template
4.17 4.07
of zeolite
The synthesis
acts both
(moles x IO”__ 0
The use of the other
ticular
of Me,N
the anionic
common
grown
are incorporated
for only a limited
(moles x 10-r) 4.25
a) a-analcium;
been shown
and Meier
Si/
of
of new
of more
of A zeolites
A with higher
therefore,
So TMA*
silica-rich
espicially
of the ratio
Na,O
many other
and
and also zeolite Nu-1
11. Influence
zeolite
there is room
Si, less Al).
(413k, 92 h). gel composition:
the extension
because
in the framework
novel structures,
and ZSM-11,
ions,
This is
especially
of the formation
and the synthesis
when large ions such as TMA+
usual,
(42) and Baerlochev
of several
formed
than
TMA
Therefore,
of zeolite n.
substance,
of the composition
the
Na+ ions,
unit structures.
organic
group,
because,
the smaller
in the synthesis
for the initiation
the Mobil
changes
Adding
have to be low (more
tralizing
the formation
series from
while
of smaller of
template
This is perhaps
is great,
range
addition
the
Dramatic
will be more silica-rich
0.06 to 0.13.
a definite
an important
been observed.
ions in the pore
would
Recently
compositions.
from
in n zeolite
for the formation
fall within
as the ZK and ZSM
of TMA’have
A produced large
is also needed
ions must
with unusual
obtained.
cavities,
is ranging
for the Me,N*
ions has become
zeolite structures presence
cages
to Me,N+
tetraalkylammonium zeolites
only when the ratio
gmelinite
provides the TPA*
cation
Landoit
(34).
Further
step of the crystallization
organizes
the initial
oxide
building
ion is obviously
tetrahedra block
it has process
into a par-
for a particular
used as a template
around
are formed.
the TPA+
It has been proved by Boxhoorn et al (44) by “C MAS ion is localized in the intersections of the channels of the ZSM-5
structure. Now it is possible started suggested
a systematic that
to synthesize study
the hydrated
ZSM-5
of ZSM-5 Na*ions
in the absence synthesis
of any organic
compounds
are able to function
as a template
(45).
Nastro et al (46) system. They for the formation of following
in the RJa, K),O.(Al,O,)x.(SiO,)y.(H,O)z
Hydrothermal
Table
12
Zeolite
TMA*
Some oolitc
synthera
type
Sodslite
Nm+tTMA+
41
synthesis and crystallization of zeolites
in prcsen~c
of orS~nic
barer (12)
C~tionl
Zeolitc
type
Li++Cs’tTMA*
Cmncrinite
Ciimmndinc
Li-ABW
SodmUte
Zcolite
Cnxrinite
Edinrtonite
Gimondine
AUlkiiC
Zeolitr
A
bsobpcs
N-A,
Offretite
cs,ZK-4) Pnisite Zeolite
TMA-E
Mnrritc
(reolites
Zeolite
EAB)
AI-V
IX-P)
(0)
Zditc
ZK-5 QtPL)
Zcolitc
ZSM-2
Gbmondine
D,ZSM-4)
(Nm,TM
A
Sod&e N~+tK++bcnrgltrimethyl~mmoninm
Erionite
GmtYpcr
Lorod
N mud Z-21)
CATMA*
ZoDlite Nu-1
Na++
Zmlite
Na++OiuO
Fu-1
Sodalitc
D&a
Lord
N~*tncopen~ltrimethyhmmonillm
Losod
Na+tCR,N+
Zeolitc
Giamondine Gmclinite
Bl%TMA+
Erionite Zeolitc L Erionitc
S++Nm++TNA*
N+CH,
Nl*tprimnrp.
n-~lkylmniner
(CI to C,J
Zeolitc
ZSM-5
Zcolite
ZSM-5
Oilretitc Na*tbopropylmninc, Chmbarite dibnlylmnine Zeolitc
ZSM-5
Zeolitc
ZSM-5
Zeolite
ZSM-5
zeo1ita
ZSM-8
Zeolitc
ZSY-11
N~*t+bcnryltriphenyl~mmonium
Zsolits
ZSM-11
N~+ttstrbotylphorphoninm
Zeolitr
ZSM-11
NH,ttelr~bntylwnmonium
Zeolite
ZSM-11
K++CH,&
Zdits
ZSM-10
Zeolito
ZSM-12
kn,
N~*tK*ttctnethyl~mmonium (with
(CI(OH),PC(OE),,AI(OH,)
(MPL)
(MEL)
ZK-5
KFL)
Lcsynite
@ dipropylmnine
or
(MFL)
Xu Qinhua and Yan Aizhen
42
Table 13.
Organic. zeolite strwtwe reltionehipaQ0)
Otgank TEA
structurs P ZSM-8
Metbyltrietbylammoulum TP.4 n-Propylamine TBA Choline
TMA+TBA
ZSM-12 ZSM-20 Mordenite ZSM-25 ZSM-12
structure type ? ZSM-5 ? FWljfl8ite Mordenite 7 I
ZSM -5 ZSM -5 ZSM-I1
ZSM-5 ZSM-5 ZSM-II
ZSM-38 ZSM-34 ZSM-43 CZH-b ZSM-39
Fe&rite Erionite-oflretitc
z&39
ZSM-39 ZSM -48
ZSM-12?
ZSM-35 ZSM-21 ZSM-23
Ferrletite Ferrierite ?
1,2-Diaminoethane
ZSM-5 ZSM-21 ZSM-35
ZSM-5 Ferrierite Ferrierite
1,3-Diaminopropane
ZSM-35 ZSM-5
Fertierite,ZSM-5
1,4-Diaminobutane
ZSM-35 ZSM-5
Ferrierite,ZSM-5
l,!i-Diaminopentane 1,6-Diaminoherane 1,7-Diamlnoheptsne
ZSM-5 ZSM-5 ZSM-11
ZSM-5 ZSM-5 ZSM-11
1,8-Disminooctane
ZSM-11 ZSM-48
ZSM-11
1,9-Diaminononane l,lO-Diaminodeonne
ZSM-11 ZSM-11
ZSM-II ZSM-11
TMA+n-propylamlne
Pyrrolidine
DDO
ZSM-10 ZK-5
ZSM-39
? ?
MD0
ZK-20
Levy&e
MQ
LZ-132 Nu-3
Levynite Levynite
TQA
ZSM-18 LOSOD
1 LOSOD
ZSM-30
?
Mordenite
M ordenlte
BP Dihexamethyleoet&mine Neopentyinmine
Hydrothermal
synthesis and crystallization of zeolites
43
It secondary building units (SBU), which arc common to ZSM-5 and also to the mordenite rtructure. was propoacd that in a suitable chemical cnvironmment Nat ions allow the very slow araembly of these units to give the precursor structures of ZSM-5 nuclei. (fig.7) Using “Si and -Al MAS NMR, IR, thermal and texture analysis Scholle and coworkers (47) have confirmed the presence of increasing amounts of TPA-ZSM-5 entities with dimensions lear than or comparable to an unit cell at early stager of the crystallization. The concentration of these. nuclei can be followed using the intensity of the lattice vibration at 550 cm-l, rpeciflc for ZSM-5 zeolite (48, 49). The change in concentration of nuclei and crystals formed during the rynthesir i8 showed in tlg. 8. It ir clear that just for any other zeolite cryrtallization, ZSM-5 also crystallizes according to the successive nucleation and growth processes.
Fig.7. The SUB of ZSM-5 zeolite.
SYNTHESIS TIME (MYS) Fig.8.
(c) nucleation
and (a) growth curves or ZSM-5
synthesis
Curve (a) represents X-ray crystnlinity and curve (b) the Ilk cryrtallanity; the nucleation curve (c) Is the difference
between
(b) and (a) (28).
44
2.4.
Xu Qinhua
and Yan Aizhen
The nature of reactants (50)
The variety
of species
When hydrated silica
zeolite
silicas
were
sodium NaX
was
used.
gismondine
used
14
Some
Charge
(51, 52), especially
formed
at 371K
“inactive”
much
sources
The “activity”
of very small amount Table
of the alumina,
silicates
The
type NaP.
as sources
silica and base
sodium
more
under
the
of the soluble
metasilicate
readily
than
appeared
14 (12).
were the source
“inactive’
of synthesis
silicates
in table
solid
and
resulted
to depend
of
colloidal
primarily
in
on the presence
of Al in them. sources
of
cations,
aluminium
and
silicon
in
zeolite
(12)
crystallization
compensating
cations
Silicon
Aluminium
-
--
--___--
Alkali
metal
Alkaline
hydroxides
earth
and
Metal
hydroxids
oxides
and
hydroxides Salts (Fluorides, halides, carbonates, phosphates, Organic
especially
Silicates A10
l
Silica
sob
Glasses
Silica
gels
Sediments Minerols,eopeoially
Silica and other synthetic gloeseo
clay
minerals,
felspathoids,
hydroxide,
and
other
glass
Silicon felspars
Tuffs
zeolites
esters and
Minerals
quaternary
volcanic including
minerals, felspars
and
Mixtures
aluminates
of two
of the
silicate
Water
bases Silicates
and
hydrates
OH
Al salts
and
ammonium
AlrO,,
Al alkoxides
sulphates,etc.)
bases
--
aluminates
Al(OH),
oxides
Other
pentahydrate,
when
conditions
sodium
has been listed
glasses clay
felspathoids, and
other
zeolites
or more
Basalts
above.
and
mineral
mixtures Sediments Combinations more
In alkaline NMR
sodium
indicates
while above
aluminate
that
pH 10.5, Al(OH);
polymeric
anions
the extent
of which
NMR plained
solutions
(AlfH,O),)‘+,
study
at pH values
is AHOH);,
and Al(OH),.nH,O In alkaline silicate
The viscosity
glass indicates
can be expected
by the presence
anion
(Al(OH),(II,O),)+ is exclusively present.
are common.
(55) of a solution
the important
to vary
of Hz0
of anionic
of water with
+ D,O
concentrations of KOH
(8M),
a high degree
of the Na silicate SiO,
“Al
A
was ex-
species as follows:
1.45M 0.22M 0.16M cyclic trimer
0.08M
linear
trimer
0.07M
linear
tetramer
0.03M
show
7,
free alkali. (0.02M)
dimer
clearly
below
of polymerization, and
(3M) and Cr”
cyclic trimer
now exsit that
or
are important species (54); solutions, on the other hand,
monomer
substituted
Data
of two
of the above
(table
15) that
the crystallization
of ZSM-5
from
the usual
gel is de-
Hydrothermal
45
synthesis and crystallization of zeolites
pendent on, among other factors, the nature of the silica source. Silica sources initially containing high amounts of the silicate monomer crystallize faster than gels in which silica is present in a higher polymeric form. It can therefore, be concluded that the rate of dissolution or of depolymerixation of As silica, which is known to be slow, intervenes in the rate-determining step for nucleation of ZSM-5. the crystal growth is influenced in the same way (table 15) this indicates that it’s rate is also determined by the concentration of monomeric silicate. Table 15. Influence of the nature of the silica source on the crystallization rate of ZSM-5 (28) Silica source
Nature
Induction period @)
“ m ’ 50% to reacn crystallinity 01)
Ref. 15
Wster glass
Monomeric
25
40
GCl
Polymeric
60
140
15
QUO
Precipitmtcd
8
12.5
62
Csbosil
Fumed
10.0
62
Ludox
ColIcAds ml
4.1
5.5
62
siucste
Monomeric
3.s
4.0
62
silica
In the synthesis of ZSM-5 zcolite, as a general rule, the TPA silicate contains a much smaller number of discrete species compared with corresponding sodium silicate system. In a real synthesis mixture form ZSM-5 with the molar composation (~PA),O),.~a,O),.(A~O~.(SiO,),,,(H,O),,,,. “Si NMR shows that many silicate species,from monomers to highly branched polysilicates, are present (56). The trans. formation of a Qissilieate species into a real precusor building of ZSM-5 reolite is shown in fig. 9 (57).
-’
\0----/
Fig. 9.
2.5
Transformation of a Qi, silicate species into a ZSM-5 building (62) Block Q2 end silicon atom; Q:Q” linear trimer silicate Qisa double five-ring silicate
Metastability
and recrystallization
The zeolites are prepared at irreversible, non-equilibrium and super saturated conditions. So the majority of these zcolites are not the true equilibrium phases. In the system Na,O, Also,, SiO,, H,O only two zeolites phases are probably equilibrium phases, these are analcime and mordenite. The exper. tmental results indicate that the less stable the phase the greater the chance in which initially it will nucleate and grow fast, but it’s chance of subsequent survival is less. Two xeolites, one which is minerial
Xu Qinhua and Yan Aizhen
46
that has formed and
and exsited
is synthesized
changes
in chemical
the ordering stable
composition.
conditions,
be transformed
that from
an aluminosilicate
depending
on crystallization
cage structures to denser
structures
growth
conditions Y and
transformation
is the predominant
diffractograms
of solid
hydroxide heating
in the liquid of zeolite
ture of molite formation found.
A and zeolite zeolite P,
This means
of intermediate
at three
appears
(14).
reaction
of zeolite work
(tr=
that the transfomation
(13) described 213 h and
However,
structural
the diffraction
of zeolite
A into zeolite
t,=
309 h).
the course
Similarly, of a mix-
of the transXI in fig. 10).
A and zeolite
place
x-ray
mr of sodium
(diffractogram
of zeolite
P takes
mol/
and
solid-solid
in the formation
during
form
peaks
nucleation
as fig. 10 that
2050
at 355 K for 213 h results
Large
into zeolite P or that
or even, that
containing
0, t,=
of zeolites, (58).
and tend to transform
indicated
A particles,
mixture
as the predominant only
results
given
This means
types
that zeolite A can be transformed
times
to another under
(14).
two or more phase
to
or equivalents.
may,
types of zeolites
in solution
VIII in fig. 10).
presented,
which
and
as related
time of crystallization)
experimental Subotic’s
of NaOH
Pt( diffractogam
diffractograms
unstable
off the reaction
A in 2.05 M soliution
process,
In all the x-ray
drawn
relatives
structure
one can obtain
at the surface
process
samples phase,
start
of disorder
and it is ease of conversion
alkalinity,
It is known
structurally
due to ordering
no mineral
more stable
S are relatively Some
have
metastable
composition,
(59).
differences
a high degree
zeolites
represent
(temperature,
media
P particles
zeolite
thermodynamically
as zeolite P (14, 59).
of zeolite
shows
of the synthetic
gel of desired
obvoiusly
chabazite
zeolites
into other
in alkaline
time, and the other which is related
exhibit
of the synthesized
synthetic
such as zeolites
into hydroxysodalite crystal
Many
Some
can
Synthesized
due to the metastability
species.
of geology
in the laboratory,
in the mineral.
This is probably more
over long periods
rapidly
without
P, can be
the formation
solids.
I% 17
18
15
14
13
12
11
10
9
8
7
6
5
t
Fig. 10.
Ifzeolite
A remains
in 3 days. zeolite
X-ray diffrecrograms solution at 355K in prolonged
Similarly,
Y to omega
zeolite
of solid samples
contact
X appears
has been observed
from 17 h to 63 h (60).
obtained
with the mother to be metastable in certain
by heating
liquor,
recrystallization
with respect
gel composition
zeolite A in 2.05M
to zeolite
when
to zeolite P (14).
the digestion
NaOH
P may occur
Conversion
of
time is extended
Hydrothermal
47
synthesis and crystallization of zeolites
The metartablility of crystalline zeolitcs was studied also by Xu Qinhur (61), and pointed out that the change of metastable zeolite to a more stable one, depends upon the intrinsic properties of the zcolite and also the chemical environment in which it exists. For example, added 50 wt% of crystalline Y zaolite, based on the amount of zeolite produced, to the aluminosilicate gel with the composition of (1.70 88H,O, which is the composition of the reaction mixture Na,O + 0.35 (Me,N),O). AGO,. lOSi0, The x-ray powder diffraction showed the characteristic pattern of the Y of omega zcolite formation. type after the gel had crystallized at 413K for 10 h, but the pattern is entirely of the omega type after the gel had crystallized for 12 h. After 24 h the process was complete (fig. 11). This means that the trans. formation of zcolite Y into omega takes place without the formation of intermediate
solids.
J
Fig. 11. X-ray diffraction pattern for Y turning to Omega zcolitc.(41) a. crystallization 10 h; b. crystallization 12 h; c. crystallization 24 h. The process and mechanism of the transformation of NaA to NaY zeolite in the chemical environment of NaY formation have been studied in ref. 20, 25 and 61. By adding NaA zeolite to the crystallization environment of NaY zeolite formation, the process of transformation or recrystallization may be represented as: Amorphous aluminosilicate gel NaY ----> ------> NaY NaA
NaA
In fact, the recrystallization of NaA zeolite does not occur until the gel crystallization is nearly complete. Scanning electron microscopic observations and the recrystallization curves are consistent with the mechanism that includes a degradation and dissolution of surface NaA crystal. The nucleation and crystal growth of NaY take place directly at the surface of NaA crystals. SEM also show that the formation of the smaller spherical NaY crystals (-0.5 pm) occurred at the surface of the larger cubic NaA crystals (.0.5 am). The nutrient for NaY crystallization is being supplied by the dissolution of NaA crystal surface. During recrystallization, the reaction process from the surface to the interior of the NaA crysals, finally the NaA crystals are converted completely to smaller NaY crystals.
Xu Qinhua and Yan Aizhen
3. MECHANISM
During
past
the
crystallization.
63-73).
two
Recently
also highly
much
siliceous
attention.
and Lechert
in the growth mechanism.
have proposed more complex
Derouane
formation
of 4 and
(monomeric
transformation
on the silica source
et al. (10) while
Roozeboom
studying
6 ring
and polymeric)
ions from
that
other
both
investigators
from
the amorphous
the
phase
ion trans-
of a solution
(22, 38, 52, 64-69) silicate
and possibly
by the dissolution
the liquid
(69)
been dis-
for crystallization
in support
phase
are important
Y synthesis indicated
systems have
et al (15, 18), Sand et
of aluminate, supplied
mechanism
system,
evidences
Zhdanov
and the gel formulation
A, X and
vs 5 ring
of zeolites
evidences
of the polymerization suggested
of zeolite
(15, 22, 38, 39, 43, 52,
the role of liquid
presented
the ions being continuously
phase
zeolite depending
and
mechanism
(38, 43) and mordenite
type zeolites.
et al. (63) and later many
phase,
the
for the synthesis
demonstrated
nuclei
et al. (43) have
and solid hydrogel
of ZSM-5
by contrast,
zeolite
Barrer
on
was studied
(70, 71) have been presented
to be the result
ions in the liquid
id gel material. mechanism
Initially
the nucleation
appeared
mechanisms
A and faujasite
(67) have,
of different
have
A and faujasite
such as the ZSM-5
different
in studying
CRYSTALLIZATION
papers
(74) and McNicol
transformation
al (22, 26), Kacirek transport
of
of zeolite
zeolites,
Two
Breck and Flanigen
by solid phase formation
a number
In most cases the synthesis
have received cussed.
decades
OF ZEOLITE
of the sol.
ion transformation
in studying
the synthesis
used.
gels, proposed
dissolution
aluminosilicate
a mechanism
of some
gels.
silicon
for the
containing
The hydroxylated
ions con-
dense with Al(OH); to form a large variety of complex aggregates. The mechanism of zeolite synthesis from alkaline aluminosilicate gels has continuously puzzled zolite chemists. The reason is the complicated reaction “skeleton”
medium
from which
with a liquid
processes
which
phase
occur
during
the zeolite crystallizes
containing the induction
during
induction
remains
an enigma
studies
on the mechanism
of zeolite
the autocatalysis role of solid
system
period,
is still being
insufflcienently
discussed
studied
the determination quantitative
phases
posed
a solid
experiments, Guth
data
no spectral
transformation
flicting. @RS),
and aluminate Roozeboom X-ray
synthesis. zeolite
crystallization,
structure in favour
(XRD) showed was
and
However,
Angel1
that the AI(
detected
of a solution
and
in the liquid
the solid
chemical some
transport
of solute
and liquid
crystallization
in NaOH phases
in the liquids species
mechanism.
(band
by Laser at 621 cm-‘)
appeared
species.
in the
But
and thus pro. similar
mechanism.
complexes
in solution
with the spectra
investigations
(Al, Si, Na) for studying
in
spectroscopic
transport
solution
in
species, and
(68), performing
of aluminosilicate
remains
of the difficulties
aluminosilicata
and Flank
of the
aluminosillcate
Some Raman
for a solution
So far, Raman
analysis solute
during
the nature
of the system
are some
of gels.
phase
ions separatly
phase
In
such as
The question
ions and silicoaluminate
the exsitence
in solution.
et al. (10) examined
diffraction
Their results
together
of the system
to kinetics,
studied.
These
structure
for the occurrence
and aluminate
are referred
of the liquid
the
of zeolite synthesis.
in the heterogenous
in the liquid
changes
evidence
ions exsiting
crystals
aluminate
and chemical
mechanism.
of silicate
quastions
gel
for tracing
The behavior
the mechanism
crystallization.
suggest
aluminosilicate
effect, on the rate of crystallization,
The composition
of spectral
indirect
the spectra
crystallization.
of zeolite
changes
sodium
to find a technique
have been insufficiently
and state of silicate,
spectroscopic
an evidence
before
alkalinity
which
with zeolite nuclei
et al. (80) reported
silicate
any
phase gave
by comparing
(75 - 79).
of zeolite
(70, 71) observed
an amorphous
It is difficult
the main
in the formation
of the structure
(80, 81) and 29Si NMR McNicol
process, effects,
in connection
determination
period
crystallization,
and seeding
and liquid
---
ions.
and is the key to understand
of the crsystallization
of the induction
similar
of
have been con-
Raman
Spectroscopy
A, X and disappeared course
Y zeolites before
of X and
Y
Hydrothermal
3.1.
Crystallization
49
synthesis and crystallization of zeolites
by solid phase transformation
mechanism
On the basis of electron microscopic studies and chemical analysis of aluminosilicate gels, the crystallization occurs from the solid gels phase (74, 75). The induction period was postulated to be a time during which the nuclei formed in the solid phase growing into a definite size. The elemental composition of the crystalline polite was almost identical to that of the initial solid phase extracted from the gel (15). During crystallizatioin neither the hydrogel phase is dissolved nor the formation and growth of crystal nuclei is occured in the liquid phase. The luminescence and Raman spectroscopy were used to study structural and chemical aspects of gel growth of A and faujasite type crystals (71). Their results are consistant with a solid-phase transformation of the solid amorphous network into zeolite crystals as shown in fig. (12 - 14). EOD)
4T,(4~
so0
1
,
400
500
I
do0
700
go0
WAVELENGTH Fig.12.
Phosphorescence
I Fig.
13. Eu’+ phosphorescence
(a) and excitation
(b) spectra of Fe’* in zeolites
-
&’ IN DEL
-
Et’ IN CRYSTALS
I
I
a00
700
WAVELENOTHCum) in gel and Linde A crystals, measured
(71)
L
ma at room
temperature
(71)
50
Xu Qinhua and Yan Aizhen
LINDE A FRAMEWORK
YlBRATIoN
7& I
I
I
750
500
250
WAVE
Fig.
14.
Raman
NUMBER
(cm-9
spectra of crystallizing gels: (CHa,N* / Linde INITIAL
ALUMINGSILICATE
A system
GEL
L-TETRADRON I,
BUILDING
BLOCK
REARRANGEMENT HYDRATED
FURTHER
ARROUND
SPBCIES
CONDENSATION
REACTION
ZBOLITE CRYSTALS
GR0WT.H OP CRYSTALS I
Pig.
15.
Schematic
rcpzescntstion
phase transformation
of the crystallization
mechanism
(82)
by a solid
(71)
51
Hydrothermal synthesis and crystallization of zeolites
Trace amount of Fe’+ ions aubstitutcd for Al% the tetrahedral eluminosilicatc gel framework exhibit characteristic phoaphorcscence spectra which have been used to follow the build up of the zeolite fremework (fig. 12). Phorphorescenee spectra of exchanged Eat’+ cations fig. 13) end Reman spectra of (CH,),N* cations present in the solid phase of gel (fig. 14) indicate that no zcolite cages exsit in this phase during the induction period. Reman spectra of the liquid pherc of the gel syrtem show only the The crystallization by e solid phase transformation presence of SiO,.(OH); end Al(OH); anions. mecheniem may be deecribed cchemetieally in fig. 15 (81). When the reactants are mixed, silicate end aluminate ions begin to link end polymerize and initial aluminosilicete gel is formed. Simultaneously, the liquid phase of gel is also formed, but the component concentrations in the liquid phase of gel would not change end remain constant, and liquid phase ion transportation would not occur on the nucleation and growth of crystalli zetion processes. The initial aluminosilicetc gel is dissolved and rearranged due to the action of OH- ions to form the basic building These primary building blocks then rearrange around the blocks of the growing zcolite crystals. More polyhedra erc futher formed and polymerize hydrated species (cations) to form e polyhedron. and combine each other to form e zeolitc crystal. Therefore, the gel formation is the major nucleation controlling process which affects the crystal size end morphology of zeolite phases. The zeolite crystals are generally uniform end smell in size (several The high microns),end are often euhcdrel. A larger number of nuclei arc formed during gel formation. degree of supersaturation with respect to the ionic species present leads to rapid heterogenous nucleation. After the initial rapid growth, continued crystal growth is extremely difticult to maintain. Seeding experiments end experiments involving repleniehment of the nutrient gel have not produced any significant increase in crystal aim. 3.2.
Crystallization
by a solution transport
mechanism
Kerr (64, 65) reported on crystallization of zeolites A end growth from solution. A direct solid-solid transformation uct does not occur. Amorphous solid diBBOlvCB rapidly in species. The concentration of this epecies remains constant
X in specificaystems in which he postulated of amorphous substrate to crystalline prodthe alkaline solution to form II soluble active during most of the growth period.
Angel1 et al. (68) have determined the mechanistic pathway in 4A synthesis, by using several different characterization techniques es a function of time. The evidence aLso supports e solution trenaport mechanism. The date obtained from the Reman spectra are summarized in table 16 and fig.16. These results, perticulnrly during the crystallization BtagC can be contreated with those reported by McNicol in previous section, who obacrved no changes with time other then the appearance of zeolite A. The spectra (ng. 16) show that conversion to framework silica occurs from precursor material. This may consist of local. ized concentrations of silica gel formed in the initial mixing of reactants, or by precipitation from e supersaturated solution, or un aluminosilicate gel with a variable degree of crosr-linking. In the early stages, i.e. during induction, the alumina concentration decreased after crystallization of zeolite A has set in. From the rerults of Reman spectroscopy end chemical Roozeboom (10) et al., aluminate disappeared from solution end ia incorporated in the solid to the silicate concentration of the liquid phase, this concentration decreaaed less drastically able low levels. Whereas, in case of both zeolite X end zeolite Y in the first 2-3 hours, i.e.
3 h when analysis by phase. As to comperbefore any
Xu Qinhua and Yan Aizhen
52
LIQUID A -INITIAL
Fig. 16.
Raman
PHASE
cm
spectra
A - INITIAL PHASE B-AFTER SHBS. DIGESTION C-AFIER 4HIlS.
i&7-- 200 of solid and liquid
I
I
,
I
1000
phases
1
,
cm+
500
I
200
in zeolite A synthesis
(68)
6 6
200
400
600
mo
FREQWENCY SHIFT (cm -9 Fig.
17.
Raman
spectra
of solid phaac in zcolite X synthesis
(IO)
Hydrothermal
synthesis and crystallization of zeolites
53
crystalliter were detected, aluminate was disappearing from rolution and incorporated in the amorphous aluminosilicate phase. In the same period an increase in silicate concentration in the liquid and a decrease in silicon concentration of the solid phase have been found. From these data it is clear that in the grst 2-3 h of the crystallization or nucleation aomc silicon containing ions (monomeric or polymeric) were dissolved from the amorphous (alumina) silicate gel. Tlttse hydroxylated iona apparently condensed with the Al(OH); ions present to form a large variety of aggregates, which may be the nuclei for crystal growth. Thus the whole mechanism of zeollte A, X and Y formation is a solution transport meohnnism. The fig. 17 and 18 give the resulting Raman spectra for the solid and liquid phases in zeolite X synthesis after 0,2,4,6, and 8 h of crystallization. Table 16 Laser Raman Spectroscopy
data as a function of
time during zcolite A synthesis aging time,h.
tryst. time,h.
band position,cm-’
(68)
band intensity and shape
-
species
liquid phase 0
0
1 2 3 4 0.5
0 0 0 0 1
0.5
2
0.5 0.5
3 4
620 400-500,700-800 unchanged unchanged unchanged unchanged 620 450,880 620 400-500,700-800 unchanged 620 400-500,700-800
rtrong,sharp very weak unchanged unchanged unchanged unchanged medium, sharp weak weak weak unchanged very weak weak, broad
-
aluminate siltcad) unchanged unchanged unchanged unchanged alum inate ailicafl) aliminate ailioad) unchanged aluminate silica(?)
solid phase 0
PICG-C
0
1 2 3 4 0.5
0 0 0 0 1
0.5 0.5 0.5
2 3 4
405 450 800 unchanged unchanged unchanged unchanged 450 800 unchanged 490 450,800 490 340,700,1040
weak
strong, broad weak, broad unchanged unchanged unchanged unchanged strong, broad weak, broad unchanged new shoulder weak, broad strong, sharp weak
silica silica silica unchanged unchanged unchanged unchanged silica silica unchanged zcolite A silica zeolitc A zelitea A
Xu Qinhua
54
and Yan Aizhen
hr
0
4 6
1
’
400
500
700
600
600
900
1
FRFLQUENCY SHIFT (Cm-‘)
Fig. In fig. 18, silicate silicate anions,
It means disappears
silicate
from
cm-‘(monosilicate)
appeared
A large
variety
nucleation.
(D-4-rings
When
silicate
while the free silica species liquid
as silica source,
species
and D-6-rings).
phase
which
in the solution
has
too
Schematic
are mixed phases
this, both
the aluminosilicate equilibrium
aluminosilicate)
actions
of polymeric
by Angel1 and Flank of their
with bands
initial
silicate
species
into
(68) that aluminate
zeolite
(for
around
450 and
A synthesis
gel.
does not give rise to any signal
800 This
in the gel when
low
prevents
the observation
concentration
representation
due
of specific Raman
to it is growth
of the crystallization
to large
peaks species
by a solution
transport
gels are formed
rapidly. Owing to
is given in fig. 19 (15).
and
aluminate
phase,
ions present
Each
the reactants
heating
are due to the depolymerization
in the separated
of complex
The solid and liquid the
(10)
silica gel.
during
mechanism
phase in zeolite X synthesis
This is also observed
ions.
the liquid
may be due to the sodium used colloidal
of liquid
species with the bands of 936, 777 and 448 cm-‘, and dimeric of 600 cm-‘, exsited in the liquid phase of X and Y zeolite after some 2-3 h
that these bands
and dimeric
621 cm-‘)
spectra
ions of monomeric
with the band
of synthesis. monomeric
Raman
18.
the gels,
depend their
the synthesis,
and silicate
product
the initial
ions are always
concentrationa
of
the
ions
of amorphous which
ions in the liquid
the ions increases,
giving
by the solution
equilibrium.
present in the liquid phase
on the composition increases,
aluminosilicate
gels are connected
solubility
and aluminosilicate between
during
of aluminosillca
(the
solubility
aluminosilicate
of aluminosilica
product
of
gels
amorphous
and temperature.
While
leads to increased concentration of the silicate, As a result, the probability of condensation rephase.
rise to the formation
of primary
aluminosilicate
blocks
(4-
Hydrothermal
synthesis and crystallization of zeolites
55
and &member rings) and crystal nuclei. The formation and growth of crystal nuclei lead to the exhausting simplest silicate, aluminate, and aluminoeilicate ions in the liquid phase, and the equilibrium state is reached by dissolving of the solid phase. The average rate of crystal growth at a given temperature depends on the concentrations and aluminate ions in the liquid phase of gel, as shown in table 17 (15).
of the silicate
HYDRATED SPECIES~ N;, OH-, AI(O (Ho&. Si CO);, (Al, 0, Si, OH) -ANIONS O.l<[Si].
IA~lXlO’<
6
(APPROXIMATELY) HEATING
osi
.A1
633HYDRATED~A~ONS 15Si:Al
hSSOLUTION OP AMORPHOUS
ONDENSATION REACITONS
RATE PHASE TEME
GROWTH IN 91735 AND
DEClkEASE OF CONCENTRATIONS
Fig. 19.
Schematic representation of the crystallization a solution transport mechanism (15).
by
Table 17 Component concentration in the liquid phase of gels(mols / 1) and rate of growth of zeolite A crystal at 363 K samples
NaOH
Al,%
SiO,
(Al)(Si)x 10’
266-l 266-2 266-3
0.560 0.460 0.428
0.0266 0.0258 0.0208
0.0103 0.0085 0.0092
5.46 4.00 3.80
3.05 2.25 1.80
266-4
0.302
0.0104
0.0108
2.25
1.25
V,,,
(15)
cc/ day
.
56
Xu Qinhua
As the nuclei and crystals of crystals
depends
grow at the expense
on that of liquid Table
phase,
18
and Yan Aizhen
of component
as confirmed
Dependence
in liquid
of gel liquid
phase
of gels, the composition 18.
(15)
Si / Al in zeolite SiO 1
X crystais
Na 1 G ,iC SiO 1
AlsO s
(mole / kg)
faujasite
phase
phase
Samples
The data
phase
of Si / Al ratio of zeolite X
on the composition
Concentrations
of the liquid
by the data given in table
Na,O
A’&‘,
SiO,
196-1
0.702
0.0124
0.397
32.0
1.75
1.63
196-2
0.825
0.0114
0.304
26.6
2.68
1.38
196-3
0.926
0.0105
0.220
21.0
4.17
1.33
196-4
0.955
0.0116
0.174
15.0
5.42
1.15
196-5
1.200
0.0202
0.117
5.8
10.05
1.07
the growth
of the Si/
given
in table
is directly
of the gels.
tion of silicates gave the evidence
18 indicates
associated Changes
and
that
with increases in component
aluminates
and
for the crystallization
in the SiO,
concentrations
from
in liquid
silica sol during
of zeolites
phase
crystallization.
by a solution
AQUEOUS BASIC SOLUTION
SILICA SGL
Al ratio
concentration
transport
in the crystal
and
Si / Al ratio
of gel were obtained These
experimental
mechanism.
BASIC ALUMINA SaUlION
1 SOL OF REACTANTS i
PRECIPITATION
1
1 AMORPHOUS GEL SOLID
1 1
L
NUCLEATION (1)
tn, NUCLEI IN GEL
NUCLEI IN SOLUTION
I
I
TRANSPORT GF MONOGLIGGMERS IN SOLUTION
GEL
REARRANGEMENT
GROWTH
4
CRYSTALS
Fig.20.
Schematic
representation
(I ) nucleation
of zeloite synthesis
from solution
of synthetic in the liquid
according
or (II ) in the gel (28)
to
from soluresults
Hydrothermal
Table 19
57
synthesis and crystallization of zeolites
Comparison
of the parameters and operating
conditions for syntheses of type A and B for ZSM-5 zeolites (43)
starting material4 Aluminium Silicon
Al metal Active SiO, (solid, polymeric)
TPA source
3.3.
Ingredient ratios(molar) Si/ Al Al/ Na (Si+Al) / TPA H,O / (Si+Al)
A-l 15.6 0.482 1.79 15.4
Synthesis conditions Pyrex tubes Temperature / k Time required for 100% crystallinity
hydrothermal 393 300-330 h
Crystallization
TPAOH
A&&SO,), 18HsG Na silicate (liquid, mono-and polysilicate anions) TPABr
A-2 13.9 0.827 1.80 15.2
B-l 45.1 0.016 9.19 21.6
l
B-2 45.2 0.016 9.80 27.6
hydrothermal 393 45-50 h
by two different mechanisms in the same system
The zcolitc crystallization may occur following two different mechanisms, in which nucleation occurs in the hydrogel phase or in the liquid phase. The successive events in both mechanisms are shown schematically in fig. 20 (28). Derouane etal. (43) were able to synthcsixc ZSM-5 by either the solution method (method A) or the gel nucleation method (method B), depending mainly on the relative conccntrations of the reactants and the nature of the silica source, to studies on the mechanism of xeolite crytallization. The comparison of the parameters and operating conditions for synthesis of type A and type B are listed in table 19. The results for type A and B measured by XRD, TGA, SEM and chemical analysis are also listed in table 20 and 21 respectively. Summarizing, synthesis of type A and B for ZSM-5 xeolite are two distinct ways from a solution or gel nucleation mechanism, respectively. It seems that a high basicity and / or the presence of foreign ions in the nutrient and also the polymeric nature of the silica source and the Si / Al ratio of the gel direct the reaction to solution nucleation, and that TPA content and degree of dilution are less important as determining parameters for the nucleation mechanism. Therefore, two different mechanisms occur in the synthesis of ZSM-5 zeolites, depending on the source of silica and the Si / Al, Al/ Na and (Si+Al)/ TPA ratios in the reaction mixture. The first is a liquid phase ion transportation process (solution transport mechanism) in which few nuclei are formed and large crystallite4 are obtained. The second is a solid hydrogel phase transformation process (solid phase transformation mechanism) in which numerous nuclei are formed, leading to polycrystalline aggregates.
Xu Qinhua
58
Table
20 Experimental
intermediate
function
(43) Conclusions
crystallinity
after
time. is achieved
Si / Al ratio
increases
The
Liquid
progressively
in the solid
crystallization analysis
as a
320 h at 393K
The
analysis
is studied of synthesis
100%
Thermal
on type A Synthesis of conclusions
Observation Crystallinity
diffraction
Chemical
observations
phases-summary
Technique X-ray
and Yan Aizhen
phase
as
proceeds.
amount
decomposed
TPA
intermediate
is progressively
incorporated
to the crystallinity
of the solid
mass
probably
occurs
of TPA
is proportional
phase
transport
phase
in the zeolite
framework; one
there
TPA
is about
entity
per
channel
intersection Scanning
Gel
electron
and
zeolite
taneously
microscopy
disapears
Table
21 Experimental
Crystallinity
phase
about The
of nuclei; mass
transport
occurs
on type B synthesis of conclusions
(43) Conclusions
is followed of synthesis
crystallinity
analysis
number
liquid probably
grow
- summary
Small
Obserbation
function
Chemical
simul
gel
observations
phases
Technique diffraction
are
The
as the crystallites
intermediate
X-ray
phases
observed.
as a
time.
is achieved
100%
after
48 h at 393 k Si/
Al ratio
the solid until
phase
near
stays
constant
during
in
the synthesis
100 % crystallinity
Crystallization
probably
occurs
by solid
of the
hydrogel
Large
amounts
transformation
is reached Thermal
analysis
The amount increases Large
amounts
present
Scanning microscopy
eletron
of TPA
with at low
An amorphous throughout Crystalhtes in near
in the solid
the crystallinity of TPA X-ray
phase
are
already
are
formed;
are
about
of small
4 TPA
entities
per unit cell in the crystalline material
is observed
Crystallites
apparent crystallized
the only material
nuclei
there
crystallinity
the synthesis. 100%
are
grow
gel by solid
of the latter
100%
within transformation
59
Hydrothermal synthesis and crystallization of zeolites
4.
KINETICS
AND GROWTH
OF ZEOLITE CRYSTALLIZATION
Kinetic studies of zeolite growth have been carried out mostly for A and X type zeolltea. The growing of A and X-type zeolltes has to be regarded as an autocatalytio process and the formation of zeolites is accelerated by nuclei already present (64, 65, 74, 83, 84). From the experiments using nuclei the growth can be regarded as a reaction of first order, whereas the nucleation is a reaction of high order. A typical kinetic experiment for the formation of zeolite A gave the rate of formation of crystalline product to be approximately close to the first-order kinetics with respect to the quantity of crystalline zeolite present (64): dz/dt
= kz
(4)
constant. where z = per cent of zeolite. in solid phase at time t and k is a proportionality The rate of any crystallization process is determined by the rate of nuclei formation and crystal growth. Equation 4 indicates the increase in either the linear rate of crystal growth or the rate of nuclei formation during the period of crystallization. The autocatalytic nature of the crystallization process cannot be attributed to a change in the crystal growth in the course of crystallization, as shown in fig. 21 (15). The rate of crystal growth is greatest at the beginning of crystallization, and gradually decreases as the process advances. The time passed from the beginning of crystallization to the formation of nuclei of crystals of each size can be calculated from the data represented by the curves in fig. 22 (15). The formation of nuclei takes place during the entire process of crystallization, but the rate of nucleation increases only during it’s initial stage. These curves can be described by a common equation in the form z * kt’. The values of the constants k and n are easily calculated from the plot of log z vs. log t as it is observed from fig. 23, the calculated points agree quite well with the experimental curves (15). The calculations confirm that the autocatalytic growth of the mass of crystals can take place during the induction period. Such an increase in the nucleation rate can be explained by postulating that not only the alumonosilieate blocks formed in the liquid phase but the similar blocks with ordered structure occurring in the gel skeleton can be the nuclei of crystals.
CRYSTAL LENGTH (r) Fig. 21.
Crystal size distribution of zeolite A in the product of crystallization (363k) (15)
Xu Qinhua and Yan Aizhen
60 loo-
aoe II
-
400- 1.0
40
0
120
go
200
160
ITMe (h) Fig.
22. Growth
of the number
of nuclei
in the rate of their formation during
crystallization
(1) and change
(2)
of xeolite A (15)
TIME (h) Fig.
Crystallization
23
of zeolites
l.Zeolite
A, 373 k
Z.Zeolite
X, 373 k@reck
3.Mordenite,
Kinetic
models
In the induction nucleation.
of crystallization
period
A kinetic
zeolite
on the crystal
where
k is related
to the composition
of the zeolite NH,-ZSM-5,
to a certain
extent.
law of the order A systematic
It was shown
2 / 3 under
study
that
the growth
of crystallization
complete
of omega
region
T-type
form Z = kt’
there
were
no spontaneous
zeolite was suggested
as
kx”’
of nucleation of the crystals growth
zeolite
calculated
system
gel, at a particular
the separation
approximately
phase
of rod-like =
of original
OPoints nucleation
or amorphous growth dx/dt
crystals
and Quoben)
curves
in non-spontaneous
of T-type
model
of time (15)
and Flanigen)
573 k(Dominc
--Experimental 4.1.
as a function
temperature
and growth
(85).
By adding
of NH,-ZSM-5
of xeolite NH,-ZSM-5
seed
was made obeys a kinetic
conditions.
in (TMA),O-Na,O-AJO,-SiO,-H,O
system
at
Hydrothermal synthesis and crystallization of zeolites
413 K was reported and the kinetics crystallization was presented.
of crystal
growth
of omega
61 zeolitc in non-spontaneous
The kinetics of the process of growth of the faujasite from amorphous aluminosilicate gel has been investigated by Lcchert et al.(67). In a wide range of crystallization, the growth obeys a kinetic law of the order 2 / 3, describing in detail as follows. Starting with the conception that the increase of the rate of crystal growth is proportional to the free surface O(x) of particles being available one obtains the equation of the kinetics of the process of zeolite growth. dx / dt = 3kx”‘xi”r,’
6)
where x = the mole fraction of the growing zeolite crystals. xs = the mole fraction of seed crystals added. rs = the average radius of seed crystals (nuclei). By rearangement and integration of x gives X xI x = xa + 3k-r + 2ka+’ ‘0
ro
+ k+’ ro
For the reaction constant or rate constant k follows that (7) k has the dimension (lenth / time), therefore, it describes the linear growth of a particle in a time unit. It can be shown that the growing of faujasite crystals from nuclei (using seed crystals) is possible in concentration ranges where nucleation of species is negligible. In a wide range of crystallization, the growth obeys a kinetic law of the order 2 / 3. From the values given by Lechert (86) it can be concluded as follows: 1. The crystallization time, t, increases with increasing Si / Al ratio in the final products, which resultx in decreasing value of k. 2. The growth of the crystals only occurs at the interface of the crystals and the solution, and a direct transition of the gel into the crystallization state must be excluded. 3. The apparent activation energy for crystal growth increases with incresing Si/ Al ratio in the final products. 4. The rate of the crystal growth is proportional to the actual surface O(x) of the crystals already present in the reaction mixture. 5. In case of the diffusion as the rate-determining step, the dependence of the crystallization rate on the surface of the crystals in the reaction mixture should not be realized. The rate-determining step seems to be given by the connection of silicate species. 4.2.
Kinetic models of crystallization
in spontaneous
nucleation system
Spontaneous nucleation system is the system without adding any seed crytals during synthesis of zcolitcs or gels. The curves showing yield of crystals against time are characteristically sigmoid in shape. This behavior has been observed for zeolitc A (83), X and Y (52, 67, 86),sodalite hydrate (27), @Ja-K)-phillipsites (23), zeolite ZK-5 (3), omega (87), Na-ferricrite (88) and mordenite (19). As a law of crystal growth for such system Meise and Schwochow (17) have formulated the expression r -j7t (8)
PICG-C’
Xu Qinhua
62
and assumed action
that
time t.
suggests
the crystals
A rough
are spherical
estimate
an e function
and Yan Aizhen
configurations
of the percent
increse
as a law of nucleation,
where
also been represented
2
r increases
of particles
in proportion
to re-
N in the reaction
time t
A and
A(eB’
3%.are cocfficicnts.
by the model-based
reaction
1)
-
The CUTYCSof crystal
growth
against
time have
(89):
= l-exp(-_,c”)
z1
The
radius
of the form dN -_ dt
Z
whose
in the number
ratio
Z,/
reaction
time
formed
is of the
Z,
mass
t the normalized
of crystal
Z,
contribution
at time (less than t) in the interval
d(Z,/
Z,)
at time d(Z,/
dr
t to the
mass
Zr) to the final
Z, in the final
yield
Z, arising
product.
only
At
from
nuclei
is
= 4 / 3n~3(t-r)3A(e”‘-l)dt
Beaction
was assumed
to be complete
all values
of r between
0 and t gave
(10) after a time t, when Z,/
1. Integration
Z,=
of equation
(10) over
(11) Experimental 5.
values
of constants
The experimental
arc illustrated
A, B, and E vs. alkalinity
and calculated
in fig. 24 (17).
The crystal
size growth
histogram
of crystal
size distribution
scope measurements
growth
reacted
for different
growth
rate for crystals
towards
zero.
intervals
Curve
of fig. 26.
This rate passes
which stant
derived
from
pendent
begins
curve
when
of crystal
A crystallization
of time
from
the beginning the average
the linear
of
results
Curve samples
The period of crystals.
to decrease.
It implies
that
growth growth
linear
the time for rate curve 2
curve
of delayed
linear
by microof nutrient
asymptotically
From
crystal
The stage
1 of fig. 25
of constant
declined
the crystallization
process.
the
was obtained
in identical
1, the rate of linear
ac-
and
3 in fig. 26, remains
con-
crystal
mass
rates
are inde.
size. kinetic
model
hydrothermal
Na,O-HMDA-M,O,-SiO,-H,) M-Si-ZSM-5
comparing
growth
mixture
curve of fig. 25 gives the nucleation
When
for the spontaneous
nucleation
- B, Al, Ga, Ti, V, Cr and Fe) has been suggested tigation
of reaction
115 h and thereafter
of the crystallization
rate has begun
observed
size distribution
and crystal
in fig. 25 (18).
size distribution
1 and 2 of fig. 25 with curve stage
nucleation
on the curve
of heating.
for about
a maximum.
point
crystals
the size distribution
through
are shown Each
largest
size extended
of each mode
all over the autocatalytic
growth
of the lo-20
both
and particle
crystallization
in the final product
2 in fig. 25 describes
of crystals
during
crystals.
are given by Meise in table
of the reaction
as time rised,
zeolite
rates of largest
of different
nucleation
that
of NaX
of diameters
and temperature
for the course
It is concluded
celerate.
and 26 gives the linear
curves
type zeolites,
(figs. 27 and 28).
synthesis at obtained
4233.
of
by Feng this
The
by calculation
system Shouhua
type
of
crystallization from
the model,
of M-Si-ZSM
-5 type zeolites
@l
et al (90) on the basis of the inveszeolites
in
kinetic
curves
agree
the for
system the
of
various
well with the experimental
Hydrothermal
synthesis
and crystallization
of zeolites
-r.T EXPERIMENTAL _--
I
TBfE
CALCULATED
63
CURVES CURVES
(min)
_‘== EXPERIMENTAL . . ..* CURVES
0
1
4
2 CRYSTAL
Fig. 24.
DlAMETER
5
6
7
tr)
Influence of alkianity on zeolite A crystallization (top) and on crystal sizedistribution (bottom), showing agreement between experimenial and calculated values (17)
64
Xu Qinhua and Yan Aizhen
TIME(h)
Fig.
25.
Crystal
size growth of Na-X
of the gel(l)
and the histogram
in the final product
zeolite during of crystal
crystallization
size distribution
(2).
80 120 loo !m
240
TIME th)
Fig.
26.
Nucleation Curved)
kinetic
for Na-X
curve(2)
and the crystal
zeolite calculated
mass
from data
growth
of Fig.25.(18)
Hydrothermal
synthesis and crystallization of zeolites
*
ee d
0
Fig. 27.
50 40 30 al 10 CRYSR+LLIZATIoW TIME (h)
Crystallization kinetic curves for B-,AlSilica sourcfzwaterglass- II B-Si-ZSM-5 0 Al-Si-ZSM-5 Q Ga-Si-ZSM-5 e
60
and Ga-Si-ZSM-5.
203040050 CRYSTALLIZATH)NTIME (h) Pig.
26.
Crystallization
kinetic
CUTYCI for
Silica souruzwaterglass-D[ Ti(m)-Si-ZSM-5, 0 V(m)-Si-ZSM-5, 0 Cr(m)-Si)ZSM-5. 0 Fe(m)-Si-ZSM-5, e
Ti(m)-;
V(m)-,Cr(m)-
and Fe(m)-Si-ZSM-5
65
Xu Qinhua and Yan Aizhen
66
The rate of nucleation
can be expressed
by the equation (12)
dz’ / dt = k,t=g,(t) where Z’is
the number
the crystallization The crystal
of nuclei;
k,is the rate constant
of nucleation;
01is a exponental
constant;
and t is
time.
mass growth
rate in the following
form is obtained (13)
assuming
the radius
of a crystal
at time t, which
and k, as a constant
related
The molar
of the crystalline
phous
fractions
solid
gel phase
g, respectively, mole number
to the crystal
per
so that
unit
of zeolite crystals,
zeolite,
volume
g,+g,+g,=
nucleated
at time z(t 2 z), as r( r, t), it’s mass as g( z, t)
shape and density
of zeolite crystal.
of the corresponding
in the
If M is the molecular
1.
the crystal
liquid
crystallization
growth
system
weight
component are
of the zcolite,
rate can be expressed
and of the amor-
expressed and
as
g,,
g, and
c n, is the total
by the equation: (14)
dgt (1)
or -
= kg,O)Y,
df
(1)
(15)
where k is the crystal
growth
As described
the course
above,
crystallization nucleation main
and crystal
growth
of zeolites
is observed growth,
the
thoretical
aspects
study
one may expect cal ingenuity
the future include
reaction
The use of nonaqueous
It was also reported
in surface
been found
zeolite
VPI-5,
not exceed
stages
in the course
linearly
increases
become
with
and
various
been suggested.
The
of zeolite synthesis.
the crystallization
proportionally
phase
The
t in the
time
to the surface
area
of
mechanism.
relatively
Because
Different
have
sophisticated,
of the almost
more novel structures,
provided
the
unlimited
quantitative
and
numbers
of zeolite,
effort
and chemi-
sufficient
phases
in the AlPO,
system
12-membered
zeolites, by other
in the synthesis
in which elements,
5% replacement
will continue
systems
recently
with 18-membered
studies
in zeolite synthesis
and the exploration
has been explored Al or Si atom
in aluminosilicate
have been synthesized
of either
Si or Al.
of widely
open
net-
(91).
(14, 90).
This might
zeolite
frame-
Framework
sub-
still be an interesting
and catalysis.
zeolite
reported
path media
replaced
chemistry
some other
has been
of many
that heteroatom
or totally
will probably
New synthetic years
has
more attention.
synthesis
systems
on the basis of liquid
synthesis
much
process.
nucleation
increases growth
for calculation.
is deployed.
challenges
subject
of crystal
almost
of zcolite
radius
introduced
is a complicated
are the two main
crystal
i.e. the rate
still require
work is partially
than
that
in the system,
Although,
stitution
synthesis
for the non-spontaneous
presented
works.
of zeolite
models
crystallization
Most
and y,(t) is the function
kinetic
conclusion
zeolites
rate constant
by Wilson
by Davies ring pores,
ring pores.
to appear;
have been described. and Flanigen
et al (94). which
Their success
They
Beside the aluminosilicate zeolite in the recent The most promissing new class of zeolites has (92, 93). have
is the first molecular will certainly
fostor
A new large-pore
synthesized
an AlPO,
zeolite
sieves with pores consisting synthesis
endeavours
material
material,
named of greater
worldwide.
Hydrothermal
67
synthesis and crystallization of zeolites
REFERENCES 1.
R.M. Batrer, J.Chem. Sot., 2158(1948).
2.
R.M. Barter, ibid, 127(1948).
3.
KM.
4.
H. Robson,
Barrer and C. Mareilly,
CHbMTGCH,
I: Chen.
A, 2735(1970).
Sot..
March 176(1978).
5.
D.E.W. Vaughan,
6.
R.M. Barrer, Stud. Surf Sci. Cofal., 24, l(1985).
84,25(1988).
7.
W. Wicker and B. Fahlke, ibid, 24, 161(1985).
8.
H. Lechert, ibid, 18, 107(1984).
9.
L.P. Rollmnnn,Zeofifc:
Chem. hng. Progress,
Science end Technology,
H.E. Robson
Mnrtinus
10.
F. Roozeboom,
11.
G.T. Kerr, Calal. Rev.-&i.
Bug., 23,281(1981).
12.
R.M. Bnrrer, Hylrothermai
Chemialryof
13.
B. Subotic,
Nijhoff Publishers,
(1984). ~109.
and S.S. Chen, ibid, (1984). ~127. Zeolites, Academic
I. Smit, 0. Mndjijn nnd L. Sckovnnnio,
14.
D.W. Break, Zeoiife Molecular
15.
S.P. Zhdanov,
press, (1980).
Zcolitcs, 2, 133(1982).
Sieves, John Wiley & Sons, New York,
(1974).
Adv. Chem. Ser., 101, 20(1971).
16.
R.M. Milton, MolecuIor Sieves, Sot. Chem. Ind., London,
17.
W. M&e nnd F.E. Schwochow,
(1968). ~199.
Molecular Sicvcq Adv. Chem. Ser., 121, (1973). ~169.
18.
S.P. Zhdnnov
19.
D. Domine and J. Quoben,
nnd N.N. Snmulevich,
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69
70
Xu Qinhua, Professor of Chemistry Department in Nanjing University (Chemistry Department, Nanjing University, Nanjing 210008, P.R. China), graduated from Jinling College in 1951. Her teaching and research are mainly in the field of Physical Chemistry She has given courses on “Colloidol Chemistry”, “Adsorption”, “Catalysis”, and “Zeolite Chemistry” in Nanjing University; and published about 70 papers on the synthesis, adsorption and catalysis of zeolites. She obtained the National Prize for Zeolite Research in 1965 and 1978. She attended the 5th, 7th and 8th International Conference on Zeolites.
Yan Aizhen is an associate Professor of Chemistry Department in Nanjing University (Chemistry Department, Nanjing University, Nanjing 210008, P.R. China), where she graduated. Her interests are Physical Chemistry, Colloidal Chemistry and Radiochemistry. She has given courses on “Inorganic Chemistry”, “Radiochemistry”, “Catalysts and Catalysis” and “Adsorbents and its Applications”. Currently her research aspects include ion-exchange, adsorption science of micropore solids, especially zeolite and aluminophosphate molecular sieves, and its application to sorption refrigeration. She has published numerous articles and presented many papers at the National Conference and International Symposium on Zeolites. She is a member ofthe Chemistry and Chemical Engineering Society, and Petroleum Engineering Society of China.