Post synthesis modification/ 1 Molecular sieves / ed.: H. G. Karge; J. Weitkamp. 3, With contrib. by P. A. Anderson

413 Pages·2002·3.55 MB·English

by  Paul A Anderson

Preface to Volume 3
For many purposes,zeolites and related materials are not utilized in the as-syn-
thesized form.Rather,they are only employed after an appropriate post-synthe-
sis modification.
Undoubtedly,the classic procedure ofzeolite treatment after synthesis is that
of ion exchange achieved through treatment of a suspension of the as-synthe-
sized (or natural) zeolite powder (usually in the sodium or potassium form) in
an aqueous solution ofa salt containing the cations to be introduced.Starting in
the 1930s,this type ofion exchange has been extensively studied,not only as a
method ofpreparation,but also with respect to thermodynamics and kinetics.
Application on an industrial scale is well developed and, because of its im-
portance,ion exchange in zeolites has been reviewed several times.Thus,the
first chapter ofVolume 3 ofthe series “Molecular Sieves – Science and Techno-
logy”,which was contributed by R.P.Townsendand R.Harjula,was able to focus
on the developments and advances made during the last decade.It emphasizes
the need for improvement of theoretical approaches,utilization of the rapidly
growing computational power,and the importance ofacquiring reliable data as
the bases for progress in fundamental studies on conventional ion exchange.
The more recent development ofsolid-state ion exchange and related modi-
fication techniques such as reactive ion exchange between solid zeolite powders
and solid or gaseous compounds containing the cations we wish to introduce is
rather exhaustively dealt with in the subsequent chapter written by H.G.Karge
and H.K.Beyer.The concept of solid-state ion exchange is explained and con-
trasted to the conventional exchange process.Experimental procedures as well
as techniques for monitoring the solid-state modification of zeolites are de-
scribed in great detail and illustrated by a large number ofinvestigated systems.
Related methods ofpost-synthesis modification,possible mechanisms,and first
approaches to study the kinetics ofsolid-state ion exchange are discussed.
Post-synthesis modification of zeolites via alteration of the aluminum con-
tent ofthe framework became a most important topic ofzeolite chemistry when,
in the mid 1960s,the effect ofstabilization through dealumination was discov-
ered.In Chapter 3,H.K. Beyer contributes a systematic review on techniques
for the dealumination of zeolites by hydrothermal treatment or isomorphous
substitution amended by a section on the reverse process,i.e.,introduction of
aluminum into and removal ofsilicon from the framework.
Methods ofpost-synthesis modification essentially different from those dis-
cussed in the first three chapters are based on the generation of extra-frame-
X Preface to Volume 3
work aggregates ofmetals (as presented in the chapter by P.Gallezot),ionic clu-
sters (as described in the contribution by P.A.Anderson),and oxides and sulfi-
des (treated in the last chapter written by J.Weitkamp et al.).One of the main
motivations for studying the generation ofsuch clusters inside the void volume
ofzeolite structures originates,ofcourse,from possible applications in catalysis.
This is most evident in the case ofmetal cluster/zeolite systems which are suc-
cessfully employed in heterogeneous catalysis ofhydrogenation,hydrocracking,
hydroisomerization, etc. However, both ionic clusters and oxidic and sulfidic
clusters hosted by the frameworks ofzeolites are interestring candidates as cata-
lysts for base-catalyzed,redox,photocatalyzed and perhaps other reactions.In
view ofcluster formation with zeolites as hosts,questions ofsize,location,dis-
tribution,interaction with the framework,and stabilization ofthe active aggre-
gates play a decisive role.Thus,in all three contributions on clusters in zeolites,
methods oftheir preparation as well as problems oftheir characterization and
utilization as catalysts and photosensitive materials,as sensors,in optics,and
electronics are extensively dealt with.These areas are still challenging for future
resarch and promising in view ofpotential applications.
However,not all important phenomena of post-synthesis modification are
covered with the present six chapters ofVolume 3 ofthe series ‘Molecular Sieves
– Science and Technology’.Topics such as,for instance,‘Incorporation of Dyes
into Molecular Sieves’,‘Preparation of Ship-in-the-Bottle Systems’,‘Secondary
Synthesis in Zeolites’, ‘Pore Size Engineering’, ‘Modification of Mesoporous
Materials’are equally important and,to a large extent,presently subject to very
active research and development.Therefore,such topics will be dealt with in one
ofthe subsequent volumes under the title ‘Post-Synthesis Modification II’.
September 2001 Hellmut G.Karge
Jens Weitkamp
Ion Exchange in Molecular Sieves
by Conventional Techniques
Rodney P.Townsend1,Risto Harjula2
1 Scientific Affairs,Royal Society ofChemistry,Burlington House,Piccadilly,
London W1J 0BA,UK;e-mail:[email protected]
2 Laboratory ofRadiochemistry,PO Box 55,00014 University ofHelsinki,Finland;
e-mail:[email protected]
Dedicated to Professor Gerhard Ertl on the occasion ofhis 65thbirthday
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 The Importance ofIon Exchange Phenomena in Molecular Sieves 2
1.2 Origin and Nature ofIon Exchange Behaviour in Molecular Sieves 5
2 The Importance and Utility ofTheoretical Approaches . . . . . . . 9
2.1 Preference,Uptake and Selectivity . . . . . . . . . . . . . . . . . . . 9
2.2 Batch and Column Exchange Operations . . . . . . . . . . . . . . . . 13
2.3 Thermodynamic Parameters,Non-Ideality and the Prediction
ofExchange Compositions . . . . . . . . . . . . . . . . . . . . . . . 16
2.4 Kinetic Processes and the Prediction ofRates ofExchange . . . . . . 20
2.4.1 Hierarchical Model ofZeolite Particle or Pellet . . . . . . . . . . . . 21
2.4.2 Intraparticular Exchange Rate Processes . . . . . . . . . . . . . . . . 22
2.5 Trace Ion Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.6 Column Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3 Experimental Approaches . . . . . . . . . . . . . . . . . . . . . . . . 29
3.1 Practical Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2 Pitfalls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2.1 Selectivity Reversal and Ion Sieving . . . . . . . . . . . . . . . . . . 31
3.2.2 Zeolite Hydrolysis Effects . . . . . . . . . . . . . . . . . . . . . . . . 32
3.2.3 Colloidal Solids in Suspension . . . . . . . . . . . . . . . . . . . . . 36
4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Molecular Sieves,Vol.3
© Springer-Verlag Berlin Heidelberg 2002
2 R.P.Townsend · R.Harjula
1
Introduction
1.1
The Importance of Ion Exchange Phenomena in Molecular Sieves
Throughout the 1990s there was a decline in the number offundamental studies
carried out on the ion exchange properties ofzeolites and related materials.One
has only to examine the content of published conference proceedings on the
subject over the last 20years to observe this trend:the situation has moved from
one where whole sessions were devoted to ion exchange studies,to one where
the subject is subsumed into sessions covering other areas.Part ofthis decline is
to be expected,as increased attention has been rightly paid to the intriguing
possibilities that can arise through the exploitation of newer alternative post-
synthesis methodologies,many ofwhich are discussed elsewhere in this volume.
Nevertheless, the fact remains that conventional ion exchange techniques
continue to be used routinely for post-synthesis modification during the prepa-
ration ofmolecular sieves for major industrial applications.Also,there are now
areas where molecular sieves find major application directly as ion exchangers
perse.In this respect the situation has changed markedly since the early 1960s,
when Helfferich,in his classic book on ion exchange,could justifiably describe
zeolites “as ion exchangers they are of little practical importance”[1]. These
direct applications are especially detergency [2–7] and also the removal of
nuclear waste [8–13] or other environmental pollutants [3]. However, it is
generally a combination of properties of a particular zeolite in addition to its
ionexchange capability that has tipped the balance in favour of its use,rather
than any intrinsic superiority per se, which the zeolite may possess as an ion
exchanger.
If,therefore,conventional ion exchange remains an important post-synthesis
preparative technique,and the materials have in addition major direct applica-
tions as ion exchangers,why have the number offundamental studies decreased?
It is certainly not because ion exchange behaviour of molecular sieves is suffi-
ciently well understood and predictable to render further fundamental research
studies unnecessary.Two causes are suggested to explain this decline:
1. Many theoretical treatments of the ion exchange reaction within zeolites
(both equilibrium and kinetic) are obscure and complicated.This has with-
out doubt rendered inaccessible the real value of the work to those many
workers who have a practical need to predict and control ion exchange be-
haviour during the industrial exploitation of molecular sieves. Although
theoretical understanding is important,it is easy to forget that the end pur-
pose of such work should be to provide information and tools that the
chemical engineer or other user ofthe molecular sieve can applysimply and
effectively.Obscurities in theoretical treatments mean that users often do not
appreciate how basic theory can be used,not just to simplify the number of
measurements which need to be made,but also to predict and control be-
haviour during application.The theory should not be an end in itself!
Ion Exchange in Molecular Sieves by Conventional Techniques 3
2. The second cause is related to the first.Even where the value oftheory for the
prediction and control of the behaviour of these materials has been recog-
nised,the utility ofthese approaches has often been greatly reduced because
of the experimental methods which have been employed or by the poor
experimental data which have been available,or both.Indeed,it is only com-
paratively recently that a proper recognition has arisen concerning the num-
ber of potential pitfalls and difficulties that can militate against the acquisi-
tion ofmeaningful and accurate experimental data.
A good example ofthis is the frequently studied Na/Ca-zeolite A system,which
has received much attention because of its importance in detergency applica-
tions.Careful and detailed experimental studies over a period spanning some
20years by different sets ofworkers [14–20] resulted in calculated values ofthe
standard free energy ofexchange (kJequiv–1*) which ranged from –0.59 [14] to
–3.09 [17]. Plots of the corrected selectivity coefficient (defined below; see
Fig.1. Plots of the logarithm of the corrected selectivity coefficient ln K [cf. KE in
G – A/B
Eq.(7b)]as determined by different workers for the Na/Ca exchange in zeolite A.E is the
Ca
equivalent fraction of calcium in the zeolite [(Eq.(3b)].BRWBarrer,Rees and Ward [14];
AAmes [15];WFWolfand Furtig [16];SWSherry and Walton [17];BRBarri and Rees [18];
WGCWiers,Grosse and Cilley [19];FTFranklin and Townsend [20].Taken from [8]
* Throughout this paper the term “equiv”denotes 1mol ofunit negative or positive charges.
4 R.P.Townsend · R.Harjula
Eq.7b) naturally show a similar diversity but also differ from each other in curve
shape and trends (Fig.1).These marked differences (particularly at the extrema
of the plots) were variously ascribed to experimental error [20], to variable
quantities of non-exchangeable sodium in the materials employed [20] (the
materials differed in their source and in their method ofpreparation [14–20])
or to variable levels ofhydronium exchange depending on the pH and other con-
ditions used [20,21].
Thus,even for this very important example,not only is some ofthe published
theoretical work difficult to interpret,but also experimental data from different
studies are frequently incompatible and incomplete.
It is essential therefore that a critical review ofadvances over the last decade
should look at the developments in the context ofthe field as a whole.This is our
intention here. After a discussion of the origin, ubiquity and nature of ion
exchange behaviour in molecular sieves,recent advances in the application of
thermodynamic and kinetic descriptions of the ion exchange process will
be described. This will demonstrate some of the shortcomings of current
approaches, together with the relative paucity of reliable literature data that
can be applied easily and practically. This whole topic has particular rele-
vance to those industrial applications where zeolites are used directly as ion
exchange materials and this will be exemplified throughout the chapter using
two main examples.The first ofthese is the application ofA- and P-type zeolites
as detergent builders,where the approach is to use a batch exchange approach
to remove hardness ions (especially calcium) as fast as is practicable before
the indigenous water hardness harms the wash performance of the detergent
product.The second concerns the treatment of nuclear waste,where a variety
of higher silica zeolites have been employed using a continuous (column)
process to remove,and subsequently store,high concentrations of monovalent
and divalent radionuclides such as caesium and strontium. For both these
major applications,in addition to selectivity,it is noteworthy that the systems
are normally multicomponent, that the kinetics of exchange are all im-
portantand that the morphology ofthe exchanger material must be controlled
carefully.
Post-synthesis modification comes into its own when preparing molecular
sieves with desirable and exploitable properties other than those of ion ex-
change,be they optical,magnetic,catalytic or adsorptive.Here it is not directly
the thermodynamic and kinetic ion exchange properties that are of prime
importance but rather which experimental,preparative methods are most com-
monly used.Thus it is important to assess what are the most appropriate exper-
imental methods of preparation,as well as to review the many pitfalls one can
fall into which can subsequently give rise to very inaccurate and inadequate
experimental data. These experimental problems can include framework
hydrolysis,hydronium exchange,dealumination,the presence ofkey trace impu-
rities,dissolution phenomena,carbonate and bicarbonate interference,colloidal
phenomena,metal ion complex formation and cation hydrolysis.
Having thus reviewed developments and advances over the last decade,the
chapter concludes with some recommendations on directions and topics for this
area ofresearch in the future.
Ion Exchange in Molecular Sieves by Conventional Techniques 5
1.2
Origin and Nature of Ion Exchange Behaviour in Molecular Sieves
Ion exchange is a characteristic property manifested by most molecular sieves.
In essence,whenever isomorphous replacement ofone cation by another ofdif-
ferent charge occurs within an initially neutral crystalline framework such as a
pure silica molecular sieve,then a net electrical charge remains dispersed over
that framework.This is neutralised through the presence,within the micropo-
rous channels,of cations of opposite charge (often referred to as counterions).
An example ofthis is seen in the introduction by direct synthesis ofsmall quan-
tities of aluminium into the silicalite framework to give the material ZSM-5.
Silicalite,the pure silica analogue ofZSM-5,is then seen to be just the end-mem-
ber of a set of isomorphous microporous molecular sieves that exhibit ion
exchange properties which are a function of the quantity and distribution of
aluminium atoms within the structurally similar frameworks.In addition,since
one can prepare,through post-synthesis modification of the framework com-
position,a variety of other isomorphous metallosilicates and metal alumino-
silicates,it is obvious that zeolites possessing ion exchange capabilities are a
common occurrence.
Pure aluminium phosphate molecular sieves are probably more common
than are pure silica analogues of zeolites.They resemble pure silica zeolites in
that they possess frameworks that are electrically neutral,but there is a signifi-
cant difference between these two classes of inorganic solids. In topological
terms both are 4:2 connected nets of T:O atoms (“T” denoting tetrahedral
framework and “O” denoting oxygen). From this it is obvious that it is only
required for the T ion to have a charge of +4 for the connectivity of the net to
give rise naturally to a neutral framework in concert with the oxide anions.This
is fulfilled for pure silicalite.In the case ofALPO molecular sieves the require-
ment is also fulfilled,but the 4:2 T:O net now comprises two types of strictly
alternating T-cations (aluminium and phosphorus,possessing respectively for-
mal positive charges of3 and 5).Providing the cations alternate strictly through-
out the framework, the 4:2 Al,P:O net holds no overall charge; however, in
contrast to a pure silica zeolite,where the formal charge at every atomic centre
is zero,within a pure AlPO the formal charge is not dispersed homogeneously,
but changes from –1 at each aluminium to +1 at each phosphorus.This greater
heterogeneity of charge distribution may in part explain the experimental
observation that ALPOs frequently exhibit poorer thermal stability than do pure
silica zeolites.
For a particular ALPO molecular sieve to possess an ion exchange capacity as
an intrinsic property,it is necessary to prepare a material where some ofthe alu-
minium and/or phosphorus framework atoms have been replaced by other
atoms ofdifferent charge.This can occur using for example silicon,to form the
so-called SAPO materials,or with metals in addition or not to silicon,to form
respectively the so-called MeAPSO and MeAPO analogues.However,it is impor-
tant to note that although silicon could in principle replace either aluminium or
phosphorus to give rise to positively or negatively charged SAPO molecular
sieves,respectively,in practice only the latter process seems to occur,or another
6 R.P.Townsend · R.Harjula
process in which two silicons replace one ofeach ofaluminium and phosphorus,
which gives rise to no net change in framework charge [22].In MeAPSOs,diva-
lent or trivalent metal ions replace the aluminiums in the framework.In this way
the charge imbalance is minimised as these isomorphous substitutions either
make no difference to the overall framework charge (T3+ for Al3+) or only
increase it by one negative charge per substitution (e.g.Mg2+for Al3+),a process
analogous to when aluminium replaces silicon in aluminosilicates [22].
Overall therefore,and in common with aluminosilicate zeolites,the norm is
for MeAPSOs and MeAPOs to possess cation exchange properties rather than
the reverse.In this respect,zeolites and ALPOs resemble many other classes of
ion exchangers that are mineralogical in origin,such as the clay minerals.These
are layered materials where a cation exchange property can arise primarily from
isomorphous replacement oftrivalent cations by divalent,or tetravalent cations
by trivalent ones,within the layers [23].However,there is a major exception:
these anionic exchangers are the double metal hydroxides,which are also lay-
ered structures but which exhibit a net positive charge across the lattice.The
“parent”material here is the mixed Mg,Al hydroxide,commonly referred to as
hydrotalcite.It would be intriguing to understand better the conditions (ifany)
under which one might expect to synthesise microporous three-dimensional
framework structures which similarly have a net positive charge dispersed over
the lattice and hence an anion exchange capacity coupled with a molecular sieve
capability.
It is important to note that,up to this point,we have been considering the zeo-
lite,ALPO,SAPO,etc.,as being described adequately as a 4:2 T:O net.This topo-
logical description,which in general terms is,as Smith points out [24],nothing
more than a mathematical construct of the human brain, does nevertheless
allow us to appreciate both the origin and magnitude ofan ion exchange capac-
ity arising from T-atoms being replaced by others ofdifferent charge.However,
this description is not sufficient to cover the observed differences in ion
exchange properties(i.e.selectivity,kinetic rate,level ofexchange) that may be
seen between various molecular sieves having similar exchange capacities.To
understand these differences,one must not only examine more closely the topo-
logical properties ofthe nets but also bring to bear structural considerations.
Considering these topological properties in more detail,it is adequate at this
point to take as read that all the T-atoms within the microporous net are joined
to each other by bridging oxygens. One can therefore concentrate on the T-
atoms only and describe molecular sieves in terms of four-connected three-
dimensional (4-conn.3D) nets ofT-atoms [25] that,in turn,can be derived from
appropriate 3-conn.2D nets [26].Considering the latter nets first,these differ
from one another in the ways the nodes (T-atoms) link to each other via net-
works ofpolygons.Any node can then be described by its “vertex symbol”,viz.
by its surrounding polygons with the number of each type of similar polygon
surrounding the node being denoted by a superscript [26].Thus the simplest
example ofa 3-conn.2D network (the hexagonal net) becomes a 63-net;a more
complicated example could be the 4.6.12-net which forms the basis for the
gmelinite structure [26].Note that all the nodes within each ofthese two sepa-
rate examples are topologically equivalent.This need not be the case.For exam-
Ion Exchange in Molecular Sieves by Conventional Techniques 7
Fig.2. Structure ofmordenite viewed along the main 8-ring and 12-ring channels parallel to
the c-axis.Four topologically distinct types of T-atoms are observed within the 3-conn.2D
(4.5.8) (4.5.12) (5212) (5.8.12) net
1 1 1 1
ple, consider the case of mordenite (Fig. 2), which is derived from a (4.5.8)
1
(4.5.12) (5212) (5.8.12) -net containing four topologically distinct types of T-
1 1 1
atoms [24].
Similar considerations apply when one considers the 4-conn.3D nets that
constitute molecular sieves.Here it is often convenient to describe the structure
in terms ofpolyhedral units or cages,with the polyhedra described topological-
ly in terms offace symbols [25] (not to be confused with vertex symbols defined
above).Thus the face symbol for the familiar sodalite unit,which is geometri-
cally a truncated octahedron,is 4668with all vertices geometrically and topolog-
ically equivalent. If these units are then linked together, for example either
through their 4-windows or half their 6-windows,one forms respectively the
zeolite A and faujasitic structures.Both these structures possess cubic symme-
try, with each structure comprising 26-hedral cages connected to each other
throughout the microporous zeolite framework,but the vertices ofthe sodalite
units are no longer all topologically equivalent.For zeolite A the sodalite units
enclose a cage which is the great rhombicuboctahedron (4126886) [25] whereas
for faujasite the cage is the so-called 26-hedron type II,denoted by the face sym-
bol 4641264124[25].
Why are these matters significant when one considers the ion exchange prop-
erties ofmolecular sieves? The answer is that these topologically non-equivalent
T-atoms combined with the overall structural properties of the three-dimen-
sional microporous framework often give rise to several very different types of
local environments which repeat themselves regularly throughout the crys-
talline structure.These different local environments,evidenced by solid state
NMR combined with X-ray crystallography [27],are distinct in themselves,dif-
fering from each other sterically and electronically,and these differences will be
8 R.P.Townsend · R.Harjula
manifested not only through their characteristic adsorptive and catalytic behav-
iour,but also through their ion exchange properties.Formally,therefore,zeolites
may be regarded as comprising a set ofcrystallographically distinct sublattices,
each having characteristic selectivities for different exchanging cations,depend-
ing on these local environments [28].The overall ion exchange behaviour of a
molecular sieve can therefore be a subtle function of the structural and topo-
logical properties combined.An important combination ofstructural and topo-
logical properties concerns the ordering of isomorphously substituted frame-
work atoms [29]:this determines what fraction ofthe overall framework charge
is found on each sublattice.Other significant structural properties can be losses
in symmetry through restricted rotation [27],and whether the sites are accessi-
ble to exchanging cations (i.e. the sizes of the micropore channels allowing
ingress and egress ofexchanging cations plus water).
A further point is worth emphasising:since site heterogeneity in a particular
zeolite is manifested through such a set of crystallographically distinct sublat-
tices,zeolites differ in this respect significantly from some other common class-
es ofion exchangers,such as the clay minerals or the resins.Whereas in zeolites
well-defined sites are repeated regularly through the crystalline matrix,in clay
minerals and resins site heterogeneity is often manifested in terms ofpatches,or
regions of the surface where the sorption energies are approximately constant
[30].Thus a statistical thermodynamic model ofion exchange for clay minerals
and resins [30] can differ markedly in character from ones developed for zeolites
[31,32].
As a consequence of all these factors combined,both the equilibrium and
kinetic aspects of selectivity and uptake of ions within molecular sieves can
rarely be understood in a straightforward manner. Phenomena which have
received either considerable attention in recent years or deserve further study
include the so-called “ion sieve effect”,behaviour of high silica materials,the
effects that framework flexibility can have on selectivity and rates ofexchange,
multicomponent ion exchange,prediction ofexchange equilibria,and the possi-
bility ofinducing phase transitions within zeolites through ion exchange.Many
ofthese are considered further below.
So far we have considered topological and internal structural factors which
give the molecular sieve particular ion exchange properties.However,an ion
exchange capacity can also be manifested which is not an intrinsic property of
the material.The source ofthis property is unsatisfied valencies occurring at the
termination of the crystal edges and faces, or at faults within the crystalline
structure.In formal terms,the origin ofthis is topological,in that this inciden-
tal and secondary property arises from disruptions in the net at interfaces,sur-
faces and faults,but the nature and extent of this incidental property depends
essentially on structural and morphological characteristics.For the former,we
can take as an example an ion exchange capacity arising either from the pres-
ence ofsilanol groups [33,34],or from hydroxyl groups attached to aluminium
atoms situated at the surface [35].In clay minerals,as much as a fifth ofthe total
exchange capacity may arise from such sources whereas in the case of zeolites
the contribution of such incidental (or secondary) ion exchange properties is
usually small compared to the intrinsic,or primary source.The exception here