|
The feldspar structures of the previous section, and shown
below,
are made by nature's crucibles - volcanoes which furnish the fiery
high temperature conditions which favor the formation of these dense "igneous"
rocks.
Feldspar structures.
Nature is, however, a wonderfully varied synthetic chemist, and
has learned to make other subtler structures by using gentler conditions.
In the deserts of Nevada in the United States of America huge deposits are
found of a low density aluminosilicate known as phillipsite.
These minerals were synthesized
millions of years ago when this land lay under water. High concentrations
of silicon and aluminium in the water at the bottom of this ancient ocean
slowly crystallized out with SiO4 and
AlO4 tetrahedra linking together
to form not a compact, dense but an open structure which is porous at the
atomic level - a microporous structure.
Phillipsite.
These natural synthetic conditions may
be reproduced, refined and varied in the laboratory. And there are now
a wide range of both synthetic and naturally occurring microporous aluminosilicates,
known as zeolites. Some of the simplest and most important structures
are built from the high symmetry cubo-octahedral cage which
may be described as a truncated octahedron, with a ring of four silicon
or aluminium atoms (linked by oxygens) replacing each of the corners and
a ring of six Si or Al making each of the faces.
These cages may themselves
be fused across the four rings as shown to generate
the structure of the natural and synthetic mineral sodalite. An alternative
and cleverer principle used by these structures is to bridge across
the four rings. In fused structures the four rings are shared between neighboring
cages; in bridged structures, four rings in one cage are linked to similar
rings in a neighboring cage by oxygen atoms which connect the tetrahedral
atoms (Si or Al) in the two rings. The resulting structure shown is
known as Zeolite A and is widely used in detergents. Bridging across
six rings generates another structure - Zeolite Y illustrated
which is used on an enormous scale in the petrochemicals industry
as a "cracking catalyst". Long chain molecules of the type present
in heavy oils may diffuse through the pores
of this structure and into the large cavity shown in where
they may be broken down into shorter chain molecules - of the right length
for petrol/gasoline.
Hydrocarbon molecules and zeolite Y.
The structures of the zeolites ZSM-5 (left) and mordenite (right).
Other structures are illustrated on the left.
Zeolite ZSM-5, shown contrasts with Zeolite
A and Y discussed above in that it is built from 5 and 10 rings.
Zeolite structural elements and zeolite structures.
A fascinating
variety of channel structures is also found. This ZSM-5 has interpenetrating
straight and wavy (sinusoidal) channels; mordenite has a
purely straight channel structure.
Indeed in these microporous structures nature seems to be exploring as
widely as possible the topological possibilities of corner shared tetrahedral
networks.
Since zeolites are aluminosilicates, they
normally contain metallic ions that neutralize the negative charge of the
aluminosilicate framework; these commonly occupy well defined sites close
to the rings in the cage structures. The image on the left highlights
the positions
occupied by cations in the Zeolite Y structure; especially interesting
is the prism position located between the bridged six rings in the structure
which provides a low energy site for many metal ions.
It is important to realize,
however, that these structures are only seen in 'dehydrated' materials
in which excess water which is always present in the structures after synthesis,
has been driven off (usually by heating). The water molecules present in
'hydrated' systems stick to and surround the metal ions which then occupy
ill-defined sites in the voids and channels. These loosely bound hydrated
metal ions can easily be taken out of the structure and exchanged for other
metal ions. And 'ion exchange' is indeed one of the oldest applications
of these minerals. For example, Zeolite A is effective at exchanging hydrated
sodium ions in its pores with calcium and magnesium ions in surrounding
water - the ions that are responsible for the 'hardness' of water as they
form precipitates (insoluble substances) with the species present in soaps
and detergents producing
the unsightly scum familiar to those (like the residents of London) whose
water supply is hard. The ion exchange process effected by Zeolite A (and
other zeolites) 'soften' the water and zeolites are now widely used in
detergents as water softeners; they are more environmentally friendly than
the alternative phosphate systems which act as nutrients for microrganisms
thereby disturbing the ecological balance of rivers and lakes into which
detergent wastes are discharged. A more spectacular and 'high tech' application
of the high ion exchange capacity of zeolites was in the use of the naturally
occurring clinoptilolile in removing radioactive metals from
contaminated water after the Three Mile Island Nuclear Accident in 1982.
This zeolite was able to extract completely a wide and diverse range of metallic
ions from this dangerously contaminated effluent.
Clinoptilolite.
Ion binding sites in the zeolite Y structure.
A simpler alternative to neutralisation
of the negative charge of the framework by cations is to incorporate protons
(or H+ ions), which stick to the bridging oxygens and provide highly acidic
groups. Zeolites may therefore
behave as exceptionally strong solid acids which allows them to initiate
a range of reactions in molecules present in their pores. The type
of reaction will, however, be influenced to a pronounced extent by the
pore architecture, which controls the shape of the molecules that can enter
and leave the pores, and the type of reaction which can take place within
them. Zeolites, owing to their acidity, are therefore potent catalysts;
but the catalysis is shape selective; it is controlled by the pore
and void structures of the zeolite. These unique catalytic properties are
widely exploited in industry. We have already mentioned the process of
catalytic cracking effected by zeolites in which acid centers attack long
chain hydrocarbon molecules, which are broken down into shorter chain species.
The reverse process we saw was effected by ZSM-5 which acts as a hydrocarbon
synthesis catalyst. Other reactions such as isomerisation (changing shape)
and alkylation (adding of carbon atoms) are also of major industrial importance.
Indeed zeolites are to the inorganic world what enzymes are to molecular
biology.
Hydrocarbon molecules packed within the pores of ZSM-5.
The microporous architecture of these materials
results in a third range of applications in gas separations. Different
shaped molecules are sorbed into and diffuse through these structures to
different extents. Zeolites may therefore be used to separate mixtures
of different organic molecules and even the nitrogen and oxygen molecules
present in our air.
Detailed understanding of both catalysis
and sorption require knowledge of the way in which molecules fit into the
pores of these zeolites. Computational methods are now providing extraordinary
insight into these processes.
The trajectory of a small hydrocarbon molecule diffusing through ZSM-5.
Computational methods also beginning to provide insights
into one of the greatest mysteries
of zeolite science - the mechanism of synthesis. It has long been known
that to synthesize certain pore architectures, it is necessary to introduce
organic bases (nitrogen containing organic molecules) to the synthesis gel.
Computational methods are now showing how these species fit inside and stabilize
the zeolite structure as shown.
Molecules can template or direct the formation of certain zeolite structures.
Microporous science has continued to diversify
with the extension of the class to include aluminophosphates (built up
of AlO4 and PO4
tetrahedra), which allow new pore architectures, like the
exceptionally wide pore material 'VPI-5' discovered by Mark
Davis at Virginia Polytechnic.
And considerable excitement has been generated
recently by the discovery of silica systems with exceptionally wide pores
(so-called mesoporous materials) whose architecture is also illustrated.
There is no doubt that the science of this extraordinary range of
materials still holds many surprises.
The microporous materials represent probably
the most sophisticated and complex of crystalline (that is, ordered) inorganic
structures. Their applications in catalysis and gas separation also illustrate
the interface behind the inorganic and organic worlds which is of such
importance in contemporary chemistry.
The structure of VPI-5.
An example of a mesoporous material.
|