Chemistry of inorganic compounds – Zeolite
Patent
1996-12-16
1999-07-20
Bell, Mark L.
Chemistry of inorganic compounds
Zeolite
423713, 423DIG28, C01B 3906, C01B 3932
Patent
active
059253290
DESCRIPTION:
BRIEF SUMMARY
This invention relates to zeolites and to a method of producing zeolites having increased amounts of metals instead of aluminium in zeolites.
Zeolites are hydrated aluminosilicate minerals which occur naturally, but which have increasingly been made synthetically. Zeolites have a three dimensional structure arising from a framework of basic units of SiO.sub.4-.sup.4 and AlO.sub.4.sup.5- tetrahedra connected through their corners of shared oxygen atoms to form polyhedra. These basic units combine to form open frameworks containing channels and cavities in which cations and water molecules are located.
Cations are held within the structure due to the slightly negative overall charge of the tetrahedra.
Much investigative work was originally carried out with synthetic zeolites to study the effect of variations in the silica/aluminium ratio. Ratios of 1600:1, i.e. silicalite and 1:1, i.e. zeolite A resulting. Subsequently, work extended into studying the substitution of atoms into the tetrahedra of the building blocks either in replacement of silica or aluminium. In particular, with regard to aluminium, it is known to replace the aluminium atoms to limited extents with titanium, iron, cobalt and other metallic atoms. Replacement of silicon to limited extents with germanium, phosphorous and other elements is also known. Such substitutions or variations are known to dramatically affect the zeolites performance both in relation to catalytic activity and/or other properties.
Zeolites are normally prepared from a silica source (such as sodium metasilicate or fume silica), a source of aluminium (for instance aluminium wire or an aluminium salt) and counter cations (usually an alkali metal hydroxide such as potassium hydroxide), together with water.
There are two basic methods by which attempts have been made to replace aluminium atoms within the structure or to produce Zeolites supplemented with other metal atoms. The first technique involves producing the aluminium/silicon zeolite in the normal manner and then stripping aluminium atoms from the structure of the crystal by chemical means; practically simultaneously metal atoms from solution are substituted in to the structure. This technique allows for only limited substitution of the aluminium, where a significant amount of aluminium is present within the structure. Attempts to substitute greater than 30% of the aluminium have resulted in the crystalline structure of the zeolite collapsing before the substitute metal atoms can be incorporated within the tetrahedra. This is particularly true of types such as zeolite L.
The second technique involves the preparation of a reaction mixture containing aluminium, silica, water and caustic soda together with a salt of a transitional metal which competes with the aluminium during the zeolite's formation. This results in the substitute metal's incorporation within the crystalline structure to a limited extent. Chlorates and nitrates of the transition metal are commonly used as the source forms. This technique also has an upper limit of 30% or so of the aluminium being replaced as is shown for instance in PCT Application WO 92/13799. Above this figure very poor crystallisation and/or poor stability of the product occurs, with the result that practically amorphous product results.
Thus whilst high level substitution has previously been claimed the present invention aims to provide a method for and materials which actually have such high levels of substitution.
It is strongly desirable to produce zeolites containing greater than 30% or so of aluminium substitutes, such as Fe, Mn, V as these zeolites potentially offer improved or even substantially new catalytic ion-exchange and/or other properties. It is the manufacture of such high content substitute metal zeolites which is the problem addressed.
According to the first aspect of the invention, we provide a zeolite in which the Si:Me ratio is 4:1 or less (for example 3:1), where Me is any metallic element other than aluminium and Me is introduced to the zeolite in tetrahedral ox
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Latham Kay
Williams Craig D.
Bell Mark L.
Sample David
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