Synthetic swelling clay minerals

Details Synthetic swelling clay minerals Synthetic swelling clay minerals

1997-11-10

2001-02-13

Wu, David W. (Department: 1713)

Catalyst, solid sorbent, or support therefor: product or process

Zeolite or clay, including gallium analogs

Clay

C502S063000, C502S084000, C502S263000

Reexamination Certificate

active

06187710

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to new, synthetic swelling clay minerals, as well as to a process for the preparation of such clay minerals.
Clay minerals are solid substances, substantially made up of metal and oxygen atoms, whose crystal lattice has a layered structure. This layered structure consists of three repeating layers. Located centrally in this elementary three-layer structure is a layer of substantially trivalent or substantially divalent metal ions (cations). Examples of clay minerals with substantially trivalent ions are montmorillonite and beidellite; examples of clay minerals with substantially divalent ions are hectorite and saponite. The metal ions present in the central layer are octahedrally surrounded by oxygen and hydroxyl ions. In a clay mineral with trivalent ions, two of the three octahedron positions are occupied by metal ions. Accordingly, this is referred to as a di-octahedral clay mineral. In a clay mineral with divalent metal ions, all three octahedron positions are occupied by metal ions; this is referred to as a tri-octahedral clay mineral. On opposite sides of this layer of octahedrally surrounded metal ions occurs a layer of tetrahedrally surrounded ions. These tetrahedrally surrounded ions are generally silicon ions, while a part of the silicon can optionally be replaced by germanium. The unit of the tetrahedrally surrounded silicon ions is Si
2
O
5
(OH). In this connection it is noted that in the tetrahedron and octahedron layers the actual point where the charge is located cannot always be indicated equally clearly. The term ′ions′ as used in this context accordingly relates to the situation where an atom, given a completely ionic structure, should possess an electrostatic charge corresponding with the oxidation state.
Essential to clay minerals is that a part of the cations present are substituted by ions of a lower valency. Thus it is possible to substitute a part of the trivalent or divalent metal ions in the octahedron layer by divalent and monovalent metal ions, respectively. With substantially trivalent metal ions, this substitution results in montmorillonite and with substantially divalent metal ions in hectorite. It is also possible to substitute the tetravalent silicon ions in the tetrahedron layers by trivalent aluminum ions. With a clay mineral with almost exclusively trivalent ions in the octahedron layer, the result is then a beidellite and with a clay mineral having almost exclusively divalent ions in the octahedron layer, the result is a saponite. Of course, substitution by an ion of lower valency leads to a deficiency of positive charge of the platelets. This deficiency of positive charge is compensated by including cations between the platelets. Generally, these cations are included in hydrated form, which leads to the swelling of the clay. The distances between the three-layer platelets is increased by the inclusion of the hydrated cations. This capacity to swell by incorporating hydrated cations is characteristic of clay minerals.
If no metal ions or silicon ions are substituted by ions of a lower valency, the platelets are not charged. The mineral then does not absorb any water into the interlayer and therefore does not swell. The mineral with exclusively aluminum in the octahedron layer and silicon in the tetrahedron layer is pyrophyllite and the mineral with exclusively magnesium in the octahedron layer and silicon in the tetrahedron layer is talc. The swelling clay minerals having a negative charge of from 0.2 to 0.6 per unit cell, −O
10
(OH)
2
, are known as smectites.
The cations in the interlayer of swollen clay minerals are strongly hydrated. As a result, these ions are mobile and can be readily exchanged. The exchange is carried out by suspending the clay mineral in a concentrated solution of the cation to be provided in the interlayer. The high concentration provides for a concentration gradient as a result of which the exchange proceeds. Upon completion of the exchange, the concentrated solution is removed by filtration or, preferably, by centrifugation and washing, whereafter, if necessary, the last metal ions not bound in the interlayer can be removed by dialysis.
The negative charge of the platelets can be compensated not only with hydrated cations, but also with (hydrated) hydrogen ions, H
3
O
+
. In this case the clay can function as a solid acid, which leads to important catalytic applications. Suspending a clay mineral in a concentrated acid does not lead without more to the provision of hydrogen ions in the interlayer. In fact, it has been found that the acid reacts with the cations of the clay structure, so that these ions are removed from the clay structure. These cations eventually end up in interlayer positions.
If it is desired to provide Brønsted-acid groups in a hydrated clay mineral, in general hydrolysing metal ions are provided in the interlayer. As a result of the hydrolysis, hydrogen ions are formed. Upon reduction of the amount of water in the interlayer, for instance through thermal desorption, the acid strength increases. Due to the lesser amount of water, the residual water molecules are polarised more strongly by the metal ions. Upon complete removal of the water, however, the Brønsted-acid groups disappear. If it is desired to impart Brønsted-acid properties to clay minerals at elevated temperatures, (hydrated) ammonium ions can be provided in the interlayer. Upon heating, the water and the ammonia escape while a proton remains behind.
Natural clay minerals have long been used for the practice of catalytic reactions in liquid and in gaseous phase. In general, the catalytic activity of clay minerals is based on the presence of Brønsted- or Lewis-acid groups in the clay minerals. In the conventional acid-catalysed reactions in the liquid phase often sulfuric acid is used. This acid yields Brønsted-acid groups while, moreover, it can dehydrate in that it has strong water-binding properties, and can take up undesired higher molecular by-products. What results, however, are large amounts of polluted sulfuric acid, acid tar, for which it is difficult to find any use. Neutralisation of large amounts of sulfuric acid used as catalyst leads to ammonium sulfate, which can be disposed of as less high-grade fertilizer, which is useful only for a business which also produces and/or sells other kinds of fertilizer.
In syntheses where Lewis-acid catalysts are needed, such as the Friedel-Crafts synthesis, metal chlorides, such as aluminum chloride, are used as catalyst. Hydrolysis of the aluminum chloride upon completion of the reaction leads to large amounts of highly corrosive suspensions of aluminum hydroxide.
Accordingly, both the use of sulfuric acid and the use of Lewis-acid catalysts, such as aluminum chloride or zinc chloride, entail drawbacks. Therefore, there is a need for solid acid catalysts that are suitable for carrying out such acid-catalysed reactions. Accordingly, one of the objects of the invention is to provide such solid acid catalysts for carrying out reactions in the liquid and/or gaseous phase, which are catalysed by Brønsted- and/or Lewis-acids.
Of great importance in this connection is the degree of hydration of the clay minerals. If water-immiscible, liquid reactants are to be processed, the presence of water on the surface of clay minerals prevents the required intensive contact between the reactants and the clay surface. The water will preferentially wet the clay surface. In many liquid phase reactions, therefore, it will be necessary to priorly dehydrate the clay mineral to be used. This must take place without any substantial reduction of the accessible clay surface. Also, the reagents used will generally have to be dried to a far-reaching extent.
Another important problem with the use of solid catalysts in liquid phase reactions is the separation of the catalyst from the reaction mixture. Generally, this is effected by filtration or centrifugation. The known, mostly natural, clay minerals generally lead to a compressible filter c

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