Separation process

Details Separation process Separation process

2000-05-05

2002-05-21

Killos, Paul J. (Department: 1623)

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

Solid sorbent

Aluminum containing

C502S080000, C562S485000, C562S493000

Reexamination Certificate

active

06391823

ABSTRACT:

This invention relates to a process for separating a compound from a mixture of different compounds.
BACKGROUND OF THE INVENTION
Processes for separating compounds from mixtures are of great importance, both in the laboratory and on an industrial scale. The purity of chemical compounds is dictated to a large extent by the purification step in which a compound is separated from other products of the reaction in which it is produced. The separation of a compound from its isomers, such as its regio and geometric isomers, can be particularly difficult to achieve. Conventional methods for separating a compound from its isomers include crystallisation and chromatography but these techniques can be relatively costly and time consuming and they do not always provide a sufficiently high degree of separation.
Layered double hydroxides are a class of compounds which comprise two metal cations and have a layered structure. A brief review of layered double hydroxides is provided in
Chemistry in Britain
, September 1997, pages 59 to 62. The hydrotalcites, perhaps the most well-known of the layered double hydroxides, have been studied for many years.
It is known that certain organic species may be intercalated into the layers in some layered double hydroxides and into clays. For Example, Ogawa et al., in
Chemistry Letters
, 1992, no. 3, p. 365-368, describe the intercalation of maleic and methylmaleic acids into the clay montmorillonite in a solid state reaction. The geometrical isomers of the acids, fumaric and methylfumaric acids, were not intercalated in the solid state reaction. However, when an ethanolic solution of the two isomers was used, the montmorillonite showed no selectivity and both isomers were intercalated.
The structure of the layered materials [LiAl
2
(OH)
6
]X, where X is Cl, Br or NO
3
, and their hydrates has been described by Besserguenev et al., in
Chem. Mater
, 1997, no. 9, p. 241-247. The materials can be produced by the reaction of gibbsite [&ggr;-Al(OH)
3
] or other forms of Al(OH)
3
, such as bayerite, nordstrandite or doyleite, with lithium salts of formula LiX. The materials can also be formed in other ways, such as by direct precipitation (see, for example, Serna et al,
Clays
&
Clay Minerals
, (1997), 25,384). The structure of the LiAl
2
(OH)
6
+
layers in the compounds is unusual amongst layered double hydroxides since it is based on an ordered arrangement of metal cations within the layers.
The synthesis of LiAl
2
(OH)
6
+
compounds is described in U.S. Pat. No. 4,348,295 and U.S. Pat. No. 4,348,297. The use of the materials for separating hydrocarbons and for gas chromatograph columns is taught in U.S. Pat. No. 4,430,097 and U.S. Pat. No. 4,321,065, respectively. In both of these latter two documents, the technology described does not involve intercalation chemistry but surface interactions with the stationary phase i.e., liquid-solid or gas-solid interactions.
Intercalates of compounds of formula LiOH.2Al(OH)
3
are described in U.S. Pat. No. 4,727,167 and U.S. Pat. No. 4,812,245. Both documents relate to uses of the intercalates as additives to organic materials such as mineral oils.
A few other layered double hydroxides having cation ordering are known. The layered double hydroxide [Ca
2
Al(OH)
6
]
2
+
SO
4
2−
is an example.
Any improvement in the degree of selectivity or efficiency of a separation process can lead to increased purity of a product and, potentially, significant cost savings. The present invention provides a separation process which can be operated under very mild conditions and is highly selective. It also has the advantage of, in some cases, having a selectivity which is solvent and/or temperature dependent and can therefore be tuned towards a particular compound in a mixture. The process is based on the use of layered materials.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a process for separating a compound from a mixture of different compounds, the compound comprising at least two negatively charged groups connected by a linker group, which process comprises treating the mixture with a material comprising layers containing at least two different types of cation disposed in an ordered arrangement within each layer, in order to separate the compound from the mixture by selective intercalation of the compound into the material. The compound may be recovered from the intercalate formed with the material.
The layered material used in the process of the present invention contains different cations within each layer in an ordered arrangement i.e., it has cation ordering. Ordered (i.e., non-random) arrangements of cations are believed to be a major factor in the selectivity of the process. The different types of cations may be cations of different metals or cations of the same metal having different oxidation states. Preferred layered materials are compounds containing layers of formula LiAl
2
(OH)
6
+
or Ca
2
Al(OH)
6
+
with the former being particularly preferred. However, other materials containing layers having cation ordering, such as other ordered layered double hydroxides, may be expected to be useful in the process of the invention.
The compound which is separated from the mixture by intercalation into the layered material can be readily recovered by treatment of the material with an anion which intercalates between the layers in the material in preference to the compound, thereby displacing the compound. Suitable anions for this purpose include inorganic anions such as carbonate and sulphate, for example, although anions which intercalate more or less strongly than carbonate can be used. Carbonate is preferred in most cases on account of its strong capacity for binding with the material which allows the guest (i.e., intercalated) compound to be recovered substantially quantitatively and since it potentially allows the material to be regenerated by calcining the carbonate intercalate and hydrating the resulting product. Typically, treatment of the material with carbonate to recover the compound may involve treating the material with an aqueous solution of a soluble carbonate salt (e.g., sodium carbonate) at about or above room temperature for up to several hours (e.g., at 20 to 80° C. for from 1 to 20 hours).
The intercalated compound may also be separated from the layered material in other ways. For example, the material, with the compound intercalated, may be treated in such a way as to break down the material and thereby free the compound (e.g., by treatment with a mineral acid). Alternatively, the intercalated compound may be removed by gradually protonating one or more of the negatively charged groups of the intercalated compound such that its retention within the material becomes energetically less favoured. Therefore, a further embodiment of the invention involves the recovery of the compound from the material by a method which comprises treatment with an acid under conditions which cause protonation and de-intercalation of the compound whilst leaving the layers of the material substantially intact.
The fact that the compound can be recovered from the material has clear advantages. Firstly, it allows the material to be regenerated, as mentioned above. Secondly, it means that a mixture of two compounds can be separated to provide each compound alone. For instance, a hypothetical mixture of A and B can be treated with the material to remove A from the mixture by intercalation into the material to leave behind B. The material with A intercalated is then removed from B, for example by filtration, and is treated with an anion, such as carbonate, to release A which can be readily separated from the carbonate intercalate of the material, for example by filtration. Alternatively, A could be released from its intercalate with the material by breaking down the material (e.g., by treatment with aqueous acid) and extraction of A into an organic solvent in a conventional biphasic extraction method.
The process of the invention preferably involves

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