A Process For The Preparation Of Magnesium Containing Non A1 Anionic Clay

Abstract

The present invention relates to a process for the preparation of magnesium containing non-A1 anionic clay wherein an aqueous suspension comprising a trivalent metal source and a magnesium source is provided for slurried starting materials and reacted under thermal or hydrothermal conditions to obtain a magnesium-containing non-A1 anionic clay, the magnesium source and the trivalent metal source being said starting materials, the ,magnesium source being an oxide, a hydroxycarbonate or a carbonate, and the trivalent metal source being an oxide, a hydroxide, a hydroxycarbonate or a carbonate.

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Specification

This invention involves the preparation of Mg-containing non-AI anionic' clays. Anionic clays have a crystal structure which consists of positively charged layers built up of specific combinations of metal hydroxides between which there are anions and water molecules. Hydrotalcite is an example of a naturally occurring anionic clay, in which carbonate is the predominant anion present. Meixnerite is an anionic day wherein OH' is the predominant anion present.
In hydrotalcite-Iike anionic clays the brucite-like main layers are built up of octahedra alternating with interlayers in which water molecules and anions, more particularly carbonate ions, are distributed. The interlayers contain

It should be noted that a variety of terms is used to describe the material which is referred to in this patent as an anionic day. HydrotaJcite-like and layered double hydroxide are interchangeably used by those skilled in the art. In this patent application we refer to the materials as anionic clays, comprising within that term hydrotalcite-Iike and layered doubie hydroxide materials.

The most commonly described anionic clays are Mg-Al anionic clays. In the prior art the emphasis is usually on this type of anionic clays, whereas the Mg-containing non-AI anionic clays are only mentioned in passing, even though the chemistry of their preparation and their properties can be very different and unpredictable. Mg-Al anionic clays are suitable for many applications in the absorbent and catalysts field, but Mg-containing non-AI anionic clays have specific applications in these fields.
The preparation of anionic clays has been described in many prior art
publications.
Two major reviews of anionic clay chemistry were published in which the
synthesis methods available for anionic clay synthesis have been
summarized, F. Cavani et ai "Hydrofalcite-type anionic ciays: Preparation,
Properties and Applications," Catalysis Today". 11 (1991) Eisevier Science
Publishers B. V. Amsterdam.
J P Besse and others "Anionic ciavs: trends in pillaring chemistry, its
synthesis and microporous solids"(1992), 2, 108, editors: M.t. Occelli, H.E.
Robson, Van Nostrand Reinhold, N.Y.
In these reviews basically two types of anionic clay preparation are described. The most conventional method is co-precipitation (in Besse this method is called the salt-base method) of a soluble divalent metal salt and a soluble trivalent metal salt, optionally followed by hydrothermal treatment or aging to increase the crystallite size, The second method is the sa/t-oxide method in which a divalent metal oxide is reacted at atmospheric pressure with a soluble trivalent metal salt, followed by aging under atmospheric pressure. This method has only been described for the use of ZnO and CuO in combination with soluble trivalent metal salts.

For work on anionic clays, reference is given to the following articles:
Helv. Chim. Acta. 25, 106-137 and 555-569 (1942)
J. Am. Ceram. Soc. 42, no. 3, 121 (1959)
Chemistry Letters (JapanV 843 (1973)
Clavsand Ciav Minerals. 23, 369 (1975)
Clavs and Clav Minerals. 28, 50 (1980)
Clays and Clav Minerals. 34, 507 (1996)
Materials Chemistry and Physics. 14, 569 (1986).
In addition there is an extensive amount of patent literature on the use of
anionic ciays and processes for their preparation.
European Patent Application 0 536 879 describes a method for introducing pH-dependent anions into the clay. The clay is prepared by the addition of
a solution of AI(N03)3 and Mg(N03)2 to a basic solution containing borate
anions. The product is then filtered, washed repeatedly with water, and dried overnight. Additionally mixtures of Zn/Mg are used.
in US 3,796,792 by Miyata et ai. entitled "Composite Metal Hydroxides" a range of materials is prepared into which an extensive range of cations is incorporated, including Sc, La, Th, in, etc. in the examples given solutions of the divalent and trivaient cations are prepared and mixed with base to cause co-precipitation. The resulting products are filtered, washed with water, and dried at 80 °C. Exampfe 1 refers to Mg and Sb and Example 3 to Mg and Bi. Other examples are given, and in each case soluble salts are used to make solutions prior to precipitation of the anionic clay at high pH.
in US 3,879,523 by Miyata entitled "Composite Metal Hydroxides" also a large number of preparation examples is outlined. The underlying chemistry, however, is again based on the co-precipitation of soluble salts

followed by washing and drying. It is important to emphasize that washing is a necessary part of such preparations, because to create a basic environment for co-precipitation of the metal ions a basic solution is needed
and this is provided by NlaOH/Na2C03 solutions. Residual sodium, for
example, can have a significant deleterious effect on the subsequent performance of the product as a catalyst or oxide support.
in US 3879525 (Miyata) very similar procedures are again described.
in US 4,351,814 to Miyata et al. a method for making fibrous hydrotaicites is described. Such materials differ in structure from the" normal piate-like morphology. The synthesis again involves soluble salts. For example, an
aqueous solution of a mixture of MgCl2 and CaCl2 is prepared and suitably
aged. From this a needle-like product Mg2(OH)3CI.4H2n precipitates. A separate solution of sodium aluminate is then reacted in an autoclave with the solid Mg2(OH)3CI.4H2n and the product is again filtered, washed with water, and dried.
In US 4,458,026 to Reichle, in which heat-treated anionic clays are described as catalysts for aldol condensation reactions, again use is made of magnesium and aluminium nitrate salt solutions. Such solutions being
added to a second solution of NaOH and Na2C03. After precipitation the
slurry is filtered and washed twice with distilled water before drying at 125 °C.
in US 4,656,156 to Misra the preparation of a novel absorbent based on mixing activated alumina and hydrotalcite is described. The hydrotalcite is

made by reacting activated MgO (prepared by activating a magnesium compound such as magnesium carbonate or magnesium hydroxide) with aqueous solutions containing aluminate, carbonate and hydroxyl ions. As
an example the solution is made from NaOH, Na2C03 and AI2O3. In
particular, the synthesis involves the use of industrial Bayer liquor as the source of Al. The resulting products are washed and filtered before drying at 105 "C.
In US 4,904,457 to Misra a method is described for producing hydrotaicites in high yield by reacting activated magnesia with an aqueous solution containing aluminate, carbonate, and hydroxyl ions.
The methodology is repeated in US 4,656,156.
In US 5,507,980 to Keikar et at at. a process is described for making novei catalysts, catalyst supports, and absorbers comprising synthetic hydrotalcite-like binders. The synthesis of the typical sheet hydrotalcite invoives reacting pseudo-boehmite to which acetic acid has been added to peptize the pseudo-boehmite. This is then mixed with magnesia. More importantly, the patent summary states clearly that the invention uses mono carboxyfic organic acids such as formic, propionic and isobutyric acid. In this patent the conventional approaches to preparing hydrotaicites are presented.
in US 6,539,861 a process is disclosed for preparing a catalysts for synthesis gas production based on hydrotaicites. The method of preparation is again based, on the co-precipitation of soiubie salts by
mixing with base, for example, by the addition of a solution of RhC^,

Mg(N03)2 and A1(N03)3 to a solution of Na2C03 and NaOH.
Also in US 5,399,537 to Bhattacharyya in the preparation of nickel-containing catalysts based on hydrotalcite use is made of the co-precipitation of soluble magnesium and aluminium salts.
In US 5,591,418 to Bhattacharyya a catalyst for removing sulfur oxides or nitrogen oxides from a gaseous mixture is made by calcining an anionic clay, said anionic clay having been prepared by co-precipitation of a
solution of Mg(N03)2, At(N03)3 and Ce(N03)3. The product again is
filtered and repeatedly washed with de-ionized water.
In US 5,114,898/WO 9110505 Pinnavaia et al. describe layered double hydroxide sorbents for the removal of sulfur oxide(s) from flue gases, which layered double hydroxide is prepared by reacting a solution of Al and Mg
nitrates or chlorides with a solution of NAOH and Na2C03. In US 5,079,203
/WO 9118670 layered double hydroxides intercaiated with polyoxo anions are described, with the parent ciay being made by co-precipitation techniques.
In US 5,578,286 in the name of Alcoa a process for the preparation of meixnerite is described. Said meixnerite may be contacted with a dicarboxylate or polycarboxylate anion to form a hydrotaIcite-iike material.
In US 4,946,581 and US 4,952,382 to van Broekhoven co-precipitation of soluble salts such as Mg(N03)2 and AI(N03)3 with, and without the incorporation of rare earth salts was used for the preparation of anionic

clays as catalyst components and additives. A variety of anions and di- and tri-valent cations are described.
As indicated in the description of the prior art given-above, there are many applications of anionic clays.
These include but are not restricted to: catalysts, adsorbents, drilling muds, catalyst supports and carriers, extenders and applications in the medical
field. In particular van Broekhoven has described their use in SOx
abatement chemistry.
Because of this wide variety of (arge-scale commercial applications for these materials, new processes utilizing alternative inexpensive raw materials are needed to provide a more cost-effective and environmentally compatible processes for making anionic clays, in particular, from the prior art described above one can conclude that the preparation process can be improved in the following ways: the use of cheaper sources of reactants, processes for easier handling of the reactants, so that there is no need for washing or filtration, eliminating the filtration problems associated with these fine-particied materials, the avoidance of alkali metals {which can be particularly disadvantageous for certain catalytic applications}: Further, in drying or calcining the anionic clay prepared by prior art processes gaseous emissions of nitrogen oxides, halogens, sulfur oxides, etc. are encountered which cause environmental pollution problems.
SUMMARY OF THE INVENTION
Our invention includes processes for producing Mg-containing non-AI anionic ciays using relatively inexpensive starting materials in a simple

process which involves reacting mixtures with or without stirring in water, optionally under hydrothermal conditions. Such processes can be operated in standard laboratory/individual equipment. More specifically, there is no need for washing or filtering, and a wide range of ratios of Mg(II)/M(III) in the reaction product is possible. The invention involves the use of a trivalent metal source and a magnesium source in aqueous suspensions, which are reacted; optionally under hydrothermal conditions and the reaction mixture results in the direct formation of a Mg-containing non-Al anionic clay. The powder X-ray diffraction pattern (PXRD) suggests that the product is comparable to anionic clays made by other standard methods. The physical and chemical properties of the product are also comparable to those anionic clays made by the other conventional methods. The overall process of this invention is very flexible, enabling a wide variety of anionic clay compositions and anionic clay-like materials involving for example carbonate, hydroxide and other anions to be prepared in an economically and environmental-friendly manner. The process may be carried our in a one-step process either in batch or in continuous mode.
This invention involves the preparation of Mg-containing non-Al anionic clays. In particular it describes a process of the preparation of an anionic clay wherein a suspension comprising a trivalent metal source and a Mg source is provided and reacted thermally or hydrothermally to obtain a Mg-containing non-Al anionic clay, the magnesium source being an oxide, a hydroxide, a hydroxycarbonate, or a carbonate.
It was found that Mg-containing non-Al anionic clays are directly obtained from the
reaction according to the invention. This is in contrast to the

coprecipitation method wherein soluble salts are first precipitated, filtered, washed to remove unwanted ions and then aged either hydrothermally or not. With the process according to the invention the presence of unwanted ions in the product can be avoided, as will be explained befow. The magnesium source is an oxide, hydroxide a hydroxy carbonate or carbonate. From this compound no ions beside hydroxide and carbonate end up in the anionic clay, which are the normal building blocks of anionic ciays. If for the trivalent metal source a compound is chosen with harmless ions such as nitrate or acetate washing and filtration of the reaction product can be avoided altogether. In fact, it was found that the reaction also takes piace when using hydroxides, oxides, hydroxycarbo nates or carbonates for both the divalent metal source and the trivalent metal source, in which case also no washing and filtration has to take place.
Since the process disclosed in this patent does not require washing of the product or filtering, there is no filtrate waste or gaseous emissions (e.g. from acid decomposition), making the process particularly environ me ntai-friendlyi and more suited to the environmental constraints which are increasingly imposed on commercial operations. The product can be spray dried directly to form microspheres or can be extruded, pelletized or beaded to form shaped bodies.
Anionic clays prepared by this method exhibit the well known properties and characteristics (e.g. chemical analysis, powder X-ray diffraction pattern, FT1R, thermal decomposition characteristics, surface area, pore volume, and pore size distribution) usually associated with anionic clays prepared by the customary and previously disclosed methods.
The anionic clay according to the invention has a layered structure

corresponding to the general formula
[Mgtll^MCIII^^OHa^^X^-bHsO
wherein m and n have a value such that m/n=l to 10, preferably 1 to 6, and b has a value
in the range of from 0 to 10, generally a value of 2 to 6 and often a value of about 4. X
may be C032" or any other anion normally present in the interlayers of anionic clays. It is
more preferred that m/n should have a value of 2 to 4, more particularly a value close to
3.
The trivalent metal source.
Suitable trivalent metal sources are compounds containing Mn3+, Fe 3+, Co3+, Ni3+,
Cr3+,Ga3+, B3+, trivalent rare earth metal cations such as La3+ and Ce3+ or mixtures of said
compounds. Preferably oxides hydroxides and carbonates of these metals are used, but
also nitrate chlorides sulfates and phosphates can be used.
Divalent metal source
Suitable Mg sources are MgO, Mg(OH)2, magnesium carbonate, and -

magnesium bicarbonate. In addition to the Mg source other divalent metal source may be added such as compounds containing Mg , Ca , Zn +r
r
Mn , Co +, Ni , Fe , Sr , Ba , Cu , and mixtures of said
compounds. Preferably oxides, hydroxides and carbonates of these metals are used, but also nitrates chlorides, sulfates and phosphates can be used.
Conditions
As mentioned above, the reaction is conducted under thermal or hydrothermai conditions. Within the context of this description hydrothermal means in the presence of water at a temperature above 100 °C at increased pressure. Thermal means at a temperature between ambient and 100 °C. Preferably the reaction takes place in water in an autoclave at a temperature above 100 °C, i.e. under autogeneous pressure. It is possibie to purge the suspension with nitrogen or inert gas if an anionic clay with predominantly hydroxide anions are desired, but in general this is not necessary. Thus, the reaction can be conducted in the presence of
C02- Said CO2 may be the C02 normally present in air or it may be added
to the reaction, for instance, by using a carbonate divalent or trivalent metal source.
Said aqueous suspension may be obtained by either combining slurries of the starting materials or adding divalent metal source to a slurry of trivalent metal source or vice versa. There is no need to wash or filter the product, as unwanted ions (e.g. sodium, ammonium, chloride, sulfate) which are frequently encountered when using other preparation methods, are absent in the product. If desired a preformed anionic clay may be added to the reaction mixture. Said preformed clay may be recycled anionic clay from

the reaction mixture or anionic clay made separately by the process according to the invention or any other process.
Because of its simplicity, this process can be carried out in a continuous mode by mixing of a first slurry comprising trivalent metal source and a second slurry comprising magnesium source passing the mixed slurry through a reactor vessel which can operate under hydrothermal conditions. Said first and/or second slurry may be subjected to a treatment prior to mixing the slurries. Said pre-treatment may involve treatment with acid, base treatment, thermal and/or hydrothermal treatment, all optionally in the presence of seeds or combinations thereof.
As mentioned-above, if desired acids and bases, for example for control of pH, may be added to the slurry before or during reaction or to the individual reactants before combining them in the slurry. The acid and bases of choice are formic acid, acetic acid, nitric acid and ammonium hydroxide, because these types of acids and bases do not introduce unwanted ions in the reaction mixture.
The most preferred combination of a divalent metal source and a trivalent metal sources in Ga-Mg, because these combinations result in Mg-containing non-Al anionic clays with specific applications in the catalyst field.
If desired, the anionic clay prepared by the process according to the invention may be subjected to ion exchange. Upon ion exchange the interlayer charge-balancing anions are replaced with other anions. Said other anions are the one commonly present in anionic clays and include pillaring anions such as V10O28* , M07O246". Said ion exchange can be

conducted before drying or after the anionic clay lormed in the slurry.
The process of the invention provides wide flexibility in preparing products with a wide range of M(i!):M{lH) ratios. The M(tl):M(lll) ratio can vary from 0.1 to 10 , preferably from 1 to 6, more preferred from 2 to 4, and especially preferred to close to 3.
For some applications it is desirable to have additives, both metals and non-metals, such as rare earth metals, Si, P, B, group VI, group VIII, alkaline earth (for instance Ca and Ba) and/or transition metals (for example Mn, Fe, Ti, Zr, Cu, Ni, Zn, Mo, Sn), present. Said metals can easily be deposited on the anionic clay. They can also be added either to the divalent metal source or the trivalent metal source or to the slurry during preparation of the anionic clay.
The present invention is illustrated by the following examples which are not to be considered limitative by any means.

EXAMPLES
Example 1
A slurry was provided of gallium nitrate and MgO with a Mg/Ga ratio of 2.3.
The pH of the slurry was adjusted to 10.02 with NH3OH, The slurry was
subjected to hydrothermal treatment in an autoclave at 120 °C for 1 hour. The product was dried at 110 °C. X-ray diffraction confirmed the formation of a Mg-Ga anionic clay.
Example 2
A slurry was provided of gallium oxide and MgO with a Mg/Ga ratio of 2.3. The pH of the slurry was adjusted to 10.2 with NH3OH. The slurry was subjected to thermal treatment at 90 °C for 18 hours. The product was dried at 110 °C. X-ray diffraction confirmed the formation of a Mg-Ga anionic ciay.
Example 3
A slurry was provided of gallium oxide and MgO with a Mg/Ga ratio of 2.3. The pH of the slurry was adjusted to 10.2 with NH3OH. The slurry was subjected to thermal treatment at 120 °C for 18 hours. The product was dried at 110 °C. X-ray diffraction confirmed the formation of a Mg-Ga anionic clay.
Example 4

A slurry was provided of gallium oxide and MgO with a Mg/Ga ratio of 2.3. The pH of the slurry was adjusted to 10.2 with NH3OH. The slurry was subjected to hydrothermal treatment at 198 "C for 1 hour. The product was dried at 110 °C. X-ray diffraction confirmed the formation of a Mg-Ga anionic clay.

We claim:
1. A process for the preparation of magnesium containing non-Al anionic clay wherein an aqueous suspension comprising a trivalent metal source and a magnesium source is provided from slurried starting materials and reacted under thermal or hydrothermal conditions to obtain a magnesium-containing non-Al anionic clay, the magnesium source and the trivalent metal source being said starting materials, the magnesium source being an oxide, a hydroxide, a hydroxycarbonate or a carbonate, and the trivalent metal source being an oxide, a hydroxide, a hydroxycarbonate or a carbonate.
2. The process as claimed in claim 1, wherein the trivalent metal source is a compound containing Mn3+, Co3+, Ni3+, Cr3+, Fe3+, Ga3+, B3+, trivalent rare earth cations such as La3+ and Ce3+ or a mixture of said compounds.
3. The process as claimed in any one of claim 1 to 2, wherein in addition to the magnesium source an additional divalent metal source is added which is a compound containing Ca2+, Zn2+, Mn2+, Co2+' Ni2+' Fe2+JSr2+, Ba2+, Cu2+ and mixtures of said compounds.
4. The process as claimed in any one of claim 1 to 3, wherein both the divalent metal source and the trivalent metal source are oxides, hydroxides or carbonates.
5. The process as claimed in any one of claim 1 to 4, wherein acid or base is present in the suspension.

6. The process as claimed in any one of the preceding claims 1 to 5, wherein the process is carried out in a continuous mode.
7. The process as claimed in any of claims 1 to 6, wherein additives are present in the suspension.
8. The process as claimed in any one of claim 1 to 7, wherein the Mg containing non-Al anionic clay is subjected to an ion exchange treatment.
9. The process as claimed in claim 8, wherein the Mg-containing non-Al anionic clay is ion exchanged with pillaring anions such as V10O28" and M07O24".
10. The process a claimed in claims 1 to 9, wherein additives are deposited on the Mg-containing non-Al anionic clay.

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