Two powder synthesis of hydrotalcite and hydrotalcite-like compounds

ES 2 140 657 T3

DESCRIPTION

Synthesis of hydrotalcite and hydrotalcite-like compounds from two powders.

This invention relates to the field of preparing mixed metal hydroxides or layered double hydroxide products. More specifically, the invention relates to an improved two-step process for preparing hydrotalcite and hydrotalcite-like compounds from dry powder constituents.

Hydrotalcite exists both naturally and synthetically. Natural deposits have been found in Snarum, Norway, and the Ural Mountains. Topical deposits are in the form of serpentines, talc-schists, or as an alteration product in which hydrotalcite forms the pseudomorph of a spinel. Like most minerals, it is almost impossible to find natural hydrotalcite in a pure state. Such deposits often contain one or more of other minerals including pennitin and muscovite.

Various processes are known to prepare synoetic hydrotalcite in product forms such as fine powder, -20 mesh granules or as 0.3175 cm diameter extrudates. A representative procedure is described in US Patent No. 3,539,306. In this an aluminum hydroxide, aluminum amino acid salt, aluminum alcoholate, water soluble aluminate, aluminum nitrate and / or aluminum sulfate are mixed with a magnesium component selected from magnesium oxide, magnesium hydroxide or salt. of magnesium soluble in water and a compound containing carbonate ion in an aqueous medium that is maintained at pH 8 or more. The resulting product can be used as an antacid for the stomach. In this topical neutralization procedure, an almost pure, very fine-sized hydrotalcite particle is formed. However, a major disadvantage of this method is the formation of a sodium salt as a by-product. This salt neutralization process to prepare hydrotalcites can also produce a brucite-like structure with anions (for example sulfate) or cations (Na⁺ ) unwanted included in it.

In US Patent No.^° 34164 from Misra Reissue, another means of synthesizing hydrotalcite is taught. The process comprises heating magnesium carbonate and / or magnesium hydroxide to form activated magnesia, then combining the activated magnesia with an aqueous solution of aluminate, carbonate, and hydroxyl ions.

Other known procedures for synthesizing hydrotalcite include: adding dry ice or ammonium carbonate to a thermal decomposition product of a magnesium nitrate-aluminum nitrate mixture, after which the intermediate product is subjected to temperatures below about 163^°C, and pressures between 13.8 and 131.9 MPa. Still another procedure, from "Properties of a Synthetic Magnesium Aluminum Carbonate Double Hydroxide and its Relationship to Magnesium-Aluminum Hydroxide Manasseite, and Hydrotalcite", The American Mineralogist, vol. 52, pp. 1036-1047 (1967), produces hydrotalcite-type materials by dosing

NaOH to a solution of MgCl2 and AlCl3 in a system without carbon dioxide. This suspension is dialyzed for 30 days at 60^°C to form a hydrated Mg-Al carbonate hydroxide having the properties of both mannaseite and hydrotalcite.

In Clay and Clay Minerals, volume 34, number 5, pages 507-10 (1986), I. Pausch et al. describe a process for preparing hydrotalcite-type compounds, but do not describe a process for using aluminum oxide to prepare meixnerite. EP-A-536879 (Amoco) describes a process for making hydrotalcite-type clays using aluminum salts, but not aluminum oxide.

A primary objective of this invention is to provide an improved means of preparing synoetic hydrotalcite and hydrotalcite-like compound from two or more relatively inexpensive dry powder compounds. Another objective of this invention is to provide an improved process for making hydrotalcite and related materials with less sodium ion contamination. Still another objective is to provide a process for synthesizing hydrotalcite without relying on the use of any aluminum gel. Still another objective is to prepare hydrotalcite and hydrotalcite-type compounds by the subsequent processing of an improved meixnerite product, prepared this by combining activated magnesia with transitional alumina with a high specific surface area.

Yet another main objective is to prepare hydrotalcite and hydrotalcite-type compounds in a more environmentally acceptable way. According to preferred embodiments, synoetic hydrotalcites prepared by procedures described hereinafter do not yield by-products other than water. Any remaining discharge water should be easy to dispose of due to its low dissolved solids content.

These objectives are achieved by the process of preparing a layered double hydroxide powder comprising: (a) reaction of at least one trivalent metal oxide powder and at least one divalent metal compound selected from the group consisting of hydroxide, oxide , carbonate and mixtures thereof, in an aqueous suspension to form a double hydroxide intermediate; (b) contacting the double hydroxide intermediate with an anion source to form a layered double hydroxide; and (c) separating the layered double hydroxide from the suspension. The process comprises reacting powdered magnesium oxide with a high surface area transition alumina in a suspension to form meixnerite or a meixnerite-type intermediate. This latter intermediate product is then contacted with a source of anions such as an acid or acid precursor, most preferably carbon dioxide, to form the layered double hydroxide compound which is separated from the suspension by filtration, centrifugation, dehydration. vacuum or by other known means. Preferably, the transitional alumina combined with activated magnesia in this way consists essentially of a powder of activated alumina.

ES 2 140 657 T3 which has a specific surface area of ​​approximately 100 m²/ g or greater. For the related double hydroxide formations, other reagents, such as bromides, chlorides, boric acids, or their salts, combine with the meixnerite intermediate to form members of the similarly structured brucite-type layered double hydroxide family.

1. Definitions

As used herein, the following terms have the meanings given below:

to. "Transitional alumina" means an alumina with a high surface area in the form of fine particles or powder. A preferred way of defining such alumina materials uses specific surface area and Loss on Calcination (LOI) measurements. More specifically, an alumina having a Brunauer-Emmett-Teller (or BET) specific surface area of ​​approximately 100 m²/ gums will be considered to have a high specific surface area and will therefore qualify as a transitional alumina for the purposes of this invention. Aluminae having a LOI weight percent of about 1.5% or more will also qualify within this definition.

A particular preferred type of transitional alumina is referred to as "rehydratable alumina". It tends to form strong hydroxyl bonds in contact with water and its rehydration reactions are highly exothermic. The mean particle sizes for such aluminas can be in the range of 0.01-200 µ, with a range between 0.1 and 10 to 20 microns being more preferred.

Some activated aluminas are more suitable than others for the purposes of this invention. Most are high surface area aluminas formed by rapid calcination of hydrated alumina at temperatures below that required for dehydration or complete calcination. Typically said aluminas are amorphous (that is, they do not have a microcrystalline structure) when analyzed by X-ray diffraction. These powders have a LOI value of 412% by weight, and a BET specific surface of 200-300 m²/ g.

b. "Activated magnesia" or activated magnesium oxide refers to the magnesium-based product activated by MgO "soft burned" at one or more temperatures between 450 and 900 ^ C. This component has a general specific surface area of ​​10-200 m²/ g, preferably 20-150 m²/ g and a LOI between 1.0 and 6.0% by weight. This criterion distinguishes this reagent from magnesia that have been completely burned or calcined. Although the latter still produces meixnerite with longer reaction times or under more drastic reaction conditions, the percent yields under said conditions are significantly lower than those preferred for the present invention.

There are numerous means to prepare an activated magnesia product for combining with transitional aluminas according to the first stage of the process of said invention. For example, commercially available magnesium carbonate can be heated to release carbon dioxide, thereby forming reactive magnesia. Magnesium oxide can also be prepared by: (a) heating synthetic or natural magnesium hydroxides or basic magnesium carbonate, at temperatures between 380 and 950 ^ C; or (b) heating MgCl2 with lime. Different known procedures can be used to generate magnesia powders of different particle sizes and / or specific surfaces.

c. "Hydrotalcite" compounds are to be understood to apply to the structural family of layered double hydroxides whose family members consist of any compound having the AwBx (OH) and Cz phaormula. nH2O, in which A represents a divalent metal cation, B a trivalent metal cation, C a polyvalent mono- a anion, yw, x, y, z and n meet the following conditions: 0 <z <x <4 <w <1 / 2y and 12> n> 1/2 (wx). Preferred embodiments of this family have been identified by the formula: A6B2 (OH) 16Cz.4H2O, in which A is selected from: Mg²⁺, Neither²⁺, Cu²⁺, Ca²⁺, Faith²⁺ yZn²⁺; Bde: Al³⁺, Faith³⁺ and CR³⁺;

and C from a list of anions that includes: OH^-, Cl^-, Br^-, NO3^-, CH3COO^-, CO3^2-, SO4^2-, PO4^3-, Fe (CN) 6^3-, Fe (CN) 6^4- and some borates, carboxylates and polyoxometalates, with 1/2 <z <2 (depending on the charge of the substituted anioan). Some background refers to any compound having the aforementioned faormules as "hydrotalcite." However, for the purpose of this invention, this family of structural compounds has been divided into different subgroups depending on the divalent and trivalent cations within their alternative brucite-type layers. For example, pyroaurites have the basic phaormula:

Mg6Fe2 (OH) 16CO3.4H2O.

Such compounds are also known as "sjogrenites". Collectively, these other members of the family have been referred to as "hydrotalcite-like" compounds.

Still another preferred definition for the term "hydrotalcite" includes any natural or synthetic compound that satisfies the formula:

Mg6Al2 (OH) 16CO3.4H2Oo

Mg4Al2 (OH) 12CO3.3H2O.

This compound has sometimes been written as:

6MgO.Al2O3.CO2.12H2O.

In its ioanic form, hydrotalcite can appear as:

[Mg6Al2 (OH) 16]²⁺. [CO3]^2-.4H2O.

The main structural unit of this compound is brucite, or magnesium hydroxide (Mg (OH) 2) in the form of an octagonal lamina with Mg ions located between multiple ions (OH) that share adjacent edges. By substituting some of the divalent magnesium ions for trivalent aluminum in this structure, magnesium and aluminum sublayers are created while still maintaining the basic type structure.

ES 2 140 657 T3 brucite sheets. To compensate for the charge imbalance due to these substitutions with aluminum ions, anions (indicated by the letter "C" in the preceding formula) and water molecules are intercalated to form (Cz.nH2O) interfaces between the brucite-type structural layers. , with 1/2 <z <2 depending on the anian interleaved in this way. The anion that has the highest affinity to combine with water in this structure to form hydrotalcite is carbonate (CO3^2-). Sulfate (SO4^2-) is another compatible anioan.

The spatial distribution of carbonate ions within hydrotalcite depends partially on how Al ions³⁺ replace Mg ions²⁺. The spacing of the brucite layers is also a function of the amount or degree of substitution of aluminum within the basic structure of hydrotalcite. As aluminum substitution increases, the interlayer spacing decreases due to increased electrostatic attraction between positive hydroxide layers and negative hydrotalcite interfaces. The thickness of the interface can also vary depending on the size and orientation of the anions that replace some or all of the carbonate ions in the hydrotalcite. In preferred embodiments a hydrotalcite material is contemplated having a Mg: Al ratio of between 2 (x = 0.33) and 3 (x = 0.25) or greater.

d. "Basic magnesium carbonate" means a dimagnesium salt containing hydroxide and carbonate anions in the same powdered product, sometimes represented by the formula Mg (OH) 2.MgCO3.

and. "Meixnerite" means a double-layered hydroxide material, of the hydrotalcite type in which all of the intercalated anions are hydroxyls.

A generic means of summarizing the reactions believed to occur in the procedure described here is as follows:

Stage 1. M_toOR_b + Al₂OR₃ GH₂Or faith MX

Stage 2. MX + HA fe Double layer hydroxide + H2O.

Most preferably, the following two steps are believed to occur for the manufacture of hydrotalcite:

Stage 1:

(1-x) MgO + x / 2Al2O3 + (1 + x / 2 + w) H2Ofe

[Mg (1-x) Alx (OH) 2] (OH) x · w H2O followed by step 2:

[Mg (1-x) Alx (OH) 2] (OH) x · wH2O + x / 2CO2 fe 2 [Mg (_X__x)To the_x(OH) 2] (CO3^2-) x / 2 ^ wH2O + xH2O

For some dry powder reagents, temperature limitations in the contacted water solution have been shown to be beneficial for overall performance. While step 1 of the above reaction can be carried out at temperatures as low as 25 ° C, for calcium-containing compounds, they are normally best carried out at one or more temperatures between 80 and 160.^°C, especially for the magnesium-containing double-layer hydroxides prepared by the process of this invention. At these temperatures, yields in excess of 75% are normally observed. Preferred reaction temperatures will generally be between about 98 and 150^°C. Although higher reaction pressures, up to about 0.81 MPa, are known to enhance the synthesis of hydrotalcite and hydrotalcite-type compounds according to this invention, the most preferred reaction pressures will generally be between ambient and ambient pressures. 0.48 MPa, determined by the vapor pressure of the water.

Suitable end uses of these hydrotalcite products prepared by this process include acid neutralizers and acceptors, especially for polypropylene and polyethylene manufacturers, adsorbents for heavy metal anions from wastewater, stabilizing components for other polymer systems such as poly (vinyl chloride), flame retardants, smoke suppressants, catalysts, catalyst supports and viscosity control agents.

Other features, objectives and advantages of the present invention will become more apparent from the detailed description of the following examples. However, it is to be understood that said examples are only representative of this invention and should not be used to limit its scope in any way.

Examples 1-7

Each of the following examples were carried out using a 1.8 liter capacity internally stirred reactor charged with 750 ml of deionized water. In each case, after the respective divalent and trivalent metal compounds had been added to the water and dispersed completely with continuous stirring, carbon dioxide was bubbled into the reactor of a pressurized cylinder. When the respective reaction times were completed, the reactor was allowed to cool and the excess carbon dioxide was gradually released into the atmosphere. The resulting suspension was then vacuum filtered using a Buchner funnel and a sample of each filtrate will be reseaked in vacuo before carrying out x-ray diffraction analyzes to determine which crystal phases were present in these dried solids. Comparative Example 1

100 grams of hydromagnesite having the formula Mg₅(CO₃)₄(OH)₂· 4Η₂Ο and 47 grams of crushed aluminum hydroxide having an average particle size of 10.0 µm. Carbon dioxide was added until the reactor pressure reached 3.47MPa. Afterwards the temperature of the reactor was kept between 2526^°C for about 4 hours. The analysis of the dry solids obtained from this reaction showed the presence of hydromagnesite and alumina as gibbsite but not hydrotalcite. Comparative Example 2

For this example, another 100 grams of hydromagnesite was charged with 41.7 grams of the same ground Al (OH) 3 as in Example 1. Liquid carbon dioxide was added until the reactor pressure reached 3.69 MPa. Afterwards the reactor temperature was kept at 48-53^°C for about 4 hours. The analysis of the dried salids obtained from this reaction again showed the presence of hydromagnesite and gibbsite.

ES 2 140 657 T3 but not from hydrotalcite.

Comparative Example 3

For this example the same amount of hydromagnesite and ground Al (OH) 3 used in Example 2 was recharged into the reactor. The reactor charged with 4.45 MPa of added carbon dioxide, for 4 hours at 90 ° C, it still showed no signs of hydrotalcite in the recovered solids.

Comparative Example 4

For this example, 100 grams of the same hydromagnesite were charged as before with 31.0 grams of rehydratable alumina with an average particle size of 2.0 µm. The suspension was stirred at room temperature for 3 hours while adding enough liquid carbon dioxide to achieve a total pressure in the reactor of 4.06 MPa. Then the whole system heated up to 50^°C for 2 hours. The dry filter cake from this reaction was found to contain significant amounts of hydrotalcite by X-ray diffraction.

Comparative Example 5

For this example, 100 grams of the same hydromagnesite as before was charged with 38.7 grams of pseudoboehmite sold by Vista Chemical Co. under the trade name Catapal.¹® SB, said material consisting of 65 µm diameter agglomerates of 0.1 µm basic particles. Enough carbon dioxide was added to bring the total reactor pressure to 4.31 MPa. Afterwards the system remained at 48-52^°C for 4 hours. X-ray diffraction analysis of the resulting filter cake showed that significant amounts of hydrotalcite were present. Comparative Example 6

For this example, the same amounts of hydromagnesite and pseudoboehmite were charged to the reactor as in example 5. With the reactor charged with 5.27 MPa of added carbon dioxide, a cake was obtained for 4 hours at 90 ° C. filter that had significant amounts of hydrotalcite (by X-ray diffraction analysis).

Comparative Example 7

The same amounts of hydromagnesite and rehydratable alumina were charged back into the reactor as in Example 4, but in this example no additional carbon dioxide was added. The system warmed up to 50^°C for 2 hours. The resulting filter cake that was tested also contained significant amounts of hydrotalcite. However, the degree of conversion of the hydromagnesite in Example 7 was less than in Example 4, based on the comparison of the X-ray diffraction peak intensities of these products.

Examples 8-13

In each of these examples, approximately 70 grams of MgO and 45.6 grams of rehydratable Al2O3 were mixed with 1200 ml of deionized water in a round bottom flask to form a suspension. Then the suspension was shaken and heated to boiling at atmospheric pressure. The area of ​​the flask above the suspension was purged with nitrogen to avoid reaction with CO2 from the air. After six (6) hours in the reactor, the samples were removed and analyzed. A considerable amount of meixnerite was found in these samples. After 22 hours of boiling, the conversion was almost complete. After several portions of this suspension were cooled below 40^°C and treated with carbon dioxide gas or atmospheric air to convert meixnerite to hydrotalcite. Samples taken from this suspension were analyzed and shown to contain significant amounts of hydrotalcite. An oxalate was formed by adding oxaolic acid to the meixnerite suspension at 26-30^°C, for Example 11. A borate form of hydrotalcite was prepared by adding booric acid to the meixnerite suspension for Example 12, and a stearate form was prepared by contacting meixnerite with steaoric acid for Example 13.