Organic compounds -- part of the class 532-570 series – Organic compounds – Rare earth containing
Reexamination Certificate
1999-04-30
2001-01-30
Bos, Steven (Department: 1754)
Organic compounds -- part of the class 532-570 series
Organic compounds
Rare earth containing
C534S016000, C556S046000, C556S049000, C556S058000, C556S061000, C556S062000, C556S112000, C556S114000, C556S121000, C556S131000, C556S140000, C556S147000, C556S027000, C556S028000, C568S851000, C568S028000, C423S593100, C423S594120, C423S595000, C423S599000, C423S600000, C423S263000
Reexamination Certificate
active
06180764
ABSTRACT:
The present invention relates to the production of hydrotalcites. Hydrotalcites are metal hydroxides having a layer lattice and belonging to the group of anionic clay minerals. Furthermore, the present invention relates to the metal oxides obtained by calcination of the metal hydroxides produced according to this invention.
Metal hydroxides are important precursors for the production of metal oxides used for instance as raw materials for refractories, ceramics, and supports for heterogeneous catalysts. In nature, metal hydroxides predominantly occur in the form of mixed metal hydroxides. There are numerous clay minerals that can be characterised by their layer lattice. The great majority of clay minerals are cationic ones. In said metal hydroxides, cations, e. g. Na
+
, Ca
2+
, etc., are located between the negatively charged layers. In anionic clay minerals which are far less common, anions are located between the positively charged layers of the metal hydroxide. A large number of said anionic clay minerals are hydroxides of metals of the main group, namely magnesium and aluminium, and hydroxides of transition metals, such as nickel, chromium, zinc, etc. The structure of said clay minerals can be derived from the brucite structure of magnesium hydroxide Mg(OH)
2
. In this structure, some of the divalent Mg(OH)
6
4−
octahedra are replaced by Al(OH)
6
3−
octahedra. Examples of said minerals are meixnerite having the idealised unit cell formula Mg
6
Al
2
(OH)
18
·4 H
2
O and hydrotalcite (Mg
6
Al
2
(OH)
16
CO
3
·4 H
2
O). According to the prior art, the magnesium : aluminium atomic ratios can be varied between 1.7 and 4.0. The metal hydroxide octahedra share adjacent edges to form layers. In addition to water, interstitial anions required for balancing the charge are located between the layers. The anion nature can be simple, e. g. OH
−
, CO
3
2−
, Cl
−
or SO
4
2−
, or complex, as for instance in large, organic or inorganic anions. Up to now, such anions have been incorporated into the layers by substitution of simple anions or by acid treatment in the presence of the desired anions.
WO-A-93 21 961 describes a process for manufacturing of stratiform, mixed metal hydroxides through controlled hydrolysis of metal oxides in a water free organic solvent with stoichiometric amounts of water. The thereby obtained metal hydroxides are gel-compositions for use as a dye carrier in dye laser applications.
According to the bottom of page 4 these metal hydroxides have the following composition
M
m
D
d
T(OH)
4(5-m+d)
* n H
2
O
wherein M, D, T are monovalent, divalent or trivalent metals, m=0 to 1, d=0 to 1 whereby m+d≠0. The hereby obtained metal hydroxides are not chemical compounds, but gel-like compositions which are unsuitable as precursors for preparing metal oxides of defined structure according to the scope of the present invention.
Numerous processes for producing stratiform, anionic clay minerals are known in the art. All of these processes employ metal salts as starting materials which are dissolved and then mixed with each other at defined pH-values. See e. g. U.S. Pat. Specification 4,539,306 describing the production of hydrotalcites for pharmaceutical use, and Reichle, W. T.,
Journal of Catalysis, Vol.
94 (1985), p. 547-557, and Nunan, J. G., et al.,
Inorganic Chemistry, Vol.
28 (1989), p. 3868-3874. Misra et al. have disclosed the production of hydrotalcites having interstitial anions which increase the interlayer spacing by exchanging the anions at elevated temperatures (cf. U.S. Pat. Specification 5,075,089). Examples of the incorporation of large, organic anions by anion exchange are given by Lagaly, G., et al. in
Inorganic Chemistry, Vol.
29 (1990), p. 5201-5207. Miyata et al. have described the production of magnesium/aluminium hydrotalcites by mixing solutions of the salts MgCl
2
and Al
2
(SO
4
)
3
and a NaOH solution (cf.
Clay and Minerals,
Vol. 25 (1977), p. 14-18). EP-A1-0 536 879 proposes the production of stratiform, anionic clay minerals by using inorganic anions which increase the interlayer spacing, such as B(OH)
4−
, V
4
O
12
4−
, V
2
O
7
4−
, or HV
2
O
7
3−
. In said publication, too, solutions of metal salts are mixed at defined pH-values with solutions of the salts that are to be incorporated. Examples of the uses of stratiform, anionic clay minerals as catalysts are given in U.S. Pat. Specification 4,774,212, U.S. Pat. Specification 4,843,168, EP-A1-0 536 879, and by Drezdon, M. in
ACS Symp. Ser.,
(
Novel Mater. Heterog. Catal.
), Vol. 437 (1990), p. 140-148.
The wide use of stratiform, anionic clay minerals has been impeded up to now by the fact that for the production starting from metal salt solutions only a time-consuming, discontinuous synthesis route is known.
Furthermore, catalyst purity is a generally accepted, essential criterion. Contaminations caused by alkali—and alkaline earth metals are particularly undesirable. However, when using metal salts, said contaminants cannot be avoided, or they can only be avoided by great efforts involving high costs. Moreover, there is no process known for producing clay minerals of the hydrotalcite type with only OH
−
—ions located in the layers without additional, subsequent ion exchange.
The most important criterion for the catalytic properties of said clay minerals is their basicity. According to the prior art, the basicity is substantially determined by the Mg:Al ratio. See McKenzie, A. L., Fishel, C. T., and Davis, R. J. in
J. Catal., Vol.
138 (1992), p. 547. Therefore, in order to adjust the catalytic characteristics of a catalyst, it is desirable to provide the widest possible variation of the Mg:Al ratio. Furthermore, it has been unknown up to now to produce stratiform, anionic clay minerals with a Mg:Al ratio of less than 1.7.
It is the object of the present invention to provide a process for producing stratiform, anionic clay minerals having the following advantages:
time-saving synthesis which can be carried out both continuously and discontinuously
use of inexpensive and readily available starting materials
high purities and low alkali concentrations of the stratiform, anionic clay minerals produced by said process
optionally, the possibility of producing stratiform, anionic clay minerals comprising only hydroxide ions as interstitial anions
production of stratiform, anionic clay minerals having sufficiently large pore volumes and surface areas required for catalysis
production of stratiform, anionic clay minerals having a Mg:Al ratio of less than 1.7.
According to the present invention there is provided a process for producing high-purity hydrotalcites which are stratiform, anionic, mixed metal hydroxides of the general formula
M
2x
2+
M
2
3+
(OH)
4x+4
·A
2
n−
·z H
2
O
wherein x ranges from 0.5 to 10 in intervals of 0.5, A is an interstitial anion, n is the charge of said interstitial anion which is up to 8, normally up to 4, and z is an integer of 1 to 6, particularly 2 to 4, wherein
(A) metal alcoholate mixtures comprising at least one or more divalent metal(s), at least one or more trivalent metal(s), and mono-, di-, or trihydric C
1
to C
40
alcoholates are used, said di- and trihydric metal alcoholates being substantially used in a molar ratio corresponding to the stoichiometry of any desired compound according to the empirical formula referred to hereinabove, and
(B) the resultant alcoholate mixture is hydrolysed with water, the water for hydrolysis being used in stoichiometric excess, referring to the reactive valences of the metals used.
The corresponding mixed metal oxide can be produced therefrom by calcination.
The metal alcoholates can be produced by reacting metals having the oxidation numbers +II or +III with mono-, di- or trihydric C
1
to C
40
alcohols. The production of the metal alcoholates can be accomplished by
(A) placing the metals jointly into the reaction vessel and then adding the alcohol, or
(B) producing the metal alcohol
Brasch Andrea
Diblitz Klaus
Noweck Klaus
Sohiefler Jan
Bos Steven
Madan Mossman & Sriram P.C.
RWE-DEA Aktiengesellschaft fur Mineraloel und Chemie
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