Chemistry of inorganic compounds – Zeolite – Isomorphic metal substitution
Reexamination Certificate
2001-02-20
2003-09-30
Sample, David (Department: 1755)
Chemistry of inorganic compounds
Zeolite
Isomorphic metal substitution
C423S326000, C423S707000, C549S531000
Reexamination Certificate
active
06627175
ABSTRACT:
The present invention relates to the manufacture of crystalline solids and in particular of titanium zeolites.
The first syntheses of titanium zeolites described in the literature form the subject-matter of U.S. Pat. Nos. 4,666,692 and 4,410,501, the latter being specific to titanium silicalites. The method of synthesis used consists of a hydrothermal treatment of a gel obtained by reaction between a hydrolysable silicon oxide source (such as an alkoxide), a titanium alkoxide, optionally an alkaline oxide, tetrapropyl-ammonium hydroxide, which acts both as mineralizing agent (which gives the OH
−
ions necessary for the hydrolysis) and as structuring agent (which influences the crystalline structure), and water. When it is desired to incorporate large amounts of titanium in the silicalite, the source of titanium oxide, more reactive than that of the silicon oxide, must be treated beforehand with an H
2
O
2
solution in order to avoid the precipitation of titanium oxide in anatase form outside the crystal lattice. This treatment and the subsequent addition of the nitrogenous organic base are carried out slowly and under cold conditions, which renders the manufacturing process laborious. In addition, before crystallization of silicalite, the alcohols resulting from the hydrolysis of the silicon and titanium alkoxides are evaporated. This evaporation takes time, consumes energy and furthermore constitutes a difficult operation to carry out on industrial scale, which additionally requires adjustment of the water content after evaporation.
Thangaraj et al. (Zeolites, 1992, Vol. 12, November/December, p. 943-950) have proposed a modified procedure for the synthesis of TS-1 which makes it possible to overcome the use of H
2
O
2
solutions and to incorporate larger amounts of titanium in the lattice. This method consists in adding a first portion of the solution of the nitrogenous organic base (TPAOH or tetra-n-propylammonium hydroxide) to STEO (silicon tetraethoxide) and in adding TTBO (titanium tetra-n-butoxide), in solution in iPrOH (isopropyl alcohol), dropwise to the combined mixture. After stirring, the remainder of the solution of the nitrogenous organic base is added and the mixture is heated to evaporate the alcohol. This method exhibits the disadvantage of using an alcoholic solution of TTBO, which is not very stable with respect to hydrolysis, and thus promotes the formation of titanium oxide precipitates. In addition, it includes a stage of evaporation of the alcohols present in the gel before crystallization.
Recently, Tuel (Catalysis Letters, 51, 1998, p. 59-63) has shown that TS-1 can be synthesized starting with the same reactants without evaporation of the isopropyl (iPrOH), ethyl (EtOH) and n-butyl (nBuOH) alcohols present in the gel, without this having an effect on the crystalline structure and the catalytic activity of the TS-1 when it is used as catalyst for reactions for the hydroxylation of phenols with H
2
O
2
. Nevertheless, this method still exhibits the disadvantage of using an alcoholic solution of the titanium alkoxide, which is not very stable with respect to hydrolysis, which solution thus has to be used rapidly and carefully after its manufacture.
The Applicant Company has found that, surprisingly, during the synthesis of crystalline solids starting from at least one source of oxide of the element from Groups IIIa, IVa and Va and from at least one transition metal oxide source, the choice of an alcohol having a pK
a
below that of water to dissolve the transition metal oxide source makes it possible to obtain a stable solution which does not result in the formation of precipitate during addition of this solution to the aqueous hydrolysis medium and which can be stored before it is used. In addition, this alcohol can remain present during the crystallization without affecting the crystalline structure and the catalytic activity of the crystalline solid obtained.
Consequently, the fact of using alcohol having a pK
a
below that of water to dissolve the source of the oxide of the transition metal makes it possible to obtain crystalline solids according to a process which is easy to employ and which makes it possible to easily reduce, indeed even to avoid, the formation of metal oxide precipitates outside the crystal lattice, such as, for example, anatase.
The present invention consequently relates to a process for the manufacture of crystalline solids comprising at least one element chosen from Groups IIIa, IVa and Va and at least one transition metal, according to which at least one source of oxide of the element from Groups IIIa, IVa and Va and at least one transition metal oxide source, which was dissolved beforehand in an alcohol having a pK
a
below that of water, are hydrolysed in an aqueous medium comprising a mineralizing agent and the gel thus obtained is crystallized in the presence of a structuring agent.
The crystalline solids according to the present invention denote all the solids where the atoms are arranged so as to define a crystal lattice and which comprise at least one element chosen from Groups IIIa, IVa and Va and at least one transition metal. The element from Groups IIIa, IVa and Va can be aluminium, silicon or phosphorus. This element is preferably silicon. The transition metal can be titanium, vanadium, zirconium, chromium, iron or cobalt. This metal is preferably titanium.
These crystalline solids are advantageously zeolites. They are preferably titanium zeolites. The term “titanium zeolite” is understood to denote a solid comprising silica which exhibits a microporous crystalline structure of zeolite type and in which several silicon atoms are replaced by titanium atoms. The titanium zeolite advantageously exhibits a crystalline structure of ZSM-5 or ZSM-11 type. It can also exhibit a crystalline structure of zeolite &bgr; type devoid of aluminium. It preferably exhibits an infrared absorption band at approximately 950-960 cm
−1
. Titanium zeolites of silicalite type are highly suitable. Those corresponding to the formula xTiO
2
(1−x)SiO
2
, in which x is from 0.0001 to 0.5, preferably from 0.001 to 0.05, have an excellent performance. Materials of this type, known under the name of TS-1, exhibit a microporous crystalline zeolite structure analogous to that of zeolite ZSM-5. The properties and the main applications of these compounds are known (B. Notari; Structure-Activity and Selectivity Relationship in Heterogeneous Catalysis; R. K. Grasselli and A. W. Sleight Editors; Elsevier; 1991; p. 243-256).
The source of oxide of the element resulting from Groups IIIa, IVa and Va is a hydrolysable compound of this element, such as, for example, the alkoxides of this element. When it is silicon, the source can be chosen from colloidal silica, alkali metal or alkaline earth metal silicates, and silicon alkoxides. The latter are particularly well suited.
The transition metal oxide source according to the invention can be a transition metal halide, such as, for example, TiCl
3
, or a transition metal alkoxide. Transition metal alkoxides are particularly well suited. Titanium alkoxides give good results.
The term “silicon or titanium alkoxides” is understood to denote compounds corresponding to the general formula M(OR)
4
where M is either silicon or titanium and R is a hydrocarbonaceous group. The silicon alkoxide is preferably a silicon tetraalkoxide. The alkoxide group advantageously comprises up to 10 carbon atoms, preferably up to 6 carbon atoms and more particularly up to 4 carbon atoms. Silicon tetraethoxide or STEO gives good results. The titanium alkoxide is preferably a titanium tetraalkoxide. The alkoxide group advantageously comprises up to 10 carbon atoms, preferably up to 6 carbon atoms and more particularly up to 4 carbon atoms. With the process according to the invention, good results are obtained when the titanium alkoxide is derived from an alcohol which has a pK
a
above that of water. Examples of such alcohols are ethanol, propanol and butanol. Titanium tetra-n-butoxide or TTBO gives excellent results
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sample David
Solvay ( Societe Anonyme)
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