Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2000-02-16
2001-12-11
Sample, David R. (Department: 1755)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C549S531000
Reexamination Certificate
active
06329537
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to molecular sieves containing silicon, tellurium, and, optionally, titanium oxides. Such molecular sieves are useful as catalysts, particularly in the liquid phase epoxidation of olefins.
BACKGROUND OF THE INVENTION
Siliceous molecular sieves or zeolites wherein certain of the silicon atoms in the crystalline framework have been replaced by titanium atoms are well-known in the art and have been extensively investigated as active and selective oxidation catalysts. For example, U.S. Pat. No. 4,401,501 describes the synthesis of “TS-1” titanium silicalite having a framework structure isomorphous with ZSM-5 (i.e., an MFI topology). Titanium silicalite catalyzes the epoxidation of olefins with hydrogen peroxide, as described in U.S. Pat. No. 4,833,260. In an effort to modify or improve the properties of such materials, the incorporation of additional metal atoms into titanium-containing zeolites has also been attempted. For instance, EP 226,257 teaches a crystalline material of zeolitic character containing silicon, titanium and aluminum oxides. Similarly, EP 266,258 teaches a zeolite material containing silicon, titanium and iron oxides. Zeolites containing oxides of silicon, titanium and gallium are described in EP 266,825. However, there have apparently been no reports to date of a successful preparation of zeolites containing silicon and tellurium oxides or silicon, titanium and tellurium oxides. The catalytic properties of such materials are unknown.
SUMMARY OF THE INVENTION
This invention provides a molecular sieve comprised of oxides of silicon, tellurium, and, optionally, titanium.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a synthetic material containing silicon oxide, tellurium oxide, and, optionally, titanium oxide having a crystalline, porous, structure of zeolitic character. While, as will be explained later in more detail, the topology of these substances may readily be controlled and varied as desired for different end-uses, this invention includes the synthesis of tellurium-containing molecular sieves characterized by framework structures having MFI, MEL, BEA, ZSM-48, MTW or MCM-41 topologies. An MFI framework structure is most preferred where the molecular sieve is to be used as an epoxidation catalyst for a relatively small olefin such as propylene in combination with hydrogen peroxide as an oxidizing agent. In its calcined and anhydrous state, these zeolite materials correspond to the empirical formula: SiO
2
:aTiO
2
:bTeO
2
wherein “a” is typically from 0 to 0.10 (in one embodiment, from 0.005 to 0.10) and “b” is typically from 0.005 to 0.2. Expressed as a percentage by weight of the calcined and anhydrous material, in preferred embodiments of the invention the Ti content ranges from 0.1 to 4.5% and the Te content ranges from 0.05 to 5%. Generally speaking, the activity of the molecular sieve as an olefin epoxidation catalyst will tend to increase as the titanium content is increased within this range.
Without wishing to be bound by theory, it is believed based on analytical characterization of these novel molecular sieves that tellurium atoms (and, where titanium is present, titanium atoms) are substituted for silicon atoms in the framework of a zeolite type framework. In certain preferred embodiments, the molecular sieves are isomorphous with either MFI (ZSM-5), MEL (ZSM-11), BEA (beta), ZSM-48, MTW (ZSM-12), or MCM-41 aluminosilicate zeolites (i.e., zeolites having aluminum and silicon, but not tellurium, atoms present in their framework lattices).
The novel molecular sieves of the invention may be prepared by a process wherein under hydrothermal conditions a derivative of silicon, a derivative of titanium (if titanium oxide incorporation is desired), a derivative of tellurium, and a nitrogenous organic base are reacted. The SiO
2
/TeO
2
molar ratio of the reactants is desirably greater than 50 but less than 600. The range of from 80 to 140 is particularly advantageous. The SiO
2
/TiO
2
molar ratio of the reactants in the embodiment where titanium is present is desirably greater than 5 but less than 450, with the range of from 10 to 40 being especially advantageous. The H
2
O/SiO
2
molar ratio of the reactants is desirably in the range of from 10 to 100, preferably within the range of 25 to 50. Although an alkali metal or alkaline earth metal compound such as an alkali metal hydroxide may also be present, generally it will be desirable to maintain the M/SiO
2
molar ratio (where M is the alkali metal or alkaline earth metal) lower than 0.1, preferably lower than 0.01, or (most preferably) equal to 0. The presence of alkali metal or alkaline earth metal cations can lead to the formation of undesirable Ti or Te phases. Preferably, all of the basic ions (e.g., hydroxide) needed in the reaction mixture are supplied by the nitrogenous organic base. Similarly, although oxides of other substances such as aluminum oxides and the like could be introduced into the molecular sieve, the amounts of such other oxides in preferred embodiments of the invention are kept low relative to the amounts of TiO
2
and TeO
2
which are present. In preferred embodiments, the molecular sieve is aluminum free or essentially aluminum free (i.e., less than 500 ppm Al). The molecular sieve may, for example, consist essentially of silicon oxides and tellurium oxides or silicon oxides, titanium oxides, and tellurium oxides.
Although the silicon derivative may be any substance capable of functioning as a source of SiO
2
in a hydrothermal synthesis, such as, for example, fumed silica, silica gel or silica sol, the silicon derivative preferably is a tetraalkyl orthosilicate such as tetramethyl orthosilicate or tetraethyl orthosilicate. Similarly, while the optional titanium derivative may be any substance capable of functioning as a source of TiO
2
in a hydrothermal synthesis, such as, for example, a titanium salt (e.g., titanium halide), in preferred embodiments of the invention the titanium derivative is a tetraalkyltitanate where the alkyl groups are the same or different and are C
1
-C
6
alkyl groups such as methyl, ethyl, n-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl and the like. Tetra n-butyl orthotitanate is a particularly preferred titanium derivative. Likewise, the tellurium derivative may be any substance capable of functioning as a source of TeO
2
in a hydrothermal synthesis. While tellurium salts such as halides and hydroxides could be utilized, it is preferred to use tellurium alkoxides where the alkoxide groups are preferably C
1
-C
6
alkoxide groups such as ethoxide, propoxide and the like. In a particularly preferred embodiment, tellurium ethoxide is utilized. Co-precipitates or co-gels comprised of Si and Ti, Si and Te, Ti and Te, or Si, Ti and Te can also be used as starting materials.
The nitrogenous organic base is preferably an alkyl ammonium hydroxide, preferably a quaternary alkyl ammonium hydroxide. The nitrogenous organic base may alternatively comprise a mixture of an alkyl ammonium halide (e.g., tetrapropyl ammonium bromide) and an organic amine (e.g., triethylamine, ethylene diamine). The NOB/SiO
2
molar ratio (where NOB=nitrogenous organic base) of the reactants is desirably maintained in the range of from 0.1 to 1, preferably from 0.2 to 0.5. The morphology of the molecular sieve may be controlled as desired by varying the structure of the nitrogenous organic base employed. Without wishing to be bound by theory, it is believed that the cation portion of the nitrogenous organic base functions as a template or structure directing agent. The size and shape of the cation appears to influence the hydrothermal crystallization process such that the framework of the resulting molecular sieve assumes either an MFI (ZSM-5), MEL (ZSM-11), BEA (beta), MTW (ZSM-12), MCM-41, ZSM-48 or other desired topology. For example, the use of tetrapropyl ammonium hydroxide leads to formation of an MFI framework. Where the cation is tetrabutyl ammonium or dialkyl 3,5-dimethyl piperidinium (s
Arco Chemical Technology L.P.
Long William C.
Sample David R.
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