Epoxidation catalyst and process

Details Epoxidation catalyst and process Epoxidation catalyst and process

2001-01-19

2001-10-30

Trinh, Ba K. (Department: 1625)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

C549S532000, C549S533000

Reexamination Certificate

active

06310224

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the epoxidation of olefins such as propylene by reaction of the olefin with hydrogen and oxygen using a metal doped noble metal on titanium silicalite catalyst, an essential feature of the invention being that reduction of the dopand is substantially avoided prior to use. In accordance with the invention the undesirable hydrogenation of the olefin during the epoxidation reaction is substantially avoided.
BACKGROUND OF THE INVENTION
It is known to epoxidze olefins such as propylene to form propylene oxide by reaction of propylene, hydrogen and oxygen using a catalyst comprised of palladium and platinum or titanium silicalite. See “Synthesis of Propylene Oxide from Propylene, Oxygen, and Hydrogen Catalyzed by Palladium-Platinum-Containing Titanium Silicalite” By: R. Meiers, U. Dingerdissen, and W. F. Holderich, Journal of Catalysis 176, 376-386 (1998). A feature of such prior procedures has been the reduction of the catalysts prior to use in the epoxidation reaction. The prior systems have been characterized by the undesirable formation of excessive amounts of propane which results from hydrogenation of the propylene reactant.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has been found that olefin hydrogenation during the epoxidation is reduced to a remarkable extent where the metal doped noble metal catalyst, eg Pd/Pt on titanium silicalite, is not completely reduced either by chemical or thermal means prior to use in the epoxidation reaction.
DETAILED DESCRIPTION OF THE INVENTION
The catalyst used in accordance with this invention comprises palladium incorporated with titanium containing zeolite, which can be prepared in accordance with known procedures. As a critical feature, the catalyst also comprises a metal doping component which is in substantially non-reduced state. Suitably at least 10% and preferably 50-100% of the doping component has a valence above zero, illustratively a valence of plus 2 in the case of platinum.
The preferred doping component is platinum although other components including silver, copper, gold and the like are useful. The doping component can be used in catalytic amounts relative to palladium, eg. a weight ratio of metal dopand to palladium of as low as 1:40 can be used, preferably the ratio is 1:30-1:1.
The preparation procedure of U.S. Pat. No. 6,063,942 is conveniently used wherein ion exchange preparation is employed and the resulting catalyst without drying or calcining is used in the epoxidation. By such a procedure reduction of the doping component is essentially avoided.
Also the doped catalyst can be isolated and dried at mild conditions and subsequently used for the epoxidation reaction.
Sequential preparation procedures wherein the catalyst components are separately incorporated with the silicalite with or without drying between stages provided substantial reduction of the dopand metal is avoided.
The catalysts to be prepared and used in the present invention are comprised of a titanium or vanadium zeolite and a noble metal (preferably an element of Group VIII of the Periodic Table). Suitable zeolites are those crystalline materials having a porous molecular sieve structure with titanium or vanadium atoms substituted in the framework. The choice of zeolite employed will depend upon a number of factors, including the size and shape of the olefin to be epoxidized. For example, it is preferred to use a relatively small pore titanium or vanadium zeolite such as a titanium silicalite if the olefin is a lower aliphatic olefin such as ethylene, propylene, or 1- butene. Where the olefin is propylene, the use of a TS-1 titanium silicalite or vanadium silicalite is especially advantageous. For a bulky olefin such as cyclohexene, a larger pore titanium zeolite such as a titanium zeolite having a structure isomorphous with zeolite beta may be preferred.
The titanium-containing zeolites useful as catalysts in the epoxidation step of the process comprise the class of zeolitic substances wherein titanium atoms are substituted for a portion of the silicon atoms in the lattice framework of a molecular sieve. Such substances are well known in the art.
Particularly preferred titanium-containing zeolites include the class of molecular sieves commonly referred to as titanium silicalites, particularly “TS-1” (having an MFI topology analogous to that of the ZSM-5 aluminosilicate zeolites), “TS-2” (having an MEL topology analogous to that of the ZSM-11 aluminosilicate zeolites), and “TS-3” (as described in Belgian U.S. Pat. No. 1,001,038). Also suitable for use are the titanium-containing molecular sieves having framework structures, isomorphous to zeolite beta, mordenite, ZSM-48, ZSM-12 and MCM41. The titanium-containing zeolite preferably contains no elements other than titanium, silicon and oxygen in the lattice framework, although minor amounts of boron, iron, aluminum, and the like may be present. Other metals such as tin or vanadium may also be present in the lattice framework of the zeolite in addition to the titanium, as described in U.S. Pat. Nos. 5,780,654 and 5,744,619.
Preferred titanium-containing zeolite catalysts suitable for use in the process of this invention will generally have a composition corresponding to the following empirical formula xTiO
2
(1−x)SiO
2
where x is between 0.0001 and 0.500. More preferably, the value of x is from 0.01 to 0.125. The molar ratio of Si:Ti in the lattice framework of the zeolite is advantageously from 9.5:1 to 99:1 (most preferably from 9.5:1 to 60:1). The use of relatively titanium-rich zeolites may also be desirable.
While any of the noble metals can be utilized (i.e., gold, silver, platinum, palladium, iridium, ruthenium, osmium), either alone or in combination, palladium is particularly desirable. Typically, the amount of noble metal present in the catalyst will be in the range of from 0.01 to 20 weight percent, preferably 0.1 to 5 weight percent.
In accordance with the invention, the zeolite in particulate form is slurried in a suitable solvent such as water or methanol or mixtures, and both the noble metal and metal dopand are incorporated into the zeolite by contact with a solution containing a soluble compound of the metals, for example, aqueous Pd tetraammine chloride and Pt tetraammine chloride with or without added ammonium hydroxide. There are no particular restrictions other then solubility regarding the choice of metal compound or complex used as the source of the noble metal. For example, suitable compounds for such purpose include the nitrates, sulfates, halides (e.g., chlorides, bromides), carboxylates (e.g. acetate), and amine complexes of noble metals and dopand metals.
Ambient temperatures are suitable for the catalyst preparation although higher or lower temperatures, eg. 0°-200° C., can be used. Generally the catalyst preparation is complete in an hour or so although longer or shorter times, eg. 5 minutes—24 hours can be used.
As a special feature of the invention, the slurry resulting from the catalyst preparation comprised of the metals in non-reduced form can be used directly for olefin epoxidation. For example, after completion of the catalyst preparation the temperature can be adjusted to the desired epoxidation temperature and oxygen, hydrogen and olefin reacted directly in the catalyst containing slurry to form epoxide. Epoxidation results achieved thereby can be better than those achieved by prior procedures where catalyst is dried and calcined before use. The effect is also achieved where the catalyst is isolated before use provided substantial reduction is avoided.
While it is preferred to prepare fresh catalyst and use the catalyst directly in the epoxidation reaction, benefits can be achieved by adding a noble metal and dopand ion exchangeable complex to a slurry which contains deactivated catalyst prepared by conventional procedures or by the procedures described above.
The olefin to be epoxidized can be any organic compound containing at least one site of ethylene unsaturation (i.e., at least

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