Direct epoxidation process using a mixed catalyst system

Details Direct epoxidation process using a mixed catalyst system Direct epoxidation process using a mixed catalyst system

2001-10-19

2002-12-24

Trinh, Ba K. (Department: 1625)

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

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

C549S532000, C549S523000

Reexamination Certificate

active

06498259

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a liquid-phase epoxidation process using a mixed catalyst system to produce epoxides from hydrogen, oxygen, and olefins wherein the liquid-phase system contains a buffer. The mixed catalyst system contains a titanium zeolite and noble metal catalyst. Surprisingly, the use of a buffer in the process results in decreased ring-opening to unwanted glycols and glycol ethers in olefin epoxidation and also typically improves the activity of the process.
BACKGROUND OF THE INVENTION
Many different methods for the preparation of epoxides have been developed. Generally, epoxides are formed by the reaction of an olefin with an oxidizing agent in the presence of a catalyst. The production of propylene oxide from propylene and an organic hydroperoxide oxidizing agent, such as ethyl benzene hydroperoxide or tert-butyl hydroperoxide, is commercially practiced technology. This process is performed in the presence of a solubilized molybdenum catalyst, see U.S. Pat. No. 3,351,635, or a heterogeneous titania on silica catalyst, see U.S. Pat. No. 4,367,342. Hydrogen peroxide is another oxidizing agent useful for the preparation of epoxides. Olefin epoxidation using hydrogen peroxide and a titanium silicate zeolite is demonstrated in U.S. Pat. No. 4,833,260. One disadvantage of both of these processes is the need to pre-form the oxidizing agent prior to reaction with olefin.
Another commercially practiced technology is the direct epoxidation of ethylene to ethylene oxide by reaction with oxygen over a silver catalyst. Unfortunately, the silver catalyst has not proved very useful in epoxidation of higher olefins. Therefore, much current research has focused on the direct epoxidation of higher olefins with oxygen and hydrogen in the presence of a catalyst. In this process, it is believed that oxygen and hydrogen react in situ to form an oxidizing agent. Thus, development of an efficient process (and catalyst) promises less expensive technology compared to the commercial technologies that employ pre-formed oxidizing agents.
Many different catalysts have been proposed for use in the direct epoxidation of higher olefins. For liquid-phase reactions, the catalysts typically contain palladium on a titanium zeolite support. For example, JP 4-352771 discloses the epoxidation of propylene oxide from the reaction of propylene, oxygen, and hydrogen using a catalyst containing a Group VIII metal such as palladium on a crystalline titanosilicate. The vapor-phase oxidation of olefins has been shown to produce epoxides over gold supported on titanium oxide (Au/TiO
2
or Au/TiO
2
—SiO
2
), see for example U.S. Pat. No. 5,623,090, and gold supported on titanosilicates, see for example PCT Intl. Appl. WO 98/00413.
Mixed catalyst systems for olefin epoxidation with hydrogen and oxygen have also been disclosed. For example, JP 4-352771 at Example 13 describes the use of a mixture of titanosilicate and Pd/C for propylene epoxidation. U.S. Pat. No. 6,008,388 also describes a catalyst in which palladium is typically added to a titanium zeolite to form a catalyst system, but additionally teaches that the palladium can be incorporated into a support before mixing with the zeolite. However, the only supports that are disclosed include silica, alumina, and activated carbon. In addition, copending Application Ser. No. 09/624,942 discloses a mixed catalyst system that is useful in olefin epoxidation comprising a titanium zeolite and a gold-containing supported catalyst.
One disadvantage of liquid-phase epoxidation reactions using these catalysts is that they are prone to ring-open under standard reaction conditions to form less desirable ring-opened products such as glycols or glycol ethers. The formation of these undesired by-products is especially likely to happen when water is used as the solvent.
In sum, new processes for the direct epoxidation of olefins are eeded. Particularly valuable processes would have good productivity nd selectivity to epoxides, while reducing the likelihood of ring-opening epoxides to glycols or glycol ethers.
SUMMARY OF THE INVENTION
The invention is an olefin epoxidation process that comprises reacting an olefin, oxygen, and hydrogen in a solvent in the presence of a catalyst mixture, wherein the solvent contains a buffer. The catalyst mixture comprises a titanium zeolite and a noble metal catalyst. The process is surprisingly found to produce much lower amounts of undesired glycol by-products compared to the process without use of the buffer. Surprisingly, the use of a buffer also typically improves the activity of the process.
DETAILED DESCRIPTION OF THE INVENTION
The process of the invention employs a catalyst mixture that comprises a titanium zeolite and a noble metal-containing supported catalyst. Suitable titanium zeolites are those crystalline materials having a porous molecular sieve structure with titanium atoms substituted in the framework. The choice of titanium 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 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 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.
Titanium zeolites 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 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 Pat. No. 1,001,038). Titanium-containing molecular sieves having framework structures isomorphous to zeolite beta, mordenite, ZSM48, ZSM-12, and MCM-41 are also suitable for use. The titanium zeolites preferably contain no elements other than titanium, silicon, and oxygen in the lattice framework, although minor amounts of boron, iron, aluminum, sodium, potassium, copper and the like may be present.
Preferred titanium zeolites will generally have a composition corresponding to the following empirical formula xTiO
2
(1−x)SiO
2
where x is between 0.0001 and 0.5000. 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.
The catalyst mixture employed in the process of the invention also contains a noble metal catalyst. While any noble metal catalyst can be utilized (i.e., gold, silver, platinum, palladium, iridium, ruthenium, osmium metal catalysts), either alone or in combination, palladium and gold are particularly desirable. Suitable noble metal catalysts include high surface area noble metals, noble metal alloys, and supported noble metal catalysts. Examples of suitable noble metal catalysts include high surface area palladium and palladium alloys. However, particularly preferred noble metal catalysts are supported noble metal catalysts comprising a noble metal and a support.
For supported noble metal catalysts, the support is preferably a porous material. Supports are well-known in the art. There are no particular restrictions on the type of support that are used. For instance, the support can be inorganic oxides, inorganic chlorides, carbon, and organic polymer resins. Preferred inorganic oxides include oxides of Group 2, 3, 4, 5, 6, 13, or 14 elements. Particularly preferred inorganic oxide supports include silica, alumina, titania, zirconia

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