Ethylene oxide catalyst

Details Ethylene oxide catalyst Ethylene oxide catalyst

2001-11-13

2004-07-13

Nguyen, Ngoc-Yen (Department: 1754)

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

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

C549S536000, C502S216000, C502S217000, C502S218000, C502S224000, C502S344000, C502S347000

Reexamination Certificate

active

06762311

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a catalyst for the oxidation of ethylene to ethylene oxide consisting of a critical combination of silver, alkali metal such as cesium, and sulfur deposited on a support such as alpha alumina and to the production of ethylene oxide using the catalyst; a fluorine or chloride component optionally can be included. The catalyst is essentially free of rhenium or transition metal components.
2. Description of the Prior Art
Processes for the production of ethylene oxide involve the vapor phase oxidation of ethylene with molecular oxygen using a solid catalyst comprised of silver on a support such as alumina. There have been great efforts by many workers to improve the effectiveness and efficiency of the silver catalyst for producing ethylene oxide. U.S. Pat. No. 5,051,395 provides a comprehensive analysis of these efforts of prior workers.
Among the many prior teachings in this area is that of U.S. Pat. No. 4,007,135 (see also UK 1,491,447) which teaches variously silver catalysts for the production of ethylene and propylene oxides comprised of a promoting amount of copper, gold, magnesium, zinc, cadmium, mercury, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium, and/or preferably barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 1-15), silver catalysts for the production of propylene oxide comprising a promoting amount of at least one promoter selected from lithium, potassium, sodium, rubidium, cesium, copper, gold, magnesium, zinc, cadmium, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium and barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 16-34), as well as silver catalysts for producing ethylene oxide or propylene oxide comprising (a) a promoting amount of sodium, cesium, rubidium, and/or potassium, and (b) magnesium, strontium, calcium and/or preferably barium in a promoting amount (column 3, lines 5-8).
U.S. Pat. No. 5,057,481, and related U.S. Pat. No. 4,908,343 are concerned with silver ethylene oxide catalysts comprised of cesium and an oxyanion of a group 3b to 7b element.
U.S. Pat. No. 3,888,889 describes catalysts suitable for the oxidation of propylene to propylene oxide comprised of elemental silver modified by a compound of an element from Group 5b and 6b. Although the use of supports is mentioned, there are no examples. The use of cesium is not mentioned.
European Publication 0 266 015 deals with supported silver catalysts promoted with rhenium and a long list of possible copromoters.
U.S. Pat. No. 5,102,848 deals with catalysts suitable for the production of ethylene oxide comprising a silver impregnated support also having thereon at least one cation promoter such as cesium, and a promoter comprising (i) sulfate anion, (ii) fluoride anion, and (iii) oxyanion of an element of Group 3b to 6b inclusive of the Periodic Table. Possibly for purposes of comparison since it is outside the scope of catalyst claimed, the patent shows at columns 21 and 22 a catalyst No. 6 comprised of Ag/Cs/S/F on a support, the Cs amount being 1096 ppm.
U.S. Pat. No. 5,486,628 describes a silver catalyst promoted with alkali metal, rhenium and a rare earth or lanthanide component.
U.S. Pat. No. 5,011,807 is concerned with an ethylene oxide catalyst comprised of silver, alkali metal, a transition metal, and sulfur on alumina support. Presented for comparative purposes are catalysts comprised of silver, alkali metal and sulfur on alumina support which provide results inferior to the transition metal containing catalysts.
In the context of the bewildering and vast number of references, many of them contradictory, applicant has discovered a novel and improved catalyst for the production of ethylene oxide.
BRIEF DESCRIPTION OF THE INVENTION
The present invention relates to an improved supported silver ethylene oxide catalyst containing a promoter combination consisting of a critical amount of both an alkali metal component, preferably cesium, together with a sulfur component, and to the catalyst preparation and use; the catalyst is essentially free of rhenium and transition metal components and optionally can contain a fluorine or chloride component.
DETAILED DESCRIPTION
Preferred catalysts prepared in accordance with this invention contain up to about 30% by weight of silver, expressed as metal, deposited upon the surface and throughout the pores of a porous refractory support. Silver contents higher than 20% by weight of total catalyst are effective, but result in catalysts which are unnecessarily expensive. Silver contents, expressed as metal, of about 5-20% based on weight of total catalyst are preferred, while silver contents of 8-15% are especially preferred.
In addition to silver, the catalyst of the invention also contains a promoter combination consisting of critical amounts of alkali metal and sulfur. The critical amount of alkali metal promoter component is at least 1000 ppm expressed as alkali metal based on the catalyst weight; preferably the catalyst contains 1200-6000 ppm, alkali metal based on the catalyst weight. Preferably the alkali metal is cesium although lithium, sodium, potassium, rubidium and mixtures can also be used. Impregnation procedures such as are described in U.S. Pat. No. 3,962,136 are advantageously employed for addition of the cesium component to the catalyst.
Necessary also for practice of the invention is the provision of sulfur as a promoting catalyst component in a critical amount relative to the alkali metal. The sulfur component can be added to the catalyst support impregnating solution as sulfate, eg. cesium sulfate, ammonium sulfate, p-toluene sulfonic acid, and the like. U.S. Pat. No. 4,766,105 describes the use of sulfur promoting agents, for example at column 10, lines 53-60, and this disclosure is incorporated herein by reference. The amount of sulfur (expressed as the element) based on the weight of catalyst in accordance with the invention is critical and must be 40-150% of the equivalent amount necessary to form the alkali metal sulfate, eg. Cs
2
SO
4
, preferably 40-100% of this amount.
It has been found that the use of relatively high amounts of alkali metal, as above indicated, which normally would produce an inactive catalyst, when used in combination with the designated amounts of sulfur produces a catalyst having outstanding activity and selectivity.
The catalyst also optionally may contain a fluorine or chlorine promoter in amount expressed as the element of 10-300 ppm, preferably 30-100 ppm by weight based on the catalyst as an optional component. Ammonium fluoride or chloride, alkali metal fluoride or chloride, and the like can be used.
The catalysts are made with supports comprising alumina, silica, silica-alumina or combinations thereof. Preferred supports are those containing principally alpha-alumina, particularly those containing up to about 15 wt % silica.
Especially preferred supports have a porosity of about 0.1-1.0 cc/g and preferably about 0.2-0.7 cc/g. Preferred supports also have a relatively low surface area, i.e. about 0.2-2.0 m
2
/g, preferably 0.4-1.6 m
2
/g and most preferably 0.5-1.3 m
2
/g as determined by the BET method. See J. Am. Chem. Soc. 60, 3098-16 (1938). Porosities are determined by the mercury porosimeter method; see Drake and Ritter, “Ind. Eng. Chem. anal. Ed.,” 17, 787 (1945). Pore and pore diameter distributions are determined from the surface area and apparent porosity measurements.
For use in commercial ethylene oxide production applications, the supports are desirably formed into regularly shaped pellets, spheres, rings, etc. Desirably, the support particles may have “equivalent diameters” in the range from 3-10 mm and preferably in the range of 4-8 mm, which are usually compatible with the internal diameter of the tubes in which the catalyst is placed. “Equivalent diameter” is the diameter of a sphere having the same exter

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