1. Field of the Invention
This invention relates to a catalyst for the production of ethylene oxide, a method for the production of the catalyst, and a method for the production of ethylene oxide. It relates more particularly to a silver catalyst which excels in catalytic activity, selectivity, and service life and permits production of ethylene oxide at high selectivity for a long time, a method for the production thereof, and a method for the production of ethylene oxide by the use of this silver catalyst.
2. Description of the Related Art
The production of ethylene oxide by the catalytic vapor phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst is widely practiced on a commercial scale. Concerning the silver catalyst to be used for the catalytic vapor phase oxidation, numerous inventions covering carriers for the catalyst, methods for depositing the catalyst on such carriers, and reaction promoters used therein have been filed as the patent.
Though the silver catalysts proposed to date are already improved enough to allow high levels of selectivity, the desirability of developing a silver catalyst capable of still higher selectivity finds recognition because an increase of the selectivity even by 1% has a high economic effect in the light of the scale of production of ethylene oxide. By the same token, the improvement in the service life or durability of a silver catalyst brings a high commercial significance.
The mechanism of the deterioration in efficiency of a silver catalyst has not been fully elucidated. A method which improves a silver catalyst effectively in service life has not yet been known to the art.
In the production on a commercial scale of ethylene oxide by the catalytic vapor phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst, it has been heretofore customary to incorporate in the feed gas for a reaction zone such a gaseous organic chlorine compound as, for example, ethylene dichloride, in an amount generally in the approximate range of 0.1 to 10 ppm for the purpose of improvement in selectivity. The present status of this practice, however, is such that the protracted use of the organic chlorine compound of this sort results in deteriorating the catalyst in performance and eventually shortening the service life of the catalyst.
Our diligent study on the cause for this deterioration, through falling short of definitely elucidating the mechanism thereof, has demonstrated that the adhesion to or the adsorption on the catalyst of the organic chlorine compound in the reaction gas induces the catalyst to suffer from the deterioration. This invention has been perfected on this knowledge.
It has been further found that the adhesion or adsorption of chlorine to or on the catalyst mentioned above can be effectively precluded by having the surface of the catalyst treated with an organic nitrogen compound. This knowledge has brought this invention to perfection.
The practice of using such an organic amine as, for example, ethanolamine, 1,3-propanediamine, ethylenediamine, or an amide in impregnating a carrier for a catalyst with a solution of a silver compound thereby effecting deposition of silver on the carrier has been heretofore known publicly (as disclosed, for example, in U.S. Pat. No. 4,769,358, JP-A-47-11,467, JP-B-59-29,291, JP-B-05-29,501, JP-A-62-114,654, etc.).
The catalyst obtained by utilizing this practice, however, is at a disadvantage in requiring the organic amine to be incorporated in the solution of the silver compound and, as a result, suffering the organic amine to decompose, volatilize, and leave behind substantially no residue in the finished catalyst when the carrier having undergone the impregnation is dried and then calcined. The practice under discussion, therefore, is not effective in preventing the catalyst from adhesion or adsorption of chlorine.
An object of this invention, therefore, is to provide a silver catalyst which excels in catalytic activity, selectivity, and service life and permits production of ethylene oxide at high selectivity for a long time.
Another object of this invention is to provide a method for the production of the silver catalyst mentioned above.
Yet another object of this invention is to provide a method for the production of ethylene oxide by the use of the silver catalyst mentioned above.