Process for producing acetic anhydride alone or acetic anhydride

Details Process for producing acetic anhydride alone or acetic anhydride Process for producing acetic anhydride alone or acetic anhydride

1995-03-22

1997-07-15

Ivy, C. Warren

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

Organic compounds

Carboxylic acid anhydrides

562519, 562890, 562517, C07C 5156, C07C 5154

Patent

active

056485310

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for producing acetic anhydride alone or acetic anhydride and acetic acid by reacting methyl acetate and/or dimethyl ether and optionally water and/or methanol with carbon monoxide alone or carbon monoxide and hydrogen in the presence of a rhodium compound and methyl iodide as the principal catalysts.


RELATED ART

Acetic acid is an essential compound necessary in many industries, including the polymer industry and the chemical industry, wherein acetic acid is used in a large amount as a starting material of acetic esters, acetic anhydride, vinyl acetate, and terephthalic acid. On the other hand, acetic anhydride not only is used in a large amount as a starting material of cellulose acetate, but also is a useful compound as a starting material of chemicals such as drugs, perfumes, and dyes. Further, acetic acid and acetic anhydride are mutually related compounds in practical uses. For example, in the cellulose acetate industry, acetic anhydride is produced from acetic acid and then reacted with cellulose to give cellulose acetate and acetic acid, which is reused.
Although acetic anhydride has heretofore been produced from ketene obtained by thermal decomposition of acetic acid, there has recently been developed a process wherein acetic anhydride is produced from methyl acetate or dimethyl ether, and carbon monoxide. A reaction involved in this process is a catalytic reaction wherein a rhodium compound and methyl iodide are used as the principal catalysts. Since the reaction rate is low when only the principal catalysts are used, however, various promoters have been proposed.
A function required of the promotors is to promote the reaction of methyl acetate or dimethyl ether with carbon monoxide to such an extent as to enable the reaction to be industrially operable. A desirable promoter is one which can exhibit a great reaction-promoting effect even when added in an amount as small as possible and helps reduce the formation of by-products, including tar.
Although various promoters have been proposed for this purpose, the formation of such by-products is not always dependent on the kind of promoter alone. A primary factor in the formation of these by-products is believed to be a rhodium hydride complex formed by a reaction of the rhodium compound as one of the principal catalysts with hydrogen present in the liquid reaction mixture. Since rhodium serves as a catalyst in the main reaction, the concentration thereof in the liquid reaction mixture must be maintained at a necessary level. On the other hand, hydrogen is necessary for enabling rhodium as one of the principal catalysts to be maintained in a highly active monovalent state, and must therefore be fed together with carbon monoxide as starting materials into a reactor according to known techniques (U.S. Pat. No. 4,374,070).
Further, as the partial pressure of hydrogen in the gaseous phase portion in the reactor is increased, the amount of the by-products formed is increased. Aside from the magnitude of the amount of the by-products formed, this tendency is always recognized, even when any promoter is used. Accordingly, it is a known fact that it is important in industrially carrying out the reaction to maintain the partial pressure of hydrogen in the gaseous phase portion in the reactor at a necessary but minimum and constant level.
In view of the foregoing, U.S. Pat. No. 4,384,907 discloses a method of removing tar formed as a by-product, wherein tar extracted with an aqueous solution of HI is further extracted with aqueous ammonia to recover an additional amount of rhodium from the tar. However, this method necessarily involves the use of ammonia and subsequent removal of the ammonia from the process stream that must be returned to the reaction. Although various other methods have been attempted (U.S. Pat. No. 4,476,238), the fact is that no methods of easily and effectively removing tar have been found yet. Further, the presence of tar sometimes decreases the catalytic activity of the catalysts, and entrap t

REFERENCES:
patent: 4102922 (1978-07-01), Price
patent: 4364907 (1982-12-01), Barnes
patent: 4374070 (1983-02-01), Larkins et al.
patent: 4476238 (1984-10-01), Palmer et al.
patent: 5362365 (1994-11-01), Niijima et al.
patent: 5380929 (1995-01-01), Erpenbach et al.

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