Catalyst for use in producing carboxylic esters

Details Catalyst for use in producing carboxylic esters Catalyst for use in producing carboxylic esters

2002-03-20

2003-10-21

Killos, Paul J. (Department: 1625)

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

Organic compounds

Carboxylic acid esters

C502S102000, C560S238000

Reissue Patent

active

RE038283

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a catalyst for use in producing a carboxylic ester from an aldehyde, an alcohol and molecular oxygen, a method for producing the catalyst, and a method for the production of a carboxylic ester by using the catalyst. More particularly, the present invention is concerned with a novel catalyst which comprises a carrier having supported thereon palladium and lead in a specific atomic ratio, and which exhibits a maximum intensity peak at a diffraction angle (2&thgr;) in a specific range in a powder X-ray diffraction pattern thereof. The present invention is also concerned with a method for producing the catalyst and a method for the continuous production of a carboxylic ester from an aldehyde, an alcohol and molecular oxygen by using the catalyst. In the production of a carboxylic ester using the catalyst of the present invention, the desired carboxylic ester can be obtained with high selectivity even when the reaction is conducted under high temperature and high aldehyde concentration. Further, the catalyst of the present invention exhibits high mechanical strength and high corrosion resistance. In the continuous method for producing a carboxylic ester using the catalyst of the present invention, the desired carboxylic ester can be produced with high efficiency and in high yield for a prolonged period of time.
2. Prior Art
Methods for producing methyl methacrylate or methyl acrylate, which is commercially useful, have already been practiced on a commercial scale. For example, a method for producing methyl methacrylate has been commercially practiced in which methacrolein is oxidized with molecular oxygen to produce methacrylic acid and then, the methacrylic acid produced is reacted with methanol to produce methyl methacrylate. As a result of various attempts for improving catalysts over many years, the yield of methacrylic acid in the above-mentioned step of oxidizing methacrolein has been improved to some extent, namely to a value in the range of from 80% to 85%, which is, however, still unsatisfactory from the commercial viewpoint. Therefore, with respect to the yield of methacrylic acid in the above step, a further improvement has been desired. Further, heteropolyacid catalysts, which are conventionally used in the above reaction for oxidizing methacrolein to produce methacrylic acid, have problems in that they have poor thermal stability, so that the heteropolyacid catalysts are gradually decomposed at the reaction temperatures. It has been reported that improvements have been made in the thermal stability of heteropolyacids. However, it is generally considered that even such improved catalysts are still unsatisfactory in catalytic life when used in commercial practice for the production of methyacrylic acid from methacrolein.
On the other hand, a new process for producing methyl methacrylate or methyl acrylate has been attracting attention, in which methacrolein or acrolein is reacted with methanol and molecular oxygen to there-by produce methyl methacrylate or methyl acrylate by a single-step process {hereinafter, methacrolein and acrolein are frequently collectively referred to as “(meth)acrolein”, and methyl methacrylate and methyl acrylate are frequently collectively referred to as “methyl(acrylate”}. In this process of reacting (meth)acrolein with methanol and molecular oxygen is essential to use a catalyst containing palladium.
Conventionally, the above-mentioned single-step process has a problem in that an aldehyde decomposition reaction occurs as a side reaction to thereby by-produce a hydrocarbon and carbon dioxide gas, so that the yield of a carboxylic ester as the desired product is low. This method is also disadvantageous in that the alcohol undergoes oxidation as a side reaction to by-produce an aldehyde which is different from the aldehyde used as a starting material, and the by-produced aldehyde causes formation of an undesired carboxylic ester which is different from the desired carboxylic ester, so that the selectivity based on the alcohol is also poor (for example, when the alcohol is methanol, methyl formate is by-produced as an undesired ester, and when the alcohol is ethanol, methyl acetate is by-produced as an undesired carboxylic ester). Further, there is also a disadvantage that the conventional catalyst cannot maintain its activity for a long period of time. Especially when a commercially valuable process for producing a carboxylic ester from an &agr;·&bgr;-unsaturated aldehyde {such as (meth) acrolein} as a starting material, is practiced, a large amount of decomposition products is produced, such as carbon dioxide gas and an olefin (which is propylene when methacrolein is used as a starting material). Thus, this process has not been successfully practiced.
In Examined Japanese Patent Application Publication (Japanese Kokoku) Nos. 57-035856, 57-035857, and 57-035859, the present inventors proposed a catalyst containing palladium and lead, and showed that, in the production of a carboxylic ester using this catalyst, the selectivity for methyl (meth)acrylate based on (meth)acrolein can be largely improved to a level as high as more than 90%. However, with this catalyst, the selectivity for methyl(meth) acrylate is low when the reaction temperature is 50° C. or more, so that it is difficult to produce methyl(meth)acrylate with high economical efficiency.
Subsequently, in Examined Japanese Patent Application Publication (Japanese Kokoku) No. 62-007902, the present inventors proposed a catalyst comprising an intermetallic compound in which palladium and lead are bonded to each other in a simple integral ratio. In this Japanese patent document, the present inventors showed that, in the production of a carboxylic ester using this catalyst, the decomposition of (meth)acrolein is almost completely suppressed, and the catalyst activity can be maintained for a prolonged period of time.
The new single-step process using these new types of catalysts has also an advantage in that the process can be performed in one step, as compared to the conventional two-step process in which a desired carboxylic ester is produced via (meth)acrylic acid and in which the yield of the desired carboxylic ester and the life of the catalyst are still unsatisfactory. Therefore, it has been desired that the new single-step process be commercialized as a new method for producing commercially useful raw materials for various valuable polymers.
However, when the production of, for example, methyl methacrylate (MMA) is conducted, using the above-mentioned new catalysts, under economically advantageous reaction conditions necessary for commercially practicing the new method, i.e., under reaction conditions such that the reaction temperature is as high as 60° C. or more and the methacrolein concentration of the reaction system is as high as 20% or more, not only does the selectivity for MMA become low, but also the by-production of methyl formate due to the oxidation of methanol is sharply increased. For example, Examined Japanese Patent Application Publication (Japanese Kokoku) No. 62-007902 shows that not only can an MMA selectivity exceeding 90% be achieved, but also the by-production of methyl formate is suppressed to a level as low as 0.03 to 0.06 mole/mole of MMA. However, these favorable results can be obtained only when the reaction is conducted under moderate reaction conditions such that the methacrolein concentration of the reaction system is as low as 10% or less and the reaction temperature is as low as 40 to 60° C. When the reaction is conducted under such moderate reaction conditions, the MMA concentration of the resultant reaction mixture is low and, hence, the recycling of a large amount of unreacted methanol inevitably becomes necessary, so that a large amount of vapor becomes necessary for recycling the unreacted methanol. In addition, in this case, the productivity of the method is low, it is necessary to use a reactor having a large volume. Therefore, this proposal is eco

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