CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage entry under § 371 of International Application No. PCT/EP2018/072419, filed on Aug. 20, 2018, and which claims the benefit of European Application No. 17188252.5, filed on Aug. 29, 2017, both of which are incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a process for producing optical molding materials on the basis of methyl methacrylate, wherein this MMA has been produced by an optimized method and the molding materials feature in particular a very low yellowness index. This MMA employed according to the invention has been produced by direct oxidative esterification of methacrolein.

In particular the present invention relates to an optimized workup of the reactor output from the oxidative esterification of methacrolein by means of which particularly discoloring byproducts are removed. This process moreover has the advantage that fewer demands than described in the prior art are placed on plant apparatus configuration.

Discussion of the Background

Methyl methacrylate is used in large amounts for preparing polymers and copolymers with other polymerizable compounds. In addition, methyl methacrylate is an important synthesis unit for a variety of specialty esters based on methacrylic acid (MAA) which can be produced by transesterification with the appropriate alcohol. There is consequently a great interest in very simple, economic and environmentally friendly processes for preparing this starting material. It is of particular interest, to provide MMA or other alkyl methacrylates which in the polymer for example for processing into the molding material result in particularly low yellow discoloration.

Methyl methacrylate (MMA) is currently produced by a variety of processes proceeding from C2, C3or C4synthesis units. In one process which is said to be particularly efficient MMA is obtained by gas-phase oxidation of isobutylene or tert-butanol with atmospheric oxygen over a heterogeneous catalyst to afford methacrolein and subsequent oxidative esterification of methacrolein using methanol. This process, developed by ASAHI, is described, inter alia, in publications U.S. Pat. Nos. 5,969,178 and 7,012,039. A particular disadvantage of this process is the very high energy requirement. In a development of the process the methacrolein is obtained from propanal and formaldehyde in the first stage. Such a process is described in WO 2014/170223.

U.S. Pat. No. 5,969,178 describes such a process for oxidative conversion of isobutene or tert-butanol into methacrolein and subsequent oxidative esterification to MMA. In this second stage a liquid mixture of methacrolein and methanol with reduced water content is reacted with molecular oxygen and a palladium catalyst, wherein said catalyst is usually in supported form as a palladium-lead catalyst. In a first distillation stage a mixture of methacrolein and methanol is then removed from the crude product of the oxidative esterification below the top of the column while low-boiling constituents are removed overhead. The MMA-containing bottoms product is then passed into a second distillation stage in which an azeotrope of methanol and saturated hydrocarbons is removed overhead. The bottoms product comprising the crude MMA is sent to a further workup while methanol is isolated from the overhead fraction by means of a phase separator and a third distillation column and passed back into the reactor. It is to be borne in mind that the methanol can contain relatively large amounts of water on account of the azeotrope formed and must therefore be sent to a dewatering.

As an alternative to this process U.S. Pat. No. 5,969,178 discloses a workup in only one column wherein in said column it is imperative that the feed be situated above the column bottom. Low-boiling constituents from the reactor output are removed from this column overhead. Remaining in the column bottom is a mixture of crude MMA and water which is to be sent to a further workup. Via a sidestream whose exact position must first be determined, said position being adjustable by addition of various sieve trays, a mixture of methacrolein and methanol intended for returning into the reactor is finally taken off from the column. U.S. Pat. No. 5,969,178 itself indicates that such a process is difficult to perform on account of a variety of azeotropes. Particularly methacrylic acid, which is always present as a byproduct, moreover plays an important role. According to this process, despite the silence of U.S. Pat. No. 5,969,178 on this issue, the methacrylic acid would be removed in a manner such that it remains in a phase to be sent for disposal and an isolation would be of only limited attractiveness. However, this results in a fall in the overall yield of methacrylic products of this process.

U.S. Pat. No. 7,012,039 discloses a workup of the reactor output from the oxidative esterification which is somewhat of a departure. Here, in a first distillation stage methacrolein is distilled overhead via sieve trays and the aqueous, MMA-containing mixture from the bottom is passed into a phase separator. In said phase separator the mixture is pH-adjusted to a pH of about 2 by addition of sulfuric acid. The separation of the sulfuric-acid-acidified water from the organic/oil phase is then effected by means of centrifuging. This oil phase is separated in a further distillation into high-boiling constituents and an MMA-containing phase withdrawn overhead. The MMA-containing phase is then separated from low-boiling constituents in a third distillation. This is even followed by a fourth distillation for final purification.

The problem with this process is the sulfuric acid which needs to be added in large amounts and can have corrosive effects on parts of the plant. Accordingly these parts, such as in particular the phase separator or else the second distillation column, must be fabricated from suitable materials. Moreover, U.S. Pat. No. 7,012,039 is silent regarding the handling of the simultaneously generated methacrylic acid or the residual methanol remaining in the product. However it can be assumed that the former is coremoved in the distillation stages while the methanol can be obtained and returned with the methacrolein only partially while the remainder is probably lost in the third distillation stage.

WO 2014/170223 describes a similar process to U.S. Pat. No. 7,012,039. The only difference is that in the actual reaction the pH is adjusted in a circuit by addition of a methanolic sodium hydroxide solution. This serves, inter alia, to protect the catalyst. Moreover, the removal of the aqueous phase in the phase separation is simpler on account of the salt content. However, another consequence is that the methacrylic acid formed is in the form of sodium salt and is later removed and disposed of with the aqueous phase. Admittedly, in the variant where sulfuric acid is added in the phase separation the free acid is recovered. However, instead, sodium (hydrogen) sulfate is generated which can lead to other problems upon disposal.

Finally, WO 2017/046110 teaches an optimized workup of the crude MMA obtained from an oxidative esterification is initially separated from a heavy phase and subsequently distilled off from this heavy phase is an alcohol-containing light phase which in turn can be recycled. What is also special about this process is that here the methacrolein has been obtained on the basis of propanal and formaldehyde, wherein the former is obtained on the basis of C2 synthesis units, for example from ethylene and synthesis gas.

However, independently of the raw material basis for the employed methacrolein all of these processes result overall in MMA or in general alkyl methacrylates that result in yellow discoloration of descendent products, for example molding materials. There is thus a need for improvement of the kind where the source of this yellow discoloration is identified and before polymerization removed from the relevant alkyl methacrylate, in particular MMA, as efficiently as possible.

SUMMARY OF THE INVENTION

In view of the prior art the problem addressed by the present invention is therefore that of providing a technically improved process for producing polymer resins having a particularly low yellow discoloration.

A particular problem addressed was that of providing the alkyl methacrylates, in particular MMA, used for producing these polymer resins on the basis of an oxidative esterification of methacrolein.

A further problem addressed by the present invention was that of identifying and efficiently removing from the alkyl methacrylate the byproducts which are formed in the oxidative esterification of methacrolein and result in yellow discoloration of descendent products.

A further particular problem addressed was that of providing a process that can be operated with the lowest possible disposal cost, in particular through reduced generation of organic constituents and acids in the waste stream.

The process shall furthermore be inexpensive, in particular in terms of the materials to be employed for construction of the plant, compared to the prior art.

