The invention provides a process for preparing methylmercaptopropionaldehyde (MMP) from gaseous acrolein (AC) and methyl mercaptan (MC). The present invention provides, more particularly, a process for preparing methylmercaptopropionaldehyde from gaseous acrolein and methyl mercaptan, in one process step, wherein simultaneously, (a) gaseous acrolein is absorbed to a mixture comprising at least one compound from the group consisting of the methylthio hemiacetal of methylmercaptopropionaldehyde, methylmercaptopropionaldehyde and methyl mercaptan, (b) in this mixture, acrolein is reacted with methyl mercaptan and/or the hemithioacetal of methylmercaptopropionaldehyde to give methylmercaptopropionaldehyde and (c) impurities and by-products are removed from this mixture.
MMP formation from methyl mercaptan and a gas mixture comprising acrolein is conventionally known.
DE 2627430 describes a two-stage process wherein, in the first stage, the AC is absorbed from a gas mixture in MMP and, in a second stage, the AC dissolved in MMP reacts with MC at temperatures between 10 and 50° C. in the presence of a catalyst. A great economic disadvantage of this separation in two stages is the necessity of MMP recycling at −10° C. in order to completely absorb AC in MMP. The MMP yield based on the MMP introduced into the absorption column in the example described is 99%. At the same time, preferably 0.1 to 0.2% hemithioacetal is established in the reaction mixture. At hemithioacetal concentrations below 0.1%, AC is lost due to incomplete conversion, whereas at hemithioacetal concentrations above 1% the yield of the MMP reaction worsens. The gas mixture formed during the catalytic oxidation of propylene is absorbed from the acrylic acid present in a solvent such as, for example, tri-n-butyl phosphate (FR 7718136), a mixture of biphenyl and diphenyl ether (FR 1393175) or water (FR 1393175), and, after performance of this process step, freed of water in a condenser at −5-0° C. This condensation step too leads to higher capital and operating costs.
According to NL-A 68/09647, it may also be possible to first contact MC with MMP in the reaction zone and to contact the mixture thus obtained with the AC-containing gas. However, an additional step is needed here for treatment of the aqueous phase (extraction), and only an MMP yield of 91% based on the AC used is achieved.
WO 9429254 describes the continuous preparation of MMP from an acrolein-containing gas mixture and MC in a “gas/liquid” reaction zone, in which uncondensable gases are additionally separated from the AC process. Hemithioacetal formation is prevented by the equimolar addition of MC and AC, monitored preferably by periodic use of gas chromatography. According to the description, it may be possible to increase the MMP formation rate by a factor of 3-10. The limitation of the AC mass transfer is minimized by turbulent conditions in the reaction system.
In all documents described above, MC worked up by distillation is used. This is evident from the fact that the principle secondary components from the MC reaction, such as dimethyl sulphide and dimethyl disulphide, are present neither in the MMP product nor in the MMP offgas (WO 9429254 and U.S. Pat. No. 4,319,047). MC is usually synthesized in the gas phase at temperatures between 300 and 500° C. and at pressures between 1 and 25 bar. One process is described, for example, in EP 850922. The product mixture of the synthesis comprises, as well as the desired MC, the water formed in the reaction and, as by-products, dimethyl sulphide, dimethyl ether, small amounts of polysulphides, and unconverted methanol, excess hydrogen sulphide, and the gases which are inert for the purposes of the reaction: nitrogen, carbon monoxide, carbon dioxide and hydrogen. The separation of the product gas mixture into its components serves for recovery of methyl mercaptan and dimethyl sulphide, for discharge of water and inert gas components, and for recycling of unconsumed methanol and hydrogen sulphide into the synthesis reactor. This gives rise, for example, to pure MC with an MC content of up to 99.6% by weight (EP 0850923 and DE 1768826). Disadvantages of this distillative workup of the complex reaction mixture are, in addition to the high capital and operating costs, the unavoidable formation of residues which require disposal and the associated loss of materials of value.
DE 10359636 describes a process which avoids the high distillation complexity to obtain pure methyl mercaptan and nevertheless uses the methyl mercaptan obtained in the catalytic reaction of H2S with methanol, without losses, for the further conversion to MMP with liquid AC. Based on the crude MC used, the isolated yield is virtually quantitative, i.e. >99.9%. This is achieved by distillative removal of the constituents from the MC synthesis still present in the MMP reaction mixture, and preferably by the supply of an inert entraining agent, for example nitrogen. This reference does not disclose the use of AC-containing gases.
U.S. Pat. No. 3,529,940 discloses that reaction temperatures in the MMP synthesis can be controlled by dividing the exothermicity of hemithioacetal formation as intermediate and, after subsequent addition of liquid AC, the MMP reaction enthalpy of the MMP reaction into 2 zones. However, the use of AC-containing gases is not disclosed.