U.S. Pat. No. 5,239,106 disclosed the separation of diphenyl carbonate (DPC) from catalyst-containing reaction solutions by crystallization of the 1:1 adduct of DPC with phenol, comprising 30.5 wt. % phenol and 69.5 wt. % diphenyl carbonate, with the aid of suspension crystallization. A disadvantage of this process that it is limited to a narrow concentration range in order to be able to isolate the 1:1 adduct in a sufficiently high yield, i.e. diphenyl carbonate concentrations of at least 50 wt. % to 70 wt. % in the reaction solution.
In order still to be able to process the resulting suspensions by filtration techniques, at least a two-stage procedure requiring expensive equipment is necessary.
Furthermore, the catalyst system in this process may not be separated off completely, since the crystals filtered off are still contaminated by adhering mother liquor and inclusions of mother liquor. During subsequent working up of the 1:1 adduct by distillation, these catalyst constituents which have not been separated off have an adverse effect due to catalysis of by-product formation and DPC decomposition. The proposed washing of the crystallization product with a mixture of 9% water and 91% phenol (see column 3, lines 14-18) reduces the yield by the dissolving of large portions of the 1:1 adduct.
This treatment moreover leads to an increase in the water content of the adduct crystals, resulting in DPC losses by hydrolysis in the subsequent distillation columns, i.e. for DPC isolation and for separating water off from the wash solution used. Furthermore, essential process parameters, e.g. relating to the nature of the reactor, the temperature program, stirrer geometry, stirrer output etc., are not disclosed in U.S. Pat. No. 5,239,106.
In order to process reaction solutions where the DPC content is less than 50 wt. % by this process, concentration by distillation is essential, with the disadvantages described above of distillation in the presence of catalyst constituents. In addition, exposure of the reaction solution to heat leads to a deactivation of the catalyst system, which requires an expensive fresh feed of the catalyst components into the process. All these disadvantages described make the process inflexible and unattractive and obstruct industrial realization.
EP-A 0 687 666 described a process for the purification of diphenyl carbonate by fractional melt crystallization of highly concentrated reaction solutions in the temperature range of 45-85° C. The diphenyl carbonate purities which may be achieved are between 97.5 and 99.5%. A disadvantage of this process is the limitation to reaction solutions with a diaryl carbonate content of greater than 70 wt. %. Reaction solutions with diaryl carbonate contents below 70 wt. % may not be processed by this process and would therefore have to be concentrated to the required contents, for example by distillation. During this exposure to heat, the catalyst system causes side reactions and is thereby deactivated itself. For these reasons the process is uneconomical and cumbersome for reaction solutions with diphenyl carbonate contents below 70 wt. %.
There is therefore interest in discovering a gentle method which allows catalyst components to be recovered in a high yield and to be recycled into the reaction without significant deactivation of the catalyst system and under economical conditions which may be realized and reproduced industrially, from solutions which comprise the hydroxyaromatic compound employed and diaryl carbonate in various compositions.