Removal of homogeneous precious metal catalysts via Organic solvent nanofiltration
In homogeneous catalysis often precious transition metal complexes are used. Ideally the used catalyst in a specific process is recovered in the downstreaming and recycled back to the reactor. In contrast to the separation of heterogeneous catalysts, the separation of homogeneous catalysts is a crucial issue: Inefficient catalyst recovery and its loss in undesired process streams may even lead to uneconomic processes. The recovery of catalytic species from these streams contributes essentially to the economic efficiency of the production process and prevents the environment from metal contaminations. Organic solvent nanofiltration (OSN) can be applied for the separation of molecules based on differences in molecular weight and steric effects. In general smaller molecules pass through a polymeric or ceramic membrane, leaving the membrane module as permeate stream, while larger molecules are retained and leave the membrane module as retentate. The difference in concentration and the pressure difference between feed and permeate side determine the driving force for the permeation. Therefore, OSN as a pressure-driven process can be operated under mild conditions and offers economic benefits over other separation techniques, since no additional mass separating agent is introduced and no heat for evaporation has to be supplied. Industrial membrane processes also have the advantage of high packing density, resulting in a compact design, while only a pulsation free pump is required in addition to the membrane module. Based on the described concept the separation of homogeneously dissolved rhodium species from different solvents and solvent mixtures originating from hydroformylation reactions will be shown. In first experiments rejections of 97 % rhodium were achieved. Furthermore the application of the organic solvent nanofiltration as a tool for the recovery of homogeneous metal catalysts in general will be discussed.