Homogeneous Catalysis with Soluble Polymer-Bound Catalysts as a Unit Operation

Homogeneous catalysis with soluble polymer-bound catalysts has been carried out by a variety of academic and industrial research groups. These investigations have been performed on a laboratory scale in a batchwise fashion for the most part. In several cases, reactions have also been performed on a pilot plant scale, however (cf. Section 7.5). From the published data, no general systematic differences from catalysis with conventional catalysts that are not polymer-bound are evident regarding the reaction conditions of catalysis. The increased solution viscosity of polymer solutions by comparison to solutions of low molecular weight compounds should usually not be a major issue in view of the rather limited concentrations required for catalysts. The high local concentration of metal centers in polymerbound catalysts represents a difference from non-polymer-bound homogeneous catalysts. This can result in an enhancement of undesirable bimolecular deactivation reactions in polymer-bound catalysts [1]. Differences in the catalytic properties by comparison to non-polymer-bound analogues, which were observed in same cases, have also been ascribed to the polymer coil representing a local different “solvent” composition. Thus, the unit operations most characteristic of catalysis with soluble polymerbound catalysts apply to the separation and recycling step rather than to the actual catalytic reaction (as a special case, both can be carried out at the same time, e.g., in a continuously operated membrane reactor; vide infra). As mentioned previously, the separation of polymer-bound catalysts makes use of properties specific to macromolecules, in order to differentiate between the polymer-bound catalyst and the low molecular weight reaction products of the catalytic reaction and unreacted substrates in the recycling step. The miscibility of polymers with low molecular weight compounds, i.e., the solubility in the reaction solvent (which can also be the neat substrate), is such a property. For most polymer/solvent combinations, a nonsolvent can easily be identified which precipitates the polymer-bound catalyst when added in sufficient amounts, while the reaction products of catalysis stay in solution. For example, the common support poly(ethylene glycol) is insoluble in diethyl ether, and polystyrene can be precipitated with methanol. However, while this approach may be useful on a laboratory scale, from a chemical engineering point of view it is not very elegant and practical, requiring the recycling and fractionation by distillation of large amounts of solvent. Alternatives are the utilization of the temperature [2– 4] or pH dependence [5, 6] of the solubility of polymers. The miscibility of many polymer/solvent combinations is strongly dependent upon temperature [7]. Thus polyethylenes are soluble in aromatic hydrocarbons only at elevated temperatures.