Membranes for the Recovery of a Homogeneous Catalyst
Desrocher, David J.
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Homogeneous catalysts demonstrate the ability to perform extremely selective organic syntheses with high yields. These catalysts are usually quite expensive and the commercial viability of processes that use homogeneous catalysts depends on the efficiency of catalyst recovery, which is normally quite complex. This obstacle often excludes the use of homogene-ous catalysts from commercial processes. This work investigates the implementation of mem-branes as the unit operation for catalyst recovery as a means to expand the use of homogeneous catalysis. The commercial polyimide, Matrimid, has been examined for its suitability as a membrane material for the homogeneous catalyst recovery of a 1-dodecene hydroformylation reaction, catalyzed by a rhodium-triphenylphosphine transition metal catalyst. This reaction occurs in the liquid phase in solution with toluene. Because of the aggressive environment of the reaction, blends of Matrimid with a crosslinkable, diacetylene-functionalized oligomer have been formed to promote polymer stability through network formation. The diacetylene groups on the oligomer and acetylene end groups can be thermally activated at 250 ??o form distributed polymer networks. Compatible blends of Matrimid and the crosslinking agent can be formed with up to 10% (w/w) crosslinking agent content. Matrimid and the blends have been investigated in the form of dense, nonporous films to evaluate their membrane performance. In terms of material stabilization, it has been found that heat treatment of the neat Matrimid at 250 ??esults in a significant suppression of the material plasticization when exposed to toluene. Addition of the crosslinking oligomer to Matrimid promotes further reduction in swelling and toluene sorption. Transport studies of the reaction components in the materials show that addition of the crosslinking oligomer results in reduced diffusion of the permeating components in the mem-brane materials. However, some increases in solute sorption occur and this is attributed to the oligomer chemistry. A 10% blend of crosslinking agent and Matrimid gave a superior catalyst rejection of 91.5%. The catalyst rejection system has been modeled using Maxwell-Stefan transport equa-tions. Through the model it was found the flux coupling significantly influences the separation characteristics, with sorption of both the solvent and solute as key factors.