(180y) Mass Transfer Limitations of Acid Catalyzed Reactions in Polymeric Membranes

Perfluorosulfonic acid (PSA) polymers have been found to catalyze a large number of industrially useful reactions but have yet to see widespread commercial application.  In particular PSA ionomers are attracting renewed interest as heterogeneous catalysts for biodiesel synthesis. A key limitation of these materials is slow reagent transport that requires substantial solvent swelling of PSA in pelletized form or dispersion of PSA within porous silica to obtain reasonable conversion rates.  PSA polymer catalyst in self-supporting membrane form is more versatile and can permit development of more efficient processes.  However, conversion rates utilizing such membranes for specialty chemical syntheses have yet to be systematically investigated.  In the present study, PSA membranes of well-defined thicknesses are produced to determine the relative contributions of mass transfer and reaction kinetics during the acid catalyzed condensation of acetone and resorcinol to produce a Flavan as the reaction product.  We also present data on the acid catalyzed acylation reaction between resorcinol and maleic anhydride to produce a quinone dye.  Thinner membranes are found to have substantially lower mass transport resistance and demonstrate that this contribution dominates the conversion rate for the reaction.  More interesting, alteration of PSA membrane morphology was found to correlate to membrane thickness and active site concentration.  Improved understanding of these phenomena provides a rational basis on the use of PSA polymer membrane catalysts in industrial chemical syntheses as well as in novel optical sensing applications.