TABLE OF CONTENTS Perfluorinated sulfonic acid ionomers have attractive properties for catalytic applications:
Very high acid strength (super-acid).
Exceptional chemical and thermal stability.
The solid state allows easy separation from the products formed; in membrane form this catalyst may actually prevent mixing of the reactants [73].
Three different types of catalysis may be considered:
Catalysis by the hydrogen ion.
Catalysis by some other cation acting as a counter-ion; some hydrogen ions may also be present.
Catalysis by some solid particles, such as metal particles, present in the ionomer.
Acid catalysis using perfluorinated ionomers in the sulfonic acid form has been studied most extensively. Olah has published a comprehensive review of this subject, including 181 references [74]. The high catalytic activity of these polymers is a result of the electron withdrawing effect of the perfluoroalkyl group on the sulfonic acid site. The acid strength of Nafion is comparable to that of 96 100% sulfuric acid. Gas phase alkylations using Nafion result in cleaner reaction products with less by-products formed. The alkylation of benzene with ethylene and propylene are given as examples with experimental procedures. When olefines are used as alkylating agents, the catalytic activity of Nafion slowly decreases, possibly due to deactivation of catalytic surface sites by some polymeric material. If alcohols are used as alkylating agents, the lifetime of the Nafion catalyst is actually improved; however, ethers may be formed as by-products.
Alkylations, disproportionations and transalkylations of alkylbenzenes typically require temperatures in excess of 160 C and the long-term stability of an ionomeric...
