Hydrogenation of cyclohexene with perfluorinated cation- and anion-exchange membranes supported platinum

A perfluorinated cation-exchange membrane (Nafion) and an anion-exchange membrane (Tosflex) supported platinum (Pt) were prepared by ion-exchanging with Pt(NH3)22+ or K2PtCl4, respectively. They were then reduced either with a large excess of NaBH4 or under hydrogen in a glass-lined stainless-steel bomb at 80°C and 20.6 MPa for 3 h. UV visible spectra indicated that 70–80% of ion exchange was completed. The rate of hydrogenation of cyclo(c)hexene in methanol with both platinised membranes showed similar trends, increasing with an increase in the Pt loading on the membrane. However, catalytic activity expressed as turnover frequency (TOF) reached a maximum(0.23 s!1) at about a 0.5% Pt loading. When the Tosflex-Pt was reduced by H2, partial Pt reduction was obtained, resulting in a low TOF. The rate of hydrogenation of c-hexene over a range of pressures indicated that the system was not diffusion limiting after 1.1 MPa.

Although it is difficult to identify the most suitable polymers for chemical reactions with polymer supported catalyst, the present work shows the great potential of perfluorinated ionexchange membranes, Nafion and Tosflex, as insoluble polymer supports for the hydrogenation of c-hexene. In particular, Nafion-Pt shows relatively high catalytic activity in the hydrogenation of neat c-hexene compared to the other types of Pt catalyst. It was found that 0.5% Pt loading offered the highest rate of hydrogenation of c-hexene. Diffusion of reactant also affects the rate of hydrogenation in both systems (i.e., c-hexene in methanol and neat c-hexene), but it is difficult to verify the catalytic behavior with different systems, since the solvent plays an important role in the chemical reaction on the polymeric sup- port. In this case of hydrogenation of c-hexene in methanol, the system is not a diffusion limiting before optimum Pt loading reaches, while an Arrhenius plot for Nafion-Pt shows that the system for the hydrogenation of neat c-hexene is diffusion limiting. Tosflex was found to be an inadequate polymeric support for the reaction requiring high temperature. However, it shows a reasonable capability as a polymeric support in the hydrogenation of c-hexene in methanol.