Acid-catalyzed oligomerization via activated proton transfer to aromatic and unsaturated monomers in Nafion membranes: a step forward in the in situ synthesis of conjugated composite membranes

Abstract

An approach to perform controlled acid-catalyzed oligomerization via vapor pressure control of the reactions inside Nafion membranes is presented. The interaction of Nafion with several classes of aromatic (pyrrol, furan, thiophene) and unsaturated (methyl-acetylene) gas phase monomers was studied as a function of contact time and temperature by in situ vibrational (FTIR) and electronic (UV-Vis) spectroscopy with the support from theoretical linear response and time dependent DFT calculations to monitor the vibrations and the effective number of conjugated double bonds. The formation of H-bonded adduct as seen from IR spectroscopy transforms the hydrogen bonded species into positively charged oligomers through an activated proton transfer mechanism where oligomerization progress through increasing contact time with the respective gas phase reactants at room temperature. The activated proton transfer oligomerization proceeds through the stepwise growth propagation cycles via carbocationic intermediates, finally leads to the formation of irreversible, conjugated charged oligomers as a product. The colored, conjugated oligomeric Nafion composite products are formed at room temperature as a function of reaction time and are irreversible after complete degassing of the gas phase reactants as well stable in ambient environment stored for many days in pure oxygen or air and cannot be extracted with common solvents, appearing strongly encapsulated inside Nafion membranes. This is crucial for future applications of the presented route for direct production of conjugated species inside Nafion and thus production and control of composite membrane materials of interest in fuel cells and catalysis.