Partial exchange of Fe(III) montmorillonite with hexadecyltrimethylammonium cation increases catalytic activity for hydrophobic substrates

Fe(III) montmorillonite clay that was partially exchanged with hexadecyltrimethylammonium (HDTMA+) cations achieved increased catalytic activity for the oxidative coupling of hydrophobic organic substrates. A series of mixed-cation organoclays were produced, where the organic cation content ranged from 6 to 50% relative to the cation-exchange capacity (CEC) of the clay, and were tested for catalytic activity using different Fe(III)-mediated oxidative coupling reactions. Enhanced catalytic activity by Fe3+/HDTMA+ montmorillonite for coupling hydrophobic substrates was observed, with maximum catalytic activity in the oxidative coupling of 2-naphthol observed at 6% HDTMA+ coverage. However, maximum catalytic activity with a more hydrophobic substrate, anthrone, was achieved with 50% HDTMA+ coverage, indicating that matching levels of organic modification to substrate hydrophobicity improves catalytic activity. The organization of the organic cations at the clay surfaces proved to be heterogeneous, as determined by scanning transmission X-ray microscopy (STXM) and powder X-ray diffraction. Results from molecular dynamics simulations supported the heterogeneous nature of the catalysts but also pointed toward large regions within the interlayers that may be filled with nonreactive hydrated Fe oxides resulting from the organic cation treatment. The exchangeable Fe content of the organic treated clays, as determined by AAS and ICP measurements, was observed to be higher than expected relative to that of Fe-saturated clay, substantiating this hypothesis. These findings have implications for the development of substrate-specific clay catalysts, where the composition and configuration of exchangeable cations can be matched to a particular substrate or reaction.