Catalytic activity of choline modified Fe(III) montmorillonite

A series of mixed-cation organoclays, containing both Fe3+ and choline+ cations, were prepared using montmorillonite and tested for catalytic activity in three Fe3+-mediated reactions: the oxidative coupling of 2-naphthol, coupling of anthrone and a Lewis-acid catalysed indole conjugate addition. Clays with different Fe3+:choline+ ratios were required to achieve maximum catalytic activity in each reaction. For coupling of both 2-naphthol and anthrone, the most effective catalysts, as measured by the turnover frequency (TOF), were mixed Fe3+/choline+ clays of varying ratios, but the catalytic efficiency of Fe3+ montmorillonite in the conjugate addition of indole with methyl vinyl ketone decreased with the addition of choline+. This result could potentially be explained by the effect of choline+ decreasing the accessibility of Lewis acid domains. The results of this study provide proof that it is possible to tune mixed-cation clays with maximum catalytic activity in specific reaction types for specific substrates. Such mixed-cation organoclays offer advantages over traditional clay-based sorbents in environmental remediation applications, as they have the potential to bind organic contaminants selectively, via interaction with surface-exchanged organic cations, and neutralise organic molecules through catalytic exchange cations that are also present on the clay surface.