Reduction of the Structural Iron in Montmorillonite by Electron Transfer from Catechol and its Derivatives

Abstract

The structural Fe(III) in montmorillonite (MMT) clay has been reduced using catechol and itsderivatives. It was found that the reduction process is pH-dependent and also depends on the ringsubstituents. If the catecholic ring has electron-donating substituents, reduction happens at high pH; ifthe catecholic ring has electron-withdrawing substituents, no reduction occurs. The process involveselectron transfer from the hydroxy groups on the compounds to the active site at the iron atoms withinthe MMT lattice. This site acts as an electron acceptor (Lewis acid). Heat treatment of the reducedsample at 100-300 oC showed an enhancement of the Fe2+/Fe3+ ratio, which is attributed to an increasein the proportion of radicalic formation induced by dehydration. The MMT sample was added to thesolutions of the catecholic compound and the slurries were stirred for 24 hours in order to reachequilibrium, then filtered, washed, and air-dried. The reactions were monitored using Mössbauerspectroscopy, x-ray powder diffraction, differential thermal analysis, electron spin resonance, infrared,and total surface area determination.