Characterisation of moderately acid-treated, size-fractionated montmorillonites using IR and MAS NMR spectroscopy and thermal analysis

Abstract

The < 2 µm fractions of Cheto (Arizona, USA) and Jelšový Potok (Slovakia) montmorillonites have been treated with 1 mol dm^–3 H₂SO₄ for periods up to 6 h at 95 °C. The materials obtained were investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), infrared (IR) spectroscopy, and ²⁹Si and ²⁷Al magic-angle spinning (MAS) NMR spectroscopy. Both clays were partially decomposed under the conditions utilized, but the Cheto montmorillonite, which contained a higher proportion of octahedral magnesium, was more susceptible to acid attack. XRD was insensitive to this level of acid attack but XRF, IR and ²⁹Si MAS NMR revealed the depopulation of the octahedral sheet. The number of acid sites was determined from the thermal desorption of cyclohexylamine and the catalytic activity was evaluated by reacting 2,3-dihydropyran with methanol to yield the tetrahydropyranyl ether. The test reaction was a more sensitive indicator of changes in clay acidity than the values determined by desorption of cyclohexylamine from acid sites.