Nanostructures of the montmorillonite-derived restructured clays K10®, HMO and the Mg2+ exchanged analogue Mg-HMO. A SANS, N2 sorption and XRPD study

Abstract

Small angle neutron scattering (SANS) of the montmorillonite (MO)-derived acid-restructured clays K10®, HMO and the Mg2+ exchanged congener of the latter, Mg-HMO, has been investigated. K10® is a strong acid-treated clay, HMO a new, mild-acid treated material with different X-ray powder diffraction (XRPD) characteristics and Mg-HMO a Mg2+ exchanged derivative; the scattering curve of the latter exhibits a peak at Q = 5.46 nm−1 ascribed to intra-layer Mg2+–Mg2+ distances. Analysis of the mid-Q region (0.2–2.6 nm−1) of the curves demonstrates that K10® and HMO differ in nanostructure. In particular, the solid matrix of K10® exhibits a mass fractal dimension, Dm = 2.53 which is absent from HMO but which is characteristic for sepiolites prepared via prolonged acid treatment. For Mg-HMO, recently found to be an efficient base catalyst for fine chemicals production, both XRPD and SANS point to a more ordered “parallel-plate in primary particle” shape. This is ascribed to both intercalation of Mg2+ between aluminosilicate sheets and enhanced correlation between platelets due to surface Mg2+ cations acting to bind clay platelet packets together face to face in agreement with the high Mg2+ loading level of HMO (10% w/w). The presence of the Mg2+ cations reduces the surface roughness and results in an increase of the length of the platelet packets in line with this enhanced ordering.