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, D_m = 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.