The interlayer collapse during dehydration of synthetic Na0.7-beidellite: A 23Na solid-state magic-angle spinning NMR study: Clays

Abstract--The dehydration and migration of the interlayer cation of the synthetic beidellite Nao.Tm14.7Si7.302o-(OH)4-nH20, were studied with solid-state 23Na and 27A1 MAS-NMR, heating stage XRD, and ther-mogravimetric analyses (TGA, DTA). The 23Na MAS-NMR of Na-beidellite at25 ~ displays a chemical shift of 0.2 ppm, which indicates a configuration comparable with that of Na § in solution. Total dehy-dration proceeds reversibly in two temperature anges. Four water molecules per Na § are gradually removed from 25 ~ to 85"C. As a result, the basal spacing decreases from 12.54/ ~ to 9.98 A and the Na § surrounded by the two remaining water molecules i relocated in the hexagonal cavities of the tetrahedral sheet. The chemical shift of 1.5 ppm exhibited after the first dehydration stage illustrates the increased influence of the tetrahedral sheet. The high local symmetry is maintained throughout the entire first dehydration stage. During the second ehydration, which proceeds in a narrow temperature range around 400 ~ the remaining two water molecules are removed reversibly without any change of the basal spacing. Key Words--Beidellite, Dehydration, Interlayer collapse, 23Na MAS-NMR