Influence Mechanism Of Heterogeneous Hydrothermal Reaction Parameters On Layer Structure And Surface Acidity On Saponite

Compared with homogeneous hydrothermal synthesis technology, heterogeneous hydrothermal synthesis can promote the nucleation and subsequently accelerate the growth of secondary minerals by providing a local environment with super-saturation of ion concentration and a relatively lower energy barrier. At a certain extent, this case is similar to “water- rock” reaction in the nature. Due to the scarce resources and the outstanding properties, saponite is usually hydrothermal synthesized. The reaction temperature, pressure and octahedral ion have a significant influence on the composition, layer structure, layer charge and surface acidity of saponite. What’s more, crystallinity and surface acidity of saponite strongly affect the catalysis, complexation, grafting and ion exchange reaction and determine the industrial application of saponite. On the other hand, understanding the influence factors and reaction mechanism of heterogeneous hydrothermal synthesis of saponite and the solid acidity of saponite can provide theoretical basis for the formation of natural saponite deposit.In the present study, series of samples were synthesized by altering Si/Al ratio(fixing the Mg:(Si+Al) =3:4), reaction temperature and time. The obtained products were characterized by XRD, FTIR, TG-DTG, 27 Al and 29 Si MAS NMR, and TEM. The conclusions are made as follows:Firstly, Characteristic reflections of phyllosilicate were recorded in XRD patterns of the resulting products with d(060) ≥ 0.153 nm, indicating a successful synthesis of saponite. The wellcrystallized saponite was obtained in the initial Si/Al ratio range of 5.43–7.89, and their crystallinities were increased with the increase of reaction temperature and time. Pure saponite could be obtained under the condition of Si/Al=5.43, 300 oC and 24 hours or even longer.Secondly, the reaction temperature had a significant effect on the reaction mechanism of heterogeneous hydrothermal synthesis of saponite. In the low temperature series(160oC), at first, Mg O was hydrated to brucite, then brucite released the Mg2+ slowly, saponite was formed with the existing Si4+ and Al3+ in the solution. In the high temperature series(300oC), the hydration of periclase formed brucite, then brucite transformed to tubular chrysolite condensed with one Si-O tetrahedron sheet, and finally formed saponite layer by condensed with another Si-O tetrahedron sheet.Thirdly, it was proved that under the high temperature, alkaline and silicon-rich condition 1:1 clay mineral could transform to 2:1 clay minerals by XRD, TEM and 27 Al MAS NMR. The transformation mechanism maybe a dissolution and reprecipitation process.Fourthly, the major part of acid sits of saponite were Lewis acid sites and most of them were weak acid sites. Br?nsted acid sites were scarce and almost all of them were medium-strength acid sites. The Br?nsted acid sites of saponite resulted from the isomorphous substitution in the tetrahedron sheet. The Lewis acid sites were derived from the broken bond on the edge.