Mn-smectites: Hydrothermal synthesis and characterization

Three varieties of Mn-smectites (hectorite, stevensite and saponite) were hydrothermally synthesized for the first time using silicic acid, MnCO3 and Al(NO3)3·9H2O mixed with aqueous solutions of NaOH or LiOH. The staring materials were treated in the temperature range of 125–200 °C for 72–168 h and the best synthesis temperature was found to be 175 °C. A lower Mn/Si molar composition than stoichiometric ideal composition was found to be most effective for the formation of almost pure and crystalline phases, apparently high Si is needed for their stability. X-ray diffraction (XRD) and thermal properties of the Mn-hectorite product have been investigated and compared to those of Zn-hectorite, which was previously synthesized by us. Manganese hectorite showed slightly larger expandability upon glycerol treatment and lower dehydroxylation temperature in the range of 300–450 °C by DTA and TG as compared to Zn-hectorite.