Hydrothermal transformation of mixed metal oxides and silicate anions to phyllosilicate under highly alkaline conditions

Mixed metal oxides (MMO) were transformed to phyllosilicate in the presence of silicate anions under hydrothermal conditions. The product was shown to be saponite with a trioctahedral layer structure by characterization with XRD , FTIR , 27 Al and 29 Si MAS NMR, HRTEM and TG-DTG. The formation of saponite was through a rehydration-dissolution-precipitation pathway, involving the critical steps such as the reconstruction of MMO to hydrotalcite (Ht), exsolution of Al 3+ from Ht, condensation of metasilicate anions with Ht, and finally crystallization of saponite. Isomorphous substitution of Al 3+ for Si 4+ in silicate oligomers is the key for the above conversion, which generated the negative charge necessary for the condensation between the silicate oligomers and Ht surface and the formation of 2:1 saponite TOT layers. High pH value (over 14.0) favored the condensation of silicate oligomers and led to crystallization of saponite. Thus, we report a novel method for the crystallization of saponite.