Characterization and catalytic activity of acid-treated, size-fractionated smectites

Five layered silicates in which the octahedral sheet contained differing amounts of Al, Mg, Fe, and Li were acid leached using acid concentrations and treatment temperatures selected to produce materials in which the octahedral sheet was depopulated in a controlled, stepwise (yet comparable) manner. SAz-1 and JP are dioctahedral smectites with octahedral compositions rich in Mg and Al, respectively. SWa-1 is a ferruginous smectite and ST an iron-rich beidellite. The fifth mineral was a trioctahedral hectorite which contains almost exclusively octahedral Mg. The Bronsted acidity and catalytic activity of the resulting materials were highest for the samples prepared with the mildest acid treatments but decreased as the octahedral sheet became increasingly depleted. Only the hectorite exhibited no catalytic activity despite the proven existence of Bronsted acid sites. The elemental composition of the starting material did not appear to make a significant contribution to the catalytic activity for the chosen test reaction although it does play a key role in determining the severity of the activation conditions required for the optimization of catalytic activity. Fourier transform infrared (FTIR) spectroscopy was found to be as sensitive to structural acid attack as Si-29 magic angle spinning NMR and the octahedral depletion (measured using FTIR) correlated well with the acidity determined from thermal desorption of cyclohexylamine.