Solid acidity of 2 : 1 type clay minerals activated by selective leaching

Various 2 : 1 type clay minerals were activated by H2SO4 treatment and their solid acidities were evaluated by NH3-temperature programmed desorption (TPD). Clay minerals with various degrees of Al substitution in the tetrahedral sheets were used as the starting materials. These included montmorillonite (ideally no tetrahedral Al), saponite (low content of tetrahedral Al), vermiculite (intermediate content of tetrahedral Al) and phlogopite (high tetrahedral Al content). All these samples showed increased solid acidity after selective leaching, the solid acidities varying in the order phlogopite < montmorillonite < vermiculite < saponite. The NH3-TPD spectra were deconvoluted into two peaks; the L-peak at < 200 °C corresponds to a weak acid site and the H-peak at > 250 °C corresponds to a strong acid site. The temperature of the H-peak was lowest in phlogopite (252 °C) and highest in saponite (305 °C). The generation of solid acidity in these samples is found to be related to the evolution of new tetrahedral Al sites showing a slightly different 27Al MAS NMR chemical shift from the original tetrahedral Al NMR resonance.