NMR investigation of octahedral cis-trans occupancy changes during I/S dehydroxylation

I/S clays (<1 μm size fraction) were analyzed by ²⁷Al NMR and infrared spectroscopy to investigate the double step dehydroxylation behavior which can be observed in many 2:1 dioctahedral clays. The intensity changes of ²⁷Al NMR peaks for tetrahedral and octahedral sites during dehydroxylation were observed. The distorted oxygen polyhedra coordinating Al in the I/S dehydroxylates caused almost complete ²⁷Al signal loss from five-fold coordinated sites and some loss from the six-fold sites. After assessing the extent of dehydroxylation of 7 samples heated at 700°C for 1 hour by IR, theoretical six-fold Al NMR peak intensities were calculated based on two previously proposed dehydroxylation mechanisms and compared with the experimental ones. The calculations suggest that there is no movement of the octahedral cations during dehydroxylation in contrast with published XRD results. NMR results showed that the different dehydroxylation temperatures for cis- and trans- vacant clays can be caused by the different OH-OH distances which required different energies for dehydroxylation.