Dehydration and rehydration of palygorskite and the influence of water on the nanopores

The dehydration and rehydration processes of the clay mineral palygorskite (PF1-1) were studied by textural analysis, thermogravimetric analysis connected with mass spectrometry (TGA-MS), and Si-29 and H-1 solid-state NMR techniques. The TGA-MS results clearly reveal weight losses at maxima of 70degreesC, 190degreesC, 430degreesC and 860degreesC. PF1-1 is characterized by a micropore area of 93 m(2)/g, corresponding to a micropore volume of 47 mm(3)/g. These values are also obtained for the sample heated up to 200degreesC for 20 h. Further heating at 300degreesC produces a collapse of the structure, as shown by the almost complete loss of microporosity. The Si-29 NMR spectra of palygorskite show two main resonances at -92.0 and -97.5 ppm, attributed to one of the two pairs of equivalent Si nuclei in the basal plane. A minor resonance at -84.3 ppm is attributed to Q(2)(Si-OH) Si nuclei. The resonance at -92.0 ppm is assigned to the central Si position, while the resonance at -97.5 ppm is assigned to the edge Si sites. It is confirmed by solid-state Si-29 and H-1 NMR that nearly complete rehydration is achieved by exposing palygorskite samples that have been partially dehydrated at 150degreesC and 300degreesC, to D2O or water vapor at room temperature. When the rehydration is accomplished with D2O, the atoms are disordered across all the protons sites.