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

Abstract

Abstract The dehydration and rehydration processes of the clay mineral palygorskite (PFl-1) were studied by textural analysis, thermogravimetric analysis connected with mass spectrometry (TGA-MS), and 29 Si and 1 H solid-state NMR techniques. The TGA-MS results clearly reveal weight losses at maxima of 70°C, 190°C, 430°C and 860°C. PFl-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 200°C for 20 h. Further heating at 300°C produces a collapse of the structure, as shown by the almost complete loss of microporosity. The 29 Si 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 bysolid-state 29 Si and 1 H NMR that nearly complete rehydration is achieved by exposing palygorskite samples that have been partially dehydrated at 150°C and 300°C, to D 2 O or water vapor at room temperature. When the rehydration is accomplished with D 2 O, the atoms are disordered across all the protons sites.