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DEHYDRATION AND REHYDRATION OF PALYGORSKITE AND THE INFLUENCE OF WATER ON THE NANOPORES

Center for Catalysis Research and Innovation and Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5

^* E-mail address of corresponding author: dete{at}science.uottawa.ca

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 ²⁹Si and ¹H 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 m²/g, corresponding to a micropore volume of 47 mm³/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 ²⁹Si 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 Q²(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 ²⁹Si and ¹H NMR that nearly complete rehydration is achieved by exposing palygorskite samples that have been partially dehydrated at 150°C and 300°C, to D₂O or water vapor at room temperature. When the rehydration is accomplished with D₂O, the atoms are disordered across all the protons sites.

Key Words: Clay Minerals • Microporosity • Palygorskite • Sepiolite • Textural Analysis • TGA-MS • ²⁹Si NMR

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