Thermal and spectroscopic characterization of N-methylformamide/Ca-, Mg-, and Na-exchanged montmorillonite intercalates

The progressive adsorption of N-methylformamide (NMF) onto Ca-, Mg-, and Na -exchanged Wyoming bentonite (Mn+ - SWy-2) together with its subsequent the rmal desorption has been studied using a variety of complementary technique s. The derivative thermograms (DTG) for the desorption of NMF from Mn+-SWy- 2 exhibit three maxima at temperatures which depend on the exchange cation. In Mg-SWy-2 these maxima occur at 130, 200, and 400 degrees C, whereas in Na-SWy-2 they occur at 100, 150, and 190 degrees C. Each of these maxima ha s been assigned to different sites and/or environments for sorbed NMF using variable temperature-X-ray diffraction (VT-XRD) and variable temperature-d iffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS). Eac h fully loaded Mn+-SWy-2/NMF complex has two layers of NMF in the gallery, which decreases to a single layer prior to the complete removal of NMF from the complex. The temperatures at which major weight losses occur coincide with decreases in the interlayer spacing. VT-DRIFTS has shown that at low t emperatures NMF was removed from NMF clusters, similar to those in liquid N MF, while at high temperatures the NMF molecules are firmly bound and direc tly coordinated to the exchangeable cations. Unusual shifts in the C-H and N-H absorption bands were observed, indicating a unique orientation of thes e groups, which probably reflects their keying into the hexagonal cavities of the tetrahedral sheet of the aluminosilicate layer.