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 thermal desorption has been studied using a variety of complementary techniques. 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 has been assigned to different sites and/or environments for sorbed NMF using variable temperature-X-ray diffraction (VT-XRD) and variable temperature-diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS). Each 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 temperatures NMF was removed from NMF clusters, similar to those in liquid NMF, while at high temperatures the NMF molecules are firmly bound and directly coordinated to the exchangeable cations. Unusual shifts in the C-H and N-H absorption bands were observed, indicating a unique orientation of these groups, which probably reflects their keying into the hexagonal cavities of the tetrahedral sheet of the aluminosilicate layer.