NMR study of kaolinite intercalation compounds with formamide and its derivatives. 1. Structure and orientation of guest molecules

Local structures and orientations of guest molecules in kaolinite intercalation compounds of kaolinite/formamide (FA), kaolinite/formamide-N,N-d(2) (FA-d(2)), kaolinite/N-methylformamide (NMF), and kaolinite/N,N-dimethylformamide (DMF) have been studied by means of solid-state NMR. Si-29 magic-angle-spinning (MAS) NMR spectra identify the original kaolinite and the intercalates, while C-13 MAS NMR spectra distinguish interlayer and outersurface adsorbed guest molecules. Quantitative analyses of Si-29 and C-13 NMR spectra combined with thermogravimetric analysis result in a formula of one interlayer guest molecule per Al2Si2O5(OH)(4) host unit for all the intercalates. Intercalation causes high-frequency shift in the C-13 resonance of the guest molecules. Strength in the guest-host interaction as revealed by changes in the C-13 chemical shift of the carbonyl group upon intercalation is in the order of FA > NMF similar to DMF. The C-13 chemical shift of the methyl group confirms a cis conformation for NMF in the kaolinite interlayer space, and changes in the methyl chemical shift upon intercalation are DMF (trans) > NMF (cis) > DMF (cis:). The magnitude of the temperature dependence in the C-13 chemical shift is on the order of FA < NMF < DMF, and the C-13 line width is FA > NMF > DMF. These facts reveal that the mobility of the guest molecules is in the order of FA < NMF < DMF. Kaolinite/NMF shows a second methyl peak whose amount increases with decreasing temperature, demonstrating the presence of molecules with a different orientation. The H-1 chemical shifts confirm hydrogen-bonding interactions between the amide protons and the kaolinite host in kaolinite/FA and no such hydrogen-bonding interactions in kaolinite/NMF at room temperature. The chemical shift value of the hydroxyl groups in the host kaolinite structure is not affected by intercalation. The H-1 line widths of carbonyls are FA > NMF > DMF, and those of methyls are NMF > DMF, which confirms again that the mobility of the guest molecules is FA < NMF < DMF.