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-d₂ (FA-d₂), kaolinite/N-methylformamide (NMF) and kaolinite/N,N-dimethylformamide (DMF) have been studied by means of solid-state NMR. ²⁹Si magic-angle-spinning (MAS) NMR spectra identify the original kaolinite and the intercalates, as shown in Fig. 1, while ¹³C MAS NMR spectra distinguish interlayer and outer-surface adsorbed guest molecules. Quantitative analyses of ²⁹Si and ¹³C 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 ¹³C resonance of the guest molecules. Strength in the guest-host interaction as revealed by changes in the ¹³C chemical shift of the carbonyl group upon intercalation is in the order of FA>NMF�`DMF. The ¹³C 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 ¹³C chemical shift is in the order of FA <NMF <DMF, and the ¹³C 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 ¹H 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 ¹H 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.

Fig. 1. ²⁹SiCP/MAS NMR spectra of (A) kaolinite, (B) kaolinite/FA, (C) kaolinite/FA-d₂, (D) kaolinite/NMF and (E) kaolinite/DMF, measured at v_L=79.50 MHz and 298 K with a spinning rate of 3.5 kHz, a contact time of 8 ms and a recycle time of 15 s. The components indicated by dashed lines show the deconvolution results, in which a fixed line shape was assumed for the residual kaolinite.