The effect of acid treatment on the structure and surface acidity of tetraalkylammonium-montmorillonites

The effect of tetrabutylammonium (Bu4N+) and tetrapentylammonium (Pe4N+) cations on the modification of the organo-montmorillonite structure upon acid-treatment was investigated. Samples were treated with HCl for various times (2–12 h). Structural changes were followed by MAS NMR spectroscopy. The 29Si MAS NMR spectra of initial Na-saturated form (Na–SAz) showed gradual decrease of the intensity of the resonance assigned to SiO4 cross-linked in the tetrahedral sheets Q3(0Al) while signals arising from the reaction product Q31OH and Q4(0Al) became more pronounced upon acid treatment. The Q3(0Al) signal almost completely disappeared for Na–SAz treated for 8 h on contrary to Bu4N–SAz and Pe4N–SAz showing signal of relatively high intensity even after 12 h. The 27Al MAS NMR measurement proved that more than one half of Al remained in the reaction product of Bu4N–SAz and Pe4N–SAz after 8 h treatment, while Al content dropped below 5% for Na–SAz. Formation of acid sites was investigated via pyridine adsorption. Only physically adsorbed and H-bonded pyridine was detected for acid-untreated samples. In contrast, the IR spectra of the samples partially decomposed in HCl revealed bands of pyridine adsorbed on Brønsted acid sites. Strongly bonded pyridine was able to bear up heating even at 230 °C.

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► Structural changes in acid treated Na-, and organo-montmorillonites were studied.
► Decomposition of the structure depended on the cation size and reaction time.
► Organo-montmorillonites were more resistant to HCl than Na-sample.
► 29Si CP MAS NMR revealed the presence of Si(OH)2 groups in the reaction product.
► IR spectra of pyridine confirmed the Brønsted sites in acid treated samples.