Studies on the solid acidity of heated and cation-exchanged montmorillonite using n-butylamine titration in non-aqueous system and diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy

摘要

The effects of heating and cation exchange on the solid acidity of montmorillonite were investigated using n-butylamine titration in non-aqueous system and diffuse reflectance Fourier transform infrared spectroscopy. The number of total, Br?nsted, and Lewis acid sites showed the same modulation tendency with increasing heating temperature, reaching a maximum at 120 °C and subsequently decreasing until it reaches a minimum at 600 °C. The Lewis acid sites result from unsaturated Al~(3+) cations, and their number increased with the heating temperature due to the dehydration and dehydroxylation of montmorillonite. The generation and evolution of Br?nsted acidity were mainly related to interlayer-polarized water molecules. Water adsorbed on the unsaturated Al~(3+) ions also acted as a Br?nsted acid. The acid strength of the Br?nsted acid sites was dependent on the polarization ability of the exchangeable cation, the amount of interlayer water, and the degree of dissociation of the interlayer water coordinated to exchangeable cations. All cation-exchanged montmorillonites exhibited different numbers of acid sites and various distributions of acid strength. Br?nsted acidity was predominant in Al~(3+)-exchanged montmorillonite, whereas the Na~+- and K~+-exchanged montmorillonites showed predominantly Lewis acidity. Moreover, Mg~(2+)- and Li~+-exchanged montmorillonites exhibited approximately equal numbers of Br?nsted and Lewis acid sites. The Br?nsted acidity of cation-exchanged montmorillonite was positively correlated with the charge-to-radius ratios of the cations, whereas the Lewis acidity was highly dependent on the electronegativity of the cations. The acid strengths of Al~(3+)- and Mg~(2+)-exchanged montmorillonites were remarkably higher than those of monovalent cation-exchanged montmorillonites, showing the highest acid strength (H 0 ≤ -3.0). Li~+- and Na~+-exchanged montmorillonites exhibited an acid strength distribution of -3.0 < H 0 ≤ 4.8, with the acid strength ranging primarily from 1.5 to 3.3 in Li~+-exchanged montmorillonite, whereas only weaker-strength acid sites (1.5 < H 0 ≤ 4.8) were present in K~+-exchanged montmorillonite. The results of the catalysis experiments indicated that montmorillonite promoted the thermal decomposition of the model organic. The catalytic activity showed a positive correlation with the solid acidity of montmorillonite and was affected by cation exchange, which occurs naturally in geological processes.