In situ determination of Bronsted/Lewis acidity on cation-exchanged clay mineral surfaces by ATR-IR

Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy has been used to determine the nature of acid sites present in Na+-, Ni2+- and Al3+-exchanged montmorillonite using the diagnostic base, pyridine. The pyridine additions were performed in situ at 25 degrees C in the presence of liquid benzene or deuterated 1,4-dioxan to ascertain whether the acidity profile was influenced by the presence of either the non-polar or oxygenated solvent. Pyridine treated Ni2+-montmorillonite exhibited absorption bands characteristic of Lewis bound base in both benzene and deuterated 1,4-dioxan indicating that the Lewis acid sites were present at low temperature and were accessible in both solvents. The presence of a strong 1537 cm(-1) band in pyridine-saturated Al3+-montmorillonite confirmed that this clay acted as a Bronsted acid in benzene. However, this diagnostic band was significantly reduced in the presence of deuterated 1,4-dioxan.