Infrared Study of Pyridine Adsorbed on Montmorillonite Surfaces

T H E NATURE of pyridine adsorption by acidand base-saturated montmorillonite from Otay, California (A.P.I, no. H-24) was studied by infrared adsorption. The spectrum for chemisorbed pyridine was found to be distinctively different from physically adsorbed pyridine; the latter gave a spectrum very similar to liquid pyridine. Upon exposure of the clay films to pyridine at low vapor pressures, only the pyridinium ion spectrum was obtained; but when the vapor pressure was increased, both chemisorbed and physically adsorbed pyridine was detected. The intensity of the spectrum for physically adsorbed molecules could be reduced by a vacuum, whereas the intensities of the peaks for the chemisorbed pyridinium ions were unaffected. Interlayer water was replaced by adsorption of pyridine. Water, in turn, replaced physically adsorbed pyridine from the clay surface but was unable to replace chemisorbed pyridine. SORPTION of gases on surfaces of colloidal solids is frequently used as a means of characterizing the acidic nature of solid surfaces. Eischens and Pliskin (1958) have given an extensive discussion of the sorption of molecules on surfaces and an interpretation of their infrared spectra. Mapes and Eischens (1954) have used the infrared spectrum of ammonia chemisorbed on cracking catalysts to differentiate between Lewis and Bronsted type acid sites. Clays, even when saturated with basic cations, act as moderately strong acids. Benesi (1956), using adsorbed color indicators, reported that sodiumand ammonium-saturated kaolinite had stronger acid sites than did montmorillonite that was saturated with the same cations. Considerable work has been done on the adsorption of ammonia on large surface area solids. Mortland et al. (1963) studied the adsorption of ammonia on montmorillonite and vermiculite extensively and found that the basic ammonia molecule reacted with adsorbed interlayer water to form ammonium ions that are chemisorbed on the clay surfaces. They were unable to obtain an infrared spectrum for physically adsorbed ammonia molecules at room temperature but were able to do so at -150°C. Ammonia, being a relatively strong base (pifj = 4.7), will react with extremely weak acid sites. By using a weaker iDase one may obtain a better * A contribution of the Texas Agricultural Experiment Station.