Characterization of the basicity of oxides through the infrared study of pyrrole adsorption

Abstract

The adsorption of pyrrole on various well characterized oxides has been studied by infrared spectroscopy; the shift of the NH stretching band due to H bonding allows the detection of basic sites on the surface and estimation of their strength.

Basic sites present on alumina are eliminated by fluorination of the alumina. The sites on alumina are appreciably stronger than those on magnesia; the basic strength of the former would be similar to that of pyridine, the strength of the latter being close to that of dimethyl sulphoxide. For silica–aluminas, basic sites are only detected at high alumina content (60 % Al₂O₃) and are attributed to the demixed alumina phase.

Data obtained for aluminas and silica–aluminas show that pyrrole may also be used to probe Lewis acid sites; a shift of bands due to ring vibration is indeed observed, as occurs for pyridine.

Tests have been made which show the pyrrole polymerization or decomposition does not affect the interpretation of spectra and the assignment of NH bands.