Determination of Integrated Molar Extinction Coefficients for Infrared Absorption Bands of Pyridine Adsorbed on Solid Acid Catalysts

Integrated molar extinction coefficients were determined for infrared absorption bands of pyridine adsorbed on acid sites in Si/Al-based catalysts. The infrared spectra of five zeolites and two amorphous silica-aluminas were recorded during quantitative dosing of pyridine gas at 150°C. The integrated molar extinction coefficients were calculated assuming that they did not depend on the catalyst or the strength of the acid site. The resulting values were 1.67 cm/μmol for the 1545-cm−1 band characteristic of pyridine on a Brønsted acid site and 2.22 cm/μmol for the 1455-cm−1 band of pyridine on a Lewis acid site. The 95% confidence limits were estimated at ±15%. Subsequent analysis of the data provided no evidence for a dependence of the integrated coefficients on the catalyst or the strength of the site. The measurements gave indications for differences between the rates of reaction of Brønsted and Lewis acid sites with pyridine.