FTIR study of the kinetics of solid-state ion exchange in zeolites using pyridine as a probe

Abstract

The solid-state interaction between CuCI and sodium forms of zeolites has been followed in situ as a function of reaction time by IR spectroscopy using pyridine as a probe. It was found that, upon heat-treatment of the mixtures of CuCI/Nay or CuCI/Na-mordenite at a temperature higher that 180 °C, the intensities of the bands of pyridine coordinated to Na⁺ cations in Zeolites decreased, while the bands of pyridine coordinated to the Cu⁺ cations on the cationic positions of the zeolites appeared and the intensities increased with reaction time. This phenomenon was explained as the ion exchange of copper(I) and Na⁺ between CuCI and the zeolites. The results indicated that the process of ion exchange between Cu⁺ in CuCI and Na⁺ in zeolites depends strongly upon the reaction temperature. A high-temperature reaction ( >300 °C), however, caused re-migration of the Cu⁺ cations away from their occupied cationic positions, probably to regenerate CuCI, and re-migration of Na⁺ cations back to those positions. Pre-activation of the CuCI/Na-mordenite samples at a temperature as high as 150 °C did not influence the ion exchange, indicating that, according to the TPD results, the zeolite-trapped water is not a prerequisite for solid-state ion exchange. The two overlapping bands at ca. 1451 and 1442 cm^–1, typical of the vibrations of pyridine–Cu⁺ and pyridine–Na⁺ coordination complexes, were deconvoluted and fitted by using a mixed Gaussian–Lorentzian function. Based upon the fitting results, the integrated absorbances of the bands of pyridine–Cu⁺ and pyridine–Na⁺ were calculated as a function of reaction time for the reactions at different temperatures and were normalised, to allow a reasonable comparison, to those which would be obtained from the identical samples at a temperature of 260 °C.