Relating Structure and Chemical Composition with Lewis Acidity in Zeolites: A Spectroscopic Study with Probe Molecules

Abstract

FTIR and EPR of adsorbed probe molecules were used to study Lewis acid sites in FAU and MFI zeolites with different nonframework Al content. Infrared spectra of adsorbed CO at low temperatures give information about all types of sites. The absorption bands were assigned to CO fixed on Brönsted sites, cations, and true Lewis acid sites with different strengths. The frequency shifts and the relative intensities of these peaks were correlated with the structure type and the nonframework Al amount. In contrast, EPR of adsorbed NO is probing only the sites strong enough to quench the orbital magnetic moment of NO molecule. However, the method can provide an estimation of the strength of the detected Lewis acid sites, via the gzz parameter. A correlation was established between the amount of nonframework Al and the spin concentrations of adsorbed NO. These absolute concentrations are lower than the expected number of Lewis acid sites. Several methods were proposed for creating Lewis acid sites in a controlled way. Among them, alumination and mild steaming were the most efficient, leading to well-distributed nonframework Al species, which act as strong Lewis acid sites. Despite its high nonframework Al content, the severely steamed zeolite Y has weak sites, only partially accessible for probe molecules, such as CO and NO.