Basic sites in zeolites followed by IR studies of NO+
The basic properties of zeolitic oxygens were examined by IR studies of NO+ ions. NO+ were formed by the disproportionation of N2O4 (N2O4 → NO+ + NO3−). NO+ were stabilized by zeolitic oxygens and NO3− by the zeolitic cations or positively charged extraframework Al species. The spectra of NO+ in zeolite NaY show three maxima that indicate the presence of oxygens of various negative charges (of various basicity). These oxygens may have various numbers of AlO4− and/or various numbers of Na+ in close vicinity. The basic oxygen content was estimated as 15 sites/u.c. which is less than 5% of all the oxygen atoms in faujasite-type zeolite. In the presence of water, N2O4 disproportionates producing: NO3−, NO2− and strongly acidic Si–OH–Al groups. Similarly, NO+ reacts with water, forming NO2− and Si–OH–Al. The presence of strongly acidic Si–OH–Al decreases the basicity of the zeolitic oxygens. The basicity of the zeolitic oxygens diminishes also with the Si/Al, due to the decreased amount of AlO4−. NO+ were also formed in zeolites NaHY but like NO3−, their amount decreased with the degree of Na/H exchange, i.e., with the amount of Na+ cations, which are supposed to stabilize NO3− ions. In zeolites NaHY the basicity of oxygens decreased with the degree of Na/H exchange, due to replacement of Na+ cations by more electronacceptor H+ ions, but this effect is smaller than the variation of Si/Al The basicity of the oxygens in dehydroxylated zeolite NaHY was found to be comparable (but somewhat lower) to that in zeolite NaY. Steamed zeolite HY (which is known to contain the most strongly acidic hydroxyls) showed the lowest basicity of the zeolitic oxygens. IR studies showed also that the basicity of the zeolitic oxygens in zeolite NaY increased in the presence of electrondonor organic molecules such as CCl4 and chloroform, which we relate to some “neutralization” of the zeolitic cations.
Graphical abstractBasic properties of zeolites of faujasite type in sodium and protonic forms as well as in zeolites steamed and dehydroxylated was investigated. NO+ was used as a probe for basic sites. Three kinds of basic sites (framework oxygens) of various basicity were found in zeolite NaY. The basicity of framework oxygens was modified by the adsorption of electrondonor organic molecules. Spectra of increasing amount of NO+ and of NO3− (spectra a–d) formed in zeolite NaY. Figure optionsDownload full-size imageDownload as PowerPoint slide
Journal: Applied Catalysis A: General - Volume 319, 1 March 2007, Pages 64–71