Zeolite basicity characterized by pyrrole chemisorption: an infrared study

TLDR: In this article, it was shown that the basic sites detected by pyrrole chemisorption in zeolites are Lewis bases, which is in agreement with the view that the most common basic sites are the framework oxygens adjacent to the cations.

Abstract: The basicity of alkali-metal cation X, Y, mordenite and ZSM-5 can be characterized by the infrared NH stretching frequency of chemisorbed pyrrole. It was shown that the basic sites detected by pyrrole chemisorption in zeolites are Lewis bases. This is in agreement with the view that the basic sites in zeolites are the framework oxygens adjacent to the cations. Moreover the coexistence of two NH-stretching bands in the spectrum of pyrrole chemisorbed in a zeolite containing two kinds of cations indicates that the basicity of these sites is determined mainly by the local environment rather than by the bulk zeolite composition. The basicity in higher Si zeolites becomes weaker both in basic strength and the density of basic sites, as indicated by the high NH-stretching frequency and the weak band intensity. A pyrrole species associated with acid sites is also detected.