Acidity measurements of zeolite Y by adsorption of several probes

Abstract

Acid strength distribution measurements of zeolite Y by a combined analysis of calorimetric and adsorption data, Cal-ad, for the reaction of zeolite Y with pyridine, 2,6-lutidine, 2,6-di-tert-butylpyridine, quinoline, tetrahydrothiophene and benzonitrile show the presence of three families of acid sites. The strongest sites (heats above −29 kcal mol⁻¹) contain a significant number of Brønsted sites (n = 0.1 mmol g⁻¹), which are located inside the large cages. The sites of intermediary strength (heats around −17 kcal mol⁻¹) are of Lewis and Brønsted types and the weaker sites (heats below −10 kcal mol⁻¹) contain significant hydrogen bonding. The strength of these sites is probably due to mutual interaction between Lewis and Brønsted sites. The accessibility on zeolite and the size of the probe molecules play fundamental role on the determination of acid site distribution. Application of the electrostatic–covalent model (ECW) to this acid provides quantitative parameters for determination of donor-acceptor contributions based on electrostatic (E_A) and covalent (C_A) intrinsic characters to each chemical bonding. The results demonstrate that zeolite Y has a stronger covalent character with E_A = 5.25, C_A = 6.89 (C_A/E_A = 1.31) consistent with softness properties suggested in the literature.