A study of the external and internal sites of 2D and 3D zeolites through the FTIR investigation of the adsorption of ammonia and pivalonitrile

Abstract

In this work absorption coefficients of the 1450 cm−1 IR maximum of ammonium ions (formed by ammonia adsorbed on BAS) and the CN band assigned to pivalonitrile interacting with BAS, are determined for MWW zeolites (MCM-22, MCM-56 and UJM-1). These three distinct types of MWW zeolites, differing in layer arrangement but not the framework (intra-layer) topology, are used to show differences in the concentration of Brønsted acid sites that are available for pivalonitrile (BAS located at the external crystal surfaces). It is shown that in the case of ammonium ions, common absorption coefficient for 10-rings zeolites (MWW, MFI) may be used (ε(NH4+) = 0.090 cm2/μmol). For pivalonitrile interacting with Brønsted acid sites in MWW zeolites the value of absorption coefficient was determined (0.05 cm2/μmol) together with re-assignment of the IR bands of pivalonitrile adsorbed in MWW based on thorough analysis of the published and obtained here spectra. The revised assignment is expected to be applicable to other layered zeolites.