Characterisation of defective silicalites

Abstract

The structure and the adsorptive properties of internal defects in two chemically very pure silicalite samples, characterised by a high and low concentration of defects (internal SiOH nests generated by Si vacancies), are investigated by means of several physical methods (neutron diffraction, IR spectroscopy of adsorbed NH₃, microcalorimetry of NH₃ adsorption and quantum chemical computation). The role of cooperative effects in hydrogen bonded chains and rings of silanols covering the internal defective nanocavities in influencing the absorptive properties of the material is elucidated. This study provides an explanation of the less hydrophobic character of defective silicalite relative to perfect silicalite and sheds light on the reason why defective silicalite acts as an efficient and selective catalyst for the gas phase Beckman rearrangement reaction where cyclohexanone oxime is converted into caprolactam.