Degree of crystallinity of dealuminated offretites determined by X-ray diffraction and by a new method based on nitrogen adsorption

Abstract

Samples obtained from thermal dealumination (in the presence and absence of water vapour) of three offretite zeolites with, respectively, 4.4, 3.4, and 2.9 aluminium atoms per unit cell, were investigated by i.r. spectroscopy. X-ray diffraction, and nitrogen adsorption. The percentage of crystallinity was then evaluated by two different methods: the standard XRD technique and a new method based on nitrogen adsorption data analysed through the α_s-method. In this case the volume of the ultramicropores (width less than 0.7 nm) was considered. Differences between the two estimates, as well as the loss of crystallinity during dealumination, which is expressed by the representation of both crystallinity evaluations, C_XRD or C_ADS, versus ν₁ (the frequency of the stretching asymmetric T-O band), were analysed for self-steamed and steamed samples. The results obtained agree within 20%, and often better. For self-steamed samples the crystallinity evaluated from XRD patterns is always smaller than that estimated from adsorption data, whereas for steamed samples the opposite situation is generally observed.