DEGREE OF CRYSTALLINITY OF DEALUMINATED OFFRETITES DETERMINED BY X-RAY-DIFFRACTION AND BY A NEW METHOD BASED ON NITROGEN ADSORPTION

Samples obtained from thermal dealumination (in the presence and absen ce 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 perce ntage of crystallinity was then evaluated by two different methods: th e standard XRD technique and a new method based on nitrogen adsorption data analysed through the alpha(s)-method. In this case the volume of the ultramicropores (width less than 0.7 nm) was considered. Differen ces between the two estimates, as well as the loss of crystallinity du ring dealumination, which is expressed by the representation of both c rystallinity evaluations, C-XRD or C-ADS, versus nu(1) (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 observ ed. (C) Elsevier Science Inc. 1997.