CHARACTERIZATION OF HIGHLY DEALUMINATED FAUJASITE-TYPE ZEOLITES - ULTRASTABLE ZEOLITE-Y AND ZSM-20

Highly dealuminated faujasite-type zeolites with different dealuminati on histories, different pore structures, and Si:Al ratios ranging from 40:1 to 300:1 have been investigated by temperature-programmed desorp tion of ammonia (NH3-TPD) and by FTIR spectroscopy: ultrastable zeolit e Y (US-Ex), dealuminated by hydrothermal treatment and a subsequent e xtraction of the extraframework aluminum; DAY, dealuminated by exchang e with SiCl4; and the recently synthesized ZSM-20, dealuminated hydrot hermally. A correlation could be established between the amount of ads orbed NH3 determined by TPD and the integrated absorbances of N-H bend ing bands in the FTIR spectra. Time-resolved FTIR spectroscopy during TPD has been used to follow the desorption behavior of the ammonia spe cies adsorbed on different sites. The analysis of the intensity decrea se of absorption bands of adsorbed ammonia and the comparison with the simultaneous increase of hydroxyl stretching bands during TPD, reveal s the following: (i) the isolated structural Bronsted acid sites have a similar acid strength in all samples, including the HZSM-20, regardl ess of the different dealumination procedure. (ii) Extraframework alum inum species formed during dealumination may block a significant part of the strong Bronsted sites. (iii) The acid framework hydroxyl groups in both the alpha-cage (causing the HF band) and the beta-cage (LF ba nd) contribute in the same way to the desorption behavior of NH3. (iv) In addition to bonding on strong Bronsted acid sites, NH3 is adsorbed on weak Bronsted and/or Lewis acid sites and on stronger Lewis acid s ites. The ammonia bonded to the stronger Lewis acid sites seems to int eract with a part of the terminal hydroxyl groups. The similarity betw een the isolated structural Bronsted acid sites in the Y zeolites stud ied and those in the HZSM-20 has been proven using rhodium dicarbonyl Rh(I)(CO)2+ as a sensitive probe molecule for the characterization of isolated acid sites.