ACIDITY CHARACTERIZATION OF ZEOLITES VIA COUPLED NH 3 -STEPWISE TEMPERATURE PROGRAMMED DESORPTION AND FT-IR SPECTROSCOPY
The effect of dealumination and pore structure on the acidity of zeolites has been examined in this study. Five different large pore zeolites were selected based upon their respective silicon to aluminum ratios (Si/Al or SAR) and their particular pore structure systems. Specifically, the samples studied were: Beta zeolite, L zeolite, Mordenite, Ultra Stable Y (USY) zeolite, and ZSM-12. The silicon to aluminum ratios (SAR's) for these samples ranged from 2.6 (for non-dealuminated USY) up to 132 (for highly dealuminated Beta zeolite). The structures ranged from one-dimensional structures (ZSM-12 and L zeolite) through a two-dimensional structure interconnected with a one-dimensional structure (Beta zeolite). Wherever possible, the similarities and differences of the five samples were considered with respect to the acidity results obtained. An amplification of the conventional Temperature Programmed Desorption technique was developed and named Stepwise Temperature Programmed Desorption (STPD). A comparison between the number of basic probe molecules measured during the STPD and the number of aluminum atoms present in each sample (as determined by ICP Spectroscopy) revealed a one-to-one relationship for most samples studied. Moreover, this technique has been shown to yield specific results for zeolites as opposed to other support materials. This proves that zeolites possess heterogeneous acid sites of discrete strengths. Deviation from this behavior was observed for samples with very low SAR (acid sites in close proximity) due to either pore blockage or adsorption of a single basic probe molecule on multiple acid sites. FT-IR spectroscopy was employed in order to determine the nature of the acidity found during the STPD experiments. The same STPD temperature profile was applied to self-supported zeolite wafers saturated with the same basic probe molecule. By combining the STPD results with the FT-IR results, it was determined that the first two STPD peaks consisted of ammonia desorbing off of Lewis acid sites and the last three STPD peaks consisted of ammonia desorbing off of Brønsted acid sites. The temperature at which each desorption step took place was also found to be proportional to the strength of the acid site in question.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:zeolite acidity temperature programmed desorption fourier transformed infrared spectroscopy
Date of Publication:01/01/2002