Comparative Study of Zeolite L Etching with Ammonium Fluoride and Ammonium Bifluoride Solutions

A commercial zeolite L is subjected to chemical etching with aqueous solutions of NH4F and NH4HF2 to generate secondary porosity. The derived samples are characterized by X-ray diffraction, N-2 adsorption, scanning, and transmission electron microscopy, Al-27 MAS NMR, thermogravimetric analysis, and their acid sites are quantified by FTIR spectroscopy of adsorbed basic probe molecules. The catalytic properties of the pristine and modified zeolites are evaluated in the dealkylation of 1,3,5-triisopropylbenzene. Tuning the etching conditions allows to generate hierarchical zeolites without losing micropore volume. Furthermore, both fluoride etchants do not change the acid properties of the zeolite substantially, which is confirmed by chemical composition analysis (ICP-OES) and by FTIR spectroscopy of pyridine. The dissolution rate of zeolite L is much faster with NH4HF2 than NH4F. The set of experimental data shows that the 1 wt% NH4HF2 solution is an efficient etchant able to generate mesopores without affecting the parent zeolite properties as lower amounts of fluoride source are required than with concentrated (20-40 wt%) NH4F solutions.