posted on 2016-02-19, 02:54authored byZ. Qin, L. Lakiss, J.-P. Gilson, K. Thomas, J.-M. Goupil, C. Fernandez, V. Valtchev
Chemical
etching with fluoride ions is a new approach for secondary
porosity engineering of aluminosilicate zeolite frameworks. We show
that diluted HF solutions extract preferentially aluminum from zeolite
frameworks. The Brønsted acidity of ZSM-5 treated in such a way
decreases, while its structure is unaffected after an HF treatment.
With higher HF concentrations, the number of undissociated HF molecules
and the concentration of HF2– ions, extracting
indiscriminately Al and Si, increase. The addition of NH4F shifts the chemical equilibria to produce more HF2–, avoiding the use of highly concentrated HF solutions;
it also suppresses HF dissociation. The etching selectivity of such
solutions is concentration-independent and extracts indiscriminately
both framework Si and Al. Zeolite dissolution in NH4F-HF
solutions starts preferentially at small intergrowth domains and goes
deeply in the crystals without a substantial increase of the external
surface area. Macropores are produced without altering zeolite acidity.
Hierarchical materials obtained by these two approaches are characterized
extensively by complementary methods and the catalytic impact illustrated
in the m-xylene conversion.