Bronsted and Lewis acid sites in microporous and mesoporous aluminosilicate catalysts: A high-resolution solid-state nuclear magnetic resonance study

The objective of this work was the characterization of Brönsted and Lewis acid sites in microporous and mesoporous aluminosilicate catalysts through the development of new solid-state NMR techniques and novel applications of existing solid-state NMR techniques.; A new solid-state NMR method for the quantitative characterization of Brönsted and Lewis acid sites in dealuminated zeolites was developed. Brönsted sites were quantified by the use of the Hahn spin-echo technique, which can distinguish between aluminum atoms of different coordination through their spin-spin relaxation time. The number of Lewis sites was determined in REDOR experiments through the extent of dephasing of the 27Al NMR signal. The dephasing was due to the 1H-27Al heteronuclear dipolar interaction between Al atoms and protons of chemisorbed ammonia at the Lewis sites. Although the framework Si/Al ratio contains all the information for a complete characterization of the Brönsted sites, it proves to be insufficient for the quantification of the Lewis sites. The exact number of Lewis sites is correlated to the preparation technique and not to the framework Si/Al ratio.; The formation of Lewis and Brönsted acid sites in “post-grafted” Al-MCM-41 mesoporous catalysts was investigated step-by-step with 2D HETCOR, a two-dimensional solid-state NMR technique. Reversible conversions of hexa-coordinated aluminum to penta-coordinated aluminum and of penta-coordinated aluminum to tetrahedral aluminum were observed. These conversions depend strongly on the cation content of the catalyst. Penta-coordinated aluminum is favored by low cation content. Two tetrahedral aluminum species are evidenced. The first type, which is inserted into the framework during the “post-grafting” process, is stable and does not depend on the cation content. The second type is produced through the treatment with ammonium chloride by transformation of pentagonal aluminum. The stability of this type depends strongly on the cation content. The precise location of every aluminum species was determined. Penta-coordinated aluminum is at the interface between hexa- and tetra-coordinated aluminum. 2D HETCOR coupled with the chemisorption of 15N-ammonia permits the direct detection of Brönsted and Lewis acid sites. The Brönsted site content increases with the aluminum content to a limit, beyond which the Lewis site content is enhanced.