Post-Synthesis Conversion of Borosilicate Zeolite Beta to an Aluminosilicate with Isolated Acid Sites: A Quantitative Distance Analysis by Solid-State NMR

Isolated acid sites were selectively generated by postsynthesis modification of a borosilicate zeolite beta. To this end, samples were prepared with pairs of adjacent boron sites balanced by Ca 2+ ions, whereas isolated boron in the zeolite framework was balanced by NH 4 + ions. To avoid undesired boron leaching, these ion exchange reactions were carried out in methanol solution rather than in water. Trigonal boron forms near the NH 4 + ions by decomposing the latter into NH 3 (and a proton), and selective extraction of the trigonal boron is achieved by water treatment, whereas the tetrahedral boron near Ca 2+ ions remains in the zeolite framework. The vacancies were refilled with aluminum by treatment with an aqueous Al(NO 3 ) 3 solution. Two Brønsted acid sites with 1 H chemical shifts of 4.0 and 5.0 ppm exist in the dehydrated samples. 1 H– 27 Al REAPDOR solid-state NMR measurements yield quantitative information on the local H–Al distances of isolated H–Al two-spin and H–Al–Al three-spin systems. The nearest H–Al distance is determined at 2.50 Å with an accuracy of 2% (or better) by fitting the oscillatory part of the REAPDOR curves, which was not observed before for zeolite acid sites. The second nearrest Al neighbors show a much larger distance of about 5 Å for the acid protons with a chemical shift of δ = 4 ppm. A second acid site at δ = 5 ppm has an approximately 50% occupation of a second Al neighbor at 3.73 Å, possibly within the same six-ring. This high resolution of dipolar interaction is not observed in a standard zeolite Al-beta prepared by direct synthesis. The method is suitable to identify well-defined local ordering in Al distributions of zeolites.