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

• Published in: Journal of physical chemistry. C 2016-05, Vol.120 (18), p.9811-9820
• Main Authors: Koller, Hubert, Senapati, Soumya, Ren, Jinjun, Uesbeck, Tobias, Siozios, Vassilios, Hunger, Michael, Lobo, Raul F
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b01680

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 Ca2+ ions, whereas isolated boron in the zeolite framework was balanced by NH4 + 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 NH4 + ions by decomposing the latter into NH3 (and a proton), and selective extraction of the trigonal boron is achieved by water treatment, whereas the tetrahedral boron near Ca2+ ions remains in the zeolite framework. The vacancies were refilled with aluminum by treatment with an aqueous Al­(NO3)3 solution. Two Brønsted acid sites with 1H chemical shifts of 4.0 and 5.0 ppm exist in the dehydrated samples. 1H–27Al 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.