Modification of Crystalline Microporous Solids

Tatsumi, Takashi

doi:10.1002/9783527618286.ch18d

Cited by 3 publications

(2 citation statements)

References 104 publications

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“…This is an important advantage over aluminosilicates, where the scattering factors of Al and Si are very similar. Tailored post-synthesis modifications have become a prominent research area, and boron can be easily exchanged by other framework elements, such as Ti or Al, ,− or for examples of Fe grafting on surface layers of delaminated materials . A post-synthetic B/Al exchange offers a pathway to catalytic materials with strong acid sites, where protons are bound to oxygen atoms bridging between Si and Al .…”

Section: Introductionmentioning

confidence: 99%

Ordered Heteroatom Siting Preserved by B/Al Exchange in Zeolites

et al. 2022

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The borosilicate zeolites SSZ-53, SSZ-55, SSZ-59, and SSZ-82 were converted to aluminosilicates by treating them with an aqueous Al(NO3)3 solution. The removal of boron atoms from the zeolite structures by acid leaching under comparable hydrothermal conditions was significantly inhibited. These findings reflect the complex post-synthesis chemistry of zeolites, where the Lewis acidic nature of Al3+ ions possibly plays a role. Density functional theory cluster calculations of 27Al chemical shifts and their comparison with experimental values were employed to determine the Al positions in the crystal structures. Al insertion takes place at the boron sites that had been previously established from X-ray diffraction data. The data suggest that making a detour via borosilicates can create aluminosilicate zeolite catalysts with ordered active sites for catalysis applications.

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Section: Introductionmentioning

confidence: 99%

Ordered Heteroatom Siting Preserved by B/Al Exchange in Zeolites

et al. 2022

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“…The classical method to prepare Brønsted acid sites in zeolites is to exchange NH 4 + for other ions in the zeolite pores (such as alkali ions) in an aqueous ion exchange process and subsequently thermally decompose the ammonium ion into NH 3 and a proton. The Brønsted proton, H + , binds to a bridging oxygen that is connected to a trivalent atom in tetrahedral framework position, TO 4/2 – , thus ensuring charge neutrality. − The most common trivalent (tetrahedral) T atom in Brønsted acid sites is Al, but Ga, B, or other trivalent ions are also possible. Nonetheless, this simple bridging hydroxyl model of an acid site is usually insufficient to explain differences in catalytic properties of different zeolite materials.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2016

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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.

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