Cation-induced transformation of boron-coordination in zeolites
Abstract
The coordination of boron atoms and their association with counterions in dehydrated zeolites B-ZSM-5, B-Beta, and B-SSZ-24 have been studied by solid state NMR methods. 11B MAS NMR spectra show that boron occurs in both trigonal (B[3]) and tetrahedral (B[4]) coordination in the zeolite framework. The isotropic 11B chemical shift (δcs) allows one to distinguish between trigonal extra-framework boron species (δcs=18–19 ppm), Bnf[3], and trigonal framework boron (δcs=9.8–10.7 ppm), Bf[3]. Extra-framework boron species can be avoided if the zeolites are completely dehydrated with care. Large counterions such as sodium, lithium and ammonium ions stabilise boron in tetrahedral coordination, whereas protons cause a transformation to trigonal boron in the framework. 11B{23Na} and 11B{1H} rotational echo double resonance (REDOR) NMR reveals that B[4] is selectively associated with sodium ions, and Bf[3] is associated with protons in mixed H,Na zeolites. 1H{11B} REDOR experiments allow one to distinguish between protons in surface or defect silanols (δ=1.7–1.8 ppm) and protons close to Bf[3] (δ=1.9–2.0, 2.2–2.4, 2.7–3.0 ppm).
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