Cation-induced transformation of boron-coordination in zeolites

The coordination of boron atoms and their association with counterions in d ehydrated zeolites B-ZSM-5, B-Beta, and B-SSZ-24 have been studied by solid state NMR methods. B-11 MAS NMR spectra show that boron occurs in both tri gonal (B-[3]) and tetrahedral (B-[4]) coordination in the zeolite framework . The isotropic B-11 chemical shift (delta(cs)) allows one to distinguish b etween trigonal extra-framework boron species (delta(cs)=18-19 ppm), B-nf([ 3]), and trigonal framework boron (delta(cs)=9.8-10.7 ppm), B-f([3]). Extra -framework boron species can be avoided if the zeolites are completely dehy drated with care. Large counterions such as sodium, lithium and ammonium io ns stabilise boron in tetrahedral coordination, whereas protons cause a tra nsformation to trigonal boron in the framework. B-11{Na-23} and B-11{H-1} r otational echo double resonance (REDOR) NMR reveals that B-[4] is selective ly associated with sodium ions, and B-f([3]) is associated with protons in mixed H,Na zeolites. H-1{B-11} REDOR experiments allow one to distinguish b etween protons in surface or defect silanols (delta=1.7-1.8 ppm) and proton s close to B-f([3]) (delta=1.9-2.0, 2.2-2.4, 2.7-3.0 ppm).