High Resolution Solid State Mas 11b-NMR Evidence of Boron Incorporation in Tetrahedral Sites of Zeolites

Abstract

High resolution solid state MAS 11B-NMR was used to detect quantitatively variously coordinated BOx structural units in zeolites. Boron stemming from Pyrex autoclaves is progressively incorporated into the tetrahedral sites of ZSM-5 and mordenite frameworks during synthesis. The final crystalline zeolite phases may contain up to 0.8 B atoms in their unit cells.

Calculated magnetic properties for the characterization of zeolite active sites

“…67 -71 These experiments show that 11 B MAS NMR signals, characteristic of tetrahedrally coordinated borons, are measured for zeolites with Na C cations and for acidic solids containing water or ammonia, whereas the 11 B signals of dehydrated acidic forms are shifted downfield by 10-15 ppm and show a characteristic quadrupolar shape, assigned to planar trigonal borons. Rehydration of dehydrated solids leads back to the NMR spectrum of tetrahedral boron, which demonstrates that the B-O(Si) bond breaking is reversible.…”

“…MAS NMR has been successfully applied to zeolites containing 7 Li, 11 B, 17 O, 23 Na, 29 Si, 27 Al and 31 P. 29 Si spectra have been particularly useful thanks to the wide range of 29 Si chemical shifts, very sensitive to the local geometry and electronic structure of the nearest neighbours. 3 -5 29 Si signals produced by crystallographically non-equivalent silicons can be obtained, as demonstrated in pioneering works on a highly siliceous silicalite 6,7 showing up to 20 well-resolved lines for the 24 sites.…”

“…3 -5 29 Si signals produced by crystallographically non-equivalent silicons can be obtained, as demonstrated in pioneering works on a highly siliceous silicalite 6,7 showing up to 20 well-resolved lines for the 24 sites. The study of elements with quadrupolar nuclei ( 27 Al, 11 B, 23 Na, 17 O) is less easy, leading to broadening and shifting of the signals. However, more recent techniques, such as two-dimensional multi-quantum (2D MQ) MAS NMR 8 and rotational echo double resonance (REDOR) NMR, 9 have been developed, providing very interesting information on the local geometry.…”

“…However, more recent techniques, such as two-dimensional multi-quantum (2D MQ) MAS NMR 8 and rotational echo double resonance (REDOR) NMR, 9 have been developed, providing very interesting information on the local geometry. In particular, the knowledge of the actual state of boron in Magnetic properties of zeolite active sites S181 the zeolite framework was provided by 11 B studies, showing the interchange, according to the experimental conditions, between tetrahedrally coordinated and trigonal boron. 10 -14 Despite the remarkable sensitivity of recent NMR techniques to detect very small changes in atomic positions, the short time-scale of NMR gives access to only dynamically averaged structures and it is often difficult to relate unequivocally the NMR spectra with specific structures.…”

Isomorphous Substitution in Zeolites

“…The framework tetrahedral boron (T) is characterized by a chemical shift of c.a. -3.9 ppm with respect to BF 3 -OEt 2 [205]. In the presence of Cs and 10 moles of H 3 BO 3 in the initial gel, up to 9.4 tetrahedral B/u.c.…”