High Aluminum Ordering in SSZ-59: Residual 1H-27Al Dipolar Coupling Effects in 1H MAS NMR Spectra of Brønsted Acid Sites in Zeolites

Schroeder C, Zones SI, Mück-Lichtenfeld C, Hansen MR, Koller H

Abstract

The extra-large pore zeolite SSZ-59 was prepared with highly ordered Al atoms in T6 positions within the double zig-zag chains (dzc) running along the 14 MR channels. Narrow 1H MAS NMR signals of bridging OH groups, which are responsible for Brønsted acidity, emerge from this Al site ordering. The theoretically predicted residual 1H-27Al dipolar effect that is caused by the large 27Al quadrupolar coupling associated with the trigonally distorted Al coordination near this OH group was observed here with an unprecedented spectral resolution. Three 1H NMR signals are separately observed at B0 = 7.0 T for proximal 27Al spins with magnetic quantum numbers of m = ±1/2, m = ±3/2 or m = ±5/2. A short 1H-27Al distance of 2.52 ± 0.02 Å was measured by means of 1H{27Al} rotational echo adiabatic passage double resonance (REAPDOR) characteristic for a Brønsted acid site. This distance predominates the initial slope of the 1H{27Al} REAPDOR evolution curve, whereas additional much weaker 1H-27Al dipolar interactions were found by a second, less steep slope at longer evolution times. The second slope can be modeled either by a second Al atom within the dzc chain located at a distance of 6.7 Å to the bridging OH group, or, alternatively, by two Al atoms in the dzc, with H-Al distances of 8.09 and 8.72 Å. The latter model with two Al is in excellent agreement with the packing of the organic structure-directing agent in the 14 MR pores of the as-made material, which places the heteroatoms - in this case boron atoms that are post-synthetically replaced by aluminum after calcination/removal of the organics - into a highly ordered pattern in the dzc. The data indicate that the packing of the organics in adjacent 14 MR channels are not independent and shifted by 1.5 unit cells along the a-direction.