aExxonMobil Research and Engineering Co., Corporate Strategic Research, 1545 Route 22E, Annandale, NJ 08801-3059, USA
bAmes Laboratory and Department of Chemistry, Iowa State University, Ames, IA 50011-3020, USA
Available online 11 April 2008.
Abstract
The recently introduced concept of the combined use of rotor assisted population transfer (RAPT) and Carr–Purcell–Meiboom–Gill (CPMG) techniques to boost the sensitivity of cross polarization (CP) based NMR experiments is applied to a synthetic zeolite (ZSM-4). The sensitivity was increased by a factor of 
4, which enabled acquisition of a high quality two-dimensional 27Al–29Si HETCOR (heteronuclear correlation) spectrum. By separating the resonances in two dimensions, through-space connectivities between spins were revealed and the effective resolution was improved in both dimensions, which allowed determination of the existing ambiguities in spectral assignments in this material. The spectra provided clear indication of random distribution of aluminum and silicon within the ZSM-4 network. Additionally, unexpected correlations were observed between different components of inhomogeneously broadened 29Si and 27Al lines, which are most likely due to differences in the second coordination sphere environments.
Keywords: ZSM-4; MAZ; Zeolite; NMR; HETCOR
Fig. 1. Pulse sequence for 27Al–29Si HETCOR experiment with sensitivity enhancement by RAPT and CPMG data acquisition.
Fig. 2. Zeolite omega structure (MAZ), viewed normal to 001, showing the position of T1 and T2 in the 4MR and 6MR of the gmelinite cage, respectively (adapted from Ref. [52]).
Fig. 3. 29Si MAS NMR spectrum of zeolite ZSM-4 recorded at 11.7 T with 4.0 kHz MAS.
Fig. 4. (a) 27Al MAS NMR spectrum of zeolite ZSM-4 recorded at 11.7 T with 13.3 kHz MAS. (b) 27Al 3QMAS NMR spectrum of zeolite ZSM-4 recorded at 11.7 T with 13.3 kHz MAS. Projections of the direct (MAS) dimension and indirect (isotropic) dimension are shown on the horizontal and vertical axes, respectively.
Fig. 5. 2D 27Al–29Si RAPT–CP–CPMG HETCOR data for zeolite ZSM-4 recorded at 14.1 T with 10 kHz MAS. The 2D spectrum was acquired in 30 rows of hypercomplex data, with 64000 scans per row, t1 increments of 100 μs, and total acquisition time of 38 h. In the right, 1D slices taken through the centers of Al1 and Al2 resonances are compared with 1D 29Si MAS spectrum.
Fig. 6. (a) Extended view of the 2D 27Al–29Si RAPT–CPMG–HETCOR spectrum of Fig. 5. (b) Cross-sections along the 29Si dimension corresponding to dashed lines in (a). The main cross-peaks are labeled 1–6.
Table 1.
Peak assignments for the 29Si MAS NMR of zeolite ZSM-4
