Template-aluminosilicate structures at the early stages of zeolite ZSM-5 formation. A combined preparative, solid-state NMR, and computational study

Species at three stages in the self-assembly of zeolite ZSM-5 have been studied with one- and two-dimensional rnagic-angle-spinning C-13, Al-27, Si-29, and H-1 NMR spectroscopy and compared with the earlier proposed structures: (1) precursor species containing 33-36 T sites around a tetrapropylammonium (TPA) cation, (2) nanoslabs consisting of a flat 4 x 3 array of such precursors, and (3) the final TPA-ZSM-5 zeolite. Synthesis was carried out in D2O to suppress the water and silanol protons. Under such conditions, the effective Si-H and AI-H distances measured with Si-29-{H-1} and Al-27-{H-1} rotational echo double resonance (REDOR) reflect the interactions between TPA cations and the surrounding aluminosilica. The Si-29-{H-1} REDOR curves for Q(4)-type silicon atoms at the three mentioned stages are closely similar, as well as the observed Al-27-H-1 REDOR curve for the precursor species compared to that for the TPA-ZSM-5. This indicates that in addition to externally attached TPA, there is also internal TPA already incorporated at ail early stage into the aluminosilicate in a similar way as in the final zeolite, in accordance with the earlier proposed MFI self-assembly pathway (Kirschhock et al. Angew. Chem. Int. Ed. 2001, 40, 2637). However, the effective distances extracted from the initial REDOR curvatures are significantly (10-15%) larger than those computed for the model. Since there is no temperature effect, we tentatively assign this difference to a reduction of the Si-29- H-1 and Al-27-H-1 interactions by multispin decoherence effects or self-decoupling caused by proton spin diffusion. By assuming the computed model distances and fitting Anderson-Weiss curves to the observed REDOR data, we obtain similar decoherence times in the order of 0. 1 ms. The observed Si-29-{H-1} REDOR dephasing for the Q(3) sites in the precursors is significantly faster than that for the Q(4) sites. This is tentatively ascribed to a partial deuteron -proton back exchange at the silanol positions.