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 magic-angle-spinning 13C, 27Al, 29Si, and 1H 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 × 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 Al-H distances measured with 29Si-{1H} and 27Al-{1H} rotational echo double resonance (REDOR) reflect the interactions between TPA cations and the surrounding aluminosilica. The 29Si-{1H} REDOR curves for Q4-type silicon atoms at the three mentioned stages are closely similar, as well as the observed 27Al-1H 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 an 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 29Si-1H and 27Al-1H 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 29Si-{1H} REDOR dephasing for the Q3 sites in the precursors is significantly faster than that for the Q4 sites. This is tentatively ascribed to a partial deuteron-proton back exchange at the silanol positions

Combined in situ 29Si NMR and small-angle X-ray scattering study of precursors in MFI zeolite formation from silicic acid in TPAOH solutions

Silicic acid powder was dissolved and polymd. in a concd. aq. tetrapropylammonium (TPA) hydroxide soln. at room temp. Two complementary techniques were employed to follow this process leading to silicalite-1 zeolite upon heating. The formation of small silicates and specific oligomers involved in the assembly of silicalite-1 nanoprecursors was investigated using 29Si NMR. Small-angle X-ray scattering (SAXS) was used to follow processes at a colloidal level. Dissoln. and polymn. of silicic acid could then be related to events occurring at both mol. and colloidal scales. The appearance of very well-defined colloidal particles was linked to a specific intermediate already obsd. in systems using an org. and monomeric silica source. In situ time-resolved ultra-small-angle X-ray scattering (USAXS) using synchrotron radiation showed a linear growth of the av. crystal diam., which was slower than of that encountered in Na+ contg. synthesis mixts. Using the results presented here, we propose a mechanism describing the TPA-mediated self-assembly of silicalite-1 from silicic acid powder as silica source. This model is in agreement with rising evidence of a common mechanism involving nanoblock aggregation for org. mediated crystn. of high-silica zeolites

Hypodermin A, a new inhibitor of human complement for the prevention of xenogeneic hyperacute rejection

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A newly established porcine aortic endothelial cell line : characterization and application to the study of human-to-swine graft rejection

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