Abstract
The dynamic behavior of tetrapropylammonium (TPA) cations in the clear precursor sols for silicalite synthesis has been investigated by 1H diffusion ordered spectroscopy (DOSY), T1, T2, and T1ρ 1H relaxation, as well as 1H→13C cross polarization (CP) nuclear magnetic resonance. The DOSY NMR experiments showed the presence of strong solute–solvent interactions in concentrated sols, which are decreasing upon dilution. Similarities in dependence of diffusion coefficients with fractional power of the viscosity constant observed for nanoparticles, TPA cations and water led to the conclusion that they aggregate as anisotropic silicate-TPA particles. Relaxation studies as well as 1H→13C CP experiments provide information on dynamic properties of ethanol, water and TPA cations, which are function of silicate aggregates. The general tendency showed that the presence of silicate as oligomers and particles decreases the relaxation times, in particular T2 and T1ρH, as a consequence of involvement of these latter in ion-pairing interactions with water-solvated TPA molecules slowing down their mobility. Furthermore, from the 1H→13C CP dynamics curve profiles a change in the CP transfer regime was observed from fast (TCH << T1ρH) for solutions without silicates to moderate (TCH~T1ρH) when silicates are interacting with the TPA cations that may reflect the occlusion of TPA into flexible silicate hydrate aggregates.
Highlights
Silicalite clear precursor sols are generally prepared from tetraethylorthosilicate (TEOS) hydrolysis and consist of a colloidal suspension of nanoparticles in water-ethanol mixture with many coexisting silicate oligomers [1,2,3,4,5,6,7,8]
In the current work we report 1 H diffusion ordered spectroscopy (DOSY) NMR studies of water, ethanol and tetrapropylammonium cations present in typical TEOS–TPAOH–H2 O system used for silicalite-1 synthesis (Figure 1)
Indicated the strong solute–solvent interactions in concentrated sols which are decreasing with the increase of the water content to reach a plateau in the values for the diffusion coefficients from the system 25 TEOS:9 TPAOH:6000 H2 O
Summary
Silicalite clear precursor sols are generally prepared from tetraethylorthosilicate (TEOS) hydrolysis and consist of a colloidal suspension of nanoparticles in water-ethanol mixture with many coexisting silicate oligomers [1,2,3,4,5,6,7,8]. Few 1 H PFG NMR studies have been conducted in typical precursor sols providing useful insights on dynamic behavior of organocations with respect to silicated nanoparticles [4,27,28,40,41,42]. Intramolecular cross-polarization experiments between the 1 H and 13 C nuclei of the template molecule demonstrated that much of the increased efficiency was a result of reduced rotational mobility of the TPA molecule [50,52] All these studies have been conducted on gel phase precursors and its application on dynamic and inhomogeneous colloidal suspension system would not be necessarily straightforward and must be undertaken to conclude to similar or different behavior. Temperature changes induce significant modulation of NMR relaxation, and allow following occlusion of tetrapropylammonium cations into nanoparticles These results are discussed in the general context of zeolite nucleation and crystal growth. Martens et al.and [1,2,4,10,14,17,20,25,26,62]
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