Si liquid NMR and in situ infrared spectroscopy was used to investigate the polycondensation process of tetraethyl orthosilicate (TEOS) in a concentrated aqueous solution of tetrapropylammonium hydroxide (TPAOH) at low temperatures. The composition was characterized by a molar hydrolysis ratio (H 2 O/TEOS) of 6 and a molar TPAOH/TEOS ratio of 0.37. The 29 Si NMR spectra and the infrared spectra of the samples recorded at different reaction times and temperatures were assigned to a limited number of specific silicate polyanions containing three and five rings. The structure directing action of tetrapropylammonium cations was evidenced by the formation of silicate polyanions with a curved hydrophobic SiO 2 surface, such as the bicyclic pentamer, pentacyclic octamer, and the tetracyclic undecamer. At room temperature, the polycondensation process leads to the selective formation of a species containing 33 Si atoms. It occluded a tetrapropylammonium molecule and had the same framework connectivity as in bulk MFI zeolite. This TEOS polycondensation process may be relevant for the first steps of the crystallization of MFI type zeolites.
The authors of the comment criticize the content of one particular paper 1 out of the series, 1-8 in which we traced the molecular steps involved in the formation of MFI-and MELtype zeosils starting from mixtures of TEOS, TPAOH, or TBAOH and water. The criticism is concerned with the following aspects: (i) zeolite crystallization mechanism via selfassembly; (ii) interpretation of FTIR spectra; (iii) experimental conditions and interpretation of 29 Si NMR spectra of silicate solutions. Our reply to the comments on these issues is as follows. Evidence for Aggregation of Silicate Oligomers That Are Structurally Related to the MFI Zeolite Framework.We provided direct experimental evidence for an aggregation mechanism, in which the smallest entities selectively condense with each other and systematically form larger and larger units until finally particles displaying Silicalite-1 or -2 type X-ray diffraction are obtained. Such an aggregation mechanism is wellknown in sol-gel science. 9 It is referred to as cluster-cluster aggregation and is the alternative for the monomer-cluster mechanism according to which a crystal grows through addition of an always present small molecular species. In silicate systems, both mechanisms may occur depending on conditions. 9 In our conditions for Silicalite-1 and Silicalite-2 synthesis, the evidence for the occurrence of an aggregation mechanism was obtained from transmission electron microscopy (TEM), X-ray scattering (XRS), small-angle X-ray scattering (SAXS), and gel permeation chromatography (GPC). These different approaches independently evidenced discrete steps of the following aggregation sequence:All silicon present in the system is engaged in this aggregation process. The nanoslabs are very specific MFI framework fragments. The nature of the precursor unit from which the nanoslabs are assembled is obvious from the nanoslab structure. The structure of the precursor unit was determined without relying on 29 Si NMR.The numbers of Si atoms in the species (eq 1) were derived from the relative volumes or weights of the entities in the sequence using the GPC, XRS, SAXS, TEM, AFM, and 29 Si MAS NMR approaches. Thus, the number of Si atoms in the different entities were determined with techniques other than 29 Si liquid NMR. 29 Si liquid NMR was only used to determine the nature of species for which the numbers of Si atoms they were composed of were known.2. Shifting of the FTIR Frequency of Five Ring Vibrations with Silicalite-1 Particle Size. Micrometer-sized Silicalite-1 and ZSM-5 samples display a framework vibration at 550 cm -1 . 10,11 So-called colloidal Silicalite-1, with a particle size of 18-100 nm shows a doublet with maxima at 555 and 570 cm -1 . 2 The obvious difference between these samples is the particle size. When the particle size is reduced further to the Silicalite-1 nanoslabs, measuring 4 × 4 × 1.3 nm, and the precursor, the IR absorption maximum occurs at 570 and 590 cm -1 , respectively. 1,3 Whereas the shifting of this topologydependent vibration is clearly rel...
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