Subcolloidal particles of a few nanometers in diameter are observed during the clear-solution synthesis of silicalite-1. These nanoparticles (3-5 nm) can be synthesized at room temperature starting from tetrapropylammonium (TPA) hydroxide, tetraethyl orthosilicate (TEOS), and water, and they have been reported to have a uniform structure identical to that of zeolite ZSM-5 (called nanoblocks or nanoslabs). To study their structure, we followed the extraction procedure proposed in the literature to obtain a dry powder of the particles. These dried particles were analyzed with powder X-ray diffraction (XRD), solid-state NMR spectroscopy, FTIR spectroscopy, thermogravimetric analysis, and N 2 adsorption isotherms. The results are compared with those obtained for colloidal size silicalite-1, amorphous silica, and the mesoporous silicate SBA-15. To obtain a better idea of the shape and structure of the particles, we conducted simulated annealing modeling to fit the particle shape to the fractions of Q n obtained from the 29 Si MAS NMR spectra. The model structures are in excellent agreement with our NMR data and suggest a poorly defined particle shape, in contrast to previous reports. The XRD patterns of samples with particle sizes in the range of the nanoparticles were simulated using the Debye formula and the SKIP algorithm. These simulations were carried out using structural models of silicalite-1 nanocrystals, the proposed nanoblock structure, and the silica particles derived from simulated annealing. We found no evidence in support of a well-defined MFI-like structure in the extracted material. The particles contain TPA, partly associated with the particles and partly as (TPA)Cl formed by the extraction process. The evidence accumulated here is in disagreement with the well-defined structure of the nanoparticles previously reported.

The valence and local symmetry of iron in framework-substituted FeZSM-5 with a high Fe dilution (Si/Fe ) 360) was studied by means of K -detected X-ray absorption spectroscopy. This technique combines highresolution (∆E ∼1 eV) fluorescence detection of the 3p to 1s (K ) transition with the X-ray absorption near-edge structure (XANES) at the Fe K-edge. An absorption-like spectrum is recorded by detecting the K fluorescence intensity as a function of the incident energy that is scanned through the K absorption edge. K -detected XANES spectra allow for a more precise separation of the weak K pre-edge structure from the main edge as compared to conventional absorption spectroscopy. Subsequent analysis and interpretation of the pre-edge spectral features therefore is more accurate. The pre-edge is sensitive to changes in the local coordination and oxidation state of Fe. Using this technique we were able to quantitatively determine the degree of iron extraction out of a zeolite framework upon steaming. With the use of appropriate reference compounds, the pre-edge analysis was used to monitor the activation of low-loaded, framework-substituted FeZSM-5 (0.3 wt % Fe). Template removal and calcination distort the zeolite framework and induce a deviation from T d symmetry for incorporated iron. The (deliberate) presence of water at high temperature (T > 500°C) facilitates the hydrolysis of the Si-O-Fe bonds and increases the formation of extraframework iron species. The amount of Fe III occupying tetrahedral sites in the MFI-type zeolite decreases to 32% and 19%, respectively, for mild-and hard-steamed samples.