Structure of the silica phase extracted from silica/(TPA)OH solutions containing nanoparticles

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 Si-29 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.