Nucleation and crystallization of silicalite-1, the all-silica zeolite with MFI framework, is typically studied using the tetrapropylammonium (TPA)-silica-water system. Here, TPA-silica sols were studied by a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and cryogenic TEM (cryo-TEM). silicalite-1 crystals, as small as 20 nm, were successfully imaged by employing high-resolution cryo-TEM. The results are consistent with the hypothesis that non-crystalline aggregates form prior to silicalite-1 crystallization, and they indicate that at least some of the aggregates contain crystalline fragments, which is suggestive of zeolite nucleation within the aggregates. It was also determined that the typical TPA-silicalite-1 crystal morphology develops when nanoparticle concentration is substantially reduced suggesting that solution mediated processes rather than nanoparticle attachment are responsible for the development of the typical faceted crystals. It was also confirmed that silicalite-1 crystal morphology as well as nucleation and crystallization kinetics are strongly dependent on the structure directing agents present. In experiments using a dimer of TPA (Bis-1,6-(tripropyalammonium)hexamethylene dihydroxide), the time at which the first evidence of crystallinity was observed and the crystal growth rate were substantially delayed. However, the sequence of events observed follow that of TPA-silicalite-1.
Bibliographical noteFunding Information:
Financial support was provided by NSF (NIRTCMMI-0707610, CBET-0522518, and MRSEC: DMR-0212302). Characterization was carried out at the Characterization Facility, UMN, which receives support from the NSF through the NNIN. The sample for microscopy characterization shown in Fig. S2 was provided by Xueyi Zhang and Pyoong-Soo Lee.