Do zeolite precursor species really exist in aqueous synthesis media?

The authors of a recent report in this journal (Houssin, et al., Phys. Chem. Chem. Phys., 2003, 5, 3518, ) claim that a tetrapropylammonium (TPA) silicate mixture with molar composition 4.41 TPAOH : 10 SiO(2) : 117 H(2)O contains high concentrations of two silicate oligomers, the prismatic double five-ring and a novel pentacyclic dodecamer. The latter species is derivative of the framework structure of silicalite-1, a MFI-type zeolite that spontaneously crystallizes from this system, and, indeed, the authors declare it to be a "nanoprecursor" in the TPA-mediated growth of silicalite-1. Using two-dimensional (29)Si COSY NMR spectroscopy to examine an equivalent mixture enriched in the (29)Si isotope, we show that the published peak assignments are erroneous. The signal assigned to the double five-ring corresponds, in reality, to the well-characterized pentacyclic octamer, while the three signals assigned to the pentacyclic dodecamer arise from three completely separate species. We find no evidence, therefore, to support the suggestion that silicalite-1 is formed by sequential condensation of precursor species any more complex than the simple orthosilicate anion.