New Evidence for Precursor Species in the Formation of MFI Zeolite in the Tetrapropylammonium Hydroxide−Tetraethyl Orthosilicate−Water System

Abstract

Tetraethyl orthosilicate was hydrolyzed and polymerized in concentrated aqueous tetrapropylammonium (TPA) hydroxide solution at 0 and 10 °C and room temperature. The formation of silicate oligomers involved in the TPA-mediated self-assembly process of colloidal Silicalite-1 was investigated using 29Si NMR. A pentacyclic dodecamer with four edge-sharing five-rings on a four-ring was detected as a new intermediate in the formation of Silicalite-1 zeolite. The formation of this pentacyclic dodecamer with a curved inner hydrophobic silicon dioxide surface is a new example of the structure-directing action of TPA. The temperature was found to be a key parameter with respect to the transformation of the intermediates with 11 and 12 Si atoms. At 0 and 10 °C, the tetracyclic undecamer having three five-rings on a four-ring was converted into the pentacyclic dodecamer by insertion of one additional silicate unit. At these low temperatures, it slowly underwent an intramolecular condensation and formation of capped double five-ring not leading to Silicalite-1 formation. At room temperature, three pentacyclic dodecamers rapidly condensed into the Silicalite-1 precursor. The involvement of the pentacyclic dodecamer in the self-assembly process offers an explanation for the absence of systematically missing T-sites in colloidal Silicalite-1.