Silicalite-1 growth from clear solution: Effect of the structure-directing agent on growth kinetics

Small-angle X-ray scattering (SAXS) has been used to quantify how perturbations of the tetrapropylammonium (TPA) cation structure affect the growth of silicalite-1 from clear solutions at 368 K. Alkyltripropylammonium (RN(C3H7)3 +OH-, R = Me, Et, Bu, and Pe), dialkyldipropylammonium (R2N(C3H7)2 +OH-, R = Et and Bu), and bis-1,6-(tripropylammonium)hexamethylene dihydroxide (TPA-dimer) cations are used as structure-directing agents (SDAs) to synthesize silicalite-1 from clear solution mixtures comparable to those that have been previously investigated for the TPAOH mediated synthesis (i.e., 1 TEOS:0.36 TPAOH:20 H2O, 368 K). All mixtures studied except those employing dialkyldipropylammonium cations lead to the formation of silicalite-1. The in-situ SAXS investigations show that TPA cations lead to the shortest reaction time as indicated by the observance of Bragg diffraction peaks (15 approximately 16.5 h) and the largest particle growth rate (1.9 +/- 0.1 nm/h). Substituting a propyl group of the TPA moiety with a different alkyl group significantly affects silicalite-1 nucleation and growth with the trend Bu > Et > Pe > Me. Synthesis mixtures containing the TPA-dimer also show a slower growth rate. All the solutions show a bimodal particle distribution throughout zeolite growth with the primary particle size being approximately 5 nm in all cases, independent of the SDA identity. Syntheses using diethyldipropylammonium hydroxide, dibutyldipropylammonium hydroxide, and 4,4'-trimethylenebis(1-methyl-1-hexyl-piperidinium) dihydroxide as the SDA do not result in silicalite-1 formation, showing that the nucleation of silicalite-1 from clear solution at 368 K is sensitive to the SDA geometry.