High-silica zeolite nucleation from clear solutions
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Understanding the mechanism of zeolite nucleation and crystallization will enable the zeolite science community to tune zeolite properties during synthesis in order to accommodate the purposes of various applications. Thus there has been considerable research effort in "deciphering" the mechanism by studying the growth course of tetrapropylammonium (TPA)-mediated silicalite-1 using several techniques, such as dynamic light scattering (DLS), small-angle X-ray/neutron scattering (SAXS/SANS), and nuclear magnetic resonance (NMR). While these studies have generated a more comprehensive picture on the silicalite-1 growth mechanism, the general application of the mechanism and how it could be applied to other zeolite systems have not been addressed. This work initially tried to apply the insights developed from the TPAsilicalite- 1 clear solution synthesis by investigating the nanoparticles formation and zeolite growth in several tetraethyl orthosilicate (TEOS)-organocation-water solutions heated at 368 K using SAXS. The results are in contrast to TEOS-TPAOH-water mixtures that rapidly form silicalite-1 at 368 K. These results imply that the developed TPA-silicalite-1 nucleation and crystallization mechanism is not universally applicable to other zeolite systems and TPA-silicalite-1 itself could be a special case. With this in mind, the next goal of this work uses in situ SAXS to revisit silicalite-1 growth kinetics prepared by using several TPA-mimic organocations and some asymmetric geometry organocations. The results clearly show the TPA cation is an extraordinarily efficient structure-directing agent (SDA) due to its moderate hydrophobicity and perfect symmetric geometry. Any perturbation of the hydrophobicity and symmetry of SDA leads to a deterioration of zeolite growth. This work further investigates the influences of alcohol identity and content on silicalite-1 growth from clear solutions at 368 K using in situ SAXS. Several tetraalkyl orthosilicates (Si(OR)4, R = Me, Pr, and Bu) are used as the alternative silica sources to TEOS in synthesizing silicalite-1. Increasing the alcohol identity hydrophobicity or lowering the alcohol content enhances silicalite-1 growth kinetics. This implies that the alcohol identity and content do affect the strength of 1) hydrophobic hydration of the SDA and 2) the water-alcohol interaction, through changing the efficiency of the interchanges between clathrated water molecules and solvated silicate species.