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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/3273 |
| Date | 12 April 2006 |
| Creators | Cheng, Chil-Hung |
| Contributors | Shantz, Daniel F. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | 6971484 bytes, electronic, application/pdf, born digital |
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