43942-AC10
Large Anisotropic Zeolite Crystals with Controllable Morphology via Microemulsion Mediated Growth
The work on this PRF grant formally began in September of 2006 with the hiring/start of Dr. Edgar Jordan to perform the work outlined in the proposal. Dr. Edgar Jordan, who obtained his PhD from the University of Muenster in Germany, is an expert in zeolite synthesis and NMR spectroscopy, particularly in the synthesis of low-silica zeolites. The main scientific accomplishments of this work include:
1. The synthesis of extremely large (> 100 μm) anisotropic crystals of Silicalite-1 formed using the bulk material dissolution method (BMD) in both anionic and cationic emulsions.
2. Investigations of whether or not this approach can be extended to the formation of and growth of other siliceous zeolites, most notably ZSM-12 and zeolite Beta.
3. Determination of the phase behavior of these of these mixtures under synthesis conditions (autogeneous pressure and high temperature).
4. Insights, albeit qualitative, about the nucleation mechanism of the silicalite-1 materials formed.
Concerning the first point, we have found that the crystal morphology is strongly sensitive to the surfactant used, with anionic surfactants favoring the formation of more needle-like crystals, whereas cationic surfactants favor the formation of the more conventional “coffin-shaped” crystals, albeit with much higher aspect ratios (longer and thinner crystals) and the lack of macroscopic twinning which is often observed in conventional silicalite-1 syntheses. We have also found that increasing the fluoride content, the mineralizing agent, tends to lead to a decrease in the aspect ratio of the crystals. In most cases crystals that are 100 – 200 μm in length are observed with aspect ratios between 10 – 20. The crystal morphology appears surprisingly insensitive to the relative amounts of oil, zeolite mixture and surfactant. Finally, most of the crystals formed were aggregates, instead of well-dispersed single crystals. We believe this indicates something about the zeolite growth mechanism and will be discussed below. Consistent with this, the use of TEOS as the silica source in lieu of quartz yield smaller, but well dispersed crystals.
Concerning the second point we were not able to extend this approach to other siliceous zeolites. While disappointing, this is consistent with the well-established precedent in the open literature that the synthesis of silicalite-1 is exceptionally robust. We did have success, however, forming large crystals of sodalite and cancrinite, materials with high aluminum content.
Phase behavior studies were also performed in the temperature range of 100 – 170 degrees Celsius. These mixtures form two-phase emulsions with the bottom (water rich) phase being the larger of the two. Under some conditions, most notably low surfactant content, three phase mixtures were observed. No silica was added to these as the glass ampules used are excellent proxies for the silica source used in the BMD syntheses (quartz). Perhaps the most interesting outcome of these experiments was the clear observation that in the case of the cationic emulsions zeolite formation occurs entirely at the surface of the ampule with many smaller crystals growing off a very large crystal, indicating nucleation is completely heterogeneous. By contrast, in anionic emulsions no crystals are observed attached to the surface of the glass ampule, indicating that nucleation does not primarily occur at the ampule surface. One way to rationalize these results is electrostatic interactions between the surfactant and the surface, which are attractive for CTAB and repulsive for SDS. The third point fed into the fourth, namely that the phase behavior studies clearly demonstrated that for low-solubility silica sources that nucleation is heterogeneous. In all syntheses employing glass as the silica source, it was observed that small crystals were observed that appeared attached to the surface of a very large crystal, or the ampule wall.
Ongoing/future work in my lab will attempt to optimize these syntheses to generate large crystals that are not fused aggregates, i.e. make well-defined/dispersed crystals. It is envisioned this will be achieved by using the secondary growth schemes developed by the Tsapatsis lab, where the different stages will employ silica sources of differing solubility.
