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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 to date include:
1. The synthesis of extremely large anisotropic crystals of Silicalite-1 formed using the bulk material dissolution method (BMD) in both anionic and cationic emulsions
2.A qualitative determination of the phase behavior of the silicalite-1 synthesis mixtures under actual reaction conditions, which also results in clear insights as to the role of heterogeneous versus homogeneous nucleation
3.The formation of highly anisotropic plate-shape crystals of low silica zeolites including cancrinite and sodalite
Relevant to point one, 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. The crystal morphology appears surprisingly insensitive to the relative amounts of oil, zeolite mixture and surfactant.
Phase behavior studies of tetrapropylammonium hydroxide/ammonium fluoride/water/surfactant/butanol/heptane mixtures were also performed in the temperature range of 100 – 170 degrees Celsius. In general the 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.
Point three was determined to due the expertise of Dr. Jordan. I had not originally intended to investigate low-silica zeolites but Dr. Jordan decided to do this on the side. We were very surprised to see that in the presence of cationic surfactant/heptane/butanol emulsions anisotropic particles were formed. This has been determined in just the last three months and ongoing work is evaluating in more detail how the presence of different anions, pH, and the like influence morphology.
Ongoing/Future Work
We are now in the process of determining the unit cell orientation of the silicalite-1 crystals, i.e. is a, b, or c the direction of preferred growth, as well as analyzing more thoroughly how the silica source solubility influences crystal size and morphology in these systems. We anticipate submitting two manuscripts, one on crystal morphology, one on phase behavior, on the silicalite-1 systems by the end of 2007. We also plan to investigate the synthesis of zeolite beta in emulsions. Our preliminary attempts at extending BMD syntheses to ZSM-12 and Beta in emulsions were not successful, however this issue will be revisited in the coming months.
