(445b) Investigations of Zeolite Nucleation and Growth Using NMR Spectroscopy

The nucleation and growth of high-silica zeolites has been extensively studied.  However, the role of the organocation in the early stages of this process is not understood.  In this talk we will present our work to date investigating optically transparent organocation ? silica nanoparticle mixtures using pulsed field gradient NMR methods (PFG-NMR) to determine the organocation self-diffusion coefficient.  From these values it is possible to determine the fraction of cations bound to the nanoparticle surface, and from this determine thermodynamic properties for the system.  Conductivity, pH and scattering measurements are also performed on these mixtures to attempt to develop a complete description of the cation ? silica nanoparticle mixtures. 

We will report results on dilute mixtures of organocation-TEOS-water using different organocations whose zeolite phase formation time is different.  In one part of the talk we will summarize our results on tripropylalkylammonium cations that we have investigated previously using X-ray scattering.¹ The goal will be to relate the cation ? silica nanoparticle interaction strength to the growth kinetics previously measured and to understand them in the context of cation charge density and hydrophobicity.

Among other results, we have obtained the diffusion coefficients of the methyltripropylammonium cation (MTPA⁺) as a function of its molar concentration. In Figure 1, we show the MTPA⁺ diffusion coefficients are almost constant when the mixtures only include MTPAOH and water. However, the diffusion coefficients decrease proportionally to the concentration of the organocation when we added TEOS to the mixtures.

Figure 1. Diffusion coefficients of MTPA⁺ in mixtures of x MTPAOH: y TEOS: 8100 H₂O where x is the variable molar concentration of MTPAOH and y is the moles of TEOS.

Ongoing work in our lab is studying mixtures containing N,N,N-trimethyl-1-adamantylammonium hydroxide and 4,4'-trimethylenebis(dimethylpiperidinium) dihydroxide.  The former is a highly non-selective cation that leads to up to eight different zeolite phases depending on the synthesis conditions.  The latter is selective to forming ZSM-12; however, in our previous work it was shown that ZSM-12 cannot be made from optically transparent mixtures with this cation.² The talk will conclude with a discussion of these results and their implications for developing a more general approach to making high-silica zeolites from optically transparent mixtures containing silica nanoparticles.

References

1.      Cheng, C.-H., Shantz, D. F. ?Silicalite-1 Growth from Clear Solution: Effect of the Structure-Directing Agent on Growth Kinetics? J. Phys. Chem. B, 2005, 109(29), 13912-13920.

2.      Cheng, C.-H., Shantz, D. F. ?Nanoparticle Formation and Zeolite Growth in TEOS/Organocation/Water Solutions? J. Phys. Chem. B, 2005, 109(15), 7266 - 7274.