Structural Investigation of Zeolites

Citation

Wagner, Paul A. (1999) Structural Investigation of Zeolites. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/t8qz-qa09. https://resolver.caltech.edu/CaltechTHESIS:03192025-175557925

Abstract

Microporous materials (including zeolites) that contain molecular-sized pores and cavities have found wide-spread use in industry as molecular sieves for chemical separations, as ion-exchangers for detergents and as heterogeneous, shape-selective catalysts. The number of unique molecular sieve structures discovered over the past few decades has burgeoned and currently is over 121.

Knowledge of the crystal structure of these microporous solids can provide important insights into their properties that can ultimately lead to the design of desirable materials. However, the structure solution of microporous materials can be challenging because they tend to form as micron or submicron sized crystals that are too small for single crystal X-ray analysis. Thus, the objective of this work is to develop and apply new techniques for solving the structures of microporous materials that tend to form micro- and nanocrystals and to utilize these structural investigations to gain a more thorough understanding of the zeolite/ organic structure directing agent (SDA) interactions that lead to the observed zeolite phase selectivity in their synthesis.

In the absence of single crystal data, structure solution and refinement have typically required the use of powder X-ray data. The difficulty in solving crystal structures from powder X-ray data is that the three dimensions of information available in a single crystal data set are collapsed into one dimension (d-spacing) in a powder X-ray data set. If the reflections in the powder X-ray data are significantly overlapping then solving the crystal structure from this data can be extremely difficult. Several techniques are applied here for solving microporous crystal structures from powder X-ray data.

The structure solution of CIT-5 (California Institute of Technology Number 5), a new high-silica molecular sieve synthesized under hydrothermal conditions in the presence of N(16) methylsparteinium and lithium cations, is obtained though an iterative process of model building and comparison of the simulated powder X-ray data with the experimental powder X-ray data. Rietveld refinement of the synchrotron powder X-ray data supports the symmetry and space group assignment for the structure as Pmn2₁(No.31) with refined unit cell parameters of a=l3.6738(8) Å, b=S.0216(3) Å and c=25.4883(7) Å (V=1750.1 ų) and confirms that CIT-5 is the first ordered zeolite to contain one-dimensional extra-large pores circumscribed by 14 tetrahedral-atoms (14 MR).

Computational techniques for solving the structures of microcrystals from powder Xray data are continuing to increase in sophistication and capability. The crystal structures of two high-silica molecular sieves, SSZ-44 and SSZ-35, are solved using Fourier recycling and represents the first application of this new computational technique for solving novel high-silica zeolite structures from powder X-ray data. Both materials contain unusual 1-dimensional pores circumscribed by 10 and 18 membered-rings, and are the first high-silica zeolites found that possess pores containing greater than 14 membered-rings.

Electron diffraction data, obtained from a transmission electron microscope (TEM), has inherent advantages over X-ray data for analyzing small crystals due to the stronger interaction between the electron beam and matter compared to X-rays. This stronger interaction allows a single crystal diffraction data set to be obtained from much smaller crystals. Provided that the interaction of the incident electron beam with the crystal is nearly kinematical direct methods can be used as a powerful tool for obtaining the phase information required to solve the crystal structure.

The development of electron diffraction methods for solving the structure of nanocrystals is described and the application of this technique to solve the structure of a large-pore, high-silica zeolite, SSZ-48, that contains an occluded organic structure directing agent is presented. The structure is confirmed by electron diffraction refinement and by high resolution transmission electron microscopy and is found to contain a one-dimensional pore system circumscribed by 12 tetrahedral atoms (12 MR). SSZ-48 is the most complex three-dimensional material to be solved at atomic resolution using electron diffraction methods and illustrates the power of electron diffraction data for resolving the structures of materials that form crystals too small for standard single crystal X-ray analysis.

These investigations into the structural details of micro- and nanocrystalline microporous materials can be utilized to gain a more thorough understanding of the zeolite/ organic structure directing agent (SDA) interactions that lead to the observed zeolite synthesis phase selectivity. Two studies are conducted to probe the relationship between the organic structure directing agent and the zeolite framework that is formed from its use.

The first study probes the interaction between the CIT-5 framework and the N(l)- methyl-α-isosparteine SDA I that is found to be a more effective structure directing agent for CIT-5 than the diastereomer N(l6)-methylsparteinium II originally used to direct this new high-silica zeolite. Molecular modeling calculations reveal that I is capable of forming a greater number of van der Waals interactions with the framework than II thereby providing a greater degree of stabilization for the CIT-5 structure as compared to II.

Finally, a study into the guest/host interactions between three new zeolite structures, SSZ-35, SSZ-36 and SSZ-39 and the 37 organic structure directing agents that are capable of directing for these zeolites is presented. The size and shape of the organic SDAs presented in this study are designed in order to obtain novel, open framework zeolites. The design effort focused on synthesizing large rigid spheroidal SDAs that will preclude the crystallization of the commonly observed clathrates and straight I-dimensional channel system zeolites that result when either small or rigid elongated molecules are employed as SDAs. Computational calculations of the organic/inorganic energy of interactions provided significant insights into the observed zeolite phase selectivity by the organic SDAs. The molecular modeling investigations presented here highlight the potential for developing a rational route to the design of desirable zeolite frameworks.

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