Electron tomography combined with electron diffraction reveals the dissolution and phase transformation of
KFI
to
CHA
zeolites
Authors: Willhammar Tom , van Tendeloo Leen , Breynaert Eric , Martens Johan , Kirschhock Christine , Bals Sara 
Published: December 2016

Introduction

Microporous alumminosilicates, zeolites, have been implemented in a large number of important industrial applications.[1] Their unique properties are to a large extent determined by their unique micropore structures with pores in the size range of small molecules. Though providing unique properties, the micropores also introduce obstacle when it comes to efficient mass transfer in the material. Introducing porosity on larger length scales will enhance the diffusion properties as well as increasing the accessibility to the chemically active micropores. In addition transformations between different zeolite phases with characteristic pore structures may be observed. Through electron tomography important knowledge about the internal pore structure can be obtained. By linking the results to information about the present crystalline phases, relationship between morphology, crystal structure and the relative orientation of intergrowing crystals from electron diffraction and high resolution TEM (HRTEM) imaging a comprehensive understanding of the material and its crystallization can be obtained.

Results and discussion

Micrometer sized single crystals of a cubic KFI zeolite were treated in in a controlled dissolution process with 1.2M Potassium hydroxide. This treatment partially dissolves the KFI crystals and creates a very peculiar sponge like morphology, see Fig 1a. The overall cubic morphology of the crystals was however preserved. Selected area electron diffraction (SAED) could confirm that the crystal despite its spongy texture remained a single crystal of KFI, Figure 1b.

In order to study the internal structure of the KFI crystals electron tomography was performed. A tilt series of 69 ADF‐STEM projection images collected in the range of ‐76 ‐ +60 degrees with a 2° tilt increment was used for the reconstruction. The reconstruction was performed using a SIRT reconstruction algorithm implemented in the ASTRA toolbox [2]. The electron tomography reconstruction clearly shows that the porosity penetrates through the entire spongy cubic crystal of KFI zeolite, see Fig 2 b‐c. Both the tomography reconstruction as well as the ADF‐STEM and HRTEM images, see Fig 1a and 2a, shows that some of the pores have a preference to run from the edges towards the center. From the HRTEM images in Fig xx it can be observed that the preferred termination surfaces are and surfaces.

Bright field TEM images shows that after the controlled dissolution process additional smaller well facetted crystals form on top of the micrometer sized spongy KFI cubes, see Fig XX. Through the use of SAED the smaller crystals could be identified to be a different trigonal zeolite, chabazite. The Potassium ions present during the dissolution direct the growth of chabazite based on the KFI structure. Interestingly the KFI and chabazite structures are intimately related as they are entirely built from the same building unit of 12 SiO4 tetrahedrons, called a double 6‐ring. The only difference between the two structures is the packing of this building unit. The SAED patterns shows that the cubic KFI zeolite grows along its directions and that the chabazite crystals tend to grow epitaxially with its [101] direction aligned along the [100] direction of the KFI crystals structure.