Zeolite frameworks with beta-cages

This study focuses on five zeolites: sodalite, zeolite A, linde type N, zeolite Y, and EMC-2, with SOD, LTA, LTN, FAU, and EMT framework topologies respectively. All of these zeolites have β-cages as the framework building units. The aims are to understand the conditions which control the formation of different zeolite phases and some of the physical properties of zeolite frameworks, mainly the framework exibility, and the capacity of β-cage to accommodate guest molecules such as water and methanol.
The hydrothermal and microwave syntheses of zeolite with no organic structure directing agents require precise control of synthesis conditions and form the fundamental part of this study. Reaction temperature and synthesis time control the phase purity in zeolite A synthesis. A transformation from zeolite A to sodalite is observed and is related to framework density and reaction temperatures. Microwave syntheses of sodalite show that the batch compositions and methods of preparation also affect the as-synthesised zeolite phases and a new sodalite morphology is found. By using microwaves instead of traditional hydrothermal methods, one of the most complex zeolites, linde type N zeolite, was synthesised. This is the first study to report this new method.
The newly defined extrinsic exibility window is studied on more depth in this work. While the intrinsic exibility is defined by the ability of an empty framework to flex with no distortions in the primary tetrahedral building units; the extrinsic exibility window is limited by the host-guest steric interactions between the framework and the extra framework contents. In zeolite Y, the extrinsic exibility window can be limited not only under compression, but also in expansion, as the β-cage in a maximally expanded framework lack the exibility to adapt to bulky contents such as a combination of methanol and water molecules. It is also found that the β-cage in zeolite Y can only accommodate a maximum number of two water molecules and one methanol molecule.
The same phenomenon is observed in sodalite under compression. In the sodium form, the framework remains within its intrinsic exibility window when fluorinert is used as pressure transmitting media, as fluorinert does not enter the zeolite pores. However, an extrinsic exibility window is observed in both the sodium and sodium bromide forms with methanol/ethanol/water mixture as pressure transmitting media.
An exception is seen in EMC-2. The intrinsic and extrinsic exibility windows are identical to each other. The presence of 18-crown-6 ether molecules in the pores does not affect the exibility window. The crown ether, despite its steric bulk, does not limit the geometric exibility of the framework since the cage of the EMT framework has enough space to accommodate the crown ether molecule and can adapt to the contraction. This shows that the extrinsic exibility window of zeolite frameworks is controlled not only by the extra framework contents, but also by the framework building units.

Keywords: beta-cages
Zeolite synthesis
Flexibility window
Sodalite cages
Zeolite frameworks
Zeolites
Advisor: Edwards, P
Sartbaeva, A