Zeolite beta is an excellent molecular sieve material that possesses the ability to selectively sort molecules and ions based primarily on size exclusion processes. This property is attributed to its well-defined and regular pore structure of molecular dimensions. Zeolite beta has a high surface area and a highly porous structure, making it an important raw material for many industries such as the petrochemicals, chemical engineering, catalysis, adsorption, and separation. The zeolite beta structure is based on a centrally symmetric tertiary building layer that makes 12-membered ring (MR) channels. These channels intersect in all three directions in the zeolite beta crystal structure. Due to the chiral nature of the channel system, chiral zeolite beta has significant potential for enantioselective reactions and chiral separation processes. However, despite the enormous interest in the chiral zeolite beta channel system, attempts to synthesize pure chiral zeolite beta with single enantiomer have been limited to date.
The structural differences between polymorphs of zeolite beta are mainly due to the translation modes of the tertiary building layers. Two MR channels can be made by translations in the a or b direction, resulting in the achiral polymorphs B and C and the chiral polymorph A. In addition, the tertiary building layer of the chiral polymorph A can make double four-membered ring (D4R) cages, which are not found in either of the achiral polymorphs. These different features allow the chiral zeolite beta to possess significant chiral applications, but only if it can be synthesized in its pure chiral form.
In order to overcome the difficulties in synthesising chiral zeolite beta, it is necessary to study its local structure and determine how the distribution of polymorphs A and B in the crystalline samples is controlled. X-ray diffraction patterns provide information about the long-range ordering of a crystal, but HRTEM can reveal the internal structure at higher resolution. It is thought that the preferred stacking sequence of the centrosymmetric tertiary building layer can be controlled by the water content in the initial reaction mixture. For example, it has been shown that TEAOH promotes formation of normal zeolite beta, whereas DMDPOH, DMCHOH, and EDMCHOH can induce the formation of polymorph A.
The simulated X-ray diffraction patterns of the six hypothetical polymorphs of zeolite beta can be obtained using the DIFFaX program. By comparing the shapes and degrees (2th) of the low-angle peaks in each of these simulated patterns, the proportions of polymorphs A and B can be accurately determined. This method can be employed to characterize the chiral zeolite beta in a more rapid and accurate way than analyzing the polymorphic composition of zeolite beta by conventional techniques. This method will also be useful for the design of new synthesis routes for chiral zeolite beta.
