PREPARATION OF CRYSTALLIZED TWISTED AND TWINNED MORPHOLOGIES FROM SUPERCRITICAL FLUIDS FOR CATALYSIS AND SEPARATION

In many areas, such as sensor development, separation processes, heterogeneous catalysis, etc., materials with a highly selective specific surface area and high porosity are required, which exhibit high permeability and thus process efficiency. Besides, they should have appropriate structure with accessible pores adapted to the target molecules. The ideal structure of such a material is therefore precisely defined already at the nanolevel dimension and should be tailored to the target application. Natural such structures are contact crystal twins, two or more crystals which are in contact according to a specific rule. Since the rules of twinning are defined by the crystal lattice of the mineral itself, such structures can occur in nanometer, micrometer as well as macrometer dimensions. In the case of multigeneration twins, self-similar structures are formed, exhibiting such pattern from atomic to macroscopic scale. While their synthesis still challenging, they are always formed under conditions of low saturation, varying key parameters that affect the thermodynamic equilibria and consequently crystallization progress. In addition to multigeneration twinning, consisting of crystals with morphology similar to individual crystals, a high specific surface can be achieved with a particular - twisted morphology. Such crystals have also been known in nature for a long time, and they are characterized by a defined angle of twisting, changing completely their morphology. Despite that, the mechanism of their formation has not yet been fully elucidated. At the nanometer level, twisted structures growing from the gas phase that follow Eshelby's twist have only recently been synthesized, while the synthesis of such macroscopic crystals is still unexplored. Naturally twisted crystals differ in twisting degree. Given that the selectivity and catalytic activity of individual crystallographic planes depends on the plane index, which determines the density of atomic steps, it can be concluded that in the case of a defined crystal twist, the density of the atomic steps would be affected too. By that it would determine surface energy of the twisted crystal plane, and consequently its catalytic activity and selectivity. In the course of the project we will try to prepare such morphologies by crystallisation in supercritical fluids (SCF) like water and CO2. The choice of supercritical conditions is based on the specific properties of such fluids, like high density (comparable to liquids) and low viscosity (comparable to gases). This is also why the faster kinetics of crystallisation does not lead to diffusion limitations and, consequently, diffusion limited crystal growth. Another, also very important SCF feature, is the significant change in density with even small changes in pressure, affecting degree of supersaturation almost instantaneously throughout the supercritical fluid. As crystallisation of twin and twisted structures requires a support from which the growth proceeds, crystallisation in supercritical fluids will be carried out on flat surfaces of varying degrees of roughness and crystallinity, as well as on highly porous polyHIPE supports of different materials and surface properties. During the crystallisation process in SCF, the pressure changes will be introduced, optionally with addition of different additives, to create conditions for the formation of twinned and twisted crystal structures. The resulting structures will be characterised for morphology and crystal structure as well as for their hydrodynamic, catalytic and separation properties. The proposed project has several objectives, which can be summarised as follows: ·       Understanding the mechanism of twisted macroscopic crystals formation ·       Understanding the crystallisation of multigeneration crystal twins from supercritical fluids ·       Preparation of twisted and multigeneration twinned crystals and determination of their catalytic activity and selectivity