By definition, zeolites are a large group of minerals consisting of hydrated aluminosilicates of sodium, potassium, calcium, and barium. These materials are of major scientific and technological importance as they have a huge commercial value and are used in a wide range of fields owing to their remarkable attributes; for instance, they can be readily dehydrated and rehydrated, and can be used as catalysts, cation exchangers or molecular sieves. Additionally, zeolites are microporous materials with high surface area and high degree of crystallinity. Therefore, comprehending their chemical structure and crystalline nature has been the forefront of many researches. Consequently, a myriad of characterization approaches has been used to study their hierarchical and crystalline structure. In particular, polarized Raman spectroscopy has been used to study zeolites, albeit mainly for the detection and orientation of molecular species hosted inside the pores but less frequently with the specific aim to assign intrinsic vibrational modes using single crystals.
Therefore, it is vital that the same approach be used to explore other properties of zeolites. To this end, researchers from the Chalmers University of Technology in Sweden: Professor Anna Martinelli, Simone Creci, Szilvia Vavra, Professor Per-Anders Carlsson and Professor Magnus Skoglundh, proposed to use polarized Raman spectroscopy to characterize the local structure in single crystals of zeotypes, namely silicalite-1 and ZSM-5, which share the MFI framework structure. Their work is currently published in the research journal Physical Chemistry Chemical Physics.
In particular, the research team demonstrated that polarized Raman spectroscopy is sensitive to the anisotropies of single crystals of zeolites and that, as a consequence, distinction between vibrational modes can be made based on the orientation of certain chemical bonds. In addition, the study of Al-free, i.e. purely siliceous crystals, was intentionally included to evidence the correlation between framework structure and vibrational modes, and show that the majority of frequencies and intensities could be maintained upon substitution of Si with Al atoms.
The interpretation of the collected Raman spectra was facilitated by the fact that, despite having a complex multiple-ring structure, the MFI framework has crystal axes orthogonal to each other and hence parallel to the laboratory coordinate system. As such, polarized and angular dependent Raman intensities were presented and used to make an assignment of the observed vibrational modes more precise than that previously available.
In summary, the study by Professor Anna Martinelli and her colleagues proved that the intrinsic anisotropy in zeolites of the MFI framework structure can be studied by polarized Raman spectroscopy, provided the study is done on isolated single crystals. The proposed assignment was supported by the good agreement between experimental and simulated polar plots, where Raman intensities were plotted as a function of the polarization angle of the incident light. In a statement to Advances in Engineering, Professor Anna Martinelli, the lead author mentioned that their work was not only intended to be a source of inspiration for further theoretical and experimental studies, but also to promote a new useful method to identify the site of metal atoms substituting for Si, a crucial aspect in catalytic materials.
Anna Martinelli, Simone Creci, Szilvia Vavra, Per-Anders Carlsson, Magnus Skoglundh. Local anisotropy in single crystals of zeotypes with the MFI framework structure evidenced by polarized Raman spectroscopy. Physical Chemistry Chemical Physics, 2020, volume 22, 1640.