In this article, we explore how electron microscopy can be used to gain deeper insights into zeolite samples.
Image Credit: Yushchuk Myroslava/Shutterstock.com
What Are Zeolites?
Zeolites are a class of materials with a very distinctive porous structure. Zeolites are made from hydrated aluminosilicates, typically with the AlO4 and SiO4 groups in linked tetrahedral structures. Depending on the relative pore to molecular size, the porous structure of the overall zeolite material allows other molecules to pass through the structure and potentially temporarily adhere to certain sites to facilitate reactions like catalytical processes.1
Zeolites are now used in a wide range of applications, including as molecular sieves, catalysts and gas exchange. As the transport properties through the zeolite are heavily dependent on pore size and structure, methods that can characterize the internal structure and pore network are incredibly important in developing new zeolite materials.
Zeolites in Nanoscience
There are now a number of synthetic methods that can be used to create zeolites with pore sizes on the nanoscale2. These nanoscale pore zeolites can be used to create zeolite materials for a number of applications, such as water filtering through either membrane processes or driving particular chemical reactions.3
Some research is even making use of nanoscience synthetic methods to create not just nanosized pores within the zeolite structures but zeolite crystals on the nanoscale.4
While zeolites are very useful for providing a large surface area for adsorption and chemical reactions, pore-clogging and slow diffusion dynamics through the zeolite are very common problems that can inhibit the overall reaction rates and efficiency of the zeolite catalyst. Minimizing the size of the overall zeolite structure may help reduce some drawbacks of more conventional zeolite technologies.
Some of the conventional methods for studying zeolite structure and function include a range of spectroscopies – e.g. infrared, UV-vis, Raman – as well as structurally sensitive methods like X-ray diffraction and NMR.5
X-ray diffraction is particularly useful for characterizing pore structures as the high penetration depth of the X-rays means it can visualize not just the external structure of the zeolite but the internal one as well.
Many of the early analytical methods were focused on the bulk structures of the zeolites, which all of the aforementioned techniques are suitable for. Many of the spectroscopic techniques struggle with zeolite studies. While techniques like Raman and infrared are very sensitive to any molecules that have become bound to a surface, visualizing the spatial arrangement of molecules means combining them with microscopy versions of the technique.
More recently, electron microscopy has shown huge promise as a method for zeolite characterization, offering superior spatial resolution over many conventional microscopy techniques so that even individual atoms can be visualized as well as potentially providing information on elemental characterization.6
Electron Microscopy Studies
The challenge for measuring zeolites has been issues with beam damage to the crystals from the intense electron beams that must be used. Now that these limitations have been overcome, researchers are using these techniques to look at really determining, with atomic precision, the spatial arrangements at boundary conditions in the zeolite.6
These boundary regions are very important in zeolite development as it is these regions that ultimately dictate how the zeolite will interact or react with any molecules that are passed through.
By using optimum bright-field scanning transmission electron microscopy, researchers have been able to produce images with a signal-to-noise ratio that was two orders of magnitude greater than with conventional techniques. Using lower electron doses also helped prevent beam damage to the sample.
Other advances in zeolite structural imaging have come from the use of low-voltage field emission scanning microscopy in combination with energy dispersive X-ray spectroscopy. The latter provides information on the elemental composition of the sample and the former was used to visualize both the outer surface and inner architecture of the zeolite.
Electron microscopy could be used to look at inner and outer defects in the zeolite structure and identify how the chemical etching being used to create the zeolite pore networks was reacting, including identifying the initial dissolution points.
Researchers could then use this information to understand the 'weak points’ in the structure that were particularly susceptible to the fluoride attack, which could be one approach to intentionally guiding the selective etching of particular structures in the future or avoid damage in unwanted regions.
In conclusion, electron microscopy methods offer a powerful new suite of tools for achieving atomic-scale spatial resolution in zeolite imaging. Being able to visualize exactly where potential molecules or ions bind or interact will help researchers refine and develop new classes of zeolite material that can exploit these interactions for applications in waste filtering, gas storage and chemical catalysis.
While beam-damage is still a potential problem, ongoing developments in electron microscopy – particularly with the application of newer lower dose methods – will mean a wider range of zeolite materials can be studied with these very high spatial resolution techniques.
References and Further Reading
Weitkamp, J. (2000). Zeolites and catalysis. Solid State Ionics, 131, pp. 175–188. doi.org/10.1016/S0167-2738(00)00632-9
Li, C., et al. (2018). Building Zeolites from Precrystallized Units : Nanoscale Architecture Angewandte. Angewandte Chemie, 57, pp. 15330–15353. doi.org/10.1002/anie.201711422
Mohmood, I., et al. (2013). Nanoscale materials and their use in water contaminants removal — a review. Environmental Science and Pollution Research, 20, pp. 1239–1260. doi.org/10.1007/s11356-012-1415-x
Mintova, S., et al. (2016). Comptes Rendus Chimie Nanosized zeolites : Quo Vadis ? Comptes Rendus - Chimie, 19(1–2), pp. 183–191. doi.org/10.1016/j.crci.2015.11.005
Vreeswijk, S. H. Van, & Weckhuysen, B. M. (2022). Emerging analytical methods to characterize zeolite-based materials. National Science Review, 9, nwac047.
Ooe, K., et al. (2023). Direct imaging of local atomic structures in zeolite using optimum bright-field scanning transmission electron microscopy. Science Advances, 9, eadf6865. doi.org/10.1126/sciadv.adf6865
Asano, N., et al. (2022). Advanced scanning electron microscopy techniques for structural characterization of zeolites. Inorganic Chemistry Frontiers, 9, pp. 4225–4231. doi.org/10.1039/d2qi00952h