New Method of Zeolitic Membrane Production for Carbon Capture Discovered

Andrew Sasser ‘23, Physical Sciences, Fall 2020

Figure 1: This is an image of a Zeolite mineral. Composed of silica and alumina molecules arranged in tetrahedral patterns, Zeolites are notable for the presence of their porosity. This makes them especially useful in catalysis and gas separation applications (Source: Wikimedia Commons).

When it comes to combatting climate change and greenhouse gas production, many efforts have focused on reducing the amount of CO₂ emissions. One of the most promising methods is pre-combustion capture of CO₂ before combustion can be completed. In this method, a synthesis gas rich in hydrogen gas (H₂) and CO₂ is produced from oxidation of coal; the CO₂ is then separated from the hydrogen for sequestration while the hydrogen is combusted². However, since this complex method is quite difficult, recent efforts to improve these processes have focused around the synthesis of zeolitic membranes, which are microporous minerals composed of AlO₄ and SiO₄ molecules⁴. Zeolitic membranes have been studied for over 30 years but have not been implemented widely for pre-combustion carbon capture because it requires a secondary growth phase⁵.

However, a team of engineers led by Kumar Agrawal from the EPFL Institute recently discovered a new method for producing these zeolitic membranes³. One prior approach to the synthesis of these nanosheets included a “bottom-up” assembly of silica and alumina molecules, which required a crystallization phase that reduced total yield. Instead, Agrawal and his team attempted to produce the nanosheets from the exfoliation of a zeolite precursor, as these were highly successful in producing the molecular-sieving pores needed for gas separation¹.

Using this method, they successfully produced RUB-15, a precursor of sodalite (a Zeolitic mineral). This precursor was chosen as it had hydrogen gas sieving rings, composed of six SiO₄ molecules arranged in hexagonal patterns¹. Through exfoliation of the precursor, the group was able to generate 0.8 nanometer thick nanosheets. These sheets were then stacked in a thin film, leaving an initial gap between them of approximately 3.6 angstroms (Å). As this gap was too big to effectively separate hydrogen from carbon dioxide, the team then condensed neighboring silanol (SiH₄O) groups on the end of the nanosheets to form Si-O-Si bond linkages¹. These closed the gaps between sheets to the 2.9 Å needed to effectively separate hydrogen and carbon dioxide. Upon testing under CO₂ and H₂ separately, the group noted that the membrane had an ideal selectivity for H₂ over CO₂ of over 100 at temperatures of 250-300˚C, which are temperatures associated with fossil fuel combustion¹. Under the addition of a 50:50 mixture of hydrogen and carbon dioxide, the selectivity for hydrogen remained above 100, while the selectivity for carbon dioxide remained below 50. Thus, compared to the crystallized membranes, the RUB-15-based membranes were more than 10 times more effective at selectively separating hydrogen gas¹.

Overall, the team noted that the “thermally robust” RUB-15 films are “very promising” for the separation of hydrogen and carbon dioxide¹. They also noted that the “scalable synthesis” of these films without the more complex crystallization step is likely to improve their reproducibility and large-scale deployment for pre-carbon capture³. While still considered an emerging technology, studies have demonstrated that this type of pre-combustion technology may be capable of capturing upwards of 80% of CO₂ emissions from a coal-fired plant, thus drastically reducing environmental impact².

References:

1. Dakhchoune, M. et al. (2020). Gas-sieving zeolitic membranes fabricated by condensation of precursor nanosheets.Nature Materials. doi:10.1038/s41563-020-00822-2
2. Jansen, D., Gazzani, M., Manzolini, G., Dijk, E. V., & Carbo, M. (2015). Pre-combustion CO2 capture.International Journal of Greenhouse Gas Control, 40, 167-187. doi:10.1016/j.ijggc.2015.05.028
3. Lego-like assembly of zeolitic membranes improves carbon capture. (2020, October 05). Retrieved October 15, 2020, from https://www.sciencedaily.com/releases/2020/10/201005112139.htm
4. Peskov, M. (n.d.). Zeolites. Retrieved October 15, 2020, from https://asdn.net/asdn/chemistry/zeolites.php
5. Sherman, J. D. (1999). Synthetic zeolites and other microporous oxide molecular sieves.Proceedings of the National Academy of Sciences, 96(7), 3471-3478. doi:10.1073/pnas.96.7.3471