(592g) Understanding the Adsorption of 4th Generation Refrigerants on Metal-Organic Frameworks By Molecular Simulations

To reduce the overall emission of greenhouse gases, the Kigali Amendment to the Montreal’s Protocol is forcing to phase-out hydrofluorocarbons (HFC) with high Global Warming Potential (GWP) and replacing them by refrigerants with low GWP.[1] In this regard, the development of refrigerants has evolved to the fourth generation, with sustainable and environmental friendly substances mainly known as hydrofluoroolefins (HFOs) [2], while part of the challenge remains on finding the best refrigerants and blends for the selected applications. For instance, taking into account that adsorption cycles can be integrated into typical refrigeration systems to enhance the cycle efficiency -by reducing the condenser temperature-, such fluorocarbon-based refrigerants have been examined recently with traditional adsorbent materials such as activated carbons and zeolites.[3]

Metal-Organic Frameworks (MOFs) comprise an important class of novel solid-state materials. MOFs are highly crystallized structures composed by the combination of metal clusters and organic ligands through the coordination bond or intermolecular interactions, possessing good thermal stability, high specific surface area and strong adsorption affinity for adsorbates, and making them promising for energy storage and gas separation.[4] Nevertheless, studies related to their adsorptive performance for HFO refrigerants is rather scarce.[5] Understanding the adsorption and phase behavior by accurately predicting the thermophysical properties of these systems with low GWP refrigerants is essential for designing and evaluating refrigeration cycle performances and determining optimal conditions and possible compositions.[6]

In recent years, molecular simulation studies have been gaining acceptance as an important complement to experiments in order to obtain reliable thermodynamic and transport properties of these new class of refrigerants.[7] Pioneering work on the use of these techniques for the adsorption of refrigerants in MOFs has been very recently published by Hu et al. [8]. In this contribution, we will present and discuss results concerning the adsorption of fourth generation refrigerant molecules (HFOs and blends) on different MOFs families, searching for the best conditions for cold thermal energy storage. Simulations were performed using the Grand Canonical Monte Carlo (GCMC) technique, to gain a molecular-level understanding of the adsorption mechanism of gases in these porous materials, and were also validated with measurements obtained experimentally in our laboratories. Focusing on the behavior at operating conditions, the effects of temperature and pressure on the adsorption and desorption (i.e., regeneration) steps were evaluated, as well as the compositions of the blends. Interfacial properties and wetting phenomena were also investigated through the molecular simulations.

We acknowledge support for this work from Khalifa University of Science and Technology (project CIRA 121)

[1] United Nations Treaty Collection. Kigali, (2016).

[2] Mota-Babiloni, J. Navarro-Esbrí, A. Barragán-Cervera, F. Molés, B. Peris, G. Verdú. Int. J. Refrigeration, 57, 186-96 (2015).

[3] D. Banker, M. Prasad, P.Dutta, K.Srinivasan. Appl.Therm.Eng., 29, 2257–64 (2009).

[4] K. Henninger, H. A. Habib, C. Janiak, J. Am. Chem. Soc., 131, 2776–77 (2009).

[5] B. Getman, Y. S. Bae, C. E. Wilmer, R. Q. Snurr. Chem. Rev., 112, 703–23 (2012).

[6] A. Fouad, L. F. Vega. AIChE J., 64, 250–62 (2018).

[7] Raabe, E. J. Maginn. J. Phys. Chem. B, 114, 10133-42 (2010).

[8] Hu, C. Liu, Q. Li, X. Shi. Int. J. Heat & Mass Transfer 125 1345–1348 (2018).