Faculty Advisor: Edward J. Maginn

Computational Investigation and Discovery of Novel Materials for the Energy-Efficient and Sustainable Phaseout of Hydrofluorocarbons

Azeotropic or near-azeotropic mixtures of hydrofluorocarbons (HFCs) are the predominant working fluids in heating, ventilation, air-conditioning, and refrigeration (HVACR) systems globally. Unfortunately, most HFCs have very high global warming potential (GWP) up to 4000 times more than CO21,2. The continued use of high GWP HFCs has been projected to result in the release of an equivalent of over 175 billion kilograms of CO2 and a global temperature rise of 0.5oC in the next few decades3–5. These scary forecasts drove leaders from 197 countries in 2016 to sign agreements to phase out high GWP HFCs. However, just signing papers to phase out HFCs does not solve the problem. What do we do with the millions of tons of these high GWP HFC mixtures already in our world considering phase-out demands? What do we use in place of these HFCs in the HVACR systems which are critical to human well-being and even survival in some contexts? How do we ensure the energy efficiency of future HVACR systems? These are the questions that define the sustainable phaseout of HFCs. These are the critical questions and challenges that my research will help address.

Rather than venting or incinerating existing HFC mixtures, we propose to separate them for reuse and recycling. The problem is that the mixtures are azeotropic or near-azeotropic, meaning conventional distillation will either not work or require tremendous amounts of energy. Ionic liquids (ILs) are effective in energy-efficient extractive distillation (ED) of HFC mixtures6,7. However, key transport and interfacial properties of binary and ternary IL/HFC systems needed for the selection of the optimal IL and the design of these energy-efficient technologies are scarce. Experimental methods are limited, expensive, and/or too time-consuming to fully explore the combinatorial design space for these IL/HFC systems. Furthermore, the underlying molecular mechanisms behind the observed macroscopic properties of these IL/HFC systems are not currently well understood. These challenges pose bottlenecks to the rational design and selection of the ‘best’ ILs and process conditions to ensure energy-efficient separation of HFC mixtures. Molecular dynamics (MD) simulations offer a feasible route to addressing this challenge. Thus, the first part of my research project is focused on the computation of key and scarce thermophysical data for IL/HFC systems and the elucidation of the underlying molecular level phenomena behind these computed properties using MD simulations. The second part of my research project is concerned with the development of novel, green refrigerant mixtures to replace high GWP HFCs in HVACR systems globally. Current HVACR systems account for 20 – 30% of U.S. building electricity usage8. Future refrigerant mixtures must be designed to reduce or at least not increase the electricity power burden of HVACR systems. The development of these green refrigerant mixtures will help to ensure the energy efficiency and sustainability of future HVACR systems.

Research Objectives

Research objectives (ROs) 1 and 2 are directed towards addressing the challenge of the sustainable separation of HFC mixtures using ILs while ROs 3 – 5 are directed towards tackling the challenge of designing green refrigerant mixtures to replace currently used HFC mixtures. When combined, ROs 1 – 5 will help achieve the overarching goal of enabling the sustainable phaseout of HFCs.

Furthermore, the outputs of ROs 1 and 2 have the potential to enable wider adoption of ILs as green solvents for use in energy-efficient separation processes in general. This will reduce the huge burden that industrial separation processes place on world energy demands. The outputs of ROs 3 – 5, will help chart a new paradigm in the design/discovery of novel molecules from scratch. The tools and methods that will be developed and/or applied for ROs 3 -5 are likely to be useful for other material discovery endeavors thereby offering further benefits relevant to sustainable energy and the environment.

Research Objective 1 (RO-1): Validation of interatomic potential models (commonly called force fields (FFs)) for HFCs.

The fidelity of MD simulation results critically hinges on the accuracy and reliability of the FF models used. To enable reliable computation of critical thermophysical properties of IL/HFC systems and elucidation of underlying molecular level phenomena, there is a need to validate developed FFs for HFCs. Recent work in our group9,10 applied machine learning (ML) to guide the tuning of some FF parameters to accurately match reference vapor-liquid equilibria (VLE) data for seven refrigerant molecules. However, the transferability of these tuned FFs was not tested for the key thermophysical properties that are lacking for IL/HFC systems. RO-1 seeks to assess and validate the transferability of the HFC FFs to transport properties such as viscosity, thermal conductivity, self-diffusivity, and interfacial properties such as surface tension. My initial work11 on RO-1 shows that the FFs tuned using ML for HFCs by our group are transferable to a plethora of thermophysical properties across multiple state points. Future work in RO-1 will be to extend the established FF validation infrastructure to validate the HFC FFs for the case of HFC mixtures. This is key since to design ED systems for separating binary (and ternary) HFC mixtures using ILs, we ultimately require transport and interfacial data of binary (and possibly ternary) HFC mixtures in ILs. MD simulations will be key to obtaining enough of these critical data and we thus require rigorous FF validation for both pure component HFCs and HFC mixtures.

Research Objective 2 (RO-2): Computation and Molecular Level Elucidation of Key Transport and Interfacial Properties of IL/HFC mixtures.

The rigorously validated FFs from RO-1 will be used to achieve RO-2 which is concerned with the computation of key and scarce properties namely, thermal conductivity, self-diffusivity, viscosity, and interfacial tension of binary and ternary IL/HFC systems. An investigation of the molecular level phenomena behind the computed properties will also be investigated. Advanced non-equilibrium molecular dynamics simulations will be used to investigate thermal and viscous transport properties. Other advanced molecular simulation techniques that will be deployed to achieve RO-2 include molecular heat flux decomposition (HFD), momentum flux decomposition (MFD), interfacial adsorption, and crossing simulations in addition to rigorous spatial and temporal liquid structure analyses. The completion of RO-2 will enable the rational design/selection of the appropriate ILs for the separation of any given HFC mixture. This will enable the energy-efficient separation of HFC mixtures using ILs in ED processes. Importantly, a fundamental, scientifically grounded understanding of the evolution of transport and interfacial properties from liquid structure and states for IL/HFC mixtures will be gleaned.

Research Objective 3 (RO-3): Exhaustive Enumeration and Pre-Screening of All Feasible Small Refrigerant Molecules.

RO-3 will be focused on the full development of FineSMILES, a tool currently in development to facilitate the exhaustive enumeration and pre-screening of candidate refrigerant molecules. Current material screening/discovery approaches either do not exhaustively navigate the chemical design space and/or are too complicated and/or require proprietary codes and/or give outputs that are not of any standard molecular representation (like SMILES) which are needed for more detailed/advanced screening. FineSMILES is intended to be a general-purpose tool for the exhaustive enumeration and pre-screening of small molecules which will then be applied specifically to the refrigerant design problem. The code when completed will be freely available, and easy to use. Its output will be well-formatted SMILES strings with cautious pre-screening using group contribution (GC) methods applied to relevant target thermophysical properties. GC methods are known to be limited in accuracy and will thus be used with caution with large tolerances applied to the properties.

Research Objective 4 (RO-4): Development of Pure Component Refrigerant Molecules.

FineSMILES from RO-3 will be used to generate a set of pre-screened SMILES which will then be used as ‘feedstock’ for a suite of previously developed ML tools12 in our group and other groups for more accurate and rigorous thermophysical, environmental, health, and safety screening. The candidate refrigerant molecules that successfully pass through all the screens will then be selected for a more detailed study using molecular simulation. This stage may involve some molecular modeling FF development and validation like what was done in RO-1 for the HFC molecules being phased out. Rigorous thermophysical calculations and other more fundamental investigations of the candidate molecules at this stage will be performed using molecular simulations. Some quantum chemical calculations may also be performed to predict, for example, enthalpies of formation and other relevant quantities which may be useful for making informed predictions on the synthesizability and stability of the candidate molecules. The candidate molecules that are found to be satisfactory considering all the factors considered and the results of the more advanced computational studies will be selected as ‘highly promising’ pure component refrigerant molecules. Previous works13,14 on refrigerant design suggest that there will be a need for future refrigerants to be designed as mixtures to satisfy the complex and often conflicting interplay of technical, environmental, health, and safety requirements that the ideal refrigerant must satisfy for a given application. The tolerance range for the screening of the individual refrigerant molecules will thus not be overly constrained to allow for more flexibility in designing the final green refrigerant mixtures which will be the thrust of RO-5.

Research Objective 5 (RO-5): Development of Green Refrigerant Mixtures.

The pure component refrigerant molecules from RO-4 will be subjected to computational miscibility tests. MD simulations can be used to predict miscibility. There are other simpler but less rigorous methods to predict the miscibility of two molecules. Once the miscible pairs are determined, MD simulations will be performed at varying compositions to find the ‘optimal’ composition(s) to give good refrigerant mixtures that will guarantee energy efficiency and optimal equipment size for given applications. Environmental and safety factors such as GWP and flammability respectively can be readily calculated using the information on the pure component GWP or flammability properties and the mixture composition. Taking all the techno-economic, environmental, health, and safety constraints into account, more stringent screening of the refrigerant mixtures will be conducted to identify a small set of refrigerant mixtures that gives a good balance for all the requirements. The successful completion of RO-5 will result in green refrigerant mixtures with low GWPs, no ozone depletion potential, low to moderate toxicity, low to moderate flammability, and highly excellent technical performance to ensure the energy efficiency and sustainability of future HVACR systems.

References

(1) United Nations. About Montreal Protocol. Ozonaction. http://www.unep.org/ozonaction/who-we-are/about- montreal-protocol (accessed 2023-06-16).

(2) United Nations. GWP-ODP Calculator. Ozonaction. http://www.unep.org/ozonaction/gwp-odp-calculator (accessed 2022-07-13).

(3) United Nations Treaty Collection. Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer. https://treaties.un.org/Pages/showDetails.aspx?objid=080000028048cd90&clang=_en (accessed 2022-07-13).

(4) United States EPA. Reducing Hydrofluorocarbon (HFC) Use and Emissions in the Federal Sector through SNAP. https://www.epa.gov/snap/reducing-hydrofluorocarbon-hfc-use-and-emissions-federal-sector-through-snap (accessed 2023-02-16).

(5) United States Environmental Protection Agency. Regulatory Impact Analysis for Phasing Down Production and Consumption of Hydrofluorocarbons (HFCs). 2022.

(6) Asensio-Delgado, S.; Pardo, F.; Zarca, G.; Urtiaga, A. Enhanced Absorption Separation of Hydrofluorocarbon/Hydrofluoroolefin Refrigerant Blends Using Ionic Liquids. Sep. Purif. Technol. 2020, 249, 117136. https://doi.org/10.1016/j.seppur.2020.117136.

(7) Monjur, M. S.; Iftakher, A.; Hasan, M. M. F. Separation Process Synthesis for High-GWP Refrigerant Mixtures: Extractive Distillation Using Ionic Liquids. Ind. Eng. Chem. Res. 2022, 61 (12), 4390–4406. https://doi.org/10.1021/acs.iecr.2c00136.

(8) U.S. Department of Energy. Quadrennial Technology Review 2015 – An Assessment of Energy Technologies and Research Opportunities. September 2015.

(9) Befort, B. J.; DeFever, R. S.; Tow, G. M.; Dowling, A. W.; Maginn, E. J. Machine Learning Directed Optimization of Classical Molecular Modeling Force Fields. J. Chem. Inf. Model. 2021, 61 (9), 4400–4414. https://doi.org/10.1021/acs.jcim.1c00448.

(10) Wang, N.; Carlozo, M. N.; Marin-Rimoldi, E.; Befort, B. J.; Dowling, A. W.; Maginn, E. J. Machine Learning- Enabled Development of Accurate Force Fields for Refrigerants. J. Chem. Theory Comput. 2023, 19 (14), 4546– 4558. https://doi.org/10.1021/acs.jctc.3c00338.

(11) Agbodekhe, B.; Marin-Rimoldi, E.; Zhang, Y.; Dowling, A. W.; Maginn, E. J. Assessment and Ranking of Difluoromethane (R32) and Pentafluoroethane (R125) Interatomic Potentials Using Several Thermophysical and Transport Properties Across Multiple State Points. J. Chem. Eng. Data 2023. https://doi.org/10.1021/acs.jced.3c00379.

(12) Abranches, D. O.; Zhang, Y.; Maginn, E. J.; Colón, Y. J. Sigma Profiles in Deep Learning: Towards a Universal Molecular Descriptor. Chem. Commun. 2022, 58 (37), 5630–5633. https://doi.org/10.1039/D2CC01549H.

(13) Kazakov, A.; McLinden, M. O.; Frenkel, M. Computational Design of New Refrigerant Fluids Based on Environmental, Safety, and Thermodynamic Characteristics. Ind. Eng. Chem. Res. 2012, 51 (38), 12537–12548. https://doi.org/10.1021/ie3016126.

(14) McLinden, M. O.; Brown, J. S.; Brignoli, R.; Kazakov, A. F.; Domanski, P. A. Limited Options for Low-Global- Warming-Potential Refrigerants. Nat. Commun. 2017, 8 (1), 14476. https://doi.org/10.1038/ncomms14476.