PNNL

Abstract

The improvement in adsorption/desorption of hydrofluorocarbons has implications for many heat transformation applications such as cooling, refrigeration, heat pumps, power generation, etc. The lack of chlorine in hydrofluorocarbons minimizes the lasting environmental damage to the ozone, with R134a (1,1,1,2-tetrafluoroethane) being used as the primary industrial alternative to commonly used Freon-12. The efficacy of novel adsorbents used in conjunction with R134a requires a deeper understanding of the host-guest chemical interaction. Metal-organic frameworks (MOFs) represent a newer class of adsorbent materials with significant industrial potential given their high surface area, porosity, stability, and tunability. In this work, we studied two benchmark MOFs, a microporous Ni-MOF-74 and mesoporous Cr-MIL-101. We employed a combined experimental and simulation approach to study the adsorption of R134a to better understand host-guest interactions using equilibrium isotherms, enthalpy of adsorption, Henry’s coefficients, and radial distribution functions. The overall uptake was shown to be exceptionally high for Cr-MIL-101 while the adsorption enthalpy in Ni-MOF-74 exceeded the enthalpy of evaporation of R134a. For both MOFs, simulation data suggest that metal sites provide preferable adsorption sites for fluorocarbon based on favorable C-F···M+ interactions between negatively charged fluorine atoms of R134a and positively charged metal atoms of the MOF framework. Molecular modeling was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Experimental portion was supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Office of Geothermal Technologies. PNNL is operated by Battelle for the U.S. Department of Energy (DOE).