Molecular Insight into Fluorocarbon Adsorption in Pore Expanded Metal-Organic Framework Analogs
Jian Zheng, Dushyant Barpaga, Benjamin A. Trump, Manish Shetty, Yanzhong Fan, Papri Bhattacharya, Jeromy J. Jenks, Cheng-Yong Su, Craig Brown, Guillaume Maurin, B. Peter McGrail, Radha Kishan Motkuri
The rapid growth in the global energy demand for space cooling requires the development of more efficient environmental chillers for which adsorption-based cooling systems can be utilized. Here in this contribution, we explore sorbents for chiller use via a pore-engineering concept to construct analogs of the 1-dimensional pore metal-organic framework MOF-74 by using elongated organic linkers and stereochemistry control. The prepared pore-engineered MOFs show remarkable equilibrium adsorption of the selected fluorocarbon refrigerant that is translated to a modeled adsorption-based refrigeration cycle and assessed using molecular simulations to understand atomic level interactions. To experimentally probe molecular level interactions at the origin of these unique adsorption properties for this series of Ni-MOFs, we combined in situ synchrotron X-ray powder diffraction, neutron powder diffraction, X-ray absorption spectroscopy, calorimetry and Fourier transform infrared techniques. Our results reveal the coordination of fluorine (of CH2F in R134a) to the nickel(II) open metal centers at low pressures for each Ni-MOF analog and provide insight into the pore filling mechanism for the full range of the adsorption isotherms. The newly designed Ni-TPM demonstrates exceptional R134a adsorption uptake compared to its parent microporous Ni-MOF-74 due to larger engineered pore size/volume. Applying this adsorption performance towards established chiller conditions yields a working capacity for Ni-TPM nearly 4.6 times that of Ni-MOF-74, which combined with kinetics directly correlates to both a higher coefficient of performance and a higher average cooling capacity generated in a modeled chiller.