Quantifying C2H2/CO2 Gas Sorption in s-Block Metal–Organic Frameworks via FTIR Spectroscopy and Periodic DFT Calculations

Abstract

The storage and separation of atmospheric and fuel gases are important chemical processes garnering attention due to their applications aimed at reducing the release of greenhouse gases into the environment. Porous materials are viewed as an effective and energy-efficient method to perform this task due to a variety of advantageous properties they possess. Three recently reported s-block metal–organic frameworks (MOFs) were investigated using quantum chemical calculations and vibrational spectroscopy for their ability to bind (and separate) CO2 and C2H2 gases under variable pressure and temperature conditions. These materials, denoted CaNCPP, NaNCP, and LiTDC, comprise a variety of metal and organic ligand combinations that exhibit a range of physical, chemical, and electronic environments within their pore cavities. Here, periodic density functional theory calculations were employed to simulate vibrational spectra, optimize guest binding positions, and derive thermochemical data. These models were then used to interpret FTIR spectra obtained from MOF samples exposed to both high vacuum and elevated guest pressures within a specialized optical transmission cell. Gas sorption efficiency calculations based on integration of guest molecule infrared bands─of novel application to porous materials─revealed that all three MOF materials preferentially adsorbed C2H2 over CO2 and that this selectivity could be quantified using IR techniques. However, data obtained by other theoretical approaches, such as calculations of the binding energy and vibrational peak shifts due to host–guest interactions, were less conclusive in determining relationships between MOF materials and their sorption properties, with the exception that LiTDC displayed no sorption affinity to CO2 being a universal outcome. This research confirms that vibrational spectroscopy, when combined with accurate computational models of specific host–guest interactions, has the potential to be refined into a readily deployable tool for passive screening of gas sorption within MOF materials, whether for laboratory analysis or for incorporation into industrial-scale applications.