Evaluation of s-block Metal-Organic Frameworks for Selective Gas Sorption

Metal-organic frameworks (MOFs) are modern materials that possess advantageous properties that are expected to find use for a wide variety of applications; most notably their high porosity leading to potential applications in selective gas sorption and trapping. Therefore, it is thought they could be employed in environmental settings, specifically by the removal of greenhouse gases from fossil fuel combustion emissions to combat climate change. Two main examples of gases relevant to this situation are CO₂ and C₂H₂, molecules with similar physical dimensions although opposite electronic surfaces.

The research discussed in this thesis aims to evaluate the potential use of s-block metal-based MOFs for gas sorption and selective separation between CO₂ and C₂H₂. Three materials were studied; CaHNCPP, NaHNCP and LiTDC, which all comprised of distinct metals and organic connecting ligands and therefore varying physical, chemical and electronic properties. Their gas sorption ability was evaluated using a combination of methods. Firstly, p-DFT calculations were employed to (i) produce theoretical FTIR spectra linked from calculated vibrational modes, (ii) optimise the predicted binding position of guests within the host framework, and (iii) calculate thermochemical information such as interaction energies. Next, FTIR spectroscopy experiments were performed on MOF sample pressed on KBr-based pellets, positioned within a HTHP (high temperature, high pressure) gas cell, where MOF spectra were recorded both under vacuum and high-pressure environments of the guest gases. The method used for these collections was designed and optimised during the study. Finally, calculations to quantify the efficiency of gas sorption were derived (inspired by quantitative astrochemical ice studies) after absorption data was modified to best represent the conditions used for porous materials. As new methods devised to answer the main research question, these procedures will also be validated to ensure they are accurate, consistent between experiments, and show good potential for further use toward similar research targets.

This combined experimental and theoretical approach yielded a wide range of results with varying levels of consistency. All three materials were shown to significantly prefer adsorbing C₂H₂ over CO₂, giving confidence all MOFs provide avenues to selectively separate the two gases. CaHNCPP and NaHNCP showed greater sorption capacity towards both C₂H₂ and CO₂ compared to LiTDC, however, a preference between the first two could not be identified. In terms of separation of C₂H₂ over CO₂ mixtures, LiTDC showed the most potential towards this goal, mainly due to the material showing no ability to adsorb CO₂. Therefore, all three MOFs have potential as new materials for selective separation of CO₂ and C₂H₂ that can be incorporated into important applications within and beyond the field of chemistry.

Overall, many of the experimental and DFT methods developed in this work appear inconsistent, casting doubt in their ability to provide significant and accurate results. However, when all of the developed methods are combined (in a composite analytical approach), they produce datasets with strong enough correlation to extract useful information towards the research question. Individually, each method still requires refinement and expansion to other host-guest systems, where the error and inconsistency displayed either within or between materials can be minimised. Therefore, we conclude that experimental and theoretical methods developed and performed as part of this thesis answered the proposed questions into host-guest interactions, but there remains much scope for improvement in future research.