Exploring Metal-Organic Frameworks for the development of innovative composites: a new frontier for CO2 separation

The primary objective of this PhD thesis has been to investigate of the role of fluorine functionalisation in Fluorinated Metal-Organic Frameworks (F-MOFs) used as potential fillers in Mixed-Matrix Membranes (MMMs) for CO2 separation. The research was conducted at the University of Torino in collaboration with multiple research institutions. A key aspect of this work has been to elucidate the impact of fluorination on the flexibility, textural properties, and gas separation performance of several F-MOFs. The extensive characterisation of fully and/or partially fluorinated MIL-53(Al) and MIL-140A(Ce) MOFs has revealed that fluorination can significantly modify the framework adsorption-induced flexibility and the interaction with CO2. For example, in Fx-MIL- 53(Al), fluorine substitution modulates the breathing capacity, namely the transition between a large and a narrow pore phase, thereby affecting its response to CO2 and H2O adsorption. In Fx-MIL-140A(Ce), the study highlighted the necessity of a complete fluorination of the organic linker to induce the adsorption-driven linker rotation mechanism, which is responsible for the peculiar S-shaped isotherm and an enhanced CO2 selectivity. Fluorine functionalisation on a UiO-66(Ce) framework, instead, revealed changes in textural properties and adsorption site heterogeneity, thereby improving the CO2 capture efficiency. This thesis also evaluated the performance of a series of per-fluorinated MOFs as fillers in MMMs. Several MOF-polymer combinations (including Matrimid®9725, Pebax®1657, and PIM-1 as polymeric matrices), were tested for CO2/N2 and CO2/CH4 separation. The testing and characterisation of the final MMMs indicated that, even if fluorinated MOFs can enhance both permeability and selectivity, the compatibility issues between MOFs and polymer matrices remain a major challenge for practical applications. Additionally, some of the results reported in this thesis derived from a collaboration with a company (2Dto3D s.r.l.). This research explored the potential of graphitic carbon nitride (g-C3N4) for CO2 adsorption and also included some attempts to develop new g-C3N4@MOF composites. The objective of this study aimed to improve the CO2 adsorption capacity of the compound and of progressing from CO2 separation to the more challenging task of CO2 utilisation (i.e. photoreduction, which is one of the major applications of g-C3N4). Preliminary results suggested that a physical mixture of g-C3N4 and MIL-53(Al) yields peculiar core-shell-like particles, wherein carbon nitride act as a protective shield for the MOF crystallites. These outcomes laid the foundation for the development of new composite materials, with potentially enhanced separation properties. On the other hand, the polymeric layer that envelops the crystallites of the MOFs may enhance the compatibility issues related to the MOF dispersion in polymeric matrices, possibly leading to the development of more efficient MMMs.