Design and synthesis of novel hybrid ultramicroporous materials for selective gas capture and separation.

Abstract

As the world is entering what is known as the “Age of gas”, our dependence on the production of chemical feedstocks is ever increasing. In this regard, the separation and purification of light hydrocarbons within the chemical industry is of paramount importance, as they serve as the foundation of many chemical commodities that we use in our everyday lives. As it stands these processes account for approximately 40 % of energy used in the chemical sector, as well as 15 % of the world’s global energy expenditure. Current technologies to achieve these separations rely heavily on techniques such as solvent extraction, cryogenic distillation and liquid amine chemisorption, all of which are hindered by extreme energy consumption, lack of recyclability and use of environmentally harmful and hazardous chemicals. This is made further challenging due to the comparable physical and chemical properties that light hydrocarbons have with one another.

To address these concerns new technologies must be implemented that can approach these challenging separations and purifications, whilst simultaneously maintaining a low energy footprint. One approach that has gained attention over the last three decades if the development of novel physisorbents, such as metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) that can enable energy efficient separation, through the use of their highly porous architectures.

Crystal engineering, is a field of chemistry that specialises in the rational design and synthesis of new crystalline solids, has emerged as a prominent approach that has realised materials capable of addressing these separations with benchmark qualities. The research reported in this thesis, focuses on using the crystal engineering approach to logically design and synthesise novel crystalline porous materials that can address the current demands for separation and purification technologies. This will be achieved through the synthesis of materials know as Hybrid ultramicroporous materials (HUMs), a class of framework materials that combine small, finely tuned pores, with strong binding sites that can potentially achieve energy efficient separations.

Chapter two details the synthesis of a series of novel HUMs, through the use of a newly implemented N-donor linker, 3,5-dimethyl-1H-pyrazol-4-ylpyridine (pypz, 21), Cu2+metal nodes and SiF62-or TiF62- anionic pillars. The newfound materials [Cu(21)2MF62-]n (M = SiF62-, TiF62) also known as SIFSIX-21-Cu and TIFSIX-21-Cu exhibits near benchmark qualities of high uptake capacity and separation selectivity for C2H2/CO2 separation, owing to a unique pore environment never before seen in the HUM platform. TIFSIX-21-Cu was found to be one of a handful of materials that can break the trade-off between adsorption capacity and selectivity prevalent in porous materials.

Chapter three reports on the incorporation of GeF62- as a pillaring anion into the newly established MFSIX-21-Cu platform, giving rise to the isostructural HUM, [Cu(21)2GeF62-]n GEFSIX-21-Cu. GEFSIX-21-Cu exhibited the same characteristics as SIFSIX-21-Cu and TIFSIX-21-Cu such as a large surface area, but with the added benefit of more finely tuned pore sizes, yielding greater adsorption capacity and excellent C2H2/CO2 separation selectivity. Furthermore, GEFSIX-21-Cu achieved separation with a relatively low heat of regeneration, as investigated using temperature programmed desorption (TPD) experiments.

Chapter four explores the synthesis and characterisation of another isostructural HUM [Cu(21)2ZrF6]n ZRFSIX-21-Cu, that exists in two ultramicroporous phases; a 2D sql α phase, and a typical 3D pcu β phase . A combination of powder X-ray diffraction and single component gas adsorption experiments identified structural relationships between ZRFSIX-21-Cu-α and ZRFSIX-21-Cu-β as well as different gas adsorption profiles, for each independent phase.

Chapter five investigates the crystal engineering of a new family of HUMs, based on the use of a novel anionic pillar. TaOF52- (TaOFFIVE). A selection of transition metal nodes (Cu2+, Ni2+, Co2+) and organic linkers (pyrazine (3), bis-pyridyl benzene (4) and pypz (21) were employed to evaluate the use of TaOF52- across a variety of HUM families with different pore chemistries and how this translates to gas adsorption and separation ability. Three HUMs, TaOFFIVE-3-Co, TaOFFIVE-3-Ni and TaOFFIVE-4-Cu-i (i = interpenetrated) were promising sorbents for the screening of CO2/N2 C2H2/CO2 and C3H4/C3H6 separations.

Chapter six details the synthesis of a novel framework material based on the well-established Hofmann clathrates family. Using an angular pyazole-based linker yielded the Hofmann-type framework CoNi-mdp. CoNi-mdp comprises of a bimetallic framework that contains open metal sandwich-like binding sites and methyl functionalised pore surface to enable selective uptake of several gases. Solid-state characterisation demonstrated the thermal and chemical robustness of CoNi-mdp, with modelling studies also providing insight into favourable binding interactions with guest gas molecules.