Crafting pore architecture of water-stable microporous MOFs for selective CO2 capture

Abstract

The sky-rocketing rise of global CO2 concentration is one of the major environmental concerns today because of its detrimental effects on climate change, species extinction, and plant nutrition. Therefore, cutting CO2 emissions is among the prime targets of the coming decades. CO2 is mainly emitted from the combustion of fossil fuels by industries and transport systems. Our daily lifestyles are almost entirely dependent on the power generated from fossil fuels and will also be so for the upcoming few decades. Consequently, the only technique to allow continued use of fossil fuels while minimizing CO2 footprint is Carbon Capture/separation and Storage (CCS technology). As CCS technology, solid-sorbents get priority over the industrially accessible liquid amine sorbents because of their lower regeneration costs and environment and plant affability. Apart from the engineering modifications, the performance of the CCS technology relies to a large extent on the sorbent capacity. Hence, finding suitable cheap designer crystalline porous material is highly needful for carbon capture technology. Metal-Organic Frameworks (MOFs) form a class of crystalline porous material made of metal nodes connected via organic linkers and have tuneable architectures and fuctionalizable pores. MOF can display excellent low-energy physisorption-based gas capture capabilities. Most industrial and power plant flue gas emissions contain substantial humidity. Pre-drying such streams for capture itself incur sizeable cost shying industries away from investing into these capture technologies. Hence being able to capture CO2 directly from the humid flue gas is desirable. For humid CO2 capture, producing humidity/water-stable porous materials is still challenging. This thesis's primary goal is to bring water stability into the framework of MOFs, simultaneously increasing the number of CO2-interacting sites into the pore architecture. This thesis proposes two strategies using the concept of "Werner's theory of coordination complexes." First, choosing high charge density metal ions (hard metals) combined with O-donor ligands (hard ligand). Second, choosing low charge density metal ions (soft metals) in conjunction with azolate (adenine) ligands. In the first strategy, the choice is tri-positive lanthanide (Ln3+) ions with polytopic carboxylic acid linkers taking advantage of the hard-acid (high charged metal)-hard acid (O-donor ligands) bonding favourability. The second strategy describes the synthesis of water-stable MOFs using adenine as an azolate ligand combined with other dicarboxylic acid ligands and comparatively softer transition metal ions. Adenine is chosen because in the ligand field strength of azolate is more than in the water. So, water is unable to destroy the framework, giving a water-stable MOF. Besides, adenine MOFs consist of multiple N-centres and can polarise CO2 for better physisorptive-selectivity. This facilitates the easy regeneration of CO2 from the framework. Moreover, being a short and inexpensive linker adenine can generate cost-effective ultra-microporous MOFs, which are the most useful for CO2 capture/separation applications. The MOFs described in this thesis are water/humid stable and have moderate to good CO2 uptakes under ambient conditions with optimum HOA for facile regeneration of CO2 and good CO2-selectivity over other gases.