Designed Ultra-microporous Metal Organic Frameworks for Selective CO2 Capture

Abstract

Carbon dioxide (CO2) is a major heat-trapping gas responsible for global warming through the greenhouse effect. Anthropogenic activities such as deforestation and burning fossil fuels account for large-scale CO2 emissions, along with some natural processes such as respiration and volcanic eruptions. According to the recent report (2017) from NASA, the atmospheric CO2 concentration is already reaching alarming levels. In fact, the Mid- Tropospheric CO2 concentration has gone up by 110 ppm (from 365ppm to 475ppm) since last one and half decade. So, there is a need for developing technologies to reduce the atmospheric CO2 concentration. Also, this can bring some economic benefits in coal-based power production. Carbon Capture and Storage (CCS) is the most promising technique for this purpose. Though the present industrial CO2 capture technology uses liquid amine-based sorbents, adsorptive separation methods employing solid sorbents are realized to be the most energy efficient and thereby cost-effective. The large-scale adsorptive separation of CO2 from industrial gas mixtures is achieved using zeolites, activated carbons, silica gels as sorbents. With an ever-growing demand for more energy-saving and thereby cost-effective, and environmentally friendly procedures for gas separation, new-generation sorbents with better efficiency are required. Metal Organic Frameworks (MOFs), made up of metal ions/metaloxo clusters connected via organic linkers, have been recognized as well-suited materials for the application mentioned above. During last decade several reports on gas separation using both Micro/Ultra-micro porous MOFs have appeared. However, MOFs with ultra-micropores (< 6 Å) seems to be the most potential candidate for the gas separation applications, especially for carbon capture. In this context, we have developed few ultra-microporous MOFs (Um-MOFs) which display very interesting CO2 capture characteristics. For example, a Um-MOF, Ni9(μ-H2O)4(H2O)2(4-PyC)18(H2O)17(CH3OH)4(C4H8O)4, IISERP-MOF1, displays the best properties suitable for pre-combustion CO2 capture and H2 purification. While, another MOF, Ni(4-PyC)2.DMF, IISERP-MOF2, exhibits lowest parasitic energy (655 KJ/kg CO2) when applied for post-combustion CO2 capture. Following this, we have developed few more 4-PyC based MOFs utilizing Mg, Mn and Cu metal ions. However, these MOFs are dense as ascertained from the crystal structure. In one of the chapters in my thesis, we show how porosity can be introduced into such non-porous frameworks by utilizing coordination flexibility. In another study, we have incorporated different basic functional moieties (triazolyl, imidazolyl and benzimidazole) to form Um-MOFs and have investigated the atomic-level details about their adsorption sites using a combined x experimental-computational approach. Our investigations include (i) the triazolyl and imidazolyl functionalized MOFs with hydrophobic methyl group-lined pores for humid CO2 capture; (ii) understanding the adsorption characteristics of MOF simultaneously lined with strong adsorptions sites of different chemistry.